Generation of the ApcCKO and ApcΔ580 Mice
To investigate the role of Apc in development of skin and its appendages, we used the Cre/loxP technology to introduce a conditional mutation of the Apc gene in mice. We constructed embryonic stem (ES) cells and mice carrying an Apc allele harboring both a pair of loxP sites flanking Apc exon 14 and a pair of FLP recognition target (FRT) sites flanking PGK-neomycin selection cassette by recombineering [12,13] (Figure 1A, ApcCKON allele, N for neomycin cassette). A PGK-neomycin cassette was inserted in the same transcriptional orientation as Apc in intron 14 of the endogenous gene. The loxP and FRT sites were used to aid unidirectional recombination [12,13]. Two mouse lines containing the same modification were generated from two independent ES clones to ensure that these two lines behave in the same way. These ApcCKON/+ mice were crossed with FLPe-deleter to generate ApcCKO/+ mice that were heterozygous for the final Apc conditional (ApcCKO) allele that removed the PGK-neomycin cassette and contains only the loxP sites in the introns flanking exon 14. To assess the effect of deleting exon 14 in mice, both lines of ApcCKO/+ mice were crossed with the Cre-deleter to generate the germline knockout line of Apc, designated ApcΔ580/+. The mutant allele (ApcΔ580) lacks exon 14 (Figure 1A). The transcript from loss of exon 14 results in a shift in the normal reading frame, resulting in a premature chain termination codon which, if utilized, would result in a truncated polypeptide that is 605 aa in length, of which the first 580 aa correspond to the normal Apc protein.
Figure 1  Generation of the Conditional Apc Allele
(A) Schematic diagram of exons 14 and 15 of the mouse Apc gene, the targeting vector, and the resulting conditional allele with 2 LoxP sites sandwiching the exon 14. The PGK-neomycin cassette was inserted within intron 14 by recombineering technique. This cassette is sandwiched by 2 FRT sites that could be removed by crossing to FLPe-expressing mice. Positions of PCR primers used for genotyping PCR (F2, R2, R4) and RT-PCR (F546 and R721) are indicated. Positions of probe used for Southern blot analysis with NdeI sites are also shown. Upon Cre-mediated recombination, exon 14 is removed and leads to truncated Apc protein, of which the first 580 aa correspond to the normal.
(B) Southern blot analysis of NdeI-digested genomic tail DNA isolated from F1 mice of various Apc mouse lines (ApcCKON, ApcΔ580), hybridized to a 600-bp probe. Tail genomic DNA from ApcCKON F1 mice derived from a modified ES clone showed a 12-kb band for the ApcCKON allele and a 10-kb band for the wild-type allele, whereas genomic DNA from the ApcΔ580 mouse was heterozygous for the ApcΔ580 allele (9.2-kb band).
(C) Kaplan-Meier survival plot of ApcCKO/+ mice (thin solid line, n = 39), ApcCKO/CKO mice (thin dotted line, n = 57), ApcΔ580/+ mice (solid line, n = 51), and wild-type littermates (broken line, n = 21). Heterozygosity of the ApcΔ580 allele led to a significantly shortened survival (p < 0.0001), whereas those of heterozygous and homozygous ApcCKO mice had no significant difference to that of wild-type littermates. Southern blot analysis of tail DNA from F1 offspring of both ApcCKON and ApcΔ580 lines confirmed the germline transmission of modified Apc allele (Figure 1B). Mice that are heterozygous for ApcΔ580 mutation are viable but have a significantly reduced lifespan (Figure 1C). These results suggested that deletion of exon 14 indeed results in either loss or abnormal function of the Apc gene product. ApcΔ580/+ mice have median survival of 5 mo of age (Figure 1C), with progressive signs of rectal bleeding and anemia. Similar to the results reported with an independently generated ApcΔ580/+ conditional mouse strain [14], ApcΔ580/+ mice had more than 100 (120 ± 37, n = 11) intestinal tumors at the time of their death (Figure S1). Inactivation of wild-type Apc is an important prerequisite for tumor development. We analyzed 30 intestinal tumors from ApcΔ580/+ mice by in vitro transcription and translation assay, but none of them showed truncated Apc products (unpublished data), indicating that the most likely mechanism of wild-type Apc inactivation is by allelic loss. The mutant allele had to be maintained and transmitted through male mice, as ApcΔ580/+ females were frequently not healthy enough to successfully nurse their own pups because of their tumor burden.
ApcCKO/+ mice were intercrossed to generate ApcCKO/CKO offspring. Approximately one-quarter of the offspring (17 of 81) were homozygous for the ApcCKO allele. These mice as well as heterozygous mice for ApcCKO allele are normal, showing no differences in their survival to the wild-type littermates (Figure 1C). We tested whether our ApcCKO allele can compliment the wild-type allele by crossing the ApcCKO/CKO female with ApcΔ580/+ male mouse. The resultant ApcCKO/Δ580 offspring were viable and born in the Mendelian ratio, suggesting that the presence of loxP sites in introns flanking exon 14 have no adverse effect on the function of the Apc gene.