Abstract
Sperm and trophoblast are among the few nucleated human cells that do not express HLA class I antigens. DNA methylation, which is proposed to be a tight mechanism of regulation, may be necessary to turn off these genes. We have investigated the transfectability of HLA class I genes and of the genes for the T-cell differentiation antigens Leu-1 (CD5) and Leu-2 (CD8) in mouse L cells by using human sperm cells and choriocarcinoma cell lines, tumors of trophoblastic origin, as sources of DNA. It was found that DNA from one choriocarcinoma line (JAR) does not transfect genes for HLA, Leu-1, or Leu-2 and that DNA from two other choriocarcinoma lines (BeWo and Ima) transfects only some of the surface markers. Sperm DNA transfects genes for all the surface antigens tested except Leu-1. DNA from control cells and from the line SCH transfects all the markers studied. Southern blots show that all cell types contain apparently intact genes encoding HLA, Leu-1, and Leu-2 and reveal differences in the DNA methylation patterns of genes from different sources of DNA. We treated JAR (the cell line with the lowest transfecting ability) with 5-azacytidine and obtained demethylation of its DNA. This demethylated DNA transfects genes for both HLA class I antigens and Leu-2. Further culture of JAR cells in the absence of 5-azacytidine results in remethylation of their DNA and decreased ability to transfect these surface antigens. These findings indicate that DNA methylation affects the efficiency of transfection of surface antigen genes in L cells.