ASGT Press Release 

Today's findings from the ASGT Annual Meeting: New research on gene therapy of inherited disorders New genetic repair model

BOSTON (Friday, June 7, 2002) -- Current gene transfer methods rely on the efficient transfer of cDNAs to effect phenotypic change. A study presented at the American Society of Gene Therapy Annual Meeting demonstrated a new approach to gene therapy. The researchers developed in vivo data supporting a novel method for genetic repair based on the correction of mutant pre-mRNA.

Christopher Walsh, MD, and Hengjun Chao, MD, at the University of North Carolina at Chapel Hill and Gary Mansfield, MD, and collaborators at Intronn Inc, Rockville, MD, made use of a natural mechanism that splices out introns and joins together the protein-coding exons. The normal process is called "cis-splicing." The researchers developed a new technology, called "Spliceosome-mediated RNA trans-splicing (SMaRT)," to correct the hemophilia phenotype in Factor VIII knock out mice. A gene therapy vector is made by inserting their "trans-splicing cassette" into a viral vector.

The researchers found that when they injected their vector into the hemophiliac mice, a significant increase in functional Factor VIII was found in the bloodstream of the mice. This factor allowed the mice to survive the bleeding trauma, which is lethal to untreated hemophilia mice. They demonstrated that the mutant FVIII RNA was repaired and that "RNA repair" occurred via trans-splicing.This novel RNA repair method can provide a new approach in gene therapy for many genetic diseases, which are difficult to deal with by current gene transfer methods.

New SCIDs treatment
BOSTON (Friday, June 7, 2002) -- The first gene therapy trial in 1990 was for ADA-deficient Severe Combined Immunodeficiency (SCID). The technology did not exist at that time to efficiently transfer the ADA gene into the blood stem cells. Researchers at the American Society of Gene Therapy Annual Meeting have developed new efficient gene therapy vectors specifically designed to allow efficient transfer of a gene (in this case the ADA gene) into blood stem cells. Fabio Candotti, MD, from the National Institutes of Health, MD, and colleagues from Children's Hospital, Los Angeles, CA, and Duke University Medical Center, Durham, NC, began a clinical trial in August 2001 to test these new improved retroviral vectors in patients with ADA-deficient SCID. Researchers found the initial results of the survey to be very encouraging in the first few months after treatment. There was increased ADA enzyme activity in peripheral blood cells and an improved T cell (immune cell) count. The long-term effects of the procedure remain to be established.A second study reports a long-term analysis of the gene transfer results from a newborn treated for ADA-deficient SCID nine years ago. The patient's umbilical cord blood had been used as the source of stem cells, which were treated with a retrovirus carrying a normal human ADA gene. It has not been possible to demonstrate that the gene transfer was beneficial to the patient, although there has been a relatively high number of gene-containing T cells in the blood since the gene transfer was performed.Denise Carbonaro, from the University of Southern California School of Medicine, Los Angeles, CA and colleagues used a new technique to track the T cells, which grew from each gene-modified stem cell. The results showed that a single stem cell that had taken up the normal ADA gene accounted for the vast majority of all of the gene-containing cells. The fact that only a single stem cell had been successfully gene-corrected and engrafted likely reflected the sub-optimal gene transfer techniques used at the time; current methods for using retroviral vectors to insert genes into human stem cells may be more effective, as used in the XSCID study of Fischer and co-workers, or the study for ADA-deficient SCID of Aiuti et al being presented at this meeting.

Treating X-linked SCID
BOSTON (Friday, June 7, 2002) -- Gene therapy has been most successful to date for infants with the X-linked form of severe combined immune deficiency (XSCID), one of the forms of the genetic disease in which children are born lacking protective immune systems. A study presented at the American Society of Gene Therapy Annual Meeting set out to improve the results of gene therapy for ADA-deficient SCID. Alessandro Aiuti, Milan, Italy, and colleagues from Italy and Jerusalem did two new maneuvers in their trial, which used a retroviral vector to insert the normal human ADA gene into stem cells from the bone marrow of patients.A moderate amount of chemotherapeutic drug (busulfan) was given to the patients before their gene-modified bone marrow cells were given back to them. The chemotherapy was intended to eliminate some of the patient's bone marrow cells and "make space" to allow the gene-modified cells to engraft. Additionally, the patients did not have access to the other "standard" therapy for ADA-deficient SCID, consisting of enzyme replacement by injections of ADA enzyme. The ADA enzyme replacement rescues the patient's own lymphocytes which are missing ADA, which eliminates the selective survival advantage that gene-corrected lymphocytes are expected to have. All previous studies of gene therapy for ADA-deficient SCID have given the patients ADA enzyme therapy as well, to avoid withholding a potentially beneficial therapy. The ADA enzyme therapy may have blunted the selective advantage of the gene-corrected lymphocytes, which was not a problem for the successful XSCID study where a similar enzyme therapy does not exist.Researchers found the results of this study promising. The two subjects described appear to be having a significant improvement in their immune systems, which is the goal of such gene therapy. They did not have any detectable problems from the chemotherapy which seems to have led to a much higher level of engraftment of gene-corrected stem cells, as intended. Therefore, based on these initial results, it appears that another form of SCID can be treated successfully by gene therapy.

Controlled gene discussion
BOSTON (Friday, June 7, 2002) -- Vectors systems have been generally based on constitutive promoters allowing stable but unregulated gene expression. In some clinical applications, hormones of therapeutic interest like growth hormone or Epo require a tight adjustment of dose delivery to prevent adverse effects.The focus of this study, presented at the American Society of Gene Therapy Annual Meeting, was evaluating a system to allow the level of expression of a transferred gene to be controlled. David Favre, DVM, PhD, of Laboratoire de Therapie Genique, Nantes, France, and colleagues used a system in which the antibiotic tetracycline can turn-on expression of a transferred gene. The erythropoietin gene under the control of the tetracycline-inducible system was inserted into monkeys carried by an adeno-associated virus (AAV) vector inserted into the monkey's muscle (similar to an approach being studied in human patients with hemophilia).The level of erythropoietin made from the transferred gene did respond when the monkeys were given tetracycline. In most animals, the system became silenced and non-responsive to tetracycline, and it appears that the reason for the loss of expression was that the monkeys developed immune responses against the proteins of the tetracycline -response system, which are derived from bacterial genes and thus are foreign to the monkey. One animal did not develop the immune response against the bacterial tetracycline control system. In this animal expression of epo remained responsive to tetracycline for over a year. Researchers concluded that this system of controlled gene expression works in a large animal model, but there is a problem in that immune responses can block the system. Newer gene expression control systems have been made using human proteins, so that this type of immune response may not occur.

The American Society of Gene Therapy is the largest medical professional organization representing researchers and scientists dedicated to discovering new gene therapies. ASGT was established in 1996, and has grown to more than 3,000 members. It is committed to promoting and fostering the general field of research involving gene therapy and to promoting professional and public education in all areas of gene therapy.

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