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Sparking interest and understanding in the epilepsies whilst bridging the communication gap between epilepsy patients, clinicians, scientists, geneticists – and the world. Hosted by Torie Robinson.
**NOT FOR CLINICAL OR PATIENT DECISION-MAKING*
Visit www.torierobinson.com & www.epilepsysparks.com.
Somatic mutations are present only in a subset of the cells. And in the context of epilepsy, they have occurred only in the brain. So they are restricted to the brain and they have not been inherited by the parents.
Could it be that they are in the brain and somewhere else? But maybe you don't know, because it's not like you have a seizure in your shoulder, for instance. Could it be both at the same time?
So. Back in the day, I just believed that if you have a genetic mutation, it’s in every cell throughout your body. I think that’s what we were all sort of really taught?! Well, the thing is: we were wrong! Well today we’re going to hear about what are called somatic genetic mutations - and they can appear just in one part of your body - like your brain - and, well… we can guess what that can lead to!
I’m Torie Robinson and today Dr. Stephanie Baulac shares her exciting research into the type of epilepsy called focal cortical dysplasia and how somatic mutations can right lead to it!.
Hello, my name is Stéphanie Baulac. I'm a researcher and my lab is based at the Paris Brain Institute within the Pitié-Salpêtrière in Paris. And over the past 25 years, my research has focused on understanding the genetic causes of epilepsy and the pathogenic mechanisms underlying epilepsy.
Terrific. So we're going to talk about, though, focal cortical dysplasia. What even is that? I mean, it's another one of those, like, long, multi-syllabled terms. What is that to a lay person?
So in children, one frequent cause of focal epilepsy is a malformation of the cerebral cortex that arises during brain development, so before birth. And one of the most common cortical malformation is focal cortical dysplasia, which often leads to drug-resistant seizures and frequently requires epilepsy surgery to achieve seizure control.
And why is it usually drug resistant?
Well, in FCD the dysplastic region contains neurons that have migrated abnormally during embryonic development, as well as characteristic enlarged abnormal neurons called dysmorphic neurons. These are intrinsically hyperexcitable and abnormally connected. And so this creates a network generating epileptic activity. But antiseizure medication often reduce neuronal excitability and restore the balance between excitation and inhibition. So they can suppress seizures, but they cannot reverse the underlying cortical malformation. They cannot reposition neurons that migrated to the wrong location during development.
And so this is genetic, right? Or is it not genetic? And is it the type of genetic epilepsy that is passed on from parents or is it just a random mutation?
So, one reason why focal cortical dysplasia was not initially thought to have a genetic origin is the absence of a family history. Those FCD 2 occurred sporadically, which long agreed against a genetic cause. So the breakthrough became came when researchers started to analyse brain tissue itself (rather than blood samples). And so we and others discovered that many patients carry mutations that are restricted to the brain. They are present only in a subset of brain cells, and therefore they are absent from the blood or other peripheral tissues.
So I think the common sort of misconception by many people (and this was certainly myself before I got into all of this), was that if you have a genetic abnormality, it affects every single cell in your body, every single gene. But this is the point, I guess, here, that you're making, is sometimes it just affects one little bit of your body and it could be in your brain, it could be in your finger, it could be in your leg, etc., right?
That's right. In contrast to inherited mutations which are present from conception, they are found in every cell of the body. Somatic mutations are present only in a subset of the cells. And in the context of epilepsy, they have occurred only in the brain. So they are restricted to the brain and they have not been inherited by (sic) the parents.
Could it be that they are in the brain and somewhere else? But maybe you don't know, because it's not like you have a seizure in your shoulder, for instance. Could it be both at the same time?
Right. Well, yes, in theory this is possible, absolutely.
Okay! So tell us about some research into this. You've got your own research, I know there are other people around the world doing all of this research. And also tell us how, you know, sometimes you only need like a limited number (I've read this anyway), a limited number of cells to identify this mutation.
That's right. So FCD varies in size. Some are extremely small, affecting only the bottom of a sulcus, but others can involve an entire lobe or hemisphere. So this variability reflects the developmental timing and clonal expansion of the underlying somatic mutation. So in small FCD2, it is true that a very small number of neurons is sufficient to trigger seizures. And there are two reasons for that. One aspect is that those mutated neurons act as abnormal drivers within that network. So they do not constitute a large proportion, but they're they are like critical nodes perturbing network excitability. But the second important aspect is that those effects are not necessarily limited to the cells carrying the mutations. We now know that mutant cells can also influence the neighbouring non-mutated cells through mechanisms that are called non-cell autonomous mechanisms.
Okay, and how does that work then? How does that happen? It almost sounds like a bit of an infection you get from one cell to another.
Well it's through molecules can that can be secreted to the neighbouring cells or just because our neurons are connected and it's about a network, so if only one neuron within the network is mutated, this creates a whole perturbation.
How does this differ from other types of epilepsy? Because, you know, say you have a - and I often use this as an example - but say you have a temporal lobe epilepsy, the most common, you can have an abnormality, some sclerosis, and then that activity does spread to other cells in the creation of a seizure. Is it similar to or the same as that?
Yes, probably very similar. Small number of cells that will, you know, will perturb the whole the whole circuit. This is this is probably something that is common to most focal epilepsies.
How many neurons do need to be affected to create the seizures?
That's a different difficult question, Torie. However, there's a a group that has shown in mouse models of focal cortical dysplasia that only 8,000 mutated neurons are sufficient to trigger spontaneous seizures.
So for perspective, 8,000 neurons for people who are not familiar, how many is that compared to the number of neurons in one's brain?
Oh, that's very little, but this is compared to the mouse brain. So in humans I I wouldn't be able to tell you. But what we know is that we can find sometimes mutations in less than 0.1% of the neurons located in the epileptogenic zone. So this is very few.
That's nuts, isn't it? Because I thought you were about to say 0.1 % of the entire brain, but you know, 0.1% of just the epileptogenic zone.
Right, because also, remember, it's what we analyse is only the surgical tissue, the one that has that has been removed that is located in the epileptogenic fossil.
Yeah, no indeed. Okay. So when it comes to mosaicism, what exactly does that mean? It sounds pretty. It actually reminds me of something beautiful, but it's not beautiful when it comes to epilepsy. So why is this.. is mosaicism a sort of important concept when it comes to epilepsy?
So, mosaicism means that not all cells in the body carry an identical genome. This challenges what many of us have learned at school, right? That every cell in an individual contains exactly the same genome. We now know that this is not strictly true because of the somatic mutations that arise naturally during cell division. And this is from the very early stages of embryonic development and throughout all life. So as a result, our brain is a mosaic of unique genomes. And this is fascinating because now each cell has a unique genome, and those somatic mutations can act as natural barcodes for all of our cells. And by tracking these barcodes, we can reconstruct cellular lineage and trace how the human brain develops from a fertilised egg. So this really gives a completely new window into human brain development just because of the somatic mutations.
That's amazing! it's almost like that these mutations create a story, if you like, and you can go back in time to see how these developed.
Absolutely. Unfortunately, in some cases, most of the somatic mutations are silent and can indeed act as natural barcodes, but some of them are pathogenic and this is the case of what is happening in focal cortical dysplasia. But you know, somatic mutations are also well known in cancer. We know that environmental carcinogens such as tobacco smoke or UV radiation also induce somatic mutations and cause cancer. However, in the case of epilepsy it's different. We don't think it this is due to carcinogens, right? This is occurring during cell divisions, resulting from errors in DNA replication or repair.
Is there any way that the work you're doing can benefit the people who have these mosaicism when it comes to mutations in cancer and vice versa?
What we can tell, now, at this stage, is more in terms of genetic counselling for the patients. Because first, identification of a somatic mutation provides an answer, right? A family often want to understand why their child developed epilepsy and we cannot explain the biological cause in many cases. But it also improves genetic counselling because most somatic mutations arise spontaneously. They're not inherited from the parents. Meaning, that the recurrence risk for future children is usually very low. And finally, of course, identifying the causal pathway opens the door toward precision medicine approaches.
And for people who aren't familiar, what is precision medicine?
It means that we can give a treatment targeting the cause. So right here, in the case of focal cortical dysplasia, somatic mutations all occur in a signalling pathway called mTOR. This is a very essential pathway that regulates cell growth, cell proliferation, metabolism, and many other key functions in the cell. And there are a number of inhibitors of this pathway. So these are kind of promising and targeted approaches that could be used.
What are the inhibitors of the pathway?
So the most known inhibitors are called everolimus, rapamycin-like drugs.
Okay, okay. And how do they work?
So they will repress the activation of the pathway. And since in focal cortical dysplasia the mutations lead to a hyperactivation of the pathway, this is exactly what we mean by “precision medicine”. The defect is an increased activation and we want to decrease it back with inhibitors.
And how does one.. because one of the most annoying issues (well, in my opinion), when it comes to treatment of epilepsy is, generally speaking, at least the first option is to go and give somebody a drug, like for instance, the one or type that you have mentioned, but that affects the entire brain. And is that how it would work with this type of epilepsy as well? Or is there a way of targeting the affected tissue? Or is the only way to do that, is that through surgery?
Mm, that's a good point. So, it's true that using rapamycin would target the whole brain. And ideally we don't want to do that. Ideally, we would like to give a drug that will target only the mutated neurons without affecting the neighbouring white type neurons. And this is exactly what we are trying to do in our lab.
So at what stage are you in your lab? Are you doing this with mice at the moment or…?
So we work with cortical organoids in vitro and as well as mouse models of FCD 2. And we are looking for biomarkers that would be specific to those abnormal cells, dysmorphic neurons that are found in focal cortical dysplasia.
Amazing. And at what stage are you? Because we have to manage people's expectations, right? I mean, we have lots of researchers who listen to our podcasts, which is amazing, but lots of clinicians who aren't involved in this and then lots of people with epilepsy who generally have no insight into this. So what's the timescale we're looking at?
Ah, that that is difficult to answer. But, we have already generated promising data in a preclinical mouse model of FCD 2. And for that we have used drugs called senolytics that act towards cellular senescence (a biomarker that we identified in those dysmorphic neurons).
Why have we only recently found these mutations, these abnormalities? Why didn't we find them before? And how recent is the discovery?
So, these discoveries are from the last 5/6 years. And it is now shown that more than 80% of FCD 2 cases carry somatic mutations in mTOR pathway genes. So this is clearly still new findings, but we still miss the mutation in some cases. And one reason is probably that those mutations are present in an extremely small fraction of cells. Sometimes below 0.1%. And they remain technically challenging to detect. But still, these discoveries have really changed our vision of FCD 2, moving from channelopathies to developmental disorders and part of the enteropathy spectrum. So, why (your question), why didn't we detect them before? Well, this is first due to the fact that we were not looking in the right tissue before, because those mutations are hidden in the brain. So, sequencing blood DNA is not useful in this case. But this is also due… So one one aspect was to have access to surgical tissue, but the second aspect is that they are very challenging to detect. And we need new sequencing technologies as well as new bioinformatic pipelines to detect the somatic mutations. And this came with the recent advances in genomic sequences.
So, normally when we look at genome sequencing, we think of whole exome sequencing, a whole genome sequencing. I mean, that won't, by the sounds of it, that won't identify these mutations. Is that correct? And so you need to do it differently.
So, what we do, usually, is that we sequence in parallel a DNA extracted from the surgical tissue and a DNA extracted from the blood sample. And we do targeted sequencing with a very deep coverage. Because rather than sequencing the whole exome, we need to sequence specifically the emptopathway genes. And that's the most, to date, reliable way to identify the somatic mutations.
This is so cool.
And you know what, we can even detect them from trice tissue that is adherent to the intracranial EEG electrodes. So, this might be a way to detect those mutations in a presurgical workup.
Stéphanie’s research, really is exciting! Because it changes the way that we think about certain genetic epilepsies! Just a few years ago, loads of these mutations were, basically, invisible to us! But now researchers can identify genetic changes that are hidden within a tiny number of brain cells and then use that information to better understand why somebodies epilepsy developed in the first place and then that could inform us how we might be able to, in the future, effectively treat them without brain surgery. Thank you so much to Stephanie for sharing her discoveries with us and how they could change the lives of so many people in the future!