The Body Covers: The 42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
Novel Approaches to the Inhibition of HIV
September 30, 2002
Bear in mind that of the first three breakthroughs in science, only the co-receptors have yet had an impact on patients and on clinical care. As Dr. Cullen warned, his work may be at least five years from having a clinical impact. But if you are interested in some exciting science, read on.
HIV has several accessory genes that differentiate it from other lentiviruses. These include tat, rev, vif, vpr, vpu and nef. Of these, only tat and rev are absolutely required for the virus to function and replicate, making them potential treatment targets. Tat is a powerful transactivator of viral transcription. What that means is that the tat protein feeds back into the nucleus and accelerates the production of HIV RNA by more than 100 fold. This, in part, leads to feedback that lets the virus switch from making a small amount of regulatory proteins to making large amounts of structural proteins so it can assemble new virus. Rev is required to make multiple spliced RNA -- an essential step in making structural proteins. Both tat and rev have multiple other proven or postulated functions.
Tat antagonists have been explored as potential drugs, as have antagonists of the complex that tat creates with cellular proteins. Work continues, but no promising candidates have emerged. Dr. Cullen's presentation looked at a totally new and different way of blocking tat and therefore blocking virus.
He focused on two phenomena found in nature, RNA interference and micro RNAs, that destroy target RNA and are used to control protein production. RNA interference occurs when a double-stranded RNA is introduced that can bind to a target mRNA (the RNA used as a template for protein synthesis). These double-stranded RNAs are cleaved by a protein called "dicer" into 22 nucleotide RNAs which inactivate the target mRNA and block protein synthesis. It turns out that RNA interference is a major antiviral defense within plants. Experimentally, he showed that this can block HIV replication if one uses RNA interference to target tat. The problem so far is that there is no practical way to introduce long double-stranded RNA into cells.
Micro RNA is a somewhat similar process. It was discovered during basic science work on development, using the worm C. elegans. Micro RNAs are used in development to turn off the production of proteins at the appropriate time. These micro RNAs are produced as complex 70 nucleotide structures called "stem loop structures." The fun begins when dicer (the same enzyme as described above) cuts the stem loops into 22 nucleotide RNA segments that target and inactivate the specific mRNA. Dr. Cullen described research in which a transfection vector was created that led to the production of custom designed stem loop structures that created the micro RNA of interest. This allows you to specifically target and knock out protein production by one gene.
He showed preliminary experiments in which this transfection vector was used to introduce micro RNA targeted at tat into HIV-infected cells. HIV production was almost totally blocked! This is one of the most exciting and practical ideas to date for using gene therapy to stop HIV production! Keep that in mind next time you wonder why the U.S. National Institutes of Health funds worm research.
Of course many steps are necessary to prove this is safe and effective. A practical and safe way of transfecting a huge number of cells in an intact animal or person must be found. Nonetheless, this is beautiful science that may lead to exciting and new approaches to controlling HIV over the coming years.
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