LIGHT CHAIN PROXIMAL TUBULOPATHY WITHOUT CRYSTALS IN A CASE OF NEWLY DIAGNOSED MULTIPLE MYELOMA PRESENTING WITH WORSENING CHRONIC KIDNEY DISEASE

J NGO1, P KIRWAN1, L HUGHES1,2, R GASIOROWSKI1,2, A RITCHIE1,2, S SEN1,2

1Concord Repatriation General Hospital, Sydney, Australia, 2The University of Sydney, Sydney, Australia

Background: Light Chain Proximal Tubulopathy (LCPT) is a rare form of paraprotein-related kidney disease (PRKD) characterised by damage to proximal renal tubular epithelial cells from intracytoplasmic deposition of monoclonal free light chains, which frequently, but not always, form crystals.
Case Report: A 71-year-old female was referred for investigation of worsening chronic kidney disease (creatinine 85-105 umol/L over two years) with proteinuria 1g/day. The patient was normotensive, with recently resolved macroscopic haematuria. A vasculitic screen and cystoscopy were unremarkable. Myeloma screen demonstrated elevated serum IgG kappa paraprotein (10 g/L) and elevated serum (663.3 mg/L) and urine (0.52 g/L) free kappa light chains. Bone marrow biopsy demonstrated 20% plasma cells, consistent with myeloma. Full blood count and serum calcium were normal and CT skeletal survey showed no lytic lesions. Renal biopsy was performed. Light microscopy showed only non-specific, focal active tubulointerstitial changes. Initial immunofluorescence staining was negative. However, electron microscopy (EM) demonstrated proximal tubule cytoplasm containing long, straight fibrils ranging in diameter from 15.1-36.9nm, in parallel arrays, with cross-section showing a central core with dense peripheral fibrils. These were predominantly within lysosomes, but also in the cytoplasm. There were no crystals. Subsequent use of an antigen retrieval process confirmed positive immunofluorescence staining for kappa in proximal tubule cytoplasm, with negative staining for lambda. These findings were consistent with a diagnosis of proximal tubulopathy without crystals, and indicative of active myeloma, requiring treatment. The patient was subsequently commenced on bortezomib/lenalidomide/dexamethasone with an improvement in creatinine to 73umol/L after 2 months.
Conclusion: We describe a case of LCPT, a rare form of PKRD with clinical significance, requiring additional pathologic testing, subsequently providing a crucial distinction from smoldering myeloma.


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