In mammalian cells, DSB recognition requires phosphorylation of histone H2AX and processing of DNA ends may involve one of several factors, which include aprataxin and polynucleotide kinase (Chappell et al. Keywords:Adenovirus, Non-homologous end joining, XRCC4, Ligase IV, E4 34k, E4orf6, E1B 55k == Introduction == DNA damage occurs in all living cells either as a result of environmental insults or normal metabolic processes and presents a major threat to the integrity of the genome. In particular, DNA doublestrand breaks (DSBs) can lead to genetic rearrangements that promote tumor formation. Eukaryotic cells repair DSBs by two mechanisms: homologous recombination, which is used for repair of DSBs that occur during late-S or G2 phases of the cell cycle, and nonhomologous end joining (NHEJ), which is used during G0 and G1 phases of the cell Benzocaine hydrochloride cycle (Lieber et al. 2004,Sonoda et al. 2006,Weterings and Chen 2008). In addition to spontaneous DSBs, somatic cell recombination processes required for immunoglobulin (Ig) production and diversification are initiated by DSBs that are repaired by NHEJ (Lieber et al. 2004,Weterings and Chen 2008). Because of the large proportion of non-cycling cells in multicellular organisms and the added role of DSBs in Ig production, the majority of DSBs in humans are repaired by NHEJ (Lieber et al. 2004,Sonoda et al. 2006). The repair of the DSBs by NHEJ involves recognition of DSBs, localization of repair enzymes to the DSB, processing EPHB2 of the DNA ends to yield ligatable termini, and ligation to reseal the break. In mammalian cells, DSB recognition requires phosphorylation of histone H2AX and processing of DNA ends may involve one of several factors, which include aprataxin and polynucleotide kinase (Chappell et al. 2002,Koch et al. 2004,Ahel et al. 2006). Mammalian NHEJ also requires the heterotrimeric DNA-dependent protein kinase (DNA-PK), a nuclear serine/threonine protein kinase belonging to the phosphatidylinositol 3-related protein kinase (PI3K) family (Smith and Jackson 1999,Weterings and Chen 2008). This heterotrimeric kinase consists of a 460 kDa catalytic subunit (DNA-PKcs) and a heterodimeric DNA binding subunit (Ku) (Smith and Jackson 1999). Ku is usually thought to be involved in the recognition of DSBs based on its abundance in nuclei and its affinity for DNA ends (Smith and Jackson 1999,Weterings and Chen 2008). In mice, disruption of genes encoding DNA-PKcs or either of the Ku subunits results in a spectrum of DSB repair and Igproduction defects (Smith and Jackson 1999,Weterings and Chen 2008). In mammalian NHEJ, Benzocaine hydrochloride the ligation event is usually catalyzed by DNA ligase IV, which forms a complex with the DNA-binding proteins XRCC4 and XLF (Bryans et al. 1999,Ahnesorg et al. 2006,Buck et al. 2006,Sonoda et al. 2006,Weterings and Chen 2008). XRCC4 makes direct contact with ligase IV and XLF, stimulates ligase IV activity and is essential for ligase IV stability (Critchlow et al. 1997,Grawunder et al. 1998,Ahnesorg et Benzocaine hydrochloride al. 2006,Buck et al. 2006,Sonoda et al. 2006,Weterings and Chen 2008). Structural studies have shown that both XRCC4 and XLF form homodimers with amazing similarity in overall fold (Junop et al. 2000,Andres et al. 2007,Li et al. 2007). Direct-binding and structural studies paint a picture of a complex composed of 1 ligase IV: 2 XRCC4: 2 XLF that is held together through strong, direct ligase IV/XRCC4 and XRCC4/XLF interactions with weaker, possibly indirect, ligase IV/XLF contacts (Deshpande and Wilson 2007). Cells lacking ligase IV, XRCC4 or XLF cannot carryout NHEJ and are therefore radiation-sensitive and unable to carry out Ig-gene rearrangement by V(D)J recombination (Barnes et al. 1998,Frank et al. 1998,Lieber et al. 2004,Buck et al. 2006,Weterings and Chen 2008). In mice, disruption of.
In mammalian cells, DSB recognition requires phosphorylation of histone H2AX and processing of DNA ends may involve one of several factors, which include aprataxin and polynucleotide kinase (Chappell et al
