Single-Nucleus RNA-Seq Reveals Dysregulation of Striatal Cell Identity Due to Huntington's Disease Mutations.

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by a trinucleotide expansion in exon 1 of the huntingtin (HTT) gene. Cell death in HD occurs primarily in striatal medium spiny neurons (MSNs), but the involvement of specific MSN subtypes and of other striatal cell types remains poorly understood. To gain insight into cell type-specific disease processes, we studied the nuclear transcriptomes of 4524 cells from the striatum of a genetically precise knock-in mouse model of the HD mutation, HttQ175/+, and from wild-type controls. We used 14- to 15-month-old male mice, a time point at which multiple behavioral, neuroanatomical, and neurophysiological changes are present but at which there is no known cell death. Thousands of differentially expressed genes (DEGs) were distributed across most striatal cell types, including transcriptional changes in glial populations that are not apparent from RNA-seq of bulk tissue. Reconstruction of cell type-specific transcriptional networks revealed a striking pattern of bidirectional dysregulation for many cell type-specific genes. Typically, these genes were repressed in their primary cell type, yet de-repressed in other striatal cell types. Integration with existing epigenomic and transcriptomic data suggest that partial loss-of-function of the polycomb repressive complex 2 (PRC2) may underlie many of these transcriptional changes, leading to deficits in the maintenance of cell identity across virtually all cell types in the adult striatum.SIGNIFICANCE STATEMENT Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder characterized by specific loss of medium spiny neurons (MSNs) in the striatum, accompanied by more subtle changes in many other cell types. It is thought that changes in transcriptional regulation are an important underlying mechanism, but cell type-specific gene expression changes are not well understood, particularly at time points relevant to the onset of disease-related symptoms. Single-nucleus (sn)RNA-seq in a genetically precise mouse model enabled us to identify novel patterns of transcriptional dysregulation because of HD mutations, including bidirectional dysregulation of many cell type identity genes that may be driven by partial loss-of-function of the polycomb repressive complex (PRC). Identifying these regulators of transcriptional dysregulation in HD can be leveraged to design novel disease-modifying therapeutics.

Prion protein lowering is a disease-modifying therapy across prion disease stages, strains and endpoints.

Lowering of prion protein (PrP) expression in the brain is a genetically validated therapeutic hypothesis in prion disease. We recently showed that antisense oligonucleotide (ASO)-mediated PrP suppression extends survival and delays disease onset in intracerebrally prion-infected mice in both prophylactic and delayed dosing paradigms. Here, we examine the efficacy of this therapeutic approach across diverse paradigms, varying the dose and dosing regimen, prion strain, treatment timepoint, and examining symptomatic, survival, and biomarker readouts. We recapitulate our previous findings with additional PrP-targeting ASOs, and demonstrate therapeutic benefit against four additional prion strains. We demonstrate that <25% PrP suppression is sufficient to extend survival and delay symptoms in a prophylactic paradigm. Rise in both neuroinflammation and neuronal injury markers can be reversed by a single dose of PrP-lowering ASO administered after the detection of pathological change. Chronic ASO-mediated suppression of PrP beginning at any time up to early signs of neuropathology confers benefit similar to constitutive heterozygous PrP knockout. Remarkably, even after emergence of frank symptoms including weight loss, a single treatment prolongs survival by months in a subset of animals. These results support ASO-mediated PrP lowering, and PrP-lowering therapeutics in general, as a promising path forward against prion disease.

High resolution time-course mapping of early transcriptomic, molecular and cellular phenotypes in Huntington's disease CAG knock-in mice across multiple genetic backgrounds.

Huntington's disease is a dominantly inherited neurodegenerative disease caused by the expansion of a CAG repeat in the HTT gene. In addition to the length of the CAG expansion, factors such as genetic background have been shown to contribute to the age at onset of neurological symptoms. A central challenge in understanding the disease progression that leads from the HD mutation to massive cell death in the striatum is the ability to characterize the subtle and early functional consequences of the CAG expansion longitudinally. We used dense time course sampling between 4 and 20 postnatal weeks to characterize early transcriptomic, molecular and cellular phenotypes in the striatum of six distinct knock-in mouse models of the HD mutation. We studied the effects of the HttQ111 allele on the C57BL/6J, CD-1, FVB/NCr1, and 129S2/SvPasCrl genetic backgrounds, and of two additional alleles, HttQ92 and HttQ50, on the C57BL/6J background. We describe the emergence of a transcriptomic signature in HttQ111/+ mice involving hundreds of differentially expressed genes and changes in diverse molecular pathways. We also show that this time course spanned the onset of mutant huntingtin nuclear localization phenotypes and somatic CAG-length instability in the striatum. Genetic background strongly influenced the magnitude and age at onset of these effects. This work provides a foundation for understanding the earliest transcriptional and molecular changes contributing to HD pathogenesis.

Transcriptional regulatory networks underlying gene expression changes in Huntington's disease.

Transcriptional changes occur presymptomatically and throughout Huntington's disease (HD), motivating the study of transcriptional regulatory networks (TRNs) in HD We reconstructed a genome-scale model for the target genes of 718 transcription factors (TFs) in the mouse striatum by integrating a model of genomic binding sites with transcriptome profiling of striatal tissue from HD mouse models. We identified 48 differentially expressed TF-target gene modules associated with age- and CAG repeat length-dependent gene expression changes in Htt CAG knock-in mouse striatum and replicated many of these associations in independent transcriptomic and proteomic datasets. Thirteen of 48 of these predicted TF-target gene modules were also differentially expressed in striatal tissue from human disease. We experimentally validated a specific model prediction that SMAD3 regulates HD-related gene expression changes using chromatin immunoprecipitation and deep sequencing (ChIP-seq) of mouse striatum. We found CAG repeat length-dependent changes in the genomic occupancy of SMAD3 and confirmed our model's prediction that many SMAD3 target genes are downregulated early in HD.

A fully humanized transgenic mouse model of Huntington disease.

Silencing the mutant huntingtin gene (muHTT) is a direct and simple therapeutic strategy for the treatment of Huntington disease (HD) in principle. However, targeting the HD mutation presents challenges because it is an expansion of a common genetic element (a CAG tract) that is found throughout the genome. Moreover, the HTT protein is important for neuronal health throughout life, and silencing strategies that also reduce the wild-type HTT allele may not be well tolerated during the long-term treatment of HD. Several HTT silencing strategies are in development that target genetic sites in HTT that are outside of the CAG expansion, including HD mutation-linked single-nucleotide polymorphisms and the HTT promoter. Preclinical testing of these genetic therapies has required the development of a new mouse model of HD that carries these human-specific genetic targets. To generate a fully humanized mouse model of HD, we have cross-bred BACHD and YAC18 on the Hdh(-/-) background. The resulting line, Hu97/18, is the first murine model of HD that fully genetically recapitulates human HD having two human HTT genes, no mouse Hdh genes and heterozygosity of the HD mutation. We find that Hu97/18 mice display many of the behavioral changes associated with HD including motor, psychiatric and cognitive deficits, as well as canonical neuropathological abnormalities. This mouse line will be useful for gaining additional insights into the disease mechanisms of HD as well as for testing genetic therapies targeting human HTT.

In vivo evaluation of candidate allele-specific mutant huntingtin gene silencing antisense oligonucleotides.

Huntington disease (HD) is a dominant, genetic neurodegenerative disease characterized by progressive loss of voluntary motor control, psychiatric disturbance, and cognitive decline, for which there is currently no disease-modifying therapy. HD is caused by the expansion of a CAG tract in the huntingtin (HTT) gene. The mutant HTT protein (muHTT) acquires toxic functions, and there is significant evidence that muHTT lowering would be therapeutically efficacious. However, the wild-type HTT protein (wtHTT) serves vital functions, making allele-specific muHTT lowering strategies potentially safer than nonselective strategies. CAG tract expansion is associated with single nucleotide polymorphisms (SNPs) that can be targeted by gene silencing reagents such as antisense oligonucleotides (ASOs) to accomplish allele-specific muHTT lowering. Here we evaluate ASOs targeted to HD-associated SNPs in acute in vivo studies including screening, distribution, duration of action and dosing, using a humanized mouse model of HD, Hu97/18, that is heterozygous for the targeted SNPs. We have identified four well-tolerated lead ASOs that potently and selectively silence muHTT at a broad range of doses throughout the central nervous system for 16 weeks or more after a single intracerebroventricular (ICV) injection. With further validation, these ASOs could provide a therapeutic option for individuals afflicted with HD.

Establishing resources and increasing awareness to advance research on Dentatorubral-pallidoluysian atrophy toward a treatment: a patient organization perspective.

Dentatorubral-pallidoluysian atrophy (DRPLA) is an ultra-rare neurodegenerative disorder characterized by ataxia, cognitive decline, myoclonus, chorea, epilepsy, and psychiatric manifestations. This article delves into the multifaceted efforts of CureDRPLA, a family-driven non-profit organization, in advancing research, raising awareness, and developing therapeutic strategies for this complex condition. CureDRPLA's inception in 2019 led to the establishment of the DRPLA Research Program, and since then have funded research projects to advance the understanding of DRPLA including but not limited to human cellular and mouse models, a natural history and biomarkers study, and a patient registry. There are currently no disease-modifying treatments for DRPLA, motivating a concerted effort on behalf of CureDRPLA to hasten their development by funding and coordinating preclinical studies of therapies in multiple modalities. Of particular interest are therapies focused on lowering the expression (or downregulation) of ATN1, the mutant gene that causes DRPLA, in hopes of tackling the pathology at its root. As with many ultra-rare diseases, a key challenge in DRPLA remains the complexity of coordinating both basic and clinical research efforts across multiple sites around the world. Finally, despite the generous financial support provided by CureDRPLA, more funding and collective efforts are still required to advance research toward the clinic and develop effective treatments for individuals with DRPLA.

Funding research projects and activities to advance research towards treatments for dentatorubral-pallidoluysian atrophy (DRPLA) This article describes the journey of CureDRPLA, a family-driven non-profit organization dedicated to making strides against dentatorubral-pallidoluysian atrophy (DRPLA), an ultra-rare brain disorder. It describes CureDRPLA's tireless efforts to understand, treat, and raise awareness about DRPLA, a condition marked by movement difficulties (ataxia), intellectual disability, uncontrollable jerky movements (myoclonus), involuntary or irregular muscle movements (chorea) and seizures. This disorder is caused by a mutation in a gene called ATN1. The gene produces a protein called atrophin-1, and when the DRPLA-causing mutation is present, the protein becomes abnormal and can build up in the brain, affecting its normal functions. Since its founding in 2019, CureDRPLA has funded research projects to unravel the mysteries of the disease and provide support for affected individuals. CureDRPLA has funded projects to create models of DRPLA using human cells and mice, which helps scientists study the disease and test potential treatments. We have started a study to learn more about how DRPLA progresses in people and are building a global database of information from individuals with DRPLA. Due to the absence of a treatment or cure, CureDRPLA is focused on testing treatments. We are particularly interested in exploring different approaches to lower the levels of the abnormal protein in the brain. CureDRPLA is actively involving the DRPLA community worldwide, raising awareness through events, conferences, and social media. We aim to connect with medical professionals, researchers, and affected families to build a strong community focused on understanding and managing DRPLA. In summary, CureDRPLA is working hard to better understand DRPLA, support affected families, and accelerate the development of treatments for this challenging condition. Our collaborative efforts and dedication underscore the importance of a united global approach to address the complexities of DRPLA.

Understanding dentatorubral-pallidoluysian atrophy (DRPLA) symptoms and impacts on daily life: a qualitative interview study with patients and caregivers.

Background: Dentatorubral-pallidoluysian atrophy (DRPLA) is a rare, neurodegenerative disorder with no disease-modifying treatments. There is a dearth of information in the literature about the patient and caregiver experience living with DRPLA. Objectives: This study aimed to (1) understand symptoms experienced by adult- and juvenile-onset DRPLA populations and their impact on daily life and (2) explore patient and caregiver treatment goals and clinical trial participation preferences. Design: The study was a qualitative interview study. Methods: Interviews were conducted remotely with adult patients with DRPLA and caregivers. Participants described patient symptoms and the impact of those symptoms on daily life, and they discussed treatment goals and potential clinical trial participation. There were 18 patients described in the interviews with two patients and seven caregivers. Some participants were caregivers to multiple patients with DRPLA. Results: Interview transcripts were coded for themes, and reported symptoms were summarized with descriptive statistics. Adult-onset patients (N = 7) experienced difficulty with ataxia (100%), cognition (100%), fine motor skills (100%), gross motor skills (100%), speech (100%), personality changes (100%), and seizures (57%). Juvenile-onset patients (N = 11) experienced difficulty with ataxia (100%), sleep (100%), speech (100%), jerking/twitching (83%), behavior (82%), cognition (82%), fine motor skills (82%), gross motor skills (82%), sensory sensitivity (75%), and seizures (64%). When considering aspects of DRPLA to target for future treatment, patients prioritized ataxia/mobility (100%), juvenile-onset caregivers prioritized ataxia/mobility (60%) and independence (60%), and adult-onset caregivers prioritized personality (60%). Almost all patients (93%) would participate in a clinical trial if given the opportunity, but travel to a clinical site could pose a participation barrier for half. Conclusion: This study found that there are symptom domains that are relevant across the DRPLA population, but there is heterogeneity within each domain based on the age of symptom onset and disease stage, which has implications for clinical trial design.

Understanding dentatorubral-pallidoluysian atrophy (DRPLA) symptoms and impacts on daily life through interviews with patients and caregivers Why was the study done? Dentatorubral-pallidoluysian atrophy (DRPLA) is a rare and progressive brain disorder. Little is known about the patient and caregiver experience living with DRPLA and this lack of information has hindered the development of patient-focused treatments and the measurement of outcomes that are most meaningful to caregivers and patients. What did the researchers do? To address this problem, researchers conducted interviews with patients and caregivers of DRPLA to (1) better understand symptoms experienced by adult- and juvenile-onset DRPLA populations and their impact on daily life and (2) explore patient and caregiver treatment goals and clinical trial participation preferences. What did the researchers find? Eighteen patients were described in the interviews. Adult-onset patients (onset at age 20 or older) experienced difficulty with coordination, cognition, motor skills, speech, personality changes, and seizures. Juvenile-onset patients (onset before age 20) experienced difficulty with coordination, sleep, speech, jerking/twitching, behavior, cognition, motor skills, sensory sensitivity, and seizures. When considering symptoms to prioritize for future treatment, patients and caregivers identified coordination/mobility, independence, and personality as important. Nearly all participants indicated they would participate in a clinical trial if given an opportunity, however half expressed that travel to a clinical site could pose a barrier. What do the findings mean? This study provides a better understanding of the symptoms experienced by DRPLA patients and their impact on daily life. Additionally, it identifies important targets for treatment and considerations when designing clinical trials for DRPLA such as the barrier caused by travel to a clinical site.

Global huntingtin knockout in adult mice leads to fatal neurodegeneration that spares the pancreas.

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expanded CAG tract in the huntingtin (HTT) gene, leading to toxic gains of function. HTT-lowering treatments are in clinical trials, but the risks imposed are unclear. Recent studies have reported on the consequences of widespread HTT loss in mice, where one group described early HTT loss leading to fatal pancreatitis, but later loss as benign. Another group reported no pancreatitis but found widespread neurological phenotypes including subcortical calcification. To better understand the liabilities of widespread HTT loss, we knocked out Htt with two separate tamoxifen-inducible Cre lines. We find that loss of HTT at 2 mo of age leads to progressive tremors and severe subcortical calcification at examination at 14 mo of age but does not result in acute pancreatitis or histological changes in the pancreas. We, in addition, report that HTT loss is followed by sustained induction of circulating neurofilament light chain. These results confirm that global loss of HTT in mice is associated with pronounced risks, including progressive subcortical calcification and neurodegeneration.

Identification of genetic modifiers of Huntington's disease somatic CAG repeat instability by in vivo CRISPR-Cas9 genome editing.

Huntington's disease (HD), one of >50 inherited repeat expansion disorders (Depienne and Mandel, 2021), is a dominantly-inherited neurodegenerative disease caused by a CAG expansion in HTT (The Huntington's Disease Collaborative Research Group, 1993). Inherited CAG repeat length is the primary determinant of age of onset, with human genetic studies underscoring that the property driving disease is the CAG length-dependent propensity of the repeat to further expand in brain (Swami et al ., 2009; GeM-HD, 2015; Hensman Moss et al ., 2017; Ciosi et al ., 2019; GeM-HD, 2019; Hong et al ., 2021). Routes to slowing somatic CAG expansion therefore hold great promise for disease-modifying therapies. Several DNA repair genes, notably in the mismatch repair (MMR) pathway, modify somatic expansion in HD mouse models (Wheeler and Dion, 2021). To identify novel modifiers of somatic expansion, we have used CRISPR-Cas9 editing in HD knock-in mice to enable in vivo screening of expansion-modifier candidates at scale. This has included testing of HD onset modifier genes emerging from human genome-wide association studies (GWAS), as well as interactions between modifier genes, thereby providing new insight into pathways underlying CAG expansion and potential therapeutic targets.

Wild-type HTT modulates the enzymatic activity of the neuronal palmitoyl transferase HIP14.

Huntington disease (HD) is caused by polyglutamine expansion in the huntingtin (HTT) protein. Huntingtin-interacting protein 14 (HIP14), one of 23 DHHC domain-containing palmitoyl acyl transferases (PATs), binds to HTT and robustly palmitoylates HTT at cysteine 214. Mutant HTT exhibits reduced palmitoylation and interaction with HIP14, contributing to the neuronal dysfunction associated with HD. In this study, we confirmed that, among 23 DHHC PATs, HIP14 and its homolog DHHC-13 (HIP14L) are the two major PATs that palmitoylate HTT. Wild-type HTT, in addition to serving as a palmitoylation substrate, also modulates the palmitoylation of HIP14 itself. In vivo, HIP14 palmitoylation is decreased in the brains of mice lacking one HTT allele (hdh+/-) and is further reduced in mouse cortical neurons treated with HTT antisense oligos (HTT-ASO) that knockdown HTT expression by ∼95%. Previously, it has been shown that palmitoylation of DHHC proteins may affect their enzymatic activity. Indeed, palmitoylation of SNAP25 by HIP14 is potentiated in vitro in the presence of wild-type HTT. This influence of HTT on HIP14 activity is lost in the presence of CAG expansion. Furthermore, in both brains of hdh+/- mice and neurons treated with HTT-ASO, we observe a significant reduction in palmitoylation of endogenous SNAP25 and GluR1, synaptic proteins that are substrates of HIP14, suggesting wild-type HTT also influences HIP14 enzymatic activity in vivo. This study describes an important biochemical function for wild-type HTT modulation of HIP14 palmitoylation and its enzymatic activity.

Huntingtin loss in hepatocytes is associated with altered metabolism, adhesion, and liver zonation.

Huntington's disease arises from a toxic gain of function in the huntingtin (HTT) gene. As a result, many HTT-lowering therapies are being pursued in clinical studies, including those that reduce HTT RNA and protein expression in the liver. To investigate potential impacts, we characterized molecular, cellular, and metabolic impacts of chronic HTT lowering in mouse hepatocytes. Lifelong hepatocyte HTT loss is associated with multiple physiological changes, including increased circulating bile acids, cholesterol and urea, hypoglycemia, and impaired adhesion. HTT loss causes a clear shift in the normal zonal patterns of liver gene expression, such that pericentral gene expression is reduced. These alterations in liver zonation in livers lacking HTT are observed at the transcriptional, histological, and plasma metabolite levels. We have extended these phenotypes physiologically with a metabolic challenge of acetaminophen, for which the HTT loss results in toxicity resistance. Our data reveal an unexpected role for HTT in regulating hepatic zonation, and we find that loss of HTT in hepatocytes mimics the phenotypes caused by impaired hepatic ß-catenin function.

Huntingtin loss in hepatocytes is associated with altered metabolism, adhesion, and liver zonation.

Huntington's disease arises from a toxic gain of function in the huntingtin ( HTT ) gene. As a result, many HTT-lowering therapies are being pursued in clinical studies, including those that reduce HTT RNA and protein expression in the liver. To investigate potential impacts, we characterized molecular, cellular, and metabolic impacts of chronic HTT lowering in mouse hepatocytes. Lifelong hepatocyte HTT loss is associated with multiple physiological changes, including increased circulating bile acids, cholesterol and urea, hypoglycemia, and impaired adhesion. HTT loss causes a clear shift in the normal zonal patterns of liver gene expression, such that pericentral gene expression is reduced. These alterations in liver zonation in livers lacking HTT are observed at the transcriptional, histological and plasma metabolite level. We have extended these phenotypes physiologically with a metabolic challenge of acetaminophen, for which the HTT loss results in toxicity resistance. Our data reveal an unexpected role for HTT in regulating hepatic zonation, and we find that loss of HTT in hepatocytes mimics the phenotypes caused by impaired hepatic ß-catenin function.

CAG expansion in the Huntington disease gene is associated with a specific and targetable predisposing haplogroup.

Huntington disease (HD) is an autosomal-dominant disorder that results from >or=36 CAG repeats in the HD gene (HTT). Approximately 10% of patients inherit a chromosome that underwent CAG expansion from an unaffected parent with <36 CAG repeats. This study is a comprehensive analysis of genetic diversity in HTT and reveals that HD patients of European origin (n = 65) have a significant enrichment (95%) of a specific set of 22 tagging single nucleotide polymorphisms (SNPs) that constitute a single haplogroup. The disease association of many SNPs is much stronger than any previously reported polymorphism and was confirmed in a replication cohort (n = 203). Importantly, the same haplogroup is also significantly enriched (83%) in individuals with 27-35 CAG repeats (intermediate alleles, n = 66), who are unaffected by the disease, but have increased CAG tract sizes relative to the general population (n = 116). These data support a stepwise model for CAG expansion into the affected range (>or=36 CAG) and identifies specific haplogroup variants in the general population associated with this instability. The specific variants at risk for CAG expansion are not present in the general population in China, Japan, and Nigeria where the prevalence of HD is much lower. The current data argue that cis-elements have a major predisposing influence on CAG instability in HTT. The strong association between specific SNP alleles and CAG expansion also provides an opportunity of personalized therapeutics in HD where the clinical development of only a small number of allele-specific targets may be sufficient to treat up to 88% of the HD patient population.

Potent and selective antisense oligonucleotides targeting single-nucleotide polymorphisms in the Huntington disease gene / allele-specific silencing of mutant huntingtin.

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by CAG-expansion in the huntingtin gene (HTT) that results in a toxic gain of function in the mutant huntingtin protein (mHTT). Reducing the expression of mHTT is therefore an attractive therapy for HD. However, wild-type HTT protein is essential for development and has critical roles in maintaining neuronal health. Therapies for HD that reduce wild-type HTT may therefore generate unintended negative consequences. We have identified single-nucleotide polymorphism (SNP) targets in the human HD population for the disease-specific targeting of the HTT gene. Using primary cells from patients with HD and the transgenic YAC18 and BACHD mouse lines, we developed antisense oligonucleotide (ASO) molecules that potently and selectively silence mHTT at both exonic and intronic SNP sites. Modification of these ASOs with S-constrained-ethyl (cET) motifs significantly improves potency while maintaining allele selectively in vitro. The developed ASO is potent and selective for mHTT in vivo after delivery to the mouse brain. We demonstrate that potent and selective allele-specific knockdown of the mHTT protein can be achieved at therapeutically relevant SNP sites using ASOs in vitro and in vivo.

Stool is a sensitive and noninvasive source of DNA for monitoring expansion in repeat expansion disease mouse models.

Repeat expansion diseases are a large group of human genetic disorders caused by expansion of a specific short tandem repeat tract. Expansion in somatic cells affects age of onset and disease severity in some of these disorders. However, alleles in DNA derived from blood, a commonly used source of DNA, usually show much less expansion than disease-relevant cells in the central nervous system in both humans and mouse models. Here we examined the extent of expansion in different DNA sources from mouse models of the fragile X-related disorders, Huntington's disease, spinocerebellar ataxia type 1 and spinocerebellar ataxia type 2. We found that DNA isolated from stool is a much better indicator of somatic expansion than DNA from blood. As stool is a sensitive and noninvasive source of DNA, it can be useful for studies of factors affecting the risk of expansion, or the monitoring of treatments aimed at reducing expansion in preclinical trials, as it would allow expansions to be examined longitudinally in the same animal and allow significant changes in expansion to be observed much earlier than is possible with other DNA sources.

Dynamics of huntingtin protein interactions in the striatum identifies candidate modifiers of Huntington disease.

Huntington disease (HD) is a monogenic neurodegenerative disorder with one causative gene, huntingtin (HTT). Yet, HD pathobiology is multifactorial, suggesting that cellular factors influence disease progression. Here, we define HTT protein-protein interactions (PPIs) perturbed by the mutant protein with expanded polyglutamine in the mouse striatum, a brain region with selective HD vulnerability. Using metabolically labeled tissues and immunoaffinity purification-mass spectrometry, we establish that polyglutamine-dependent modulation of HTT PPI abundances and relative stability starts at an early stage of pathogenesis in a Q140 HD mouse model. We identify direct and indirect PPIs that are also genetic disease modifiers using in-cell two-hybrid and behavioral assays in HD human cell and Drosophila models, respectively. Validated, disease-relevant mHTT-dependent interactions encompass mediators of synaptic neurotransmission (SNAREs and glutamate receptors) and lysosomal acidification (V-ATPase). Our study provides a resource for understanding mHTT-dependent dysfunction in cortico-striatal cellular networks, partly through impaired synaptic communication and endosomal-lysosomal system. A record of this paper's Transparent Peer Review process is included in the supplemental information.

Natural history of disease in the YAC128 mouse reveals a discrete signature of pathology in Huntington disease.

Models of Huntington disease (HD) recapitulate some neuropathological features of the disease. However, a global natural history of neuroanatomy in a mouse expressing full-length huntingtin has not been conducted. We investigated neuropathological changes in the YAC128 murine model of HD using magnetic resonance imaging (MRI). Structures affected in human HD are reduced in the YAC128 mice both in absolute terms and in terms of percentage of brain volume. Structures resistant to degeneration in HD, including the cerebellum and hippocampus, are spared in the YAC128 mice. Segmentation of major white matter structures confirms specific, progressive, loss of white matter in HD. In parallel with their specific volume loss, the YAC128 mice also show progressive increases in total ventricular volume, similarly to human HD patients. Cortical atrophy in the YAC128 mice is layer specific, which is the observed pattern of cortical loss in human HD patients. Finally, we have used a classification tree analysis to maximize separation of genotypes using all 62 structure volumes in an objective manner. This analysis demonstrates that sub-cortical gray matter structures (striatum, globus pallidus, thalamus) and cerebral white matter structures (corpus callosum, anterior commisure, fimbria) are the most discriminatory. The high resolution of the current study enables robust measurement of subtle early pathological changes. The use of mice furthermore enables us to address questions difficult to address in humans, including the sequential changes of HD from baseline and the relation between MRI and stereological measures.

Huntingtin structure is orchestrated by HAP40 and shows a polyglutamine expansion-specific interaction with exon 1.

Huntington's disease results from expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene, producing an aberrantly functioning form of HTT. Both wildtype and disease-state HTT form a hetero-dimer with HAP40 of unknown functional relevance. We demonstrate in vivo and in cell models that HTT and HAP40 cellular abundance are coupled. Integrating data from a 2.6 Å cryo-electron microscopy structure, cross-linking mass spectrometry, small-angle X-ray scattering, and modeling, we provide a near-atomic-level view of HTT, its molecular interaction surfaces and compacted domain architecture, orchestrated by HAP40. Native mass spectrometry reveals a remarkably stable hetero-dimer, potentially explaining the cellular inter-dependence of HTT and HAP40. The exon 1 region of HTT is dynamic but shows greater conformational variety in the polyglutamine expanded mutant than wildtype exon 1. Our data provide a foundation for future functional and drug discovery studies targeting Huntington's disease and illuminate the structural consequences of HTT polyglutamine expansion.

Age-dependent alterations of corticostriatal activity in the YAC128 mouse model of Huntington disease.

Huntington disease is a genetic neurodegenerative disorder that produces motor, neuropsychiatric, and cognitive deficits and is caused by an abnormal expansion of the CAG tract in the huntingtin (htt) gene. In humans, mutated htt induces a preferential loss of medium spiny neurons in the striatum and, to a lesser extent, a loss of cortical neurons as the disease progresses. The mechanisms causing these degenerative changes remain unclear, but they may involve synaptic dysregulation. We examined the activity of the corticostriatal pathway using a combination of electrophysiological and optical imaging approaches in brain slices and acutely dissociated neurons from the YAC128 mouse model of Huntington disease. The results demonstrated biphasic age-dependent changes in corticostriatal function. At 1 month, before the behavioral phenotype develops, synaptic currents and glutamate release were increased. At 7 and 12 months, after the development of the behavioral phenotype, evoked synaptic currents were reduced. Glutamate release was decreased by 7 months and was markedly reduced by 12 months. These age-dependent alterations in corticostriatal activity were paralleled by a decrease in dopamine D(2) receptor modulation of the presynaptic terminal. Together, these findings point to dynamic alterations at the corticostriatal pathway and emphasize that therapies directed toward preventing or alleviating symptoms need to be specifically designed depending on the stage of disease progression.