Atypical haemolytic uremic syndrome (aHUS), primary membranoproliferative glomerulonephritis type I (MPGN I), dense deposit disease, (DDD) and C3-glomerulonephritis (C3GN) are disorders characterized by excessive activation of the alternative complement pathway (AP). HUS is a rare disease of microangiopathic haemolysis, thrombocytopenia and renal failure. The most common form, associated with infection by shiga-toxin producing Escherichia coli, has a good prognosis. The rarer atypical form (aHUS) has a poorer outcome, with 10-15% acute mortality and 50% of cases progressing to ESRD. Sixty percent of cases are related to mutations in genes encoding complement regulatory proteins, factor H (CFH), membrane-cofactor protein (MCP), factor I (CFI) and thrombomodulin (THBD), or the alternative pathway (AP) C3 convertase components C3 and factor B (FB). Hybrid CFH-CFHR1 genes have been reported. Homozygous CFHR1-CFHR3 deletion is associated with an autoimmune form with inhibitory anti-CFH autoantibodies. Such abnormalities cause complement hyperactivation in particular on the surface of microvascular endothelium and platelets leading to endothelial injury and thrombus formation in the kidney and in other organs. However, not all mutation carriers develop the disease. Multiple gene mutations/variants and environmental triggers may be required for disease manifestation. The type of genetic defect strongly impacts on outcome and recurrence rate after renal transplantation. Major therapeutic progress has been made recently with the discovery of Eculizumab, an anti-C5 antibody as effective treatment of aHUS.

MPGN I, DDD and C3GN are characterized by glomerular inflammation, proteinuria and progressive renal failure. Affected children develop end stage renal disease during late childhood or adolescence. These conditions are characterized by glomerular C3 deposition. C3NeF, an autoantibody stabilizing the AP C3-convertase, is found in 50-75% of patients. Auto-antibodies against CFH and factor B and complement gene mutations have been also reported. There is no specific treatment. Eculizumab reduced proteinuria in a few patients with MPGN I and DDD, but others did not respond. Several crucial issues remain unsolved that ask for innovative and integrated studies. The underlying cause has not been identified in about 40% of aHUS and 70% of MPGN I, DDD and C3GN patients. The formation of anti-CFH antibodies cannot be attributed only to CFHR1/CFHR3 deletion since the deletion is also present in 5% of normal individuals. The origin, mechanism of action and role of C3NeF are still unclear. The incomplete penetrance in mutation carriers and the highly heterogeneous phenotypic expression (patients with the same mutation may develop either aHUS, MPGN I, DDD or C3GN) indicate that multiple predisposing factors are involved and define the need to establish disease-specific genetic/molecular/proteomic signatures.

Search for genetic initiators of complement-related disorders
In patients without identified abnormalities, exome/whole genome sequencing will be used to identify new disease-associated genes. Candidate gene variants will be validated by sequencing controls matched for ethnicity. New CFH/CFHRs gene rearrangements/deletions will be searched by MLPA, array CGH and sequencing Cell and tissue localization of novel gene products and the effect of mutations will be assessed in kidney biopsies from patients and controls. Functional studies and protein-protein interaction assays will be carried out using recombinant proteins or proteins isolated from patients. For in vivo studies and drug discovery purposes Xenopus morpholino knockdown and KI/KO mouse models of novel disease-associated genes will be studied (if already available) or generated de novo.