Genetic variability of human taste receptor genes: an evolutionary adaptation to human complex traits

Bitter is one of the five main taste classes currently known (the other four are sour, salty, sweet and umami). In vertebrates, bitter perception is perceived through the bitter taste receptor family (T2Rs) that comprises over 25 G protein–coupled receptors expressed on the surface of taste buds. Bitter taste is largely genetically determined and is influenced by a relatively small number of polymorphisms showing a large effect. Therefore, these single nucleotide polymorphisms (SNPs) can be used to predict the ability to taste bitterness. These receptors are expressed even in the gut, testis, ecc… and several studies found associations between the genetic variability of taste receptor genes and extremely different phenotypes: from susceptibility to cancer to male infertility. A major hypothesis today is that taste genes are associated with infections and bacterial colonization. Only a few studies investigated the relationship between host genetic variability and microbiome diversity. All of them were conducted on adult individuals. Generally, the results did not replicate across studies due to the preponderance of the environment over genetics (as one of the main consequences). A potential way to strongly limit the environmental effects on the gut microbiome could be to study the neonatal meconium microbiome. The meconium is the first “stool” sample of mammal newborns, and despite the fact that it was previously considered sterile, recent studies challenged the traditional view by suggesting that instead meconium has its own microbiome. The aim of this study is to analyze the genetic variability associated with the bitter taste perception, in relation to the meconium microbiome of newborns. The study samples consisted of 380 healthy term newborns. Biological samples of blood and meconium were collected at the Neonatology Unit of Santa Chiara Hospital in Pisa. For each subject and its respective mother, anthropometric and clinical data were also collected. SNPs associated with bitter taste perception with a P-Value of at least 5x10-8 were selected from the GWAS Catalog database. Genomic DNA was extracted from blood samples through an automated extraction process and stocked in 384-well plates. Genotyping was done using an allele-specific PCR with fluorescent probes. The protocol for microbial DNA extraction was properly modified because of the texture and consistency of the meconium samples and the difficulty of homogenization. The V3-V4 sub-regions of the 16S gene were amplified by PCR and gel-purified. Sequencing was then performed using a paired-end approach. Additionally, four negative controls were sequenced to verify that no contamination occurred during the whole procedure, from the extraction to the sequencing process. No association was statistically significant when considering Bonferroni's threshold neither for alpha nor for beta diversity. However, we observed an association between the C bitter perception allele of the SNP TAS2R19-rs1868769 with a decreased microbial variability in three over four alpha diversity indexes. We also observed a difference in microbiome beta diversity basing on the TAS2R38 genotypes for the three SNPs rs1726866, rs10246939 and rs713598, the former two being statistically significant, and the latter being almost statistically significant. In detail, the G allele of the SNP rs1726866, which is associated with higher bitter taste perception and the C allele of rs10246939, which is associated with higher bitter taste perception, explained both a variation of about 1% in beta diversity. These results seems to suggest a potential association between the SNPs involved in bitter taste perception and meconium microbiome diversity, and suggest that the microbiome composition may be regulated by host genetic factors even prior to birth