MCM6 Versus Ki-67 in Diagnosis of Luminal Molecular Subtypes of Breast Cancers

Background: Currently, 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) can be taken as a surrogate marker to differentiate molecular subtypes and aid in more precise grading of tumors. Methods: We 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 relevant Cancer Institute database. Results: MCM6 could discriminate between different histologic grades. The luminal B subtype exhibited a higher MCM6 score in comparison to luminal A (P=0.01). There were significantly higher MCM6 scores in the hormone receptor (HR) negative, in comparison to luminal breast cancers (P<0.001). MCM6 score had a significant correlation with the mitotic count (P<0.001). Conclusion: MCM6 can reliably differentiate luminal A and luminal B subtypes and was correlated with the mitotic counts. 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.