Rare Variants in the Complement Factor H-Related Protein 5 Gene Contribute to Genetic Susceptibility to IgA Nephropathy

A recent genome wide association study of IgA nephropathy (IgAN) identified 1q32, which contains multiple complement regulatory genes, including the complement factor H (CFH) gene and the complement factor H related (CFHRs) genes, as an IgAN susceptibility locus. Abnormal complement activation caused by a mutation in CFHR5 was shown to cause CFHR5 nephropathy, which shares many characteristics with IgAN. To explore the genetic effect of variants in CFHR5 on IgAN susceptibility, we recruited 500 patients with IgAN and 576 healthy controls for genetic analysis. We sequenced all exons and their intronic flanking regions as well as the untranslated regions of CFHR5 and compared the frequencies of identified variants using the sequence kernel association test. We identified 32 variants in CFHR5, including 28 rare and four common variants. The distribution of rare variants in CFHR5 in patients with IgAN differed significantly from that in controls (P=0.002). Among the rare variants, in silico programs predicted nine as potential functional variants, which we then assessed in functional assays. Compared with wild-type CFHR5, three recombinant CFHR5 proteins, CFHR5-M (c.508G>A/p.Val170Met), CFHR5-S (c.533A>G/p. Asn178Ser), and CFHR5-D (c.822A>T/p.Glu274Asp), showed significantly higher C3b binding capacity (CFHR5-M: 109.67%+/- 3.54%; P=0.02; CFHR5-S: 174.27%+/- 9.78%; P<0.001; CFHR5-D: 127.25%+/- 1.75%; P<0.001), whereas another recombinant CFH R5 (c.776T>A/p.Leu259Termination) showed less C3b binding (56.89%+/- 0.57%; P<0.001). Our study found that rare variants in CFHR5 may contribute to the genetic susceptibility to IgAN, which suggests that CFHR5 is an IgAN susceptibility gene.