Anti-Contactin-1-Associated Membranous Nephropathy Presenting With Subsequent Autoimmune Nodopathy

A 70-year-old man presented with biopsy-proven phospholipase A2 receptor--negative membranous nephropathy (MN). Shortly thereafter, he presented with progressive sensorimotor polyneuropathy, initially consistent with an inflammatory demyelinating polyneuropathy. Laboratory testing revealed strongly positive anti-contactin-1 (CNTN1) autoantibodies, a rare antigen target recently implicated in a subset of MN patients with concurrent inflammatory demyelinating polyneuropathy, reclassifying it as an autoimmune nodopathy. Intensive immunosuppressive treatment including methylprednisolone, cyclophosphamide, plasmapheresis, intravenous immunoglobulins, and rituximab led to significant neurological improvement and seronegativity for anti-CNTN1 antibodies. Nephrotic syndrome improved gradually with partial remission observed 2 years after treatment initiation. This case highlights the autoimmune overlap of MN and autoimmune nodopathy mediated by anti-CNTN1 antibodies, targeting shared epitopes in podocytes and the paranodal region of myelinated peripheral nerves. It underscores the clinical importance of screening for novel antibodies in phospholipase A2 receptor--negative MN patients, particularly anti-CNTN1 when neurological symptoms develop. This report supports aggressive immunosuppression tailored to antibody-mediated disease and suggests anti-CNTN1 titers as a biomarker for monitoring therapeutic response.