The goal of this page is to highlight some of the recent scientific findings on TSC. Currently, a large number of scientists are working on understanding the cellular function of the TSC1 and TSC2 gene products. Here, we will make an effort to translate their groundbreaking findings and emphasize how they may help those with TSC.
What are the TSC genes and gene products?
TSC1 and TSC2 are two distinct genes located on two different chromosomes. Each of these genes gives rise to a protein called hamartin and tuberin, respectively. Within a cell, these two proteins bind to one another and work together. That is why a mutation in the tuberin gene and a mutation in the hamartin gene results in a similar disease in people. The loss of either tuberin or hamartin results in the formation of numerous benign tumors (hamartomas). Thus, the TSC1 and TSC2 genes are tumor suppressor genes. Until recently, the precise functions of these two genes were unclear. Research published since 2002 has provided evidence that tuberin and hamartin are involved in a key pathway in the cell that regulates protein synthesis and cell size.
How do cells naturally regulate growth?
One of the ways that cells use to regulate their growth is by controlling the rate of protein synthesis. A protein called mTOR was identified as one of the master regulators of cell growth. mTOR, in turn, is controlled by rheb, a small G-protein (also known as GTPase). When rheb is activated, the protein synthesis machinery is turned on, most likely via mTOR, and the cell grows in size.
How are tuberin and hamartin involved in cell growth?
Tuberin and hamartin were recently recognized as regulators of the rheb - mTOR - protein synthesis pathway. Several groups have confirmed that tuberin and hamartin work together to inhibit rheb function, which then leads to the inactivation of the protein synthesis machinery. Therefore, under normal circumstances tuberin and hamartin keep cell growth under check. If you do not have tuberin or hamartin, cells grow beyond their normal size, leading to TSC.
Akt and AMPK-dependent regulation of TSC genes.
Previous studies have shown that one of the major cellular functions of the TSC1/TSC2 protein complex is to inhibit protein synthesis. Growth factors such as insulin activate an enzyme called Akt, which inhibits TSC1/TSC2 function and thus increases protein synthesis. A paper in the journal Cell demonstrated that the TSC1/TSC2 complex is also regulated by cellular energy levels (Inoki et al., Cell 2003). This is due to the function of another enzyme called AMPK, also known as the "cellular fuel gauge." When the energy levels in the cell drop, AMPK is active and in turn activates TSC2. This inhibits protein synthesis in response to energy deprivation.
So far, two different pathways (Akt and AMPK) that regulate TSC1/TSC2 have been identified. They both use phosphorylation at different sites on TSC2 to regulate the function of TSC2 gene products. These findings not only shed light on the molecular basis of cellular energy metabolism, but also provide a better understanding of the role of TSC1/TSC2 proteins in the cell. In turn, such an understanding may ultimately provide a context for the development of rational therapeutic interventions for TSC. At the same time, each new report raises new questions to be addressed: How do the two pathways (Akt and AMPK) interact in cells? What is the role of energy metabolism in patients with TSC?
Do treatments such as the ketogenic diet affect cellular functions of TSC1/TSC2?
What is being developed to prevent TSC?
The importance of these findings is that they identify new targets for therapeutic intervention in abnormal cell growth. In TSC, when tuberin/ hamartin are not functioning normally, rheb and mTOR become overly active. It is now believed that if we can find ways to block the activity of rheb and/or mTOR, we may be able to stop the abnormal growth of cells seen in TSC. In fact, clinical trials are now underway using an inhibitor of mTOR, a drug called rapamycin. Rapamycin (sirolimus) is a commercially available immunosuppressant shown to inhibit the ability of mTOR to phosphorylate downstream targets, such as S6K and 4EBP. Since mTOR has been show to be constitutively active when there is a mutation in TSC1 or TSC2, an mTOR inhibitor is a novel treatment option for TSC. Preliminary trials using rapamycin in TSC patients are promising. One would expect similar efforts to be made to inhibit rheb in the future. Once again, these studies reinforce the belief that our ability to develop novel treatment options for TSC depends on advancing our understanding of the cellular functions of the tuberin and hamartin proteins.
Rapamycin-induced regression of SEGAs in TSC
Five to 15% of individuals affected with TSC present with subependymal giant cell astrocytomas (SEGAs), low-grade astrocytomas that usually do not respond to radiation therapy or chemotherapy. Since their presence can produce significant neurological effects due to the production of hydrocephalus, mass effect and their characteristic position near the foramen of Monro, resection is usually the best treatment of problematic SEGAs. A recent study by David Franz and colleagues (Franz et al. Ann Neurol, 2006) suggests that therapy with oral rapamycin may induce regression of SEGAs. Five individuals with clinically definite TSC were treated with oral rapamycin at standard immunosuppressive doses (serum levels 5-15ng/ml) for several months and all lesions exhibited regression, and one, necrosis. These lesions regrew in one patient whose therapy was interrupted, only to regress again when therapy was resumed. While these findings are preliminary, they strongly imply the efficacy of rapamycin in the treatment of SEGAs in patients with TSC. Rapamycin is usually well tolerated in patients, although its use does have reported side-effects, including aphthous ulcers, acne-like rash, diarrhea, impaired wound healing, hypercholesterolemia and arthralgias. The use of rapamycin in patients with TSC may also be complicated by the frequent use of anti-epileptic drugs, which may decrease the potency of rapamycin. Nonetheless, these initial findings indicate rapamycin as a potential therapy for TSC patients in the future.
Rapamycin-insensitive roles of mTOR
Rapamycin is a known inhibitor of mTOR, and is a potential therapeutic treatment of TSC, as well as a current treatment used in some cancer therapies. Studies from two groups (David Sabatini at MIT and Michael Hall at Basel, Switzerland) indicate that mTOR may also have some functions in the cell that are not inhibited by rapamycin. In this rapamycin-independent form, mTOR seems to regulate cellular skeleton, not protein synthesis. It is not yet clear whether the TSC1/TSC2 complex is also involved in these rapamycin-independent functions of mTOR. However, if it is, one may have to find other inhibitors in addition to rapamycin to reverse the effects of TSC1 or TSC2 mutations.
Role of TSC1 and TSC2 in neuronal morphology and neurological symptoms of TSC
A recent paper by Bernardo Sabatini and colleagues (Tavazoie et al. Nature Neuroscience, 2005) shows a role for the TSC pathway in regulating growth and synapse function in neurons, disturbances of which likely contribute to the pathogenesis of the neurological symptoms seen in TSC patients. They showed that loss of the Tsc1/Tsc2 complex in mice resulted in multiple changes in neuronal morphology, including increased soma and dendritic spine size, and decreased dendritic spine density. Treatment of neurons deficient in Tsc1 or Tsc2 with rapamycin restored neuronal soma and dendritic spine head sizes to normal levels, suggesting that overexpression of mTOR may be responsible for the aberrant neuronal morphology seen in TSC patients. TSC patients often present with altered cortical structure, especially tubers, disorganized regions of the brain that have distorted lamination with increased numbers of astrocytes and sparse neurons. Traditionally, it has been thought that disruptions in cortical architecture are responsible for the neurological deficits seen in TSC patients, chiefly due to the strong correlation seen between the number of cortical tubers in a patient and severity of seizures. These new findings, however, suggest that neuronal defects in individual neurons may also contribute to the neurological symptoms seen in TSC patients. This could have implications in the treatment of seizures and other neurological symptoms of TSC.
Are tubers different from focal cortical dysplasias?
Two papers in the October 2004 issue of Annals of Neurology (Peter Crino's group at University of Pennsylvania and Harry Vinters' group at UCLA) address the question of whether mTOR pathway activation is specific to tuberous sclerosis or whether it is also found in isolated focal cortical dysplasias commonly found in patients with epilepsy. Unlike TSC, focal cortical dysplasia is a sporadic and not a genetically inherited condition. The authors asked whether the balloon cells in focal cortical dysplasia and the giant cells in tubers of the tuberous sclerosis complex (TSC) share immunoreactivity for the same antigens. They found that phospho-S6 is present in both balloon cells and giant cells, but phospho-S6kinase is specific for giant cells of tuberous sclerosis specimens. These data suggest that while focal cortical dysplasias and tubers associated with TSC look similar pathologically, they are probably formed via distinct molecular mechanisms.
Unusually mild form of TSC, associated with TSC2 R905Q mutation
A recent paper by Jansen and colleagues (Jansen et al. Ann Neurol 2006) describes an unusually mild form of tuberous sclerosis associated with a missense mutation at amino acid position 905, a relatively common mutation in the TSC2 gene. Individuals with this TSC2 R905Q mutation, where a basic amino acid, arginine, is replaced with a neutral amino acid, glutamine, at the 905th residue, present with a much milder form of the disease, characterized most often by hypomelanotic macules or focal seizures that respond well to medication or remit spontaneously. These findings may have implications for a large number of patients who present with limited clinical symptoms of TSC and may change the way genetic counseling is conducted in these individuals.
