Genetic Changes in Atypical Hemolytic Uremic Syndrome
Atypical HUS involves genetic mutations in complement regulatory and activating proteins in approximately 60% of cases, with the most critical genes being CFH, CFHR1-5, C3, CD46, CFI, THBD, DGKE, CFB, and WT1, though 40-50% of patients have no identifiable genetic mutation. 1
Primary Genetic Alterations
Complement-Related Genes (Most Common)
The genetic landscape of aHUS centers on dysregulation of the alternative complement pathway. Genetic testing should include next-generation sequencing of the following genes: CFH, CFHR1-5, C3, CD46, CFI, THBD, DGKE, and CFB, plus multiplex ligation-dependent probe amplification for CFH and CFHR1-5. 1
These mutations fall into two functional categories:
Loss-of-function mutations affecting complement regulators:
- Factor H (CFH) - most common and associated with poorest prognosis (45% 1-year kidney survival without complement blockade) 2
- Factor H-related proteins (CFHR1-5)
- Membrane cofactor protein (CD46)
- Factor I (CFI)
- Thrombomodulin (THBD)
Gain-of-function mutations affecting complement activators:
- Factor B (CFB)
- Complement component 3 (C3)
Non-Complement Genes (Age-Specific Consideration)
When aHUS presents in the first year of life, mutations in complement-unrelated genes (DGKE, WT1) must be considered. 1 These ultra-rare forms require different diagnostic and therapeutic approaches.
Additionally, inborn errors in cobalamin metabolism (MMACHC gene) can cause methylmalonic acidemia with homocystinuria cblC type, presenting as metabolism-associated HUS. 1
Critical Clinical Implications
Genetic Detection Rates and Autoantibodies
Gene variants appear more commonly than autoantibodies in adults with aHUS, while factor H autoantibodies are detected in 53% of pediatric patients. 1, 2 Among pediatric patients with factor H autoantibodies, those with homozygous CFHR1 deletion exhibit significantly higher relapse rates. 2
Important Caveats
The absence of identifiable mutations does not exclude aHUS diagnosis - 40-50% of patients have no detectable genetic or immunologic abnormality. 1 This represents a critical diagnostic pitfall. The disease can still result from complement dysregulation through mechanisms not yet fully characterized.
Mutations may also occur in coagulation proteins and enzymes affecting the coagulation cascade indirectly, though these are less common. 1
Genetic Testing Strategy
When to Test
Genetic testing should be performed once aHUS diagnosis is confirmed clinically (presence of microangiopathic hemolytic anemia, thrombocytopenia, and renal involvement after excluding other TMAs). 1
Post-Testing Management
Once diagnosis is confirmed, patients must be informed about possible genetic transmission and provided access to genetic counseling. 1 This is particularly crucial for:
- Family planning decisions
- Living-related kidney donor evaluation (to protect both donor and recipient from developing aHUS) 4
- Risk stratification for family members
- Treatment duration decisions with complement inhibitors
Actionable Genetic Information
The genetic background directly impacts clinical management decisions, particularly regarding duration of eculizumab therapy and kidney transplantation risk. 5, 4 CFH mutations carry the highest risk for poor outcomes and disease recurrence post-transplant, requiring indefinite complement blockade in most cases.
Population-Specific Considerations
Recent data from Chinese pediatric cohorts show only 20% have identifiable genetic mutations, suggesting ethnic variation in genetic penetrance. 2 The CFHR5 p.V170M variant appears enriched in some aHUS populations. 2
Genetic workup remains essential despite variable detection rates across populations, as it fundamentally alters prognosis prediction and treatment planning.