A pediatric case of membranous-like glomerulopathy with masked IgG kappa deposits

Membranous-like glomerulopathy with masked Immunoglobulin G (IgG) kappa deposits (MGMID) is a recently described rare entity. MGMID is characterized by a membranous pattern of kidney injury with monoclonal IgG kappa restriction and is recognized and "unmasked" by pronase digestion on formalin-fixed paraffin-embedded tissue using immunofluorescence staining. This technique is necessary to identify peculiar forms of glomerular immune complex deposition, which is essential for diagnosing MGMID. Patients with MGMID are usually young adult women. We report the case of a 9-year-old boy who was referred for evaluation of proteinuria and hematuria. The patient initially showed isolated microscopic hematuria. However, following an infectious episode, macrohematuria developed, leading to the decision to perform a kidney biopsy. A kidney biopsy showed a membranous pattern of injury characterized by immune deposits distributed segmentally in the subepithelial region, forming spikes in the glomerular basement membrane in all glomeruli, and segmental mesangial hypercellularity on light microscopy. Only segmental deposits of C3 were present along the glomerular basement membrane, as shown by immunofluorescence performed on fresh frozen tissue. Paraffin immunofluorescence showed positive IgG and kappa deposition. Only IgG1 deposits were positive among the IgG subclasses. Electron microscopy showed segmental subepithelial electron-dense deposits. Eventually, the patient was diagnosed with MGMID. To the best of our knowledge, this case of MGMID in a 9-year-old boy is the youngest case documented to date. The findings from our case suggest that immunofluorescence staining with pronase digestion on formalin-fixed paraffin-embedded tissue is required in pediatric patients with a membranous pattern of kidney injury showing only C3 staining by routine immunofluorescence on fresh frozen tissue.