Katherine A. Vernon

A Hybrid CFHR3-1 Gene Causes Familial C3 Glomerulopathy

Controlled activation of the complement system, a key component of innate immunity, enables destruction of pathogens with minimal damage to host tissue. Complement factor H (CFH), which inhibits complement activation, and five CFH-related proteins (CFHR1-5) compose a family of structurally related molecules. Combined deletion of CFHR3 and CFHR1 is common and confers a protective effect in IgA nephropathy. Here, we report an autosomal dominant complement-mediated GN associated with abnormal increases in copy number across the CFHR3 and CFHR1 loci. In addition to normal copies of these genes, affected individuals carry a unique hybrid CFHR3-1 gene. In addition to identifying an association between these genetic observations and complement-mediated kidney disease, these results provide insight into the protective role of the combined deletion of CFHR3 and CFHR1 in IgA nephropathy.

Acute Presentation and Persistent Glomerulonephritis Following Streptococcal Infection in a Patient With Heterozygous Complement Factor H-Related Protein 5 Deficiency

Acute poststreptococcal glomerulonephritis is a common cause of acute nephritis in children. Transient hypocomplementemia and complete recovery are typical, with only a minority developing chronic disease. We describe a young girl who developed persistent kidney disease and hypocomplementemia after a streptococcal throat infection. Kidney biopsy 1 year after presentation showed isolated glomerular complement C3 deposition, membranoproliferative changes, and subendothelial, intramembranous and occasional subepithelial electron-dense deposits consistent with C3 glomerulopathy. Complement gene screening revealed a heterozygous single nucleotide insertion in exon 4 of the complement factor H–related protein 5 gene (CFHR5), resulting in a premature stop codon. This variant was not detected in 198 controls. Serum CFHR5 levels were reduced. The mother and sister of the index patient were heterozygous for the sequence variant, with no overt evidence of kidney disease. We speculate that this heterozygous CFHR5 sequence variant is a risk factor for the development of chronic kidney disease after streptococcal infection.

Loss of Properdin Exacerbates C3 Glomerulopathy Resulting from Factor H Deficiency

Complement factor H (CFH) is a negative regulator of the alternative pathway of complement, and properdin is the sole positive regulator. CFH-deficient mice (CFH(-/-)) develop uncontrolled C3 activation and spontaneous renal disease characterized by accumulation of C3 along the glomerular basement membrane, but the role of properdin in the pathophysiology is unknown. Here, we studied mice deficient in both CFH and properdin (CFH(-/-).P(-/-)). Although CFH(-/-) mice had plasma depleted of both C3 and C5, CFH(-/-).P(-/-) animals exhibited depletion of C3 predominantly, recapitulating the plasma complement profile observed in humans with properdin-independent C3 nephritic factors. Glomerular inflammation, thickening of the capillary wall, and glomerular C3 staining were significantly increased in CFH(-/-).P(-/-) compared with CFH(-/-) mice. We previously reported that exogenous CFH ameliorates C3 staining of the glomerular basement membrane and triggers the appearance of mesangial C3 deposits in CFH(-/-) mice; here, we show that these effects require properdin. In summary, during uncontrolled activation of C3 driven by complete CFH deficiency, properdin influences the intraglomerular localization of C3, suggesting that therapeutic inhibition of properdin would be detrimental in this setting.

Recurrence of complement factor H-related protein 5 nephropathy in a renal transplant.

Complement factor H‐related protein 5 (CFHR5) nephropathy is a familial renal disease endemic in Cyprus. It is characterized by persistent microscopic hematuria, synpharyngitic macroscopic hematuria and progressive renal impairment. Isolated glomerular accumulation of complement component 3 (C3) is typical with variable degrees of glomerular inflammation. Affected individuals have a heterozygous internal duplication in the CFHR5 gene, although the mechanism through which this mutation results in renal disease is not understood. Notably, the risk of progressive renal failure in this condition is higher in males than females. We report the first documented case of recurrence of CFHR5 nephropathy in a renal transplant in a 53‐year‐old Cypriot male. Strikingly, histological changes of CFHR5 nephropathy were evident in the donor kidney 46 days post‐transplantation. This unique case demonstrates that renal‐derived CFHR5 protein cannot prevent the development of CFHR5 nephropathy.

ATYPICAL HEMOLYTIC UREMIC SYNDROME AND GENETIC ABERRATIONS IN THE COMPLEMENT FACTOR H RELATED 5 GENE

Atypical hemolytic uremic syndrome (aHUS) is a severe renal disorder that is associated with mutations in genes encoding proteins of the alternative complement pathway. Previously, we identified pathogenic variations in genes encoding complement regulators (CFH, CFI and MCP) in our aHUS cohort. In this study, we screened for mutations in the alternative pathway regulator CFHR5 in 65 aHUS patients by means of PCR on genomic DNA and sequence analysis. Potential pathogenicity of genetic alterations was determined by published data on CFHR5 variants, evolutionary conservation and in silico mutation prediction programs. Detection of serum CFHR5 was performed by western blot analysis and enzyme-linked immunosorbent assay. A potentially pathogenic sequence variation was found in CFHR5 in three patients (4.6%). All variations were located in short consensus repeats that might be involved in binding to C3b, heparin or C-reactive protein. The identified CFHR5 mutations require functional studies to determine their relevance to aHUS, but they might be candidates for an altered genetic profile predisposing to the disease.

Partial Complement Factor H Deficiency Associates with C3 Glomerulopathy and Thrombotic Microangiopathy

The complement-mediated renal diseases C3 glomerulopathy (C3G) and atypical hemolytic uremic syndrome (aHUS) strongly associate with inherited and acquired abnormalities in the regulation of the complement alternative pathway (AP). The major negative regulator of the AP is the plasma protein complement factor H (FH). Abnormalities in FH result in uncontrolled activation of C3 through the AP and associate with susceptibility to both C3G and aHUS. Although previously developed FH-deficient animal models have provided important insights into the mechanisms underlying susceptibility to these unique phenotypes, these models do not entirely reproduce the clinical observations. FH is predominantly synthesized in the liver. We generated mice with hepatocyte-specific FH deficiency and showed that these animals have reduced plasma FH levels with secondary reduction in plasma C3. Unlike mice with complete FH deficiency, hepatocyte-specific FH-deficient animals developed neither plasma C5 depletion nor accumulation of C3 along the glomerular basement membrane. In contrast, subtotal FH deficiency associated with mesangial C3 accumulation consistent with C3G. Although there was no evidence of spontaneous thrombotic microangiopathy, the hepatocyte-specific FH-deficient animals developed severe C5-dependent thrombotic microangiopathy after induction of complement activation within the kidney by accelerated serum nephrotoxic nephritis. Taken together, our data indicate that subtotal FH deficiency can give rise to either spontaneous C3G or aHUS after a complement-activating trigger within the kidney and that the latter is C5 dependent.

Experimental models of membranoproliferative glomerulonephritis, including dense deposit disease.

Membranoproliferative glomerulonephritis (MPGN) is characterised by mesangial expansion and hypercellularity and capillary wall thickening with capillary wall and mesangial deposits of immunoglobulin and/or complement. Two main forms are described in humans: MPGN type I with subendothelial and mesangial electron-dense deposits on electron microscopy, and MPGN type II, or dense deposit disease, with electron dense transformation of the glomerular capillary wall. Spontaneous MPGN type I has been described in dogs and sheep in association with C3 deficiency. Induced models of MPGN type I have been described in mice with cryoglobulinaemia. Glomerulonephritis resembling MPGN type II has occurred spontaneously in pigs that have a genetic deficiency of the complement control protein factor H. The animals develop capillary wall deposits of C3 before birth. Mice have been genetically engineered with a deficiency of factor H and similarly develop glomerular capillary wall C3 with MPGN. This model has been used to study both pathogenesis and therapeutic interventions. In particular, MPGN associated with factor H deficiency is absolutely dependent on both the ability to activate C3 and on the ability of factor I to cleave C3b. There is an important role for C5 activation in the development of glomerular inflammation in this model. Factor H dysfunction is associated with an increased susceptibility to complement-activating nephrotoxic insults and in these scenarios C5 activation appears to play a major role in mediating glomerular injury.