Synergistic Signaling of KRAS and Thyroid Hormone Receptor β Mutants Promotes Undifferentiated Thyroid Cancer through MYC Up-Regulation

Undifferentiated thyroid carcinoma is one of the most aggressive human cancers with frequent RAS mutations. How mutations of the RAS gene contribute to undifferentiated thyroid cancer remains largely unknown. Mice harboring a potent dominant negative mutant thyroid hormone receptor beta, TR beta PV (Thrb(PV/PV)), spontaneously develop well-differentiated follicular thyroid cancer similar to human cancer. We genetically targeted the Kras(G12D) mutation to thyroid epithelial cells of Thrb(PV/PV) mice to understand how Kras(G12D) mutation could induce undifferentiated thyroid cancer in Thrb(PV/PV)Kras(G12D) mice. Thrb(PV/PV)Kras(G12D) mice exhibited poorer survival due to more aggressive thyroid tumors with capsular invasion, vascular invasion, and distant metastases to the lung occurring at an earlier age and at a higher frequency than Thrb(PV/PV) mice did. Importantly, Thrb(PV/PV)Kras(G12D) mice developed frequent anaplastic foci with complete loss of normal thyroid follicular morphology. Within the anaplastic foci, the thyroid-specific transcription factor paired box gene 8 (PAX8) expression was virtually lost and the loss of PAX8 expression was inversely correlated with elevated MYC expression. Consistently, co-expression of KRAS(G12D) with TR beta PV upregulated MYC levels in rat thyroid pccl3 cells, and MYC acted to enhance the TR beta PV-mediated repression of the Pax8 promoter activity of a distant upstream enhancer, critical for thyroid-specific Pax8 expression. Our findings indicated that synergistic signaling of KRAS(G12D) and TR beta PV led to increased MYC expression. Upregulated MYC contributes to the initiation of undifferentiated thyroid cancer, in part, through enhancing TR beta PV-mediated repression of the Pax8 expression. Thus, MYC might serve as a potential target for therapeutic intervention.