Allele-specific Expression of a Bitter Taste Receptor

The Basic Biology of Bitter Taste. The perception of bitter taste is thought to have evolved
as a mechanism to protect against the ingestion of toxic materials, and is the result of
ligand-activation of one of more than 25 different bitter taste receptors, so-called T2Rs
(1). These receptors are found on the tongue in what are called fungiform papillae,
mushroom-like structures that contain taste buds with receptors responding to a variety of
tastes including sweet, salty, sour, umami, and bitter. A prototypical example of one of
these bitter ligands is phenylthiocarbamide (PTC), which actives the T2R38 receptor. While
initially identified in type II taste cells, T2R38 is also expressed in nasal epithelium,
where it participates in innate immune defense responses to invading bacteria (2-6).

TAS2R38: A Model System for Genotype-Phenotype Studies. Prior studies have identified two
main forms of T2R38, active and inactive, which are characterized by three genetic variants
in the TAS2R38 gene. These variants result in three amino acid changes, proline (P) to
alanine (A) at position 49, alanine (A) to valine (V) at position 262, and valine (V) to
isoleucine (I) at position 296 in the T2R38 receptor. Individuals who are homozygous for the
active (PAV/PAV) form detect bitterness in compounds containing a thiourea (-N-C=S) moiety,
including PTC, 6-n-propylthiouracil (PROP), and the plant compound goitrin, common in foods
such as green vegetables (7-9). They also respond to acyl-homoserine lactones (AHLs), a class
of compounds produced as signaling molecules by certain bacteria, triggering a rapid defense
reaction consisting of increased ciliary beat frequency (CBF) to facilitate mucociliary
clearance, and generation of nitric oxide (NO), a gas that can diffuse into the airway and
kill bacteria (4). In contrast, those who are homozygous for all three variants (AVI/AVI)
consume these compounds without perceiving them as bitter and do not appear to respond to
AHLs (10). The frequency of both the active and inactive forms of TAS2R38 is at a near
balance of 50:50 in many human racial groups, including Americans of European and African
descent.

The Heterozygote Hypothesis. Interestingly, individuals heterozygous for the active form of
the receptor (AVI/PAV) exhibit highly variable phenotypes, with some people very sensitive to
bitter compounds, and others needing high concentrations to taste them at all (11). While the
investigators know that taste papillae density plays at least some role in this variability

, our preliminary taste data suggest that the range of response is tied to how much mRNA is
expressed from the active (PAV) form of the receptor, a concept called allele-specific
expression (12). For example, this is the case when analyzing caffeine consumption, which
strongly correlates with active mRNA expression (12). The investigators therefore hypothesize
that the abundance of active TAS2R38 mRNA in heterozygous individuals also predicts the
biologically significant change in magnitude of defensive responses in the presence of AHLs
(13-15). The proposed study will determine whether this is in fact the case, and whether
those people who have high mRNA abundance in taste tissue (fungiform papillae) also have
correspondingly high abundance in nasal epithelium, or whether regulation is
tissue-dependent. This will allow us to determine whether taste tests could provide a
reliable representation of receptor function in other tissues and cell types. Should mRNA
abundance prove to be a key factor, the investigators will determine whether high expressers
sustain this expression over time.

Of note, a study performed by Dr. Reed and collaborators found that the population in
Philadelphia contained 18% of individuals in the homozygous nontaster (AVI) group, 17% in the
homozygous taster (PAV) group and 37% in the common heterozygous group (AVI/PAV) (16). The
remaining 28% were distributed among ten other less common genotypes, which will not be
analyzed in this study. Thus, as the majority of the population is heterozygous, a thorough
understanding of their ability to fight infection is clinically important.

Clinical Significance. As one of the most common chronic conditions in the United States,
chronic rhinosinusitis invokes a direct treatment cost of $3.5-5 billion annually. Its
incidence is 146 per 1,000 and increasing (17). Our prior studies have shown that individuals
with two copies of the active form of T2R38 have nasal epithelium that defends very
effectively against certain bacteria, such as Pseudomonas aeruginosa, and are less likely to
develop severe chronic rhinosinusitis requiring surgery, while those with two inactive forms
cannot defend themselves as effectively, and are more likely to develop severe chronic
rhinosinusitis requiring surgical intervention. Because the treatment of chronic
rhinosinusitis involves multiple rounds of antibiotics and often surgical management through
functional endoscopic sinus surgery (FESS), this research has significant implications for
antibiotic stewardship, surgical morbidity and mortality, and health care expenditures.

Inclusion Criteria:

- Key inclusion criteria include age 21-50 years

- English speaking, and plans to undergo a sinonasal procedure for reconstructive
purposes or other reasons.

Exclusion Criteria:

- Key exclusion criteria include a history of chronic rhinosinusitis

- Plans to undergo a procedure for reasons other than reconstruction

- Oral disease

- Pregnancy, or any condition that would prevent psychophysical testing.

- Subjects showing signs of oral disease, including tongue lesions or xerostomia, would
be excluded from tongue sampling, and therefore excluded from the study.

- Subjects will not be excluded because of economic status, gender, race or ethnicity.