DiagnosisClinical DiagnosisHemolytic-uremic syndrome (HUS) is characterized by hemolytic anemia, thrombocytopenia, and renal failure caused by platelet thrombi in the microcirculation of the kidney and other organs.Typical (acquired) HUS is triggered by infectious agents such as strains of E. coli (Stx-E. coli) that produce powerful Shiga-like exotoxins and manifests with diarrhea (D⁺HUS), often bloody. However, approximately 25% of children with typical HUS do not have diarrhea. Typical HUS may subside when the underlying condition has been treated or removed. Typical HUS is not known to be associated with any genetic predisposition. Atypical HUS (aHUS) can be genetic, acquired, or idiopathic (of unknown cause). Individuals with aHUS frequently relapse even after complete recovery from the presenting episode; thus, aHUS is sometimes referred to as recurrent or relapsing HUS. Relapsing HUS is more likely to be genetic. The final outcome of aHUS is usually death or permanent renal or neurologic impairment. Atypical HUS is considered genetic in the following situations:Two or more members of the same family are affected by the disease at least six months apart and exposure to a common triggering infectious agent has been excluded;
OR A disease-causing mutation(s) is identified in one of the six genes known to be associated with aHUS, irrespective of familial history. Genetic atypical HUS can be multiplex (i.e., familial; two or more affected family members) or simplex (i.e., a single occurrence in a family).Atypical HUS is considered acquired when an underlying environmental factor such as drugs, systemic disease, viral agents, or bacterial agents that do not result in Shiga-like exotoxins (Stx) can be identified.Atypical HUS is considered idiopathic when no trigger (genetic or environmental) is evident.TestingLaboratory testing Typical and atypical HUS. The following are laboratory hallmarks of both typical HUS and atypical HUS: Thrombocytopenia that is usually severe Platelet count should be less than 150,000/mm³ to establish the diagnosis. In most cases, platelet counts are below 60,000/mm³. Platelet survival time is reduced, reflecting enhanced platelet disruption in the circulation. Giant platelets may be seen in the peripheral smear, a finding consistent with secondary activation of thrombocytopoiesis. Microangiopathic hemolytic anemia that is usually severe Hemoglobin concentrations lower than 10 mg/dL are reported in 99% of cases and lower than 6.5 mg/dL in 40% of cases. Serum lactate dehydrogenase (LDH) concentrations are increased (>460 U/L), often at very high levels, reflecting not only hemolysis, but also diffuse tissue ischemia. Hyperbilirubinemia (mainly unconjugated), reticulocytosis, circulating free hemoglobin, and low or undetectable haptoglobin concentrations are additional nonspecific indicators of accelerated red cell disruption and production. Detection of fragmented red blood cells (schistocytes) with the typical aspect of burr or helmet cells in the peripheral smear together with a negative Coombs test (with the exception of Streptococcus pneumoniae-associated HUS) are needed to confirm the microangiopathic nature of the hemolysis. Acute renal insufficiency Serum concentration of creatinine greater than 97th centile for age Serum concentration of urea (BUN) greater than 97th centile for age Typical HUS. The following are characteristic findings during acute illness in typical but not in atypical HUS: Shiga toxins in the stools (by the Vero cell assay)
AND/OR Serum antibodies against Shiga toxin (by enzyme-linked immunosorbent assay [ELISA]) and/or LPS (lipopolysaccharides) (O157, O26, O103, O111, and O145, by ELISA). The detection of free fecal STEC (Shiga toxin-producing E. coli) can be made by commercial immunoassays and requires only a few hours [Gianviti et al 2003]. Note: STEC isolation and detection of LPS antibodies are not routinely available and require a few days to complete.Complement studies Serum C3 and C4 concentrations can be used to monitor complement activation or dysregulation; however, these markers are not disease specific. Plasma concentrations of Bb (cleaved Factor B) can be used to evaluate the activation of the alternative pathway of complement. Plasma levels of the soluble membrane attack complex (sMAC) that includes sC5b-9 can be measured as an index of activation of the terminal complement cascade.CFH, CFI, and CFB serum concentrations and surface expression of membrane cofactor protein (MCP) (encoded by CD46) in peripheral leukocytes should also be evaluated as they may give an indication of the underlying genetic mechanism.Serum anti-CFH IgG autoantibodies . Five percent to 10% of individuals with aHUS have serum anti-CFH IgG autoantibodies even though plasma CFH antigen levels and analysis of CFH are normal. In about 90% of such individuals, deletion of the adjacent genes CFHR1 and CFHR3 were detected on both chromosomes. These genes encode factor H-related proteins 1 and 3, respectively, which share structural and functional similarities to factor H. The mechanism underlying the association between deletion of CFHR1 and CFHR3 and factor H antibody formation is not understood [Dragon-Durey et al 2004, Zipfel et al 2007]. Three affected individuals positive for anti-CFH autoantibodies were recently found to have no copies of CFHR1 and a single copy of CFHR3 rather than none and a novel deletion incorporating CFHR1 and CFHR4. Thus, these individuals had a CFHR3/CFHR1 deletion on one allele and a CFHR1/CFHR4 deletion on the other allele [Moore et al 2010].This finding suggests that the complete deficiency of factor H-related protein 1 is probably the significant factor associated with the production of factor H autoantibodies in aHUS.Renal histology (typical and atypical HUS). The common microvascular lesions of HUS consist of vessel (capillary and arteriole) wall thickening with endothelial swelling, which allows accumulation of proteins and cell debris in the subendothelial layer, creating a space between endothelial cells and the underlying basement membrane of affected microvessels. Both the widening of the subendothelial space and intraluminal platelet thrombi lead to a partial or complete obstruction of the vessel lumen. The partial occlusion of the lumen probably disrupts erythrocytes by mechanical trauma, which explains the hemolysis and presence of fragmented and distorted erythrocytes in the blood smear. In D⁺HUS the glomeruli are large; the capillaries are distended by red cells and platelet fibrin thrombi that may extend proximally into the afferent arteriole, suggesting that the thrombus is initiated in the glomerular capillaries themselves. Arterial lesions and mesangial changes are not reported, even long after the initial episode of D⁺HUS [Taylor et al 2004].In D^–HUS glomerular thrombosis, intracapillary foamy cells, endocapillary swelling, endocapillary hypercellularity, mesangiolysis, and doubled basement membranes are observed. Arterioles have thrombosis, endothelial swelling, or fibrinoid necrosis. Arteries have intimal swelling with various amounts of hypercellularity and thrombosis [Taylor et al 2004].Note: (1) In children younger than age two years the lesion is mainly confined to the glomerular tuft and is noted in an early phase of the disease. Glomerular capillary lumina are reduced or occluded. In patent glomerular capillaries packed with red blood cells and fibrin, thrombi occasionally are seen. (2) Examination of biopsies taken several months after disease onset shows that most glomeruli are normal; 20% eventually became sclerotic. (3) Arterial thrombosis does occur but is uncommon and appears to be a proximal extension of the glomerular lesion. (4) In the acute phase, tubular changes include foci of necrosis of proximal tubular cells and presence of red blood cells and eosinophilic casts in the lumina of distal tubules. Occasionally fragmented red blood cells can be detected in the distal tubular lumina. (5) In adults and older children, glomerular changes are different and more heterogeneous than in infants, and the classic pattern of thrombotic microangiopathy is less evident. Molecular Genetic TestingGenes. Evidence is emerging that 50%-60% of the atypical HUS (aHUS) is associated with genetically determined alterations of the complement system. Mutations have been found in the following genes [Noris & Remuzzi 2009, Noris et al 2010]: CFH (encoding complement factor H). Intragenic CFH mutations, mostly missense, account for 30% of aHUS (also known as aHUS1). Gene rearrangements that inactivate CFH: A CFH mutant allele with two mutations in cis configuration, p.[Ser1191Leu;Val1197Ala], has been occasionally reported in persons with aHUS. It has been demonstrated that this allele arose by gene conversion between CFH and CFHR1 (formerly known as CFHL1) [Heinen et al 2006] (see Molecular Genetics). A heterozygous CFH-CFHR1 hybrid allele was found in five persons with aHUS [Venables et al 2006] (see Molecular Genetics). The frequency of this heterozygous hybrid allele in aHUS is estimated to be approximately 3%-5% and it is detectable by deletion/duplication analysis CD46 (MCP) (encoding membrane cofactor protein). Mutations in CD46 account for an estimated 12% of aHUS (also known as aHUS2). In one child, complete paternal uniparental isodisomy of chromosome 1 with homozygosity for a splice defect of exon 10 resulted in severe deficiency of CD46 expression [Fremeaux-Bacchi et al 2007].CFI (encoding complement factor I). Mutations in CFI account for an estimated 5%-10% of aHUS (also known as aHUS3). CFB (encoding complement factor B). Mutations in CFB have been reported in affected individuals from two Spanish families [Goicoechea de Jorge et al 2007]. Both are gain-of-function mutations that result in either enhanced formation of C3bBb convertase or increased resistance to inactivation by complement regulators. In an US cohort CFB mutations accounted for 4% of aHUS [Maga et al 2010b] (also known as aHUS4).C3 (encoding the third component of complement C3). Mutations in C3 account for an estimated 5% of aHUS (also known as aHUS5). THBD (encoding the anticoagulant protein thrombomodulin). Mutations in THBD account for 3%-5% of aHUS [Delvaeye et al 2009] (also known as aHUS6).CFHR3, CFHR1, and CFHR4 are contiguous genes (in the order shown) in the regulators of complement activation (RCA) cluster (see Molecular Genetics). Deletion of either CFHR3 and CFHR1 or CFHR1 and CFHR4 is associated with atypical aHUS [Zipfel et al 2007, Dragon-Durey et al 2009, Moore et al 2010]. The frequency of the CFHR1 and CFHR3 deletion is 26.5% among individuals with aHUS [Moore et al 2010].The frequency of the CFHR1 and CFHR4 deletion is 1.4% among individuals with aHUS [Moore et al 2010]. Digenic inheritanceAbout 3% of affected individuals have mutations in two or even three of the following genes that encode complement: CFH and CD46 [Caprioli et al 2006, Richards et al 2007]; CD46 and CFI [Caprioli et al 2006, Esparza-Gordillo et al 2006]; CFH and CFI or CFH and THBD [Noris et al 2010]; and other combinations [Bienaime et al 2010]. A few affected individuals with homozygous deletion of CFHR1 also have a mutation in CFH, CFI, CD46, or C3. This suggests that multiple concurrent susceptibility factors are necessary in some individuals for aHUS to manifest [Moore et al 2010].Clinical testing Sequence analysis of the entire coding regions, including all exons and adjacent intronic sequences of CFH, CD46, CFI, CFB, C3, and THBD is possible (see Table 1). Deletion/duplication analysis of CFH can detect the CFH/CFHR1 hybrid allele and the CFHR3/CFHR1 and CFHR1/CFHR4 deletions (see Table 1).
SomNo deletions or duplications involving either CD46 or CFI as causative of aHUS have been reported. The clinical utility of such testing is unknown.Table 1. Summary of Molecular Genetic Testing Used in Atypical Hemolytic-Uremic SyndromeView in own windowGene Symbol (Phenotype Designation)Proportion of aHUS Attributed to Mutations in This GeneTest Method Mutations DetectedTest AvailabilityCFH (aHUS1)30% Sequence analysis Sequence variants ^1, 2 ClinicalDeletion / duplication analysis ^{3Exonic or whole-gene deletions3%-5% CFH/CFHR1 hybrid allele 4 CD46 (aHUS2)12% Sequence analysis Sequence variants 1 ClinicalDeletion / duplication analysis 3Exonic or whole-gene deletionsCFI (aHUS3)5%-10% Sequence analysisSequence variants 1 ClinicalDeletion / duplication analysis 3Exonic or whole-gene deletionsCFB (aHUS4)1%-4%Sequence analysisSequence variants 1 ClinicalC3 (aHUS5)5%Sequence analysisSequence variants 1ClinicalTHBD (aHUS6)3%-5%Sequence analysisSequence variants 1ClinicalCFHR3, CFHR15%-10%Deletion / duplication analysis 3Deletions involving CFHR3 and CFHR1 5ClinicalCFHR1, CFHR4Deletion / duplication analysis 3Deletions involving CFHR1 and CFHR4 6Research1. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected.2. Sequence analysis does not detect the CFH/CFHR1 hybrid allele that accounts for approximately 3%-5% of all aHUS.3. Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; included in the variety of methods that may be used are: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and chromosomal microarray (CMA) that includes this gene/chromosome segment. 4. This hybrid allele, resulting from crossing over between intron 21 of CFH and intron 4 of CFHR1, consists of the first 21 exons of CFH and the last two exons of CFHR1. Deletion analysis detects the hybrid allele, which is not detected by sequence analysis of CFH.5. The allele frequency of the CFHR1 and CFHR3 deletion is 26.5% [Moore et al 2010].6. The allele frequency of the CFHR1 and CFHR4 deletion is 1.4% among individuals with aHUS [Moore et al 2010].Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Testing StrategyTo confirm/establish the diagnosis of genetic aHUS in a probandMeasurement of the concentration of CFH and CFI by ELISA methods, measurement of CFB by nephelometry, and evaluation of CD46 protein expression on peripheral blood leukocytes by FACS; recommended in all affected individuals prior to molecular genetic testing because the results of these tests may provide insight into which gene is likely to be mutated.Molecular genetic testing of CFH, CD46, CFI, C3, the CFH/CFHR1 hybrid allele, THBD, and CFB (in the order shown) in all individuals in whom clinical evaluation and laboratory testing supports the diagnosis of aHUS
Note: (1) No one laboratory test is correlated with the presence or absence of a mutation in one of the genes encoding a complement factor. (2) If only one mutation is identified in CFH, CD46, CFI, and C3, complete sequencing of each gene should be considered to determine if a mutation is present in a second gene, a finding that supports the possibility of digenic inheritance.Search for CFH autoantibodies by ELISA in all affected individuals whether or not a mutation has been identified because CFH autoantibodies have been found in both instances.
Note: The CFHR3/CFHR1 deletion on both alleles is found in 90% of persons with CFH autoantibodies [Moore et al 2010]. Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Predictive testingPredictive testing for at-risk asymptomatic adult family members requires prior identification of the disease-causing mutations in the family.Prior to kidney transplantation, predictive testing is important because individuals with mutations in CFH, CFI, CFB, C3, and THBD tend to have recurrence of aHUS after renal transplantation [Bresin et al 2006, Noris et al 2010]. Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) DisordersCFHDense deposit disease / membranoproliferative glomerulonephritis type II (DDD/MPGN II). CFH mutations have been described in DDD/MPGN II: Ault et al [1997] reported a Native American boy with two heterozygous mutations in CFH. In four persons with MPGN II, Dragon-Durey et al [2004] identified three different homozygous mutations. Among 22 persons with biopsy-proven MPGN II, Abrera-Abeleda et al [2006] found an association between four SNPs in CFH and three SNPs in CFHR5. See also Differential Diagnosis.Glomerulonephritis with isolated C3 deposits (glomerulonephritis C3). CFH and CFI mutations have been described in glomerulonephritis C3 [Servais et al 2007]: Two persons had heterozygous mutations in CFH. One person had a heterozygous mutation in CFI. Age-related macular degeneration (AMD). CFH normal variants have been described in association with AMD in different cohorts [Thakkinstian et al 2006]. CD46No allelic disorders are knownCFI Complement factor I deficiency. CFI homozygous or compound heterozygous mutations have been described in persons with complement factor I deficiency [Baracho et al 2003, Vyse et al 1996].Glomerulonephritis with isolated C3 deposits (glomerulonephritis C3). CFH and CFI mutations have been described in glomerulonephritis C3 [Servais et al 2007]: Two persons had heterozygous mutations in CFH. One person had a heterozygous mutation in CFI. CFBCFB normal variants have been described in association with age-related macular degeneration (AMD) [Gold et al 2006, Maller et al 2006]. C3C3 deficiency. C3 homozygous mutations have been described in persons with C3 deficiency [Botto et al 1990, Tsukamoto et al 2005]. Botto et al [1992] also identified partial-gene deletion as the molecular basis of C3 deficiency in an Afrikaans patient. Age-related macular degeneration (AMD). C3 normal variants have been described in association with AMD [Maller et al 2007, Yates et al 2007, Bergeron-Sawitzke et al 2009].THBDThrombophilia due to thrombomodulin defect. THBD heterozygous mutations have been described in association with thombophilia [Ohlin & Marlar 1995, Faioni et al 2002].Myocardial infarction. THBD heterozygous mutations have been described in association with myocardial infarction [Doggen et al 1998, Kunz et al 2000].}

Natural HistoryAtypical HUS (aHUS) comprises genetic aHUS, acquired (sporadic) aHUS, and idiopathic (of unknown cause) aHUS.Onset of atypical HUS ranges from prenatal to adulthood [Constantinescu et al 2004, Taylor et al 2004, Sellier-Leclerc et al 2007, Noris et al 2010].Collectively, aHUS is associated with poor outcome. Fifty percent of acquired aHUS and 60% of genetic aHUS progresses to end-stage renal disease (ESRD) [Ruggenenti et al 2001, Caprioli et al 2003, Caprioli et al 2006, Noris et al 2010].Genetic (Multiplex and Simplex) Atypical HUSCurrently, genetic atypical HUS accounts for an estimated 60%-70% of all aHUS. Note: The remaining 30%-40% may also be genetic; however, causative mutations in other genes have not yet been identified. Genetic atypical HUS frequently relapses even after complete recovery following the presenting episode [Ruggenenti et al 2001, Taylor et al 2004], with death or permanent neurologic or renal sequelae being the final outcome in the majority of cases.It is likely that mutations in CFH, CD46, CFI, CFB, C3, and THBD confer a predisposition to develop aHUS, rather than directly causing the disease, and that a second mutational event in the remaining normal allele is required for full-blown manifestation of the disease. Conditions that trigger complement activation either directly (bacterial and viral infections or sepsis) or indirectly (drugs or certain systemic diseases that cause endothelial insult) may precipitate an acute event in those with the predisposing genetic background [Caprioli et al 2006, Noris et al 2010].Multiplex aHUS (i.e., more than one affected individual in the family) accounts for approximately 10% of all aHUS. Both autosomal dominant and autosomal recessive forms of aHUS have been noted.In autosomal recessive aHUS the onset is usually early in childhood. The prognosis is poor, with a mortality rate of 60%-70%. Episodes of aHUS recur frequently. In autosomal dominant aHUS the onset is usually in adulthood. The prognosis is poor, with a cumulative incidence of death or ESRD of 50%-90%. Sporadic (Acquired) aHUSTriggers for acquired aHUS include non-enteric bacterial infections, viruses, drugs, malignancies, transplantation, pregnancy, and other underlying medical conditions (scleroderma, anti-phospholipid syndrome, and systemic lupus erythematosus [SLE]). Triggering agents for acquired (sporadic) aHUS differ from those of typical HUS (see Differential Diagnosis, Distinguishing typical HUS from atypical HUS): Infection caused by Streptococcus pneumoniae accounts for 40% of aHUS and 5% of all causes of HUS in children in US. Neuroaminidase produced by Streptococcus pneumoniae removes sialic acids from the cell membranes, exposing Thomsen-Friedenreich antigen to preformed circulating IgM antibodies which bind to this neoantigen on platelet and endothelial cells and cause platelet aggregation and endothelial damage. The clinical picture is usually severe, with respiratory distress, neurologic involvement, and coma; the mortality rate is 12.3% [Copelovitch & Kaplan 2008]. Drugs that have been most frequently reported to induce aHUS include the following [Dlott et al 2004]: Anti-cancer agents (mitomycin, cisplatin, bleomycin, gemcitabine). The risk of developing aHUS after use of mitomycin is 2%-10%. The onset is delayed, occurring almost one year after starting treatment. The prognosis is poor, with up to 75% mortality at four months [Dlott et al 2004]. Immunotherapeutic agents (cyclosporine, tacrolimus, OKT3, interferon, and quinidine) Antiplatelet agents (ticlopidine, clopidogrel) A variety of common medications, including oral contraceptives and anti-inflammatory agents Cancer-associated aHUS complicates almost 6% of cases of metastatic carcinoma. Gastric cancer alone accounts for approximately half of such cases. Post-transplantation aHUS is being reported with increasing frequency [Ruggenenti et al 2001]. It may occur for the first time in individuals who have not experienced aHUS before (de novo post-transplantation aHUS) or may affect those whose primary cause of ESRD was aHUS (recurrent post-transplantation aHUS; see Management, Treatment of Manifestations). De novo post-transplantation aHUS may occur in individuals receiving transplants of kidneys or other organs because of the use of calcineurin inhibitors or because of humoral (C4b-positive) rejection. Post-renal transplantation aHUS occurs in 5%-15% of patients receiving cyclosporine and in approximately 1% of those receiving tacrolimus. Pregnancy-associated aHUS may occasionally develop as a complication of preeclampsia. Some women progress to a life-threatening variant of preeclampsia with severe thrombocytopenia, microangiopathic hemolytic anemia, renal failure, and liver involvement (HELLP syndrome). Complete remission usually follows prompt delivery. Post-partum aHUS usually manifests in women within three months of delivery. The outcome is usually poor with 50%-60% mortality; residual renal dysfunction and hypertension are the rule in those who survive the acute episode. Underlying medical conditions. Autoimmune disease is one of the underlying medical conditions; autoantibodies to CFH are present in an estimated 6%-10% of individuals [Dragon-Durey et al 2005, Jozsi et al 2007, Noris et al 2010].

Genotype-Phenotype CorrelationsThe phenotype of aHUS ranges from mild with complete recovery of renal function to severe resulting in ESRD or death [Noris & Remuzzi 2005]. Although genotype-phenotype correlations are not always straightforward, analysis of published reports reveals that the course and outcome of the disease are influenced by the gene in which mutations occur [Caprioli et al 2003, Neumann et al 2003, Noris & Remuzzi 2005, Caprioli et al 2006, Sellier-Leclerc et al 2007, Noris et al 2010].CFH. Atypical HUS associated with CFH mutations presents early in childhood in approximately 70% of affected individuals and in adulthood in approximately 30%. Irrespective of the pattern of inheritance, the clinical course is characterized by a high rate of relapse and a 60%-80% rate death or ESRD following the presenting episode or as a consequence of relapse. CD46. Atypical HUS associated with CD46 mutations presents mostly in childhood; the acute episode is in general milder than that associated with CFH mutations. Eighty percent of affected individuals experience complete remission. Recurrences are frequent but have little effect on long-term outcome; an estimated 60%-70% of individuals remain dialysis-free even after several recurrences. A subgroup of individuals, however, loses renal function either during the first episode or later in life. CFI. Atypical HUS associated with CFI mutations is variable. The onset is in childhood in 50% of affected individuals. Fifty-eight percent develop ESRD over the long term. CFB. Atypical HUS associated with CFB mutations is poorly understood, as few affected individuals have been reported. C3. Atypical HUS associated with C3 mutations presents in childhood in about 50% of individuals. More than 60% of affected individuals develop ESRD over the long term. THBD. Atypical HUS associated with THBD mutations presents in childhood in about 90% of individuals. More than 50% of patients develop ESRD over the long term.Digenic inheritance. About 3% of individuals have mutations in two or even three genes encoding complement regulatory proteins. Reports of digenic inheritance include the following:CFH and CD46 [Caprioli et al 2006, Richards et al 2007] CD46 and CFI [Caprioli et al 2006, Esparza-Gordillo et al 2006] CFH and CFI [Noris et al 2010] CFH and CFI, CFH and THBD, and CFI and C3 [Bienaime et al 2010, Noris et al 2010] These findings indicate that CFH, CD46, CFI, and C3 genetic variants could have an additive effect in determining the aHUS phenotype, since the proteins encoded by CFH, CD46, CFI, and C3 interact with each other to control complement activation on host cells.A few affected individuals with combined homozygous deletion of CFHR3 and CFHR1 and mutations in CFH, CFI, CD46, or C3 have also been reported [Moore et al 2010]. A number of common normal allelic variants described in the regulator of complement activation (RCA) cluster (see Molecular Genetic Pathogenesis) may predispose to aHUS both in individuals with CFH/CD46/CFI mutations and in those without identifiable mutations [Caprioli et al 2003, Esparza-Gordillo et al 2005, FH aHUS Mutation Database].Understanding of genotype-phenotype correlations could potentially optimize treatment (see Management).

Differential DiagnosisDistinguishing typical HUS from atypical HUS (aHUS). Typical HUS is triggered by infective agents such as certain strains of E. coli that produce the Shiga-like powerful exotoxins (Stx-E. coli). Typical HUS triggered by Stx-E. coli manifests with a prodrome of diarrhea (D⁺HUS), often bloody. However, approximately 25% of typical HUS is diarrhea negative. During an acute episode, identification of Shiga toxins in the stools (by the Vero cell assay) and/or serum antibodies against Shiga toxin (by enzyme-linked immunosorbent assay [ELISA]) and/or LPS (O157, O26, O103, O111, and O145, by ELISA) distinguishes typical HUS (D⁺HUS or D^–Stx⁺HUS) from aHUS (D^–Stx^–HUS).In its most common presentation, typical HUS manifests as an acute disease and 80%-90% of individuals recover without sequelae, either spontaneously (as in most cases of childhood typical HUS) or after plasma infusion or exchange (as in adult or severe forms of typical HUS) [Ruggenenti et al 2001].Typical HUS usually subsides when the underlying condition is treated or removed.Distinguishing atypical HUS from thrombotic thrombocytopenic purpura (TTP). Atypical HUS and TTP share a common pathologic lesion (thrombotic microangiopathy) but have different clinical manifestations. In aHUS the lesions and clinical symptoms are mainly localized in the kidney, whereas the pathologic changes of TTP are more extensively distributed, probably reflecting the systemic nature of the underlying defect. Clinically, TTP manifests mainly with central nervous system symptoms, but renal insufficiency has been reported. Approximately 80% of TTP is triggered by deficient activity of ADAMTS13, a plasma metalloprotease that cleaves von Willebrand factor (VWF) multimers soon after their secretion by endothelial cells. ADAMTS13 deficiency can be constitutive, as a result of homozygous or compound heterozygous mutations in ADAMTS13; or acquired, as a result of the presence of an inhibitory antibody. Evaluation of ADAMTS13 activity in serum or plasma is performed by several specialized laboratories using different tests based on the capability of the protease to cleave standard VWF multimers in vitro. One such test is the collagen binding assay. In large clinical studies, deficiency of ADAMTS13 activity was found in individuals with TTP but not in those with either typical or atypical HUS [Galbusera et al 2006]. This observation generated the hypothesis that TTP is caused by a deficiency of ADAMTS13 activity, whereas atypical HUS is unrelated to mutation of ADAMTS13.The exception to the above hypothesis occurs when ADAMTS13 and CFH mutations are observed in the same individual.Of two sisters with thrombotic microangiopathy, one presented with neurologic symptoms only and the other with superimposed severe renal impairment [Noris et al 2005]. Both had complete ADAMTS13 deficiency resulting from two heterozygous ADAMTS13 mutations; however, the sister who developed chronic renal failure also had a heterozygous CFH mutation that was not present in her sister, who had neurologic symptoms only. Thus, it was hypothesized that CFH haploinsufficiency had a role in determining the renal complications superimposed on the systemic disease caused by ADAMTS13 deficiency. Other affected individuals with both ADAMTS13 and CFH mutations have been reported [Zimmerhackl et al 2007]. Distinguishing aHUS from dense deposit disease / membranoproliferative glomerulonephritis type II (DDD/MPGN II). DDD/MPGN II typically occurs in children age five to 15 years who manifest with one of the following: hematuria, proteinuria, hematuria and proteinuria, acute nephritic syndrome, or nephrotic syndrome. DDD/MPGN II is associated with alternative pathway complement activation usually caused by C3 nephritic factors, IgG autoantibodies that stabilize the alternative C3 convertase (C3bBb). Diagnosis of DDD/MPGN II requires electron microscopy and immunofluorescence studies of a renal biopsy [Walker et al 2007]. Electron microscopy demonstrates dense transformation of the glomerular basement membrane (GBM) that occurs in a segmental, discontinuous, or diffuse pattern in the lamina densa. The precise composition of these altered areas remains unknown. Immunofluorescence should be positive for C3, usually in the absence of immunoglobulin deposition. Spontaneous remissions are uncommon in DDD/MPGN II. Approximately half of affected individuals develop ESRD within ten years of diagnosis. Other findings can include visual impairment late in the disease, acquired partial lipodystrophy, and other autoimmune diseases including diabetes mellitus type 1 and celiac disease. Mutations in CFH, C3, FHR5, and LMNA have been implicated in the pathogenesis of DDD/MPGN II.DDD/MPGN II has been reported in individuals with CFH deficiency. In contrast to individuals with aHUS, persons with DDD/MPGN II generally have homozygous or compound heterozygous CFH mutations that cause severely reduced CFH protein levels [Dragon-Durey et al 2004]. However, the rare cases of aHUS associated with homozygous mutations in CFH and very low levels of circulating CFH protein can blur the distinction between HUS and DDD/MPGN. Furthermore, this overlap in phenotypes is evident in those few individuals who have a mixed diagnosis of aHUS and DDD/MPGN in the same biopsy or in biopsies taken at different points in time [Bresin et al 2007].Distinguishing aHUS from cobalamin C (cblC) disease. Cobalamin C (cblC), caused by mutations in MMACHC, is characterized by abnormal vitamin B₁₂ metabolism, manifest as metabolic acidosis, methylmalonic aciduria, homocystinuria, hematologic abnormalities, and, on occasion, aHUS [Geraghty et al 1992, Van Hove et al 2002]. Inheritance is autosomal recessive [OMIM 277400]. See Disorders of Intracellular Cobalamin Metabolism.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

ManagementEvaluations Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with atypical hemolytic-uremic syndrome (aHUS), the following evaluations are recommended:Renal function Creatinine clearance (i.e., glomerular filtration rate [GFR]) Serum concentration of creatinine Urinalysis Hematologic status Platelet count Erythrocyte count Search for schistocytes in the blood smear Leukocyte count Other Serum LDH concentration Haptoglobin Serum C3 and C4 concentrations Plasma concentrations of Bb and sC5b-9Measure serum concentrations of CFH and CFI Assessment of CD46 expression on leukocytes Testing for CFH autoantibodies because affected individuals who have autoantibodies could benefit from an immunosuppressive therapy (see Treatment of Manifestations).Treatment of ManifestationsPlasma infusion or exchange. The mortality rate for aHUS dropped from 50% to 25% after plasma manipulation (plasma infusion or exchange) was introduced [Lara et al 1999]. A consistent number of individuals with aHUS respond to plasma treatment [Lara et al 1999, Caprioli et al 2006]. Debate continues as to whether plasma infusion and/or plasma exchange is effective in the treatment of acute episodes. It has been proposed that plasma exchange is more effective than plasma infusion because it removes potentially toxic substances from the blood; in one study the efficacy of plasma exchange was shown to be superior to that of plasma infusion [Ruggenenti et al 2001]. However, persons treated with plasma exchange were given larger amounts of plasma than those treated with plasma infusion alone; when equivalent volumes of plasma were given, the two treatments appeared to be equally effective. In situations that limit the amount of plasma that can be provided with infusion alone (e.g., renal insufficiency, heart failure), plasma exchange should be considered the therapy of choice [Ruggenenti et al 2001].Plasma exchange usually involves exchanging one plasma volume (40 mL/kg) per session. Treatment can be intensified by increasing the volume of plasma replaced. Twice-daily exchange of one plasma volume is probably the treatment of choice for those with refractory disease in order to minimize the recycling of infused plasma.Plasma infusion is the first-line therapy when plasma exchange is not available. In plasma infusion 30-40 mL/kg of plasma is administered initially, followed by 10-20 mL/kg/day. Plasma infusion should be used to treat or prevent recurrent episodes.Platelet count and serum LDH concentration are the most sensitive markers for monitoring response to plasma therapy. Plasma treatment should be continued until platelet count and serum LDH concentration remain normal after therapy is discontinued. Discontinuation of plasma therapy is the only way to know if complete remission has been achieved. Immediate exacerbation of disease activity, principally manifested by falling platelet count that requires the resumption of daily plasma therapy, occurs in 29%-82% of patients after treatment is discontinued. Thus, many cycles of stopping and resuming plasma therapy may be required.Genetic characterization of persons with aHUS has the potential to optimize the treatment: CFH. Plasma infusion or exchange has been used in patients with aHUS and CFH mutations with the rationale of providing normal CFH to compensate for the genetic deficiency, as CFH is a circulating plasma protein. In published studies, some patients with CFH mutations did not respond at all to plasma therapy and died or developed ESRD. Others required infusion of plasma at weekly intervals in order to raise CFH plasma levels enough to maintain remission [Landau et al 2001].
Stratton and Warwicker [2002] were able to induce sustained remission in a patient with a CFH mutation by three months of weekly plasma exchange in conjunction with intravenous immunoglobulins. One year after discontinuation of plasma therapy, the patient remained disease free and dialysis independent.
In the authors' series [Caprioli et al 2006, Noris et al 2010], approximately 60% of patients with CFH mutations treated with plasma underwent either complete or partial remission (hematologic normalization with renal sequelae). However, the remaining patients did not respond at all to plasma and 20% died during the acute episode. CD46. The rationale for using plasma in patients with CD46 mutations is not so clear, since the CD46 protein (also known as MCP) is a transmembrane protein and, theoretically, plasma infusion or plasma exchange would not compensate for the MCP defect. Published data indicate that the majority (80%-90%) of patients undergo remission following plasma infusion or exchange [Richards et al 2003, Caprioli et al 2006]; however, complete recovery from the acute episode was also observed in 100% of patients not treated with plasma [Noris et al 2010]. The decision whether or not to treat with plasma should be based on the clinical severity of the acute episode. CFI. Theoretically one should expect a good response to plasma therapy in patients with CFI mutations because CFI (like CFH) is a circulating protein; the results, however, suggest that a larger quantity of plasma is required to provide sufficient wild-type CFH or CFI to compensate for the genetic deficiency [Caprioli et al 2006]. Indeed, remission was achieved in only 25% of episodes treated with plasma in persons with CFI mutations [Noris et al 2010].CFB. Few data are available on response of individuals with CFB mutations to treatment with plasma. Only one of three treated individuals underwent remission [Goicoechea et al 2007].C3. Response to plasma treatment in persons with C3 mutations was comparable (57%) to that in persons with CFH mutations [Noris et al 2010]. It is hypothesized that plasma exchange could remove mutant hyperactive C3 and also provide regulatory plasma proteins to counteract complement activation induced by mutant C3.THBD. Plasma treatment induced disease remission in about 80% of acute episodes in persons with THBD mutations [Noris et al 2010]. CFH- autoantibodies. In individuals with anti-CFH autoantibodies, plasma treatment induced complete or partial remission (normalization of hematologic parameters with renal sequelae) of 75% of episodes [Noris et al 2010]. Persons with anti-CFH autoantibodies benefit from treatment with steroids or other immunosuppressants in conjunction with plasma exchange.Treatment with ACE inhibitors or angiotensin receptor antagonists helps to reduce renal disease progression to end-stage renal failure, while at the same time controlling blood pressure levels.Bilateral nephrectomy has been performed on rare occasion in a small number of individuals with extensive microvascular thrombosis at renal biopsy, refractory hypertension, and signs of hypertensive encephalopathy, in whom conventional therapies including plasma manipulation are not adequate to control the disease (i.e., persistent severe thrombocytopenia and hemolytic anemia). Follow-up has been excellent in some patients [Remuzzi et al 1996]. Plasma-resistant/plasma-dependent disease. Some patients with aHUS are plasma resistant (i.e., they do not achieve remission despite plasma therapy); some become plasma dependent, experiencing disease relapse as soon as plasma infusion or exchange is stopped. The following treatments are ineffective:Splenectomy, which induces remission in some persons with plasma-resistance, is ineffective and actually increased morbidity and mortality in others. Other treatments including antiplatelet agents, prostacyclin, heparin or fibrinolytic agents, steroids, and intravenous immunoglobulins have been attempted in both plasma resistance and plasma dependence with no consistent benefit [Ruggenenti et al 2001]. Renal transplantation is not necessarily an option for aHUS in contrast to typical HUS. An estimated 50% of individuals with aHUS who underwent renal transplantation had a recurrence of the disease in the grafted organ [Artz et al 2003, Noris & Remuzzi 2005]. Recurrences occur at a median time of 30 days after transplantation (range 0 days to 16 years). There is no effective treatment of recurrences. Reported recurrences have so far invariably ended with loss of the kidney [Ruggenenti et al 2001, Artz et al 2003]. Molecular genetic tests could help to define graft prognosis; thus, all patients should undergo such testing prior to transplantation.CFH. In patients with CFH mutations the graft outcome is poor. Recurrence rate ranges from 30% to 100% (depending on the survey) and is significantly higher than in patients without CFH mutations [Neumann et al 2003, Noris & Remuzzi 2005, Bresin et al 2006, Noris et al 2010]. As CFH is mainly produced by the liver, kidney transplantation does not correct the CFH genetic defect in these patients.
Simultaneous kidney and liver transplantation has been performed in two young children with aHUS and CFH mutations with the objective of correcting the genetic defect and preventing disease recurrence [Noris & Remuzzi 2005]. However, following transplantation both children experienced premature irreversible liver failure. The first child recovered after a second uneventful liver transplantation. This child, who had had monthly recurrences of aHUS before transplantation, had no symptoms of aHUS for more than two years following transplantation. The second child expired after primary non-function of the liver graft followed by multi-organ failure.
In six other patients with CFH mutations and in one child with digenic mutations (one in CFH and one in CFI) who received simultaneous kidney and liver transplantation [Saland et al 2006, Saland et al 2009, Noris et al 2010], good renal and liver function were recorded at two-year follow-up. In these individuals extensive plasma exchange was given prior to surgery to provide enough normal CFH to prevent damage to the liver graft.CD46. Kidney graft outcome is favorable in patients with CD46 mutations: four patients have been successfully transplanted with no disease recurrence [Noris & Remuzzi 2005, Noris et al 2010]. The strong theoretical rationale is that because the CD46 protein (MCP) is a transmembrane protein that is highly expressed in the kidney, transplantation of a kidney expressing normal MCP corrects the defect. CFI, C3, and CFB. As CFI, CFB, and C3 are plasma proteins, one could speculate that aHUS may recur in the transplanted kidney, resulting in graft failure. The few data available support this hypothesis as graft failures secondary to recurrences occurred in 70% patients with CFI mutations and in one patient with a CFB mutation. The percentage of graft failure was slightly lower (50%) in patients with C3 mutations [Noris et al 2010]. THBD. One patient with THBD mutations had disease recurrence in the kidney graft, which is unexpected because thrombomodulin, like CD46, is an endothelial transmembrane protein. However, a soluble thrombomodulin form circulates in plasma and has functional activities similar to those of membrane-bound thrombomodulin. It is possible that the grafts were not sufficiently protected against complement activation because of dysfunctional soluble thrombomodulin in persons with THBD mutations [Noris et al 2010].CFH autoantibodies. The transplant outcome is rather good in those patients: of five persons with anti-CFH autoantibodies, only one had disease recurrence and lost the graft [Noris & Remuzzi 2010].Prevention of Primary ManifestationsPlasma infusion or plasma exchange has been shown to induce remission of disease symptoms in 50%-90% of persons with aHUS depending on the underlying genetic defect (see Treatment of Manifestations). Plasma exchange prophylaxis has been shown to prevent disease recurrences in persons with mutation of CFH [Davin et al 2008]. SurveillanceIndividual with known aHUS. Measure serum concentration of hemoglobin, platelet count, and serum concentrations of creatinine, LDH, C3, and C4, and haptoglobin: Every month in the first year after an aHUS episode, then every three to six months in the following years, particularly for persons with normal renal function or chronic renal insufficiency as they are at risk for relapse. Note: Individuals with ESRD usually do not relapse. Every two weeks for those rare individuals with homozygous CFH mutations that result in very low or undetectable levels of the CFH protein Note: The proposed time intervals for checking hemoglobin, platelet count, and serum concentrations of creatinine, LDH, C3, C4, and haptoglobin are suggestions (based on the Authors' experience); each center may follow different guidelines based on their own experience.At-risk relative of an individual with aHUS Offer molecular genetic testing to at-risk family members of persons in whom disease-associated mutations have been identified. For relatives who are mutation-positive (i.e., have the family-specific mutation), monitor hemoglobin, platelet count, and serum concentrations of creatinine, LDH, C3 and C4, and haptoglobin) when exposed to potential triggering events such as severe infections, inflammation, and pregnancy. For relatives who are mutation-negative (i.e., do not have the family-specific mutation), no monitoring is needed. For family members of persons in whom disease-associated mutations have NOT been identified, no monitoring is needed. Agents/Circumstances to AvoidDiscontinue cyclosporine or tacrolimus when aHUS develops following challenge with the medication.Fresh frozen plasma should be avoided (i.e., plasma therapy is contraindicated) in persons with aHUS induced by Streptococcus pneumoniae because plasma from an adult contains antibodies against the Thomsen-Friedenreich antigen, which may exacerbate the disease. It is preferable to transfuse washed red blood cells or platelets. There is no evidence that plasmapheresis is of value [Copelovitch & Kaplan 2008]. Individual with known aHUS. An individual with known aHUS should avoid if possible the following known possible precipitants of aHUS, especially any that are known to have triggered aHUS (see Clinical Description, Sporadic aHUS). PregnancyDrugs: Some anti-cancer molecules, including mitomycin C, cisplatin, daunorubimicin, cytosine arabinoside Immunotherapeutic agents, including cyclosporin and tacrolimus Antiplatelet agents, including ticlopidine and clopidogrel Some common medications such as oral contraceptives, anti-inflammatory agents Unaffected mutation-positive relatives of an individual with aHUS should avoid known precipitants of aHUS (see Clinical Description, Sporadic aHUS. Evaluation of Relatives at RiskMolecular genetic testing should be offered to at-risk family members of persons in whom disease-predisposing mutations have been identified.Note: Testing family members needs to be done with caution because the family-specific mutations are predisposing (not causative) and, thus, are only one of several risk factors required for disease causation. Predictions based on a single risk factor in unaffected individuals are unreliable. From currently available data, the penetrance of disease for all mutations is approximately 50%. The degree of penetrance is thought to be determined by: (1) common normal allelic variants of CFH and CD46 [Caprioli et al 2003]; (2) risk haplotypes in RCA (see Molecular Genetic Pathogenesis) cluster [Esparza-Gordillo et al 2005]; and (3) exposure to environmental triggers (e.g., infection and drugs. Therefore, the risk cannot be quantified for a given individual. For relatives who are mutation positive (i.e., who have the family-specific predisposing mutation), the following are appropriate:Monitoring when exposed to potential triggering events such as severe infections, inflammation, and pregnancy (see Surveillance) Avoiding known precipitants of aHUS (see Clinical Description, Sporadic aHUS) See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationResearch efforts are aimed at identifying more specific approaches that may interfere with the primary cause of microangiopathy in the different forms of aHUS.Eculizumab, a human anti-C5 monoclonal antibody which has been registered for the treatment of paroxysmal nocturnal hemoglobinuria [Brodsky et al 2008], effectively induced remission of acute episodes of aHUS refractory to plasma therapy [Gruppo & Rother 2009].
In 17 adults and adolescents with plasma resistant forms of aHUS (see ClinicalTrials.gov: NCT00844844 and NCT00844545), the preliminary analysis of Phase II trials showed that early and continuing treatment with eculizumab stopped the systemic thrombotic microangiopathy in 65% of cases. Five out of seven persons on dialysis showed improved renal function and were able to stop dialysis. No severe adverse effects or deaths were reported.
In 20 adults and adolescents with plasma-dependent aHUS (see ClinicalTrials.gov: NCT00844428 and NCT00838513), the analysis of Phase II trials showed that treatment with eculizumab stopped the systemic thrombotic microangiopathy and stabilized renal function, allowing them to discontinue plasma therapy. No severe adverse effects or deaths were reported.
Trials of eculizumab in adults with aHUS and in chidren with aHUS who are newly diagnosed, have previously been diagnosed, or are post-kidney transplantation are ongoing. See ClinicalTrials.gov: NCT01193348 and NCT01194973).For aHUS associated with CFH mutations:Specific replacement therapies with recombinant CFH protein could become a viable alternative to plasma treatment. Efforts are also ongoing to isolate plasma fractions enriched in CFH protein that could provide the patient with sufficient active molecules while minimizing the risk of allergy and fluid overload associated with standard plasma infusion therapy. A CFH protein concentrate under development for clinical use has recently achieved Orphan Drug designation by European Medicines Agency (EMEA) and FDA.The discovery of mutations in three different complement-regulatory genes provides sufficient evidence to undertake clinical trials using complement inhibitors that block the activation of C3 [Kirschfink 2001]. Studies on other complement-regulatory genes would help to clarify the molecular determinants underlying the pathogenesis of aHUS and potentially improve management and therapy. Advances in vector safety and transfection efficiency may eventually make gene therapy a realistic option.Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Atypical Hemolytic-Uremic Syndrome: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDCFH1q31​.3Complement factor HResource of Asian Primary Immunodeficiency Diseases (RAPID)CFHCD461q32​.2Membrane cofactor proteinCD46 homepage - Mendelian genesCD46CFI4q25Complement factor IResource of Asian Primary Immunodeficiency Diseases (RAPID)
CFI homepage - Mendelian genesCFICFB6p21​.33Complement factor BCFB homepage - Mendelian genesCFBC319p13​.3Complement C3Resource of Asian Primary Immunodeficiency Diseases (RAPID)
C3 homepage - Mendelian genesC3THBD20p11​.21ThrombomodulinTHBD homepage - Mendelian genesTHBDCFHR31q31​.3Complement factor H-related protein 3CFHR3 homepage - Mendelian genesCFHR3CFHR11q31​.3Complement factor H-related protein 1CFHR1 homepage - Mendelian genesCFHR1CFHR41q31​.3Complement factor H-related protein 4 CFHR4Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Atypical Hemolytic-Uremic Syndrome (View All in OMIM) View in own window 120700COMPLEMENT COMPONENT 3; C3 120920CD46 ANTIGEN; CD46 134370COMPLEMENT FACTOR H; CFH 134371COMPLEMENT FACTOR H-RELATED 1; CFHR1 138470COMPLEMENT FACTOR B; CFB 188040THROMBOMODULIN; THBD 217030COMPLEMENT FACTOR I; CFI 235400HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 1; AHUS1 605336COMPLEMENT FACTOR H-RELATED 3; CFHR3 605337COMPLEMENT FACTOR H-RELATED 4; CFHR4 612922HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 2; AHUS2 612923HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 3; AHUS3 612924HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 4; AHUS4 612925HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 5; AHUS5 612926HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 6; AHUS6Molecular Genetic PathogenesisIn 1998 Warwicker et al studied three families with aHUS and established linkage in the affected individuals to the regulator of complement activation (RCA) gene cluster on human chromosome 1q32, which encodes two complement-regulatory proteins (CFH [Warwicker et al 1998] and CD46 [Noris et al 2003, Richards et al 2003]) and five factor H-related proteins (CFHR1-5 [Monteferrante et al 2007, Zipfel et al 2007, Moore at al 2010]).Because an association between familial HUS and CFH abnormalities had been reported previously, the first examined candidate gene in this region was factor H (CFH). CFH is a plasma glycoprotein that plays an important role in the regulation of the alternative pathway of complement. It serves as a cofactor for the C3b-cleaving enzyme, factor I (encoded by CFI) in the degradation of newly formed C3b molecules and controls decay, formation, and stability of the C3b convertase C3bBb. The CFH glycoprotein consists of 20 homologous short consensus repeats (SCRs). The complement-regulatory domains needed to prevent fluid phase alternative pathway amplification have been localized within the N-terminal SCR1-4 [Rodriguez de Cordoba et al 2004].The inactivation of surface-bound C3b is dependent on the binding of the C-terminal domain of CFH protein to polyanionic molecules that increases CFH protein affinity for C3b and exposes its complement-regulatory N-terminal domain. The C-terminal domain contains two C3b-binding sites, located in SCR12-14 and SCR19-20, and three polyanion-binding sites, located in SCR7, SCR13, and SCR19-20 [Jozsi et al 2004]. However, the C3b- and the polyanion-binding sites located in SCR19-20 are required for CFH to inactivate surface-bound C3b, since deletion of this portion of the molecule renders CFH protein incapable of blocking complement activation on sheep erythrocytes.Abnormalities in two additional genes encoding for complement-regulatory proteins have been recently involved in predisposition to aHUS. Two independent reports described mutations in CD46, encoding membrane cofactor protein (MCP), in affected individuals of four families [Noris et al 2003, Richards et al 2003]. MCP is a widely expressed transmembrane glycoprotein that serves as a cofactor for CFI protein to cleave C3b and C4b deposited on the host cell surface [Goodship et al 2004]. MCP has four extracellular N-terminal SCRs important for their inhibitory activity, followed by a serine-threonine-proline rich domain, a transmembrane domain, and a cytoplasmic tail. To date, about 50 CD46 mutations in aHUS have been reported, with a mutation frequency of 10%-15% among all aHUS [FH aHUS Mutation Database]. Evaluation of mutant protein expression and function showed either severely reduced protein expression on the cell surface or reduced C3b-binding capability and/or capacity to block complement activation [Caprioli et al 2006, Noris et al 2010].About 30 mutations in CFI, which encodes a plasma serine protease that cleaves and inactivates C3b and C4b, have been reported in individuals with aHUS, with a frequency of 5%-10% depending on the study [Fremeaux-Bacchi et al 2004, Kavanagh et al 2005, Caprioli et al 2006, Noris et al 2010]. All are heterozygous mutations, 80% cluster in the serine-protease domain and may either cause reduced protein secretion or result in mutant proteins with decreased cofactor activity. However, studies on the p.Gly261Asp mutation revealed no alteration of CFI serum concentration or functional defect in CFI [Nilsson et al 2007].Gain-of-function mutations in the gene encoding complement factor B (CFB), a zymogen that carries the catalytic site of the complement alternative pathway convertase, have been found in two families from a Spanish HUS cohort [Goicoechea de Jorge et al 2007]. Mutants have excess C3b affinity and form a hyperactive C3 convertase that is resistant to dissociation, enhancing C3b formation. About 5% of persons have heterozygous mutations in C3, usually with low C3 levels [Fremeaux-Bacchi et al 2008, Noris et al 2010]. Most mutations reduce C3b binding to CFH and MCP, which severely impairs degradation of mutant C3b. More recently, about 5% of persons with aHUS have been found to have heterozygous mutations in THBD [Noris et al 2010, Delvaeye 2009]. Cells expressing these variants are less efficient in degrading C3b and in generating activated thrombin-activatable fibrinolysis inhibitor (TAFI), a plasma carboxypeptidase B that cleaves C3a and C5a. Genetic variants of CFHR5 that have been identified may play a secondary role in the pathogenesis of HUS [Monteferrante et al 2007].Complete absence of both CFHR1 and CFHR3 proteins was detected in about 10% of aHUS [Zipfel et al 2007, Moore at al 2010]. CHR1/CFHR3 plasma deficiency is associated in the large majority of cases with the formation of anti-CFH autoantibodies. These antibodies bind both to CFHR1 and to the CFH C-terminal [Moore et al 2010], reduce CFH binding to C3b and enhance alternative pathway dependent lysis of sheep erythrocytes without influencing fluid-phase cofactor activity. The causal link between CFHR1-R3 deletion and CFH autoantibodies is still unknown. More recently, three affected individuals positive for anti-CFH autoantibodies were found to have no copies of CFHR1 but a single copy of CFHR3 and a novel deletion incorporating CFHR1 and CFHR4. In these individuals a CFHR3/CFHR1 deletion was present on one allele and a CFHR1/CFHR4 deletion on the other allele, suggesting that the complete deficiency of factor H-related protein 1 was probably the significant factor associated with the production of factor H autoantibodies in aHUS [Moore et al 2010].CFH Normal allelic variants. CFH is approximately 100 kb long. It comprises 23 exons. Pathologic allelic variants. See Table 2. Table 2. Selected CFH Pathologic Allelic Variants View in own windowDNA Nucleotide ChangeProtein Amino Acid ChangeReference Sequencesc.3572C>Tp.Ser1191LeuNM_000186​.2
NP_000177​.2 c.3590T>Cp.Val1197AlaCFH and CFHR1 hybrid alleleSee Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org). The list of published and unpublished mutations within CFH is continuously updated in the FH aHUS Mutation Database. Since the first report by Warwicker et al [1998], a number of studies have been performed, describing more than 100 different CFH mutations in individuals with aHUS [Saunders et al 2006]. The vast majority of CFH mutations in individuals with aHUS are heterozygous and cause either single amino acid changes or premature translation terminations that primarily cluster in the C-terminus domains and are commonly associated with normal CFH protein plasma levels. A minority of the mutations result in the production of a truncated protein or impaired secretion of protein [Perez-Caballero et al 2001, Richards et al 2001, Caprioli et al 2006].A heterozygous hybrid allele of CFH and CFHR1, derived from a crossing over between intron 21 of CFH and intron 4 of CFHR1 (CFH-related 1), was found in five persons with aHUS [Venables et al 2006]. The hybrid allele consists of the first 21 exons of CFH (encoding short consensus repeats [SCRs] 1-18 of CFH) and the last two exons of CFHR1 (encoding SCR4 and SCR5 of CFHR1). The frequency of this heterozygous hybrid allele in aHUS is estimated to be approximately 3%-5%. A novel heterozygous hybrid allele of CFH and CFHR1, derived from a crossing over between intron 22 of CFH and intron 5 of CFHR1 has been recently found in two persons with aHUS [Maga et al 2010a]. The novel hybrid allele consists of the first 22 exons of CFH (encoding SCRs 1-19) and the last exon of CFHR1 (encoding SCR5 of CFHR1). The frequency of this novel heterozygous hybrid allele in aHUS is estimated to be 1.5% [Maga et al 2010a].Because CFH exon 22 (SCR19) and CFHR1 exon 5 (SCR4) encode identical proteins, the deletion found by Venables and the deletion found by Maga produce identical fusion proteins despite different non-homologous allelic recombination (NHAR) sites.Normal gene product. Mainly synthesized by the liver, the complement factor H (CFH) protein is a 150-kd single-chain plasma glycoprotein and consists of 20 homologous structural domains called SCRs (short consensus repeats), each of which comprises approximately 60 amino acids. Abnormal gene product. Expression and functional studies demonstrated that CFH proteins with aHUS-associated mutations (deriving from point mutations, gene conversion, and a hybrid allele) have a severely reduced ability to interact with polyanions and with surface-bound C3b [Jozsi et al 2004], resulting in a lower density of mutant CFH molecules bound to endothelial cell surface and a diminished complement-regulatory activity on the cell membrane [Jozsi et al 2004]. In contrast these mutants have a normal capacity to control activation of the complement in plasma, as indicated by their retention of normal cofactor activity in the proteolysis of fluid-phase C3b. The majority of CFH mutations are heterozygous and cluster in the exons that encode for the C-terminal portion of the protein. A minority of the mutations result in the production of a truncated protein or impaired secretion of protein [Perez-Caballero et al 2001, Richards et al 2001, Caprioli et al 2006].Interestingly, the protein product of the hybrid CFH/CFHR1 is identical to another CFH mutant allele with two mutations in cis configuration, p.[Ser1191Leu;Val1197Ala], which arises by gene conversion between CFH and CFHR1 and whose protein product lacks surface complement-regulatory activity. See Molecular Genetic Pathogenesis.CD46 Normal allelic variants. CD46 is an estimated 43 kb long. It comprises 14 exons. Pathologic allelic variants. The majority of CD46 mutations are heterozygous and cluster in the exons encoding the four N-terminal extracellular short consensus repeats (SCRs). No deletions or duplications involving either CD46 or CFI as causative of aHUS have been reported. The list of published and unpublished mutations within CD46 is continuously updated in the FH aHUS Mutation Database.Normal gene product. CD46 encodes the membrane cofactor protein (MCP), with is a widely expressed transmembrane glycoprotein composed of four extracellular SCRs followed by a serine-threonine-proline rich region, a transmembrane domain, and a cytoplasmic tail. Abnormal gene product. CD46 mutations generally result in either reduced MCP expression or impaired C3b binding capability [Noris et al 2003, Richards et al 2003, Caprioli et al 2006]. See Molecular Genetic Pathogenesis. CFI Normal allelic variants. CFI is approximately 63 kb long. It comprises 13 exons. Pathologic allelic variants. See Table 3. The majority of CFI mutations cluster in the exons that encode the serine-protease domain. No deletions or duplications involving either CD46 or CFI as causative of aHUS have been reported.Table 3. Selected CFI Pathologic Allelic Variants View in own windowDNA Nucleotide ChangeProtein Amino Acid ChangeReference Sequencesc.782G>Ap.Gly261AspNM_000204​.2
NP_000195​.2 See Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org).The list of published and unpublished mutations within CFI is continuously updated in the FH aHUS Mutation Database.Normal gene product. Mainly produced by the liver, complement factor I (CFI) protein is a 88-kd plasma serine-protease with a modular structure. It is a heterodimer and consists of a non-catalytic 50-kd heavy chain linked to a catalytic 38-kd light chain by a disulphide bond. Abnormal gene product. Approximately 60% of the mutations result in low CFI levels or low CFI activity, the functional significance of the others remains to be determined [Fremeaux-Bacchi et al 2004, Caprioli et al 2006]. See Molecular Genetic Pathogenesis. CFB Normal allelic variants. CFB is an estimated 6 kb long. It comprises 18 exons. Pathologic allelic variants. See Table 4.Table 4. Selected CFB Pathologic Allelic Variants View in own windowDNA Nucleotide ChangeProtein Amino Acid ChangeReference Sequencesc.858C>Gp.Phe286LeuNM_001701​.2
NP_001701​.2 c.967A>Gp.Lys323GluSee Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org).Two heterozygous mutations in affected members of two Spanish pedigrees, c.858C>G and c.967A>G, have been reported [Goicoechea de Jorge et al 2007]. Normal gene product. CFB encodes complement factor B, a 90-kd protein consisting of three domains: a three-module complement control protein, a von Willebrand factor A domain, and a C-terminal serine protease domain that adopts a default inactive (zymogen) conformation. Abnormal gene product. In affected members of two Spanish pedigrees, two gain-of-function heterozygous mutations, p.Phe286Leu and p.Lys323Glu, were found to result in enhanced formation of the C3bBb convertase and increased resistance to inactivation by complement regulators, respectively [Goicoechea de Jorge et al 2007]. C3Normal allelic variants. C3 is an estimated 42.8 kb long. It comprises 41 exons. Pathologic allelic variants. The majority of C3 mutations are heterozygous. Mutations are spread all over the gene; however, a hot spot is evident in the thioester-containing (TED) domain.Normal gene product. C3 encodes complement component C3. Mainly produced by the liver, C3 is the pivotal component of the complement system. The mature protein is made by 1641 amino acids that form a beta chain and an alpha chain. C3 is processed by proteolytic cleavage by enzymatic complexes, the C3 convertases, into the anaphylatoxin C3a and the C3b fragment that deposits on cell surfaces causing the activation of the complement cascade. The alpha and beta chains form 13 domains including eight homologous domains called macroglobulin (MG) domains, the linker domain (LNK), the anaphylatoxin (ANA) domain, the CUB domain, the thioester-containing (TED) domain, and finally the C345c domain.Abnormal gene product. Most mutations reduce C3b binding to CFH and MCP, which severely impairs degradation of mutant C3b [Fremeaux-Bacchi 2008]See Molecular Genetic Pathogenesis.THBDNormal allelic variants. THBD is approximately 4.03 kb long; it comprises a single exon. Pathologic allelic variants. All THBD mutations are heterozygous and cluster in the lectin-like domain and in the serine-threonine (ST)-rich peptide. Normal gene product. Thrombomodulin is a 557-amino-acid endothelial glycoprotein that is anchored to the cell by a short cytoplasmic tail and a single transmembrane domain that is followed by a ST-rich domain. A series of six epidermal growth factor-like repeats are required for thrombin-mediated generation of activated protein C, which has anticoagulant and cytoprotective properties, and the generation of activated TAFI, which has C3a degrading and C5a-degrading properties. Farthest from the transmembrane domain is the lectin-like domain, which confers resistance to proinflammatory stimuli, including endotoxin. Thrombomodulin facilitates complement inactivation by CFI in the presence of CFH.Abnormal gene product. Cells expressing these variants are less efficient in degrading C3b and in generating activated TAFI, a plasma carboxypeptidase B that cleaves C3a and C5a [Delvaeye et al 2009]. CFHR3, CFHR1, and CFHR4Normal allelic variants. CFHR3, CFHR1, and CFHR4 (previously known as CFHL3, CFHL1, and CFHL4, respectively) are contiguous genes that occur in this relative order on chromosome 1 at 1q31-q32.1. These genes are in the regulators of complement activation (RCA) cluster (see Molecular Genetic Pathogenesis). Each comprises six exons; reference sequences are NM_021023.5, NM_002113.2, and NM_006684.4.Pathologic allelic variants. Deletion involving CFHR3 and CFHR1 or CFHR1 and CFHR4 is associated with atypical aHUS [Moore et al 2010, Zipfel et al 2007]. Deletions occur by non-allelic homologous recombination between regions of high sequence identity [Moore et al 2010]. Normal gene product. Like the human complement factor H (CFH), the factor H-related proteins, CFHR3, CFHR1, and CFHR4 are exclusively composed of highly-related short consensus repeats (SCRs), each of which contains four cysteine residues and additional conserved amino acids.CFHR3 encodes a secreted protein which belongs to the complement factor H-related protein family.CFHR1 encodes a secreted protein which belongs to the complement factor H protein family. It binds to Pseudomonas aeruginosa elongation factor Tuf together with plasminogen, which is proteolytically activated.Both CFHR3 and CFHR4 enhance the cofactor activity of factor H in C3b inactivation.Abnormal gene product. Individuals with aHUS who are homozygous for these alleles typically have factor H autoantibodies [Moore et al 2010].