RHEB/mTOR-hyperactivity causing cortical malformations drives seizures through increased axonal connectivity

Dominant-active mutations in Ras Homolog Enriched in Brain 1 (RHEB), such as the recently identified RHEBp.P37L mutation, can cause malformations of cortical development (MCD) with associated epilepsy and intellectual disability through a yet unknown mechanism. We found that focal expression of RHEBp.P37L in mouse somatosensory cortex results in an MCD-like phenotype, with increased mammalian target of rapamycin (mTOR) signaling, ectopic localization of neurons and generalized seizures. In addition, the RHEBp.P37L expressing neurons showed increased axonal length and branching. By temporally controlling RHEBp.P37L expression, we found that the cortical malformation by itself was neither necessary nor sufficient to generate seizures. Rather, seizures were contingent on persistent mTOR activation and enhanced axonal connectivity of RHEBp.P37L expressing neurons, causing hyperexcitability of distally connected neurons. These results provide new evidence of the extent of anatomical and physiological abnormalities caused by mTOR hyperactivity, beyond local malformations, that can lead to generalized epilepsy.