Mutations in a key section of a human sex chromosome appear to be able to cause mental retardation, an Australian researcher said Thursday.
A change involving duplication of a small part of the 'X' chromosome was discovered in six families out of more than 550 in a worldwide study of mental health problems, geneticist Jozef Gecz said.
He said that although the figure sounded relatively low, because of the complexity of the change the number was actually 'very high'.
Another three families where a gene had been lost from the same section of the chromosome also suffered mental retardation, said Gecz, of the University of Adelaide.
The study, conducted with Katholieke Universiteit Leuven in Belgium and the Wellcome Trust Sanger Institute in Cambridge, England, is published in February's edition of the American Journal of Human Genetics.
The 'X' chromosome is one of two chromosomes that determine gender. Women normally have two, while men have one plus a smaller 'Y' chromosome.
As a result, problems with the 'X' chromosome appear more frequently in boys than girls, although women may be silent 'carriers' who can pass the defective genes on to their sons.
Mental disability is about 30 percent more common in men than in women.
The finding adds weight to arguments that the 'X' chromosome plays a key role in human brain function, Gecz said.
Around 30 individuals were affected, mostly boys or men, Gecz told AFP. They appeared normal, but showed subtle deficits.
'To some extent they can dress and feed themselves. They don't necessarily require 24/7 care but they still need supervision,' he added.
'Visually these people look exactly like you and I. They just don't have the learning and memory working properly to allow them to lead a normal life.'
The duplicated part of DNA contained two genes known by the numbers HSD17B10 and HUWE1, the latter a so-called 'tumour suppressor gene' whose roles include regulating the renewal of nerve cells.
'Clearly the amount you produce of this gene has to be just right,'Gecz said. 'We are making an educated guess that this is in control of growth of cells in the human brain.'
A test has now been developed which will allow women to find out before becoming pregnant if they carry the defective DNA. Researchers also plan to look into possible future treatments.
So far about 85 genes have been discovered on the 'X' chromosome which are involved in various forms of mental disability. There are about 800 genes on the chromosome, Gecz said.