Activity of the adenosine deaminase promoter in transgenic mice

The promoter of the human gene for adenosine deaminase (ADA) is extremely G/C-rich, contains several G/C-box motifs (GGGCGGG) and lacks any apparent TATA or CAAT boxes. These features are commonly found in promoters of genes that lack a strong tissue specificity, and are referred to as "housekeeping genes". Like other housekeeping genes, the ADA gene is expressed in all tissues. However, there is a considerable variation in the levels of expression of the ADA protein in different tissues. In order to study the activity of the ADA promoter, transgenic mice were generated that harbor a chimeric gene composed of the ADA promoter linked to a reporter gene encoding the bacterial enzyme Chloramphenicol Acetyl Transferase (CAT). These mice reproducibly showed CAT expression in all tissues examined, including the hempietic organs (spleen, thymus and bone marrow). However, examination of the actual cell types expressing the CAT gene revealed the ADA promoter to be inactlve in the hempoietic cells. This was substantiated by a transplantation experiment in which bone marrow from ADA-CAT transgenic mice was used to reconstitute the hempoietic compartment of lethally irradiated mice. The engrafted recipients revealed strongly reduced CAT activity in their hemopietic organs. The lack of expression in hempietic cells was further shown to be correlated with a hypermethylated state of the transgene. Combined, our data suggest that the ADA promoter sequences tested can direct expression in a wide variety of tissues as expected for a regular housekeeping gene promter. However, the actlvity of the ADA prormter fragment did not reflect the tissuespecific variations in expression levels of the endcqenous ADA gene. Additionally, regulatory elements are needed for expression in the hemopoietic cells.