A technique of genetic engineering based on the use of restriction endonucleases in order to modify the genome. The originality of this technique results in the high specificity of nucleation of the double strand of DNA by nucleases, unlike previous techniques where the modifications were largely random. Four families of nucleases (“molecular scissors”) are used to cut DNA to the desired level, to add or remove a particular sequence.
The modified double strand of DNA is joined together by homologous recombination or by joining non-homologous ends. The insertion of genetic sequences by homologous recombination after double strand cutting of DNA makes it possible to correct, modify or inhibit the target genetic sequence.
Zinc finger nucleases
These are hybrid proteins created by fusion of genes encoding zinc finger proteins, interacting with a defined DNA sequence, and the gene encoding the restriction enzyme.
Each zinc finger recognizes a short specific sequence (three nucleotides), the association of several suitable zinc finger domains makes it possible to uniquely target the sequence of interest.
Effector nucleases of transcription activator type
These are artificial enzymes created by merging a specific DNA binding domain (succession of TAL effectors) and the catalytic domain of the enzyme. The TAL effectors are proteins secreted by the phytopathogenic bacterium Xanthomonas from the Pseudomonadaceae family (Cf Pseudomonadaceae). The protein sequence of an effector TAL varies at the level of two amino acids involved in the specific recognition of a particular nucleotide in the genome.
Consequently, by combining different TAL effectors, the DNA binding domain of the TALENs can recognize a genomic sequence of interest.
Unlike ZFN and TALEN, meganucleases recognize very long DNA sequences. They split the double strand of DNA at highly specific sites, but these sequences are naturally defined and do not lend themselves to modification by genetic engineering; they can therefore only be used in very specific cases.
Screenings of random constructions of meganucleases have been used to select the most specific of target genes.
The CRISPR – Cas9 system
In this genetic construction, three sequences have a specific role: 1- the Cas 9 DNA sequence which allows the cell to produce the Cas9 enzyme; 2- the Tracr sequence (trans-activating crRNA) which will produce a small non-coding RNA; 3- the Crispr sequence which will produce the guide RNAs. The latter, by hybridizing with Tracr RNA and by clinging to the Cas9 enzyme, will guide the protein to the place of the genome to be cut.
The guide RNAs are complementary to a target sequence (protospacer) located upstream of a short PAM sequence (Protospacer adjacent motif) which is itself placed upstream of the cut point of the double strand of DNA.
This tool has several advantages compared to the best ZFN and TALEN nucleases: simplicity, speed (developed for a particular gene in less than two weeks) and cost (approximately ten times less).
Applications of genome editing systems
Genetic editing constructs must be carried to and within cells. The vectors used are often modified viruses added to the cultures of the target cells or injected in vivo.
Editing the genome makes it possible to build modified stable cell lines: abolition, gene fusion, point mutations, gene substitutions. Transfers are permanent and transferable.
Selection genes are no longer needed. These methods can be applied to all living species, as well as somatic cells as well as germ cells. In the agronomic field, genomic editing is used to genetically modify species of rice, tobacco, wheat, sorghum, corn, tomato, orange, etc. CRISPR / Cas9 is increasingly used for the simultaneous modification of several copies of genes in many cultivated species for which genome duplication events are relatively frequent (eg tetra- or hexaploid wheats).
Using the same principle, genome editing systems allow the elimination of different copies of susceptibility genes (for example, downy mildew), in order to limit the use of fungicides and pesticides.
Genome editing is widely used in animal transgenesis. All species are concerned, down to the closest to humans, both as research tools and as agri-food applications. Thus, CRISPR / Cas9 has been used to produce pigs in which one of the genes necessary for infection by a virus (PRRSv, porcine reproductive and respiratory syndrome virus) has been inactivated, making these pigs resistant to this infection which represents a major veterinary health problem.
Therapeutic approaches are also developed in experimental models (notably murine) of genetic or acquired pathologies. The first medical applications are in the clinical trials stage. This is particularly the case with the modification of stem cells in the bone marrow of patients suffering from AIDS and made resistant to the HIV virus by abolition of the CCR5 receptor gene by zinc finger nucleases.
Controversial work, announced by a Chinese team, even relates to human embryos (for the moment not viable).