B Cell Activating Factor and c-Myc Regulate the Progression of Chronic Lymphocytic Leukemia.

Abstract 359

Chromosomal translocations involving c-Myc are frequently found in high grade lymphoma and multiple myeloma. In contrast, c-Myc translocations rarely occur in low-grade lymphomas/leukemias like chronic lymphocytic leukemia (CLL), but when present they are associated with rapid disease progression and bad prognosis. Overexpression of c-myc may also be the result of increased transcription by several proto-oncogene transcription factors, including NF-kB. Mice with c-Myc de-regulation at different stages of B cell development develop either aggressive B cells lymphomas or plasma cell neoplasm. So far, no c-Myc mouse model developed low-grade lymphoma/leukemia. iMyc^Ca mice develop an expansion of CD5⁺ peritoneal B1 cells, as compared with WT littermates mice. These mice have a normal life-span and very rarely develop B cell lymphoma at older age. Interestingly, in iMyc^Ca mice mature B cells, but not plasma cells,could be rescued from apoptosis by administration of B cell-activating factor belonging to the TNF family (BAFF). To our surprise, double transgenic iMyc^Ca/Baff-Tg (Myc/Baff) mice developed a disease resembling human CLL, with dramatically shorter mean survival than parental strains, due to early onset and rapid clonal expansion of a mature CD5⁺B220^low B cell population. Those cells transferred the disease into Baff-Tg (Baff) mice with marked infiltration in lymphoid organs and bone marrow. Gene-expression analyses revealed that among the genes altered in Myc/Baff CD5⁺B220^lowleukemia cells were those with known relevance to human CLL disease, including elevated anti-apoptotic Bcl2 family members. Apart from studies on individual genes, sub-network analysis was performed which showed enrichment of apoptosis-related and stress-induced gene sets in Myc/Baff CD5⁺CD3^- leukemia cells. The NF-kB gene set, a major target downstream of BAFF signaling, was also enriched in Myc/Baff CD5⁺CD3^- leukemia cells. We observed a continuum in levels of c-MYC mRNA in 166 samples using Affymetrix array analyses. Changes in c-Myc protein expression were confirmed by immunoblot analyses and correlated with disease progression. In accordance with the functions of c-Myc as a promoter of cell cycle progression, as well as apoptosis, we found enhanced spontaneous cell death in vitro in CLL cells expressing high levels of c-Myc, which could be abrogated by co culture with BAFF expressing nurse-like cells (NLC) or recombinant BAFF. In addition to its anti-apoptotic role, BAFF treatment of primary human CLL cells led to dramatically enhanced expression of c-Myc through the IKK/NF-kB pathway. Inhibition of the NF-kB pathway significantly reduced viability of both Myc/Baff CD5⁺CD3^- leukemia cells and human CLL cells co-cultured with NLC. Also it significantly lowered CD5⁺B220^low leukemia cell population in blood and spleen, and prevented the infiltration of leukemia cells into lymph nodes and bone marrow of transplanted mice. This study demonstrates a potential pathologic role for c-Myc, in the pathogenesis and progression of CLL.