Nailing the AIDS virus with an effective vaccine or anti-viral isthe goal of researchers on every continent. What makes theirmission a little like nailing Jell-O to the wall is the slipperynature of HIV, an elusive quick-change artist.
A molecular silver bullet aimed at one or another viral subunittarget may hit a bull's eye today, but will be history tomorrow,when the virus changes its genomic spots. That is why much ofthe ongoing research seeks segments of HIV's genetic makeupthat remain relatively conserved -- that don't change veryoften from one strain to another. When such a non-movingtarget is pinpointed, the trick will be to find a technique thatcan disable it and stop the virus dead.
The current Proceedings of the National Academy of Science(PNAS) carries a report by Mang Yu, Flossie Wong-Staal, et al.of the University of California, San Diego (UCSD), titled "Ahairpin ribozyme inhibits expression of diverse strains ofhuman immunodeficiency virus type 1." In 1984, Wong-Staalwas the first person to clone the AIDS virus and among thefirst to solve its molecular architecture.
Ribozymes, as the portmanteau word suggests, are bits orpieces of ribosomal nucleic acid that behave like enzymes. Thatis, they cleave other RNA molecules.
"Ribozymes have enormous potential as anti-viral agents," saidWong-Staal. She and her co-workers, led by the paper's firstauthor, molecular biologist Mang Yu, constructed a murineretrovirus carrying the gene for a catalytic ribozyme controlledby a specific promoter that targets some of HIV's conservedAchilles-heel RNA.
They equipped their hairpin ribozyme (named after itsgenomic contour) to seek out and sever a specific site on the 5-prime long terminal repeat leader sequence of HIV-1. To givethe ribozyme gene this specificity, they put it under thecontrol of a polymerase III promoter.
When they inoculated cultures of HIV-infected human Helacells with this package, it duly chopped the target sequence intwo, with up to 95 percent success in inhibiting viralreplication.
What's more, the technique worked on many diverse strains ofthe AIDS virus, including the Zairean Eli version. Only in theNorth American MN strain did the 5' leader vary, by a one-base mutation, which somewhat diminished viral inhibition.
Ribozymes come in two main configurations, hairpin andhammerhead. Most of the academic and industrial playersvying to sic their pit-bull ribozymes onto HIV, said Yu, usehammerheads. His group prefers the hairpin model, which, hetold BioWorld, "is closer to in vivo conditions and might confergreater stability. Most important of all, it works."
Since submitting their results to PNAS last January, the SanDiegans have taken several giant steps toward eventual humangene therapy. These involve replacing their laboratory Hela cellline with T cells from fresh human blood, and laboratory HIV-1strains with viruses from actual AIDS patients
As Yu explained, these viruses are already mixtures of manystrains, so it is possible to test the ability of ribozymes tocleave an array of conserved sites with a single experiment.
Yu quotes Wong-Staal, a full professor of biology and medicineat UCSD, who told the Ninth International AIDS Conference inBerlin, "We have no time to waste." Ergo, she and her team arepushing for early clinical trials of their gene-therapy technique.
They plan to ask the Recombinant DNA Advisory Committee(RAC) of the National Institutes of Health, which meets inSeptember, for permission to conduct preliminary clinical trialsto gauge toxicity
Their trial design is similar to that of the very first RAC-authorized ex vivo human gene therapy experiment, whichtreated two young girls with SCID. The ribozyme genes, inretroviral vectors, will incubate blood cells taken from thesubjects, then reinfused. At a later stage, Yu said, "our moreambitious plan is to perform the ex vivo therapy on bone-marrow stem cells rather than mature T cells.
The UCSD group is very excited about its data, he told BioWorld,but also very anxious not to raise any false hopes among AIDSpatients.
One of the major non-academic players in the ribozyme assaulton HIV is Ribozyme Pharmaceuticals Inc. of Boulder, Colo. Cellbiologist Ken Draper, a senior scientist at RPI, pointed to themost salient advance in the "very positive paper in PNAS." Hetold BioWorld, "Its most important point is that, given thegenetic drift of HIV, they actually showed efficacy againstmultiple strains."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.