Functional evaluation of complement factor I variants by immunoassays and SDS-PAGE.

Factor I (FI) is an essential regulator of the complement system. Together with co-factors, FI degrades C3b, which inhibits further complement activation. Genetic mutations in FI are associated with pathological conditions like age-related macular degeneration and atypical hemolytic uremic syndome. Here, we evaluated eight recombinant FI genetic variants found in patients. We assessed FI's co-factor activity in the presence of two co-factors; Factor H and soluble CR1. Different analytical assays were employed; SDS-PAGE to evaluate the degradation of C3b, ELISA to measure the generation of fluid phase iC3b and the degradation of surface-bound C3b using a novel Luminex bead-based assay. We demonstrate that mutations in the FIMAC and SP domains of FI led to significantly reduced protease activity, whereas the two analyzed mutations in the LDLRA2 domain did not result in any profound changes in FI's function. The different assays employed displayed a strong positive correlation, but differences in the activity of the genetic variants Ile55Phe and Gly261Asp could only be observed by combining different methods and co-factors for evaluating FI activity. In conclusion, our results provide a new perspective regarding available diagnostic tools for assessing the impact of mutations in FI.

Complement Factor I Gene Variant in an Atypical Hemolytic Uremic Syndrome Triggered by Hypereosinophilia Syndrome.

Atypical hemolytic uremic syndrome (aHUS) is a condition characterized by acute kidney injury (AKI), thrombocytopenia, and microangiopathic hemolytic anemia secondary to complement pathway dysregulation. Several triggers have been identified as causing aHUS in genetically susceptible patients; however, hypereosinophilia syndrome (HES)-triggered aHUS has not been reported. In this article, we present a case of aHUS presented with generalized urticarial rashes and angioedema. The initial investigations revealed hypereosinophilia (maximal absolute eosinophil count of 6,840 cells/µL) with normal bone-marrow analyses; hence, idiopathic HES was diagnosed. During hospitalization, the patient developed convulsion, stuporous, and full-blown thrombotic microangiopathy (TMA), with AKI requiring temporary hemodialysis. A kidney biopsy confirmed the existence of renal TMA. Next-generation sequencing of the coding regions of aHUS-related genes was performed, revealing an underlying complement factor I (CFI) deficiency, a heterozygous variant p.P64L of CFI gene. The patient was successfully treated with high-dose steroids and extended duration of plasmapheresis.

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic.

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

Mutations in atypical hemolytic uremic syndrome provide evidence for the role of calcium in complement factor I.

Atypical hemolytic uremic syndrome (aHUS) is a rare thrombotic microangiopathy. Genetic variants in complement proteins are found in ≈60% of patients. Of these patients, ≈15% carry mutations in complement factor I (CFI). Factor I (FI) is a multidomain serine protease that cleaves and thereby inactivates C3b and C4b in the presence of cofactor proteins. Crystal structures have shown that FI possesses 2 calcium-binding domains, low-density lipoprotein receptor class A (LDLRA) 1 and LDLRA2. Yet, the role of calcium in FI is unknown. We determined that 9 genetic variants identified in aHUS (N151S, G162D, G188A, V230E, A240G, G243R, C247G, A258T, and Q260D) cluster around the calcium-binding site of LDLRA1. Using site-directed mutagenesis, we established that the synthesis of all, except A258T, was impaired, implying defective protein folding, perhaps due to loss of calcium binding. To further explore this possibility, we generated 12 alanine mutants that coordinate with the calcium in LDLRA1 and LDLRA2 (K239A, D242A, I244A, D246A, D252A, E253A, Y276A, N279A, E281A, D283A, D289A, and D290A) and are expected to perturb calcium binding. Except for K239A and Y276A, none of the mutants was secreted. These observations suggest that calcium ions play key structural and functional roles in FI.

Complement Factor I Gene Variant as a Treatable Cause of Recurrent Aseptic Neutrophilic Meningitis: A Case Report.

Mutations in the complement factor I (CFI) gene have previously been identified as causes of recurrent CNS inflammation. We present a case of a 26-year-old man with 18 episodes of recurrent meningitis, who had a variant in CFI(c.859G>A,p.Gly287Arg) not previously associated with neurologic manifestations. He achieved remission with canakinumab, a human monoclonal antibody targeted at interleukin-1 beta.

Pearls & Oy-sters: Homozygous Complement Factor I Deficiency Presenting as Fulminant Relapsing Complement-Mediated CNS Vasculitis.

A 36-year-old man presented multiple times with fever, headache, alteration of mental status, and focal neurologic deficits. MRI revealed extensive white matter lesions that were partially reversed between episodes. Workup revealed persistently low complement factor C3, low factor B, and absent alternative complement pathway activity. Biopsy revealed neutrophilic vasculitis. Genetic testing revealed a homozygous variant in complement factor I (CFI), which was thought to be pathogenic. CFI regulates complement-mediated inflammation, and deficiency in this factor leads to unchecked alternative pathway activity and decrease in C3 and factor B through consumption. The patient has remained stable since starting IL-1ß inhibition. Complement factor I is a rare disorder that should be considered in patients with atypical relapsing neurologic disease associated with neutrophilic pleocytosis.

Rapid genome sequencing identifies novel variants in complement factor I.

Complement factor I deficiency (CFID; OMIM #610984) is a rare immunodeficiency caused by deficiencies in the serine protease complement factor I (CFI). CFID is characterized by predisposition to severe pneumococcal infection, often in infancy. We report a previously healthy adolescent male who presented with respiratory failure secondary to pneumococcal pneumonia and severe systemic inflammatory response. Rapid genome sequencing (rGS) identified compound heterozygous variants in CFI in the proband, with a novel maternally inherited likely pathogenic variant, a single nucleotide deletion resulting in premature stop (c.1646del; p.Asn549ThrfsTer25) and a paternally inherited novel likely pathogenic deletion (Chr 4:110685580-110692197del).

Functional and Expressional Analyses Reveal the Distinct Role of Complement Factor I in Regulating Complement System Activation during GCRV Infection in Ctenopharyngodon idella.

Complement factor I (CFI), a complement inhibitor, is well known for regulating the complement system activation by degrading complement component 3b (C3b) in animal serum, thus becoming involved in innate defense. Nevertheless, the functional mechanisms of CFI in the complement system and in host-pathogen interactions are far from being clarified in teleost fish. In the present study, we cloned and characterized the CFI gene, CiCFI, from grass carp (Ctenopharyngodon idella) and analyzed its function in degrading serum C3b and expression changes after grass carp reovirus (GCRV) infection. The open reading frame of CiCFI was found to be 2121 bp, encoding 706 amino acids with a molecular mass of 79.06 kDa. The pairwise alignments showed that CiCFI shared the highest identity (66.9%) with CFI from Carassius gibelio and the highest similarity (78.7%) with CFI from Danio rerio. The CiCFI protein was characterized by a conserved functional core Tryp_SPc domain with the catalytic triad and substrate binding sites. Phylogenetic analysis indicated that CiCFI and the homologs CFIs from other teleost fish formed a distinct evolutionary branch. Similar with the CFIs reported in mammals, the recombinant CiCFI protein could significantly reduce the C3b content in the serum, demonstrating the conserved function of CiCFI in the complement system in the grass carp. CiCFI mRNA and protein showed the highest expression level in the liver. After GCRV infection, the mRNA expressions of CiCFI were first down-regulated, then up-regulated, and then down-regulated to the initial level, while the protein expression levels maintained an overall downward trend to the late stage of infection in the liver of grass carps. Unexpectedly, the protein levels of CiCFI were also continuously down-regulated in the serum of grass carps during GCRV infection, while the content of serum C3b proteins first increases and then returns to the initial level, suggesting a distinct role of CiCFI in regulating complement activation and fish-virus interaction. Combining our previous results that complement factor D, a complement enhancer, shows continuously up-regulated expression levels in grass carps during GCRV infection, and this study may provide the further essential data for the full picture of complex complement regulation mechanism mediated by Df and CFI of the grass carp during pathogen infection.

Potential Urine Proteomic Biomarkers for Focal Segmental Glomerulosclerosis and Minimal Change Disease.

Primary focal segmental glomerulosclerosis (FSGS), along with minimal change disease (MCD), are diseases with primary podocyte damage that are clinically manifested by the nephrotic syndrome. The pathogenesis of these podocytopathies is still unknown, and therefore, the search for biomarkers of these diseases is ongoing. Our aim was to determine of the proteomic profile of urine from patients with FSGS and MCD. Patients with a confirmed diagnosis of FSGS (n = 30) and MCD (n = 9) were recruited for the study. For a comprehensive assessment of the severity of FSGS a special index was introduced, which was calculated as follows: the first score was assigned depending on the level of eGFR, the second score-depending on the proteinuria level, the third score-resistance to steroid therapy. Patients with the sum of these scores of less than 3 were included in group 1, with 3 or more-in group 2. The urinary proteome was analyzed using liquid chromatography/mass spectrometry. The proteome profiles of patients with severe progressive FSGS from group 2, mild FSGS from group 1 and MCD were compared. Results of the label free analysis were validated using targeted LC-MS based on multiple reaction monitoring (MRM) with stable isotope labelled peptide standards (SIS) available for 47 of the 76 proteins identified as differentiating between at least one pair of groups. Quantitative MRM SIS validation measurements for these 47 proteins revealed 22 proteins with significant differences between at least one of the two group pairs and 14 proteins were validated for both comparisons. In addition, all of the 22 proteins validated by MRM SIS analysis showed the same direction of change as at the discovery stage with label-free LC-MS analysis, i.e., up or down regulation in MCD and FSGS1 against FSGS2. Patients from the FSGS group 2 showed a significantly different profile from both FSGS group 1 and MCD. Among the 47 significantly differentiating proteins, the most significant were apolipoprotein A-IV, hemopexin, vitronectin, gelsolin, components of the complement system (C4b, factors B and I), retinol- and vitamin D-binding proteins. Patients with mild form of FSGS and MCD showed lower levels of Cystatin C, gelsolin and complement factor I.

An Infant Case of Streptococcus Pneumoniae-Associated Thrombotic Microangiopathy with Heterozygous CFI Mutation and CFHR3-CFHR1 Deletion.

Thrombotic microangiopathy (TMA) is a disease that causes organ damage due to microvascular hemolytic anemia, thrombocytopenia, and microvascular platelet thrombosis. Streptococcus pneumoniae-associated TMA (spTMA) is a rare complication of invasive pneumococcal infection. In addition, atypical hemolytic uremic syndrome (aHUS) is TMA associated with congenital or acquired dysregulation of complement activation. We report the case of a nine-month-old boy with refractory nephrotic syndrome complicated by spTMA in the setting of heterozygous complement factor-I (CFI) gene mutation and CFHR3-CFHR1 deletion. He repeatedly developed thrombocytopenia, anemia with schistocytes, hypocomplementemia, and abnormal coagulation triggered by infection, which manifested clinically with convulsions and an intraperitoneal hematoma. Eculizumab (a monoclonal humanized anti-C5 antibody) provided transient symptomatic benefit including improvement in thrombocytopenia; however, he developed unexplained cardiac arrest and was declared brain dead a few days later. In this report, we highlight the diagnostic challenges of this case and the causal relationship between spTMA and complement abnormalities and consider the contribution of heterozygous mutation of CFI and CFHR3-CFHR1 deletion.

An assessment of prevalence of Type 1 CFI rare variants in European AMD, and why lack of broader genetic data hinders development of new treatments and healthcare access.

PURPOSE: Advanced age-related macular degeneration (AAMD) risk is associated with rare complement Factor I (FI) genetic variants associated with low FI protein levels (termed 'Type 1'), but it is unclear how variant prevalences differ between AMD patients from different ethnicities. METHODS: Collective prevalence of Type 1 CFI rare variant genotypes were examined in four European AAMD datasets. Collective minor allele frequencies (MAFs) were sourced from the natural history study SCOPE, the UK Biobank, the International AMD Genomics Consortium (IAMDGC), and the Finnish Biobank Cooperative (FINBB), and compared to paired control MAFs or background population prevalence rates from the Genome Aggregation Database (gnomAD). Due to a lack of available genetic data in non-European AAMD, power calculations were undertaken to estimate the AAMD population sizes required to identify statistically significant association between Type 1 CFI rare variants and disease risk in different ethnicities, using gnomAD populations as controls. RESULTS: Type 1 CFI rare variants were enriched in all European AAMD cohorts, with odds ratios (ORs) ranging between 3.1 and 7.8, and a greater enrichment was observed in dry AMD from FINBB (OR 8.9, 95% CI 1.49-53.31). The lack of available non-European AAMD datasets prevented us exploring this relationship more globally, however a statistical association may be detectable by future sequencing studies that sample approximately 2,000 AAMD individuals from Ashkenazi Jewish and Latino/Admixed American ethnicities. CONCLUSIONS: The relationship between Type 1 CFI rare variants increasing odds of AAMD are well established in Europeans, however the lack of broader genetic data in AAMD has adverse implications for clinical development and future commercialisation strategies of targeted FI therapies in AAMD. These findings emphasise the importance of generating more diverse genetic data in AAMD to improve equity of access to new treatments and address the bias in health care.

Heme Interferes With Complement Factor I-Dependent Regulation by Enhancing Alternative Pathway Activation.

Hemolysis, as a result of disease or exposure to biomaterials, is characterized by excess amounts of cell-free heme intravascularly and consumption of the protective heme-scavenger proteins in plasma. The liberation of heme has been linked to the activation of inflammatory systems, including the complement system, through alternative pathway activation. Here, we investigated the impact of heme on the regulatory function of the complement system. Heme dose-dependently inhibited factor I-mediated degradation of soluble and surface-bound C3b, when incubated in plasma or buffer with complement regulatory proteins. Inhibition occurred with factor H and soluble complement receptor 1 as co-factors, and the mechanism was linked to the direct heme-interaction with factor I. The heme-scavenger protein hemopexin was the main contaminant in purified factor I preparations. This led us to identify that hemopexin formed a complex with factor I in normal human plasma. These complexes were significantly reduced during acute vasoocclusive pain crisis in patients with sickle cell disease, but the complexes were normalized at their baseline outpatient clinic visit. Hemopexin exposed a protective function of factor I activity in vitro, but only when it was present before the addition of heme. In conclusion, we present a mechanistic explanation of how heme promotes uncontrolled complement alternative pathway amplification by interfering with the regulatory capacity of factor I. Reduced levels of hemopexin and hemopexin-factor I complexes during an acute hemolytic crisis is a risk factor for heme-mediated factor I inhibition.

Sequential Increase in Complement Factor I, iC3b, and Cells Expressing CD11b or CD14 in Cutaneous Vasculitis.

Mast cells contribute to the pathogenesis of cutaneous vasculitis through complement C3 that is cleaved to C3b and then to iC3b by complement factor I. The receptor of iC3b, CD11b, is expressed on neutrophils and monocytes and CD14 on monocytes. Their role in vasculitis is obscure. In this study, frozen skin biopsies from the nonlesional skin, initial petechial lesion, and palpable purpura lesion from 10 patients with immunocomplex-mediated small vessel vasculitis were studied immunohistochemically for complement factor I, iC3b, CD11b, and CD14. Peripheral blood mononuclear cells from 5 healthy subjects were used to study cell migration and cytokine secretion. Already, the nonlesional skin revealed marked immunostaining of complement factor I, iC3b, CD11b, and CD14, and their expression increased sequentially in initial petechial and palpable purpura lesions. Mast cell C3c correlated to iC3b, and both of them correlated to CD11b+ and CD14+ cells, in the nonlesional skin. The stimulation of mononuclear cells with 0.01-0.1 µg/ml iC3b induced cell migration in the transwell assay. C3a stimulated slightly interleukin-8 secretion, whereas 1 µg/ml iC3b inhibited it slightly, in 4/5 subjects. In conclusion, the C3-C3b-iC3b axis is activated already in the early vasculitis lesion leading to progressive accumulation of CD11b+ and CD14+ cells.

Post-Transplant Thrombotic Microangiopathy due to a Pathogenic Mutation in Complement Factor I in a Patient With Membranous Nephropathy: Case Report and Review of Literature.

Thrombotic microangiopathy (TMA) is characterized by microangiopathic hemolytic anemia, thrombocytopenia and organ injury occurring due to endothelial cell damage and microthrombi formation in small vessels. TMA is primary when a genetic or acquired defect is identified, as in atypical hemolytic uremic syndrome (aHUS) or secondary when occurring in the context of another disease process such as infection, autoimmune disease, malignancy or drugs. Differentiating between a primary complement-mediated process and one triggered by secondary factors is critical to initiate timely treatment but can be challenging for clinicians, especially after a kidney transplant due to presence of multiple confounding factors. Similarly, primary membranous nephropathy is an immune-mediated glomerular disease associated with circulating autoantibodies (directed against the M-type phospholipase A2 receptor (PLA2R) in 70% cases) while secondary membranous nephropathy is associated with infections, drugs, cancer, or other autoimmune diseases. Complement activation has also been proposed as a possible mechanism in the etiopathogenesis of primary membranous nephropathy; however, despite complement being a potentially common link, aHUS and primary membranous nephropathy have not been reported together. Herein we describe a case of aHUS due to a pathogenic mutation in complement factor I that developed after a kidney transplant in a patient with an underlying diagnosis of PLA2R antibody associated-membranous nephropathy. We highlight how a systematic and comprehensive analysis helped to define the etiology of aHUS, establish mechanism of disease, and facilitated timely treatment with eculizumab that led to recovery of his kidney function. Nonetheless, ongoing anti-complement therapy did not prevent recurrence of membranous nephropathy in the allograft. To our knowledge, this is the first report of a patient with primary membranous nephropathy and aHUS after a kidney transplant.

Complement Factor I Variants in Complement-Mediated Renal Diseases.

C3 glomerulopathy (C3G) and atypical hemolytic uremic syndrome (aHUS) are two rare diseases caused by dysregulated activity of the alternative pathway of complement secondary to the presence of genetic and/or acquired factors. Complement factor I (FI) is a serine protease that downregulates complement activity in the fluid phase and/or on cell surfaces in conjunction with one of its cofactors, factor H (FH), complement receptor 1 (CR1/CD35), C4 binding protein (C4BP) or membrane cofactor protein (MCP/CD46). Because altered FI activity is causally related to the pathogenesis of C3G and aHUS, we sought to test functional activity of select CFI missense variants in these two patient cohorts. We identified 65 patients (16, C3G; 48, aHUS; 1 with both) with at least one rare variant in CFI (defined as a MAF < 0.1%). Eight C3G and eleven aHUS patients also carried rare variants in either another complement gene, ADAMTS13 or THBD. We performed comprehensive complement analyses including biomarker profiling, pathway activity and autoantibody testing, and developed a novel FI functional assay, which we completed on 40 patients. Seventy-eight percent of rare CFI variants (31/40) were associated with FI protein levels below the 25th percentile; in 22 cases, FI levels were below the lower limit of normal (type 1 variants). Of the remaining nine variants, which associated with normal FI levels, two variants reduced FI activity (type 2 variants). No patients carried currently known autoantibodies (including FH autoantibodies and nephritic factors). We noted that while rare variants in CFI predispose to complement-mediated diseases, phenotypes are strongly contingent on the associated genetic background. As a general rule, in isolation, a rare CFI variant most frequently leads to aHUS, with the co-inheritance of a CD46 loss-of-function variant driving the onset of aHUS to the younger age group. In comparison, co-inheritance of a gain-of-function variant in C3 alters the phenotype to C3G. Defects in CFH (variants or fusion genes) are seen with both C3G and aHUS. This variability underscores the complexity and multifactorial nature of these two complement-mediated renal diseases.

Generation of an iPSC line (SCTCi014-A) and isogenic control line (SCTCi014-A-1) from an age-related macular degeneration patient carrying the variant c.355G>A in the CFI gene.

Age-related macular degeneration (AMD) is a common eye disease among the elderly in the Western world. AMD is a multifactorial disease, with a strong association with genetic variation in the complement system. One of the AMD-associated variants is the c.355G>A (p.Gly119Arg) variant in complement factor I (CFI), a central regulator of complement activation. Here, we report the generation of an iPSC line and its isogenic wildtype control derived from peripheral blood mononuclear cells of a male AMD-affected individual carrying the heterozygous variant c.355G>A (p.Gly119Arg). The line can be utilized to study the effects of this variant in disease-specific cell types.

Generation of an iPSC line (SCTCi015-A) and isogenic control line (SCTCi015-A-1) from an age-related macular degeneration patient carrying the variant c.355G>A in the CFI gene.

Age-related macular degeneration (AMD) is a common eye disease among the elderly in the Western world. AMD is a multifactorial disease, with a strong association with genetic variation in the complement system. One of the AMD-associated variants is the c.355G>A (p.Gly119Arg) variant in complement factor I (CFI), a central regulator of complement activation. Here, we report the generation of an iPSC line and its isogenic wildtype control derived from peripheral blood mononuclear cells of a female AMD-affected individual carrying the heterozygous variant c.355G>A (p.Gly119Arg). This line can be utilized to study the effects of this variant in disease-specific cell types.

Functional analysis of rare genetic variants in complement factor I in advanced age-related macular degeneration.

Factor I (FI) is a serine protease inhibitor of the complement system. Heterozygous rare genetic variants in complement factor I (CFI) are associated with advanced age-related macular degeneration (AMD). The clinical impact of these variants is unknown since a majority have not been functionally characterized and are classified as 'variants of uncertain significance' (VUS). This study assessed the functional significance of VUS in CFI. Our previous cross-sectional study using a serum-based assay demonstrated that CFI variants in advanced AMD can be categorized into three types. Type 1 variants cause a quantitative deficiency of FI. Type 2 variants demonstrate a qualitative deficiency. However, Type 3 variants consist of VUS that are less dysfunctional than Types 1 and 2 but are not as biologically active as wild type (WT). In this study, we employed site-directed mutagenesis followed by expression of the recombinant variant and a comprehensive set of functional assays to characterize nine Type 3 variants that were identified in 37 individuals. Our studies establish that the expression of the recombinant protein compared with WT is reduced for R202I, Q217H, S221Y and G263V. Further, G362A and N536K, albeit expressed normally, have significantly less cofactor activity. These results led to re-categorization of CFI variants R202I, Q217H, S221Y and G263V as Type 1 variants and to reclassification of N536K and G362A as Type 2. The variants K441R, Q462H and I492L showed no functional defect and remained as Type 3. This study highlights the utility of an in-depth biochemical analysis in defining the pathologic and clinical implications of complement variants underlying AMD.

Sex and Age-Related Differences in Complement Factors Among Patients With Intermediate Age-Related Macular Degeneration.

Purpose: Age-related macular degeneration (AMD) is an acquired degenerative disease of the retina classified into early, intermediate, and advanced AMD. A key factor in the pathogenesis of AMD is the complement system. The interaction of age and sex with the complement system may affect the risk of developing AMD. The purpose of this study was to determine if there were sex-specific differences in levels of complement factors among patients with the intermediate phenotype of AMD (iAMD) and explore the correlation between age and complement proteins. Methods: We studied complement factors in patients with iAMD and controls without AMD. Nonparametric, rank-based linear regressions including a sex by AMD interaction were used to compare levels for each analyte. Correlations between age and complement proteins were evaluated using the Spearman rank correlation coefficient. Results: We found significantly higher levels of factor B and factor I in females compared with males with iAMD, whereas no differences were seen in complement levels in male and female controls. The ratios of Ba/factor B, C3a/C3, C4b/C4, and C5a/C5 were not different in males and females with iAMD. Conclusions: We demonstrate disparities in a subset of systemic complement factors between females and males with iAMD, but apparent complement turnover as measured by ratios of activation fragments to intact molecules was not different between these groups. The results suggest that complement system levels, including complement regulator factor I, exhibits sex-related differences in patients with iAMD and highlights that stratification by sex might be helpful in the interpretation of clinical trials of anticomplement therapy.

Rare complement factor I variants associated with reduced macular thickness and age-related macular degeneration in the UK Biobank.

To evaluate potential diagnostic and therapeutic biomarkers for age-related macular degeneration (AMD), we identified 8433 UK Biobank participants with rare complement Factor I gene (CFI) variants, 579 with optical coherence tomography-derived macular thickness data. We stratified these variants by predicted gene expression and measured their association with retinal pigment epithelium-Bruch's membrane (RPE-BM) complex and retinal thicknesses at nine macular subfields, as well as AMD risk, using multivariable regression models adjusted for the common complement Factor H gene (CFH) p.Y402H and age-related maculopathy susceptibility protein 2 gene (ARMS2) p.A69S risk genotypes. CFI variants associated with low Factor I levels predicted a thinner mean RPE-BM (95% confidence interval [CI] -1.66 to -0.37 µm, P = 0.002) and retina (95% CI -5.88 to -0.13 µm, P = 0.04) and a higher AMD risk (odds ratio [OR] = 2.26, 95% CI 1.56 to 3.27, P < 0.001). CFI variants associated with normal Factor I levels did not impact mean RPE-BM/retinal thickness (P = 0.28; P = 0.99) or AMD risk (P = 0.97). CFH p.Y402H was associated with a thinner RPE-BM (95% CI -0.31 to -0.18 µm, P < 0.001 heterozygous; 95% CI -0.62 to -0.42 µm, P < 0.001 homozygous) and retina (95% CI -0.73 to -0.12 µm, P = 0.007 heterozygous; 95% CI -1.08 to -0.21 µm, P = 0.004 homozygous). ARMS2 p.A69S did not influence RPE-BM (P = 0.80 heterozygous; P = 0.12 homozygous) or retinal thickness (P = 0.75 heterozygous; P = 0.07 homozygous). p.Y402H and p.A69S exhibited a significant allele-dose response with AMD risk. Thus, CFI rare variants associated with low Factor I levels are robust predictors of reduced macular thickness and AMD. The observed association between macular thickness and CFH p.Y402H, but not ARMS2 p.A69S, highlights the importance of complement dysregulation in early pathogenesis.