Human microcephaly ASPM protein is a spindle pole-focusing factor

Nonsense mutations in the ASPM gene have been most frequently identified among familial microcephaly patients. Depletion of ASPM causes spindle pole unfocusing during mitosis in multiple cell types of Drosophila. However, it remains unknown whether human ASPM has a similar function. Here, using CRISPR-based gene knockout (KO) and RNA interference combined with chemical inhibitors and auxin-inducible degron, we show that ASPM functions in spindle pole organisation redundantly with the centrosomes and the kinesin-14 motor HSET in human tissue culture cells. Deletion of the ASPM gene alone did not affect spindle morphology or mitotic progression. However, when CDK5RAP2, the activator of centrosomal microtubule nucleation, was degraded in ASPM KO cells, the spindle poles were unfocused and mitosis was significantly delayed. HSET inhibition in ASPM KO cells also resulted in synthetic pole disorganisation. Similarly, a hypomorphic mutation identified in microcephaly patients caused spindle pole unfocusing in the absence of CDK5RAP2, suggesting this spindle pole defect as a possible cause of microcephaly.