New York University School of Medicine researchers have discovered that a gene normally found in the bladder may contribute to a common birth defect that is the leading cause of kidney failure in children. The surprising finding may lead to earlier detection of the condition, called vesicoureteral reflux, than is now possible. It also sheds light on the function of a remarkable gene that apparently plays a key role in maintaining a healthy bladder and urinary system, according to the NYU researchers.
Tung-Tien Sun, Ph.D., the Rudolf L. Baer Professor of Dermatology and Professor of Pharmacology and Urology at NYU, and co-workers report in a new study that they have, for the first time, knocked out the gene called uroplakin III in mice. In the study, published in the Nov. 27 issue of the Journal of Cell Biology, they show that the absence of the gene compromised the function of the bladder, and affected kidney function.
The uroplakin genes were first identified in the 1990s by Dr. Sun and Xue-Ru Wu, M.D., Assistant Professor of Urology and Microbiology at NYU, who were studying the urothelium, the cellular lining of the bladder. They discovered that the surface of the bladder is covered by numerous, rigid plaques made up of only four proteins, dubbed uroplakins, which form a crystalline sheath that is unique in the body. They found the four separate, but related genes that encode the proteins, and they reported that these genes are present in mammals, including humans. But the function of these genes was largely unknown.
"We believed that the uroplakins formed a barrier that prevented noxious material in the urine from spilling into the bladder's interior, and therefore may prevent bladder cancer, among other functions, but we didn't have definitive proof," says Dr. Sun. The bladder is an impermeable sac, yet it can stretch enormously and then shrink and stretch again without tearing. These exceptional properties, the NYU researchers believed, were due to the uroplakins.
In the new study, the researchers show that mice lacking the uroplakin III gene did indeed have leaky bladders. A blue dye injected into the gene-depleted bladder readily penetrated the urothelium, which would not occur in normal animals. The orderly rows of rigid plaques that cover the bladder were gone, replaced by a distorted layer of cells with abnormally small plaques interspersed with large, plaque-free areas. Moreover, the openings where the ureters are connected to the bladder were enlarged, which allowed urine to flow backward into the kidney. (Urine normally flows only away from the kidneys through the ureters.) "The function of the bladder was totally compromised," says Dr. Sun.
The reverse flow of urine into the kidney is the hallmark of vesicoureteral reflux, which can lead to scarring of the kidney, and eventually, to kidney failure. This condition, which runs in families, is most likely caused by more than one gene. But the NYU researchers believe that the animal model they have devised for the condition, implicating the uroplakin III gene, closely mirrors the human disease. Additional studies are underway to determine if the gene actually causes the human disease, says Dr. Sun. Vesicoureteral reflux (VUR) Up to half of the young children who are brought to pediatricians because of urinary infections and fever are found to have VUR, says Ellen Shapiro, M.D., Professor of Urology and Director of the Pediatric Urology & Reconstructive Surgery Program at NYU and one of the authors of the new study.
Infants with VUR are highly susceptible to urinary tract infections, which trigger additional urologic problems. The reflux of infected urine into the kidneys can cause acute inflammation and subsequent renal scarring (which compromises kidney function), hypertension, and end-stage renal disease.
"All patients with reflux must be treated with antibiotics to prevent recurrent infections," says Dr. Shapiro. Some patients, however, require surgery. Many children with VUR eventually grow out of the condition and require no further intervention. "Telltale symptoms in infants include fever, irritability, and failure to feed and grow properly. Young children will have a burning sensation and the need to urinate frequently, as well as pain, most often in the belly, but sometimes in the back or flank," says Dr. Shapiro.
Fortunately, the condition can be detected even before birth, through ultrasound examination. Still, many cases are missed. As Dr. Sun notes, "For a significant number of kids, when reflux is first diagnosed, they already have kidney scarring. So it is very important for us to develop better means of detecting children who are at risk, allowing us to make earlier diagnoses and give them prophylactic antibiotic treatment or, in serious cases, surgery." Dr. Sun and his colleagues plan to create additional knockout mice in order to determine the roles of the other uroplakin genes. "My suspicion is that defects in these genes, and others too, may all cause reflux. This may provide the first insights that will allow us to subclassify this very important group of hereditary defects," he says. VUR occurs in approximately one-half to one percent of all newborns.
The animal experiments in the study were conducted by Ping Hu, a graduate student in pharmacology at NYU School of Medicine. Other authors on the study include Fang-Ming Deng, M.D., and Feng-Xia Liang, M.D., who are post-doctoral fellows in Dr. Sun's laboratory, Anna Auerbach of NYU School of Medicine, and Bechara Kachar, M.D., Chief of the Laboratory of Cellular Biology, National Institute on Deafness and Other Communication Disorders.
Journal
Journal of Cell Biology