Expanded huntingtin CAG repeats disrupt the balance between neural progenitor expansion and differentiation in human cerebral organoids

Abstract

Huntington disease (HD) manifests in both adult and juvenile forms. Mutant HTT gene carriers are thought to undergo normal brain development followed by a degenerative phase, resulting in progressive clinical manifestations. However, recent studies in children and prodromal individuals at risk for HD have raised the possibility of abnormal neurodevelopment.Although key findings in rodent models support this notion, direct evidence in the context of human physiology remains lacking. Using a panel of isogenic HD human embryonic pluripotent stem cells and cerebral organoids, we investigated the impact of mutant HTT on early neurodevelopment. We find that ventricular zone-like neuroepithelial progenitor layer expansion is blunted in an HTT CAG repeat length-dependent manner due to premature neurogenesis in HD cerebral organoids, driven by cell intrinsic processes. Transcriptional profiling and imaging analysis revealed impaired cell cycle regulatory processes, increased G1 length, and increased asymmetric division of apical progenitors, collectively contributing to premature neuronal differentiation. We demonstrate increased activity of the ATM-p53 pathway, an up-stream regulator of cell cycle processes, and show that treatment with ATM antagonists partially rescues the blunted neuroepithelial progenitor expansion in HD organoids. Our findings suggest that CAG repeat length regulates the balance between neural progenitor expansion and differentiation during early neurodevelopment. Our results 1/26further support the view that HD, at least in its early-onset forms, may not be a purely neurodegenerative disorder, and that abnormal neurodevelopment may be a component of HD pathophysiology.

A Progressive Loss of phosphoSer138-Profilin Aligns with Symptomatic Course in the R6/2 Mouse Model of Huntington’s Disease: Possible Sex-Dependent Signaling

“…The critical role for HTT in the brain has also led to the suggestion that HD is a neurodevelopmental disorder, rather than simply an adult neurodegenerative disorder (Wiatr et al 2018) and has also led to the assumption that mHTT carriers experience normal brain development, but that an emerging degenerative phase ultimately leads to the appearance of clinical symptoms. A recent study based on human organoids suggests that the CAG/glutamine repeat length in HTT regulates neurogenesis during early development (Zhang et al 2019), while a repeat length below the disease threshold benefits brain structure and general intelligence among children aged 6-18 years of age (Lee et al 2017). These same authors also reported that a higher repeat length (as long as it is below disease threshold) gives females an advantage on cortical thickness and intelligence.…”

Recent Overview of the Use of iPSCs Huntington’s Disease Modeling and Therapy

“…A similar hypothesis that mHTT may cause early defect during neurodevelopment and neurogenesis led the research team of Zhang et al [52] to study mention impact of mHTT and CAG repeat length using isogenic HD human ESC/HD iPSC-derived brain organoids. Obtained results from organoids examination showed a lower number of neuroepithelial structures, reduced size, and abnormal development of large radial structures together with the disorganization of neural progenitors.…”

Molecular Components of Store-Operated Calcium Channels in the Regulation of Neural Stem Cell Physiology, Neurogenesis, and the Pathology of Huntington’s Disease

“…The dysfunction of NSCs/NPCs under conditions of HD pathology, including both juvenile- and adult-onset, is illustrated. Juvenile-onset HD, in which mutant HTT (mHTT) contains over 60 CAG repeats, is characterized by a decrease in the differentiation of NSCs/NPCs in different in vitro models, with the exception of data that were published by Zhang et al (2019) who discovered premature neurogenesis and neuronal differentiation (the reference is indicated in the figure by an up arrow). Furthermore, NSCs/NPCs exhibited decreases in neurogenesis and disruption in cell organization in juvenile-onset HD.…”

“…All illustrated HD organoids, including two region-specific brain organoids that consist of the three-dimensional (3D) human striatal spheroid (hStrS) and human cortical spheroid (hCS) that were used to create the assembloid by Miura et al (2020) , originated from induced pluripotent stem cells (iPSCs). Cortical organoids ( Conforti et al, 2018 ; Zhang et al, 2019 ) and the chimeric organoids ( Zhang et al, 2019 ) represent 3D cell models of Huntington’s disease. Motor nerve organoids extended axons as a result of their culture in the special chamber with microchannels ( Kawada et al, 2017 ).…”