This episode reviews a study using hiPSC-derived 2D cortical neurons and 3D cortical organoids from individuals with del(17)p11.2 (Smith-Magenis syndrome) to map chromatin, transcriptional, developmental, and electrophysiological consequences of the deletion.
0:06Welcome 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. So imagine you are tasked with studying a complex, uniquely human brain disorder.
0:34Right, a massive challenge. Exactly. Naturally, you look to a traditional scientific model, like a mouse. But here is the problem. You are trying to study a uniquely human condition. And the developmental blueprints, well, they simply do not match.
0:50No, they really don't. I mean, mice have cortical development cycles that are 3 times shorter than humans. Three times shorter. Yeah, so because of this accelerated timeline, they completely miss the prolonged expansion phase.
1:01And that critical window of neural progenitor cell division is what makes human brain so incredibly complex and folded right. Exactly. That is the exact phase that gives us our unique brain architecture.
1:14So if the animal models don't have the same brain architecture as you do, how can we possibly uncover the molecular mysteries of human neurodevelopment? It's a huge bottleneck, like how do you even begin to understand what's going wrong?
1:26It's a fundamental problem in neuroscience that we've wrestled with for decades, honestly. When it comes to the intricate, uniquely human steps of cortex development, you know, the prolonged subcycles, the specific types of radial glial cells, the massive spatial expansion of the neocortex, a mouse model just can't capture the full picture, the underlying biology is just too divergent.
1:50Which brings us to today. Today, we celebrate the work of a research team, led by Yuju Li, Yating Chang, and Wei Chang Huang at McGill University, who have advanced our understanding of Smith Magenta Syndrome.
2:02Yeah, they published this incredible deep dive in the American Journal of Human Genetics, and they decided to completely bypass the limitations of Animal Models to uncover the etiology of Smith Magenis syndrome.
2:13Right, by building human brains in a dish. Exactly. They engineered human stem cell derived cortical organoids. What the media often calls mini brains, along with 2D neuronal models. Okay, so before we get into the mini brains, let's talk about the syndrome itself.
2:27Smith Magenus syndrome or SMS, it's a profound genomic disorder, right? Yes, it is. It's caused by a heterozygous deletion of a specific chromosomal region. Region 17 P 11.2 for those keeping track. Right, which encompasses a really critical gene called RAI1.
2:44And individuals with this deletion experience a wide array of severe clinical symptoms. Like intellectual disability, profound sleep disturbances, metabolic defects, and epilepsy. Yes, all of those. But what makes SMS particularly fascinating from a nerve developmental perspective is its relationship with autism spectrum disorder.
3:04Yeah, this part really stood out to me because about 90% of individuals with SMS are diagnosed with autism, that the demographics are flipped. Right, they're completely reversed. In typical autism diagnoses, you generally see a male to female ratio of about 4 to one.
3:18But in SMS, it's reversed. 3 to one, female to male. Which is so interesting. And the researchers took this clinical reality into account, which I love, they made a very deliberate choice in their methodology to primarily use cells derived from female patients to build their models.
3:32Which is exactly how precision modeling should be done. You have to tailor your cellular models to the clinical reality of this specific syndrome. You really do. And to understand why this human centric methodology is so vital, we have to look at what physically happens in the brain of someone with SMS.
3:49Neuroanatomically speaking. Right. Patients often present with reduced gray matter volume and something called mild ventricula megaly. Ventricula megaly. So enlarged ventricles. Exactly. The fluid filled spaces in the brain, the ventricles are abnormally enlarged.
4:06And this isn't just something seen later in life. You know, it can actually be detected on prenatal fetal MRIs. Oh, wow. So it starts incredibly early in development. And this leads right back to our opening problem with the mice.
4:18The mice don't show it. Right. Researches have previously engineered mice with this exact 17 P11.2 genetic deletion. And those mouse models do successfully replicate some of the systemic symptoms, like the obesity and the sleep disruptions.
4:33And even the epilepsy to an extent. But when you look at their actual brain structure, their cortices remain perfectly intact. There is no enlarged ventricular space, no reduced gray matter. The structural defects is just entirely missing in the animal.
4:48Why is that? Well, because the mouse simply doesn't have the developmental runway to show the defect. It all comes down to those cell cycle differences we mentioned earlier. The prolonged expansion phase.
4:59Yes. Human neural progenitor cells, the foundational building blocks that divide to create the entire cortex have a much longer proliferative phase. And we also have entirely different chromatin spatial organization compared to mice.
5:12So if a genetic error primarily disrupts these uniquely human developmental timelines, a mouse model will simply mask the structural deficits. Precisely. The biology moves too fast for the error to compound into a structural collapse.
5:27It's like, well, the host analogy earlier was great. The baking bread. Yeah. If you don't let the yeast proof for that crucial prolonged window before baking, You don't get the complex airy structure of the loaf.
5:38You just get a dense brick. And the mouse model, the essentially rushes the baking process. Exactly, which means the McGill team had to build a human brain in a dish. So let's talk about that core methodology.
5:48They took human fiber blasts of individuals with SMS, and they reprogrammed them into induced pluripotent stem cells or hit PSCs. Right, a standard but incredibly powerful technique. And then they used highly specialized chemical cues to coax these stem cells to grow into 2 different models.
6:06First, they made 2D cortical neurons spread flat on a plate to study pure electrical activity. And second, they grew 3D patterned dorsal cortical organoids. These are the mini brains used to study tissue structures, spatial organization, and growth over time.
6:22Right. And human cortical expansion requires a delicate, highly regulated orchestration of cell cycle timing, specific gene expression, and a massive physical expansion of space over months. So by growing these organoids for up to 75 days, they could finally watch the exact human specific errors unfold in real time.
6:41Block by block as the cells attempted to build that complex structure. Okay, let's unpack this because once they had these human mini brains growing in the lab, they didn't just look at the overall shape first.
6:51No, they zoomed all the way into the nucleus. Right. To look at the physical blueprint of the DNA. Yes, utilizing high C sequencing. And this isn't just about reading the genetic sequence like a string of letters to see what's missing.
7:03Right, it's much more spatial. Exactly. High C captures the physical three-dimensional spatial organization of the entire genome inside the nucleus. Imagine your DNA as a massive, impossibly complex piece of origami packed into a microscopic sphere.
7:19Okay, DNA origami. like that. How it folds dictates which genes are physically touching each other, and which regulatory elements like enhancers can interact with specific promoters to turn genes on or off.
7:31So proximity matters just as much as the code itself. Huge amounts. The genome is partitioned into localized neighborhoods called topologically associated domains or tads. Wait, I need to clarify something here because this seems a bit counterintuitive.
7:45Sure, go ahead. The deletion in SMS is localized entirely to chromosome 17. The RII1 gene and the surrounding chunk are just gone. But the high sea sequencing revealed local fusions in global reorganization of those tasks.
7:58Yes it did. So how are genes on entirely different chromosomes getting dysregulated? Is the missing chunk on chromosome 17 somehow destabilizing the physical structure of the entire nucleus? It is, actually.
8:10It's a profound concept, and geneticists refer to this as a trans effect. Trans Effect. Okay. You're right that the physical deletion is restricted to chromosome 17. However, the nucleus is a densely packed, highly interactive environment.
8:25The loss of that specific segment doesn't just mean those local genes are missing. It changes the structure of everything else. Right. It alters the physical tension and the structural scaffolding of the promatin around it.
8:35This leads to aberrant folding patterns that literally cascade outward. The researchers found that an astonishing 70 to 77% of the dysregulated genes in these cells were located completely outside the deleted chromosome 17 region.
8:50So missing one piece of the origami causes the entire structure to fold incorrectly. It brings the wrong pieces of paper into contact with each other across the whole genome. Precisely. And because genome folding dictates gene expression, just as much as the DNA sequence itself, this 3D miswiring disrupts the regulatory landscape globally.
9:10Promoters might lose contact with their enhancers. Or they might be exposed to the wrong ones entirely. The physical architecture of chromatin is absolutely essential. When it's miswired like this, the downstream transcriptional programs required for precise neural development just fall apart.
9:26That is fascinating. Okay, so we've established that the blueprint itself is physically tangled. We've seen how the structural architecture fails on a molecular level. But how does that translate to the actual growing brain tissue?
9:37To find out, the team looked inside the 3D mini brains analyzing them at 25 days and then heavily at 75 days of growth. And they used single nucleus RNA sequencing or SNRNA sec. This allowed them to isolate and examine the gene expression of 1000s of individual cells within these organoids.
9:55And the morphological findings were pretty striking, weren't they? Very. The SMS organoids were physically smaller overall compared to the control lines. Their individual cells had reduced nuclear sizes, and critically, the organoids developed clear, fluid filled cavity.
10:10In large ventricles. Exactly. Mild ventriculomegaly, just like we see in the patients. Okay, I had to push back here for a 2nd because the scale we were talking about is hard to reconcile. Okay, what do you mean?
10:21We're talking about a microscopic clump of cells floating in a dish, maybe the size of a lentil, right? Yeah roughly. Is it truly scientifically sound to equate the tiny fluid spaces inside a 75 day old organoid to macroscopic MRI findings, like enlarged ventricles in a human fetus.
10:39Can a mini brain really replicate something that massive? It's totally valid skepticism, but the single nucleus RNA sequencing provides the mechanistic bridge that proves the scale translation is valid.
10:51How so? Well, the sequencing revealed exactly why those cavities form in the organoid, and the biology is identical to what happens in the fetal brain. They found a severe dysregulation in genes related to cell cycle progression and metabolic pathways.
11:04Ah, right. Specifically within a crucial type of neural progenitor cell called a truncated radial glia, or TRG. And radio glia are essentially the scaffolding and the stem cells that build the cortex, right?
11:17Exactly. These TRG cells contact the ventricles and act as the architectural basis for brain expansion. They have to divide at a very specific prolonged rate to generate enough neurons to populate the cortex.
11:30But because the DNA is miswired globally, these specific progenitor cells have impaired proliferation. Yes, they simply aren't dividing correctly. And if your foundational building blocks fail to multiply at the required rate.
11:44The structural integrity of the developing tissue is compromised. So you get reduced overall growth. And the spaces where the tissue should be filling in remain empty, accumulating fluid. Exactly. Whether that happens in a millimeter wide organoid or a developing human fetus, The underlying cellular failure is exactly the same.
12:03That makes a lot of sense. The organoid isn't just mimicking the shape. It's suffering from the exact same architectural failure at the cellular level. Right. It's finocopying the disease perfectly. And the Sessan RAC revealed even more about the specific genes driving this.
12:18Because they were using a human model, they could detect changes in primate specific genes. Genes that literally do not exist in mice. Yes. Specifically, they found severe down regulation in genes called poti in potef.
12:32Which highlights, again, why the mouse models failed to show these structural phenotypes. Right. If the disease mechanism heavily involves genes that evolve specifically in primates to manage our massive cortical expansion, a nonprimate model is completely blind to that pathology.
12:47That's incredible. And they also found transcriptomic signatures that mirrored other diseases, right? Yeah, the pattern of which genes were turned up or down, strongly mirrored the signature scene in other complex neuropsychiatric diseases, particularly autism spectrum disorder and schizophrenia.
13:00The overlap with autism brings us right back to that 90% diagnosis rate in SMS patients, and the reversed female to male ratio. It provides a clear molecular rationale for those clinical observations. It suggests that while the root genetic cause of Smith Magena syndrome is unique, the downstream consequences funnel into common pathways of neurodevelopmental vulnerability.
13:22So the secondary effects converge on the same biological networks that are compromised in idiopathic autism and schizophrenia. Exactly. Okay, so the 3D organoids beautifully modeled the structural and transcriptomic deficits.
13:36But a building can have the right or wrong walls and still have completely faulty electrical wiring. That's great way to put it. A major debilitating symptom of SMS is epilepsy. To understand how the electrical function of the brain is altered.
13:49The researchers had to zoom in on the individual neurons using their 2D cortical models. Right, because structure and function are intimately linked, but they require different tools to measure. To assess the electrical signaling, the team utilized patch clamp electrophysiology.
14:04This technique is wild. It involves attaching a microscopic glass pipe pet to the membrane of a single living neuron. Yep, to monitor the tiny electrical currents flowing across it. Normal human cortical neurons have a highly regulated rhythm.
14:20They receive a signal, they fire an action potential, and then they reset. But the patch clamp data showed that the SMS cortical neurons behaved very differently. Very differently. They were distinctly hyper excitable.
14:32Here's where it gets really interesting. The data show that these SMS neurons fire action potentials at a significantly lower threshold. It takes far less stimulation to trigger them. Not only that, but the waveforms of the electrical spikes were sharper and narrower.
14:47This means the neuron can repolarize. Essentially reset itself to baseline, and fire again much faster than a healthy neuron. Yes, it's firing too easily, and it's firing too fast. And the researchers identified the exact molecular defect causing this, didn't they?
15:02They did. The hyper excitability was driven by a significant reduction in voltage gated potassium channel conductance. They saw deficits in both the fast inactivating and the slow, delayed rectifier potassium currents.
15:15Okay, so in cellular neurophysiology, potassium channels are essentially the brakes, right? Exactly. They are the brakes. When a neuron fires, sodium rushes in to create the spike. To calm the cell down and reset it, potassium channels open, allowing positive ions to leave the cell.
15:31So if you have a reduction in potassium conductance, you have worn out brakes. Warnut Brakes is the perfect analogy. The cell stays depolarized longer, or in this case, the membrane dynamics are altered, so it stays in a highly excitable state, ready to fire prematurely.
15:47And they also cross reference this with bulk RNA sequencing of these 2D neurons, which showed altered expression of the SLC family of genes. Right. Those salute carrier genes act as ion exchangers across the membrane.
16:00Which further compounds the instability of the neuron's electrical gradient. But it wasn't just the mature firing that was altered. They noticed something strange about how these neurons grew initially, too.
16:09Yes. Early in development, there was an abnormal acceleration and dendritic growth, and they found a massive overexpression of a specific synaptic gene called LMO7. LMO seven. What does that do? It's a crucial piece of the puzzle.
16:22LMO7 encodes a cell adhesion molecule that localizes to synapses and interacts with the actin cytoskeleton. Basically it promotes excitatory synapse function. Oh, wow. So look at the comprehensive picture of this neuron here.
16:36pretty grim. Early in its life. It is growing more complex dendrites, and overexpressing synaptic proteins that enhance excitatory connections. At the exact same time, it is failing to express sufficient potassium channels to slow down its electrical firing.
16:51Exactly. It is wired to receive more excitatory input, and it lacks the internal mechanism to dampen that input. If you've ever known someone dealing with a severe epilepsy associated with the SMS, or if you've seen the epileptiform EEGs that are so common in these patients, this is the exact molecular mechanism behind those seizures.
17:09It is. The threshold for electrical cascades is perilously low. And by using this human derived model, the team successfully bridged the gap from a missing piece of DNA, all the way up to a tangible, debilitating clinical symptom that impacts the listeners' reality.
17:24It is a remarkable synthesis of molecular biology and clinical neurology. Though, you know, the study does have some limitations that we should acknowledge as all science does. Of course. What should we keep in mind?
17:36Well, they focus their modeling on the 90% of SMS cases that present with a full chromosomal deletion rather than the 10% of cases that arise from isolated point mutations in the RAI one gene alone. Which makes sense, since the deletion cases are much more common.
17:51True, but understanding the point mutations could help isolate the exact role of RAI 1 versus the surrounding genes in the region. Furthermore, while utilizing female deride cells was a brilliant methodological choice to capture the specific autism phenotype in SMS, future studies will absolutely need to incorporate male hit PSC lines.
18:11Right, to provide a complete comparative picture of the syndrome across sexes. Exactly. So bringing this all together. What is the core takeaway here? Smith Magina syndrome is not just a simple story of one missing gene causing one specific problem.
18:23It is a cascading, multiscale failure of neurodevelopment. Yes. It starts in the tightly packed nucleus with altered 3D DNA folding. The genome's orgami gets tangled. This trans effect disrupts the expression of hundreds of genes across different chromosomes, severely impacting cell cycle regulation in radial glia.
18:43And that cellular failure leads to macrostructural brain deficits, like reduce cortex volume and enlarged ventricles. And on a functional level, the tangled genetics alter the expression of critical ion channels, ultimately resulting in a network of hyper excitable neurons prone to seizures.
18:58What does this mean for the future of how we approach this? Well, this study highlights the absolute non-negotiable necessity of human specific models for complex neurological conditions. Mouse models are deeply useful for many systemic traits, but to truly understand and eventually cure human brain disorders, we have to study human brain cells.
19:19And the groundwork laid by this McGill team leaves us with a highly provocative thought to take away. Traditionally, if we think about gene therapy for a genomic disorder like SMS, the instinct is to try and replace the missing piece, right?
19:30Right, to somehow deliver the RA1 gene back into the cells. But if the root cause of the epilepsy and the structural defects is actually driven by this global 3D chrome miswiring and specific downstream failures, like the worn out potassium brakes, could future therapies pivot entirely?
19:48What if, instead of trying to replace a missing gene in every cell of the brain, medicine focused on directly untangling the 3D DNA structure, or even more immediately, targeted neuropharmacology to fix those faulty cellular potassium channels to stop the seizures at their functional source?
20:04That is exactly the kind of critical paradigm shifting question these human models allow us to ask. It changes the entire therapeutic landscape from simply trying to replace what was lost, to actively mapping and repairing the functional downstream damage.
20:17This 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. If you enjoyed this, follow or subscribe in your podcast app and leave a five-star rating.
20:31If you'd like to support our work, use the donation link in the description. Now stay with us for an original track created especially for this episode, and inspired by the article you've just heard about.
20:42Thanks for listening and join us next time as we explore more science base by base. In addition, the dawn, the blueprints bend and slide. Rooms in the genome open walls don't align. Loops start talking to strangers across the line.
21:27In the map we trusted, we draw us overnight. Presented us slow down, stuck in the waiting room. G1 hold the door like a quiet, heavy tomb. Small worlds don't grow the way they're meant to doom. But the signal keeps climbing through the gloom.
21:48Miss white and 3D. But I can still feel the beat, combating, crossing lights, making new streets. Fast grown branches, sparks, after, snap sweet when the K plus won't hold, the fight won't sleep. Miss wired and 3D.
22:10Hear my heart beat. Da, da, da, da, da, da, da, da, da, da, da, da, da, da, da, da, then trickles widen like a hollow, bright frame, fewer hands dividing, but the questions stay the same. Every cell tie carries part of the change name, a chorus of transcription in the rain.
22:43Turned on the noise from the channel that's gone. If the current back is comic, the brakes hold on. The cycle camera dark If the timing comes strong, baby, miss wiring learns softer song. But I can still feel the beef roll, the truth from the loops to the firing sheet, the organ or shadows to a clear retreat, name the pathway, trace the heat.
23:07Miss Wired in 3D. Now we're closer to, really.
