White matter abnormalities are a nearly universal pathological feature of neurodegenerative disorders including Huntington disease (HD). A long-held assumption is that this white matter pathology is simply a secondary outcome of the progressive neuronal loss that manifests with advancing disease. Using a mouse model of HD, here we show that white matter and myelination abnormalities are an early disease feature appearing before the manifestation of any behavioral abnormalities or neuronal loss. We further show that selective inactivation of mutant huntingtin (mHTT) in the NG2+ oligodendrocyte progenitor cell population prevented myelin abnormalities and certain behavioral deficits in HD mice. Strikingly, the improvements in behavioral outcomes were seen despite the continued expression of mHTT in nonoligodendroglial cells including neurons, astrocytes, and microglia. Using RNA-seq and ChIP-seq analyses, we implicate a pathogenic mechanism that involves enhancement of polycomb repressive complex 2 (PRC2) activity by mHTT in the intrinsic oligodendroglial dysfunction and myelination deficits observed in HD. Our findings challenge the long-held dogma regarding the etiology of white matter pathology in HD and highlight the contribution of epigenetic mechanisms to the observed intrinsic oligodendroglial dysfunction. Our results further suggest that ameliorating white matter pathology and oligodendroglial dysfunction may be beneficial for HD.

Fragile X syndrome (FXS) is an incurable neurodevelopmental disorder with no effective treatment. FXS is caused by epigenetic silencing of FMR1 and loss of FMRP expression. To investigate the consequences of FMRP deficiency in the context of human physiology, we established isogenic FMR1 knockout (FMR1KO) human embryonic stem cells (hESCs). Integrative analysis of the transcriptomic and proteomic profiles of hESC-derived FMRPdeficient neurons revealed several dysregulated pathways important for brain development including processes related to axon development, neurotransmission, and the cell cycle. We functionally validated alterations in a number of these pathways, showing abnormal neural rosette formation and increased neural progenitor cell proliferation in FMR1KO cells. We further demonstrated neurite outgrowth and branching deficits along with impaired electrophysiological network activity in FMRP-deficient neurons. Using isogenic FMR1KO hESC-derived neurons, we reveal key molecular signatures and neurodevelopmental abnormalities arising from loss of FMRP. We anticipate that the FMR1KO hESCs and the neuronal transcriptome and proteome datasets will provide a platform to delineate the pathophysiology of FXS in human neural cells..