PLK1-mediated phosphorylation of WDR62/MCPH2 ensures proper mitotic spindle orientation.

Primary microcephaly (MCPH) is an autosomal recessive disorder characterized by congenital reduction of head circumference. Here, we identified compound heterozygous mutations c.731 C > T (p.Ser 244 Leu) and c.2413 G > T (p.Glu 805 X) in the WDR62/MCPH2 gene, which encodes the mitotic centrosomal protein WDR62, in two siblings in a Japanese family with microcephaly using whole-exome sequencing. However, the molecular and cellular pathology of microcephaly caused by WDR62/MCPH2 mutation remains unclear. To clarify the physiological role of WDR62, we used the CRISPR/Cas9 system and single-stranded oligonucleotides as a point-mutation-targeting donor to generate human cell lines with knock-in of WDR62/MCPH2 c.731 C > T (p.Ser 244 Leu) missense mutation. In normal metaphase, the mitotic spindle forms parallel to the substratum to ensure symmetric cell division, while WDR62/MCPH2-mutated cells exhibited a randomized spindle orientation caused by the impaired astral microtubule assembly. It was shown that a mitotic kinase, Polo-like kinase 1 (PLK1), is required for the maintenance of spindle orientation through astral microtubule development. In this study, we demonstrated that WDR62 is a PLK1 substrate that is phosphorylated at Ser 897, and that this phosphorylation at the spindle poles promotes astral microtubule assembly to stabilize spindle orientation. Our findings provide insights into the role of the PLK1-WDR62 pathway in the maintenance of proper spindle orientation.