What Is a Differentially Methylated Region (DMR)?

A differentially methylated region is a genomic interval where methylation differs between two conditions, groups, or alleles by more than a stated threshold, over a stated number of CpGs, with statistical support.

The definition has three moving parts — effect size, CpG density, and statistical test — and every DMR list is a consequence of choices about them. A DMR is therefore a finding about a comparison, not a property of the genome.

Confusing the first two with the last three is common: an imprinting DMR has a defined allelic pattern, whereas a tissue-specific DMR is a population average.

The pipeline is consistent across tools even when parameters differ:

1. Per-CpG methylation from bisulfite or EM-seq, with coverage per site.
2. Filtering for minimum coverage and removal of unreliable sites.
3. Site-level testing with a model appropriate to the design (beta-binomial is common; simple t-tests on proportions are not).
4. Regional aggregation — smoothing, hidden Markov models, or windowed scanning — to combine neighbouring sites into intervals.
5. Thresholding on mean difference and, often, on the number of CpGs.

Choices at steps 2, 4, and 5 dominate the final list. Two labs analysing the same data with different callers can produce substantially different DMRs, which is why methods sections must name the caller and parameters.

A DMR claim should state:

• • Coverage at each CpG (and how low-coverage sites were handled).
• • Mean methylation difference across the region, in percentage points.
• • Number of CpGs in the region and their spacing.
• • Statistical support, corrected for multiple testing, with the test named.
• • Consistency, e.g., the difference being present in a stated fraction of samples or replicates.
• • Comparison group definition, including sex, age, and cell composition where relevant.

The last point matters most in tissue studies: a DMR between "tumour" and "normal" may reflect differing cell composition rather than a regulatory change.

"A DMR means the region is regulatory." A DMR marks a difference in methylation. Whether it affects transcription depends on location, CpG density, and factor binding — and many DMRs sit in regions with no detectable regulatory function.

"A DMR between groups indicates a causal exposure effect." Cross-sectional designs cannot distinguish cause from consequence, and cell composition differences produce DMRs without any epigenetic response.

"Hypermethylation of a promoter means the gene is silenced." Frequently true for CpG-island promoters, but not universally, and rarely the whole story: residual expression, alternative promoters, and biallelic patterns all occur.

A defensible sequence:

1. Annotate DMRs against promoters, enhancers, CTCF sites, and repeats.
2. Check composition with deconvolution, especially in blood or tissue.
3. Prioritize a small set with a plausible target gene and a functional annotation.
4. Test causality by targeted editing — demethylate with dCas9–TET1 or methylate with a DNMT3A fusion — and measure expression with matched controls.
5. Report the negative ones too, since a DMR that does not change expression is informative about the relationship.

Step 4 is the only one that turns a DMR into a mechanism, and it is the step most often absent.

EDITGENE builds the follow-up systems: targeted methylation and demethylation lines at nominated DMRs, isogenic controls, and expression readouts with neighbour-gene checks.

Send us the DMR coordinates and the candidate gene. Contact EDITGENE to request a project consultation or quote.

DMR calls depend on coverage, caller, thresholds, and covariates; comparisons across studies require matched processing and should be interpreted with the method reported.

A DMR indicates a methylation difference, not a regulatory effect. Causal claims require perturbation and functional readouts.

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• • Park Y, Wu H. Differential methylation analysis for BS-seq data under general experimental design. Bioinformatics. 2016;32:1446–1453.
• • Greenberg MVC, Bourc'his D. The diverse roles of DNA methylation in mammalian development and disease. Nature Reviews Molecular Cell Biology. 2019;20:590–607.
• • Loyfer N, et al. A DNA methylation atlas of normal human cell types. Nature. 2023;613:355–364.