Bearing in mind that temperamental traits
Bearing in mind that temperamental traits are genetically determined, in the present study we have chosen to investigate COMT and OPRM1 gene polymorphisms related to stimulation of the nervous system, which is extremely important in combat sports.
The μ-opioid receptor gene OPRM1 is located on the chromosome 6 at [6q25.2]. It consists of 4 exons. The OPRM1 polymorphism (rs1799971) studied is a substitution of A with G (A118G) located in exon 1 and results in a substitution of asparagine for aspartic needs your attention at codon 40, in the extracellular N-terminal domain of the μ-opioid receptor (van den Wildenberg et al., 2007).
The Catechol-O-methyltransferase COMT gene is located on chromosome 22 at [22q11.21]. It consists of 6 exons.
The Val(108/158)Met functional polymorphism in the COMT gene is located in exon 4 and results in replacing guanine with adenine at position 472 in the long mRNA and at position 322 in the short mRNA. The polymorphism results in a substitution of valine with methionine in the membrane-bound long form (MB-COMT) at codon 158 and in the soluble short form (S-COMT) at codon 108 (Liang et al., 2012, Männisto and Kaakkola, 1999). The substitution of valine with methionine results in a reduced COMT activity.
The association between temperamental traits measured with the FCB-TI test and genetic determinants was studied by Oniszczenko et al. (2003) in 1500 twins, showing that the genetic influence was the highest for the FCB-TI scales in self- and peer reports. Peciña, Love, Stohler, Goldman, and Zubieta (2015) studied the association of OPRM1 rs1799971 polymorphism and personality. Their results point to a potentially significant role of OPRM1 A118G substitution in diseases characterized by vulnerability to physical and emotional stressors, e.g. persistent pain conditions, addictions and mood disorders.
In relation to the literature on genetic determinants of temperamental traits, particularly concerning genes encoding neurotransmitter system receptors in the brain, in the present study we have chosen to study the association of two functional polymorphisms in the COMT and OPRM1 genes with the FCB-TI scores to determine their impact on temperamental traits in athletes.
Results Temperamental profiles of both groups are presented in Table 1. Differences between the groups have been observed for 3 out of 6 measured traits and point to higher temperament strength and resilience in combat athletes. Perseveration and emotional reactivity in athletes was significantly lower in comparison with the control group, while endurance of the nervous system was significantly higher. The frequencies of both OPRM1 rs1799971 and COMT rs4680 alleles and genotypes were similar among athletes and controls (Table 2, Table 4). Due to the low frequency of OPRM1 GG homozygotes (only 2 subjects), AG and GG genotypes were pooled together for further analyses (minor allele carriers). When analysing the OPRM1 genotype in relation to temperamental traits, the only significant genotype-dependent association was observed for sensitivity in athletes: subjects carrying at least one minor G allele were characterized by higher sensitivity (14.5±3.45) when compared with AA wild-type homozygotes (13.08±3.47). However, such an association was not observed in the controls (Table 3). In the case of the COMT rs4680 variant, several significant differences were identified between subjects stratified by the COMT genotype (Table 5). Among the athletes, homozygous subjects with the G allele (GG) were characterized by lower sensitivity, compared with the A allele carriers (AA or AG). Instead, AA homozygotes presented lower endurance than G allele carriers (GG or AG). The data suggest that the G allele may be associated with lower sensitivity and higher endurance among the athletes. The G allele may be also associated with increased briskness because in AG heterozygotes that parameter was significantly higher in comparison with AA subjects (16.63±2.63 vs 15.51±2.86). However, that association is uncertain, as there were no significant differences between GG and AA homozygotes. Similarly, the difference in reactivity was significant only in heterozygotes compared to GG homozygotes. With regard to the control group, the COMT GG genotype was associated with lower perseveration, compared with A allele carriers (AA+AG, p=0.0498). Among the students, subjects with the GG allele also showed lower reactivity, compared to AA homozygotes (p=0.026), as well as AG heterozygotes (p=0.018). No further significant associations were observed for the remaining traits.