Emerging Therapies Target DNA Expansion to Treat Huntington’s Disease

Huntington’s disease, a fatal neurodegenerative condition caused by a genetic mutation in the HTT gene, may soon face new treatment strategies focused on “somatic expansion.” Recent research suggests that the lifelong accumulation of CAG DNA repeats—rather than just the resulting toxic protein—drives the disease’s progression. Scientists are now developing therapies to stabilize these genetic repeats, potentially offering a way to delay or halt the neurological decline that characterizes the condition for over 41,000 symptomatic patients in the U.S.

The Role of Somatic Expansion in Disease Progression

For decades, the primary focus of Huntington’s research was the huntingtin protein. The disease is caused by a CAG repeat expansion in the HTT gene; once an individual inherits more than 40 repeats, the development of the disease is considered inevitable. However, researchers have long observed that the length of these repeats is not static. Throughout a patient’s life, these DNA sequences continue to expand within specific neurons, a process known as somatic expansion.

According to findings published in Cell in February 2025 by neurogeneticist Steve McCarroll and his team, there is a clear “tipping point” in this process. By examining post-mortem human brain cells, researchers identified that once CAG repeats reach approximately 80 copies, their growth accelerates. When the count exceeds 150, the neurons begin a rapid decline toward death. This discovery provides a new framework for intervention: if medical professionals can stabilize the repeat length before a patient hits this threshold, they may preserve vital brain tissue.

Shifting Therapeutic Strategies: From Protein to DNA

Initial efforts to treat Huntington’s focused on lowering the levels of the toxic huntingtin protein. While this remains a significant area of study, the field is increasingly looking “upstream” to the DNA repair machinery that facilitates repeat expansion. A key target in this effort is the MSH3 gene.

Research led by Sarah Tabrizi at University College London, published in Lancet Neurology, identified that natural variations in MSH3 influence the rate at which CAG repeats grow. By suppressing MSH3 activity, researchers hope to slow the genetic “stutter” that makes the disease progressively worse over time.

Clinical Outlook and Future Directions

The landscape of Huntington’s treatment is evolving from a single-target approach to a more nuanced strategy. While the 2021 failure of the drug tominersen created a significant hurdle for the field, the 2024 FDA decision to support a new-drug application for a gene therapy from the company uniQure has renewed optimism. This therapy, which involves a single infusion to dial down huntingtin production, demonstrated the ability to lower neurofilament light levels—a biomarker for neuronal death.

Key Facts About Huntington’s Disease

• Prevalence: Huntington’s disease affects approximately 1 in 20,000 people globally.
• Genetic Risk: Each child of an affected parent has a 50% chance of inheriting the mutated HTT gene.
• Mechanism: The disease is driven by CAG repeat expansion, which triggers a toxic cascade in the brain’s striatum, the area responsible for motor control and decision-making.
• Current Research: New clinical trials are investigating both small-molecule pills and direct-to-brain gene therapies to target the MSH3 pathway and stabilize DNA repeats.