Dopamine D5 receptor gene (DRD5)

Although several amino acid substitutions with different agonist binding capacities have been described for the DRD5 (Cravchik and Gejman, 1999, also see Figure 2A), investigations of the coding-region SNPs have been hindered by two pseudogenes (Housley et al., 2009). Therefore, a marker dinucleotide repeat polymorphism located 18.5 kb from the 5 end has been studied most often in relation to psychiatric disorders. This polymorphism was identified at the time the gene was cloned (Sherrington et al., 1993), and the 12 alleles were named based on their length, which ranged from 134 to 156 bp, with the most common allele being 148 bp. To our knowledge, no functional study on this polymorphism has been published yet.

2.3.2. Dopamine synthesis: the tyrosine hydroxylase gene

The rate-limiting step of dopamine biosynthesis is the conversion of tyrosine into dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase (TH), which is then turned into dopamine by aromatic amino acid decarboxylase (Figure 1). A 4 bp repeat sequence (TCAT tetranucleotide repeat) in the first intron has been thoroughly investigated by Meloni s laboratory. Their latest in vitro study showed a quantitative silencing effect of the TCAT-repeat on TH gene expression, with the 3-, 5-, 8-, and 10-repeat alleles exhibiting a step-wise inhibition on transcriptional activity (Albanese et al., 2001). Association studies refer to these intronic microsatellite alleles as K1–K5, numbering them in a decreasing order, i.e., from K1 = 10-repeat to K5 = 6-repeat (other alleles are rare). Dopamine is converted to norepinephrine by dopamine beta hydroxylase (DBH) in the synaptic vesicles of noradrenergic neurons; therefore, polymorphisms of the DBH gene belong to the noradrenergic system. Interestingly, the DBH enzyme can be measured in the plasma, which allows for in vivo measurements of its genetic variants (reviewed by Gizer et al., 2009).

2.3.3. Dopamine clearance: the dopamine and norepinephrine transporter genes

Dopamine neurotransmission is efficiently terminated via dopamine transporter (DAT) in subcortical regions by taking back dopamine from the synapse to the presynaptic neuron (Figure 1A). DAT is the site of action of stimulant drugs such as cocaine, amphetamine, and methylphenidate, which is used in ADHD treatment. Coding region variants of the DAT gene (DAT1, official symbol SLC6A3) are rare; therefore, VNTRs and SNPs of the non-coding regions have been studied in association studies. There is a common 40 bp VNTR in the 3′ untranslated region (UTR) with repeat numbers between 3 and 13. The most frequent allele has 10 repeats, followed by the 9-repeat allele; the others are rare variants. The results of comparative studies of the most frequent variants are controversial using either reporter gene assays (Fuke et al., 2001; Miller and Madras, 2002; Mill et al., 2005a) or post-mortem brain expression data (Mill et al., 2002a; Wonodi et al., 2009). SPECT analyses appear to have more congruent results: Two independent studies have reported a similar difference in striatal transporter density when studying large (N = 96 and N = 79), healthy populations. Participants with at least one copy of the 9-repeat allele (9/9 and 9/10 genotype) had a significantly higher transporter density compared to the 10/10 genotype group (van Dyck et al., 2005; van de Giessen et al., 2009). Studying a 30 bp VNTR in intron 8 (the most frequent alleles were originally named as 2-repeat and 3-repeat, but recent association studies refer to them as 5-repeat and 6-repeat), Guindalini et al. (2006) showed that the 3-repeat allele had a reduced basal expression compared to the 2-repeat allele, however, this allele had a 3-fold higher induction compared to the 2-repeat allele in response to a KCl and forskolin challenge. The higher level of transcriptional activity of the 3-repeat allele was also demonstrated in post-mortem midbrain tissues (Brookes et al., 2007). Constructing haplotypes from the two DAT1 VNTRs are preferred in recent ADHD studies (Asherson et al., 2007; Rommelse et al., 2008; Stevens et al., 2009; Franke et al., 2010).

As mentioned in the previous section, cortical DAT availability is very low compared to that of the striatum, however, dopamine can be taken up by the norepinephrine transporter (NET) in the cortex, resulting in a different dopamine elimination in this brain region (Figure 1B). Similarly to the DAT1 gene, the NET gene (official symbol SLC6A2) does not contain many common non-synonymous polymorphisms, therefore, synonymous SNPs from exons (e.g., rs5569, also called as G1287A from exon 9) and intronic SNPs (e.g., rs2242447 from intron 13) have been selected to cover the whole gene in comprehensive association studies. Unfortunately, the different workgroups did not select the same sets of SNPs, making the comparison of single marker and haplotype analyses hard (Gizer et al., 2009). The only SNP indicated to have functional relevance is located 3081 bp upstream from the transcription start site (−3081 A/T, rs28386840). The −3081 T-allele showed decreased promoter activity compared to the A-allele (Kim et al., 2006).

2.3.4. Dopamine inactivation: the monoamine oxidase and the catechol-O-methyltransferase genes

The monoamine oxidase (MAO) together with the aldehyde dehydrogenase and the COMT enzymes convert dopamine into homovanillic acid (HVA) in two consecutive steps (see Figure 1 legend). MAO is localized in the outer mitochondrial membrane in monoaminergic neurons and glial cells (Shih, 2004), while COMT is localized mainly in the rough endoplasmic reticulum in postsynaptic neurons and glial cells (Männistö and Kaakkola, 1999).

There are two MAO isoforms (MAOA and MAOB) that are encoded by different genes in the central nervous system, and both of these isoforms can degrade dopamine. The specificity of these enzymes is based on differential expression. MAOA is predominantly found in catecholaminergic neurons, whereas MAOB is more abundant in serotonergic and histaminergic neurons and glial cells. Data from studies using knockout mice suggest that MAOA is also important for serotonin breakdown. Increased levels of norepinephrine and dopamine were accompanied by increased serotonin level in MAOA knockout mice compared to wild-type mice, whereas MAOB knockout mice showed only increased phenylethylamine level compared to wild-type mice (reviewed by Shih, 2004). The MAOA and MAOB genes are closely aligned on the X chromosome; therefore, males have only one copy of these genes. For the association studies using females, it is important to note that the MAOA gene likely has a monoallelic expression (Hendriks et al., 1992; Nordquist and Oreland, 2006; Stabellini et al., 2009), although data arguing for an escape from inactivation have been also published (Carrel and Willard, 2005; Pinsonneault et al., 2006).

A 30 bp VNTR in the 5′ UTR of the MAOA gene, located 1.2 kb upstream from exon 1 (therefore called upstream VNTR or uVNTR), has been widely studied and has been repeatedly shown to affect gene expression. The most common alleles contain 3 or 4 repeats, and the less frequent variants are the 2-, 3.5-, and 5-repeat alleles. The 3-repeat allele had a lower transcriptional activity compared to the 3.5- and 4-repeat alleles in the in vitro experiments (Sabol et al., 1998; Deckert et al., 1999) and in fibroblasts (Denney et al., 1999). In post-mortem brain samples, a tendency towards the same difference between the 3- and 4-repeat alleles was observed in the level of enzyme activity (Balciuniene et al., 2002). Since there is a controversy about the activity of the 5-repeat allele (Sabol et al., 1998; Deckert et al., 1999), it is advisable to leave out the rare variants from the analyses, however, most genetic association studies refer to the 3.5- and 4-repeat alleles as “high activity”, whereas the 2-, 3-, and 5-repeat alleles are grouped together as “low activity” according to the first in vitro study (Sabol et al., 1998).

The availability of the COMT enzyme in cortical areas makes its gene a first-choice candidate gene for neuropsychological studies. COMT also has two isoforms, but these are coded from the same gene by two alternative promoters. The longer, membrane-bound enzyme differs from the shorter, soluble form by 50 additional amino acids forming a hydrophobic, membrane-spanning region. In addition, a human post-mortem study identified two variants of the membrane-bound COMT which probably differ in post-translational modifications (Tunbridge et al., 2006b). The membrane-bound form is expressed predominantly in the brain (Tenhunen et al., 1994; Tunbridge et al., 2007). A human specific G/A SNP (rs4680) in the 158^th codon of the membrane-bound form causes a valine-methionine substitution (Val158Met), whereas in the soluble COMT form, the same polymorphism is located in the 108^th codon. The Met-form (A-allele) has a lower stability at 37°C resulting in a 20–25% reduction in enzyme activity compared to the Val-variant that is coded by the G-allele (Lotta et al., 1995; Lachman et al., 1996; Chen et al., 2004). Significantly lower protein level and 30–40% lower enzyme activity was measured in postmortem PFC tissues and lymphocytes in Met/Met homozygotes compared to Val/Val homozygotes, whereas heterozygotes had intermediate level of activity (Chen et al., 2004). Significant difference in protein levels (but not in enzyme activity) was confirmed between the Val/Val and Val/Met genotypes (Tunbridge et al., 2007). Since COMT is one of the major determinants of dopamine action in the PFC (Käenmäki et al., 2010), the Val158Met polymorphism might cause significant differences in the cortical dopamine level. Based on the above data, Met/Met homozygotes likely have the highest dopamine levels in the PFC compared to the Val/Met and Val/Val genotypes, and this level seems to be the optimal level for cognitive functions under normal conditions (Tunbridge et al., 2006a). One workgroup showed that the Met-form was associated with better performance of PFC functions (Egan et al., 2001), and after amphetamine treatment, which elevated the dopamine level, cognitive functioning in the Met/Met homozygotes decreased (possibly as a result of too high dopamine level in the PFC), whereas this treatment was beneficial for the Val/Val homozygotes in the same experiment (Mattay et al., 2003). Taking into account the elevated dopamine concentration in stressful situations (Arnsten, 2009) and the association of COMT Val158Met genotypes with PFC-mediated cognitive functions (reviewed by Dickinson and Elvevag, 2009), it is probable that the effect of COMT genotypes on cognitive functions depends on various factors influencing the actual dopamine level in the PFC. In certain circumstances, it is the Val-form which is associated with better cognitive flexibility (Bilder et al., 2004).

Recently, other non-coding and synonymous SNPs of the COMT genes are advised to be investigated in addition to the Val158Met (rs4680), because an in vitro study indicated reduced protein translation efficiency due to secondary mRNA structure (Nackley et al., 2006). The rs6269A-rs4633C-rs4818C-rs4680G(Val) haplotype showed significantly reduced COMT protein level and enzyme activity compared to the rs6269G-rs4633C-rs4818G-rs4680G(Val). The third most frequent haplotype rs6269A-rs4633T-rs4818C-rs4680A(Met) had normal protein level but reduced enzyme activity (probably due to the cMet-allele), and hence referred to as intermediate-activity haplotype.

Date: 2016-01-03; view: 1021