title: Full-length HIV-1 Gag determines protease inhibitor susceptibility within in vitro assays.
abstract: OBJECTIVE
  There is evidence that gag contributes to protease inhibitor susceptibility in treatment-experienced patients. Moreover, protease inhibitor resistance-associated mutations can arise in gag in the absence of protease mutations in vitro. We wished to assess the contribution of full-length Gag to protease inhibitor susceptibility in viruses unexposed to protease inhibitors, in particular from the most common HIV-1 subtypes, namely subtype A and C.
  
  
  METHODS
  We compared the drug resistance profiles of subtype A and C cognate gag-protease (from viruses not previously exposed to protease inhibitor) to protease combined with a generic subtype B gag as in routine phenotypic testing.
  
  
  METHODS
  We amplified gag-protease sequences from plasma-derived virus or molecular clones, and used a single cycle transfection-based drug resistance assay to compare the fold changes in the concentration of drug required to inhibit 50% of viral replication of these viruses to a generic subtype B. We made a series of chimeras to explore phenotypes further.
  
  
  RESULTS
  In some cases, use of protease sequences without the cognate gag overestimated susceptibility to protease inhibitors, in particular to lopinavir. We provide evidence that gag sequences from wild-type viruses can contribute as much as 14-fold reduction in susceptibility to lopinavir, and that cognate protease can balance this by partially restoring susceptibility.
  
  
  CONCLUSIONS
  Our findings demonstrate the importance of considering protease inhibitor susceptibility in the context of full-length gag, particularly with respect to the range of HIV-1 subtypes circulating worldwide.
authors.affiliation: MRC Centre for Medical Molecular Virology, University College London, Windeyer Institute, London, UK.
authors.forename: Ravindra K
authors.initials: RK
authors.lastname: Gupta
authors.forename: Arinder
authors.initials: A
authors.lastname: Kohli
authors.forename: Adele L
authors.initials: AL
authors.lastname: McCormick
authors.forename: Greg J
authors.initials: GJ
authors.lastname: Towers
authors.forename: Deenan
authors.initials: D
authors.lastname: Pillay
authors.forename: Chris M
authors.initials: CM
authors.lastname: Parry
mesh_terms: D024882:Drug Resistance, Viral / Q000235:genetics; D015750:Genes, gag; D016333:HIV Protease / Q000235:genetics; D017320:HIV Protease Inhibitors / Q000494:pharmacology*; D015497:HIV-1 / Q000187:drug effects / Q000235:genetics* / Q000502:physiology; D006801:Humans; D061466:Lopinavir; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D009154:Mutation; D011744:Pyrimidinones / Q000494:pharmacology; D014779:Virus Replication / Q000187:drug effects / Q000235:genetics
chemical_list: D017320:HIV Protease Inhibitors; D011744:Pyrimidinones; D061466:Lopinavir; D016333:HIV Protease