Role of complement genetic variants in inflammatory diseases by an interactive database and protein structure modelling

The rare diseases atypical haemolytic uraemic syndrome (aHUS) and C3 glomerulopathy (C3G) are associated with dysregulation of complement activation. It is unclear which genes most frequently predispose to aHUS or C3G. Accordingly, a six- centre analysis of 610 rare genetic variants in 13 mostly complement genes from >3500 patients with aHUS and C3G was performed. A new interactive Database of Complement Gene Variants was developed to extract allele frequencies for these 13 genes using the Exome Aggregation Consortium server as the reference genome. For aHUS, significantly more protein-altering rare variation was found in the five genes CFH, CFI, CD46, C3 and DGKE than in ExAC. For C3G, an association was only found for rare variants in C3 and the N-terminal C3b-binding or C-terminal non-surface-associated regions of factor H (FH). FH is the major regulator of C3b and its Tyr402His polymorphism is an age-related macular degeneration risk-factor. To better understand FH complement binding, the solution structures of both allotypes were studied. Starting from known FH short complement regulator domains and glycan structures, small angle X-ray scattering data were fitted using Monte Carlo methods to determine atomistic structures for monomeric FH. The analysis of 29,715 physically realistic but randomised FH conformations resulted in 100 similar best-fit FH structures for each allotype. Two distinct molecular structures resulted; an extended N-terminal domain arrangement with a folded-back C-terminus, or an extended C-terminus and folded-back N-terminus. To clarify FH functional roles in host protection, crystal structures for the FH complexes with C3b and C3dg revealed that the extended N-terminal conformation accounted for C3b fluid phase regulation, the extended C-terminal conformation accounted for C3d binding, and both conformations accounted for bivalent FH binding to the host cell-surface. Finally, statistical analyses indicated that the structural location of rare variants in complement may predict the occurrences of aHUS or C3G