Current knowledge of miRNAs as biomarkers in breast cancer

Breast cancer (BC) is the most common malignancy in women worldwide. Breast cancer related mortality has dropped significantly since the widespread adoption of mammographic screening. Approximately, 5% of the patients with BC carry germline mutations that are responsible for their condition. Women carrying a BRCA 1 or 2 mutation have a 57% and 49% lifetime risk of developing breast cancer, respectively. For BRCA1/2 patients reluctant to a prophylactic surgery, the only risk reducing strategy remains an increased imaging and clinical surveillance. Whereas a closer screening mammogram program is helpful in detecting BC at an earlier stage, no laboratory markers exist. New markers would be helpful in identifying BC, once the mutations are identified, and in monitoring the cancer behavior and response to treatment, once the cancer is diagnosed. MicroRNAs (miRNAs) are key regulatory molecules operating in a post-transcriptional level by regulating gene expression. Aberrant miRNA expression was documented in several pathological conditions, including solid tumors, suggesting their involvement in tumorigenesis. MiRNAs can be detected in human fluids and could be used as biomarkers. A different pattern expression of miRNA in biological fluids of BC patients as compared to healthy control is currently under consideration in many clinical trials. The biologic mechanism of miRNAs, a rationale of its use as biomarker in cancer, and a literature review of the most significant results achieved about miRNAS in BC will be reported and discussed in this chapter.