P53-Independent Apoptosis Limits DNA Damage-Induced Aneuploidy
McNamee, Laura M., Brodsky, Michael H., Genetics
DNA damage or unprotected telomeres can trigger apoptosis via signaling pathways that directly sense abnormal DNA structures and activate the p53 transcription factor. We describe a p53-independent mechanism that acts in parallel to the canonical DNA damage response pathway in Drosophila to induce apoptosis after exposure to ionizing radiation. Following recovery from damage-induced cell cycle arrest, p53 mutant cells activate the JNK pathway and expression of the pro-apoptotic gene hid. Mutations in grp, a cell cycle checkpoint gene, and puc, a negative regulator of the JNK pathway, sensitize p53 mutant cells to ionizing radiation (IR)-induced apoptosis. Induction of chromosome aberrations by DNA damage generates cells with segmental aneuploidy and heterozygous for mutations in ribosomal protein genes. p53-independent apoptosis limits the formation of these aneuploid cells following DNA damage. We propose that reduced copy number of haploinsufficient genes following chromosome damage activates apoptosis and helps maintain genomic integrity.
EUKARYOTIC cells employ diverse mechanisms to preserve the structure and function of their genome following chromosome damage. Unicellular organisms rely on DNA repair systems and cell cycle arrest to prevent propagation of genome damage, while multicellular organisms can also activate programmed cell death pathways to eliminate cells following damage (Sancar et al. 2004; Roos and Kaina 2006; Bartek and Lukas 2007; Harper and Elledge 2007). In response to double-strand DNA breaks (DSBs), the Mre11, Rad50, and Nbs1 (MRN) complex helps process the damage and activates the ATM and ATR kinases. ATM and ATR phosphorylate many substrates including the downstream kinases Chk1 and Chk2, which regulate cell cycle arrest and apoptosis. The p53 transcription factor plays an evolutionarily conserved role, connecting the DNA damage signaling pathway to the core apoptotic machinery (Murray-Zmijewski et al. 2006; Helton and Chen 2007). Mammalian p53 is directly phosphorylated by ATM and Chk2 and activates many targets genes including pro-apoptotic Bcl-2 family members and the cell cycle regulator p21 (Riley et al. 2008). The p53 paralogs p63 and p73 also contribute to p53-dependent apoptosis by helping p53 bind the promoters of pro-apoptotic genes in some, but not all cell types (Flores et al. 2002; Senoo et al. 2004).
While p53 is required for normal induction of cell death following DNA damage, p53-independent mechanisms can also activate a reduced or delayed response ( Yuan et al. 1999; Irwin et al. 2003; Urist et al. 2004; Ozaki and Nakagawara 2005; reviewed in Roos and Kaina 2006). Several studies have demonstrated that p53-independent death can be enhanced by inactivating checkpoints that block cell cycle progression in the presence of unrepaired DNA damage. In one example, cancer cells lacking p53 required ATM, ATR, Chk1, and p38MAPK/MK2 signaling for cell cycle arrest; loss of this response led to caspase-3 activation and mitotic catastrophe following DNA damage (Reinhardt et al. 2007). In another case, zebrafish embryos lacking p53 and Chk1 function required ATM and ATR to activate an unusual apoptotic response requiring caspase-2, but not caspase-9 or caspase-3 (Sidi et al. 2008).
In Drosophila, rapid induction of apoptosis by ionizing radiation (IR) or unprotected telomeres requires homologs of ATM, Chk2 (TEFU, MNK, respectively), and p53 (Brodsky et al. 2000a, 2004; Ollmann et al. 2000; Xu et al. 2001; Peters et al. 2002; Oikemus et al. 2004; Silva et al. 2004; Song et al. 2004). G2 arrest requires ATR, ATRIP, and Chk1 (MEI-41, MUS304, and GRP, respectively) (Hari et al. 1995; Fogarty et al. 1997; Brodsky et al. 2000b, 2004; De Vries et al. 2005). Transcriptional targets of p53 include the pro-apoptotic genes reaper (rpr), head involution defective (hid), and sickle (skl ) (Brodsky et al. 2000a, 2004; Christich et al. 2002; Akdemir et al. 2007). hid is essential for the rapid induction of apoptosis, while rpr may play a less significant role (Brodsky et al. …