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A Giant Step for Gene Therapy

St. Jude researchers offer the first proof that gene therapy can reduce the symptoms of hemophilia B.

A Giant Step for Gene Therapy

Chris Potwora doesn’t remember life without hemophilia.

He was just a little tyke when doctors realized that his blood failed to clot properly. For Potwora, a skinned knee could be dangerous. A tumble from a bicycle could be deadly. Football or other contact sports? Out of the question. As a result, the boy endured frequent injections of factor IX, a protein that helped his blood clot.

“I used to go camping with my dad, and we’d take factor IX along in the cooler,” Potwora recalls. “It’s something I’ve always dealt with, so I didn’t know any different.”

All that changed when Potwora spent a week at St. Jude Children’s Research Hospital earlier this year. On Valentine’s Day of 2012, he became one of the first people in the world to receive a novel kind of gene therapy.

After that, Potwora discontinued his regular injections of factor IX.

A factor of nine

Hemophilia is an inherited disorder that primarily affects males. Because they lack an essential protein needed to help the blood clot, individuals with the disease may bleed profusely after minor injuries or may spontaneously bleed into their joints. About one in 30,000 boys inherits hemophilia B, which is caused by a mutated F9 gene. These individuals lack sufficient amounts of clotting factor IX. People with severe hemophilia require regular transfusions of the protein to prevent bleeding episodes. Besides being painful and inconvenient, the frequent injections are expensive, costing as much as $400,000 a year.

Enter gene therapy.

For years, researchers believed that hemophilia B could be cured by adding a normal F9 gene to replace the function of the faulty one. The healthy gene would then command cells to produce the missing factor IX protein and — voilà! —the patient’s blood would clot normally.

St. Jude hematologist Arthur Nienhuis, MD, and his colleagues have spent decades seeking the best method for delivering healthy genes into the body. The therapeutic gene could not be inserted directly into a cell; instead, it must hitch a ride with a carrier that could transport its cargo to the desired location. The logical vehicle would be a virus, which has a natural tendency to infect cells and replicate within them. If a virus could be engineered so that it could infect cells and introduce its genetic material but not replicate, then the normal F9 gene could be introduced into a target cell. This genetically engineered virus is called a vector.

One obstacle to using a viral vector is that a patient’s immune system might attack and destroy the foreign invader. Scientists homed in on what is called an adeno-associated virus (AAV), a benign virus that does not cause disease in humans. When injected into the bloodstream, the AAV tends to travel directly to the liver, the site where factor IX is normally produced.

Designing a unique vector

In the 1990s, St. Jude postdoctoral fellow Amit Nathwani, MD, PhD, and surgeon Andrew Davidoff, MD, began collaborating with Nienhuis to develop a gene therapy approach for hemophilia. The relationship continued when Nathwani later moved to University College London (UCL).

Davidoff, now St. Jude Surgery chair, says the success of the subsequent gene therapy trial is due in part to a vector design that is unlike any other. The research team, which included John Gray, PhD, of St. Jude Experimental Hematology, chose a virus called AAV8 to deliver the genetic material into the liver.

“Our vector design is unique in certain aspects,” Davidoff explains. “All clinical trials before ours that used adeno-associated virus used a type called AAV2. Almost all humans have been exposed to that virus and have some degree of immunity against it. We chose AAV8, because only 5 to 10 percent of humans have been exposed to it. To qualify for our trial, patients could not have had prior exposure to AAV8.” Because the participants had never encountered AAV8, their immune systems would be less likely to recognize and attack it.

The vector used in the study was produced at the Children’s GMP, LLC, which is located on the St. Jude campus. St. Jude was the first pediatric research center to have an on-site Good Manufacturing Practices facility. The Children’s GMP produces biopharmaceutical products under government-approved manufacturing guidelines.

Because the gene therapy treatment must be perfected on adults before it could be used on children, the team approached UCL hemophilia expert Edward G.D. Tuddenham, MD, PhD, to recruit adult patients. Six men enrolled in the first phase of the study, which occurred in London. Each participant received a one-time gene therapy treatment that was delivered via a simple intravenous infusion.

Of the initial group of participants, two received low doses of the vector, two received intermediate doses and two received higher doses. Factor IX levels rose in all of the men, with the ones receiving the most vector enjoying the highest increases. One high-dose recipient experienced a mild immune response in the liver, which was successfully quelled with short-term steroid treatment.

Read all about it

A New England Journal of Medicine report on the study generated international headlines. The clinical trial had offered the first proof that gene therapy could reduce the symptoms of hemophilia B. Four patients discontinued their regular protein injections altogether; the two who received the lowest doses required injections much less frequently than before the study.

The participants’ lifestyles changed dramatically. For the first time in their lives, they could run marathons or play soccer without bleeding into their joints.

“Clearly it was a life-transforming experience, because they went from acquiring factor IX infusions two or three times a week and in constant risk of bleeding to essentially being clinically normal,” Nienhuis observes.

Although any level above 1 percent is therapeutically significant, the eventual goal of the hemophilia B trial is to achieve and sustain factor levels exceeding 15 percent. The study’s participants continue to maintain levels ranging from 2 to 6 percent. Most importantly, those results have persisted for more than two-and-a-half years.

“This is clearly the first study that has documented long-term expression of a therapeutic transgene,” Davidoff says.

Looking ahead

The gene therapy study continues to accrue adult patients, all of whom receive high doses of the vector. Potwora was the first patient to receive his treatment at St. Jude. Ulrike Reiss, MD, of St. Jude Hematology notes that Potwora experienced a minor liver inflammation, which was treated with steroids. Potwora now takes factor IX only after incurring an injury, such as when he recently broke his toe.

The trial’s next phase will use a vector in which researchers have removed the viral particles that lack DNA. Davidoff believes this improvement will further reduce participants’ immune reactions and will enable clinicians to deliver more genetic material into the liver. The modified vector is currently in production in the Children’s GMP, LLC. Eventually the St. Jude team hopes to extend gene therapy to children with hemophilia B, as well as to individuals with Gaucher disease, hemophilia A and galactosialidosis.

For Nienhuis, a pioneer in the field of gene therapy, these trials are the culmination of his life’s work.

“It’s a grand experience, to see it come to fruition,” admits Nienhuis, former president of the American Society of Gene Therapy. “If you had asked me 30 years ago whether we could ever treat a disease by simply injecting a virus into the bloodstream, I would have said, ‘It’s not likely.’ But clearly the biology was very different from what we expected. It definitely works.”

Potwora is adjusting to life without regular injections of factor IX.

“It was weird getting used to it at first,” admits the recent college graduate and future medical student. “I was interested in the clinical study because I didn’t want to take the factor anymore, but as a pre-med student I also had an academic interest in gene therapy. I knew they need these kinds of studies in order for the field of gene therapy to move forward. I think it’s really interesting from a scientific perspective and really helpful from a clinical one. When I’m a doctor, I’d really like to see this be an option for treatment, because there are a lot of conditions it could help.”

Reprinted from Promise Autumn 2012

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