Investigation Bolsters Use of Variant Protein in Hemophilia Gene Therapy

Using gene therapy to produce a mutant human protein with unusually high blood-clotting power, scientists have successfully treated animals 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’s leader, Valder R. Arruda, MD, PhD, a hematology researcher at The Children’s Hospital of Philadelphia (CHOP) and an associate professor of Pediatrics at the Perelman School of Medicine at the University of Pennsylvania. Dr. Arruda and colleagues published their results in the recent print edition of Blood.

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, research teams 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.

Dr. Arruda and colleagues therefore investigated gene therapy that used lower dosages of vector produce a more potent clotting factor — a variant protein called FIX-Padua.

In 2009, Dr. Arruda was part of a team that discovered this variant protein in a young Italian man who had thrombosis, excessive clotting that can dangerously obstruct blood vessels. A mutation produced the mutant clotting factor (named FIX-Padua after the patient’s city of residence), 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. Therefore in the current study, the researchers needed to strike a balance: to relieve severe hemophilia 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 Dr. Arruda, “by adapting this variant protein found in one patient to benefit other patients with the opposite disease.”

The recent Blood investigation tested the safety of FIX-Padua in animals, all with naturally occurring types of hemophilia B very similar to that found in people. The researchers found the gene therapy injections changed their hemophilia from severe to mild, with no bleeding episodes for up to two years. The animals did not develop inhibitory antibodies, nor was there evidence of thrombosis.

Another set of preclinical safety studies supported the safety and efficacy of gene therapy using FIX-Padua. However, Dr. Arruda noted additional work is needed to confirm these encouraging early results.

In the meantime, at least one clinical trial is making use of FIX-Padua in adult patients with hemophilia B — at the University of North Carolina at Chapel Hill, under Paul Monahan, MD. Leaders of a separate trial being planned at Spark Therapeutics in Philadelphia, under Katherine A. High, MD, are contemplating using FIX-Padua as well.