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Title: The T2R38 bitter taste receptor as a modifier of host response to Pseudomonas aeruginosa infection in cystic fibrosis
Author: Turnbull, Andrew Richard
ISNI:       0000 0004 7963 7644
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Rationale: Heterogeneity in cystic fibrosis (CF) lung disease is greater than can be explained by variability at the cystic fibrosis transmembrane conductance regulator (CFTR) locus, suggesting the involvement of modifier genes. The T2R38 (taste receptor 2, member 38) bitter taste receptor on respiratory epithelia detects Pseudomonas aeruginosa (Pa) N-acyl-L-homoserine lactones (AHLs). T2R38 activation leads to increased ciliary beat frequency (CBF) and thus mucociliary clearance, dependent on polymorphisms in the TAS2R38 gene. In the sinonasal airway, TAS2R38 genotype is proposed to be a modifier of host response to Pa; homozygosity for the functional allele (proline-alanine-valine; PAV) is proposed to be protective compared to homozygosity for the non-functional allele (alanine-valine-isoleucine; AVI) or compound heterozygosity. I hypothesised that the T2R38 receptor would be localised in the CF airway where it would be a modifier of host response to Pa; I also hypothesised that polymorphisms in the TAS2R38 gene would be associated with clinically important outcomes in patients with CF. Methods: T2R38 localisation was evaluated by immunocytochemistry in respiratory epithelia from CF and non-CF subjects. Attempts were made to develop an assay to measure change in CBF in airway epithelial cells in suspension or cultured at air liquid interface (ALI), exposed to AHLs and other chemical stimulants of CBF. AHL concentrations in Pa clinical isolates from CF patients of different TAS2R38 genotypes were quantified using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Finally, a large cohort of children and adults with CF were genotyped for the common TAS2R38 polymorphisms. Prevalence of intermittent or chronic Pa infection was analysed by logistic regression. Results: In CF nasal and bronchial epithelium, T2R38 localised to the ciliary rootlet in the same cellular distribution as in non-CF cells. Development of an in vitro assay to measure chemically-induced changes in CBF was limited by cell culture failure rates and assay sensitivity; change in CBF could be demonstrated with cellular exposure to the ciliary stimulant adenosine 5'-triphosphate (ATP), but not to Pa AHLs. Quantification of AHLs in a small sample of clinical Pa isolates revealed high within-sample variability, but the proportion of AHL-deficient Pa isolates did not differ between patients of different TAS2R38 genotypes. In the clinical cohort, 225 patients had AVI/AVI (33%), AVI/PAV (49%) or PAV/PAV (18%) TAS2R38 genotypes and well-defined Pa infection status. Only age, but not TAS2R38 genotype, was associated with Pa infection status. Lung function was not different in Pa-infected patients of the differing genotypes. Conclusions: TAS2R38 genotype does not influence prevalence or consequences of Pa infection in CF patients. This is not related to differences in T2R38 localisation in the CF airway, or to predominance of AHL-deficient Pa strains in subgroups. My data indicate that if there is an impact of TAS2R38 polymorphisms on the host response to Pa AHLs, this is not clinically relevant in CF. These findings suggest there to be no prognostic value of TAS2R38 genotyping in patients with CF, and do not indicate the T2R38 receptor to be a promising drug target in CF mucosal immunity.
Supervisor: Davies, Jane ; Alton, Eric ; Bush, Andy Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral