title: Inhibition of homologous recombination by variants of the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs).
abstract: Two major DNA double-strand break repair pathways exist in all eukaryotes, nonhomologous DNA end joining (NHEJ) and homologous recombination (HR). Although both pathways can function throughout the cell cycle, NHEJ predominates in G0/G1) (when a replicated sister chromatid is unavailable), whereas HR makes a more substantial contribution in S and G2. How a cell chooses between these two important DNA repair pathways is largely unknown. DNA-dependent protein kinase (DNA-PK) is critical for NHEJ. Here, we describe two conserved splice variants of a catalytic subunit of DNA-PK (DNA-PKcs) that are expressed predominately in nondividing cells. Although both encode stable products, neither reverses the NHEJ defects in DNA-PKcs-deficient cells. In fact, cells expressing one of the DNA-PKcs variants are slightly more radiosensitive than cells completely deficient in DNA-PKcs. We investigated whether cells expressing the DNA-PKcs variants had any other DNA repair deficits and found that these cells are considerably more sensitive to both etoposide and mitomycin C than cells that express no DNA-PKcs at all. Because repair of DNA damage induced by these two agents requires intact HR, we tested whether the NHEJ-defective variants of DNA-PKcs inhibit double-strand break-induced HR in an integrated substrate. In cells expressing the NHEJ-defective variants, HR was markedly reduced. Because the splice variants are expressed highly only in nondividing cells, quiescent cells would be afforded a mechanism to inhibit repair by means of HR when sister chromatids are not available as templates for accurate repair with low risk of genome rearrangement, thereby enhancing genome stability.
authors.affiliation: Department of Pathobiology and Diagnostic Investigation, Michigan State University, East Lansing, MI 48824, USA.
authors.forename: Erin
authors.initials: E
authors.lastname: Convery
authors.forename: Euy Kyun
authors.initials: EK
authors.lastname: Shin
authors.forename: Qi
authors.initials: Q
authors.lastname: Ding
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Pauline
authors.initials: P
authors.lastname: Douglas
authors.forename: Laurie S
authors.initials: LS
authors.lastname: Davis
authors.forename: Jac A
authors.initials: JA
authors.lastname: Nickoloff
authors.forename: Susan P
authors.initials: SP
authors.lastname: Lees-Miller
authors.forename: Katheryn
authors.initials: K
authors.lastname: Meek
mesh_terms: D017398:Alternative Splicing; D020134:Catalytic Domain; D002453:Cell Cycle / Q000235:genetics; D002455:Cell Division; D002460:Cell Line; D045744:Cell Line, Tumor; D017124:Conserved Sequence; D051747:DNA-Activated Protein Kinase; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000528:radiation effects; D014644:Genetic Variation; D006801:Humans; D009687:Nuclear Proteins; D010957:Plasmids; D017346:Protein Serine-Threonine Kinases / Q000235:genetics* / Q000528:radiation effects; D021122:Protein Subunits / Q000235:genetics; D011839:Radiation, Ionizing; D011995:Recombination, Genetic; D014162:Transfection
chemical_list: D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D021122:Protein Subunits; D051747:DNA-Activated Protein Kinase; C497263:PRKDC protein, human; D017346:Protein Serine-Threonine Kinases