Low-level microsatellite instability colorectal carcinomas: do they really belong to a "gray zone" between high-level microsatellite instability and microsatellite-stable cancers?
Colorectal carcinomas demonstrating low-level microsatellite instability (MSI-L) may form a distinct group differing both from high-level MSI (MSI-H) and microsatellite-stable (MSS) tumors. In a retrospective series of 172 colorectal carcinomas the microsatellite status was examined based on DNA extracted from archival blocks. Three groups - MSS ( n=100), MSI-L ( n=37), MSI-H ( n=35) - were compared with respect to clinical data, stage, histology, and immunoexpression of Ki-67, and P53. Compared to MSS and MSI-H carcinomas the MSI-L tumors were exceptionally rarely right-sided, and demonstrated the lowest proliferation fraction. There was a trend for less frequent high-grade histology, more frequent intermediate P53 expression, and prominent mucinous histology. Features of MSI-L colorectal carcinomas are not necessarily located between their MSS and MSI-H counterparts. The MSI-L category may contain a group of tumors belonging to a distinct carcinogenetic pathway.