hyper-IgM Syndrome Type Research Articles

A Novel Heterozygous Variant in AICDA Impairs Ig Class Switching and Somatic Hypermutation in Human B Cells and is Associated with Autosomal Dominant HIGM2 Syndrome

B cells and their secreted antibodies are fundamental for host-defense against pathogens. The generation of high-affinity class switched antibodies results from both somatic hypermutation (SHM) of the immunoglobulin (Ig) variable region genes of the B-cell receptor and class switch recombination (CSR) which alters the Ig heavy chain constant region. Both of these processes are initiated by the enzyme activation-induced cytidine deaminase (AID), encoded by AICDA. Deleterious variants in AICDA are causal of hyper-IgM syndrome type 2 (HIGM2), a B-cell intrinsic primary immunodeficiency characterised by recurrent infections and low serum IgG and IgA levels. Biallelic variants affecting exons 2, 3 or 4 of AICDA have been identified that impair both CSR and SHM in patients with autosomal recessive HIGM2. Interestingly, B cells from patients with autosomal dominant HIGM2, caused by heterozygous variants (V186X, R190X) located in AICDA exon 5 encoding the nuclear export signal (NES) domain, show abolished CSR but variable SHM. We herein report the immunological and functional phenotype of two related patients presenting with common variable immunodeficiency who were found to have a novel heterozygous variant in AICDA (L189X). This variant led to a truncated AID protein lacking the last 10 amino acids of the NES at the C-terminal domain. Interestingly, patients’ B cells carrying the L189X variant exhibited not only greatly impaired CSR but also SHM in vivo, as well as CSR and production of IgG and IgA in vitro. Our findings demonstrate that the NES domain of AID can be essential for SHM, as well as for CSR, thereby refining the correlation between AICDA genotype and SHM phenotype as well as broadening our understanding of the pathophysiology of HIGM disorders.

A Novel AICDA Splice-Site Mutation in Two Siblings with HIGM2 Permits Somatic Hypermutation but Abrogates Mutational Targeting

Hyper-IgM syndrome type 2 (HIGM2) is a B cell intrinsic primary immunodeficiency caused by mutations in AICDA encoding activation-induced cytidine deaminase (AID) which impair immunoglobulin class switch recombination (CSR) and somatic hypermutation (SHM). Whereas autosomal-recessive AID-deficiency (AR-AID) affects both CSR and SHM, the autosomal-dominant form (AD-AID) due to C-terminal heterozygous variants completely abolishes CSR but only partially affects SHM. AR-AID patients display enhanced germinal center (GC) reactions and autoimmune manifestations, which are not present in AD-AID, suggesting that SHM but not CSR regulates GC reactions and peripheral B cell tolerance. Herein, we describe two siblings with HIGM2 due to a novel homozygous AICDA mutation (c.428-1G > T) which disrupts the splice acceptor site of exon 4 and results in the sole expression of a truncated AID variant that lacks 10 highly conserved amino acids encoded by exon 4 (AID-ΔE4a). AID-ΔE4a patients suffered from defective CSR and enhanced GC reactions and were therefore indistinguishable from other AR-AID patients. However, the AID-ΔE4a variant only partially affected SHM as observed in AD-AID patients. In addition, AID-ΔE4a but not AD-AID patients revealed impaired targeting of mutational hotspot motives and distorted mutational patterns. Hence, qualitative defects in AID function and altered SHM rather than global decreased SHM activity may account for the disease phenotype in these patients.

Activation-induced deaminase is critical for the establishment of DNA methylation patterns prior to the germinal center reaction.

Activation-induced deaminase (AID) initiates antibody diversification in germinal center B cells by deaminating cytosines, leading to somatic hypermutation and class-switch recombination. Loss-of-function mutations in AID lead to hyper-IgM syndrome type 2 (HIGM2), a rare human primary antibody deficiency. AID-mediated deamination has been proposed as leading to active demethylation of 5-methycytosines in the DNA, although evidence both supports and casts doubt on such a role. In this study, using whole-genome bisulfite sequencing of HIGM2 B cells, we investigated direct AID involvement in active DNA demethylation. HIGM2 naïve and memory B cells both display widespread DNA methylation alterations, of which ∼25% are attributable to active DNA demethylation. For genes that undergo active demethylation that is impaired in HIGM2 individuals, our analysis indicates that AID is not directly involved. We demonstrate that the widespread alterations in the DNA methylation and expression profiles of HIGM2 naïve B cells result from premature overstimulation of the B-cell receptor prior to the germinal center reaction. Our data support a role for AID in B cell central tolerance in preventing the expansion of autoreactive cell clones, affecting the correct establishment of DNA methylation patterns.

CD40L acts as an allosteric activator of integrins for signal transduction independent of inside-out signaling

Abstract CD40L plays a major role in immune response and is a target for inflammatory disease therapy. Besides CD40, CD40L binds to several integrins but their role in signaling is unclear. We showed that integrins αvβ3 and α5β1 bind to the CD40L trimeric interface through classical ligand binding site of integrins (site 1). Several CD40L mutants from HIGM1 (hyper-IgM syndrome type 1) patients were clustered in trimeric interface and defective in integrin binding, and in NF-κB and B cell activation, but still bound CD40 and acted as antagonists of CD40L signaling. Thus, integrins play a critical role in CD40L signaling. Previous studies showed that CX3CL1 and sPLA2-IIA can activate integrins by binding to a recently identified allosteric site (site 2) of integrins. Using cell-free assays, we found that soluble (s)-CD40L activated soluble integrins, α4β1, αvβ3 and α5β1, and induced their binding to known specific ligands independent of inside-out signaling. Also, sCD40L activated cell-surface integrins in CHO cells lacking CD40. Finally, sCD40L bound to a cyclic peptide derived from site 2. These findings indicate that CD40L acts as an allosteric activator of integrins by binding to site 2. Docking simulation predicted that the integrin site 2-binding site of CD40L is located outside of CD40L trimer. Importantly, four HIGM1 mutations are clustered in the predicted site 2-binding site of CD40L. The K143T and G144E mutants were the most defective in integrin activation, indicating that integrin binding occurs around these residues of the predicted site 2-binding site. We propose that allosteric integrin activation by CD40L plays a role in CD40L signaling and that defective integrin site 2-binding may be causal for defective CD40L signaling in HIGM1.

Soluble CD40L activates soluble and cell-surface integrin αvβ3, α5β1, and α4β1 by binding to the allosteric ligand-binding site (site 2)

CD40L is a member of the TNF superfamily that participates in immune cell activation. It binds to and signals through several integrins, including αvβ3 and α5β1, which bind to the trimeric interface of CD40L. We previously showed that several integrin ligands can bind to the allosteric site (site 2), which is distinct from the classical ligand-binding site (site 1), raising the question of if CD40L activates integrins. In our explorations of this question, we determined that integrin α4β1, which is prevalently expressed on the same CD4+ T cells as CD40L, is another receptor for CD40L. Soluble (s)CD40L activated soluble integrins αvβ3, α5β1, and α4β1 in cell-free conditions, indicating that this activation does not require inside-out signaling. Moreover, sCD40L activated cell-surface integrins in CHO cells that do not express CD40. To learn more about the mechanism of binding, we determined that sCD40L bound to a cyclic peptide from site 2. Docking simulations predicted that the residues of CD40L that bind to site 2 are located outside of the CD40L trimer interface, at a site where four HIGM1 (hyper-IgM syndrome type 1) mutations are clustered. We tested the effect of these mutations, finding that the K143T and G144E mutants were the most defective in integrin activation, providing support that this region interacts with site 2. We propose that allosteric integrin activation by CD40L also plays a role in CD40L signaling, and defective site 2 binding may be related to the impaired CD40L signaling functions of these HIGM1 mutants.

Examining the Structure of Activation‐Induced Cytidine Deaminase (AID)

In order to respond to any foreign antigen that might present itself, B cells must produce antibodies of great variety. Activation‐Induced Cytidine Deaminase (AID) is a protein that plays a critical role in the diversification of antibodies.1 Discovered by T. Honjo et al. in 2000, AID is a protein consisting of 198 amino acids that creates a U:G mismatch by deaminating cytosine to uracil in transcribed Ig DNA2. AID helps in the process of binding site differentiation, known as somatic hypermutation (SHM), and class‐switching, known as CSR (class switch DNA recombination).3 The Governor’s Academy MSOE Center for BioMolecular Modeling SMART team used 3‐D modeling and printing technology to better understand how several aspects of the structure of AID play a critical role in accomplishing the functions of SHM and CSR. Our model highlights Histidine 56, Cysteine 87 and Cysteine 90, which bind zinc ions forming a zinc finger, and the hotspot recognition loop, all of which are involved in DNA binding.4 The catalytic domain, which is similar to the other members of the apolipoprotein B RNA‐editing cytidine deaminase enzymes (APOBEC) is also shown.5 Unfortunately, two of the residues that are most critical for AID functioning could not be shown on our model. Serine 38, which is an important phosphorylation site, was cut out during the isolation and crystallization process and thus is not included in the protein data bank files. Glutamic Acid 58 is the residue most critical for the deamination reaction and was mutated to an Alanine in order to prevent the reaction from occurring so that AID could be isolated with DNA bound.6 Lastly, we highlighted residues R112 and R24. Mutation of either of these Arginines disrupts the configuration of AID, which undermines the ability of AID to undergo SHM and CSR, causing an immunodeficiency disorder called Hyper‐IgM syndrome type 2 (HIGM‐2)

Integrin Binding to the Trimeric Interface of CD40L Plays a Critical Role in CD40/CD40L Signaling.

CD40L plays a major role in immune response and is a major therapeutic target for inflammation. Integrin α5β1 and CD40 simultaneously bind to CD40L. It is unclear if α5β1 and CD40 work together in CD40/CD40L signaling or how α5β1 binds to CD40L. In this article, we describe that the integrin-binding site of human CD40L is predicted to be located in the trimeric interface by docking simulation. Mutations in the predicted integrin-binding site markedly reduced the binding of α5β1 to CD40L. Several CD40L mutants defective in integrin binding were defective in NF-κB activation and B cell activation and suppressed CD40L signaling induced by wild-type CD40L; however, they still bound to CD40. These findings suggest that integrin α5β1 binds to monomeric CD40L through the binding site in the trimeric interface of CD40L, and this plays a critical role in CD40/CD40L signaling. Integrin αvβ3, a widely distributed vascular integrin, bound to CD40L in a KGD-independent manner, suggesting that αvβ3 is a new CD40L receptor. Several missense mutations in CD40L that induce immunodeficiency with hyper-IgM syndrome type 1 (HIGM1) are clustered in the integrin-binding site of the trimeric interface. These HIGM1 CD40L mutants were defective in binding to α5β1 and αvβ3 (but not to CD40), suggesting that the defect in integrin binding may be a causal factor of HIGM1. These findings suggest that α5β1 and αvβ3 bind to the overlapping binding site in the trimeric interface of monomeric CD40L and generate integrin-CD40L-CD40 ternary complex. CD40L mutants defective in integrins have potential as antagonists of CD40/CD40L signaling.

The experience of hematopoietic stem cell transplantation in primary immunodeficiencies in the Russian Children's Clinical Hospital

Primary immunodeficiencies (PID) include a group of congenital diseases, many of which are associated with a high risk of developing life-threatening infectious and non-infectious complications. Many of PIDs require hematopoietic stem cell transplantation (HSCT), which can lead to a complete cure of the disease. The article presents more than 20 years of experience in conducting HSCT with PID in the Russian Children's Clinical Hospital for the period from 1997 to 2018. 88 HSCTs were performed in 80 patients (64 boys and 16 girls) with various PIDs: severe combined immune deficiency (SCID, n = 34), hemophagocytic lymphohistiocytosis (HLH, n = 12), chronic granulomatous disease (CGD, n = 11), Wiskott–Aldrich syndrome (WAS, n = 10), congenital agranulocytosis (n = 4), hyper IgM syndrome type 1 (n = 3), Nijmegen breakage syndrome (n = 2), lymphoproliferative syndrome (n = 2), Chediak–Higashi syndrome (n = 1), leukocyte adhesion deficiency (n = 1). Оverall survival (OS) and event-free survival (EFS) after HSCT with PID was 63.1% and 49.3%. OS after HSCT with SCID was 65.5%, EFS – 48.4%. The article presents the results of HSCT taking into account the type of HSCT, the source of hematopoietic stem cells (HSC) and the type of graft manipulation, conditioning regimen. Growth of positive results of HSCT in patients with PID in recent years is associated with the improvement of accompanying therapy (improving the quality of infection control, the introduction of new drugs for the prevention and treatment of hepatic veno-occlusive disease); technology application TcRα+β+/CD19+ depletion at haploidentical transplantation; optimization of conditioning regimens; successes in the prevention and treatment of the graftversus- host disease (antithymocyte globulin and rituximab administration during the period of conditioning, post-transplant administration of cyclophosphamide at haploidentical HSCT). The study was approved by the Independent Ethics Committee of Russian Children's Clinical Hospital.

TYPICAL RECURRENT CHILDHOOD INFECTIONS OR PRIMARY IMMUNODEFICIENCY?

Introduction Hyper IgM syndromes involve lack of class switch recombination leading to high levels of serum IgM and low levels of IgG, IgA, and IgE. While the absence of high affinity antibodies predisposes patients with X-linked Hyper IgM syndrome to opportunistic infections, this is not so for the less common Hyper IgM syndrome type 2, often making diagnosis difficult. Case Description A 20-month old girl with a six month history of recurrent viral infections presented with persistent high-grade fevers, rash, intermittent diarrhea, and decreased appetite. Physical exam revealed left sided erythematous maculopapular rash, cervical lymphadenopathy and erythematous posterior pharynx. A thorough infectious workup was negative and the patient was initially diagnosed with systemic onset Juvenile Idiopathic Arthritis. Afterwards, a humoral immune panel revealed: IgM 1590 mg/dL,IgA. Discussion Hyper IgM type 2 patients have three common findings: absence of immunoglobulin class switch recombination, lack of immunoglobulin somatic hypermutations, and lymph node hyperplasia secondary to AID gene expression in germinal center B cells. Our patient presented with all of these findings, but given the complexity of this condition, suffered a significant delay in diagnosis. It is important to include primary immunodeficiency in the differential diagnoses of patients with autoimmune manifestations and recurrent infections.

The RNA-binding protein ROD1/PTBP3 cotranscriptionally defines AID-loading sites to mediate antibody class switch in mammalian genomes

Activation-induced cytidine deaminase (AID) mediates class switching by binding to a small fraction of single-stranded DNA (ssDNA) to diversify the antibody repertoire. The precise mechanism for highly selective AID targeting in the genome has remained elusive. Here, we report an RNA-binding protein, ROD1 (also known as PTBP3), that is both required and sufficient to define AID-binding sites genome-wide in activated B cells. ROD1 interacts with AID via an ultraconserved loop, which proves to be critical for the recruitment of AID to ssDNA using bi-directionally transcribed nascent RNAs as stepping stones. Strikingly, AID-specific mutations identified in human patients with hyper-IgM syndrome type 2 (HIGM2) completely disrupt the AID interacting surface with ROD1, thereby abolishing the recruitment of AID to immunoglobulin (Ig) loci. Together, our results suggest that bi-directionally transcribed RNA traps the RNA-binding protein ROD1, which serves as a guiding system for AID to load onto specific genomic loci to induce DNA rearrangement during immune responses.

Genetic Analysis of Patients with Two Different Types of Hyper IgM Syndrome

ABSTRACTBackground: Hyper IgM Syndrome (HIGM) is a rare primary immunodeficiency in which impairment of class switching recombination (CSR) and somatic hyper-mutation (SHM) leads to recurrent infections.Objectives: The aim of this study is to report the clinical and genetic features of six Iranian HIGM patients.Methods: Six patients, who suspected to have HIGM based on two clinical findings, including recurrent infections and low levels of IgG and IgA and normal or elevated levels of IgM, were entered this study to undergo genetic studies. Sanger sequencing was applied to detect pathogenic mutations in CD40L and AID genes causing two most common forms of HIGM, which known as HIGM type 1 and 2, respectively.Results: All patients who entered the study were males from unrelated families with a median age of 3.8 years. The most frequent clinical manifestation was recurrent pneumonia. Genetic studies of the patients revealed six different mutations, including five mutations in CD40L besides one mutation in AID. Two mutations in CD40L (p.F31fsX5 and p.C84S) were novel and three mutations (p. G219R, p.D62fsX18, and p.Q186X) have been previously reported. The mutation found in AID (p.E122X) was also previously described.Conclusion: The study results may provide valuable information for prenatal diagnosis and also for genetic counseling especially for those who have a history of primary immunodeficiency in their family.

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.

Clinical, molecular, and T cell subset analyses in a small cohort of Chinese patients with hyper-IgM syndrome type 1

Type 1 hyper-IgM syndrome (HIGM1) is a rare primary immunodeficiency disease caused by mutations in the CD40L gene. Patients often present with recurrent infections and autoimmune manifestations. We investigated the clinical and molecular characteristics of HIGM1 in thirteen patients from the Chinese mainland and examined the proportion of CD4+CD25+FoxP3+Treg, Th17, and Th1 cells in the peripheral blood. We identified ten distinct CD40L mutations in eleven patients: one missense mutation, one nonsense mutation, one insertion mutation (in frame), and seven deletions. Six of these mutations were novel. We observed the percentage of Tregs in the peripheral blood of HIGM1 patients decreased markedly compared with that in healthy controls, but no statistically significant differences was found in the percentages of Th17 and Th1. The identified mutations reflect the heterogeneity of the CD40L gene in HIGM1. Precise genetic diagnosis of HIGM1 will enable appropriate therapeutic interventions, reliable detection of carriers, and genetic counseling. Skewed Treg, Th17/Treg, and Th1/Treg profiles may be associated with immune responses to autoimmunity or infection, which requires replication in larger studies.

A novel activation-induced cytidine deaminase (AID) mutation in Brazilian patients with hyper-IgM type 2 syndrome

Activation-induced cytidine deaminase (AID) is a DNA editing protein that plays an essential role in three major events of immunoglobulin (Ig) diversification: somatic hypermutation, class switch recombination and Ig gene conversion. Mutations in the AID gene (AICDA) have been found in patients with autosomal recessive Hyper-IgM (HIGM) syndrome type 2. Here, two 9- and 14-year-old Brazilian sisters, from a consanguineous family, were diagnosed with HIGM2 syndrome. Sequencing analysis of the exons from AICDA revealed that both patients are homozygous for a single C to G transversion in the third position of codon 15, which replaces a conserved Phenylalanine with a Leucine. To our knowledge, this is a new AICDA mutation found in HIGM2 patients. Functional studies confirm that the homologous murine mutation leads to a dysfunctional protein with diminished intrinsic cytidine deaminase activity and is unable to rescue CSR when introduced in Aicda(-/-)stimulated murine B cells. We briefly discuss the relevance of AICDA mutations found in patients for the biology of this molecule.

A Structural Basis for the Biochemical Behavior of Activation-induced Deoxycytidine Deaminase Class-switch Recombination-defective Hyper-IgM-2 Mutants

Hyper-IgM syndrome type 2 stems from mutations in activation-induced deoxycytidine deaminase (AID) that abolish immunoglobulin class-switch recombination, causing an accumulation of IgM and absence of IgG, IgA, and IgE isotypes. Although hyper-IgM syndrome type 2 is rare, the 23 missense mutations identified in humans span almost the entire gene for AID resulting in a recessive phenotype. Using high resolution x-ray structures for Apo3G-CD2 as a surrogate for AID, we identify three classes of missense mutants as follows: catalysis (class I), substrate interaction (class II), and structural integrity (class III). Each mutant was expressed and purified from insect cells and compared biochemically to wild type (WT) AID. Four point mutants retained catalytic activity at 1/3rd to 1/200th the level of WT AID. These "active" point mutants mimic the behavior of WT AID for motif recognition specificity, deamination spectra, and high deamination processivity. We constructed a series of C-terminal deletion mutants (class IV) that retain catalytic activity and processivity for deletions ≤18 amino acids, with ΔC(10) and ΔC(15) having 2-3-fold higher specific activities than WT AID. Deleting 19 C-terminal amino acids inactivates AID. WT AID and active and inactive point mutants bind cooperatively to single-stranded DNA (Hill coefficients ∼1.7-3.2) with microscopic dissociation constant values (K(A)) ranging between 10 and 250 nm. Active C-terminal deletion mutants bind single-stranded DNA noncooperatively with K(A) values similar to wild type AID. A structural analysis is presented that shows how localized defects in different regions of AID can contribute to loss of catalytic function.

ARDS as Presenting Symptom in an Infant with CD40L Deficiency (Hyper-IgM Syndrome Type 1)

We report on a 4 month old male infant with respiratory syncytial virus (RSV) infection leading to acute respiratory distress syndrome (ARDS). A diagnostic algorithm including extended infectiological and immunological work-up revealed absence of CD40-ligand. ARDS was treated successfully with a complex respiratory therapy plus intravenous immunoglobulin substitution. Molecular analysis detected mutations in the CD40L gene (Hyper-IgM syndrome Type 1). The case underlines the importance of an extended diagnostic work-up in an uncommonly severe course of respiratory infection in early infancy.

Cryptosporidium Infection in Patients With Primary Immunodeficiencies

Cryptosporidium species infection is usually self-limited in immunocompetent populations, but can be severe and life-threatening among immunocompromised individuals, particularly in patients with AIDS and in these patients with primary immunodeficiencies (PIDs). A group of 5 patients with genetically confirmed hyper-IgM syndrome type 1 (XHIM) and one patient with primary CD4 lymphopenia were enrolled in the study. At least 2 stool samples and a bile sample in one patient were examined for Cryptosporidium oocysts by a modified Ziehl-Neelsen technique, by immunofluorescence assay using a commercial kit, as well as by molecular analysis followed by genotyping. Immunological status at the time of PID diagnosis and the complex picture of disease are presented. Chronic cryptosporidiosis was confirmed in 3 patients with XHIM and in one patient with primary CD4 lymphopenia. Molecular diagnosis showed the presence of C parvum, C hominis, and C meleagridis in analyzed specimens. Cryptosporidium infection with serious clinical symptoms observed in patients with hyper-IgM syndrome calls for regular, repeated screening in this group of patients.

Juvenile recurrent parotitis and immunodeficiency: Report of two cases

Recurrent self-limiting parotid inflammation is known as Juvenile recurrent parotitis (JRP). Although congenital malformations, viral or bacterial infections, dehydration, hypersensitivity, autoimmunity have been blamed the pathogenesis remains unclear. Only a few patients who have RP in association with B cell immunodeficiency have been reported. We report two patients with JRP associated with immunodeficiency. One with Hyper IgM syndrome type II (HIGM2) and another with hypogammaglobulinemia of unknown origin suggestive for common variable immunodeficiency (CVID) by low serum IgG and IgM levels with normal B cell number. Although we could not find any evidence of autoimmunity or infection, it is impossible to exclude the infections or autoimmunity as causative factors in our patients since they have immunodeficiency.

Female hyper IgM syndrome type 1 with a chromosomal translocation disrupting CD40LG

Hyper-IgM syndrome type 1 (HIGM1) is a primary immunodeficiency characterized by recurrent bacterial and opportunistic infections, associated with normal or high serum level of IgM and decreased serum levels of IgG, IgA and IgE due to the defect of class switch recombination. CD40LG, located in Xq26, has been reported to be mutated in male HIGM1 patients. Here, we report the second case of a female HIGM1 with the defect of CD40 ligand (CD40L) expression and of soluble serum CD40L. Clinical course and HIGM phenotype was indistinguishable from that of male HIGM1 including severe neutropenia. High-resolution chromosome banding revealed that this patient's karyotype is 46, X, t(X;14)(q26.3;q13.1), and FISH analysis demonstrated that the break point of the chromosomal translocation is within CD40LG. Using four chimeric cDNA clones obtained by 3' RACE method, the break point was identified within the intron 4 of CD40LG on X chromosome and non-coding region of chromosome 14. We also found an extremely skewed X-chromosome inactivation pattern by methylation-specific PCR. Thus, the reciprocal translocation caused the disruption of CD40LG, resulting in defective CD40L expression in the female patient with an extremely skewed X-inactivation pattern in T cells leading to the HIGM1 phenotype.

Analysis of class switch recombination and somatic hypermutation in patients affected with autosomal dominant hyper-IgM syndrome type 2

Autosomal recessive form of hyper-IgM syndrome type 2 (AR-HIGM2) is secondary to mutations affecting both alleles of AICDA gene encoding activation-induced cytidine deaminase, characterized by defects of immunoglobulin class switch recombination (CSR) and somatic hypermutation (SHM) in most of the patients. We herein report the immunological phenotype of seven patients carrying a single heterozygous R190X mutation in AICDA. Variable defect in in vivo CSR inherited as an autosomal dominant (AD) trait strongly suggests that this heterozygous AICDA mutation causes HIGM (AD-HIGM2). In AD-HIGM2 B cells, CSR was consistently found impaired in vitro. However, in contrast to AR-HIGM2, the CSR-induced double-stranded DNA breaks in the switch region of IgM heavy chain gene were detected. The SHM frequency in V regions of IgM heavy chain gene in B cells was normal in all (but one patient). The characteristics of the AD-HIGM2 phenotype indicate that the AID C-terminal region may be involved in DNA repair machinery required for CSR.