Augmenting Antitumor Immune Responses with Epigenetic Modifying Agents

The paternal and maternal genomes are not equivalent and both are required for mammalian development. The difference between the parental genomes is believed to be due to gamete-specific differential modification, a process known as genomic imprinting. The study of transgene methylation has shown that methylation patterns can be inherited in a parent-of-origin-specific manner, suggesting that DNA methylation may play a role in genomic imprinting. The functional significance of DNA methylation in genomic imprinting was strengthened by the recent finding that CpG islands (or sites) in three imprinted genes, H19, insulin-like growth factor 2 (Igf-2), and Igf-2 receptor (Igf-2r), are differentially methylated depending on their parental origin. We have examined the expression of these three imprinted genes in mutant mice that are deficient in DNA methyltransferase activity. We report here that expression of all three genes was affected in mutant embryos: the normally silent paternal allele of the H19 gene was activated, whereas the normally active paternal allele of the Igf-2 gene and the active maternal allele of the Igf-2r gene were repressed. Our results demonstrate that a normal level of DNA methylation is required for controlling differential expression of the paternal and maternal alleles of imprinted genes.

5-Azacytidine was first synthesized almost 40 years ago. It was demonstrated to have a wide range of anti-metabolic activities when tested against cultured cancer cells and to be an effective chemotherapeutic agent for acute myelogenous leukemia. However, because of 5-azacytidine's general toxicity, other nucleoside analogs were favored as therapeutics. The finding that 5-azacytidine was incorporated into DNA and that, when present in DNA, it inhibited DNA methylation, led to widespread use of 5-azacytidine and 5-aza-2'-deoxycytidine (Decitabine) to demonstrate the correlation between loss of methylation in specific gene regions and activation of the associated genes. There is now a revived interest in the use of Decitabine as a therapeutic agent for cancers in which epigenetic silencing of critical regulatory genes has occurred. Here, the current status of our understanding of the mechanism(s) by which 5-azacytosine residues in DNA inhibit DNA methylation is reviewed with an emphasis on the interactions of these residues with bacterial and mammalian DNA (cytosine-C5) methyltransferases. The implications of these mechanistic studies for development of less toxic inhibitors of DNA methylation are discussed.