This is the last blog in the DNA Series (boo!), and we’re delving into the world of gene editing. For this series, the blog posts will be:
Humans have been engineering organisms for decades, what propel the advancement of the technologies used for it. Gene editing techniques, also known as New Breeding Techniques (NBTs), are highly precise tools that can identify specific regions of DNA and introduce targeted breaks in the DNA double-helix. Among the most well-known gene-editing techniques are Zinc Finger Nucleases (ZFNs), TALENs, and CRISPR.
All of these techniques have one important feature in common: they rely on molecular “scissors” called nucleases. These nucleases can be made of proteins or, in some systems, involve RNA-guided mechanisms that direct an enzyme to a specific DNA sequence. Because these nucleases are site-specific, they can recognize and cut DNA at a particular region, allowing researchers to make targeted genetic modifications. Those cuts enable the removal, insertion or replacement of nucleotides in the site.
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats and is the most used gene editing technique in the world. CRISPR is a record of past genetic attacks on the cell of some single-celled organisms (mainly bacteria and archeae), allowing it to recognize and respond to similar attacks in the future, fuctioning as a immune mechanism against invading viruses.
The discovery of this technology was awarded a Nobel Prize in Chemistry in 2020 to two women, for the first time in history: Emmanuelle Charpentier and Jennifer Doudna. Although being very recent, the contributions that led to the discovery of CRISPR started early on, with Ishino et al. 1987, Mojica 1993, Jansen 2002, Barrangou and Horvath 2007, Marraffini and Sontheimer 2008, and Brouns et al. 2008, In 2011, Charpentier’s group made a huge discovery about the CRISPR system that facilitated its application. In 2013, Doudna’s group published one of the first CRISPR applications for gene editing.
A bacteriophage is a type of viruses that are specialized and will only infect bacteria. Bacteriophage infects bacteria by inserting their DNA inside the bacteria cell. They need to do so because viruses do not have their own means for replication, so they need to use the bacterial machinery to replicate its genome material and body parts. If the bacteria is not able to defend itself from the virus, the viruses parts will be replicated as many times as possible, so the bacterial cell “pops” like a balloon, releasing the new phages (as bacteriophages are called). This is called the lytic cycle. Another way virus can behave with bacteria is the lysogenic cycle, where the viral DNA starts integrating the bacterial DNA and it stays latent until the conditions are favourable to start the lytic cycle.
Bacteria then developed a way to fight viral infection, by identifying and cutting foreign DNA inside the bacterial cell. This cutted DNA will be stored inside the bacterial DNA, separed by palindromic sequences, and will work as a library for the bacteria to remeber previous infections and rapdly recognize new infections and act fast to cut the virus DNA as soon as it is recognize by the CRISPR system by the Cas9 nuclease. Here, we are using the Cas9 as an example because it is widely used now, but there are other types of Cas.
CRISPR became a powerful gene-editing tool when scientists discovered that its natural components could be adapted and programmed to target specific sequences of DNA. By designing the guide RNA, researchers can direct the CRISPR system to a chosen location in the genome. The CRISPR-Cas9 gene-editing system has two main components: guide RNA (gRNA) and the Cas9 enzyme. The guide RNA is designed to recognize a particular DNA sequence and acts as a molecular guide. Once it finds the matching sequence, it brings the Cas9 enzyme to that location. Cas9 then acts as molecular scissors, making a cut in the DNA. The cell recognizes the break as damage and activates its natural DNA repair mechanisms to fix it. Scientists can take advantage of these repair processes to disrupt, remove, or introduce changes to a gene.
In this way, CRISPR-Cas9 allows researchers to make targeted changes to an organism’s DNA by combining a programmable guide (gRNA) or single guide (sgRNA) with a DNA-cutting enzyme (Cas9).
There are already many gene edited products on the market in Canada, even though the technology is partially new, because of its simple and cheap mechanism.
Organism | Characteristic | Food use | In the Market? |
|---|---|---|---|
Corn | High amylopectin starch | Starch | No |
Improved resistance to northern corn leaf blight, gray leaf spot, anthracnose stalk rot, and southern corn rust | Oil, starch, flakes, flour, grits, meal, sweetener, beverage alcohol, etc. | In 2028 | |
Alfalfa | Modified forage quality through altered lignin polymer composition | Not intended for food use | Since 2023 |
Potato | Higher tuber set | Fresh and processed food | Since 2024 |
Reduced polyphenol oxidase (PPO) in tubers) | Since 2025 | ||
Mustard Greens | Reduced pungency to improve flavour | Leafy greens food usage | Since 2023 |
Reduced pungency and trichomes | Since 2024 | ||
Strawberry | Longer harvest season | Fresh and processed food | Since 2025 |
Soybean | Increased yield | Same food use as other soybean varieties in the market | Since 2026 |
Herbicide tolerance | In 2027 | ||
Tomato | Extended field holding, high yield, longer shelf life, less grey mold infection | Processing | In 2028 |
Increased gamma-aminobutyric acid (GABA) level | Fresh and processed food | Since 2026 | |
Blackberry | Compact growth, seedless, and/or thornless | Direct use as human food | In 2028 |
Pig | Porcine respiratory and reproductive syndrome virus (PRRSV)-resistant | Fresh and processed food | In 2027 |
Not only in Canada, but around the world there are many products available on the market that were developed with gene editing, such as non-browning potato in Argentina, extra starch corn, easily digestible soybean, higher sugar content sugarcane, heat tolerant cattle, and drought tolerant soybean in Brazil, fungus resistant apple in Chile, bacteria-resistant rice in Colombia, various fishes have been also edited for improved growth in different countries, and many plants have been edited for virus resistance worldwide.
Gene editing is transforming the way scientists approach the improvement of plants, animals, and microorganisms. What began as a natural defense mechanism in bacteria has been adapted into a precise and programmable technology that allows researchers to make targeted changes to DNA. CRISPR-Cas9, in particular, has accelerated the development of organisms with characteristics that can benefit agriculture, food production, animal health, and potentially many other areas. As the number of gene-edited products continues to grow around the world, understanding how these technologies work is becoming increasingly important not only for scientists, but for society as a whole.
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