Enabling high-precision gene manipulation, the CrisprCas9 gene scissors have raised hopes for finding effective therapies for hereditary diseases. And fueled dreams of human enhancement too.
With the CrisprCas9 gene scissors, scientists now have the potential to rewrite the code of life – with a method that is quick, easy, and efficient. People in the life sciences and medicine are talking about a quantum leap. Is that right?
Anita Rauch: Yes, you could say that. Although physicists might argue that a quantum leap is almost the tiniest thing that exists at all (laughs). CrisprCas9 makes a lot of things easier and therefore represents an important milestone in the ongoing development of genetics.
What is new about these gene scissors?
Rauch: With Crispr we can genetically modify cells relatively easily and efficiently. The gene scissors are getting better and better almost by the day – also because many research teams around the world are working on their development. Crispr works quite well for some regions of the genome but is more difficult to use in others.
What's the difference between the genetic scissors and conventional gene technology?
Rauch: It's in the field of gene therapy that Crispr is particularly promising. In traditional gene therapy, a virus is delivered into the cell carrying a replacement gene – and there it integrates with the intact gene into the human DNA. This is not without its problems because the viruses can also cause damage in the cells – in the worst case, the result can be cancer. In the meantime, safer variants of this technique have been developed. Another problem is the fact that very large genes cannot be transported by viruses. With Crispr, on the other hand, we can target a particular gene and specifically modify it. The advantage here is that we can repair large genes on the spot and even prevent the growth of gene products that have become harmful as a result of a mutation.
And what can Crispr be used for today?
Rauch: Crispr enables us to produce better disease models in the lab. This has helped us gain a better understanding of the consequences of genetic mutations – which is an important area of application. We can now work with human cell lines, as we now have pluripotent cells – almost like human stem cells – available to us. From these we can make neurons, for example, and analyze which cellular consequences of a gene mutation play a role in mental disability or dementia. If we can understand the molecular mechanisms of diseases, we can then develop more specific therapies. This doesn't necessarily need to be a gene therapy, though, as it would be ludicrously expensive at the moment to create a tailor-made gene therapy for each patient.
Are there alternatives to gene therapies?
Rauch: Hypercholesterolemia, a pathologically high blood cholesterol level, for example, can be cured if a certain gene is turned off. This finding has enabled a drug to be developed that prevents the overall impact of this gene.
What role does Crispr play in your daily work as an ethicist, Ms. Krones?
Tanja Krones: Media interest in Crispr is very high. However, in the clinical practice at the UniversityHospital Zurich, gene therapy still doesn't play much of a role as it is still mainly the subject of basic research. Nevertheless, questions on its societal impact naturally arise.
What kind of questions do the gene scissors raise?
Krones: From an ethical point of view, Crispr is nothing more than an instrument, a sharp knife. And every instrument can be used for good or for bad. But this leads us on to a whole range of fundamental questions: What does it mean to change the human genome? In what kind of society do we want to live? What is sickness, what is health? Can genetics free mankind from all evils? Many promises of salvation in medicine have not come true. In the field of stem cell research for example, although much has been achieved, it is by no means as far as had been hoped for ten years ago.
The image of using gene scissors or a sharp knife to cut out and replace genes is very graphic. But is it actually true? Is it really that easy to replace bad genes with good ones?
Rauch: I think the image of the scissors is very apt. But the use of these scissors still needs to be developed and improved. A child must also first learn how to use scissors. Our genome is a tricky organ, and not every part is equally easy to cut. Some are difficult to get at due to their chemical structure. There is still a lot of research to be done.
Do we know where the tricky places are?
Rauch: Some we know, some we find out through trial-and-error experimentation. What can also occur, however, is that the gene scissors don't cut where you want them to or they only repair a part of the cells.
This is when we talk about "mosaics".
Rauch: Exactly, it sometimes happens that these mosaics don't achieve the desired result or even end up being more harmful than the original disease. There are some organs, such as the liver, blood or retina, that the gene scissors can easily access. The brain, on the other hand, is very difficult to reach because this means crossing the blood-brain barrier.
Where do the gene scissors function best?
Rauch: They seem to work best in germline therapy, which already starts with fertilization. At this stage, the cells undergo a particularly large number of repairs. American researchers have shown that the gene exchange functions best when, during fertilization, the gene scissors are injected into an egg cell at the same time as the sperm cell. This means that the embryo is healed and there is a corrected gene variant in every cell. In Switzerland, however, such interventions in the germline are prohibited.
Interventions in the germline also raise ethical questions. What's your view on this, Ms. Krones?
Krones: We must ask ourselves what the advantages and disadvantages of such gene therapies are. And we must think very carefully about whether there is a sense in making such interventions in the germline. After all, this doesn't guarantee that the couple will have a healthy child afterwards. And besides, other alternatives exist. A couple who have a genetic predisposition to severe hereditary diseases can choose artificial insemination. With the help of preimplantation diagnostics, the embryos can be tested, and one that has not inherited the disease can then be selected for implantation. Even in the case of severe hereditary diseases, usually only a maximum of 50 percent of embryos are affected, often even less. This means that gene therapy isn't even necessary.
Do you agree, Ms. Rauch?
Rauch: Preimplantation diagnostics is a good and now proven method to help such couples give birth to a healthy child. However, there are rare constellations where a germline therapy with Crispr would be the only way for a couple to have their own biological child. In addition, there are cases where older women no longer have enough egg cells available for selection.
Do you consider a Crispr intervention desirable in certain clearly defined cases, such as to prevent the birth of a sick child?
Rauch: I think that this would be an application I could understand and support. I'm afraid, however, that it opens up the field for questionable uses of this method. What I find much more problematic is the idea of using germline therapy to enhance the human race.
Ms. Krones, what do you say about interventions in the germline - yes or no?
Krones: As already mentioned, alternatives exist. In very rare cases and when all these alternatives have been exhausted, such an intervention could be considered. But even then, the risks must not be withheld from the parents and alternatives, such as sperm donation or adoption, must also be thoroughly discussed.
Ms. Rauch, you have indicated that the gene scissors could be used to enhance human beings. What makes you think so?
Rauch: Internationally there seems to be considerable support for enhancement projects and with the discovery of the gene scissors, it seems that the obvious method has been found.
That sounds worrying. Is the research not justified simply by the fact that we can help people who suffer from severe genetic defects?
Rauch: Indeed, most researchers want to cure serious diseases and reject the idea of "enhancement". However, there are always forces in all societies wanting to enhance mankind, and researchers who are open to this. One known – and rightly condemned – example was the Chinese scientist who is said to have used Crispr last year to turn off the HIV gene of twin babies, apparently wanting to protect the children from AIDS. Apart from the fact that, in view of current scientific findings, this was irresponsible and reprehensible, studies with mice suggest that their learning skills improve when this gene is knocked out. So the researcher may have been interested in doing something quite different from protecting the children's health, i.e. in making them more intelligent. However, our biological nature is subject to a sensitive balance – and such supposed advantages may come with unknown disadvantages.
What do you say to this, Ms. Krones?
Krones: From an ethical point of view, the idea of enhancement has several dimensions. It raises questions about our image of mankind, for example, and about justice. The discussion has been going on for as long as genetics has existed: In the past it was about preimplantation diagnostics, today it's about Crispr. There will always be the concern that scientists will change human beings so much that societies with egalitarian and liberal values will no longer be able to support this image of mankind.
Where do you draw the line?
Krones: It's most important that there is a link to a disease. The idea is to use Crispr when someone is sick, and not because we prefer brown to blue eyes. But the discussion about designer babies is more of a media hype. Most people don't want designer babies, because it means undergoing artificial insemination; they'd rather have a child naturally. In any case, most of the world's population are struggling with completely different problems. In comparison, the proportion of people for whom the idea of enhancement is even a topic is negligible. But, of course, we still have to talk about it. The subject is important because it stirs something within us. It all comes from our belief in the power of technology, the belief that we can eradicate diseases and eliminate risks. But we can't do that. We will always be mortal.
Is it not the case that the idea of optimizing the human body through technical means – whether prostheses or medical interventions – is becoming more and more socially acceptable?
Krones: The question is whether nature is good per se. It's not, or course. Ever since scientific knowledge has existed, people have been looking for ways to improve us. But not everyone sees the sense in this. Take the example of prenatal diagnostics: Many women or couples deliberately reject the tests because they do not wish to know about the genetic risks. Many people also can't see the point in us fallible, mortal beings chasing the dream of our own enhancement.
Rauch: At the moment, the designer baby is a distant vision, and will maybe even remain a complete utopia given the complexity of our development. Most personal characteristics result from innumerable genetic factors coming together. There won't be a master switch, for example, that makes one feature big or beautiful without side effects.
Krones: We also assumed that if we could once decipher the genome, we would then have understood the human being. But the more we understand, the more we realize how complex our biology actually is.
Let's now look to the future: Can we be hopeful for Crispr or should we be worried?
Krones: Both, I think. As is often the case with new technologies, the issue is ambivalent. As humans, our responsibilities grow the more we can do. That's why it's essential to talk to each other about how we want to live. And to think about how our children and future generations are to survive in this world.
Rauch: At the moment gene therapies have been developed for just a handful of diseases. In the coming years, Crispr will hopefully provide many more. But there certainly won't be such a therapy for every disease overnight and for some it may remain a dream. However, for certain diseases – like retinal diseases or cancer – new therapies will be available soon.
Bacteria with a memory
When researchers Emmanuelle Charpentier and Jennifer Doudna developed the CrisprCas9 gene scissors in 2012, they harnessed the memory capacity of the bacterial immune system: When a bacterium survives a viral attack, it incorporates short pieces of the viral DNA into its genome and is then able to use its "molecular memory" to fend off a renewed attack more quickly.
In the process, the bacterium inserts the viral DNA fragments into short, repeating gene sequences called Crispr (Clustered Regularly Interspaced Short Palindromic Repeats). A protein called Cas9 plays an important role in the bacterial immune system. This protein is initially responsible for cutting the viral DNA during the initial infection so that the fragments can be inserted into the Crispr sections of the bacterium. If the virus attacks again, the Crispr sections are rewritten into ribonucleic acid (RNA); these RNA molecules attach themselves to Cas9, serving as a "probe" to recognize the penetrating viral sequences. Cas9 then cuts the viral sequence open – making it harmless. This is how the term "gene scissors" has become the established term for Cas9 in scientific communications.
Just one year later, in 2013, scientists discovered that CrisprCas9 could not only be used on bacteria, but also on much more complex plant and animal cells. The researchers also found that the gene scissors could be directed to virtually any part of the genetic material using a specifically produced guide RNA. Cas9 cuts the DNA double strand at the desired location. Depending on how the subsequent repair is carried out, different objectives can be achieved – from immobilizing the affected gene to the exchange of individual DNA building blocks and the insertion of additional gene sequences.