A NICKNAME can sometimes be the kiss of death. It wasn鈥檛 so long ago that scientists at the US Department of Agriculture were crowing over a bit of genetic engineering they called the Technology Protection System, a way to make crop seeds sterile and so stop genetically engineered genes from spreading.
But anti-GM groups gave it a more sinister name: Terminator. They pointed out that the technology would put an end to the age-old tradition of saving seed from this year鈥檚 crop to plant next year, forcing poor farmers to buy new seed every year. Soon companies and researchers couldn鈥檛 move fast enough to distance themselves from the technology. But Pim Stemmer doesn鈥檛 seem too worried by the label an anti-GM group recently gave his new idea. 鈥淭hey call it Exorcist?鈥� he laughs. 鈥淣ow that鈥檚 good. I like it.鈥�
Exorcist is also a genetic trick, but it鈥檚 more cunning than Terminator. Rather than killing the seeds of GM plants, Exorcist splices out and destroys every bit of foreign DNA inside the seeds and fruit before the crop is harvested. The crop benefits from a GM trait but the food it yields is, in theory, GM free.
Advertisement
Stemmer bets that for a lot of people the stigma of GM food would vanish along with those foreign genes. And as vice-president of research at the biotech company Maxygen in Redwood City, California, he is confident he could turn that bit of molecular magic into reality.
鈥淚 think this is a compromise between two reasonable policies,鈥� says Stemmer. 鈥淔armers get to use GM plants and consumers get GM-free food.鈥� And he says the benefits would go way beyond making food more palatable to people simply spooked by the thought of GM. Consumer concerns over allergic or toxic reactions to proteins from the newly introduced genes will evaporate. Farmers could plant their crops without worrying that some errant gene will escape via pollen into other crops or plants. And since the valuable genes would vanish at the end of the crop鈥檚 life cycle, companies would get extra protection against theft of the GM crops they created.
And while Exorcist already has its foes, it has fans as well. 鈥淭his is revolutionary in a lot of ways,鈥� says William Muir, a geneticist at Purdue University who studies gene flow between GM and non-GM organisms. 鈥淚n one fell swoop it seems to address a lot of concerns people have about GM plants.鈥� And according to Stemmer, giving all sides new benefits is the perfect way to bring the GM food debate to a close.
Stemmer and his Maxygen colleague, Robert Keenan, first floated their idea a few months ago in a commentary in the journal Nature Biotechnology (vol 20, p 215). They point out that in GM agriculture as it is currently practised, genetic changes are wholesale and permanent. Engineers insert DNA containing a foreign gene into a plant cell, where it finds a home by slipping into a chromosome. If that cell gets used to make a new plant, the gene and the protein it produces will be in every tissue of that plant. And once it is there, the foreign gene is in every descendant that inherits the altered chromosome.
Critics say that those genes and proteins could trigger nightmarish disasters such as toxic side effects or allergies in people who eat them. And those fears have been fed by reports like those from Arpad Pusztai, a plant researcher in Scotland who announced four years ago that rats who dined on GM potatoes suffered irregularities in their immune system and changes in their internal organs. However, other researchers found serious flaws in Pusztai鈥檚 experiments and few now believe his claims.
More recently, another furore erupted when StarLink, a type of GM corn only approved for animal feed, turned up in grocery stores in taco shells and other products intended for human consumption. The worry was that the gene introduced into the corn, for an insecticidal protein called Cry9c, might trigger allergies in some people. Again, there was no evidence that anyone had been hurt. But the incidents weren鈥檛 good publicity for GM food safety.
Exorcist, by banishing all foreign DNA from the plant or just its fruit or seed, would make those fears moot, says Stemmer. 鈥淚 personally believe GM foods are very safe, so this would be a cosmetic change,鈥� he adds. 鈥淏ut if it made people more comfortable with GM products, it would be worth it.鈥�
A much more serious concern to Stemmer is the potential negative environmental and commercial impact of gene leakage from transgenic plants into unmodified crops or even wild plants. The most recent fiasco involved a report in Nature that claimed transgenic genes had infiltrated traditional maize crops in Mexico and were causing genetic havoc (麻豆传媒, 15 June, p 14). This research has been questioned, but plant companies concede that their transgenes may have inadvertently slipped into crops, even in their own greenhouses. Here too, Exorcist would solve that problem by removing foreign genes from pollen before it matured.
Indeed, Exorcist incorporates three features that were at the heart of a 2000 report by the Advisory Committee on Releases to the Environment (ACRE), which advises the British government (麻豆传媒, 28 Oct 2000, p 4). It recommended that companies engineer plants to limit gene flow, switch genes on only where they are needed and incorporate as little foreign DNA as possible. 鈥淚f you can excise all the DNA that may cause the problems, so much the better,鈥� says Brian Johnson, biotechnology adviser with English Nature and an adviser to ACRE. 鈥淚t鈥檚 encouraging that people are beginning to think this way.鈥�
Here鈥檚 how Exorcist works. At its heart is a little protein called Cre, borrowed from a virus that usually infects bacteria. Cre is an enzyme called a 鈥渞ecombinase鈥�, a pair of molecular scissors that can snip out any DNA that lies between two copies of a short marker sequence called loxP. Cre and enzymes like it are already widely used to manipulate DNA in plants and animal cells. But it was Stemmer鈥檚 insight that it could be used to excise all the genes that plant biologists were splicing into crops.
He suggested that alongside the gene for a trait you want to engineer in, say, herbicide resistance, you also insert a gene for Cre and flank the entire piece with loxP markers. Finally you insert a regulatory DNA site, or promoter, to turn Exorcist on exactly where and when it鈥檚 needed (see Diagram). In the edible part of the crop that usually means at the early stage of development, while in the reproductive organs it might be before the pollen starts to ripen.
To achieve such exquisite control over the timing and location of the exorcism, Stemmer suggests using a tissue-specific promoter. These are naturally-occurring sequences which switch genes on only in a particular tissue at a particular time.
For example, to delete a foreign gene from fruit, you could choose a promoter that normally activates genes involved in the fruit鈥檚 early development. As those first few cells start to differentiate into fruit cells, the Exorcist system also springs into action, excising all the DNA between the recognition sites. All that鈥檚 left in the fruit cell chromosome is a tiny footprint34 DNA letters long 鈥� a loxP site 鈥� but no active genes making foreign proteins. The excised DNA forms a loop that usually breaks down and disappears in a few days, or is simply diluted into insignificance by cell division. By the time it鈥檚 harvested the fruit is GM-free.
Tissue-specific promoters aren鈥檛 the only option. If you prefer more hands-on control, you can use a promoter that is inactivated by an externally-applied chemical such as ethanol. Under normal conditions, the promoter switches on the Cre gene and Exorcist does its work in every tissue of the crop plant. But spray the field with ethanol and you shut down the system and the foreign genes stay in place. That way, farmers could remove foreign genes from plants destined for the dinner table, but hang onto them in plants they want to use for seed crop. And if those genes should escape from that seed crop in pollen, they would simply splice themselves out of their new host and disappear.
Stemmer suggests that labs could even evolve the Cre recombinase to recognise native plant DNA sequences instead of loxP. That would mean you could even remove that tiny DNA footprint, leaving nothing but natural DNA. 鈥淚t really surprises me that no one had proposed this before,鈥� says Stemmer.
On paper the system looks fairly simple. The necessary components, such as Cre, all exist as research tools, or, like the tissue-specific promoters, are something plant biotech companies are eager to develop.
But will it really work? Stemmer cites the work of Nam-Hai Chua of New York鈥檚 Rockefeller University who showed the Cre excision is already almost 100 per cent successful in plant cells (Nature Biotechnology, vol 19, p 157). But Chua is sceptical about Exorcist. He points out that in his research, excision from plant tissues was very efficient. But in the seeds it was incomplete, ranging from 22 to 69 per cent. 鈥淎nd out in the field, the plant is going to be subjected to all sorts of unpredictable insults,鈥� he says. 鈥淚n the lab, they are taken care of like babies. I鈥檓 not saying it鈥檚 not going to work, but they have to prove it does.鈥�
David Ow of the US Department of Agriculture鈥檚 Plant Gene Expression Center in Albany, California, one of the pioneers in using the Cre/loxP system in plants, adds an even more bizarre twist. He says that in some of his experiments the excised DNA doesn鈥檛 disappear but mysteriously persists inside plant cells in a form Ow hasn鈥檛 yet been able to figure out. 鈥淵ou are going to need some elegant and fine genetic engineering to make this really reliable,鈥� he says. 鈥淏ut if it does work, it could have real benefits for people who are anti-GM. Someone is going to give it a try.鈥�
Stemmer is undeterred. He points out that many of the components of Exorcist are already being worked on for other reasons, such as controlling expression of genes in different parts of plants. He also points out there are other recombinases more likely to leave the DNA in a form that is more amenable to breaking down. While he agrees that nothing can be guaranteed at 100 per cent, he thinks some GM residue, offending genes in less than 1 per cent of cells, for example, could be tolerated in much the way as some trace of pesticide or dirt is tolerated now.
In fact, there is hardly a technical problem that Stemmer hasn鈥檛 got a fix for. For instance, you might think that removing the genes from the seeds or fruit might rule out improving their nutritional content, a fundamental goal of so-called 鈥渟econd-generation鈥� GM technology. Not so, says Stemmer. Because seeds draw most of their nutrients from elsewhere in the plant, it would still be possible to boost vitamins and other nutrients. What about genes you want to stay in the seed during growth such as those that might protect it from insects? He would put those under a 鈥渄esiccation-inducible鈥� promoter so the gene would be exorcised as the grain dries after harvest. 鈥淭he technical part of this is will be the simplest part,鈥� he says.
The hardest part will be convincing GM鈥檚 biggest critics. 鈥淚t鈥檚 an attempt to 脭greenwash鈥� the issue, rather deceptive,鈥� says Sue Mayer of Genewatch UK. To her, the technology is really just Terminator 2, an attempt to pass off a technology that protects intellectual property as a food and environmental safety tool. 鈥淚 just had to laugh when I saw it.鈥� And she doesn鈥檛 trust Exorcist to behave as advertised. She points out that engineering plants has already had some unintended consequences. Recently, for instance, GM potatoes designed to be resistant to sap-sucking bugs became more susceptible to other types of insects (麻豆传媒, 1 June, p 17). Putting more genes and clever switches into plants is only going to increase the range of unforeseeable consequences, she says.
鈥淲hy should society accept a new, unproven technology to fix a defective one?鈥� agrees Hope Shand of action group ETC (formerly the Rural Advancement Foundation International), whose team coined the term Exorcist and were among the most vocal opponents of Terminator. She feels that Stemmer鈥檚 offer to banish the evil spirit of spliced genes is a tacit admission that transgenics are dangerous in the first place.
But Stemmer is convinced that if he can remove every detectable trace of engineered genes from food, he will convert 鈥渞easonable鈥� people to GM. 鈥淚f we go all this way to address their concerns, the people who continue to reject it will be shown for what they are, non-rational and anti-technology and it will make them look stupid. That鈥檚 really good.鈥�
It would be Exorcist鈥檚 best trick if it could split the GM opposition as effectively as it snips out foreign DNA. But there are still some major hurdles to overcome. As both Shand and Mayer point out, it鈥檚 a mistake to define the GM debate in purely technical terms. One of the most powerful objections to GM technology is that it makes farmers increasingly dependent on large seed companies. That鈥檚 something the Exorcist system 鈥� which Maxygen has filed a patent on 鈥� cannot solve.
Stemmer may have an even tougher sell when it comes to the agribusiness giants. If the big companies aren鈥檛 willing to put their money down to develop the technology, Exorcist will remain exactly where it is now: a clever idea on a piece of paper. So far the big companies aren鈥檛 beating a path to Stemmer鈥檚 door. 鈥淭his approach is astronomically unfeasible right now. I鈥檇 rather take the money and perfect golden rice [vitamin A-enriched rice],鈥� says Mary-Dell Chilton of the crop biotechnology giant Syngenta. Her company feels that by simply educating the public more about GM food, consumer support for these products will grow.
Exorcist鈥檚 creator says he often encounters a similar attitude among other plant scientists. 鈥淐ompanies think if they can get over this current hump of opposition, the crops they have right now are going to win over the public,鈥� says Stemmer. 鈥淏ut what if they don鈥檛 get over that hump? They haven鈥檛 got an alternative.鈥� For that reason, he prefers a different name for Exorcist. He calls it 鈥淧lan B鈥�.