Abstract
DNA methylation changes are reliable biomarkers of aging, but the driving mechanisms remain poorly understood. Here we present SCARLET (Stem Cells and Age-ReLated Epigenetic Trajectories), a parsimonious mathematical model that describes how methylation changes in blood arise and propagate through hematopoietic stem cell divisions. Using a large human cohort, we demonstrate that seemingly distinct age-related methylation patterns can be explained by a unifying mechanistic model. We show that SCARLET captures known drivers of epigenetic aging, with accelerated individuals showing reduced ratios of stem cell pool size to division rate (N/s). Applying SCARLET to methylation data from 11 mammalian species reveals that N/s scales with maximum lifespan, suggesting that evolutionary adjustments to stem cell dynamics, rather than epigenetic maintenance efficiency, drive the previously observed relationship between methylation rates and lifespan. Our findings provide a quantitative framework for understanding epigenetic aging and suggest that stem cell dynamics may be a key driver of aging across mammals.
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Data availability
The mammal methylation dataset used was created by the Mammalian Methylation Consortium37 and is publicly available via the Gene Expression Omnibus under accession number GSE223748. Data on maximum lifespan were taken from the AnAge database (https://genomics.senescence.info/species/index.html)59. Human data were from the Generation Scotland cohort. According to the terms of consent for Generation Scotland participants, access to data must be reviewed by the Generation Scotland Access Committee. Applications should be made to access@generationscotland.org.
Code availability
All code used for the analysis (conducted in Python v.3.11.4) is available via GitFront at https://gitfront.io/r/scrofts/znVKdYUPtz66/SCARLET/.
References
Hannum, G. et al. Genome-wide methylation profiles reveal quantitative views of human aging rates. Mol. Cell 49, 359–367 (2013).
Koch, C. M. & Wagner, W. Epigenetic-aging-signature to determine age in different tissues. Aging 3, 1018–1027 (2011).
Bocklandt, S. et al. Epigenetic predictor of age. PLoS ONE 6, e14821 (2011).
Horvath, S. DNA methylation age of human tissues and cell types. Genome Biol. 14, R115 (2013).
Seale, K., Horvath, S., Teschendorff, A., Eynon, N. & Voisin, S. Making sense of the ageing methylome. Nat. Rev. Genet. 23, 585–605 (2022).
Slieker, R. C. et al. Age-related accrual of methylomic variability is linked to fundamental ageing mechanisms. Genome Biol. 17, 191 (2016).
Wang, Y., Pedersen, N. L. & Hägg, S. Implementing a method for studying longitudinal DNA methylation variability in association with age. Epigenetics 13, 866–874 (2018).
Seale, K., Teschendorff, A., Reiner, A. P., Voisin, S. & Eynon, N. A comprehensive map of the aging blood methylome in humans. Genome Biol. 25, 240 (2024).
Vershinina, O., Bacalini, M. G., Zaikin, A., Franceschi, C. & Ivanchenko, M. Disentangling age-dependent DNA methylation: deterministic, stochastic, and nonlinear. Sci. Rep. 11, 9201 (2021).
Johnson, N. D. et al. Non-linear patterns in age-related DNA methylation may reflect CD4+ T cell differentiation. Epigenetics 12, 492–503 (2017).
Levine, M. E., Higgins-Chen, A., Thrush, K., Minteer, C. & Niimi, P. Clock work: deconstructing the epigenetic clock signals in aging, disease, and reprogramming. Preprint atbioRxiv https://doi.org/10.1101/2022.02.13.480245 (2022).
Bell, C. G. et al. DNA methylation aging clocks: challenges and recommendations. Genome Biol. 20, 249 (2019).
Jung, M. & Pfeifer, G. P. Aging and DNA methylation. BMC Biol. 13, 7 (2015).
Koch, Z., Li, A., Evans, D. S., Cummings, S. & Ideker, T. Somatic mutation as an explanation for epigenetic aging. Nat. Aging 5, 709–719 (2025).
de Magalhães, J. P. Ageing as a software design flaw. Genome Biol. 24, 51 (2023).
Lu, A. T. et al. Universal DNA methylation age across mammalian tissues. Nat. Aging 3, 1144–1166 (2023).
Morandini, F., Seluanov, A. & Gorbunova, V. Slow and steady lives the longest. Nat. Aging 4, 7–9 (2024).
Lu, Y. R., Tian, X. & Sinclair, D. A. The information theory of aging. Nat. Aging 3, 1486–1499 (2023).
Crofts, S. J. C., Latorre-Crespo, E. & Chandra, T. DNA methylation rates scale with maximum lifespan across mammals. Nat. Aging 4, 27–32 (2024).
Horvath, S., Zhang, J., Haghani, A., Lu, A. T. & Fei, Z. Fundamental equations linking methylation dynamics to maximum lifespan in mammals. Nat. Commun. 15, 8093 (2024).
Cagan, A. et al. Somatic mutation rates scale with lifespan across mammals. Nature 604, 517–524 (2022).
Simons, B. D. & Clevers, H. Strategies for homeostatic stem cell self-renewal in adult tissues. Cell 145, 851–862 (2011).
Till, J. E., Mcculloch, E. A. & Siminovitch, L. A stochastic model of stem cell proliferation, based on the growth of spleen colony-forming cells. Proc. Natl Acad. Sci. USA 51, 29–36 (1964).
Barile, M. et al. Hematopoietic stem cells self-renew symmetrically or gradually proceed to differentiation. Preprint at bioRxiv https://doi.org/10.1101/2020.08.06.239186 (2020).
Loeffler, D. et al. Asymmetric lysosome inheritance predicts activation of haematopoietic stem cells. Nature 573, 426–429 (2019).
Lee-Six, H. et al. Population dynamics of normal human blood inferred from somatic mutations. Nature 561, 473–478 (2018).
Walker, R. M. et al. Data resource profile: whole-blood DNA methylation resource in Generation Scotland (MeGS). Int. J. Epidemiol. 54, dyaf091 (2025).
Smith, B. H. et al. Generation Scotland: the Scottish Family Health Study; a new resource for researching genes and heritability. BMC Med. Genet. 7, 74 (2006).
Dabrowski, J. K. et al. Probabilistic inference of epigenetic age acceleration from cellular dynamics. Nat. Aging 4, 1493–1507 (2024).
Watson, C. J. et al. The evolutionary dynamics and fitness landscape of clonal hematopoiesis. Science 367, 1449–1454 (2020).
Mitchell, E. et al. Clonal dynamics of haematopoiesis across the human lifespan. Nature 606, 343–350 (2022).
Körber, V. et al. Detecting and quantifying clonal selection in somatic stem cells. Nat. Genet. 57, 1718–1729 (2025).
Catlin, S. N., Busque, L., Gale, R. E., Guttorp, P. & Abkowitz, J. L. The replication rate of human hematopoietic stem cells in vivo. Blood 117, 4460–4466 (2011).
Pfeifer, G. P., Steigerwald, S. D., Hansen, R. S., Gartler, S. M. & Riggs, A. D. Polymerase chain reaction-aided genomic sequencing of an X chromosome-linked CpG island: methylation patterns suggest clonal inheritance, CpG site autonomy, and an explanation of activity state stability. Proc. Natl Acad. Sci. USA 87, 8252–8256 (1990).
Riggs, A. D. & Xiong, Z. Methylation and epigenetic fidelity. Proc. Natl Acad. Sci. USA 101, 4–5 (2004).
Meyer, D. H. & Schumacher, B. Aging clocks based on accumulating stochastic variation. Nat. Aging 4, 871–885 (2024).
Haghani, A. et al. DNA methylation networks underlying mammalian traits. Science 381, eabq5693 (2023).
Montazid, S. et al. Adult stem cell activity in naked mole rats for long-term tissue maintenance. Nat. Commun. 14, 8484 (2023).
Whittemore, K., Vera, E., Martínez-Nevado, E., Sanpera, C. & Blasco, M. A. Telomere shortening rate predicts species life span. Proc. Natl Acad. Sci. USA 116, 15122–15127 (2019).
Kapadia, C. D. et al. Clonal dynamics and somatic evolution of haematopoiesis in mouse. Nature 641, 681–689 (2025).
Robertson, N. A. et al. Age-related clonal haemopoiesis is associated with increased epigenetic age. Curr. Biol. 29, R786–R787 (2019).
Evans, M. A. & Walsh, K. Clonal hematopoiesis, somatic mosaicism, and age-associated disease. Physiol. Rev. 103, 649–716 (2023).
Slieker, R. C., Relton, C. L., Gaunt, T. R., Slagboom, P. E. & Heijmans, B. T. Age-related DNA methylation changes are tissue-specific with ELOVL2 promoter methylation as exception. Epigenetics Chromatin 11, 25 (2018).
Wang, Y., Grant, O. A., Zhai, X., Mcdonald-Maier, K. D. & Schalkwyk, L. C. Insights into ageing rates comparison across tissues from recalibrating cerebellum DNA methylation clock. GeroScience 46, 39–56 (2024).
Réu, P. et al. The lifespan and turnover of microglia in the human brain. Cell Rep. 20, 779–784 (2017).
von Bartheld, C. S., Bahney, J. & Herculano-Houzel, S. The search for true numbers of neurons and glial cells in the human brain: a review of 150 years of cell counting. J. Comp. Neurol. 524, 3865–3895 (2016).
Haluza, Y. et al. Axolotl epigenetic clocks offer insights into the nature of negligible senescence. Preprint at bioRxiv https://doi.org/10.1101/2024.09.09.611397 (2024).
Gerber, T. et al. Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration. Science 362, eaaq0681 (2018).
Moqri, M. et al. PRC2-AgeIndex as a universal biomarker of aging and rejuvenation. Nat. Commun. 15, 5956 (2024).
Higham, J. et al. Local CpG density affects the trajectory and variance of age-associated DNA methylation changes. Genome Biol. 23, 216 (2022).
Werner, B. et al. Reconstructing the in vivo dynamics of hematopoietic stem cells from telomere length distributions. eLife 4, e08687 (2015).
Jörg, D. J., Kitadate, Y., Yoshida, S. & Simons, B. D. Stem cell populations as self-renewing many-particle systems. Annu. Rev. Condens. Matter Phys. 12, 135–153 (2021).
Loeffler, D. et al. Asymmetric organelle inheritance predicts human blood stem cell fate. Blood 139, 2011–2023 (2022).
Zagkos, L., Roberts, J. & Auley, M. M. A mathematical model which examines age-related stochastic fluctuations in DNA maintenance methylation. Exp. Gerontol. 156, 111623 (2021).
Renshaw, E. A survey of stepping-stone models in population dynamics. Adv. Appl. Probab. 18, 581–627 (1986).
Bailey, N. T. J. The Elements of Stochastic Processes with Applications to the Natural Sciences (Wiley, 1991).
Abril-Pla, O. et al. PyMC: a modern, and comprehensive probabilistic programming framework in Python. PeerJ Comput. Sci. 9, e1516 (2023).
Tomusiak, A. et al. Development of an epigenetic clock resistant to changes in immune cell composition. Commun. Biol. 7, 934 (2024).
Tacutu, R. et al. Human Ageing Genomic Resources: new and updated databases. Nucleic Acids Res. 46, D1083–D1090 (2018).
Acknowledgements
We thank all the Generation Scotland (GS) study participants and research team members who have contributed, and continue to contribute, to ongoing studies. This research was funded in whole, or in part, by the Wellcome Trust grant 218492/Z/19/Z. For the purpose of open access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. S.J.C.C., C.M.G. and T.C. were supported by the Mayo Clinic Robert and Arlene Kogod Center on Aging. C.M.G. was supported by National Institutes of Health (NIH) Training grant T32 GM145408/GM/NIGMS, awarded to Mayo Clinic. GS received core support from the Chief Scientist Office of the Scottish Government Health Directorates (CZD/16/6) and the Scottish Funding Council (HR03006). E.L.-C. was partially supported by funding from the University of Edinburgh and the Medical Research Council (MC_UU_00009/2) and by an EHA Bilateral grant to K. Kirschner (BCG-202209–02649). E.L.C. has been funded by the Spanish Research Agency (AEI), through the Severo Ochoa (CEX2020-001049-S, MCIN/AEI/10.13039/501100011033) and Maria de Maeztu Program for Centers and Units of Excellence in R&D (CEX2020–001084-M). E.L.-C. thanks CERCA Program/Generalitat de Catalunya for institutional support. Research into this publication has been partially carried out in the Barcelona Collaboratorium for Modelling and Predictive Biology. DNA methylation profiling of the GS samples was carried out by the Genetics Core Laboratory at the Wellcome Trust Clinical Research Facility, Edinburgh, UK, and was funded by the Medical Research Council UK, the Brain & Behavior Research Foundation (ref. 27404) and the Wellcome Trust (Wellcome Trust Strategic Award ‘STratifying Resilience and Depression Longitudinally’ ((STRADL) ref. 104036/Z/14/Z)). R.E.M. was supported by BBSRC grant (UKRI/BB/C001941/1).
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E.L.-C. and T.C. conceived and supervised the study. S.J.C.C., C.M.G., R.E.M., E.L.-C. and T.C. wrote the paper. S.J.C.C., E.L.-C. and T.C. conducted data analysis.
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Extended data
Extended Data Fig. 1 Site parameters derived from SCARLET across CpG categories.
a, Comparisons of model fits across site categories, across varying sample sizes and numbers of selected sites. Models compared using expected log pointwise predictive density using leave-one-out cross-validation. Points represent means and error bars indicate standard deviations. Estimates use the approximately optimal value of \(N/s\)=10,000, while \(s\) is fixed at a plausible value of 1. b, The absolute difference between the inferred starting methylation value (at 0 years post sexual maturity, p) and steady-state methylation level approached as age increases (η) against the sum of epigenetic error probabilities, \({\,P}_{M\to U}\) and \({\,P}_{U\to M}\). Estimates use the approximately optimal value of \(N/s\)=10,000, while \(s\) is fixed at a plausible value of 1. c, Boxplots (showing median and interquartile range) of epigenetic maintenance efficiencies when \(N/s\) is the optimal value shown in Fig. 2b (10,000) and the rate of stem cell division, \(s\), is fixed at a realistic value of 1 division per stem cell per year. Sites are split into those that increase or decrease in mean methylation with age (note that for VMPs, this categorization will be essentially arbitrary). Estimates taken from 200 sites and 200 samples for each category.
Extended Data Fig. 2 Sensitivity analyses and further model validation for Fig. 2.
a, Sensitivity analysis for human \(N/s\) estimates by group (Fig. 2c), with age-related sites selected based on variance change (White test) rather than mean change. Each group comprises 500 participants individually age-matched to the other cohorts within each plot. Each estimate is derived from the top 500 age-related sites. Smoking and non-smoking defined by a binarised ‘weighted smoking’ variable (see Methods for details). High epigenetic age acceleration defined as the top 500 accelerated participants as measured according to Dabrowski et al. (2024)29 (see Methods for further details). Estimates displayed as mean of the posterior, with error bars indicating the 95% credible interval. b, Sensitivity analysis of the effect of number of methylation sites used in the inference of \(N/s\). Sample size fixed at 200 participants. Sites selected based on the top age-related sites (Spearman’s rank correlation coefficient). Estimates displayed as mean of the posterior, with error bars indicating the 95% credible interval. c, Sensitivity analysis of the effect of number of participants analysed in the inference of \(N/s\). Number of CpGs fixed at 200. Estimates displayed as mean of the posterior, with error bars indicating the 95% credible interval.
Extended Data Fig. 3 Sensitivity analyses for Fig. 3.
a, Sensitivity analysis on the effect of different analysed timespans (a proxy for lifespans) on estimates of \(N/s\). For each estimate, the human cohort was trimmed to the stated maximum age after adulthood. The top 100 age-related sites (Spearman’s rank correlation coefficient) were then recalculated on this restricted dataset, and the model was then fit. Each estimate was calculated from a sample of 150 participants (approximately the maximum sample size in the smallest age range), uniformly sampled within the given age range. Estimates displayed as mean of the posterior, with error bars indicating the 95% credible interval (log scale). Included in red is the regression line we find from our scaling analysis, i.e., what we would expect if our results were due solely to the fact that animals of longer lifespan are generally observed over longer timespans. b, As in a), except with sites selected based on variance change (White test) rather than mean change. c, Sensitivity analysis on the effect of site selection on the scaling of \(N/s\) with maximum lifespan across mammals. Each inference based on the top 500 sites that change in variance (White test) instead of mean change, and using the maximum sample size available for each mammal (minimum 110, capped at 1000 for humans). d, Example in mice of a CpG fit using a single \(N/s\) value across all mammals. e, The same data shown in d, fit using an \(N/s\) value that scales with lifespan across mammals. f, The same data shown in d, fit using the \(N/s\) values determined independently for each mammal. g, Example in humans of a CpG fit using a single \(N/s\) value across all mammals. h, The same data shown in g), fit using an \(N/s\) value that scales with lifespan across mammals. i, The same data shown in g), fit using the \(N/s\) values determined independently for each mammal.
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Mathematical derivation of SCARLET and Supplementary Table 1.
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Crofts, S.J.C., Grenko, C.M., Marioni, R.E. et al. A unifying model of stem cell dynamics explains age-related methylation patterns across mammals. Nat Aging (2026). https://doi.org/10.1038/s43587-026-01125-y
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DOI: https://doi.org/10.1038/s43587-026-01125-y