Tiopronin-induced Membranous Nephropathy Presenting with IgG4-predominant Staining Pattern

Tiopronin is a key drug used to treat cystinuria. A 41-year-old Japanese woman with cystinuria presented with eyelid edema and weight gain after the administration of tiopronin. Her serum albumin was 1.8 g/dL and her urinary protein level was 5.5 g/gCre. After cessation of tiopronin, she achieved remission of nephrotic syndrome (NS). Secondary NS due to tiopronin was evident based on the clinical course and laboratory values. A kidney biopsy showed membranous nephropathy (MN), and an immunofluorescence analysis revealed strong deposition of immunoglobulin G4 (IgG4). However, a previous case report of tiopronin-induced MN showed staining for IgG1 and IgG3. This case report suggests a novel etiology for tiopronin-induced MN.