In the past 18 months, 11 gene editing research and development projects have entered the clinical development stage in the United States or the European Union, 6 of which are based on the CRISPR-Cas gene editing system. Recently, a review published in Nature Reviews Drug Discovery conducted an in-depth inventory of the gene editing R&D pipeline.
When it comes to gene editing technology, the first will be the CRISPR-Cas gene editing system which utlizes guide RNA to allow Cas enzymes to identify specific sequences in the genome, thereby accurately cutting DNA or RNA sequences. Because of its simplicity and programmability, the CRISPR-Cas gene editing system has been widely used in academia and R&D since its introduction. However, CRISPR is not the only tool for gene editing. At present, a number of R&D projects based on other gene editing technology platforms have also undergone clinical development, one of them is Zinc-finger nucleases (ZFNs). These endonucleases do not rely on RNA, but rely on zinc finger proteins to recognize specific DNA sequences.
Sangamo has accumulated a lot of experience in developing gene editing systems based on zinc finger nucleases. Because zinc finger proteins need to be modified to change the specificity of ZFN's targeting sequence, zinc finger nuclease-based gene editing systems are not as simple as CRISPR gene editing systems. However, the ZFN gene editing system also has its own advantages. For example, it is not a protein derived from bacteria, so it may be less likely to stimulate the body's immune response when used in the body. Moreover, the protein domain of ZFN recognition DNA sequence is smaller than other gene editing systems, and it is easier to fit into the vector for delivery of gene editing systems.
In addition to CRISPR and zinc finger nucleases, other gene editing technology platforms include TALEN, meganuclease, and megaTAL technology that integrates TALEN and meganuclease construction. They all have their own advantages and disadvantages, and can also play a role in different application scenarios.
The initial gene editing R&D project focused on editing cells in vitro. Editing the genome of cells in vitro can avoid many obstacles that plague gene editing. Taking CRISPR as an example, a major obstacle to gene editing is how to deliver larger gene editing systems into cells, while in vitro gene editing can use electroporation to open the barrier of the cell membrane, allowing the gene editing system to enter the cell relatively easily.
Two other hurdles that CRISPR gene editing technology needs to overcome are preventing the "off-target effect" of gene editing and the immunogenicity of Cas protein. Gene editing of cells in vitro can circumvent the immune response of the human body, and can detect the edited cells to find and remove cells that have "off-target effect", improving the safety of potential treatments. At present, in vitro gene editing R&D projects are mainly concentrated in the two major fields of hemoglobin disease and cancer immunology. Whether it is hemoglobin disease or blood cancer, the success of existing gene therapy and cell therapy has paved the way for the development of innovative therapies based on gene editing. Meanwhile, they also represent the unmet medical needs of patients.
In vivo gene editing therapy requires the gene editing system to be delivered into the patient's body to complete the editing of specific sequences in the genome. Compared with in vitro gene editing, it needs to overcome more obstacles, and its application fields are also more diverse.
In the gene editing R&D pipeline, the diseases treated by many projects are rare diseases. This is because as an emerging technology, the safety of gene editing has not been fully verified. Therefore, the benefits of treating rare patients without existing therapies or risk ratio are more reasonable. However, as more clinical trial results verify its safety, gene editing technology is likely to expand to more common diseases with a larger number of patients.
In next two years, multiple gene editing clinical projects will announce the results. These clinical results will have a great impact on the short-term future of gene editing therapy. At the same time, researchers have made important progress in further enriching the "toolbox" of gene editing. For example, Professor David Liu of the Broad Institute developed a single-base editor based on the CRISPR gene editing system, which can modify any base in the gene sequence into other bases. This single-base editing system can be used to treat a variety of genetic diseases caused by single-base mutations.
Another research and development direction for gene editing systems is to use its ability to recognize genomic sequences, instead of cutting DNA sequences, but make it carry regulatory factors that can regulate gene expression. For example, the CRISPR-dCas9 system has no activity to cut DNA sequences, but it can be fused with effectors that regulate gene expression to regulate the expression of specific genes.
Since the advent of gene editing technology, people have made many ideas about what innovations it can bring to the treatment of diseases. After years of hard work, the results of clinical trials showing the potential of gene editing therapy may be announced in the near future.