Characterization of human AMP deaminase 2 (AMPD2) gene expression reveals alternative transcripts encoding variable N-terminal extensions of isoform L

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Abstract
AMP deaminase (AMPD) is a highly regulated enzymic activity and multiple isoforms of this enzyme are coded for by a multigene family in mammalian species, including man. Isoform L (liver) is the main activity present in adult human liver and is the protein product of the AMPD2 gene, which is widely expressed in non-muscle tissues and cells. A previous report described almost the full-length cDNA sequence and part of the human AMPD2 gene and also presented Northern blot evidence for multiple transcripts in brain. This study was performed to further characterize the AMPD2 gene and its expression in human tissues. AMPD2 genomic and human cerebellum cDNA clones were isolated, sequenced and used as probes in RNase protection analyses which together demonstrated the following: (1) an intervening sequence near the 5′-end of the published AMPD2 cDNA, which affects the predicted N-terminal amino acid sequence of isoform L; (2) alternative transcripts resulting from exon shuffling at, or near, the 5′-end of the AMPD2 gene that exhibit tissue-specific patterns of relative abundance; (3) predicted usage of three different initiation codons to confer variable N-terminal extensions on isoform L polypeptides; and (4) an extension of a 3′ untranslated sequence in some AMPD2 transcripts. In addition, reverse transcriptase PCR and additional RNase protection analyses were used to map the 5′-ends of two mutually-exclusive exon 1 sequences, both of which contain multiple transcription-initiation sites. These results are discussed in relation to predicted isoform L diversity across human tissues and cells.