Multiple APC messenger RNA isoforms encoding exon 15 short open reading frames are expressed in the context of a novel exon 10A-derived sequence.

Intragenic splice mechanisms affecting the coding exons 8 to 15 of the human adenomatous polyposis coli (APC) gene have been analyzed. Three mechanisms within this gene area were found to contribute to mRNA heterogeneity: (i) facultative expression of exon 9-encoded sequences; (ii) in-frame insertion of a 54-nucleotide sequence encoded by a novel exon located 1.6 kb down-stream from exon 10, provisionally designated APC exon 10A; (iii) skipping of exon 14, resulting in a novel exon 13/15 connection. Interestingly the latter event provided the mRNA with a novel open reading frame, which was terminated after 19 codons of exon 15-derived sequences. Combinatorial joining of these segments yielded 7 different transcripts in addition to an mRNA species resulting from an exon 10/15 connection, as determined by cloning and sequence analysis. RT-PCR expression analyses were carried out to demonstrate that this complexity of splice variants is indeed synthesized in cell lines derived from various tissues. Furthermore, in accordance with our findings at the transcript level, we provide Western blot analyses demonstrating that moderate steady-state levels of genuine APC-specific low m.w. polypeptide chains exist. These APC "light chains", however, are not identical with polypeptide chains, which have been reported to accompany apoptosis and necrosis, since the molecules described here are definitively co-expressed with p300apc at the transcript and protein levels.