Cell-type-specific CAG repeat expansions and toxicity of mutant Huntingtin in human striatum and cerebellum

Brain region-specific degeneration and somatic expansions of the mutant Huntingtin ( mHTT ) CAG tract are key features of Huntington’s disease (HD). However, the relationships among CAG expansions, death of specific cell types and molecular events associated with these processes are not established. Here, we used fluorescence-activated nuclear sorting (FANS) and deep molecular profiling to gain insight into the properties of cell types of the human striatum and cerebellum in HD and control donors. CAG expansions arise at mHTT in striatal medium spiny neurons (MSNs), cholinergic interneurons and cerebellar Purkinje neurons, and at mutant ATXN3 in MSNs from SCA3 donors. CAG expansions in MSNs are associated with higher levels of MSH2 and MSH3 (forming MutSβ), which can inhibit nucleolytic excision of CAG slip-outs by FAN1. Our data support a model in which CAG expansions are necessary but may not be sufficient for cell death and identify transcriptional changes associated with somatic CAG expansions and striatal toxicity. Fluorescence-activated nuclear sorting combined with deep profiling shows that Huntington’s disease repeat expansions arise in specific cell types and are associated with elevated MSH2 and MSH3, which promote expansions in vitro by inhibiting excision of CAG slip-outs by FAN1.