PARP1 is required for chromosomal translocations

Abstract

• Chromosomal translocations are mediated by PARP1 and can be suppressed by the clinical PARP1 inhibitors.Chromosomal translocations are common contributors to malignancy, yet little is known about the precise molecular mechanisms by which they are generated. Sequencing translocation junctions in acute leukemias revealed that the translocations were likely mediated by a DNA double-strand break repair pathway termed nonhomologous endjoining (NHEJ). There are major 2 types of NHEJ: (1) the classical pathway initiated by the Ku complex, and (2) the alternative pathway initiated by poly ADP-ribose polymerase 1 (PARP1). Recent reports suggest that classical NHEJ repair components repress translocations, whereas alternative NHEJ components were required for translocations. The rate-limiting step for initiation of alternative NHEJ is the displacement of the Ku complex by PARP1. Therefore, we asked whether PARP1 inhibition could prevent chromosomal translocations in 3 translocation reporter systems. We found that 2 PARP1 inhibitors or repression of PARP1 protein expression strongly repressed chromosomal translocations, implying that PARP1 is essential for this process. Finally, PARP1 inhibition also reduced both ionizing radiation-generated and VP16-generated translocations in 2 cell lines. These data define PARP1 as a critical mediator of chromosomal translocations and raise the possibility that oncogenic translocations occurring after high-dose chemotherapy or radiation could be prevented by treatment with a clinically available PARP1 inhibitor. (Blood. 2013;121(21):4359-4365) IntroductionChromosomal translocations both classify types of malignancies and are required for the origin of those malignancies.1,2 Because of this trend, translocations have been widely studied, but their precise molecular mechanism remains poorly understood. It is intuitively and experimentally clear that simultaneous DNA doublestrand breaks (DSBs) must occur in distinct chromosomes for a translocation to occur.3 DNA DSBs can be repaired by 3 pathways: (1) homologous recombination (HR), where sequence integrity is preserved; or (2) single-strand annealing (SSA) and (3) nonhomologous end-joining (NHEJ), both of which generate deletions. 4 Sequencing junctions of leukemic translocations in patient samples revealed that these junctions often had deleted sequences, indicating that these translocations predominantly arose by SSA or NHEJ. 5 Therefore, HR is not thought to play a significant role in chromosomal translocations.1,2 SSA and NHEJ can be distinguished by the presence of repeated sequences adjacent to the junction site that could mediate the annealing in SSA. Thus, when long repeated sequences are adjacent to DSBs, translocations occur more frequently via SSA vs NHEJ.3 When no repeated sequences are present, then translocations are mediated by NHEJ.There are 2 major NHEJ pathways: the dominant, classical (cNHEJ) pathway; and the alternative (aNHEJ) pathway.4 cNHEJ begins when the Ku70/80 complex recognizes free DNA ends and recruits D...

Alternative end-joining pathway(s): Bricolage at DNA breaks

“…Concomitantly to the finding that PARP inhibitors increase the sensitivity of DNA-PK-deficient cells to radiomimetic-induced DSBs [45], biochemical experiments [45,48,49] and plasmid assays in Kudeficient cells [61] substantiated the involvement of PARP1 in a non [62,71,82,95,100,116,118,[149][150][151][152]154,160,161,163,168,169,171,174,180,182,184,185,264,[305][306][307] Translocation IR, etoposide, Zn-finger nucleases [94,240,245,303,308] RAG, AID, spontaneous [62,95,100,152,154,162,163,171,240,261,269,…”

Inhibition of Homologous Recombination and Promotion of Mutagenic Repair of DNA Double-Strand Breaks Underpins Arabinoside–Nucleoside Analogue Radiosensitization

“…B-NHEJ causes more extensive deletions at DSB junctions than D-NHEJ and, importantly, markedly promotes chromosome translocations (15)(16)(17). Thus, B-NHEJ is now widely considered as a main source of genomic instability in cells of higher eukaryotes.…”

Normal ABL1 is a tumor suppressor and therapeutic target in human and mouse leukemias expressing oncogenic ABL1 kinases

“…The absence of ABL1 did not affect cell cycle distribution of BCR-ABL1 cells in response to DNA damage (supplemental Figure 5), but transcriptional microarray analysis revealed that the presence of ABL1 in BCR-ABL1 cells is associated with expression of numerous genes whose products regulate DNA damage response and mitotic spindle assembly checkpoint ( Figure 6G-H Figure 6). 41 Genes involved in kinetochore/spindle/centrosome regulation (Pcid, Csnk2a2, Zwint, Plk2, Nek1) and sister chromatid segregation (Smc1a) are downregulated in BCR-ABL1 Abl1 2/2 cells compared with BCR-ABL1 Abl1 1/1 cells, which may be responsible for abundant aneuploidy in the former cells.…”