New research shows that virus-like particles can help ferry cutting-edge CRISPR therapies to specific cells.
In new work published today in Cell Reports, IGI Founder Jennifer Doudna and IGI Director of Human Health Alex Marson and their teams show how virus-like particles can deliver genome-editing tools to specific cells. In May, co-first authors Jenny Hamilton and Connor Tsuchida talked with IGI science writer Hope Henderson about the findings and what it means for developing the next generation of CRISPR therapies.
What is the main idea of your paper?
Jenny: Amazing disease treatment options have been built that rely on CRISPR-Cas9 genome editing tools, but there’s still a major challenge to be able to deliver these tools so that they can effectively and safely function as therapeutics. Some delivery strategies rely on viruses and some strategies use non-viral delivery. What we do here is combine the things that are really great about viruses and the things that are really great about non-viral options and bring them together into something called a virus-like particle.
What are the big challenges in delivery for CRISPR delivery?
Connor: Jenny and I are both most interested in using CRISPR specifically in humans as a therapy. One of the really big challenges with delivery is specificity: getting the genome-editing tools just to the cell you want edits in. Keeping edits only to the cells where they are needed reduces the risk of off-target effects in unintended tissues or organs. The virus-like particles really have that specificity.
Jenny: Once the CRISPR-Cas9 molecules are inside the right cell, you also have to think about hitting the correct DNA target and not hitting other sites within the genome. Right now, the main method for delivering CRISPR-Cas9 tools in the body requires encoding the instructions for them in DNA, packaged in an adeno-associated virus. When that gets inside of the cell, the cell uses the DNA instructions to make your editor and your guide RNA continuously. It’s going to be really good at making the change you want, but you’re going to be much more likely to make an off-target edit just because the editing components are always around.
Connor and I are really excited about this new work because instead of delivering genetic instructions to make Cas9 and the guide RNA sequence, we’re actually delivering the already-made protein and guide inside the virus-like particle. It does its thing and then is cleared from the cell within a couple days. Others have shown that using Cas9-guide RNA complexes minimizes unwanted off-target edits. Because of this, we think we can reduce off-target editing by using virus-like particles, compared to a strategy where you’re continuously making the editor components.
What’s the difference between a virus and a virus-like particle?
Connor: The important thing to understand is that viruses have two parts: the outer membrane which you can think of like a shell, and the cargo inside that shell. The shell has components that help the virus get into specific types of cells — that’s the part that we’re co-opting here. The cargo inside the virus is what has the viral genome, makes a virus replicate, and potentially cause you harm. Our virus-like particles lack the instructions required for viral replication.
We used proteins from HIV to make our virus-like particles. HIV is really good at getting into a specific type of immune cell called a CD4 T cell. We’re using part of its shell to get into those cells, but just the shell, none of the viral cargo. In our case, the cargo being delivered is Cas9, and there’s no viral genome so our virus-like particles don’t replicate within the body. When you bring up HIV, people understandably might feel a little hesitant, but there is no possible way to contract HIV from this approach.
What do you do with the virus-like particles in your new research?
Jenny: This our first proof of concept of making virus-like particles that look like one virus or another to take advantage of the natural ability of viruses to get into specific kinds of cells. Like Connor said, we created particles that look like HIV on the outside. HIV gets into a kind of immune cell known as a helper T cells. We showed that in a mixed population of immune cells, we could use these virus-like particles to specifically edit the helper T cells and not edit the other kinds of immune cells.
We did this in a test tube using human immune cells, but there’s no reason to think that it would be any different inside of the body. Our next experiments will be trying it in mice, with immune cells that circulate in the blood system.
One reason this is really exciting and important is that T cells are being used as a strategy for fighting cancer. You may have heard of CAR-T cells. With CAR-T therapies, T cells are taken out of the body and reprogrammed with genomic tools so that they attack cancer cells. It’s a lengthy and expensive process right now — taking blood, filtering the T cells out, editing them, putting them back in. We think our virus-like particles could be used to actually make CAR-T cells by simply giving a patient a treatment with injection or by IV. This would be quicker, cheaper, and easier.
Could other kinds of viral-like particles be used to get CRISPR into other kinds of cells?
Connor: Yes, definitely. There’s a number of naturally occurring viruses that are really good at getting into specific types of cells. For instance, the rabies virus is really good at infecting neuronal cells. So if we wanted to deliver a CRISPR therapy to neurons, we might be able to use some proteins from the shell of the rabies virus to get inside those cells. There’s also been a lot of research using antibodies or other engineered molecules that bind to specific types of cells to direct where things are delivered. Potentially any cell type in the body could be specifically targeted through these approaches.