Microsatellite Instability Statuses and Clinicopathological Characteristics of Colorectal Carcinomas in a Sub-Saharan African Population

Context: Colorectal carcinomas (CRC) in Africans tend to occur at an earlier age and have unique clinicopathologic as well as putatively distinctive molecular characteristics. We recently undertook a study of the microsatellite instability (MSI) analysis of a cohort of colectomies in our practice to examine this issue further. Methods: Fifty-five (55) colectomies done for colorectal carcinoma were evaluated. The pathological features of the tumours were recorded. Microsatellite Instability (MSI) analysis was carried out using immunohistochemistry for the DNA mismatch repair proteins, MLH1 and MSH2. Results: The average age was 49.2 years and the highest number of cases was seen in the 6th decade of life. Nineteen (34.5%) of the tumours in this study were found to be MSI-positive. Fourteen (73.7%) of the MSI tumours were found in patients below 50 years of age. There was a significant statistical association between age and MSI-status (p=0.004). Eleven (58%) out of the MSI tumours were located in the right colon while 8 (42%) were located in the left. On the other hand, 24 (77%) of the tumours with a normal MMR immuno profile were located on the left and 10 (23%) on the right. There was a statistically significant association between the site of tumour and microsatellite instability status (p=0.012). Adenocarcinoma was the commonest histological type making up 65.5% of all the tumours. This is closely followed by mucinous carcinomas (32.7%) and signet ring carcinomas (1.8%). Eleven of the 18 mucinous carcinomas were found in patients under 50 years of age. Nine (50%) of the mucinous carcinomas were MSI-positive although this was not statistically significant. There was no case of generalised polyposis found in this study. Conclusion: Our findings is consonant with a lower average age (49.2 years) for colorectal carcinomas in similar settings as ours and contrasts with high-incidence CRC in Caucasians in who the reported average age is consistently above 60 years. A significant number of tumours where deficient in mismatch repair proteins and these tumours tended to be right-sided, mucinous and to occur at an earlier age. These findings signal the need for further investigation of alternative pathways for the aetiology and pathogenesis of colorectal carcinomas in our region.