Serrated adenoma.

Serrated adenomas are polypoid lesions present in the colon that are characterized by saw-toothed or serrated crypts with dysplasia, and are distinguished from classical adenomas and hyperplastic polyps by their histologic appearance. Serrated adenomas were recognized as a distinct entity in 1990, and have also been referred to as mixed hyperplastic adenomatous polyps. It has been reported that serrated adenomas make up ∼1% to 2% of all colorectal polyps. Serrated adenomas have been reported to have infrequent mutations of APC, frequent methylation of the promoters of putative tumor suppressor genes, and other genetic alterations. There has not been a systematic search for patterns of genomic instability in these lesions, and the relevance of these lesions to a multistep carcinogenesis pathway remains uncertain. Hyperplastic polyps are generally regarded as nonneoplastic lesions, however, it has been reported that some of these polyps have hyperproliferative activity, overexpression of p53, some form of genomic instability, and K-ras mutations. Serrated adenomas and hyperplastic polyps have some similarities in their genetic mutational signatures, including frequent 18q allelic imbalance and infrequent 5q allelic imbalance. Serrated adenomas have histologic features that resemble hyperplastic polyps, such as the saw-toothed or serrated crypts. Several reports have noted the histologic and genetic similarities between hyperplastic polyps and serrated adenomas. Serrated adenomas are significantly more likely to have an LOH event on 18q and are significantly less to experience allelic imbalance at 5q, or have K-ras mutations, compared with other adenomatous polyps in the colon. Serrated adenomas may evolve through a different genetic pathway than other types of adenomas in the colon.

Am J Surg Pathol. 1990 Jun;14(6):524–537.