Concordant CpG island methylation in hyperplastic polyposis

Abstract

The CpG island methylator phenotype (CIMP) is a newly described mechanism for carcinogenesis in colorectal carcinomas and adenomas characterized by methylation of multiple CpG islands. The causes of CIMP are unknown. We studied CIMP in hyperplastic polyps (HPs), with emphasis on patients with multiple HPs (5 to 10 HPs), large HPs (one HP >1 cm) or hyperplastic polyposis (>20 HPs). Methylation of p16, MINT1, MINT2, MINT31, and hMLH1 was analyzed by methylation-specific polymerase chain reaction in 102 HPs, 8 serrated adenomas, 19 tubular adenomas, and 9 adenocarcinomas from 17 patients, with multiple/large HPs or hyperplastic polyposis and in 16 sporadic HPs from 14 additional patients. Sporadic HPs were CIMP-negative (not methylated at any locus), but 43% of HPs from multiple/large HPs, or hyperplastic polyposis were CIMP-high (two or more methylated loci, P = 0.00001). Methylation among the four loci was correlated within HPs (odds ratio, 3.41; P = 0.002), and the methylation status of HPs within the same patient was also correlated (odds ratio, 5.92; P = 0.0001). CIMP-high HPs were present primarily in patients with a predominance of HPs in the right colon and/or serrated adenomas (P = 0.0009) and were associated with the absence of K-ras proto-oncogene mutations (odds ratio, 5.08; P = 0.03). Our findings of concordant CpG island methylation of HPs in multiple/large HPs or hyperplastic polyposis supports the concept that some patients have a hypermethylator phenotype characterized by methylation of multiple HPs and other colorectal lesions. The hypermethylator phenotype is related to patient-specific factors, such as carcinogenic exposure or genetic predisposition.

Figure 1.

Methylation analysis of CpG islands in HPs. Methylation of p16, MINT1, MINT2, and MINT31 was evaluated by methylation-specific PCR using primers for methylated (M) and unmethylated (U) alleles of bisulfite-treated DNA. Loci examined and HP numbers are indicated above each gel. DNA from RKO, a colon cancer cell line, was used as a positive control, and dH₂O without DNA was used as negative control.

Figure 2.

A: Prevalence of methylation at p16, MINT1, MINT2, and MINT31 in HPs, serrated adenomas, tubular adenomas, and carcinomas. B: CIMP status of sporadic HPs and of HPs, serrated adenomas, tubular adenomas, and carcinomas from patients with multiple HPs, large HPs, and hyperplastic polyposis. C: CIMP status of HPs from patients with multiple HPs, large HPs, and hyperplastic polyposis.

Figure 3.

Methylation status of HPs, serrated adenomas, tubular adenomas, and carcinomas from patients with multiple HPs, large HPs, and hyperplastic polyposis.

Figure 4.

CIMP, MSI-high, and hMLH1 methylation status of HPs, serrated adenomas, tubular adenomas, and carcinomas from patients with MSI-high lesions. Concordant methylation of hMLH1, CIMP-high, and MSI-high was present in an adenoma and carcinoma from one patient and a carcinoma from another patient, but methylation of hMLH1 was not present in all other MSI-negative HPs, serrated adenomas, tubular adenomas, or carcinomas.