Exosomes are small vesicles (50–100 nm) of endocytic origin, which are released in the extra-cellular milieu by several cell types. It is known that cell-to-cell communication is partially mediated by exosomes. The role of exosomes has been shown in tumor progression, due to their ability to carry and transfer microRNAs (miRNAs) to the recipient cells. In this study, we sought to examine the role of circulating exosomes in modulating transition from a monoclonal gammopathy of undermined significance (MGUS) stage to a smoldering myeloma (SMM) stage.
Exosomes were collected from peripheral blood obtained from healthy individuals (n=4), MGUS patients (n=4) and SMM patients (n=4), using ultracentrifugation; and further studied by using electron microscopy and immunogold labeling for the detection of CD63 and CD81. miRNA profiling has been performed using nCounter miRNA expression assay (Nanostring® Technologies, Seattle WA). Bioinformatic software tools (TargetScan, MIRDB) were used to predict the target genes of identified miRNA and define their function.
Circulating exosomes were studied at ultrastructural level showing positivity for CD81 and CD63, as demonstrated by immunogold labeling and electron microscopy. We identified 16 miRNAs differentially expressed in circulating exosome obtained from MGUS patients compared to healthy subjects (p < 0.05): specifically, higher expression of miR-450a, -30e, -125a, -300 and lower expression of miR-185, -150, -98 were observed in MGUS- compared to heatly individual-derived circulating exosomes. Interestingly, miR-30e and -150 are important for modulation of NK cell activity by targeting perforin and c-Myb, respectively. We furthermore compared the miRNA expression profiling between MGUS and SMM circulating exosomes; and found 11 miRNAs differentially expressed (p < 0.05). Specifically, higher expression of miR-107 and lower expression of miR-28, -32, -548a, -939, -99a, -345, -125a, -587, -323b and -92a were observed in SMM- compared to MGUS-derived circulating exosomes. Among the de-regulated miRNAs, miR-99a, -345, -92a and -28 are known to act as tumor suppressors in prior publications. Moreover, predicted targets for miR-107 include genes involved in molding the bone marrow microenvironment. Indeed, miR-107 is known to decrease hypoxia-inducible factor-1 β (HIF-1β), miR-125a correlated with the expression level of matrix metalloproteinase 11 (MMP11), and vascular endothelial growth factor A (VEGF-A) and miR-548a regulates the expression of MMP2.
These findings indicate that circulating exosomes differ between normal, MGUS and SMM patients, and could potentially be involved in modulating the host microenvironment for specific homing of clonal plasma cells to the bone marrow; thus providing a better understanding of the epigenetic changes responsible for the transition from an MGUS stage to a SMM stage.
Ghobrial:Millennium: Advisory Board Other; Novartis: Advisory Board, Advisory Board Other.
Asterisk with author names denotes non-ASH members.