Loss of Tsc2 in Neonatal V-SVZ Neural Stem Cells Causes Rare Malformations Tsc2 Mutation Causes Rare Malformations

Tuberous Sclerosis Complex (TSC) is a genetic disorder caused by mutations that inactivate TSC1 or TSC2 genes. TSC1 or TSC2 mutations activate the mammalian target of rapamycin complex 1 (mTORC1) protein kinase pathway. Although many patients inherit a single copy of a mutant TSC gene, somatic mutations that cause loss of heterozygosity in inhibitory neural progenitor cells are hypothesized to be one cause of abnormal development. This may lead to cortical malformations or benign growths along the ventricular-subventricular zone (V-SVZ), cortex, olfactory tract, and olfactory bulbs (OB). This idea is supported by focal single-cell knockout experiments that induce CRE-mediated recombination following neonatal electroporation of conditional Tsc2 or Tsc1 mice. Loss of Tsc2 causes mTORC1 pathway activation and the formation of striatal hamartomas composed of ectopic clusters of abnormal cells and cytomegalic neurons including within the OB. Neural phenotypes in this model can be partially rescued with Rapalink-1, a bisteric mTOR inhibitor, demonstrating the importance of mTOR in pathogenesis. We previously demonstrated that global V-SVZ neural stem cell (NSC) Tsc2 mutation induced by nestin-CRE-ERT2 causes transcriptional and translational errors that are accompanied by mTORC1 pathway activation. While we previously described the OB granule cells from these mice, we did not thoroughly describe changes outside this region. Here we provide evidence that removal of Tsc2 from neonatal V-SVZ NSCs causes subtle and rare brain malformations. This is exemplified by ectopic clusters of cytomegalic neurons or glia and mTORC1 activation. Heterotopic clusters retain elevated levels of NSC proteins including Sox2, supporting a model wherein loss of Tsc2 leads to excessive mTORC1, persistent expression of stem proteins, and the inability of a subset of cells to differentiate at the appropriate time. These data support the use of this model to study TSC, but the rarity and stochastic nature of lesions make the use challenging for identifying mechanisms and testing therapies.