Up-Regulated WRN and DNA Ligase IIIα Are Involved in Alternative NHEJ Repair Pathway of DNA Double Strand Breaks (DSB) in Chronic Myeloid Leukemia (CML).

The oncogenic BCR-ABL in CML produces increased reactive oxygen species (ROS) leading to DSB and aberrant repair. We have previously shown that CML cells demonstrate an increased frequency of errors of non homologous end-joining (NHEJ). DSB are repaired by two major pathways, homologous recombination (HR) and NHEJ, the dominant pathway in eukaryotic cells, also known as DNA-PK dependent NHEJ (D-NHEJ). Recent reports have identified alternative or “back-up” NHEJ pathways (B-NHEJ) that are highly error-prone, and may explain the altered DSB repair reported in CML. To determine the mechanism for the aberrant NHEJ repair in CML, we examined steady state levels of D-NHEJ proteins, including Ku70/86, DNA-PKcs, Artemis and DNA Ligase IV/XRCC4 in four different BCR-ABL positive CML cell lines compared with three lymphoblastoid cell lines established from normal individuals and one BCR-ABL negative CML cell line. We find that two key components of D-NHEJ, Artemis (4–7 fold) and DNA Ligase IV (2–3 fold) are down-regulated, compared with controls. These data suggest that D-NHEJ repair is compromised in CML. To determine whether alternative NHEJ repair plays a role in the aberrant repair of DSB in CML cells, we next examined expression levels of DNA Ligase IIIα/XRCC1, PARP and other proteins known to be associated with NHEJ repair, such as the protein found to be deleted in Werner’s syndrome, WRN. We find that WRN and DNA Ligase IIIα are increased (3–6 fold) in BCR-ABL-positive CML compared with control cell lines. Importantly, DNA Ligase IIIα/XRCC1 forms a complex with WRN, suggesting that it may be a new member of the alternative repair pathway. To confirm that up-regulation of DNA Ligase IIIα and WRN are elicited by BCR-ABL, we examined the levels of these proteins in primary samples (N=4) from patients with different levels of BCR-ABL, following treatment with the tyrosine kinase inhibitor Gleevec. WRN and DNA Ligase IIIα are down regulated in patient samples where BCR-ABL levels are significantly decreased. Furthermore, we confirmed that these up-regulated proteins are involved in DSB repair in CML cells because they co-localize to induced DSB in BCR-ABL-positive cell lines stably transfected with DSB-containing DRneo plasmid, using fluorescence in situ hybridization (FISH) co-immunostaining. Importantly we show that siRNA down-regulation of WRN and DNA Ligase IIIα leads to elevated levels of unrepaired DSB and a decreased frequency of DSB repair efficiency in CML cells. In addition siRNA down-regulation of WRN leads to large deletions at the site of repair, while siRNA down-regulation of DNA Ligase IIIα results in an increased frequency of misrepair. Finally, we determined whether “correction” of main NHEJ pathway proteins in CML can lead to a decrease in the frequency of errors of end-joining repair. Over-expression of Artemis using pcDNA constructs in CML cells leads to more correct end-joining, compared with vector transfected controls. We conclude that down-regulation of Artemis and DNA Ligase IV leads to compensatory up-regulation of alternative repair pathways in BCR-ABL-positive CML cells, and suggest a role for a new protein complex in CML, in protecting and joining DNA ends, thus ensuring the survival of CML cells. Inhibition of alternative NHEJ repair may be explored in combination with other agents as a therapeutic strategy in CML.