Female germline mosaicism in tuberous sclerosis confirmed by molecular genetic analysis

We have investigated a family in which three siblings with the autosomaldominant disorder tuberous sclerosis had unaffected parents. The familywere typed for polymorphic markers spanning the two genes known to causetuberous sclerosis located at 9q34 (TSC1) and 16p13.3 (TSC2). TSC1 markersshowed different maternal and paternal haplotypes in affected children,excluding a mutation in TSC1 as the cause of the disease. For the TSC2markers all the affected children had the same maternal and paternalhaplotypes, as did three of their unaffected siblings. Mutation screeningby RT-PCR and direct sequencing of the TSC2 gene identified a 4 bpinsertion TACT following nucleotide 2077 in exon 18 which was present inthe three affected children but not in five unaffected siblings or theparents. This mutation would cause a frameshift and premature terminationat codon 703. Absence of the mutation in lymphocyte DNA from the parentswas consistent with germline mosaicism and this was confirmed by ourfinding of identical chromosome 16 haplotypes in affected and unaffectedsiblings, providing unequivocal evidence of two different cell lines in thegametes. Molecular analysis of the TSC2 alleles present in the affectedsubjects showed that the mutation had been inherited from the mother. Thisis the first case of germline mosaicism in tuberous sclerosis proven bymolecular genetic analysis and also the first example of female germlinemosaicism for a characterized autosomal dominant gene mutation apparentlynot associated with somatic mosaicism.