Release date: 2017-10-26
When we talk about life, we are talking about chemical molecules. Whether DNA or protein is the atomic rearrangement of these biological macromolecules, it has spawned countless biochemical reactions that bring life to the earth.
Professor David Liu, principal of the study (Source: Broad Institute)
Today, Professor David Liu, a Chinese scholar at the Broad Institute, published an amazing study! His team developed a "base editor" that uses a simple chemical reaction in the cell to rearrange a base of DNA into another base. Unlike popular genetic editing methods such as CRISPR-Cas9, this technology can perform precise editing of genes without breaking DNA. The study was published in the leading academic journal Nature.
Nearly half of the pathogenic mutations are derived from changes in CG combinations to AT combinations (Source: Nature)
To understand this research, let's take a look at the DNA itself. We know that the double helix of DNA consists of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). They are paired with A and T, and C and G are paired, just like letters, writing human genetic information. However, due to the problem of chemical structure, the letter C is not stable, and it is prone to spontaneous deamination mutations, turning the original good CG combination into an AT combination. It is estimated that 100-500 such mutations occur in every cell of humans every day. Up to half of the pathogenic single base variants known to humans belong to this mutation.
A suitable deamination reaction can convert adenine into inosine that is structurally similar to guanine (Source: Nature)
In other words, if we can fix these genetic mutations at a fixed point and change the AT back to CG, it is expected to correct many of the human genetic diseases from the roots. This is the research idea of ​​Professor Liu's team. In the laboratory, they observed an interesting phenomenon - adenine (A) becomes a molecule called inosine after deamination, and it is very close to the structure of guanine (G). It can also successfully fool the DNA polymerase in the cell. After a few rounds of DNA replication, the AT combination can change back to CG.
But scientists have a tough problem—there is no enzyme in nature that can catalyze the deamination of adenine in DNA.
If there is no ready road, open up one! In the human body, scientists have discovered an enzyme called TadA that catalyzes the transfer of adenine (A) from RNA to deamination. Although the subject of catalysis is different, Professor Liu's team believes that it has sufficient application potential. So, using the power of evolution, scientists have transformed TadA. They introduced the gene encoding TadA into E. coli and hoped that the enzyme could mutate the ability to catalyze DNA adenine in the rapid propagation of E. coli.
In this study, the mechanism of the base editor (Source: Nature)
At the same time, scientists also think that there are a lot of adenines on DNA, and they can't always convert them all into cockles. Therefore, the specific catalysis of a certain base is the key to the practical application of this system. Professor Liu thought of his laboratory neighbor Professor Zhang Feng, a Chinese scholar known for his CRISPR gene editing technology. If we use the accuracy of the CRISPR-Cas9 system, but do not allow it to cut double-stranded DNA, it may be possible to atomically rearrange adenine to make it another base. To this end, scientists have also introduced a special CRISPR-Cas9 system for immobile DNA in the process of screening TadA enzyme for precise positioning.
Hard work pays off! Although this system is extremely complicated, after a long 7-generation screening, Professor Liu finally developed a new "base editor" whose core is the TadA enzyme that can effectively target DNA. Whether in bacteria or in human cells, this editor works well. In human cells, its editing efficiency exceeds 50%!
This system can be effectively used in human cells (Source: Nature)
Although the system utilizes the CRISPR-Cas9 system, scientists have pointed out in this paper that the technology they developed and the CRISPR-Cas9 system have their own merits. It is more effective and "cleaner" than the CRISPR-Cas9 system in correcting single-base mutations. It causes almost no mutations such as random insertion and deletion, and the off-target effect in the whole genome is better than the CRISPR-Cas9 technique. Be aware that this is one of the biggest concerns about the security of the CRISPR-Cas9 technology.
Previously, researchers have also developed methods for editing other bases. At present, Professor Liu's team has the tools to turn C into T, turn A into G, turn T into C, and turn G into A. Admittedly, these tools are currently far from human clinical applications. However, it is known that it only involves atomic rearrangement of bases, without the need for DNA double-strand breaks, thereby reducing the risk of gene therapy. In addition, many genetic diseases are single-gene mutations, and treatment with these tools is also more targeted.
We thank Professor Liu's team for bringing us such exciting new tools for gene editing. There is no doubt that the era of genetic editing has arrived. Are you ready for the impact?
Reference material
[1] Programmable base editing of A?T to G?C in genomic DNA without DNA cleavage
Source: Academic Jingwei
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