The 180 splice variant of NCAM-containing exon 18-is specifically expressed in small cell lung cancer cells

The Neural Cell Adhesion Molecule (NCAM) is a glycoprotein expressed as 120, 140 and/or 180 kDa isoforms, all derived through alternative splicing of a single gene. NCAM 120 contains no intracellular domain, whereas NCAM 140 and 180 have different intracellular domains determined by alternative splicing of exon 18. NCAM has been described as a biomarker to discriminate small cell lung cancer (SCLC) from non-SCLC (NSCLC). However, peripheral blood mononuclear cells (PBMC) also express NCAM. We studied the expression of NCAM splice variants in cell lines, tumor tissues and control cells.

Using reverse transcriptase-PCR we evaluated the expression of NCAM exon 18 splice variants in lung cancers cell lines, control cell lines, PBMC of healthy controls and SCLC tissue. In addition we studied the expression of the NCAM exon 18 encoded protein (E18) in SCLC by immunocytochemistry and flow cytometry using an E18-specific monoclonal antibody obtained by hybridoma fusion of E18-immunized mouse spleen cells. Finally we looked at immune responses to E18 in mice.

We found expression of RNA encoding the NCAM 180 variant in all SCLC cell lines. NCAM exon 18 was not expressed in 23/28 (82%) of the other tumor and leukemia cell lines tested and PBMC. Next, we also evaluated the expression of NCAM exon 18 in human SCLC tissue. Expression of NCAM exon 18 in 8 of the 10 (80%) SCLC biopsy samples was found. The newly raised E18-specific antibodies stained NCAM at the adherent junctions between adjacent cells in SCLC cell lines. The data demonstrate the intracellular location of E18 in SCLC. Furthermore, a specific cytotoxic T cell (CTL) response and significant antibody titers were found in mice upon immunization with recombinant E18 and its encoding DNA.

The results of this study can be applied in the diagnosis and immunotherapy of SCLC. A larger study investigating E18 as a marker for SCLC is indicated.