Study bolsters ‘turbocharged’ protein as a promising tool in hemophilia gene therapy

Using mutant protein, CHOP hematologist safely removes unwanted antibodies, reverses hemophilia in dog model of bleeding disease

Using gene therapy to produce a mutant human protein with unusually high blood-clotting power, scientists have successfully treated dogs with the bleeding disorder hemophilia, without triggering an unwanted immune response. In addition, the "turbocharged" clotting factor protein eliminated pre-existing antibodies that often weaken conventional treatments for people with hemophilia.

"Our findings may provide a new approach to gene therapy for hemophilia and perhaps other genetic diseases that have similar complications from inhibiting antibodies," said the study leader, Valder R. Arruda, M.D., Ph.D., a hematology researcher at The Children's Hospital of Philadelphia (CHOP).

Arruda and colleagues published their animal study results in the print edition of Blood on March 5.

Hemophilia is an inherited bleeding disorder that famously affected European royal families descended from Queen Victoria. Most commonly occurring in two types, hemophilia A and hemophilia B, the disease impairs the blood's ability to clot, sometimes fatally. When not fatal, severe hemophilia causes painful, often disabling internal bleeding and joint damage.

Doctors treat hemophilia with frequent intravenous infusions of blood clotting proteins called clotting factors, but these treatments are expensive and time-consuming. Moreover, some patients develop inhibiting antibodies that negate the effectiveness of the infusions.

For more than two decades, many research teams, including at CHOP, have investigated gene therapy strategies that deliver DNA sequences carrying genetic code to produce clotting factor in patients. However, this approach has been frustrated by the body's immune response against vectors--the non-disease-causing viruses used to carry the DNA. Those responses, which defeated initial benefits seen in experimental human gene therapy, were dose-dependent: higher amounts of vectors caused more powerful immune responses.

Arruda and colleagues therefore investigated gene therapy that used lower dosages of vector (adeno-associated viral-8 vector, or AAV-8 vector) to produce a more potent clotting factor--a variant protein called FIX-Padua.

Arruda was part of a scientific team in 2009 that discovered FIX-Padua in a young Italian man who had thrombosis, excessive clotting that can dangerously obstruct blood vessels. A mutation produced the mutant clotting factor, called FIX-Padua, named after the patient's city of residence. This was the first mutation in the factor IX gene found to cause thrombosis. All previously discovered FIX mutations lead to hemophilia, the opposite of thrombosis.

FIX-Padua is hyperfunctional--it clots blood 8 to 12 times more strongly than normal, wild-type factor IX. In the current study, the researchers thus needed to strike a balance--to relieve severe hemophilia in dogs, by using a dose strong enough to allow clotting, but not enough to cause thrombosis or stimulate immune reactions. "Our ultimate goal is to translate this approach to humans," said Arruda, "by adapting this variant protein found in one patient to benefit other patients with the opposite disease."

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Study bolsters 'turbocharged' protein as a promising tool in hemophilia gene therapy