Microsatellite Instability Analysis in Colorectal Cancer Tissue Samples from Patients at Lagos State University Teaching Hospital

Microsatellite instability (MSI) is a molecular phenotype arising from defective DNA mismatch repair (MMR) mechanisms and serves as a predictive biomarker for immunotherapy response and prognostic stratification in colorectal cancer (CRC). MSI data from Nigerian CRC patients are sparse, limiting the application of precision oncology approaches in the country. This study analyzed microsatellite instability in colorectal cancer tissue samples from patients at Lagos State University Teaching Hospital (LASUTH), Ikeja, Lagos State, Southwest Nigeria. Formalin-fixed paraffin-embedded (FFPE) tissue blocks from 50 histologically confirmed CRC cases were retrieved. DNA was extracted using a FFPE-specific extraction kit. MSI analysis was performed by fluorescent multiplex PCR targeting the standard Bethesda panel of five microsatellite loci (BAT-25, BAT-26, D2S123, D5S346, D17S250). MSI-High status was detected in 22% of CRC cases, a proportion consistent with West African MSI epidemiology data. All MSI-H cases were confirmed to have reduced MLH1 protein expression by immunohistochemistry. Age at diagnosis was significantly lower in MSI-H patients compared to microsatellite stable (MSS) patients. These findings support the feasibility and clinical relevance of MSI testing for CRC patient management at tertiary hospitals in Lagos. Keywords: microsatellite instability, colorectal cancer, mismatch repair, Bethesda panel, Lagos.