Atypical Hemolytic-Uremic Syndrome

[Familial Hemolytic-Uremic Syndrome, Hereditary Hemolytic-Uremic Syndrome, aHUS, Atypical HUS]

Clinical Diagnosis

Hemolytic-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 four 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.

Testing

Laboratory 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 less than 10 mg/dL are reported in 99% of cases and less 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 disregulation; however, these markers are not disease specific.

• CFH, CFI, and CFB serum concentrations should also be evaluated as they may give an indication of the underlying genetic background.

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 Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Genes. 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:

• CFH (encoding complement factor H). Mutations in CFH account for 30% of aHUS.

• CD46(MCP) (encoding membrane cofactor protein). Mutations in CD46 account for an estimated 12% of aHUS.

• CFI (encoding complement factor I). Mutations in CFI account for an estimated 5%-10% of aHUS.

• CFB (encoding complement factor B). Mutations in CFB have been reported in affected individuals from two Spanish families [Goicoechea de Jorge et al 2007].

Digenic inheritance. A few individuals have been described with variants in both CFH and CD46 [Caprioli et al 2006, Richards et al 2007] and in both CD46 and CFI [Caprioli et al 2006, Esparza-Gordillo et al 2006].

Uniparental disomy (UPD). In one child, complete paternal uniparental isodisomy of chromosome 1 resulted in severe deficiency of MCP expression. UPD was identified after homozygosity for a novel mutation (IVS10+2T>C) in the splice-donor of exon 10 was detected in an affected child and heterozygosity for the same mutation was found in her unaffected father, but not in her unaffected mother [Fremeaux-Bacchi et al 2007].

Gene conversion between CFH and CFHL1. A CFH mutant allele with two mutations in cis, p.[Ser1191Leu;Val1197Ala], has been occasionally reported in persons with aHUS. Gene conversion between CFH and CFHL1 has been clearly demonstrated [Heinen et al 2006].

Clinical testing

• Sequencing of the entire coding regions, including all exons and adjacent intronic sequences of CFH, CD46, and CFI. Note: Although these tests are not routinely available around the world, they are of particular importance prior to transplantation [Bresin et al 2006] (see Management).

• Deletion analysis by multiplex ligation-dependent probe amplification (MLPA) for selected exons of CFH. A heterozygous hybrid allele derived from a crossing over between intron 21 of the gene CFH and intron 4 of the gene 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 6%.
  
  MLPA may be used to detect the hybrid CFH/CFHR1 gene, as it is not detected by sequence analysis of CFH. Interestingly, the protein product of the hybrid CFH/CFHR1 gene is identical to another CFH mutant allele with two mutations in cis, p.[Ser1191Leu;Val1197Ala], which arises by gene conversion between CFH and CFHR1 and whose protein product lacks surface complement-regulatory activity.

• Deletion/duplication analysis of CD46 and CFI genes is available clinically, but the mutation detection frequencies are unknown and may be quite low.

• Sequencing of the entire coding region of CFB, including all exons and adjacent intronic sequences of CFB. Two CFB mutations associated with aHUS have recently been reported. Both are gain-of-function mutations that result in either enhanced formation of C3bBb convertase or increased resistance to inactivation by complement regulators [Goicoechea de Jorge et al 2007].

Table 1 summarizes molecular genetic testing for this disorder.

Table 1. Molecular Genetic Testing Used in Atypical Hemolytic-Uremic Syndrome

Gene Symbol	Test Method	Mutations Detected	Proportion of aHUS Attributed to Mutations in This Gene	Mutation Detection Frequency by Gene and Test Method	Test Availability
CFH	Sequence analysis	Sequence variants 1	30%	99% 2	Clinical
CFH/CFHR1 hybrid allele	Deletion analysis	CFH/CFHR1 hybrid allele 3	6%	100%
CD46	Sequence analysis	Sequence variants 1	12%	99%	Clinical
	Deletion/duplication analysis 4	Deletion or duplication of partial or whole gene	Unknown 5	Unknown 5
CFI	Sequence analysis	Sequence variants 1	5%-10%	99%	Clinical
	Deletion/duplication analysis 4	Deletion or duplication of partial or whole gene	Unknown 5	Unknown 5
CFB	Sequence analysis	Sequence variants 1	Unknown	99%	Clinical

1. Proportion of affected individuals with a mutation(s) as classified by gene and test method

1. Nonsense mutations, missense mutations, frameshift mutations, splicing mutations, small deletions or insertions.

2. Sequence analysis does not detect the CFH/CFHR1 hybrid allele that accounts for approximately 6% of aHUS.

3. 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.

4. Testing that identifies deletions/duplications not detectable by sequence analysis of genomic DNA; a variety of methods including quantitative PCR, real-time PCR, multiplex ligation dependent probe amplification (MLPA), and array CGH (see ) may be used.

5. No deletions or duplications involving either the CD46 or CFI gene as causative of aHUS disorder have been reported. Therefore, the mutation detection rate is unknown and may be very low. However, newly available deletion/duplication testing methods may identify mutations in affected individuals who tested negative by sequence analysis.

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

To confirm the diagnosis of genetic aHUS in a proband

• Molecular genetic testing of CFH, CD46, CFI, and CFB in all individuals in whom clinical evaluation and laboratory testing supports the diagnosis of aHUS, because no one laboratory test is correlated with the presence or absence of a mutation in one of the genes encoding a complement factor.

• CFH and CFI circulating concentration measurement by ELISA methods and evaluation of CD46 protein expression on peripheral blood leukocytes by FACS are recommended in all affected individuals.

• Search for CFH autoantibodies in individuals in whom no CFH, CD46, and CFI mutation is identified as CFH autoantibodies have only been found in individuals who do not have an identifiable mutation in one of the genes encoding a complement factor.

• Testing strategy may need to consider reports of digenic inheritance. Complete sequencing of CFH, CD46, and CFI should be performed on all affected individuals.

Prognostication. Molecular genetic testing is particularly important prior to transplantation [Bresin et al 2006] as individuals with mutations in CFH or CFI tend to have recurrence of aHUS after renal transplantation (see Management, Other).

Predictive testing for at-risk asymptomatic family members requires prior identification of the disease-causing mutation in the family.

Prenatal diagnosis for at-risk pregnancies requires prior identification of the disease-causing mutations (or mutation) in the family.

Genetically Related (Allelic) Disorders

Dense 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].

Natural History

Atypical 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].

Collectively, aHUS is associated with poor outcome. Fifty percent of acquired aHUS and 60% of genetic aHUS progresses to ESRD [Ruggenenti et al 2001, Caprioli et al 2003, Caprioli et al 2006].

Genotype-Phenotype Correlations

The phenotype of aHUS is variable ranging from mild forms, with complete recovery of renal function, to severe forms, with end-stage renal failure (ESRD) or death being the final outcome [Noris & Remuzzi 2005]. Although genotype-phenotype correlations are not always straightforward, analysis of published reports [Caprioli et al 2003, Neumann et al 2003, Noris & Remuzzi 2005, Caprioli et al 2006, Sellier-Leclerc et al 2007] reveals that the course and outcome of the disease are influenced by the gene involved.

CFH. Atypical HUS associated with CFH mutations presents early in childhood in approximately 70% of cases and in adulthood in approximately 30% of cases. Irrespective of the pattern of inheritance and location of the mutations, 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, lose renal function either during the first episode or later in life.

CFI. Atypical HUS associated with CFI mutations is variable in phenotype. 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 with CFB mutations have been reported.

Digenic inheritance

CFH and CD46 or CD46 and CFI. A few individuals have been identified with variants in both CFH and CD46 [Caprioli et al 2006, Richards et al 2007] and in both CD46 and CFI [Caprioli et al 2006, Esparza-Gordillo et al 2006] indicating that CFH, CD46, and CFI genetic variants could have an additive effect in determining the aHUS phenotype, since the proteins encoded by CFH, CD46, and CFI interact with each other to control complement activation on host cells.

Moreover, a number of common polymorphic variants described in the RCA (regulator of complement activation) 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].

Genotype-phenotype correlations could potentially optimize treatment (see Management, Other).

Penetrance

The incomplete penetrance of HUS often found in those with CFH and CD46 mutations indicates that such mutations confer a predisposition to develop aHUS, rather than directly causing the syndrome and that multiple genetic and/or environmental hits are required for full-blown disease manifestation.

CFH. A substantial number of individuals with CFH mutations never develop aHUS. Overall the penetrance of the disease in those with CFH mutations is 59%. Conditions that trigger complement activation, either directly (bacterial and viral infections) or indirectly by causing endothelial insult (drugs, systemic diseases, pregnancy), precipitate the acute event in approximately 60% of those with CFH mutations [Caprioli et al 2006]. The reasoning is that in these individuals suboptimal CFH activity is sufficient to protect the host from the effects of complement activation in physiologic conditions; however, suboptimal CFH activity is not sufficient to prevent C3b from being deposited on vascular endothelial cells when exposure to an agent that activates complement results in production of higher than normal amounts of C3b.

CD46. For those with CD46 mutations penetrance is 54% [Caprioli et al 2006].

Nomenclature

Typical HUS is also referred to as Shiga-like toxin associated HUS (Stx-HUS) and D⁺HUS (diarrhea-positive HUS), although 25% of cases do not manifest diarrhea. Thus, at a minimum a search for free-fecal Shiga toxin (by commercial immunoassays) is recommended even in D^–HUS.

Atypical HUS (aHUS) is also referred to as non-Shiga-like toxin-associated HUS (non-Stx-HUS) and D^–HUS (diarrhea-negative HUS). However, the term diarrhea-negative HUS to refer to aHUS is not completley accurate, as 25% of typical HUS does not manifest diarrhea.

Prevalence

Atypical HUS is less common than typical HUS and accounts for only 5%-10% of individuals presenting with findings of HUS [Ruggenenti et al 2001].

According to a recent US study the incidence of aHUS in children is approximately one tenth that of typical HUS, corresponding to an estimated annual incidence of 2:1,000,000 population.

Evaluations Following Initial Diagnosis

To 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

• Molecular genetic testing of CFH, CD46, and CFI

• Measure serum concentrations of CFH and CFI

• Assessment of CD46 expression on leukocytes

• Testing for CFH autoantibodies in individuals in whom no CFH, CD46, and CFI mutations have been identified

Treatment of Manifestations

Plasma 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 patients 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; the efficacy of plasma exchange was shown in one study to be superior to that of plasma infusion [Ruggenenti et al 2001]. However, patients 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 (e.g., renal insufficiency, heart failure) that limit the amount of plasma that can be provided with infusion alone, plasma exchange should be considered the therapy of choice [Ruggenenti et al 2001].

In plasma exchange, usually one plasma volume (40 mL/kg) is exchanged 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 patients 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 whether 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 after treatment discontinuation in 29%-82% of patients. Thus, many cycles of stopping and resuming plasma therapy may be required.

Plasma infusion or exchange has been used in patients with aHUS caused by CFH mutations with the rationale of providing normal CFH to compensate for the genetic deficiency (see Genotype-Phenotype Correlations).

Plasma infusion or plasma exchange was used to treat acute episodes in patients with CD46 (MCP) mutations; however, remission generally occurred in both plasma-treated episodes and non-treated episodes, suggesting that plasma infusion or exchange does not have a great impact on the outcome of aHUS in persons with CD46 mutations [Caprioli et al 2006]. These findings can be explained with the reasoning that the protein encoded by CD46 is membrane-bound and theoretically plasma infusion or exchange would not correct the defect.

Bilateral nephrectomy has been performed in a small number of rare 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].

Surveillance

Individual 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 patients with normal renal function or chronic renal insufficiency as they are at risk for relapse (Patients 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, deriving from their own experience.

At-risk relative of an individual with aHUS

• Offer molecular genetic testing to at-risk family members of patients in whom disease-associated mutations have been identified.

  • For relatives who are mutation-positive (i.e., have the family-specific mutation), monitoring of 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 patients in whom disease-associated mutations have NOT been identified, no monitoring is needed.

Agents/Circumstances to Avoid

Discontinue cyclosporine or tacrolimus when aHUS develops following challenge with the medication.

Individual with known aHUS. An individual with known aHUS should avoid if possible the following known precipitants of aHUS, especially any that are known to have triggered aHUS (see Clinical Description, Sporadic (acquired) aHUS).

• Drugs

  • 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

• Pregnancy

Unaffected mutation-positive relatives of an individual with aHUS should avoid known precipitants of aHUS (see Clinical Description, Sporadic (acquired) aHUS (acquired) aHUS).

Testing of Relatives at Risk

Molecular genetic testing should be offered to at-risk family members of patients in whom disease-associated mutations have been identified.

Note: Testing for predisposing mutations is a challenging issue. The problem with predictive testing based on a predisposing factor (as opposed to a causative mutation) is that it is one of only 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 polymorphic 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 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 (acquired) aHUS)

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Research efforts are aimed at identifying more specific approaches that may interfere with the primary cause of microangiopathy in the different forms of aHUS.

• For aHUS associated with CFH mutations, specific replacement therapies with recombinant CFH could become a viable alternative to plasma treatment. Efforts are also ongoing to isolate plasma fractions enriched in CFH that could provide the patient with sufficient active molecules while minimizing the risk of allergy and fluid overload associated with standard plasma infusion therapy.

• 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 fully 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.

Other

Patients with Streptococcus pneumoniae-induced aHUS. Fresh frozen plasma should be avoided (i.e., plasma therapy is contraindicated) in patients with aHUS induced by Streptococcus pneumoniae because adult plasma 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].

Plasma-resistant/plasma-dependent patients. 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.

• Splenectomy induces remission in some plasma-resistant cases, but 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-resistant and plasma-dependent patients 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]. Recurrences occur at a median time of 30 days after transplantation (range 0 days to 16 years). There is no effective treatment of recurrences and cases reported so far invariably ended with loss of the kidney [Ruggenenti et al 2001, Artz et al 2003].

Precipitation of aHUS in renal donors. Renal transplantation for individuals with aHUS from a live related donor has been shown to precipitate disease onset in the healthy donor relative in two families [Donne et al 2002]. Subsequent molecular genetic testing revealed that one of the donors had a CFH mutation that put him at risk for aHUS. Thus, molecular genetic testing should be particularly recommended before live related donation to avoid the risk of triggering disease in the donors.

Influence of genotype in determining treatment. Genetic characterization of patients has the potential to optimize the treatment of aHUS:

• Plasma therapy
  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. 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] and unpublished data, approximately 50% 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.
  
  Similar results were observed in patients with CFI mutations [Caprioli et al 2006]. Theoretically one should expect a good response to plasma therapy in patients with CFH and CFI mutations because CFH and CFI are circulating proteins; 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].
  
  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 [Richards et al 2003, Caprioli et al 2006] indicate that the majority (70%-80%) of patients undergo remission following plasma infusion or exchange; however, complete recovery from the acute episode was also observed in 70%-80% of patients not treated with plasma. The decision whether or not to treat with plasma should be based on the clinical severity of the acute episode.

• Kidney transplantation. Whether kidney transplantation is an appropriate treatment in patients with aHUS who have progressed to ESRD is debatable. An estimated 50% of patients who undergo renal transplantation have a recurrence of the disease in the grafted organ [Noris & Remuzzi 2005]. 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]. As CFH is mainly produced by the liver, kidney transplantation will 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, both cases treated with this procedure were complicated by premature irreversible liver failure. The first patient recovered after a second uneventful liver transplantation. The child, who before transplantation had monthly recurrences before transplantation, has had no symptoms of aHUS for more than two years' follow-up. The second case had a fatal, primary nonfunction of the liver graft followed by multi-organ failure and death.
  
  In two other patients with CFH mutations with combined kidney and liver transplantation [Saland et al 2006], good renal and liver function were recorded at two-year follow-up. In those two cases extensive plasma exchange was given prior to surgery to provide enough normal CFH to prevent liver graft damage.
  
  CFI and CFB. As CFI and CFB are plasma proteins, one could speculate that aHUS recurrence may take place in the transplanted kidney, resulting in graft failure. The few data available support this hypothesis as graft failures secondary to recurrences occurred in 15 of 18 patients with CFI mutations and in one patient with a CFB mutation.
  
  CD46. Kidney graft outcome is favorable in patients with CD46 mutations; four patients have been successfully transplanted with no disease recurrence [Noris & Remuzzi 2005]. The strong theoretical rationale is that because the CD46 protein (MCP) is a transmembrane protein highly expressed in the kidney, transplantation of a kidney expressing normal MCP corrects the defect.

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

Predisposition to atypical HUS (aHUS) is inherited in an autosomal recessive manner or in an autosomal dominant manner with incomplete penetrance [Caprioli et al 2006].

Rare digenic inheritance occurs [Esparza-Gordillo et al 2006] and one case of uniparental isodisomy has been reported [Fremeaux-Bacchi et al 2007].

Risk to Family Members — Autosomal Recessive Inheritance

Parents of a proband

• The parents of a child with autosomal recessive aHUS are obligate heterozygotes and therefore carry a single copy of a disease-causing mutation.

• Heterozygotes (carriers) are usually asymptomatic. Rare cases of carriers who have developed aHUS in adulthood have also been reported [Caprioli et al 2006].

Sibs of a proband

• At conception, each sib of an individual with autosomal recessive aHUS has a 25% chance of inheriting two disease-causing mutations, a 50% chance of inheriting one mutation and being a carrier, and a 25% chance of inheriting neither mutation.

• Heterozygotes (carriers) are usually asymptomatic.

• Clinical severity and disease phenotype often differ among individuals with the same mutations; thus, age of onset and/or disease progression and outcome cannot be predicted.

Offspring of a proband. The offspring of an individual with autosomal recessive aHUS are obligate heterozygotes (carriers) for a disease-causing mutation and will likely be asymptomatic.

Note: Because individuals known to be homozygous have not yet reached reproductive age, whether offspring of these individuals will be affected is currently unknown.

Carrier Detection

Carrier testing for family members at risk for autosomal recessive aHUS is available on a clinical basis once the disease-causing mutations have been identified in the proband.

Risk to Family Members — Autosomal Dominant Inheritance

Parents of a proband

• Some individuals diagnosed with autosomal dominant aHUS have an affected parent or other close relative, but the majority of cases are simplex (i.e., a single occurrence in a family).

• In simplex forms of aHUS the CFH mutation was either inherited from a healthy parent or, more rarely (only 4 cases reported) occurred as a de novo mutation in the proband [Perez-Caballero et al 2001, Neumann et al 2003].

• Family history may be negative because of reduced penetrance of the disease-causing mutation in an asymptomatic parent, early death of a parent, late onset in a parent (or close relative), or a de novo mutation in the proband.

• If both parents are unaffected and a disease-causing mutation is identified in the proband, molecular genetic testing should be offered to both parents. If a disease-causing mutation is identified in a parent, the parent is at risk of developing aHUS and of transmitting the disease-causing mutation to other offspring.

• Clinical severity and disease phenotype often differ among individuals with the same mutations; thus, age of onset and/or disease progression and outcome cannot be predicted.

Sibs of a proband

• The risk to the sibs of the proband depends on the genetic status of the parents.

• If a parent of the proband is affected, has a positive family history, or is found to have a disease-causing mutation, the risk to the sibs of inheriting the mutation is 50%.

• Clinical severity and disease phenotype often differ among individuals with the same mutations; thus, age of onset and/or disease progression and outcome cannot be predicted.

• If the disease-causing mutation found in the proband cannot be detected in DNA extracted from the leukocytes of either parent, the risk to the sibs is low but greater than that of the general population because of the possibility of germline mosaicism.

Offspring of a proband

• Each child of an individual with autosomal dominant aHUS has a 50% chance of inheriting the mutation.

• Clinical severity and disease phenotype often differ among individuals with the same mutations; thus, age of onset and/or disease progression and outcome cannot be predicted.

Other family members of a proband. The risk to other family members depends on the status of the proband's parents. If a parent is found to be affected or is a carrier of a disease-causing mutation, his or her family members may be at risk and molecular genetic testing should be offered.

Risk to Family Members — Digenic Inheritance

Digenic aHUS is caused by the simultaneous presence of one mutation in each of two complement-regulatory genes. Individuals with mutations in CFH and CD46 [Caprioli et al 2006, Richards et al 2007] and individuals with mutations in CD46 and CFI [Caprioli et al 2006, Esparza-Gordillo et al 2006] have been reported.

Parents of a proband

• The parents are obligate heterozygotes.

• Heterozygotes (carriers) are usually asymptomatic.

Sibs of a proband

• At conception, each sib has a 25% chance of inheriting both mutations, a 50% chance of inheriting one mutation and being a carrier, and a 25% chance of inheriting neither mutation.

• Heterozygotes (carriers) are usually asymptomatic.

Offspring of a proband. All offspring are obligate carriers of one of the mutations.

Other family members of a proband. Other family members may be at risk and molecular genetic testing should be offered.

Carrier Detection

Carrier testing for family members at risk for digenic aHUS is available on a clinical basis once the disease-causing mutations have been identified in the family.

Related Genetic Counseling Issues

See Testing of Relatives at Risk for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant in situations in which the sensitivity of currently available testing is less than 100%. See for a list of laboratories offering DNA banking.

Prenatal Testing

Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. The disease-causing allele(s) must be identified in the family before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutation(s) have been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

In 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 for several complement-regulatory proteins [Warwicker et al 1998].

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, 43 CD46 mutations in aHUS have been reported, with a mutation frequency of 10%-15% [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].

More recently, two 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].

The protein product of a CFH familial mutation has complement-inhibitory activity, and similar to normal CFH protein, binds surface proteoglycans. 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 aHUS [Zipfel et al 2007]. CHR1/CFHR3 plasma deficiency may contribute to defective regulation of complement activation on cell and tissue surfaces.

CFH

Normal allelic variants. CFH is approximately 100 kb long. It comprises 23 exons.

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 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].

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 comprising approximately 60 amino acids.

Abnormal gene product. Expression and functional studies demonstrated that CFH proteins carrying HUS-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].

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).

The list of published and unpublished mutations within CD46 is continuously updated in the FH aHUS Mutation Database.

Normal gene product. The CD46 gene 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 cytoplasmatic 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.

Table 3. CFI Pathologic Allelic Variants Discussed in This GeneReview

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 40% of the mutations result in partial CFI deficiency, 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.

Two heterozygous mutations in affected members of two Spanish pedigrees, resulting in c.858C>G and c.967A>G changes in CFB 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].

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Author Notes

Web site: www.marionegri.it

Revision History

• 20 November 2008 (cd) Revision: deletion/duplication testing for CFI and CD46 available clinically

• 17 December 2007 (cd) Revision: sequence analysis available for CFB

• 16 November 2007 (me) Review posted to live Web site

• 27 March 2007 (mn) Original submission