We plant virologists – or almost-former plant virologists, in my case – have a slightly cynical attitude towards the all-new, all-encompassing craze among cell biologists and mainstream virologists that is siRNA, and all its purported applications.
This is because the phenomenon of RNA silencing was first discovered in the context of unexpected resistance in transgenic plants to the homologous virus, mediated by transgenes that either produced no measurable amount of protein, or mRNAs that were not translatable. The phenomenon was known as “post-transcriptional gene silencing” back then, among plant molecular biologists and virologists – until, that is, it was taken up by the mammalian and insect cell biology folk, whereupon its origins were quickly forgotten, and the Nobels went to…well, let us just say, not to whom some folk thought they ought.
But I digress: suffice it to say that siRNA has now been amply demonstrated to be not only the eukaryotic cell’s (and especially those of plants) adaptive nucleic acid-based immune response to virus infection, but also a widely used means of regulation of gene expression (see here). Needless to say, its potential uses for gene and disease therapy are also multiplying daily – which is when people forget the roots of the science. At first sight, the New Scientist issue of 23rd March – which has an excellent article on the use of siRNA-based strategies to combat insect pests – goes some way to redressing that, given that the science has found its way back to plants. It is especially interesting that delivering siRNA-eliciting constructs in insects can be achieved by simply feeding them dsRNA of the appropriate sequence, rather than by use of chemically-altered or encapsulated material.
However, and here’s where one can see that no-one except plant virologists reads the plant virology literature (the converse being untrue, naturally), a problem crops up when the article goes on to discuss whether or not dsRNA is safe. In a side box in the article, this is said:
Is it safe?
Using gene silencing, or RNA interference (RNAi) to target specific pests while leaving other species unharmed sounds like an enormous step forward. But can we be sure that the key ingredient – double-stranded RNA (dsRNA) – won’t have unexpected side effects in people?
Although most RNA in cells is single-stranded, all plants and animals also produce dsRNA to regulate the activity of their own genes. “There are lots of dsRNAs in the plant and animal products that we eat every day,” says Michael Czech, a molecular biologist at the University of Massachusetts Medical School in Worcester, who is exploring ways to use RNAi to treat type II diabetes. “Those RNA molecules are rapidly chopped up by the enzymes in our gut and are non-toxic.”
There is also a second line of defence in the form of enzymes in our blood that break down dsRNA. “You could inject dsRNA into primates at moderate doses and nothing would happen, [my emphasis]” says Daniel Anderson of the David H. Koch Institute for Integrative Cancer Research at the Massachusetts Institute of Technology, who is designing drugs based on RNAi.
Ummmm…sorry, that simply isn’t true: as long ago as the early 1970s, plant and fungal virologists had hit on the idea of using antibodies specific for dsRNA to detect the molecules in extracts of hosts infected with dsRNA (and even ssRNA) viruses. Here’s two examples of papers from then:
Detection of mycoviruses using antiserum specific for dsRNA.
Moffitt EM, Lister RM. Virology. 1973 Mar;52(1):301-4.Immunochemical detection of double-stranded ribonucleic acid in leaves of sugar cane infected with Fiji disease virus.
Francki RI, Jackson AO. Virology. 1972 Apr;48(1):275-7.
And of course, using antibodies to dsRNA implies that one can raise them in the first place…which, as I recall (my career started shortly afterwards, and I read these papers), was as a consequence of raising antisera to dsRNA-containing phytoreovirus and cryptovirus virions. It was later found that sera to synthetic ds oligoribonucleic acids also detect dsRNAs – all of which means that injection of dsRNAs is likely to elicit antibodies against them. With who knows what corollaries…because not too many plant virologists were too worried about long-term effects in the mainly bunnies that they injected.
Of course, the route to dsRNA pesticide effects is via the insect gut – and the same New Scientist article has this to say about dosing humans thus:
“There are lots of dsRNAs in the plant and animal products that we eat every day,” says Michael Czech, a molecular biologist at the University of Massachusetts Medical School in Worcester, who is exploring ways to use RNAi to treat type II diabetes. “Those RNA molecules are rapidly chopped up by the enzymes in our gut and are non-toxic.”
Ye-es…they would say that, wouldn’t they? And no-one knew you could make Abs to dsRNA before someone noticed phytoreovirus antiserum bound dsRNA – so it would be a very interesting exercise to assay sera from individuals or animals previously exposed to multiple rounds of dsRNA rotavirus infection, and see whether these contained dsRNA-specific Abs, wouldn’t it? The NS article negates some of its own reassurances by saying:
So there is good reason to think sprays containing dsRNAs lethal to insects, or plants modified to produce them, will pass all the safety tests. However, if the RNA was altered in a way that allows it to get into human cells, perhaps as a result of changes intended to make it persist longer in the environment, it might cause problems. “If you modify the dsRNA – by encapsulating it or changing the RNA molecule – then you are imposing a new chemistry that could have toxic effects on humans,” Czech cautions.
Yeah – like encapsidating it like a virus does….
And it is all probably a bit of a sideshow, in that we are in fact exposed to dsRNAs in our diet all the time: especially if one eats organic, the fresh fruits and uncooked vegetables will be rife with dsRNA-containing fungal viruses; even material containing ss+RNA viruses contains significant amounts of replicative form dsRNA (including insect material, BTW) – so a little more used as pesticide probably wouldn’t hurt.
We hope.
ViroBlogy 