Polyps and colorectal cancer in serrated polyposis syndrome: contribution of the classical adenoma-carcinoma and serrated neoplasia pathways

INTRODUCTION: Patients with serrated polyposis syndrome (SPS) have an increased risk to develop colorectal cancer (CRC). Due to an abundance of serrated polyps, these CRCs are assumed to arise mainly through the serrated neoplasia pathway rather than through the classical adenoma-carcinoma pathway. We aimed to evaluate the pathogenetic routes of CRCs in patients with SPS.METHODS: We collected endoscopy and pathology data on CRCs and polyps of patients with SPS under treatment in our center. Our primary end point was the proportion of BRAF^V600E mutated CRCs, indicating serrated pathway CRCs (sCRCs). CRCs lacking BRAF^V600E most likely inferred a classical adenoma-carcinoma origin (aCRCs). We assessed patient, polyp, and CRC characteristics and stratified for BRAF^V600E mutation status.RESULTS: Thirty-five patients with SPS harbored a total of 43 CRCs. Twenty-one CRCs (48.8%) carried a BRAF^V600E mutation, 10 of which lacked MLH1 staining and 17 (81%) were located in the proximal colon. Twenty-two CRCs (51.1%) did not carry a BRAF^V600E mutation and were MLH1 proficient. Of these 22 putatively aCRCs, 17 (77.3%) were located distally and one-third (36.4%) harbored a pathogenic KRAS or NRAS mutation. In patients with BRAF^wt-CRCs, a higher ratio of the median number of conventional adenomas versus serrated polyps was found (4 vs 13) than patients with BRAF^V600E-CRCs (1 vs 14).DISCUSSION: Our study indicates that in patients with SPS, the ratio of sCRCs:aCRCs on average is 50:50. This elevated sCRC:aCRC ratio in patients with SPS, when compared with non-SPS patients, correlates well with the differences in the ratios of the numbers of sessile serrated lesions and conventional adenomas in patients with SPS and non-SPS patients, respectively.