Researchers at Scripps Research, Florida have developed a special molecular switch that could possibly be embedded into gene therapies to facilitate investigator’s controlled dosing. Reported in Journal Nature Biotechnology, the breakthrough offers gene therapy designers a potential technique to adjust the activity levels of therapeutic genes—potentially solving a major safety issue and impediment to more broad use of gene therapies.
Today, there has been a lack of a safety mechanism to control dose levels for gene therapy administration. Although gene therapy holds tremendous potential, gene therapies have been viewed as inherently risky as once they are delivered to a patient’s cells, they cannot be turned off or modulated. Consequently, few actual gene therapies have been approved by the FDA thus far.
Potential Practical Way to Regulate Gene Therapy Dose
Published recently in Journal Nature Biotechnology, the Scripps team led by principal investigator Michael Farzan evidenced the power of their new switching technique by incorporating it into a gene therapy that generates the hormone erythropoietin, used as a treatment for anemia. This team showcased evidence of being able to suppress expression of its gene to low levels with a specially embedded molecule and thereafter could boost or increase the gene’s expression over a wide and dynamic range by using injected controlled molecules called morpholinos—these are approved by the FDA.
Details: A Transgene Switch
The study team developed a transgene switch from a family of ribonucleic acid (RNA) molecules called hammerhead ribozymes. These ribozymes have the remarkable property that they cut themselves in two as soon as they are copied out into RYNA from the DNA that encodes them.
The therapeutic transgene containing the DNA of such a ribozyme will be copied out in cells into strands of RNA—transcripts—that will tend to separate into two pieces prior before they can be translated into proteins. The self-cleaving action of the ribozyme can be blocked by RNA-like morpholinos that latch onto the ribozyme’s active site; if this happens, the transgene transcript remains intact and will be more likely to be translated into the therapeutic protein.
Hence, the ribozyme effectively serves as an “off switch” for the transgene, whereas the matching morpholinos, injected into the tissue where the transgene resides, can effectively turn the transgene back “on” again—to a degree that depends on the morpholino dose.
Starting with a hammerhead ribozyme called N107 that had ben used as an RNA switch in prior studies, they found that the difference in production of a transgene-encoded test protein between the “off” and “on” state was too modest for this ribozyme to be useful in gene therapies. But over many months of experimentation, they could modify the ribozyme until it had a dynamic range that was dozens of times wider.
Preclinical Study in Mice Model
The team demonstrated the ribozyme-based switch in a mouse model of an EPO gene therapy not yet approved by the FDA. However, it is considered potentially better than current methods for treating anemia associated with severe kidney disease. They injected an EPO transgene into muscle tissue in live mice, and showed that the embedded ribozyme suppressed EPO production to a very low level. Injection of a small dose of the morpholino molecules into affected tissue strongly reversed that suppression, enabling EPO production to rise by a factor of more than 200—and stay there for more than a week, compared to the half-life of a few hours for EPO delivered by a standard injection. Those properties make the ribozyme-based switch potentially suitable for real clinical applications.
Call to Action: Farzan and his colleagues are now working to adapt their ribozyme switch technique so that it can be used as a gene therapy failsafe mechanism, deactivating errant transgenes permanently.