BCR/ABL Regulates the Expression and Interacts with Werner Syndrome Helicase/Exonuclease To Modulate Its Biochemical Properties.

Abstract

A genome-wide screen suggested that BCR/ABL kinase might stimulate WRN, a member of the RecQ-like DNA helicases family. The Werner syndrome protein (WRN) exerts DNA helicase and 3′-5′ exonuclease activities. Inactivating mutations in the WRN gene causes Werner syndrome, characterized by premature aging, genomic instability and cancer predisposition. The WRN helicase unwinds unusual DNA structures, which can occur physiologically, or can be accidentally generated during DNA repair (double-stranded DNA with mismatched tails, bimolecular G4 quartets and Holliday junctions). In addition, WRN physically interacts with components of two major systems for DNA double-strand breaks (DSBs) repair: non-homologous end-joining (NHEJ) and homologous recombination (HR). Here we demonstrated that BCR/ABL regulates the expression of WRN mRNA and protein in CML primary cells and BCR/ABL-transformed cell lines. BCR/ABL kinase-induced WRN expression is mediated by c-MYC, but not STAT5 - dependent transcription as well as by inhibition of caspases-dependent cleavage. In addition, immunoprecipitation and pull-down studies indicated that BCR/ABL interacts directly with WRN resulting in its tyrosine phosphorylation. Mutation analysis revealed that multiple domains/amino acid residues of BCR/ABL and WRN are involved in the interaction. BCR/ABL-positive leukemia cells exerted an enhanced WRN-dependent helicase activity. In addition, immunoprecipitation and double-immunofluorescence co-localization studies demonstrated an elevated interaction between WRN and RAD51 in BCR/ABL cells undergoing genotoxic stress in comparison to parental counterparts. Altogether, it is likely that WRN is involved in DSBs repair by HR in leukemia cells. More detailed studies are underway to pinpoint the role of WRN in DNA damage response in BCR/ABL-transformed cells.