Production of low-lactose milk by ectopic expression of intestinal lactasein the mouse mammary gland

We have investigated, in mice, an in vivo method for producing low-lactose milk, based on the creation of transgenic animals carrying a hybrid gene in which the intestinal lactase-phlorizin hydrolase cDNA was placed under the control of the mammary-specific alpha-lactalbumin promoter. Transgenic fem ales expressed lactase protein and activity during lactation at the apical side of mammary alveolar cells. Active lactase was also secreted into milk, anchored in the outer membrane of fat globules. Lactase synthesis in the m ammary gland caused a significant decrease in milk lactose (50-85%) without obvious changes in fat and protein concentrations. Sucklings nourished wit h low-lactose milk developed normally. Hence, these data validate the use o f transgenic animals expressing lactase in the mammary gland to produce low -lactose milk in vivo, and they demonstrate that the secretion of an intest inal digestive enzyme into milk can selectively modify its composition.