From the Salk Institute in La Jolla, CA came an announcement last week that the four factors previously identified to turn ordinary cells into stem cells (in cell cultures) was successfully used as a rejuvenation procedure in live mice. The results provide important new evidence for the hypothesis that aging is under epigenetic control, and a proof of principle that we might slow aging by modifying the chromosome markers and attachments that determine gene expression. But the way in which this was done involved genetic engineering before birth, and there is no obvious way to translate the results quickly into an anti-aging teatment for living humans.
Ten years ago, Shinya Yamanaka’s Kyoto University laboratory announced that just four proteins could turn an ordinary, differentiated cell back into the stem cell from whence it came. The four were transcription factors, high level switches that turn whole systems of genes on and off with one signal, and the “Yamanaka factors” became known by their initials, OSKM.
Last week, Izpisua Delmonte’s laboratory at the Salk Institute announced a success in rejuvenating whole animals, live mice, using the same OSKM.
Whether this is the germ of a potent new rejuvenation treatment remains to be seen; but the immediate message is a dramatic affirmation of the new paradigm in anti-aging medicine: aging can be reversed by signaling, without artificially-engineered repair of damage. A bold form of this paradigm is the epigenetic hypothesis—now just 4 years old—which says that aging is controlled by gene expression. It is the set of genes that are turned on and off, and the genes’ levels of expression that determine the state of the body’s age. (This idea does not deny that tissues and biomolecules suffer damage with age; but the hypothesis says that the body knew how to repair this damage at one time, and is capable of repairing the damage again, when the signal molecules engage repair mechanisms appropriately for a young individual.)
Gene expression is controlled, in turn, by markers on the chromosome and on the histone spool around which the DNA is wrapped like thread on a spool. These markers are known to change with age in a characteristic pattern. Shifting epigenetic markers program all the stages of development, and (according to the hypothesis), the program continues, and causes the body to pass through the stages of aging.
But in nature this clock never goes backward. The epigenetic clock is reset to zero as the genome is wiped clean and reprogrammed. This happens twice: once in the creation of germ cells, the sperm and egg; and a second time after sperm and egg join to make a zygote. (This is a simplification; some traits are epigenetically inherited, implying that some genome markers are retained and pass across generations.)
If we buy the epigenetic hypothesis, then the holy grail of anti-aging medicine would be to reset all the epigenetic markers, say from age 60 to age 20, but not all the way back to zero. This must be “possible” in some sense of the word; but if it depends on us to read (for example) the methylation of a 20-year-old’s chromosomes and write the results onto the chromosomes of a 60-year-old, in every cell of the body, without otherwise disrupting the living organism, then the task is daunting. And methylation is just one of about 100 known epigenetic modifications of the chromatin.
Nature knows how to reset the epigenetic clock all the way to age 0, but there is no precedent for a partial reset. All tissue differentiation is lost, and the growth rate is pedal-to-the-metal high. It should be no surprise that previous attempts to rejuvenate the living mouse by resetting the clock with OSKM have led to cancer disasters [ref, ref]. In this new report, the Salk researchers used short, intermittent exposure to OSKM to “partially reset” the epigenome. If this really works, and if the epigenetic hypothesis continues to pan out, then this is indeed a week to celebrate.
The procedure was first tested in human cell cultures, demonstrating “partial de-differentiation”, where functional cells were rejuvenated without returning them completely to stem cell status. This was an important proof of concept. The authors cite previous experiments suggesting that OSKM re-setting is a multi-step process, so it is possible in principle to halt the process after partial re-programming, and hope for a somewhat younger state.
Next, the procedure was tested on mice with genetically short life spans, living an average of just 18 weeks. Mice with the OSKM treatment lived 24 weeks. Impressively, the treatment was tuned so that it did not increase cancer rates. The particular life-shortening gene was defective LMNA (Lamin A). Lamin A is important for structure and function in the cell nucleus (details hazy), and there is no way to distinguish from this experiment whether the OSKM treatment merely counteracts the deficiency of Lamin A or whether it slows aging generally. Finally, the procedure was tested in normal, aged mice. They showed signs of improved healing, nerve regrowth, and mitochondrial chemistry typical of younger mice. But the mice were sacrificed to determine these things, so there was no demonstration of increased lifespan for genetically normal mice. Sometimes “publish or perish” pressure leads researchers to kill the goose that lays the golden egg…
But the big asterisk on this new result is that the delivery system was through a gene added to the mice at the egg stage. It is easy to modify genes at the egg stage, when there is just one cell, and then the modification is copied in every cell of the adult; but to do gene therapy for adult humans is still at an early experimental stage. Every cell in their bodies had an extra copy of each of the 4 genes for the OSKM factors, and these genes were so configured that they could be turned on and off with a drug (doxycycline). Administration of the drug was arranged to be just right to reprogram the cells, but not all the way. Optimum was found to be a low dose, just two days a week.
Bench scientists have learned to adopt the epigenetic hypothesis in practice, but some are still bound to the obsolete theoretical ideas, inherited from sclerotic thinking about “selfish genes”. Belmonte was quoted as saying the epigenome becomes “damaged” late in life “At the end of life there are many marks and it is difficult for the cell to read them.”
But studies of aging epigenomics show that in addition to random changes in the epigenetic state, there are definite, programmed changes—enough to make an accurate epigenetic clock—and that some of these changes turn down cell repair functions and turn up inflammation. Aging involves a loss of order (“damage”), but it also entails a set of programmed changes. It is the latter that we may hope to address through a streamlined, signaling approach to anti-aging medicine, and, if we are lucky, the body may take up the ball from here and undo part or all of the damage.
The bottom line
This is an important new confirmation of the epigenetic hypothesis. Previous confirmations were
- in parabiosis experiments, but the experiment could not be continued long enough to be sure that lifespan was extended
- in “methylation clock” measurements, but there has been no way to distinguish whether epigenetic changes were a cause or a result of aging.
This new experiment shows that epigenetic changes can extend lifespan. But the experiment offers no clear extrapolation to life extension for humans. The treatment depends on four large molecules that need to be delivered to every cell nucleus in the body. These molecules cannot be taken orally because they will not survive digestion, and even intravenous delivery will not get the molecules to cell nuclei where they are needed.
In cell cultures with the OSKM factors applied externally, stem cell yields are still just a few percent, even after ten years of experience.
The Salk researchers got around this by inserting the OSKM genes into the cell, but for already-living humans this is not a possibility. The best we know how to do is to modify some of the body’s cells with gene therapy; CRISPR in living humans is itself a new technique in its experimental phase.
So for the foreseeable future, I see a two-pronged approach to cell-level rejuvenation. One is to remove senescent cells (senolytics); and the other is stem cell removal, rejuvenation, multiplication in vitro, and return to the body. OSKM may be useful in this second step, rejuvenation of stem cells in vivo.