Protein Degradation Strategies in Huntington Disease: Mechanistic Insights and Therapeutic Advances

Abstract

Huntington disease is a devastating, inherited neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and psychiatric symptoms. A central pathogenic feature is the accumulation of mutant huntingtin protein (mHTT) aggregates, which disrupt neuronal homeostasis and drive neurotoxicity. Recent research has focused on enhancing protein degradation pathways, particularly the ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway, as promising therapeutic strategies to mitigate mHTT-mediated toxicity. This review synthesizes current evidence on protein degradation mechanisms in Huntington disease, highlights recent advances in targeted therapies, and discusses clinical implications for patient care.

Introduction

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder, primarily affecting the central nervous system. It arises from an expanded CAG trinucleotide repeat in the HTT gene, resulting in the production of a mutant huntingtin protein with an elongated polyglutamine tract. The mutant protein is prone to misfolding and aggregation, leading to neuronal dysfunction and cell death, especially in the striatum and cortex. Despite advances in understanding its molecular pathology, effective disease-modifying therapies remain elusive. The exploration of protein degradation pathways represents a critical area of investigation for the development of targeted, mechanism-based treatments.

Epidemiology / Disease Burden

HD has a prevalence of approximately 5 to 10 per 100,000 individuals in populations of European descent. The disease typically manifests in mid-adulthood, although juvenile-onset cases exist. The progressive nature of HD leads to significant morbidity, reduced quality of life, and a shortened lifespan, with death commonly occurring 15–25 years after onset. The burden extends beyond patients to families and healthcare systems, given the inherited nature and chronic course of the disorder.

Pathophysiology

The pathogenesis of HD centers on the toxic gain-of-function effects of mHTT. The mutant protein undergoes aberrant post-translational modifications, misfolds, and aggregates within neuronal cells, impairing intracellular transport, transcriptional regulation, mitochondrial function, and synaptic integrity. Protein homeostasis (proteostasis) is disrupted, overwhelming endogenous degradation systems. Impaired UPS and autophagy-lysosome pathway function contribute to the intracellular accumulation of toxic mHTT species, which correlates with neuronal dysfunction and cell death. Enhancing these protein degradation pathways has thus emerged as a potential therapeutic strategy for HD.

Risk Factors

The primary risk factor for HD is the presence of an expanded CAG repeat (≥36) in the HTT gene. The length of the repeat inversely correlates with age of onset and disease severity. Other potential modifiers include genetic background, epigenetic factors, and environmental influences, which may affect disease penetrance, progression, and response to therapeutic interventions.

Clinical Features

HD presents with a triad of motor, cognitive, and psychiatric symptoms. Motor manifestations include chorea, dystonia, rigidity, and impaired voluntary movements. Cognitive deficits encompass executive dysfunction, memory impairment, and eventual dementia. Psychiatric symptoms range from depression and irritability to psychosis. The clinical course is progressive, with increasing disability over time. Early identification of symptoms is critical for timely intervention and genetic counseling.

Diagnosis

Diagnosis of HD is based on clinical assessment and confirmed by genetic testing for the HTT CAG repeat expansion. Neuroimaging, particularly MRI, may reveal characteristic striatal atrophy. Biomarkers, including mutant huntingtin levels in cerebrospinal fluid and imaging markers of neurodegeneration, are under investigation for disease monitoring and therapeutic response assessment.

Treatment & Management

Currently, there is no cure for HD. Management is symptomatic and multidisciplinary, involving pharmacological therapies for motor and psychiatric symptoms, physical and occupational therapy, and psychosocial support. Tetrabenazine and deutetrabenazine are approved for chorea, while antipsychotics and antidepressants address psychiatric manifestations. Genetic counseling and predictive testing are integral for at-risk individuals and family planning.

Recent Advances / Emerging Therapies

Therapies targeting protein degradation pathways have gained momentum in HD research. Enhancing UPS activity through small molecules or gene therapy has shown preclinical promise in promoting mHTT clearance. Autophagy inducers, such as rapamycin analogs and trehalose, facilitate the removal of aggregated proteins and improve neuronal survival in animal models. Emerging approaches include antisense oligonucleotides (ASOs) and RNA interference targeting HTT mRNA, thereby reducing the production of mHTT. Proteolysis-targeting chimeras (PROTACs) represent a novel class of molecules designed to harness the cell\"s ubiquitin-proteasome machinery for selective degradation of mHTT. Early-phase clinical trials of ASOs (e.g., tominersen) have demonstrated target engagement and reduction of mHTT levels in cerebrospinal fluid, though safety and efficacy in clinical outcomes remain under investigation.

Guideline Recommendations

Current guidelines emphasize a multidisciplinary approach for HD management, prioritizing early diagnosis, symptom control, and supportive care. While disease-modifying therapies are not yet established, patients may be enrolled in clinical trials evaluating novel agents targeting protein degradation. Genetic counseling is recommended for at-risk individuals, and regular monitoring of functional, cognitive, and psychiatric status is advised to guide therapy adjustments. Ongoing research and clinical trial participation are encouraged to advance therapeutic options.

Conclusion

Protein degradation strategies represent a promising frontier in the quest for disease-modifying therapies in Huntington disease. By targeting the pathogenic accumulation of mutant huntingtin, these approaches aim to ameliorate neurotoxicity and alter disease progression. Ongoing research into UPS modulation, autophagy induction, and innovative molecular therapies continues to illuminate new possibilities for intervention. A collaborative, evidence-based approach integrating basic science, translational research, and clinical care will be essential to realize the potential of protein degradation therapies for Huntington disease in the coming years.