Gene Therapy Basics
When I’ve written about therapeutics in the past, I mainly focused on small molecules and peptides. But another branch of therapeutics is quickly becoming an important approach for treating disease: gene therapy. At its most basic description, gene therapy involves transferring genetic material (DNA or RNA) to a patient to treat disease. Compared to small molecules, gene therapy is a relatively recent development, with the first clinical study occurring in 1990. Since then, multiple gene therapy products have been approved by the FDA and are benefiting patients who wouldn’t otherwise have had a treatment.
Within gene therapy drug development, there are three major approaches to treating a disease. First, there’s gene augmentation. This is the approach for patients that have a non-functional gene, due to a mutational defect that leads to a disease pathology. An example of such a disease is the retinal condition Leber’s congenital amaurosis (LCA), which can be caused by a mutation in the RPE65 gene and results in a non-functional protein and loss of vision. To treat this type of disease using augmentation gene therapy, a gene is delivered into a patient and is taken up by the cells. The transferred gene contains the corrected gene sequence, providing the proper blueprint for the protein. The correct protein can then be made, restoring function and preventing the disease pathology.
The second type of gene therapy involves the suppression of an aberrantly expressed gene within a patient. Huntington’s disease is an example of where this type of gene therapy approach could be used. In this disease, patients have a mutation in the huntingtin gene, and the expression of this mutated gene results in a defective protein that accumulates and gradually damages brain cells. Gene therapy could be used to introduce inhibitory RNA sequences into the patient’s cells. Once inside the cell, these RNA sequences would silence the expression of the mutated gene and suppress the production of the aberrant protein.
The third major gene therapy approach emerged more recently and is called genome editing, an approach that you may have heard about in the news or in discussions around CRISPR gene editing. In this context, the gene therapy approach generally involves using molecular scissors to cut out the mutated gene, which can then be replaced with a corrected sequence. This contrasts with the other two types of gene therapy, which both involve the transferred DNA/RNA sequence existing in parallel to the mutated gene. Only in the gene editing approach is the mutated DNA actually removed and replaced.
While gene therapy can be used in different ways, a key component of these approaches is the delivery of the gene therapy product. This often occurs with specially designed viral vectors, which can be broadly categorized into predominantly integrating and predominantly nonintegrating vectors. Integrating vectors means that the new genetic material is incorporated into the cell’s genome. This can be desired if you’re trying to introduce the new gene into rapidly dividing precursor cells, like stem cells. By incorporating the new DNA into the genome of parental cells, you ensure that it will be passed down to the subsequent cells during cell division. In contrast, nonintegrating vectors wouldn’t be copied during the cell division process and therefore wouldn’t be passed down to progeny cell. So, nonintegrating vectors are often used to treat non-dividing cells, where genome incorporation isn’t necessary.
While there are a variety of viral vectors, including retroviral, lentiviral, adenoviral, and adeno-associated viral (AAV), the gene therapy field has been predominantly coalescing around two: lentiviral vectors and AAV vectors. Lentiviral vectors have RNA genomes and are integrating vectors. In contrast, AAV vectors have DNA genomes and are generally nonintegrating. The choice between lentiviral vectors and AAV vectors often comes down to the context of the gene transfer and whether it occurs ex vivo or in vivo.
Ex vivo approaches occur outside the body and involve giving the gene therapy vector to cells collected from the patient. Oftentimes, these cells are hematopoietic stem cells, the precursors for all the blood cell lineages. Because these cells readily divide and proliferate, an integrating vector is generally desired to ensure the new gene sequence is propagated through all the subsequent daughter cells.
Prominent examples of ex vivo gene therapy are the CAR T cell therapies for treating certain cancers. These approaches involve modifying a patient’s own T cells, a key immune cell type, so that they are able to identify and kill the patient’s specific cancer cells. During this process, T cells are initially collected from a patient, and then a new genetic sequence is transferred into them using an integrating vector. This new sequence encodes a specific chimeric antigen receptor (CAR), which is then expressed and displayed on the T cells’ surface. The CAR specifically recognizes and binds to patterns on the patient’s cancer cells, allowing the patient’s immune system to identify and eliminate the malignant cells. These highly customized gene therapies, including KYMRIAH® and YESCARTA®, have been approved by the FDA for treating certain types of lymphoma and leukemia.
In contrast, in vivo gene therapy approaches involve giving the treatment directly to a patient. The best example of an in vivo gene therapy is LUXTURNA®, which was the first in vivo gene therapy to be approved by the FDA. It was approved in 2017 for the treatment of a type of Leber’s congenital amaurosis (LCA). As mentioned above, this type of LCA involves a mutation in the RPE65 gene that results in a non-functional RPE65 protein. RPE65, as its name suggests, is found in retinal pigment epithelium (RPE), a cell layer in the retina involved in the visual cycle. Without a functional RPE65, the retina is unable to properly transduce light into electrical signals, resulting in progressive blindness. LUXTURNA® introduces a corrected RPE65 gene sequence to the RPE cells, resulting in the production of functional protein and improving the vision of the patient.
In addition to the approved gene therapies mentioned here, there are a host of in vivo and ex vivo gene therapies under development for treating a wide range of disease, including cancers, central nervous system disorders, immunologic conditions, neuromuscular diseases, and retinal degenerations.
As with other therapeutics, there are risks and challenges surrounding gene therapy. Some adverse events have been noted with CAR T cells, including off-target effects or on-target but off-tumor toxicity. Most prevalently, CAR T cells can lead to serious systemic inflammatory conditions. More generally, because integrating vectors insert new DNA into a cell’s genome, the process can result in complications if the newly inserted gene interrupts the function of another vital gene in the genome.
Additional challenges arise from the nature of the viral vectors themselves. Even though these vectors are modified and are non-dividing, they are still derived from naturally occurring viruses and therefore have the potential to trigger an immune response in the patient. This may lead to inflammation within the patient, but it could also render the gene therapy ineffective. If the body’s immune response eliminates the viral vectors before the gene can be transferred, there won’t be any therapeutic effect. Ongoing research is working to better understand our body’s relationship with these viral vectors, to identify the contexts where the vector induces an immune response and those where it’s immunologically silent.
Even with its challenges, gene therapy is an exciting field in drug development. Continued success with gene therapies will have major and long-lasting impact on the lives of patients across a multitude of diseases.
Special thanks to Qiagen and Todd Festerling for sponsoring the blog.