Features of microsatellite instability in colorectal cancer: Comparison between colon and rectum

In one third of colorectal cancer patients, tumours occur in the rectum. Un ique aetiologies may underlie the increased carcinogenesis in this region o f the colorectum. Microsatellite instability (MSI) was analysed in specimen s obtained from 121 colorectal carcinoma patients, using five dinucleotide markers and a new fluorescent system. The incidence of microsatellite alter ations in the proximal colon, the distal colon and the rectum was 44.4% (16 /36), 37.2% (16/43) and 23.8% (10/42), respectively. Patterns of microsatel lite alterations could be classified into two subtypes, one showing relativ ely small changes within 6 bases (type A) and the other exhibiting drastic changes over 8 bases (type B). All the changes observed in tumours in the r ectum were type A, and no type B mutation was noted. There was a close corr elation between type B mutations and high-frequency MSI (greater than or eq ual to2 markers), MSI-H, and between type A mutations and low-frequency MSI (1 marker), MSI-L. The type B/MSI-H phenotype significantly correlated wit h the proximal localisation of tumours. In the rectum, there was no tumour with the type B/MSI-H phenotype. These findings suggest that cancers occurr ing in the colon and the rectum have a differential molecular background fo r carcinogenesis. Copyright (C) 2001 S. Karger AG, Basel.