Multiple copies of beta-lactoglobulin promoter do not function as LCR

Duplication of Stat5-binding sites within the β-lactoglobulin promoter compromises transcription in vivo

Achieving high and reliable levels of expression is essential for biotechnological application of transgene technology. In an attempt to maximise transgene expression, we analysed the effect of adding extra Stat5-binding sites, while maintaining their spatial arrangement, to the beta-lactoglobulin promoter. A 195 bp-fragment comprising the three Stat5-binding sites in the beta-lactoglobulin promoter was duplicated directly upstream, 88 bp or 177 bp upstream from the original site. We assessed the effect that addition of extra Stat5-binding sites had in transcriptional activation and chromatin structuring of the promoter, both in vitro in HC11 cells and in vivo in transgenic animals. Duplication of Stat5-binding sites in the beta-lactoglobulin promoter did favour chromatin remodelling in vivo but unexpectedly did not enhance transcriptional activation of the transgene, despite the higher basal and induced expression levels achieved in vitro in HC11 cells. Furthermore, the well-described positive relationship between beta-lactoglobulin transgene copy number and mammary expression was lost in the new lines. These results indicate that chromatin remodelling upon Stat5 binding is not sufficient to activate transgene expression, denote that strategies to achieve transgenic transcriptional activation are not obvious.

BLG-e1 - a novel regulatory element in the distal region of the beta-lactoglobulin gene promoter

beta-Lactoglobulin (BLG) is a major ruminant milk protein. A regulatory element, termed BLG-e1, was defined in the distal region of the ovine BLG gene promoter. This 299-bp element lacks the established cis-regulatory sequences that affect milk-protein gene expression. Nevertheless, it alters the binding of downstream BLG sequences to histone H4 and the sensitivity of the histone-DNA complexes to trichostatin A treatment. In mammary cells cultured under favorable lactogenic conditions, BLG-e1 acts as a potent, position-independent silencer of BLG/luciferase expression, and similarly affects the promoter activity of the mouse whey acidic protein gene. Intragenic sequences upstream of BLG exon 2 reverse the silencing effect of BLG-e1 in vitro and in transgenic mice.

On the use of post-transcriptional processing elements in transgenes

RNA processing events modulate final productivity of a given transgene. We have evaluated a series of RNA elements for their ability to enhance α1-antitrypsin production in mammary cells. Our results indicate the need for a case-by-case assessment of each construct design and the occurrence of gene silencing events in vivo.

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

Effect of the rabbit alphas1-casein gene distal enhancer on the expression of a reporter gene in vitro and in vivo

Several gene constructs containing the firefly luciferase gene and the herpes simplex virus thymidine kinase gene promoter (TK) were used to evaluate the transcriptional activity of the distal enhancer (-3442, -3285) of the rabbit alphas1-casein gene. Six copies of the enhancer (6i) were added upstream of the TK-luciferase construct in the presence or absence of the chicken beta-globin 5'HS4 insulator. The activity of the constructs was tested by transient transfection in CHO cells and in rabbit primary mammary cell cultured on plastic or on floating collagen. Constructs were also tested in stably transfected mouse mammary HC11 cells. In all cell types the multimerized alphas1-casein enhancer strongly stimulated luciferase gene expression in the presence of lactogenic hormones. It was also sensitive to the extracellular matrix in rabbit primary mammary cells. The constructs were used to generate transgenic mice. The 6i TK transgenic animals expressed the luciferase gene at very low levels irrespectively of the physiological state. No preferential expression in the mammary gland was observed. Addition of 5'HS4 insulator to the 6i TK construct did not prevent silencing in most of the transgenic lines. However, two lines expressed high luciferase levels specifically in the mammary gland. Our data suggest that 6i may confer, when insulated properly, a higher and mammary-specific expression to the TK promoter.