Functional impact of genetic background on variable expressivity in neurodevelopmental disorders

Disease-associated variants can lead to variable phenotypic outcomes in neurodevelopmental disorders, but the biological mechanisms underlying this variability remain poorly understood. Here, we develop a framework to investigate this phenomenon using the 16p12.1 deletion as a paradigm of variable expressivity. Using induced pluripotent stem cell models from affected families and CRISPR-edited lines with the 16p12.1 deletion, we find that the deletion and rare variants in the genetic background jointly influence chromatin accessibility and expression of neurodevelopmental genes. Cellular analyses identify family-specific phenotypes, including altered inhibitory neuron production and neural progenitor cell proliferation, which correlate with head-size variation. CRISPR activation of individual 16p12.1 genes variably rescue these defects by modulating key developmental signaling pathways. Integrative analyses further identify regulatory hubs, including transcription factors FOXG1 and JUN, as mediators of these effects. Our study provides a functional framework for investigating how individual genetic architectures contribute to phenotypic variability in neurodevelopmental disorders.