The current understanding of serrated polyps of the colorectum indicates that HPs and SSAs are molecularly distinct entities with contrasting potential for neoplastic progression but with overlapping morphologic features (23
). Our data support these prior observations, in that we found BRAF
mutations in all serrated polyps with neoplastic potential. By contrast, BRAF
mutations were found in a subset of HPs. Both BRAF
mutations in HPs have been described (12
), with KRAS
mutations relatively more common among the goblet cell variant (19
). Although only a small number of HPs were sequenced in this study, we nonetheless believe our data regarding the frequency and spectrum of BRAF
mutations in serrated polyps are in keeping with those observations.
Our data now also confirm prior observations of aberrant β-catenin expression in SSAs (27
), and further implicate disruption of the Wnt signaling pathway as an additional molecular feature that segregates these two classes of serrated polyps. In human colon cancer, inactivating mutations of the adenomatous polyposis coli gene (APC
) form the basis for the hereditary colon cancer syndrome familial adenomatous polyposis (14
). Biallelic inactivation of APC
is found in the majority of sporadic colon cancers as well, and in both instances inactivation of APC
leads to the inappropriate stabilization of β-catenin and activation of Wnt target genes. By contrast, activating mutations in CTNNB1
, and not APC
genetic inactivation, are frequent features of microsatellite-instability (MSI+) colon cancers that similarly cause stabilization of β-catenin and aberrant Wnt pathway activation (17
). While we found frequent β-catenin nuclear labeling in SSAs, we found no mutations in CTNNB1
as a mechanism for Wnt activation. However, as CTNNB1
mutations have been described as a specific feature of HNPCC-associated but not sporadic MSI+ colon cancers (11
), our findings support claims that SSAs are a precursor of sporadic MSI+ colon cancer (1
). Of note, Suzuki et al
provided compelling evidence of Wnt pathway activation in colorectal cancer cell lines by hypermethylation of Wnt pathway antagonists SFRP1, SFRP2
). Thus, it is conceivable that loss of expression of Wnt antagonists by promoter hypermethylation, and not genetic alterations of APC
, may be an underlying mechanism of Wnt pathway activation in SSAs. Additional studies will be required to address this possibility.
In addition to the confirmation of β-catenin nuclear labeling in serrated polyps with neoplastic potential, we also now report that aberrant nuclear labeling, and hence Wnt pathway activation, is a pervasive feature of SSAs that have progressed to development of conventional epithelial dysplasia (SSAD). However, there was no relationship of the degree of abnormal β-catenin nuclear accumulation to the presence of conventional dysplasia in these polyps, nor was abnormal β-catenin nuclear accumulation specific to the regions of conventional dysplasia in SSADs, indicating Wnt activation preceded the development of conventional dysplasia. This is consistent with the observations by Wu et al (27
) who found prominent β-catenin nuclear accumulation (now classified as Abnormal Pattern #2) in an SSA without conventional epithelial dysplasia ( in that paper). These patterns of abnormal β-catenin expression differ from that previously reported for MLH1 protein expression in SSAs in which loss of expression was exclusively seen in regions of conventional low or high grade epithelial dysplasia and/or infiltrating carcinoma, but not in the adjacent SSA (21
). Thus, the identification of nuclear β-catenin labeling may aid identification of SSAs with neoplastic potential that have not yet lost MLH1 expression.
Of interest, we also found that the molecular features of TSAs are distinct from SSAs in that all TSAs had a KRAS
mutation whereas BRAF
mutations were only seen in SSAs. This concept is not novel (19
), yet conclusive studies of the molecular genetics of TSAs have been limited by the heterogeneity of polyps classified in this category. By contrast, we have utilized the criteria recently reported by Torlakovic et al
for classification of TSAs (24
) resulting in a highly homogeneous population of polyps with classic features of traditional serrated adenoma. Moreover, while the number of TSAs analyzed was not extensive, our results nonetheless indicate that Wnt pathway activation, seen as nuclear labeling for β-catenin, is also a feature of a subset of TSAs. However, whereas nuclear labeling was most frequent in SSAs with conventional epithelial dysplasia, there was no correlation of nuclear labeling to the presence or absence of low grade conventional epithelial dysplasia in TSAs. Of interest, Yamamoto et al
also evaluated β-catenin expression in serrated polyps and concluded that β-catenin does not play a role (28
). However, review of the immunostaining presented in of that paper does in fact indicate scattered positive nuclear labeling consistent with the Abnormal Pattern #1/weak labeling pattern that we also observed in TSAs.
In summary, we now provide compelling evidence for frequent Wnt pathway activation in SSAs, and in doing so further define the molecular progression of this class of colorectal polyp. These data also indicate that, in a minority of cases, the presence of abnormal β-catenin nuclear labeling may provide support for recommendations regarding the most appropriate management of patients with an isolated and problematic serrated polyp encountered at endoscopy.