A role for Sprouty2 in human and mouse lymphoma

B cell lymphoma is the most common immune system malignancy. The over-expression or ectopic expression of the non-enzymatic oncoprotein, T cell leukemia/lymphoma I (TCL1), is implicated in the transformation of mature B cells in humans and transgenic mice. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B- and T-cell leukemia and lymphoma subtypes, demonstrating an oncogenic role for TCL1. However the precise mechanism for TCL1-mediated B cell transformation has yet to be resolved. TCL1 can bind to and augment AKT pathway signaling, yet tumorigenic activity of TCL1 involves more than augmentation of AKT signaling. Increasing AKT activity in B-cells to levels higher than are detected in TCL1-tg B-cells by deleting Pten fails to cause B cell transformation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg B-cell tumors to discover tumor-associated epigenetic changes, and identified hypermethylation of Sprouty2. Sprouty proteins are context-dependent negative or positive regulators of RAS-RAF-MEK-ERK pathway signaling (ERK pathway), but their roles in B-cell physiology or pathology are unknown. Activation of the ERK pathway is critical for progenitor B cell maturation and proliferation, mature B cell proliferation and germinal center B cell maturation into memory B cells. Here we show that repression of Sprouty2 in mouse and human B cell lines, and in primary TCL1-tg mouse and human B-cell lymphomas, is associated with DNA hypermethylation and was reversed by inhibition of DNA methyltransferases. Sprouty2 expression is induced in splenic B-cells by ERK pathway activation, including CD40 and BCR co-stimulation, and regulates a negative feedback loop that represses ERK pathway signaling. We also demonstrate that TCL1 can enhance ERK pathway signaling resulting in enhanced Sprouty2 induction. Over-expression of Sprouty2 in B cells leads to decreased survival via a reduction of ERK pathway signaling, while loss of Sprouty2 increases ERK-dependent B cell proliferation and survival, which gives support for a tumor suppressor role for Sprouty2 in B-cell lymphoma progression. These data suggest that Sprouty2 null mice should have an increase in the number of B cells, yet surprisingly we see the opposite effect. Paradoxically, Sprouty2 null mice have a decreased number of splenic B cells, due to a decrease in the production of progenitor B cells caused by cell extrinsic mechanisms related to the multiple developmental defects of Sprouty2 null mice. In support of this conclusion, mice that lose Sprouty2 expression at 4 weeks of age have an increased production of B cell progenitors, likely due to increased ERK pathway signaling. Combined, these results implicate epigenetic silencing of Sprouty2 expression in B cell tumor progression and support a tumor suppressor role for Sprouty2 in B cell malignancies.