517 Disseminated Disease Despite Low Mutant Allele Fractions: the Variable Phenotype of Mosaic Tuberous Sclerosis Complex

Mosaicism has been suggested to explain tuberous sclerosis complex (TSC) manifesting as unilateral angiofibromas or as mild disease and no mutation identified using standard genetic testing. To identify mosaicism and its phenotypic spectrum in TSC, we performed next generation sequencing (NGS) on DNA isolated from TSC skin tumors, including angiofibromas, ungual fibromas, shagreen patch, oral fibroma, and fibrous cephalic plaque. Mosaicism was identified in 17/30 patients, first by identification of mutations in TSC2 in DNA from 28 skin tumors from the 17 patients, of which 16 had two mutations consistent with Knudson’s two-hit hypothesis. The mutant allele fractions in whole tumors or cultured tumor cells ranged from 1 to 54%. A shared mutation in TSC2 was identified in every case where more than one tumor was studied from a single patient. The median mutant allele fraction was 5% in blood samples (range 0-19, n=11) and 1.4% in other control tissues (range 0-13, n=13), well below the 50% allelic fraction expected for germline mutations. Examination of these mosaic adult women with TSC revealed that 4 had unilateral or asymmetric angiofibromas, 3 had fewer than 20 angiofibromas, and 10 had numerous bilateral facial angiofibromas. Overall these results show that: 1) analysis of DNA from whole skin tumors or cultured tumor cells enables identification of low-level mosaicism using NGS, and 2) the phenotype of mosaic TSC patients varies from those with unilateral angiofibromas and mild disease to those with a disseminated phenotype that may be indistinguishable from those with germline mutations.