Background: Microsatellite instability (MSI) is seen in 10%-15% of sporadic colorectal cancers mostly in the right colon, but the precursors of cancers with MSI remain unknown. We examined whether sporadic cancers with MSI arise from pre-existing benign proliferative lesions (such as hyperplastic polyps or serrated adenomas [together denoted as "serrated polyps"]).
Methods: The frequency of benign epithelial lesions (serrated polyps and conventional adenomas) was determined by histologic review of resection specimens from individuals (n = 29) with sporadic colorectal cancer with MSI and from a matched control group (n = 29) with cancer showing microsatellite stability (MSS). MSI status, expression of mismatch repair enzyme (product of the human mut-L homologue 1 [hMLH1] gene), and hMLH1 gene promoter methylation in the benign lesions were determined. Data were analyzed by the chi-square test, by Wilcoxon's rank-sum test, and by conditional logistic regression as appropriate, and a two-sided probability less than.05 was considered to be statistically significant.
Results: Individuals with cancers showing MSI were more likely to harbor at least one serrated polyp than individuals with cancers showing MSS (odds ratio = 4.0; 95% confidence interval = 1.1 to 14.2; P =.03), but the frequency of conventional adenomas was the same in both groups (P =.52, Mann-Whitney test). Loss of hMLH1 protein expression was seen in lesions from 10 of 13 patients with MSI, but no loss was seen in lesions from four patients with MSS (P =.02, Fisher's exact test). Loss of hMLH1 protein expression was associated with MSI in assessable lesions. The hMLH1 promoter was methylated in all assessable serrated polyps from patients with cancers showing MSI but in none of the lesions from patients with MSS cancers.
Conclusions: Some right-sided hyperplastic polyps may give rise to sporadic colorectal carcinomas with MSI. Methylation of the hMLH1 gene promoter within neoplastic cell subpopulations may be a critical step in the progression to carcinoma. The frequency with which benign lesions progress to cancer with MSI is unknown.