The GAP-related domain of tuberin, the product of the TSC2 gene, is a target for missense mutations in tuberous sclerosis

Tuberous sclerosis is an autosomal dominant trait in which thedysregulation of cellular proliferation and differentiation results in thedevelopment of hamartomatous growths in many organs. The TSC2 gene is oneof two genes determining tuberous sclerosis. Inactivating germlinemutations of TSC2 in patients with tuberous sclerosis and somatic loss ofheterozygosity at the TSC2 locus in the associated hamartomas indicate thatTSC2 functions as a tumour suppressor gene and that loss of function iscritical to expression of the tuberous sclerosis phenotype. The TSC2product, tuberin, has a region of homology with the GTPase activatingprotein rap1GAP and stimulates the GTPase activity of rap1a and rab5a invitro. Here we show that the region of homology between tuberin and humanrap1GAP and the murine GAP mSpa1 is more extensive than previously reportedand spans approximately 160 amino acid residues encoded within exons 34-38of the TSC2 gene. Single strand conformation polymorphism analysis of theseexons in 173 unrelated patients with tuberous sclerosis and directsequencing of variant conformers together with study of additional familymembers enabled characterisation of disease associated mutations in 14cases. Missense mutations, which occurred in exons 36, 37 and 38 wereidentified in eight cases, four of whom shared the same recurrent changeP1675L. Each of the five different missense mutations identified was shownto occur de novo in at least one sporadic case of tuberous sclerosis. Thehigh proportion of missense mutations detected in the region of the TSC2gene encoding the GAP-related domain supports its key role in theregulation of cellular growth.