Recent advances in understanding and treating nephrotic syndrome

Introduction

The glomerulus is the filtration unit of the kidney and allows the passage of vast amounts of water and small solutes (180 l per day in an adult human) into the urinary space while preventing the passage of almost any protein (<30 mg/day). This highly specialised property is achieved by the glomerular filtration barrier (GFB), comprising endothelial cells, a unique basement membrane, and podocytes on the urinary side.

Damage to the GFB can come from many potential sources, including genetic defects (primarily affecting podocytes), paracrine events (for example, affecting endothelial-podocyte cross-talk or from adjacent mesangial cells), or systemic circulating insults. The last of these can take many forms, including circulating immune complexes, metabolic disturbances (most commonly, diabetes), infections, toxins (for example, shiga toxin in haemolytic uraemic syndrome) and drugs.

The clinical manifestation is of oedema, low serum albumin, and massive proteinuria. This leads to numerous consequences, related to circulatory/dynamic effects, and loss of essential circulating proteins.

Idiopathic, or primary, nephrotic syndrome is often used to describe the group of patients for whom no specific cause has been identified, and the histology is relatively non-specific. These patients will usually receive immunosuppression without knowledge of the mechanism and be categorised according to response. So the challenge is to understand and categorise the underlying injury at a molecular level and therefore adapt treatments according to the likely mechanism.

Idiopathic nephrotic syndrome current classification

Current classification of idiopathic nephrotic syndrome (INS) is based on observational characteristics. This can be according to response to steroids, which is the usual first-line response, or according to light microscopy patterns of injury on renal biopsy. The commonest biopsy finding, particularly in children, is of ‘minimal change’ nephrotic syndrome (MCNS). This means that light microscopy is entirely normal, importantly not even showing any signs of immune infiltration or upregulation of immune markers. Pathology is seen only at the level of electron microscopy, where effacement or flattening of podocyte foot processes is the characteristic finding in any patient with nephrosis. The next most common biopsy finding (between 10% and 20% of patients) is of focal segmental glomerulosclerosis (FSGS), a description of chronic, fibrotic damage in the glomerulus. Response to therapy often (but not always) correlates with these biopsy findings, in that most patients with MCNS will be sensitive to steroids and most with FSGS will be resistant. However, there are many shades in between these phenotypes, and patients frequently become relapsing, steroid-dependent, calcineurin-responsive, and so on. This reflects our lack of knowledge of the underlying mechanisms and the need for a different approach to this disease.

Genetics

The most concrete advance in establishing the underlying mechanistic cause of INS in recent years has been the discovery that a substantial proportion of patients with steroid-resistant nephrotic syndrome (SRNS) have a single-gene mutation causing their disease (Table 1). To date, at least 53 different genes have been implicated¹, almost all causing structural or functional defects in the podocyte. Some mutations cause isolated kidney disease, others are part of a syndromic condition. Most are autosomal-recessive, though a few present as X-linked or autosomal-dominant, the latter usually presenting later in life. Some genes have been linked with steroid or cyclosporine sensitivity, such as EMP2² or KANK genes³, though the data are still sporadic and need consistent verification in other pedigrees.

The incidence of genetic disease in the population varies with age, and over 80% of patients presenting under the age of a year have an identifiable mutation^4,5. Overall, in childhood, the incidence in an unselected UK national cohort of SRNS has been reported as 26.5%¹. The incidence in adulthood remains to be ascertained, and phenotype even within individual pedigrees, such as ACTN4 mutations⁶, is often highly variable.

With the advent of next-generation sequencing, the ability to screen for genetic causes in a clinical setting, despite the ever-increasing number of genes to be screened, has become practical, cost-effective and rapid⁷ and is changing clinical practice. Gene panels such as the Bristol SRNS panel (https://www.nbt.nhs.uk/severn-pathology/pathology-services/bristol-genetics-laboratory-bgl) can now yield a result within 4 weeks, thus potentially obviating the need for a biopsy and allowing consideration of early withdrawal of immunosuppression if a positive result is found.

Circulating factor disease

Patients who screen negative for the known SRNS genes, as well as those with steroid-sensitive disease, fall into a category wherein a substantial proportion will have an immune-mediated, circulating factor disease (CFD). The evidence for this remains circumstantial, and the most compelling clinical scenario is that of post-transplant recurrence of disease⁸. This is a situation where patients with SRNS who eventually reach established renal failure are then transplanted. Between 30% and 50% of patients suffer from rapid (most commonly, hours or days post-transplant) recurrence of massive proteinuria, and biopsy of the new kidney shows classic foot process effacement. This is presumed to be due to a circulating factor, and a fascinating recent case report described a patient with early recurrence, whose newly transplanted kidney was then removed and re-transplanted into a recipient without SRNS, and the kidney recovered completely⁹. It is not known where the circulating factor or factors originate, but the fact that immunosuppression is often effective, and relapses are often triggered by viral infections, points to the immune system (Table 2). Both T cells and B cells have been implicated¹⁰; for example, anti-CD20 B cell-depleting monoclonal antibodies are highly effective in some patients¹¹, and a T helper 17/regulatory T (Th17/Treg) cell imbalance has been described in minimal change disease¹². Another clue to a circulating factor pathogenesis in steroid-sensitive nephrotic syndrome, where the biopsy shows minimal change, is the lack of immune cell infiltration or activation within the glomerulus itself.

A significant gap in our knowledge is whether there is one common circulating factor that causes all forms of the disease, or several distinct factors, or possibly a family of related factors. There are no indications at present, either laboratory or clinical, that help to resolve this question, though we have made progress in some areas of prediction of CFD.

Clinical biomarkers

There have been no consistent clinical cues to either whether a patient with nephrotic syndrome has the risk of becoming steroid-resistant in the future or whether they will suffer recurrence post-transplant. There are weak clinical associations with recurrence (for example, age at onset of disease^13,14, race¹⁵, serum albumin at diagnosis¹³ or time to first dialysis/transplant^13,15,16). Interestingly, the last two features may point to the possibility that CFD has a more aggressive presentation and natural course, compared with the monogenic or ‘other’ groups. To address the question of whether there are clinical features that pertain to CFD, we hypothesised that patients with the archetypal CFD, those with post-transplant recurrence, would have distinct early clinical features regarding their initial response to immunosuppression. If a patient has initial steroid sensitivity (otherwise described as secondary steroid resistance), they are likely to have an immune-mediated circulating factor causing their underlying disease and therefore high risk of recurrence. On retrospective review of 150 transplanted patients with SRNS, we found that 93% of patients with initial steroid sensitivity recurred post-transplant (odds ratio = 30, P <0.0001), making this by far the strongest clinical biomarker for recurrent disease yet found¹⁷.

Laboratory biomarkers

Over many decades, researchers have searched for the elusive circulating factor or factors, but there has been no consistent answer to date¹⁸. Clearly, if the factor were identified, this would be the ideal biomarker to both diagnose and monitor disease activity. There are other rational ways to approach biomarker discovery, based on the fact that circulating blood from patients with active disease is likely to carry the active factor and that we know the most likely target cell of such a factor, the podocyte. Therefore, an approach of stimulating human podocytes in vitro with plasma or serum from disease has been used by ourselves and others, and podocyte damage has been assayed in various ways. For example, we have shown that plasma exchange fluid from patients being treated for post-transplant recurrence causes relocalisation of key podocyte slit diaphragm proteins¹⁹ and abnormal signalling and cell motility²⁰. Torban et al. showed a tumour necrosis factor-alpha (TNFα) pathway-dependent change in podocyte cytoskeletal changes²¹ and also changes in podocyte focal adhesion complexes²². The challenge is to show consistency and disease specificity by using these types of assays, before they can be introduced into clinical practice.

Newer therapies

The mainstay of current therapy is immunosuppression, which is appropriate for the immune-mediated group of diseases, but there is very limited evidence of efficacy in monogenic disease. There are studies that support a direct effect of some immunosuppressive drugs on the podocyte^23–27, though the majority of clinical evidence points to efficacy being achieved via effects on the immune system.

Some newer therapies have been proposed on the basis of direct targeting of either the immune system or podocyte signalling pathways. The most prominent of these are the use of anti-CD20 monoclonal antibodies²⁸, which deplete B cells, and anti-B7-1 monoclonal antibody therapy²⁹. The latter has been proposed following the observation that in certain experimental and human glomerular diseases the T-cell co-stimulatory molecule B7-1 has been noted to be upregulated on podocytes³⁰. This can be targeted by the drug abatacept and is the subject of current trials in larger numbers of patients.

Future stratification and personalised medicine

Our understanding of INS at the molecular, cell biology and genetic levels is advancing rapidly, and the information gained will be critical in stratification and re-categorisation of patients into clinically useful mechanistic categories. The advent of rapid second- and now third-generation sequencing technologies is already changing clinical practice. Exome/genome sequencing, side by side with powerful population sequencing databases such as the exome aggregation consortium (ExAC)³¹, means that novel genes will continue to be discovered, even from sporadic cases, to complete our knowledge of the extent of heritable disease in this population.

A challenge of CFD is to discover whether this is a single entity, or separate mechanistic diseases, and target the most appropriate therapies to the individual groups. For example, is relapsing non-progressive disease different from secondary steroid resistance, which usually leads to renal failure (and recurrence post-transplant)?

Novel therapies based on common podocyte signalling pathways are already on the horizon, and the availability of large national patient registries currently being developed will greatly accelerate the ability to trial these in appropriately chosen patient groups.