Lessons from Comprehensive Analysis of a Factor I (FI) Variant in Atypical Hemolytic Uremic Syndrome (aHUS)
Category: Glomerular Diseases
- 1202 Glomerular Diseases: Immunology and Inflammation
- Java, Anuja, Washington University in St. Louis, Saint Louis, Missouri, United States
- Atkinson, John P., Washington University, St. Louis, Missouri, United States
aHUS is characterized by microangiopathic hemolytic anemia, thrombocytopenia and acute kidney injury. Mutations in genes encoding complement proteins have been identified in ~60% of patients. Majority of these variants are variants of uncertain significance (VUS). Herein, we report functional and structural analyses on a FI variant identified in a patient with HUS.
Clinical history: 22 y/o male had h/o ESRD secondary to E. coli-HUS. The patient underwent a renal transplant in 2006. In 2007, he developed non-Hodgkin’s lymphoma and his immunosuppression was reduced. Later that year, he lost his kidney to acute cellular rejection. During evaluation for a 2nd kidney transplant, genetic testing for HUS was performed which demonstrated a VUS (R406H) in FI. He received eculizumab post-transplant. One year later, after eculizumab was discontinued, the patient lost his kidney again to acute cellular and antibody-mediated rejection. He is now being evaluated for a 3rd transplant.
Genetic analysis: The FI variant has been analyzed by several prediction models and due to lack of consensus among these tools, was classified as a VUS.
Antigenic analysis: The secretion of the variant protein (ELISA) was comparable to wild type (WT). Patient had a borderline low serum FI level.
Functional analysis: Cofactor assays were performed using purified FI, C3b and with membrane cofactor protein (MCP), Factor H (FH) or Complement Receptor 1 (CR1) as the cofactor protein. WT and variant cDNAs were transfected into human 293T cells. The variant had defective cofactor activity (CA) with FH and CR1 as evident by decreased rate of cleavage of the α-chain of C3b. No defect was observed with MCP.
Structural analysis: The side chain of R406 in FI forms a salt bridge with the carboxyl group of E123 of FH, thereby stabilizing the FI-FH binding interface. Residue E123 is a unique signature of FH. It is lacking in MCP, possibly accounting for why the R406H mutation impairs FH- but not MCP-mediated CA.
Summary/Lessons Learned: It is imperative to define the functional activity of a VUS. The variant R406H is pathogenic based on defective CA. Given his clinical course, borderline low FI antigenic levels and these new functional data, we believe that the patient is at high risk of developing a thrombotic microangiopathy or an accelerated rejection in the future.