Tumor lysis or tubular lysis?

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A patient presented to the outpatient service with progressive renal insufficiency and sub-nephrotic range proteinuria. He had a long history of CMML with few blasts in his bone marrow aspirate but a peripheral white cell count that was persistently in the 40s. There are many ways in which leukemia can cause renal dysfunction the commonest being infiltration of the kidney, tumor lysis, acute or chronic TMA and toxicity related to the treatment including chemotherapy and radiotherapy. It was not initially obvious what the specific cause of the renal impairment was in this case so he was referred for a renal biopsy.
CMML (chronic myelomonocytic leukemia) is classified as a myelodysplastic/myeloproliferative disorder as it has characteristics of both and primarily is a disease of the elderly. It is characterized by the presence of increased number of peripheral monocytes and usually causes renal damage through infiltration or amyloid deposition. Under normal circumstances, monocytes and macrophages produce lysozyme. This protein is stored in these cells and used to lyse the cell walls of certain bacteria. In some patients with CMML, there is marked overproduction of lysozyme some of which is released into the circulation. It is subsequently filtered by the glomerulus and then reabsorbed by proximal tubular cells where it causes direct toxicity. There may also be an effect related to the fact that it is positively charged thus leading to potassium wasting. Interestingly, previous case reports have found that lysozyme is the predominant protein found in the urine of patients with this disorder and measuring urine and serum lysozyme levels could obviate the need for a renal biopsy in some cases.
The figure above is taken from a case report of lysozyme nephropathy, showing the normal appearing glomeruli, degenerative tubules and many protein reabsorption droplets on EM. Panel C shows strong staining for lysozyme in proximal tubular cells by immunohistochemistry. Treatment is generally supportive and aimed at treating the underlying disorder, thus reducing the production of lysozyme

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