Strong genetic evidence of DCDC2 as a susceptibility gene for dyslexia

Abstract

We searched for linkage disequilibrium (LD) in 137 triads with dyslexia, using markers that span the most-replicated dyslexia susceptibility region on 6p21-p22, and found association between the disease and markers within the VMP/DCDC2/KAAG1 locus. Detailed refinement of the LD region, involving sequencing and genotyping of additional markers, showed significant association within DCDC2 in single-marker and haplotype analyses. The association appeared to be strongest in severely affected patients. In a second step, the study was extended to include an independent sample of 239 triads with dyslexia, in which the association--in particular, with the severe phenotype of dyslexia--was confirmed. Our expression data showed that DCDC2, which contains a doublecortin homology domain that is possibly involved in cortical neuron migration, is expressed in the fetal and adult CNS, which--together with the hypothesized protein function--is in accordance with findings in dyslexic patients with abnormal neuronal migration and maturation.

Figures

Figure 1

Genomic, transcript, and LD maps for the DYX2 locus. A, The two DYX2 gene clusters: the distal cluster with VMP/DCDC2/KAAG1 and the more-proximal, with KIAA0319/TTRAP/THEM2. B and C, Genomic structure of the DCDC2 gene implicated by association study. Transcripts produced by alternative initiation and splicing from DCDC2. Gray boxes denote untranslated exons. D, LD between markers genotyped in this study. The distal and proximal gene cluster are not in LD, consistent with their different levels of association to dyslexia. Also, VMP and DCDC2 show only moderate LD.

Figure 2

Expression of DCDC2. A, Expression analysis of DCDC2 “long” by RT-PCR of human tissue samples with use of primers DCDC2-2F and DCDC2-2R located in exons 5 and 9, respectively. PCR products of 577 bp were separated by electrophoresis on agarose gels and were visualized by UV and ethidium-bromide staining. B, Expression analysis of DCDC2 “short” by RT-PCR of human tissue samples with use of primers DCDC2-smallF and DCDC2-probeR located in the 5′ extension of exon 9 and in exon 11, respectively, which generate an 844-bp PCR product. C, Northern-blot analysis of the DCDC2 transcript of human brain tissues, which indicate an expression of an ∼2-kb transcript in most brain tissues. D, Northern-blot analysis of DCDC2 of human fetal tissues, which shows strong expression of high molecular transcripts in fetal kidney.

Figure 3

Expression analysis of KAAG1 by RT-PCR of human tissue samples with use of primers KAAG1-F2 and KAAG1-R2 located 3′ and 5′ of DCDC2 exon 2, respectively. PCR products of 837 bp were separated by electrophoresis on agarose gels and were visualized by UV and ethidium-bromide staining.