The genomes of many organisms, including flies, mice, and humans, contain several genes derived from retroviruses. In 2018, it was shown that one of these, Arc, can package mRNA and transfer it between cells. At the time, I, like many other biologists, thought this was super cool and should be used for gene editing. I even wrote about it as part of a class project.
But having the idea and actually executing it are two very different things. Now, a team at the Broad Institute (from Feng Zhang’s lab) has reported that they’ve got it to work, at least in cultured cells.
To start, they identified several retroviral-like proteins encoded in the mouse genome that had features suggesting they could package RNA. They next overexpressed these proteins in a human cell line, and checked whether they were secreted from the cells. Top hits included Arc and PEG10, as well as two other proteins. PEG10 performed the best in these experiments, and the researchers additionally showed that PEG10 encapsulates its own mRNA. After some additional followup experiments to confirm secretion of PEG10 virus-like particles, the researchers next moved on to exploiting this for gene therapy.
To test this out, the researchers put the 5′ and 3′ untranslated regions of PEG10 mRNA (which contain packaging signals) onto mRNA encoding Cre recombinase. The Cre recombinase would act as a switch to turn off GFP expression in a reporter cell lines. They then co-expressed this mRNA with PEG10 protein, either by itself, or with a virus-derived “fusogenic” protein attached to it to help it enter target cells. They observed that the fusogenic PEG10 particles successfully delivered the Cre mRNA to the target cells.1
Still, the researchers wanted to get around the requirement for a viral fusogenic protein. Therefore, they found a mammalian one, mouse syncytin A. Expressing SYNA, PEG10, and the cargo mRNA resulted in the production of virus-like particles that could enter target cells. Importantly, the researchers showed that they could package Cas9 mRNA as well as gRNA to achieve gene editing in cultured mouse and human cells.
So, why is this better than viral methods? Because PEG10 is a human protein, it won’t trigger an immune response the way a virus does. This means that it could potentially be used at higher doses and achieve more efficient editing without immune-mediated side effects. I’m looking forward to seeing if this works in vivo.
Note: Don’t worry, the herpesvirus series is still in progress. Those viruses aren’t going away any time soon.
They also did some control experiments to rule out that the effect was caused by packaged Cre protein.