SUGP2 p.(Arg639Gln) variant is involved in the pathogenesis of hemochromatosis via the CIRBP/BMPER signaling pathway

Abstract Pathogenic variants in HFE and non‐ HFE genes have been identified in hemochromatosis in different patient populations, but there are still a certain number of patients with unexplained primary iron overload. We recently identified in Chinese patients a recurrent p.(Arg639Gln) variant in SURP and G‐patch domain containing 2 (SUGP2), a potential mRNA splicing‐related factor. However, the target gene of SUGP2 and affected iron‐regulating pathway remains unknown. We aimed to investigate the pathogenicity and underlying mechanism of this variant in hemochromatosis. RNA‐seq analysis revealed that SUGP2 knockdown caused abnormal alternative splicing of CIRBP pre‐mRNA, resulting in an increased normal splicing form of CIRBP V1, which in turn increased the expression of BMPER by enhancing its mRNA stability and translation. Furthermore, RNA‐protein pull‐down and RNA immunoprecipitation assays revealed that SUGP2 inhibited splicing of CIRBP pre‐mRNA by a splice site variant at CIRBP c.492 and was more susceptible to CIRBP c.492 C/C genotype. Cells transfected with SUGP2 p.(Arg639Gln) vector showed up‐regulation of CIRBP V1 and BMPER expression and down‐regulation of pSMAD1/5 and HAMP expression. CRISPR‐Cas9 mediated SUGP2 p.(Arg622Gln) knock‐in mice showed increased iron accumulation in the liver, higher total serum iron, and decreased serum hepcidin level. A total of 10 of 54 patients with hemochromatosis (18.5%) harbored the SUGP2 p.(Arg639Gln) variant and carried CIRBP c.492 C/C genotype, and had increased BMPER expression in the liver. Altogether, the SUGP2 p.(Arg639Gln) variant down‐regulates hepcidin expression through the SUGP2/CIRBP/BMPER axis, which may represent a novel pathogenic factor for hemochromatosis.