There is increased incidence of
microsatellite instability (MSI) in patients who develop multiple primary
colorectal cancers (CRC), although the association with
hereditary nonpolyposis colon cancer (HNPCC) is unclear. This study aims to evaluate the underlying genetic cause of MSI in these patients.
Microsatellite instability was investigated in 111
paraffin-embedded
CRCs obtained from 78 patients with metachronous and synchronous
cancers, and a control group consisting of 74
cancers from patients with a single CRC. Tumours were classified as high level (MSI-H), low level (MSI-L) or stable (MSS). MLH1, MSH2 and MSH6 gene expression was measured by immunohistochemistry. Methylation of the MLH1 promoter region was evaluated in MSI-H
cancers that failed to express MLH1, and mutational analysis performed in MSI-H samples that expressed MLH1, MSH2 and MSH6
proteins. The frequency of MSI-H was significantly greater in the multiple, 58 out of 111 (52%), compared to the single
cancers, 10 out of 74 (13.5%), P < 0.01. Of the 32 patients from whom two or more
cancers were analysed, eight (25%) demonstrated MSI-H in both
cancers, 13 (41%) demonstrated MSI-H in one
cancer and 11 (34%) failed to demonstrate any MSI-H. MSI-H single
cancers failed to express MLH1 or MSH2 in seven out of nine (78%) cases and MSI-L/MSS
cancers failed to express MLH1 or MSH2 in one out of 45 (2.2%) cases, all
cancers expressed MSH6. MSI-H multiple
cancers failed to express MLH1 or MSH2 in 21 out of 43 (48%) cases and MSI-L/MSS
cancers failed to express MLH1 or MSH2 in four out of 32 (12.5%) cases. MSH6 expression was lost in five MSI-H multiple
cancers, four of which also failed to express MLH1 or MSH2. Loss of expression of the same mismatch repair (MMR) gene was identified in both
cancers from six out of 19 (31%) patients. Methylation was identified in 11 out of 17 (65%) multiple and three out of six (50%) single MSI-H
cancers that failed to express MLH1. Mutational analysis of 10 MSI-H multiple
cancers that expressed MLH1, MSH2 and MSH6 failed to demonstrate mutations in the MLH1 or MSH2 genes. We suggest that, although MSI-H is more commonly identified in those with multiple
colorectal cancers, this does not commonly arise from a classical HNPCC pathway.