(Epi)genetic changes in Lynch syndrome-associated adenomas with intact mismatch repair protein expression and in recurrent adenomas

Satu Mäki-Nevala1, Satu Valo1, Ari Ristimäki1,2, Virinder Sarhadi1, Sakari Knuutila1, Minna Nyström1, Maarit Ahtiainen3, Toni Seppälä2, Laura Renkonen-Sinisalo2, Anna Lepistö2, Jukka-Pekka Mecklin4,3, Päivi Peltomäki1

1University of Helsinki, Helsinki, Finland. 2Helsinki University Hospital, Helsinki, Finland. 3Central Finland Central Hospital, Jyväskylä, Finland. 4University of Jyväskylä, Jyväskylä, Finland


Objectives: DNA mismatch repair (MMR) defects generally drive colorectal tumorigenesis in Lynch syndrome (LS). Some LS-associated colorectal adenomas have proficient MMR, i.e. intact MMR protein expression and/or microsatellite stable (MSS) phenotype, suggesting the involvement of other mechanisms in tumorigenesis. Our aim was to investigate DNA methylation changes and somatic mutations in LS-associated colorectal adenomas retaining MMR protein expression. Moreover, studies are underway to address these molecular mechanisms as possible contributors to excessive polyposis present in some LS patients.

Methods: MMR-deficient (MMR-D, n=49) and MMR-proficient (MMR-P, n=18) adenomas from LS patients as well as adenomas & carcinomas (n=17) derived from a LS patient with a history of recurrent adenomas were assayed by methylation-specific multiplex ligation-dependent probe amplification and targeted sequencing.

Results: Promoter methylation of CpG island methylator phenotype (CIMP)-associated marker genes and selected CRC-associated tumor suppressor genes (TSGs) increased, and LINE-1 methylation decreased from normal mucosa to MMR-P adenomas to MMR-D adenomas. Methylation differences were statistically significant when either adenoma group was compared with normal mucosa, but not between MMR-P and MMR-D adenomas. Significantly increased methylation was found in multiple CIMP marker genes (IGF2, NEUROG1, CRABP1, CDKN2A) and TSGs (SFRP1, SFRP2) in MMR-P adenomas already. Furthermore, certain CRC-associated somatic mutations, such as KRAS, were prevalent in MMR-P adenomas.

Conclusions: DNA methylation changes and somatic mutations of cancer-associated genes might serve as an alternative pathway accelerating, or possibly initiating, LS-associated tumorigenesis in the presence of proficient MMR.