Two novel polymorphisms (c954T>C and c1038A>G) in exon8 of NPHS2 gene identified in Taiwan Chinese

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.

Identification of podocin (NPHS2) gene mutations in African Americans with nondiabetic end-stage renal disease

BACKGROUND

Podocin, encoded by NPHS2 and mapped to 1q25.2, is an integral membrane protein exclusively expressed in glomerular podocytes. Mutations in the NPHS2 gene cause autosomal-recessive nephrotic syndrome and have been associated with proteinuria in several populations. Evidence for linkage of end-stage renal disease (ESRD) to chromosome 1q25-31 in the region of NPHS2 has been identified in a genome-wide scan in African American (AA) siblings.

METHODS

To investigate the potential role of this gene in ESRD, we sequenced all coding regions and approximately 2 kb of upstream promoter sequence of NPHS2 in 96 unrelated AA nondiabetic ESRD cases and 96 healthy population-based AA controls, and assessed several single nucleotide polymorphisms (SNPs) for association in a larger case-control sample.

RESULTS

Fifty-five variants were identified with minor allele frequencies ranging from <1% to 44%. Twenty-three polymorphisms were located in the promoter region, 11 were exonic, 13 were intronic, and 8 were in the 5' and 3'- untranslated regions. Two novel nonsynonymous coding SNPs were identified (A44E and A61V). An insertion polymorphism in intron 3, IVS3+9insA, was detected in 6 ESRD patients and in no controls. This variant, and 4 other common SNPs, were evaluated in a larger sample of 288 AA ESRD cases and 278 AA controls. The overall minor allele frequencies for the insertion allele were 0.018 in cases and 0.002 in controls. Significant evidence of association of IVS3+9insA was observed (P= 0.012), and the haplotype containing the insertion allele in cases was also associated.

CONCLUSION

These results suggest that uncommon variants of the NPHS2 gene may play a role in the development of nondiabetic ESRD in AAs.

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.

NPHS2 mutations in sporadic steroid-resistant nephrotic syndrome in Japanese children

Podocin is an integral membrane protein encoded by NPHS2, which is mapped to 1q25-31 and is exclusively expressed in glomerular podocytes. 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 (age less than 6 years) and rapid progression to chronic renal insufficiency. This gene mutation is also responsible for an adolescent/adult onset form of autosomal recessive familial FSGS with heavy proteinuria. It has been demonstrated that sporadic SRNS and heavy proteinuria are also due to NPHS2 gene mutations. We isolated genomic DNA from 36 Japanese children with chronic renal insufficiency caused by SRNS or heavy proteinuria, and analyzed all eight exons and exon-intron boundaries of NPHS2 using the polymerase chain reaction and direct sequencing. The age at onset of disease was 3.9+/-0.5 years. There were 29 patients with SRNS and 7 with heavy proteinuria without nephrotic syndrome at the onset, but all patients developed chronic renal insufficiency 4.6+/-0.8 years after the onset. A new homozygous missense variant of NPHS2, G34E (G101A) in exon 1, was detected in 1 of 36 patients. However, this homozygous variant was also found in 1 of 44 normal controls, suggesting that the mutation is a polymorphism. Two silent variants (T954C and A1038G) in exon 8 of this gene were also identified in some of the patients and normal controls, indicating that the silent variants are also polymorphisms. There was no significant difference in the genotypic and allelic frequencies of T954C and A1038G polymorphisms between the patients and normal controls. In conclusion, NPHS2 gene mutations are not a major cause of chronic renal insufficiency caused by sporadic SRNS or heavy proteinuria in Japanese children.

The genetic basis of FSGS and steroid-resistant nephrosis

Studies of Mendelian forms of focal segmental glomerulosclerosis (FSGS) and nephrotic syndrome have provided new insights into the mechanism of these diseases. Congenital nephrotic syndrome and familial forms of FSGS form a spectrum of podocyte diseases of varying severity and age of onset. Mutations in both nephrin gene (NPHS1) alleles lead to congenital nephrosis, podocyte foot process efacement, and loss of slit-diaphragm structure. Mutations in both podocin gene (NPHS2) alleles lead to a wide range of human disease, from childhood-onset steroid-resistant FSGS and minimal change disease to adult-onset FSGS. Dominantly inherited mutations in ACTN4, the alpha-actinin-4 gene, can lead to a slowly progressive adult-onset form of FSGS. In addition, FSGS is observed as part of several rare multisystem inherited syndromes. Here we review recent progress in understanding the genetic basis of FSGS in humans.

NPHS2 mutations in late-onset focal segmental glomerulosclerosis: R229Q is a common disease-associated allele

Mutations in NPHS2, encoding podocin, have been identified in childhood onset focal and segmental glomerulosclerosis (FSGS). The role of NPHS2 in adult disease is less well defined. We studied 30 families with FSGS and apparent autosomal recessive inheritance and 91 individuals with primary FSGS. We screened family members for NPHS2 mutations. NPHS2 mutations appeared to be responsible for disease in nine of these families. In six families, the affected individuals were compound heterozygotes for a nonconservative R229Q amino acid substitution. This R229Q variant has an allele frequency of 3.6% in a control population. In these families, R229Q was the only mutation identified on one of the two disease-associated NPHS2 alleles. We used in vitro-translated podocin and purified nephrin to investigate the effect of R229Q on their interaction and found decreased nephrin binding to the R229Q podocin. These data suggest that this common polymorphism contributes to the development of FSGS. Chromosomes bearing the R229Q mutation share a common haplotype defining an approximately 0.2-Mb region. R229Q appears to enhance susceptibility to FSGS in association with a second mutant NPHS2 allele. Identification of R229Q mutations may be of clinical importance, as NPHS2-associated disease appears to define a subgroup of FSGS patients unresponsive to corticosteroids.