The glutamate metabotropic receptor 4 ( GRM4 ) locus is linked to susceptibility to human osteosarcoma, through unknown mechanisms. We show that Grm4 − / − genetargeted mice demonstrate accelerated radiation-induced tumor development to an extent comparable with Rb1 +/ − mice. GRM4 is expressed in myeloid cells, selectively regulating expression of IL23 and the related cytokine IL12. Osteosarcoma-conditioned media induce myeloid cell Il23 expression in a GRM4-dependent fashion, while suppressing the related cytokine Il12 . Both human and mouse osteosarcomas express an increased IL23:IL12 ratio, whereas higher IL23 expression is associated with worse survival in humans. Consistent with an oncogenic role, Il23 −/− mice are strikingly resistant to osteosarcoma development. Agonists of GRM4 or a neutralizing antibody to IL23 suppressed osteosarcoma growth in mice. These fi ndings identify a novel, druggable myeloid suppressor pathway linking GRM4 to the proinfl ammatory IL23/IL12 axis.
SIGNIFICANCE:Few novel systemic therapies targeting osteosarcoma have emerged in the last four decades. Using insights gained from a genome-wide association study and mouse modeling, we show that GRM4 plays a role in driving osteosarcoma via a non-cell-autonomous mechanism regulating IL23, opening new avenues for therapeutic intervention.Osteosarcoma is an aggressive primary malignant tumor of the bone and a signifi cant cause of cancer-related death in the young. Patients are commonly treated with multimodal approaches, including surgery and adjuvant chemotherapy. Few effective systemic therapies have emerged in the last four decades for relapsed or metastatic osteosarcoma ( 1, 2 ). Osteosarcomas represent a promising indication for strategies that target the immune system ( 2 ); however, a recent clinical trialResearch.on July 16, 2020. Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): M. Kansara, K. Thomson, P. Pang, D.M. Thomas Study supervision: M. Kansara, D.M. ThomasResearch.on July 16, 2020.