Roles of functional catechol-O-methyltransferase genotypes in Chinese patients with Parkinson’s disease

Parkinson’s disease (PD) is a common progressive neurodegenerative disease in middle-aged and elderly people that consists of interactions between environmental and genetic factors and contributes to the dysfunction of dopaminergic neurotransmission in the central nervous system (CNS).

Genetic polymorphisms in enzymes, which regulate the biosynthesis or degradation of dopamine and its metabolites, may play a role in the susceptibility to PD and levodopa treatment. Catechol-O-methyltransferase (COMT) is one of the most important enzymes in the metabolism of drugs and neurotransmitters, such as L-dopa, dopamine, noradrenaline, etc. [1]; COMT-mediated O-methylation inactivates biologically active or toxic catechol and some hydroxylated metabolites [2]. Blocking of COMT activity further reduces peripheral levodopa degradation, as it prolongs plasma half-life of levodopa and elevates delivery of levodopa to the brain [3]. The human COMT gene is located on chromosome 22q11.21, and polymorphisms in COMT are associated with high, intermediate and low levels of enzyme activity, which might affect the risk and treatment of PD [4].

Most studies focused on the rs4680 single nucleotide polymorphisms (SNPs), which is a G to A substitution in the fourth exon of the COMT gene, leading to the substitution of valine 158 with a methionine (Val158Met) and resulting in low COMT enzyme activity, which is regarded as the L (low activity) allele, in contrast to the H (high activity) allele [5]. The COMT protein encoded by the L allele is thermolabile, which influences individual variations in the therapeutic response to levodopa and susceptibility to PD [6‐9]. Moreover, studies [1, 10‐12] have focused on three other common haplotypes comprising combinations of four SNPs in the COMT gene: rs6269 A > G, rs4633 C > T (His62His), rs4818 C > G (Leu136Leu) and rs4680 G > A (Val158Met). The haplotype structure formed by all four SNPs affects COMT enzymatic activity more than the single rs4680 SNP, which was regarded as a key determinant of COMT enzymatic activity in previous studies. COMT activity has been reported to depend on the presence of haplotypes formed by the four analyzed SNPs: A_C_C_G-low activity-L, A_T_C_A –medium activity-M, and G_C_G_G-high activity-H [11]. One study in Poland reported a high frequency of patients with late onset PD who were carriers of the G_C_G_G (high activity) haplotype, and the COMT haplotype seemed to have little influence on the development of levodopa-induced dyskinesia [1].

Additionally, some of the SNPs that have been studied with respect to PD risk exhibit linkage disequilibrium, although interethnic differences exist. According to the 1000 genome datasets (http://​browser.​1000genomes.​org/​Homo_​sapiens/​Search/​Results?​site=​ensembl&​q=​comt), Caucasian individuals show higher linkage values: the SNPs rs4680 and rs4633 exhibit complete linkage (D’ = 1.00), the SNPs rs4680 and rs4818 exhibit partial linkage (D’ = 0.685), and the SNPs rs4633 and rs4818 also exhibit partial linkage (D’ = 0.685). However, for Oriental individuals, all linkages showed lower values: D’ = 0.927 for the SNPs rs4680 and rs4633, D’ = 0.240 for the SNPs rs4680 and rs4818, and D’ = 0.223 for the SNPs rs4633 and rs4818 [4]. To our knowledge, no study has examined the associations between the two linked COMT SNPs (rs4680-rs4633 or rs6269-rs4818) with PD risk, clinical characteristics, and drug treatment. However, of the studies that have estimated the association between the combined two linked or four combined COMT SNPs and PD risk, the clinical symptoms and pharmacotherapy have not yet been investigated in China.

In this case–control study, we investigated the association of the most common functional COMT gene haplotypes (formed by SNPs: rs6269 A > G; rs4633C > T; rs4818 C > G; and rs4680 G > A), whether single, two linked or four combined SNPs, with PD risk in China. In addition, we examined the association of single, two linked or four combined COMT SNPs with the patients’ clinical features and pharmacotherapy, particularly the complications of levodopa therapy in patients with PD.

The COMT enzyme is a natural candidate that has been implicated in the pathogenesis of PD. In this study, we investigated the association between functional COMT haplotypes and PD susceptibility. We found that the frequencies of rs4633 T carriers, rs4680 A carriers and rs4633-rs4680 T/A carriers were significantly higher in the EOPD group than in the healthy control group, suggesting that rs4633 and rs4680 polymorphisms are associated with susceptibility to EOPD. Considering the sample size is small, larger cohort studies focus EOPD patients are needed. However, no other significant difference in the risk of the individual COMT gene variants for PD pathogenesis was observed. The results of previous studies addressing the possible association between SNPs in the COMT gene and the risk of developing PD are controversial, particularly for rs4680, which primarily determines COMT activity. One meta-analysis examined 24 studies of 9,719 patients with PD and 14,634 controls and concluded that the rs4680 polymorphism is not a major determinant of the risk for PD [4]. Based on another meta-analysis, the Val158Met polymorphism may be a risk factor associated with Parkinson’s disease in Asian rather than Caucasian populations [16]. Chuan, Gao [17] examined 13 studies including 1,834 patients and 2,298 controls and showed that the Val158Met polymorphism may be associated with PD in Japanese rather than Chinese populations. In conclusion, there is a remarkable heterogeneity in COMT gene variants in different ethnic groups.

Moreover, one study identified a decreased risk for PD in homozygous carriers of the COMT rs4680G and rs4633C alleles [12], whereas other studies focusing on the rs6269 [1], rs4818 [1, 18] and rs4633 SNPs [1, 19] did not detect a significant influence on PD susceptibility. However, Kimchi-Sarfaty [20] suggested that the determination of haplotypes is more appropriate than SNPs for the analysis of genetic variations in patients with pain, and Nackley, Shabalina [11] found that COMT haplotypes might have more influence on COMT activity than single SNPs. Few studies have focused on the four linked SNPs (rs6269, rs4818, rs4680 and rs4633) regarding pain intensity [21] or cognitive impairment in patients with PD [12]. Studies from Portland [1, 12] (including 322 patients with PD and 357 controls) did not demonstrate an association between functional COMT haplotypes in patients with PD and the controls; the frequencies of low (A_C_C_G) and medium (A_T_C_A) or without high (G_C_G_G) activity haplotypes tended to be slightly lower among patients with PD than among the controls. Unfortunately, our results did not identify an association of the four-combined functional COMT SNPs with patients and controls. The frequencies of low and without high haplotypes tended to be slightly higher among patients with PD, although the difference was not significant. Considering the ethnic factors, this result requires further evaluation.

In our results, both the linkage of rs6269-rs4818 and rs4680-rs4633 in the PD group were higher than that in the control group, such as D’ = 1.00 for rs4680 and rs4633 in control group, D’ = 0.91 for rs4680 and rs4633 in PD group. While according to the online data from the International HapMap Project (http://​hapmap.​ncbi.​nlm.​nih.​gov/​index.​html.​en) in 2016, the linkages of rs4633 with rs4680 are D’ = 1.0, LOD = 14.07, r2 = 0.887, and the linkage of rs6269 with rs4818 has not been shown in Chinese people. In particular, our results were different from data of Oriental individuals concluded from 1000 genome datasets in 2015 [4]. These data of the Oriental individuals were concluded from the northern and southern part in China (Beijing, Yunnan, Hunan and Fujian), while Shanghai, as the eastern part of China, was not involved in that program (http://​www.​internationalgen​ome.​org/​cell-lines-and-dna-coriell). According to the different geographic and host genetic factors, larger sample sizes and multi-center studies are needed in the future. Although no report has yet addressed the issue of dopamine catabolism in relation to the PD risk using a two-SNP model, we speculated that the difference might be associated with PD progression.

The patients with the T allele of rs4633 or the A allele of rs4680 and the TT/AA alleles of the two linked SNPs rs4633-rs4680 had a tendency to experience the “wearing-off” phenomenon, had a longer disease duration and larger daily LED and had higher UPDRS scores, suggesting that patients with rs4633 and rs4680 polymorphisms had more severe disease. Only one study focused on the association of rs4633 with medication and reported that the rs4633 polymorphism was associated with the UPDRS score but not with L-dopa medication [19]. Both rs4633 TT and rs4680 AA encode the low activity COMT enzyme, which may decrease COMT activity and dopamine degradation [8, 22]. The rs4680 SNP has been an area of intense research for several years. Our results were similar to those of other studies. A meta-analysis published in 2015 concluded that allele A of rs4680 was correlated with a risk of PD “wearing-off” [23]. As shown in the study by Watanabe, Harada [22], the “wearing-off” phenomenon tended to occur in patients with PD carrying the COMT rs4680 (AA) SNP. The authors proposed that decreased COMT activity might result in increased neuromelanin metabolism, which might subsequently promote the formation of cytotoxic radicals released upon neuromelanin interactions, contributing to neuronal degeneration. The possible effects of COMT SNPs on PD-correlated neuropharmacological variables are controversial. Bialecka, Kurzawski [1] reported a study of COMT haplotypes in Poland that did not identify a significant impact of COMT haplotypes on the development of the “wearing-off” phenomenon, longer disease duration and larger LED. Hao, Shao [9] found that the rs4680 GG allele may be a risk factor for the “wearing-off” phenomenon. The reasons for the discrepancy with our results may be that the patients involved in our study presented different disease severities. The mean disease duration of our patients was 6.0 years, which was longer than the 3 years of the patients in the study of Hao, Shao [9]. The mean disease duration of the patients in the study by Watanabe, Harada [22] was 9.4 years, which established the same conclusion as our study. In patients with low COMT activity, the prolonged duration and increased quantities of levodopa and dopamine in the plasma may lead to an increased accumulation, which may accelerate the neurodegenerative process. In addition to the “wearing-off” phenomenon, a recent prospective study showed that the homozygosity in the AA allele of the COMT Val158Met polymorphism increases the risk of dyskinesia, which also suggests that the AA genotype may decrease L-dopa metabolism [24]. Our study did not arrive at the same conclusion because of the limited number of patients who presented dyskinesia. Finally, there were no significant effects of rs6269 and rs4818 on the morbidity and pharmacotherapy of PD, and the results are consistent with other currently available studies [4].

However, our results concerning the association of COMT SNPs with PD susceptibility and medication were based on a small sample size, and further investigations, particularly a larger cohort study including patients with different disease severities, should investigate the role of combined functional COMT haplotypes in Chinese Han patients. Additionally, the limitation of technology (e.g., low throughput, only sequenced by segments, needed manual comparison) used in this research could not be ignored, and the novel technologies of high-throughput sequencing become more popular and easily available in the future.

In conclusion, we provide the first report highlighting the possible association of functional COMT haplotypes with the risk of PD in Chinese patients. The combined COMT genotype also showed a possible influence on the motor response to levodopa and disease severity, particularly the duration, the “wearing-off” phenomenon, daily LED and UPDRS in patients with PD, which may be useful in instituting individualized therapy for patients with PD.