Post translational modifications in the complement system associated with idiopathic membranous nephropathy

Abstract Objective: To explore the molecular pathogenesis of idiopathic membranous nephropathy (IMN) at the level of post-translational modifications (PTMs) using the shotgun proteomics and a modified open search analysis. Methods: Serum proteins from 41 IMN patients and 92 healthy people were analyzed by high resolution mass spectrometry. The mass differences between the coding and observed amino acid residues crossing the proteomes, termed the amino acid variations or polymorphisms, were determined with the open search engine followed by multivariate clustering, Gaussian regression and amino acid localization. The biological functions and interaction networks of these amino acid variants were correlated with the pathogenesis of IMN using bioinformatics analysis. Results: A total of 2627 clustered delta masses were identified, spreading over 27597 sites on 457 proteins. Among them, the levels of amino acid variants at 159 proteins were upregulated in patients with IMN, and 20 downregulated. The proteins with dysregulated amino acid variants were predominantly related to LXR/RXR activation, acute phase response signaling, coagulation system as well as the complement system. Markedly, the complement factors such as C3, C4B, C4BPA, C8g and CFH were the top proteins with Leu/Ile>Val and Asn>Asp substitutions, binding with calcium ions, and unknown PTMs, such as C4BPA@372N>D, C4B@141L>V, C7@389S>T, and CFH@339X>T. The functional analysis of proteins showed that nephrotoxicity (Global injury, renal inflammation, renal nephritis, kidney failure, renal damage) was associated with kidney disease, involving 28 kinds of proteins, including C4A/C4B. Through IPA analysis, we found that C4A/C4B interacted with many molecules, such as extracellular, cytoplasmic and nuclear. Conclusion: Complement pathway plays an important role in the pathogenesis of membranous nephropathy. The post-translational modification of C4B may be the most important pathogenesis of membranous nephropathy in the complement pathway.