FUNCTIONAL DISSECTION OF THE CONTRIBUTION OF BAZ1B TO NEURAL CREST DYSREGULATION IN NEURODEVELOPMENTAL DISORDERS CAUSED BY SYMMETRICAL CNVS AT 7Q11.23

The hemizygous deletion and duplication of 28 genes in the 7q11.23 region is responsible for two neurodevelopmental disorders: Williams-Beuren syndrome (WBS) and 7q11.23 duplication syndrome (7dup), respectively. These two syndromes exhibit both shared and symmetrically opposite clinical traits, thus pointing to a dosage-sensitive impact of a small subset of genes, mostly affecting cognition, sociality and Neural Crest (NC)-mediated craniofacial and cardiovascular development. Despite some studies on mouse models and patients with smaller deletions have established possible associations between specific genes in the 7q11.23 region and specific features of the disease, the vast majority of the genes remains without a clear connection. BAZ1B, a transcriptional regulator and chromatin remodeler included in the region, is a prime candidate to study disease-associated NC alterations, because of its critical role in the migration of Neural Crest Stem Cells (NCSCs) in Xenopus and in the acquisition of craniofacial defects in mice lacking this protein. Intriguingly, WBS patients bearing a partial deletion of the region that spares few genes, including BAZ1B, display milder craniofacial dysmorphisms, further pointing to its involvement in this specific phenotype. In order to define the BAZ1B-dependent alterations responsible for NC-related defects in the two genetic conditions, we integrated transcriptomic analysis with enhancer profiling, given the critical role of their regulatory architecture in NCSC development and function. First, we derived bona fide NCSCs from a large and uniquely informative cohort of patient- specific induced pluripotent stem cells (iPSCs), including some derived from atypical BAZ1B-sparing deletions. Our results suggest that gene dosage alterations at the 7q11.23 region do not affect the cell-specific differentiation potential and the differentiation process itself. In addition, RNA profiling of NCSCs indicates dysregulation of genes specifically involved in NC-related biological processes and functions. Next, I selectively downregulated BAZ1B levels by RNA interference in both control- and patient- NCSC lines and I performed RNA-sequencing analysis to dissect its contribution to transcriptional dysregulation. I uncovered a subset of genes whose levels followed BAZ1B levels either in a direct or inverse fashion, indicating clear BAZ1B dosage- dependent transcriptional alterations. These genes were mostly involved in pathways related to bone and cardiovascular system development and cell migration and motility. Moreover, the observed impact of BAZ1B knock-down on human NCSC migration confirmed the evidence in animal models and shed new lights into BAZ1B-dependent mechanisms responsible for the acquisition of NC-related phenotypes. Finally, preliminary analyses indicate a limited, but potentially crucial, BAZ1B-dependent remodeling on NCSC putative active enhancer regions. Taken together, these results provide new inroads into a functional partitioning of BAZ1B transcriptional and epigenetic roles in NCSCs and they increase our understanding of these two debilitating neurodevelopmental disorders which will eventually pave the way for the development of novel effective therapies.