0:00Welcome to Base by Base, the papercast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. Imagine this. You're trying to use probably the most precise tool in the world, CRISPER, to find the exact genes that let cancer spread.
0:17Metastasize. So you load up your genetic library, you put it into your model, and you wait. But when you look the results, the tool itself has completely wrecked the experiment. And not because it failed to edit the genome?
0:30No, not at all. It's because the immune system of the host is busy fighting your editing tool, not the cancer. It's the ultimate experimental blind spot. I mean, we rely on CRISPR screening to discover these cancer vulnerabilities, but when you do those screens in Vivo inside a living body with a real immune system, The um, the standard bacterial parts we use are just immediately flagged as foreign.
0:52And soon as they're flagged, the immune system just launches a T cell response. It does. It selectively kills off the tumor cells that have the CRISPR machinery in them. So it's a totally artificial reaction.
1:03It's an anti-tool response, which just it skews all the data. It causes what the researchers call, and I think this is a great term, iatrogenic clonal dropout. It's a perfect description. It means the intervention itself is hiding the very things you're trying to find.
1:18Exactly. And if we can't study metastasis in an environment that actually reflects human immunity, then we can't really trust our findings, can we? The real question becomes, how do we run these powerful, large scale genetic screens in immunocompetent models without triggering this huge immune backlash against the research itself?
1:37And that is our mission for this deep dive, exploring how one team made crisper, essentially invisible to the immune system. Today, we're celebrating the work of Massimosaini, Frances Castrogina and Nicola Essetto, and their team at the Swiss Federal Institute of Technology eats Zurich.
1:54Their breakthrough introduces a systemic, you know, an immunogen free platform that really advances our ability to conduct accurate genetic screens. Letting us expose these metastasis regulators that were up until now completely concealed.
2:07Okay, so let's unpack why the standard way of doing things. Got us into this mess in the 1st place. For, what, decades? The easiest path in cancer research was just to bypass the immune system completely.
2:18That's the classic model problem. To study human tumors in mice, researchers have traditionally used immunodeficient models, like NSG mice, they just can't mount a functional immune response. Which makes the experiment much simpler, right?
2:32The host won't reject the tumor or the foreign CRISPR tools. It's simpler, but it comes at a massive cost. You eliminate the single most important factor in how cancer actually works in humans, that dynamic interplay between the tumor and the immune system.
2:47You can't understand how cancer evades the body if the body isn't even fighting back. Right. So when researchers try to move these screens into immunocompetent systems, mice with healthy immune systems, the whole thing just fell apart.
2:59Immediately. The host recognized 2 key things as foreign. First, the bacterial cast 9 nucleus, which often comes from streptococcus pyogenous. A common bacterium. And second, the selection markers we use to track the edited cells, things like GFP or the pure mice and resistance gene purer.
3:17So every single time a tumor cell successfully took up the CRISPR vector, it basically painted a giant target on its own back for the T cells. That is a perfect analogy. The T cells launched a really rapid response, something we call immun editing.
3:34And this just fundamentally distorted the results. The successful tumor cell is the ones with the knockouts you actually wanted to study, were just selectively eliminated. And that led to this drastic contraction and the genetic diversity of the screen.
3:46When you say drastic, what kind of numbers are we talking about? Well, standard screens that used cast 9 in that pure marker, they saw a nearly tenfold reduction in the number of unique single guide RNAs, SGRNAs they recovered.
3:59Denpole. Yeah. I mean think about that. 90% of your initial genetic library, which has all your potential targets, was just lost. Gone, simply because the immune system was attacking your tools. You were almost guaranteed to miss critical targets.
4:11That really sets the stage for a necessary invention. The team realized they couldn't just tweak the system. They had to rebuild it from the ground up to be immunologically inert, and they called their platform stealthy.
4:23Stealthy wise hole guiding principle was to get rid of every single foreign protein that might trigger a response. The goal was to enable screens in any immunocompetent setting. Singenic mice, humanized models, you name it, and they attack the problem with 3 really distinct, and I think, ingenious, strategies.
4:44Okay, let's start with the markers. If you can't use a bacterial resistance gene or, you know, bright green protein to track your cells, what do you use instead? Strategy one was what they called immune stealth reporters.
4:55They just replaced those foreign markers with something the host immune system would ignore. Species autologous reporters. So something that hosts body already recognizes as self. Exactly. For their mouse models, they used owl forms of phi one, which is a surface protein naturally found in mice.
5:11The host already tolerates these alliforms, so the tumor cells don't trigger that initial immune flag just for expressing the reporter. Okay, that handles the permanent tag. But the big bacterial cast 9 protein.
5:22That's the most immunogag part, right? If you express it constantly, it's like a huge alarm bell. And that brings us to strategy too. Hit and run delivery. Instead of integrating the cast 9 gene into the genome for stable, continuous expression.
5:36Which is how it's usually done. Right? Instead, they transiently delivered the purified cast 9 protein itself. They call it OpoCast 9 transfection. It's basically the genetic equivalent of a smash and grab operation.
5:48So the protein gets in, it cuts the DNA, achieves the edit really quickly, and then what? And then it's gone. The Epocastine protein is rapidly cleared by the cell, usually within 48 to 60 hours. That's long before a robust, long-term T cell response can even get going.
6:03It gets the job done without leaving behind that highly visible foreign flag. That's brilliant. But hang on. If the cast 9 protein just disappears. How do the researchers know which cells were successfully edited and which weren't?
6:17You'd need a way to purify your population before you inject them. And that's exactly what Strategy 3 addresses. They call it the Harakiri selection mechanism. It's a way to purify the cells ex vivo outside the body, to make sure only the truly edited ones make it into the host.
6:32Okay, walk us through that. How does that work? They build a vector with a self-targeting guide RNA right alongside the guide RNA for the gene they actually want to knockout. This self-targeting guide is designed to hit a selectable surface reporter, like a truncated CD 8 protein.
6:47see. So when the transient cast 9 protein shows up. It does 2 things at once. It knocks out the gene of interest, and it edits and destroys that surface reporter. So the successful edit is signaled by the disappearance of the tag.
6:59Precisely. The cells that lost the surface tag are your pure knockout cells. The researchers can then use magnetic isolation to just pull out this perfectly edited, pure pool of cells before they ever go into the animal.
7:11And that guarantees that 100% of the injected cells are the ones you want to study. 100% with no lingering cast 9 DNA or protein to set off the immune system. Okay, so that whole sequence, the stealth markers, the hit and run protein, and the external purification, that's what makes stealthier work.
7:28Once they applied it, the difference in their results must have been night and day. Oh, it was startling. When they ran the screens in the immunocompetent models, stealthy Y immediately preserved that clonal diversity, the full SGRNA library that the old cast 9 pure controls just decimated.
7:44They proved the integrity of the screen was completely restored. So with all that experimental noise finally gone, what did they find? What hidden targets started to emerge? Well, they started finding these genuine metastasis relevant genes that were totally shielded by the background noise before?
8:00Things like receptor tyracine Kynas's Erb 3, egg fur and parts of the TGF beta pathway? But the real surprise came from a pathway that had been really entirely unappreciated in this context. The surprising developmental axis.
8:14That would be the consistent identification of the AMHAMHR 2 axis. That's anti-mallurian hormone and its receptor. This pathway, which is mostly known for sex differentiation during development was consistently one of the most critical drivers of metastasis.
8:28And what was the magnitude of that effect? Was this a small discovery? Far from it. When they knocked out the receptor, AMHR2, it caused the most significant drop in metastatic potential they saw, a staggering 20 fold reduction in metastases.
8:4320 fold. Yeah, that's not a marginal effect. is nearly eliminating the tumor's ability to spread. A 20 fold reduction. That's a major vulnerability. And that leads us straight to the translational side.
8:54Does this finding hold up in human models? What does it mean for therapy? This is where it gets really interesting. The AMH MHR 2 axis presents this novel and clinically actionable pathway. First, the researchers confirmed that this cast and immunogenicity problem was also happening in humanized NSG mice, the Hunt NSGs.
9:14So models that have human immune components. Exactly. Standard CRISPR with compromising screens, even in these really valuable human models. Only the Stelk Y platform with a human reporter called Huth Y1 allowed for faithful human metastasis modeling.
9:29So this technical problem wasn't just in lab mice. It was blinding us in our best human translational models too. Absolutely Then they validated the clinical relevance using patient data from the cancer genome Atlas DCGA.
9:42And across different human cancers, they found a clear link. High expression of AMH correlated with a poor prognosis, shorter survival, earlier relapse, particularly in patients with breast cancer. So now they know it's a real human vulnerability.
9:56Yeah. How did they try to target this pathway therapeutically? They leveraged a really elegant natural mechanism. There's a dominant negative splice variant of the receptor called AMHR2 Delta, this decoy receptor.
10:08It lacks the intracellular part needed to send a signal. So it's like a broken lock. A perfect analogy. When they administered this decoy, it effectively acted like a sponge. It's soaked up all the AMH ligened and blocked the pro metastatic signal.
10:21And did it work? Do the decoy holt metastas in the human models? It worked dramatically well. Inhumanized, patient derived xenographed, or PDX models of breast cancer. The decoy suppressed the main tumor volume by over 80%, and it essentially abolished all measurable metastasis.
10:36That sounds like a huge success. But this is the critical moment, right? The moment that validates the entire reason for building stealthy why in the 1st place. This is the ultimate validation. That dramatic therapeutic effect, the tumor suppression, the end of metastasis, it only happened in the immunocompetent Hunnish G mice.
10:55When they ran the exact same experiment in the immunodeficient NFT mice, the decoy did almost nothing. That is just a staggering finding. It means the way this therapy works isn't just about stopping a growth signal and the tumor cell.
11:08It depends on re-engaging the host's own immune system to fight the cancer. Precisely. Blocking this axis is essentially an indirect immunotherapy. They dug into the immune mechanism and found that when you block AMHR2 signaling, it causes this massive shift in the immune cells inside the tumor.
11:25What kind of changes did they see? What flips the switch on the tumor? They saw a big drop in t-regulatory cells? Those are the ones that suppress the immune attack, and a corresponding increase in activated macrophages and cytotoxic T cells?
11:37You know, the anti-tumor fighters? So the MHR 2 axis is normally acting as an immune evasion mechanism. Yes, it's a fundamental one. It subtly buffers all these tumor suppressive signals. The tumor was using it to hold the immune system back.
11:50And when you block it, you release the brake. That's exactly right. And removing that brake triggers this upregulation of powerful anti-tumor signals, especially type I interference genes, and activation of the stat one pathway.
12:04It primes the host's own immunity to fight the cancer effectively. This research really goes far beyond just discovering this one target. It gives us a critical new resource. It dictates how all future and vivo genetic screens really should be conducted.
12:19For sure. The stealthwide platform establishes that the immunological context of your preclinical model isn't just a factor. It is the single most important factor that determines whether you can find these genuine vulnerabilities.
12:30If your tool is immunogenic, you will find false negatives. You will miss targets. So what does this all mean for future research and for you, our listener? It means we have to be incredibly critical of the tools we use.
12:41This study didn't just fix a tool. It revealed a hidden immune evasion mechanism. The AMHR 2 axis, that we can now target to help the body's own immune system suppress metastasis. And this raises, I think a really provocative question.
12:57Considering how many 1000s of CRISPR screens have been run in immunocompetent systems over the last decade, could all those previously discarded results, the experiments that just seem too noisy or failed to yield clear targets, could they be revisited using these new non-immunogenic methods?
13:14Potentially unlocking a whole hidden cache of clinically relevant targets that were just completely overlooking right now. That's definitely something for us to mull over. This episode was based on an open access article under the CCBY 4.0 license.
13:27You can find a direct link to the paper and the license and our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating. If you'd like to support our work, use the donation link in the description.
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