Our study has shown that genetic variation within HTR3B is associated with AUD comorbid with ASPD in the Finnish population. Consistently with our a priori hypothesis, no significant differences emerged when controls were compared with non-ASPD alcoholics, indicating that 5-HTR3 receptor might be specifically involved in the etiology of early onset alcoholics with antisocial behavior. HTR3B markers were also significantly associated with alpha power in both the Bethesda and Plains Indians populations. Among both of the two mainly Caucasian populations (United States and Finnish), most of the signal appeared to come from an intronic HTR3B SNP, namely rs3782025. This SNP significantly predicted AUD with comorbid ASPD in the Finnish population. In the Bethesda population, the same allele was also strongly associated with low alpha power and accounted for 8% of the variance of this resting EEG trait. Significant associations between alpha power and HTR3B markers were found among Plains Indians but with different SNPs as compared to Caucasians. No significant findings were reported for HTR3A.
Our findings suggest that genetic variation within HTR3B
may influence vulnerability to develop AUD with comorbid ASPD. Consistent with this idea, the 5-HTR3R antagonist ondansetron appears to be effective in the treatment of early onset alcoholics who frequently have ASPD. In a 12-week double-blind, placebo-controlled clinical trial conducted in 321 alcohol-dependent subjects, ondansetron was effective in reducing alcohol consumption and craving in early onset alcoholics (<25 years) but not in late onset (Johnson et al., 2000
). Also, ondansetron has been shown to reduce alcohol-heightened aggressive behavior in mice (McKenzie-Quirk et al., 2005
). Genetic variation within HTR3B
has been shown to influence vulnerability to bipolar disorder (Frank et al., 2004
) and major depression (Yamada et al., 2006
), but to our knowledge this is the first time that an association between AUD with co-morbid ASPD and the HTR3B
gene has been reported. Most of the in vitro electrophysiological studies conducted so far on 5-HT3R have focused on the A subunit. However, more recently, two studies have explored the roles of both subunits and have found that ethanol enhances the activation of HTR3A homomeric receptors, but has no effect on HT3A/B hetoromeric receptors (Hayrapetyan et al., 2005
; Stevens et al., 2005
). Thus, genetic variation within HTR3B
might influence level of expression of this subunit and in turn the proportion of HTR3A homomeric Vs HTR3A/B heteromeric receptors. Alteration in the subunit composition of 5-HTR3R in the brain could in turn affect risk of developing alcoholism moderating ethanol response via this receptor that is known to regulate reward to alcohol and other drugs (Minabe et al., 1991
; Rasmussen et al., 1991
). Interestingly, a deletion within HTR3B
has been shown to moderate response to ondansetron in the treatment of nausea (Tremblay et al., 2003
). Similarly, one could speculate that genetic variation within HTR3B might moderate response to ondansetron in the treatment of alcoholism.
Alterations in brain electrical activity have been described in both alcoholics and ASPD patients (Arentsen and Sindrup, 1963
; Coger et al., 1978
; Ehlers and Phillips, 2007
; Enoch et al., 1999
; Jones and Holmes, 1976
, Lindberg et al., 2005
). In this report, HTR3B
was found to be associated with low alpha power in the resting EEG, an intermediate phenotype for AUD and ASPD (Lindberg et al., 2005
). Our finding indicates that genetic variation within HTR3B
might be a modulator of neuronal electrical activity and might contribute to the imbalance between excitation and inhibition that characterizes the brain of alcoholics. Consistently, in a whole genome linkage study recently conducted in the Plains Indians sample, an LOD score (2.2) suggestive of linkage to alpha power was found in the same chromosome region where HTR3A
genes are located (Enoch et al., submitted for publication
). 5-HT3R is an ion-channel receptor whose activation results in rapid neuronal depolarization. One of the possible mechanisms through which 5-HT3R might influence resting EEG is through the modulation of GABAergic neurons. 5-HT3R is expressed in the subpopulation of GABAergic interneurons that are located within the neocortex, hippocampus, and amygdala (Bloom and Morales, 1998
). Activation of GABAergic interneurons via 5-HT3R has been shown to result in a GABA-mediated inhibition of pyramidal cell of the rat hippocampus (Ropert and Guy, 1991
) and similar mechanisms are likely to exist in other brain regions.
Associations that we found with both ASPD and alpha power were strongest with marker rs3782025, and stronger than associations with any haplotype. However, a significant association was reported also at the haplotype level. Haplotype GAGT, located in the second HTR3B block, was less frequent among alcoholics with ASPD in the Finnish population and was associated with increased alpha power in the Bethesda population. Our results suggest that the HTR3B association signal originates from the 3′ end of the HTR3B gene, which is also upstream of the HTR3A gene. Interestingly, SNP rs3782025 is located in a region which appears to be highly conserved across species.
A significant association between HTR3B
and alpha power was also found among Plains Indians. However, the signal in this case came from different SNPs, namely rs2276307 that is in LD with rs3782025 and rs11606194 that is in haplotype block 1. Also, among Plains Indians haplotype GGAT was significantly associated with reduced alpha power. This result overall supports the idea that genetic variation within HTR3B
might be important in moderating alpha power. However, the pattern of association among Plains Indians appears to be different from that observed in the Bethesda and Finnish populations. A possible explanation for these discrepancies might be related to differences in the ascertainment of the Bethesda/Finnish samples as compared to the Plains Indian population. In the latter population, family members of alcoholics’ probands were also recruited. As a consequence, many nonalcoholics had a family history of alcoholism, and alcoholism was not associated with alpha power in this population (Enoch et al., submitted for publication
). In contrast, association between alpha power and alcoholism were found both in the Finnish (Lindberg et al., 2005
) and Bethesda (Enoch et al., 1999
) populations. Another possible explanation might be related to genetic heterogeneity; therefore, functional polymorphisms might be different across different populations.
This study has strengths as well as important limitations. Strengths include the use of three relatively large and independent samples, use of clinical phenotypes as well as intermediate phenotypes to increase power to detect gene effects (Goldman and Ducci 2007
), and use of ancestry informative markers to rule out ethnic stratification, Limitations include the lack of one large population with measures of both alpha EEG power and a sufficient number of antisocial alcoholics to perform case-control analysis. P
values presented in the present study were not corrected for multiple testing. Nevertheless, the same SNP and haplotypes were associated with two related phenotypes (ASPD and alpha power) in the same direction in two independent samples of Caucasians. Furthermore, even after applying Bonferroni correction for the number of SNPs tested (n
= 13), associations between rs3782025 with AUD + ASPD in the Finns and alpha power in the Bethesda population remain significant. Of note, applying Bonferroni correction for the number of SNPs is a very conservative approach because the SNPs tested were not independent (see ).
In conclusion, our findings indicate that genetic variation within HTR3B may influence vulnerability to alcoholism with comorbid ASPD and may contribute to the low alpha power trait detected among alcoholics as a group. Further research is needed to confirm these findings and, if confirmed, to identify a functional locus or loci in the HTR3B/HTR3A region and to explore the molecular biological mechanisms whereby 5-HT3R influences alpha power and vulnerability to disease.