Genetics of atypical haemolytic uraemic syndrome

Abstract

Atypical haemolytic uraemic syndrome (aHUS) is a life threatening renal disease, caused by deregulation of the alternative complement pathway. Several genes within this pathway are associated with aHUS. At the outset of this project, the genetic cause had been identified in 45% of familial cases in the Newcastle aHUS cohort. The aim of this project was to identify the genetic cause of disease in the remaining 55%. Complement Factor H (CFH) and Complement Factor H-related (CFHRs) are found on chromosome 1. This area contains several low copy repeats, the result of genomic duplications that occurred early in evolution. This causes genomic instability, which can lead to gene conversions or rearrangements. Sanger sequencing will not always detect these abnormalities, therefore patients were also screened using multiplex ligation-dependent probe amplification and western blotting. A novel hybrid CFH/CFHR3 gene was described, which arose by microhomology-mediated end joining. Functional analysis demonstrated that it was defective at regulating complement at the cell surface, which was predicted to predispose this patient to disease. Review of all patients in the Newcastle aHUS cohort with CFH abnormalities, identified a third of patients had a genomic rearrangement between CFH and CFHRs. The relative frequency of genomic rearrangements emphasised the importance of undertaking copy number analysis in aHUS diagnostic testing, because often they are not detected by Sanger sequencing. Whole exome sequencing was then undertaken in Newcastle familial cohort with an unknown genetic aetiology. Pathogenic sequence variants were identified in genes, known to be associated with thrombotic microangiopathies. Sequence variants that were predicted to be pathogenic, were found in three genes not previously associated with disease. Two of these genes were located outside of the complement system, indicating that complement-directed therapies may be contraindicated. In this project, a genetic cause of disease was found in 54% of familial cases tested.