MTOR HYPERACTIVATION IN EPILEPTIC MUTANT MICE REVEALS CRITICAL ROLES IN CORTICAL AND HIPPOCAMPAL DEVELOPMENT

The cerebral cortex is assembled through tightly regulated developmental programs that coordinate progenitor proliferation, neuronal migration, and laminar organization within defined temporal windows. Disruption of one or more of these processes underlies a broad spectrum of neurodevelopmental disorders. Focal cortical dysplasia type II (FCDII) is a severe developmental epileptogenic condition caused by somatic mutations in the mTOR pathway and is histopathologically characterized by cortical dyslamination and dysmorphic neurons. To investigate how pathogenic MTOR variants perturb telencephalic development, we generated conditional knock-in mouse models expressing two recurrent MTOR mutations (p.T1977K and p.S2215F) in the Emx1 lineage. These models reproduce key features associated with mTOR-related cortical pathology, including lamination defects, spontaneous seizures assessed by video-EEG, and autism-related behavioral alterations. In addition to cortical abnormalities, mutant mice exhibit hippocampal disorganization—characterized by a double CA1 layer, dispersion of CA3 neurons, and dentate gyrus heterotopia—as well as subcortical heterotopic bands within the corpus callosum. These structural alterations are accompanied by abnormal persistence of a subset of Cajal–Retzius cells, consistent with a disruption of developmental timing. Using spatial transcriptomics (MERSCOPE) and high-density multielectrode array recordings, we show that ectopic neurons retain their canonical molecular identity and that telencephalic regions displaying structural disorganization exhibit altered electrophysiological activity, with network hyperexcitability. Together, these findings demonstrate that mTOR hyperactivation driven by patient MTOR variants disrupts telencephalic patterning primarily through altered neuronal positioning rather than fate specification.