# Patent

## Title
Nucleic acid silencing sequences

## Abstract

The present invention features compositions and methods for introducing, into cells, nucleic acids whose expression results in chromosomal silencing. The nucleic acids are targeted to specific chromosomal regions where they subsequently reduce the expression of deleterious genes, or cause the death of deleterious cells. Where the nucleic acid sequence is a silencing sequence, it may encode an Xist RNA or other non-coding, silencing RNA. Accordingly, the present invention features, inter alia, nucleic acid constructs that include a transgene (e.g., a silencing sequence encoding an Xist RNA or other non-coding RNA that silences a segment of a chromosome); first and second sequences that direct insertion of the silencing sequence into a targeted chromosome; and, optionally, a selectable marker.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 13/483,240, filed on May 30, 2012, now U.S. Pat. No. 8,574,900, issued on Nov. 5, 2013, which is a continuation of U.S. application Ser. No. 12/512,964, filed on Jul. 30, 2009, now U.S. Pat. No. 8,212,019, issued on Jul. 3, 2012, which claims the benefit of the filing date of U.S. Application No. 61/084,918, filed on Jul. 30, 2008, each of which is incorporated herein by reference in its entirety.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support under Grant Nos. GM053234, GM068138, HD007439, and GM096400 awarded by the National Institutes of Health. The U.S. Government has certain rights in the invention.


TECHNICAL FIELD
This invention relates to compositions and methods for silencing gene expression from a whole chromosome or chromosome segment, and more particularly to the use of an Xist gene or other chromosomal silencing RNA to silence expression from trisomic, translocated, duplicated, or partially duplicated genomic sequences.
BACKGROUND
Naturally occurring chromosomal imbalances are an exceptionally important clinical problem, in part because they are extremely common. Almost 1% of all live births and a much higher percentage of conceptions are affected. Many of the abnormalities involve “extra” chromosomal material, and many of these are so deleterious that they cause spontaneous abortion. Trisomies, in which the fetus carries three of a given chromosome rather than a pair, are usually lethal. Some are not; trisomy 13 (Patau syndrome), trisomy 18 (Edward syndrome), trisomy 21 (Down syndrome), triple-X syndrome, as well as duplications of the X and Y chromosomes (e.g., XXY and XYY) are seen in live births, although babies born with Patau syndrome or Edward syndrome usually do not live more than a year or two.
Down syndrome is extremely common relative to other severe genetic disorders. In the United States alone, over 350,000 people are living with the severe handicaps typical of Down syndrome, and there are millions of affected people around the world. Although Down syndrome children are often happy and highly loved, their disorder greatly impacts them, their entire families, and society. Mental retardation, with poor verbal functioning, is the most debilitating outcome, but there are also other medical issues, including much greater risks of early onset Alzheimer's disease, leukemia, and cardiac defects. Because Down syndrome individuals often are at or just below the threshold of independent functioning, even small increases in function could have significant positive consequences for them and their families.
Although the incidence of Down syndrome increases with the mother's age, 80% of Down syndrome babies are born to women under 35 who are not currently subject to prenatal screening. Upon birth of the Down syndrome baby, the whole family is faced with the enormous challenges associated with caring for and nurturing such a child. For older mothers who do have pre-natal screening, the parents are faced with the heart-wrenching decision of birthing a mentally retarded child or terminating the pregnancy, with no hope of systemic therapy. We believe that whole chromosome therapy would result in a paradigm shift in the minds of many scientists, families, and clinicians, who currently presume that gene therapy for this multi-gene disorder, with such pleiotropic effects, is just not possible.
SUMMARY
The present invention features compositions and methods for introducing, into cells, nucleic acids whose expression results in chromosomal silencing. The nucleic acids are targeted to specific chromosomal regions where they subsequently reduce the expression of deleterious genes, or cause the death of deleterious cells. Where the nucleic acid sequence is a silencing sequence, it may encode an Xist RNA or other non-coding, silencing RNA. Accordingly, the present invention features nucleic acid constructs that include a transgene (e.g., a silencing sequence encoding an Xist RNA or other non-coding RNA that silences a segment of a chromosome); first and second sequences that direct insertion of the silencing sequence into a targeted chromosome; and, optionally, a selectable marker. Below, we may refer to the first and second sequences that direct insertion of the silencing sequence into a targeted chromosome as “first and second targeting elements.” These sequences or elements can be readily selected and inserted into the nucleic acid constructs using methods well known in the art.
In the present application, we use the term “Xist” to refer to an Xist gene or the encoded Xist RNA regardless of the origin of the sequence. For example, the present compositions can include, and the present methods can be carried out with, an Xist gene encoding an Xist RNA from humans or another mammal (e.g., a rodent such as a mouse, dog, cat, cow, horse, sheep, goat, or another mammalian or non-mammalian animal). We mention this as the scientific literature has adopted a loose convention whereby the term is fully capitalized (XIST) when referring to a human sequence but not fully capitalized (Xist) when referring to the murine sequence. That convention is not used here, and we wish to make clear that human and non-human sequences may be used as described herein.
The “silencing sequence” is a nucleotide sequence that encodes an RNA that silences a chromosome or a segment or region thereof. While the invention is not limited to the use of silencing sequences that work by any particular molecular mechanism, silencing sequences are believed to encode RNA that binds across the chromosome or chromosome segment and induces repressive changes to chromatin that silence gene expression at the level of transcription. The silencing sequence can include, but is not limited to, a naturally occurring DNA sequence, and “silencing” is a term of art that is understood to refer to a significant reduction in the level of transcription of a gene within the silenced or targeted region of a chromosome.
The silencing sequence can be a full-length Xist gene sequence, a sequence encoding another full-length silencing RNA (examples of which are provided below), or any biologically active fragment or other biologically active variant thereof. The sequence is “biologically active” where its activity is sufficient to effect a therapeutically beneficial outcome. The level of activity of a biologically active fragment or other variant may vary so long as a useful chromosomal silencing RNA is produced. Xist RNA is referred to as a chromosomal silencing RNA because it silences by binding across the chromosome or chromosome segment, and therefore silences at the level of transcription, by inducing repressive changes to chromatin. While Xist RNA is a well studied example of a chromosomal silencing RNA, other non-coding RNAs can silence specific clusters of imprinted genes or segments of a chromosome. These other chromosomal silencing RNAs include Air RNA, HOTAIR RNA, and Kcnq1ot1 RNA (see Goodrich and Kugel, Crit. Rev. Biochem. and Mol. Biol. 44:3-15, 2009), any of which can be formulated and used as described herein for Xist. Other intergenic noncoding RNAs, which may be useful in the present nucleic acid constructs and the silencing methods described herein are described by Khalil et al. (Proc. Natl. Acad. Sci. USA 106:11675-11680, 2009).
The silencing sequence can exclude one or more introns (wholly or partially) or one or more exons (wholly or partially). However, the silencing sequence cannot exclude all exons. For example, the silencing sequence can be an Xist gene sequence exclusive of one or more introns or one or more exons (but not all exons). For example, the silencing sequence can include about 6 kb to about 10 kb of exon 1 of an Xist gene sequence (e.g., about 6-7 kb, 7-8 kb, 8-9 kb, 6.5-8.5 kb, or about 7.5 kb). More specifically, the silencing sequence can be or can include the Xist cDNA sequence having accession number M97168 or a biologically active fragment or other variant thereof (SEQ ID NO:1).
The silencing sequence can be a mammalian sequence (e.g., a human sequence) and can further include a regulatory sequence (e.g., a regulatory sequence that promotes expression of the Xist RNA). More specifically, the regulatory sequence can include a promoter, which may be constitutively active, inducible, tissue-specific, or a developmental stage-specific promoter. Enhancers and polyadenylation sequences can also be included.
The targeted chromosome can be any autosome or an X or Y chromosome. For example, the targeted chromosome can be chromosome 13, chromosome 18, or chromosome 21. The targeted chromosome can be the third chromosome within a trisomic cell or any region of a chromosome that is aberrant (e.g., a gene or genetic sequence that is duplicated, partially duplicated and/or translocated).
Numerous selectable markers can be incorporated in the nucleic acid constructs. These markers are discussed further below and many such markers will be known to one of ordinary skill in the art. Sequences including a selectable marker can, for example, upon transcription and translation, confer resistance to a toxin or encode proteins that produce an observable characteristic. Thus, expression of the selectable marker sequence allows one to distinguish or “select” genetically modified cells from non-modified cells.
Numerous vectors are also known in the art, and any vector (including viral and non-viral vectors) can be used to deliver the present nucleic acids to a patient or a cell in culture. Accordingly, the invention features vectors and isolated or cultured cells that include any of the nucleic acid constructs described herein.
In another embodiment, the invention features compositions (e.g., pharmaceutically acceptable compositions) that include the nucleic acid constructs or vectors described above (and elsewhere herein) and, alternatively or in addition, a vector that facilitates delivery of the transgene to a cell and/or incorporation of the transgene into the targeted chromosome. Thus, the invention encompasses pharmaceutically acceptable compositions that include nucleic acid constructs carrying a transgene (e.g., a silencing sequence), first and second sequences that direct insertion of the silencing sequence into a targeted chromosome and, optionally, a selectable marker. The targeted integration may be facilitated by inclusion in the construct of sequences homologous to the site of desired chromosomal integration (i.e., the first and second sequences or targeting elements), coupled with the transgene (e.g., sequence encoding Xist RNA or other chromatin-associated silencing RNA).
As noted, the invention features compositions that also include vectors that facilitate delivery of the transgene. Well established targeting methods that rely on homologous recombination can be made more efficient by use of zinc finger nucleases to direct integration at specific sites. Thus, the present compositions can include a cleavage vector comprising a sequence encoding a first chimeric zinc finger nuclease (ZFN) or an adeno associated virus, which can specifically integrate the transgene and deliver it to cells. We describe the ZFNs as “chimeric” as they include at least one zinc finger DNA binding domain effectively linked to at least one nuclease capable of cleaving DNA. Ordinarily, cleavage by a ZFN at a target locus results in a double stranded break (DSB) at that locus.
Various combinations of the constructs and vectors described herein can also be formulated as pharmaceutical compositions. For example, the present compositions can include an adeno associated virus into which a silencing sequence has been inserted or a combination of (a) a nucleic acid construct or vector that silences a targeted chromosomal region or induces cell death following targeted chromosomal integration and (b) a cleavage vector encoding a chimeric ZFN.
The cleavage vector can include more than one chimeric ZFN, any of which can include a DNA binding domain and a cleavage domain. The DNA binding domain binds a genomic sequence that is present in each of the two strands of the targeted chromosome such that the cleavage domain generates a double stranded break in the targeted chromosome at a site into which the first and second sequences will direct insertion of the silencing sequence.
The same cleavage vector that encodes a first chimeric ZFN can include one or more additional sequences encoding one or more additional chimeric ZFNs (e.g., two, three, four, or more ZFNs). Alternatively, additional chimeric ZFNs can be carried on a separate vector. The second and any subsequent chimeric ZFNs can include a DNA binding domain and a cleavage domain. The first chimeric ZFN and the second chimeric ZFN bind, respectively, to distinct sequences in each of the two strands of the targeted chromosome such that the respective cleavage domains generate a double stranded break in the targeted chromosome at a site into which the first and second sequences within the nucleic acid construct will direct insertion of the silencing sequence (or other sequence (e.g., a sequence that causes cell death)). As when the nucleic acid constructs or vectors, including adeno associated vectors, are used alone, the ZFNs can target an autosome. For example, ZFNs can target chromosome 13, chromosome 18, or chromosome 21.
Also within the scope of the invention are RNAs and proteins encoded by the cleavage vector and compositions that include them (e.g., lyophilized preparations or solutions, including pharmaceutically acceptable solutions or other pharmaceutical formulations).
In another embodiment, the invention features cells that include the nucleic acid constructs, vectors (e.g., an adeno associated vector), and compositions described herein. The cell can be isolated in the sense that it can be a cell within an environment other than that in which it normally resides (e.g., the cell can be one that is removed from the organism in which it originated). The cell can be a germ cell, a stem cell (e.g., an embryonic stem cell, an adult stem cell, or an induced pluripotent stem cell (iPS cell or IPSC)), or a precursor cell. Where adult stem cells are used, the cell can be a hematopoietic stem cell, a cardiac muscle stem cell, a mesenchymal stem cell, or a neural stem cell. The cell can also be a differentiated cell (e.g., a fibroblast or neuron).
The methods of the invention can be used to treat patients who have a birth defect, genetic disease, or cancer associated with a genetic aberration (e.g., a trisomy, partial duplication of a chromosomal region, translocation, or ring X-chromosome). Any of the methods can include the step of identifying a patient in need of treatment; any of the patients can be human; and any of the methods can be carried out by either administering the present compositions to the patient or removing cells from the patient, treating the cells, and “readministering” those cells. For example, the invention features methods of treating a genetic disorder associated with a trisomic chromosome by identifying a patient in need of treatment; and administering to the patient a nucleic acid construct, vector, and/or cleavage vector as described herein. The targeted chromosome can be the trisomic chromosome, and the amount of the construct or vector administered will be an amount sufficient to improve a condition associated with the disorder. Where cells are harvested from a patient to treat a condition or disorder described herein (or an associated symptom), the methods can include the steps of identifying a patient in need of treatment; harvesting cells from the patient; transfecting the cells with one or more of the types of constructs and/or vectors described herein; and administering to the patient a sufficient number of the transfected cells to treat the condition or improve a condition or symptom associated with the disorder. The symptoms associated with many birth defects and other conditions are well known. For example, individuals having Down Syndrome often experience mental retardation, hypotonia, cardiac defects, Alzheimer's Disease, hematological abnormalities and leukemia (see Antonarakis and Epstein, Trends Mol. Med. 12:473-479, 2006). As noted above, treatment can also be carried out in vivo by administering present compositions to the patient via pharmaceutically acceptable compositions.
The cells can include differentiated cells (e.g., white blood cells or fibroblasts) and/or undifferentiated cells (e.g., stem cells or precursor cells). The cells can also be differentiated cells that are induced, ex vivo, into iPS cells, or multi-potent stem cells or stem cells of particular lineage, such as neural stem cells. The condition can be a neurological or blood disorder such as Alzheimer's Disease and leukemia, respectively, or a muscular defect, including defects of the heart. Where the condition is myelodysplastic disease which leads to leukemia, it can be an acute lymphocytic leukemia, an acute myelogenous leukemia, or an acute megakaryoblastic leukemia.
In any of the present methods, cells can be transfected with a cleavage vector that includes a sequence encoding a first chimeric zinc finger nuclease (ZFN) having a DNA binding domain and a cleavage domain. As in other embodiments, the DNA binding domain binds a genomic sequence that is present in each of the two strands of the targeted chromosome such that the cleavage domain generates a double stranded break in the targeted chromosome at a site into which the first and second sequences will direct insertion of the silencing sequence or other therapeutically useful sequence (e.g., a toxin or pro-apoptotic protein). Where desirable, the transgene (e.g., a silencing sequence encoding an Xist RNA) can be targeted to a polymorphic sequence that is present in just one chromosome (e.g., one of a set of trisomic chromosomes). Additionally, integration of the Xist transgene can be targeted so as to directly disrupt a particularly deleterious gene, such as the APP gene, over-expression of which leads to the exceptionally high rate and early onset of Alzheimer's Disease among Down Syndrome individuals.
The invention also includes compositions and methods for the silencing of a duplicated genomic region or trisomic chromosome using an approach of random transgene integration followed by cell selection, where the silencing of the trisomic chromosomal material provides a significant selective advantage as compared to silencing of other disomic chromosomes. As a result, patient cells in which the deleterious extra chromosomal material has been silenced by a large non-coding RNA may have a selective advantage; thus, even where Xist transgenes have been inserted at random into the genome of patient cells, cells in which the trisomic chromosome has been silenced may be selected for over those cells in which a disomic chromosome has been silenced (since the latter would generate a functional monosomy that is likely lethal to the cell). In the case of translocated chromosomes, targeting of the transgene can be directed to the unique site generated at the translocation junction, to selectively silence that abnormal chromosome, or to introduce a sequence encoding a toxin, pro-apoptotic protein, or other factor that results in cell death of deleterious cells. For example, using the present methods, one can introduce a sequence encoding a toxin, pro-apoptotic protein, or other factor that results in cell death into a translocated chromosome associated with cancer. This approach can be extended to silencing of duplicated regions of specific chromosomes that are associated with genetic conditions, such as duplication of segments of Chromosome 15q11-13 in Autism. This duplication, which is thought to be the most frequent cytogenetic abnormality in autism, can be targeted by the present methods and is described further in Nakatani et al. (Cell 137:1236-1246, 2009). This approach can also be extended to silencing the inappropriate expression of imprinted regions in genetic disease, as in Prader-Willi/Angelman syndrome, also on Chr 15 and associated with autism.
To illustrate a particular application, Xist mediated chromosomal therapy could be used to ameliorate transient myeloproliferative disorder (TMD) in Down Syndrome children and possibly prevent the later development of acute leukemia. Successful bone marrow transplants for diseases like leukemia depend upon immune compatibility, to avoid Graft versus Host Disease (GVHD). To avoid graft rejection, the patient's own cells can be used and transgenically modified prior to transplant. There are two scenarios to acquire and modify stem cells for bone marrow transplant. In the first, the patient's own bone marrow stem cells can be obtained and an Xist transgene can be introduced and targeted to chromosome 21. When Xist expression silences the trisomic chromosome, these cells can then be transplanted back into the patient following standard bone transplant procedures following the destruction of the patient's bone marrow using irritation. Alternatively modified patient bone marrow cells can be transplanted without first irradiating the patient to destroy the unmodified bone marrow. This would produce a situation where the patient's bone marrow would be mosaic for trisomy 21 (a mixture of modified and unmodified cells). We expect that the modified cells would have a growth advantage over the non-modified fully trisomic cells, and the modified cells would eventually out grow the “diseased” cells (see Douillard-Guilloux et al., J. Gene Med. 11:279-287, 2009). In the second approach, the patient's fibroblast (skin) cells can be used to produce iPS cells, into which a transgenic Xist gene is inserted and targeted to chromosome 21. IPS cells that silence one of the three trisomic chromosomes will then be differentiated into adult hemopoietic stem cells and introduced back into the patient as stated above.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating the process of chromosomal inactivation. Three chromosomes are shown in the cell initially. Following targeted insertion of an Xist transgene, Xist RNA exerts its effect on chromosomal sequences that are cis to the Xist transgene, resulting in inactivation of one chromosome and a functionally disomic cell.
FIG. 2 is an image of chromosomes from an XXX mouse cell showing that Xist RNA (green in color and brighter in black-and-white) coats two X-chromosomes and inactivates both.
FIG. 3A is a photograph showing Xist RNA localized to an inactive X chromosome in a cell (upper photograph) and the corresponding DAPI dense Barr Body in the same cell (lower photograph).
FIG. 3B is a photograph showing Xist RNA localized to an inactive human Chr 4 carrying a transgenic Xist gene (upper photograph) and the lower panel shows the corresponding DAPI dense Barr Body in the same cell (lower photograph). Inactivation of the autosome occurred in an adult somatic cell (derived from a fibrosarcoma).
FIG. 4 is a schematic diagram of a karyotype of chromosomes involved in X;9 translocation, in which Xist RNA was shown to silence the duplicated Chromosome 9 material in the transgene, thereby avoiding the deleterious effects of partial Chr. 9 trisomy.
FIG. 5 is a panel of four images of the same cell showing, from left to right, Xist RNA localization, hnRNA, macroH2A localization, and H3K27me localization.
FIGS. 6A-E are a series of five contiguous panels showing the complete exon sequence of the Homo sapiens X (inactive)-specific transcript (Xist), GenBank Accession No. M97168.1, SEQ ID NO:1.
FIGS. 7A-J are a series of 10 contiguous panels showing the Homo sapiens DNA sequence from clone RP13-216E22 on chromosome Xq13.1-21.1 with the X (inactive)-specific transcript (non-protein coding) (Xist), non-coding RNA, GenBank Accession No. AL353804.22, SEQ ID NO:2.



DETAILED DESCRIPTION
Down Syndrome, caused by trisomy of chromosome 21, is the leading cause of mental retardation in newborns, impacting one in every 600-700 live births in the U.S. and across the world. Many families with Down Syndrome children find them loving, happy children, but they are challenged with mild to moderate mental retardation and frequently have a number of medical conditions requiring treatment or surgery. Despite the enormous clinical importance of Down Syndrome and related chromosomal imbalances, there has been little hope or effort for gene therapy in Down Syndrome. Historically, devising therapeutic strategies for trisomies has been particularly challenging because more is involved than a single defective gene or even several defective genes. Down syndrome, for example, involves a quantitative imbalance in tens or hundreds of genes across a 50 Mb chromosome, the most important of which are still not yet well understood (Antonarakis and Epstein, Trends Mol. Med. 12:473-479, 2006). Thus, unlike other genetic diseases in which silencing of an individual gene might produce an effective therapy, genetic therapy for trisomies such as Down syndrome is much more challenging. This is because chromosomal trisomies (and segmental duplications or translocations) involve the over-expression of potentially hundreds of genes across a  ̃50 Mb or larger chromosome, rendering ineffective standard approaches to gene therapy which treat single gene defects.
This invention makes possible an approach to “chromosome therapy” for trisomies or segmental duplications by reducing the challenge of individually correcting over-expression of tens or hundreds of genes, to the much more tractable approach of introducing just one gene, for example Xist, which then silences the whole chromosome.
We set out to develop an innovative approach to chromosome therapy that would translate the system nature devised to dosage compensate the X chromosome in females. Nature assures proper “gene dosage” of X-linked genes between females (XX) and males (XY) via the Xist gene, which produces a large, non-coding RNA (Brown et al., Cell, 71:527-542, 1992; Clemson et al., J. Cell Biol. 132:259-275, 1996) that is expressed from and accumulates exclusively on the inactive X chromosome (Xi), “painting” the whole structure of the interphase chromosome territory (reviewed in Hall and Lawrence, Semin. Cell Dev. Biol. 14:369-378, 2003). Brown et al. discloses the cDNA sequence of Xist, which can be used in the design and construction of the nucleic acid constructs described herein. FIG. 2 shows mouse Xist RNA on two mitotic X chromosomes in a cell with X trisomy. Nature has also devised a counting mechanism such that all but one X chromosome is silenced (thus trisomy X has essentially normal gene dosage and is viable). Once Xist RNA coats the chromosome, a series of chromatin modifications occurs which are key hallmarks of the inactive X, including histone H3K27 methylation, H2A ubiquitination, macroH2A, and hypoacetylation of histone H4 (e.g., FIG. 5). Importantly, although Xist RNA is essential to initially enact this silencing process, once formed, the heterochromatic chromosome remains largely inactivated, even if Xist expression is later experimentally silenced (reviewed in Hall and Lawrence, Semin. Cell Dev. Biol. 14:369-378, 2003).
The new system would result in the silencing of additional or duplicated material (e.g., trisomies or segmental duplications) or translocated genomic material (e.g., the translocation of chromosomal arms that sometimes gives rise to birth defects or the translocations seen in certain cancers). The silencing would not be targeted to an intact X chromosome but could be targeted to an abnormal X chromosome lacking the Xist gene.
Where the intention is to “turn off” an extra chromosome, one can incorporate a silencing sequence using the compositions and methods described herein. The silencing sequence (e.g., an Xist gene of a human or other mammal such as a mouse or another silencing sequence described herein) is targeted to the region to be silenced (e.g., to the trisomic chromosome).
In another embodiment, where the intention is to kill a genetically aberrant cell (e.g., a cell in which a cancer-related translocation has occurred), one can incorporate a silencing sequence at the unique site of the translocation using the compositions and methods described herein. Silencing of the translocation would create a functional monosomy for the involved autosomal material which, depending on the chromosomal region silenced and the extent of the chromosomal region silenced would induce cell death or impede cell proliferation. In addition to incorporating a silencing sequence or as an alternative to incorporating a silencing sequence, one can use the present nucleic acid constructs and methods to target a “cell death” gene to the site of the translocation. For example, the nucleic acid construct can include an Xist gene and/or a gene encoding a toxin or pro-apoptotic factor. The toxins that can be expressed may include Shiga toxins 1 and 2 (Stx1 and Stx2); botulinum toxin from Clostridium botulinum; a virulence factor produced by Bacillus anthracis (e.g., a tripartite exotoxin referred to as anthrax toxin); a Vibrio cholerae multifunctional-autoprocessing RTX toxin; pertussis toxin from Bordetella pertussis; VacA from Helicobacter pylori: diphtheria exotoxin from Corynebacterium diphtheriae; ricinus communis; pasteurella multocida toxin (PMT); β-toxin-like peptide (named BmKBT) and two MkTx I homologues (named MkTx II and MkTx III), from a venom gland cDNA library of the Chinese scorpion Buthus martensii Karsch; haemorrhagic toxin acutolysin A from Agkistrodon acutus: Bacillus thuringiensis (Bt) toxin Cry1A and Cry1A(b); and Type A Clostridium perfringens cpb2. The pro-apoptotic proteins that can be expressed include, for example, Bcl-2-associated X protein (BAX), BH3 interacting domain death agonist (BID), Bcl-2 homologous antagonist killer (BAK), and Bcl-2-associated death promoter (BAD).
As noted, the translocation or duplication may be associated with a birth defect or may be a translocation associated with a cancer. The targeted chromosomal regions are then regions at or near the site of the translocation or duplication. These sites can be determined through genetic and sequence analysis, and several examples are known in the art. For example, one can target the following known sites of translocations: t(2;5)(p23;q35), which is associated with anaplastic large cell lymphoma; t(8;14), which is associated with Burkitt's lymphoma (c-myc); t(9;22)(q34;q11)/Philadelphia chromosome, which is associated with CML and ALL; t(11;14), which is associated with Mantle cell lymphoma (Bcl-1); t(11;22)(q24;q11.2-12), which is associated with Ewing's sarcoma; t(14;18)(q32;q21), which is associated with follicular lymphoma (Bcl-2); t(17;22), which is associated with dermatofibrosarcoma protuberans; t(15;17), which is associated with acute promyelocytic leukemia; t(1;12)(q21;p13), which is associated with acute myelogenous leukemia; t(9;12)(p24;p13), which is associated with CML and ALL (TEL-JAK2); t(X;18)(p11.2;q11.2), which is associated with synovial sarcomas; t(12;15)(p13;q25)—(TEL-TrkC), which is associated with acute myeloid leukemia, congenital fibrosarcoma, and secretory breast carcinoma.
Where the transgene is delivered to the patient, it will be useful to use a delivery system such as adeno associated virus, which also has the potential for site-specific integration.
Thus, the strategies described herein are applicable to trisomies, translocations, or any partial duplication of genomic DNA or aberration. For example, the present compositions and methods can be used to silence fragment or ring X-chromosomes associated with severe mental retardation.
Our strategies are based on our belief that trisomies can be treated by silencing essentially the whole trisomic chromosome, which would obviate the need to decipher the complex pathologies of the resulting syndromes and determining which specific genes are most “important” (if that is even possible at any practical level) (Antonarakis et al., Trends Mol. Med. 12:473-479, 2006). The silencing can be carried out in a variety of cell types which naturally represent or are induced to represent a variety of developmental stages (e.g., in early embryonic stem cells or induced pluripotent stem cells, fetal cells, neonatal cells, and other types of stem cells including hematopoietic cells, neural stem cells, mesenchymal stem cells, and other types of stem cells). In preparation for human therapies, the silencing can be carried out in a mouse model of Down syndrome. Several such models are available and known in the art. One could use any of the nucleic acid constructs described herein to determine if either the abnormal phenotype (or any aspect thereof) or embryonic lethality of a Down Syndrome mouse model could be rescued or ameliorated.
Although the potential therapeutic use of Xist transgenes for chromosomal trisomies has not been envisioned in the art, we believe that the mechanism whereby nature silences one X-chromosome in females can be extended to autosomal material, based on analysis of patients that carry naturally occurring X:autosome translocations. Two patients carrying X-autosome translocations in the context of trisomy for that autosome avoided otherwise devastating clinical consequences due to silencing of autosomal regions by endogenous Xic regions (Hall et al., Proc. Natl. Acad. Sci. USA, 99:8677-8682, 2002) (FIG. 4). In addition, we have shown that it is possible for an Xist transgene randomly integrated into a human autosome to induce silencing in somatic cells, when it had previously been believed that it could do so only in mouse embryonic stem cells (Hall et al., Hum. Mol. Genet., 11:3157-3165, 2002) (e.g., see “Chr. 4 Barr Body in FIG. 3). We have cultured and used the transgenic cells described herein, and we have observed highly stable Xist expression and autosome silencing. We have also found evidence that two cultured (neoplastic) cell lines support chromosome silencing to a large degree at most of the random integration sites tested (Chow et al., Proc. Natl. Acad. Sci. USA 104:10104-10109, 2007). A caveat to these studies is that they were done in transformed cell lines, leaving open the possibility that the neoplastic changes somehow allow formation of Xi heterochromatin. However, several recent studies from our lab and others demonstrate that cancer cells most commonly lose heterochromatin, including the inactive X (Pageau et al., 2007), thus we believe this is unlikely to be the explanation. No prior studies have described the use of a transgenic construct that is integrated into an autosome in a site-specific manner. The present nucleic acid constructs include targeting elements (e.g., first and second targeting elements) that drive integration of a silencing sequence (e.g., an Xist transgene) into a selected or targeted region of the genome (e.g., into a trisomic chromosome). The first and second sequences and/or the first and second targeting elements are nucleic acid sequences that share sequence homology (including sequence identity) with a chromosomal site and, due to base pairing between the first and second sequences (and/or the first and second targeting elements) and sequences present at the chromosomal site, promote site-specific integration of all or part of a nucleic acid construct of which they are a part into the chromosomal site.
Integrated Mouse Xist or human Xist transgenes can silence an autosome, as shown by studies in mouse embryonic stem cells (Wutz and Jaenisch, Mol. Cell, 5:695-705, 2000; Savarese et al., Mol. Cell Biol. 26:7167-7177, 2006) and in human somatic (fibrosarcoma) cells (FIG. 3; Hall et al., Hum. Mol. Genet. 11:3157-3165, 2002; Chow et al., Proc. Natl. Acad. Sci. USA 104:10104-10109, 2007). Natural autosomal silencing by Xist was also shown in patient cells, with an autosomal trisomy due to X; autosome translocations (Hall et al., Proc. Natl. Acad. Sci. USA 99:8677-8682, 2002; (FIG. 4)). Although the silencing of autosomal material may not be quite as complete or may vary somewhat between autosomal regions, autosomes studied to date are largely if not entirely silenced in response to Xist RNA. While Chr. 21 has not been directly tested, its small ( ̃50 Mb) acrocentric (essentially no short arm) structure is favorable, since Xist RNA would not need to spread far or across a centromere.
The fact that, to our knowledge, targeted inactivation of autosomes has never been discussed in the literature, possibly due to the belief that an Xist transgene cannot silence a chromosome outside a very narrow and early embryonic window and/or because the use of Xist transgenes for chromosomal therapeutic purposes was not envisioned. However, our studies of human Xist transgenes in adult cells and in differentiated embryonic cells (induced to express Xist post-differentiation) lead us to believe that the potential for Xist-mediated chromosome therapy in somatic cells is significant.
The present compositions and methods are applicable to abnormalities involving duplication of chromosomal material. Duplication of even a small chromosome fragment has severe clinical consequences. For example, Turner's syndrome (45, X) females have only one X-chromosome but typically have a quite mild phenotype, with normal intelligence but primary amenorrhea and sterility. However, Turner syndrome fetuses often have a fragment of the second X chromosome, which can result in either a very severe phenotype or the Turner-like mild one. A key to whether this fragment will be deleterious is whether or not it contains the Xist gene (Nussbaum et al., Thompson and Thompson Genetics in Medicine, Philadelphia, pA, Saunders/Elsevier, 2007). If the Xist gene is present, the chromosome fragment is silenced and the deleterious effects are avoided. Thus, Xist could be inserted into an abnormal chromosome that lacks Xist sequences. Another category of duplication events arises via imbalanced translocations, and we have previously characterized two examples in which the imbalance was rescued by the Xist gene on the translocated chromosome. FIG. 4 shows the karyotype of an individual with a normal phenotype, even though they carried a Chr. 9 trisomy which would otherwise be lethal. Instead, the extra chr. 9 material was silenced by the Xist gene on the translocated chromosome. We showed that Xist RNA coated much of the Chr. 9 material, as it did Chr. 14 material in an analogous example, and in both cases Xist nullified what would have been a devastating trisomy (reviewed in Hall and Lawrence, Semin. Cell Dev. Biol. 14:369-378, 2003). Similarly, Xist transgenes could potentially silence any rearrangement which creates duplication (partial trisomy) for part of a chromosome.
Nucleic Acid Constructs:
Accordingly, the present invention features nucleic acid constructs that include a silencing sequence and one or more targeting sequences (e.g., first and second sequences that flank the silencing sequence and direct insertion of the silencing sequence into a targeted chromosome). The silencing sequence can be or can include the sequence of an XIC (X inactivation complex) locus or any portion thereof that encodes an RNA capable of silencing the chromosome into which it has been inserted. For example, the constructs can include an XIC locus lacking the sequences 3′ to Xist that trigger the “counting” mechanism. Other constructs can include the Xist gene, with or without some or all of the intronic sequences, or a biologically active variant of the Xist gene (e.g., a fragment or other mutant). For information regarding the structure of XIC, one can consult Wutz and Gribnau (Curr. Opin. Genetics Dev. 17:387-393, 2007).
The silencing sequence (e.g., an Xist transgene) can silence the expression of one or more genes located within a trisomic and/or translocated chromosomal region located in cis to the integrated Xist transgene. In certain embodiments, the targeting elements are sequences homologous to those that occur naturally in the trisomic and/or translocated chromosomal region and will promote integration of the silencing sequence (e.g., an Xist transgene) to the corresponding trisomic and/or translocated chromosomal region. The targeted region may be a polymorphic region (i.e., a region where corresponding sequences differ between paired chromosomes in an individual). Whether the present nucleic acid constructs are used alone or in combination with a second moiety that enhances or facilitates homologous recombination (e.g., a zinc finger nuclease), the targeted region can be one having only one or more polymorphic sites, such as single nucleotide polymorphisms (SNPs). Zinc finger domains can recognize and target highly specific chromosomal sequences, including SNPs, which can be used to facilitate targeted integration of the transgene to particular alleles in just one of the homologous chromosomes. As noted, a vector that may facilitate both insertion of a transgene and delivery to a cell is an adeno associated virus, but delivery of the transgene to cells in vitro may be done by commonly used transfection methods, without the use of any adeno-associated, lenti or other virus.
In certain other embodiments, the targeting elements are homologous to non-naturally occurring sequences that have been introduced into a trisomic and/or translocated region by recombinant methods. In these embodiments, the targeting elements will promote integration of the transgene at a site defined by the non-naturally occurring sequences, such as FRT sequences, which can promote integration into that site.
Regardless of whether the silencing sequence is inserted with the assistance of a polymorphism on the targeted chromosome or whether the nucleic acid constructs are used in combination with a second moiety that enhances or facilitates homologous recombination, the present compositions and methods can be designed to target just one copy of a chromosome if desired. These methods can also be used to target one or more than one site on a targeted chromosome (e.g., two, three, or four sites), which may or may not be in close proximity to one another. While it is our expectation that the RNA encoded by the silencing sequence or transgene will silence most if not all of the genes residing on the targeted chromosome, one can nevertheless target specific genes (e.g., genes associated with Alzheimer's Disease (e.g., APP), leukemias (e.g., RUNX1) or other conditions that occur with increased frequency in patients with trisomies or translocation).
As would be understood in the art, the term “recombination” is used to indicate the process by which genetic material at a given locus is modified as a consequence of an interaction with other genetic material. Homologous recombination indicates that recombination has occurred as a consequence of interaction between segments of genetic material that are homologous or identical. In contrast, “non-homologous” recombination indicates a recombination occurring as a consequence of the interaction between segments of genetic material that are not homologous (and therefore not identical). Non-homologous end joining (NHEJ) is an example of non-homologous recombination.
As used herein, an Xist transgene refers to a nucleic acid sequence having the sequence of all or part of a naturally occurring Xic region so long as it (a) includes an Xist RNA coding sequence or a biologically active variant thereof and (b) is functional (e.g., the Xist transgene is capable of silencing the expression of one or more genes in cis when integrated into a chromosome). The Xist transgene may carry one or more regulatory elements found in the Xic region that are not a part of the Xist coding sequence. For example, deletion of the DXPas34 locus found 3′ to the Xist coding sequence eliminates Xist expression in mammalian embryonic stem cells as described in Debrand et al. (Mol. Cell. Bio., 19:8513-8525, 1999) herein incorporated by reference. As a further example, silencing by mouse Xist transgenes have been shown to require a conserved repeat sequence located at the 5′ end of Xist (Wutz et al., Nat. Genetics, 30:167-174, 2002).
The Xist transgene need not include the whole of the Xist gene sequence, although it may. For example, the Xist transgene may be derived from an Xist cDNA cloned from one of multiple naturally occurring splice variants. This cDNA may lack sequences corresponding to one or more introns or exons or portions thereof. Additionally, the Xist transgene may include non-naturally occurring Xist coding sequences. For example, the Xist coding sequence may be mutated (e.g., truncated) or otherwise variant with respect to naturally occurring Xist coding sequences so long as it includes sequences that are required for transgene function. For example, deletion analysis demonstrates that the first exon of human Xist is sufficient for both transcript localization and the induction of silencing (Chow et al., Proc. Natl. Acad. Sci. USA 104:10104-10109, 2007). Thus, smaller Xist constructs can be generated that are more easily manipulated but still biologically active.
Non-limiting examples of Xist transgenes (derived from mouse and human sequences) that are useful in this invention are described in the following references which are herein incorporated by reference: Chow et al. (Proc. Natl. Acad. Sci. USA 104:10104-10109, 2007); Hall et al. (Proc. Natl. Acad. Sci. USA 99:8677-8682, 2002); Chow et al. (Genomics, 82:309-322, 2003); and Wutz et al. (Nat. Genet., 2002, 30:167-174, 2002).
The nucleic acid constructs of this invention include targeting sequences or elements that promote sequence specific integration of an Xist transgene into a chromosomal site (e.g., by homologous recombination). Methods for achieving site-specific integration by ends-in or ends-out targeting are known in the art and in the nucleic acid constructs of this invention, the targeting elements are selected and oriented with respect to the Xist transgene according to whether ends-in or ends-out targeting is desired. In certain embodiments, two targeting elements flank the Xist transgene.
As described previously, the targeting element may be identical in sequence to a naturally occurring sequence found in a trisomic and/or translocated chromosomal region. For example, a targeting element may be identical in sequence to a sequence found in any one of human chromosomes 9, 13, 14, 18, or 21 (as described in Hattori et al., Nature, 405:311-319, 2000) or in any other chromosome. In another example, a targeting element may be identical in sequence to a sequence found in any one of mouse chromosomes 16, 17, or T(1716)65Dn.
A targeting sequence or element may vary in size. In certain embodiments, a targeting element may be at least or about 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000 bp in length (or any integer value in between). In certain embodiments, a targeting element is homologous to a sequence that occurs naturally in a trisomic and/or translocated chromosomal region, including a polymorphic sequence which may be present on just one of the homologous chromosomes.
Accordingly, in one aspect, the invention provides an isolated or purified nucleic acid construct that includes a silencing sequence (e.g., an Xist transgene) and one or more targeting sequences that are oriented with respect to each other in such a way that the Xist transgene can be integrated in a site-specific fashion into a chromosomal site when the nucleic acid construct is introduced into a cell.
The construct elements as described here may be variants of naturally occurring sequences. Preferably, any construct element (e.g., an Xist transgene, other non-coding, silencing RNA, or a targeting element) includes a nucleotide sequence that is at least 60% identical to its corresponding naturally occurring sequence (its reference sequence, e.g., an Xist coding region, a human Chr 21 sequence, or any duplicated or translocated genomic sequence). More preferably, the silencing sequence or the sequence of atargeting element is at least 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% identical to its reference sequence (e.g., SEQ ID NO:1 or SEQ ID NO:2).
As used herein, “% identity” of two nucleic acid sequences is determined using the algorithm of Karlin and Altschul (Proc. Natl. Acad. Sci. USA, 87:2264-2268, 1990), modified as in Karlin and Altschul, Proc. Natl. Acad. Sci. USA, 90:5873-5877, 1993). Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (J. Mol. Biol. 215:403-410, 1990). BLAST nucleotide searches are performed with the NBLAST program, score=100, wordlength=12. BLAST protein searches are performed with the XBLAST program, score=50, wordlength=3. To obtain gapped alignment for comparison purposes GappedBLAST is utilized as described in Altschul et al. (Nucl. Acids Res., 25:3389-3402, 1997). When utilizing BLAST and GappedBLAST programs the default parameters of the respective programs (e.g., XBLAST and NBLAST) are used to obtain nucleotide sequences homologous to a nucleic acid molecule of the invention.
The nucleic acid constructs of the invention can be prepared by recombinant methods that are known in the art.
Moreover, the present invention provides a vector containing one or more of the nucleic acid constructs described herein. The vector may be useful for propagating the nucleic acid construct or may contain elements useful for integrating the Xist transgene into a chromosome once the vector has been introduced into a mammalian cell. For example, the vector may be an expression vector designed to express a recombinase, such as FokI recombinase, coupled with a zinc finger nuclease, designed to aid in the integration of the silencing sequence (e.g., an Xist transgene) into a chromosome at a specific site.
For expression in animal cells, such as embryonic stem cells, adult bone marrow stem cells, CHO, COS, and NIH3T3 cells, the expression vector must have a promoter such as an SV40 promoter (Mulligan et al., Nature 277:108, 1979), MMLV-LTR promoter, EF1α promoter (Mizushima et al., Nucl. Acids Res. 18:5322, 1990), and CMV promoter. The vector may also carry an inducible promoter, for example, a doxycycline inducible promoter, or a promoter that may be activated to express Xist RNA by excision of intervening sequences, using a Cre-lox system. More generally, the nucleic acid construct that includes a silencing sequence can also include one or more control elements that facilitate expression of the silencing sequences. These control elements include promoter, enhancer, and termination sequences, and the promoter may be a constitutively active, inducible, tissue-specific or developmental stage-specific promoter.
Our findings further support the conclusion that chromosome inactivation also occurs in mature cells (e.g., in differentiated cells), but at a slower rate than in embryonic cells. Regarding developmental competence, we note that, in addition to the requirement for expression of an integrated Xist transgene, the cells must respond to Xist to support initiation of chromosome inactivation, which normally occurs during the earliest transition of pluripotent embryonic stem cells to more committed/differentiated cells. While all somatic cells in adult females are competent to maintain the silenced state, it has been believed that only early embryonic stem cells can initiate chromosome silencing in response to Xist. A study in mouse ES cells reported that if expression of Xist was delayed just two days after differentiation, the cells had lost the competence to initiate chromosome inactivation (Wutz et al. 2000, Molecular Cell). However, our study in human somatic cells (Hall et al., 2002, PNAS) first showed that a randomly integrated Xist transgene was able to initiate chromosome inactivation of the autosome (carrying the ectopic Xist gene) in human HT1080 cells, derived from an adult male fibrosarcoma. Our studies in Chow et al. (PNAS 2007) confirmed this for HT1080 cells and human 293 cells; however, because both of these somatic cell lines have neoplastic origins, it has been thought that their capacity to initially form the heterochromatic chromosome may not reflect the capacity of fully normal cells. However, in other work, we and others have shown that cancer cells tend to lose heterochromatin, including Xi (Pageau et al., 2007, Nature Reviews Cancer).
To further investigate whether normal, non-neoplastic somatic cells are competent to initiate chromosome inactivation post-differentiation, we differentiated mouse ES cells carrying an inducible-Xist transgene, essentially the same experimental system used in Wutz et al. Our findings demonstrate that these normal murine ES cells are able to support chromosome silencing post-differentiation. ES cells were differentiated for several (4-9) days prior to induction of Xist expression with doxycycline, and then cells were evaluated at various intervals up to two weeks following Xist expression. Whereas Wutz et al. waited just 2 days after Xist expression to evaluate whether chromosome silencing had taken place, we surmised that the multi-step inactivation process may occur more slowly in somatic cells. Indeed, our findings demonstrate that the vast majority of cells in these differentiated cultures had supported chromosome silencing, between 7-14 days after Xist expression. Thus, these findings indicate that the successful chromosome silencing in Hall et al. (2002) was likely not due to the neoplastic nature of the cells used, but to the long, ten-day time-frame over which the cells were evaluated.
While naturally occurring stem cells may have an enhanced competence to respond to Xist to initiate chromosome silencing, differentiated somatic cells, such as fibroblasts, can also be induced to form induced pluripotent stem cells (iPS cells) by introduction of specific genes that control developmental programs. The iPS cells have properties essentially like those of ES cells, and thus would be competent to not only initiate X-inactivation in response to Xist, but to form a variety of stem cells committed to specific cell types, such as neural, hematopoeitic, cardiac myoblasts, etc., which may enhance their therapeutic utility.
The present nucleic acid constructs can be used to integrate a silencing sequence (e.g., the Xist transgene) into a chromosome in murine or human embryonic, iPS, or adult stem cells (for example, see Zhang, J. Hematotherapy & Stem Cell Research 12:625-634, 2003, herein incorporated by reference). For example, bone marrow stem cells and induced pluripotent stem cells may be used. Pluripotency can be induced as described above by the methods of Wernig et al. (Nature 448:318-325, 2007); Shi et al. (Cell Stem Cell. 2:525-528, 2008); and Nakagawa et al. (Nature Biotechnol., 26:101-106, 2008), all of which are incorporated by reference herein. In addition, neural precursor cells as described in Zhang et al. (Nature Biotechnology, 19:1129-1133, 2001) may be used. For example, the following steps could be used to generate a population of corrected patient stem cells of a particular type that will not be subject to immune rejection (because they are isogenic to the patient's DNA), but which can provide therapeutic value. 1) providing fibroblasts or lymphocytes or other cells from a patient with trisomy 21 (Down Syndrome); 2) treating these cells with reprogramming factors shown to generate induced pluripotent stem cells or early developmental cells; 3) introducing into these cells a zinc finger nuclease (with FokI recombinase) specifically designed to promote efficient integration of exogenous DNA at a specific location; 4) introducing an Xist transgene flanked by sequences homologous to the desired site of integration under the control of a promoter designed to be expressed as desired, and verifying that Xist is expressed and silences the chromosome; and 6) culturing the Xist-transgenic iPS cells under conditions that promote the generation of neural, hematological, cardiac or other desired stem cells. The corrected stem cells (in which the deleterious chromosome or region has been silenced) can then be reintroduced into the patient's body so as to achieve therapeutic benefit, by introducing the appropriate type of stem cells into the appropriate tissue or organ. For example, in Down Syndrome there is thought to be loss of neurons or neural function that appears to be progressive with age that contributes to mental retardation. Similarly, Alzheimer's Disease is associated with loss of proper neuron function. As has been shown in mouse models of another neurological disease, intracranial injection of normal neural stem cells can provide therapeutic benefit (Lee et al., Nature Med. 13:439-447, 2007) and, more generally, cell implantation methods, including via intracranial surgery, are known in the art. Similarly, Down Syndrome is associated with hematological abnormalities that could be treated by correcting patient cells that are natural bone marrow stem cells or induced bone marrow stem cells (from iPS cells or other mesenchymal stem cells). In a patient with TMD (transient myeloproliferative disorder, which often precedes leukemia) or leukemia, the corrected bone marrow stem cells could be introduced into the patient's blood, to repopulate the bone marrow with more normal stem cells. Similarly, babies with Down Syndrome have a high rate of congenital heart defects, which in some cases could be treated by the use of cardiac stem cell therapy, where the stem cells used would be isogenic with the patient's DNA but would be corrected by silencing the trisomic chromosome.
In addition, the vector may contain a marker for the selection of transfected cells (for instance, a drug resistance gene for selection by a drug such as neomycin, hygromycin, and G418). Such vectors include pMAM, pDR2, pBK-RSV, pBK-CMV, pOPRSV, pOP13, and so on. More generally, the term “marker” refers to a gene or sequence whose presence or absence conveys a detectable phenotype to the host cell or organism. Various types of markers include, but are not limited to, selection markers, screening markers, and molecular markers. Selection markers are usually genes that can be expressed to convey a phenotype that makes an organism resistant or susceptible to a specific set of environmental conditions. Screening markers can also convey a phenotype that is a readily observable and distinguishable trait, such as green fluorescent protein (GFP), GUS or β-galactosidase. Molecular markers are, for example, sequence features that can be uniquely identified by oligonucleotide probing, for example RFLP (restriction fragment length polymorphism), or SSR markers (simple sequence repeat). To amplify the gene copies in host cell lines, the expression vector may include an aminoglycoside transferase (APH) gene, thymidine kinase (TK) gene, E. coli xanthine guanine phosphoribosyl transferase (Ecogpt) gene, dihydrofolate reductase (dhfr) gene, and such as a selective marker.
In vivo expression of the DNA of the invention may be performed by constructing the DNA into an appropriate vector and transfecting the construct into the body using retrovirus, liposome, cationic liposome, adeno-associated virus (particularly where chromosome 19 is targeted), lentivirus, electroporation and so on. It is possible to use such a construct to perform gene therapy for diseases resulting from chromosomal trisomies and/or translocations or duplications. Examples of vectors used for this purpose include retrovirus vector (such as pZIPneo), but are not limited thereto. General manipulations, such as insertion of the DNA into the vector, may be performed by using standard methods (Molecular Cloning, 5.61-5.63). The vector may be administered to the patient through in vivo administration or by way of methods that are carried out, at least partially, ex vivo. For example, the vector may be administered to cells harvested from a patient and maintained in culture. The construct-carrying or vector-carrying cells can then be introduced to the patient. For example, stem cells or hematopoietic cells can be harvested from a patient or obtained from another source, modified in culture as described here to include insertion of a silencing sequence into a targeted site, and administered to the patient. To facilitate the method, the recipient patient may be subjected to bone marrow ablation.
Facilitating Targeting with Zinc Finger Nucleases:
Targeting the present “silencing” constructs to particular chromosomes or regions of chromosomes can be facilitated by introducing chimeric zinc finger nucleases (ZFNs) into a cell. These nucleases exploit endogenous cellular mechanisms for homologous recombination and repair of double stranded breaks in genetic material. ZFNs can be used to target a wide variety of endogenous nucleic acid sequences in a cell or organism. The present compositions include cleavage vectors that target a ZFN to a region within a trisomic chromosome or within a translocated sequence, and the methods include transfection or transformation of a host cell or organism by introducing a cleavage vector encoding a ZFN (e.g., a chimeric ZFN), or by introducing directly into the cell the mRNA that encodes the recombinant zinc finger nuclease, or the protein for the ZFN itself. One can then identify a resulting cell or organism in which a selected endogenous DNA sequence is cleaved and exhibits a mutation or DNA break at a specific site, into which the transgene will become integrated.
To help clarify the nucleic acid to which we are referring, we tend to use the term “nucleic acid construct” to describe a nucleic acid that includes the silencing sequence and the term “cleavage vector” to describe a nucleic acid that encodes the ZFN. It is to be understood, however, that both are, or include, nucleic acid sequences (e.g., DNA); both can be properly referred to as constructs; and both can be properly referred to as vectors, particularly when they include nucleic acid sequences that facilitate entry into a host cell.
The methods can include construction of a vector or isolation of an mRNA encoding a chimeric ZFN by, for example, selecting a zinc finger DNA binding domain capable of preferentially binding to a specific host DNA locus to be mutated; further selecting a non-specific DNA cleavage domain capable of cleaving double-stranded DNA when operatively linked to the binding domain and introduced into the host cell; further selecting a promoter region capable of inducing expression in the host cell; and further operatively linking DNA encoding the binding domain and the cleavage domain and the promoter region to produce a DNA construct. Elements are operatively linked when they work in concert. For example, a control element and a transgene are operatively linked when the control element alters the expression of the transgene. To bring a transgene under the control of a promoter, one positions the 5′ end of the transcription initiation site of the transcriptional reading frame generally between about 1 and about 50 nucleotides “downstream” (i.e., 3′) of the chosen promoter. The “upstream” promoter stimulates transcription of the DNA.
The nucleic acid (e.g., DNA) construct is then introduced into a target host cell and at least one host cell exhibiting recombination at the target locus in the host DNA is identified.
The ZFN can be a chimeric protein molecule that directs targeted genetic recombination or targeted mutation in a host cell by causing a double stranded break at the target locus. For example, a ZFN can include a DNA-binding domain that includes at least one zinc finger, and that binding domain can be operatively linked to a DNA-cleavage domain. The DNA-binding domain is at the N-terminus of the chimeric protein molecule, and the DNA-cleavage domain is located at the C-terminus of the molecule.
The ZFN can include multiple (e.g., at least three (e.g., 3, 4, or 5)) zinc fingers in order to improve its target specificity. The zinc finger domain can be derived from any class or type of zinc finger. For example, the zinc finger domain can include the Cys2His2 type of zinc finger that is very generally represented, for example, by the zinc finger transcription factors TFIIIA or Sp1. In a preferred embodiment, the zinc finger domain comprises three Cys2His2 type zinc fingers.
The DNA recognition and/or the binding specificity of a ZFN can be altered in order to accomplish targeted genetic recombination at any chosen site in cellular DNA. Such modification can be accomplished using known molecular biology and/or chemical synthesis techniques. ZFNs comprising zinc fingers having a wide variety of DNA recognition and/or binding specificities are within the scope of the present invention.
The ZFN DNA-cleavage domain can be derived from a class of non-specific DNA cleavage domains, for example the DNA-cleavage domain of a Type II restriction enzyme such as FokI. Thus, a chimeric ZFN useful in the present methods can include three Cys2His2 type zinc fingers and a DNA-cleavage domain derived from the Type II restriction enzyme FokI. In this event, each zinc finger contacts three consecutive base pairs of DNA creating a 9 bp recognition sequence for the ZFN DNA binding domain. The DNA-cleavage domain of the embodiment requires dimerization of two ZFN DNA-cleavage domains for effective cleavage of double-stranded DNA. This imposes a requirement for two inverted recognition (target DNA) sites within close proximity for effective targeted genetic recombination. If all positions in the target sites are contacted specifically, these requirements enforce recognition of a total of 18 base pairs of DNA. There may be a space between the two sites. The space between recognition sites for ZFNs may be equivalent to 6 to 35 bp of DNA. The region of DNA between the two recognitions sites may be referred to as the “spacer.”
A linker, if present, between the cleavage and recognition domains of the ZFN can be a sequence of amino acid residues that result in a flexible linker is flexible, although linkerless constructs tend to improve target site specificity. A linkerless construct has a strong preference for binding to and then cleaving between recognition sites that are 6 bp apart. However, with linker lengths of between 0 and about 18 amino acids in length, ZFN-mediated cleavage occurs between recognition sites that are between 5 and 35 bp apart. For a given linker length, there will be a limit to the distance between recognition sites that is consistent with both binding and dimerization. As noted, there may be no linker between the cleavage and recognition domains, and the target locus can include two nine nucleotide recognition sites in inverted orientation with respect to one another, separated by a six nucleotide spacer.
To target genetic recombination or mutation, two 9 bp zinc finger DNA recognition sequences are identified in the host DNA. These recognition sites will be in an inverted orientation with respect to one another and separated by about 6 bp of DNA. ZFNs are then generated by designing and producing zinc finger combinations that bind DNA specifically at the target locus, and then linking the zinc fingers to a cleavage domain of a Type II restriction enzyme.
A silencing sequence flanked by sequences (typically 400 bp-5 kb in length) homologous to the desired site of integration can be inserted (e.g., by homologous recombination) into the site cleaved by the endonuclease, thereby achieving a targeted insertion. When used in combination with a ZFN construct, the silencing sequence may be referred to as “donor” nucleic acid or DNA.
The various active sequences, including the silencing sequence and the sequence encoding a chimeric ZFN can be introduced into a host cell on the same vector or separately (e.g., on separate vectors or separate types of vectors at the same time or sequentially). Methods for introducing the various nucleic acids, constructs, and vectors are discussed further below and are well known in the art.
The nucleic acid constructs including a silencing sequence, whether used alone or in combination with a ZFN can either introduce a therapeutic sequence or disrupt a targeted sequence, gene, or chromosome in a somatic cell or in a germ cell. In some cases, a therapeutic Xist transgene may be inserted in such a way as to simultaneously disrupt a deleterious gene, such as the APP gene that leads to high incidence of Alzheimer's Disease. Cells with such disruption in the targeted gene can be “selected for” in order to create an organism without a functioning target sequence or for administration to a patient. Accordingly, the constructs, other compositions, and methods of the present invention are applicable to a wide range of cell types and organisms. While our own intention is to develop therapies for human patients, the silencing methods we have discovered can be carried out with a single celled or multicellular organism; an oocyte; a gamete; a germline cell in culture or in a host organism; a somatic cell in culture or in a host organism; an insect cell, including an insect selected from the group consisting of Coleoptera, Diptera, Hemiptera, Homoptera, Hymenoptera, Lepidoptera, or Orthoptera, including a fruit fly, a mosquito and a medfly; a plant cell, including a monocotyledon cell and a dicotyledon cell; a mammalian cell, including but not limited to a cell of a mouse, rat, pig, sheep, cow, dog, cat, or human; an avian cell, including, but not limited to a cell of a chicken, turkey, duck or goose; or a fish cell, including, but not limited to zebrafish, trout and salmon.
DNA encoding an identifiable marker can also be included with either the nucleic acid construct including the silencing sequence or the vector carrying the ZFN-encoding sequence. Such markers may include a gene or sequence whose presence or absence conveys a detectable phenotype to the host cell or organism. Various types of markers include, but are not limited to, selection markers, screening markers and molecular markers. Selection markers are usually genes that can be expressed to convey a phenotype that makes an organism resistant or susceptible to a specific set of environmental conditions. Screening markers can also convey a phenotype that is a readily observable and distinguishable trait, such as Green Fluorescent Protein (GFP), beta-glucuronidase (GUS) or beta-galactosidase. Markers may also be negative (e.g., codA) or positive selectable markers. Molecular markers are, for example, sequence features that can be uniquely identified by oligonucleotide probing, for example RFLP (restriction fragment length polymorphism), or SSR markers (simple sequence repeat).
The compositions and methods described herein can be used to accomplish germline gene therapy in mammals.
The frequency of homologous recombination in any given cell is influenced by a number of factors. Different cells or organisms vary with respect to the amount of homologous recombination that occurs in their cells and the relative proportion of homologous recombination that occurs is also species-variable. The length of the region of homology between donor and target affects the frequency of homologous recombination events, the longer the region of homology, the greater the frequency. The length of the region of homology needed to observe homologous recombination is also species specific. However, differences in the frequency of homologous recombination events can be offset by the sensitivity of selection for the recombinations that do occur. It will be appreciated that absolute limits for the length of the donor-target homology or for the degree of donor-target homology cannot be fixed but depend on the number of potential events that can be scored and the sensitivity of the selection for homologous recombination events. Where it is possible to screen 109 events, for example, in cultured cells, a selection that can identify 1 recombination in 109 cells will yield useful results. Where the organism is larger, or has a longer generation time, such that only 100 individuals can be scored in a single test, the recombination frequency must be higher and selection sensitivity is less critical. Random integration is discussed elsewhere herein. We note here, however, that random integration can be used in combination with selection for cells that have targeted the desired gene or chromosome.
Transformation can be carried out by a variety of known techniques which depend on the particular requirements of each cell or organism. Such techniques have been worked out for a number of organisms and cells and are readily adaptable. Stable transformation involves DNA entry into cells and into the cell nucleus. For single-celled organisms and organisms that can be regenerated from single-cells (which includes all plants and some mammals), transformation can be carried out in culture, followed by selection for transformants and regeneration of the transformants. Methods often used for transferring DNA or RNA into cells include forming DNA or RNA complexes with cationic lipids, liposomes or other carrier materials, micro-injection, particle gun bombardment, electroporation, and incorporating transforming DNA or RNA into virus vectors. Other techniques are well known in the art.
Where silencing is limited to less than an entire chromosome, boundary elements or sequences associated with escape from inactivation can be used to help impede the spread of the silencing RNA. For a description of a unique sequence feature of the X-chromosome that always escapes inactivation, see McNeil et al. (Genome Res. 16:477-484, 2006). This sequence is among those that can be used to confer “escape” from silencing. See also Filippova et al. (Dev. Cell. 8:31-42, 2005).
In the following paragraphs, we describe some delivery systems useful in practicing the present invention.
Liposomal formulations: In certain embodiments of the invention, the oligo- or polynucleotides and/or expression vectors containing silencing sequences and/or ZFNs may be entrapped in a liposome. Liposomes are vesicular structures characterized by a phospholipid bilayer membrane and an inner aqueous medium. Multilamellar liposomes have multiple lipid layers separated by aqueous medium. They form spontaneously when phospholipids are suspended in an excess of aqueous solution. The lipid components undergo self-rearrangement before the formation of closed structures and entrap water and dissolved solutes between the lipid bilayers. Also contemplated are cationic lipid-nucleic acid complexes, such as lipofectamine-nucleic acid complexes. Lipids and liposomes suitable for use in delivering the present constructs and vectors can be obtained from commercial sources or made by methods known in the art.
Microinjection: Direct microinjection of DNA into various cells, including egg or embryo cells, has also been employed effectively for transforming many species. In the mouse, the existence of pluripotent embryonic stem (ES) cells that can be cultured in vitro has been exploited to generate transformed mice. The ES cells can be transformed in culture, then micro-injected into mouse blastocysts, where they integrate into the developing embryo and ultimately generate germline chimeras. By interbreeding heterozygous siblings, homozygous animals carrying the desired gene can be obtained.
Viral Vectors as Expression Constructs:
Other viral vectors may be employed as expression constructs in the present invention. Vectors derived from, for example, vaccinia virus, adeno-associated virus (MV), and herpes viruses may be employed. Extensive literature is available regarding the construction and use of viral vectors. For example, see Miller et al. (Nature Biotechnol. 24:1022-1026, 2006) for information regarding adeno associated viruses. Defective hepatitis B viruses, may be used for transformation of host cells. In vitro studies show that the virus can retain the ability for helper-dependent packaging and reverse transcription despite the deletion of up to 80% of its genome. Potentially large portions of the viral genome can be replaced with foreign genetic material. The hepatotropism and persistence (integration) are particularly attractive properties for liver-directed gene transfer. The chloramphenicol acetyltransferase (CAT) gene has been successfully introduced into duck hepatitis B virus genome in the place of the viral polymerase, surface, and pre-surface coding sequences. The defective virus was cotransfected with wild-type virus into an avian hepatoma cell line, and culture media containing high titers of the recombinant virus were used to infect primary duckling hepatocytes. Stable CAT gene expression was subsequently detected.
Non-Viral Methods:
Several non-viral methods are contemplated by the present invention for the transfer into a host cell of DNA constructs encoding ZFNs and, when appropriate, donor DNA. These include calcium phosphate precipitation, lipofectamine-DNA complexes, and receptor-mediated transfection. Some of these techniques may be successfully adapted for in vivo or ex vivo use.
In one embodiment of the invention, the expression constructs may simply consist of naked recombinant DNA, or in some cases mRNA for the recombinant ZFN. Transfer of the construct may be performed by any of the nuclei acid transfer methods mentioned above which physically or chemically permeabilize the cell membrane. For example, polyomavirus DNA in the form of CaPO4 precipitates was successfully injected into liver and spleen of adult and newborn mice which then demonstrated active viral replication and acute infection. In addition, direct intraperitoneal injection of CaPO4 precipitated plasmid expression vectors results in expression of the transfected genes.
EXAMPLES
Silencing a Trisomic Chromosome in Human Somatic Cells and in a Trisomic Mouse Model of DS
We will introduce an Xist transgene into human and mouse trisomic cells, and demonstrate silencing of the trisomic chromosome in culture. We believe that human Xist transgenes can: (1) initiate silencing outside of the normal very early development window in normal (non-neoplastic) human somatic cells and/or stem cells; (2) be targeted to and effectively silence an autosome (e.g., trisomic human chromosome 21) in human cultured cells; and with higher efficiency techniques, (3) stably silence the trisomic chromosome in mouse ES cells and mice in an established mouse model of Down Syndrome, thereby ameliorating the deleterious phenotype. Trisomic mouse models of Down Syndrome are available and can be used to test both chromosome silencing and amelioration of the phenotype of DS mice (see below).
Experiments in Mouse ES or iPS Cells and a Mouse Model of Down Syndrome:
The goal of these studies is to first target an Xist transgene into ES, iPS or bone marrow cells derived from one or more mouse models of Down Syndrome and verify that the trisomic chromosome is effectively silenced. Two available mouse DS models carry trisomy chr. 16 (syntenic to human 21) and one carries an actual human Chr 21 as the third chromosome (further detailed below).
Trisomic Mouse Models of DS:
Mouse chromosome 16 is largely syntenic to human chromosome 21, and mouse models of DS have been developed that are either trisomic for mouse chr. 16, or carry as the third chromosome an actual human chromosome 21 (reviewed in Reeves Trends Mol. Med. 12:237-240, 2006). There are mouse strains with segmental trisomies, such as Ts65Dn, which carries a segment (15.6-Mb) of mouse chromosome 16 on a marker chromosome (Reeves et al., Nat. Genet. 11:177-184, 1995). The extra T(1716)65Dn marker chromosome produces a trisomy for the mouse orthologs of about half of the human genes on chr. 21. Mice display a number of DS-like phenotypes, including an overall reduction in size and growth rate, altered noradrenergic transmission in the hippocampus and cerebral cortex and degeneration of basal forebrain cholinergic neurons, and defects in cranial bone development (Hill et al., J. Anat. 210:394-405, 2007; Olson et al., Hum. Mol. Genet. 16:774-782, 2007). In addition, trisomic females have smaller and fewer litters and trisomic males display hypospermia. Another mouse model, Ts16, carries an essentially complete third copy of mouse chr. 16, involving a Robertsonian translocation of chr 16 (Epstein et al., Ann. NY Acad. Sci. 450:157-168, 1985). As with other mouse full trisomies, these mice die during fetal development, so all studies are done on the fetal mice or cells derived from fetuses. However, for our purposes the severity of the phenotype would potentially make the therapeutic benefits clearer, if we should see an increase in viability upon inactivation of one copy of chromosome 16. The Rb(6.16)24Lub/Rb(16.17)7BnrF1 Robertsonian translocation mouse strain (used to generate Ts16) and the Ts65Dn mouse strain are available from the Jackson Laboratory.
Another interesting mouse DS model (Tcl) carries an almost complete human chromosome 21, and exhibits several characteristics that are reminiscent of Down Syndrome (O'Doherty et al., Science 309:2033-2037, 2005). However, this model is less attractive as it is not clear how well human Xist would silence a human chromosome in an otherwise mouse nucleus; our prior study of mouse/human hybrids showed that human Xist RNA may not localize properly. Thus, we have decided to begin with the Ts65n model which carries a partial chr 16 trisomy and is more straightforward. We summarize the approach for this system, but other models can be tested similarly.
Generation of Mouse Down's Syndrome ES Cells and iPS Cells:
The Ts65Dn mouse model (B6EiC3Sn a/A-Ts(1716)65Dn) has been obtained from the Jackson Laboratory (stock number 001924) (Roper et al., Genetics 172:437-443, 2006). Female T(1716)65Dn mice have been mated to 129SV/EV males, and a number of litters produced. DS pups have been identified by karyotyping cytogenetic preps, and using FISH on interphase cells from tail tip Fibroblasts and whole blood.
Generation of Trisomic ES Cells:
Blastocysts will be harvested from the mating of Ts65Dn females with 129SV/EV males, to derive ES cells, using standard procedures, DS embryos can also be generated by somatic cell nuclear transfer of a fibroblast from a Ts65Dn mouse into an enucleated normal mouse egg.
Generation of Trisomic iPS Cells:
Fibroblasts from DS pups have been isolated and iPS cells generated using lentivirus expression of 4 pluripotency genes (mOct4, mSox2, mKlf4, mc-Myc). Twenty hours post-transduction, the virally transduced cells were resuspended in ES cell growth medium and re-seeded. The following day the pluripotency genes were induced with doxycycline. Colonies that appear (ours started showing up by day 3), are replated onto inactivated feeder MEFs. Doxycycline is removed once colonies are verified to express the other pluripotency markers (SSEA-1, Nanog, etc), between 14 and 22 days. Resulting colonies are then cultured on feeders in the absence of dox, similar to normal mouse ES cells. These iPS cells can now be used to target Xist to chromosome 16.
Targeting the Xist Transgene to a Trisomic Chromosome in Mouse Cells and Assessing Silencing of Chr 16:
The Xist transgene will be targeted into a specific region of chromosome 16 in the trisomic iPS cells. We will target the region critical for Down's syndrome on chromosome 16, using as preferred targeting sites some of the genes important to the pathology of Down's syndrome (e.g., Runx-1, APP, Dyrk1A, etc). The transgene can have a constitutive or inducible promoter. We have begun to construct a targeting transgene which incorporates Xist cDNA sequences from an Xist cDNA that we have obtained by Anton Wutz (Research Institute of Molecular Pathology, Vienna) (Savarese et al., Mol. Cell Biol. 26:7167-7177, 2006). This Xist transgene contains an inducible promoter and Xist cDNA, but lacks the 3′ sequence necessary for counting. Thus, it will not be susceptible to random inactivation in the female ES cells. We are modifying this transgene with appropriate promoters, homologous targeting sequences, and inducible system to make it appropriate for targeting the Xist transgene to the specific chromosome in iPS cells. It will be expressed during cell differentiation, for optimal silencing.
We will be using established mouse gene targeting protocols on our iPS cells. We are currently designing an Xist targeting construct using a vector previously validated to efficiently target the Runx1 gene. Runx1 is located in the critical region of chr16, has been linked to leukemia, and likely plays a role in the hematological abnormalities seen in DS individuals (mouse and human). The targeting vector can also include a GFP mini-gene to facilitate identification of transgenic cells and to confirm silencing of this gene. Selectable markers will facilitate identification of properly targeted clones, using known procedures. Similarly, targeting can be to the APP gene which is known to be important to Alzheimer's Disease or other gene important in other diseases.
In mouse models of DS, we can not only manipulate mouse ES or iPS cells, which are known to support chromosome silencing, but also test two critical points in the proof-of-principle: that stable silencing of the third chromosome (carrying an Xist transgene) can be achieved, and that this can have ameliorating effects on the disorder, at the whole organism level. Our plan is to initially target the Xist transgene into a trisomic chromosome of “Down Syndrome” mouse ES/iPS cells, induce silencing and confirm that it silences, and then generate mice (or chimeric mice) from these engineered ES/iPS cells. If this procedure successfully silences and mitigates the phenotype, we can use an inducible promoter to induce Xist expression at later stages of development, to determine how late effective silencing and mitigation of the disorder can be achieved. Also, the original TS64DN non-modified mice may also be used to examine the effects of reintroducing bone marrow, after chromosome modification, on the hematological abnormalities in DS mice (these studies are further detailed below).
Validation of Targeting and Silencing:
As we have previously published, single cell analysis by molecular cytological methods (immunofluorescence and FISH), will be used to validate targeting and the extent of chromosomal silencing in differentiated iPS cell cultures (FIG. 5). RNA FISH for single genes or real-time PCR or microarrays will be used to examine gene expression levels in targeted versus non-targeted iPS cells. We will initially determine by fluorescence in situ hybridization whether the Xist gene is expressing and producing a localized accumulation that “paints” the chromosome. This is a very distinctive relationship of RNA to the chromosome that is almost always correlated with silencing, which our lab first discovered and established (Clemson et al., J. Cell Biol. 132:259-275, 1996). If Xist RNA coated chromosomes are observed, we will then use other Xi hallmarks (e.g., hybridization to hnRNA, immunofluorescence to H3K27 methylation or macroH2A) to further validate the silencing, as shown in FIG. 5 and in our published papers.
In addition, as stated above, we will generate mice from the modified mouse iPS (or ES) cells to test the prediction that the trisomic chromosome can be silenced and the deleterious phenotype will be substantially ameliorated (this is further detailed below).
Experiments in Human Primary or iPS Down Syndrome Cells:
Several DS human cell types are available to generate DS iPS lines from, including DS patient fibroblasts and bone marrow. We have acquired three primary DS fibroblast lines and two bone marrow cell lines, and have TERT immortalized one female DS fibroblast line. We plan to generate iPS cells from both primary as well a TERT immortalized DS lines, using lentivirus expression of 4 pluripotency genes similar to the methodology used in developing the mouse DS iPS cell lines. We have also acquired a DS iPS line from Harvard Stem Cell Institute.
Studies in Human Primary Cells:
Although iPS cells can now be made from primary lines, we can also test the developmental competence of terminally differentiated primary human cells to support silencing in response to an inducible Xist transgene integrated into an autosome. This may help define whether more differentiated cell types may also be available for chromosome therapy in the future. Some of our studies have been aimed at determining whether somatic or only stem cells can support chromosome silencing of an autosome. A prior study of transgenic mouse ES cells concluded that Xist did not induce silencing just a few days after the earliest embryonic differentiation (Wutz et al.), however this study examined silencing just two days after Xist expression. We have shown that adult human somatic cells can initiate silencing of an autosome carrying an Xist transgene (Hall et al., Proc. Natl. Acad. Sci. USA 99:8677-8682). However, the study indicated this took at least 10 days to occur in somatic cells. Thus, we will examine chromosome silencing after two or more weeks and are hopeful that somatic cells will largely retain the ability to induce chromosome silencing, albeit more slowly than in ES cells. Our recent findings indicate that human ES cells (hESCs) also support silencing of the normal X chromosome in culture similar to that seen for the mouse. We have also derived a sub-line of neural stem cells (from hESCs), and we will test their competence to support silencing, which may be particularly relevant to future DS therapeutic applications. It may also be possible to partially reprogram somatic cells, such that they more closely resemble adult stem cells instead of ES/iPS cells, and assess the competence of these partially reprogrammed cells to inactivate.
Wutz lab (Savarese et al., Mol. Cell Biol. 26:7167-7177, 2006) suggests that bone marrow stem cells may retain capacity to support chromosome silencing post-embryonic differentiation, but our close reading of this data suggests to us that some or many more cell-types in fetal or adult mice still retain the capacity for X-inactivation. For reasons summarized below, we are particularly interested in testing this strategy in DS bone marrow cells. Thus, for these experiments, rather than use transformed cell lines, we will test several different primary somatic cell types, including DS patient fibroblasts and bone marrow cells, primary diploid myoblasts, epithelial cells, and trisomic amniocytes (available from Coriell Cell Repositories, Camden, N.J.). The developmental competence of these cells will be compared to TERT immortalized primary DS lines, and embryonic and neural stem cells, including those derived from induced pluripotent stem (iPS) cells generated from patient somatic cells.
These questions regarding the developmental competence of cells to enact chromosome silencing do not require targeted integration (we will use the methodology that provides the highest efficiency of integration), and is similar to work we have done successfully with transformed cell lines.
Incorporation of the ZFN Technology to Target the Xist Transgene to a Trisomic Chromosome in Human Cells and Assess Silencing of Chr 21 (or Chr 13):
We will test the best strategies to target human Chr 21 (or Chr 13) and to confirm the effectiveness of silencing. Conventional targeting strategies as well as a new zinc-finger based methodology will be used for targeting the Xist transgene to Chr 21. This will be assessed both with and without the use of selection, and by using the endogenous or an inducible promoter (as in our papers). Selection might be utilized with future ES/iPS methodologies, but would not be an option in vivo; see Moehle et al. (Proc. Natl. Acad. Sci. USA, 104:3055-3060, 2007). The constructs lack the sequences 3′ to Xist that trigger the “counting” mechanism, so this will not complicate results.
We will use site-specific targeting, using zinc finger nucleases (ZFNs). This approach provides much higher integration efficiency, without a requirement for selection. More specifically, this method uses the cells own machinery for double strand break repair to improve the efficiency of gene targeting. The zinc finger motifs can be engineered to recognize almost any sequence, and we will engineer ZFN transgenes that target two or more sites of Chr. 21. The transgene encoding the ZFN can be introduced along with a vector carrying the gene to be inserted flanked by a few hundred by of DNA homologous to the target site. Recent studies have achieved targeted integration rates of about 5 to 20% without selection with integration of up to 8 kb of DNA (Urnov et al., Nature 435:646-651, 2005; Moehle et al., Proc. Natl. Acad. Sci. USA 104U; 3055-3060, 2007). If very high efficiency is obtained, it is possible that we could get integration into two copies of the chromosome in some cells. If this occurs, we will target polymorphic sites. In addition, these methods have shown particular promise for use in human ES cells.
We will begin these studies in 293 cancer cells that transfect at very high efficiency, and then move on to TERT immortalized trisomy 21 fibroblasts, and iPS cells generated from these DS lines. We anticipate that most autosomal material is competent to be inactivated in response to Xist RNA. However, because there is some sequence specificity to this process, we will determine the effectiveness of chromosome 21 silencing specifically using molecular cytological assays. Presuming this shows silencing, we will use microarray analysis to determine the profile of gene expression for Chr. 21 genes, in comparison to the trisomic cells (without the transgene) and normal cells. In addition to the trisomy 21 cells, one of the human ES cell lines approved by the NIH carries a trisomy for chr. 13. We will study Chr. 13 inactivation in these cells, both as undifferentiated ES cells and as cells differentiated along a neuronal pathway.
Utilizing Random Integration of Xist:
In some studies, we will use the ZFN technology described above, but analyses can also be done using our protocol for random integration with the same constructs and transfection approach that we have successfully used before (Hall et al. 2002b; Chow et al. 2007). In these experiments, transgenes can be designed to select against cells in which the trangene has not integrated. If integration is into a disomic chromosome which is then silenced, this would create a functional monosomy, which would reduce or severely reduce cell viability. In contrast, if the random integration involved the trisomic chromosome in a DS cell line, there is likely to be a growth advantage to these “corrected” cells within the population, and this, coupled with selection against cells where integration generated a functional monosomy, may generate a strong selection for the desired cells in which the random integration was into the trisomic chromosome even without targeting methodologies. This same natural selective disadvantage of functionally monosomic cells may provide a natural protection against the less frequent occurrence that two of the trisomic chromosomes are silenced, as such cells would be selected against as they are in human development. (In individuals mosaic for Down Syndrome where a non-disjunction even generates a trisomic cell-line and a monsomic cell line, the monosomic cell line is not see; these cells die). However, we have also shown that transfectants can be selected for drug expression (prior to chromosome inactivation), and the integration site and impact on chromosome silencing can be determined in several different clones or in pooled populations of random integrants.
Further Studies Using the Down's Syndrome Mouse Models
Use Trisomic Mouse ES/iPS Cells Carrying the Xist-Transgene to Generate Mice and Assess Phenotype:
The targeted ES/iPS cells can be injected into blastocysts of albino C57Bl/6 E3.5 to generate mice in which coat color changes can be used to assess the degree of chimerism or into the blastocysts of trisomic Ts65n blastocysts. The latter mice would be “mosaic” for the Xist-transgenic trisomic cells and uncorrected trisomic cells, providing a good model of partial correction. In addition, routine methods using blastocysts that have undergone tetraploid fusion can generate mice completely derived from the modified ES cells.
To assess the corrective effects of Xist-mediated silencing of the T(1716)65Dn chromosome in this model for DS, we will measure birth weight and growth throughout the postnatal period, as well as correction of defects in craniofacial skeletal formation. Bone defects have been analyzed in mice using X-rays and microCT analysis (Lengner et al., J. Cell Biol. 172:909-921, 2006), and we will also examine hematopoietic properties and make appropriate crosses to determine whether the Xist transgene corrects the deficiency in male fertility. In addition, maintenance of gene silencing will be assayed in a variety of organs by both qPCR for genes present of the T(1716)65Dn chromosome as well as by analysis of GFP expression, and by standard molecular cytological methods.
Relevance of Trisomy 21 to Hematological Abnormalities and Bone Marrow Stem Cells:
We will test this strategy to silence the trisomic chromosome in bone marrow cells. There is a direct and measurable clinical impact of trisomy on bone marrow function. DS children develop hematological abnormalities, ranging from mild to severe. For example, neonates with DS commonly develop a transient myeloproliferative disorder (TMD; also termed transient leukemia). Although TMD often is self-resolving, it is highly predictive of later development of acute leukemia, can result directly in significant morbidity, and may be fatal in 10-20% of affected infants. Most importantly, it is well established that DS children have a greatly increased risk of progressing to leukemia, such that 2% of childhood leukemia patients have DS. The incidence of ALL and AML is 20 fold higher than normal, and the normally very rare AMKL (acute megakaryoblastic leukemia) is increased a remarkable 500 fold (Lange, Br. J. Haematol. 110:512-524, 2000). Finally, most DS children have MCV (mean corpuscular volume) above the 97th percentile. While not a significant clinical concern, this is indicative of disordered hematopoiesis and provides a prevalent “marker” of the syndrome (also in the mouse DS model), that could be readily evaluated in “corrected” cells.
Regarding the Ts65Dn model (above), an important report just appeared which demonstrates that it also provides a good model for many of the hematological abnormalities seen in human DS (Kirsammer et al., Blood 111(2), 2008). The DS mice exhibit a highly penetrant myeloproliferative disease, as well as macrocytosis, dysplastic megakaryocyte morphology, and myelofibrosis. Therefore, we will test the chromosome silencing strategy in bone marrow stem cells from the Ts65Dn mouse. In addition to the clinical impact of trisomy on hematological abnormalities, the study of bone marrow is advantageous because it is an accessible source of cells that can be genetically manipulated, and then replaced. Furthermore, a study of the Xist mechanism, using random integration of inducible Xist transgenes, provided evidence that hematopoeitic precursor cells of adult mice are most readily competent to support chromosome silencing (Savarese et al., Mol. Cell Biol. 26:7167-7177, 2006). (This study was unrelated to any concept regarding therapeutic use of Xist transgenes) We will use inducible mouse Xist transgene constructs as described in Savarese (supra), and we can include in these constructs GFP, which will facilitate sorting and enrichment of cells carrying the Xist transgene. Ultimately, the silencing of the transgenic chromosome would be verified using procedures described above and the modified cells will be reintroduced, after bone marrow ablation by radiation. At various intervals after “chromosome correction”, hematological analysis will be carried out to determine whether or to what extent the aforementioned anomalies of the DS mouse, particularly the myeloproliferative disease and the prevalent macrocytosis, have been ameliorated.
Use of Inducible Xist Transgene to Test Phenotypic Benefits Later in Development:
To determine the extent of the therapeutic benefit achieved when the trisomic chromosome is silenced later in development, we will generate ES/iPS cells that contain the doxycyclin inducible Xist transgene, or another indicuble Xist construct (e.g., Cre/Lox mediated removal of drug resistance gene generating a new construct where Xist is transcribed using the drug resistance promoter). The inducible Xist transgene can then be turned on in both undifferentiated ES/iPS cells as well as at different times during differentiation and in terminally differentiated cells to assess the developmental competence of inactivation.
The same inducible transgenic iPS/ES cells can be used to induce the “rescue” later in fetal development, allowing the evaluation of phenotypic benefits in more differentiated cells. Addition of doxycyclin to the drinking water of the pregnant dams bearing the chimeric mice for 14 days should result in activation of the tet promoter controlling Xist expression, and Xist-mediated silencing of the T(1716)65Dn chromosome. A similar strategy has been used previously in mice to regulate Xist expression in hematopoietic precursor cells (Savarese et al., Mol. Cell Biol. 26:7167-7177, 2006).
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

## Claims

1. A method of reducing gene expression from a trisomic human chromosome 13, 18, or 21 or mouse chromosome 16, the method comprising:
identifying a subject with a trisomic human chromosome 13, 18, or 21 or mouse chromosome 16;
harvesting hematopoietic stem cells from the subject;
transfecting the hematopoietic stem cells with a vector comprising a nucleic acid construct comprising:
a silencing sequence encoding an XIST/Xist RNA; and
first and second sequences homologous to a site of desired integration in human chromosome 13, 18, or 21 or mouse chromosome 16 that specifically direct insertion of the silencing sequence into human chromosome 13, 18, or 21 or mouse chromosome 16 by homologous recombination; and

administering to the subject a sufficient number of the transfected hematopoietic stem cells to reduce gene expression from the trisomic chromosome.

2. The method of claim 1, wherein the silencing sequence is a full-length XIST/Xist gene sequence.

3. The method of claim 1, wherein the silencing sequence is an XIST/Xist gene sequence exclusive of one or more introns.

4. The method of claim 1, wherein the silencing sequence comprises about 6 kb to about 10 kb of exon 1 of an XIST/Xist gene sequence.

5. The method of claim 4, wherein the silencing sequence comprises the XIST/Xist cDNA sequence having accession number M97168 or a biologically active fragment or other variant thereof.

6. The method of claim 1, wherein the silencing sequence comprises a biologically active fragment or other biologically active variant of a naturally occurring XIST/Xist gene sequence.

7. The method of claim 1, further comprising a regulatory sequence.

8. The method of claim 7, wherein the regulatory sequence is a constitutively active, inducible, tissue-specific, or developmental stage-specific promoter.

9. The method of claim 1, wherein the first and second sequences direct insertion of the silencing sequence into a polymorphic region of the targeted chromosome.

10. The method of claim 1, wherein the first and second sequences direct insertion of the silencing sequence into an APP gene.

11. The method of claim 1, wherein the vector comprising the nucleic acid construct further comprises a selectable marker.

12. The method of claim 1, wherein the subject is a human.