This episode summarizes a study that genotyped the CCR5Δ32 deletion in ancient and modern human genomes, compared genotyping methods for low‑coverage ancient DNA, reconstructed CCR5 haplotypes, and modeled the deletion's spatiotemporal frequency and selection history. The work benchmarks HAPI (with informed priors) against GATK and VG, maps haplotype distributions (A,B,C), and infers allele frequency trajectories and selection signals across ancestries and time periods.
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 you're walking around right now with a biological shield built directly into your DNA, a shield that protects you against one of the most notorious, devastating viruses in modern human history, which is HIV.
0:21Yeah, it sounds like science fiction. Right. And the crazy thing is, this shield isn't some complex engineered piece of synthetic biology or, you know, a futuristic vaccine. It is literally a microscopic genetic typo.
0:35Just 32 missing letters of DNA code out of the 3000000000 letters that make up your genome. That is all it takes to build a fortress around your immune cells. It really is incredible when you think about the scale of it.
0:46It is. So how did this microscopic deletion become so incredibly common in certain parts of the world today? Was it driven by the horrors of the Black Death? Was it spread across the seas by Viking conquerors?
0:56And what really happens when we try to artificially edit this ancient shield into human embryos using modern tools like CRISPR? Well, those are the exact questions that have kept geneticists arguing for decades, actually.
1:07Because when you're looking at an anomaly that is a literal blank space in the human genome, tracing its history becomes a monumental task. Which brings us to today's deep dive. Today we celebrate the work of a massive collaborative team from the University of Copenhagen, including the Novo Nordis Foundation Centers and the Globe Institute, along with the Lundbeck Foundation Geogenetics Center, who have advanced our understanding of our own genetic evolutionary history.
1:33Yeah, it's a huge undertaking. The scale of what this collaborative group managed to pull off is just a massive leap forward. For sure. The specific paper we're looking at is titled tracing the evolutionary history of the CCR 5 Delta 32 deletion via ancient and modern genomes, and it was authored by Christine Raven, Leonardo Cabuccio, and colleagues, published in the journal cell in July 2025.
1:55They essentially built a computational time machine out of ancient crumbling DNA to settle a debate that has, you know, fractured the scientific community for years. Okay, let's unpack this. To understand the mystery, we really have to understand what this typo actually does inside your body.
2:11The gene in question is called CCR 5. Right. And normally, this gene provides the instructions to build a protein that acts like a communications antenna on the outside your immune cells. Its day job is to listen for chemical signals and help regulate inflammation when you get sick. Exactly.
2:30But when you have the specific typo, the CCR 5 Delta 32 dilution 32 letters of the instruction manual are just, well, they're gone. Just totally missing. Yeah, which means the cell reads the instructions, hits a premature stop sign, and builds a broken antenna.
2:45That broken receptor never actually makes it to the surface of the immune cell. And here's where he gets fascinating from a medical standpoint. HIV one, you know, the virus that causes AIDS acts like a burglar that specifically looks for that CCR 5 antenna to pick the lock and break into your immune cells.
3:00The virus needs that specific handle to get inside. Right. So if you inherited this deletion from both of your parents, meaning you are homozygous for it, You don't have the antenna, the burglar has no way in, you are naturally resistant to HIV.
3:14Doctors have even taken stem cells from donors who naturally have this mutation, transplanted them into patients with HIV and leukemia, and effectively cured them. It is a remarkable medical reality, but, uh, if we connect this to the bigger picture, we have to look at the collateral effects.
3:31Genetics is rarely a simple one-to-one relationship. Oh, definitely not. The CCR 5 gene is highly clear tropic. Think of it like a master electrical switch in a house that isn't just wired to one room, but somehow connects to the heating, the plumbing and the security system all at once.
3:47That's a great way to put it. Yeah, because this gene regulates fundamental immune inflammation, having a broken antenna affects your susceptibility to a lot of other things, like it modulates how severely your body reacts to COVID-19.
4:00It plays a role in autoimmune diseases, like inflammatory bowel disease. It's implicated in murological disorders. Wait, really? Neurological disorders too. Yeah, and it even affects tumor progression in certain cancers.
4:11So if you are someone who carries this deletion, your entire immune baseline is just operating on a totally different frequency, which brings us to the great geographical mystery. If you're listening to us from northern Europe right now, there is up to a 16% chance, you carry this allel.
4:29Which is surprisingly high. It is. But if we look at populations in Africa or East Asia or indigenous populations in the Americas who don't have recent European ancestry, it is virtually non-existent. Why would an immune system tweak be so heavily concentrated in one specific corner of the globe?
4:46Well, that geographical concentration is the classic hallmark of a severe evolutionary bottleneck, something terrible swept through Europe, and selectively spared the people who had this broken antenna.
4:56And theories have been wild. I mean, for a long time, the dominant guess was the bubonic plague in the 1300s. Right, the Black Deck. Yeah. And others pointed to smallpox outbreaks. Some even said it originated in Scandinavia, and the Vikings ceded it along their rating routes.
5:11The estimates for when this tigo 1st happened, ranged anywhere from, like, 700 years ago to over 5000 years ago. It was all over the map. Exactly. So to solve it, we'd need to look at ancient DNA. But doing so feels like an impossible hurdle.
5:28I was trying to picture the reality of the lab work here. Working with ancient DNA isn't like taking a cheek swab from a living person. It's like trying to read a shredded water damaged manuscript that has been buried in the dirt for millennia.
5:42That is a very accurate way to visualize it. When DNA degrades over 1000s of years, it breaks into tiny fragments, often only 40 or 50 base pairs long. And the researchers are looking for a hole that is 32 base pairs long.
5:55How do you find a missing paragraph in a book that's already been run through a woodchipper? You don't look for the missing paragraph at all. You look for the unique ink splatters on the surviving pages that were sitting right next to it.
6:06Oh, okay. Yeah, the team at Copenhagen recognize that genetic mutations don't occur in a vacuum. When this 32 letter deletion 1st happened 1000s of years ago. It happened inside the body of one specific human being.
6:20And that human being had their own unique pattern of other single letter typos surrounding that gene. Because DNA is a physical structure. The gene sitting next to each other on the chromosome physically travel together down the family tree.
6:34Exactly. In genetics, we call this linkage to equilibrium. It just means that certain variants are passed down together as a block because they're physically too close to be easily separated during reproduction.
6:45makes sense. So the researchers discovered that the CCR 5 deletion is surrounded by a massive entourage. It sits on a specific genetic block they named hapletype A, flanked by 84 other genetic variants, which we call tag SNPs.
6:59Okay, so if the researchers can find those 84 other surrounding typos, they can constantly assume the massive deletion is sitting right in the middle of them, even if the DNA fragment containing the deletion itself has completely rotted away.
7:12That underlying logic led them to build a custom computational tool specifically for this study called API. API. Yeah, it stands for Apple type aware, probabilistic model for Indels. It functions entirely on probability.
7:25The software uses 4 of those highly linked tag SMPs as a prior clue. If the software detects those 4 specific variants in a fragmented ancient genome, it mathematically amplifies the probability that the deletion is there.
7:40Acting like a highly sensitive genetic metal detector. And they needed a sensitive metal detector because the ancient DNA they were looking at was just incredibly degraded. The paper notes. They were successfully identifying this deletion in genomes with as little as .3X coverage.
7:55What does that actually look like for a geneticist staring at a screen? It means you're looking at a genome where roughly 70% of the data is just completely missing. Wow. Yeah. Traditional sequencing tools look at a .3X coverage sample and just output statistical noise.
8:09They require multiple overlapping reads to confirm a variant. But HAPI bridges those dark gaps in the data by leveraging the entourage effect. If the data is that fragmentary, you would need an enormous sample size to draw any real conclusions.
8:23How many skeletons are we talking about here? They analyzed an unprecedented 934 ancient genomes, cross-referencing them against 2504 modern genomes from the 1000 genomes project. That massive. We are talking about spanning 1000s of years of human history across the globe, rather than just looking at a handful of bones from a single medieval graveyard.
8:47So when they run this massive data set through their new mathematical metal detector, what do they actually find? Where did this typo come from? Here's where it gets really interesting. They completely rewrote the timeline.
9:00The data definitively shows this mutation is vastly older than the black death or the Viking age. They tracked it all the way back to the Western Eurasian step. originating at least 6,700 years ago. Over 6000 years ago.
9:13Yeah, they anchored this finding on a specific skeleton from a site called Karagash, in modern day Kazakhstan, dating back nearly 5000 years. A Bronze Age pastoralist, living on the steps. And crucially, that specific skeleton from Caragash yielded a highly preserved genome with 26 times coverage.
9:31Because the preservation was so good, the researchers could read the entire genetic sequence without relying entirely on probability. So they didn't just guess it was there. No, they physically saw the entire entourage, all 86 tag variants of hapletype A traveling right alongside the deletion.
9:48It proves an unbroken genetic lineage from a pastoralist on the ancient steppe directly to modern populations in northern Europe. So we have the origin. But a single person getting a random mutation on the steps doesn't explain how it ended up in 16% of Northern Europeans today.
10:05Nature had to actively reward the people carrying this broken antenna. Right. The team used statistical modeling to map how the alleles spread across space and time. They found a massive signature of positive selection, but it didn't happen in the Middle Ages.
10:18The explosive growth of this mutation happened between 8000 and 2000 years ago, spanning the late Neolithic period and the Bronze Age. What was happening during that specific window that would make a broken immune antenna so incredibly valuable.
10:33It coincides with a monumental shift in how humans lived. This was the era when populations began adopting agriculture, building denser settlements, and most importantly, living in very close proximity to newly domesticated animals like cattle, sheep, and pigs.
10:48Oh, zoonotic spillover. The animals were carrying pathogens that humans had never encountered before. A barrage of them. The selection coefficients the researchers calculated are staggeringly high. Some unknown ancient pathogen, or perhaps a series of pathogens, was violently sweeping through these dense early farming communities.
11:08So it was a survival of the fittest situation. Exactly. If you didn't have the CCR 5 deletion, you were highly susceptible, if you had it, your immune system didn't overreact, and you survived to pass the hapletype A entourage to your children.
11:21Wait, I want to clarify how this impacts the old theories. If the statistical models show the rapid spread of this mutation plateaued about 2000 years ago, that completely reshapes how we view history.
11:31It mathematically rules out the bubonic plague and the Viking expansions as the primary drivers. By the time the Black Death ravaged Europe in the 14th century, the genetic frequency of this deletion was already firmly established in the population.
11:46The Vikings may have moved it around locally, but they didn't create the massive evolutionary surge. That makes so much sense, but it also brings to mind a totally different paper that came out recently.
11:57Didn't Linen colleagues publish a widely circulated pre-print, claiming there was no evidence of ancient selection for this gene at all? Yes, and the Copenhagen team directly addressed that. The older study by Lenin colleagues relied on a fundamentally flawed methodology.
12:12Oh really? Yeah. Because they were using a DNA capture technique that struggled to see the physical 32 base paradeletion, they relied on a proxy. They looked for a single single letter variant called RS7333033.
12:26Assuming that whenever they saw that variant, the deletion must be sitting next to it. So Len colleagues thought they were tracking a specific footprint for the CCR 5 deletion, but they were actually tracking a completely different shoe.
12:37That is a perfect way to conceptualize it. Imagine trying to track a rare wolf migrating across Europe by looking for very specific paw print. But years later, you realize that exact same paw print actually belongs to a common wild dog that lives all over Africa.
12:52Oh, I see. If you count the dog prints as wolf prints, your tracking data is going to be a complete mess. The new research shows this proxy marker is highly common in African populations where the actual 32 base paradeletion literally does not exist.
13:07It belongs to much older, different genetic lineages. So Lanning colleagues were tracking the wrong footprint, which diluted their data and entirely masked the explosive evolutionary signal that the API tool uncovered.
13:19That is wild, and tracking the correct footprint with API didn't just reveal the ancient steppe origins. It also uncovered some fascinating modern migration patterns. The paper notes that they found this exact step butation, along with its specific hapletype A entourage in Latin America today.
13:36How does a Bronze Age step mutation end up in modern Colombia or Puerto Rico? It serves as a stark genetic map of the post-Columbian exchange. When European, African, and Native American populations began mixing after 1492, this ancient mutation was carried across the Atlantic by Spanish colonizers, the researchers were even able to identify homologous recombination of the haplotype in Latin American individuals.
14:01Okay, for those of us who aren't geneticists, what does homologous recombination mean in this context? Think of your DNA like a deck of cards. When a person reproduces, their body shuffles the genetic deck they got from their mother with the deck they got from their father before passing half of it to their child.
14:16Over generations, long blocks of DNA get broken up and mixed. The researchers found segments of this specific hapletype A entourage in Latin America that had physically crossed over and shuffled with other genetic lines, but still maintain the core sequence.
14:30It acts like an undeniable genetic fingerprint pointing directly back to post 1492 Spanish colonization. So what does this all mean? We have a microscopic typo born on the Eurasian step over 6000 years ago.
14:43It was forged into a biological shield by forgotten Bronze Age plagues. It carries a massive entourage of 84 other genetic variants. And today, it grants modern humans resistance to HIV. How should this ancient history impact modern medicine?
14:59What's fascinating here is realizing that the CCR 5 Delta Coma 2 deletion is not just an isolated off switch, for decades, a medical community has viewed it as a simple binary problem, if the gene is broken, you don't get HIV, let's find a way to break it in, everyone.
15:13Sure, that seems logical on the surface. But this research proves that this deletion arose on a pre-existing hapletype. Those 84 other surrounding variants are part of the package. Meaning when nature gives you the ATIV shield, it also hands you 84 other genetic tweaks.
15:28And those tweaks aren't silent. When the researchers queried massive genetic databases, they found that those 84 tag variants are linked to many of the exact same complex immune traits as the deletion itself.
15:42Some of these variants actually sit inside other nearby immune genes, like CCR2 and CCR3. So they might be secretly tweaking the volume dials on our entire immune system in ways we haven't fully appreciated.
15:54That is the danger of ignoring evolutionary history. The natural shield we see in northern Europeans works within a specific balanced biological environment that evolved over 6 millennia. Which brings us to the elephant in the room, human gene editing.
16:09A few years ago, a scientist controversially used CRISPR to edit human embryos. He used molecular scissors to snip out those exact 32 letters, trying to recreate this deletion and make those babies immune to HIV.
16:22And this paper elegantly demonstrates why that was so incredibly reckless. If you just take a molecular scissor and snip out 32 letters in a sterile lab, you are completely ignoring the 84 other variants that naturally evolved alongside it to balance the immune system.
16:36You're moving the context. Exactly. You are artificially dropping a massive genetic deletion into a foreign genetic background, a context that has never existed in human history. We have no idea what clinical side effects that might have on a child's immune system as they grow up.
16:51You cannot decouple the genotype from the evolutionary haptotype. It is a profoundly humbling reminder of how complex our biology is. We can't just play a molecular mechanic without reading the whole manual.
17:04But to be fair to the scientific process, the research is that Copenhagen do point out some limitations in their own work, don't they? They are very transparent about the caveats. Ancient DNA is inherently problematic.
17:15Even with a brilliant probabilistic tool like happy eye, postmortem DNA damage is a reality. As DNA degrades in the soil, the chemical bonds break down, which can cause the sequencing machines to misread the letters.
17:27It's like trying to read that shredded manuscript, but the ink is also run, so a letter C looks like a T. That chemical damage can lead to slight misclassifications. Furthermore, they highlight the challenge of uneven spatial temporal sampling.
17:40Meaning we only have data from places where people actually bothered to dig up skeletons. Precisely. The archaeological record is heavily biased. We have massive databases of sequence skeletons from Neolithic Denmark and Germany, but far fewer high coverage genomes from other parts of the world during the same time periods.
17:59This uneven map means there is always a risk that our models of exactly how fast or where the allele spread are slightly skewed by where we happen to point our shovels, but even with those caveats, the sheer volume of data and the clear linkage to Apple type A make the overarching narrative undeniable.
18:18It really does. The CCR 5 Delta 32 deletion is not a recent medieval anomaly that an ancient genetic shield born on the Eurasian steppe over 6700 years ago, driven to high frequency by forgotten pathogens during the Bronze Age.
18:31This deletion travels with a massive entourage of other genetic variants that shape our immune systems today. What does this mean for our modern attempts to edit the human genome? When a single typo is actually part of a much larger ancient evolutionary story?
18:43It reminds us that evolution isn't just about the words that are missing. It is about the entire sentence that remains. 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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