This episode examines a study showing that insertion/deletion mutations (indels) in the v1 loop of the antiviral protein TRIM5a can create new viral specificities in a single step, whereas missense mutations often cannot. The authors used saturation missense mutagenesis, combinatorial libraries, and a novel deep indel scanning approach to compare evolutionary potential.
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. You know, when we usually think about evolution, we tend to picture this incredibly slow, majestic escalator.
0:16Yeah, like tiny incremental changes gradually building up over 1000000s of years. Exactly. But there is a place where that escalator is replaced by like a high-speed roller coaster, and that is the evolutionary arms race between our immune system and viruses.
0:31Oh, definitely, because viruses replicate so rapidly, they mutate constantly to just slip past our defenses. Right, and our cells basically have to evolve just as fast to catch them. But this race is a really compelling question for you to ponder today.
0:44What happens when our cells face a viral threat that is mathematically almost impossible to defeat through normal, slow genetic evolution? It creates a terrifying situation for the host. Traditional adaptation basically hits a brick wall.
0:56Exactly. What if the virus has jumped so far ahead that catching up would require rolling a biological 6 sided die and getting a 6 50 times in a row? The organism is just left completely vulnerable to infection.
1:09Right. But here is the hook for today's deep dive. What if a genetic type of specifically, an insertion or a deletion of DNA that is almost always catastrophic for a protein could actually be a secret evolutionary shortcut?
1:23It sounds so counterintuitive. It really does. I mean, how could a destructive, chaotic mutation that usually breaks a biological machine actually save us from a completely new virus? Today, we celebrate the work of the research teams at UCSF and the Fred Hutchinson Cancer Center, who have advanced our understanding of how antiviral proteins evolve via insertion and deletion mutations.
1:44We are taking a deep dive into a June 2025 paper published in cell genomics by 10th 3 in colleagues. And it completely reframes how we think about genetic variation, right? Oh, absolutely. It changes structural biology and immune defense fundamentally.
1:58So to really grasp the magnitude of what this research team uncovered, we need to, um, set the stage with the specific frontline defenders in our cells. We were talking about restriction factors. Right.
2:10Restriction factors are innate immune proteins. You can think of them as like the cells internal security guards. Just floating around looking for trouble. Basically, yeah. Their entire job is to flow around the side of plasm, recognize the signatures of incoming viral invaders, and neutralize them before they can travel to the nucleus and hijack the cell's replication machinery.
2:30Okay. And the specific security guard the study focuses on is a protein called trim 5, or human trim 5 alpha. Yes, it's an antiviral protein specifically tailored to target retroviruses. Like HIV and SIV, the simian immunodeficiency virus.
2:47Exactly. And trim 5 has a very specific biological weapon for this job. It uses this, um, highly variable, largely unstructured segment of its anatomy called the V1 loop. Right. The V1 loop. It uses this loop to physically grab onto the capsids.
3:02Which are those geometric protective outer shells of the incoming viruses? Yes. But what is crucial to understand about this V1 loop is just how disordered it is. What do you mean by disordered? Well, most of a protein is folded into a very strict, rigid, predictable 3D structure.
3:18The V1 loop is not. It is highly flexible. Oh, so it acts almost like a loose, floppy net that the trim 5A protein just throws over the viral capsid. That's a great analogy. But because viruses are constantly mutating the surface of their capsids to evade this net, the V1 loop has to evolve constantly.
3:37To change the shape of its netting to keep catching them. Exactly. So if the virus is constantly changing its protective shell, how is trim 5A supposed to keep up? Traditionally, we've assumed it uses mis sense mutations.
3:49Right, which are the small tweaks. Yeah. In genomics, if we think of DNA as a string of letters making up words, a misense mutation is just swapping a single letter, like changing the word cat to bat. It alters the meaning slightly, yeah.
4:01Right. It might change one single amino acid in the protein, but it's a small, manageable structural change. And most research focuses heavily on these misense mutations. But today we are focusing on something entirely different.
4:14Indels. Indels, that is short for insertions or deletions. So instead of gently swapping a single letter, you are violently cramming an entirely new chunk of letters into the genetic sequence, or just ripping a chunk out.
4:27To put that into perspective for you, Indols actually account for about 10 to 20% of human genetic variation, but they are usually absolutely disastrous for the function of proteins. They really are. I mean, if a protein is a precise clockwork mechanism, an indel is like suddenly duplicating a random gear or just yanking a vital gear out, it almost always jams the entire machine.
4:49You can see why indels are heavily selected against in nature. They wildly distort the secondary structure of proteins. They create weird bulges or they misalign all the other working parts. But the authors of this paper wanted to know if these usually destructive indels could somehow be uniquely useful against a specific terrifying global health threat.
5:09Right. Zenotic spillovers. Which is when a virus jumps from an animal reservoir into the human population. Precisely. They focus their research on a virus called SIVSEB. It is a virus endemic to Sabeus Monkeys, which are a species of African green monkey.
5:25Now, if you look at Reese's Monkeys, they possess a version of Trim Fiva that easily catches and neutralizes CIVSAP. But human trim 5 though. It is completely blind to it. The human V1 loop cannot restrict SIVSEB at all.
5:40And understanding how our human immune proteins could potentially evolve to stop such novel animal viruses is absolutely critical for predicting and preventing future pandemics. It's basically the whole ball game.
5:52So the researchers have their clear target. They want to see if they can mutate human trem fiva, so it can successfully catch this monkey virus, SIVSab. Right. And they started with the classic state of the art approach to this problem, which is deep mutational scanning or DMS.
6:07But I really want to dig into what that actually looks like in a laboratory because it's not just a computer simulation, is it? No, it is a massive physical brute force biological experiment. So what did they actually do?
6:18Deep mutational scanning means they took the genetic code for the V1 loop of Human Trem 5A. And they synthesize a massive library of physical DNA plasmids. They systematically created every single possible misense mutation, right?
6:32Yes. Every possible single letter genetic swap, resulting in every single possible amino acid change across that entire loop. That is wild. So you just have this huge soup of 1000s of slightly different human trim 5A genetic blueprints.
6:49What do you do with them? You introduce them into a massive population of human cells growing in a dish. Okay. So that each cell takes up just one mutated blueprint and starts manufacturing that specific variant of cream 5A?
7:02And then you subject all those cells to a brutal test? Exactly. You infect the entire dish with a modified version of SCSab. But here is the trick. Um, they engineered the virus to carry a gene for a green fluorescent protein.
7:16Right, so the infection becomes visible. If the virus successfully slips past the mutated trim fiva and hijacks the cell, it forces the cell to manufacture that fluorescent protein, and the cell literally glows green.
7:27But if the treatment 5 variant successfully intercepts the virus, the cell stays dark. That is so elegant. It really is. Then they run 1000000s of these cells through a machine called a flow cytometer.
7:38And that uses lasers to look at every single cell, one by one. Right, yes. The machine physically sorts the cells, separating the glowing green, infected cells from the dark surviving cells. And by sequencing the DNA of the surviving cells, you can see exactly which single letter mutations save them.
7:56Exactly. And we know this incredibly complex method works beautifully for other viruses. Right, because previous studies showed that almost half of all possible single mis sense mutations in this V one loop actually give human tree 5A some ability to fight HIV one.
8:12Right. So they run this massive flow psychometry experiment against a VSAB. They test every single single letter change. And the result? Exactly zero. Wait, really? Out of all possible single amino acid changes, not a single one help human train 5A fight cybisec.
8:28Not a single one. It was a spectacular failure of the missing's paradigm. SIV set proved to be an incredibly formidable evolutionary challenge. It showed that a single gentle mutation simply wasn't going to cut it.
8:39The shape of the viral capsule was just too different. So when single changes failed, the team had to get creative. They built what's called a combinatorial library. Right. They knew the Reese's Monkey version of Trimmo 5 could stop the virus.
8:52So instead of random single mutations, they started intentionally mixing and matching human and Reese's V1 loop parts. They wanted to map out the exact evolutionary path the human protein would need to take to finally grab the SIVSab capsid.
9:10And the results of that chimera experiment really illustrated the problem, didn't they? Oh, vividly. To get human trim Myfod to work against SIVSAV via misinsputations, it required an incredibly specific combination.
9:23How specific? It needed exactly 5 Reese's like mutations right in the center of the V1 loop, plus a 6th modifier mutation, sitting just outside the loop. Six specific simultaneous amino acid changes. Exactly. That is not a gentle evolutionary slope.
9:38Getting 6 specific mutations is an impassable canyon. Let me put it this way for you. It's like trying to open a 6 style combination lock. like that analogy. Evolution works when every correct number you guess gives you a slight survival advantage, right?
9:51You lock in the 1st number, you're a bit stronger, so you survive to guess the 2nd number. That's the classic model. But here, getting one mutation or 2 or 3 or 5 did absolutely nothing to stop the virus.
10:03There was 0 survival advantage along the way. Right. An organism would basically have to guess all 6 numbers on the combination lock at the exact same time by pure random chance to get any benefit. Which is just mathematically absurd.
10:18In evolutionary biology, we call that a flat fitness landscape. Yeah. The organism is wandering blindly through multiple useless mutations before ever seeing a reward. It's practically impossible on any realistic biological time scale.
10:31Exactly. The human population would be decimated by the virus long before that combination was randomly rolled. So the gentle misense route is a complete dead end. And this is where the team introduces their breakthrough method.
10:43Deep Indel scanning or diss? Instead of swapping single letters, they intentionally created a library of genetic stutters. Right. They engineered duplications and deletions ranging from one to 9 amino acids long, sweeping across the entire V1 loop.
10:58They were mimicking a natural biological phenomenon called DNA polymery slippage. Which is what, exactly? Well, when our cellular machinery copies our DNA. It's not perfect. The polymery enzyme sometimes stutters on the track, accidentally copying small genetic sequence twice, or just skipping over a chunk entirely.
11:17Wait, I'm looking at this methodology and something feels, um, a bit too clean. They purposefully designed this experiment to only test what they call in frame indels, right? Yes, they did. So they only inserted or deleted chunks of DNA in exact multiples of 3 letters.
11:33But in nature, polymerary slippage is sloppy. It doesn't care about the rules of reading frames. That is an excellent point and a vital distinction in genetic mechanics. DNA is read by the cell in triplets called codons, right?
11:46Right. each triplet equals one amino acid. So if a natural stutter inserts just one or 2 DNA letters, it causes a frame shift. It pushes the entire downstream reading frame out of alignment. Exactly. Every single piece of the protein code past that typo becomes complete gibberish.
12:02That almost always results in a totally destroyed, truncated non-functional protein. So by forcing the library to stick to multiples of three, these in-frame indols. The researchers avoided scrambling the rest of the trim 5 a protein.
12:14Precisely. They basically just smoothly added or removed an intact gear from the middle of the mechanism. to see what would happen specifically to the local shape of the V1 loop. Right. They run this deep Indel scanning library through the exact same flow cytometry gauntlet.
12:31They infect the dish with the green fluorescent SIV sub. And they wait to see if any of these gear breaking in frame stutters actually save the cell from glowing green. And this is where the massive aha moment of the paper happens.
12:42The disk library revealed that duplicating just one single amino acid. Specifically, a phenylinine at position 339. Right, which they refer to as the F 339 Julebut mutation. That instantly gave human trim 5A potent robust ability to restrict SIV sub.
13:00It is an astonishing leap. One stutter, 3 little nucleotides naturally copied twice during a polymerary slip, and suddenly the protein achieves what 6 impossible misends mutations couldn't. It bypasses that impassable evolutionary canyon entirely.
13:15totally bypasses it. It's incredibly counterintuitive. I mean, you're throwing an extra gear into the system, and instead of breaking, it instantly calibrates to a completely new threat. And it wasn't a weak response either.
13:27Not at all. The F 339 duplication provided an antiviral restriction capability that was equivalent to completely swapping in the entire Rees' Monkey V1 loop. Which, as we established, represents a vast evolutionary distance.
13:41But there was a bonus benefit here that I found, frankly, staggering. Usually in these host virus arms races, if a protein mutates its shape to stop a brand new virus, it loses its structural ability to stop an old one.
13:55It's a strict trade-off. That is the standard assumption yes. Plearchopy usually dictates a cost. But the researchers found that the F339 depth mutation didn't just stop the new threat, SIV SAB. It also enhanced the protein's ability to restrict HIV one and another virus called SIVCTZ.
14:11And it did all of this without impairing any of its existing cellular functions. So there was absolutely no apparent evolutionary cost to this massive structural leap. None that they could find. Three nucleotides copied, one evolutionary leap.
14:24It's like throwing an extra gear into a mechanical watch. And not only does it continue keeping perfect time, but it also suddenly starts perfectly tracking the phases of the moon. It's a complete biological shortcut.
14:35It really is. But of course, listeners might be wondering, you know, sure, a genetically engineered insertion works in a highly controlled flow cytometry lab setup, but does nature actually use this cheat code in the wild?
14:47Well, that is the crucial next step, the researchers took, to validate that this isn't just lab trickery. They look deep into the evolutionary phylogenetic history of primate trim fiva. They map the genetics of various monkey species, right?
15:02Yes, and they found that primate trim fiva has naturally relied on these exact indel mutations at least 3 separate times in its evolutionary history. Wow. So nature has been running its own deep indel scanning experiment for 1000000s of years.
15:15Exactly. For example, they found that the ancestors of modern macaques and baboons evolved a 2 amino acid insertion right in this loop. Okay, that's one. And all new world monkeys share a distinct 9 amino acid deletion in the exact same region.
15:29They just ripped a chunk out. Right. But perhaps the most striking historical example involves the African green monkeys. They naturally possess a massive 20 amino acid duplication in their V1 loop. 20 amino acids.
15:43That is a massive chunk of genetic material to just suddenly duplicate. It is huge. And the researchers experimentally proved that this specific natural historical duplication is exactly what allows those African green monkeys to fiercely fight off SA Mac.
15:58Which is a potent virus endemic to captive macaques. Exactly. Without that massive messy stutter, they would be vulnerable. Okay, we need to synthesize what this means for biology as a whole. We started by stating that indels are disastrous, gear breaking mutations that usually cause disease.
16:14Have we been looking at this fundamental mechanism all wrong? It represents a major paradigm shift in structural biology. We need to stop viewing Indel's purely as destructive genetic errors. What are they then?
16:25The reality is that they are a high risk, high reward evolutionary strategy. It is true that an indel will absolutely shatter a rigid, highly ordered protein core. Right, the clockwork part. Exactly. But unstructured regions, like the V1 loop of Prime of 5 are fundamentally different.
16:44Because they are intrinsically disordered like a loose net, they have the physical tolerance to absorb these massive structural leaps. They take the shock of the Indel and use it to instantly reconfigure their entire interaction surface.
16:57I really want to dig into the biophysics here. I want to know how this is like actually working on a molecular level. Okay, let's break it down. If misense mutations are like changing the electrical charge of the net to make it magnetically stick to the virus.
17:11What is an extra finale and amino acid actually doing mechanically to the net? Well, this brings up the concept of the entropic penalty. Remember, an unstructured loop is floppy. To successfully grab the viral capsid.
17:23It basically has to freeze into a very specific rigid shape that matches the capsid surface. In thermodynamics, forcing a chaotic, floppy structure into a highly ordered, frozen state takes a massive amount of energy.
17:36That energy caused as the entropic penalty. Oh, so, um, it's like trying to wrangle a wet noodle into a very specific, tight lasso shape. It takes a lot of work to overcome the floppiness. is a perfect analogy.
17:49Now, consider the specific amino acid they duplicated. Finiling. Okay. Phenoline is a bulky, highly hydrophobic molecule. It fiercely repels water. Meaning it hates the watery environment of the cell. It wants to hide.
18:03The theory proposed by the authors is that to hide this new hydrophobic bulky gear, the V1 loop is forced to prefold around it. Oh, wow, that makes so much sense. By trying to hide the water hating finale in the loop accidentally starches itself.
18:17Exactly. It restricts his own floppiness and naturally falls into a pre-folded shape that just happens to be much more compatible with the viral capsid. It drastically reduces the energy needed to form that lasso.
18:28Precisely. It alters the mechanical flexibility of the loop, bypassing the need to perfectly tweak the electrical charges through 6 different misense mutations. It is a beautiful biophysical shortcut. But, you know, to be fair to the scientific process, we do need to point out the study's imitations.
18:46They couldn't directly observe every aspect of this wet noodle lassoing the virus, right? True. The researchers objectively note that they couldn't directly measure the exact chemical binding affinity of the mutated human trim fita to the SIV sab capsids in a test tube.
19:03Why is it so hard to measure? Because the natural affinity is notoriously low and hard to measure. Trim feeder relies on multiple very weak interactions clustering together across a vast lattice to work.
19:14Right. That makes it incredibly difficult to isolate and measure a single protein's exact binding strength using current biochemical assays. And there's also that limitation with the Indels themselves that I brought up earlier, the whole clean library issue.
19:28Yes, the in phase versus out of phase Indel problem. They purposefully designed their laboratory steaders to insert whole words intact, three-letter codons. Right. They only tested in phase indels to keep the rest of the protein pristine.
19:40But nature is messy. A polymerase slippage can happen after the 1st or 2nd letter of a code on. Which creates an out of phase indel. Exactly. These out of phase indales might eventually restore the overall reading frame down the line, but at the boundary of the stutter, they create a messy genetic collision that causes adjacent misense mutations right next to the indo.
20:02Right, because the letters are suddenly grouped differently at the edges of the type. Exactly. By designing a clean, perfect library. The researchers basically ignored about 66% of the indels that might naturally occur during DNA replication.
20:15Those messy out of phase indoles might hold even more powerful evolutionary secrets, or they might introduce structural flaws that completely destroy the benefits we just talked about. We simply don't know yet because they weren't scanned.
20:27Which perfectly highlights where this field needs to go next. I mean, the obvious next step is expanding this indel scanning technique to include those messy, out of phase natural stutters. Absolutely.
20:39But more importantly, applying this to other innate immune proteins with unstructured loops like MXA or MXB to see what other hidden defenses we might unlock. To synthesize all of this, the central insight of the tensory paper is profound.
20:54Indel mutations, long dismissed as primarily destructive genetic errors actually serve as crucial evolutionary shortcuts. They allow host immune proteins to instantly cross impassable fitness landscapes.
21:07Right, and acquire complex new defenses against rapidly evolving viruses in a single leap. It completely changes how we view adaptation. Before we wrap up, I just can't stop thinking about that broken mechanical watch analogy.
21:19Yeah, the extra gear. We spend so much time in biology and medicine, trying to gently tune the mechanism, polishing the gears, swapping a tiny spring here and there to make the immune system tick perfectly against a new virus.
21:31The mis sense approach. Right. But sometimes, survival doesn't belong to the perfect timekeeper. Sometimes, survival belongs to the system that survived having a massive wrench thrown into its gears, and somehow adapted to perfectly track something entirely new.
21:46What does this mean for our future? If a single genetic stutter can leapfrog 1000000s of years of slow evolution, how might we use these inbells to engineer synthetic immune proteins capable of stopping the next pandemic before it starts?
22:00That is the ultimate question we should all be asking. This episode was based on an open access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description.
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