Minichromosome maintenance protein 7 in colorectal cancer: implication of prognostic significance

Minichromosome maintenance (MCM) proteins are essential components for DNA replication, and also prognostic markers for various human tumors. MCM-positive but Ki67-negative cells (e.g. primary oocytes) are thought to be licensed non-proliferating populations, and are significantly correlated with the clinicopathological profiles of some human tumors. In the present study, we evaluated the expression levels of MCM7, MCM2 and Ki67 in colorectal cancer to clarify their pathobiological significance. We carried out Western blot analyses of 5 human colorectal cancer cell lines and performed immunohistochemistry on 202 surgically removed colorectal cancers of Dukes' B and C stages. Double-labeling immunofluorescence was also carried out on the cancer specimens to identify MCM-positive but Ki67-negative tumor cells. MCM proteins were detected in all the 5 cell lines examined. MCM7 and MCM2 were coexpressed in almost the same populations of tumor cells, whereas MCM7-negative but Ki67-positive tumor cells were absent in the double-labeled specimens, except for mitotic cells. The mean positive tumor labeling indexes (LIs) for MCM7, MCM2 and Ki67 were 58.1, 57.1 and 40.6%, respectively. The mean LI for MCM7-positive but Ki67-negative tumor cells was 17.6%, and significantly correlated with the N status (P=0.01), distant metastasis (P=0.01) and UICC stage (P=0.02). The high LI of >58.1% for MCM7 were independent prognostic factors in multivariate Cox regression analysis (relative risk = 2.12; P=0.02). Our results indicate that MCM7 expression is an independent prognostic factor for human colorectal cancer, and MCM7-positive but Ki67-negative tumor cells are correlated with tumor metastases.