Duodenal Schwannoma as a Rare Association With Membranous Nephropathy: A Case Report

Membranous nephropathy (MN) associated with malignancies is a well-known entity. However, its association with benign neoplasm is not broadly recognized. A 69-year-old man with recurrent nephrotic syndrome presented with pedal edema and proteinuria of 5 months' duration. Laboratory results showed hypoalbuminemia and hyperlipidemia. Proteinuria was estimated to be protein excretion of 3.5g/d. Studies were negative for viral hepatitis, syphilis, human immunodeficiency virus, autoimmune diseases, and paraproteinemia. Kidney biopsy disclosed MN with negative phospholipase A₂ receptor (PLA₂R) staining, favoring a secondary form of MN. Computed tomography detected a 7.6-cm duodenal schwannoma. Elective surgical resection was performed. Pathologic study showed that THSD7A (thrombospondin type 1 domain-containing 7A) was positive in both glomeruli and schwannoma. Commonly, secondary MN is related to underlying conditions, including lupus, hepatitis, and neoplasm, and can be medication induced. The risk for developing a concomitant neoplasm among patients with PLA₂R-negative MN is up to 12 times higher than in the general population. Most of these neoplasms are malignancies, and the presence of autoantibodies directed at similar tissue targets is hypothesized as the potential mechanism. In our case, THSD7A may be the autoantibody that has linked the schwannoma and the development of MN. Although benign tumors rarely produce renal manifestations, effective treatment may lead to resolution of nephrotic syndrome.