Epistasis between polymorphisms in COMT, ESR1, and GCH1 influences COMT enzyme activity and pain

Abstract

Abnormalities in the enzymatic activity of catechol-O-methyltransferase (COMT) contribute to chronic pain conditions, such as temporomandibular disorders (TMD). Thus, we sought to determine the effects of polymorphisms in COMT and functionally related pain genes in the COMT pathway (estrogen receptor 1 [ESR1], guanosine-5-triphosphate cyclohydrolase 1 [GCH1], methylenetetrahydrofolate reductase [MTHFR]) on COMT enzymatic activity, musculoskeletal pain, and pain-related intermediate phenotypes among TMD cases and healthy control subjects. Results show that the COMT rs4680 (val(158)met) polymorphism is most strongly associated with outcome measures, such that individuals with the minor A allele (met) exhibit reduced COMT activity, increased TMD risk, and increased musculoskeletal pain. Epistatic interactions were observed between the COMT rs4680 polymorphism and polymorphisms in GCH1 and ESR1. Among individuals with the COMT met allele, those with 2 copies of the GCH1 rs10483639 minor G allele exhibit normalized COMT activity and increased mechanical pain thresholds. Among individuals with the COMT val allele, those with 2 copies of the ESR1 rs3020377 minor A allele exhibit reduced COMT activity, increased bodily pain, and poorer self-reported health. These data reveal that the GCH1 minor G allele confers a protective advantage among met carriers, whereas the ESR1 minor A allele is disadvantageous among val carriers. Furthermore, these data suggest that the ability to predict the downstream effects of genetic variation on COMT activity is critically important to understanding the molecular basis of chronic pain conditions.

Keywords: Catecholamines; Fibromyalgia; Gene expression; Gene–gene interaction; Temporomandibular disorders.

Fig. 1. COMT genotype effects COMT activity

A. COMT val¹⁵⁸met genotype effects COMT activity, such that individuals with 1 or 2 copies of the met allele exhibit reduced activity. Data represent mean ± s.e.m. ***p < 0.0001. B. Linkage disequilibrium plot of the genotyped SNPs in the COMT gene locus. The gene diagram indicates the location of each SNP with respect to exons (boxes) and introns of the gene. The color of each box in the LD plot depicts the strength of LD between aligned SNPs according to D’ (darker red indicates stronger LD), and the number in the box is the corresponding r² value. C. COMT activity levels of each genotype are shown for all COMT SNPs tested. Symbols and lines are colored green if the respective minor allele is associated with increased COMT activity, and red with decreased COMT activity; the rs4680 val¹⁵⁸met is shown in black. In the top panel genotype effects of each SNP are unadjusted; in the bottom panel each SNP effect is adjusted for val¹⁵⁸met genotype. D. COMT activity level of each genotype of rs737865, stratified by val¹⁵⁸met genotype, shows that the minor G allele of this SNP is associated with decreased COMT activity after the confounding effect of val¹⁵⁸met is removed.

Fig. 2. ESR1 SNP effects on COMT activity

All three ESR1 SNPs showed significant (p<0.05) association with COMT activity using a codominant model, although the dominant model provides a better fit for the data for rs3020377. Data represent mean ± s.e.m.

Fig. 3. COMT × ESR1 effects on COMT activity and pain phenotypes

For genotypes of the ESR1 SNP rs3020377, all carriers of the minor G allele were combined (dominant test) and stratified by carriers of COMT alleles val and met. Significance level shown is that of the test of the G allele carriers against the reference of the major allele homozygote group, within val carriers only. Among val carriers, the ESR1 rs3020377 SNP produced effects on A. COMT enzymatic activity, B. physical functioning subscale of the SF12 questionnaire, C. pain intensity rating using the Gracely scale of the SPSR, D. pain unpleasantness rating using the Gracely Pain Scale of the SPSR, and E. count of the number of bodily sites painful upon palpation by clinical examiner. For each phenotype, rs3020377 modulates the effect of the val allele, but not the met allele, of COMT. The G allele of rs3020377 decreased COMT activity and physical functioning, and increased nociceptive phenotypes of pain intensity, unpleasantness, and distribution across body sites. Data are quantitative trait means ± s.e.m. † p < 0.1 * p< 0.05 ** p<0.01.

Fig. 4. COMT × GCH1 effects on COMT activity and pain phenotypes

The GCH1 SNP rs10483639 showed a recessive mode of inheritance, and was stratified by carriers of the COMT alleles val and met. Significance level shown is that of the test of the C allele homozygotes against the reference of the major allele carriers, within met carriers only. Among met carriers, the GCH1 rs10483639 SNP produced effects on A. COMT enzymatic activity, B. algometer pressure at which pain is detected, measured at the temporalis muscle, C. depression subscale of the SCL90 questionnaire, D. hostility subscale of the SCL90, E. elated-depressed axis of the POMS, F. energetic-tired axis of the POMS. G. distress subscale of the PSS, and H. dysfunction in daily activities due to emotional factors, measured by the SF12 questionnaire. Carrying two copies of the rs10483639 C allele raised COMT activity in met carriers to the level of val carriers, but had no effect in the high-activity val carriers. The C allele was associated with higher algometer threshold in both val and met carriers, but was associated with more negative psychological characteristics in met carriers only. Data are quantitative trait means ± s.e.m. * p< 0.05.

Fig. 5. Three-way interaction between COMT, ESR1, and GCH1 SNPs on COMT activity and TMD risk

A. COMT enzyme activity (means ± s.e.m.) and B. TMD frequency is plotted for each genotype. The GCH1 rs10483639 C allele increases COMT activity and reduces TMD risk among met carriers, while the ESR1 rs3020377 G allele decreases COMT activity and increases TMD risk among val carriers with only 0 or 1 copy of the GCH1 rs10483639 C allele.