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Chromosomal Abnormalities Cause of Some Unexplained Mental Retardation

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Nov. 12, 1999 (New York) -- Subtle alterations to the ends of pieces of genetic material may explain a significant number of cases of moderate to severe mental retardation that currently have no known cause. In the Nov. 13 issue of the journal The Lancet, British researchers conclude that testing for these chromosomal abnormalities is warranted in children for whom there is no apparent explanation for their mental handicap.

In moderate to severe cases of mental retardation, 30% to 40% can be explained by known genetic or chromosomal disorders and 10% to 30% can be explained by environmental factors, leaving about 40% of cases unexplained. Evidence has suggested that minor changes on chromosomes, which are responsible for transmitting genetic information, may be an important cause of mental retardation, but much of this evidence has come from case reports or small studies.

In the new study, researchers from John Radcliffe Hospital in Oxford, England, used a modification of an existing laboratory test, called the fluorescence in-situ hybridization or FISH, to analyze chromosome ends in about 450 mentally retarded children and 75 normal men. The children's retardation varied from mild to severe.

Among the 284 children with moderate to severe mental retardation, the frequency of chromosomal abnormalities detected was 7.4%. Among children with mild retardation, the frequency was 0.5% The normal adult male controls had no chromosomal abnormalities. In approximately 50% of children with the detected chromosome alterations, a family link was established by performing the same testing on parents, siblings, or other family members.

Study author Samantha J.L. Knight, PhD, and colleagues suggest that the family association is important since it may provide explanations in some families for children with unexplained retardation in which another cause, such as brain damage, has previously been given. The authors estimate that the population prevalence of these chromosomal abnormalities is 2.1 per 10,000 people. They recommend this type of testing, the multiprobe FISH, for children with mild to severe unexplained mental retardation. Such children have a variety of disabilities and generally have an IQ below 50. An average IQ is considered to be about 100.

In an editorial accompanying the study, John L. Hamerton, DSc, and Leonie Stranc of the University of Manitoba in Winnipeg, say the prevalence of the subtle chromosomal abnormalities makes them second only to Down's syndrome as a leading cause for mental retardation. Close inspection of the data suggests that the alterations may provide an explanation for about 20% of cases of moderate to severe retardation in which the cause currently is unknown. Hamerton and Stranc also say parents and other immediate family members of affected children may wish to consider prenatal testing if they are found to carry the chromosomal rearrangement that can result in retardation in offspring.

"If you talk to parents of children with mental retardation, they often want to know, and there is a feeling of relief when they have a firm diagnosis because it helps them understand the situation." Kenneth Mack, MD, tells WebMD in an interview seeking objective commentary on the study.

Mack, who is associate professor of pediatrics and neurology at the University of Wisconsin - Madison, says the new test has definite potential for immediate use. "I'm not aware of any place in the United States that currently does this type of testing, but I would predict that the technology would be readily available such that places could begin doing it within a three- to six-month period of time," he tells WebMD. The estimated cost of the multiprobe FISH test is about $400 per test.

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