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Some women could develop breast cancer because of the way hormones affected their body during puberty, researchers have suggested.
Exposure to high levels of female hormones - oestrogens - due to early menstruation or late menopause are already suspected to affect breast cancer risk in women.
This latest US study of twins suggests some women may have an inherited sensitivity to hormones that surge through the body during puberty, causing physical development and menstruation.
These may affect immature breast cells as they develop, but the damage may not manifest itself as cancer until several decades later.
The researchers suspect that genes responsible for fixing damaged DNA might not work as well in these women as they should.
They say their finding could lead to the discovery of other genes which might explain the causes of hereditary breast cancer.
But UK experts say the study's findings could be explained by the recognised risk linked to exposure to high levels of oestrogens.
Twin study
Scientists from the Keck School of Medicine of the University of Southern California looked at 1,811 pairs of identical and fraternal female twins, one or both of whom had breast cancer, recruited between 1980 and 1991.
They were asked questions including at what age they had their first period, when they had their first child, how many children that had, when they went through the menopause.
In identical twins where both had had breast cancer, the twin who started her periods first was more than five times as likely as her twin to get breast cancer first.
Those who began menstruating before they were12 were especially likely to develop breast cancer first.
Later first pregnancy, having fewer children and late menopause were linked to a higher breast cancer risk in identical and fraternal twins, where only one had the disease.
Although the scientists are not certain which genes are responsible, they say the finger points towards those responsible for fixing damaged DNA.
Genetic susceptibility
The team from the University of Southern California suggest these genes might not work as well as they should in some women, so they cannot mend the damage ovarian hormones cause during puberty.
Professor Ann Hamilton, who led the study, said: "We don't know all the causes of breast cancer, and this study provides some insights into another pathway that could lead to the discovery of additional genes that might help explain the causes of hereditary breast cancer.
"All breast cancer might not be due to hormone exposure over a lifetime."
But Dr Emily Banks, deputy director of Cancer Research UK's Epidemiology Unit in Oxford, told BBC News Online: "We have known for some time that the younger a woman is when she starts her periods, the higher her risk of developing breast cancer later in life.
"The younger a woman is, the longer her overall exposure to high levels of oestrogens."
She said this was what the US study had concluded, but it was simply suggesting a different mechanism - genetic damage during puberty - without proving that was responsible for the higher risk.
"The difference is how they are interpreting the data," she said.
The research is published in the New England Journal of Medicine.
Content provided by ArmMed Media
Revision date: 12 December 2007
Last revised by Amalia K. Gagarina, M.S., R.D.
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