John C DeFries
Article Spotlight
DEVELOPMENTAL DYSLEXIA: GENETIC DISSECTION OF A COMPLEX COGNITIVE TRAIT
Simon E. Fisher & John C. DeFries
Preface
Developmental dyslexia, a specific impairment of reading ability despite adequate intelligence and educational opportunity, is one of the most frequent childhood disorders. Since the first documented cases at the beginning of the last century, it has become increasingly apparent that the reading problems of people with dyslexia form part of a heritable neurobiological syndrome. As for most cognitive and behavioural traits, phenotypic definition is fraught with difficulties and the genetic basis is complex, making the isolation of genetic risk factors a formidable challenge. Against such a background, it is notable that several recent studies have reported the localization of genes that influence dyslexia and other language-related traits. These investigations exploit novel research approaches that are relevant to many areas of human neurogenetics.
Summary
Despite decades of multidisciplinary investigation, the biological basis of dyslexia — a specific impairment of reading ability — remains obscure. But a series of recent studies has emphasized the contribution of genetic factors to this disorder.
Dyslexia runs in families, and studies of monozygotic and dizygotic twins have provided valuable insights into the heritability of the condition. Methods developed for these studies have also aided in the genetic mapping of this reading disability.
For several reasons, the genetic analysis of dyslexia is complex. For example, there is no straightforward correspondence between genotype and phenotype, and phenotypic variations can depend on the developmental stage of the subject. Similarly, there is a lack of consensus on the definition of dyslexia, and on whether it is a single trait or a cluster of traits with distinct aetiologies.
Successful localization of genes that influence dyslexia has been aided by innovations in three areas. First, methods have been developed for mapping genes that contribute to quantitative variability in reading performance. Second, researchers are dissecting the phenotypic profile into distinct but related components for genetic study. Third, it is now possible to scan all chromosomes of the genome when searching for genes that influence complex traits such as dyslexia.
Targeted linkage studies of dyslexia have provided strong evidence that two chromosomal regions — 15q21 and 6p21 — are involved in this syndrome. Similarly, genome-wide scans have identified further regions on chromosomes 2, 3 and 18 that seem to be linked to dyslexia in multiple independent sets of families.
Although the linkage results highlight chromosomal regions that are involved in dyslexia susceptibility, finding individual genes that are affected remains a daunting task. So far, no specific dyslexia gene has been identified, but studies of speech and language deficits have found a gene — FOXP2 — that is responsible for a rare form of the disorder.
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