PASADENA, Calif.- As a young medical student, Matthew Porteus recalls his frustration when admitting his first patient with chronic pain caused by sickle-cell anemia. There was little medicine could do to help her in a sustained and meaningful way. The experience influenced his decision to study the basic mechanisms of "gene targeting," one possible way to cure such diseases, which are caused by a single mutation in a cell. Now Porteus will be assisted in his studies thanks to a grant from the Burroughs Wellcome Fund.
The Career Award in the Biomedical Sciences is a five-year, $500,000 grant that consists of two parts – a two-year postdoctoral portion, and a three-year faculty portion. The program is intended to develop biomedical researchers early in their careers as they make the transition to becoming independent investigators.
Porteus is currently a postdoctoral researcher in the lab of Caltech president David Baltimore. Porteus's work is focused on developing techniques to increase the efficiency and specificity of gene targeting in human somatic cells. Gene targeting is a technique in which a gene is introduced into a cell to replace a specific gene that's been damaged.
Porteus notes there are now scores of diseases that are caused by mutations in single genes. Sickle-cell anemia, for example, is caused by a single mutation in the protein that carries oxygen in the blood. Cystic fibrosis, in which there is a small mutation in a channel that regulates salt and water flux into cells, is another.
Gene targeting is one promising approach to curing such diseases. Researchers like Porteus can introduce genetic change into a cell by creating subtle differences between the introduced gene and the target gene. Gene targeting is now widely used in the generation of "knockout" mice, in which genes are "knocked out" or disrupted, then replaced by genes from other organisms that are inserted into a particular genetic location. But gene targeting has had an extremely low success rate in human somatic cells. Porteus's work is focused on understanding the mechanism and regulation of gene targeting, with the goal of developing it as a technique for gene correction therapy.
To study gene targeting, Porteus and his colleagues have developed a system in which they can correct a mutation in a specific gene called GFP. From this, they have established several of the key parameters that regulate gene targeting in human somatic cells. The first major parameter is the role of DNA double-stranded breaks. As the name implies, these are a form of DNA damage in which both strands of the DNA molecule are broken. Porteus confirmed what others have shown, that the presence of a DNA double-stranded break in the target gene can stimulate the process of gene targeting by over 10,000-fold; that increases the success rate from one in a million to 3 to 5 percent. Porteus is currently working on ways to generate double-stranded breaks in a target gene.
The second major parameter is the role of random integration. When a gene is introduced into a cell it can either be integrated into the genome of the cell in a random fashion, or replace a specific gene. In human somatic cells the relative rate of gene targeting is very low, in part because the random integration rate is high. Porteus is working to understand the factors behind this, since, in other cell types, he's found that the relative rate of gene targeting is high because random integration is low. Besides studying the factors that affect the rate of random integration, he is developing techniques to block it as well.
Porteus believes that such focused study will eventually lead to gene targeting becoming a tool for gene correction therapy, and for a potential cure for people suffering from diseases that are caused by mutations in single genes.
The Burroughs Wellcome Fund is an independent private foundation dedicated to advancing the medical sciences by supporting research and other scientific and educational activities. Within this broad mandate, BWF's general strategy is to help scientists early in their careers develop as independent investigators, and to support investigators who are working in or entering fields in the basic medical sciences that are undervalued or in need of encouragement.-