RET-regulated microRNAs as Recurrence Biomarkers and Therapeutic Targets in Medullary Thyroid Carcinoma

Abstract

Medullary thyroid carcinoma (MTC) is an aggressive malignancy which accounts for 3 – 5 % of all thyroid cancers. MTC originates from calcitonin-producing parafollicular C-cells of the thyroid gland. Genetically, gain-of-function mutations of the RET tyrosine kinase is known to be the key driver of MTC tumourigenesis. RET has been targeted by tyrosine kinase inhibitors (TKIs), however the efficacy has been modest. MicroRNAs are small non-coding RNAs and are known to suppress gene expression. Deregulation of miRNAs in cancer is often associated with the progression of malignancy, and targeting miRNA expression can modify cancer behaviour. We aimed to identify miRNAs whose expressions are altered by RET in MTC, investigate their cellular and molecular effects on tumourigenesis and on modulation of TKI (cabozantinib) responses, and ultimately establish a novel miRNA therapeutic target for MTC. Small RNA-Sequencing was performed in MTC cells before and after RET inhibition to identify RET-regulated miRNAs. Expressions of potential miRNAs were validated in a large cohort of clinical MTC tissue samples. We identified a specific miRNA, miR-153-3p which was under-expressed in MTC. In vitro gain-of-function studies demonstrated the tumour suppressive roles of miR-153-3p in MTC. Restoration of miR-153-3p reduced cell proliferation, migration and invasion, while enhancing apoptosis. miR-153-3p repressed the expression of ribosomal protein S6 kinase B1 (RPS6KB1) of mTOR signalling and further reduced downstream phosphorylation of Bcl-2 associated death promoter (BAD). Finally, miR-153-3p alone and/or in combination with cabozantinib was tested in mouse xenograft models. miR-153-3p delivery alone significantly impeded the xenograft growth. Combined treatment resulted in greater growth inhibition. Furthermore, miR-153-3p appeared to enhance cell responses to cabozantinib in the xenografts. We report for the first time how RET contributes to tumour behaviour through regulation of specific miRNAs in MTC. We have identified miR-153-3p, which offers potential as a therapeutic target and biomarker for recurrence and disease progression. This study highlights potential for improved therapeutic efficacy with a novel combined treatment of miRNA plus TKIs, especially for patients with advanced, metastatic MTC.