| Summary |
Attention-deficit hyperactivity disorder (ADHD) affects ~3%-5% of children in the United States. In the current psychiatric nomenclature, ADHD comprises three sub-types: inattentive, hyperactive-impulsive, and combined. In this study, they used four analytic strategies to examine the association and linkage of the dopamine transporter gene (DAT1) and ADHD. The sample included 122 children referred to psychiatric clinics for behavioral and learning problems that included but were not limited to ADHD, as well as their parents and siblings. Within-family analyses of linkage disequilibrium, using the transmission disequilibrium test (TDT), confirmed the 480-bp allele as the high-risk allele. In between-family association analyses, levels of hyperactive-impulsive symptoms but not inattentive symptoms were related to the number of DAT1 high-risk alleles. Siblings discordant for the number of DAT1 high-risk alleles differed markedly in their levels of both hyperactive-impulsive and inattentive symptoms, such that the sibling with the higher number of high-risk alleles had much higher symptom levels. Within-family analyses of linkage disequilibrium, using the TDT, suggested association and linkage of ADHD with DAT1 and that this relation was especially strong with the combined but not the inattentive subtype. The relation of DAT1 to ADHD increased monotonically, from low to medium to high levels of symptom severity. These results replicate and extend previous findings of the association between the DAT1 gene and childhood ADHD. This represents one of the first replicated relations of a candidate gene and a psychiatric disorder in children. |
| Total Sample |
The sample consisted of 122 families in which there was a child (i.e., a proband) who had been assessed and/or treated for attention-deficit disorders, related behavioral disorders, and/or learning problems at a specialty clinic or by psychiatrists in private practice. The siblings in 41 of these families also participated in the study, as did available and cooperating parents in all of the families. The total sample for analyses contained 117 families of clinic-referred children (by combining the Tucson and Atlanta sites). All but one of the probands met the criteria for an ADHD diagnostic subtype based on our questionnaire, and the remaining proband met criteria for ODD. About two-thirds (68%) of the probands were of full Caucasian ethnicity, with 12% being of full African USAn ethnicity, 4% of full Hispanic ethnicity, and the remaining 16% of mixed ethnicity. The proband and sibling samples were very similar in age, sex, and ethnicity. |
| Sample Collection |
Data collection in this study was conducted at two research sites: Tucson, Arizona, and Atlanta, Georgia. The study protocol was approved by the institutional review boards of both the University of Arizona and Emory University, and appropriate informed consent was obtained for all subjects. At the Tucson site, boys who had ADHD and/or a related disorder were identified through psychiatrists in private practice. Whenever possible, a brother of the clinic-referred child was also sampled. At the Atlanta site, clinic-referred male and female children (i.e., probands) were sampled through the Center for Learning and Attention Deficit Disorders at the Emory University School of Medicine. At this site, brothers and sisters of the probands were also sampled whenever possible. |
| Diagnosis Description |
Probands and their siblings were rated by their parents on the Emory Diagnostic Rating Scale to assess symptoms of the major DSM-IV (USAn Psychiatric Association 1994) childhood psychiatric disorders. These include symptoms of the disruptive behavior disorders (i.e., CD, ODD, and the inattention and hyperactivity-impulsivity symptom dimensions of ADHD), as well as symptoms of internalizing disorders. Each symptom of these disorders was translated into a rating-scale item on which children were rated by their parents on a 0-4 scale, with 0 meaning not at all characteristic of their child and 4 meaning very much characteristic of their child. In this study, scores on the hyperactive-impulsive and inattentive symptom dimensions of ADHD were analyzed. The 0-4 scores for each symptom were summed for each of the items making up the hyperactive-impulsive and inattentive symptom dimensions, yielding symptom-scale scores for each proband and sibling. For purposes of comparison, similar symptom-scale scores were created for ODD, CD, and depression/dysthymia. |
| Technique |
DNA collection, extraction, and amplification of the DAT1 locus were performed by use of previously published procedures (Vandenbergh et al. 1992; Rowe et al. 1998). Buccal cells were collected. The DAT1 locus was amplified on an MJ PTC100 thermal cycler (MJ Research) in a two-step protocol with an initial 1 min denaturing step at 94oC, followed by 28 cycles of 10 s at 94oC and 30 s at 74oC and a final extension of 2 min at 72oC using the primers 5'-TGT GGT GTA GGG AAC GGC CTG AG-3' and 5'-CTT CCT GGA GGT CAC GGC TCA AGG-3' (Vandenbergh et al. 1992). Genotypes were determined from pictures of UV-illuminated stained gels by at least two researchers. Ambiguous or unidentifiable results were reamplified and rescored, as were a random sample of 5% of the probands. Samples that continued to amplify poorly were eliminated from the study population. |
| Analysis Method |
They examined the association between DAT1 and ADHD using both between-family and within-family analytic methods. The between-family association methods consisted of examination of the relation between the number of high-risk alleles (i.e., 0, 1, or 2) and the levels of ADHD symptoms. They relied on within-family analyses using the TDT as a primary source of evidence regarding the association and linkage of DAT1 and ADHD. They also analyzed data from two-parent families only, omitting the TDT families with data from only one parent, to avoid biases incurred by inclusion of such families (Curtis and Sham 1995). The conventional TDT is a 1-df McNemar X2 test, as follows: X2=(b-c)2/(b+c), where b is the number of times the high-risk allele was transmitted and c is the number of times the low-risk allele was transmitted. |
| Result Description |
Between-Family Association of DAT1 and ADHD Symptoms: The number of DAT1 high-risk alleles was significantly related to the number of hyperactive-impulsive symptoms but not to the number of inattentive symptoms. The numbert of DAT1 high-risk alleles explained 3.6% of the variance in hyperactive-impulsive symptoms and 1.1% of the variance in inattentive symptoms. Controlling for ethnicity had virtually no impact on the results. Within-Family Analyses of DAT1 and ADHD Symptoms, Using Genetically Discordant Siblings: In 10 of the 12 sibling pairs, the sibling with the greater number of DAT1 high-risk alleles had higher scores on both symptom dimensions than the sibling with fewer DAT1 high-risk alleles. These genetically discordant sibling pairs differed markedly in levels of hyperactive-impulsive symptoms and inattentive symptoms. Within-Family Analyses of Linkage Disequilibrium between DAT1 and ADHD, Using the TDT: the results suggested linkage disequilibrium between DAT1 and any ADHD diagnosis at each level of symptom severity, with the degree of linkage disequilibrium becoming stronger as levels of symptom severity increased. Specifically, the TDT for the low level of symptom severity fell just short of significance at the 0.05 level, whereas the TDTs for medium and high levels of symptom severity both were significant. The odds of transmission of a high-risk versus a low-risk allele increased monotonically with increasing symptom severity level. Within-Family Analyses of the Genetic Model for DAT1 and ADHD: Neither the results for recessive nor for dominant transmission were as strong as the corresponding TDTs. In addition, results of these analyses for any ADHD diagnosis and for the combined subtype were somewhat inconsistent. There was no evidence for either recessive or dominant transmission for the inattentive subtype, consistent with the TDT results. |
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