Netrin G1 is a Novel Target Antigen in Primary Membranous Nephropathy

Background: Primary membranous nephropathy (MN) is caused by circulating autoantibodies (ab) binding to antigens on the podocyte surface. PLA₂R1 is the main target antigen in 70-80% of cases, but the pathogenesis is unresolved in 10-15% of patients. Methods: We used native Western blotting to identify IgG4-ab in the serum of the index MN patient, which binds an antigen endogenously expressed on podocyte membranes. These IgG4-ab were used to immunoprecipitate the target antigen and mass spectrometry used to identify Netrin G1 (NTNG1). Native Western blot and ELISA analyzed NTNG1-ab in cohorts of 888 patients with MN or other glomerular diseases. Results: NTNG1 was identified as a novel target antigen in MN. It is a membrane protein expressed in healthy podocytes. Immunohistochemistry confirmed granular NTNG1 in subepithelial glomerular immune deposits. In prospective and retrospective MN cohorts, we identified three patients with NTNG1-associated MN, who showed IgG4-dominant circulating NTNG1-ab, enhanced NTNG1 expression in the kidney, and glomerular IgG4 deposits. No NTNG1-ab were identified in 561 PLA2R1-ab positive patients, 27 THSD7A-ab positive patients, and 77 patients with other glomerular diseases. In two patients with available followup of 2 and 4 years, both NTNG1-ab and proteinuria persisted. Conclusions: NTNG1 expands the repertoire of target antigens in patients with MN. The clinical role of NTNG1-ab remains to be defined.