New developments in the neurobiology of the tuberous sclerosis complex

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Abstract
Objective: To outline recent developments in the neurobiology of the tuberous sclerosis complex (TSC). Background: TSC may be associated with neuropsychiatric disorders including epilepsy, mental retardation, and autism. The uncontrolled growth of subependymal giant cell astrocytomas may lead to hydrocephalus and death. The recent identification of mutations in two genes (TSC1 and TSC2) that cause TSC has led to rapid progress in understanding the molecular and cellular pathogenesis of this disorder. How distinct mutations lead to the varied clinical phenotype of TSC is under intense investigation. Results: We report the recent diagnostic criteria for TSC and provide an overview of the molecular genetics, molecular pathophysiology, and neuropathology of TSC. Important diagnostic criteria for TSC include facial angiofibromas, ungual fibromas, retinal hamartomas, and cortical tubers. Both familial and sporadic TSC cases occur. Approximately 50% of TSC families show genetic linkage to TSC1 and 50% to TSC2. Among sporadic TSC cases, mutations in TSC2 are more frequent and often accompanied by more severe neurologic deficits. Multiple mutational subtypes have been identified in the TSC1 and TSC2 genes. The TSC1 (chromosome 9) and TSC2 (chromosome 16) genes encode distinct proteins, hamartin and tuberin, respectively, which are widely expressed in the brain and may interact as part of a cascade pathway that modulates cellular differentiation, tumor suppression, and intracellular signaling. Tuberin has a GTPase activating protein–related domain that may contribute to a role in cell cycle passage and intracellular vesicular trafficking. Conclusion: Identification of tuberous sclerosis complex (TSC) gene mutations has fostered understanding of how brain lesions in TSC are formed. Further characterization of the roles of hamartin and tuberin will provide potential therapeutic avenues to treat seizures, mental retardation, and tumor growth in TSC.