High-level expression of human lactoferrin in milk of transgenic mice using genomic lactoferrin sequence

In our previous study, transgenic mice were generated that expressed human lactoferrin (hLF) in milk using cDNA under control of the 2 kb bovine beta-casein promoter. The expression level of the protein in milk of 7 mice ranged from 1 to 200 microg/ml; 1 to 34 microg/ml in 6 mice and 200 microg/ml in 1 mouse. With the aim of inducing higher expression of the protein, we constructed an expression cassette comprised of 10 kb of the bovine beta-casein gene promoter and the hLF genomic sequence in place of the cDNA. The hLF genomic sequence of about 27 kb, spanning 23 kb of the entire coding region and 4 kb of the 3'-flanking sequence, was placed downstream the bovine beta-casein promoter. In total, 8 transgenic mice were generated from 31 mice (transgenic rate of 25.8%) born from the embryos microinjected with the 40-kb hLF expression cassette. Mammary-specific expression of the transgene was addressed by performing Northern hybridization of the total RNAs from various tissues of transgenic mice. Immunoblot analysis showed that the recombinant protein expressed in milk has the same molecular weight as the native protein. The amount of the protein in milk of 5 mice ranged from 60 to 6,600 microg/ml when judged by ELISA analysis. Three mice expressed the protein at the level higher than 500 microg/ml. These data suggest that the genomic lactoferrin sequence represents a valuable element for the efficient expression of the protein in milk of transgenic animals.

Structure of the human lactoferrin gene and its chromosomal localization

The complete gene encoding the human lactoferrin (hLf) was isolated from a cosmid library and its structure was characterized. A detailed restriction map and the complete exon-intron structure of the gene are presented. The transcript is assembled from 17 exons like the bovine (bLf) and murine lactoferrin (mLf) genes, which are separated by introns ranging in size from about 300 bp to 3.3 kb. The total length of the gene is about 24.5 kb, estimated by adding up the sizes of the exons and introns. A comparison of the structure of the hLf gene with that of other species revealed that the hLf gene is more closely related to bLf than to mLf. The positions of the introns were well matched with those of the bLf gene except only for exons 14 and 15, wherein exon 14 of the hLf gene is one nt shorter while exon 15 is one nt longer than the bLf gene. All the exon-intron boundaries of the gene conformed to the GT/AG rule. In situ fluorescence hybridization indicated that the hLf gene was mapped in the region 3p21.3.