Neoplastic progression occurs through mutator pathways in hyperplastic polyposis of the colorectum

Abstract

Aim: Colorectal cancer has been described in association with hyperplastic polyposis but the mechanism underlying this observation is unknown. The aim of this study was to characterise foci of dysplasia developing in the polyps of subjects with hyperplastic polyposis on the basis of DNA microsatellite status and expression of the DNA mismatch repair proteins hMLH1, hMSH2, and hMSH6.

Materials and methods: The material was derived from four patients with hyperplastic polyposis and between one and six synchronous colorectal cancers. Normal (four), hyperplastic (13), dysplastic (13), and malignant (11) samples were microdissected and a PCR based approach was used to identify mutations at 10 microsatellite loci, TGFbetaIIR, IGF2R, BAX, MSH3, and MSH6. Microsatellite instability-high (MSI-H) was diagnosed when 40% or more of the microsatellite loci showed mutational bandshifts. Serial sections were stained for hMLH1, hMSH2, and hMSH6.

Results: DNA microsatellite instability was found in 1/13 (8%) hyperplastic samples, in 7/13 (54%) dysplastic foci, and in 8/11 (73%) cancers. None of the MSI-low (MSI-L) samples (one hyperplastic, three dysplastic, two cancers) showed loss of hMLH1 expression. All four MSI-H dysplastic foci and six MSI-H cancers showed loss of hMLH1 expression. Loss of hMLH1 in MSI-H but not in MSI-L lesions showing dysplasia or cancer was significant (p<0.001, Fisher's exact test). Loss of hMSH6 occurred in one MSI-H cancer and one MSS focus of dysplasia which also showed loss of hMLH1 staining.

Conclusion: Neoplastic changes in hyperplastic polyposis may occur within a hyperplastic polyp. Neoplasia may be driven by DNA instability that is present to a low (MSI-L) or high (MSI-H) degree. MSI-H but not MSI-L dysplastic foci are associated with loss of hMLH1 expression. At least two mutator pathways drive neoplasia in hyperplastic polyposis. The role of the hyperplastic polyp in the histogenesis of sporadic DNA microsatellite unstable colorectal cancer should be examined.

Figure 1

(A) Focus of dysplasia (right) arising in a hyperplastic polyp from patient No 1 (haematoxylin-eosin). (B) There is loss of nuclear staining for hMLH1 in the dysplastic area (ABC technique). (C) Bandshifts are shown for MYCL, BAT25, BAT26, and hMSH6. N, normal mucosa; T, tumour.

Figure 2

(A) Junction of a hyperplastic polyp (right) and tubular adenoma (left) from patient No 2 (haematoxylin-eosin). (B) Loss of nuclear MLH1 is evident in the dysplastic tubules (ABC technique). (C) Bandshifts are shown for MYCL, BAT40, BAX, and hMSH3. N, normal mucosa; T, tumour.

Figure 3

(A) Focus of serrated dysplasia (right) arising in a hyperplastic polyp (haematoxylin-eosin). (B) There is accentuation of nuclear staining for hMLH1 (ABC technique). (C) The lesion shows instability at MYCL only. N, normal mucosa; T, tumour.

Figure 4

Three possible pathways of neoplastic progression in hyperplastic polyposis. The adenoma (occurring either as a coincidental lesion or as a component of a mixed polyp) is shown as following the suppressor pathway. Hyperplastic polyps may evolve through the mild mutator/suppressor pathway or the high mutator pathway. Molecular characteristics of the three pathways are reported elsewhere.²⁹ MMR, mismatch repair, MSI-H, MSI-L, microsatellite instability-high, -low; MSS, microsatellite stable.