Distinct B-Cell Specific Transcriptional Contexts of the BCL2 Oncogene Impact Pre-Malignant Development in Mouse Models

Simple Summary Beyond the classical t(14;18) translocation associated with follicular lymphoma, BCL2 is deregulated in multiple B-cell malignancies, including some cases of myeloma, and through diverse genetic anomalies. It is currently unclear how the various deregulation patterns mechanistically impact the phenotype of theses malignancies. We designed two different BCL2 deregulation models in transgenic mice, whereby the oncogene was either associated with the IgH3 ' RR superenhancer, as in t(14;18), or inserted into the kappa light chain locus. We compared the impact of these models on B-cell fate and lymphoid tissues. Linkage to the IgH superenhancer showed a quite specific impact on germinal center B cell populations. The Ig kappa model was much less specific and strongly boosted the plasma cell in-flow and the accumulation of long-lived plasma cells. Upregulated expression of the anti-apoptotic BCL2 oncogene is a common feature of various types of B-cell malignancies, from lymphoma to leukemia or myeloma. It is currently unclear how the various patterns of deregulation observed in pathology eventually impact the phenotype of malignant B cells and their microenvironment. Follicular lymphoma (FL) is the most common non-Hodgkin lymphoma arising from malignant germinal center (GC) B-cells, and its major hallmark is the t(14:18) translocation occurring in B cell progenitors and placing the BCL2 gene under the control of the immunoglobulin heavy chain locus regulatory region (IgH 3 ' RR), thus exposing it to constitutive expression and hypermutation. Translocation of BCL2 onto Ig light chain genes, BCL2 gene amplification, and other mechanisms yielding BCL2 over-expression are, in contrast, rare in FL and rather promote other types of B-cell lymphoma, leukemia, or multiple myeloma. In order to assess the impact of distinct BCL2 deregulation patterns on B-cell fate, two mouse models were designed that associated BCL2 and its full P1-P2 promoter region to either the IgH 3 ' RR, within a 3 ' RR-BCL2 transgene mimicking the situation seen in FL, or an Ig light chain locus context, through knock-in insertion at the Ig kappa locus (Ig kappa-BCL2 model). While linkage to the IgH 3 ' RR mostly yielded expression in GC B-cells, the Ig kappa-driven up-regulation culminated in plasmablasts and plasma cells, boosting the plasma cell in-flow and the accumulation of long-lived plasma cells. These data demonstrate that the timing and level of BCL2 deregulation are crucial for the behavior of B cells inside GC, an observation that could strongly impact the lymphomagenesis process triggered by secondary genetic hits.

Automated summary

BCL2 is a commonly deregulated gene in various B-cell malignancies, and its deregulation may have different impacts on B-cell fate. This study designed two transgenic mouse models to compare the effects of BCL2 association with IgH3'RR and Ig kappa gene loci on B-cell fate and lymphoid tissues. The results showed that BCL2 association with IgH3'RR primarily impacted germinal center B-cell populations, while its association with Ig kappa locus primarily boosted the influx of plasma cells and the accumulation of long-lived plasma cells.