Crispr/Cas9 is a new method to manipulate the genome. It promises great advances in the medical field and very positive concrete cases have already been published. However, such scientific progress could be used for less noble purposes.
We will describe the principle of this new method which allows to modify the genomes in a targeted way. Thanks to it, it is possible, in a simple and rapid way to neutralize genes or even to add new ones within any living organism. Medical applications are very promising and the mouse model has allowed the cure of genetic diseases like Duchenne myopathy. Pernicious effects reside in the modification of human germ cells leading to new transmissible traits or to the modification of microorganisms in order to make them resistant to antibiotics or to immune system with direct applications in the field of bioterrorism.
A disturbing observation}
In the bacterial genome there are sequences of letters (A, T, C and G) which can be read indifferently in one direction or the other: these palindromes are named “Clustered Regularly Interspaced Palindromic Repeats = CRISPR”. Between the palindromes strange sequences have been identified: they are not of bacterial origin but correspond to viral sequences (bacterial virus are named bacteriophages).
Why are viral sequences stored in the bacterial genome?
It has been found that these sequences belong to viruses which have already infected the bacterium: the latter thus keeps in reserve a DNA trace of this bacteriophage. When the same virus wants to infect the bacterium again, its DNA is quickly detected by the CRISPR/Cas9 complex thanks to a piece of RNA which is complementary to the DNA stored by the bacterium (RNA guide). Cas9 is a specialized enzyme that cleaves the viral DNA (endonuclease). This phenomenon is similar to those found in immune defenses: the bacterium that has been infected by a virus keeps it in its memory in order to destroy it very effectively on a second contact.
In summary, when the guide RNA detects a sequence which is complementary to it on a genomic DNA, the CRISPR/Cas9 complex cuts this DNA. Therefore, it has been constituted a genetic tool for the laboratories formed by the protein Cas and its guide RNA. When the RNA is positioned on the complementary part of the DNA of a genome, the protein Cas cuts the gene or even replace it by another one. It is then possible to program a given RNA sequence to inactivate or replace the corresponding gene on a genome.
It is therefore quite possible, for example, to correct mutations in the genome by replacing the defective sequence.
This new tool replaces much less efficient systems that already existe.
The first applications and projects
Understanding diseases and gene therapy
- T lymphocytes unable of recognizing and therefore attacking lung cancer cells are currently being modified by this tool in order to give them back their activity: after injection into the patient, they could eliminate the tumor;
- similar applications in the fields of AIDS, cancers, cystic fibrosis, hemophilia or Alzheimer’s disease are possible and under study.
Modification of vectors
- a mosquito has been made resistant to malaria: released in the wild it could transmit this resistance gene to its congeners, contributing to decrease the incidence of the disease;
In the field of vegetation
Vegetable applications are also being studied: wheat resistant to powdery mildew has already been produced.
How far can we go?
Do modified plants fall into the category of GMOs?
If changes in somatic cells (blood, liver, etc.) don’t seem to be a major problem, the modification of the germ cells, giving new transmissible characters, can guide the evolution of the human species (eugenics?). This becomes a societal problem.
Note that the use of this method does not make it possible to show that the genome has been manipulated.
Terrorist use
What kind of weapon?
One can imagine the development of a virus able of cutting, modifying, suppressing or activating a particular gene of the infected person that can leads to cellular dysfunction and therefore physiological problem leading to death. Researchers have already developed a murine poxvirus able to kill a normally immune mouse to this virus. Other harmful applications, such as the development of bacteria that are resistant to all antibiotics, or bacteria and/or viruses with enhanced pathogenicity, are conceivable. We can also play on the vector of diseases: for exemple, the mosquito responsible for the transmission of malaria or the flee responsible of the plague …
Obama’s science advisors letter
By the end of November 2016 President Obama’s PCAST (President’s Council of Advisors on Science and Technology) provided him a letter to warn him of the emergence of a new form of terrorism. They are worried by the new genetic engineering technologies such as CRISPR. They think that they should be better taken into consideration. Such technologies in the hands of malicious users could be very dangerous! The authors recall that recent infections of Zika, Ebola or H1N1, although unintentional, have been dramatic because they have emerged in an overall context of rapid urbanization, globalization or climate change, these factors currently beeing not under control. They point out that the US is clearly unprepared for these new biological threats.
What to do ?
Experts recommend a significant increase in “biomonitoring” defined as “the active process of collecting, analyzing and interpreting biosphere data” that may be related to threats to human health. Integrated surveillance must be able to demonstrate in real time the emergence of a disease in order to take the right measures against intentional or unintentional threats. Biomonitoring must integrate humans, animals and plants.
Conclusion
The review “Nature” of November 15, 2016 reports the case of a Chinese patient with a cancer who was injected cells which genes had been modified by the CRISPR/Cas9 technique. The patient has improved.
This is the positive side of this new technique.
Its misuse leading to the development of biological weapons is possible. In all cases, manipulation of genomes can only be done in equipped laboratories and by experienced scientists.
Auteur : Professeur François Renaud