Abstract 2761: Activating RET mutations promotes osteoblastic bone metastasis by inhibiting osteoclastogenesis and stimulating osteoblast activity via regulation of osteoprotegerin

Abstract Bone metastases (BM) are a source of increased mortality and significant morbidity in patients with medullary thyroid cancer (MTC), with ~50% survival at five years post-diagnosis. MTC causes osteoblastic, osteolytic, or mixed lesions, yet the underlying mechanisms are unknown. The RET proto-oncogene encodes a transmembrane tyrosine kinase receptor and is the driver oncogene in ~60% of MTC cases where a germline or somatic activating mutation results in ligand-independent constitutive activation of the receptor. However, the role of RET mutations in the genesis of bone metastasis lesions is unknown. We show that activating RET mutations promotes an osteoblastic phenotype in mice implanted with patient-derived MTC cells by increasing bone formation and decreasing bone resorption. We found that the radiographic appearance of osteoblastic lesions differed between the types of RET mutations (C634W and M918T). MTC cells with RETC634W mutation showed a four-fold increase in bone mineral density, trabecular bone volume, number, and thickness with a decreased number of osteoblasts compared to the non-tumor-bearing femur. TRAP staining revealed that osteoclast numbers per long bone were reduced in the injected femur compared to the control. In contrast, the MTC-RETM918T mutation showed an increased cortical thickness and porosity with reduced medullary area and increased bone formation rate compared to the non-tumor bearing femur. MTC cells formed tumors with extensive mineralized tissue, composed of bone matrix surrounded by osteoblast-like cells and osteoid. The knockdown of RET in the MTC-RETM918T model significantly reduced cortical thickness and increased the number of osteoclasts per bone surface. We observed increased osteoprotegerin (OPG) expression in MTC-bearing femurs compared to controls, suggesting decreased bone resorption. The factors secreted by MTC cells, including OPG, inhibited osteoclast formation from primary bone marrow cells and Raw264 cells. In addition, RET knockdown by shRNA in MTC cells decreased OPG expression. Moreover, the MTC-conditioned media (CM) increased osteoblast differentiation and mineralization of the primary culture of pre-osteoblasts isolated from calvarial bone, long bones, and MC3T3 cells. The conditioned media of RET-depleted cells inhibits osteoblast differentiation and mineralization. The expression of alkaline phosphatase, osteocalcin, OPG, and osterix increased in pre-osteoblast incubated with MTC-CM. Compared to primary non-metastatic tumors, the OPG expression increases in tissues of sporadic MTC bone metastases harboring RETM918T mutation. These results suggest that MTC with RET-activating mutations induces osteoblastic bone metastasis due to decreased bone resorption and increased bone formation. Citation Format: Rozita Bagheri-Yarmand, Joseph L. Kidd, Gabriel M. Pagnotti, Trupti Trivedi, Leah Guerra, Xinhai Wan, Jian Song, Sue-Hwa Lin, Theresa A Guise. Activating RET mutations promotes osteoblastic bone metastasis by inhibiting osteoclastogenesis and stimulating osteoblast activity via regulation of osteoprotegerin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2761.