What is genome editing? How does it work?

Definition: Genome editing (also known as genome engineering or gene editing) was first pioneered in 1990 and defined as:

‘a technique used for modification of DNA within a cell by cutting at particular sequence by an enzyme (engineered nuclease), which leads to a change in physical traits.’

This technological advancement allows genetic material to be added, modified, or removed.

Approaches to Gene Editing:

The latest methodology of gene editing is known as CRISPR. In the research world, the CRISPR-Cas9 system has created a lot of enthusiasm since it is faster and has a lower cost. Moreover, it is more precise, reliable, and efficient than other previous genome editing techniques. It was discovered in bacteria for destroying invading viruses.

How does Gene Editing Work?

1. An engineered nuclease cuts the DNA from a specific site. 
2. The cell starts to repair naturally.
3. Scientists can modify this repair process to make changes in the desired location.   

Steps and Working:

The gene-editing method can be summarized in the following steps:

Double-strand break repair:

Two main pathways repair DNA strand break (DSB)

1. Non-homologous end joining (NHEJ) 
2. Homology-directed repair (HDR) 

NHEJ uses a range of enzymes to connect the DNA ends directly. At the same time, HDR is more precise, and it uses a homologous sequence as a model for the regeneration of missing specific DNA sequences at the end of the DNA. It will lead to a particular transformation being implanted at the DSB site. Recombination frequency is enhanced by at least three orders of magnitude, while HDR-based gene editing is analogous to homologous recombination-based genome targeting.

Engineering of nucleases:

Three classes of nucleases have been discovered in gene engineering, which cut the DNA from a particular site. 

These are: 

1. Zinc finger Nucleases (ZFN)
2. Transcription activator-like effector nucleases (TALNEs)
3. Clustered regularly interspaced short palindromic repeats (CRISPR)

How do Zinc-finger Nucleases (ZFN) work?

ZFN was used in the early 1990s to reduce off–target edits. Scientists engineered these proteins to bind with DNA for its cutting to allow the researcher to add, delete, or modify it with a new sequence. Its success rate was not very high, leading scientists back to further research other technologies.

How do transcription activator-like effector nucleases (TALENs) work?

TALENs were the new class of protein discovered in 2003. So, it was more effective than ZFN but with some high-cost rates. However, TALENs are simple and skillfully engineered. 

How does the Clustered regularly interspaced short palindrome repeats (CRISPR) work?

Cas9 protein is a protein with DNA-cutting skills that work with CRISPR technology. With CRISPR, researchers can create short RNA segments that match the desired DNA. Creating the RNA template is very easily comparable to the other two approaches. DNA and RNA of matched sequences combine. RNA portion guides the Cas9 protein to cut the genome. CRISPR makes deletion or insertion in the genome with six times the improved rate of ZFN and TALENs. 

References

“What Are Genome Editing and CRISPR-Cas9?: MedlinePlus Genetics.” MedlinePlus, U.S. National Library of Medicine, 18 Sept. 2020, medlineplus.gov/genetics/understanding/genomicresearch/genomeediting/. 

“What Is Genome Editing?” Facts, The Public Engagement Team at the Wellcome Genome Campus, 23 Aug. 2017, www.yourgenome.org/facts/what-is-genome-editing. 

“What Is Genome Editing?” Genome.gov, www.genome.gov/about-genomics/policy-issues/what-is-Genome-Editing.