A duodenum-specific enhancer regulates expression along three axes in the small intestine

Adenosine deaminase (ADA) is expressed at high levels in the epithelium of proximal small intestine. Transgenic mice were used to characterize the reg ulatory region governing this activation. A duodenum-specific enhancer is l ocated in intron 2 of the human ADA gene at the central site among a cluste r of seven DNase I-hypersensitive sites present in duodenal DNA. Flanking D NA, including the remaining hypersensitive sites, is required for consisten t high-level enhancer function. The enhancer activates expression in a patt ern identical to endogenous ADA along both the anterior-posterior axis of t he small intestine and the crypt-villus differentiation axis of the intesti nal epithelium. Timing of activation by the central enhancer mimics endogen ous mouse ADA activation, occurring at 2-3 wk of age. However, two upstream DNA segments, one proximal and one distal, collaborate to change enhancer activation to a perinatal time point. Studies with duodenal nuclear extract s identified five distinct DNase I footprints within the enhancer. Protecte d regions encompass six putative binding sites for the transcription factor PDX-1, as well as proposed CDX, hepatocyte nuclear factor-4, and GATA-type sites.