Low in vivo levels of human ANX7 (annexin VII) gene expression are due to endogenous inhibitory promoter sequences

ANX7 is a Ca²⁺/GTP-dependent membrane fusion protein, which is involved in regulating exocytotic secretion. While the nullizygous ANX7 (-/-) knockout mutation in the mouse is lethal, ANX7 is nonetheless the least expressed member of the annexin gene family in mammalian and lower organisms. To investigate the basis for low ANX7 expression level we have studied the human ANX7 promoter. In our early cloning studies we learned that the human ANX7 promoter lacks TATA and CCAAT boxes, but is GC rich and contains several SP1 binding sites. To study how expression of the ANX7 gene is controlled, we fused nested deletion mutants of the 5'-flanking region to a luciferase reporter gene. We find that the maximal level of luciferase activity is obtained by a construct comprising the domain -889/+34. However, a repressor effect observed in the region between -240 and + 34 is dramatically eliminated either by the inclusion of 177 bp upstream sequences, or by the presence of an upstream SV40 enhancer sequences. ANX7 gene expression is also cell type specific and is subject to different transcriptional controls in different cellular environments. The fact that the SV40 enhancer sequences not only eliminates the suppression of luciferase activity in the inhibitory region of the ANX7 promoter, but also reverses the inhibitory effect of phorbol ester suggests that endogenous cis-acting elements or transacting factors may be responsible for the physiologically low level of ANX7 protein expression. (C) 2000 Academic Press.