The team from the University of Washington, headed by medical geneticists Dr. Wendy Raskind and Ellen Wijsman and developmental psychologist Virginia Berninger, cautioned that the new findings do not mean that scientists have found "the gene" responsible for dyslexia.
"Just as with heart disease, no single gene will provide the answer to what causes dyslexia," said Raskind. "When you look at something that is inherited there could be multiple genes, perhaps as many as a hundred, that contribute to it. And when you look at any characteristic of a person, you must consider the environmental background. There are other factors besides genes that could modify a behavior."
The study, published in the March issue of the journal Molecular Psychiatry, is noteworthy for two reasons. First, it points to a new location containing genes that contribute to dyslexia. Second, the gene or genes at that location are involved in speed of decoding - changing written words into spoken words without clues to their meaning - a basic and persistent component of dyslexia.
Building on previous findings by the UW team on familial patterns of accuracy and rates of decoding, the research also provides the first evidence identifying separate genetic influences on these abilities.
"In other published and in-press research we have shown that accuracy of decoding may be a bridge to reading at one stage of learning to read and rate of decoding may be a bridge at a later stage. Effective instructional techniques for each bridge are not necessarily the same and typically require more than teaching as usual," said Berninger, who heads the UW's Learning Disabilities Center.
The UW researchers used pseudowords, or what Berninger calls "jibberwacky," which are similar to the nonsense syllables in Lewis Carroll's poem "Jabberwocky." Examples of these pseudowords are "chimwoggle," "meb" and "crong." The "jibberwacky" was used to test 108 first- through ninth-grade children diagnosed with dyslexia and their families - parents, grandparents, siblings at least 6 ½ years old, aunts and uncles. In all, 874 people were involved in the study.
Raskind and Wijsman said they used three different analytic approaches to search for genetic influences affecting how fast and accurately people could pronounce nonwords. For accuracy alone, they found five different potential contributing locations on four different chromosomes. When they examined influences on speed and accuracy, they found three other locations on different chromosomes. However, the signal from chromosome 2 was the most robust, particularly when the researchers only looked at speed and not accuracy in decoding. They said evidence points to a possible combination of genes on chromosomes 2, 10 and 11 affecting speed.
The researchers focused on how dyslexics translate written words into spoken words without meaning cues because of a large body of evidence pointing to it as a hallmark deficit in dyslexia. The UW group also has published a finding that this deficit is detectable in children and adults.
Dyslexia is a complex learning disorder that affects 5 to 10 percent of school-age children in the United States. It typically is characterized by early difficulties in learning to name letters and associate sounds with letters. Later on, children with dyslexia have difficulty pronouncing real words and pseudowords when they are not in a sentence or do not have meaning cues. Such children can have problems with fluent reading and spelling as well.
The results of the new study provide further evidence that dyslexia has biological roots and is a learning disorder that requires specialized instruction. Earlier UW research has shown that there are chemical differences in brain function and how the brains of dyslexic and non-dyslexic children use oxygen during sound processing tasks.
That research also showed that dyslexics can be successfully treated. A set of specialized instructional approaches developed at the UW not only helped dyslexic children improve their reading skills, but also showed that their brains change in response to instruction designed for this genetic influence on learning to decode quickly. These approaches show children the regularity of decoding written English words and how to apply this regularity quickly, efficiently and strategically.
"It was not phonics as usual," said Berninger, "but could be implemented in schools so that parents do not have to seek costly treatment outside the schools."
The National Institute of Child Health and Development supported the research. Co-authors of the UW study are Robert Igo, Nicola Chapman and Mark Matsushita, medical genetics research scientists; Dr. Zoran Brkanac, acting assistant professor of psychiatry and behavioral sciences; Jennifer Thompson, former clinical coordinator; Ted Holzman, computer research consultant; and Mary Brown, undergraduate student.