Researchers identify the role of a protein in hemoglobin gene silencing

Virginia Commonwealth University Massey Cancer Center researchers have identified the role of a protein in hemoglobin gene silencing that may one day be a potential target for the treatment of genetic blood disorders like sickle-cell anemia and beta-thalassemia on the molecular level.

In the April issue of the journal Proceedings of the National Academy of Sciences, researchers reported for the first time that the protein, MBD2, mediates silencing of the fetal gamma-globin gene through DNA methylation, a process that chemically modifies DNA. Researchers used a transgenic mouse model containing the human hemoglobin gene locus to show that MBD2 interprets the DNA methylation "signal" throughout the genome, which determined how the pattern of methylation effected the expression of specific genes.

"Understanding how these epigenetic switches turn specific genes on and off, and identifying the important proteins involved, could lead to more targeted ways to reactivate genes and determine if there is a therapeutic benefit for particular diseases," said Gordon D. Ginder, M.D., director of the VCU Massey Cancer Center and lead author of the study.

Epigenetics refers to the study of the modifications of DNA and the surrounding proteins found in chromosomes that turn genes on and off and that can be passed on after cell division in an individual. Traditionally, researchers have focused their attention on changes to the DNA base code as being responsible for altered gene expression in disease.

Previous clinical studies have shown that increased gamma-globin gene expression has a positive effect in those with sickle-cell anemia or beta-thalassemia. "The gamma-globin genes normally become silent in adult hemoglobin expressing red blood cells. If we can find a specific and safe mechanism to reactivate the gamma-globin gene, we may be able to overcome the underlying molecular defect in sickle-cell anemia and beta-thalassemia," Ginder said.

Gene silencing is important for the differentiation of many different types of cells to take place. In humans, there are five beta-type globin genes clustered on chromosome 11 in the order in which they are "turned on," or expressed, during development. These genes include the embryonic epsilon-globin gene, two gamma-globin genes and the adult delta- and beta-globin genes. During fetal development, the embryonic epsilon-globin gene is active first, followed by the gamma-globin genes, and finally the major adult form, beta-globin, becomes the dominant expressed gene following birth.

According to Ginder, regulation of many genes and other molecular processes require DNA methylation. He said that DNA methylation is associated with the silencing of many types of genes, including tumor suppressor genes found in cancer cells. Scientists now know that DNA methylation plays a significant role in the development and progression of several forms of cancer.

Currently, the only therapeutic approach to relieving methylation-mediated gene silencing that has been tested in humans is through blocking the methylating enzymes non-specifically throughout the cell. Although this approach may have the desired effect on the specific gene or genes involved, it can also have an undesirable effect by turning on the wrong genes, he said.

"The more targeted the approach the better, because there is less likelihood of producing any unintended negative side-effects. For example, there is some specificity of how some proteins, such as MBD2, act to silence only certain sets of methylated genes," Ginder said.

Mutations of hemoglobin genes play a role in genetic blood disorders such as sickle-cell anemia and beta-thalassemia.