Abstract
Olfactory receptors (OR) belong to the large family of proteins with seven transmembrane (TM) domains. They detect extracellular ligands according to their specificity, resulting in signal transduction into the cell lumen. There are hints that HLA-linked OR are involved in the perception of individual-specific body odours, resulting in odour dependent preferences for potential mating partners. It was shown for rodents that products of the major histocompatibility complex (MHC) influence the individual-specific body odour, and it is possible, that the configuration of the human MHC, the HLA-complex, can be smelled also by humans. To support this hypothesis, specific OR-haplotypes should be linked to specific HLA-haplotypes. This was analysed by testing a total of 13 potential functional genes and 9 OR pseudogenes in ten different HLA-homozygous or -hemizygous cell lines. All analysed genes with open reading frame (ORF) were polymorphic. Silent mutations as well as conservative and non conservative amino acid substitutions were found in all OR domains, with exception of EC4 and TM7. The analysis of the ten cell lines and the 13 HLA-linked OR genes with ORF led to at least 13 different OR haplotypes, which could be the basis for individual odour preferences.
Three of the analysed genes show in some cell lines intact alleles and in other cell lines non functional alleles. Individuals, who are homozygous for such an intact allele, could show symptoms of a specific anosmia. So far it is unknown, if amino acid substitutions influence the ligand specificity or the interaction with proteins of the signal transduction cascade, because no specific ligands could be identified so far.
OR are not only present in the olfactory epithelium, but also in other organs. The expression of six OR genes in 50 tissues was tested with a Northern blot. Two of the tested genes show a specific expression only in three specific organs, whereas the other four genes are present in numerous organs. Some organs transcribe only specific HLA-linked OR genes and others a broader spectrum. The analysis of three cDNA libraries from testis, lung and kidney has shown, that in testis 13 of 14 analysed HLA-linked OR genes are transcribed, in contrast to lung and kidney with only 4 or 5, respectively.
To get hints about the promoter region of HLA-linked OR genes, the genomic structure of several OR genes was characterised. The 3' and 5' ends of some OR transcripts in testis, kidney and lung were detected with RACE (Rapid Amplification of cDNA Ends). Several results were obtained, some of which were novel for G-protein coupled receptors:
- only five of these genes show additional 5' non coding exons
- three of these genes (hs6M1-21, -27 and -18) show one common first 5' non coding exon; which is located, e.g. in the case of hs6M1 21, ~110 kb upstream of the start codon
- the distance between the first 5' exon of hs6M1 16, which is located centromeric of these three genes and exhibits an opposite transcription direction, and the first common exon of the neighbouring three genes (hs6M1-21, -27 and -18) is only 80 bp. Therefore, it is tempting to speculate that these 80 bp contain a bidirectional promoter
- hs6M1 16 exhibits exons in the 3' UTR
- some of the genes show splicing within the coding region, probably leading to truncated proteins
- some of the genes show premature polyadenylation within the coding region
Some of these results indicate a complex gene regulation of HLA-linked OR genes. Together with several genes, which are functional in only some individuals, and numerous conservative and non conservative variations in the coding region, an unexpected complexity was found with respect to the HLA-linked ORs, which could explain individual odour preferences. |
