A small molecule chaperone rescues keratin-8 mediated trafficking of misfolded podocin to correct genetic Nephrotic Syndrome.

Podocin is a key membrane scaffolding protein of the kidney podocyte essential for intact glomerular filtration. Mutations in NPHS2, the podocin-encoding gene, represent the commonest form of inherited nephrotic syndrome (NS), with early, intractable kidney failure. The most frequent podocin gene mutation in European children is R138Q, causing retention of the misfolded protein in the endoplasmic reticulum. Here, we provide evidence that podocin R138Q (but not wild-type podocin) complexes with the intermediate filament protein keratin 8 (K8) thereby preventing its correct trafficking to the plasma membrane. We have also identified a small molecule (c407), a compound that corrects the Cystic Fibrosis Transmembrane Conductance Regulator protein defect, that interrupts this complex and rescues mutant protein mistrafficking. This results in both the correct localization of podocin at the plasma membrane and functional rescue in both human patient R138Q mutant podocyte cell lines, and in a mouse inducible knock-in model of the R138Q mutation. Importantly, complete rescue of proteinuria and histological changes was seen when c407 was administered both via osmotic minipumps or delivered orally prior to induction of disease or crucially via osmotic minipump two weeks after disease induction. Thus, our data constitute a therapeutic option for patients with NS bearing a podocin mutation, with implications for other misfolding protein disorders. Further studies are necessary to confirm our findings.

Paediatric rhabdomyolysis: A UK centre's 10-year retrospective experience.

AIMS: To describe the aetiologies of paediatric rhabdomyolysis and explore the medium-term renal consequences. METHODS: Retrospective, single-centre review of children with rhabdomyolysis. RESULTS: Two hundred and thirty-two children met inclusion criteria for the analysis. Mean age at presentation was 8.4 (SD ± 5.5) years. The commonest aetiology was infection (28%), with viral myositis making up the clear majority (75%). Trauma was identified as a cause in 18% of children, seizures in 10% and immune-mediated mechanisms in 8%. Acute kidney injury (AKI) was present in 32% of the cases overall. Children with AKI tended to be younger, with higher peak creatine kinase (CK) and active urinary sediment on urinalysis at presentation. AKI and the need for renal replacement therapy (RRT) were associated with a prolonged hospital stay (15 (interquartile range, IQR 6.5-33) vs. 2 (IQR 0-7) days). A total of 18 children and young people required RRT, with a mean duration of 7.1 ± 4.3 days. Those who received RRT were more likely to have abnormalities on urinalysis at presentation (46% vs. 5%). Over the period of the study, 9% of children died and 2% met criteria for a diagnosis of chronic kidney disease. CONCLUSIONS: This large paediatric rhabdomyolysis case series provides new and unique insights into the condition. Our results highlight the common aetiologies and provide evidence of good renal recovery overall, even in the most severely affected cases. Abnormalities of urinalysis appear to be important in predicting the development of AKI and the need for RRT.

Genomic and clinical profiling of a national nephrotic syndrome cohort advocates a precision medicine approach to disease management.

Steroid Resistant Nephrotic Syndrome (SRNS) in children and young adults has differing etiologies with monogenic disease accounting for 2.9-30% in selected series. Using whole exome sequencing we sought to stratify a national population of children with SRNS into monogenic and non-monogenic forms, and further define those groups by detailed phenotypic analysis. Pediatric patients with SRNS were identified via a national United Kingdom Renal Registry. Whole exome sequencing was performed on 187 patients, of which 12% have a positive family history with a focus on the 53 genes currently known to be associated with nephrotic syndrome. Genetic findings were correlated with individual case disease characteristics. Disease causing variants were detected in 26.2% of patients. Most often this occurred in the three most common SRNS-associated genes: NPHS1, NPHS2, and WT1 but also in 14 other genes. The genotype did not always correlate with expected phenotype since mutations in OCRL, COL4A3, and DGKE associated with specific syndromes were detected in patients with isolated renal disease. Analysis by primary/presumed compared with secondary steroid resistance found 30.8% monogenic disease in primary compared with none in secondary SRNS permitting further mechanistic stratification. Genetic SRNS progressed faster to end stage renal failure, with no documented disease recurrence post-transplantation within this cohort. Primary steroid resistance in which no gene mutation was identified had a 47.8% risk of recurrence. In this unbiased pediatric population, whole exome sequencing allowed screening of all current candidate genes. Thus, deep phenotyping combined with whole exome sequencing is an effective tool for early identification of SRNS etiology, yielding an evidence-based algorithm for clinical management.

Initial steroid sensitivity in children with steroid-resistant nephrotic syndrome predicts post-transplant recurrence.

Of children with idiopathic nephrotic syndrome, 10%-20% fail to respond to steroids or develop secondary steroid resistance (termed initial steroid sensitivity) and the majority progress to transplantation. Although 30%-50% of these patients suffer disease recurrence after transplantation, with poor long-term outcome, no reliable indicator of recurrence has yet been identified. Notably, the incidence of recurrence after transplantation appears reduced in patients with steroid-resistant nephrotic syndrome (SRNS) due to monogenic disorders. We reviewed 150 transplanted patients with SRNS to identify biomarkers that consistently predict outcome of SRNS after transplantation. In all, 25 children had genetic or familial SRNS and did not experience post-transplant recurrence. We reviewed phenotypic factors, including initial steroid sensitivity, donor type, age, ethnicity, time to ESRD, and time on dialysis, in the remaining 125 children. Of these patients, 57 (45.6%) developed post-transplant recurrence; 26 of 28 (92.9%) patients with initial steroid sensitivity recurred after transplantation, whereas only 26 of 86 (30.2%) patients resistant from the outset recurred (odds ratio, 30; 95% confidence interval, 6.62 to 135.86; P<0.001). We were unable to determine recurrence in two patients (one with initial steroid sensitivity), and nine patients did not receive initial steroids. Our data show that initial steroid sensitivity is highly predictive of post-transplant disease recurrence in this pediatric patient population. Because a pathogenic circulating permeability factor in nephrotic syndrome remains to be confirmed, we propose initial steroid sensitivity as a surrogate marker for post-transplant recurrence.

Adeno-associated virus gene therapy prevents progression of kidney disease in genetic models of nephrotic syndrome.

Gene therapy for kidney diseases has proven challenging. Adeno-associated virus (AAV) is used as a vector for gene therapy targeting other organs, with particular success demonstrated in monogenic diseases. We aimed to establish gene therapy for the kidney by targeting a monogenic disease of the kidney podocyte. The most common cause of childhood genetic nephrotic syndrome is mutations in the podocyte gene NPHS2, encoding podocin. We used AAV-based gene therapy to rescue this genetic defect in human and mouse models of disease. In vitro transduction studies identified the AAV-LK03 serotype as a highly efficient transducer of human podocytes. AAV-LK03-mediated transduction of podocin in mutant human podocytes resulted in functional rescue in vitro, and AAV 2/9-mediated gene transfer in both the inducible podocin knockout and knock-in mouse models resulted in successful amelioration of kidney disease. A prophylactic approach of AAV 2/9 gene transfer before induction of disease in conditional knockout mice demonstrated improvements in albuminuria, plasma creatinine, plasma urea, plasma cholesterol, histological changes, and long-term survival. A therapeutic approach of AAV 2/9 gene transfer 2 weeks after disease induction in proteinuric conditional knock-in mice demonstrated improvement in urinary albuminuria at days 42 and 56 after disease induction, with corresponding improvements in plasma albumin. Therefore, we have demonstrated successful AAV-mediated gene rescue in a monogenic renal disease and established the podocyte as a tractable target for gene therapy approaches.

Big data and stratified medicine: what does it mean for children?

Stratified medicine in paediatrics is increasingly becoming a reality, as our understanding of disease pathogenesis improves and novel treatment targets emerge. We have already seen some success in paediatrics in targeted therapies such as cystic fibrosis for specific cystic fibrosis transmembrane conductance regulator variants. With the increased speed and decreased cost of processing and analysing data from rare disease registries, we are increasingly able to use a systems biology approach (including '-omics') to screen across populations for molecules and genes of interest. Improving our understanding of the molecular mechanisms underlying disease, and how to classify patients according to these will lead the way for targeted therapies for individual patients. This review article will summarise how 'big data' and the 'omics' are being used and developed, and taking examples from paediatric renal medicine and rheumatology, demonstrate progress being made towards stratified medicine for children.

Current concepts of the podocyte in nephrotic syndrome.

Nephrotic syndrome is a disorder of the glomerular filtration barrier, and central to the filtration mechanism of the glomerular filtration barrier is the podocyte. We are starting to better understand how this cell, with its unique architectural features, fulfils its exact filtration properties. The multiprotein complex between adjacent podocyte foot processes, the slit diaphragm, is essential to the control of the actin cytoskeleton and cell morphology. Many of the proteins within the slit diaphragm, including nephrin, podocin, transient receptor potential-6 channel, and α-actinin-4, have been identified via genetic studies of inherited nephrotic syndromes. Signaling from slit diaphragm proteins to the actin cytoskeleton is mediated via the Rho GTPases. These are thought to be involved in the control of podocyte motility, which has been postulated as a focus of proteinuric pathways. Nephrotic syndrome is currently treated with immunosuppressive therapy, with significant adverse effects. These therapies may work in nephrotic syndrome due to specific effects on the podocytes. This review aims to describe our current understanding of the cellular pathways and molecules within the podocyte relevant to nephrotic syndrome and its treatment. With our current knowledge of the cellular biology of the podocyte, there is much hope for targeted therapies for nephrotic syndromes.