Tel1p and Mec1p Regulate Chromosome Segregation and Chromosome Rearrangements in <italic>Saccharomyces cerevisiae</italic>
Cancer cells often have elevated frequencies of chromosomal aberrations, and it is likely that loss of genome stability is one driving force behind tumorigenesis. Deficiencies in DNA replication, DNA repair, or cell cycle checkpoints can all contribute to increased rates of chromosomal duplications, deletions and translocations. The human ATM and ATR proteins are known to participate in the DNA damage response and DNA replication checkpoint pathways and are critical to maintaining genome stability. The <italic>Saccharomyces cerevisiae</italic> homologues of ATM and ATR are Tel1p and Mec1p, respectively. Because Tel1p and Mec1p are partially functionally redundant, loss of both Tel1p and Mec1p in haploid yeast cells (<italic>tel1 mec1</italic> strains) results in synergistically elevated rates of chromosomal aberrations, including terminal duplications, chromosomal duplications, and telomere-telomere fusions. To determine the effect of Tel1p and Mec1p on chromosome aberrations that cannot be recovered in haploid strains, such as chromosome loss, I investigated the phenotypes associated with the <italic>tel1 mec1</italic> mutations in diploid cells. In the absence of induced DNA damage, <italic>tel1 mec1</italic> diploid yeast strains exhibit extremely high rates of aneuploidy and chromosome rearrangements. There is a significant bias towards trisomy of chromosomes II, VIII, X, and XII, whereas the smallest chromosomes I and VI are commonly monosomic.
The telomere defects associated with <italic>tel1 mec1</italic> strains do not cause the high rates of aneuploidy, as restoring wild-type telomere length in these strains by expression of the Cdc13p-Est2p fusion protein does not prevent cells from becoming aneuploid. The <italic>tel1 mec1</italic> diploids are not sensitive to the microtubule-destabilizing drug benomyl, nor do they arrest the cell cycle in response to the drug, indicating that the spindle assembly checkpoint is functional. The chromosome missegregation phenotypes of <italic>tel1 mec1</italic> diploids mimic those observed in mutant strains that do not achieve biorientation of sister chromatids during mitosis.
The chromosome rearrangements in <italic>tel1 mec1</italic> cells reflect both homologous recombination between non-allelic Ty elements, as well as non-homologous end joining (NHEJ) events. Restoring wild-type telomere length with the Cdc13p-Est2p fusion protein substantially reduces the levels of chromosome rearrangements (terminal additions and deletions of chromosome arms, interstitial duplications, and translocations). This result suggests that most of the rearrangements in <italic>tel1 mec1</italic> diploids are initiated by telomere-telomere fusions. One common chromosome rearrangement in <italic>tel1 mec1</italic> strains is an amplification of sequences on chromosome XII between the left telomere and rDNA sequences on the right arm. I have termed this aberration a "schromosome." Preliminary evidence indicates that the schromosome exists in the <italic>tel1 mec1</italic> cells as an uncapped chromosome fragment that gets resected over time.
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