Abstract ED04-01: Aging and cancer: are telomeres and telomerase the connection?

ED04-01

Telomeres are composed of repetitive DNA sequences at the end of linear chromosomes. Human telomeres contain thousands of repeats of the six nucleotide sequence, TTAGGG (1). Telomere length is maintained by a balance between processes that lengthen telomeres [telomerase] and processes that shorten telomeres [the end-replication problem]. Telomerase (TEE-LÓM-ER-ACE) is a ribonucleoprotein enzyme complex (a cellular reverse transcriptase) that stabilizes telomere length by adding hexameric (TTAGGG) repeats onto the telomeric ends of the chromosomes, thus compensating for the erosion of telomeres that occurs in its absence. The enzyme is expressed in adult reproductive cells, but is undetectable in normal somatic cells except for proliferative cells of renewal tissues (e.g. bone marrow cells, basal cells of the epidermis, proliferative endometrium, and intestinal crypt cells). In all non-reproductive cells, including proliferative cells of renewal tissues, progressive telomere shortening is observed, eventually leading to greatly shortened telomeres and to a limited ability to continue to divide. Telomere shortening is a molecular clock mechanism that counts the number of times a cell has divided and when telomeres are short, cellular senescence (growth arrest) occurs (2-3). It is believed that shortened telomeres in mitotic (dividing) cells functions as an initial brake against the development of cancer (4-6) and may be responsible for some of the changes we associate with normal aging. The telomere-telomerase hypothesis of cancer is based on the findings that immortal cancer cells have engaged a mechanism to maintain stable telomere lengths almost always by reactivating or up-regulating telomerase activity (7).

Aging and cancer are two ends of the same spectrum:

Cellular senescence may have evolved, in part, to protect long-lived organisms, such as humans, against the early development of cancer. It takes many divisions to accumulate all of the mutations (alterations) needed to become a cancer cell. Cells that use up their replicative life span become senescent. However, most preneoplastic tissues have accumulated several mutations and have greatly shortened telomeres. In many instance these preneoplastic cells remain pre-malignant and do not progress to cancer, suggesting that short telomeres may limit the proliferation of pre-malignant cells (8). It has been proposed that up-regulation or re-expression of telomerase may be a critical event responsible for continuous tumor cell growth. In contrast to normal cells, tumor cells show no net loss of average telomere length with cell division, suggesting that telomere stability may be required for cells to escape from replicative senescence and proliferate indefinitely. Most, but not necessarily all, malignant tumors may need telomerase to sustain their growth. Immortalization may occur through a mutation of a gene in the telomerase repression pathway. Thus, up-regulation or reactivation of telomerase activity may be a rate-limiting step required for the continuing proliferation of advanced cancers. Since telomerase activity is present in most human tumors, but not in tissues adjacent to the tumors, much research is currently focused on the development of methods for the accurate diagnosis of cancer using telomerase (8) and on novel anti-telomerase cancer therapeutics (9-13). Importantly, cancer stem cells are telomerase positive so inhibiting telomerase is likely to also affect cancer stem cells.

Gradual shortening of telomeres coincide with the long term aging process:

Under normal conditions human tissues can function adequately for a typical life span. However, with the improvement in sanitation, the development of antibiotics, vaccines, and modern pharmaceutical drugs, humans are living longer and consequently there is an increase in aged-related cellular decline in normal people who live to an exceptionally old age, while in the past problems from a limited cellular proliferative capacity was only observed in disease states. However in older individuals without diseases, there is an increased incidence of chronic ulcers, wearing down of the vascular endothelium leading to arteriosclerosis, proliferative decline of retinal pigmented epithelial cells leading to age-related blindness, immunological deficiencies which may lead to an increased incidence of cancer, and several other diseases of tissues that have an intrinsic capacity for replacement (14).

Many cells (even in tissues noted for division) are not completely senescent in centenarians. This does not contradict the role of senescent cells in aging. Senescent cells may interfere with function in otherwise normal somatic tissues. There is evidence that senescent cells secrete proteases and cytokines that may contribute to the aging phenotype by disrupting tissue homeostasis. Some of these secreted factors appear to stimulate pre-malignant epithelial cells to form tumors. Populations of aging cells may equilibrate somewhat and yet must still vary considerably: old people will always have cells that can divide or they could not survive. The fact that cells can grow out of tissues obtained from elderly donors does not exclude the presence of senescent cells in that specimen (4). Every tissue has cells that are not senescent or the tissue would have failed long before. It is unlikely that senescent cells would comprise even a majority of cells in any tissue. In summary, it is believed, but not yet proven, that in some aged-related disorders telomere decline in a threshold of cells in specific tissues and organs may contribute to aging vulnerability (Table 1).

Correlative evidence in support of the telomere hypothesis of cellular aging:

1. Telomeres in normal human cells from young individuals progressively shorten when grown in cell culture.

2. Telomeres are shorter in most somatic tissues from older individuals compared to younger individuals.

3. Telomeres are shorter in somatic cells than in germ line cells.

4. Individuals with genetic diseases due to mutations in telomerase (e.g. idiopathic pulmonary fibrosis, dyskeratosis congenita, and sporadic bone marrow failure) have shorter telomeres compared to age-matched controls (17-19).

Introduction of the telomerase catalytic protein component (hTERT) into normal human cells without detectable telomerase results in restoration of telomerase activity (15). Normal human cells stably expressing transfected telomerase demonstrate extension of life span, providing more direct evidence that telomere shortening controls cellular aging. The cells with introduced telomerase maintain a normal chromosome complement and continue to grow in a normal manner (16). These observations provide more direct evidence for the hypothesis that telomere length determines the proliferative capacity of human cells.

Future Directions:

While at the present time there are many correlations between short telomeres and aging and cancer, future experiments will involve reversing telomere decline (by telomere elongation or slowing down the rate of telomeres loss) in cells and tissues and demonstrating that these cells can continue to participate in normal tissue function (17). For example, taking mobilized bone marrow stem cells from patients with dyskeratosis congenita and rejuvenating the telomeres ex vivo (using an adenoviral hTERT) and demonstrating that this slows down or reverses the disease process would be an important proof of principal that there is a cause and effect relationship between telomeres and aging (18). One question to contemplate is what would happen in early preneoplasia if telomere decline was slowed down or stopped (20, 21). Would this trigger more rapid transition to cancer or slow the process down?

Citation Information: Cancer Prev Res 2008;1(7 Suppl):ED04-01.