Chromatin structure and gene expression in germ line and somatic cells

The human growth hormone gene is regulated by a multicomponent locus control region

The five-member human growth hormone (hGH)/chorionic somatomammotropin (hCS) gene cluster encodes the pituitary-specific hGH-N gene and four highly related genes (hGH-V, hCS-A, hCS-B, and hCS-L) that are expressed only in the placenta. When the hGH-N or hCS-A gene, together with all previously identified cis-acting regulatory sequences, was integrated into the mouse genome, it was expressed only sporadically and at low levels in the transgenic target organs. DNase I mapping of chromatin from expressing and nonexpressing cell types was used to identify a pituitary-specific set of DNase I-hypersensitive sites (HS) and a set of HS common to both the pituitary and placenta, centered approximately 15 and 30 kb 5' of hGH-N, respectively. When contained on a cosmid insert in their native genomic configuration, these HS consistently directed high-level, pituitary-specific expression of hGH-N in transgenic mice and appeared to define a locus control region required for hGH-N expression. Individually, each set of HS was able to mediate position-independent hGH-N expression in the pituitary but demonstrated loss of physiologic control and loss of tissue specificity. The gene-proximal set of HS contained a potent enhancer activity in the pituitary, while the more distal set appeared to function primarily to establish site-of-integration independence. These data indicate that synergistic interactions among multiple elements are required to restrict hGH-N transcription to the pituitary and generate appropriate levels of expression. In addition, these results suggest a role for both shared and unique regulatory sequences in locus control region-mediated expression of the hGH/hCS gene cluster in the pituitary and possibly the placenta.

Dissecting a locus control region: facilitation of enhancer function by extended enhancer-flanking sequences

Using transgenic mice, we have defined novel gene regulatory elements, termed "facilitators." These elements bilaterally flank, by up to 1 kb, a 200-bp T-cell-specific enhancer domain in the human adenosine deaminase (ADA) gene. Facilitators were essential for gene copy-proportional and integration site-independent reporter expression in transgenic thymocytes, but they had no effect on the enhancer in transfected T cells. Both segments were required. Individual segments had no activity. A lack of facilitator function caused positional susceptibility and prevented DNase I-hypersensitive site formation at the enhancer. The segments were required to be at opposed ends of the enhancer, and they could not be grouped together. Reversing the orientation of a facilitator segment caused a partial loss of function, suggesting involvement of a stereospecific chromatin structure. trans-acting factor access to enhancer elements was modeled by exposing nuclei to a restriction endonuclease. The enhancer domain was accessible to the 4-cutter DpnII in a tissue- and cell-type-specific fashion. However, unlike DNase I hypersensitivity and gene expression, accessibility to the endonuclease could occur without the facilitator segments, suggesting that an accessible chromatin domain is an intermediate state in the activational pathway. These results suggest that facilitators (i) are distinct from yet positionally constrained to the enhancer, (ii) participate in a chromatin structure transition that is necessary for the DNase I hypersensitivity and the transcriptional activating function of the enhancer, and (iii) act after cell-type-specific accessibility to the enhancer sequences is established by factors that do not require the facilitators to be present.