Colorectal carcinoma occurring via the adenoma–carcinoma pathway in patients with serrated polyposis syndrome

Background Although serrated polyposis syndrome (SPS) is associated with an increased colorectal cancer (CRC) risk, the carcinogenic mechanisms remain unknown. We investigated clinicopathological characteristics and genetic abnormalities in colorectal polyps and CRC to elucidate carcinogenic mechanisms in SPS. Methods We retrospectively analyzed clinicopathological features of colorectal polyps in 44 SPS patients, and examined mutations of genes including APC, RAS, BRAF, and TP53, and microsatellite instability (MSI) in CRC tissues. Results Of the 44 patients, 25 (56%) fulfilled WHO criterion 1, 11 (25%) fulfilled criterion 2, and 8 (18%) fulfilled both. A total of 956 polyps were observed; 642 (67%) hyperplastic polyps (HP), 204 (21%) sessile serrated lesions (SSL), 10 (1%) traditional serrated adenoma (TSA), and 100 (11%) adenomas. The median numbers of polyps (/patient) were 10.5 (IQR 2.75–23) HPs, 4.0 (2.0–6.0) SSLs, 0 (0–0) TSA, and 1 (0–3.3) adenoma. SSL and HP located preferentially in the proximal and distal colon, respectively. Twenty-two CRCs were found in 18 patients. Based on the histological coexistence of SSL/TSA, BRAF mutation and MSI, 5 CRCs (26%) were classified as serrated-neoplasia pathway. Conversely, based on the coexistence of adenoma, APC/RAS and TP53 mutations, 11 CRCs (58%) were classified as adenoma–carcinoma pathway. The remaining three were unclassifiable. Most CRCs through adenoma–carcinoma pathway were located in the left-side colorectum and patients bearing those met criterion 2, characterized by many HP and advanced adenomas. Adenoma was a significant risk factor for CRC. Conclusions Our results suggest that more than half of the CRCs, particularly those in the left-side colorectum, developed through the adenoma–carcinoma pathway in SPS patients. Adenoma was a risk factor for CRCs, suggesting its importance in colorectal carcinogenesis.