Success stories are hard to come by, but will, I am sure, become more common. Discovered by chemists at the Swiss company Novartis, Gleevec works against a blood cancer called chronic myeloid leukemia (CML) by specifically blocking the growth-stimulating activity of membrane receptor proteins that are overproduced by cancerous cells of this type. If given early in the course of CML, Gleevec generally leads to long disease-free remissions, and hopefully in many cases to true cures. For some unlucky individuals, though, the disease reappears when new mutations in the gene encoding the membrane-receptor proteins render Gleevec ineffective.
One of the most important anticancer-drug target proteins may be the receptor for epidermal growth factor (EGFR). This receptor frequently shows up in much higher quantities in cancer cells (particularly in breast and lung cancers) than in normal ones, which suggests that it may well be a winner as a drug target. Several potent drugs that specifically block EGFR action are now in late-stage clinical testing. But while the arrival of target-specific drugs will certainly introduce big new guns in the war against cancer, the likelihood is that, after initial remission, many patients will suffer a relapse as resistance to the new drugs evolves among the cancer cells colonizing the body.
For this reason, many have come to believe that a better long-term way of fighting cancer cells may involve targeting their nutritional lifelines. They, like all cells in the body, need nutrients to grow, and they receive these nutrients from blood vessels that grow near them. If you block the growth of blood vessels into tumors, you can eventually starve to death the cancer cells they serve. The idea that small tumors become dangerous only once they are infiltrated by newly formed blood vessels (a process called "angiogenesis") first occurred to Judah Folkman in the early 1960s while he was doing his military service in the Naval Medical Research Institute outside Washington, D.C. The precocious son of an Ohio rabbi, Folkman was the first graduate of Ohio State University to enter Harvard Medical School. By the time he went to high school he had already assisted in surgery on a dog, and in college he invented a surgical device to cool the liver when its blood supply was temporarily cut off. At thirty-four, he became the youngest professor of surgery in the history of Harvard University. Folkman's anti-angiogenesis ideas could not, however, be explored therapeutically until the recent discovery of three specific growth factors that play vital roles in the growth of "endothelial" cells, those that line blood vessels. Inhibitors developed against these growth factors – anti-angiogenesis drugs – might very well prove effective against many forms of cancer. Some forty years after Folkman's original insight, we may at last be able in the foreseeable future to cure most cancers, including those that have become resistant to the best conventional anticancer drugs.
Already Sugen, a firm outside San Francisco, has developed two highly specific small-molecule drugs that work against distinct angiogenesis growth factors and inhibit tumors in model animal systems. Neither drug given separately has yet proved effective against advanced human cancers. However, preliminary data from experiments with cancer-prone mice done by Doug Hanahan at UCSF suggest the Sugen drugs might have worked had they been administered in tandem. Unfortunately the future of onco-mouse experiments at UCSF and elsewhere is jeopardized by the ongoing dispute provoked by Du Pont's aggressive onco-mouse licensing policies.
Blood vessel infiltration into mouse tumors has also been prevented by a newly discovered group of proteins that are likely naturally occurring inhibitors of blood vessel formation. Two such proteins, angiostatin and endostatin, isolated by Michael O'Reilly in Judah Folkman's lab, are currently in clinical trials. While neither is present in blood in amounts large enough to be extracted for human testing, recombinant DNA procedures permit both proteins to be made in yeast cells in quantities sufficient for clinical use. And while neither angiostatin nor endostatin alone has yet demonstrated miracle-like anticancer effects in humans, mouse experiments suggest that, as with Sugen's drugs, an efficacious combination of the two may soon be discovered. Over the next decade, a virtual armada of small-molecule and protein inhibitors will probably be ready to sail through the systems of cancer sufferers, thwarting blood vessel formation before tumors have a chance to become lethal. And if tumor growth can indeed be curtailed in this way, we may come to regard cancer as we do diabetes, as a disease that can be controlled rather than completely cured outright.
Since recombinant technologies allow us to harness cells to produce virtually any protein, the question has logically arisen: Why limit ourselves to pharmaceuticals? Consider the example of spider silk. So-called dragline silk, which forms the radiating spokes of a spider web, is an extraordinarily tough fiber. By weight, it is five times as strong as steel. Though there are ways spiders can be coaxed to spin more than their immediate needs require, unfortunately, attempts to create spider farms have foundered because the creatures are too territorial to be reared en masse. Now, however, the silk-protein-producing genes have been isolated and can be inserted into other organisms, which can thus serve as spider-silk factories. This very line of research is being funded by the Pentagon, which sees Spiderman in the U.S. Army's future: soldiers may one day be clad in protective suits of spider-silk body armor.
Another exciting new frontier in biotechnology involves improving on natural proteins. Why be content with nature's design, arrived at by sometimes arbitrary and now irrelevant evolutionary pressures, when a little manipulation might yield something more useful? Starting with an existing protein, we now have the ability to make slight alterations in its amino acid sequence. The limitation, unfortunately, is in our knowledge of what effect altering even a single amino acid in the chain is likely to have on the protein's properties.
Here we can return to nature's example for a solution: a procedure known as "directed molecular evolution" effectively mimics natural selection. In natural selection new variants are generated at random by mutation and then winnowed by competition among individuals; successful – better adapted – variants are more likely to live and contribute to the next generation. Directed molecular evolution stages this process in the test tube. After using biochemical tricks to introduce random mutations into the gene for a protein, we can then mimic genetic recombination to shuffle the mutations to create new sequences. From among the resulting new proteins our system selects the ones that perform best under the conditions specified. The whole cycle is repeated several times, each time with the "successful" molecules from the previous cycle competing in the next.
For a nice example of how directed molecular evolution can work, we need look no farther than the laundry room. Here disasters occur when a single colored item finds its way accidentally into a load of whites: some of the dye inevitably leaches out of that red T-shirt and before you know it every sheet in the house is a pale pink. It so happens that a peroxidase enzyme naturally produced by a toadstool – the inkcap mushroom, to be specific – has the property of decolorizing the dyes that have leached out of clothing. The problem, however, is that the enzyme cannot function in the hot soapy environment of a washing machine. By using directed molecular evolution, however, it has been possible to improve the enzyme's capacity for coping with these conditions: one specially "evolved" enzyme, for instance, demonstrated an ability to withstand high temperatures 174 times greater than that of the toadstool's own enzyme. And such useful "evolutions" do not take long. Natural selection takes eons, but directed molecular evolution in the test tube does the job in just hours or days.
Genetic engineers realized early that their technologies could also have a positive impact on agriculture. As the biotech world now knows all too well, the resulting genetically modified (GM) plants are now at the center of a firestorm of controversy. So it's interesting to note that an earlier contribution to agriculture – one that increased milk production – also led to an outcry.
Bovine growth hormone (BGH) is similar in many ways to human growth hormone, but it has an agriculturally valuable side effect: it increases milk pro
duction in cows. Monsanto, the St. Louis-based agricultural chemical company, cloned the BGH gene and produced recombinant BGH. Cows naturally produce the hormone, but, with injections of Monsanto's BGH, their milk yields increased by about 10 percent. In late 1993 the FDA approved the use of BGH, and by 1997 some 20 percent of the nation's 10 million cows were receiving BGH supplements. The milk produced is indistinguishable from that produced by nonsupplemented cows: they both contain the same small amounts of BGH. In fact, a major argument against labeling milk as "non-BGH-supplemented" versus "BGH-supplemented" is that it is impossible to distinguish between milk from supplemented and nonsupplemented cows, so there is no way to determine whether or not such advertising is fraudulent. Because BGH permits farmers to reach their milk production targets with fewer cattle, it is in principle beneficial to the environment because it could result in a reduction in the size of dairy herds. Because methane gas produced by cattle contributes significantly to the greenhouse effect, herd reduction may actually have a long-term effect on global warming. Methane is twenty-five times more effective at retaining heat than carbon dioxide, and on average a grazing cow produces six hundred flatulent liters of the stuff a day – enough to inflate forty party balloons.
At the time I was surprised that BGH provoked such an outburst from the anti-DNA lobby. Now, as the GM food controversy drags on, I have learned that professional polemicists can make an issue out of anything. Jeremy Rifkin, biotechnology's most obsessive foe, was launched on his career in naysaying by the U.S. Bicentennial in 1976. He objected. After that he moved on to objecting to DNA. His response in the mid-1980s to the suggestion that BGH would not likely inflame the public was, "I'll make it an issue! I'll find something! It's the first product of biotechnology out the door, and I'm going to fight it." Fight it he did. "It's unnatural" (but it's indistinguishable from "natural" milk). "It contains proteins that cause cancer" (it doesn't, and in any case proteins are broken down during digestion). "It'll drive the small farmer out of business" (but, unlike with many new technologies, there are no up-front capital costs, so the small farmer is not being discriminated against). "It'll hurt the cows" (nearly nine years of commercial experience on millions of cows has proved this not to be the case). In the end, rather like the Asilomarera objections to recombinant techniques, the issue petered out when it became clear that none of Rifkin's gloom-and-doom scenarios were realistic.
The spat over BGH was a taste of what was to come. For Rifkin and like-minded DNA-phobes, BGH was merely the appetizer: genetically modified foods would be the protesters' main course.
CHAPTER SIX
TEMPEST IN A CEREAL BOX:
GENETICALLY MODIFIED AGRICULTURE
In June 1962, Rachel Carson's book Silent Spring created a sensation when it was serialized in The New Yorker. Her terrifying claim was that pesticides were poisoning the environment, contaminating even our food. At that time I was a consultant to John Kennedy's President's Scientific Advisory Committee (PSAC). My main brief was to look over the military's biological warfare program, so I was only too glad to be diverted by an invitation to serve on a subcommittee that would formulate the administration's response to Carson's concerns. Carson herself gave evidence, and I was impressed by her careful exposition and circumspect approach to the issues. In person, too, she was nothing like the hysterical ecofreak she was portrayed as by the pesticide industry's vested interests. An executive of the American Cyanamid Company, for instance, insisted that "if man were to faithfully follow the teachings of Miss Carson, we would return to the Dark Ages, and the insects and diseases and vermin would once again inherit the earth." Monsanto, another giant pesticide producer, published a rebuttal of Silent Spring, called The Desolate Year, and distributed five thousand copies free to the media.
My most direct experience of the world Carson described, however, came a year later when I headed a PSAC panel looking into the threat posed to the nation's cotton crop by herbivorous insects, especially the boll weevil. Touring the cotton fields of the Mississippi Delta, West Texas, and the Central Valley of California, one could hardly fail to notice the utter dependence of cotton growers on chemical pesticides. En route to an insect research laboratory near Brownsville, Texas, our car was inadvertently doused from above by a crop duster. Here billboards featured not the familiar Burma-Shave ads but pitches for the latest and greatest insect-killing compounds. Poisonous chemicals seemed to be a major part of life in cotton country.
Whether Carson had gauged the threat accurately or not, there had to be a better way to deal with the cotton crop's six-legged enemies than drenching huge tracts of country with chemicals. One possibility promoted by the U.S. Department of Agriculture scientists in Brownsville was to mobilize the insects' own enemies – the polyhedral virus, for instance, which attacks the bollworm (soon to become a greater threat to cotton than the boll weevil) – but such strategies proved impracticable. Back then, I could not have conceived of a solution that would involve creating plants with built-in resistance to pest insects: such an idea would simply have seemed too good to be true. But these days that is exactly how farmers are beating the pests while at the same time reducing dependence on noxious chemicals.
Genetic engineering has produced crop plants with onboard pest resistance. The environment is the big winner because pesticide use is decreased, and yet paradoxically organizations dedicated to protecting the environment have been the most vociferous in opposing the introduction of these so-called genetically modified (GM) plants.
As with genetic engineering in animals, the tricky first step in plant biotechnology is to get your desired piece of DNA (the helpful gene) into the plant cell, and afterwards into the plant's genome. As molecular biologists frequently discover, nature had devised a mechanism for doing this eons before biologists even thought about it.
Crown gall disease results in the formation of an unattractive lumpy "tumor," known as a gall, on the plant stem. It is caused by a common soil bacterium called Agrobacterium tumefaciens, which opportunistically infects plants where they are damaged by, say, the nibbling of a herbivorous insect. How the bacterial parasite carries out the attack is remarkable. It constructs a tunnel through which it delivers a parcel of its own genetic material into the plant cell. The parcel consists of a stretch of DNA that is carefully excised from a special plasmid and then wrapped in a protective protein coat before being shipped off through the tunnel. Once the DNA parcel is delivered, it becomes integrated, as a virus's DNA would be, into the host cell's DNA. Unlike a virus, however, this stretch of DNA, once lodged, does not crank out more copies of itself. Instead, it produces both plant growth hormones and specialized proteins, which serve as nutrients for the bacterium. These promote simultaneous plant cell division and bacterial growth by creating a positive feedback loop: the growth hormones cause the plant cells to multiply more rapidly, with the invasive bacterial DNA being copied at each cell division along with the host cell's, so that more and more bacterial nutrients and plant growth hormones are produced.
For the plant the result of this frenzy of uncontrolled growth is a lumpy cell mass, the gall, which for the bacterium serves as a kind of factory in which the plant is coerced into producing precisely what the bacterium needs, and in ever greater quantities. As parasitic strategies go, Agrobacterium's is brilliant: it has raised the exploitation of plants to an art form.
The details of Agrobacterium's parasitism were worked out during the 1970s by Mary-Dell Chilton at the University of Washington in Seattle and by Marc van Montagu and Jeff Schell at the Free University of Ghent, Belgium. At the time the recombinant DNA debate was raging at Asilomar and elsewhere. Chilton and her Seattle colleagues later noted ironically that, in transferring DNA from one species to another without the protection of a P4 containment facility, Agrobacterium was "operating outside the National Institutes of Health guidelines."
Chilton, van Montagu, and Schell soon were not alone in their fascination with Agrobacterium. In the early eighties M
onsanto, the same company that had condemned Rachel Carson's attack on pesticides, realized that Agrobacterium was more than just a biological oddity. Its bizarre parasitic lifestyle might hold the key to getting genes into plants. When Chilton moved from Seattle to Washington University, St. Louis, Monsanto's hometown, she found that her new neighbors took a more than passing interest in her work. Monsanto may have made its entry late in the Agrobacterium stakes, but it had the money and other resources to catch up fast. Before long both the Chilton and the van Montagu/Schell laboratories were being funded by the chemical giant in return for a promise to share their findings with their benefactor.
Monsanto's success was built on the scientific acumen of three men, Rob Horsch, Steve Rogers, and Robb Fraley, all of whom joined the company in the early eighties. Over the next two decades they would engineer an agricultural revolution. Horsch always "loved the smell of [the soil], the heat of it" and, even as a boy, wanted "always to grow things better than what I could find at the grocery store." He instantly saw a job at Monsanto as an opportunity to follow that dream on an enormous scale. By contrast, Rogers, a molecular biologist at Indiana University, initially discarded the company's letter of invitation, viewing the prospect of such work as "selling out" to industry. Upon visiting, however, he discovered not only a vigorous research environment but also an abundance of one key element that was always in short supply in academic research: money. He was converted. Fraley was possessed early on by a vision for agricultural biotechnology. He came to the company after approaching Ernie Jaworski, the executive whose bold vision had started Monsanto's biotechnology program. Jaworski proved not only a visionary but also an affable employer. He was unfazed by his first encounter with the new man when they were both passing through Boston's Logan Airport: Fraley announced that one of his goals was to take over Jaworski's job.
Dna: The Secret of Life Page 15
