Characterization of the human AMPD3 gene reveals that 5′ exon useage is subject to transcriptional control by three tandem promoters and alternative splicing

Abstract

Previous work has identified multiple human AMPD3 transcripts proposed to differ by mutually exclusive alternative splicing of three exons located at, or near, the 5′ end of the gene. In this study, we perform a more comprehensive evaluation of human AMPD3 gene expression. Combined Northern blot and RNase protection analyses show that alternative mRNAs are widely expressed in human tissues and cells, but at variable relative abundances. Sequencing of human genomic clones, together with human-mouse somatic cell hybrid analysis, demonstrates that the entire gene is comprised of seventeen exons spanning approx. 60 kilobases on the short arm of chromosome 11 in the region p13-pter. Together, RT-PCR and additional RNase protection analyses establish that exons 1a, 1b, and 1c are 5′ terminal sequences in alternative transcripts. Transient transfection experiments show fusion constructs containing proximal flanking and 5′ untranslated sequence from each of these exons are able to direct expression of a reporter luciferase gene in mammalian cell lines. These combined results reveal that AMPD3 gene expression is subject to transcriptional control by three tandem promoters. Differential regulation of the exon 1b promoter in skeletal myocytes, as compared to retinal pigment epithelial cells, is proposed to be mediated by skeletal muscle-specific basic helix-loop-helix protein/E-box interactions. Finally, an internal splice acceptor site in exon 1c is shown to be used alternatively to retain the 3′ portion of this exon in mature AMPD3 transcripts initiating upstream in exon 1b.