Isolated Crystalloid Podocytopathy With Focal Segmental Glomerulosclerosis in Renal Allograft: An Unusual Presentation of Post-Transplant Monoclonal Gammopathy of Renal Significance—A Case Report

Background Monoclonal gammopathy of renal significance denotes a spectrum of hematologic disorders that cause direct or indirect renal damage. Case presentation A 51-year-old man had received a living-donor kidney transplant from his wife in 2008. He had gradual increased proteinuria 4 years later. His renal biopsy results revealed cytoplasmic crystalloid inclusions in the podocytes. No crystalloid inclusion was found in other renal cells. Despite that immunofluorescent examination failed to show light-chain deposition, the serum immuno-electrophoresis revealed monoclonal immunoglobulin-Gκ. Bone marrow biopsy showed interstitial infiltration of plasma cells of approximately 10%. A follow-up renal biopsy was performed in 2016. Light microscopy showed focal segmental glomerulosclerosis. The immunofluorescent examination remained negative for light chain, but κ-light chain could be demonstrated after antigen retrieval. Similar to previous biopsy results, cytoplasmic inclusions were found only in podocytes without involving other renal cells. Conclusions To the best of our knowledge, this is the first report of monoclonal gammopathy of renal significance presenting as isolated crystalloid podocytopathy in the allograft kidney. The mechanism of preferential podocyte deposition of crystalloid immunoglobulin remains unclear. The inherent features of crystalloid podocytopathy may mislead the pathologic diagnosis.