Genetic etiology of reading difficulties in boys and girls

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.

The utility of chromosomal microarray analysis in developmental and behavioral pediatrics

Chromosomal microarray analysis (CMA) has emerged as a powerful new tool to identify genomic abnormalities associated with a wide range of developmental disabilities including congenital malformations, cognitive impairment, and behavioral abnormalities. CMA includes array comparative genomic hybridization (CGH) and single nucleotide polymorphism (SNP) arrays, both of which are useful for detection of genomic copy number variants (CNV) such as microdeletions and microduplications. The frequency of disease-causing CNVs is highest (20%-25%) in children with moderate to severe intellectual disability accompanied by malformations or dysmorphic features. Disease-causing CNVs are found in 5%-10% of cases of autism, being more frequent in severe phenotypes. CMA has replaced Giemsa-banded karyotype as the first-tier test for genetic evaluation of children with developmental and behavioral disabilities.

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).

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.

Genetic and environmental influences on word recognition and spelling deficits as a function of age

Previous twin studies have suggested a possible developmental dissociation between genetic influences on word recognition and spelling deficits, wherein genetic influence declined across age for word recognition, and increased for spelling recognition. The present study included two measures of word recognition (timed, untimed) and two measures of spelling (recognition, production) in younger and older twins. The heritability estimates for the two word recognition measures were .65 (timed) and .64 (untimed) in the younger group and .65 and .58 respectively in the older group. For spelling, the corresponding estimates were .57 (recognition) and .51 (production) in the younger group and .65 and .67 in the older group. Although these age group differences were not significant, the pattern of decline in heritability across age for reading and increase for spelling conformed to that predicted by the developmental dissociation hypothesis. However, the tests for an interaction between genetic influences on word recognition and spelling deficits as a function of age were not significant.