The promise of CRISPR

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In a first, researchers from the Oregon Health and Science University along with colleagues in California, China and South Korea repaired an error in the DNA sequence (mutation) in human embryos by using a gene-editing tool called CRISPR-Cas9.

The mutation seen in the MYBPC3 gene causes a common heart condition called hypertrophic cardiomyopathy, which is marked by thickening of the heart muscle. The mutation is seen in about one in 500 people and can lead to sudden death later in life. It is an inherited cardiac disease and the presence of even one copy of the gene can cause symptoms, which usually manifest as heart failure.

Correcting the mutation in the embryo will ensure that the child is born healthy and also prevent the defective gene from being passed on to future generations. Existing treatment can only ease the symptoms but cannot cure the patients.

How does the gene-editing tool work?

CRISPR-Cas9 is a system used by bacterial cells to recognise and destroy viral DNA as a form of adaptive immunity. Using components of the CRISPR system, researchers can remove, add or alter specific DNA sequences in the genome of higher organisms.

The gene editing tool has two components — a single-guide RNA (sgRNA) that contains a sequence that can bind to DNA, and the Cas9 enzyme which acts as a molecular scissor that can cleave DNA. The genetic sequence of the sgRNA matches the target sequence of the DNA that has to be edited. In order to selectively edit a desired sequence in DNA, the sgRNA is designed to find and bind to the target.

Gene editing - photo
Individual blastomeres within the early embryos two days after co-injection with sperm and CRISPR-Cas9. As a result of initiating the repair process at the time of fertilization, a new study revealed that each new cell in the developing embryos was uniformly free of the disease-causing mutation. – Photo: OHSU

Upon finding its target, the Cas9 enzyme swings into an active form that cuts both strands of the target DNA. One of the two main DNA-repair pathways in the cell then gets activated to repair the double-stranded breaks. While one of the repair mechanisms result in changes to the DNA sequence, the other is more suitable for introducing specific sequences to enable tailored repair.

In theory, the guide RNA will only bind to the target sequence and no other regions of the genome. But the CRISPR-Cas9 system can also recognize and cleave different regions of the genome than the one that was intended to be edited. These “off-target” changes are very likely to take place when the gene-editing tool binds to DNA sequences that are very similar to the target one.

Though many studies have found few unwanted changes suggesting that the tool is probably safe, researchers are working on safer alternatives.

How successful was it?

Along with sperm from a man with hypertrophic cardiomyopathy, the gene-editing tool was also introduced into eggs from 12 healthy women before fertilisation. In normal conditions, a piece of DNA with the correct sequence serves as a template for the repair to work, although the efficiency can be significantly low. It was observed that instead of the repair template that was provided by the researchers, the cells used the healthy copy of the DNA from the egg as a template. This came as a big surprise.

If sperm from a father with one mutant copy of the gene is fertilized in vitro with normal eggs, 50% of the embryos would inherit the condition. When the gene-editing tool was used, 42 out of the 58 embryos did not carry the mutation. The remaining 16 embryos had unwanted additions or deletions of DNA. Thus the probability of inheriting the healthy gene increased from 50 to 72.4%. Except the place where the mutation was present, the gene-editing tool did not snip off the DNA at any other site of the genome.

According to Nature, “the edited embryos developed similarly to the control embryos, with 50% reaching an early stage of development known as the blastocyst, in which the embryos contain different cell types. This indicates that editing does not block development.”

What the future holds

In a first, researchers moved a step closer to xenotransplantation by using CRISPR to make dozens of genetic changes to produce 37 piglets. Researchers crippled all 25 copies of “PERV” genes — DNA in the pig genome that makes potentially dangerous viruses that could infect anyone who receives a pig organ. This would enable swine to become a source of organs for human transplants.

In April 2015, a Chinese team became the first to use CRISPR/Cas9 to modify the gene responsible for beta-thalassaemia in human embryos. The same year, a little girl was treated with gene-edited immune cells that eliminated all signs of the leukaemia. In China, clinical trials are under way to use this tool for editing immune cells to treat cancer.

In May this year, researchers showed in mice that it is possible to completely shut down HIV-1 replication and even eliminate the virus from infected cells using CRISPR/Cas9. A month later, the U.S. approved a clinical trial to use CRISPR-Cas9 for cancer therapy. The tool is already being used in agriculture. A new breed of crops that are gene-edited rather than genetically modified will become available in the market in a few years. In February this year, the National Academy of Sciences (NAS) and the National Academy of Medicine said scientific advances make gene editing in human reproductive cells “a realistic possibility that deserves serious consideration.”

Published in The Hindu on August 12, 2017

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