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Musculoskeletal pain conditions, such as fibromyalgia and low back pain, tend to coexist in affected individuals and are characterized by a report of pain greater than expected based on the results of a standard physical evaluation. The pathophysiology of these conditions is largely unknown, we lack biological markers for accurate diagnosis, and conventional therapeutics have limited effectiveness. Growing evidence suggests that chronic pain conditions are associated with both physical and psychological triggers, which initiate pain amplification and psychological distress; thus, susceptibility is dictated by complex interactions between genetic and environmental factors. Herein, we review phenotypic and genetic markers of common musculoskeletal pain conditions, selected based on their association with musculoskeletal pain in previous research. The phenotypic markers of greatest interest include measures of pain amplification and 'psychological' measures (such as emotional distress, somatic awareness, psychosocial stress and catastrophizing). Genetic polymorphisms reproducibly linked with musculoskeletal pain are found in genes contributing to serotonergic and adrenergic pathways. Elucidation of the biological mechanisms by which the markers contribute to the perception of pain in these patients will enable the development of novel effective drugs and methodologies that permit better diagnoses and approaches to personalized medicine. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Pain is emotionally detrimental and consciously avoided; however, it is absolutely crucial for our survival. Pain perception is one of the most complicated measurable traits because it is an aggregate of several phenotypes associated with peripheral and central nervous system dynamics, stress responsiveness and inflammatory state. As a complex trait, it is expected to have a polygenic nature shaped by environmental pressures. Here we discuss what is known about these contributing genetic variants, including recent discoveries that show a crucial role of voltage-gated sodium channel Nav1.7 in pain perception and how we can advance our understanding of the pain genetic network. We propose how both rare deleterious genetic variants and common genetic polymorphisms are mediators of human pain perception and clinical pain phenotypes. [Copyright &y& Elsevier]

Abstract: Variations in the gene encoding catechol-O-methyltransferase (COMT) are linked to individual differences in pain sensitivity. A single nucleotide polymorphism (SNP) in codon 158 (val 158 met), which affects COMT protein stability, has been associated with the human experience of pain. We recently demonstrated that three common COMT haplotypes, which affect the efficiency of COMT translation, are strongly associated with a global measure of pain sensitivity derived from individuals’ responses to noxious thermal, ischemic, and pressure stimuli. Specific haplotypes were associated with low (LPS), average (APS), or high (HPS) pain sensitivity. Although these haplotypes included the val 158 met SNP, a significant association with val 158 met variants was not observed. In the present study, we examined the association between COMT genotype and specific pain-evoking stimuli. Threshold and tolerance to thermal, ischemic, and mechanical stimuli, as well as temporal summation to heat pain, were determined. LPS/LPS homozygotes had the least, APS/APS homozygotes had average, and APS/HPS heterozygotes had the greatest pain responsiveness. Associations were strongest for measures of thermal pain. However, the rate of temporal summation of heat pain did not differ between haplotype combinations. In contrast, the val 158 met genotype was associated with the rate of temporal summation of heat pain, but not with the other pain measures. This suggests that the val 158 met SNP plays a primary role in variation in temporal summation of pain, but that other SNPs of the COMT haplotype exert a greater influence on resting nociceptive sensitivity. Here, we propose a mechanism whereby these two genetic polymorphisms differentially affect pain perception. [Copyright &y& Elsevier]

p53 tumor suppressor gene controls cell response to a variety of stresses inducing growth arrest or apoptosis in damaged cells. It largely determines the sensitivity of tumor and normal cells to radiation and chemotherapy, and, therefore, defines both the efficacy and limitations of anti-cancer treatment. To determine molecular mechanisms of p53-dependent stress response in normal tissues we identified and compared the spectra of radiation-responsive genes in cells of different origin and p53 status using a cDNA array hybridization technique. The majority of genes identified were p53-dependent and cell type specific. Several of the new p53 responders encode known secreted growth inhibitory factors. This suggests that p53, in addition to its intrinsic antiproliferation activity, can cause `bystander effect' by inducing export of growth suppressive stimuli from damaged cells to neighboring cells. Consistently, a p53-dependent accumulation of factors, which causes growth inhibitory effects in a variety of cell lines, was found after gamma irradiation in the media from established and primary cell cultures and in the urine of irradiated mice. Moreover, p53-dependent factors released by normal human fibroblasts potentiated the cytotoxic effect of a chemotherapeutic drug on co-cultivated tumor cells. This suggests a previously unknown role for normal cells in chemo- and radiation therapy of cancer. [ABSTRACT FROM AUTHOR]

Intracellular calcium is critical in orchestrating neuronal excitability and analgesia. Carbonic anhydrase-8 (CA8) regulates intracellular calcium signaling through allosteric inhibition of neuronal inositol trisphosphate receptor 1 (ITPR1) to produce profound analgesia. Recently, we reported the "G" allele at rs6471859 represents cis-eQTL regulating alternative splicing of a 1697 bp transcript (CA8-204G) with a retained intron, alternative polyadenylation site and a new stop codon producing a functional 26 kDa peptide with an extended exon 3. In this study we show the reversion mutation (G to C) at rs6471859 within the CA8-204G expression vector also produced a stable 1697 bp transcript (CA8-204C) coding for a smaller peptide (~ 22 kDa) containing only the first three CA8 exons. Surprisingly, this peptide inhibited ITPR1 (pITPR1) activation, ITPR1-mediated calcium release in vitro; and produced profound analgesia in vivo. This is the first report showing CA8-204C codes for a functional peptide sufficient to regulate calcium signaling and produce profound analgesia. [ABSTRACT FROM AUTHOR]

Genetic variation in melanocortin-1 receptor (MC1R) has a known role in red hair. Studies on responses to noxious stimuli in red-haired individuals have also been conducted, with mixed findings. To investigate a possible divergence between variants responsible for red hair and pain sensitivity, we performed a genewide association analysis in the Orofacial Pain: Prospective Evaluation and Risk Assessment cohort. All genotyped (17) MC1R variants were tested for association with heat pain temporal summation and sensitivity. Our analyses showed an association for pain sensitivity with the 5'-UTR, tagged by rs3212361, and 1 missense variant, rs885479 (R163Q), previously shown to be weakly associated with red hair. For both variants, the minor allele was protective. These results were validated in the 500,000-person UK Biobank cohort, where the minor alleles of rs3212361 and rs885479 were associated with a reduced count of persistent pain conditions as well as individual pain conditions. Haplotype association analysis revealed a possible joint effect from the 2 individual variants. The 5'-UTR variant rs3212361 was further identified as an expression quantitative trait locus, associated with reduced transcript levels of MC1R in the brain and in the peripheral tibial nerve. Hair colour association analysis of the loss-of-function 5'-UTR rs3212361 allele identified association with red hair, and red hair colour itself was associated with a reduced count of persistent pain conditions. Together, our results suggest that primarily different mechanisms-affecting expression levels vs protein activity-mediated by different genetic variants in the MC1R locus contribute to red hair and pain. [ABSTRACT FROM AUTHOR]

Temporomandibular disorders (TMDs), collectively representing one of the most common chronic pain conditions, have a substantial genetic component, but genetic variation alone has not fully explained the heritability of TMD risk. Reasoning that the unexplained heritability may be because of DNA methylation, an epigenetic phenomenon, we measured genome-wide DNA methylation using the Illumina MethylationEPIC platform with blood samples from participants in the Orofacial Pain: Prospective Evaluation and Risk Assessment (OPPERA) study. Associations with chronic TMD used methylation data from 496 chronic painful TMD cases and 452 TMD-free controls. Changes in methylation between enrollment and a 6-month follow-up visit were determined for a separate sample of 62 people with recent-onset painful TMD. More than 750,000 individual CpG sites were examined for association with chronic painful TMD. Six differentially methylated regions were significantly (P < 5 × 10-28) associated with chronic painful TMD, including loci near genes involved in the regulation of inflammatory and neuronal response. A majority of loci were similarly differentially methylated in acute TMD consistent with observed transience or persistence of symptoms at follow-up. Functional characterization of the identified regions found relationships between methylation at these loci and nearby genetic variation contributing to chronic painful TMD and with gene expression of proximal genes. These findings reveal epigenetic contributions to chronic painful TMD through methylation of the genes FMOD, PM20D1, ZNF718, ZFP57, and RNF39, following the development of acute painful TMD. Epigenetic regulation of these genes likely contributes to the trajectory of transcriptional events in affected tissues leading to resolution or chronicity of pain. [ABSTRACT FROM AUTHOR]

G protein-coupled receptor 37-like 1 (GPR37L1) is an orphan GPCR with largely unknown functions. Here, we report that Gpr37l1/GRP37L1 ranks among the most highly expressed GPCR transcripts in mouse and human dorsal root ganglia (DRGs) and is selectively expressed in satellite glial cells (SGCs). Peripheral neuropathy induced by streptozotoxin (STZ) and paclitaxel (PTX) led to reduced GPR37L1 expression on the plasma membrane in mouse and human DRGs. Transgenic mice with Gpr37l1 deficiency exhibited impaired resolution of neuropathic pain symptoms following PTXand STZ-induced pain, whereas overexpression of Gpr37l1 in mouse DRGs reversed pain. GPR37L1 is coexpressed with potassium channels, including KCNJ10 (Kir4.1) in mouse SGCs and both KCNJ3 (Kir3.1) and KCNJ10 in human SGCs. GPR37L1 regulates the surface expression and function of the potassium channels. Notably, the proresolving lipid mediator maresin 1 (MaR1) serves as a ligand of GPR37L1 and enhances KCNJ10- or KCNJ3-mediated potassium influx in SGCs through GPR37L1. Chemotherapy suppressed KCNJ10 expression and function in SGCs, which MaR1 rescued through GPR37L1. Finally, genetic analysis revealed that the GPR37L1-E296K variant increased chronic pain risk by destabilizing the protein and impairing the protein's function. Thus, GPR37L1 in SGCs offers a therapeutic target for the protection of neuropathy and chronic pain. [ABSTRACT FROM AUTHOR]

The article presents translational research called OPPERA in the genomic era. Topics discussed include the coexistence of temporomandibular disorder (TMD) with the other chronic pain conditions, the association of single single-nucleotide polymorphisms with TMD risk, and the attribution of pain to the biological mechanism markers.

Carbonic anhydrase-8 (CA8) is an intracellular protein that functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) critical to intracellular Ca++ release, synaptic functions and neuronal excitability. We showed previously that murine nociception and analgesic responses are regulated by the expression of this gene in dorsal root ganglion (DRG) associated with a cis-eQTL. In this report, we identify an exon-level cis-eQTL (rs6471859) that regulates human DRG CA8 alternative splicing, producing a truncated 1,697bp transcript (e.g., CA8-204). Our functional genomic studies show the “G” allele at rs6471859 produces a cryptic 3’UTR splice site regulating expression of CA8-204. We developed constructs to study the expression and function of the naturally occurring CA8-204G transcript (G allele at rs6471859), CA8-204C (C allele at rs6471859 reversion mutation) and CA8-201 (full length transcript). CA8-204G transcript expression occurred predominantly in non-neuronal cells (HEK293), while CA8-204C expression was restricted to neuronal derived cells (NBL) in vitro. CA8-204G produced a stable truncated transcript in HEK293 cells that was barely detectable in NBL cells. We also show CA8-204 produces a stable peptide that inhibits pITPR1 and Ca++ release in HEK293 cells. These results imply homozygous G/G individuals at rs6471859, which are common in the general population, produce exclusively CA8-204G that is barely detectable in neuronal cells. CA8 null mutations that greatly impact neuronal functions are associated with severe forms of spinal cerebellar ataxia, and our data suggest G/G homozygotes should display a similar phenotype. To address this question, we show in vivo using AAV8-FLAG-CA8-204G and AAV8-V5-CA8-201 gene transfer delivered via intra-neural sciatic nerve injection (SN), that these viral constructs are able to transduce DRG cells and produce similar analgesic and anti-hyperalgesic responses to inflammatory pain. Immunohistochemistry (IHC) examinations of DRG tissues further show CA8-204G peptide is expressed in advillin expressing neuronal cells, but to a lesser extent compared to glial cells. These findings explain why G/G homozygotes that exclusively produce this truncated functional peptide in DRG evade a severe phenotype. These genomic studies significantly advance the literature regarding structure-function studies on CA8-ITPR1 critical to calcium signaling pathways, synaptic functioning, neuronal excitability and analgesic responses. [ABSTRACT FROM AUTHOR]

Numerous studies have shown associations between genetic variants and neuropathic pain disorders. Rare monogenic disorders are caused by mutations of substantial effect size in a single gene, whereas common disorders are likely to have a contribution from multiple genetic variants of mild effect size, representing different biological pathways. In this review, we survey the reported genetic contributors to neuropathic pain and submit them for validation in a 150,000-participant sample of the U.K. Biobank cohort. Successfully replicated association with a neuropathic pain construct for 2 variants in IL10 underscores the importance of neuroimmune interactions, whereas genome-wide significant association with low back pain (P = 1.3e-8) and false discovery rate 5% significant associations with hip, knee, and neck pain for variant rs7734804 upstream of the MAT2B gene provide evidence of shared contributing mechanisms to overlapping pain conditions at the molecular genetic level. [ABSTRACT FROM AUTHOR]

Chronic pain is a prevalent disease with increasing clinical challenges. Genome-wide association studies in chronic pain patients have identified hundreds of common pathogenic variants, yet they only explained a portion of individual variance of chronic pain. With the advances in next-generation sequencing technologies, it is now feasible to conduct rarer variants studies in large-scale databases. Here, we performed gene-based rare variant analyses in 200,000 human subjects in the UK biobank whole-exome sequencing database for investigating 9 different chronic pain states and validated our findings in 3 other large-scale databases. Our analyses identified the SLC13A1 gene coding for sodium/sulfate symporter associated with chronic back pain and multisite pain at the genome-wide level and with chronic headache, knee, and neck and shoulder pain at the nominal level. Seven loss-of-function rare variants were identified within the gene locus potentially contributing to the development of chronic pain, with 2 of them individually associated with back pain and multisite pain. These 2 rare variants were then tested for replication in 3 other biobanks, and the strongest evidence was found for rs28364172 as an individual contributor. Transcriptional analyses of Slc13a1 in rodents showed substantial regulation of its expression in the dorsal root ganglia and the sciatic nerve in neuropathic pain assays. Our results stress the importance of the SLC13A1 gene in sulfate homeostasis in the nervous system and its critical role in preventing pain states, thus suggesting new therapeutic approaches for treating chronic pain in a personalized manner, especially in people with mutations in the SLC13A1 gene. [ABSTRACT FROM AUTHOR]

Transferring fibromyalgia patient immunoglobulin G (IgG) to mice induces pain-like behaviour, and fibromyalgia IgG binds mouse and human satellite glia cells (SGCs). These findings suggest that autoantibodies could be part of fibromyalgia pathology. However, it is unknown how frequently fibromyalgia patients have anti-SGC antibodies and how anti-SGC antibodies associate with disease severity. Here, we quantified serum or plasma anti-SGC IgG levels in 2 fibromyalgia cohorts from Sweden and Canada using an indirect immunofluorescence murine cell culture assay. Fibromyalgia serum IgG binding to human SGCs in human dorsal root ganglia tissue sections was also assessed by immunofluorescence. In the cell culture assay, anti-SGC IgG levels were increased in both fibromyalgia cohorts compared with control group. Elevated anti-SGC IgG was associated with higher levels of self-reported pain in both cohorts, and higher fibromyalgia impact questionnaire scores and increased pressure sensitivity in the Swedish cohort. Anti-SGC IgG levels were not associated with fibromyalgia duration. Swedish fibromyalgia (FM) patients were clustered into FMsevere and FM-mild groups, and the FM-severe group had elevated anti-SGC IgG compared with the FM-mild group and control group. Anti-SGC IgG levels detected in culture positively correlated with increased binding to human SGCs. Moreover, the FMsevere group had elevated IgG binding to human SGCs compared with the FM-mild and control groups. These results demonstrate that a subset of fibromyalgia patients have elevated levels of anti-SGC antibodies, and the antibodies are associated with more severe fibromyalgia symptoms. Screening fibromyalgia patients for anti-SGC antibodies could provide a path to personalized treatment options that target autoantibodies and autoantibody production [ABSTRACT FROM AUTHOR]

Transient receptor potential vanilloid 1 (TRPV1) is a polymodal receptor activated by capsaicin, heat, and acid, which plays critical roles in thermosensation and pain. In addition, TRPV1 also contributes to multiple pathophysiological states in respiratory, cardiovascular, metabolic, and renal systems. These contributions are further supported by evidence that variations in the human TRPV1 (hTRPV1) gene are associated with various physiological and pathological phenotypes. However, it is not well understood how the variations in hTRPV1 affect channel functions. In this study, we examined functional consequences of amino acid variations of hTRPV1 induced by 5 nonsynonymous single-nucleotide polymorphisms (SNPs) that most commonly exist in the human population. Using electrophysiological assays in HEK293 cells, we examined 9 parameters: activation, Ca permeation, and desensitization after activation by capsaicin, acid, and heat. Our results demonstrated that the 5 SNPs differentially affected functional properties of hTRPV1 in an agonist-dependent manner. Based upon the directionality of change of each phenotype and cumulative changes in each SNP, we classified the 5 SNPs into 3 presumptive functional categories: gain of function (hTRPV1 Q85R, P91S, and T469I), loss of function (I585V), and mixed (M315I). These results reveal a spectrum of functional variation among common hTRPV1 polymorphisms in humans and may aid mechanistic interpretation of phenotypes associated with nonsynonymous hTRPV1 SNPs under pathophysiological conditions. [ABSTRACT FROM AUTHOR]

Activation of microglia in the spinal cord dorsal horn after peripheral nerve injury contributes to the development of pain hypersensitivity. How activated microglia selectively enhance the activity of spinal nociceptive circuits is not well understood. We discovered that after peripheral nerve injury, microglia degrade extracellular matrix structures, perineuronal nets (PNNs), in lamina I of the spinal cord dorsal horn. Lamina I PNNs selectively enwrap spinoparabrachial projection neurons, which integrate nociceptive information in the spinal cord and convey it to supraspinal brain regions to induce pain sensation. Degradation of PNNs by microglia enhances the activity of projection neurons and induces pain-related behaviors. Thus, nerve injury-induced degradation of PNNs is a mechanism by which microglia selectively augment the output of spinal nociceptive circuits and cause pain hypersensitivity. [ABSTRACT FROM AUTHOR]

Abstract: Individual vulnerability factors influencing the function of the hypothalamic-pituitary-adrenal axis may contribute to the risk of the development of persistent musculoskeletal pain after traumatic stress exposure. The objective of the study was to evaluate the association between polymorphisms in the gene encoding FK506 binding protein 51, FKBP5, a glucocorticoid receptor co-chaperone, and musculoskeletal pain severity 6weeks after 2 common trauma exposures. The study included data from 2 prospective emergency department-based cohorts: a discovery cohort (n=949) of European Americans experiencing motor vehicle collision and a replication cohort of adult European American women experiencing sexual assault (n=53). DNA was collected from trauma survivors at the time of initial assessment. Overall pain and neck pain 6weeks after trauma exposure were assessed using a 0–10 numeric rating scale. After adjustment for multiple comparisons, 6 FKBP5 polymorphisms showed significant association (minimum P <0.0001) with both overall and neck pain in the discovery cohort. The association of rs3800373, rs9380526, rs9394314, rs2817032, and rs2817040 with neck pain and/or overall pain 6weeks after trauma was replicated in the sexual assault cohort, showing the same direction of the effect in each case. The results of this study indicate that genetic variants in FKBP5 influence the severity of musculoskeletal pain symptoms experienced during the weeks after motor vehicle collision and sexual assault. These results suggest that glucocorticoid pathways influence the development of persistent posttraumatic pain, and that such pathways may be a target of pharmacologic interventions aimed at improving recovery after trauma. [Copyright &y& Elsevier]

Abstract: 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 proinflammatory cytokine monocyte chemotactic protein (MCP-1) and antiinflammatory cytokine interleukin (IL)-1ra, whereas TMD+WPT cases had elevated levels of proinflammatory 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 antiinflammatory cytokine transforming growth factor β1 (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 antiinflammatory 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. [Copyright &y& Elsevier]

Chronic pain is often present at more than one anatomical location, leading to chronic overlapping pain conditions. Whether chronic overlapping pain conditions represent a distinct pathophysiology from the occurrence of pain at only one site is unknown. Using genome-wide approaches, we compared genetic determinants of chronic single-site versus multisite pain in the UK Biobank. We found that different genetic signals underlie chronic single-site and multisite pain with much stronger genetic contributions for the latter. Among 23 loci associated with multisite pain, nine loci replicated in the HUNT cohort, with the DCC netrin 1 receptor (DCC) as the top gene. Functional genomics identified axonogenesis in brain tissues as the major contributing pathway to chronic multisite pain. Finally, multimodal structural brain imaging analysis showed that DCC is most strongly expressed in subcortical limbic regions and is associated with alterations in the uncinate fasciculus microstructure, suggesting that DCC-dependent axonogenesis may contribute to chronic overlapping pain conditions via corticolimbic circuits. [ABSTRACT FROM AUTHOR]

The introduction of sex-as-a-biological-variable policies at funding agencies around the world has led to an explosion of very recent observations of sex differences in the biology underlying pain. This review considers evidence of sexually dimorphic mechanisms mediating pain hypersensitivity, derived from modern assays of persistent pain in rodent animal models. Three well-studied findings are described in detail: the male-specific role of spinal cord microglia, the female-specific role of calcitonin gene-related peptide (CGRP), and the female-specific role of prolactin and its receptor. Other findings of sex-specific molecular involvement in pain are subjected to pathway analyses and reveal at least one novel hypothesis: that females may preferentially use Th1 and males Th2 T cell activity to mediate chronic pain. • Many sex differences in the biological mediation of chronic pain have been recently reported. • There is much evidence for sex-dependent pain processing involving microglia, CGRP, and prolactin. • Pathway analyses implicate sex-dependent involvement of Th1 T cells in females and Th2 cells in males. [ABSTRACT FROM AUTHOR]

The opioid receptor (OPR) family comprises the mu-, delta-, and kappa-opioid, and nociceptin receptors that belong to the superfamily of 7-transmembrane spanning G protein-coupled receptors (GPCRs). The mu-opioid receptor is the main target for clinically used opioid analgesics, and its biology has been extensively studied. The N-terminally truncated 6TM receptors isoform produced through alternative splicing of the OPRM1 gene displays unique signaling and analgesic properties, but it is unclear if other OPRs have the same ability. In this study, we have built a comprehensive map of alternative splicing events that produce 6TM receptor variants in all the OPRs and demonstrated their evolutionary conservation. We then obtained evidence for their translation through ribosomal footprint analysis. We discovered that N-terminally truncated 6TM GPCRs are rare in the human genome and OPRs are overrepresented in this group. Finally, we also observed a significant enrichment of 6TM GPCR genes among genes associated with pain, psychiatric disorders, and addiction. Understanding the biology of 6TM receptors and leveraging this knowledge for drug development should pave the way for novel therapies. [ABSTRACT FROM AUTHOR]

Background and Purpose: The μ-opioid receptor (μ receptor) is the primary target for opioid analgesics. The 7-transmembrane (TM) and 6TM μ receptor isoforms mediate inhibitory and excitatory cellular effects. Here, we developed compounds selective for 6TM- or 7TM-μ receptors to further our understanding of the pharmacodynamic properties of μ receptors.Experimental Approach: We performed virtual screening of the ZINC Drug Now library of compounds using in silico 7TM- and 6TM-μ receptor structural models and identified potential compounds that are selective for 6TM- and/or 7TM-μ receptors. Subsequently, we characterized the most promising candidate compounds in functional in vitro studies using Be2C neuroblastoma transfected cells, behavioural in vivo pain assays using various knockout mice and in ex vivo electrophysiology studies.Key Results: Our virtual screen identified 30 potential candidate compounds. Subsequent functional in vitro cellular assays shortlisted four compounds (#5, 10, 11 and 25) that demonstrated 6TM- or 7TM-μ receptor-dependent NO release. In in vivo pain assays these compounds also produced dose-dependent hyperalgesic responses. Studies using mice that lack specific opioid receptors further established the μ receptor-dependent nature of identified novel ligands. Ex vivo electrophysiological studies on spontaneous excitatory postsynaptic currents in isolated spinal cord slices also validated the hyperalgesic properties of the most potent 6TM- (#10) and 7TM-μ receptor (#5) ligands.Conclusion and Implications: Our novel compounds represent a new class of ligands for μ receptors and will serve as valuable research tools to facilitate the development of opioids with significant analgesic efficacy and fewer side-effects. [ABSTRACT FROM AUTHOR]

Abstract: Traditional classification and prognostic approaches for chronic pain conditions focus primarily on anatomically based clinical characteristics not based on underlying biopsychosocial factors contributing to perception of clinical pain and future pain trajectories. Using a supervised clustering approach in a cohort of temporomandibular disorder cases and controls from the Orofacial Pain: Prospective Evaluation and Risk Assessment study, we recently developed and validated a rapid algorithm (ROPA) to pragmatically classify chronic pain patients into 3 groups that differed in clinical pain report, biopsychosocial profiles, functional limitations, and comorbid conditions. The present aim was to examine the generalizability of this clustering procedure in 2 additional cohorts: a cohort of patients with chronic overlapping pain conditions (Complex Persistent Pain Conditions study) and a real-world clinical population of patients seeking treatment at duke innovative pain therapies. In each cohort, we applied a ROPA for cluster prediction, which requires only 4 input variables: pressure pain threshold and anxiety, depression, and somatization scales. In both complex persistent pain condition and duke innovative pain therapies, we distinguished 3 clusters, including one with more severe clinical characteristics and psychological distress. We observed strong concordance with observed cluster solutions, indicating the ROPA method allows for reliable subtyping of clinical populations with minimal patient burden. The ROPA clustering algorithm represents a rapid and valid stratification tool independent of anatomic diagnosis. ROPA holds promise in classifying patients based on pathophysiological mechanisms rather than structural or anatomical diagnoses. As such, this method of classifying patients will facilitate personalized pain medicine for patients with chronic pain. [ABSTRACT FROM AUTHOR]

Abstract: Sex differences for chronic back pain (cBP) have been reported, with females usually exhibiting greater morbidity, severity, and poorer response to treatment. Genetic factors acting in an age-specific manner have been implicated but never comprehensively explored. We performed sex- and age-stratified genome-wide association study and single nucleotide polymorphism-by-sex interaction analysis for cBP defined as "Back pain for 3+ months" in 202,077 males and 237,754 females of European ancestry from UK Biobank. Two and 7 nonoverlapping genome-wide significant loci were identified for males and females, respectively. A male-specific locus on chromosome 10 near SPOCK2 gene was replicated in 4 independent cohorts. Four loci demonstrated single nucleotide polymorphism-by-sex interaction, although none of them were formally replicated. Single nucleotide polymorphism-explained heritability was higher in females (0.079 vs 0.067, P = 0.006). There was a high, although not complete, genetic correlation between the sexes (r = 0.838 ± 0.041, different from 1 with P = 7.8E-05). Genetic correlation between the sexes for cBP decreased with age (0.858 ± 0.049 in younger people vs 0.544 ± 0.157 in older people; P = 4.3E-05). There was a stronger genetic correlation of cBP with self-reported diagnosis of intervertebral disk degeneration in males than in females (0.889 vs 0.638; P = 3.7E-06). Thus, the genetic component of cBP in the UK Biobank exhibits a mild sex- and age-dependency. This provides an insight into the possible causes of sex- and age-specificity in epidemiology and pathophysiology of cBP and chronic pain at other anatomical sites. [ABSTRACT FROM AUTHOR]

Alterations in cellular energy metabolism have been implicated in chronic pain, suggesting a role for mitochondrial DNA. Previous studies reported associations of a limited number of mitochondrial DNA polymorphisms with specific pain conditions. In this study, we examined the full mitochondrial genomes of people with a variety of chronic pain conditions. A discovery cohort consisting of 609 participants either with or without a complex persistent pain conditions (CPPCs) was examined. Mitochondrial DNA was subjected to deep sequencing for identification of rare mutations, common variants, haplogroups, and heteroplasmy associated with 5 CPPCs: episodic migraine, irritable bowel syndrome, fibromyalgia, vulvar vestibulitis, or temporomandibular disorders. The strongest association found was the presence of the C allele at the single nucleotide polymorphism m.2352T>C (rs28358579) that significantly increased the risk for fibromyalgia (odds ratio [OR] = 4.6, P = 4.3 × 10). This relationship was even stronger in women (OR = 5.1, P = 2.8 × 10), and m.2352T>C was associated with all other CPPCs in a consistent risk-increasing fashion. This finding was replicated in another cohort (OR = 4.3, P = 2.6 × 10) of the Orofacial Pain: Prospective Evaluation and Risk Assessment study consisting of 1754 female participants. To gain insight into the cellular consequences of the associated genetic variability, we conducted an assay testing metabolic reprogramming in human cell lines with defined genotypes. The minor allele C was associated with decreased mitochondrial membrane potential under conditions where oxidative phosphorylation is required, indicating a role of oxidative phosphorylation in pathophysiology of chronic pain. Our results suggest that cellular energy metabolism, modulated by m.2352T>C, contributes to fibromyalgia and possibly other chronic pain conditions. [ABSTRACT FROM AUTHOR]

Older individuals have an elevated risk for chronic pain as half of all individuals over 65 years old have at least one chronic pain condition. Unfortunately, relevant assessment tools and recommendations for chronic pain management targeting older adults are lacking. This study explores changes in response to pain between young (2–3 months old) and geriatric (20–24 months old) ages using mice. Although cutaneous thresholds to brisk stimuli (von Frey and radiant heat assays) were not affected, behavioral responses to tonic stimuli (acetone and capsaicin assays) were more pronounced in geriatric animals. After nerve injury, geriatric mice present an altered neuropathic pain profile with hypersensitivity to mechanical stimuli but not acetone and an impairment in conditioned noxious stimuli avoidance. This altered behavioral response pattern was associated with an abnormal monoaminergic signature in the medial prefrontal cortex, suggesting decreased COMT function. We conclude that young and geriatric mice exhibit different behavioral and physiological responses to the experience of pain, suggesting that knowledge and practices must be adjusted for geriatric populations. • Baseline cutaneous thresholds are not different between young and geriatric mice. • Geriatric mice are hyperreactive to tonic pain stimuli. • Nerve-injured geriatric mice are hypersensitive to mechanical but not cooling stimuli. • Geriatric mice display decreased aversive responses to chronic pain. • Geriatric mice display altered supraspinal plasticity during chronic pain. [ABSTRACT FROM AUTHOR]

It has recently been shown that epidermal growth factor receptor (EGFR) contributes to the pathogenesis of pain. We scanned genetic markers within genes coding for receptors of the EGFR family (EGFR, ERBB2, ERBB3, and ERBB4) and their ligands (AREG, BTC, EGF, EPGN, EREG, HBEGF, MUC4, NRG1, NRG2, NRG3, NRG4, and TGFA) for association with self-reported pain intensity in patients with chronic facial pain who participated in the Orofacial Pain: Prospective Evaluation and Risk Assessment (OPPERA) cohort. We found that only epiregulin (EREG) was associated with pain. The strongest effect was observed for a minor allele at rs6836436 in EREG, which was associated with lower chronic pain intensity. However, the same allele was associated with higher facial pain intensity among cases with recent onset of facial pain. Similar trends were observed in an independent cohort of UK Biobank (UKB) where the minor allele at rs6836436 was associated with a higher number of acute pain sites but a lower number of chronic pain sites. Expression quantitative trait loci analyses established rs6836436 as a loss-of-function variant of EREG. Finally, we investigated the functional role of EREG using mouse models of chronic and acute pain. Injecting mice with an EREG monoclonal antibody reversed established mechanosensitivity in the complete Freund's adjuvant and spared nerve injury models of chronic pain. However, the EREG monoclonal antibody prolonged allodynia when administered during the development of complete Freund's adjuvant-induced mechanosensitivity and enhanced pain behavior in the capsaicin model of acute pain. [ABSTRACT FROM AUTHOR]

Many genetic markers have been associated with variations in treatment response to analgesics, but none have been assessed in the context of combination therapies. In this study, the treatment effects of nortriptyline and morphine were tested for an association with genetic markers relevant to pain pathways. Treatment effects were determined for single and combination therapies. A total of 24 functional single nucleotide polymorphisms were tested within the gene loci of mu-opioid receptor (OPRM1) gene locus, ATP-Binding Cassette B1 Transporter (ABCB1), Cytochrome P450 gene family (CYP2C19 and CYP2D6), catecholamine inactivator Catechol-O-Methyl Transferase (COMT), and serotonin receptor 2A (HTR2A). Genotyping was performed in a population of neuropathic pain patients who previously participated in a clinical trial. For monotherapy, neither nortriptyline nor morphine responses were associated with single nucleotide polymorphisms. However, for nortriptyline + morphine combination therapy, the single nucleotide polymorphism rs1045642 within the drug efflux pump ABCB1 transporter significantly predicted analgesic response. The presence of the C allele accounted for 51% of pain variance in this subgroup in response to combination treatment. The T-allele homozygotes demonstrated only 20% improvement in pain scores, whereas the C-allele homozygotes 88%. There was no significant contribution of rs1045642 to the medication side effects under all treatment conditions. The UK Biobank data set was then used to validate this genetic association. Here, patients receiving similar combination therapy (opioid + tricyclic antidepressant) carrying the C allele of rs1045642 displayed 33% fewer body pain sites than patients without that allele, suggesting better pain control. In all, our results show a robust effect of the rs1045642 polymorphism in response to chronic pain treatment with a nortriptyline + morphine combination. [ABSTRACT FROM AUTHOR]

Objective: Heightened somatic symptoms are reported by a wide range of patients with chronic pain and have been associated with emotional distress and physical dysfunction. Despite their clinical significance, molecular mechanisms leading to their manifestation are not understood.Methods: We used an association study design based on a curated list of 3,295 single nucleotide polymorphisms mapped to 358 genes to test somatic symptoms reporting using the Pennebaker Inventory of Limbic Languidness questionnaire from a case-control cohort of orofacial pain (n = 1,607). A replication meta-analysis of 3 independent cohorts (n = 3,189) was followed by functional validation, including in silico molecular dynamics, in vitro enzyme assays, and measures of serotonin (5-HT) plasma concentration.Results: An association with the T allele of rs11575542 coding for an arginine to glutamine substitution in the L-aromatic amino acid decarboxylase (AADC) enzyme was replicated in a meta-analysis of 3 independent cohorts. In a combined meta-analysis of all cohorts, this association reached p = 6.43 × 10-8 . In silico studies demonstrated that this substitution dramatically reduces the conformational dynamics of AADC, potentially lowering its binding capacity to a cofactor. in vitro enzymatic assays showed that this substitution reduces the maximum kinetic velocity of AADC, hence lowering 5-HT levels. Finally, plasma samples from 90 subjects showed correlation between low 5-HT levels and heightened somatic symptoms.Interpretation: Using functional genomics approaches, we identified a polymorphism in the AADC enzyme that contributes to somatic symptoms through reduced levels of 5-HT. Our findings suggest a molecular mechanism underlying the pathophysiology of somatic symptoms and opens up new treatment options targeting the serotonergic system. ANN NEUROL 2019;86:168-180. [ABSTRACT FROM AUTHOR]

Background. When patients first develop a painful temporomandibular disorder (TMD) and seek care, 1 priority for clinicians is to assess prognosis. The authors aimed to develop a predictive model by using biopsychosocial measures from the Diagnostic Criteria for Temporomandibular Disorders (DC-TMD) to predict risk of developing TMD symptom persistence. Methods. At baseline, trained examiners identified 260 participants with first-onset TMD classified by using DC-TMD-compliant protocols. After follow-up at least 6 months later, 72 (49%) had examiner-classified TMD (persistent cases), and 75 (51%) no longer had examiner-classified TMD (transient cases). For multivariable logistic regression analysis, the authors used blocks of variables selected using minimum redundancy maximum relevance to construct a model to predict the odds of TMD persistence. Results. At onset, persistent cases had multiple worse TMD clinical measures and, among Axis II measures, only greater baseline pain intensity (odds ratio [OR], 1.5; 95% confidence interval [Cl], 1.04 to 2.2; P = .030) and more physical symptoms (OR, 1.8; 95% Cl, 1.2 to 2.9; P = .004) than did transient cases. A multivariable model using TMD clinical measures showed greater discriminative capacity (area under the receiver operating characteristic curve, 0.74; 95% Cl, 0.73 to 0.75) than did a model involving psychosocial measures (area under the receiver operating characteristic curve, 0.63; 95% Cl, 0.62 to 0.64)- Conclusions. Clinical measures that clinicians can assess readily when TMD first develops are useful in predicting the risk of developing persistent TMD. Psychosocial measures are important predictors of onset but do not add meaningfully to the predictive capacity of clinical measures. Practical Implications. When TMD first develops, clinicians usefully can identify patients at higher risk of developing persistence by using clinical measures that they logically also could use in treatment planning and for monitoring outcomes of intervention. [ABSTRACT FROM AUTHOR]

Chronic postmastectomy pain (PMP) imposes a major burden on the quality of life of the ever-increasing number of long-term survivors of breast cancer. An earlier report by Nissenbaum et al. claimed that particular polymorphisms in the gene CACNG2 are associated with the risk of developing chronic PMP after breast surgery (Nissenbaum J, Devor M, Seltzer Z, Gebauer M, Michaelis M, Tal M, Dorfman R, Abitbul-Yarkoni M, Lu Y, Elahipanah T, delCanho S, Minert A, Fried K, Persson AK, Shpigler H, Shabo E, Yakir B, Pisante A, Darvasi A. Susceptibility to chronic pain following nerve injury is genetically affected by CACNG2. Genome Res 2010;20:1180-90). This information is important because in principle, it can inform the surgical, radiological, and chemotherapeutic decision-making process in ways that could mitigate the increased risk of chronic pain. In this study, we revisited this claim by independently evaluating the proposed marker haplotype using 2 different patient cohorts recruited in different research settings. Meta-analysis of these new postmastectomy cohorts and the original cohort confirmed significant association of the CACNG2 haplotype with PMP. In addition, we tested whether the same markers would predict chronic postsurgical pain in men who underwent surgery for inguinal hernia repair, and whether there is significant genetic association with cutaneous thermal sensitivity in postmastectomy and postherniotomy patients. We found that the biomarker is selective because it did not predict pain after laparoscopic hernia repair and was not associated with pain sensitivity to experimentally applied noxious thermal stimuli. We conclude that the A-C-C haplotype at the 3 single-nucleotide polymorphisms (rs4820242, rs2284015, and rs2284017) in the CACNG2 gene is associated with increased risk of developing PMP. This information may advance current knowledge on pathophysiology of PMP and serve as a step forward in the prediction of clinical outcomes and personalized pain management. [ABSTRACT FROM AUTHOR]

The Human Pain Genetics Database (HPGDB) is a comprehensive variant-focused inventory of genetic contributors to human pain. After curation, the HPGDB currently includes 294 studies reporting associations between 434 distinct genetic variants and various pain phenotypes. Variants were then submitted to a comprehensive analysis. First, they were validated in an independent high-powered replication cohort by testing the association of each variant with 10 different pain phenotypes (n = 1320-26,973). One hundred fifty-five variants replicated successfully (false discovery rate 20%) in at least one pain phenotype, and the association P values of the HPGDB variants were significantly lower compared with those of random controls. Among the 155 replicated variants, 21 had been included in the HPGDB because of their association with analgesia-related and 13 with nociception-related phenotypes, confirming analgesia and nociception as pathways of vulnerability for pain phenotypes. Furthermore, many genetic variants were associated with multiple pain phenotypes, and the strength of their association correlated between many pairs of phenotypes. These genetic variants explained a considerable amount of the variance between different pairs of pain phenotypes, indicating a shared genetic basis among pain phenotypes. In addition, we found that HPGDB variants show many pleiotropic associations, indicating that genetic pathophysiological mechanisms are also shared among painful and nonpainful conditions. Finally, we demonstrated that the HPGDB data set is significantly enriched for functional variants that modify gene expression, are deleterious, and colocalize with open chromatin regions. As such, the HPGDB provides a validated data set that represents a valuable resource for researchers in the human pain field. [ABSTRACT FROM AUTHOR]

It has been reported consistently that many female chronic pain sufferers have an attenuation of symptoms during pregnancy. Rats display increased pain tolerance during pregnancy due to an increase in opioid receptors in the spinal cord. Past studies did not consider the role of non-neuronal cells, which are now known to play an important role in chronic pain processing. Using an inflammatory (complete Freund's adjuvant) or neuropathic (spared nerve injury) model of persistent pain, we observed that young adult female mice in early pregnancy switch from a microglia-independent to a microglia-dependent pain hypersensitivity mechanism. During late pregnancy, female mice show no evidence of chronic pain whatsoever. This pregnancy-related analgesia is reversible by intrathecal administration of naloxone, suggesting an opioid-mediated mechanism; pharmacological and genetic data suggest the importance of δ-opioid receptors. We also observe that T-cell-deficient (nude and Ray/-null mutant) pregnant mice do not exhibit pregnancy analgesia, which can be rescued with the adoptive transfer of CD4 + or CD8 + T cells from late-pregnant wild-type mice. These results suggest that T cells are a mediator of the opioid analgesia exhibited during pregnancy. [ABSTRACT FROM AUTHOR]

Carbonic anhydrase-8 ( Car8 mouse gene symbol) is devoid of enzymatic activity, but instead functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) to regulate this intracellular calcium release channel important in synaptic functions and neuronal excitability. Causative mutations in ITPR1 and carbonic anhydrase-8 in mice and humans are associated with certain subtypes of spinal cerebellar ataxia (SCA). SCA mice are genetically deficient in dorsal root ganglia (DRG) Car8 expression and display mechanical and thermal hypersensitivity and susceptibility to subacute and chronic inflammatory pain behaviors. In this report, we show that DRG Car8 expression is variable across 25 naïve-inbred strains of mice, and this cis-regulated eQTL (association between rs27660559, rs27706398, and rs27688767 and DRG Car8 expression; P < 1 × 10) is correlated with nociceptive responses in mice. Next, we hypothesized that increasing DRG Car8 gene expression would inhibit intracellular calcium release required for morphine antinociception and might correlate with antinociceptive sensitivity of morphine and perhaps other analgesic agents. We show that mean DRG Car8 gene expression is directly related to the dose of morphine or clonidine needed to provide a half-maximal analgesic response (r = 0.93, P < 0.00002; r = 0.83, P < 0.0008, respectively), suggesting that greater DRG Car8 expression increases analgesic requirements. Finally, we show that morphine induces intracellular free calcium release using Fura 2 calcium imaging in a dose-dependent manner; V5-Car8 overexpression in NBL cells inhibits morphine-induced calcium increase. These findings highlight the 'morphine paradox' whereby morphine provides antinociception by increasing intracellular free calcium, while Car8 and other antinociceptive agents work by decreasing intracellular free calcium. This is the first study demonstrating that biologic variability associated with this cis-eQTL may contribute to differing analgesic responses through altered regulation of ITPR1-dependent calcium release in mice. [ABSTRACT FROM AUTHOR]

Nociception is protective and prevents tissue damage but can also facilitate chronic pain. Whether a general principle governs these two types of pain is unknown. Here, we show that both basal mechanical and neuropathic pain are controlled by the microRNA-183 (miR-183) cluster in mice. This single cluster controls more than 80% of neuropathic pain–regulated genes and scales basal mechanical sensitivity and mechanical allodynia by regulating auxiliary voltage-gated calcium channel subunits α2δ-1 and α2δ-2. Basal sensitivity is controlled in nociceptors, and allodynia involves TrkB+ light-touch mechanoreceptors. These light-touch–sensitive neurons, which normally do not elicit pain, produce pain during neuropathy that is reversed by gabapentin. Thus, a single microRNA cluster continuously scales acute noxious mechanical sensitivity in nociceptive neurons and suppresses neuropathic pain transduction in a specific, light-touch–sensitive neuronal type recruited during mechanical allodynia. [ABSTRACT FROM AUTHOR]

It is generally believed that immune activation can elicit pain through production of inflammatory mediators that can activate nociceptive sensory neurons. Emerging evidence suggests that immune activation may also contribute to the resolution of pain by producing distinct pro-resolution/anti-inflammatory mediators. Recent research into the connection between the immune and nervous systems has opened new avenues for immunotherapy in pain management. This review provides an overview of the most utilized forms of immunotherapies (e.g., biologics) and highlight their potential for immune and neuronal modulation in chronic pain. Specifically, we discuss pain-related immunotherapy mechanisms that target inflammatory cytokine pathways, the PD-L1/PD-1 pathway, and the cGAS/STING pathway. This review also highlights cell-based immunotherapies targeting macrophages, T cells, neutrophils and mesenchymal stromal cells for chronic pain management. [ABSTRACT FROM AUTHOR]

The article reports on a study, according to which variations in the arginine vasopressin receptor gene, AVPR1A, are associated with pain sensitivity in a stress- and sex-specific manner in both mice and men. The key to these findings is that the association between the gene and pain response was almost exclusively observed in males, and only when the individuals were stressed. The study shows interaction can affect the contribution of a gene, and genetic variant, to the pain state.

α2A adrenergic receptor (α2A-AR) activation has been shown in animal models to play an important role in regulating the balance of acute pain inhibition vs facilitation after both physical and psychological stress. To our knowledge, the influence of genetic variants in the gene encoding α2A-AR, ADRA2A, on acute pain outcomes in humans experiencing traumatic stress has not been assessed. In this study, we tested whether a genetic variant in the 3'UTR of ADRA2A, rs3750625, is associated with acute musculoskeletal pain (MSP) severity following motor vehicle collision (MVC, n = 948) and sexual assault (n = 84), and whether this influence was affected by stress severity. We evaluated rs3750625 because it is located in the seed binding region of miR-34a, a microRNA (miRNA) known to regulate pain and stress responses. In both cohorts, the minor allele at rs3750625 was associated with increased musculoskeletal pain in distressed individuals (stress*rs3750625 P = 0.043 for MVC cohort and P = 0.007 for sexual assault cohort). We further found that (1) miR-34a binds the 3'UTR of ADRA2A, (2) the amount of repression is greater when the minor (risk) allele is present, (3) miR-34a in the IMR-32 adrenergic neuroblastoma cell line affects ADRA2A expression, (4) miR-34a and ADRA2A are expressed in tissues known to play a role in pain and stress, (5) following forced swim stress exposure, rat peripheral nerve tissue expression changes are consistent with miR-34a regulation of ADRA2A. Together, these results suggest that ADRA2A rs3750625 contributes to poststress musculoskeletal pain severity by modulating miR-34a regulation. [ABSTRACT FROM AUTHOR]

Background: Mild traumatic brain injury (mTBI) often results in post-concussion symptoms, chronic pain, and sleepiness. Genetic factors are thought to play an important role in poor prognosis. Aims: The aims of this study are to (1) document the prevalence of pain and post-concussion symptoms in mTBI patients in acute and chronic phases (2) determine whether candidate genes predispose to post-concussive symptoms and pain. Methods: Posttraumatic symptoms, evaluated using the Rivermead Post-Concussion Symptoms Questionnaire, and pain were assessed in 94 mTBI patients in the acute phase as well as in 22 healthy controls. Assessment was repeated in 36 patients after one year who agreed to participate in the follow-up visit. Gene polymorphisms and expression were assessed in mTBI patients and healthy controls. Results: In the acute phase, mTBI patients with pain (69%) presented more psychological symptoms and sleepiness and were less able to return to work than those without pain. At one year, 19% of mTBI patients had persistent pain and psychological distress. Two haplotypes (H2 and H3) in the brain-derived neurotrophic factor (BDNF) gene were shown to be respectively deleterious and protective against post-concussion symptoms and pain in both acute and chronic phases. Protective haplotype H3 was associated with a decreased expression of the anti-sense of BDNF (BDNF-AS). Deleterious haplotype H2 predicted the development of chronic pain at one year, whereas H3 was protective. Conclusions: This pilot study suggests a protective mechanism of a multilocus effect in BDNF, through BDNF-AS, against post-concussion symptoms and pain in the acute phase and possibly chronic pain at one year post-mTBI. The role of antisense RNA should be validated in larger cohorts. [ABSTRACT FROM AUTHOR]

The classification of most chronic pain disorders gives emphasis to anatomical location of the pain to distinguish one disorder from the other (eg, back pain vs temporomandibular disorder [TMD]) or to define subtypes (eg, TMD myalgia vs arthralgia). However, anatomical criteria overlook etiology, potentially hampering treatment decisions. This study identified clusters of individuals using a comprehensive array of biopsychosocial measures. Data were collected from a case-control study of 1031 chronic TMD cases and 3247 TMD-free controls. Three subgroups were identified using supervised cluster analysis (referred to as the adaptive, pain-sensitive, and global symptoms clusters). Compared with the adaptive cluster, participants in the pain-sensitive cluster showed heightened sensitivity to experimental pain, and participants in the global symptoms cluster showed both greater pain sensitivity and greater psychological distress. Cluster membership was strongly associated with chronic TMD: 91.5% of TMD cases belonged to the pain-sensitive and global symptoms clusters, whereas 41.2% of controls belonged to the adaptive cluster. Temporomandibular disorder cases in the pain-sensitive and global symptoms clusters also showed greater pain intensity, jaw functional limitation, and more comorbid pain conditions. Similar results were obtained when the same methodology was applied to a smaller case-control study consisting of 199 chronic TMD cases and 201 TMD-free controls. During a median 3-year follow-up period of TMD-free individuals, participants in the global symptoms cluster had greater risk of developing first-onset TMD (hazard ratio = 2.8) compared with participants in the other 2 clusters. Cross-cohort predictive modeling was used to demonstrate the reliability of the clusters. [ABSTRACT FROM AUTHOR]

Catecholamine-O-methyltransferase (COMT) is a polymorphic gene whose variants affect enzymatic activity and pain sensitivity via adrenergic pathways. Although COMT represents one of the most studied genes in human pain genetics, findings regarding its association with pain phenotypes are not always replicated. Here, we investigated if interactions among functional COMT haplotypes, stress, and sex can modify the effect of COMT genetic variants on pain sensitivity. We tested these interactions in a cross-sectional study, including 2 cohorts, one of 2972 subjects tested for thermal pain sensitivity (Orofacial Pain: Prospective Evaluation and Risk Assessment) and one of 948 subjects with clinical acute pain after motor vehicle collision (post-motor vehicle collision). In both cohorts, the COMT high-pain sensitivity (HPS) haplotype showed robust interaction with stress and number of copies of the HPS haplotype was positively associated with pain sensitivity in nonstressed individuals, but not in stressed individuals. In the post-motor vehicle collision cohort, there was additional modification by sex: the HPS-stress interaction was apparent in males, but not in females. In summary, our findings indicate that stress and sex should be evaluated in association studies aiming to investigate the effect of COMT genetic variants on pain sensitivity. [ABSTRACT FROM AUTHOR]

N 6 -Methyladenosine (m 6 A) is a reversible and abundant internal modification of messenger RNA (mRNA) and long noncoding RNA (lncRNA) with roles in RNA processing, transport, and stability. Although m 6 A does not preclude Watson–Crick base pairing, the N 6 -methyl group alters the stability of RNA secondary structure. Since changes in RNA structure can affect diverse cellular processes, the influence of m 6 A on mRNA and lncRNA structure has the potential to be an important mechanism for m 6 A function in the cell. Indeed, an m 6 A site in the lncRNA metastasis associated lung adenocarcinoma transcript 1 (MALAT1) was recently shown to induce a local change in structure that increases the accessibility of a U 5 -tract for recognition and binding by heterogeneous nuclear ribonucleoprotein C (HNRNPC). This m 6 A-dependent regulation of protein binding through a change in RNA structure, termed “m 6 A-switch”, affects transcriptome-wide mRNA abundance and alternative splicing. To further characterize this first example of an m 6 A-switch in a cellular RNA, we used nuclear magnetic resonance and Förster resonance energy transfer to demonstrate the effect of m 6 A on a 32-nucleotide RNA hairpin derived from the m 6 A-switch in MALAT1. The observed imino proton nuclear magnetic resonance resonances and Förster resonance energy transfer efficiencies suggest that m 6 A selectively destabilizes the portion of the hairpin stem where the U 5 -tract is located, increasing the solvent accessibility of the neighboring bases while maintaining the overall hairpin structure. The m 6 A-modified hairpin has a predisposed conformation that resembles the hairpin conformation in the RNA–HNRNPC complex more closely than the unmodified hairpin. The m 6 A-induced structural changes in the MALAT1 hairpin can serve as a model for a large family of m 6 A-switches that mediate the influence of m 6 A on cellular processes. [ABSTRACT FROM AUTHOR]

Opioid and α 2 -adrenoceptor (AR) agonists are analgesic when administered in the spinal cord and show a clinically beneficial synergistic interaction when co-administered. However, α 2 -AR antagonists can also inhibit opioid antinociception, suggesting a complex interaction between the two systems. The α 2A -AR subtype is necessary for spinal adrenergic analgesia and synergy with opioids for most agonist combinations. Therefore, we investigated whether spinal opioid antinociception and opioid-adrenergic synergy were under allosteric control of the α 2A -AR. Drugs were administered intrathecally in wild type (WT) and α 2A -knock-out (KO) mice and antinociception was measured using the hot water tail immersion or substance P behavioral assays. The α 2A -AR agonist clonidine was less effective in α 2A -KO mice in both assays. The absence of the α 2A -AR resulted in 10–70-fold increases in the antinociceptive potency of the opioid agonists morphine and DeltII. In contrast, neither morphine nor DeltII synergized with clonidine in α 2A -KO mice, indicating that the α 2A AR has both positive and negative modulatory effects on opioid antinociception. Depletion of descending adrenergic terminals with 6-OHDA resulted in a significant decrease in morphine efficacy in WT but not in α 2A -KO mice, suggesting that endogenous norepinephrine acts through the α 2A -AR to facilitate morphine antinociception. Based on these findings, we propose a model whereby ligand-occupied versus ligand-free α 2A -AR produce distinct patterns of modulation of opioid receptor activation. In this model, agonist-occupied α 2A -ARs potentiate opioid analgesia, while non-occupied α 2A -ARs inhibit opioid analgesia. Exploiting such interactions between the two receptors could lead to the development of better pharmacological treatments for pain management. [ABSTRACT FROM AUTHOR]