Chronic active B-cell-receptor signalling in diffuse large B-cell lymphoma

Abstract

A link between B-cell-receptor (BCR) signalling and human B-cell lymphomas has been inferred from the study of immunoglobulin genes in human lymphomas and from work on mouse models, but more evidence is required to confirm an oncogenic role. New work from Davis et al. shows that chronic activation of BCR signalling is required for the survival of the ABC DLBCL subset of B-cell lymphomas. Somatic mutations in CD29A and CD79B found in these lymphomas appear to contribute to BCR activation. The role of B-cell-receptor (BCR) signalling in human B cell lymphomas has been a long-standing question, with genetic and functional evidence for its oncogenic role in human lymphomas lacking. Here, a form of 'chronic active' BCR signalling that is required for cell survival in the activated B-cell-like subtype of diffuse large B-cell lymphoma is described and analysed, with potential implications for future therapeutic strategies. A role for B-cell-receptor (BCR) signalling in lymphomagenesis has been inferred by studying immunoglobulin genes in human lymphomas1,2 and by engineering mouse models3, but genetic and functional evidence for its oncogenic role in human lymphomas is needed. Here we describe a form of ‘chronic active’ BCR signalling that is required for cell survival in the activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL). The signalling adaptor CARD11 is required for constitutive NF-κB pathway activity and survival in ABC DLBCL4. Roughly 10% of ABC DLBCLs have mutant CARD11 isoforms that activate NF-κB5, but the mechanism that engages wild-type CARD11 in other ABC DLBCLs was unknown. An RNA interference genetic screen revealed that a BCR signalling component, Bruton’s tyrosine kinase, is essential for the survival of ABC DLBCLs with wild-type CARD11. In addition, knockdown of proximal BCR subunits (IgM, Ig-κ, CD79A and CD79B) killed ABC DLBCLs with wild-type CARD11 but not other lymphomas. The BCRs in these ABC DLBCLs formed prominent clusters in the plasma membrane with low diffusion, similarly to BCRs in antigen-stimulated normal B cells. Somatic mutations affecting the immunoreceptor tyrosine-based activation motif (ITAM) signalling modules6 of CD79B and CD79A were detected frequently in ABC DLBCL biopsy samples but rarely in other DLBCLs and never in Burkitt’s lymphoma or mucosa-associated lymphoid tissue lymphoma. In 18% of ABC DLBCLs, one functionally critical residue of CD79B, the first ITAM tyrosine, was mutated. These mutations increased surface BCR expression and attenuated Lyn kinase, a feedback inhibitor of BCR signalling. These findings establish chronic active BCR signalling as a new pathogenetic mechanism in ABC DLBCL, suggesting several therapeutic strategies.