Recombination and the Tel1 and Mec1 checkpoints differentially effect genome rearrangements driven by telomere dysfunction in yeast

Abstract

In telomerase-deficient Saccharomyces cerevisiae, telomeres are maintained by recombination. Here we used a S. cerevisiae assay for characterizing gross chromosomal rearrangements (GCRs) to analyze genome instability in post-senescent telomerase-deficient cells. Telomerase-deficient tlc1 and est2 mutants did not have increased GCR rates, but their telomeres could be joined to other DNAs resulting in chromosome fusions. Inactivation of Tel1 or either the Rad51 or Rad59 recombination pathways in telomerase-deficient cells increased the GCR rate, even though telomeres were maintained. The GCRs were translocations and chromosome fusions formed by nonhomologous end joining. We observed chromosome fusions only in mutant strains expressing Rad51 and Rad55 or when Tel1 was inactivated. In contrast, inactivation of Mec1 resulted in more inversion translocations such as the isochromosomes seen in human tumors. These inversion translocations seemed to be formed by recombination after replication of broken chromosomes.Telomeres function in replication and maintenance of chromosome ends, to prevent DNA ends from being inappropriately joined to each other and to prevent chromosome ends from activating checkpoints 1,2 . Telomeres are maintained by telomerase, which consists of the Est2 catalytic subunit, the Tlc1 RNA and other subunits 2 .Telomere maintenance also requires other proteins. These include the Tel1 protein kinase that functions in telomere protection and length regulation and proteins such as Cdc13 and Ku that target telomerase to telomeres and protect telomeres from degradation 2 . Proteins such as Pif1 help regulate telomere length 3 and prevent telomerase from adding telomeres to broken DNAs 3,4 . In telomerase-deficient S. cerevisiae cells telomeres are maintained by recombination 5,6 . Most mammalian cells lack telomerase 7 and have a limited lifespan. Immortalization and cancer progression require increased telomere maintenance capacity, either through upregulation of telomerase activity 7 or through the alternative lengthening of telomere pathway 8 .Recombination and the Tel1 and Mec1 checkpoints differentially effect genome rearrangements driven by telomere dysfunction in yeast L E T T E R S 612VOLUME 36 | NUMBER 6 | JUNE 2004 NATURE GENETICS RDKY5233 is a tlc1∆ type II strain. Additional relevant GCR rates include the tlc1∆ type I strain, RDKY5232 (3.1 × 10 -10 (0.9)); lig4∆ strain, RDKY3641 (1.6 × 10 -9 (9); ref. 10); tel1∆ lig4∆ strain, RDKY5238 (4.2 × 10 -9 (12)); and tel1∆ lig4∆ est2∆ strain, RDKY5240 (3.5 × 10 -9 (10)). ND, not determined.

Inositol pyrophosphates regulate cell death and telomere length through phosphoinositide 3-kinase-related protein kinases

“…The PI3K-related protein kinases Tel1 and Mec1 control DNA damage checkpoints during cell cycle so that their deletion or inhibition elicits hypersensitivity to DNA-damaging agents (35)(36)(37). Earlier, we showed that inositol pyrophosphates are required for DNA hyper recombination in certain yeast (38).…”

Smc5–Smc6 complex suppresses gross chromosomal rearrangements mediated by break-induced replications

“…Several studies using this approach have demonstrated that there are eight pathways for suppressing these chromosomal aberrations, while six pathways promote GCR formation. The suppression mechanisms include cell cycle checkpoints [7][8][9][10][11][12], post-replication [13,14] and mismatch repair [15,16], recombination pathways, an anti-de novo telomere addition mechanism [17,18], chromatin assembly factors [11,19], mechanisms that prevent end-to-end chromosome fusions [17,18,20] and a pathway detoxifying reactive oxygen species [14,21,22]. In contrast, the promoters of GCRs include telomerase-related factors [17,23], a mitotic checkpoint network [24], the Rad1-Rad10 endonuclease [25], non-homologous end-joining proteins including Lig4 and Nej1 [17], a pathway generating inappropriate recombination via sumoylation and the Srs2 helicase [13] and the Bre1 ubiquitin ligase [13].…”

Phosphorylation of Sae2 Mediates Forkhead-associated (FHA) Domain-specific Interaction and Regulates Its DNA Repair Function

“…Mutations of MEC1 and TEL1 are known to cause substantial increases in gross chromosomal rearrangements (GCRs) (10 -12). Although telomere fusion is thought to contribute to chromosomal rearrangements observed in the mec1⌬ tel1⌬ mutant, loss of telomerase alone does not cause similar chromosomal rearrangements (11,12), indicating that defective DNA repair might also be involved. Despite the fact that many substrates of Mec1 and Tel1 have been identified, a major challenge has been to identify and characterize those Mec1/ Tel1 substrates whose phosphorylation specifically regulates DNA repair and genome maintenance.…”