The HNF4A R76W mutation causes atypical dominant Fanconi syndrome in addition to a ß cell phenotype.

BACKGROUND: Mutation specific effects in monogenic disorders are rare. We describe atypical Fanconi syndrome caused by a specific heterozygous mutation in HNF4A. Heterozygous HNF4A mutations cause a beta cell phenotype of neonatal hyperinsulinism with macrosomia and young onset diabetes. Autosomal dominant idiopathic Fanconi syndrome (a renal proximal tubulopathy) is described but no genetic cause has been defined. METHODS AND RESULTS: We report six patients heterozygous for the p.R76W HNF4A mutation who have Fanconi syndrome and nephrocalcinosis in addition to neonatal hyperinsulinism and macrosomia. All six displayed a novel phenotype of proximal tubulopathy, characterised by generalised aminoaciduria, low molecular weight proteinuria, glycosuria, hyperphosphaturia and hypouricaemia, and additional features not seen in Fanconi syndrome: nephrocalcinosis, renal impairment, hypercalciuria with relative hypocalcaemia, and hypermagnesaemia. This was mutation specific, with the renal phenotype not being seen in patients with other HNF4A mutations. In silico modelling shows the R76 residue is directly involved in DNA binding and the R76W mutation reduces DNA binding affinity. The target(s) selectively affected by altered DNA binding of R76W that results in Fanconi syndrome is not known. CONCLUSIONS: The HNF4A R76W mutation is an unusual example of a mutation specific phenotype, with autosomal dominant atypical Fanconi syndrome in addition to the established beta cell phenotype.

Genetic susceptibility to experimental autoimmune glomerulonephritis in the Wistar Kyoto rat.

In experimental autoimmune glomerulonephritis (EAG), a model of Goodpasture's disease, Wistar Kyoto (WKY) rats immunized with collagenase-solubilized glomerular basement membrane (GBM) or the recombinant NC1 domain of the α3 chain of type IV collagen [α3(IV)NC1] develop anti-GBM antibodies and focal necrotizing glomerulonephritis with crescent formation. However, Lewis (LEW) rats, which share the same major histocompatibility complex (MHC) haplotype, are resistant to EAG development. A genome-wide linkage analysis of backcrossed animals with EAG revealed a major quantitative trait locus (QTL) on rat chromosome 13 (LOD = 3.9) linked to the percentage of glomerular crescents. To investigate the role of this QTL in EAG induction, reciprocal congenic rats were generated (LEW.WCrgn1 congenic and WKY.LCrgn1 congenic), immunized with recombinant rat α3(IV)NC1, and assessed for EAG development. WKY.LCrgn1 rats showed a marked reduction in albuminuria, severity of crescentic nephritis, and number of glomerular macrophages compared with WKY controls. No reduction in antibody levels was observed. However, LEW.WCrgn1 rats were resistant to EAG development, as were LEW controls. Macrophage activation in vitro was assessed in parental and congenic rat bone marrow-derived macrophages (BMDMs). WKY.LCrgn1 BMDMs showed a significant reduction in Fc receptor-mediated oxidative burst, phagocytosis of opsonised polystyrene beads, and LPS-induced levels of MCP-1 secretion and iNOS mRNA expression compared with WKY rats. These results confirm the importance of Crgn1 on chromosome 13 in EAG susceptibility, mediated partly through differences in Fc receptor-mediated macrophage activation.

Copy number polymorphism in Fcgr3 predisposes to glomerulonephritis in rats and humans.

Identification of the genes underlying complex phenotypes and the definition of the evolutionary forces that have shaped eukaryotic genomes are among the current challenges in molecular genetics. Variation in gene copy number is increasingly recognized as a source of inter-individual differences in genome sequence and has been proposed as a driving force for genome evolution and phenotypic variation. Here we show that copy number variation of the orthologous rat and human Fcgr3 genes is a determinant of susceptibility to immunologically mediated glomerulonephritis. Positional cloning identified loss of the newly described, rat-specific Fcgr3 paralogue, Fcgr3-related sequence (Fcgr3-rs), as a determinant of macrophage overactivity and glomerulonephritis in Wistar Kyoto rats. In humans, low copy number of FCGR3B, an orthologue of rat Fcgr3, was associated with glomerulonephritis in the autoimmune disease systemic lupus erythematosus. The finding that gene copy number polymorphism predisposes to immunologically mediated renal disease in two mammalian species provides direct evidence for the importance of genome plasticity in the evolution of genetically complex phenotypes, including susceptibility to common human disease.

Effect of hemolysis on insulin determination by the Beckman Coulter Unicell DXI 800 immunoassay analyzer.

OBJECTIVES: To evaluate the effect of hemolysis on the measurement of insulin using the Beckman Coulter Unicell DXI 800 immunoassay analyzer. DESIGN AND METHODS: Insulin concentrations were measured using the Beckman Coulter Unicell DXI 800. Interference studies were conducted by mixing plasma of known insulin concentrations with hemolysates of increasing hemoglobin concentration. Hemolysates were prepared by osmotic shock. A change of >10% from baseline results was taken as evidence of significant interference. RESULTS: Hemolysates mediated a concentration-dependent reduction in insulin concentration. CONCLUSIONS: A concentration-dependent negative bias in the measurement of insulin is associated with increasing degrees of hemolysis on the Beckman Coulter Unicell DXI 800.

Segregation of experimental autoimmune glomerulonephritis as a complex genetic trait and exclusion of Col4a3 as a candidate gene.

Experimental autoimmune glomerulonephritis (EAG), an animal model of Goodpasture's disease, can be induced in Wistar-Kyoto (WKY) rats (RT1-l) by immunization with rat glomerular basement membrane (GBM) in adjuvant. The model in this rat strain is characterized by anti-GBM antibody production accompanied by focal necrotizing glomerulonephritis with crescent formation. The main autoantigen in humans and rats has been identified as the non-collagenous domain of the alpha3 chain of type IV collagen (alpha3(IV)NC1). By contrast, Lewis (LEW) rats with the same MHC background (RT1-l), immunized with the same antigen, develop similar levels of circulating anti-GBM antibodies, but no histological evidence of nephritis. In order to investigate the genetic basis of susceptibility to EAG, we examined the response of both F1 (WKY x LEW) and backcross (BC1; WKY x F1) rats to immunization with rat GBM. F1 animals were completely resistant to the development of EAG, while BC1 animals showed a range of responses from severe crescentic glomerulonephritis to no histological evidence of disease. The results indicate that EAG is inherited as a complex trait under the control of WKY genes unlinked to the MHC. cDNA sequence analysis of alpha3(IV)NC1 in the two parental strains was identical, indicating no predicted amino acid sequence variation in the alpha3(IV)NC1 domain between these strains. Radiation hybrid mapping, using two separate PCR amplicons from rat alpha3(IV)NC1, localized rat Col4a3 to a region of chromosome 9. Since Col4a3 (encoding the autoantigen) is a candidate for susceptibility to EAG, we screened the region of rat chromosome 9 where Col4a3 is localized, using polymorphic microsatellite markers in segregating BC1 progeny. No significant linkage was detected. These results exclude Col4a3 as a recessive susceptibility gene for EAG in the BC1 progeny.