title: Targeted expression of herpes simplex virus thymidine kinase in adenovirus-infected cells reduces virus titers upon treatment with ganciclovir in vitro.
abstract: BACKGROUND
  Adenoviruses are a frequent cause of life-threatening infections in immunocompromised patients. Available therapeutics still cannot completely prevent fatal outcomes. By contrast, herpes viruses are well treatable with prodrugs such as ganciclovir (GCV), which are selectively activated in virus-infected cells by virus-encoded thymidine kinases. This effective group of prodrugs is not applicable to adenoviruses and other DNA viruses because they lack those kinases.
  
  
  METHODS
  To render adenoviruses amenable to GCV treatment, we generated an adenoviral vector-based delivery system for targeted expression of herpes simplex virus thymidine kinase (HSV-TK) in wild-type adenovirus 5 (wt Ad5)-infected cells. HSV-TK expression was largely restricted to wt virus-infected cells by transcription of the gene from the Ad5 E4 promoter. Its activity is dependent on the adenoviral E1A gene product which is not produced by the vector but is only provided in cells infected with wt adenovirus. The anti-adenoviral effect of HSV-TK expression and concomitant treatment with GCV was assessed in vitro in four different cell lines or primary cells.
  
  
  RESULTS
  E4 promoter-mediated HSV-TK background expression was sufficiently low to prevent cytotoxicity in the presence of low-levels GCV in cells not infected with wt Ad5. However, expression was several-fold increased in wt Ad5-infected cells and treatment with low levels of GCV efficiently inhibited wt Ad5 DNA replication. Genome copy numbers and output of infectious particles were reduced by up to > 99.99% and cell viability was greatly increased.
  
  
  CONCLUSIONS
  We extended the concept of enzyme/prodrug therapy to adenovirus infections by selectively sensitizing adenovirus-infected cells to treatment with GCV.
authors.affiliation: Children's Cancer Research Institute, St. Anna Kinderkrebsforschung, Department of Pediatrics, Medical University of Vienna, Austria.
authors.forename: Mirza
authors.initials: M
authors.lastname: Ibrišimović
authors.forename: Ulrike
authors.initials: U
authors.lastname: Nagl
authors.forename: Doris
authors.initials: D
authors.lastname: Kneidinger
authors.forename: Margit
authors.initials: M
authors.lastname: Rauch
authors.forename: Thomas
authors.initials: T
authors.lastname: Lion
authors.forename: Reinhard
authors.initials: R
authors.lastname: Klein
mesh_terms: D000256:Adenoviridae / Q000201:enzymology / Q000235:genetics / Q000502:physiology*; D000257:Adenoviridae Infections / Q000821:virology*; D017498:Adenovirus E1 Proteins / Q000378:metabolism; D016923:Cell Death / Q000187:drug effects; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D015774:Ganciclovir / Q000494:pharmacology*; D015316:Genetic Therapy / Q000379:methods*; D005822:Genetic Vectors / Q000235:genetics; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D038101:Insulator Elements / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D017384:Sequence Deletion / Q000235:genetics; D018139:Simplexvirus / Q000187:drug effects / Q000201:enzymology* / Q000502:physiology; D013937:Thymidine Kinase / Q000235:genetics* / Q000627:therapeutic use; D019562:Viral Load / Q000187:drug effects*; D014779:Virus Replication / Q000187:drug effects
chemical_list: D017498:Adenovirus E1 Proteins; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D013937:Thymidine Kinase; D015774:Ganciclovir