Genome engineering
Genome engineering refers to the strategies and techniques developed in recent years for the targeted, specific modification of the genetic information – or genome – of living organisms.
It represents a very active field of research because of the wide range of possible applications, particularly in the areas of human health - the correction of a gene carrying a harmful mutation, the production of therapeutic proteins, the elimination of persistent viral sequences - agricultural biotechnology - the development of new generations of genetically modified plants - and for the development of research tools - for example, to explore the function of a gene.
Various modifications can be performed:
- Insertion involves introducing a gene into a chromosome to obtain a new function (for example to obtain a better drought-resistant plant) or to compensate for a defective gene, particularly by making it possible to manufacture a functional protein if the protein produced by the patient is defective (such as factor VIII in hemophilia A).
- Inactivation, or “knock-out”, is today mainly used in fundamental research to shed light on the function of a gene by observing the anomalies that occur as a result of its inactivation. It can also have other applications, for example to remove a persistent viral sequence from infected cells, or in agriculture to eliminate the irritant or allergenic properties of a plant.
- Correction aims to remove and replace a defective gene sequence with a functional sequence. This correction can be performed on a very short sequence, sometimes just a few nucleotides, such as in the case of drepanocytosis (sickle cell anemia). In plants, this manipulation can also help improve the properties of a species without the addition of foreign DNA, as in the work carried out by the company Cibus.
How does it differ from previous genome modification technologies?
Early technologies developed to insert a gene into a living cell, such as transgenesis, are limited by the random nature of the insertion of the new sequence into the genome. The new gene is positioned blindly, and may inactivate or disturb the functioning of other genes or even cause severe unwanted effects; it may trigger a process of cancerization, for example. Furthermore, these technologies offer no degree of reproducibility, as there is no guarantee that the new sequence will be inserted at the same place in two different cells.
The major advantage of genome engineering, which uses more recent knowledge and technology, is that it enables a specific area of the DNA to be modified, thereby increasing the precision of the correction or insertion, preventing any cell toxicity and offering perfect reproducibility.
Genome engineering and synthetic genomics (designing artificial genomes) are currently among the most promising technologies in terms of applied biological research and industrial innovation.
General principles
Early approaches to genome engineering involved modifying genetic sequences using only homologous recombination. Using a homologous sequence located on another strand as a model can lead this natural DNA maintenance mechanism to repair a DNA strand. It is possible to induce homologous recombinations between a cellular DNA strand and an exogenous DNA strand inserted in the cell by researchers, using a vector such as the modified genome of a retrovirus. The recombination phenomenon is flexible enough for a certain level of change (addition, suppression or modification of a DNA portion) to be introduced to the targeted homologous area.
In the 1980s, Mario R. Capecchi and Oliver Smithies worked on the homologous recombination of DNA as a “gene targeting” tool; in other words, as an instrument for the inactivation or modification of specific genes. Working with Martin J. Evans, they developed a process for the modification of the mouse genome by modifying the DNA of mouse embryonic stem cells in culture and injecting these modified stem cells into mouse embryos. Genetically modified mice generated using this method make useful laboratory models to study human diseases. This tool is now commonly used in medical research. The three researchers were awarded the 2007 Nobel Prize in Medicine for their work. [1]
However, modifying genomes using only homologous recombination remains a long and random process, with limited results and restricted potential for modifications. Researchers wishing to modify a gene in a specific way, but with a greater degree of efficiency and scope, are increasingly opting for “molecular scissors”. These are enzymes with specific properties which enable them to cut the long DNA strand precisely along the sequence to be modified, thereby triggering the homologous recombination process at the required location. Some of these enzymes are also able to stimulate the homologous recombination process at the required location.
Restriction enzymes commonly used in molecular biology to cut DNA interact with sequences of 1 to 10 nucleotides. These sequences, which are very short and generally palindromic, often occur at several sites in the genome (the human genome comprises 6.4 billion bases). Restriction enzymes are therefore likely to cut the DNA molecule several times. In their efforts to find a genome surgery approach offering a higher degree of accuracy and security, scientists therefore turned to more precise tools.
Targeted genome engineering can be performed by using enzymes which are able to recognize and interact with DNA sequences that are sufficiently long so as to occur only once, in all probability, in any given genome. The DNA modification therefore takes place precisely at the site of the target sequence. With recognition sites of over 12 base pairs, meganucleases and zinc-finger nucleases offer this degree of precision.
Once the DNA has been cut, natural DNA repair mechanisms and homologous recombination enable the incorporation of a modified sequence or a new gene.
The success of these different stages (recognition, cleavage and recombination) depends on various factors, including the efficacy of the vector that introduces the enzyme into the cell, the enzyme cleavage activity, the cell’s capacity for homologous recombination and probably the state of the chromatin at the given locus.
Meganuclease-based Engineering
Meganucleases, discovered in the late 1980s, are enzymes in the endonuclease family which are characterized by their capacity to recognize and cut large DNA sequences (from 12 to 40 base pairs)[2]. The most widespread and best known meganucleases are the proteins in the LAGLIDADG family, which owe their name to a conserved amino acid sequence.
These enzymes were identified in the 1990s as promising tools for genome engineering. However, even though they occur in nature and each one exhibits slight variations in its DNA recognition site, there is virtually no chance of finding the exact meganuclease required to act on a specific DNA sequence. Each new genome engineering target therefore requires an initial protein engineering stage to produce a custom meganuclease.
There are two possible methods for creating custom meganucleases:
- Mutagenesis involves generating collections of variants using a meganuclease with properties similar to the desired enzyme, then selecting these variants using high-throughput screening. This procedure can be optimized by adopting what are known as “semi-rational” methods, in which the structural data is electronically processed in order to focus the mutagenesis to the part of the enzyme that interacts with DNA and triggers the cleavage[3]
- Combinatorial assembly is a method whereby protein subunits from different enzymes can be associated or fused.[4]
These two approaches can be combined. Scientists from the French biotechnology company Cellectis have identified in the structure of several meganucleases the areas responsible for DNA cleavage and the areas that interact with specific DNA sites. By acting on these recognition sites, they have been able to generate variants that interact with different DNA sequences from those of the initial meganucleases, while retaining their ability to cut the DNA and their high degree of specificity.
A large bank containing several tens of thousands of protein units has been created. These units can be combined to obtain chimeric meganucleases that recognize the target site, thereby providing research and development tools that meet a wide range of needs (fundamental research, health, agriculture, industry, energy, etc.).
This technique has enabled the development of several meganucleases specific for sequences in the genomes of viruses, plants, etc., and the industrial-scale production of two meganucleases able to cleave the human XPC gene; mutations in this gene result in Xeroderma pigmentosum, a severe monogenic disorder that predisposes the patients to skin cancer and burns whenever their skin is exposed to UV rays.[5]
Another approach involves using computer models to try to predict as accurately as possible the activity of the modified meganucleases and the specificity of the recognized nucleic sequence[6]. The Northwest Genome Engineering Consortium, a US consortium funded by the National Institutes of Health, has adopted this approach with the aim of treating leukemia by modifying hematopoietic stem cells. The model’s prediction has been verified and guided by means of directed mutagenesis and in vitro biochemical analysis.
Zinc-finger nuclease-based Engineering
Zinc-finger proteins occur in several transcription factors. The zinc ion, found in 8% of all human proteins, plays an important role in the organization of their three-dimensional structure. In transcription factors, it is most often located at the protein-DNA interaction sites, where it stabilizes the finger-shaped motif. The C-terminal part of each finger is responsible for the specific recognition of the DNA sequence.
The recognized sequences are short, made up of around 3 base pairs, but by combining 6 to 8 zinc fingers whose recognition sites have been characterized, it is possible to obtain specific proteins for sequences of around 20 base pairs. It is therefore possible to control the expression of a specific gene. It has been demonstrated that this strategy can be used to promote a process of angiogenesis in animals[7]. It is also possible to fuse a protein constructed in this way with the catalytic domain of an endonuclease in order to induce a targeted DNA break, and therefore to use these proteins as genome engineering tools[8].
The method generally adopted for this involves associating two proteins – each containing three specifically chosen zinc fingers – with the catalytic domain of the Fok I endonuclease. The two proteins recognize two DNA sequences that are a few nucleotides apart. Linking the two zinc-finger proteins to their respective sequences brings the two endonucleases associated with them closer together. This means that they can be dimerized and then cut the DNA molecule.
Several approaches are used to design specific zinc-finger nucleases for the chosen sequences. The most widespread involves combining zinc-finger units with known specificities (modular assembly). Various selection techniques, using bacteria, yeast or mammal cells have been developed to identify the combinations that offer the best specificity and the best cell tolerance. A difficulty often encountered with zinc-finger nucleases is that they are highly sensitive to the actual genomic environment in which they are required to act. Unwanted cuts in the genome occur quite frequently[9].
Zinc-finger nucleases are research and development tools that have already been used to modify a range of genomes, in particular by the laboratories in the Zinc Finger Consortium. The US company Sangamo BioSciences uses zinc-finger nucleases to carry out research into the genetic engineering of stem cells and the modification of immune cells for therapeutic purposes[10], [11]. Modified T lymphocytes are currently undergoing phase I clinical trials to treat a type of brain tumor (glioblastoma) and in the fight against AIDS.11
References
- ^ " The 2007 Nobel Prize in Physiology or Medicine - Press Release" Nobelprize.org 16 Sep 2010
- ^ Stoddard BL (2006). Homing endonuclease structure and function. Quartely Reviews in Biophysics; 38(1): 49-95.
- ^ Seligman LM, Chisholm KM, Chevlier BS, Chadsey MS, Edward ST, Savage JH, Veillet AL (2002). Mutations altering the cleavage specificity of a homing endonuclease. Nucleic Acids Research; 30: 3870-3879.
- ^ Arnould S, Chams P, Perez C, Lacroix E, Duclert A, Epinat JC, Stricher F, Petit AS, Patin A, Guillier S, Rolland S, Prieto J, Blanco FJ, Bravo J, Montaya G, Serrano L, Duchateau P, Pâques F (2006). Engineering of large numbers of highly specific homing endonucleases that induce recombination to novel DNA targets. Journal of Molecular biology; 355: 443-458.
- ^ Redondo P, Prieto J, Munoz IG, Alibès A, Stricher F, Serrano L, Cabaniols J-P, Daboussi F, Arnould S, Perez C, Duchateau P, Pâques F, Blanco FJ, Montoya G (2008). Molecular basis of xeroderma pigmentosum group C DNA recognition by engineered meganucleases. Nature; 456(7218): 107-111.
- ^ Ashworth J, Taylor GK, Havranek JJ, Quadri SA, Stoddard BL, Baker D (2010). Computational reprogramming of homing endonuclease specificity at multiple adjacent base pairs. Nucleic Acid Research; 38(16): 5601-5608.
- ^ Ashworth J, Taylor GK, Havranek JJ, Quadri SA, Stoddard BL, Baker D (2010). Computational reprogramming of homing endonuclease specificity at multiple adjacent base pairs. Nucleic Acid Research; 38(16): 5601-5608.
- ^ Kim H-G, Cha J, Chandrasegaran (2007). Hybrid restriction enzymes : Zinc finger fusions to Fok I cleavage domain. Proceedings of National Academy of Sciences of the United States of America; 93: 1156-1160
- ^ Ramirez CL et al (2008). Unexpected failure rates for modular assembly of engineered zinc fingers. Nat. Methods; 5: 374-375
- ^ [Reik A et al (2008). Zinc finger nucleases targeting the glucocorticoid receptor allow IL-13 zetakine transgenic CTLs to kill glioblastoma cells in vivo in the presence of immunosuppressing glucocorticoids. Mol. Ther.; 16:S13-S14.]
- ^ Holt N et al (2010). Human hematopoitic stem/progenitor cells modified by zinc-finger nucleases targeted to CCR5 control HIV-1 in vivo. Nature Biotech.; 28:839-847.