Worldwide tests on more than 1,000 people from 43 national and ethnic groups show that Huntington's disease, a fatal degenerative nerve disorder, is almost always caused by a short stretch of genetic code that repeats itself dozens of times.
The study, published in the current issue of The New England Journal of Medicine, seems to resolve the question of whether more than one genetic defect is responsible for Huntington's disease and whether that defect varies between populations or within families.
The finding, according to an accompanying editorial, "extends previous studies, demonstrating that virtually all patients in whom Huntington's disease is diagnosed have an expanded stretch" of genetic material and that such an expansion is specific to the disorder." The editorial was written by Dr. James Gusella and Dr. Marcy MacDonald of Massachusetts General Hospital.
The study also gives doctors more assurance about the accuracy of the Huntington's disease test, which looks for the defect and helps predict when the illness will strike among those who carry it.
The defect was uncovered last year on the body's fourth largest chunk of genetic material, known as chromosome 4.
To see if it crossed national and ethnic boundaries, a team led by Dr. Berry Kremer of the University of British Columbia in Vancouver studied 1,007 people from 565 families who had been found to have the disease.
In addition, they looked for the telltale portion of genetic code in 113 people with a family history of other nerve-related diseases whose initial symptoms might mimic Huntington's disease.
The Kremer team found that in 995 of 1,007 patients, the crucial chunk of genetic code was repeated 36 to 121 times. The number of repetitions was not influenced by the person's nationality or ethnic group.
None of the people without a family history of Huntington's had the repeated chunks of genetic code, which means that the test can be used "to differentiate Huntington's disease from the other illness," the Kremer team said. But just because virtually everyone with Huntington's disease has the defect does not mean that everyone with the defect is destined to develop the disease, particularly if there is no family history.