Hershey, Pennsylvania: Prader-Willi syndrome (PWS), a rare, nightmarish disorder in which people can literally eat themselves to death, is finally yielding its genetic secrets.
PWS, which afflicts roughly 1 in 10,000, has been notoriously difficult to identify. Many of its symptoms mimic other illnesses. and standard genetic tests may reveal little out of the ordinary, often leaving physicians and families frustrated.
Now, using an array of sophisticated molecular techniques, scientists at Penn State's Milton S. Hershey Medical Center and the University of Florida College of Medicine have been able to identify the disease in nearly 90 percent of the suspected cases they examined, linking nearly every one to a telltale genetic anomaly.
PWS, the most common form of inherited obesity associated with mental retardation, is characterized by low intelligence quotient (IQ), behavioral disorders, poor muscle tone, and an insatiable appetite. The disease usually occurs when a particular segment of the 15th chromosome is missing.
Chromosomes are collections of genes, the biochemical carriers of inherited characteristics such as height and eye color. Chromosomes normally occur in 23 matching pairs -- one from each parent -- in the nucleus of every human cell.
Some two thirds of all children with PWS are missing genetic material from chromosome 15 contributed by the father. Most other cases of the disease stem from inheriting either two complete copies of one of the mother's chromosome 15 pairs or one copy of each of the pair. In roughly 5 to 10 percent of PWS cases, scientists cannot find anything wrong, detecting chromosomes inherited equally from both parents.
Many children suspected of having PWS can be identified with a blood test. Using the microscope, researchers can spot the missing portion of chromosome 15. But in the remaining cases, up to 60 percent will show both chromosomes coming from the mother, though routine testing shows the children's chromosomes to be normal.
Without supervision, children with PWS can balloon to two or three times their normal weight, putting them at risk for respiratory and heart problems. Along with a voracious appetite, they may have varying degrees of mental retardation and learning disabilities. They tend to be short, with small hands and feet and large almond-shaped eyes.
The work also sheds light on a phenomenon known as genomic imprinting, in which a gene's expression is different, depending on which parent contributes it. Although imprinting is thought to be rare, the researchers believe that it may play a greater role than previously assumed in conditions such as Huntington's disease.
During the next year, Hershey will open an outpatient clinic for families to address their medical and social needs and is already offering molecular diagnosis.
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