DNA methylation at the mu-1 opioid receptor gene (OPRM1) promoter predicts preoperative, acute, and chronic postsurgical pain after spine fusion

Abstract

Introduction: The perioperative pain experience shows great interindividual variability and is difficult to predict. The mu-1 opioid receptor gene (OPRM1) is known to play an important role in opioid-pain pathways. Since deoxyribonucleic acid (DNA) methylation is a potent repressor of gene expression, DNA methylation was evaluated at the OPRM1 promoter, as a predictor of preoperative, acute, and chronic postsurgical pain (CPSP).

Methods: A prospective observational cohort study was conducted in 133 adolescents with idiopathic scoliosis undergoing spine fusion under standard protocols. Data regarding pain, opioid consumption, anxiety, and catastrophizing (using validated questionnaires) were collected before and 2-3 months postsurgery. Outcomes evaluated were preoperative pain, acute postoperative pain (area under curve [AUC] for pain scores over 48 hours), and CPSP (numerical rating scale >3/10 at 2-3 months postsurgery). Blood samples collected preoperatively were analyzed for DNA methylation by pyrosequencing of 22 CpG sites at the OPRM1 gene promoter. The association of each pain outcome with the methylation percentage of each CpG site was assessed using multivariable regression, adjusting for significant (P<0.05) nongenetic variables.

Results: Majority (83%) of the patients reported no pain preoperatively, while CPSP occurred in 36% of the subjects (44/121). Regression on dichotomized preoperative pain outcome showed association with methylation at six CpG sites (1, 3, 4, 9, 11, and 17) (P<0.05). Methylation at CpG sites 4, 17, and 18 was associated with higher AUC after adjusting for opioid consumption and preoperative pain score (P<0.05). After adjusting for postoperative opioid consumption and preoperative pain score, methylation at CpG sites 13 and 22 was associated with CPSP (P<0.05).

Discussion: Novel CPSP biomarkers were identified in an active regulatory region of the OPRM1 gene that binds multiple transcription factors. Inhibition of binding by DNA methylation potentially decreases the OPRM1 gene expression, leading to a decreased response to endogenous and exogenous opioids, and an increased pain experience.

Keywords: DNA methylation; OPRM1; chronic postsurgical pain; epigenetics; pain.

Figure 1

Depiction of the OPRM1 promoter region (HG19; Chr 6: 154360587 to 154360838) and the location of the CpG sites. The knobs represent each CpG site, and the primers are indicated in brackets below. The red-colored knob at +117 indicates the CpG site (CpG17) associated with the variant A118G. The arrows indicate sites that have been described as Sp1 transcription factor-binding sites in previous studies (CpG sites 9, 10, 12, 16, 21, and 23 at −18, −14, 12, +84, +145, +150, and +159 from ATG site). Abbreviations: OPRM1, mu-1 opioid receptor gene; TSS, transcription start site.

Figure 2

Recruitment timeline for the spine surgery study cohort is delineated. Of the 261 eligible patients who satisfied the inclusion/exclusion criteria, reasons for not enrolling and derivation of final cohorts included in the study with preoperative, acute, and chronic pain outcomes are described.

Figure 3

The probability of developing CPSP based on DNA methylation at CpG 13 and 22, derived from the regression model, is depicted. The probabilities were estimated using median preoperative pain scores (0), median morphine consumption (1.7 mg/kg), and 2.5%, 25%, 50%, 75%, and 97.5% of the methylation data of each of the two sites. The 97.5% values for DNA methylation in the data are 40% for CpG13 and 57% for CpG22. The nongenetic covariates are already adjusted for in the regression model. Hence, the probability of CPSP holding other variables constant increases with increased methylation at these sites. Abbreviation: CPSP, chronic postsurgical pain.