CRISPR gene editing
| CRISPR gene editing |
(technology, genetic engineering, Biological weapon/Research, research)
|Interest of||Feng Zhang|
|Breakthrough technology making gene editing easy and fast.|
CRISPR gene editing (pronounced "crisper") is a genetic engineering technique in molecular biology by which the genomes of living organisms may be modified. It is based on a simplified version of the bacterial CRISPR-Cas9 antiviral defense system. By delivering the Cas9 nuclease complexed with a synthetic guide RNA (gRNA) into a cell, the cell's genome can be cut at a desired location, allowing existing genes to be removed and/or new ones added in vivo (in living organisms).
The technique is considered highly significant in biotechnology and medicine as it allows for the genomes to be edited in vivo with extremely high precision, cheaply and with ease. It can be used in the creation of new medicines, agricultural products, and genetically modified organisms, or as a means of controlling pathogens and pests. It also has possibilities in the treatment of inherited genetic diseases as well as diseases arising from somatic mutations such as cancer. However, its use in human germline genetic modification is highly controversial.
Working like genetic scissors, the Cas9 nuclease opens both strands of the targeted sequence of DNA to introduce the modification by one of two methods. Knock-in mutations, facilitated via homology directed repair (HDR), is the traditional pathway of targeted genomic editing approaches. This allows for the introduction of targeted DNA damage and repair. HDR employs the use of similar DNA sequences to drive the repair of the break via the incorporation of exogenous DNA to function as the repair template. This method relies on the periodic and isolated occurrence of DNA damage at the target site in order for the repair to commence. Knock-out mutations caused by CRISPR-Cas9 result in the repair of the double-stranded break by means of non-homologous end joining (NHEJ). NHEJ can often result in random deletions or insertions at the repair site, which may disrupt or alter gene functionality. Therefore, genomic engineering by CRISPR-Cas9 gives researchers the ability to generate targeted random gene disruption. Because of this, the precision of genome editing is a great concern. Genomic editing leads to irreversible changes to the genome.
CRISPR-Cas9 genome editing techniques have many potential applications, including in medicine and agriculture. The use of the CRISPR-Cas9-gRNA complex for genome editing was the AAAS's choice for Breakthrough of the Year in 2015. Many bioethical concerns have been raised about the prospect of using CRISPR for germline editing, especially in human embryos.
CRISPR-Cas technology has been proposed as a treatment for multiple human diseases, especially those with a genetic cause. Its ability to modify specific DNA sequences makes it a tool with potential to fix disease-causing mutations. Early research in animal models suggest that therapies based on CRISPR technology have potential to treat a wide range of diseases, including cancer, progeria, beta-thalassemia, sickle cell disease,hemophilia, cystic fibrosis,Duchenne's muscular dystrophy, Huntington's disease, and heart disease. CRISPR has also been used to cure malaria in mosquitos, which could eliminate the vector and the disease in humans. CRISPR may also have applications in tissue engineering and regenerative medicine, such as by creating human blood vessels that lack expression of MHC class II proteins, which often cause transplant rejection.
In 2016, researchers demonstrated that CRISPR from an ordinary mouth bacterium could be used to edit RNA. The researchers searched databases containing hundreds of millions of genetic sequences for those that resembled CRISPR genes. They considered the fusobacteria Leptotrichia shahii. It had a group of genes that resembled CRISPR genes, but with important differences. When the researchers equipped other bacteria with these genes, which they called C2c2, they found that the organisms gained a novel defense.
Many viruses encode their genetic information in RNA rather than DNA that they repurpose to make new viruses. HIV and poliovirus are such viruses. Bacteria with Cas13 make molecules that can dismember RNA, destroying the virus. Tailoring these genes opened any RNA molecule to editing.
Gene Drive Extinction
A gene drive is an existing technology of genetic engineering that is able to propagate a particular suite of genes throughout a population by altering the probability that a specific allele will be transmitted to offspring (instead of the Mendelian 50% probability). It application is particularly suited for creating an irreversible species extinction.
Prime editing (or base editing) is a CRISPR refinement to accurately insert or delete sections of DNA. The CRISPR edits are not always perfect and the cuts can end up in the wrong place. Both issues are a problem for using the technology in medicine. Prime editing does not cut the double-stranded DNA but instead uses the CRISPR targeting apparatus to shuttle an additional enzyme to a desired sequence, where it converts a single nucleotide into another. The new guide, called a pegRNA, contains an RNA template for a new DNA sequence to be added to the genome at the target location. That requires a second protein, attached to Cas9: a reverse transcriptase enzyme, which can make a new DNA strand from the RNA template and insert it at the nicked site. Those three independent pairing events each provide an opportunity to prevent off-target sequences, which significantly increases targeting flexibility and editing precision. Prime editing was developed by researchers at the Broad Institute of MIT and Harvard in Massachusetts. More work is needed to optimize the methods.
Creating a ‘Superbug’ - Rewrite life as it exists on Earth
In 2021, team at the University of Cambridge under DR. Jason Chin used CRISPR to engineer a New ‘Superbug’ that is Invincible to all viruses. They used CRISPR to replace over 18,000 codons (DNA triplet codes), with synthetic amino acids that don’t exist anywhere in the natural world. The result was a bacteria that’s virtually resistant to all viral infections, because it lacks the normal protein “door handles” that viruses need to infect the cell.
“Because all of biology uses the same genetic code, the same 64 codons and the same 20 amino acids, that means viruses also use the same code…they use the cell’s machinery to build the viral proteins to reproduce the virus,” explained Chin. Now that the bacteria cell can no longer read nature’s standard genetic code, the virus can no longer tap into the bacterial machinery to reproduce—meaning the engineered cells are now resistant to being hijacked by almost any viral invader.
Until now, scientists have only been able to slip one designer amino acid into a living organism. The new work opens the door to hacking multiple existing codons at once, copyediting at least three synthetic amino acids at the same time. And when it’s 3 out of 20, that’s enough to fundamentally rewrite life as it exists on Earth.
Next up, the team is looking to potentially further reprogram our natural biological code to encode even more synthetic protein building blocks into bacterial cells. They’ll also move towards other cells—mammalian, for example, to see if it’s possible to compress our genetic code.
- A New Gene Editing Tool Could Make CRISPR More Precise. Lila Thulin, The Smithsonian Magazine. 21 October 2019.
- New 'prime' genome editor could surpass CRISPR. Jon Cohen, Science. 21 October 2019.
- New "Prime Editing" Method Makes Only Single-Stranded DNA Cuts. Emma Yasinski, The Scientist. 21 October 2019.
- Prime editing: DNA tool could correct 89% of genetic defects. James Gallagher, BBC News. 21 October 2019.