Epigenetic activation of the human growth hormone gene cluster during placental cytotrophoblast differentiation

The hGH cluster contains a single human pituitary growth hormone gene (hGH-N) and four placenta-specific paralogs. Activation of the cluster in both tissues depends on 5' remote regulatory elements. The pituitary-specific locus control elements DNase I-hypersensitive site I (HSI) and HSII, located 14.5 kb 5' of the cluster (position -14.5), establish a continuous domain of histone acetylation that extends to and activates hGH-N in the pituitary gland. In contrast, histone modifications in placental chromatin are restricted to the more 5'-remote HSV-HSIII region (kb -28 to -32) and to the placentally expressed genes in the cluster, with minimal modification between these two regions. These data predict distinct modes of hGH cluster gene activation in the pituitary and placenta. Here we used cell culture models to track structural changes at the hGH locus through placental-gene activation. The data revealed that this process was initiated in primary cytotrophoblasts by histone H3K4 di- and trimethylation and H4 acetylation restricted to HSV and to the individual placental-gene repeat (PGR) units within the cluster. Later stages of transcriptional induction were accompanied by enhancement and extension of these modifications and by robust H3 acetylation at HSV, at HSIII, and throughout the placental-gene regions. These data suggested that elements restricted to HSIII-HSV regions and each individual PGR might be sufficient for activation of the hCS genes. This model was tested by comparing hCS transgene expression in the placentas of mouse embryos carrying a full hGH cluster to that in placentas in which the HSIII-HSV region was directly linked to the individual hCS-A PGR unit. The findings indicate that the HSIII-HSV region and the PGR units, although targeted for initial chromatin structural modifications, are insufficient to activate gene expression and that this process is dependent on additional, as-yet-unidentified chromatin determinants.