SPROUTY 2: A Novel Attenuator of B Cell Receptor and MAPK Signaling in Chronic Lymphocytic Leukemia

Clinical heterogeneity is a major barrier to effective treatment of Chronic Lymphocytic Leukemia (CLL). Emerging evidence suggests that constitutive activation of various signaling pathways plays a role in the heterogeneous clinical outcome of CLL patients. MAPK-Erk signaling represents one such pathway with a demonstrated role in CLL pathogenesis. In this study, we have investigated the role of Sprouty2 (SPRY2) as a negative regulator of receptor and non-receptor tyrosine kinase signaling in the pathogenesis of CLL. We show that SPRY2 expression is significantly decreased in CLL cells, particularly from poor prognosis patients compared to those from good prognosis patients. Over-expression of SPRY2 in CLL cells from poor prognosis patients decreased their proliferation while increasing their apoptosis. Conversely, down-regulation of SPRY2 in CLL cells from good prognosis patients resulted in increased proliferation. Furthermore, CLL cells with low SPRY2 expression grew more rapidly in a xenograft model of CLL. Strikingly, B-cell specific transgenic over-expression of spry2 in mice led to a decrease in the frequency of B1 cells, the precursors of CLL cells in rodents. Mechanistically, we show that SPRY2 attenuates the BCR and MAPK-Erk signaling by binding to and antagonizing the activities of RAF1, BRAF and SYK in normal B cells and CLL cells. We identified that SPRY2 is targeted by miR-21 which in turn leads to increased activity of Syk and Erk in CLL cells. We also show that the activation of miR-21 is mediated by IL-10 induced STAT3 signaling in CLL cells. Taken together, these results establish SPRY2 as a critical negative regulator of BCR-mediated MAPK-Erk signaling in CLL, thereby providing one of the molecular mechanisms to explain the clinical heterogeneity of CLL