Absence of glucocorticoid receptor-beta in mice

Two human glucocorticoid receptor (GR) isoforms, GRalpha and GRbeta, are derived from the same gene by alternative splicing involving exon 9 of the GR locus. The non-ligand binding isoform GRbeta was proposed to act as a transdominant negative inhibitor of GRalpha, thus modulating glucocorticoid responsiveness of target tissues. To study GRbeta in mice we characterized the genomic region around exon 9 of the murine GR gene. Sequence analysis revealed that the presumed exon 9beta contained an open reading frame of 59 amino acids. In contrast, human exon 9beta encoded only 15 amino acids. Using reverse transcriptase polymerase chain reaction the absence of GRbeta mRNA was demonstrated in all adult mouse tissues examined. To exclude the possibility that the polymerase chain reaction conditions employed were not suitable for the amplification of GRbeta mRNA, we synthesized an artificial template corresponding to the presumed GRbeta mRNA spanning exons 7, 8, and 9beta. Various amounts of this template were added to brain cDNA preparations and as little as 25 molecules were detectable under the polymerase chain reaction conditions chosen. Since GRbeta is not conserved across species its physiological significance in humans appears questionable.