Telomere maintenance mechanisms in cancer: clinical implications

Abstract

The presence of immortal cell populations with an up-regulated telomere maintenance mechanism (TMM) is an almost universal characteristic of cancers, whereas normal somatic cells are unable to prevent proliferation-associated telomere shortening and have a limited proliferative potential. TMMs and related aspects of telomere structure and function therefore appear to be ideal targets for the development of anticancer therapeutics. Such treatments would be targeted to a specific cancer-related molecular abnormality, and also be broad-spectrum in that they would be expected to be potentially applicable to most cancers. However, the telomere biology of normal and malignant human cells is a relatively young research field with large numbers of unanswered questions, so the optimal design of TMM-targeted therapeutic approaches remains unclear. This review outlines the opportunities and challenges presented by telomeres and TMMs for clinical management of cancer.

Fig. (1)

Telomeres undergo gradual attrition during cellular proliferation. Telomeres (lighter bars; darker bars represent non-telomeric DNA) contain tandemly repeated arrays of the hexameric sequence, 5'-TTAGGG-3'. Telomeres are mostly double-stranded, but they terminate in a region of single-stranded (usually G-rich) DNA. In cultured human fibroblasts, telomeres shorten by approximately 50-150 base pairs per cell division. This ultimately results in a DNA damage response (DDR) focus, and, when a sufficient number of such foci accumulate, the cell undergoes permanent withdrawal from the cell cycle (i.e., becomes senescent).

Fig. (2)

Telomeres can adopt at least three different conformational states. The fully capped state inhibits both the DDR and DNA repair by nonhomologous end-joining (NHEJ) which would result in end-to-end joining of chromosomes and therefore genomic instability. Normal telomere shortening results in an intermediate-state telomere which inhibits NHEJ but not DDR. When the threshold number of telomeric DDR foci is exceeded, the cell continues through mitosis and then arrests in G1 phase. An abnormal cell state, such as excessively slow transit through mitosis, also elicits intermediate-state telomeres (ISTs) by an unknown mechanism, with the result that the cell continues to proceed through mitosis until it reaches G1 and can then undergo an orderly exit from the cell cycle. Excessive telomere shortening results in fully uncapped telomeres which elicit both DDR and NHEJ and result in cell death; when most of the cells in a culture enter this state, this is referred to as "culture crisis".

Fig. (3)

Telomerase up-regulation in cancer cells can occur by a variety of mechanisms. The telomerase catalytic subunit (TERT) and the RNA template molecule (TR) are both present at very low copy number in normal cells, and it is likely that the expression of both must be increased in order to express sufficient telomerase to prevent telomere shortening in cancer cells. Increased transcriptional activation (for both TERT and TR), and loss of transcriptional repressors (for TERT) have been found in some cancer cells. Many telomerase-positive cancer cells have a mutation in the TERT gene promoter/enhancer region, which appears to result in increased trans-activation of this gene. The activity of TERT may also be increased by certain kinases. Increased copy number of TERT and/or TERC has been identified in many cancers.

Fig. (4)

Telomerase and ALT both result in de novo synthesis of telomeric DNA. Unlike telomerase, which reverse transcribes new telomeric sequence from the template region (black bar) of its RNA subunit, ALT involves formation of a DNA recombination intermediate and the use of a DNA template for synthesis of new telomeric sequence; see text for details.

Fig. (5)

Multiple factors contribute to telomere length dynamics in normal and cancer cells. Telomeres undergo proliferation-associated attrition, which may be counteracted by lengthening via telomerase and/or ALT. Over-lengthening may be corrected rapidly by telomere trimming. In immortalized cells, telomere length is maintained within a range around a maintenance set point, whereas in normal, mortal cells telomere length is not maintained overall and may decrease sufficiently to induce senescence. Cells in which tumor suppressor pathways, p53 and pRb, are inactivated may fail to enter senescence and their telomeres may continue shortening until they are no longer able to inhibit DNA repair by end-joining, at which point (crisis) they undergo cell death.

Fig. (6)

The action of telomerase may be inhibited at multiple points in its "life cycle". These include synthesis of the individual components of active telomerase (TERT, dyskerin and TR), assembly into the enzyme complex, transport to a telomere, docking with the telomere, extending the telomere via its catalytic function, and movement to another telomere.

Fig. (7)

Telomeric chromatin is altered in ALT cells. Variant repeat DNA sequences which are normally only abundant in the proximal region of the telomere (i.e., the portion closest to the centromere) become spread throughout the remainder of the telomere. This results in increased binding of other proteins such as nuclear receptors, and a relative decrease in the number of shelterin molecules.

Fig. (8)

Telomerase (TEL) and ALT may both need to be targeted for cancer therapy, either simultaneously or sequentially. A significant minority of cancers have both telomerase and ALT activity, either because they contain a mixture of cells with either TMM or, perhaps, because they contain cells which have both TMMs. Treatment of telomerase-positive cancers with potent telomerase inhibitors may be expected to exert a strong selection pressure for the cells to activate ALT, and treatment of ALT cancers with ALT inhibitors will potentially select for activation of telomerase.