Diffuse Large B-cell Lymphoma (DLBCL) is the most common lymphoid malignancy in adults; in the United States alone 30.000 new cases are diagnosed every year. This tumor is clinically and molecularly heterogeneous and only half of the patients will survive their disease. MicroRNAs (miRNA) are non-protein coding RNAs which control gene expression by pairing to the 3'UTR of target transcripts. Although miRNAs are often disrupted in cancer, their role in the pathogenesis of DLBCL remains unclear. MiRNA-155 (miR- 155) is overexpressed in aggressive subtypes of DLBCL. The oncogenic nature of this miRNA was confirmed in E-miR-155 transgenic mice, whereas its key role in lymphocyte biology was shown in loss of function animals. However, the mechanisms by which miR-155 contributes to lymphomagenesis are still unknown. Using genome-wide approaches and confirmatory strategies we uncovered that miR-155 directly targets the transcription factor SMAD5 and significantly impairs the TGF/BMP-mediated induction of ID2, a key negative regulator of the oncogeneic transcription factor PAX5. Furthermore, we found that DLBCL cell lines genetically modified to overexpress miR-155 or a SMAD5 shRNA become resistance to the growth inhibitory effects of TGF1 in association with a block in p21 expression. The overall objective of this proposal is to elucidate the role of miR-155 in DLBCL and test the hypothesis that SMAD5 targeting, by disrupting multiple downstream effectors of the TGF/BMP signaling module, is at the core of the miR-155 lymphomagenesis. Our specific aims are: 1) Establish the interplay between miR-155, SMAD5 and TGF/BMP signals in primary human DLBCLs and mature B-cells from miR-155-/- mice, 2) Characterize in vitro and in vivo the contribution of a defective ID2 regulation to miR-155-mediated lymphomagenesis and, 3) Define the mechanisms by which SMAD5 regulates p21 expression, and establish in vivo the role of PAX5 in the lymphomas associated with miR-155 overexpression and SMAD5-specific knockdown. The hitherto unexplored connection between miR-155 and the TGF pathway is highly relevant. These studies could forge a role for SMAD5 in cancer and highlight a novel mechanism by which cancer cells escape the tumor suppressing TGF signals. Complete characterization of the downstream components of these responses should improve our understanding of normal and malignant lymphocyte biology and uncover novel opportunities for therapeutic manipulation of DLBCLs overexpressing miR-155.