Biomedical Frontiers: Winter/Spring 1996, Vol.3, No.2
Research Advance
Methylation Therapy

Researchers are developing therapies for AIDS and cancer that rely on a naturally occurring defense system within cells. This defense system, which is based on methylation of cytosines in DNA, is being used to inactivate deleterious genes.

Normally, cytosine methylation of promoters acts to restrict proliferation of parasitic sequence elements, says Dr. Timothy Bestor, associate professor of genetics and development. The process, in which methyl groups are attached to cytosine residues in DNA, effectively shuts off the methylated gene. Without such a defense system, parasitic sequence elements would proliferate unchecked and might impose a lethal mutagenic or cytotoxic burden.

Molecular graphics of methylated DNA.

In his work, for which a patent application has been filed, Dr. Bestor cloned and sequenced mammalian DNA methyltransferase and used gene disruption techniques to show that DNA methyltransferase protein is required for mammalian development. Dr. Bestor based the targeted methylation concept on studies of DNA methyltransferase, which has separate regulatory and catalytic domains. Furthermore, in preliminary work with targeted methylation, Dr. Bestor, Dr. Guo-Liang Xu, a postdoctoral fellow, and Michael Livstone, a graduate student, are developing CpG-specific DNA methyltransferase of novel sequence specificity that can be directed to a single promoter that controls the expression of viral DNA or other deleterious genes.

Bacterial DNA methyltransferase in the act of catalysis.

"Methylation holds great promise for reducing the severity and incidence of a number of diseases."

This technology holds great promise for reducing the severity and incidence of a number of diseases by shutting off the genes that are involved, says Dr. Bestor. For instance, only a single promoter is required for the replication of HIV; methylation of this DNA, therefore, could effectively shut down the virus.

Targeted methylation has several advantages over alternative methods of shutting off the expression of a specific gene. First, methylation stimulates an existing host defense system and does not depend on a highly artificial system to work. "It has the virtues of a vaccine, rather than of a foreign chemical," says Dr. Bestor. "Vaccines have been more important than any other development in the treatment of infectious diseases." In addition, once a new methylation pattern is imposed on a promoter, the cellular mechanism keeps it intact. Therefore, any therapeutic agent used to methylate a target sequence would only have to be present for a brief period. Other approaches require therapeutic agents to be present indefinitely, making these methods impractical and potentially toxic or harmful. Alternative methods include antisense inhibitors, ribozymes, and intermolecular triplex oligonucleotides. Dr. Bestor foresees targeted methylation as a potential treatment for AIDS, hepatitis B, and human papilloma virus infection.