Association analysis of two candidate phospholipase genes that map to the chromosome 15q15.1-15.3 region associated with reading disability

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.

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.

Dysbindin (DTNBP1) and the biogenesis of lysosome-related organelles complex 1 (BLOC-1): main and epistatic gene effects are potential contributors to schizophrenia susceptibility

BACKGROUND

The DTNBP1 gene, encoding dysbindin, has been strongly implicated in schizophrenia (SZ) susceptibility by a series of independent genetic association and gene expression studies. Among its known functions, dysbindin is part of a protein complex, termed the biogenesis of lysosome-related organelles complex 1 (BLOC-1), the molecular components of which might be involved in the regulation of vesicular trafficking and dendrite branching.

METHODS

A systematic investigation of the other seven BLOC-1 genes (MUTED, PLDN, CNO, SNAPAP, BLOC1S1, BLOC1S2, and BLOC1S3) for evidence of association with SZ was undertaken in a sample of 373 SZ cases and 812 control subjects. Possible epistasis between combinations of BLOC-1 genes, including DTNBP1, was tested with a novel method of investigating for gene-gene interaction. Quality control measures were incorporated into genotyping strategy, and all results were corrected for multiple testing to prevent false positive results.

RESULTS

We identified significant evidence of association between BLOC1S3 and SZ (odds ratio = 1.45, confidence interval = 1.13-1.86, p = .0028, corrected p = .0389). We also report evidence for epistatic interaction between DTNBP1 and MUTED contributing to SZ in the absence of a significant main effect at MUTED (p = .0009, corrected p = .0252). Single marker and epistasis results remained significant after correction for multiple testing.

CONCLUSIONS

Together these data provide evidence for the involvement of the BLOC-1 protein complex in SZ pathogenesis.

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.

The genetics of developmental dyslexia

Reading reflects the complex integration of several cognitive processes and proves more difficult to achieve for a significant proportion of the population. Developmental dyslexia (DD), or specific reading disability, is influenced by genes, a fact that has led several research groups to attempt to identify susceptibility genes through the sequential analysis of genetic linkage and association. Strong evidence has now emerged for the presence of genes influencing DD at several chromosomal loci and for at least one of these, there is evidence implicating specific genes. In this review, we present the evidence for a genetic contribution to DD and its component processes and review the current status of molecular genetic research aimed at identifying susceptibility genes for this common, complex disorder.

No evidence for association between dyslexia and DYX1C1 functional variants in a group of children and adolescents from Southern Italy

Recently, DYX1C1, a candidate gene for developmental dyslexia, encoding a nuclear tetratricopeptide repeat domain protein dynamically regulated in brain, has been characterized through a translocation breakpoint in a Finnish family. Two putatively functional variants, -3G/A and 1249G/T, have been reported to be significantly associated with dyslexia in this population. Further studies, conducted on different ethnic groups (English and Canadian), have not confirmed a role for DYX1C1 variants in increasing the risk for dyslexia. We investigated the role of these variants in dyslexic children and adolescents from Southern Italy. No significant evidence for association between dyslexia and these DYX1C1 putative functional variants has been shown. We argue that the different DYX1C1 allele frequencies shown among Italian and Finnish subjects with dyslexia could be attributable to the different linkage disequilibrium structure of these populations.