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 range. 

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 said. 

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)