Common haplotypes at the CFH locus and low-frequency variants in CFHR2 and CFHR5 associate with systemic FHR concentrations and age-related macular degeneration

Age-related macular degeneration (AMD) is the principal cause of blindness in the elderly population. A strong effect on AMD risk has been reported for genetic variants at the CFH locus, encompassing complement factor H (CFH) and the complement-factor-H-related (CFHR) genes, but the underlying mechanisms are not fully understood. We aimed to dissect the role of factor H (FH) and FH-related (FHR) proteins in AMD in a cohort of 202 controls and 216 individuals with AMD. We detected elevated systemic levels of FHR-1 (p = 1.84 × 10-6), FHR-2 (p = 1.47 × 10-4), FHR-3 (p = 1.05 × 10-5) and FHR-4A (p = 1.22 × 10-2) in AMD, whereas FH concentrations remained unchanged. Common AMD genetic variants and haplotypes at the CFH locus strongly associated with FHR protein concentrations (e.g., FH p.Tyr402His and FHR-2 concentrations, p = 3.68 × 10-17), whereas the association with FH concentrations was limited. Furthermore, in an International AMD Genomics Consortium cohort of 17,596 controls and 15,894 individuals with AMD, we found that low-frequency and rare protein-altering CFHR2 and CFHR5 variants associated with AMD independently of all previously reported genome-wide association study (GWAS) signals (p = 5.03 × 10-3 and p = 2.81 × 10-6, respectively). Low-frequency variants in CFHR2 and CFHR5 led to reduced or absent FHR-2 and FHR-5 concentrations (e.g., p.Cys72Tyr in CFHR2 and FHR-2, p = 2.46 × 10-16). Finally, we showed localization of FHR-2 and FHR-5 in the choriocapillaris and in drusen. Our study identifies FHR proteins as key proteins in the AMD disease mechanism. Consequently, therapies that modulate FHR proteins might be effective for treating or preventing progression of AMD. Such therapies could target specific individuals with AMD on the basis of their genotypes at the CFH locus.

Complement Factor H Levels Associate With Plasmodium falciparum Malaria Susceptibility and Severity

Background

Plasmodium falciparum may evade complement-mediated host defense by hijacking complement Factor H (FH), a negative regulator of the alternative complement pathway. Plasma levels of FH vary between individuals and may therefore influence malaria susceptibility and severity.

Methods

We measured convalescent FH plasma levels in 149 Gambian children who had recovered from uncomplicated or severe P. falciparum malaria and in 173 healthy control children. We compared FH plasma levels between children with malaria and healthy controls, and between children with severe (n = 82) and uncomplicated malaria (n = 67). We determined associations between FH plasma levels and laboratory features of severity and used multivariate analyses to examine associations with FH when accounting for other determinants of severity.

Results

FH plasma levels differed significantly between controls, uncomplicated malaria cases, and severe malaria cases (mean [95% confidence interval], 257 [250 to 264], 288 [268 to 309], and 328 [313 to 344] µg/mL, respectively; analysis of variance P < .0001). FH plasma levels correlated with severity biomarkers, including lactate, parasitemia, and parasite density, but did not correlate with levels of PfHRP2, which represent the total body parasite load. Associations with severity and lactate remained significant when adjusting for age and parasite load.

Conclusions

Natural variation in FH plasma levels is associated with malaria susceptibility and severity. A prospective study will be needed to strengthen evidence for causation, but our findings suggest that interfering with FH binding by P. falciparum might be useful for malaria prevention or treatment.

Mutations in Complement Factor H Impair Alternative Pathway Regulation on Mouse Glomerular Endothelial Cells in Vitro

Complement factor H (FH) inhibits complement activation and interacts with glomerular endothelium via its complement control protein domains 19 and 20, which also recognize heparan sulfate (HS). Abnormalities in FH are associated with the renal diseases atypical hemolytic uremic syndrome and dense deposit disease and the ocular disease age-related macular degeneration. Although FH systemically controls complement activation, clinical phenotypes selectively manifest in kidneys and eyes, suggesting the presence of tissue-specific determinants of disease development. Recent results imply the importance of tissue-specifically expressed, sulfated glycosaminoglycans (GAGs), like HS, in determining FH binding to and activity on host tissues. Therefore, we investigated which GAGs mediate human FH and recombinant human FH complement control proteins domains 19 and 20 (FH19-20) binding to mouse glomerular endothelial cells (mGEnCs) in ELISA. Furthermore, we evaluated the functional defects of FH19-20 mutants during complement activation by measuring C3b deposition on mGEnCs using flow cytometry. FH and FH19-20 bound dose-dependently to mGEnCs and TNF-α treatment increased binding of both proteins, whereas heparinase digestion and competition with heparin/HS inhibited binding. Furthermore, 2-O-, and 6-O-, but not N-desulfation of heparin, significantly increased the inhibitory effect on FH19-20 binding to mGEnCs. Compared with wild type FH19-20, atypical hemolytic uremic syndrome-associated mutants were less able to compete with FH in normal human serum during complement activation on mGEnCs, confirming their potential glomerular pathogenicity. In conclusion, our study shows that FH and FH19-20 binding to glomerular endothelial cells is differentially mediated by HS but not other GAGs. Furthermore, we describe a novel, patient serum-independent competition assay for pathogenicity screening of FH19-20 mutants.

A meningococcal vaccine antigen engineered to increase thermal stability and stabilize protective epitopes

Factor H binding protein (FHbp) is part of two vaccines recently licensed for prevention of sepsis and meningitis caused by serogroup B meningococci. FHbp is classified in three phylogenic variant groups that have limited antigenic cross-reactivity, and FHbp variants in one of the groups have low thermal stability. In the present study, we replaced two amino acid residues, R130 and D133, in a stable FHbp variant with their counterparts (L and G) from a less stable variant. The single and double mutants decreased thermal stability of the amino- (N-) terminal domain compared with the wild-type protein as measured by scanning calorimetry. We introduced the converse substitutions, L130R and G133D, in a less stable wild-type FHbp variant, which increased the transition midpoint (Tm) for the N-terminal domain by 8 and 12 °C; together the substitutions increased the Tm by 21 °C. We determined the crystal structure of the double mutant FHbp to 1.6 Å resolution, which showed that R130 and D133 mediated multiple electrostatic interactions. Monoclonal antibodies specific for FHbp epitopes in the N-terminal domain had higher binding affinity for the recombinant double mutant by surface plasmon resonance and to the mutant expressed on meningococci by flow cytometry. The double mutant also had decreased binding of human complement Factor H, which in previous studies increased the protective antibody responses. The stabilized mutant FHbp thus has the potential to stabilize protective epitopes and increase the protective antibody responses to recombinant FHbp vaccines or native outer membrane vesicle vaccines with overexpressed FHbp.