|Scientists Find Bitter Taste Gene|
|By Melissa Lee Phillips |
Neuroscience for Kids Consultant
July 15, 2003
In 1931, a chemist named Arthur Fox sat at his DuPont Company lab bench, mixing a powdered chemical. He accidentally let a bit of the powder blow into the air around him. Fox and another scientist got some of the chemical into their mouths. Dr. Fox's colleague exclaimed how bitter the powder tasted. Fox was surprised -- he had been much closer to the chemical, but he tasted nothing at all. Both men tasted the chemical again. Again, Fox said the chemical was tasteless, but his co-worker insisted it was very bitter. Fox handed out crystals of the chemical, which were not poisonous, to his friends, family members and fellow scientists and asked them if they tasted anything. Some people, like Fox, tasted nothing; others found the chemical somewhat bitter or intensely bitter.
Dr. Fox's chemical, called phenylthiocarbamide (PTC), has been used widely since its discovery to detect genetic variation in tasting abilities. Studies soon after the incident in Fox's lab showed that there is a genetic component that influences how PTC tastes. Scientists found that people were much more likely to find PTC bitter if other members of their family also found it bitter. The evidence was so strong for a genetic link that PTC tasting ability was used as evidence in paternity tests before DNA tests were available.
More than 70 years later, a single gene underlying this variation was discovered and reported by Un-kyung Kim and colleagues from the National Institute on Deafness and Other Communication Disorders. This was something of a surprise, because for many years, scientists suspected that more than one -- possibly many -- genes were responsible for PTC taste sensitivity. Geneticists argued for years about how many genes might be responsible for this variation and where these genes might be located on our chromosomes.
Using molecular genetics techniques with a variety of families, Kim isolated an area on chromosome 7 that was likely to contain a gene affecting PTC tasting ability. This region, however, also contained more than 150 other genes. Of these, nine were known to produce proteins for bitter taste receptors on the tongue. To narrow down their search, the researchers figured out the DNA sequences of all nine of these genes. They looked to see if different people had different versions of the same gene for any of these, and, if so, if any gene variations correlated with PTC sensitivity.
The researchers found a single gene for a bitter taste receptor that completely explains different PTC tasting abilities. There are actually three versions of this gene that differ from one another only slightly. This small difference in the gene, and in the protein that it makes, eventually forms a tongue taste receptor that has a different shape from a "normal" bitter taste receptor. This altered shape means that the person's receptors will not respond to PTC and the person will not think the PTC tastes bitter. Since all people have two copies of every gene, different combinations of the bitter taste gene (two copies of form 1; one copy of form 1 and one of form 2; two copies of form 3; etc.) determine whether someone finds PTC intensely bitter, somewhat bitter, or without taste at all.
Some scientists are now interested in potential health applications of genetic taste studies. For example, people who find PTC bitter are suspected to find the taste of cigarettes bitter, which could make strong tasters less likely to smoke. There are also possible correlations between the ability to taste PTC and preferences for certain foods, which could influence a person's diet. These types of correlations are very uncertain, however: it's going to be a while before a phenylthiocarbamide gene provides an excuse for why some of us think that broccoli is just too bitter to eat.
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