Familial IgA nephropathy in southeastern Kentucky

IgA vasculitis with nephritis: update of pathogenesis with clinical implications

IgA vasculitis with nephritis (IgAVN) shares many pathogenetic features with IgA nephropathy (IgAN). The purpose of this review is to describe our current understanding of the pathogenesis of pediatric IgAVN, particularly as it relates to the four-hit hypothesis for IgAN. These individual steps, i.e., hits, in the pathogenesis of IgAN are (1) elevated production of IgA1 glycoforms with some O-glycans deficient in galactose (galactose-deficient IgA1; Gd-IgA1), (2) generation of circulating IgG autoantibodies specific for Gd-IgA1, (3) formation of pathogenic circulating Gd-IgA1-containing immune complexes, and (4) kidney deposition of the Gd-IgA1-IgG immune complexes from the circulation and induction of glomerular injury. Evidence supporting the four-hit hypothesis in the pathogenesis of pediatric IgAVN is detailed. The genetics, pediatric outcomes, and kidney histopathologic features and the impact of these findings on future treatment and potential biomarkers are discussed. In summary, the evidence points to the critical roles of Gd-IgA1-IgG immune complexes and complement activation in the pathogenesis of IgAVN. Future studies are needed to characterize the features of the immune and autoimmune responses that enable progression of IgA vasculitis to IgAVN.

Pathogenesis of immunoglobulin A nephropathy

PURPOSE OF REVIEW

In this article, we review recent findings on the pathogenesis and genetics of immunoglobulin A (IgA) nephropathy.

RECENT FINDINGS

During the past 2 years, the understanding of the pathogenesis of IgA nephropathy has evolved as a result of progress in technology and new tools that have been developed. Since 1968, when IgA nephropathy was described as an IgA-IgG immune-complex disease, the knowledge base expanded to allow definition of IgA nephropathy as an autoimmune disease with a multihit pathogenetic process. Specifically, galactose-deficient immunoglobulin A1 (IgA1) is recognized by unique autoantibodies, resulting in the formation of pathogenic immune complexes that ultimately deposit in the glomerular mesangium and induce renal injury. New approaches using high-resolution mass spectrometry have provided unique insight at the molecular level into IgA1 O-glycosylation. Cutting-edge genome-wide association studies revealed multiple disease-associated risk loci and have mapped their geographic and racial distribution.

SUMMARY

Recent studies of molecular and genetic defects operating in IgA nephropathy can define new biomarkers specific for the disease that can be developed into clinical assays to aid in the diagnosis, assessment of prognosis, and monitoring of disease progression. Moreover, disease-specific targets are being discovered that may lead to development of new approaches for treatment.

IgG-Fc N-glycosylation at Asn297 and IgA O-glycosylation in the hinge region in health and disease

Immunoglobulins (Igs) are the major molecules secreted by B lymphocytes during an adaptive immune response. They are glycoproteins with distinctive glycosylation patterns, resulting in wide variations in the number, type and location of their oligosaccharides in each isotype and subclass. The sugars play specific structural roles, maintaining and modulating effector functions of Igs. Aberrant glycosylation might contribute to disease pathogenesis. This review will focus on the glycosylation of IgG and IgA because they have been studied more extensively than other immunoglobulins. Rheumatoid arthritis and IgA nephritis are used to describe the association of glycosylation aberration and disease pathogenesis.

Glycosylation of IgA1 and pathogenesis of IgA nephropathy

IgA nephropathy, described in 1968 as IgA-IgG immune-complex disease, is an autoimmune disease. Galactose-deficient IgA1 is recognized by unique autoantibodies, resulting in the formation of pathogenic immune complexes that ultimately induce glomerular injury. Thus, formation of the galactose-deficient IgA1-containing immune complexes is a critical factor in the pathogenesis of IgA nephropathy. Studies of molecular defects of IgA1 can define new biomarkers specific for IgA nephropathy that can be developed into clinical assays to aid in the diagnosis, assessment of prognosis, and monitoring of disease progression.

Pathogenesis of IgA nephropathy

Since its first description in 1968, IgA nephropathy has remained the most common form of idiopathic glomerulonephritis leading to chronic kidney disease in developed countries. The exact pathogenesis of IgA nephropathy is still not well defined. Current data implicate an important genetic factor, especially in promoting the overproduction of an aberrant form of IgA1. The immunochemical aberrancy of IgA nephropathy is characterized by the undergalactosylation of O-glycans in the hinge region of IgA1. However, such aberrant glycosylation alone does not cause renal injury. The next stage of disease development requires the formation of glycan-specific IgG and IgA antibodies that recognize the undergalactosylated IgA1 molecule. These antibodies often have reactivity against antigens from extrinsic microorganisms and might arise from recurrent mucosal infection. B cells that respond to mucosal infections, particularly tonsillitis, might produce the nephritogenic IgA1 molecule. With increased immune-complex formation and decreased clearance owing to reduced uptake by the liver, IgA1 binds to the glomerular mesangium via an as yet unidentified receptor. Glomerular IgA1 deposits trigger the local production of cytokines and growth factors, leading to the activation of mesangial cells and the complement system. Emerging data suggest that mesangial-derived mediators following glomerular deposition of IgA1 lead to podocyte and tubulointerstitial injury via mesangio-podocytic-tubular crosstalk. This Review summarizes the latest findings in the pathogenesis of IgA nephropathy.

Familial C3 glomerulopathy associated with CFHR5 mutations: clinical characteristics of 91 patients in 16 pedigrees

BACKGROUND AND OBJECTIVES

Complement factor H and related proteins (CFHR) are key regulators of the alternative complement pathway, where loss of function mutations lead to a glomerulopathy with isolated mesangial C3 deposits without immunoglobulins. Gale et al. (12) reported on 26 patients with the first familial, hematuric glomerulopathy caused by a founder mutation in the CFHR5 gene in patients of Cypriot descent living in the United Kingdom. CFHR5 nephropathy is clinically characterized by continuous microscopic hematuria whereas some patients present with additional episodes of synpharyngitic macrohematuria, associated with infection and pyrexia. A subgroup of patients, particularly men, develop additional proteinuria, hypertension, and chronic renal disease or ESRD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We herewith expand significantly on the study by Gale et al., reporting on histologic, molecular, and clinical findings in 91 patients from 16 families with the same founder mutation.

RESULTS

Eighty-two patients (90%) exhibited microscopic hematuria; 51 (62%), exhibited only microscopic hematuria, whereas the remaining 31 additionally had proteinuria (38%); 28 proteinuric patients developed chronic renal failure (CRF). Among carriers of CFHR5 mutation aged >50 years, 80% of the men and 21% of the women developed CRF; 18 developed ESRD (14 men [78%], 4 women [22%]).

CONCLUSIONS

The diagnosis of CFHR5-related, isolated C3 glomerulopathy was established in 2009 using newly described mutation analysis after decades of follow-up with unclear diagnoses, occasionally confused with IgA nephropathy. This larger patient cohort establishes the clinical course, significant variable expressivity, and marked gender difference regarding the development of CRF and ESRD.

Aberrant glycosylation of IgA1 is inherited in both pediatric IgA nephropathy and Henoch-Schönlein purpura nephritis

Serum galactose-deficient immunoglobulin A1 (Gd-IgA1) is an inherited risk factor for adult IgA nephropathy (IgAN). In this paper, we determined the heritability of serum Gd-IgA1 levels in children with IgAN and Henoch-Schönlein purpura nephritis (HSPN), two disorders with clinical phenotypes sharing common pathogenic mechanisms. Serum Gd-IgA1 concentrations were quantified using a Helix aspersa-lectin-based enzyme-linked immunosorbent assay. As a group, 34 children with either disorder (20 with HSPN and 14 with IgAN) had significantly higher Gd-IgA1 levels compared with 51 age- and ethnicity-matched pediatric controls. Serum levels of Gd-IgA1 were also elevated in a large fraction of 54 first-degree relatives of pediatric IgAN and HSPN patients compared with 141 unrelated healthy adult controls. A unilineal transmission of the trait was found in 17, bilineal transmission in 1, and sporadic occurrence in 5 of 23 families when both parents and the patient were analyzed. There was a significant age-, gender-, and household-adjusted heritability of serum galactose-deficient IgA1 estimated at 76% in pediatric IgAN and at 64% in HSPN patients. Thus, serum galactose-deficient IgA1 levels are highly inherited in pediatric patients with IgAN and HSPN, providing support for another shared pathogenic link between these disorders.