INTRO TO CRISPR TECHNOLOGY

You may have heard of CRISPR, the revolutionary gene editing technology, used in various types of biomedical research, clinics or also agriculture aiming diverse goals. This technique has integrated many fields, from improving bacterial immunity against viruses to evaluating genes normally targeted by chemo and radiotherapies or also the manipulation of crop species and development of more sustainable agricultural systems. But what exactly is CRISPR? And how does it work?

Every part of your body is made up of millions of cells that grow and function by following the instructions encoded in genes; these functional units make up threadlike structures of DNA called chromosomes. Our genetic make-up contributes greatly to how the immune system fights off diseases; when viruses invade cells, an immune response is triggered to a particular antigen on the virus which allows antigen-specific genes to interfere by destroying the virus’s genetic material, thus protecting the body. However, the immune system still has an efficiency boundarie; many infectious agents can mutate and develop structures that go unseen by the defense system, or in other cases, the infection can’t be fought against once it has reached a certain stage of development. This “immune deficiency”, so to speak, is what leads to severe inflammations, uncontrolled spreading, and tragic outcomes. However, not all hope must be lost; those dreadful consequences are what motivated scientists to dig more into that mechanism and later on discover the editable characteristic of the CRISPR bacterial defense system.

The CRISPR immune system intervenes when bacterial cells are invaded by viruses (you may also be familiar with the term “bacteriophage”); this system creates 2 types of short RNA sequences, with one of them matching a specific sequence from the viral genome perfectly. These 2 RNAs form a complex with a protein Cas-9 (CRISPR-associated protein 9), a specific enzyme that uses CRISPR DNA sequence to identify and cleave the complementary DNA strands of interest. When the matching RNA (AKA guiding RNA) finds its matching target sequence, Cas-9 cuts the viral DNA, thus disabling the virus. The steps of this pathway can be summarized in the following illustration:

The CRISPR immune system works to protect the body from repeated viral attacks in 3 simple steps:

1) Adaptation: the CRISPR sequence is composed of short DNA repeats (shown as the black forms seen in the figure) and spacers (shown as the colored boxes in the figure). These spacers are derived from the DNA of viruses that have previously attacked the host bacterium. Therefore, spacers serve as a ‘genetic memory’ of previous infections. When a previously unseen virus infects bacteria, a new spacer derived from the virus is inserted amongst the existing spacers. In other words the DNA from the invading virus is processed into short sequences that are incorporated into the CRISPR sequence as new spacers.

2) Production of CRISPR RNA: The CRISPR sequence is then transcribed to form short CRISPR RNAs. The CRISPR RNA associates with the bacterial machinery and guides it to the matching target sequence in the invading agent.

3) Targeting: The bacterial machinery cuts up and destroys the invading viral genome, protecting the host bacterium.

However, scientists did not limit the application of CRISPR to only cutting viral genome, but to cut any DNA by simply changing the sequence of the guiding RNA to match the one of the target genome. And we could see some fascinating results such as applications in cancer treatment. As tested recently by Israeli scientists in Tel Aviv University, this technology is considered to be more elegant and effective than chemotherapy as it will cut the DNA of the cancerous cell without destroying the neighboring beneficial organisms; not only this, but the CRISPR application will only require 3 sessions of treatment, unlike the long periods of chemotherapeutic protocols.

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Last Edited:14.03.2021