Results of this double-blind, placebo-controlled ADHD pharmacogenetic trial of psychostimulant-naïve school-aged children suggest DAT and DRD4 variations may be associated with unique MPH dose-response curves. Children lacking the DAT 10-repeat allele and those with the DRD4 4-repeat allele had a more robust methylphenidate response compared to alternate genotypes, consistent with an improved response for the ADHD susceptibility low risk alleles.
Our findings suggest possible MPH dose-response differences based on DAT genotype, with those lacking the 10-repeat showing improved MPH effects on hyperactive-impulsive symptoms and a trend toward improvements in inattentive scores across doses. This is consistent with a prior meta-analysis and several individual studies13–16
showing diminished rates of MPH response for 10-repeat homozygotes, although other studies have linked the 9-repeat allele to blunted response.17–19
Our findings are in line with studies demonstrating functional effects of the DAT VNTR polymorphisms after methylphenidate administration, including greater increases in basal ganglia DAT density15
and a failure to increase short interval cortical inhibition (measured via transcranial magnetic stimulation)60
for 10-repeat homozygotes compared to other groups, although it should be noted that, unlike these studies, we did not observe differences between 10-repeat homozygotes and heterozygotes.
We found that the DRD4 4-repeat allele may be associated with improved MPH response across doses compared to other VNTR lengths, congruent with observations of the 4-repeat’s enhanced receptor expression61
and increased sensitivity to dopamine62
in some studies, although other studies have not demonstrated major pharmacological differences for the DRD4 variants.63
Our findings echo Cheon et al33
and McGough et al,32
who also used teacher ratings for outcome assessment. In contrast, two additional studies found no relationship between the DRD4 4-repeat and MPH effects on ADHD symptoms, but unlike our study, neither considered teacher ratings.20,24
McGough et al studied the effects of MPH on math performance,20
and found that those lacking a 4-repeat allele showed an improved response to methylphenidate at lower doses but deterioration at higher doses. Although the DRD4 variants’ math performance MPH dose-response curves in this sample differed from our findings regarding hyperactive-impulsive symptoms, this is not unexpected given studies identifying MPH dose-response differences for learning compared to social behavior outcomes.64
It is uncertain why the DRD4 effects on MPH response that we observed were limited to the hyperactive-impulsive domain, as some ADHD etiology studies have suggested a predominant effect of this VNTR on inattentive symptoms,65
while others have suggested a preferential effect on hyperactive-impulsive symptoms.66
We observed a main effect of ADRA2A genetic variants on MPH response such that the G allele was associated with significantly higher ratings of hyperactive-impulsive symptoms on placebo and across doses. This is consistent with studies showing an association between increased ADHD symptomatology and the G allele, although prior studies found a stronger association with inattentive rather than hyperactive-impulsive ratings.67–70
ADRA2A*dose interactions did not meet our threshold for significance. Prior studies observing a significant association between the G allele and improved MPH response on inattentive symptoms36,37
or on total symptoms (but not inattentive symptoms alone, with borderline significant effects on hyperactive-impulsive symptoms)38
evaluated response to a single MPH dose and did not evaluate ADRA2A*dose interactions.36–38
COMT was not significantly associated with MPH response, although our pattern of results suggest that val homozygotes experienced greater improvements in hyperactive-impulsive symptoms with increasing doses compared to other groups (p=0.09). This pattern is consistent with 2 prior studies25,40
which found significant effects of the COMT genotypes on responder/nonresponder status25,40
or change in ADHD symptom scores after MPH treatment,25
but did not evaluate gene*dose interactions.
Our study has a number of limitations, including that we adjusted our level of significance by a factor of two (p=0.025) in recognition that we ran two analytic models (one for each outcome), but did not correct for the number of polymorphisms evaluated. Had we made Bonferroni corrections for both the number of models/outcomes and polymorphisms evaluated, the adjusted α would be 0.006. None of our gene*dose interactions met the 0.006 level of significance, although the DAT*dose effect on hyperactive-impulsive symptoms would have been borderline (p=0.008). Additionally, our sample size, while larger than three18,20,32
of the four19
prior pediatric ADHD pharmacogenetic controlled trials, is modest. Due to sample size considerations, we were unable to conduct a genome-wide association study, which can be used to identify genetic variants that have not previously been hypothesized to influence stimulant response (e.g., the glutamate receptor 7 gene).71
Although we recognize the possibility of elevated false positives when using a candidate gene approach,20
corroboration of our DRD432,33
findings in prior studies provides some reassurance that our results are not false positives. However, we cannot exclude the possibility that other polymorphisms in linkage disequilibrium with the DRD4 and/or DAT variants may be responsible for the observed effects.
Additional limitations include the restricted duration of follow-up and the heterogeneity of our sample due to recruitment from a variety of sources. Further, our sample had more inattentive (52%) than combined subtype (48%) participants. Although consistent with the subtype distribution in many epidemiologic samples,72–76
this differs from clinic settings where combined type is most common.
Our study is further limited because we did not evaluate the relationship between the catecholamine-related genes and MPH side effects despite increasing evidence of the important role that allelic variants may play in predicting methylphenidate tolerability.20,32
There is also growing recognition that genetic factors are unlikely to act in isolation, but limited sample size precluded our evaluation of gene-gene and gene-environment interactions on methylphenidate dose-response. We found effects of moderate size for the individual DRD4*dose and DAT*dose interactions, but were unable to determine the magnitude of their likely larger combined effects. As we move toward personalized ADHD treatment, pharmacogenetic studies with larger samples and a range of outcomes (i.e., efficacy and
side effects) are needed to determine the clinical utility of genomic information.