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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 in5% 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.

Mutation screening of the breast and ovarian cancer predisposition genes BRCA1 and BRCA2 is becoming an increasingly important part of clinical practice. Classification of rare non-truncating sequence variants in the BRCA1 and BRCA2 genes is problematic because it is not known whether these subtle changes alter function sufficiently to predispose cells to cancer development. The BRCA1 331G > A substitution mutation, which occurs at the last nucleotide of exon 5, results in an Arg-to-Lys change at codon 71 (R71K). cDNA analysis indicated that the R71K mutation significantly increases the level of a transcript, characterized by a 22 bp deletion in exon 5, which putatively produces a truncated BRCA1 protein of 63 amino acids. The mutation completely abolishes normal splicing as the mutant allele does not generate any normal transcript. Analysis of a tumor specimen indicates loss of heterozygosity. These results support the conclusion that BRCA1 331G > A (R71K) is a deleterious mutation.

Shah MA, Salo-Mullen E, Stadler Z, Ruggeri JM, Mirander M, Pristyazhnyuk Y, Zhang L. De novo CDH1 mutation in a family presenting with early-onset diffuse gastric cancer. In this report, we describe the first concluded case of a de novo germline mutation in CDH1 in a hereditary diffuse gastric cancer (HDGC) kindred. The incident case was a woman with a personal history of Hodgkin's lymphoma and diffuse gastric cancer, who was then confirmed to have a CDH1 mutation (c.1792 C>T (R598X)). The patient's mother was found to have the same CDH1 germline mutation; however, neither maternal grandparent was found to carry the mutation, thus leading to a conclusion that the proband's mother's mutation is of de novo origin. This case highlights the importance of recognition of the HDGC syndrome and of testing for CDH1 germline mutations in young individuals with diffuse gastric cancer without a family history of the disease.