Immunohistochemical Mismatch Repair Proteins Expression in Colorectal Cancer

Analysis of DNA mismatch repair proteins expression and BRAF V600E mutation in a subset of early- and late-onset colorectal carcinoma patients in Mexico

BACKGROUND AND AIMS

A third of colorectal carcinomas (CRC) affect patients <50 years of age. Fifteen percent of CRC cases with microsatellite instability are due to inherited germ-line mutations in DNA mismatch repair genes. The rest have an epigenetic hypermethylation of the MLH1 promoter in whom the BRAF V600E mutation is a common hallmark. Immunohistochemistry helps to classify colorectal cancers with 100% specificity and 92% sensitivity. We undertook this study to determine if age is a risk factor for defective MMR protein expression and BRAF mutations in our population and to compare these results with the histopathological tumor features.

METHODS

Immunohistochemistry for MLH1 and MSH2 and RT-PCR BRAF V600E mutation was performed on tissue specimens from 57 patients <50 years of age. Data on age, gender, tumor location, histology, depth of infiltration, and the presence of metastatic lymph nodes were collected. Forty eight patients >50 years of age were used as a control group. A statistical analysis using ANOVA, χ(2), and Spearman's rho test were performed.

RESULTS

Absent MMR protein expression was more prevalent in patients <50 years of age. No BRAF V600E mutations were detected in either group. Medullary and mucinous types were more prevalent among young patients, whereas intestinal type was more frequent in older patients (p = 0.0008). No differences were found regarding clinicopathological stages between groups.

CONCLUSIONS

We found an association between young age and defective MMR expression. No V600E BRAF mutations were detected in either group.

Clinicopathological and molecular characterization of colorectal micropapillary carcinoma

Invasive micropapillary carcinoma is associated with frequent lymph node metastasis and adverse clinical outcome. Initially described as a variant of breast and ovarian carcinoma, it has subsequently been found in other organs, most recently the colon. Reports of colorectal micropapillary carcinoma to date are limited in number, and their molecular profile has not been established. The aims of the present study were to analyze their clinicopathological features and molecular profile, and compare them with those of conventional adenocarcinoma. Clinicopathological features of a cohort of 379 patients with primary colorectal cancer were retrospectively reviewed for the presence of the pattern characteristic of micropapillary carcinoma. We also assessed the expression of KRT7, KRT20, CEACAM5, MUC1 (EMA, clone E29), MUC1 (clone MA695), MLH1, MSH2, MSH6 and TP53 by immunohistochemistry. Genetic assessments of microsatellite instability, chromosomes 17p and 18q, and mutations in TP53, BRAF and KRAS were performed using DNA extracted from formalin-fixed, paraffin-embedded sections. In all, 60 of the reviewed cases (16%) had a micropapillary component that ranged from 5 to 95% of the tumor, characterized by a higher frequency of an infiltrative pattern, lymphovascular and perineural invasion, a higher depth of invasion and more positive lymph nodes than conventional adenocarcinoma. Immunohistochemistry for MUC1 (clone MA695) and MUC1 (EMA, clone E29) enhanced the characteristic inside-out staining pattern of the micropapillary carcinoma component, whereas the rest of the tumor showed luminal staining patterns. KRT7 expression was slightly increased in micropapillary carcinoma, but did not reach significance (17-3%, P=0.1967). The molecular parameters showed a higher frequency of TP53 alterations and a low incidence of microsatellite instability and RER phenotype (loss of mismatch repair protein) in micropapillary carcinoma. With regard to the histological parameters, micropapillary carcinoma appears to be more aggressive than conventional colorectal adenocarcinoma. The molecular profile supports the hypothesis that micropapillary carcinoma carcinogenesis develops through the classical chromosomal instability pathway.