The frequency of DRD2 rs1076560 and OPRM1 rs1799971 in substance use disorder patients from the United Arab Emirates

The dopaminergic and opioid systems are part of a network involved in rewarding response following the consumption of opioids and other psychoactive substances [1, 2]. The dopamine system has been central to theories in reward of substance use disorder (SUD) that has been debated for several decades [3]. The consumption of addictive substances stimulates the release of dopamine into nucleus accumbens (NAc) elevating the dopamine level to above basal levels [4]. There are different mechanisms of action and target molecules in the dopamine system for the variety of substances that are commonly consumed. This dopaminergic system comprises an array of dopamine receptors, transporters, and substance-metabolising enzymes. Members of the family of the dopamine receptor genes, DRD1, DRD2, DRD3, DRD4, and DRD5, have been widely studied as risk factors for SUD [2]. The dopamine D2 receptor is a part of G protein-coupled receptors (GPCRs) that is encoded by the DRD2 gene. It is located on chromosome 11q23, spanning a region of 65.56 kilobases and comprises 8 exons separated by 7 introns. During the splicing process of the DRD2 mRNA precursor, two alternative subtypes of the D2 receptors are formed: a 443 amino acid D2L or a 414 amino acid D2S form. The longer D2L form is more common [5]. This 29 amino acid difference between the two isoforms does not appear to affect the pharmacological properties of the dopamine D2 receptor. The D2L/S variation changes the localization of the third intracellular loop of the receptor that interacts with the G coupled protein; hence, it affects the intracellular signalling mechanism. The mechanisms of dopamine receptor signal transduction and regulation are not only mediated via G protein signalling, but also involve G protein independent signalling events [6].

Understanding the reward and the treatment responses highlight the necessity of reviewing the relation between the genetic variants of these dopaminergic genes and SUD [7]. Patriquin et al. [7] reviewed the correlation of dopaminergic genes to SUD. The genetic variants of DRD2 have been a focus of intense research to determine their link to SUD. Two single-nucleotide polymorphisms (SNPs) of the DRD2 loci; rs2283265 in intron 5 and rs1076560 in intron 6 have been reported to be associated with cocaine use [8]. A DRD2 variant (rs1076560) has also been studied in various populations. Clark et al. [9] reported the association between rs1076560 and opioid use in African Americans (AA) (p = 0.03) and European Americans (EA) (p = 0.02). These findings introduced insights into the possible roles of these dopaminergic variants on SUD. However, the extent of genetic variations acting as a risk factor for SUD is still not understood.

The opioid receptor gene family has been extensively studied to identify if there are any associations with SUD. There three subtypes are μ, κ, and δ, encoded by the OPRM1, OPRK1, and OPRD1 genes, respectively. The product of the OPRM1 gene plays a role in facilitating the analgesia and euphoria effects of opioids. The G protein-coupled mu opioid receptor encoded by the OPRM1 gene is a multiple trans-membrane protein that has a high affinity for endogenous and exogenous opioids [10]. The OPRM1 gene consists of 9 exons which encode over 100 variants that produce between 19 and 39 splice forms of the protein [11]. The rs1799971 (A118G) site is an SNP that is located in exon 1 of the OPRM1 gene. This variant encodes a missense change in OPRM1 at position 40 resulting in a change from an asparagine to an aspartate (Asn40Asp) in the extracellular domain of the receptor. This substitution eliminates an N-glycosylation site in the extracellular domain, which affects endogenous opioid binding and receptor activity [12]. The role of the rs1799971 in SUD remains in dispute [13, 14]. The effect of the variants of rs1799971 on different classes of substance has been extensively studied in various populations [10, 11, 13, 15–20]. However, only two studies [21, 22] have looked into rs1799971 variants in the Arab population. Several studies have reported associations between A118G with different substances of use in patients from different ethnic groups [10, 11, 13, 15–20], with others not finding significant associations with SUD [23, 24].

This study is the first study to report on the allele frequency for the rs1076560 SNP of DRD2 and the rs1799971 SNP of OPRM1 in individuals with SUD from the United Arab Emirates (UAE). This case–control study investigated the genetic association between the DRD2 rs1076560 SNP and OPRM1 rs1799971 SNP and SUD in the UAE population. The allele frequencies of DRD2 rs1076560 SNP in the UAE population were compared to other global populations. The allele frequencies of the OPRM1 rs1799971 SNP in this UAE study were compared with SUD cohorts in other Arab populations.

The results of this UAE study were compared with published data that included association studies between the DRD2 rs1076560 SNP and the use of different substances (e.g., alcohol, cocaine, opioids, and poly-substances) in a number of different populations (e.g., Caucasians, African Americans, Asians, and Jordanian Arabs) (Table 1). Six relevant publications matched the selection criteria described in the “Methods” section. All the studies identified were case versus control studies. Clark et al. [9] studied a relatively large population of EA and AA (999 EA cases versus 656 EA controls and 278 AA cases versus 750 AA controls) and showed that the DRD2 rs1076560 SNP was significantly associated with opioid use in both populations with p values of 0.02 and 0.03, respectively. The MAF of the DRD2 rs1076560 SNP was higher in EA case group (17.0%) when compared to EA control group (14.0%) as well as in AA cases (12.0%) versus AA controls (9.0%) [9]. Moyer et al. [28] studied cocaine users in the same two ethnic groups (EA and AA) and showed that the DRD2 rs1076560 SNP was associated with cocaine use in EA (p = 0.003, OR = 2.74), but not in AA (p = 0.53, OR = 0.66). A polish study of European alcohol users reported results that were not significant (p = 0.14, OR = 1.34). In a study of Japanese patients, the risk allele “A” of the DRD2 rs1076560 SNP was associated with alcohol use (p = 0.03, OR = 1.30) [29]. To date, there has only been one other study of patients of Middle Eastern descent. Al-Eitan et al. [30] found the DRD2 rs1076560 SNP to be associated with poly-substance use in a Jordanian Arab population (p = 0.03, OR = 1.53).

In the UAE cohort represented in this study, there was no significant genetic association between the DRD2 rs1076560 SNP (p = 0.52) and SUD. The MAF of the DRD2 rs1076560 SNP was higher in the controls (13.2%) when compared to the substance users (11.8%). This was similar to the observations made in an AA population studied by Moyer et al. [28], where the MAF in cocaine users (6.0%) was lower than in the control group at 8.0%. In general, Moyer et al. [28] reported an association between the DRD2 rs1076560 SNP and cocaine use, in EA but not the AA. The overall odds ratio of 1.94 in population (n = 214) was attributed to an artefact arising from the small sample size that was studied [9]. In a more recent study, Clark et al. [9] replicated the Moyer et al. [28] study by increasing sample size. The DRD2 rs1076560 SNP was found to be associated with opioid use disorder in the two populations examined in this subsequent study (EA: p = 0.02, AA: p = 0.03), but not cocaine use (EA: p = 0.23, AA: p = 0.19). The MAF of the DRD2 rs1076560 SNP, the “A allele”, was found to be higher in the cases when compared to controls in both populations: EA at 17.0% versus 14.0%, respectively, and in AA at 12.0% versus 9.0%, respectively [9].

The different association outcomes between the studies may account for the differences in the substance of use or the pattern of use in the cohorts that were studied. Table 1 summarises the type of substances in each study, which included alcohol, opioid, cocaine, and poly-substance use. Stratifying these studies based on the type or pattern of substance used is important to identify more specific genetic risk variants [31]. Iacono et al. [32] suggested that specific substances influenced the nature of the genes that are involved in the pharmacodynamics and pharmacokinetics of that substance [31, 32]. However, it is a challenge to stratify patients according to substance of use, as often there is no single substance that is used by patients and there are overlaps between the substances are used. Clark et al. [9] investigated associations with a single substance. However, their study was plagued with difficulties related to overlap between different types of the used substances [9]. In addition, the differences in the genetic architecture between populations could dictate whether a variant is associated or not (Table 1). For example, the association between DRD2 rs1076560 SNP and alcohol use was statistically significant (p = 0.03, OR = 1.30) in a Japanese population [29]. However, the same SNP was not statistically significant in the Polish patients with alcohol use disorder (p = 0.14, OR = 1.30) [33]. Even though the findings of Malecka et al. [33] were not significant, the MAF “A allele” in the group of alcohol users was higher in the cases (21.0%) than in the controls (17.0%). In contrast, the MAF “A allele” in this UAE study and the AA group in Moyer et al. [28] were opposite, where the MAF “A allele” was higher in controls than in cases (Table 1).

This study found no significant genetic association between the OPRM1 rs1799971 SNP (p = 0.12) with SUD among patients from the UAE population. The association between the OPRM1 rs1799971 SNP and various phenotypes of SUD has been studied and includes being a risk factor to different types of substances of use including tobacco consumption [34], alcohol use and sensitivity [13, 19, 34, 35], and opioid use [16, 36]. Other studies looked into inducing clinical symptoms or mediating responses to therapeutic treatment [11, 17, 37, 38]. The association between the OPRM1 rs1799971 SNP and SUD failed to reach statistical significance in our study (p = 0.12) and in a previous study by Enabah et al. [22] (p = 0.54). In addition, the MAF of the OPRM1 rs1799971 SNP or “G allele” in our study (case = 15.4%, control = 18.9%) was distributed in a similar pattern to Enabah et al. [22] (case = 5.2%, control = 7.0%), where the MAF “G allele” was lower in cases than in the controls. However, by combining the two cohorts, as shown in Table 3, the increase in numbers resulted in a significant association between the OPRM1 rs1799971 SNP and substance use. This suggests that a larger population size in future studies is required. The OPRM1 rs1799971 SNP association varies and appears to depend on the study population and the nature of the substance of use. For example, Chen et al. [35] examined the association between the OPRM1 rs1799971 SNP and alcohol use disorder in two different populations (Asian and Caucasian) in a meta-analysis study. They reported an association with the OPRM1 rs1799971 SNP in Asians (p ≤ 0.001) but not in Caucasian (p = 0.76). Other studies have reported a lack of association with alcohol use [24] and with heroin and/or cocaine use [16, 23]. Since the OPRM1 rs1799971 SNP has been widely studied in different populations, we focused on compiling data based on the Arab studies (Table 2). Another Arab study by Al-Eitan et al. [21] investigated the role of OPRM1 variants including the rs1799971 SNP on the outcomes of therapeutic treatment for opioids. This association between OPRM1 rs1799971 SNP and the possibility of an increased chance of relapse in patients undergoing Naltrexone treatment for opioid use disorder in Jordanian patients was not significant (p = 0.55). The variability of the findings from the range of studies conducted to date highlights some contribution by the OPRM1 rs1799971 SNP. However, the variability in associations found to date requires further study to understand the contribution of this SNP.

This study is the first to examine if there is any association between combined genotypes of two genes (DRD2 rs1076560 SNP and OPRM1 rs1799971 SNP) and the susceptibility to SUD in patients of Arabian ancestry. There was no significant association found between the combined genotype frequencies of the two SNPs and disease in this case–control study (cases p = 0.88 and controls p = 0.23). Although some studies support the combined effect of variants of these two genes (DRD2 and OPRM1), the exact mechanism remains elusive. This may suggest the involvement of other genetic variants within or in the vicinity of the DRD2 and OPRM1 genes. For example, Zhang et al. [10] examined 13 SNPs in the OPRM1 gene using haplotype analysis in an association study involving substance use patients from two populations: European and Russian ancestries [10]. They reported the involvement of the intronic variants of OPRM1 (rs511435, rs534731, rs3823010, rs2075572, and rs609148) in increasing risk to SUD. Some of these SNPs were located in linkage disequilibrium (LD) with OPRM1 rs1799971 SNP and others have been postulated to be involved in transcription regulation or alternative gene splicing. The findings in Zhang et al. [10] highlighted the limitation of selecting a single SNP of a candidate gene to examine the genetic association with SUD.

Interaction between the DRD2 and OPRM1 genes with other genes has been examined across different substances of use. For instance, Lechner et al. [39] examined the combined effect of the OPRM1 rs1799971 SNP and the DRD4 exon 3 VNTR variants on cigarette craving after alcohol consumption. The study reported that the presence of the G allele is associated with an increase in cigarette craving after alcohol consumption. However, no significant association between to the exon 3 VNTR variants of the DRD4 were found with the condition [39]. In addition, Sullivan et al. [40] reported a gene–gene interaction between the dopamine receptors gene (DRD2) and the dopamine transporter gene (DAT) in cocaine users. The interaction between the regulatory variant of DRD2 (rs2283265) and dopamine transporters gene altered DAT protein activity, supporting the possibility that variants being a risk factors for cocaine use [40].

This study provides insights into two major genes that are thought to be risk factors of substance use. Specifically, the DRD2 rs1076560 SNP and the OPRM1 rs1799971 SNP were studied in substance use patients from the UAE population. No significant association between the DRD2 rs1076560 SNP, the OPRM1 rs1799971 SNP, and the combined genotype of the two SNPs and SUD was identified in this cohort. Nevertheless, future studies must consider stratification of the disease phenotype to assess possible association with DRD2 and OPRM1. In addition, the findings of this study might suggest the involvement of other variants or genes in the mechanism of the disorder. Haplotype analyses for DRD2 and OPRM1 variants can be considered in future studies to evaluate the interaction of variants on each gene. Alternatively, genome-wide association study (GWAS) could be more objective strategy, since it does not rely on any previous conclusions from other populations.