Table 1.

Functional implications of circulating microRNAs and their characteristic features in breast cancer patients

miRNA	signaling pathways	target genes	source	characteristic BC features	references
miR-21	apoptosis; EGFR	PDCD4, PTEN, BCL-2, HER2, FAS, TPM1	serum	↗ in primary BC, correlation to tumor size and lymph node status	[10,15,16,25-29]
miR-34 a	vascularization; EGFR, β-Catenin	VEGF, MYC, BCL2, WNT, p53	serum	↗ in metastatic BC compared to primary BC and controls	[13,30]
miR-125b	apoptosis; EGFR	HER2, p53, BAK1	serum	↗ in primary BC, prediction of chemotherapy resistance	[17,18,31-33]
miR-155	Akt; apoptosis; morphogenesis; EMT	VHL, VEGF, p53, TGF- β	serum	↗ in primary BC; ↘ after surgery and chemotherapy	[20,21,34,35]
miR-195	apoptosis	BCL 2, CDK6	whole blood	↗ in primary BC	[12,19]
			serum	↘ in primary BC	[22]
miR-200b	EMT	ZEB1/2, E-Cadherin	plasma	↗ in metastatic BC; correlation to CTC status	[18,36]
miR-200c	EMT	ZEB1/2, E-Cadherin	plasma	↗ in metastatic BC; correlation to CTC status	[18,37]
miR-375	apoptosis	14-3-3 Protein	plasma	↗ in metastatic BC	[14,18]
miR-451	MDR	MDR1, 14-3-3 Protein	plasma, serum	↗ in primary BC	[24,38,39]

MiRNA specimen pre-selection for this study was based on previous investigations elucidating functional features and interrelations of miRNA expression in regard to breast carcinogenesis.

↗: increased, ↘: decreased expression levels of miRNAs in comparison to healthy controls; EMT: epithelial-mesenchymal-transition; CTC: circulating tumor cells; MDR: multi-drug resistance.