title: Epstein-Barr virus BamHI W repeat number limits EBNA2/EBNA-LP coexpression in newly infected B cells and the efficiency of B-cell transformation: a rationale for the multiple W repeats in wild-type virus strains.
abstract: The genome of Epstein-Barr virus (EBV), a gammaherpesvirus with potent B-cell growth-transforming ability, contains multiple copies of a 3-kb BamHI W repeat sequence; each repeat carries (i) a promoter (Wp) that initiates transformation by driving EBNA-LP and EBNA2 expression and (ii) the W1W2 exons encoding the functionally active repeat domain of EBNA-LP. The W repeat copy number of a virus therefore influences two potential determinants of its transforming ability: the number of available Wp copies and the maximum size of the encoded EBNA-LP. Here, using recombinant EBVs, we show that optimal B-cell transformation requires a minimum of 5 W repeats (5W); the levels of transforming ability fall progressively with viruses carrying 4, 3, and 2 W repeats, as do the levels of Wp-initiated transcripts expressed early postinfection (p.i.), while viruses with 1 copy of the wild-type W repeat (1W) and 0W are completely nontransforming. We therefore suggest that genetic analyses of EBV transforming function should ensure that wild-type and mutant strains have equal numbers (ideally at least 5) of W copies if the analysis is not to be compromised. Attempts to enhance the transforming function of low-W-copy-number viruses, via the activity of helper EBV strains or by gene repair, suggested that the critical defect is not related to EBNA-LP size but to the failure to achieve sufficiently strong coexpression of EBNA-LP and EBNA2 early postinfection. We further show by the results of ex vivo assays that EBV strains in the blood of infected individuals typically have a mean of 5 to 8 W copies, consistent with the view that evolution has selected for viruses with an optimal transforming function.
authors.affiliation: School of Cancer Sciences, Birmingham Cancer Research UK Centre, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. R.J.Tierney@bham.ac.uk
authors.forename: Rosemary J
authors.initials: RJ
authors.lastname: Tierney
authors.forename: Kuan-Yu
authors.initials: KY
authors.lastname: Kao
authors.forename: Jasdeep K
authors.initials: JK
authors.lastname: Nagra
authors.forename: Alan B
authors.initials: AB
authors.lastname: Rickinson
mesh_terms: D001402:B-Lymphocytes / Q000378:metabolism / Q000821:virology*; D002472:Cell Transformation, Viral; D002478:Cells, Cultured; D020031:Epstein-Barr Virus Infections / Q000235:genetics / Q000378:metabolism / Q000821:virology*; D019309:Epstein-Barr Virus Nuclear Antigens / Q000235:genetics / Q000378:metabolism*; D004854:Herpesvirus 4, Human / Q000235:genetics* / Q000472:pathogenicity; D006801:Humans; D012367:RNA, Viral / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics*; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: C402641:EBNA-2 protein, Human herpesvirus 4; C402640:EBNA-LP protein, Human herpesvirus 4; D019309:Epstein-Barr Virus Nuclear Antigens; D012367:RNA, Viral; D014764:Viral Proteins