A novel human GnRH receptor homolog gene: abundant and wide tissue distribution of the antisense transcript

Gonadotropin releasing hormone (GnRH) regulates the reproductive system thr ough a specific G-protein-coupled receptor (GPCR) in pituitary gonadotropes . The existence of two (or more) forms of GnRH in most vertebrates suggeste d the existence of GnRH receptor subtypes (I and II). Using sequence inform ation for extracellular loop 3 of a putative Type II GnRH receptor from a r eptile species, we have looked for a Type II GnRH receptor gene in the huma n genome EST (expressed sequence tag) database. A homolog was identified wh ich has 45% and 41% amino acid identity with exons 2 and 3 of the known hum an GnRH pituitary receptor (designated Type I) and much lower homology with all other GPCRs. A total of 27 contiguous ESTs was found and comprised a c ontinuous sequence of 1642 nucleotides. The EST sequences were confirmed in the cloned human gene and in PCR products of cDNA from several tissues. Al l EST transcripts detected were in the antisense orientation with respect t o the novel GnRH receptor sequence and were highly expressed in a wide rang e of human brain and peripheral tissues. PCR of cDNA from a wide range of t issues revealed that intronic sequence equivalent to intron 2 of the Type I GnRH receptor was retained. The failure to splice out putative intron sequ ences in transcripts which spanned exon-intron boundaries is expected in an tisense transcripts, as candidate donor and acceptor sites were only presen t in the gene when transcribed in the orientation encoding the GnRH recepto r homolog. No transcripts extended 5' to the sequence corresponding to intr on 2 of the Type I GnRH as the antisense transcripts terminated in poly A d ue to the presence of a polyadenylation signal sequence in the putative int ron 2 when transcribed in the antisense orientation. These findings suggest that a Type II GnRH receptor gene has arisen during vertebrate evolution a nd is also present in the human. However, the receptor may have become vest igial in the human, possibly due to the abundant and universal tissue trans cription of the opposite DNA strand to produce antisense RNA.