THE question of why women live several years longer than men has puzzled
biologists for decades. But a team of Scandinavian scientists now think it may
be because every woman’s body contains two slightly different cell lines, each
the product of the two different X chromosomes they inherited from their
parents. From their studies of twins the researchers suggest women’s bodies
choose the more vigorous of these cell lines during the course of their lives,
giving them an edge over men in longevity.
“The work is first-rate and utilises twins in a completely novel and very
interesting way,” says Tom Johnson of the University of Colorado, Boulder. “It
is pretty clear that there is some effect that allows one cell type to win
dzܳ.”
Every cell in a woman’s body contains two X chromosomes, one from her father
and one from her mother. In each cell, one of the two X chromosomes is switched
off. Hence every woman’s body has two different cell lines: one with the
paternal X chromosome expressed, and another with the maternal X chromosome up
and running.
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Kaare Christensen of Odense University in Denmark and a group headed by Karen
Helene Ørstavik at Ulleval University Hospital in Oslo wanted to know if one of
the two cell lines predominated. They found that they could easily tell the two
lines apart by analysing parts of the androgen receptor gene on the X
chromosome.
In young women, this analysis showed that the two cell lines were split
roughly 50:50—in other words, X inactivation was random. When they looked
at older women, however, they found that their bodies favoured one cell line
over the other. And in a study of 71 pairs of female identical twins aged
between 73 and 93 the same cell line predominated in both twins.
“This suggests that one of the cell lines is `better’ than the other,” the
researchers say. “Males only have one cell line,” Christensen points out. “So
they have only one chance.” He believes that having the choice of cell lines
could help women live longer.
The team has produced “intriguing data” and “an appealing and interesting
hypothesis”, says Judy Campisi of the Lawrence Berkeley National Laboratory.
“[Their] findings are certainly compatible with the data.” But she cautions that
“this is a correlative study . . . so the conclusions are `soft’.”
Animal studies back up their hypothesis: in mammals, males have two different
sex chromosomes (XY), and generally have a shorter life span. In birds, however,
it’s the females that have different sex chromosomes (ZW), and the research team
cites Russian scientists who claim that male birds, with two Z chromosomes, live
longer.
Not everyone agrees with their conclusions. “The suggestion that this may be
responsible for the difference in life span is uncertain and almost impossible
to validate,” says Johnson. Steven Austad of the University of Idaho agrees.
“The data are quite convincing that one of the two X-inactivated cell lines
takes over in later life,” he admits. “[But] the speculation that the ability to
select the better X chromosome is a general explanation for greater female
longevity seems far-fetched to me.”
Austad adds: “A key test of this idea would be in marsupials, in which it is
always the paternal X which is inactivated in females. This should presumably
negate any effect of selection for the better X chromosome.”
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Source:
Blood (vol 95, p 2449)
