Diagnosis of FGP can present several challenges particularly if there is no known family history or clinical suspicion and no serum analysis provided, as was the case with our patient. Our initial routine histochemical investigations showed the presence of mesangial deposits extending into the capillary loops that were negative with Congo red staining, suggestive of FGP. Periodic acid-Schiff (PAS) which is generally strong for fibronectin [5
] in this case was weakly positive and it was unclear whether the staining was associated with the deposits. Immunofluorescence microscopy with our routine antibody panel showed positive staining for IgM, C3c and fibrinogen only which also may be seen in fibronectin glomerulopathy [2
Electron microscopy showed moderately electron dense deposits that were globular in shape containing mainly amorphous or granular material with occasional filaments. The moderate electron density of the deposits compared to adjacent basement membrane was consistent with fibronectin deposits described in a previous ultrastructural study [8
]. However, attempts to measure the diameter of the filaments gave variable dimensions ranging between 9-16 nm. Fibronectin filament dimensions have been reported in the range of 10 to 16 nm [3
] and this dimension must then be distinguished from 10 nm amyloid filaments and the larger amyloid fibril [10
] and 18-22 nm fibrils of fibrillary glomerulopathy [11
]. Ultimately, because of the paucity of filamentous structure in this case we felt ultrastructural morphometry was inadequate to make an unequivocal diagnosis of FGP, prompting us to investigate immunoelectron microscopy as an alternative approach.
Initial immunohistochemical staining with antifibronectin was variably positive in capillary loops and the mesangium but it was not clear if the positive staining was due to deposit formation or thickening of the basement membrane. Immunoelectron microscopy was carried out using the same antibody. This showed positive labelling of distinct globular deposits adjacent to the GBM and mesangial matrix providing confirmation of the diagnosis of FGP. The resolution of immunoelectron microscopy allowed specific localisation of fibronectin to the actual deposits rather than just the region of the capillary loop including both GBM and deposit. However, we were unable to quantify the amount of fibronectin present in the deposits or determine other constituents.
The source of the fibronectin in the deposits of FGP remains unknown. Fibronectin is a normal constituent of the GBM predominantly along the endothelial aspect as well as the mesangium [14
]. It has been suggested that abnormalities in the metabolism of circulating fibronectin may play a pathogenetic role in the disease [1
] and studies of family members diagnosed with hereditary fibronectin glomerulopathy show massive glomerular deposits in the subendothelial space with strong positive staining with antibody to common fibronectin [9
] or plasma fibronectin (pFN) [2
]. However, locally produced cellular fibronectin (cFN) is widely distributed in the extracellular matrix [12
] and is known to be secreted by mesangial cells [13
]. The deposits in our case showed positive staining with a monoclonal antibody raised against an isotype of fibronectin secreted by cultivated human fibroblasts, a cFN isotype. We could not find data to indicate whether our antibody to the cell-attachment domain of fibronectin was a universal marker that would also label soluble pFN but specificity testing of an antibody to the 120K collagenbinding fragment of fibronectin found it to recognise human fibronectin and to show cross species reactivity [15
] suggesting that these epitopes are conserved.
The process of deposit formation in fibronectin glomerulopathy is also poorly understood. Experimental studies in animals have identified some conditions in which fibronectin staining in the glomerulus is increased [16
] and generally this was found to be associated with thickening of the basement membrane. So, it is important in any assessment of fibronectin deposition firstly to confirm that the increased staining is localised to a deposit and not thickened GBM or mesangial matrix. In our case the deposits were clearly adjacent the lamina densa of the GBM and mesangial matrix. We do not consider that thickening of the GBM or mesangial matrix initiated the deposition of fibronectin as deposit formation occurred in the absence of obvious thickening or folding of basement membrane. We speculate, as have others [12
], that changes either in the structure of the basement membrane or in circulating pFN may lead to restriction of pFN but cannot rule out the possibility that locally produced mesangial cell cFN may also contribute to these deposits, as may occur in IgA nephropathy [18
In the present case, we were able to determine that at least one close relative had previously died from renal disease suggesting that we were looking at a hereditary case. We have localised fibronectin to globular deposits adjacent the GBM and mesangial matrix though identification of the source of fibronectin will require markers that can discriminate the various isotypes of pFN and cFN. There is currently no definitive treatment for fibronectin glomerulopathy and prognosis is uncertain, in some cases leading to chronic renal failure requiring renal dialysis or renal transplantation. This study adds another case to the small series already reported and is one of the first to confirm the diagnosis by immunoelectron microscopy.