Abstract 3081: Methylation changes and somatic mutations as early events in Lynch syndrome-associated colorectal cancer

Lynch syndrome (LS) is a hereditary cancer predisposition syndrome caused by germline mutations in the DNA mismatch repair (MMR) genes resulting in an increased risk of colorectal cancer (CRC) and other malignancies. Those germline defects with other genetic and epigenetic changes accelerate multistep tumorigenesis. However, early events leading to polyp formation and the timing and order of the molecular “hits” remain unknown. Studies on sporadic CRC have indicated that promoter hypermethylation leading to gene silencing can act as an alternative mechanism to mutations at early stages of tumorigenesis, but its importance in hereditary CRC is obscure. In this study we aimed to define 1) methylation changes that occur at different stages of LS-associated colorectal tumor progression, and 2) somatic mutations that occur in adenomas comparing the status of low- and high-grade dysplasia and MMR deficiency (MMR-D) and proficiency (MMR-P). Colorectal biopsies including normal mucosae, adenomas and carcinomas were prospectively collected from 104 LS patients during colonoscopy surveillance, supplemented with retrospective tumor specimens from 56 patients. Promoter methylation was analyzed using the methylation-specific multiplex ligation-dependent probe amplification test (MS-MLPA) including selected tumor suppressor genes (TSGs) associated with early colon oncogenesis. Hypermethylation tendency was evaluated based on the frequency of CpG island methylator phenotype (CIMP) according to the methylation status of eight established CIMP marker genes. LINE-1 methylation was studied as a surrogate marker for global hypomethylation. Immunohistochemistry was used to detect MMR protein expression in neoplastic lesions. As an ongoing study mutation status of adenomas are studied using the Ion AmpliSeq TM Colon and Lung Cancer Panel targeting mutational hot spots in 22 genes. Results indicate that the expression of the MMR protein corresponding to the gene mutated in the germline decreases along with dysplasia but occurs as a relatively late event in the tumor progression, suggesting the presence of other somatic events that drive neoplastic transformation. Taken together, methylation of TSGs and frequency of CIMP increased and LINE-1 methylation decreased in adenomas and carcinomas along with dysplasia. However, a significant increase of methylation in some genes was detected already in adenomas with low-grade dysplasia compared to normal mucosae. In addition, a proportion of low-grade adenomas could already be classified CIMP positive. When comparing MMR-P and MMR-D adenomas, LINE-1 methylation decreased along the loss of MMR protein expression, and interestingly, higher methylation of SFRP1 was observed in MMR-P adenomas compared to MMR-D cases and normal mucosae. These findings emphasize the importance and early appearance of epigenetic changes in LS-associated tumorigenesis. Citation Format: Satu Maki-Nevala, Satu Valo, Ari Ristimaki, Laura Renkonen-Sinisalo, Anna Lepisto, Jukka-Pekka Mecklin, Paivi Peltomaki. Methylation changes and somatic mutations as early events in Lynch syndrome-associated colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3081.