Human Genetics Unit, Level 1, Molecular Medicine Centre; Western General Hospital
The upstream region of the GNAS1 gene contains two oppositely imprinted first exons, which encode two unrelated polypeptides and are separated by only 11 kb of genomic sequence. The 5'-most of these, encoding NESP55, is expressed from the maternal chromosome and methylated on the paternal allele; conversely the other exon, encoding XLas, is expressed from the paternal chromosome and methylated on the maternal one. The differentially methylated CpG-rich region associated with the XLas exon extends approximately 3 kb upstream of its putative initiation codon and here we describe another transcript originating in this region. This transcript, which has an orientation antisense to XLas, has multiple alternative splice forms and terminates approximately 20 kb 5' to the NESP55 exon. Although this RNA lacks coding potential, its sequence is very highly conserved. Nonetheless, the identification of one rare single-nucleotide polymorphism. allowed us to establish that this antisense transcript is imprinted. Furthermore, this transcript is not detected by RT-PCR using RNA from the parthenogenetic lymphoblastoid cell line FD, although it is present in normal female lymphoblastoid cell RNA. Antisense transcripts have been described for other imprinted genes and suggested to play a role in regulating in cis the coding (sense) transcripts. Traversing as it does the maternally expressed NESP55 promoter, the paternally expressed GNAS1 antisense transcript could play a major role in the regulation of this complex gene.