Muznah Khan
The World Health Organization has proclaimed the current outbreak of the Zika virus a public health emergency with potential repercussions around the world. This troubling virus is known to be transmitted from person to person through the bite of a mosquito, and at the moment there is no solid prevention method.
However, what if there was a way to stop this outbreak from spreading by genetically removing a mosquito’s ability to carry the virus, thereby ending its ability to transfer it to humans? This option would have been improbable a few years ago, but a gene-editing tool called CRISPR has made it a viable solution.
Overview of Genome Editing
Genome editing, the process of editing or changing an organism’s DNA (genetic makeup), has been around for many decades now. Various gene-editing methods have been successfully utilized to modify genes in animals and agricultural crops, even in humans. However, no past method compares to the efficiency of the CRISPR/Cas9 immune system process. CRISPR stands for “Clustered Regularly Interspaced Short Palindromic Repeats” and it is the newest and most promising gene editing method yet.
Zinc finger nucleases and TALENs were the main gene editing methods in use before CRISPR. Both of these work in similar ways to CRISPR but have significant drawbacks.
Zinc fingers are engineered proteins that can be programmed to target specific genes in an organism. These proteins have two ends: one end is tailored to recognize a specific DNA sequence while the other end cuts the DNA at a certain point. Each DNA edit done using this process required scientists to create a new zinc finger protein designed specifically for the targeted DNA sequence. This was a long, costly process that didn’t always produce positive results.
Soon after zinc fingers, the TALEN (Transcription Activator-Like Effector Nucleases) method was created. TALENs are proteins that work similarly to zinc fingers. While an improvement over zinc fingers, TALENs are too large to work with and take more time to construct making the method ineffective in time-sensitive scenarios as ones presented by disease outbreaks such as the Zika virus outbreak.
What Makes CRISPR Different?
In 2012, researchers studying bacteria observed their unique ability to fight viral infections using an immune system they called CRISPR. The researchers realized they could use the CRISPR system as a tool for highly targeted genetic engineering in other organisms.
Unlike previous methods, CRISPR does not require a series of steps that, if performed incorrectly, could be quite harmful. To target a new DNA sequence, scientists only need to create a new RNA strand (genetic cousin to DNA that attaches to DNA at its bases), not a whole new protein as needed for other gene-editing processes. In a study conducted by Spencer Knight and his team, it was shown that even though the Cas9 protein complex within CRISPR can attach to the wrong site at times, it is very uncommon for cleaving, the cutting of DNA, to occur. The protein quickly moves onto the next DNA segment not giving it enough time to cleave.
Simplified Cas9/CRISPR complex
How Does CRISPR Work in an Organism?
To understand how CRISPR works, let’s use the example of its interaction with viruses. With the CRISPR immune system, virally infected bacteria are able to use a protein called Cas9 which can detect, cut, and then degrade viral DNA. After viral DNA enters the bacterial cell, the cell creates complementary RNA, the genetic cousin of DNA, which matches the viral DNA at each of its bases. The RNA and Cas9 protein create a complex that searches through the cell’s entire DNA to find and cut matching viral DNA, inactivating the virus.
The CRISPR complex is customizable so researchers can program it, by creating new RNA strands, to recognize ANY DNA sequence in an organism’s cells, making it an important genome engineering method for all organisms, not just an immune system in bacteria.
Real World Applications
The flexibility and efficiency of CRISPR presents itself to many useful applications, especially in the fight against infectious diseases. In one of a number of successful projects, researchers have been able to use CRISPR to create mosquitoes resistant to the malaria parasite. They were able to remove a segment of mosquito DNA that allowed it to carry the parasite and replace it with a DNA sample synthetically engineered in the lab. 100% of these mutated mosquitoes’ offspring are resistant to the malaria-causing parasite that they otherwise would transfer to humans.
Similar to malaria, the Zika virus is spread by mosquitoes. Currently there is no conclusive way of stopping the spread of the virus.
But maybe there is.
Researchers have actually started considering CRISPR as a solution. Scientists could potentially remove the gene segment that gives mosquitos’ their ability to carry the Zika virus. Similar to the malaria experiment, these mosquitoes could become genetically unable to carry and transmit Zika virus.
Proving CRISPR’s versatility, further studies show CRISPR being successfully used in the editing of both plants and animals. Scientists in China have created goats with deletions in the gene that inhibits muscle and hair growth. These goats are better able to support the country’s meat and wool industries.
Also in China, Gao Caixia’s group has used CRISPR to disable four rice genes with positive results and removed a gene in wheat that could lead to plants resistant to powdery mildew. These plants are the first of many shown to be responsive to the Cas9/CRISPR technology.
CRISPR also promises significant advancements in disease control and gene therapy—the prevention of disease through alterations in an organism’s genome. From studies on animals, CRISPR could be used to treat blindness, HIV and many other diseases in humans. In addition to treating diseases, CRISPR could be used to enhance the human body.
Ethical Concerns
There have been ethical concerns raised about gene editing right from the earliest days of the technology, and CRISPR is no different. With the technology moving so quickly, many scientists feel it’s necessary to discuss the ethical implications of CRISPR.
One major concern is in editing the human genome. Is it right to edit human embryos and alter genes for generations to come? Is it even safe to do such a thing?
Another concern is the environmental impact genetically engineered organisms can have. Genetically modified organisms can be harmful to humans and other organisms if they are not watched carefully. Furthermore, mutated insects (i.e mosquitoes) can cause an imbalance in the environment and be detrimental to species around them.
Ethical concerns will always be a concern for every new technology as societies grapple with the pros and cons of scientific advancements. However, carefully considered regulations and guidelines can eliminate many safety and ethical concerns.
Like any new technology, more studies need to be conducted before widespread use of CRISPR is seen. Researchers are still trying to better understand the CRISPR system, but from what they have gathered, CRISPR is the best gene editing technology yet. And with millions of lives at stake, we can at least consider CRISPR’s promising role in combatting debilitating diseases such as malaria and Zika virus.
References:
- Doudna, Jennifer. “We Can Now Edit Our DNA. But Let’s Do It Wisely.” TED. TED Conferences, Sept. 2015. Web. 26 Jan. 2016.
- Pennisi, E. “The CRISPR Craze.” Science 341.6148 (2013): 833-36. Web. 25 Jan. 2016.
- Caplan, A. L., B. Parent, M. Shen, and C. Plunkett. “No Time to Waste–the Ethical Challenges Created by CRISPR.” EMBO Reports 16.11 (2015): 1421-426. Web.