MCM6 versus Ki-67 in diagnosis of luminal molecular subtypes of breast cancers.

backgroundCurrently, breast cancers are divided into four major molecular subtypes. The distinction between the luminal A and luminal B subtypes is mainly based on the cellular proliferation indices and is assessed by the Ki-67 scoring. Due to the limitations in the assessment and expression of Ki-67, we hypothesized that minichromosome maintenance protein 6 (MCM6) might be taken as a surrogate marker to differentiate molecular subtypes and aid in more precise grading of tumors.

methodsWe performed a retrospective, cross-sectional study on 124 samples of breast cancer and 40 samples of normal breast tissue. Relevant clinical information was retrieved from the Cancer Institute database.

resultsMCM6 could discriminate between various categories of histologic grades, tubule formation, mitotic indices, and nuclear pleomorphism (P = 0.002 for tubule formation and P < 0.001 for other). Moreover, the MCM6 score exhibited a significant correlation with the mitotic count (P < 0.001). However, the Ki-67 score could not discriminate subgroups of the mitotic index and nuclear pleomorphism. Compared to the luminal A subtype, luminal B exhibited a higher MCM6 score (P = 0.01). Besides, MCM6 scores were higher for certain subtypes with more aggressive behaviors, such as hormone receptor (HR)-negative disease, and human epidermal growth factor receptor 2 (HER2)-enriched and triple-negative breast cancers, as there was a significantly higher MCM6 mean score in the HR-negative in comparison to the luminal breast cancers (P < 0.001). Similarly, higher MCM6 scores were observed among samples with more advanced nuclear grades, tubule formation, and overall grades.

conclusionMCM6 can differentiate luminal A and luminal B subtypes and is correlated with mitotic counts. However, this study was unable to prove the superiority of MCM6 in differentiating between molecular subtypes compared to the Ki-67 score. Nevertheless, in our study, MCM6 was superior to Ki-67 in exhibiting correlations with the mitotic grade, tubule formation, and nuclear grades. More studies are needed to standardize its assessment methods, determine more robust cut-off values, and evaluate its associations with prognostic features of breast cancer.