Cloning of embryonal stem cell-specific genes: characterization of the transcriptionally controlled gene esg-1.

We have isolated, by differential library screening, eight cDNAs representing genes that are specifically expressed in the embryonal stem cell line IMT-11, when compared to the parietal endoderm-like cell line PYS-2 or to NIH3T3 fibroblasts. One of these genes, embryonal stem cell gene 1 (esg-1), was analyzed in detail. esg-1 mRNA is found at high levels in both IMT-11 and F9 embryonal carcinoma cells and disappears during the differentiation of the stem cells. Furthermore, expression of the gene was found to be extremely low in, or absent from, oocytes and fertilized eggs, but it is strongly induced at the 2-cell stage, reaching maximum levels at the 4-cell stage. In contrast, esg-1 expression is detectable neither in midgestation embryos nor in neonatal tissues. These results strongly suggest that esg-1 is expressed specifically or at least predominantly in embryonal stem cells. Antibodies directed against a glutathione S-transferase-esg-1 fusion product detect a protein of M(r) approximately 14,000 in F9 embryonal carcinoma cells, but not in differentiated cells. Apart from the esg-1 gene, which contains two introns, there are at least seven esg-1-related pseudogenes in the mouse genome that differ from the esg-1 gene by the presence of multiple point mutations, by the lack of intervening sequences, and/or by the presence of a polyadenylated stretch at the 3' end. The esg-1 gene is under stringent transcriptional control in differentiating and differentiated cells, as shown by both nuclear run-on assays and the transient F9 stem cell-specific expression of constructs consisting of esg-1 upstream sequences fused to a luciferase reporter gene.