Analysis of the basic genetic defect in tuberous sclerosis would be greatly expedited by definitive determination of the chromosomal location of the TSC gene or genes.
The identification of the TSC1 gene on chromosome 9q, along with functional studies and mutational analyses of both TSC genes, will likely provide fascinating insights into the pathogenesis of TSC.
Analysis of TSC-associated hamartomas has shown loss of heterozygosity for the regions of chromosomes 9 and 16 known to harbour TSC genes, consistent with the occurrence of somatic 'second-hit' mutations.
A proportion of hamartomas from patients with TSC show loss of heterozygosity (LOH) for DNA markers in the region of either the TSC1 gene on chromosome 9q34 or the TSC2 gene on 16p13.3.
Tuberous sclerosis complex is an autosomal dominant disorder with loci on chromosome 9q34 (TSC1) and chromosome 16p13.3 (TSC2).The TSC2 gene has been isolated.
To investigate the molecular mechanisms of tuberous sclerosis (TSC) histopathologic lesions, we have tested for loss of heterozygosity the two TSC loci (TSC1 and TSC2) and seven tumor suppressor gene-containing regions (TP53, NF1, NF2, BRCA1, APC, VHL, and MLM) in 20 hamartomas from 18 TSC patients.
These data support the hypothesis that both the TSC genes act as tumour suppressors and that the manifestations of TSC in patients with germline TSC mutations rise from "second hit" somatic mutations inactivating the remaining normal copy of the TSC gene.
TSC has been shown to be genetically heterogeneous, with one causative gene mapping to chromosome 9q (denoted TSC1) and at least one other gene on chromosome 16p (denoted TSC2).The TSC2 gene was recently cloned.
The recently reported loss of heterozygosity (LOH) at the regions of the TSC1 or TSC2 locus in hamartomas obtained from different organs of patients with established tuberous sclerosis, including cortical tubers, stimulated us to examine epilepsy-associated tuberous sclerosis-like glioneuronal malformations with respect to LOH at the TSC1 and TSC2 loci of chromosomes 9q34 and 16p 13.3, respectively.
We conclude that there is a reduced risk of mental retardation in TSC1 as opposed to TSC2 disease and that consequent ascertainment bias, at least in part, explains the relative paucity of TSC1 mutations in sporadic TSC.
Diagnostic testing will be difficult because of the genetic heterogeneity of TSC (which has at least two causative genes: TSC1 and TSC2), the large size of the TSC2 gene, and the variety of mutations.
Because the 9q breakpoint was located in the same region as the tuberous sclerosis type I locus (TSC1), which is associated with renal tumors, we performed FISH with two TSC1 flanking cosmids that were mapped proximal to the 9q breakpoint, thus excluding its involvement.