PrP-LIKE GENE

Abstract: The present invention provides nucleic acids encoding the Doppel ("Dpl") protein, Dpl peptides, and assays utilizing the Dpl nucleic acids and/or peptides. In related aspects the invention features expression vectors and host cells comprising nucleic acids that encode a human Dpl polypeptide. The present invention also relates to antibodies that bind specifically to a human Dpl polypeptide, methods for producing human Dpl polypeptides, methods for identifying cells expressing Dpl, methods for using the Dpl gene and the Dpl polypeptide to alter cellular function and prion infectivity in culture or <i>in vivo</i>, and identification of individuals at risk for prion disorders by detecting alteration in Dpl coding and regulatory sequences and Dpl expression levels.

Description: PrP-LIKE GENE FIELD OF THE INVENTION This invention relates to nucleic acids, proteins encoded by such nucleic acids, and assays involving use of the these nucleic acids and/or proteins BACKGROUND OF THE INVENTION Pπons are infectious pathogens that cause central nervous system spongiform encephalopathies m humans and animals Pnons are distmct from bacteπa, viruses and viroids The predominant hypothesis at present is that no nucleic acid component is necessary for infectivity of prion protem Further, a pπon which infects one species of animal (e g , a human) will not readily infect another (e g , a mouse) A major step in the study of pπons and the diseases that they cause was the discovery and puπfication of a protem designated pπon protem ("PrP") [Bolton et al , Science 218 1309-11 (1982), Prusmer et al , Biochemistry 21 6942-50 (1982), McKinley et al , CeU 35 57-62 (1983)] Complete pπon protem-encoding genes have smce been cloned, sequenced and expressed m transgenic animals PrP c is encoded by a smgle-copy host gene [Basler et al , Cell 46417-28 (1986)] and is normally found at the outer surface of neurons A leading hypothesis is that prion diseases result from conversion of PrP c mto a modified form called PrP Sc It appears that the scrapie isoform of the prion protem (PrP Sc ) is necessary for both the transmission and pathogenesis of the transmissible neurodegenerative diseases of animals and humans See Prusmer. S B . "Molecular biology of pπon disease," Science 252 1515-1522 (1991) The most common pπon diseases of animals are scrapie of sheep and goats and bovine spongiform encephalopathy (BSE) of cattle [Wilesmith, J and Wells, Microbiol Immunol 772 21-38 (1991)] Four pπon diseases of humans have been identified (1) kuru, (2) Creutzfeldt- Jakob Disease (CJD), (3) Gerstaann-Strassler-Sche nker Disease (GSS), and (4) fatal familial insomnia (FFI) [Gajdusek, D C , Science 197 943 -960 ( 1977), Medoπ et al , N Engl J Med 326444-449 ( 1992)] The presentation of human pπon diseases as sporadic, genetic and infectious illnesses initially posed a conundrum which has been explamed by the cellular genetic oπgm of PrP Most CJD cases are sporadic, but about 10-15% are rnheπted as autosomal dominant disorders that are caused by mutations mthe human PrP gene [Hsiao et al , Neurology 40 1820-1827 (1990), Goldfarb et al , Science 258 806-808 (1992). Kitamoto et al , Proc R Soc Lond 343 391- 398 Iatrogemc CJD has been caused by human growth hormone deπved from cadaveric pituitanes as well as dura mater grafts [Brown et al , Lancet 340 24-27 (1992)] Despite numerous attempts to link CJD to an infectious source such as the consumption of scrapie infected sheep meat, none has been identified to date [Harnes-Jones et al , J Neurol Neurosurg Psychiatry 51 1113-1119 (1988)] except in cases of latrogemcally mduced disease On the other hand, kuru, which for many decades devastated the Fore and neighboring tnbes of the New Gumea highlands, is believed to have been spread by infection dunng πtuahstic cannibalism [Alpers, M P , Slow Transmissible Diseases of the Nervous System. Vol 1, S B Prusmer and W J Hadlow. eds (New York Academic Press), pp 66- 90 (1979)] The initial transmission of CJD to experimental primates has a nch history beginning with William Hadlow's recognition of the similaπty between kuru and scrapie In 1959, Hadlow suggested that extracts prepared from patients dymg of kuru be inoculated into nonhuman primates and that the animals be observed for disease that was predicted to occur after a prolonged mcubation peπod [Hadlow, W J , Lancet 2 289-290 (1959)] Seven years later, Gajdusek, Gibbs and Alpers demonstrated the transmissibihty of kuru to chimpanzees after incubation peπods ranging form 18 to 21 months [Gajdusek et al , Nature 209 794-796 (1966)] The similaπty of the neuropathology of kuru with that of CJD [Klatzo et al , Lab Invest 8 799-847 (1959)] prompted similar experiments with chimpanzees and transmissions of disease were reported in 1968 [Gibbs, Jr et al , Science 161 388-389 (1968)] Over the last 25 years, about 300 cases of CJD, kuru and GSS have been transmitted to a vaπety of apes and monkeys The expense, scarcity and often perceived inhumanity of such experiments have restricted this work and thus limited the accumulation of knowledge While the most reliable transmission data has been said to emanate from studies usmg nonhuman primates, some cases of human prion disease have been transmitted to rodents but apparently with less regularity [Gibbs, Jr et al , Slow Transmissible Diseases of the Nervous System. Vol 2, S B Prusmer and W J Hadlow, eds (New York Academic Press), pp 87-110 (1979), Tateishi et al , Prion Diseases of Humans and Animals. Prusmer et al , eds (London Ellis Horwood), pp 129-134 (1992)] The importance of understanding the conversion of PrP c into PrP Sc has been heightened by the possibility that bovine pπons have been transmitted to humans who developed vaπant Creutzfeldt- Jakob disease (vCJD), G Chazot, et al , Lancet 347 1181 (1996), R G Will, et al , Lancet 347921- 925 (1996) Earlier studies had shown that the N-terminus of PrP Sc could be truncated without loss of scrapie infectivity, S B Prusmer. et al , Biochemistry 21 6942-6950 (1982), S B Prusmer, et al , Cell 38 127-134 (1984) and coπespondingly, the truncation of the N-terminus of PrP Sc still allowed its conversion mto PrP bc M Rogers, et al , Proc Natl Acad Sci USA 90 3182-3186 ( 1993) Recent studies have advanced the ability to visualize the structural transition of PrP c to PrP So at a molecular level For example, the N-terminal portion is relatively unstructured and flexible, but assists m stabilizing structural elements withm the C-terminal portion D G Donne et al , Proc Natl Acad Sci USA 94 13452-13457 (1997) Furthermore, immunological studies have demonstrated that N-terminal epitopes are cryptic in PrP Sc , supporting the idea that this region undergoes profound conformational change dunng pπon propagation Peretz et al , J Mol Biol 273 614-622 (1997) Despite these advances, the understanding of the structural biology of the pathogenic conversion process remams incomplete in many wa> s For example, it is unknown exactly which structural regions of PrP c are necessary or sufficient for conformational change to occur It is also unknown which regions of PrP Sc are necessary or sufficient for infectivity Evidence mdicates that pnon stram phenomena and species barπers are encoded by alternative PrP conformations, but the precise structural determinants of these conformations have not yet been precisely identified Telling et al Science 274 2079-2082 (1996), Billeter, et al , Proc Natl Acad Sci USA 94 7281-7285 (1997) Recent studies have identified four residues of mouse PrP (MoPrP) that appear to mteract with protem X, a putative factor postulated to facilitate the conformational change from PrP c to PrP Sc Telling, et al Cell 83 79-90 (1995) All four ammo acids come together to form the putative protem X bmding site in the tertiary structure of recombinant PrIP 90-231 and PrIP 29-23 I D G Donne et al , Proc Natl Acad Sci USA 94 13452-13457 (1997), T L James et al , Proc Natl Acad Sci USA 94 10086-10091 (1997) However, despite several reports of protems which bmd PrP c , the identity of protem X remams elusive Finally, although the structures of refolded, recombinant PrP molecules may resemble PrP c , a structural solution for PrP Sc remams lacking One strategy for determining PrP function is through the identification of protems similar to PrP, and the elucidation of the function of such protems The identification and study of pπon-related genes may offer insight into the general biology and progression of neurodegenerative disorders, as well as offering insight into the mechanistic alterations that result m pπon-mediated disorders There is thus a need m the art for the identification and study of genes encoding protems with similar structure and/or function SUMMARY OF THE INVENTION The present mvention provides nucleic acids encoding the Doppel ("Dpi") protem, Dpi peptides, and assays utilizing the Dpi nucleic acids and/or peptides In a particular aspect, the Dpi protem is encoded by the nucleotide sequence of SEQ ID NO 1 and degenerate sequences thereof In addition, the invention features isolated nucleic acid sequences compπsmg a Dpi promoter, as well as nucleic acid sequences that hybπdize under strmgent conditions to SEQ ID NO 1 In related aspects the invention features expression vectors and host cells compnsmg nucleic acids that encode a human Dpi polypeptide The present invention also relates to antibodies that bmd specifically to a human Dpi polypeptide, methods for producing human Dpi polypeptides, methods for identifying cells expressmg Dpi, methods for usmg the Dpi gene and the Dpi polypeptide to alter cellular function and pπon -->- infectivity in culture or in vivo, and identification of individuals at nsk for degenerative disorders by detecting alteration m Dpi coding and regulatory sequences and Dpi expression levels A primary object of the mvention is to provide an isolated human Dpi polypeptide-encoding nucleic acids for use m expression of human Dpi (e g, m a recombinant host cell) and for use m, for example, identification of human Dpi polypeptide bmdmg compounds (especially those compounds that affect human Dpi polypeptide-mediated activity, which compounds can be used to modulate Dpi activity) Another object of the invention is to provide an isolated human Dpi polypeptide-encoding nucleic acid for use m generation of non-human transgemc animal models for Dpi gene function, particularly "knock-in" Dpi non-human transgemc animals characteπzed by excess or ectopic expression of the Dpi gene In particular, Dpi appears to have a synergistic interaction with the products of the PrP locus, and mutations in the Dpi locus decreases the incubation period of animals infected with the scrapie form of PrP, PrP Sc DNA encodmg Dpi is also disclosed for use of the DNA expression product m an assay to modulate the mcubation time needed for pπon-mediated disorders, as well as to identify compounds that modulate the mcubation time necessary for the measurable onset of symptoms of pπon infection The mvention also features DNA sequences that hybπdize under strmgent conditions to DNA encodmg Dpi, or to DNA complementary to DNA encoding Doppel Such DNA sequences are preferably at least 25 nucleotides in length, more preferably 50 nucleotides m length Peptide fragments of Dpi, e g those useful m assays, are preferably at least 10 ammo acids m length, more preferably at least 30 ammo acids m length The mvention also features chimeπc Dpi genes having sequences from both a host animal and a genetically diverse animal, and transgemc animals containing such genes An object of the mvention is to provide a nucleotide sequence encodmg a novel Dpi protem and its functional equivalents Another object is to provide a cell lme genetically engmeered to express the nucleotide sequence encodmg Dpi Another object is to provide an antibody which selectively binds to the Dpi protem or peptides thereof Another object is to provide a method whereby a compound or library of compounds can be assayed for their ability to activate or block the activity of the protem expressed by the Dpi nucleotide sequence An advantage of the present mvention is that mutations that mcrease Dpi activity and/or expression accelerate pπon infection, thus allowmg an earlier identification of pπon infectivity from an moculum in animal assays, e g m transgemc HuPrP mouse assays An aspect of the invention is transgemc mice expressmg (1) human Dpi, (2) human Dpi and human PrP, (3) human/mouse chimenc genes for Dpi and PrP, (4) bovme Dpi, (5) bovme Dpi and bovme PrP, (6) bovine/mouse chimeπc genes for Dpi and PrP, (7) combmation of (1-6) with an ablated endogeneous Dpi and/or PrP gene, (7) sheep Dpi, (8) goat Dpi, (9) elk Dpi, and (10) deer Dpi Transgemc mice that may be used m the present mvention are descπbed m U S Patents 5,565, 186 issued October 15, 1996, 5,763.740 issued June 9, 1998, 5,789,655 issued August 4, 1998. and 5,792,901, issued August 11, 1998, which are incorporated by reference herem These and other objects, advantages and features of the present mvention will become apparent to those skilled m the art upon reading the disclosure BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a schematic diagram illustrating the genomic structure and codmg regions of the Dpi (PrnD) gene locus Figure IB illustrates the splice vaπants of Dpi, as well as identified ESTs m this region Figure 2A-2E is the nucleotide and coπespondmg ammo acid sequence of the mouse Dpi gene Figure 3 is a schematic diagram illustrating the alternative splice donor sites of Exon 1 Figure 4 is a structural overview of Prn gene region, with the most abundant forms Prnp and PrnD spliced mRNAs (thin lmes) shown at the top of the figure Chimeπc cDNAs are shown m bold lmes, with the positions of ohgonucleotide primers (arrows) used for RT-PCR shown above the chromosomal gene Figure 5 is a schematic diagram illustrating the structure of Dpi cDNA vaπeties Prnp exon 2 and PrnD exon 2 sequences common to all cDNAs are shown by bold cross-hatch and open boxes, respectively Figure 6 is a schematic diagram illustrating the structures of PrP gene disruptions in four lmes of Prnp 0/0 mice The large black arrows indicate the alleles which delete the exon 3 splice acceptor and are associated with the development of a late-onset ataxia A small vertical aπow indicates the position of the exon 3 splice acceptor deleted m the Ngsk and Rcm alleles The open box is the PrP codmg region and the gray box is the PrP UTR Enzymes are as follows E, Eco Rl, X, Xba I, K, Kpn I -->- Figure 7 and 8 illustrate multiple sequence alignment of human (SEQ ID NO 2), mouse (SEQ ID NO 3) and rat (SEQ ID NO 4) Dpi with representative PrP molecules showmg the regions of identity and conservation Figure 9 is a schematic of the Dpi and PrP protein structure showmg the positions of the predicted features of each Three alpha helices found m PrP are shown as grey boxes with A, B and C representmg the three predicted Dpi helices The white box with the + symbols indicate a cluster of basic residues DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Before the present DNA molecule, protems, and methods of use are descπbed, it is to be understood that this mvention is not limited to particular molecules descnbed, as such may, of course, vary It is also to be understood that the terminology used herem is for the purpose of descπbmg particular embodiments only, and is not mtended to be limiting, smce the scope of the present invention will be limited only by the appended claims Unless defined otherwise, all technical and scientific terms used herem have the same meaning as commonly understood by one of ordinary skill m the art to which this mvention belongs Although any methods and matenals similar or equivalent to those descπbed herem can be used m the practice or testmg of the present invention, the prefened methods and matenals are now descπbed All publications mentioned herem are incorporated herem by reference to disclose and descπbe the methods and/or matenals m connection with which the publications are cited The publications discussed herem are provided solely for their disclosure pπor to the filing date of the present application Nothing herem is to be construed as an admission that the present mvention is not entitled to antedate such publication by virtue of pπor mvention Further, the dates of publication provided may be different from the actual publication dates which may need to be mdependently confirmed DEFINITIONS The term "nucleic acid" as used herem refers to an ohgonucleotide, nucleotide, and fragments or portions thereof, as well as to peptide nucleic acids (PNA), fragments, portions or antisense molecules thereof, and to DNA or RNA of genomic or synthetic oπgm which can be smgle- or double-stranded, and represent the sense or antisense strand Where "nucleic acid" is used to refer to a specific nucleic acid sequence (e g a Dpi polypeptide-encoding nucleic acid), "nucleic acid" is meant to encompass nucleic acids that encode a polypeptide that is functionally equivalent to the recited polypeptide, e g , nucleic acids that are degenerate vanants, or nucleic acids that encode biologically active vanants or fragments of the recited polypeptide Similarly, "polypeptide" as used herem refers to an ohgopeptide, peptide, or protem Where "polypeptide" is recited herem to refer to an ammo acid sequence of a naturally-occurring protem molecule, "polypeptide" and like terms are not meant to limit the ammo acid sequence to the complete, native ammo acid sequence associated with the recited protem molecule By "antisense nucleic acid" is mean a nucleic acid havmg a nucleotide sequence complementary to a given nucleic acid sequence (e g, a nucleic acid sequence encodmg a Dpi polypeptide) mcludmg nucleic acid sequences associated with the transcnption or translation of the given nucleic acid sequence (e g , a promoter of a nucleic acid encodmg a Dpi polypeptide), where the antisense nucleic acid is capable of hybndizmg to a Dpi polypeptide-encoding nucleic acid sequence Of particular mterest are antisense nucleic acids capable of inhibiting transcnption and/or splicing and/or translation of a Dpl-encodmg nucleic acid either in vitro or in vivo "Peptide nucleic acid" as used herem refers to a molecule which compπses an ohgomer to which an ammo acid residue, such as lysme, and an ammo group have been added These small molecules, also designated anti-gene agents, stop transcπpt elongation by bmdmg to their complementary (template) strand of nucleic acid (Nielsen et al , Anticancer Drug Des 8 53-63 (1993)) By a "polypeptide" is meant any cham of ammo acids, regardless of length or post- translational modification (e g , glycosylation) As used herem, "Dpi polypeptide" refers to an ammo acid sequence of a recombmant or nonrecombinant polypeptide havmg an ammo acid sequence of l) a native Dpi polypeptide, u) a biologically active fragment of a Dpi polypeptide, m) biologically active polypeptide analogs of a Dpi polypeptide, or iv) a biologically active variant of a Dpi polypeptide Dpi polypeptides of the mvention can be obtained from any species, e g , mammalian or non-mammalian (e g , reptiles, amphibians, avian (e g , chicken)), particularly mammalian, mcludmg human, rodent (e g . murine or rat), bovme, ovme, porcine, murme, or equine, preferably rat or human, from any source whether natural, synthetic, semi-synthetic or recombmant "Human Dpi polypeptide" refers to the ammo acid sequences of isolated human Dpi polypeptide obtained from a human, and is meant to mclude all naturally-occurring allehc vaπants, and is not meant to limit the ammo acid sequence to the complete, native ammo acid sequence associated with the recited protem molecule As used herem, "antigenic ammo acid sequence" means an ammo acid sequence that, either alone or m association with a earner molecule, can elicit an antibody response in a mammal A "vanant" of a human Dpi polypeptide is defined as an am o acid sequence that is altered by one or more ammo acids The vanant can have "conservative" changes, wherem a substituted ammo acid has similar structural or chemical properties, e g , replacement of leucme with isoleucme More rarely, a vanant can have "nonconservative" changes, e g , replacement of a glycme with a tryptophan Similar minor variations can also include amino acid deletions or insertions, or both Guidance in determining which and how many amino acid residues may be substituted, inserted or deleted without abolishing biological or immunological activity can be found using computer programs well known in the art, for example, DNAStar software. A "deletion" is defined as a change in either amino acid or nucleotide sequence in which one or more amino acid or nucleotide residues, respectively, are absent as compared to an amino acid sequence or nucleotide sequence of a naturally occurring Dpi polypeptide. An "insertion" or "addition" is that change in an amino acid or nucleotide sequence which has resulted in the addition of one or more amino acid or nucleotide residues, respectively, as compared to an amino acid sequence or nucleotide sequence of a naturally occurring Dpi polypeptide. A "substitution" results from the replacement of one or more amino acids or nucleotides by different amino acids or nucleotides, respectively as compared to an amino acid sequence or nucleotide sequence of a naturally occurring Dpi polypeptide. The term "biologically active" refers to human Dpi polypeptide having structural, regulatory, or biochemical functions of a naturally occuπing Dpi polypeptide. Likewise, "immunologically active" defines the capability of the natural, recombinant or synthetic human Dpi polypeptide, or any oligopeptide thereof, to induce a specific immune response in appropriate animals or cells and to bind with specific antibodies. The term "derivative" as used herein refers to the chemical modification of a nucleic acid encoding a human Dpi polypeptide or the encoded human Dpi polypeptide. Illustrative of such modifications would be replacement of hydrogen by an alkyl, acyl, or amino group. A nucleic acid derivative would encode a polypeptide which retains essential biological characteristics of a natural Dpi polypeptide. As used herein the term "isolated" is meant to describe a compound of interest (e.g. , either a nucleic acid or a polypeptide) that is in an environment different from that in which the compound naturally occurs. "Isolated" is meant to include compounds that are within samples that are substantially enriched for the compound of interest and/or in which the compound of interest is partially or substantially purified. As used herein, the term "substantially purified" refers to a compound (e.g., either a nucleic acid or a polypeptide) that is removed from its natural environment and is at least 60% free, preferably 75% free, and most preferably 90% free from other components with which it is naturally associated. "Stringency" typically occurs in a range from about Tm-5°C (5°C below the Tm of the probe) to about 20°C to 25 °C below Tm. As will be understood by those of skill in the art, a hybridization stringency can be manipulated to identify or detect identical nucleic acid sequences or to identify or detect similar or related nucleic acid sequences. The term "hybπdization" as used herem shall include "any process by which a strand of nucleic acid joins with a complementary strand through base pairing" (Coombs, Dictionary of Biotechnology. Stockton Press, New York NY (1994)) Amplification as earned out m the polymerase cham reaction technologies is descnbed m Dieffenbach et al , PCR Pnmer. a Laboratory Manual. Cold Spnng Harbor Press, Plamview NY (1995) By "transformation" is meant a permanent or transient genetic change, preferably a permanent genetic change, mduced m a cell following incorporation of new DNA (/ e , DNA exogenous to the cell) Genetic change can be accomplished either by incorporation of the new DNA mto the genome of the host cell, or by transient or stable maintenance of the new DNA as an episomal element Where the cell is a mammalian cell, a permanent genetic change is generally achieved by mtroduction of the DNA mto the genome of the cell By "construct" is meant a recombmant nucleic acid, generally recombmant DNA, that has been generated for the purpose of the expression of a specific nucleotide sequence(s), or is to be used m the construction of other recombmant nucleotide sequences By "operably linked" is meant that a DNA sequence and a regulatory sequence(s) are connected in such a way as to permit gene expression when the appropπate molecules (e g , transcπptional activator protems) are bound to the regulatory sequence(s) By "operatively inserted" is meant that a nucleotide sequence of mterest is positioned adjacent a nucleotide sequence that directs transcnption and translation of the mtroduced nucleotide sequence of mterest (i e , facilitates the production of, e g , a polypeptide encoded by a Dpi sequence) By "Dpi associated disorder" is meant a physiological condition or disease associated with altered Dpi function (e g , due to abeπant Dpi expression or a defect m Dpi expression or m the Dpi protein) Such Dpi associated disorders can mclude, but are not necessanly limited to, pπon-like disorders or other similar disorders that mvolve neurotoxicity and/or neurodegeneration The term "transgene" is used herem to descnbe genetic mateπal which has been or is about to be artificially inserted mto the genome of a mammalian, particularly a mammalian cell of a living ammal By "transgemc organism" is meant a non-human organism (e g , smgle-cell organisms (e g , yeast), mammal, non-mammal (e g , nematode or Drosophila)) havmg a non-endogenous (i e , heterologous) nucleic acid sequence present as an extrachromosomal element m a portion of its cells or stably mtegrated mto its germ lme DNA By "transgemc ammal" is meant a non-human animal, usually a mammal, havmg a non- endogenous (. e , heterologous) nucleic acid sequence present as an extrachromosomal element m a portion of its cells or stably mtegrated mto its germ lme DNA (i e , m the genomic sequence of most or all of its cells) Heterologous nucleic acid is mtroduced mto the germ lme of such transgemc animals by genetic manipulation of, for example, embryos or embryonic stem cells of the host animal A mouse is a prefened transgemc animal A "knock-out" of a target gene means an alteration m the sequence of the gene that results m a decrease of function of the target gene, preferably such that target gene expression is undetectable or msigmficant A knock-out of a Dpi gene means that function of the Dpi gene has been substantially decreased so that Dpi expression is not detectable or only present at msigmficant levels "Knock-out" transgenics of the mvention can be transgemc animals havmg a heterozygous knock-out of the Dpi gene or a homozygous knock-out of the Dpi gene "Knock-outs" also include conditional knock-outs, where alteration of the target gene can occur upon, for example, exposure of the animal to a substance that promotes target gene alteration, mtroduction of an enzyme that promotes recombmation at the target gene site (e g , Cre m the Cre-lox system), or other method for directmg the target gene alteration postnatally A "knock-in" of a target gene means an alteration m a host cell genome that results in altered expression (e g , increased (includmg ectopic) or decreased expression) of the target gene, e g , by mtroduction of an additional copy of the target gene, or by operatively insertmg a regulatory sequence that provides for enhanced expression of an endogenous copy of the target gene "Knock-in" transgenics of the mvention can be transgemc animals havmg a heterozygous knock-in of the Dpi gene or a homozygous knock-in of the Dpi gene "Knock-ins" also encompass conditional knock-ins The term "Prnp 0/0 or Prnp-Abl" refers to a transgemc ammal which has its PrP gene ablated with the " 0/0 " mdicatmg that both alleles are ablated whereas o/+ indicates only one is ablated Specifically, the ammal bemg refeπed to is generally a transgemc mouse which has its PrP gene ablated i e , a PrP knockout mouse In that the PrP gene is disrupted, no mouse PrP protem is expressed The term "pπon" shall mean an infectious particle known to cause diseases (spongiform encephalopathies) m animals mcludmg cows and humans The term "pnon" is a contraction of the words "protem" and "infection" and the particles are compnsed largely if not exclusively of PrP Sc molecules encoded by a PrP gene Pnons are distmct from bacteπa, viruses and viroids Known pnons include those which infect animals to cause scrapie, a transmissible, degenerative disease of the nervous system of sheep and goats as well as bovme spongiform encephalopathies (BSE) or "mad cow" disease and fel e spongiform encephalopathies of cats Four pπon diseases known to affect humans are (1) kuru, (2) Creutzfeldt-Jakob Disease (CJD), (3) Gerstmann-Straussler-Scheinker Disease (GSS), and (4) fatal familial insomnia (FFI) As used herein pπon mcludes all forms of pπons causmg all or any of these diseases or others m any animals used — and m particular m humans, cows and other domesticated farm animals The terms "PrP gene" and "Prnp gene" are used interchangeably herein to describe genetic material which expresses proteins (for example those shown in Figures 3-5 of U.S. Patent 5,565,186 issued October 15, 1996) and polymorphisms and mutations such as those listed herein under the subheading "Pathogenic Mutations and Polymorphisms." The PrP gene can be from any animal including the "host" and "test" animals described herein and any and all polymorphisms and mutations thereof, it being recognized that the terms include other such PrP genes that are yet to be discovered. The term "artificial PrP gene" is used herein to encompass the term "chimeric PrP gene" as well as other recombinantly constructed genes which when included in the genome of a host animal (e.g., a mouse) will render the mammal susceptible to infection from prions which naturally only infect a genetically diverse test mammal, e.g., human, bovine or ovine. In general, an artificial gene will include the codon sequence of the PrP gene of the mammal being genetically altered with one or more (but not all, and generally less than 40) codons of the natural sequence being replaced with a different codon — preferably a coπesponding codon of a genetically diverse mammal (such as a human). The genetically altered mammal being used to assay samples for prions which only infect the genetically diverse mammal. Examples of artificial genes are mouse PrP genes encoding the sequence as shown in Figures 3, 4 and 5 of U.S. Patent 5,565,186 with one or more different replacement codons selected from the codons shown in these Figures for humans, cows and sheep replacing mouse codons at the same position, with the proviso that not all the mouse codons are replaced with differing human, cow or sheep codons. Artificial PrP genes of the invention can include not only codons of genetically diverse animals but may include codons and codon sequences associated with genetic prion diseases such as CJD and codons and sequences not associated with any native PrP gene but which, when inserted into an animal render the ammal susceptible to infection with prions which would normally only infect a genetically diverse animal. The terms "chimeric gene ' "chimeric PrP gene" and the like are used interchangeably herein to mean an artificially constructed gene containing the codons of a host animal such as a mouse with one or more of the codons being replaced with coπesponding codons from a genetically diverse test animal such as a human, cow or sheep. In one specific example the chimeric gene is comprised of the starting and terminating sequence (i.e., N- and C- terminal codons) of a PrP gene of a mammal of a host species (e.g. a mouse) and also containing a nucleotide sequence of a coπesponding portion of a PrP gene of a test mammal of a second species (e.g. a human). A chimeric gene will, when inserted into the genome of a mammal of the host species, render the mammal susceptible to infection with prions which normally infect only mammals of the second species. The prefened chimeric gene disclosed herein is MHu2M which contains the starting and terminating sequence of a mouse PrP gene and a non-terminal sequence region which is replaced with a coπesponding human sequence which differs from a mouse PrP gene m a manner such that the protem expressed thereby differs at nine residues The terms "host animal" and "host mammal" are used to descnbe animals which will have their genome genetically and artificially manipulated so as to mclude genetic matenal which is not naturally present within the animal For example, host animals mclude mice, hamsters and rats which have their endogenous PrP gene altered by the insertion of an artificial gene of the present mvention or by the insertion of a native PrP gene of a genetically diverse test animal The terms "host animal" and "host mammal" are used to descnbe animals which will have their genome genetically and artificially manipulated so as to mclude genetic matenal which is not naturally present withm the ammal For example, host animals mclude mice, hamsters and rats which have their endogenous Dpi gene altered In a preferred embodiment, these host animals also have a PrP gene altered by the insertion of an artificial gene or by the insertion of a native PrP gene of a genetically diverse test animal The terms "test animal" and "test mammal" are used to describe the animal which is genetically diverse from the host animal m terms of differences between the PrP gene of the host ammal and the PrP gene of the test animal The test animal may be any animal for which one wishes to run an assay test to determine whether a given sample contains pπons with which the test a mal would generally be susceptible to infection For example, the test ammal may be a human, cow, sheep, pig, horse, cat, dog, turkey or chicken, and one may wish to determine whether a particular sample mcludes pnons which would normally only mfect the test animal This is done by mcludmg PrP gene sequences of the test animal mto the host ammal and moculatmg the host animal with pnons which would normally only infect the test ammal The terms "genetically diverse animal" and "genetically diverse mammal" are used to descnbe an an mal which mcludes a native PrP codon sequence of the host ammal which differs from the genetically diverse test animal by 17 or more codons, preferably 20 or more codons, and most preferably 28-40 codons Thus, a mouse PrP gene is genetically diverse with respect to the PrP gene of a human, cow or sheep, but is not genetically diverse with respect to the PrP gene of a hamster The terms "ablated PrP gene", "disrupted PrP gene" and the like are used interchangeably herem to mean an endogenous PrP gene which has been altered (e g , add and/or remove nucleotides) m a manner so as to render the gene moperative Examples of non-functional PrP genes and methods of making such are disclosed in Bueler, et al, Nature 356577-582 (1992) which is incorporated herem by reference Both alleles of the genes are preferably disrupted The terms "ablated Dpi gene", "disrupted Dpi gene" and the like are used interchangeably herem to mean an endogenous Dpi gene which has been altered (e g , add and/or remove nucleotides) m a manner so as to render the gene moperative The terms "hybnd animal", "transgemc hybnd animal" and the like are used interchangeably herem to mean an ammal obtained from the cross-breeding of a first animal havmg altered Dpi activity and a second animal which mcludes (1) an ablated endogenous PrP gene (1) a chrmenc or artificial PrP gene and/or (3) a PrP gene from a genetically diverse animal For example a hybπd mouse is obtained by cross-breedmg a mouse havmg a Dpi transgene knock-in with a mouse containing (1) bovme PrP genes (which may be present m high copy numbers) alone or with (2) chimeπc PrP genes In another example, a hybnd mouse can be obtained by cross-breedmg a PrP 00 mouse with unregulated Dpi activity and a mouse havmg an mducible exogenous human PrP gene The term hybnd mcludes any offsprmg of a hybπd mcludmg mbred offsprmg of two hybnds provided the resulting offsprmg is susceptible to infection with pnons with normal infect only a genetically diverse species and the symptoms of the infection are observable m about 350 days or less, preferably 250 or less The term "mcubation time" shall mean the time from inoculation of an animal with a pπon until the time when the a mal first develops detectable symptoms of disease resultmg from the infection A reduced mcubation time is preferably about 200 days ± 50 days or less, more preferably about 50 days ± 20 days or less By "antibody" is meant an immunoglobulin protem which is capable of bmdmg an antigen Antibody as used herem is meant to mclude the entire antibody as well as any antibody fragments (e g F(ab') 2 , Fab', Fab, Fv) capable of bmdmg the epitope, antigen or antigemc fragment of mterest Antibodies of the invention are lmmunoreactive or lmmunospecific for and therefore specifically and selectively bmd to a Dpi protem Antibodies for Dpi are preferably rmmunospecific — i e , not substantially cross-reactive with related matenals Although the term "antibody" encompasses all types of antibodies (e g , monoclonal) the antibodies of the mvention are preferably produced usmg the phage display methodology descnbed herem The prefened antibody of the mvention is a punfied antibody By puπfied antibody is meant one which is sufficiently free of other protems, carbohydrates, and hpids with which it is naturally associated Such an antibody "preferentially binds" to a Dpi protem (or an antigemc fragment thereof), i e , does not substantially recognize and bmd to other antigemcally-unrelated molecules By "specifically activates", as used herem, is meant an agent which activates Dpi or a fragment or analog thereof to initiate Dpl-mediated biological events as descπbed herem, but which does not substantially bmd other molecules m a sample, e g , a biological sample By "specifically inhibits", as used herem, is meant an agent which inhibits activation of Dpi or a polypeptide, fragment or analog thereof Preferably, the agent activates or inhibits the biological activity in vivo or in vitro of the protem to which it binds By "substantial increase" is meant an increase m activity or other measurable phenotypic charactenstic that is at least approximately a 2-fold mcrease over control level (where control assays are performed m the absence of activator), preferably at least approximately a 5 -fold mcrease, more preferably at least approximately a 10-fold mcrease m activity over a control assay By "substantial decrease" or "substantial reduction" is meant a decrease or reduction m activity or other measurable phenotypic charactenstic that is approximately 80% of the control level, preferably reduced to approximately 50% of the control level, or more preferably reduced to approximately 10% or less of the control level The terms "screening method" and "assay method" are used to descnbe a method of screemng a candidate compound for its ability to act as an activator or suppressor of 1) Dpi activity and/or 2) pnon infection mcubation time The terms "treatment", "treating", "treat" and the like are used herem to generally mean obtaining a desired pharmacologic and/or physiologic effect The effect may be prophylactic in terms of completely or partially preventmg a disease or symptom thereof and/or may be therapeutic m terms of a partial or complete cure for a disease and/or adverse effect attributable to the disease "Treatment" as used herem covers any treatment of a disease in a mammal, particular a human, and mcludes (a) preventmg the disease or symptom from occurring m a subject which may be predisposed to the disease or symptom but has not yet been diagnosed as having it, (b) inhibiting the disease symptom, i e , anestmg its development, or (c) relieving the disease symptom, i e , causing regression of the disease GENERAL ASPECTS OF THE INVENTION Descnbed herem is a novel 179 residue protem designated Dpi (Dpi) with -25% identity to all known pπon protems (PrP) Smce database searches to identify PrP -like genes failed to be informative, and hybndization studies were not fruitful m such efforts (Westaway et al , Nucleic Acids Res 14 2035-2044 (1986)), the sequencmg of large cosmid clones containing the PrP gene was undertaken (Lee et al , Genome Res 8 1022-1037 (1998)) While most vertebrate genes of related function are not ananged m clusters, some related genes are In studies of the regions around the human, sheep and mouse PrP genes, no additional open readmg frames (ORF) were identified Only when the sequencmg of a cosmid clone isolated from a Prnp b/b (ILN/J) mouse was extended downstream of PrP was a novel ORF found The Dpi locus, called "Prnd" is 16Kb downstream of the PrP gene Prnp and produces two major transcripts of 1 7 and 2 7 Kb as well as some unusual chimeπc transcπpts generated by mtergemc splicmg with Prnp Polycistromc mRNA between PrP and Dpi have also been identified Like PrP, Dpi mRNA is expressed dunng embryogenesis but m contrast to PrP, it is expressed at low levels m the CNS but at high levels m the testis Dpi is unregulated m the CNS of the two Prnp 0/0 lmes that develop a late-onset ataxia and Purkinje cell degeneration, but not m a Prnp 00 lme that does not develop ataxia The Dpi may be neurotoxic which explams why some lmes of Prnp 0/0 mice develop neurodegeneration Dpi may also be toxic to other non neuronal cell types Moreover, the substantial homology between Dpi and PrP suggests these two protems may share some biological properties and as such, Dpi may compnse a new element for understanding pπon biology By identifying Prnd as the first candidate for a PrP-hke gene, we have begun to define the Prn gene family Interestingly, overexpression of Dpi, like some mutant and foreign PrPs, produces neurodegeneration m the CNS of mice In addition, Dpi and PrP may mteract directly or indirectly by competing as ligands for a common receptor protem The mvention descnbed herem provides a means for testmg these hypotheses Dpi Nucleic Acid The terms "Dpi gene" and "Prnd" are used geneπcally to designate the codmg region of Dpi "Dpi gene" and "Prnd" are also mtended to mean the open readmg frame encodmg specific Dpi polypeptides, mtrons, and adjacent 5 ' and 3 ' non-codmg nucleotide sequences mvolved m the regulation of expression, up to about 1 kb beyond the transcnbed region, but possibly further m either direction The DNA sequences encodmg Dpi may be cDNA or genomic DNA or a fragment thereof The gene may be introduced mto an appropπate vector for extrachromosomal maintenance or for mtegration mto the host The term "cDNA" as used herem is mtended to mclude all nucleic acids that share the anangement of sequence elements found m native mature mRNA species, where sequence elements are exons (e g , sequences encoding open readmg frames of the encoded polypeptide) and 3' and 5' non- codmg regions Normally mRNA species have contiguous exons, with the mtervemng mtrons removed by nuclear RNA splicmg, to create a contmuous open readmg frame encodmg the Dpi polypeptide While other genomic Dpi sequences of other sources may have non-contiguous open readmg frames (e g , where mtrons interrupt the protem codmg regions), the human genomic Dpi sequence has no mtrons interrupting the codmg sequence A genomic sequence of mterest compπses the nucleic acid present between the initiation codon and the stop codon, as defined m the listed sequences, mcludmg all of the mtrons that are normally present m a native chromosome It may further mclude the 3 ' and 5 ' untranslated regions found m the mature mRNA It may further include specific transcπptional and translational regulatory sequences, such as promoters, enhancers, etc , mcludmg about 1 kb, but possibly more, of flanking genomic DNA at either the 5' or 3' end of the transcnbed region The genomic DNA may be isolated as a fragment of 100 kbp or smaller; and substantially free of flanking chromosomal sequence. The sequence of this 5' region, and further 5' upstream sequences and 3' downstream sequences, may be utilized for promoter elements, including enhancer binding sites, that provide for expression in tissues where Dpi is expressed. The sequences of the Dpi promoter elements of the invention can be based on the nucleotide sequences of any species (e.g., mammalian or non- mammalian (e.g., reptiles, amphibians, avian (e.g., chicken)), particularly mammalian, including human, rodent (e.g., murine or rat), bovine, ovine, porcine, murine, or equine, preferably rat or human) and can be isolated or produced from any source whether natural, synthetic, semi-synthetic or recombinant. The tissue specific expression of Dpi is useful for determining the pattern of expression, and for providing promoters that mimic the native pattern of expression. Naturally occurring polymorphisms in the promoter region are useful for deteπnining natural variations in expression, particularly those that may be associated with disease. Alternatively, mutations may be introduced into the promoter region to determine the effect of altering expression in experimentally defined systems. Methods for the identification of specific DNA motifs involved in the binding of transcriptional factors are known in the art, e.g. sequence similarity to known binding motifs, gel retardation studies, etc. For examples, see Blackwell et al, Mol Med 7:194-205 (1995); Mortlock et al., Genome Res. 6: 327-33 (1996); and Joulin et al., Eur J Biochem 232: 620-626 (1995). In one embodiment, the promoter is used to modulate Dpi expression. As discussed below, Dpi is expressed in embryonic cardiac tissue and a subset of neurons in the adult brain. Thus, the developmentally timed expression directed by the Dpi promoter can be exploited to facilitate expression of heterologous genes operably linked to the Dpi promoter. Exemplary genes of interest that can be expressed from the Dpi promoter include, but are not necessarily limited to: marker genes (e.g. , for marking the neuronal cells expressing Dpi) and reporter genes (e.g. , luciferase, CAT, etc. ) Such marker and reporter genes can be used to aid in elucidation of Dpi' s normal physiological function, in identifying mechanisms for regulating Dpi and/or to search for bioactive agents (e.g. , candidate pharmaceutical agents) that regulate Dpi expression, and the like. The regulatory sequences may be used to identify cis acting sequences required for transcriptional or translational regulation of Dpi expression, especially in different tissues or stages of development, and to identify cis acting sequences and trans acting factors that regulate or mediate Dpi expression. Such transcriptional or translational control regions may be operably linked to a Dpi gene or other genes in order to promote expression of wild type or altered Dpi or other proteins of interest in cultured cells, or in embryonic, fetal or adult tissues, and for gene therapy. Dpi transcriptional or translational control regions can also be used to identify extracellular signal molecules that regulate Dpi promoter activity, and thus regulate Dpi expression The nucleic acid compositions used m the subject mvention may encode all or a part of the Dpi polypeptides as appropπate The Dpi sequences may be directed to forms of Dpi associated with disease states and/or to naturally occurring vaπants of the protem Fragments may be obtamed of the DNA sequence by chemically synthesizing ohgonucleotides m accordance with conventional methods, by restnction enzyme digestion, by PCR amplification, etc For the most part, DNA fragments will be of at least about ten contiguous nucleotides, usually at least about 15 nt, more usually at least about 18 nt to about 20 nt, more usually at least about 25 nt to about 50 nt Such small DNA fragments are useful as primers for PCR, hybndization screemng, etc Larger DNA fragments, . e greater than 100 nt are useful for production of the encoded polypeptide For use m amplification reactions, such as PCR, a pair of primers will be used The exact composition of the primer sequences is not cntical to the mvention, but for most applications the primers will hybπdize to the subject sequence under strmgent conditions, as known m the art It is preferable to choose a pair of primers that will generate an amplification product of at least about 50 nt, preferably at least about 100 nt Algonthms for the selection of primer sequences are generally known, and are available m commercial software packages Amplification primers hybndize to complementary strands of DNA, and will prime towards each other The Dpi gene is isolated and obtamed m substantial punty, generally as other than an intact mammalian chromosome Usually, the DNA will be obtamed substantially free of other nucleic acid sequences that do not mclude a Dpi sequence or fragment thereof, generally bemg at least about 50%, usually at least about 90% pure and are typically "recombmant", / e flanked by one or more nucleotides with which it is not normally associated on a naturally occurring chromosome The DNA sequences are used in a vanety of ways They may be used as probes for identifying other genes encodmg pnon-like protems, or for identifying Dpi homologs m vaπous species Mammalian homologs have substantial sequence similanty to one another, . e at least 75%, usually at least 90%, more usually at least 95% sequence identity Sequence similanty is calculated based on a reference sequence, which may be a subset of a larger sequence, such as a conserved motif, codmg region, flanking region, etc A reference sequence will usually be at least about 18 nt long, more usually at least about 30 nt long, and may extend to the complete sequence that is bemg compared Algonthms for sequence analysis are known m the art, such as BLAST, descnbed m Altschul et al , J Mol Biol 275 403-10 (1990) Nucleic acids havmg sequence similanty are detected by hybπdization under low stringency conditions, for example, at 50°C and 6XSSC (0 9 M sahne/0 09 M sodium citrate) and remam bound when subjected to washing at 55°C m 1XSSC (0 15 M sodium chloπde/0 015 M sodium citrate) Sequence identity may be determined by hybπdization under high stringency conditions, for example, at 50°C or higher and 0 1XSSC (15 mM sahne/0 15 mM sodium citrate) By usmg probes, particularly labeled probes of DNA sequences, one can isolate homologous or related genes The source of homologous genes may be any species, e g primate species, particularly human, rodents, such as rats and mice, canines, felmes, bovmes, ovmes, eqmnes, yeast, Drosophύa, Caenhorabditis, etc The Dpl-encodmg DNA may also be used to identify expression of the gene in a biological specimen The manner m which one probes cells for the presence of particular nucleotide sequences, as genomic DNA or RNA. is well established m the literature and does not require elaboration here mRNA is isolated from a cell sample mRNA may be amplified by RT-PCR, usmg reverse transcπptase to form a complementary DNA strand, followed by polymerase cham reaction amplification using primers specific for the subject DNA sequences Alternatively, mRNA sample is separated by gel electrophoresis, transfened to a suitable support, e g nitrocellulose, nylon, etc , and then probed with a fragment of the subject DNA as a probe Other techniques, such as ohgonucleotide ligation assays, in situ hybπdizations, and hybridization to DNA probes anayed on a solid chip may also find use Detection of mRNA hybπdizmg to a Dpi sequence is indicative of Dpi gene expression m the sample The Dpi nucleic acid sequence may be modified for a number of purposes, particularly where they will be used mtracellularly, for example, by bemg joined to a nucleic acid cleaving agent, e g a chelated metal ion, such as iron or chromium for cleavage of the gene, or the like The sequence of the Dpi locus, mcludmg flanking promoter regions and codmg regions, may be mutated m various ways known m the art to generate targeted changes m promoter strength, sequence of the encoded protem, etc The DNA sequence or product of such a mutation will be substantially similar to the sequences provided herem, i e will differ by at least one nucleotide or ammo acid, respectively, and may differ by at least two but not more than about ten nucleotides or ammo acids The sequence changes may be substitutions, insertions or deletions Deletions may further mclude larger changes, such as deletions of a domam or exon Other modifications of mterest mclude epitope tagging, e g with the FLAG system, HA, etc For studies of subcellular localization, fusion protems with green fluorescent proteins (GFP) may be used Such mutated genes may be used to study structure-function relationships of Dpi polypeptides with other polypeptides (e g , Nkx-6 1, which is co-expressed with Dpi), or to alter properties of the protems that affect their function or regulation Such modified Dpi sequences can be used to, for example, generate the transgemc animals Techniques for in vitro mutagenesis of cloned genes are known Examples of protocols for scanning mutations may be found m Gustm et al , Biotechniques 14 22 (1993), Barany, Gene 57 111- 23 (1985), Cohcelli et al , Mol Gen Genet 199 537-9 (1985), and Prentki et al , Gene 29 303-13 (1984) Methods for site specific mutagenesis can be found m Sambrook et al , Molecular Cloning A Laboratory Manual. CSH Press, pp 15 3-15 108 (1989). Werner et al , Gene 72635-41 (1993), Sayers et al , Biotechmques 13 592-6 (1992), Jones et al , Biotechmques 72 528-30 (1992), Barton et al , Nucleic Acids Res 18 7349-55 (1990), Marotti et al , Gene Anal Tech 667-70 (1989), and Zhu, Anal Biochem 777 120-4 (1989) Dpi gene compnsmg portions of the host animal e g , end portions of a host animal and a middle portion of a genetically diverse test animal wherem the middle portion mcludes a specific alterations designed to match that of a disease state of such a host Further, the mvention mcludes, a transgemc animal containing the artificial gene or modulated expression of a Dpi gene from a genetically diverse animal, hybnd transgemc animals which are the offspring of different transgemc animals with each other or with a transgemc ammal that has an ablated endogenous pπon protem gene, a standardized pπon preparation and assay methodology which uses the preparation and genetically altered animals to detect pathogemc pπons in a sample The artificial gene mcludes a sequence such that when it is inserted mto the genome of an animal (such as a mouse), the ammal is rendered susceptible to mfection with pπons which normally would mfect only a specific species of genetically diverse animal (such as a human, cow, sheep, pig, chicken, cat or dog) The artificial Dpi gene may be compπsed partially or completely of an artificial polynucleotide sequence, l e codon sequences not present m any native Dpi gene sequence Alternatively, the artificial gene may be compnsed of the codon sequence of a host animal with one or more codon substitutions bemg made wherem the substitutions are preferably conespondmg Dpi gene codons from a genetically diverse animal, meaning that Dpi gene differs from the Dpi gene of the host animal by 20 or more codons Transgemc ammals containing elevated levels of expression of the Dpi gene which can be obtamed for example, by over expression of the gene with an enhanced promoter and/or with high copy numbers of the natural Dpi gene of a genetically diverse animal are also disclosed Hybπd transgemc ammals mclude animals resultmg from a cross between two transgemc animals and m particular a cross between a transgemc animal containing the entire pnon protem gene of a genetically diverse animal (e g , a mouse containing a human pnon protem gene) and an ammal with its endogenous pπon protem gene disrupted (e g , a mouse with an ablated pπon protem gene) Hybπds also specifically include crossmg a transgemc animal having a chimeric pnon protem gene with an animal with its endogenous pnon protem gene ablated Dpi Transgemc Animals The Dpl-encoding nucleic acids can be used to generate genetically modified non-human ammals or site specific gene modifications in cell lmes The term "transgemc" is mtended to encompass genetically modified animals havmg a deletion or other knock-out of Dpi gene activity, havmg an exogenous Dpi gene that is stably transmitted m the host cells, "knock-in" havmg altered Dpi gene expression, or havmg an exogenous Dpi promoter operably linked to a reporter gene Of particular mterest are homozygous and heterozygous knock-outs of Dpi Transgemc animals may be made through homologous recombmation, where the Dpi locus is altered Alternatively, a nucleic acid construct is randomly mtegrated mto the genome Vectors for stable mtegration mclude plasmids, retroviruses and other animal viruses, YACs, and the like Of mterest are transgemc mammals, preferably a mammal from a genus selected from the group consistmg of Mus (e g , mice), Rattus (e g , rats), Oryctologus (e g , rabbits) and Mesocπcetus (e g , hamsters) More preferably the animal is a mouse which is defective or contains some other alteration m Dpi gene expression or function A "knock-out" animal is genetically manipulated to substantially reduce, or eliminate endogenous Dpi function, preferably such that target gene expression is undetectable or msigmficant Different approaches may be used to achieve the "knock-out" A chromosomal deletion of all or part of the native Dpi homolog may be induced Deletions of the non-codmg regions, particularly the promoter region, 3' regulatory sequences, enhancers, or deletions of gene that activate expression of the Dpi genes A functional knock-out may also be achieved by the mtroduction of an anti-sense construct that blocks expression of the native Dpi gene (for example, see Li et al , Cell 85 319-329 (1996)) Conditional knock-outs of Dpi gene function can also be generated Conditional knock-outs are transgemc ammals that exhibit a defect m Dpi gene function upon exposure of the animal to a substance that promotes target gene alteration, mtroduction of an enzyme that promotes recombmation at the target gene site (e g , Cre m the Cre-loxP system), or other method for directing the target gene alteration For example, a transgemc ammal having a conditional knock-out of Dpi gene function can be produced usmg the Cre-loxP recombmation system (see, e g , Kilby et al , Trends Genet 9 413-421 (1993)) Cre is an enzyme that excises the DNA between two recognition sequences, termed loxP This system can be used m a vanety of ways to create conditional knock-outs of Dpi For example, two mdependent transgemc mice can be produced one transgemc for a Dpi sequence flanked by loxP sites and a second transgemc for Cre The Cre transgene can be under the control of an mducible or developmentally regulated promoter (Gu et al , Cell 73 1155-1164 (1993), Gu et al , Science 265 103- 106 (1994)), or under control of a tissue-specific or cell type-specific promoter (e g , a neuron-specific promoter or bram tissue-specific promoter) The Dpi transgemc is then crossed with the Cre transgemc to produce progeny deficient for the Dpi gene only in those cells that expressed Cre durmg development Transgemc ammals may be made havmg an exogenous Dpi gene For example, the transgemc animal may compnse a "knock-in" of a Dpi gene, such that the host cell genome contains an alteration that results in altered expression (e g , mcreased (mcludmg ectopic) or decreased expression) of a Dpi gene, e g , by mtroduction of an additional copy of the target gene, or by operatively inserting a regulatory sequence that provides for enhanced expression of an endogenous copy of the target gene "Knock-in" transgenics can be transgemc animals having a heterozygous knock-in of the Dpi gene or a homozygous knock-in of the Dpi "Knock-ins" also encompass conditional knock-ins The exogenous gene introduced mto the host cell genome to produce a transgemc ammal is usually either from a different species than the ammal host, or is otherwise altered in its codmg or non- codmg sequence The mtroduced gene may be a wild-type gene, naturally occurring polymoφhism, or a genetically manipulated sequence, for example those previously descnbed with deletions, substitutions or insertions m the codmg or non-codmg regions The mtroduced sequence may encode a Dpi polypeptide, or may utilize the Dpi promoter operably linked to a reporter gene Where the mtroduced gene is a codmg sequence, it is usually operably linked to a promoter, which may be constitutive or mducible, and other regulatory sequences required for expression m the host ammal Specific constructs of mterest mclude, but are not limited to, anti-sense Dpi, or a πbozyme based on a Dpi sequence, which will block Dpi expression, as well as expression of dominant negative Dpi mutations, and over-expression of a Dpi gene A detectable marker, such as lac Z may be mtroduced mto the Dpi locus, where upregulation of expression of the conespondmg Ngn gene will result m an easily detected change m phenotype Constructs utilizing a promoter region of the Dpi genes m combination with a reporter gene or with the codmg region of Dpi are also of mterest Constructs having a sequence encodmg a truncated or altered (e g, mutated) Dpi are also of mterest The modified cells or animals are useful m the study of function and regulation of Dpi and other protems mvolved the CNS and embryonic development Such modified cells or ammals are also useful m, for example, the study of the function and regulation of neurotoxic and/or neurodegenerative processes affected by Dpi, as well as the study of the pnon family of genes in vivo Thus, the transgemc ammals of the mvention are useful m identifying both downstream targets of Dpi and potentially a receptor for Dpi, as such may have a role in the phenotypes associated with defects m Dpi Ammals may also be used m functional studies, drug screemng, etc , e g to determine the effect of a candidate drug on embryonic development, neurodegeneration, or on symptoms associated with disease or conditions associated with Dpi defects (e g , on symptoms associated with neurotoxicity A seπes of small deletions and/or substitutions may be made m the Dpi genes to determine the role of different polypeptide-encoding regions m DNA bmdmg, transcnptional regulation, etc By providing expression of Dpi protem in cells m which it is otherwise not normally produced (e g , ectopic expression), one can mduce changes m cell behavior These ammals are also useful for exploring mechanistic models of neurodegeneration, and m particular the mechanisms mvolved m plaque formation, e g PrP Sc plaques m the brain DNA constructs for homologous recombmation will compπse at least a portion of the Dpi gene with the desired genetic modification, and will mclude regions of homology to the target locus DNA constructs for random mtegration need not mclude regions of homology to mediate recombmation Convemently, markers for positive and negative selection are mcluded Methods for generatmg cells havmg targeted gene modifications through homologous recombmation are known m the art For various techniques for transfectmg mammalian cells, see Keown et al , Methods in Enzvmologv 185 527-537 (1990) For embryonic stem (ES) cells, an ES cell lme may be employed, or embryonic cells may be obtamed freshly from a host, e g mouse, rat, guinea pig, etc Such cells are grown on an appropnate fibroblast-feeder layer or grown m the presence of appropπate growth factors, such as leukemia inhibiting factor (LIF) When ES cells have been transformed, they may be used to produce transgemc animals After transformation, the cells are plated onto a feeder layer m an appropπate medium Cells containing the construct may be detected by employing a selective medium After sufficient time for colomes to grow, they are picked and analyzed for the occuπence of homologous recombmation or mtegration of the construct Those colomes that are positive may then be used for embryo manipulation and blastocyst injection Blastocysts are obtamed from 4 to 6 week old superovulated females The ES cells are trypsimzed, and the modified cells are injected mto the blastocoel of the blastocyst After injection, the blastocysts are returned to each uterine horn of pseudopregnant females Females are then allowed to go to term and the resultmg litters screened for mutant cells havmg the construct By providing for a different phenotype of the blastocyst and the ES cells, chimeπc progeny can be readily detected The chimeric animals are screened for the presence of the modified gene Chimeπc animals havmg the modification (normally chimenc males) are mated with wildtype animals to produce heterozygotes, and the heterozygotes mated to produce homozygotes If the gene alterations cause lethality at some point in development, tissues or organs can be maintained as allogeneic or congemc grafts or transplants, or m in vitro culture Production of an overt morphological defect m smgle or double gene ablated mice may pave the way for precise assignment of Dpi and PrP c functions The role of Dpi overexpression m Purkime cell degeneration or alternatively, of closely linked genes present withm extant Prn BAC and YAC clones (Westaway et al , Proc Natl Acad Sci USA 91, 6418-6422 (1994)) can be assessed by use of heterologous promoter elements It will be of mterest to test the susceptibility of mice, which overexpress Dpi or which harbor null or mutant alleles of PrnD to infection with prions Investigation of genetic function may also utilize non-mammalian models, particularly usmg those organisms that are biologically and genetically well-characterized, such as C elegans, D melanogaster and S cerevisiae For example, transposon (Tel) insertions m the nematode homolog of a Dpi gene or a promoter region of a Dpi gene may be made The Dpi gene sequences may be used to knock-out or to complement defined genetic lesions m order to determme the physiological and biochemical pathways mvolved m function of pnon-like protems It is well known that human genes can complement mutations in lower eukaryotic models Production of Dpi Polypeptides Dpl-encodmg nucleic acid may be employed to synthesize full-length Dpi polypeptides or fragments thereof, particularly fragments conespondmg to functional domains, DNA bmdmg sites. etc , and mcludmg fusions of the subject polypeptides to other protems or parts thereof For expression, an expression cassette may be employed, providing for a transcnptional and translational initiation region, which may be mducible or constitutive, where the codmg region is operably linked under the transcnptional control of the transcnptional initiation region, and a transcnptional and translational termination region Vaπous transcnptional initiation regions may be employed that are functional m the expression host The polypeptides may be expressed m prokaryotes or eukaryotes m accordance with conventional ways, depending upon the purpose for expression For large scale production of the protem, a unicellular orgamsm, such as E coli, B subtihs, S cerevisiae, or cells of a higher orgamsm such as vertebrates, particularly mammals, e g COS 7 cells, may be used as the expression host cells In many situations, it may be desirable to express the Dpi genes m mammalian cells, especially where the encoded polypeptides will benefit from native folding and post-translational modifications Small peptides can also be synthesized m the laboratory With the availability of the polypeptides m large amounts, by employmg an expression host, the polypeptides may be isolated and punfied m accordance with conventional ways A lysate may be prepared of the expression host and the lysate punfied usmg HPLC, exclusion chromatography, gel electrophoresis, affinity chromatography, or other puπfication technique The puπfied polypeptide will generally be at least about 80% pure, preferably at least about 90% pure, and may be up to and mcludmg 100% pure Pure is intended to mean free of other protems, as well as cellular debns The Dpi polypeptides can be used for the production of antibodies, where short fragments provide for antibodies specific for the particular polypeptide, and larger fragments or the entire protem allow for the production of antibodies over the surface of the polypeptide Antibodies may be raised to the wild-type or vanant forms of Dpi, mcludmg forms of Dpi that may be associated with disease states or naturally occurring allehc vaπants Antibodies may be raised to isolated peptides conespondmg to these domains, or to the native protem, e g by immunization with cells expressmg Dpi, immunization with hposomes havmg Dpi polypeptides inserted m the membrane, etc Antibodies are prepared m accordance with conventional ways, where the expressed polypeptide or protem is used as an lmmunogen, by itself or conjugated to known immunogenic earners, e g KLH, pre-S HBsAg, other viral or eukaryotic protems, or the like Vanous adjuvants may be employed, with a senes of injections, as appropnate For monoclonal antibodies, after one or more booster injections, the spleen is isolated, the lymphocytes immortalized by cell fusion, and then screened for high affinity antibody bmdmg The immortalized cells, i e hybndomas, producmg the desired antibodies may then be expanded For further descnption, see Monoclonal Antibodies A Laboratory Manual, Harlow and Lane eds , Cold Spnng Harbor Laboratones, Cold Spnng Harbor, New York, 1988 If desired, the mRNA encodmg the heavy and light chains may be isolated and mutagemzed by cloning m E coli, and the heavy and light chains mixed to further enhance the affinity of the antibody Alternatives to in vivo immunization as a method of raising antibodies mclude bmdmg to phage "display" libraries, usually in conjunction with in vitro affinity maturation Isolation of Dpi Allelic Vanants and Homologues in Other Species Other mammalian Dpi genes can be identified and their function charactenzed usmg the Dpi genes used m the present invention Dpi genes of mterest mclude, but are not limited to, mammalian (e g , human, rodent (e g, muπne, or rat), bovine, felme, canine, and the like) and non-mammalian (e g , chicken, reptile, and the like) Methods for identifymg, isolatmg, sequencmg, and characteπzmg an unknown gene based upon its homology to a known gene sequence are well known m the art (see, e g , Sambrook et al , Molecular Cloning A Laboratory Manual. CSH Press 1989) Drug Screening The animal models of the invention, as well as methods usmg the Dpi polypeptides in vitro, can be used to identify candidate agents that affect Dpi expression (e g , by affecting Dpi promoter function) or that mteract with Dpi polypeptides Agents of mterest can include those that enhance, inhibit, regulate, or otherwise affect Dpi activity and/or expression Agents that alter Dpi activity and/or expression can be used, for example to treat or study disorders associated with mcreased Dpi activity (e g , neurodegenerative disorders), to treat vaπous degenerative or developmental conditions, e g mappropπate neural outgrowth or other conditions that are altered by a change m Dpi expression, activity, and/or conformation, and to facilitate development of PrP Sc either in vitro or in vivo Candidate agents mclude synthetic molecules (e g , small molecule drugs, peptides, or other synthetically produced molecules or compounds, as well as recombinantly produced gene products) as well as naturally-occurring compounds (e g , polypeptides. endogenous factors, plant extracts, and the like) Drug Screenmg Assays Of particular mterest m the present mvention is the identification of agents that have activity m affectmg Dpi expression and/or function Such agents are candidates for development of treatments for, for example, mcreased and/or mappropnate neural outgrowth, or other condition that may be associated with altered Dpi activity Drug screemng identifies agents that provide a replacement or enhancement for Dpi function m affected cells Conversely, agents that reverse or inhibit Dpi function may provide a means to regulate neurodegeneration or neurotoxicity, especially m Purkinje cells Of particular mterest are screemng assays for agents that have a low toxicity for human cells The term "agent" as used herein descπbes any molecule, e g protem or pharmaceutical, with the capability of altering or mimicking the expression or physiological function of Dpi Generally a plurality of assay mixtures are run m parallel with different agent concentrations to obtain a differential response to the various concentrations Typically, one of these concentrations serves as a negative control, i e at zero concentration or below the level of detection Candidate agents encompass numerous chemical classes, though typically they are orgamc molecules, preferably small orgamc compounds havmg a molecular weight of more than 50 and less than about 2,500 daltons Candidate agents compnse functional groups necessary for structural mteraction with protems, particularly hydrogen bonding, and typically include at least an amine, carbonyl, hydroxyl or carboxyl group, preferably at least two of the functional chemical groups The candidate agents often compnse cyclical carbon or heterocychc structures and/or aromatic or polyaromatic structures substituted with one or more of the above functional groups Candidate agents are also found among biomolecules mcludmg. but not limited to peptides, sacchandes, fatty acids, steroids, puπnes, pynmidmes, denvatives, structural analogs or combmations thereof Candidate agents are obtained from a wide vaπety of sources mcludmg libraπes of synthetic or natural compounds For example, numerous means are available for random and directed synthesis of a wide vanety of orgamc compounds and biomolecules, mcludmg expression of randomized ohgonucleotides and ohgopeptides Alternatively, libraπes of natural compounds m the form of bactenal, fungal, plant and animal extracts are available or readily produced Additionally, natural or synthetically produced libraπes and compounds are readily modified through conventional chemical, physical and biochemical means, and may be used to produce combmatonal libraπes Known pharmacological agents may be subjected to directed or random chemical modifications, such as acylation, alkylation, esteπfication, amidification, etc to produce structural analogs Screemng of Candidate Agents In Vivo Agents can be screened for their ability to affect Dpi expression or function or to mitigate an undesirable phenotype (e g . a symptom) associated with an alteration m Dpi expression or function In a prefened embodiment, screemng of candidate agents is performed in vivo m a transgemc animal descnbed herem Transgemc animals suitable for use m screemng assays mclude any transgemc ammal havmg an alteration m Dpi expression, and can mclude transgemc animals havmg, for example, an exogenous and stably transmitted human Dpi gene sequence, a reporter gene composed of a (removed human) Dpi promoter sequence operably linked to a reporter gene (e g , β-galactosidase, CAT, or other gene that can be easily assayed for expression), or a homozygous or heterozygous knockout of a Dpi gene The transgemc ammals can be either homozygous or heterozygous for the genetic alteration and, where a sequence is mtroduced mto the animal's genome for expression, may contain multiple copies of the mtroduced sequence Where the in vivo screemng assay is to identify agents that affect the activity of the Dpi promoter, the Dpi promoter can be operably linked to a reporter gene (e g , luciferase) and mtegrated mto the non-human host animal's genome or mtegrated mto the genome of a cultured mammalian cell lme The candidate agent is administered to a non-human, transgemc ammal havmg altered Dpi expression, and the effects of the candidate agent determined The candidate agent can be administered m any manner desired and/or appropπate for delivery of the agent m order to effect a desired result For example, the candidate agent can be administered by injection (e g , by injection intravenously, intramuscularly, subcutaneously, or directly mto the tissue m which the desired affect is to be achieved), orally, or by any other desirable means Normally, the in vivo screen will mvolve a number of animals receiving varying amounts and concentrations of the candidate agent (from no agent to an amount of agent hat approaches an upper limit of the amount that can be delivered successfully to the animal), and may mclude delivery of the agent m different formulation The agents can be administered smgly or can be combmed m combmations of two or more, especially where administration of a combmation of agents may result m a synergistic effect The effect of agent administration upon the transgemc animal can be momtored by assessmg Dpi function as appropnate (e g , by examining expression of a reporter or fusion gene), or by assessmg a phenotype associated with the Dpi expression For example, where the transgemc animal used m the screen contains a defect in Dpi expression (e g , due to a knock-out of the gene), the effect of the candidate agent can be assessed by determining levels of neurodegeneration m the control mouse relative to the levels produced in the Dpi defective transgemc mouse and/or m wildtype mice (e g, by assessmg levels of ataxia m the treated mice) Methods for assaymg neurological defects m rodent are well known m the art Where the in vivo screemng assay is to identify agents that affect the activity of the Dpi promoter and the non-human transgemc ammal (or cultured mammalian cell lme) compπses a Dpi promoter operably linked to a reporter gene, the effects of candidate agents upon Dpi promoter activity can be screened by, for example, monitoring the expression from the Dpi promoter (through detection of the reporter gene) and conelation of altered Dpi promoter activity with neuronal activity and/or degeneration Alternatively or m addition, Dpi promoter activity can be assessed by detection (qualitative or quantitative) of Dpi mRNA or protein levels Where the candidate agent affects Dpi expression, and/or affects a Dpl-associated phenotype, m a desired manner, the candidate agent is identified as an agent suitable for use m therapy of a Dpl-associated disorder or a PrP -associated disorder Screemng of Candidate Agents In Vitro In addition to screemng of agents m Dpi transgemc ammals, a wide vaπety of in vitro assays may be used for this p pose, mcludmg labeled in vitro protem-protem bmdmg assays, protem-DNA bmdmg assays, electrophoretic mobility shift assays, immunoassays for protem bmdmg, and the like For example, by providmg for the production of large amounts of Dpi protem, one can identify ligands or substrates that bmd to, modulate or mimic the action of the protems The puπfied protem may also be used for determination of three-dimensional crystal structure, which can be used for modelmg intermolecular mteractions, transcnptional regulation, etc The screemng assay can be a bmdmg assay, wherem one or more of the molecules may be jomed to a label, and the label directly or indirectly provide a detectable signal Vaπous labels mclude radioisotopes, fluorescers, chenulummescers, enzymes, specific bmdmg molecules, particles, e g magnetic particles, and the like Specific bmdmg molecules mclude pairs, such as biotm and streptavidm, digoxin and antidigoxin etc For the specific bmdmg members, the complementary member would normally be labeled with a molecule that provides for detection, m accordance with known procedures A vaπety of other reagents may be mcluded m the screemng assays descnbed herem Where the assay is a bmdmg assay, these mclude reagents like salts, neutral protems, e g albumin, detergents, etc that are used to facilitate optimal protem-protem bmdmg, protem-DNA bmdmg, and/or reduce nonspecific or background mteractions Reagents that improve the efficiency of the assay, such as protease inhibitors, nuclease inhibitors, anti-microbial agents, etc may be used The mixture of components are added m any order that provides for the reqmsite bmdmg Incubations are performed at any suitable temperature, typically between 4 and 40°C Incubation penods are selected for optimum activity, but may also be optimized to facilitate rapid high-throughput screemng Typically between 0 1 and 1 hours will be sufficient Other assays of mterest detect agents that mimic Dpi function For example, candidate agents are added to a cell that lacks functional Dpi, and screened for the ability to reproduce Dpi activity in a functional assay. In a particular embodiment, agents that mimic unregulated Dpi activity are screened for the ability to decrease incubation time of prion disorders. Many mammalian genes have homologs in yeast and lower animals. The study of such homologs' physiological role and interactions with other proteins in vivo or in vitro can facilitate understanding of biological function. In addition to model systems based on genetic complementation, yeast has been shown to be a powerful tool for studying protein-protein interactions through the two hybrid system described in Chien et al., Proc. Natl. Acad. Sci. USA 55:9578-9582 (1991). Two- hybrid system analysis is of particular interest for exploring transcriptional activation by Dpi proteins and to identify cDNAs encoding polypeptides that interact with Dpi. Binding to Dpi may influence the function/activity of particular target proteins or be itself functionally modulated and may bear relevance for both normal or specific disease states. Identified Candidate Agents The compounds having the desired pharmacological activity may be administered in a physiologically acceptable carrier to a host for treatment of a condition attributable or in part modulated by a defect in normal Dpi function (e.g. , a neurological disorder associated with unregulated Dpi levels. The compounds may also be used to enhance Dpi function. The therapeutic agents may be administered in a variety of ways, orally, topically, parenterally e.g. subcutaneously, intraperitoneally, by viral infection, intravascularly, etc. Inhaled treatments are of particular interest. Depending upon the manner of introduction, the compounds may be formulated in a variety of ways. The concentration of therapeutically active compound in the formulation may vary from about 0J-100 wt.%. The pharmaceutical compositions can be prepared in various forms, such as granules, tablets, pills, suppositories, capsules, suspensions, salves, lotions and the like. Pharmaceutical grade organic or inorganic carriers and/or diluents suitable for oral and topical use can be used to make up compositions containing the therapeutically-active compounds. Diluents known to the art include aqueous media, vegetable and animal oils and fats. Stabilizing Agents, wetting and emulsifying Agents, salts for varying the osmotic pressure or buffers for securing an adequate pH value, and skin penetration enhancers can be used as auxiliary agents. Pharmacogenetics Pharmacogenetics is the linkage between an individual's genotype and that individual's ability to metabolize or react to a therapeutic agent. Differences in metabolism or target sensitivity can lead to severe toxicity or therapeutic failure by altering the relation between bioactive dose and blood concentration of the drug. In the past few years, numerous studies have established good relationships between polymoφhisms in metabolic enzymes or drug targets, and both response and toxicity. These relationships can be used to individualize therapeutic dose administration. Genotyping of polymoφhic alleles is used to evaluate whether an individual will respond well to a particular therapeutic regimen. The polymoφhic sequences are also used in drug screening assays, to determine the dose and specificity of a candidate therapeutic agent. A candidate Dpi polymoφhism is screened with a target therapy to determine whether there is an influence on the effectiveness in treating, for example, a neurological disorder. Drug screening assays are performed as described above. Typically two or more different sequence polymoφhisms are tested for response to a therapy. Where a particular sequence polymoφhism coπelates with differential drug effectiveness, diagnostic screening may be performed. Diagnostic methods have been described in detail in a preceding section. The presence of a particular polymoφhism is detected, and used to develop an effective therapeutic strategy for the affected individual. Detection of Dpi Associated Disorders Detection of Dpi levels and/or the presence of different conformations of Dpi can provide a means for diagnosing neurodegenerative disorders associated with Dpi. Iin particular, detection of changes in Dpi expression and/or changes in Dpi conformation may be associated with disorders involving Purkinje cell degeneration, such as hereditary cerebellar cortical atrophy, generalized epilepsy with tonic-clonic seizures (GTCS), spinocerebellar ataxia type 6, and the like. Measurement of Dpi levels and/or different conformations of the protein may also be used for other neurodegenerative disorders, as will be apparent to one skilled in the art upon reading the present disclosure. Diagnosis of Dpl-associated disorders is performed by protein, DNA or RNA sequence and/or hybridization analysis of any convenient sample from a patient, e.g. biopsy material, blood sample, scrapings from cheek, etc. A nucleic acid sample from a patient having a disorder that may be associated with Dpi is analyzed for the presence of a predisposing polymoφhism in Dpi. A typical patient genotype will have at least one predisposing mutation on at least one chromosome. The presence of a polymoφhic Dpi sequence that affects the activity or expression of the gene product, and confers an increased susceptibility to a Dpi associated disorder (e.g, a neurodegenerative disorder) is considered a predisposing polymoφhism. Individuals are screened by analyzing their DNA or mRNA for the presence of a predisposing polymoφhism, as compared to sequence from an unaffected individual(s). Screening may also be based on the functional or antigenic characteristics of the protein. Immunoassays designed to detect predisposing polymoφhisms in Dpi proteins may be used in screemng Where many diverse mutations lead to a particular disease phenotype, functional protem assays can be effective screemng tools Biochemical studies may be performed to determine whether a candidate sequence polymoφhism m the Dpi codmg region or control regions is associated with disease For example, a change m the promoter or enhancer sequence that affects expression of Dpi may result m predisposition to neurological disorders, e g pnon-mediated disorders Expression levels of a candidate vanant allele are compared to expression levels of the normal allele by vaπous methods known m the art Methods for determining promoter or enhancer strength mclude quantitation of the expressed natural protem. insertion of the vanant control element mto a vector with a reporter gene such as β-galactosidase, luciferase, chloramphemcol acetyltransferase, etc that provides for convement quantitation, and the like The activity of the encoded Dpi protem may be determined by companson with the wild-type protem A number of methods are available for analyzing nucleic acids for the presence of a specific sequence Where large amounts of DNA are available, genomic DNA is used directly Alternatively, the region of mterest is cloned mto a smtable vector and grown in sufficient quantity for analysis Cells that express Dpi genes may be used as a source of mRNA, which may be assayed directly or reverse transcnbed mto cDNA for analysis The nucleic acid may be amplified by conventional techniques, such as the polymerase cham reaction (PCR), to provide sufficient amounts for analysis The use of the polymerase cham reaction is descπbed m Saiki, et al , Science 239 487 (1985), a review of cunent techniques may be found in Sambrook, et al , Molecular Clomng A Laboratory Manual. CSH Press 1989, pp 14 2-14 33 Amplification may also be used to determine whether a polymoφhism is present, by using a primer that is specific for the polymoφhism Alternatively, vanous methods are known in the art that utilize ohgonucleotide ligation as a means of detecting polymoφhisms, for examples see Riley et al , Nucl Acid Res 18 2887-2890 (1990), and Delahunty et al , Am J Hum Genet 58 1239-1246 (1996) A detectable label may be mcluded m an amplification reaction Smtable labels mclude fluorochromes, e g fluorescern lsothiocyanate (FITC), rhodamine, Texas Red, phycoerythnn, allophycocyanin, 6-carboxyfluoresceιn (6-FAM), 2',7'-dmιethoxy-4'J'-dιchloro-6-carboxyfluorescem (JOE), 6-carboxy-X-rhodamιne (ROX), 6-carboxy-2',4'J',4J-hexachlorofluorescem (HEX), 5-carboxyfluorescem (5-FAM) or N,N,N',N'-tetramethyl-6-carboxyrhodamme (TAMRA), radioactive labels, e g 32 P, 33 P, 35 S, 3 H, etc The label may be a two stage system, where the amplified DNA is conjugated to biotm, haptens, etc havmg a high affinity bmdmg partner, e g avidin, specific antibodies, etc . where the bindmg partner is conjugated to a detectable label The label may be conjugated to one or both of the primers Alternatively, the pool of nucleotides used m the amplification is labeled, so as to incoφorate the label mto the amplification product The sample nucleic acid, e g amplified or cloned fragment, is analyzed by one of a number of methods known m the art The nucleic acid may be sequenced by dideoxy or other methods, and the sequence of bases compared to either a neutral Dpi sequence (e g , a Dpi sequence from an unaffected individual) Hybπdization with the vanant sequence may also be used to determine its presence, by Southern blots, dot blots, etc The hybndization pattern of a control and vanant sequence to an anay of ohgonucleotide probes immobilized on a solid support, as descnbed m US 5,445,934, or m WO 95/35505, may also be used as a means of detectmg the presence of vanant sequences Smgle strand conformational polymoφhism (SSCP) analysis, denaturing gradient gel electrophoresis (DGGE), mismatch cleavage detection, and heteroduplex analysis m gel matnces are used to detect conformational changes created by DNA sequence vaπation as alterations m electrophoretic mobility Alternatively, where a polymoφhism creates or destroys a recogmtion site for a restπction endonuclease (restnction fragment length polymoφhism, RFLP), the sample is digested with that endonuclease, and the products size fractionated to determme whether the fragment was digested Fractionation is performed by gel or capillary electrophoresis, particularly acrylamide or agarose gels The hybndization pattern of a control and vanant sequence to an anay of ohgonucleotide probes immobilized on a solid support, as descπbed m US 5,445,934, or m WO 95/35505, may be used as a means of detectmg the presence of vanant sequences In one embodiment of the mvention, an anay of ohgonucleotides are provided, where discrete positions on the anay are complementary to at least a portion of mRNA or genomic DNA of the Dpi locus Such an anay may compπse a series of ohgonucleotides, each of which can specifically hybπdize to a nucleic acid sequence, e g, mRNA, cDNA, genomic DNA, etc from the Dpi locus Usually such an anay will mclude at least 2 different polymoφhic sequences, i e polymoφhisms located at umque positions within the locus, usually at least about 5, more usually at least about 10, and may mclude as many as 50 to 100 different polymoφhisms The ohgonucleotide sequence on the anay will usually be at least about 12 nt in length, may be the length of the provided polymoφhic sequences, or may extend mto the flanking regions to generate fragments of 100 to 200 nt m length For examples of anays, see Hacia et al , Nature Genetics 14 441-447 (1996), Lockhart et al , Nature Biotechnol 14 1675-1680 (1996), and De Risi et al , Nature Genetics 14 457-460 (1996) Antibodies specific for Dpi polymoφhisms may be used m screemng rmmunoassays A reduction or mcrease m Dpi and/or presence of a Dpi disorder associated polymoφhism is indicative that the suspected disorder is Dpl-associated A sample is taken from a patient suspected of havmg a Dpl-associated disorder Samples, as used herem, include tissue biopsies, biological fluids, organ or tissue culture deπved fluids, and fluids extracted from physiological tissues, as well as denvatives and fractions of such fluids The number of cells m a sample will generally be at least about 10 3 , usually at least 10 4 more usually at least about 10 s The cells may be dissociated, m the case of solid tissues, or tissue sections may be analyzed Alternatively a lysate of the cells may be prepared Diagnosis may be performed by a number of methods The different methods all determme the absence or presence or altered amounts of normal or abnormal Dpi m patient cells suspected of havmg a predisposmg polymoφhism m Dpi For example, detection may utilize stainmg of cells or histological sections, performed m accordance with conventional methods The antibodies of mterest are added to the cell sample, and mcubated for a penod of time sufficient to allow bmdmg to the epitope, usually at least about 10 minutes The antibody may be labeled with radioisotopes, enzymes, fluorescers, chemilummescers, or other labels for direct detection Alternatively, a second stage antibody or reagent is used to amplify the signal Such reagents are well known m the art For example, the pπmary antibody may be conjugated to biotm, with horseradish peroxidase-conjugated avidin added as a second stage reagent Final detection uses a substrate that undergoes a color change m the presence of the peroxidase The absence or presence of antibody bmdmg may be determined by vaπous methods, mcludmg flow cytometry of dissociated cells, microscopy, radiography, scintillation counting, etc If the particular disease state is associated with conformational transition of Dpi, diagnosis may rely on the recognition of the disease-associated conformation of the protem The disease- associated form the protem may be distinguished from other forms of Dpi usmg charactenstics of the disease-associated conformation, e g insolubility, resistance to protease digestion, change m epitope availability and the like An alternative method for diagnosis depends on the n vitro detection of bmdmg between antibodies and Dpi m a lysate Measuring the concentration of Dpi bmdmg m a sample or fraction thereof may be accomplished by a vaπety of specific assays A conventional sandwich type assay may be used For example, a sandwich assay may first attach Dpl-specific antibodies to an insoluble surface or support The particular manner of bmdmg is not crucial so long as it is compatible with the reagents and overall methods of the mvention They may be bound to the plates covalently or non- covalently, preferably non-covalently The insoluble supports may be any compositions to which polypeptides can be bound, which is readily separated from soluble matenal, and which is otherwise compatible with the overall method The surface of such supports may be solid or porous and of any convenient shape Examples of smtable insoluble supports to which the receptor is bound include beads, e g magnetic beads, membranes and microtiter plates These are typically made of glass, plastic (e g polystyrene), polysacchandes, nylon or nitrocellulose Microtiter plates are especially convement because a large number of assays can be earned out simultaneously, usmg small amounts of reagents and samples Patient sample lysates are then added to separately assayable supports (for example, separate wells of a microtiter plate) containing antibodies Preferably, a senes of standards, containing known concentrations of normal and/or abnormal Dpi is assayed m parallel with the samples or aliquots thereof to serve as controls Preferably, each sample and standard will be added to multiple wells so that mean values can be obtamed for each The mcubation tune should be sufficient for bmdmg, generally, from about 0 1 to 3 hr is sufficient After mcubation, the insoluble support is generally washed of non-bound components Generally, a dilute non-ionic detergent medium at an appropnate pH, generally 7-8, is used as a wash medium From one to six washes may be employed, with sufficient volume to thoroughly wash non-specifically bound protems present m the sample After washing, a solution containing a second antibody is applied The antibody will bmd Dpi with sufficient specificity such that it can be distinguished from other components present The second antibodies may be labeled to facilitate direct, or indirect quantification of bmdmg Examples of labels that permit direct measurement of second receptor bmdmg mclude radiolabels, such as 3 H or 125 I, fluorescers, dyes, beads, chemiluminescers, colloidal particles, and the like Examples of labels which permit mdirect measurement of bmdmg mclude enzymes where the substrate may provide for a colored or fluorescent product In a prefened embodiment, the antibodies are labeled with a covalently bound enzyme capable of providing a detectable product signal after addition of suitable substrate Examples of smtable enzymes for use m conjugates include horseradish peroxidase, alkaline phosphatase, malate dehydrogenase and the like Where not commercially available, such antibody- enzyme conjugates are readily produced by techniques known to those skilled m the art The mcubation time should be sufficient for the labeled ligand to bmd available molecules Generally, from about 0 1 to 3 hr is sufficient, usually 1 hr sufficing After the second bmdmg step, the insoluble support is agam washed free of non-specifically bound matenal The signal produced by the bound conjugate is detected by conventional means Where an enzyme conjugate is used, an appropπate enzyme substrate is provided so a detectable product is formed Other immunoassays are known in the art and may find use as diagnostics Ouchterlony plates provide a simple determination of antibody bmdmg Western blots may be performed on protem gels or protem spots on filters, usmg a detection system specific for Dpi as desired, convemently usmg a labeling method as descnbed for the sandwich assay Other diagnostic assays of mterest are based on the functional properties of Dpi protems Such assays are particularly useful where a large number of different sequence changes lead to a common phenotype For example, a functional assay may be based on the changes m the transcnptional repertoire of cells mediated by addition of mediated by Dpi gene products Other assays may, for example, detect conformational changes, size changes resulting from insertions, deletions or truncations, or changes in the subcellular localization of Dpi proteins. In a protein truncation test, PCR fragments amplified from the Dpi gene or its transcript are used as templates for in vivo transcription/translation reactions to generate protein products. Separation by gel electrophoresis is performed to determine whether the polymoφhic gene encodes a truncated protein, where truncations may be associated with a loss of function. Diagnostic screening may also be performed for polymoφhisms that are genetically linked to a predisposition for prion-mediated disorders such as CJD, particularly through the use of microsatellite markers or single nucleotide polymoφhisms. Frequently the microsatellite polymoφhism itself is not phenotypically expressed, but is linked to sequences that result in a disease predisposition. However, in some cases the microsatellite sequence itself may affect gene expression. Microsatellite linkage analysis may be performed alone, or in combination with direct detection of polymoφhisms, as described above. The use of microsatellite markers for genotyping is well documented. For examples, see Mansfield et aL.Genomics 2 :225-233 (1994); Ziegle et al., Genomics 74:1026-1031 (1992); Dib et al., supra. Microsatellite loci that are useful in the subject methods have the general formula: U (R) n U', where U and U' are non-repetitive flanking sequences that uniquely identify the particular locus, R is a repeat motif, and n is the number of repeats. The repeat motif is at least 2 nucleotides in length, up to 7, usually 2-4 nucleotides in length. Repeats can be simple or complex. The flanking sequences U and U' uniquely identify the microsatellite locus within the human genome. U and U' are at least about 18 nucleotides in length, and may extend several hundred bases up to about 1 kb on either side of the repeat. Within U and U', sequences are selected for amplification primers. The exact composition of the primer sequences are not critical to the invention, but they must hybridize to the flanking sequences U and U', respectively, under stringent conditions. Criteria for selection of amplification primers are as previously discussed. To maximize the resolution of size differences at the locus, it is preferable to chose a primer sequence that is close to the repeat sequence, such that the total amplification product is between 100-500 nucleotides in length. The number of repeats at a specific locus, n, is polymoφhic in a population, thereby generating individual differences in the length of DNA that lies between the amplification primers. The number will vary from at least 1 repeat to as many as about 100 repeats or more. The primers are used to amplify the region of genomic DNA that contains the repeats. Conveniently, a detectable label will be included in the amplification reaction, as previously described. Multiplex amplification may be performed in which several sets of primers are combined in the same reaction tube This is particularly advantageous when limited amounts of sample DNA are available for analysis Convemently, each of the sets of pnmers is labeled with a different fluorochrome After amplification, the products are size fractionated Fractionation may be performed by gel electrophoresis, particularly denatuπng acrylamide or agarose gels A convement system uses denatuπng polyacrylamide gels m combmation with an automated DNA sequencer, see HunkapiUar et al , Science 254 59-74 (1991) The automated sequencer is particularly useful with multiplex amplification or pooled products of separate PCR reactions Capillary electrophoresis may also be used for fractionation A review of capillary electrophoresis may be found m Landers, et al , BioTechniques 14 98-111 (1993) The size of the amplification product is proportional to the number of repeats (n) that are present at the locus specified by the pnmers The size will be polymoφhic m the population, and is therefore an allehc marker for that locus Diagnosis and therapeutic treatment of PrP -associated disorders Animals with increased expression of Dpi activity, such as the two PrP 0/0 lmes descnbed herem, also display a reduced mcubation penod for development of the symptoms of pnon mfection Based on this observation, the present mvention provides assay methods for determining the pnon infectivity of a sample usmg animals that have an altered expression of Dpi The method determines whether a sample is infected with prions by moculatmg a Dpi transgemc or hybrid mammal of the mvention with a sample to be tested and observing the mammal for a penod of time sufficient to determme if they develop symptoms of a disease normally associated with prions The animals may have an mcreased level of Dpi expression due to 1) a natural endogenous Dpi mutation, e g a mutation m the Dpi locus identified m a genetic screen, 2) a Dpi mutation produced m a transgemc animal, e g a deletion of the PrP locus that results m mcreased expression of the Dpi gene product, and/or 3) the mtroduction of an exogenous transgene that encodes a Dpi polypeptide The Dpi animals of the present invention can also be used m a method for determining the cause of death of an ammal Such a method mvolves moculatmg a Dpi transgemc or hybnd ammal of the mvention with body fluid or tissue such as extracted bram tissue from the animal which has died (and preferably moculatmg control animals with a standardized preparation of pπons) and observing the transgemc or hybrid ammal (and control animals) m order to determme if the ammal develops symptoms of pnon infections In a prefened embodiment, the ammals of the invention with altered Dpi activity also have a genome that is altered with respect to the PrP locus Exemplary animals mclude, but are not limited to (1) animals with ablated endogenous PrP genes, as disclosed m U S Pat No 5,698,763, (2) animals with an ablated endogenous PrP gene and an exogenous PrP transgene from a genetically diverse ammal, as disclosed m U S Pat No 5,792,901 and U S S N 08/935,363, and (3) animals with an ablated endogenous PrP gene and an mducible PrP transgene, as disclosed m U S S N 09/052,963 Prefened host ammals are mice and hamsters, with mice bemg most prefened m that there exists considerable knowledge on the production of transgemc ammals Other possible host animals mclude those belongmg to a genus selected from Mus (e g mice), Rattus (e g rats), Oryctolagus (e g rabbits), and Mesocncetus (e g hamsters) and Cavia (e g , guinea pigs) In general mammals with a normal full grown adult body weight of less than 1 kg which are easy to breed and maintain can be used The host PrP gene can be changed to mclude codons from genetically diverse PrP genes from test ammals belongmg to a genus selected from Bos, Ovis, Sus and Homo Preferably, a mouse host PrP gene is changed to mclude codons from a human, cow or sheep PrP gene, with cow bemg most prefened Cows are prefened because an important object of the invention is to use the animal to test a statistically significant number of cows in a herd of cows to determine if the cows are infected with pnons which cause BSE, known as "mad cow" disease The present mvention also provides a method of testmg the efficacy of a drug m the treatment of disease developed as a result of infection with prions compnsmg administering a drug to be tested to a transgemc or hybrid ammal infected with pnons (preferably a standardized pnon preparation) and observing and/or testmg the mammal to determine if the drug aids m treatmg or slowmg the progress of the disease or its symptoms Such methods are described m U S Pat No 5,792,901 and U S S N 08/935,363, which are both mcoφorated herem by reference for this puφose The present mvention also provides a method for treatmg pnon disease by administration of a compound that downregulates Dpi expression and/or decreases Dpi activity Smce increased Dpi activity is associated with a reduction in the mcubation penod for pnon disease, and smce PrP c can rescue the mcreased Dpi phenotype (suggestmg they may either mteract or compete in a pathway), decreasmg Dpi activity can slow or halt the progression of pπon disorder, for example by prevention of the degeneration of neurons m response to PrP Sc Therapeutic Uses of Dpl-Encod g Nucleic Acid Dpl-encodmg nucleic acid can be mtroduced mto a cell to accomplish transformation of the cell, preferably stable transformation Introduction of the Dpl-encodmg nucleic acid mto the cell can be accomplished accordmg to methods well known m the art (e g , through use of electroporation, micromjection, lipofection mfection with a recombinant (preferably replication-deficient) virus, and other means well known m the art) Preferably, the Dpl-encodmg nucleic acid is operably linked to a promoter that facilitates a desired level of Dpi polypeptide expression (e g , a promoter deπved from CMV, SV40, adenovirus, or a tissue-specific or cell type-specific promoter) Transformed cells containmg the Dpl-encodmg nucleic acid can be selected and/or ennched via, for example, expression of a selectable marker gene present m the Dpl-encodmg construct or that is present on a plasmid that is co-transfected with the Dpl-encodmg construct Typically selectable markers provide for resistance to antibiotics such as tetracycline, hygromycm, neomycm, and the like Other markers can mclude thymidine krnase and the like The ability of the transformed cells to express the Dpl-encodmg nucleic acid can be assessed by vanous methods known m the art For example, Dpi expression can be examined by Northern blot to detect mRNA which hybπdizes with a DNA probe deπved from the relevant gene Those cells that express the desired gene can be further isolated and expanded m in vitro culture usmg methods well known m the art The host cells selected for transformation with Dpl-encodmg DNA will vary with the puφose of the ex vivo therapy (e g , Dpi peptide production), the site of implantation of the cells, and other factors that will vary with a vanety of factors that will be appreciated by the ordmaπly skilled artisan Methods for in vivo delivery of a nucleic acid of interest for expression m a target cell are known m the art For example, in vivo methods of gene delivery normally employ either a biological means of mtroducmg the DNA mto the target cells (e g , a virus containmg the DNA of mterest) or a mechamcal means to introduce the DNA mto the target cells (e g , direct injection of DNA mto the cells, hposome fusion, pneumatic injection usmg a "gene gun," and the like) The amount of DNA and/or the number of mfectious viral particles effective to mfect the targeted tissue, transform a sufficient number of cells, and provide for production of a desired level of Dpi can be readily determined based upon such factors as the efficiency of the transformation in vitro and the susceptibility of the targeted cells to transformation Generally, the amounts of DNA can be extrapolated from the amounts of DNA effective for delivery and expression of the desired gene m an animal model For example, the amount of DNA for delivery m a human is roughly 100 times the amount of DNA effective m a rat Regardless of whether the Dpl-encodmg DNA is mtroduced in vivo or ex vivo, the DNA (or cells expressmg the DNA) can be administered m combmation with other genes and other agents In addition, Dpl-encodmg DNA (or recombmant cells expressmg Dpi DNA) can be used therapeutically for disorders associated with, for example, a decrease m Dpi production, but which are not associated with an alteration m Dpi function per se EXAMPLES The following examples are put forth so as to provide those of ordinary skill m the art with a complete disclosure and descnption of how to make and use the present invention, and are not mtended to limit the scope of what the inventors regard as their mvention nor are they intended to represent that the experiments below are all or the only experiments performed Efforts have been made to ensure accuracy with respect to numbers used (e g amounts, temperature, etc ) but some experimental enors and deviations should be accounted for Unless mdicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is m degrees Centigrade, and pressure is at or near atmospheric EXAMPLE 1 Identification of the Prnd Locus Previous large-scale sequencmg studies of phage and cosmid molecular clones encompassmg human, sheep, and the "a" allele of the mouse PrP gene, Prnp . failed to reveal any evidence of additional codmg regions, either immediately adjacent to PrP or withm mtromc sequences (Lee et al , Genome Res 8 1022-1037(1998)) However, sequencing of the mouse Prnp 0 cosmid clone I/LnJ-4 (Westaway et al , Neuron 759-68 (1991)), which extends further m a 3' direction and analysis with the "XGrail 1 3c" program (Uberbacher et al, ORNL/TM 11741 (1991)) revealed a candidate coding region at nt 36119 to 36,751 (Figure 1A) Cosrmds and YAC clones were propagated by standard procedures and have been descnbed previously (D Westaway, Cell 76 117-29 (1994)) A 129Sv/J BAC clone 321L17 was identified from a library kmdly provided by Millenium Pharmaceuticals The predicted ammo acid sequence of this ORF (Figure 2) exhibits homology to mammalian, marsupial and avian PrP We refer to the putative gene as Prnd (pπon gene complex, downstream) and the protem product as "Dpi" (downstream, pπon protein-hke German, "double") Southern blot analyses mdicated that the putative codmg region of Prnd was a smgle-copy sequence m the mouse genome To exclude the Prnd ORF as an ldiosyncracy of I/LnJ mice, we sequenced the conespondmg chromosomal region from a C57B6-deπved YAC clone (Westaway et al , Proc Natl Acad Sci USA 91 6418-6422 (1994)) and a 129/SvJ BAC clone 321L17 Both revealed an identical ORF Furthermore, related ORFs are present m rats and humans, with the human PrnD gene positioned 27 kb 3 ' to PRNP These data indicate that PrnD genes will be found to be present m all mammals EXAMPLE 2 Structure of the Prnd Locus Prnd cDNAs were isolated to demonstrate the splicmg of mRNAs transcnbed from this region and to define appropriate codmg exons The complimentary approaches of cDNA library screenmg and rapid amplification of cDNA ends (RACE) were undertaken usmg wild type adult mouse brain cDNA as starting matenal RACE clomng of PrnD RNAs For 5' RACE analysis, "Marathon" mouse brain cDNA (Clontech, Palo Alto, CA) was amplified with adapter primer AP 1 and the Prnd anti-sense strand pnmer DW112, 5'-CGGTTGGTCCACGGCGACCCGAA -3' (SEQ ID NO 6) (0 2 μM), usmg a Perkin-Elmer 2400 thermocycler and "touchdown" PCR conditions of 94 °C 20 seconds, 5 cycles of 94 °C, 5 sec and 72°C, 360 seconds, 5 cycles of 94°C, 5 seconds and 70°C 360 seconds, and 30 cycles of 94°C, 5 seconds and 68°C, 360 seconds Taq polymerase was used m conjunction with "TaqStart" antibody (Clontech, Palo Alto, CA) Size-fractionated reaction products were resuspended m Tncme buffer and a second-round of PCR was performed usmg a nested primer set AP2 universal pnmer and the Prnd pnmer ORFP-R2 (0 2 μM), 5'-GCAGATCTCTTTGATCAGCC-3' (SEQ ID NO 7) (94° C, 60 seconds, and 20 cycles of 94 °C 10 seconds, 57 °C, 20 seconds, 72 °C 150 seconds Subsequent to venfication by hybπdization to a lαnase-labeled internal pnmer DW95 5'-CAGATCCACCGAAGCTCGGG-3' (SEQ ID NO 8), PCR reaction products were either sequenced directly or subsequent to clomng mto the plasmid vector pCRJ 1TOPO (Invitrogen) A similar strategy was adopted for clomng of 3' RACE products, usmg the nested pnmers DW111, 5'-TGGTGACCAGCTGCGTCAACGCCA-3' (SEQ ID NO 9)and DW192, 5'-TGGGAAGGCCCTGAGCGACAACCGTG-3' (SEQ ID NO 10) RT-PCR of Py-wrf mRNAs Total RNA was prepared by the acid-phenol method and polyA-selected on ohgo dT-latex (Quiagen) First-strand cDNA synthesis was earned out usmg lOOng of polyA+ RNA primed with ohgo dT or random hexamer pnmers (as noted) and MuLV reverse transcnptase, as recommended by the manufacturer (Stratagene. La Jolla, CA) 2ng ahquots of cDNA synthesis reactions mcubated plus or minus reverse transcnptase were either amplified directly, or following concentration and de-salting by ultrafiltration In the latter mstance, a cDNA first-strand synthesis reaction of 40ul (equivalent to 160 ng cDNA) was diluted with 450 ul water and mtroduced mto a 50.000 molecular weight cut-off "Eluticon" microconcentrator (Owl Scientific) Subsequent to spinning at 14,000xg, the membrane was washed with 500 ul water, spun, inverted and eluted with 10 ul water 2 5 ul of this preparation was used per PCR reaction Alternatively, 10 ug of total RNA was reverse-transcπbed m a total volume of 50 ul usmg Superscnpt reverse transcnptase (Life Technologies) with 2 5 ul used per amplification reaction "Hot-start" amplification (Platmum Taq". Life Technologies or Advantage KlenTaq, Clontech) was performed m standard reaction buffers, as noted below Pnmer sets were as follows Prnd exon la to PrnD exon 2 DW189 = 5'-GCTCCAAGCTTCAGAGGCCACAGTAGCA-3' (SEQ ID NO 11) and DW96, 5'-TTACTTCACAATGAACCAAACGAAAC-3' (SEQ ID NO 12) (1 uM) 94°C, 180 seconds, 40 cycles of 94°C, 15 seconds, 65 °C 30 seconds, 72°C, 75 seconds, usmg Platinum Taq polymerase and MgCL at a final concentration of 2 mM Intergemc exon 2 to PrnD exon 2 DW117, 5'-GAGTGGAGGTCTTCGCGCA-3* (SEQ ID NO 13) and DW96 (0 5 uM) 95 °C 300 seconds, and 45 cycles of 95 °C, 10 seconds, 55 °C 20 seconds, 72°C 150 seconds, usmg Platmum Taq polymerase, and MgCl 2 at a final concentrations of 2 5 mM Prnp exon 2 to PrnD exon 2 5'UT 3 (Westaway, Neuron 7 59-68 (1991)) and DW96 (0 2 uM) 95°C 300 seconds, and 40 cycles of 94°C, 15 seconds, 60°C 30 seconds, 72°C 120 seconds, usmg Platmum Taq polymerase and MgCl 2 and DMSO at a final concentrations of 2 5 mM and 4% respectively Rat Prnp exon 2 to the 3 ' end of PrnD (defined by ESTAJ136375) DW213 5'-TCAAAACTGAACCATTTCAACCCAACTGAAGlATTCTGCC-3' (SEQ ID NO 14) and DW214 5'-ACCCAGCGTTCTGGCCCGGTATTAGGATT-3' (SEQ ID NO 15) (mismatches to the mouse gene sequences are underlined) 94 °C for 120 seconds followed by 40 cycles of 94 °C, 15 seconds, and 72 °C 240 seconds β-Actin these were used as recommended by the manufacturer (Statagene, La Jolla, CA) Results from the RACE and RT-PCR analyses were used to search for EST matches from public databases and a number were retπeved from mouse, human and rat (Figure IB) PrnD cDNAs of 1 5 and 1 7 Kb were retrieved from a Balb/C testis cDNA library These clones corresponded to the major Dpi mRNA species observed on Northern blots A larger transcπpt of 2 7 Kb was also apparent on northern blots and 3' RACE provided good evidence for alternative splicmg of exon 2, which could account for this mRNA species Examination of the two major cDNA clones mdicated that they were nearly full length, as they contained polyA tracts at the 3' end and partial sequence from alternately spliced 5' exons 5' RACE analysis was m close agreement with the sequence of these clones indicating splicmg events from short 5' exons, denoted exons la and lb respectively, startmg at either nt 34,124 or nt 34,277 to a common splice acceptor lymg immediately 5' to exon 2, the Prnd ORF, at nt 36.204 (Figure IB, Figure 2A, 2B) This was also m agreement with the sequence of a mouse Dpi EST AA796652, which is generated by splicmg of exon lb to exon 2 (Figure IB) The 5' boundary of the PrnD 5 " untranslated region exon was located withm an interval of approximately 30 nucleotides by RT-PCR reactions with different pnmers (Figure 2A) Pnmer extension reactions and nuclease protection assays were employed for more precise mapping Pπmer-extension reactions with pnmers DW197, (5'-CCAGCCGGTTCTTCATGGTGAATCTCGG-3' (SEQ ID NO 16), hybndization temperature 65 °C), and DW123a, (5'-CATGGTGAATCTCGGTTCTC-3' (SEQ ID NO 17). hybndization temperature 55 °C), were carned-out as before (Westaway et al , 1987) except that pnmers were labeled to a specific activity of JE 9 x IO 7 dpm/pmole usmg 6000Cι/mmol gamma 32 P -ATP ("Kmase Max", Ambion Inc NEN) A 42-mer ohgonucleotide labeled m the same manner was used for a nuclease protection assay ("Multi-NPA", Ambion Inc ) Results of these mappmg experiments were m close agreement, defining a cluster of mRNA start-sites 13-18 nucleotides upstream of 5' termim defined by RACE cDNAs and EST gbAA796652 (from mouse mammary gland) These assignments are also m accord with the structure of human Prnd cDNAs Because of the alternative 3' boundaπes of this 5' non-codmg exon, ansing from the use of two alternative splice donors, it is denoted "exon la/lb" Notably, a similar anangement has been observed m the bovme PrP gene (Horuichi et al , Biochem Biophys Res Commun 233 660-654 (1997)) Smce neither exon la or lb contains ATG codons in-frame with the chromosomal ORF, these cDNAs (and others described below) mdicate that the initiation codon for Prnd lies 3 ' to the splice acceptor at nucleotide 36,212 This ATG is conserved and conforms closely to the Kozak consensus for the initiation of eukaryotic mRNAs (consensus GCCGCCa/gCCATGG (SEQ ID NOS 18, 19), PrnD AGATTCACCATGA (SEQ ID NO 20)) (Kozak, Nucleic Acids Res 15 8125-8148 1987) The position of the PrnD start site is organized m a similar fashion to Prn-p itself, where the ATG codon lies 10 nucleotides 3' to the splice acceptor of exon 3 (Westaway et al , Cell 57 651-662 (1987)) The internal structures of mdependently-denved cDNAs indicated that the Prnd mRNA 3'UTR is encoded withm separate exons and subject to alternative splicmg (Figure IB) Thus one splice donor 28 nucleotides 3' of the ORF termination codon (position 36,779) codon and another further downstream at 37,472 supply a splice acceptor at position ~38,006, suggestmg an explanation for the ~1 7 and 2 7 kb RNAs detected by Northern blots (below) cDNA clones containmg polyA tracts were found to terminate at positions 39,099 and 39J 15 m the I/LnJ-4 cosmid While the more 3' site is preceded by a consensus polyadenylation signal, AATAAA, a similar motif is lacking from the 5' site Of note, a sequence CTTAAA is located 27 nucleotides 5' of the polyA tail This 3' gene architecture is unusual as (l) mtrons withm 3'UTR codmg sequences are rare, (n) splice sites at positions 37,472 and 38,006 differ from the accepted consensus (such that definitive positioning of the exact mtron boundanes are complicated by nucleotide redundancies whereby the boundary may be shifted by +1 or +2 from these coordmates) or a stram polymoφhism, and (m), mtron 2 defines the putative protem codmg exon of Prnd as being an internal, rather than a termmal exon, as m the case of Prnp. With a size of either 575 or 1J68 nucleotides (depending upon the selection of splice donor site) this exon contravenes the rule that 99% of internal exons are less than 400 bases in length (Berget, J. Biol. Chem. 270:2411-2414 (1995)). EXAMPLE 3: Expression of Transcripts from the Prnd Locus Northern blot analyses were performed using the entire 570 bp PrnD ORF fragment derived from the ILn/J-4 cosmid using total (50 g loading) or oligo-dT selected RNA (7 g loading). Total RNA was made using Trizol reagent (Gibco) following the manufacturers instructions. Poly A+ RNA was isolated using a Oligotex mRNA kit (Qiagen). RNA samples were heated for 30 minutes to 50 deg C in glyoxal sample buffer and electrophoresed on 1.2% agarose gels as recommended by the manufacturer (Ambion Inc., Woodland ,TX). RNA was transfened onto Hybond N+ (Amersham) in 5XSSC lOmM NaOH for 2 hours using a turboblotter (Schleicher and Schuell), rinsed in 5XSSC, UN fixed (Statagene, La Jolla, CA) then prehybridized (6xSSC, 1% SDS, 3% (w/v) dextran sulfate, lOug/ml sonicated herring sperm DΝA) for 1 hour at 65 °C prior to the addition of a 540bp Dpi ORF PCR a-dCTP 32 random-primed probe (Feinberg et al., Anal. Biochem. 732:6-13 (1983)) generated with the primers RM1 (5'- ATGAAGAACCGGCTGGGTAC-3') (SEQ ID NO.: 21) and DW96 with 129Sv/J genomic DNA template. Membranes were hybridized for 16 hours then rinsed for 5 minutes at room temperature in 2XSSC/1% (w/v) SDS, washed 30 minutes at 65 deg C in 2XSSC/l%(w/v) SDS then 30 mins in 1XSSC/1% (w/v) SDS at 65 deg C. Blots were exposed at -80 deg C for 3 days. This revealed Prnd mRNAs of approximately 1.7 and 2.7 kb in testis and heart polyA+ RNA of wild type mice. No Prnd mRNA was detected in brain. Using RT-PCR, mRNA in wild type mice produced by splicing from exon la to exon 2 could be detected in adult brain cDNA and 14-day embryos (Figure 3). Additional cDNA species, one of which reflects inclusion of exon lb, were detected in some adult brain samples. Some preliminary evidence of Dpi mRNA expression can be derived from the tissue origin of the Prnd EST matches (Figure IB): mouse ESTs AA796652, AA104098 and AA190150 were derived from adult mammary gland, embryonic heart and adult spleen respectively. In sum, Prnd mRNAs are expressed in embryos, in peripheral tissues, and at low levels in the adult CNS. As predicted from the DNA sequence data, species of similar electrophoretic mobility were apparent in brain RNA of Tg(MoPrP-B)2091 mice, harboring a high copy-number anay of an ILn/J-4 cosmid transgene within which Dpi was first noted. A coπesponding product was undetectable in wt control mouse brain samples as noted above. EXAMPLE 4 Computer Analysis of Dpi Sequences Narious non-redundant combinations of the SWISSPROT and TREMBL (A Bairoch et al , Nucleic Acids Res 19 Suppl 2247-9 (1991), Stoesser, et al , Biochemistry 377185-7193 (1998)) sequence databases were searched with the sequences of human and mouse Dpi usmg the alignment programs FASTA 3 (Pearson et al , Proc Νatl Acad Sci USA 85 2444-2448 (1988)) and PSI_BLAST (S F Altschul et al , Nucleic Acids Res 25 3389-402 (1997) The Dpi protems show distant but significant homology (24-26% identity over -160 residues) to many PrP sequences (FASTA_3 score E() =0 00015, where E() is the expected number of sequence matches by chance, PSI-BLAST E() =2X10(-41) for alignment with domestic dog PrP) Possible transmembrane segments were predicted usmg TopPred 2 (G von Heijne, JMol Biol 225 487-94 (1992)) and TMPred (See e g D S Millican et al , Endocr Res 24 387-90 (1998) Ν-terminal signal peptide cleavage sites for human and MoDop were predicted to be at the positions mdicated usmg the SignalP program (H Nielsen, Int J Neural Syst 8 581-99 (1997)) This program locates eukaryotic signal peptides with -70% accuracy However, 50% of Type-II N-terminal transmembrane segments are over-predicted as signal peptides The GPI attachment site (denoted w) and the succeedmg two residue positions (w+1 and w+2) have distinct residue preferences and aversions, e g preferences for seπne and aspartic acid at position w (Y Furukawa Biochim Biophys Acta 1328 185-96 (1997), P Harnson, unpublished data) and requires a stretch of mostly hydrophobic residues (8-31 residues long) m the cleaved C-termmal signal peptide This region is separated from w+2 by a 2-8 residue segment of greater hydrophilicity Both signals are found m the Dpi sequences The most likely GPI-anchor attachment site for Dpi ( {w. w+1, w+2} = G155A(G or A) ) was determined from an examination of the literature on GPI anchors, the sequence homology between the Dpls and PrPs at the C-termmus and from a representative set of GPI-anchored protems culled from sequence databases (P Harnson, unpublished data) (Figures 2 and 5 The GPI anchor site suggested by the multiple sequence alignment ( {w, w+1, w+2} = G157LR ) is unlikely (Fig 2) as w+1 is almost never a large aliphatic hydrophobic or aromatic (97%, 61/63 examples) and w+2 is almost never charged (or large aliphatic hydrophobic or aromatic) (98%, 62/63) (P Harnson, unpublished data) Other possible attachment sites m this region can be discounted for similar reasons, leavmg only the suggested site, (w, w+1, w+2} = G155A(G or A) EXAMPLE 5 Chrmenc Prnp/PrnD mRNAs In parallel to RACE and cDNA clomng, the gene-finding program "GRAIL" was also used to identify possible 5' exons of PrnD Analyses of the relevant area of the I/LnJ-4 cosmid sequence did not detect exon la lb, but mstead two putative sense-strand exons (nt 29,589-29,672, "Grail exon 6" and nt 29,798-29,832 , "Grail exon 7") between the Prnp and PrnD ORFs Usmg a pnmer located m "grail exon 6", cDNA could be amplified from adult wild-type mouse bram RNA, with DNA sequencmg confirming splicmg from a consensus splice donor at nucleotide 29,671 to the PrnD exon 2 splice acceptor (Figure 4) These RT-PCR analyses defined grail exon 6 as a new 5' exon (Figure 4, mtergene exon 2), but our failure to identify cDNAs mcludmg PrnD exon la/lb (or "Grail exon 7") suggested that conserved sequences 5 ' to exon la constitute the PrnD promote proper The possibility that the PrnD mRNAs mcludmg "Grail exon 6" ongmate from a more 5 ' promoter was then examined Smce the Prnp promoter is located only 23 Kb away and positioned m the conect transcnptional orientation, this was a strong candidate for the PrnD promoter as well Usmg a 5 ' pnmer located m the Prnp exon 2 and a 3' pnmer m PrnD exon 2, a product was obtamed m RT-PCR analyses of bram cDNA from wild-type mice (Figure 4) Sequence analysis of these cDNAs confirmed their chimeπc nature, that is, startmg withm the Prnp transcription unit at exon 2 and finishing withm the PrnD transcnption umt commencing at exon 2a (Figure 5) Some cDNAs mcluded the "Grail 6" exon inteφosed between Prnp exon 2 and PrnD exon 2a and thereby defined the 5 ' boundary of this exon as a consensus splice acceptor site at nucleotide 29,671 Other cDNAs mcluded an additional exon (mtergene exon 2), located between nucleotides 25,482 and 26004, m a sub-set of cDNAs Alternative use of these two mtergene exons results m a vanety of cmmenc mRNAs (Figure 5) Smce there is only one precedent for mtergemc splicmg in the chromosomal genes of a higher mammal (Magrangeas et al , J Biol Chem 273 16005-16010 (1998)). we sought to confirm our observations m a second species Usmg the sequence of rat Prnp exon 2 and EST All 36375 homologous to the 3' boundary of mouse PrnD exon 3, RT-PCR experiments were performed for rat tissues As predicted, analogous chimeπc cDNAs mcludmg two permutations of mtergene exons were detected m both bram and testis Smce the two mtergene exons are poorly conserved between mouse and rat (not presented) and fail to exhibit ATG codons m-frame with the PrnD ORF, we infer that they do not serve a protem codmg function EXAMPLE 6 Establishment of cell lmes expressmg Dpi In order to better study the activity and/or cellular mteractions of Dpi, a Dpi transgene was transfected mto an established neuronal cell lme Preparation of mammalian Dpl-expression vector A PrnD cDNA cassette was prepared by amplification of the B6-9 YAC clone with the ohgonucleotide pnmer pair DW174 = 5'-CGGAATTCCAGCCTTTCCCTTGCCGATTCAC-3' and DW175 - 5'-GCTCTAGAACTGGGCTACCTCTGTCTACCT-3' The 5' pnmer of this pair alters the exon 2a splice acceptor site Subsequent to digestion with EcoRl and Xba the resultmg cDNA cassette was gel-punfied and cloned mto the mammalian expression vector pcDNA3 0 (Invitrogen) Establishment of stable Dpi cell lmes pcDNA3 0_Dpl (l-5g) was added to 0 lmL Opti-MEM (Life Technologies), mixed with 15L LipofectAMINE reagent (Life Technologies) and then mcubated at room temperature for 40 minutes Neuro-2a cells [N2a] (ATCC), cultured m high glucose DMEM (Life Technologies) supplemented with 10% fetal bovme serum [FBS] (Life Technologies), were grown to 60% confluency m 6cm dishes Cells were washed twice with Hank's balanced salt solution [HBBS] and 2mL Opti-MEM was added to each dish The hpid/DNA mixture was added dropwise to each dish with gentle mixing and cells were mcubated for 5 hours at 37 °C Cells were also transfected with pcDNA3 0 lacking the PrnD insert, and Opti- MEM/hpid mixture containing no DNA as negative controls Cells were then fed with lmL DMEM containmg 20% FBS and mcubated overnight at 37°C Trypsimzed cells and seeded 10cm plates with DMEM containing 10% FBS and Img/mL G418 Individual colomes were selected and expanded after 2 weeks of selection, and maintained m DMEM containing 10% FBS and 0 3 mg/mL G418 Preparation of RNA from cell lmes Cells were grown to -80% confluency in 75 cm flasks, washed three times m 12mL HBBS Cells were scraped mto 5mL ice-cold HBBS and pelleted at 1,000 x g for 5 mm at 4°C Cells were resuspended m 1 mL ice-cold HBBS, transfened to a RNA-free microfuge tube (Ambion) and pelleted at 14,000 rpm for 1 minute at 4°C The pellet was resuspended m 0 6mL freshly-prepared denaturing solution (4M guanidinium thiosulphate. 25mM sodium citrate, pH 7 0, 0 5% Sarkosyl, 0 1M β-mercaptoethanol) Next 60L 2M sodium acetate, pH4 0, 0 6mL acid phenol, and 0 13 mL chloroform: isoamyl alcohol (24: 1) were added and the contents of the tube were mixed vigorously by shaking. The sample was incubated on ice of 20 min and then spun at 14,000 φm for 20 minutes at 4°C. The aqueous phase was transfened to a new RNA- free microfuge tube and 0.6 mL ice-cold isopropanol was added. The tube was mixed by shaking and incubated at -20 °C for 1 hr. The tube was then centrifuged at 14,000 φm at 4°C for 20 min and the isopropanol was removed. The pellet was washed in 75% ethanol, made up using DEPC-treated ddH 2 0, and incubated at -20 °C for 20 min. The tube was then spun at 14,000 φm at 4°C for 20 min and the ethanol was removed. After air-drying, the pellet was resuspended in 30L DEPC-ddH 2 0, IL was in order to determine the RNA concentration by absorbence at 260nm, and the sample was immediately snap frozen on dry-ice and stored at -80 °C. Northern blot analysis of Dpi cell lines RNA samples were thawed on ice and brought to 2 g/L using DEPC-ddH 2 0. 5L of each sample was analyzed. Total RNA was fractionated on a 1% agarose gel containing formaldehyde using the NorthemMax kit (Ambion). The gel was then transfened to Nytran-plus membrane (S&S) also using the NorthemMax kit. A 400bp Dpi ORF fragment was prepared by PCR amplification of the PrnD cDNA cassette using primers DW95 and DW96, and random primed using the DECAprime II kit (Ambion). The membrane was hybridized using ExpressHyb solution (Clontech) and exposed to X-ray film at -80 °C for 72 hr. EXAMPLE 7: Dpi regulation and Ataxic Behavior in Prnp 0/0 mice. The alteration of Prnd expression in Prnp 0/0 lines was determined to be responsible for the ataxia observed in two PrP knockout lines of mice lines (Sakaguchi et al., Nature 350:528-531 (1996)). cDNAs prepared from different lines of Prnp 010 mice were shown to behave differently in semi-quantitative RT-PCR analyses. The differences conesponded with both the structure of the ablated allele design and the PrP deficient phenotype. The structures of the 4 independently generated alleles is shown in Figure 6, and fit into two main classes: those that create in internal insertion/deletion within PrP exon 3 (Bύeler et al., Nature 355:577-582 (1992); Manson et al., Mol. Neurobiol. 5:121-127 (1994)), which do not develop a late-onset ataxia, and two alleles which remove the entire PrP ORF but also a -1Kb region 5' to exon 3, including the exon 3 splice acceptor site. Thus Zτc Prnp 0 ' 0 mice (Bϋeler et al., Nature 35(5:577-582 (1992)) with an intact splice acceptor gave no signal while the Rcm and Ngsk lines of Prnp 0/0 mice with an ablated exon 3 splice acceptor site showed a potent increase in chimeric mRNA relative to wild-type mice. Control PCR reactions performed with alternative primer sets were used to exclude trivial differences between cDNA preparations. Northern blot analyses showed that the expression levels of Dpi mRNA in brain increases dramatically in the two Prnp 0/0 lines developing ataxia. Thus, overexpression of PrnD mRNA in brain is associated with the development of ataxia in Ngsk and Rcm Prnp 0 ' 0 mice while the low levels of PrnD mRNA in brains of Zrch Prnp m mice is not accompanied by cerebellar dysfunction. The following table summarizes these results: Table 1: Cerebellar ataxia and Dpi expression in lines of Prnp ° mice Profound overexpression of Prnd occurs in Ngsk and Rcm lines of Prnp m mice, both of which develop ataxia. Equally striking is the finding that Purkinje cells remain healthy in Zrch Prnp 010 mice, which do not overexpress Prnd . These findings suggest that overexpression of Prnd is toxic for Purkinje cells. Because expression of chimeric mRNAs is dependent upon the Prnp promoter, it is of interest to note that this promoter is active in Purkinje cells (Nishida et al., in press), with a putative Purkinje cell-specific enhancer element infened to exist either within Prnp intron 2 or 3' of a Sal I restriction site in the PrnplPrnd intergene region (Fischer et al., EMBO J. 75: 1255-1264 (1996)). EXAMPLE 8: Predicted features of the Dpi proteins Comparison of available Dpi peptide sequences reveals a protein which is highly conserved, providing evidence that Dpi is a functional gene product under selection pressure (Figures 7 and 8). Mouse and rat Dpi proteins share >90% identity whilst mouse with the human Dpi ORF reveals 76% sequence identity. The majority of the sequence differences between mouse and human (94% , 32/34) are conservative amino acid replacements. Homology between Dpls and mouse PrP are estimated at between 20 and 24%, slightly lower than that observed between chicken and mammalian PrPs (31-34%) (Gabriel et al., Molecular cloning and structural analysis of a candidate chicken prion protein. In Prion Diseases of Humans and Animals, S. B. Prusiner, J. Collinge, J. Powell and B. Anderton, eds. (London: Ellis Horwood), pp. 407-431 (1992); Harris et al., Proc. Nat. Acad. Sci USA 55:7664-7668 (1991)). Dpls from all three species have a predicted N-terminal signal peptide cleavage site indicating that they are synthesized, like PrP c , in the secretory pathway (Figure 9). PrP has a cluster of basic residues rich in arginine and lysine adjacent to a predicted N-terminal signal peptide cleavage site and Dpi shares a similar feature between residues 25 to 38 m which 7 residues (50%) are basic Dpi lacks a convmcmg His/Pro/Gly πch octarepeat region which is present m PrP and which is thought to bmd Cu(II) ions in vivo (Brown et al , Nature 390 684-687 (1997), Stockel et al , Biochemistry 377185-7193 (1998). Niles et al . Proc Νatl Acad Sci USA 962042-2047) However, mouse Dpi has a short PSSGGQ motif between codons 41-46 very similar to a PSSGGS motif m chicken PrP codons 103-108 The significance of this observation is uncertain because we are unsure whether Dpi or PrP is the original molecule I e whether Dpi was generated from a recent gene duplication of PrP or vice versa Either way, it could be argued that this motif represents a vestigial repeat which has been deleted or, alternately, a prototypic umt which has been amplified to form the repeat structure withm PrP Dpi lacks a region with significant homology to the AGAAAAGA motif completely conserved m all known PrP sequences and conespondmg to mouse PrP codons 112-119 (Bamborough et al , Cold Spring Harb Svmp Quant Biol 61 495-509 (1996), Schatzl et al , J Mol Biol 245 362- 374 (1995)) This difference may mdicate a significant functional divergence between Dpi and PrP because this region of PrP has been shown to be cπtical for PrP topology m the ER membrane (Hegde et al , Science 279 827-834 (1998)) and the synthetic PrP peptide 106-126 has also been shown to be neurotoxic to primary cultures exposed to the synthetic peptide 106-126 (Forlorn et al , Nature 3(52 543-546 (1993)) PrP has a glycosylphosphatidylmositol (GPI) anchor attached at its C-terminus at serme 231 (Stahl et al , Ceil 57 229-240 (1987)) In common with PrP, Dpi has a hydrophobic C-termmal region and we predict that this may be compatible with either addition of a GPI anchor, or a smgle transmembrane domam Although Dpi lacks a seπne residue conespondmg to the hamster PrP codon 231 we predict that a GPI may be attached at glycine 155 GPI attachment at a glycme has been demonstrated for the α-folate receptor (W Yan and M Ratnam, Biochemistry 34 14594-600 (1995) It is interesting to note here that examination of rabbit PrP sequence mdicates that it has no seπne at residue 231 suggesting that either rabbit PrP is not GPI anchored or that it and may by GPI anchored by a glycme residue rather than a serme Conservation of residues on the surface of two evolutionary related protems is also a useful mdicator of functionally important elements (Lichtarge et al , J Molec Biol 257 342-358 (1996)) PrP has two consensus N-linked glycosylation sites of the form N-x-(S or T) The N-hnked glycosylation site analogous to N 181 on helix B in MoPrP is maintained m Dpi but the site on helix C is lost (Figures 8 and 9) That this is the only conserved exposed residue on helix B argues for some functional significance of this complex-type glycosylation An additional conserved consensus N-hnked glycosylation site is found 13 residues ammo terminal of this position (MoDop N-72 NVT) m an exposed surface loop A minority of N-glycosylated protems (-23%) have one or more unglycosylated consensus sites and only -10% of consensus sites in N-glycosylated protems are unglycosylated (G von Heyne, JMol Biol 225 487-94 (1992)) indicating that this site is also likely to be glycosylated Electrostatic surface potential, evaluated usmg the GRASP algoπthm (Nicholls et al , Proteins 11 281-296 (1991)), is distinct for Dpi and PrP PrP has positively and negatively charged patches on opposmg faces of the molecule (R Riek, Nature 382 180-2), suggestmg an onentation for membrane bmdmg Dpi mamtams the positively charged patch seen m PrP although the boundaπes of these areas are not equivalent but lacks a similar negatively charged patch (data not shown), havmg mstead a neutral/positively-charged area In general, the surface of Dpi has more positive charge m areas which, m the case of PrP, are thought not to be conformationally heterogeneous (i e , residues 121-231) It seems likely that this disposition of charged surfaces might play a role m multimeπzation if are found to mteract Whether Dpi and PrP compete for bmdmg to a common receptor remams to be established Smce PrnD mRNAs are expressed at low levels in the CNS of wild-type ammals, we sought evidence for Dpi protem in transfected cells expressmg PrnD cDNAs under the control of a constitutive cytomegalovirus promoter Untransfected N2A cells and stably transfected lmes containing the CMV/Dpl plasmid and grown m the presence of selectable marker were harvested for RNA, which was assessed by Northern blot analysis Whereas 1 7 and 2 8 kb PrnD mRNAs were detected m a positive control (testis RNA), they were undetectable m untransfected N2A cells Of 8 stable transfected lmes, 5 displayed abundant levels of the anticipated 0 9 kb mRNA denved from the expression plasmid EXAMPLE 9 Detection of Dpi by Western Blot Two clones with abundant mRNA were selected for further analysis by western blotting, usmg an antiserum directed against the Dpl-2 synthetic peptide A 15 residue Dpi peptide 2 (NH-CFGAEGNRYYAANYY-COOH) (SEQ ID NO 5) conespondmg to residues 71 to 84 was synthesized by FMOC chemistry and conjugated to maleimide activated KLH via a free sulfhydryl group The conjugated peptide was mixed with RIBI pπor to subcutaneous inoculation of rabbits Cells were grown to approximately 80% confluency m 10cm dishes and washed three times with lOmL HBBS Cells were then scraped mto 5mL HBBS and pelleted at 1,000 x g for 5 mm at 4°C The pellet was resuspended in O l mL cell lysis buffer (20 mM Tπs-HCl, pH8 0. 150 mM NaCl, 0 5% Tnton X-100, 0 5% sodium deoxycholate, 1 Complete mini protease inhibitor tablet (Boehπnger Mannheim)), transfened to a steπle microfuge tube and agitated at 4°C for 10 mmutes Samples were pelleted at 14,000 φm at 4°C for 10 mmutes and supernatant was transfened to a clean microfuge tube Protem concentration was determined by Bradford assay as recommended by the manufacturer (Bio-Rad) Samples were brought up m 4X Laemmli buffer and stored at -80 °C Both lmes exhibited an mtense heterodisperse signal with an apparent M r of 30-36 KDa A comparable signal was absent from untransfected cells and clones not expressmg PrnD mRNA Normalized gel-loadings were confirmed by probmg with an anti-actrn antiserum Smce the predicted size of the full-length and N- and C-terminally processed Dpi is 20 4 and 14 9 KDa, respectively, we mfer that Dpi is heavily glycosylated This conclusion is m accord with the presence of two consensus sites for N-hnked glycosylation sites and offers a strong parallel to the case of PrP c itself, where a 20 KDa polypeptide cham is mcreased m size to 33-35 KDa by the addition of carbohydrate chains EXAMPLE 10 Rescue of Dpi defect by overexpression of PrP transgene The rescue of Ngsk Prnp 0/0 mice by high level expression of a MoPrP-A encoded by a transgene anay argues that Dpi overexpression m these mice can be overcome by high levels of PrP c (Nishida et al, m press) In these Tg(P)Ngsk/Rrft/-> 0/0 mice that were rescued by overexpression of a wild type MoPrP-A, the transgene was constructed usmg the Synan hamster Cos Tet vector, which does not contain the Dpi ORF (Scott et al , Protem Sci 7 986-997 (1992)) The levels of PrnD mRNA m Tg(P NgsklPrnp 0/0 mice did not differ from Ngsk Prnp 010 httermates lacking this transgene Thus, if cerebellar degeneration is directly attributable to overexpression of PrnD, "rescue" by overexpression of PrP must proceed by a postranscnptional mechanism presumably PrP c titrates-out the toxic effects of Dpi, perhaps by a direct physical interaction or competition for a common receptor While the present invention has been descπbed with reference to the specific embodiments thereof, it should be understood by those skilled m the art that vaπous changes may be made and equivalents may be substituted without departing from the tme spmt and scope of the mvention In addition, many modifications may be made to adapt a particular situation, matenal, composition of matter, process, process step or steps, to the objective, spmt and scope of the present mvention All such modifications are mtended to be within the scope of the claims appended hereto

Claims: CLAIMS What is claimed is: 1. An isolated doppel (Dpi) polypeptide. 2. The human Dpi polypeptide of claim 1, comprising an amino acid sequence selected from the group consisting of SEQ ID NOJ, SEQ ID NO:3, and SEQ ID NO:4. 3. An isolated nucleic acid sequence or complement thereof comprising a nucleic acid sequence encoding a Dpi polypeptide of claim 1. 4. An isolated nucleic acid sequence of claim 3 comprising a nucleic acid sequence of SEQ ID NOJ . 5. A recombinant expression vector comprising the nucleic acid sequence of claim 3. 6. An isolated recombinant host cell containing the nucleic acid sequence of claim 1. 7. A transgenic animal, comprising a genome wherein at least one allele of an endogenous Dpi gene is altered. 8. The transgenic animal of claim 7, wherein the alteration results in enhanced expression from the Dpi gene. 9. The transgemc animal of claim 7, wherein the animal further comprises a genome wherein at least one endogenous PrP allele is ablated. 10. The transgenic animal of claim 9, wherein said genome having operatively inserted therein an exogenous PrP gene from a genetically diverse species. 11. An isolated antibody that specifically binds a human Dpi polypeptide of claim 1. 12. A method for facilitating the detection of prion infectivity, the method comprising the steps of: obtaining sample tissue from an animal to be tested; inoculating the transgenic animal of claim 7 with the sample; and observing the transgenic animal for symptoms of prion disease.

Inventors: HOOD LEROY E, LEE INYOUL, MOORE RICHARD, PRUSINER STANLEY B, TREMBLAY PATRICK, WESTAWAY DAVID, HOOD, LEROY, E., LEE, INYOUL, MOORE, RICHARD, PRUSINER, STANLEY, B., TREMBLAY, PATRICK, WESTAWAY, DAVID
Applicants: HOOD LEROY E, LEE INYOUL, MOORE RICHARD, PRUSINER STANLEY B, TREMBLAY PATRICK, UNIV CALIFORNIA, UNIV TORONTO, UNIV WASHINGTON, WESTAWAY DAVID, GOVERNING COUNCIL OF THE UNIVERSITY OF TORONTO, HOOD, LEROY, E., LEE, INYOUL, MOORE, RICHARD, PRUSINER, STANLEY, B., THE REGENTS OF THE UNIVERSITY OF CALIFORNIA, THE UNIVERSITY OF WASHINGTON, TREMBLAY, PATRICK, WESTAWAY, DAVID
Application year: 2000
Main classification: C12N15
IPC codes: A01K67, C07K14, C07K16, C12N1, C12N5, C12N15, G01N33