A New Approach to mRNA Delivery
Lentivirus, adenovirus, adeno-associated virus (AAV), and lipid nanoparticle (LNP) are currently the main delivery vectors used for human gene therapy, but some of these delivery vectors can randomly integrate into the genome, some are ineffective, and some can trigger unintended immune responses.
The entire biomedical community is striving to develop new and more powerful molecular therapies, but delivering these therapies precisely and effectively to cells is fraught with challenges. A novel answer to these challenges was brought, by attempting to solve the biggest problem facing gene editing/gene therapy—delivery.
On August 19, 2021, Feng Zhang’s team published a research paper entitled: Mammalian retrovirus-like protein PEG10 packages its own mRNA and can be pseudotyped for mRNA delivery in the leading international academic journal Science.
The research team developed a novel RNA delivery platform, SEND (selective endogenous eNcapsidation for cellular delivery), which, with its core, the retroviral-like protein—PEG10, is able to bind to its own mRNA and forms a spherical protective capsule around it. The team adapted its design for the encapsulation and delivery of RNA.
The team used the SEND system to deliver the CRISPR-Cas9 gene editing system to mouse and human cells and successfully edited the target genes. This will provide a novel delivery vector for gene therapy, as the SEND system is self-assembled into virus-like particles using human components, which induces less immune response and is safer than other delivery vectors.
Feng Zhang said SEND technology could complement existing viral delivery vectors and lipid nanoparticles to expand the toolbox for delivering genes to cells and gene-editing therapies.
“Inspired by retrotransposons.”
The PEG10 protein is naturally present in humans and is derived from “retrotransposons,” virus-like genetic elements that integrated themselves into the genomes of human ancestors millions of years ago. Over time, PEG10 has been taken up by the human body and has become part of the protein pool important for life. In January 2018, Jason Shepherd’s lab at the University of Utah published a paper in Cell, which identified a retrotransposon-derived protein, ARC, that forms virus-like structures and participates in the transfer of RNA between cells. This study showed that it was possible to design retrotransposon proteins as a delivery platform, but at the time, these proteins had not been successfully used to package and deliver specific RNAs in mammalian cells.
To explore the potential of retrotransposons as a gene delivery platform, Feng Zhang’s team began a systematic search of the human genome for this class of proteins, looking for proteins that could form protective vesicles. After preliminary analysis, they found 48 human genes encoding proteins that may have this ability, 19 of which present in both mice and humans.
Feng Zhang’s team discovered that PEG10 was the most prominent of these proteins in terms of potential, and that cells were able to release PEG10 particles, with the majority of these PEG10 particles also containing their own mRNA, implying that PEG10 may be able to package specific RNA molecules.
Feng Zhang’s team modified PEG10 to make it a delivery platform. They first identified the sequence in PEG10’s mRNA sequence that recognizes and packages its RNA, and then modified the PEG10 protein and this mRNA sequence so that PEG10 can selectively package the RNA. The PEG10 protein was then modified with a fusion protein to facilitate fusion with the cell membrane and better entry into the cell. Through this series of modifications, PEG10 is expected to target specific kinds of cells, tissues or organs with RNA delivery.
Because the SEND system is made up of proteins that occur naturally in the body, it may not elicit an immune response. If further research confirms this, SEND could become a reusable gene therapy delivery vehicle with minimal side effects.