Traip Mitotic Function Controls Brain Size

Microcephaly is a developmental failure to achieve proper brain size and neuron number. Mutations in diverse genes are linked to microcephaly, including several with DNA damage repair (DDR) functions; however, it is not well understood how these DDR gene mutations limit brain size. One such gene is TRAIP, which has multiple known functions in DDR. We characterized the Drosophila ortholog Traip, finding that loss of Traip causes a brain-specific defect in the Mushroom Body (MB). Traip mutant (traip-) MBs had reduced size and fewer neurons, but no neurodegeneration, consistent with human primary microcephaly disorders. Reduced neuron numbers in traip- were explained by premature caspase-dependent cell death of MB neuroblasts (MB-NBs). Many traip- MB-NBs had prominent chromosome bridges in anaphase, along with polyploidy, aneuploidy, or micronuclei. We found no evidence for an interphase DNA repair role for Traip in MB-NBs; instead, proper MB development requires Traip function during mitosis, where Traip localizes to centrosomes and mitotic spindles. Our results suggest that proper brain size is ensured by the recently described role for TRAIP in unloading stalled replication forks in mitosis, which suppresses DNA bridges and neural stem cell death to promote proper neuron number. Further, the mitotic nature of traip- MB-NB defects and Traip localization suggest a closer etiological link between DDR microcephaly genes like Traip and the centrosome/spindle-related genes more commonly associated with microcephaly.