Dysregulated mTOR signaling and tissue-specific phenotypes in Tuberous Sclerosis Complex

Abstract

Tuberous Sclerosis Complex (TSC) is a multi-organ hamartomatous disease caused by loss of function mutations in either the TSC1 or TSC2 genes. Despite involvement of multiple organs such as the kidneys, lungs, and skin, neurological aspects are usually the most severe due to a very high prevalence of cognitive impairment, autism and epilepsy. The protein products of TSC1 and TSC2, hamartin and tuberin respectively, regulate the mTOR kinase signaling pathway. Current models of TSC propose that hamartoma formation is secondary to a loss of heterozygosity at either the TSC1 or TSC2 loci, and subsequent hyperactivation of mTOR Complex 1 (mTORC1). In this dissertation I explore the underlying mechanisms of organ specific pathogenesis in TSC.

In the first half of my dissertation, I demonstrate that loss of Tsc1 in the distal convoluted tubule of the kidney results in cystogenesis. Cyst formation in these kidneys is due to a mTORC1 but not mTORC2 dependent process. I then show that cystic changes in these kidneys may be due to ciliary defects.

While a loss of heterozygosity has clearly been reported in the kidney and other organ system, second hit mutations in neural lesions have only rarely been identified. Thus, to begin to define the role of the heterozygosity of TSC1 or TSC2 during the pathogenesis of TSC in the brain, we generated induced pluripotent stem cells (iPSC) from patients with TSC. Deep sequencing of these patents revealed that all of our patient derived lines are heterozygous for TSC2 mutations. I then provide evidence that these heterozygous iPSCs are abnormal with increased cell survival and enhanced maintenance of pluripotency. These changes may be due to slight changes in mTORC1 signaling.

The work presented in this dissertation increases our understanding of the tissue specific phenotypes and underlying mechanisms of TSC pathogenesis. This research may lead to the identification of new therapeutic targets for TSC and associated comorbidities.