Summations
• Dopamine is a chemical produced by the brain that acts as a neurotransmitter.
• Alcohol consumption increases dopamine release in the nucleus accumbens from ventral tegmental neurons.
• Alcohol activates dopamine system and contributes to an individual's susceptibility to alcohol dependence by damaging the key components of the ‘brain reward system’.
Considerations
• Alcohol dependence is a result of interaction between environmental and genetic factors altering multiple brain pathways.
• Several studies have also linked alcohol dependence to polymorphisms in certain genes that are exclusively involved in the dopamine pathway.
Introduction
Alcohol dependence is one of the major elements that significantly influence drinking pattern that provokes the alcohol-induced organ damage. Alcohol dependence is a complex and multi-factorial disorder arising from interactions between genetic and environmental risk factors (Reference Li, Yin, Crabb, O'connor and Ramchandani1). There are several independent lines of evidence for genetic contributions to vulnerability to alcohol dependence (Reference Edenberg and Kranzler2). Alcohol dependence leads to the development of an array of undesirable physical, mental, and social effects. Although the prevalence of alcohol dependence differs significantly between cultures, as well as between the two sexes within the same culture, the vulnerability to alcohol dependence corresponds partly to interindividual differences in pharmacokinetics and the pharmacodynamic response to alcohol (Reference Oroszi and Goldman3). Our knowledge about alcohol's actions in the brain is very little. Unlike other drugs such as cocaine, marijuana, and heroin that interact with specific cellular proteins, alcohol is a relatively simple compound that has the potential to interact with many such proteins in the brain and show broad biological and pharmacological actions (Reference Al-Housseini, Sivanandam, Bradbury, Tannenberg, Dodd and Gu4). The susceptibility to alcohol dependence among individuals is mainly attributed to the variations in genes that encode these proteins.
Evidence for genetic influences on alcohol dependence
Evidence from twin, adoption and family studies show that alcohol dependence is genetically influenced, with a heritability of 50–60% (Reference Kendler, Prescott, Neale and Pedersen5). Using standard alcohol dependence diagnostic criteria in both male and female twins, the Australian twin registry found a consistent heritability of 64% (Reference Heath, Bucholz and Madden6). By considering the intersection samples between the Swedish twin registry and Swedish temperance board registrations, the calculated prevalence was similar in monozygotic (MZ) and dizygotic (DZ) twins, but the concordance rate for such registration was significantly higher in MZ twins (Reference Kendler, Prescott, Neale and Pedersen5). As alcohol dependence co-occurs with other externalising disorders such as conduct disorder, antisocial personality disorder, and drug dependence, this common liability showed a heritability of 80% (Reference Hicks, Krueger, Iacono, Mcgue and Patrick7). Hence, alcohol dependence shares genetic vulnerability with nicotine dependence and the genes that influence risk for nicotine dependence may also increase risk for alcohol dependence and vice versa (Reference Dick, Agrawal and Wang8).
Dopaminergic reward system
Mesolimbic dopamine has been shown to play a crucial role in mediating the reinforcing effects of alcohol. Dopaminergic neurons are present chiefly in the ventral tegmental area (VTA) of the midbrain and projected into the nucleus accumbens in the ventral striatum, and also innervate the amygdala, bed nucleus of stria terminalis (BNST), lateral septal area, and lateral hypothalamus. The VTA is in close proximity to the substantia nigra, another dopamine rich nucleus. VTA dopaminergic projections also influence the prefrontal cortex. In turn, the prefrontal cortex can provide feedback to the VTA either directly or through the nucleus accumbens. Magnetic resonance imaging scans revealed structural and neurophysiological abnormalities in the frontal lobes of chronic alcoholics (Reference Moselhy, Georgiou and Kahn9). Alcohol may inhibit the action of enzyme monoamine oxidase, which is responsible for the breakdown of dopamine in the synaptic cleft (Reference Tabakoff, Lee, De Leon-Jones and Hoffman10). As the dopamine is not efficiently brokendown, its action persists on the post-synaptic neuron for a longer period than usual and exhibits increased pleasure. Individuals desire to continue to experience the pleasure caused by dopamine, and therefore continue to drink alcohol and finally become addicts. Furthermore, addiction to alcohol may alter the regular production of dopamine, and hence discontinuation of alcohol consumption after addiction may trigger withdrawal symptoms (Reference Robbins and Everitt11). As dopamine is considered as a pleasure molecule, any reduction in its levels leads to reward deficiency, which results in aberrant substance-seeking behaviour (Reference Comings and Blum12). It is well established that the synthesis, storage, and catabolism of dopamine is under the tight genetic control. These candidate genes involved in dopaminergic pathway are of immense interest to the researchers engaged in the study of a wide range of addictive disorders.
Dopaminergic system candidate genes
In brief, the dopaminergic pathway includes dopamine synthesis, release, receptor activation, re-uptake and catabolism. Various candidate genes involved in dopaminergic neurotransmission have been characterised and they have identified polymorphisms, including those for tyrosine hydroxylase (TH), dopamine transporter (DAT1 or SLC6A3), dopamine receptors (DRD1-DRD5), dopamine β-hydroxylase (DBH), sulfotransferase 1A3 (SULT1A3) catechol-O-methyltransferase (COMT), and monoamine oxidase A and B (MAOA and MAOB) (Fig. 1). The simple molecule, dopamine, has been at the centre of interest for both the scientists and the clinicians alike for many years, as it is associated with a number of diseases such as Parkinson's disease, schizophrenia, psychoses, drug abuse, hyperprolactinemia, and reproductive disturbances. From the broad literature, this review summarises specific polymorphisms of dopaminergic genes that are involved in alcohol dependence.
Schematic representation of dopamine synthesis and breakdown pathways. DA, dopamine; TH, tyrosine hydroxylase; DBH, dopamine β-hydroxylase; DAT, dopamine transporter; MAOA, monoaminoxidase type A; MAOB, monoaminoxidase type B; COMT, catechol-O-methyltransferase; SULT1A3, sulfotransferase 1A3; DRD1–DRD5, dopamine receptor D1–D5.
Dopamine synthesis
The initial step involves hydroxylation of tyrosine to dihydroxyphenylalanine (DOPA), catalysed by the enzyme TH. Once formed, DOPA is rapidly decarboxylated to dopamine by aromatic l-amino-acid decarboxylase. Through the entire pathway, tyrosine hydroxylation is a rate limiting reaction and thus controls the rate of dopamine synthesis (Reference Jonsson, Sedvall and Brene13). Human TH cDNA encompasses 1921 bp coding for a protein of 528 amino acids and a molecular mass of 58.5 kDa. The human TH gene consists of 14 exons and various isoforms resulting from alternative splicing of pre-mRNA (Reference Kobayashi, Kaneda and Ichinose14). TH is mainly expressed in brain and adrenal gland. A penta-allelic tetranucleotide (TCAT)n repeat polymorphism of the first intron and three other non-synonymous SNPs (Val81Met, Leu205Pro, and Val468Met) are known to alter the functional activity of TH protein (Reference Wei, Ramchand and Hemmings15). Tetranucleotide repeat polymorphism exhibits five different alleles known as K1, K2, K3, K4, and K5 with a size of 260, 256, 252, 248, and 244 bp, respectively (Reference Polymeropoulos, Xiao, Rath and Merril16). The presence of atleast one copy of the K1 allele had shown reduced homovanillic acid (HVA) levels in the cerebrospinal fluid of healthy individuals (Reference Jonsson, Sedvall and Brene13). Furthermore, allelic variations of the tetranucleotide repeats were found to be involved in quantitative and qualitative changes in the binding ZNF191, a zinc finger protein, and these repeats may regulate transcription (Reference Albanese, Biguet, Kiefer, Bayard, Mallet and Meloni17).
Analysis of penta-allelic polymorphism at the TH gene in Scandinavian chronic alcoholics and control subjects did not show any significant association with alcohol dependence (Reference Geijer, Jonsson and Neiman18). Systematic screening of all the exons, the exon–intron boundaries, and the 5′ promoter region of the TH gene in Japanese also failed to indicate a positive association between TH gene and alcohol dependence (Reference Ishiguro, Arinami and Saito19). In a small proportion of alcohol-dependent Germans subjects, the rare allele containing 10-repeats (A10) of a polymorphic tetranucleotide motif mediated the vulnerability to alcohol withdrawal delirium (Reference Sander, Harms, Rommelspacher, Hoehe and Schmidt20).
Dopamine receptors
In the central nervous system (CNS) of vertebrates, dopamine receptors are a class of metabotropic G protein-coupled receptors that play a key role in controlling motivation, learning, and fine motor movement. For dopamine receptors, the neurotransmitter dopamine is the primary endogenous ligand. Five subtypes of dopamine receptor have been so far identified and cloned. The D1 and D5 receptors are closely related; coupled with Gs alpha, they stimulate adenylyl cyclase activity. In contrast, the D2, D3, and D4 receptors couple with Gi alpha and inhibit the formation of cAMP. Impaired dopaminergic neurotransmission in CNS has been implicated in multiple neuropsychiatric disorders. Polymorphisms of each of the dopamine receptor genes (DRD1, DRD2, DRD3, DRD4, and DRD5) have been identified and examined in association studies of various conditions influenced by dopaminergic neurotransmission.
DRD1
The gene coding for DRD1 is located at chromosome 5q35.1 and contains two exons separated by a small intron in the 5′-untranslated region (UTR). Four polymorphic sites of DRD1 were initially identified by single-strand conformational analysis (SSCA). The DdeI polymorphism is an A–G transition in the 5′-UTR (A-48G), one of the four polymorphisms initially identified by SSCA of DRD1 (Reference Cichon, Nothen, Erdmann and Propping21). The other three are BstN1 (G-94A) in the 5′-UTR, rare and synonymous PvuI (G1263A) for codon 421, and Bsp1286I (T1403C; rs686) in the 3′-UTR. Although several other polymorphisms around DRD1 were identified later, one resulted in a change of an amino-acid residue. Systematic screening of the 5′-flanking regulatory region of DRD1 gene did not reveal any polymorphism that influenced the transcriptional activity (Reference Cichon, Nothen and Stober22). By using an in vitro reporter assay, a recent study demonstrated that the allele variants of polymorphism rs686, located in 3′-UTR of DRD1, produced different expression levels of the chimeric luciferase gene, indicating rs686 to be a functional polymorphism that affected the expression of DRD1 (Reference Huang, Ma, Payne, Beuten, Dupont and Li23). A population-based association study failed to show any association between Bsp1286I RFLP of the DRD1 gene and alcohol dependence (Reference Sander, Harms and Podschus24). Alcoholics did not exhibit any significant difference in apomorphine-induced growth hormone release among different genotypes and treatment responses between different alleles of the Bsp1286I RFLP at the DRD1 locus (Reference Heinz, Sander and Harms25). In a relatively homogenous genetic population of male Finns, no association was found between alcohol dependence and the D1 receptor gene EcoRI restriction fragment length polymorphism alleles (Reference Hietala, Pohjalainen, Heikkila-Kallio, West, Salaspuro and Syvalahti26). The non-polymorphic structure of the DRD1 gene among the Tourette's syndrome and the alcohol-dependent populations suggests that coding region mutations of the DRD1 gene are unlikely to contribute to the inheritance of these disorders (Reference Thompson, Comings, Feder, George and O'dowd27). Analysis of DRD1 DdeI genotypes and the Zuckerman scores, exhibited a significant association only in men, indicating a male-limited association between the DRD1 gene polymorphism and sensation-seeking score in alcohol-dependent subjects (Reference Limosin, Loze, Rouillon, Ades and Gorwood28). The same polymorphism has not shown any significant association in both the male and the female subjects analysed (Reference Szczepankiewicz, Dmitrzak-Weglarz and Skibinska29). Furthermore, this polymorphism has shown a significant association with severity and temperament of alcohol dependence in Koreans (Reference Kim, Park and Yoon30). Analysis of DRD1 gene rs686 and rs4532 polymorphisms revealed that the rs686T allele and rs686*T-rs4532*G haplotype was more precisely associated with alcohol dependence (Reference Batel, Houchi, Daoust, Ramoz, Naassila and Gorwood31).
DRD2
Animal and human studies on addiction indicate the critical role played by the D2 dopamine receptor (DRD2) in the mechanism of reward and reinforcement behaviour. Any genetic polymorphism that affects the expression of D2 dopamine receptors may be a possible candidate for susceptibility to alcohol dependence (Reference Vandenbergh, Thompson and Cook32). The DRD2 gene is localised on human chromosome 11 at q22–q23, extends over 270 kb, and has eight exons (Reference Grandy, Litt and Allen33). This transmembrane receptor is a member of the rhodopsin family. To date, the D2 dopamine receptor (DRD2) gene has been among the stronger candidate genes implicated in alcohol dependence and other substance-use disorders. A search for functional variants in the coding region (exons 2–8) of DRD2 has identified three infrequent variants that altered the amino-acid sequence of the receptor (Ser311Cys, Val96Ala, and Pro310Ser) (Reference Cravchik, Sibley and Gejman34,Reference Gejman, Ram and Gelernter35). Two polymorphisms were identified, one in the promoter region where the DRD2 gene was further identified as deletion allele −141C Ins/Del, by in vitro functional assays (Reference Arinami, Gao, Hamaguchi and Toru36). Furthermore, the intronic region of the DRD2 contained three restriction-site polymorphisms (RSPs), TaqIA, TaqIB, and TaqID, and two dinucleotide short tandem repeat polymorphisms (STRPs), CA, and CT repeats (Reference Kidd, Morar and Castiglione37,Reference Bhaskar, Thangaraj, Non, Singh and Rao38).
The bulk of research published to date has used the Taq1 allele system, Ser311Cys polymorphism, and −141C Ins/Del polymorphism. The ‘A1’ allele of Taq1A in DRD2 gene has been associated with alcohol dependence (Reference Blum, Noble and Sheridan39). Since then, several studies have been published and an increase of ‘A1’ frequency in alcoholics has been reported in more severe forms of alcohol dependence (Reference Berggren, Fahlke and Aronsson40). Eriksson et al. (Reference Eriksson, Berggren, Blennow, Fahlke, Mansson and Balldin41) reported that alcoholics with ‘A1’ allele have low-platelet monoamino oxidase-B activity than those with A2 allele. The ‘A1’ allele is associated with high-dopamine transporter density in detoxified alcoholics (Reference Laine, Ahonen, Rasanen and Tiihonen42). Moreover, it should be noted that this variant of the DRD2 gene has functional significance, as subjects carrying the DRD2 Taq1 ‘A1’ allele had a significantly decreased dopamine D2 receptor B max in the basal ganglia (Reference Morganti, Sweeney, Albanese, Burak, Hosea and Connolly43). Eriksson et al. (Reference Eriksson, Berggren, Blennow, Fahlke, Mansson and Balldin41) reported that alcoholics with the A allele have lower platelet monoamino oxidase-B activity than those with the ‘A2’ allele. The ‘A1’ allele is associated with high-dopamine transporter density in detoxified alcoholics (Reference Laine, Ahonen, Rasanen, Pohjalainen, Tiihonen and Hietala44). The association between DRD2 TaqIA allelic variation and the alcohol dependence remains controversial (Reference Dick, Agrawal and Wang8). In fact, DRD2 TaqIA polymorphism is not located in the DRD2 gene, but rather within a protein-coding region, of the adjacent ANKK1 gene that is located downstream of DRD2 gene (Reference Neville, Johnstone and Walton45).
DRD3
DRD3 is localised in limbic and cortical brain areas and probably affects locomotion as well as emotional and cognitive processes (Reference Schwartz, Diaz, Pilon and Sokoloff46). The gene encoding DRD3 was mapped to chromosome 3q13.3 and its polymorphic site in exon 1 with a serine 9 to glycine substitution has been postulated as a useful investigating tool in psychiatric disorder (Reference Rietschel, Nothen and Lannfelt47). Multiple studies have also been conducted to assess the role of the Ser9Gly polymorphism in alcohol dependence (Reference Sander, Harms and Podschus24,Reference Gorwood, Limosin, Batel, Duaux, Gouya and Ades48–Reference Thome, Weijers and Wiesbeck52). This supported possible involvement of the DRD3 gene polymorphism in the development of addiction to alcohol.
DRD4
The human D4 receptor gene has been localised to chromosome 11 (11q15.5). The third exon of DRD4 contains an unusually polymorphical 48 bp variable number of tandem repeats (VNTR) (16 amino acids), and physiologic differences in ligand binding have been observed between the short (two to five repeats; four repeats is most common) and long (6–10 repeats; seven repeats is most common) forms of this repeat. The short alleles (two to five VNTR) frequencies showed significant differences between controls and alcoholics with delirium tremens (DT) or alcohol seizures. Furthermore, higher frequency of short alleles among individuals with an early age of onset of alcohol dependence was observed (Reference Grzywacz, Kucharska-Mazur and Samochowiec53). A recent study has demonstrated the genotypes without the seven-repeat allele to be the risk factors for alcohol dependence in Mexican Americans (Reference Du, Yang, Yeh and Wan54). Other expressed polymorphisms in DRD4 have also been known to occur, some of which have been examined in association studies, including a 12 bp duplication polymorphism and a 13 bp deletion, both located in exon 1 (Reference Chang and Kidd55). Analysis of dopamine D4 receptor gene (DRD4) using six polymorphisms, both separately and as haplotypes in Taiwanese populations did not show any association with alcohol dependence (Reference Chang, Ko, Lu, Pakstis and Kidd56).
DRD5
The human D5 receptor gene has been localised to chromosome 4 (4p15.1–p15.3). A common, silent polymorphism (Pro-326) has been reported (Reference Sommer, Sobell and Heston57). Five polymorphisms coding for amino acid changes have recently been identified for the D5 receptor; one of these, a rare nonsense change predicted to result in a 50% diminution of functional protein (Reference Sobell, Lind and Sigurdson58). A polymorphic dinucleotide repeat in the DRD5 promoter has also been identified, but is not believed to be of functional significance (Reference Beischlag, Nam, Ulpian, Seeman and Niznik59). A few of DRD5 gene polymorphisms were identified at Leu88Phe, Ala269Val, Pro330Gln, Asn351Asp, Ser453Cys, and Cys335Stop, which altered the protein configuration by amino-acid substitutions. The two amino-acid substitutions in the transmembrane domains had an effect on agonist binding to the human D5 dopamine receptor. Leu88Phe resulted in a small increase in dopamine-binding affinity. Asn351Asp resulted in an ∼10-fold decrease in dopamine and threefold decrease in R(+)-SKF-38393 binding affinities (Reference Cravchik and Gejman60). However, no study has been conducted to ensure the association of these polymorphisms with alcohol dependence.
Dopamine reuptake
Dopamine transporter (DAT1) mediates the active re-uptake of DA from the synapses and recycles DA back into the presynaptic terminal, thus terminating synaptic firing (Reference Giros and Caron61,Reference Schultz62). The gene encoding DAT1 (gene symbol SLC6A3) consists of 15 exons spanning 60 kb on chromosome 5p15.32 [MIM*126455] (Reference Vandenbergh, Persico and Hawkins63). A polymorphic 40 bp VNTR with repeats ranging from three to 13 copies was observed in the 3′ untranslated region of the DAT gene (Reference Vandenbergh, Persico and Hawkins63). The functional effects of different DAT VNTR genotypes investigated intensely in experimental settings yielded conflicting results. Individuals with 10/10 genotype exhibited a 13% decrease in the efficiency of binding to the transporter (Reference Jacobsen, Staley and Zoghbi64). However, the in vitro and in situ studies showed that the nine-repeat sequence can enhance transcription (Reference Michelhaugh, Fiskerstrand, Lovejoy, Bannon and Quinn65). Furthermore, 10-repeat allele exhibited increased gene expression compared with the seven or nine-repeat alleles (Reference Fuke, Suo, Takahashi, Koike, Sasagawa and Ishiura66). In contrast, nine-repeat allele showed increased transcription relative to the 10-copy allele (Reference Miller and Madras67). Population differences in the DAT1 allele frequency distribution patterns have been described (Reference Bhaskar, Thangaraj and Mulligan68).
DAT1 VNTR allele and genotype frequencies did not show significant differences between alcoholics and control subjects in five different ethnic groups of Taiwan (Reference Chen, Chen and Huang69). A population-based association study showed increased number of allele A of DAT1 G2319A in homozygous carriers of the 10-repeat allele of DAT1 with severe alcohol withdrawal symptoms (AWS) and DT in Germans (Reference Wernicke, Smolka, Gallinat, Winterer, Schmidt and Rommelspacher70). Several studies showed that an increased number of nine-repeat allele in alcoholics with either DT or AWS compared with healthy controls (Reference Gorwood, Limosin, Batel, Hamon, Ades and Boni71,Reference Le Strat, Ramoz and Pickering72). Furthermore, nine-repeat allele is associated with sex-specific vulnerability to serious alcohol-related problems and its withdrawal complications in alcoholics (Reference Limosin, Loze and Boni73,Reference Vaske, Beaver, Wright, Boisvert and Schnupp74). However, in western Europeans, the allele nine-repeat allele is strongly associated with alcohol dependence but not with withdrawal symptoms or daily alcohol intake (Reference Kohnke, Batra and Kolb75). In contrast, no association between DAT VNTR and alcohol dependence was observed in a series of studies (Reference Preuss, Zill, Koller, Bondy and Sokya76–Reference Anghelescu, Klawe and Singer79). The A9 allele of the DAT1 VNTR was significantly associated with alcohol dependence in Badaga population but not in Kota population of India, which indicated that these associations are population specific (Reference Bhaskar, Thangaraj, Wasnik, Singh and Raghavendra Rao80).
Dopamine catabolism
In all primates the major end product of dopamine metabolism is HVA. Enzymes responsible for metabolism of dopamine to HVA are COMT, MAOA, and MAOB. An alternative metabolic pathway for dopamine leads to the formation of norepinephrine; the dopamine β-hydroxylase (DBH) enzyme catalyses this conversion. Furthermore, a supergene family of enzymes called sulfotransferases (SULTs) is involved in the catabolism of dopamine. Human SULT1A3 has been shown to be the major sulfotransferase that sulfonates dopamine.
COMT
COMT is one of the enzymes involved in the degradation of dopamine, norepinephrine, and epinephrine via a transmethylation reaction using S-adenosyl-methionine as coenzyme, and it exists in a soluble and membrane-bound form (Reference Mannisto and Kaakkola81). The gene encoding COMT protein is located on chromosome 22q11.21–q11.23 (Reference Grossman, Emanuel and Budarf82). In humans, the COMT gene consists of six exons that encode 1.3 and 1.5 kb transcripts [membrane bound (MB) and soluble (S) COMT] (Reference Tenhunen, Salminen, Lundstrom, Kiviluoto, Savolainen and Ulmanen83). A common G–A transition in the fourth exon of the COMT gene (rs4680), results in a valine to methionine substitution at codon 158 of the MB form (Val108Met in the S form) (Reference Lachman, Papolos, Saito, Yu, Szumlanski and Weinshilboum84). The allele encoding methionine is associated with low enzyme activity and thermolability compared with the valine allele (Reference Lachman, Papolos, Saito, Yu, Szumlanski and Weinshilboum84), and exhibits tissue-specific expression (Reference Zhu, Lipsky and Xu85). This polymorphism did not show any significant difference in the allelic distributions in Japanese alcoholics and controls indicating that the COMT gene is unlikely to play a significant role in alcohol dependence (Reference Ishiguro, Haruo Shibuya, Toru, Saito and Arinami86). Analysis of 5 COMT loci including Val158Met in community-ascertained American Indians showed that both male and female alcoholics were more likely to have at least one Val158 allele compared with non-alcoholics (Reference Enoch, Waheed, Harris, Albaugh and Goldman87). High-activity COMT allele is associated with alcohol dependence in the Czech male but not in female population indicating the sex-specific differences in the influence of COMT Val158Met on alcohol dependence (Reference Sery, Didden, Mikes, Pitelova, Znojil and Zvolsky88). Family-based and case–control study-based analysis using 100 Polish families with alcohol dependence and 196 ethnically and gender matched healthy controls failed to show statistically significant differences in allele transmission or association between COMT Val158met and alcohol dependence (Reference Samochowiec, Kucharska-Mazur and Grzywacz89). Analysis of 18 SNPs in 219 multiplex alcohol-dependent families of European American descent from the Collaborative Study on the Genetics of Alcoholism did not support the association between alcohol dependence and variations in COMT (Reference Foroud, Wetherill and Dick90). Analysis of COMT Val158Met in 122 alcoholics and 150 controls of polish origin did not show significant association with alcohol dependence when the whole group of alcoholics and the controls were compared. Similarly, there were no differences between either the Lesch type I or type II alcoholics and the control subjects (Reference Samochowiec, Kucharska-Mazur and Grzywacz77). Gene polymorphisms in several genes that tested as predictors of relapse revealed that the met allele of the COMT Val158Met polymorphism is more likely to relapse than in patients without this allele (Reference Wojnar, Brower and Strobbe91). Screening of COMT Val158Met in 395 male alcoholics, and 326 healthy male controls of Russians showed that the frequency of Met allele was significantly higher in alcoholics with positive family history compared with the controls suggesting that the functional Val158Met COMT polymorphism is one of the significant markers of genetic predisposition to alcohol dependence (Reference Kibitov, Voskoboeva, Brodianskii, Chuprova and Smirnova92).
MAOA and MAOB
Two different forms of monoamine oxidases, known as MAOA and MAOB, are found bound to the outer membrane of mitochondria in most cell types in the body. These two forms differ slightly in their localisation and their ability to metabolise dopamine and related biogenic amines. The genes encoding MAO-A and MAO-B are located side-by-side on the short arm of the X chromosome (Xp11.3). MAOA and MAOB genes span at least 60 kb, consist of 15 exons, and exhibit identical exon–intron organisation. Exon 12 codes for the covalent FAD-binding site, and it is the most conserved exon and has about 70% sequence similarity (Reference Grimsby, Chen, Wang, Lan and Shih93). Lower-platelet monoamine oxidase (MAO) activity has been associated with alcohol dependence (Reference Alexopoulos, Lieberman and Frances94,Reference Dolinsky, Shaskan and Hesselbrock95). However, the finding of low-platelet MAO activity in alcoholics has not been consistently replicated (Reference Sherif, Hallman and Oreland96,Reference Farren, Clare, Tipton and Dinan97). With regard to MAO-B activity changes during the withdrawal period, previous studies have yielded equivocal results. Some studies showed a transitory increase in platelet MAO activity during withdrawal (Reference Alexopoulos, Lieberman, Frances and Stokes98,Reference Alling, Balldin, Bokstrom, Gottfries, Karlsson and Langstrom99), whereas others showed a decrease (Reference Berggren, Fahlke and Balldin100,Reference Rommelspacher, May, Dufeu and Schmidt101). However, platelet MAO activity in recovered alcoholics after long-term abstinence is not significantly different from that of the controls (Reference Giller and Hall102).
Several lines of evidence suggest that the MAO enzyme activity is under genetic control of a single major locus (Reference Devor, Cloninger, Hoffman and Tabakoff103). The MAOA gene presents a 30-bp VNTR in the promoter region with different repeat sizes (3, 3.5, 4, or 5 copies) and is in linkage disequilibrium with other MAOA and MAOB polymorphisms. This polymorphism selectively influences the protein transcription and alters the enzymatic activity (Reference Denney, Koch and Craig104). Alleles with 3.5 or 4 copies of the repeat sequence are transcribed 2–10 times more efficiently (MAOA high) than those with three or five copies of the repeat (MAOA low), suggesting an optimal length for the regulatory region (Reference Sabol, Hu and Hamer105). A dinucleotide repeat polymorphism in intron 2 (MAO-A CA repeat) is in linkage disequilibrium with a regulatory VNTR (Reference Black, Chen, Craig and Powell106). There are two groups of CA repeats in the MAOA gene (short with one to three repeats; long with four to eight repeats) (Reference Manuck, Flory, Ferrell, Mann and Muldoon107). Association of the MAOA dinucleotide repeat length polymorphism with alcohol dependence has now been demonstrated in Euro-Americans (Reference Parsian, Suarez and Tabakoff108,Reference Vanyukov, Moss, Yu, Tarter and Deka109) and Han Chinese (Reference Hsu, Loh, Chen, Chen, Yu and Cheng110). In Caucasians the low-activity three-repeat alleles of the VNTR polymorphism were significantly high in antisocial alcoholics compared with control subjects (Reference Samochowiec, Lesch and Rottmann111). In Brazilians, the functional VNTR polymorphism is associated with female subjects but not the male subjects; the genotype containing at least one three-repeat allele was significantly more frequent among alcoholics than controls (Reference Guindalini, Scivoletto and Ferreira112). Among the Swedish students, girls with long MAOA u-VNTR variant showed a higher risk of alcohol consumption. On the contrary boys with short allele were related to higher alcohol consumption suggesting sex difference in the G × E interaction (Reference Nilsson, Comasco, Aslund, Nordquist, Leppert and Oreland113).
The MAOB gene harbours a polymorphic repeat region in the intron 13, and includes a transition (A–G) polymorphism, but it is not associated with the MAO-B enzyme activity in platelets (Reference Girmen, Baenziger and Hotamisligil114). Individuals with A-allele in intron 13 showed lower MAO-B enzyme activity in blood (Reference Garpenstrand, Ekblom, Forslund, Rylander and Oreland115). Conversely, the G allele is linked to lower enzyme activity in postmortem human brains (Reference Balciuniene, Emilsson, Oreland, Pettersson and Jazin116). MAOB dinucleotide repeat length polymorphism did not show any significant association with alcohol dependence in five different ethnic groups in Taiwan (Reference Hsu, Loh, Chen, Chen, Yu and Cheng110). In Americans, a MAOB haplotype has shown significant association with alcohol dependence independently from antisocial personality disorder (Reference Ducci, Enoch and Hodgkinson117). In contrast, the Monoamine oxidase-B polymorphism (MAOB-A/G) failed to reveal any association with alcohol dependence in Croatians (Reference Mokrovic, Matosic and Hranilovic118).
DBH
Dopamine β-hydroxylase is present in the synaptic vesicles of postganglionic sympathetic neurons and catalyses the oxidative hydroxylation of dopamine to norepinephrine (Reference Kim, Zabetian and Cubells119). DBH is a 290 kDa copper-containing oxygenase consisting of four identical subunits, and its activity requires ascorbate as a cofactor (Reference Rush and Geffen120). The gene coding for DBH is localised to 9q34. The DBH gene, ∼23 kb in length, is composed of 12 exons (Reference Kobayashi, Kurosawa, Fujita and Nagatsu121). Several mutations in the DBH gene have been found to reduce dopamine β-hydroxylase activity. A ‘C’ to ‘T’ polymorphism at nucleotide −1021 in the five-prime region of the DBH gene was related to dopamine β-hydroxylase activity levels in plasma accounting for 35–52% of the variation in plasma DBH activity in several ethnically different populations (Reference Zabetian, Anderson and Buxbaum122,Reference Kohnke, Zabetian and Anderson123). The IVS1 + 2T > C is a T-to-C transition in the donor splice site of exon 1 of the DBH gene and is found to be in linkage disequilibrium with −1021C-T polymorphism thereby interfering with the normal processing of dopamine β-hydroxylase (Reference Kim, Zabetian and Cubells119). Analysis of unrelated adults of African American, European American, and German origin and African American mothers and infants revealed the frequency of the IVS1 + 2T > C (Reference Zabetian, Romero and Robertson124). This splice site mutation was also observed as a compound heterozygote along with two coding mutations such as Val87Met and Asp100Glu (Reference Kim, Zabetian and Cubells119).
After hospitalisation, mean serum DBH levels in alcoholics were not statistically significant when compared with control groups, indicating that serum DBH activity is not an important parameter to screen alcohol susceptibility (Reference Sullivan, Stanfield, Schanberg and Cavenar125). The plasma DBH activity levels among nonalcoholic young men with or without family history demonstrated a 20% lower level of DBH in men with positive family history; however, a statistical difference could not be seen (Reference Schuckit, O'connor, Duby, Vega and Moss126). Although the plasma DBH activities in chronic non-abstinent alcoholics and controls were not significantly different, the uncontrolled alcoholics showed a significant reduction of DBH activity when compared with the controlled alcoholic subgroup (Reference Lykouras, Moussas and Markianos127). Further, functional studies using in vitro and in vivo models demonsrated a fivefold to sixfold increase in DBH mRNA levels in response to ethanol, thus confirming DBH as a major ethanol-responsive gene (Reference Thibault, Lai and Wilke128). In German alcoholics, the plasma DBH activity was significantly lower than the controls, but this reduction is independent of DBH-1021C > T genotype (Reference Kohnke, Zabetian and Anderson123). This is supported by a study in Brazilians, where the distribution of DBH-1021C > T allele and genotype frequencies are not statistically significant between alcoholics and controls (Reference Freire, Hutz and Bau129). The DBH*444G/A polymorphism of the dopamine-β-hydroxylase gene is associated with alcohol dependence but not with alcohol withdrawal seizures and DT (Reference Kohnke, Kolb, Kohnke, Lutz, Schick and Batra130).
SULT1A3
SULT1A3 catalyses the sulphate conjugation of dopamine and structurally related drugs. Two copies of the gene coding for cytosolic sulfotransferase SULT1A3 is located on chromosome 16 and both genes are known to produce mRNA transcripts. The SULT1A3 gene is 8.4 kb in length and consists of 10 exons, the first three of which are untranslated (Reference Wood, Aksoy, Aksoy and Weinshilboum131). Several known and novel SNPs were identified, and studies on functional genomics showed that some of these SNPs resulted in decreased SULT1A3 activity (Reference Hildebrandt, Salavaggione and Martin132,Reference Thomae, Rifki, Theobald, Eckloff, Wieben and Weinshilboum133). So far no study has been conducted reliably to evaluate the association between SULT1A3 gene polymorphisms and alcohol dependence.
Ethnicity and alcohol
Alcohol consumption is governed by socio-cultural practices that are imparted through an individual's cultural and ethnic contextual experiences. In addition, differences in drinking patterns and consequences of alcohol use may exist among various ethnic and cultural groups. Overlooking these influences can lead to false views on appropriateness and inappropriateness of alcohol consumption and concomitant behaviours (Reference Heath134). Further genetic difference between ethnic groups occurs among other enzymes involved in metabolising alcohol in the liver. According to the HapMap SNP database, gene polymorphisms are seen in highly varying percentages in different populations worldwide. The genetic demographic studies demonstrate variation in the frequency of DAT1 nine-repeat allele in worldwide populations (Reference Bhaskar, Thangaraj and Mulligan68). DRD4 exon III VNTR seven-repeat allele also showed striking variations among world populations in their frequency ranging from 0% to 2% of South Asians to 78% of native South Americans (Reference Nikolaidis and Gray135).
Gene–environment (G × E) interactions and alcohol dependence
Alcohol dependence and related diseases are the result of gene to gene and gene to environment interactions. Although, several genetic factors that are associated with alcohol-related diseases have been identified, the environmental factors involved in modifying the risk of alcohol-related diseases remain undefined. The possible environment factors that modulate alcohol abuse or alcohol dependence include the exposure to ethanol and other drugs, cultural factors, and life stage events. With reference to alcohol dependence, the most studied environments include various kinds of physical and social stressors, or early drug exposures (Reference Barr, Newman and Lindell136,Reference Stevens, Leckman, Coplan and Suomi137). Initial gene–environment interaction studies demonstrated that the genetic influences on alcohol use can be moderated by marriage (Reference Heath, Jardine and Martin138), religion (Reference Koopmans, Slutske, Van Baal and Boomsma139), parental monitoring (Reference Dick, Viken, Purcell, Kaprio, Pulkkinen and Rose140) and friends (Reference Dick, Pagan and Holliday141). Hence, studies on gene–environment interactions will help in understanding how genes influence alcohol abuse and alcohol dependence behaviours.
Conclusions
From the current literature review, no evidence emerged to indicate a strong association between alcohol dependence and polymorphisms of dopamine pathway genes. Ethnic differences, variations in socio-cultural practices, genetic difference among populations, gene–gene interactions, gene–environment interactions, and different linkages to the polymorphisms might have influenced heterogeneity of alcohol dependence risk.
Future directions
Only a fraction of the genetic risk is yet accounted for by known loci, but a growing number of replicable loci are encouraging. Similarly encouraging are the facts that these loci coincide with protein products that could be related to alcohol-dependence risk and that they were identified by the application of standard methods. Admittedly, though, the proof for a specific variant to be related to a disease risk on the basis of studies that take into account the function of allelic variants, statistical evidence from linkage, and association studies is not sufficient. Despite the advances, there is a long way to go to fully characterise the genetic basis for alcohol dependence, and while it is likely that linkage and candidate gene or targeted association analysis will not take us the rest of the way, it is also now the case that these methods are rapidly being augmented by newer methodologies. Identifying various environmental conditions that alter the epigenetic status of neurotransmitters and their receptor genes may help us to understand the role of environmental factors that are critical in the development of alcohol-related diseases.
Acknowledgements
The authors are grateful to Mrs. Merin Selvanayagam for English correction to the paper.
Conflicts of Interest
There are no conflicts of interest.
