October 28, 1998
Scientists now can predict, with 99 percent accuracy, carriers of the gene for adrenoleukodystrophy (ALD), the disease featured in the movie "Lorenzo's Oil." Unlike existing tests that depend on changes in blood chemistry, the new procedure developed at Johns Hopkins -- DNA carrier-based testing -- directly analyzes a woman's genes for specific mutations.
"Because of this test, women who've delayed having children because of uncertainty about passing on the disease can have a new confidence," says geneticist Garry R. Cutting, M.D., one of the researchers and head of Hopkins' DNA Diagnostic Laboratory. The scientists present the technique this week at The American Society of Human Genetics meetings in Denver.
ALD is an inherited disorder that, in its most devastating form, causes death of the brain's "white matter" -- nerve cells that transport information within the brain and from brain to spinal cord. The adrenal glands also are destroyed. Boys with ALD appear healthy and normal until early childhood, when they begin to regress mentally. Thinking deteriorates, along with the ability to move or see. The severe form, which affects about 40 percent of all male ALD patients, kills most children by age 10. "Families are severely traumatized by this downhill path," says neurologist Hugo W. Moser, M.D., also on the Hopkins research team.
ALD results from an altered recessive gene carried on the X chromosome, one of the sex chromosomes. Other "sex-linked" diseases include Duchenne muscular dystrophy and hemophilia. If a male inherits the one of his mother's two X chromosomes that bears the altered gene, he's likely to get the disorder. Most women who have the gene are unaware that they're carriers.
In 1981, Moser introduced the first test for both carriers and those suspected of having ALD, based on high amounts of a key fatty molecule in their blood plasma. "The plasma test is 100 percent accurate in diagnosing the disease in males," says Moser, "but unfortunately, it has only 85 percent accuracy in telling if a woman is a carrier, and 15 percent may have false negatives. That figure doesn't go down well with women: many in families with ALD refuse to marry or have children." Worse, he says, "is that some women mistakenly interpret the numbers. They say, the odds are only 15 percent,' take that risk and produce children who suffer and die."
The new test, however, focuses directly on patients' genes. It employs both molecular biology techniques to pinpoint important parts of target genes and amplify them, and a new, high-accuracy gene sequencer/software system to reveal mutations. "The most definitive way to see if someone's a carrier is to analyze the gene," says Cutting. Nearly 200 mutations have been identified in the ALD gene, and the techniques should reveal more, he adds. "For the first time, a test allows us to exclude carrier status with certainty; we lift a weight from thousands of women," says Moser.
The test, which requires only a blood sample, also lends itself to pre-implantation diagnosis, where expendable cells from days-old embryos are analyzed.
The purchase of the new $65,000 gene-analyzing system was made possible by a charity golf tournament hosted by major league baseball umpire John Hirschbeck and his wife. The Hirschbecks have two sons with the disease.
The new techniques also should speed research in treating those with the disorder by helping to show precisely where abnormal genes go wrong. ALD patients have abnormally high levels of molecules called very long chain fatty acids (VLCFAs) in their blood and other tissues. Until recently, researchers assumed a defective gene engineered faulty enzymes that prevented VLCFAs from being broken down in the body, says Cutting. When the gene's defective, the fatty acids clog cells. Their buildup is probably a key source of problems in adrenoleukodystrophy.
But more recent studies show instead that the normal gene instructs cells to make a carrier protein -- called ALD protein -- that eases VLCFAs through the membranes of cell storage chambers rich in enzymes, where they're digested. In the disease, the protein carrier fails and fatty acids build up. A similar protein goes awry in cystic fibrosis.
The "Lorenzo's Oil" originally suggested as a cure for the disease is a blend of dietary oils that tricks body chemistry into lowering the amount of VLCFAs in the blood. "Its usefulness in treating the disease is still in question," says Moser. The movie, "Lorenzo's Oil," was based on a past Hopkins patient.
ALD affects more than one in 50,000 people of all races, the researchers say. The new technique was presented at the United Leukodystrophy Foundation annual meeting this summer. A journal article is in press. The technique is available only at Johns Hopkins.
Hopkins geneticist Samuel S. Chong, Ph.D., is a key developer of the technique. Also on the team is Corinne D. Boehm, M.S.