Abstract
Multiple myeloma is the second most frequent hematological cancer after lymphoma and remains an incurable disease. The pervasive support provided by the bone marrow microenvironment to myeloma cells is crucial for their survival. Here, an unbiased assessment of receptor tyrosine kinases overexpressed in myeloma identified ROR2, a receptor for the WNT noncanonical pathway, as highly expressed in myeloma cells. Its ligand, WNT5A is the most abundant growth factor in the bone marrow of myeloma patients. ROR2 mediates myeloma cells interactions with the surrounding bone marrow and its depletion resulted in detachment of myeloma cells from their niche in an in vivo model, triggering apoptosis and thus markedly delaying disease progression. Using in vitro and ex vivo 3D-culture systems, ROR2 was shown to exert a pivotal role in the adhesion of cancer cells to the microenvironment. Genomic studies revealed that the pathways mostly deregulated by ROR2 overexpression were PI3K/AKT and mTOR. Treatment of cells with specific PI3K inhibitors already used in the clinic reduced myeloma cell adhesion to the bone marrow. Together, our findings support the view that ROR2 and its downstream targets represent a novel therapeutic strategy for the large subgroup of MM patients whose cancer cells show ROR2 overexpression.