By Eileen Feng, ’17

DNA, the Instructions of Life

Deoxyribonucleic acid, more commonly known as DNA, carries the blueprints for life, dictating how the cells in our body should work together to make life possible. When this instruction is altered, things often go awry. Mutations, or permanent changes in DNA, occur as a result of cellular replication errors or external environmental factors such as chemical exposure and radiation. While these mutations are beneficial in some cases, others are harmful and can lead to debilitating genetic disorders.

Genetic disorders encompass a wide variety of diseases, but are generally difficult to cure as rarely can mutations be removed from the DNA. Often times, such as in the treatment of cancer, damaged cells are targeted for elimination to prevent further proliferation. However, these treatments only slow down the progression of the disease, rather than actually eradicating its root causes. As a result, researchers are always actively searching for new therapies that may be able to permanently cure genetic disorders.

How CRISPR Works

In 2012, a novel technology named CRISPR (Clustered Regularly-Interspaced Short Palindromic Repeats) broke through for its potential to be a gene-editing tool in human cells. Originally an immune defense mechanism in bacteria, CRISPR acts to protect bacteria from infections by exogenous viruses (Jinek et al., 2012). When viral DNA enters bacteria, the bacteria’s immune system triggers the production of two RNA strands, one of which is a guide RNA that is sequence specific to the DNA of the invading virus. These two RNA strands form a complex with Cas9, a DNA cutting enzyme, allowing the guide RNA to then scan for and bind to its complementary viral DNA in the bacteria. When this occurs, Cas9 cuts the target DNA, disabling the virus, thus protecting the bacteria from imminent infection (McGovern Institute for Brain Research at MIT, 2014).

Scientists quickly recognized that this clever system could be adapted and used as a versatile tool for genome editing in essentially any organism by simply designing the guide RNA sequence to match the DNA target of interest. However, there is one caveat. Because Cas9 is only found in prokaryotes, it needs to be introduced into eukaryotic cells. This can be implemented using plasmid technology, which allows both Cas9 and the designed guide RNA to be inserted into the cell of interest. This results in a system that can very precisely and efficiently manipulate any piece of DNA.

What Can CRISPR Really Do?

The potential for CRISPR to genetically modify any DNA sequence has garnered international attention over the past four years. Current research shows that CRISPR not only allows deletion of targeted DNA sequences, but can also be used to insert any desired piece of DNA. Cutting out the “bad” gene and replacing it with its “normal” copy may be able to rescue the phenotype of a disorder. As a result, current scientific research is moving toward using CRISPR as a therapeutic for serious genetic diseases (Wu et al., 2013).

One of the applications of CRISPR currently being investigated utilizes the system for the treatment of AIDS. AIDS is a major public health issue that currently affects 36.9 million people worldwide (World Health Organization, 2015). Affected individuals are infected by the HIV retrovirus, which stays in the body and weakens the immune system. With treatment, the average life expectancy of a person with AIDS can reach 51.4 years, but without treatment, the number drops to just about 3 years post diagnosis, highlighting the crucial nature of finding effective treatments for this debilitating condition (Samji et al., 2013; AIDS.gov, 2015). In the search for a cure, researchers are turning to CRISPR. The idea is to silence HIV viral gene expression so that effects of the virus would essentially be eliminated from the body. In 2014, a group from the National Institute of Allergy and Infectious Diseases excised a 9709 base pair fragment of the HIV virus genome, rendering it inactive with no toxicity or off-target editing (Hu et al., 2014). Just this February, a lab successfully used CRISPR to eliminate the entire HIV-1 genome in infected T cells, the first to do so in human cells (Kaminski et al., 2016). What is perhaps even more exciting about this finding is that continued expression of Cas9 and the guide RNA in these cells can prevent future infections from the HIV-1 virus. With the potential to have both effective and long-term outcomes, these observations are just few examples of how CRISPR can transform the fields of biology and medicine.

Because of its versatility, CRISPR can be easily adapted as a treatment option in response to novel outbreaks. The recent Zika Virus epidemic has been declared a global health emergency by the WHO as more than 4000 cases of microcephaly cases (and counting) have been reported in Brazil that is likely linked to the outbreak (Charner, 2016; Tavernise & Mcneil, 2016). Compared to the 147 cases reported in 2014, this surge in microcephaly represents a 27-fold increase. Spread through infected Aedes mosquito species, the virus has reached as many as 20 countries including the United States. Scientists are now quickly responding to treatment demands for this disease, and are looking to CRISPR for this task. Biologist Anthony James at the University of California, Irvine, expressed that a CRISPR treatment can be developed “easily within a year” (Relegado, 2016). Likewise, other experts are optimistic about gene therapies, suggesting that they are likely to develop quicker than vaccines (Relegado, 2016). However, gene-driven therapies such as CRISPR are so potent that they may drive the Aedes species to extinction, an issue that raises substantial ethical concerns from many.

Ethical Concerns

In less than four years, the development of CRISPR has proven to “transform labs around the world” says Jung-Ruey Joanna Yeh, a chemical biologist at Massachusetts General Hospital (Waltz, 2016). An easy-to-use technology, CRISPR is expanding our ability to fiddle with human nature. Consequently, this has sparked several ethical debates about its use. As some enthusiasts faithfully wait for the first human clinical trial using CRISPR, others caution its development. Should we be allowed to select out the “bad” genes? Can we eventually produce CRISPR babies? At what point should we stop? These are all valid questions with no definite answers. What we do know so far at least, is that CRISPR can have a profound effect on the future of medicine and has the potential to improve the health outcomes of generations to come.

 

References

AIDS.gov (2015). How does HIV progress in your body? Retrieved from https://www.aids.gov/hiv-aids-basics/just-diagnosed-with-hiv-aids/hiv-in-your-body/stages-of-hiv/

Charner, F. (2016, January 28). Zika virus victims: Children born with disorders. CNN. Retrieved from http://www.cnn.com/2016/01/27/health/the-children-of-zika/

McGovern Institute for Brain Research at MIT. (2014, November 5). Genome Editing with CRISPR-Cas9 [Video file]. Retrieved from http://mcgovern.mit.edu/news/videos/genome-editing-with-crispr-cas9/

Jinek, M., Chylinski, K., Fonfara, I., Hauer, M., Doudna, J. A., & Charpentier, E. (2012). A programmable dual-RNA–guided DNA endonuclease in adaptive bacterial immunity. Science337(6096), 816-821.

Hu, W., Kaminski, R., Yang, F., Zhang, Y., Cosentino, L., Li, F., … & Mo, X. (2014). RNA-directed gene editing specifically eradicates latent and prevents new HIV-1 infection. Proceedings of the National Academy of Sciences111(31), 11461-11466.

Kaminski, R., Chen, Y., Fischer, T., Tedaldi, E., Napoli, A., Zhang, Y., … & Khalili, K. (2016). Elimination of HIV-1 Genomes from Human T-lymphoid Cells by CRISPR/Cas9 Gene Editing.Scientific reports6.

Regalado, A. (2016, February 8). We Have the Technology to Destroy All Zika Mosquitoes. Technology Review. Retrieved from https://www.technologyreview.com/s/600689/we-have-the-technology-to-destroy-all-zika-mosquitoes/

Samji, H., Cescon, A., Hogg, R. S., Modur, S. P., Althoff, K. N., Buchacz, K., … & Justice, A. (2013). Closing the gap: increases in life expectancy among treated HIV-positive individuals in the United States and Canada. PloS one8(12), e81355.

Tavernise, S., & Mcneil, D. G. (2016, February 01). Zika Virus a Global Health Emergency, W.H.O. Says. The New York Times. Retrieved from http://www.nytimes.com/2016/02/02/health/zika-virus-world-health-organization.html?_r=0

Waltz, E. (2016, February 22). Software Helps Gene Editing Tool CRISPR Live Up to Its Hype. IEEE. Retrieved from http://spectrum.ieee.org/biomedical/diagnostics/software-helps-gene-editing-tool-crispr-live-up-to-its-hype

World Health Organization (2015). HIV/AIDS. Retrieved from http://www.who.int/mediacentre/factsheets/fs360/en/

Wu, Y., Liang, D., Wang, Y., Bai, M., Tang, W., Bao, S., . . . Li, J. (2013). Correction of a Genetic Disease in Mouse via Use of CRISPR-Cas9. Cell Stem Cell, 13(6), 659-662.

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