Secondary mutation in a coding mononucleotide tract in MSH6 causes loss of immunoexpression of MSH6 in colorectal carcinomas with MLH1/PMS2 deficiency

Abstract

Immunohistochemical staining for DNA mismatch repair proteins may be affected by various biological and technical factors. Staining variations that could potentially lead to erroneous interpretations have been recognized. A recently recognized staining variation is the significant reduction of staining for MSH6 in some colorectal carcinomas. The frequency and specific characteristics of this aberrant MSH6 staining pattern, however, have not been well analyzed. In this study of 420 colorectal carcinoma samples obtained from patients fulfilling the Revised Bethesda Guidelines, we detected 9 tumors (2%) showing extremely limited staining for MSH6 with positive staining present in <5% of the tumor cells. Our analyses showed that these tumors belonged to two distinct categories: (1) MLH1 and/or PMS2 protein-deficient carcinomas (n=5, including 1 with a pathogenic mutation in PMS2); and (2) MLH1, PMS2 and MSH2 normal but with chemotherapy or chemoradiation therapy before surgery (n=4). To test our hypothesis that somatic mutation in the coding region microsatellite of the MSH6 gene might be a potential underlying mechanism for such limited MSH6 staining, we evaluated frameshift mutation in a (C)8 tract in exon 5 of the MSH6 gene in seven tumors that had sufficient DNA for analysis, and detected mutation in four; all four tumors belonged to the MLH1/PMS2-deficient group. In conclusion, our data outline the main scenarios where significant reduction of MSH6 staining is more likely to occur in colorectal carcinoma, and suggest that somatic mutations of the coding region microsatellites of the MSH6 gene is an underlying mechanism for this staining phenomenon in MLH1/PMS2-deficient carcinomas.

Differences in Microsatellite Instability Profiles between Endometrioid and Colorectal Cancers

“…However, IHC can be intact in MSI-H tumors with germline mutations when the specific mutation(s) does not disrupt antibody-reactive epitope, 19 and IHC might not be the ideal method for detecting EMC in treated patients, as chemoradiation can reduce MMR protein expression in otherwise MSS tumors. 20 There are several possible mechanisms underlying the difference in MSI profiles seen in MSI-H EMCs and CRCs that, in part, may be attributable to differences in the biology of the two different types of epithelium. 21,22 First, like normal colonic and endometrial epithelium, MSI-H CRCs may have more rapid turnover and more rounds of DNA replication compared with those of EMCs, consequently leading to a larger nucleotide shift compared with that in MSI-H EMCs.…”

Evaluation of colorectal cancers for Lynch syndrome: practical molecular diagnostics for surgical pathologists

“…This is due to the fact that there is a coding mononucleotide repeat in MSH6 that may become unstable and inactivate MSH6 in a mismatch repair-deficient tumor; those areas of the tumor in which the MSH6 mononucleotide repeat is unstable will lose immunostaining, while those areas in which the mononucleotide repeat does not become unstable will show intact staining. 9 Importantly, this 'clonal' MSH6 loss typically occurs in tumors that are mismatch repair deficient due to causes unrelated to MSH6, including sporadic mismatch repair-deficient tumors or tumors with germline mutations of mismatch repair genes besides MSH6.…”

Template for Reporting Results of Biomarker Testing of Specimens From Patients With Carcinoma of the Colon and Rectum

“…One should also keep in mind that nucleolar staining or complete loss of MSH6 staining has been described in colorectal cancer cases with prior radiation or chemotherapy, 3,4 and a significant reduction of MSH6 staining has been described in a small percentage of colorectal carcinomas with somatic mutations of the coding region microsatellites of the MSH6 gene in MLH1/PMS2-deficient carcinomas. 5 B: MLH1 Promoter Hypermethylation Analysis and BRAF Mutational Analysis.-Defective MMR in sporadic colorectal cancer is most often due to inactivation of the MLH1 gene promoter by hypermethylation (epigenetic silencing). The V600E mutation of the BRAF gene may be present in up to 70% of tumors with hypermethylation of the MLH1 promoter.…”