Vascular Progenitor Cells in Multiple Myeloma: Genomic Characterization and Clinical Significance

Abstract

Multiple myeloma (MM) is a bone marrow (BM) cancer of neoplastic plasma cells, and remains incurable despite recent advances in treatment. While the etiology of neoplastic transformation of the plasma cell clone remains unknown, genetic alterations observed in neoplastic plasma cells, disease-defining tumor gene expression patterns, and paracrine cross-regulation between MM tumor cells and their BM microenvironment are clearly involved in the tumorigenesis of MM. The growth and survival of MM cells is dependent on increased vascularization of the BM microenvironment. Our laboratory first showed that in MM patients, endothelial progenitor cell (EPC) levels in blood covary with disease progression, and that these MM EPCs contribute to tumor neoangiogenesis. Furthermore, we and others have demonstrated that, similar to MM tumor cells, MM EPCs display evidence of genetic instability, suggesting that they are an integral part of the neoplastic process in MM, and that EPC markers could provide additional targets to improve current treatment strategies. However, the molecular profile of MM EPC-driven neoangiogenesis and its potential for therapeutic intervention in MM has not been fully defined. My thesis work is a translational study in which I explored the hypothesis that EPCs in MM patients display genomic instability, bear genomic similarity to the neoplastic plasma cell clone, and express candidate biomarkers that may serve as important treatment targets. Results of my experiments demonstrate strong evidence for myeloma-specific genomic instability in MM EPCs, as evidenced by: (1) high-throughput array comparative genomic hybridization showing tumor-similar and EPC-unique genome-wide chromosomal gains and losses in MM EPCs; (2) X-chromosome inactivation studies indicating that, as in MM tumor cells, MM EPCs are clonally restricted; (3) quantitative PCR analysis indicating clonal immunoglobulin gene rearrangement in MM EPCs; and (4) RNA microarray profiling showing disease-relevant gene expression patterns in MM EPCs. Importantly, each of these findings was confirmed as being distinct from normal control endothelial cells. Furthermore, bioinformatic integration of genomic data presented in my thesis shows for the first time that the EPC genome contains tumor-like and tumor-divergent gene expression profiles distinct from those of normal human endothelial cells. Many gene pathways in either EPC profile have not previously been characterized in MM, but include cancer biomarkers that have been implicated in other cancers. In fact, one of the pathways that is commonly dysregulated in MM tumor and endothelial cells includes genes involved in the unfolded protein response (UPR), and therefore, as an initial application of this bioinformatic approach to disease treatment, I present preclinical data demonstrating the ability of a novel small molecule inhibitor of the molecular chaperone heat shock protein 70 to induce strong synergistic cytotoxic effects in combination with other agents that augment the UPR in MM tumor cells and EPCs. These novel results shed light on previously unknown mechanisms that facilitate neovasculogenesis in MM, and lead to the important conclusion that genomically unstable MM EPCs in the tumor microenvironment represent a new pathogenic aspect of MM disease that can inform potential treatment strategies. Thus, MM EPCs warrant further investigation as a key component of the process that results in neoplastic transformation.