PhD student University of Melbourne, Victoria, Australia
This abstract has been invited to present during the Better Patient Outcomes through Diversity Platform poster session
Rationale: Tuberous sclerosis complex (TSC) is a genetic disorder associated with neurological, renal, dermatological and other anomalies. The majority of patients have germline mutations in TSC1 or TSC2 genes, but 10–15% patients have no variants identified by clinical genetic testing. Previous studies suggest mutation-negative TSC patients may have low-frequency mosaicism missed by routine testing. We studied 32 patients with mild TSC where germline heterozygous pathogenic variants were reported as absent, and searched for low-level mosaic pathogenic variants in DNA derived from different tissues. Methods: Blood was collected from all patients, with skin, urine, saliva, buccal and brain tissue samples also obtained for a subset. High-depth TSC1/2 gene panels (1000x-2000x) were utilized to detect candidate variants. Droplet digital PCR (ddPCR) assays were designed to validate candidate variants and test the level of mosaicism including in different tissues. Results: We found candidate variants in 21/32 (66%) patients, of which 16/21 (76%) were mosaic variants undetectable on initial germline analyses, and 5/21 (24%) were heterozygous germline variants missed on prior clinical screening. For the 16 mosaic patients, the variant allele frequency (VAF) varied from 2%-30% with 13 variants being below 20%. Most of the 16 mosaic variants (n=14, 88%) were in the TSC2 gene, while only 2 (12%) were in the TSC1 gene. In 3 patients with mosaic variants we determined variant allele frequency in different tissues: one patient had mosaicism at a similar level in different tissues (1.3-2.2% VAF); in the two others the mosaicism was more variable between tissues (1.1-5.2% VAF and 0.06-2.5% VAF, respectively). Conclusions: Our findings confirm that low-level mosaicism in TSC1/2 are responsible for a significant proportion of mild TSC cases. The level of mosaicism varies between tissues derived from different germ layers likely correlating with the timing of mutagenesis. Importantly, mosaic variants, especially low level ones, are not normally reported from clinical labs. These findings have important implications for genetic and reproductive counseling. Funding: Please list any funding that was received in support of this abstract.: This study is supported by a Sanming Project of Medicine in Shenzhen, China (SZSM201812005) to J.L. and I.E.S., and National Health and Medical Research Council Program Grant (1091593) to I.E.S. and S.F.B., a Project Grant (1129054) to S.F.B., a Project Grant (1079058) to M.S.H., a Practitioner Fellowship (1006110) to I.E.S., a Senior Research Fellowship (1102971) to M. B., and a R.D Wright Career Development Fellowship (1063799) to M.S.H.