2025-01-31
Gene editing is transforming treatment for hereditary diseases, with new therapies gaining approval. As clinical trials advance, careful risk assessments are crucial to ensure safety and long-term success in genetic disorder treatments.
Gene editing using CRISPR technology is a groundbreaking approach to gene therapy that allows for precise changes to the human genome.
The field has advanced at an astonishing rate. Numerous clinical trials are underway, and the first treatment was recently approved in both the U.S. and the EU.
Due to technical challenges, research has primarily focused on blood and liver disorders.
Gene editing can be performed in different ways, and it remains to be seen which methods will prove most effective, with the fewest and least severe side effects.
Gene editing refers to the process of altering the genetic material in a predefined manner. It is a form of precision therapy that enables changes to be made to the genes in somatic cells. Gene editing in germ cells is not currently a focus, and the need for it is limited since preimplantation genetic diagnosis (PGD) allows for the selection of healthy embryos in nearly all cases. The most widely used technology for gene editing today is based on the 2020 Nobel Prize-winning discovery of the ”gene scissors” (CRISPR/Cas). Over the past few years, there has been an explosive development in the field, and there is no indication that the pace will slow down. In the U.S., several pharmaceutical and tech companies have been founded with a focus on gene editing. A growing number of clinical trials are underway, and approved treatments for blood disorders are already available.
Gene editing is a technology-intensive field, and this article provides an overview of the available techniques today, along with a look into potential future advancements.
As early as 2005, researchers, in collaboration with the American company Sangamo Therapeutics, demonstrated that it was possible to repair a defective gene responsible for severe combined immunodeficiency. The technique used was based on a modular system of genetically modified proteins, where specific amino acid sequences form finger-like structures called zinc fingers by coordinating a zinc atom. Zinc fingers bind to double-stranded DNA, and the technique has been studied in several diseases. However, it is complex, labor-intensive, and has not yet led to an approved drug.
Another gene editing method, called TALEN (Transcription Activator-Like Effector Nucleases), is based on bacteria that infect plant cells. TALEN is easier to use than zinc fingers, and shortly after its study began, the discovery of the CRISPR/Cas system was made. As CRISPR is much simpler to use, research quickly shifted almost entirely to that technology, and TALEN never gained the momentum many had anticipated. While development continues for both zinc fingers and TALEN, the progress is modest, so these technologies are not covered further in this article.
The CRISPR/Cas system, as we and others have previously described, originated in bacteria. For a more comprehensive review of precision techniques for genetic modification, including the Nobel Prize-winning technology that specifically deactivates genes, refer to our previous work. Bacteria’s worst enemies are viruses called bacteriophages, which, together, are the most abundant organisms on Earth. To defend against bacteriophages, bacteria have developed several mechanisms, one of which is the CRISPR/Cas system. Cas is an enzyme that cuts both strands of DNA, while RNA guides the system to the matching area in the genome. In clinical gene editing, RNA is replaced by synthetic guide RNA (gRNA).
In recent years, three new CRISPR-based gene editing technologies have emerged, described schematically in Figure 1. The first method (Figure 1A) closely resembles the bacterial defense mechanism and involves cutting both DNA strands in a chromosome using the CRISPR ”gene scissors.” When the chromosome breaks, the cell’s repair enzymes often introduce genetic changes, which can be beneficial if the goal is to deactivate a gene. Intellia Therapeutics has recently developed a CRISPR-based technology for the treatment of Transthyretin Amyloidosis, which has already been used in clinical trials, including for Swedish patients.
The next method, called base editing (Figure 1B), targets only one DNA strand to create a “nick,” leaving the other strand intact. This is done by using a fusion of catalytically inactive Cas and the enzyme deaminase. Cytosine deaminase changes cytosine to uracil, which is then converted to thymidine in the DNA, i.e., C→T. Similarly, adenosine deaminase changes adenosine to guanine, i.e., A→G. However, base editing is currently limited in the types of nucleotide changes it can produce. Intense research is underway to expand its capabilities.
The latest technology, called prime editing (Figure 1C), does not require a double-strand break but can be much more efficient when both DNA strands are cut. By combining a primer with the enzyme reverse transcriptase, prime editing could theoretically achieve almost any sequence change. The main limitation is the length of the new DNA that can be synthesized, currently about 50 nucleotides. Prime editing is still in its early stages, and its clinical effectiveness remains to be fully determined.
An additional possibility is to correct disease-related cellular processes by introducing epigenetic changes, known as epigenome editing. This method uses modified gene scissors without cutting DNA or altering nucleotide sequences. Epigenetics plays a central role in gene expression regulation, and gene editing that targets chemical modifications of chromatin could influence cells. These changes may be preserved after cell division, although there is still limited understanding of what happens when these modifications are introduced through epigenome editing.
Gene editing is not only a form of treatment on its own but also a tool to improve and develop existing therapies. Chimeric Antigen Receptor (CAR)-T cell therapies are already used to treat B-cell malignancies, including myeloma. Gene editing can enhance CAR-T cells by introducing the chimeric receptor more precisely into the genome. An exciting new development is allogeneic CAR-T cells, which do not require the patient’s own T cells. Once fully developed, this technology will speed up treatment delivery and likely reduce costs.
As we reported earlier, the first clinical study using gene editing for hereditary diseases was published in the New England Journal of Medicine in January 2021, involving patients with sickle cell anemia and thalassemia. Vertex Pharmaceuticals, in collaboration with Crispr Therapeutics, was the first to have a gene editing-based drug approved, receiving approval in the UK on November 16, 2023, from the Medicines and Healthcare products Regulatory Agency (MHRA) for the treatment of sickle cell anemia and thalassemia. The treatment was approved in the U.S. in December 2023 and in the EU in February 2024. The drug, Casgevy (exagamglogen-autotemcel), works by deactivating the patient’s BCL11A gene in hematopoietic stem cells, allowing the production of fetal gamma-globin, which prevents the disease.
In Sweden, gene editing is already making an impact. Patients with transthyretin amyloidosis have been treated at Norrlands University Hospital with drugs produced by Intellia Therapeutics. Gene editing complexes are delivered into the blood, where they are absorbed by liver cells for editing. In March 2024, the first patient in a Phase 3 study of transthyretin amyloidosis and cardiomyopathy was treated. Several research projects on gene editing are underway at universities and hospitals, including Karolinska Institute, where a focus is on developing gene editing treatments for hereditary immunodeficiencies and enabling cost-effective production of treatments for sickle cell anemia.
Despite its promise, gene editing is still a young technology, and a comprehensive clinical risk assessment is not yet possible. While the number of treated patients is increasing rapidly, observation periods remain short. However, no acute toxicity has been reported. One potential risk is off-target edits in the genome. With large-scale DNA sequencing, we can study these risks in advance, but it is difficult to generalize due to genetic variations among individuals. Double-strand breaks in the DNA may lead to faulty repairs, and if multiple breaks occur simultaneously, it can cause chromosomal translocations. Some of these translocations could reduce the cell’s survival, while others may lead to cancer. Techniques like base editing and prime editing, which typically do not induce double-strand breaks, may present fewer risks. Continued research and follow-up studies are necessary to evaluate the safety of different gene editing methods. The safety concerns must also be weighed against the urgency of delivering life-saving treatments for serious diseases.
C I Edvard Smith, Professor, Senior Consultant, Karolinska ATMP Center; Department of Laboratory Medicine, Karolinska Institute
Rula Zain, Associate Professor, Center for Rare Diseases (CSD), Clinical Genetics and Genomics, Karolinska University Hospital; Karolinska ATMP Center; Department of Laboratory Medicine, Karolinska Institute
Pontus Blomberg, Associate Professor, Director, Karolinska Center for Cell Therapy (KCC), Karolinska University Hospital, Stockholm; Karolinska ATMP Center; Department of Laboratory Medicine, Karolinska Institute