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The Scientific Trenches: An Insider's Perspective

How a Nobel Prize in Physiology Influences Tomorrow’s Varroa Control

- January 1, 2018 - Alison McAfee - (excerpt)

Every beekeeper has wrestled with the Varroa destructor mite. Every beekeeper also knows we’re running out of weapons. With abounding resistance to conventional miticides, researchers at Monsanto are instead trying to use biotechnology to fight the mite – it could work, but is another mite-killing agent what the industry really needs?

Recently, I had the chance to listen to talks from two members of Monsanto’s bee research team: Jerry Hayes (America Bee Journal’s own respected author of The Classroom) and Alex Inberg. Hayes was a keynote speaker at the British Columbia Honey Producers’ Association annual general meeting in Kelowna last October, and Inberg spoke at the Entomological Society of America conference in Denver last November. Both highlighted how Monsanto is developing a new method of varroa control using a biotechnology technique called “RNA interference” (RNAi for short). By feeding honey bee colonies with syrup containing specific RNA molecules, they can suppress the growth of mite populations. It is not yet ready for market and it’s no silver bullet, but it’s an intriguing endeavor for many reasons (scientifically and otherwise).

RNAi took molecular biology by storm over the last few decades, earning its discoverers (Andrew Fire and Craig Mello) a Nobel Prize in 2006. In the late 1990’s, they found that injecting small worms (Caenorhabditis elegans) with a certain kind of RNA molecule could change the worms’ behavior, and deduced that this happened because they were disrupting the regular flow of genetic information.1 Cells contain DNA in the nucleus, which is normally transcribed into smaller molecules called messenger RNA (mRNA) that travel outside the nucleus to be translated into proteins (Figure 1A). DNA is double-stranded (think of the famous double helix structure) whereas mRNA is single-stranded (we call it the “sense” strand), but it too has the potential to form a double-strand if it’s given the right complimentary partner (the “antisense” strand). Fire and Mello found that when the sense and antisense RNA strands were combined to form double-stranded RNA and injected into the worm, the gene corresponding to the sense sequence (the one that’s needed to create a functional protein) was silenced and the worms began to twitch. So what was going on?

The gene that Fire and Mello were targeting was called unc22, which is normally produced in muscle cells and is important for muscle contraction and motility. Under regular conditions, the unc22 DNA sequence is transcribed into the mRNA messenger and translated into the unc22 protein (the lack of italics here isn’t a mistake – it’s a weird convention in the field that gene names are italicized, and protein names aren’t). This is the normal flow of information for any gene.

Injecting double-stranded unc22 potently suppressed expression of that gene, causing the worms to lose muscle control (hence the twitching). After Fire and Mello’s seminal paper in 1998, many other researchers participated in elucidating the key molecular players (Figure 1B) – as it turns out, the double-stranded RNA is cut up into smaller pieces by a protein called, fittingly, “Dicer,” and incorporated into a bigger protein complex called “RISC.” RISC uses these bits of RNA like puzzle piece detectors to seek and destroy normal mRNA molecules that have the matching piece, or complimentary RNA sequence. When the normal mRNA molecule is destroyed, it can’t be translated into a protein and the function it normally serves is lost. So, when unc22 double-stranded RNA is injected into worms, the end result is suppression of the unc22 protein production and a subsequent loss of muscle function.

This technology hasn’t escaped Monsanto’s attention for applications in pest control. Except for prokaryotes, nearly every living thing has the molecular machinery RNAi requires, including many plants, insects, and mammals. However, this doesn’t mean that the RNA treatments are also universally toxic – that would be bad for business. Since the researchers control the double-stranded RNA sequence, they also control the genes they decide to shut down. To target one organism over another (say, varroa, but not honey bees), we just need to find an essential gene sequence that one species has, but the other doesn’t.

In the case of varroa, there’s a gene expressed in the foundress mite which is needed for reproduction. Sometimes there can be surprising off-target effects, so it all requires rigorous testing, but we are pretty good at predicting what genes will and will not be affected.

“The RNA isn’t toxic to humans,” Hayes told the crowd at the Kelowna meeting. “You eat it – we all eat RNA in our regular food. And if you spill it on the ground, it quickly breaks down.” RNAi treatments are often marketed as “non-synthetic,” but that description is imprecise; the double-stranded RNA molecules are synthetic – large amounts of purified enzymes are used to synthesize it in controlled chemical reactions. The difference between RNAi and conventional treatments is in the molecule’s properties and the mechanism of action.

The formulation isn’t corrosive, like conventional acid varroa treatments, nor should it accumulate in the wax of the hives. Risk of ….