A study published in Science Advances demonstrates the effectiveness of two CRISPR-based gene editing technologies for correcting Duchenne Muscular Dystrophy (DMD) mutations in mice.
Prof Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said:
“This is a complex story, due to the complex nature of the gene encoding Dystrophin and the nature of the mutations within this DMD gene that lead to Duchenne muscular dystrophy, a devastating disease where muscles degenerate and affected individuals (usually boys as it is an X chromosome-linked gene) rarely survive into their 20s and are often incapacitated well before this. But it is also a story that deserves attention. The authors make inventive use of relatively new methods of genome editing, termed Base editing and Prime editing, in order to promote either exon* skipping or reframing, both of which can restore production of a functional Dystrophin protein when the underlying mutation would otherwise lead to a truncated and therefore inactive protein. (After exon skipping, the Dystrophin protein made is a little smaller, but it still functions if the affected part of the protein is within a region containing multiple redundant repeats, which is often the case in Duchenne muscular dystrophy patients.) Because the goal is to treat patients, it was necessary to make use of a viral vector system to introduce the genome editing components. However, the necessary proteins are too large to fit into single AAV (‘adeno-associated viral’) vectors, which are commonly used in gene therapy. The authors made clever use of a newly developed system that allows proteins made by separate vectors to be joined together within the cell, and this was demonstrated to give functional Base and Prime editing proteins. The authors tested the methods in mice and in human muscle cells and could demonstrate that both were able to restore Dystrophin levels sufficiently to rescue muscle fibres to a degree that would be promising as a therapy for patients. However, the authors clearly state that the methods are not there yet, especially the method of delivery of the genome editing components. The amount of AAV vectors they had to use was at least a hundred times higher than that which would be safe. The authors are therefore suitably restrained in their conclusions. A lot more work need to be done to develop the technology before it can be applied to patients – but this paper does represent a very promising step.