Modeling DEPDC5 Mutations in Epilepsy Using Patient-Derived Cells and a Genetic Mouse Model

Abstract

Mutations in the DEPDC5 gene cause focal epilepsies. DEPDC5 encodes an essential component of the GATOR1 complex, a negative regulator of the mTORC1 pathway. Dysregulation of mTORC1 signaling due to DEPDC5 mutation likely has numerous consequences for brain development and neuronal function, however, the underlying mechanisms of how DEPDC5 mutations result in epilepsies are not clear. We generated induced pluripotent stem cells (iPSCs) from epilepsy patients harboring loss-of-function mutations in DEPDC5 to investigate the contribution of gene dosage to the underlying neuropathology of DEPDC5-related epilepsies. Patient-derived iPSCs and cortical neurons demonstrated elevated mTORC1 activation. Consistent with this, we observed increased neuronal soma size that was rescued by treatment with rapamycin. These data indicate that heterozygous DEPDC5 loss-of-function mutations result in haploinsufficiency for control of mTORC1 signaling. Our findings suggest that human pathology differs from mouse models of DEPDC5-related epilepsies, which do not show phenotypic differences in heterozygous neurons, and support the need for human-based models of DEPDC5-related epilepsy. However, generation of epilepsy animal models to define mechanisms of epileptogenesis remains vital for development and testing of novel therapies. We also generated a conditional knockout (CKO) mouse with deletion of Depdc5 in cortical neurons and astrocytes. Depdc5-CKO mice have a greatly reduced lifespan with terminal seizures, display evidence of mTORC1 hyperactivation in both neurons and astrocytes, and demonstrate dysplastic cortical neurons with an altered response to GABAergic input. These results support that an altered response to inhibitory neuronal inputs may be a feature of DEPDC5-related epilepsies. Postnatal treatment with rapamycin prolonged lifespan and prevented seizures, even after its withdrawal. Overall, these data support a primary role for mTORC1 hyperactivation in epilepsy due to DEPDC5 mutations. Future directions should focus on determining the functional contributions of various phenotypes induced by mTORC1 hyperactivation due to loss of DEPDC5.