Science blockbuster: new mRNA delivery SEND, opening up a new method of molecular therapy delivery

At the beginning of 2020, the new crown epidemic is raging around the world.

Compared with traditional vaccines, RNA vaccines seem to be specially prepared for the new crown epidemic

After the RNA vaccine is delivered to the human body, it can continuously produce viral proteins in human cells, which is equivalent to transforming the body into a "vaccine factory" and training the immune system to recognize virus invasion

Now, the limitations of the use of RNA vaccines are expected to be broken

The research team led by Professor Zhang Feng, a Chinese scientist from the Massachusetts Institute of Technology, has successfully developed a new RNA delivery platform-SEND

The new RNA delivery platform SEND (Source: MIT)

The research was published in the latest issue of Science with the title "Mammalian retrovirus-like protein PEG10 packages its own mRNA and can be pseudotyped for mRNA delivery"

(Source: Science)

Regarding this research result, Zhang Feng, a pioneer of CRISPR gene editing technology, a core research member of the Broad Institute and a researcher of the McGovern Institute, said, “The biomedical community has been developing powerful molecular therapies, but how to deliver them accurately and effectively? Target cells are still challenging

All protein-level diseases can be solved using RNA therapy

Most of the targets of small molecule drugs currently on the market are proteins.

But new drug developers are not satisfied with this

RNA is such a potential target

Targeted RNA also has many benefits: because it is located upstream of the protein, the targeted RNA is expected to directly up-regulate or down-regulate the translation efficiency of the protein, solving the problem of protein "unable to be a drug"; RNA is extremely abundant in the human genome, resulting in non-coding The RNA sequence accounts for 70% of the genome, and its abundance is an order of magnitude higher than that of the protein-encoding sequence

However, in the past few decades, because RNA molecules are easily degraded and have a short half-life in the body, they have been considered unable to be "therapeutic drugs"

Until recent years, with technological advances and improvements in stability chemistry, RNA molecules with shorter half-lives have become clinical new favorites, gradually attracting the attention of the industry and entering a stage of explosive growth

As a new type of therapy, RNA drugs have a short R&D cycle, simple production process, low cost, strong effect, rapid expansion of production capacity, and better safety, which are their natural advantages

At present, the application prospect of RNA therapy is very broad, including vaccines, tumor immunotherapy, monoclonal antibody drug replacement, protein drug replacement, assisted reproduction, and so on

The biggest obstacle to RNA drugs: delivery

Although the application prospect of RNA drugs is very broad, the current research and development of RNA drugs is also facing a huge challenge, that is, the problem of RNA delivery
.

　　Nucleic acid drugs want to enter the body, there are three main difficulties: the molecular weight and negative charge of nucleic acid make it impossible to freely pass through biological membranes; RNA is easily degraded by RNase enzymes in plasma and tissues, and is quickly cleared by the liver and kidneys and recognized by the immune system.
; After entering the cell, the "card" cannot function in the endocytic body
.

　　The above-mentioned technical obstacles facing the development of RNA drugs-drug delivery, have not been resolved
.
At present, there are two main methods to solve the delivery problem: one is to modify nucleic acid molecules to stabilize and avoid recognition by the immune system; the other is to use drug delivery systems, such as lipid nanoparticles (LNP) and carrier viruses
.

　　The principle of RNA delivery by nanoliposomes is not yet fully understood, but it is generally believed that nanoliposomes bind to cell membranes through non-covalent affinity and are taken up by endocytosis.
After entering the cell, RNA escapes the endocytic vesicles and is released.
To express the target protein in the cytoplasm
.
Nanoliposomes can also be expelled from the cell through the opposite exocytosis, which is also the point that needs attention in the delivery of RNA through nanoliposomes
.

　　At present, RNA is mainly delivered by nano-preparations.
Due to the limitation of nano-liposomes, RNA therapy is only suitable for liver and spleen targeted therapy, and other tissues are difficult to target
.
At the same time, the low transmembrane of mRNA drugs also leads to great individual differences.
If the transmembrane property of the drug is 1%, then a 1% individual difference will cause a difference in the effective drug concentration twice, but if the transmembrane property is 50%, then An individual difference of 1% is irrelevant
.

　　The current strategy of the industry is to first choose a project with a larger safety window such as a vaccine, but if it expands to more complex targets, the industry needs to find biomarkers that can monitor drug response
.

Breaking the dilemma of RNA therapy

　　The PEG 10 protein naturally exists in the human body and is derived from a virus-like genetic element called a "retrotransposon"
.
PEG 10 protein was integrated into the genome of human ancestors millions of years ago.
Over time, PEG 10 has been integrated with the human genome and played important functions in the human body
.

　　Previously, researchers discovered that another retrotransposon-derived protein ARC can form a virus-like structure and participate in the transfer of RNA between cells
.
The results of this study indicate that retrotransposon-related proteins may be used as RNA delivery platforms for RNA therapy, but scientists have not successfully used the ARC protein to deliver RNA in mammalian cells
.

　　In order to further explore the functions of retrotransposon proteins, Professor Zhang Feng led the research team to conduct a systematic search for retrotransposon proteins in the human genome, looking for proteins that can potentially transport RNA
.

　　Preliminary analysis revealed that 48 genes in the human genome may encode retrotransposon proteins
.
Among them, 19 proteins exist in both mice and humans
.

　　In in vitro studies, the researchers found that the retrotransposon protein PEG 10 is an efficient RNA carrier protein
.
Compared with other retrotransposon proteins, PEG 10 is more penetrating in mammalian cells, and it itself participates in RNA transport
.

　　Subsequently, the researchers found the molecular sequence that recognizes and packaged RNA in the mRNA of the PEG 10 protein
.
By modifying the FEG 10 protein mRNA molecular packaging sequence and the PEG 10 protein, the researchers tried to make the PEG 10 protein carry different RNAs and target different cells
.

　　In the end, the researchers developed two different protein-modified PEG 10 proteins and achieved target cell RNA delivery in cell experiments
.

　　In this regard, Professor Zhang Feng said, “Our research shows that by modifying the RNA packaging component and recognition component of PEG 10 protein, it is theoretically possible to provide a modular platform for the treatment of different diseases
.
”

　　Since the RNA carriers used in the SEND platform are all derived from natural proteins in the body, this means that this system will not trigger the body's immune response, and side effects will be greatly reduced
.
In the future, SEND technology may replace nanoliposomes and viral vectors and become the most suitable vector for gene editing therapy
.

　　In the next step, the team will test SEND in animals and further design and develop more retrotransposon proteins to deliver more RNA to various tissues and cells
.

　　Reference materials:

　　https://science.
sciencemag.
org/content/373/6557/882

　　https://news.
mit.
edu/2021/send-peg10-drug-delivery-0819

　　https://