Fibromyalgia (FM) represents a complex disorder characterized by widespread pain and tenderness and frequently accompanied by additional somatic and cognitive/affective symptoms. Genetic risk factors are known to contribute to the etiology of the syndrome, but few specific genetic variants have been identified to date and still require replication. In this study, a large scale candidate gene approach was used to examine over 350 genes for association with FM.
Four hundred ninety-six FM patients were included in the study as cases with a total of 348 chronic pain-free controls. Genotyping was performed using a dedicated gene array chip, the Pain Research Panel, which assays variants characterizing over 350 genes known to be involved in biological pathways relevant to nociception, inflammation, and mood. Association testing was performed using logistic regression.
Significant differences in allele frequencies between cases and controls were observed for three genes: GABRB3 (rs4906902, p = 3.65×10−6), TAAR1 (rs8192619, p = 1.11×10−5) and GBP1 (rs7911, p = 1.06×10−4). These three genes, and seven other genes with suggestive evidence for association, were examined in a second, independent cohort of FM patients and controls genotyped using the Perlegen 600K platform. Evidence of association in the replication cohort was observed for TAAR1, RGS4, CNR1, and GRIA4 genes.
Variation in these genes may serve as a basis for development of new diagnostic approaches, and these genes’ products may contribute to the pathophysiology of FM and represent potential targets for therapeutic action.
Evidence suggests that the effect of cigarette smoking on chronic pain is stronger in younger than older adults. This case control study investigated whether age modified an effect of smoking on temporomandibular disorder (TMD) in 299 females aged 18–60 years. It also investigated the extent to which this relationship was explained by psychological profile, inflammatory response and allergy. Cases were defined using the Research Diagnostic Criteria for Temporomandibular Disorders based on clinical examination. Psychological profile was evaluated using standardized instruments. Inflammatory response was evaluated with 11 cytokines isolated in plasma. History of allergy conditions was self-reported. Odds ratios (OR) for the effect of smoking were calculated using binary logistic regression. Stratified analyses and the likelihood ratio test examined effect modification by smoking. Compared to non-smokers, ever smokers aged <30 years had higher odds of TMD (OR =4.14, 95% CI: 1.57, 11.35) than older adults (OR =1.23, 95% CI: 0.55, 2.78) (P (effect modification) =0.038). Adjustment for psychological profile, cytokines and history of allergy-like conditions attenuated the effect by 45% to statistical non-significance. The main finding was reproduced with secondary analyses of two nationally-representative surveys of adults conducted in the U.S. and Australia.
Epidemiology; case-control study; temporomandibular joint disorder; psychological stress; allergy and hypersensitivity, cytokines
The ability to perceive noxious stimuli is critical for an animal's survival in the face of environmental danger, and thus pain perception is likely to be under stringent evolutionary pressure. Using a neuronal-specific RNAi knock-down strategy in adult Drosophila, we recently completed a genome-wide functional annotation of heat nociception that allowed us to identify α2δ3 as a novel pain gene. Here we report construction of an evolutionary-conserved, system-level, global molecular pain network map. Our systems map is markedly enriched for multiple genes associated with human pain and predicts a plethora of novel candidate pain pathways. One central node of this pain network is phospholipid signaling, which has been implicated before in pain processing. To further investigate the role of phospholipid signaling in mammalian heat pain perception, we analysed the phenotype of PIP5Kα and PI3Kγ mutant mice. Intriguingly, both of these mice exhibit pronounced hypersensitivity to noxious heat and capsaicin-induced pain, which directly mapped through PI3Kγ kinase-dead knock-in mice to PI3Kγ lipid kinase activity. Using single primary sensory neuron recording, PI3Kγ function was mechanistically linked to a negative regulation of TRPV1 channel transduction. Our data provide a systems map for heat nociception and reinforces the extraordinary conservation of molecular mechanisms of nociception across different species.
Nociception is the perception of noxious, potentially damaging stimuli; and this pain or its equivalent behavioral readout is evolutionarily conserved from fruit flies to humans. Using genetic techniques in the fruit fly, we have been able to evaluate the potential functional contribution of every gene in the fruit fly genome for a role in avoidance of high noxious temperatures (heat pain-like responses). Using this functional genomics data set, we have developed a conserved network map of heat pain/nociception that predicts numerous conserved genes and pathways as novel pain pathways, including phospholipid signaling. Studies in multiple mutant mice confirmed a role for lipid signaling in pain perception, and more specifically we identify the critical lipid kinase (PI3Kγ) as a negative regulator of TRPV1 (receptor for noxious heat and capsaicin, the active component in chili peppers) signaling. This finding shows that our fly-based genetic pain network map is a valuable tool for the discovery of novel “nociception genes” in mammals.
For reasons unknown, temporomandibular disorder (TMD) can manifest as localized pain or in conjunction with widespread pain. We evaluated relationships between cytokines and TMD without or with widespread palpation tenderness (TMD−WPT or TMD+WPT, respectively), at protein, transcription factory activity, and gene levels. Additionally, we evaluated the relationship between cytokines and intermediate phenotypes characteristic of TMD and WPT. In a case-control study of 344 females, blood samples were analyzed for levels of 22 cytokines and activity of 48 transcription factors. Intermediate phenotypes were measured by quantitative sensory testing and questionnaires asking about pain, health, and psychological status. Single nucleotide polymorphisms (SNPs) coding cytokines and transcription factors were genotyped. TMD−WPT cases had elevated protein levels of pro-inflammatory cytokine MCP-1 and anti-inflammatory cytokine IL-1ra, whereas TMD+WPT cases had elevated levels of pro-inflammatory cytokine IL-8. MCP-1, IL-1ra, and IL-8 were differentially associated with experimental pain, self-rated pain, self-rated health, and psychological phenotypes. TMD−WPT and TMD+WPT cases had inhibited transcription activity of the anti-inflammatory cytokine TGFβ1. Interactions were observed between TGFβ1 and IL-8 SNPs: an additional copy of the TGFβ1 rs2241719 minor T allele was associated with twice the odds of TMD+WPT among individuals homozygous for the IL-8 rs4073 major A allele and half the odds of TMD+WPT among individuals heterozygous for rs4073. These results demonstrate how pro- and anti-inflammatory cytokines contribute to the pathophysiology of TMD and WPT in genetically-susceptible people. Furthermore, they identify MCP-1, IL-1ra, IL-8, and TGFβ1 as potential diagnostic markers and therapeutic targets for pain in patients with TMD.
Epistasis; Gene polymorphism; Interleukin-1 receptor antagonist (IL-1ra); Interleukin-8 (IL-8); Monocyte chemotactic protein-1 (MCP-1); Transforming growth factor β1 (TGFβ1)
Opioid drugs are among the most commonly used and effective human analgesics. To date, the clinical benefits of opioid analgesics have not been fully realized due to substantial individual variations in the responses to opioids, insufficient drug dosing, and a high rate (up to 66%) of adverse events. As such, there is a substantial need to identify the genetic and molecular biological mechanisms that mediate individual responses to opioid therapy. Recent discoveries show that genetic variations in the μ-opioid receptor (OPRM1) gene locus play an essential role in inter-individual responses. The majority of genetic association studies have focused on the A118G polymorphism, which codes for a non-synonymous change in OPRM1 exon 1. In addition to the A118G polymorphism, another functional SNP (rs563649), which is located within an alternatively-spliced OPRM1 isoform (MOR-1K), has been identified. The MOR-1k isoform codes for 6TM OPRM1 isoforms that display excitatory rather than the inhibitory cellular effects, which are characteristic of the canonical 7TM isoforms. Thus, stimulation of the 6TM isoforms may engage the molecular mechanisms mediating opioid-dependent hyperalgesia, tolerance and dependence. Future clinical and basic studies that seek to identify the functional genetic variants within OPRM1 locus, and associated molecular mechanisms, will result in a better understanding of individual responses to opioid therapy and ultimately to the development new pharmacotherapeutics and diagnostic tools.
analgesic response to opioids; mu-opioid receptor (MOR); opioid receptor gene (OPRM1) structure; genetic association study; 6 transmembrane (6TM) MOR
Opioids that stimulate the μ-opioid receptor (MOR1) are the most frequently prescribed and effective analgesics. Here we present a structural model of MOR1. Molecular dynamics simulations show a ligand-dependent increase in the conformational flexibility of the third intracellular loop that couples with the G-protein complex. These simulations likewise identified residues that form frequent contacts with ligands. We validated the binding residues using site-directed mutagenesis coupled with radioligand binding and functional assays. The model was used to blindly screen a library of ~1.2 million compounds. From the thirty-four compounds predicted to be strong binders, the top three candidates were examined using biochemical assays. One compound showed high efficacy and potency. Post hoc testing revealed this compound to be nalmefene, a potent clinically used antagonist, thus further validating the model. In summary, the MOR1 model provides a tool for elucidating the structural mechanism of ligand-initiated cell signaling and screening for novel analgesics.
Several case-control studies have been conducted that examine the association between autonomic variables and persistent pain conditions; however, there is a surprising dearth of published studies in this area that have focused on temporomandibular disorders (TMD). The current study presents autonomic findings from the baseline case-control study of the OPPERA (Orofacial Pain: Prospective Evaluation and Risk Assessment) cooperative agreement. Measures of arterial blood pressure, heart rate, heart rate variability, and indirect measures of baroreflex sensitivity were assessed at rest and in response to a physical (orthostatic) and psychological (Stroop) stressors in 1633 TMD-free controls and 185 TMD cases. In bivariate and demographically adjusted analyses, greater odds of TMD case status were associated with elevated heart rates, reduced heart rate variability, and reduced surrogate measures of baroreflex sensitivity across all experimental procedures. Principal component analysis was undertaken to identify latent constructs revealing five components. These findings provide evidence of associations between autonomic factors and TMD. Future prospective analyses in the OPPERA cohort will determine if the presence of these autonomic factors predicts increased risk for developing new onset TMD.
Temporomandibular Disorders; Heart Rate Variability; Blood Pressure; Heart Rate; Baroreflex Sensitivity; Stroop; Orthostatic
Genetic factors play a role in the etiology of persistent pain conditions, putatively by modulating underlying processes such as nociceptive sensitivity, psychological well-being, inflammation, and autonomic response. However, to date, only a few genes have been associated with temporomandibular disorders (TMD). This study evaluated 358 genes involved in pain processes, comparing allelic frequencies between 166 cases with chronic TMD and 1442 controls enrolled in the OPPERA (Orofacial Pain: Prospective Evaluation and Risk Assessment) study cooperative agreement. To enhance statistical power, 182 TMD cases and 170 controls from a similar study were included in the analysis. Genotyping was performed using the Pain Research Panel, an Affymetrix gene chip representing 3295 single nucleotide polymorphisms, including ancestry-informative markers that were used to adjust for population stratification. Adjusted associations between genetic markers and TMD case status were evaluated using logistic regression. The OPPERA findings provided evidence supporting previously-reported associations between TMD and two genes: HTR2A and COMT. Other genes were revealed as potential new genetic risk factors for TMD, including NR3C1, CAMK4, CHRM2, IFRD1, and GRK5. While these findings need to be replicated in independent cohorts, the genes potentially represent important markers of risk for TMD and they identify potential targets for therapeutic intervention.
Pain genetics; temporomandibular joint disorders (TMD); association study; HTR2A; COMT; NR3C1; CAMK4; CHRM2; IFRD1; GRK5
The subcutaneous and systemic injection of serotonin reduces cutaneous and visceral pain thresholds and increases responses to noxious stimuli. Different subtypes of 5-hydroxytryptamine (5-HT) receptors are suggested to be associated with different types of pain responses. Here we show that serotonin also inhibits catechol O-methyltransferase (COMT), an enzyme that contributes to modultion the perception of pain, via non-competitive binding to the site bound by catechol substrates with a binding affinity comparable to the binding affinity of catechol itself (Ki = 44 μM). Using computational modeling, biochemical tests and cellular assays we show that serotonin actively competes with the methyl donor S-adenosyl-L-methionine (SAM) within the catalytic site. Binding of serotonin to the catalytic site inhibits the access of SAM, thus preventing methylation of COMT substrates. The results of in vivo animal studies show that serotonin-induced pain hypersensitivity in mice is reduced by either SAM pretreatment or by the combined administration of selective antagonists for β2- and β3-adrenergic receptors, which have been previously shown to mediate COMT-dependent pain signaling. Our results suggest that inhibition of COMT via serotonin binding contributes to pain hypersensitivity, providing additional strategies for the treatment of clinical pain conditions.
Genetic variations in the catechol-o-methyltransferase (COMT) gene have been associated with experimental pain and risk of chronic pain development, but no studies have examined genetic predictors of neck pain intensity and other patient characteristics after motor vehicle collision (MVC). We evaluated the association between COMT genotype and acute neck pain intensity and other patient characteristics in 89 Caucasian individuals presenting to the emergency department (ED) after MVC. In the ED in the hours after MVC, individuals with a COMT pain vulnerable genotype were more likely to report moderate to severe musculoskeletal neck pain (76% vs. 41%, RR = 2.11 (1.33 - 3.37)), moderate or severe headache (61% vs. 33%, RR = 3.15 (1.05 – 9.42), and moderate or severe dizziness (26% vs. 12%, RR = 1.97 (1.19 – 3.21)). Individuals with a pain vulnerable genotype also experienced more dissociative symptoms in the ED, and estimated a longer time to physical recovery (median 14 vs. 7 days, p = .002) and emotional recovery (median 8.5 vs. 7 days, p = .038). These findings suggest that genetic variations affecting stress response system function influence the somatic and psychological response to MVC, and provide the first evidence of genetic risk for clinical symptoms after MVC.
Worldwide, acute and chronic pain affects 20% of the adult population and represents an enormous financial and emotional burden. Using genome-wide neuronal-specific RNAi knock-down in Drosophila, we report a global screen for an innate behavior and identify hundreds of novel genes implicated in heat nociception, including the α2δ-family calcium channel subunit straightjacket (stj). Mice mutant for the stj ortholog CACNA2D3 (α2δ3) also exhibit impaired behavioral heat pain sensitivity. In addition, in humans, α2δ3 SNP variants associate with reduced sensitivity to acute noxious heat and chronic back pain. Functional imaging in α2δ3 mutant mice revealed impaired transmission of thermal pain evoked signals from the thalamus to higher order pain centers. Intriguingly, in α2δ3 mutant mice thermal pain and tactile stimulation triggered strong cross-activation or synesthesia of brain regions involved in vision, olfaction, and hearing.
Not all patients with nerve injury develop neuropathic pain. The extent of nerve damage and age at the time of injury are two of the few risk factors identified to date. In addition, preclinical studies show that neuropathic pain variance is heritable. To define such factors further, we performed a large-scale gene profiling experiment which plotted global expression changes in the rat dorsal root ganglion in three peripheral neuropathic pain models. This resulted in the discovery that the potassium channel alpha subunit KCNS1, involved in neuronal excitability, is constitutively expressed in sensory neurons and markedly downregulated following nerve injury. KCNS1 was then characterized by an unbiased network analysis as a putative pain gene, a result confirmed by single nucleotide polymorphism association studies in humans. A common amino acid changing allele, the ‘valine risk allele’, was significantly associated with higher pain scores in five of six independent patient cohorts assayed (total of 1359 subjects). Risk allele prevalence is high, with 18–22% of the population homozygous, and an additional 50% heterozygous. At lower levels of nerve damage (lumbar back pain with disc herniation) association with greater pain outcome in homozygote patients is P = 0.003, increasing to P = 0.0001 for higher levels of nerve injury (limb amputation). The combined P-value for pain association in all six cohorts tested is 1.14 E−08. The risk profile of this marker is additive: two copies confer the most, one intermediate and none the least risk. Relative degrees of enhanced risk vary between cohorts, but for patients with lumbar back pain, they range between 2- and 3-fold. Although work still remains to define the potential role of this protein in the pathogenic process, here we present the KCNS1 allele rs734784 as one of the first prognostic indicators of chronic pain risk. Screening for this allele could help define those individuals prone to a transition to persistent pain, and thus requiring therapeutic strategies or lifestyle changes that minimize nerve injury.
neuropathic pain; phenotype; molecular genetics; axonal injury; gene expression
Methyltransferases possess a homologous domain that requires both a divalent metal cation and S-adenosyl-L-methionine (SAM) to catalyze its reactions. The kinetics of several methyltransferases has been well characterized; however, the details regarding their structural mechanisms have remained unclear to date. Using catechol O-methyltransferase (COMT) as a model, we perform discrete molecular dynamics and computational docking simulations to elucidate the initial stages of cofactor binding. We find that COMT binds SAM via an induced-fit mechanism, where SAM adopts a different docking pose in the absence of metal and substrate in comparison to the holoenzyme. Flexible modeling of the active site side-chains is essential for observing the lowest energy state in the apoenzyme; rigid docking tools are unable to recapitulate the pose unless the appropriate side-chain conformations are given a priori. From our docking results, we hypothesize that the metal reorients SAM in a conformation suitable for donating its methyl substituent to the recipient ligand. The proposed mechanism enables a general understanding of how divalent metal cations contribute to methyltransferase function.
Catechol-O-methyltransferase (COMT) is a major enzyme controlling catecholamine levels that plays a central role in cognition, affective mood and pain perception. There are three common COMT haplotypes in the human population reported to have functional effects, divergent in two synonymous and one nonsynonymous position. We demonstrate that one of the haplotypes, carrying the non-synonymous variation known to code for a less stable protein, exhibits increased protein expression in vitro. This increased protein expression, which would compensate for lower protein stability, is solely produced by a synonymous variation (C166T) situated within the haplotype and located in the 5′ region of the RNA transcript. Based on mRNA secondary structure predictions, we suggest that structural destabilization near the start codon caused by the T allele could be related to the observed increase in COMT expression. Our folding simulations of the tertiary mRNA structures demonstrate that destabilization by the T allele lowers the folding transition barrier, thus decreasing the probability of occupying its native state. These data suggest a novel structural mechanism whereby functional synonymous variations near the translation initiation codon affect the translation efficiency via entropy-driven changes in mRNA dynamics and present another example of stable compensatory genetic variations in the human population.
Three common haplotypes in the gene encoding catechol-O-methyltransferase (COMT) have been associated with pain modulation and the risk of developing chronic musculoskeletal pain, namely temporomandibular disorder (TMD). Haplotypes coding for higher enzymatic activity were correlated with lower pain perception. Rodent studies showed that COMT inhibition increases pain sensitivity via β2/3-adrenergic receptors. We hypothesized that the non-selective β-adrenergic antagonist propranolol will reduce clinical and experimental pain in TMD patients in a manner dependent on the subjects’ COMT diplotype.
40 female Caucasian participants meeting the Research Diagnostic Criteria for TMD were genotyped for COMT polymorphisms and completed a randomized, double–blind, placebo-controlled, two-period crossover pilot study. Each period consisted of a baseline assessment week followed by an intervention week (propranolol or placebo). Changes in clinical pain ratings, psychological status, and responses to heat and pressure stimuli between baseline and intervention weeks were compared across periods.
The number of patients reporting a reduction in pain intensity rating was greater during propranolol treatment (p=0.014) compared with placebo. Propranolol significantly reduced a composite pain index (p=0.02) but did not decrease other clinical and experimental pain ratings. When stratified by the COMT high activity haplotype, a beneficial effect of propranolol on pain perception was noted in subjects not carrying this haplotype, a diminished benefit was observed in the heterozygotes, and no benefit was noted in the homozygotes.
COMT haplotypes may serve as genetic predictors of propranolol treatment outcome, identifying a subgroup of TMD patients who will benefit from propranolol therapy.
propranolol; β-blockers; pharmacogenetics; chronic pain; temporomandibular joint disorder (TMD); catechol-O-methyltransferase (COMT); polymorphism
The β3-adrenergic receptor (β3AR) is an essential regulator of metabolic and endocrine functions. A major cellular and clinically significant consequence of β3AR activation is the substantial elevation in interleukin-6 (IL-6) levels. Although the β3AR-dependent regulation of IL-6 expression is well established, the cellular pathways underlying this regulation have not been characterized. Using a novel method of homogenous reporters, we assessed the pattern of activation of 43 transcription factors in response to the specific β3AR agonist CL316243 in adipocytes, cells that exhibit the highest expression of β3ARs. We observed a unique and robust activation of the CRE-response element, suggesting that IL-6 transcription is regulated via the Gs-protein/cAMP/protein kinase A (PKA) but not nuclear factor kappa B (NF-κB) pathway. However, pretreatment of adipocytes with pharmacologic inhibitors of PKA pathway failed to block β3AR-mediated IL-6 up-regulation. Additionally, stimulation of adipocytes with the exchange protein directly activated by cAMP (Epac) agonist did not induce IL-6 expression. Instead, the β3AR-mediated transcription of IL-6 required activation of both the p38 and PKC pathways. Western blot analysis further showed that transcription factors CREB and ATF-2 but not ATF-1 were activated in a p38- and PKC-dependent manner. Collectively, our results suggest that while stimulation of the β3AR leads to a specific activation of CRE-dependent transcription, there are several independent cellular pathways that converge at the level of CRE-response element activation, and in the case of IL-6 this activation is mediated by p38 and PKC but not PKA pathways.
Cytokine; p38; PKC; obesity; inflammation
Opioids are the most widely used analgesics for the treatment of clinical pain. They produce their therapeutic effects by binding to μ-opioid receptors (MORs), which are 7 transmembrane domain (7TM) G-protein-coupled receptors (GPCRs), and inhibiting cellular activity. However, the analgesic efficacy of opioids is compromised by side-effects such as analgesic tolerance, dependence and opioid-induced hyperalgesia (OIH). In contrast to opioid analgesia these side effects are associated with cellular excitation. Several hypotheses have been advanced to explain these phenomena, yet the molecular mechanisms underlying tolerance and OIH remain poorly understood.
We recently discovered a new human alternatively spliced isoform of MOR (MOR1K) that is missing the N-terminal extracellular and first transmembrane domains, resulting in a 6TM GPCR variant. To characterize the pattern of cellular transduction pathways activated by this human MOR1K isoform, we conducted a series of pharmacological and molecular experiments. Results show that stimulation of MOR1K with morphine leads to excitatory cellular effects. In contrast to stimulation of MOR1, stimulation of MOR1K leads to increased Ca2+ levels as well as increased nitric oxide (NO) release. Immunoprecipitation experiments further reveal that unlike MOR1, which couples to the inhibitory Gαi/o complex, MOR1K couples to the stimulatory Gαs complex.
The major MOR1 and the alternative MOR1K isoforms mediate opposite cellular effects in response to morphine, with MOR1K driving excitatory processes. These findings warrant further investigations that examine animal and human MORK1 expression and function following chronic exposure to opioids, which may identify MOR1K as a novel target for the development of new clinically effective classes of opioids that have high analgesic efficacy with diminished ability to produce tolerance, OIH, and other unwanted side-effects.
In genetic mapping of complex traits, scored haplotypes are likely to represent only a subset of all causal polymorphisms. At the extreme of this scenario, observed polymorphisms are not themselves functional, and only linked to causal ones via linkage disequilibrium (LD). We will demonstrate that due to such incomplete knowledge regarding the underlying genetic mechanism, the variance of a trait may become different between the scored haplotypes. Thus, unequal variances between haplotypes may be indicative of additional functional polymorphisms affecting the trait. Methods accounting for such haplotype-specific variance may also provide an increased power to detect complex associations. We suggest ways to estimate and test these haplotypic variance contrasts, and incorporate them into the haplotypic tests for association. We further extend this approach to data with unknown gametic phase via likelihood-based simultaneous estimation of haplotypic effects and their frequencies. We find our approach to provide additional power, especially under the following types of models: (a) where scored and unobserved variants are epistatically interacting with each other; and (b) under heterogeneity models, where multiple unobserved mutations are linked to nonfunctional observed polymorphisms via LD. An illustrative example of usefulness of the method is discussed, utilizing analysis of multilocus effects within the catechol-O-methyl transferase (COMT) gene.
haplotype association; complex disease; epistasis; gene-gene interactions
Catechol-O-methyltransferase (COMT) is an enzyme that plays a key role in the modulation of catechol-dependent functions such as cognition, cardiovascular function, and pain processing. Three common haplotypes of the human COMT gene, divergent in two synonymous and one nonsynonymous (val158met) position, designated as low (LPS), average (APS), and high pain sensitive (HPS), are associated with experimental pain sensitivity and risk of developing chronic musculoskeletal pain conditions. APS and HPS haplotypes produce significant functional effects, coding for 3- and 20-fold reductions in COMT enzymatic activity, respectively. In the present study, we investigated whether additional minor single nucleotide polymorphisms (SNPs), accruing in 1 to 5% of the population, situated in the COMT transcript region contribute to haplotype-dependent enzymatic activity. Computer analysis of COMT ESTs showed that one synonymous minor SNP (rs769224) is linked to the APS haplotype and three minor SNPs (two synonymous: rs6267, rs740602 and one nonsynonymous: rs8192488) are linked to the HPS haplotype. Results from in silico and in vitro experiments revealed that inclusion of allelic variants of these minor SNPs in APS or HPS haplotypes did not modify COMT function at the level of mRNA folding, RNA transcription, protein translation, or enzymatic activity. These data suggest that neutral variants are carried with APS and HPS haplotypes, while the high activity LPS haplotype displays less linked variation. Thus, both minor synonymous and nonsynonymous SNPs in the coding region are markers of functional APS and HPS haplotypes rather than independent contributors to COMT activity.
Catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines, has recently been implicated in the modulation of pain. Specifically, low COMT activity is associated with heightened pain perception and development of musculoskeletal pain in humans as well as increased experimental pain sensitivity in rodents.
We report that the proinflammatory cytokine tumor necrosis factor α (TNFα) downregulates COMT mRNA and protein in astrocytes. Examination of the distal COMT promoter (P2-COMT) reveals a putative binding site for nuclear factor κB (NF-κB), the pivotal regulator of inflammation and the target of TNFα. Cell culture assays and functional deletion analyses of the cloned P2-COMT promoter demonstrate that TNFα inhibits P2-COMT activity in astrocytes by inducing NF-κB complex recruitment to the specific κB binding site.
Collectively, our findings provide the first evidence for NF-κB-mediated inhibition of COMT expression in the central nervous system, suggesting that COMT contributes to the pathogenesis of inflammatory pain states.
The μ-opioid receptor (OPRM1) is the principal receptor target for both endogenous and exogenous opioid analgesics. There are substantial individual differences in human responses to painful stimuli and to opiate drugs that are attributed to genetic variations in OPRM1. In searching for new functional variants, we employed comparative genome analysis and obtained evidence for the existence of an expanded human OPRM1 gene locus with new promoters, alternative exons and regulatory elements. Examination of polymorphisms within the human OPRM1 gene locus identified strong association between single nucleotide polymorphism (SNP) rs563649 and individual variations in pain perception. SNP rs563649 is located within a structurally conserved internal ribosome entry site (IRES) in the 5′-UTR of a novel exon 13-containing OPRM1 isoforms (MOR-1K) and affects both mRNA levels and translation efficiency of these variants. Furthermore, rs563649 exhibits very strong linkage disequilibrium throughout the entire OPRM1 gene locus and thus affects the functional contribution of the corresponding haplotype that includes other functional OPRM1 SNPs. Our results provide evidence for an essential role for MOR-1K isoforms in nociceptive signaling and suggest that genetic variations in alternative OPRM1 isoforms may contribute to individual differences in opiate responses.
Adrenergic receptor β2 (ADRB2) is a primary target for epinephrine. It plays a critical role in mediating physiological and psychological responses to environmental stressors. Thus, functional genetic variants of ADRB2 will be associated with a complex array of psychological and physiological phenotypes. These genetic variants should also interact with environmental factors such as physical or emotional stress to produce a phenotype vulnerable to pathological states. In this study, we determined whether common genetic variants of ADRB2 contribute to the development of a common chronic pain condition that is associated with increased levels of psychological distress and low blood pressure, factors which are strongly influenced by the adrenergic system. We genotyped 202 female subjects and examined the relationships between three major ADRB2 haplotypes and psychological factors, resting blood pressure, and the risk of developing a chronic musculoskeletal pain condition - Temporomandibular Joint Disorder (TMD). We propose that the first haplotype codes for lower levels of ADRB2 expression, the second haplotype codes for higher ADRB2 expression, and the third haplotype codes for higher receptor expression and rapid agonist-induced internalization. Individuals who carried one haplotype coding for high and one coding for low ADRB2 expression displayed the highest positive psychological traits, had higher levels of resting arterial pressure, and were about 10 times less likely to develop TMD. Thus, our data suggest that either positive or negative imbalances in ADRB2 function increase the vulnerability to chronic pain conditions such as TMD through different etiological pathways that imply the need for tailored treatment options.
adrenergic receptor β2; haplotype; SNPs; chronic pain; blood pressure; somatization; depression; anxiety; negative moods
Traditionally, four groups of factors have been identified in the etiology of temporomandibular disorder (TMD): anatomical variation in the masticatory system; psychosocial characteristics; pain in other body regions; and demographics. Orthodontic treatment has been variously cited both as a protective and harmful factor in TMD etiology. Recently, a search has begun for a genetic influence on TMD etiology. Genetic markers can be of additional value in identifying gene-environment interactions, that is, isolating population sub-groups, defined by genotype in which environmental influences play a relatively greater or lesser etiological role. This paper reviews concepts and study design requirements for epidemiological investigations into TMD etiology. Findings are presented from a prospective cohort study of 186 females that illustrate an example of gene-environment interaction in TMD onset. Among people with a variant of the gene encoding catechol-O-methyl-transferase, an enzyme associated with pain responsiveness, risk of developing TMD was significantly greater for subjects who reported a history of orthodontic treatment compared with subjects who did not (P=0.04). While further studies are needed to investigate TMD etiology, this genetic variant potentially could help to identify patients whose risk of developing TMD is heightened following orthodontic treatment, hence serving as a risk marker useful in planning orthodontic care.