Abstract
Breast cancer is the most common malignancy in women. Bearing in mind these circumstances, a review of new molecular mechanisms underlying breast cancer progression, resistance and various aspects of existing therapeutic methods would lead to new insights for biologists and clinicians. In this regard, we conducted a study covering recent advances in breast cancer biology with a focus on the p21 protein. The study included 147 patients diagnosed with invasive breast cancer. The presence of non-invasive lesions was noted in each invasive breast cancer and surrounding tissue. p21 expression was determined by reading the percentage of nuclear expression in epithelial cells of invasive breast cancer and non-invasive lesions. Results showed that expression of p21 increases with the progression of cytological changes in the epithelium; it is significantly higher in invasive breast cancer compared to non-invasive lesions (p<0.001). There is a difference in p21 expression between different molecular subtypes of breast cancer (p=0.004). Statistically significantly higher values of p21 expression were observed in those breast cancers that showed overexpression of HER2 compared to HER2-negative tumors (p=0.001). Depending on Ki67 expression, the highest p21 expression is in the group with high Ki67 expression values (p=0.019). The increase in p21 expression in tumor cells was accompanied by a statistically significantly reduced expression of ER (p=0.015, ρ=-0.225) and PR (p=0.027, ρ−0.205). p21 protein plays an important role in proliferation, malignant transformation, as well as in progression from non-invasive lesions to invasive breast cancer.