Exon 14-skipping of the adenomatous polyposis coli gene in purified epithelial cells of colonic mucosa and tumors

Adenomatous polyposis coli (APC) exon 14-skipped transcripts encode putative APC proteins of low molecular weight. To prove that exon 14-skipped mRNA variants do not simply represent tissue culture artifacts, expression of these APC transcript variants was demonstrated in native colorectal epithelium. Fresh surgical specimens of human colon were processed and epithelial cells were affinity-purified with the dynabead-immobilized monoclonal antibody Ber-EP4. Epithelium derived cDNA was PCR-amplified in the range of linear accumulation. RT-PCR products with and without APC exon 14 were evaluated by densitometry. Analyses of normal mucosa (n=8) and matching mucosa - tumor samples (n=4) revealed a consistent 4 to 1 ratio of APC exon 14-positive to exon 14-negative mRNA levels. We conclude, a) that APC exon 14-skipped transcripts are physiologically expressed in native human colon mucosa, and b) that ratios of exon 14-negative to exon 14-positive isoforms were not altered when colorectal tumor cells were compared with matching normal mucosa (p = 0.80).