3.1.1 CNR1 and CNR2
In 1990 and in 1993, the first and second cannabinoid receptors (CB1, CB2) were identified and cloned, which contributed to a significant increase in the understanding of cannabinoids. The CB1 receptor, encoded by the
CNR1 gene (cannabinoid receptor type 1, OMIM: 114610) [
5], is located on the human chromosome 6q14-15 and consists of four exons, of which the latter is the largest and most commonly expressed in brain tissue [
6]. Evolutionary conservatism is a characteristic feature of the CB1 receptor. It has been observed that amino acid sequences are similar in humans, mice, and rats to a degree of 97–99% [
7]. Activation of the CB receptor stimulates the appetite and has antiemetic, analgesic, and sedative effects [
8]. Association studies between the occurrence of single nucleotide polymorphism (SNP) in the
CNR1 gene and marijuana addiction, also including the trinucleotide repeat locus (AAT)
n
and insertion–deletion (-3180T) polymorphism, revealed contradictory results [
9]. Comings et al. showed a correlation between (AAT)
>5 repeats with drug dependence in 206 non-Hispanic Caucasians (92 subjects and 114 controls) [
10], but most studies on trinucleotide repeats (AAT)
n
demonstrated negative results [
11‐
13].
In an investigation performed on the group of 1923 individuals, a correlation of two SNPs in
CNR1 gene rs806368 (
p = 0.05) and rs806380 (
p = 0.009) with cannabis dependence were identified [
9]. Haplotype analysis revealed stronger association by rs806380 [
9]. In turn, Hopfer and his group analyzed five SNPs variants (rs2273512, rs6454674, rs806380, rs806377, and rs104935) in
CNR1 gene and demonstrated that rs806380 was significantly associated with cannabis dependence symptoms [
14]. However, Herman et al. have not confirmed these observations [
15]. The same conclusions were reached by Australian and American researchers [
16,
17]. Moreover, Hartman group showed an association (
p = 0.029), of rs1049353 in the
CNR1 gene with cannabis dependence symptoms in case–control samples (
n = 224) [
17]. On the other hand, Bühler et al. indicated non-statistically significant correlation of
CNR1 rs1049353, rs806368, rs6454674, and rs7766029 with cannabis use in a group composed of young individuals (
n = 91) [
18]. In case of rs806368, Zuo and co-workers analyzed 451 healthy control subjects and 550 substance dependence (including European Americans and African Americans) and showed that substance dependence increased with the number of G alleles in European Americans significantly [
19]. In studies of rs2023239
CNR1 gene variant among 105 daily marijuana smokers—students from the University of Colorado were tested [
20]. Participants with CT or CC genotype were more susceptible to the negative impact of the cannabinoids action (greater withdrawal, negative affect, and higher levels of craving to smoke more). Moreover, heterozygous had 20% higher marijuana dependence checklist scores than TT subjects [
20]. Di Marzo et al. demonstrated that marijuana abuse by carriers of the c.*2394A>G (rs12720071) variant of the
CNR1 gene contributes to deficits in the volume of white matter in the brains of schizophrenia patients [
8].
The second gene selected through GWAS is
CNR2 encoding CB2 (OMIM: 605051). It has been actively investigated for its role in osteoporosis, inflammatory responses, leukemia, and certain forms of cancer. However, investigations on the
CNR2 gene in the context of addiction are rarely described in the literature. The Ishiguro research group showed a link between the polymorphism c.188A>G (p.Gln63Arg, rs2501432) and alcohol dependence in the Japanese population [
21]. In turn, Carrasquer et al. introduced Gln63Arg, His316Tyr, and Gln63Arg/His316 mutations in recombinant human CB2 receptors. Mutant CB2 receptors were than transfected into HEK293 cells. Through this study, it was demonstrated that the CB2 polymorphic receptors at both positions 63 and 316 are able to bind cannabinoid ligands and mediate signal transduction which may contribute to the etiology of certain diseases [
22].
3.1.2 Trpv1
The
TRPV1 gene encodes the transient receptor potential action channel subfamily V member 1 (OMIM: 602076). So far, there have been no association studies that can demonstrate a relationship between the occurrence of
TRPV1 gene polymorphism and addiction to marijuana. Most research on the functioning of the endocannabinoid system (ECS) focuses on two well-characterized receptors: CB1 and CB2. In recent years, scientists have shown that this mechanism is much more complex and the ECS consisting of other types of receptors, which may affect the activity of ligands in many physiological processes [
23]. Arnold et al. have described that Δ
9tetrahydrocannabinol (Δ
9THC) binds to CB1 and CB2 receptors with similar affinities and behaves as a partial agonist, but it does not bind TRPV1 receptors. Cannabidiol (CBD) showed a more complex profile—simultaneous blockade of CB2 and TRPV1 significantly inhibited CBD-induced
MDR1 (multidrug resistance, also known as
ABCB1 gene, which was described in the next section) mRNA expression and both CB2 and TRPV1 antagonist were not effective when tested individually. CBD in its two enantiomeric forms showed no affinity for the CB1 and CB2 receptors (-)enantiomer or showed significant affinity for both receptor subtypes. It is, therefore, possible that the effectiveness of the CBD is related to inhibition of fatty acid amide hydrolase (FAAH) and elevates the level of anandamide (AEA). This would imply that AEA’s simultaneous activation of CB2 and TRPV1 is necessary for increased
MDR1 mRNA expression following CBD exposure [
23].
3.1.3 Gpr55
The human gene that encodes GPR55 protein is located on the long arm of chromosome 2 (2q37.1), encompasses 53,910 bases, and contains four exons (GeneCards, GeneBank, and Ensemble). The GPR55 receptor is consists of 319 amino acids and 7 hydrophobic domains. It belongs to a G-protein-coupled receptor superfamily, precisely to the rhodopsin-like (ClassA) family of GPCRs. The molecular mass of the protein is 36,637 Da. It is an integral membrane protein and its structure reminds structure of cannabinoid receptor. The GPR55 is de-orphanized as a CB receptor [
24]. The amino acid sequence is the most similar to the GPR35 (27%), P2Y (29%), GPR23 (30%), and CXCR4 (26%) but also to CB2 (14.4%) and CB1 (13.5%) receptors [
24]. It has been suggested that GPR55 is a novel CB receptor [
25].
Expression of
GPR55 (OMIM: 04107) in certain areas of the human brain is responsible for binding exo- and endogenous cannabinoids [
26,
27]. From the functional analysis of the sole missense polymorphism c.584G>T (p.Gly195Val, rs3749073) in the
GPR55 gene, which have a binding affinity for endocannabinoids, an association with anorexia nervosa has been observed [
28].
3.1.4 OPRM1 and GABRA2
Many reports in the literature describe the possible association of polymorphisms with cannabis dependence [
27,
29‐
32]. Particular attention is drawn to genes encoding opioid receptor (
OPRM1, OMIM: 600018) and gamma-aminobutyric acid (GABA) receptors (
GABRA2, OMIM: 137140).
Agraval et al. suggest cannabis dependence association with SNPs in
GABRA2 gene [
33], in opposite to Lind group results [
30]. They genotyped 11 SNPs within or flanking
GABRA2 in 4597 subjects and showed no correlation with alcohol, smoking, or cannabis dependence [
30].
OPRM1 gene encoding the mu opioid receptor (MOR) is also considered as candidate gene for cannabis use side effect. Gelernter and co-workers analyzed two polymorphisms affecting protein sequence in exon 1, Ala6Val and Asp40Asn in African American (AA), European American (EA), Hispanic, Japanese, Ethiopians, Bedouins, and Ashkenazi Jews populations. The entire study group consisted of 891 subjects presented differences in the frequency of alleles between the AA and EA populations have been observed. For other studied subjects, they did not observe significant heterogeneity among. For all analyzed populations, neither polymorphism appears to be a direct risk factor for substance dependence [
31].