Genetics of specific reading disability

Pathogenesis of Comorbid ADHD and Chinese Developmental Dyslexia: Evidence From Eye-Movement Tracking and Rapid Automatized Naming

BACKGROUND

ADHD and Chinese developmental dyslexia (DD) have a very high comorbidity rate; however, which cognitive deficits characterize the comorbidity and when they occur during cognitive processing are still under debate.

METHODS

Rapid automatic naming (RAN) tasks with eye-movement tracking were conducted with 75 children who were typically developing, had comorbid ADHD and DD, had only ADHD, and had only DD.

RESULTS

The clinical groups had longer first fixation durations than the control for RAN digits. Temporal eye-movement measures, such as gaze duration and total reading time, were found to vary between the comorbidity and ADHD groups. Spatial eye-movement measures, such as regression probability and incoming saccade amplitude, differed between the comorbidity and DD groups.

CONCLUSIONS

These results indicate that investigation with eye-movement measures combined with RAN tasks can strengthen the understanding of the pathogenesis of comorbid ADHD and DD.

Current Research Informing the Conceptualization, Identification, and Treatment of Dyslexia Across Orthographies: An Introduction to the Special Series

This introduction to the special series summarizes evidence for the genetic and brain bases for dyslexia and cognitive-behavioral indicators (including ones that can be measured even before the onset of reading instruction) that attest to meaningful differences between children with dyslexia and their non-dyslexic peers. Authors review controversies that have surrounded approaches to dyslexia identification and treatment during the last few decades. Finally, they introduce the findings of the articles in the special series and discuss potential implications for dyslexia identification and treatment.

[Dyslexia as a multideficit disorder]

The article provides a review of research on dyslexia. Various views on the role of genetic and environmental factors in the etiology and pathogenesis of this disorder are examined. The results of neurophysiological and neuropsychological studies are presented, indicating a disturbance of some higher mental functions in dyslexia. The main neurocognitive deficits observed in dyslexia are considered: a disturbance of certain parameters of attention and working memory, a decrease in the speed of information processing, and insufficient automation of new skills. Based on the data presented, dyslexia appears to be a multifactorial and multideficit disorder.

Neuromagnetic speech discrimination responses are associated with reading-related skills in dyslexic and typical readers

Poor neural speech discrimination has been connected to dyslexia, and may represent phonological processing deficits that are hypothesized to be the main cause for reading impairments. Thus far, neural speech discrimination impairments have rarely been investigated in adult dyslexics, and even less by examining sources of neuromagnetic responses. We compared neuromagnetic speech discrimination in dyslexic and typical readers with mismatch fields (MMF) and determined the associations between MMFs and reading-related skills. We expected weak and atypically lateralized MMFs in dyslexic readers, and positive associations between reading-related skills and MMF strength. MMFs were recorded to a repeating pseudoword /ta-ta/ with occasional changes in vowel identity, duration, or syllable frequency from 43 adults, 21 with confirmed dyslexia. Phonetic (vowel and duration) changes elicited left-lateralized MMFs in the auditory cortices. Contrary to our hypothesis, MMF source strengths or lateralization did not differ between groups. However, better verbal working memory was associated with stronger left-hemispheric MMFs to duration changes across groups, and better reading was associated with stronger right-hemispheric late MMFs across speech-sound changes in dyslexic readers. This suggests a link between neural speech processing and reading-related skills, in line with previous work. Furthermore, our findings suggest a right-hemispheric compensatory mechanism for language processing in dyslexia. The results obtained promote the use of MMFs in investigating reading-related brain processes.

Dyslexia associated functional variants in Europeans are not associated with dyslexia in Chinese

Genome-wide association studies (GWAS) of developmental dyslexia (DD) often used European samples and identified only a handful associations with moderate or weak effects. This study aims to identify DD functional variants by integrating the GWAS associations with tissue-specific functional data and test the variants in a Chinese DD study cohort named READ. We colocalized associations from nine DD related GWAS with expression quantitative trait loci (eQTL) derived from brain tissues and identified two eSNPs rs349045 and rs201605. Both eSNPs had supportive evidence of chromatin interactions observed in human hippocampus tissues and their respective target genes ZNF45 and DNAH9 both had lower expression in brain tissues in schizophrenia patients than controls. In contrast, an eSNP rs4234898 previously identified based on eQTL from the lymphoblastic cell lines of dyslexic children had no chromatin interaction with its target gene SLC2A3 in hippocampus tissues and SLC2A3 expressed higher in the schizophrenia patients than controls. We genotyped the three eSNPs in the READ cohort of 372 cases and 354 controls and discovered only weak associations in rs201605 and rs4234898 with three DD symptoms (p < .05). The lack of associations could be due to low power in READ but could also implicate different etiology of DD in Chinese.

N1 lateralization and dyslexia: An event-related potential study in children with a familial risk of dyslexia

The rapid automatic specialized processing of printed words is signalled by the left-lateralization of the N1 component in the visual event-related potential (ERP). In the present study, we have investigated whether differences in N1 lateralization can be observed between Dutch children with and without (a familial risk of) dyslexia around the age of 12 years using a linguistic judgement task. Forty-five participants were included in the ERP analysis, 18 in the low familial risk group without dyslexia, 15 in the high familial risk group without dyslexia, and 12 in the high familial risk group with dyslexia. The results showed that although the N1 peaked slightly earlier in the left hemisphere, the N1 amplitude was right-lateralized in all groups. Moreover, there were no group differences in N1 amplitude or latency, and there was no relationship between reading (related) test scores and N1 characteristics. The results of the present study and our previous findings in adults suggest that print-tuning lateralization is a process that is still developing in adolescence. Because other studies did find N1 lateralization in younger readers with a print versus nonprint contrast, the current results seem to indicate that differences in N1 lateralization also depend on the experimental paradigm.

Neurobiological Sex Differences in Developmental Dyslexia

Understanding sex differences at the neurobiological level has become increasingly crucial in both basic and applied research. In the study of developmental dyslexia, early neuroimaging investigations were dominated by male-only or male-dominated samples, due at least in part to males being diagnosed more frequently. While recent studies more consistently balance the inclusion of both sexes, there has been little movement toward directly characterizing potential sex differences of the disorder. However, a string of recent work suggests that the brain basis of dyslexia may indeed be different in males and females. This potential sex difference has implications for existing models of dyslexia, and would inform approaches to the remediation of reading difficulties. This article reviews recent evidence for sex differences in dyslexia, discusses the impact these studies have on the understanding of the brain basis of dyslexia, and provides a framework for how these differential neuroanatomical profiles may develop.

Predictors for grade 6 reading in children at familial risk of dyslexia

The present study investigates whether grade 6 reading outcomes, reading fluency, and reading comprehension can be predicted by grade 3 reading fluency, familial risk of dyslexia (FR), and grade 3 reading related skills: rapid automatized naming (RAN), phonological awareness (PA), and vocabulary. In a sample of 150 children, of whom 83 had a parent with dyslexia, correlation and regression analyses were performed. FR, measured on a continuous scale, was by itself related to all outcomes. However, FR did not explain any variance on top of grade 3 reading fluency. Grade 3 reading fluency strongly predicted grade 6 reading fluency and was also related to reading comprehension. RAN improved the prediction of grade 6 reading fluency, though the additional explained variance was small. Vocabulary and PA fully explained the variance that grade 3 reading fluency explained in grade 6 reading comprehension. Vocabulary explained a substantial amount of variance in grade 6 reading comprehension making it an interesting clinical target. As we used continuous measures of reading fluency and FR, our findings are not biased by distinct diagnostic criteria.

Examining the relationship between home literacy environment and neural correlates of phonological processing in beginning readers with and without a familial risk for dyslexia: an fMRI study

Developmental dyslexia is a language-based learning disability characterized by persistent difficulty in learning to read. While an understanding of genetic contributions is emerging, the ways the environment affects brain functioning in children with developmental dyslexia are poorly understood. A relationship between the home literacy environment (HLE) and neural correlates of reading has been identified in typically developing children, yet it remains unclear whether similar effects are observable in children with a genetic predisposition for dyslexia. Understanding environmental contributions is important given that we do not understand why some genetically at-risk children do not develop dyslexia. Here, we investigate for the first time the relationship between HLE and the neural correlates of phonological processing in beginning readers with (FHD+, n = 29) and without (FHD-, n = 21) a family history of developmental dyslexia. We further controlled for socioeconomic status to isolate the neurobiological mechanism by which HLE affects reading development. Group differences revealed stronger correlation of HLE with brain activation in the left inferior/middle frontal and right fusiform gyri in FHD- compared to FHD+ children, suggesting greater impact of HLE on manipulation of phonological codes and recruitment of orthographic representations in typically developing children. In contrast, activation in the right precentral gyrus showed a significantly stronger correlation with HLE in FHD+ compared to FHD- children, suggesting emerging compensatory networks in genetically at-risk children. Overall, our results suggest that genetic predisposition for dyslexia alters contributions of HLE to early reading skills before formal reading instruction, which has important implications for educational practice and intervention models.

Reading difficulties in blind, braille-reading children

An account is given of a new research project that is concerned with examining the problems encountered by some young blind children as they learn to read braille. The research to be conducted will look at various developmental and learning theories that are used to explain the reading difficulties of sighted readers, with a view to ascertaining their relevance to the teaching and learning of braille.

The Colorado Longitudinal Twin Study of Reading Difficulties and ADHD: Etiologies of Comorbidity and Stability

Approximately 60% of children with reading difficulties (RD) meet criteria for at least one co-occurring disorder. The most common of these, attention deficit-hyperactivity disorder (ADHD), occurs in 20-40% of individuals with RD. Recent studies have suggested that genetic influences are responsible. To assess the genetic etiologies of RD and the comorbidity of RD and two ADHD symptom dimensions -- inattention (IN) and hyperactivity/impulsivity (H/I) -- we are conducting the first longitudinal twin study of RD and ADHD. Data from twin pairs in which at least one member of the pair met criteria for proband status for RD at initial assessment, and were reassessed 5 years later, were subjected to DeFries-Fulker (DF) analysis. Analyses of reading composite data indicated that over 60% of the proband deficit at initial assessment was due to genetic influences, and that reading deficits at follow-up were due substantially to the same genetic influences. When a bivariate DF model was fitted to reading performance and IN data, genetic influences accounted for 60% of contemporaneous comorbidity and over 60% of the longitudinal relationship. In contrast, analysis of the comorbidity between reading performance and H/I indicated that common genetic influences accounted for only about 20% of the contemporaneous and about 10% of the longitudinal relationships. Results indicate that (1) genetic influences on RD are substantial and highly stable; (2) the comorbidity between RD and IN is due largely to genetic influences, both contemporaneously and longitudinally; and (3) genetic influences contribute significantly less to the comorbidity between RD and H/I.

Evaluation of ocular movements in patients with dyslexia

The aims of this study were to analyze the relationship between dyslexia and eye movements and to assess whether this method can be added to the workup of dyslexic patients. The sample was comprised of 11 children with a diagnosis of dyslexia and 11 normal between 8 and 13 years of age. All subjects underwent orthoptic evaluation, ophthalmological examinations, and eye movement analysis, specifically, stability analysis on fixating a still target, tracking saccades, analysis of fixation pauses, speed reading, saccades, and regressions through the reading of a text. Stability analysis on fixating a still target showed a significant (p < 0.001) difference between the two groups showing an increased amount of loss of fixation among dyslexic subjects (5.36 ± 2.5 s and 0.82 ± 2.1, respectively). Tracking saccades (left and right horizontal axis) did not show a significant difference. When reading parameters were looked into (number of saccades, number of regressions, reading time through the reading of a text), a significant (p < 0.001) difference was found between the groups. This study supports the belief that the alteration of eye movement does not depend on oculo-motor dysfunction but is secondary to a defect in the visual processing of linguistic material. Inclusion of assessment of this defect might prove beneficial in determining the presence of dyslexia in young children at a younger age, and an earlier intervention could be initiated.

Association of the ROBO1 gene with reading disabilities in a family-based analysis

Linkage studies have identified a locus on chromosome 3 as reading disabilities (RD) and speech and sound disorder (SSD) susceptibility region, with both RD and SSD sharing similar phonological processing and phonological memory difficulties. One gene in this region, roundabout homolog 1 (ROBO1), has been indicated as a RD candidate and has shown significant association with measures of phonological memory in a population-based sample. In this study, we conducted a family-based association analysis using two independent samples collected in Toronto and Calgary, Canada. Using the two samples, we tested for association between ROBO1 single nucleotide polymorphisms (SNPs) and RD, along with quantitative measures for reading, spelling and phonological memory. One SNP, rs331142, which was selected based on its correlation with ROBO1 expression in brain tissue, was found to be significantly associated with RD in the Toronto sample with over transmission of the minor C allele (P = 0.001), correlated with low expression. This SNP is located ~200 bp from a putative enhancer and results for a marker within the enhancer, rs12495133, showed evidence for association with the same allele in both the Toronto and Calgary samples (P = 0.005 and P = 0.007). These results support previous associations between ROBO1 and RD, as well as correlation with low gene expression, suggesting a possible mechanism of risk conferred by this gene.

Xq27 FRAXA locus is a strong candidate for dyslexia: evidence from a genome-wide scan in French families

Dyslexia is a frequent neurodevelopmental learning disorder. To date, nine susceptibility loci have been identified, one of them being DYX9, located in Xq27. We performed the first French SNP linkage study followed by candidate gene investigation in dyslexia by studying 12 multiplex families (58 subjects) with at least two children affected, according to categorical restrictive criteria for phenotype definition. Significant results emerged on Xq27.3 within DYX9. The maximum multipoint LOD score reached 3,884 between rs12558359 and rs454992. Within this region, seven candidate genes were investigated for mutations in exonic sequences (CXORF1, CXORF51, SLITRK2, FMR1, FMR2, ASFMR1, FMR1NB), all having a role during brain development. We further looked for 5'UTR trinucleotide repeats in FMR1 and FMR2 genes. No mutation or polymorphism co-segregating with dyslexia was found. This finding in French families with Dyslexia showed significant linkage on Xq27.3 enclosing FRAXA, and consequently confirmed the DYX9 region as a robust susceptibility locus. We reduced the previously described interval from 6.8 (DXS1227-DXS8091) to 4 Mb also disclosing a higher LOD score.

Dense-map genome scan for dyslexia supports loci at 4q13, 16p12, 17q22; suggests novel locus at 7q36

Analysis of genetic linkage to dyslexia was performed using 133,165 array-based SNPs genotyped in 718 persons from 101 dyslexia-affected families. Results showed five linkage peaks with lod scores >2.3 (4q13.1, 7q36.1-q36.2, 7q36.3, 16p12.1, and 17q22). Of these five regions, three have been previously implicated in dyslexia (4q13.1, 16p12.1, and 17q22), three have been implicated in attention-deficit hyperactivity disorder (ADHD, which highly co-occurs with dyslexia; 4q13.1, 7q36.3, 16p12.1) and four have been implicated in autism (a condition characterized by language deficits; 7q36.1-q36.2, 7q36.3, 16p12.1, and 17q22). These results highlight the reproducibility of dyslexia linkage signals, even without formally significant lod scores, and suggest dyslexia predisposing genes with relatively major effects and locus heterogeneity. The largest lod score (2.80) occurred at 17q22 within the MSI2 gene, involved in neuronal stem cell lineage proliferation. Interestingly, the 4q13.1 linkage peak (lod 2.34) occurred immediately upstream of the LPHN3 gene, recently reported both linked and associated with ADHD. Separate analyses of larger pedigrees revealed lods >2.3 at 1-3 regions per family; one family showed strong linkage (lod 2.9) to a known dyslexia locus (18p11) not detected in our overall data, demonstrating the value of analyzing single large pedigrees. Association analysis identified no SNPs with genome-wide significance, although a borderline significant SNP (P = 6 × 10(-7)) occurred at 5q35.1 near FGF18, involved in laminar positioning of cortical neurons during development. We conclude that dyslexia genes with relatively major effects exist, are detectable by linkage analysis despite genetic heterogeneity, and show substantial overlapping predisposition with ADHD and autism.

Gene by environment interactions influencing reading disability and the inattentive symptom dimension of attention deficit/hyperactivity disorder

BACKGROUND

Reading disability (RD) and attention deficit/hyperactivity disorder (ADHD) are comorbid and genetically correlated, especially the inattentive dimension of ADHD (ADHD-I). However, previous research indicates that RD and ADHD enter into opposite gene by environment (G × E) interactions.

METHODS

This study used behavioral genetic methods to replicate these opposite G × E interactions in a sample of same-sex monozygotic and dizygotic twin pairs from the Colorado Learning Disabilities Research Center (CLDRC; DeFries et al., 1997) and to test a genetic hypothesis for why these opposite interactions occur.

RESULTS

We replicated opposite G × E interactions for RD (bioecological) and ADHD-I (diathesis-stress) with parental education in the same sample of participants. The genetic hypothesis for this opposite pattern of interactions is that only genes specific to each disorder enter into these opposite interactions, not the shared genes underlying their comorbidity. To test this hypothesis, we used single models with an exploratory three-way interaction, in which the G × E interactions for each disorder were moderated by comorbidity. Neither three-way interaction was significant. The heritability of RD did not vary as a function of parental education and ADHD-I. Similarly, the heritability of ADHD-I did not vary as a function of parental education and RD.

CONCLUSIONS

We documented opposite G × E interactions in RD and ADHD-I in the same overall twin sample, but the explanation for this apparent paradox remains unclear. Examining specific genes and more specific environmental factors may help resolve the paradox.

Etiology of the Stability of Reading Difficulties: The Longitudinal Twin Study of Reading Disabilities

Results obtained from previous longitudinal studies of reading difficulties indicate that reading deficits are generally stable. However, little is known about the etiology of this stability. Thus, the primary objective of this first longitudinal twin study of reading difficulties is to provide an initial assessment of genetic and environmental influences on the stability of reading deficits. Data were analyzed from a sample of 56 twin pairs, 18 identical (monozygotic, MZ) and 38 fraternal (dizygotic, DZ), in which at least one member of each pair was classified as reading-disabled in the Colorado Learning Disabilities Research Center, and on whom follow-up data were available. The twins were tested at two time points (average age of 10.3 years at initial assessment and 16.1 years at follow-up). A composite measure of reading performance (PIAT Reading Recognition, Reading Comprehension and Spelling) was highly stable, with a stability correlation of .84. Data from the initial time point were first subjected to univariate DeFries-Fulker multiple regression analysis and the resulting estimate of the heritability of the group deficit (h2g) was .84 (± .26). When the initial and follow-up data were then fitted to a bivariate extension of the basic DF model, bivariate heritability was estimated at .65, indicating that common genetic influences account for approximately 75% of the stability between reading measures at the two time points.

Genetic and environmental etiologies of reading difficulties: DeFries-Fulker analysis of reading performance data from twin pairs and their nontwin siblings

Reading performance data from 254 pairs of identical (MZ) and 420 pairs of fraternal (DZ) twins, 8.0 to 20.0 years of age, were subjected to multiple regression analyses. An extension of the DeFries-Fulker (DF) analysis (DeFries & Fulker, 1985, 1988) that facilitated inclusion of data from 303 of their nontwin siblings was employed. In addition to providing estimates of heritability, this analysis yields a test of the difference between shared environmental influences for twins versus siblings (Astrom et al., 2011). Results suggest that proband reading deficits are due substantially to genetic factors (.67 ± .07, p < .001), and that shared environmental influences are significantly higher for members of twin pairs than for those of twins and their nontwin siblings (viz., .25 versus .17, p = .02).

Learning disabilities, dyslexia, and vision

Learning disabilities constitute a diverse group of disorders in which children who generally possess at least average intelligence have problems processing information or generating output. Their etiologies are multifactorial and reflect genetic influences and dysfunction of brain systems. Reading disability, or dyslexia, is the most common learning disability. It is a receptive language-based learning disability that is characterized by difficulties with decoding, fluent word recognition, rapid automatic naming, and/or reading-comprehension skills. These difficulties typically result from a deficit in the phonologic component of language that makes it difficult to use the alphabetic code to decode the written word. Early recognition and referral to qualified professionals for evidence-based evaluations and treatments are necessary to achieve the best possible outcome. Because dyslexia is a language-based disorder, treatment should be directed at this etiology. Remedial programs should include specific instruction in decoding, fluency training, vocabulary, and comprehension. Most programs include daily intensive individualized instruction that explicitly teaches phonemic awareness and the application of phonics. Vision problems can interfere with the process of reading, but children with dyslexia or related learning disabilities have the same visual function and ocular health as children without such conditions. Currently, there is inadequate scientific evidence to support the view that subtle eye or visual problems cause or increase the severity of learning disabilities. Because they are difficult for the public to understand and for educators to treat, learning disabilities have spawned a wide variety of scientifically unsupported vision-based diagnostic and treatment procedures. Scientific evidence does not support the claims that visual training, muscle exercises, ocular pursuit-and-tracking exercises, behavioral/perceptual vision therapy, "training" glasses, prisms, and colored lenses and filters are effective direct or indirect treatments for learning disabilities. There is no valid evidence that children who participate in vision therapy are more responsive to educational instruction than children who do not participate.

DeFries-Fulker analysis of longitudinal reading performance data from twin pairs ascertained for reading difficulties and from their nontwin siblings

Reading difficulties are both heritable and stable; however, little is known about the etiology of this stability. Results from a preliminary analysis of data from 56 twin pairs who participated in the Colorado Longitudinal Twin Study of Reading Disability (Astrom et al., Twin Res Hum Genet 10:434-439, 2007) suggested that about two-thirds of the proband deficit at follow-up was due to genetic factors that also influenced deficits at their initial assessment. Although our proband sample is now nearly twice as large, it is still relatively small; thus, to increase power, we subjected data from probands, co-twins and their nontwin siblings to a novel extension of DeFries-Fulker analysis (DeFries and Fulker, Behav Genet 15:467-473, 1985; DeFries and Fulker, Acta Genet Med Gemellol, 37, 205-216, 1988). In addition to providing estimates of univariate and bivariate heritability, this analysis facilitates a test of the difference between shared environmental influences for twins versus siblings. Longitudinal composite reading performance scores at 10.6 and 15.5 years of age, on average, were analyzed from 33 MZ and 64 DZ twin pairs in which at least one member of each pair had reading difficulties, and from 44 siblings of the probands. Scores were highly stable (.86 ± .03, across probands, co-twins and siblings) and heritability of the group deficit at initial assessment was .67 ± .22. Longitudinal bivariate heritability was .59 ± .21, suggesting that nearly 60% of the proband reading deficit at follow-up is due to genetic factors that influenced reading difficulties at the initial assessment. However, tests for special twin environmental influences were nonsignificant.

Moving closer to a public health model of language and learning disabilities: the role of genetics and the search for etiologies

Continued progress in language and learning disabilities (LDs) research requires a renewed focused on issues of etiology. Genetics research forms a central tenet of such an agenda and is critical in clarifying relationships among oral language development, acquisition of literacy and mathematics, executive function skills, and comorbid conditions. For progress to be made, diversified efforts must continue to emphasize molecular and behavioral genetics (including quantitative genetics) approaches, in concert with multi-disciplinary and multi-modal projects, to provide an integrated understanding of the behavioral and biological manifestations of language and learning disabilities. Critically, increased efforts to include ethnic, socio-economic, and linguistically diverse participant samples across a range of developmental stages is required to meet the public health needs of learners in the US and across the world. Taken together, this body of work will continue to enhance our understanding of LDs and help us move toward a truly prevention based approach to language and learning disabilities.

Identification of candidate genes for dyslexia susceptibility on chromosome 18

BACKGROUND

Six independent studies have identified linkage to chromosome 18 for developmental dyslexia or general reading ability. Until now, no candidate genes have been identified to explain this linkage. Here, we set out to identify the gene(s) conferring susceptibility by a two stage strategy of linkage and association analysis.

METHODOLOGY/PRINCIPAL FINDINGS

Linkage analysis: 264 UK families and 155 US families each containing at least one child diagnosed with dyslexia were genotyped with a dense set of microsatellite markers on chromosome 18. Association analysis: Using a discovery sample of 187 UK families, nearly 3000 SNPs were genotyped across the chromosome 18 dyslexia susceptibility candidate region. Following association analysis, the top ranking SNPs were then genotyped in the remaining samples. The linkage analysis revealed a broad signal that spans approximately 40 Mb from 18p11.2 to 18q12.2. Following the association analysis and subsequent replication attempts, we observed consistent association with the same SNPs in three genes; melanocortin 5 receptor (MC5R), dymeclin (DYM) and neural precursor cell expressed, developmentally down-regulated 4-like (NEDD4L).

CONCLUSIONS

Along with already published biological evidence, MC5R, DYM and NEDD4L make attractive candidates for dyslexia susceptibility genes. However, further replication and functional studies are still required.

Analysis of dyslexia candidate genes in the Raine cohort representing the general Australian population

Several genes have been suggested as dyslexia candidates. Some of these candidate genes have been recently shown to be associated with literacy measures in sample cohorts derived from the general population. Here, we have conducted an association study in a novel sample derived from the Australian population (the Raine cohort) to further investigate the role of dyslexia candidate genes. We analysed markers, previously reported to be associated with dyslexia, located within the MRPL19/C2ORF3, KIAA0319, DCDC2 and DYX1C1 genes in a sample of 520 individuals and tested them for association with reading and spelling measures. Association signals were detected for several single nucleotide polymorphisms (SNPs) within DYX1C1 with both the reading and spelling tests. The high linkage disequilibrium (LD) we observed across the DYX1C1 gene suggests that the association signal might not be refined by further genetic mapping.

Etiologies and molecular mechanisms of communication disorders

Quantitative behavioral genetic studies have made it clear that communication disorders such as reading disability, language impairment, and autism spectrum disorders follow some basic principles: (1) complex disorders have complex causes, in which each clinical disorder is influenced by a number of separate genes; and (2) at least some behaviorally related disorders are influenced by the same genes. Recent advances in molecular and statistical methods have confirmed these principles and are now leading to an understanding of the genes that may be involved in these disorders and how their disruption may affect the development of the brain. The prospect is that the genes involved in these disorders will define a network of interacting neurologic functions and that perturbations of different elements of this network will produce susceptibilities for different disorders. Such knowledge would clarify the underlying deficits in these disorders and could lead to revised diagnostic conceptions. However, these goals are still in the future. Identifying the individual genes in such a network is painstaking, and there have been seemingly contradictory studies along the way. Improvements in study design and additional functional analysis of genes are gradually clarifying many of these issues. When combined with careful phenotypic studies, molecular genetic studies have the potential to refine the clinical definitions of communication disorders and influence their remediation.

Differential genetic etiology of reading difficulties as a function of IQ: an update

In order to test the hypothesis that the genetic etiology of reading disability differs as a function of IQ, composite reading performance data from 308 pairs of identical (monozygotic, MZ) twins and 440 pairs of fraternal (dizygotic, DZ) twins (254 same-sex and 186 opposite-sex) in which at least one member of each pair was classified as reading-disabled were subjected to multiple regression analysis (DeFries and Fulker, Behav Genet 15:467-473, 1985; Acta Genet Med Gemellol 37:205-216, 1988). In the total sample, heritability of the group deficit in reading performance (h(g)(2)) was .61 (±.06). However, results of fitting an extended regression model to reading performance and IQ data suggested that the genetic etiology of reading disability differs as a linear function of IQ (p ≤ .04). When the basic regression model was fitted separately to data from twin pairs with Wechsler (Examiner's manual: Wechsler intelligence scale for children-revised, 1974; Examiner's manual: Wechsler adult intelligence scale-revised, 1981) Full Scale IQ scores in the upper and lower 25% of the sample, resulting estimates of h(g)(2) were .75 (±.12) and .50 (±.10), respectively (p ≤ .045). These results suggest that reading difficulties in children with a higher IQ are due substantially to genetic influences and may require intensive remediation efforts.

Genetics of developmental dyslexia

Developmental dyslexia is a highly heritable disorder with a prevalence of at least 5% in school-aged children. Linkage studies have identified numerous loci throughout the genome that are likely to harbour candidate dyslexia susceptibility genes. Association studies and the refinement of chromosomal translocation break points in individuals with dyslexia have resulted in the discovery of candidate genes at some of these loci. A key function of many of these genes is their involvement in neuronal migration. This complements anatomical abnormalities discovered in dyslexic brains, such as ectopias, that may be the result of irregular neuronal migration.

No clear genetic influences on the association between dyslexia and anxiety in a population-based sample of female twins

Individuals with dyslexia are at an increased risk for anxiety disorders (e.g. generalized anxiety disorder, stress disorders, panic disorder). The extent to which this association is mediated by genetic and/or environmental influences is unclear. The current study explored the relationship between these two phenotypes using a large population-based twin sample. In total, 940 monozygotic and 903 dizygotic female twin pairs were included in the analyses. The presence of dyslexia and anxiety was determined by self-report of diagnosis by a health professional. Tetrachoric correlations confirmed an association between the two phenotypes, but suggested that there was no evidence for shared genetic risks. Bivariate twin modelling corroborated this finding and indicated the relationship between dyslexia and anxiety is mediated by shared environmental factors. Future research should seek to identifying the environmental factors that increase the vulnerability of individuals with dyslexia to emotional problems should be a priority for future research.

Two functions of early language experience

The unique human ability of linguistic communication, defined as the ability to produce a practically infinite number of meaningful messages using a finite number of lexical items, is determined by an array of "linguistic" genes, which are expressed in neurons forming domain-specific linguistic centers in the brain. In this review, I discuss the idea that infants' early language experience performs two complementary functions. In addition to allowing infants to assimilate the words and grammar rules of their mother language, early language experience initiates genetic programs underlying language production and comprehension. This hypothesis explains many puzzling characteristics of language acquisition, such as the existence of a critical period for acquiring the first language and the absence of a critical period for the acquisition of additional language(s), a similar timetable for language acquisition in children belonging to families of different social and cultural status, the strikingly similar timetables in the acquisition of oral and sign languages, and the surprisingly small correlation between individuals' final linguistic competence and the intensity of their training. Based on the studies of microcephalic individuals, I argue that genetic factors determine not only the number of neurons and organization of interneural connections within linguistic centers, but also the putative internal properties of neurons that are not limited to their electrophysiological and synaptic properties.

[Psychocognitive and psychiatric disorders associated with developmental dyslexia: A clinical and scientific issue]

INTRODUCTION

Dyslexia is a complex neurodevelopemental disorder that affects 5 to 10% of school-age children. This condition consists in a specific learning disability with a neurological origin. These learning difficulties are unexpected in relation to other cognitive abilities and the provision of efficient classroom instruction. A range of neurobiological investigations suggests that disruption of the parieto-temporo-occipital systems underlies a failure of skilled reading to develop. The observation that dyslexia is both a familial and heritable problem was made early on and was confirmed by twin studies. They also suggested that both genetic and environmental factors are involved. Several loci have been implicated in dyslexia, notably on chromosomes 2, 3, 6, 15 and 18 and some candidate genes have been proposed, but no functional mutation has yet been identified.

LITERATURE REVIEW

Dyslexia seldom appears isolated and dyslexic people are very likely to present other kinds of learning disabilities or psychiatric disorders. Specific language impairment, often with a mild outcome, is the most frequently associated with dyslexia. Indeed, late language development is often reported by dyslexic patients and also occurs more frequently among their siblings. Genetic linkage studies suggest some common genetic factor underlying this comorbidity. Dyscalculia is associated with dyslexia in 25% of cases, but most people with dyscalculia do not have any sign of dyslexia. The question of whether dyscalculia associated with dyslexia and dyscalculia itself rely on the same cognitive impairment is still controversial. Impaired motor development is also a common feature that affects nearly 50% of dyslexics and dyslexia is frequent among dyspraxic patients. This association raises the discussion on the role of motor impairment in dyslexia's physiopathology and the cerebellar theory of dyslexia. Beyond its link with other learning disorders, the study of dyslexia's comorbidity highlights psychopathological issues. ADHD is the most frequent psychiatric disorder associated with dyslexia. Underpinnings of this link between the two disorders seem to rely on common cognitive and genetic factors. Some authors have proposed a candidate gene ADRA2A to determine the condition including ADHD and dyslexia. In addition, dyslexics are exposed to a higher risk of anxiodepressive and behavioural disorders. Dyslexic children experience three times more behavioural disorders and one third of children with behavioural problems turn out to be affected by dyslexia. The literature study reveals inconsistent findings about depressed mood among dyslexics, but evidence of a persistent increase in the rate of anxiety disorders. The authors put forward the impact of environmental factors to explain these psychiatric comorbidities.

CONCLUSION

This review emphasizes dyslexia's comorbidities because they represent an important issue, both from a scientific and clinical point of view. Indeed, for clinicians, children showing multiple learning disabilities have specific reeducation and educational needs and dyslexics have a higher risk of emotional and behavioural disorders. On the other hand, dyslexia's comorbidity study provides a powerful method for researchers to investigate the still unknown physiopathology of dyslexia.

A common variant associated with dyslexia reduces expression of the KIAA0319 gene

Numerous genetic association studies have implicated the KIAA0319 gene on human chromosome 6p22 in dyslexia susceptibility. The causative variant(s) remains unknown but may modulate gene expression, given that (1) a dyslexia-associated haplotype has been implicated in the reduced expression of KIAA0319, and (2) the strongest association has been found for the region spanning exon 1 of KIAA0319. Here, we test the hypothesis that variant(s) responsible for reduced KIAA0319 expression resides on the risk haplotype close to the gene's transcription start site. We identified seven single-nucleotide polymorphisms on the risk haplotype immediately upstream of KIAA0319 and determined that three of these are strongly associated with multiple reading-related traits. Using luciferase-expressing constructs containing the KIAA0319 upstream region, we characterized the minimal promoter and additional putative transcriptional regulator regions. This revealed that the minor allele of rs9461045, which shows the strongest association with dyslexia in our sample (max p-value = 0.0001), confers reduced luciferase expression in both neuronal and non-neuronal cell lines. Additionally, we found that the presence of this rs9461045 dyslexia-associated allele creates a nuclear protein-binding site, likely for the transcriptional silencer OCT-1. Knocking down OCT-1 expression in the neuronal cell line SHSY5Y using an siRNA restores KIAA0319 expression from the risk haplotype to nearly that seen from the non-risk haplotype. Our study thus pinpoints a common variant as altering the function of a dyslexia candidate gene and provides an illustrative example of the strategic approach needed to dissect the molecular basis of complex genetic traits.

Dyslexia, or reading disability, is a common disorder caused by both genetic and environmental factors. Genetic studies have implicated a number of genes as candidates for playing a role in dyslexia. We functionally characterized one such gene (KIAA0319) to identify variant(s) that might affect gene expression and contribute to the disorder. We discovered a variant residing outside of the protein-coding region of KIAA0319 that reduces expression of the gene. This variant creates a binding site for the transcription factor OCT-1. Previous studies have shown that OCT-1 binding to a specific DNA sequence upstream of a gene can reduce the expression of that gene. In this case, reduced KIAA0319 expression could lead to improper development of regions of the brain involved in reading ability. This is the first study to identify a functional variant implicated in dyslexia. More broadly, our study illustrates the steps that can be utilized for identifying mutations causing other complex genetic disorders.

Heritability of high reading ability and its interaction with parental education

Moderation of the level of genetic influence on children's high reading ability by environmental influences associated with parental education was explored in two independent samples of identical and fraternal twins from the United States and Great Britain. For both samples, the heritability of high reading performance increased significantly with lower levels of parental education. Thus, resilience (high reading ability despite lower environmental support) is more strongly influenced by genotype than is high reading ability with higher environmental support. This result provides a coherent account when considered alongside results of previous research showing that heritability for low reading ability decreased with lower levels of parental education.

Genome scan of a nonword repetition phenotype in families with dyslexia: evidence for multiple loci

To understand the genetic architecture of dyslexia and identify the locations of genes involved, we performed linkage analyses in multigenerational families using a phonological memory phenotype--Nonword Repetition (NWR). A genome scan was first performed on 438 people from 51 families (DS-1) and linkage was assessed using variance components (VC), Bayesian oligogenic (BO), and parametric analyses. For replication, the genome scan and analyses were repeated on 693 people from 93 families (DS-2). For the combined set (DS-C), analyses were performed with all three methods in the regions that were identified in both samples. In DS-1, regions on chromosomes 4p, 6q, 12p, 17q, and 22q exceeded our initial threshold for linkage, with 17q providing a parametric LOD score of 3.2. Analysis with DS-2 confirmed the locations on chromosomes 4p and 12p. The strongest VC and BO signals in both samples were on chromosome 4p in DS-C, with a parametric multipoint LOD(max) of 2.36 for the 4p locus. Our linkage analyses of NWR in dyslexia provide suggestive and reproducible evidence for linkage to 4p12 and 12p in both samples, and significant evidence for linkage to 17q in one of the samples. These results warrant further studies of phonological memory and chromosomal regions identified here in other datasets.

Neuropsychologic outcomes in a case series of twins discordant for perinatal stroke

Perinatal stroke may affect cognitive development, but few studies have addressed the details of cognitive function after perinatal stroke. The present study was designed to compare the neuropsychologic features of five sets of twins discordant for perinatal stroke. All of the affected children had unilateral middle cerebral artery infarction (two left, three right); four of the five infarcts were large-branch, affecting the entire M1 territory. Three of the five affected children had comorbid epilepsy. Measures of intelligence, memory, language, attention, executive function, visual-motor integration, and fine motor skills were administered to children at a median age of 5 years (range, 5-8 years). Relative to their unaffected co-twins, the twins with perinatal stroke exhibited lower levels of full scale (p=0.005), verbal (p=0.006), and nonverbal (p=0.005) intelligence. Children with perinatal stroke also showed significant deficits on tests of verbal memory (p=0.041), receptive language (p=0.011), verbal fluency (p=0.019), and visual attention (p=0.011), compared with their unaffected co-twins. Twin gestation may be a risk factor for poor cognitive outcome after perinatal stroke. Large infarct size and comorbid epilepsy may have contributed to some of the poor cognitive outcomes in this cohort.

The Genetic Lexicon of Dyslexia

Reading abilities are acquired only through specific teaching and training. A significant proportion of children fail to achieve these skills despite normal intellectual abilities and an appropriate opportunity to learn. Difficulty in learning to read is attributable to specific dysfunctions of the brain, which so far remain poorly understood. However, it is recognized that the neurological basis for dyslexia, or reading disability, is caused in large part by genetic factors. Linkage studies have successfully identified several regions of the human genome that are likely to harbor susceptibility genes for dyslexia. In the past few years there have been exciting advances with the identification of four candidate genes located within three of these linked chromosome regions: DYX1C1 on chromosome 15, ROBO1 on chromosome 3, and KIAA0319 and DCDC2 on chromosome 6. Functional studies of these genes are offering new insights about the biological mechanisms underlying the development of dyslexia and, in general, of cognition.

Association of the dopamine receptor D1 gene, DRD1, with inattention symptoms in families selected for reading problems

Twin studies have provided evidence for shared genetic influences between attention-deficit/hyperactivity disorder (ADHD) and specific reading disabilities (RD), with this overlap being highest for the inattentive symptom dimension of ADHD. Previously, we found evidence for association of the dopamine receptor D1 gene (DRD1) with ADHD, and with the inattentive symptom dimension in particular. This, combined with evidence for working memory (WM) deficits in individuals with RD or ADHD, and the importance of D1 receptors in attentional processes and WM function, suggests that DRD1 may be a common genetic influence underlying both disorders. Here, in a study of 232 families ascertained through probands with reading problems, we tested for association of the DRD1 gene with RD, as a categorical trait, and with quantitative measures of key reading component skills, WM ability, and inattentive symptoms. Although no associations were found with RD, or with reading component skills or verbal WM, we found evidence for association with inattentive behaviour. Specifically, DRD1 Haplotype 3, the haplotype previously found to be associated with inattentive symptoms in ADHD, is also associated with parent- and teacher-reported symptoms of inattention in this sample selected for reading problems (P=0.023 and 0.004, respectively). Together, the replicated finding of Haplotype 3 association with inattentive symptoms in two independent study samples strongly supports a role for DRD1 in attentional ability. Furthermore, the association of DRD1 with inattention, but not with RD, or the other reading and reading-related phenotypes analysed, suggests that DRD1 contributes uniquely to inattention, without overlap for reading ability.

Genes, language development, and language disorders

Genetic factors are important contributors to language and learning disorders, and discovery of the underlying genes can help delineate the basic neurological pathways that are involved. This information, in turn, can help define disorders and their perceptual and processing deficits. Initial molecular genetic studies of dyslexia, for example, appear to converge on defects in neuronal and axonal migration. Further study of individuals with abnormalities of these genes may lead to the recognition of characteristic cognitive deficits attributable to the neurological dysfunction. Such abnormalities may affect other disorders as well, and studies of co-morbidity of dyslexia with attention deficit disorder and speech sound disorder are helping to define the scope of these genes and show the etiological and cognitive commonalities between these conditions. The genetic contributions to specific language impairment (SLI) are not as well defined at this time, but similar molecular approaches are being applied to identify genes that influence SLI and comorbid disorders. While there is co-morbidity of SLI with dyslexia, it appears that most of the common genetic effects may be with the language characteristics of autism spectrum disorders rather than with dyslexia and related disorders. Identification of these genes and their neurological and cognitive effects should lay out a functional network of interacting genes and pathways that subserve language development. Understanding these processes can form the basis for refined procedures for diagnosis and treatment.

Neuropsychological deficits and neural dysfunction in familial dyslexia

We report the neuropsychological profile and the pattern of brain activity during reading tasks in a sample of familial dyslexics. We studied our subjects with an in-depth neuropsychological assessment and with functional neuroimaging (fMRI) during word and pseudoword reading and false font string observations (baseline condition). The neuropsychological assessment revealed that familial dyslexia, in both persistent and compensated forms, is often associated with deficits in verbal short-term memory, phonological awareness and automatization abilities. The functional results showed a lack of activation in the posterior areas of the reading network. This study, together with the previously published VBM study (Brambati, S.M., Termine, C., Ruffino, M., Stella, G., Fazio, F., Cappa, S.F. and Perani, D., Regional reductions of gray matter volume in familial dyslexia, Neurology, 63 (2004) 742-5), provides a multiple modality evaluation of familial dyslexia. The neuropsychological assessment showed cognitive deficits associated with dyslexia that persist also in subjects with compensated reading deficit. Both the anatomical and the functional study point out a deficit in the posterior areas of the reading network.

“Scientific roots” of dualism in neuroscience

Although the dualistic concept is unpopular among neuroscientists involved in experimental studies of the brain, neurophysiological literature is full of covert dualistic statements on the possibility of understanding neural mechanisms of human consciousness. Particularly, the covert dualistic attitude is exhibited in the unwillingness to discuss neural mechanisms of consciousness, leaving the problem of consciousness to psychologists and philosophers. This covert dualism seems to be rooted in the main paradigm of neuroscience that suggests that cognitive functions, such as language production and comprehension, face recognition, declarative memory, emotions, etc., are performed by neural networks consisting of simple elements. I argue that neural networks of any complexity consisting of neurons whose function is limited to the generation of electrical potentials and the transmission of signals to other neurons are hardly capable of producing human mental activity, including consciousness. Based on results obtained in physiological, morphological, clinical, and genetic studies of cognitive functions (mainly linguistic ones), I advocate the hypothesis that the performance of cognitive functions is based on complex cooperative activity of "complex" neurons that are carriers of "elementary cognition." The uniqueness of human cognitive functions, which has a genetic basis, is determined by the specificity of genes expressed by these "complex" neurons. The main goal of the review is to show that the identification of the genes implicated in cognitive functions and the understanding of a functional role of their products is a possible way to overcome covert dualism in neuroscience.

Genetic influences on early word recognition abilities and disabilities: a study of 7-year-old twins

BACKGROUND

A fundamental issue for child psychology concerns the origins of individual differences in early reading development.

METHOD

A measure of word recognition, the Test of Word Reading Efficiency (TOWRE), was administered by telephone to a representative population sample of 3,909 same-sex and opposite-sex pairs of 7-year-old twins. Analyses allowing for sex differences in aetiology were used to estimate the extent to which genetic and environmental influences contribute to normal variation in word recognition and word recognition difficulties, defined by scores below the 5th and 10th percentiles of the unselected sample.

RESULTS

Both normal variation in word recognition and impaired word recognition abilities were substantially heritable (h2 = .65-.67; h(g)2 = .37-.72). Environmental influences were primarily shared between twins, rather than specific to each individual, and small to moderate in magnitude. There was evidence for qualitative sex differences. Quantitative sex differences were also suggested at the extremes, with genetic influences being more important as a cause of reading difficulties in boys than in girls.

CONCLUSIONS

These findings indicate that early individual differences and impairments in word recognition are principally influenced by genetic factors and may involve partly distinct genetic or environmental effects in boys and girls. Crucially, they also provide evidence that reading impairments are linked genetically to the normal distribution. Genetic risk for early impairments in word recognition is continuous rather than discrete.

Neuropsychological Analyses of Comorbidity Between Reading Disability and Attention Deficit Hyperactivity Disorder: In Search of the Common Deficit

Measures of component reading and language skills, executive functions, and processing speed were administered to groups of children with attention deficit hyperactivity disorder (ADHD; n = 113), reading disability (RD; n = 109), both RD and ADHD (n = 64), and neither RD nor ADHD (n = 151). Groups with RD exhibited pronounced deficits on all measures of component reading and language skills, as well as significant weaknesses on measures of verbal working memory, processing speed, and response inhibition. Groups with ADHD exhibited weaknesses on all response-inhibition and processing speed tasks and were impaired on some measures of component reading skills and verbal working memory. The group with comorbid RD and ADHD exhibited the combination of the deficits in the RD-only and ADHD-only groups, providing evidence against the phenocopy and cognitive subtype hypotheses as explanations for the co-occurrence of RD and ADHD. Slow and variable processing speed was characteristic of all 3 clinical groups, suggesting that measures of this domain may be useful for future studies that search for the common genes that increase susceptibility to RD and ADHD.

The effect of consanguineous marriage on reading disability in the Arab community

The present study examined the effect of consanguineous marriage in the Arab community on reading disabilities of offspring. It examined whether the rate of reading disabilities was higher among offspring of first-cousin parents than offspring of unrelated parents; and whether reading-disabled children of first-cousin parents were more disabled in phonological awareness and phonological decoding than reading-disabled children of unrelated parents and normally reading younger children. These questions were investigated among 814 pupils of the 4th, 5th, and 6th grades, using word recognition and reading comprehension tests. Two experimental groups were chosen from this population. These were a reading-disabled group of 22 pupils who were children of first-cousin marriages and 21 pupils who were children of unrelated parents. A control group was also selected, consisting of 21 younger normally reading pupils at the same reading level. All the groups were tested on non-words, real words, phonological, orthographic and working memory measures. The results indicated that the rate of reading disabilities among children of first-cousin parents was higher than that of with children of second-cousin parents, distantly related parents, or unrelated parents. Further, no differences were found in phonological awareness and decoding between the two reading-disabled groups. Moreover, the results indicate a significant advantage of the younger normal readers over the reading-disabled children in the measures of phonological awareness, decoding, and orthographical knowledge that requires spelling. However, in reading common words and choosing words in context, the performance of the reading-disabled groups and the normally reading group were similar. It has been suggested that further research is needed to evaluate the role of intelligence, nevertheless our results provide new evidence for a genetic basis to reading disabilities.

Developmental dyslexia

Developmental dyslexia, or specific reading disability, is a disorder in which children with normal intelligence and sensory abilities show learning deficits for reading. Substantial evidence has established its biological origin and the preponderance of phonological disorders even though important phenotypic variability and comorbidity have been recorded. Diverse theories have been proposed to account for the cognitive and neurological aspects of dyslexia. Findings of genetic studies show that different loci affect specific reading disability although a direct relation has not been established between symptoms and a given genomic locus. In both children and adults with dyslexia, results of neuroimaging studies suggest defective activity and abnormal connectivity between regions crucial for language functions--eg, the left fusiform gyrus for reading--and changes in brain activity associated with performance improvement after various remedial interventions.

The development of reading impairment: a cognitive neuroscience model

This review discusses recent cognitive neuroscience investigations into the biological bases of developmental dyslexia, a common disorder impacting approximately 5 to 17 percent of the population. Our aim is to summarize central findings from several lines of evidence that converge on pivotal aspects of the brain bases of developmental dyslexia. We highlight ways in which the approaches and methodologies of developmental cognitive neuroscience that are addressed in this special issue-including neuroimaging, human genetics, refinement of cognitive and biological phenotypes, neural plasticity and computational model-can be employed in uncovering the biological bases of this disorder. Taking a developmental perspective on the biological bases of dyslexia, we propose a simple cascading model for the developmental progression of this disorder, in which individual differences in brain areas associated with phonological processing might influence the specialization of visual areas involved in the rapid processing of written words. We also discuss recent efforts to understand the impact of successful reading interventions in terms of changes within cortical circuits associated with reading ability.

Developmental Dyslexia and Specific Language Impairment: Same or Different?

Developmental dyslexia and specific language impairment (SLI) were for many years treated as distinct disorders but are now often regarded as different manifestations of the same underlying problem, differing only in severity or developmental stage. The merging of these categories has been motivated by the reconceptualization of dyslexia as a language disorder in which phonological processing is deficient. The authors argue that this focus underestimates the independent influence of semantic and syntactic deficits, which are widespread in SLI and which affect reading comprehension and impair attainment of fluent reading in adolescence. The authors suggest that 2 dimensions of impairment are needed to conceptualize the relationship between these disorders and to capture phenotypic features that are important for identifying neurobiologically and etiologically coherent subgroups.

The genetic basis of dyslexia

Dyslexia, a disorder of reading and spelling, is a heterogeneous neurological syndrome with a complex genetic and environmental aetiology. People with dyslexia differ in their individual profiles across a range of cognitive, physiological, and behavioural measures related to reading disability. Some or all of the subtypes of dyslexia might have partly or wholly distinct genetic causes. An understanding of the role of genetics in dyslexia could help to diagnose and treat susceptible children more effectively and rapidly than is currently possible and in ways that account for their individual disabilities. This knowledge will also give new insights into the neurobiology of reading and language cognition. Genetic linkage analysis has identified regions of the genome that might harbour inherited variants that cause reading disability. In particular, loci on chromosomes 6 and 18 have shown strong and replicable effects on reading abilities. These genomic regions contain tens or hundreds of candidate genes, and studies aimed at the identification of the specific causal genetic variants are underway.