Targeting expression to the mammary gland: intronic sequences can enhance the efficiency of gene expression in transgenic mice

We are studying the tissue-specific expression of the sheep milk-whey protein gene, β-lactoglobulin. We have used sequences derived from this gene to target the expression of biomedical proteins into milk with the intention to exploit this technology in transgenic sheep as a means of protein production. In the present study, a series of β-lactoglobulin hybrid genes and β-lactoglobulin minigenes were evaluated for expression in the mammary gland of transgenic mice. In particular, we have assessed whether there is a requirement for introns for efficient transgene expression in the mammary gland, since the coding sequences of many candidate proteins are available only as cDNAs. The results suggest that the inclusion of natural introns in constructs can enhance the efficiency of transgene expression. Thus, a hybrid construct comprising 4.3 kb of the immediate 5′ flanking sequences of β-lactoglobulin fused to a genomic minigene encoding human α-antitrypsin (α₁AT) was expressed much more efficiently than an α₁AT-cDNA construct containing the same β-lactoglobulin segment. Similarly, the intact β-lactoglobulin gene was expressed more efficiently than the corresponding intronless β-lactoglobulin minigene. This effect was not seen in transient expression expriments in baby hamster kidney cells when β-lactoglobulin-α₁AT constructs were driven by SV40 enhancer sequences. The effect cannot be explained by a simple requirement for splicing, since the inclusion of the first β-lactoglobulin intron into cDNA constructs encoding human α₁AT or β-lactoglobulin itself failed to enhance the efficiency of transgene expression. It is concluded that sequence elements within introns may interact with the upstream 5′ flanking sequences of β-lactoglobulin and enable the latter to function efficiently in the mammary gland of transgenic mice.