Hallmark 2 of 12
Telomere attrition
Telomeres are protective caps at the ends of chromosomes. They shorten a little with every cell division until the cell can no longer divide. The image of a biological hourglass made this hallmark famous and simplified it heavily at the same time. Human data paint a considerably more awkward picture, in which short telomeres carry risks and long ones do too.
- Scientific term
- Telomere attrition
- Group
- Primary causes
- Hallmarks of Aging
- Described in 2013
The hallmarks of aging are a research framework, not a diagnostic tool. They describe processes that occur during ageing, but they do not prove causation and are not a basis for medical decisions.
What lies behind it
In humans, telomeres consist of thousandfold repeats of the TTAGGG sequence plus a protein complex called shelterin. Their job is to package the chromosome end so the cell does not mistake it for a double-strand break. Without that protection the repair machinery would fuse chromosomes together.
At every replication a piece is lost at the end of the lagging strand because DNA polymerase cannot work all the way to the edge. This end-replication problem leads to a loss of roughly fifty to two hundred base pairs per division. Once a telomere falls below a critical length, the cell triggers a DNA damage response and enters senescence or programmed cell death.
The enzyme telomerase can lengthen telomeres again. In most body cells it is switched off; it remains active in germ cells, stem cells and a subset of immune cells. Around ninety per cent of tumours reactivate telomerase, which grants them unlimited division capacity. This dual role explains why blanket telomere lengthening is not a harmless idea.
Beyond division count, other factors influence telomere length. Oxidative stress preferentially damages the guanine-rich telomeric sequences, and a substantial share of the differences between people is inherited.
How robust is the evidence
Moderate evidence
The mechanism is well documented; human data are largely observational.
That telomeres shorten with age is uncontested and has been measured in many tissues. What is contested is what length says about an individual. The spread between people of the same age is larger than the average loss across a decade, which makes single measurements weakly informative.
Genetic studies carry the most weight. An analysis of 472,174 UK Biobank participants linked inherited predisposition for telomere length to disease risk and found associations spanning neoplastic, vascular and inflammatory conditions. People whose telomere length at age forty was more than one standard deviation below the mean had a life expectancy 2.5 years lower than the group with particularly long telomeres.
Which direction the risk takes depends on the disease. A Mendelian randomisation across several consortia found that genetically longer telomeres carried markedly higher risk for several cancers, most pronounced for glioma, and also for lung adenocarcinoma and melanoma. The same analysis found longer telomeres associated with lower risk of interstitial lung disease and of abdominal aortic aneurysm. Short and long both carry disadvantages, just different ones.
Rare inherited conditions support the causal reading at the lower end. In dyskeratosis congenita and related telomeropathies, defects in the telomerase complex lead to very short telomeres and subsequently to bone marrow failure, pulmonary fibrosis and liver damage. This shows that extreme shortening does cause disease. It does not follow that moderate differences within the normal range reflect the same mechanism.
Findings in humans
- Telomere length declines with age on average, but the spread between people of the same age is large.
- Genetically long telomeres were associated with higher risk of several cancers, most strongly for glioma.
- The same analysis found long telomeres associated with lower risk of interstitial lung disease and abdominal aortic aneurysm.
- Telomeropathies such as dyskeratosis congenita cause bone marrow failure and pulmonary fibrosis, demonstrating causality at the extreme end.
- Around ninety per cent of tumours reactivate telomerase in order to divide without limit.
What of this is measurable at YEARS
Not measurable in preventive careYEARS does not determine telomere length, and that is a deliberate decision. Commercial telomere tests usually measure average length in white blood cells. This average obscures what matters biologically, namely the share of critically short telomeres. Measurement methods vary considerably between laboratories, and a single value cannot meaningfully be translated into a recommendation for an individual.
Context
If a provider derives a biological age from a telomere measurement, scepticism is warranted. For pace of ageing, epigenetic methods are currently better validated. Telomere length measurement has its place in the workup of rare telomeropathies, meaning in the hands of specialists with a concrete suspicion, not in general preventive care.
Limits of this hallmark
The hourglass image does not carry far. It suggests that every cell has a fixed division budget that can be read off. In reality, telomere lengths differ between tissues, between chromosomes within the same cell and between people of the same age to a degree that leaves an average value with little informative value.
The two-sidedness of the risk is the key point. Short is associated with organ failure, long with cancer. There is no established target value to aim for, and no study showing that lengthening telomeres in healthy people leads to better outcomes. Supplements marketed with telomere protection have no robust basis for that claim.
In humans, telomere attrition is also a weaker driver of ageing than in short-lived laboratory animals. Mice have considerably longer telomeres than humans yet a far shorter lifespan. That argues against reading telomere length as a universal clock.
Frequently asked questions
What are telomeres and why do they shorten?→
Telomeres are repeat sequences at the end of every chromosome, together with a protein complex that protects the chromosome end. Without that protection the cell would treat the open end as a DNA break and join chromosomes incorrectly. At every cell division a piece is lost because the copying machinery cannot start at the very end. That amounts to roughly fifty to two hundred base pairs per division. Oxidative stress accelerates the loss further. Once a telomere falls below a critical length, the cell stops dividing permanently. A substantial share of the differences between people is, however, inherited and not explained by lifestyle.
Is a telomere test worthwhile?→
For general preventive care, currently not. Commercial tests determine mean telomere length in white blood cells. That average hides precisely what matters biologically, namely how many telomeres are already critically short. Results fluctuate between laboratories and between measurement occasions more than the annual ageing effect amounts to. On top of this there is no established target value: short telomeres are associated with organ failure, long ones with several cancers. No meaningful recommendation can therefore be derived from a single value. One exception is a justified suspicion of a rare telomeropathy, which belongs in specialist hands.
Can telomeres be lengthened?→
In the laboratory yes, via the enzyme telomerase. In healthy people it is not an established goal, for a concrete reason: around ninety per cent of tumours switch telomerase back on because it gives them unlimited division capacity. Genetic studies also show that inherited longer telomeres go along with higher risk of certain cancers. No clinical trial demonstrates that lengthening telomeres in healthy people leads to less disease or longer life. Supplements advertised on this basis rest on cell culture data, not on human endpoint trials.
Does lifestyle influence telomere length?→
Probably to some degree, but more weakly than often portrayed. Observational studies link smoking, marked obesity and chronic stress with shorter telomeres, and physical activity with longer ones. These associations are usually small statistically and vulnerable to confounding, because people with healthier lifestyles differ in many other characteristics. Intervention studies with hard endpoints are largely lacking. The practical consequence barely changes: not smoking, exercise and sleep are well justified independently of telomeres. Using telomere length as a progress check for lifestyle change is not currently defensible.
Sources
- López-Otín C, Blasco MA, Partridge L, Serrano M, Kroemer G. Hallmarks of aging: An expanding universe. Cell. 2023;186(2):243–278.
- Blackburn EH, Epel ES, Lin J. Human telomere biology: A contributory and interactive factor in aging, disease risks, and protection. Science. 2015;350(6265):1193–1198.
- Codd V, Wang Q, Allara E, et al. Polygenic basis and biomedical consequences of telomere length variation. Nature Genetics. 2021;53:1425–1433.
- Telomeres Mendelian Randomization Collaboration, Haycock PC, Burgess S, et al. Association between telomere length and risk of cancer and non-neoplastic diseases: a Mendelian randomization study. JAMA Oncology. 2017;3(5):636–651.
- Aviv A, Shay JW. Reflections on telomere dynamics and ageing-related diseases in humans. Philosophical Transactions of the Royal Society B. 2018;373(1741):20160436.
- Armanios M, Blackburn EH. The telomere syndromes. Nature Reviews Genetics. 2012;13(10):693–704.
- Gems D, de Magalhães JP. The hoverfly and the wasp: A critique of the hallmarks of aging as a paradigm. Ageing Research Reviews. 2021;70:101407.
Related hallmarks
Hallmark 1 of 12
Genomic instability
DNA is damaged thousands of times a day. Whatever repair misses remains as a mutation.
Hallmark 8 of 12
Cellular senescence
Cells stop dividing permanently but do not die. From this in-between state they reshape their surroundings.
Hallmark 9 of 12
Stem cell exhaustion
Tissue reserves for renewal shrink and slow down. The cells themselves are not always the problem.