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I am sure that you are aware of this paper. It illustrates that hypermutation does not always lead to an efficient or escape mutants.
http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1002550
APOBEC3G-Induced Hypermutation of Human Immunodeficiency Virus Type-1 Is Typically a Discrete “All or Nothing” Phenomenon
Andrew E. Armitage1, Koen Deforche2, Chih-hao Chang1, Edmund Wee1, Beatrice Kramer3, John J. Welch4, Jan Gerstoft5, Lars Fugger1,6, Andrew McMichael1, Andrew Rambaut7*, Astrid K. N. Iversen1,8* 1 MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, Oxford University, Oxford, United Kingdom, 2 Rega Institute for Medical Research, Katholieke Universiteit Leuven, Leuven, Belgium, 3 Department of Infectious Diseases, King's College London School of Medicine, London, United Kingdom, 4 Department of Genetics, University of Cambridge, Cambridge, United Kingdom, 5 Department of Infectious Diseases, Rigshospitalet, The National University Hospital, Copenhagen, Denmark, 6 Department of Clinical Neurology, John Radcliffe Hospital, Oxford University, Oxford, United Kingdom, 7 Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, United Kingdom, 8 The Peter Medawar Building for Pathogen Research, Nuffield Department of Medicine, Oxford University, Oxford, United Kingdom Abstract TopThe rapid evolution of Human Immunodeficiency Virus (HIV-1) allows studies of ongoing host–pathogen interactions. One key selective host factor is APOBEC3G (hA3G) that can cause extensive and inactivating Guanosine-to-Adenosine (G-to-A) mutation on HIV plus-strand DNA (termed hypermutation). HIV can inhibit this innate anti-viral defense through binding of the viral protein Vif to hA3G, but binding efficiency varies and hypermutation frequencies fluctuate in patients. A pivotal question is whether hA3G-induced G-to-A mutation is always lethal to the virus or if it may occur at sub-lethal frequencies that could increase viral diversification. We show in vitro that limiting-levels of hA3G-activity (i.e. when only a single hA3G-unit is likely to act on HIV) produce hypermutation frequencies similar to those in patients and demonstrate in silico that potentially non-lethal G-to-A mutation rates are ~10-fold lower than the lowest observed hypermutation levels in vitro and in vivo. Our results suggest that even a single incorporated hA3G-unit is likely to cause extensive and inactivating levels of HIV hypermutation and that hypermutation therefore is typically a discrete “all or nothing” phenomenon. Thus, therapeutic measures that inhibit the interaction between Vif and hA3G will likely not increase virus diversification but expand the fraction of hypermutated proviruses within the infected host.
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