TDT-association analysis of EKN1 and dyslexia in a Colorado twin cohort

The Influence of Dyslexia Candidate Genes on Reading Skill in Old Age

A number of candidate genes for reading and language impairment have been replicated, primarily in samples of children with developmental disability or delay, although these genes are also supported in adolescent population samples. The present study used a systematic approach to test 14 of these candidate genes for association with reading assessed in late adulthood (two cohorts with mean ages of 70 and 79 years). Gene-sets (14 candidates, axon-guidance and neuron migration pathways) and individual SNPs within each gene of interest were tested for association using imputed data referenced to the 1000 genomes European panel. Using the results from the genome-wide association (GWA) meta-analysis of the two cohorts (N = 1217), a competitive gene-set analysis showed that the candidate gene-set was associated with the reading index (p = .016) at a family wise error rate corrected significance level. Neither axon guidance nor neuron migration pathways were significant. Whereas individual SNP associations within CYP19A1, DYX1C1, CNTNAP2 and DIP2A genes (p < .05) did not reach corrected significance their allelic effects were in the same direction as past available reports. These results suggest that reading skill in normal adults shares the same genetic substrate as reading in adolescents, and clinically disordered reading, and highlights the utility of adult samples to increase sample sizes in the genetic study of developmental disorders.

The influence of DCDC2 risk genetic variants on reading: Testing main and haplotypic effects

Developmental dyslexia (DD) is a complex neurodevelopmental heritable disorder. Among DD candidate genes, DCDC2 is one of the most replicated, with rs793862, READ1 and rs793842 likely contribute to phenotypic variability in reading (dis)ability. In this study, we tested the effects of these genetic variants on DD as a categorical trait and on quantitative reading-related measures in a sample of 555 Italian nuclear families with 930 offspring, of which 687 were diagnosed with DD. We conducted both single-marker and haplotype analyses, finding that the READ1-deletion was significantly associated with reading, whereas no significant haplotype associations were found. Our findings add further evidence to support the hypothesis of a DCDC2 contribution to inter-individual variation in distinct indicators of reading (dis)ability in transparent languages (i.e., reading accuracy and speed), suggesting a potential pleiotropic effect.

Genetic factors contributing to human primary ciliary dyskinesia and male infertility

Primary ciliary dyskinesia (PCD) is an autosomal-recessive disorder resulting from the loss of normal ciliary function. Symptoms include neonatal respiratory distress, chronic sinusitis, bronchiectasis, situs inversus, and infertility. However, only 15 PCD-associated genes have been identified to cause male infertility to date. Owing to the genetic heterogeneity of PCD, comprehensive molecular genetic testing is not considered the standard of care. Here, we provide an update of the progress on the identification of genetic factors related to PCD associated with male infertility, summarizing the underlying molecular mechanisms, and discuss the clinical implications of these findings. Further research in this field will impact the diagnostic strategy for male infertility, enabling clinicians to provide patients with informed genetic counseling, and help to adopt the best course of treatment for developing directly targeted personalized medicine.

Evaluation of visual motion perception ability in mice with knockout of the dyslexia candidate susceptibility gene Dcdc2

Developmental dyslexia is a heritable disability characterized by difficulties in learning to read and write. The neurobiological and genetic mechanisms underlying dyslexia remain poorly understood; however, several dyslexia candidate risk genes have been identified. One of these candidate risk genes-doublecortin domain containing 2 (DCDC2)-has been shown to play a role in neuronal migration and cilia function. At a behavioral level, variants of DCDC2 have been associated with impairments in phonological processing, working memory and reading speed. Additionally, a specific mutation in DCDC2 has been strongly linked to deficits in motion perception-a skill subserving reading abilities. To further explore the relationship between DCDC2 and dyslexia, a genetic knockout (KO) of the rodent homolog of DCDC2 (Dcdc2) was created. Initial studies showed that Dcdc2 KOs display deficits in auditory processing and working memory. The current study was designed to evaluate the association between DCDC2 and motion perception, as these skills have not yet been assessed in the Dcdc2 KO mouse model. We developed a novel motion perception task, utilizing touchscreen technology and operant conditioning. Dcdc2 KOs displayed deficits on the Pairwise Discrimination task specifically as motion was added to visual stimuli. Following behavioral assessment, brains were histologically prepared for neuroanatomical analysis of the lateral geniculate nucleus (LGN). The cumulative distribution showed that Dcdc2 KOs exhibited more small neurons and fewer larger neurons in the LGN. Results compliment findings that DCDC2 genetic alteration results in anomalies in visual motion pathways in a subpopulation of dyslexic patients.

Deficits in learning and memory in mice with a mutation of the candidate dyslexia susceptibility gene Dyx1c1

Dyslexia is a learning disability characterized by difficulty learning to read and write. The underlying biological and genetic etiology remains poorly understood. One candidate gene, dyslexia susceptibility 1 candidate 1 (DYX1C1), has been shown to be associated with deficits in short-term memory in dyslexic populations. The purpose of the current study was to examine the behavioral phenotype of a mouse model with a homozygous conditional (forebrain) knockout of the rodent homolog Dyx1c1. Twelve Dyx1c1 conditional homozygous knockouts, 7 Dyx1c1 conditional heterozygous knockouts and 6 wild-type controls were behaviorally assessed. Mice with the homozygous Dyx1c1 knockout showed deficits on memory and learning, but not on auditory or motor tasks. These findings affirm existing evidence that DYX1C1 may play an underlying role in the development of neural systems important to learning and memory, and disruption of this function could contribute to the learning deficits seen in individuals with dyslexia.

Tackling the 'dyslexia paradox': reading brain and behavior for early markers of developmental dyslexia

Developmental dyslexia is an unexplained inability to acquire accurate or fluent reading that affects approximately 5-17% of children. Dyslexia is associated with structural and functional alterations in various brain regions that support reading. Neuroimaging studies in infants and pre-reading children suggest that these alterations predate reading instruction and reading failure, supporting the hypothesis that variant function in dyslexia susceptibility genes lead to atypical neural migration and/or axonal growth during early, most likely in utero, brain development. Yet, dyslexia is typically not diagnosed until a child has failed to learn to read as expected (usually in second grade or later). There is emerging evidence that neuroimaging measures, when combined with key behavioral measures, can enhance the accuracy of identification of dyslexia risk in pre-reading children but its sensitivity, specificity, and cost-efficiency is still unclear. Early identification of dyslexia risk carries important implications for dyslexia remediation and the amelioration of the psychosocial consequences commonly associated with reading failure.

An informatics approach to integrating genetic and neurological data in speech and language neuroscience

A number of heritable disorders impair the normal development of speech and language processes and occur in large numbers within the general population. While candidate genes and loci have been identified, the gap between genotype and phenotype is vast, limiting current understanding of the biology of normal and disordered processes. This gap exists not only in our scientific knowledge, but also in our research communities, where genetics researchers and speech, language, and cognitive scientists tend to operate independently. Here we describe a web-based, domain-specific, curated database that represents information about genotype-phenotype relations specific to speech and language disorders, as well as neuroimaging results demonstrating focal brain differences in relevant patients versus controls. Bringing these two distinct data types into a common database ( http://neurospeech.org/sldb ) is a first step toward bringing molecular level information into cognitive and computational theories of speech and language function. One bridge between these data types is provided by densely sampled profiles of gene expression in the brain, such as those provided by the Allen Brain Atlases. Here we present results from exploratory analyses of human brain gene expression profiles for genes implicated in speech and language disorders, which are annotated in our database. We then discuss how such datasets can be useful in the development of computational models that bridge levels of analysis, necessary to provide a mechanistic understanding of heritable language disorders. We further describe our general approach to information integration, discuss important caveats and considerations, and offer a specific but speculative example based on genes implicated in stuttering and basal ganglia function in speech motor control.

Imaging-genetics in dyslexia: connecting risk genetic variants to brain neuroimaging and ultimately to reading impairments

Dyslexia is a common pediatric disorder that affects 5-17% of schoolchildren in the United States. It is marked by unexpected difficulties in fluent reading despite adequate intelligence, opportunity, and instruction. Classically, neuropsychologists have studied dyslexia using a variety of neurocognitive batteries to gain insight into the specific deficits and impairments in affected children. Since dyslexia is a complex genetic trait with high heritability, analyses conditioned on performance on these neurocognitive batteries have been used to try to identify associated genes. This has led to some successes in identifying contributing genes, although much of the heritability remains unexplained. Additionally, the lack of relevant human brain tissue for analysis and the challenges of modeling a uniquely human trait in animals are barriers to advancing our knowledge of the underlying pathophysiology. In vivo imaging technologies, however, present new opportunities to examine dyslexia and reading skills in a clearly relevant context in human subjects. Recent investigations have started to integrate these imaging data with genetic data in attempts to gain a more complete and complex understanding of reading processes. In addition to bridging the gap from genetic risk variant to a discernible neuroimaging phenotype and ultimately to the clinical impairments in reading performance, the use of neuroimaging phenotypes will reveal novel risk genes and variants. In this article, we briefly discuss the genetic and imaging investigations and take an in-depth look at the recent imaging-genetics investigations of dyslexia.

DYX1C1 is required for axonemal dynein assembly and ciliary motility

DYX1C1 has been associated with dyslexia and neuronal migration in the developing neocortex. Unexpectedly, we found that deleting exons 2-4 of Dyx1c1 in mice caused a phenotype resembling primary ciliary dyskinesia (PCD), a disorder characterized by chronic airway disease, laterality defects and male infertility. This phenotype was confirmed independently in mice with a Dyx1c1 c.T2A start-codon mutation recovered from an N-ethyl-N-nitrosourea (ENU) mutagenesis screen. Morpholinos targeting dyx1c1 in zebrafish also caused laterality and ciliary motility defects. In humans, we identified recessive loss-of-function DYX1C1 mutations in 12 individuals with PCD. Ultrastructural and immunofluorescence analyses of DYX1C1-mutant motile cilia in mice and humans showed disruptions of outer and inner dynein arms (ODAs and IDAs, respectively). DYX1C1 localizes to the cytoplasm of respiratory epithelial cells, its interactome is enriched for molecular chaperones, and it interacts with the cytoplasmic ODA and IDA assembly factor DNAAF2 (KTU). Thus, we propose that DYX1C1 is a newly identified dynein axonemal assembly factor (DNAAF4).

Alleles of a polymorphic ETV6 binding site in DCDC2 confer risk of reading and language impairment

Reading disability (RD) and language impairment (LI) are common learning disabilities that make acquisition and utilization of reading and verbal language skills, respectively, difficult for affected individuals. Both disorders have a substantial genetic component with complex inheritance. Despite decades of study, reading and language, like many other complex traits, consistently evade identification of causative and functional variants. We previously identified a putative functional risk variant, named BV677278 for its GenBank accession number, for RD in DCDC2. This variant consists of an intronic microdeletion and a highly polymorphic short tandem repeat (STR) within its breakpoints. We have also shown this STR to bind to an unknown nuclear protein with high specificity. Here, we replicate BV677278's association with RD, expand its association to LI, identify the BV677278-binding protein as the transcription factor ETV6, and provide compelling genetic evidence that BV677278 is a regulatory element that influences reading and language skills. We also provide evidence that BV677278 interacts nonadditively with KIAA0319, an RD-associated gene, to adversely affect several reading and cognitive phenotypes. On the basis of these data, we propose a new name for BV677278: "READ1" or "regulatory element associated with dyslexia 1."

The zebrafish orthologue of the dyslexia candidate gene DYX1C1 is essential for cilia growth and function

DYX1C1, a susceptibility gene for dyslexia, encodes a tetratricopeptide repeat domain containing protein that has been implicated in neuronal migration in rodent models. The developmental role of this gene remains unexplored. To understand the biological function(s) of zebrafish dyx1c1 during embryonic development, we cloned the zebrafish dyx1c1 and used morpholino-based knockdown strategy. Quantitative real-time PCR analysis revealed the presence of dyx1c1 transcripts in embryos, early larval stages and in a wide range of adult tissues. Using mRNA in situ hybridization, we show here that dyx1c1 is expressed in many ciliated tissues in zebrafish. Inhibition of dyx1c1 produced pleiotropic phenotypes characteristically associated with cilia defects such as body curvature, hydrocephalus, situs inversus and kidney cysts. We also demonstrate that in dyx1c1 morphants, cilia length is reduced in several organs including Kupffer’s vesicle, pronephros, spinal canal and olfactory placode. Furthermore, electron microscopic analysis of cilia in dyx1c1 morphants revealed loss of both outer (ODA) and inner dynein arms (IDA) that have been shown to be required for cilia motility. Considering all these results, we propose an essential role for dyx1c1 in cilia growth and function.

A family-based association analysis and meta-analysis of the reading disabilities candidate gene DYX1C1

Reading disabilities (RD) have a significant genetic basis and have shown linkage to multiple regions including chromosome 15q. Dyslexia susceptibility 1 candidate gene 1 (DYX1C1) on chromosome 15q21 was originally proposed as a candidate gene with two potentially functional polymorphisms at the -3G/A and 1249G/T positions showing association with RD. However, subsequent studies have yielded mixed results. We performed a literature review and meta-analysis of the -3G/A and 1249G/T polymorphisms, including new unpublished data from two family-based samples. Ten markers in DYX1C1 were genotyped in the two independently ascertained samples. Single marker and -3G/A:1249G/T haplotype analyses were performed for RD in both samples, and quantitative trait analyses using standardized reading-related measures was performed in one of the samples. For the meta-analysis, we used a random-effects model to summarize studies that tested for association between -3G/A or 1249G/T and RD. No significant association was found between the DYX1C1 SNPs and RD or any of the reading-related measures tested after correction for the number of tests performed. The previously reported risk haplotype (-3A:1249T) was not biased in transmission. A total of 9 and 10 study samples were included in the meta-analysis of the -3G/A and 1249G/T polymorphisms, respectively. Neither polymorphism reached statistical significance, but the heterogeneity for the 1249G/T polymorphism was high. The results of this study do not provide evidence for association between the putatively functional SNPs -3G/A and 1249G/T and RD.

The genetics of reading disabilities: from phenotypes to candidate genes

This article provides an overview of (a) issues in definition and diagnosis of specific reading disabilities at the behavioral level that may occur in different constellations of developmental and phenotypic profiles (patterns); (b) rapidly expanding research on genetic heterogeneity and gene candidates for dyslexia and other reading disabilities; (c) emerging research on gene-brain relationships; and (d) current understanding of epigenetic mechanisms whereby environmental events may alter behavioral expression of genetic variations. A glossary of genetic terms (denoted by bold font) is provided for readers not familiar with the technical terms.

Three dyslexia susceptibility genes, DYX1C1, DCDC2, and KIAA0319, affect temporo-parietal white matter structure

BACKGROUND

Volume and integrity of white matter correlate with reading ability, but the underlying factors contributing to this variability are unknown.

METHODS

We investigated single nucleotide polymorphisms in three genes previously associated with dyslexia and implicated in neuronal migration (DYX1C1, DCDC2, KIAA0319) and white matter volume in a cohort of 76 children and young adults from the general population.

RESULTS

We found that all three genes contained polymorphisms that were significantly associated with white matter volume in the left temporo-parietal region and that white matter volume influenced reading ability.

CONCLUSIONS

The identified region contained white matter pathways connecting the middle temporal gyrus with the inferior parietal lobe. The finding links previous neuroimaging and genetic results and proposes a mechanism underlying variability in reading ability in both normal and impaired readers.

Association of the DYX1C1 dyslexia susceptibility gene with orthography in the Chinese population

Several independent studies have supported the association of DYX1C1 with dyslexia, but its role in general reading development remains unclear. Here, we investigated the contribution of this gene to reading, with a focus on orthographic skills, in a sample of 284 unrelated Chinese children aged 5 to 11 years who were participating in the Chinese Longitudinal Study of Reading Development. We tested this association using a quantitative approach for Chinese character reading, Chinese character dictation, orthographic judgment, and visual skills. Significant or marginally significant associations were observed at the marker rs11629841 with children's orthographic judgments at ages 7 and 8 years (all P values<0.020). Significant associations with Chinese character dictation (all P values<0.013) were also observed for this single-nucleotide polymorphism (SNP) at ages 9, 10, and 11 years. Further analyses revealed that the association with orthographic skills was specific to the processing of specific components of characters (P values<0.046). No association was found at either SNP of rs3743205 or rs57809907. Our findings suggest that DYX1C1 influences reading development in the general Chinese population and supports a universal effect of this gene.

The rs3743205 SNP is important for the regulation of the dyslexia candidate gene DYX1C1 by estrogen receptor β and DNA methylation

Estrogen is involved in numerous physiological processes such as growth, differentiation, and function of the male and female reproductive tissues. In the developing brain, estrogen signaling has been linked to cognitive functions, such as learning and memory; however, the molecular mechanisms underlying this phenomenon are poorly understood. We have previously shown a link between developmental dyslexia and estrogen signaling, when we studied the functional interactions between the dyslexia candidate protein DYX1C1 and the estrogen receptors α (ERα) and β (ERβ). Here, we investigate the 17β-estradiol (E2)-dependent regulation of dyslexia susceptibility 1 candidate 1 (DYX1C1) expression. We demonstrate that ERβ, not ERα, binds to a transcriptionally active cis-regulatory region upstream of DYX1C1 transcriptional start site and that DYX1C1 expression is enhanced by E2 in a neuroblastoma cell line. This regulation is dependent on transcription factor II-I and liganded ERβ recruitment to this region. In addition, we describe that a single nucleotide polymorphism previously shown to be associated with dyslexia and located in the cis-regulatory region of DYX1C1 may alter the epigenetic and endocrine regulation of this gene. Our data provide important molecular insights into the relationship between developmental dyslexia susceptibility and estrogen signaling.

Approach to epigenetic analysis in language disorders

Language and learning disorders such as reading disability and language impairment are recognized to be subject to substantial genetic influences, but few causal mutations have been identified in the coding regions of candidate genes. Association analyses of single nucleotide polymorphisms have suggested the involvement of regulatory regions of these genes, and a few mutations affecting gene expression levels have been identified, indicating that the quantity rather than the quality of the gene product may be most relevant for these disorders. In addition, several of the candidate genes appear to be involved in neuronal migration, confirming the importance of early developmental processes. Accordingly, alterations in epigenetic processes such as DNA methylation and histone modification are likely to be important in the causes of language and learning disorders based on their functions in gene regulation. Epigenetic processes direct the differentiation of cells in early development when neurological pathways are set down, and mutations in genes involved in epigenetic regulation are known to cause cognitive disorders in humans. Epigenetic processes also regulate the changes in gene expression in response to learning, and alterations in histone modification are associated with learning and memory deficits in animals. Genetic defects in histone modification have been reversed in animals through therapeutic interventions resulting in rescue of these deficits, making it particularly important to investigate their potential contribution to learning disorders in humans.

Dissection of genetic associations with language-related traits in population-based cohorts

Recent advances in the field of language-related disorders have led to the identification of candidate genes for specific language impairment (SLI) and dyslexia. Replication studies have been conducted in independent samples including population-based cohorts, which can be characterised for a large number of relevant cognitive measures. The availability of a wide range of phenotypes allows us to not only identify the most suitable traits for replication of genetic association but also to refine the associated cognitive trait. In addition, it is possible to test for pleiotropic effects across multiple phenotypes which could explain the extensive comorbidity observed across SLI, dyslexia and other neurodevelopmental disorders. The availability of genome-wide genotype data for such cohorts will facilitate this kind of analysis but important issues, such as multiple test corrections, have to be taken into account considering that small effect sizes are expected to underlie such associations.

DCDC2, KIAA0319 and CMIP are associated with reading-related traits

BACKGROUND

Several susceptibility genes have been proposed for dyslexia (reading disability; RD) and specific language impairment (SLI). RD and SLI show comorbidity, but it is unclear whether a common genetic component is shared.

METHODS

We have investigated whether candidate genes for RD and SLI affect specific cognitive traits or have broad effect on cognition. We have analyzed common risk variants within RD (MRPL19/C2ORF3, KIAA0319, and DCDC2) and language impairment (CMIP and ATP2C2) candidate loci in the Avon Longitudinal Study of Parents and Children cohort (n = 3725), representing children born in southwest England in the early 1990s.

RESULTS

We detected associations between reading skills and KIAA0319, DCDC2, and CMIP. We show that DCDC2 is specifically associated with RD, whereas variants in CMIP and KIAA0319 are associated with reading skills across the ability range. The strongest associations were restricted to single-word reading and spelling measures, suggesting that these genes do not extend their effect to other reading and language-related skills. Inclusion of individuals with comorbidity tends to strengthen these associations. Our data do not support MRPL19/C2ORF3 as a locus involved in reading abilities nor CMIP/ATP2C2 as genes regulating language skills.

CONCLUSIONS

We provide further support for the role of KIAA0319 and DCDC2 in contributing to reading abilities and novel evidence that the language-disorder candidate gene CMIP is also implicated in reading processes. Additionally, we present novel data to evaluate the prevalence and comorbidity of RD and SLI, and we recommend not excluding individuals with comorbid RD and SLI when designing genetic association studies for RD.

Association of the rs3743205 variant of DYX1C1 with dyslexia in Chinese children

Background

Dyslexia is a learning disability that is characterized by difficulties in the acquisition of reading and spelling skills independent of intelligence, motivation or schooling. Studies of western populations have suggested that DYX1C1 is a candidate gene for dyslexia. In view of the different languages used in Caucasian and Chinese populations, it is therefore worthwhile to investigate whether there is an association of DYX1C1 in Chinese children with dyslexia.

Method and Results

Eight single nucleotide polymorphisms (SNPs) were genotyped from three hundred and ninety three individuals from 131 Chinese families with two which have been reported in the literature and six tag SNPs at DYX1C1. Analysis for allelic and haplotypic associations was performed with the UNPHASED program and multiple testing was corrected using false discovery rates. We replicated the previously reported association of rs3743205 in Chinese children with dyslexia (p_corrected = 0.0072). This SNP was also associated with rapid naming, phonological memory and orthographic skills in quantitative trait analysis.

Conclusion

Our findings suggest that DYX1C1 is associated with dyslexia in people of Chinese ethnicity in Hong Kong.

A theoretical molecular network for dyslexia: integrating available genetic findings

Developmental dyslexia is a common specific childhood learning disorder with a strong heritable component. Previous studies using different genetic approaches have identified several genetic loci and candidate genes for dyslexia. In this article, we have integrated the current knowledge on 14 dyslexia candidate genes suggested by cytogenetic findings, linkage and association studies. We found that 10 of the 14 dyslexia candidate genes (ROBO1, KIAA0319, KIAA0319L, S100B, DOCK4, FMR1, DIP2A, GTF2I, DYX1C1 and DCDC2) fit into a theoretical molecular network involved in neuronal migration and neurite outgrowth. Based on this, we also propose three novel dyslexia candidate genes (SLIT2, HMGB1 and VAPA) from known linkage regions, and we discuss the possible involvement of genes emerging from the two reported genome-wide association studies for reading impairment-related phenotypes in the identified network.

Molecular genetics and molecular biology of dyslexia

Developmental dyslexia has been recognized as a distinct entity among learning disabilities as early as the late 1800s and its partially genetic nature has been firmly established by family and twin studies. The application of genetic mapping and molecular cloning methods has revealed specific genes that contribute to the genetic risk, but those known now do not yet suffice for explaining all of it. More importantly, the first genes, some of them found by the study of rare families, have indicated specific neurodevelopmental processes important for the development of dyslexia, including control of neuronal migration for the DYX1C1, DCDC2, and KIAA0319 genes, and a role of axonal and dendritic guidance suggested by the ROBO1 gene. I anticipate that forthcoming research within only a few years will yield molecular networks with fundamental roles in the molecular biology of dyslexia, and may aid in resolving relationships between comorbid disorders. WIREs Cogni Sci 2011 2 441-448 DOI: 10.1002/wcs.138 For further resources related to this article, please visit the WIREs website.

In search of the perfect phenotype: an analysis of linkage and association studies of reading and reading-related processes

Reading ability and specific reading disability (SRD) are complex traits involving several cognitive processes and are shaped by a complex interplay of genetic and environmental forces. Linkage studies of these traits have identified several susceptibility loci. Association studies have gone further in detecting candidate genes that might underlie these signals. These results have been obtained in samples of mainly European ancestry, which vary in their languages, inclusion criteria, and phenotype assessments. Such phenotypic heterogeneity across samples makes understanding the relationship between reading (dis)ability and reading-related processes and the genetic factors difficult; in addition, it may negatively influence attempts at replication. In moving forward, the identification of preferable phenotypes for future sample collection may improve the replicability of findings. This review of all published linkage and association results from the past 15 years was conducted to determine if certain phenotypes produce more replicable and consistent results than others.

Genetic variance in a component of the language acquisition device: ROBO1 polymorphisms associated with phonological buffer deficits

The region containing ROBO1 (Chromosome 3p12.3) has been implicated as a susceptibility gene for reading disorder and language deficit by translocation and linkage data. No association studies have yet been reported supporting any candidate gene. Here we report the first association of this gene with language deficits, specifically with phonological buffer deficits (a phenotype implicated in language acquisition, Specific Language Impairment and Speech Sound Disorder) and dyslexia (reading and spelling ability traits) in an unselected sample of adolescent twins and their siblings. Family-based analyses were performed on 144 tag SNPs in ROBO1, typed in 538 families with up to five offspring and tested for association with a developmental marker of language impairment (phonological buffer capacity, assessed using non word repetition). A reading and spelling ability measure--based on validated measures of lexical processing (irregular word) and grapheme-phoneme decoding (pseudo word)--and measures of short-term and working memory were also analysed. Significant association for phonological buffer capacity was observed for 21 of 144 SNPs tested, peaking at 8.70 × 10(-05) and 9.30 × 10(-05) for SNPs rs6803202 and rs4535189 respectively for nonword repetition, values that survive correction for multiple testing. Twenty-two SNPs showed significant associations for verbal storage (forward digit span)--a trait linked to phonological span. By contrast, just 5 SNPs reached nominal significance for working-memory, not surviving correction, and, importantly, only one SNP in the 144 tested reached nominal significance (0.04) for association with reading and spelling ability. These results provide strong support for ROBO1 as a gene involved in a core trait underpinning language acquisition, with a specific function in supporting a short-term buffer for arbitrary phonological strings. These effects of ROBO1 appear to be unrelated to brain mechanisms underpinning reading ability, at least by adolescence. While replication will be critical, the present results strongly support ROBO1 as the first gene discovered to be associated with language deficits affecting normal variation in language ability. Its functional role in neuronal migration underlying bilateral symmetry and lateralization of neuronal function further suggests a role in the evolution of human language ability.

Investigation of dyslexia and SLI risk variants in reading- and language-impaired subjects

Dyslexia (or reading disability) and specific language impairment (or SLI) are common childhood disorders that show considerable co-morbidity and diagnostic overlaps and have been suggested to share some genetic aetiology. Recently, genetic risk variants have been identified for SLI and dyslexia enabling the direct evaluation of possible shared genetic influences between these disorders. In this study we investigate the role of variants in these genes (namely MRPL19/C20RF3, ROBO1, DCDC2, KIAA0319, DYX1C1, CNTNAP2, ATP2C2 and CMIP) in the aetiology of SLI and dyslexia. We perform case-control and quantitative association analyses using measures of oral and written language skills in samples of SLI and dyslexic families and cases. We replicate association between KIAA0319 and DCDC2 and dyslexia and provide evidence to support a role for KIAA0319 in oral language ability. In addition, we find association between reading-related measures and variants in CNTNAP2 and CMIP in the SLI families.

Investigation of dyslexia and SLI risk variants in reading- and language-impaired subjects

Dyslexia (or reading disability) and specific language impairment (or SLI) are common childhood disorders that show considerable co-morbidity and diagnostic overlaps and have been suggested to share some genetic aetiology. Recently, genetic risk variants have been identified for SLI and dyslexia enabling the direct evaluation of possible shared genetic influences between these disorders. In this study we investigate the role of variants in these genes (namely MRPL19/C20RF3, ROBO1, DCDC2, KIAA0319, DYX1C1, CNTNAP2, ATP2C2 and CMIP) in the aetiology of SLI and dyslexia. We perform case-control and quantitative association analyses using measures of oral and written language skills in samples of SLI and dyslexic families and cases. We replicate association between KIAA0319 and DCDC2 and dyslexia and provide evidence to support a role for KIAA0319 in oral language ability. In addition, we find association between reading-related measures and variants in CNTNAP2 and CMIP in the SLI families.

Dyslexia and DYX1C1: deficits in reading and spelling associated with a missense mutation

The status of DYX1C1 (C15q21.3) as a susceptibility gene for dyslexia is unclear. We report the association of this gene with reading and spelling ability in a sample of adolescent twins and their siblings. Family-based association analyses were carried out on 13 single-nucleotide polymorphisms (SNPs) in DYX1C1, typed in 790 families with up to 5 offspring and tested on 6 validated measures of lexical processing (irregular word) and grapheme-phoneme decoding (pseudo-word) reading- and spelling-based measures of dyslexia, as well as a short-term memory measure. Significant association was observed at the misssense mutation rs17819126 for all reading measures and for spelling of lexical processing words, and at rs3743204 for both irregular and nonword reading. Verbal short-term memory was associated with rs685935. Support for association was not found at rs3743205 and rs61761345 as previously reported by Taipale et al., but these SNPs had very low (0.002 for rs3743205) minor allele frequencies in this sample. These results suggest that DYX1C1 influences reading and spelling ability with additional effects on short-term information storage or rehearsal. Missense mutation rs17819126 is a potential functional basis for the association of DYX1C1 with dyslexia.

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.

Understanding the complex etiologies of developmental disorders: behavioral and molecular genetic approaches

OBJECTIVE

This article has 2 primary goals. First, a brief tutorial on behavioral and molecular genetic methods is provided for readers without extensive training in these areas. To illustrate the application of these approaches to developmental disorders, etiologically informative studies of reading disability (RD), math disability (MD), and attention-deficit hyperactivity disorder (ADHD) are then reviewed. Implications of the results for these specific disorders and for developmental disabilities as a whole are discussed, and novel directions for future research are highlighted.

METHOD

Previous family and twin studies of RD, MD, and ADHD are reviewed systematically, and the extensive molecular genetic literatures on each disorder are summarized. To illustrate 4 novel extensions of these etiologically informative approaches, new data are presented from the Colorado Learning Disabilities Research Center, an ongoing twin study of the etiology of RD, ADHD, MD, and related disorders.

CONCLUSIONS

RD, MD, and ADHD are familial and heritable, and co-occur more frequently than expected by chance. Molecular genetic studies suggest that all 3 disorders have complex etiologies, with multiple genetic and environmental risk factors each contributing to overall risk for each disorder. Neuropsychological analyses indicate that the 3 disorders are each associated with multiple neuropsychological weaknesses, and initial evidence suggests that comorbidity between the 3 disorders is due to common genetic risk factors that lead to slow processing speed.

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.

Further evidence for DYX1C1 as a susceptibility factor for dyslexia

OBJECTIVE

Dyslexia-susceptibility-1-candidate-1 (DYX1C1) was the first gene associated with dyslexia. Since the original report of 2003, eight replication attempts have been published reporting discordant results. As the dyslexia community still considers the role of DYX1C1 unsettled, we explored the contribution of this gene in a sample of 366 trios of German descent.

METHODS

To the common four markers used in previous studies, we added two new single nucleotide polymorphisms found by resequencing both the putative regulatory and coding region of the gene in randomly selected cases and controls. As linkage disequilibrium blocks of the region were not easy to define, we approached the association problem by running a transmission disequilibrium test over sliding windows of dimension 1 to 6 on consecutive markers. The significance of this test was calculated generating the empirical distribution of the global P value by simulating the data. As our study sample had a large female proband content, we also stratified our analysis by sex.

RESULTS

We found statistically significant association with global corrected P value of 0.036. The three-marker haplotype G/G/G spanning rs3743205/rs3743204/rs600753 was most associated with a P value of 0.006 and odds ratio 3.7 (95% confidence interval: 1.4-9.6) in female probands. A detailed haplotype-phenotype analysis revealed that the dyslexia subphenotype short-term memory contributed mainly to the observed findings. This is in accordance with a recent short-term memory-DYX1C1 association in an independent sample of dyslexia.

CONCLUSION

As significant association was proved in our sample, we could also conclude that denser maps, sex information, and well-defined subphenotypes are crucial to correctly determine the contribution of DYX1C1 to dyslexia.

Progress towards a cellular neurobiology of reading disability

Reading Disability (RD) is a significant impairment in reading accuracy, speed and/or comprehension despite adequate intelligence and educational opportunity. RD affects 5-12% of readers, has a well-established genetic risk, and is of unknown neurobiological cause or causes. In this review we discuss recent findings that revealed neuroanatomic anomalies in RD, studies that identified 3 candidate genes (KIAA0319, DYX1C1, and DCDC2), and compelling evidence that potentially link the function of candidate genes to the neuroanatomic anomalies. A hypothesis has emerged in which impaired neuronal migration is a cellular neurobiological antecedent to RD. We critically evaluate the evidence for this hypothesis, highlight missing evidence, and outline future research efforts that will be required to develop a more complete cellular neurobiology of RD.

Association of ADHD and the Protogenin gene in the chromosome 15q21.3 reading disabilities linkage region

Twin studies indicate genetic overlap between symptoms of attention deficit hyperactivity disorder (ADHD) and reading disabilities (RD), and linkage studies identify several chromosomal regions possibly containing common susceptibility genes, including the 15q region. Based on a translocation finding and association to two specific alleles, the candidate gene, DYX1C1, has been proposed as the susceptibility gene for RD in 15q. Previously, we tested markers in DYX1C1 for association with ADHD. Although we identified association for haplotypes across the gene, we were unable to replicate the association to the specific alleles reported. Thus, the risk alleles for ADHD are yet to be identified. The susceptibility alleles may be in a remote regulatory element, or DYX1C1 may not be the risk gene. To continue study of 15q, we tested a coding region change in DYX1C1, followed by markers across the gene Protogenin (PRTG) in 253 ADHD nuclear families. PRTG was chosen based on its location and because it is closely related to DCC and Neogenin, two genes known to guide migratory cells and axons during development. The markers in DYX1C1 were not associated to ADHD when analyzed individually; however, six markers in PRTG showed significant association with ADHD as a categorical trait (P = 0.025-0.005). Haplotypes in both genes showed evidence for association. We identified association with ADHD symptoms measured as quantitative traits in PRTG, but no evidence for association with two key components of reading, word identification and decoding was observed. These findings, while preliminary, identify association of ADHD to a gene that potentially plays a role in cell migration and axon growth.

The human lexinome: genes of language and reading

Within the human genome, genetic mapping studies have identified 10 regions of different chromosomes, known as DYX loci, in genetic linkage with dyslexia, and two, known as SLI loci, in genetic linkage with Specific Language Impairment (SLI). Further genetic studies have identified four dyslexia genes within the DYX loci: DYX1C1 on 15q, KIAA0319 and DCDC2 on 6p22, and ROBO1 on 13q. FOXP2 on 7q has been implicated in the development of Speech-Language Disorder. No genes for Specific Language Impairment have yet been identified within the two SLI loci. Functional studies have shown that all four dyslexia genes play roles in brain development, and ongoing molecular studies are attempting to elucidate how these genes exert their effects at a subcellular level. Taken together, these genes and loci likely represent only a fraction of the human lexinome, a term we introduce here to refer to the collection of all the genetic and protein elements involved in the development of human language, expression, and reading.

LEARNING OUTCOMES

The reader will become familiar with (i) methods for identifying genes for complex diseases, (ii) the application of these methods in the elucidation of genes underlying disorders of language and reading, and (iii) the cellular pathways through which polymorphisms in these genes may contribute to the development of the disorders.

[Phonological awareness, working memory, reading and writing performances in familial dyslexia]

BACKGROUND

familial dyslexia.

AIM

to characterize and compare the phonological awareness, working memory, reading and writing abilities of individuals whose family members are also affected.

METHOD

in this study 10 familial nuclei of natural family relationship of individuals with dyslexia were analyzed. Families of natural individuals living in the west region of the state of São Paulo were selected. Inclusion criteria were: to be a native speaker of the Brazilian Portuguese language, to have 8 years of age or more, to present positive familial history for learning disabilities, that is, to present at least one relative with difficulties in learning. Exclusion criteria were: to present any neurological disorder genetically caused or not, in any of the family members, such as dystonia, extra pyramidal diseases, mental disorder, epilepsy, attention deficit and hyperactivity disorder (ADHA); psychiatric symptoms or conditions; or any other pertinent conditions that could cause errors in the diagnosis. As for the diagnosis of developmental dyslexia, information about the familial history of the adolescents and children was gathered with the parents, so that a detailed pedigree could be delineated. Neurological, psychological, speech-language, and school performance evaluations were made with the individuals and their families.

RESULTS

the results of this study suggest that the dyslexic individuals and their respective relatives, also with dyslexia, presented lower performances than the control group in terms of rapid automatic naming, reading, writing and phonological awareness.

CONCLUSION

deficits in phonological awareness, working memory, reading and writing seem to have genetic susceptibility that possibly determine, when in interaction with the environment, the manifestation of dyslexia.

From human genetics and genomics to pharmacogenetics and pharmacogenomics: past lessons, future directions

A brief history of human genetics and genomics is provided, comparing recent progress in those fields with that in pharmacogenetics and pharmacogenomics, which are subsets of genetics and genomics, respectively. Sequencing of the entire human genome, the mapping of common haplotypes of single-nucleotide polymorphisms (SNPs), and cost-effective genotyping technologies leading to genome-wide association (GWA) studies - have combined convincingly in the past several years to demonstrate the requirements needed to separate true associations from the plethora of false positives. While research in human genetics has moved from monogenic to oligogenic to complex diseases, its pharmacogenetics branch has followed, usually a few years behind. The continuous discoveries, even today, of new surprises about our genome cause us to question reviews declaring that "personalized medicine is almost here" or that "individualized drug therapy will soon be a reality." As summarized herein, numerous reasons exist to show that an "unequivocal genotype" or even an "unequivocal phenotype" is virtually impossible to achieve in current limited-size studies of human populations. This problem (of insufficiently stringent criteria) leads to a decrease in statistical power and, consequently, equivocal interpretation of most genotype-phenotype association studies. It remains unclear whether personalized medicine or individualized drug therapy will ever be achievable by means of DNA testing alone.

Gene × Environment interactions in speech sound disorder predict language and preliteracy outcomes

Few studies have investigated the role of gene × environment interactions (G × E) in speech, language, and literacy disorders. Currently, there are two theoretical models, the diathesis–stress model and the bioecological model, that make opposite predictions about the expected direction of G × E, because environmental risk factors may either strengthen or weaken the effect of genes on phenotypes. The purpose of the current study was to test for G × E at two speech sound disorder and reading disability linkage peaks using a sib-pair linkage design and continuous measures of socioeconomic status, home language/literacy environment, and number of ear infections. The interactions were tested using composite speech, language, and preliteracy phenotypes and previously identified linkage peaks on 6p22 and 15q21. Results showed five G × E at both the 6p22 and 15q21 locations across several phenotypes and environmental measures. Four of the five interactions were consistent with the bioecological model of G × E. Each of these four interactions involved environmental measures of the home language/literacy environment. The only interaction that was consistent with the diathesis–stress model was one involving the number of ear infections as the environmental risk variable. The direction of these interactions and possible interpretations are explored in the discussion.

A review of association and linkage studies for genetical analyses of learning disorders

Learning disorders (LD) commonly comprise of a heterogeneous group of disorders manifested by unexpected problems in some children's experiences in the academic performance arena. These problems especially comprise of a variety of disorders which may be subclassified to attention-deficit hyperactivity disorder (ADHD), reading disability (RD), specific language impairment (SLI), speech-sound disorder (SSD), and dyspraxia. The aim of this review is to summarize the current molecular studies and some of the most exciting recent developments in molecular genetic research on LD. The findings for the association and linkage of LD with candidate genes will help to set the research agendas for future studies to follow.

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.

Neuropsychology and genetics of speech, language, and literacy disorders

The authors review the neuropsychology, brain bases, and genetics of three related disorders of language development: reading disability, or developmental dyslexia (RD); language impairment (LI); and speech sound disorder (SSD). Over the past three decades, cognitive analysis has demonstrated that the reading difficulties of most children who have RD result from phonologic impairments (difficulties processing the sound structure of language). Although understanding of LI and SSD is somewhat less developed, both disorders are also associated with phonologic impairments, which may account for their comorbidity with RD. Research across levels of analysis is progressing rapidly to promote understanding not only of each disorder by itself but also of the relationships of the three disorders to each other.

Genetics of dyslexia: the evolving landscape

Dyslexia is among the most common neurodevelopmental disorders, with a prevalence of 5-12%. At the phenotypic level, various cognitive components that enable reading and spelling and that are disturbed in affected individuals can be distinguished. Depending on the phenotype dimension investigated, inherited factors are estimated to account for up to 80%. Linkage findings in dyslexia are relatively consistent across studies in comparison to findings for other neuropsychiatric disorders. This is particularly true for chromosome regions 1p34-p36, 6p21-p22, 15q21 and 18q11. Four candidate genes have recently been identified through systematic linkage disequilibrium studies in linkage region 6p21-p22, and through cloning approaches at chromosomal breakpoints. Results indicate that a disturbance in neuronal migration is a pathological correlate of dyslexia at the functional level. This review presents a summary of the latest insights into the genetics of dyslexia and an overview of anticipated future developments.

Association of short-term memory with a variant within DYX1C1 in developmental dyslexia

A substantial genetic contribution in the etiology of developmental dyslexia (DD) has been well documented with independent groups reporting a susceptibility locus on chromosome 15q. After the identification of the DYX1C1 gene as a potential candidate for DD, several independent association studies reported controversial results. We performed a family-based association study to determine whether the DYX1C1 single nucleotide polymorphisms (SNPs) that have been associated with DD before, that is SNPs '-3GA' and '1249GT', influence a broader phenotypic definition of DD. A significant linkage disequilibrium was observed with 'Single Letter Backward Span' (SLBS) in both single-marker and haplotype analyses. These results provide further support to the association between DD and DYX1C1 and it suggests that the linkage disequilibrium with DYX1C1 is more saliently explained in Italian dyslexics by short-term memory, as measured by 'SLBS', than by the categorical diagnosis of DD or other related phenotypes.

Disruption of neuronal migration by RNAi of Dyx1c1 results in neocortical and hippocampal malformations

The brains of individuals with developmental dyslexia have neocortical neuronal migration abnormalities including molecular layer heterotopias, laminar dysplasias, and periventricular nodular heterotopias (PNH). RNA interference (RNAi) of Dyx1c1, a candidate dyslexia susceptibility gene, disrupts neuronal migration in developing embryonic neocortex. Using in utero electroporation, we cotransfected cells in the rat neocortical ventricular zone (VZ) at E14/15 with short hairpin RNA vectors targeting Dyx1c1 along with either plasmids encoding enhanced green fluorescent protein or plasmids encoding monomeric red fluorescent protein only. RNAi of Dyx1c1 resulted in pockets of unmigrated neurons resembling PNH. The pattern of migration of transfected neurons was bimodal, with approximately 20% of the neurons migrating a short distance from the VZ and another 40% that migrated past their expected lamina. Approximately 25% of the transfected brains had hippocampal pyramidal cell migration anomalies. Molecular layer ectopias, which were not related to injection site artifacts, were also seen in 25% of the animals. These results support the hypothesis that targeted disruption of the candidate dyslexia susceptibility gene, Dyx1c1, results in neuronal migration disorders similar to those seen in the brains of dyslexics.

Genes, environment, and dyslexia. The 2005 Norman Geschwind Memorial Lecture

This article presents an overview of some methods and results from our continuing studies of genetic and environmental influences on dyslexia, and on individual differences across the normal range that have been conducted over the past 25 years in the Colorado Learning Disabilities Research Center (CLDRC) and in related projects. CLDRC investigators compare the similarities of identical twin pairs who share all their genes and fraternal twins who share half their segregating genes to assess the balance of genetic, shared family environment, and nonshared environment influences on dyslexia and on individual differences across the normal range. We have learned that among the children we have studied in Colorado, group deficits in reading (dyslexia) and individual differences in reading across the normal range are primarily due to genetic influences, and these genetic influences are often shared with some of the same genetic influences on deficits and individual differences in language and ADHD. We have also learned from our molecular-genetic linkage studies that there are regions on several chromosomes likely to contain genes that influence dyslexia. Several specific genes within these regions have been tentatively identified through molecular-genetic association analyses, but much more research is needed to understand the pathways among specific genes, regions of noncoding DNA that regulate the activity of those genes, the brain, and dyslexia. I conclude with a discussion of our research on individual differences in early reading development, on the role of early learning constraints in dyslexia, and on how genetic influences are expressed through their interaction and correlation with the environment.

Developmental dyslexia: an update

Dyslexia is the most common and carefully studied of the learning disabilities in school-age children. It is characterized by a marked impairment in the development of reading skills, and affects a large number of people (5-10%). Reading difficulties may also arise from poor vision, emotional problems, decreased hearing ability, and behavioral disorders, such as attention-deficit hyperactivity (ADHD). Although many areas of the brain are involved in reading, analysis of postmortem brain specimens by a variety of imaging techniques most consistently suggests that deficiency within a specific component of the language system - the phonologic module - in the temporo-parietal-occipital brain region underlies dyslexia. It is a highly familial and heritable disorder with susceptibility loci on chromosomes 1, 2, 3, 6, 11, 13, 15 and 18. Recently, four candidate genes (KIAA 0319, DYX1C1, DCDC2 and ROBO1) are shown to be associated with dyslexia. Although some of these results are controversial because of the genetic heterogeneity of the disorder, the available evidence suggests that dyslexia could be due to the abnormal migration and maturation of neurons during early development. Interestingly, in spite of genetic heterogeneity, the pathology appears to involve common phonological coding deficits. The condition can be managed by a highly structured educational training exercise.

Zur Genetik der Lese-Rechtschreibschwäche

Zusammenfassung: Die Lese-Rechtschreibstörung (LRS) ist eine der häufigsten Entwicklungsstörungen. Die Ursachen dieser komplexen Störung sind bisher nur kaum verstanden. Familienuntersuchungen zeigen, dass die LRS familiär gehäuft auftritt und dass das Risiko für ein Geschwisterkind, eine LRS zu entwickeln, ca. 3,5fach erhöht ist. Verschiedene kognitive Fähigkeiten sind mit der LRS korreliert. Hierzu gehören die phonologische Bewusstheit, orthographisches Wissen, phonologisches Dekodieren, auditives Kurzzeitgedächtnis und schnelles Benennen. Eine familiäre Häufung dieser mit der LRS korrelierten Dimensionen und eine hohe Erblichkeit (Heritabilität) wurden wiederholt gefunden. Die Heritabilität für die Lesefähigkeit liegt zwischen 50-60%, für die Rechtschreibstörung zwischen 50 und 70%. Durch genomweite Kopplungsuntersuchungen wurden bisher 9 Kandidatengenregionen (DYX1-9) identifiziert. Vier Kandidatengene, DCDC2, KIAA0319, ROBO1 und DYX1C1 wurden kürzlich beschrieben. Diese beeinflussen die neuronale Migration und sind daher funktionell aussichtsreiche Kandidatengene für die LRS. Allerdings konnte bisher keine funktionell relevante Mutation gefunden werden. Die Komorbidität zwischen LRS und ADHD sowie LRS und Sprachentwicklungsstörungen könnte zum Teil durch gemeinsame genetische Faktoren erklärt werden. In der Zukunft wird es für die Ursachenforschung der LRS entscheidend sein, möglichst alle ursachenrelevanten Dimensionen gemeinsam an ausreichend großen Stichproben zu untersuchen. Neben den relevanten neurobiologischen Faktoren sollten auch Umweltfaktoren und die verschiedenen Interaktionen, wie z.B. Gen-Umwelt und Gen-Gen-Interaktionen untersucht werden. In einem europäischen, kollaborativen Forschungsvorhaben (NeuroDys) wird weltweit die größte Stichprobe von Kindern mit einer LRS gesammelt und untersucht, um durch ein verbessertes Ursachenverständnis unter Einschluss der Identifikation von genetischen Risikofaktoren die Komplexität des Störungsbildes besser zu verstehen und perspektivisch spezifische Therapien zu entwickeln.