Hamartin and tuberin function as a complex which is involved in the control of cell growth and cell division. (The complex appears to be a Rheb GTPase which suppresses mTOR signalling, part of the growth factor (insulin) signalling pathway.) Thus, mutations at the TSC1 and TSC2 loci result in a loss of control of cell growth and cell division, and therefore a predisposition to forming tumors.TSC affects tissues from different germ layers. Cutaneous and visceral lesions may occur, including adenoma sebaceum, cardiac rhabdomyomas, and renal angiomyolipomas. The central nervous system (CNS) lesions seen in this disorder include hamartomas of the cortex, hamartomas of the ventricular walls, and subependymal giant cell tumors, which typically develop in the vicinity of the foramina of Monro.
Molecular genetic studies have defined at least two loci for TSC. In TSC1, the abnormality is localized on chromosome 9q34, but the nature of the gene protein, called hamartin, remains unclear. No missense mutations occur in TSC1. In TSC2, the gene abnormalities are on chromosome 16p13. This gene encodes tuberin, a guanosine triphosphatase–activating protein. The specific function of this protein is unknown. In TSC2, all types of mutations have been reported; new mutations occur frequently. Few differences have yet been observed in the clinical phenotypes of patients with mutation of one gene or the other.
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