The human growth hormone locus control region mediates long-distance transcriptional activation independent of nuclear matrix attachment regions

Expression of the human growth hormone (hGH-N) transgene in the mouse pituitary is dependent on a multicomponent locus control region (LCR). The primary determinant of hGH LCR function maps to the pituitary-specific DNase I hypersensitive sites (HS) HSI,II, located 15 kb 5' to the hGH-N gene. The mechanism by which HSI,II mediates long-distance activation of the hGH locus remains undefined. Matrix attachment regions (MARs) comprise a set of AT-rich DNA elements postulated to interact with the nuclear scaffold and to mediate long-distance interactions between LCR elements and their target promoters. Consistent with this model, sequence analysis strongly predicted a MAR determinant in close proximity to HSI,II. Surprisingly, cell-based analysis of nuclear scaffolds failed to confirm a MAR at this site, and extensive mapping demonstrated that the entire 87 kb region encompassing the hGH LCR and contiguous hGH gene cluster was devoid of MAR activity. Homology searches revealed that the predicted MAR reflected the recent insertion of a LINE 3'-UTR segment adjacent to HSI,II. These data point out discordance between sequence-based MAR predictions and in vivo MAR function and predict a novel MAR-independent mechanism for long-distance activation of hGH-N gene expression.