A few years ago in Japan, an oddball strain of rat turned up in an animal-breeding colony. The rodents had outsized kidneys and livers, which massively swelled their abdomens. Breeders recognized the signs and symptoms of a major pediatric malady autosomal recessive polycystic kidney disease, or ARPKD.
So did molecular geneticist Peter Harris, at the Mayo Clinic in Rochester, Minn. “ARPKD is one of the most important childhood diseases of the kidneys,” he observed. “Also known as infantile PKD, it affects one in 20,000 births worldwide, and results in the development of multiple fluid-filled cysts in the kidneys, plus fibrosis in the liver. It’s often associated with poor lung development and neonatal death.”
Harris continued: “The physical, psychological and financial effects ARPKD has on the entire family are devastating. This is a recessively inherited disease, so if both carrier parents have one defective copy of the gene, there’s a 4-in-1 chance that each conception will result in a child with the disease. Improved respiratory treatment has increased neonatal survival,” he went on, “but death still occurs in infancy, in roughly 30 percent of affected children. But if they can get beyond that, many of them succumb to renal failure in the next 20 or 30 years. About half of those who survive the neonatal period progress to end-stage renal disease in the first decade of life, so require organ transplantation or lifelong kidney dialysis.”
Harris is senior author of an article in the March 2002 issue of Nature Genetics. Its title: “The gene mutated in autosomal recessive polycystic kidney disease encodes a large, receptor-like protein.”
“With all of these gene clonings,” he told BioWorld Today, “it’s an important first step toward identifying what the recessive polycystic gene does. Now that we know what the gene is, it allows us to try to determine the normal role of the protein encoded by that gene and the pathogenesis of the disorder.”
Harris and his co-authors took as their point of departure the Japanese PKD-mimicking rat. Unlike most animal models, this rodent didn’t derive from transgenic or knockout cloning tricks. Rather, it arrived on the research scene two or three years ago as an unpremeditated product of nature. “This was a spontaneous mutation in rats bred in Japan,” Harris pointed out. “Then they were inbred over several generations, and it was realized the disease was inherited in recessive form.”
Inbred Japanese Rats Model Kidney Disease
“One of our team members,” Harris recounted, “brought the rat here to Mayo. When we did genetic linkage analysis on it, we found that the same gene mutated in this animal’s genome was one that causes the human gene and the human disease.
“It’s a very large gene,” Harris noted, “spread over nearly half a million base pairs of DNA. It has 66 protein-forming exons. The part that encodes proteins is about 12 kilobases long, making a predicted protein of 4,074 amino acids. A gene that size is quite complicated to screen for diagnostics in this disorder. And the work we’ve done on it up to now suggested a wide range of different mutations in different individuals.
“The protein, which we dubbed fibrocystin,” Harris went on, “is not similar to any other protein that we know. But it does have some motifs found in other proteins, in particular, an immunoglobulin-like structural fold that’s found in such proteins as hepatocyte growth-factor receptors.”
The Mayo team then turned from rats to humans. “In these patients,” Harris recounted, “we wanted to show that this gene we had identified in the rat was associated with human disease. To do that we looked at a number of probands with autosomal recessive polycystic disease who had survived the neonatal stage. On the whole, they represented probably not the most extreme form of the disease because many died in the perinatal period. Some of these had severe disease, and some had already had transplants, but on the whole they were a little less severe than the worst form of PKD.
“Then we screened all of their gene sequences,” Harris recounted, “looking for mutations found only in PKD-affected patients. And we found a variety of different changes. Because this PKD is a recessive disease, in many of the patients we discovered two different mutations one inherited from father, one from mother.
“Only when you have two such mutations,” Harris explained, “does the infant or individual get the disease. And we found quite a wide range of different changes sufficient to prove that this was in fact the autosomal recessive gene not the long-ago identified dominant form of the disease. So then we went on to try and predict from the gene what its protein would look like. That was when we came up with the idea that it might be some type of receptor, probably involved in fetal development of the kidney, particularly of that organ’s collecting ducts and the biliary tract in the liver. These are the two parts that are defective in this disorder. Ultimately, we hope to be able to develop some kind of rational therapy for the disease.”
Harris and his co-authors are now “raising antibodies to the protein toward identifying what its function might be. We will go on to do more protein biochemistry once we have these antibody reagents in hand. We’re also trying to look at a larger number of patients to see if there’s a correlation between where the mutations are and what the disease is like.”
First Clinical Application: Prenatal Diagnosis
“Prenatal diagnosis is one putative clinical application,” he said. “I think the other prospects are longer term, when we can determine what the role of this protein is. Then hopefully we’ll be able to develop some types of therapy, which will be able to ameliorate the disease. But that will still be a little bit down the road measured in years.”
Mayo has filed a patent application, with Harris as principal inventor. “It claims identification of the gene and its application primarily to diagnostics. We identified a protein now involved in normal kidney development not previously known,” Harris concluded. “This could be an important player in kidney therapy, and a whole range of associated diseases yet to be discovered.”