Organization and evolution of the human growth hormone receptor gene 5′-flanking region

Previous studies have identified eight variant human GH receptor (hGHR) messenger RNA (mRNAs; V1-V8), that differ in their 5 ' -untranslated regions (5 ' UTRs) but splice into the same site just upstream of the translation start site in exon 2; thus, they encode the same protein. Here we report a novel variant, V9, and describe the mapping of all nine 5 ' UTR sequences within 40 kb upstream of exon 2. A cluster of three sequences, V2-V9-V3 (termed module A), lies furthest 5 ', and approximately 16 kb downstream is a second cluster of four exons, V7-V1-V4-V8 (module B). V6 is midway between modules A and B. Module B is about 18 kb upstream of V5, which lies adjacent to exon 2. hGHR expression is under developmental- and tissue-specific regulation, and expression of the variant mRNAs is related to their position within the 5 ' -flanking region; whereas module A (V2,V9,V3) and V5 variants are widely expressed, module B (V7,V1,V4,V8) and V6 variant mRNAs are detectable only in postnatal liver. Transcriptional start sites for V1 and V9 (representing the two different modules) were identified, showing that postnatal liver-specific expression of V1 is driven from two TATA boxes, whereas the ubiquitous V9 transcript has a single start site and a TATA-less promoter. V9 promoter activity was shown by in vivo and in. vitro transfection assays, and an NF-Y binding site was demonstrated by electromobility shift assay. Thus, the regulatory regions of the hGHR gene are complex, and the clustering of seven 5 ' UTR exons within two modules with distinctly different mRNA expression patterns is the most striking feature.