Regulation of the Human Adenosine Deaminase Gene by First Intron Sequences: A T-Cell Enhancer

Abstract

Adenosine deaminase (ADA) is expressed ubiquitously in mammalian cells and tissues but the levels vary greatly according to tissue and species. In humans the thymus exhibits levels of ADA up to 100-fold higher than most other tissues. In the large first intron of the human ADA gene, our laboratory previously identified a complex cis regulatory region 4–10 kb downstream of the first exon (Aronow et al., 1989). This region exhibits an array of DNase I hypersensitive sites, some of which are tissue specific. Inclusion of intronic fragments encompassing this region in hybrid transgene constructions conveyed generalized expression of the transgene in all tissues and very high level transgene expression in thymus. Transfection-transient assay experiments with plasmids containing hybrid gene constructions indicated that this intronic region contains a lymphoid specific enhancer that functions in a potent manner in T cells that express high levels of ADA. Studies indicated that this putative enhancer lay within a 1.3 kb intronic fragment that encompasses hypersensitive sites II and III. In the studies described here, we have continued to utilize hybrid gene constructions that contain the chloramphenicol acetyltransferase (CAT) coding sequence (as a reporter gene) and human ADA gene fragments to promote expression. These constructions have been used in transfection studies to characterize the enhancer.