The Second Coming of Gene Therapy

The Second Coming of Gene Therapy
For years, gene therapy produced tons of hype but no results. Recently, though, new approaches have yielded its first successes: breakthrough treatments for blindness, cancer, and the deadly bubble boy disease.
by Jill Neimark

“For the first two years of her life, my daughter, Katlyn, was knocking on heaven’s door every day,” says Daisy Demerchant, a 26-year-old mom living in Centreville, New Brunswick, just north of Maine. “Two months after she was born she started getting sick, and she never got better.” At six months Katlyn was diagnosed with “bubble boy” disease, formally known as severe combined immunodeficiency (SCID), which robs the immune system of the ability to fight infection. There are many causes of this disorder; in Katlyn’s case it was lack of the enzyme adeno­sine deaminase, or ADA, which rids the body of a natural toxin called deoxyadenosine. When the toxin builds up, it destroys T and B lymphocytes, the body’s infection-fighting immune cells. As a result, Katlyn’s immune cells were dying.

Treatment options ranged from risky to grim. One was a bone marrow transplant, in which imported donor cells could manufacture healthy T cells to fight invading germs. But bone marrow transplants can have lethal complications and often require drugs that further inhibit the patient’s immune system, leaving a window of vulnerability until the transplant kicks in. Another potential treatment involved injections of the ADA enzyme itself. But there was a risk Katlyn would develop antibodies to the drug, rendering it useless. Without any treatment at all, she would simply die.

While weighing their options, doctors put the little girl on protective antimicrobials and sent her to a hospital eight hours from her home. She became another fragile bubble baby sequestered from the world. “My husband quit his job building fire trucks, and we lived with Katlyn in the hospital for 15 months,” Demerchant says. The parents had to wear sterile gowns, booties, masks, and gloves, and the urge to touch their child—let alone hug and kiss her—had to be put on hold.

Just when it seemed as if Katlyn’s life might never improve, science and fate intervened. Her specific condition, called ADA-SCID, had long tantalized researchers seeking to repair genetic defects with a technique called gene therapy. Rare, deadly, and caused by a single gene mutation, it was a perfect proof-of-principle condition for anyone seeking to replace damaged DNA with genes that did the job. With all her troubles, little Katlyn Demerchant had been almost made to order for Fabio Candotti, a senior investigator at the National Human Genome Research Institute at the National Institutes of Health in Bethesda, Maryland.

Before Katlyn showed up at NIH, the doctors there were already well prepared: They had inserted healthy human ADA genes into a modified mouse retrovirus—a type of virus that can enter human cells and transfer new genetic material right into the DNA strands in their nuclei.

Once Katlyn arrived in May 2007, Candotti and his team removed stem cells from her bone marrow and exposed them to the engineered retrovirus, creating a human-virus hybrid. Then they injected the hybrid cells back into Katlyn. Like heat-seeking missiles, the retooled stem cells automatically found their way back home to the marrow. There, they began to specialize, creating all of the secondary or “daughter” cells that such stem cells normally produce—including healthy T cells with functioning ADA genes.

Everybody waited while Katlyn, still stuck inside the bubble, learned to walk on the floor of her sterile isolation room and to play through the protective window with a visiting dog named Toffee. On September 3, blood tests showed Katlyn’s immune system was being populated with robust, functioning T cells. She was so restored, in fact, that her parents were able to take her outside for the first time since she was an infant. “The first day we took her out she was really quiet and a little terrified,” Daisy Demerchant says. “The second time she started running around and asking us a million questions. She’d point to the sun, clouds, leaves, cars, everything imaginable, and ask us what it was. Ever since that day, she has never wanted to stay inside.”

Six months after her gene therapy transplant, Katlyn was so healthy that doctors let her return home to Canada. It can take a year or longer for the immune system to reconstitute itself in full, so Katlyn still takes antimicrobials as a precaution, but today she plays outside, even in the dirt, and is resistant enough to fly on a commercial plane.

The new DNA treatments for Katlyn Demerchant and other bubble babies are nothing short of remarkable, the culmination of a major push to perfect gene therapy for the disease, Candotti says. Across the ocean, in Italy, bubble babies with ADA-SCID are also being cured: A trial led by Alessandro Aiuti, a molecular biologist at San Raffaele Telethon Institute for Gene Therapy in Milan, restored the immune system in eight of ten children, while a ninth had significant improvement.

And bubble babies are far from alone. In Europe and the United States, gene therapists have restored vision in individuals suffering from a rare genetic disorder that inevitably leads to blindness. In Texas, a team has manipulated genes in order to put deadly cancers into complete remission. Building on these successes, gene therapy may soon be used to correct hereditary genetic diseases like cystic fibrosis, hemophilia, and Tay-Sachs and to activate the immune response against a wide variety of infectious diseases and cancers. Gene therapy and its adjuncts may help us trick the body into growing new tissue to rejuvenate arthritic joints, fix injured hearts, and speed the healing of wounds….Continued

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