I have been enthusiastic about telomerase therapies for anti-aging since 2003. But if I can’t change my mind as new data appears, what’s the point of being a scientist? I still believe that lengthening telomeres is a net benefit, but the potential for added years is modest, and there are probably risks and tradeoffs. The study that has most influenced me is this one, implying that telomerase affects epigenetics (through methylation) in ways that accelerate aging. My theory is that the unexpected relationship between telomerase and methylation is an example of antagonistic pleiotropy, but pleiotropy in a very different sense from the standard evolutionary theory.
Do people with longer telomeres have longer life expectancy? In 2003, Richard Cawthon of University of Utah first addressed this question experimentally with a study that was clever, innovative and courageous. It was innovative in that he introduced a fast and convenient way to measure telomere length from very small quantities of DNA, using the Polymerase Chain Reaction. It was clever in that, instead of a “prospective study” measuring telomere length in his subjects and then following 20 years to see what would happen to them, he did the experiment retrospectively, using historic samples of blood that had been taken from people twenty years earlier and kept in frozen storage by a local hospital. And it was courageous in that everyone believed at the time that extending life could not be so easy as just lengthening telomeres, or else the body would already be doing it! That is to say, no one would fund the study because they thought they knew how it had to come out.
But they were wrong. Even with Cawthon’s small sample of only 143 subjects, the relationship between telomere length and diseases of old age jumped out of the statistics. The quartile with the shortest telomeres had suffered two times higher mortality and three times greater incidence of heart disease in the intervening 20 years than those with the longest telomeres.
Red blood cells have no DNA, hence no telomeres, but white blood cells are constantly dividing to target specific bacterial, so the telomeres in white blood cells are a sensitive measure of immune health. Cawthon reported that the group with shortest telomeres had suffered 9 times the rate of infectious disease compared to the longest telomere group.
At the time of Cawthon’s study, there was a great deal of skepticism, based purely on theory. The standard hypothesis was that all animals are evolved to live as long as possible, all else being equal, and if telomerase were being held back, there must be a powerful downside associated with it. I was already marching to the beat of a different drummer in 2003, and I didn’t believe that evolution was always going for the longest lifespan available. Because I believe that aging is an evolutionary program, it was easy for me to see telomere shortening as part of the program. The biggest clue in my mind was the evolutionary origin of telomere shortening in single-celled protozoans. In the ciliates (e.g. paramecium), telomerase is not expressed in mitosis (when the cell copies itself), but only when it conjugates (recombining genes with other individuals) with another. Hence, a cell that just goes on reproducing as fast as possible without sharing its genes was doomed to die of cell senescence. A billion years ago, telomeres were already a means of enforcing the communal imperative, Share your genes! It is easy to imagine that the same evolutionary imperative has persisted through the aeons, and that telomere shortening insures death in many higher organisms. Indeed, since Cawthon, it has been demonstrated that short telomeres are a mode of aging in dogs, cats, and horses, (but not cows, pigs or mice).
Three years ago, I reported on a Danish study that replicated Cawthon’s results on a huge scale. In 60,000 subjects, Rode associated short telomeres with all-cause mortality, heart disease, diabetes, and some cancers.
Telomere shortening leads to senescence and higher disease risk by three known mechanisms. First, stem cells with the shortest telomeres stop reproducing, hence the body’s tissues don’t renew as efficiently. Second, senescent cells are not just dead weight, they actually emit chemical signals (cytokines) that increase inflammation. This has been called SASP, for Senescence-Associated Secretory Phenotype.) Third, senescence in the bone marrow that generates new white blood cells is especially damaging to the immune system, because it prevents the body from responding effectively when challenged with new infections.
Telomeres and cancer
If short telomeres cause all these problems, why would the body ever allow its telomeres to become short? It was recognized early in the game that production of telomerase entails no substantial metabolic cost, so the question challenges the conventional theory that individual animals are evolved to live as long as possible. Of course, for us who believe that aging is programmed, there is no problem with this. But the first suggestion of an answer within the conventional paradigm came from Carol Greider, one of the original discoverers of telomerase, and independently from Ruth Sager. Telomerase is needed to make cells immortal. 90% of cancer cells have found ways to bypass the suppression of telomerase in order to continue proliferating unabated. Greider and Sager proposed that keeping telomerase under lock and key constitutes one of the barriers that keeps cells from going rogue as tumors. Telomere shortening helps to prevent cancer.
This argument never made any sense to me. First, what good was it to suppress cancer if the net effect was to shorten lifespan? And second, I believe that the body’s principal defense against cancer is the immune system, and if short telomeres can cripple the immune system, that was likely to do more to promote cancer than to prevent it.
Nevertheless, the idea that telomerase is rationed to protect against cancer persisted in the biomedical community for 20 years based on theory alone, even as it was moderated by the discovery of SASP.
Experimental link between telomeres and cancer
I came into this field very skeptical of the idea that long telomeres could cause cancer. But as the evidence has accumulated, I’m compelled to reconsider. Just last summer, I blogged critically about the largest genetic study to date, linking genetic predisposition for longer telomeres with cancer rates later in life. I noted that the measured effect is actually quite small, but is reported blown up to alarming proportions by exponential extrapolation. But that didn’t mean it was necessarily wrong, only that it was unconvincing. Shortly afterward, I became aware of observational studies, based on measured telomere length rather than the genetic predisposition. These are harder to refute.
In this study from the Moffitt Cancer Center, short telomeres (as measured directly, not imputed from genetic variation) are associated with higher risk of squamous cell skin cancer, but long telomeres are associated with higher risk of melanoma skin cancer. Same methodology, same authors. Why would I believe one and disbelieve the other? Melanoma tends to occur at younger ages than squamous cell carcinoma, this supporting the Greider hypothesis that telomere shortening should be especially important for cancer prevention while we are still in a fertile stage of life. The Moffitt results on melanoma were confirming a finding reported earlier from Harvard Med School.
In this study, people with the longest telomeres had nearly twice the risk of lung cancer compared to people with short telomeres, after adjustment for age and smoking status. There are 25 co-authors, and Cawthon is #2. In this study, short telomeres protect against (devastatingly lethal) pancreatic cancer, and in this one, there is an elevated risk of breast cancer associated with long telomeres.
There are studies contradicting each of these findings. Overall, the field seems to be more of a confused mess even than most areas of epidemiology. But for lung cancer, melanoma, and pancreatic cancer, the predominance of the evidence says that longer telomeres are associated with higher risk.
Longer telomeres uncontroversially protect against heart disease and stroke. There is no contradiction of this finding in sight, and there has been no contradiction of the major finding (by Rode and Cawthon) that short telomeres increase all-cause mortality. Perhaps that’s all we need to know.
Stop the Presses
Just a few weeks ago, I learned of this new study linking telomerase to the epigenetic changes that the methylation clock associates with aging. The implication is that telomerase accelerates aging. It began with an investigation by Steve Horvath’s group (about which I reported last month) asking, what genetic variations are associated with people who age faster or slower than average, according to the Horvath methylation clock? They did a genome-wide search for statistical correlates and the standout association was telomerase. People who have small genetic variations that support greater telomerase expression tend to have longer telomeres, but they also tend to age faster, as measured by the Horvath clock.
It’s been known for a long time that telomerase has other effects in addition to lengthening telomeres. But this is the first time that telomerase has been reported to affect DNA methylation. So it seems we are presented with a tradeoff, or pleiotropy, or Catch-22, or “damned if you do, damned if you don’t.”
The association between telomerase and accelerated aging (measured by methylation) was found in the genetic statistics, and then confirmed in a cell culture. When telomerase was artificially activated in the cell culture, the methylation patterns changed in the cells consistent with older age according to the Horvath clock. In fact (and remarkably in my opinion) they found no Horvath aging at all in the cell cultures that lacked telomerase. Could it be that telomerase is the one and only driver of epigenetic aging at the cellular level?
Telomere length and the Horvath methylation clock are both correlated with age, but they are not otherwise correlated with each other. The Horvath clock is a combination of 353 methylation levels that is optimized to correlate maximally with age. The observed correlation is 0.95. Telomere length is not statistically optimized but measured as nature offers it, and its correlation is much weaker (~0.4 according to my estimate, as I have not found this number in print). Thus Horvath clock is an excellent measure of chronological age, and combining information about telomere length can make it potentially a little more accurate yet. But the telomere clock on its own is a very unreliable measure of age.
The Horvath group designed an experiment to separate the direct effect of telomerase on methylation from an indirect effect (telomerase ⇒ telomere length ⇒ methylation age). They found no indirect effect. Telomerase itself affects methylation aging, but telomere length does not.
This raises (what is for me) an uncomfortable question. Many “good” life habits have been associated with telomerase expression, including exercise, meditation, and social integration. Could it be that these habits are simultaneously slowing our telomere aging, while hastening our epigenetic aging?
“While the paradoxical finding cannot be disputed on scientific grounds, its biological interpretation remains to be elucidated.” [Lu et. al, 2018, the same study I’ve been talking about]
(Another finding of this same study: Earlier menopause is associated with epigenetic age acceleration in women, but this is mitigated by hormone replacement therapy. HRT modestly slows aging, as measured by the Horvath clock.)
Antagonistic Pleiotropy turned Upside Down
So, what’s going on? My inclination is always to think in evolutionary terms.
Antagonistic Pleiotropy is the standard explanation for aging, though I have long argued that it doesn’t fit the data. The theory says that some genes enhance fertility and survival early in life, but have detrimental effects late in life. These genes are selected in a Darwinian process because their benefits outweigh their costs. Even though they die younger, those individuals carrying the pleiotropic genes leave more offspring, and that’s what counts for evolution. The crux of the theory is that nature is caught between Scylla and Charybdis, forced by limitations of the available genes to choose either high fertility with short lifespan or low fertility with longer lifespan. Crucial to the theory is the assumption that it is biologically impossible to separate the benefits of these pleiotropic genes (fertility) from their costs, so that there is no way evolution could engineer higher fertility without triggering later senescence.
This theory was formulated by George Williams in 1957, long before anyone had heard of epigenetics. He assumed that if you have a gene, you’re stuck with it for life. We can’t blame Williams for the frame of mind that he brought to the evolutionary question, but we now know that this is very much not the case. The body turns genes on and off in individual tissues and at specific times with exquisite precision. In fact, most of the euklaryotic genome is devoted not to genes, but to epigenetic controls of one kind or another.
The fact is that genes are turned on that dial up fertility and promote robust replacement cell growth early in life, and aging at that time occurs quite slowly. Later in life, these growth and fertility genes are dialed way back, and that is the era in which aging comes at us with a vengeance. This, to me, is a direct refutation of Antagonistic Pleiotropy as a theory.
Nevertheless, many examples pleiotropic genes have been found in studies of aging. The above story of telomerase seems to be a conspicuous example. Telomerase promotes epigenetic aging, while lack of telomerase promotes cellular senescence. “If the ’gaitors don’t getcha then the ’skeeters will.”[ref]
My interpretation of pleiotropy is in my book and some of my academic papers. It is this: Aging has been built into our genomes by natural selection for the sake of the community. Fixed lifespan, (especially when modified conditions of food stress) is helpful in preventing population overshoot that can lead to famines, epidemics, and extinction. But whenever a trait is good for the community and bad for the individual, there is a temptation for the individual to cheat (“cheating” is actually the term used by evolutionary theorists). In this case, cheating would mean evolving a longer lifespan via selfish genes that spread rapidly through the population, because they are more successful at the lowest level of Darwin’s competition.
Individual competition would erase aging if left unchecked. The results would be great for individual fitness, but soon would be disastrous for the population. Overpopulation would ensue, followed by the famines and epidemics mentioned above. Evolution has learned (over a very long expanse of time) to protect the communal interest, placing barriers in the way of individual selection for ever longer lifespan. This is the evolutionary significance of pleiotropy. It provides that no simple mutation can substantially extend any aspect of lifespan without adversely affecting another aspect of lifespan or of fertility. The aging clock has been “purposely” configured so as to be spread out over several different mechanisms, tied not just to other pro-aging mechanisms but to fertility as well. Aging is hard to get rid of “by design”.
In the standard theory that I don’t believe, antagonistic pleiotropy is a precondition, and evolution has had to make the best of a bad deal. In my version, antagonistic pleiotropy has been crafted by natural selection in its long-term mode. Limiting lifespan has been so important to the viability of the population that evolution has arranged to protect it from leaking away due to cheating, and antagonistic pleiotropy is one of the ways in which this is arranged. I have modeled this process in numerical simulations of evolution.
My guess is that the connection between telomerase and epigenetic aging is an example of antagonistic pleiotropy in this latter sense–certainly not in the sense of Williams, because on their face telomerase and methylation have little to do with one another.
Bad news for life extension strategies
But whatever the theoretical origins, the pleiotropic connection between telomerase and epigenetic aging complicates any strategy we might devise for slowing the progression of human aging.
I believe that the preponderance of evidence still indicates that activating telomerase has a net benefit for lifespan, but that probably we can add at most a few years by this route. I think that epigenetics is much closer to the core, the origin of aging, and that interventions to modify epigenetic aging will eventually be our holy grail. The caveat is that telomeres are simple, but methylation is complicated, and methylation is just one of many epigenetic mechanisms.