Mutation comicHIV mutates at extraordinary speed, allowing it to throw out mutants that are at least temporarily invisible to cytotoxic T lymphocytes. We also know, of course, that anti-retroviral drugs (if used alone) are often only temporarily effective against HIV, until the virus mutates and develops resistance to the drug. On the other hand, there are apparently some HIV infections, long-term non-progressors, in which the virus isn’t able to find an effective immune escape strategy; the roads to evasion in these people travel over too many mountains for the virus to scale. When looking at potential new anti-HIV therapy, one of the questions has to be how quickly the virus can discover a path to drug resistance.

One of the relatively recent exciting findings in retrovirology1 is the APOBEC gene family’s role in blocking viral infection. The APOBECs are a largish family of enzymes, of which a few seem to exist mainly to prevent retrovirus replication. In humans, APOBEC3G (as well as some other APOBEC family members) seems to be a fairly important anti-retroviral protein; but it doesn’t help against HIV much, because HIV has evolved an immune evasion gene that blocks APOBEC3G’s effect. (A nice review is at my colleague Yong-Hui’s site.)

This is the vif (“viral infectivity”) protein. vif prevents APOBEC3G from damaging the viral genome by forcing it (APOBEC3G) to be destroyed by the proteasome;2 this is yet another instance of the immune evasion arms race between host and virus, in which each side alternately develops new weapons and defenses. (It’s also a reason for host restriction, since HIV vif doesn’t work against APOBECs from many non-human species.)

APOBEC3GWithout vif, newly formed HIV virions end up incorporating APOBEC into their capsids as they form. APOBEC forces hypermutation of the HIV genome, which is enough to destroy the virus (some mutation is good, but massive hypermutation is bad).

Mutant HIV that don’t have vif are profoundly defective in cells that contain APOBEC3G,3 That makes the vif/APOBEC interaction an attactive drug target.4 If you can prevent vif from destroying APOBECs, then you allow APOBECs to do their jobs and prevent HIV from replicating and spreading.

But will it work? Or is this just another bump in the road for HIV, a little hill that the virus can mutate around by developing resistance to APOBEC even without vif?

APOBEC + vif (Kao et al 2004)The answer5 seems to be that it has a reasonable chance of working, though there’s no guarantee.

The experiment was to grow vif-negative HIV in a cell line that has APOBEC3G and see how often resistant viruses grew out; also, how long it took to see the resistant set, and what mutations enabled the virus to resist APOBEC3G activity.

Just three resistant viruses, out of 48 attempts, grew out by about five weeks of culture. That’s a reasonably good success rate, especially as all three of the mutants turned out to still be susceptible to other APOBECs. That is, although vif confers resistance to APOBEC3G and 3F, these mutants were resistant only to 3G. An anti-vif therapeutic would presumably be even more effective, because 3F would still be active. 6

Our studies therefore provide strong additional justifications for efforts to develop Vif-neutralizing therapeutics.

The interesting thing was how these mutants had evolved resistance. Rather than reactivating vif or some parallel pathway, they had apparently discovered an altogether new way of reducing APOBEC3G damage.

It took a double mutation to escape. One of the mutatations seems to be simply making more virus. APOBEC3G acts when it’s packaged in viral particles; more viral particles means the APOBEC3G is diluted more, so there’s less per virus.

The other mutation truncated another viral gene, vpr, raising the possibility that vpr actually increases HIV susceptibility to APOBEC3G:

These unexpected results suggested that Vpr might actually facilitate APOBEC3G-dependent restriction by, for instance, packaging a cellular factor.

Finally, the authors raise the possibility that the mutant HIV might have discovered a solution already in use by other viruses:

the APOBEC3G “resistance by tolerance” mechanism described here might well apply to many viruses. … Therefore, the ability to tolerate and perhaps even regulate APOBEC3-dependent restriction has clear implications for virus evolution, immune escape, and drug resistance.

  1. Sheehy AM, Gaddis NC, Choi JD, Malim MH (2002) Isolation of a human gene that inhibits HIV-1 infection and is suppressed by the viral Vif protein. Nature 418:646-650.[]
  2. Sheehy AM, Gaddis NC, Malim MH (2003) The antiretroviral enzyme APOBEC3G is degraded by the proteasome in response to HIV-1 Vif. Nat Med 9:1404-1407.[]
  3. Lecossier D, Bouchonnet F, Clavel F, Hance AJ (2003) Hypermutation of HIV-1 DNA in the absence of the Vif protein. Science 300:1112.[]
  4. For example: Izumi T, Shirakawa K, Takaori-Kondo A (2008) Cytidine deaminases as a weapon against retroviruses and a new target for antiviral therapy. Mini Rev Med Chem 8:231-238.[]
  5. Hache G, Shindo K, Albin JS, Harris RS (2008) Evolution of HIV-1 Isolates that Use a Novel Vif-Independent Mechanism to Resist Restriction by Human APOBEC3G. Curr Biol 18:819-824 []
  6. On the other hand, a therapeutic might be easier to escape from by mutating vif away from, say, the drug binding site. This experiment wasn’t designed to test that, but rather the underlying question of whether attacking vif is a good long-term strategy in the first place.[]