Huntington's Disease Monkeys Research Articles

CAG Repeat Instability in the Peripheral and Central Nervous System of Transgenic Huntington's Disease Monkeys.

Huntington’s Disease (HD) is an autosomal dominant disease that results in severe neurodegeneration with no cure. HD is caused by the expanded CAG trinucleotide repeat (TNR) on the Huntingtin gene (HTT). Although the somatic and germline expansion of the CAG repeats has been well-documented, the underlying mechanisms had not been fully delineated. Increased CAG repeat length is associated with a more severe phenotype, greater TNR instability, and earlier age of onset. The direct relationship between CAG repeat length and molecular pathogenesis makes TNR instability a useful measure of symptom severity and tissue susceptibility. Thus, we examined the tissue-specific TNR instability of transgenic nonhuman primate models of Huntington’s disease. Our data show a similar profile of CAG repeat expansion in both rHD1 and rHD7, where high instability was observed in testis, liver, caudate, and putamen. CAG repeat expansion was observed in all tissue samples, and tissue- and CAG repeat size-dependent expansion was observed. Correlation analysis of CAG repeat expansion and the gene expression profile of four genes in different tissues, clusterin (CLU), transferrin (TF), ribosomal protein lateral stalk subunit P1 (RPLP1), and ribosomal protein L13a (RPL13A), showed a strong correlation with CAG repeat instability. Overall, our data, along with previously published studies, can be used for studying the biology of CAG repeat instability and identifying new therapeutic targets.

CAG repeat instability in embryonic stem cells and derivative spermatogenic cells of transgenic Huntington’s disease monkey

The expansion of CAG (glutamine; Q) trinucleotide repeats (TNRs) predominantly occurs through male lineage in Huntington's disease (HD). As a result, offspring will have larger CAG repeats compared to their fathers, which causes an earlier onset of the disease called genetic anticipation. This study aims to develop a novel in vitro model to replicate CAG repeat instability in early spermatogenesis and demonstrate the biological process of genetic anticipation by using the HD stem cell model for the first time. HD rhesus monkey embryonic stem cells (rESCs) were cultured in vitro for an extended period. Male rESCs were used to derive spermatogenic cells in vitro with a 10-day differentiation. The assessment of CAG repeat instability was performed by GeneScan and curve fit analysis. Spermatogenic cells derived from rESCs exhibit progressive expansion of CAG repeats with high daily expansion rates compared to the extended culture of rESCs. The expansion of CAG repeats is cell type-specific and size-dependent. Here, we report a novel stem cell model that replicates genome instability and CAG repeat expansion in in vitro derived HD monkey spermatogenic cells. The in vitro spermatogenic cell model opens a new opportunity for studying TNR instability and the underlying mechanism of genetic anticipation, not only in HD but also in other TNR diseases.

Progressive Polyglutamine Repeat Expansion in Peripheral Blood Cells and Sperm of Transgenic Huntington's Disease Monkeys.

The expanded CAG repeat results in somatic mosaicism and genetic anticipation in Huntington’s disease (HD). Here we report a longitudinal study examining CAG repeat instability in lymphocytes and sperm of three HD monkeys throughout their whole life-span that encompass the prodromal to symptomatic stages of HD. We demonstrate a progressive increase in CAG repeat length in lymphocytes and sperm as the animals aged. We also examined the impact of CAG repeat length on expansion rate, which showed a clear linear correlation up to 62Q, and high instability after. Our findings stress the importance of further investigation in CAG instability in peripheral blood cells longitudinally.

Amelioration of Huntington's disease phenotype in astrocytes derived from iPSC-derived neural progenitor cells of Huntington's disease monkeys.

Huntington’s disease (HD) is a devastating monogenic, dominant, hereditary, neurodegenerative disease. HD is caused by the expansion of CAG repeats in exon 1 of the huntingtin (HTT) gene, IT15, resulting in an expanded polyglutamine (polyQ) residue in the N-terminus of the HTT protein. HD is characterized by the accumulation of mutant HTT (mHTT) in neural and somatic cells. Progressive brain atrophy occurs initially in the striatum and extends to different brain regions with progressive decline in cognitive, behavioral and motor functions. Astrocytes are the most abundant cell type in the brain and play an essential role in neural development and maintaining homeostasis in the central nervous system (CNS). There is increasing evidence supporting the involvement of astrocytes in the development of neurodegenerative diseases such as Parkinson’s disease (PD), Huntington’s disease (HD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). We have generated neural progenitor cells (NPCs) from induced pluripotent stem cells (iPSCs) of transgenic HD monkeys as a model for studying HD pathogenesis. We have reported that NPCs can be differentiated in vitro into mature neural cells, such as neurons and glial cells, and are an excellent tool to study the pathogenesis of HD. To better understand the role of astrocytes in HD pathogenesis and discover new therapies to treat HD, we have developed an astrocyte differentiation protocol and evaluated the efficacy of RNAi to ameliorate HD phenotypes in astrocytes. The resultant astrocytes expressed canonical astrocyte-specific markers examined by immunostaining and real-time PCR. Flow cytometry (FACS) analysis showed that the majority of the differentiated NPCs (95.7%) were positive for an astrocyte specific marker, glial fibrillary acidic protein (GFAP). Functionalities of astrocytes were evaluated by glutamate uptake assay and electrophysiology. Expression of mHTT in differentiated astrocytes induced cytosolic mHTT aggregates and nuclear inclusions, suppressed the expression of SOD2 and PGC1, reduced ability to uptake glutamate, decreased 4-aminopyridine (4-AP) response, and shifted I/V plot measured by electrophysiology, which are consistent with previous reports on HD astrocytes and patient brain samples. However, expression of small-hairpin RNA against HTT (shHD) ameliorated and reversed aforementioned HD phenotypes in astrocytes. This represents a demonstration of a novel non-human primate (NHP) astrocyte model for studying HD pathogenesis and a platform for discovering novel HD treatments.

Progressive cognitive deficit, motor impairment and striatal pathology in a transgenic Huntington disease monkey model from infancy to adulthood.

One of the roadblocks to developing effective therapeutics for Huntington disease (HD) is the lack of animal models that develop progressive clinical traits comparable to those seen in patients. Here we report a longitudinal study that encompasses cognitive and motor assessment, and neuroimaging of a group of transgenic HD and control monkeys from infancy to adulthood. Along with progressive cognitive and motor impairment, neuroimaging revealed a progressive reduction in striatal volume. Magnetic resonance spectroscopy at 48 months of age revealed a decrease of N-acetylaspartate (NAA), further suggesting neuronal damage/loss in the striatum. Postmortem neuropathological analyses revealed significant neuronal loss in the striatum. Our results indicate that HD monkeys share similar disease patterns with HD patients, making them potentially suitable as a preclinical HD animal model.

Germline transmission in transgenic Huntington's disease monkeys

Transgenic nonhuman primate models are an increasingly popular model for neurologic and neurodegenerative disease because their brain functions and neural anatomies closely resemble those of humans. Transgenic Huntington's disease monkeys (HD monkeys) developed clinical features similar to those seen in HD patients, making the monkeys suitable for a preclinical study of HD. However, until HD monkey colonies can be readily expanded, their use in preclinical studies will be limited. In the present study, we confirmed germline transmission of the mutant huntingtin (mHTT) transgene in both embryonic stem cells generated from three male HD monkey founders (F0) and in second-generation offspring (F1) produced via artificial insemination by using intrauterine insemination technique. A total of five offspring were produced from 15 females that were inseminated by intrauterine insemination using semen collected from the three HD founders (5 of 15, 33%). Thus far, sperm collected from the HD founder (rHD8) has led to two F1 transgenic HD monkeys with germline transmission rate at 100% (2 of 2). mHTT expression was confirmed by quantitative real-time polymerase chain reaction using skin fibroblasts from the F1 HD monkeys and induced pluripotent stem cells established from one of the F1 HD monkeys (rHD8-2). Here, we report the stable germline transmission and expression of the mHTT transgene in HD monkeys, which suggest possible expansion of HD monkey colonies for preclinical and biomedical research studies.

Reversal of Cellular Phenotypes in Neural Cells Derived from Huntington’s Disease Monkey-Induced Pluripotent Stem Cells

SummaryHuntington’s disease (HD) is a dominant neurodegenerative disorder caused by the expansion of glutamine residues in the N-terminal region of the huntingtin (HTT) protein. The disease results in progressive neuronal loss, leading to motor, cognitive, and psychiatric impairment. Here, we report the establishment of neural progenitor cell (NPC) lines derived from induced pluripotent stem cells (iPSCs) of transgenic HD monkeys. Upon differentiation to neurons, HD neural cells develop cellular features of HD, including the formation of nuclear inclusions and oligomeric mutant HTT (mHTT) aggregates, as well as increased apoptosis. These phenotypes are rescued by genetic suppression of HTT and pharmacological treatment, demonstrating the ability of our HD cell model to respond to therapeutic treatment. The development and reversal of HD-associated phenotypes in neural cells from HD monkeys provides a unique nonhuman primate (NHP) model for exploring HD pathogenesis and evaluating therapeutics that could be assessed further in HD monkeys.

MicroRNA-128a dysregulation in transgenic Huntington's disease monkeys.

BackgroundHuntington’s Disease (HD) is a progressive neurodegenerative disorder with a single causal mutation in the Huntingtin (HTT) gene. MicroRNAs (miRNAs) have recently been implicated as epigenetic regulators of neurological disorders, however, their role in HD pathogenesis is not well defined. Here we study transgenic HD monkeys (HD monkeys) to examine miRNA dysregulation in a primate model of the disease.ResultsIn this report, 11 miRNAs were found to be significantly associated (P value < 0.05) with HD in the frontal cortex of the HD monkeys. We further focused on one of those candidates, miR-128a, due to the corresponding disruption in humans and mice with HD as well as its intriguing lists of gene targets. miR-128a was downregulated in our HD monkey model by the time of birth. We then confirmed that miR-128a was also downregulated in the brains of pre-symptomatic and post-symptomatic HD patients. Additionally, our studies confirmed a panel of canonical HD signaling genes regulated by miR-128a, including HTT and Huntingtin Interaction Protein 1 (HIP1).ConclusionOur studies found that miR-128a may play a critical role in HD and could be a viable candidate as a therapeutic or biomarker of the disease.

A two years longitudinal study of a transgenic Huntington disease monkey

BackgroundA two-year longitudinal study composed of morphometric MRI measures and cognitive behavioral evaluation was performed on a transgenic Huntington’s disease (HD) monkey. rHD1, a transgenic HD monkey expressing exon 1 of the human gene encoding huntingtin (HTT) with 29 CAG repeats regulated by a human polyubiquitin C promoter was used together with four age-matched wild-type control monkeys. This is the first study on a primate model of human HD based on longitudinal clinical measurements.ResultsChanges in striatal and hippocampal volumes in rHD1 were observed with progressive impairment in motor functions and cognitive decline, including deficits in learning stimulus-reward associations, recognition memory and spatial memory. The results demonstrate a progressive cognitive decline and morphometric changes in the striatum and hippocampus in a transgenic HD monkey.ConclusionsThis is the first study on a primate model of human HD based on longitudinal clinical measurements. While this study is based a single HD monkey, an ongoing longitudinal study with additional HD monkeys will be important for the confirmation of our findings. A nonhuman primate model of HD could complement other animal models of HD to better understand the pathogenesis of HD and future development of diagnostics and therapeutics through longitudinal assessment.

61 TRANSGENIC HUNTINGTON'S DISEASE MONKEY SPERM HAS A LOWER CRYOTOLERANCE

Cryopreservation is an important tool routinely used for preserving sperm for artificial reproductive technologies (ART), as well as genetic preservation of unique animal models. The cryopreservation process is harsh and detrimental to the fragile gametes, and damage to the sperm is not only known, but inevitable. This study presents new data in which sperm from 3 transgenic Huntington's disease (HD) monkeys (rhesus macaques) are compared with 3 wild-type (WT) rhesus sperm donors. Currently, there are no data comparing HD versus WT sperm viability and cryotolerance in humans. The goal of this study was to investigate differences between fresh and frozen semen by quantitative analysis on sperm viability based on (1) motility, (2) membrane integrity, and (3) acrosome integrity. Sperm motility was determined by visual evaluation. Membrane and acrosome integrity were assessed simultaneously by Hoechst 33342, propidium iodide (PI), and fluorescein isothiocyanate-peanut agglutinin (FITC-PNA) triple staining. Sperm viability analysis was divided into 3 groups: (1) fresh HD versus fresh WT, (2) fresh versus cryopreserved-thawed WT, (3) and fresh HD versus cryopreserved-thawed HD sperm. Interestingly, fresh HD sperm had a lower percentage of membrane-damaged cells (38.57 ± 3.15) compared with WT (49.67 ± 3.56; P &lt; 0.03). However, after cryopreservation and subsequent thawing, HD sperm had a significantly higher percentage increase in damaged membranes than WT sperm (27.91 ± 2.93 v. 8.27 ± 8.28; P &lt; 0.001), respectively. No significant difference in acrosome damage between groups was identified in either fresh or cryopreserved sperm populations. Motility significantly declined in both cryopreserved populations [HD: 89.7 to 43.4% (P &lt; 0.001) and WT: 90.0 to 45.6% (P &lt; 0.001)]. There was no significant difference between either freeze-thawed group. These data illustrate that HD sperm have a lower cryotolerance than WT sperm. Our findings suggest that the optimization of the HD sperm cryopreservation method and investigation on biochemical differences (e.g. membrane lipid composition) are necessary to improve post-thaw survival. This in turn is important for the establishment of a sperm cryobank and future derivation of a unique animal model such as HD monkey. Our study also suggests that HD monkey could be a useful model for optimizing cryopreservation method for HD patients.

Cryopreservation of transgenic Huntington's disease rhesus macaque sperm-A Case Report.

The cryoprotective effects of glycerol in three different semen freezing extenders, Tris-citrate (TRIS), TEST, and Tes-Tris-Egg yolk (TTE), on wild-type (WT) rhesus monkey (Macaca mulatta) sperm cryopreservation have been tested. Sperm motility and viability were examined to evaluate the integrity of frozen-thawed sperm, and the best extender was selected to cryopreserve sperm from transgenic Huntington's disease (HD) rhesus monkey. The results showed no post-thaw motility difference among the freezing extender tested (P>0.05). However, sperm membrane integrity in TEST and TTE were significantly better than in TRIS extender (P<0.05). TEST was chosen for HD rhesus monkey sperm cryopreservation. The results showed that post-thawed HD sperm motility and viability was not different compared with WT control group (P>0.05). The present study demonstrates that TEST and TTE were excellent extenders and suitable for rhesus monkey sperm cryopreservation and no detectible differences of post-thaw sperm motility and viability between HD and WT rhesus monkeys resulted from TEST extender.

Pathogenic Cellular Phenotypes are Germline Transmissible in a Transgenic Primate Model of Huntington's Disease

A transgenic primate model for Huntington's Disease (HD) first reported by our group that (HD monkeys) carry the mutant Huntingtin (HTT) gene with expanded polyglutamine (CAG) repeats and, develop chorea, dystonia, and other involuntary motor deficiencies similar to HD [ 1 ]. More recently, we have found that longitudinal magnetic resonance imaging of the HD monkey brain revealed significant atrophy in regions associated with cognitive deficits symptomatic in HD patients, providing the first animal model which replicates clinical phenotypes of diagnosed humans. Here we report germline transmission of the pathogenic mutant HTT in HD monkey by the production of embryos and subsequent derivation of HD monkey embryonic stem cells (rHD-ESCs) using HD monkey sperm. rHD-ESCs inherit mutant HTT and green fluorescent protein (GFP) genes through the gametes of HD monkey. rHD-ESCs express mutant HTT and form intranuclear inclusion, a classical cellular feature of HD. Notably, mosaicism of the pathogenic polyQ region in the sperm as well as derived ESCs were also observed, consistent with intraindividual and intergenerational reports of mosaic CAG repeats [ 2 , 3 ]and CAG expansion in HD patients [ 4-7 ]. The confirmation of transgene inheritability and development of pathogenic HD phenotype in derived rHD-ESCs reported in this study is a milestone in the pursuit of a transgenic primate model with inherited mutant HTT for development of novel disease biomarkers and therapeutics.

NEURODEGENERATIVE DISEASES

In the past few decades, a tremendous amount of effort has been invested in developing gene and cell therapies for inherited genetic diseases such as Huntington's disease (HD). However, progress in their clinical application has been very limited. One of the major barriers is the lack of appropriate animal models that allow precise prediction patterns in human patients. Most of the animal models used for gene and cell therapy study are primarily focused on safety and toxicity evaluation, while therapeutic efficacy cannot be fully addressed because they do not carry the same human diseases. Although mouse models of human diseases are available and have been widely used for the development of new therapies, mice are not good predictors for humans because of the fundamental differences (genome composition, body size, life span and metabolic mechanism) between humans and rodents. Although monkeys are one of the best models for studying pharmacokinetics and overall impact of treatment, they are primarily used for safety and toxicity evaluation. Even HD monkey models, created by chemical induction or focal gene transfer in the brain, develop similar cellular pathology, therapeutic efficacy and systemic evaluation cannot be determined, which is one of the major barriers in drug and therapeutic development. The development of transgenic HD monkeys has opened the door for a new paradigm of animal modeling for the advancement of novel gene and cell therapy. HD monkeys not only carry the genetic defect that leads to human HD, they also develop clinical features comparable to humans that no other animal model does. While testing in HD monkeys has yet to be achieved until a cohort of well characterized HD monkeys was established, iPS cell lines derived from HD monkeys with a board spectrum of HD pathology and clinical features are a unique in vitro model for studying HD pathogenesis and the development of novel therapeutic approaches. New knowledge and treatments generated from iPS cells can next be translated and applied in HD monkeys from whom the stem cells were derived, thus the goal of personalized medicine can also be evaluated. This work was funded by a grant from NCRR/NIH (R24RR018827).

IMPACT OF LARGE MAMMALS MODELS IN IMMUNOLOGY

Immune responses that control tolerance to self, tolerance to conceptus during pregnancy, and defense against cancer and pathogens are governed by dendritic cells (DC), that are key immune cell types that integrate environment signals and direct T and B cell immune responses. Different DC subtypes exist in mice that each trigger a specific type of immune response, such as cytotoxicity, antibody production, and regulatory processes. Interestingly in mice, it is possible to target specific DC subtypes with antigenised antibodies and obtain a desired type of immune response. However this conceptual breakthrough in vaccinology and immune regulation manipulation may only be valid for laboratory mice, as unfortunately often encountered in the process of bench to bed side translation. Furthermore, whether DC subsets knowledge and manipulation can be translated to human and to animal of socio-economical importance is still not known. We adressed this question in pigs and ruminants. The interest of these species over mice are that (1) they are the direct target species for vaccines, (2) they present genetic diversities and live in an open environment, (3) they present physiological similarities with human such as skin for pigs with skin being a main site for vaccination, (4) skin migrated DC and DC subsets can be collected from lymph after surgical catheterisation of lymph ducts in these species, and not in mice. We studied the molecular characterisation profiles of DC subsets from skin in ruminant and swine and evaluated how they compare to mouse and human DC subsets, based on comparative transcriptomic analyses. We assessed whether ruminant and swine DC subsets share functional similarities and differences with the corresponding murine subsets, and whether these properties translate into novel vaccine developements. Overall our work unravel conserved molecular and functional features that allow characterization of dendritic cell subtypes across mammals and possibly across vertebrates. In the past few decades, a tremendous amount of effort has been invested in developing gene and cell therapies for inherited genetic diseases such as Huntington's disease (HD). However, progress in their clinical application has been very limited. One of the major barriers is the lack of appropriate animal models that allow precise prediction patterns in human patients. Most of the animal models used for gene and cell therapy study are primarily focused on safety and toxicity evaluation, while therapeutic efficacy cannot be fully addressed because they do not carry the same human diseases. Although mouse models of human diseases are available and have been widely used for the development of new therapies, mice are not good predictors for humans because of the fundamental differences (genome composition, body size, life span and metabolic mechanism) between humans and rodents. Although monkeys are one of the best models for studying pharmacokinetics and overall impact of treatment, they are primarily used for safety and toxicity evaluation. Even HD monkey models, created by chemical induction or focal gene transfer in the brain, develop similar cellular pathology, therapeutic efficacy and systemic evaluation cannot be determined, which is one of the major barriers in drug and therapeutic development. The development of transgenic HD monkeys has opened the door for a new paradigm of animal modeling for the advancement of novel gene and cell therapy. HD monkeys not only carry the genetic defect that leads to human HD, they also develop clinical features comparable to humans that no other animal model does. While testing in HD monkeys has yet to be achieved until a cohort of well characterized HD monkeys was established, iPS cell lines derived from HD monkeys with a board spectrum of HD pathology and clinical features are a unique in vitro model for studying HD pathogenesis and the development of novel therapeutic approaches. New knowledge and treatments generated from iPS cells can next be translated and applied in HD monkeys from whom the stem cells were derived, thus the goal of personalized medicine can also be evaluated. This work was funded by a grant from NCRR/NIH (R24RR018827).

Reprogramming Huntington Monkey Skin Cells into Pluripotent Stem Cells

Induced pluripotent Huntington's disease monkey stem cells (rHD-iPSCs) were established by the overexpression of rhesus macaque transcription factors (Oct4, Sox2, and Klf4) in transgenic Huntington's monkey skin fibroblasts. The rHD-iPSCs were pluripotent and capable of differentiating into neuronal cell types in vitro and developed teratoma in immune compromised mice. We also demonstrated the upregulation of endogenous Oct4 and Sox2 after successful reprogramming to pluripotency in rHD-iPSCs, which was not expressed in skin fibroblasts. rHD-iPSCs also developed cellular features comparable to Huntington's disease (HD), including the accumulation of mutant huntingtin (htt) aggregate and the formation of intranuclear inclusions (NIs) paralleling neural differentiation in vitro. Induced pluripotent stem cells from transgenic HD monkeys open a new era of nonhuman primate modeling of human diseases. rHD-iPSCs that develop key HD cellular features and parallel neural differentiation can be a powerful platform for investigating the developmental impact on HD pathogenesis and developing new therapies, which can be evaluated in HD monkeys from whom the rHD-iPSCs were derived.

Monkey hybrid stem cells develop cellular features of Huntington's disease

BackgroundPluripotent stem cells that are capable of differentiating into different cell types and develop robust hallmark cellular features are useful tools for clarifying the impact of developmental events on neurodegenerative diseases such as Huntington's disease. Additionally, a Huntington's cell model that develops robust pathological features of Huntington's disease would be valuable for drug discovery research.ResultsTo test this hypothesis, a pluripotent Huntington's disease monkey hybrid cell line (TrES1) was established from a tetraploid Huntington's disease monkey blastocyst generated by the fusion of transgenic Huntington's monkey skin fibroblast and a wild-type non-transgenic monkey oocyte. The TrES1 developed key Huntington's disease cellular pathological features that paralleled neural development. It expressed mutant huntingtin and stem cell markers, was capable of differentiating to neural cells, and developed teratoma in severely compromised immune deficient (SCID) mice. Interestingly, the expression of mutant htt, the accumulation of oligomeric mutant htt and the formation of intranuclear inclusions paralleled neural development in vitro , and even mutant htt was ubiquitously expressed. This suggests the development of Huntington's disease cellular features is influenced by neural developmental events.ConclusionsHuntington's disease cellular features is influenced by neural developmental events. These results are the first to demonstrate that a pluripotent stem cell line is able to mimic Huntington's disease progression that parallels neural development, which could be a useful cell model for investigating the developmental impact on Huntington's disease pathogenesis.

Towards a transgenic model of Huntington’s disease in a non-human primate

Non-human primates are valuable for modelling human disorders and for developing therapeutic strategies; however, little work has been reported in establishing transgenic non-human primate models of human diseases. Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor impairment, cognitive deterioration and psychiatric disturbances followed by death within 10-15 years of the onset of the symptoms. HD is caused by the expansion of cytosine-adenine-guanine (CAG, translated into glutamine) trinucleotide repeats in the first exon of the human huntingtin (HTT) gene. Mutant HTT with expanded polyglutamine (polyQ) is widely expressed in the brain and peripheral tissues, but causes selective neurodegeneration that is most prominent in the striatum and cortex of the brain. Although rodent models of HD have been developed, these models do not satisfactorily parallel the brain changes and behavioural features observed in HD patients. Because of the close physiological, neurological and genetic similarities between humans and higher primates, monkeys can serve as very useful models for understanding human physiology and diseases. Here we report our progress in developing a transgenic model of HD in a rhesus macaque that expresses polyglutamine-expanded HTT. Hallmark features of HD, including nuclear inclusions and neuropil aggregates, were observed in the brains of the HD transgenic monkeys. Additionally, the transgenic monkeys showed important clinical features of HD, including dystonia and chorea. A transgenic HD monkey model may open the way to understanding the underlying biology of HD better, and to the development of potential therapies. Moreover, our data suggest that it will be feasible to generate valuable non-human primate models of HD and possibly other human genetic diseases.