The Bitter Truth About PTC Tasting

Asking students to taste PTC-soaked paper is a classic classroom exercise to demonstrate a simple inherited trait. Some grimace, others look puzzled. "PTC perception is arguably one of the most studied human traits," says Sun-Wei Guo, a professor of pediatrics and biostatistics at the Medical College of Wisconsin in Milwaukee. A new investigation reveals more to chew on: Rare individuals who are not quite sure whether they taste phenylthiocarbamide (PTC) have provided a hint that the inheritance might not be straightforward. "PTC tasting was considered a Mendelian trait for 50 years. Only in the past 25 years has this model begun to fray," says Dennis Drayna, special expert at the National Institute on Deafness and Other Communication Disorders. Drayna and colleagues from Stanford University and the University of Utah identified a major gene controlling PTC taste sensitivity. They described haplotypes that account for the differing proportions of tasters and nontasters in human populations.1 This new way of looking at inheritance of PTC-tasting ability also provides a tool to follow the trait in the primate cousins. In 1931 DuPont chemist A.L. Fox synthesized phenylthiocarbamide (similar compounds are in cabbage, broccoli, Brussels sprouts and cauliflower) while researching artificial sweeteners. "He discovered accidentally that some people found the chemical intensely bitter, although he himself found it tasteless as chalk," relates Guo. "This taste dimorphism later was found to be hereditary." Danielle R. Reed, an associate member of the Monell Chemical Senses Center in Philadelphia, investigates the inheritance of PTC tasting with Guo.2 "PTC is a member of a large class of compounds characterized by their antithyroid properties and chemical structure, which includes an N-C=S group," she explains. The aforementioned veggies share this chemical group. Courtesy of Danielle R. Reed YUCK! This child is tasting a solution of PROP, a chemical relative of phenylthiocarbamide. It obviously doesn't taste like strawberry ice cream. Guided by past studies linking PTC taster status to part of chromosome 7 that houses other bitter taste-receptor genes, Drayna and coworkers searched for SNP patterns that correlate with the phenotype among families in the CEPH (Centre 'Etude du Polymorphisme Humain) Genotype Database at the University of Utah. "They designed experiments to reveal nuances in tasting ability," says Neil Risch, professor of genetics at Stanford University Medical Center. "We used a classic test of sorting six cups [of water], three with PTC and three without. The test is repeated at serial dilutions of one-half until the cups are not sorted properly. This is the taste threshold. There is considerable overlap between the taster and nontaster distributions." In a 150-kilobase region, the researchers identified three common SNPs that alter amino acid sequence in a gene they dubbed PTC. Specifically, changes at amino acid positions 49, 262, and 296 define haplotypes that combine in different ways to confer tasting ability, or lack thereof. An NIH-based database confirmed the associations. Among the Europeans in the samples, taster/nontaster status was about evenly divided, but among Asians, 70% were tasters. The few exceptions--tasters with nontaster haplotypes and vice versa--implicate other genes. "While the different haplotypes completely explain the bimodal distribution, they do not explain all the variance in PTC taste ability," says Drayna, who has found another candidate gene on chromosome 16.3 The ability to taste PTC varies widely in different populations. "It ranges from 35% tasters in the Kondhs or Orissa populations in India, to 98.6% in the Papago Indians in Arizona," says Guo. About 90% of African Americans are tasters, and all populations have some nontasters, he adds. Such variance of a trait among populations could echo back to the hunter-gatherers. "It is not clear why the nontaster allele has risen to such high frequency in some populations. It could be simply genetic drift; possibly only a small band of founders emerged out of Africa, and only the major taster and nontaster alleles colonized the non-African world," speculates Drayna. Selection is equally plausible, Risch and Drayna say, but could have worked either way: Tasting may have prompted people to spit out bitter poisons, but nontasting would have promoted a more eclectic and healthful diet. And so a balanced polymorphism scenario might have arisen--each haplotype offering an advantage--that has maintained both alleles in populations today. Ricki Lewis (rickilewis@nasw.org) is a freelance writer based in Scotia, NY. She is a nontaster. References 1. U. Kim et al., "Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenyl- thiocarbamide," Science, 29:1221-4, Feb. 21, 2003. 2. S.W. Guo, D.R. Reed, "The genetics of phenylthiocarbamide perception," Ann Hum Biol, 28111-42, 2001. 3. D. Drayna et al., "Genetic analysis of a complex trait in the Utah Genetic Reference Project: A major locus for PTC taste ability on chromosome 7q and a secondary locus on chromosome 16p," Hum Genet, 112:567-72, May 2003. function sendData() { document.frm.pathName.value = location.pathname; result = false if (document.frm.score[0].checked) result = true; if (document.frm.score[1].checked) result = true; if (document.frm.score[2].checked) result = true; if (document.frm.score[3].checked) result = true; if (document.frm.score[4].checked) result = true; if (!result) alert("Please select") return result; } .myradio{background-color : #94bee5} Please indicate on a 1 - 5 scale how strongly you would recommend this article to your colleagues? Not recommended 1 2 3 4 5 Highly recommended Please register your vote

The Bitter Truth About PTC Tasting

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