Alternative mRNA splicing in colon cancer causes loss of expression of neural cell adhesion molecule

Background. The neural cell adhesion molecule (NCAM) has numerous isoforms resulting from alternative splicing of mRNA. The 3 major isoforms found in adult tissue are (1) a 120-kDa protein that is linked to the plasma membran e by glycosylphosphatidylinositol; (2) a 140-kDa form that has a transmembr ane component and a cytoplasmic tail with unknown function; and (3) a 180-k Da isoform that has an intracellular protein that binds the cytoskeleton. N CAM is capable of homotypic binding and therefore plays a role in cell-cell adhesion for cells expressing the 180-kDa isoform by anchoring groups of c ells into epithelial sheets. NCAM-180 is the isoform found in colonocytes, and loss of expression is associated with clinically aggressive colon cance rs. Methods. Western blotting and reverse transcriptase-polymerase chain reacti on were used to screen commercially, available cell lines for NCAM-180 expr ession. For cell-line pairs with differential. NCAM-180 expression, exon an alysis was performed with reverse transcriptase-polymerase chain reaction t o determine where the molecule was spliced, culminating in failed expressio n. These results were confirmed with exon analysis in colon cancers harvest ed at the time of laparotomy. Results. Analysis of a SW480 cell line (derived from a patient's primary co lon cancer lesion) revealed, NCAM-180 expression, whereas no expression was found in the SW620 cell line (derived from a metastatic lesion from the sa me patient). Exon analysis of NCAM mRNA transcripts from SW620 revealed tha t the transcripts were truncated after exon 12. This region correlates to a n area between 2 fibronectin-III domains on the NCAM protein. Conclusions. The most common site for NCAM alternative splicing is between the 2 fibronectin-III domains corresponding to the border between exons 12 and 13 of the NCAM gene. Loss of NCAM-180 expression in aggressive colon ca rcinoma results from a splice defect in the same area, which may result in defective intracellular adhesion between colonocytes.