University of Virginia Health System

U.VA. SCIENTISTS UNCOVER KEY FACTORS INVOLVED IN PROCESS THAT PLAYS CRITICAL ROLE IN TURNING GENES OFF

Scientists at the University of Virginia have determined the key components and signals of a biochemical reaction involved in turning genes off. This reaction is crucial to development in organisms ranging from yeasts to mammals. The findings, which appear in the current issue of Science, could have implications for developing treatments for cancer and other diseases.

In a series of experiments using proteins from the fruitfly Drosophila, Sepideh Khorasanizadeh, assistant professor of biochemistry and molecular genetics at the University of Virginia School of Medicine, and U.Va. graduate student Steven A. Jacobs determined how a protein known as heterochromatin-associated protein 1 (HP1) interacts with a histone protein, which functions as part of a spool that DNA wraps around. Although it was previously known that HP1 binds histones, this is the first time researchers have discovered the crucial factors involved in the interaction.

Khorasanizadeh and Jacobs used various methods, such as X-ray diffraction, to discern the characteristics of the interaction and get a clear picture of how it occurs. They found that the process depends on methylation - the addition of methyl groups - on a particular amino acid in the histone called lysine. They were able to visualize, at the resolution of one-millionth of a millimeter, the way in which the methylation mark is specifically recognized by HP1, and characterize the energetics associated with this recognition.

The methylation induces molecular and chemical changes that facilitate binding between the proteins. Once the lysine has been methylated, it is a signal for the HP1 protein to come and bind to the histone, Khorasanizadeh said, noting that additional experiments showed that when the lysine is not methylated the HP1 protein will not bind to the histone. This protein is not going to carry out its function until methylation has occurred, she said.

This binding process appears to be very important in the functioning of organisms because if it is disrupted, it profoundly impedes development. Experiments, in which the researchers deliberately induced mutations into the surface of HP1 where it contacts the histone, were lethal in Drosophila. One reason we picked Drosophila is because its HP1 and histone proteins are very similar to the human versions, so we could ask what happens when there are mutations in these proteins at or near the site necessary for recognition of methylation mark, Khorasanizadeh said. Flies don't even develop if they have these mutations, she said, noting that this would probably hold true in humans as well.

The reason for this is that the protein interaction is involved in a very important process called epigenetic silencing. This is the body's mechanism for turning off different genes in different cells. Every cell in the body contains the DNA for every gene, but not every gene is necessary in every cell, and the use of this information is selected according to what is needed, Khorasanizadeh said. In the brain, certain genes have to be expressed, whereas in the hands other genes have to be expressed. Epigenetic silencing is the process of turning the right genes off in the desired locations.

Prior research, mainly by David Allis and his co-workers at the U.Va. Department of Biochemistry and Molecular Genetics, has shown that gene silencing reaction works in part like this: Once the histone is methylated, this sends a signal that recruits HP1 to bind to histone and this in turn triggers an accumulation of more HP1, which turns off all the genes in that area. So if there is a mutation that affects either HP1, the histone or methylation, the entire epigenetic silencing event is disrupted.

Any disruption of these events can lead to malfunction in the body, Khorasanizadeh said. This has implications for all kinds of diseases because it is the basis of what leads to human diseases and cancer, she added. In cancer cells and other disease syndromes, it may be the epigenetic control that has become misdirected. For example, breast cancer can occur upon lack of expression of HP1 protein.

By shedding light on how the epigenetic silencing process works, the research may ultimately lead to treatments for various diseases. Those are the ultimate goals of what we are trying to present in this paper, Khorasanizadeh said. Then [scientists] can design things, such as gene therapy, to correct underlying defects in this process and potentially cure diseases.

February 21, 2002