Multiple Myeloma (MM) is the second most prevalent hematological malignancy and it remains incurable, with a median survival of 3-5 years. One possible explanation for the changes that occur in normal plasma cells that lead to proliferation into malignant MM cells is epigenetic alterations, such as the regulation of micro-RNA (miRNAs). miRNAs have recently been shown to be involved in the pathogenesis of MM. In this grant, we chose to focus on the role of miR-15a and -16-1 in the progression of MM. miR-15a and -16-1 are present in a cluster located at chromosome 13q14, which is commonly deleted in over 50% of patients with MM. We have recently shown that the expression of miR-15a and -16-1 is low in MM cells. However, what is lacking is a better understanding of the mechanisms by which these miRNAs regulate tumor progression and dissemination, and how these miRNAs regulate interactions between the tumor clone and the BM microenvironment. We hypothesize that miR-15a and -16-1 are critical regulators of tumor progression and dissemination in MM, through regulations that occur in the MM clone and its interaction with the BM microenvironment. In Specific aim 1, we will examine the role of the miR-15a/-16-1 cluster as tumor suppressors in the MM clone during disease progression. This will be performed using samples from patients during MM progression, and through transgenic mouse models of MM and DLEU2/miR15a/16-1 knockout mice. In Aim 2, we will determine the role of the miR-15-a/16-1 cluster on the interaction of MM cells and mesenchymal cells in the BM milieu. Here, we will examine whether the interaction of the MM clone and the mesenchymal stem cells/stromal cells is through transfer of miRNA in exosomes. Finally, in Aim 3, we determine the role of miR15a/16-1 on cell dissemination and drug-resistance in MM. Here, we will examine the role of miR15a/16-1 in the regulation of cell metastasis and invasion in MM in vitro and in vivo. We will also determine the role of miR15a/16-1 in regulating sensitivity/resistance of MM cells to therapeutic agents such as bortezomib and dexamethasone. These studies will lead to the application of miR-15a/-16-1 as prognostic markers clinically used in practice. In addition, the development of pre-miRNA mimetics that can be used in clinical trials for patients may lead to a paradigm shift in the treatment of MM, and may allow individualized therapy based on the presence of specific miRNA expression. Therefore, this proposal will likely have significant therapeutic and prognostic applications for patients with MM.