title: Tax impairs DNA replication forks and increases DNA breaks in specific oncogenic genome regions.
abstract: BACKGROUND
  Human T-cell leukemia virus type 1 (HTLV-I) is a human retrovirus associated with adult T-cell leukemia (ATL), an aggressive CD4 T-cell proliferative disease with dismal prognosis. The long latency preceding the development of the disease and the low incidence suggests that the virus itself is not sufficient for transformation and that genetic defects are required to create a permissive environment for leukemia. In fact, ATL cells are characterized by profound genetic modifications including structural and numerical chromosome alterations.
  
  
  RESULTS
  In this study we used molecular combing techniques to study the effect of the oncoprotein Tax on DNA replication. We found that replication forks have difficulties replicating complex DNA, fork progression is slower, and they pause or stall more frequently in the presence of Tax expression. Our results also show that Tax-associated replication defects are partially compensated by an increase in the firing of back-up origins. Consistent with these effects of Tax on DNA replication, an increase in double strand DNA breaks (DDSB) was seen in Tax expressing cells. Tax-mediated increases in DDSBs were associated with the ability of Tax to activate NF-kB and to stimulate intracellular nitric oxide production. We also demonstrated a reduced expression of human translesion synthesis (TLS) DNA polymerases Pol-H and Pol-K in HTLV-I-transformed T cells and ATL cells. This was associated with an increase in DNA breaks induced by Tax at specific genome regions, such as the c-Myc and the Bcl-2 major breakpoints. Consistent with the notion that the non-homologous end joining (NHEJ) pathway is hyperactive in HTLV-I-transformed cells, we found that inhibition of the NHEJ pathway induces significant killing of HTLV-I transformed cells and patient-derived leukemic ATL cells.
  
  
  CONCLUSIONS
  Our results suggest that, replication problems increase genetic instability in HTLV-I-transformed cells. As a result, abuse of NHEJ and a defective homologous repair (HR) DNA repair pathway can be targeted as a new therapeutic approach for the treatment of adult T-cell leukemia.
authors.forename: Hassiba
authors.initials: H
authors.lastname: Chaib-Mezrag
authors.forename: Delphine
authors.initials: D
authors.lastname: Lemaçon
authors.forename: Hélène
authors.initials: H
authors.lastname: Fontaine
authors.forename: Marcia
authors.initials: M
authors.lastname: Bellon
authors.forename: Xue Tao
authors.initials: XT
authors.lastname: Bai
authors.forename: Marjorie
authors.initials: M
authors.lastname: Drac
authors.forename: Arnaud
authors.initials: A
authors.lastname: Coquelle
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Kansas Medical Center, 3901 Rainbow Boulevard, Kansas City, KS 66160, USA. cnicot@kumc.edu.
authors.forename: Christophe
authors.initials: C
authors.lastname: Nicot
mesh_terms: D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded; D059766:DNA End-Joining Repair; D004261:DNA Replication; D016356:Gene Products, tax / Q000378:metabolism*; D015894:Genome, Human; D042822:Genomic Instability; D057809:HEK293 Cells; D015368:Human T-lymphotropic virus 1 / Q000502:physiology*; D006801:Humans; D015459:Leukemia-Lymphoma, Adult T-Cell / Q000235:genetics* / Q000821:virology; D009569:Nitric Oxide / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics; D000097782:NF-kappaB-Inducing Kinase
chemical_list: D016356:Gene Products, tax; D009569:Nitric Oxide; D017346:Protein Serine-Threonine Kinases; D016271:Proto-Oncogene Proteins c-myc; D000097782:NF-kappaB-Inducing Kinase; C489427:MYC protein, human