Proliferation and Molecular Risk Score of Low Risk Myeloma Cells Are Increased in High Risk Microenvironment Via Augmented Bioavailability of Growth Factors

Introduction: Multiple myeloma (MM) cells from patients with smoldering MM (SMM) and low-risk (LR) MM harbor genetic alterations typically seen in patients with high-risk (HR) disease. To test whether the bone marrow (BM) microenvironment plays a role in controlling growth of LR MM cells, we established an experimental model that mimics a HR microenvironment by co-culturing normal mesenchymal stem cells (MSCs) with HR MM cells. We previously have shown that MSC conditioned media (CM) promotes growth of MM cells more effectively than cell-cell contact, as adhesion to MSCs often promotes survival at the expense of proliferation. Therefore, we utilized CM and hypothesized that MSC CM is enriched with bioactive growth factors that facilitate proliferation of LR MM cells. The aim of the study was to test the effect of CM from unprimed and primed MSCs on the survival, growth, and molecular properties of LR MM cells, and identify molecular pathways that mediate these effects. Methods: Primed MSCs were prepared by co-culturing normal MSCs with BM-dependent MM lines for 5 days. MSCs were trypsinized, replated for 40 min followed by serial washing to remove MM cells. Molecularly classified CD138-selected LR MM cells from 8 newly diagnosed patients were treated with 50% primed CM or unprimed CM, or growth media (CONT) for 5 days. Growth and survival of primary MM cells was assessed by MTT assay and detection of annexin V/PI and KI67 by flow cytometry. Microarrays were performed on primed and unprimed MSCs (n=7) and on primary LR MM cells treated with primed and unprimed MSCs CM (n=3). Pathways were analyzed using Ingenuity. Ultra low depth WGS was performed to assess copy number variation. Protein arrays were performed to test levels of secreted factors in CM (n=7). Results: Growth of primary LR MM cells (n=8) was increased by primed CM 5.1±0.05 (p