Our DNA Changes as we get Older
Cells Use Genes Differently as People Age
June 26, 2008
A study just out shows that as we get older, our DNA changes. A lot.
Researchers in Iceland and the U.S. showed that over a period of 10-16 years, some people's DNA changed as much as 20%. These differences aren't in the famous A, T, C, and G's of DNA though. Instead, they are changes in something called DNA methylation.
Methylation can affect how genes are used. So a change in methylation might cause a change in how someone's genes are used.
How big is a number like 20%? There are millions of spots on human DNA that are methylated. So a 20% change represents tens or even hundreds of thousands of changes.
These changes also tended to cluster in families. In other words, if a dad had less methylation as he got older, chances are his son or daughter would have less methylation too. So people pass on whether or not their methylation will change with age. And if it does, they also pass on whether their kids are more likely to end up with more or less methylation.
These findings matter because changes in methylation have been linked to diseases like cancer and autoimmune disorders. It might be that some cases of these diseases are the result of this change in methylation. This would also mean that some people might inherit an increased risk for these diseases.
People are Different Partly Because of DNA Differences
DNA is made up of a repeating set of chemicals usually abbreviated as A, G, C, and T. These letters are the raw stuff of genes. Each gene has a certain number of these letters in a certain order.
A gene has the instructions for making a specific protein. Each of these proteins then does a specific job in the cell. They carry oxygen, help digest our food, and let us see, hear, breathe and even think.
One way that people are different is in the letters of their DNA. On average, one person has around 6 million different letters (out of 6 billion) compared to someone else.
If one of these differences is in a gene, it can cause the protein to be made to act differently. For example, this kind of change can cause one person to make a protein that causes brown eyes. Or it can cause another person to make a protein that gives him or her blue eyes.
Another reason people are different is in how they use their genes. Some people might have a certain gene turned way up so that it makes lots of protein. Others might have the same gene turned down so they make less protein.
Part of this can come from differences in letters. But part can come from differences in methylation too.
Methylation can hide a gene from the cell making it harder to read. This means that a methylated gene gets read less often or not at all. Which means little or even no protein gets made. So changes in methylation might cause changes in how some genes are used.
How DNA Can Change
The letters of DNA that we are born with don't change much over our lifetime. There is an occasional change but it is pretty rare.
Methylation is thought to be a different matter though. Scientists think that methylation can change a lot in the DNA of any cell.
DNA letters can change like this.
Methylation can change as well.
Scientists know that as a fertilized egg develops into a human, different genes get methylated and unmethylated. The new patterns of methylation are a big part of determining whether a cell turns into skin, muscle, nerve, etc.
Scientists also know that at least in mice, what mom eats can affect how a pup's DNA is methylated. The same is thought to be true in people too.
So there are definitely methylation changes that can happen in the womb. But what about afterwards?
A recent study showed that each twin in an identical twin pair has different DNA methylation. What this suggests is that the environment we live in can affect how our DNA works. It also suggests that as we age, our DNA methylation pattern changes. Now this new report confirms it.
The researchers showed that methylation can change with age by looking at two different groups. The first was a group of 111 Icelanders. The second was 126 people from Utah.
The 111 Icelanders were part of a group who gave DNA samples in 1991 and then again sometime between 2002 and 2005. What the researchers found was that when they looked at an average of all 111, it looked like there were little or no methylation differences over time. It was a different story when they looked at each person's DNA separately though.
Of the 111 people, 70 showed at least a 5% change in methylation. Another 33 showed at least a 10% change and 9 showed a whopping 20%. Some people had more methylation and others had less. Which is why the average was zero.
So DNA methylation can definitely change with age. The researchers next looked at people from Utah to confirm these results in another group.
They found that some of these people also experienced big changes in DNA methylation over time. They also found that the tendencies of these changes ran in families too.
Methylation (shown in green)
can change as we age.
The researchers looked at the DNA from a group of 126 people from Utah (Utahns). This group contained many multigenerational families.
The Utahns were sampled twice over a period of 16 years. The researchers found similar results to those found with the Icelanders.
Of the 126 Utahns, 50 showed at least a 5% change in methylation. Another 23 showed at least a 10% change and 13 showed a 20% change. Again, some people had more methylation and others had less.
When the researchers looked at families, they found that if a parent's methylation went up, the kids' usually did too. And if the parent's went down, the kids' often went down as well.
Many of these were families who did not live together over the 16 year period (grown children, for example). So it is unlikely that the environment would have similar effects on their methylation. It is more likely that something about their genes causes their methylation to increase or decrease with age.
How to Inherit Methylation Tendencies
Methylation really just means putting a methyl group on a C in DNA. (Usually the C is next to a G.) That methyl group is put there by certain proteins. And can also be removed by different proteins.
Like any other protein, the instructions for these methylating proteins are found in genes. These genes can also come in different versions like other genes.
Imagine that someone has a gene for a powerful methylating protein. This person might end up with more methylation than someone with a weaker version. This person could also pass the gene to his or her kids. Now the kids would end up with more methylation too.
A big part of who we are is in our genes. Differences in how our genes are used are going to affect this. Perhaps this is one reason why we change so much as we get older. Or perhaps what we do causes the DNA changes.
Another important detail is that some diseases are caused by changes in methylation. For example, cancer is caused by changes in how genes are used.
Sometimes the cancer results from a change in the letters of DNA. And sometimes they are caused by changes in methylation.
It is well known that older people get cancer more often than younger people. Part of this almost certainly comes from environmental abuse of their DNA over time. This abuse changes the letters of DNA to turn on genes that should be off and vice versa.
Methylation changes can do the same thing. It might be that the changes in methylation turn on a gene that should be off and that causes cancer. And maybe that increased risk was passed down from a person's parents.