Hybrid baculovirus treats hemophilia in mouse models
By University of Iowa
Aug 23, 2005, 21:13
A virus that typically infects insects could help with the development of gene therapy treatment for Hemophilia A, a condition in which even a bump on the knee can cause serious internal bleeding in people.
Researchers at the University of Iowa Roy J. and Lucille A. Carver College of Medicine improved a vector -- a vehicle that delivers gene therapy to cells -- in two ways to create a sustained, partial correction to bleeding problems in mice engineered to have Hemophilia A, which is also known as factor VIII deficiency. The findings appear in the Sept. 1 issue of the journal Blood (published Aug. 19 online).
The team adapted the outer layer, or "coat," from a baculovirus, a virus that infects butterflies and moths, onto another modified virus. This hybrid vehicle could more easily attach to certain liver cells and allow the genes within the vehicle to enter the cells. The genes then caused the liver cells to make the protein that prevents bleeding.
The researchers also modified the vehicle so that it would express these therapeutic genes only in liver cells, thus reducing the likelihood of negative side effects.
The laboratory findings have significant potential for developing improved treatment for hemophilia but are not yet applicable to people, cautioned Paul McCray, UI professor of pediatrics and the study's corresponding author. "It's an exciting finding, but we are still many steps away from a possible gene therapy for people with hemophilia," he said.
Hemophilia A is the leading sex-linked bleeding disorder, affecting one in 5,000 to 10,000 males. People with the condition have a genetic mutation that leaves them with little to no factor VIII protein to prevent uncontrolled bleeding. Individuals with the severe form of the disease have less than 1 percent of the normal amount of protein. However, only a relatively small amount of the normal protein level is needed to make the problem milder and, thus, less life threatening.
"Hemophilia is considered an ideal candidate for correction with gene therapy because if you could just raise the factor VIII activity from less than 1 percent of normal to within 5 to 10 percent of normal, the tendency for spontaneous bleeding and need for hospitalization would diminish dramatically," McCray said.
"In the mouse model in our study, we were able to achieve levels of gene expression that converted the hemophilia A in the mouse from a severe to a mild form. The correction lasted 30 weeks -- the duration of the study," he added.
One of the current treatments for hemophilia involves intravenously delivering recombinant (genetically engineered) human factor VIII protein to prevent bleeding episodes. However, the weekly to bi-weekly preventive treatments are extremely expensive, costing up to $500,000 per year. In addition, over time some patients may develop antibodies to the protein, making the treatments less effective.
In earlier studies, McCray's team, which includes Yubin Kang, M.D., at the time a UI assistant research scientist in pediatrics (now a UI resident in internal medicine), targeted the liver because its main functional cells, called hepatocytes, can make the factor VIII protein and secrete it into the bloodstream. However, the investigators recognized the need to target the liver more effectively.
"It has been difficult to conclusively identify the cells that normally make factor VIII," McCray said. "Hepatocytes may not be the main source of this protein, but they are relatively easy to target. So we aimed to find a way to get these cells to make more of it. In effect, we're using the hepatocytes as a factory to make this protein and secrete it into the bloodstream."
To better target the hepatocytes in the mice, the team took the disabled protein coat from the baculovirus Autographa californica and put it on to a modified type of lentivirus called feline immunodeficiency virus (FIV). FIV causes leukemia in cats but no disease in humans.
The hybrid vehicle efficiently bound to receptors on the liver cells because the modified baculovirus coat serves as a "key" that fits into the "lock," or receptor. The percentage of liver cells that took up the virus increased from approximately 5 percent to 20 percent.
The team also modified the part of the FIV that expresses the therapeutic gene so that its promoter that causes gene expression worked only when it was in a liver cell.
"Even if this FIV modified virus goes to other organs of the body, it won't express well because its promoter is liver-specific," McCray said. "This modification helps prevent negative side effects. For example, if the gene were expressed in immune cells instead of liver cells, it could lead to a damaging immune response."
McCray said the team now is studying additional ways to make the hybrid vector express the protein even better.
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