“CRISPR, kick-starting the revolution in drug discovery” or “A year after the first CRISPR babies, stricter regulations are now in place.” read some of the recent headlines. CRISPR, a new gene editing technology, is making waves around the world and Israel is no exception. The Israeli startup eggXTt is preparing to use CRISPR-tech to mark chicken eggs by gender in an effort to reduce waste in the poultry industry, and research labs at institutes around the country regularly make use of CRISPR-tech to make groundbreaking discoveries in the biological sciences.
But how does CRISPR actually work, and what are the limitations of this new technology? CRISPR is often touted by scientists and science journalists as a ‘pair of molecular scissors’ allowing us to ‘edit our genomes at will’ in a ‘point-and-click’ fashion. Although it is tempting to believe these buzzwords, they are not particularly accurate, and can be misleading for the public and policymakers considering the potential impacts of this new technology. After all, our DNA is not a tiny Microsoft Word document that can be altered however we see fit. In this article we will dive into exactly what CRISPR is, what it can and cannot do, and why we might not be seeing designer CRISPR babies for a few more decades (or centuries).
First of all, CRISPR is not a pair of molecular scissors. It is a system of proteins that evolved in bacteria to protect them against viruses. Proteins can take all shapes and sizes, and CRISPR proteins look something like the wire cleaning scrubbers you can find in many kitchens. The oft-mentioned analogy that CRISPR are molecular scissors is doubly misleading, because scissors imply that someone (ie: scientists) are somehow wielding them in a precise manner to cut and paste DNA as they please. This gives the false impression that scientists are the sole possessors of CRISPR knowledge, bestowing upon them the power to alter our genomes at will.
In reality, CRISPR proteins slide along DNA strands, recognizing specific areas by their unique feel. More specifically, the proteins move along the DNA until they find a spot on the DNA that matches perfectly with their recognition site, and then they squeeze down and cause the DNA to break at that point. This is similar to how your handprint fits well into its imprint in the sand. When you think about the wide variety of proteins in the human body (over 100,000) it makes sense that few other proteins would make the same match (a rubber duck or iron nail would not fit well into your handprint either). When the CRISPR proteins move along the DNA, they are only able to make the DNA break at these specific points. Scientists are able to take advantage of this tendency of CRISPR proteins, and can manipulate them to make breaks in DNA at the area they want removed or altered in their experiments. The CRISPR system also consists of a few other components, including a set of guide RNAs that help the CRISPR proteins match up with the DNA of their choice.
Unfortunately, CRISPR proteins are not perfect, and DNA is a very long and repetitive molecule, so it is possible for mistakes to occur. Other areas of DNA may look the same to the CRISPR proteins due to similar or identical sequences, causing the CRISPR proteins to break the DNA at undesired places. Recent research has noted that CRISPR can have a high frequency of off-target DNA breaks, up to 50% in many model systems. These issues mean that once CRISPR is released into a living organism it is sometimes hard to predict where these off target effects will occur. The challenge of off-target effects is one of the reasons CRISPR babies are likely a long way off. As a result a number of institutions and many scientists, including the World Health Organization, have called for a comprehensive ban on genetic modifications to reproductive or germline tissues. Despite this, a team of researchers in China recently managed to create a set of genetically altered twins, resulting in significant controversy. The ethical questions surrounding CRISPR in humans are another compelling reason to wait, particularly because edits of germline tissues like eggs and sperm could result in permanent changes to the human genome.
Another issue with the CRISPR system is that it needs to be inserted into living cells using a viral vector. This means the CRISPR system has to be translated into DNA, coded into a type of non-deadly virus, and injected into cells, which then produce the CRISPR proteins themselves. These viral systems are never 100% successful, and sometimes only enter 15-20% of all cells, which is not ideal for medical-grade treatments.
Despite these barriers there are several medical treatments in development using CRISPR-tech to address difficult-to-treat diseases. One of the most advanced is a CRISPR-based treatment for Duchenne Muscular Dystrophy (DMD), a rare and incurable muscle degenerative disease predominantly affecting children. DMD is caused by mutations in the dystrophin gene and is always fatal with an average patient lifespan of 26 years. Recent studies in mouse models and human heart cells in petri dishes have shown that CRISPR can cause reduction in muscular degeneration symptoms, which are the hallmark of this disease. Because DMD is caused by mutations in one specific region in the genome, scientists and clinicians can take advantage of CRISPR’s targeted DNA-breakage effects to chop the affected section out of the genome by targeting two RNA guide probes, one to each side of the mutant piece of DNA. In most cases simply excising the mutant piece of DNA is not sufficient to remove symptoms of a disease. However, in this rare case removing the mutant DNA section allows for a partial improvement in some muscle cells, which is why this treatment has shown promise for clinical applications.
Many of the future CRISPR-based treatments will need to insert a new, healthy piece of DNA in addition to removing the mutant DNA. This is obviously many times more difficult as in addition to mitigating risk from off-target CRISPR effects, it will also be necessary to reduce the risk of the new piece of DNA inserting into the wrong portion of the genome and causing undesirable effects. Nevertheless, trials are now underway to translate this treatment method to the clinic in studies investigating the use of CRISPR for Sickle-Cell Anemia, Cystic Fibrosis and non-Hodgkin’s Lymphoma.
Although the major benefits of CRISPR-tech are likely decades away, CRISPR is already having significant impacts in the scientific, medical and biotech spheres. As long as this technology is used responsibly, we have much to gain from a world where we could one day become the masters of our own genomes.
This is an article in the series Science & Technology in the Holy Land, a regular column on innovations in science, tech, start-ups and futurism by Jamie Magrill, an MSc, Biomedical Sciences Candidate at the Hebrew University of Jerusalem.