Partial remission with cyclosporine A in a patient with nephrotic syndrome due to NPHS2 mutation

How immunosuppressive drugs may directly target podocytes in glomerular diseases

Podocytes are the direct target of immunologic injury in many immune-mediated glomerular diseases, leading to proteinuria and subsequent kidney failure. Immunosuppressive agents such as steroids, calcineurin inhibitors, and rituximab are the commonly used treatment strategies in this context for their immunotherapeutic or anti-inflammatory properties. However, in recent years, studies have demonstrated that immunosuppressive agents can have a direct effect on podocytes, introducing the concept of the non-immunologic mechanism of kidney protection by immunomodulators. In this review, we focus on the mechanisms by which these agents may directly target the podocyte independent of their systemic effects and examine their clinical significance.

Current understandings in treating children with steroid-resistant nephrotic syndrome

Steroid-resistant nephrotic syndrome (SRNS) remains a challenge for paediatric nephrologists. SRNS is viewed as a heterogeneous disease entity including immune-based and monogenic aetiologies. Because SRNS is rare, treatment strategies are individualized and vary among centres of expertise. Calcineurin inhibitors (CNI) have been effectively used to induce remission in patients with immune-based SRNS; however, there is still no consensus on treating children who become either CNI-dependent or CNI-resistant. Rituximab is a steroid-sparing agent for patients with steroid-sensitive nephrotic syndrome, but its efficacy in SRNS is controversial. Recently, several novel monoclonal antibodies are emerging as treatment option, but their efficacy remains to be seen. Non-immune therapies, such as angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers, have been proven efficacious in children with SRNS and are recommended as adjuvant agents. This review summarizes and discusses our current understandings in treating children with idiopathic SRNS.

Treatment of steroid-resistant nephrotic syndrome in the genomic era

The pathogenesis of steroid-resistant nephrotic syndrome (SRNS) is not completely known. Recent advances in genomics have elucidated some of the molecular mechanisms and pathophysiology of the disease. More than 50 monogenic causes of SRNS have been identified; however, these genes are responsible for only a small fraction of SRNS in outbred populations. There are currently no guidelines for genetic testing in SRNS, but evidence from the literature suggests that testing should be guided by the genetic architecture of the disease in the population. Notably, most genetic forms of SRNS do not respond to current immunosuppressive therapies; however, a small subset of patients with monogenic SRNS will achieve partial or complete remission with specific immunomodulatory agents, presumably due to non-immunosuppressive effects of these agents. We suggest a pragmatic approach to the therapy of genetic SRNS, as there is no evidence-based algorithm for the management of the disease.

TRPC channels: Regulation, dysregulation and contributions to chronic kidney disease

Mutations in the gene encoding canonical transient receptor potential-6 (TRPC6) channels result in severe nephrotic syndromes that typically lead to end-stage renal disease. Many but not all of these mutations result in a gain in the function of the resulting channel protein. Since those observations were first made, substantial work has supported the hypothesis that TRPC6 channels can also contribute to progression of acquired (non-genetic) glomerular diseases, including primary and secondary FSGS, glomerulosclerosis during autoimmune glomerulonephritis, and possibly in type-1 diabetes. Their regulation has been extensively studied, especially in podocytes, but also in mesangial cells and other cell types present in the kidney. More recent evidence has implicated TRPC6 in renal fibrosis and tubulointerstitial disease caused by urinary obstruction. Consequently TRPC6 is being extensively investigated as a target for drug discovery. Other TRPC family members are present in kidney. TRPC6 can form a functional heteromultimer with TRPC3, and it has been suggested that TRPC5 may also play a role in glomerular disease progression, although the evidence on this is contradictory. Here we review literature on the expression and regulation of TRPC6, TRPC3 and TRPC5 in various cell types of the vertebrate kidney, the evidence that these channels are dysregulated in disease models, and research showing that knock-out or pharmacological inhibition of these channels can reduce the severity of kidney disease. We also summarize several areas that remain controversial, and some of the large gaps of knowledge concerning the fundamental role of these proteins in regulation of renal function.

Genetic diagnosis of steroid-resistant nephrotic syndrome in a longitudinal collection of Czech and Slovak patients: a high proportion of causative variants in NUP93

BACKGROUND

Steroid-resistant nephrotic syndrome (SRNS) has a heterogeneous spectrum of monogenic causes that substantially differ among populations. The aim of this study was to analyse the genetic aetiology of SRNS in Czech and Slovak paediatric patients.

METHODS

We analysed clinical data from 74 patients (38 boys) with congenital (15%), infant (14%), and childhood-onset (71%) SRNS collected from the Czech Republic and Slovakia from 2000 to 2017 (inclusive). The DNA samples were first analysed by Sanger sequencing (genes NPHS2, NPHS1, and WT1) and then by next generation sequencing (NGS) using a targeted panel of 48 genes previously associated with SRNS. Family segregation of the causative variants was confirmed by Sanger sequencing when possible.

RESULTS

Genetic diagnosis was established in 28/74 patients (38%) based on findings of pathogenic or likely pathogenic causative variants in genotypes conforming to the expected mode of inheritance. Sanger sequencing diagnosed 26% of patients, whereas second-tier testing by a targeted NGS panel diagnosed a further 12%. Frequent causative genes were NPHS2 (15%), WT1 (9.5%), and surprisingly NUP93 with four (5.4%) unrelated cases. Additional causative genes included COQ2 (two patients), NPHS1, INF2, DGKE, and LMX1B (one patient each).

CONCLUSIONS

Compared with outright use of NGS, our tiered genetic testing strategy was considerably more rapid and marginally less expensive. Apart from a high aetiological fraction of NPHS2 and WT1 genes, our study has identified an unexpectedly high frequency of a limited set of presumably ancestral causative mutations in NUP93. The results may aid in tailoring testing strategies in Central European populations.

Long-Term Outcome of Steroid-Resistant Nephrotic Syndrome in Children

We investigated the value of genetic, histopathologic, and early treatment response information in prognosing long-term renal outcome in children with primary steroid-resistant nephrotic syndrome. From the PodoNet Registry, we obtained longitudinal clinical information for 1354 patients (disease onset at >3 months and <20 years of age): 612 had documented responsiveness to intensified immunosuppression (IIS), 1155 had kidney biopsy results, and 212 had an established genetic diagnosis. We assessed risk factors for ESRD using multivariate Cox regression models. Complete and partial remission of proteinuria within 12 months of disease onset occurred in 24.5% and 16.5% of children, respectively, with the highest remission rates achieved with calcineurin inhibitor-based protocols. Ten-year ESRD-free survival rates were 43%, 94%, and 72% in children with IIS resistance, complete remission, and partial remission, respectively; 27% in children with a genetic diagnosis; and 79% and 52% in children with histopathologic findings of minimal change glomerulopathy and FSGS, respectively. Five-year ESRD-free survival rate was 21% for diffuse mesangial sclerosis. IIS responsiveness, presence of a genetic diagnosis, and FSGS or diffuse mesangial sclerosis on initial biopsy as well as age, serum albumin concentration, and CKD stage at onset affected ESRD risk. Our findings suggest that responsiveness to initial IIS and detection of a hereditary podocytopathy are prognostic indicators of favorable and poor long-term outcome, respectively, in children with steroid-resistant nephrotic syndrome. Children with multidrug-resistant sporadic disease show better renal survival than those with genetic disease. Furthermore, histopathologic findings may retain prognostic relevance when a genetic diagnosis is established.

Application of next-generation sequencing technology to diagnosis and treatment of focal segmental glomerulosclerosis

A broad range of genetic and non-genetic factors can lead to kidney injury that manifests as focal segmental glomerulosclerosis (FSGS), which can be classified into primary (idiopathic) and secondary forms. Previous genetic approaches to familial or sporadic cases of FSGS or steroid-resistant nephrotic syndrome identified causal mutations in a subset of genes. Recently, next-generation sequencing (NGS) approaches are becoming a part of a standard assessment in medical genetics. Current knowledge of the comprehensive genomic information is changing the way we think about FSGS and draws attention not only to identification of novel causal genes, but also to potential roles for combinations of mutations in multiple genes, mutations with complex inheritance, and susceptibility genes with variable penetrance carrying relatively minor but significant effects. This review provides an update on recent advances in the genetic analysis of FSGS and highlights the potential as well as the new challenges of NGS for diagnosis and mechanism-based treatment of FSGS.

Podocytes from the diagnostic and therapeutic point of view

The central role of podocytes in glomerular diseases makes this cell type an interesting diagnostic tool as well as a therapeutic target. In this review, we discuss the current literature on the use of podocytes and podocyte-specific markers as non-invasive diagnostic tools in different glomerulopathies. Furthermore, we highlight the direct effects of drugs currently used to treat primary glomerular diseases and describe their direct cellular effects on podocytes. A new therapeutic potential is seen in drugs targeting the podocytic actin cytoskeleton which is essential for podocyte foot process structure and function. Incubation of cultured human podocyte cell lines with sera from patients with active glomerular diseases is currently also used to identify novel circulating factors with pathophysiological relevance for the glomerular filtration barrier. In addition, treatment of detached urinary podocytes from patients with substances that restore their cytoskeleton might serve as a novel personalized tool to estimate their potential for podocyte recovery ex vivo.

Podocyte directed therapy of nephrotic syndrome-can we bring the inside out?

Several of the drugs currently used for the treatment of glomerular diseases are prescribed for their immunotherapeutic or anti-inflammatory properties, based on the current understanding that glomerular diseases are mediated by immune responses. In recent years our understanding of podocytic signalling pathways and the crucial role of genetic predispositions in the pathology of glomerular diseases has broadened. Delineation of those signalling pathways supports the hypothesis that several of the medications and immunosuppressive agents used to treat glomerular diseases directly target glomerular podocytes. Several central downstream signalling pathways merge into regulatory pathways of the podocytic actin cytoskeleton and its connection to the slit diaphragm. The slit diaphragm and the cytoskeleton of the foot process represent a functional unit. A breakdown of the cytoskeletal backbone of the foot processes leads to internalization of slit diaphragm molecules, and internalization of slit diaphragm components in turn negatively affects cytoskeletal signalling pathways. Podocytes display a remarkable ability to recover from complete effacement and to re-form interdigitating foot processes and intact slit diaphragms after pharmacological intervention. This ability indicates an active inside-out signalling machinery which stabilizes integrin complex formations and triggers the recycling of slit diaphragm molecules from intracellular compartments to the cell surface. In this review we summarize current evidence from patient studies and model organisms on the direct impact of immunosuppressive and supportive drugs on podocyte signalling pathways. We highlight new therapeutic targets that may open novel opportunities to enhance and stabilize inside-out pathways in podocytes.

Rapid Response to Cyclosporin A and Favorable Renal Outcome in Nongenetic Versus Genetic Steroid-Resistant Nephrotic Syndrome

BACKGROUND AND OBJECTIVES

Treatment of congenital nephrotic syndrome (CNS) and steroid-resistant nephrotic syndrome (SRNS) is demanding, and renal prognosis is poor. Numerous causative gene mutations have been identified in SRNS that affect the renal podocyte. In the era of high-throughput sequencing techniques, patients with nongenetic SRNS frequently escape the scientific interest. We here present the long-term data of the German CNS/SRNS Follow-Up Study, focusing on the response to cyclosporin A (CsA) in patients with nongenetic versus genetic disease.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Cross-sectional and longitudinal clinical data were collected from 231 patients with CNS/SRNS treated at eight university pediatric nephrology units with a median observation time of 113 months (interquartile range, 50-178). Genotyping was performed systematically in all patients.

RESULTS

The overall mutation detection rate was high at 57% (97% in CNS and 41% in SRNS); 85% of all mutations were identified by the analysis of three single genes only (NPHS1, NPHS2, and WT1), accounting for 92% of all mutations in patients with CNS and 79% of all mutations in patients with SRNS. Remission of the disease in nongenetic SRNS was observed in 78% of patients after a median treatment period of 2.5 months; 82% of nongenetic patients responded within 6 months of therapy, and 98% of patients with nongenetic SRNS and CsA-induced complete remission (normalbuminemia and no proteinuria) maintained a normal renal function. Genetic SRNS, on the contrary, is associated with a high rate of ESRD in 66% of patients. Only 3% of patients with genetic SRNS experienced a complete remission and 16% of patients with genetic SRNS experienced a partial remission after CsA therapy.

CONCLUSIONS

The efficacy of CsA is high in nonhereditary SRNS, with an excellent prognosis of renal function in the large majority of patients. CsA should be given for a minimum period of 6 months in these patients with nongenetic SRNS. In genetic SRNS, response to CsA was low and restricted to exceptional patients.

Nonimmunologic targets of immunosuppressive agents in podocytes

Proteinuria is a characteristic finding in glomerular diseases and is closely associated with renal outcomes. In addition, therapeutic interventions that reduce proteinuria improve renal prognosis. Accumulating evidence has demonstrated that podocytes act as key modulators of glomerular injury and proteinuria. The podocyte, or glomerular visceral epithelial cell, is a highly specialized and differentiated cell that forms interdigitated foot processes with neighboring podocytes, which are bridged together by an extracellular structure known as the "slit diaphragm" (SD). The SD acts as a size- and charge-selective barrier to plasma protein. Derangement of SD structure or loss of SD-associated protein results in podocyte injury and proteinuria. During the past decades, several immune-modulating agents have been used for the treatment of glomerular diseases and for the reduction of proteinuria. Interestingly, recent studies have demonstrated that immunosuppressive agents can have a direct effect on the SD-associated proteins and stabilize actin cytoskeleton in podocyte and have therefore introduced the concept of nonimmunologic mechanism of renoprotection by immunomodulators. This review focuses on the evidence that immuno-modulating agents directly target podocytes.

Heterogeneous genetic alterations in sporadic nephrotic syndrome associate with resistance to immunosuppression

In children, sporadic nephrotic syndrome can be related to a genetic cause, but to what extent genetic alterations associate with resistance to immunosuppression is unknown. In this study, we designed a custom array for next-generation sequencing analysis of 19 target genes, reported as possible causes of nephrotic syndrome, in a cohort of 31 children affected by sporadic steroid-resistant nephrotic syndrome and 38 patients who exhibited a similar but steroid-sensitive clinical phenotype. Patients who exhibited extrarenal symptoms, had a familial history of the disease or consanguinity, or had a congenital onset were excluded. We identified a genetic cause in 32.3% of the children with steroid-resistant disease but zero of 38 children with steroid-sensitive disease. Genetic alterations also associated with lack of response to immunosuppressive agents in children with steroid-resistant disease (0% of patients with alterations versus 57.9% of patients without alterations responded to immunosuppressive agents), whereas clinical features, age at onset, and pathologic findings were similar in steroid-resistant patients with and without alterations. These results suggest that heterogeneous genetic alterations in children with sporadic forms of nephrotic syndrome associate with resistance to steroids as well as immunosuppressive treatments. In these patients, a comprehensive screening using such an array may, thus, be useful for genetic counseling and may help clinical decision making in a fast and cost-efficient manner.

Actin-associated Proteins in the Pathogenesis of Podocyte Injury

Podocytes have a complex cellular architecture with interdigitating processes maintained by a precise organization of actin filaments. The actin-based foot processes of podocytes and the interposed slit diaphragm form the final barrier to proteinuria. The function of podocytes is largely based on the maintenance of the normal foot process structure with actin cytoskeleton. Cytoskeletal dynamics play important roles during normal podocyte development, in maintenance of the healthy glomerular filtration barrier, and in the pathogenesis of glomerular diseases. In this review, we focused on recent findings on the mechanisms of organization and reorganization of these actin-related molecules in the pathogenesis of podocyte injury and potential therapeutics targeting the regulation of actin cytoskeleton in podocytopathies.

Treatment of steroid-resistant nephrotic syndrome in children: new guidelines from KDIGO

Kidney Disease: Improving Global Outcomes (KDIGO) recently published the clinical practice guideline on glomerulonephritis (GN) to assist the practitioner caring for patients with GN. Chapter 4 of the guideline focuses on managing children aged 1-18 years with steroid-resistant nephrotic syndrome (SRNS), defined by an inability to achieve complete remission with corticosteroid therapy. Guideline development followed a thorough evidence review, and management recommendations and suggestions were based on the best available evidence. Limitations of the evidence, including the paucity of large-scale randomized controlled trials, are discussed. This article provides both the guideline recommendations and a brief review of relevant treatment trials related to each recommendation. This précis serves as a summary of the complete guidelines recently published.

The KDIGO practice guideline on glomerulonephritis: reading between the (guide)lines--application to the individual patient

The KDIGO guideline for glomerulonephritis is designed to assist health-care providers in treating patients with glomerular diseases. A guideline is not a set of rules but is intended to allow the practitioner to make an informed decision based on the available evidence. Due to its general nature and the variability of strength of the available studies, it is often difficult to directly apply a guideline to the care of an individual patient. This commonly relates to the limited generalizability of the evidence, i.e., does not cover every clinical scenario. To underscore this point, we have introduced within the context of the glomerulonephritis guideline cases with specific features to illustrate the constant need for clinical judgment. These vignettes are intended to demonstrate how the best treatment plans should be individualized and take into account patient preference and clinical acumen, as well as the best available evidence.

The podocyte as a target: cyclosporin A in the management of the nephrotic syndrome caused by WT1 mutations

Children with steroid-resistant nephrotic syndrome secondary to WT1-associated glomerulopathies (WT1-GP) were considered unresponsive to cyclosporin A (CsA). This assumption is challenged by the findings of recent studies. The patients of these studies had different types of WT1 mutations and varying clinical presentations. However, all of them were of young age and the favourable response to CsA might be the result of treatment at an early stage of the disease. The additional administration of angiotensin-converting enzyme inhibitors may have contributed to the positive outcome. We review recent data on the role of WT1 in the development of WT1-GP and discuss putative therapeutic targets explaining the therapeutic effect of CsA.

Effect of cyclosporin A on proteinuria in the course of glomerulopathy associated with WT1 mutations

Denys-Drash syndrome (DDS) is characterized by progressive glomerulopathy caused by diffuse mesangial sclerosis (DMS), genitourinary defects, and a higher risk of developing Wilms' tumor. It is commonly assumed that the DMS is unresponsive to any medications. In this report, we present a patient with Denys-Drash syndrome, in whom the cyclosporine A (CsA) was found to induce total remission. This observation and observations of other authors confirm that in genetic forms of nephrotic syndrome, the proteinuric effect of CsA may be due to a non-immunologic mechanism. We confirm the beneficial effect of CsA treatment in DDS; however, the potential nephrotoxicity of this drug will probably not allow long-term use.

Frasier syndrome: early gonadoblastoma and cyclosporine responsiveness

Frasier syndrome is characterized by progressive glomerulopathy that is unresponsive to corticosteroids, male pseudohermaphroditism, and an increased risk of genitourinary tumors. Of 21 girls with steroid-resistant nephrotic syndrome secondary to focal segmental glomerulosclerosis (FSGS) who were screened for mutations in the WT1 gene, two showed Frasier syndrome. Both patients had donor splice-site mutations in intron 9 of the WT1 gene and a male karyotype (46, XY). Long-term therapy with cyclosporine resulted in partial remission in both cases. One patient showed foci of gonadoblastoma in the excised dysgenetic gonads. This report highlights the need for screening for mutations in the WT1 gene in girls with steroid-resistant FSGS. Patients with Frasier syndrome might benefit from early gonadectomy.

Hereditary nephrotic syndrome: a systematic approach for genetic testing and a review of associated podocyte gene mutations

Several genes have been implicated in genetic forms of nephrotic syndrome occurring in children. It is now known that the phenotypes associated with mutations in these genes display significant variability, rendering genetic testing and counselling a more complex task. This review will focus on the recent clinical findings associated with those genes known to be involved in isolated steroid-resistant nephrotic syndrome in children and, thereby, propose an approach for appropriate mutational screening. The recurrence of proteinuria after transplantation in patients with hereditary forms of nephrotic syndrome will also be discussed.

Immunosuppression and renal outcome in congenital and pediatric steroid-resistant nephrotic syndrome

BACKGROUND AND OBJECTIVES

Mutations in podocyte genes are associated with steroid-resistant nephrotic syndrome (SRNS), mostly affecting younger age groups. To date, it is unclear whether these patients benefit from intensified immunosuppression with cyclosporine A (CsA). The aim of this study was to evaluate the influence of podocyte gene defects in congenital nephrotic syndrome (CNS) and pediatric SRNS on the efficacy of CsA therapy and preservation of renal function.

DESIGN, SETTINGS, PARTICIPANTS, & MEASUREMENTS

Genotyping was performed in 91 CNS/SRNS patients, irrespective of age at manifestation or response to CsA.

RESULTS

Mutations were identified in 52% of families (11 NPHS1, 17 NPHS2, 11 WT1, 1 LAMB2, 3 TRPC6). Sixty-eight percent of patients with nongenetic SRNS responded to CsA, most of them achieved complete remission. In contrast, none of the patients with genetic CNS/SRNS experienced a complete remission and only two (17%) achieved a partial response, both affected by a WT1 mutation. Preservation of renal function was significantly better in children with nongenetic disease after a mean follow-up time of 8.6 years (ESRD in 29% versus 71%).

CONCLUSIONS

The mutation detection rate in our population was high (52%). Most patients with genetic CNS/SRNS did not benefit from CsA with significantly lower response rates compared with nongenetic patients and showed rapid progression to end-stage renal failure. These data strongly support the idea not to expose CNS/SRNS patients with inherited defects related to podocyte function to intensified immunosuppression with CsA.

Non-immunologic mechanisms of calcineurin inhibitors explain its antiproteinuric effects in genetic glomerulopathies

It has been reported (this issue Pediatric Nephrology) that cyclosporine A (CyA) therapy in combination with corticosteroids, angiotensin-converting enzyme inhibitor, and an angiotensin receptor blocker decreased proteinuria in three patients with nephrotic syndrome (NS) due to WT1 mutations. Treatment with calcineurin inhibitors were found to induce a partial remission of proteinuria in several other children with genetic forms of NS, such as mutation in the podocine and in the phospholipase C epsilon gene. CyA therapy has also been reported to be beneficial to patients with Alport syndrome. Recent data have shown that the antiproteinuric effect of CyA in these cases may be due to a non-immunologic mechanism. CyA exerts an antiproteinuria effect by preventing the degradation of the actin organizing protein synaptodpodin and by a downregulation of TRPC6. This mechanism leads to the stabilization of the actin cytoskeleton in the kidney podocytes. This beneficial effect of CyA is interesting, but long-term results regarding function and nephrotoxicity are still missing.