New system uses MRI for non-invasive monitoring of gene expression, treatment effect
A research team including NIBIB-funded scientists has developed a simplified approach for delivering and monitoring gene therapy for brain disorders. The group used eye drops to deliver the gene for a growth factor called granulocyte colony stimulating factor (G-CSF) in a mouse model of brain ischemia, which refers to a lack of blood and hence, oxygen, to the brain. The treatment led to a significant reduction in brain atrophy, neurological deficits, and death in the mice. The research team also devised a system to monitor the success of the gene delivery using MRI. The combination of simple delivery and non-invasive monitoring has the potential to contribute to improved studies of experimental gene therapy in animal models of stroke, Alzheimer’s dementia, Parkinson’s disorder, and ALS. The system also offers the intriguing possibility that acute brain injury may someday be treated by emergency medical workers through the simple delivery of eye drops carrying a therapeutic gene.
In past studies in a number of animal models of brain disorders including stroke, Alzheimer’s, and Parkinson’s, G-CSF held promise as a potential treatment. However, when tested in a human trial in more than 400 stroke patients, G-CSF did not improve treatment outcomes, so enthusiasm about it diminished.
In the studies conducted in humans, it was not possible to determine whether G-CSF was successfully expressed in the brains of the patients, which could explain the discrepancy in results in humans versus mice. To thoroughly test the efficacy of any gene therapies in humans, it is critical to establish that the gene is expressed in the test participants.
Recognizing that the lack of a technology to verify gene expression in humans was a central problem in such studies, Philip K. Liu, Ph.D., of the Martinos Center for Biomedical Imaging at Harvard Medical School, and his collaborators, Drs. H. Prentice and J. Wu of Florida Atlantic University, developed a novel system for G-CSF delivery and monitoring of treatment. MRI was used to confirm successful administration and expression of G-CSF delivered to the brain using gene therapy. MRI was also used to monitor the effectiveness of the treatment in experimentally induced brain ischemia. The work is reported in the July issue of Gene Therapy.1
“This new, rapid, non-invasive administration and evaluation of gene therapy has the potential to be successfully translated to humans,” says Richard Conroy, Ph.D., Director of the NIBIB Division of Applied Science and Technology. “The use of MRI to specifically image and verify gene expression, now gives us a much clearer picture of how effective the gene therapy is. The dramatic reduction in brain atrophy in mice, if verified in humans, could lead to highly effective emergency treatments for stroke and other diseases that often cause brain damage such as heart attack.”
The first goal of Liu’s team was to develop a simplified technique that would allow rapid delivery of G-CSF to the brain without elaborate technologies requiring highly trained staff and equipment. This is particularly important for stroke and cardiac arrest, where treatment within a few hours is critical. The second goal was to devise a non-invasive method to demonstrate that G-CSF was delivered to the brain and expressed at therapeutic levels. The overall aim is to successfully demonstrate a system in animals that can be translated to humans.
Gene delivery in a mouse model of brain ischemia
The researchers used a mouse model of brain damage caused by blocking oxygen to the brain. The G-CSF was delivered by inserting the G-CSF gene into an adenovirus that is safe in humans and known to infect brain cells efficiently. The adenovirus was then administered through eye drops. An advantage of this simple eye drop method is the ability to easily give multiple treatments. Delivering the G-CSF gene at multiple time points after the induced blockage led to significant reduction in deaths, cerebral atrophy, and neurological deficits as measured by behavioral testing of the treated mice.
Verification of gene expression and treatment success with MRI
The researchers used MRI to confirm that G-CSF was expressed in treated mouse brains and to visualize the differences in brain tissue damage in both control and treated mice. For control mice, which did not receive G-CSF in eye drops, the MRI technique identified brain areas with reduced metabolic activity and eventual cerebral atrophy as the result of ischemia. Mice treated with the G-CSF gene therapy retained normal levels of metabolic activity and did not reveal any areas of cerebral atrophy. On average, following ischemia, mouse brain striatum size was reduced more than 3-fold, from 15 square millimeters in normal mice to less than 5 square millimeters. In contrast, G-CSF-treated mice retained an average striatum volume of more than 13 square millimeters—close to normal volume.
To confirm G-CSF gene expression, the researchers used a contrast agent attached to a piece of DNA that targets the G-CSF gene. The combination molecule enabled MRI imaging of G-CSF gene expression in the brains of treated mice. Control mice treated with the same adenovirus carrying the contrast agent bound to a different control gene produced no MRI signal in the brain.
“We are very excited about the potential of this system for eventual use in the clinic,” says Liu, “The eye drop administration allows us to do additional treatments with ease when necessary. The MRI allows us to track gene expression and treatment success over time. The fact that both methods are non-invasive increases the ability to develop, and successfully test gene therapy treatments in humans.”
The group is now taking steps necessary for a clinical trial, which includes obtaining FDA approval of the use of the G-CSF gene in humans and collaborating with physicians to develop a clinical trial protocol.
The work was supported by the National Institutes of Health through grant R01EB013768 from the National Institute of Biomedical Imaging and Bioengineering, grant R01DA029889 from the National Institute on Drug Abuse, grant P30DK057521 from the National Institute of Diabetes and Digestive and Kidney Diseases, and grant NS045776 from the National institute of Neurological Disorders and Stroke. The Wellcome Trust and State of Florida also supported this research.