One of the major themes in aging science of the last 15 years has been that there is natural variation in telomere length, and individuals with longer telomeres have lower disease risk and longer life expectancy than those with shorter telomeres. A paper last week in Nature Structural and Molecular Biology found that stem cell telomeres are actively maintained at a target length, not just by elongation (with telomerase or ALT) when they get too short but by active trimming when they get “too long”. We know what “too short” means: short telomeres lead to cellular senescence. Cells with short telomeres are not just falling down on the job; they are toxic. But what does it mean for telomeres to be “too long”?
The headline in MedicalXpress says “Scientists find that for stem cells to be healthy, telomere length has to be just right”. The story underneath includes the claim that “really long telomeres caused telomeric fragility, which can lead to initiation of cancer”. But now that I’ve read the research article on which it is based and some of the references in that article, I see that the part about cancer was tacked onto a (new and interesting) research finding. I’m convinced that the meme “long telomeres lead to cancer” has been resounding in an academic echo chamber for 25 years; that it never has had an experimental foundation, and its theoretical foundation is just wrong.
What is new and interesting is this: researchers at Salk Institute have discovered a mechanism for trimming telomeres. In our previous understanding, telomeres lose length every time a chromosome is copied (every time a new cell is created). Telomeres are partially rebuilt by the enzyme telomerase, or by a less direct mechanism called ALT. Telomeres are fully rebuilt only when new life is created, in a germ cell or a fertilized egg. In the previous understanding, shortening of telomeres is passive, while lengthening is active. The new study documents an active mechanism for shortening telomeres.
Part of each telomere is unpaired, a single strand of DNA extending past the end of the chromosome, and folded back over the main (double-stranded) part. Single-stranded DNA normally means a problem, and the cell nucleus has multiple means to repair or degrade it. To protect the telomere from being attacked (to prevent fixing of what ain’t broke) the telomere is chaperoned by various protective proteins, most famously shelterin.
We have known that stem cells can express telomerase to counteract telomere shortening, though (in humans) there is not enough to keep telomeres from shortening progressively through a lifetime. The new finding is that when a stem cell detects that telomeres are “too long”, there is a way to trim them back. A strand of DNA is manufactured that is complementary to the telomere’s repeated sequence TTAGGG. The complementary strand (that would be AATCCC, repeated) has an affinity for the telomere repeats, and it finds and binds to a segment of telomere, then circles, “bites its tail”, and breaks off a ringlet of double-stranded telomere-stuff (called a T-circle), effectively shortening the telomere.
What’s wrong with extra-long telomeres?
The obvious question: why is the cell doing this? What is the danger of telomeres that are too long? One natural place to look is in the telomere position effect (TPE). Telomeres fold back over the chromosome in such a way as to silence genes near the ends. We might expect that the right genes must be silenced at the right times, and that silencing too many genes with an extra-long telomere would cause problems. My own best guess is that this is the right answer.
Another hypothesis is that extra-long telomeres are inherently unstable and unmanageable. But the present studies were done with human cell cultures, where telomeres are ~10,000 BP in length; mice commonly have telomeres ten times that long without causing problems.
The conventional hypothesis is that telomeres are trimmed to prevent cancer, and that is the spin put on the findings by the authors of the paper in their press release. “We were surprised to find that forcing cells to generate really long telomeres caused telomeric fragility, which can lead to initiation of cancer.” In the paper itself, they were more circumspect about this explanation, as is academically appropriate. These people are masters at what they do (Since spending time at NIBS in Beijing, I have an expanded awe for the experimental virtuosi who are able to infer reliable data about the inner workings of cells.) But they are not theorists and they trust the community of biological theorists to supply the theoretical framework for interpreting their result.
In this case, the trust is misplaced. The idea that telomeres are kept short to prevent cancer was originally proposed by (Nobel laureate) Carol Greider (1990), and has been promoted most explicitly by Judith Campisi (1999) before she became convinced by experimental data that the situation is more complicated, that short telomeres are more likely to cause than to prevent cancer (2013). Through strength in numbers the cancer/telomere hypothesis has achieved “echo chamber” status–many researchers cite each other’s secondary statements on the subject, until tracing the empirical support for the hypothesis becomes unnecessary. It is common knowledge.
One of the authorities cited in the original paper is this article which is actually about deletion of a gene for a shelterin-related protein that binds to telomeres. When this gene is deleted, telomeres become unstable and cancer rates rise. But the article is not about telomeres that are “too long”. Another authority for the hypothesis that is cited in the original paper is this book chapter. The chapter is not about cancer, but it does peripherally cite this study from NCI, which finds that cancer is associated with short telomeres, but not long telomeres.
Contrary evidence: health benefits from extra-long telomeres
Just last spring, Maria Blasco’s group at the Spanish National Cancer Research Centre gave us this study, in which stem cells with hyper-long telomeres (up to 300,000 BP) were introduced into mice (in which telomeres normally are already 10 times as long as humans’). “Mice with hyper-long telomeres…accumulate fewer cells with short telomeres and less DNA damage with age, and express lower levels of p53….We further show that wound-healing rates in the skin are increased in chimaeric mice.” No life span data is reported, but cancer risk was lower in the mice with hyper-long telomeres.
This study from another research group at the same institution linked the extraordinary healing and regeneration capacity of very young mice to their extra-long telomeres.
In this study from UCSF, published just this fall, heart patients whose telomeres were lengthening over the four-year span of the study had 1/3 the mortality rate of matched patients whose telomeres were shortening over the same time span.
Are short telomeres a symptom or a cause of age-related disease?
This is a controversial question only because the causal hypothesis is in direct opposition to standard evolutionary theory. So much the worse for standard evolutionary theory.
One causal mechanism which is incontrovertible is that cells become senescent when their telomeres shorten beyond a critical length. Senescent cells are not just non-functional, they are toxic. Removing senescent cells from the body has been shown to lengthen life span in mice, and senolytic agents are being developed for human use. Many of us in the life extension movement regard senolytics as the #1 most promising strategy for major life extension in the near term.
The question of causality can be answered definitively by intervening to make telomeres longer or shorter “by hand”. If short telomeres are a mere marker of past stress, then this should make little difference in the trajectory of aging; but if short telomeres are a cause of aging, then we expect that lengthening telomeres should lengthen life expectancy and lower the (age-adjusted) risk of disease. In fact, this experimental model has been realized several times in mouse studies, two of which are referenced just a few paragraphs above [#1, #2]. The most dramatic success was in dePinho’s Harvard lab, but there are also impressive results from Blasco’s group in Madrid.
If lengthening telomeres is an effective life extension strategy for mice, it should be all the more so for humans, who have shorter telomeres, longer life spans, and less telomerase than mice.
In the face of this evidence, there are still some influential researchers and advocates in the anti-aging community who opine that “On the whole telomere length looks a lot like a marker of aging rather than the cause of problems: the groups that primarily seek to engineer longer telomeres in search of a way to slow aging are probably putting the cart before the horse.” [quoted today at FightAging.org] Meanwhile, Michael Fossel has initiated a clinical trial of telomerase gene therapy to treat dementia. Cancer scares from the echo chamber are spooking the venture capital that would be so welcome for startups that are seeking to bring telomerase therapy to the public.