A mouse genetic approach to dissect the novel role of the calcineurin/NFAT signaling pathway in lymphomagenesis. – CalciNFAT

Cell surface receptors coupled to the activation of phospholipase C-g (receptor-type tyrosine kinases, immunoreceptors) or phospholipase C-b (G protein-coupled receptors) lead to the increase in intracellular calcium ion concentration through the sequential opening of specific calcium channels. The increase in intracellular calcium concentration induces the activation of calcium- and calmodulin (CaM)-dependent protein kinases including CaMKII, CaMKIV and that of calcineurin, a CaM-dependent serine/threonine-specific protein phosphatase. Calcineurin (PP2B) plays a pivotal role in many important biological processes, including the development and function of the immune and nervous systems, patterning of the embryonic vasculature, muscle growth and development and heart valve morphogenesis. Sustained signaling through calcium/calcineurin result in the activation of transcription factors of the NFAT family (NFAT : Nuclear Factors of Activated T cells), and mouse genetic studies have demonstrated a strong epistatic relationship between calcineurin and NFATs activation and function in many developmental processes. In lymphoid cells, the calcineurin/NFAT signaling pathway is esssential to specific aspects of T cell development and plays a central role in the activation of the immune response and in its homeostatic control. The critical role of the calcineurin/NFAT signaling module in the immune response is illustrated (i) by the fact that the two most effective immunosuppressants used in human medicine, FK506 (Prograf) and cyclosporin A (CsA) are inhibitors of calcineurin; (ii) by the fact that patients with a rare form of hereditary severe combined immunodeficiency (SCID) show a selective defect in calcineurin/NFAT activation. Despite their central role in T cell development, immune function and homeostasis, deregulation of calcineurin and of the calcineurin/NFAT pathway have not been shown so far to have a pro-oncogenic role in lymphomagenesis. We have recently shown that (i) several pre-clinical mouse models of human lymphoma present the persistent activation of the calcineurin/NFAT pathway through mechanism(s) that require the maintenance of lymphoma cells in their in vivo cellular context ; (ii) that treatment of mice carrying lymphoma induced by completely distinct initiating mechanisms (e.g. TEL-JAK2- and NotchIC-induced lymphoma) with either CsA or FK506 result both in inactivation of the calcineurin/NFAT pathway in lymphoma cells and in severe inhibition of tumor progression. These observations show that the calcineurin/NFAT pathway is critical to lymphomagenesis and is a target of therapeutic interest in the control of these diseases. The objectives of this research proposal are, mostly through the use of genetically modified mice (i) to demonstrate that calcineurin is required in a tumor cell autonomous fashion in lymphomagenesis ; (ii) to analyze whether, in keeping with its essential role as a calcineurin effector in normal T cells, NFATs are also essential effectors of calcineurin in lymphomas ; (iii) to analyze whether the different factors of the NFAT family found to be activated in these cells (NFAT1, NFAT2, NFAT4) function in a specific or redundant fashion to favour lymphomagenesis ; (iv) to identify the calcineurin/NFAT-specific transcriptome in lymphomas to gain insight into the molecular pathway(s) controled by calcineurin/NFAT in these pathological situations.