03 November 2008
Scientists Developing Cancer-Fighting Purple Tomatoes
Genetically modified tomatoes contain high levels of anti-oxidants
Washington — One red tomato plant plus two genes from a snapdragon flower equals a purple tomato that might offer protection against a broad range of human diseases.
Researchers at the John Innes Centre in the United Kingdom added the snapdragon genes to produce a genetically modified tomato that contains high levels of anthocyanins, chemicals that might protect against certain cancers and cardiovascular disease, according to an October 26 report in the journal Nature Biotechnology.
The study showed that cancer-prone mice lived longer when fed diets supplemented with purple tomato powder, which appears to be nontoxic.
Anthocyanins are present at high levels in berries such as blackberries and cranberries. Scientists are investigating ways to increase the levels of health-promoting compounds in commonly eaten fruits and vegetables.
“This is one of the first examples of metabolic engineering that offers the potential to promote health through diet by reducing the impact of chronic disease,” said Professor Cathie Martin, lead author of the study. She added that tomatoes are an important food crop around the world.
Researchers long have thought that anti-oxidants promote health and discourage disease. The U.S. Centers for Disease Control and Prevention (CDC), the Department of Health and Human Services, and the National Cancer Institute recommend that people eat at least five portions of fruits and vegetables per day. However, only 23 percent of the U.S. population follows these recommendations, according to the CDC.
THE SCIENCE BEHIND PURPLE TOMATOES
Anthocyanins are small, water-soluble pigments that color plants red, blue or purple depending on their acid content — blueberries, purple eggplants and blood oranges derive their characteristic colors, in part, from these chemicals. Anthocyanins also act as anti-oxidants, meaning that they slow or prevent oxidation reactions from occurring.
In a metabolic paradox, oxygen is required for life but also can damage cells by engaging in oxidation reactions that produce highly reactive free radicals, molecules that are often toxic.
Foods rich in water- and fat-soluble anti-oxidants are thought to be the best protection against disease. Tomatoes are high in lycopene, a fat-soluble anti-oxidant, but contain small amounts of water-soluble anti-oxidants, most of which are concentrated in the tomato peel.
Martin and colleagues boosted water-soluble anti-oxidant levels by adding two genes from the snapdragon Antirrhinum majus to the MicroTom tomato variety.
Cells produce anthocyanins through a multistep process in which a chemical, the amino acid phenylalanine, is successively modified — much the same way that a quilt is assembled by connecting and modifying individual panels.
Each step of the modification is activated by certain genes; more than a dozen regulate the entire pathway. Normally these genes are not on in tomatoes, so few anthocyanins are produced. The two snapdragon genes researchers added, however, are master regulators of anthocyanin production: they turn on the genes, normally off in tomatoes, necessary to produce anthocyanins.
The purple color is proof that the genetic modification worked. Researchers confirmed this by measuring levels of anthocyanins in both the peel and the flesh. Purple tomatoes averaged 2.83 milligrams of anthocyanin per gram of fruit. In unmodified tomatoes, anthocyanins were “virtually undetectable,” according to the report.
COULD PURPLE TOMATOES PREVENT CANCER?
In the laboratory, anti-oxidants display health-promoting effects, but scientists have yet to demonstrate concrete benefits in human studies.
Martin and colleagues asked whether their purple tomatoes actually could help prevent a chronic disease. In a pilot study, researcher took an established strain of cancer-prone mice, genetically engineered to rapidly develop spontaneous tumors, and fed them diets supplemented with tomato powder.
Cancer-prone mice on a standard diet or one containing 10 percent red tomato powder had an average life span between 142 and 146 days. On diets supplemented with purple tomato power, mice lived about 30 percent longer, 182 days on average.
Researchers are now testing whether purified anthocyanins produce a similar effect.
The anthocyanin-rich purple tomato is the first example of a genetically modified plant with a trait that “really offers a potential benefit for all consumers,” Martin said. The next step will be to perform studies with humans “to see if we can promote health through dietary preventive medicine strategies.”
The United States supports advances in food technology, such as genetically modified crops, to boost food production, Secretary of State Condoleezza Rice has said.
According to Dan Price, deputy national security advisor for international economic affairs, genetically modified crops lead to higher yields, and discouraging their use in developing nations deprives those nations of higher yields domestically and for export markets. (See “Long-term Approach to Food Aid Problems Necessary, Rice Says” and Background Briefing on Bush’s Announcement on Global Food Aid.)
For more information on the health benefits of fruits and vegetables, see the CDC’s Fruits & Veggies — More Matters Web site.
