Symposium 9 Monday Sept. 24th 9 am–10.30 am; Room: Lecture Hall 1
Genetic approaches to alcohol dependency: Chairpersons: Wernicke C (Germany), Anghelescu I (Germany)
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Abstract |
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Presentation S9-1
The hangover gene defines a cellular stress pathway required for ethanol tolerance in Drosophila melanogaster
Scholz H (Germany)
Alcohol dependence is a complex behavior with a clear genetic component involving multiple genes. Individuals with alcohol dependence develop ethanol tolerance, which is most simply defined as a reduction in the behavioral response to the same dose of ethanol. Very little is known about what genes and mechanisms underlie ethanol tolerance and how this behavior contributes to alcoholism. To understand the neuronal and molecular genetic basis underlying ethanol tolerance, we have developed a behavioral paradigm in Drosophila melanogaster.
To identify novel genes mediating ethanol tolerance we screened for mutants with altered tolerance. We identified 15 transposon induced mutants with altered ethanol induced behavior. We focused on the analysis of one mutant, hangover (hang), because it displayed strongly reduced tolerance. The hang mutants are also impaired in their response to oxidative stress and heat shock, suggesting a role for hangover in cellular stress-response pathways. The hangover gene encodes for a nuclear zinc finger protein that do not bind DNA and properly has a function in RNA processing.
Through detailed analyses of mutants we have determined that at least two different mechanisms contribute to tolerance, one acting at the level of a neuronal circuit to modulate brain function in response to ethanol, and the other acting at the cellular level, perhaps to protect the CNS from ethanol-induced damage (Scholz et al., 2005). It has been shown recently that changes of the transcriptional level of a putative mammalian homolog of the hangover gene can be correlated with alcohol dependence (Riley et al., 2006). Therefore Drosophila melanogaster might be a useful tool to discover new genes and mechanism underlying ethanol induced behavior in humans.
Presentation S9-2
Haplotypes of candidate genes in alcohol dependence
Wernicke C, Kraschewski A, Reese J, Anghelescu I, Rommelspacher H (Germany)
Aims. Animal studies suggest that low mesolimbic dopamine activity is associated with high ethanol consumption and neuroadaptive changes of this pathway are conceived to underlie the transition from controlled to uncontrolled use of alcohol and other drugs. Dopamine exerts its actions by activating five distinct membrane receptor subtypes, from which the dopamine receptor D2 (DRD2) is highly expressed in the CNS. Two isoforms of DRD2 exist derived from alternative splicing of exon 6 (Ginrich and Caron, 1993). The long isoform is preferentially involved in postsynaptic response, whereas the short isoform predominantly excerts presynaptic autoinhibition of dopamine synthesis and release (Mercuri et al., 1997; Khan et al., 1998; Usiello et al., 2000). It was shown, that the overexpression of DRD2 in the nucleus accumbens reduces ethanol intake in ethanol preferring rats (Thanos, 2001). These and other data indicate the DRD2 as a candidate gene influencing the vulnerability to alcohol dependence. Several association studies were published with contradictory results. Most of them dealed with the Taq 1A polymorphism, which lies more than 10 kb downstream of the gene and a deletion (Ins-141Del) in the promoter region which is thought to reduce the transcription rate. The G1385A polymorphism in the 3' UTR (exon 8) was repeatedly found to be associated with a higher anxiety/depression in alcoholics (Finckh et al., 1997; Lucht et al., 2001). Recently, a synonymou SNP in exon 7 (C957T) was reported to influence the mRNA stability (Duan et al., 2003). Furthermore, an association between this SNP and striatal DRD2 binding in vivo was shown (Hirvonen et al., 2004).
Methods. We investigated these four SNPs (Ins-141Del-C957T-G1385A-TaqA2/A1) and additionally a microsatellite located in intron 2 in about 400 alcoholics and 400 controls and in an independent family based cohort of alcoholics (trios).
Results. Our haplotype estimation revealed Ins-T-A-A2, Ins-C-G-A2, and Ins-C-A-A1 as three major haplotypes (50-13%). Minor haplotypes (3.5–2%) were Del-T-A-A2, Del-C-G-A2, and Ins-C-A-A2. Other haplotypes were not or randomly observed. We found a significant difference in the haplotype distribution between alcoholics and controls. Additionally to the earlier published association between the exon 8 polymorphism and anxiety/depression (Finckh et al., 1997), we found an interaction of the exon 7 polymorphism on this association. There is evidence, that the exon 8 polymorphism influences the mRNA stability (Finckh, oral communication). Own cell culture investigations point to a possible association of the exon 7 polymorphism and alternativ splicing of DRD2.
Conclusions. The gene of the DRD2 contributes to the vulnerability of alcohol dependence. Several polymorphisms may influence the expression of the receptor in different ways (e.g.: transcription, splicing, mRNA stability). Therefore, the investigation of haplotypes is important for our understanding of the interaction of several mechanisms influencing DRD2 availability and function.
Presentation S9-3
Polymorphisms in ADH/ALDH genes and the phenotypic correlation
Yin S-J (Taiwan)
Aims. Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the principal enzymes responsible for metabolism of ethanol. Both ADH and ALDH exhibit genetic polymorphisms among racial populations. Functional variant alleles of ADH1B*2 and ALDH2*2 have been consistently replicated to show protection against developing alcohol dependence. Functional allele ADH1C*1, which is in linkage disequilibrium with ADH1B*2, appears insignificant in protecting against the disease as evidenced by relative haplotype analyses.
Methods. In conjunction with kinetic parameters and contents of hepatic ADH1B and ADH1C allozymes, a functional window has been proposed for assessing validity of potential candidate alcoholism genes in the ADH family.
Results. Multiple logistic regression analyses suggest that ADH1B*2 and ALDH2*2 may independently influence the risk for alcoholism. Kinetic studies with recombinant normal and mutant ALDH2 and their three-dimensional structures support a partial dominance model for loss of the enzyme activity.
Conclusions. Pharmacokinetic and pharmacodynamic studies of individuals with different ALDH2 genotypes following an oral alcohol challenge suggest that protection against alcoholism by the ALDH2*2 variant gene allele can be attributed to alcohol sensitivity reactions accompanied by a prolonged and large accumulation of acetaldehyde in blood.