Timothy H. Bestor, Ph.D.

Research Summary

DNA Modification and the control of parasitic DNA in the mammalian genome.

Transcription of mammalian genes is prevented by the methylation of cytosines within promoter elements. It was previously thought that reversible promoter methylation might be involved in gene regulation during development, but recent data indicate that the primary function of DNA methylation is the suppression of intragenomic parasites and of proviral DNA. DNA methylation also has crucial roles in genomic imprinting and X chromosome inactivation in females.

Transposons threaten the structure and orderly expression of the genome; they act only in their own interest and provide no benefit to the host. Mammals have developed a sophisticated nuclear host defense system that allows them to recognize and irreversibly inactivate nearly every transposon. Inactivation involves the attachment of methyl groups to the 5 position of cytosine residues by an enzyme called DNA (cytosine-5)-methyltransferase (Dnmt1). We were the first to purify this enzyme, to clone the gene that encodes it, and to use gene disruption technology to show that cytosine methylation is required for the normal development of mammals. More recently we have found that certain transposable elements show unrestrained activity when Dnmt1 is removed from mouse embryos, confirming our hypothetical host defense role for cytosine methylation.

The nuclear host defense hypothesis has significance for human health in two important areas. First, a loss of cytosine methylation is almost always seen in human cancer, and cancer cells show a reduced capacity to control their transposable elements. This is almost certainly involved in the genomic instability characteristic of tumors cells. The ability to identify inborn abnormalities of the DNA methylating system would identify individuals at risk for the early development of cancer.

Second, cytosine methylation is part of a nuclear host defense system, and it is possible to stimulate this system to target the irreversible inactivation of deleterious genes. The incidence or severity of many diseases (including many types of cancer) is proportional to the level of expression of single genes or small groups of genes. We are developing new technologies for the targeted methylation of deleterious genes. This approach has the virtues of a vaccine in that it stimulates an existing host defense system. Competing technologies (largelyantisense inhibition and ribozyme cleavage) do not take advantage of the existing biology and are crude by comparison. The first target of this targeted methylation therapy is HIV-1, the virus that causes AIDS. In addition to this work of direct relevance to human health we are working to understand the genetic and biochemical mechanisms that allow the cell to recognize and inactivate parasitic sequences of all types.