Osteoclast Gene Expression Profiling in Multiple Myeloma

Multiple myeloma (MM) is a fatal hematologic malignancy associated with clonal expansion of malignant plasma cells within the bone marrow and the development of a destructive osteolytic bone disease. The principal cellular mechanisms involved in the development of myeloma bone disease are an increase in osteoclastic bone resorption, and a reduction in bone formation. Myeloma cells (MMC) are found in close association with sites of active bone resorption, and the interactions between myeloma cells and other cells within the specialized bone marrow microenvironment are essential, both for tumor growth and the development of myeloma bone disease. In order to investigate the gene expression profile (GEP) of osteoclastic cells, we compare GEP of osteoclastic cells (7 samples) with normal B cells (7 samples), normal bone marrow plasma cells (7 samples), bone marrow stromal cells (5 samples), bone marrow CD3 cells (5 samples), CD14 cells (7 samples), CD15 cells (7 samples), CD34 cells (7 samples) and primary MMC (123 samples). Using SAM analysis, a set of 552 genes was overexpressed in osteoclasts compared to others cell subpopulations with a FDR ≤ 1% and a ratio ≥ 2. Osteoclasts specifically overexpressed genes coding for chemokines (CCL2, CCL7, CCL8, CCL13, CCL18, CXCL5 and CCL23) and MMC growth factors (IGF-1, APRIL and IL-10). Anti- IGF-1 receptor and TACI-Fc inhibit MMC growth induced by osteoclasts. Among the chemokines overexpressed by osteoclasts, the majority of them have a common receptor: CCR2 expressed by MMC. Anti-CCR2 MoAb inhibits migration of the CCR2⁺ HMCL in response to osteoclasts. Expression data of purified MMC were analyzed by supervised clustering of group with higher (CCR2^high) versus lower (CCR2^low) CCR2 expression level. Patients in the CCR2^high group are characterized by a higher bone disease. A set of 176 genes was differentially expressed between CCR2^high and CCR2^low MMC. CCR2^high displayed a gene signature linked to the dependency of MMC on the interactions with the BM osteoclastic subpopulation and the osteoclastic bone resorption.

Taken together, our findings suggest addition of chemokine antagonists to current treatment regimens for MM should result in better therapeutic responses because of the loss of both the protective effect of the bone marrow environment on the MMC and the osteoclastic cells activity.