This chapter is the mechanism, in as much detail as the evidence supports — which is now considerably more than it was, and still less than a full explanation.
1. The study, in full
| Kerrebijn and colleagues, Nature Medicine, 28 July 2026 | |
|---|---|
| Size | 2,563,755 individuals — 54,629 cases and 2,509,126 controls — across 11 cohorts, multi-ancestry, about 90% of European ancestry |
| Found | 26 risk loci, the first identified for fibromyalgia |
| Strongest signal | rs149109767-A, an in-frame glutamic acid deletion in exon 58 of the 67-exon HTT gene. Odds ratio 1.09, p = 2.2 × 10−12 |
| Other prioritised genes | GPR52, which regulates huntingtin, plus CAMKV, DCC, DRD2/NCAM1, MDGA2 and CELF4 — all with roles in neural development, synaptic function or pain processing |
| Where the heritability sits | Exclusively enriched in brain tissue and neural cell types, with peak enrichment in hippocampal dentate gyrus neurons and enteric neurons |
| Genetic correlation | Chronic low back pain 0.75, post-traumatic stress disorder 0.78, irritable bowel syndrome 0.70 — and positive but much weaker correlation with autoimmune conditions: Sjögren's 0.39, rheumatoid arthritis 0.33, hypothyroidism 0.21 |
Strong for the conclusion that the heritable risk is neural. Emerging for anything about mechanism beyond that — a GWAS tells you which organ system carries the genetic signal, not how the pain happens.
The limitations the authors state
- The cohort is predominantly European, which limits how far the findings generalise.
- Cases were identified from electronic health record diagnostic codes — ICD-10 M79.7 and equivalents — which the authors give as their principal limitation. Some people who do not meet full syndromic criteria may be included, and undiagnosed people are necessarily excluded — which, given the diagnostic history in section 3, is not a random exclusion.
2. The HTT finding, and why it is interesting rather than alarming
Same gene, different part, different consequence entirely
- Huntington's disease is caused by a CAG repeat expansion in exon 1 of the HTT gene. The fibromyalgia variant sits in exon 58 of 67 — a different region, a different kind of change, and a different biological consequence.
- The effect size is 9% higher odds. For comparison, that is the kind of shift a common variant produces in almost any complex condition, and it is nowhere near diagnostic. Most carriers do not have fibromyalgia; most people with fibromyalgia are not carriers.
- It does not raise anyone's risk of Huntington's disease, and the two conditions do not resemble each other clinically in any respect.
- And the authors tested that directly rather than asserting it, which is the part worth knowing. The variant does sit on the Huntington-disease-associated 'A1' haplotype found in Europeans, and it did show a raw statistical association with Huntington's disease. That association disappeared once the analysis was conditioned on the variant that tracks the repeat expansion — meaning it was the haplotype tagging along, not the fibromyalgia variant carrying risk. Replicated in an Icelandic dataset. So the reassurance in the line above is a finding, not a hope.
- Why it matters anyway: huntingtin is a protein with a central role in neuronal function — axonal transport, synaptic function, protecting cells from self-destruction. A signal there points at neuronal housekeeping rather than at inflammation or muscle. And GPR52, which regulates huntingtin, is already being investigated as a drug target in Huntington's disease — which is why the authors raise the possibility of therapeutic repurposing. That is a hypothesis, not a plan.
3. Why the sex ratio was wrong
Fibromyalgia has been described for decades as up to nine times commoner in women. That figure came substantially from how it was diagnosed.
- The 1990 criteria required tender points — a physical examination counting how many of 18 specified sites were painful under pressure. Women, on average, report tenderness at lower pressure thresholds than men, so a rule built on tender points selected women.
- The 2016 criteria dropped it in favour of widespread pain and symptom severity. Under those criteria the ratio narrows sharply — reported figures cluster around three to one, and some clinicians who see large numbers estimate lower still.
- The genetics point the same way, and quantitatively. The genetic correlation between men and women was 1.03 — statistically indistinguishable from a perfect correlation, meaning the same variants act the same way in both. That is very difficult to reconcile with a ninefold difference in the biology and very easy to reconcile with a difference in who gets diagnosed.
- The consequence for men has been real. Men presenting with widespread pain, fatigue and unrefreshing sleep have been told the diagnosis does not fit them, on the strength of a ratio produced by the measuring instrument.
4. Nociplastic pain — the idea that ties it together
Pain is now usually divided three ways:
| Type | What is happening | Example |
|---|---|---|
| Nociceptive | Tissue is being damaged and the alarm is working correctly | A broken bone, a burn |
| Neuropathic | The nerve itself is damaged | Sciatica, diabetic neuropathy |
| Nociplastic | The processing system has changed — amplifying, sustaining and generalising signals that would not otherwise produce this much pain | Fibromyalgia, and a family of related conditions |
The genetic correlations above 0.7 with low back pain, IBS and PTSD are exactly what you would expect if these share a mechanism — and it explains something clinicians have long observed without being able to justify: the same treatments help across all of them. Low-dose antidepressants, graded activity, sleep-focused treatment and cognitive approaches work in fibromyalgia, IBS, tension headache and bladder pain syndrome. That was never coincidence. It is the same volume control.
The language problem, which matters more than it should. “Central
sensitisation” and “the brain is amplifying the signal” are accurate and are routinely
heard as you are imagining it. They mean the opposite. Amplification is a
physical process in a physical organ, the pain produced is real pain, and a system stuck at high gain is
a mechanical fault rather than a failure of character. The brain is where pain is made in
everybody — that is not a special claim about this condition. What is specific here is
that the setting is wrong.