DNA excision in liver by an albumin-Cre transgene occurs progressively with age

We have previously described a line of albumin-Cre (Alb-Cre) transgenic mice [C57BL6-TgN(AlbCre)21Mgn] in which the promoter and upstream enhancer of the rat albumin gene (Fig. 1) were used to direct liver-specific expression of Cre (Postic et al., 1999). When used in combination with a loxed glucokinase gene allele (gk super(lox)), the Alb-Cre trangene led to a liver-specific knockout of this enzyme (Postic et al., 1999). Southern blot analysis of DNAs isolated from 8-week-old gk super(lox/lox)+Alb-Cre mice showed more than 80% conversion of the gk super(lox) allele to a deleted allele (gk super(del)) in liver with no recombination in other tissues (Postic et al., 1999). At this age, Western blot analysis and immunocytochemistry of liver samples showed no detectable glucokinase expression, suggesting that recombination in hepatocytes was essentially complete. To examine the efficiency of recombination as a function of age, Southern blot analysis was performed using liver genomic DNA from gk super(lox/lox)+Alb-cre mice isolated at times ranging from 1 day to 12 weeks. Surprisingly, the efficiency of recombination was only about 40% immediately after birth but increased to 60% by 1 week and to 75% at weaning (3 weeks of age) (Fig. 2A). Recombination appeared to be complete by 6 weeks of age (Fig. 2A). The thoroughness of recombination in liver was addressed using ROSA26 reporter mice (Soriano, 1999). As shown in Fig. 2B, more than 99% of hepatocytes from a 12-day-old Rosa26+Alb-Cre mouse showed positive beta galactosidase staining, although this experiment only assessed the effect of Cre on a single Rosa26 reporter allele.