Immunoglobulin heavy chain switch region gene polymorphisms in glomerulonephritis

Genome-wide scan in a novel IgA nephropathy model identifies a susceptibility locus on murine chromosome 10, in a region syntenic to human IGAN1 on chromosome 6q22-23

Genetic factors are considered to be involved in the initiation and progression of IgA nephropathy (IgAN) on the basis of racial differences in the prevalence and familial aggregation. The ddY mouse is a spontaneous animal model of human IgAN, with a highly variable incidence and extent of glomerular injury as a result of the heterogeneous background, resembling the human situation. It was hypothesized that susceptibility genes for IgAN can be detected by a genome-wide scan using this model. First, serial renal biopsies were performed at 20, 40, and 60 wk of age in 361 ddY mice. The ddY mice were classified into three groups on the basis of the onset of glomerular injury: Early onset at 20 wk (31.9%), late onset at 40 wk (37.9%), and quiescent even at 60 wk (30.2%). The severity of glomerular lesions in both onset groups correlated with the intensity of glomerular IgA deposition but not with serum IgA level. The genome-wide scan with 270 microsatellite markers identified three chromosomal regions on chromosomes 1, 9, and 10, which were significantly associated with the glomerular injuries. Surprisingly, the peak marker D10MIT86 on chromosome 10 is located on the region syntenic to human 6q22-23 with IGAN1, which is the responsible candidate of familial IgAN. In addition, D1MIT16 on chromosome 1 was very closely located at the locus of selectin gene, which is a known candidate of human IgAN. In conclusion, the three-group ddY mouse model can be a useful tool for identifying the susceptibility genes and also to examine their roles in the pathogenesis of IgAN.

Polymorphism of the human alpha1 immunoglobulin gene 3' enhancer hs1,2 and its relation to gene expression

We studied the hs1,2 transcriptional enhancer identified downstream of the human alpha1 gene of the immunoglobulin H (IgH) locus, for which two different allelic configurations (a and b) were previously reported by Southern blotting. By using a polymerase chain reaction (PCR) method we amplified minisatellites within the hs1,2 core enhancer, with variable numbers of tandem repeats (VNTR) defining three 'PCR alleles' alpha1A, alpha1B and alpha1C (including one, two and three repeats, respectively). Five different alpha1 h1,2 genotypes were encountered in a population of 513 donors, representing 13.8, 34.5, 49.7, 1.3 and 0.6% for the AA, BB, AB, AC and BC genotypes, respectively. Luciferase assays showed that increasing the number of minisatellites increased the transcriptional strength of the alpha1 hs1,2 enhancer. Simultaneous determination of Southern blot alleles and VNTR alleles only showed a partial linkage between both types of polymorphism, altogether defining at least six different allelic forms of the 3'alpha1 region. In conclusion, the present study further demonstrates the genetic instability of the 3'alpha region, for which multiple alleles have been generated through inversions and internal deletions and/or duplications. This study also strengthens the hypothesis that the polymorphism at the IgH 3' regulatory region of the alpha1 gene could play a role in the outcome of diseases involving immunoglobulin secretion.

Alleles of the alpha1 immunoglobulin gene 3' enhancer control evolution of IgA nephropathy toward renal failure

BACKGROUND

IgA nephropathy is the most common glomerular disease. Mechanisms leading to its occurrence and controlling the evolution of the disease remain largely unknown. Various genetic factors have been found, mostly implicating immunologically relevant genes (IgH, TCR, human lymphocyte antigen, and complement loci). A regulatory region recently identified downstream, the alpha1 gene of the IgH locus, was a likely candidate for the control of IgA1 production in patients. Alleles of this region, differing by size, sequence, and orientation of the alpha1 hs1,2 transcriptional enhancer, were first identified through Southern blot hybridization.

METHODS

We established a polymerase chain reaction (PCR) method suitable for routine testing that amplifies minisatellites within the alpha1 hs1, 2 enhancer, with variable numbers of tandem repeats (VNTR) defining the two alleles. This assay allowed the typing of 104 patients with IgAN and 83 healthy volunteers. Results from typing of alpha1 hs1,2 alleles were compared with long-term clinical outcome in patients. Enhancer alleles were compared in a luciferase reporter gene assay.

RESULTS

The alpha1 hs1,2 alleles do not constitute a predictive factor for IgA nephropathy, since similar allelic frequencies were observed in healthy individuals and in unrelated European patients. In contrast, among patients, homozygosity for the weakest enhancer allele (AA genotype) was significantly correlated with a milder form of the disease, whereas the allele B was associated with severe evolution. The minisatellite region within the alpha1 hs1,2 enhancer carried potential transcription factor-binding sites, and its duplication increased the transcriptional strength of the alpha1 hs1, 2 allele B over that of allele A.

CONCLUSION

Altogether, these alleles may constitute a risk factor for the prognosis of IgA nephropathy.

Induction of IgA against Haemophilus parainfluenzae antigens in tonsillar mononuclear cells from patients with IgA nephropathy

Much evidence suggests that IgA production in vivo and in vitro is enhanced in patients with IgA nephropathy (IgAN). We have demonstrated glomerular deposition of the outer membranes of Haemophilus parainfluenzae (HP) antigens (OMHP) and the presence of HP-specific IgA in the serum of patients with IgAN. In this study, we investigated the production of IgA and several cytokines by tonsillar mononuclear cells (TMC) from IgAN patients induced by stimulation with OMHP. The spontaneous production of total IgA and TGF-beta by TMC from IgAN patients was higher than that by TMC from patients with chronic tonsillitis (CT) (P < 0.05). Stimulation with OMHP in vitro enhanced the production of HP-specific IgA by TMC from IgAN patients (P < 0.01), but not by TMC from CT patients. OMHP stimulation also enhanced the production of TGF-beta and IL-10 by TMC from IgAN patients (P < 0.001). These results suggest that the infection of HP in the tonsil may be involved in the etiology of IgAN.

Polymorphism in the Iα1 Germ-Line Transcript Regulatory Region and IgA Productivity in Patients with IgA Nephropathy

Enhanced in vivo and in vitro production of IgA has been reported in patients with IgA nephropathy (IgAN) and their family members. It is generally considered that IgA1 is a prominent subclass of IgA in IgAN. Although genetic mechanisms of IgA class switch recombination in IgAN have been studied enthusiastically, the critical factors that induce IgA1-specific class switching in IgAN have yet to be elucidated. A large body of data indicates that the germ-line transcript of Ig constant region (CH) genes that precedes actual class switching has regulatory effects on class switch recombination. To analyze structural abnormalities in the Iα1 germ-line transcript regulatory gene, a region about 1000 bp long located upstream of Iα1 exons was surveyed by the PCR-single strand conformation polymorphism method, and the polymorphism detected was confirmed by subsequent DNA sequencing. Three hot spots for point mutation were detected upstream of the promoter region of the Iα1 germ-line transcript, and the mutations were observed more frequently in patients than in controls. Patients with the mutations showed higher levels of serum IgA and higher in vitro IgA synthesis. In the luciferase assay, the regulatory gene with the mutations showed a potent effect for induction of the Iα1 germ-line transcript. The polymorphism in the Iα1 regulatory region possibly causes enhanced IgA production in some patients with IgAN.

The polymorphism of the immunoglobulin heavy chain switch region gene in IgA nephropathy, Henoch-Schönlein nephritis and idiopathic nephrotic syndrome

It is well known genetic predisposition may play an important role in the pathogenesis of renal diseases. Recently, there has been some controversy about the possible role of the polymorphism of the immunoglobulin heavy chain switch region gene. We have studied this gene by SstI restriction fragment length polymorphisms using DNA from 41 children with IgA nephropathy, 44 with Henoch-Schönlein nephritis and 60 with idiopathic nephrotic syndrome. There was no association of specific genotype with these diseases, in contrast to previous reports. These results are probably due to the differing genetic background of Japanese and Caucasoid patients as far as the switch region is concerned; no switch region genotype constitutes a genetic risk factor for the Japanese in these diseases.