Abstract

The pursuit of rationale, targeted therapies relies on a detailed understanding of the mechanisms that subvert normal growth control and lead to development of Multiple Myeloma (MM). To further define the mechanistic steps that contribute to MM pathogenesis, we examined mRNA expression profiles of CD138+ plasma cells obtained from normal, Monoclonal Gammopathy of Unknown Significance (MGUS), and MM patient samples. Using genomic results in combination with molecular and cellular-based assays, we demonstrate a critical role for UBC9 and the sumoylation pathway in myeloma cell growth and survival. Notably, Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) demonstrated a ten-fold induction in UBC9; a gene that encodes the sole Sumo-conjugating enzyme in human cells. We demonstrated an elevation of UBC9 in both MM primary patient cells and in a number of patient-derived MM cell lines. In addition, a number of other sumoylation pathway components were induced in primary MM cells at the gene and protein level relative to normal plasma cells. We believe that induction of UBC9 is an early genetic event in the pathogenesis of MM since the induction was observed at the gene and protein level in plasma cells from patients with MGUS. Importantly, UBC9 induction was functionally significant since a different pattern of sumoylation was observed in total cell lysate from MM patient plasma cells relative to that of normal plasma cells. Furthermore, overexpresion of a mutant form of UBC9 deficient in Sumo-conjugating activity increased the sensitivity of plasma cells to apoptosis by chemotherapeutic agents and these cells were impaired in other essential functions that included cellular proliferation, DNA synthesis, resistance to apoptosis and adhesion to bone marrow stroma. Immunoblotting of MM patient cell lysates also demonstrated a similar induction of the UBC9 gene product (Ubc9) as well as induction of the Sumo ligases Nse2 and PIAS1. These studies identify UBC9 as a target upregulated early in the pathogenesis of MM and indicate a critical role for sumoylation in disrupting the controls that govern normal plasma cell growth. To further develop prognostically relevant molecular signatures and classifications of MM subtypes, we analyzed the survival outcome of patients that expressed induced levels of UBC9, as well as other sumoylation components, and demonstrate significantly reduced survival of such patients following current treatment modalities. The results provide evidence for critical role of UBC9 and sumoylation in MM pathogenesis. Furthermore, sumoylation pattern may serve as a therapeutic target in MM, help stratify clinical management and provide a framework for the identification of sumoylation pathway targets that govern MM cell growth and progression.

Author notes

Disclosure: No relevant conflicts of interest to declare.