Comprehensive genetic analysis of complement and coagulation genes in atypical hemolytic uremic syndrome

Plasmin-cleaved von Willebrand factor as a biomarker for microvascular thrombosis

ABSTRACT

von Willebrand factor (VWF) is an essential contributor to microvascular thrombosis. Physiological cleavage by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) limits its prothrombotic properties, explaining why ADAMTS13 deficiency leads to attacks of microthrombosis in patients with thrombotic thrombocytopenic purpura (TTP). We previously reported that plasminogen activation takes place during TTP attacks in these patients. Furthermore, stimulation of plasminogen activation attenuates pathogenesis in preclinical TTP models in vivo. This suggests that plasmin is an endogenous regulator of VWF thrombogenicity, in particular when ADAMTS13 falls short to prevent microvascular occlusions. VWF cleavage by plasmin is biochemically distinct from cleavage by ADAMTS13. We hypothesized that plasmin-cleaved VWF (cVWF) holds value as a biomarker of microvascular thrombosis. Here, we describe the development of a variable domain of heavy-chain-only antibody (VHH)-based bioassay that can distinguish cVWF from intact and ADAMTS13-cleaved VWF in plasma. We validate this assay by tracking cVWF release during degradation of microthombi in vitro. We demonstrate that endogenous cVWF formation takes place in patients with TTP during acute attacks of thrombotic microangiopathy but not in those in remission. Finally, we show that therapeutic plasminogen activation in a mouse model of TTP amplifies cVWF formation, which is accompanied by VWF clearance. Our combined findings indicate that cVWF is released from microthrombi in the context of microvascular occlusion.

Complement dysregulation associated with a genetic variant in factor H-related protein 5 in atypical hemolytic uremic syndrome

BACKGROUND

Atypical hemolytic uremic syndrome (aHUS) can be associated with mutations, deletions, or hybrid genes in factor H-related (FHR) proteins.

METHODS

A child with aHUS was investigated. Genetics was assessed by Sanger and next generation sequencing. Serum FHR5 was evaluated by immunoblotting, ELISA, and by induction of rabbit red blood cell hemolysis in the presence/absence of recombinant human rFHR5. Mutagenesis was performed in HEK cells.

RESULTS

A heterozygous genetic variant in factor H-related protein 5 (CFHR5), M514R, was found in the child, who also had a homozygous deletion of CFHR3/CFHR1, and antibodies to factor H, as well as low levels of C3. Patient serum exhibited low levels of FHR5. In the presence of rabbit red blood cells, patient serum induced hemolysis which decreased when rFHR5 was added at physiological concentrations. Similar results were obtained using serum from the father, bearing the CFHR5 variant without factor H antibodies. Patient FHR5 formed normal dimers. The CFHR5 M514R variant was expressed in HEK cells and minimal secretion was detected whereas the protein level was elevated in cell lysates.

CONCLUSIONS

Decreased secretion of the product of the mutant allele could explain the low FHR5 levels in patient serum. Reduced hemolysis when rFHR5 was added to serum suggests a regulatory role regarding complement activation on red blood cells. As such, low levels of FHR5, as demonstrated in the patient, may contribute to complement activation.

Differential contributions of the C5b-9 and C5a/C5aR pathways to microvascular and macrovascular thrombosis in complement-mediated thrombotic microangiopathy patients

To clarify the role of the C5a/C5aR (C5a receptor) and C5b-9 pathways in macrovascular thrombosis (MAT) and renal microthrombosis (MIT), 73 renal biopsy-proven complement-mediated thrombotic microangiopathy (C-TMA) patients were enrolled; 9 patients with pure MAT and 13 patients with pure MIT were selected for further study. Twenty-five external C-TMA patients were selected as the validation cohort. Plasma C5a and sC5b-9 (soluble C5b-9) levels were significantly higher in patients with MAT than in those with MIT (P = 0.008, P = 0.041, respectively). The mean optical density of C5aR1 in the kidney was significantly higher in MAT patients than in those with MIT (P < 0.001). Both urinary sC5b-9 levels (MIT: P < 0.001, MAT: P = 0.004) and renal deposition of C5b-9 (MIT: P < 0.001, MAT: P = 0.001) were significantly higher in C-TMA patients compared to normal control, but were similar between MAT and MIT groups. In the correlation analysis within 22C-TMA patients, urinary sC5b-9 levels and renal deposition of C5b-9 were positively correlated to renal MIT formation (P = 0.009 and P = 0.031, respectively). Furthermore, the renal citrullinated histone H3 (CitH3)- and neutrophil elastase (NE)-positive area ratios were both significantly higher in the MAT group than in the MIT group (P = 0.006 and P = 0.020, respectively). Therefore, the local C5b-9 and C5a/C5aR1 pathways might have differential contributions to MIT and MAT formation in the disease.

Defective C3d caused by C3 p.W1034R in inherited atypical hemolytic uremic syndrome

INTRODUCTION

Atypical hemolytic uremic syndrome (aHUS) is a rare form of thrombotic microangiopathy. Personal genome analyses have revealed numerous aHUS-causing variants, mainly complement-related genes. However, not all aHUS-causing variants have been functionally validated.

METHODS

An exome sequence analysis of a Japanese multiplex family composed of three patients diagnosed with aHUS in infancy and showing frequent relapses clustered in a dominant transmission mode was performed. Protein interaction between the C3d and C-terminal domains of factor H was analyzed using a quartz crystal microbalance.

RESULTS

Following filtering by heterozygous variants, amino acid substitutions, and allele frequency, the analysis revealed eight rare variants shared by the affected individuals. Variant prioritization listed C3 p.W1034R as the most likely candidate gene mutation in the affected individuals, despite being classified as a variant of uncertain significance. Binding of recombinant C3d harboring 1034R to recombinant short consensus repeats 15 to 20 of factor H was significantly suppressed compared with that of C3 with 1034W.

CONCLUSION

C3 p.W1034R results in an inherited form of aHUS that often presents with recurrent episodes, possibly because of impaired interactions between the C3d and C-terminal domains of factor H. Following comprehensive genomic analysis, functional validation of C3 p.W1034R strengthens the molecular basis for aHUS pathophysiology.

Pregnancy-Associated Atypical Hemolytic Uremic Syndrome: A Case Report with MCP Gene Mutation and Successful Eculizumab Treatment

Pregnancy-associated atypical hemolytic uremic syndrome (P-aHUS) is a rare condition characterized by microangiopathic hemolytic anemia and kidney injury from thrombotic microangiopathy. P-aHUS occurs in approximately 1 in 25,000 pregnancies and is strongly related to complement dysregulation and pregnancy-related disorders, such as preeclampsia, eclampsia, and hemolysis, elevated liver enzymes, low platelet (HELLP) syndrome, resulting in adverse perinatal and fetal outcomes. Complement dysregulation in P-aHUS is commonly attributed to genetic mutations or autoantibodies affecting complement factors, including CFH , CFI , and MCP. We present a case of a 25-year-old primigravida who experienced severe preeclampsia and HELLP syndrome followed by the development of complicated P-aHUS during the early postpartum period. The patient exhibited severe clinical manifestations, including hypertensive emergency, central nervous system involvement, renal impairment, and microangiopathic hemolytic anemia. Timely initiation of eculizumab therapy resulted in successful disease remission. Further genetic analysis revealed a likely rare pathogenic MCP gene variant.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Identification of complement factor H variants that predispose to pre-eclampsia: A genetic and functional study

OBJECTIVE

The objective of the study was to investigate the role of genetic variants in complement proteins in pre-eclampsia.

DESIGN

In a case-control study involving 609 cases and 2092 controls, five rare variants in complement factor H (CFH) were identified in women with severe and complicated pre-eclampsia. No variants were identified in controls.

SETTING

Pre-eclampsia is a leading cause of maternal and fetal morbidity and mortality. Immune maladaptation, in particular, complement activation that disrupts maternal-fetal tolerance leading to placental dysfunction and endothelial injury, has been proposed as a pathogenetic mechanism, but this remains unproven.

POPULATION

We genotyped 609 pre-eclampsia cases and 2092 controls from FINNPEC and the national FINRISK cohorts.

METHODS

Complement-based functional and structural assays were conducted in vitro to define the significance of these five missense variants and each compared with wild type.

MAIN OUTCOME MEASURES

Secretion, expression and ability to regulate complement activation were assessed for factor H proteins harbouring the mutations.

RESULTS

We identified five heterozygous rare variants in complement factor H (L3V, R127H, R166Q, C1077S and N1176K) in seven women with severe pre-eclampsia. These variants were not identified in controls. Variants C1077S and N1176K were novel. Antigenic, functional and structural analyses established that four (R127H, R166Q, C1077S and N1176K) were deleterious. Variants R127H and C1077S were synthesised, but not secreted. Variants R166Q and N1176K were secreted normally but showed reduced binding to C3b and consequently defective complement regulatory activity. No defect was identified for L3V.

CONCLUSIONS

These results suggest that complement dysregulation due to mutations in complement factor H is among the pathophysiological mechanisms underlying severe pre-eclampsia.

Genetic investigation of Nordic patients with complement-mediated kidney diseases

Background

Complement activation in atypical hemolytic uremic syndrome (aHUS), C3 glomerulonephropathy (C3G) and immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) may be associated with rare genetic variants. Here we describe gene variants in the Swedish and Norwegian populations.

Methods

Patients with these diagnoses (N=141) were referred for genetic screening. Sanger or next-generation sequencing were performed to identify genetic variants in 16 genes associated with these conditions. Nonsynonymous genetic variants are described when they have a minor allele frequency of <1% or were previously reported as being disease-associated.

Results

In patients with aHUS (n=94, one also had IC-MPGN) 68 different genetic variants or deletions were identified in 60 patients, of which 18 were novel. Thirty-two patients had more than one genetic variant. In patients with C3G (n=40) 29 genetic variants, deletions or duplications were identified in 15 patients, of which 9 were novel. Eight patients had more than one variant. In patients with IC-MPGN (n=7) five genetic variants were identified in five patients. Factor H variants were the most frequent in aHUS and C3 variants in C3G. Seventeen variants occurred in more than one condition.

Conclusion

Genetic screening of patients with aHUS, C3G and IC-MPGN is of paramount importance for diagnostics and treatment. In this study, we describe genetic assessment of Nordic patients in which 26 novel variants were found.

Concurrent Cobalamin C and Plasminogen Deficiencies in a Patient with Chronic Thrombotic Microangiopathy

BACKGROUND

Although most patients with atypical hemolytic uremic syndrome (aHUS) have variants in genes participating in alternative complement pathways, rare variants in non-complement pathway-related genes, including DGKE, INF2, MMACHC, PLG, and THBD, have also been described.

CASE PRESENTATION

We report an 18-year-old male patient with renal biopsy-proven chronic thrombotic microangiopathy that raised suspicion of aHUS. Whole-exome sequencing revealed a novel pathogenic homozygous MMACHC c.484G>T (p.Gly162Trp) variant. Subsequently, clinical and laboratory findings confirmed cobalamin C (Cbl C) deficiency. Also, homozygous missense c.1112C>T PLG (p.Thr371Ile) variant was detected (it had been reported as a variant of unknown significance). However, the low serum plasminogen (PLG) activity proved the pathogenicity of c.1112C>T. Hence, the patient was diagnosed with concurrent Cbl C and PLG deficiencies. Segregation analysis revealed that the mother and father had the same heterozygous PLG and MMACHC variants. PLG variants have generally been described in aHUS patients concomitant with complement gene variants in the literature; therefore, the association between aHUS and PLG variants is controversial. The possible contribution of PLG deficiency to thrombotic microangiopathy was also discussed in this case.

CONCLUSION

Non-complement-mediated aHUS is an exceptional disorder. A limited number of genes are involved in this entity. To our knowledge, this is the first aHUS patient diagnosed with both Cbl C and PLG deficiencies in the literature.

Genotypic analysis of a large cohort of patients with suspected atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal impairment. Complement and coagulation gene variants have been associated with aHUS susceptibility. We assessed the diagnostic yield of a next-generation sequencing (NGS) panel in a large cohort of Canadian patients with suspected aHUS. Molecular testing was performed on peripheral blood DNA samples from 167 patients, collected between May 2019 and December 2021, using a clinically validated NGS pipeline. Coding exons with 20 base pairs of flanking intronic regions for 21 aHUS-associated or candidate genes were enriched using a custom hybridization protocol. All sequence and copy number variants were assessed and classified following American College of Medical Genetics guidelines. Molecular diagnostic results were reported for four variants in three individuals (1.8%). Twenty-seven variants of unknown significance were identified in 25 (15%) patients, and 34 unique variants in candidate genes were identified in 28 individuals. An illustrative patient case describing two genetic alterations in complement genes is presented, highlighting that variable expressivity and incomplete penetrance must be considered when interpreting genetic data in patients with complement-mediated disease, alongside the potential additive effects of genetic variants on aHUS pathophysiology. In this cohort of patients with suspected aHUS, using clinical pipelines for genetic testing and variant classification, pathogenic/likely pathogenic variants occurred in a very small percentage of patients. Our results highlight the ongoing challenges in variant classification following NGS panel testing in patients with suspected aHUS, alongside the need for clear testing guidance in the clinical setting. KEY MESSAGES: • Clinical molecular testing for disease associated genes in aHUS is challenging. • Challenges include patient selection criteria, test validation, and interpretation. • Most variants were of uncertain significance (31.7% of patients; VUS + candidates). • Their clinical significance may be elucidated as more evidence becomes available.  • Low molecular diagnostic rate (1.8%), perhaps due to strict classification criteria. • Case study identified two likely pathogenic variants; one each in MCP/CD46 and CFI.

Dysregulation of Immune Cell Subpopulations in Atypical Hemolytic Uremic Syndrome

Atypical hemolytic uremic syndrome (aHUS) is a rare, life-threatening thrombotic microangiopathy. Definitive biomarkers for disease diagnosis and activity remain elusive, making the exploration of molecular markers paramount. We conducted single-cell sequencing on peripheral blood mononuclear cells from 13 aHUS patients, 3 unaffected family members of aHUS patients, and 4 healthy controls. We identified 32 distinct subpopulations encompassing 5 B-cell types, 16 T- and natural killer (NK) cell types, 7 monocyte types, and 4 other cell types. Notably, we observed a significant increase in intermediate monocytes in unstable aHUS patients. Subclustering analysis revealed seven elevated expression genes, including NEAT1, MT-ATP6, MT-CYB, VIM, ACTG1, RPL13, and KLRB1, in unstable aHUS patients, and four heightened expression genes, including RPS27, RPS4X, RPL23, and GZMH genes, in stable aHUS patients. Additionally, an increase in the expression of mitochondria-related genes suggested a potential influence of cell metabolism on the clinical progression of the disease. Pseudotime trajectory analysis revealed a unique immune cell differentiation pattern, while cell-cell interaction profiling highlighted distinctive signaling pathways among patients, family members, and controls. This single-cell sequencing study is the first to confirm immune cell dysregulation in aHUS pathogenesis, offering valuable insights into molecular mechanisms and potential new diagnostic and disease activity markers.

Genetic evaluation of living kidney donor candidates: a review and recommendations for best practices

The growing accessibility and falling costs of genetic sequencing techniques has expanded the utilization of genetic testing in clinical practice. For living kidney donation, genetic evaluation has been increasingly used to identify genetic kidney disease in potential candidates, especially in those of younger ages. However, genetic testing on asymptomatic living kidney donors remains fraught with many challenges and uncertainties. Not all transplant practitioners are aware of the limitations of genetic testing, are comfortable with selecting testing methods, comprehending test results, or providing counsel, and many do not have access to a renal genetic counselor or a clinical geneticist. Although genetic testing can be a valuable tool in living kidney donor evaluation, its overall benefit in donor evaluation has not been demonstrated and it can also lead to confusion, inappropriate donor exclusion, or misleading reassurance. Until more published data become available, this practice resource should provide guidance for centers and transplant practitioners on the responsible use of genetic testing in the evaluation of living kidney donor candidates.

[Thrombotic microangiopathy]

In the emergency room, patients with anemia and thrombocytopenia are common. Although these findings can often be explained by the medical situation, thrombotic microangiopathy is an important differential diagnosis. In this case, occlusion of the smallest vessels consequently leads to functional impairment of the affected organs. Patients generally present with symptoms of organ dysfunction, e.g., in the kidney or brain. Characteristically, Coombs-negative fragmentation of erythrocytes with hemolysis occurs in the area of the occluded vessels. Lactate dehydrogenase levels are elevated, and platelets and haptoglobin are reduced. Differential diagnoses beyond thrombotic microangiopathy that should be considered are numerous and diverse in their pathophysiology. Rapid workup is needed, because sometimes a specific treatment must be initiated rapidly. For example, thrombotic thrombocytopenic purpura leads to death in about 90% of patients if left untreated. However, by reconstitution of the underlying deficiency of the so-called ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) by plasma separation, survival can be ensured in most cases. Complement-mediated hemolytic uremic syndrome should also be considered and, if suspected, treated with complement inhibition. In many cases, however, thrombotic microangiopathy reflects a disorder elsewhere and may be a manifestation of severe hypertension or a coagulation disorder, such as disseminated intravascular coagulation or antiphospholipid syndrome. It can also be observed as a consequence of drug therapies or metabolic derangement. Systemic workup is therefore necessary for rapid clarification of differential diagnoses.

Genetic Susceptibility to Chronic Kidney Disease: Links, Risks and Management

Chronic kidney disease (CKD) is associated with significant morbidity and mortality worldwide. In recent years, our understanding of genetic causes of CKD has expanded significantly with several renal conditions having been identified. This review discusses the current landscape of genetic kidney disease and their potential treatment options. This review will focus on cystic kidney disease, glomerular disease with genetic associations, congenital anomalies of kidneys and urinary tract (CAKUT), autosomal dominant-tubulointerstitial kidney disease (ADTKD), inherited nephrolithiasis and nephrocalcinosis.

Differentiating and Managing Rare Thrombotic Microangiopathies During Pregnancy and Postpartum

The most common thrombotic microangiopathy (TMA) of pregnancy is the well-recognized syndrome of preeclampsia with hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome. However, rare TMAs, including thrombotic thrombocytopenic purpura, complement-mediated hemolytic-uremic syndrome, and catastrophic antiphospholipid syndrome, may occur during pregnancy or postpartum and present with features similar to those of preeclampsia with severe features. Early recognition and treatment of these infrequently encountered conditions are key for avoiding serious maternal morbidities with long-term sequelae and possible maternal or fetal death. Differentiating between preeclampsia with severe features and these rare TMAs is diagnostically challenging as there is significant overlap in their clinical and laboratory presentation. Given the rarity of these TMAs, high-quality evidence-based recommendations on diagnosis and management during pregnancy are lacking. Using current objective information and recommendations from working groups, this report provides practical clinical approaches to diagnose and manage these rare TMAs. This report also discusses how to manage individuals with a history of these rare TMAs who are planning to conceive. To optimize favorable outcomes, a multidisciplinary approach including obstetricians, maternal-fetal medicine specialists, hematologists, and nephrologists alongside close clinical and laboratory monitoring is vital.

The role of the complement system in kidney glomerular capillary thrombosis

The complement system is part of the innate immune system. The crucial step in activating the complement system is the generation and regulation of C3 convertase complexes, which are needed to generate opsonins that promote phagocytosis, to generate C3a that regulates inflammation, and to initiate the lytic terminal pathway through the generation and activity of C5 convertases. A growing body of evidence has highlighted the interplay between the complement system, coagulation system, platelets, neutrophils, and endothelial cells. The kidneys are highly susceptible to complement-mediated injury in several genetic, infectious, and autoimmune diseases. Atypical hemolytic uremic syndrome (aHUS) and lupus nephritis (LN) are both characterized by thrombosis in the glomerular capillaries of the kidneys. In aHUS, congenital or acquired defects in complement regulators may trigger platelet aggregation and activation, resulting in the formation of platelet-rich thrombi in the kidneys. Because glomerular vasculopathy is usually noted with immunoglobulin and complement accumulation in LN, complement-mediated activation of tissue factors could partly explain the autoimmune mechanism of thrombosis. Thus, kidney glomerular capillary thrombosis is mediated by complement dysregulation and may also be associated with complement overactivation. Further investigation is required to clarify the interaction between these vascular components and develop specific therapeutic approaches.

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

Systematic review of atypical hemolytic uremic syndrome biomarkers

BACKGROUND AND OBJECTIVES

Observing biomarkers that affect alternative pathway dysregulation components may be effective in obtaining a new and more rapid diagnostic portrayal of atypical hemolytic uremic syndrome. We have conducted a systematic review on the aHUS biomarkers: C3, C5a, C5b-9, factor B, complement factor B, H, and I, CH50, AH50, D-dimer, as well as anti-CFH antibodies.

METHODS

An exhaustive literature search was conducted for aHUS patient population plasma/serum, collected/reported at the onset of diagnosis. A total of 60 studies were included with the data on 837 aHUS subjects, with at least one biomarker reported.

RESULTS

The biomarkers C3 [mean (SD): 72.1 (35.0), median: 70.5 vs. reference range: 75-175 mg/dl, n = 752]; CH50 [28.3 (32.1), 24.3 vs. 30-75 U/ml, n = 63]; AH50 [27.6% (30.2%), 10% vs. ≥ 46%, n = 23]; and CFB [13.1 (6.6), 12.4, vs. 15.2-42.3 mg/dl, n = 19] were lower among aHUS subjects as compared with the reference range. The biomarkers including C4 [mean (SD): 20.4 (9.5), median: 20.5 vs. reference range: 14-40 mg/dl, n = 343]; C4d [7.2 (6.5), 4.8 vs. ≤ 9.8 μg/ml, n = 108]; CFH [40.2 (132.3), 24.5 vs. 23.6-43.1 mg/dl, n = 123 subjects]; and CFI [8.05 (5.01), 6.55 mg/dl vs. 4.4-18.1 mg/dl, n = 38] were all observed to be within the reference range among aHUS subjects. The biomarkers C5a [mean (SD): 54.9 (32.9), median: 48.8 vs. reference range: 10.6-26.3 mg/dl, n = 117]; C5b-9 [466.0 (401.4), 317 (186-569.7) vs. ≤ 250 ng/ml, n = 174]; Bb [2.6 (2.1), 1.9 vs. ≤ 1.6 μg/ml, n = 77] and D-dimer [246 (65.05), 246 vs. < 2.2 ng/ml, 2, n = 2 subjects] were higher among patients with aHUS compared with the reference range.

CONCLUSION

If a comprehensive complement profile were built using our data, aHUS would be identified by low levels of C3, CH50, AH50, and CFB along with increased levels of C5a, C5b-9, Bb, anti-CFH autoantibodies, and D-dimer. A higher resolution version of the Graphical abstract is available as Supplementary information.

Endothelial-specific loss of Krüppel-Like Factor 4 triggers complement-mediated endothelial injury

Thrombotic microangiopathy (TMA) in the kidney represents the most severe manifestation of kidney microvascular endothelial injury. Despite the source of the inciting event, the diverse clinical forms of kidney TMA share dysregulation of endothelial cell transcripts and complement activation. Here, we show that endothelial-specific knockdown of Krüppel-Like Factor 4 (Klf4)ΔEC, an anti-inflammatory and antithrombotic zinc-finger transcription factor, increases the susceptibility to glomerular endothelial injury and microangiopathy in two genetic murine models that included endothelial nitric oxide synthase knockout mice and aged mice (52 weeks), as well as in a pharmacologic model of TMA using Shiga-toxin 2. In all models, Klf4ΔEC mice exhibit increased pro-thrombotic and pro-inflammatory transcripts, as well as increased complement factors C3 and C5b-9 deposition and histologic features consistent with subacute TMA. Interestingly, complement activation in Klf4ΔEC mice was accompanied by reduced expression of a key KLF4 transcriptional target and membrane bound complement regulatory gene, Cd55. To assess a potential mechanism by which KLF4 might regulate CD55 expression, we performed in silico chromatin immunoprecipitation enrichment analysis of the CD55 promotor and found KLF4 binding sites upstream from the CD55 transcription start site. Using patient-derived kidney biopsy specimens, we found glomerular expression of KLF4 and CD55 was reduced in patients with TMA as compared to control biopsies of the unaffected pole of patient kidneys removed due to kidney cancer. Thus, our data support that endothelial Klf4 is necessary for maintenance of a quiescent glomerular endothelial phenotype and its loss increases susceptibility to complement activation and induction of prothrombotic and pro-inflammatory pathways.

Tacrolimus-Induced Thrombotic Microangiopathy After Orthotopic Heart Transplant: A Case Report

Tacrolimus (FK 506) is a calcineurin inhibitor and is commonly used as an immunosuppressant after a solid organ transplant. One of the serious adverse effects of tacrolimus is thrombotic microangiopathy (TMA) which has a high mortality rate. TMA leads to vascular thrombosis, eventually leading to ischemia of end organs. It is diagnosed clinically and based on the laboratory parameters. Early detection of TMA and prompt treatment can change the course. Drug-induced TMA has a poor prognosis as compared to idiopathic TMA.

We present here the case of a 47-year-old male who developed tacrolimus-induced TMA after an orthotopic heart transplant and he was treated with the currently available treatment. He ultimately died after 40 days of hospitalization. 

The causes and consequences of paediatric kidney disease on adult nephrology care

Adult nephrologists often look after patients who have been diagnosed with kidney disease in childhood. This does present unique challenges to the adult nephrologist, who may be unfamiliar with the underlying cause of kidney disease as well as the complications of chronic kidney disease (CKD) that may have accumulated during childhood. This review discusses common causes of childhood CKD, in particular congenital anomalies of the kidney and urinary tract (CAKUT), autosomal dominant tubulointerstitial kidney disease (ADTKD), polycystic kidney disease, hereditary stone disease, nephrotic syndrome and atypical haemolytic uraemic syndrome. The long-term consequences of childhood CKD, such as the cardiovascular consequences, cognition and education as well as bone health, nutrition and growth are also discussed.

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.

The Role of Complement in Microangiopathic Lesions of IgA Nephropathy

Introduction

Methods

Results

Conclusion

A novel missense mutation in complement factor I predisposes patients to atypical hemolytic uremic syndrome: a case report

Background

Atypical hemolytic uremic syndrome, also called the nondiarrheal form of hemolytic uremic syndrome, is a rare disease characterized by the triad of thrombocytopenia, Coomb’s test-negative microangiopathic hemolytic anemia, and acute renal failure. Approximately 60% of cases of atypical hemolytic uremic syndrome are associated with deficiencies of the complement regulatory protein, including mutations in complement factor H, complement factor I, or the membrane co-factor protein.

Case presentation

We report the case of a 26-year-old Asian man who presented with pulmonary infection, elevated blood pressure, microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Renal biopsy revealed diffuse capillary fibrin deposition, endothelial swelling, and arteriole narrowing like “onion skinning” consistent with thrombotic microangiopathy. Bidirectional sequencing of CFH, CFHR5, CFHR1, CFI, DGKE, CFB, and MCP confirmed that the patient was heterozygous for a novel missense mutation, p.Cys67Phe, in CFI. This patient had rapid evolution to end-stage renal disease and needed renal replacement therapy. Plasma exchange seemed inefficacious in this patient.

Conclusions

This report confirms the importance of screening patients with atypical hemolytic uremic syndrome for mutations in genes involved in complement system to clarify the diagnosis and demonstrates the challenges in the management of these patients.

Complement Blockade Is a Promising Therapeutic Approach in a Subset of Critically Ill Adult Patients with Complement-Mediated Hemolytic Uremic Syndromes

Thrombotic microangiopathy (TMA) gathers consumptive thrombocytopenia, mechanical haemolytic anemia, and organ damage. Hemolytic uremic syndromes (HUS) are historically classified as primary or secondary to another disease once thrombotic thrombocytopenic purpura (TTP), Shiga-toxin HUS, and cobalamin C-related HUS have been ruled out. Complement genetics studies reinforced the link between complement dysregulation and primary HUS, contributing to reclassifying some pregnancy- and/or post-partum-associated HUS and to revealing complement involvement in severe and/or refractory hypertensive emergencies. By contrast, no firm evidence allows a plausible association to be drawn between complement dysregulation and Shiga-toxin HUS or other secondary HUS. Nevertheless, rare complement gene variants are prevalent in healthy individuals, thus providing an indication that an investigation into complement dysregulation should be carefully balanced and that the results should be cautiously interpreted with the help of a trained geneticist. Several authors have suggested reclassifying HUS in two entities, regardless of they are complement-mediated or not, since the use of eculizumab, an anti-C5 antibody, dramatically lowers the proportion of patients who die or suffer from end-stage renal disease within the year following diagnosis. Safety and the ideal timing of eculizumab discontinuation is currently under investigation, and the long-term consequences of HUS should be closely monitored over time once patients exit emergency departments.

Case series: coronavirus disease 2019 infection as a precipitant of atypical hemolytic uremic syndrome: two case reports

Background

Atypical hemolytic uremic syndrome is an exceedingly rare thrombotic microangiopathy caused by accelerated activation of the alternative complement pathway.

Case presentation

Here, we report two cases of patients presenting with suspected atypical hemolytic uremic syndrome precipitated by coronavirus disease 2019 infection. The first patient, a 25-year-old Hispanic male, had one prior episode of thrombotic microangiopathy presumed to be atypical hemolytic uremic syndrome precipitated by influenza A, and re-presented with thrombocytopenia, microangiopathic hemolytic anemia, nonoliguric renal failure, and normal ADAMTS13 activity, with confirmed coronavirus disease 2019 positivity. The second patient, a 31-year-old Caucasian female, had no personal history of thrombotic microangiopathy, though reported a family history of suspected atypical hemolytic uremic syndrome. She presented with similar laboratory derangements, oliguric renal failure requiring hemodialysis, and confirmed coronavirus disease 2019 positivity. Both patients were treated with eculizumab with complete resolution of their hematologic and renal complications.

Conclusion

To our knowledge, this represents the largest case series of atypical hemolytic uremic syndrome precipitated by coronavirus disease 2019 in adults.

Genetic abnormalities in biopsy-proven, adult-onset hemolytic uremic syndrome and C3 glomerulopathy

Abstract

Atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G) have been linked to mutations in many of the proteins that are involved in alternative complement pathway activation. Age and etiology confounded, the prevalence of such mutations has been reported to be over 30 to 50% in these diseases. However, the cohorts studied included many children or individuals with a familial history of complement-related disorders and genetic tests were usually limited to exome sequencing of known causative or risk-associated genes. In this study, a retrospective adult cohort of 35 patients with biopsy-proven thrombotic microangiopathy (the largest in Canada) and 10 patients with C3 glomerulopathy was tested through an extended exome panel to identify causative defects in associated or candidate genes including those of the alternative and terminal complement pathways. A variant of unknown significance was also analyzed for pathogenicity through in vitro studies. To our surprise, the prevalence of known causative or risk-associated variants in either of these cohorts was found to be less than ~ 15% overall. However, the panel used and analyses carried out allowed to identify novel variants of potential clinical significance and a number of candidate genes. The prevalence of known genetic defects in adult-onset aHUS and C3G is thus probably much lower than 30 to 50%. Our results also point towards the importance of investigating diseases of the alternative complement pathway through extended exome panels and in vitro analyses.

Key messages

The alternative complement pathway plays a major role in the pathogenesis of hemolytic uremic syndrome and C3 glomerulopathy.

Based on previous studies, both disorders have been commonly linked to variants in the various intermediates that sustain or regulate this pathway.

The prevalence of such mutations in the adult-onset and sporadic forms of these diseases is probably much lower than expected based on larger series.

The sporadic forms of complementopathies are likely to involve additional genes that are yet to be uncovered.

Supplementary information

The online version contains supplementary material available at 10.1007/s00109-021-02102-1.

FHR-5 Serum Levels and CFHR5 Genetic Variations in Patients With Immune Complex-Mediated Membranoproliferative Glomerulonephritis and C3-Glomerulopathy

Background

Factor H-related protein 5 (FHR-5) is a member of the complement Factor H protein family. Due to the homology to Factor H, the main complement regulator of the alternative pathway, it may also be implicated in the pathomechanism of kidney diseases where Factor H and alternative pathway dysregulation play a role. Here, we report the first observational study on CFHR5 variations along with serum FHR-5 levels in immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) and C3 glomerulopathy (C3G) patients together with the clinical, genetic, complement, and follow-up data.

Methods

A total of 120 patients with a histologically proven diagnosis of IC-MPGN/C3G were enrolled in the study. FHR-5 serum levels were measured in ELISA, the CFHR5 gene was analyzed by Sanger sequencing, and selected variants were studied as recombinant proteins in ELISA and surface plasmon resonance (SPR).

Results

Eight exonic CFHR5 variations in 14 patients (12.6%) were observed. Serum FHR-5 levels were lower in patients compared to controls. Low serum FHR-5 concentration at presentation associated with better renal survival during the follow-up period; furthermore, it showed clear association with signs of complement overactivation and clinically meaningful clusters.

Conclusions

Our observations raise the possibility that the FHR-5 protein plays a fine-tuning role in the pathogenesis of IC-MPGN/C3G.

Thrombotic microangiopathy in aHUS and beyond: clinical clues from complement genetics

Studies of complement genetics have changed the landscape of thrombotic microangiopathies (TMAs), particularly atypical haemolytic uraemic syndrome (aHUS). Knowledge of complement genetics paved the way for the design of the first specific treatment for aHUS, eculizumab, and is increasingly being used to aid decisions regarding discontinuation of anti-complement treatment in this setting. Complement genetic studies have also been used to investigate the pathogenic mechanisms that underlie other forms of HUS and provided evidence that contributed to the reclassification of pregnancy- and postpartum-associated HUS within the spectrum of complement-mediated aHUS. By contrast, complement genetics has not provided definite evidence of a link between constitutional complement dysregulation and secondary forms of HUS. Therefore, the available data do not support systematic testing of complement genes in patients with typical HUS or secondary HUS. The potential relevance of complement genetics for distinguishing the underlying mechanisms of malignant hypertension-associated TMA should be assessed with caution owing to the overlap between aHUS and other causes of malignant hypertension. In all cases, the interpretation of complement genetics results remains complex, as even complement-mediated aHUS is not a classical monogenic disease. Such interpretation requires the input of trained geneticists and experts who have a comprehensive view of complement biology.

von Willebrand factor variants in C3 glomerulopathy: A Chinese cohort study

C3 glomerulopathy (C3G) is a rare renal disease characterized by predominant glomerular C3 staining. Complement alternative pathway dysregulation due to inherited complement defects is associated with C3G. To identify novel C3G-related genes, we screened 86 genes in the complement, coagulation and endothelial systems in 35 C3G patients by targeted genomic enrichment and massively parallel sequencing. Surprisingly, the most frequently mutated gene was VWF. Patients with VWF variants had significantly higher proteinuria levels, higher crescent formation and lower factor H (FH) levels. We further selected two VWF variants to transiently express the von Willebrand factor (vWF) protein, we found that vWF expression from the c.1519A > G variant was significantly reduced. In vitro results further indicated that vWF could regulate complement activation, as it could bind to FH and C3b, act as a cofactor for factor I-mediated cleavage of C3b. Thus, we speculated that vWF might be involved in the pathogenesis of C3G.

A Novel Homozygous In-Frame Deletion in Complement Factor 3 Underlies Early-Onset Autosomal Recessive Atypical Hemolytic Uremic Syndrome - Case Report

Background and Objectives

Atypical hemolytic uremic syndrome (aHUS) is mostly attributed to dysregulation of the alternative complement pathway (ACP) secondary to disease-causing variants in complement components or regulatory proteins. Hereditary aHUS due to C3 disruption is rare, usually caused by heterozygous activating mutations in the C3 gene, and transmitted as autosomal dominant traits. We studied the molecular basis of early-onset aHUS, associated with an unusual finding of a novel homozygous activating deletion in C3.

Design, Setting, Participants, & Measurements

A male neonate with eculizumab-responsive fulminant aHUS and C3 hypocomplementemia, and six of his healthy close relatives were investigated. Genetic analysis on genomic DNA was performed by exome sequencing of the patient, followed by targeted Sanger sequencing for variant detection in his close relatives. Complement components analysis using specific immunoassays was performed on frozen plasma samples from the patient and mother.

Results

Exome sequencing revealed a novel homozygous variant in exon 26 of C3 (c.3322_3333del, p.Ile1108_Lys1111del), within the highly conserved thioester-containing domain (TED), fully segregating with the familial disease phenotype, as compatible with autosomal recessive inheritance. Complement profiling of the patient showed decreased C3 and FB levels, with elevated levels of the terminal membrane attack complex, while his healthy heterozygous mother showed intermediate levels of C3 consumption.

Conclusions

Our findings represent the first description of aHUS secondary to a novel homozygous deletion in C3 with ensuing unbalanced C3 over-activation, highlighting a critical role for the disrupted C3-TED domain in the disease mechanism.

CFH and CFHR Copy Number Variations in C3 Glomerulopathy and Immune Complex-Mediated Membranoproliferative Glomerulonephritis

C3 Glomerulopathy (C3G) and Immune Complex-Mediated Membranoproliferative glomerulonephritis (IC-MPGN) are rare diseases characterized by glomerular deposition of C3 caused by dysregulation of the alternative pathway (AP) of complement. In approximately 20% of affected patients, dysregulation is driven by pathogenic variants in the two components of the AP C3 convertase, complement C3 (C3) and Factor B (CFB), or in complement Factor H (CFH) and Factor I (CFI), two genes that encode complement regulators. Copy number variations (CNVs) involving the CFH-related genes (CFHRs) that give rise to hybrid FHR proteins also have been described in a few C3G patients but not in IC-MPGN patients. In this study, we used multiplex ligation-dependent probe amplification (MLPA) to study the genomic architecture of the CFH-CFHR region and characterize CNVs in a large cohort of patients with C3G (n = 103) and IC-MPGN (n = 96) compared to healthy controls (n = 100). We identified new/rare CNVs resulting in structural variants (SVs) in 5 C3G and 2 IC-MPGN patients. Using long-read single molecule real-time sequencing (SMRT), we detected the breakpoints of three SVs. The identified SVs included: 1) a deletion of the entire CFH in one patient with IC-MPGN; 2) an increased number of CFHR4 copies in one IC-MPGN and three C3G patients; 3) a deletion from CFHR3-intron 3 to CFHR3-3'UTR (CFHR3_{4 - 6} Δ) that results in a FHR3-FHR1 hybrid protein in a C3G patient; and 4) a CFHR3_{1 - 5}-CFHR4₁₀ hybrid gene in a C3G patient. This work highlights the contribution of CFH-CFHR CNVs to the pathogenesis of both C3G and IC-MPGN.

Inherited Kidney Complement Diseases

In the past 20 years, we have witnessed tremendous advances in our ability to diagnose and treat genetic diseases of the kidney caused by complement dysregulation. Staggering progress was realized toward a better understanding of the genetic underpinnings and pathophysiology of many forms of atypical hemolytic uremic syndrome (aHUS) and C3-dominant glomerulopathies that are driven by complement system abnormalities. Many of these seminal discoveries paved the way for the design and characterization of several innovative therapies, some of which have already radically improved patients' outcomes. This review offers a broad overview of the exciting developments that have occurred in the recent past, with a particular focus on single-gene (or Mendelian), complement-driven aHUS and C3-dominant glomerulopathies that should be of interest to both nephrologists and kidney researchers. The discussion is restricted to genes with robust associations with both aHUS and C3-dominant glomerulopathies (complement factor H, complement component 3, complement factor H-related proteins) or only aHUS (complement factor B, complement factor I, and membrane cofactor protein). Key questions and challenges are highlighted, along with potential avenues for future directions.

Inherited Kidney Complement Diseases

In the past 20 years, we have witnessed tremendous advances in our ability to diagnose and treat genetic diseases of the kidney caused by complement dysregulation. Staggering progress was realized toward a better understanding of the genetic underpinnings and pathophysiology of many forms of atypical hemolytic uremic syndrome (aHUS) and C3-dominant glomerulopathies that are driven by complement system abnormalities. Many of these seminal discoveries paved the way for the design and characterization of several innovative therapies, some of which have already radically improved patients' outcomes. This review offers a broad overview of the exciting developments that have occurred in the recent past, with a particular focus on single-gene (or Mendelian), complement-driven aHUS and C3-dominant glomerulopathies that should be of interest to both nephrologists and kidney researchers. The discussion is restricted to genes with robust associations with both aHUS and C3-dominant glomerulopathies (complement factor H, complement component 3, complement factor H-related proteins) or only aHUS (complement factor B, complement factor I, and membrane cofactor protein). Key questions and challenges are highlighted, along with potential avenues for future directions.

Therapeutic Targeting of the Complement System: From Rare Diseases to Pandemics

The complement system was discovered at the end of the 19th century as a heat-labile plasma component that "complemented" the antibodies in killing microbes, hence the name "complement." Complement is also part of the innate immune system, protecting the host by recognition of pathogen-associated molecular patterns. However, complement is multifunctional far beyond infectious defense. It contributes to organ development, such as sculpting neuron synapses, promoting tissue regeneration and repair, and rapidly engaging and synergizing with a number of processes, including hemostasis leading to thromboinflammation. Complement is a double-edged sword. Although it usually protects the host, it may cause tissue damage when dysregulated or overactivated, such as in the systemic inflammatory reaction seen in trauma and sepsis and severe coronavirus disease 2019 (COVID-19). Damage-associated molecular patterns generated during ischemia-reperfusion injuries (myocardial infarction, stroke, and transplant dysfunction) and in chronic neurologic and rheumatic disease activate complement, thereby increasing damaging inflammation. Despite the long list of diseases with potential for ameliorating complement modulation, only a few rare diseases are approved for clinical treatment targeting complement. Those currently being efficiently treated include paroxysmal nocturnal hemoglobinuria, atypical hemolytic-uremic syndrome, myasthenia gravis, and neuromyelitis optica spectrum disorders. Rare diseases, unfortunately, preclude robust clinical trials. The increasing evidence for complement as a pathogenetic driver in many more common diseases suggests an opportunity for future complement therapy, which, however, requires robust clinical trials; one ongoing example is COVID-19 disease. The current review aims to discuss complement in disease pathogenesis and discuss future pharmacological strategies to treat these diseases with complement-targeted therapies. SIGNIFICANCE STATEMENT: The complement system is the host's defense friend by protecting it from invading pathogens, promoting tissue repair, and maintaining homeostasis. Complement is a double-edged sword, since when dysregulated or overactivated it becomes the host's enemy, leading to tissue damage, organ failure, and, in worst case, death. A number of acute and chronic diseases are candidates for pharmacological treatment to avoid complement-dependent damage, ranging from the well established treatment for rare diseases to possible future treatment of large patient groups like the pandemic coronavirus disease 2019.

Risk of Atypical HUS Among Family Members of Patients Carrying Complement Regulatory Gene Abnormality

Introduction

Atypical hemolytic uremic syndrome (aHUS) is mainly due to complement regulatory gene abnormalities with a dominant pattern but incomplete penetrance. Thus, healthy carriers can be identified in any family of aHUS patients, but it is unpredictable if they will eventually develop aHUS.

Methods

Patients are screened for 10 complement regulatory gene abnormalities and once a genetic alteration is identified, the search is extended to at-risk family members. The present cohort study includes 257 subjects from 71 families: 99 aHUS patients (71 index cases + 28 affected family members) and 158 healthy relatives with a documented complement gene abnormality.

Results

Fourteen families (19.7%) experienced multiple cases. Over a cumulative observation period of 7595 person-years, only 28 family members carrying gene mutations experienced aHUS (overall penetrance of 20%), leading to a disease rate of 3.69 events for 1000 person-years. The disease rate was 7.47 per 1000 person-years among siblings, 6.29 among offspring, 2.01 among parents, 1.84 among carriers of variants of uncertain significance, and 4.43 among carriers of causative variants.

Conclusions

The penetrance of aHUS seems a lot lower than previously reported. Moreover, the disease risk is higher in carriers of causative variants and is not equally distributed among generations: siblings and the offspring of patients have a much greater disease risk than parents. However, risk calculation may depend on variant classification that could change over time.

Atypical hemolytic uremic syndrome and acute tubular necrosis induced by complement factor B gene (CFB) mutation: A case report

RATIONALE

Atypical hemolytic uremic syndrome (aHUS) is an uncommon and serious disease that manifests hemolytic anemia, thrombocytopenia, and acute kidney injury. Genetic complement abnormalities have been shown to be responsible. Compared with the aHUS caused by other mutated genes, aHUS secondary to CFB mutation in adults is extremely rare. We report an adult with CFB mutation developing aHUS.

PATIENT CONCERNS

A 56-year-old man was admitted for 4-day history of nausea and fatigue, anuria for 2 days, and unconsciousness for 10 hours.

DIAGNOSES

The patient presented with life-threatening anemia, thrombocytopenia, acute kidney injury, and nervous system abnormalities. The patient had schistocytes on the peripheral blood smear, increased lactate dehydrogenase (LDH), and plasma-free hemoglobin levels. The patient was later found to harbor a pathogenic variant in the CFB gene (C.1598A>G), and was diagnosed with aHUS and acute kidney injury.

INTERVENTION

The patient was treated by plasmapheresis, continuous renal replacement therapy, blood transfusion, and anti-infective and antihypertensive treatment.

OUTCOMES

After the treatment, the patient's consciousness returned to normal, and the hemoglobin, platelet, and serum creatinine recovered. The disease activity remained quiescent during the follow-up.

LESSONS

A rare heterozygous variant c.1598A>G p.Lys 533Arg in the CFB gene, which was associated with adult-onset aHUS, was described and successfully treated. This case can help in understanding the early diagnosis and effective therapies of this rare disease.

Australia and New Zealand renal gene panel testing in routine clinical practice of 542 families

Genetic testing in nephrology clinical practice has moved rapidly from a rare specialized test to routine practice both in pediatric and adult nephrology. However, clear information pertaining to the likely outcome of testing is still missing. Here we describe the experience of the accredited Australia and New Zealand Renal Gene Panels clinical service, reporting on sequencing for 552 individuals from 542 families with suspected kidney disease in Australia and New Zealand. An increasing number of referrals have been processed since service inception with an overall diagnostic rate of 35%. The likelihood of identifying a causative variant varies according to both age at referral and gene panel. Although results from high throughput genetic testing have been primarily for diagnostic purposes, they will increasingly play an important role in directing treatment, genetic counseling, and family planning.

Clinical characterization and identification of rare genetic variants in atypical hemolytic uremic syndrome: A Swedish retrospective observational study

Complement-mediated atypical hemolytic uremic syndrome (aHUS) is an ultra-rare renal disease primarily caused by genetic alterations in complement proteins. The genetic work-up required for confirmation of diagnosis is complicated and not always logistically accessible. The aim of the present study was to apply a diagnostic scheme compliant with the American College of Medical Genetics and Genomics guidelines to investigate the prevalence of complement-mediated aHUS among subjects formerly included in a retrospective cohort of clinically suspected aHUS. Clinical outcomes and genetic correlations to complement analyses were assessed. Subjects were investigated with medical record reviewing, inquiries, and laboratory analyses composed of whole genome sequencing; enzyme-linked immunosorbent assays for factor I, factor H, and factor H-specific antibodies; nephelometry for complement components three of four; flow cytometry for CD46 surface expression and immunoblotting for the presence of factor H-related protein 1. In total, 45% (n = 60/134) of the subjects were deceased at the time of study. Twenty of the eligible subjects consented to study participation. Based on genetic sequencing and clinical characteristics, six were categorized as definite/highly suspected complement-mediated aHUS, 10 as non-complement-mediated aHUS and four as having an HUS-like phenotype. In the complement-mediated aHUS group, two subjects had not received an aHUS diagnosis during the routine clinical management. Disease-contributing/likely disease-contributing genetic variants were identified in five subjects, including a novel missense variant in the complement factor H gene (c.3450A>G, p.I1150M). This study illustrates the risk for misdiagnosis in the management of patients with complement-mediated aHUS and the importance of a comprehensive assessment of both phenotype and genotype to reach a diagnosis.

Genomic investigation of inherited thrombotic microangiopathy-aHUS and TTP

Thrombotic microangiopathies (TMA) are a heterogeneous group of red cell fragmentation syndromes characterized by a tendency for thrombosis and pathognomonic red cell fragments in peripheral blood, which results in thrombosis in the microvasculature due to endothelial damage. Genomic investigations into inherited TMAs are of diagnostic, prognostic and therapeutic value. Here, we present two cases that capture the importance of performing genomic testing in rare disorders. Treatment options for these conditions, such as plasma exchange and monoclonal antibodies against complement factors, are intensive and expensive health care interventions. The results of genomic investigation into rare TMAs can better inform the clinicians and their patients of prognosis and suitable personalized treatment options.

Distinct genetic profile with recurrent population-specific missense variants in Korean adult atypical hemolytic uremic syndrome

INTRODUCTION

Atypical hemolytic uremic syndrome (aHUS) is a rare thrombotic microangiopathy (TMA), characterized by micro-angiopathic hemolytic anemia, thrombocytopenia, and renal failure. In more than half of cases, genetic defects leading to overactivation of the alternative complement system have been identified. In this study, we investigated genetic defects in Korean adult patients with aHUS.

MATERIALS AND METHODS

Sixty-six Korean adult patients with aHUS were ascertained from the Korean TMA Registry. Genetic variants of 15 aHUS-related genes (eight core genes [CFH, CFB, CFI, CD46, C3, THBD, PLG, and DGKE] and seven candidate genes [CFP, C4BPA, and CHFR1-5]) were analyzed from exome sequencing data. Multiplex ligation-dependent probe amplification of CFH and related genes was performed to detect hybrid genes or large deletions.

RESULTS

Thirty patients (45%) had at least one aHUS-related variant (s) in eight core genes (total 40 variant alleles). The most frequently affected gene was CFH (13/40, 32%), followed by THBD (8/40, 20%) and CD46 (7/40, 18%). The two most common variants were Asp486Tyr of THBD (N = 7) and Tyr1058His-Val1060Leu of CFH (N = 5, linked on the same allele), accounting for 30% (12/40). In seven candidate genes, 19 variants were detected. When combined, 40 patients (61%) had at least one variant in 15 core or candidate genes. No patients had anti-CFH Ab or hybrid gene/CFHR1 homozygous deletions.

CONCLUSIONS

The genetic profile of Korean adult aHUS was unique with recurrent missense variants, demonstrating ethnicity- and age-dependent differences in the genetic background of aHUS.

A life-threatening case of pregnancy-related atypical Haemolytic uremic syndrome and successful treatment with Eculizumab

BACKGROUND

Pregnancy-related Atypical Haemolytic Uremic Syndrome (P-aHUS) is a rare condition affecting genetically predisposed women during pregnancy. It is often difficult to diagnose and has a significant impact on maternal and foetal outcomes. It is characterised by microangiopathic haemolytic anaemia and kidney injury from thrombotic microangiopathy.

CASE PRESENTATION

A 27-year-old female of Lebanese descent presented at 36 weeks' gestation with foetal death in-utero (FDIU) with placental abruption on a background of previously normal antenatal visits. She was coagulopathic and anaemic with anuric acute kidney injury, requiring emergency Caesarean section, intubation and dialysis. Her coagulopathy rapidly resolved, however, her anaemia and renal dysfunction persisted. A diagnosis of P-aHUS was made, and she was empirically treated with Eculizumab. Her ADAMTS13 level was normal, effectively excluding thrombotic thrombocytopenic purpura. Within 2 weeks of treatment her haematological parameters improved, and her renal function began to recover and within 2 months she became dialysis independent.

CONCLUSION

This case highlights the challenges of a timely diagnosis of P-aHUS from other pregnancy-related diseases. Although our patient is dialysis-independent, her risk of relapse remains high with subsequent pregnancies. Currently we are awaiting her genetic sequencing to complete her assessment for underlying mutations and are determining the safest approach to a future planned pregnancy.

Molecular Studies and an ex vivo Complement Assay on Endothelium Highlight the Genetic Complexity of Atypical Hemolytic Uremic Syndrome: The Case of a Pedigree With a Null CD46 Variant

Atypical hemolytic uremic syndrome (aHUS) is an ultra-rare disease characterized by microangiopathic hemolysis, thrombocytopenia, and renal impairment and is associated with dysregulation of the alternative complement pathway on the microvascular endothelium. Outcomes have improved greatly with pharmacologic complement C5 blockade. Abnormalities in complement genes (CFH, CD46, CFI, CFB, C3, and THBD), CFH–CFHR genomic rearrangements, and anti-FH antibodies have been reported in 40–60% of cases. The penetrance of aHUS is incomplete in carriers of complement gene abnormalities; and multiple hits, including the CFH–H3 and CD46_GGAAC risk haplotypes and the CFHR1^*B risk allele, as well as environmental factors, contribute to disease development. Here, we investigated the determinants of penetrance of aHUS associated with CD46 genetic abnormalities. We studied 485 aHUS patients and found CD46 rare variants (RVs) in about 10%. The c.286+2T>G RV was the most prevalent (13/485) and was associated with <30% penetrance. We conducted an in-depth study of a large pedigree including a proband who is heterozygous for the c.286+2T>G RV who experienced a severe form of aHUS and developed end-stage renal failure. The father and paternal uncle with the same variant in homozygosity and six heterozygous relatives are unaffected. Flow cytometry analysis showed about 50% reduction of CD46 expression on blood mononuclear cells from the heterozygous proband and over 90% reduction in cells from the proband's unaffected homozygous father and aunt. Further genetic studies did not reveal RVs in known aHUS-associated genes or common genetic modifiers that segregated with the disease. Importantly, a specific ex vivo test showed excessive complement deposition on endothelial cells exposed to sera from the proband, and also from his mother and maternal uncle, who do not carry the c.286+2T>G RV, indicating that they share a circulating defect that results in complement dysregulation on the endothelium. These results highlight the complexity of the genetics of aHUS and indicate that CD46 deficiency may not be enough to induce aHUS. We hypothesize that the proband inherited from his mother a genetic abnormality in a complement circulating factor that has not been identified yet, which synergized with the CD46 RV in predisposing him to the aHUS phenotype.

A critical role for plasminogen in inflammation

Plasminogen and its active form, plasmin, have diverse functions related to the inflammatory response in mammals. Due to these roles in inflammation, plasminogen has been implicated in the progression of a wide range of diseases with an inflammatory component. In this review, we discuss the functions of plasminogen in inflammatory regulation and how this system plays a role in the pathogenesis of diseases spanning organ systems throughout the body.

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.

Practical Considerations for APOL1 Genotyping in the Living Kidney Donor Evaluation

BACKGROUND

Association between the apolipoprotein L1 gene (APOL1) and nephropathy has altered the epidemiology of chronic kidney disease. In addition, donor APOL1 genotypes play important roles in the time to allograft failure in kidneys transplanted from deceased donors and the safety of living kidney donation.

METHODS

This article reviews genetic testing for inherited kidney disease in living kidney donors to improve donor safety. APOL1 genotyping in donors with recent African ancestry is considered.

RESULTS

Based on current data, transplant physicians should discuss APOL1 genotyping with potential living kidney donors self-reporting recent African ancestry. Until results from APOL1 Long-term Kidney Transplant Outcomes Network ancillary studies are available, we present practical approaches from our experience for considering APOL1 genotyping in the living donor evaluation.

CONCLUSIONS

Transplant physicians should inform potential living kidney donors at risk for APOL1-associated nephropathy about the gene and possibility of genetic testing early in the donor evaluation, well before scheduling the donor nephrectomy. Transplant programs must weigh risks of performing a donor nephrectomy in those with 2 APOL1 renal risk variants (high-risk genotypes), particularly younger individuals. Our program counsels kidney donors with APOL1 high-risk genotypes in the same fashion as with risk genotypes in other nephropathy genes. Because most African American kidney donor candidates lacking hypertension, proteinuria and reduced kidney function after workup will not possess APOL1 high-risk genotypes, genetic testing is unlikely to markedly increase donor declines and may reassure donors with regard to their long-term kidney outcomes, potentially increasing the number of African American donors.

Atypical hemolytic uremic syndrome in Brazil: clinical presentation, genetic findings and outcomes of a case series in adults and children treated with eculizumab

Background

Atypical hemolytic uremic syndrome (aHUS) is characterized by microangiopathic hemolytic anemia, thrombocytopenia and kidney injury caused by a dysregulation of the alternative complement pathway.

Methods

We conducted a multicenter nonregistry study aimed at collecting clinical, laboratory and genetic information of patients with aHUS in Brazil. Demographic data, genetic findings, treatments and outcomes are presented.

Results

Thirty-four patients were included, 62% were female and 67% were Caucasian. Half of the patients had the first manifestation of aHUS before the age of 18 years (pediatric group). Among the 17 patients who had the first manifestation after the age of 18 years (adult group), 6 were kidney transplant patients. Overall, 22 patients (65%) received plasma exchange/plasma infusion (PE/PI) and 31 patients (91%) received eculizumab. Eculizumab was started later in the adult group compared with the pediatric group. Two patients stopped dialysis after PE/PI and 19 patients stopped dialysis after eculizumab despite a late start. A pathogenic/likely pathogenic variant was found in 24.3% of patients. A coexisting condition or trigger was present in 59% of patients (infections, pregnancy, hypertension, autoimmune disease and transplant), especially in the adult group. There was a 30% relapse rate after stopping eculizumab, irrespective of genetic status.

Conclusion

This is the largest case series of aHUS in Brazil involving a wide range of patients for which eculizumab was the main treatment. Although eculizumab was started later than advised in the guidelines, most patients were able to stop dialysis at variable intervals. Discontinuation of eculizumab was associated with a 30% relapse of aHUS.