LONDON - It may one day be possible to harvest neural stem cells from the brain of someone who has developed a brain tumor, insert the gene for a potent antitumor cytokine, grow the cells up and then replace them in the brain to treat the tumor, a study of gene therapy for brain tumors in rats suggests.
Although a clinical application such as this is a long way off, research carried out in Italy gives hope that it may be possible. Gaetano Finocchiaro, director of the Laboratory of Neuro-Oncology and Gene Therapy at the National Neurological Institute in Milan, Italy, told BioWorld International that the approach has had "promising" results in animal studies.
"This could be a very powerful technique," he said. "It could be used, for example, to express factors such as endostatin and angiostatin, which inhibit the formation of blood vessels in tumors." Such a strategy could be helpful in the treatment of cancer, he said, as tumors without their own blood supply are limited in size to about 2 millimeters.
Finocchiaro and his colleagues report their latest studies in the April issue of Nature Medicine, in a paper titled "Gene Therapy of Experimental Brain Tumors Using Neural Progenitor Cells."
Their aim has been to develop a gene therapy for human brain tumors such as glioblastomas, for which the prognosis is very poor. These tumors tend to spread throughout the central nervous system, making complete surgical removal impossible. Cytotoxic drugs have limited effect, partly because of their inability to penetrate the blood-brain barrier effectively.
Finocchiaro's group wanted to improve on a gene therapy technique it already had developed, which involved using a retroviral vector. Among the problems was that of delivering the viral vector to the dispersed malignant cells. Secondly, when the malignant cells infiltrate the surrounding tissue, they are not reproducing, so there is no opportunity for the genes from the retroviral vector to integrate into the cellular DNA.
The team therefore decided to concentrate on genetic manipulation of neural progenitor cells, also known as stem cells. They chose these cells partly because there is some evidence that they can move along pathways of the nervous system similar to those used by infiltrating malignant cells, Finocchiaro said.
For the experiments described in the Nature Medicine paper, the researchers transferred the gene for interleukin-4 (IL-4) into mouse neural progenitor cells and injected these into the sites of induced brain tumors of mice. Mice injected in this way survived significantly longer than control mice: six out of seven injected mice were alive at 90 days, compared to none of six controls.
Interestingly, mice injected with stem cells that had been genetically manipulated to carry a marker gene also survived significantly longer than controls. This indicates, says the paper, that brain neural progenitor cells themselves may produce some factor with an antitumor activity.
Other studies were carried out in rats using rat neural progenitor cells that were immortalized by transfection with a temperature-sensitive variant of the SV40 large T antigen. These cells also were genetically manipulated to carry either the gene for IL-4 or a marker gene.
Normally, when healthy rats are injected into the brain with cells from a highly malignant glioblastoma cell line, 80 percent to 85 percent of them die. However, when these malignant cells were injected along with the genetically manipulated rat progenitor cells containing the IL-4 gene, there was long-term survival of 50 percent of the animals. Likewise, when these manipulated progenitor cells were injected into the site of induced glioblastomas in rats, the survival of 11 out of 17 animals was significantly prolonged, compared with control rats (p< 0.004).
Tests showed that the neural progenitor cells were present in the brains of the rats up to 20 days after their injection. Levels of IL-4 were increased in the injected side of the animals' brains up to one month after the injections.
The authors conclude in their paper: "Overall, our results here have shown that neural progenitor cells engineered to release high levels of IL-4 can have a strong antitumor effect and be more effective than retrovirus-mediated, in vivo transfer of IL-4." They predict that their results may constitute "a new paradigm for [the therapeutic use of neural progenitor cells] in primary or secondary brain cancers."
Finocchiaro said there is some evidence that stem cells are present in the adult human brain. "This raises the possibility that you could harvest these cells during surgery to remove the tumor, grow them in culture and engineer in the gene for IL-4, for example, and then put them back in at the site of the tumor," Finocchiaro said. "But we don't know if stem cells from another individual would necessarily be rejected."
Commenting on the paper, Mark Noble, of the Huntsman Cancer Institute at the University of Utah in Salt Lake City, said one of its "intriguing findings" is "the possibility that the ability of neural stem cells to migrate extensively when transplanted into the adult central nervous system may enable such cells to deliver therapeutic agents widely throughout this tissue. If the migration paths used by the neural stem cells are the same as those used by the tumor cells themselves, then this approach might allow effective delivery of therapeutic agents over broad distances and in relevant locations."
One question raised by the technique, he wrote, is how to turn off the production of the therapeutic agent. "It is difficult to imagine that the introduction into the central nervous system of a constituitively produced compound that was not part of normal central nervous system function would be completely harmless."
Finocchiaro said one of his team's future aims is to test the technique in rats that develop brain tumors spontaneously, in order to mimic more closely the clinical picture of human brain tumors. "We also want to improve the control of expression of the IL-4 gene," he said. "In addition, we want to collaborate with researchers who will help us to grow stem cells in abundance in vitro, because if you grow these cells for too long they begin to differentiate."