Polygenicity in a box: Copy number variants, neural circuit development, and neurodevelopmental disorders

Clinically defined neurodevelopmental disorders (cd-NDDs), including Autistic Spectrum Disorder (ASD) and Schizophrenia (Scz), are primarily polygenic: Multiple risk genes distributed across the genome, in potentially infinite combinations, account for variable pathology. Polygenicity raises a fundamental question: Can "core" cd-NDD pathogenic mechanisms be identified given this genomic complexity? With the right models and analytic targets, a distinct class of polygenic mutations-Copy Number Variants (CNVs): contiguous gene deletions or duplications associated with cd-NDD risk-provide a singular opportunity to define cd-NDD pathology. CNVs orthologous to those that confer cd-NDD risk have been engineered in animals as well as human stem cells. Using these tools, one can determine how altered function of multiple genes cause serial stumbles over cell biological steps typically taken to build optimal "polygenic" neural circuits. Thus, cd-NDD pathology may be a consequence of polygenic deviations-stumbles-that exceed limits of adaptive variation for key developmental steps.