Inhibiting HIV-1's Pathogenic Efffects

APOBEC3G Inhibits Elongation of HIV-1 Reverse Transcripts is a PLOS Pathogens paper authored by Kate N. Bishop, Mohit Verma, Eun-Young Kim, Steven M. Wolinsky and Michael H. Malim. The abstract:

APOBEC3G (A3G) is a host cytidine deaminase that, in the absence of Vif, restricts HIV-1 replication and reduces the amount of viral DNA that accumulates in cells. Initial studies determined that A3G induces extensive mutation of nascent HIV-1 cDNA during reverse transcription. It has been proposed that this triggers the degradation of the viral DNA, but there is now mounting evidence that this mechanism may not be correct. Here, we use a natural endogenous reverse transcriptase assay to show that, in cell-free virus particles, A3G is able to inhibit HIV-1 cDNA accumulation not only in the absence of hypermutation but also without the apparent need for any target cell factors. We find that although reverse transcription initiates in the presence of A3G, elongation of the cDNA product is impeded. These data support the model that A3G reduces HIV-1 cDNA levels by inhibiting synthesis rather than by inducing degradation.

Humans and other host organisms are able to draw on their own viral defense resources in order to inhibit the replication of the human immunodeficiency virus known as HIV-1. The question is how is this accomplished? Researchers focused on an enzyme dubbed APOBEC3G (A3G) to get answers. A3G belongs to a class of enzymes called deaminases which catalytically cause the removal of an amino group from a compound, generally by hydrolysis. But what strategy does A3G employ in inhibiting viral replication? Does it induce the degradation of viral DNA or does it inhibit synthesis. Researchers, authoring this paper, believe the latter occurs.

Replication of HIV-1 occurs through a process called reverse transcription. Retroviruses, including HIV-1, coopt host cell resources to enable their own viral functions. The replication of their own genome is accomplished with cellular assistance and a viral enzyme known as reverse transcriptase which makes reverse transcription possible. During the process an RNA viral genome is transcribed to complementary DNA (cDNA). The authors reveal in their paper their prior belief that viral cDNA would be stripped of uracil nucleotides by cellular enzymes called DNA glycosylases. Resulting abasic sites, sometimes referred to as hypermutation, would induce cleaving by cellular endonucleases causing subsequent degradation of cDNA. Yet the discovery that the inhibition of viral replication can take place in the absence of hypermutation led researchers to suspect that something more was afoot. They posed the logical question: what would account for cDNA degradation if not enzymatic editing?

The answer to the question appears to lie with a disruption of reverse transcription. Reverse transcription is multi-step process. A3G seems able to, at least, partially disrupt some steps thereby inhibiting viral replication. In the words of the authors:

Together with existing data, our results support the proposal that A3G inhibits the elongation of HIV-1 DNA by reverse transcriptase, probably by steric hindrance, rather than promoting the degradation of viral cDNA.