Structure of the glucocorticoid receptor (NR3C1) gene 5' untranslated region: identification, and tissue distribution of multiple new human exon 1.

The 5' untranslated region (UTR) of the glucocorticoid receptor (GR) plays a key role in determining tissue-specific expression and protein isoforms. Analysis of the 5' UTR of the human GR (hGR) has revealed 11 splice variants of the hGR exon 1, based on seven exon 1s, four of which (1-D to 1-F and 1-H) were previously unknown. All of the exon 1 variants have unique splice donor sites and share a common exon 2 splice acceptor site. Due to an upstream in-frame TGA stop codon the predicted translation from all splice variants is identical. The four new exon 1s show remarkable similarity with their rat homologues. Exon 1-D starts and finishes 17 and 36 bp upstream of the corresponding ends of the rat exon 1(4). Exon 1-E is only 6 bp longer than its homologue exon 1(5). Exon 1-F contains two short inserts of 11 and 6 bp when compared with the rat 1(7). 1-H is 18 bp longer than the corresponding rat 1(11). In addition to these new exons, we found that the human exon 1-C occurs as three distinct splice variants, covering the region homologous to the rat exons 1(9) and 1(10). All of the alternative hGR exons 1s presented here were found to be transcribed in human tissue. The human hippocampus expresses mRNA of all the exon 1 variants, while the expression of the other exon 1s seems to be tissue specific. While exon 1-D is only in the hippocampus, exons 1-E and 1-F are also detected in the immune system, and exon 1-H additionally in the liver, lung and smooth muscle. The 5' region of the hGR is more complex than previously thought, and we suggest that each of these untranslated first exons have a distinct proximal promoter region, providing additional depth to the mechanisms available for tissue-specific expression of the hGR isoforms.