Abstracts

Rationale: Tuberous Sclerosis Complex (TSC) is a multisystem genetic disorder characterized by mutations in Tsc1 or Tsc2 leading to mammalian target of rapamycin (mTOR) hyperactivity. 80-90% of TSC individuals suffer from intractable seizures resulting from cortical malformations (called tubers) which form during embryonic life. Understanding how these lesions form and lead to hyper-excitability has been limited by the absence of an animal model exhibiting tubers. Methods: To address this limitation, we used in utero electroporation of Cre recombinase-containing vector in transgenic mice carrying a floxed and a mutant Tsc1 allele for knocking out Tsc1 in selected neuronal populations at a precise developmental time-point. Results: Single-cell knockout of Tsc1 led to increased mTOR activity and soma size of Tsc1 null neurons. This approach generated heterotopic nodules above or in the white matter and discrete cortical tuber-like lesions displaying a mosaic of cell size and phospho-S6 immunoreactivity. The electroporation time-point determined the severity of the malformations with late-born cortical structures being the most affected. Tuber-like lesions display ectopic neurons resulting in loss of cortical architecture, cytomegalic and multinucleated neurons with abnormal dendritic trees resembling giant cells. No gliosis was visible and phospho-pS6 immunoreactivity was surprisingly not up-regulated in Tsc1 null astrocytes despite a lower seizure threshold. Conclusions: These data suggest that a double-hit strategy to eliminate Tsc1 in discrete neuronal populations generates TSC-associated cortical lesions providing a model to uncover the mechanisms of lesion formation and hyper-excitability.