Wyns, ArneHendrix, JolienVan Campenhout, JenteBuntinx, YantheXiong, Huan-YuDe Bruyne, ElkeGodderis, LodeNijs, JoRice, DavidChiang, DanielPolli, Andrea2026-02-012026-02-012026-01-14Int J Mol Sci, ISSN: 1422-0067 (Print); 1422-0067 (Online), MDPI AG, 27(2), 826-. doi: 10.3390/ijms270208261422-00671422-0067http://hdl.handle.net/10292/20571Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and fibromyalgia (FM) are debilitating disorders with overlapping symptoms such as chronic pain and fatigue. Dysregulation of the endogenous opioid system, particularly µ-opioid receptor function, may contribute to their pathophysiology. This study examined whether epigenetic modifications, specifically µ-opioid receptor 1 gene (OPRM1) promoter methylation, play a role in this dysfunction. Using a repeated-measures design, 28 ME/CFS/FM patients and 26 matched healthy controls visited the hospital twice within four days. Assessments included blood sampling for epigenetic analysis, a clinical questionnaire battery, and quantitative sensory testing (QST). Global DNA (hydroxy)methylation was quantified via liquid chromatography-tandem mass spectrometry, and targeted pyrosequencing was performed on promoter regions of OPRM1, COMT, and BDNF. ME/CFS/FM patients reported significantly worse symptom outcomes. No differences in global (hydroxy)methylation were found. Patients showed significantly higher OPRM1 promoter methylation, which remained after adjusting for symptom severity and QST findings. Across timepoints, OPRM1 methylation consistently correlated with BDNF Promoter I and Exon III methylation. This is, to the best of our knowledge, the first study examining OPRM1 methylation in ME/CFS/FM. Increased OPRM1 methylation in patients, independent of symptoms or pain sensitivity measures, supports the hypothesis of dysregulated opioidergic signaling in these conditions.© 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.OPRM1bisulfite conversiondescending modulationepigeneticsfibromyalgiamyalgic encephalomyelitis/chronic fatigue syndromeopioids3101 Biochemistry and Cell Biology31 Biological Sciences3404 Medicinal and Biomolecular Chemistry34 Chemical Sciences3107 MicrobiologyMinority HealthHealth Disparities and Racial or Ethnic Minority Health ResearchPain ResearchChronic Fatigue Syndrome (ME/CFS)Clinical ResearchHealth DisparitiesOpioidsFibromyalgiaGeneticsWomen's HealthNeurosciencesChronic Pain2.1 Biological and endogenous factorsDisputed aetiology and other0399 Other Chemical Sciences0604 Genetics0699 Other Biological SciencesChemical Physics3101 Biochemistry and cell biology3107 Microbiology3404 Medicinal and biomolecular chemistryHumansFatigue Syndrome, ChronicReceptors, Opioid, muDNA MethylationFemaleFibromyalgiaMaleMiddle AgedAdultPromoter Regions, GeneticEpigenesis, GeneticBrain-Derived Neurotrophic FactorCase-Control StudiesJournal ArticleOpenAccess10.3390/ijms27020826