From the Economist: The long and the short of it

[Guy Berger]

Ageing

The long and the short of it

Jul 12th 2007

From The Economist print edition

Children may inherit their lifespans from their fathers, not their mothers

BIOLOGISTS have long been befuddled by the causes of ageing. The underlying principle is known as the disposable soma. This is the idea that a hen is merely an egg's way of making another egg. Given that the hen is likely, sooner or later, to meet a hungry fox, there is little point in it having a body which can repair itself perfectly—particularly if the resources for that repair come at the expense of egg-laying. Hence that body (or soma, in Greek) gradually runs down. How this general principle plays out in particular, though, is the subject of eager study.

Wen-Chi Hsueh of the University of California, San Francisco, has been looking at one important aspect of disposable-soma theory. This is the role of structures called telomeres that cap the ends of chromosomes. Every time a cell divides, its telomeres get shorter. When they shrink beyond a certain point, that cell can no longer divide. The reason for this is believed to be to stop cancer in its tracks by preventing tumour cells from dividing more than a certain number of times. (Successful cancers often have a special enzyme that repairs telomeres.) The cost is that, eventually, even healthy tissue cannot renew itself. So the longer your telomeres are, the longer you can expect to live. But what remains unknown is why some people have longer telomeres than others. And it is this question that Dr Hsueh addresses in a paper just published in the Proceedings of the National Academy of Sciences.

Old Order Amish—the most traditional of the Pennsylvania Dutch communities—are unusual in many ways. They eat the same food together, farm that food together and do not marry outsiders. They do not smoke, drink or drive. And, almost uniquely, their menfolk have the same average lifespan as their women—71 years. For that last reason, and also because the Amish have come to welcome medical researchers who wish to study them for the edification of less well-disciplined examples of humanity, Dr Hsueh focused her endeavours in Lancaster county, Pennsylvania, where the Amish live.

She and her colleagues asked volunteers to give blood samples. They then extracted DNA from cells in those samples and measured the average length of the 356 male volunteers' and 551 female volunteers' telomeres. Next to that number, they recorded the volunteers' ages.

Their first observation was that, adjusted for age, the telomeres of men and women were the same length. (In the outside world, men generally have shorter telomeres than their female contemporaries.) That correlates nicely with the fact that among the Amish, men and women are equally long lived. And, since it is unlikely (though not impossible) that Amish men are genetically different from others, it also suggests that men's shorter telomeres in the outside world are a result of the way they conduct their lives, rather than an inevitable property of being male.

Of course, a healthy way of life is well known to prolong existence. But it is also true, as every actuary knows, that the best way to live for a long time is to pick your parents carefully. Long-lived parents tend to have long-lived offspring. With that in mind, the researchers started comparing telomere lengths within families.

Tel-tales

What they found was a bit of a shock. Children's telomeres did, indeed, correlate in length with those of their parents—but they correlated only with one parent, the father. Moreover, it was not merely the father's telomere length that correlated with that of his offspring, but also his actual lifespan. But here, too, there was a sex-linked wrinkle. The correlation between paternal lifespan and children's telomere length applied only to daughters. Moreover, the age at which men and women became parents also influenced their offspring's average telomere length: the older the parents, the longer the offspring's telomeres.

There is, frankly, no good explanation for these results at the moment. Dr Hsueh suggests that part of the answer may lie in a conflict between maternal and paternal genes. Such conflicts have been observed in the womb, when genes derived from the two parents compete to control the amount of nutrition a fetus draws from its mother. It is hard, though, to see a parent's evolutionary interest in controlling its offspring's lifespan by making some somas more disposable than others—nor why evolution would favour longevity in the children of older parents. Biologists, therefore, are still befuddled. But actuaries might care to take note.