Ku regulates the non-homologous end joining pathway choice of DNA double-strand break repair in human somatic cells

The repair of dna double-strand breaks (dsbs) is critical for the maintenance of genomic integrity and viability for all organisms. mammals have evolved at least two genetically discrete ways to mediate dna dsb repair: homologous recombination (hr) and non-homologous end joining (nhej). in mammalian cells,most dsbs are preferentially repaired by nhej. recent work has demonstrated that nhej consists of at least two sub-pathways - the main ku heterodimerdependent or classic nhej (c-nhej) pathway and an alternative nhej (a-nhej) pathway,which usually generates microhomology-mediated signatures at repair junctions. in our study,recombinant adeno-associated virus knockout vectors were utilized to construct a series of isogenic human somatic cell lines deficient in the core c-nhej factors (ku,dna-pkcs,xlf,and ligiv),and the resulting cell lines were characterized for their ability to carry out dna dsb repair. the absence of dna-pkcs,xlf,or ligiv resulted in cell lines that were profoundly impaired in dna dsb repair activity. unexpectedly,ku86-null cells showed wild-type levels of dna dsb repair activity that was dominated by microhomology joining events indicative of a-nhej. importantly,a-nhej dna dsb repair activity could also be efficiently de-repressed in ligiv-null and dna-pkcs-null cells by subsequently reducing the level of ku70. these studies demonstrate that in human cells c-nhej is the major dna dsb repair pathway and they show that ku is the critical c-nhej factor that regulates dna nhej dsb pathway choice. © 2010 fattah et al.