From the science fiction of "Jurassic Park" to the reality of genetically modified foods, the idea of customizing plants, animals and even human genomes has held our fascination for decades. This obsession is not surprising considering the remarkable impact of genetic engineering on our lives, and the promise it holds for our future. Enhanced “golden rice,” which provides your daily dose of Vitamin A, modified poplar plants that help reduce pollution, and engineered human insulin for diabetics are a few ways genetic engineering has improved the quality of human life.
But, as you would imagine, modifying the genes of an organism is not easy. It’s not as simple as removing the parts you do not like with an eraser. It involves making changes at a molecular level that are both complex and challenging.
Historically, this has been the problem with genetic engineering: it is tedious, time consuming and not very efficient. “Genome editing” represents a newer branch of genetic engineering that attempts to correct these problems. It’s much simpler and far more efficient than the traditional methods of genetic engineering.
Genome editing utilizes “nucleases,” which are proteins that function like molecular scissors and have the ability to cut DNA at specific locations. Each nuclease enzyme has a unique sequence of DNA that it recognizes and cuts. This allows scientists to tailor the genetic material by adding, removing or substituting parts of the genome. Using this technology, one could remove unhealthy, bad parts of the genome and add in better, healthy versions of the DNA, revolutionizing the way we treat diseases in humans.
Scientists have endeavored to develop safe and reliable approaches to genome editing to aid scientific research, as well as medicine. Recently, researchers at Stanford University, Harvard University, the Massachusetts Institute of Technology and other leading scientific institutions around the world have developed a novel method of genome editing called CRISPR-Cas.
CRISPR, which stands for Clustered Regularly Interspaced Short Palindromic Repeats, utilizes a nuclease protein called Cas9. What is special about the Cas9 nuclease? It is smarter than other nucleases! The region of DNA that Cas9 binds to depends on another molecule called the guide RNA. Think about the position on the genome that Cas9 needs to be transported to as an address, which is represented by a DNA sequence. The guide RNA has this DNA address programmed into it and functions as a GPS to direct the Cas9 to this location on the DNA. What is the advantage of this peculiarity of Cas9? It allows scientists to choose where Cas9 gets targeted to on the DNA. By changing the DNA address on the guide RNA, we can now direct our Cas9 to any desired location on the genome.
Let's say we are trying to cure a disease that we know is caused by a defective gene. We can specifically target our Cas9 using the appropriate guide RNA to the defective gene on the DNA where it can be made to either stop the gene from functioning or replace the gene with a healthy copy. This is possible because, once at the desired target site, the Cas9 protein cleaves the DNA on both strands. Immediately, the cell recruits its repair machinery to come attach the cleaved DNA back together.
However, before the repair is made, the system can then be manipulated to remove a few nucleotides at that site, which would lead to loss in the ability of the gene to function. Or, the system can insert/replace DNA sequences in the gene. In either case, the end result would be a genetically modified “tailored” organism and, hopefully, a cured patient.
Though the technology is not yet advanced enough to fully cure diseases, the CRISPR system promises great things for the future of stem-cell based gene therapy and regenerative medicine. Following the discovery of CRISPR-Cas, a company in Massachusetts called Editas Medicine is actively attempting to adapt this technology for use in a clinical setting to cure human diseases. Scientists have already begun working towards this by performing such editing experiments in human cell culture with great success.
The Cowan and Rossi research groups at Harvard have managed to edit human blood stem cells in such a way that they can no longer be infected with the HIV virus. The scientists involved in this research hope that one day these engineered cells can be used to cure patients of HIV. While much work still needs to be done for this dream to be realized, a future where a patient’s genome is directly edited to remove the disease-causing gene or treating patients with modified stem cells is not far away.
There have also been several immediate benefits of the CRISPR-Cas technology. It has provided scientists with an easy tool to make genetically modified organisms (transgenic animals).
Model organisms are integral to successful scientific research. Like the name suggests, they allow scientists to model disease symptoms to understand the cell and molecular processes that underlie them. How do scientists do this? They induce mutations in animals that resemble ones that cause disease in humans. This provides them with a “model” to study physiological, molecular and behavioral changes that go hand-in-hand with the disease, bringing them one step closer to curing it.
However, making these mutations was very laborious until the CRISPR-Cas system was discovered. Now, scientists can easily induce mutations in their gene of interest and use it to study the effects, even in model organisms like frogs and chimpanzees where it was previously almost impossible to make transgenic animals.
In a similar manner, CRISPR-Cas stands to make an enormous impact on agriculture owing to the effortlessness with which one can make genetically modified crops. Recently, a researcher in China successfully used the CRISPR-Cas system to make a genetically modified species of wheat that is resistant to fungal infection, thus eradicating the need to use poisonous and environmentally harmful fungicide.
The unprecedented ease and efficiency of the CRISPR-Cas technology has truly revolutionized the field of genome editing providing a powerful tool for genetic manipulation for basic research, as well as clinical and agricultural applications.
But, with a technology that is developing so rapidly, caution and restraint need to be exercised with use. Higher the ease of use, greater is the fear and possibility of misuse. As with any genetic engineering technique, CRISPR-Cas has raised several ethical concerns in both the scientific community as well as general public. The greatest concern is that what starts with editing human genomes for curing disease can quickly degenerate to editing for convenience or pleasure.
One particular concern among scientists is modifications made in human germline cells – i.e. the sperm or the egg. These are genetic changes that would actually be passed on through generations and scientists fear there could be unexpected side effects to this. Additionally, there are the ethical concerns of people misusing this technology to intentionally modify the genome to make “designer babies” with enhanced characteristics. These are some real concerns that are causing scientists to pause and weigh the pros and cons of use of this technology.
In spite of these disputes, no one can deny that CRISPR-Cas holds more promise than any previous technology for successful genome editing. Though we may not yet have a race of superhuman babies or dinosaurs raised from the dead, it seems that genome editing is slowly turning science fiction into a reality.