Analysis of NPHS1, NPHS2, ACTN4, and WT1 in Japanese patients with congenital nephrotic syndrome

BACKGROUND

Congenital nephrotic syndrome (CNS) causes significant renal failure, and is classified into two types: (1) Finnish type; and (2) other, including diffuse mesangial sclerosis. Mutations of NPHS1 and NPHS2, which encode the slit diaphragm components nephrin and podocin, cause CNS and autosomal-recessive familial steroid-resistant nephrotic syndrome, respectively. Most patients with Finnish-type CNS in Europe and the United States have NPHS1 mutations. However, NPHS2 mutations have been detected in some cases. Mutations in ACTN4, encoding alpha-actinin-4, cause an autosomal-dominant focal segmental glomerulosclerosis. alpha-actinin-4 stabilizes the podocyte cytoskeleton structure, connecting with actin filaments. WT1 mutations, causing Wilm's tumor, have been demonstrated in some CNS patients with diffuse mesangial sclerosis. Systematic investigation of genes for CNS in Japan has never been performed.

METHODS

To clarify the role of mutations in these four genes, we used polymerase chain reaction (PCR) and direct sequencing to investigate all exons and exon-intron boundaries for these genes in 13 unrelated CNS patients from regional pediatric kidney disease centers in Japan.

RESULTS

A novel homozygous nonsense mutation of NPHS1, E246X in exon 7, and a novel homozygous deletion mutation of NPHS1, 2156_2163del in exon 16 were detected in one patient each. A novel homozygous nonsense mutation of NPHS2, R196X in exon 5, was found in one patient, and the same heterozygous nonsense mutation was detected in another. No ACTN4 or WT1 mutations were detected.

CONCLUSION

These studies demonstrate that mutation of NPHS1 is not a major cause of CNS in Japanese patients, and that mutation of NPHS2 can be responsible for CNS in this population.

Congenital nephrotic syndrome responsive to angiotensin-converting enzyme inhibition

Congenital nephrotic syndrome is a severe disorder caused by increased permeability of the glomerular capillary leading to massive proteinuria. Typically, this disorder presents in the first three months and is caused by inherited mutations in genes encoding structural proteins of the podocyte slit membrane and, as such, is usually irreversible. Medical management is often insufficient to stem the enormous losses of protein, and the patients require nephrectomies. Here, we present results for a patient with congenital nephrotic syndrome of unknown etiology which responded to treatment with an angiotensin-converting enzyme inhibitor alone. The patient's proteinuria relapsed when the medication was stopped, but went into complete remission after restarting treatment. This remarkable response is discussed in the light of recent investigations into the effect of angiotensin II on podocyte integrity.

Respiratory chain deficiency presenting as congenital nephrotic syndrome

Nephrotic syndrome (NS) in infancy includes NS of Finnish type (mutation of the nephrin gene), diffuse mesangial sclerosis (idiopathic or linked to WT1 mutation), idiopathic NS, most often steroid resistant, and NS related to infections during pregnancy (virus, syphilis, toxoplasmosis). Later in life, NS has a large variety of etiologies. It has been described in association with neuromuscular symptoms, deafness, and diabetes in a few children and adults with respiratory chain (RC) disorders. To date, however, NS has never been observed in neonates with RC disorders. Here, we report RC deficiency in one infant with certain congenital NS and two siblings with acute neonatal cardiac and renal disease with probable NS. Although clinical and histopathological presentations were initially close to congenital NS of Finnish type, clinical outcome was atypical and nephrin mutation was excluded. Mitochondrial RC complex II+V deficiency was identified in the three patients. Based on these observations, we suggest that RC disorders should be considered in patients with congenital NS.

Congenital and infantile nephrotic syndrome in Thai infants

Congenital and infantile nephrotic syndrome reported from the Eastern world is rare and might be a different entity from that in the West. In a retrospective review of 10 nephrotic syndrome in Thai infants (5 girls and 5 boys), 7 were diagnosed with congenital nephrotic syndrome and 3 with infantile nephrotic syndrome. Two had congenital nephrotic syndrome secondary to congenital syphilis. All had edema, ascites, and failure to thrive. Of the 3 patients tested for thyroid function, all showed hypothyroidism. Two patients developed renal failure. Renal tissue was examined from 4 patients from 3 biopsies and 2 autopsies; only 1 patient showed tubular microcysts. Symptomatic therapy was performed concurrently with penicillin therapy in 2 patients having congenital syphilis. Prednisolone, cyclophosphamide, captopril, and enalapril were tried in some patients, with little effect. Five patients died from respiratory failure complicated by later infection, 1 patient died from renal failure, and 4 patients were lost to follow-up. Nephrotic syndrome in the first year of life in the Eastern world is rare. Prognosis of nephrotic syndrome in Thai infants at this time is still poor.

Syndromes néphrotiques congénitaux et infantiles

Congenital nephrotic syndrome is present at birth or appears during the first three months of life and infantile nephrotic syndrome during the first year. Finnish type congenital nephrotic syndrome is an autosomal recessive disease. Nephrotic syndrome is present at birth, severe and does not respond to therapy. Infectious and nutritional complications are frequent. Renal function deteriorates necessitating a dialysis-transplantation program. Between age five and eight. The disease does not recur after transplantation. Diffuse mesangial sclerosis is the second cause of congenital and infantile nephrotic syndrome. It may be isolated or part of a Denys-Drash syndrome (association of the nephropathy with male pseudohermaphroditism and Wilm's tumor). Nephrotic syndrome is resistant to therapy. Renal failure develops in early childhood. Therapy is aimed to prevent oedema, denutrition, infections and thrombosis. Proteinuria does not recur after renal transplantation. Other causes are less frequent.

Podocytes are firmly attached to glomerular basement membrane in kidneys with heavy proteinuria

Glomerular epithelial cells (podocytes) play an important role in the pathogenesis of proteinuria. Podocyte foot process effacement is characteristic for proteinuric kidneys, and genetic defects in podocyte slit diaphragm proteins may cause nephrotic syndrome. In this work, a systematic electron microscopic analysis was performed of the structural changes of podocytes in two important nephrotic kidney diseases, congenital nephrotic syndrome of the Finnish type and minimal-change nephrotic syndrome (MCNS). The results showed that (1) podocyte foot process effacement was present not only in proteinuric glomeruli but also in nonproteinuric MCNS kidneys; (2) podocytes in proteinuric glomeruli did not show detachment from the basement membrane or cell membrane ruptures; (3) the number of pinocytic membrane invaginations in the basal and apical parts of the podocytes was comparable in proteinuric and control kidneys; (4) in proteinuric kidneys, the podocyte slit pore density was decreased by 69 to 80% and up to half of the slits were so "tight" that no visible space between foot processes was seen; thus, the filtration surface area between podocytes was dramatically reduced; and (5) in the narrow MCNS slit pores, nephrin was located in the apical part of the podocyte foot process, indicating vertical transfer of the slit diaphragm complex in proteinuria. In conclusion, these results suggest that protein leakage in the two nephrotic syndromes studied occurs through defective podocyte slits, and the other structural alterations commonly seen in electron microscopy are secondary to, not a prerequisite for, the development of proteinuria.

Congenital nephrotic syndrome of Finnish type: detection of new nephrin mutations and prenatal diagnosis in an Italian family

OBJECTIVES

Congenital nephrotic syndrome of the Finnish type (CNF) is a rare autosomal recessive disorder, caused by mutations in the NPHS1 gene, coding for nephrin. The aim of this work was to investigate the disease mutations in a CNF Italian family and to perform genetic prenatal diagnosis in the second pregnancy.

METHODS

Polymerase chain reaction (PCR) and automatic sequence analysis were used to screen the CNF Italian family for NPHS1 mutations.

RESULTS

Two novel heterozygous mutations, including a single nucleotide insertion (c.248insA) and a missense mutation (p.S572N), were detected in the proband. Molecular prenatal diagnosis was performed on fetal DNA sample: the fetus resulted compound heterozygous for the same proband mutations.

CONCLUSION

To the best of our knowledge, this is the first report of a molecular prenatal diagnosis performed in an Italian family with congenital nephrotic syndrome of Finnish type (CNF). Our findings indicate that, even though CNF is not very common outside Finland, availability and reliability of DNA diagnostics are important issues to confirm the AFP results in prenatal diagnosis.

Molecular basis of steroid-resistant nephrotic syndrome

The identification of the underlying gene defect in some cases of steroid resistant nephrotic syndrome (SRNS) has recently led to a critical breakthrough in the understanding of the pathogenesis of nephrotic syndromes. The more severe form of hereditary nephrotic syndromes is the congenital nephrotic syndrome of the Finnish type (CNF). The causative gene, NPHS1, encodes a novel protein, nephrin which is a transmembrane protein belonging to the immunoglobulin superfamily specifically expressed in the podocyte at the slit diaphragm. Using a positional cloning approach, our group identified a gene, NPHS2, involved in a specific entity of familial SRNS characterized by early onset, complete steroid-resistance, rapid progression to ESRD and no recurrence after renal transplantation. NPHS2 encodes a novel membrane protein named podocin localized at the cytoplasmic part of the slit diaphragm. Familial autosomal dominant cases of primary FSGS have been described in adulthood. Two corresponding genes have been mapped to date, one to 19q13 and the second to 11q21-22. The former has been identified as ACTN4, the gene encoding the actin-binding protein, a-actinin 4. Other genes involved in the slit-diaphragm or the nephrotic syndrome are CD2-associated protein (CD2AP), FAT1, WT1, LMX1B, SMARCAL1. Altogether, these data demonstrate the pivotal role of the podocyte in the development and the maintenance of the glomerular filtration barrier and the crucial role of the genetic factors in the development of SRNS.

[Responsible genes for proteinuria and concept of the treatment for proteinuria]

Proteinuria has been demonstrated to be not only a representative sign of renal lesion but also a risk factor for the progression to renal failure through its injurious effects on tubulointerstitium. The responsible gene for Finnish type congenital nephrotic syndrome was identified and its product was named 'nephrin' which is located on slit membrane between foot processes of glomerular epithelial cells and is considered to be concerned also in the induction of acquired renal lesions with proteinuria. The monoclonal antibody against rat nephrin can induce proteinuria. These facts suggest that the important role for final barrier against macromolecules is played by the slit membrane. Understanding the proteinuria mechanism at molecular level is expected to lead to the establishment of appropriate treatments. Nephrin is regarded as the most promising and attractive molecule for the development of new therapeutic strategy. Many nephrologists are now much interested in the intimate relationship between nephrin and angiotensin II.

[Congenital nephrotic syndrome]

Although congenital nephrotic syndromes (CNS) form a heterogenous group of disease characterized by proteinuria shortly after birth, the most common type of CNS is the congenital NS of the Finnish type (CNF). CNF is an autosomal recessive disease, and caused by mutations in the gene (NPHS1) for nephrin which is a key component of the podocyte slit diaphragm. In this review, some special issues concerning clinical and molecular diagnosis for CNS and optimal management of CNF patients were briefly summarized.

Two cases of congenital nephrotic syndrome

The clinical course and renal histological changes in two patients with congenital nephrotic syndrome are presented. Both patients developed edema and proteinuria at 2 days of age. Steroids and immunosuppressive drugs were not used in either patient. One patient showed gradual disappearance of nephrotic syndrome and proteinuria during the 16-month follow-up. In the other patient, the nephrotic state persisted and nephrotic complications necessitated bilateral nephrectomy when she was 12 months old. Renal biopsy showed minimal change lesions in one patient, while the nephrectomized kidneys showed mild mesangial proliferation and matrix increase in the glomeruli. No tubular dilatation or microcysts were observed in either patient.

Tissue expression of nephrin in human and pig

Nephrin is a major component of the glomerular filtration barrier. Mutations in the nephrin gene (NPHS1) are responsible for congenital nephrotic syndrome of the Finnish type (NPHS1). Nephrin was at first thought to be podocyte specific, but recent studies have suggested that nephrin is also expressed in nonrenal tissues such as pancreas and CNS. We studied the expression of nephrin in human and porcine tissues at different stages of development and correlated these findings to clinical characteristics of NPHS1 children. Immunofluorescence staining and Western blotting were used to detect nephrin protein in frozen tissue samples. Polyclonal antibodies against the intracellular part of nephrin were used in these analyses. In situ hybridization was used to detect nephrin mRNA in specimens from normal human subjects and patients with NPHS1. Nephrin protein was not detected in nonrenal tissues obtained from human and porcine fetuses, newborns, and infants. Likewise, nephrin mRNA expression was not observed outside kidney glomerulus in normal or NPHS1 children. The phenotype analysis of NPHS1 children with severe nephrin gene mutations supported the findings in the tissue expression studies and revealed no impairment of the neurologic, testicular, or pancreatic function in a great majority of the patients. The studies suggest that nephrin has no major clinical significance outside the kidney.

Recent advances in congenital nephrotic syndrome

PURPOSE OF REVIEW

This review provides a concise update of the most recent literature related to the diagnosis and care of patients with congenital nephrotic syndrome. This topic is of particular interest in light of the rapidly growing body of literature regarding mutations of proteins such as nephrin and podocin that are expressed at or near the podocyte slit diaphragm.

RECENT FINDINGS

The phenotypic variance of patients with congenital nephrotic syndrome with nephrin and podocin mutations resulting from triallelic mutations represents an important advance in our understanding of the effect of multiple genetic mutations on clinical disease expression. Clinically, the management of patients with unilateral nephrectomy, rather than the classic bilateral nephrectomy, represents an efficacious alternative management strategy and may impart better chances of graft survival by allowing later transplantation. Identification of a subset of patients with congenital nephrotic syndrome at increased risk of recurrence who also have antinephrin antibodies may enhance our understanding of recurrent disease in congenital nephrotic syndrome after transplantation.

SUMMARY

Exciting recent findings in the genotypic/phenotypic correlations of patients with congenital nephrotic syndrome may not only modify our understanding of this disease but may also help to revolutionize our understanding of human genetics. Promising outcomes with unilateral nephrectomy in patients with congenital nephrotic syndrome have permitted transplantation to be delayed and may potentially decrease the risk of complications. New findings regarding recurrence of nephrotic syndrome in patients with congenital nephrotic syndrome after transplantation may lead to improved survival in future renal transplantations.

A case of atypical congenital nephrotic syndrome

We present a female newborn with the nephrotic syndrome of intrauterine onset and a unique set of extrarenal abnormalities, as well as atypical renal lesions. The extrarenal anomalies comprised a soft tissue hemangioma in the frontotemporal region, unilateral microphthalmia (with persistent hyperplastic corpus vitreous and detachment of the retina), and glaucoma in the other eye. Immature glomeruli and/or glomeruli with large cellular crescents were found in renal biopsy specimens in the 3rd week of life. On autopsy, 7 weeks later, diffuse mesangial sclerosis (DMS) was the predominant type of glomerular lesion. In addition, dilations of tubules, forming microcysts, as well as clusters of infiltrating cells in the interstitium, were found both in renal biopsy and autopsy specimens. Although the symptoms observed in our patient did not match any reported in association with the known forms of the congenital nephrotic syndrome (CNS), the most probable diagnosis seemed to be CNS due to DMS of intrauterine onset, with superimposed drug-related tubulointerstitial nephritis.

[The genetic basis of childhood nephrotic syndrome]

Great progress has been made of late in understanding the mechanisms of proteinuria, the structure and function of the slit diaphragm, and the genetic background of congenital nephrotic syndromes in new borns and infants. This paper presents recent achievements of molecular genetics in unraveling the causes of inherited disorders, e.g. Finnish-type nephrotic, Denys-Drash and Frasier's syndromes, as well as sporadic focal-segmental glomerulosclerosis. A change in the routine policy used in evaluating the causes of childhood nephrotic syndrome is discussed.

Denys-Drash syndrome

We report a case of Denys-Drash syndrome, a disorder characterized by male pseudohermaphroditism, congenital nephrotic syndrome, and early renal failure. The patient received dialysis therapy from 15 days of age until his death at the age of 6 months. DNA analysis was performed on the WT1 gene, and a missense point mutation was detected in exon 8 (R366H). After prenatal confirmation of normal WT1 gene in the family's next child, they had a healthy baby 14 months after the patient's death.

The contribution of ultrasound for the differential diagnosis of congenital and infantile nephrotic syndrome

The aim of this study was to determine whether high-resolution ultrasound is able to differentiate between the various diseases associated with nephrotic syndrome (NS). We reviewed the US features of 15 patients less than 1 year presenting a NS whose exact type was defined by pathology nephrotic syndrome of Finnish type (NSFT, n=2); focal and segmental hyalinosis (FSH, n=3); minimal-change glomerular disease (MCGD, n=2); neonatal glomerulonephritis (n=1), and diffuse mesangial sclerosis (DMS, n=7). The US features studied included the size of the kidneys, cortical echogenicity, cortico-medullary differentiation (CMD), and borders. The images were reviewed on hard copies by two observers unaware of the final diagnosis. In each case a diagnosis was proposed based on the reading of the US features. Six patients with DMS displayed a peculiar US pattern: mild increase of renal size; and inhomogeneous (patchwork-like) parenchymal hyperechogenicity that included areas of the cortex and medulla. The NSFT and neonatal glomerulonephritis displayed some of the same US features: increased kidney size (+2 SD) and had homogeneous cortical hyperechogenity with persistent cortico-medullar differentiation. The kidneys in the 3 patients with SFH were sonographically normal (n=1) or displayed a mild cortical hyperechogenicity (n=2). Inhomogeneous parenchymal hyperechogenicity involving only segments of the cortex and medulla seems to be a specific US pattern for DMS. Ultrasound is less specific for the other types of CNS.

Genetic basis of congenital nephrotic syndrome

A young girl presented at 1 month of age with nephrotic syndrome. By incorporating clinical and pathologic data, the diagnosis of congenital nephrotic syndrome, Finnish type, was made. The differential diagnosis of early onset nephrotic syndrome, as it pertains to this patient is discussed. This article highlights recently discovered glomerular filtration proteins and their relationship to the pathophysiology of inherited kidney disease.

Congenital nephrotic syndrome: report of an infant with diffuse mesangial sclerosis

A case of congenital nephrotic syndrome (CNS) caused by diffuse mesangial sclerosis (DMS) is presented. A female baby weighting 2,680 gm was delivered at 35 weeks' gestation. She had early onset of generalized edema, heavy proteinuria, oligouria, uncorrectable hypoalbuminemia, and rapid progression to renal failure. Even after being treated with strong antibiotics (teicoplanin and ceftazidine), the infant died of septic shock with Enterobacter cloacea, only sensitive to imipenem, at the age of 7 days. The necropsy showed diffuse mesangial sclerosis. This case demonstrates that prematurity, low birth body weight and early onset of symptoms are not pathognomonic of the congenital nephrotic syndrome of the Finnish type (CNF). It can also occur in DMS. Besides, empirical antibiotics should be started promptly and should cover the major hospital strains of bacteria if the patient is not well.

Molekulare Genetik in der Nephrologie

Genetische Untersuchungen in der Nephrologie mussten sich bislang darauf beschränken, zur Identifikation von meist seltenen Erkrankungen zu dienen, ohne jedoch Einblick in die molekulare Pathogenese dieser Entitäten zu liefern oder gar Behandlungsmöglichkeiten derselben aufzuzeigen. Dies hat sich in den letzten Jahren dramatisch zu ändern begonnen. Im vorliegenden Beitrag soll die Wandlung der molekularen Genetik in der Nephrologie zu einer klinisch relevanten und wissenschaftlich hoch aktuellen Disziplin am Beispiel des neuen Verständnisses familiärer Erkrankungen des glomerulären Filters der Niere illustriert werden.

Podocyte differentiation and hereditary proteinuria/nephrotic syndromes

The study of familial nephrotic syndromes (NS) and the analysis of murine models of glomerular diseases resulted in major progresses in the knowledge of podocyte physiology and pathology. Numerous proteins participating in the composition of the slit diaphragm region have been identified. The importance of several of them (nephrin, podocin, CD2AP, and Neph1) in the maintenance of the glomerular filtration barrier has been demonstrated by the occurrence of massive proteinuria when they are defective. The role of the cytoskeleton has been revealed by the development of proteinuria/NS in patients with ACTN4 mutation and the occurrence of early and severe NS in alpha-actinin-4-deficient mice. Given the genetic heterogeneity of familial NS and the many other genes to be identified, further insights in the molecular basis of the role of the podocyte in the maintenance of the glomerular filtration barrier may be expected in the near future.