Activating RET Mutations Promotes Osteoblastic Bone Metastases in Medullary Thyroid Cancer

Development of bone metastases increases mortality in patients with medullary thyroid cancer (MTC), with ∼50% survival at 5 years after diagnosis, but the underlying mechanisms are unknown. We show that patient-derived MTC cells ( RET ^C634W mutant TT cells and RET ^M918T mutant MZCRC1 cells) promote an osteoblastic phenotype due to reduced bone resorption. Mechanistically, activated RET increases the expression of osteoprotegerin (OPG), an inhibitor of bone resorption, leading to decreased osteoclast differentiation. Furthermore, RET knockdown or pharmacological RET inhibition attenuates tumor burden and osteoblastic lesions in MTC-bearing femurs. Circulating levels of OPG were increased in the plasma of MTC patients who developed bone metastases and this was associated with poor overall survival. Patients who were treated with multi-kinase inhibitors have lower circulating levels of OPG. These novel findings identify a link between the RET signaling pathway and abnormal osteoblastic bone formation and suggest OPG as a potential biomarker of MTC bone metastases.