Since antiretroviral therapy requires a lifelong treatment regimen, research to find more permanent cures for HIV infection is currently underway. It is possible to synthesize zinc finger nucleotides with zinc finger components that selectively (almost selectively) bind to specific portions of DNA. Conceptually, targeting and editing could focus on host cellular co-receptors for HIV or on proviral HIV DNA.
Host cellular co-receptors for HIV It has also been observed that 20% of the Caucasian population possess a mutation, called CCR5-Δ32 (frequency of 0.0808 for homozygous allele), that prevents the CCR5 chemokine receptor protein, which is the main means of viral access into the cell, from being expressed on the surface of their CD4+ T-cells. Individuals who are homozygous for this mutation are immune to HIV strains that use the CCR5 receptor to access the cell, while those that are heterozygous for this mutation have been found to reduce plasma viral load and delay progression of AIDS. By combining these facts, researchers have proposed a novel method of treatment for HIV. This method attempts to treat the infection by disrupting the CCR5 gene, such as introducing the CCR5-Δ32 mutation using a recombinant adenoviral vector or forcing DNA repair by nonhomologous end joining, which is prone to error and results in a non-functional gene. As a consequence, resulting in the expression of nonfunctional CCR5 co-receptors on CD4+ T cells, providing immunity against infection. The zinc finger nucleases that have been synthesized for this treatment are manufactured by combining FokI Type II restriction endonucleases with engineered zinc fingers. The number of zinc fingers attached to the endonuclease controls the specificity of the ZFN since they are engineered to preferentially bind to specific base sequences in DNA. Each ZFN is made up of multiple zinc fingers and one nuclease enzyme.
Proviral HIV DNA A recent and unique application of ZFN-technology to treat HIV has emerged whose focus is to target not the host genome, but rather proviral HIV DNA, for mutagenesis. The authors of this work have drawn their inspiration from the innate defense mechanism against bacteria-infecting-viruses called bacteriophages, present amongst those bacteria endowed with restriction modification (R-M) systems. These bacteria secrete a restriction enzyme (REase) that recognizes and repetitively cleaves around palindromic sequences within the xenogenic DNAs of the bacteriophages or simply phages, until the same is disabled. Further support for this approach resides in the fact that, the human genome comprises in large part remnants of retroviral genomes that have been inactivated by several mechanisms, some of whose action resembles that of ZFN. It should not be surprising, therefore, that the initial work leading to the application of ZFN technology in this manner revolved around and involved the isolation and testing of HIV/SIV targeting bacteria-derived REases, whose non-specificity (due to their short recognition sequences) unfortunately, rendered them toxic to the host genome. The latter-potential host-genome toxicity posed by the raw bacteria-derived REases limited their application to ex-vivo modalities for HIV prevention, namely synthetic or live microbicides. Subsequently, however, the unique specificity offered by ZFNs was quickly recognized and harnessed, paving way for a novel strategy for attacking HIV in-vivo (through target mutagenesis of proviral HIV DNA) that is similar to the manner by which bacteria equipped with R-M systems do, to disable the foreign DNAs of in-coming phage-genomes. Because latent proviral HIV DNA resident in resting memory CD4 cells forms the major barrier to the eradication of HIV by highly active antiviral therapy (HAART), it is speculated that this approach may offer a 'functional cure" for HIV. Both ex-vivo (manipulation of stem or autologous T cell precursors) and in-vivo delivery platforms are being explored. It is also hoped that, when applied to non-HIV infected persons, this strategy could offer a genomic vaccine against HIV and other viruses. Similar work is ongoing for high-risk HPVs (with the intent of reversing cervical neoplasia) as well as with HSV-2 (with the goal of achieving a complete cure for genital herpes)
… excerpt ends here. Continue reading the full article.
