In 2012, Shinya Yamanaka and John Gurdon were awarded the Nobel Prize for Physiology or Medicine for the discovery that mature somatic cells can be converted to stem cells by introducing just four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc). These so-called induced pluripotent stem cells (iPSCs) have properties that are very similar to embryonic stem cells; that is, they can be turned into any specialized somatic cell, such as a skin cell or a heart cell.
However, for our purpose here, the most interesting feature of iPSCs is that the age of these reprogrammed cells is reversed. For instance, according to the current most accurate aging biomarker, the Horvath clock, the age of iPSCs is essentially 0. Likewise, the telomere length is restored as well.
David Sinclair (and others) want to use this remarkable feature of iPSCs to reverse the age of cells not only in vitro but also in vivo, that is, in multicellular organisms. Obviously, you can’t just turn specialized cells into stem cells in a living organism. If you turn a heart cell into a stem cell, it can no longer do its job in the heart and instead will cause cancer. However, Sinclair believes that it is possible to avoid this problem by working with a subset of the Yamanaka factors (without c-Myc) and by limiting the reprogramming time.
The rejuvenation procedure that Sinclair proposes works like this:
- With the help of an engineered adeno-associated virus (AAV), the DNA of all cells receive new genes (a combination of the Yamanaka factors). However, these genes are inactive at first.
- In addition to the Yamanaka factors, a failsafe switch is inserted that allows the activation of the Yamanaka factors by administering a certain molecule, such as doxycycline.
- To start the rejuvenation process, doxycycline is given for a certain time. This activates the combination of the Yamanaka factors, which reverses the age of all cells without turning them into stem cells. After this, the treatment with doxycycline is discontinued.
- The rejuvenated cells restart the aging process. Thus, after the cells reach a certain age, you must take doxycycline again.
This sounds like science fiction; however, the technologies used (iPSCsC technology gene therapy and controlled gene expression) are not really that new and are more or less well established. What’s new is to use this combination of these technologies for reversing the age of multicellular organisms. In fact, Sinclair’s team successfully applied these technologies in the optic nerves of aging mice to restore their vision and Juan Carlos Izpisua Belmonte was even able to prolong the lifespan of mice.
Reprogramming of cells in living tissue cries for cancer because even if it is possible to prevent somatic cells from turning into stem cells, errors during reprogramming in a small number of cells should significantly increase the likelihood of further mutations that eventually cause cancer.
However, through fine-tuning of the procedure, the risk can probably be lowered. With the rapid progress in cancer research thanks to recent advances in cell biology, new treatments such as immunotherapy will probably eliminate the terror of cancer in the not-too-distant future.
Yamanaka and Sinclair’s theory of aging
I was a little surprised to find that Sinclair believes cell reprogramming is the most promising approach to finding a cure for aging. His information theory of aging claims that information loss in the epigenome caused by the wear and tear of the repair processes that fix double-strand breaks in DNA leads to the loss of cell identity and cellular dysfunction. If the reprogramming of cells can reverse their age, this information cannot be lost entirely because the information is required to restore the epigenome to its original state.
Sinclair is quite aware of this problem. He solves this contradiction by claiming that cells have a “backup” of the epigenome, or what he calls “a biological correction device” (with reference to Shannon’s information theory). Let me quote this because I believe this the most crucial part of Sinclair’s theory (David Sinclair, Lifespan: Why We Age―and Why We Don’t Have To, pp. 169–170):
If adult cells in the body, even old nerves, can be reprogrammed to regain a youthful epigenome, the information to be young cannot all be lost. There must be a repository of correction data, a backup set of data or molecular beacons, that is retained through adulthood and can be accessed by the Yamanaka factors to reset the epigenome using the cellular equivalent of TCP/IP. What those youth markers are, we’re still not sure. They are likely to involve methyl tags on DNA, which are used to estimate an organism’s age, the so-called Horvath clock. They likely also involve something else: a protein, an RNA, or even a novel chemical attached to DNA that we haven’t yet discovered. But whatever they are made of, they are important, for they would be the fundamental correcting data that cells retain over a lifetime that somehow direct a reboot.
I must have read several thousand pages about cell biology in the last few years, and I’ve learned that many amazing things happen down there at the cellular level. However, the claim that the epigenome stores a backup of its original state in the epigenome (methyl tags, unknown chemical attached to DNA, etc.) deeply confused me because it contradicts everything I understood about the cellular development processes.
Puzzling questions for Sinclair
In the philosophy of science, we would call this the introduction of hidden variables. An obvious contradiction between a theory and empirical data is explained away by referring to hidden processes that have not yet been discovered. This most famously happened when physicists were unable to accept the probabilistic nature of quantum physics. Einstein wasted a lot of time trying to find these hidden variables (because God does not play dice).
Of course, this doesn’t mean that Sinclair’s hidden backup device doesn’t exist. However, his claim about the existence of a correcting device raises more questions for me than it answers:
- If aging is information loss in the epigenome and the backup is also stored in the epigenome, why is this backup information not affected by the wear and tear of DNA repairs? Or, in other words, why doesn’t the backup lose information to entropy but the rest of the epigenome does?
- We already have a theory (although not well-established) that explains how reprogramming works in early embryogenesis and gametogenesis. For instance, after a sperm cell fertilizes an ovum, DNA demethylation first removes the methyl tags and then de novo methylation adds the methyl groups in a precisely programmed way. It is mostly DNA (through transcription factors), signals from other cells in the organism, and environmental factors (and not some epigenomic backup device) that control cell differentiation and therefore the structure of the epigenome. So why would this be any different with iPSCs? Obviously, the Yamanaka factors exist for a natural reason. Without the ability of cells to reprogram and reverse their age, life could not exist because all cells have progenitors that lived millions or even billions of years ago. Thus, cells must be able divide into daughter cells with age 0 because otherwise offspring would be born with the same biological age as their parents.
Yamanaka and the wear and tear theory of aging
Even more puzzling to me is that any theory that claims the wear of tear of metabolism causes aging could survive after Yamanaka’s discovery. What happened to all the oxidative and free radical damage to DNA, epigenome, mitochondria, proteins, and lipids? How can just four transcription factors (or three, for that matter) repair all this damage that has been accumulated in decades in the blink of an eye? In fact, all wear and tear theories of aging need a myriad of Sinclair’s backup devices to restore all the damaged cell ingredients to a youthful state during reprogramming.
On the other hand, if reprogramming decreases age, why not simply assume that the age of cells increases over time due to programming? If aging is a programmed process accompanied by all the other programming that happens during cell differentiation, no hidden backup device is needed. Aging, therefore, would then be just the programmed change of gene expression patterns. Sounds crazy? Not if you admit that aging has an important function in biological evolution.
Note that the damage theory of aging contradicts a variety of empirical findings. A good summary can be found in this article.
What I really like about Sinclair’s book is that he at least addresses the problems that his theory faces. Many other scientists who believe in the wear and tear theory seem to simply ignore the contradicting evidence. The fact that an advocate of the wear and tear theory feels the need to introduce hidden variables, is a god sign that we are on the verge of a paradigm shift in geroscience. The good thing is that Sinclair’s research does not seem to be determined by his theoretical views. Instead of actively searching for his correcting device, he focuses on reprogramming instead. In the end, it doesn’t really matter if Sinclair’s information theory of aging is correct or not, as long as he finds a way to program the right age into our cells.
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- How and when Mother Nature resets the aging clock and why the wear and tear theory of aging is worn out – August 11, 2021
- Book review of Jean Hebert’s Replacing Aging,why gerontology is not the solution – December 29, 2020