Chromosome double-strand breaks (DSBs) threaten the integrity of the genome. Cells respond to DSBs by activating the DNA damage checkpoint that blocks cells prior to mitosis, allowing more time for the repair of damaged DNA. When the DSB can be repaired, the cell cycle checkpoint is turned off so that cells can resume cell cycle progression, a process termed recovery. If the DSB remains unrepaired, G2/M arrest persists for a long time but eventually cells adapt and – despite the persistent DNA damage – complete mitosis and divide. Much of our understanding of the DNA damage response has come from the study of the budding yeast Saccharomyces cerevisiae, where it is possible to create DSB damage synchronously in all cells of the population. This can be accomplished either by uncapping telomeres, exposing their normally protected ends or by creating a single, defined DSB by inducing the site-specific HO endonuclease. From such studies, it was possible to identify a highly evolutionarily conserved DNA damage sensing and signaling cascade that is initiated by Mec1, the yeast homolog of mammalian ATR protein kinase (reviewed in Ref. (1)). Yeast genetic approaches revealed a number of adaptation-defective mutants, a subset of which also are recovery-defective. Previous studies also demonstrated that triggering the DNA damage checkpoint affects not only mitosis and the efficiency of DNA repair within the nucleus; it also affects cytoplasmic responses (2, 3). In a new paper from the Haber lab published in PNAS, we uncovered mutations in the Golgi-Associated Retrograde Protein (GARP) complex that are adaptation-defective. We show that the defect in these mutants can be mimicked by activating the cytoplasm-to-vacuole (CVT) pathway of autophagy that prevents the nuclear accumulation of separase, Esp1, in the nucleus, thus preventing the cells both adapting and recovering from DSB damage.

In budding yeast, a single unrepaired double-strand break (DSB) triggers the Mec1-dependent cell cycle arrest prior to anaphase for 12-15 before they adapt. Adaptation is accompanied by the loss of hyperphosphorylation of Rad53, yeast’s Chk2 homolog.  Rad53 remains phosphorylated in a number of adaptation-defective mutations, including deletion of the two PP2C phosphatases, ptc2D ptc3D, that normally dephosphorylate Rad53.  Adaptation is also blocked by ablating a number of proteins with diverse roles in DSB repair, including srs2D, rdh54D as well as by a mutation in yeast’s polo kinase cdc5-ad.

In our paper, we find that hyperactivation of the cytoplasm-to-vacuole (CVT) autophagy pathway causes the permanent G2/M arrest of cells with a single DSB that is reflected in the nuclear exclusion of both separase, Esp1, and its chaperone/inhibitor, securin, Pds1(See figure).  Autophagy in response to DNA damage can be induced in three different ways: (1) by deleting members of the Golgi-Associated Retrograde Protein complex (GARP) such as vps51D; (2) by adding rapamycin; or (3) by overexpressing a dominant-negative ATG13-8SA mutation.  The permanent checkpoint-mediated arrest in any of these three conditions can be overcome in three ways: (1) by blocking autophagy with mutations such as atg1D, atg5D or atg11D; (2) by deleting the vacuolar protease Prb1 or its activator, Pep4; or (3) by driving Esp1 into the nucleus with a SV40 nuclear localization signal.  In contrast, these same alterations fail to suppress the adaptation defects of ptc2D ptc3D or cdc5-ad.  Transient accumulation of Pds1 in the vaucole is also seen in wild type cells lacking PEP4 after induction of a DSB.  Unlike other adaptation-defective mutations, G2/M arrest persists even as the DNA damage-dependent phosphorylation of Rad53 diminishes, suggesting that cells have become unable to activate separase to initiate anaphase after DNA damage.  In addition, we have found that cells fail to recover when VPS51 is deleted or when ATG13-8SA is overexpressed.

Increased autophagy causes the delocalization of both Pds1 (securin) and Esp1 (separase) from the nucleus in checkpoint-arrested budding yeast cells. A. GFP-tagged Pds1 and Esp1 localize to the nucleus at the neck of G2/M-arrested wild type (WT) cells that have suffered a single unrepaired chromosome double-strand break (DSB). Both rdh54Δ and vps51Δ prevent cells from adapting and resuming cell cycle progression, but only ablating Vps51 – part of the Golgi-associated retrograde protein (GARP) complex – causes the mislocalization of Pds1 and Esp1 and the partial degradation of Pds1 by vacuolar proteases. Preventing degradation of Pds1 (and possibly other mitotic regulators) results in the suppression of permanent arrest and the relocalization of sufficient Esp1 into the nucleus to release cells from their pre-anaphase arrest. A similar suppression of arrest in vps51Δ cells is obtained by disabling autophagy (not shown). B. Induction of autophagy by overexpression of ATG13-8SA (6) prevents adaptation in wild type cells. Expression of ATG13-SA was induced at the same time that a single, unrepairable DSB was created. Whereas normal cells adapt by 24 h, increased autophagy prevents cells from progressing beyond the G2/M stage of the cell cycle. Deletion of the PEP4 gene that activates vacuolar proteases or ATG1 that is required for autophagy suppresses the arrest and allows cells to divide and resume cell cycle progression.

Taken together with other recent results (4, 5), these observations emphasize that the DNA damage response can trigger the mislocalisation and cytoplasmic proteolysis of important nuclear proteins that regulate DNA repair and cell cycle progression. These results broaden our perspective on the ways in which cells respond to DNA damage and delay cell cycle progression while such damage persists.

Ex MCB grad Farokh Dotiwala, current MCB grad Vinay Eapen and ex-postdoc Jake Harrison were the co-first authors on this paper. Assistant professor Satoshi Yoshida also contributed significantly to this project.

Dotiwala F(*), Eapen VV(*), Harrison JC(*), Arbel-Eden A, Ranade V, Yoshida S & Haber JE (2012) DNA damage checkpoint triggers autophagy to regulate the initiation of anaphase, PNAS (Published online before print November 19, 2012, doi: 10.1073/pnas.1218065109)

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