DEPDC5 haploinsufficiency drives increased mTORC1 signaling and abnormal morphology in human iPSC-derived cortical neurons.

Mutations in the DEPDC5 gene can cause epilepsy, including forms with and without brain malformations. The goal of this study was to investigate the contribution of DEPDC5 gene dosage to the underlying neuropathology of DEPDC5-related epilepsies. We generated induced pluripotent stem cells (iPSCs) from epilepsy patients harboring heterozygous loss of function mutations in DEPDC5. Patient iPSCs displayed increases in both phosphorylation of ribosomal protein S6 and proliferation rate, consistent with elevated mTORC1 activation. In line with these findings, we observed increased soma size in patient iPSC-derived cortical neurons that was rescued with rapamycin treatment. These data indicate that human cells heterozygous for DEPDC5 loss-of-function mutations are haploinsufficient for control of mTORC1 signaling. Our findings suggest that human pathology differs from mouse models of DEPDC5-related epilepsies, which do not show consistent phenotypic differences in heterozygous neurons, and support the need for human-based models to affirm and augment the findings from animal models of DEPDC5-related epilepsy.

Prevention of premature death and seizures in a Depdc5 mouse epilepsy model through inhibition of mTORC1.

Mutations in DEP domain containing 5 (DEPDC5) are increasingly appreciated as one of the most common causes of inherited focal epilepsy. Epilepsies due to DEPDC5 mutations are often associated with brain malformations, tend to be drug-resistant, and have been linked to an increased risk of sudden unexplained death in epilepsy (SUDEP). Generation of epilepsy models to define mechanisms of epileptogenesis remains vital for future therapies. Here, we describe a novel mouse model of Depdc5 deficiency with a severe epilepsy phenotype, generated by conditional deletion of Depdc5 in dorsal telencephalic neuroprogenitor cells. In contrast to control and heterozygous mice, Depdc5-Emx1-Cre conditional knockout (CKO) mice demonstrated macrocephaly, spontaneous seizures and premature death. Consistent with increased mTORC1 activation, targeted neurons were enlarged and both neurons and astrocytes demonstrated increased S6 phosphorylation. Electrophysiologic characterization of miniature inhibitory post-synaptic currents in excitatory neurons was consistent with impaired post-synaptic response to GABAergic input, suggesting a potential mechanism for neuronal hyperexcitability. mTORC1 inhibition with rapamycin significantly improved survival of CKO animals and prevented observed seizures, including for up to 40 days following rapamycin withdrawal. These data not only support a primary role for mTORC1 hyperactivation in epilepsy following homozygous loss of Depdc5, but also suggest a developmental window for treatment which may have a durable benefit for some time even after withdrawal.