HTT CAG Repeats Surpass 150 Threshold in Neocortex, Extending Somatic Expansion Toxicity to Second Major Brain Region in Huntington's

The Broad-Harvard-McLean team used single-cell DNA and RNA sequencing on postmortem neocortex tissue. Cells showed abrupt gene-expression collapse only after repeat length crossed the 150 threshold, with nuclear huntingtin inclusions appearing exclusively in those high-repeat, pre-apoptotic neurons. This places inclusions as late markers of terminal dysfunction rather than protective aggregates.

Regional kinetics differ sharply: neocortical cells require decades longer to reach the toxic length, leaving most below threshold at death and explaining milder cortical pathology relative to striatum. Distinct downstream transcriptional programs in cortex versus striatum further account for variable clinical phenotypes across brain areas.

Therapeutic programs now targeting somatic expansion could therefore address both primary sites of degeneration. Because the mechanism is shared, a single drug class might slow progression in multiple networks rather than requiring region-specific interventions.

Next steps include longitudinal repeat-length measurements in living patients via emerging blood or CSF assays to test whether expansion rate predicts individual disease tempo and therapeutic windows.