Telomeres

Terminology

• A telomere is the repetitive DNA at a chromosome end together with the proteins bound to it. Human telomeres are thousands of repeats of TTAGGG.¹⁵ The strand that runs 5′ to 3′ towards the chromosome end is rich in G and extends beyond the other strand as a single-stranded G-overhang.¹⁶
• Shelterin is a complex of six proteins, TRF1, TRF2, POT1, TIN2, TPP1 and Rap1, that binds telomeric DNA and lets cells tell telomeres from DNA breaks.¹
• Telomerase is a reverse transcriptase that adds telomeric repeats to chromosome ends, copying them from a template in its own RNA. Its two core parts are the protein TERT and the RNA TERC.^17181920
• The end-replication problem is the inability of the DNA-copying machinery to copy a linear molecule to its very end, so that each round of replication would shorten it.²¹²²
• Replicative senescence is the permanent arrest of division that human cells enter after a limited number of divisions in culture, the Hayflick limit (see hayflick-moorhead-experiment and cellular-senescence).
• Alternative lengthening of telomeres (ALT) is the maintenance of telomere length by recombination between telomeres rather than by telomerase.⁶⁷

Structure and maintenance

The chromosome end

Hermann Muller named the telomere in 1938, from his work on irradiated Drosophila, and argued that a chromosome could not persist without its natural end.²³ Barbara McClintock showed in maize that broken chromosome ends fuse with each other, which starts cycles of breakage, fusion and bridging between dividing cells, while natural ends do not.²⁴

The first telomeric sequence was found by Elizabeth Blackburn and Joseph Gall at the ends of small DNA molecules in the protozoan Tetrahymena: a six-base repeat present 20 to 70 times.²⁵ In 1982 Jack Szostak and Blackburn showed that these ends allow a linear plasmid to be maintained in yeast.²⁶ Robert Moyzis and colleagues identified the human repeat, TTAGGG, at the ends of all human chromosomes in 1988.¹⁵

Telomeric DNA is not a simple tail. In human and mouse cells the G-overhang can tuck back into the double-stranded repeats to form a large loop, the t-loop, which hides the end.²⁷ Shelterin binds along the repeats: TRF1, TRF2 and POT1 recognise the sequence, and TIN2, TPP1 and Rap1 link them together.¹ Without this protection, a chromosome end looks like a double-strand break to the cell’s DNA-damage response, which then arrests the cell or joins the end to another chromosome.

Why telomeres shorten

In 1972 James Watson pointed out that the machinery that copies DNA cannot finish the end of a linear molecule; Alexey Olovnikov had made the same argument in 1971 and proposed that the resulting shortening counts cell divisions.²¹²² The original form of the argument predicts an overhang at only half of chromosome ends. Victor Makarov, Yumiko Hirose and Jack Langmore found long G-overhangs, averaging 130 to 210 bases, on more than 80% of human telomeres, and concluded that the C-rich strand is actively trimmed at both ends, which adds to the loss.¹⁶

Telomerase

Carol Greider and Blackburn found an activity in Tetrahymena extracts that adds telomeric repeats to the end of a telomere-like DNA strand,²⁸ and later showed that it carries its own RNA, part of which is the template for the repeats.¹⁷ The enzyme, telomerase, solves the end-replication problem by adding back what replication loses. Blackburn, Greider and Szostak shared the 2009 Nobel Prize in Physiology or Medicine “for the discovery of how chromosomes are protected by telomeres and the enzyme telomerase”.²⁹

Telomeres and cell division

Calvin Harley, Bruce Futcher and Greider found in 1990 that the amount and length of telomeric DNA in human fibroblasts decreases as the cells are passaged in culture.³⁰ Whether this caused senescence was then open. Andrea Bodnar and colleagues answered it in 1998: human cells given the TERT gene lengthened their telomeres, kept a normal set of chromosomes and “exceeded their normal life-span by at least 20 doublings”.² The arrest is triggered by the shortest telomeres, not by the average. In mice, Michael Hemann and colleagues found that loss of telomere function occurred preferentially on the chromosomes with critically short telomeres, even when only half the chromosomes had short ones.³

In the human body, telomere length is usually measured in white blood cells. Telomeres shorten fastest in early childhood, by more than 1,000 base pairs a year, then much more slowly.³¹ Across cross-sectional studies of adults, blood-cell telomeres are about 22 base pairs shorter for each year of age; the few longitudinal studies found losses of 32 to 46 base pairs a year.³² Lily Daniali and colleagues measured telomeres in blood, muscle, skin and fat from the same 87 adults. Figure 1 shows the result: the tissues differ in length, but all shorten at about 25 base pairs a year. The differences between tissues are therefore set early in life.³³

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "description": "Figure 1. Telomere length in four tissues from the same 87 adults aged 19 to 77 (Daniali et al. 2013). The first panel shows mean telomere length with its standard deviation; the second, how much shorter telomeres are for each additional year of age, with its reported uncertainty. Blood cells have the shortest telomeres and muscle the longest, but all four tissues shorten at about the same rate.",
  "data": {
    "values": [
      {"tissue": "Blood", "order": 1, "length": 7.02, "lsd": 0.86, "rate": 26, "rse": 6},
      {"tissue": "Skin", "order": 2, "length": 7.29, "lsd": 0.9, "rate": 23, "rse": 7},
      {"tissue": "Fat", "order": 3, "length": 8.18, "lsd": 0.93, "rate": 25, "rse": 7},
      {"tissue": "Muscle", "order": 4, "length": 8.55, "lsd": 0.94, "rate": 24, "rse": 7}
    ]
  },
  "hconcat": [
    {
      "width": 290,
      "height": 170,
      "transform": [{"calculate": "datum.length - datum.lsd", "as": "lo"}, {"calculate": "datum.length + datum.lsd", "as": "hi"}],
      "layer": [
        {"mark": {"type": "rule", "style": "model", "strokeWidth": 2}, "encoding": {"x": {"field": "lo", "type": "quantitative"}, "x2": {"field": "hi"}}},
        {"mark": {"type": "circle", "style": "model-point", "size": 80}, "encoding": {"x": {"field": "length", "type": "quantitative"}}}
      ],
      "encoding": {
        "y": {"field": "tissue", "type": "nominal", "sort": {"field": "order"}, "axis": {"title": null, "ticks": false, "domain": false, "minExtent": 62, "maxExtent": 62, "labelLimit": 62}},
        "x": {"type": "quantitative", "scale": {"domain": [5, 10], "nice": false}, "axis": {"title": "Mean telomere length, kilobases", "values": [5, 6, 7, 8, 9, 10]}}
      }
    },
    {
      "width": 290,
      "height": 170,
      "transform": [{"calculate": "datum.rate - datum.rse", "as": "lo"}, {"calculate": "datum.rate + datum.rse", "as": "hi"}],
      "layer": [
        {"mark": {"type": "rule", "style": "alt", "strokeWidth": 2, "strokeDash": []}, "encoding": {"x": {"field": "lo", "type": "quantitative"}, "x2": {"field": "hi"}}},
        {"mark": {"type": "circle", "style": "alt", "opacity": 1, "size": 80}, "encoding": {"x": {"field": "rate", "type": "quantitative"}}}
      ],
      "encoding": {
        "y": {"field": "tissue", "type": "nominal", "sort": {"field": "order"}, "axis": {"title": null, "ticks": false, "domain": false, "minExtent": 62, "maxExtent": 62, "labelLimit": 62}},
        "x": {"type": "quantitative", "scale": {"domain": [0, 40], "nice": false}, "axis": {"title": "Shortening, base pairs per year", "values": [0, 10, 20, 30, 40]}}
      }
    }
  ]
}

Telomeres and cancer

Nam Kim and colleagues developed a sensitive assay for telomerase in 1994. Telomerase activity was present in 98 of 100 immortal cell populations and in none of 22 mortal ones, and in 90 of 101 tumour biopsies from 12 tumour types but in none of 50 normal somatic tissues.⁵ Some tumours and tumour cell lines without telomerase maintain their telomeres by recombination between telomeres.⁶ In 2013, mutations in the promoter of the TERT gene, which raise its activity, were found in 50 of 70 melanomas examined by Francis Huang and colleagues,³⁴ and a germline promoter mutation was found in a family prone to melanoma by Susanne Horn and colleagues.³⁵

Mice illustrate both sides of the trade-off. Maria Blasco and colleagues made mice without telomerase RNA. Their telomeres shortened by about 4.8 kilobases each generation, and from the fourth generation some chromosome ends had no detectable telomeric repeats and fused with other chromosomes.³⁶ Karl Lenhard Rudolph and colleagues found that late generations of these mice had shorter lives, a reduced capacity to respond to stresses such as wounding and ablation of the blood-forming system, and more spontaneous cancers, but loss of telomere function “did not elicit a full spectrum of classical pathophysiological symptoms of aging”.³⁷ In mice that also carried a p53 mutation, critically short telomeres drove chromosome rearrangements and epithelial cancers.³⁸ Laboratory mice are a weak model of human telomere biology: their telomeres are far longer than human ones,³⁹ and cells from mice lacking telomerase become immortal about ten million times more often than human cells.⁴⁰

In 15 rodent species, Andrei Seluanov and colleagues found telomerase repressed in the large species and active in the small ones, including the long-lived naked mole-rat.⁴¹ Across more than 60 mammals, Nuno Gomes and colleagues found telomere length inversely related to lifespan, and inferred that short telomeres with repressed telomerase are the ancestral mammalian state.¹³ Michael Pepke and Dan Eisenberg reanalysed the Gomes data and confirmed that telomere length falls with lifespan; they also found that it falls with body mass, but found only weak and non-robust evidence that telomerase activity tracks body mass. In a separate analysis of 22 species, longer telomeres predicted higher cancer risk, and they suggested that short telomeres were selected in large, long-lived species “probably as a mechanism to suppress cancer”.⁴²

In people, Mendelian randomisation compares individuals who differ, from conception, in genetic variants that set telomere length. In a study covering 22 cancers and 32 other diseases, genetically longer telomeres raised the odds of 9 of the cancers and lowered the odds of 6 of the other diseases, including coronary heart disease (Figure 2); the cancer associations were strongest for rarer cancers.⁸ Among 261,837 UK Biobank participants aged 60 or over by the end of follow-up, each 250 base pairs of genetically longer telomere, about a decade of average adult shortening, raised the odds of cancer by 11% and lowered the odds of coronary heart disease by 5%, and showed little association with parental lifespan.¹¹ Most carriers of loss-of-function mutations in POT1, a shelterin gene, have telomeres longer than the 99th percentile for their age, did not show the usual shortening over two years of follow-up, and develop clonal expansions of blood cells and a wide range of benign and malignant tumours.⁴³ Mary Armanios summarised that long telomeres “are now appreciated to be the most common shared germline risk factor for cancer in population studies”.⁹

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "description": "Figure 2. Odds ratios for disease per standard deviation of genetically longer telomeres, from the Telomeres Mendelian Randomization Collaboration (Haycock et al. 2017). The upper nine are cancers, the lower four other diseases. Bars are 95% confidence intervals; the axis is logarithmic, and odds ratios above 1 mean that longer telomeres raise the risk. Shown are the nine cancers and four of the non-neoplastic diseases with significant associations; most common cancers, such as colorectal cancer, showed little or no association.",
  "width": 560,
  "height": 468,
  "data": {
    "values": [
      {
        "order": 1,
        "disease": "Glioma",
        "or": 5.27,
        "lo": 3.15,
        "hi": 8.81,
        "kind": "Cancer"
      },
      {
        "order": 2,
        "disease": "Low-malignant-potential ovarian",
        "or": 4.35,
        "lo": 2.39,
        "hi": 7.94,
        "kind": "Cancer"
      },
      {
        "order": 3,
        "disease": "Lung adenocarcinoma",
        "or": 3.19,
        "lo": 2.4,
        "hi": 4.22,
        "kind": "Cancer"
      },
      {
        "order": 4,
        "disease": "Neuroblastoma",
        "or": 2.98,
        "lo": 1.92,
        "hi": 4.62,
        "kind": "Cancer"
      },
      {
        "order": 5,
        "disease": "Bladder cancer",
        "or": 2.19,
        "lo": 1.32,
        "hi": 3.66,
        "kind": "Cancer"
      },
      {
        "order": 6,
        "disease": "Melanoma",
        "or": 1.87,
        "lo": 1.55,
        "hi": 2.26,
        "kind": "Cancer"
      },
      {
        "order": 7,
        "disease": "Testicular germ cell",
        "or": 1.76,
        "lo": 1.02,
        "hi": 3.04,
        "kind": "Cancer"
      },
      {
        "order": 8,
        "disease": "Kidney cancer",
        "or": 1.55,
        "lo": 1.08,
        "hi": 2.23,
        "kind": "Cancer"
      },
      {
        "order": 9,
        "disease": "Endometrial cancer",
        "or": 1.31,
        "lo": 1.07,
        "hi": 1.61,
        "kind": "Cancer"
      },
      {
        "order": 10,
        "disease": "Coronary heart disease",
        "or": 0.78,
        "lo": 0.67,
        "hi": 0.9,
        "kind": "Other"
      },
      {
        "order": 11,
        "disease": "Abdominal aortic aneurysm",
        "or": 0.63,
        "lo": 0.49,
        "hi": 0.81,
        "kind": "Other"
      },
      {
        "order": 12,
        "disease": "Coeliac disease",
        "or": 0.42,
        "lo": 0.28,
        "hi": 0.61,
        "kind": "Other"
      },
      {
        "order": 13,
        "disease": "Interstitial lung disease",
        "or": 0.09,
        "lo": 0.05,
        "hi": 0.15,
        "kind": "Other"
      }
    ]
  },
  "encoding": {
    "y": {
      "field": "disease",
      "type": "nominal",
      "sort": {
        "field": "order"
      },
      "axis": {
        "title": null,
        "labels": false,
        "ticks": false,
        "domain": false,
        "grid": false,
        "minExtent": 62,
        "maxExtent": 62
      }
    }
  },
  "layer": [
    {
      "transform": [
        {
          "filter": "datum.order == 1"
        }
      ],
      "mark": {
        "type": "rule",
        "style": "guide"
      },
      "encoding": {
        "x": {
          "datum": 1,
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          },
          "axis": {
            "title": "Odds ratio per SD longer telomeres",
            "values": [
              0.05,
              0.1,
              0.2,
              0.5,
              1,
              2,
              5,
              10
            ],
            "format": "~g"
          }
        },
        "y": null
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Cancer'"
        }
      ],
      "mark": {
        "type": "rule",
        "style": "model",
        "strokeWidth": 2,
        "yOffset": 6
      },
      "encoding": {
        "x": {
          "field": "lo",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        },
        "x2": {
          "field": "hi"
        }
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Cancer'"
        }
      ],
      "mark": {
        "type": "circle",
        "style": "model-point",
        "size": 60,
        "yOffset": 6
      },
      "encoding": {
        "x": {
          "field": "or",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        }
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Other'"
        }
      ],
      "mark": {
        "type": "rule",
        "style": "alt",
        "strokeWidth": 2,
        "strokeDash": [],
        "yOffset": 6
      },
      "encoding": {
        "x": {
          "field": "lo",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        },
        "x2": {
          "field": "hi"
        }
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Other'"
        }
      ],
      "mark": {
        "type": "circle",
        "style": "alt",
        "opacity": 1,
        "size": 60,
        "yOffset": 6
      },
      "encoding": {
        "x": {
          "field": "or",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        }
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Cancer'"
        },
        {
          "calculate": "0.042",
          "as": "tx"
        },
        {
          "calculate": "datum.disease + ' ' + format(datum.or, '.2f')",
          "as": "label"
        }
      ],
      "mark": {
        "type": "text",
        "style": "model-label",
        "fontWeight": "normal",
        "align": "left",
        "yOffset": -8
      },
      "encoding": {
        "x": {
          "field": "tx",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        },
        "text": {
          "field": "label"
        }
      }
    },
    {
      "transform": [
        {
          "filter": "datum.kind == 'Other'"
        },
        {
          "calculate": "0.042",
          "as": "tx"
        },
        {
          "calculate": "datum.disease + ' ' + format(datum.or, '.2f')",
          "as": "label"
        }
      ],
      "mark": {
        "type": "text",
        "style": "alt-label",
        "fontWeight": "normal",
        "align": "left",
        "yOffset": -8
      },
      "encoding": {
        "x": {
          "field": "tx",
          "type": "quantitative",
          "scale": {
            "type": "log",
            "domain": [
              0.04,
              12
            ],
            "nice": false
          }
        },
        "text": {
          "field": "label"
        }
      }
    }
  ]
}

Telomeres and ageing

Short telomere syndromes

Inherited defects in telomere maintenance cause a family of diseases. The first gene linked to telomerase was DKC1, mutated in the X-linked form of dyskeratosis congenita. Its product, dyskerin, binds telomerase RNA, and patients’ cells have less telomerase and shorter telomeres.⁴⁴ The autosomal dominant form is caused by mutations in the telomerase RNA gene itself.⁴⁵ Mutations in telomerase genes were found in 6 of 73 families with idiopathic pulmonary fibrosis, most without the classic features of dyskeratosis congenita.⁴⁶ Patients with sporadic pulmonary fibrosis also have short telomeres, about one in ten below the first percentile for their age.⁴⁷ Armanios describes the short telomere syndromes as “the most prevalent premature aging disorders, with prominent phenotypes affecting the lung and hematopoietic system”.⁹ Among 180 patients, 12.8% developed a cancer other than of the skin, three-quarters of them myelodysplastic syndrome or acute myeloid leukaemia.⁴⁸

Telomere length and mortality

Short blood-cell telomeres are consistently but modestly associated with higher mortality.⁴⁹⁵⁰ Genetic studies support a causal effect of shorter telomeres on coronary heart disease, but not on overall survival, because the same variants that lengthen telomeres raise the risk of cancer.¹⁰⁸¹¹

• Early study. Among 143 Utah residents aged 60 or over, those with shorter telomeres had poorer survival, with mortality rates 3.18 times higher from heart disease and 8.54 times higher from infectious disease; the confidence interval for infection ran from 1.5 to 48.⁵¹ A study of 598 people aged 85 or over found no association,⁵² and a meta-analysis of 121,749 people found a hazard ratio of 1.26 for the shortest against the longest quarter, smaller above age 80.⁴⁹
• Cardiovascular disease. In a meta-analysis of 24 studies and 43,725 participants, people in the shortest third of telomere length had 1.54 times the risk of coronary heart disease of those in the longest third; the figure was 1.40 in prospective studies and 1.34 after correction for publication bias.⁵³
• Observational and genetic estimates compared. Among 64,637 Danes, those in the shortest tenth of telomere length had a 40% higher mortality than those in the longest tenth. Genetic variants that shorten telomeres, by about 69 base pairs per allele, were not associated with all-cause mortality and were associated with lower cancer mortality.¹⁰
• UK Biobank. Among 472,174 participants with measured telomere length, men whose telomeres were more than one standard deviation shorter than average had an estimated life expectancy at 40 that was 2.5 years lower than men whose telomeres were at least one standard deviation longer; the estimate for women was similar. Cardiovascular disease and cancer together accounted for less than a fifth of the difference, which points to confounding by other conditions.⁵⁴

The usual reading of the difference between the observational and genetic estimates is that much of the observational association reflects conditions that shorten telomeres, or that accompany short telomeres, rather than an effect of telomere length itself. As a marker of biological age, telomere length “can only allow a rough estimate of aging rate and can hardly be regarded as a clinically important risk marker for age-related pathologies and mortality”, in the assessment of Alexander Vaiserman and Dmytro Krasnienkov.¹² Part of the reason is measurement error. Most large studies measure telomere length by quantitative PCR, whose between-run coefficient of variation was 6.45% in a blinded comparison, against 1.74% for Southern blotting.⁵⁵ A 6% error in a 7-kilobase telomere is about 400 base pairs, more than a decade of average adult shortening, so single measurements cannot track ageing in an individual, and much of the apparent lengthening seen in longitudinal studies is measurement error.⁵⁶

Across species

Long-lived mammals do not have long telomeres; across more than 60 species, telomere length is inversely related to lifespan.¹³ Kurt Whittemore and colleagues measured blood-cell telomeres at different ages in mice, dolphins, goats, reindeer, flamingos, griffon vultures, gulls and elephants, and found that “the telomere shortening rate, but not the initial telomere length alone, is a powerful predictor of species life span”. Mouse telomeres shortened about 100 times faster than human telomeres. The study was cross-sectional, with one measurement per animal and as few as 7 animals in a species.¹⁴ Whether faster shortening shortens life, or both reflect faster cell turnover, the comparison cannot say.

Telomerase as an intervention

Delivering telomerase to adult mice with a virus extended their median lifespan, by 24% when given at one year of age and 13% at two years, without increasing cancer, in groups of 14 to 23 mice from a single laboratory.⁵⁷ Mice made with hyper-long telomeres, without extra telomerase, had a 12.75% longer median lifespan and fewer tumours.⁵⁸ Telomerase overexpression extended lifespan in mice carrying extra copies of tumour suppressors,⁵⁹ but without that protection it made mice more prone to skin tumours after chemical carcinogenesis and led to mammary carcinomas in ageing females.⁶⁰⁶¹ The human evidence that long telomeres raise cancer risk, including at old ages, makes the mouse results a weak guide to human therapy. (See Antagonistic pleiotropy.)

History

Discovery

Muller named the telomere in 1938, and McClintock described the instability of broken chromosome ends in 1941.²³²⁴ Olovnikov and Watson identified the end-replication problem in the early 1970s, and Olovnikov proposed that it limits the number of times a cell can divide.²²²¹ Blackburn and Gall found the first telomeric repeat in 1978, Szostak and Blackburn showed that telomeres protect linear DNA in 1982, and Greider and Blackburn discovered telomerase in 1985.²⁵²⁶²⁸ Moyzis and colleagues identified the human repeat in 1988.¹⁵

Telomeres, senescence and cancer

Harley, Futcher and Greider linked telomere shortening to senescence in cultured human cells in 1990,³⁰ and Kim and colleagues linked telomerase to cancer in 1994.⁵ In 1997 Blasco and colleagues made telomerase-deficient mice,³⁶ and Bryan and colleagues described telomerase-independent maintenance in tumours,⁶ which Dunham and colleagues showed in 2000 to work by recombination.⁷ Bodnar and colleagues showed in 1998 that telomerase extends the division capacity of human cells,² and the t-loop was described in 1999.²⁷ Mutations in telomerase components were found in dyskeratosis congenita in 1999 and 2001.⁴⁴⁴⁵

Telomere length in populations

From the 2000s, telomere length in blood was studied as a predictor of disease and death.⁵¹⁵³ Mendelian randomisation, from the mid-2010s, separated the effects of telomere length from those of the conditions that shorten it, and showed that longer telomeres raise cancer risk.¹⁰⁸⁵⁴ Families carrying POT1 mutations that lengthen telomeres, described in detail in 2023, showed the same cancer risk within single pedigrees.⁴³

Data and figures

Figure 1 reproduces the means and rates of shortening reported by Daniali and colleagues.³³ Figure 2 reproduces published estimates from the Telomeres Mendelian Randomization Collaboration.⁸