Atypical hemolytic uremic syndrome: a syndrome in need of clarity

Characterization of patients with aHUS and associated triggers or clinical conditions: A Global aHUS Registry analysis

INTRODUCTION

Atypical haemolytic uremic syndrome (aHUS) is a rare form of thrombotic microangiopathy (TMA) associated with complement dysregulation; aHUS may be associated with other 'triggers' or 'clinical conditions'. This study aimed to characterize this patient population using data from the Global aHUS Registry, the largest collection of real-world data on patients with aHUS.

METHODS

Patients enrolled in the Global aHUS Registry between April 2012 and June 2021 and with recorded aHUS-associated triggers or clinical conditions prior/up to aHUS onset were analysed. aHUS was diagnosed by the treating physician. Data were classified by age at onset of aHUS (< or ≥18 years) and additionally by the presence/absence of identified pathogenic complement genetic variant(s) and/or anti-complement factor H (CFH) antibodies. Genetically/immunologically untested patients were excluded.

RESULTS

1947 patients were enrolled in the Global aHUS Registry by June 2021, and 349 (17.9%) met inclusion criteria. 307/349 patients (88.0%) had a single associated trigger or clinical condition and were included in the primary analysis. Malignancy was most common (58/307, 18.9%), followed by pregnancy and acute infections (both 53/307, 17.3%). Patients with an associated trigger or clinical condition were generally more likely to be adults at aHUS onset.

CONCLUSION

Our analysis suggests that aHUS-associated triggers or clinical conditions may be organized into clinically relevant categories, and their presence does not exclude the concurrent presence of pathogenic complement genetic variants and/or anti-CFH antibodies. Considering a diagnosis of aHUS with associated triggers or clinical conditions in patients presenting with TMA may allow faster and more appropriate treatment.

Recurrent microangiopathic hemolysis after recovery from complement-mediated hemolytic uremia syndrome during chemotherapy for a CFH-mutated patient with T-lymphoblastic lymphoma

Complement-mediated hemolytic uremic syndrome (CM-HUS) following chemotherapy for pediatric acute lymphoid neoplasms has rarely been reported. We report the case of an 8-year-old boy with T-lymphoblastic lymphoma (T-LBL) who developed CM-HUS with complement factor H (CFH) mutations (S1191L, V1197A) during induction therapy. Safe administration of chemotherapy after CM-HUS recovery was challenging. By closely monitoring hemolytic and renal parameters during the 2-year treatment period, we observed four episodes of microangiopathic hemolytic anemia (MAHA) with hypocomplementemia and low haptoglobin but no renal dysfunction or thrombocytopenia. Here, we describe the MAHA and CM-HUS episodes in the hopes of elucidating the complex pathophysiology of disorders associated with CFH mutation.

The Role of Complement in HSCT-TMA: Basic Science to Clinical Practice

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA) is a common complication occurring post-HSCT and is associated with substantial morbidity and mortality if not promptly identified and treated. Emerging evidence suggests a central role for the complement system in the pathogenesis of HSCT-TMA. The complement system has also been shown to interact with other pathways and processes including coagulation and inflammation, all of which are activated following HSCT. Three endothelial cell-damaging “hits” are required for HSCT-TMA genesis: a genetic predisposition or existing damage, an endothelial cell-damaging conditioning regimen, and additional damaging insults. Numerous risk factors for the development of HSCT-TMA have been identified (including primary diagnosis, graft type, and conditioning regimen) and validated lists of relatively simple diagnostic signs and symptoms exist, many utilizing routine clinical and laboratory assessments. Despite the relative ease with which HSCT-TMA can be screened for, it is often overlooked or masked by other common post-transplant conditions. Recent evidence that patients with HSCT-TMA may also concurrently present with these differential diagnoses only serve to further confound its identification and treatment. HSCT-TMA may be treated, or even prevented, by removing or ameliorating triggering “hits”, and recent studies have also shown substantial utility of complement-targeted therapies in this patient population. Further investigation into optimal management and treatment strategies is needed. Greater awareness of TMA post-HSCT is urgently needed to improve patient outcomes; the objective of this article is to clarify current understanding, explain underlying complement biology and provide simple tools to aid the early recognition, management, and monitoring of HSCT-TMA.

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.

Treatment of Rare Inflammatory Kidney Diseases: Drugs Targeting the Terminal Complement Pathway

The complement system comprises the frontline of the innate immune system. Triggered by pathogenic surface patterns in different pathways, the cascade concludes with the formation of a membrane attack complex (MAC; complement components C5b to C9) and C5a, a potent anaphylatoxin that elicits various inflammatory signals through binding to C5a receptor 1 (C5aR1). Despite its important role in pathogen elimination, priming and recruitment of myeloid cells from the immune system, as well as crosstalk with other physiological systems, inadvertent activation of the complement system can result in self-attack and overreaction in autoinflammatory diseases. Consequently, it constitutes an interesting target for specialized therapies. The paradigm of safe and efficacious terminal complement pathway inhibition has been demonstrated by the approval of eculizumab in paroxysmal nocturnal hematuria. In addition, complement contribution in rare kidney diseases, such as lupus nephritis, IgA nephropathy, atypical hemolytic uremic syndrome, C3 glomerulopathy, or antineutrophil cytoplasmic antibody-associated vasculitis has been demonstrated. This review summarizes the involvement of the terminal effector agents of the complement system in these diseases and provides an overview of inhibitors for complement components C5, C5a, C5aR1, and MAC that are currently in clinical development. Furthermore, a link between increased complement activity and lung damage in severe COVID-19 patients is discussed and the potential for use of complement inhibitors in COVID-19 is presented.

Postoperative Atypical Hemolytic Uremic Syndrome in a 67-Year-Old Woman Associated with Hemolytic Anemia, Thrombocytopenia, and Acute Renal Failure

Patient: Female, 67-year-old

Final Diagnosis: Atypical hemolytic uremic syndrome

Symptoms: Anemia • renal failure • thrombocytopenia

Medication: —

Clinical Procedure: —

Specialty: Hematology • General and Internal Medicine • Pathology

Atypical Hemolytic Uremic Syndrome: New Challenges in the Complement Blockage Era

Atypical hemolytic uremic syndrome (aHUS) is a rare cause of thrombotic microangiopathy (TMA), characterized by microangiopathic hemolytic anemia, consumptive thrombocytopenia, and multisystem end organ involvement, most commonly affecting the kidney. Diagnosis is clinical, after exclusion of other TMA causes. Primary aHUS arises from genetic abnormalities, resulting in uncontrolled complement activity, while a variety of clinical scenarios cause secondary aHUS, including infection, pregnancy, malignancy, autoimmune disease, and medications. They can also induce a temporary complement deregulation with an overlap between both scenarios, which can make differential diagnosis difficult. Primary aHUS can be sporadic or familial and is associated with a high rate of progression to ESRD. Many aHUS patients relapse in the native or transplanted kidneys, leading to kidney failure. The introduction of eculizumab has changed the prognosis of aHUS, by inducing hematologic remission, improving or stabilizing kidney functions, and preventing graft failure. The early institution of appropriate therapy can prevent multiorgan damage, so is essential to recognize and differentiate the TMA syndromes. Eculizumab is considered now the first-line treatment, and it is recommended lifelong therapy. However, the high cost of therapy has led to make efforts to develop precise complement functional and genetic studies that help physicians to determine the appropriate duration of eculizumab therapy. Nowadays, more studies are needed to select candidates to adjustment of therapy.

A Case Report of Atypical Hemolytic Uremic Syndrome in a Two-Month-Old Infant With a Negative Reported Genetic Profile and Five-Year Follow-Up on Eculizumab

Atypical hemolytic uremic syndrome (aHUS) is a rare but life-threatening pediatric disease caused by uncontrolled activation of the alternative complement pathway related to genetic mutations and carries a worse prognosis. In the last decade, a monoclonal antibody against complement C5, eculizumab, has dramatically improved the disease outcomes. The complement mutations in aHUS are detected only in 60%-70% of cases in previous studies. We report a severe presentation of aHUS diagnosed in a two-month-old child who presented with seizures, renal failure with anuria, and microangiopathic hemolytic anemia and required peritoneal dialysis soon after admission. The patient was clinically diagnosed having aHUS and was started on eculizumab on day 4 of hospital admission. The genetic study for major known complement mutations causing aHUS was reported negative. He had a major episode of disease relapse associated with seizures four weeks after eculizumab therapy and required prolonged peritoneal dialysis over more than two months at the time of initial admission. He developed dilated cardiomyopathy and oro-motor dysfunction as complications of aHUS. At five-year follow-up, the patient has stage 3 chronic kidney disease (CKD), proteinuria, hypertension, and required G-tube for feeds. This report discussed the long-term outcome of an infant diagnosed with aHUS and tested negative for common complement mutations on eculizumab therapy. More research is needed to identify novel genes and antibodies contributing to aHUS. While the eculizumab is expensive, and the duration of treatment is not definite, the clinical severity of the disease, relapses, and presence of long-term renal complications are essential factors to decide treatment continuation.

Epidemiology of Atypical Hemolytic Uremic Syndrome: A Systematic Literature Review

Atypical hemolytic uremic syndrome (aHUS) is a rare but severe disorder that frequently has a genetic component and results from the overactivation of the alternative complement pathway. As research moves toward improved diagnosis and therapy of aHUS, it will be important to better understand its epidemiology. Our objective was to conduct a systematic literature review to assess the incidence and prevalence estimates of aHUS globally. A comprehensive literature search was conducted in Embase and MEDLINE. Additionally, practice guidelines, databases of national/international organizations, and regulatory agencies were searched. From 2960 publications identified via MEDLINE and Embase, 105 publications were eligible for full-text screening, and a total of eight full-text articles met eligibility criteria for inclusion. Regional epidemiologic estimates were obtained for Europe and Oceania. Country-specific data were available for France, Norway, Australia, and Italy. Four of the identified studies reported on the prevalence of aHUS, prevalence in the age group of 20 years or younger was ranging from 2.2 to 9.4 per million population, while the only study that reported prevalence in all ages showed a prevalence of 4.9 per million population. Six studies reported on the incidence of aHUS, annual incidence in the age group of 20 years or younger was ranging from 0.26 to 0.75 per million population, and for all ages, annual incidence was ranging from 0.23 to 1.9 per million population. To our knowledge, this is the first systematic review conducted to provide a comprehensive overview of global incidence and prevalence estimates of aHUS. In general, incidence estimates were similar across all the studies; however, prevalence data were found to be more variable. Study limitations were related to inconsistencies in the definitions of aHUS between studies and also a dearth of epidemiological studies assessing incidence and prevalence of aHUS outside of Europe.

Diagnostic Testing for Differential Diagnosis in Thrombotic Microangiopathies

Thrombotic microangiopathies (TMAs) are multiple disease entities with different etiopathogeneses, characterized by thrombocytopenia, microangiopathic hemolytic anemia (MAHA) with schistocytosis, variable symptoms including fever, and multi-organ failure such as mild renal impairment and neurological deficits. The two paradigms of TMAs are represented on one hand by acquired thrombotic thrombocytopenic purpura (TTP) and on the other by hemolytic uremic syndrome (HUS). The differential diagnosis between these two paradigmatic forms of TMA is based on the presence of either frank renal failure in HUS or a severe deficiency (<10%) of the zinc-protease ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) in TTP. ADAMTS13 is an enzyme involved in the proteolytic processing of von Willebrand factor (vWF), and its deficiency results in formation of high-molecular-weight vWF-rich microthrombi in the environment of the microvasculature. The presence of these ultra-large vWF multimers in the microcirculation can recruit platelets, promoting multi-organ ischemic lesions. The presence of ADAMTS13 activity at >10% could rule out the presence of a TTP form. However, it is often difficult to differentiate either a TTP or HUS clinical scenario presenting with typical symptoms of TMA. There are in fact several additional diagnoses that should be considered in patients with ADAMTS13 activity of >10%. Widespread inflammation with endothelial damage and adverse reactions to drugs play a central role in the pathogenesis of several forms of TMA, and in these cases, the differential diagnosis should be directed at the underlying disease. Hence, a correct etiologic diagnosis of TMA should involve a critical illness, cancer-associated TMA, drug-induced TMA, and hematopoietic transplant-associated TMA. A complete assessment of all the possible etiologies for TMA symptoms, including acquired or congenital TTP, will allow for a more accurate diagnosis and application of a more appropriate treatment.

An updated classification of thrombotic microangiopathies and treatment of complement gene variant-mediated thrombotic microangiopathy

Conditions presenting with signs of thrombotic microangiopathies (TMAs) comprise a wide spectrum of different diseases. While pathological hallmarks are thrombosis of arterioles and capillaries, clinical signs are mechanical haemolysis, thrombocytopenia and acute renal injury or neurological manifestations. The current classification of various syndromes of TMA is heterogeneous and often does not take the underlying pathophysiology into consideration. Therefore we propose a simplified classification based on the aetiology of different syndromes leading to TMA. We propose to categorize different TMA syndromes in hereditary and acquired forms and classify them based on the genetic background or underlying conditions. Of course, this classification is not always distinctly applicable in each case and from time to time reassessment of the established diagnosis is strongly recommended. The recommended treatment of TMA in the past was plasma exchange (PE). However, recently, the terminal complement inhibitor eculizumab became commercially available and has shown promising results in different open-label studies and case series. In our centre, first-line therapy is PE; however, patients are instantly switched to complement inhibitory therapy in case of treatment failure or intolerance.