Cloning and characterization of a novel gene encoding a putative transmembrane protein with altered expression in some human transformed and tumor-derived cell lines

Identification and characterization of genes expressed in normal cells and decreased in their malignant counterparts is an important method for detecting candidate tumor suppressors. Using differential display of mRNAs from nontumorigenic infinite life span human fibroblast cell strain MSU-1.1 and an isogenic fibrosarcoma-derived cell line, 6A/SB1, which was derived from chemical carcinogen transformed MSU-1.1 cells, we identified a novel gene, ST7, showing sixfold lower expression in 6A/SB1 cells compared with parental MSU-1.1 cells. Molecular cloning of a near full-length cDNA revealed that the novel gene encodes a putative transmembrane protein composed of 859 amino acids: the 492 N-terminal amino acids including a fivefold cysteine-rich repeat of 40 amino acids homologous to the ligand binding repeat of the known low density lipoprotein receptor, a 24 hydrophobic amino acid stretch spanning the plasma membrane, and a C-terminal domain of 343 residues. ST7 is located on human chromosome 8, band q22.2-23.1, the same locus as the genes involved in acute myeloid leukemia and a locus of high polymorphism in cancer biopsies. The ST7 gene is widely expressed in normal human tissues and is particularly abundant in human heart and skeletal muscle. Northern analysis of 15 tumor cell lines derived from patients and 16 cell lines established from tumors formed in athymic mice by MSU-1.1 cells transformed in culture by various methods showed that 16 of the 31 cell lines have low or undetectable levels of ST7 mRNA. Furthermore, Western blotting analysis using a specific anti-peptide antibody demonstrated that the level of ST7 protein is high in normal fibroblasts and low in 12 sarcoma-derived cell lines tested. Altered expression of ST7 appears to occur at both the transcriptional and post-transcriptional levels. These studies are a first step in characterizing a novel putative receptor protein, whose expression is downregulated in some malignantly transformed cells, and which may play an important role in the transformation process of these cells.