In aging science, there are two kinds of “unexpected” results. The first disagrees with theory. These have become commonplace and are no longer really a surprise. Aging “theory” has become so disconnected from experimental results that when contradictions arise, the theorists respond only that “aging is complicated”. But the second is a disagreement with what we thought we knew in the laboratory. These are truly unexpected. In honor of Chinese New Year, I offer this week “Two from Group A and One from Group B”.
Aging is not caused by mutations in the cellular DNA accumulating over a lifetime*
An old theory of aging says that there is a chance of DNA mutation with every cell replication. The “germ line” consists of egg and sperm cells that will be passed to the next generation and on and on, so natural selection has been strong to assure replication is super-accurate. But within a single lifetime there are cells that are used to build the body. Skin and muscle and blood cells will all die with the body, so there is less evolutionary pressure to keep their DNA pristine. It’s more important to the individual’s success that the replication take place fast and efficiently, even at the expense of accuracy (so the theory goes). So we might expect that little inaccuracies might accumulate over a lifetime, leading to dysfunction.
This theory dates all the way back to 1959, proposed by a nuclear physicist named Leó Szilárd**. Szilárd assumed that all cells in the body make copies of themselves – skin cells make more skin cells and liver cells make more liver cells. Stem cells had not yet been discovered. Like a good physicist, he worked out the mathematical consequences of the theory in the abstract. Almost nothing was known about mutation rates at the time, so he could fill in parameters that made the theory work.
So why does aging proceed only gradually for most of a lifetime, and then just at the time when an old individual is slowed down, both in activity and in cell replication, aging begins to proceed at an accelerating pace, becoming catastrophic in the late stages? A few years after Szilárd, Leslie Orgel answered this question and the theory of somatic mutations became known thereafter as Orgel’s Hypothesis. What Orgel added to the Szilárd theory was that some mutations are more important than others. Indeed, some mutations affect replication itself, and they cause more inaccuracy. Thus inaccuracy in the particular genes that affect DNA replication cause ever more mutations, and more inaccuracies in a vicious cycle. This was Orgel’s explanation for the fact that aging does not proceed steadily over a lifetime, but starts slow and then accelerates, plunging toward death.
Of course, once stem cells were discovered (just a few years after Orgel’s paper), there was no longer any reason for copying errors to beget copying errors, and no basis for the accelerating schedule that characterizes aging. (Indeed, this is thought to be the reason that the body uses specialized stem cells.) This should have been a mortal blow to the theory. And further, the theory also conflicted directly with experiment. Cells from old mice and young were infected with a virus. The virus commandeers the host cell’s machinery to copy and transcribe its own DNA; so it was thought that if this machinery becomes less efficient with age, then the virus would spread more slowly through the old cells than the young. No such effect was found. [Rabinovitch & Martin, 1982] Another lab set out very deliberately to test the Orgel hypothesis by culturing cells over a long period, then counting the transcription errors in younger and older cell lines. They found no difference. [Harley et al, 1980]
So how surprised can we be with this week’s results? Genomes were compared for pairs of identical twins 40 years old and 100 years old. The assumption is that, at birth, the genomes of each pair were identical, but that a lifetime of random mutations could cause the twins’ genomes to diverge. The result was no detectable mutations in the 40-year-old pair, and only 8 mutations out of 3 billion base pairs in the centenarians. 5 of the 8 were in non-coding regions of DNA. Mutations at this level are likely to be utterly insignificant.
For me, the interesting question is why theories like this are still considered viable, though they have been falsified on multiple occasions.
Cancer begins not with a single rogue cell, but with a weak or toxic metabolic environment.
Just a few years ago, the mainstream view of cancer was that it was the result of a rare accident, a cell that just happened to mutate in such a way as to make it reproduce out of control. Once the mutation took place (so the theory went), any single cell would become an unstoppable parasite, spreading through the body and halting only with the patient’s death.
It has come to light more recently that the body has many defenses against cancer cells, even after they have become malignant. Hence, cancer should be regarded as a systemic disease, not a cellular anomaly. Now, many cancer biologists believe that these mutations to a cancerous state are common occurrences, but that most cells are smart enough to detect their own diseased state, and this triggers apoptosis=programmed cell death. For those few that escape apoptosis, a healthy immune system is able to detect them and attack them in the same way foreign microbes are targeted and killed [ref]. Hence immune therapies for cancer have become the most promising line of research in oncology today. (It was Science Magazine’s “breakthrough of the year” a few weeks ago.)
In an Italian study published last week, rats were treated with a chemical that reliably causes liver cancer. Half of them developed the disease. Another group of rats was treated with the same chemical, but they were also injected with 8 million normal liver cells (= a fraction of a ml — a lot for a rat). These were not stem cells, just end-differentiated liver cells. (It is known that such cells find their way to the liver – we don’t know how.) Butnone of the treated rats developed cancer, compared to half the untreated animals.
Increasing evidence indicates that carcinogenesis is dependent on the tissue context in which it occurs, implying that the latter can be a target for preventive or therapeutic strategies. We tested the possibility that re-normalizing a senescent, neoplastic-prone tissue microenvironment would exert a modulatory effect on the emergence of neoplastic disease.
Rats were exposed to a protocol for the induction of hepatocellular carcinoma (HCC). [One] group of animal was then delivered 8 million normal hepatocytes, via the portal circulation. Hepatocytes transplantation resulted in a prominent decrease in the incidence of both pre-neoplastic and neoplastic lesions. At the end of 1 year 50% of control animals presented with HCC, while no HCC were observed in the transplanted group.
Extensive hepatocyte senescence was induced by the carcinogenic protocol in the host liver; however, senescent cells were largely cleared following infusion of normal hepatocytes.
Note the last sentence. Senescent cells are cells with short telomeres, and they are known to be a risk factor for all diseases of aging. In a 2001 experiment from Mayo Clinic, a genetically-engineered trigger was introduced to allow the experimenters to eliminate senescent cells at will; the result was that disease was avoided and life span extended. This week’s experiment raises the possibility that simply introducing healthy, young cells can signal the body to eliminate the bad actors. How does this work? Is it through chemical signaling that can be induced without the cells? This is a promising avenue for research.
An easy path to stem cells?
Background: The cells in our bodies are mostly specialized to perform a task. Nerve cells or muscle cells or skin cells or blood cells do their particular jobs. There are also stem cells, whose job is to grow more of every other kind of cell. Stem cells are important for renewing body tissues and for healing. Specialized stem cells can grow into several kinds of tissue, for example white or red blood cells. The most powerful stem cells are completely undifferentiated, and they have the potential to grow into any other kind of cell the body might happen to need. They are called pluripotent stem cells.
Before the Republicans, with their superior ethics, took over the rules of research at NSF, there was an abundant supply of stem cells for research, gleaned from aborted and stillborn fetal tissue. At the beginning of the GWBush Administration, that source was cut off (for reasons that seemed more political than moral). But then the Law of Unintended Consequences kicked in. There was a surge of interest in the basic science of stem cells, with a practical focus on the question: Is it possible to turn an ordinary differentiated cell back into the stem cell from which it was derived? There ensued a race to create IPS cells in the lab. (IPS stands for “INDUCED pluripotent stem cells”.)
The race was won by a Korean lab, which first created IPS cells by careful doctoring of ordinary skin cells. The next breakthrough came from a Japanese lab. Their technique was refined and streamlined until, a few years ago, it was common to say that just 4 chemicals are needed to turn a skin cell back into a stem cell.
In Nature this week is an article about another Japanese group that claims an even easier path to create stem cells, just by starting with ordinary skin cells and reading them the Riot Act. No specific chemical treatment was used, but the cell did the job itself in response to stress. They report that the specific stress that seems to work best is an acid bath.
IPS cells created in this way were introduced into an embryo, in order to demonstrate that they have the ability to grow into any of the body’s tissues. They were able to produce achimeric mouse, meaning that different cells in the mouse’s body were derived from different parents.
This result was truly unexpected. Many labs have worked for years to come up with a reliable technique for creating stem cells. None of them has reported that it was easy. Whythis should work is yet a deeper mystery. It is reminiscent of certain species of jellyfish andbeetles that revert to their larval state when stressed, and, when conditions improve, can begin life anew with a full life span ahead of them.
* Confusingly, this hypothesis has nothing to do with the “Mutation Accumulation Theory of Aging”. The latter concerns random, detrimental mutations that (hypothetically) only affect the organism late in life, after most have died of other causes. Therefore these become invisible to natural selection, and they accumulate as part of what is called “genetic load”. The present article is about a metabolic theory, not a genetic theory. It is the hypothesis about mutations that accumulate over a single lifetime, not in the genetic material but in the stem cells that renew the body’s tissues.
**Szilárd was a Hungarian-American who first suggested the possibility that nuclear fission could be realized as a self-sustaining chain reaction, which is the basis for both nuclear bombs and nuclear power. As a Jewish refugee, a brilliant physicist, and a grateful American, Szilárd worked on the Manhattan Project during World War II; but he personally urged President Truman not to use the superweapon he had helped create against the Japanese people, but to demonstrate it instead in Tokyo Harbor. After the war, he helped found the Council for a Livable World—an early, high-profile disarmament advocacy group—as he spent the last years of his life studying not physics but biochemistry.