Activation-induced Cytidine Deaminase Deficiency Research Articles

A single-center study points to diverse features and outcome in patients with Hyperimmunoglobulin M Syndrome and Class- Switch Recombination defects.

Isotype class-switch recombination (CSR), somatic hypermutation (SHM), B cell signalling and DNA repair mechanisms defects are responsible for high IgM. The hyperimmunoglobulin M (HIGM) phenotype and CSR-related defects are now classified under primary antibody defects, combined immunodeficiencies or syndromic immunodeficiencies groups. The aim of the study is to evaluate the diverse phenotypic/genotypic/laboratory characteristics and outcome of patients with CSR defects and HIGM-related defects. We enrolled 50 patients. The most common gene defect was Activation-induced cytidine deaminase (AID) deficiency (n=18), followed by CD40 Ligand (CD40L) (n=14) and CD40 (n=3) deficiency. Median ages at first symptom and diagnosis were significantly lower in CD40L deficiency (8.5 and 30 months, respectively) than AID deficiency (30 and 114 months, respectively) (p=.001 and p=.008, respectively). Frequent clinical symptoms were recurrent (66%) and severe (14.9%) infections, and/or autoimmune/non-infectious inflammatory features (48.4%). Eosinophilia and neutropenia were at a higher rate in CD40L deficiency patients (77.8%, p=.002 and 77.8%, p=.002, respectively) when compared to AID deficiency. Median serum IgM level was low in 28.6% of CD40L deficiency patients. It was significantly lower when compared to AID deficiency (p < 0.001). Six patients (CD40L deficiency n=4, CD40 deficiency n=2) underwent hematopoietic stem cell transplantation. Five were alive at the last visit. Four patients two patients with CD40L deficiency, one with CD40 deficiency and one with AID deficiency had novel mutations. In conclusion; patients with CSR defects and HIGM phenotype may present with a wide range of clinical manifestations and laboratory findings. Low IgM, neutropenia and eosinophilia were prominent in patients with CD40L deficiency. Characterization of genetic defect-specific clinical and laboratory features may ease the diagnosis, prevent the underdiagnoses of patients and ameliorate the outcome.

Patterns of Immune Dysregulation in Primary Immunodeficiencies: A Systematic Review

Immune dysregulation is as important as susceptibility to infection in defining primary immunodeficiencies (PIDs). Because of the variability and nonspecificity of the symptoms of PIDs, diagnosis can be delayed-especially if a patient presents with immune dysregulation. Diagnosis is then based on certain combinations of symptoms and relies on the clinician's ability to recognize a pattern. So far there is no large report linking patterns of immune dysregulations to the underlying genetic defects. To identify immune dysregulatory patterns associated with PIDs and to help clinicians to detect an underlying PID in certain patients with noninfectious inflammatory diseases. A systematic literature review was performed. We included 186 articles that reported on n= 745 patients. The most common immune dysregulation category was "autoimmunity" (62%, n= 463), followed by "intestinal disease" (38%, n= 283) and "lymphoproliferation" (36%, n=268). Most patients (67%) had 1 or more symptoms of immune dysregulation. Autoimmune hemolytic anemia, the most common autoimmune phenotype, was most frequently reported in patients with LPS responsive beige-like anchor protein deficiency (when combined with hypogammaglobulinemia or gastrointestinal symptoms), activation-induced cytidine deaminase deficiency (when combined with autoimmune hepatitis), or RAG1 deficiency (when it was the only symptom of immune dysregulation). Eczema, allergies, and asthma were reported in 34%, 4%, and 4% of the patients, respectively. Patterns of immune dysregulation may help the physician to recognize specific PIDs. This systematic review provides clinicians with an overview to better assess patients with immune dysregulation.

Diversity of Immunoglobulin (Ig) Isotypes and the Role of Activation-Induced Cytidine Deaminase (AID) in Fish.

The disparate diversity in immunoglobulin (Ig) repertoire has been a subject of fascination since the emergence of prototypic adaptive immune system in vertebrates. The carboxy terminus region of activation-induced cytidine deaminase (AID) has been well established in tetrapod lineage and is crucial for its function in class switch recombination (CSR) event of Ig diversification. The absence of CSR in the paraphyletic group of fish is probably due to changes in catalytic domain of AID and lack of cis-elements in IgH locus. Therefore, understanding the arrangement of Ig genes in IgH locus and functional facets of fish AID opens up new realms of unravelling the alternative mechanisms of isotype switching and antibody diversity. Further, the teleost AID has been recently reported to have potential of catalyzing CSR in mammalian B cells by complementing AID deficiency in them. In that context, the present review focuses on the recent advances regarding the generation of diversity in Ig repertoire in the absence of AID-regulated class switching in teleosts and the possible role of T cell-independent pathway involving B cell activating factor and a proliferation-inducing ligand in activation of CSR machinery.

Activation-induced cytidine deaminase targets SUV4-20-mediated histone H4K20 trimethylation to class-switch recombination sites

Activation-induced cytidine deaminase (AID) triggers antibody diversification in B cells by catalysing deamination and subsequently mutating immunoglobulin (Ig) genes. Association of AID with RNA Pol II and occurrence of epigenetic changes during Ig gene diversification suggest participation of AID in epigenetic regulation. AID is mutated in hyper-IgM type 2 (HIGM2) syndrome. Here, we investigated the potential role of AID in the acquisition of epigenetic changes. We discovered that AID binding to the IgH locus promotes an increase in H4K20me3. In 293F cells, we demonstrate interaction between co-transfected AID and the three SUV4-20 histone H4K20 methyltransferases, and that SUV4-20H1.2, bound to the IgH switch (S) mu site, is replaced by SUV4-20H2 upon AID binding. Analysis of HIGM2 mutants shows that the AID truncated form W68X is impaired to interact with SUV4-20H1.2 and SUV4-20H2 and is unable to bind and target H4K20me3 to the Smu site. We finally show in mouse primary B cells undergoing class-switch recombination (CSR) that AID deficiency associates with decreased H4K20me3 levels at the Smu site. Our results provide a novel link between SUV4-20 enzymes and CSR and offer a new aspect of the interplay between AID and histone modifications in setting the epigenetic status of CSR sites.

Opposing impact of AID deficiency on BCL-2 and IL-6 driven B-lymphoma development in BALB/c mice

Activation-induced cytidine deaminase (AID), an essential enzymatic activity required for somatic hypermutation and immunoglobulin class switch recombination in the course of normal B-lymphocyte development, has been implicated in the initiation and promotion of malignant B-cell tumors by virtue of a complex mechanism that includes the generation of oncogene-activating genomic rearrangements and the introduction of point mutations in cancer genes. Here, we use transgenic mouse models of B-cell lymphoma driven by the pro-inflammatory cytokine, interleukin 6 (IL-6), or the survival-enhancing oncoprotein, B-cell leukemia 2 (BCL-2), to evaluate the impact of loss of AID on neoplastic B-cell development. We show that AID deficiency accelerates BCL-2 induced lymphoma but delays IL-6 induced lymphoma. This led us to conclude that AID may function as tumor suppressor or tumor promoter, depending on the genetic context. Elucidating the mechanism of AID's dual function during malignant B-cell transformation may be important for new approaches to tumor treatment and prevention.

Enrichment of rare variants in population isolates: single AICDA mutation responsible for hyper-IgM syndrome type 2 in Finland

Antibody class-switch recombination and somatic hypermutation critically depend on the function of activation-induced cytidine deaminase (AID). Rare variants in its gene AICDA have been reported to cause autosomal recessive AID deficiency (autosomal recessive hyper-IgM syndrome type 2 (HIGM2)). Exome sequencing of a multicase Finnish family with an HIGM2 phenotype identified a rare, homozygous, variant (c.416T>C, p.(Met139Thr)) in the AICDA gene, found to be significantly enriched in the Finnish population compared with other populations of European origin (38.56-fold, P<0.001). The population history of Finland, characterized by a restricted number of founders, isolation and several population bottlenecks, has caused enrichment of certain rare disease-causing variants and losses of others, as part of a phenomenon called the Finnish Disease Heritage. Accordingly, rare founder mutations cause the majority of observed Finnish cases in these mostly autosomal recessive disorders that consequently are more frequent in Finland than elsewhere. Screening of all currently known Finnish patients with an HIGM2 phenotype showed them to be homozygous for p.(Met139Thr). All the Finnish p.(Met139Thr) carriers with available data on their geographic descent originated from the eastern and northeastern parts of Finland. They were observed to share more of their genome identity by descent (IBD) than Finns in general (P<0.001), and they all carried a 207.5-kb ancestral haplotype containing the variant. In conclusion, the identified p.(Met139Thr) variant is significantly enriched in Finns and explains all thus far found AID deficiencies in Finland.

Novel and recurrent AID mutations underlie prevalent autosomal recessive form of HIGM in consanguineous patients.

Immunoglobulin class switch recombination deficiencies (Ig-CSR-D) are characterized by normal or elevated serum IgM level and absence of IgG, IgA, and IgE. Most reported cases are due to X-linked CD40L deficiency. Activation-induced cytidine deaminase deficiency is the most frequent autosomal recessive form, whereas CD40 deficiency is more rare. Herein, we present the first North African study on hyper IgM (HIGM) syndrome including 16 Tunisian patients. Phenotypic and genetic studies allowed us to determine their molecular basis. Three CD40LG mutations have been identified including two novels (c.348_351dup and c.782_*2del) and one already reported mutation (g.6182G>A). No mutation has been found in another patient despite the lack of CD40L expression. Interestingly, three AICDA mutations have been identified in 11 patients. Two mutations were novel (c.91T>C and c.389A>C found in one and five patients respectively), and one previously reported splicing mutation (c.156+1T>G) was found in five patients. Only one CD40-deficient patient, bearing a novel mutation (c.109T>G), has been identified. Thus, unlike previous reports, AID deficiency is the most frequent underlying molecular basis (68%) of Ig-CSR-D in Tunisian patients. This finding and the presence of specific recurrent mutations are probably due to the critical role played by inbreeding in North African populations.

Absence of Activation-induced Cytidine Deaminase, a Regulator of Class Switch Recombination and Hypermutation in B Cells, Suppresses Aorta Allograft Vasculopathy in Mice.

Antibody-mediated rejection is caused in part by increasing circulation/production of donor-specific antibody (DSA). Activation-induced cytidine deaminase (AID) is a key regulator of class switch recombination and somatic hypermutation of immunoglobulin in B cells, yet its role in antibody-mediated transplant rejection remains unclear. We show here that AID deficiency in mice enables suppression of allograft vasculopathy (AV) after aorta transplantation, a DSA-mediated process. Splenocytes from C57BL/6 J (B6) AID(−/−) mice were used for determining in vitro proliferation responses, alloreactivity, cell surface marker expression, and antibody production. BALB/c mouse aortas were transplanted into B6 AID(−/−) mice with or without FK506 treatment. Blood and aorta grafts were harvested on day 30 after transplantation and were subjected to DSA, histological, and immunohistological analyses. The AID(−/−) splenocytes were comparable to wild type splenocytes in proliferation responses, alloreactivity, and expression of cell surface markers in vitro. However, they completely failed to produce immunoglobulin G, although they were not impaired in immunoglobulin M production relative to controls. Furthermore, BALB/c aorta grafts from B6 AID(−/−) recipient mice on day 30 after transplantation showed reduced signs of AV compared to the grafts from B6 wild type recipient mice which had severe vascular intimal hyperplasia, interstitial fibrosis, and inflammation. Treatment with FK506 produced a synergistic effect in the grafts from AID(−/−) recipients with further reduction of intimal hyperplasia and fibrosis scores. The AID deficiency inhibits DSA-mediated AV after aorta transplantation in mice. We propose that AID could be a novel molecular target for controlling antibody-mediated rejection in organ transplantation.

First Report of the Hyper-IgM Syndrome Registry of the Latin American Society for Immunodeficiencies: Novel Mutations, Unique Infections, and Outcomes

Hyper-IgM (HIGM) syndrome is a heterogeneous group of disorders characterized by normal or elevated serum IgM levels associated with absent or decreased IgG, IgA and IgE. Here we summarize data from the HIGM syndrome Registry of the Latin American Society for Immunodeficiencies (LASID). Of the 58 patients from 51 families reported to the registry with the clinical phenotype of HIGM syndrome, molecular defects were identified in 37 patients thus far. We retrospectively analyzed the clinical, immunological and molecular data from these 37 patients. CD40 ligand (CD40L) deficiency was found in 35 patients from 25 families and activation-induced cytidine deaminase (AID) deficiency in 2 unrelated patients. Five previously unreported mutations were identified in the CD40L gene (CD40LG). Respiratory tract infections, mainly pneumonia, were the most frequent clinical manifestation. Previously undescribed fungal and opportunistic infections were observed in CD40L-deficient patients but not in the two patients with AID deficiency. These include the first cases of pneumonia caused by Mycoplasma pneumoniae, Serratia marcescens or Aspergillus sp. and diarrhea caused by Microsporidium sp. or Isospora belli. Except for four CD40L-deficient patients who died from complications of presumptive central nervous system infections or sepsis, all patients reported in this study are alive. Four CD40L-deficient patients underwent successful bone marrow transplantation. This report characterizes the clinical and genetic spectrum of HIGM syndrome in Latin America and expands the understanding of the genotype and phenotype of this syndrome in tropical areas.

High-content cytometry and transcriptomic biomarker profiling of human B-cell activation

Primary antibody deficiencies represent the most prevalent, although very heterogeneous, group of inborn immunodeficiencies, with a puzzling complexity of cellular and molecular processes involved in disease pathogenesis. We aimed to study in detail the kinetics of CD40 ligand/IL-21-induced B-cell differentiation to define new biomarker sets for further research into primary antibody deficiencies. We applied high-content screening methods to monitor B-cell activation on the cellular (chip cytometry) and transcriptomic (RNA microarray) levels. The complete activation process, including stepwise changes in protein and RNA expression patterns, entry into the cell cycle, proliferation and expression of activation-induced cytidine deaminase (AID), DNA repair enzymes, and post-class-switch expression of IgA and IgG, was successfully monitored during in vitro differentiation. We identified a number of unknown pathways engaged during B-cell activation, such as CXCL9/CXCL10 secretion by B cells. Finally, we evaluated a deduced set of biomarkers on a group of 18 patients with putative or proved intrinsic B-cell defects recruited from the European Society for Immunodeficiencies database and successfully predicted 2 AID defects and 1 DNA repair defect. Complete absence of class-switched B cells was a sensitive predictor of AID deficiency and should be further evaluated as a diagnostic biomarker. The biomarkers found in this study could be used to further study the complex process of B-cell activation and to understand conditions that lead to the development of primary antibody deficiencies.

LINE-1 retroelements complexed and inhibited by activation induced cytidine deaminase.

LINE-1 (abbreviated L1) is a major class of retroelements in humans and mice. If unrestricted, retroelements accumulate in the cytoplasm and insert their DNA into the host genome, with the potential to cause autoimmune disease and cancer. Retroviruses and other retroelements are inhibited by proteins of the APOBEC family, of which activation-induced cytidine deaminase (AID) is a member. Although AID is mainly known for being a DNA mutator shaping the antibody repertoire in B lymphocytes, we found that AID also restricts de novo L1 integrations in B- and non-B-cell lines. It does so by decreasing the protein level of open reading frame 1 (ORF1) of both exogenous and endogenous L1. In activated B lymphocytes, AID deficiency increased L1 mRNA 1.6-fold and murine leukemia virus (MLV) mRNA 2.7-fold. In cell lines and activated B lymphocytes, AID forms cytoplasmic high-molecular-mass complexes with L1 mRNA, which may contribute to L1 restriction. Because AID-deficient activated B lymphocytes do not express ORF1 protein, we suggest that ORF1 protein expression is inhibited by additional restriction factors in these cells. The greater increase in MLV compared to L1 mRNA in AID-deficient activated B lymphocytes may indicate less strict surveillance of retrovirus.

Molecular mechanism of colitis-associated colorectal carcinogenesis

The development of colorectal cancer caused by chronic inflammatory bowel disease (IBD) is the representative example of inflammation-associated carcinogenesis. The mechanism underlying the development of colorectal cancers through chronic inflammation, however, is not known. Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutation in the immunoglobulin gene. We recently demonstrated that the mutagenic activity of AID expression links colonic inflammation to the development of colitis associated colorectal cancers. Immunohistochemistry revealed enhanced expression of endogenous AID protein not only in the inflamed colonic mucosa of ulcerative colitis patients, but also in tumor lesions of colitis-associated colorectal cancers. Pro-inflammatory cytokine TNF-α and/or T helper cell-2-driven cytokines IL-4 and IL-13 induced strong aberrant expression of AID in human colonic epithelial cells. In vivo, aberrant AID expression in the inflamed colon is associated with the accumulation of somatic mutations in tumor suppressor Trp53 gene, and AID deficiency resulted in a reduced incidence of colitis-associated colon cancers. These findings suggested that pro-inflammatory cytokine-mediated aberrant expression of AID in colonic epithelial cells plays a role as a genotoxic factor that enhances genetic instability during chronic colonic inflammation, leading to colitis-associated colorectal cancer development.

Case of a 10-Year-Old Boy with Hyper–Immunoglobulin M Immunodeficiency Caused by Activation-Induced Cytidine Deaminase Deficiency

We report the case of a 10-year-old boy, with a history of recurrent sinopulmonary infections, presenting with pneumococcal meningitis. Immune workup found low immunoglobulin G (IgG), IgA, and IgE and elevated IgM. Specific antibodies to pneumococcus, tetanus, and mumps were low. Genetic testing identified CD40L and uracil-DNA glycosylase as normal. Sequence analysis for mutations in activation-induced cytidine deaminase (AID) gene on chromosome 12p13 revealed a single missense mutation in exon 3 caused by a point mutation. AID is an enzyme that is specifically present in activated B cells and necessary for isotype switching. This case highlights the significance of AID deficiency in hyper-IgM immunodeficiency.

Activation-induced cytidine deaminase deficiency causes organ-specific autoimmune disease.

Activation-induced cytidine deaminase (AID) expressed by germinal center B cells is a central regulator of somatic hypermutation (SHM) and class switch recombination (CSR). Humans with AID mutations develop not only the autosomal recessive form of hyper-IgM syndrome (HIGM2) associated with B cell hyperplasia, but also autoimmune disorders by unknown mechanisms. We report here that AID−/− mice spontaneously develop tertiary lymphoid organs (TLOs) in non-lymphoid tissues including the stomach at around 6 months of age. At a later stage, AID−/− mice develop a severe gastritis characterized by loss of gastric glands and epithelial hyperplasia. The disease development was not attenuated even under germ-free (GF) conditions. Gastric autoantigen -specific serum IgM was elevated in AID−/− mice, and the serum levels correlated with the gastritis pathological score. Adoptive transfer experiments suggest that autoimmune CD4+ T cells mediate gastritis development as terminal effector cells. These results suggest that abnormal B-cell expansion due to AID deficiency can drive B-cell autoimmunity, and in turn promote TLO formation, which ultimately leads to the propagation of organ-specific autoimmune effector CD4+ T cells. Thus, AID plays an important role in the containment of autoimmune diseases by negative regulation of autoreactive B cells.

Hyper IgM Syndrome Due To AID Deficiency In Two Patients From The Dominican Republic

Hyper IgM (HIGM) syndrome is a primary immunodeficiency characterized by a defect in immunoglobulin switch recombination that produces normal or high concentrations of IgM with low or absent IgG, IgA and IgE. The inheritance of this defect can be X-linked or recessive. Activation-Induced Cytidine Deaminase deficiency (AID) is an autosomal recessive form of HIGM, that has been described in patients from Europe, Africa, Asia and North America.

Case studies in immunology: A clinical companion, 5th Ed. by Raif Geha and the late Fred Rosen

Raif Geha and the late Fred Rosen, Garland Science, New York and London, 2008, 344 pp., ISBN 0-8153-4145-8/9-780815-341451, $49.95. E. J. Wood*, * Centre for Bioscience, The Higher Education Academy, University of Leeds, Leeds LS2 9JT, United Kingdom. This case studies volume is aimed mostly at the more advance medical students but would also be of interest to those taking immunology courses. It contains 47 case studies of a variety of immunological diseases both common and rare, and, the authors say, is updated and has some new cases compared with the previous editions. Each case gives initially some background, then presents the case (based on real patients—the names are changed to protect the innocent), and then a description is given followed by 4–6 questions for students to self-test their understanding. Answers are given at the back of the book. Many cases have pictures of the patients plus histology and other clinical information. Most cases are accompanied by excellent diagrams. This is not simple immunology (perhaps immunology is never simple anyway), but the cases are complicated and required a great deal of insight and knowledge. To give just one example: “Deficiency of the C8 component of complement,” the introduction gives details of the lytic part of the complement cascade with diagrams and electron micrographs, and the following description, after the case has been presented, tells us about the frequency of complement component deficiencies in various ethnic populations along with the known molecular biology of the mutations for C8 deficiency. One of the questions at the end is, “Hemolytic complement levels were determined by measuring the CH50. What is that and how is the test performed?” This is not stated in the text and indicates the level of sophistication and background knowledge expected. There is a lot of biochemistry in the book, but of course the emphasis is mainly clinical/immunological. Some examples of cases presented are as follows: interleukin-1 receptor-associated kinase deficiency; activation-induced cytidine deaminase deficiency; multiple myeloma; mixed essential cryoglobulinemia; lepromatous leprosy; AIDS; contact sensitivity to poison ivy; hemolyic disease of the newborn; graft-versus-host disease. The book will be useful to medical students toward the end of their clinical training and to those on a clinical immunology track, but those teaching immunology courses will also find that the content will provide useful, real, examples to illustrate their lectures (or use in problem-based sessions).

AID is required for germinal center–derived lymphomagenesis

Most human B cell non-Hodgkin's lymphomas (B-NHLs) derive from germinal centers (GCs), the structure in which B cells undergo somatic hypermutation (SHM) and class switch recombination (CSR) before being selected for high-affinity antibody production. The pathogenesis of B-NHL is associated with distinct genetic lesions, including chromosomal translocations and aberrant SHM, which arise from mistakes occurring during CSR and SHM. A direct link between these DNA remodeling events and GC lymphoma development, however, has not been demonstrated. Here we have crossed three mouse models of B cell lymphoma driven by oncogenes (Myc, Bcl6 and Myc/Bcl6; refs. 5,6) with mice lacking activation-induced cytidine deaminase (AID), the enzyme required for both CSR and SHM. We show that AID deficiency prevents Bcl6-dependent, GC-derived B-NHL, but has no impact on Myc-driven, pre-GC lymphomas. Accordingly, abrogation of AID is associated with the disappearance of CSR- and SHM-mediated structural alterations. These results show that AID is required for GC-derived lymphomagenesis, supporting the notion that errors in AID-mediated antigen-receptor gene modification processes are principal contributors to the pathogenesis of human B-NHL.

A Novel Mechanism of Ig and Myc Translocations in AID-Deficient Mice.

Consistent with the role of activation induced cytidine deaminase (AID) as a major “catalyst” of aberrant translocations between the Ig switch regions and c-myc, AID-sufficient Bcl-xL transgenic mice rapidly develop transplantable plasmacytomas with classical T(12;15) translocations. Unexpectedly, we found that Bcl-xL transgenic BALB/cAn mice deficient for AID (designated pBxAicda−/− mice) also developed plasma cell tumors but with a lower frequency (24% vs. 62%) and with a longer mean latency (108 d vs. 36 d) than AID-sufficient controls. Six out of nine of primary tumors were shown by interphase FISH to contain a T(12;15) translocation and one other had a T(6;15). pBxAicda−/− tumors did not transplant well because they were presumably in early stages of neoplastic development or had not progressed to full malignancy including association with ascites. Nevertheless, two tumors (4885 and 4961) were successfully transplanted and established as stable cell lines. They exhibited mature plasma cell phenotype (CD45−, CD138+, PC-1+, CD19−, CD23−) and secreted IgM. Gene expression profiling showed no significant difference from control plasma cell tumors of AID-sufficient mice. Detailed molecular and cytogenetic analysis of 4885 uncovered an unusual unbalanced T(12;15) translocation with IgH Cμ and Pvt-1 in a head to tail orientation at the breakpoint, resulting in elevated c-myc expression as detected by qPCR. In contrast, 4961, a T(12;15) negative cell line, had elevated N-myc expression as a result of paracentric inversion of Chr. 12. These rearrangements had no direct association with RAG activity. We conclude that rapid development of malignant plasma cell tumors with reciprocal T(12;15) does require AID, and that in AID deficiency a novel less efficient mechanism can be utilized to bring c-myc and Ig genes into juxtaposition.

Activation-induced cytidine deaminase (AID) promotes B cell lymphomagenesis in Emu-cmyc transgenic mice

Activation-induced cytidine deaminase (AID), which is essential to both class switch recombination and somatic hypermutation of the Ig gene, is expressed in many types of human B cell lymphoma/leukemia. AID is a potent mutator because it is involved in DNA breakage not only of Ig but also of other genes, including proto-oncogenes. Recent studies suggest that AID is required for chromosomal translocation involving cmyc and Ig loci. However, it is unclear whether AID plays other roles in tumorigenesis. We examined the effect of AID deficiency on the generation of surface Ig-positive B cell lymphomas in Emu-cmyc transgenic mice. Almost all lymphomas that developed in AID-deficient transgenic mice were pre-B cell lymphomas, whereas control transgenic mice had predominantly B cell lymphomas, indicating that AID is required for development of B but not pre-B cell lymphomas from cmyc overexpressing tumor progenitors. Thus, AID may play multiple roles in B cell lymphomagenesis.

Characterization of immunoglobulin mutations in humans with activation-induced cytidine deaminase deficiency

Somatic hypermutation (SHM) is initiated in germinal center (GC) B cells expressing high levels of activation-induced cytidine deaminase (AID), which targets WRC (W = A/T, R = A/G) motifs generating a uracil:guanidine mismatch. During SHM, 73% of the mutations are attributed to AID and error-prone polymerase eta (POLη). To characterize the nature of other mutations, 316 genomic nonproductive V(D)J rearrangements amplified and sequenced from three AID-/- patients were analyzed. The mutation frequency was 20-fold less than normals (0.09% versus 2%) but ~5 × 104-fold more than non-B cells. Reduced RGY (6% versus 27%) and WRC (6% versus 15%) motif mutations and decreased replacements in complementarity determining regions were attributed to the lack of AID. Reduced WA motif mutations (12% versus 23%) suggested POLη activity was decreased. Prominent G>C and A>T biases suggested that mutation and repair mechanisms occurred preferentially on one DNA strand. A high percentage of the mutations were G substitutions on the sense strand (43% versus 29% in normals), reflecting C mutational targeting on the opposite strand. These were primarily G to A transitions, suggesting that UNG activity was reduced even though UNG is upregulated in normal GC B cells and large GCs occur in AID-/- patients. The mutation pattern suggests that AID-independent C substitutions contribute a small proportion of SHM but lack the targeting provided by AID. Finally, a low level of SHM clearly can develop in the absence of AID; however, this mechanism is inefficient at altering the binding capacity of immunoglobulin heavy chain genes.