Abstract 3534: The study of Lynch syndrome in a special population reveals a strong founder effect and an unusual mutational mechanism in familial adenomatous polyposis

Abstract

Abstract Lynch syndrome (LS) is the leading cause of hereditary colorectal cancer (CRC). Germline mutations in DNA mismatch-repair (MMR) genes (MLH1, PMS2, MSH2, and MSH6) are known to cause LS in the heterozygous state. Biallelic mutations in these genes cause mismatch repair cancer syndrome (MMRCS), a very rare childhood cancer syndrome that predisposes to CRC among other cancer types. In this study, we aimed to characterize the mutational landscape of LS in a large CRC cohort (n=1,207) from a population with high rates of consanguinity. The overall contribution of germline mutations in LS gene was comparable to that observed in outbred populations (26/1207). Remarkably, however, we also observed the unusual homozygous occurrence of a founder PMS2 mutation. The homozygous individual presented with massive adenomatous polyposis of the colon at age 8 years, and mutation analysis of multiple polyps revealed the occurrence of multiple independent somatic APC mutations. Our study highlights a previously unrecognized role of LS genes in triggering APC-driven tumors, a phenomenon facilitated by consanguinity. Citation Format: Abdul K. Siraj, Sandeep K. Parvathareddy, Rong Bu, Tariq Masoodi, Khadija Alobaisi, Maha Alrasheed, Saeeda O. Ahmed, Zeeshan Qadri, Padmanaban Annaiyappanaidu, Allianah Benito, Dionne R. Rala, Fowzan Al-Kuraya, Khawla S. Al-Kuraya. The study of Lynch syndrome in a special population reveals a strong founder effect and an unusual mutational mechanism in familial adenomatous polyposis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3534.