Lynch syndrome (LS) is the most prevalent cancer predisposition syndrome in which germline mutation in one of four DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2 causes significantly increased lifetime risks of colorectal, endometrial, ovarian, and other cancers. Whether or not breast cancer is part of LS tumor spectrum is currently debated. Moreover, the reasons for organ selectivity in germline mutation carriers are unknown. We have reported that breast carcinoma from LS individuals resembles common breast carcinoma in many respects, but differs with respect to genomic instability (Lotsari et al. 2012, Cancer Res.). To extend these preliminary observations, we are now carrying out an in-depth investigation into the spectra of somatic mutations in LS and sporadic breast carcinomas. All available breast carcinomas among 300 LS families from the nation-wide Hereditary Colon Cancer Registry of Finland have been identified and the tumors investigated for MMR status, DNA methylator phenotype, mechanisms of two-allele inactivation for MMR genes, and somatic mutation status. For the latter, we use massive parallel sequencing as described in our recent publication (Porkka et al. 2017, Oncotarget). To define the unique vs. shared molecular features of LS breast tumorigenesis, profiles of somatic alterations in LS breast carcinomas are compared to those of established LS spectrum carcinomas (colorectal and ovarian) as well as sporadic breast carcinomas with sequence information available from large electronic data-sets. Of 14 LS breast carcinoma samples, 9 (64 %) were MMR-deficient and 5 (36 %) MMR-proficient. This contrasts with LS colorectal and ovarian carcinomas, all of which were MMR-deficient (p=0.0012). MMR-deficient LS breast carcinomas harbored an average of 783 non-synonymous mutations and MMR-proficient tumors 555 mutations (statistically non-significant). In contrast to colorectal and ovarian carcinomas, in which loss of heterozygosity (LOH) or somatic MMR gene mutation provided a second hit consistent with two-hit inactivation in 89 % (25/28) of the tumors, a detectable second hit (mainly LOH) was present in only 43 % (6/14) of breast carcinomas (p=0.0025). Genes harboring high-frequency mutations in at least one-third of tumors were regarded candidates for driver genes. There were 133 such genes in LS colorectal carcinomas, 10 in ovarian carcinomas, and 18 in breast carcinomas. Genes involved in epigenetic regulation were significantly enriched in all three tumor types. In addition, a significant enrichment of NOTCH signaling associated genes characterized LS breast carcinomas. This research is likely to shed light to the mechanisms of organ-specific cancer susceptibility in germline carriers of MMR gene mutations. Our results are expected to guide the surveillance and other clinical management of LS individuals with breast carcinoma.

Citation Format: Noora Porkka, Alisa Olkinuora, Satu Mäki-Nevala, Samuli Eldfors, Päivi Peltomäki. Tumorigenesis in Lynch syndrome: Somatic mutation profiles compared to sporadic counterparts [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 5369.