Functional analysis of a novel, thyroglobulin-embedded microRNA gene deregulated in papillary thyroid carcinoma

MicroRNAs, non-coding regulators of gene expression, are known culprits of thyroid cancer. Using next-generation sequencing, we identified a novel microRNA gene, encoded within an important thyroid regulator - thyroglobulin, and analyzed its functionality in the thyroid gland. In vitro and in silico analyses proved that the novel miR-TG is processed from the precursor, and co-expressed with thyroglobulin. Both genes are specific for thyroid tissue and downregulated in papillary thyroid carcinoma by 44% (p = 0.04) and 48% (p = 0.001), respectively. Putative target genes for miR-TG were identified using in silico tools, which pinpointed MAP4K4, an oncogene upregulated in thyroid cancer. Analysis of transcriptome by RNA-seq revealed that overexpression of miR-TG in PTC-derived cell line led to downregulation of several genes, including MAP4K4 (fold change 0,82; p = 0.036). The finding was confirmed by SQ-PCR (fold change 071; p = 0.004). Direct interaction between miR-TG and MAP4K4 was confirmed in the luciferase assay (p = 0.0006). Functional studies showed increase proliferation in K1 cell line transfected with miR-TG. We propose that in normal thyroid miR-TG plays a fine-tuning effect on the maintenance of MAPK pathway, inhibiting the expression of miR's target MAP4K4. This regulation is disturbed in cancer due to downregulation of the novel, thyroglobulin-embedded microRNA, characterized in this study.