Renal Crystal-Storing Histiocytosis: Transition from Monoclonal Gammopathy of Unknown Significance to Monoclonal Gammopathy of Renal Significance

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Introduction: Crystal-storing histiocytosis (CSH) is a rare condition where immunoglobulins accumulate in histiocytes, causing a variety of manifestations depending on the affected organ systems. We present a case of a man with MGUS which transitioned to MGRS evidenced by renal CSH. Case Description: A 71-year-old man was diagnosed with MGUS from the workup of normocytic anemia 10 years ago. Urine protein electrophoresis showed a monoclonal spike, serum-free kappa level was 563 mg/dL, and immunofixation showed a κ light chain without corresponding heavy chain and faint IgG κ monoclonal protein. Bone marrow revealed 5-10% of plasma cells. Urinary protein: creatinine ratio (UPCR) had been stable at 400 mg/g of creatinine (Cr). Six years later, he presented with non-nephrotic-range proteinuria. A 24-hour-urine protein was 1,436 mg/day with a stable creatinine of 1.1-1.3 mg/dL. Given the increase in proteinuria with an increase in levels of IgG, kappa light chain, and M-protein and worsening anemia, a kidney biopsy was performed which showed kappa light chain proximal tubulopathy, crystallin type, crystal storing histiocytosis. Segmental glomerulosclerosis, with podocyte crystalline inclusions. Low-grade segmental lambda light chain mesangial deposits. Mildly thickened glomerular basement membranes, mild global glomerulosclerosis, arterial and mild arteriolar sclerosis. He has been treated with bortezomib and dexamethasone. His serum Cr is stable at 1.1-1.2 mg/dL and a 24-hour-urine protein decreased to 257 mg/day over 4 years despite without antiproteinuric agents. Discussion: Our patient had MGUS for 4 years before developing MGRS secondary to renal CSH. Renal involvement in MGUS is usually due to AL amyloidosis, light-chain deposition disease, or chronic kidney disease secondary to paraproteinemia. Nephropathy associated with CSH is extremely rare. Patients diagnosed with MGUS can develop multiple myeloma or other related diseases including MGRS which increases the risk for morbidity and mortality. Therefore, surveillance for transition from MGUS to MGRS is warranted to to early detect and promptly treat for evolving kidney diseases.

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