PO-059 IBTK promotes B cell lymphomagenesis in Eμ-MYC transgenic mice conferring resistance to apoptosis

Introduction IBtk acts as substrate receptor of a Cullin 3-dependent E3 ligase, and promotes proteasomal degradation of Pdcd4, a translation inhibitor. There are evidences that IBtk is involved in cell survival. RNA interference of IBtk reduced viability of Ras-positive colorectal cancer cells and mouse embryonic fibroblasts under reticulum stress. Differential methylation of the IBtk genomic region was reported for U-CLL (Unmutated-Chronic Lymphocytic Leukaemia) and M-CLL (Mutated-Chronic Lymphocytic Leukaemia),suggesting a possible correlation between the IBtk expression levels and aggressiveness of disease. Recently, we provided the evidence of IBtk as a prognostic marker of CLL progression. Material and methods Ibtk -/- mice were bred with congenic Em-myc transgenic mice, a pre-clinical mouse model of Non-Hodgkin’s Lymphoma, to generate Ibtk -/- Em-myc mice, which were monitored for survival and tumour development. We used flow-cytometry for B cells immunophenotyping. Primary murine B cells were compared using different approaches: proliferation, cell viability and cell death assays. Results and discussions Our data provide the evidence that Ibtk gene increases survival and delays tumour onset in Eμ-myc mice. Ibtk -/- Em-myc mice mostly developed pre-B lymphoma and to a lesser extent mature B lymphoma, which was consistent with the tumour phenotype of Em-myc mice.Loss of Ibtk substantially reduced the number of premalignant B-lymphoid cells without affecting their proliferation rate. In particular, pre-cancerous immature B cells were reduced in bone marrow and spleen of Ibtk -/- Em-myc compared to Ibtk +/+ Em  myc mice.In Em-myc mice, the pre-cancerous state is characterised by aberrant proliferation of B-lymphoid cells, which is initially offset by pro-apoptotic action of c-Myc.We have previously shown that the enhanced expression of IBtk in CLL cells down regulate the expression of pro-apoptotic genes, thus counteracting apoptosis.According to this hypothesis, we found an increased spontaneous apoptosis of pre-cancerous Ibtk -/- Em-myc B cells ex vivo and in vitro without added cytokines. Conclusion c-Myc is abnormally expressed in a great majority of human cancers. The evidence that IBtk promotes the survival of c-Myc–driven premalignant B cells could have general implications for oncogenesis. Our findings support a synergistic role of IBtk in Myc-driven B-lymphomagenesis conferring resistance to apoptosis.In summary, this study provides the rationale for novel therapeutic approaches of B-lymphoma.