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by Dr. Mark L. Batshaw, M.D.
Currently, we treat metabolic disorders using dietary restriction, and in some cases, also daily medication. Gene therapy offers the potential of a permanent correction of the enzyme deficiency, with resumption of a normal diet and life-style. The basic idea underlying gene therapy is that inserted copies of a normal gene can take over the function of a defective one.
Genes direct the production of the enzymes deficient in PKU and other inborn errors of metabolism. In its current state of development, gene therapy has assumed two forms. The first is called ex vivo gene therapy. This technique requires surgical removal of a small portion of a body organ, such as the liver, insertion of copies of a new gene into cells from the organ, and then reinfusion of the genetically modified cells into the patient. This approach has been used with limited success to treat familial hypercholesterolemia, a liver-based metabolic disorder causing early heart disease. It also has been used to treat "bubble baby syndrome," a severe immune deficiency in which the bone marrow is targeted. The disadvantage of this approach is that it is not very efficient. We know that fewer than 5% of the cells in an organ can be altered in this way.
Dr. Mark Batshaw and his collegues are studying the potential for gene therapy of Ornithine Transcarbamylase Deficience, another inborn error of metabolism
A second approach is called in vivo gene therapy. It is based on direct injection into the bloodstream of new genes encased in a virus. The virus is used as a "taxi" to direct the gene to the correct organ. A genetically modified adenovirus (a cause of the common cold) is currently used for liver-based diseases. It is being used in a human trial of gene therapy in ornithine transcarbamylase deficiency (OTCD), the focus of this article. The advantage of in vivo gene therapy is that it is very efficient, adding the new gene to virtually all cells in an organ. Its disadvantage is that the correction is short-lived because the body’s defense system can destroy the virus and the new gene it contains.
OTCD is the most common of the inherited urea cycle disorders. With an incidence of about 1 in 40,000 births, it is about one-third as common as PKU. The urea cycle is critical for the metabolism of the protein we eat. A defect in any of its five enzymes leads to accumulation of ammonia, which is severely toxic to the brain.
Like PKU, OTCD is a liver-based disease. So it is likely that any gene therapy methodology that proves useful in OTCD also may prove helpful in treating PKU. Unlike PKU, a recessively inherited disorder in which both parents are carriers and males and females are equally at risk for the disease, OTCD is a sex-linked disorder. Like hemophilia and muscular dystrophy, OTCD is inherited from the mother, and mainly affects male offspring. While most carrier girls and women have no symptoms of the disease, about 10% will have clinical symptoms of vomiting, lethargy, and coma. PKU is a "silent" disease. That is, if unrecognized and untreated, it insidiously leads to mental retardation in childhood. But OTCD commonly announces its presence in the first week of life; if unrecognized and untreated, affected males with OTCD generally die in the first week of life. Even with currently available dietary and medical treatment, only about a quarter of boys with newborn-onset disease survive. Most children with OTCD who are rescued from hyperammonemic coma sustain severe brain injury and have mental retardation and other developmental disabilities.
Because of the devastating consequences of OTCD, new treatment approaches are urgently needed. One recent heroic measure has been liver transplantation. It has been successful in correcting the metabolic defect in a number of these patients. But due to its expense and potentially life-threatening complications, liver transplantation is considered a transitional therapy, useful only until something better and safer becomes available. However, the success of liver transplantation in "curing" the disease suggests that liver-based gene therapy also may be of value.
For a number of reasons, many scientists think that OTCD is an ideal candidate to develop gene therapy, as a model for other liver-based inherited metabolic diseases. First, there are hundreds of affected children with this disease in the United States whose families are very anxious to try this new technique because of the disease’s high morbidity and mortality. Second, the OTC gene has been identified and cloned, a necessity for gene therapy. Finally, there is an animal model for OTCD, the sparse fur mouse. This mouse has permitted investigators to try gene therapy in animals before beginning human trials.
With this background, let me tell you a bit about our research team in Philadelphia. I am a developmental pediatrician and metabolic specialist, and lead the team with James Wilson, the director of the Institute for Human Gene Therapy at the University of Pennsylvania. Our team includes a research staff of over 20. It is supported by a grant from the National Institute of Child Health and Human Development as well as private foundation funding. For the past quarter century, since I was in training, I have dedicated my research efforts to developing new approaches to treating inborn errors of metabolism. For me, this project is most exciting and important as it offers the possibility of permanently correcting a disorder I have fought these many years.
Our work started four years ago with attempts to correct OTCD in the sparse fur mouse. We used many modifications of the adenovirus containing the OTC gene before we found one that, when injected into the blood stream of these mice, improved the metabolic abnormalities in the disease for about three months (see Figure 1 below). We then developed a means of reinjecting the virus without having the body marshal its immune mechanism to destroy the virus. We also found that the virus worked within 24 hours of its injection, so that it could potentially be helpful in treating these critically ill children. Finally, we tested it for toxicity, in the mouse and in monkeys. We found it safe in doses we proposed to use in human subjects.
Figure 1. In this study, a group of OTCD sparse fur (spf) mice were injected with a genetically engineered adenovirus containing the OTC gene. We followed metabolic abnormalities for three months. This graph shows changes in blood glutamine levels, which are elevated, in the untreated spf mice (solid line) versus normal (dotted lines). These levels fell to normal after gene therapy and remained normal for almost three months (line with dots).
Our next step was to present our data to the Food and Drug Administration (FDA) to obtain permission for a Phase 1 safety study. There were a number of unique features that we proposed. This was to be one of the first human gene therapy trials in which the gene was given directly into the bloodstream, rather than being injected into a tumor or into cells from a body organ. It was also the first time healthy subjects were to be enrolled. We had chosen mainly to study carrier women who, unaffected themselves, were the mothers of affected children. Because OTCD is a sex-linked disorder, these women have metabolic abnormalities we can follow to determine if they are corrected following gene therapy. As the goal of this first study was to find an effective and safe dose rather than to treat the disease, we thought it more ethical to enroll adults who could give informed consent rather than enrolling their children, who would gain little at this stage of research.
The Phase I study involves enrolling eighteen adults (six groups of three) who receive gradually increasing doses of the adenovirus containing the OTC gene. At the writing of this article, we are one-third of the way through the trial. The six hardy souls who are participating have suffered through being prodded and probed, punctured and examined, with good humor and fortitude. Fortunately, there have been no significant complications. Our hope is to complete this study in the next 12-18 months and to be able to then start a Phase II treatment trial in children. It is likely that this trial will employ a more advanced virus "taxi" than the one we are using currently.
At this point we feel a bit like the Wright Brothers. We have gotten off the ground, but just a few feet, and for a short distance. If gene therapy is to prove successful in OTCD and other liver-based inborn errors of metabolism such as PKU, we are still a number of years away from knowing—but we are working very hard to find an answer.
After writing this article, Dr. Batshaw joined the Children’s National Medical Center at George Washington University School of Medicine in Washington, DC. (July 1998). Previously, he was Prof. of Pediatrics and Neurology at the U. of Pennsylvania School of Medicine; Physician-in-Chief of Children’s Seashore House; and Chief of the Division of Child Development and Rehabilitation Medicine at The Children’s Hospital, Philadelphia. He is an international authority on inborn errors of metabolism.
Editor’s Note: Gene therapy research specific to PKU also continues at Mt. Sinai School of Medicine’s Institute for Genetic Research in New York, under the direction of Dr. Savio Woo; and at the University of Wisconsin, Madison under the direction of Dr. Cary Harding. We will bring you news of progress in these laboratories as it becomes available.