Membranoproliferative glomerulonephritis with deposition of monoclonal IgG evolved from polyclonal IgG: a case report with two consecutive renal biopsies

BACKGROUND: Proliferative glomerulonephritis with monoclonal Immunoglobulin (G) deposits (PGNMID) is a rare kind of MGRS with intact monoclonal IgG deposition. Seventy percent of PGNMID patients were negative for M-spike. CASE PRESENTATION: A 51-year-old Chinese woman presented with 16-month history of chronic nephritic syndrome. Her first biopsy showed a MPGN pattern, and the IF showed polyclonal IgG deposition but with IgG3λ dominance, MGRS was highly suspected. But the serum/urine IFE and bone marrow examination was negative for monoclonal gammopathy. She was treated with RAS inhibitors, and monitored carefully in the outpatient clinic. When the proteinuria was not controlled by RAS inhibitors, immunosuppressive agents were initiated. The second biopsy was done due to her acute kidney injury 9 months later, showing a MPGN pattern with acute tubulointerstitial disease, but the IF showed monoclonal IgG3λ deposition. The κ light chain, IgG1, IgG2 and IgG4 were absent. Electron microscopic examination revealed electron-dense deposits in the mesangial, subendothelial and subepithelial area which is the same as the first renal biopsy. The final diagnose of this patient was PGNMID (IgG3λ) with non-organized deposits. Repeated serum/urine IFE and free light chain still failed to identify monoclonal gammopathy. The patient was treated with steroid and cyclophosphamide, and her serum creatinine decreased. CONCLUSIONS: Some of the PGNMID patients may be derived from polyclonal immune complex mediated glomerulonephritis.