The robust gene editing tool CRISPR has conferred a never-before-experienced freedom in selectively modifying genes with a high degree of precision. It has opened up new possibilities for curing devastating diseases such as amyotrophic lateral sclerosis (ALS) and Huntington’s disease, enabling novel immunotherapies, generating tailor-made model organisms, and improving crop yields, to name just a few potential applications. As CRISPR has been in the news due to the controversial gene editing of human embryos in late November 2018, let’s discuss its applications and challenges, along with an outlook on the future of this technology.
A message of hope:
Here are just some of the examples of countless CRISPR applications on the horizon.
Treatment of blood disorders
Inching one step closer to the treatment of rare blood disorders including sickle cell anemia and β-thalassemia, CRISPR clinical trials have begun in Europe. The trials will be carried out in Germany and are sponsored by Boston-based Vertex Pharmaceuticals and the Swiss biopharmaceutical company CRISPR Therapeutics, which has labs in Cambridge, MA. The success of these trials could signal a much broader use of CRISPR in medicine. Patients with sickle cell anemia have inflexible red cells that can burst easily, leading to anemia, shortage of red blood cells in the system, and fatigue. These diseases have no known cure and require blood transfusions as the only available mode of treatment.
The experimental therapy known as CTX001 is designed to investigate whether CRISPR-modified cells can lead to increased production of hemoglobin. Increased levels of hemoglobin should mitigate the effects of these disorders. The therapy will be tested ex vivo, which means that blood cells will be removed from the patient, modified using CRISPR, and then reinserted in the patient. These clinical trials have raised a worldwide interest in the application of CRISPR to treat human diseases, and other likely candidates for successful trials could be eye disease and liver disease. These trials offer hope for millions of people and could serve as harbingers of similarly revolutionary treatments for other heritable diseases.
Therapy for cancer and immune disorders
Cancer immunotherapy has attained limited success due to the time-consuming efforts in making modified viruses to deliver genetic material in cells. The most advanced method involves engineering the immune system’s T cells, which can kill tumor cells in the host body. In a significant discovery, scientists have developed a non-viral CRISPR-Cas9 genome-targeting system, that uses electrical fields to deliver genetic material in human T cells. Preclinical evidence demonstrates that the technique is rapid and flexible for treating a variety of cancers. Using this strategy, the scientists engineered T cells to recognize human melanoma cells. In mice carrying the human tumor cells, the engineered T cells identified tumor cells and killed them. The technique was also used in the lab to correct a rare mutation in cells from patients with autoimmune disease. The researchers plan to return these edited cells to patients to restore normal cellular function. The technique may also be used to treat HIV in patients. Since the HIV virus infects T cells, CRISPR could be used to make T cells resistant to the virus. The engineered cells would replace the infected ones and an HIV infection would not progress to AIDS.
In another promising approach to treating cancer, CAR T cell therapy could receive a boost by combination with CRISPR. CAR T cell therapy begins by harnessing the patient’s T cells, engineering them in a lab to kill cancer cells, and implanting them back into the patient’s body. However, several challenges have limited the clinical development of the approach against cancer, including difficulties harvesting non-malignant T cells from affected patients and the “self-killing” of CAR T cells. CRISPR has the potential to resolve the challenges in the current CAR T cell treatment. Upcoming clinical trials in 2018 intend to combine CRISPR with CAR T therapy for tumor destruction in 18 patients with myeloma, sarcoma, and melanoma, leading the way for a more potent cancer cure in clinics. Thus, the combination of CRISPR-modified T cells without viral delivery could be a boon to patients suffering from cancer and immune disorders.
Malaria-resistant mosquitoes
Malaria is a leading cause of mortality in developing countries and is classified by the World Health Organization as a health crisis. Researchers have used CRISPR to delete a gene called FREP1 from the genome of malaria-transmitting mosquitoes. Experiments have demonstrated that the malarial parasites did not multiply within the modified mosquitoes. However, the results also showed a slow maturation of FREP1-modified mosquito larvae to adults. The team is now working on inactivating FREP1 in adult mosquitoes and investigating other genes that could make them a good host for malarial parasites. This could eventually lead to a gene drive, an engineered genetic system that could bypass the traditional rules of sexual reproduction and increase the chances of rendering a new population of edited mosquitoes that are unable to transmit malaria to a host.
ALS and other neurodegenerative diseases
ALS, also known as Lou Gehrig’s disease, is a progressive motor neuron disease with no known cure. A team of researchers have used CRISPR to systematically knock out genes in the human genome to identify potential drug candidates for ALS. The researchers identified a knock-out of the gene TMX2 that increased the survival rate of mouse neurons with ALS. TMX2 is therefore a promising drug candidate for treating ALS.
In another experiment, researchers used CRISPR to edit and disable the mutant SOD1 gene in the spinal cord of mice with ALS. SOD1 mutations are a leading genetic cause of familial ALS. This approach resulted in increased survival of the mice with ALS. They are currently working to program a self-destruct mechanism for the Cas9 protein, to prevent any off-target effects or immune response after editing the target gene. Another group of researchers used CRISPR to identify a deficiency in the protein STIM1 in brain tissue of patients with Alzheimer’s. These examples show how CRISPR is being employed to identify drug candidates and develop a model to reflect the mechanisms involved in the progression of various neurodegenerative diseases.
Food and agriculture
The global population is expected to double by 2050, which would put huge demands on the current agricultural pipeline to pump out more food for the masses. CRISPR can make farming more efficient and increase crop yields. Companies like Mars Inc. are using CRISPR to make disease-resistant cacao. Research groups have used CRISPR to increase the yield of tomatoes, corn, and wheat. Moreover, these crops are safe for consumption as no foreign DNA gets introduced into the plants. Recently, scientists used CRISPR to make water-efficient plants without compromising their yield. Researchers modified a gene to increase the expression of the photosynthetic protein PsbS in tobacco plants. The modified plants will also yield more crops than nonmodified plants in case of water shortage. PsbS is found in all plants and therefore this modification should function similarly in other crops. The team is currently testing to improve the water efficiency of other food crops.
A message of caution:
With the revelation by the Chinese scientist He Jiankui on November 26, 2018, of using CRISPR to make the world’s first genetically edited human embryos, questions on the regulation of germline editing have risen to the forefront of debate. The scientist altered a gene called CCR5 that confers HIV resistance in the embryos before implanting them in the mother’s womb. The research hasn’t been published in any journal and the data haven’t been validated yet. However, the news has generated a storm in the world of biotechnology.
The technique is also under the scanner for inadvertently altering other genes when introduced in cells and possibly causing cancer by inactivating a programmed cell-death pathway in healthy cells. Two studies published in Nature Medicine earlier this year have raised concerns that CRISPR-edited cells are at an increased risk of developing cancer. Further, another recent study found that a majority of human beings contain antibodies against bacterial Cas9 proteins, possibly due to frequent bacterial infections during a human being’s lifetime. This discovery has raised concerns about the applicability of CRISPR for treating patients in the clinic.
What lies ahead?
CRISPR applications are only going to grow in the near future, attracting more investments and possibly tighter regulation. Despite ongoing ethical and technical challenges surrounding applications of this technology, it will be no surprise if, in the years to come, CRISPR-altered food products arrive on grocery shelves, novel therapies for various diseases continue to expand, and new applications arise that no one has even imagined yet. But one thing is for sure: CRISPR will make an indelible impact on our society.
