Revolutionising medicine one gene at a time

Gene therapy to combat disease has long been talked about but is now becoming a reality. With the advent of CRISPR, a catchy name for Clustered Regularly Interspaced Short Palindromic Repeats, there is now the promise of editing genes in living cells. In order to think about CRISPR let’s consider Duchenne Muscular Dystrophy (DMD).

Muscular Dystrophy occurs in many forms, all of which involve weakening and wasting of the muscles. DMD is an inherited dystrophy affecting muscles including the heart, and most people living with DMD die from heart or respiratory failure before or during their 30s. In DMD there is a fault or mutation in the gene encoding dystrophin, a protein that links the cytoskeleton of muscle to the surrounding connective tissues. Dystrophin acts like a shock absorber and, when absent, muscle lacks strength and stability and is easily damaged. In DMD dystrophin is completely absent or is not effective. The gene for dystrophin resides on the X chromosome, as a result DMD affects ~1 in every 35,000 boys and 1 in 50 million girls.

Thousands of mutations in the dystrophin gene have been identified. The dystrophin gene itself is large, more than 2.5 million base pairs comprising 79 exons (the coding part of DNA). Replacing the whole gene is unrealistic, so the opportunity to precisely edit the dystrophin gene to remove mutations is very attractive. Within the gene there are clustered ‘hotspots’ of mutations between exons 2–10 and exons 45–55 and this gives pointers to areas where editing of the gene might be most effective. Imagine if you could repair the dystrophin gene to kick start production? That’s where CRISPR comes in.

 

CRISPR – how it works?

Scientists create a short genetic sequence called Guide RNA, the code of which matches the DNA you want to modify. This RNA is combined with a protein called Cas 9, which acts like scissors to cut DNA and delivered to cell. The created guide RNA homes in on the target gene for modification and the Cas 9 protein cuts out the mutated or damaged DNA so that the RNA you created can be swapped in its place. Bring in enzyme repair mechanisms and the new RNA is neatly patched into the gene.

 

 

The promise of CRISPR emerging as a reality

In August 2018, it was reported that beagle puppies with a canine form of DMD could have their condition partially reversed through CRISPR repair of the dystrophin gene. The repair was targeted at exon 51, a known hotspot for errors. Dogs undergoing CRISPR experienced improvements in dystrophin production between 3% and 90% depending on the muscle type. In skeletal muscle, dystrophin levels were boosted by ~60% of normal, whilst in heart muscle dystrophin levels were boosted by 92%. It has been estimated that an increase of dystrophin levels of even 15% of normal would provide substantial benefits to patients with DMD, so you can see why these results in dogs is so exciting. The team behind the research suggest that if replicated in humans correcting the errors at exon 51 with CRISPR could benefit 13% of DMD sufferers.

There are some notes of caution for CRISPR as there are for all gene technologies. It cannot be known yet if there are ‘off target’ effects – inadvertent edits to other genes, or if manipulation upregulates proteins involved in cancer, but early signals from animal studies suggest not.

Clinical trials for CRISPR are ongoing, and not just for DMD, but for gene technology to have a real impact in a condition like DMD where there are upwards of 4,000 disease causing different mutations in the dystrophin gene, the challenge will be to identify a method that can target mutations to provide effective correction in a wide number of patients.

 

CRISPR – applications in humans

In November 2018, He Jiankui based in China announced with some ceremony the birth of genetically edited twin girls. The girls’ genomes were edited when they were just a single cell in order to protect them in the future from HIV. By editing embryos, the effect of the modification will go forward for generations into their children and their children’s children. Whilst a profound leap for science, this advance raises questions of ethics and even eugenics, so that society’s view of gene editing now needs to do a fast catch-up to be sure such technologies are used responsibly.

Jennifer Doudna, from the University of California at Berkeley the co-inventor of CRISPR said in an interview “This work is a break from the cautious and transparent approach of the global scientific community’s application of CRISPR-Cas9 for human germline editing,”. and in late January 2019 an investigation by the Chinese government determined that He Jiankui “seriously violated” state laws in pursuit of “personal fame and fortune.” And the story rolls on, a second woman in China has been identified as carrying a gene edited baby, as well as news that scientists at Stanford University knew about the work of He Jiankui and on the same day news that the patent war over CRISPR technology may be set to come to a peaceful conclusion.

The search continues, but gene therapy is here to stay and with it will come the need for rational, responsible discussion about its application and safety. The genie really is out of the bottle now, and we need to understand our responsibility in ensuring this technology brings only benefits.