MTAP Deletions in Cancer Create Vulnerability to Targeting of the MAT2A/PRMT5/RIOK1 Axis.

Homozygous deletions of p16/CDKN2A are prevalent in cancer, and these mutations commonly involve co-deletion of adjacent genes, including methylthioadenosine phosphorylase (MTAP). Here, we used shRNA screening and identified the metabolic enzyme, methionine adenosyltransferase II alpha (MAT2A), and the arginine methyltransferase, PRMT5, as vulnerable enzymes in cells with MTAP deletion. Metabolomic and biochemical studies revealed a mechanistic basis for this synthetic lethality. The MTAP substrate methylthioadenosine (MTA) accumulates upon MTAP loss. Biochemical profiling of a methyltransferase enzyme panel revealed that MTA is a potent and selective inhibitor of PRMT5. MTAP-deleted cells have reduced PRMT5 methylation activity and increased sensitivity to PRMT5 depletion. MAT2A produces the PRMT5 substrate S-adenosylmethionine (SAM), and MAT2A depletion reduces growth and PRMT5 methylation activity selectively in MTAP-deleted cells. Furthermore, this vulnerability extends to PRMT5 co-complex proteins such as RIOK1. Thus, the unique biochemical features of PRMT5 create an axis of targets vulnerable in CDKN2A/MTAP-deleted cancers.

RANK and RANK ligand expression in primary human osteosarcoma.

Receptor activator of nuclear factor kappa-B ligand (RANKL) is an essential mediator of osteoclast formation, function and survival. In patients with solid tumor metastasis to the bone, targeting the bone microenvironment by inhibition of RANKL using denosumab, a fully human monoclonal antibody (mAb) specific to RANKL, has been demonstrated to prevent tumor-induced osteolysis and subsequent skeletal complications. Recently, a prominent functional role for the RANKL pathway has emerged in the primary bone tumor giant cell tumor of bone (GCTB). Expression of both RANKL and RANK is extremely high in GCTB tumors and denosumab treatment was associated with tumor regression and reduced tumor-associated bone lysis in GCTB patients. In order to address the potential role of the RANKL pathway in another primary bone tumor, this study assessed human RANKL and RANK expression in human primary osteosarcoma (OS) using specific mAbs, validated and optimized for immunohistochemistry (IHC) or flow cytometry. Our results demonstrate RANKL expression was observed in the tumor element in 68% of human OS using IHC. However, the staining intensity was relatively low and only 37% (29/79) of samples exhibited≥10% RANKL positive tumor cells. RANK expression was not observed in OS tumor cells. In contrast, RANK expression was clearly observed in other cells within OS samples, including the myeloid osteoclast precursor compartment, osteoclasts and in giant osteoclast cells. The intensity and frequency of RANKL and RANK staining in OS samples were substantially less than that observed in GCTB samples. The observation that RANKL is expressed in OS cells themselves suggests that these tumors may mediate an osteoclastic response, and anti-RANKL therapy may potentially be protective against bone pathologies in OS. However, the absence of RANK expression in primary human OS cells suggests that any autocrine RANKL/RANK signaling in human OS tumor cells is not operative, and anti-RANKL therapy would not directly affect the tumor.

RANKL inhibition blocks osteolytic lesions and reduces skeletal tumor burden in models of non-small-cell lung cancer bone metastases.

INTRODUCTION: Bone metastasis is a serious complication in patients with lung cancer, occurring in up to 40% of patients. Tumor cell-mediated osteolysis occurs ultimately through induction of RANK ligand (RANKL) within the bone stroma although this hypothesis has not been tested extensively in the setting of non-small-cell lung cancer (NSCLC). By using two novel NSCLC bone metastasis mouse models, we examined the effects of RANKL inhibition on osteolysis and tumor progression. METHODS: We treated mice bearing skeletal NSCLC tumors with osteoprotegerin-Fc (OPG-Fc) to assess whether osteoclast inhibition through RANKL inhibition would affect bone metastases at early or late stages of bone colonization. Progression of skeletal tumor was determined by radiography, longitudinal bioluminescent imaging, and histological analyses. RESULTS: OPG-Fc reduced development and progression of radiographically evident osteolytic lesions and also significantly reduced skeletal tumor progression in both NSCLC bone metastasis models. In the H1299 human NSCLC bone metastasis model, OPG-Fc plus docetaxel in combination resulted in significantly greater inhibition of skeletal tumor growth compared with either single agent alone. The observed ability of RANKL inhibition to reduce NSCLC osteolytic bone destruction or skeletal tumor burden was associated with decreases in tumor-associated osteoclasts. CONCLUSIONS: These results demonstrate that RANKL is required for the development of tumor-induced osteolytic bone destruction caused by NSCLC cells in vivo. RANKL inhibition also reduced skeletal tumor burden, presumably through the indirect mechanism of blocking tumor-induced osteoclastogenesis and resultant production of growth factors and calcium from the bone microenvironment. RANKL inhibition also provided an additive benefit to docetaxel treatment by augmenting the reduction of tumor burden.

RANK expression on breast cancer cells promotes skeletal metastasis.

RANK ligand (RANKL), acting through its cognate receptor RANK, is a key factor for bone remodeling and metastasis by regulating the differentiation, survival and activation of osteoclasts. RANKL is also crucial for the development of mouse mammary glands during pregnancy and has been recently linked to the etiology of breast cancer via its direct activity on RANK-expressing normal or transformed breast epithelial cells, leading to increased mitogenesis, enhanced regenerative potential of mammary stem cells, and increased invasion and migration. We demonstrate that higher RANK expression in MDA-MB-231 breast cancer cells (MDA-231-RANK cells) is sufficient to confer a significantly greater metastatic growth rate in the bone compared with MDA-MB-231 cells which do not express high levels of RANK. Blockade of osteoclastic bone resorption, achieved with treatment by either RANKL inhibition or zoledronic acid, did reduce skeletal tumor progression of MDA-231-RANK cells suggesting that the vicious cycle contributes to metastatic growth. However, RANKL inhibition reduced skeletal growth of MDA-231-RANK tumors to a significantly greater extent than zoledronic acid, indicating that skeletal growth of RANK-positive tumors is also driven by direct RANKL effects. RANKL stimulated the expression of multiple genes associated with cell invasive behavior, including several matrix metalloproteinases and other genes previously defined as part of a bone metastasis gene signature. These data indicate that RANKL provokes breast cancer bone metastases via two distinct, but potentially overlapping mechanisms: stimulation of tumor-associated osteoclastogenesis and stimulation of RANK-expressing tumor cells.