Clinicopathologic characteristics of colorectal cancer with microsatellite instability.

Colorectal cancer (CRC) can be classified according to the level of microsatellite instability (MSI) exhibited by the tumor. The aim of this study was to determine MSI status in CRC from Tunisia and to identify clinical and pathological characteristics of MSI-H tumors. Microsatellite status was determined by polymerase chain reaction amplification using standard markers (BAT25, BAT26, D2S123, D5S346 and D17S250, the Bethesda panel) in 44 CRC cases. Molecular results were correlated with pathological and clinical features. Six CRC cases (13.8%) showed high-level instability (MSI-H), 14 cases had low level instability (MSI-L), and the remainders were stable (MSS). Immunohistochemical analysis showed loss of MSH2 protein in 3 cases among the 6 MSI-H tumors, whereas no silencing of MLH1 or MSH6 was found in any case. Significant differences in age and family history of cancers were observed between MSI-H and MSS/MSI-L groups (p=0.01 and p=0.002). However, statistical analysis showed that there were no significant differences between MSI-H and MSS/MSI-L tumors in terms of tumor location, lymph node involvement and stage of disease. Regarding histological features, MSI-H tumors were more likely to be poorly differentiated (p=0.003), to have a medullary pattern (p=0.005), and to harbor increased numbers of peritumoral lymphocytes (p=0.001). These findings indicate that careful observation of the tumor morphology can assist in the identification of unstable colorectal cancers requiring molecular investigations.