Why some people get MS and some people don't

A/Prof David Booth talks about why some people get MS and some people don't - covering both genetics and environmental influences. 

Transcript of Presentation

The topic I’ve been given is to explain why some people get MS and some people don’t. There are some things that we can definitely answer here. Like Michael, I’ll start off with something we’re absolutely sure of, its genes and environment. That's a very safe remark. How much is genes and how much environment is more difficult to answer. But for example, one in a thousand people get MS. Yet, of the people who do have MS, one in five have a close relative with MS. So there’s a very big increase if you’re related to someone with MS.

I’ll get onto the genes in a sec, but let me just talk about environment as well, as Michael mentioned, the big ones for environmental component are so far vitamin D and EBV. But there’s also suggestions of other viruses being involved. We know that if you live in the tropics, you have a lower chance, things like that. The big problem with environmental causes is you don’t know what came first. So for example, is it the vitamin D, or is it the sunshine, which also raises the level of vitamin D that protects you from MS; you don’t know which of those two or is it something else. It’s just there’s an association just like diet and heart disease; it’s very hard to track down which direction it goes, and similarly with EBV. We know that virtually everyone with MS has EBV, or has antibodies to EBV. But then 90% of the people who don’t have MS also have EBV or antibodies to EBV. So in those people with MS, did the EBV precipitate the MS, or is it a co-marker of susceptibility; they can’t clear EBV as well as other people and that's why they got the MS; it’s not the EBV.

So it’s very difficult to work out with environmental factors but with genetics I think it’s a bit different. If you find the genes unequivocally associated with MS, then you would have to say that that genetic variant which increases your risk, affects pathogenesis of MS.

Michael showed this slide; it’s a very famous slide in MS research. This is the Australian part of the theory, but it’s found all around the world that the further away from the equator you go, the higher your risk of getting MS. It depends also on timing, so if you grew up elsewhere and at the age of fifteen migrated to a nice place up in Cairns let’s say, you actually carry the risk from where you grew up, not the risk from where you go to. So timing matters for that. Apologies to the New Zealanders, I just wacked that on at the end, but they have a nice latitude gradient as well. You have it in South Africa, you have it in America, and you have it in Europe. It’s a worldwide phenomenon.

This is evidence that it’s quite compelling that there’s a genetic component, which usually is spoken of as a disease which is about 30% heritable. If you have a twin with MS, then your chances of having MS are about one in four, or one in five, depending on where you live, and depending on the study. That's an awful lot more than one in a thousand, which is what it is for people who are unrelated. And for twins, it doesn't matter if you grew up together or whether you were split up at birth, as a big Canadian study showed. Those twins who were split up and grew up in different environments, have exactly that same risk, one in four of getting MS if one of the other twins has got MS.

If you’re not quite so related, so twins are 100% related; completely identical genetically; if you go to a first degree relative and now you’re 50% related; 50% identical genetically, and the risk drops dramatically to 1 in 50, compared to 1 in 1000 for the normal population. So that's quite compelling evidence for a genetic involvement.

And this is quite similar to other autoimmune diseases; you see the same scenario. Your chances of getting Type 1 diabetes are 30% if your identical twin has diabetes. 7% if your dizygotic twin, or brother or sister has got it, compared to the normal population of 0.5%.

MS is not quite as heritable as Type 1 diabetes, so it’s down at 21%. And similarly, these other diseases have that same cline, which is typical of a disease with a genetic component.

So, we’re expecting that by finding the genes which cause MS, or the genetic variance which cause MS, we’ll be able to identify aspects of the pathogenic process, which would then allow us to develop therapies. It may be the case, and we know that people vary greatly in their response to any drugs, but this also occurs in MS, and its very likely to have a genetic component. So we’re hoping the genetics will also help us work out who’s going to respond better to which drug.

We also are hoping that the genetics will tell us who’s going to go through which clinical course; whether is going to be primary progressive, secondary progressive, relapsing remitting, or how severe the disease is going be. That's a hope. I can tell you so far that’s not panning out. Also, because MS is a very heterogeneous condition, and the parameters that are being used to describe it at the moment may not be the best parameters. It may be that you can use genetics to subdivide the different types of MS. And that might be more useful in trying to understand how to treat it.

So those are the things that genetics might give us. The first gene that was discovered, nailed down in 1988, was HLADRB1, the variant 15-01 increases you risk of getting MS. So if you’ve got 15-01 variant, then you’re three times as likely to get MS. Or to put it in another way, 20% of the population have this gene and don’t have MS, whereas of the MS population, 60% have this gene.

It means that 40% don’t have this gene, who have MS. And that's a point that I really want to make; it’s not deterministic. So a lot of genetic diseases, if you’ve got the genetic variant, you get the disease, 100%. They’re called mendelian diseases; diseases like cystic fibrosis, or Huntington disease, or the amyloidosis. MS is not like that at all. What happens is, it looks like it’s more a case of common variants, increase your risk. But it’s only increased risk, it’s not deterministic.
But this first gene was a very good gene to find because it really only has, compared to many genes [which] have lots of functions so it’s very hard to work out how they contribute to the disease process. But this gene, it’s a beauty. It really only has one function. Here it is on its cells which produce it called antigen presenting cells, and the gene HLADRV01 presents little bits of protein to t-cells, and as a result, the t-cells get activated or are controlled in some other way.
So it’s that process which says t-cells are important in MS, as are antigen presenting cells. There’s strong support to the idea that it’s an autoimmune condition, or at least an immune condition.

And this protein here is called peptide, it’s a very small protein that would typically come from a virus if your immune system is trying to protect you against something you’ve been infected with or it could be from your own body, in which case you would be getting tolerised to it. And the idea behind the glateramer-acetate is, it is actually based on what that particular gene binds; what sort of peptide it binds. So that's a drug which would, it actually come through a different pathway, how they chose to use this drug, they were trying to give mice EAE or not, but either way, the fact that this drug works at least partially in MS, and its involved in the gene function is a nice consistency.

Overall, the fact that this gene is involved in MS supports the idea that T-cell modulation is going to work as a treatment. It’s being very well supported by other data. So one gene on 31st July 2007. The next day three genes, and soon after, six genes, eighteen genes, and now we know 57 genes associated with MS.
Very rapid progress and it has all come due to the human genome project and other related things. The right experiments were able to be done. So large DNA banks; people with MS donated their blood to make these DNA banks, healthy controls did as well, so that around the world, the number of individuals who have given DNA to these DNA banks is around about 30 000.

That allowed the right experiments to be done. Also we got knowledge of genetic variation from the human genome and from the Hapmap projects. We’ve got ways of genotyping people; new ways, new techniques, dramatically increasing the number of genotypes you can do per individual. Cost dropped, and that led to the era of the genome wide analyses, which has allowed the identification of the genes which affect autoimmune diseases and a number of other diseases; the genes we couldn’t find for years, we now know them.

And it was a huge collaboration, it wasn’t just ANZgene, this mob here from Australia, but all around the world people contributed their samples. And this was the experiment: you get 10 000 DNA samples from people with MS, you compare that with controls, and you work out the genetic variants, 600 000 genetic variants for every individual, and then you see which variants are more common in MS, compared to healthy controls. And as a result of that giant experiment; it took about three years to do, and it took about a year to analyse, but we got the first analysis in February last year, and the final analysis has been completed, and this is the study which has given us the 57 genes.

Before I go onto what those 57 genes tell us, I’ll just go back to pre-July 31st 2007 MS paradigm, which was that antigen presenting cells activate T-cells which cross the blood-brain barrier and orchestrate an immune response on the myelin sheath of neurones, killing the oligodendrocytes and the neurones and ultimately causing MS.

We look at the genes which have been identified are saying exactly the same thing; that the vast majority of the genes are either expressed or act on antigen presenting cells and T-cells, and other immune cells. A few of them are expressed in neuronal cells, but in each of those cases, it’s not sure that that's where they have their effect. So the genetic information is strongly supporting that the genetic susceptibility of MS is due to the genes of the immune system. If you look at the list of the 57 genes and see what they do, there’s one particular pathway that is massively overrepresented; it is the T-cell differentiation pathway. So it’s where antigen presenting cells talk to t-cells, and the t-cells then differentiate into TH2’s, TH1’s, TH17’s or T-regs; they’re the four core t-cell subsets, and they’re the ones which seem to be the culprits, the main culprits at least, in MS.

If we just look at the usual suspects, they are already incriminating from lots of other evidence, including the drugs that work at least for relapsing remitting MS. You can divide t-cells up in many ways, there are many different types of t-cells because they have many different functions, but these are the main t-cells, and particularly if you go to that guy, he’s good; he stops MS; he stops autoimmune diseases, he stops t-cell activation.

As opposed to virtually all the others, which are bad, in the sense that they orchestrate an attack in autoimmune diseases on the myelin sheath, or in MS on the myelin sheath? What we now have to do, is find out exactly why these genes favour the bad ones over the good ones in people with MS. Consistent with that, because it’s telling us t-cells generically, we want to find out specifically which t-cells subsets and why. It’s also if you look at people with MS, they’ve got an expression signature in their whole blood which indicates that t-cells are over-activated. That's what this slide shows. So this is an ANZgene project study, where we looked at every; that’s 150 columns and around about 300 rows, where each column is an individual. So these columns are all control individuals, these columns are all people with MS, and these are the; the red means the gene is ‘turned on’ too much and the green means it’s turned on not enough, at least compared to controls.

Then you can see that MS have all the genes unregulated and they’re t-cell activation genes. But you’ll see that there are some people with MS that don’t have that signature, like that one, and you’ll see some control with the MS signature. So this is probably something that actually fluctuates over time, but on average, people with MS are more often having this signature; their t-cells are a little bit too active. This is untreated MS by the way. So that's consistent with the message from the genetics, and we’re hoping that this could be used as a biomarker to see whether drugs are working, whether it removes this signature, or whether it could be used to predict who will respond to therapies targeting t-cells’ activation.

I should point out too that one of the peculiar things is that primary progressive MS and relapsing remitting MS, secondary progressive MS, they all have the same genes associated with them. This was one of things we were hoping to find out what was the difference with PPMS. And we can’t find it with genetic susceptibility; they appear to be identical.

And same with the gene expression signature; that’s more than half of them are people with primary progressive MS and they’ve got exactly the same gene expression signature; too much t-cell activation, as we get in relapsing remitting.  So far that puzzle remains as Michael suggested, and as I think Suzanne will talk about more, is that the drugs that currently work in at least relapsing remitting MS are modulatory, and they are the drugs you would expect from the genetic pattern; the genetic susceptibility pattern.

So nearly finishing, just two points to make. Firstly, on this slide I'm talking about the fact that these risk factors are all small, so that you have a 3-fold higher risk factor if you have one first gene DRB1. The reason it took us so long to find others was they only increase your risk by about 20%, as opposed to 300%. So it took a long time to find them; it took a lot of samples to be compared to do it. But the fact that the risk factor is small doesn't mean that the significance of the gene is small. The gene could be just as important as the DRB1 gene in disease progression.

What it tells us is that the genetic variant only changes your risk slightly but the gene could be 100% important. So that's why it’s now very valuable to go to that next step; find out why these genes affect MS.  I can just tell you from that 57, there’s a couple of interesting ones. Straight away, VCAM1, that's a gene which is expressed on the blood-brain barrier and it’s how t-cells cross the blood-brain barrier; they first bind to VCAM1 with a molecule called CD49, and that is actually the target of natalizumab. So the prediction from the genetics is that this is a pathway that matters.

As it happens, the drug companies got there first, and this drug was developed before the gene was known, but this is an example of the way it works; knowing a pathway can tell you a therapeutic approach. Another thing is, with the environment, the problem is always association; how do you go from working out whether things are just associated or whether it really is involved in pathogenesis. And very excitingly, amongst those 57 genes, two of them are involved with vitamin D metabolism. One, cyp27B1 activates vitamin D, and the other one, cyp24A1 deactivates it. So why are those two genes associated with MS? This is really supporting the idea that vitamin D is important.

But we actually have to go through and do those experiments and see whether these genetic variants do increase the effective activation of vitamin D, and that will really strongly support an involvement of it in genetic susceptibility.

In summary, 57 genes are associated with MS, unequivocally. A very exciting follow on experiment is now being done; I told you this was 10 000 individuals, the next experiment is 20 000 individuals, it’s called the immunochip experiment. 20 000 with MS, 20 000 with each of 9 other autoimmune diseases, so its 200 000 individuals. They’re being genotyped for 200 000 genetic variants. The expectation from this is that we’ll increase that number from 57 to probably around 100, or maybe even more, which will further track down processes which are important in MS pathogenesis.

So we expect from the immunochip experiment; the experiment has already been done and we’re now moving into analysis, but analysis takes a long time with these huge experiments, so towards the end of the year/early next year. There may be a gold nugget in there which straight away points to something which would be ideal for the disease in therapeutic intervention.

I already mentioned that middle point, and we know are trying to find out why these genes; that's the next step now; find out why these genes affect susceptibility, to come up with new therapies. And the question everybody wants to know is how soon in the clinic, and the answer to that of course is, how long is a piece of string? Because we know that, for example VTLA4 has already supported, is consistent with drugs that have come up, and you can go back and say why drugs that have already been used should work by looking at this list. What new ones that will come from this list, takes a long time, typically. So overall, it’s giving great impetus to the idea of interfering with particular aspects of the immune system, particularly the t-cells.