Congenital nephrotic syndrome of the Finnish type is not associated with the Pax-2 gene despite the promising transgenic animal model

Genetic kidney diseases

Knowledge of the primary cause of a disease is essential for elucidation of its mechanisms, and for adequate classification, prognosis, and treatment. Recently, the causes of many kidney diseases have been shown to be single-gene defects-eg, steroid-resistant nephrotic syndrome, which is caused by podocin mutations in about 25% of children and nearly 15% of adults with the disease. Knowledge of a disease-causing mutation in a single-gene disorder represents one of the most robust diagnostic examples of personalised medicine because the mutation conveys an almost 100% risk of developing the disease by a defined age. Whereas single-gene diseases are rare disorders, polygenic risk alleles arise in common adult-onset diseases. In this Review, I will discuss prominent renal single-gene kidney disorders, and polygenic risk alleles of common disorders. I delineate how emerging techniques of total exome capture and large-scale sequencing will assist molecular genetic diagnosis, prognosis, and specific treatment, and lead to an improved elucidation of disease mechanisms, thus enabling development of new targeted drugs.

Positional cloning uncovers mutations in PLCE1 responsible for a nephrotic syndrome variant that may be reversible

Nephrotic syndrome, a malfunction of the kidney glomerular filter, leads to proteinuria, edema and, in steroid-resistant nephrotic syndrome, end-stage kidney disease. Using positional cloning, we identified mutations in the phospholipase C epsilon gene (PLCE1) as causing early-onset nephrotic syndrome with end-stage kidney disease. Kidney histology of affected individuals showed diffuse mesangial sclerosis (DMS). Using immunofluorescence, we found PLCepsilon1 expression in developing and mature glomerular podocytes and showed that DMS represents an arrest of normal glomerular development. We identified IQ motif-containing GTPase-activating protein 1 as a new interaction partner of PLCepsilon1. Two siblings with a missense mutation in an exon encoding the PLCepsilon1 catalytic domain showed histology characteristic of focal segmental glomerulosclerosis. Notably, two other affected individuals responded to therapy, making this the first report of a molecular cause of nephrotic syndrome that may resolve after therapy. These findings, together with the zebrafish model of human nephrotic syndrome generated by plce1 knockdown, open new inroads into pathophysiology and treatment mechanisms of nephrotic syndrome.

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.

mRNA differential display analysis of nephrotic kidney glomeruli

BACKGROUND

Differential display RT-PCR (DDRT-PCR) is a new powerful technique for identification and characterization of altered gene expression in eukaryotic cells and tissues. We studied here changes in kidney glomerular gene expression in patients with congenital nephrotic syndrome of the Finnish type (CNF), an inherited kidney disease with heavy proteinuria already in utero.

METHODS

Using the DDRT-PCR approach and isolated glomeruli from removed human kidneys, we compared the gene expression patterns of normal human and CNF glomeruli. Differential expression of candidate genes was verified by Northern blotting, and the corresponding PCR fragments were sequenced and compared to known sequences in databanks.

RESULTS

We found several genes and sequence tags with altered expression in nephrotic glomeruli including fragments with close homologies to cytochrome c oxidase subunit I, integrin-linked kinase, insulin-like growth factor II receptor and eotaxin, and also clones resembling anchyrin and cadherin-like consensus sequences.

CONCLUSION

All the sequences identified are of interest in respect to pathogenesis of proteinuria. Furthermore, this study reveals potentially new members to known gene families with tissue and cell type-specific expression.

Morphologic changes suggesting abnormal renal differentiation in congenital nephrotic syndrome

Retrograde differentiation (or dedifferentiation) has recently been proposed as a pathogenetic mechanism involved also in various renal diseases. Here we studied whether evidence of these mechanisms can be found in the kidneys of patients with congenital nephrotic syndrome of the Finnish type (CNF). These patients show isolated massive proteinuria but no primary symptoms from any other organ systems. For the analysis we used antibody markers of early (fibronectin, stem cell factor, Wilms' tumor gene product, cytokeratin) and later (laminin, midgestation and kidney, heparin binding growth-associated molecule) stages of nephron differentiation as well as for apoptosis (acridine orange staining), rescue from apoptosis (anti-Bcl-2 antibodies) and cell proliferation (antibodies to proliferating cell nuclear antigen). In the peritubular spaces atypically organized areas were found which appeared positive with markers of low stages of differentiation, but neither abnormal cell proliferation nor activation of the apoptotic pathway could be detected. As morphologic signs of abnormal tissue organization, we found clusters of tightly compacted and large glomeruli corresponding to the size of two to three normal glomeruli. However, all individual glomerular cell compartments (mesangial, endothelial, visceral epithelial cells) appeared balanced in relative cell numbers. Together these results may indicate abnormal early mesenchymoepithelial tissue interaction leading to excessive and poorly organized formation of glomeruli. This could be causally related also to the serious functional immaturity of CNF kidneys presented as isolated proteinuria.

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.

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.

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

The congenital nephrotic syndrome of the Finnish type (CNF) is an autosomal recessive disease characterized by massive proteinuria already at birth. The gene locus defective in CNF was searched for using polymorphic markers of candidate genes coding for components of the basement membrane (BM). The linkage analyses in 17 Finnish CNF families demonstrated exclusion of obligatory recombination events between the disease and eight genes coding for BM components. The genes coding for the alpha 1(IV), alpha 2(IV), alpha 3(IV) and alpha 4(IV) chain of type IV collagen, the B1e, B2e and B2t chains of laminin, as well as the BM heparan sulfate proteoglycan core protein were all excluded in this Finnish family material. Since the defect is not in any of the genes coding for major components of BM, the identification of the gene defect will most probably reveal a new gene important for the development and function of the glomerular basement membrane.