Altered expression of microRNA miR-21, miR-155, and let-7a and their roles in pulmonary neuroendocrine tumors

MicroRNA (miRNA) has a critical effect on tumorigenesis through post-transcriptional modification and is considered to be potential biomarkers for cancer diagnosis and treatment monitoring. We evaluated the expression pattern of three selected miRNAs (miR-21, miR-155, and let-7a) to evaluate their potential roles by quantitative reverse transcription-polymerase chain reaction using formalin-fixed and paraffin-embedded tissues of 63 surgically resected pulmonary neuroendocrine (NE) tumors (19 typical carcinoids (TCs), 6 atypical carcinoids (ACs), 19 large cell NE carcinomas (LCNECs), and 19 small cell lung carcinomas (SCLCs). Control amplification for U6 small nuclear RNA (U6) was performed in all samples. Normalized Ct values were calculated (Ct_{Experimental miRNA}-Ct_U6) for each case and recorded. The expression levels of miR-21 and miR-155 were significantly higher in high-grade NE carcinomas (LCNECs and SCLCs) than in carcinoid tumors (TCs and ACs) (each P < 0.001). The expression level of miR-21 in carcinoid tumors with lymph node metastasis was significantly higher than in carcinoid tumors without lymph node metastasis (P= 0.010). To the best of our knowledge, the present study is the first to examine the expression patterns of miR-21 and miR-155 as an adjunctive diagnostic tool or clinically relevant biomarkers for pulmonary NE tumors.