Recessive Steroid-resistant Nephrotic Syndrome Research Articles

Overcoming the challenges associated with identification of deep intronic variants by whole genome sequencing.

Whole-genome sequencing (WGS) now allows identification of multiple variants in non-coding regions. The large number of variants identified by WGS however complicates their interpretation. Through identification of the first deep intronic variant in NPHS2, which encodes podocin, a protein implicated in autosomal recessive steroid resistant nephrotic syndrome (SRNS), we compare herein three different tools including a newly developed targeted NGS-based RNA-sequencing to explore the splicing effect of intronic variations. WGS identified two different variants in NPHS2 eventually involved in the disease. Through RT-PCR, exon-trapping Minigene assay and targeted RNA sequencing, we were able to identify the splicing defect in NPHS2 mRNA from patient kidney tissue. Only targeted RNA-seq simultaneously analyzed the effect of multiple variants and offered the opportunity to quantify consequences on splicing. Identifying deep intronic variants and their role in disease is of utmost importance. Alternative splicing can be predicted by in silico tools but always requires confirmation through functional testing with RNA analysis from the implicated tissue remaining the gold standard. When several variants with potential effects on splicing are identified by WGS, a targeted RNA sequencing panel could be of great value.

MO044: Cellular mechanism of the exceptional dominant transmission in NPHS2-associated glomerulopathy

Abstract BACKGROUND AND AIMS Mutations of NPHS2 cause autosomal recessive steroid-resistant nephrotic syndrome (SRNS). The encoded podocin, a key component of the slit diaphragm, homo-oligomerizes through two C-terminal oligomerization sites. We formerly found that specific podocin variants may exert a dominant negative effect against R229Q podocin variant secondary to an altered oligomerization [1, 2]. Recently, two families with autosomal dominant focal segmental glomerulosclerosis were found to carry a de novo heterozygous NPHS2 last-exon frameshift mutations affecting the second oligomerization site (either c.989_992delTGTC, p.L330Pfs*17 or c.988_989delCT, p.L330Vfs*15) [3]. We hypothesized that these podocin variants exert a dominant negative effect against wild type (WT) podocin. METHODS We coexpressed eGFP-tagged WT podocin with either hemagglutinin-tagged (HA) WT, p.L330Pfs*17 or p.L330Vfs*15 podocin variants in cultured podocytes. Plasma membrane was stained with CF405M-conjugated wheat germ agglutinin (WGA), and immunohistochemical staining was performed with primary anti-HA and secondary AlexaFluor568-conjugated antibodies. All analysis was done in a blinded fashion. The membrane-targeting of podocin was assessed by the ratio of the WGA-labeled perimembranous area that colocalized with podocin (Mander's overlap coefficients, MOC) and the colocalization of the coexpressed variants were estimated as Pearson correlation coefficients (PCC) by FiJi software on confocal microscopic images. MOC and PCC were compared by Holmes–Bonferroni adjusted Wilcoxon signed-rank test. RESULTS The frameshift podocin variants were not membranous in monoexpression [MOC: HA-p.L330Pfs*17 (n = 26): 0.08 (0.03–0.22) median (interquartile ranges); HA-p.L330Vfs*15 (n = 24): 0.07 (0.02–0.16)]. The GFP-tagged WT podocin strongly colocalized with the HA-tagged WT or frameshift podocin variants [PCC: WT-eGFP + HA-WT (n = 19): 0.64 (0.49–0.71); WT-eGFP + HA-L330Pfs*17 (n = 17): 0.54 (0.35–0.58); WT-eGFP + HA-L330Vfs*15 (n = 17): 0.44 (0.29–0.66)]. The GFP-WT podocin was retained in intracellular compartments in the presence of any of the two frameshift podocin variants [MOCWT(WT) = 0.58 (0.46–0.69)], MOCWT(L330Pfs*17) = 0.01 (0.002–0.029), P = 2x10-6 versus WT(WT); MOCWT(L330Vfs*15) = 0.003 (0.001–0.013), P = 2 x 10−6 versus WT(WT)]. CONCLUSIONS The dominant podocin variants retain WT podocin in intracellular compartments explaining the exceptional dominant transmission of NPHS2-associated glomerulopathy. This is in accordance with our former results showing the primary role of C-terminal hetero-oligomerization in exerting interallelic interactions of NPHS2 alleles. FUNDING MTA-SE Lendulet Research Grant (LP2015-11/2015), OTKA KH125566 and K135798, and ÚNKP-20–3-I-SE-29 from the source of the National Research, Development and Innovation Fund.

The podocin V260E mutation predicts steroid resistant nephrotic syndrome in black South African children with focal segmental glomerulosclerosis

In black African children with focal segmental glomerulosclerosis (FSGS) there are high rates of steroid resistance. The aim was to determine genetic associations with apolipoprotein L1 (APOL1) renal risk variants and podocin (NPHS2) variants in 30 unrelated black South African children with FSGS. Three APOL1 variants were genotyped and the exons of the NPHS2 gene sequenced in the cases and controls. APOL1 risk alleles show a modest association with steroid sensitive nephrotic syndrome (SSNS) and steroid resistant nephrotic syndrome (SRNS). The NPHS2 V260E variant was present in SRNS cases (V/V = 5; V/E = 4; E/E = 11), and was absent in SSNS cases. Haplotype analysis suggests a single mutation origin for V260E and it was associated with a decline in kidney function over a 60-month period (p = 0.026). The V260E variant is a good predictor of autosomal recessive SRNS in black South African children and could provide useful information in a clinical setting.

Nephrotic Syndrome With Mutations in NPHS2: The Role of R229Q and Implications for Genetic Counseling

Mutations in the NPHS2 gene, which encodes the podocyte slit diaphragm protein podocin, cause autosomal recessive steroid-resistant nephrotic syndrome (Online Mendelian Inheritance in Man [OMIM] #600995). Basic research and clinical studies have provided important insights about genotype-phenotype correlations. This knowledge allows personalized genetic (risk) counseling and should lead to changes in the advice given to patients. A patient who carries the R229Q variant (which has a high allele frequency of 3.7% in the European population) in combination with a pathogenic variant in exon 7 or 8 is at high risk for developing nephrotic syndrome that may not manifest before adulthood, whereas a patient with 2 pathogenic variants will develop congenital or childhood-onset nephrotic syndrome. In contrast, a patient who carries the R229Q variant in combination with a pathogenic variant in exons 1 to 6 is unlikely to develop nephrotic syndrome. In this article, we review the emerging knowledge about the NPHS2 gene and translate these findings from the bench to practical advice for the clinical bedside.

MAGI2 Mutations Cause Congenital Nephrotic Syndrome.

Steroid-resistant nephrotic syndrome (SRNS), a heterogeneous disorder of the renal glomerular filtration barrier, results in impairment of glomerular permselectivity. Inheritance of genetic SRNS may be autosomal dominant or recessive, with a subset of autosomal recessive SRNS presenting as congenital nephrotic syndrome (CNS). Mutations in 53 genes are associated with human SRNS, but these mutations explain ≤30% of patients with hereditary cases and only 20% of patients with sporadic cases. The proteins encoded by these genes are expressed in podocytes, and malfunction of these proteins leads to a universal end point of podocyte injury, glomerular filtration barrier disruption, and SRNS. Here, we identified novel disease-causing mutations in membrane-associated guanylate kinase, WW, and PDZ domain-containing 2 (MAGI2) through whole-exome sequencing of a deeply phenotyped cohort of patients with congenital, childhood-onset SRNS. Although MAGI2 has been shown to interact with nephrin and regulate podocyte cytoskeleton and slit diaphragm dynamics, MAGI2 mutations have not been described in human SRNS. We detected two unique frameshift mutations and one duplication in three patients (two families); two siblings shared the same homozygous frameshift mutation, whereas one individual with sporadic SRNS exhibited compound heterozygosity. Two mutations were predicted to introduce premature stop codons, and one was predicted to result in read through of the normal translational termination codon. Immunohistochemistry in kidney sections from these patients revealed that mutations resulted in lack of or diminished podocyte MAGI2 expression. Our data support the finding that mutations in the MAGI2 gene are causal for congenital SRNS.

NUP107 mutations in children with steroid-resistant nephrotic syndrome.

NUP107 is a novel gene associated with autosomal recessive steroid-resistant nephrotic syndrome (SRNS) with focal segmental glomerulosclerosis (FSGS) in children. The frequency of NUP107 mutations in children with SR-FSGS remains unknown. Nine families with two siblings affected by childhood-onset SRNS or proteinuria were recruited. FSGS was confirmed by a kidney biopsy in at least one affected sibling in all families. Additionally, 69 sporadic pediatric cases with biopsy-proven SR-FSGS who had not responded to any treatment were included. All coding exons with flanking introns of the NUP107 gene were amplified using polymerase chain reaction and directly sequenced. Biallelic NUP107 mutations were detected in four pairs (44.4%) of siblings from the familial cases and three (4.3%) sporadic cases. All affected patients harbored the p.Asp831Ala mutation in one allele and a truncating or abnormal splicing mutation in the other allele. NUP107 mutation-positive patients showed an earlier onset age (39.4 ± 13.1 versus 76.8 ± 50.0 months, P= 0.027) and more rapid progression to end-stage renal disease (at the ages of 58.9 ± 23.4 versus 123.1 ± 62.7 months, P < 0.001) compared with mutation-negative patients. None of the eight mutation-positive cases, who underwent kidney transplantation, showed recurrence of FSGS in the graft kidney, while 35.3% of mutation-negative cases showed recurrence of FSGS. An unexpectedly high incidence of NUP107 mutations was observed in Korean children with SR-FSGS. Initial genetic screening of children with SR-FSGS should include the NUP107 gene, at least in Korea. Further studies are necessary to determine the incidences of NUP107 mutations in other countries.

Identificación de variantes del gen NPHS2 en niños con síndrome nefrótico corticorresistente

Podocin is a protein located in the glomerular slit diaphragm where it takes part in the regulation of glomerular filtration. Mutations of the NPHS2 gene that codes podocin are the main cause of autosomal recessive steroid resistant nephrotic syndrome (SRNS). ObjectivesTo identify the NPHS2 mutations in Chilean children with SRNS, and to determine the prevalence of the most common variants in a group of healthy adults. Patients and methodsMutation analysis of NPHS2 in 34 Chilean children with SRNS. Once the two most common variants of NPHS2 were identified, screening for these mutations was performed on 233 healthy adults. The mutation analysis was performed by the direct sequencing of the eight coding exons by polymerase chain reaction amplification. The DNA sequencing was performed using a fluorometric method, and then evaluated with SeqPilot™ software. The relationship between the presence of NPHS2 variants and SRNS was calculated by comparing the allele frequency between patients with SRNS and those of the healthy volunteers using the exact Fisher test. A P<.05 was considered significant. ResultsPathogenic NPHS2 mutations were detected in 7 (21%) of the 34 patients studied, of which 6 were heterozygotes for p.R229Q and p.A284V. The presence of p.R229Q was 2.46% in the healthy volunteers. ConclusionsThis study shows that p.R229Q and p.A284V are the most frequent variants in Chilean children with SRNS. It is the first time that this relationship has been reported in Chilean children. Based on this, a screening strategy is proposed for the genetic study in patients with SRNS and their families. A parallel or sequential search strategy for p.R229Q and p.A284V in these patients is proposed.

Variability in phenotype induced by the podocin variant R229Q plus a single pathogenic mutation.

BackgroundMutations in podocin (NPHS2) are the most common cause of childhood onset autosomal recessive steroid-resistant nephrotic syndrome (SRNS). The disease is characterized by early-onset proteinuria, resistance to immunosuppressive therapy and rapid progression to end-stage renal disease. Compound heterozygous changes involving the podocin variant R229Q combined with another pathogenic mutation have been associated with a mild phenotype with disease onset often in adulthood.MethodsWe screened 19 families with early-onset SRNS for mutations in NPHS2 and WT1 and identified four disease-causing mutations (three in NPHS2 and one in WT1) prior to planned whole-exome sequencing.ResultsWe describe two families with three individuals presenting in childhood who are compound heterozygous for R229Q and one other pathogenic NPHS2 mutation, either L327F or A297V. One child presented at age 4 years (A297V plus R229Q) and the other two at age 13 (L327F plus R229Q), one with steadily deteriorating renal function.ConclusionsThese cases highlight the phenotypic variability associated with the NPHS2 R229Q variant plus pathogenic mutation. Individuals may present with early aggressive disease.

NPHS2 variation in Chinese southern infants with late steroid-resistant nephrotic syndrome

NPHS2 mutations are responsible for autosomal recessive familial steroid-resistant nephrotic syndrome (SRNS) with minor glomerular abnormalities or focal segmental glomerulosclerosis (FSGS), which is characterized by early childhood onset and rapid progression to chronic renal insufficiency. This gene mutation is also responsible for an adolescent onset form of autosomal recessive familial FSGS with heavy proteinuria. Many infants with late steroid-resistant nephrotic syndrome (late SRNS) are prone to an implicated clinical and therapeutic course. The etiopathogenesis and the long-term prognosis of late SRNS remain obscure. Considering the similar steroid resistance between late and initial SRNS, analysis of NPHS2 variation was performed in 70 sporadic Chinese infants with late SRNS and 70 controls in the present study to investigate the possible role of NPHS2 gene in late SRNS. The variation analysis revealed three polymorphisms (288C > T heterozygous in exon 2, 954T > C heterozygous and homozygous, and 1038A > G heterozygous in exon 8) in 22 out of 70 patients studied. In conclusion, NPHS2 gene mutations are not a major cause of chronic renal insufficiency caused by late SRNS in Chinese southern infants.

Rapid detection of monogenic causes of childhood-onset steroid-resistant nephrotic syndrome.

In steroid-resistant nephrotic syndrome (SRNS), >21 single-gene causes are known. However, mutation analysis of all known SRNS genes is time and cost intensive. This report describes a new high-throughput method of mutation analysis using a PCR-based microfluidic technology that allows rapid simultaneous mutation analysis of 21 single-gene causes of SRNS in a large number of individuals. This study screened individuals with SRNS; samples were submitted for mutation analysis from international sources between 1996 and 2012. For proof of principle, a pilot cohort of 48 individuals who harbored known mutations in known SRNS genes was evaluated. After improvements to the method, 48 individuals with an unknown cause of SRNS were then examined in a subsequent diagnostic study. The analysis included 16 recessive SRNS genes and 5 dominant SRNS genes. A 10-fold primer multiplexing was applied, allowing PCR-based amplification of 474 amplicons in 21 genes for 48 DNA samples simultaneously. Forty-eight individuals were indexed in a barcode PCR, and high-throughput sequencing was performed. All disease-causing variants were confirmed via Sanger sequencing. The pilot study identified the genetic cause of disease in 42 of 48 (87.5%) of the affected individuals. The diagnostic study detected the genetic cause of disease in 16 of 48 (33%) of the affected individuals with a previously unknown cause of SRNS. Seven novel disease-causing mutations in PLCE1 (n=5), NPHS1 (n=1), and LAMB2 (n=1) were identified in <3 weeks. Use of this method could reduce costs to 1/29th of the cost of Sanger sequencing. This highly parallel approach allows rapid (<3 weeks) mutation analysis of 21 genes known to cause SRNS at a greatly reduced cost (1/29th) compared with traditional mutation analysis techniques. It detects mutations in about 33% of childhood-onset SRNS cases.

New perspectives on the renal slit diaphragm protein podocin

Podocin is a critical component of the glomerular filtration barrier, its mutations causing recessive steroid-resistant nephrotic syndrome. A GenBank analysis of the human podocin (NPHS2) gene resulted in the possible existence of a new splice variant of podocin in the kidney, missing the in-frame of exon 5, encoding the prohibitin homology domain. Using RT–polymerase chain reaction and immunoblotting followed by sequence analysis, we are for the first time able to prove the expression of a novel podocin isoform (isoform 2), exclusively and constitutively expressed in human podocytes. Furthermore, we reveal singular extrarenal podocin expression in human and murine testis. Our data show the Sertoli cells of the seminiferous tubules to be the origin of testicular podocin. Confocal laser microscopy illustrates the co-localization of podocin with filamentous actin within Sertoli cells, suggesting a role of podocin in the blood/testis barrier. These results led to the rationale to examine podocin expression in testes of men with Sertoli cell-only syndrome, a disorder characterized by azoospermia. Interestingly, we observed a complete down-regulation of podocin mRNA in Sertoli cell-only syndrome, indicating a possible role of podocin in the pathogenesis of this germinal aplasia. Men with Sertoli cell-only syndrome show normal renal podocin expression, suggesting an alternate regulation of the testicular promoter. Our findings may change the perception of podocin and give new insights into the ultrastructure of glomerular slit diaphragm and the blood/testis barrier.

Exome sequencing identified MYO1E and NEIL1 as candidate genes for human autosomal recessive steroid-resistant nephrotic syndrome

To identify gene loci associated with steroid-resistant nephrotic syndrome (SRNS), we utilized homozygosity mapping and exome sequencing in a consanguineous pedigree with three affected siblings. High-density genotyping identified three segments of homozygosity spanning 33.6 Mb on chromosomes 5, 10, and 15 containing 296 candidate genes. Exome sequencing identified two homozygous missense variants within the chromosome 15 segment; an A159P substitution in myosin 1E (MYO1E), encoding a podocyte cytoskeletal protein; and an E181K substitution in nei endonuclease VIII-like 1 (NEIL1), encoding a base-excision DNA repair enzyme. Both variants disrupt highly conserved protein sequences and were absent in public databases, 247 healthy controls, and 286 patients with nephrotic syndrome. The MYO1E A159P variant is noteworthy, as it is expected to impair ligand binding and actin interaction in the MYO1E motor domain. The predicted loss of function is consistent with the previous demonstration that Myo1e inactivation produces nephrotic syndrome in mice. Screening 71 additional patients with SRNS, however, did not identify independent NEIL1 or MYO1E mutations, suggesting larger sequencing efforts are needed to uncover which mutation is responsible for the phenotype. Our findings demonstrate the utility of exome sequencing for rapidly identifying candidate genes for human SRNS.

A novel mutation in NPHS2 gene identified in a Chinese pedigree with autosomal recessive steroid-resistant nephrotic syndrome

Steroid-resistant nephrotic syndrome (SRNS) is an inherent deficiency of podocyte caused by mutations of genes encoding slit diaphragm proteins. Mutations in NPHS2, encoding podocin, have been identified as responsible for childhood-onset familial SRNS. The present study revealed the genotype of a Chinese pedigree with autosomal recessive (AR) SRNS and reported a novel disease-causing NPHS2 mutation. A Chinese pedigree with AR-SRNS was enrolled in the study. All eight exons and exon-intron boundaries of NPHS2 genes were amplified from the genomic DNA of the family members and analysed by direct sequencing. The deficient expression of the mutant protein was illustrated by indirect immunofluorescence. A compound heterozygous NPHS2 mutation (c.211C > T /c.460dupT) was found in the proband. The paternal c.211C > T is a novel point mutation, resulting in an immediate stop codon (p.Arg71X). The maternal c.460dupT is a frameshift mutation introducing an earlier stop codon (p.Phe156AspfsX10). Both mutations could be expected to lead to truncated protein of podocin. Abnormal expression and distribution of the mutated protein were also exhibited in the patient. The compound heterozygous mutation in NPHS2 may explain the development of SRNS in this family. p.Arg71X is a novel disease-causing mutation leading to a deficient expression of podocin.

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

Autosomal recessive steroid-resistant nephrotic syndrome (NS) is a rare, genetically determined nephropathy caused mainly by a mutation in the NPHS2 gene. This type of NS is usually resistant to other immunosuppressive therapy as well, but a few cases of cyclosporine A-induced partial remission of inherited NS have been reported. We present a boy that developed NS at the age of 18 months. There was no decrease of proteinuria on standard prednisolone therapy, and a diagnosis of steroid-resistant NS was established. However, the proteinuria decreased significantly following the initiation of cyclosporine A therapy (from 1280 to 380 mg/m(2) per day) without any negative effects on renal function (stable glomerular filtration rate 130-150 ml/min per 1.73 m(2)). The molecular genetic test revealed a homozygous R138Q mutation in the NPHS2 gene. Our case demonstrates that cyclosporine A can induce partial remission in patients with genetic forms of NS without influencing the glomerular filtration rate. However, its long-term effect and safety in children with hereditary forms of nephrotic syndrome have yet to be investigated.

Clinical and epidemiological assessment of steroid-resistant nephrotic syndrome associated with the NPHS2 R229Q variant

Mutations of NPHS2, encoding podocin, are the main cause of autosomal recessive steroid-resistant nephrotic syndrome (NS) presenting in childhood. Adult-onset steroid-resistant NS has been described in patients heterozygous for a pathogenic NPHS2 mutation together with the p.R229Q variant. To determine the frequency and the phenotype of patients carrying the p.R229Q variant, we sequenced the complete coding region of NPHS2 in 455 families (546 patients) non-responsive to immunosuppressive therapy or without relapse after transplantation. Among affected Europeans, the p.R229Q allele was significantly more frequent compared to control individuals. Thirty-six patients from 27 families (11 families from Europe and 14 from South America) were compound heterozygotes for the p.R229Q variant and one pathogenic mutation. These patients had significantly later onset of NS and end stage renal disease than patients with two pathogenic mutations. Among 119 patients diagnosed with NS presenting after 18 years of age, 18 patients were found to have one pathogenic mutation and p.R229Q, but none had two pathogenic mutations. Our study shows that compound heterozygosity for p.R229Q is associated with adult-onset steroid-resistant NS, mostly among patients of European and South American origin. Screening for the p.R229Q variant is recommended in these patients along with further NPHS2 mutation analysis in those carrying the variant.

Maternal Environment Interacts with Modifier Genes to Influence Progression of Nephrotic Syndrome

Mutations in the NPHS2 gene, which encodes podocin, are responsible for some cases of sporadic and familial autosomal recessive steroid-resistant nephrotic syndrome. Inter- and intrafamilial variability in the progression of renal disease among patients bearing NPHS2 mutations suggests a potential role for modifier genes. Using a mouse model in which the podocin gene is constitutively inactivated, we sought to identify genetic determinants of the development and progression of renal disease as a result of the nephrotic syndrome. We report that the evolution of renal disease as a result of nephrotic syndrome in Nphs2-null mice depends on genetic background. Furthermore, the maternal environment significantly interacts with genetic determinants to modify survival and progression of renal disease. Quantitative trait locus mapping suggested that these genetic determinants may be encoded for by genes on the distal end of chromosome 3, which are linked to proteinuria, and on the distal end of chromosome 7, which are linked to a composite trait of urea, creatinine, and potassium. These loci demonstrate epistatic interactions with other chromosomal regions, highlighting the complex genetics of renal disease progression. In summary, constitutive inactivation of podocin models the complex interactions between maternal and genetically determined factors on the progression of renal disease as a result of nephrotic syndrome in mice.

Specific Podocin Mutations Correlate with Age of Onset in Steroid-Resistant Nephrotic Syndrome

Mutations in the gene encoding podocin (NPHS2) cause autosomal recessive steroid-resistant nephrotic syndrome (SRNS). For addressing the possibility of a genotype-phenotype correlation between podocin mutations and age of onset, a worldwide cohort of 430 patients from 404 different families with SRNS were screened by direct sequencing. Recessive podocin mutations were present in 18.1% (73 of 404) of families with SRNS, and 69.9% of these mutations were nonsense, frameshift, or homozygous R138Q. Patients with these mutations manifested symptoms at a significantly earlier age (mean onset <1.75 years) than any other patient group, with or without podocin mutations, in this study (mean onset >4.17 yr). All but one patient affected by truncating or homozygous R138Q mutations developed SRNS before 6 yr of age. Patient groups with other recessive podocin mutations, with single heterozygous podocin mutations, with sequence variants, and with no podocin changes could not be distinguished from each other on the basis of age of onset. In conclusion, nephrotic syndrome in children with truncating or homozygous R138Q mutations manifests predominantly before 6 yr of life, and the onset of disease is significantly earlier than for any other podocin mutations. Because the age of onset can vary by several years among those with identical mutations, additional factors may modify the phenotype.

Role of podocyte slit diaphragm as a filtration barrier (Review Article)

Although the role of glomerular basement membrane has been emphasised as the barrier for retaining plasma proteins in the past three decades, some recent studies have demonstrated that the slit diaphragm of the glomerular epithelial cell (podocyte) is the structure likely to be the barrier in the glomerular capillary wall. Nephrin and podocin were identified as gene products mutated in Finnish-type congenital nephrotic syndrome and autosomal recessive steroid-resistant nephrotic syndrome, respectively. Nephrin s located at the outer leaflet of plasma membranes of the slit diaphragm. Podocin is reported to have an interaction with nephrin. The anti-nephrin antibody is capable of inducing massive proteinuria, which indicates that nephrin is a key functional molecule in the slit diaphragm. The expression of nephrin and podocin was reduced in glomeruli of minimal change nephrotic syndrome, which suggested that the altered expression of these molecules contributes to the development of proteinuria also in acquired diseases. Some recent studies demonstrated that CD2-associated protein (CD2AP) is also a functional molecule in the slit diaphragm, and its expression is altered in membranous nephropathy. These observations suggested that alteration of the molecular arrangement in the slit diaphragm is involved in the development of proteinuria in several kinds of glomerular diseases.

Organization of the pronephric filtration apparatus in zebrafish requires Nephrin, Podocin and the FERM domain protein Mosaic eyes

Podocytes are specialized cells of the kidney that form the blood filtration barrier in the kidney glomerulus. The barrier function of podocytes depends upon the development of specialized cell–cell adhesion complexes called slit-diaphragms that form between podocyte foot processes surrounding glomerular blood vessels. Failure of the slit-diaphragm to form results in leakage of high molecular weight proteins into the blood filtrate and urine, a condition called proteinuria. In this work, we test whether the zebrafish pronephros can be used as an assay system for the development of glomerular function with the goal of identifying novel components of the slit-diaphragm. We first characterized the function of the zebrafish homolog of Nephrin, the disease gene associated with the congenital nephritic syndrome of the Finnish type, and Podocin, the gene mutated in autosomal recessive steroid-resistant nephrotic syndrome. Zebrafish nephrin and podocin were specifically expressed in pronephric podocytes and required for the development of pronephric podocyte cell structure. Ultrastructurally, disruption of nephrin or podocin expression resulted in a loss of slit-diaphragms at 72 and 96 h post-fertilization and failure to form normal podocyte foot processes. We also find that expression of the band 4.1/FERM domain gene mosaic eyes in podocytes is required for proper formation of slit-diaphragm cell–cell junctions. A functional assay of glomerular filtration barrier revealed that absence of normal nephrin, podocin or mosaic eyes expression results in loss of glomerular filtration discrimination and aberrant passage of high molecular weight substances into the glomerular filtrate.

A novel mutation of NPHS2 identified in a Chinese family.

Since the identification of the NPHS2 gene,which encodes podocin, several groups from European, Middle Eastern, and North American countries have reported NPHS2 mutations in families with steroid-resistant nephrotic syndrome (SRNS) or focal segmental glomerulo sclerosis (FSGS). Families with SRNS have also been reported in China with a population of more than1.3 billion. However, to our knowledge, there is no mutational analysis of the NPHS2 gene in familial SRNS orFSGS in China. We identified a novel mutation of NPHS2(467_468insT and 503G>A) in a Chinese family with autosomal recessive SRNS using polymerase chain re-action, denaturing high-performance liquid chromatography, and DNA sequencing techniques. The results demonstrate that there is also NPHS2 mutation in Chinese familial SRNS. Therefore, Chinese SRNS patients with a familial history of NS should also be screened for possible mutations of NPHS2. We also detected clearly decreased staining with a specific podocin C-terminal antibody(P35) and negative staining with a specific podocin N-terminal antibody (P21). These results were contrary to those predicted from the mutated sites. Further studies are needed to explore the mechanism and impact of the mutant gene on the expression and localization of the relevant protein.