Microsatellite instability and high tumor mutational burden detected by next generation sequencing are concordant with loss of mismatch repair proteins by immunohistochemistry.

Impairment of DNA mismatch repair function in neoplasms can be assessed by DNA-based methods to assess for high microsatellite instability (MSI-High) or immunohistochemical (IHC) analysis to assess for deficiency of mismatch repair proteins (dMMR). Neoplasms with mismatch repair deficiency often have high tumor mutational burden (TMB-High). MSI-High, dMMR, and TMB-High are all histology agnostic biomarkers for potential therapy using immune checkpoint inhibitors (ICI). In this single center, retrospective study, our primary aim was to assess if NGS-based positive TMB/MSI findings are concordant with patient matched concurrent MMR IHC studies. In addition, we determined if positive TMB/MSI findings are attributable to genetic/epigenetic alterations of MMR genes. Finally, we explored potential associations between IHC, TMB and MSI findings and specific tumor types We screened 4,258 patients in our database who had tumor-normal-testing with our institutional high-throughput NGS-based CLIA assay between Apr 1, 2021-August 31, 2022 for TMB and MSI. We identified 65 patients who had neoplasms with documented TMB-High/MSI-High (n = 59) or TMB-High/MSI-Undetermined (n = 6) results as well as concurrent IHC results for MMR proteins [colorectal (n = 25), endometrial (n = 28), prostatic (n = 7), urothelial (n = 3), other (n = 5)]. The concordance between positive NGS TMB/MSI and MMR results was 98 %. Genetic/epigenetic alterations of MMR genes were documented in 78 % of the neoplasms. IHC studies for dMMR proteins revealed loss of MLH1/PMS2 (n = 33), MSH2/MSH6 (n = 14), MLH1/MSH2/PMS2 (n = 1), MLH1 (n = 1), MSH2 (n = 2), MSH6 (n = 6) and PMS2 (n = 6). All six prostatic neoplasms with dMMR had loss of MSH2/MSH6 (p < 0.0001). We conclude that neoplasms with positive results for TMB/MSI are highly concordant with positive dMMR results. Genetic/epigenetic alterations in the MMR genes are an underlying reason for most positive findings. The association of MSH2/MSH6 loss with prostatic neoplasms is of in-terest, but sample size is limited, and further studies are warranted to address this association.