Serum amyloid P deposition is a sensitive and specific feature of membranous-like glomerulopathy with masked IgG kappa deposits

Abstract

Membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) is a recently described pattern of glomerulonephritis with a unique histopathology. The pattern is characterized by subepithelial and/or mesangial immune deposits that are "masked", to immunoglobulin staining by routine immunofluorescence but strongly stain for IgG and kappa light chain after protease digestion. Patients with this pattern of glomerulonephritis are most commonly young females presenting with proteinuria and a vague history of autoimmune disease such as low titer antinuclear antibodies. Here we compared the mass spectrometry profile of laser capture microdissected glomeruli from nine MGMID renal biopsies with eight biopsies showing other patterns of membranous glomerulopathy. The protein most significantly increased in MGMID was serum amyloid P. Immunostaining showed serum amyloid P colocalized with IgG in the glomeruli of MGMID but not with PLA2R-associated membranous glomerulopathy. Serum amyloid P was positive in the glomeruli of all 32 MGMID biopsies but negative in biopsies of other types of membranous glomerulopathies such as those associated with PLA2R and THSD7A. There were four biopsies with glomerular serum amyloid P staining among the 173 biopsies that did not fulfill criteria for MGMID or amyloidosis. All four of these biopsies with positive serum amyloid P staining had a membranous pattern of glomerulopathy with IgG kappa deposits that only differed from MGMID by the lack of "masking". Thus, positive staining within glomerular deposits for serum amyloid P identifies a unique form of glomerulonephritis likely sharing a common pathophysiologic mechanism of disease.

Keywords: glomerulonephritis; masked deposits; mass spectrometry; membranous-like glomerulopathy; subepithelial deposits.

Figure 1 |. Serum amyloid P–component (SAP) is significantly enriched in microdissected glomeruli from patients with membranous-like glomerulopathy with masked IgG kappa deposits.

Normalized intensity-based absolute quantification values from MaxQuant (Max Planck Institute of Biochemistry, Planegg, Germany) were used for statistical analysis. Dashed lines depict heuristic cutoffs for fold change and P value. (a) Volcano plot comparing 3 phospholipase A2 receptor (PLA2R)–positive membranous cases with others identifies 2 proteins that are clear outliers in the top right quadrant. (b) Volcano plot comparing 2 thrombospondin type 1 domain containing 7A (THSD7A)–positive membranous cases with others identifies 2 proteins that are clear outliers with THSD7A showing the greatest fold change. (c) Volcano plot comparing samples of membranous-like glomerulopathy with masked IgG kappa deposits with all other types of membranous cases identifies 6 proteins that are clear outliers in the top right quadrant. C6, complement component 6; C7, complement component 7; CFHR1, complement factor-H-related protein 1; HP1BP3, heterochromatin protein 1 binding protein 3; HPRT1, hypoxanthine phosphoribosyltransferase 1; IGHG3, Ig heavy constant gamma 3.

Figure 2 |. Colocalization of IgG and serum amyloid P–component (SAP).

(a–c) Immunofluorescence experiments performed on a renal biopsy sample from a patient diagnosed with membranous-like glomerulopathy with masked IgG kappa deposits shows granular glomerular basement membrane staining for (a) SAP and (b) IgG. (c) There is strong colocalization of SAP and IgG in the glomerular basement membrane deposits. (d–f) Immunofluorescence experiments performed on a renal biopsy sample from a patient with 3 phospholipase A2 receptor– positive membranous glomerulopathies shows (d) negative glomerular basement membrane staining for SAP and (e) positive granular glomerular basement membrane staining for IgG. (f) There is lack of colocalization in this patient with 3 phospholipase A2 receptor–associated membranous glomerulopathies. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 3 |. Renal biopsy findings in membranous-like glomerulopathy with masked IgG kappa deposits.

Photomicrographs from the biopsy shown in Figure 2 are shown here. (a) Segmental spikes are present along the glomerular basement membranes, by Jones methenamine silver stain (original magnification × 400). (b) Electron microscopy reveals subepithelial deposits present along the glomerular basement membrane (original magnification × 12,000). (c) Glomeruli stain negative for IgG by routine immunofluorescence on fresh tissue (direct immunofluorescence; original magnification × 400). (d) Glomeruli from the same case stain positive on paraffin-embedded tissue after protease digestion (direct immunofluorescence; original magnification × 400). (e) Glomeruli show staining for kappa and (f) not lambda on the protease-digested paraffin-embedded tissue (direct immunofluorescence; original magnification × 400). To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.