Bitter Taste and Allele-specific Expression of the Human TAS2R38 Gene

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.