TRPC6 mutational analysis in a large cohort of patients with focal segmental glomerulosclerosis

Cost-effective PKHD1 genetic testing for autosomal recessive polycystic kidney disease

BACKGROUND

Genetic diagnosis of autosomal recessive polycystic kidney disease (ARPKD) is challenging due to the length and allelic heterogeneity of the PKHD1 gene. Mutations appear to be clustered at specific exons, depending on the geographic origin of the patient. We aimed to identify the PKHD1 exons most likely mutated in Spanish ARPKD patients.

METHODS

Mutation analysis was performed in 50 ARPKD probands and nine ARPKD-suspicious patients by sequencing PKHD1 exons arranged by their reported mutation frequency. Haplotypes containing the most frequent mutations were analyzed. Other PKD genes (HNF1B, PKD1, PKD2) were sequenced in PKHD1-negative cases.

RESULTS

Thirty-six different mutations (concentrated in 24 PKHD1 exons) were detected, giving a mutation detection rate of 86%. The screening of five exons (58, 32, 34, 36, 37) yielded a 54% chance of detecting one mutation; the screening of nine additional exons (3, 9, 39, 61, 5, 22, 26, 41, 57) increased the chance to 76%. The c.9689delA mutation was present in 17 (34%) patients, all of whom shared the same haplotype. Two HNF1B mutations and one PKD1 variant were detected in negative cases.

CONCLUSIONS

Establishing a PKHD1 exon mutation profile in a specific population and starting the analysis with the most likely mutated exons might significantly enhance the efficacy of genetic testing in ARPKD. Analysis of other PKD genes might be considered, especially in suspicious cases.

254C>G: a TRPC6 promoter variation associated with enhanced transcription and steroid-resistant nephrotic syndrome in Chinese children

BACKGROUND

Mutations in canonical transient receptor potential channel 6 (TRPC6) have been identified as responsible for the development of focal segmental glomerulosclerosis, a proteinuric disease with steroid resistance and poor prognosis. This study explores the prevalence of TRPC6 variants in Chinese children with idiopathic nephrotic syndrome (INS), the genotype/phenotype correlation of TRPC6 variants, the therapeutic response, and the underlying molecular mechanism.

METHODS

Fifty-one children with sporadic INS were enrolled: 23 steroid-sensitive cases and 28 steroid-resistant cases Polymerase chain reaction was used to amplify 13 exons and the promoter sequences of TRPC6 before sequencing. The expression of TRPC6 in renal tissues was illustrated by immunohistochemistry staining. The transcriptional activity of variants in TRPC6 promoter was measured by the luciferase assay.

RESULTS

Three variants (-254C>G, rs3824934; +43C/T, rs3802829; and 240 G>A, rs17096918) were identified. The allele frequency of the -254C>G single-nucleotide polymorphism (SNP) in the steroid-resistant nephrotic syndrome (SRNS) patients (40.5%) was higher than that in the steroid-sensitive nephrotic syndrome subjects (27.1%; P = 0.046). The -254C>G SNP enhanced transcription from TRPC6 promoter in vitro and was associated with increased TRPC6 expression in renal tissues of SRNS patients.

CONCLUSION

-254C>G, a SNP underlying enhanced TRPC6 transcription and expression, may be correlated with the development of steroid resistance in Chinese children with INS.

Genetics of steroid-resistant nephrotic syndrome: a review of mutation spectrum and suggested approach for genetic testing

Identification of genes, associated mutations and genotype-phenotype correlations in steroid-resistant nephrotic syndrome (SRNS) is being translated to clinical practice through genetic testing. This review provides an update on the genes and mutations associated with SRNS along with a suggested approach for genetic testing in patients with SRNS.

CONCLUSION

The number of indentified genes associated with SRNS is increasing along with our understanding of their impact on treatment response and risk of recurrence. A systematic approach to genetic testing in patients with SRNS might aid the physician in selecting appropriate treatment.

A novel TRPC6 mutation in a family with podocytopathy and clinical variability

BACKGROUND

Mutation in several podocyte-specific genes have been noted to result in phenotypic heterogeneity. Herein, we report a novel, autosomal dominant TRPC6 mutation in a family with disease ranging from asymptomatic minimal change disease to end-stage kidney disease.

CASE PRESENTATION

A 35 year old woman developed asymptomatic, nephrotic range proteinuria during pregnancy that did not resolve after delivery. Her mother had end-stage kidney disease of unknown etiology and her brother had asymptomatic proteinuria. Kidney biopsy revealed minimal change disease in both the proband and her brother. Genetic testing was performed in the proband and mother, revealing a novel frameshift mutation in TRPC6, D873fsX878. The proband continues to have subnephrotic range proteinuria and normal creatinine but her brother has since developed progressive chronic kidney disease.

CONCLUSIONS

The current case report underscores the heterogeneity of disease in podocytopathies and related genes. Genetic testing of podocyte genes is useful in order to understand the pathophysiologic processes underlying these overlapping diseases.

Vitamin D down-regulates TRPC6 expression in podocyte injury and proteinuric glomerular disease

The transient receptor potential cation channel C6 (TRPC6) is a slit diaphragm protein expressed by podocytes. TRPC6 gain-of-function mutations cause autosomal dominant focal segmental glomerulosclerosis. In acquired proteinuric renal disease, glomerular TRPC6 expression is increased. We previously demonstrated that acquired increased TRPC6 expression is ameliorated by antiproteinuric angiotensin receptor blockers and angiotensin-converting enzyme inhibitors. Vitamin D also has an antiproteinuric effect. We hypothesized that vitamin D reduces proteinuria by affecting TRPC6 expression in podocytes. Adriamycin-induced nephropathy increased TRPC6 mRNA and protein expression and induced proteinuria in rats. Treatment with 1,25-dihydroxyvitamin D3 (1,25-D3) normalized TRPC6 expression and reduced proteinuria. In vitro, podocyte injury induced by adriamycin exposure in cultured podocytes increased TRPC6 expression. Treatment of injured podocytes with 1,25-D3 dose dependently reduced adriamycin-induced TRPC6 expression. Chromatin immunoprecipitation analysis demonstrated that the vitamin D receptor directly binds to the TRPC6 promoter. Moreover, 1,25-D3 reduced TRPC6 promoter activity in a luciferase reporter assay. In 1,25-D3-deficient 25-hydroxy-1α-hydroxylase knockout mice, TRPC6 expression was increased, accompanied by podocyte foot process effacement and proteinuria. 1,25-D3 supplementation normalized TRPC6 expression, podocyte morphology, and proteinuria in these mice. These results demonstrate that vitamin D down-regulates the enhanced TRPC6 expression in in vivo and in vitro podocyte injury, possibly through a direct effect on TRPC6 promoter activity. This TRPC6 down-regulation could contribute to the antiproteinuric effect of vitamin D.

New TRPC6 gain-of-function mutation in a non-consanguineous Dutch family with late-onset focal segmental glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a leading cause of steroid-resistant nephrotic syndrome. Hereditary FSGS is frequently caused by mutations in important structural podocyte proteins, including the slit diaphragm-associated transient receptor potential channel C6 (TRPC6).

METHODS

In five patients with biopsy-proven autosomal-dominant FSGS from five different Dutch families, all 13 exons of TRPC6 were sequenced. Upon identification of a novel TRPC6 sequence variant, the resultant amino acid change was introduced in the wild-type TRPC6 protein and functionally tested using patch-clamp analyses and cell-surface biotinylation experiments.

RESULTS

None of the previously described TRPC6 mutations were found in our cohort. In one family, we identified a novel c.524G>A sequence variant resulting in a p.Arg175Gln (R175Q) substitution in the TRPC6 protein. This sequence variant was absent in 449 control subjects and from public SNP databases. The mutation was located in the third ankyrin repeat domain (ANK3) in the cytoplasmic N-tail of TRPC6, important for protein-protein interaction and regulation of ion channel activity. Patch-clamp analyses of the mutant channel indeed showed an increased TRPC6 channel-mediated current. However, cell-surface expression of the mutant channel was not increased.

CONCLUSIONS

We identified a novel TRPC6 p.Arg175Gln gain-of-function mutation that shows increased TRPC6-mediated current, which is not due to altered cell-surface expression. This is the first mutation identified in ANK3 of the TRPC6 N-tail and is most likely responsible for the late-onset autosomal dominant FSGS in this family.

A spectrum of novel NPHS1 and NPHS2 gene mutations in pediatric nephrotic syndrome patients from Pakistan

BACKGROUND

Mutations in the NPHS1 and NPHS2 genes are among the main causes of early-onset and familial steroid resistant nephrotic syndrome respectively. This study was carried out to assess the frequencies of mutations in these two genes in a cohort of Pakistani pediatric NS patients.

METHODS

Mutation analysis was carried out by direct sequencing of the NPHS1 and NPHS2 genes in 145 nephrotic syndrome (NS) patients. This cohort included 36 samples of congenital or infantile onset NS cases and 39 samples of familial cases obtained from 30 families.

RESULTS

A total of 7 homozygous (6 novel) mutations were found in the NPHS1 gene and 4 homozygous mutations in the NPHS2 gene. All mutations in the NPHS1 gene were found in the early onset cases. Of these, one patient has a family history of NS. Homozygous p.R229Q mutation in the NPHS2 gene was found in two children with childhood-onset NS.

CONCLUSIONS

Our results show a low prevalence of disease causing mutations in the NPHS1 (22% early onset, 5.5% overall) and NPHS2 (3.3% early onset and 3.4% overall) genes in the Pakistani NS children as compared to the European populations. In contrast to the high frequency of the NPHS2 gene mutations reported for familial SRNS in Europe, no mutation was found in the familial Pakistani cases. To our knowledge, this is the first comprehensive screening of the NPHS1 and NPHS2 gene mutations in sporadic and familial NS cases from South Asia.

Genetic causes of focal segmental glomerulosclerosis: implications for clinical practice

Focal segmental glomerulosclerosis (FSGS) is a common cause of steroid-resistant nephrotic syndrome in children and adults. Although FSGS is considered a podocyte disease, the aetiology is diverse. In recent years, many inheritable genetic forms of FSGS have been described, caused by mutations in proteins that are important for podocyte function. In the present commentary, we review these genetic causes of FSGS and describe their prevalence in familial and sporadic FSGS. In routine clinical practice, the decision to perform the costly DNA analysis should be based on the assessment if the results affect the care of the individual patient with respect to the evaluation of extra-renal manifestations, treatment decisions, transplantation and genetic counselling.

TRPC6 mutations in children with steroid-resistant nephrotic syndrome and atypical phenotype

BACKGROUND AND OBJECTIVES

Mutations in the TRPC6 gene have been recently identified as the cause of late-onset autosomal-dominant focal segmental glomerulosclerosis (FSGS). To extend the screening, we analyzed TRPC6 in 33 Italian children with sporadic early-onset SRNS and three Italian families with adult-onset FSGS.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

TRPC6 mutation analysis was performed through PCR and sequencing. The effects of the detected amino acid substitutions were analyzed by bioinformatics tools and functional in vitro studies. The expression levels of TRPC6 and nephrin proteins were evaluated by confocal microscopy.

RESULTS

Three heterozygous missense mutations (c.374A>G_p.N125S, c.653A>T_p.H218L, c.2684G>T_p.R895L) were identified. The first new mutation, p.H218L, was found in a 18-year-old boy who presented a severe form of FSGS at the age of 8 years. The second, p.R895L, a new de novo mutation, was identified in a girl with collapsing glomerulosclerosis at the age of 2 years. The former mutation, p.N125S, was found in two siblings with early-onset steroid-resistant nephrotic syndrome (SRNS) at the ages of 4 and 14 years. Renal immunofluorescence revealed upregulated expression of TRPC6 and loss of nephrin in glomeruli. The intracellular calcium concentrations were significantly higher in the cells expressing all mutant TRPC6 channels compared with cells expressing wild-type TRPC6.

CONCLUSIONS

Our findings suggest that TRPC6 variants can also be detected in children with early-onset and sporadic SRNS (4 of 33 patients). Moreover, in one patient a new de novo TRPC6 mutation was associated with a rare severe form of childhood collapsing glomerulosclerosis with rapid progression to uremia.

Angiotensin II contributes to podocyte injury by increasing TRPC6 expression via an NFAT-mediated positive feedback signaling pathway

The transient receptor potential channel C6 (TRPC6) is a slit diaphragm-associated protein in podocytes involved in regulating glomerular filter function. Gain-of-function mutations in TRPC6 cause hereditary focal segmental glomerulosclerosis (FSGS), and several human acquired proteinuric diseases show increased glomerular TRPC6 expression. Angiotensin II (AngII) is a key contributor to glomerular disease and may regulate TRPC6 expression in nonrenal cells. We demonstrate that AngII regulates TRPC6 mRNA and protein levels in cultured podocytes and that AngII infusion enhances glomerular TRPC6 expression in vivo. In animal models for human FSGS (doxorubicin nephropathy) and increased renin-angiotensin system activity (Ren2 transgenic rats), glomerular TRPC6 expression was increased in an AngII-dependent manner. TRPC6 expression correlated with glomerular damage markers and glomerulosclerosis. We show that the regulation of TRPC6 expression by AngII and doxorubicin requires TRPC6-mediated Ca(2+) influx and the activation of the Ca(2+)-dependent protein phosphatase calcineurin and its substrate nuclear factor of activated T cells (NFAT). Accordingly, calcineurin inhibition by cyclosporine decreased TRPC6 expression and reduced proteinuria in doxorubicin nephropathy, whereas podocyte-specific inducible expression of a constitutively active NFAT mutant increased TRPC6 expression and induced severe proteinuria. Our findings demonstrate that the deleterious effects of AngII on podocytes and its pathogenic role in glomerular disease involve enhanced TRPC6 expression via a calcineurin/NFAT positive feedback signaling pathway.

Clinical utility of genetic testing in children and adults with steroid-resistant nephrotic syndrome

BACKGROUND AND OBJECTIVES

The increasing number of podocyte-expressed genes implicated in steroid-resistant nephrotic syndrome (SRNS), the phenotypic variability, and the uncharacterized relative frequency of mutations in these genes in pediatric and adult patients with SRNS complicate their routine genetic analysis. Our aim was to compile the clinical and genetic data of eight podocyte genes analyzed in 110 cases (125 patients) with SRNS (ranging from congenital to adult onset) to provide a genetic testing approach.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Mutation analysis was performed by sequencing the NPHS1, NPHS2, TRPC6, CD2AP, PLCE1, INF2, WT1 (exons 8 and 9), and ACTN4 (exons 1 to 10) genes.

RESULTS

We identified causing mutations in 34% (37/110) of SRNS patients, representing 67% (16/24) familial and 25% (21/86) sporadic cases. Mutations were detected in 100% of congenital-onset, 57% of infantile-onset, 24 and 36% of early and late childhood-onset, 25% of adolescent-onset, and 14% of adult-onset patients. The most frequently mutated gene was NPHS1 in congenital onset and NPHS2 in the other groups. A partial remission was observed in 7 of 26 mutation carriers treated with immunosuppressive agents and/or angiotensin-converting enzyme inhibitors. Patients with NPHS1 mutations showed a faster progression to ESRD than patients with NPHS2 mutations. None of these mutation carriers relapsed after kidney transplantation.

CONCLUSIONS

We propose a genetic testing algorithm for SRNS based on the age at onset and the familial/sporadic status. Mutation analysis of specific podocyte-genes has a clinical value in all age groups, especially in children.

Mutations in INF2 are a major cause of autosomal dominant focal segmental glomerulosclerosis

The recent identification of mutations in the INF2 gene, which encodes a member of the formin family of actin-regulating proteins, in cases of familial FSGS supports the importance of an intact actin cytoskeleton in podocyte function. To determine better the prevalence of INF2 mutations in autosomal dominant FSGS, we screened 54 families (78 patients) and detected mutations in 17% of them. All mutations were missense variants localized to the N-terminal diaphanous inhibitory domain of the protein, a region that interacts with the C-terminal diaphanous autoregulatory domain, thereby competing for actin monomer binding and inhibiting depolymerization. Six of the seven distinct altered residues localized to an INF2 region that corresponded to a subdomain of the mDia1 diaphanous inhibitory domain reported to co-immunoprecipitate with IQ motif-containing GTPase-activating protein 1 (IQGAP1). In addition, we evaluated 84 sporadic cases but detected a mutation in only one patient. In conclusion, mutations in INF2 are a major cause of autosomal dominant FSGS. Because IQGAP1 interacts with crucial podocyte proteins such as nephrin and PLCε1, the identification of mutations that may alter the putative INF2-IQGAP1 interaction provides additional insight into the pathophysiologic mechanisms linking formin proteins to podocyte dysfunction and FSGS.

Lack of association between transient receptor potential cation channel 6 polymorphisms and primary membranous glomerulonephritis

Membranous glomerulonephritis (MGN) is one common cause of idiopathic nephrotic syndrome. Transient receptor potential cation channel 6 (TRPC6) has been identified as causing a familial form of progressive focal and segmental glomerulosclerosis. The objective was to clarify the relationship between TRPC6 polymorphisms and MGN. We recruited a cohort of 134 biopsy-diagnosed MGN patients and 265 healthy subjects. Genotyping of TRPC6 polymorphisms was performed using allele-specific polymerase chain reaction methods. We then analyzed associations between TRPC6 gene polymorphisms and clinical manifestations and pathogenesis of MGN. There was no statistically significant difference of TRPC6 gene rs3824935 C/T, rs17096918 C/T, and rs4326755 A/G polymorphisms between controls and patients with MGN. There was no statistical significance of allele frequencies in these two groups. The characteristics of clinical parameters in TRPC6 gene (rs3284935) C/T polymorphism revealed no difference except proteinuria (p < 0.0005) between CC and non-CC genotype in MGN patients. Besides, no apparent statistically significant differences of rs17096918 C/T (TT and non-TT) and rs4326755 A/G (AA and non-AA) polymorphisms between genotypes were found in the clinical parameters. There is no different genotype distribution between normal controls and patients with MGN of TRPC6 gene. The data also show that TRPC6 gene may not be associated with disease clinical course of MGN.

Hereditary kidney diseases: highlighting the importance of classical Mendelian phenotypes

A Mendelian inheritance underlies a nonnegligible proportion of hereditary kidney diseases, suggesting that the encoded proteins are essential for maintenance of the renal function. The identification of genes involved in congenital anomalies of the kidney and in familial forms of nephrotic syndrome significantly increased our understanding of the renal development and kidney filtration barrier physiology. This review will focus on the classical phenotype and clinical heterogeneity observed in the monogenic forms of these disorders. In addition, the role of susceptibility genes in kidney diseases with a complex inheritance will also be discussed.

Clinical value of NPHS2 analysis in early- and adult-onset steroid-resistant nephrotic syndrome

BACKGROUND AND OBJECTIVES

To date, very few cases with adult-onset focal segmental glomerulosclerosis (FSGS) carrying NPHS2 variants have been described, all of them being compound heterozygous for the p.R229Q variant and one pathogenic mutation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Mutation analysis was performed in 148 unrelated Spanish patients, of whom 50 presented with FSGS after 18 years of age. Pathogenicity of amino acid substitutions was evaluated through an in silico scoring system. Haplotype analysis was carried out using NPHS2 single nucleotide polymorphism and microsatellite markers.

RESULTS

Compound heterozygous or homozygous NPHS2 pathogenic mutations were identified in seven childhood-onset steroid-resistant nephrotic syndrome (SRNS) cases. Six additional cases with late childhood- and adult-onset SRNS were compound heterozygotes for p.R229Q and one pathogenic mutation, mostly p.A284V. p.R229Q was more frequent among SRNS cases relative to controls (odds ratio=2.65; P=0.02). Significantly higher age at onset of the disease and slower progression to ESRD were found in patients with one pathogenic mutation plus the p.R229Q variant in respect to patients with two NPHS2 pathogenic mutations.

CONCLUSIONS

NPHS2 analysis has a clinical value in both childhood- and adult-onset SRNS patients. For adult-onset patients, the first step should be screening for p.R229Q and, if positive, for p.A284V. These alleles are present in conserved haplotypes, suggesting a common origin for these substitutions. Patients carrying this specific NPHS2 allele combination did not respond to corticoids or immunosuppressors and showed FSGS, average 8-year progression to ESRD, and low risk for recurrence of FSGS after kidney transplant.

Podocyte-specific overexpression of wild type or mutant trpc6 in mice is sufficient to cause glomerular disease

Mutations in the TRPC6 calcium channel (Transient receptor potential channel 6) gene have been associated with familiar forms of Focal and Segmental Glomerulosclerosis (FSGS) affecting children and adults. In addition, acquired glomerular diseases are associated with increased expression levels of TRPC6. However, the exact role of TRPC6 in the pathogenesis of FSGS remains to be elucidated. In this work we describe the generation and phenotypic characterization of three different transgenic mouse lines with podocyte-specific overexpression of the wild type or any of two mutant forms of Trpc6 (P111Q and E896K) previously related to FSGS. Consistent with the human phenotype a non-nephrotic range of albuminuria was detectable in almost all transgenic lines. The histological analysis demonstrated that the transgenic mice developed a kidney disease similar to human FSGS. Differences of 2-3 folds in the presence of glomerular lesions were found between the non transgenic and transgenic mice expressing Trpc6 in its wild type or mutant forms specifically in podocytes. Electron microscopy of glomerulus from transgenic mice showed extensive podocyte foot process effacement. We conclude that overexpression of Trpc6 (wild type or mutated) in podocytes is sufficient to cause a kidney disease consistent with FSGS. Our results contribute to reinforce the central role of podocytes in the etiology of FSGS. These mice constitute an important new model in which to study future therapies and outcomes of this complex disease.

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.

Renal TRPathies

Many ion channels and transporters are involved in the filtration, secretion, and resorption of electrolytes by the kidney. In recent years, the superfamily of transient receptor potential (TRP) ion channels have received deserved attention because mutated TRP channels are linked to human kidney diseases. This review focuses on two TRP members--TRPC6 and TRPM6--and their functions in the kidney. Gain-of-function mutations in TRPC6 are the cause for progressive kidney failure with urinary protein loss such as FSGS. Thus, TRPC6 is an essential signaling component in a functional slit diaphragm formed by podocytes around the glomerular capillaries. Loss-of-function mutations in TRPM6 are a molecular cause of hypomagnesemia with secondary hypocalcemia, suggesting that TRPM6 is critically involved in transcellular Mg2+ transport in the kidney. Here, we highlight how recent studies analyzing function and expression of these channels in the kidney improve our mechanistic understanding of TRP channel function in general and pave the way to new, promising therapeutic strategies to target kidney diseases such as FSGS and hypomagnesemia with secondary hypocalcemia.

Nephrin mutations cause childhood- and adult-onset focal segmental glomerulosclerosis

Mutations in the NPHS1 gene cause congenital nephrotic syndrome of the Finnish type presenting before the first 3 months of life. Recently, NPHS1 mutations have also been identified in childhood-onset steroid-resistant nephrotic syndrome and milder courses of disease, but their role in adults with focal segmental glomerulosclerosis remains unknown. Here we developed an in silico scoring matrix to evaluate the pathogenicity of amino-acid substitutions using the biophysical and biochemical difference between wild-type and mutant amino acid, the evolutionary conservation of the amino-acid residue in orthologs, and defined domains, with the addition of contextual information. Mutation analysis was performed in 97 patients from 89 unrelated families, of which 52 presented with steroid-resistant nephrotic syndrome after 18 years of age. Compound heterozygous or homozygous NPHS1 mutations were identified in five familial and seven sporadic cases, including one patient 27 years old at onset of the disease. Substitutions were classified as 'severe' or 'mild' using this in silico approach. Our results suggest an earlier onset of the disease in patients with two 'severe' mutations compared to patients with at least one 'mild' mutation. The finding of mutations in a patient with adult-onset focal segmental glomerulosclerosis indicates that NPHS1 analysis could be considered in patients with later onset of the disease.