Immunohistochemical (IHC) staining for mismatch repair (MMR) proteins and PCR-based detection for microsatellite status are routinely performed on colorectal carcinoma (CRC) surgical samples. However, the concordance of the two detections, which is related to the quality control of our molecular pathology laboratory, is unknown so far. So the main aim of this study is to compare the differences between the two analyses and to improve our work.

IHC analyzed the expression of MLH1, PMS2, MSH2 and MSH6 which was performed on 368 cases of formalin-fixed paraffin-embedded (FFPE) CRC tissues. If any one protein is negative in all of cancer cells but positive in normal colorectal mucosa, the IHC staining was reported as mismatch repair defective (dMMR). If the four MMR proteins are expressed in the nucleus of one or more cancer cells, the IHC result was interpreted as mismatch repair proficient (pMMR). All of the 37 cases of dMMR and selected 28 cases of pMMR were tested by PCR-based MSI analysis. Paired normal and cancer DNA samples isolated from the FFPE tissues were tested for MSI using Bethesda recommended 5 markers (BAT25, BAT26, D2S123, D5S346, D17S250). At last the results of IHC and PCR were compared and their concordance were analyzed.

IHC analyses were performed on 368 cases of CRC, among which 37 cases were dMMR and 331 cases were pMMR. After excluding 2 cases from the 37 samples, the remained 35 samples were tested for MSI, among which 32 samples were high-level microsatellite instability (MSI-H) and 3 samples were microsatellite stable (MSS). In addition, 28 cases of pMMR samples were selected to be tested by PCR for MSI, among which 27 cases were MSS but one case was MSI-H. The sensitivity and specificity of immunohistochemistry was 97.0% and 90.0%, separately, the sensitivity and specificity of PCR was 91.4% and 96.4%, separately, and the total concordance of the two detections achieved 93.7%. The most common original site of dMMR CRC was right hemicolon (occupying 48.6%), and the most common pathological features included mucous adenocarcinoma, poor differentiated adenocarcinoma with lymphocytes infiltration, and pathologic TNM stage Ⅱ and stage Ⅲ.

The total concordance of the immunohistochemistry for MMR proteins and PCR-based MSI testing achieved 93.7%, and the former is an economic and quick screening method which is deserved to be popularized in China. Moreover, we have to emphasize the important role of intra and external laboratory quality control of the two methods and then to improve the process, so as to increase the testing accuracy.

Colorectal neoplasms; Immunohistochemistry; Microsatellite instability; Mismatch repair proteins