Protein modifications associate with idiopathic membranous nephropathy diagnosis and pathogenesis

Abstract Idiopathic membranous nephropathy (IMN) is an organ-specific autoimmune disease. However, the pathogenic causes and risk factors underlying its molecular mechanism remain unclear. In this study, we aimed to characterize the differentially expressed protein modifications in the circulating blood of IMN patients to identify potential risk factors associated with the pathogenesis and progression of the disease. Serum samples were collected from patients with IMN (N = 40) and healthy controls (N = 48) for the analysis of protein modifications through label-free proteomic profiling using LC–MS/MS. Subsequently, open search and bioinformatics analyses were carried out. The proteins present in the bloodstream of patients with IMN exhibited extensive post-translational modifications, with several showing significant differential expression. The most prominently up-regulated modification was the cysteinylation of haptoglobin at residue 266, a cysteine residue (Cys + 119.006, > 17-fold increase, p = 0.0002). Among the most significantly down-regulated modifications were the deamidation of the fibrinogen alpha chain (FGA) at residue 296, an asparagine residue (Asn + 0.984, > 98-fold increase, p = 0.005), and an unidentified modification of the fibrinogen beta chain at residue 30, a serine residue (Ser − 104.057, > 108-fold increase, p = 0.01). Proteins exhibiting differentially expressed modifications were significantly enriched in pathways associated with thrombosis. Additionally, the modification of haptoglobin (Cys266 + 119.00692) was identified as the top discriminatory marker (AUC = 0.8188) according to the Mann–Whitney U (MWU) test. Our data revealed up-regulation of haptoglobin cysteinylation at Cys266 and down-regulation of FGA deamidation at Asn296. The cysteinylation of haptoglobin is associated with oxidative stress, while the deamidation of fibrinogen is linked to thrombosis. Further investigation into these post-translational modifications may facilitate the identification of potential risk factors for idiopathic membranous nephropathy (IMN) and its associated hypercoagulability complications.