CD24 is a novel oncogene in colorectal cancer, detected by microarray profiling of cell transformation, and is a target for immunotherapy of cancer.

B136

Aims:To identify genes that play a role in colorectal cancer (CRC) carcinogenesis by analysis of differential gene expression of normal and transformed CRC cell lines. Methods : Gene expression array analysis, (RG-U34) Genechip^®, was performed in normal and transformed rat intestinal epithelial cells before and after exposure to celecoxib (Pfizer, New York, NY, USA). In particular, we were looking for: a. altered gene expression in the transformed cells that reverts to normal following exposure to a selective COX2 inhibitor of. b. Novel genes. c. Genes encoding membrane receptors or ligands. As a validation of the results and for human patients, immunohistochemistry was performed on 398 biological samples from the GI tract (normal, polyps and adenocarcinomas). Human cancer cell lines were tested for their response to anti-CD24 monoclonal antibodies. Three different antibodies were added separately to the medium at different concentrations or durations; viability was measured by Methylene blue assay. CD24 cellular levels in HT29 CRC cell line were stably down regulated using siRNA technology and changes in their proliferation capacities were examined. Results : 1081 genes were differently expressed following malignant transformation; 71 genes showed alter expression that reverted to normal following treatment with celecoxib, among them was the gene to CD24. Immunohistochemistry confirmed that increased expression of CD24 is an early event in CRC carcinogenesis. It was expressed in 90.7% of adenomas and 86.3% of CRC. Very low expression was seen in normal epithelium (16.6%). Human cancer cell lines showed growth inhibition in response to the antibodies, according to their expression levels of CD24 and in dose and time dependent manners. These results were repetitive for three different antibodies. The derivatives of HT29 that showed reduced levels of CD24 following transfections with two siRNA expression vectors that differ in their binding sites, showed a significant reduction in growth rate when compared to parental HT29 and HT29 with a control-siRNA and a higher level of apoptosis in FACS analysis. Conclusions : CD24 is a new gene that is overexpressed in the colonic mucosa, already at an early stage of carcinogenesis and might play a role in tumor progression. Therefore, it is likely to be a useful biomarker for early detection and might be of use as a new target in CRC therapy.