It's a proven fact that antisense constructs are able to inhibitgene expression by binding to nucleic acids, particularly single-stranded RNA, where they interfere with its ability to directthe synthesis of proteins.
But the high concentrations of oligodeoxynucleotides normallyneeded to achieve this result can elicit non-specific as well asspecific effects. Researchers at Gilead Sciences Inc.(NASDAQ:GILD) of Foster City, Calif., have improved theproperties of those "first-generation" antisense constructs bysynthesizing a new class of oligodeoxynucleotides that are morepotent and bind more tightly to their targets.
The potency in vitro is about 100-fold more than the earliergenerations of generally available oligos, according to MichaelRiordan, president and chief executive officer of Gilead. "Thatpotency is primarily a function of the fact that the newconstructs bind very tightly to the target RNA," he added.
Reporting in last Friday's issue of Science, Gilead researcherRichard Wagner and his colleagues described the modifiedantisense constructs -- C-5 propyne-substitutedphosphorothioate oligodeoxynucleotides -- and their ability toinhibit gene expression in a sequence-specific manner.
The researchers analyzed the modified constructs' ability toinfluence the expression of two different genes, the SV40 largeT antigen and E. coli beta galactosidase, in cultured mammaliancells. Not only did they determine that the modified oligos weregene-specific, but they also determined that the potency ofinhibition of expression depended on the length of theconstruct.
Both the activity and the affinity deminished with decreasingmolecule length. Still, the modified constructs can be shorterthan "first-generation" antisense molecules. "With this newclass we've shown we can make them shorter than before andretain sufficient binding affinity," Riordan told BioWorld. That'sbecause the modified bases have a higher affinity for thetarget.
Even so, a mismatch of even one base out of 15 on theantisense construct vis-a-vis its target sequence can reduce theability of that construct to inhibit expression. And after that,"there's a rapid fall-off in activity (as the number ofmismatches increases)," explained Riordan.
That a slight degree of base mismatch still allows someinhibition of gene expression might actually be a positive factorwhen it comes to considering the therapeutic use of antisensecompounds in humans. "The flexibility might be preferable if agene has two alleles that differ by one base. We could hit bothof them this way," Riordan said.
These new antisense molecules are obviously not ready forclinical applications yet, but they could provide the means tospecifically inhibit disease-causing genes, as long as thesequences of those genes are known. In fact, "many hundredsof clinically relevant genes have already been sequenced,including for pathogens, oncogenes and growth regulators,"Riordan explained.
Gilead is developing this new class of antisense molecules aspart of its codeblocker research program, a collaboration withGlaxo Inc. that is aimed at blocking the genetic code of disease-causing genes, including those involved in cancer.
-- Jennifer Van Brunt Senior Editor
(c) 1997 American Health Consultants. All rights reserved.