No Association Between Novelty Seeking and the Type 4 Dopamine Receptor Gene (DRD4) in Two New Zealand Samples

We studied two groups of subjects: 1) 86 subjects with a current major depressive illness of at least moderate severity who were studied in the context of a randomized clinical trial that compared the long-term efficacy of the antidepressants fluoxetine and nortriptyline and 2) 181 subjects who were members of 14 multiplex alcoholic pedigrees who were ascertained from clinical sources and studied according to a protocol designed to maximize comparability to a larger-scale set of American studies. Both studies were ethically reviewed and approved in advance, and all subjects provided written informed consent.

Psychiatric diagnoses were determined by using structured diagnostic interviews (10, 11) administered by trained raters. All subjects completed the self-report Temperament and Character Inventory (12, 13). This instrument contains essentially the same 34 items used by Ebstein et al. (5) and Malhotra et al. (8). (Benjamin et al. [6] computed novelty seeking scores from another personality measure.) To maximize comparability to the previous reports, the total novelty seeking scores reported here were summed across these 34 items.

Material from subjects in the 14 pedigrees dense with alcoholism (N=181) was in the form of lymphoblastoid cell lines generated by Epstein-Barr virus transformation of white blood cells. DNA was extracted from 2×10⁶ frozen cells of each cell line (14). A different method was used to extract DNA from peripheral blood samples of subjects from the depression treatment trial (N=86). Peripheral blood (5 ml in an EDTA or lithium-heparin tube) was mixed with 45 ml of lysis buffer (0.32 M sucrose, 10 mM Tris pH 7.5, 5 mM MgCl₂, 1% Triton X-100). Leukocytes were recovered by centrifugation and resuspended in lysis solution (4 M guanidine isothiocyanate, 25 mM sodium acetate, 0.84% β-mercaptoethanol) to release DNA. An equal volume of isopropanol was added to precipitate the DNA, which was recovered by centrifugation and washed three times in cold 70% ethanol. The DNA was then resuspended and dissolved in 10 mM Tris pH 8.0–1 mM EDTA (0.5 ml) and stored at 4°C.

For maximal comparability to previous reports (5, 6, 8), we stratified our samples by the presence or absence of the DRD4 7-repeat allele. All p values reported are two-tailed.

Because novelty seeking is negatively correlated with age and may have gender differences (13), age and gender were included as covariates in analysis of variance models. Effect sizes were calculated according to the method of Cohen (16): an effect size of 0.2 was considered a small effect and an effect size of 0.5 was considered a medium effect.

To take into account the genetic relationships in the subjects from the 14 pedigrees dense with alcoholism (N=181), we used a sibling pair approach. We constructed a data set consisting of all possible pairs of siblings (N=133) and then selected the 72 pairs discordant for the presence of the DRD4 7-repeat allele. We compared the novelty seeking scores of these discordant sibling pairs by using Student's t test. In these analyses, a given sibling may be included more than once. Such an inclusion, however, is “anticonservative” in that the p value obtained tends to be smaller than the true p value.

As shown in table 1, substantial proportions of each sample had lifetime DSM-III-R diagnoses of major depression and alcohol dependence. The mean novelty seeking total score for each sample was similar to that reported by Ebstein et al. (5). The DRD4 7-repeat allele was more prevalent in these two New Zealand samples than in the Israeli sample of Ebstein et al. (7-repeat allele frequency=0.274) (5) or in the U.S. sample of Benjamin et al. (7-repeat allele frequency=0.167) (6).

For DRD4, there was no statistically significant association in either sample between the 7-repeat allele and the behavioral trait of novelty seeking. Moreover, the effect sizes (16) we observed were considerably less than those of Ebstein et al. (effect size=0.51) (5) or Benjamin et al. (effect size=0.39) (6).The analyses in table 1 for the subjects from 14 pedigrees dense with alcoholism (N=181) consider each subject as an independent observation even though they are genetically related. To take these relationships into account, we compared the novelty seeking scores of the 72 sibling pairs discordant for the presence of the DRD4 7-repeat allele (from 133 possible sibling pairs). Again, there was no significant association between novelty seeking and the presence or absence of the DRD4 7-repeat allele (paired t test=0.569, p=0.51; mean novelty seeking score with 7-repeat allele present=16.2, SD=4.5, with 7-repeat allele absent=16.7, SD=6.0). These analyses ignore the fact that a given sibling may be included more than once in these discordant pairs. However, these analyses are “anticonservative” in that the p value obtained tends to be smaller than the true p value. Given that p=0.52 for the t test above, it is unlikely that these data contain a significant relationship between novelty seeking and the DRD4 7-repeat allele.

We also conducted several preplanned secondary analyses. 1) There were no significant associations between any of the four novelty seeking subscales and the presence or absence of the DRD4 7-repeat allele in either of the two samples (p>0.20 for all eight comparisons). 2) For the depressed sample, novelty seeking scores for each subject were available from an informant and from the subject 6 months after the initial determination of novelty seeking. The use of multiple informants and longitudinal measurements is an important way to distinguish the “stable” part of novelty seeking from the error inherent in self-reported behavioral traits (17). We used principal components analysis of these three different novelty seeking scores (i.e., from the subject initially, from the subject after 6 months, and from an informant) to extract an index of “stable” novelty seeking; the first principal component accounted for 79.2% of the variance in novelty seeking scores. Similar to the analyses reported in table 1, the presence or absence of the DRD4 7-repeat allele was not significantly associated with the stable component of novelty seeking (p=0.54, controlling for age and sex). 3) Clinical samples such as the ones in the present report are likely to be heterogeneous. It is possible that such heterogeneity could mask a true association of DRD4 with novelty seeking. When we stratified each of the two samples by the presence of certain “impulsive” traits (i.e., the presence of DSM-III-R alcohol dependence, bulimia nervosa, childhood conduct disorder, and adult antisocial personality disorder), there were again no significant associations between novelty seeking and the DRD4 7-repeat allele.