Postnatal DEPDC5 Loss Without Disruption of Cortical Migration is Sufficient to Cause Epilepsy and Seizure-Induced Death

Introduction: Focal cortical dysplasia type II (FCDII) is a major cause of refractory epilepsy and is often associated with pathogenic variants of the mTOR pathway genes including DEPDC5, the most common cause of familial focal epilepsy. Surgical resection of the epileptogenic focus can often control FCDII-associated seizures. However, its utility is muted by the frequent absence of a detectable lesion on MRI, necessitating novel therapeutic options.

The mechanisms of epileptogenesis associated with FCDII and hyperactive mTOR signaling remain unclear. Determination of whether in utero defects in cortical development or later neuronal dysfunction of mTOR signaling mechanisms lead to epilepsy after loss of DEPDC5 is critical for developing future therapies.

Methods: We developed a postnatal, adeno-associated virus (AAV)-mediated focal cortical Depdc5 knockout mouse model. Postnatal day 0-1 Depdc5-floxed mouse pups were injected in the motor cortex of one hemisphere with either AAV2/8-Cre or control AAV2/8-GFP. We analyzed cortical layers and mTOR activity at postnatal day 10. Neuropathological defects, glial abnormalities, and pentylenetetrazol(PTZ)-induced seizures were assessed at 6-10 weeks of age.

Results: The AAV-Cre injected hemisphere of Depdc5-floxed mice had decreased DEPDC5 levels with increased mTOR activity compared to the contralateral hemisphere and control AAV-GFP injected mice. Cortical lamination was not disrupted by postnatal cortical DEPDC5 loss. Injection of PTZ (65 mg/kg) led to seizures in 100% (22/22) of focal Depdc5 knockout mice compared to 72% (16/22) of control mice (p=0.04, Fisher’s exact test). Mice with postnatal cortical DEPDC5 loss also had an increased prevalence of focal seizures (p=0.0006, Fisher’s exact test), multiple seizures (p=0.03, Fisher’s exact text), and seizure-induced death (p=0.048, Fisher’s exact test) compared to control mice. Pathologic hallmarks of FCDII were still identified in the hemisphere with postnatal DEPDC5 loss, including increased SMI-311 neurofilament staining, hypomyelination, astrogliosis, and microglial activation.

Conclusion : This study demonstrates that postnatal DEPDC5 loss leads to ongoing mTOR hyperactivation without disruption of cortical migration and is sufficient to cause epilepsy and seizure-induced death. Therefore, restoration of DEPDC5 function represents a viable treatment approach.