The melanoma differentiation associated gene-7 (mda-7) was identified in HO-1 melanoma cells induced to terminally differentiate by treatment with fibroblast interferon (IFN-^), and the protein kinase C activator mezerein (MEZ) (1). The differentiated and growth arrested HO-1 melanoma cells mRNAs were used to generate a cDNA library; another library was generated from proliferating HO-1 cells. Differentiation induction subtraction hybridization (DISH) of these two yielded a temporally spaced subtracted cDNA library enriched for genes activated during HO-1 terminal differentiation (2,3). The underlying hypothesis of the above approach was that cancer cells would stop or significantly reduce expression of genes regulating growth control or differentiation, and that treatment with IFN-^ and MEZ would reactivate them. mda-7 was identified as a gene with minimal or absent expression in proliferating melanoma cells, high expression in normal melanocytes, and inducible expression in terminally differentiated melanoma cells (4-6). Studies on patient-derived specimens reported that MDA-7 protein expression inversely correlated with melanoma progression and that transfection of metastatic human melanoma cells with a vector encoding for mda-7/IL-24 reduced their colony formation capabilities (6,7). The lack of MDA-7 protein expression in cancer cells did not result from mutations in the gene but rather from permanent defects in the signaling pathways or mRNA/protein stability (8,9).
Subsequent studies on mda-7 have shown that it encodes an evolutionarily conserved protein of 206 amino acids, with a predicted size of 23.8 kDa (see Fig. 1A) (1,9,10). Two orthologs have been identified: the rat c49a and the mouse mob5 genes. The rat c49a gene, with 78% nucleotide homology to human mda-7, was initially identified by differential display polymerase chain reaction (PCR) as a gene overexpressed in wound healing and was associated with proliferation of fibroblasts (8); mob5 was identified as a ras-induced gene involved in intestinal epithelia neoplasia (11). The mouse mda-7 ortholog was later identified as a Th2-specific cytokine, and named IL-4 induced secreted protein (FISP) (12).
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