For release: Tuesday, February 28, 2006
A recent study shows that variations in a gene called DCDC2 may disrupt the normal formation of brain circuits that are necessary for fluent reading, leading to dyslexia. After further research, genetic screening for these variations could identify affected children early in their lives and possibly prevent the misdiagnosis of other learning disabilities that resemble dyslexia.
“We have good statistical data that variations of the DCDC2 gene are strongly associated with reading disability, also known as dyslexia. These results reconfirm that dyslexia is strongly genetic and is not a consequence of just environmental factors,” says lead investigator Jeffrey Gruen, M.D., Associate Professor of Pediatrics at Yale University School of Medicine in New Haven, Connecticut. This study was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS) and appeared in the November 22, 2005, issue of Proceedings of the National Academy of Sciences.*
Dyslexia is a common disorder that causes people to have difficulties with accurate and/or fluent word recognition and hinders the development of reading skills. The brain-based learning disability specifically impairs a person's ability to read. These affected individuals typically read at levels significantly lower than expected despite having normal intelligence. “Unfortunately, people assume that if you read poorly that correlates with having a low IQ. This study confirms that dyslexic children can be typically smart and can have strong IQs. The reading disability is not a global effect on entire brain function,” says Dr. Gruen.
The researchers examined variations in a gene, doublecortin domain containing 2 (DCDC2) and found that specific variations including a large deletion may be inherited in up to 20 percent of people affected with dyslexia. The study demonstrates that DCDC2 is involved in neuronal migration (the movement of neurons to their target regions during early brain development). The researchers tested the hypothesis that down-regulation of DCDC2 affects brain or neuronal circuits using embryonic rats. Brain sections of these rats showed an abnormal migration pattern of neurons, in which the neurons failed to reach their target destination.
In humans, the DCDC2 gene is strongly expressed in the same brain regions (cortex, hypothalamus, amygdala and hippocampus) of normal and dyslexic readers, suggesting that changes in the gene's function, rather than a deletion of the entire gene, cause the disorder. The gene in people with dyslexia is altered but still somewhat functional, which may explain why dyslexia is frequently associated with subtle changes and not extreme learning disability.
Previous clinical studies have shown that up to 50 percent of children of dyslexic parents have dyslexia themselves. These and other studies of heritability led researchers to hunt for genes that influence the disorder. In recent years, two other genes have also been linked to dyslexia, EKN1 and KIAA0319. KIAA0319 is also involved in neuronal migration.
The DCDC2 gene and its strong relationship to dyslexia have already been replicated in an independent study in
Although the disorder varies from person to person, common characteristics among people with dyslexia include difficulty with phonological processing (the manipulation of sounds) and/or rapid visual-verbal responding. Researchers estimate that 10 -17 percent of the population in the
“Traditionally, many people are under the impression that dyslexia is an issue with visualizing letters differently. However, we now know that dyslexics just learn to read differently - most learn to read by sight memorization not phonetically,” says Dr. Gruen.. Anatomically, dyslexics have normal brains; however, the brain areas activated in reading may be different. Several studies using fMRI, an imaging technique that studies activity in the brain, have shown that dyslexics display different patterns of brain activity than other people when they read.
“Some kids just learn differently. Not all children learn to read with our current one-size fits all methods,” says Dr. Gruen. “The earlier we can identify children at risk, the earlier we can start intervention when studies have shown that remediation works best. Even with the future capabilities of genetic testing for dyslexia, the biggest problem is still the lack of infrastructure for early intervention in schools.”
Dr. Gruen cautions that although we can now draw a path from the disease to the gene, the reverse is not necessarily proven yet. That is, predicting the outcome for a person who has the gene and determining if the person will have the disorder is still not possible. The next steps of the research will examine other variations including deletions or changes in this particular gene family. The investigators will also examine brain imaging in children and adults given reading tasks and then see how that relates to DCDC2 variations. Ultimately, identifying dyslexia genes may provide opportunities for early identification of the disorder and help to distinguish dyslexia from other learning disabilities.
The NINDS is a component of the National Institutes of Health (NIH) in
*Meng H, Smith S, Hager K, Held M, Liu J, Olson R, Pennington B, DeFries J, Gelernter J, O’Reilly-Pol T, Somlo S, Skudlarski P, Shaywitz S, Shaywitz B, Marchione K, Wang Y, Murugan P, LoTurco J, Grier P, Gruen J. “DCDC2 is associated with reading disability and modulates neuronal development in the brain.” Proceedings of the National Academy of Sciences, November 22, 2005, Vol.102, pp.17053-17058.
**Schumacher J, Anthoni H, Dahdouh F, Konig I, Hillmer A, Kluck N, Manthey M, Plume E, Warnke A, Remschmidt H, Hulsmann J, Cichon S, Lindgren C, Propping P, Zuccheli M, Ziegler A, Peyrard-Janvid M, Schulte-Korne G, Nothen M, Kere J. “Strong genetic evidence of DCDC2 as a susceptibility gene for dyslexia.” The American Journal of Human Genetics, January 2006, Vol. 78, pp.52 – 62.
-By Michelle D. Jones-London, Ph.D.
Last Modified January 31, 2007