Altered splicing of CEACAM1 in breast cancer: Identification of regulatory sequences that control splicing of CEACAM1 into long or short cytoplasmic domain isoforms

We show that CEACAM1 is expressed in a tissue specific manner with significant differences in the ratios of its short (CEACAM1-S) and long (CEACAM1-L) cytoplasmic domain splice variants. Importantly, we find dramatic differences between the ratios of S:L isoforms in normal breast tissues versus breast cancer specimens, suggesting that altered splicing of CEACAM1 may play an important role in tumorogenesis. Furthermore, we have identified two regulatory cis-acting elements required for the alternative splicing of CEACAM1. Replacement of these regulatory elements by human β-globin exon sequences resulted in exon 7-skipped mRNA as the predominant product. Interestingly, while insertion of exon 7 in a β-globin reporter gene resulted in its skipping, exon 7 along with the flanking intron sequences recapitulated the alternative splicing of CEACAM1.Our results indicate that a network of regulatory elements control the alternative splicing of CEACAM1. These findings may have important implications in therapeutic modalities of CEACAM1 linked human diseases.Alternative splicing is a process by which a single pre-mRNA can generate multiple mRNA isoforms by differential joining of 5' and 3' splice sites (ss) [1,2]. Mapping of the human genome suggests that more than 70% of genes encode for transcripts that undergo alternative splicing [3-5], and most alternative splicing events affect coding capacity of genes [6,7]. Given the importance of mRNA splicing in gene expression, it is not surprising that ~50% of human genetic diseases may be caused by mutations of the splice junctions or auxiliary regulatory sequences [8,9]. However, sequencing of many cancer specific alternatively spliced genes has indicated that genomic mutations are not the only cause of aberrant splicing. In fact, deregulated expression of splicing factors has been shown to affect alternative splicing, which is often accompanied by a shift in the ratio of alternatively spliced mRNA isoforms [10-15]. Accordingly,