Telomerase- and recombination-independent immortalization of budding yeast

Abstract

It is generally assumed that there are only two ways to maintain the ends of chromosomes in yeast and mammalian nuclei: telomerase and recombination. Without telomerase and recombination, cells enter senescence, a state of permanent growth arrest. We found that the decisive role in preventing senescent budding yeast cells from dividing is played by the Exo1 nuclease. In the absence of Exo1, telomerase-and recombination-defective yeast can resume cell cycle progression, despite degradation of telomeric regions from many chromosomes. As degradation progresses toward internal chromosomal regions, a progressive decrease in viability would be expected, caused by loss of essential genes. However, this was not the case. We demonstrate that extensive degradation and loss of essential genes can be efficiently prevented through a little-studied mechanism of DNA double-strand-break repair, in which short DNA palindromes induce formation of large DNA palindromes. For the first time, we show that large palindromes form as a natural consequence of postsenescence growth and that they become essential for immortalization in the absence of telomerase activity.[Keywords: PAL-mechanism; telomere; Exo1; palindrome] Supplemental material is available at http://www.genesdev.org.

Rif1 and Exo1 regulate the genomic instability following telomere losses

“…The fraction of isogenic strains proliferating at specific times is depicted in Figure 2(B–E). As previously reported (Maringele & Lydall, 2004b), an exo1∆ mutation allowed 50% of tlc1∆ rad52∆ strains to divide indefinitely, whereas an mre11∆ mutation had no effect on its own, yet raised the fraction of proliferating tlc1∆ rad52∆ exo1∆ strains to 100% (Fig. 2B).…”

“…1B). The succession of these events in PALs was previously described (Maringele & Lydall, 2004b) and is summarized in a diagram (Fig. 1C), showing that PAL cells emerge from senescence and proliferate while losing chromosome ends.…”

“…Relevant mutations are indicated above columns. The yku70∆ mre11∆ is TLC 1+ , yet senesce and escape as described (Maringele & Lydall, 2004b). (B) Several newly germinated rif1∆ strains (on the right half of each plate) and the same number of RIF 1+ controls (left half), were propagated every 5 days on a succession of plates and photographed at the indicated passage.…”

“…This is a good model for human somatic cells (lacking telomerase activity and rarely undergoing telomere recombination). Despite losing telomeres, PAL cells continue to proliferate for many generations with uncapped (free) chromosome ends (Maringele & Lydall, 2004b; Lee et al ., 2008). A plausible hypothesis, explaining how cell division continues in the absence of telomeres, is that checkpoint pathways are inactivated.…”

“…To do so, they should be able to tolerate telomere losses and the associated genomic instability, and also to escape the DNA damage checkpoint control, through yet unknown mechanisms. One of the best models to identify such mechanisms is the PAL system, consisting of budding yeast cells unable to maintain telomeres, similarly to most human somatic cells (Maringele & Lydall, 2004b; Deshpande et al ., 2011). In this system, cells escape replicative senescence and proliferate indefinitely without telomeres, accumulating genomic deletions and amplifications, for example palindromic duplications (Maringele & Lydall, 2004b; Lee et al ., 2008), hence the name.…”