Telomere Extension by Telomerase
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Telomere Extension by Telomerase
Unlimited replicative potential and widespread genomic disarray are among the most common characteristics exhibited by human cancer cells. Although several distinct molecular pathways regulate specific aspects of each of these phenotypes, specialized chromosomal terminal structures, termed telomeres act as essential regulators of both cell life span and chromosomal integrity (Ref.1).

Telomeres are dynamic DNA-protein complexes that cap the ends of linear chromosomes, preventing detrimental chromosome rearrangements and defending against genomic instability and the associated risk of cancer. Telomeres shorten every time a cell divides because of incomplete DNA replication and DNA end processing. When telomere length reaches a critical point, cells stop dividing and undergo replicative senescence (Ref.2). Mammalian telomeres are composed of tandem repeats of the TTAGGG sequence and an array of associated proteins. Telomeres end in a 3 overhang, known as the G-strand overhang, which bends back on itself and anneals with the complementary sequences in the 5 end of the opposite strand. This displaces part of the 5 end in a D-loop and creates a telomere or T-loop, which is stabilized by a set of specialized proteins. T-loops facilitate the formation of a higher order structure that mediate end capping by masking telomeric DNA ends from recognition by the DNA repair system. Loss of telomeric capping leads to chromosome end-to-end fusion and triggers cell cycle arrest and apoptosis that contribute to the aging process (Ref.3). When they are not being replicated telomeres are bound by a protein or protein complex whose function is to protect the telomere from degradation.

Telomerase, also called telomere terminal transferase, prevents telomere shortening by using its integral RNA component as a template to add hexameric repeats TTAGGG sequences to the mammalian telomeres to compensate for the loss of basepairs that occurs after subsequent rounds of DNA replication (Ref.2). It is found in fetal tissues, adult germ cells, and tumor cells and has a very low, almost undetectable activity in somatic cells. Telomerase has two essential components, an RNA molecule called TERC (Telomerase RNA Component; hTERC in humans), and a catalytic subunit called TERT (Telomerase Reverse Transcriptase; hTERT in humans). TERC, the RNA subunit act in concert to elongate telomeres by reading from the RNA template sequence carried by the RNA subunit and synthesizing a complementary DNA strand (Ref.3). The mechanism of telomerase synthesis involves telomerase first recognizing the 3 overhanging telomeric sequence that exists at the chromosome ends. The telomerase RNA template sequence basepairs with the terminal TTAGGG repeat to initiate elongation of the 3 DNA end. The RNA template has only 11 bases that match the TTAGGG repeat sequence, such that only one repeats of the sequence can be added in a single elongation. Synthesis terminates with the circularly permuted sequence GGTTAG. Telomerase can continue to synthesize telomeric repeats on the same DNA strand by unwinding the DNA from the DNA-RNA hybrid, holding the DNA end while the RNA slides down 6 bases to allow proper alignment and base pairing. Coordination between C-strand and G-strand synthesis is required for proper telomere length maintenance (Ref.4). The ability of telomerase to elongate telomeres is regulated by several other factors. In mammals, the telomere-binding protein TRF1 (Telomeric Repeat Binding Factor-1)/Pin2, TRF2, TANK (TRF1-interacting, Ankyrin-related ADP-ribose polymerase, also known as Tankyrase), TIN2 (TRF1-Interacting Nuclear Factor-2), and heterogeneous nuclear ribonucleoproteins such as HNRPA1 (Heterogeneous Nuclear Ribonucleoprotein-A1), HNRPA2B1 (Heterogeneous Nuclear Ribonucleoprotein-A2/B1) affect telomere maintenance (Ref.5). TRF1 is a negative regulator of telomere length, whereas TRF2 plays an essential role in protecting telomeric integrity. The TRF1 complex interacts with POT1 (Protection Of Telomeres-1; a single-stranded telomeric DNA-binding protein) and controls telomerase-mediated telomere elongation. TRF2 assists in the formation of the T-loop and helps to maintain the secondary structure of the telomere. TRF2 also interacts with several proteins, including the human RAP-1 (Repressor Activator Protein-1)/TERF2IP (Telomeric Repeat Binding Factor 2 Interacting Protein) and the MRE11 (Meiotic Recombination-11) complex, composed of MRE11, Rad50, and the NBS1 (Nijmegen Breakage Syndrome-1) protein, which is implicated in the cellular response to agents that damage DNA. In addition to the MRE11 complex, the Ku complex, involved in certain types of DNA double-stranded break repair, localizes to the telomere. Thus, the physiologic maintenance of the telomere requires complex interactions among these proteins, telomeric DNA, and other cellular factors (Ref.1).

Beyond their role in replication and capping, telomeres participate in meiotic chromosome pairing, meiotic and mitotic chromosome segregation, and in the organization of the nucleus. Telomeres and telomerase are very important determinants of cell fate and cell life span. Without telomerase, cells can carry out only a limited number of cell divisions before the failure to synthesize the ends of chromosomes. Telomere integrity is also essential for chromosome numerical and positional stability, and telomere shortening facilitates the evolution of cancer cells by promoting chromosome end-to-end fusions and the development of aneuploidy (Ref.6). Inhibition of telomerase in immortal cancer-cell lines by genetic, antisense, or pharmacological methods results in telomere shortening and eventually halts cell proliferation (Ref.1).