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Haploinsufficiency of BAZ1B contributes to Williams syndrome through transcriptional dysregulation of neurodevelopmental pathways

ABSTRACT: Williams syndrome (WS) is a neurodevelopmental disorder caused by a genomic deletion of ~28 genes that results in a cognitive and behavioral profile marked by overall intellectual impairment with relative strength in expressive language and hypersocial behavior. Advancements in protocols for neuron differentiation from induced pluripotent stem cells allowed us to elucidate the molecular circuitry underpinning the ontogeny of Williams syndrome. In patient-derived stem cells and neurons, we determined the expression profile of the Williams-Beuren Syndrome Critical Region deleted genes and the genome-wide transcriptional consequences of the hemizygous genomic microdeletion at 7q11.23. Derived neurons displayed disease-relevant hallmarks and indicated novel aberrant pathways in WS neurons including over-activated Wnt signaling accompanying an incomplete neurogenic commitment. We show that haploinsufficiency of the ATP-dependent chromatin remodeler, BAZ1B, which is deleted in WS, significantly contributes to this differentiation defect. Chromatin-immunoprecipitation (ChIP-seq) revealed BAZ1B target gene functions are enriched for neurogenesis, neuron differentiation, and disease-relevant phenotypes. BAZ1B haploinsufficiency caused widespread gene expression changes in neural progenitor cells, and together with BAZ1B ChIP-seq target genes, explained 42% of the transcriptional dysregulation in WS neurons. BAZ1B contributes to regulating the balance between neural precursor self-renewal and differentiation and the differentiation defect caused by BAZ1B haploinsufficiency can be rescued by mitigating over-active Wnt signaling in neural stem cells. Altogether, these results reveal a pivotal role for BAZ1B in neurodevelopment and implicate its haploinsufficiency as a likely contributor to the neurological phenotypes in WS.

ORGANISM(S): Homo sapiens

PROVIDER: GSE71664 | GEO | 2016/01/19

SECONDARY ACCESSION(S): PRJNA291760

REPOSITORIES: GEO

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Project description:Kabuki Syndrome (KS) is a multisystemic rare disorder, characterized by growth delay, distinctive facial features, intellectual disability, and rarely autism spectrum disorder. This condition is mostly caused by de novo mutations of KMT2D, encoding a catalytic subunit of the COMPASS complex involved in enhancer regulation. KMT2D catalyzes the deposition of histone-3-lysine-4 mono-methyl (H3K4Me1) that marks active and poised enhancers. To assess the impact of KMT2D mutations in the chromatin landscape of KS tissues, we have generated patient-derived induced pluripotent stem cells (iPSC), which we further differentiated into neural crest stem cells (NCSC), mesenchymal stem cells (MSC) and cortical neurons (iN). In addition, we further collected blood samples from 5 additional KS patients. To complete our disease modeling cohort we generated an isogenic KMT2D mutant line from human embryonic stem cells, which we differentiated into neural precursor and mature neurons. Micro-electrode-array (MEA)-based neural network analysis of KS iNs revealed an altered pattern of spontaneous network-bursts in a Kabuki-specific pattern. RNA-seq profiling was performed to relate this aberrant MEA pattern to transcriptional dysregulations, revealing that dysregulated genes were enriched for neuronal functions, such as ion channels, synapse activity, and electrophysiological activity. Here we show that KMT2D haploinsufficiency tends to heavily affect the transcriptome of cortical neurons and differentiated tissues while sparing multipotent states, suggesting that KMT2D has a most prevalent role in terminally differentiated cell and activate transcriptional circuitry unique to each cell type. Moreover, thorough profiling of H3K4Me1 unveiled the almost complete uncoupling between this chromatin mark and the regulatory effects of KMT2D on transcription, which is instead reflected by a defect of H3K27Ac. By integrating RNA-seq with ChIP-seq data we defined TEAD and REST as the master effectors of KMT2D haploinsufficiency. Also, we identified a subset of genes whose regulation is controlled by the balance between KMT2D and EZH2 dosage. Finally, we identified the bona fide direct targets of KMT2D in healthy and KS mature cortical neurons and TEAD2 as the main proxy of KMT2D dysregulation in KS. Overall, our study provides the transcriptional and epigenomic characterization of patient-derived tissues as well as iPSCs and differentiated disease-relevant cell types, as well as the identification of KMT2D direct target in cortical neurons, together with the identification of a neuronal phenotype of the spontaneous electrical activity.

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Haploinsufficiency of BAZ1B contributes to Williams syndrome through transcriptional dysregulation of neurodevelopmental pathways

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