E 2 F 1 promotes the recruitment of DNA repair factors to sites of DNA double-strand breaks

Cellular DNA is constantly being damage by both exogenous and endogenous agents. Failure to repair DNA damage in a timely and efficient manner can lead to mutations, chromosomal instability and life threatening diseases such as cancer. Cells initiate an elaborate signaling network in response to DNA damage to activate cell cycle checkpoints and coordinate DNA repair mechanisms. At the apex of the DNA damage response are several members of the PI3 kinase family, including ataxia telangiectasia mutated (ATM), which is activated in response to doublestrand breaks (DSB) and ATM and Rad3 related (ATR), which is activated in response to stalled replication and transcription forks. Protein targets of these kinases include NBS1, SMC1, p53, Chk1 and 2, BRCA1 and 2, and many other proteins involved in cell cycle checkpoints, DNA repair, apoptosis and cellular senescence. Inherited genetic defects in the DNA damage response lead to chromosomal instability syndromes and a predisposition to cancer. For example, mutations in the ATM gene cause ataxia telagiectasia (AT), which is marked by immunodeficiency, progressive cerebellar ataxia and a predisposition to leukemia and lymphoma. Similarly, hypomorphic mutations in NBS1 cause Nijmegen breakage syndrome (NBS), which shares many characteristics with AT. Defects in other downstream targets of ATM, including p53, BRCA1 and BRCA2, are also strongly associated with cancer development. The E2F1 transcription factor is another component of the DNA damage response and a direct target of the ATM and ATR the e2F1 transcription factor is post-translationally modified and stabilized in response to various forms of DNA damage to regulate the expression of cell cycle and pro-apoptotic genes. e2F1 also forms foci at DNA double-strand breaks (DSBs) but the function of e2F1 at sites of damage is unknown. Here we demonstrate that the absence of e2F1 leads to spontaneous DNA breaks and impaired recovery following exposure to ionizing radiation. e2F1 deficiency results in defective NBS1 phosphorylation and foci formation in response to DSBs but does not affect NBS1 expression levels. Moreover, an increased association between NBS1 and e2F1 is observed in response to DNA damage, suggesting that e2F1 may promote NBS1 foci formation through a direct or indirect interaction at sites of DNA breaks. e2F1 deficiency also impairs RpA and Rad51 foci formation indicating that e2F1 is important for DNA end resection and the formation of single-stranded DNA at DSBs. these findings establish new roles for e2F1 in the DNA damage response, which may directly contribute to DNA repair and genome maintenance. E2F1 promotes the recruitment of DNA repair factors to sites of DNA double-strand breaks