Defining Nephritic Factors as Diverse Drivers of Systemic Complement Dysregulation in C3 Glomerulopathy.

Introduction: C3 glomerulopathy (C3G) is an ultrarare renal disease characterized by deposition of complement component C3 in the glomerular basement membrane (GBM). Rare and novel genetic variation in complement genes and autoantibodies to complement proteins are commonly identified in the C3G population and thought to drive the underlying complement dysregulation that results in renal damage. However, disease heterogeneity and rarity make accurately defining characteristics of the C3G population difficult. Methods: Here, we present a retrospective analysis of the Molecular Otolaryngology and Renal Research Laboratories C3G cohort. This study integrated complement biomarker testing and in vitro tests of autoantibody function to achieve the following 3 primary goals: (i) define disease profiles of C3G based on disease drivers, complement biomarkers, and age; (ii) determine the relationship between in vitro autoantibody tests and in vivo complement dysregulation; and (iii) evaluate the association between autoantibody function and disease progression. Results: The largest disease profiles of C3G included patients with autoantibodies to complement proteins (48%) and patients for whom no genetic and/or acquired drivers of disease could be identified (43%). The correlation between the stabilization of convertases by complement autoantibodies as measured by in vitro modified hemolytic assays and systemic biomarkers that reflect in vivo complement dysregulation was remarkably strong. In patients positive for autoantibodies, the degree of stabilization capacity predicted worse renal function. Conclusion: This study implicates complement autoantibodies as robust drivers of systemic complement dysregulation in approximately 50% of C3G but also highlights the need for continued discovery-based research to identify novel drivers of disease.

Iptacopan in Idiopathic Immune Complex-Mediated Membranoproliferative Glomerulonephritis: Protocol of the APPARENT Multicenter, Randomized Phase 3 Study.

Introduction: Immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) is an ultra-rare, fast-progressing kidney disease that may be idiopathic (primary) or secondary to chronic infection, autoimmune disorders, or monoclonal gammopathies. Dysregulation of the alternative complement pathway is implicated in the pathophysiology of IC-MPGN; and currently, there are no approved targeted treatments. Iptacopan is an oral, highly potent proximal complement inhibitor that specifically binds to factor B and inhibits the alternative pathway (AP). Methods: This randomized, double-blind, placebo-controlled phase 3 study (APPARENT; NCT05755386) will evaluate the efficacy and safety of iptacopan in patients with idiopathic (primary) IC-MPGN, enrolling up to 68 patients (minimum of 10 adolescents) aged 12 to 60 years with biopsy-confirmed IC-MPGN, proteinuria ≥1 g/g, and estimated glomerular filtration rate (eGFR) ≥30 ml/min per 1.73 m2. All patients will receive maximally tolerated angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and vaccination against encapsulated bacteria. Patients with any organ transplant, progressive crescentic glomerulonephritis, or kidney biopsy with >50% interstitial fibrosis/tubular atrophy, will be excluded. Patients will be randomized 1:1 to receive either iptacopan 200 mg twice daily (bid) or placebo for 6 months, followed by open-label treatment with iptacopan 200 mg bid for all patients for 6 months. The primary objective of the study is to evaluate the efficacy of iptacopan versus placebo in proteinuria reduction measured as urine protein-to-creatinine ratio (UPCR) (24-h urine) at 6 months. Key secondary end points will assess kidney function measured by eGFR, patients who achieve a proteinuria-eGFR composite end point, and patient-reported fatigue. Conclusion: This study will provide evidence toward the efficacy and safety of iptacopan in idiopathic (primary) IC-MPGN.

Congenital Cytomegalovirus Testing Outcomes From the ValEAR Trial.

OBJECTIVE: To determine the positivity rate of congenital cytomegalovirus (cCMV) testing among universal, hearing-targeted CMV testing (HT-cCMV) and delayed targeted dried blood spot (DBS) testing newborn screening programs, and to examine the characteristics of successful HT-cCMV testing programs. STUDY DESIGN: Prospective survey of birth hospitals performing early CMV testing. SETTING: Multiple institutions. METHODS: Birth hospitals participating in the National Institutes of Health ValEAR clinical trial were surveyed to determine the rates of cCMV positivity associated with 3 different testing approaches: universal testing, HT-cCMV, and DBS testing. A mixed methods model was created to determine associations between successful HT-cCMV screening and specific screening protocols. RESULTS: Eighty-two birth hospitals were surveyed from February 2019 to December 2021. Seven thousand six hundred seventy infants underwent universal screening, 9017 infants HT-cCMV and 535 infants delayed DBS testing. The rates of cCMV positivity were 0.5%, 1.5%, and 7.3%, respectively. The positivity rate for universal CMV screening was less during the COVID-19 pandemic than that reported prior to the pandemic. There were no statistically significant drops in positivity for any approach during the pandemic. For HT-cCMV testing, unique order sets and rigorous posttesting protocols were associated with successful screening programs. CONCLUSION: Rates of cCMV positivity differed among the 3 approaches. The rates are comparable to cohort studies reported in the literature. Universal CMV prevalence decreased during the pandemic but not significantly. Institutions with specific order set for CMV testing where the primary care physician orders the test and the nurse facilitates the testing process exhibited higher rates of HT-cCMV testing.

Renin and renin blockade have no role in complement activity.

Renin, an aspartate protease, regulates the renin-angiotensin system by cleaving its only known substrate angiotensinogen to angiotensin. Recent studies have suggested that renin may also cleave complement component C3 to activate complement or contribute to its dysregulation. Typically, C3 is cleaved by C3 convertase, a serine protease that uses the hydroxyl group of a serine residue as a nucleophile. Here, we provide seven lines of evidence to show that renin does not cleave C3. First, there is no association between renin plasma levels and C3 levels in patients with C3 Glomerulopathies (C3G) and atypical Hemolytic Uremic Syndrome (aHUS), implying that serum C3 consumption is not increased in the presence of high renin. Second, in vitro tests of C3 conversion to C3b do not detect differences when sera from patients with high renin levels are compared to sera from patients with normal/low renin levels. Third, aliskiren, a renin inhibitor, does not block abnormal complement activity introduced by nephritic factors in the fluid phase. Fourth, aliskiren does not block dysregulated complement activity on cell surfaces. Fifth, recombinant renin from different sources does not cleave C3 even after 24 hours of incubation at 37 °C. Sixth, direct spiking of recombinant renin into sera samples of patients with C3G and aHUS does not enhance complement activity in either the fluid phase or on cell surfaces. And seventh, molecular modeling and docking place C3 in the active site of renin in a position that is not consistent with a productive ground state complex for catalytic hydrolysis. Thus, our study does not support a role for renin in the activation of complement.

Advances in cochlear gene therapies.

PURPOSE OF REVIEW: Hearing loss is the most common sensory deficit and in young children sensorineural hearing loss is most frequently genetic in etiology. Hearing aids and cochlear implant do not restore normal hearing. There is significant research and commercial interest in directly addressing the root cause of hearing loss through gene therapies. This article provides an overview of major barriers to cochlear gene therapy and recent advances in preclinical development of precision treatments of genetic deafness. RECENT FINDINGS: Several investigators have recently described successful gene therapies in many common forms of genetic hearing loss in animal models. Elegant strategies that do not target a specific pathogenic variant, such as mini gene replacement and mutation-agnostic RNA interference (RNAi) with engineered replacement, facilitate translation of these findings to development of human therapeutics. Clinical trials for human gene therapies are in active recruitment. SUMMARY: Gene therapies for hearing loss are expected to enter clinical trials in the immediate future. To provide referral for appropriate trials and counseling regarding benefits of genetic hearing loss evaluation, specialists serving children with hearing loss such as pediatricians, geneticists, genetic counselors, and otolaryngologists should be acquainted with ongoing developments in precision therapies.

Single-cell RNA-sequencing of stria vascularis cells in the adult Slc26a4-/- mouse.

BACKGROUND: The primary pathological alterations of Pendred syndrome are endolymphatic pH acidification and luminal enlargement of the inner ear. However, the molecular contributions of specific cell types remain poorly characterized. Therefore, we aimed to identify pH regulators in pendrin-expressing cells that may contribute to the homeostasis of endolymph pH and define the cellular pathogenic mechanisms that contribute to the dysregulation of cochlear endolymph pH in Slc26a4-/- mice. METHODS: We used single-cell RNA sequencing to identify both Slc26a4-expressing cells and Kcnj10-expressing cells in wild-type (WT, Slc26a4+/+) and Slc26a4-/- mice. Bioinformatic analysis of expression data confirmed marker genes defining the different cell types of the stria vascularis. In addition, specific findings were confirmed at the protein level by immunofluorescence. RESULTS: We found that spindle cells, which express pendrin, contain extrinsic cellular components, a factor that enables cell-to-cell communication. In addition, the gene expression profile informed the pH of the spindle cells. Compared to WT, the transcriptional profiles in Slc26a4-/- mice showed downregulation of extracellular exosome-related genes in spindle cells. Immunofluorescence studies in spindle cells of Slc26a4-/- mice validated the increased expression of the exosome-related protein, annexin A1, and the clathrin-mediated endocytosis-related protein, adaptor protein 2. CONCLUSION: Overall, cell isolation of stria vascularis from WT and Slc26a4-/- samples combined with cell type-specific transcriptomic analyses revealed pH-dependent alternations in spindle cells and intermediate cells, inspiring further studies into the dysfunctional role of stria vascularis cells in SLC26A4-related hearing loss.

Assessing variants of uncertain significance implicated in hearing loss using a comprehensive deafness proteome.

Hearing loss is the leading sensory deficit, affecting ~ 5% of the population. It exhibits remarkable heterogeneity across 223 genes with 6328 pathogenic missense variants, making deafness-specific expertise a prerequisite for ascribing phenotypic consequences to genetic variants. Deafness-implicated variants are curated in the Deafness Variation Database (DVD) after classification by a genetic hearing loss expert panel and thorough informatics pipeline. However, seventy percent of the 128,167 missense variants in the DVD are "variants of uncertain significance" (VUS) due to insufficient evidence for classification. Here, we use the deep learning protein prediction algorithm, AlphaFold2, to curate structures for all DVD genes. We refine these structures with global optimization and the AMOEBA force field and use DDGun3D to predict folding free energy differences (∆∆GFold) for all DVD missense variants. We find that 5772 VUSs have a large, destabilizing ∆∆GFold that is consistent with pathogenic variants. When also filtered for CADD scores (> 25.7), we determine 3456 VUSs are likely pathogenic at a probability of 99.0%. Of the 224 genes in the DVD, 166 genes (74%) exhibit one or more missense variants predicted to cause a pathogenic change in protein folding stability. The VUSs prioritized here affect 119 patients (~ 3% of cases) sequenced by the OtoSCOPE targeted panel. Approximately half of these patients previously received an inconclusive report, and reclassification of these VUSs as pathogenic provides a new genetic diagnosis for six patients.

Assessing Variants of Uncertain Significance Implicated in Hearing Loss Using a Comprehensive Deafness Proteome.

Hearing loss is the leading sensory deficit, affecting ~ 5% of the population. It exhibits remarkable heterogeneity across 223 genes with 6,328 pathogenic missense variants, making deafness-specific expertise a prerequisite for ascribing phenotypic consequences to genetic variants. Deafness-implicated variants are curated in the Deafness Variation Database (DVD) after classification by a genetic hearing loss expert panel and thorough informatics pipeline. However, seventy percent of the 128,167 missense variants in the DVD are "variants of uncertain significance" (VUS) due to insufficient evidence for classification. Here, we use the deep learning protein prediction algorithm, AlphaFold2, to curate structures for all DVD genes. We refine these structures with global optimization and the AMOEBA force field and use DDGun3D to predict folding free energy differences (∆∆G Fold ) for all DVD missense variants. We find that 5,772 VUSs have a large, destabilizing ∆∆G Fold that is consistent with pathogenic variants. When also filtered for CADD scores (> 25.7), we determine 3,456 VUSs are likely pathogenic at a probability of 99.0%. These VUSs affect 119 patients (~ 3% of cases) sequenced by the OtoSCOPE targeted panel. Approximately half of these patients previously received an inconclusive report, and reclassification of these VUSs as pathogenic provides a new genetic diagnosis for six patients.

Protein therapeutics and their lessons: Expect the unexpected when inhibiting the multi-protein cascade of the complement system.

Over a century after the discovery of the complement system, the first complement therapeutic was approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH). It was a long-acting monoclonal antibody (aka 5G1-1, 5G1.1, h5G1.1, and now known as eculizumab) that targets C5, specifically preventing the generation of C5a, a potent anaphylatoxin, and C5b, the first step in the eventual formation of membrane attack complex. The enormous clinical and financial success of eculizumab across four diseases (PNH, atypical hemolytic uremic syndrome (aHUS), myasthenia gravis (MG), and anti-aquaporin-4 (AQP4) antibody-positive neuromyelitis optica spectrum disorder (NMOSD)) has fueled a surge in complement therapeutics, especially targeting diseases with an underlying complement pathophysiology for which anti-C5 therapy is ineffective. Intensive research has also uncovered challenges that arise from C5 blockade. For example, PNH patients can still face extravascular hemolysis or pharmacodynamic breakthrough of complement suppression during complement-amplifying conditions. These "side" effects of a stoichiometric inhibitor like eculizumab were unexpected and are incompatible with some of our accepted knowledge of the complement cascade. And they are not unique to C5 inhibition. Indeed, "exceptions" to the rules of complement biology abound and have led to unprecedented and surprising insights. In this review, we will describe initial, present and future aspects of protein inhibitors of the complement cascade, highlighting unexpected findings that are redefining some of the mechanistic foundations upon which the complement cascade is organized.

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).

Genetic diagnosis and renal biopsy findings in the setting of a renal genetics clinic.

As genetic testing becomes more available, its utilization as an early diagnostic tool in nephrology is more common. The objective of the study is to examine diagnostic agreement between the renal biopsy findings and genetic diagnoses. A retrospective study was conducted in February 2022. A total of 28 patients had both genetic diagnosis and histologic results (n = 1 nephrectomy, n = 27 biopsy). We collected clinical, renal biopsy findings, and genetic information. The relationship between the histologic findings and the genetic diagnoses was classified as: concordant, nonspecific, and discordant. A total of 15 males and 13 females were included (mean age = 9.6 years). Clinical suspicion of Alport syndrome was the most common reason for referral (n = 11, 39.3%), followed by nephrotic syndrome (n = 8, 28.5%), "other" (n = 6, 21.4%), cystic kidney disease (n = 1, 3.6%), isolated hematuria (n = 1, 3.6%), and non-nephrotic proteinuria (n = 1, 3.6%). The overall concordance rate between renal histologic and genetic diagnoses was 71.4% (20/28), nonspecific biopsy results were observed in 17.9% (5/28), and discordant results were observed in 10.7% (3/28). All patients referred for suspected Alport Syndrome had pathogenic/likely pathogenic variants in one of the COL4A genes. Two cases of Lowe syndrome and one of PAX2-associated nephropathy had discordant histology findings. Agreement between renal histologic findings and genetic results varies based on the reason for referral. There was a complete agreement for patients referred for Alport Syndrome; However, there were examples that renal biopsy showed secondary findings that were not specifically associated with the underlying genetic results.

De novo variants are a common cause of genetic hearing loss.

PURPOSE: De novo variants (DNVs) are a well-recognized cause of genetic disorders. The contribution of DNVs to hearing loss (HL) is poorly characterized. We aimed to evaluate the rate of DNVs in HL-associated genes and assess their contribution to HL. METHODS: Targeted genomic enrichment and massively parallel sequencing were used for molecular testing of all exons and flanking intronic sequences of known HL-associated genes, with no exclusions on the basis of type of HL or clinical features. Segregation analysis was performed, and previous reports of DNVs in PubMed and ClinVar were reviewed to characterize the rate, distribution, and spectrum of DNVs in HL. RESULTS: DNVs were detected in 10% (24/238) of trios for whom segregation analysis was performed. Overall, DNVs were causative in at least ∼1% of probands for whom a genetic diagnosis was resolved, with marked variability based on inheritance mode and phenotype. DNVs of MITF were most common (21% of DNVs), followed by GATA3 (13%), STRC (13%), and ACTG1 (8%). Review of reported DNVs revealed gene-specific variability in contribution of DNV to the mutational spectrum of HL-associated genes. CONCLUSION: DNVs are a relatively common cause of genetic HL and must be considered in all cases of sporadic HL.

Alternative Complement Pathway Inhibition With Iptacopan for the Treatment of C3 Glomerulopathy-Study Design of the APPEAR-C3G Trial.

Introduction: Complement 3 glomerulopathy (C3G) is a rare kidney disease characterized by dysregulation of the alternative pathway (AP) of the complement system. About 50% of patients with C3G progress to kidney failure within 10 years of diagnosis. Currently, there are no approved therapeutic agents for C3G. Iptacopan is an oral, first-in-class, potent, and selective inhibitor of factor B, a key component of the AP. In a Phase II study, treatment with iptacopan was associated with a reduction in proteinuria and C3 deposit scores in C3G patients with native and transplanted kidneys, respectively. Methods: APPEAR-C3G (NCT04817618) is a randomized, double-blind, and placebo-controlled Phase III study to evaluate the efficacy and safety of iptacopan in C3G patients, enrolling 68 adults with biopsy-confirmed C3G, reduced C3 (<77 mg/dl), proteinuria ≥1.0 g/g, and estimated glomerular filtration rate (eGFR) ≥30 ml/min per 1.73 m2. All patients will receive maximally tolerated angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and vaccination against encapsulated bacteria. Patients with any organ transplantation, progressive crescentic glomerulonephritis (GN), monoclonal gammopathy of undetermined significance, or kidney biopsy with >50% interstitial fibrosis/tubular atrophy, will be excluded. Patients will be randomized 1:1 to receive either iptacopan 200 mg twice daily or placebo for 6 months, followed by open-label treatment with iptacopan 200 mg twice daily for all patients for 6 months. The primary objective is to evaluate the efficacy of iptacopan versus placebo on proteinuria reduction urine protein:creatinine ratio (UPCR) (24 h urine). Key secondary endpoints will assess kidney function measured by eGFR, histological disease total activity score, and fatigue. Conclusion: This study aims to demonstrate the clinical benefits of AP inhibition with iptacopan in C3G.

Haemolytic uraemic syndrome.

Haemolytic uraemic syndrome (HUS) is a heterogeneous group of diseases that result in a common pathology, thrombotic microangiopathy, which is classically characterised by the triad of non-immune microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury. In this Seminar, different causes of HUS are discussed, the most common being Shiga toxin-producing Escherichia coli HUS. Identifying the underlying thrombotic microangiopathy trigger can be challenging but is imperative if patients are to receive personalised disease-specific treatment. The quintessential example is complement-mediated HUS, which once carried an extremely high mortality but is now treated with anti-complement therapies with excellent long-term outcomes. Unfortunately, the high cost of anti-complement therapies all but precludes their use in low-income countries. For many other forms of HUS, targeted therapies are yet to be identified.

International Pediatric Otolaryngology Group (IPOG) consensus recommendations: Evaluation and management of congenital tracheal stenosis.

OBJECTIVES: To outline an expert-based consensus of recommendations for the diagnosis and management of pediatric patients with congenital tracheal stenosis. METHODS: Expert opinions were sought from members of the International Pediatric Otolaryngology Group (IPOG) via completion of an 18-item survey utilizing an iterative Delphi method and review of the literature. RESULTS: Forty-three members completed the survey providing recommendations regarding the initial history, clinical evaluation, diagnostic evaluation, temporizing measures, definitive repair, and post-repair care of children with congenital tracheal stenosis. CONCLUSION: These recommendations are intended to be used to support clinical decision-making regarding the evaluation and management of children with congenital tracheal stenosis. Responses highlight the diverse management strategies and the importance of a multidisciplinary approach to care of these patients.

C3 glomerulopathy: Understanding an ultra-rare complement-mediated renal disease.

C3 glomerulopathy (C3G) describes a pathologic pattern of injury diagnosed by renal biopsy. It is characterized by the dominant deposition of the third component of complement (C3) in the renal glomerulus as resolved by immunofluorescence microscopy. The underlying pathophysiology is driven by dysregulation of the alternative pathway of complement in the fluid-phase and in the glomerular microenvironment. Characterization of clinical features and a targeted evaluation for indices and drivers of complement dysregulation are necessary for optimal patient care. Autoantibodies to the C3 and C5 convertases of complement are the most commonly detected drivers of complement dysregulation, although genetic mutations in complement genes can also be found. Approximately half of patients progress to end-stage renal disease within 10 years of diagnosis, and, while transplantation is a viable option, there is high risk for disease recurrence and allograft failure. This poor outcome reflects the lack of disease-specific therapy for C3G, relegating patients to symptomatic treatment to minimize proteinuria and suppress renal inflammation. Fortunately, the future is bright as several anti-complement drugs are currently in clinical trials.

Results from a nationwide retrospective cohort measure the impact of C3 and soluble C5b-9 levels on kidney outcomes in C3 glomerulopathy.

C3 glomerulopathy (C3G) is a rare complement-mediated disease. Specific treatments are not yet available and factors predictive of kidney survival such as age, kidney function and proteinuria are not specific to C3G. The prognostic value of biomarkers of complement activation, which are pathognomonic of the diseases, remains unknown. In a large cohort of 165 patients from the French National registry, we retrospectively assess the prognostic value of C3, soluble C5b-9 (sC5b-9), C3 nephritic factor, and rare disease-predicting variants in complement genes in predicting clinical outcome of patients. By multivariate analysis age (adult onset), reduced kidney function (defined by estimated glomerular filtration rate under 60ml/min) and presence of rare disease-predicting variants in complement genes predicted risk of progression to kidney failure. Moreover, by multivariate analysis, normal C3/high sC5b-9 levels or low C3/normal sC5b-9 levels remained independently associated with a worse kidney prognosis, with the relative risk 3.7- and 8-times higher, respectively. Subgroup analysis indicated that the complement biomarker profiles independently correlated to kidney prognosis in patients with adult but not pediatric onset. In this subgroup, we showed that profiles of biomarkers C3 and/or sC5b-9 correlated with intra glomerular inflammation and may explain kidney outcomes. In children, only the presence of rare disease-predicting variants correlated with kidney survival. Thus, in an adult population, we propose a three-point C3G prognostic score based on biomarker profiles at risk, estimated glomerular filtration rate at presentation and genetic findings, which may help stratify adult patients into subgroups that require close monitoring and more aggressive therapy.

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.