LONDON - Study of the genetics underlying a rare inherited kidney disorder has led to the discovery of a new protein present in the kidney, which appears to play a crucial role in the filtration of urine. Studies are under way to find out if the gene encoding the new protein, which has been called podocin, is mutated in sporadic cases of kidney malfunction.
Corinne Antignac, director of research at Inserm, based at the Necker Hospital in Paris, told BioWorld International: "Once we had found this gene, which is called NPHS, we wanted to find out which tissues it was expressed in. Interestingly, we found that it was expressed only in certain cells found in the glomerulus of the kidney. These are called podocytes, and they are involved in the filtration process. If podocin is absent, you have proteinuria - leakage of protein from the plasma."
Podocytes are specialized epithelial cells in the glomeruli of the kidney. These cells are found at the outer surface of the capillary walls. The glomeruli carry out ultrafiltration of the plasma, forming "primary urine."
The podocytes gain their name from the "foot processes" that they bear. These processes interdigitate and are connected at the capillary wall by a mesh of proteins known as the slit diaphragm. One of the first changes that occurs in some types of kidney disease is loss of the podocytes' "feet" and of the slit diaphragm.
Antignac and her colleagues report their data in a paper in the April issue of Nature Genetics titled, "NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome."
A pediatric nephrologist, Antignac turned to research more than 10 years ago, specializing in the study of inherited kidney diseases. One of the conditions she focused on was the disorder known as nephrotic syndrome.
She said, "Nephrotic syndrome is a relatively common kidney disorder in childhood, but most cases are sporadic and respond to steroid therapy. If you give steroids, the proteinuria disappears. The child may have relapses, but does not usually go on to develop end-stage renal disease."
In fewer than 20 percent of cases, however, steroids have no effect, the function of the kidneys deteriorates and end-stage renal failure develops, for which the only treatment is hemodialysis or a kidney transplant.
She and her team collected a subgroup of families in which this form of nephrotic syndrome appeared to be inherited. "We had 10 families," she said, "and we did a genome-wide search and found the location of the gene on the long arm of chromosome 1. That was reported in 1995. After that, we collected more families and did positional cloning, which allowed us to identify the causative gene in the smallest genetic interval defined by studying the families."
Part of the search involved analyzing many genes in the area of interest, as well as many expressed sequence tags, to find out if they were expressed in the kidney. Many were expressed in the kidney, including one, NPHS2, which was expressed only in the kidney and not in other tissues. "This was a very good candidate gene for this disorder," Antignac said, "as the only defect in the patients is the kidney disease."
In all, they found 10 different mutations in NPHS2, which included nonsense, frameshift and missense mutations. In the families studied, these mutations cosegregated with the disease. "When we found that, we were sure then that this gene was the one we were looking for, and that it was involved with the disease," Antignac said.
The finding is important for patients and their families, she added. "It will make it possible to identify cases of steroid-resistant nephrotic syndrome and avoid subjecting these patients to unnecessary immunosuppressive therapy that will not have any effect. In the event that a transplant becomes necessary, the disease will not recur after a graft."
Having found the gene, the next question was what protein did it encode? It turned out to be a previously unknown protein, which the team has called podocin. Sequence homology suggests it is a member of the family of proteins known as stomatins. Antignac and her colleagues speculate in their paper that podocin may play a role in stabilizing interactions between other proteins involved in filtration of urine at the glomerulus and in anchoring these to the cytoskeleton of the podocyte.
Commenting on the paper in a News and Views article in the same issue of Nature Genetics, Stefan Somlo, of Yale University School of Medicine in New Haven, Conn., and Peter Mundel, of Albert Einstein College of Medicine in the Bronx, N.Y., point out that the discovery of the gene encoding podocin is one of a series of discoveries of genes that, when mutated, result in various kidney disorders.
In their article, titled "Getting a foothold in nephrotic syndrome," they predict that the identification of these new proteins will open new avenues for the study of molecular mechanisms underlying podocyte function, the development of proteinuria and nephrotic syndromes. They conclude: "Our rapidly increasing knowledge of the molecular players should facilitate the analysis of the signal transduction pathways regulating podocyte structure and function and, eventually, lead to therapeutic targets for a host of kidney diseases."
Antignac added, "The reason why everyone is interested in the podocytes of the glomeruli is because they hold the key to the development of the common condition known as focal segmental glomerular sclerosis, or FSGS. This condition is seen in diabetic nephropathy and HIV nephropathy, for example. One question that springs to mind is whether there are some polymorphisms of the podocin gene that increase someone's risk of developing nephropathy or, conversely, protect them from it."
She and her colleagues currently are conducting studies to find out if people with sporadic nephrotic syndrome have mutations in their podocin genes.