MIPseq and exome sequencing of 11,555 human congenital heart disease probands implicate 60 dominant CHD genes, with NOTCH1 cysteine‑altering and transmitted MYH6 missense variants driving distinct defects.
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. It's great to be back. So today, we're diving into a medical mystery.
0:12And it's one that has been hiding in plain sight, really, for decades. It has. It's about the most common type of birth defect in the world, something that affects what, over one% of all babies? That's right.
0:25Between one and almost 2% of all live births. It's a huge number. But the mystery here isn't really about the defect itself. I mean, we've gotten very good at the plumbing, so to speak. Oh, absolutely.
0:37Surgeons have gotten incredibly good at repairing the heart structure. Exactly. But here's the puzzle. You can have 2 babies with the exact same heart defect. They get the exact same surgery. One grows up, you know, perfectly healthy.
0:49Maybe becomes an athlete. But the other one, the other one struggles. Severe learning disabilities, maybe autism, other developmental delays their whole life. And for years, the explanation for that was just bad luck.
1:02Bad luck. Maybe something went wrong in the operating room. A random roll of the dice. That was the prevailing wisdom, yes. It's a convenient explanation when you don't have the data to point you somewhere else.
1:13But what if it's not luck at all? What if the answer was written in their DNA the whole time, but we just, we couldn't read it. Because it was just too expensive to look. Exactly. And that's the breakthrough.
1:24We're talking about a technology that dropped the cost of this kind of genetic detective work by 85%. An incredible drop. It took it from this high end research tool to something that costs about as much as, you know, a couple of coffees.
1:37And that lower cost is what finally let researchers see the pattern. It turns out we've been diagnosing these kids based on how they look on the outside. while completely missing the real story happening on the inside in their genes.
1:51It's a huge story. It's about technology, biology, and really how we even define what a disease is. It is. So before we get into the details, we absolutely have to give some credit here, I mean, who are the people behind this massive undertaking?
2:04This was a colossal effort? We really have to celebrate the work of the pediatric cardiac genomics consortium, the PC and the pediatric heart network. A huge collaboration. A huge one. The paper we're looking at was led by Michael Cyrent, Martina Bruchner and Richard Lifton.
2:22Some of the biggest names in the field. Without a doubt. They're working out of Yale, the Rockefeller University and, you know, a whole host of other amazing institutions. Their mission was, Well, it was simple to say, but incredibly hard to do.
2:35Which was? To stop guessing about the genetics of congenital heart disease and to actually map it out base by base in the largest group of patients ever put together. Okay, so let's set the stage a bit.
2:47Congenital heart disease or CHD. We know it's common, and we know it's serious. It's the leading cause of infant mortality from birth defects, period. About a 3rd of these infants need a life-saving intervention like surgery in their 1st year.
3:00The good news, though, is that now about 90% of them survive into adulthood. Which is incredible progress. But I think you're hinting that survival isn't quite the same as thriving. That is the crucial distinction.
3:13As this generation of survivors grows up, we see really high rates of other problems. Comorbidities. Exactly. Heart failure, arrhythmias, and the one that's most distressing for families, neurodevelopmental disabilities, cognitive issues that can really impact their quality of life.
3:32And we've known for a while what was a genetic piece to this puzzle, right? Oh yes, it runs in families. But finding the specific genes has just been a nightmare. A needle in a haystack kind of problem.
3:43A classic needle in a haystack. The old way of doing this, whole XM sequencing was powerful, but it used to cost about $170 per person. Which sounds manageable until you realize you don't need 10 people, you need 10,000.
3:56Precisely. To find these rare mutations, you need massive, massive sample sizes. And at that price point, it just wasn't feasible. That's why previous studies had only really nailed down about 7 genes.
4:07Seven. That is barely scratching the surface. So they had to break that cost barrier. They had to. And this brings us to the methodology, which I have to say is just incredibly cool. They use something called MIC.
4:19Molecular inversion probe sequencing. Let's break that down because the paper says this is how they got the cost down to around $25 a sample. How does that work? It's a brilliant bit of engineering. So imagine you want to read a few specific sentences in a giant encyclopedia, but you don't want to pay to photocopy the entire thing.
4:37Okay. Standard sequencing is like photocopying the whole book. Exactly. M I sick is different. You create a custom probe. It's a single strand of DNA with 2 arms on the end. And these arms are specific.
4:49They're designed to match the beginning and end of the exact gene target you want to look at. You mix these probes with the patient's DNA, the arms grab onto their target, and the probe snaps into a loop.
5:00So it lassos the gene you care about. It lasses it perfectly. Then an enzyme comes in and fills the gap. Copying only the DNA inside the loop. Now you have a closed circle of DNA. Okay. And here's the really clever bit.
5:14You add another enzyme and exonucles. Which is like a molecular shredder. It's a shredder, yeah. It chews up any DNA that's in a straight line. So all the original patient DNA, any probes that didn't find a target.
5:27It all gets destroyed. But it can't chew up the circles. It can't touch the circles. So you're left with the pure sample of only the targets you wanted to read. You're not wasting a single cent sequencing the 1000000000s of DNA letters you don't care about.
5:42That is so smart. It's like searching for a needle in a haystack by just burning all the hay. That's a surprisingly good analogy, actually. And that efficiency is how they got the price down to 25 bucks.
5:53Which let them assemble this massive group. 11,555 patients, a scale that we've just never seen before in this via. Okay, so they have the tech, they have the patients. They targeted 248 genes that previous research hinted might be involved.
6:07What did they find? They blew the doors off it. They identified 60 genes with a significant burden of damaging variants. So they went from 7 known genes to 60. It's a landmark discovery. And these 60 genes explain the cause of the heart defect in over 10% of all the patients in the study.
6:26That's one in 10 kids who now have a specific molecular answer. Yes. And even more exciting, 13 of these genes had never, ever been linked to congenital heart disease in humans before. I want to dig into some of these, because the paper tells a few really distinct stories.
6:43Let's start with what you could call the heart specific story, the gene MYH6. Right. This is one of the most compelling parts of the paper. MYH 6 makes a protein called alpha myosin heavy chain. It's the engine of the heart muscle cell.
6:58It's the protein that physically contracts to pump blood, and critically, it's pretty much only expressed in the heart. So it's a specialist. is not doing anything in the brain or the liver. Correct. And they found that the damaging mutations in this gene were often transmitted.
7:11They were inherited from a parent. Okay, wait. If a parent pass it down, why wasn't the parent in the hospital with a heart defect? That's the $1000000 question. It's a concept we call incomplete penetrance.
7:23The parent carries the variant, but for whatever reason, other protective genes, maybe environment, they never develop a serious problem, but they're a carrier. And if they pass their on? The risk to their child is huge.
7:37They calculated the risk ratio at six. So a sixfold higher chance of having a defect. That must be terrifying for a parent. You feel fine. You have no family history, and then your child is born with a serious issue.
7:49It is, but, and this is a really important, but there's some good news here, relatively speaking. Because MYH6 only works in the heart, the damage is contained there. These kids mostly had isolated heart defects, like holes between the chambers.
8:04So things a surging can fix. Yes. And the risk of having a neurodevelopmental delay was incredibly low, around 4%. So if a parent gets a genetic report that says the cause is MYH 6, they can breathe a huge sigh of relief about their child's future cognitive development.
8:19For the most part, yes. It predicts a structural problem, not a systemic one. Okay, so that's one path, the isolated heart problem. But then there's the other group of genes, the chromatin genes. And this seems to be the much heavier side of the story.
8:32It is. the other side of the coin. They found 10 genes involved in what was called chromatin modification. Genes like KMT2D and CHD7. Remind us what chromatin is. Chromatin is the packaging for your DNA.
8:45Your DNA is incredibly long. So to fit inside a cell nucleus, It has to be wound up really tightly around these protein spools. Chromatin genes control how tightly or loosely that DNA is wound. If it's wound too tight, a gene can't be read.
8:59If it's too loose, it might be red when it shouldn't be. They're master regulators. So they're like the librarians of the genome, deciding which books get open. That's a great way to put it. And because every cell needs to organize its library, these genes are expressed everywhere.
9:13In the heart, yes, but also critically in the brain. And I think I see where this is going. If you break the librarian, you don't just mess up one section of the library, you mess up the whole thing. The correlation was just stark.
9:26Unlike the MYH6 kids, patients with these chromatin gene mutations had very high rates of problems in other organs and severe neurodevelopmental delay. How severe are we talking? Let's take the genes CHD 7.
9:39If a child had a damaging mutation in that gene, their risk of neurodevelopmental delay wasn't 4%. What was it? It was about 95%. 95. That's that's not a risk factor anymore. That's a near certainty. It's an overwhelmingly strong predictor.
9:54And that knowledge completely changes the conversation for that family and their doctors. You're not just treating her anymore. You're preparing for a lifetime of cognitive challenges. That's, wow. Okay, this all leads to what might be the most important part of the paper, the discussion about hidden syndromes.
10:11This is where it gets really practical for doctors. So we've known about certain genetic syndromes for a long time, things like Kabuki syndrome from the KMT2D gene or charge syndrome from CHT7. And doctors are trained spot, these, right?
10:24They look for a specific set of physical features. Exactly. They have a checklist. Does the child have the characteristic facial features? Are the ears low set? For charge syndrome, a key sign is a colaboma, a defect in the eye.
10:37So they're diagnosing with their eyes. They are, but this study found a huge number of patients who had, say, the CHD 7 mutation for charge syndrome, but were never diagnosed with it. Why not? Because they didn't look the part.
10:50They didn't have the classic features. Take the I defect. The kids with the CHD 7 mutation, who didn't have a coloboma, were almost never clinically diagnosed. So the doctor looks at the baby, sees the eyes are normal, and says, well, it can't be chart syndrome.
11:03must just be an isolated heart defect. Exactly that. But the child has the mutation. And that means they are at massive risk for all the other invisible parts of the syndrome, the learning disabilities, the hearing loss, the growth problems.
11:17That's just, it's heartbreaking. You can imagine a parent going from doctor to doctor for years thinking all these different problems are just a string of bad luck. When, in reality, it was all one thing.
11:29It was CHD 7 all along, but it was missed because the child didn't fit the textbook picture. It really shows the limits of that kind of physical diagnosis. It does. We're moving into an era where the molecular diagnosis.
11:42The DNA is the ground truth. It forces us to ask, what is a syndrome? Is it the way you look? Or is it the gene you have? And this paper argues pretty strongly that the gene is what matters. Because the gene is what predicts the future.
11:56If you sequence these babies at birth, which now only costs $25, you're not just explaining the heart defect. You're predicting their odds of struggling in school. And that lets you intervene. That's the whole point.
12:07If you know a newborn has a CHD 7 mutation, you don't wait for them to fall behind. You start speech therapy, physical therapy, hearing tests right away. You can change that child's entire life trajectory before they even show a symptom.
12:19You can. And on the flip side, with the MYH 6 kids, you can provide incredible reassurance. You can tell the parents, look, your child needs heart surgery, but their brain is going to be fine. Let's focus on the heart.
12:31The power of that certainty is just immense. Now, they found 60 genes, but this was a huge study. Is the map complete now? Are we done? Oh, not even close. This is just the 1st really big piece of the puzzle.
12:45Based on their statistical models. They predict. They're probably around 297 total genes that are major risk factors for CHD. So if they found 60. There's still more than 200 out there left to find. About 230, yes.
12:59We're seeing the tip of the iceberg. But for the 1st time, we have the tools and the statistical power to go find the rest. So if we pull all of this together, what's the big take-home message? I think the message is that we are fundamentally shifting how we think about this disease.
13:14We're moving from describing heart defects by their anatomy to describing them by their molecular cause. Because the molecular cause is a much better predictor of a patient's entire life, not just their heart.
13:25It's the difference between describing a car crash by the dents and the fender, versus knowing if the brakes failed, or if the steering column broke. One tells you what happened, the other tells you what's likely to happen next.
13:37Which really leads to a big question, doesn't it? What does this mean for the very definition of a syndrome when a patient has the gene? But not the classic look? Are we in the process of redefining the diseases themselves?
13:51I think we are. We're redefining them based on the blueprint, not just the building, and that's a profound shift. It absolutely is Well, that gives us plenty to think about. This episode was based on an open access article under the CCBY 4.0 license.
14:05You can find a direct link to the paper and a license in 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.
14:16Now stay with us for an original track created, especially for this episode, and inspired by the article you've just heard about. Thanks for listening, and join us next time as we explore more science base by base.
