Breast cancer (BC) remains a significant global health issue, necessitating advanced molecular approaches for early detection and prevention. This review delves into the roles of microRNAs (miRNAs) and circular RNAs (circRNAs) in BC, highlighting their potential as non-invasive biomarkers. Utilizing in silico tools and databases, we propose a novel methodology to establish mRNA/circRNA/miRNA axes possibly indicative of early detection and possible prevention. We propose that during early tumor initiation, some changes in oncogene or tumor suppressor gene expression (mRNA) are mirrored by alterations in corresponding circRNAs and reciprocal changes in sponged miRNAs affecting tumorigenesis pathways. We used two Gene Expression Omnibus (GEO) datasets and identified five mRNA/circRNA/miRNA axes as early possible tumor initiation biomarkers. We further validated the proposed axes through a Kaplan–Meier (KM) plot and enrichment analysis of miRNA expression using patient data. Evaluating coupled differential expression of circRNAs and miRNAs in body fluids or exosomes provides greater confidence than assessing either, with more axes providing even greater confidence. The proposed methodology not only improves early BC detection reliability but also has applications for other cancers, enhancing preventive measures.

CircRNAs are key regulators of gene expression. The functional roles of exonic versus exon–intron circRNAs in early breast cancer remain unclear. Connexin-43 (GJA1) generates exonic circRNA (ecircRNA) hsa_circ_0077755 (circ_755) and exon–intron circRNA (EIcircRNA), hsa_circ_0077753 (circ_753), both linked to increased breast cancer risk. We examined their structural, molecular, and phenotypic roles in transient and stable knockdown S1 breast epithelial cells, using RT-qPCR, dual-luciferase, lumen formation and invasion assays, and extracellular vesicle (EV) analysis. Knockdown of cytoplasm-enriched circ_755 reduced Connexin-43 transcript and protein, whereas knockdown of nuclear-enriched circ_753 preserved GJA1. Both induced lumen disruption, β-catenin disorganization, increased invasion, and regulated onco-miR-182. More pronounced miR-182 upregulation and broader repression of tumor-suppressive targets (BCL2, FOXO1, RECK) were observed in circ_753 knockdown cells, alongside concurrent depletion of hsa_circ_0008932 (circ_8932), which is predicted to interact with miR-182. Similar reciprocal changes in circRNA/miR-182 were observed in released EVs. These data highlight circRNA-miR-182 networks with distinct effects on tumor initiation and potential value as breast cancer biomarkers.