Authors: Armand Marie Leroi
If superoxide dismutase moonlights, then the argument proposed above is predicated on a false premise. And with it goes one of the few good reasons for believing the whole free radical theory of ageing. The proponents of this theory (and among scientists they surely number in the thousands) may well feel that this is a harsh assessment of the only mechanistic account of the origin of ageing that has any pretensions to generality. It is certainly still possible that superoxide dismutase’s seemingly beneficial effects on ageing are mostly due to free radical scavenging, but this remains to be shown. For the time being, however, few would disagree that superoxide dismutase can be struck from the list of elixirs that might one day stave off the decline of our later years.
A WRINKLE
Even if free radicals are not the sole, or even major, source of mutations, mutations may still cause at least some aspects of ageing. Mutations may be especially destructive in those tissues, such as skin, whose cells divide continually throughout life. Some of us keep relatively youthful complexions well into old age, while others wrinkle when young. This variety partly depends on the exposure to the elements, sun most obviously, that each of us has received; ultraviolet light is a powerful
mutagen. But even sheltered skin ages. And for all the parasols, veils and sun-block in the world, no thirty-five-year-old’s skin has ever glowed as it glowed when she was fifteen.
Wrinkling is a manifestation of a deeper inability of epidermal cells to replace themselves and maintain the integrity of the connective tissue of our skins. It is a problem that pervades our bodies. This is evident from people whose skins and connective tissues age with unusual, indeed catastrophic, rapidity. An inherited disorder called Werner’s syndrome causes its victims to go grey and bald when still in their teens. In their twenties, the testicles atrophy in men as the ovarian follicles do in women – a kind of premature menopause. In their thirties sufferers need lens transplants to cure cataracts, and their arteries stiffen and become covered in fat deposits. In their forties they die, usually from heart attacks.
Werner’s syndrome is one of a group of inherited rapid-ageing disorders called ‘progerias’. The disorder is caused by mutations that disable a protein that maintains the integrity of DNA during replication. Cells that lack the protein have very high mutation rates. This barrage of mutations causes the cells to die instead of proliferating, or else to produce abnormal proteins. Tissues, such as skin, which rely on large numbers of dividing cells in order to maintain their integrity, fall apart. Perhaps something similar happens to us all, only at a much slower rate.
As we age, vitality slips away from our cells. This can be seen in the laboratory. It has long been possible to grow human cells in
petri-dishes by means of elaborate and delicate protocols. No matter how salubrious their environment, however, freshly harvested cells will divide only a certain number of times and then divide no more. Their decline is gradual, and is caused by some intrinsic limit. Many have suggested that this cellular senescence is not merely a consequence of the ageing body but its direct cause.
Supporting this idea, cells taken from human foetuses can divide for about twice as many generations as can those from ninety-year-olds before sinking into decline. Perhaps, then, elderly people have many cells that are closer to the end of their replicative lifespans and which are, therefore, unable to contribute to repairing the wear and tear of everyday life as well as they might. When, therefore, in 1998, the molecular cause of the limit to cell division was discovered, and then broken, the thrill was tangible. If cellular senescence could be cured, perhaps so could ageing.
Each time a cell divides, its chromosomes must be replicated as well. But the enzymes that replicate chromosomal DNA are unable to replicate the ends of the chromosomes. These ends are, therefore, protected by sequences, thousands of base-pairs long, called telomeres that are gradually whittled away over the course of many cell divisions at a rate of about a hundred base-pairs per cell division. When the telomeres are gone, the cell can no longer divide and it dies. It is the rate of whittling that sets the fundamental clock of ageing. Or so the argument goes.
What is needed, then, is a way to prevent the attrition of telomeres. Not all cells lose their telomeres. The germ cells that give rise to eggs and sperm possess a complex enzyme called
telomerase that maintains their telomeres and so confers upon them the immortality that they must necessarily have. The loss of telomeres that occurs in the rest of the body’s cells is precisely due to the fact that they do not contain this enzyme. If telomerase is engineered into cells that normally lack the enzyme, their telomeres are preserved division after division. The cells also became immortal.
If the route to cellular immortality is so easy, why have we not taken it? The reason is quite simple: immortality is a property of cancers. Nearly all tumor cells have, somewhere in their history, undergone mutations that cause them to have telomerase where other cells do not. The absence of telomerase in our cells is probably one of the first defences we have against the multiplication of rogue cells. Besides, there is still little to show that short telomeres do, in fact, cause ageing. Only one experiment has addressed the problem directly: an experiment in which telomerase-defective mice were engineered and then bred for six generations.
Mice, it seems, can get by without telomerase for at least a while. The first generation of telomerase-defective mice that was ever produced showed no signs of premature ageing. In a way this is not surprising. These mice had telomeres as long as those of any other mice, for mice, like us, inherit their telomeres from their parents, and their parents were normal. For want of telomerase in their germ cells, however, each successive generation of these mutant mice started life with ever shorter telomeres. The effects became apparent by the fourth generation when the male mice proved to have few viable sperm. By the
sixth generation they had none at all. Females were not sterile, but they produced fewer eggs than normal, and those they did produce often gave rise to defective embryos. By the sixth generation, too, male and female mice alike began to age prematurely. Like humans, mice go bald and grey with age, and the sixth-generation mice did so while still young.
These results provide at best mixed support for the idea that a want of telomeres causes ageing. Sufficiently short telomeres can clearly cause premature ageing; but since this happens only after six generations of attrition, they cannot be the cause of normal ageing in mice. While it is tempting to dismiss the whittling away of telomeres as an explanation of ageing in humans, it is probably too soon to do so. Laboratory mice have extraordinarily long telomeres – far longer than ours. If our telomeres are rather short at the start of our lives and must, by virtue of our greater size and longevity, undergo far more attrition than a mouse’s, it remains quite possible that they matter to us.
One way to prove the point would be to clone a human. Clones should start life with abnormally short telomeres, for they are produced without the aid of germ cells and so their telomeres are never renewed. Successive generations of clones should have shorter and shorter telomeres and age with increasing rapidity – all the more so if the clone-donors are elderly. What with the global ban on human cloning this experiment is not likely to be carried out soon – unless by UFO cultists or renegade Italian obstetricians. But, of course, it has been done in animals. Sheep 6LL3, a.k.a ‘Dolly’, got her chromosomes from the udder-cells of a six-year-old Finn Dorset. She therefore
began life with substantially worn-down telomeres. Many thought that she would age fast. Some arthritis aside, however, she was quite healthy; there was nothing exotic about the viral disease that prompted her euthanasia at the age of six. Clones of other animals such as cattle and mice often suffer from a variety of health problems such as obesity, but none have been reported to be progeric. Still, these are early days.
Telomerase-mutant humans would be informative too. There is another progeria, rarer than Werner’s but even more severe, in which catastrophic ageing begins in childhood. The victims of this disorder usually die by the age of twelve or so, again from heart attacks, by which time they are to all appearances very small octogenarians. Their symptoms suggest defective telomeres. Even if this grim disease can be explained by too-rapid cellular senescence, we will have penetrated only a small way into ageing’s mysteries. For while the progerias hasten some aspects of physical decline, they leave the minds of their victims untouched.
MAKING A CENTURY
In the last ten years there has been a revolution in the study of ageing. Much of it has come from the study of the nematode worm
Caenorhabditis elegans.
This worm is only about 1 millimetre long, and it is possible to grow thousands of them in petri-dishes. They are perfectly transparent. Under a powerful microscope it is possible to see every single one of the 959 cells in their living bodies. For whatever reason, it has been
especially easy to identify worm mutants that are extraordinarily long-lived. Some of these mutant worms live twice as long as normal worms do: forty-two days – in human terms, about 150 years.
So far, at least a hundred genes have been identified in worms that, when mutated, cause them to live longer. Many of these mutations disable the worm’s insulin-like growth-factor-signalling pathway. As a consequence of doing so, the whole physiology of the worm changes. Mutant worms that are defective for IGF signalling reproduce less, store large amounts of fat and sugars, and activate a whole battery of genes that encode for stress-resistance proteins, among them superoxide dismutase. The result is worms that radiate health even as their normal contemporaries wither in their petri-dishes.
We have come across insulin-like growth factor before. It is the lack of this hormone that makes pygmies small and its excess that makes Great Danes large. It is also one of the hormones that, when inactivated in mice, cause them to be dwarf and long-lived. In worms, IGF does not seem to control body size (something of a surprise since it does so in so many other creatures, including fruit flies). Even so, taking these findings from worms together with what is known about IGF in mice, flies and many other creatures, it is possible to sketch an account of a mechanism, perhaps universal to all animal life, that allows animals to live longer when they need to.
Worms are not frightfully bright. The nervous system of any one worm, including what passes for its brain, contains only 302 neurons; a human brain has around a billion-fold more. Even so,
a worm has nous enough to know how much food it has. When a worm perceives that it is about to starve, neuronal signals from sense organs in its head signal the rest of the body and IGF signalling is shut off. A change in environment mimics what many mutants do, and the result is the same: the worm lives longer.
This should sound familiar. It is, in effect, what happens in caloric restriction in mice and rats. And it suggests an interpretation for how and why
la vita sobria
has its beneficial effects. Far from being an odd laboratory phenomenon of interest only to gerontologists and diet gurus pursuing dreams of immortality, the caloric restriction response is probably a device that has evolved to allow animals to cope with the vicissitudes of life. Perceiving that it is in for hard times, a young animal alters its mode of life. Instead of investing resources in growing large and reproducing soon, it switches to survival mode. It remains small and ceases to reproduce, in effect gambling that sooner or later better times will come. If this view of caloric restriction is correct, then its enthusiasts are attempting nothing less than the revival of devices evolved to cope with the deprivation that was surely our lot for millennia of prehistory (and surely a lot of history too). Though they do not know it, when they calculate their foods to the last calorie, surround themselves with bottled vitamins, and monitor, as they must, their bone density by the month, they are playing the part of civilisation’s most dedicated discontents.
Can longevity genes be found in humans? Many scientists think so. In France, Britain, Holland, Japan, Finland and the United States gerontologists are busily compiling lists of centenarians
and analysing their DNA in order to find out why they live so long. They do so not in the expectation that there is any one mutation or polymorphism that all these centenarians have in common – and they fully accept that some centenarians will have made their century by a combination of good luck and virtuous living. Rather, the approach is to scan many genes which, for one reason or another, are believed to contribute to the diseases of old age and to search for those variants that are more common in geriatric survivors relative to the rest of the population.
One of the first longevity genes to be identified in this way was apolipoprotein E (APOE). The protein encoded by this gene comes in several polymorphic variants called ?2, ?3 and ?4. About 11 per cent of Frenchmen and women under the age of seventy carry at least one copy of the ?4 variant, but in French centenarians this number drops to 5 per cent, the difference being made up by the ?2 variant, which becomes more common. This implies that should you wish to see your hundredth birthday, you should hope to have at least one copy of ε2 but none of ε4.
This is because the APOE gene, which encodes a protein involved in cholesterol transport, has been implicated in Alzheimer’s disease. About one in ten people aged sixty-five or over will contract Alzheimer’s, but the odds are skewed drastically if you are an ?4 carrier. One copy of ?4 relative to none increases your risk of Alzheimer’s three-fold; two copies increases your risk eight-fold. Were this not enough, ?4 also predisposes to cardiovascular disease. With this sort of molecular double jeopardy it is easy to see why ?4 carriers rarely survive to a great age.
All this seems to matter less if you are black. Surveys of APOE genes have shown that ?4 is very common in sub-Saharan Africa. Nearly half of African pygmies carry at least one copy. Does this really mean that Alzheimer’s disease is rampant among the Efe? The short answer is that we don’t know. No studies on the epidemiology of Alzheimer’s seem to have been carried out on pygmies, and they would be hard to do since a high rate of death due to infection and accidents means that few pygmies survive to an age when Alzheimer’s might be seen. This, in itself, may explain why ?4 is so common among them, but a more likely explanation is that it is less dangerous to Africans than it is to Europeans. Several studies have sought, and failed, to find an increased risk of Alzheimer’s in Nigerians and African Americans who carry the ?4 variant. Why this should be so is something of a mystery.