MULTIPLE CHANGES IN CHROMATIN STRUCTURE PRECEDE THE TRANSCRIPTIONAL ACTIVATION OF THE HUMAN GROWTH-HORMONE LOCUS IN PLACENTAL CELLS

In addition to the growth hormone gene (hGH-N) itself, the human growt h hormone (hGH) locus contains four related genes, namely hGH-V and hC S-L, -A and -B, which have appeared very recently in evolution and are specifically expressed in placenta. With the aim of identifying the r egulatory elements responsible for this placental-specific expression, we have mapped the DNaseI hypersensitive sites present at the hGH gen e cluster in a placental cell line (BeWo) that expresses the hGH-V and hCS genes. Our results reveal a complex pattern of hypersensitive sit es distributed along the hGH locus, most of which appear to be cell ty pe-specific. Thus, we have identified placental-specific hypersensitiv e sites within the first intron of the hGH-N and hGH-V genes, but not in the equivalent regions of the hCS genes. In addition, we have found several placental-specific hypersensitive sites downstream of the hCS -L and hCS-A genes, which might reflect the presence of enhancer eleme nts similar to that located downstream of the hCS-B gene (Walker et al . (1990) J. Biol. Chem. 265, 12940). Comparison of BeWo cells with a p lacental cell line (JEG-3) which does not express the hGH-V and hCS ge nes revealed a very similar pattern of hypersensitive sites, suggestin g that the sites detected are established before the onset of transcri ption. Our results indicate that the transition to an active hGH locus in placental cells requires multiple alterations in chromatin structu re, and provide a framework for the molecular analysis of the regulato ry elements and mechanisms mediating such processes.