Permanent repair of a faulty gene has long been a goal of researchers working to develop gene-based therapies. But many current gene modification strategies that have entered clinical trials have been based on temporary forms of gene correction — treatments that will need to be given frequently throughout a person's life.
Now, a team of scientists from the U.S., Canada and France, led by Charles Gersbach in the Department of Biomedical Engineering at Duke University, says permanent gene repair may be closer to reality than previously believed. They've specifically demonstrated this with respect to the dystrophin gene, which, when mutated, causes Duchenne muscular dystrophy (DMD).
Working with cells taken from boys with DMD, David Ousterout, a graduate student at Duke University, and colleagues, used laboratory-engineered proteins called TALENs to repair genes for the muscle protein dystrophin, publishing their results online June 4, 2013, in Molecular Therapy.
The repairs were accomplished at the level of DNA, the "master copy" of the instructions for all the body's proteins. Current gene modification strategies in DMD that have reached the clinical trial stage operate at the RNA level; examples are exon skipping and read-through of premature stop codons.
RNA is continuously produced from DNA, so errors in it must be continuously corrected. Repairing DNA is analogous to correcting an original document used to make photocopies; repairing RNA is analogous to correcting each photocopy as it's produced.
The research team created a TALEN — transcription activator-like effector nuclease — that targets a part of the dystrophin gene called exon 51. (That's the same part of the gene that's being targeted by two experimental exon-skipping drugs — eteplirsen and drisapersen— that seek to modify dystrophin at the RNA level.)
One part of a lab-designed TALEN targets two specific DNA sequences, while another part cuts the DNA between the two sequences, stimulating cellular DNA repair mechanisms.
The TALEN the team developed for these experiments — dubbed TN3/8— edited exon 51 in such a way that shorter-than-normal, but probably functional, dystrophin proteins were made by many of the cells the researchers took from DMD patients.
The researchers say the TALEN strategy could be used to edit the DNA in muscle stem cells taken from someone with DMD, after which the stem cells could be injected back into the patient, where they (and all cells derived from them) would carry the correction. Or, alternatively, methods could be developed to deliver TALENs directly to muscle cells in the body.
|Pictured: A laboratory-engineered TALEN protein finds its target site in the human genome by binding to DNA, shown in green, with an engineered DNA-recognition protein, shown in orange. Once the protein finds its target, the DNA is modified by the enzyme part of the protein, shown in blue.|
"Future studies are warranted to investigate the therapeutic efficacy of this approach and similar permanent gene-editing strategies," the researchers write. "Genome editing is a powerful approach for creating custom alterations to the genome [all genes]."
The investigators say the approach they used "may provide a versatile therapy for DMD when frame restoration [correcting the way genes are "read" by the cell] is predicted to permanently correct the native gene and restore protein function."
They also say the approach could be applied to other disorders, specifically noting that the Fukuyama type of congenital muscular dystrophy (CMD) and the type 2B form of limb-girdle muscular dystrophy (LGMD) might be good candidates for study.
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