Do you want to know how CRISPR gene editing works? Read this article to take a deep dive into gene editing and how it could benefit us in the near future.
Introduction
From unicellular organisms to multicellular organisms, the features of living things are determined by their genes. A gene is a sequence of DNA that determines the genotype and phenotype of an organism. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Sometimes, mutation can occur to DNA and the bases of DNA are altered. Some of the mutations are very severe and this leads to many genetic diseases.
In 2012, American scientist Jennifer Doudna and French scientist Emmanuelle Charpentier discovered a new method to edit the organisms' genes. This is also known as CRISPR. At this point in time, it is the most precise and easiest way to edit the genome of organisms. Hence, CRISPR is widely used in producing genetically modified crops and organisms.
Where does CRISPR come from?
CRISPR is a naturally occurring process in bacteria which they use to defend against the invasion of viruses. Naturally occurring CRISPR contains two main components: short repetitive sequences of DNA called clustered regularly interspaced and short palindromic repeats also known as CRISPR and CRISPR-associated proteins. When a virus invades a bacterium, Cas-protein cut out a segment of viral DNA and inserts it into the bacterium’s DNA in a particular pattern to create segments called CRISPR arrays. CRISPR arrays produce short pieces of RNA. These RNA then bind to Cas9 protein. When the bacterium is infected by the same virus, RNA recognizes the viral DNA and Cas9 destroys the viral DNA.
How does CRISPR work in gene editing?
The modern gene-editing process uses two main components: single-guide RNA (sgRNA) and Cas9 protein. sgRNA is a combination of crRNA and tracrRNA. crRNA contains the sequence that matches the part of DNA which is needed to edit and tracrRNA allows Cas9 to combine with sgRNA to produce an active complex. sgRNA helps Cas9 to reach the target sequence and Cas9 protein cuts the DNA sequence. Once the DNA is cut, the cell will try to repair itself by mechanisms: the error-prone non-homologous end joining (NHEJ) pathway or the homology-directed repair (HDR). However, NHEJ can introduce random insertion or deletion with missing or extra bases. The resulting gene is inactivated and turned off. In contrast, HDR can introduce precise genomic modifications at the target gene. Therefore, scientists need to use a homologous DNA donor template to modify the new DNA sequence.
Figure 1: Overview of CRISPR-Cas9 plasmid construction (source: Wikipedia)
Figure 2: DNA repairs after a double-stranded break (National Library of Medicine)
What's next?
In the future, scientists hope that CRISPR can be used to cure genetic diseases such as cancer, cystic fibrosis and sickle allele anaemia. Yet, there are still drawbacks to CRISPR. One of the concerns is that CRISPR will be only available to wealthy people and they will use it for non-therapeutic reasons. Another concern is that embryos and future generations may be affected by undesirable consequences of CRISPR as their genes are edited without their consent.
Citations
1. MedlinePlus. “What Are Genome Editing and CRISPR-Cas9?” Medlineplus.gov, Medlineplus, 22 Mar. 2022, medlineplus.gov/genetics/understanding/genomicresearch/genomeediting/.
2. Gostimskaya, Irina. “CRISPR–Cas9: A History of Its Discovery and Ethical Considerations of Its Use in Genome Editing.” Biochemistry (Moscow), vol. 87, no. 8, 15 Aug. 2022, pp. 777–788, www.ncbi.nlm.nih.gov/pmc/articles/PMC9377665/#:~:text=CRISPR%20%E2%80%93%20clustered%20regularly%20interspaced%20short,from%20Osaka%20University%20(Japan)., https://doi.org/10.1134/s0006297922080090.
3. Xu, Yuanyuan, and Zhanjun Li. “CRISPR-Cas Systems: Overview, Innovations and Applications in Human Disease Research and Gene Therapy.” Computational and Structural Biotechnology Journal, vol. 18, 8 Sept. 2020, pp. 2401–2415, www.ncbi.nlm.nih.gov/pmc/articles/PMC7508700/pdf/main.pdf, https://doi.org/10.1016/j.csbj.2020.08.031.
4. Wikipedia Contributors. “CRISPR Gene Editing.” Wikipedia, Wikimedia Foundation, 6 May 2019, en.wikipedia.org/wiki/CRISPR_gene_editing.
Comments