Academic journal article Genetics

Xlf1 Is Required for DNA Repair by Nonhomologous End Joining in Schizosaccharomyces Pombe

Academic journal article Genetics

Xlf1 Is Required for DNA Repair by Nonhomologous End Joining in Schizosaccharomyces Pombe

Article excerpt

ABSTRACT

The accurate repair of DNA double-strand breaks is essential for cell survival and maintenance of genome integrity. Here we describe xlf1^sup +^, a gene in the fission yeast Schizosaccharomyces pombe that is required for repair of double-strand breaks by nonhomologous end joining during G^sub 1^ phase of the cell cycle. Xlf1 is the ortholog of budding yeast Nej1 and human XLF/Cernunnos proteins.

DNA double-strand breaks (DSBs), which can arise from endogenous or exogenous sources, particularly ionizing radiation (IR), are considered among the most lethal and genome-destabilizing types of DNA damage (PIERCE et al. 2001; WILLERS et al. 2004). Efficient and accurate repair of DSBs is essential to preserve genome integrity. The options for repair of DSBs depend on the circumstances in which the DNA damage is suffered (PAQUES and HABER 1999; VAN GENT et al. 2001; WEST 2003; KROGH and SYMINGTON 2004; SANCAR et al. 2004). DSBs that arise in G^sub 2^ phase of a haploid organism can be effectively repaired by homologous recombination (HR), an error-free mechanism that uses the undamaged sister chromatid as a template for repair of the broken chromosome. In contrast, when DSBs occur in G^sub 1^ phase of a haploid cell, the sister chromatid is unavailable as a template for DNA repair. In this circumstance, the only option for repair of DSBs in unique DNA sequences is the mechanism of nonhomologous end joining (NHEJ). This error-prone mechanism of DNA repair, which was originally discovered in mammalian cells, has since been shown to be broadly conserved among eukaryotes, including the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe.

In mammalian cells, NHEJ depends on the Ku70/80 complex that is thought to function as an end-to-end synapsis factor, onDNA-dependent protein kinase (DNA-PK), on Artemis endonuclease, and on the XRCC-DNA ligase IV complex, which carries out the final step of NHEJ (SANCAR et al. 2004; WETERINGS and VAN GENT 2004). NHEJ in S. cerevisiae requires Ku70/80, the Mre11-Rad50-Xrs2 (MRX) complex that is also essential for repair of DSBs by homologous recombination, DNA ligase IV in a complex with theXRCC4-like protein Lif1, and a protein known as Nej1 (FRANK-VAILLANT and MARCAND 2001; KEGEL et al. 2001; VALENCIA et al. 2001; DUDASOVA et al. 2004; DALEY et al. 2005). The only known NHEJ factors in fission yeast are the Ku70/80 complex and DNA ligase IV (MANOLIS et al. 2001; MIYOSHI et al. 2003; PRUDDEN et al. 2003; FERREIRA and COOPER 2004). Curiously, S. pombe does not have a homolog of XRCC4/Lif1, nor does it require the Mre11-Rad50-Nbs1 (MRN) complex forNHEJ (MANOLIS et al. 2001). Similarly, current evidence suggests that the MRN complex is not required for NHEJ in metazoan organisms (DI VIRGILIO and GAUTIER 2005). These data suggest that a core group of proteins are required for NHEJ in all species (Ku70/80 and DNA ligase IV), with a larger group of accessory factors (DNA-PK, Artemis, XRCC4/Lif1, and MRN/MRX) that are required for NHEJ in only a subset of organisms.

An exciting recent development in the field of DNA repair was the discovery of a novel human NHEJ factor known as XLF/Cernunnos and the realization that is an ortholog of S. cerevisiae Nej1 (AHNESORG et al. 2006; BUCK et al. 2006a; CALLEBAUT et al. 2006; SEKIGUCHI and FERGUSON 2006). Cernunnos patients display a set of phenotypes, including microcephaly and immune deficiency, which closely resemble the characteristics of patients defective for DNA ligase IV (O'DRISCOLL et al. 2001; BEN-OMRAN et al. 2005; BUCK et al. 2006b). These data are consistent with the evidence thatXLF/Cernunnos interacts with the XRCC4-DNA ligase IV complex, suggesting that XLF/Cernunnos is a third essential subunit of this complex. We had previously noted the existence of a potential Nej1/Xlf1/Cernunnos-like gene in the genome of fission yeast, as also recently reported by others (CALLEBAUT et al. 2006). …

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