DISEASE CHARACTERISTICS:

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, whereas atypical HUS (aHUS) can be genetic, acquired, or idiopathic (of unknown cause). Onset of atypical HUS ranges from prenatal to adulthood. Individuals with genetic atypical HUS frequently experience relapse even after complete recovery following the presenting episode. Sixty percent of genetic aHUS progresses to end-stage renal disease (ESRD).

DIAGNOSIS/TESTING:

Atypical HUS is considered genetic when 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 when a disease-causing mutation(s) is identified in one of the nine genes in which mutations are known to be associated with aHUS, irrespective of familial history. The nine genes are: CFH (encoding complement factor H), accounting for an estimated 30% of aHUS; CD46 (MCP) (encoding membrane cofactor protein) accounting for approximately 12% of aHUS; CFI (encoding complement factor I), accounting for an estimated 5%-10% of aHUS; C3 (encoding the third component of complement C3) accounting for 5% of aHUS; rarely, CFB (encoding complement factor B); and THBD (encoding thrombomodulin) accounting for about 5% of aHUS. Deletions involving CFHR1 and CFHR3 or CFHR1 and CFHR4 account for 5%-15% of aHUS.

MANAGEMENT:

Treatment of manifestations: Plasma manipulation (plasma infusion or exchange) to reduce mortality; however, plasma resistance or plasma dependence is possible. Bilateral nephrectomy when extensive renal microvascular thrombosis, refractory hypertension, and signs of hypertensive encephalopathy are not responsive to conventional therapies including plasma manipulation. Surveillance: Serum concentration of hemoglobin, platelet count, and serum concentrations of creatinine, LDH, C3, C4, and haptoglobin: (1) every month in the first year after an aHUS episode, then every three to six months in the following years, particularly for those with normal renal function or chronic renal insufficiency as they are at risk for relapse; and (2) in mutation-positive relatives following exposure to potential triggering events. Agents/circumstances to avoid: Those with known aHUS should avoid if possible pregnancy and the following drugs that are known precipitants of aHUS: anti-cancer molecules (including mitomycin C, cisplatin, daunorubimicin, cytosine arabinoside); immunotherapeutic agents (including cyclosporin and tacrolimus); and antiplatelet agents (including ticlopidine and clopidogrel). Plasma therapy is contraindicated in those with aHUS induced by Streptococcus pneumoniae because antibodies in the plasma of adults may exacerbate the disease. Evaluation of relatives at risk: While it is appropriate to offer molecular genetic testing to at-risk family members of persons in whom disease-associated mutations have been identified, predictive testing based on a predisposing factor (as opposed to a causative mutation) is problematic as it is one of only several risk factors required for disease causation. Other: Live-related renal transplantation for individuals with aHUS should also be avoided in that disease onset can be precipitated in the healthy donor relative. Evidence suggests that kidney graft outcome is favorable in those with CD46 mutations but not in those with CFH, CFI, or CFB mutations; however, simultaneous kidney and liver transplantation in young children with aHUS and CFH mutations may correct the genetic defect and prevent disease recurrence.

GENETIC COUNSELING:

Predisposition to atypical HUS (aHUS) is inherited in an autosomal recessive or autosomal dominant manner with incomplete penetrance. Rarely digenic inheritance and uniparental isodisomy are observed. Autosomal recessive inheritance: Heterozygotes (carriers) are usually asymptomatic; however, rarely carriers have developed aHUS in adulthood. 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. Autosomal dominant inheritance: Some individuals diagnosed with autosomal dominant aHUS have an affected parent or an affected close relative, but in the majority the family history is 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. Each child of an individual with autosomal dominant aHUS has a 50% chance of inheriting the mutation. In both genetic types, 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. Prenatal diagnosis for pregnancies at increased risk is possible if the disease-associated mutation(s) has (have) been identified in the family.

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