IgM monoclonal gammopathy with heavy-and-light-chain amyloidosis resembling fibrillary glomerulonephritis determined by tandem mass spectrometry: a case report

BACKGROUND: Fibrillary glomerulonephritis (FGN) is distinguished from amyloidosis by thicker fibrils and the lack of staining with histochemical dyes typically reactive with amyloid. However, congophilic FGN has been proposed recently and adding laser microdissection followed by mass spectrometry (LMD/MS) to conventional pathological methods would be helpful to diagnose FGN. Here, we report a patient initially diagnosed with FGN whose final pathological diagnosis was changed to immunoglobulin heavy-and-light-chain amyloidosis (AHL) after LMD/MS. CASE PRESENTATION: A 75-year-old male developed nephrotic syndrome. Protein electrophoresis showed IgM κ type M proteinemia and he was diagnosed with IgM monoclonal gammopathy. A renal biopsy was performed and pathological examination showed marked periodic acid-Schiff-positive enlargement of the mesangial region and silver stain positivity, but weak direct fast scarlet staining. Immunofluorescence analysis showed monoclonal deposition of IgM-κ chain in the glomerulus. Under electron microscopy, the fibrils were about 20 nm in diameter, which was thicker than typical amyloid fibrils. Based on these findings, the patient was diagnosed with FGN. Although cyclophosphamide and prednisolone were administered, his renal function deteriorated and progressed to end stage renal disease requiring maintenance hemodialysis. As congophilic FGN has been recognized since 2018, Congo red staining and LMD/MS were performed. The Congo red staining was positive and LMD/MS results indicated that this was a case of AHL. CONCLUSIONS: We reported a case of μ and κ chain AHL resembling FGN requiring LMD/MS for definitive diagnosis. Since FGN and amyloidosis exhibit pathological findings, even if Congo red staining is positive, LMD/MS needs to be considered in cases atypical pathological findings, such as silver stain positivity or thicker fibrils.