X-linked intellectual disability is caused by defective genes on the X chromosome. As males only have one X chromosome and the disease is passed on in a recessive manner, the disorder mainly occurs in boys.
Women are affected only if both their X chromosomes carry the defective genes. Women with one healthy and one mutated X chromosome are usually healthy but have a 50 per cent chance of passing the mutated X chromosome on to their offspring.
An international research team headed by Vera Kalscheuer from the Max Planck Institute for Molecular Genetics in Berlin has now analysed 405 families, in which cases of X-linked intellectual disability occur.
The researchers have discovered changes in a number of genes that were already known to be related to the disorder.
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In addition, they discovered that X-linked intellectual disability can also be caused by mutations in seven other genes that, until now, were not associated with the disorder.
This technology allows to sequence a large number of DNA segments simultaneously and to more easily identify genetic defects.
Using this method, the scientists investigated all DNA regions of the X chromosome containing protein-relevant information.
"In addition to known disease-related genes, we have discovered seven novel genes as the cause of X-linked intellectual disability and analysed what signalling pathways in the cells each protein is involved in," said Kalscheuer.
According to the researchers, the clinical presentation and severity of the disorder depend on the responsible gene and the nature of the mutation.
With the help of systematic re-sequencing of all X-linked genes, the responsible genetic defect can be identified in around 60 per cent of families with X-linked intellectual disability.