Rhonda: So, I was thinking, to start this off, we could start talking about aging more as a concept. And maybe you can explain to people how to differentiate between age as a number, so, chronological aging, versus biological aging, phenotypic aging, and functional aging.

Dr. Levine: Yeah. So, I think, even in the field of aging, there isn't a good definition for what we're actually talking about when we're saying, "We're working on aging." I think most people in the public think of aging in terms of just chronological time, so, whatever age your driver's license says or your passport. But really what we care about is this thing that we would call biological aging, or even phenotypic or functional aging, and that is all the changes that your body undergoes as a function of this time, usually.

So, aging in our society has kind of a negative connotation. But it's not the years you've been alive that's actually the problem, it's kind of how your body has changed over that time. And not everyone's body changes in the same degree or at the same pace. So that's the really important thing is can we figure out what's changed, how that's going to affect your risk of disease, your functioning ability, or any of these things.

So, as you mentioned, we can talk about this in terms of biological age, so, we can measure it in terms of molecules and cells, how those have changed in your body. We talk about maybe phenotypic or functional, which I think is kind of the stuff you can feel and see in your body. Are you able to, you know, run a mile as quickly as you used to or get up a flight of stairs? And these are the things that people actually worry about losing over their life course. And it's really important to try and quantify these, so, we can understand what drives these changes and, potentially, how you would slow that or, hopefully, like some people are interested in, even reverse it.

Rhonda: As you mentioned, you know, people do think about chronological age often. For me, when I think about aging, I often will think about functional aging. I mean, it's more personal for me, I'm worried about becoming demented or losing my cognitive function or, as you said, I'm not able to walk as well, I'm just physically declining. But as a scientist, biological aging is much more interesting because it's more fundamental, would you agree?

Dr. Levine: Yeah, and it's really where we think it all starts. So, we kind of take a perspective of what we would consider these biological levels of organization. So, you have all the kind of molecules and atoms in your body that kind of feed into, you know, cells that make up tissues that make up whole organ systems and then the whole organism. And we think the aging process, all these changes are starting at these lower levels, so, you have changes in molecules and cells, but we don't see that until you feel it at these higher levels, until you feel, you know, you have weaker muscles or you're not thinking as well as you used to. And that's really, once it's reached a certain level. But if we talk about understanding what's driving these and where to intervene, you have to do it at the lower levels if you want to affect all those things that we see and feel every day.

Rhonda: So, there's some pretty well-defined hallmarks of aging. And, as you know, these are things like genomic instability, telomere, you know, shortening, cellular senescence, mitochondrial dysfunction, epigenetic alterations, nutrient sensing problems or dysfunction, stem-cell depletion. So, there's quite a few of these hallmarks that are sort of accepted within most, you know, of the scientific community, you know, they together seem to drive the aging process in a way, right, as you mentioned, at the molecular level, cellular level. I'm sort of curious what your thoughts are on what you think some of the major drivers of aging are, caveats included, or why it's important to really understand what those drivers of aging are.

Dr. Levine: Yeah. I think, you know, there's a big push in the field to figure out what's causal in aging, like what are the things that are changing, that are really pushing this aging process and driving all the other changes? And I don't think we have a good idea about that, you know, what's actually causal versus just correlative, it's just an outcome of aging that we can observe. In my lab, we're really interested in epigenetics, and that's mostly because I think of the epigenetic system as the operating system of the cell. So, most of the cells in your body have, essentially, the same DNA, but what makes something a neuron or a, you know, brain cell or a skin cell is the epigenetic program. So, it gives the cells the ability to respond to stress, it gives them almost their kind of physical form and all of the things that they are supposed to do.

The problem is that this program gets completely rewritten with aging. And we don't know exactly why, whether it's errors or whether it's just the program kind of having glitches along the way. But we think that this then produces cells that are not adapting correctly to their environment or maybe doing things they shouldn't be doing. And potentially, this is something we think might cause aging, although there's still some debate on whether it's truly causal in the aging process. And I always say, "No one really knows yet what the true causes are."

Rhonda: Since your research really does focus on the epigenetic alterations, can you explain to people what epigenetic aging clocks are, generally speaking? And then maybe we can get into some of the differences between the first-generation epigenetic aging clocks, like corvette clock, and then second-generation like Levine, or PhenoAge it's also called, and GrimAge? So...

Dr. Levine: Yeah, absolutely. So, there are a bunch of different types of epigenetic modifications but the type that these clocks are based on is something called CpG methylation or DNA methylation. And really what that means is you can look across, you know, one strand of DNA, and we know we have A, C, G, and T, but you have these regions which we call CpG sites, and that's where you basically just have a C right next to a G. And these tend to be located in, you know, specific regulatory regions of the genome. But what happens is the CpGs can become methylated. Some of them are supposed to be methylated from the beginning. But what we find with aging is that the ones that we expect to have methylation lose methylation with aging, and the ones that shouldn't have methylation gain methylation with aging.

And the methylation in this is basically turning on or off different parts of your genome. So, when you have methylation, we can, essentially, assume that part is repressed. So that, wherever it is in the genome, it's not accessible, you're not expressing the genes in that region. Versus, when you remove the methylation, we consider this more an active region.

So, epigenetic clocks look across either hundreds or hundreds of thousands of these sites and just say, "Does the pattern of whether you have methylation or not resemble someone of a given age typically?" So, it would say, "Oh, your pattern looks like someone who is 40-years-old," even though maybe you're 50-years-old chronologically. And what we find is that that kind of difference is biologically meaningful.