OBJECTIVE: Defects in mismatch repair genes or microsatellite instability (MSI) are seen in colorectal cancers (CRCs) both in sporadic and more predominantly in hereditary cases. Loss of MutS homolog (MSH)-6 as a mismatch repair gene may be seen in the CRC. We aimed at evaluating the expression of MSH-6 as a marker of MSI in a common tumor with hereditary and familial features for preventive, diagnostic, and therapeutic purposes.

PATIENTS AND METHODS: Paraffin blocks of 103 patients who underwent colonoscopy or excisional biopsy with a pathologic diagnosis of CRC were selected and immunohistochemistry (IHC) with MSH-6 marker was done.

RESULTS: Of all patients, 53 (51.45%) were males and the age range was 29-87 years. Of these, 96 (93.2%) were positive for the MSH-6 marker and 7 (6.8%) were negative. Of the 103 patients, 96 had adenocarcinoma. No significant relationship was observed between MSH-6 marker and gender, age group, cancer location in the colon, and cancerous type (p-values: 0.261, 0.343, 0.75, and 0.401, respectively). Out of 7 cases with MSH-6 loss, female gender, early-onset (≤50 years), mucinous or poorly differentiated, proximal location, and absence of lymph node involvement were seen in 5, 4, 3, 3, and 5 cases, respectively.

CONCLUSIONS: According to the results and literature, MSH-6 IHC on CRC specimens is recommended especially in young age females, with right-sided tumors, poor differentiation, and mucinous component. First-degree relatives of the patients with MSH-6 loss may be trained for strictly following the guidelines of colorectal cancer screening.