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Recent research from UCLA Health has identified a crucial role for DNA mismatch repair (MMR) genes in Huntington’s disease (HD), revealing how they drive neuronal vulnerability and disease progression. Published in Cell, the study provides mechanistic insights linking genetic modifiers to selective neuronal degeneration, offering potential therapeutic targets.
Huntington’s disease, an inherited neurodegenerative disorder, results from an expanded CAG repeat in the huntingtin gene. While individuals with fewer than 35 repeats remain unaffected, those with 40 or more develop the disease, with earlier onset correlating with longer repeats. Despite huntingtin being expressed in all cells, HD primarily affects striatal medium spiny neurons (MSNs), a longstanding mystery in neurodegeneration.
Genome-wide association studies (GWAS) have identified genetic modifiers influencing HD onset, particularly within DNA repair pathways. Several MMR genes—Msh3, Pms1, Msh2, and Mlh1—are implicated, but their mechanistic role in selective neuronal vulnerability remained unclear. To address this, researchers used an HD mouse model (Q140) with 140 CAG repeats and systematically disrupted these MMR genes.
Loss of Msh3 or Pms1 substantially mitigated disease phenotypes, preventing huntingtin aggregation, correcting gene expression dysregulation, and improving locomotor and synaptic function. The effects persisted for 20 months—equivalent to 60 human years—highlighting the sustained therapeutic potential. Notably, Msh3-deficient mice exhibited a near-complete halt in CAG repeat expansion within MSNs, which otherwise increased at a rate of +8.8 repeats/month, surpassing 220 repeats by 12 months.
The findings underscore that MMR genes actively drive disease by accelerating CAG repeat expansion, ultimately leading to neuronal dysfunction. The study provides critical links between patient-derived modifier genes, MMR-driven repeat instability, and MSN-specific degeneration. Importantly, targeting Msh3 and Pms1 did not cause notable off-target effects, strengthening their viability as therapeutic targets.
Given the involvement of DNA repeat instability in over 30 neurological disorders, these insights extend beyond HD, offering a framework for broader therapeutic applications. Future research will focus on developing targeted interventions that modulate this minor MMR complex to delay disease onset or slow progression in HD and related disorders.
