University Children's Hospitals of Freiburg and Heidelberg, Germany
Introduction
Nephrotic syndrome (NS) usually occurs as a sporadic disease, but familial cases have frequently been reported. The first observation in six families with 13 affected siblings was reported by Fanconi et al. [1]. Larger population studies have been published since 1970 [24]; whereas only some of the reported families presented with steroid-sensitive NS, the majority presented with steroid-resistant NS. In most of the patients with primary steroid-resistant NS, focal segmental glomerulosclerosis (FSGS) was demonstrated by renal biopsy. Despite uniform renal histological findings, heterogeneous pathogenesis had been suspected, but it is only during the last 5 years that several genes have been localized which have allowed a more precise classification of FSGS and steroid-resistant NS. Recently the gene for steroid-resistant NS (NPHS2) was cloned [5], which is an important step in understanding the pathogenesis of NS in general and NPHS2 in particular.
Familial idiopathic nephrotic syndrome
Familial idiopathic NS in childhood is mostly inherited as an autosomal recessive trait [3,6,7], whereas in adulthood autosomal dominant forms are more frequently observed [810]. By the localization of a gene, namely NPHS2, responsible for steroid-resistant NS, to chromosome 1q25, the existence of a distinct genetic subgroup of autosomal recessive nephrosis was demonstrated for the first time [7]. In this population, the disease occurred within the first year(s) of life, with rapid progression to end-stage renal disease (ESRD). In most of the patients, FSGS was identified by renal biopsies and recurrence of the disease in the renal allograft was not observed. The NPHS2 gene was cloned in 2000 [5]. It is predicted to be an integral membrane protein (podocin), which is exclusively expressed in glomerular podocytes. Podocin may interact with nephrin and CD2AP, suggesting a function within the dynamic regulation of the actin cytoskeleton [11].
Autosomal dominant variants of FSGS, with later onset in adulthood, have been shown to be linked to chromosomes 19q13 (FSGS1 [10,12]) and 11q21-q22 (FSGS2 [13]). FSGS1 is now known to be caused by defective -actinin-4 (ACTN4), which is a structural gene of the glomerular filtration barrier [14]. The gene for FSGS2 has not yet been identified.
Congenital nephrotic syndrome of the Finnish type
The gene locus for congenital nephrotic syndrome (CNF) (NPHS1) was mapped on chromosome 19q13.1 in a large Finnish population [15]. Only four different haplotypes were found in 90% of the Finnish CNF-families, allowing for a prenatal diagnosis based on haplotype analysis in this population [16]. However, NPHS1 was also found to be involved in non-Finnish CNF-families [17]. The NPHS1 gene is 26 kb in size and contains 29 exons [18]. It codes for a putative transmembrane protein (nephrin), containing eight immunoglobulin-C2-like motifs, a fibronectin type III-like domain, a transmembrane domain and a cytosolic domain. It is structurally related with proteins of the immunoglobulin superfamily, which are involved in cell adhesion processes. By co-localization studies nephrin was found to be specifically expressed at the slit membrane of podocytes [19]. This is also true for the CD2 associated protein CD2AP, the mouse homolog of the human CMS protein, which has been shown to be associated with nephrin. CD2AP-deficient mice develop a congenital nephrotic syndrome and die from renal failure at 67 weeks of age [20].
Up to now, 36 mutations in NPHS1, including deletions, insertions, nonsense, missense and splicing mutations have been reported [21]. Two mutations (Finmajor and Finminor) have been found in most of the Finnish chromosomes [18], whereas these two mutations are rare in other populations.
The diagnostic procedure in CNF has been significantly improved by using DNA-based methods. Especially in the Finnish population the diagnosis is possible on the basis of haplotype analysis in nearly 95% of the cases, whereas in non-Finnish CNF-patients most of the mutations can be detected by mutational analysis.
Nephrotic syndrome and WT1 mutations
Diffuse mesangial sclerosis (DMS) is characterized by early onset proteinuria with progression into renal failure within the first year(s) of life. Morphological alterations in the kidney are characteristic and include hypertrophy of the podocytes, thickening of the glomerular basement membrane, dilated tubules and a tubulointerstitial fibrosis [22]. DMS can occur in association with a male pseudo-hermaphroditism and/or Wilms tumour (WT), classifying the DenysDrash syndrome (DDS) [23,24]. However, incomplete variants of DDS have been described, presenting without WT, or in genotypic females with nephropathy and WT.
The Wilms tumour gene WT1 was cloned in 1990 and is located on chromosome 11p13. Exons 16 code for an amino terminal proline- and glutamine-rich domain; exons 710 encode four zinc fingers within the DNA-binding domain. Due to alternative splice sites in exon 5 and 9 (KTS±) and an alternative start codon, different mRNAs are transcribed. WT1 plays a role in the embryonic development of the kidney and the gonads. During embryogenesis WT1 is expressed in the mesonephron, where it is essential for its differentiation. In the adult kidney WT1 persists in the glomerular podocytes. The other main expression sites are the gonads. WT1 deficiency in knock-out mice result in aplasia of both the kidneys and the gonads.
In patients with DDS, numerous WT1-mutations, including missense, nonsense and splice site mutations have been described (for review see [25]). Most of them have been detected in exon 8 or 9 with a hot spot mutation (R394W) in exon 9 at position 394. Furthermore, WT1-mutations have also been found in patients with isolated DMS and FSGS [26,27].
WT1 mutational analysis should be considered in males with ambiguous genitalia and patients whose renal biopsy shows DMS, in order to determine the WT risk [26,28]. WT1 missense mutations usually cause DDS with rapid progression to ESRD and an elevated risk of developing WT, whereas patients with the KTS splice site mutation do not develop WT and renal failure progresses slowly. However, this characteristic mutation can also be found in phenotypic females with progressive renal failure and FSGS.
Recently, Denamur et al. [29] investigated the possibility that some cases of primary steroid-resistant NS associated with FSGS may be caused by WT1 splice site-mutations. Within 37 children with non-familial primary steroid-resistant NS they found one boy with FSGS and associated pathologies (diaphragmatic hernia, proximal hypospadias and unilateral testicular ectopia) who carried the heterozygous splice site-mutation identical to that reported in patients with Frasier syndrome. By this, the study added to the already reported heterogeneity of primary steroid-resistant NS. The authors suggested that the reported mutations are not likely to be a common cause of isolated steroid-resistant NS and recommended a WT1 exon 9/intron 9 splice-site study in children with primary steroid-resistant NS if genital or diaphragmatic anomalies are associated.
Like DDS, the Frasier syndrome is defined by complete XY gonadal dysgenesis with nephrotic syndrome. However, progression into renal failure is usually slower than in DDS and the histological findings often show FSGS rather than DMS. Patients are at high risk to develop gonadoblastoma.
Frasier syndrome is caused by mutations in the donor splice site in intron 9 of the WT1-gene, which leads to haploinsufficiency of the alternative splice product KTS+. It has been suggested that this imbalance between the KTS-isoforms might be responsible for the development of podocytes and gonads (KTS+) or WT (KTS-) [30].
Lessons from the genetic studies in steroid resistant NS
From the recent progress in the research on molecular biology and genetics of NS we can draw several conclusions.
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Notes
Correspondence and offprint requests to: O. Mehls, Division of Paediatric Nephrology, University Children's Hospital, Inm Neuenheimer Feld 150, D-69120 Heidelberg, Germany.
References