Fig. 1.
Conditional truncation of the APC gene at exon 468 to generate APClox468 mice. (A) A cosmid clone spanning exons 10 to 14 of the mouse APC gene was modified to encode a loxp flanked neoR gene cassette upstream of exon 11 and a third loxP sequence carrying an SphI site immediately downstream of APC exon 12. The entire cassette was excised and transfected into TC1 (129/SV) embryonic stem cells (46), and NeoR clones were picked. An external (thick blue line) and an internal (thick red line) probe were used to detect homologous recombination by DNA Southern blot, as revealed by SphI or EcoRI fragments spanning upstream sequences (2.3 kb and 6.5 kb for wt and recombinant alleles, respectively; red lines) or downstream (11 kb and 8.5 kb for wt and recombinant alleles, respectively; blue lines). Loss of the third loxP site produced a smaller SphI fragment of 1.3 kb. The neoR cassette was removed by crossing to synaptonemal-Cre mice and screening. Mice were backcrossed for more than 20 generations to C57BL/6J. (B) Schematic representation of the truncation at exon 468 compared with other truncations from previously reported mouse models of polyposis. (C) Distribution of polyps in the GI tract of TS4Cre × cAPClox468 mice. Pictures were taken at a magnification of ×16 and stitched by using Mosaic J plug-in of ImageJ. (D) Compiled frequencies of polyps from six TS4Cre × cAPClox468 mice at 4 mo of age.