Dyslexia occurs in 5%-17% of the general population, depending on the diagnostic criteria, and has been linked with speech and language disorders, as well as ADHD, Catherine Doust, PhD, of the University of Edinburgh and colleagues wrote.
However, previous studies of the genetics of dyslexia are limited, corresponding author Michelle Luciano, PhD, said in an interview. “So much progress has been made in understanding the genetics of behavior and health, but only a small genomewide study of dyslexia existed before ours.”
Currently, genetic testing for dyslexia alone is not done.
“You couldn’t order a genetic test for dyslexia unless it were part of another genetic panel,” according to Herschel Lessin, MD, of Children’s Medical Group, Poughkeepsie, N.Y.
There are also known associations with some genes and autism, but none are definitive, and testing requires a workup of which a genetic panel may be a part. Such tests are expensive, and rarely covered by insurance, the pediatrician explained.
Experts recommend genetic screening for every child with developmental delay, but most insurance won’t cover it, Dr. Lessin continued.
In the new genomewide association study published in Nature Genetics, the researchers reviewed data from 51,800 adults aged 18 years and older with a self-reported dyslexia diagnosis and 1,087,070 controls. All study participants are enrolled in ongoing research with 23andMe, the personal genetics company.
The researchers investigated the genetic correlations with reading and related skills and evaluated evidence for genes previously associated with dyslexia. The mean ages of the dyslexia cases and controls were 49.6 years and 51.7 years, respectively.
The researchers identified 42 independent genetic variants (genomewide significant loci) associated with dyslexia; 15 of these loci were in genes previously associated with cognitive ability and educational attainment, and 27 were newly identified as specifically associated with dyslexia. The researchers further determined that 12 of the newly identified genes were associated with proficiency in reading and spelling in English and European languages, and 1 in a Chinese-language population.
A polygenic risk score is a way to characterize an individual’s risk of developing a disease, based on the total number of genetic changes related to the disease; the researchers used this score to validate their results. Dyslexia polygenic scores were used to predict reading and spelling in additional population-based and reading disorder–enriched samples outside of the study population; these genetic measures explained up to 6% of variance in reading traits, the researchers noted. Ultimately, these scores may be a tool to help identify children with a predisposition for dyslexia so reading skills support can begin early.
The researchers also found that many of the genes associated with dyslexia are also associated with ADHD, (24% of dyslexia patients reporting ADHD vs. 9% of controls), and with a moderate correlation, which suggests possible shared genetic components for deficits in working memory and attention.
The study findings were limited by the inability to prove causality, and by the potential bias in the study sample, but were strengthened by the large study population, the researchers noted.