LAUSR

Background: Colorectal cancer (CRC) incidence and mortality in African Americans (AA) is higher compared to Caucasians (CAU). It has also been reported that the disparity is due to differences in the mutation spectrum particularly germline mutations in the DNA mismatch repair (MMR) genes (such as hMSH6). Aim: We aimed to determine MSH6 mutation profile in AAs with CRC. Methods: We performed RNA sequencing in 8 paired CRC samples (AA, n=4 and CAU, n=4) to identify the unique transcript and novel pathogenic mutation in AA compared to CAU by comparing CRC matched to corresponding normal epithelia. In silico analysis of TCGA COAD RNA sequencing data was performed. Results: We found non-synonymous germline mutations (p.G1139S) in exon 5 of MSH6 gene (one of the MMR genes) in AA CRC samples. Interestingly, this mutation is present in the active site of ATP binding (1135 to 11442 residue) motif of MSH6 protein. In silico analysis was used to determine the novelty and pathogenicity of this mutation by comparison to various publicly available databases. Also, the deleterious effect of p.G1139S missense mutation on the structure of MSH6 protein revealed that p.G1139S is destabilizing MSH6 protein. Based on Ramachandran plot analysis, we found lot of clash and outliers in the mutant type compared to wild type MSH6 protein. Structural analysis revealed that serine substitution results in a wide ATP binding site compared to glycine present in wild-type MSH6. In addition, we found a loss of hydrogen bonding or gain of ionic interaction between wild-type and mutant MSH6 protein. Conclusion: Based on our in silico analysis, the deleterious effect of the detected missense mutation in the active site of ATP binding motif of MSH6 affects the structure and function of MSH6. This may result in the accumulation of mono-, or di-nucleotide repeats instability in the genome, leading to genomic alterations that associate with carcinogenesis in African American CRC patients. Citation Format: Mudasir Rashid, Rumaisa Rashid, Sudhir Varma, Zaki Sherif, John Carethers, Hassan Brim, Hassan Ashktorab. Unique and novel deleterious MSH6 mutation among African American patients with colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1907.