Steve Horvath – Aging and the Epigenetic Clocks


The Undoing Aging conference, a joint effort between the SENS Research Foundation and Michael Greve’s Forever Healthy Foundation, took place on March 15-17 in Berlin, gathering many aging researchers, advocates, investors, and other important members of the longevity community in one place.

Steve Horvath is a Professor of Human Genetics and Biostatistics at UCLA. His research sits at the intersection of biostatistics, bioinformatics, computational biology, cancer research, genetics, epidemiology, epigenomics, machine learning, and systems biology. His research team applies these methods to study a wide range of topics, including aging research, cancer, cardiovascular disease, HIV, Huntington’s disease, and neurodegenerative diseases.

What is the epigenetic clock?

It is a bit like rust on a car. The DNA molecules experience chemical changes that could be interpreted as rust. So, as we age, things change on the molecule, and these changes are methylation, so by measuring the amount of “rust”, you can measure the age.


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Another metaphor is that different locations on DNA are very much like hourglasses. Think of it like 353 different hourglasses in different locations, and I measure the height of sand in each to find an age estimate.

You use 353 locations on DNA to determine biological age; can you explain why you use this amount, and how did you choose them?

These measurements are of 353 locations on the DNA molecules. They were chosen through a mathematical procedure so that they would optimally measure age, and it was a mathematical algorithm that selected them.

Sometimes, doctors say that we can determine our biological aging through measurements such as organ function, arterial stiffness, lung capacity and so on. Why is the epigenetic clock more accurate than these traditional biomarkers?

The epigenetic clock is more accurate for measuring age; for example, if I take a blood sample from you [to assess your epigenetic clock – auth.], I can tell how old you are. These other clinical biomarkers are far more important than age, because if somebody has high glucose levels, they might have diabetes and it should be treated. So these really are two different kinds of measurement; one is measuring age, and the other measurement is to measure health condition, to measure disease.


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Still, the stiffness of arteries can also tell how old you are, right?

The stiffness of arteries certainly increases with age, and it gives you a rough estimate of how old someone is. However, you can also have someone who is 80 years old but does not have stiff arteries; for example, in Brazil, there is a tribe living near the Amazon River, and when people looked at their arteries, they appeared perfectly young. The stiffness of arteries can also work the other way; you can have a young person with very stiff arteries due to a disease.

So your system provides a more accurate way to find the correlation between a specific number of biomarkers and age?

Yes, exactly.

Could we use the epigenetic clock as a diagnostic for diseases?


Not yet. It is not a diagnostic for any disease. The hope is that we will develop anti-aging medicines and then it could be used as a diagnostic. But, right now, we have no therapies, so it is not currently. In the future, it may prove useful to see if someone is epigenetically older and can be given rejuvenation.

Why is the epigenetic clock more accurate than measuring telomere length?

Yes, it is far more accurate, there is no comparison. Why is a good question. In my opinion, it shows that epigenetic changes are far more important for aging than telomere maintenance. People have studied telomeres for many years, including me, but telomere shortening alone does not explain aging. You may know that mice have perfect telomeres, but they only live three years.

It’s known that cells in our body are renewed at different speeds; why does your clock measure the age of the tissue or whole organ and not the age of specific cells?

Actually, it does measure the age of specific cells. You can have liver cells, and the epigenetic clock works beautifully. It also works very well for neurons and glial cells. Even in blood, you can have sorted blood, for example T cells or B cells, and the clock works on those cells.

Certain types of cell only live for a few days. What does their clock show, are they any different by age?

In the blood, you have some cell types that live only a week or two weeks, and there are some that live six months. When I analyse the ages of these cells using my clock, they are all roughly the same age and are actually the age of the donor. There are cells called neutrophils that live for two weeks, and they would all have your age, the reason is because the epigenetic clock probably measures the hematopoietic stem cells, which are blood stem cells located in the bone marrow that create neutrophils, so my clock measures the properties of the stem cells and not the individual cells they produce. The stem cells kind of keep the age of the person, and these cells differentiate into all the other cells, but, fundamentally, the epigenetic clock measures the age of the hematopoietic stem cells.

Some people store their cord blood from when they are younger so that it can be used later when they are older in case they need treatment. How would the age of these stored cells differ from the age of the patient?

I think that if they preserve these cells in some way, such as being frozen, then the cells will not age according to my clock. If someone thirty years old freezes their stem cells, then the age of the cells would be halted.

During your talk, you mentioned that when there is a transfusion of cells from a younger donor to an older patient, then the transfused cells will also be younger than the patient. What happens in the case when someone uses their cord blood and transfuses it into themselves when they are older, does that mean these cells would remain younger?

In principle, yes. My results indicate that it might be a good strategy to bank your own cells when you are young and use them decades later, such as using blood to replace your old blood stem cells. The problem is this blood replacement therapy is dangerous; it’s called hematopoietic stem cell therapy, and it is used for leukemia. It is a last resort because it is so dangerous and people die from it. The way the treatment happens is that you are irradiated and get chemotherapy to destroy all your blood cells, and then you have to wait a couple of weeks without any immune cells, and then you get the replacement cells, but some people die because they have no immune system.

Also, when you have blood from other people, then the body fights back; this is called graft vs host disease, which adds more complications. So, although my studies show in theory that this procedure definitely works to rejuvenate you, the side effects are the problem.

Does your clock represent aging?

This is a good question with two answers. One way to ask this question is to ask if methylation changes cause aging. And we honestly don’t know; there is no data. The other question to ask is if the epigenetic clock is the indicator of a biochemical process that plays a role in aging. Which I think it is; it is a biomarker of a process. There is no question that this process that underlies the clock, that if you target this process, you slow aging; this, we know.

What is going to happen if we influence this methylation process?

With the methylation process, we don’t know. Imagine that you have a clock; there is the clock face with the dials, and then there is the clockwork. The discussion with the epigenetic clock is whether methylation is part of the dial or is it part of the clockwork. There is no doubt that it is part of the dial, and if you interfere with the clockwork, there is no question you that rejuvenate people. But it could be that the clockwork might not be the same as methylation; we are not sure.

With a clock face, you can just take the hands and move them, but it may do nothing to actual time. Behind the clock, there is the clockwork, and we don’t completely understand the clockwork. A lot of people are asking about it, but we just don’t know yet.

Is it true that our organs age at different speeds?

Generally, most organs age at roughly the same speed. However, for example, female breast tissue ages faster. We analyzed breast tissue from women aged 25-30, and already, their breast tissue was older than their blood. We also analyzed a woman who was 112 and looked at 30 parts of her body; it turned out that the cerebellum at the back of the brain, which helps with motion and balance, was the youngest part. I studied a lot of people over 100 and found that the cerebellum aged slower than the rest of the body.

Does it have to do with the evolution of our brain?

The cerebellum has the highest concentration of neurons; there are very special kinds of neurons there. On one hand, it could be evolutionary, but it could also be the fact that most of the cells are neurons, while other parts of the brain contain other cell types. So it could be a cell type difference.

In general, we believe that our brain ages slower than our body, is that true?

Yes, I find that true in mice. When I studied the 112-year-old woman, her brain was younger than all other parts of her body. Her brain was perhaps five years younger, and the cerebellum was about fifteen years younger, roughly. On average, her brain was aging much slower.

Why do organs like breasts age faster?

I think it must be hormones; the female breast has a lot of estrogen to make it grow, and this exposure to hormones is one possible reason. It could also be that this aging effect is protective, because cellular senescence is a way to stop cancer, so it may be that the epigenetic aging of the breast is made to protect it from cancer.

Do men have an organ that ages faster?

I haven’t found an organ in men that stands out as being different despite considerable study.

Can we find out anything about our cancer risk?

As an aging researcher, I know that faster aging of blood carries a weak increase of cancer risk; the effect is very small even if statistically significant. It’s interesting because it shows that the epigenetic clock relates on some level to all age-related conditions. So, we can predict mortality from various diseases using it.

How can we predict mortality without seeing specific risk for diseases?

It relates to this root cause of aging or biological aging; you can imagine that if someone is aging faster, they will have many problems later on, but we don’t know what kind of problems they will have. Imagine if you have an 85-year-old man, you know he has a high risk of mortality, but you really don’t know what disease he will die from; you only know his general risk of death. It’s similar with the epigenetic clock, if it tells you that someone is aging faster, you know it is bad, but you do not know exactly what they will die of.

But we can determine what tissue or organ is aging faster?

Unfortunately, you can only measure blood from a living person, or perhaps buckle cells, or skin. It would be nice to measure the epigenetic age of kidneys, lungs, and so on, but people do not take biopsies of this kind from living people. In principle, it would be possible, though. I study such tissues from people who have died, like people who died from Alzheimer’s disease, and see that their brains have aged faster. But it is limited by how invasive it is in living people.

However the good news is that the epigenetic age of blood relates to a lot of organs, so this is a new epigenetic clock that is all based on blood. The age of blood relates to risk of dying from many diseases, but it also relates to your frailty, physical and even cognitive ability. So when you have a person in their 80s, but according to their epigenetic log, their blood is younger, they are cognitively better.

If we want to make blood analysis more accurate to determine the biological age of a whole organism, what must we do?

When taking a blood sample we should measure everything a doctor does, such as kidney function, glucose level, cholesterol, liver function, and more; you should do all these things. And then from blood, you can also also measure blood cell count to see if there is inflammation; all these things are very important.

Then you measure methylation; this helps those other biomarkers by adding more information. So, you can have a person who is perfectly healthy according to the other biomarkers, but their methylation age could be younger or older than expected, so it adds information. For example, in the US Mexican population, when you do traditional medical evaluations, they look like high-risk patients, they have high inflammation and high cholesterol, and they should die sooner according to regular blood tests. However, Hispanics are slightly younger epigenetically than Europeans and tend to live longer; this is called the Hispanic paradox. The epigenetic clock shows that they age more slowly and helps to explain why such a paradox happens when traditional tests would put them at high risk. In the US, Hispanics can live, on average, four years longer than Europeans.

If someone smokes what happens to their epigenetic clock?

We have a new kind of epigenetic clock called the pheno-clock. The original clock was published in 2013, and this new clock was discovered in 2018 and very much detects smoking. If you smoke, the new clock shows you have a much older age, but the original clock does not detect it.

What is the difference between your original clock and the new pheno-clock?

They were built in different ways for different purposes. The original clock was built to measure age in all organs and tissues. However, the new clock was built to predict how long you will live. The new clock would very much relate to your smoking habits because smoking is a large mortality risk.

So, smoking increases biological age; what if you quit?

I do not know if it reverses; I have not studied that yet. I do know that obesity and losing weight afterwards does not reverse epigenetic changes in the liver, but we will have to find out about smoking. We only looked 9 months after weight loss, so it may eventually be that it does reverse in perhaps five years, but we don’t have data.

What about pregnancy; is that the same?

I looked at women who had many children to see if it influenced epigenetic age, and it did not. A woman being pregnant is not the same as being obese, so while they gain weight, it is not the same as being obese. Certainly, this is the case in the 10,000 or so women I have examined to see if the number of pregnancies has a relation to epigenetic aging of blood, and it does not. It would be interesting to test this in breast tissue, and we will be doing so in the future.

Also, if you do breast feeding, it could be that it slows the epigenetic clock because breastfeeding actually lowers your risk for breast cancer. So, we are looking at this now and should know in a year’s time, but it could be that it is protective against aging. I have also found that women who enter menopause earlier, say around 35 instead of 50, can have blood that is epigenetically a couple of years older.

How accurate are commercial kits that measure the epigenetic clock?

I am not directly involved, but there is a company called Zymo, and they offer a test. My employer, UCLA, has the patent which they license to them, so I get a little bit of royalty, but I am not involved directly, so I do not know how they measure things. They offer two urine and blood analyses; the German police have used them. There was a refugee who claimed he was a teenager, but they did not believe him, so they needed a test. They used Zymo and showed that this person was like 25 and much older than he claimed.

How does short-term stress affect aging?

It turns out that short-term stress does not age you according to my clock. So, in other words, if someone has a stress event, like a divorce or a soldier with PTSD. However, for people who suffer lifelong stress, for example, abused children, they have cumulative lifetime stress, and it turns out that their epigenetic age is older. But the good news is that short-term stress does not age you.

Is there a period in life when we age quicker?

In a healthy person after age 21, there is no change; aging is constant. Interestingly, during development, the epigenetic clock is aging faster; remember, the epigenetic clock applies to children, and in children, it is not actually aging, but it measures development.

Nobody wants to be old when you are fifty; you want to be young. However, when you are two years old, you want to be older because being older means being more developed. So, when you are old, you want to be young, and when you are young, you want to be old. It turns out that children who are bigger at birth have an older epigenetic age because they are more developed, so initially being older is better. Children grow very fast, and so the epigenetic clock goes faster, because early on, it measures development, and only later does it measure bad things.

When do we start aging initially?

From birth, we start aging. However, it could be older than 21, certainly later. The epigenetic clock first relates to development, then it relates to a mechanism that protects you from cancer. It could be that in your 20s, 30s or even 40s it protects you from cancer, and only in your 50s could it become bad for you; it is hard to be sure exactly when it changes from good to bad.

What about sunlight and aging?

My original clock from 2013 does not detect effects from sunlight. But, using our new skin clock, it shows that radiation damage from the sun ages you.

What are the factors aside from sunlight that age our skin?

I cannot tell you. We have not analyzed that. In general, using sunscreen and moisturizer is a good idea.

What about food, if we eat turkey and chicken compared to red meat?

I looked at that carefully. We looked at 4000 women, and we knew the amount of red meat they ate. Honestly, the effect was negligible; there was a small difference but not a lot overall. It did not seem to affect the epigenetic clock a great amount. However, our studies show that eating vegetables and fish are good for you; it shows that these are beneficial but it does not show that red meat is bad for you per se.

What about the epigenetic clock in mice; who has made the best clock?

I have not done a fair comparison, but I have to say I really admire the work of Vadim Gladyshev; it’s phenomenal and should have been in Science Magazine. I have high hopes that he will use his clock to answer the questions we all want to know, to find drugs against aging. He is a superb scientist, so there is a good chance that he will find them.

Can we slow down aging now?

I want to tell you that I am very optimistic and that we will have treatments against aging in a few years. I could be wrong, and I want to be cautious, but I want to tell you that I am very optimistic because we already have encouraging results. We already have treatments that have a huge effect, like the Yamanaka factors in mice, but also in human cells. If you use Yamanaka factors on human cells, it completely reverses their age. The problem is how to make them safe.

My hope is that maybe even our generation will benefit from it; certainly, my daughter should benefit from it. I would be absolutely shocked if the next generation does not live twenty years longer. On that level, I am very optimistic.

If you ask me right now what you should do, I can only tell you boring things; immediately stop smoking, avoid obesity, avoid diabetes; if you are a diabetic, manage it; avoid high blood pressure, and if you have it, take action. It is boring, but all my studies show that this is the best thing we can do now.

Could we stop aging or even reverse it?

I think slowing aging will be much easier, but also I think we will be able to reverse the age of many organs. There have been promising results at this conference where people stimulate stem cells in organs, and that rejuvenates the organ. So in my view, absolutely that will influence the epigenetic age.

What is the danger of using Yamanaka factors?

In a word, cancer. That is the number one risk. However, there are four Yamanaka factors, and if you use four it rejuvenates the epigenetic clock, but the question now is maybe three factors are enough to rejuvenate the clock, or even one. The challenge now is to test these factors and see which of these factors work, and by not using all four, it may minimize the risk. Also there are chemical interventions that dedifferentiate cells so maybe some of these may also be able to rejuvenate the epigenetic clock.

Do you have the data from the Salk Institute, which used Yamanaka factors in mice?

I don’t have the data but I have no doubt they are working on this question. I met some of the research team, and they mentioned they want to look at that. Many people are working on Yamanaka factors, including companies; even my lab is going to apply it to human cells and mice. There are a lot of people working on this, so it’s a bit of a competition.

There are also a lot of alternatives like chemical interventions that have pretty much the same effect as Yamanaka factors. It may be that one of these alternatives could be much safer and work for humans.

What are the main challenges in your research in aging?

Scientific challenges, honestly I don’t have them. Because there is so much work to do and I have a good plan, it is not a problem. Financially, there is a challenge; research is expensive, especially human trials. I have a very exciting collaboration with a company which has an anti-aging treatment, and to test it will cost three million dollars. So, as you can imagine, money is the challenge.

How can we solve the funding problem?

The good thing is that many people are working on it and are good at raising money, especially companies. What needs to happen is that industry and companies need to really finance these trials, so it is really a community effort.

What is the role of state funding?

It really is not good. The NIH in the US have supported me, but, to be honest, it is not enough. The reason is the epigenetic clock is a bit controversial because some researchers criticize it because it does not work in C.elegans worms. This is because C.elegans do not have the same type of methylation as we do; some aging researchers only study worms, and they suggest because a worm does not have an epigenetic clock, how can it be aging? So, there is some debate about this among researchers, and this can make grant applications harder.

Ironically we have around 5000 cures against aging in the worm; we have solved aging in the worm. There are 1000 genes in the worm that control aging. I always say that we have cured aging in the worm, and now, with the epigenetic clock, we have the unique opportunity to cure aging in humans. So, these studies should be funded. These are my problems as a researcher.

Do you have a parting message for our readers?

Well, I have no doubt that their children will be much healthier and stay much younger, and if everything goes well, then actually many of us will still benefit. I just turned fifty, and I am hopeful that even my generation will benefit. The greatest would be if my parents would benefit too; my father is 84, and I am a bit frustrated. I mean, these epigenetic clocks were developed in 2013, and progress has been revolutionary in five years with major insights. I think that in another five years, there could be very promising anti-aging treatments, but as a scientist, one must be cautious. But, honestly, I am quite optimistic.

About the author

Steve Hill

Steve serves on the LEAF Board of Directors and is the Editor in Chief, coordinating the daily news articles and social media content of the organization. He is an active journalist in the aging research and biotechnology field and has to date written over 600 articles on the topic, interviewed over 100 of the leading researchers in the field, hosted livestream events focused on aging, as well as attending various medical industry conferences. His work has been featured in H+ magazine, Psychology Today, Singularity Weblog, Standpoint Magazine, Swiss Monthly, Keep me Prime, and New Economy Magazine. Steve is one of three recipients of the 2020 H+ Innovator Award and shares this honour with Mirko Ranieri – Google AR and Dinorah Delfin – Immortalists Magazine. The H+ Innovator Award looks into our community and acknowledges ideas and projects that encourage social change, achieve scientific accomplishments, technological advances, philosophical and intellectual visions, author unique narratives, build fascinating artistic ventures, and develop products that bridge gaps and help us to achieve transhumanist goals. Steve has a background in project management and administration which has helped him to build a united team for effective fundraising and content creation, while his additional knowledge of biology and statistical data analysis allows him to carefully assess and coordinate the scientific groups involved in the project.