Rubén Martínez-Barricarte

Identification of a mutation in complement factor H-related protein 5 in patients of Cypriot origin with glomerulonephritis.

Summary Background Complement is a key component of the innate immune system, and variation in genes that regulate its activation is associated with renal and other disease. We aimed to establish the genetic basis for a familial disorder of complement regulation associated with persistent microscopic haematuria, recurrent macroscopic haematuria, glomerulonephritis, and progressive renal failure. Methods We sought patients from the West London Renal and Transplant Centre (London, UK) with unusual renal disease and affected family members as a method of identification of new genetic causes of kidney disease. Two families of Cypriot origin were identified in which renal disease was consistent with autosomal dominant transmission and renal biopsy of at least one individual showed C3 glomerulonephritis. A mutation was identified via a genome-wide linkage study and candidate gene analysis. A PCR-based diagnostic test was then developed and used to screen for the mutation in population-based samples and in individuals and families with renal disease. Findings Occurrence of familial renal disease cosegregated with the same mutation in the complement factor H-related protein 5 gene (CFHR5). In a cohort of 84 Cypriots with unexplained renal disease, four had mutation in CFHR5. Overall, we identified 26 individuals with the mutation and evidence of renal disease from 11 ostensibly unrelated kindreds, including the original two families. A mutant CFHR5 protein present in patient serum had reduced affinity for surface-bound complement. We term this renal disease CFHR5 nephropathy. Interpretation CFHR5 nephropathy accounts for a substantial burden of renal disease in patients of Cypriot origin and can be diagnosed with a specific molecular test. The high risk of progressive renal disease in carriers of the CFHR5 mutation implies that isolated microscopic haematuria or recurrent macroscopic haematuria should not be regarded as a benign finding in individuals of Cypriot descent. Funding UK Medical Research Council and Wellcome Trust.

Human C3 mutation reveals a mechanism of dense deposit disease pathogenesis and provides insights into complement activation and regulation

Dense deposit disease (DDD) is a severe renal disease characterized by accumulation of electron-dense material in the mesangium and glomerular basement membrane. Previously, DDD has been associated with deficiency of factor H (fH), a plasma regulator of the alternative pathway (AP) of complement activation, and studies in animal models have linked pathogenesis to the massive complement factor 3 (C3) activation caused by this deficiency. Here, we identified a unique DDD pedigree that associates disease with a mutation in the C3 gene. Mutant C(3923ΔDG), which lacks 2 amino acids, could not be cleaved to C3b by the AP C3-convertase and was therefore the predominant circulating C3 protein in the patients. However, upon activation to C3b by proteases, or to C3(H₂O) by spontaneous thioester hydrolysis, C(3923ΔDG) generated an active AP C3-convertase that was regulated normally by decay accelerating factor (DAF) but was resistant to decay by fH. Moreover, activated C(3b923ΔDG) and C3(H₂O)(923ΔDG) were resistant to proteolysis by factor I (fI) in the presence of fH, but were efficiently inactivated in the presence of membrane cofactor protein (MCP). These characteristics cause a fluid phase-restricted AP dysregulation in the patients that continuously activated and consumed C3 produced by the normal C3 allele. These findings expose structural requirements in C3 that are critical for recognition of the substrate C3 by the AP C3-convertase and for the regulatory activities of fH, DAF, and MCP, all of which have implications for therapeutic developments.

Common polymorphisms in C3, factor B, and factor H collaborate to determine systemic complement activity and disease risk

Common polymorphisms in complement alternative pathway (AP) proteins C3 (C3R102G), factor B (fBR32Q), and factor H (fHV62I) are associated with age-related macular degeneration (AMD) and other pathologies. Our published work showed that fBR32Q influences C3 convertase formation, whereas fHV62I affects factor I cofactor activity. Here we show how C3R102G (C3S/F) influences AP activity. In hemolysis assays, C3102G activated AP more efficiently (EC50 C3102G: 157 nM; C3102R: 191 nM; P < 0.0001). fB binding kinetics and convertase stability were identical, but native and recombinant fH bound more strongly to C3b102R (KD C3b102R: 1.0 μM; C3b102G: 1.4 μM; P < 0.0001). Accelerated decay was unaltered, but fH cofactor activity was reduced for C3b102G, favoring AP amplification. Combining disease “risk” variants (C3102G, fB32R, and fH62V) in add-back assays yielded sixfold higher hemolytic activity compared with “protective” variants (C3102R, fB32Q, and fH62I; P < 0.0001). These data introduce the concept of a functional complotype (combination of polymorphisms) defining complement activity in an individual, thereby influencing susceptibility to AP-driven disease.

C3 glomerulopathy–associated CFHR1 mutation alters FHR oligomerization and complement regulation

C3 glomerulopathies (C3G) are a group of severe renal diseases with distinct patterns of glomerular inflammation and C3 deposition caused by complement dysregulation. Here we report the identification of a familial C3G-associated genomic mutation in the gene complement factor H–related 1 (CFHR1), which encodes FHR1. The mutation resulted in the duplication of the N-terminal short consensus repeats (SCRs) that are conserved in FHR2 and FHR5. We determined that native FHR1, FHR2, and FHR5 circulate in plasma as homo- and hetero-oligomeric complexes, the formation of which is likely mediated by the conserved N-terminal domain. In mutant FHR1, duplication of the N-terminal domain resulted in the formation of unusually large multimeric FHR complexes that exhibited increased avidity for the FHR1 ligands C3b, iC3b, and C3dg and enhanced competition with complement factor H (FH) in surface plasmon resonance (SPR) studies and hemolytic assays. These data revealed that FHR1, FHR2, and FHR5 organize a combinatorial repertoire of oligomeric complexes and demonstrated that changes in FHR oligomerization influence the regulation of complement activation. In summary, our identification and characterization of a unique CFHR1 mutation provides insights into the biology of the FHRs and contributes to our understanding of the pathogenic mechanisms underlying C3G.

The disease-protective complement factor H allotypic variant Ile62 shows increased binding affinity for C3b and enhanced cofactor activity

Mutations and polymorphisms in the gene encoding factor H (CFH) have been associated with atypical haemolytic uraemic syndrome, dense deposit disease and age-related macular degeneration. The disease-predisposing CFH variants show a differential association with pathology that has been very useful to unravel critical events in the pathogenesis of one or other disease. In contrast, the factor H (fH)-Ile(62) polymorphism confers strong protection to all three diseases. Using ELISA-based methods and surface plasmon resonance analyses, we show here that the protective fH-Ile(62) variant binds more efficiently to C3b than fH-Val(62) and competes better with factor B in proconvertase formation. Functional analyses demonstrate an increased cofactor activity for fH-Ile(62) in the factor I-mediated cleavage of fluid phase and surface-bound C3b; however, the two fH variants show no differences in decay accelerating activity. From these data, we conclude that the protective effect of the fH-Ile(62) variant is due to its better capacity to bind C3b, inhibit proconvertase formation and catalyze inactivation of fluid-phase and surface-bound C3b. This demonstration of the functional consequences of the fH-Ile(62) polymorphism provides relevant insights into the complement regulatory activities of fH that will be useful in disease prediction and future development of effective therapeutics for disorders caused by complement dysregulation.

Human TYK2 deficiency: Mycobacterial and viral infections without hyper-IgE syndrome

Kreins et al. report the identification and immunological characterization of a group of TYK2-deficient patients.

The mutation significance cutoff: gene-level thresholds for variant predictions

Next-generation sequencing (NGS) has made it possible to identify about 20,000 variants in the protein-coding exome of each individual, of which only a few are likely to underlie a genetic disease. Variant-level methods such as PolyPhen-2, SIFT and CADD are useful for obtaining a prediction as to whether a given variant is benign/damaging1–3 or tolerant/intolerant1–3 (we hereafter use the terms benign/deleterious). These methods are commonly interpreted in a binary manner for filtering out benign variants from NGS data, with a single significance cutoff value across all protein-coding genes. PolyPhen-2 and SIFT integrate the fixed cutoff in the software. CADD proposed (but did not recommend for categorical usage) the fixed value of 15 (or another value between 10 and 20). Gene-level methods, such as RVIS, de novo excess and GDI are also useful4–6. Combining fixed gene-level and variant-level cutoffs is also applied in the RVIS hot zone approach4. However, owing to the diversity of medical and population genetic features between human genes and across populations, a uniform cutoff is unlikely to be accurate genome-wide.

The human gene damage index as a gene-level approach to prioritizing exome variants

Significance The protein-coding exome of a patient with a monogenic disease contains about 20,000 variations, of which only one or two are disease causing. When attempting to select disease-causing candidate mutation(s), a challenge is to filter out as many false-positive (FP) variants as possible. In this study, we describe the gene damage index (GDI), a metric for the nonsynonymous mutational load in each protein-coding gene in the general population. We show that the GDI is an efficient gene-level method for filtering out FP variants in genes that are highly damaged in the general population. The protein-coding exome of a patient with a monogenic disease contains about 20,000 variants, only one or two of which are disease causing. We found that 58% of rare variants in the protein-coding exome of the general population are located in only 2% of the genes. Prompted by this observation, we aimed to develop a gene-level approach for predicting whether a given human protein-coding gene is likely to harbor disease-causing mutations. To this end, we derived the gene damage index (GDI): a genome-wide, gene-level metric of the mutational damage that has accumulated in the general population. We found that the GDI was correlated with selective evolutionary pressure, protein complexity, coding sequence length, and the number of paralogs. We compared GDI with the leading gene-level approaches, genic intolerance, and de novo excess, and demonstrated that GDI performed best for the detection of false positives (i.e., removing exome variants in genes irrelevant to disease), whereas genic intolerance and de novo excess performed better for the detection of true positives (i.e., assessing de novo mutations in genes likely to be disease causing). The GDI server, data, and software are freely available to noncommercial users from lab.rockefeller.edu/casanova/GDI.

Complement Factor H Is Expressed in Adipose Tissue in Association With Insulin Resistance

OBJECTIVE Activation of the alternative pathway of the complement system, in which factor H (fH; complement fH [CFH]) is a key regulatory component, has been suggested as a link between obesity and metabolic disorders. Our objective was to study the associations between circulating and adipose tissue gene expressions of CFH and complement factor B (fB; CFB) with obesity and insulin resistance. RESEARCH DESIGN AND METHODS Circulating fH and fB were determined by enzyme-linked immunosorbent assay in 398 subjects. CFH and CFB gene expressions were evaluated in 76 adipose tissue samples, in isolated adipocytes, and in stromovascular cells (SVC) (n = 13). The effects of weight loss and rosiglitazone were investigated in independent cohorts. RESULTS Both circulating fH and fB were associated positively with BMI, waist circumference, triglycerides, and inflammatory parameters and negatively with insulin sensitivity and HDL cholesterol. For the first time, CFH gene expression was detected in human adipose tissue (significantly increased in subcutaneous compared with omental fat). CFH gene expression in omental fat was significantly associated with insulin resistance. In contrast, CFB gene expression was significantly increased in omental fat but also in association with fasting glucose and triglycerides. The SVC fraction was responsible for these differences, although isolated adipocytes also expressed fB and fH at low levels. Both weight loss and rosiglitazone led to significantly decreased circulating fB and fH levels. CONCLUSIONS Increased circulating fH and fB concentrations in subjects with altered glucose tolerance could reflect increased SVC-induced activation of the alternative pathway of complement in omental adipose tissue linked to insulin resistance and metabolic disturbances.

The mutation significance cutoff: gene-level thresholds for variant predictions [Letter]

Next-generation sequencing (NGS) has made it possible to identify about 20,000 variants in the protein-coding exome of each individual, of which only a few are likely to underlie a genetic disease. Variant-level methods such as PolyPhen-2, SIFT and CADD are useful for obtaining a prediction as to whether a given variant is benign/damaging1–3 or tolerant/intolerant1–3 (we hereafter use the terms benign/deleterious). These methods are commonly interpreted in a binary manner for filtering out benign variants from NGS data, with a single significance cutoff value across all protein-coding genes. PolyPhen-2 and SIFT integrate the fixed cutoff in the software. CADD proposed (but did not recommend for categorical usage) the fixed value of 15 (or another value between 10 and 20). Gene-level methods, such as RVIS, de novo excess and GDI are also useful4–6. Combining fixed gene-level and variant-level cutoffs is also applied in the RVIS hot zone approach4. However, owing to the diversity of medical and population genetic features between human genes and across populations, a uniform cutoff is unlikely to be accurate genome-wide.