Simple IHC reveals complex MMR alternations than PCR assays: Validation by LCM and next‐generation sequencing

Evaluation of the status of mismatch repair (MMR) in tumors is crucial for determining the application of immune checkpoint inhibitors (ICIs). Conventional PCR (MSI‐PCR) is the gold standard for confirming the MMR status. However, it requires visual confirmation and presents difficulties in determining MMR status. Immunohistochemistry (IHC) is a simple method and can confirming MMR protein expression in the whole tumor. We aim to investigate IHC is more suitable for evaluating MMR status in the tumor. We compared MSI‐PCR and IHC by testing 319 samples from 284 patients across 14 cancer types. In discordant cases, we performed laser‐capture microdissection and microsatellite instability assay by next‐generation sequencing (MSI‐NGS). The concordance rate between IHC and MSI‐PCR testing was 98.1% (313/319). Two reasons for these discrepancies were ambiguous MSI‐PCR results and heterogeneous MSI status within the tumor. Among six cases (1.9%), three were judged as MSI‐H by MSI‐PCR but with proficient MMR by IHC. The results of MSI‐NGS revealed microsatellite stable in these three cases. The remaining three cases, two of three were MSI‐H and one was MSS in whole tumor in MSI‐PCR. IHC showed a “mosaic” pattern containing both proficient MMR and deficient MMR portions by IHC in all three cases. We performed microdissection and MSI‐PCR and found intratumoral heterogeneity of MMR status. These results indicated the advantages of IHC and performed expanded samples (n = 1082) and two additional mosaic cases were identified. Our results clearly indicated that simple IHC is the best choice for determining MMR alterations in critical cases for ICIs treatment.