Year: 2010
Type: article
Abstract: Antigen triggering of the B-cell receptor (BCR) has long been assumed to provide a critical growth signal in the multistep process of lymphomagenesis. However, the evidence for this was based on the characteristics of the rearranged immunoglobulin (Ig) genes expressed by the tumor cells and on engineered mouse models, rather than on the presence of structural and functional alterations in human samples. A recent study by Davis et al.1 published in Nature demonstrates that chronic activation of BCR signaling is required for the survival of tumor cells in a distinct subset of lymphomas characterized by unfavorable prognosis, and provides a pathogenetic basis for these findings by identifying the presence of somatic mutations in genes encoding for the proximal BCR subunits CD79B and CD79A. These data offer the rationale for the development of targeted therapies aimed at inhibiting BCR signaling in this lymphoma subtype. The tumor cells of most B-cell non-Hodgkin's lymphomas (B-NHLs) express BCRs on their cell surface.2 The pattern of somatic hypermutation in their rearranged Ig genes, and the observation that translocations of proto-oncogenes into the Ig heavy chain locus are generally targeted to non-functional alleles, has long suggested that the tumor cells or their precursors have been positively selected for the expression of a functional BCR; this implies a role for a yet unidentified antigen in tumor growth. Tumor cells may also acquire growth-promoting signals through the BCR in an antigen-independent manner, as tonic BCR signaling has been shown to be required for mature B-cell survival.3 More recently, experimental evidence for a role of BCR signaling in lymphoma has been obtained from the observation that a subset of diffuse large B-cell lymphomas (DLBCLs) exhibits coordinate overexpression of BCR signaling components,4 and that pharmacological inhibition of the BCR pathway in these tumors can block proliferation by inducing apoptosis.5 Nevertheless, as of now, a causative role for abnormal BCR activation in lymphomagenesis has not been formally demonstrated. DLBCL, the most common type of B-NHL, represents a heterogeneous diagnostic category that can be broadly separated into at least two phenotypic subtypes differing in molecular features and clinical presentation and reflecting the origin from distinct stages of B-cell differentiation.6 These tumors also show largely distinct genetic alterations, indicating their reliance on separate oncogenic pathways. The less curable activated B-cell type (ABC)-DLBCL is most commonly associated with chromosomal translocations deregulating the germinal center master regulator BCL6 and with mutations inactivating the PRDM1 gene, which encodes a transcriptional repressor required for plasma cell differentiation. Structural alterations of BCL6 and PRDM1 are mutually exclusive and are thought to promote lymphomagenesis by blocking terminal differentiation of germinal center B cells.7 One particularly relevant signaling pathway in the pathogenesis of ABC-DLBCL is the one mediated by the nuclear factor (NF)-κB transcription complex, as demonstrated by the dependence of these cells on NF-κB for proliferation and survival.8 Indeed, genetic alterations affecting multiple components of the NF-κB signaling pathway have been recently identified in more than 50% of ABC-DLBCL patients, where these lesions promote constitutive NF-κB activation, thereby contributing to lymphoma growth (Figures 1a and b). These include oncogenic mutations of CARD11 (∼10% of the cases), a lymphocyte-specific cytoplasmic scaffolding protein that—together with MALT1 and BCL10—forms the BCR signalosome, which relays signals from antigen receptors to the downstream mediators of NF-κB activation.9,10 An even larger fraction of cases (∼30%) carry biallelic genetic lesions inactivating the negative NF-κB regulator A20.10,11 The study by Davis et al. adds another important player to this scenario, and provides for the first time genetic evidence for DNA mutations in genes encoding proximal BCR signaling components (Figure 1c).1 The authors report two main observations: (1) a subset of ABC-DLBCL cell lines requires constitutive or ‘chronic’ active BCR signaling for survival; and (2) in this subset, which typically lacks CARD11 mutations, BCR activation is associated with the presence of somatic mutations of the BCR-associated signaling molecules CD79B (Igβ) and, less frequently, CD79A (Igα). The initial observation by Davis et al. derives from an RNA interference screen aimed at identifying genes with oncogenic potential in ABC-DLBCL. The authors had previously reported that all ABC-DLBCL cell lines depend on CARD11 to activate NF-κB, yet most of these cases carry an intact CARD11 gene. A role for proximal BCR signaling components in these cases was revealed by a subsequent screen showing that short hairpin RNA-mediated downregulation of the BCR-associated Bruton's tyrosine kinase was specifically toxic to cell lines with wild-type CARD11, but not to a cell line with mutated CARD11 or to other lymphoma subtypes. This finding, together with the observation that other components of this signaling pathway (including IgM, Syk, BLNK and PKC-β) were variably required for survival of the same cell lines, suggested the existence of distinct mechanisms by which ABC-DLBCL may activate NF-κB. Moreover, all ABC-DLBCL cell lines as well as a number of patient biopsies showed prominent BCR clusters on their surface, which are reminiscent of those forming in normal B cells upon antigen recognition. This suggests a role for chronic rather than tonic (antigen-independent) BCR signaling. Support for this notion comes from the independent observation that activity of NF-κB is not required for the antigen-independent maintenance of mature B cells.12 Davis et al. then asked whether the active BCR signaling in ABC-DLBCL could be attributed to genetic lesions in BCR signaling components. Notably, sequence analysis of over 150 ABC-DLBCL primary cases and cell lines revealed the presence of somatic changes affecting the CD79B ITAM region in ∼20% of the samples, but not in other B-NHL, indicating a specific role for these lesions in this disease subtype. When reintroduced in BCR-inactive cell lines, the mutant CD79B alleles were associated with higher levels of surface IgM expression and reduced kinase activity of the negative regulator Lyn, which is required for receptor internalization. Based on these findings, the authors propose a model in which CD79B mutations may maintain surface BCR despite chronic active stimulation by attenuating this negative feedback circuit. Thus, CD79B mutations may be selected in the early stages of tumor development to allow the pre-malignant clone to respond abnormally well to antigen. Although additional work will be required to elucidate the precise pathogenetic mechanism by which mutations in CD79B affect BCR activation, the elegant work by Davis et al. identifies a genetically defined lymphoma subset accounting for up to one-fifth of all ABC-DLBCL, and provides the mechanistic basis for the development of anti-BCR specific therapies. Interestingly, although none of the ABC-DLBCL cell lines included in the study carry inactivating mutations of the NF-κB negative regulator A20, analysis of a larger cohort of DLBCL patients revealed that CD79B ITAM mutations, activating mutations of CARD11 and biallelic loss of A20 are mutually exclusive, together accounting for ∼50% of all ABC-DLBCL with constitutive NF-κB activation (L Pasqualucci et al., unpublished observations). In future work, it will be important to test whether the presence of molecular lesions in A20 also confers resistance to BCR signaling inhibition. The relative contribution of BCR and NF-κB signaling to ABC-DLBCL pathogenesis has relevant therapeutic implications. Engagement of the BCR activates NF-κB as well as a number of growth-promoting signal transduction routes, including the phosphoinositide (PI)3-kinase and ERK mitogen-activated protein kinase signaling pathways. Future studies will be needed to address whether the pathogenetic role of chronic active BCR signaling is mediated by activation of NF-κB, or whether it also involves NF-κB-independent downstream pathways. If the latter is the case, one may expect that the combination of therapeutic strategies aimed at inhibiting NF-κB and other BCR-induced tyrosine kinases will have synergistic effects in killing DLBCL cells with chronic active BCR signaling. This possibility is supported by the preliminary experiments by Davis et al. and by the recent observation that signaling of the BCR through PI3-kinase is critical for the survival of mature B cells, independent of NF-κB.11 Overall, the findings of Davis et al. represent an important and long-awaited step in the quest of tracking down the role of BCR in lymphomagenesis. Model for the physiological and pathological activation of the BCR signaling pathway in B cells. (a) In normal B cells, engagement of the BCR by antigen activates the NF-κB signaling pathway via a multiprotein complex composed of CARD11, MALT1 and BCL10. Termination of NF-κB response is then ensured by simultaneous induction of the negative NF-κB regulator A20. BCR engagement also induces activation of PI3K and ERK mitogen-activated protein kinase signaling. (b) In a significant fraction of ABC-DLBCL, mutually exclusive oncogenic mutations of CARD11 and genetic loss of A20 contribute to lymphomagenesis by promoting constitutive NF-κB activation. (c) Mutations of the CD79A (Igα) and CD79B (Igβ) ITAM motifs in a different subset of ABC-DLBCL promote chronic, active BCR signaling as well as NF-κB activation, providing survival signals to the tumor cells.
Source: Immunology and Cell Biology
DOI: 10.1038/icb.2010.36
Authors: Ulf Klein, Laura Pasqualucci
Language: English
Subfield: Pathology and Forensic Medicine
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W1977053390