The Genom of Homo sapiens.pdf

368 DRAYNA ET AL.Table 1. Summary of Polymorphisms in the PTC GeneSNP Amino acid Location inposition (b.p.) allele frequency position (a.a.) a.a. encoded predicted proteinC .36 proline 1st145 G .64 49 alanine intracellularloop785 C .38 262 alanine 6th transmembraneT .62 valine886 G .38 296 valine 7th transmembraneA .62 isoleucineIn addition to the common PAV and AVI haplotypes, athird haplotype, designated AAV, was observed at lowerfrequency. Note this haplotype differs from the PAV haplotypeby a single amino acid difference, a proline versusalanine at residue 49. This creates a significant differencein phenotype. In a simple dichotomous phenotypic classification,AVI/AAV heterozygotes are almost evenly dividedbetween tasters and non-tasters. In comparison, wewould expect AVI/PAV heterozygotes, which differ fromAVI/AAV heterozygotes by one amino acid difference inone copy of the gene, to be all tasters. Further analysisshowed that all of the bimodality of the taste thresholddistribution was explained by these haplotypes, and thatthese haplotypes explained from 55% (in the CEPH sample)to 85% (in the NIH sample) of the total variance intaste threshold score.Overall, these data indicate that this TAS2R gene is themajor determinant of PTC taste threshold. However,other factors are clearly involved. For example, Figure 3shows three individuals who are homozygous for the nontasterhaplotype AVI and have taste thresholds of 8 andabove, clearly in the taster range. Conversely, among theAVI/PAV heterozygotes is one individual whose tastethreshold is under 4, well into the non-taster range. Ourlinkage studies in the Utah CEPH families support theview that additional genes outside of chromosome 7qcontribute to this phenotype, and that one such gene mayreside on the short arm of chromosome 16.Gene RelationshipsThe TAS2R bitter taste receptor family is highly divergent.Among the 24 functional TAS2R genes predictedin the human genomic sequence, the averageamino acid identity between any two members is ~25%.The major non-taster allele of the human PTC receptorgene is identical to the human TAS2R38 sequence (GenBank accession number AF494231), with the exceptionof nucleotide 557, which is an A (encoding Asn-186) inTAS2R38 and a T (encoding Ile-186) in the PTC receptor.The next closest relative is human TAS2R37, whichdisplays 30% amino acid identity with the PTC receptor.Worldwide DistributionThe PTC taste phenotype has been studied in hundredsof populations worldwide. In an effort to measure correlationsbetween phenotypes and genotypes in diversepopulations, we studied variation in the PTC receptorgene in seven major populations: European, sub-SaharanAfrican, Pakistani, Chinese, Korean, Japanese, andSouthwest American Indians. In this study, we determinedhaplotypes in unrelated individuals by using PCRto amplify both copies of the PTC receptor gene from genomicDNA and cloning the PCR product en masse intoa plasmid vector. This was then transformed into competentcells, and 10 transformants were picked from eachPCR. The inserts in these clones, which each representedone allele or the other, were then sequenced. In doing so,we identified two additional haplotypes, designated AAIand PVI. Both of these haplotypes were observed only inindividuals of sub-Saharan origin. The frequency of eachof these five haplotypes in different populations is shownin Figure 4.The worldwide distribution of haplotypes confirms thephenotype data gathered over the past 60 years and providesa number of additional insights into the history ofthese variants. Outside of sub-Saharan Africa, most pop-Figure 3. Correlation of haplotypes with PTCtaste threshold phenotype. PTC taste thresholdsof 90 subjects from the NIH population sortedby haplotype. AVI/PAV heterozygotes (meanthreshold 8.81) display a significantly reducedPTC sensitivity compared to PAV homozygotes(mean threshold 10.00), indicating a heterozygoteeffect.