Tools and Targets of Gene Therapy

Researchers use several methods to introduce therapeutic genes into cells. Healing DNA is given linked to viruses that have had their disease-causing genes removed; in fatty bubbles called liposomes or complexed with other lipid molecules; "shot" along with metal particles into cells; and as "naked" preparations of DNA alone. The challenge in any nonheritable gene therapy is to target sufficient numbers of affected cells for a long enough time to exert a noticeable effect. A look at some specific gene therapy approaches provides a nice ending to our survey of human genetics. Figure 24.16 summarizes gene therapies.

Bone Marrow

Because bone marrow tissue includes the precursors of all mature blood cell types, it provides a route to treat blood disorders and immune deficiencies. Certain stem cells in bone marrow can also travel to other sites, such as muscle, liver, and brain, and differentiate there into, respectively, muscle, liver, or neural cells. Many new gene therapy targets might be reached via bone marrow.

Skin

Skin grafts can be genetically bolstered to secrete missing proteins, such as clotting factors, growth factors, or enzymes.

Endothelium

Endothelium, a tissue which forms capillaries and lines the interiors of other blood vessels, can be altered to secrete a substance directly into the bloodstream. Engineered endothelium might secrete insulin to treat diabetes mellitus or a clotting factor to treat hemophilia.

Liver

The liver is a very important focus of gene therapy because it controls many bodily functions and because it can regenerate. A gene therapy that corrects just 5% of the 10 trillion cells of the liver could produce an effect. For example, a liver gene therapy targets heart disease. Normal liver cells have low-density lipoprotein (LDL) receptors on their surfaces, which bind cholesterol in the bloodstream and bring it into the cell. When liver cells lack LDL receptors, cholesterol accumulates on artery interiors. Liver cells genetically altered to have more LDL receptors can relieve the cholesterol buildup. Such gene therapy could be lifesaving for children who have inherited familial hypercholesterolemia (see fig. 24.7).

Lungs-

cystic fibrosis hereditary emphysema

Liver

Aerosol

Implantation familial hypercholesterolemia

Cell implants

Viruses

Liposomes

Muscle-

muscular dystrophies

Figure 24.16

Sites of gene therapy and the methods used to introduce normal DNA.

Muscle Implants For Muscular Dystrophy

Brain

Alzheimer disease Huntington disease neurotransmitter imbalances glioma

Skin melanoma

Blood sickle cell disease

Endothelium (blood vessel lining)

hemophilias diabetes mellitus pituitary dwarfism

Bone marrow

Gaucher disease Hurler disease severe combined immune deficiency

Lungs

The respiratory tract is a prime candidate for gene therapy because an aerosol can directly reach its lining cells, making it unnecessary to remove cells, alter them, and reimplant them. Once inhaled, lung-lining cells take up the gene and produce the protein missing or mutant in the inherited illness. For example, gene therapy can provide an enzyme whose absence causes a hereditary form of emphysema.

Nerve Tissue

Gene therapy on neurons presents a problem, because these cells do not normally divide. Altering other cell types can circumvent this obstacle, such as glia or fibro-

blasts that secrete nerve growth factor. Another route to nerve cell gene therapy is to send in a valuable gene attached to the herpes simplex virus, which remains in nerve cells after infection. Such a herpes gene carrier could alter a neuron's ability to secrete neurotransmitters.

Gene Therapy Against Cancer

Viruses may provide a treatment for a type of brain tumor called a glioma, which affects glia. Cancerous glia divide very rapidly, usually causing death within a year even with aggressive treatment. A gene therapy approach infects fibroblasts with a virus bearing a gene from a herpes virus that makes the cells sensitive to a drug called ganciclovir. The altered fibroblasts are implanted near the tumor. There, the doctored virus infects nearby

Gene Therapy Successes and Setbacks

Any new medical treatment or technology begins with creative minds and then brave volunteers. The first people to take new vaccines or to undergo new treatments know that they may give their lives in the process. Gene therapy, however, is unlike conventional drug therapy. It alters the genotype in a part of the body that has failed, and because the potentially therapeutic gene is usually delivered with other DNA, the body's reactions are unpredictable. Following are the stories of a few of the young people who have pioneered gene therapy.

phase in the research was to "fix" umbilical-cord stem cells in newborns diagnosed prenatally with the condition. These cells remain in the circulation longer. Three infants were treated, and as they've grown, gradually T cells capable of producing ADA have taken over. Their infections are easily controlled. Treating ADA deficiency was an early success for gene therapy.

adenosine deaminase deficiency-Early

Success

Ashanti DaSilva was the first child to receive gene therapy. Shortly after noon on September 14, 1990, the four-year-old sat in a bed at the National Institutes of Health hospital in Bethesda, watching her own T cells, given copies of a missing gene, drip into her arm. Lack of the liver enzyme adenosine deaminase (ADA) caused an intermediate compound to accumulate that destroyed her T cells, toppling both her cellular and humoral immunity. Enzyme supplements had recently helped Ashanti avoid life-threatening infections, but gene therapy might offer a longer-lasting treatment. Eight-year-old Cynthia Cutshall joined the experiment a few months later. Each girl continued to receive the enzyme to prevent infection, but success was seen at the cellular level — gradually, more T cells contained the healing gene.

Ashanti and Cynthia's gene therapy had to be repeated often, because T cells die quickly. The next

Ornithine Transcarbamylase deficiency-a setback

Jesse Gelsinger underwent gene therapy almost nine years to the day that Ashanti DaSilva received her treatment. But his story stands in sharp contrast to the success stories of the children with ADA deficiency and sparked a reevaluation of the technology in general. In September 1999, the eighteen-year-old died just days after receiving gene therapy, of an overwhelming and unanticipated immune system reaction.

Jesse had an inborn error of metabolism called ornithine transcarbamylase deficiency (OTC). In this X-linked recessive disorder, one of five enzymes required to break down amino acids lib erated from dietary proteins is absent. The nitrogen from the amino acids combines with hydrogen to form ammonia (NH3), which rapidly accumulates in the bloodstream and travels to the brain. The condition usually causes irreversible coma within seventy-two hours of birth. Half of affected babies die within a month, and another quarter, by age five. The survivors can control their symptoms by eating a special low-protein diet and taking drugs that bind ammonia.

Jesse wasn't diagnosed until he was two, because some of his cells could produce the enzyme, so his symptoms were milder. Still, when he went into a coma in December 1998 after a few days of not taking his medications, he and his father

Ada Deficiency Symptoms

Ashanti DaSilva rides her bike three years after she began receiving periodic gene therapy for ADA deficiency, an inherited lack of immunity. She is the first person to receive gene therapy— and it worked, although to a limited extent.

began to consider volunteering for a gene therapy trial they had read about. The next summer, four days after Jesse turned eighteen, he underwent testing at the University of Pennsylvania gene therapy center and was admitted to the trial. He knew he would not directly benefit soon, but he had wanted to try to help affected newborns.

The gene therapy consisted of an adenovirus, which causes the common cold, that carried a functional human OTC gene but had the genes that cause disease removed. The virus had already been used, usually safely, in about a quarter of the 330 gene therapy experiments done on more than 4,000 patients since 1990. Three groups of six patients were to receive three different doses. The trial would identify the lowest dose that would fight the disease, but not cause dangerous side effects.

Jesse entered the hospital on Monday, September 13, after the seventeen others in the trial had already been treated and suffered nothing worse than a fever and aches and pains. Several billion engineered viruses were sent into an artery leading into his liver. That night, Jesse developed a high fever. By morning, the whites of his eyes were yellow, indicating a high bilirubin level as his liver struggled to dismantle the hemoglobin bursting from shattered red blood cells. The ammonia level in his liver soared, as his blood clotting faltered. Jesse became disoriented, then comatose. By Wednesday, his lungs began to fail, and Jesse was placed on a ventilator. Thursday, other

vital organs began to shut down, and by Friday, he was brain dead. His father turned off the life support.

It isn't entirely clear why the gene therapy killed Jesse Gelsinger. Perhaps underlying medical conditions had not been detected, such as a past infection with parvovirus, that may have led his immune system to attack the adenovirus. In the liver, the adenovirus had gone not to the targeted hepatocytes, but to a different cell type, the macrophages that trigger an immune response. The autopsy also revealed that the engineered virus had spread beyond the liver to the spleen, lymph nodes, bone marrow, and elsewhere. In addition, Jesse's bone marrow lacked erythroid progenitor cells, indicating an underlying and undetected problem in hematopoiesis. Finally, examination of the adenovirus in Jesse's bloodstream revealed genetic changes — the gene therapy vector may have mutated.

Development of DNA microar-ray technology, which can analyze many genes at once, will make gene therapy safer by enabling researchers to select patients based on more complete genetic information, including underlying conditions and how their immune systems would react to certain viruses. Many researchers compare gene therapy to organ transplantation, which also began slowly and with notable failures until the advent of immunosup-pressant drugs and better ways to match donor and recipient transformed the technology into a standard medical practice. So too will gene therapy probably become, someday, a common part of health care. ■

Jesse Gelsinger died at age eighteen of gene therapy to treat an inborn error of metabolism, ornithine transcarbamylase deficiency.

cancer cells, but not the healthy neurons, because they do not divide. When the patient takes ganciclovir, only the cells harboring the virus die—not healthy brain cells.

Another genetic approach to battling cancer is to enable tumor cells to produce immune system biochemi-cals, or to mark them so that the immune system more easily recognizes them. This approach is called a cancer vaccine. A treatment for the skin cancer melanoma, for example, alters tumor cells to display an antigen called HLA-B7, which stimulates the immune system to attack the cell.

99 What are the two basic types of gene therapy? ^9 How does gene therapy work?

^9 What are some of the ways that nonheritable gene therapy is being conducted?

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