Figure 2. The chromosomal location of the human dopamine D2
receptor (DRD2) gene. The DRD2 gene is located at chromosome
11 q22-23 (from Blum et al., 1990).
The candidate gene approach involves selecting a gene that may be involved in the known physiology of the disease. Since several brain neurotransmitter receptors are involved in alcohol action, plausible candidates are neurotransmitter receptor genes or neurotransmitter synthesis genes (Gordis et al., 1990).
The central catecholamines, especially dopamine are thought to play a major role in central reward processes (Kornetsky et al., 1988; Di Chiara and Imperato 1988; Koob and Bloom, 1988; Wozniak et al. 1991). Alcohol can influence neuronal function through actions on the membrane proteins, including neurotransmitter receptors (Bloom, 1991). However, unlike other drugs of abuse such as opiates, stimulants and hallucinogens, there is no receptor molecule specifically for binding ethanol (Mason et al., 1991).
Brain stimulation reward may be a result of a direct or indirect effect of a substance on the dopaminergic mesocortico-limbic system. Abuse substances that either increase the presynaptic release of, or block uptake of dopamine may cause brain reward. However, the effect of ethanol on brain stimulation reward is not as obvious as that seen with other abused drugs (Kornetsky et al., 1988).
There are at least six dopamine receptors (D1, D2a, D2b, D3, D4, and D5). D2 dopamine receptors are expressed in neurons of the midbrain, caudate, and limbic systems, specifically in the nucleus accumbens, the amygdala, the hippocampus, and parts of the cerebral cortex. D2 receptors have a high affinity for antipsychotic drugs and the therapeutic action of these drugs are thought to occur at these sites (Kandel, 1991).
Any genetic polymorphism that affects the expression of D2 dopamine receptors may be a possible candidate for susceptibility to alcoholism (Parsian et al., 1991b; Uhl, Persico and Smith, 1992). Some defect in dopaminergic systems could make individuals more susceptible to the reinforcing actions of psychostimulants (Koob and Bloom, 1988). This would be expected in early onset alcoholics who consume alcohol for its euphorigenic effects, rather than in alcoholics who drink for its anti-anxiety effects (Parsian et al., 1991b).
Grandy et al. (1989b) used rat brain cDNA previously isolated and identified by Bunzow et al. (1988) as a probe to isolate human D2 receptor DNA from a human genomic library. The genomic clone isolated, lhD2G1, had a 1.6 kb fragment that encoded the last 64 amino acids of the human D2 receptor and 1.2 kb of 3' non-coding sequence. It was found that the fragment occurred only once in the human genome. Another clone, lhD2G2, was found that overlapped the first by 400 bp. The two clones were found to span 34 kb of the human dopamine D2 receptor gene - DRD2. Figure 1 shows the structure of the human dopamine D2 receptor (DRD2).
Figure 1. The human dopamine D2 receptor (DRD2) gene. The
DRD2 gene contains eight exons, spans 270 kb and includes an intron
of approximately 250 kb separating the first exon from the exons
encoding the receptor protein (Eubanks et al., 1992).
Exons 2-8 span 14 kb. The transcript undergoes alternative splicing
of exon 6 to produce two mRNAs. The translation products differ
by 29 amino acids (Gandelman et al., 1991). Also shown
are the location of other polymorphic markers (see discussion).
Grandy et al. (1989a) located the DRD2 gene to chromosome 11 q22-23 (Figure 2) using in situ hybridisation of lhD2G1 to metaphase chromosomes. They also searched for RFLPs associated with DRD2 and found a two-allele TaqI RFLP (Figure 3). The lhD2G1 probe hybridised to a 6.6 kb TaqI fragment in the A1 allele, and two fragments of 2.9 kb and 3.7 kb in the A2 allele. Constant bands of 2.3 kb and 10.5 kb were found in all individuals. The frequencies of A1 and A2 were measured in 43 unrelated Caucasians and found to be 0.24 for A1 and 0.76 forA2.
Figure 2. The chromosomal location of the human dopamine D2
receptor (DRD2) gene. The DRD2 gene is located at chromosome
11 q22-23 (from Blum et al., 1990).
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