Multiple Myeloma With Renal Pathological Findings of Both Light Chain-Only Variant of Proliferative Glomerulonephritis With Monoclonal Immunoglobulin Deposits (PGNMID-LC) and Light Chain Deposition Disease (LCDD)

Light chain-only variant of proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID-LC) and light chain deposition disease (LCDD) are both renal disorders caused by the overproduction of monoclonal immunoglobulin light chains and their deposition in renal tissues. However, the renal pathological features of these two entities are characteristically distinct. We report a rare case of multiple myeloma presenting with renal pathology exhibiting overlapping features of both PGNMID-LC and LCDD. A 54-year-old woman presented with nephrotic syndrome. Renal biopsy revealed glomerular lesions resembling type III membranoproliferative glomerulonephritis on light microscopy. Immunofluorescence microscopy demonstrated κ light chain deposition along the glomerular capillary walls as well as in the mesangial areas, Bowman's capsule, and tubular basement membranes. C3 was also deposited in the glomeruli, while IgG and λ light chain staining were negative. Electron microscopy revealed dense deposits in the mesangial, subepithelial, and subendothelial areas, along with powdery deposits within the glomerular basement membrane, tubular basement membranes, and Bowman's capsule. Monoclonal κ light chains were detected in both serum and urine, and bone marrow examination showed 30.6% monoclonal plasma cells. The patient was diagnosed with multiple myeloma-associated kidney disease due to monoclonal light chain deposition and was treated with bortezomib and dexamethasone, resulting in improved renal function and a marked reduction in proteinuria. This case highlights a rare pathological overlap between PGNMID-LC and LCDD, underscoring the importance of comprehensive renal pathological evaluation in patients with monoclonal gammopathies.