Down-regulation of drs mRNA in human colon adenocarcinomas

We have previously reported that the drs gene, whose mRNA expression is down-regulated by retroviral oncogenes such as v-src and v-K-ras, has the ability to suppress transformation by v-src and v-K-ras in the rat cell line F2408. We have also isolated a human homolog of this gene (h-drs) and shown that expression of h-drs mRNA is markedly reduced in a variety of human cancer cell lines, including those of carcinomas of the colon, bladder, and ovary, suggesting that down-regulation of drs mRNA is correlated with the development of human cancers. To clarify the correlation between down-regulation of the drs gene and malignant tumor formation in human cancer tissues, we examined expression of drs mRNA in human normal tissues, colon adenoma, and adenocarcinoma tissues by in situ hybridization. Expression of drs mRNA was detected in most normal tissues tested, including those of the colon, bladder, and ovary. However, drs mRNA was hardly expressed in any of the colon adenocarcinoma tissues examined. Northern blot analyses confirmed these results. Neither gross deletion nor re-arrangement of the drs genome was detected by Southern blot hybridization in these adenocarcinoma tissues. drs mRNA was significantly expressed in colon adenoma with mild atypia but down-regulated in adenomas with moderate atypia and focal carcinoma. Our results indicate that down-regulation of drs mRNA is closely correlated with development of colon adenocarcinoma, suggesting a tumor-suppressor function of the drs gene in human cancers.