Written by | Wang Cong
Throughout evolution, retroviruses and retroviruses have inserted their genetic code into the mammalian genome. Although many of these integrated virus-like sequences pose a threat to the integrity of the host genome, some have been recombined by mammalian cells to play an important role in development. About 8% of the human genome is the remnant of retroviruses that infected humans in ancient times.
At present, the delivery vectors used in human gene therapy mainly include lentiviruses, adenoviruses, adeno-associated viruses (AAVs), and lipid nanoparticles (LNPs), but some of these delivery vectors will be randomly integrated into the genome, some are inefficient, and some will lead to unnecessary immune responses.
The entire biomedical community is working to develop new and stronger molecular therapies, but delivering these therapies precisely and efficiently into cells is challenging. Today, Zhang Feng brings a whole new answer to these challenges.
On August 19, 2021, Zhang's team published a research paper titled: Mammalian retrovirus-like protein PEG10 packages its own mRNA and can be pseudotyped for mRNA delivery.
The research team has developed a new RNA delivery platform, SEND (Selective Endogenous eNcapsidation for cellular Delivery), whose core is the retroviral-like protein PEG10, which binds to its own mRNA and forms a spherical protective sac around it. The research team adapted it to package and deliver RNA.
The research team used the SEND system to deliver the CRISPR-Cas9 gene-editing system to mouse and human cells and successfully edited the target gene. This will provide a completely new delivery vector for gene therapy, a SEND system that uses components within humans to self-assemble into virus-like particles that cause fewer immune responses and are safer than other delivery vectors.
Zhang said send technology could complement existing viral delivery vectors and lipid nanoparticles to expand the toolbox for delivering genes to cells and editing therapies.
Inspired by reverse transcription transposons
The PEG10 protein is naturally found in humans and derives from "retrotransposons" – virus-like genetic elements that integrated themselves into the genomes of their ancestors millions of years ago. Over time, PEG10 has been absorbed by the human body as part of a protein library that is important for life.
Four years ago, researchers discovered that another reverse transcription-derived protein, ARC, was able to form a virus-like structure and was involved in the transfer of RNA between cells. Although these studies suggest the potential to design reverse transcription transposon proteins as delivery platforms, scientists have yet to successfully use these proteins to package and deliver specific RNAs in mammalian cells.
To explore the potential of retrotransposons as a platform for gene delivery, Zhang's team began a systematic search for this class of proteins in the human genome for proteins that can form protective sacs. After preliminary analysis, Zhang's team found that 48 genes in human genes encode proteins that may have this ability, 19 of which are present in both mice and humans.
In in vitro cell experiments, Zhang's team found that the most prominent potential of these proteins is PEG10, the cell is able to release PEG10 particles, and most of these PEG10 particles also contain their own mRNA, suggesting that PEG10 may be able to package specific RNA molecules.
Develop a delivery platform
To develop PEG10 as a delivery platform, Zhang's team engineered PEG10, first, they found the sequence in PEG10's mRNA sequence that recognized and packaged its RNA, and then modified the PEG10 protein and the mRNA sequence so that PEG10 could selectively package RNA. The research team then modified the PEG10 protein with fusion protein to facilitate its fusion with the cell membrane and help it better enter the cell.
Through this series of modifications, PEG10 is expected to target specific types of cells, tissues or organs and perform RNA delivery.
Advance gene therapy
The SEND system is made up of proteins that are naturally produced in the human body, which means it may not trigger an immune response. If further studies confirm this, send will hopefully be a reusable gene therapy delivery vector with minimal side effects.
Zhang's team used SEND to successfully deliver the CRISPR-Cas9 system into human and mouse cells in the form of mRNA and edit specific genes. Next, Zhang's team will further test SEND in animals and further engineer it so that the system can deliver mRNA to individual tissues and cells.
Michael Segel, the paper's first author and a postdoc in Feng Zhang's lab, said PEG10 is not the only person with the ability to transfer RNA. This is also where it's exciting, and this study suggests that there may be other RNA transfer systems in humans that could also be used for therapeutic purposes.
Zhang Feng said that this study shows that we can use PEG10 and other similar proteins in the human body to design new delivery vectors and develop new gene therapies.
This new delivery platform, SEND, is able to work effectively in cell models and, as it evolves further, could open up a new class of delivery methods for a wide range of molecular drugs – including for gene editing and gene replacement. SEND technology will also complement viral delivery vectors and lipid nanoparticles to further expand the toolbox for delivering genes to cells and editing therapies.
Thesis Link:
https://science.sciencemag.org/content/373/6557/882