Microsatellite Status Detection in Gastrointestinal Cancers: PCR/NGS Is Mandatory in Negative/Patchy MMR Immunohistochemistry

Abstract

Background: Microsatellite instability (MSI) is a predictive biomarker for immune checkpoint inhibitors. The main goal was to investigate the discordance between IHC and PCR/NGS for MSI testing in gastrointestinal cancers. Methods: Two series were analyzed through IHC for mismatch-repair-system proteins (MMRP) and PCR, with one series of 444 colorectal cancers (CRC) and the other of 176 gastric cancers (GC). All cases with discordant results between IHC and PCR were analyzed by NGS. IHC staining was evaluated as follows: proficient MMR (pMMR), with all MMR positive; deficient MMR (dMMR), with the loss of one heterodimer; and cases with the loss/patchy expression of one MMR (lo-paMMR). Cases with instability in at least two markers by PCR were MSI-high (MSI-H) and with instability in one marker, MSI-low (MSI-L). Cases without instability were evaluated as microsatellite-stable (MSS). Results: In the CRC cohort, 15 out of 444 cases were dMMR and 46 lo-paMMR. Among the 15 dMMR, 13 were MSI-H and 2 MSS. Among the 46 lo-paMMR, 13 were MSI-H and 33 were MSS. In the GC cohort, 13 out of 176 cases were dMMR and 6 cases lo-paMMR. Among the 13 dMMR, 12 were MSI-H and only 1 was MSS. All six lo-paMMR cases were MSS. All NGS results were in agreement with PCR. Conclusions: In clinical practice, MMR–IHC could be used as a screening test and additional molecular analysis is mandatory exclusively in cases carrying loss/patchy MMR-IHC.

An unusual non‑metastatic, mismatch repair‑deficient primary gastric squamous cell carcinoma presenting as a large, exophytic, bleeding tumor: A case report

“…Furthermore, based on recent reports, weak, heterogeneous nuclear expression of MSH2 protein requires testing for MSI mutations; in our case, the MSH6 protein showed only focally weak nuclear expression in a few cells. It should be emphasized that molecular testing is mandatory in cases with loss/patchy MMR-IHC in daily clinical practice, to avoid misdiagnosis and consequently inadequate therapeutic choices ( 31 ).…”

Implementation of microsatellite instability testing for the assessment of solid tumors in clinical practice

“…MMR–IHC analysis could address the limitations of PCR‐based MSI testing, particularly TAT 26 . However, PCR‐based MSI testing might be useful in cases that cannot be diagnosed via MMR–IHC analysis alone 27 . Based on demand, the combination of MSI testing and MMR–IHC analysis could be feasible in clinical practice.…”

Assessment of the DNA Mismatch Repair System Is Crucial in Colorectal Cancers Necessitating Adjuvant Treatment: A Propensity Score-Matched and Win Ratio Analysis

“…Heterogeneous expression was defined as strongly immunoreactive cells admixed with negative cells, and patchy expression was defined as confluent areas of staining loss. In these cases, as previously described, additional molecular analysis by polymerase chain reaction (PCR) was used to unequivocally categorize cases into microsatellite stable (MSS) and microsatellite unstable (MSI) tumors [ 23 ]. MSI was determined on tumor DNA using EasyPGX ® , including the following mononucleotide repeats: BAT25, BAT26, NR21, NR22, NR24, NR27, CAT25, and MONO27.…”