Mouse model of endemic Burkitt translocations reveals the long-range boundaries of Ig-mediated oncogene deregulation.

Human Burkitt lymphomas are divided into two main clinical variants: the endemic form, affecting African children infected with malaria and the Epstein-Barr virus, and the sporadic form, distributed across the rest of the world. However, whereas sporadic translocations decapitate Myc from 5' proximal regulatory elements, most endemic events occur hundreds of kilobases away from Myc. The origin of these rearrangements and how they deregulate oncogenes at such distances remain unclear. We here recapitulate endemic Burkitt lymphoma-like translocations in plasmacytomas from uracil N-glycosylase and activation-induced cytidine deaminase-deficient mice. Mapping of translocation breakpoints using an acetylated histone H3 lysine 9 chromatin immunoprecipitation sequencing approach reveals Igh fusions up to ∼350 kb upstream of Myc or the related oncogene Mycn. A comprehensive analysis of epigenetic marks, PolII recruitment, and transcription in tumor cells demonstrates that the 3' Igh enhancer (Eα) vastly remodels ∼450 kb of chromatin into translocated sequences, leading to significant polymerase occupancy and constitutive oncogene expression. We show that this long-range epigenetic reprogramming is directly proportional to the physical interaction of Eα with translocated sites. Our studies thus uncover the extent of epigenetic remodeling by Ig 3' enhancers and provide a rationale for the long-range deregulation of translocated oncogenes in endemic Burkitt lymphomas. The data also shed light on the origin of endemic-like chromosomal rearrangements.

AID expression levels determine the extent of cMyc oncogenic translocations and the incidence of B cell tumor development.

Immunoglobulin (Ig) isotype switching is a recombination event that changes the constant domain of antibody genes and is catalyzed by activation-induced cytidine deaminase (AID). Upon recruitment to Ig genes, AID deaminates cytidines at switch (S) recombination sites, leading to the formation of DNA breaks. In addition to their role in isotype switching, AID-induced lesions promote Igh-cMyc chromosomal translocations and tumor development. However, cMyc translocations are also present in lymphocytes from healthy humans and mice, and thus, it remains unclear whether AID directly contributes to the dynamics of B cell transformation. Using a plasmacytoma mouse model, we show that AID(+/-) mice have reduced AID expression levels and display haploinsufficiency both in the context of isotype switching and plasmacytomagenesis. At the Ig loci, AID(+/-) lymphocytes show impaired intra- and inter-switch recombination, and a substantial decrease in the frequency of S mutations and chromosomal breaks. In AID(+/-) mice, these defects correlate with a marked decrease in the accumulation of B cell clones carrying Igh-cMyc translocations during tumor latency. These results thus provide a causality link between the extent of AID enzymatic activity, the number of emerging Igh-cMyc-translocated cells, and the incidence of B cell transformation.

Signaling assemblies formed in mast cells activated via Fcepsilon receptor I dimers.

Although aggregation of the Fcepsilon receptor I (FcepsilonRI) is necessary for Ag-mediated mast cell triggering, the relationship between the extent of the FcepsilonRI aggregation and subsequent biochemical and topographical events is incompletely understood. In this study, we analyzed the activation events induced by FcepsilonRI dimers, elicited by binding of anti-FcepsilonRI mAb to rat basophilic leukemia cells. We found that, in contrast to extensively aggregated FcepsilonRI, receptor dimers (1) induced a less extensive association of FcepsilonRI with detergent-resistant membranes, (2) delayed the tyrosine phosphorylation and membrane recruitment of several signaling molecules, (3) triggered a slower but more sustained increase in concentration of free cytoplasmic calcium, (4) induced degranulation which was not inhibited at higher concentrations of the cross-linking mAb, and (5) failed to produce clusters of FcepsilonRI, Syk kinase and Grb2 adapter in osmiophilic membranes, as detected by immunogold electron microscopy on membrane sheets. Despite striking differences in the topography of FcepsilonRI dimers and multimers, biochemical differences were less pronounced. The combined data suggest that FcepsilonRI-activated mast cells propagate signals from small signaling domains formed around dimerized/oligomerized FcepsilonRI; formation of large FcepsilonRI aggregates in osmiophilic membranes seems to promote both strong receptor triggering and rapid termination of the signaling responses.

Involvement of filamentous actin in setting the threshold for degranulation in mast cells.

Previous studies using cytochalasins and latrunculin B, inhibitors of actin polymerization, showed that filamentous (F)-actin had a negative regulatory role in Fc epsilon receptor I (Fc epsilon RI) signaling. How F-actin is involved in regulating the activation of mast cells is unknown. In this study we investigated the role of F-actin in mast cell activation induced by aggregation of the glycosylphosphatidylinositol (GPI)-anchored proteins Thy-1 and TEC-21, and compared it to activation via Fc epsilon RI. Pretreatment of rat basophilic leukemia cells with latrunculin B inhibited the Thy-1-induced actin polymerization and elevated the Thy-1-mediated secretory and calcium responses. Inhibition of actin polymerization followed by Thy-1 aggregation resulted in an increased tyrosine phosphorylation of Syk, phospholipase C gamma (PLC gamma), Gab2 and linker for activation of T cells (LAT) adapters, and some other signaling molecules. Enzymatic activities of phosphatidylinositol 3-kinase, PLC gamma, and phosphatase SHP-2 were also up-regulated, but tyrosine phosphorylation of ezrin was inhibited. Similar changes were observed in Fc epsilon RI-activated cells. Significant changes in intracellular distribution, tyrosine phosphorylation, and/or enzymatic activities of signaling molecules occurred in latrunculin-pretreated cells before cell triggering. The combined data suggest that actin polymerization is critical for setting the thresholds for mast cell signaling via aggregation of both Fc epsilon RI and GPI-anchored proteins.

Positive and negative regulation of Fc epsilon receptor I-mediated signaling events by Lyn kinase C-terminal tyrosine phosphorylation.

Although it is known that the Src family tyrosine kinase Lyn initiates Fc epsilon receptor I (Fc epsilon RI) signaling by phosphorylation of the receptor subunits, regulation of Lyn kinase activity and its consequences for receptor signaling are incompletely understood. Using a phospho-Lyn-specific antiserum, we show an increased phosphorylation of the Lyn C-terminal regulatory tyrosine and decreased Lyn kinase activity during Fc epsilon RI-mediated mast cell activation. Mutant Lyn, defective in the C-terminal tyrosine, constitutively phosphorylated several substrates in resting cells, but did not cause Fc epsilon RI internalization or spontaneous degranulation. Fc epsilon RI-induced signaling in the presence of constitutively active Lyn exhibited enhanced phosphorylation of the receptor subunits, Syk, LAT, Gab2, phospholipase C (PLC)gamma 1 and PLC gamma 2, and production of phosphatidylinositol 3,4,5-trisphosphate. Although enzymatic activities of PLC gamma 1 and PLC gamma 2 were also up-regulated, amounts of inositol 1,4,5-trisphosphate, mobilization of intracellular calcium and degranulation were suppressed. Additionally, constitutively active Lyn was strikingly less efficient than wild-type Lyn in restoring the receptor-mediated calcium responses in bone marrow mast cells derived from Lyn(-/-) mice. These findings pinpoint the tight regulation of Lyn kinase activity as a critical event in mast cell degranulation.

Exogenous administration of gangliosides inhibits Fc epsilon RI-mediated mast cell degranulation by decreasing the activity of phospholipase C gamma.

Gangliosides released from tumor cells, as well as administered exogenously, suppress the immune responses by largely unknown mechanisms. We show here that a pretreatment of rat basophilic leukemia cells with isolated brain gangliosides inhibited the release of preformed secretory mediators from cells activated via FcepsilonRI but not Thy-1 glycoprotein. Exogenously administered gangliosides also affected the cell-substrate adhesion and the levels of polymeric filamentous actin in Ag-activated cells. Although the production of phosphoinositides was also decreased, enzymatic activity of phosphatidylinositol 3-kinase was not inhibited. Gangliosides had no or only marginal effect on the association of aggregated FcepsilonRI with glycosphingolipid-enriched membranes and on tyrosine phosphorylation of FcepsilonRI and the linker for activation of T cells. Though pretreatment with gangliosides did not inhibit the association of linker for activation of T cells with phospholipase C (PLC)gamma1 and PLCgamma2, tyrosine phosphorylation of these enzymes, as well as their enzymatic activities and association with detergent-insoluble signaling assemblies were reduced. This resulted in a decreased production of inositol 1,4,5-trisphosphate and an inhibition of Ca(2+) mobilization. The combined data support the concept that exogenously administered gangliosides interfere with those properties of glycosphingolipid-enriched membranes that are important for the formation of plasma membrane-associated signaling assemblies containing PLCgamma but not for initial tyrosine phosphorylation of FcepsilonRI subunits.