Targeting Genetic Disorders with CRISPR

Scientists have the ability to genetically engineer all organisms including bacteria, animals, and plants. A method of DNA editing called CRISPR, which stands for clustered regularly interspaced short palindromic repeats, allows scientists to edit DNA quickly and more efficiently (Stovicek 2015). CRISPR is found naturally in most bacteria and archaea and is part of their immune system (Horvath 2010). CRISPR is a set of DNA repeats that are separated by variable sequences called spacers (Horvath 2010). These spacers have the same DNA sequences of viruses or other foreign invaders that have invaded the organism before. When the same virus attacks again, RNA, which is transcribed from DNA and contains CRISPR, will go to the virus and cut up the part of the virus’ DNA that is complementary to the DNA sequence in a spacer, killing the virus (Gratz 2013). If a new virus attacks, a new spacer is created in case the organism needs to fight off the same virus again. Many researchers are interested in CRISPR because if it is manipulated, sections of DNA can be cut at precise locations and undesired DNA can be eliminated. Desired DNA can be inserted at precise sections. There is so much potential in using CRISPR. It is being researched for uses in medicines and gene therapy (Xue 2016). Even germline gene therapy, which is currently forbidden, can become a reality if CRISPR is deemed safe to use.

CRISPR is a very recent discovery in genetic engineering. It was first identified in 1987 in Escherichea coli, and later became known as being part of the immune system (Gratz 2013). CRISPR RNA can break the double helix of foreign DNA at a specific location that corresponds to DNA sequences in spacers. To manipulate this system, researchers need to change the sequences of spacers to complement DNA sequences that they want CRISPR RNA to cut. Instead of changing the genome of an organism, they can change the sequences of CRISPR RNA. In one experiment, when spacers were deleted, the cell lost its resistance to the viruses that it used to be resistant to and when spacers were added, the cell gained resistance against viruses it never encountered (Barrangou 2007). Fortunately, CRISPR can be used in eukaryotes even though it is naturally found in most bacteria and archaea, probably due to the similarity in genomes. Experiments have shown that CRISPR can be used in yeast cells and human stem cells (Gratz 2013). Plants can also be genetically modified using CRISPR (Bortesi 2014).

Figure 1- Editing and correction of gene with CRISPR in mouse with DMD

There have been significant advancements in using CRISPR for gene therapy. There were three studies on treating mice with Duchenne muscular dystrophy (DMD), which is caused by a mutation (Xue 2016). As shown in Figure 1, in the experiments, CRISPR was manipulated to remove the mutation in DNA. It was injected into the mice via a virus vector for somatic gene therapy. The mutations were corrected and the mice were observed. The mice regained some muscle control (Xue 2016). The ratio of muscles that do not have the mutation increased, which showed that the corrected gene is being expressed (Xue 2016). These experiments show promising results, but not all genetic disorders are caused by a single mutation. There can be complex causes to these disorders. In addition, editing cells can have unknown consequences in cellular functions and in how different cells communicate (Xue 2016). Edited cells may also work too well compared to unedited ones. It may not necessarily be good if edited cells have greater fitness and can survive longer than unedited ones. The same disadvantages apply to germline gene therapy using CRISPR, although the problems magnify because they are inheritable. Researchers do have the ability to manipulate stem cells, but it is strictly forbidden to provide germline gene therapy so they cannot research the use of CRISPR on germline gene therapy. Like many treatments, CRISPR has advantages and disadvantages that researchers need to explore. As researchers uncover more facts about CRISPR, perhaps gene therapy using CRISPR can become a regular treatment.

Works Cited

Barrangou R, Fremaux C, Deveau H, Richards M, Boyaval P, Moineau S, Romero DA, Horvath P. Science 2007; 315: 1702-1712.

Gratx SJ, Cummings AM, Nguyen JN, Hamm DC, Donohue LK, Harrison MM, Wildonger J, O’Conner-Giles KM. Genome Engineering of Drosophila with the CRISPR RNA-Guided Cas9 Nuclease. GENETICS 2013; 194: 2019-1035.

Horvath P, Barrangou R. CRISPR/Cas, the Immune System of Bacteria and Archaea. Science 2010; 327: 167.

Stovicek V, Borodina I, Forster J. CRISPR-Cas system enables fast and simple genome editing of industrial Saccharomyces cerevisiae strains. Metabolic Engineering Communications 2015; N/A: 13-22.

Xue HY, Zhang X, Wang Y, XIaojie L, Dai WJ, Xu Y. In vivo gene therapy potentials of CRISPR- Cas9. Gene Therapy 2016; 23: 557-559.