In a setback for a novel approach to curing HIV, researchers involved in a technique that uses enzymes to remove viral genes from the DNA of infected cells have found that HIV rapidly develops resistance to the guide molecules that target the correct part of the DNA sequence. The resistant viruses that develop can, in some cases, replicate even faster than viruses not exposed to the gene therapy (though they are still susceptible to conventional antiretroviral (ARV) drugs).
Furthermore, the researchers suggest, the way the gene therapy works may actually promote the development of resistance, as it actively creates small mutations at the site where it bisects the cellular DNA.
Resistance arises rapidly – within 8-10 days of the therapy first starting to work, according to aidsmap.com
This does not mean the whole gene-splicing approach is doomed to failure, but it does imply that the gene-degrading enzyme would need to be attached to a variety of different gene probes, designed to attach to a number of different points on the viral DNA hidden within the human DNA inside infected cells.
The gene technology involves transporting a DNA-degrading enzyme called CRISPR or cas9 into the heart of the nucleus of human cells. The cas9 enzyme, which was originally found within bacteria as a natural defence against viruses, is attached to a single-strand length of ‘guide RNA’ (sgRNA) that guides the cas9 to the particular piece of rogue DNA that needs to be removed.
The concept is not dissimilar to the versatile gene therapy called short-interfering RNA (siRNA), which is being investigated for a number of diseases including chronic hepatitis B. But whereas siRNA targets and degrades the RNA messenger and component molecules that act as the replication machinery of viruses within the main part of the cell (cytoplasm), sgRNA/cas9 targets integrated DNA, the ‘master template’ for viral manufacture that retroviruses like HIV insert into a cell’s core genetic instructions, and which exists in the nucleus, not the surrounding cytoplasm.
According to Gus Cairns of AIDSMAP:
“.. the sgRNA/cas9 itself that was causing the resistance in these cases. The cas9 was deranging the DNA at the cleavage site in such a way that it was producing resistant viruses itself.
In this case, resistance was not being caused by viral turnover in the presence of low levels of drug, that exert a selective evolutionary pressure – it was being caused directly by the drug itself. In short, sgRNA/cas9 was acting as a mutagen, a direct driver of viral mutation”.
REF: Wang Z et al. CRISPR/Cas9-derived mutations both inhibit HIV-1 replication and accelerate viral escape. Cell Reports, 15, pages 1-9. DOI: http://dx.doi.org/10.1016/j.celrep.2016.03.042. April 2016.