Allele-specific Expression of a Bitter Taste Receptor
Condition: Rhinosinusitis
Study type: Observational

Summary: 
      This single-site, within-subject experimental basic research study is designed to analyze the
      hypothesis that allele-specific expression of the bitter taste receptor T2R38 in taste tissue
      of individuals heterozygous for the taste receptor gene TAS2R38 correlates with that in nasal
      epithelium, and is responsible for differences in acyl-homoserine lactone-induced respiratory
      defenses. Subjects will include 100 predominantly European adults without chronic
      rhinosinusitis who will be undergoing a sinonasal procedure for reconstructive purposes. All
      subjects will provide saliva samples for genotyping, from which 25 subjects heterozygous for
      TAS2R38 (AVI/PAV) will be identified. These individuals will be asked to complete a beverage
      frequency questionnaire and taste test prior to the procedure that will evaluate for a number
      of compounds, among them bitter ligands specific to T2R38. Their tongue will also be
      photographed to evaluate the anatomy of their fungiform papillae, the mushroom-like
      structures on the tongue which contain taste buds. Subjects will subsequently provide nasal
      epithelium and taste tissue, which will be processed to 1) evaluate for allele-specific
      expression of TAS2R38 mRNA in both the taste and nasal tissue, with the nasal tissue
      concurrently being cultured in an air-liquid interface system to 2) assess the AHL-induced
      respiratory defenses of ciliary beat frequency (CBF) and nitric oxide (NO) production. Should
      subjects require a subsequent sinonasal procedure for clinically-determined reasons, taste
      and nasal tissue will again be obtained and analyzed for TAS2R38 mRNA, allowing for 3)
      longitudinal evaluation of mRNA expression level.
    

Detailed description: 
      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.
    

Intervention name(s): Observational study
Intervention type(s): Other
Observational model: Cohort
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        One hundred subjects will be tested for TAS2R38 genotype. Our typical subject population is
        approximately 82% of European descent and 35% female. All subjects will be recruited from
        those seen in the department of otorhinolaryngology's rhinology division and planning to
        undergo a sinonasal procedure for reconstructive purposes. The initial interview will occur
        in the clinic setting. From these 100 subjects, 25 who are heterozygous for TAS2R38 will be
        asked to perform a taste test, complete a beverage frequency questionnaire, have their
        tongue photographed, and provide nasal epithelium and taste tissue during their procedure.
        Key exclusion criteria include a history of chronic rhinosinusitis, oral disease, and any
        condition that would prevent them from completing the psychophysical testing.
      
Biospecimen description: 
      In part one of the study, each subject will provide a saliva sample. DNA will be isolated,
      purified, and quantitated for alleles of TAS2R38, a bitter taste receptor gene. Based on the
      results of the genetic screen, 25 individuals with the heterozygous genotype will be invited
      to return for part two of the study. In part two of the study, subject will provide nasal
      epithelium and taste tissue in the form of fungiform papillae. One half of each set of tissue
      will be placed in saline on ice in preparation for cell culture and assessment of AHL-induced
      defensive responses, while the second half will be placed in RNA preservative for mRNA
      isolation.
    
Keywords: chronic rhinosinusitis; airway physiology; taste receptors; tas2r; epithelial biology; upper respiratory disease