Apoptotic DNA fragmentation and DNA double-strand break repair promote the formation of chromosome translocations

Chromosome translocations are speculated to play a pivotal role in leukemogenesis, because these types of DNA rearrangements are found in 65% of all acute leukemias. The treatment of childhood acute lymphocytic leukemia is considered to be one of the greatest successes in the fight against cancer, as 5-year survival rates approach 70%. Although this is true, leukemias associated with a translocation involving the MLL gene have a 5-year survival rate of less than 5%. Translocations involving MLL are a frequent finding in both de novo and therapy-related leukemia. To date, the mechanisms responsible for the generation of chromosome translocations are unknown. Breaks in MLL are clustered in an 8.3 kb segment of DNA, termed the breakpoint cluster region (bcr). Site-specific cleavage of the bcr is observed in human TK6 lymphoblastoid cells that are undergoing apoptosis. This cleavage event is not specific to any individual apoptotic stimulus, as both radiation and Fas receptor engagement produce cleavage. MLL is not cleaved in WIL2-NS lymphoblastoid cells that are known to be resistant to radiation induced apoptosis. Induction of apoptosis through Fas receptor engagement in WIL2-NS cells restores MLL cleavage, demonstrating that MLL cleavage is the direct result of apoptosis. Further research identified that this break is the result of high molecular weight (HMW) DNA fragmentation, which occurs in the early stages of the apoptotic process. Additionally inhibition of apoptosis, by caspase inhibitors, prevents HMW DNA fragmentation and consequently MLL cleavage. Sequence data obtained by analyzing translocations isolated from patient samples, suggests that nonhomologous end-joining (NHEJ) repair had occurred at the translocation breakpoint. NHEJ repair is the primary mechanism for repairing blunt-end DNA double-strand breaks in mammals. These are the same types of breaks introduced into the genome during apoptosis. Inhibition of NHEJ repair with a known inhibitor, or by using cell lines deficient in NHEJ results in the accelerated appearance of MLL cleavage. These findings demonstrate that NHEJ repair is active at the MLL bcr during apoptosis. Taken together these data suggest a novel model in which NHEJ repair of apoptotic DNA fragments promotes the formation of a chromosome translocation.