Dyslexia is known to run in families – partly because of genetic factors – but, until now, little was known about the identities of the genes involved.
The new study, led by the University of Edinburgh, represents the largest molecular genetic investigation of dyslexia to date. Previous studies linking dyslexia to individual genes have been carried out with much smaller numbers of families and the evidence was unclear, the research team says.
The team analysed DNA from more than 50,000 adults who have been diagnosed with dyslexia and more than one million adults who have not, identified via collaboration with the US company, 23andMe, Inc.
“Over several decades of earlier research, more limited genetic investigations of dyslexia gave us tantalizing first clues to how DNA may be involved.” notes Simon Fisher, director of the Language and Genetics department at the Max Planck Institute. “Now, largescale genomic studies of this kind promise to transform understanding of how our genes help us learn to read and write.”
Researchers screened the entire genome, testing the association between millions of common DNA variants with dyslexia status, and found 42 genetic loci that were significantly related to the trait. Some of these are associated with other neurodevelopment conditions, such as language delay, and with thinking skills and academic achievement. Many, however, are novel and could represent genes that more specifically associate with processes essential for learning to read.
Many of the genes associated with dyslexia are also associated with attention deficit hyperactivity disorder. A much smaller overlap of the genes associated with dyslexia was found for psychiatric, lifestyle and health conditions.
The genetic findings were successfully validated in an independent sample of almost 34,000 people tested for word reading skills, collected by GenLang, an international consortium led by researchers at the Max Planck Institute for Psycholinguistics in Nijmegen. Several of the associated genetic variants were also significant in a Chinese speaking sample, suggesting that there are cognitive processes in learning to read that are not dependent on the type of language that people use.
The researchers also used the DNA findings for investigation of four different cohorts, together with teams at the QIMR Berghofer Medical Research Institute in Australia, Oxford University in the UK, and the University of Colorado in the USA. They found that genetic information from the study could give an approximate index of how well children and adults can read and spell, but not with the accuracy that would be needed for diagnostic use.
Lead researcher Michelle Luciano, of the University of Edinburgh’s School of Philosophy, Psychology and Language Sciences, says the study sheds light on many unanswered questions around dyslexia.
“Our findings show that common genetic differences have very similar effects in boys and girls, and that there is a genetic link between dyslexia and ambidexterity,” says Dr Luciano. “Our results also suggest that dyslexia is very closely genetically related to performance on reading and spelling tests reinforcing the importance of standardised testing in identifying dyslexia.”