PhD Student Hebrew University of Jerusalem, Israel
Abstract: Huntington's disease (HD) is a genetic neurodegenerative disorder, caused by a CAG repeat expansion (>39) in the poly-glutamine (polyQ) tract of the Huntingtin (HTT) gene. Despite its relatively late onset, HD has been shown to cause abnormal neurodevelopment during embryogenesis and early childhood. Moreover, DNA methylation was found to play an important role as an epigenetic mechanism in HD among adults. Therefore, we sought to explore early neurodevelopmental alterations in DNA methylation using cerebral organoids from juvenile forms (72Q and 180Q) of isogenic HD pluripotent cells. Additionally, to differentiate between gain- and loss-of-function phenotypes in HD, we generated HTT-KO organoids for comparison. HD, but not HTT-KO organoids, displayed widespread DNA hypomethylation, in a CAG-repeat length dependant manner. Bulk RNA-seq revealed neuronal differentiation-related pathways shared between the KO and HD systems, supporting a loss-of-function, as well as pathways uniquely altered in the 72Q-iPSCs organoids, with the latter being associated with methylation changes in the relevant promoters. We further found a highly significant enrichment for the DNMT3B motif among the differentially methylated positions. Using multiple methods, we demonstrate polyQ-length-dependent interactions between DNMT3B and HTT, suggesting the sequestering of DNMT3B by mutated HTT. Our findings reveal neurodevelopmental and epigenetic defects during early neurogenesis in HD models, and highlight DNA methylation as a central pathway.
