This retrospective cohort analysis demonstrated that about two thirds of native kidney biopsy samples across two institutions that were deemed to have primary FSGS underwent subsequent electron microscopy. Of those that did, two were reported to have characteristics that might be consistent with an underlying collagen IV disorder. In both samples electron microscopy revealed a diffusely thin GBM, with the second additionally identifying early focal splitting of the GBM. The first sample was suggested to be consistent with TBMN whereas there was no pathological comment made about the second. FSGS (NOS) was the most common lesion described in this study, which is consistent with prior reports (2). Importantly close to one in three cases of primary FSGS were not proceeding to electron microscopy despite an indication to do so and 1 in 20 cases within our cohort had structural changes that were consistent with an underlying collagen IV variant. Whilst some samples were unable to undergo electron microscopy due to a lack of glomeruli in the biopsy core, in the majority of the others it is unclear why subsequent electron microscopy did not occur. The annualised rate of biopsy samples not subjected to electron microscopy varied by year, but on average around one in three samples were not subjected to electron microscopy despite receiving a histological diagnosis of FSGS.
Curiously, neither of the two patients in whom electron microscopy was suggestive of an underlying collagen IV disorder was noted to have haematuria on their urinalysis at the time of presentation. Both of these samples were noted to have a thin basement membrane on electron microscopy, with focal splitting noted in one with an average thickness of 230.72nm given. Unfortunately, due to the retrospective nature of this study we were unable to send any samples for immunostaining of collagen IV.
There is an increasing body of evidence indicating that inheritable variants in collagen IV genes may underlie a proportion of cases of FSGS, with up to 12.5% cases of autosomal dominant FSGS attributable to COL4A3 in some cohorts (14). Not subjecting these renal biopsy samples to electron microscopy represents a potential gap in the investigation and subsequent management of such patients given they are much less likely to respond to immunosuppressive therapy (15) which has otherwise been classically indicated.
This study was designed as a retrospective cohort study looking at the number of samples sent for electron microscopy as well as any potential changes which might be consistent with a collagen IV glomerular basement membrane disorder. It is important to recognise that not all groups have found the characteristic changes associated with the collagen IV disorders such as Alport’s Syndrome or TBMN on electron microscopy. One study described the typical pathological changes of FSGS but not the glomerular basement membrane abnormalities characterising Alport syndrome or TBMN in patients known to have variants in either COL4A3 or COL4A4 (7). It is thus possible that a lack of classical findings for a collagen IV glomerular basement membrane disorder may have accounted for the low number of those with GBM features on electron microscopy consistent with a collagen 4 disorder noted within our study. Suggesting against this however another study suggested that biopsy samples from patients with the classical features of Alport Syndrome or TBMN showed podocyte detachment which might be expected and subsequently cause FSGS type lesions (4). Other studies which have looked at electron microscopy in FSGS cases have similarly found low numbers of abnormalities that may be consistent with an underlying collagen 4 disorder (3, 8) which suggests the overall number of abnormalities to be found via electron microscopy may be low.
The process by which variants within the collagen IV genes might cause FSGS remains unclear, particularly given their clear association with Alport Syndrome and TBMN. One proposal is that the ultrastructural changes induced by the collagen IV variants, perhaps under the influence of modifier genes such as laminin, result in impaired podocyte attachment to the glomerular basement membrane which leads to accelerated podocyte detachment, subsequent foot process effacement as a response to the increased shear stress induced by the denuded basement membrane and at a critical level of podocyte loss collapse of the capillary network with the appearance of the classical segmental sclerotic lesion (2, 4, 16). It also remains unclear as to whether the changes of FSGS are a secondary process occurring in those with TBMN or whether the collagen 4 variants are capable of causing primary FSGS (7, 17). FSGS occurring as a secondary process to other basement membrane abnormalities may explain why immunosuppressive therapy has traditionally been less effective in inherited forms of FSGS, although there are case reports of the successful use of the calcineurin inhibitor cyclosporine for some patients harbouring inheritable collagen 4 disorders (6).
In summary, this study has found that not all biopsy samples that had primary FSGS as a histological diagnosis were subjected to subsequent electron microscopy. This may have potentially led to inadvertently overlooking characteristic basement membrane abnormalities which may suggest an underlying and heritable collagen IV disorder. These findings reflect an opportunity to change practice in order to better investigate, counsel and provide clinical management to these and future patients.