Variant B cell receptor isotype functions differ in hairy cell leukemia with mutated BRAF and IGHV genes.
A functional B-cell receptor (BCR) is critical for survival of normal B-cells, but whether it plays a comparable role in B-cell malignancy is as yet not fully delineated. Typical Hairy Cell Leukemia (HCL) is a rare B-cell tumor, and unique in expressing multiple surface immunoglobulin (sIg) isotypes on individual tumor cells (mult-HCL), to raise questions as to their functional relevance. Typical mult-HCL also displays a mutated BRAF V(600)E lesion. Since wild type BRAF is a primary conduit for transducing normal BCR signals, as revealed by deletion modelling studies, it is as yet not apparent if mutated BRAF alters BCR signal transduction in mult-HCL. To address these questions, we examined BCR signalling in mult-HCL cases uniformly displaying mutated BRAF and IGHV genes. Two apparent functional sets were delineated by IgD co-expression. In sIgD(+ve) mult-HCL, IgD mediated persistent Ca(2+) flux, also evident via >1 sIgH isotype, linked to increased ERK activation and BCR endocytosis. In sIgD(-ve) mult-HCL however, BCR-mediated signals and downstream effects were restricted to a single sIgH isotype, with sIgM notably dysfunctional and remaining immobilised on the cell surface. These observations reveal discordance between expression and function of individual isotypes in mult-HCL. In dual sIgL expressing cases, only a single sIgL was fully functional. We examined effects of anti-BCR stimuli on mult-HCL survival ex-vivo. Significantly, all functional non-IgD isotypes increased ERK1/2 phosphorylation but triggered apoptosis of tumor cells, in both subsets. IgD stimuli, in marked contrast retained tumor viability. Despite mutant BRAF, BCR signals augment ERK1/2 phosphorylation, but isotype dictates functional downstream outcomes. In mult-HCL, sIgD retains a potential to transduce BCR signals for tumor survival in-vivo. The BCR in mult-HCL emerges as subject to complex regulation, with apparent conflicting signalling by individual isotypes when co-expressed with sIgD. This suggests the possibility that mutant BRAF by-passes BCR constraints in mult-HCL.
Tumour Immunogenetics Group, Cancer Sciences Academic Unit, Faculty of Medicine, University of Southampton, Southampton, United Kingdom.
Nicola J
NJ
Weston-Bell
Haematology Oncology Group, Cancer Sciences Academic Unit, Faculty of Medicine, University of Southampton, Southampton, United Kingdom.
Francesco
F
Forconi
University Medical Center Groningen, Department of Internal Medicine-Haematology, Division of Haematology, Groningen, The Netherlands.
Hanneke C
HC
Kluin-Nelemans
Tumour Immunogenetics Group, Cancer Sciences Academic Unit, Faculty of Medicine, University of Southampton, Southampton, United Kingdom.
Surinder S
SS
Sahota
D017209:Apoptosis; D001402:B-Lymphocytes / Q000276:immunology / Q000378:metabolism*; D020013:Calcium Signaling; D002478:Cells, Cultured; D004252:DNA Mutational Analysis; D004705:Endocytosis; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006801:Humans; D007143:Immunoglobulin Heavy Chains / Q000235:genetics*; D007943:Leukemia, Hairy Cell / Q000235:genetics* / Q000276:immunology / Q000473:pathology; D010766:Phosphorylation; D020033:Protein Isoforms / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics*; D011971:Receptors, Immunologic / Q000378:metabolism*
D007143:Immunoglobulin Heavy Chains; D020033:Protein Isoforms; D011971:Receptors, Immunologic; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D048049:Extracellular Signal-Regulated MAP Kinases