Four independent studies from the United States, Germany, and England implicate two genes in fostering dyslexia. The genes contribute to early brain development.
Dyslexia, a learning disorder that afflicts at least 5 percent of elementary school children, is characterized by difficulties in perceiving sounds within words, spelling and reading problems, and troubles with written and oral expression.
Both of the newly implicated genes normally trigger production of proteins that assist neurons in migrating to appropriate destinations during brain formation. Each gene lies in a section of chromosome 6 that previous studies linked to dyslexia.
“Genetic testing for susceptibility to dyslexia is a realistic possibility in the future,” says pediatrician Jeffrey R. Gruen of Yale University School of Medicine in New Haven, Conn.
A team led by Gruen found that variants of a gene known as DCDC2 frequently occur in individuals diagnosed with a serious reading disability but not in their immediate-family members who don’t have a reading problem. The researchers studied 536 parents and siblings in 153 families, each of which contained at least one person with severe difficulties reading and spelling.
The new findings appear in an upcoming Proceedings of the National Academy of Sciences.
Gruen’s group examined the DNA sequence within the crucial chromosome-6 region. DNA alterations specific to reading disability appeared only in DCDC2.
The gene’s precise function remains unclear. After injecting neurons of rat embryos with a substance that inhibits DCDC2 activity, the investigators found that, as the animals’ brains grew, those neurons migrated shorter distances than corresponding cells in healthy rat embryos did.
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In laboratory tests on preserved human-brain tissue, Gruen’s team observed high concentrations of DCDC2‘s protein products in areas that brain-scan studies had identified as active during both fluent and poor reading (SN: 4/30/05, p. 280: Read All about It).
A group led by geneticist Juha Kere of the Karolinska Institute in Stockholm has also found a strong association between variations within DCDC2 and severe dyslexia, as indicated by a pronounced spelling disorder. The team’s study, slated to appear in the American Journal of Human Genetics, included 111 German families.
A pair of studies, both conducted in England, link dyslexia to variants of a second gene in the same chromosome-6 region. Laboratory tests indicate that this gene, KIAA0319, also influences early stages of neurons’ migration.
A group led by psychologist Julie Williams of Cardiff University reported that specific variations of KIAA0319 frequently appeared in 223 youngsters with dyslexia but not in 273 children without reading problems. The scientists found no association between DCDC2 and dyslexia. They published their findings in the April American Journal of Human Genetics.
A comparably large study just completed by University of Oxford researchers has independently confirmed those results, Oxford geneticist Anthony P. Monaco told Science News.
In contrast, the U.S. and German studies uncovered no role for KIAA0319 in dyslexia. Differences among the populations with dyslexia that were studied or in criteria for diagnosing it may account for the divergent genetic results, Gruen says.
Several genes on chromosome 6 and elsewhere in the genome probably contribute to dyslexia, comments neuroscientist Guinevere Eden of Georgetown University in Washington, D.C. The involvement of DCDC2 and KIAA0319, with their role in brain development, makes sense in light of the subtle disruptions of neural activity that accompany dyslexia, Eden says.