Fanconi's syndrome, kappa light-chain myeloma, non-amyloid fibrils and cytoplasmic crystals in renal tubular epithelium.

A 59-year-old woman with kappa light-chain myeloma had Fanconi's syndrome characterized by renal glycosuria, generalized aminoaciduria, bicarbonaturia and decrease of phosphorus and uric acid reabsorption. A bone marrow biopsy showed the presence of 27% of dystrophic plasma cells; the cytoplasm of these cells was intensely stained with anti-kappa light-chain monoclonal antibodies. By light microscopy, the renal biopsy revealed a tubulointerstitial nephritis without glomerular lesions and with intratubular casts. By immunofluorescence, no deposits were observed along the glomerular and tubular basement membranes, but a positivity with anti-kappa light chain was noticed in some tubular epithelia and casts. By electron microscopy, fibrils (35-nm diameter) were observed in the cytoplasm of proximal tubular cells. These fibrils were situated in vesicles (100- to 600-nm diameter) in the luminal side of tubular cells. In the basal pole of the cell, fibrils seemed to group in crystals (120- to 200-nm diameter). Only kappa light-chain protein was demonstrated in these fibrils and crystals by an immunoelectron microscopic technique. These data suggested the pathogenic role of the fibrils and crystals present in tubular epithelium in the tubular proximal syndrome.