SHMT2 deficiency disrupts transcriptional regulation through homocysteine-mediated suppression of histone lactylation in Huntington's disease models.

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and striatal neuron degeneration, primarily affecting medium spiny neurons (MSNs). Despite extensive research, the underlying metabolic vulnerabilities contributing to HD pathogenesis remain poorly understood. In this study, we employed RNA-seq and metabolomics analyses to identify marked dysregulation of 1-carbon metabolism in HD. We validated that SHMT2, a key mitochondrial enzyme in the mitochondrial 1-carbon pathway, was substantially downregulated in HD patient-derived iPSC-differentiated human striatal organoids (hSOs) and YAC128 mice. Functionally, pharmacologic inhibition or genetic deletion of SHMT2 exacerbated mutant huntingtin aggregation, induced MSN degeneration in hSOs, and impaired motor function in WT mice. Conversely, SHMT2 overexpression attenuated MSN degeneration in HD-hSOs and improved motor performance in YAC128 mice. Mechanistically, SHMT2 deficiency led to accumulation of homocysteine, which interacted with AARS1 and suppressed histone lactylation, thereby perturbing transcriptional regulation and associating with neurodegenerative phenotypes. Finally, we demonstrated that the HD clinical drug haloperidol modulated SHMT2 expression and restored histone lactylation, providing a pharmacologic tool to probe SHMT2-dependent metabolic and epigenetic regulation in HD models. These findings highlight a metabolic-epigenetic axis as a promising therapeutic target for HD.