Colorectal polyps with extensive absorptive enterocyte differentiation: histologically distinct variant of hyperplastic polyps

An update on the morphology and molecular pathology of serrated colorectal polyps and associated carcinomas

Our understanding of serrated colorectal polyps has increased dramatically over the past two decades and has led to a modern classification scheme for these lesions. Sessile serrated polyps with dysplasia represent the most clinically significant serrated polyp; however, the morphologic heterogeneity of dysplasia in sessile serrated polyps has only recently been recognized and correlated with MLH1 immunohistochemistry. Detailed morphologic analysis of traditional serrated adenomas has led to the recognition of flat and early forms of this polyp. Robust data on the risk of metachronous lesions in patients with serrated polyps are also beginning to emerge. This review will summarize our current understanding of serrated polyps and associated carcinomas with a focus on diagnostic criteria, morphologic heterogeneity, molecular findings, and natural history. Controversial issues in the diagnosis and classification of these polyps are also discussed.

Diagnostic and reporting issues of preneoplastic polyps of the large intestine with early carcinoma

Premalignant polyps of the large intestine are common specimens in surgical pathology. They consist of several different subtypes identifiable by histological criteria that are associated with different molecular characteristics and with the development of different types of colorectal carcinoma. The most common of these is the conventional adenoma, which most commonly leads to carcinomas with a low degree of methylation (CIMP-L) that are microsatellite stable. In Lynch syndrome patients these polyps lead to CIMP-L carcinomas that are microsatellite instable. The second most common is the sessile serrated adenoma, which leads to carcinomas with a high degree of methylation (CIMP-H) that may be either microsatellite stable or instable. The least common premalignant polyp is the traditional serrated adenoma, which can lead to either CIMP-L or CIMP-H carcinomas, most often microsatellite stable. This paper will review the histological features of these lesions, discuss problems in diagnosis and discuss the role of histology in management.

Morphologic reappraisal of serrated colorectal polyps

The "hyperplastic polyp" is considered a benign lesion with no malignant potential, whereas "serrated adenoma" is a precursor of adenocarcinoma. The morphologic complexity of the serrated adenoma varies from being clearly adenomatous to being difficult to distinguish from hyperplastic polyp, which creates a need for more detailed morphologic analysis of all serrated polyps. We evaluated 24 morphologic variables in 289 serrated polyps from the colon and rectum. Cluster analysis and discriminant analysis were performed. A subset of polyps was immunostained for hMLH1 and hMSH2. Major differences were found between right-sided and left-sided polyps. A distinct group of serrated polyps with abnormal proliferation was identified throughout the colon and rectum. These polyps demonstrated decreased expression of hMHL1 and hMSH2 compared with polyps with normal proliferation. Left-sided serrated polyps with normal proliferation further clustered into three groups: vesicular cell-type, goblet cell-type, and mucin-poor-type. We recommend evaluation of the localization, size, and morphologic features when serrated polyps are included in colorectal carcinogenesis research. Polyps with abnormal proliferation are similar to the polyps in "hyperplastic polyposis" and, because of their decreased expression of hMLH1 and hMSH2, may be the subset of polyps associated with the development of colorectal carcinoma via the microsatellite instability pathway.

Hyperplastic polyps and DNA microsatellite unstable cancers of the colorectum

Although the scientific and clinical rationale for classifying colorectal cancer according to mechanisms underlying genetic instability is well supported, little is known of the early morphogenesis of sporadic cancer showing high levels of DNA microsatellite instability (MSI-H). Evidence is accumulating that the traditional adenoma-carcinoma sequence may not apply to sporadic MSI-H colorectal cancer. The serrated pathway comprising hyperplastic polyps, mixed polyps and serrated adenomas may serve as the missing link. This review relates the recently described CpG island methylator phenotype (CIMP) to the serrated pathway. Two rate-limiting genetic steps may underlie the neoplastic pathway associated with CIMP. A transmembrane receptor expressed by pericryptal myofibroblasts (HPP1) may be implicated in the transition from normal to hyperplasia whereas inactivation of hMLH1 is responsible for the conversion of hyperplasia to dysplasia through loss of DNA mismatch repair. These mechanisms fit with clinical observations relating to sporadic MSI-H colorectal cancer, specifically proximal location, multiplicity, higher frequency among females and rapid evolution of early cancer.