BITTER TASTE: TASTE TESTING

(PHENOTYPE) AND PCR

(GENOTYPE)Donna C. Sullivan, PhD

Division of Infectious Diseases

University of Mississippi Medical Center

FIVE BASIC TASTES Sweet Sour Bitter Salty Umami (the taste of monosodium

glutamate).

WHY DO WE HAVE THESE? Sweet: Identify energy-rich nutrients Umami: Recognize amino acids Salt: Ensures proper dietary electrolyte

balance Sour and bitter: Warn against the intake

of potentially noxious or poisonous chemicals

PHENYLTHIOCARBAMIDE (PTC) TASTE RECEPTOR The inability to taste certain compounds is

usually due to simple, recessive Mendelian inheritance.

Dozens of taste and odorant receptors have been cloned and sequenced in the last 20 years.

The TAS2R28 gene encodes a bitter taste receptor that enables humans to taste the compound PTC.

The PTC (TAS2R28) gene has a single coding exon, for a polypeptide chain with 333 amino acids.

Three common single nucleotide polymorphisms (SNPs) are associated with PTC sensitivity.

Each SNP results in a change to the amino acid sequence of the PTC receptor.

Table 1. Polymorphisms within the PTC gene

Position (bp)

Position (amino acid)

SNP Allele

Amino Acid Encoded

145 49 C or G Pro or Ala785 262 C or T Ala or Val886 296 G or A Val or Ile

PTC taste receptor, continued

THE SNPS ARE USUALLY INHERITED TOGETHER IN CERTAIN COMBINATIONS, E.G., HAPLOTYPES

Table 2. SNP haplotypes of the PTC gene within two study groups (named for the first letter of the amino acid present at positions 49, 262 and 296)Haplotype European

Freq.East Asian Freq.

PAV 49% 70%

AVI 47% 30%

AAV (from recomb. at aa 49) 3% -

A later screen identified two additional haplotypes, PVI and AAI, which were found only in individuals of sub-Saharan African ancestry. The AVI haplotype was found in all populations except Southwest Native Americans (Kim et al., 2003).

CERTAIN HAPLOTYPES ARE GENERALLY CORRELATED WITH TASTER STATUS

Table 3. Genotype association with taste phenotypes (by haplotypes)

Genotype (diploid) Nontasters TastersAVI/AVI (73) 81% 19%AVI/AAV (21) 52% 48%*/PAV (170) 2% 98%

*= PAV, AVI or AAV. The total number of PTC genotypes observed was 5, as no AAV homozygotes were observed in the study group (Kim et al, 2003).

PTC taste sensitivity displays a broad and continuous distribution (e.g., it behaves like a quantitative trait).

On average, PTC taste sensitivity is highest for the PAV/PAV (taster) homozygotes, slightly but significantly lower for the PAV heterozygotes, and lowest by far for the AVI/AVI (non-taster) homozygotes.

More rare AVI/AAV heterozygotes have a mean PTC score slightly, but significantly, higher than the AVI/AVI homozygotes.

All non-human primates examined to date are homozygous for the PAV (taster) haplotype. Thus, the AVI nontaster haplotype arose after humans diverged from the most recent common primate ancestor.

There are non-taster chimps: same gene, but different mutation than humans => molecular convergent evolution!!

More Fun Informaton about PTC Receptor

DNA ISOLATION

PCR AMPLIFICATION

DIGEST WITH HAEIII

GEL ELECTROPHORESIS

RESULTS PCR amplification

and restriction digestion identifies the G-C polymorphism in the TAS2R38 gene.

The “C” allele, on the right, is digested by HaeIII and correlates with PTC tasting.

RESULTS