Loss of novel mda-7 splice variant (mda-7s) expression is associated with metastatic melanoma.

Expression of melanoma differentiation associated gene-7 (mda-7) also known as interleukin 24 (IL-24) decreases during melanoma cell differentiation and induces apoptosis in melanoma cells but not in melanocytes. Here we identify a novel splice variant of the cancer growth suppressor gene mda-7/IL-24 (mda-7s) that is differentially expressed in RNA preparations from normal human melanocytes, transformed melanocytes, nevi, subcutaneous metastasis, lymph node metastasis, and melanoma cell lines. The 450 bp mda-7s mRNA encodes a protein of 63 residues with a molecular weight of 12 kDa. mda-7s lacks exons 3 and 5 of the full-length transcript and contains only 14 amino acids of homology to MDA-7 located within the signal peptide region of the wild-type sequence. Despite minimal homology, MDA-7S coprecipitates full length MDA-7 and reduces secretion of cotransfected MDA-7. mda-7 and mda-7s are coexpressed in all RNA preparations other than subcutaneous and lymph node metastasis where mda-7s expression is lacking. mda-7s expression is therefore linked to a non-metastatic phenotype.