Defective mismatch repair proteins and microsatellite instability in young Mexican patients with colorectal cancer.

Abstract

e14708 Background: Colorectal carcinoma (CRC) is prevalent malignancy and a third of the cases affect young patients. 15% of CRC have microsatellite instability (MSI) due to disruptions in mismatch repair (MMR) genes, like germline mutations (3%) and hypermethylation of the MLH1 gene promoter associated to the BRAF V600E mutation (12%). The aim of this work was to assess MMR abnormalities in tumors of Mexican CRC patients under 50 years old. Methods: CRC paraffin-embedded tissues of 47 patients with available demographic/clinical data were studied by immunohistochemistry (IHC) for MLH1/MSH2, qPCR with specific probes/sequencing for the BRAF V600E mutation, and conventional PCR (5 markers) for MSI analysis. Results: Female:Male ratio was 0.81:1. Most of the cases were classified as TNM Stage II, were located in the cecum, invaded the serous coat, and showed intestinal-type histology. 20 samples showed alterations in MMR protein expression. MLH1, MSH2, and combined deficiency of both proteins were detected in 17, 4, and 4 tumors, respectively. No BRAF mutations were detected. MSI analysis restricted to the 20 altered IHC samples showed MSI in 10 tumors (3 MSI-low and 7 MSI-high tumors). The four cases with MLH1/MSH2 deficiency, showed MSI-high pattern. Conclusions: We found 42.6% cases with defective MMR expression. No epigenetic abnormalities associated to BRAF V600E mutation were registered. The lack of MSI in ten tumors with deficient MMR may be due to alternate DNA repair mechanisms. Acknowledgments. Work supported by the CHIBCHA Project (European Commission7FP grant #223678).