Gene gold turning to dust?
Governments are sinking further billions into genomics and related research but a new study finds no sign of revolution in healthcare.
ISIS Press Release 05/04/05
Controversy over Gene Therapy
‘Breakthrough’
A ’precision’ gene therapy turns out to have significant
off-target effects Dr. Mae-Wan Ho
The source for this
article is posted on ISIS members’ website.
Details here
Collateral damage from ‘precision’ gene therapy
A gene therapy technique, hailed as 2002’s ‘breakthrough of
the year’ in its ability to shut down specifically and
precisely any chosen gene, has been found not to be so specific or
precise after all. The technique involves RNA interference ("Subverting the
genetic text", SiS
24), the ability of a short specific duplex sequence of RNA to target the
transcript of gene, thereby shutting it down. Unfortunately, there are
"off-target" effects on other genes and proteins.
The technique depends on a perfect match between the siRNA (small
interfering RNA) introduced and its complementary sequence in the gene
transcript. Only sequences of 19-21 base pairs are generally used, as longer
sequences induce nonspecific immune reactions.
However, various mismatches between the siRNA and its target appear to
be tolerated, so that other transcripts with similar sequences are also
affected.
Peter Linsley, executive director of cancer biology for Rosetta
Inpharmatics, a company based in Seattle, Washington, USA, and a subsidiary of
drug giant Merck, was using siRNAs to design more targeted drugs. The plan was
to use siRNA to shut down a particular gene, and then add a compound that also
targets the gene to see if additional genes are affected.
But his team found that the siRNAs were shutting down more genes than
just the one intended. "The siRNAs were dirtier than our compounds," Linsley
said. They kept finding the same results, and finally concluded that the siRNA
could "cross-react" with other genetic targets. They had trouble convincing
reviewers to get their results eventually published in Nature
Biotechnology in June 2003.
At first skeptical of the findings, the RNA interference community was
gradually prodded by Linsley’s work to look more carefully at their own
findings. "We saw more and more unexplained phenomena," admitted Rene Benards,
a cancer geneticist at the Netherlands Cancer Institute in Amsterdam.
Similarly, Phillip Zamore, a biochemist at the University of Massachusetts
Medical School in Worcester, now believes that the limitation of the technique
should have been obvious, and it was "incredibly unreasonable" to have presumed
absolute specificity.
Using microarrays to screen for off-target effects, researchers are
finding in general that a dozen genes may be affected by a single siRNA;
although Linsley has recorded on average at least 40genes affected. But
microarrays only show the effect on RNA transcripts, and not on proteins (and
microarrays themselves are proving unreliable, see "Biotech wonder tool in
disarray", this series). So the off-target effects could be even more
extensive.
This is especially understandable in hindsight, as geneticists have
discovered numerous species of microRNAs interfering naturally and copiously in
gene and protein functions. Putting in siRNAs is rather like throwing a monkey
wrench randomly into the incredibly complicated and sophisticated machinery
that RNA interference has turned out to be.
Predictably, proponents remain hopeful that such off-target effects may
not matter, and could be addressed by further research.
The first clinical trial of siRNA therapy was launched in October 2004
by the Philadelphia company Acuity Pharmaceuticals on macular degeneration, a
breakdown of the small area at the back of the eye that’s responsible for
acuity in vision, which causes blindness. Because the treatment is restricted
to the eye, it is hoped that the risk of off-target effects is of less concern.
Another candidate for treatment is hepatitis B, where the hope is that the
siRNA could disable the virus without causing too much off-target damage.
But it would be irresponsible to proceed in the absence of further
research, given the sorry record of gene therapy thus far ("Gene therapy woes", this series).
| 
