Failure to elongate very short telomeres, as occurs in cells lacking telomerase, induces a short telomere checkpoint. As this growth arrest can be bypassed in human cells by the expression of transforming genes without the re-expression of telomerase, this arrest represents a true cell cycle checkpoint (Harley and Villeponteau, 1995). In cultures of human and yeast cells, the majority of cells arrest. In yeast, this arrest appears as a prolonged delay at the G2/M boundary of the cell cycle (Enomoto et al., 2002; IJpma and Greider, 2003). However, rare cells can undergo mutagenic events that allow them to grow out of these cultures. For transformed human cells, these rare cells go on to form cell lines. In yeast, these mutant cells that can grow without telomerase are often referred to as "survivors."
At this point it is worth asking: is the senescence observed in yeast due to short telomeres a true senescence, or is it cell death? Senescence is defined as a quiescent but metabolically active state. In yeast, short telomere-induced senescence has been demonstrated by the ability to return these nongrowing cells to growth by reintrodu-cing active telomerase. While defining the minimal portions of yeast telomerase RNA required for function, Livengood et al. (2002) grew cells lacking the telomerase RNA gene to the point of senescence and prepared these cells for transformation. Transforming cells with an empty vector yielded very few transformants, while transforming cells with a vector bearing a functional telomerase RNA gene gave many transformants. These data suggest that these arrested cells can be returned to the cell cycle and normal growth by taking up a new gene, transcribing and processing the RNA and assembling an active telomerase enzyme. These activities suggest that many of the senescent cells are metabolically active. A more defined set of experiments where telomere lengthening is re-established in senescent cells would provide the opportunity to analyze the reversible cellular changes associated with this growth arrest.
Yeasts lacking telomerase are constructed by deletion of the gene for a telomerase component such as yeast TERT (yTERT) or yeast TER (yTER or the TLC1 gene). Since the haploid yeast have no copy of the telomerase component in its genome, the only way to escape senescence and death due to loss of telomere repeats is to find an alternative mechanism to maintain these repeats that does not involve telomerase. In the budding yeast S. cerevisiae, there are two types of survivors that can replicate telomeres, called Type I and Type II. Type I survivors are yeast that have rearranged their genomes such that each telomere contains copies of the sub-telomeric repeat called Y0 followed by a short stretch of TG1-3. These telomeres are thought to use nonreciprocal recombination to replicate their TG1-3 repeats. The generation of Type I survivors requires a number of recombination genes including RAD52, RAD51, RAD54 and RAD57. In contrast, Type II survivors contain long stretches of TG1-3 at the chromosome end and require the recombination genes RAD52, RAD50 and SGS1 (Chen et al., 2001; Huang et al., 2001; Teng et al., 2000; Teng and Zakian, 1999).
Postsenescent survivors have also been isolated in the fission yeast, S. pombe. Cells lacking the TERT subunit can give rise to two types of survivors: a recombination-driven mode of growth with amplification of subtelomeric sequences to give telomeres of heterogeneous lengths, and a chromosome circularization mode in which the 3 linear S. pombe chromosomes are converted to circular versions (Nakamura et al., 1997). The S. pombe genome is similar in size to that of S. cerevisiae, but organized into 3 chromosomes instead of the 16 in budding yeast. This reduced chromosome number apparently allows shortening telomeres on the same chromosome to undergo head-to-head fusions and generate circular chromosomes that allow stable growth at a detectable frequency. In contrast, the 16 chromosomes of S. cerevisiae provide 32 telomeres that may compete with each other for telomere-telomere fusion events and reduce the chance of forming a cell where all of the cell's chromosomes are circularized. However, circularized chromosomes can be detected in some survivor strains of budding yeast, indicating that telomere-telomere fusions of this type are a general phenomenon (Liti and Louis, 2003).
In contrast to yeast deletion mutants, human cells that lack telomerase activity have often repressed the expression of the gene for the hTERT subunit. Cells that grow out of cultures that have bypassed senescence to form cell lines have often reactivated this gene and now express telomerase activity. Increased telomerase activity is also found in many, but not all, human tumors (Kim et al., 1994). However, telomere replication in the absence of telomerase activity has also been found in human cells and some human tumors (reviewed by Henson et al. (2002)). This Alternative Lengthening of Telomeres, or ALT, pathway also appears to involve recombination pathways that involve copying sequences from one telomere to another, as well as more complex recombination events (Fasching et al., 2005; Londono-Vallejo et al., 2004). Thus, both telomerase-mediated and telomerase-independent modes of telomere replication are conserved in yeasts and humans, strengthening the use of yeast as a model system for telomere function.
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