Transgenic Plants Produce Pharmaceutical, Chemical Products

Contents
Introduction
Two Crops from One
Blood (Products) from Turnips?
Not a Panacea
Specialty Chemicals

Introduction (Back to Top)
Even with the obvious advantages of using every component of a crop for something—anything—it's rare to see a single plant grown for two significant products. Now a group at Pacific Northwest Laboratory (PNL; Richland, WA) has discovered a way to make a single plant do double duty. By controlling genes transplanted into the plant, researchers can direct production of a desired chemical in the leaf while using the root for its traditional use—food—thus generating two very different crops from a single plant.

With a large leaf mass and very large roots, potatoes are an ideal plant for trying out this idea. PNL's first target protein was cellulase, an enzyme that breaks down non-digestible (to humans) cellulose, which is found in practically all plants, but the lab has other enzymes in mind. "The process can be adapted to create additional enzymes such as lipases and proteases used in pharmaceuticals, specialty chemical and industrial products," said Brian Hooker, a biochemical engineer at PNL.

Industrial enzymes are usually produced in fermenters through a process that is labor-, time-, and capital-intensive. Using plants as "bioreactors" to grow the enzymes is much easier and cheaper. While fermentation process costs range from $50 to $250 per gram of raw product, PNL estimates that growing the same enzymes in plants would cost less than a penny per gram for finished cellulase product, including processing costs.

A purified enzyme for a penny a gram? What ever happened to the high costs of downstream processing?

"The particular cellulases for which we were designing the finishing process and ultimately developing production/process economics were CBH1 from T. reesei and E1 from Acidothermus cellulolyticus," Hooker stated. "Both enzymes are fairly highly thermally stable and required fairly easy purification steps since most competing proteins extracted from potato tops would be denatured at 55–60°C. This just happens to be the optimal activity temperature for CBH1. The E1 enzyme's is even higher—80°. So no affinity purification is needed for either protein. Merely heat treat and discard the insoluble fraction."

Two Crops from One (Back to Top)
The idea of a combined food/chemical crop isn't limited just to potato plants. Other plants, especially corn, can be modified to produce enzymes in the non-edible portions. With more than 120 million dry tons of corn stalks and 4 million dry tons of potato foliage produced per year, corn represents a vast, untapped resource for industrial chemicals.

Additionally, the process could be a boon to farmers who would be able to sell two crops for the cost of growing one. Pacific Northwest researchers estimate that by selling the potato tubers for food and vines for enzymes, farmers could increase their profits by as much as $100 to $200 per acre.

In future research, Pacific Northwest plans to use unique promoters that would turn on the enzyme-producing gene in the foliage after harvest. The reason for trying this nifty trick is public opinion: Consumers may find dual-crop veggies more palatable if the industrial chemicals are produced after the edible portion is removed. Of course, even the tubers of potato plants would contain the DNA for producing the protein, so the time-delay strategy will not be without its skeptics.

Blood (Products) from Turnips? (Back to Top)
Blood products are an area where transgenic plants could provide clear-cut benefits over traditional harvesting and processing from donated blood. PNL is also actively pursuing a program for making blood proteins and tissue growth agents in turnips. Blood clotting agents produced in transgenic turnips would be safer and far less expensive for hemophiliacs. These "factors" are currently made from human blood plasma or by cultivating mammalian cells. But as history has shown, treatments derived from human or animal tissues are risky. About 80% of hemophiliacs in the United States over the age of 10 have been infected with HIV from receiving clotting factors produced from donated blood, before screening programs were put into place.

Even with screening programs some viral products such as HIV, Epstein-Barr, Hepatitis B and C, and flu can be transmitted through blood products. Using plants to produce human blood proteins eliminates the potential for transmitting infectious agents.

"In addition to the obvious health benefits, we expect the cost of synthesizing blood factors in transgenic or genetically modified plants to be just one-tenth as high as current methods," Hooker said of this project. "And unlike human blood donors or mammalian cells, plants provide a stable production source and yield much higher amounts of the desired blood factors."

Realizing the potential for making safe, inexpensive blood products from plants, Hookers's group is using gene splicing to transplant human genes into tobacco plants to make blood clotting factors. Patents are pending on the production and composition of plant-derived human blood coagulation factors. PNL researchers have thus far produced coagulation factor VIII, which is critical to hemophilia therapies, as well as factor XIII and thrombin—all clotting enzymes that aid in healing wounds and offer an alternative to sutures and other surgical sealants.

PNL's commercialization manager, Daniel Anderson, indicated it would likely be several years before the blood products will be available for humans. Anderson notes Pacific Northwest researchers currently are using a similar technique to grow valuable industrial enzymes in non-edible portions of common agricultural crops.

Not a Panacea (Back to Top)
Plants aren't capable of performing all the chemistry needed by human therapeutics, however. "Right now the major constraints on plant based recombinant enzymes
are that, for one, plants do not gamma-carboxylate proteins," Hooker stated. "This excludes such therapeutics as Protein C, Factor IX, osteocalcin, prothrombin, and many others and. Second, plants' glycosylation patterns vary slightly from humans. For example, plants can't provide sialic acid caps but instead use such non-human residues such as xylose and 1,3-fucose linkages. Given these constraints, there is still a myriad of enzymes and therapeutic proteins/growth factors that can feasibly be produced in transgenic plants."

Specialty Chemicals (Back to Top)
Due to the high production costs associated with making therapeutic enzymes and the dangers of tissue culture sources, most researchers transgenic plant work today focuses on producing pharmaceutical products. As gene splicing goes making transgenic plants is relatively simple, requiring only a single inserted "transgene."

But plant-based production won't end with very high value-added pharmaceutical products, according to Hooker. "Other value-added chemicals such as industrial or consumer enzymes will soon enjoy an initial high market appeal due to pure manufacturing economics. But in the distant future, even commodity chemicals may be extracted from transgenic plants portions. However, the routine processing of plant structural materials such as cellulose, lignin, xylan, etc. into commodity chemicals such as simple sugars, phenolics, acids, etc.) in will require multiple transgene insertions as well as new pathways that may compromise the health of plants. I believe that we'll eventually get there. However, this is a high volume, low margin market which is not economically attractive right now, even for larger agbiotech corporations.

For more information: Brian Hooker, Biochemical Engineer, Pacific Northwest Laboratory, MS K2-10, PO Box 999, Richland, WA 99352. Tel: 509-375-4420. Fax: 509-372-4660. Email: brian.hooker@pnl.gov.

By Angelo DePalma