Exclusion of eight genes as mutated loci in congenital nephrotic syndrome of the Finnish type

A novel role for the adaptor molecule CD2-associated protein in transforming growth factor-beta-induced apoptosis

CD2-associated protein (CD2AP) is an adaptor molecule involved in T cell receptor signaling and podocyte homeostasis. CD2AP-deficient mice develop nephrotic syndrome and renal failure caused by glomerulosclerosis. Here we report that increased transforming growth factor-beta1 (TGF-beta1) expression and apoptosis were present in podocytes at the onset of albuminuria and were followed by depletion of podocytes associated with progressive focal-segmental glomerulosclerosis in CD2AP-/- mice. Conditionally immortalized podocytes derived from CD2AP-/- mice were more susceptible to TGF-beta-induced apoptosis compared with CD2AP+/+ podocytes. Reconstitution of CD2AP rescued CD2AP-/- podocytes from TGF-beta-induced apoptosis. CD2AP was required for early activation of anti-apoptotic phosphatidylinositol 3-kinase (PI3K)/AKT and extracellular signal-regulated kinase 1/2 by TGF-beta. In contrast, activation of pro-apoptotic p38 MAPK by TGF-beta was accelerated and enhanced in the absence of CD2AP. CD2AP was not required for PI3K/AKT activation by insulin and epidermal growth factor, indicating that CD2AP is a selective mediator of anti-apoptotic TGF-beta signaling. In summary, we identified CD2AP as a novel mediator for selective activation of survival pathways and repression of apoptosis signaling by TGF-beta in podocytes. Together, our in vitro and in vivo findings suggest that TGF-beta-induced podocyte apoptosis is an early pathomechanism in mice developing focal-segmental glomerulosclerosis associated with functional impairment of CD2AP.

Lipid peroxidation in human proteinuric disease

BACKGROUND

While metabolically generated oxidants are produced locally in experimental glomerular diseases, little is still known of their significance and the respective scavenger systems in human glomerular diseases.

METHODS

Here we studied kidneys from patients with congenital nephrotic syndrome of the Finnish type (CNF), a human model disease of isolated proteinuria. Expression of specific mRNAs for a major antioxidant system against lipoperoxidation [phospholipid hydroperoxide glutathione peroxidase (PHGPx)] and for mitochondrial proteins were studied in Northern blotting together with analysis of PHGPx in semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). The respective proteins and lipoperoxide (LPO) adducts malonyldialdehyde (MDA) and 4-hydroxynonenal (4-HNE) were analyzed in immunohistochemistry.

RESULTS

PHGPx and the mitochondrially encoded subunits of cytochrome-c-oxidase were distinctly down-regulated within the glomeruli of CNF kidneys. These changes were confirmed in semiquantitative RT-PCR. Increases of lipoperoxidation products MDA and 4-HNE were constantly found in the glomeruli of CNF. In agreement with findings in CNF, similar results were obtained in biopsies from other human glomerular diseases.

CONCLUSIONS

These findings suggest that local mitochondrial damage initiates LPO, which then causes deposition of the cytotoxic LPO products in glomeruli, as seen especially in CNF kidneys. Together with down-regulation of the local antioxidant protection, these may be important pathophysiologic mechanisms in human glomerular disease.

Genetics of the nephrotic syndrome

There are a large number of glomerular diseases that may be responsible for a nephrotic syndrome, the most frequent in childhood being minimal change disease. In the past few years, the molecular genetic basis of several conditions that may cause a nephrotic syndrome have been identified. Denys-Drash syndrome and Frasier syndrome are related diseases caused by mutations in the WT1 gene. Familial forms of idiopathic nephrotic syndrome with focal and segmental glomerular sclerosis/hyalinosis have been identified with an autosomal dominant or recessive mode of inheritance and linkage analysis have allowed to localize several genes on chromosomes 1, 11 and 17. The gene responsible for the Finnish type congenital nephrotic syndrome has been identified. This gene, named NPHS1, codes for nephrin, which is located at the slit diaphragm of the glomerular podocytes and is thought to play an essential role in the normal glomerular filtration barrier.

Improved prognosis for congenital nephrotic syndrome of the Finnish type in Irish families

Congenital nephrotic syndrome of the Finnish type is a rare autosomal recessive disease with a high infant mortality without aggressive treatment. The biochemical basis of the disease is not understood fully but the disease locus has been mapped recently to chromosome 19q12-q13.1 in Finnish families. This paper describes the clinical features and outcome of 20 patients in Ireland with congenital nephrotic syndrome of the Finnish type who have presented since 1980. Before 1987, all infants died by the age of 3 years. After the introduction of daily intravenous albumin infusion, nutritional support, elective bilateral nephrectomy, and renal transplantation, mortality in the past decade has fallen to 30%, with no deaths in the past five years. Genetic linkage analysis was performed in six families in whom DNA was available and the locus responsible was mapped to the same region on chromosome 19 as in Finnish families, suggesting that Irish families share the same disease locus.

Positionally cloned gene for a novel glomerular protein--nephrin--is mutated in congenital nephrotic syndrome

Congenital nephrotic syndrome of the Finnish type (NPHS1) is an autosomal-recessive disorder, characterized by massive proteinuria in utero and nephrosis at birth. In this study, the 150 kb critical region of NPHS1 was sequenced, revealing the presence of at least 11 genes, the structures of 5 of which were determined. Four different mutations segregating with the disease were found in one of the genes in NPHS1 patients. The NPHS1 gene product, termed nephrin, is a 1241-residue putative transmembrane protein of the immunoglobulin family of cell adhesion molecules, which by Northern and in situ hybridization was shown to be specifically expressed in renal glomeruli. The results demonstrate a crucial role for this protein in the development or function of the kidney filtration barrier.

Congenital nephrotic syndrome of the Finnish type: linkage to the locus in a non-Finnish population

Congenital nephrotic syndrome of the Finnish type (CNF) is inherited as an autosomal recessive trait. The biochemical basis of the disease is unknown, although a lesion in the glomerular basement membrane is strongly suggested. Recently, the CNF locus was assigned to chromosome 19q12-q13.1 on the basis of linkage analysis in Finnish families. The high incidence of the disease in Finland, as well as the demonstration of linkage disequilibrium in the Finnish study, strongly suggests a founder effect based on a common ancient mutation in this population. We confirm linkage of the CNF locus to the same chromosomal region in seven non-Finnish CNF families without evidence of linkage disequilibrium. Our results show that the same gene seems to be affected in both Finnish and non-Finnish CNF populations. However, in the latter the mutation-carrying chromosomes descend from different ancestors without evidence of a founder effect.

Glomerular antigens in severe hereditary nephrosis

In search of the basic defect and cell type responsible for the massive treatment-resistant proteinuria of congenital nephrotic syndrome of the Finnish type (CNF), we examined tissue samples of CNF kidneys using established antibody and lectin markers of various glomerular cell types. Markers of vascular endothelium (antibodies to factor VIII and a human homologue of podocalyxin (anti-PHM5) and UEA I lectin) showed no qualitative changes in the endothelial cells of glomeruli or peritubular areas in CNF as compared with controls. Markers of glomerular mesangial cells (antibodies to desmin, smooth muscle actin, RCA I lectin) revealed a secondary increase in mesangial reactivity reflecting the sclerosis and expansion of the mesangial areas in CNF. Markers of visceral epithelial cells (antibodies to a human homologue of podocalyxin, C3b receptor, vimentin, common lymphocytic leukemia antigen, gp44, and the WGA, LFA and, after neuraminidase treatment, PNA lectin) failed to show appreciable qualitative changes in CNF kidney samples. Interestingly, the alpha 2 beta 1 integrins appeared greatly reduced in all CNF samples studied, possibly explaining the mechanisms of CNF-associated proteinuria.

Functional rescue of the glomerulosclerosis phenotype in Mpv17 mice by transgenesis with the human Mpv17 homologue

The germ line insertion of a defective retrovirus into the Mpv17 gene of mice is associated with a recessive phenotype. Mice homozygous for the integration develop glomerulosclerosis at a young age. The phenotype resembles human glomerulosclerosis in its physiological parameters as well as in histology. A human homologue of the Mpv17 gene has been identified, isolated and analyzed. We here show that this gene, which has a role in the production of reactive oxygen species, can rescue the phenotype of Mpv17 deficient mice when introduced by transgenesis. This provides formal proof for the hypothesis that the phenotype is caused by the loss of function of the Mpv17 gene. It also provides evidence for the functional conservation of the Mpv17 gene in mammals and points to a potential role of this gene in human kidney disease.

Management of congenital nephrotic syndrome of the Finnish type

Congenital nephrotic syndrome of the Finnish type (CNF) is a rare autosomal recessively inherited disease characterised by intrauterine onset of massive urinary loss of proteins, 90% of which is albumin. The CNF gene has been localised to the long arm of chromosome 19, but the pathogenesis remains unclear. Historically, all CNF patients died, usually within the first 6 months of life. Today, a normal life can be achieved for a child with CNF by correcting the protein deficiency and normalising nutrition. This is accomplished by early intravenous albumin supplementation, nutritional support, aggressive treatment of complications and early renal transplantation, after bilateral nephrectomy and peritoneal dialysis. In the present article current treatment strategies are reviewed, and our own experience with 43 CNF patients during the last 10 years is presented.