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CRISPR explained: The revolutionary tool that transforms genetic engineering



We are in the midst of a gene-editing revolution.

For four decades, scientists have tinkered with our genes. Since the 1970s, they have experimentally switched them on and off, uncovering their functions; mapped their location within our genome; and even inserted or deleted in animals, plants and human beings.

And in November 2018, and a Chinese scientist claimed to have created the world's first genetically modified human beings.

Though scientists have made great inroads into human genetics, editing our genes has remained a complex process requiring imprecise, expensive technology, years of expertise, and just a little luck.

In 201

2, a pair of scientists developed a new tool to modify genes, reshaping the entire field of gene-editing forever: CRISPR. Often described as "a pair of molecular scissors," CRISPR is widely considered the most accurate, cost-effective and quickest way to edit genes. Its potential applications are far-reaching, affecting conservation, agriculture, drug development and how we could fight genetic diseases. It could even alter the entire gene pool of a species.

The field of CRISPR research is still remarkably young, but we have already seen how it could be used to fight HIV infection, combat invasive species and destroy antibiotic-resistant bacteria. Many unknowns remain, however, including how CRISPR could damage DNA, leading to pathogens such as cancer.

Such a monumental leap in genetic engineering is full of complexities that ask big, often philosophical questions about science, ethics, how we conduct research, and the future of humanity itself. With the confirmation that two human embryos were modified using CRISPR and carried to term, those questions have come sharply into focus. The future of gene-editing seemingly arrived overnight.

But what exactly is CRISPR and what are the outstanding concerns about such a powerful tool?

Let's break it all down.

 Embryo selection for IVF light micrograph

CRISPR has the potential to be used in editing human embryos to create "designer babies."


Science Photo Library / Getty Images
                                                

What is CRISPR

Few predicted how important CRISPR would become for gene editing upon its discovery 30 years ago.

As early as 1987, researchers at Osaka University studying the function of Escherichia coli genes first noticed a set of short, repeated DNA sequences, but they did not understand the significance.

Six years later, another microbiologist, Francisco Mojica, noted the sequences in a different single-celled organism,
Haloferax mediterranei. In 2002, unusual DNA structures were given a name: Clustered regularly interspaced short palindromic repeats.

CRISPR.

Studying the sequences more intensely revealed that CRISPR forms an integral part of the "immune system" in bacteria, allowing them to fight off invading viruses. When a virus enters the bacteria, it fights back by cutting the virus' DNA. This kills the virus and the bacteria stores some of the DNA leftover.

The leftover DNA is like a fingerprint, stored in the CRISPR database. If invaded again, the bacteria produce an enzyme called Cas9 that acts like a fingerprint scanner. Cas9 uses the CRISPR database to match the stored fingerprints with those of the new invader. If it can find a match, Cas9 is able to chop up the invading DNA.

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Eric Mack
                                                

How is CRISPR used to edit genes

Nature often provides great templates for technological advances. For example, the nose of a Japanese bullet train is modeled on the kingfisher's beak because the latter is expertly "designed" by evolution to minimize noise as the bird dives into a stream to catch fish.

In a similar way, CRISPR / Cas9's ability to efficiently locate specific genetic sequences, and cut them, inspired a team of scientists to ask whether that ability could be mimicked for other purposes.

The answer will change forever.

In 2012, pioneering scientists Jennifer Doudna, from UC Berkeley and Emmanuelle Charpentier, at Umea University Sweden, showed CRISPR could be hijacked and modified. Essentially, they turned CRISPR from a bacterial defense mechanism into a DNA-seeking missile strapped to a pair of molecular scissors. Their modified CRISPR system worked wonderfully well, finding and cutting any genes they chose.

 CRISPR-Cas9 gene editing complex

An illustration of the CRISPR-Cas9 gene editing complex. The Cas9 nuclease protein (white and green) uses a RNA (red) sequence to cut DNA (blue) at a complementary site.


Molekull / Science Photo Library / Getty
                                                

Several research groups followed up on the original work, showing that the process was possible in yeast and cultured mouse and human cells.

The floodgates were opened, and CRISPR research, which had long been the domain of molecular microbiologists, skyrocketed. The number of articles referencing CRISPR in the preeminent research journal Nature has increased by over 6,000 percent between 2012 and 2018.

While other gen-editing tools are still in use, CRISPR provides a gigantic leap because of its precision and reliability. It's really good at finding genes and making accurate cuts. That allows genes to be cut out with ease, but it also provides the opportunity to insert new genes into the gap. Previous gene-editing tools could do this too, but not with the ease that CRISPR can do.

Another huge advantage CRISPR has over alternative gene-editing techniques is its expense. While the previous techniques may cost a laboratory upward of $ 500 to edit a single gene, CRISPR kit can do the same thing for under $ 100.

What can CRISPR do

The CRISPR / Cas9 system has been adapted to enable gene editing in organisms including yeast, fungi, rice, tobacco, zebrafish, mice, dogs, rabbits, frogs, monkeys, mosquitoes and, course, human – so its potential applications are enormous.

For research scientists, CRISPR is a tool that provides better, faster tinkering with genes, allowing them to create models of disease in human cell lines and mouse models with much higher proficiency. With better models of say, cancer, researchers are able to fully understand the patology and how it develops, and that could lead to improved treatment options.

One particular leap in cancer therapy options is the genetic modification of T cells, a type of white blood cell that is critical to the human immune system. A Chinese clinical trial extracted T cells from patients used CRISPR to delete a gene that acts as an immune system brake and then reintroduced them into the patients in an effort to combat lung cancer. And that's just one of many ongoing trials using CRISPR edited cells to fight particular types of cancer.

Beyond cancer, CRISPR has the potential to treat diseases caused by a mutation in a single gene, such as sickle cell anemia or Duchenne muscular dystrophy. Correcting and defective genes are known as gene therapy, and CRISPR is potentially the most powerful way to perform it. Using mouse models, researchers have demonstrated the efficacy of such treatments, but human gene therapies using CRISPR remain untested.

 Close-up of a mosquito

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