Abstract
Chromosomal double strand breaks (DSBs) are usually repaired in mammalian cells through either of two pathways: end-joining (EJ) or homologous recombination (HR). To clarify the relative contribution of each pathway and the ensuring genetic changes, we developed a system to trace the fate of DSBs in the human genome by introducing restriction enzyme I-SceI site into thymidine kinase gene (TK) of human lymphoblastoid cell line (TK6). A DSB in the TK gene stimulated EJ as well as inter-chromosomal HR, but EJ contributed to the repair of DSBs over 100 times more frequently than HR. Molecular analysis revealed that EJ mainly causes small deletions limited to the TK gene. Seventy percent of the small deletion mutants analyzed showed 100 to 4,000 bp deletions with a 0 to 6-bp homology at the joint. Another 30%, however, were accompanied by complicated DNA rearrangements, presumably the result of sister chromatid fusion and breakage-fusion-bridge (BFB) cycle. BFB cycle could be an important mechanism for explaining delayed chromosomal instability caused by DSBs. HR, on the other hand, always resulted in non-crossing-over gene conversion. [J Radiat Res 44:375 (2003)]
