Transcript Bitter Taste: Taste Testing (Phenotype) and PCR

Donna C. Sullivan, PhD
Division of Infectious Diseases
University of Mississippi Medical Center
 Sweet
 Sour
 Bitter
 Salty
 Umami
(the taste of monosodium
glutamate).
 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
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.

PTC taste receptor, continued
 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
Position
SNP
Amino Acid
(bp)
(amino acid) Allele
Encoded
145
49
C or G
Pro or Ala
785
262
C or T
Ala or Val
886
296
G or A
Val or Ile
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).
Table 3. Genotype association with taste phenotypes (by haplotypes)
Genotype (diploid)
AVI/AVI (73)
AVI/AAV (21)
*/PAV (170)
Nontasters
81%
52%
2%
Tasters
19%
48%
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).
More Fun Informaton about PTC Receptor
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!!

 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.