Coupled homologous and nonhomologous repair of a double-strand break preserves genomic integrity in mammalian cells

DNA double-strand breaks (DSBs) may. be caused by normal metabolic processe s or exogenous DNA damaging agents and can promote chromosomal rearrangemen ts, including translocations, deletions, or chromosome loss. In mammalian c ells, both homologous recombination and nonhomologous end joining (NHEJ) ar e important DSB repair pathways for the maintenance of genomic stability. U sing a mouse embryonic stem cell system, we preciously demonstrated that a DSB in one chromosome can be repaired by recombination with a homologous se quence on a heterologous chromosome, without any evidence of genome rearran gements (C. Richardson, M E. Moynahan, and M. Jasin, Genes Dev., 12:831-384 2, 1998). To determine if genomic integrity would be compromised if homolog y were constrained, we have now examined interchromosomal recombination bet ween truncated but overlapping gene sequences. Despite these constraints, r ecombinants were readily recovered when a DSB was introduced into one of th e sequences. The overwhelming majority of recombinants showed no evidence o f chromosomal rearrangements. Instead, events were initiated by homologous invasion of one chromosome end and completed by NHEJ to the other chromosom e end, which remained highly preserved throughout the process. Thus, genomi c integrity was maintained by a coupling of homologous and nonhomologous re pair pathways. Interestingly, the recombination frequency, although not the structure of the recombinant repair products, was sensitive to the relativ e orientation of the gene sequences on the interacting chromosomes.