Corticostriatal synaptic function in mouse models of Huntington's disease: early effects of huntingtin repeat length and protein load

Huntington's disease (HD) is an autosomal dominant, late onset, neurodegenerative disease characterized by motor deficits and dementia that is caused by expansion of a CAG repeat in the HD gene. Clinical manifestations result from selective neuronal degeneration of predominantly GABAergic striatal medium-sized spiny neurons (MSNs). A growing number of studies demonstrate that personality, mood and cognitive disturbances are some of the earliest signs of HD and may reflect synaptic dysfunction prior to neuronal loss. Previous studies in striatal MSNs demonstrated early alterations in NMDA-type glutamate receptor currents in several HD mouse models, as well as evidence for presynaptic dysfunction prior to disease manifestations in the R6/2 HD fragment mouse model. We have compared corticostriatal synaptic function in full-length, human HD gene-carrying YAC transgenic mice expressing a non-pathogenic CAG repeat (YAC18; control) with three increasingly severe variants of pathogenic HD gene-expressing mice (YAC72 and two different lines of YAC128), at ages that precede any detectable disease phenotype. We report presynaptic dysfunction and a propensity towards synaptic depression in YAC72 and YAC128 compared to YAC18 mice, and, in the most severe model, we also observed altered AMPA receptor function. When normalized to evoked AMPAR currents, postsynaptic NMDAR currents are augmented in all three pathogenic HD YAC variants. These findings demonstrate multiple perturbations to corticostriatal synaptic function in HD mice, furthering our understanding of the early effects of the HD mutation that may contribute to cognitive dysfunction, mood disorders and later development of more serious dysfunction. Furthermore, this study provides a set of neurophysiological sequelae against which to test and compare other mouse models and potential therapies in HD.     

Compensatory changes in the ubiquitin-proteasome system, brain-derived neurotrophic factor and mitochondrial complex II/III in YAC72 and R6/2 transgenic mice partially model Huntington's disease patients

Intraneuronal protein aggregates of the mutated huntingtin in Huntington's disease (HD) brains suggest an overload and/or dysfunction of the ubiquitin-proteasome system (UPS). There is a general inhibition of the UPS in many brain regions (cerebellum, cortex, substantia nigra and caudate-putamen) and skin fibroblasts from HD patients. In the current experiment, the widely used mutant huntingtin-exon 1 CAG repeat HD transgenic mice model (R6/2) (with 144 CAG repeat and exon 1) during late-stage pathology, had increases in proteasome activity in the striatum. However, this discrepancy with HD patient tissue was not apparent in the mutant CAG repeat huntingtin full-length HD (YAC72) transgenic mouse model during post-symptomatic and late-stage pathology, which then also showed UPS inhibition similar to HD patients' brains. In both types of HD model mice, we determined biochemical changes, including expression of brain-derived neurotrophic factor (BDNF) and mitochondrial complex II/III (MCII/III) activities related to HD pathology. We found increases of both BDNF expression, and MCII/III activities in YAC72 transgenic mice, and no change of BDNF expression in R6/2 mice. Our data show that extreme CAG repeat lengths in R6/2 mice is paradoxically associated with increased proteasome activity, probably as a cellular compensatory biochemical change in response to the underlying mutation. Changes in HD patients for UPS function, BDNF expression and MCII/III activity are only partially modeled in R6/2 and YAC72 mice, with the latter at 16 months of age being most congruent with the human disease.     

Selective degeneration and nuclear localization of mutant huntingtin in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult onset neurodegenerative disorder that predominantly affects the striatum and cortex despite ubiquitous expression of mutant huntingtin (htt). Here we demonstrate that this pattern of selective degeneration is present in the YAC128 mouse model of HD. At 12 months, YAC128 mice show significant atrophy in the striatum, globus pallidus and cortex with relative sparing of the hippocampus and cerebellum (striatum: -10.4%, P<0.001; globus pallidus: -10.8%, P=0.04; cortex: -8.6%, P=0.001; hippocampus: +0.3%, P=0.9; cerebellum: +2.9%, P=0.6). Similarly, neuronal loss at this age is present in the striatum (-9.1%, P<0.001) and cortex of YAC128 mice (-8.3%, P=0.02) but is not detected in the hippocampus (+1.5%, P=0.72). Mutant htt expression levels are similar throughout the brain and fail to explain the selective neuronal degeneration. In contrast, nuclear detection of mutant htt occurs earliest and to the greatest extent in the striatum-the region most affected in HD. The appearance of EM48-reactive mutant htt in the nucleus in the striatum at 2 months coincides with the onset of behavioral abnormalities in YAC128 mice. In contrast to YAC128 mice, the R6/1 mouse model of HD, which expresses exon 1 of mutant htt, exhibits non-selective, widespread atrophy along with non-selective nuclear detection of mutant htt at 10 months of age. Our findings suggest that selective nuclear localization of mutant htt may contribute to the selective degeneration in HD and that appropriately regulated expression of full-length mutant htt in YAC128 mice results in a pattern of degeneration remarkably similar to human HD.     

Whole body cholesterol metabolism is impaired in Huntington's disease

We previously reported impaired cholesterol biosynthesis in rodent Huntington Disease (HD) models and HD patients' fibroblasts and post mortem brains. We also found that plasma levels of 24S-hydroxycholesterol (24OHC), the brain specific elimination product of cholesterol considered a marker of brain cholesterol turnover, were significantly reduced in HD patients at any disease stage. In the present study we analysed by mass spectrometry the fasting plasma levels of cholesterol, its biosynthetic precursors lanosterol and lathosterol, of the whole-body elimination products 27-hydroxycholesterol and of brain 24OHC in a cohort of premanifest and HD patients at different disease stages. We found that the cholesterol precursors lanosterol and lathosterol (both index of whole body cholesterol synthesis), the levels of the bile acid precursor 27-hydroxycholesterol, and of the brain specific 24OHC, were all significantly reduced in manifest HD patients, suggesting that whole-body and brain cholesterol homeostasis are both impaired in HD.     

Human huntingtin derived from YAC transgenes compensates for loss of murine huntingtin by rescue of the embryonic lethal phenotype

Huntington disease (HD) is caused by expansion of a CAG trinucleotide repeat in exon 1 of a novel gene. The HD protein (huntingtin) plays a critical role in early embryonic development since homozygous targeted disruption of the murine HD gene results in embryonic lethality by day 7.5. To rescue this phenotype by transgene based huntingtin expression it is therefore essential to express the protein early enough in development in the appropriate cells. Since YAC based transgenes are known to be regulated in an appropriate temporal and tissue-specific manner, we sought to rescue the embryonic lethality by breeding YAC transgenic mice expressing human huntingtin with mice heterozygous for the targeted disruption. We generated viable offspring homozygous for the disrupted murine HD gene but expressing human huntingtin derived from the YAC. This result clearly shows that YAC transgene based expression of huntingtin occurs prior to 7.5 days gestation. Additionally, we show that human huntingtin expression in YAC transgenic mice follows an identical tissue distribution and subcellular localisation pattern as that of the murine endogenous protein and that expression levels of 2-3 times endogenous can be achieved. This shows that human huntingtin under the influence of its native promoter, despite differences to the murine protein, is functional in a murine background and can compensate for loss of the murine protein. These results show that YAC transgenic approaches are a particularly promising route to producing an animal model for disorders associated with CAG expansion.      

Selective striatal neuronal loss in a YAC128 mouse model of Huntington disease

An expanded CAG repeat is the underlying genetic defect in Huntington disease, a disorder characterized by motor, psychiatric and cognitive deficits and striatal atrophy associated with neuronal loss. An accurate animal model of this disease is crucial for elucidation of the underlying natural history of the illness and also for testing experimental therapeutics. We established a new yeast artificial chromosome (YAC) mouse model of HD with the entire human HD gene containing 128 CAG repeats (YAC128) which develops motor abnormalities and age-dependent brain atrophy including cortical and striatal atrophy associated with striatal neuronal loss. YAC128 mice exhibit initial hyperactivity, followed by the onset of a motor deficit and finally hypokinesis. The motor deficit in the YAC128 mice is highly correlated with striatal neuronal loss, providing a structural correlate for the behavioral changes. The natural history of HD-related changes in the YAC128 mice has been defined, demonstrating the presence of huntingtin inclusions after the onset of behavior and neuropathological changes. The HD-related phenotypes of the YAC128 mice show phenotypic uniformity with low inter-animal variability present, which together with the age-dependent striatal neurodegeneration make it an ideal mouse model for the assessment of neuroprotective and other therapeutic interventions.     

Marked differences in neurochemistry and aggregates despite similar behavioural and neuropathological features of Huntington disease in the full-length BACHD and YAC128 mice

The development of animal models of Huntington disease (HD) has enabled studies that help define the molecular aberrations underlying the disease. The BACHD and YAC128 transgenic mouse models of HD harbor a full-length mutant huntingtin (mHTT) and recapitulate many of the behavioural and neuropathological features of the human condition. Here, we demonstrate that while BACHD and YAC128 animals exhibit similar deficits in motor learning and coordination, depressive-like symptoms, striatal volume loss and forebrain weight loss, they show obvious differences in key features characteristic of HD. While YAC128 mice exhibit significant and widespread accumulation of mHTT striatal aggregates, these mHTT aggregates are absent in BACHD mice. Furthermore, the levels of several striatally enriched mRNA for genes, such as DARPP-32, enkephalin, dopamine receptors D1 and D2 and cannabinoid receptor 1, are significantly decreased in YAC128 but not BACHD mice. These findings may reflect sequence differences in the human mHTT transgenes harboured by the BACHD and YAC128 mice, including both single nucleotide polymorphisms as well as differences in the nature of CAA interruptions of the CAG tract. Our findings highlight a similar profile of HD-like behavioural and neuropathological deficits and illuminate differences that inform the use of distinct endpoints in trials of therapeutic agents in the YAC128 and BACHD mice.     

(Over)correction of FMR1 deficiency with YAC transgenics: behavioral and physical features

Fragile X syndrome is a common cause of mental retardation involving loss of expression of the FMR1 gene. The role of FMR1 remains undetermined but the protein appears to be involved in RNA metabolism. Fmr1 knockout mice exhibit a phenotype with some similarities to humans, such as macroorchidism and behavioral abnormalities. As a step toward understanding the function of FMR1 and the determination of the potential for therapeutic approaches to fragile X syndrome, yeast artificial chromosome (YAC) transgenic mice were generated in order to determine whether the Fmr1 knockout mouse phenotype could be rescued. Several transgenic lines were generated that carried the entire FMR1 locus with extensive amounts of flanking sequence. We observed that the YAC transgene supported production of the human protein (FMRP) which was present at levels 10 to 15 times that of endogenous protein and was expressed in a cell- and tissue-specific manner. Macro-orchidism was absent in knockout mice carrying the YAC transgene indicating functional rescue by the human protein. Given the complex behavioral phenotype in fragile X patients and the mild phenotype previously reported for the Fmr1 knockout mouse, we performed a more thorough evaluation of the Fmr1 knockout phenotype using additional behavioral assays that had not previously been reported for this animal model. The mouse displayed reduced anxiety-related responses with increased exploratory behavior. FMR1 YAC transgenic mice overexpressing the human protein did produce opposing behavioral responses and additional abnormal behaviors were also observed. These findings have significant implications for gene therapy for fragile X syndrome since overexpression of the gene may harbor its own phenotype.     

Role of NR2B-type NMDA receptors in selective neurodegeneration in Huntington disease

N-Methyl-D-aspartate receptor (NMDAR)-mediated excitotoxicity has been proposed to play a role in Huntington disease (HD), caused by expansion of a polyglutamine tract in the protein huntingtin. HD is characterized by selective neurodegeneration most severely affecting striatal medium-sized spiny projection neurons (MSNs), where expression of the NMDAR subunit NR2B is increased relative to other NR2 subunits. Here, we review our data that NR2B-type NMDAR currents are selectively potentiated by mutant huntingtin in transfected non-neuronal cells and acutely dissociated striatal MSNs from the YAC72 transgenic mouse model of HD. As well, we report increased striatal MSN NMDAR-mediated synaptic currents in corticostriatal slice recordings from YAC72 compared with wild-type mice. This effect was associated with a larger NMDAR- to AMPAR-mediated current ratio, suggesting specific potentiation of postsynaptic NMDARs. Enhanced NMDAR current likely involves increased surface receptor numbers or activity, since we observed no differences between genotypes in striatal NR2B expression. Potentiation of NR2B-containing NMDAR current in striatal MSNs expressing mutant huntingtin may help explain the exquisite vulnerability of these neurons to degeneration in HD.     

Huntingtin phosphorylation on serine 421 is significantly reduced in the striatum and by polyglutamine expansion in vivo

Huntington disease (HD) results from polyglutamine expansion in the huntingtin protein (htt). Despite the widespread tissue expression pattern of htt, neuronal loss is highly selective to medium spiny neurons of the striatum. Huntingtin is phosphorylated on serine-421 (S421) by the pro-survival signaling protein kinase Akt (PKB) and this has been previously shown to be protective against the toxicity of polyglutamine-expanded htt in cell culture. Using an antibody specific for htt phosphorylated on S421, we now demonstrate that htt phosphorylation is present at significant levels under normal physiological conditions in human and mouse brain. Furthermore, htt phosphorylation shows a regional distribution with the highest levels in the cerebellum, less in the cortex, and least in the striatum. In cell cultures and in YAC transgenic mice, the endogenous phosphorylation of polyglutamine-expanded htt is significantly reduced relative to wild-type htt. The presence and pattern of significant htt phosphorylation in the brain indicates that this dynamic post-translational modification is important for the regulation of htt and may contribute to the selective neurodegeneration seen in HD.     

Low plasma total cholesterol in patients with Huntington's disease and first-degree relatives

Recent studies indicate altered cholesterol homeostasis in Huntington's disease (HD) after it was found that cultured human and mice cells expressing mutant huntingtin show reduced mRNA of cholesterol biosynthetic enzymes. Plasma total cholesterol (TC) levels have been connected to degenerative disorders, but data for HD are lacking. We estimated plasma TC in three groups of HD related subjects: (a) patients with overt symptomatology, (b) subjects with expanded CAG repeat number in the Huntington gene before disease onset, and (c) siblings or descendants of HD patients, with normal CAG repeat number. Compared to TC levels of age-matched controls, all three groups had significantly lower plasma TC levels. The expected positive correlation of TC to age, present in the control group, was absent in the whole group or the three subgroups of the HD subjects. TC of the ApoE genotype subgroups showed small, non-significant differences. In the group of patients, TC levels were not related to severity of illness, duration of illness, and presence of depression or dementia in their symptomatology, while lower TC levels were found in patients with psychotic features. The results indicate altered cholesterol homeostasis in members of families with HD patients. Low TC levels have been connected to increased suicide risk in several studies, and high suicidal ideation has been reported in both HD gene carriers and non-carriers. Although low plasma TC levels do not necessarily imply alterations in brain cholesterol levels, a more detailed study of plasma lipids in HD patients and their first-degree relatives, as well as the search for genetic factors regarding cholesterol synthesis and disposition, are warranted.     

Altered palmitoylation and neuropathological deficits in mice lacking HIP14

Huntingtin interacting protein 14 (HIP14, ZDHHC17) is a huntingtin (HTT) interacting protein with palmitoyl transferase activity. In order to interrogate the function of Hip14, we generated mice with disruption in their Hip14 gene. Hip14-/- mice displayed behavioral, biochemical and neuropathological defects that are reminiscent of Huntington disease (HD). Palmitoylation of other HIP14 substrates, but not Htt, was reduced in the Hip14-/- mice. Hip14 is dysfunctional in the presence of mutant htt in the YAC128 mouse model of HD, suggesting that altered palmitoylation mediated by HIP14 may contribute to HD.     

Calpain and STriatal-Enriched protein tyrosine phosphatase (STEP) activation contribute to extrasynaptic NMDA receptor localization in a Huntington's disease mouse model

In Huntington's disease (HD), the mutant huntingtin (mhtt) protein is associated with striatal dysfunction and degeneration. Excitotoxicity and early synaptic defects are attributed, in part, to altered NMDA receptor (NMDAR) trafficking and function. Deleterious extrasynaptic NMDAR localization and signalling are increased early in yeast artificial chromosome mice expressing full-length mhtt with 128 polyglutamine repeats (YAC128 mice). NMDAR trafficking at the plasma membrane is regulated by dephosphorylation of the NMDAR subunit GluN2B tyrosine 1472 (Y1472) residue by STriatal-Enriched protein tyrosine Phosphatase (STEP). NMDAR function is also regulated by calpain cleavage of the GluN2B C-terminus. Activation of both STEP and calpain is calcium-dependent, and disruption of calcium homeostasis occurs early in the HD striatum. Here, we show increased calpain cleavage of GluN2B at both synaptic and extrasynaptic sites, and elevated extrasynaptic total GluN2B expression in the YAC128 striatum. Calpain inhibition significantly reduced extrasynaptic GluN2B expression in the YAC128 but not wild-type striatum. Furthermore, calpain inhibition reduced whole-cell NMDAR current and the surface/internal GluN2B ratio in co-cultured striatal neurons, without affecting synaptic GluN2B localization. Synaptic STEP activity was also significantly higher in the YAC128 striatum, correlating with decreased GluN2B Y1472 phosphorylation. A substrate-trapping STEP protein (TAT-STEP C-S) significantly increased VGLUT1-GluN2B colocalization, as well as increasing synaptic GluN2B expression and Y1472 phosphorylation. Moreover, combined calpain inhibition and STEP inactivation reduced extrasynaptic, while increasing synaptic GluN2B expression in the YAC128 striatum. These results indicate that increased STEP and calpain activation contribute to altered NMDAR localization in an HD mouse model, suggesting new therapeutic targets for HD.     

α-Synuclein accumulates in huntingtin inclusions but forms independent filaments and its deficiency attenuates early phenotype in a mouse model of Huntington's disease

Huntington's disease (HD) is the most common of nine inherited neurological disorders caused by expanded polyglutamine (polyQ) sequences which confer propensity to self-aggregate and toxicity to their corresponding mutant proteins. It has been postulated that polyQ expression compromises the folding capacity of the cell which might affect other misfolding-prone proteins. α-Synuclein (α-syn) is a small neural-specific protein with propensity to self-aggregate that forms Parkinson's disease (PD) Lewy bodies. Point mutations in α-syn that favor self-aggregation or α-syn gene duplications lead to familial PD, thus indicating that increased α-syn aggregation or levels are sufficient to induce neurodegeneration. Since polyQ inclusions in HD and other polyQ disorders are immunopositive for α-syn, we speculated that α-syn might be recruited as an additional mediator of polyQ toxicity. Here, we confirm in HD postmortem brains and in the R6/1 mouse model of HD the accumulation of α-syn in polyQ inclusions. By isolating the characteristic filaments formed by aggregation-prone proteins, we found that N-terminal mutant huntingtin (N-mutHtt) and α-syn form independent filamentous microaggregates in R6/1 mouse brain as well as in the inducible HD94 mouse model and that N-mutHtt expression increases the load of α-syn filaments. Accordingly, α-syn knockout results in a diminished number of N-mutHtt inclusions in transfected neurons and also in vivo in the brain of HD mice. Finally, α-syn knockout attenuates body weight loss and early motor phenotype of HD mice. This study therefore demonstrates that α-syn is a modifier of polyQ toxicity in vivo and raises the possibility that potential PD-related therapies aimed to counteract α-syn toxicity might help to slow HD.     

A Huntington's disease CAG expansion at the murine Hdh locus is unstable and associated with behavioural abnormalities in mice

Huntington's disease (HD) is a dominant disorder characterized by premature and progressive neurodegeneration. In order to generate an accurate model of the disease, we introduced an HD-like mutation (an extended stretch of 72-80 CAG repeats) into the endogenous mouse Hdh gene. Analysis of the mutation in vivo reveals significant levels of germline instability, with expansions, contractions and sex-of-origin effects in evidence. Mice expressing full-length mutant protein display abnormal social behaviour in the absence of acute neurodegeneration. Given that psychiatric changes, including irritability and aggression, are common findings in HD patients, our data are consistent with the hypothesis that some clinical features of HD may be caused by pathological processes that precede gross neuronal cell death. This implies that effective treatment of HD may require an understanding and amelioration of these dysfunctional processes, rather than simply preventing the premature death of neurons in the brain. These mice should facilitate the investigation of the molecular mechanisms that underpin the pathway from genotype to phenotype in HD.     

Increased PKA signaling disrupts recognition memory and spatial memory: role in Huntington's disease

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. However, the molecular events involved in this cognitive decline are still poorly understood. Here, using three different paradigms, the novel object recognition test, the T-maze spontaneous alternation task and the Morris water maze, we detected severe cognitive deficits in the R6/1 mouse model of HD before the onset of motor symptoms. When we examined the putative molecular pathways involved in these alterations, we observed hippocampal cAMP-dependent protein kinase (PKA) hyper-activation in na?ve R6/1 mice compared with wild-type (WT) mice, whereas extracellular signal-regulated kinase 1/2 and calcineurin activities were not modified. Increased PKA activity resulted in hyper-phosphorylation of its substrates N-methyl-D-aspartate receptor subunit 1, Ras-guanine nucleotide releasing factor-1 and striatal-enriched protein tyrosine phosphatase, but not cAMP-responsive element binding protein or the microtubule-associated protein tau. In correlation with the over-activation of the PKA pathway, we found a down-regulation of the protein levels of some phosphodiesterase (PDE) 4 family members. Similar molecular changes were found in the hippocampus of R6/2 mice and HD patients. Furthermore, chronic treatment of WT mice with the PDE4 inhibitor rolipram up-regulated PKA activity, and induced learning and memory deficits similar to those seen in R6 mice, but had no effect on R6/1 mice cognitive impairment. Importantly, hippocampal PKA inhibition by infusion of Rp-cAMPS restored long-term memory in R6/2 mice. Thus, our results suggest that occlusion of PKA-dependent processes is one of the molecular mechanisms underlying cognitive decline in R6 animals.     

Enhanced striatal NR2B-containing N-methyl-D-aspartate receptor-mediated synaptic currents in a mouse model of Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disease caused by expansion of a polyglutamine tract near the N terminus of the protein huntingtin, leading to dramatic loss of striatal medium-sized spiny GABAergic projection neurons (MSNs). Evidence suggests overactivation of N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs) contributes to selective degeneration of MSNs in HD. Striatal MSNs are enriched in NR2B, and whole cell current and excitotoxicity mediated predominantly by the NR2B subtype of NMDARs is increased with expression of mutant huntingtin in transfected cell lines and striatal MSNs from mice models. To test whether synaptic NMDAR current is altered by mutant huntingtin expression, we recorded striatal MSN excitatory postsynaptic currents (EPSCs) evoked by stimulation of cortical afferents in corticostriatal slices from YAC72 mice and their wild-type (WT) littermates at age 21-31 days. The ratio of NMDAR- to AMPAR-mediated EPSC amplitude was significantly increased in YAC72 compared to WT mice. Furthermore, using a paired-pulse stimulation protocol as a measure of presynaptic glutamate release probability, we found no significant differences between YAC72 and WT striatal MSN responses. These data suggest selective potentiation of postsynaptic NMDAR activity at corticostriatal synapses in YAC72 mice. Measurements of EPSC decay kinetics, as well as the effects of NR2B-subtype selective antagonists and glycine concentration on EPSC amplitude, are consistent with the majority of postsynaptic NMDARs being triheteromers of NR1/NR2A/NR2B in both WT and YAC72 mice. Together with previous results, our data suggest that enhanced activity of NR2B-containing NMDARs is one of the earliest changes leading to neuronal degeneration in HD.     

Loss of wild-type huntingtin influences motor dysfunction and survival in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult-onset neurodegenerative disease caused by a toxic gain of function in the huntingtin (htt) protein. The contribution of wild-type htt function to the pathogenesis of HD is currently uncertain. To assess the role of wild-type htt in HD, we generated YAC128 mice that do not express wild-type htt (YAC128-/-) but express the same amount of mutant htt as normal YAC128 mice (YAC128+/+). YAC128-/- mice perform worse than YAC128+/+ mice in the rotarod test of motor coordination (P = 0.001) and are hypoactive compared with YAC128+/+ mice at 2 months (P = 0.003). Striatal neuropathology was not clearly worse in YAC128-/- mice compared with YAC128+/+ mice. There was no significant effect of decreased wild-type htt on striatal volume, neuronal counts or DARPP-32 expression but a modest worsening of striatal neuronal atrophy was evident (6%, P = 0.03). The testis of YAC128+/+ mice showed atrophy and degeneration, which was markedly worsened in the absence of wild-type htt (P = 0.001). YAC128+/+ mice also showed a male specific deficit in survival compared with WT mice which was exacerbated by the loss of wild-type htt (12-month-male survival, P < 0.001). Overall, we demonstrate that the loss of wild-type htt influences motor dysfunction, hyperkinesia, testicular degeneration and impaired lifespan in YAC128 mice. The mild effect of wild-type htt on striatal phenotypes in YAC128 mice suggests that the characteristic striatal neuropathology in HD is caused primarily by the toxicity of mutant htt and that replacement of wild-type htt will not be an adequate treatment for HD.