May 9, 2012
As Mother’s Day approaches, let’s take a moment to celebrate grandmothers. Grandmothers have traditionally been important members of the family who help their daughters raise children. Some anthropologists have suggested that the evolutionary benefits of grandmothering may explain why women have such long post-menopausal lives. You don’t see that in other primates. The idea is controversial, but it has been the center of numerous research studies.
Now, Jack da Silva of Australia’s University of Adelaide adds a new twist to the grandmother effect: It may have helped keep harmful breast cancer mutations at bay.
Mutations in the BRCA1 and BRCA2 genes are risk factors for breast cancer. The genes normally keep a cell’s growth in check. When certain mutations arise in these genes, cells grow out of control and cancer develops in the breasts or reproductive organs. These mutations are among the main causes of hereditary breast cancer. According to the National Cancer Institute, about 12 percent of women in the general population get breast cancer compared to 60 percent of women carrying BRCA1 or BRCA2 mutations.
Last fall, a study published in the Proceedings of the Royal Society B reported an unexpected benefit of the BRCA1 and BRCA2 mutations. In a sample of women born in Utah before 1930, those who carried the mutated genes had greater fertility than those who didn’t. Carriers had on average 6.22 children, while non-carriers had 4.19 kids. That’s almost a 50 percent increase in fertility. Exactly how these mutations improve fertility is not known, but women carrying the mutations had more reproductive years and shorter intervals between births.
In a paper published online today in Proceedings of the Royal Society B, da Silva considers the paradox of the BRCA1 and BRCA2 mutations—that they are both good and bad from an evolutionary perspective. They are an example of what biologists call antagonistic pleiotropy. Pleiotropy occurs when a gene influences more than one trait. Antagonistic pleiotropy explains why otherwise harmful mutations can stick around in the gene pool. In the game of evolution, the goal is to pass on your DNA. Any mutation that helps an individual reproduce will be selected for, even if that mutation is harmful later in life. That seems to be what happens with these breast cancer mutations, which tend to cause cancer after a woman’s reproductive years are over.
Based on estimated mutation rates and the mutations’ reproductive benefits, da Silva calculates that the BRCA1 and BRCA2 mutations should be much more common (one estimate suggests the BRCA1 mutations occur in about 1 in 3,000 women in the United States). This is where grandmothers come in.
Grandmothers can help ensure the survival of their grandchildren (and by extension, the survival of their own DNA) by helping raise them. A study of Finnish and Canadian women living on farms in the 18th and 19th centuries found that a woman produced an extra 0.2 grandchild for every year that she lived beyond age 50. If grandmothering is really that vital, then it might give women who don’t carry the breast cancer mutations an evolutionary edge over women who do and are therefore less likely to live as long.
Taking into account several factors about women’s reproductive lives and the effects of grandmothering, and with a little bit of math, da Silva argues that grandmothering would have limited the spread of the BRCA1 and BRCA2 mutations in the distant past, when more people lived in traditional hunter-gatherer societies. Based on this, he calculates that the mutations should occur in 0.275 percent of the population. He points out that that’s close to the worldwide average of 0.227 percent.
To get to this conclusion, da Silva made many assumptions about fertility, life span and the usefulness of grandmothers in hunter-gatherers. Those assumptions need to be validated by data from a variety of real-world groups for his conclusions to hold up.
Grandmothers’ effects on breast cancer mutations are smaller today because many people live in societies where birth control, fertility treatments, day care, nannies, etc. play big roles in reproduction and child rearing (and where breast cancer can be treated). But even if grandmothers had only a small part in limiting the spread of BRCA1 and BRCA2 mutations, it’s still one more reason to be thankful for them this Mother’s Day.
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