DNA SEQUENCES ENCODING DYSTROPHIN MINIGENES AND METHODS OF USE THEREOF

Abstract: The present invention provides a series of novel dystrophin minigenes that retain the essential biological functions. The expression of the dystrophin minigenes may be controlled by a regulatory element along with a small polyadenylation signal. The entire gene expression cassettes may be readily packaged into a viral vector, preferably an AAV vector. The present invention further defines the minimal functional domains of dystrophin and provides ways to optimize and create new versions of dystrophin minigenes. Finally, the present invention provides a method of treatment for Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD).

Description: DNA sequences encoding dystrophin minigenes and methods of use thereof TECHNICAL FIELD The present invention relates to novel dystrophin minigenes that retain the essential biological functions of a full length dystrophin gene, and methods of treatment for Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) in a mammalian subject using the dystrophin minigenes. BACKGROUND OF INVENTION Duchenne muscular dystrophy (DMD) is an X-linked genetic muscle disease affecting 1 of every 3,500 newborn males (Kunkel et al. Nature (London) 322,73-77 [1986]). The progressive muscle degeneration and weakness usually confine the patients to wheelchairs by their early teens, and lead to death by their early twenties. DMD is caused by recessive mutations in the dystrophin gene, the largest gene known to date, which spans nearly 3 million base-pairs on the X-chromosome with 79 exons, a coding sequence of about 1 1 kb, and a high rate of de novo mutations. (Koenig et al. Cell 50, 509-517 [1987]). Dystrophin is an enormous rod-like protein of 3,685 amino acids (aa) localized beneath the inner surface of muscle cell membrane (Watkins, S. C. et al. Nature 333, 863-866 [1988]). It functions through four major structural domains: a N-terminal domain ( 1 -756 aa), a central rod domain (757-3122 aa), a cysteine rich (CR) domain (3123-3409aa), and a distal C-terminal domain (3410-3685 aa). The N-terminal domain binds to the F-actin of cytoskeletal structures, while the CR domain along with the distal C-terminal domain anchors to the cell membrane via dystrophin-associated protein (DAP) complexes, thus, dystrophin crosslinks and stabilizes the muscle cell membrane and cytoskeleton. The central rod domain contains 24 triple-helix rod repeats (R1-R24) and 4 hinges (H1-H4). Each repeat is approximately 109 aa long. (Koenig et al. 7 Biol Chem 265, 4560-4566 [1990]). The central rod domain presumably functions as a "shock absorber" during muscle contraction. Dystrophin crosslinks and stabilizes the muscle cell membrane and cytoskeleton. The absence of a functional dystrophin results in the loss of DAP complexes and causes instability of myofiber plasma membrane. These deficiencies in turn lead to chronic muscle damage and degenerative pathology. The vast majority of DMD mutations disrupt the dystrophin mRNA reading frame or introduce a stop codon that prematurely ends protein translation (Monaco et al. Genomics 2, 90-95 [1988]). In the less severe allelic form of the disease, Becker muscular dystrophy (BMD), dystrophin gene mutations are usually such that the mRNA reading frame is maintained. Thus in BMD patients, some functional gene product, albeit of reduced quantity and/or quality, is synthesized that contributes to the milder phenotype (Hoffman et al. N. Engl. J. Med. 318, 1363-1368 [1988]). The mdx mouse (Bulfield et al. Proc. Natl. Acad. Sci. USA 81, 1189-1 192 [1984]) is an animal model of DMD. The genetic lesion in the mdx dystrophin gene is a nonsense mutation at base 3185 of the mRΝA that causes premature termination of translation within exon 23. This nonsense mutation precludes synthesis of a functional protein. Due to the lack of effective treatment for DMD, novel genetic approaches including cell therapy and gene therapy have been actively explored. However, clinical trials of myoblast transplantation have met with little success owing to the poor survival of the transplanted cells (Gussoni et al., Nature Med 3, 970-977 [1997]). It was recently reported that gentamicin treatment in mdx mice led to the suppression of the premature stop codon in the dystrophin gene, and the subsequent expression and localization of functional dystrophin to the cell membrane (Barton-Davis et al. J Clin Invest. 104, 375- 381 [1999]). This treatment could prove effective in up to 15% of patient with DMD. Somatic gene transfer offers a new approach to replace the defective dystrophin gene. A preferred approach for introducing genetic material encoding a gene product into an organ or a tissue is by use of a viral vector. In this situation, the genetic material encoding the gene product is inserted into the viral genome (or a partial viral genome). The regulatory elements directing the expression of the gene product can be included with the genetic material inserted into the viral genome (i.e., linked to the gene inserted into the viral genome) or can be provided by the viral genome itself, for example, a retrovirus long terminal repeat (LTR) or an adeno-associated virus (AAN) inverted terminal repeat (ITR). Infection of cells with a viral vector has the advantage that molecules encoded within the viral vector, e.g., by a cDΝA contained in the viral vector, are expressed efficiently in cells which have taken up viral vector nucleic acid and viral vector systems can be used in vivo. Different viral vectors are described separately in the subsections below. /. Adenovirus vectors: The genome of an adenovirus can be manipulated such that it encodes and expresses a gene product of interest but is inactivated in terms of its ability to replicate in a normal lytic viral life cycle (Curiel , Ann N Y Acad Sci 886, 158-71 [1999]). Suitable adenoviral vectors derived from the adenovirus strain Ad type 5 dl324 or other strains of adenovirus (e.g., Ad2, Ad3, Ad7 etc.) are well known to those skilled in the art. Recombinant adenoviruses are advantageous in that they do not require dividing cells to be effective gene delivery vehicles and can be used to infect a wide variety of cell types, including airway epithelium, endothelial cells and muscle cells. Additionally, introduced adenoviral DNA (and foreign DNA contained therein) is not integrated into the genome of a host cell but remains episomal, thereby avoiding potential problems that can occur as a result of insertional mutagenesis in situations where introduced DNA becomes integrated into the host genome (e.g., retroviral DNA). Moreover, the carrying capacity of the adenoviral genome for foreign DNA is large (up to 8 kilobases) relative to other gene delivery vectors (Haj-Ahmand et al. J. Virol. 57, 267-273 [1986]). Most replication-defective adenoviral vectors currently in use are deleted for all or parts of the viral El and E3 genes but retain as much as 80% of the adenoviral genetic material. Adenoviral vectors deleted for all viral coding regions are also described by Kochanek et al. and Chamberlain et al. (U.S. Pat. No. 5,985,846 and U.S. Pat. No. 6,083,750). Adenovirus vectors have been successfully tested in dystrophic animal models (Ragot et al. Nature 361, 647-50 [1993]; Howell et al. Hum Gene Ther 9, 629-34 [1998]). Nonetheless, the immunogenicity and inefficiency of infecting mature muscle cells remain major hurdles to overcome before the adenovirus vectors can be safely used in humans. 2. Herpes simplex virus (HSV) vectors: The main feature of an HSV vector is that it has very large packaging capacity, is usually replication defective and does not integrate into the host genome. HSV infects cells of the nervous system (Fink et al. Annu Rev Neurosci 19, 265-287, [1996]). The virus contains more than 80 genes, one of which (IE3) can be replaced to create the vector. The generation of HSV vectors with deletions in many of the immediate early gene products has resulted in vectors with reduced toxicity and antigenicity, as well as prolonged expression in vivo. However, these modifications also result in a lower virus yield. Construction of HSV vectors is described in U.S. Pat. No. 5,661,033. 3. Retrovirus vectors: Defective retroviruses are well characterized for use in gene transfer for gene therapy purposes (Miller AD Blood 76, 271-278 [1990]). The members of the family Retroviridae are characterized by the presence of reverse transcriptase in their virions. There are several genera included within this family, including Cisternavirus A, Oncovirus A, Oncovirus B, Oncovirus C, Oncovirus D, Lentivirus, and Spumavirus. A recombinant retrovirus can be constructed having a nucleic acid encoding a gene product of interest inserted into the retroviral genome. Additionally, portions of the retroviral genome can be removed to render the retrovirus replication defective. The replication defective retrovirus is then packaged into virions which can be used to infect a target cell through the use of a helper virus by standard techniques. Protocols for producing recombinant retroviruses and for infecting cells in vitro or in vivo with such viruses can be found in "Current Protocols in Molecular Biology, Ausubel, F. M. et al. (eds.) Greene Publishing Associates, (1989), Sections 9.10-9.14" and other standard laboratory manuals. Examples of suitable retroviruses include pLJ, pZJP, pWE and pEM which are well known to those skilled in the art. Examples of suitable packaging virus cell lines include .psi. Crip, .psi.Cre, .psi.2 and .psi.Am. Retroviruses have been used to introduce a variety of genes into many different cell types, including epithelial cells, endothelial cells, lymphocytes, myoblasts, hepatocytes, hematopoietic stem cells, in vitro, and/or in vivo (U.S. Pat. No. 4,868,116; U.S. Pat. No. 5,449,614 and U.S. Pat. No. 6,207,455). Retroviral vectors require target cell division in order to be integrated into the host genome to stably introduce nucleic acid into the cell. Thus, it may be necessary to stimulate replication of the target cell. Successful transductions of hematopoietic stem or progenitor cells with retroviral vectors in an ex vivo setting have been reported. However, Recombinant retroviral vectors can only accommodate about 8 kb to 10 kb of foreign DNA. This packaging capacity also limits its use in the genetic treatment of DMD. 4. Lentivirus vectors. Lentiviruses also belong to the retrovirus family, but they can infect both dividing and non-dividing cells. The best-known lentivirus is the human immunodeficiency virus (HIV), which has been disabled and developed as a vector for in vivo gene delivery. Like the simple retroviruses, HIV has three genes termed gag, pol and env, it also carries genes for six accessory proteins termed tαt, rev, vpr, vpu, nef and vif. Using the retrovirus vectors as a model, lentivirus vectors have been made, with the transgene enclosed between the LTRs and a packaging sequence (Naldni et al. Science 272, 263-267 [1996]). Some of the accessory proteins can be eliminated without affecting production of the vector or efficiency of infection. When lentiviral vectors are injected into rodent brain, liver, muscle, eye or pancreatic islet cells, they give sustained expression for over six months. Little is known about the possible immune problems associated with lentiviral vectors. Furthermore, there seems to be no potent antibody response. A major concern about lentiviral vector is its safety in human applications. However, recent development in producing the third generation lentiviral vectors with more deletion in viral genes and improved safety may allow for the general application of lentiviral vectors to in vivo gene therapy. 5. Adeno-associated viruses (AA V) vectors: AAV is a naturally occurring defective virus that requires another virus, such as an adenovirus or a herpes virus, as a helper virus for efficient replication and a productive life cycle (Muzyczka et al. Curr. Topics in Micro, and Immunol. 158, 97-129 [1992]). AAV vector is the only viral vector system that is based on a non-pathogenic and replication defective virus. It is also one of the few viruses that may integrate its DNA into non-dividing cells, and exhibits a high frequency of stable integration (Flotte et al. Am. J Respir. Cell. Mol. Biol. 1, 349-356 [1992]; Samulski et al. J. Virol. 63, 3822-3828 [1989]). Vectors containing as little as 300 base pairs of AAV DNA can be packaged. AAV vectors have been successfully used to establish efficient and long-term gene expression in vivo in a variety of tissues without significant immune response or toxicity (Xiao et al. J. Virol. 70, 8098-108 [1996]; Kessler et al. Proc Natl Acad Sci USA 93, 14082-7 [1996]; Xiao et al. J Virol 72, 10222-6 [1998]). Unlike other viral vectors, AAV readily bypasses extracellular barriers due to its small viral particle size (20 nm) that facilitates efficient transduction of muscle myofibers of various maturity (Pruchnic et al. Hum Gene Ther 1 1, 521-36 [2000]). AAV can also be delivered into a large number of muscle groups via the blood vessels (Greelish et al. Nat. Med. 5, 439-443 [1999]) The unparalleled efficiency and safety have led to an increasing interest in AAV-mediated gene therapy for genetic muscle disorders, as well as for metabolic diseases. However, a major obstacle for AAV vectors is the limited packaging size that only allows for genes smaller than 4.7 kb (Song et al. Proc Natl Acad Sci USA 95, 14384-8 [1998]; Kay et al. Nat Genet 24, 257-261 [2000]), therefore precludes such large gene as dystrophin with a cDNA of 14 kb. Other viral vector systems that may have application in the subject invention have been derived from vaccinia virus (Chen et al. J Immunother 24, 46-57 [2001]), and several RNA viruses. The plus-strand RNA viridae, such as polio (Bledsoe et al. Nat Biotechnol. 18, 964-9 [2000]), hepatitis A (Romano G. Stem CellsJS, 19-39 [2000]), and sindbis virus (Wahlfors et al. Gene Ther 1, 472-80 [2000]) are being developed for high- level gene expression, following either viral infection or delivery of nucleic acids using a nonviral system. These viruses express a replicase protein that can specifically replicate the viral RΝA. By inserting a transgene in place of the viral capsid gene(s), it is possible to generate a chimeric RΝA that replicates autonomously in the cell and expresses a high level of protein from the plus-coding strand of RΝA. These viral vectors are well suited for immunization strategies in which high, transient gene expression is needed to induce an immune response to the transduced cells. In addition to the viral gene transfer vectors, powerful non-viral gene transfer vectors have also become available for clinical application in the past several years (Ropert et al. Braz J Med Biol Res. 32,163-9 [1999]; Lee RJ et al. Crit Rev Ther Drug Carrier Syst 14, 173-206 [1997]). These vectors rely on normal mechanisms used by mammalian cells for the uptake and intracellular transport of macromolecules to deliver genetic materials into cells. These vectors include cationic and other liposomes, DΝA- viral conjugates, RΝA/DΝA oligonucleotides and, surprisingly, naked DΝA molecules. Physical procedures, such as hydrodynamics-based and electroporation-based procedures have been used to improve gene transfer efficiency of some non-viral vectors (Zhang G. et al. Gene Ther 7, 1344-9 [2000]; Yamashita et al. Cancer Res. 61, 1005-12 [2001]). Recently, it was also reported that intraperitoneal injection of a β-galactosidase fused to the protein transduction domain from the human immunodeficiency virus TAT protein resulted in delivery of the fusion protein to all tissues in mice (Schwarze et al. Science, 3, 1569-1572 [1999]) Somatic gene transfer using non-viral vectors carrying dystrophin gene have been attempted [Acsadi et al. Nature 352, 815-818 [1991]; Rando et al. Proc. Natl. Acad. Sci USA 97, 5363-5368 [2000]). Transgene expression was achieved with only very limited efficiency. Previous attempts to generate dystrophin minigenes that were shorter than 1/2 of the full-length dystrophin failed to preserve the essential protective functions. Cox et al. and Greenberg et al. reported that expression of Dp 71 , a 71 kD non-muscle product of the dystrophin gene that consists of the cysteine-rich and C-terminal domains of dystrophin (exon 63-79), in the skeletal muscle of dystrophin deficient mdx mice restored normal levels dystrophin associated proteins (DAPs). However, expression of Dp71 failed to alleviate symptoms of muscle degeneration [Cox et al. Nature Genet 8, 333-339 [1994]; Greenburg et al. Nature Genet 8, 340-344 [1994]). Similarly, Yuasa et al (Yuasa et al. EE5S Lett 425, 329-336 [1998]; Yamamoto et al. Hum Gene Ther 1 1, 669-80 [2000]) demonstrated that expression of dystrophin minigenes with both intact N- and C- terminal domains and 1 to 3 central rod repeats in mouse skeletal muscle was sufficient to restore DAP complexes but insufficient to restore myofiber morphology and to prevent dystrophic pathology. SUMMARY OF THE INVENTION The present invention provides dystrophin minigenes that are significantly reduced in size without compromising essential functions in protecting muscles from dystrophic phenotypes. The present invention also provides viral vectors carrying the dystrophin minigenes that are capable of mediating efficient and stable correction of both biochemical and physiological defects in a mammalian subject. Furthermore, the present invention provides a method that is more convenient and less time-consuming to discern the dystrophin functional domains in vivo and to optimize the minigenes for DMD gene therapy. Finally, the present invention provides a method for treatment of muscular dystrophy. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the construction of highly truncated dystrophin minigenes and AAV vectors carrying the dystrophin minigene. FIG. 2a shows immunofluorescent (IF) analysis of the dystrophin and DAP complexes in gastrocnemius muscle of mdx muscle at 3-months after treatment with construct AAV-MCK-Δ3849 or AAV-MCK-Δ3990. FIG. 2b shows IF analysis of the dystrophin and DAP complexes in gastrocnemius muscle from 15-week old normal C57/B 10 mice, from mdx mice treated either with vector AAV-MCK-Δ3849, AAV-MCK-Δ3990 or AAV-MCK-Δ4173, or from untreated mdx mice. FIG. 3 a shows mini-dystrophin expression in mdx mice treated with AAV-MCK- Δ3849 at 10 days of age. The animals were sacrificed 6 months post viral injection. FIG. 3b shows dystrophin expression in untreated 6-month-old mdx mice. FIG. 3c shows mini-dystrophin expression in mdx mice treated with AAV-MCK- Δ3849 as adult. The animals were sacrificed 2-months post viral injection. FIG. 3d shows mini-dystrophin expression in mdx mice treated with AAV-MCK- Δ3990 as adult. The animals were sacrificed 2 months post viral injection. FIG. 3e shows mini-dystrophin expression in mdx mice treated with AAV-MCK- Δ3849 as adult. The animals were sacrificed 4 months post viral injection. FIG. 3f shows mini-dystrophin expression in mdx mice treated with AAV-MCK- Δ3990 as adult. The animals were sacrificed 4 months post viral injection. FIG. 3g shows mini-dystrophin expression in mdx mice treated with AAV-CMV- Δ3849 as adult. The animals were sacrificed 6 months post viral injection. FIG. 3h shows mini-dystrophin expression in mdx mice treated with AAV-CMV- Δ3990 as adult. The animals were sacrificed 6 months post viral injection. FIG. 4a shows protection of muscle plasma membrane integrity by dystrophin minigenes in mdx mice treated at 10 days of age. FIG. 4b shows protection of muscle plasma membrane integrity by dystrophin minigenes in mdx mice treated as adult. FIG.5a shows the IF analysis of mini-dystrophin expression from construct Δ2796, which contains two rods (rodl & rod24, see FIG.l). Note that the muscle cell moφhology and central nucleation were not improved after its injection into young mdx mice. FIG.5b. shows the EF analysis of mini-dystrophin expression from construct CMV-M3, which contains 1 rod (a hybrid rod between rod RI & rod R24, see FIG.l and Yuasa et al). Note that the muscle cell moφhology and central nucleation were not improved after its injection into young mdx mice. FIG.6. shows muscle force improvement after AAV-MCK-Δ3990 vector injection into the TA muscle of adult mdx mice. After 10 cycles of lengthening activation the untreated mdx TA muscles (n=8) had only 23% of the force output remaining, while the AAV treated mdx TA muscles (n=8) had nearly 40% of the force output remaining. DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION The practice of the present invention will employ, unless other wise indicated, conventional methods of histology, virology, microbiology, immunology, and molecular biology within the skill of the art. Such techniques are explained fully in the literature. All publications, patents and patent applications cited herein, whether supra or infra, are hereby incoφorated by reference in their entirety. As used in this specification and the appended claims, the singular forms "a," "an" and "the" include plural references unless the content clearly dictates other wise. A. Definitions In describing the present invention, the following terms will be employed, and are intended to be defined as indicated below. "Gene transfer" or "gene delivery" refers to methods or systems for reliably introducing a particular nucleotide sequence (e.g., DNA) into targeted cells. The introduced nucleotide sequences may persist in vivo in episomal forms, or integrate into the genome of the target cells. Gene transfer provides a unique approach for the treatment of acquired and inherited diseases, and a number of systems have been developed in the art for gene transfer into mammalian cells. See, e.g., U.S. Pat. No. 5,399,346. As used herein, the term "effective amount" refers to a level of infection which brings about at least partially a desired therapeutic or prophylactic effect in an organ or tissue infected by the method of the present invention. The infection with an effective amount of the vector carrying genetic material of interest can then result in the modification of the cellular activities, e.g., a change in phenotype, in an organ or a tissue that has been infected by the method of the present invention. In a preferred embodiment, the infection with an effective amount of the vector carrying genetic material of interest results in modulation of cellular activity in a significant number of cells of an infected organ or a tissue. A gene transfer "vector" refers to any agent, such as a plasmid, phage, transposon, cosmid, chromosome, liposome, DNA-viral conjugates, RNA/DNA oligonucleotides, virus, bacteria, etc., which is capable of transferring gene sequences into cells. Thus, the term includes cloning and expression vehicles, as well as viral and non-viral vectors. A vector may be targeted to specific cells by linking a target molecule to the vector. A targeting molecule is any agent that is specific for a cell or tissue type of interest, including for example, a ligand, antibody, sugar, receptor, or other binding molecule. The invention is also intended to include such other forms of vectors which serve equivalent functions and which become known in the art subsequently hereto. An "AAV vector" refers to vectors derived from an adeno-associated virus serotype, including human AAV-1 , AAV-2, AAV-3, AAV-4, AAV-5, AAV-6, avian AAV, ovian AAV, etc., and to vectors derived from more than one AAV serotype (hybrid AAV vectors). For example, a hybrid AAV vector may contain DNA sequences derived from both AAV-1 and AAV-2. An AAV vectors can have one or more of the AAV wild- type genes deleted in whole or part, preferably the rep and/or cap genes, but retain functional flanking ITR sequences. AAV vectors can be constructed using recombinant techniques that are known in the art to include one or more heterologous nucleotide sequences flanked on both ends (5' and 3') with functional AAV ITRs. In the practice of the invention, an AAV vector can include at least one AAV ITR and a suitable promoter sequence positioned upstream of the heterologous nucleotide sequence and at least one AAV ITR positioned downstream of the heterologous sequence. A "recombinant AAV vector plasmid" refers to one type of recombinant AAV vector wherein the vector comprises a plasmid. As with AAV vectors in general, 5' and 3' ITRs flank the selected heterologous nucleotide sequence. AAV vectors can also include transcription sequences such as polyadenylation sites, as well as selectable markers or reporter genes, enhancer sequences, and other control elements which allow for the induction of transcription. Such control elements are described more fully below. In addition, an "AAV vector" can be stably introduced into a cell line or cell lines for the puφose of viral particle production. Such a cell line is usually termed as AAV packaging cell line. As used herein, the term "recombinant AAV", "recombinant AAV particle" or "recombinant AAV virion" is defined as an infectious, replication-defective virus composed of an AAV protein shell encapsidating (i.e., surrounding with a protein coat) a heterologous nucleotide sequence, which in turn is flanked 5' and 3' by AAV ITRs. In this regard, single-stranded AAV nucleic acid molecules (either the sense/coding strand or the antisense/anticoding strand as those terms are generally defined) can be packaged into an AAV virion; both the sense and the antisense strands are equally infectious. When the recombinant AAV DNA is equal to or smaller than 50% of the full length viral genome (about 5,000 nucleotides), it can also be packaged as double-stranded haiφin- like DNA into AAV virion. Such virion is also fully infectious. The term "recombinant AAV particle" or "recombinant AAV virion"" also refers to a hybrid AAV particle in which the AAV protein shell and the encapsulated nucleotide sequence may be derived from AAVs of different serotype. For example, a hybrid AAV particle may contain AAV- 1 capsid proteins and AAV-2 ITRs, or vice versa. It is also possible to create hybrid AAV capsid proteins using coding sequences from two or more AAV capsid genes. In addition, the capsid protein of a recombinant AAV may be manipulated by mutation, deletion, and/or insertion of amino acid sequence in order to modify the tropism of the recombinant AAV (Wu et al. J. Virol 74, 8635-47 [2000];Girod et al. Nat Med 5, 1052-1056 [1999]). A number of techniques for constructing recombinant AAV are known in the art. See, e.g., U.S. Pat. No. 5,173,414, Lebkowski et al. Mol Cell Biol 8, 3988-3996 [1988]; Carter BJ, Current Opinion in Biotechnology 3, 533-539 [1992]; Muzyczka N, cited supra; and Zhou et al. J. Exp. Med. 179, 1867-1875 [1994]; Xiao et al. J. Virol. 72, 2224- 32 [1998]. The term "expression control element" or "regulatory element" refers collectively to promoter sequences, polyadenylation signals, transcription termination sequences, upstream regulatory domains, origins of replication, internal ribosome entry sites ("IRES"), enhancers, and the like, which collectively provide for the replication, transcription and translation of a coding sequence in a recipient cell. Not all of these control sequences need always be present so long as the selected coding sequence is capable of being replicated, transcribed and translated in an appropriate host cell. The term "promoter" is used herein in its ordinary sense to refer to a DNA regulatory sequence that are sufficient for RNA polymerase recognition, binding and transcription initiation. Additionally, a promoter includes sequences that modulate the recognition, binding and transcription initiation activity of RNA polymerase. Such sequences may be cis acting or may be responsive to trans acting factors. Depending upon the nature of the regulation, promoters may be constitutive or regulated. Examples of promoters are SP6, T4, T7,SV40 early promoter, cytomegalovirus (CMV) promoter, mouse mammary tumor virus (MMTV) steroid-inducible promoter, Moloney murine leukemia virus (MMLV) promoter, phosphoglycerate kinase (PGK) promoter, muscle creatine kinase (MCK) promoter, myosin promoter, α-actin promoter and the like. Alternatively, the modified versions of the above promoters and even the synthetic muscle promoters (Li et al. Nat Biotechnol 17, 241-245, [1999]) may be included. Finally, the promoter may be an endogenous AAV promoter or AAV inverted terminal repeat (ITR). The term "transduction" denotes the delivery of a DNA molecule to a recipient cell either in vivo or in vitro, via a replication-defective viral vector, such as via a recombinant AAV virion. The term "muscle cell" or "tissue" refers to a cell or group of cells derived from muscle, including but not limited to cells and tissue derived from skeletal muscle; cardiac muscle, smooth muscle, e.g., from the digestive tract, urinary bladder and blood vessels.. The term captures muscle cells both in vitro and in vivo. Thus, for example, an isolated cardiomyocyte would constitute a "muscle cell" for puφoses of the present invention, as would a muscle cell as it exists in muscle tissue present in a subject in vivo. The term also encompasses both differentiated and nondifferentiated muscle cells, such as myocytes, myotubes, myoblasts, cardiomyocytes and cardiomyoblasts, and progenitor cells, for example, the muscle derived stem cells or the bone marrow derived stem cells that can become muscle cells after differentiation "Operably linked" refers to an arrangement of elements wherein the components so described are configured so as to perform their usual function. Thus, control elements operably linked to a coding sequence are capable of effecting the expression of the coding sequence. The control elements need not be contiguous with the coding sequence, so long as they function to direct the expression thereof. Thus, for example, intervening untranslated yet transcribed sequences can be present between a promoter sequence and the coding sequence and the promoter sequence can still be considered "operably linked" to the coding sequence. The term "dystrophin minigene" refers to the novel dystrophin constructs created by extensive deletions in the central rod domain plus extensive deletion in the C-terminal domain of the human dystrophin cDNA. In addition, the dystrophin minigenes may contain a modified N-terminal domain in which DNA sequences surrounding the original protein translation initiation codon ATG are modified. The modified sequences enhance the mini-dystrophin protein synthesis. Alternatively, the dystrophin minigene may be a hybrid gene in which some of the domains are substituted with homologous domains from utrophin or spectrin genes (Tinsley et al, Nature 360, 591-593 [1992]; Koenig et al. Cell 53, 219-216 [1988]). In particular, utrophin is highly homologous to dystrophin in both structure and functions, so that their major domains should be interchangeable (Tinsley et al, Nature. 384, 349-353 [1996]; Deconinck et al, Nat Med. 3, 1216-21 [1997]; Rafael et al Nat Genet.19, 79-82 [1998];). For example, the N-terminal and/or the C-terminal domains of dystrophin may be substituted with the utrophin counteφarts in the dystrophin minigenes. Similarly, the central rod domain may consist of rod repeats from utrophin or spectrin genes. The dystrophin minigenes are smaller than the 5 kb packaging limit of AAV viral vectors. Furthermore, it is also plausible to construct a minigene of utrophin in a similar fashion as of the dystrophin minigene described in this invention. Because some DMD patients completely lack the dystrophin protein, the dystrophin minigene product may be a neo-antigen. Substitution of dystrophin domains with those of utrophin may lower immune responses. The term "mini-dystrophin" refers to the polypeptides encoded by the dystrophin minigenes. Most importantly, the mini-dystrophins harbor biological functions that can protect the muscle from dystrophic pathology and symptoms. The symbol "Δ" (delta) is a prefix for the dystrophin minigenes that contain deletions as described above. By "mammalian subject" is meant any member of the class Mammalia including, without limitation, humans and nonhuman primates such as chimpanzees and other apes and monkey species; farm animals such as cattle, sheep, pigs, goats and horses; domestic mammals such as dogs and cats; laboratory animals including rodents such as mice, rats and guinea pigs, and the like. The term does not denote a particular age or sex. Thus, adult and newborn subjects, as well as fetuses, whether male or female, are intended to be covered. The term "expression cassette" refers to a construct of genetic material that contains coding sequences and enough regulatory information to direct proper transcription and translation of the coding sequences in a recipient cell. B. Detailed description of the invention To explore the feasibility of using viral vectors for DMD gene therapy, we have devised strategies to create novel dystrophin minigenes, which are small enough to be packaged into retrovirus or AAV vectors, and yet retain the essential functions needed for protecting muscle from the pathological symptoms. We have created minigenes in which up to 75% of the central rod domain (20 of the 24 rods; 2 of the 4 hinges), as well as nearly all the C-terminal domain (exons 71-78), are deleted (FIG. 1). These novel dystrophin minigenes, as small as only one third (1/3) of the 1 1 kb full-length dystrophin coding sequence, are significantly smaller than the 6.3 kb Becker-form mini-dystrophin gene (England et al. Nature 343, 180-2 [1990]) that was widely used in transgenic and gene therapy studies in mdx mice. The minigene comprises the N-terminus sequence of the dystrophin gene, the C-terminal cysteine-rich (CR) domain of the dystrophin gene, at least hinges HI and H4 of dystrophin gene, and at least four rod repeats. The rod repeats may be chosen from the rod repeats of dystrophin, utrophin or spectrin genes, preferably from the 24 rod repeats of dystrophin gene, and most preferably from the group consisting of rod repeats RI, R2, R3, R22, R23 and R24 of dystrophin gene. The N- terminus of the dystryphin minigene may be modified to improve expression efficiency without affecting the functionality of the gene product. For example, the original sequence surrounding the translation initiation ATG codon of the dystrophin gene may be substituted by the Kozak sequence to increase the efficiency of protein synthesis. In one embodiment of the current invention, the three nucleotides upstream of the coding sequence may be changed from "AAA" to "CCA" and the fourth nucleotide in the coding sequence may be changes from "C" to "G". In addition, a portion or the entire N- terminus may be substituted by its counteφart of the utrophin gene. Similarly, the CR domain of the dystrophin minigene can also be substituted by its counteφart of the utrophin gene. The dystrophin minigenes may be introduced into a mammalian subject using a variety of methods. It may be introduced into the subject in an expression cassette as a naked DNA with or without hydrodynamic-based or electroporation-based proceduces. It may be introduced into the subject using non-viral vectors such as liposomes or virus- liposome complexes, or with viral vectors such as adenovirus, HSV, baculovirus, retrovirus, lentivirus, and preferably AAV. Expression of the dystrophin minigenes may be controlled by a number of regulatory elements, including but not limited to, AAV inverted terminal repeat (ITR), retrovirus long terminal repeat (LTR), cytomeglovirus (CMV) immediate early promoter and/or enhancer, CMV enhancer and chicken β-actin promoter (CB promoter), α-actin promoter, myosin promoter, muscle-specific creatine kinase (MCK) promoter and/or enhancer, and the like. Alternatively, the modified versions of the above promoters and the synthetic muscle promoters (Li et al. cited supra) etc. may also be used. Expression of dystrophin minigene may be detected by immunofluorescent staining and immunoblotting (Western blotting). The functionality of mini-dystrophin may be examined by determining whether the mini-dystrophins are capable of restoring the missing DAP complexes on the myofiber plasma membrane, including the sarcoglycan complex which is not found in untreated dystrophic muscle due to the primary deficiency of dystrophin. To further investigate the functionality of the novel mini-dystrophins, it is essential to demonstrate that they can protect muscle from the pathological phenotypes. The onset of the pathology in mdx mice starts at around three weeks of age with massive waves of myofiber degeneration/regeneration. This process is characterized by the presence of central nuclei in myofibers, a primary pathological sign of muscular dystrophies. The absence or reduction of central nucleation after gene therapy would suggest that the therapy is successful. The position of nuclei in a muscle fiber may be determined by DAPI staining or H & E staining. Muscle cryosections of 8 μm thickness may be immunofluorescently stained with the Mouse-on-Mouse Kit from the Vector Laboratories (Burlingame, CA) according to the manufacturer's protocol, except that the cryosections may be immediately treated with the blocking buffer without the fixation step (Li et al. Gene Ther 6, 74-82 [1999]). Monoclonal antibodies against dystrophin (NCL-Dys3 and NCL-Dys2) and against α-, β- , and γ-sarcoglycans (NCL-α-S ARC, NCL-β-SARC and NCL-γ-SARC) may be purchased from Novocastra Laboratories Ltd (Burlingame, CA). Muscle cell nuclei may be counterstained with 0.01% DAPI (Sigma, St. Louis, MO) for 10 minutes. Photographs may be taken with a Nikon TE-300 fluorescent microscope. Plasma membrane damage and leakage in dystrophic muscle is a major physiological defect and also a major pathological cause. To determine whether AAV mini-dystrophin treatment would be effective in protecting plasma membrane from mechanical damage, myofiber membrane integrity test may be performed by intravenous injection of Evans Blue dye. Evans Blue is a widely used vital red-fluorescent dye that is excluded by the healthy myofibers, but is taken up by the dystrophic myofibers containing leaky cell membrane due to contractile damages. A previous study of mdx mice revealed that the apoptotic myonuclei were mostly found in Evans Blue dye positive myofibers, thus correlating plasma membrane leakage and muscle cell apoptosis (Matsuda et al. J Biochem (Tokyo) 1 18, 959-64 [1995]). Evans Blue dye (10 mg/ml in PBS) may be injected into the tail vein of C57/B10 mice, mdx mice, and AAV vector-treated mdx mice at the dose of 0.1 mg/gram of body weight. Following dye injection, mice may be allowed continuous swimming for 20 minutes. At 15 hours after Evans Blue injection, muscles may be collected and cryosectioned. Evans Blue dye positive myofibers may be observed under the fluorescent microscope with Rhodamine filters. Muscle constrctile force improvement was evaluated in the mdx mice after treatment with AAV vectors containing the dystrophin minigene. Tibialis anterior (TA) muscles of 2 to 3 month old mdx mice were injected with AAV-MCK-3990 vector. The injection was given in such a way that one leg was treated while the other leg in the same animal was left untreated. The latter was used as a control. At 6 months after AAV treatment, the mdx mice were anesthetized with pentobarbital sodium (70mg/kg, i.p.) and the entire TA muscle was removed and mounted in a vertical tissue chamber for in vitro force measurement. The muscle was stimulated (Grass model S-88 stimulator and current amplifier) by use of monophasic rectangular pulses of cathodal current (1.0-ms duration). Maximum tetanic force (P 0 ) was assessed using a stimulation frequency of 75 pps delivered in a 500 ms duration train. Following the determination of P 0 , the ability of the TA muscle to sustain force generation during repetitive lengthening activations (which should induce maximal damage to the muscle) was assessed. Peak force measured prior to lengthening was termed P IS0 . Subsequently, the muscle was lengthened at a constant velocity of 1.0 L/s from 100 to 110% L υ . Stimulus trains were repeated every 2-min (duty cycle 0.004) for a total of 10 cycles. Changes in P ISO were used to index impairment of muscle function associated with the damages caused by lengthening activations. C. Preferred embodiments The following examples are meant merely to exemplify several embodiments, and should not be inteφreted as limiting the scope of the claims, which are delimited only by the specification. Example 1 Dystrophin minigenes and AA V vectors carrying the minigenes This example describes the construction of highly truncated dystrophin minigenes and AAV vectors carrying the minigenes. The dystrophin minigene constructs were made mainly by PCR cloning method using Pfu polymerase (Stratagene, CA) and human dystrophin cDNA (GenBank # NM 004006) as the template. For consistency, the numbering of the nucleotide only includes the 11 ,058 bp dystrophin protein coding sequence (SEQ ID NO: l). As depicted in FIG. 1, dystrophin minigene Δ4173 (SEQ ID NO:2) contains nucleotides 1-1992 (N-terminus, hinge HI and rods RI, R2 & R3, SEQ ID NO:3) and 8059-10227 (rods R22, R23 & R24, hinge H4 and CR domain, SEQ ED NO:4) and 1 1047-1 1058 (the last 3 amino acids of dystrophin, SEQ ID NO:5). Dystrophin minigene Δ3990 (SEQ ED NO:6) contains nucleotides 1-1668 (N- terminus, hinge HI and rods RI & R2, SEQ ED NO:7), 7270-7410 (hinge H3, SEQ ED NO:8), 8059-10227 (rods R22, R23 & R24, hinge H4 and CR domain, SEQ ED NO:4) and 1 1047-1 1058 (the last 3 amino acids of dystrophin, SEQ D NO:5). Dystrophin minigene Δ3849 (SEQ ED NO:9) contains nucleotides 1-1668 (N- terminus, hinge HI and rods RI & R2, SEQ ED NO:7), 8059-10227 (rods R22, R23 & R24, hinge H4 and CR domain, SEQ ED NO:4), and 11047-11058 (the last 3 amino acids of dystrophin, SEQ ED NO:5). Dystrophin minigene Δ3531 (SEQ ED NO: 10) contains nucleotide 1-1341 (N- terminus, hinge HI and rods RI, SEQ ED NO: 11), 8059-10277 (rods R22, R23 & R24, hinge H4 and CR domain, SEQ ED NO:4), and 1 1047-11058 (the last 3 amino acids of dystrophin, SEQ ED NO:5). Dystrophin minigene Δ3510 (SEQ ED NO: 12) contains nucleotide 1-1668 (N- terminus, hinge HI and rods RI & R2, SEQ ED NO:7), 8407-10277 (rods R23 & R24, hinge H4 and CR domain, SEQ ID NO: 13) and 1 1047-1 1058 (the last 3 amino acids of dystrophin, SEQ ED NO:5). Dystrophin minigene Δ3447 (SEQ ED NO: 14) contains nucleotide 1-1992 (N- terminus, hinge HI and rods RI , R2 & R3, SEQ ED NO:3), 8794-10277 (rod R24, hinge H4 and CR domain, SEQ ED NO: 15) and 1 1047-1 1058 (the last 3 amino acids of dystrophin, SEQ ED NO:5). The above constructs were made by blunt-end ligation of the Pfu amplified PCR products of each individual segment, so that all the protein coding sequences are precisely spliced together in frame. The PCR primers used in the reactions are listed in Table 1 : Table 1. PCR primers used in the cloning of dystrophin fragments Primer Names Primer Sequence(5'-3') Sequence position Forward primers: Fl (ATG- A) ATTTTCACCATGGTTTGGTGGGAAGAAG 1-19 (28bp) SEQ ID NO: 16 F2 (H3- 1 ) CAGCCTGACCTAGCTCCTGGACTGA 7270-7294 (25bp) SEQ ID NO: 17 F3 (R22-1 ) ACTCATAGATTACTGCAACAGTTCC 8059-8083 (25bp) SEQ ID NO: 18 F4 (R23-1) AGTTCTGACCAGTGGAAGCG 8407-8427 (20 bp) SEQ ID NO: 19 F5 (R24-1) ACCCTTGAAAGACTCCAGGAAC 8794-8816 (22 bp) SEQ ID NO:20 Reverse primers: RI (Rl-2) TCTATGTAAATTGCTTTGTT 1341-1361 (20 bp) SEQ ID NO:21 R2 (R2-2) GTCTTGTAAAAGAACCCAGCGGTCT 1668-1644 (25bp) SEQ ID NO:22 R3 (R3-2) CTGTGCTGTACTCΓTTTCAAGTTTT 1992-1968 (25bp) SEQ ID NO:23 R4 (H3-2) AGGTACCTCCAACATCAAGGAAGAT 7410-7386 (25bp) SEQ ID NO:24 R5 (Tail-2A) CTACATTGTGTCGGGAGTTTCCATGTTGTC 1 1058-1 1047, SEQ ID NO:25 10227-10210(30bp) The dystrophin minigenes were then subcloned into an AAV vector plasmid (SEQ ED NO:26) containing an MCK promoter, a 595 bp Hind IU BstE II fragment from plasmid p(+enh206) 358MCKCAT ( Shield et al. Mol Cell Biol 16, 5058-68 [1996]), and a 60 bp small polyA signal sequence, resulting in AAV vector constructs AAV-MCK- Δ4173 (SEQ ED NO:27), AAV-MCK-Δ3990 (SEQ ED NO:28), AAV-MCK-Δ3849 (SEQ ED NO:29), AAV-MCK-3531 (SEQ ED NO:30), AAV-MCK-3510 (SEQ ED NO:31) and AAV-MCK-3447 (SEQ ED NO:32). Similarly, the dystrophin minigenes were also cloned into an AAV vector plasmid (SEQ ED NO:33) containing a CMV promoter (620 bp) and the small polyA signal sequence, resulting in AAV vector constructs AAV-CMV-Δ3990 (SEQ ED NO:34), AAV-CMV-Δ3849 (SEQ ED NO:35). In addition, the dystrophin minigene Δ3849 was cloned into an AAV vector plasmid containing an MCK enhancer, a CMV promoter, and the small polyA signal sequence, resulting AAV vector construct AAV-E-CMV-3849 (SEQ ED NO:36). The recombinant viral vector stocks were produced precisely according to the three-plasmid co-transfection method as described by Xiao et al. (cited supra). The AAV viral vectors were subsequently purified twice through CsCl density gradient ultracentrifugation using the previously published protocols (Snyder et al. in Current Protocols in Human Genetics, eds. Dracopoli et al. [John Wiley & Sons Ltd., New York], pp. 12.1.1-12.2.23. [1996]). The vector titers of viral particle number were determined by DNA dot blot method (Snyder et al. cited supra), and were approximately 5 x 10 12 genome copies (GC) per ml. Example 2 Restoration of DAP complexes This example describes whether dystrophin minigene products still retain the major biochemical functionality including submembrane localization and interaction with dystrophin associated protein (DAP) complexes. Healthy C57/B10 mice and dystrophic mdx mice were purchased from The Jackson Laboratory (Bar Harbor, Maine). Ten-day old mdx pups or 50-day old mdx adult mice were injected into the hindleg gastrocnemius muscle with 50 μl (5 x 10 10 GC) of different AAV mini-dystrophin vectors. At three months and six months after vector injection, the muscles were collected for evaluation of mini-dystrophin expression and biochemical restoration of the DAP complexes, which were absent due to the primary deficiency of dystrophin. EF staining on thin sections of AAV treated muscles, using an antibody (Dys3) specific to human dystrophin, revealed widespread vector transduction and correct submembrane location of the mini-dystrophins in a majority of the myofibers, especially in muscles treated with AAV vectors containing dystrophin minigene Δ3849 or Δ3990 (FIG. 2a & 2b; FIG. 3a). As expected, the equivalent muscle from the age-matched healthy C57/B10 mice showed indistinguishable dystrophin staining pattern, when stained with an antibody (Dys2) that recognizes both mouse and human dystrophin C-terminal region (FIG. 2b). As expected, this antibody (Dys2) failed to stain the AAV treated mdx muscle due to deletion of the C- terminal region in our dystrophin minigenes (data not shown). This result further confirmed the identity of mini-dystrophins that were derived from the AAV vectors. Consistently, the untreated mdx control muscle showed no dystrophin staining (FIG. 2b) except the very few somatic revertant myofibers recognized by Dys2 antibody. Furthermore, injection of AAV mini-dystrophin vectors into the adult mdx muscle (gastrocnemius) showed similar results when examined for dystrophin expression at 2 and 4 months after injection of AAV MCK vectors (FIG. 3c-3f), or at 6 months after injection of AAV CMV vectors (FIG. 3g and 3h). Importantly, there was no cytotoxic T- lymphocyte (CTL) destruction against the myofibers that persistently expressed mini- dystrophins of human origin from AAV vectors, either driven by a CMV promoter or by a muscle-specific MCK promoter. Immunofluorescent staining using three antibodies against α, β, and γ sarcoglycans respectively, showed positive results in all of the consecutive thin sections adjacent to those stained with dystrophin antibodies (FIG. 2b). These results provided evidence of biochemical functionality of the mini-dystrophins, which lack the C-terminal domain but are still capable of interacting with the DAP complexes. Example 3 Amelioration of dystrophic pathology This example demonstrates that dystrophin minigene products can protect muscle from the pathological phenotypes. The onset of the pathology in mdx mice starts at around three weeks of age with massive waves of myofiber degeneration/regeneration. This process is characterized by the presence of central nuclei in myofibers, a primary pathological sign of muscular dystrophies. The absence or reduction of central nucleation after gene therapy would suggest that the therapy is successful. Therefore, we initially chose to test the AAV mini-dystrophin constructs in young mdx mice (10-day old) before the onset of central nucleation, to see whether muscle degeneration regeneration can be prevented. Histological examination of the mdx muscles at 3 and 6 months after AAV mini- dystrophin (containing more than 2 rod domains) treatment, which was prior to the onset of central nucleation, showed nearly exclusive (-98%) peripheral nucleation in the mini- dystrophin positive myofibers, as revealed by dystrophin immunostaining and myonuclei counterstaining with DAPI (FIG.2a, FIG. 2b first column; FIG.3a and Table 2). The mutual exclusivity between mini-dystrophin expression and central nucleation in the vector treated mdx muscle precisely mirrored that of the normal muscle (FIG. 2b and Table 2). In addition, the myofibers positive for mini-dystrophin expression also exhibited consistent myofiber sizes and polygonal shapes indistinguishable from those of the normal muscle (FIG. 2a and 2b). By contrast, the untreated mdx muscle showed extensive (75.4%) central nucleation (Table 2)), with additional signs of dystrophic pathology including wide variation of myofiber sizes, round myofiber shapes, and fibrosis (FIG. 2b). Noticeably, the mdx muscle treated with constructs Δ2796 and M3 (containing 2 rods and 1 rod, respectively, see FIG.l.) showed the same moφhology as the untreated mdx muscle, except for the positive EF staining of the mini-dystrophin, which is not functional in terms of improving muscle moφhology, pathology and preventing muscle degeneration/regeneration and central nucleation (FIG.5a & 5b and Table 2). Hence, treatment of dystrophic muscle by AAV vector with minigenes containing more than 2 rods (See FIG.1.)prevented dystrophic pathology and led to normal histology in terms of peripheral nucleation, consistent myofiber size and lack of fibrosis in the mini-dystrophin positive areas. These results unequivocally demonstrated the absence of muscle degeneration due to the therapeutic effects of the novel mini- dystrophins in young mdx mice. We subsequently tested AAV vectors containing dystrophin minigenes with more than 2 rods in treating adult mdx mice (45 days of age) after the onset of massive waves of degeneration/regeneration, to see whether the pathological process can be stopped or reversed. At the time of vector injection, a majority (-75%) of the myofibers already underwent degeneration/regeneration process and displayed central nucleation. At 2 months, 4 months and 6 months after AAV mini-dystrophin injection, widespread dystrophin expression was observed accompanied by normal myofiber moφhology and lack of fibrosis in the dystrophin positive areas (FIG. 3a and 3b). By contrast, muscle of untreated mdx mice (FIG. 3b), or areas of treated muscle without successful vector gene transfer, manifested progressive degeneration and fibrosis. In addition, a reduction of central nucleation in mini-dystrophin positive myofibers was observed (from approximately 75% before vector treatment to 35-50% after vector treatment; see Table 2). The partial reversal of central nucleation was also observed in healthy mouse muscle, where a majority of the myonuclei remained centrally located once experiencing a transient pathology such as myotoxin treatment (Martin et al. Muscle Nerve 1 1 :588-96 [1988]). Persistence of central nucleation was also observed after treatment of adult mdx muscle with a gutless adenovirus vector containing the full-length dystrophin cDNA. Based on the above observations, our novel mini-dystrophin genes (containing more than 2 rods) demonstrated therapeutic effects in ameliorating dystrophic pathology in both young and adult mdx muscles. Example 4 Protection of myofiber membrane integrity This example demonstrates that mini-dystrophins containing more than 2 rods (see FIG.1.) are capable of protect myofiber membrane integrity. Initially, Evans Blue was administered into the tail vein of mdx mice that were treated at young age (10-day old) with AAV vectors three months before. The age-matched untreated mdx mice and healthy C57/B 10 mice were used as controls. To induce mechanical stress, the mice were allowed to exercise by continuous swimming for 20 minutes. Muscles were then collected and examined for dystrophin expression as well as for Evans Blue dye uptake. As expected, muscle from healthy mice revealed uniform dystrophin staining across the muscle sections and no uptake of the dye by the myofibers (FIG. 4a, top row). The AAV vector treated mdx muscle showed results consistent with the healthy muscle, thus mutual exclusivity of dystrophin expression and dye uptake (FIG. 4a, second to fourth rows). Dye uptake (red fluorescence) was found only in myofibers that lacked mini-dystrophin expression in the areas not transduced by AAV vectors (FIG. 4a, 2 nd , 3 rd & 4 th rows). By contrast, the untreated mdx muscle revealed absence of dystrophin and extensive dye uptake (FIG. 4a, bottom row). More importantly, AAV mini-dystrophin treatment of adult mdx muscle also achieved similar results in protecting myofibers from plasma membrane leakage when analyzed at 2 months and 6 months after vector injection (FIG. 4b). These results unequivocally demonstrated the physiological functionality of the novel mini-dystrophins in maintaining membrane integrity and protecting myofibers from mechanical damages in both young and adult mdx mice. Table 2. AAV mini-dystrophin gene transfer in young and adult mdx mice Animals * n Age at Vector Months post % Dystrophin % Central nuclei** & vectors injection injection positive fibers mdx+ A3510 4 12 days 3 35-80 1.06(68/6413) mdx+A353\ 4 12 days 3 25-40 1.72(37/2140) mdx+A3S49 4 10 days 3 56-88 1.02(72/7098) mdx+A3990 4 10 days 3 50-80 0.99 (56/5652) mdx+A4H3 4 10 days 3 15-25 0.93 (26/2791) mdx+ Δ3849 4 10 days 6 40-60 2.80(51/1824) mdx+A3990 2 10 days 6 35-45 2.30 (34/1478) α +Δ3849 2 50 days 2 35-50 34.76(510/1467) mαj+Δ3990 2 50 days 2 35-40 34.18(685/2004) mαj+Δ3849 4 50 days 4 20-25 44.24 (615/1390) mdx+A3990 4 50 days 4 20-30 46.18(695/1505) C57/B10 4 No injection N/A 100 1.45(56/3860) mdx 4 No injection N/A <1 75.4(2382/3160) mdx+A2196 4 12 days 3 30-45 72 (3888/5400) mdx+M3 8 10-12 days 3 20-65 81 (5589/6900) Note: * Untreated control mdx and C57/B 10 mice were about 3 months old at the endpoints of experiments. AAV vectors were driven by a MCK promoter. ** All numbers were collected from dystrophin-positive myofibers which were photographed following immunofluorescent staining and DAPI counterstaining, except in untreated mdx mice which had extensive central nucleation and very few dystrophin- positive Example 5 Restoration of muscle strength This example demonstrates that mini-dystrophins restore muscle strength in mdx mice.Muscle constrctile force improvement was evaluated in the mdx mice after treatment with AAV vectors containing the dystrophin minigene. Tibialis anterior (TA) muscles of 2 to 3 month old mdx mice were injected with AAV-MCK-3990 vector. The injection was given in such a way that one leg was treated while the other leg in the same animal was left untreated. The latter was used as a control. At 6 months after AAV treatment, the mdx mice were anesthetized and the entire TA muscle was removed and mounted in a vertical tissue chamber for in vitro force measurement. The muscle was stimulated by the use of monophasic rectangular pulses of cathodal current (1.0-ms duration). Maximum tetanic force (P o ) was assessed using a stimulation frequency of 75 pps delivered in a 500 ms duration train. Following the determination of P o , the ability of the TA muscle to sustain force generation during repetitive lengthening activations (which should induce maximal damage to the muscle) was assessed. Peak force measured prior to lengthening was termed P lso . Subsequently, the muscle was lengthened at a constant velocity of 1.0 LVs from 100 to 1 10% L 0 . Stimulus trains were repeated every 2- min (duty cycle 0.004) for a total of 10 cycles. Changes in P IS0 were used to index impairment of muscle function associated with the damages caused by lengthening activations. As shown in Figure 6, after 10 cycles of lengthening activations the untreated TA muscles (n=8) had only 23% of the force output remaining, while the AAV treated TA muscles (n=8) had nearly 40% of the force output remaining. This result strongly indicates that the dystrophin minigene can protect the muscle from mechanical force induced damage, therefore, restore the muscle strength. In summary, these examples demonstrate that dystrophin gene can be successfully reduced to one third (1/3) of its 1 1 kb full-length coding sequence, without compromising essential functions in protecting muscles from dystrophic phenotypes. Moreover, we have demonstrated for the first time that intramuscular injection of AAV vectors carrying the novel human dystrophin minigenes can achieve efficient and long- term therapeutic effects in a mammalian animal model. Long-term correction of both biochemical and physiological defects in the dystrophic muscles was realized by the persistent mini-dystrophin expression from AAV vectors, and the apparent lack of CTL immune response against myofibers expressing human dystrophin. Example 6 Construction and testing of retroviral vectors Retrovirus based gene transfer vectors are widely used to introduce transgenes permanently into the in vitro cultured cells. Those target cells may be stable cell lines or primary cell cultures derived from freshly isolated tissues or bone marrow. Some of the primary cell cultures may contain progenitor cells or stem cells, for example, hematopoietic stem cells and muscle derived stem cells. Those stem cells possess the capability of differentiating into mature muscle cells, i.e. myotubes and myofibers. Therefore, ex vivo gene transfer of the dystrophin minigenes via the retroviral vector may be a useful method to treat muscular dystrophin by infecting the stem cells isolated from the patients, who lack the dystrophin protein due to mutations in the dystrophin gene. To examine the usefulness of retroviral vectors, we cloned dystrophin minigene Δ3990 or Δ3849 (PCR product) into the Stu I site of a retroviral vector plasmid pLNCX (Clontech, California, USA). Two retroviral vector plasmids were obtained respectively carrying dystrophin minigene Δ3990 or Δ3849 under the control of a CMV promoter. The retroviral vector particles were produced by transfecting the vector plasmid pLNCX- Δ3990 or pLNCXΔ3849 into the packaging cell line AmphoPack 293 (Clontech, California, USA). The above retrovirus particles were used to infect the myoblast cells isolated from the muscle tissue of mdx mice. Selection drug G418 was used to kill the cells not infected by the retroviral vector, which carried a Neo' gene to confer the G418 resistance. The G418 resistant myoblast cells containing the Δ3990 or Δ3849 minigene were induced to differentiation into myotubes by culturing with 2% horse serum in DMED media. The differentiated myotubes were subjected to immunofluorescent staining using monoclonal antibody (Dys-3) against the minigene protein product. A majority of the myotubes were stained positive for the dystrophin minigene expression, demonstrating the minigene can be successfully introduced into muscle progenitor cells by retroviral vectors. Such retroviral vector infected progenitor cells or stem cells may be used for the puφose of ex vivo gene therapy for Duchenne and Becker muscular dystrophies.

Claims: What is claimed is: An isolated nucleotide sequence encoding a dystrophin minigene comprising: (a) a N-terminal domain; (b) four to six rod repeats; (c) an HI domain of a dystrophin gene and an H4 domain of the dystrophin gene; and (d) a cysteine-rich domain, wherein the N-terminal domain is selected from a group consisting of a N- terminal domain of the dystrophin gene, a modified N-terminal domain of the dystrophin gene, and a N-terminal domain of a utrophin gene; the rod repeats are selected from a group consisting of rod repeats in the dystrophin gene, rod repeats in the utrophin gene, and rod repeats in a spectrin gene; the cysteine-rich domain is a cysteine-rich domain of the dystrophin gene or a cysteine-rich domain of the utrophin gene. 2. The isolated nucleotide sequence of claim 1 , further comprising a last three amino acids of a C-terminal domain of the dystrophin gene. 3. An isolated nucleotide sequence encoding a dystrophin minigene comprising: (a) a N-terminal domain of a dystrophin gene or a modified N-terminal domain of the dystrophin gene; (b) four to six rod repeats of the dystrophin gene; (c) an H 1 domain of the dystrophin gene and an H4 domain of the dystrophin gene; and (d) a cysteine-rich domain of the dystrophin gene, wherein said nucleotide sequence has fewer than 5,000 nucleotides. 4. The isolated nucleotide sequence of claim 3, further comprising an H2 domain of the dystrophin gene and/or an H3 domain of the dystrophin gene. 5. The isolated nucleotide sequence of claim 3, containing four rod repeats of the dystrophin gene. 6. The isolated nucleotide sequence of claim 3, containing five rod repeats of the dystrophin gene. 7. The isolated nucleotide sequence of claim 3, containing six rod repeats of the dystrophin gene. 8. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO:2, or a substantially complementary strand of SEQ ED NO:2. 9. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO:6, or a substantially complementary strand of SEQ ED NO:6. 10. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO:9, or a substantially complementary strand of SEQ ED NO:9. 1 1. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO: 10, or a substantially complementary strand of SEQ ED NO: 10. 12. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO: 12, or a substantially complementary strand of SEQ D NO: 12. 13. The isolated nucleotide sequence of claim 3, consisting of SEQ ED NO: 14, or a substantially complementary strand of SEQ ED NO: 14. 14. A recombinant adeno-associated virus vector, comprising the nucleotide sequence of claim 1 operably linked to an expression control element. 15. The recombinant adeno-associated virus vector of claim 14, wherein the expression control element is an MCK promoter or a CMV promoter. 16. A recombinant adeno-associated virus vector, comprising any one of the nucleotide sequence of claim 8, 9, 10, 1 1, 12 and 13, operably linked to an expression control element. 17. The recombinant adeno-associated virus vector of claim 16, wherein the control element is an MCK promoter or a CMV promoter. 18. A method of treating Duchenne muscular dystrophy and Becker muscular dystrophy in a mammalian subject, comprising: (i) providing a vector comprising the dystrophin minigene of claim 1 operably linked to an expression control element; (ii) administering an effective amount of said vector to the mammalian subject under conditions that result in the expression of the dystrophin minigene at a level that provides a therapeutic effect in said mammalian subject. 19. The method of claim 18, wherein the vector is a recombinant adeno- associated virus. 20. The method of claim 18, wherein the vector is a retrovirus. 21. A method of treating Duchenne muscular dystrophy and Becker muscular dystrophy in a mammalian subject, comprising: (i) providing a vector comprising a dystrophin minigene operably linked to an expression control element; (ii) administering an effective amount of said vector to the mammalian subject under conditions that result in the expression of the dystrophin minigene at a level that provides a therapeutic effect in said mammalian subject, wherein the dystrophin minigene has fewer than 5,000 nucleotides comprising: (a) a N-terminal domain of a dystrophin gene or a modified N-terminal domain of the dystrophin gene; (b) four to six rod repeats of the dystrophin gene; (c) an HI domain of a dystrophin gene and an H4 domain of the dystrophin gene; and (d) a cysteine-rich domain of the dystrophin gene. 22. The method of claim 21 , wherein the vector is a recombinant adeno- associated virus. 23. The method of claim 21, wherein the vector is a retrovirus.

Inventors: XIAO XIAO, XIAO, XIAO
Applicants: XIAO XIAO, XIAO, XIAO
Application year: 2001
Main classification: C12N
IPC codes: A61K38, C07K14