Genetic Therapy of Cystic Fibrosis: The Treatment of Tomorrow
Author: Ivana Mišová, PhD.
Published at: 12/16/2019
Genetic therapy is the greatest hope of people with genetic diseases, cystic fibrosis (CF) included. CF is caused by various mutations in the CFTR gene, which results in a defective CFTR protein. If we could get the correct CFTR protein into the affected cells, the problem would be solved. There are many laboratory techniques used to add or change genes in model organisms, so why not in humans? Fixing the problem in one's DNA might sound simple in theory, yet there are many challenges to overcome.
To restore the function of a mutated CFTR protein, there are two main genetic therapeutic approaches being developed at the moment. In one (gene editing), the mutated DNA template itself can be modified to produce the correct protein. The other (gene therapy) often leaves the original mutated DNA template intact and introduces an additional template, which produces the correct protein.
Preparing the appropriate CFTR template is not the issue. However, getting it into the cells is more challenging. First, the template needs a delivery vector to enter the cells. For this, either viruses or small particles are used. Viral vectors no longer have their original harmful properties but instead carry the template, which is introduced into the cells upon “infection”. Small liposomal particles have a similar composition to the cell membrane, so when the two come into contact, they merge and the template gets inside the cell.
Second, it is important that the vector carrying the template gets to the appropriate cells. Genetic therapy is beneficial only to the cells that received the correct template. When using viral vectors, it is important to identify those that can specifically infect the type of cells we need. In CF, it is important to get it to the cells in the airways, but the thick, sticky mucus typical for this disease makes getting through difficult. Furthermore, it would be best to target stem cells that give rise to other airway cells1.
Gene editing aims to modify the original defective DNA sequence. The leading approach to gene editing is via the CRISPR system. This system combines a guide molecule, a DNA-cutting enzyme, and the desired DNA template. The guide molecule finds and marks the defective gene location for editing, the enzyme cuts the DNA, and then the gap in the DNA needs to be repaired. In most cases, the ends are joined together, which is not the desired way of repair for gene editing. The minor way of repair uses the introduced DNA template, which results in an edited gene2.
Unlike CFTR modulators that are useful only for some mutations, CRISPR could be customized for any of the CF-causing mutations. A huge appeal of gene editing lies in the possibility of a one-time treatment to alleviate the health of CF patients. Despite the initial doubts, this method can be used also in cells that no longer divide, which is valuable to the diversely dividing airway cells2,3.
A major disadvantage of this approach is that the dominant repair of the DNA gap could cause undesired mutations, which can convert a druggable CFTR mutation into non-druggable2. It would be a huge step back for the CF mutations that can be targeted with small-molecule drugs (see Available CFTR modulator drugs). Another risk when making permanent changes to the DNA is that the system must be very precise.
So, what is the current status of gene editing? It is being explored in the research of CF, hemophilia or sickle cell disease in pre-clinical studies4. One of them has shown that CRISPR was effective in restoring CFTR in several splicing mutations5. However, it is a relatively young branch of research and it will take some time till gene editing becomes available to CF patients.
Gene therapies are based on introducing the correct template to the cell – the template can be designed to integrate into the DNA (integrating gene therapy), or stay apart (non-integrating gene therapy and RNA therapy). These therapies differ by the duration of the therapeutic effect and by the risks they present. An undeniable appeal of gene therapy is that it would be universal – no matter your CF mutation, you could benefit from it!
Integrating gene therapy aspires to replace the faulty CFTR gene with a correct one. The correct DNA template is introduced to the cell and should integrate into the DNA. This way, it becomes a permanent part of the DNA. On one hand, it is advantageous from the therapeutic point of view – no repeated treatments. On the other hand, there is a risk of improper integration of the CFTR template, which could disrupt the genes around it and ultimately increase the risk of cancer. Non-integrating gene therapy brings a correct DNA template that stays apart from the rest of the DNA. Because it does not integrate, it does not disrupt any genes and there is no elevated risk of cancer. However, for the same reason, it is not permanent and frequent treatments would be required. RNA therapy skips a step by directly introducing the RNA formed from the correct DNA template. Similar to non-integrating gene therapy, RNA therapy does not disrupt the genome and would also require repeated treatments.
So how close are we to benefiting from gene therapy? Gene therapy for CF is not ready for patients yet, although many clinical trials are underway1. However, several gene therapy products targeting various conditions, ranging from a specific type of blindness to cancer, are already a reality for patients in the USA and Europe7-9.
- 1 - M. Donnelley and D. W. Parsons, “Gene therapy for cystic fibrosis lung disease: Overcoming the barriers to translation to the clinic,” Frontiers in Pharmacology, vol. 9, no. NOV. Frontiers Media S.A., 27-Nov-2018.
- 2 - C. A. Hodges and R. A. Conlon, “Delivering on the promise of gene editing for cystic fibrosis,” Genes and Diseases, vol. 6, no. 2. Chongqing University, pp. 97–108, 01-Jun-2019.
- 3 - J. Nishiyama, T. Mikuni, and R. Yasuda, “Virus-Mediated Genome Editing via Homology-Directed Repair in Mitotic and Postmitotic Cells in Mammalian Brain,” Neuron, vol. 96, no. 4, pp. 755-768.e5, Nov. 2017.
- 4 - https://ghr.nlm.nih.gov/primer/genomicresearch/genomeediting
- 5 - D. J. Sanz, J. A. Hollywood, M. F. Scallan, and P. T. Harrison, “Cas9/gRNA targeted excision of cystic fibrosis-causing deep-intronic splicing mutations restores normal splicing of CFTR mRNA,” PLoS One, vol. 12, no. 9, Sep. 2017.
- 6 - https://www.fda.gov/news-events/press-announcements/fda-approves-novel-gene-therapy-treat-patients-rare-form-inherited-vision-loss
- 7 - https://www.ema.europa.eu/en/news/new-gene-therapy-treat-rare-inherited-blood-condition
- 8 - https://www.fda.gov/vaccines-blood-biologics/cellular-gene-therapy-products/approved-cellular-and-gene-therapy-products