Epigenetics: Why ME/CFS and Fibromyalgia Differ

Epigenetics: Why ME/CFS and Fibromyalgia Look Different

Fibromyalgia and ME/CFS overlap so heavily — widespread pain, profound fatigue, unrefreshing sleep, cognitive fog — that many patients carry both diagnoses, and clinicians often struggle to separate them. A study published in the Journal of Translational Medicine on 22 August 2026 found that the two conditions leave partly distinct marks on DNA methylation, a chemical modification that changes how genes are read without altering the DNA sequence itself.

The finding does not yet have clinical use. It does support the idea that the overlap is real but not complete.

The Study

This was a cross-sectional study of 188 women: 73 healthy controls, 71 with ME/CFS, and 44 with fibromyalgia. The researchers profiled DNA methylation in peripheral blood mononuclear cells (PBMCs) using the Illumina MethylationEPIC v2 array, then applied principal component analysis, region-level methylation analysis, and functional enrichment.

The work comes from the Pain in Motion Research Group at the Vrije Universiteit Brussel and KU Leuven in Belgium.

Two Distinct Methylation Axes

The analysis identified two independent components that separated the groups in different ways:

PC5 separated ME/CFS from both fibromyalgia and healthy controls. This axis aligned mainly with post-exertional malaise and cognitive symptoms — the features that most sharply distinguish ME/CFS clinically.

PC6 separated both patient groups from healthy controls, but not from each other. This axis tracked broader pain-related and multisystem burden: widespread pain, pain impact, sleep disturbance, visceral symptoms, and temporal summation.

That pattern is the interesting part. One signal is specific to ME/CFS. Another is shared across both conditions and scales with how broadly symptoms are felt. Region-level analysis identified 591 PC5-associated and 54 PC6-associated high-confidence differentially methylated regions.

Which Pathways Appeared

Functional enrichment implicated neuroimmune, metabolic, cytokine, NF-κB, JAK-STAT, and TGF-β pathways, along with immune-regulatory processes. Several of these — NF-κB and the cytokine pathways in particular — overlap with the inflammation findings reported elsewhere in fibromyalgia research. TGF-β has drawn particular interest from patient communities following this paper.

What This Means and What It Does Not

Methylation differences are associations, not causes. The study cannot say whether these patterns contribute to the conditions, result from them, or reflect something else that differs between the groups.

Four further limitations matter:

• Blood is not the tissue of interest. The study measured methylation in immune cells circulating in blood, which is accessible but not where pain is processed. Whether these patterns reflect what happens in the nervous system is unknown.
• The sample is modest, and the two patient groups differ in size (71 versus 44).
• It was cross-sectional. A single time point cannot show whether methylation changes over time or with symptom flares.
• The study was not designed to be diagnostic. It identifies biological differences between groups, not a test that can classify an individual patient.

The authors state that independent replication and longitudinal studies are needed before these findings have clinical utility.