Varicella as a trigger of atypical haemolytic uraemic syndrome associated with complement dysfunction: two cases

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.

Complement in Secondary Thrombotic Microangiopathy

Thrombotic microangiopathy (TMA) is a condition characterized by thrombocytopenia and microangiopathic hemolytic anemia (MAHA) with varying degrees of organ damage in the setting of normal international normalized ratio and activated partial thromboplastin time. Complement has been implicated in the etiology of TMA, which are classified as primary TMA when genetic and acquired defects in complement proteins are the primary drivers of TMA (complement-mediated TMA or atypical hemolytic uremic syndrome, aHUS) or secondary TMA, when complement activation occurs in the context of other disease processes, such as infection, malignant hypertension, autoimmune disease, malignancy, transplantation, pregnancy, and drugs. It is important to recognize that this classification is not absolute because genetic variants in complement genes have been identified in patients with secondary TMA, and distinguishing complement/genetic-mediated TMA from secondary causes of TMA can be challenging and lead to potentially harmful delays in treatment. In this review, we focus on data supporting the involvement of complement in aHUS and in secondary forms of TMA associated with malignant hypertension, drugs, autoimmune diseases, pregnancy, and infections. In aHUS, genetic variants in complement genes are found in up to 60% of patients, whereas in the secondary forms, the finding of genetic defects is variable, ranging from almost 60% in TMA associated with malignant hypertension to less than 10% in drug-induced TMA. On the basis of these findings, a new approach to management of TMA is proposed.

Gastrointestinal pathogens in anti-FH antibody positive and negative Hemolytic Uremic Syndrome

BACKGROUND

Prodromal symptoms are frequently reported in the atypical form of Hemolytic uremic syndrome (aHUS) suggesting implication of infectious triggers. Some pathogens may also play a role in the mechanisms of production of autoantibody directed against Factor H (FH), a complement regulator, leading to aHUS.

METHODS

The presence of 15 gastrointestinal (GI) pathogens was investigated by using xTAG-based multiplex PCR techniques on stools collected at the acute phase in a cohort of Indian HUS children classified according to the presence or absence of anti-FH autoantibodies.

RESULTS

Prevalence of pathogens in patients with anti-FH antibody (62.5%) was twice that in those without (31.5%). Different pathogens were detected, the most frequent being Clostridium difficile, Giardia intestinalis, Salmonella, Shigella, Rotavirus, Norovirus and Entamoeba histolytica. No stool was positive for Shigatoxin.

CONCLUSION

This study reveals a higher prevalence of GI pathogens in anti-FH positive than in negative patients. No single pathogen was implicated exclusively in one form of HUS. These pathogens may play a role in the disease initiation by inducing complement activation or an autoimmune response.

Acute kidney injury in a patient with hemolytic anemia and thrombocytopenia

Atypical hemolytic uremic syndrome (HUS) is clinically difficult to distinguish from HUS and thrombotic thrombocytopenic purpura. Atypical HUS results from dysregulation of complement activation causing thrombotic microangiopathy affecting multiple organ systems. Atypical HUS is associated with high morbidity and mortality, making early recognition and appropriate therapy necessary to improve patient outcomes.

Atypical hemolytic uremic syndrome triggered by varicella infection

Varicella Zoster Virus (VZV) is a well-known virus that belongs to the Herpesviridae family which induces a self-limited disease except in specific cases in particular among stem cell transplant patients. This virus is not known however to trigger atypical Hemolytic Uremic Syndrome (aHUS). Here we report the case of a six-year-old boy who was hospitalized with fever and abdominal pains associated to pruritic and vesicular rash, thrombocytopenia and acute renal failure. He was diagnosed with aHUS precipitated by varicella virus. He was treated by an association of antimicrobials against potential superinfections, plasmapheresis and eculizumab for curative aHUS treatment. This was effective but after 6 months the kidney function remained poor. The current case describes an aHUS associated to varicella infection as demonstrated by the simultaneous occurrence of the viral infection and aHUS manifestations. Apart from typical Hemolytic Uremic Syndrome which is triggered by bacteria mostly Shiga toxin producing Echerichia coli and Streptococcus pneumoniae or Shigella, aHUS may be linked to viral infections such as HIV, EBV and enteroviruses, but very rarely by varicella. This case highlights a possible even rare complication of varicella infection a very common childhood disease. This complication could be avoided by to anti-VZV vaccination.

Occurrence of atypical HUS associated with influenza B

Hemolytic uremic syndrome (HUS) is a disease characterized by thrombotic microangiopathy with a triad of non-immune hemolytic anemia, thrombocytopenia, and renal impairment. Approximately 10% of cases of HUS are classified as atypical (aHUS). While today many genetically forms of aHUS pathology are known, only about 50% of carriers precipitate the disease. The reason remains unclear, and triggering events like intercurrent infections have been postulated. In rare cases, influenza A is the known trigger of aHUS; however, no cases of influenza B have been reported.

CONCLUSION

We describe for the first time that influenza B strain as a trigger for aHUS in children with primary hereditary forms. We also showed in our three cases that immunization appears to be safe; however, this needs to be confirmed in a larger cohort. What is Known: • Known triggers of aHUS are infectious specimen. • Influenza A-associated aHUS cases are rarely published. What is New: • aHUS can be triggered by influenza B virus infection. • Influenza vaccination of patients with aHUS appears safe.

Anti-complement-factor H-associated glomerulopathies

Atypical haemolytic uraemic syndrome (aHUS), an important cause of acute kidney injury, is characterized by dysregulation of the complement pathway, frequent need for dialysis, and progression to end-stage renal disease. Autoantibodies against complement factor H (FH), the main plasma regulatory protein of the alternative pathway of the complement system, account for a considerable proportion of children with aHUS. The autoantibodies are usually associated with the occurrence of a homozygous deletion in the genes encoding the FH-related proteins FHR1 and FHR3. High levels of autoantibodies, noted at the onset of disease and during relapses, induce functional deficiency of FH, whereas their decline, in response to plasma exchanges and/or immunosuppressive therapy, is associated with disease remission. Management with plasma exchange and immunosuppression is remarkably effective in inducing and maintaining remission in aHUS associated with FH autoantibodies, whereas terminal complement blockade with eculizumab is considered the most effective therapy in other forms of aHUS. Anti-FH autoantibodies are also detected in a small proportion of patients with C3 glomerulopathies, which are characterized by chronic glomerular injury mediated by activation of the alternative complement pathway and predominant C3 deposits on renal histology.

The alternative pathway of complement and the thrombotic microangiopathies

Thrombotic microangiopathies (TMA) are disorders defined by microangiopathic hemolytic anemia, non-immune thrombocytopenia and have multi-organ involvement including the kidneys, brain, gastrointestinal, respiratory tract and skin. Emerging evidence points to the central role of complement dysregulation in leading to microvascular endothelial injury which is crucial for the development of TMAs. This key insight has led to the development of complement-targeted therapy. Eculizumab is an anti-C5 monoclonal antibody, which has revolutionized the treatment of atypical hemolytic uremic syndrome. Several other anti-complement therapeutic agents are currently in development, offering a potential armamentarium of therapies available to treat complement-mediated TMAs. The development of sensitive, reliable and easy to perform assays to monitor complement activity and therapeutic efficacy will be key to devising an individualized treatment regime with the potential of safely weaning or discontinuing treatment in the appropriate clinical setting.

The Genetics of Ultra-Rare Renal Disease

The complement-mediated renal diseases are a group of ultra-rare renal diseases that disproportionately affect children and young adults and frequently lead to irreversible renal failure. Genetic mutations in alternate pathway of complement genes are pathomechanistically involved in a significant number of these unique diseases. Here, we review our current understanding of the role of genetics in the primary complement-mediated renal diseases affecting children, with a focus on atypical hemolytic uremic syndrome and C3 glomerulopathy. Also, included is a brief discussion of the related diseases whose relationship to complement abnormality has been suspected but not yet confirmed. Advances in genetics have transformed both treatment and outcomes in these historically difficult to treat, highly morbid diseases.

The role of ADAMTS-13 activity and complement mutational analysis in differentiating acute thrombotic microangiopathies

ESSENTIALS: Molecular diagnostics has improved the differentiation of acute thrombotic microangiopathys (TMAs). Atypical hemolytic uremic syndrome may have features mimicking thrombotic thrombocytopenic purpura. We identified novel complement mutations and a high incidence of CD46, with favorable long term outcomes. Complement mutation analysis in TMA where the diagnosis is unclear and ADAMTS-13 activity is >10%.

BACKGROUND

Differentiation of acute thrombotic microangiopathy (TMA) at presentation has historically been dependent on clinical parameters. Confirmation of thrombotic thrombocytopenic purpura (TTP) is increasingly reliant on demonstrating deficient ADAMTS-13 activity. The identification of alternative complement pathway abnormalities in atypical hemolytic uremic syndrome (aHUS), along with the proven efficacy of terminal complement inhibitors in treatment, has increased the need for rapid differentiation of TTP from aHUS.

OBJECTIVES

We describe the clinical phenotype and nature of complement mutations in a cohort of aHUS patients referred as acute TMAs.

PATIENTS/METHODS

Fourteen consecutive aHUS patients were screened for mutations in C3, CD46, CFH, CFI, and CFB, as well as factor H (FH) antibodies. All aHUS patients had ADAMTS-13 activity > 10%.

RESULTS

Of 14 aHUS patients, 11 (79%) had platelet counts < 30 × 10(9) /L during the acute phase. Median presenting creatinine level was 295 μmol L(-1) , while five (36%) of 14 presented with a serum creatinine level < 200 μmol L(-1) . Alternative complement pathway mutations were detected in 9 (64%) of 14 patients, including CD46 mutations in five (36%) of 14 patients. Patients were identified with novel mutations in CFB and C3 that have not been previously reported.

CONCLUSIONS

We demonstrate that diagnostic differentiation based on platelet count and renal function is insufficient to predict an underlying complement mutation in some aHUS cases. Specifically, we demonstrate a high frequency of functionally significant CD46 mutations which may mimic TTP. ADAMTS-13 activity > 10% in a patient with a TMA should necessitate genetic screening for complement abnormalities.

Complement regulator CD46: genetic variants and disease associations

Membrane cofactor protein (MCP; CD46) is an ubiquitously expressed complement regulatory protein that protects host cells from injury by complement. This type-I membrane glycoprotein serves as a cofactor for the serine protease factor I to mediate inactivation of C3b and C4b deposited on host cells. More than 60 disease-associated mutations in MCP have now been identified. The majority of the mutations are linked to a rare thrombotic microangiopathic-based disease, atypical hemolytic uremic syndrome (aHUS), but new putative links to systemic lupus erythematosus, glomerulonephritis, and pregnancy-related disorders among others have also been identified. This review summarizes our current knowledge of disease-associated mutations in this complement inhibitor.

Atypical aHUS: State of the art

Tremendous advances in our understanding of the thrombotic microangiopathies (TMAs) have revealed distinct disease mechanisms within this heterogeneous group of diseases. As a direct result of this knowledge, both children and adults with complement-mediated TMA now enjoy higher expectations for long-term health. In this update on atypical hemolytic uremic syndrome, we review the clinical characteristics; the genetic and acquired drivers of disease; the broad spectrum of environmental triggers; and current diagnosis and treatment options. Many questions remain to be addressed if additional improvements in patient care and outcome are to be achieved in the coming decade.

Atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. The atypical form of HUS is a disease characterized by complement overactivation. Inherited defects in complement genes and acquired autoantibodies against complement regulatory proteins have been described. Incomplete penetrance of mutations in all predisposing genes is reported, suggesting that a precipitating event or trigger is required to unmask the complement regulatory deficiency. The underlying genetic defect predicts the prognosis both in native kidneys and after renal transplantation. The successful trials of the complement inhibitor eculizumab in the treatment of atypical HUS will revolutionize disease management.

Hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy defined by thrombocytopenia, nonimmune microangiopathic hemolytic anemia, and acute renal failure. The most frequent form is associated with infections by Shiga-like toxin-producing bacteria (STEC-HUS). Rarer cases are triggered by neuraminidase-producing Streptococcus pneumoniae (pneumococcal-HUS). The designation of aHUS is used to refer to those cases in which an infection by Shiga-like toxin-producing bacteria or S. pneumoniae can be excluded. Studies performed in the last two decades have documented that hyperactivation of the complement system is the pathogenetic effector mechanism leading to the endothelial damage and the microvascular thrombosis in aHUS. Recent data suggested the involvement of the complement system in the pathogenesis of STEC-HUS and pneumococcal-HUS as well. Clinical signs and symptoms may overlap among the different forms of HUS; however, pneumococcal-HUS and aHUS have a worse prognosis compared with STEC-HUS. Early diagnosis and identification of underlying pathogenetic mechanism allows instating specific support measures and therapies. In clinical trials in patients with aHUS, complement inhibition by eculizumab administration leads to a rapid and sustained normalization of hematological parameters with improvement in long-term renal function. This review summarizes current concepts about the epidemiological findings, the pathological and clinical aspects of STEC-HUS, pneumococcal-HUS, and aHUS, and their diagnosis and management.

Long-term remission of atypical HUS with anti-factor H antibodies after cyclophosphamide pulses

BACKGROUND

Anti-complement factor H (CFH) autoantibody (Ab)-associated atypical hemolytic uremic syndrome (aHUS) has a poor prognosis, but no consensus exists on its treatment.

METHODS

We report the follow-up of four children with anti-CFH Ab (8,000 to >32,000 arbitrary units)-associated aHUS after plasma exchanges (PEs), prednisone, and cyclophosphamide pulse therapy with the evolution of anti-CFH Ab titers and kidney function.

RESULTS

Patient 1 received PEs + prednisone + cyclophosphamide pulses after two relapses following PEs and then PEs + rituximab. The other three patients were treated with PEs + prednisone + cyclophosphamide pulses as a first-line therapy. In our four patients, the induction protocol combining PEs + prednisone + cyclophosphamide pulses led to a rapid and sustained remission up to 6 years, 4 years and 4 months without any maintenance therapy. Kidney function was normal and anti-CFH Ab titer decreased, but remained detectable during remission without any clinical or biological signs of relapse.

CONCLUSIONS

We demonstrate the long-term efficiency and safety of cyclophosphamide pulses combined with PEs and prednisone in anti-CFH Ab-associated aHUS leading to a prolonged decrease in anti-CFH Ab titers and prevention of relapses without the need for maintenance therapy.

Rapid recovery of membrane cofactor protein (MCP; CD46) associated atypical haemolytic uraemic syndrome with plasma exchange

Atypical haemolytic uraemic syndrome (aHUS), unlike typical HUS is due to complement dysregulation. At least one abnormality of the complement system can be identified in 70% of patients. aHUS is associated with a poor prognosis with 25% mortality and 50% progress to end-stage renal disease. Genetic abnormalities in the complement system, proteins including CFH, CFI, CFB, C3, CFHR1/3 and MCP (CD46) lead to uncontrolled complement activation in aHUS. We presented the second reported case of aHUS associated with a heterozygous c.191G > T mutation in exon 2 of MCP who responded rapidly to plasma exchange.

Viral-associated thrombotic microangiopathies

Thrombotic microangiopathies encompass a group of disorders characterized by microangiopathic hemolytic anemia, thrombocytopenia associated with hyaline thrombi (comprised primarily of platelet aggregates in the microcirculation), and varying degrees of end-organ failure. Many primary (genetic) and secondary etiological predisposing factors have been described-namely pregnancy, autoimmune disorders, cancer, drugs and antineoplastic therapy, bone marrow transplantation/solid organ transplantation, and infections. In the setting of infectious diseases, the association with Shiga or Shiga-like exotoxin of Escherichia coli 0157:h7 or Shigella dysenteriae type 1-induced typical hemolytic uremic syndrome is well known. Recently however, an increasing body of evidence suggests that viruses may also play an important role as trigger factors in the pathogenesis of thrombotic microangiopathies. This is a comprehensive review focusing on the current understanding of viral associated/induced endothelial stimulation and damage that ultimately leads to the development of this life-threatening multisystemic disorder.

Novel developments in thrombotic microangiopathies: is there a common link between hemolytic uremic syndrome and thrombotic thrombocytic purpura?

Thrombotic microangiopathies (TMA) represent a spectrum of related disorders associated with newly formed thrombi that block perfusion and thus affect the function of either renal or neurological organs and tissue. Recent years have seen a dramatic development in the field of TMA and for the two major forms hemolytic uremic syndrome (HUS) and thrombocytopenic purpura (TTP), new genetic causes and also autoimmune forms have been identified. This development indicates a similar pathophysiology and suggests that the two acute disorders are based on common principles. HUS is primarily a kidney disease and TTP also develops in the kidney and at neurological sites. In HUS thrombi formation is likely due to a deregulated complement activation and inappropriate platelet activity. In TTP thrombi formation occurs because of inappropriate processing of released multimers of von Willebrand Factor (vWF). Defining both the similarities and the unique features of each disorder will open up new ways and concepts that are relevant for diagnosis, for therapy, and for the prognostic outcome of kidney transplantations. Here we summarize the most relevant topics and timely issues that were presented and discussed at the 4th International Workshop on Thrombotic Microangiopathies held in Weimar in October 2009 (www.hus-ttp.de).

Atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is defined by the triad of mechanical hemolytic anemia, thrombocytopenia and renal impairment. Atypical HUS (aHUS) defines non Shiga-toxin-HUS and even if some authors include secondary aHUS due to Streptococcus pneumoniae or other causes, aHUS designates a primary disease due to a disorder in complement alternative pathway regulation. Atypical HUS represents 5 -10% of HUS in children, but the majority of HUS in adults. The incidence of complement-aHUS is not known precisely. However, more than 1000 aHUS patients investigated for complement abnormalities have been reported. Onset is from the neonatal period to the adult age. Most patients present with hemolytic anemia, thrombocytopenia and renal failure and 20% have extra renal manifestations. Two to 10% die and one third progress to end-stage renal failure at first episode. Half of patients have relapses. Mutations in the genes encoding complement regulatory proteins factor H, membrane cofactor protein (MCP), factor I or thrombomodulin have been demonstrated in 20-30%, 5-15%, 4-10% and 3-5% of patients respectively, and mutations in the genes of C3 convertase proteins, C3 and factor B, in 2-10% and 1-4%. In addition, 6-10% of patients have anti-factor H antibodies. Diagnosis of aHUS relies on 1) No associated disease 2) No criteria for Shigatoxin-HUS (stool culture and PCR for Shiga-toxins; serology for anti-lipopolysaccharides antibodies) 3) No criteria for thrombotic thrombocytopenic purpura (serum ADAMTS 13 activity > 10%). Investigation of the complement system is required (C3, C4, factor H and factor I plasma concentration, MCP expression on leukocytes and anti-factor H antibodies; genetic screening to identify risk factors). The disease is familial in approximately 20% of pedigrees, with an autosomal recessive or dominant mode of transmission. As penetrance of the disease is 50%, genetic counseling is difficult. Plasmatherapy has been first line treatment until presently, without unquestionable demonstration of efficiency. There is a high risk of post-transplant recurrence, except in MCP-HUS. Case reports and two phase II trials show an impressive efficacy of the complement C5 blocker eculizumab, suggesting it will be the next standard of care. Except for patients treated by intensive plasmatherapy or eculizumab, the worst prognosis is in factor H-HUS, as mortality can reach 20% and 50% of survivors do not recover renal function. Half of factor I-HUS progress to end-stage renal failure. Conversely, most patients with MCP-HUS have preserved renal function. Anti-factor H antibodies-HUS has favourable outcome if treated early.

Atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is defined by the triad of mechanical hemolytic anemia, thrombocytopenia and renal impairment. Atypical HUS (aHUS) defines non Shiga-toxin-HUS and even if some authors include secondary aHUS due to Streptococcus pneumoniae or other causes, aHUS designates a primary disease due to a disorder in complement alternative pathway regulation. Atypical HUS represents 5 -10% of HUS in children, but the majority of HUS in adults. The incidence of complement-aHUS is not known precisely. However, more than 1000 aHUS patients investigated for complement abnormalities have been reported. Onset is from the neonatal period to the adult age. Most patients present with hemolytic anemia, thrombocytopenia and renal failure and 20% have extra renal manifestations. Two to 10% die and one third progress to end-stage renal failure at first episode. Half of patients have relapses. Mutations in the genes encoding complement regulatory proteins factor H, membrane cofactor protein (MCP), factor I or thrombomodulin have been demonstrated in 20-30%, 5-15%, 4-10% and 3-5% of patients respectively, and mutations in the genes of C3 convertase proteins, C3 and factor B, in 2-10% and 1-4%. In addition, 6-10% of patients have anti-factor H antibodies. Diagnosis of aHUS relies on 1) No associated disease 2) No criteria for Shigatoxin-HUS (stool culture and PCR for Shiga-toxins; serology for anti-lipopolysaccharides antibodies) 3) No criteria for thrombotic thrombocytopenic purpura (serum ADAMTS 13 activity > 10%). Investigation of the complement system is required (C3, C4, factor H and factor I plasma concentration, MCP expression on leukocytes and anti-factor H antibodies; genetic screening to identify risk factors). The disease is familial in approximately 20% of pedigrees, with an autosomal recessive or dominant mode of transmission. As penetrance of the disease is 50%, genetic counseling is difficult. Plasmatherapy has been first line treatment until presently, without unquestionable demonstration of efficiency. There is a high risk of post-transplant recurrence, except in MCP-HUS. Case reports and two phase II trials show an impressive efficacy of the complement C5 blocker eculizumab, suggesting it will be the next standard of care. Except for patients treated by intensive plasmatherapy or eculizumab, the worst prognosis is in factor H-HUS, as mortality can reach 20% and 50% of survivors do not recover renal function. Half of factor I-HUS progress to end-stage renal failure. Conversely, most patients with MCP-HUS have preserved renal function. Anti-factor H antibodies-HUS has favourable outcome if treated early.

Is complement a culprit in infection-induced forms of haemolytic uraemic syndrome?

Haemolytic uraemic syndrome (HUS) accounts for the most common cause of childhood acute renal failure. Characterized by the classical triad of a microangiopathic haemolytic anaemia, thrombocytopaenia and acute renal failure, HUS occurs as a result of Shiga-toxin producing microbes in 90% of cases. The remaining 10% of cases represent a heterogeneous subgroup in which inherited and acquired forms of complement dysregulation have been described in up to 60%. Emerging evidence suggests that microbes associated with HUS exhibit interaction with the complement system. With the advent of improved genetic diagnosis, it is likely that certain cases of infection-induced HUS may be attributed to underlying defects in complement components. This review summarises the interplay between complement and infection in the pathogenesis of HUS.

Clinical features of anti-factor H autoantibody-associated hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a rare form of thrombotic microangiopathy that associates, in 70% of cases, with genetic or acquired disorders leading to dysregulation of the alternative pathway of complement. Autoantibody directed against Factor H causes at least 6% to 10% of aHUS cases, but only a few clinical reports are available. Here, we describe the clinical, biologic, genetic features, treatment, and outcome of 45 patients who presented with aHUS associated with anti-FH autoantibody. We found that this form of aHUS primarily affects children between 9 and 13 years old but it also affects adults. It presents with a high frequency of gastrointestinal symptoms and with extrarenal complications and has a relapsing course. Activation of the alternative pathway of complement at the onset of disease portends a poor prognosis. Early specific treatment may lead to favorable outcomes. These data should improve the recognition and diagnosis of this form of aHUS and help identify patients at high risk of a poor outcome.

Thrombotic microangiopathies: new insights and new challenges

PURPOSE OF REVIEW

Thrombotic microangiopathies (TMAs) manifest as a spectrum of related disorders in the form of thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). New data on both diseases support more and more the relatedness of the disorders and reveal related pathomechanisms, which, however, manifest in different organs. TTP develops primarily at neurological sites, and also in the kidney, and HUS is a kidney disease. In TTP thrombi formation occurs subsequently to the release of multimers of von Willebrand factor (vWF), and in HUS endothelial cell damage is considered the reason for complement and platelet activation leading to thrombus formation.

RECENT FINDINGS

Genetic mutations are associated with both disorders: in TTP the ADAMTS13 gene, the vWF cleaving protease, is affected, and in HUS several complement genes are mutated. In addition autoimmune forms, with acquired, de-novo generated inhibitors in the form of autoantibodies exist for both disorders, affecting ADAMTS13 in TTP or the central complement inhibitor factor H in HUS. In HUS autoantibodies can develop in the context of a specific mostly homozygous chromosomal deletion that represents a new subform of the disease, which is termed DEAP-HUS (deficient for CFHR proteins and autoantibody positive HUS).

SUMMARY

As the underlying disease mechanisms of TMA are now being better understood new options for a more precise diagnosis, improved therapy and prognosis for kidney transplantation become available for the benefit of patients. Here we summarize the recent developments in this rapidly progressing field.

DEAP-HUS: deficiency of CFHR plasma proteins and autoantibody-positive form of hemolytic uremic syndrome

DEAP-HUS [Deficiency of CFHR (complement factor H-related) plasma proteins and Autoantibody Positive form of Hemolytic Uremic Syndrome] represents a novel subtype of hemolytic uremic syndrome (HUS) with unique characteristics. It affects children and requires special clinical attention in terms of diagnosis and therapy. DEAP-HUS and other atypical forms of HUS share common features, such as microangiopathic hemolytic anemia, acute renal failure, and thrombocytopenia. However, DEAP-HUS has the unique combination of an acquired factor in the form of autoantibodies to the complement inhibitor Factor H and a genetic factor which, in most cases, is the chromosomal deletion of a 84-kbp fragment within human chromosome 1 that results in the absence of the CFHR1 and CFHR3 proteins in plasma. Special attention is required to diagnose and treat DEAP-HUS patients. Most patients show a favorable response to the reduction of autoantibody titers by either plasma therapy, steroid treatment, and/or immunosuppression. In addition, in those DEAP-HUS patients with end-stage renal disease, the reduction of autoantibody titers prior to transplantation is expected to prevent post-transplant disease recurrence by aiming for full complement control at the endothelial cell surface in order to minimize adverse complement and immune reactions.

Pulse cyclophosphamide therapy and clinical remission in atypical hemolytic uremic syndrome with anti-complement factor H autoantibodies

We report 3 children with atypical hemolytic uremic syndrome associated with anti-complement factor H (CFH) autoantibodies who presented with sustained remission with low antibody titers and normal kidney function after plasma exchanges (PEs) and cyclophosphamide pulses. The 3 children initially presented with acute vomiting, fatigue, gross hematuria, hypertension, hemolytic anemia, thrombocytopenia, nephrotic syndrome, and acute kidney injury. C3 levels were normal in patients 1 and 3 and low in patient 2 (0.376 mg/mL [0.376 g/L]). CFH antibody titers were increased (15,000 to > 32,000 arbitrary units [AU]). Patient 1, an 11-year-old boy, was treated with 12 PEs, leading to a decrease in CFH antibody titer (to 800 AU). A first relapse 1 month later was treated with 6 PEs and 4 rituximab infusions. A second relapse 3 months later required 5 PEs, and the patient received oral steroids (0.5 mg/d/kg body weight) and 5 cyclophosphamide pulses (1 g/1.73 m(2)), leading to sustained remission with normal kidney function (estimated glomerular filtration rate [eGFR], 120 mL/min/1.73 m(2) [2.0 mL/s/1.73 m(2)]) and a stable decrease in CFH antibody titer (to 2,000 AU) 3 years later. Patient 2, a 5-year-old boy, required dialysis therapy for 2 weeks. He received 3 plasma infusions without remission. Six PEs associated with 2 cyclophosphamide pulses (0.5 g/1.73 m(2)) and steroids (1 mg/d/kg body weight) led to rapid remission, with eGFR of 107 mL/min/1.73 m(2) [1.78 mL/s/1.73 m(2)] and a prolonged decrease in CFH antibody titer after 15 months (1,300 AU). Patient 3, a 16-month-old boy, was treated with oral steroids (1 mg/d/kg body weight), 2 PEs, and 2 cyclophosphamide pulses (0.5 g/1.73 m(2)), resulting in a stable decrease in CFH antibody titer to 276 AU. Kidney function quickly normalized (eGFR, 110 mL/min/1.73 m(2) [1.83 mL/s/1.73 m(2)]) and has remained normal after 14 months. All 3 patients show a homozygous deletion mutation of the CFHR1 and CFHR3 genes. Cyclophosphamide pulses with PE may lead to a prolonged decrease in CFH antibody titers and a favorable outcome of atypical hemolytic uremic syndrome and kidney function.