From the Lab: Biotechnology

Fatty-Acid Factories
Engineered seeds produce healthful oils

Results: Canadian researchers have engineered mustard seeds to make very-long-chain polyunsaturated fatty acids such as omega-3 fatty acids that are known to reduce the risk of death from heart attacks and strokes. By transplanting genes from six sources, including marine fungi and marigolds, into the mustard plant, the researchers built new metabolic pathways that enabled the plants’ seeds to convert two fatty acids they ordinarily make into two omega-3 fatty acids they don’t normally make: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The seeds produced a commercially viable amount of EPA, which topped out at 15 percent of the total fatty acids in the seed oil, and a detectable but not commercially viable amount of DHA, at 1.5 percent.

Why It Matters: Omega-3 acids are found primarily in fish oils, and food manufacturers have recently been extracting them and adding them to eggs, milk, juice, butter, breads, and other foods. But while these acids are good for people’s health, their source, fish oil, can contain high levels of toxic chemicals such as mercury. By turning mustard seeds–chosen because they normally produce large amounts of oils and can easily incorporate new genes–into fatty-acid factories, the researchers hope to produce a safe source of omega-3 acids not limited by the supply of fish. Previously engineered plants produced low levels of EPA and no DHA. Here, an engineered mustard seed produced DHA for the first time and high enough levels of EPA to make it a potential commercial source.

Methods: Led by Xiao Qiu of Bioriginal Food and Science in Saskatoon, Saskatchewan, the researchers introduced three to nine genes from plants and microrganisms into the cells of India mustard seedlings, then analyzed the oils in the seeds those plants produced. The new genes produced enzymes that in a series of steps transformed the two normally present fatty acids into the omega-3 fatty acids. By adding genes successively in a series of experiments, the researchers could see how each gene changed fatty-acid production, which allowed them to understand in detail the metabolic pathways involved and to try different genes that would produce higher yields of the target fatty acids.

Next Step: The researchers are attempting to increase the seeds’ production of DHA by adding more genes to the omega-3 fatty-acid pathways. – By Kevin Bullis

Source: Wu, G., et al. 2005. Stepwise engineering to produce high yields of very long-chain polyunsaturated fatty acids in plants. Nature Biotechnology 23:1013-1017.

Results: In a step toward enlisting small interfering RNA molecules (siRNAs) as drugs that turn off specific disease-causing genes, researchers from Harvard Medical School have for the first time induced siRNAs to enter only targeted cells in lab animals. They tagged the siRNAs with an antibody fragment that binds specifically to a cell-surface receptor. In one lab-dish experiment, they found that their tagged siRNAs were absorbed only by mouse melanoma cells engineered to carry this receptor and not by normal melanoma cells. Next, they injected the siRNAs–designed to shut down known cancer-causing genes–into mice that had had the engineered melanoma cells implanted in them. They found that after nine days the tumors were about half the weight of those in control-group mice.

Why It Matters: By turning off certain genes in a process called RNA interference, small interfering RNA molecules could become a new class of drugs for a wide range of diseases, such as cancer, cystic fibrosis, and certain infectious illnesses. So far, the biggest research challenges have been delivering these fragile molecules throughout the body in a stable form and ensuring that they home in on specific cells. While other researchers had stabilized siRNAs so that they remained intact in the bloodstream of lab animals, the Harvard researchers, led by Judy Lieberman, have taken the next big step: shuttling the molecules to specific cells.

Methods: In one set of experiments, the researchers engineered mouse melanoma cells to produce a receptor normally found on the surface of the HIV virus. They used this receptor because an antibody tag specific to it had already been shown to deliver DNA to HIV-infected cells, and they wanted to see if the same tag would also work with siRNAs. They fused a fragment of an antibody specific to that receptor to another protein, which they then bound to three different siRNA molecules, each of which shut down a different cancer gene. They introduced the antibody-tagged siRNA molecules, alone and in combination, into lab dishes containing the engineered melanoma cells and measured the effect on cell division. Then they implanted the cancer cells in the flanks of mice, under the skin; injected the mice with the siRNAs; and measured the effects on tumor volume and weight.

Next Step: The researchers need to show that they can target siRNAs to receptors naturally present on tumor cells. And they need to check that their siRNA molecules don’t elicit undesirable inflammatory responses in lab animals and are not rapidly degraded by blood enzymes. – By Corie Lok

Source: Song, E., et al. 2005. Antibody mediated in vivo delivery of small interfering RNAs via cell-surface receptors. Nature Biotechnology 23:709-717.