Abstracts

Rationale:
Epileptic encephalopathies (EE) are severe early-onset epilepsies, often refractory to therapies, with associated cognitive deficits and early lethality. Our group has recently identified recessive mutations in the PIGB gene, encoding the GPI mannosyltransferase 3 protein, a member of the glycosylphosphatidylinositol (GPI)-anchor biosynthesis pathway, in patients with global developmental delay, early epilepsy and axonal neuropathy. GPI anchors permit the anchoring of various receptors, ion channels and cell adhesion proteins on cell membranes. However, the exact mechanisms by which deficits in GPI anchors result in such a striking neurodevelopmental phenotype are unknown. Recent data from our group suggest that impairments in the early development or function of inhibitory GABAergic interneurons (INs) result in epilepsy and cognitive deficits in mice, suggesting a role for cortical disinhibition in the pathophysiology of specific genetic forms of EE. We postulate that GPI anchor deficit results in an EE phenotype by  impairing INs development.
Method:
To investigate this hypothesis, we generated different murine models carrying targeted Pigb mutations in distinct neuronal populations. We combined high-resolution cell imaging, immunohistochemistry, neuronal quantification and morphological reconstitution, together with video-EEG recordings and behavioral analysis to characterize the models and investigate the underlying mechanisms.
Results:
While constitutive knock-out and pan-neuronal conditional mutant mice are not viable, mice with GPI deficiency carrying a targeted deletion of Pigb in MGE-derived INs are viable and  develop spontaneous seizures as well as behavioral deficits reminiscent of those observed in patients. Furthermore, neuronal quantification at different key developmental stages (e13.5, e15.5, P0) revealed a delay in INs migration,  with reduced number of INs at the migratory front embryonically (e13.5 and e15.5) resulting in a reduction of INs numbers at postnatal stages (P14 and P21). Notably, 3D reconstruction of e13.5 migrating INs revealed striking perturbations in IN morphology, including increased neurite length, number and complexity of leading and trailing processes. Moreover, live imaging of MGE explants suggests perturbations in cytoskeletal remodeling and migration kinetics in INs.
Conclusion:
Our results reveal that the loss of GPI anchors, through Pigb recessive mutations, impair IN migration, resulting in epilepsy and cognitive deficits. Subsequent studies will help unveil the specific GPI-anchored proteins required to sustain IN migration, further clarifying the underlying disease mechanisms.
Funding:
:Savoy Foundation, Canadian Institutes of Health Research