ABSTRACT
Affinity maturation of the humoral immune response depends on somatic hypermutation (SHM) of immunoglobulin (Ig) genes, which is initiated by targeted lesion introduction by activation-induced deaminase (AID), followed by error-prone DNA repair. Stringent regulation of this process is essential to prevent genetic instability, but no negative feedback control has been identified to date. Here we show that poly(ADP-ribose) polymerase-1 (PARP-1) is a key factor restricting AID activity during somatic hypermutation. Poly(ADP-ribose) (PAR) chains formed at DNA breaks trigger AID-PAR association, thus preventing excessive DNA damage induction at sites of AID action. Accordingly, AID activity and somatic hypermutation at the Ig variable region is decreased by PARP-1 activity. In addition, PARP-1 regulates DNA lesion processing by affecting strand biased A:T mutagenesis. Our study establishes a novel function of the ancestral genome maintenance factor PARP-1 as a critical local feedback regulator of both AID activity and DNA repair during Ig gene diversification.
Subject(s)
Cytidine Deaminase/genetics , Genes, Immunoglobulin/genetics , Immunoglobulin Variable Region/genetics , Poly (ADP-Ribose) Polymerase-1/genetics , Somatic Hypermutation, Immunoglobulin/genetics , Animals , B-Lymphocytes/metabolism , B-Lymphocytes/pathology , Cell Line, Tumor , Cells, Cultured , Cytidine Deaminase/metabolism , DNA Damage , DNA Repair , Humans , Mice , Mutation , Poly (ADP-Ribose) Polymerase-1/metabolismABSTRACT
Activation-induced cytidine deaminase (AID) initiates immunoglobulin diversification in germinal center B cells by targeted introduction of DNA damage. As aberrant nuclear AID action contributes to the generation of B cell lymphoma, the protein's activity is tightly regulated, e.g. by nuclear/cytoplasmic shuttling and nuclear degradation. In the present study, we asked whether DNA damage may affect regulation of the AID protein. We show that exogenous DNA damage that mainly activates base excision repair leads to prevention of proteasomal degradation of AID and hence its nuclear accumulation. Inhibitor as well as knockout studies indicate that activation of poly (ADP-ribose) polymerase (PARP) by DNA damaging agents promotes both phenomena. These findings suggest that PARP inhibitors influence DNA damage dependent AID regulation, with interesting implications for the regulation of AID function and chemotherapy of lymphoma.
Subject(s)
Cytidine Deaminase/metabolism , Lymphoma/metabolism , Poly(ADP-ribose) Polymerases/metabolism , Animals , Cell Line, Tumor , Cell Nucleus/metabolism , DNA Damage/drug effects , DNA Damage/physiology , DNA Repair/drug effects , DNA Repair/physiology , Enzyme Activation/physiology , Humans , Lymphoma/pathology , Poly(ADP-ribose) Polymerase Inhibitors/pharmacologyABSTRACT
The pathogenesis of nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) and its relationship to other lymphomas are largely unknown. This is partly because of the technical challenge of analyzing its rare neoplastic lymphocytic and histiocytic (L&H) cells, which are dispersed in an abundant nonneoplastic cellular microenvironment. We performed a genome-wide expression study of microdissected L&H lymphoma cells in comparison to normal and other malignant B cells that indicated a relationship of L&H cells to and/or that they originate from germinal center B cells at the transition to memory B cells. L&H cells show a surprisingly high similarity to the tumor cells of T cell-rich B cell lymphoma and classical Hodgkin lymphoma, a partial loss of their B cell phenotype, and deregulation of many apoptosis regulators and putative oncogenes. Importantly, L&H cells are characterized by constitutive nuclear factor kappaB activity and aberrant extracellular signal-regulated kinase signaling. Thus, these findings shed new light on the nature of L&H cells, reveal several novel pathogenetic mechanisms in NLPHL, and may help in differential diagnosis and lead to novel therapeutic strategies.
Subject(s)
Gene Expression Profiling , Hodgkin Disease , Lymphocytes/immunology , Lymphoma, Follicular , B-Lymphocytes/cytology , B-Lymphocytes/immunology , Biomarkers/metabolism , Diagnosis, Differential , Enzyme Activation , Extracellular Matrix/metabolism , Extracellular Signal-Regulated MAP Kinases/metabolism , Germinal Center/cytology , Hodgkin Disease/genetics , Hodgkin Disease/immunology , Hodgkin Disease/pathology , Lymph Nodes/cytology , Lymphoma, Follicular/genetics , Lymphoma, Follicular/immunology , Lymphoma, Follicular/pathology , Microarray Analysis , NF-kappa B/metabolism , Phenotype , Reproducibility of ResultsABSTRACT
To identify genes involved in the pathogenesis of classical Hodgkin lymphoma (cHL), we performed serial analysis of gene expression (SAGE) and array-based comparative genomic hybridization (aCGH). Comparison of SAGE libraries of cHL cell lines L428 and L1236 with that of germinal centre B cells revealed consistent overexpression of only 14 genes. In contrast, 141 genes were downregulated in both cHL cell lines, including many B cell and HLA genes. aCGH revealed gain of 2p, 7p, 9p, 11q and Xq and loss of 4q and 11q. Eighteen percent of the differentially expressed genes mapped to regions with loss or gain and a good correlation was observed between underexpression and loss or overexpression and gain of DNA. Remarkably, gain of 2p and 9p did not correlate with increased expression of the proposed target genes c-REL and JAK2. Downregulation of many genes within the HLA region also did not correlate with loss of DNA. FSCN1 and IRAK1 mapping at genomic loci (7p and Xq) that frequently showed gain were overexpressed in cHL cell lines and might be involved in the pathogenesis of cHL.
