Rhonda: Hello, everyone. I'm sitting here with Dr. Steve Horvath, who is a professor of genetics and biostatistics at UCLA. Probably one of Steve's most well-known contributions to biology is the development of what's known as the Horvath Aging Clock. And since then he's gone on to develop even more accurate aging clocks, which I'm so excited to talk about. I have talked about your work multiple times on multiple podcasts to multiple scientists. So, thank you for spending time to talk with me today.

Dr. Horvath: Thank you for your interest. Thank you for visiting me.

Rhonda: Well, Steve, maybe we could kind of start at the beginning with what is this Horvath Aging Clock that you had developed? Like, can you explain what it is? 

Dr. Horvath: Well, the Horvath Aging Clock is what I sometimes call the so-called pan tissue epigenetic clock. And so, it is the most accurate molecular measure of age. It applies to all cells in the bodies. Certainly, all cells that have DNA, all tissues, all organs. It measures age in prenatal samples, in children all the way to supercentenarians, people who are over 110 years old. And it measures age. So, if somebody provides, for example, DNA from blood, or DNA from neurons, or DNA extracted from saliva or urine, I can very accurately estimate their age. 

Rhonda: Their chronological age, like how old they are in years?

Dr. Horvath: Exactly. And that's already a deep question. So, the clock does measure chronological age. However, it's of course, not a perfect measure of chronologic age. There's always an error. For example, if I analyze the blood from a 50-year-old, the epigenetic clock may say well, this person is actually 55 or 45. And so, there is a small error, and this error is actually biologically meaningful, you know. It's not just noise, but rather it is in part related to what people call biological age.

Rhonda: Hello, everyone. I'm sitting here with Dr. Steve Horvath, who is a professor of genetics and biostatistics at UCLA. Probably one of Steve's most well-known contributions to biology is the development of what's known as the Horvath Aging Clock. And since then he's gone on to develop even more accurate aging clocks, which I'm so excited to talk about. I have talked about your work multiple times on multiple podcasts to multiple scientists. So, thank you for spending time to talk with me today.

Dr. Horvath: Thank you for your interest. Thank you for visiting me.

Rhonda: Well, Steve, maybe we could kind of start at the beginning with what is this Horvath Aging Clock that you had developed? Like, can you explain what it is? 

Dr. Horvath: Well, the Horvath Aging Clock is what I sometimes call the so-called pan tissue epigenetic clock. And so, it is the most accurate molecular measure of age. It applies to all cells in the bodies. Certainly, all cells that have DNA, all tissues, all organs. It measures age in prenatal samples, in children all the way to supercentenarians, people who are over 110 years old. And it measures age. So, if somebody provides, for example, DNA from blood, or DNA from neurons, or DNA extracted from saliva or urine, I can very accurately estimate their age. 

Rhonda: Their chronological age, like how old they are in years?

Dr. Horvath: Exactly. And that's already a deep question. So, the clock does measure chronological age. However, it's of course, not a perfect measure of chronologic age. There's always an error. For example, if I analyze the blood from a 50-year-old, the epigenetic clock may say well, this person is actually 55 or 45. And so, there is a small error, and this error is actually biologically meaningful, you know. It's not just noise, but rather it is in part related to what people call biological age.

Rhonda: That's super interesting there, that basically the error was actually related to biological aging because that was the next thing I was going to say was, you know, people age at different rates, like even, you know, obviously chronologically they could be the same age, but if you look at a variety of biomarkers...In fact, there was a paper published a few years ago in "PNAS" that looked at, like, 18 different biomarkers. They looked at glycated hemoglobin, so HbA1c, VO₂ max, triglycerides, telomere lengths, immunosenescence, a lot of the, you know, typical biomarkers that are clinically used to, like, determine health status. And it was basically found that people, you know, aged at very different rates based on those biomarkers. So, some people biologically appeared much younger than their chronological age, and some people appeared much older than their chronological age. So, now you've developed a different clock that can actually predict...

Dr. Horvath: That's right. Well, let me comment first on the term biologic age. It's a very intuitive term. Most people have a vague understanding of it. It somewhat relates to morbidity risk or mortality risk and also aging. But strictly speaking, it's not well defined because different researchers have different ways of measuring biological age. Some people use clinical markers that you mentioned, various markers of glucose levels or lipid levels, and so on. Now, my take to measuring biologic aging is based on a chemical modification of the DNA molecule. It's DNA methylation, you know. And I mention it because depending on how you measure biologic age, you might get different answers. So, a person might look bad in terms of glucose levels, and you would say, well, they age faster than they should. However, it could turn out that according to methylation, they are actually in pretty good shape, you know. So, yeah.

Rhonda: Have you seen that before where you can see people have, for example, like other clinical biomarkers that are unhealthy like higher fasting blood glucose or maybe elevated triglycerides, elevated C-reactive protein, a marker of inflammation? Do you find that those typically correlate well with the epigenetic age or...?

Dr. Horvath: I wouldn't say it correlates well. It correlates, you know, so people who have high levels of inflammation and what you mentioned, the epigenetic clock goes a little bit faster, but the word is there's a weak relationship, you know, because it is quite possible that somebody turns out to be in good shape according to epigenetic aging rates. And the number one example I want to mention in this context are actually people of Hispanic ancestry. Unfortunately, Hispanics often have a higher risk for diabetes, the higher metabolic syndrome. However, according to the epigenetic clock, they actually age more slowly, you know, and so there is this really this disconnect. And this is actually an interesting disconnect because there's something known as Hispanic mortality paradox, you know. Hispanics, as I mentioned, have often a disadvantageous risk profile according to clinical biomarkers. But it turns out, on average, they live much longer than expected. They actually live longer lives than people of European ancestry, you know. And that association is paradoxical to a clinician who looks at clinical biomarkers. But according to the epigenetic clock, it's not paradoxical because, as I mentioned, we have found that Hispanics age more slowly according to the epigenetic clock. 

Rhonda: That's very interesting. So, in this case, the epigenetic clock correlates more closely with lifespan than with clinical biomarkers and health status?

Dr. Horvath: That's right. Yeah. 

Rhonda: Well, in a way that's kind of good because you mean...

Dr. Horvath: Well, it shows it adds something. And also, the epigenetic clock is actually very much under genetic control. Some people just inherit a genome that makes...or DNA that really allows the epigenetic clock to progress more slowly. And so, the heritability is about 40%, you know. So, in this sense, it's not just lifestyle factors. By contrast, some of the clinical biomarkers you mentioned are very much under the influence of lifestyle, you know. So, you can probably cure high glucose levels by just avoiding carbs, right? And also, high lipid levels by taking statins, you know. So, there are a lot of clinical markers that can be influenced with lifestyle interventions and pills. By contrast, we don't have many interventions that allow us to reverse the epigenetic aging rate.

Rhonda: But do we know that? We haven't been testing that though, right? 

Dr. Horvath: Yeah. I mean, not really. I mean, in my lab, we clearly want to find interventions that slow the epigenetic clock, and by now many people are working on it, you know. And it's a gold rush who comes up with an intervention that affects the epigenetic clock, you know.

Rhonda: So, you mentioned that the heritability was about 40%. Is that of the epigenetic aging clock or...?

Dr. Horvath: Yes.

Rhonda: Okay. Because I recall from your looking at the epigenetic clock of the semi-supercentenarians, people that are like 105 years old, they had epigenetic aging clocks that were like 8.6 years younger than their actual chronological age.

Dr. Horvath: Yeah. That's true. It's true. That's a manifestation of that. So, if you have a parent who lived until age 100 or 105, then chances are that your blood is actually younger than the blood of a person of the same age, same gender, same everything, but whose parents didn't live until age 100, you know. So, the offspring of centenarians obviously have a genetic advantage, hopefully, but also that is manifested in the epigenetic clock. 

Rhonda: Yeah. So, you see that their epigenetic aging seems to be slower. 

Dr. Horvath: Yes. So, that's one line of evidence, but there's another. So, people have these longitudinal epidemiological studies, and they may have collected a blood sample from a person when they were, let's say 40 years old, and then 15 years later, they get a second blood sample. And so, you can then ask the question whether a person who was aging quickly at the first blood draw, did they still age quickly at the second blood draw? And the answer is yes. And conversely, you observe the same for people who age more slowly, you know. And, in my opinion, this could already be observed when you study, let's say a person at a young age, age 20. Draw their blood, then if you follow them for 60 years, you know, you would find this consistency that people who are slow agers at age 20, they are also slow agers at age 60 or 80. 

Rhonda: So, that does seem to imply that genetic...

Dr. Horvath: Yes. 

Rhonda: I mean unless someone dramatically changes their lifestyle. How stable are these changes in methylation, these methylation patterns that are so-called, you know, the aging clock? How stable are they over a person's lifetime?

Dr. Horvath: I mean, they are remarkably stable. So, when we compare to any other genomic measurement, I mean, they would be far more stable than anything I'm aware of. They're more stable, far more stable than gene expression, proteomics, metabolomics measurements. All the omics are less stable, you know, and that's really the biological reason why these epigenetic clocks are the most accurate measures of aging. It's just that methylation is so stable. And it's stable not just in vivo, but also when people collect DNA which...So, we've collected DNA, and then we didn't store the blood tubes properly, and so they melted, you know, and we extracted DNA and the measurements were perfect, you know. So, even on a technical level, they're very stable.

Rhonda: That's interesting because I have done experiments, intentionally though, almost the same, where we were collecting blood samples from participants in a trial, and I was measuring DNA damage as biomarked by Gamma-H2AX. And I wanted to know how long I could have blood at room temperature before I started to get artifactual DNA damage happening. And so, I did time course and, you know, found that after two hours of blood being at room temperature, there was just tons of DNA damage had started to increase. That was...

Dr. Horvath: I see. Yeah. That's scary. That's very scary.

Rhonda: Right? So...

Dr. Horvath: That wouldn't be the case with DNA methylation. I know because I hired a phlebotomist here in L.A. to visit families to collect blood, and this person didn't have an air conditioner in his car and it was the hottest day in L.A. ever. So, the blood tube melted, you know, that was my experiment. And then I just couldn't send this phlebotomist out to go back to the families to collect blood, you know. I felt sorry for the families. So, that's why we did this experiment, you know, and all I can tell you, we got beautiful data. And so, I see that over and over, and people sometimes ask me, "What if you have a forensic sample?" So, let's say a bloodstain, you know, that was, let's say, in a room for a couple of weeks, or even let's say a bone sample collected, you know, a couple of months later. All of this data, in my opinion, would still lead to very good methylation measurements for the purpose of measuring aging.

Rhonda: Yeah, because it's so stable.

Dr. Horvath: It's just so stable. 

Rhonda: Do you find that...Because these methylation patterns obviously are changing with age, you were able to predict first chronological age with 96% or so accuracy. And so, you know, obviously, while they're stable, at the same time they are changing, but do they change...like, is there a pattern of change? Like, do they change, you know, every few years all of a sudden things rearrange? Or is there, like, a pattern you can see where things start to change every block of time? Do you know what I'm saying? 

Dr. Horvath: Yeah, I do. It's a good question, you know. So, let me start slowly and say the epigenetic clocks typically track several hundred locations in the genome, you know. For example, the pan tissue clock is based on 353 locations in the genome. And a question is whether each locus changes, each location gains methylation in a continuous fashion, you know. Probably not, you know. I think what happens is some locations gain methylation, others lose methylation, and it's a bit random. But on aggregate, once you average hundreds of sites, you know, you kind of average out the noise, the variability, and that gives rise then to this very accurate age estimate. 

Rhonda: Okay. I definitely want to jump into some questions on mechanism and stuff too. But before I kind of go into the woods, this predicting the biological age, your DNA methylation PhenoAge was if I recall from reading your papers, which I read recently, was able to predict all-cause mortality, disease-specific mortality, like cardiovascular disease-related mortality. I mean, that was pretty interesting to me.

Dr. Horvath: Yeah. I mean, just to set the stage, so we have some epigenetic clocks whose purpose is really to measure chronologic age, period. But then we have other epigenetic clocks that are really defined to be lifespan predictors or they are meant to predict time to death or time to major onset of a disease or what people call health span, how long are you healthy? And as you mentioned, we have actually two biomarkers. One is called DNA methylation PhenoAge, but also another one that was named after the grim reaper, so it's called DNA methylation GrimAge. Now, these biomarkers were developed really for that purpose of predicting health span and lifespan as opposed to measuring aging. 

And the reason why we have these different tools is that we, of course, plan to use them in human clinical trials of anti-aging interventions, you know, and in that context, you want to see whether an intervention actually resets an epigenetic clock and that resetting then has a benefit in terms of delaying risk for various diseases. And you mentioned heart disease, GrimAge is a pretty good predictor of time to coronary heart disease. Surprisingly, these clocks even predict time to cancer. And that is surprising because it's a measurement based on blood, you know, and so you could ask why would a measurement in blood be predictive of the onset of various types of cancers in other solid tissues? But then...

Rhonda: You can predict it before other clinical diagnostics in some cases?

Dr. Horvath: Yeah. I mean, let me start out by saying I'm not sure whether this biomarker is clinically useful, okay? Because I'm very scared people think they can now measure their blood and I will predict you will get cancer in 10 years. It's not at that level. However, if you have, for example, a study of 1,000 women and somebody collected their blood in the 1990s, you know, and so for each woman you have follow-up information, whether she developed breast cancer or when she developed breast cancer. And if then analyze the data, you will find that biomarkers such as GrimAge and other biomarkers actually do predict onset to cancer in a statistical fashion. The P-value would be quite significant, you know. But as I said, I wouldn't claim that this is right now ready for prime time in a clinical setting for finding high-risk individuals because the effect sizes are too small. When I predict, for example, that you will develop heart disease in 15 years, you know, there would be a big error bar associated with it, plus, minus six years or so. So, for the individual, it might not be useful. 

Rhonda: That's a significant amount of time for a person, six years. 

Dr. Horvath: Yes, exactly. 

Rhonda: What about...So disease stage, you mentioned cancer. People with cancer or Alzheimer's disease or Parkinson's disease, like, have you measured the epigenetic age of these individuals and does it look like it's accelerated aging?

Dr. Horvath: Yes. So, we looked at blood samples from Parkinson's cases and controls, and there's no question, there's an age acceleration effect in blood. It's minor. It's one or two years, you know, but it is there. Alzheimer's disease, we looked at prefrontal cortex samples from the Religious Order Study, you know, and again, we found age acceleration in the prefrontal cortex. When it comes to blood samples from Alzheimer's disease, I think there might be a signal, but if there is a signal, it's very weak. And also...what other disease did you mention? 

Rhonda: Cancer. 

Dr. Horvath: Cancer. Yeah.

Rhonda: I'm wondering...you know, cancer is a beast. I mean, there's so many different types and...

Dr. Horvath: Yeah, cancer is complicated. So, the exciting insight is that yes, blood methylation data indicate that blood samples collected before the person develop cancer show a slight epigenetic age acceleration. So, that supports the view that faster epigenetic aging is predictive of future onset of cancer. And that finding has been validated by many groups. Problem is this association is weak, you know, you need really a couple of thousand people to observe it. But the question is, what about if we, for example, measured...?

Rhonda: Tumor tissue.

Dr. Horvath: Well, tumor tissue, the signal is huge. So, for example, when I analyzed malignant breast tissue samples from women with so-called luminal breast cancer, the epigenetic age acceleration is off the chart. So, their breast tissue is much older than expected. But it's complicated because...

Rhonda: Have you compared it to their blood? Like, is tumor tissue the same?

Dr. Horvath: No, no. It would be different. I mean, I would say the...I mean, yeah, let's say in breast tissue we find 10, 15 years age acceleration in malignant tissue, but in blood, I mean, the effect would be much smaller if at all.

Rhonda: It's very interesting because you mentioned Alzheimer's disease blood also...

Dr. Horvath: Very weak effect.

Rhonda: ...very weakening signal, but if you measured actual brain tissue, which is where, you know, it's a neurological disorder and find a signal, the interesting thing is you said Parkinson's disease, you do measure the signal in the blood.

Dr. Horvath: Yes, that's true. 

Rhonda: It'd be really interesting to know if the immune system is playing a role in Parkinson's disease. 

Dr. Horvath: I can tell you the following. So, we did this study of Parkinson's people. Why? Because I was interested in epigenetic aging. However, my software also produces estimates of blood cell counts. And so, it turned out that the blood cell counts, in particular neutrophils, were really highly elevated in Parkinson's disease, huge effect, you know. And so, in certain ways, this was completely surprising to me, but this finding has now been validated over and over. So, yes, PD cases have highly elevated neutrophil counts. 

Rhonda: That's very interesting. 

Dr. Horvath: Yes. So, yes, immune system plays a role. We don't know the causal direction, you know, is it first immune dysregulation that gives rise to PD or is it the other way around? 

Rhonda: There's been some interesting links to gut, the relationship of gut and Parkinson's, and of course, the immune system's involved in the gut and all that. So, the reason I asked about the cancer tissue though was because, you know, another biomarker of aging, telomere length, the longer the telomeres are, it's associated with basically better biological aging in a way because your telomeres get shorter with age. So, it's assumed that longer telomeres means younger, right? 

Dr. Horvath: Yes. 

Rhonda: But some cancers find a way to, like, reactivate enzymes, like, involved in building telomeres like telomerase, for example, and so some cancers have longer telomeres. 

Dr. Horvath: Absolutely.

Rhonda: And so, if you just were looking at that one biomarker, you'd look at that tissue sample and think, "Oh, this is...," you know.

Dr. Horvath: Yeah. Let me make a few comments about telomere length. And so, as you said, by now we know that there is a U-shape behavior. You don't want telomeres that are too short, and you don't want to have telomeres that are too long, you know. And so that's the first statement. The second statement is telomere length per se is actually not a good biomarker for predicting onset of various diseases. Most diseases don't have a strong relationship with telomere length. In particular, when it comes to predicting lifespan, you know, telomere length is actually a shockingly weak predictor of lifespan. For many years, people wrote articles where they claim there is no relationship to lifespan. By now, the field has moved on to say, well, if you have very large data, you do see a relationship to lifespan. But if we compared telomere length versus an epigenetic clock such as GrimAge when it comes to predicting lifespan, time to cancer, time to coronary heart disease, I mean, there would be no comparison, you know. In this sense, telomere length plays a very important role in certain disorders, you know, but it's just not a broad biomarker for aging. 

Rhonda: Right. How does the epigenetic clock, whether we're talking about the PhenoAge or DNA GrimAge, relate to other biomarkers of aging? So, does it usually correlate? Like, if you have...So, there's immunosenescence which, you know, is associated with aging, DNA damage, inflammation, there's telomere length. Does it correlate typically, like, in the same direction? 

Dr. Horvath: Yeah, it would. So, talking about GrimAge or PhenoAge, they would correlate in a consistent fashion, you know, so they would have a weak correlation with telomere length to give you a number correlation 0.1. So, it's actually a weak correlation but, yes, if you have a thousand people, you pick it up. In general, telomere biology is really a different hallmark of aging compared to epigenetic changes, you know. So, they measure different aspects of aging, but yeah, there's consistency. 

Rhonda: Telomere biology has been...I interviewed Dr. Elissa Epel on the podcast. She works closely with Dr. Elizabeth Blackburn. She's at UCSF, and she has published some studies showing that, like, stress plays a big role...it plays a pretty good role in tumor biology. So, you can find that, like, different types of stress can actually affect telomere length. So, lifestyle factors that affect the epigenetic clocks, so for example, diet, exercise, smoking, or even, you know...

Dr. Horvath: Education. 

Rhonda: Education, exactly. So, how do those lifestyle factors, in general, affect epigenetic aging? 

Dr. Horvath: Yeah. Everything your grandmother ever told you about living a healthy lifestyle is kind of corroborated by our epigenetic clocks, you know. So, for example, people who eat vegetables, people who exercise, also actually educational level, you know, to some extent even alcohol consumption show a beneficial effect. Now, the problem is these effects are weak, you know. Again, you need a couple of thousand people, then you pick it up. In terms of statistical significance, there's no debate. Clearly, these associations are there. But for the individual, the question is what if I follow the perfect lifestyle, do I make a big dent on epigenetic aging? And the answer is, unfortunately, not really, you know. I mean, I'm as much of a health nut as many other people in Southern California, you know, so I am trying to have a healthy lifestyle. 

So, yes, you want to avoid diabetes and all of that, you know, and certainly, you don't want to smoke, but the truth is a lifestyle intervention will never have a profound impact on aging at the population level. Because what I would like to do is I would like to increase healthspan by 10, 15 years, you know, and in my opinion, lifestyle interventions won't get us there, in healthy people, okay? So, let's say you have a friend who smokes and is obese, and yes, tell your friend to adopt a healthier lifestyle because, for this person, it will have a huge effect. But let's say you take somebody like me who is reasonably slender, doesn't smoke, and now you're telling me, what about if you become a vegetarian? Or what if you double the amount of exercise you do? Will you have a strong effect on my lifespan? And the answer is no, not really.

Rhonda: According to the epigenetic clock?

Dr. Horvath: According to the epigenetic clock. 

Rhonda: So, the physical activity was only, like, a weak...?

Dr. Horvath: Yeah. Physical activity, exactly, unfortunately weak. So, I want to say correlation 0.08 for people who know what that means. That's a very weak correlation in blood, though. Because the question is maybe if we studied heart tissue or muscle, maybe we would find a much more pronounced effect. At least in blood, we didn't see it.

Rhonda: Well, that sort of brings the question about tissue types too as well. I mean, you know...

Dr. Horvath: Yeah. Example is the effect of obesity on epigenetic aging. Turns out obese people age faster in blood. However, the strongest effect can be found in liver tissue. So, obesity greatly accelerates the epigenetic age of liver tissue, you know. And so, I think a lot of stress factors really have an organ-specific effect. Conversely, anti-aging interventions also have an organ-specific effect. So, for example, when we evaluated the effect of postmenopausal hormone therapy in women, we found no beneficial effect in blood. However, interestingly, the buccal epithelial cells or the cells inside of your mouth, they actually revealed that women who took hormone therapy were aging more slowly in these cells.

Rhonda: Oh, interesting. 

Dr. Horvath: Yes. 

Rhonda: Many cell types are also epithelial cells. I mean, many of your organs have epithelial cells. 

Dr. Horvath: Yes.

Rhonda: Blood cells are a little different, I mean. So...

Dr. Horvath: And now the finding made sense because blood cells don't have as many estrogen receptors as buccal epithelial cells, so yes. Obviously, if you have a hormone intervention, you want to study cells that are susceptible to it, you know, but yeah. 

Rhonda: Right. So, that really does highlight the importance of measuring different tissue types to see because...I mean, sometimes it's easy to kind of think, well, whatever fundamental mechanism is regulating this aging process would do it in all the tissues but doesn't necessarily mean...It wouldn't be the same either, right?

Dr. Horvath: Exactly.

Rhonda: So, it's tricky. 

Dr. Horvath: Yeah. I mean, it would be fantastic if a blood measurement is really a surrogate for all other tissues and organs, you know. But it seems to be not the case, at least for stress factors, as I mentioned.

Rhonda: [inaudible 00:31:26] environmental [inaudible 00:31:27].

Dr. Horvath: Environmental...but also even genetic factors, you know. So, some people inherit a genetic variant that accelerates the epigenetic age in blood but not really in brain tissue, you know.

Rhonda: Oh, really?

Dr. Horvath: Even there, it's complicated. Yeah.

Rhonda: So, there's genes that are regulating the epigenetics instead of vice versa?

Dr. Horvath: Yeah. 

Rhonda: I kind of lost track of...I was going to ask you something, Geez, it was important too but...oh, I know what it was. I had read a study, I think it was one of your really good reviews that you published where you talked about bone marrow transplants. Because the question is, you know, there's lots of these studies coming out with parabiosis where you can transplant young blood from animals into animals that are older and sort of have this rejuvenation effect. So, the question is, if you take cells from a younger recipient and put it into an older donor, like an...sorry. If you take cells from a younger donor and put it into an older recipient, so you're basically taking young cells and putting them into an older person, so the young cells will have a younger epigenetic age, obviously, than what was already there. The environment around the cells, the niche, so to speak, does that play a role in...do those blood cells, like, have more of an accelerated aging? Do they...?

Dr. Horvath: Yeah. So, just to be clear, I want to distinguish parabiosis from these hematopoietic stem cell transplantations. I can comment on both, but let me start out with a hematopoietic stem cell transplantation. Some people have a very severe form of leukemia, and therefore their bone marrow stem cells have to be removed, and the procedure is dangerous, I'll start out with that. So, that's why I say only severe forms of leukemia patients get that treatment. But yes, let's say you take a 50-year-old and you give this person a bone marrow transplant from a 20-year-old. And so, after the transplant, the blood in the recipient reconstitutes itself. The person has now new blood. And the question is, what's the age of the blood? Does the blood have the age of the 20-year-old donor or the age of the 50-year-old recipient? And you can make a case for both scenarios. 

But there are now several scientific papers that really give an unequivocal answer, which is the reconstituted blood in the recipient has the age of the donor, you know, and that effect persists for decades, you know. So, if you take...again, the 50-year-old got a bone marrow transplant from a 20-year-old, you follow this 50-year-old, 30 years, now he's 80. Question is how old is his blood? Well, the age would be now 50, because 30 years have passed and you add that to the age of the donor, you know. And one would think that the stem cell niche in the bone marrow could possibly affect the aging rate, but it just isn't the case. And so, that's on some level a very exciting finding because it kind of hints to an idea that you could possibly rejuvenate people through transplantation. The reason why this is not yet a viable strategy is because people who get a transplant often get so-called graft versus host disease. So, there are all sorts of complications, you know. It's a dangerous procedure, but in theory, you know, it could work. 

And talking about parabiosis where people have connected two mice, you know, where one mouse is much older than the other one or much younger than the mouse. We just recently analyzed parabiosis mice, you know, it's unpublished, and we found two results. One corroborates things and one refutes it, okay? But just to explain it, so we looked at cortex and also subventricular zone, deep white matter in the brain, and we found that a young mouse that was connected to an old mouse actually aged faster according to an epigenetic clock in mice. So, that part confirmed these parabiosis experiments. So, in other words, you can age a young mouse. But that's not what people are interested in. They're interested in the opposite. You take an old mouse and you connect it to a young mouse, and then you study the brain of the old mouse, and you want to see that the brain is rejuvenated, you know. And for that scenario, actually, our results were disappointing. We didn't see a rejuvenation effect, you know. And so, now we're trying to get additional data because our first study was underpowered, but one of these months we will have a definitive answer.

Rhonda: Yeah. You know, some of these animal studies are really good for trying to understand mechanism, and all this data suggest, you know, you've got a clock that can predict chronological age. You've got a clock that can look at your biological age and also predict time to death, lifespan, the GrimAge. I mean, something clearly is changing these methylation patterns. So, the question is, what is that? Is there a chronic signal that's doing it or is it just completely under genetic control? And maybe they're related, right? So, what are your thoughts on the aging process and even [crosstalk 00:37:44]?

Dr. Horvath: Yeah. Well, when it comes to these epigenetic clocks, this is the number one weakness of these clocks that we don't completely understand the molecular mechanism. And coming back to telomere length, that's a great advantage of telomere biology. We really understand very well what regulates telomere length, you know. But yeah, with the epigenetic clocks, this is a very active area of research. Top biologists and labs are working on that very question, you know, and there are many theories. Some people think stem cell biology plays an important role and that's probably true for many tissues, you know. In certain ways, it could measure aspects of stem cells, for example, how often a stem cell divided. The problem with that interpretation is that the epigenetic clocks work beautifully in neurons, you know, which really don't rejuvenate over the lifespan.

And yes, another group thinks that epigenetic clocks might relate to circadian rhythm, so there have been some theories. Now, I believe that also processes that play a role in development must be playing a role here and the reason is because my original pan tissue epigenetic clock works actually beautiful in prenatal brain samples. It works beautifully in various in vitro studies of so-called three-dimensional brains, you know, or in retina samples. So, really, it captures aging of gestational age during development, you know, and during development, there's really no noise. This is a highly coordinated process. And so, yes, these processes also must play a role. 

Rhonda: Yeah. Certainly. One thing I had thought about, and I want to get back to development because of the stem cell thing, but one thing that came to my mind with this chronic signal is one particular gene that is methylated during early age but then becomes demethylated as a person ages is this p16INK4a gene. And it plays a role in cell cycle progression, meaning, like, it basically stops the cell from going on to the next cycle. So, obviously, you don't want it to be active during development or early age because your cells are growing. So, when it becomes active, it stops stem cells, like hematopoietic stem cells, they aren't growing. So, there's, like, this kind of role in aging where it's sort of basically stopping the stem cell from growing. It's going to have a negative impact on aging, although it can be positive for cancer because...well, positive for the person that has cancer because it can stop a cancer cell from growing, right? 

So, the question is, there's a group of demethylases that can take off the methyl group that become active and take it off of this gene, and they're activated by inflammation. So, what I'm wondering is, is anyone looking at...Obviously, these methyl groups are changing, and so the enzymes that are pulling off methyl groups and the demethylase is the enzymes that are putting them on their methyltransferases, they must be doing something. 

Dr. Horvath: Yes. And people are looking at it. Yeah. I mean...

Rhonda: Okay. What's changing those enzymes? Like, is there a signal there or is it a gene? Is it genetic control or what is it, you know, that can change? 

Dr. Horvath: Yeah. You make very good points. So, if you want to understand the epigenetic clock clearly, you start with so-called DNA methyltransferases or these TET enzymes. Why? Because they, on the one hand, add methyl groups or remove methyl groups. So, that's a low hanging fruit. And just recently, we and others, several groups have actually found no doubt when you interfere with these enzymes you affect epigenetic age. There are very exciting findings where people studied certain developmental disorders where mutations deactivated DNA methyltransferase or mutation rendered it overactive, you know. And sure enough, all of these mutations in humans, you know, affect epigenetic age. And so, at that level, we know it has an effect as expected, and the effect is pronounced. It could add 5 or 10 years to a person or the opposite, you know. 

Rhonda: And does that correlate with the life span of whatever disease?

Dr. Horvath: You see, we don't know exactly. That's the question. So, some of these children have a developmental disorder, and there is various syndromes, you know, one is the so-called Sotos syndrome. Anyways. And so, we see strong deviations in blood in both directions plus five years, minus five years. So, at that level, it's all confirmed, plus right now, we do mouse crosses, you know, where we knock out these DNA methyltransferases just to very carefully study it in a very controlled setting. But I can already tell you you'll find strong effects. But the question you really ask is, well, on this upstream of those, you know, what regulates the clockwork? What tells the DNA methyltransferase, "Go to this location and deposit and work your magic," you know? And that's where we don't have an understanding yet. 

Rhonda: And that's why I mentioned the Jumonji demethylases because that is just one group of demethylase that particularly plays a role in taking off the methyl groups for the INK4a locus. And I remember trying to figure out, you know, inflammation plays a role in aging, and so I had seen studies showing, sure enough, you know, TNF alpha, these things play a role in activating those demethylases. And so, it's like, well, is that something that just over time chronic, you know, that there's a threshold or what? You know, I don't know.

Dr. Horvath: Absolutely.

Rhonda: I've never seen the experiment done with that particular class of enzymes. 

Dr. Horvath: I mean, I've seen results from other groups, you know, that look at that issue, chronic inflammation or even looking at these...sorry, what are these...transposons, you know. So, there's some very exciting results that shows that some transposons become active in older tissues, you know, and so I've seen some preliminary data where people said this was associated with increased epigenetic aging. And also, our finding that HIV is very much associated with accelerated epigenetic aging also points again to this idea of a viral component, you know. So, clearly there must be a connection, you know, and it will be very interesting to tease it out. I don't have a good sense how strong that effect is, you know, for example, chronic inflammation, or do these enzymes that you mentioned...how strong is their effect if we perturb it? Do they explain 30% of the variability or just 10%? You know. So, that sense summarize the question. 

Rhonda: Have you measured the epigenetic age in stem cells versus already differentiated cells? Is there a different pattern or different...?

Dr. Horvath: Yeah. There is a different pattern. So, if you take, for example, a stem cell and iPS cell and then differentiate it into a more mature cell, for example, a more mature neuron, you will find that the epigenetic age increases. The issue is it doesn't increase by a lot, you know, so a more differentiated cell, a more mature cell, it may be one or two years older than the stem cell. The question is, how do we age a cell by 30 years? Because when you study neurons in a dish, you want to perhaps understand neurodegeneration, and so you want to study very old neurons. And so, when you deal with neurons derived from stem cells, we don't have a good way to age them. But nowadays, people pursue this idea called transdifferentiation. So, you take, for example, a skin cell and you add certain factors, maybe microRNAs or what have you, and then turn the skin cell into a neuron, and this transdifferentiation protocol actually preserves the epigenetic age. And so, we have shown in collaboration with several groups that yes, the resulting neuron has the epigenetic age of the skin cell it started with.

Rhonda: Oh, that's interesting. So, what about when you take a skin cell, and you mentioned induced pluripotent stem cells, and you basically can add, you know, like four transcription factors, I think even less now, but the original from Shinya Yamanaka, four transcription factors and make this into like a pluripotent stem cell that can become any type of cell in the body, the epigenome, what happens to the epigenome?

Dr. Horvath: You completely reset it.

Rhonda: To a young...completely young?

Dr. Horvath: Completely young. 

Rhonda: So, that's like the ultimate reversing aging, right? I mean...

Dr. Horvath: It's true. Because, in the past, people were wondering, you know, are there interventions that actually reset the epigenetic clock? And the number one proof of principle study is really the administration of these Yamanaka factors because it completely resets the age actually to a prenatal stage, you know.

Rhonda: And that's completely genetic, right? I mean, you're manipulating genes.

Dr. Horvath: Yeah. I'm not sure. I mean, it's really...So, also messenger RNA can do it, you know.

Rhonda: I guess what I mean is it's something that's kind of under our genetic control, like, ultimately. 

Dr. Horvath: Yes. I mean, right now there is an idea in the aging field to rejuvenate people by leveraging this fundamental insight. It's called reprogramming. You can take an old cell, you administer certain factors as you mentioned, four factors or three factors or other variants, you know, and by administering this cocktail of factors for just a few days, not too long...because if you administer it for too long you greatly increase the risk of cancer, you know, because the cells lose their identity, the skin cell forgets that it's a skin cell. It thinks it's a stem cell, you know. But if you do it briefly for let's say 5 days, you get the benefit of rejuvenation, you may have rejuvenated the cell by 5 years or 10 years, you know, but it still remembers its identity. 

Rhonda: So, does the epigenome reset a little bit? Like, it only...

Dr. Horvath: Yes. Yes. So, there are a couple of groups that are working on it and have already shown that effect, you know.

Rhonda: That is fascinating. That is [crosstalk 00:49:51]. 

Dr. Horvath: Yes. So, I analyzed fibroblasts and endothelial cells from such an intervention. It's called sometimes interrupted reprogramming or transient reprogramming, you know, and sure enough, you know, that idea worked. It reset the age, but the cells still remembered their identity and therefore arguably will not become malignant. 

Rhonda: Right. Yeah. That's super interesting. 

Dr. Horvath: Yeah, I have to say that's true. And the epigenetic clock is the ideal biomarker for that kind of a study, in my opinion, because as I said, it very much tracks the state of the cell, but we will see. Hopefully, in 5 or 10 years, it turns out that this strategy turns into a viable anti-aging intervention that can be used in the clinic. But at this point, these are all proof of concept studies that have been done in the dish or in mouse studies, you know.

Rhonda: Do you personally think that the epigenetic clock plays a causal role in aging? Do you think...?

Dr. Horvath: Yeah, let me answer it in two ways. It clearly relates to a process that plays a causal role, and so to use a metaphor, is it the face of the clock, you know, or is it the clockwork of aging, you know? And no doubt the epigenetic clock must relate to at least one causal process because it predicts lifespan. If it didn't relate to a causal process, it wouldn't be able to predict how long you live. But the real question is perhaps what if you changed the methylome, you know, if you had an intervention, you change DNA methyltransferases, you kind of...I want to call it the superficial way of perturbing the clock, would that have a benefit? And personally, I don't have an answer to it yet. 

Rhonda: Yeah. Have you ever looked at how...So, you mentioned obesity and how obesity is associated with an accelerated epigenetic age. Have you ever come across data with respect to the opposite of that, like fasting or even calorie restriction, if that slows the epigenetic aging? 

Dr. Horvath: Yeah. I mean, in mice, the answer is clear cut. Definitely caloric restriction slows the epigenetic clock in mice. And we know that because several groups have looked at it, including my group. All of us arrived at the same answer. Conversely, by the way, high-fat diet, you know, accelerates the epigenetic age of mice, you know. So, that's all clear cut. The question though is really what about humans? And I'm not aware of a study where people really looked at caloric restriction versus epigenetic aging, but let me share some thoughts. When it comes to any intervention, including dietary intervention that prevents anything related to metabolic syndrome or diabetes, that will be detectable. And also, in other words, if you have an intervention that takes a very obese person to a lean person, in opinion, GrimAge will pick that up. It has to, you know. 

However, when you ask the question, what about if you take a relatively lean person, healthy body mass index 23, and this person is so motivated to live 10 years longer and they pursue now a lifestyle where their BMI is, let's say, 17, you know, almost at the point of being unhealthy. And so, if you compare a healthy person versus a semi-starved person, would that have a benefit? And, in my opinion, that may not be the case. And I'm just speculating here, but even according to other biomarkers. Personally, I haven't seen convincing studies, you know. Otherwise, I think my by BMI would be 17 right now, you know, so yes. 

Rhonda: There's an acquaintance of mine who has...he runs a pretty popular aging blog. His name is Josh Middledorf. He's reached out to me, I don't know, a few months ago because he's trying to organize a study where people that are basically already practicing some type of fasting, whether it's an intermittent fasting, they're doing, you know, a 16 or 24-hour fast or sometimes maybe they do prolonged fasting, which in some cases can be longer than 48 hours, 48 hours or more. People are doing this stuff, you know, it's happening. 

Dr. Horvath: I'm trying to too, trying to do this intermittent fasting, but I don't have much self-discipline, but yeah. We will now analyze mice, you know, from the lab, from Joe Takahashi who did various interventions, and probably in a couple of weeks, we will have some answers, you know, whether these strategies have a strong effect in various tissues in mice. But yeah, when it comes to human data that's really the prize. 

Rhonda: It is. And as you mentioned, you know, you have someone that's starting off with already what you would think is healthy. Someone who's not obese, someone who exercises, doesn't smoke. Someone who is, you know, eating a relatively good diet. If you take that person who's already pretty healthy and then do an intervention, so that intervention is going to be, whether it's like intermittent fasting...So, I mentioned Josh Middledorf because he wants to track...You know, there is a company that does, based off of your clock, I don't know which clock, the Horvath original clock or the PhenoAge, but they're called myDNAge. 

Dr. Horvath: That's right. 

Rhonda: I just ordered it. It just arrived. I'm going to get my blood tested. 

Dr. Horvath: I'd say original clock of mine, yeah. 

Rhonda: It's the original. Okay. So that's...

Dr. Horvath: Except they have modified it a little bit, you know, I think, yeah. 

Rhonda: So, that's probably better at predicting chronological age rather than biological age. 

Dr. Horvath: Well, I think nobody knows, you know...

Rhonda: Nobody knows. Right. Because...

Dr. Horvath: ...because these biomarkers should really be evaluated in a prospective study. 

Rhonda: Yeah. So, that's the question. It's like, well, if you have these types of interventions on already healthy people, will they make any difference? At least according to epigenetic aging, right...

Dr. Horvath: Yes.

Rhonda: ...does it mean that? I mentioned before we started filming, I mentioned to you a pretty recent randomized controlled clinical trial with vitamin D supplementation in a population that started off unhealthy. So, these were obese African Americans, which were very low in vitamin D. African Americans tend to be the lowest in vitamin D because they have a natural sunscreen. And so, they were given 4,000 IUs of vitamin D a day and after, I can't remember how long the trial was, a certain period of time, their epigenetic clocks were measured at baseline and after, and their epigenetic clocks, Horvath, the Horvath Clock was used, it basically was reduced by 1.8 years or something like that. And you mentioned the significance is a little...the sample size is small, and so it's kind of like, well, it's a start.

Dr. Horvath: I mean, it's an exciting finding. I think it was based on 51 people, and it's a nice finding. It would be brilliant if a simple intervention as taking a vitamin D supplement actually affected the epigenetic age. I agree with the authors. We just need larger studies to validate it, you know, but in general, as you know, clinical trials are very expensive and that's a real bottleneck in all sorts of anti-aging interventions. We really wish, on the one hand, private industry would find merit in investing in clinical trials. Obviously, also the government plays a role, but what we really need is really dozens if not hundreds of clinical trials. Why? Just to have a chance for serendipity. Maybe it is as simple as a vitamin D supplement, but maybe you need something much more radical like, for example, a modification of the Yamanaka cocktail, you know. Or it could be plasma transfusions or so, you know. We as a field need to experiment, you know, with what kind of interventions work. It could be hormones, by the way, you know. It could be a hormone intervention, and so on. 

Rhonda: When you mention, obviously, do you see...? I'm going to change topics here, but I don't want to because, you know, the intervention trials are important. As you mentioned, it may have some effect, it may not have a big effect. But, you know, even if a combination of factors like, you know, getting your fish intake, which you've shown to also...seems to be related to...

Dr. Horvath: Yeah. Interestingly, according to GrimAge, we did find that people who used omega-3 supplements of fish oil, they were actually aging more slowly, and we thought this was a nice little insight. 

Rhonda: Oh, that's interesting because I take fish oil supplements.

Dr. Horvath: Me too. Because what happened is, I want to say six months ago, there was a publication, really very large-scale clinical trials looking at fish oil supplementation, and they did not observe any benefits. And I said, "Oh, my God, I wasted all these years eating fish oil." But then we analyzed really an observational study, and that's our problem, our study was an epidemiological study, I want to say the Women's Health Initiative, and there we did see this association that women who took fish oil supplements were aging more slowly according to GrimAge, you know.

Rhonda: Wow. And I'll just add a note to that study. You mentioned the big VITAL D study, which was a vitamin D and omega-3, a huge study. There was no effect on the primary outcome, which was looking at all combined cardiovascular-related events. However, when you specifically looked at like heart attack versus stroke, there was a strong effect.

Dr. Horvath: I see. Interesting.

Rhonda: It's always, you know, the secondary outcome. Like, however you're designing the trial, you know, there's a primary outcome. If that primary outcome is negative, then it's like, well...you know. 

Dr. Horvath: The problem with the primary outcome is that it's far removed from the processes that are being targeted by the molecules, you know, and that is really the benefit of having what I call a surrogate marker. Something like GrimAge is arguably much closer to the mechanisms, is perhaps less susceptible to confounding factors, you know. And so, maybe that's why these surrogate markers pick it up. 

Rhonda: Right. Yeah. I mean, obviously the methylation patterns, these are molecular changes that happen before someone has a heart attack, before someone has high blood pressure. I mean, these are accumulated, you know, changes at the molecular level that are occurring.

Dr. Horvath: Yes. And also, when you think about heart disease, it probably relates to many confounders such as psychosocial stress. You go through a divorce, you know, you get fired, and various substance abuse, you know. A lot of processes that are...and certainly stress factors, but they are not part of what I would call innate aging processes, you know, whereas these biomarkers such as an epigenetic clock is hopefully much closer to an innate aging process. And when we want to cure aging, we want to cure innate aging, we don't want to...It's not my goal to reduce people's stress levels so that they should do yoga or any other intervention. 

Rhonda: Yeah. Right. Sleep. I mean, obviously sleep is another thing associated with heart attacks. And I don't know if sleep is even usually one of the confounding factors that it's adjusted for, right? I don't even think that's something I usually see.

Dr. Horvath: You know, we looked at women who have sleep disturbances in the Women's Health Initiative and sure enough, their epigenetic age of blood was slightly accelerated.

Rhonda: That makes sense. Yeah. I talked to a sleep expert not long ago, Dr. Matthew Walker, he runs the Human Sleep Center at UC Berkeley, and he just talked about all the, you know, basically various diseases and all-cause mortality and how everything just goes up when sleep quality goes down. So, I certainly feel like [inaudible 01:03:13]...

Dr. Horvath: I always hate these studies because I don't sleep well. I like the study of the so-called super sleepers, you know, who sleep only five hours at night and still are perfectly healthy. So, I like more the optimistic spin on things. But also, just to tell you, so the effect of sleep quality versus epigenetic aging and blood, these associations were statistically very significant, but again, they were weak, you know, and that makes me hopeful. 

Rhonda: Yeah. Compared to the semi-supercentenarians, which had an epigenetic age of 8.6 years younger than their chronological...that's pretty robust, right? That would be a robust...or more strong, I guess. 

Dr. Horvath: Yes. I want to say it was maybe not eight years. I want to say it was five years. So, it depends on what you compare.

Rhonda: Not the offspring, but the actual person.

Dr. Horvath: Yeah, yeah, that's right. You're right on. So, if you analyze the blood from a centenarian or supercentenarian, it's true, our age estimates are really way below the chronologic ages, could be 15 years younger, and also there's a real leveling-off effect, you know.

Rhonda: Yeah. What about studying how other...like there's other biological processes that, at least in animals, when you perturb them are known to regulate aging, for example, and you can mutate certain mitochondrial factors and have accelerated aging or cellular senescence. You can also have a certain mouse where you can accelerate aging or the opposite where you lower growth hormone levels and they'll live longer. Have you or anyone looked at the epigenetic aging, how that relates to these other...?

Dr. Horvath: To some extent, yeah. What you mentioned, these growth hormone knock out mice that are known to live longer, sure enough, according to the epigenetic clocks in mice, they really age more slowly. So, it was a beautiful validation, you know, of the fact that epigenetic clocks measure biologic age because growth hormone receptor knock out mice. That's really a gold standard anti-aging intervention, and you want that the clock picks that up. And when it comes to other strategies, senescence, right now it's a hot topic, the so-called senolytics that remove senescent cells, you know. And we are about to analyze data collected by James Clement who did clinical trials of these senolytic strategies, and hopefully, we'll have an answer in a couple of weeks, you know, but I just don't know whether removing senescent cells has an effect on epigenetic age. 

In general, the relationship between senescence and epigenetic age is complicated because when it comes to inducing senescence, there's several ways of inducing senescence. One is simply what is known as replicative senescence, you passage cells, you split them and let them grow, and grow, and grow. And that form of senescence is somewhat related to epigenetic clocks. Then there are other forms, so-called radiation-induced senescence, you radiate the cell. And that form of senescence doesn't seem to accelerate the epigenetic clocks, you know. So, it's complicated. And conversely, there are ways of immortalizing cells by overexpressing the component of the telomerase, the TERT. So, immortalizing a cell actually it doesn't stop epigenetic aging. You can even immortalize cells that you can passage for decades, but the epigenetic age keeps going up in those cells.

Rhonda: Really?

Dr. Horvath: Yes. So, do you see? Epigenetic clocks are not simply markers of cellular senescence. They really pick up a different aspect of biology. 

Rhonda: The radiation and DNA damage is kind of surprising because I think in one of your papers looking at...and this was another question I wanted to ask was, where are these methylation patterns occurring in the genome? Are there genes that are really particularly known to be involved in the aging process, or basically in just health in general? Like, are these like the metabolism genes or DNA repair and things like that?

Dr. Horvath: Yeah. I wouldn't make that claim. So, let me start out by saying...So, my original clock used 353 loci. When we look at GrimAge from Ake Lu, scientist in my lab, it uses over 1,000 locations in the genome. Now, one question is, what if we remove these locations from our data and just build a new clock? Would we still get another good clock? And the answer is yes, you know. I could have built alternative clocks using other locations in the genome, you know. On that level, these locations are not unique, you know, and when you look at the genome, we have, in principle, 28 million locations in the genome as cytosines, you know, and I want to say a quarter of them change with age. Some of them gain methylation, some of them lose methylation. So, these methylation changes like almost globally, you know. And in that sense, epigenetic clocks look at perfect representatives of the entire what is known methylome. They represent everything that's going on. But you can see that maybe looking at only 300 locations is not ideal, you know. 

Having said this, we certainly did look at it and say, "Are these locations enriched, you know, with certain pathways?" And no doubt they are. So, sites that gain methylation with aging are known to be located in so-called polycomb group protein target sites, so certain proteins that play a very important role in maintaining stem cells, you know, or conversely, sites that play a role in cell differentiation and development. So, these sites tend to gain methylation with aging, you know. The sites that lose methylation also are enriched with certain themes, for example, so-called enhancer regions, you know. So, the field of epigenetics has very much characterized the genome, which parts change with aging. There is wonderful review articles on it.

Rhonda: And then, I just got on the stem cell thing. That's just so interesting that a lot of those are regulating stem cell functioning because it's just...

Dr. Horvath: Yeah. Coming back to the mechanism of the clock, that's really a profound insight, you know, that when you look at the data, you keep seeing themes related to development, tissue differentiation, organ development. And it is a profound insight because if you had asked an aging researcher five years ago whether developmental processes matter in aging, they would have said no. Many people think of aging as noise, right, wear and tear, you know, but these epigenetic clocks have really linked development to tissue dysfunction in a direct manner. An epigenetic clock is a continuous readout that links prenatal tissues directly to very old samples.

Rhonda: It's really blowing my mind. It's actually that is...I never would have thought...

Dr. Horvath: It combines two fields, you know.

Rhonda: It does. Yeah.

Dr. Horvath: Because many people obviously study development, but these are not aging researchers, you know, but these clocks really point to commonalities, you know, of these fields.

Rhonda: Right. Yeah. Wow, it's amazing. It's very interesting. I just want to thank you so much for doing all the work that you're doing, and we'll continue to follow, you know, your work.

Dr. Horvath: Thank you for your interest. Yeah.

Rhonda: People that want to learn more about your research, probably the best place is Wikipedia, we talked about. I mean, they can...

Dr. Horvath: Yeah. I've written a review article in "Nature Reviews Genetics." People, have written Wikipedia pages. Then you mentioned the blog by the Josh Middledorf, you know, so there are various forms of learning about them, you know.

Rhonda: Well, thank you, Steve, so much. I've really enjoyed this conversation. 

Dr. Horvath: Great. Thanks.