A gene thought to be associated with dyslexia is also connected with reading ability in the general population, according to research funded by the Wellcome Trust. A study to be published online today in the journal American Journal of Psychiatry suggests that a common variant of this gene, carried by more than one in seven people, is associated with poor reading ability.
Dyslexia is a learning difficulty which affects the development of literacy and language related skills, such as reading and spelling, but does not affect overall IQ. It is believed that as many as one in ten people are affected by dyslexia to some degree.
Previous research has identified at least six candidate genes that appear to affect susceptibility to developing dyslexia. However, scientists have often failed to replicate these findings, suggesting that their role in causing dyslexia is debatable.
The most promising of the candidates is the gene KIAA0319, which lies on chromosome 6. This gene has been reported in at least three independent studies. Researchers at the Wellcome Trust Centre of Human Genetics, University of Oxford, have previously identified a haplotype - a particular DNA sequence, which spans part of this gene - associated with dyslexia in both a large samples of UK families and a samples of twins in the US. This association was also identified independently by researchers at Cardiff University.
Now, working with a cohort of over 6,000 seven to nine-year old children from Children of the 90s, also know as the Avon Longitudinal Study of Parents and Children (ALSPAC), researchers have looked at the association between this particular haplotype, which is carried by 15% of the population, and general reading ability.
"On average, people carrying this common genetic variant tended to perform poorly on tests of reading ability," says Dr Silvia Paracchini from the Wellcome Trust Centre for Human Genetics, lead author of the study. "However, it's important to note that this is only true for reading ability and not for IQ, so it doesn't appear to be connected to cognitive impairment."
Dr Paracchini and colleagues have previously shown that the same haplotype is associated with reduced expression of the KIAA0319 gene during development of the foetus - in other words, it acts like a dimmer switch, reducing the power of the gene to do its normal job as the foetus grows. This affects development of the cerebral cortex, the area of the brain responsible for thought processes. In animal studies, switching off KIAA0319 affects neuronal migration, the process that enables nerve cells created in the inner layer of the cerebral cortex area to migrate outwards to their destination.
"This is clearly only part of the jigsaw puzzle that explains why some people have poorer reading ability than others or develop dyslexia," says Dr Paracchini. "There are likely to be many other contributing factors, but our research provides some valuable clues. We need to carry out studies into the exact role that this gene plays in brain development and how this affects people's reading ability."
BBC News Online 1 October: http://news.bbc.co.uk/1/hi/health/7643760.stm
Reuters 1 October: http://uk.reuters.com/article/domesticNews/idUKTRE49012H20081001
Daily Mirror 1 October: P27 and www.mirror.co.uk/news/top-stories/2008/10/01/dyslexia-gene-hits-reading-115875-20763608/
Oxford Mail 1 October: www.oxfordmail.co.uk/news/3717902.Genes_link_to_reading_skills/
PhysOrg.com 1 October: www.physorg.com/news142091390.html
Science News 3 October: www.sciencenews.org/view/generic/id/37222/title/Genetic_link_to_dyslexia
Times Online 25 October: www.timesonline.co.uk/tol/life_and_style/education/article5006527.ece