New Cause Of Mental Retardation Identified

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New Cause Of Mental Retardation Identified

Friday, November 12, 1999

Reuters Health -- In about 40% of cases, the cause of mental

retardation is unknown. However, new study findings suggest that

rearrangements at the ends of chromosomes may be responsible for some cases

of unexplained mental retardation in children.

Co-author of the study Dr. Flint and colleagues report that

" subtle chromosomal rearrangements are the second most common cause of

moderate to severe mental retardation after Down's syndrome. " The study

results are published in the November 13th issue of The Lancet.

Chromosomes, found in the nucleus or control center of the cell,

contain genetic information in the form of DNA. The order of material on

chromosome strands is important to its function, namely determining which

proteins are to be produced by the cell.

In an interview with Reuters Health, Flint said there is no cure or

therapy for mental retardation caused by the chromosomal defects.

Flint, of the Institute of Molecular Medicine in Oxford, UK, and

colleagues examined the integrity of chromosome ends in 466 children with

unexplained mental retardation ranging from mild to severe.

The investigators found chromosomal abnormalities in 7.4% of children

with moderate to severe retardation. Approximately 0.5% of children with

mild retardation had the abnormalities. One of the parents carried the

abnormalities in about half the cases.

" Our results show that subtle chromosomal rearrangements are common in

children with mental retardation, that the rearrangements cause the

children's disabilities, and that half of the rearrangements are

familial " -- carried by one of the parents, Flint and colleagues explain.

The investigators suggest that " all children with moderate to severe

unexplained mental retardation " should be screened for chromosomal

abnormalities.

In an interview with Reuters Health, Flint said that screening is

important to assure distraught parents that the mental retardation is not

their fault. Screening also allows for identification of parents and other

family members that might carry the abnormalities in order to notify them

that they risk passing the defect onto their children, he said.

One other reason for screening, Flint said, is for long-term prognosis

of these children. Due to uncertainties about the children's life span, some

physicians may be reluctant to treat medical conditions in children with

unexplained mental retardation and this can result in problems later on, he

noted. He added that the life span of children with these chromosomal

abnormalities remains unknown.

In an accompanying commentary, Drs. Hamerton and Leonie Stranc of

the University of Manitoba in Winnipeg, Canada, write that the study " has

important implications for the counseling of both immediate and extended

family members -- particularly for those carrying the (abnormalities) who

may wish to consider prenatal diagnosis in future pregnancies. "

SOURCE: The Lancet 1999;354:1659-1660, 1676-1681.

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