Site specific insertion of a transgene into the murine α-casein (CSN1S1)
gene results in the predictable expression of a recombinant protein in
Gene loci of highly expressed genes provide ideal sites for transgene
expression. Casein genes are highly expressed in mammals leading to the
synthesis of substantial amounts of casein proteins in milk. We have
assessed the α-casein (CSN1S1) gene as a site of transgene expression in
transgenic mice and a mammary gland cell line. A transgene encoding an
antibody light chain gene (A1L) was inserted into the α-casein gene
using sequential homologous and site-specific recombination. Expression
of the inserted transgene is directed by the α-casein promoter, is
responsive to lactogenic hormone activation, leads to the synthesis of a
chimeric α-casein/A1L transgene mRNA and secretion of the recombinant
A1L protein into milk. Transgene expression is highly consistent in all
transgenic lines, but much lower than that of the α-casein gene (4%).
Recombinant A1L protein accounted for 0.5% and 1.6% of total milk
protein in heterozygous and homozygous transgenic mice, respectively.
The absence of the α-casein protein in homozygous A1L transgenic mice
leads to a reduction of total milk protein and delayed growth of the
pups nursed by these mice. Overall, the data demonstrate that the
insertion of a transgene into a highly expressed endogenous gene is
insufficient to guarantee its abundant expression.