Okay, welcome back. This is the supplemental lecture for Unit 7. And what I want to talk about here today in this module is epigenetics and twins. And actually there's a phenomenal amount of research, if you Google epine, genetics and, and twins. You pull up a lot of listings. It's a very, very active area of research. So I thought I'd just give you some flavor for what the nature of this research is. And there are two major areas that twins are being used in in epigenetic research today. The first is to estimate the heritability of epigenetic markings. Just like we use twins to estimate the heritability of any phenotype. Epigenetic markings are a phenotype. So people have begun to use twin studies to try to estimate whether or not epigenetic markings are heritable. And I'll come back in just a second, why that might be an interesting question to ask, or answer. The second is that, epigenetics is thought to be a potential source for understanding why monozygotic twins who inherit the same genome, might be discordant for mental health problems. So those are the two things I'll touch on. But let's first talk about the heritability of epigenetic markings. If you go back to Unit 4, we talked about methylation. And I'm going to focus on methylation here. It's, it's the most widely studied epigenetic process, and it's I think for us easiest to conceptually understand. So if we go back to Unit 4, we talked about that often upstream of a gene they'll be what are called cb, CpG islands. C being a, a DNA base, cytosine, G being a DNA base, guanine. And just p representing the phosphate bind bond between the cytosine and, and guanine basis. The CpG islands represent opportunities for methyl groups being attached to the DNA sequence at that particular point in the sequence. That attachment of a methyl group to the CpG island is called methylation. These islands are often in the region of the promoter of the gene. And as we discussed back in Unit 4, if you have methyl groups attached, the gene is said to be methylated, and it doesn't get transcribed. It's, it blocks the transcription of the gene. So, you don't produce the gene product. If the gene is unmethylated, the methyl groups are not attached, the transcription machinery has access to the gene. It gets tran, transcribed, you get the gene product. So, why might there be heritable influences on methylation? Well, obviously one, it's the only one, it's not the only one, but it's the only one I'm going to talk about here. Obviously, if methyl groups are preferentially attached to regions of the genome where you have a C followed by a G, if you, if you inherit different bases of DNA. Suppose some people have C followed by C, and then other people have C followed by G. Then the person having C followed by C, a difference in their DNA sequence, they're not going to have a target for a methyl group being attached. So, differences in the sequence we inherit can actually affect the likelihood that an epigenetic phenomena can occur, that a methyl group can be attached to that sequence. And that can vary bet, among people the, the nature of the sequence of course. And what heritability seeks to understand is the extent of which those differences in sequence might relate to differences in the phenotype. Here the phenotype being methylation. So I'm just going to talk about one study. There aren't a lot of studies at this point that are published, or at least a lot of large scale studies. There, there's maybe a dozen or so smaller studies. This is a study coming out of the University of Queensland by Allan McRae and his colleagues. And they took 117 families consisting of twins, the siblings of the twins, and the parents of the twins, total of over 600 individuals. And then they actually put them through a methylation array. And what the methylation array does is in this case, or, at over 400,000 sites across the whole genome where methylation could occur. Basically what they're measuring in this study is the percent of times that methylation actually occurred at that site. So we can think of the, the phenotype here as, and they are actually four, over 400,000 phenotypes, because they're doing it on a genome wide level. Is the number of sites where, or the percentage of sites of each site where you had methylation. The first thing they report is the family correlations for methylation at each of the 400,000 sites. And in this case, they average the correlations over the 400,000 sites. So these are average phenotypic correlations for how much methylation there is over 400,000 plus sites. What you can see is that there's very little correlation among the genetically unrelated mother-father pair, the correlation's only 0.02. First degree relatives are correlated about 0.1. It's not a gigantic correlation, but it's a highly significant correlation, given the number of sites that they're looking at. And monozygotic twins are correlated more similarly than dizygotic twins, in fact, about twice as similar, correlation of 0.2. This pattern suggests that there's some heritable influences on methylation. They subsequently, in a graph in this article, report the the actual heritability estimates across the 400,000 plus probes. On average, the heritability is 20%, which is very consistent with the average correlations from the previous slide. But this gives the distribution. Some sites are highly heritable, how methylated they are. Other sites are not heritable at all. So the first thing that's coming out of twin studies of epigenetics is that epigenetic phenomena are probably, in part, heritable. Some might actually be strongly heritable like the ones up here, others only weakly heritable. But they're likely, to some degree many of the methyl, many of the epigenetic phenomena also have a genetic basis to them. And why do they have a genetic basis? Probably because differences in our DNA sequences provide different opportunities for epigenetic phenomena. The second thing I want to touch on is that there's actually a lot of interest in looking at epigenetics as a source for why monozygotic twins might be discordant for psychiatric and indeed a lot of different diseases, cancer, cardiovascular disease. This is an old example. In fact, this is a, a pair of twins who were studied in the 1930s. They're actually reared-apart twins, male, monozygotic, reared-apart twins studied by James Shields in England. And the thing I'm, I'm sure you note is that they're actually quite different in size. The one on the right here is quite a bit bigger than his genetically identical co-twin. Why is that? Even back in the 1930s, the 1940s, when people would look at this pair of twins, they'd speculate, well, they inherited the same DNA sequence. We know how big we are is largely influenced by the genes that we inherit, why is it that they differ so much? The speculation has been for many, many years is that it likely is involved in how those genes were expressed in epigenetic phenomenon. But of course back in the 1930's or 40's or 1950's, they had no way of studying epigenetics. They didn't have the technical tools to actually explore whether or not they could account for this difference epigenetically. Now we do have the tools. And given the tools, can we begin to find that epigenetic differences in monozygotic twins explain phenotypic differences in those twins. Well there's a classic example of this called Beckwith-Wiedmann Syndrome. I might be pronouncing, mispronouncing that. What Beckwith-Wiedemann syndrome is, it's an overgrowth syndrome. What happens with, it's rare, it occurs in 1 out of every 13, approximately every 13,000 births. So it's very rare. But if you have this syndrome, what happens is you just grow more rapidly than other individuals. So, people with Beckwith-Wiedemann Syndrome are actually born, they weight a lot. They're longer than other babies. And they continue to grow into maybe early childhood. But actually, ultimately they have normal height in adulthood and normal weight. And for the most part Beckwith-Wiedemann Syndrome doesn't have medical problems associated with it. There are some problems associated with it. There's some increased risk of cancer in childhood that's usually very treatable. But apart from that, what happens with people with this syndrome is they grow very rapidly early in life, and then later in life, they, they don't grow as much, so they, they end up with normal height. Most people with Beckwith Wie, Wiedemann syndrome are what are considered sporadic. That is, they're the only one in their family that has the syndrome. It turns out that that's even true with monozygotic twins. Here's a pair of girls that are about three and a half years old, where one has Beckwith-Wiedermann syndrome and her genetically identical sister does not. They have the same genomes. And you can see, right, this, this girl looks, you know, quite a bit bigger, especially in her face, than her genetically identical sister. And again, they're three and a half years old. They have the same genomes, they're monozygotic twins. And but yet, they end up with a different syndrome. What explains that? It turns out that what explains it is an epigentic phenomenon. What's happened with this girl here is a gene on chromosome 11 has been hypomethylated, that is hypomethylated means the methyl groups being removed. The gene being expressed. And it's the expression of that gene, it's not too important for our purposes what the gene is. But it's the expression of that gene that leads her to grow rapidly early in life. Her sister doesn't have that gene hypomethylated. They have the same DNA sequence. One had the gene, the methyl group's removed from the gene. The other did not. One ends up with Beckman Wiedemann Syndrome, Beckwith-Wiedemann Syndrome, the other does not. What, the interesting thing in this, I, I don't think it's still understand, understood why. This actually occurs with some frequency in monozygotic twins, and in particular in female monozygotic twins. There are actually many cases of female monozygotic twins. It, I wouldn't say it's common, but it's still rare. But there are multiple cases of female monozygotic twins where one member of the pair, one of the girls had this gene, the methyl groups remove, she overgrew early, and her sister did not. So, in any case a beautiful ill, illustration of how epigenetics can lead to discordances in monozygotic twins, even though they have the same sequences. Another important study along these lines is a study that was published in 2005 by Mario Fraga and his research group from Spain. And what they're interested in is just to what extent epigenetic differences can arise in the lifetime of a pair of monozygotic twins. And they studied a sample of monozygotic twins, only two pairs of which I'm going to highlight here. And what I'm going to show you here is a chromosome 1 for each member of, of two pairs of monozygotic twins. And the chromosome 1's are labeled. If it's labeled red, then it's hypomethylated. So the methyl groups aren't there. The gene, that, those genes are being expressed. If it's green, it's hypermethylated. The methyl groups are attached, and so, the gene is not being expressed. If it's yellow, yellow is red plus green in the labeling. So you're getting a balance between being hypomethylated, expressed, and hypermethylated, not expressed. So in the Fraga study, here's actually, these, this is, a pair of twins. So this is one twin, and this is the other twin, chrom, just chromosome 1. And what you see here is a couple things. One is that, you know, most of it is yellow so you're seeing a balance between hypo and hypermethylation. But the other thing to, to note and maybe the most important thing here, is that the two twins, the methylation patterns look almost identical at age 3. They're very, very similar. So early in life it appears that the monozygotic twins have very similar patterns of methylation, at least across their chromosome 1. But what happens as they get older? Here's a pair, it's not the same twins obviously. This is a cross sectional study, but here's another pair of 50-year-old monozygotic twins. Red again, hypomethalation, green hyper, and the yellow being the combination of the two. Two things to note. There's quite a bit of difference between what the chromosomes look like in terms of these epigenetic markings at, at age 3 versus age 50. Our epigenetic markings can change over time, over our life span. That's sometimes called epigenetic drift. They don't always change, but sometimes they do change, and you are seeing that here. The second thing to note is that now that you begin to see differences between the two monozygotic twins. Sometimes they're similar, but in other regions of their chromosome, you actually begin to see differences in the markings. So, even though they have the same DNA sequence when they're 50, they're expressing genes at different levels because of the different methylation patterns. That's why this shifting of epigenetic profiles over age is why psychiatric geneticists are very excited about studying epigenetic phenomena. [SOUND] So what have we learned to date from epigenetic studies of psychiatric illness? Well the first thing to say is that we, to be honest, we haven't learned a lot. If you're really interested in this, I've given you a citation here. It's actually an open access citation so anybody can get the, the paper. And it's a review of all relevant studies in this area, where they've looked at discordant twins and then tried to relate that to discordance in their epigenetic profiles. And I think if you read that paper it's a fair characterization that I'm giving you here. We're at the very early stages. There're some studies looking at discordant twins for, discordant monozygotic twins for schizophrenia or autism, but there just aren't enough of them to draw solid conclusions at this time. It's hard to collect the samples, but certainly researchers are trying to do that. And I think over the next three or four years, I think we're, we're going to see some pretty exciting results from this area. I'll end though, with noting, and you can get this also from this article, a couple of the challenges is that people doing this research have. So what they're trying to do is identify twins. Let's say, twins, monozygotic twins were discordant for schizophrenia. And then they're going to look at their profile of methylation across the genome to see if there are any methylation differences. The first issue that they have is what tissue should they sample to look for those methylation differences. Recall our cells are differentiated. What does that mean? Blood cells are not the same as liver cells, are not the same the same as neurons. And they're not the same because they have different epigenetic profiles. So if you're studying a psychiatric illness, what you'd like to do is study neurons. But it's, you're not going to you, you, in living participants is, you're not going to get, you're not going to be able to sample their neurons. So one of the things, the most common tissue to sample is usually blood. And there's a lot of debate as to whether or not epigenetic markings in blood can tell you about epigenetic markings in the brain. That's one issue that people researchers in this issue field are grappling with now. The second thing is differentiating cause from effect. They correlate differences in whether or not they have schizophrenia with differences in their epigenetic profile. They'd like to say that the differences in the epigenetic profile led to differences in schizophrenia. But it might also be the other way around. If you develop schizophrenia, your diet changes. You're probably a smoker. You probably don't keep good health good care of your health. Maybe those lifestyle changes are, or even the treatment, the pharmacological treatments for schizophrenia, change the epigenetic profile of your DNA in a way that creates differences with your unaffected monozygotic cotwin. So it's very tricky to try to sorts out those cause and effect issues. The final thing is the issue of chorionicity. And this actually gets back to the Beckwith-Wiedemann Syndrome example. There's some concern that, in monozygotic twins if they share a chorion versus they don't share a chorion share a placenta versus they don't share a placenta. That actually, that that could potentially have differential effects on the epigenetic profile of the twins. And maybe that's why you get Beckwith-Wiedemann Syndrome, something about this chorionicity is at least a hypothesis. If that's the case, then sorting out chorionicity, which is often hard to do with monozygotic twins unless you've actually studied their placenta tissue, placental tissue. It might be very important in trying to interpret the results of these epigenetic studies. Regardless, this is a very exciting area of research now, wedding twin studies with epigenetics. And I'm sure over the next three or four years researchers are going to solve some of these problems I've highlighted here, and we'll see some exciting results from this field. Thank you very much. [SOUND]
