ScienceWeek

MOLECULAR BIOLOGY: ON TELOMERES

The following points are made by Vicki Lundblad (Nature 2003 423:926):

1) In every organism, maintaining the integrity of the genome is a crucial endeavor. One aspect of genome maintenance involves protecting telomeres, the natural ends of linear chromosomes. This task is achieved by a suite of specialized protein complexes, which are anchored to chromosome ends through their association with further proteins that bind directly to telomeric DNA. The resulting structure prevents events that would be catastrophic for the genome, such as the loss of terminal DNA sequences or end-to-end chromosome fusions.

2) One of the complexes involved in telomere maintenance is an enzyme called "telomerase", which adds DNA back to telomeres that have become eroded. Several other proteins also regulate this complex. But how the different proteins talk to one another -- to keep telomeres the right length, to protect them, and to replicate them during cell division -- is poorly understood.

3) Telomeric DNA is composed of G-rich repeats -- reiterations of a short DNA sequence that does not code for protein and is high in guanine (G) nucleic-acid bases. It also has a single-stranded stretch that overhangs the end of the double-stranded (duplex) telomeric region. This overhang is the substrate for telomerase, which elongates chromosome ends by adding G-rich repeats. The importance of telomerase is evident from studies of yeast and human cells in which reductions in telomerase levels produce a steady decline in telomere length that eventually blocks cell division. Not surprisingly, then, telomerase is highly active in systems such as the blood and reproductive system, which rely on continuous replenishment through cell proliferation(3). Much to the interest of cancer biologists, telomerase levels are also increased in most human tumors, providing a potential target for the development of anticancer drugs(4).

4) In normal cells, telomerase activity is carefully controlled by several mechanisms. For instance, subunits that are part of the telomerase complex itself can positively regulate the enzyme, for example by mediating recruitment of the complex to chromosome ends(5). Surprisingly, proteins that bind to the duplex region of the telomere can also be potent regulators, even though they do not appear to associate physically with telomerase. These duplex-binding proteins -- which include Rap1 in budding yeast and the TRF1 and TRF2 proteins in human cells -- can "count" the number of G-rich repeats and, when telomeres become overly long, inhibit further telomerase activity. Missing from this elegant proposal for telomere-length regulation, however, is an explanation for how information from the duplex portion of the telomere is relayed to the very tip of the chromosome -- the site of telomerase action(1,2).

References (abridged):

1. Loayza, D. & de Lange, T. Nature 423, 1013-1018 (2003)

2. Colgin, L. M., Baran, K., Baumann, P., Cech, T. R. & Reddel, R. R. Curr. Biol. 13, 942-946 (2003)

3. Lee, H.-W. et al. Nature 392, 569-574 (1998)

4. Damm, K. et al. EMBO J. 20, 6958-6968 (2001)

5. Evans, S. K. & Lundblad, V. Science 286, 117-120 (1999)

Nature http://www.nature.com/nature

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SENESCENCE: DOES IT ALL HAPPEN AT THE ENDS?

The following points are made by S.A. Stewart and R.A. Weinberg (Oncogene 2002 21:627):

1) Over 60 years ago Barbara McClintock [1902-1992] described the telomere and suggested that it protected the chromosome from illegitimate or end-to-end fusion, thus functioning to protect the genome. Since that time we have discovered that the telomere is a complex structure composed of both DNA and a growing list of associated proteins that together serve to regulate the length of the telomere and, as predicted by McClintock, protect genomic integrity.

2) In addition to its protective role, the telomere has also been hypothesized to serve as a molecular clock that tallies the number of cell divisions and limits further divisions at a predetermined point. However, the precise role of telomeres in predicting and limiting cellular lifespan remains a matter of much debate.

Oncogene http://www.nature.com/onc/

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TELOMERASE AND SENESCENCE.

The following points are made by S. E. Artandi et al (Proc. Nat. Acad. Sci. 2002 99:8191):

1) Telomerase, the reverse transcriptase that synthesizes telomeric repeats, is present at low levels in human tissue stem cells, progenitor cells, and germ cells, and is undetectable in the vast majority of adult somatic tissues. Insufficient telomerase activity and the inability of DNA polymerase to replicate the extreme ends of chromosomes lead to telomere attrition with each round of cell division in the setting of organ renewal with advancing age (1), high-turnover disease states (2), and passage in culture (3). During human tumorigenesis, telomerase becomes reactivated by transcriptional up-regulation of TERT, the catalytic subunit of telomerase (4). One critical function served by TERT reactivation during cancer progression is to avert the adverse consequences of telomere shortening and loss of chromosomal capping function (5). Less clear, however, is whether the pro-oncogenic activity of telomerase extends beyond its role in maintaining telomere function.

2) In human cells, progressive shortening of telomeres precipitates replicative senescence after 60-80 divisions of primary human fibroblasts (3) and crisis after extended division of cells expressing viral oncoproteins. Introduction of telomerase into primary human cells stabilizes telomeres, prevents both senescence and crisis, and endows cells with unlimited proliferative potential. The ability of telomerase to rescue cells from the adverse consequences of telomere dysfunction is likely critical for its role in facilitating malignant transformation of primary human cells and in maintaining the viability of established cancer cells.

3) In summary: Telomerase is up-regulated in the vast majority of human cancers and serves to halt the progressive telomere shortening that ultimately blocks would-be cancer cells from achieving a full malignant phenotype. In contrast to humans, the laboratory mouse possesses long telomeres and, even in early generation telomerase-deficient mice, the level of telomere reserve is sufficient to avert telomere-based checkpoint responses and to permit full malignant progression. These features in the mouse provide an opportunity to determine whether enforced high-level telomerase activity can serve functions that extend beyond its ability to sustain telomere length and function. The authors report the generation and characterization of transgenic mice that express the catalytic subunit of telomerase (mTERT) at high levels in a broad variety of tissues. Expression of mTERT conferred increased telomerase enzymatic activity in several tissues, including mammary gland, splenocytes, and cultured mouse embryonic fibroblasts. In mouse embryonic fibroblasts, mTERT overexpression extended telomere lengths but did not prevent culture-induced replicative arrest, thus reinforcing the view that this phenomenon is not related to occult telomere shortening. Robust telomerase activity, however, was associated with the spontaneous development of mammary intraepithelial neoplasia and invasive mammary carcinomas in a significant proportion of aged females. The authors suggest these data indicate that enforced mTERT expression can promote the development of spontaneous cancers even in the setting of ample telomere reserve.

References (abridged):

1. Hastie, N. D., Dempster, M., Dunlop, M. G., Thompson, A. M., Green, D. K. & Allshire, R. C. (1990) Nature (London) 346, 866-868.

2. Rudolph, K. L., Chang, S., Millard, M., Schreiber-Agus, N. & DePinho, R. A. (2000) Science 287, 1253-1258.

3. Harley, C. B., Futcher, A. B. & Greider, C. W. (1990) Nature (London) 345, 458-460.

4. Kim, N. W., Piatyszek, M. A., Prowse, K. R., Harley, C. B., West, M. D., Ho, P. L., Coviello, G. M., Wright, W. E., Weinrich, S. L. & Shay, J. W. (1994) Science 266, 2011-2015.

5. Blackburn, E. H. (2000) Nature (London) 408, 53-56.

Proc. Nat. Acad. Sci. http://www.pnas.org

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