MicroRNA-21 and long non-coding RNA MALAT1 are overexpressed markers in medullary thyroid carcinoma

Non-coding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), are well-recognized post-transcriptional regulators of gene expression. This study examines the expression of microRNA-21 (miR-21) and lncRNA MALAT1 in medullary thyroid carcinomas (MTCs) and their effects on tumor behavior.

Tissue microarrays (TMAs) were constructed using normal thyroid (n=39), primary tumors (N=39) and metastatic MTCs (N=18) from a total of 42 MTC cases diagnosed between 1987 and 2016. In situ hybridization with probes for miR-21 and MALAT1 was performed. PCR quantification of expression was performed in a subset of normal thyroid (N=10) and primary MTCs (N=32). An MTC-derived cell line (MZ-CRC-1) was transfected with small interfering RNAs (siRNAs) targeting miR-21 and MALAT1 to determine the effects on cell proliferation and invasion.

In situ hybridization (ISH) showed strong (2+ to 3+) expression of miR-21 in 17 (44%) primary MTCs and strong MALAT1 expression in 37 (95%) primary MTCs. Real-time PCR expression of miR-21 (P<0.001) and MALAT1 (P=0.038) in primary MTCs were significantly higher than in normal thyroid, supporting the ISH findings. Experiments with siRNAs showed inhibition of miR-21 and MALAT1 expression in the MTC-derived cell line, leading to significant decreases in cell proliferation (P<0.05) and invasion (P<0.05).

There is increased expression of miR-21 and MALAT1 in MTCs. This study also showed an in vitro pro-oncogenic effect of MALAT1 and miR-21 in MTCs. The results suggest that overexpression of miR-21 and MALAT1 may regulate MTC progression.