title: Widespread ectopic expression of olfactory receptor genes.
abstract: BACKGROUND
  Olfactory receptors (ORs) are the largest gene family in the human genome. Although they are expected to be expressed specifically in olfactory tissues, some ectopic expression has been reported, with special emphasis on sperm and testis. The present study systematically explores the expression patterns of OR genes in a large number of tissues and assesses the potential functional implication of such ectopic expression.
  
  
  RESULTS
  We analyzed the expression of hundreds of human and mouse OR transcripts, via EST and microarray data, in several dozens of human and mouse tissues. Different tissues had specific, relatively small OR gene subsets which had particularly high expression levels. In testis, average expression was not particularly high, and very few highly expressed genes were found, none corresponding to ORs previously implicated in sperm chemotaxis. Higher expression levels were more common for genes with a non-OR genomic neighbor. Importantly, no correlation in expression levels was detected for human-mouse orthologous pairs. Also, no significant difference in expression levels was seen between intact and pseudogenized ORs, except for the pseudogenes of subfamily 7E which has undergone a human-specific expansion.
  
  
  CONCLUSIONS
  The OR superfamily as a whole, show widespread, locus-dependent and heterogeneous expression, in agreement with a neutral or near neutral evolutionary model for transcription control. These results cannot reject the possibility that small OR subsets might play functional roles in different tissues, however considerable care should be exerted when offering a functional interpretation for ectopic OR expression based only on transcription information.
authors.affiliation: Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel. ester.feldmesser@weizmann.ac.il
authors.forename: Ester
authors.initials: E
authors.lastname: Feldmesser
authors.forename: Tsviya
authors.initials: T
authors.lastname: Olender
authors.forename: Miriam
authors.initials: M
authors.lastname: Khen
authors.forename: Itai
authors.initials: I
authors.lastname: Yanai
authors.forename: Ron
authors.initials: R
authors.lastname: Ophir
authors.forename: Doron
authors.initials: D
authors.lastname: Lancet
mesh_terms: D000818:Animals; D020224:Expressed Sequence Tags; D020869:Gene Expression Profiling; D023281:Genomics; D006801:Humans; D008297:Male; D051379:Mice; D009831:Olfactory Mucosa / Q000378:metabolism; D020411:Oligonucleotide Array Sequence Analysis; D011544:Pseudogenes; D012333:RNA, Messenger / Q000378:metabolism; D018035:Receptors, Odorant / Q000235:genetics* / Q000378:metabolism; D013094:Spermatozoa / Q000378:metabolism; D013737:Testis / Q000378:metabolism; D014018:Tissue Distribution; D014158:Transcription, Genetic
chemical_list: D012333:RNA, Messenger; D018035:Receptors, Odorant