Relationship between MLH1, PMS2, MSH2 and MSH6 gene‐specific alterations and tumor mutational burden in 1057 microsatellite instability‐high solid tumors

Microsatellite instability‐high (MSI‐H) and tumor mutational burden (TMB) are predictive biomarkers for immune‐checkpoint inhibitors (ICIs). Still, the relationship between the underlying cause(s) of MSI and TMB in tumors remains poorly defined. We investigated associations of TMB to mismatch repair (MMR) protein expression patterns by immunohistochemistry (IHC) and MMR mutations in a diverse sample of tumors. Hypothesized differences were identified by the protein/gene affected/mutated and the tumor histology/primary site. Overall, 1057 MSI‐H tumors were identified from the 32 932 tested. MSI was examined by NGS using 7000+ target microsatellite loci. TMB was calculated using only nonsynonymous missense mutations sequenced with a 592‐gene panel; a subset of MSI‐H tumors also had MMR IHC performed. Analyses examined TMB by MMR protein heterodimer impacted (loss of MLH1/PMS2 vs. MSH2/MSH6 expression) and gene‐specific mutations. The sample was 54.6% female; mean age was 63.5 years. Among IHC tested tumors, loss of co‐expression of MLH1/PMS2 was more common (n = 544/705, 77.2%) than loss of MSH2/MSH6 (n = 81/705, 11.5%; P