Medullary thyroid carcinoma (MTC) is a rare cancer which originates from a small population of neuroendocrine C-cells of thyroid gland. Surgery remains the only curative treatment. The human REarranged during Transfection (RET) proto-oncogene encodes a receptor tyrosine kinase, and its mutation is recognised as the key event of MTC tumorigenesis. RET in MTC has been targeted by tyrosine kinase inhibitors (TKIs) but with modest efficacy. Growing evidence shows that cancer biology can be modified by targeting miRNA expression. Progress of miRNA studies in MTC has been hampered due to the lack of normal C-cell tissue as a differential expression comparator.
Aims & Hypothesis:
We aimed to identify miRNAs whose expression is altered by RET proto-oncogene in MTC. We propose that specific RET-associated miRNAs play fundamental roles in MTC tumorigenesis and modulation of the TKI responses, with the ultimate goal of establishing systemic miRNA-based treatment as a novel therapeutic regimen.
RET of human MTC cells were silenced with TKI, Cabozantinib, or siRNA. Enriched miRNA samples were prepared for small RNA sequencing performed by Australian Genome Research Facility. MTC cells were transfected with miRNA mimics and MTS assay was performed to assess the miRNA effects on cell proliferation.
A list of differentially expressed miRNAs common to both RET blockade have been first identified with small RNA sequencing then validated in a large cohort of MTC tissues using RT-qPCR. Restoration of under-expressed miR-1277-5p and miR-153-3p significantly reduced cell proliferation in MTC cells. Combined treatment of miR-1277-5p and Cabozantinib caused greater cell proliferation inhibition when compared to individual treatment. Furthermore, mRNA and protein expression of the miRNA target - ribosomal protein S6 kinase B1 (RPS6KB1) was significantly reduced with the increased expression of miR-1277-5p and miR-153-3p.
To our knowledge, this is the first study identifying RET-related miRNAs in MTC. Functional characterisation of under-expressed miRNAs as tumour suppressors in MTC offers potential to establish novel miRNA therapy in MTC.