Clinical features and mutational survey of NPHS2 (podocin) in Japanese children with focal segmental glomerulosclerosis who underwent renal transplantation

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.

The amino acid mutations of the podocin in proteinuria: a meta-analysis

While many previous studies have reported an association between the single-nucleotide polymorphisms (SNPs) of the podocin and proteinuria occurred, a conclusive relationship has not been defined in every oligoallelic state of amino acid (AA) mutations in podocin. In this study, we performed a meta-analysis of the published data to investigate the impact of the oligoallelic AA mutations of the podocin on proteinuria; a total 16 AA mutations were investigated for oligoallelic pathogenicity. Despite significant heterogeneity within some of the comparisons, the results revealed significantly higher risks of proteinuria in early-onset (onset age <16) individuals for five mutations (P118L, R138Q, R168H, V180M, and V260E), and in all onset ages individuals for five mutations (R138Q, G140X, R229Q, V260E, and V290M) compared to non-variant individuals. We also tested the steroid response in individuals with R229Q and E237Q. No statistically significant differences in the two mutations carrier rate were observed between steroid resistance patients and controls. No AA mutation was selected for meta-analysis on the recurrence of proteinuria after renal transplantation as lack of control data. In conclusion, our meta-analysis tested the pathogenicity of the oligoallelic AA mutations in podocin and suggested the potential causative mutations, and the alleles showing an association with protein susceptibility. The sensitivity and specificity of each causative mutation are pending further testing.

Decreased glomerular filtration as the primary factor of elevated circulating suPAR levels in focal segmental glomerulosclerosis

BACKGROUND

Circulating factor(s) has been thought to be the underlying cause of focal segmental glomerulosclerosis (FSGS), and recent studies foster this idea by demonstrating increased soluble urokinase receptor (suPAR) levels in the serum of FSGS patients.

METHODS

To explore the possible contribution of suPAR in FSGS pathogenesis, we analyzed serum suPAR levels in 17 patients with FSGS and compared them with those in patients with steroid-sensitive nephrotic syndrome, chronic glomerulonephritis, or non-glomerular kidney diseases.

RESULTS

Serum suPAR levels in patients with FSGS were higher than those in patients with steroid-sensitive nephrotic syndrome or chronic glomerulonephritis, but not higher than those in patients with non-glomerular kidney diseases. suPAR levels negatively correlate with estimated glomerular filtration rate and were decreased after renal transplantation in patients with FSGS as well as in those with non-glomerular kidney diseases. Furthermore, 6 FSGS patients with post-transplant recurrence demonstrated that suPAR levels were not high during the recurrence.

CONCLUSIONS

Based on our results, elevated suPAR levels in FSGS patients were attributed mainly to decreased glomerular filtration. These data warrant further analysis for involvement of possible circulating factor(s) in FSGS pathogenesis.

The expression of cytoskeletal proteins in kidney specimens of children with primary focal segmental glomerulosclerosis

Several studies have evaluated cytoskeletal proteins as prognostic factors for some types of nephrotic syndrome. However, studies concerning children with FSGS are scarce. This study was done to evaluate the glomerular, tubular, and interstitial expression of vimentin, desmin, and alpha smooth muscle actin (α-SMA) in kidney specimens of children with FSGS. Clinical and histologic data of 31 children with FSGS were reviewed. Thirty one formalin-fixed, paraffin-embedded kidney biopsy sections (3 μm) were selected for immunohistochemical staining. Double immunohistochemistry using a microwave-based two-color staining was applied. The mean age at onset in male and female was 56.3 ± 41.4 and 78.0 ± 60.4 months, respectively. The duration of follow-up was 46.3 ± 56.5 months. Interstitial fibrosis and tubular atrophy were reported in 42% and 54% of the patients, respectively. The latest evaluated mean blood pressure was significantly correlated with the expression of both vimentin and α-SMA in the interstitium (P < 0.05). However, we were not able to demonstrate any cytoskeletal protein expression as an independent predictor for renal survival. Further studies with larger sample size and longer follow-up periods are warranted to investigate the prognostic values of other histopathologic features in pediatrics with FSGS.

Pretransplantation combined therapy with plasmapheresis and rituximab in a second living-related kidney transplant pediatric recipient with a very high risk for focal segmental glomerulosclerosis recurrence

Prophylactic PP can provide some protection against post-transplantation recurrences of FSGS, but it cannot prevent recurrences in all cases. Therefore, new preventive therapies are needed. We report on a 7.9-yr-old girl treated with pretransplantation prophylactic combined therapy consisting of four sessions of PP and one dose of rituximab before a second living-related KTX. The patient had a very high risk of post-transplantation FSGS recurrence because this had occurred after the first KTX. During the 36 months since the second transplantation, she has had no evidence of proteinuria or significant infectious complications. Although our experience is too preliminary to draw any generalizable conclusions, pretransplantation combined therapy with PP and rituximab might be a possible option for the prevention of FSGS recurrence in very high-risk recipients undergoing living-donor KTXs.

Preemptive plasmapheresis and recurrence of focal segmental glomerulosclerosis in pediatric renal transplantation

  FSGS has a high recurrence rate after renal transplantation. To examine the effects of the use of preemptive and post-transplant PP on recurrence and graft outcome, we conducted a retrospective study on 34 pediatric patients (mean age 13±5 yr) with biopsy-proven pretransplant FSGS and who underwent a renal transplantation between 1996 and 2007. Recurrence was defined as a serum albumin level of <3.0g/L in the presence of nephrotic-range proteinuria (>40mg/m(2) /h). Total response to PP therapy was defined as the resolution of the nephrotic-range proteinuria and partial response as persistent proteinuria despite PP but not in the nephrotic range. Fifteen patients received a LD renal transplantation and 19 patients received a DD renal transplantation. Nineteen patients received CsA and 14 patients received tacrolimus. Nineteen patients (56%) had FSGS recurrence. There was no difference in the recurrence rate between patients receiving CsA vs. tacrolimus. Among the 15 LD patients, 13 received preemptive PP (1-10 sessions) and seven patients (47%) had subsequent FSGS recurrence. Among the 19 DD patients, four received preemptive PP and 12 (63%) had FSGS recurrence. The number of preemptive PP did not affect the recurrence rate. In a group of patients with a previous graft loss secondary to recurrence, the rate of recurrence was lower than expected (40%) and two of the three patients who did not recur had three or more sessions of preemptive PP. Of the 19 patients with recurrence, 17 were treated with PP therapy and 88% of the patients fully or partially responded. Only five patients had graft loss at three yr post-transplant: two from FSGS recurrence and three from non-compliance. These results suggest that preemptive PP does not decrease the rate of recurrence after transplantation but might be beneficial in treating high-risk patients with documented recurrence. Patients with FSGS recurrence post-transplant can achieve good graft survival with both LD and DD transplantation.

Potential donor-recipient MYH9 genotype interactions in posttransplant nephrotic syndrome after pediatric kidney transplantation

Recurrence of focal segmental glomerulosclerosis (FSGS) with nephrotic syndrome is relatively common after kidney transplantation in young recipients whose predialysis course consists of heavy proteinuria, hypertension and subacute loss of kidney function. The gene(s) mediating this effect remain unknown. We report an unusual circumstance where kidneys recovered from a deceased African American male donor with MYH9-related occult FSGS (risk variants in seven of eight MYH9 E1 haplotype single nucleotide polymorphisms) were transplanted into an African American male child with risk variants in four MYH9 E1 risk variants and a European American female teenager with two MYH9 E1 risk variants. Fulminant nephrotic syndrome rapidly developed in the African American recipient, whereas the European American had an uneventful posttransplant course. The kidney donor lacked significant proteinuria at the time of organ procurement. This scenario suggests that donor-recipient interactions in MYH9, as well as other gene-gene and gene-environment interactions, may lead to recurrent nephrotic syndrome after renal transplantation. The impact of transplanting kidneys from donors with multiple MYH9 risk alleles into recipients with similar genetic background at high risk for recurrent kidney disease needs to be determined.

Increase of integrin-linked kinase activity in cultured podocytes upon stimulation with plasma from patients with recurrent FSGS

Recurrent focal segmental glomerulosclerosis (FSGS) is a major challenge in the field of transplantation. Integrin-linked kinase (ILK) has emerged as a key mediator of podocyte-glomerular basement membrane (GBM) interactions. To clarify the involvement of plasma factors in FSGS recurrence, we examined the effects of plasma from FSGS patients with or without posttransplant recurrence on cultured podocytes, focusing particularly on ILK activity. Podocytes from a conditionally immortalized mouse podocyte cell line were treated with plasma from 11 FSGS patients, and ILK activity was determined using an immune complex kinase assay. Treatment with plasma from three patients with recurrence induced an increase in ILK activity. In contrast, no increase in ILK activity was observed in cultured podocytes treated with plasma from the remaining three patients with recurrence and five patients without recurrence. Cultured podocytes treated with plasma that induced ILK activity showed alterations of focal contact and detachment from the laminin matrix. In conclusion, this preliminary study provides experimental evidence suggesting the possible presence of circulating toxic factors in the plasma of some patients with recurrent FSGS, which induce an increase in podocyte ILK activity that may lead to the detachment of podocytes from the GBM.