Natural selection and molecular evolution in PTC, a bitter-taste receptor gene

The Remarkable Diversity of Vertebrate Bitter Taste Receptors: Recent Advances in Genomic and Functional Studies

Bitter taste perception is crucial for animal survival. By detecting potentially harmful substances, such as plant secondary metabolites, as bitter, animals can avoid ingesting toxic compounds. In vertebrates, this function is mediated by taste receptors type 2 (T2Rs), a family of G protein-coupled receptors (GPCRs) expressed on taste buds. Given their vital roles, T2Rs have undergone significant selective pressures throughout vertebrate evolution, leading to frequent gene duplications and deletions, functional changes, and intrapopulation differentiation across various lineages. Recent advancements in genomic and functional research have uncovered the repertoires and functions of bitter taste receptors in a wide range of vertebrate species, shedding light on their evolution in relation to dietary habits and other ecological factors. This review summarizes recent research on bitter taste receptors and explores the mechanisms driving the diversity of these receptors from the perspective of vertebrate ecology and evolution.

Taste and oral somatosensation: Role of PTC bitter sensitivity, gender, and age

Sensitivity to the bitterness of phenylthiocarbamide (PTC) is considered to be linked to general taste perception, which is higher in women, and exhibits a slight decrease with age. Additionally, PTC sensitivity may contribute to somatosensory perception of spiciness and astringency. However, controversial data have been reported. This study investigates the relation between PTC perception, age, and gender on taste and somatosensory sensitivity using the Seven-iTT. A total of 200 healthy participants were classified based on their PTC perception, and subjects underwent an extended version of Seven-iTT to identify and rate the intensity of twelve stimuli. Results indicated that individuals classified as Tasters (T) exhibited higher taste scores compared to Non-Tasters (NT) for PTC, with age serving as a significant factor (p ≤ 0.009). Women outperformed men only among NT (p = 0.019). Older T demonstrated higher taste sensitivity than NT, suggesting that PTC status might act as a protective factor of gustatory function in aging. While PTC taster status significantly affected the intensity of sweet, salty, and bitter sensations (p ≤ 0.032), it had minimal impact on the intensity of somatosensory sensations (capsaicin and tannins). The combined use of Seven-iTT and the investigation of PTC status show promise for applications in research and clinical practice, offering a comprehensive approach to understanding taste perception and its implications for diet, nutrition, and aging.

Association of the bitter taste genes TAS2R38 and CA6 and breast cancer risk; a case-control study of Polish women in Poland and Polish immigrants in USA

It is known that the perception of bitterness is mediated by type 2 bitter taste receptors (TAS2Rs). However, recent reports have suggested that the carbonic anhydrase 6 (CA6) gene may also influence bitterness sensing. Genetic variants in these genes could influence dietary intake of brassica vegetables, whose increased consumption has been observed in the literature, though inconsistently, to decrease breast cancer (BC) risk. We hypothesized that the estimated odds ratios (ORs) for the association between BC and taster diplotype (PAV/PAV) and/or genotype A/A, will be in the direction of increased BC risk, potentially due to reduced consumption of brassica vegetables. Using a case-control study of BC in Polish women in Poland (210 cases and 262 controls) and Polish immigrant women to USA (78 cases and 170 controls) we evaluated the association of the taster diplotypes in TAS2R38 gene and genotypes in the CA6 gene and BC risk in these two populations individually and jointly. No significant increase in risk was observed for the TAS2R38 PAV/PAV diplotype (tasters) in each population individually or in the joint population. For the CA6 gene, in the joint population, we observed an increased BC risk for the combined G/A and G/G genotypes (non-tasters) vs A/A (tasters), OR = 1.41 (95% CI 1.04-1.90, p = 0.026) which after adjustment for False Discovery Rate (FDR), was not significant at p≤0.05 level. However, for the joint population and for the combined genotype of the two genes AVI/AVI+G* (non-tasters) vs. PAV/*+A/A (tasters), we observed a significant increase in BC risk, OR = 1.77 (95%CI 1.47-2.74, p = 0.01), for the non-tasters, which remained significant after FDR adjustment. In conclusion for the joint population and the joint effect for the two bitter sensing genes, we observed an increase in BC risk for the bitterness non-tasters, association which is in the opposite direction to our original hypothesis.

TAS2R38 bitter taste perception in the Koṅkaṇī Sārasvata Brahmin population

BACKGROUND

The TAS2R38 gene carries markers for phenylthiocarbamide (PTC) sensitivity. Various studies have investigated the genotype-phenotype association pattern for bitter tasting ability and other factors in different populations. However, a paucity of such information for endogamous Indian populations is the reason behind this study.

OBJECTIVE

To study the association of phenylthiocarbamide (PTC) sensitivity with TAS2R38 gene variations in Koṅkaṇī Sārasvata Brahmin population.

METHODS

We studied the association of the alleles rs714598, rs1726866, rs10246939 with PTC sensitivity and other factors in the Koṅkaṇī Sārasvata Brahmin population. DNA was extracted from 114 individuals belonging to the Koṅkaṇī Sārasvata Brahmin community. The TAS2R38 gene was sequenced to find the genotype distribution pattern. The association between genotype and phenotype was checked using the Chi-Square test and multifactorial logistical regression.

RESULTS

We observed a 58.8% frequency of the AVI haplotype, which is the most prevalent in European populations. A higher number of non-taster haplotypes and diplotypes were observed in Koṅkaṇī Sārasvata Brahmins, with the allele rs10246939 showing a significant association with PTC bitter taste sensitivity in both allelic (p = 8.6 × 10-4; Allele-G, OR = 3.57 [95% CI = 1.66-7.69]) and genotype-based (p = 6.9 × 10-4; genotype-AG, OR = 3.11 [95% CI = 0.73-13.20]; genotype-GG, OR = 40 [95% CI = 3.58-447.03]) tests.

CONCLUSION

Our results are in line with earlier studies, which report an association between PTC sensitivity and the TAS2R38 gene in different populations. In the global context, Koṅkaṇī Sārasvata Brahmins, who are mostly distributed along the southwestern coast of India, show a PTC sensitivity pattern slightly similar to that of West Eurasian populations. Our findings suggest ancestry specific selection in TAS2R38 gene variations for taste sensitivity at global level.

Activation Profile of TAS2R2, the 26th Human Bitter Taste Receptor

SCOPE

To avoid ingestion of potentially harmful substances, humans are equipped with about 25 bitter taste receptor genes (TAS2R) expressed in oral taste cells. Humans exhibit considerable variance in their bitter tasting abilities, which are associated with genetic polymorphisms in bitter taste receptor genes. One of these variant receptor genes, TAS2R2, is initially believed to represent a pseudogene. However, TAS2R2 exists in a putative functional variant within some populations and can therefore be considered as an additional functional bitter taste receptor.

METHODS AND RESULTS

To learn more about the function of the experimentally neglected TAS2R2, a functional screening with 122 bitter compounds is performed. The study observes responses with eight of the 122 bitter substances and identifies the substance phenylbutazone as a unique activator of TAS2R2 among the family of TAS2Rs, thus filling one more gap in the array of cognate bitter substances.

CONCLUSIONS

The comprehensive characterization of the receptive range of TAS2R2 allows the classification into the group of TAS2Rs with a medium number of bitter agonists. The variability of bitter taste and its potential influences on food choice in some human populations may be even higher than assumed.

Automatic extraction of ranked SNP-phenotype associations from text using a BERT-LSTM-based method

Extraction of associations of singular nucleotide polymorphism (SNP) and phenotypes from biomedical literature is a vital task in BioNLP. Recently, some methods have been developed to extract mutation-diseases affiliations. However, no accessible method of extracting associations of SNP-phenotype from content considers their degree of certainty. In this paper, several machine learning methods were developed to extract ranked SNP-phenotype associations from biomedical abstracts and then were compared to each other. In addition, shallow machine learning methods, including random forest, logistic regression, and decision tree and two kernel-based methods like subtree and local context, a rule-based and a deep CNN-LSTM-based and two BERT-based methods were developed in this study to extract associations. Furthermore, the experiments indicated that although the used linguist features could be employed to implement a superior association extraction method outperforming the kernel-based counterparts, the used deep learning and BERT-based methods exhibited the best performance. However, the used PubMedBERT-LSTM outperformed the other developed methods among the used methods. Moreover, similar experiments were conducted to estimate the degree of certainty of the extracted association, which can be used to assess the strength of the reported association. The experiments revealed that our proposed PubMedBERT-CNN-LSTM method outperformed the sophisticated methods on the task.

Automated identification of the genetic variants of TAS2R38 bitter taste receptor with supervised learning

Several studies were focused on the genetic ability to taste the bitter compound 6-n-propylthiouracil (PROP) to assess the inter-individual taste variability in humans, and its effect on food predilections, nutrition, and health. PROP taste sensitivity and that of other chemical molecules throughout the body are mediated by the bitter receptor TAS2R38, and their variability is significantly associated with TAS2R38 genetic variants. We recently automatically identified PROP phenotypes with high precision using Machine Learning (mL). Here we have used Supervised Learning (SL) algorithms to automatically identify TAS2R38 genotypes by using the biological features of eighty-four participants. The catBoost algorithm was the best-suited model for the automatic discrimination of the genotypes. It allowed us to automatically predict the identification of genotypes and precisely define the effectiveness and impact of each feature. The ratings of perceived intensity for PROP solutions (0.32 and 0.032 mM) and medium taster (MT) category were the most important features in training the model and understanding the difference between genotypes. Our findings suggest that SL may represent a trustworthy and objective tool for identifying TAS2R38 variants which, reducing the costs and times of molecular analysis, can find wide application in taste physiology and medicine studies.

Variations in the TAS2R38 gene among college students in Hubei

BACKGROUND

The bitter taste receptor gene TAS2R38 is a member of the human TAS2R gene family. Polymorphisms in TAS2R38 affect the ability to taste the bitterness of phenylthiourea (PTC) compounds, thus affecting an individual's food preference and health status.

METHODS

We investigated polymorphisms in the TAS2R38 gene and the sensitivity to PTC bitterness among healthy Chinese college students in Hubei province. The association of TAS2R38 polymorphisms and PTC sensitivity with body mass index (BMI), food preference, and health status was also analyzed. A total of 320 healthy college students were enrolled (male: 133, female: 187; aged 18-23 years). The threshold value method was used to measure the perception of PTC bitterness, and a questionnaire was used to analyze dietary preferences and health status. Polymerase chain reaction (PCR) was used to analyze polymorphisms at three common TAS2R38 loci (rs713598, rs1726866, and rs10246939).

RESULTS

In our study population, 65.00% of individuals had medium sensitivity to the bitterness of PTC; in contrast, 20.94% were highly sensitive to PTC bitterness, and 14.06% were not sensitive. For the TAS2R38 gene, the PAV/PAV and PAV/AAI diplotypes were the most common (42.19% and 40.63%, respectively), followed by the homozygous AVI/AVI (8.75%) and PAV/AVI (5.00%) diplotypes.

CONCLUSION

There was a significant correlation between the sensitivity to PTC bitterness and sex, but there was no correlation between the common diplotypes of TAS2R38 and gender. Polymorphisms in the TAS2R38 gene were associated with the preference for tea, but not with one's native place, BMI, health status, or other dietary preferences. There was no significant correlation between the perception of PTC bitterness and one's native place, BMI, dietary preference, or health status. We hope to find out the relationship between PTC sensitivity and TAS2R38 gene polymorphisms and dietary preference and health status of Chinese population through this study, providing relevant guidance and suggestions for dietary guidance and prevention of some chronic diseases in Chinese population.

Associations between Sweet Taste Sensitivity and Polymorphisms (SNPs) in the TAS1R2 and TAS1R3 Genes, Gender, PROP Taster Status, and Density of Fungiform Papillae in a Genetically Homogeneous Sardinian Cohort

Individual differences in sweet taste sensitivity can affect dietary preferences as well as nutritional status. Despite the lack of consensus, it is believed that sweet taste is impacted by genetic and environmental variables. Here we determined the effect of well-established factors influencing the general taste variability, such as gender and fungiform papillae density, specific genetic variants (SNPs of TAS1R2 and TAS1R3 receptors genes), and non-specific genetic factors (PROP phenotype and genotype), on the threshold and suprathreshold sweet taste sensitivity. Suprathreshold measurements showed that the sweet taste response increased in a dose-dependent manner, and this was related to PROP phenotype, gender, rs35874116 SNP in the TAS1R2 gene, and rs307355 SNP in the TAS1R3 gene. The threshold values and density of fungiform papillae exhibited a strong correlation, and both varied according to PROP phenotype. Our data confirm the role of PROP taste status in the sweet perception related to fungiform papilla density, show a higher sweet sensitivity in females who had lower BMI than males, and demonstrate for the first time the involvement of the rs35874116 SNP of TAS1R2 in the sweet taste sensitivity of normal weight subjects with body mass index (BMI) ranging from 20.2 to 24.8 kg/m2. These results may have an important impact on nutrition and health mostly in subjects with low taste ability for sweets and thus with high vulnerability to developing obesity or metabolic disease.

Global population genetics and diversity in the TAS2R bitter taste receptor family

Bitter taste receptors (TAS2Rs) are noted for their role in perception, and mounting evidence suggests that they mediate responses to compounds entering airways, gut, and other tissues. The importance of these roles suggests that TAS2Rs have been under pressure from natural selection. To determine the extent of variation in TAS2Rs on a global scale and its implications for human evolution and behavior, we analyzed patterns of diversity in the complete 25 gene repertoire of human TAS2Rs in ∼2,500 subjects representing worldwide populations. Across the TAS2R family as a whole, we observed 721 single nucleotide polymorphisms (SNPs) including 494 nonsynonymous SNPs along with 40 indels and gained and lost start and stop codons. In addition, computational predictions identified 169 variants particularly likely to affect receptor function, making them candidate sources of phenotypic variation. Diversity levels ranged widely among loci, with the number of segregating sites ranging from 17 to 41 with a mean of 32 among genes and per nucleotide heterozygosity (π) ranging from 0.02% to 0.36% with a mean of 0.12%. F_ST ranged from 0.01 to 0.26 with a mean of 0.13, pointing to modest differentiation among populations. Comparisons of observed π and F_ST values with their genome wide distributions revealed that most fell between the 5th and 95th percentiles and were thus consistent with expectations. Further, tests for natural selection using Tajima’s D statistic revealed only two loci departing from expectations given D’s genome wide distribution. These patterns are consistent with an overall relaxation of selective pressure on TAS2Rs in the course of recent human evolution.

TAS2R38 polymorphisms, Helicobacter pylori infection and susceptibility to gastric cancer and premalignant gastric lesions

BACKGROUND

Gastric cancer is worldwide the fourth more common cancer type by incidence, and the third by mortality. We analyzed three missense variants of TAS2R38 gene: rs713598 (A49P), rs1726866 (V262A), and rs10246939 (I296V). These variants and their combination in haplotypes (proline, alanine and valine/tasters or alanine, valine and isoleucine/nontasters) and diplotypes are responsible for individual differences in bitter perception. The single-nucleotide polymorphisms and the related phenotypes are known to be associated with susceptibility to Gram-negative bacterial infections, such as Helicobacter pylori , and with risk of various cancer types. An association between intermediate tasters (as defined by TAS2R38 diplotypes) and increased risk of gastric cancer was reported in a Korean population.

METHODS

We analyzed 2616 individuals of Latin American origin, representing the whole spectrum of lesions from gastritis to gastric cancer.

RESULTS

Comparing cancer cases vs. noncancers we observed a decrease in risk associated with heterozygous carriers of rs10246939 ( P  = 0.006) and rs1726866 ( P  = 0.003) when compared with homozygotes of the more common allele. Also, the analysis of diplotypes/phenotypes reflected the same association, with super-tasters showing a borderline increased risk of developing gastric cancer compared to medium-tasters [odds ratio (OR) = 1.63; 95% confidence interval (CI), 1.04-2.56; P  = 0.033]. Also, nontasters showed an increased risk when compared to medium-tasters although not reaching statistical significance (OR = 1.58; 95% CI, 0.80-2.87; P  = 0.203). We also tested the interactions between the TAS2R38 genotypes and H. pylori cagA status in a subset of samples and found no interaction.

CONCLUSION

In conclusion, our results suggest only a modest contribution of TAS2R38 gene genetic variability in gastric cancer etiology.

Association of phenylthiocarbamide perception with anthropometric variables and intake and liking for bitter vegetables

Phenylthiocarbamide (PTC) sensitivity, a sensory trait mediated by the bitter taste receptor 38 (TAS2R38), has been described as a promising biomarker of health status or disease risk. The aim of this cross-sectional study was to evaluate the influence of PTC phenotypes on (1) individual anthropometric and clinical history variables; (2) other basic taste recognition thresholds (RTs), and (3) the hedonic perception and habitual intake of Brassicaceae vegetables in a young adult population (18.9 ± 1.7 years old). The PTC phenotype was determined by the quantitative measure of the PTC recognition threshold (non-tasters, 24.1%; tasters, 52.3%; and super tasters, 23.6%). No significant differences in smoking habits, oral and nasal disorders, family antecedents of diseases related to metabolic syndrome, and Brassicaceae vegetable hedonic perception and consumption were found between the PTC phenotype groups. The average BMI of super-taster females and males was significantly lower compared to non-tasters. In addition, the PTC taster status was a predictor of lower scores for other basic taste RTs. Overall, the defined PTC super-taster cohort could be differentiated from the non-tasters by variables related to weight control such as BMI and sucrose RT.

Population Genomics Approaches for Genetic Characterization of SARS-CoV-2 Lineages

The genome of the Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2), the pathogen that causes coronavirus disease 2019 (COVID-19), has been sequenced at an unprecedented scale leading to a tremendous amount of viral genome sequencing data. To assist in tracing infection pathways and design preventive strategies, a deep understanding of the viral genetic diversity landscape is needed. We present here a set of genomic surveillance tools from population genetics which can be used to better understand the evolution of this virus in humans. To illustrate the utility of this toolbox, we detail an in depth analysis of the genetic diversity of SARS-CoV-2 in first year of the COVID-19 pandemic. We analyzed 329,854 high-quality consensus sequences published in the GISAID database during the pre-vaccination phase. We demonstrate that, compared to standard phylogenetic approaches, haplotype networks can be computed efficiently on much larger datasets. This approach enables real-time lineage identification, a clear description of the relationship between variants of concern, and efficient detection of recurrent mutations. Furthermore, time series change of Tajima's D by haplotype provides a powerful metric of lineage expansion. Finally, principal component analysis (PCA) highlights key steps in variant emergence and facilitates the visualization of genomic variation in the context of SARS-CoV-2 diversity. The computational framework presented here is simple to implement and insightful for real-time genomic surveillance of SARS-CoV-2 and could be applied to any pathogen that threatens the health of populations of humans and other organisms.

Re-evaluating how sweet-liking and PROP-tasting are related

Past research has identified distinct phenotypic differences in responses to sweet taste, although the origins of these differences remain unclear. One possibility is that these individual differences in sweet-liking are a manifestation of the more widely known differences in sensitivity to the bitter tastant 6-n-propylthiouracil (PROP), which has been related to wider differences in food liking and preference. However, previous studies exploring the relationship between sweet-liking and PROP-tasting have had mixed outcomes. This is possibly due to older studies using a more simplistic dichotic characterisation of sweet likers, whereas recent research suggests three sweet-liking phenotypes (extreme sweet likers, ESL; moderate sweet likers, MSL; and sweet dislikers, SD). To re-assess how sweet-liking and PROP tasting are inter-related, 236 volunteers evaluated their liking for 1.0 M sucrose and the intensity of three concentrations of each NaCl and PROP. Using three different methods for classifying PROP taster status, our analysis confirmed that all three sweet-liking phenotypes were represented in all three PROP taster groups (super-tasters, ST; medium tasters, MT; and non-tasters, NT), but relatively few ESL were classified as ST, or SD as NT. Overall, these data suggest that while PROP tasting and sweet-liking are not causally related, the SD phenotype may partly be explained by a broader tendency for anhedonia.

Automated Classification of 6-n-Propylthiouracil Taster Status with Machine Learning

Several studies have used taste sensitivity to 6-n-propylthiouracil (PROP) to evaluate interindividual taste variability and its impact on food preferences, nutrition, and health. We used a supervised learning (SL) approach for the automatic identification of the PROP taster categories (super taster (ST); medium taster (MT); and non-taster (NT)) of 84 subjects (aged 18-40 years). Biological features determined from subjects were included for the training system. Results showed that SL enables the automatic identification of objective PROP taster status, with high precision (97%). The biological features were classified in order of importance in facilitating learning and as prediction factors. The ratings of perceived taste intensity for PROP paper disks (50 mM) and PROP solution (3.2 mM), along with fungiform papilla density, were the most important features, and high estimated values pushed toward ST prediction, while low values leaned toward NT prediction. Furthermore, TAS2R38 genotypes were significant features (AVI/AVI, PAV/PAV, and PAV/AVI to classify NTs, STs, and MTs, respectively). These results, in showing that the SL approach enables an automatic, immediate, scalable, and high-precision classification of PROP taster status, suggest that it may represent an objective and reliable tool in taste physiology studies, with applications ranging from basic science and medicine to food sciences.

Worldwide diversity, association potential, and natural selection in the superimposed taste genes, CD36 and GNAT3

CD36 and GNAT3 mediate taste responses, with CD36 acting as a lipid detector and GNAT3 acting as the α subunit of gustducin, a G protein governing sweet, savory, and bitter transduction. Strikingly, the genes encoding CD36 and GNAT3 are genomically superimposed, with CD36 completely encompassing GNAT3. To characterize genetic variation across the CD36-GNAT3 region, its implications for phenotypic diversity, and its recent evolution, we analyzed from ~2,500 worldwide subjects sequenced by the 1000 Genomes Project (1000GP). CD36-GNAT3 harbored extensive diversity including 8,688 single-nucleotide polymorphisms (SNPs), 414 indels, and other complex variants. Sliding window analyses revealed that nucleotide diversity and population differentiation across CD36-GNAT3 were consistent with genome-wide trends in the 1000GP (π = 0.10%, P = 0.64; FST = 9.0%, P = 0.57). In addition, functional predictions using SIFT and PolyPhen-2 identified 60 variants likely to alter protein function, and they were in weak linkage disequilibrium (r2 < 0.17), suggesting their effects are largely independent. However, the frequencies of predicted functional variants were low (P¯ = 0.0013), indicating their contributions to phenotypic variance on population scales are limited. Tests using Tajima's D statistic revealed that pressures from natural selection have been relaxed across most of CD36-GNAT3 during its recent history (0.39 < P < 0.67). However, CD36 exons showed signs of local adaptation consistent with prior reports (P < 0.035). Thus, CD36 and GNAT3 harbor numerous variants predicted to affect taste sensitivity, but most are rare and phenotypic variance on a population level is likely mediated by a small number of sites.

Bitter taste receptors: Genes, evolution and health

Bitter taste perception plays vital roles in animal behavior and fitness. By signaling the presence of toxins in foods, particularly noxious defense compounds found in plants, it enables animals to avoid exposure. In vertebrates, bitter perception is initiated by TAS2Rs, a family of G protein-coupled receptors expressed on the surface of taste buds. There, oriented toward the interior of the mouth, they monitor the contents of foods, drinks and other substances as they are ingested. When bitter compounds are encountered, TAS2Rs respond by triggering neural pathways leading to sensation. The importance of this role placed TAS2Rs under selective pressures in the course of their evolution, leaving signatures in patterns of gene gain and loss, sequence polymorphism, and population structure consistent with vertebrates' diverse feeding ecologies. The protective value of bitter taste is reduced in modern humans because contemporary food supplies are safe and abundant. However, this is not always the case. Some crops, particularly in the developing world, retain surprisingly high toxicity and bitterness remains an important measure of safety. Bitter perception also shapes health through its influence on preference driven behaviors such as diet choice, alcohol intake and tobacco use. Further, allelic variation in TAS2Rs is extensive, leading to individual differences in taste sensitivity that drive these behaviors, shaping susceptibility to disease. Thus, bitter taste perception occupies a critical intersection between ancient evolutionary processes and modern human health.

A New World Monkey Resembles Human in Bitter Taste Receptor Evolution and Function via a Single Parallel Amino Acid Substitution

Bitter taste receptors serve as a vital component in the defense system against toxin intake by animals, and the family of genes encoding these receptors has been demonstrated, usually by family size variance, to correlate with dietary preference. However, few systematic studies of specific Tas2R to unveil their functional evolution have been conducted. Here, we surveyed Tas2R16 across all major clades of primates and reported a rare case of a convergent change to increase sensitivity to β-glucopyranosides in human and a New World monkey, the white-faced saki. Combining analyses at multiple levels, we demonstrate that a parallel amino acid substitution (K172N) shared by these two species is responsible for this functional convergence of Tas2R16. Considering the specialized feeding preference of the white-faced saki, the K172N change likely played an important adaptive role in its early evolution to avoid potentially toxic cyanogenic glycosides, as suggested for the human TAS2R16 gene.

Contrasting Patterns of Gene Duplication, Relocation, and Selection Among Human Taste Genes

In humans, taste genes are responsible for perceiving at least 5 different taste qualities. Human taste genes’ evolutionary mechanisms need to be explored. We compiled a list of 69 human taste-related genes and divided them into 7 functional groups. We carried out comparative genomic and evolutionary analyses for these taste genes based on 8 vertebrate species. We found that relative to other groups of human taste genes, human TAS2R genes have a higher proportion of tandem duplicates, suggesting that tandem duplications have contributed significantly to the expansion of the human TAS2R gene family. Human TAS2R genes tend to have fewer collinear genes in outgroup species and evolve faster, suggesting that human TAS2R genes have experienced more gene relocations. Moreover, human TAS2R genes tend to be under more relaxed purifying selection than other genes. Our study sheds new insights into diverse and contrasting evolutionary patterns among human taste genes.

Using the Integrated Genome Viewer to reveal amplicon-derived polymorphism enriched at the phenylthiocarbamide locus in the teaching lab

Due to its distinct phenotype and relatively simple inheritance pattern, the phenylthiocarbamide (PTC) loci is frequently utilized in teaching laboratories to demonstrate genetic concepts such as Mendelian inheritance and population genetics. We have developed a next-generation sequencing and bioinformatics approach to analyze the PTC gene locus to reveal single nucleotide polymorphism (SNP) variation at nucleotide position 785 that predicts tasting ability in humans. Here students purify DNA from their own cheek cells, perform polymerase chain reaction (PCR) amplification of the PTC gene followed by cleaved amplified polymorphic sequence (CAPS) testing. Students perform a second PCR on the PTC loci using high-fidelity Taq to create bar-coded amplicons for next-generation sequencing on the Ion Torrent Personal Genome Machine. Bioinformatic verification reveals polymorphic variation by aligning the entire class PTC PCR fragment sequence to the human gene using Bowtie2 and visualizing the results in the Integrated Genome Viewer. This exercise presents a learning opportunity for students to use next-generation sequencing to predict their own PTC taste sensitivity phenotype coupled with the standard CAPS method. This approach brings the PTC teaching method into the genomics era.

Association of single nucleotide polymorphisms with taste and food preferences of the Hungarian general and Roma populations

It is reasonable to suppose that poor diet underlies the unfavorable health status of the Roma population of Europe. Previously in the framework of a complex health survey, fruit and vegetable consumption, quantity of sugar added, salting frequency; bitter, salty, sweet and fat taste preferences were evaluated of Hungarian (HG, n = 410) and Roma (HR, n = 387) populations. In the present study the associations of taste and food preferences with TAS1R3, CD36, SCNN1B, TRPV1, TAS2R38, TAS2R19 and CA6 polymorphisms were tested in the same samples. Genotype frequencies did not differ significantly between the two populations. Although we initially observed associations between certain genetic polymorphisms and taste and food preferences in our study samples, none of the p values remained significant after the multiple test correction. However, some of our results could be considered promising (0.05<corrected p < 0.20), which showed potential ethnicity-specific effects (CA6 rs2274333 with salty taste and raw kohlrabi preference, CD36 rs1527483 with fat taste preference, TAS2R19 rs10772420 with grapefruit preference, and TAS2R38 rs713598 with quantity of sugar added). Our results may suggest that genetics may mediate food preferences, and individuals with different ethnic background may require personalized interventions to modify diet. Further investigations with greater sample sizes are essential to explore the effect of these genetic variants on taste and food preferences.

Allyl Isothiocyanate: A TAS2R38 Receptor-Dependent Immune Modulator at the Interface Between Personalized Medicine and Nutrition

Understanding individual responses to nutrition and medicine is of growing interest and importance. There is evidence that differences in bitter taste receptor (TAS2R) genes which give rise to two frequent haplotypes, TAS2R38-PAV (functional) and TAS2R38-AVI (non-functional), may impact inter-individual differences in health status. We here analyzed the relevance of the TAS2R38 receptor in the regulation of the human immune response using the TAS2R38 agonist allyl isothiocyanate (AITC) from Brassica plants. A differential response in calcium mobilization upon AITC treatment in leucocytes from healthy humans confirmed a relevance of TAS2R38 functionality, independent from cation channel TRPV1 or TRPA1 activation. We further identified a TAS2R38-dependence of MAPK and AKT signaling activity, bactericidal (toxicity against E. coli) and anti-inflammatory activity (TNF-alpha inhibition upon cell stimulation). These in vitro results were derived at relevant human plasma levels in the low micro molar range as shown here in a human intervention trial with AITC-containing food.

Changes of Taste, Smell and Eating Behavior in Patients Undergoing Bariatric Surgery: Associations with PROP Phenotypes and Polymorphisms in the Odorant-Binding Protein OBPIIa and CD36 Receptor Genes

Bariatric surgery is the most effective long-term treatment for severe obesity and related comorbidities. Although patients who underwent bariatric surgery report changes of taste and smell perception, results from sensory studies are discrepant and limited. Here, we assessed taste and smell functions in 51 patients before, one month, and six months after undergoing bariatric surgery. We used taste strip tests to assess gustatory function (including sweetness, saltiness, sourness, umaminess, bitterness and oleic acid, a fatty stimulus), the “Sniffin’ Sticks” test to assess olfactory identification and the 3-Factor Eating Questionnaire to assess eating behavior. We also explored associations between these phenotypes and flavor-related genes. Results showed an overall improvement in taste function (including increased sensitivity to oleic acid and the bitterness of 6-n-propylthiouracil (PROP)) and in olfactory function (which could be related to the increase in PROP and oleic acid sensitivity), an increase in cognitive restraint, and a decrease in disinhibition and hunger after bariatric surgery. These findings indicate that bariatric surgery can have a positive impact on olfactory and gustatory functions and eating behavior (with an important role of genetic factors, such PROP tasting), which in turn might contribute to the success of the intervention.

Ecological Sensing Through Taste and Chemosensation Mediates Inflammation: A Biological Anthropological Approach

Ecological sensing and inflammation have evolved to ensure optima between organism survival and reproductive success in different and changing environments. At the molecular level, ecological sensing consists of many types of receptors located in different tissues that orchestrate integrated responses (immune, neuroendocrine systems) to external and internal stimuli. This review describes emerging data on taste and chemosensory receptors, proposing them as broad ecological sensors and providing evidence that taste perception is shaped not only according to sense epitopes from nutrients but also in response to highly diverse external and internal stimuli. We apply a biological anthropological approach to examine how ecological sensing has been shaped by these stimuli through human evolution for complex interkingdom communication between a host and pathological and symbiotic bacteria, focusing on population-specific genetic diversity. We then focus on how these sensory receptors play a major role in inflammatory processes that form the basis of many modern common metabolic diseases such as obesity, type 2 diabetes, and aging. The impacts of human niche construction and cultural evolution in shaping environments are described with emphasis on consequent biological responsiveness.

Insights into the Function and Evolution of Taste 1 Receptor Gene Family in the Carnivore Fish Gilthead Seabream (Sparus aurata)

A plethora of molecular and functional studies in tetrapods has led to the discovery of multiple taste 1 receptor (T1R) genes encoding G-protein coupled receptors (GPCRs) responsible for sweet (T1R2 + T1R3) and umami (T1R1 + T1R3) taste. In fish, the T1R gene family repertoires greatly expanded because of several T1R2 gene duplications, and recent studies have shown T1R2 functional divergence from canonical mammalian sweet taste perceptions, putatively as an adaptive mechanism to develop distinct feeding strategies in highly diverse aquatic habitats. We addressed this question in the carnivore fish gilthead seabream (Sparus aurata), a model species of aquaculture interest, and found that the saT1R gene repertoire consists of eight members including saT1R1, saT1R3 and six saT1R2a-f gene duplicates, adding further evidence to the evolutionary complexity of fishT1Rs families. To analyze saT1R taste functions, we first developed a stable gene reporter system based on Ca²⁺-dependent calcineurin/NFAT signaling to examine specifically in vitro the responses of a subset of saT1R heterodimers to L-amino acids (L-AAs) and sweet ligands. We show that although differentially tuned in sensitivity and magnitude of responses, saT1R1/R3, saT1R2a/R3 and saT1R2b/R3 may equally serve to transduce amino acid taste sensations. Furthermore, we present preliminary information on the potential involvement of the Gi protein alpha subunits saGαi1 and saGαi2 in taste signal transduction.

MinProtMaxVP: Generating a minimized number of protein variant sequences containing all possible variant peptides for proteogenomic analysis

Identifying single-amino-acid variants (SAVs) from mass spectrometry-based experiments is critical for validating single-nucleotide variants (SNVs) at the protein level to facilitate biomedical research. Currently, two approaches are usually applied to convert SNV annotations into SAV-harboring protein sequences. One approach generates one sequence containing exactly one SAV, and the other all SAVs. However, they may neglect the possibility of SAV combinations, e.g., haplotypes, existing in bio-samples. Therefore, it is necessary to consider all SAV combinations of a protein when generating SAV-harboring protein sequences. In this paper, we propose MinProtMaxVP, a novel approach which selects a minimized number of SAV-harboring protein sequences generated from the exhaustive approach, while still accommodating all possible variant peptides, by solving a classic set covering problem. Our study on known haplotype variations of TAS2R38 justifies the necessity for MinProtMaxVP to consider all combinations of SAVs. The performance of MinProtMaxVP is demonstrated by an in silico study on OR2T27 with five SAVs and real experimental data of the HEK293 cell line. Furthermore, assuming simulated somatic and germline variants of OR2T27 in tumor and normal tissues demonstrates that when adopting the appropriate somatic and germline SAV integration strategy, MinProtMaxVP is adaptable to labeling and label-free mass spectrometry-based experiments. SIGNIFICANCE: We present MinProtMaxVP, a novel approach to generate SAV-harboring protein sequences for constructing a customized protein sequence database, which is used in database searching for variant peptide identification. This approach outperforms the existing approaches in generating all possible variant peptides to be included in protein sequences and possibly leading to identification of more variant peptides in proteogenomic analysis.

TAS2R38 Haplotype Predicts 24-Hour Urinary Sodium Excretion in Patients With Heart Failure and Their Family Caregivers

BACKGROUND

Adherence to a low-sodium diet is essential to self-care of heart failure (HF). Genetic determinants of preference for high-sodium foods may impede adherence but have not been well-studied.

OBJECTIVE

Our purpose was to examine if TAS2R38 haplotype predicted salt taste sensitivity and dietary sodium intake among patients with HF.

METHOD

This pilot study used baseline data from a large interventional randomized control trial to support adherence to a low-sodium diet in patients with HF and their family caregivers. Participants were tested for salt taste sensitivity and provided a 24-hour urinary sodium sample and a blood sample for DNA analysis at baseline. Fungiform papillae were counted. χ2 Test and 1-way analysis of variance were used to compare haplotype groups. Linear regression was performed to examine predictors of salt taste sensitivity and 24-hour urinary sodium excretion, controlling for age, gender, ethnicity, smoking status, and fungiform papillae density.

RESULTS

There were 42 patients with HF and their family caregivers (age, 64.6 ± 13.4 years, 46.5% male, 97.7% white, and 90.7% nonsmoker). Pronine-alanine-valine homozygous haplotype predicted lower urinary sodium excretion (b = -1780.59, t41 = -2.18, P = .036), but genotype was not a significant predictor of salt taste sensitivity.

CONCLUSIONS

The results of our study partially supported our hypothesis that PAV homozygous haplotype predicts 24-hour urinary sodium excretion. With our small sample size, more research is needed. Understanding genetic influences on taste can lead to development of educational interventions tailored to patients with HF and their family caregivers to better support dietary adherence.

The roles of genes in the bitter taste

The aims of this review were to understand the roles of bitter taste genes in humans. Some of the peoples have the capacity to taste some chemical substance such as phenylthiocarbamide (PTC) while others cant not based on the dietary hazards and food preferences. There are two alleles responsible to express these phenotypes which are homozygous recessive. In human TAS2R38 genes located on the chromosome number 7 and consist of different nucleotide polymorphism that related to detection of the phenotype of different chemical compounds such as 6-n-propylthiouracil (PROP) and phenylthiocarbamide bitterness and this Gene is the member of the TAS2R genes which are eleven pseudogenes and twenty that has roles in many biological processes.

There are many factors that affect the bitter taste such as food, age, sex, and different diseases. The mechanism of food bitter taste and genotype of TAS2R38 until know not well understood due to that the proof of relation between bitter taste sensitivity and food is harmful. there are many different diseases can impact the influence of taste such as neoplasm and lifestyle such as consumption of alcohol along with the use of medication, head trauma, upper tract infections. On the other hand, A relation between TAS2R38 genotype and meal preferences has been observed among children, however, no associations have been mentioned among older adults. Some previous research proved some vital points that show an association between type 1 of diabetes and phenylthiocarbamide (PTC) but other studies cannot demonstrate that. However, of other disease such as obesity is controversial but other studies reported to the relationship between them.

Genetic Background of Taste Perception, Taste Preferences, and Its Nutritional Implications: A Systematic Review

Background: The rise in nutrition-related morbidity and mortality requires public health intervention programs targeting nutritional behavior. In addition to socio-economical, socio-cultural, psychological determinants, taste is one of the main factors that influence food choices. Differences in taste perception and sensitivity may be explained by genetic variations, therefore the knowledge of the extent to which genetic factors influence the development of individual taste preferences and eating patterns is important for public policy actions addressing nutritional behaviors. Our aim was to review genetic polymorphisms accounting for variability in taste and food preferences to contribute to an improved understanding of development of taste and food preferences.

Methods: The electronic databases PubMed, Scopus, and Web of Science were searched using MeSH in PubMed and free text terms for articles published between January 1, 2000 and April 13, 2018. The search strategy was conducted following the PRISMA statement. The quality of the included studies was assessed by the validated Q-Genie tool.

Results: Following the PRISMA flowchart, finally 103 articles were included in the review. Among the reviewed studies, 43 were rated to have good quality, 47 were rated to have moderate quality, and 13 were rated to have low quality. The majority of the studies assessed the association of genetic variants with the bitter taste modality, followed by articles analyzing the impact of polymorphisms on sweet and fat preferences. The number of studies investigating the association between umami, salty, and sour taste qualities and genetic polymorphisms was limited.

Conclusions: Our findings suggest that a significant association exists between TAS2R38 variants (rs713598, rs1726866, rs10246939) and bitter and sweet taste preference. Other confirmed results are related to rs1761667 (CD36) and fat taste responsiveness. Otherwise further research is essential to confirm results of studies related to genetic variants and individual taste sensitivity. This knowledge may enhance our understanding of the development of individual taste and related food preferences and food choices that will aid the development of tailored public health strategy to reduce nutrition-related disease and morbidity.

A role for taste receptors in (neuro)endocrinology?

The sense of taste is positioned at the forefront when it comes to the interaction of our body with foodborne chemicals. However, the role of our taste system, and in particular its associated taste receptors, is not limited to driving food preferences leading to ingestion or rejection before other organs take over responsibility for nutrient digestion, absorption and metabolic regulation. Taste sensory elements do much more. On the one hand, extra-oral taste receptors from the brain to the gut continue to sense nutrients and noxious substances after ingestion and, on the other hand, the nutritional state feeds back on the taste system. This intricate regulatory network is orchestrated by endocrine factors that are secreted in response to taste receptor signalling and, in turn regulate the taste receptor cells themselves. The present review summarises current knowledge on the endocrine regulation of the taste perceptual system and the release of hunger/satiety regulating factors by gastrointestinal taste receptors. Furthermore, the regulation of blood glucose levels via the activation of pancreatic sweet taste receptors and subsequent insulin secretion, as well as the influence of bitter compounds on thyroid hormone release, is addressed. Finally, the central effects of tastants are discussed briefly.

Is there adaptation in the human genome for taste perception and phase I biotransformation?

Background

During the modern human expansion, new environmental pressures may have driven adaptation, especially in genes related to the perception of ingested substances and their detoxification. Consequently, positive (adaptive) selection may have occurred in genes related to taste, and in those related to the CYP450 system due to its role in biotransformation of potentially toxic compounds. A total of 91 genes (taste receptors and CYP450 superfamily) have been studied using Hierarchical Boosting, a powerful combination of different selection tests, to detect signatures of recent positive selection in three continental human populations: Northern Europeans (CEU), East Asians (CHB) and Africans (YRI). Analyses have been refined with selection analyses of the 26 populations of 1000 Genomes Project Phase 3.

Results

Genes related to taste perception have not been positively selected in the three continental human populations. This finding suggests that, contrary to results of previous studies, different allele frequencies among populations in genes such as TAS2R38 and TAS2R16 are not due to positive selection but to genetic drift. CYP1 and CYP2 genes, also previously considered to be under positive selection, did not show signatures of selective sweeps. However, three genes belonging to the CYP450 system have been identified by the Hierarchical Boosting as positively selected: CYP3A4 and CYP3A43 in CEU, and CYP27A1 in CHB.

Conclusions

No main adaptive differences are found in known taste receptor genes among the three continental human populations studied. However, there are important genetic adaptations in the cytochrome P450 system related to the Out of Africa expansion of modern humans. We confirmed that CYP3A4 and CYP3A43 are under selection in CEU, and we report for the first time CYP27A1 to be under positive selection in CHB.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1366-7) contains supplementary material, which is available to authorized users.

Modeling Associations between Chemosensation, Liking for Fats and Sweets, Dietary Behaviors and Body Mass Index in Chronic Smokers

Chronic smokers have a greater risk for altered chemosensation, unhealthy dietary patterns, and excessive adiposity. In an observational study of chronic smokers, we modeled relationships between chemosensation, fat/carbohydrate liking, smoking-associated dietary behaviors, and body mass index (BMI). Also tested in the model was liking for sweet electronic cigarette juice (e-juice). Smokers (n = 135, 37 ± 11 years) were measured for: Taste genetics (intensity of 6-n-propylthiouracil-PROP); taste (NaCl and quinine intensities) and olfactory (odor identification) function; liking for cherry e-juice; and weight/height to calculate BMI. Smokers survey-reported their food liking and use of smoking for appetite/weight control. Structural equation models tested direct and indirect relationships between chemosensation, fat/carbohydrate liking, dietary behaviors, and BMI. In good-fitting models, taste intensity was linked to BMI variation through fat/carbohydrate liking (greater PROP intensity→greater NaCl intensity→greater food liking→higher BMI). Olfactory function tended to predict sweet e-juice liking, which, in turn, partially mediated the food liking and BMI association. The path between smoking-associated dietary behaviors and BMI was direct and independent of chemosensation or liking. These findings indicate that taste associates with BMI in chronic smokers through liking of fats/carbohydrates. Future research should determine if vaping sweet e-juice could improve diet quality and adiposity for smokers.

Taste disorders are partly genetically determined: Role of the TAS2R38 gene, a pilot study

OBJECTIVES/HYPOTHESIS

Taste sensitivity varies greatly among individuals influencing eating behavior and health, consequently the disorders of this sense can affect the quality of life. The ability to perceive the bitter of thiourea compounds, such as phenylthiocarbamide (PTC), has been largely reported as a marker of the general taste sensitivity, food preferences, and health. PTC sensitivity is mediated by the TAS2R38 receptor and its genetic common variants. We study the role of the TAS2R38 receptor in taste disorders with the aim of understanding if these can be genetically determined.

STUDY DESIGN

Prospective cohort study.

METHODS

Differences in the PTC responsiveness between the patients cohort and healthy controls were assessed. All subjects received standardized tests for smell and taste function and were genotyped for the TAS2R38 gene.

RESULTS

PAV/PAV homozygous patients gave high PTC ratings, whereas PAV/AVI genotypes reported lower values, which are similar to those determined in AVI/AVI or rare genotypes. In addition, the patients cohort did not meet the Hardy-Weinberg equilibrium at the TAS2R38 locus, showing a very low frequency of subjects carrying the PAV/AVI diplotype. Independently, in healthy controls who were in equilibrium at the locus, PAV/PAV homozygous and heterozygous rated PTC bitterness higher compared to AVI/AVI or rare genotypes.

CONCLUSIONS

Our findings, by showing that an only taster haplotype (PAV) is not sufficient to evoke high responses of TAS2R38 receptor in patients with taste disorders, suggest that the genetic constitution may represent a risk factor for the development of taste disorders.

LEVEL OF EVIDENCE

2c Laryngoscope, 129:E307-E312, 2019.

Signatures of Long-Term Balancing Selection in Human Genomes

Balancing selection maintains advantageous diversity in populations through various mechanisms. Although extensively explored from a theoretical perspective, an empirical understanding of its prevalence and targets lags behind our knowledge of positive selection. Here, we describe the Non-central Deviation (NCD), a simple yet powerful statistic to detect long-term balancing selection (LTBS) that quantifies how close frequencies are to expectations under LTBS, and provides the basis for a neutrality test. NCD can be applied to a single locus or genomic data, and can be implemented considering only polymorphisms (NCD1) or also considering fixed differences with respect to an outgroup (NCD2) species. Incorporating fixed differences improves power, and NCD2 has higher power to detect LTBS in humans under different frequencies of the balanced allele(s) than other available methods. Applied to genome-wide data from African and European human populations, in both cases using chimpanzee as an outgroup, NCD2 shows that, albeit not prevalent, LTBS affects a sizable portion of the genome: ∼0.6% of analyzed genomic windows and 0.8% of analyzed positions. Significant windows (P < 0.0001) contain 1.6% of SNPs in the genome, which disproportionally fall within exons and change protein sequence, but are not enriched in putatively regulatory sites. These windows overlap ∼8% of the protein-coding genes, and these have larger number of transcripts than expected by chance even after controlling for gene length. Our catalog includes known targets of LTBS but a majority of them (90%) are novel. As expected, immune-related genes are among those with the strongest signatures, although most candidates are involved in other biological functions, suggesting that LTBS potentially influences diverse human phenotypes.

Taste Perception of Antidesma bunius Fruit and Its Relationships to Bitter Taste Receptor Gene Haplotypes

It was shown more than 40 years ago that the ability to perceive the bitterness of the fruit of the Antidesma bunius tree is inversely correlated with the ability to perceive the well-studied bitter tastant phenylthiocarbamide (PTC). To determine if variants of the TAS2R38 gene, which encodes the PTC taste receptor, or variants in any of the other TAS2R bitter or TAS1R sweet receptor genes account for Antidesma taste perception, we recruited an independent subject sample and examined associations between these taste receptor gene haplotypes and Antidesma perception. Consistent with previous findings, almost none of our subjects who reported Antidesma juice as bitter was a PTC "responder" by previous definitions (i.e. a PTC taster). In our study, of the 132 individuals who perceived PTC as bitter, 15 perceived Antidesma as bitter, although these 15 subjects had very weak bitterness perception scores. Examination of TAS2R38 gene haplotypes showed that, of the subjects who perceive Antidesma as bitter, all carried at least one copy of the TAS2R38 AVI (PTC non-taster) haplotype. However, 86 subjects carried at least one AVI haplotype and failed to perceive Antidesma as bitter. No other TAS2R or TAS1R gene variants showed an association with Antidesma bitter, sweet, or sour perception. Our results show that TAS2R38 haplotypes are associated with differential perception of Antidesma berry juice bitterness, and that all those who perceive this bitterness carry at least one AVI haplotype. This indicates that the AVI haplotype is necessary for this perception, but that additional variable factors are involved.

Genes from the TAS1R and TAS2R Families of Taste Receptors: Looking for Signatures of Their Adaptive Role in Human Evolution

Taste perception is crucial in monitoring food intake and, hence, is thought to play a significant role in human evolution. To gain insights into possible adaptive signatures in genes encoding bitter, sweet, and umami taste receptors, we surveyed the available sequence variation data from the 1000 Genomes Project Phase 3 for TAS1R (TAS1R1-3) and TAS2R (TAS2R16 and TAS2R38) families. Our study demonstrated that genes from these two families have experienced contrasting evolutionary histories: While TAS1R1 and TAS1R3 showed worldwide evidence of positive selection, probably correlated with improved umami and sweet perception, the patterns of variation displayed by TAS2R16 and TAS2R38 were more consistent with scenarios of balancing selection that possibly conferred a heterozygous advantage associated with better capacity to perceive a wide range of bitter compounds. In TAS2R16, such adaptive events appear to have occurred restrictively in mainland Africa, whereas the strongest evidence in TAS2R38 was detected in Europe. Despite plausible associations between taste perception and the TAS1R and TAS2R selective signatures, we cannot discount other biological mechanisms as driving the evolutionary trajectories of those TAS1R and TAS2R members, especially given recent findings of taste receptors behaving as the products of pleiotropic genes involved in many functions outside the gustatory system.

6-n-propylthiouracil taste disruption and TAS2R38 nontasting form in Parkinson's disease

BACKGROUND

The few studies that evaluated taste function in Parkinson's disease (PD) showed inconsistent results. The inherited ability to taste the bitter compound of 6-n-propylthiouracil has been considered to be a paradigm of general taste perception. 6-n-propylthiouracil taste perception is mediated by the TAS2R38 receptor, and reduced 6-n-propylthiouracil sensitivity has been associated with several diseases not typically related to taste function.

OBJECTIVES

We evaluated the 6-n-propylthiouracil taste perception and the TAS2R38 gene as genetic risk factors for the development of idiopathic PD in PD patients and healthy controls (HC).

METHODS

The 6-n-propylthiouracil taste perception was assessed by testing the responsiveness, and the ability to recognize, 6-n-propylthiouracil and sodium chloride. The participants were classified for 6-n-propylthiouracil taster status and genotyped for the TAS2R38 gene.

RESULTS

A significant increase in the frequency of participants classified as 6-n-propylthiouracil nontasters and a reduced ability to recognize bitter taste quality of 6-n-propylthiouracil were found in PD patients when compared with healthy controls. The results also showed that only 5% of PD patients had the homozygous genotype for the dominant tasting variant of TAS2R38, whereas most of them carried the recessive nontaster form and a high number had a rare variant.

CONCLUSIONS

Our results show that 6-n-propylthiouracil taster status and TAS2R38 locus are associated with PD. The 6-n-propylthiouracil test may therefore represent a novel, simple way to identify increased vulnerability to PD. Moreover, the presence of the nontasting form of TAS2R38 in PD may further substantiate that disease-associated taste disruption may represent a risk factor associated with the disease. © 2018 International Parkinson and Movement Disorder Society.

Vertebrate Bitter Taste Receptors: Keys for Survival in Changing Environments

Research on bitter taste receptors has made enormous progress during recent years. Although in the early period after the discovery of this highly interesting receptor family special emphasis was placed on the deorphanization of mainly human bitter taste receptors, the research focus has shifted to sophisticated structure-function analyses, the discovery of small-molecule interactors, and the pharmacological profiling of nonhuman bitter taste receptors. These findings allowed novel perspectives on, for example, evolutionary and ecological questions that have arisen and that are discussed.

Recognition of phenylthiocarbamide (PTC) in taste test is related to blood group B phenotype, females, and risk of developing food allergy: a cross-sectional Brazilian-based study

Anti-nutritional factors, including hemagglutinins, are natural substances that reduce nutritional bioavailability and/or generate adverse physiological effects. Most are bitter toxic compounds, but present chemo-protective properties at low concentrations. Responses to phenylthiocarbamide (PTC) allow for an evaluation of humans' perception of bitter taste, a perception that has evolutionary advantages. Therefore, we hypothesized that relationships between food preference, dietary exposures and disease risk could reflect possible associations not only with the recognition threshold for the bitter taste of PTC, but also with ABO/Rh blood group phenotypes. To test this hypothesis, 375 volunteers of both genders, aged 16-49 years, were recruited. Data were obtained from laboratory tests and questionnaires. PTC test followed literature; blood typing used commercially available sera. Allele frequencies calculated from phenotypes were: T=0.51, t=0.49 (PTC); I^A=0.22, I^B=0.08, i=0.70 (ABO); D=0.57, d=0.43 (Rh). Associations with the recognition threshold for bitter taste were found for blood group B, females, and risk of developing food allergy for bitter taste at PTC dilution 1 (the highest concentration) (OR=3.862; 95%CI=1.387-10.756; p=0.016); for each more diluted PTC solution, the chance of food allergy fell 25.2% (95%CI = 0.764-0.836), while for each more concentrated solution the chance of food allergy increased 20.1% (p=0.000). There were also nominally significant differences among PTC tasting, ABO/Rh, genders and age-groups in relation to food preferences. Results demonstrated that the ability to recognize PTC in taste test is related to blood group B, females, and risk of developing food allergy, thus confirming the research hypothesis, and presenting original and important associations.

Probing the Evolutionary History of Human Bitter Taste Receptor Pseudogenes by Restoring Their Function

Lineage-specific gene losses can be driven by selection or environmental adaptations. However, a lack of studies on the original function of species-specific pseudogenes leaves a gap in our understanding of their role in evolutionary histories. Pseudogenes are of particular relevance for taste perception genes, which encode for receptors that confer the ability to both identify nutritionally valuable substances and avoid potentially harmful substances. To explore the role of bitter taste pseudogenization events in human origins, we restored the open reading frames of the three human-specific pseudogenes and synthesized the reconstructed functional hTAS2R2, hTAS2R62 and hTAS2R64 receptors. We have identified ligands that differentially activate the human and chimpanzee forms of these receptors and several other human functional TAS2Rs. We show that these receptors are narrowly tuned, suggesting that bitter-taste sensitivities evolved independently in different species, and that these pseudogenization events occurred because of functional redundancy. The restoration of function of lineage-specific pseudogenes can aid in the reconstruction of their evolutionary history, and in understanding the forces that led to their pseudogenization.

An automated system for the objective evaluation of human gustatory sensitivity using tongue biopotential recordings

The goal of this work is to develop an automatic system for the evaluation of the gustatory sensitivity of patients using an electrophysiological recording of the response of bud cells to taste stimuli. In particular, the study aims to evaluate the effectiveness and limitations of supervised classifiers in the discrimination between subjects belonging to the three 6-n-propylthiouracil (PROP) taster categories (supertasters, medium tasters, and non-tasters), exploiting features extracted from electrophysiological recordings of the tongue. Thirty-nine subjects (equally divided into the three PROP status classes by standard non-objective scaling methods) underwent a non-invasive, differential, biopotential recording of their tongues during stimulation with PROP by using a custom-made, flexible, silver electrode. Two different classifiers were trained to recognize up to seven different features extracted from the recorded depolarization signal. The classification results indicate that the identified set of features allows to distinguish between PROP tasters and non-tasters (average accuracy of 80% ± 18% and up to 94% ± 15% when only supertasters and non-tasters are considered), but medium tasters were difficult to identify. However, these apparent classification errors are related to uncertainty in the labeling procedures, which are based on non-objective tests, in which the subjects provided borderline evaluations. Thus, using the proposed method, it is possible, for the first time, to automatically achieve objective PROP taster status identification with high accuracy. The simplicity of the recording technique allows for easy reproduction of the experimental setting; thus the technique can be used in future studies to evaluate other gustatory stimuli. The proposed approach represents the first objective and automatic method to directly measure human gustatory responses and a milestone for physiological taste studies, with applications ranging from basic science to food tasting evaluations.