Cerebral organoids expressing mutant actin genes reveal cellular mechanism underlying microcephalic cortical malformation

Pathogenic ACTB and ACTG1 gene variants, encoding the actin isoforms βCYA and γCYA, respectively, are frequently associated with the Baraitser-Winter-CerebroFrontoFacial syndrome (BWCFF-S) that includes malformations of cortical development. Here we explore whether cerebral organoids grown from BWCFF-S patient-derived induced pluripotent stem cells can provide insight into the pathogenesis underlying the cortical malformations of these patients. Cerebral organoids expressing either an ACTB or an ACTG1 gene variant, each with a point mutation resulting in a single amino acid substitution, are reduced in size, showing smaller ventricle-like structures with a thinner ventricular zone (VZ). This decrease in VZ- progenitors is associated with a striking change in the orientation of their cleavage plane from predominantly vertical (control) to predominantly horizontal (BWCFF-S), which is incompatible with increasing VZ-progenitor abundance. The underlying cause appears to be an altered subcellular tubulin localization due to the actin mutations that affects mitotic spindle positioning of VZ-progenitors in BWCFF-S.