Researchers look for genetic clues to rare pediatric disorder
Dr. Marshall Summar is studying a rare ammonia-buildup disorder that strikes some children. (Photo by Donna Jones Bailey).
For babies born with a rare genetic disorder that causes ammonia to build up in their bodies, the first few days after birth are the toughest hurdle.
Vanderbilt Children's Hospital researchers ‹ the only group in the country studying this condition ‹ have taken a significant step toward a genetic treatment.
Late last year, they successfully cloned the gene that has gone awry in these children and synthesized the enzyme they are missing.
"The newborn period is very tough on these children," said Dr. Marshall L. Summar, associate professor of Pediatrics.
These babies are missing an important enzyme, carbamyl phosphate synthetase (CPS), which controls the urea cycle. The urea cycle is the pathway that converts waste nitrogen to urea for excretion in the urine. Nitrogen is a by-product of the breakdown of protein that travels in the bloodstream as ammonia.
The resulting buildup of ammonia in the babies' blood is toxic to a number of organs but most importantly to the brain.
"The brain starts to swell and the babies gradually go into a coma," Summar said.
"As they become more and more lethargic, they eat less and less, so their body begins to break down its own protein for energy, which produces more nitrogen/ammonia, and a vicious cycle sets in.
"These children get into a spiral very fast and can die quickly."
Summar's previous work with this gene led to development of a prenatal diagnosis, an area now led by Cindy L. Vnencek-Jones, Ph.D., associate professor of Pathology and Pediatrics. However, the defect is too rare for the screening to be done routinely ‹ it occurs in only about one in 50,000 births. Currently, screening is only done for expectant mothers who have had previous children with the defect.
Even if the children survive the newborn period with dialysis and other techniques, medical management is quite challenging. The children must eat a very regimented low-protein, high-calorie diet. Any infection or other problem that "stresses" their system or reduces their appetite may result in admission to the hospital to be put on "ammonia-scavenging" drugs.
"Normal day-to-day life is tough, the diet is tough," he said. "And everytime these kids get sick, you worry. It's a disease that we need to do something about.
"In the near future, we would like to target them with a temporary genetic therapy to replace the enzyme for two or three weeks so that we can get them through the rough newborn period and give medical management a chance to work."
Ultimately, researchers would like to develop a "gene therapy" to permanently replace the defective copy of the gene with a normal one. However, delivery systems have not yet been developed that will replace a gene in the body for longer than a brief time, Summar said.
"Someday, someone here or elsewhere will develop an effective delivery mechanism and we want to be ready for it," Summar said. "In the meantime, though, even transient replacement of the gene would be beneficial."
The researchers would like to target a child who has been identified through prenatal diagnosis as having the deficiency. The goal would be to administer the gene immediately after birth so that some of the enzyme has been produced by the time the ammonia would begin to build up in their bloodstream, he said.
Summar began researching the CPS gene about eight years ago. Since then, he has collaborated with a colleague in England and in Spain, but no other researchers in the United States are working on this gene.
Recently, his work has broadened to include more common implications of defects in the gene.
"When we look at ammonia responses to different stresses ‹from cirrhosis to different drug exposures to chemotherapy ‹ we see that certain groups have a more toxic response than others," Summar said.
"We've speculated that there might be trivial changes in the CPS gene that might account for this. Let's say that 100 percent enzyme activity is normal, and this change results in 50-60 percent activity. That would be enough day-to-day, but under certain circumstances, it might tip you over the edge."
Last year, Summar and his colleagues discovered a subtle change in the CPS gene that is widely distributed in the population. About 25 percent of people, they have estimated, carry two copies of this change.
That incidence mirrors the proportion of people who experience a build up of ammonia in response to chemotherapy or other stresses, he said.
Summar is collaborating with Dr. Brian W. Christman, associate professor of Medicine, to look for this subtle change in the CPS gene in patients undergoing bone marrow transplantation.
"We are looking at patients who have elevations in their ammonia or difficulties with the urea cycle, and checking to see the distribution of this change," he said.
The researchers will test their theory by expressing this gene in cell cultures and comparing the normal protein directly with the mutated protein under various conditions.
Another potential implication is among patients taking a seizure medication called valproic acid, which causes ammonia elevation in about 25 percent of people taking it. Summar is collaborating with Dr. Anthony W. Kilroy, associate professor of Neurology and Pediatrics, and Dr. Sheila P. Dawling, associate professor of Pathology, to determine whether ammonia buildup relates in these patients to mutation of the CPS gene.
"It's been rewarding to take a very rare and somewhat unusual metabolic disease, work on the gene for that and now see if what we learned applies to the rest of the population," Summar said.