Feb. 6, 2003 A gene known for its ability to form blood vessels has been found to be a key player in a chromosomal abnormality that causes potentially devastating birth defects in the heart and throughout the body.
In a study published in the February 2003 issue of Nature Medicine, a group of collaborators from across the globe reports that abnormalities in vascular endothelial growth factor, or VEGF, is a cause of DiGeorge syndrome. The syndrome can cause a wide range of heart defects, many of which are vascular in nature, as well as problems with the thymus and parathyroid gland, craniofacial abnormalities and mental retardation.
"We have found one of the downstream target genes," said Dr. Simon J. Conway, developmental biologist at the Medical College of Georgia and a senior author on the Nature Medicine paper along with Dr. Peter Carmeliet, director of The Center for Transgenic Technology at Katholieke Universiteit Leuven in Belgium. One next step would be to find why these VEGF defects occur with an ultimate goal of trying to prevent them, Dr. Conway said.
Researchers found this target "downstream" of human chromosome 22, which is known to be deleted in 60 to 70 percent of people with DiGeorge syndrome. Deletion of chromosome 22 removes a group of 24 genes as well, many of which are transcription genes known to control many downstream targets. Although the targets remain largely unknown, it's believed that these 24 genes control hundreds, maybe thousands, of downstream genes, which helps explain the complexity of the syndrome that can result when the chromosome is deleted, Dr. Conway said.
One of those 24 genes is Tbx1, which is widely considered the primary gene involved in DiGeorge syndrome, he said. In 2001, several research groups published their findings on mice models lacking Tbx1; those mice had some, but not all, the defects found in humans. In fact, although Tbx1 seems to have a role in DiGeorge and it may be the primary contributor to the heart defects that occur, no patient with DiGeorge syndrome has been found to have only a deletion of this gene.
That's one of the things that led scientists to look downstream of this master gene. Dr. Conway's lab, which focuses on normal and abnormal heart development, already was studying these downstream target genes, including VEGF.
"It struck us that in the mouse model for DiGeorge syndrome, a lot of the heart defects are vascular in nature, so we looked at one of the major, vascular genes, VEGF," he said. "If you knock out VEGF from a mouse model, it dies very, very early, before you get a heart, a head, even before you see an embryo proper."
Dr. Carmeliet in Belgium was the first to produce a knockout mouse in which VEGF was removed and replaced with the three most interesting of the five versions of the gene to try and determine the role of each. In the Nature Medicine study, researchers found that only one of the five versions, VEGF 164/164, was critical to DiGeorge; if it is absent or abnormal, the syndrome would occur. Both mice that had one of the other versions had the classic defects of DiGeorge syndrome. "That means if you change the contributions of the different types of VEGF and you don't have enough 164, you get what looks like DiGeorge syndrome in a mouse," Dr. Conway said. "It means it's a downstream target that is playing a role in DiGeorge." And it also means that one of the functions of the 24 genes deleted in DiGeorge is to ensure that the body has enough of VEGF 164/164, he said.
The researchers collected data in animal models, including the mouse and zebra fish - in collaboration with Mermaid Pharmaceuticals in Germany - then collaborated with other researchers, including those at the University of Pennsylvania School of Medicine, Albert Einstein College of Medicine and the Institute of Child Health in London, to examine human DNA from patients with DiGeorge.
They sequenced the human DNA to look for any abnormalities in the VEGF gene and found problems with the VEGF promoter, the piece of DNA that tells VEGF to be expressed and how to be expressed.
That means that people who have a deletion of chromosome 22 and problems with the VEGF promoter are at greatest risk of complex, congenital defects, Dr. Conway said. It also means that something on chromosome 22 is working with VEGF, and the researchers believe that's Tbx1, and that a reduction in Tbx1 and a corresponding reduction in VEGF are enough to produce DiGeorge.
"We have shown that we get all these defects and that there is an association with Tbx1," Dr. Conway said. "What we still don't know is why they occur. Our (next) job is to try and work out the cause."
Collaborators in Dr. Conway's lab included Dr. Jian Wang, graduate student Paige Kneer and research assistant Rhonda H. Rogers. The studies at MCG were funded in part by Dr. Conway's four grants from the National Institutes of Health and one from the American Heart Association.
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