Australian actor Chris Hemsworth recently had a genetic test which flagged him as being at higher risk of developing Alzheimer’s disease. We spoke to Sir Peter Donnelly, Founder and CEO of Genomics PLC and Professor of Statistical Sciences at the University of Oxford, about what genetic testing can tell us about our health – and what we can do to stay healthy. good health, whatever our genes.
What happens when someone undergoes one of these tests?
Usually this would involve taking a biological sample from the individual. Usually either a blood sample, which is a little more common, or a saliva sample. Then this would be sent to a lab which would take DNA from the sample and then analyze it for the piece of genetic information that the test is trying to find.
What is DNA?
DNA is the chemical material that contains all the information our cells use to do their jobs – to make the proteins that make them work and to build tissues and organs. We get one copy of our DNA from our mother and one from our father.
All of the DNA is called our genome – it’s just one word for all of the DNA. DNA itself is a long chemical made up of different components. You can think of it as a long list, and at each position there is one of four possibilities that start with the letters A, G, C, and T.
In total, we receive 3 billion letters of DNA from our mother and 3 billion letters of DNA from our father. Each of us, in every cell of our body, we have 6 billion letters.
What is a gene?
A gene is a piece of our DNA whose letters contain explicit instructions that help our cells make a protein. They may differ in length. A gene may only contain a few hundred or thousands of letters of DNA, but some genes are much longer than that.
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If we look at all of our DNA, only about 1% of it is made up of genes. The rest was called junk DNA before we figured out what it was doing. We now understand that he has other information. For example, information telling a particular gene when it should make a protein.
All of our cells have all of our genes. There may be a gene that makes a protein that’s really important in the retina, but you absolutely don’t need that protein in your tongue. And so there are instructions in the DNA that will be able to tell this gene, I want you to make this protein if you’re sitting in a retina, but the same protein in your tongue won’t be activated.
What influence can a single gene have on our body?
We have about 20,000 genes in total. In the case of Chris Hemsworth, he talked about a particular gene. We know some things about what it does, but there are a lot of mysteries, like there are with a lot of human biology. We all have two copies of this gene – one from our mother and one from our father. The problem is that there may be slight differences between the copies.
Many of these differences make no difference to us, but some of them have consequences. Sometimes these consequences can be very serious. In conditions like cystic fibrosis, for example, where if you inherit a mutated copy of a gene that doesn’t work as expected, you can end up getting really sick. Sometimes if you have a copy of a gene that doesn’t work, you’re fine. Because your mom’s isn’t working, but your dad’s is fine.
Hemsworth learned that he was about 10 times more likely to develop Alzheimer’s disease due to this genetic factor. How important is it?
There are certain diseases where if you inherit the genetic change you will get sick. There are other examples, and this is one, where if you inherit a particular genetic change, you may be more likely, sometimes a little more likely, to develop the condition.
For most common conditions, such as heart disease, diabetes, and many common cancers, genetics accounts for a large part of the risk. But it’s not a change or two changes. There are millions of positions that each contribute a tiny bit to this risk. So his example is in the middle where he’s probably about ten times more likely to develop disease. And although we are all very aware of diseases like Alzheimer’s disease, it is quite rare.
There is a big difference between relative risk, which is how much more likely you are to get the disease than someone else, and absolute risk, which is whether you will actually catch it. So the important point is that it does not determine whether or not he will get the disease. It only increases the risk for him.
What would you say to people who would rather not know their risk because it would only make them anxious?
It depends a bit on the disease and it depends a bit on the magnitude of the impact. So in the case of Alzheimer’s disease, the gene called APOE that was checked for Chris Hemsworth, has a pretty big impact on his risk of getting the disease. At the moment there is not much you can do about it.
I think different people will have different points of view. Some prefer to know and some prefer not to know. And it absolutely depends on the individual. But I think research on Alzheimer’s disease is progressing very quickly. So it could be that before too long there are things you could do and there could be medications to help reduce the risk or catch it early enough to slow its progression. I think it’s a slightly different question for different illnesses.
Will we ever see a day when entire populations undergo tests like these on a regular basis?
I think it will happen. First of all, it should depend on the individuals. No one should force individuals into this. But for most common diseases, genetics is a risk factor. What if we knew, instead of just saying “here are the ten or 20 diseases that should worry you the most. And here are some generic tips, we could say that in your case, you are particularly at risk for heart disease.
We can actually tell you this when you’re in your twenties, so you should work even harder on diet and lifestyle. Perhaps it would be appropriate to take medication to lower your cholesterol a little earlier in life. We might do this because we have this special information about you.
About our expert: Sir Peter Donnelly
Peter Donnelly is Professor of Statistical Sciences at the Wellcome Center for Human Genetics at the University of Oxford. He is also co-founder and CEO of genetic health services provider Genomics PLC.
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