Complement-mediated kidney diseases: Genotype, phenotype and inhibition studies

Abstract: Complement-mediated kidney diseases are ultrarare conditions characterized by excess complement activation related in most cases to complement gene variants or circulating autoantibodies. These conditions are chronic and can lead to kidney failure. They include atypical hemolytic uremic syndrome (aHUS), C3 glomeru-lopathy (C3G), and, to a certain extent, immune complex-membranoproliferative glomerulonephritis (IC-MPGN). In this thesis, a panel of genes was studied including CFH, C3, FB, FI, CD46/MCP, C5, CFHR1-5, CFP, CLU, DGKE, THBD and PLG. The phenotype of three heterozygous CFB variants was characterized in Paper I in which one variant (D371G) was shown to have gain-of-function properties and form excess C3 convertase. The phenotype was compared to a well-characterized CFB variant, D279G, and studies showed that a factor D inhibitor, Danicopan, could inhibit cleavage of factor B and excess complement activation as determined by hemolysis of rabbit red blood cells and release of C5b-9 from human glomerular endothelial cells. In Paper II the CFB D371G variant was further studied in a large pedigree in which three family members were affected by aHUS and seven were carriers of the variant. Two of the carriers were adult monozygotic twins but only one was affected by the disease. As they did not carry other variants this suggests that the CFB D371G variant predisposes but is not the sole factor associated with the development of the aHUS phenotype. In Paper III a large cohort (n=141) of Nordic patients with the three kidney diseases was investigated. Patients (72% aHUS and 38% C3G) were found to have genetic variants with a minor allele frequency In Paper IV the phenotype of a heterozygous variant in CFHR5, M514R, was investigated. The variant was found in a child with aHUS with a deletion of CFHR3/CFHR1 as well as antibodies to factor H. The variant was minimally secreted from cells and the patient had low levels of circulating factor H-related protein 5 (FHR5). The addition of FHR5 to patient serum reduced hemolysis of rabbit red blood cells, and at higher concentrations, this even occurred in normal sera. We, therefore, suggest that this genetic variant could contribute to complement activation.In summary, this thesis describes many novel variants in genes encoding complement proteins associated with aHUS, C3G and IC-MPGN, and describes the phenotype of several variants to better understand how they cause disease. Furthermore, a factor D inhibitor was effectively shown to block factor B degradation and down-stream complement activation