Microcephaly-causative ASPM mutations disrupt mitotic spindle orientation and timely specification of human outer radial glia cells

Mutations in the gene abnormal spindle-like, microcephaly-associated (ASPM) are the most common cause of primary microcephaly (MCPH). The mechanisms underlying MCPH have remained unclear, as ASPM disruption leads to species-specific phenotypes. Using human cortical organoids, we observed premature mitotic spindle randomization in ASPM-mutant ventricular cortical progenitors, followed by their early delamination to the basal compartment. Similar delamination following mitotic spindle misorientation could be induced by overexpression of Inscuteable, further linking the two phenotypes. Delaminated progenitors resembled outer radial glial cells (oRGCs), which are selectively amplified in human corticogenesis, but they failed to adopt a bona fide oRGC identity, displaying downregulation of key oRGC genes required for their amplification and maintenance. Our findings link the evolutionarily conserved requirement of ASPM for proper mitotic spindle orientation to the species-specific timing of delamination and proper specification of human oRGC, shedding light on the origins of species-specific vulnerability to ASPM mutations.