Project description:DNA double-strand break (DSB) repair by homologous recombination is confined to the S and G2 phases of the cell cycle partly due to 53BP1 antagonizing DNA end resection in G1 phase and non-cycling quiescent (G0) cells where DSBs must be repaired by non-homologous end joining (NHEJ). Unexpectedly, we uncovered extensive MRE11- and CtIP-dependent DNA end resection at DSBs in G0 mammalian cells. A whole genome CRISPR/Cas9 screen revealed the DNA-dependent kinase (DNA-PK) complex as a key factor in promoting DNA end resection in G0 cells. In agreement, depletion of FBXL12, which promotes ubiquitylation and removal of the KU70/KU80 subunits of DNA-PK from DSBs, promotes even more extensive resection in G0 cells. In contrast, a requirement for DNA-PK in promoting DNA end resection in cycling cells at the G1 or G2 phase cells was not observed. Our findings establish that DNA-PK uniquely promotes DNA end resection in G0, but not in G1 or G2 phase cells, and has important implications for DNA DSB repair in quiescent cells.
Project description:DNA double-strand break (DSB) repair by homologous recombination is confined to the S and G2 phases of the cell cycle partly due to 53BP1 antagonizing DNA end resection in G1 phase and non-cycling quiescent (G0) cells where DSBs must be repaired by non-homologous end joining (NHEJ). Unexpectedly, we uncovered extensive MRE11- and CtIP-dependent DNA end resection at DSBs in G0 mammalian cells. A whole genome CRISPR/Cas9 screen revealed the DNA-dependent kinase (DNA-PK) complex as a key factor in promoting DNA end resection in G0 cells. In agreement, depletion of FBXL12, which promotes ubiquitylation and removal of the KU70/KU80 subunits of DNA-PK from DSBs, promotes even more extensive resection in G0 cells. In contrast, a requirement for DNA-PK in promoting DNA end resection in cycling cells at the G1 or G2 phase cells was not observed. Our findings establish that DNA-PK uniquely promotes DNA end resection in G0, but not in G1 or G2 phase cells, and has important implications for DNA DSB repair in quiescent cells.
Project description:Most DNA double-strand breaks (DSBs) are harmful to genome integrity. However, some forms of DSBs are essential to biological processes, such as meiotic recombination and V(D)J recombination. DSBs are also required for programmed DNA elimination (PDE) in ciliates and nematodes. In nematodes, the DSBs are healed with telomere addition. While telomere addition sites have been well-characterized, little is known regarding the DSBs that fragment nematode chromosomes. Here, we used embryos from the nematode Ascaris to study the timing of PDE breaks and examine the DSBs and their end processing. Using END-seq, we characterize the DSB ends and demonstrate that DNA breaks are introduced before mitosis, followed by extensive end resection. The resection profile is unique for each break site, and the resection generates 3’ overhangs before the addition of telomeres. Interestingly, telomere healing occurs much more frequently on retained DSB ends than on eliminated ends. This biased repair of the DSB in Ascaris is likely due to the sequestration of the eliminated DNA into micronuclei, preventing their ends from telomere healing. Additional DNA breaks occur within the eliminated DNA in both Ascaris and Parascaris, ensuring chromosomal breakage and providing a fail-safe mechanism for nematode PDE.
Project description:DNA double-strand breaks (DSBs) represent a threat to the genome because they can lead to loss of genetic information and chromosome rearrangements. The DNA repair protein p53 binding protein 1 (53BP1) protects the genome by limiting nucleolytic processing of DSBs by a mechanism that requires its phosphorylation, but whether it does so directly is not known. Here we identify Rapl-interacting factor 1 (Rif1) as an Ataxia-Telangiectasia Mutated (ATM) phosphorylation-dependent interactor of 53BP1, and show that absence of Rif1 results in 5M-bM-^@M-^Y-3M-bM-^@M-^Y DNA end resection in mice. Consistent with enhanced DNA resection, Rif1 deficiency impairs DNA repair in the G1 and S phases of the cell cycle, interferes with class switch recombination (CSR) in B lymphocytes, and leads to accumulation of chromosome DSBs. Study of Rif1 DNA-end protection activity against resection via analysis of single-stranded DNA binding protein RPA and Rad51 accumulation at sites of AID-induced DNA damage by ChIP-seq. All samples shown in Fig. 4 are included (controls and test samples, 7 samples in total).
Project description:In non-homologous end joining repair of DNA double strand breaks, DNA dependent protein kinase catalytic subunit (DNA-PKcs) and Ku70/80 binds the free DNA ends forming the holoenzyme DNA-PK. DNA-PK synapses across the break to tether the broken ends in the initial long range synaptic complex. We generated an integrative structural model of DNA-PK synapsis at a precision of 13.5Å with crosslinking mass spectrometry (XL-MS) restraints. While our hydrogen deuterium exchange (HX) analysis revealed an allosteric axis in DNA-PK connecting DNA binding (including the plug domain) to the kinase domain. Our model presents a symmetrical synapsis primarily through head to head interactions and protection of the DNA by previously uncharacterized a plug domain. The offset of the DNA in our model allows access to downstream processing enzymes, while the combination of DNA binding and kinase loading creates a tensed state that could have roles in the re-arrangement/dissociation of DNA-PKcs as the repair process progresses.
Project description:DNA double-strand breaks (DSBs) represent a threat to the genome because they can lead to loss of genetic information and chromosome rearrangements. The DNA repair protein p53 binding protein 1 (53BP1) protects the genome by limiting nucleolytic processing of DSBs by a mechanism that requires its phosphorylation, but whether it does so directly is not known. Here we identify Rapl-interacting factor 1 (Rif1) as an Ataxia-Telangiectasia Mutated (ATM) phosphorylation-dependent interactor of 53BP1, and show that absence of Rif1 results in 5’-3’ DNA end resection in mice. Consistent with enhanced DNA resection, Rif1 deficiency impairs DNA repair in the G1 and S phases of the cell cycle, interferes with class switch recombination (CSR) in B lymphocytes, and leads to accumulation of chromosome DSBs.
Project description:DNA double-strand breaks (DSBs) can be repaired by several pathways. In eukaryotes, repair pathway choice – the cellular decision making underlying DSB repair – occurs at the level of DSB resection and is controlled by the cell cycle. Upon cell cycle-dependent activation, cyclin-dependent kinases (CDKs) phosphorylate resection proteins and thereby stimulate DSB resection and repair by homologous recombination (HR). Here, we identify Dbf4-dependent kinase (DDK) as a second major cell cycle regulator of DNA end resection. Using inducible genetic and chemical inhibition of DDK in budding yeast and human cells, we show that DNA resection and HR require activation by DDK. Mechanistically, DDK catalyzes phosphorylation of at least two resection nucleases. Via phosphorylation of the Mre11 activator Sae2 it promotes activation of resection initiation, via phosphorylation of the Dna2 nuclease it promotes long-range resection. Notably, synthetic activiation of DDK allows limited resection and HR in G1 cells, suggesting that DDK is a key component of DSB repair decision making.