ELSC Seminar Series

Modelling Huntington’s disease using human pluripotent stem cells

Huntington’s disease (HD) is a genetic neurodegenerative disorder, caused by an expansion of CAG repeats (>39) coding for poly-glutamine (polyQ) tract in the Huntingtin (HTT) gene. The mutant polyQ-expanded HTT protein is insoluble and forms cellular polyQ aggregates. Although HD is late onset, it was shown to have an early neurodevelopmental component. I will discuss two running projects in the lab where we sought to unveil early pre-symptomatic alterations using juvenile forms of HD human induced pluripotent stem cells (iPSCs). First, using cerebral organoids, we found widespread DNA demethylation in the mutant cells (72Q and 180Q), which also displayed a polyQ length-dependent increase in the predicted epigenetic age. We show that the demethylated positions are highly enriched in DNMT3B motif, and using structural modelling, co-IP, and FRET imaging analyses, we demonstrate interactions between DNMT3B and HTT depending on polyQ length. In the second project, we have developed human iPSC-derived neuronal progenitors (NPCs) with polyQ aggregates by either ectopically expressing mutant GFP-105Q-HTT fusion protein in healthy iPSCs or by expressing a WT (18Q) version of the same GFP fusion protein (GFP-18Q-HTT) in iPSC-derived NPCs naturally containing 180Q repeats. Comparing genetically identical NPCs grown together, with and without aggregates, we identified a neuroinflammatory response in the aggregates-containing cells, and, seeking the mechanism by which neuroinflammation genes are induced, we identified ATF3 as the transcription factor involved. ATF3 directly binds and activates neuroinflammation genes in aggregates-containing cells, and knocking-out ATF3 in NPCs prevents polyQ aggregate formation. These results highlight ATF3 as operating upstream to neuroinflammation in HD. Taken together, our findings reveal fundamental neurodevelopmental and epigenetic defects in the early stages of neurogenesis in HD models, highlighting DNA methylation and neuroinflammation as a central pathway in early HD.