Toward Identifying a Cellular Determinant of Telomerase Repression

Normal human cells undergo an irreversible growth arrest after a limited number of cell divisions ( 1 , 2 ). In contrast, a hallmark of most cancer cells is their ability to divide an unlimited number of times. Lately, the importance of counteracting the limitations of normal cell growth as a cellular requirement for cancer progression has become appreciated ( 3 - 5 ). There is evidence for a genetic basis of cellular aging. Thus, somatic cell hybrids between immortal cancer cells and normal cells are mortal, demonstrating that cellular aging/senescence(definition) is dominant over immortality ( 6 ). These findings have been pursued in an attempt to identify specific genes regulating these processes. Microcell-mediated chromosome transfer has provided mounting evidence that there are several senescence-specific genetic pathways ( 7 ). In some instances, the introduction of specific chromosomes or genes into proliferating cells results in a rapid growth arrest, suggesting that a senescence-like stress response may quickly induce cell cycle checkpoints. In other instances, there is a significant delay after chromosome transfer until the growth arrest. Recent progress in understanding the basis for this latter observation is the subject of this editorial. The limited proliferative capacity of normal cells is now thought to be controlled by a generational clock that resides at the chromosome ends, the telomeres. The hexameric sequence, TTAGGG, is repeated several thousand times and comprises what is known as telomeric DNA ( 8 ). Due to the cell's inability to complete the replication of linear DNA [end replication problem ( 9 )], some telomeric DNA is lost with each cell division. Telomeres thus provide a buffer of expendable, noncoding DNA to accommodate the end replication problem ( 10 ). Telomeres protect chromosome ends from degradation and recombination ( 11 ) and also promote correct mitotic separation of sister chromatids ( 12 ). In addition, telomeres serve as a platform for telomere(definition) binding proteins ( 13 ). When a critically shortened telomere length is reached, the senescence program is initiated, perhaps via a DNA damage checkpoint pathway ( 14 ). Indeed, there is increasing evidence that the sequential shortening of telomeric DNA may be an important molecular timing mechanism. Correlative evidence in support of this includes the following: 1) Telomeres are shorter in somatic tissues from older individuals when compared with younger individuals ( 15 ); 2) telomeres are shorter in somatic (body) tissues than comparable germline (reproductive) cells ( 16 ); 3) normal cells generally have longer telomeres than those in tumor cells obtained from the same individual ( 17 ); 4) children born with certain early aging syndromes have shorter telomeres than age-matched control subjects ( 18 ); and 5) telomeres in normal cells from young individuals progressively shorten when grown in cell culture ( 19 ). At birth, telomeres in human cells consist of about 15 000 base pairs of repeated TTAGGG DNA sequences ( 17 ). Every time a cell divides, it loses 25-200 DNA base pairs from the telomere ends ( 18 , 19 ). Once this pruning has occurred about 100 times, a cell undergoes senescence and does not continue dividing. Reproductive tissues must have a mechanism to compensate for the progressive telomere erosion, otherwise the species would soon be lost. Nature's solution for germline cells is telomere terminal transferase (telomerase), a ribonucleoprotein reverse transcriptase enzyme ( 20 - 22 ). Telomerase uses its RNA component containing an internal template complementary to the telomeric TTAGGG repeats to bind to telomeres. Its catalytic protein component synthesizes telomeric DNA directly onto the ends of chromosomes by the process of reverse transcription of the RNA template ( 20 - 26 ). Telomerase is present in most fetal tissues, normal adult male reproductive cells, inflammatory cells, proliferative cells of renewal tissues, and in most tumor cells ( 27 - 31 ). Overall, the telomere/telomerase hypothesis of aging and cancer can be summarized as follows: 1) Progressive telomere loss in all somatic cells including stem cells of renewal tissues is normal and is the “ clock” or timing mechanism that regulates cellular senescence ( 32 , 33 ); 2) cellular senescence occurs when only a single telomere or perhaps a few telomeres shorten to a critical length; 3) telomerase is repressed in most somatic tissues, and change(s) in the normal telomerase repression pathway cause the enzyme to increase in activity or reactivate; and 4) either telomerase activity or another mechanism to maintain telomere stability in cancer cells is a hallmark of all immortalized cancer cell populations. During the past 3 years, more than 500 papers have been published on the topic of telomerase. These have almost universally shown that telomerase is detected in primary tumors but not in adjacent cleared margins ( 28 - 31 ). Importantly, while there is a strong association between the presence of telomerase and cancer, this does not indicate that telomerase is either an oncogene or that it causes cancer ( 34 ). Because telomerase activity is detected in almost all advanced tumors, it is hoped that scientists may be able to develop a therapy that inhibits telomerase activity and interferes with the growth of many types of cancer ( 35 ). Following conventional treatments (surgery, radiotherapy, and chemotherapy), antitelomerase agents would most likely be given to limit the proliferative capacity of the rare surviving tumor cells in the hope that this would prevent cancer recurrence. In addition, telomerase inhibitors could also be used as chemopreventive agents in high cancer-susceptibility individuals or in early stage cancer to prevent overgrowth of metastatic cells. One important consideration with this proposed treatment regimen is the prolonged time potentially required for a telomerase inhibitor to be effective (Fig. 1). Since th