Telomeres and Telomerase A Brief Introduction

Telomeres are the specialized structures at the tips of linear chromosomes. They prevent the natural ends from being recognized as damaged DNA, and consequently protect them from enzymatic modification, such as nucleolytic resection and fusion. Telomeres are nucleoprotein complexes consisting of G-rich repeats of DNA, bound by sets of polypeptides that interact with the repeats and each other. The very terminus of telomeres is not blunted, but consists of a single-stranded 3' protrusion of the G-rich strand, called a G-tail or G-overhang. These overhangs have been observed in yeast, humans, mice, ciliates, plants, and trypanosomes, demonstrating that they are evolutionary conserved and an essential feature of telomeres (Dionne and Wellinger 1996, Hemann and Greider 1999, Jacob et al. 2001, Makarov et al. 1997, Munoz-Jordan et al. 2001, Riha et al. 2000, Wellinger et al. 1996, Wright et al. 1997). Studies in human and mouse cells led to a telomeric loop-model, which suggests that the G-rich telomeric single-stranded overhang can loop back and invade homologous double-stranded telomeric tracts, resulting in a large lasso-like structure, termed telomeric loop, or t-loop (Griffith et al. 1999). This structure provides an attractive mechanism by which chromosome ends can be distinguished from broken DNA ends, and subsequently, in addition to humans and mice, t-loops were observed in trypanosomes, ciliates, plants and in yeast species with artificially long telomeres (de Lange 2004).

Mammalian telomeres are associated with the shelterin complex, an assembly of interdependent telomeric core proteins, consisting of TRF1, TRF2, Tin2, Rapl, TPP1, and POT1 (de Lange 2005). It is becoming increasingly clear that the proteins in this complex display multiple interactions, and disruption of individual members leads to disintegration of the complex and telomere dysfunction. In many model organisms loss of telomere function has multiple consequences, such as loss of the telomeric G-rich overhang, resection of the C-rich strand, increased levels of recombination at chromosome ends, altered gene expression patterns, fusion of chromosomes, genome instability, growth arrest and premature senescence, or cell death. In addition to shelterin, mammalian telomeres interact with a plethora of other factors that can influence chromosome end integrity and dynamics, such as Tankyrase 1 and 2, PARP, the MRN complex, the RecQ helicases WRN and BLM, Ku70, Ku86, DNAPK, ATM, ERCC1, XPF and RAD51D (de Lange 2005), pointing to the repair and recombination machineries as important contributors to telomere function.

To counteract replication-associated telomere shortening, a specialized reverse transcriptase complex evolved, which was named terminal telomere transferase at the time of its discovery in the ciliate Tetrahymena thermophila (Greider and Blackburn 1985), and is now known simply as telomerase. Telomerase is capable of adding G-rich telomeric repeats to the very ends of chromosomes using its own RNA, named TERC, as a template (Greider and Blackburn 1987, Greider and Blackburn 1989). The identification of the catalytic telomerase subunits in Saccharomyces cerevisiae (Lendvay et al. 1996), the ciliate Euplotes aediculatus (Lingner and Cech 1996), and humans (Bodnar et al. 1998, Meyerson et al. 1997) suggests a common reverse transcriptase-based mechanism for telomere length stabilization in most organisms with linear genomes (Lingner et al. 1997). Telomerase is active in the germ line and during early development, ensuring that replicative telomere shortening is suppressed, and telomere length is kept constant. However, in most somatic human cells telomerase is not expressed, consequently subjecting chromosome ends in such cells to shortening every time the cell divides (Harley et al. 1990). Progressive telomere shortening ultimately leads to the generation of telomeres that are too short to fulfill their protective and regulatory properties at chromosome ends. Such dysfunctional telomeres are detected by the cell-internal DNA damage machinery, and the cell responds either with death, or by entering a terminally differentiated state, termed replicative senescence (de Lange 2002; see also Allsopp, this volume). Senescent cells are metabolically active, but most likely irreversibly arrested (Fig. 1.1).

As a result, telomere shortening limits the natural replicative life span of somatic human cells, representing a powerful tumor suppressive mechanism (de Lange and Jacks 1999). The finding that all cancer cells maintain a constant telomere length, mostly via activation of telomerase (Kim et al. 1994), points to telomerase-based telomere length stabilization as a fundamental requirement for immortality (see Rudolph, this volume; Blasco, this volume, Fig. 1.1).

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