Genetic Engineering in Human Beings: CRISPR/Cas9

8-cellembryoOn April 22nd, the journal Protein and Cells published a paper describing the first known example of genetic engineering of a human embryo. It was an exploratory study conducted on non-viable embryos (they would have never been able to give rise to an adult human being) and the experiments were only partially successful. Regardless, the publication of this study is immensely relevant because it means we have now crossed a line, and applied genetic engineering to humans.

The technology that has allowed this major scientific breakthrough is called CRISPR/Cas9 and it has revolutionized the world of genetic engineering. The aim of genetic engineering is to generate genetically modified organisms (GMOs) by manipulating their DNA to eliminate, modify or introduce genes. These changes are permanent, and will be passed on to the descendants. Genetic engineering is broadly applied nowadays to generate plants fortified with vitamin A, for example, or plants resistant to herbicides. In health science, GMOs allow scientists to reproduce and study the biology and treatments of a disease by reproducing it in mice and other animals (for a concrete example see “Additional explanations 1”, below). However, the enormous complexity and technical limitations of the process have historically precluded its direct application to human health. Until now.

The CRISPR/Cas9 technology allows us to quickly and easily modify one or multiple genes at the same time in an efficient and specific way. This is in stark contrast to the traditional method of creating GMOs, which required months to generate one with a single genetic modification and years to model a complex disease like cancer, which often requires the combination of several modifications. With CRISPR/Cas9 what used to take years can now be done in a matter of weeks. This is why CRISPR/Cas9 brings the possibility of genetically modifying human beings much closer. The potential therapeutic applications to genetic diseases are endless. We could get rid of damaged genes or add genes which could protect us from infections or cancer. In the study I mentioned above, for example, CRISPR/Cas9 was applied to replace the damaged genes associated with a blood disease called thalassemia with functional ones, which could potentially cure the disease. (See the schematics and the additional explanations below if you are curious about the fundaments of CRISPR/Cas9).

Considering the speed with which this technology is developing, it is inevitable that this approach will be repeated in the coming months and years. Already our capabilities with this technology have outstripped our understanding of it, and of the human genome. Our current knowledge of the function of the human genome is still very limited, and so even straightforward modifications could have unanticipated consequences. Taking into account that these modifications would be passed to our children, our responsibility is overwhelming.

Reaction to this study has been swift and vocal. Prior to its publication, several scientists and bioethicists wrote an open letter (published in the journal Science), titled “A prudent path forward for genomic engineering and germline gene modification”. The main conclusion was that research using CRISPR/Cas9 should be encouraged as long as there is transparency, control and discussion every step of the way. Other mainstream media outlets were more critical calling for stringent controls. I can’t encourage you enough to read these articles and get familiar with this debate, because the question of how to take advantage of new biotechnologies in a prudent, rational and ethical way it is certain to be at the centre of political and bioethical discussion for years to come.

 

Additional explanations

  1. GMOs and pancreatic cancer.

The study of GMOs has provided an enormous amount of information about the biology of many diseases which used to be relatively unknown. For example, pancreatic cancer is often diagnosed at a very late stage. Unfortunately, only a very small percentage of patients can even undergo surgery. So scientists could only study human tumours at late stages, when they have already accumulated many mutations, making it very difficult to understand which mutation/s originally cause the disease. Analysis of the DNA of those tumours, however, had showed that there was a single gene, (called Kras), which was mutated in more than 90% of the patients. In 2003 a lab generated a genetically modified mouse which had that same mutation in its pancreas. And the mice developed pancreatic cancer, showing that that Kras mutation on its own can cause the disease, it is the “driver” and so the ideal candidate for chemotherapy. Unfortunately, Kras is extremely difficult to switch off so no Kras-based therapies are available so far. However, this GMO of pancreatic cancer is now used worldwide and has provided us with invaluable information about how the disease starts and develops.

  1. The story of CRISPR/Cas9.

CRISPR/Cas9 is based on the immunological system of primitive organisms (bacteria and archaea) which collect bits of the genetic material of the viruses they encounter and keeps them in a very long gene (known as a CRISPR). This gene CRISPR is passed on to their descendants, who add to it as they encounter new viruses. If an archaea comes under attack, it has a history of all the viruses that it and its ancestors have encountered. It can then activate an enzyme called Cas9, which reads the CRISPR and attacks and destroys any genetic material which matches it (see scheme 1. The archaea is the big green cell, with its CRISPR containing material of many viruses (orange, purple, red, yellow,…). Yellow and red viruses are invading the cell and Cas9 is reading the CRISPR and looking for matching viruses to destroy them).
This system was only discovered in 2012, and scientists immediately saw the potential to apply it to the generation of GMOs. (see scheme 2)They could create a CRISPR which instead of bits of virus gathered bits of as many genes as we want to modify (in this case, the red and the green genes). This CRISPR could then be introduced into a mouse zygote (one cell embryo, blue cell in the scheme) together with Cas9, which will read the CRISPR, identify the genes and destroy them all in one go. The embryo will give rise to a mouse with the modifications in both genes.GMO(1 and 2, second go)

  1. The traditional method of generating GMOs (See scheme 3)

It started by building a false gene with the new genetic information (red gene). This false gene would be put into mouse embryonic cells (blue cells in a). A small subset of these cells would incorporate “by mistake” the new gene into their DNA (red cells in b). Those mutated cells would then put back into early mouse embryos (c). Some of them would get integrated in the embryo, a subset of which would survive and give rise to an adult animal, which will be a mix of mutated and normal cells (d). Some of these mixed mice (chimaeras) will have the mutation in their gametes- eggs and sperm, so they would be able to pass the mutated gene to their descendants, who finally will inherit the gene normally (e) . To create a complex GMO which combined several genomic changes (for example, red and green) this process would be repeated for each of the genes (f) and then the mice would be bred mice amongst them (g), requiring three generations to get each gene in both chromosomes in 1 out of 16 mice (h).GMO(3)

 

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