Telomeres and human disease: ageing, cancer and beyond

Abstract

Telomere length and telomerase activity are important factors in the pathobiology of human disease. Age-related diseases and premature ageing syndromes are characterized by short telomeres, which can compromise cell viability, whereas tumour cells can prevent telomere loss by aberrantly upregulating telomerase. Altered functioning of both telomerase and telomere-interacting proteins is present in some human premature ageing syndromes and in cancer, and recent findings indicate that alterations that affect telomeres at the level of chromatin structure might also have a role in human disease. These findings have inspired a number of potential therapeutic strategies that are based on telomerase and telomeres.

Key Points

  • Telomeres are specialized chromatin structures at the ends of chromosomes, which consist of tandem DNA repeats (of TTAGGG) and associated proteins. Telomeres are also bound by nucleosome arrays, which contain epigenetic modifications that are characteristic of constitutive heterochromatin.

  • Telomeres protect chromosome ends from repair and degradation activities. This function is impaired by both the shortening of TTAGGG repeats to below a critical length, and the loss of telomere-binding proteins.

  • Dysfunctional telomeres trigger a DNA damage response, which results in cell-cycle arrest or apoptosis. Telomere dysfunction can also lead to end-to-end chromosome fusions, and can interfere with the repair of DNA lesions in non-telomeric regions, resulting in hypersensitivity to various genotoxic agents.

  • Telomerase is a cellular reverse transcriptase that synthesizes de novo telomeric repeats at chromosome ends. Most somatic tissues lack telomerase activity and show progressive telomere shortening coupled to cell division.

  • Various diseases associated with ageing, including cancer, as well as a number of premature ageing syndromes, are characterized by critically short telomeres. Telomere shortening and the absence of telomerase in normal tissues is a tumour-suppression mechanism. By contrast, tumours aberrantly upregulate telomerase, which elongates short telomeres and allows continuous growth.

  • Mice that lack telomerase activity age prematurely and are more resistant to cancer. Human premature ageing syndromes that are characterized by short telomeres are recapitulated in the mouse only when in the context of telomerase-deficiency and short telomeres.

  • The telomerase core components telomerase reverse transcriptase (TERT) and telomerase RNA component (TERC), as well as telomerase-interacting protein dyskeratosis congenita 1, dyskerin (DKC1), are mutated in the premature ageing human diseases dyskeratosis congenita and aplastic anaemia.

  • Several telomere-binding proteins are altered in human cancer and premature ageing syndromes that are characterized by chromosomal instability.

  • Telomerase and telomere-binding proteins are new potential targets for anti-cancer and anti-ageing therapies.

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Figure 1: Telomere structure and telomerase activity.
Figure 2: Epigenetic regulation of telomeric chromatin and implications for disease.
Figure 3: Telomerase and telomere length in tumorigenesis.

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Acknowledgements

I would like to thank P. Muñoz, S. Gonzalo and I. Flores for critical reading of this manuscript. M.A.B.'s laboratory is funded by the Spanish Ministery of Culture and Science, the Regional Government of Madrid, the European Union and the Josef Steiner Cancer Research Award 2003.

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DATABASES

Entrez gene

BLM

DKC1

MRE11

NBN

Tert

Trf1

WRN

Omim

Ataxia telangiectasia

Ataxia-telangiectasia-like disorder

Bloom syndrome

Dyskeratosis congenita

Fanconi anaemia

Nijmegen breakage syndrome

Rett syndrome

Werner syndrome

Xeroderma pigmentosum syndrome

SwissProt

ATM

HP1α

POT1

RB1

RBL1

RBL2

SUV39H1

SUV39H2

TIN2

TRF1

TRF2

FURTHER INFORMATION

Scientific progammes at the Centro Nacional de Investigaciones Oncológicas

Telomeres Information Center web site

The Science of Aging Knowledge Environment home page

EMBO Journal

Glossary

NUCLEOSOME

The structure responsible in part for the compactness of a chromosome. Each nucleosome consists of a sequence of DNA wrapped around a histone core.

HETEROCHROMATIN

Chromosomal material that is tightly coiled and inactive in terms of gene expression.

SATELLITE REPEATS

DNA that contains many tandem repeats of a short basic repeating unit. Both the major and minor satellite repeats are located at pericentric heterochromatin.

RETINOBLASTOMA PROTEINS

These are a family of tumour supressor proteins that share a similar structure and function. Mutations in RB1 have been associated with retinoblastoma, a malignant eye tumour in children.

RETT SYNDROME

An X-linked dominant neurological disorder that affects girls only and is one of the most common causes of mental retardation in females. Rett syndrome is due to a mutation in the MECP2 gene (methyl-CpG-binding protein 2).

ANTICIPATION

A phenomenon in which a genetic disease appears earlier in the lifetime of an individual with each successive generation.

MITOTIC CATASTROPHE

The loss of cell viability that results from the attempted aberrant chromosome segregation in the presence of severe cellular damage (for example, short telomeres).

snoRNA

Small nucleolar RNAs. The functions of these snoRNAs include RNA cleavage reactions, as well as specifying sites of ribose methylation and pseudouridylation. Mutations in DKC1 (dyskeratosis congenita 1, dyskerin) result in defective pseudouridylation of the H/ACA box class of snoRNAs.

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