Isolation and functional analysis of the genes involved in suppression of transformation in primary cells

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The drs gene was originally isolated as a suppressor gene against v-src transformation from rat primary embryo fibroblast cDNA library. The Drs protein has one transmembrane domain, a short intracellular domain in the C terminus, and three consensus repeats (Sushi motifs). We have shown that expression of drs mRNA was markedly reduced in a variety of human cancer cell lines and malignant tissues, including those of the colon, bladder, ovary, lung, and prostate. Furthermore, introduction of drs cDNA by retrovirus vector into these cancer cell lines caused suppression of anchorage-independent growth and tumorigenicity. These findings indicate that downregulation of drs mRNA is closely correlated with expression of malignant phenotypes in development of human cancers. Analyses with deletion mutants of the drs gene revealed that both the C-terminal region inside the transmembrane domain and three consensus repeats (CR) in the N-terminal region are essential for the suppression of anchorage-independent growth of the cells. An Rb-independent downregulation of cyclin A mRNA was involved in the suppression of anchorage-independent growth by the drs gene in human cancer cells. We also isolated a novel variant cDNA of mouse drs (mDRS-2) which contains two CRs in addition to a mouse homologue of drs (mDRS-1) which contains three CRs. Both types of drs mRNA were expressed in normal mouse tissues. The mDRS-1 had the ability to suppress anchorage-independent growth of these cells whereas mDRS-2 did not, indicating that the lack of one CR is critical for suppression of anchorage-independent growth. Furthermore, we isolatede a mouse genomic clone for gene targetting and construction of drs-knockout mouse is in progress.