New Cause of Mental Retardation Identified
CHROMOSOME ABNORMALITIES PINPOINTED
One third of the cases of mental retardation have historically been
categorized as being of unknown cause. An accurate determination of the precise
cause for an individual case of mental retardation is very important to the
families involved. These parents’ inability to obtain some knowledge about
whether the cause was genetic or environmental often intimidated and frustrated
them with regards to having future children.
In a recent issue of The Lancet, a British medical publication, scientists
reported their findings that a very abstrusive rearrangement of DNA located at
the tips of the chromosomes, may be the cause of numerous cases of moderate to
severe mental retardation. Their work was done at the John Radcliffe Hospital,
which is located in Oxford, England, and involved using a very mature technique
known as Fluorescence In-Situ Hybridization (FISH).
Using this FISH system, the researchers have extrapolated that this method may
be capable of providing a casual explanation for about 20% of the cases of
moderate to severe mental handicap for whom the cause was previously unknown. As
part of their study, they had examined the integrity of chromosome ends in 466
children who had had unexplained mental retardation within the mild to severe
As an extension of the study, they examined the parents of those children found
with these type of chromosome abnormalities. These researchers found that about
half of the children with this type of chromosome damage had parents who
exhibited a similar genetic rearrangement.
Sometimes the tip of a chromosome falls off and reattaches to another
chromosome. When this occurs, a condition will develop where a child inherits
the abnormal copy of a parent’s chromosome that is missing genetic information
on the end.
These findings would make the defect second only to Down’s syndrome as the
cause for mental handicap, according to John Hamerton of the University of
Manitoba, who reviewed the work of the researchers. “This new technology
should now allow scientists to examine the tiniest submicroscopic segment of the
chromosome and may eventually provide an explanation for about 20 percent of
previously unexplainable cases of moderate to serve retardation,” Hamerton
The type of information reported in this paper has the potential to result in
providing advice about reproductive options not previously available to such
patients and their families.
(Subtle chromosomal rearrangements in children with unexplained mental
retardation; The Lancet; November 13, 1999; Volume 354, Number 9191)