From Washington University School of Medicine
Cancers appear in mice treated with adeno-associated virus, a gene therapy vector
St. Louis, ó Studying one type of gene therapy in mice, researchers made an unexpected and unsettling discovery: Six animals eventually developed cancer. The results of the NIH-funded study are described in two scientific reports published in the current issue of the journal Gene Therapy.
The research team, headed by Mark S. Sands, Ph.D., an assistant professor of medicine and of genetics at Washington University School of Medicine in St. Louis, used adeno-associated virus (AAV) to insert a human gene into 59 newborn mice. The gene coded for the enzyme beta-glucuronidase, which the mice were unable to produce because their own beta-glucuronidase genes were mutated.
Their condition mimicked Sly's syndrome, one of more than 40 lysosomal storage diseases that in aggregate affect approximately 1 in 5,000 babies. Lysosomes are cellular compartments that dismantle complex molecules into constituent pieces. When one of their enzymes is missing, the large molecules pile up, damaging cells in many tissues.
In these mice, just one injection of the gene-carrying virus raised production of beta-glucuronidase enzyme to functional levels for at least a year. The treated mice gained near-normal amounts of weight, their bones grew to almost normal lengths, and they didn't develop retinal problems.
Therefore, gene therapy just after birth prevented many of the symptoms associated with lysosomal storage disease. When the surviving mice were checked at 18 months of age, three of the five had signs of liver cancer. Examination of additional mice that were either sacrificed or spontaneously died between 8 and 18 months identified three more animals with tumors. None of eight surviving untreated mice had cancer.
Moreover, the researchers have never seen these tumors in the many mice they have treated in other ways for beta-glucuronidase deficiency. The Washington University study was not designed to determine whether AAV might be linked to cancer. It set out to test the long-term efficacy of gene therapy for mice lacking beta-glucuronidase.
Possible explanations for the findings include:
- Gene therapy with the particular recombinant AAV used by Sands' group may cause cancer in mice;
- Gene therapy with AAV may cause cancer in mice if performed during the neonatal period;
- Gene therapy with AAV may cause cancer in mice that lack beta-glucuronidase because these mice are immunocompromised and have other organ problems;
- Gene therapy with AAV may cause cancer in mice if the vector is injected intravenously;
- Overexpression of the human beta-glucuronidase gene in mice may cause cancer, regardless of the vector;
- Gene therapy with AAV may cause cancer in mice;
- The disease MPS VII may predispose these animals to malignancies.
Further studies will be needed to distinguish among these possibilities, the researchers stress. Sandsí group currently is repeating the experiment to determine if the results are reproduceable.
The agent used for gene transfer in these experiments, adeno-associated virus, has been used in human gene-therapy trials. However, those human trials employed a localized method of delivery, rather than the systemic, intravenous approach used in this special mouse model. At the time the tumors were discovered, Washington University investigators informed the U.S. Food and Drug Administration and the National Institutes of Health of their findings. The research was discussed in a public meeting sponsored by NIHís Recombinant DNA Activity Committee, held March 7 in Rockville, Maryland.
An editorial regarding this research can be found in the August issue of the journal Molecular Therapy. The editorial was written by the journalís editor-in-chief, Inder M. Verma, Ph.D., professor of genetics at the Salk Institute.
Editor's note: Humans who lack beta-glucuronidase have Sly's syndrome (also called beta-glucuronidase deficiency or mucopolysaccharidosis type VII). They are unable to break down certain large carbohydrates (dermatan sulfate, heparan sulfate and chondroitin sufates) and can have short stature, mental retardation, abnormal facial features and numerous other problems. No patient with Sly's syndrome or other lysosomal storage disease has undergone gene therapy with adeno-associated virus. The clinical trials involving AAV have focused on other diseases, such as cystic fibrosis and hemophilia.