Delayed forebrain excitatory and inhibitory neurogenesis in STRADA-related megalencephaly via mTOR hyperactivity.

Biallelic pathogenic variants in STRADA (STE20-related adaptor alpha), an upstream regulator of the mechanistic target of rapamycin (mTOR) pathway, result in megalencephaly, drug-resistant epilepsy, and severe intellectual disability. This study explores how mTOR pathway hyperactivity alters cell fate specification in dorsal and ventral forebrain development using STRADA knockout human stem cell-derived brain organoids. In both dorsal and ventral forebrain STRADA knockout organoids, neurogenesis is delayed, with a predilection for progenitor renewal, increased proliferation and an expanded outer radial glia population. Ventrally, interneuron subtypes shift to an increase in neuropeptide Y-expressing cells. Inhibition of the mTOR pathway with rapamycin rescues most phenotypes. When mTOR pathway variants are present in all cells of the developing brain, overproduction of interneurons and altered interneuron cell fate may underlie mechanisms of megalencephaly, epilepsy, and cognitive impairment. Our findings suggest that mTOR inhibition during fetal brain development could be a potential therapeutic strategy in STRADA deficiency.