Bx42 directs neural stem cell exit from quiescence through Prospero

Microcephaly is a rare neurodevelopmental disorder characterized by a severely reduced head and brain size in children and is accompanied by a myriad of debilitating side effects including cognitive and developmental impairments. While many cases of microcephaly arise from genetic mutations, the molecular mechanism linking variants to disease remain poorly understood. We previously identified Bx42 as a microcephaly-causing gene from a patient-informed study using human brain organoid modeling and functional studies in Drosophila melanogaster. Here, we demonstrate that loss of Bx42 leads to microcephaly by reducing neural stem cell proliferation early in development. Mechanistically, our data suggest that Bx42 promotes the transition from quiescence to proliferation through regulation of the neural differentiation factor Prospero. Reduction of Bx42 results in reduced neural stem cell division due to a prolonged quiescent state. Importantly, we demonstrate that two patient-derived variants in the human ortholog, SNW1, are nonfunctional or hypomorphic, providing strong evidence that these variants are pathogenic and causative of microcephaly. Together, our findings define a previously unrecognized role for the Bx42/SNW1 pathway in regulating neural stem cell activation and brain growth, offering new mechanistic insight into the pathogenesis of genetically driven microcephaly.