Dopamine D2 receptor gene (DRD2)

The D2-receptor family genes contain introns that can give rise to alternative splicing and other protein sequence variations in these receptors. For example, DRD2 has two alternative splicing forms (short and long), which differ in 29 amino acid sequence in the third cytoplasmic loop of the protein and show slight differences in signal transduction efficiency (Fraeyman and Vermis, 2003). Based on different subcellular localization of the two DRD2 splice forms at specific neurons in the monkey brain (Khan et al., 1998), as well as on DRD2-long knockout mice experiments (Usiello et al., 2000), the DRD2 short form is proposed to act as autoreceptor, whereas the DRD2 long form acts primarily as postsynaptic receptor. Interestingly, intronic SNPs (rs2283265 in intron 5 and rs1076560 in intron 6, which are in strong linkage disequilibrium) have been shown to affect the expression of the DRD2 short splice variant relative to DRD2 long one: The minor T alleles of these SNPs favor inclusion of exon 6 (the grey box on the DRD2 gene picture at Figure 2A), resulting in a significant reduction of the DRD2 short splice form, compared to the G alleles (Zhang et al., 2007). Other known functional polymorphisms of the DRD2 gene involve rare SNPs in the coding region, like Pro310Ser and Ser311Cys (Cravchik et al., 1996), but synonymous SNPs and non-coding polymorphisms are used more often in association studies.

Several restriction fragment length polymorphisms (RFLPs) located in non-coding regions of the DRD2 gene have been used as markers, such as TaqIA and TaqIB, which were genotyped with the TaqI enzyme. Because DRD2-specific ligands allow for in vivo DRD2 analyses, the gene expression effect of these SNPs can be demonstrated in neuroimaging studies. The most widely studied TaqIA SNP (rs1800497) was identified during the chromosomal localization of the gene. Recently, it became clear that this SNP, which is 10 kb downstream from the DRD2 gene, is located in the neighboring ANKK1 gene where it causes an amino acid substitution (Glu713Lys) (Neville et al., 2004). Nevertheless, independent studies have reported reduced D2 receptor density in the minor A1-allele carriers in SPECT (single photon emission computed tomography) or PET (positron emission tomography) studies. Four studies showed significant differences, and one study showed a trend towards a reduction of striatal DRD2 binding in A1-allele carriers compared to subjects with the A2/A2 genotype (reviewed by Noble, 2003). Therefore, it seems that the DRD2/ANKK1 TaqIA SNP is either in linkage with another functional DRD2 SNP or that it is indirectly involved in DRD2 gene expression. Data exist supporting both possibilities.

On the one hand, the TaqIA SNP is in linkage with the TaqIB SNP (rs1079597) and the C957T SNP (rs6277, Pro319Pro) within the DRD2 gene, and these SNPs have also been associated with altered striatal DRD2 density: The minor B1-allele (in linkage with the A1-allele) has been repeatedly shown to be associated with a low DRD2 density (Jonsson et al., 1999; Ritchie and Noble, 2003). As for the C957T SNP, a detailed PET study showed that the increased binding potential of the 957 T-allele was more pronounced than the slightly increased DRD2 density, making the striatal DRD2 availability of the T-allele higher compared to the C-allele (T/T>C/T>C/C for the 3 genotype groups) (Hirvonen et al., 2009a). The authors also reported that the 957 C-allele carriers had a significantly higher A1-allele frequency, which is in accordance with the notion that A1-allele carriers have lower striatal DRD2 availability. Haplotype analyses also showed that subjects with the A2/A2 and 957 T/T genotypes had the highest DRD2 availability, whereas A1-allele and 957 C-allele carriers had the lowest DRD2 availability, subjects with the A2/A2 and 957 C/C or C/T genotypes had intermediate levels (Hirvonen et al., 2009a). It is important to mention that a recent PET study reported opposite effects for the TaqIA and C957T SNPs in extrastriatal DRD2 availability (Hirvonen et al., 2009b). In this PET study, the 957 T-allele was associated with lower DRD2 availability in a step-wise fashion (T/T<C/T<C/C) throughout the cortex, thalamus, amygdala and hippocampus. The A1-allele carriers had similar, but only marginally, higher extrastriatal DRD2 availability when compared to the A2/A2 group. This finding illustrates the importance of variations in brain region-specific dopamine transmission. Interestingly, the TaqIA SNP is also in linkage with the intronic SNPs, which were shown to affect the DRD2-short isoform expression (Zhang et al., 2007).

On the other hand, the TaqIA SNP shows strong linkage disequilibrium with several non-synonymous SNPs of the ANKK1 gene. An in vitro study demonstrated that a neighboring ANKK1 SNP (rs273849, Arg490His) altered NF-κB function, which in turn may affect DRD2 expression (Huang et al., 2009). Therefore, ANKK1 SNPs might indirectly influence DRD2 function. Interestingly, the ANKK1 protein has been recently detected in human astrocytes and in mouse radial glial cells. The peak mRNA expression of ANKK1 corresponded to that of DRD2 in mouse embryonic brain samples, suggesting that the interaction of ANKK1 and DRD2 may be relevant in brain development (Hoenicka et al., 2010). This workgroup also showed that a non-synonymous ANKK1 polymorphism (rs7118900, Ala239Thr), which is in strong linkage disequilibrium with TaqIA, had an impact on the ANKK1 protein level (Garrido et al., 2010).

In addition to the previously mentioned SNPs, the DRD2 promoter −141C Ins/Del polymorphism has been also frequently studied; however, the functional role of this polymorphism is less clear. The results of an in vitro reporter gene experiment showed a lower expression for the −141C Del-allele compared to the −141C Ins-allele (Arinami et al., 1997), whereas in vivo studies could not detect any significant differences in striatal or extrastriatal DRD2 availability (Ritchie and Noble, 2003; Hirvonen et al., 2009b). In contrast to these results, a higher striatal dopamine receptor density was shown for the Del-allele in a SPECT study (Jonsson et al., 1999).

Genetic association studies variably used the above mentioned DRD2 polymorphisms with other, mostly non-coding variants from the 11q chromosomal region (containing not only the DRD2 gene but the ANKK1 and other neighboring genes as well). The SNP combinations used in haplotype analyses conducted by different workgroups are also diverse (Xu et al., 2004; Yang et al., 2007; Huang et al., 2009; Kraschewski et al., 2009).

Date: 2016-01-03; view: 814