Have you heard? A revolution has seized the scientific community. Within only a few years, research labs worldwide have adopted a new technology that facilitates making specific changes in the DNA of humans, other animals, and plants. Compared to previous techniques for modifying DNA, this new approach is much faster and easier. This technology is referred to as “CRISPR,” and it has changed not only the way basic research is conducted, but also the way we can now think about treating diseases [1,2].
What is CRISPR
CRISPR is an acronym for Clustered Regularly Interspaced Short Palindromic Repeat. This name refers to the unique organization of short, partially palindromic repeated DNA sequences found in the genomes of bacteria and other microorganisms. While seemingly innocuous, CRISPR sequences are a crucial component of the immune systems  of these simple life forms. The immune system is responsible for protecting an organism’s health and well-being. Just like us, bacterial cells can be invaded by viruses, which are small, infectious agents. If a viral infection threatens a bacterial cell, the CRISPR immune system can thwart the attack by destroying the genome of the invading virus . The genome of the virus includes genetic material that is necessary for the virus to continue replicating. Thus, by destroying the viral genome, the CRISPR immune system protects bacteria from ongoing viral infection.
How does it work?
Figure 1 ~ The steps of CRISPR-mediated immunity. CRISPRs are regions in the bacterial genome that help defend against invading viruses. These regions are composed of short DNA repeats (black diamonds) and spacers (colored boxes). When a previously unseen virus infects a bacterium, a new spacer derived from the virus is incorporated amongst existing spacers. The CRISPR sequence is transcribed and processed to generate short CRISPR RNA molecules. The CRISPR RNA associates with and guides bacterial molecular machinery to a matching target sequence in the invading virus. The molecular machinery cuts up and destroys the invading viral genome. Figure adapted from Molecular Cell 54, April 24, 2014 .
Interspersed between the short DNA repeats of bacterial CRISPRs are similarly short variable sequences called spacers (FIGURE 1). These spacers are derived from DNA of viruses that have previously attacked the host bacterium . Hence, spacers serve as a ‘genetic memory’ of previous infections. If another infection by the same virus should occur, the CRISPR defense system will cut up any viral DNA sequence matching the spacer sequence and thus protect the bacterium from viral attack. If a previously unseen virus attacks, a new spacer is made and added to the chain of spacers and repeats.
The CRISPR immune system works to protect bacteria from repeated viral attack via three basic steps :
Step 1) Adaptation – DNA from an invading virus is processed into short segments that are inserted into the CRISPR sequence as new spacers.
Step 2) Production of CRISPR RNA – CRISPR repeats and spacers in the bacterial DNA undergo transcription, the process of copying DNA into RNA (ribonucleic acid). Unlike the double-chain helix structure of DNA, the resulting RNA is a single-chain molecule. This RNA chain is cut into short pieces called CRISPR RNAs.
Step 3) Targeting – CRISPR RNAs guide bacterial molecular machinery to destroy the viral material. Because CRISPR RNA sequences are copied from the viral DNA sequences acquired during adaptation, they are exact matches to the viral genome and thus serve as excellent guides.
The specificity of CRISPR-based immunity in recognizing and destroying invading viruses is not just useful for bacteria. Creative applications of this primitive yet elegant defense system have emerged in disciplines as diverse as industry, basic research, and medicine.
What are some applications of the CRISPR system?
The inherent functions of the CRISPR system are advantageous for industrial processes that utilize bacterial cultures. CRISPR-based immunity can be employed to make these cultures more resistant to viral attack, which would otherwise impede productivity. In fact, the original discovery of CRISPR immunity came from researchers at Danisco, a company in the food production industry [2,3]. Danisco scientists were studying a bacterium called Streptococcus thermophilus, which is used to make yogurts and cheeses. Certain viruses can infect this bacterium and damage the quality or quantity of the food. It was discovered that CRISPR sequences equipped S. thermophilus with immunity against such viral attack. Expanding beyond S. thermophilus to other useful bacteria, manufacturers can apply the same principles to improve culture sustainability and lifespan.
In the Lab
Beyond applications encompassing bacterial immune defenses, scientists have learned how to harness CRISPR technology in the lab  to make precise changes in the genes of organisms as diverse as fruit flies, fish, mice, plants and even human cells. Genes are defined by their specific sequences, which provide instructions on how to build and maintain an organism’s cells. A change in the sequence of even one gene can significantly affect the biology of the cell and in turn may affect the health of an organism. CRISPR techniques allow scientists to modify specific genes while sparing all others, thus clarifying the association between a given gene and its consequence to the organism.
Rather than relying on bacteria to generate CRISPR RNAs, scientists first design and synthesize short RNA molecules that match a specific DNA sequence—for example, in a human cell. Then, like in the targeting step of the bacterial system, this ‘guide RNA’ shuttles molecular machinery to the intended DNA target. Once localized to the DNA region of interest, the molecular machinery can silence a gene or even change the sequence of a gene (Figure 2)! This type of gene editing can be likened to editing a sentence with a word processor to delete words or correct spelling mistakes. One important application of such technology is to facilitate making animal models with precise genetic changes to study the progress and treatment of human diseases.
Figure 2 ~ Gene silencing and editing with CRISPR. Guide RNA designed to match the DNA region of interest directs molecular machinery to cut both strands of the targeted DNA. During gene silencing, the cell attempts to repair the broken DNA, but often does so with errors that disrupt the gene—effectively silencing it. For gene editing, a repair template with a specified change in sequence is added to the cell and incorporated into the DNA during the repair process. The targeted DNA is now altered to carry this new sequence.
With early successes in the lab, many are looking toward medical applications of CRISPR technology. One application is for the treatment of genetic diseases. The first evidence that CRISPR can be used to correct a mutant gene and reverse disease symptoms in a living animal was published earlier this year . By replacing the mutant form of a gene with its correct sequence in adult mice, researchers demonstrated a cure for a rare liver disorder that could be achieved with a single treatment. In addition to treating heritable diseases, CRISPR can be used in the realm of infectious diseases, possibly providing a way to make more specific antibiotics that target only disease-causing bacterial strains while sparing beneficial bacteria . A recent SITN Waves article discusses how this technique was also used to make white blood cells resistant to HIV infection .
The Future of CRISPR
Of course, any new technology takes some time to understand and perfect. It will be important to verify that a particular guide RNA is specific for its target gene, so that the CRISPR system does not mistakenly attack other genes. It will also be important to find a way to deliver CRISPR therapies into the body before they can become widely used in medicine. Although a lot remains to be discovered, there is no doubt that CRISPR has become a valuable tool in research. In fact, there is enough excitement in the field to warrant the launch of several Biotech start-ups that hope to use CRISPR-inspired technology to treat human diseases .
Ekaterina Pak is a Ph.D. student in the Biological and Biomedical Sciences program at Harvard Medical School.
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