Huntington’s disease (HD) causes progressive loss of function, cognition, and motor control, with no approved therapy yet shown to slow disease progression. In the PROOF-HD phase 3 trial, pridopidine did not meet the primary or key secondary outcomes in the overall population, but participants who remained off antidopaminergic medications (ADMs) showed benefits compared to placebo during the double-blind phase. Whether such benefits continue with longer duration treatment and how they compare with expected natural-history trajectories remains unknown. We evaluated outcomes through Week 104 from baseline in participants who received continuous pridopidine (45 mg twice daily) and remained off-ADMs throughout the double-blind and open-label extension period (n=90). External comparators from ENROLL-HD and TRACK-HD were constructed using propensity-score weighting methods. Least-squares mean changes from baseline to Week 104 were estimated using mixed-effects models for repeated measures across outcomes. At two-years, pridopidine treatment was associated with benefits versus propensity-score weighted natural-history comparators across multiple outcomes. Relative to ENROLL-HD, participants receiving pridopidine showed slowing of progression over 104 weeks, expressed as percent slowing across cUHDRS, TFC, SWR, SDMT, and TMS outcomes (39.5–88.3% slowing). Similar patterns were observed relative to TRACK-HD across the same measures (48.5 – 81.5% slowing), including quantitative motor performance assessed by Q-Motor FT-IOI (77.8% slowing). Exploratory analyses including participants receiving concomitant ADMs showed similar directional patterns as the primary analyses. In a two-year follow-up, continuous pridopidine treatment in participants remaining off-ADMs was associated with slower clinical progression relative to expected natural-history trajectories. (Clinical Trials Identifier: NCT04556656 )

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene, which leads to a mutant protein that destroys neurons in the brain. Despite intense effort, there remains no approved disease-modifying therapy for HD. Here we develop a pan-HTT-targeting CRISPR-Cas9 system that, when delivered to the striatum of R6/2 and YAC128 mice by AAV5, lowered mutant HTT mRNA and protein by 55-80% via its induction of frameshift-inducing indel mutations in HTT exon 1. Cas9 targeting improved motor coordination and locomotor activity, decreased anxiety-like deficits, reduced clasping and weight loss, limited striatal atrophy, and decreased the formation of intranuclear inclusions immunoreactive for the mutant HTT protein. In Hu21/21 mice, which carry the wild-type human HTT gene in lieu of the mouse ortholog, Cas9 lowered the HTT protein by 44% but induced no measurable behavioral deficits and had no adverse effect on neuronal viability, though its targeting was associated with neuroinflammation. Altogether, our results demonstrate the ability for a newly developed pan-HTT-targeting Cas9 system to affect HD-related phenotypes across models and provides insights into its tolerability.

Background Gene silencing is widely recognized as a promising therapeutic approach for dominant monogenic disorders. Current silencing strategies, many of which are transient, utilize RNA interference. Gene silencing may also be achieved through directed epigenetic editing using a CRISPR/dCas9 effector fused to DNA methyltransferase 3A (dCas9-DNMT3A). We used this system to direct DNA methylation to HTT, the causal gene underlying the autosomal dominant neurodegenerative disorder Huntington's disease, to assess the translational potential of this strategy for treating a genetic neurological disease. Objective To characterize the regulatory effect of targeted dCas9-DNMT3A-mediated DNA methylation at HTT. Methods We exploited DNA methylation profiles of high and low HTT-expressing tissues and targeted hypomethylated regions of HTT associated with high levels of HTT expression. Results De novo DNA methylation of loci within defined upstream, promoter, intragenic and downstream regions of HTT resulted in robust, acute silencing of HTT. The best long-term silencing of HTT, which persisted up to 30 days, was observed when targeted DNA methylation was directed to the 5'UTR and promoter regions of HTT. Conclusions HTT gene silencing may be achieved via targeted de novo DNA methylation within hypomethylated regulatory regions at the HTT locus. DNA methylation editing may be an attractive therapeutic approach for Huntington disease due to its potential for long-term silencing and reversibility.

The sigma-1 receptor (S1R) is an endoplasmic reticulum (ER)-resident protein enriched at the mitochondria-associated ER membranes (MAMs) that supports ER homeostasis, preserves mitochondrial function, and enhances cell survival under stress. Disruptions of MAM integrity and prolonged ER stress are well-recognized pathological features of amyotrophic lateral sclerosis (ALS), contributing to motor neuron dysfunction and degeneration. In this study, we evaluated the protective effects of pridopidine, a highly selective and potent S1R agonist currently in clinical development for Huntington’s disease (HD) and ALS, using neural progenitor cells (NPCs) derived from induced pluripotent stem cells (iPSCs) from a patient with sporadic ALS. Exposure of ALS NPCs to the ER stressor tunicamycin increased the ER stress markers binding immunoglobulin protein (BiP) and C/EBP homologous protein (CHOP), disrupted mitochondrial membrane potential, upregulated expression of the mitochondrial apoptotic marker, BAX, increased caspase-3 activation, and reduced cell viability. Pridopidine significantly attenuated tunicamycin-induced BiP and CHOP expression in a biphasic, dose-dependent manner (with maximal efficacy at 1 µM), consistent with the typical pharmacology of S1R agonists. Pridopidine restored mitochondrial membrane potential, reduced mitochondrial apoptotic signaling, shown by decreased BAX expression and caspase-3 activation, and improved survival of ALS-NPCs under ER stress. Co-treatment with the selective S1R antagonist, NE-100, attenuated these effects, supporting an S1R-mediated mechanism of action for pridopidine. Together, these results demonstrate that S1R activation by pridopidine mitigates ER-stress-induced mitochondrial dysfunction and cell loss in ALS-NPCs, resulting in enhanced survival of NPCs supporting the therapeutic potential of pridopidine in ALS.

Background: Sensitive biomarkers that objectively stage Huntington disease (HD) are needed to improve participant stratification and facilitate the enrichment of clinical trials with biologically and clinically homogeneous populations. The HDClarity study, an international longitudinal biofluid collection initiative for HD, provides a unique resource for large-scale proteomic profiling of matched CSF and serum samples spanning the disease spectrum. Here, we leveraged baseline proteomic data from HDClarity to characterize protein signatures associated with HD stage and clinical severity, compare measurements across analytical platforms and biofluid compartments, and identify candidate multi-protein panels for disease staging. Methods: Baseline proteomic data generated using Olink Explore (~3,000 proteins) and SomaScan v4.1 (~7,000 proteins) were analyzed in matched CSF and serum samples from 315 HD gene-expansion carriers and 92 non-HD controls. A total of 2,119 proteins overlapped between Olink and SomaScan, enabling assessment of cross-platform concordance, while CSF-serum relationships were evaluated using all available protein measurements within each assay. Covariate-adjusted linear regression models were used to assess disease stage-associated differences in protein abundance, while partial correlation analyses evaluated relationships between protein abundance, clinical severity in HD gene-expansion carriers, and estimated years to disease onset in premanifest participants. A nested machine-learning pipeline incorporating univariate feature ranking, penalized regression-based feature selection, and repeated cross-validation was used to derive compact multi-protein classifiers for HD staging. Results: Cross-platform and CSF-serum correlations were highly protein-dependent, with some analytes showing strong concordance and others exhibiting weak or inverse relationships. These findings highlight substantial heterogeneity in biomarker behaviour across analytical platforms and biofluids. Adjusted models identified both known HD-associated markers (NEFL, GFAP, CHI3L1) and less well-characterized proteins in CSF and serum whose baseline abundance differed across HD-Integrated Staging System (HD-ISS) and clinical stages. Partial correlation analyses revealed additional candidate biomarkers associated with clinical severity and estimated time to disease onset. Machine-learning models derived compact CSF and serum protein panels that accurately classified participants across HD-ISS stages 0 and 1, as well as the transition from premanifest to early manifest disease. Conclusions: This study provides the first large-scale orthogonal comparison of matched CSF and serum proteomes in HDClarity, establishing robust baseline proteomic signatures across the HD continuum. Our findings demonstrate the importance of considering both analytical platform and biofluid when interpreting protein biomarkers and identify compact protein panels with potential utility for objective disease staging, patient stratification, and clinical trial enrichment in HD.

Age of onset in Huntington disease (HD) is influenced by cis-acting genetic variants, particularly the loss of interrupting codons in the HTT CAG and CCG repeats (CAG-CCG LOI variant). The CAG-CCG LOI variant is not detectable by current diagnostic assays, leading to underestimation of CAG repeat length, misdiagnosis, and inaccurate prediction of risk of symptom onset in the reduced penetrance range. In clinical trials, unidentified CAG-CCG LOI variants may affect interpretation of results, particularly for small trials. Accurate ascertainment and reporting of the CAG-CCG LOI genotype therefore has important implications for HD diagnosis, genetic counselling, and clinical trial design.

Huntington’s disease (HD) is a rare, fatal, chronic progressive neurodegenerative disorder with a significant unmet medical need for effective treatments. Pridopidine is a novel, first-in-class, highly selective and potent sigma-1 receptor (S1R) agonist in development for HD. Pridopidine has been extensively studied in adult HD across the full spectrum of disease severity and age ranges, and its safety profile has been characterized in approximately 1600 participants across multiple studies and a broad range of doses. The specific objective of this study was to gain an in-depth understanding of pridopidine’s safety profile at the recommended human dose of 45 mg twice daily (bid) in patients with HD. An integrated safety analysis of pooled data from 1067 patients with HD enrolled in four double-blind, placebo-controlled studies was performed. The safety profile of pridopidine was compared with placebo. Pridopidine was found to be generally safe and well tolerated with an adverse event (AE) profile comparable to that of placebo. Moreover, there were no significant differences observed in the safety profile of pridopidine compared with placebo when analyzed by age, sex, baseline total functional capacity (TFC), cytosine–adenine–guanine (CAG) repeat length, use of antidopaminergic medications (ADMs), and region. The integrated analysis replicated and corroborated the good safety profile observed in the individual studies. Despite the larger sample size, no new safety signals emerged. Long-term exposure to pridopidine, up to 6.5 years in open-label extension studies, revealed no new safety concerns, supporting its potential for long-term use in patients with HD.

Huntington disease (HD) is a progressive and devastating neurodegenerative disease caused by expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene above a critical threshold of 35 repeats resulting in expression of mutant HTT (mHTT). A promising treatment approach being tested in clinical trials is HTT lowering, which aims to reduce levels of the mHTT protein. Target engagement of these therapies in the brain are inferred using antibody-based assays that measure mHTT levels in the cerebrospinal fluid (CSF). These levels are typically reported as the absolute concentration of mHTT concentration, derived from a standard curve generated using a single protein standard. However, patient biofluids are a complex milieu containing different mHTT protein species, suggesting that absolute quantitation is challenging. As a result, a single recombinant protein standard may not be sufficient to interpret assay signal as molar mHTT concentration. In this study, we used immunoprecipitation and flow cytometry (IP-FCM) to investigate different factors that influence mHTT detection assay signal. Our results show that HTT protein fragmentation, protein–protein interactions, affinity tag positioning, oligomerization and polyglutamine tract length affect assay signal intensity. These findings indicate that absolute HTT quantitation in heterogeneous biological samples is not possible with current technologies using a single standard protein. We also explore the binding specificity of the MW1 anti-polyglutamine antibody, commonly used in these assays as a mHTT-selective reagent and demonstrate that mHTT binding is preferred but not specific. Furthermore, we find that MW1 depletion of mHTT for quantitation of wildtype HTT is not only incomplete, leaving residual mHTT, but also non-specific, resulting in pull down of some wildtype HTT protein. Based on these observations, we recommend that mHTT detection assays report only relative mHTT quantitation using normalized arbitrary units of assay signal intensity, rather than molar concentrations, in the assessment of central nervous system HTT lowering in ongoing clinical and preclinical studies. Further, we recommend that MW1-depletion not be used as a method for quantifying wildtype HTT protein and that detergent be consistently added to samples during testing.

Disruption of autophagy has emerged as a common feature in many neurodegenerative diseases. Autophagy is a membrane-dependent pathway that requires many key regulators to quickly localize on and off membranes during induction, promoting membrane fusion. Previously, our bioinformatic approaches have shown that autophagy and Huntington disease (HD) are enriched in palmitoylated proteins. Palmitoylation involves the reversible addition of long-chain fatty acids to promote membrane binding. Herein, we show that inhibition of palmitoylation regulates the abundance of several key regulators of autophagy and leads to a partial block of autophagic flux. We confirm that the autophagy receptor SQSTM1/p62 (sequestosome 1) is palmitoylated and directed to the lysosome. Importantly, we report that SQSTM1 palmitoylation is significantly reduced in HD patient and mouse model brains. This finding reveals a novel mechanism contributing to the generation of empty autophagosomes previously seen in HD models and patient-derived cells.

BACKGROUND:Antidopaminergic medications (ADM) are often used for symptom management of Huntington's disease (HD). Evidence from past research suggests that ADMs are associated with worse clinical outcomes in HD, but their impact on various domains remains underexplored. OBJECTIVE:We used causal inference analysis to understand the impact of ADM use on measures of clinical progression in HD across multiple domains over 2 years. METHODS:We used the Enroll-HD database with a new-user design, which compared a cohort that initiated ADM use after the first visit with an unexposed cohort that remained off ADMs. To control for 27 covariates, we used a doubly robust targeted maximum likelihood estimation and conducted two analyses. First, we analyzed ADM treatment 2 years post-baseline and separately for 12 outcome measures. Second, we examined the association of ADM dose with measures of clinical outcomes. RESULTS:The ADM-exposed group exhibited faster change in measures of clinical outcome compared with the off-ADM group, which was statistically reliable in cognitive and functional outcome measures, and the composite Unified Huntington's Disease Rating Scale (cUHDRS). Motor domain analyses showed faster change in bradykinesia in the ADM-exposed group versus off-ADM but no difference in chorea or total motor score (TMS). Higher ADM doses also showed greater differences compared to the off-ADM group. CONCLUSIONS:ADM use was associated with more rapid change in clinical measures, particularly in cognitive and functional domains. However, assumptions required to establish causation between ADM use and disease progression may not have been fully met, and further research is warranted. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Background: Antidopaminergic medications (ADMs), including vesicular monoamine transporter-2 (VMAT2) inhibitors and antipsychotics, are frequently used to manage Huntington's disease (HD) symptoms. Prior studies suggest that ADMs may be associated with worsening on measures of outcome in HD clinical trials. The PROOF-HD placebo arm ([NCT04556656][1]) provided a controlled, double-blind setting to evaluate ADM impacts on measures of HD progression. Objective: Assess the association between ADM exposure and change in clinical outcomes in the placebo arm of PROOF-HD. Methods: Placebo-arm participants (n=247) were categorized as on vs. off ADMs. Overall main analyses were corroborated by propensity-score weighting (PSW)-adjusted analyses. Unadjusted analyses examined exposure by ADM class and dose. Outcomes included Total Functional Capacity (TFC), composite Unified Huntington's Disease Rating Scale (cUHDRS), Stroop Word Reading (SWR), Symbol Digit Modalities Test (SDMT), and Total Motor Score (TMS). Results: Group differences (Delta) favored off ADMs in cUHDRS (Weeks 39-78) and TFC (Weeks 26-78); at Week 52, cUHDRS had Delta=0.66 (95% CI 0.31-1.01; p=0.0002) and TFC had Delta=0.85 (95% CI 0.47-1.22; p<0.0001) as compared with on ADMs. Other outcomes were significant or directionally favored off-ADM participants beyond Week 39. All TMS subdomain scores, except for chorea, directionally favored off ADMs at all visits. Antipsychotic-only and higher-dose ADMs were associated with worse cUHDRS and TFC vs. off ADMs. Conclusions: In this post hoc study, ADM use was associated with greater worsening of measures of global, functional, cognitive, and motor outcomes versus off ADMs. Accounting for ADM exposure and dose is essential for interpretation of results from HD trials. ### Competing Interest Statement Karen Elta Anderson Contributed clinical interpretation, insight, and editorial support. Financial Disclosures: Scientific Advisor to Prilenia, Site Investigator for PROOF-HD study, Site Investigator for PRIDE-HD. Andrew M. Tan Contributed to clinical data analysis and interpretation; drafted the manuscript and supervised its final production and submission with all authors. Financial Disclosures: Employee of Prilenia Therapeutics B.V. with stock options. Andrew Feigin Served as the North American Principal Investigator; led clinical oversight and coordination in PROOF-HD (North America). Financial Disclosures: Received grant support from Prilenia through NYU for his role as principal investigator for North American study sites. Ralf Reilmann Conceived and designed the study; served as European Principal Investigator; led clinical oversight and coordination in PROOF-HD (Europe). Financial Disclosures: Founding director/owner of the George-Huntington-Institute (GHI) and QuantiMedis; has provided consulting, advisory, clinical trial operations, and Q-Motor analyses for Prilenia; principal investigator for European PROOF-HD sites; European coordinating investigator for PRIDE-HD. Anne E. Rosser Conceived and designed the study; served as European co-lead; led clinical oversight and coordination in PROOF-HD (Europe). Financial Disclosures: Served as European co-lead for the PROOF-HD study. Lynn A. Raymond Contributed clinical interpretation, insight, and editorial support. Financial Disclosures: None declared. Sandra K. Kostyk Served as North American co-lead; led clinical oversight and coordination in PROOF-HD (North America). Contributed clinical interpretation, insight, and editorial support. Financial Disclosures: Medical Director of the HDSA Center of Excellence at The Ohio State University; Co-Chair, Huntington Study Group Executive Membership Committee; North American Co-PI of PROOF-HD. Carsten Saft Contributed clinical interpretation, insight, and editorial support. Financial Disclosures: Declares no conflicts of interest related to this study. Kelly Chen Contributed statistical analysis and methodology, clinical data analysis and interpretation. Financial Disclosures: Employee of Prilenia Therapeutics B.V. with stock options. Randal Hand Contributed to data interpretation; data analysis, and visual presentations in the manuscript. Financial Disclosures: Employee of Prilenia Therapeutics B.V. with stock options. Michal Geva Conceived and designed the study; contributed to clinical data analysis and interpretation; editorial support in manuscript development; involved in funding acquisition and study sponsorship. Financial Disclosures: Employee of Prilenia Therapeutics B.V. with stock options. Michael R. Hayden Conceived and designed the study; contributed to clinical data analysis and interpretation; involved in funding acquisition and study sponsorship. Supervised study and manuscript development. Financial Disclosures: CEO and scientific co-founder of Prilenia Neurotherapeutics B.V.; Physician Scientist and University Killam Professor at the University of British Columbia; serves on the Boards of Ionis Pharmaceuticals, 89Bio, and AbCellera. ### Clinical Trial NCT04556656 ### Funding Statement This research was fully supported by Prilenia Therapeutics B.V. and/or its subsidiaries, which provided the necessary resources and funding. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This analysis used de-identified data from the Phase 3 PROOF-HD trial (25), which was conducted in accordance with the Declaration of Helsinki and ICH-GCP guidelines, with IRB/IEC approval at all participating sites and written informed consent from all participants. As this study involved secondary analysis of existing de-identified data, no new ethical approval was required from the ethics committees for the current report. 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I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes De-identified participant data (IDP) with data dictionaries, along with key documents, the protocol, SAP, and informed-consent template, will be available to qualified researchers starting six months after publication and for five years thereafter. Access is limited to investigators at academic or non-profit institutions pursuing scientifically sound, ethics-approved analyses that align with the study aims or address closely related questions. Because of ethical and legal constraints on participant privacy (e.g., GDPR), data are not placed in a public repository. To request access, contact the Sponsor at info{at}prilenia.com. Requests are reviewed by the Sponsor or its data access committee, with a decision provided within 90 days. Approved users enter a Data Use Agreement that prohibits re-identification, disallows unauthorized data sharing, and limits use to the agreed purposes. Authorship or acknowledgment follows the International Committee of Medical Journal Editors (ICMJE) Recommendations (2024). A DUA template is available on request. No third-party proprietary datasets were used; all data were collected and analyzed by the investigators and the Sponsor as described in Methods. [1]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT04556656&atom=%2Fmedrxiv%2Fearly%2F2025%2F11%2F03%2F2025.10.30.25339054.atom

Adeno-associated viral (AAV) vectors are an ideal platform for gene therapy due to their ability to deliver therapeutic cargos safely and effectively across various target organs. Their low immunogenicity contributes to long-lasting therapeutic effects. However, recent insights highlight the significance of CpG content within AAV vectors, where unmethylated CpG dinucleotides can trigger a TLR9-mediated immune response, leading to the rapid elimination of transduced cells. Clinical evidence indicates an inverse relationship between CpG content and therapeutic success, with lower CpG counts correlating with sustained transgene expression. Here, we sought to optimize a novel, CpG-rich AAV8 vector, referred to as pVR59, designed for treating lipoprotein lipase deficiency (LPLD). We strategically reduced CpG levels in pVR59, resulting in the development of pNC182, a CpG-depleted vector that maintains therapeutic efficacy. A single intramuscular injection of pNC182 demonstrated comparable effectiveness to pVR59 in normalizing lipemia and hypertriglyceridemia in LPLD mouse models, with a 38% reduction in total CpG count. These findings support the clinical application of pNC182 as a safe, long-lasting AAV gene therapy for LPLD and provide a framework for future AAV vector designs aimed at maximizing therapeutic efficacy while minimizing immunogenic responses in human settings.

Huntington's disease (HD) is a rare, neurodegenerative disorder for which only symptomatic treatments are available. The PROOF-HD study was a randomized, double-blind, placebo-controlled phase 3 trial evaluating the efficacy and safety of pridopidine, a selective Sigma-1 receptor agonist, in HD. The primary and key secondary endpoints, change in total functional capacity (TFC) and composite Unified Huntington's Disease Rating Scale (cUHDRS) score at week 65, were not met in the overall population. The TFC least-squares mean difference between pridopidine and placebo was -0.18 (95% confidence interval -0.49 to 0.14; P = 0.26). The cUHDRS least-squares mean difference between pridopidine and placebo was -0.11 (95% confidence interval -0.40 to 0.18; P = 0.45). Sensitivity analysis in a subgroup of participants not treated with antidopaminergic medications at any time demonstrated a consistent pattern favoring pridopidine across multiple measures, including TFC and cUHDRS. Notably, pridopidine 45 mg twice daily demonstrated a favorable safety and tolerability profile. Taken together, pridopidine has the potential to address a critical unmet need in HD. ClinicalTrials.gov identifier: NCT04556656 .

Huntington's disease (HD) is a progressive neurodegenerative disorder marked by motor, cognitive, and behavioral impairments. Antidopaminergic medications (ADMs), such as VMAT2 inhibitors and antipsychotics, are commonly used to manage HD motor disturbances and behavioral disorders. For patients and caregivers, ADMs are an important tool for managing symptoms that negatively affect daily life. However, the impact of ADM use in HD is not firmly understood due to a lack of robust, systematic studies that assessed their overall effect on HD disease. A mounting body of evidence suggests these medications may be associated with worse clinical measures of cognitive function and functional impairment. While regulatory guidelines highlight adverse effects like sedation, cognitive dysfunction, and extrapyramidal symptoms, it is unclear whether ADMs directly impact disease progression or if the side effects mimic or exacerbate measures of HD symptoms in clinical trials. Given ADM effects on the central nervous system and biological uncertainty within HD outcomes, clinical trial designs should recognize the impact of ADMs on key outcomes, as measured by acceptable scales including Total Functional Capacity, Stoop Word Reading, Symbol Digit Modality Test, and the composite Unified Huntington's Disease Rating Scale. The development of novel HD interventions requires consideration of concomitant ADM use that may influence measures of disease presentation. In this review, we highlight the role of ADMs in HD management, their symptomatic benefits and potential risks, especially with high dose associated side effects, interactions with CYP2D6 inhibitors, and the individualized need for careful dose monitoring for clinical care and trial design.