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Black women develop more aggressive, lethal breast cancers at a younger age than do white women, said Martha McClintock, PhD, during a session at APA's 2007 Annual Convention. Because 70 percent to 80 percent of women with breast cancer have no evidence of an inherited mutation, she hypothesized that black women's social behavior and environment may spark genetic changes leading to breast cancer.

"We're interested in a model where race is defined as being perceived and self-identifying as an African-American woman," said McClintock, the director of the Institute for Mind and Biology at the University of Chicago. McClintock believes that this identity in some African American women puts them in social environments and certain psychological states that change their hormone function. These hormonal changes may then lead to acquired genetic mutations and mammary tumor growth.

McClintock's theory was inspired by black women living in Chicago neighborhoods where crime and poverty-two known stressors-force the women to live in a state of vigilance that may increase their bodies' production of stress hormones. Violence and financial instability often force the women to move frequently, which erodes their social support network and can make them feel lonely and isolated, said McClintock. The isolation, she noted, may kick off the series of physiological changes that precede tumor growth.

To test her hypothesis, McClintock ran a series of experiments in which she mimicked the women's stressful conditions by socially isolating Sprague-Dawley rats-which spontaneously develop mammary tumors-and measuring how the solitude influenced tumor growth. Rats are naturally social animals, said McClintock, however certain individuals-called sentinels or pariahs-isolate themselves near the entrance to the group burrow. They keep watch and determine when it's safe for the rest of the group to venture outside, but they pay the price of increased stress, hypervigilance and isolation. McClintock found that the median lifespan for the isolated rats was 550 days, versus 850 days for rats housed in groups.

A main cause of rat death was naturally occurring mammary tumors, and the socially housed rats were tumor free for 550 days, on average, but half of the isolated rats had developed tumors by day 400. McClintock also euthanized certain 17-month-old rats and found that at that age, isolated females averaged 25 grams of single or multiple tumors, but the group-housed rats were virtually tumor free.

McClintock believes her approach is unique because most cancer models are created by injecting cancerous cells into lab animals instead of examining the spontaneously developing tumors that plague humans.

"Our understanding of tumor biology comes from very insightful cell and carcinogen-based models, but they are very artificial models," said McClintock. "[Our] animal model of spontaneous tumor development, I believe, is a very powerful one for making conclusions comparatively to humans."

Through her research, McClintock is collaborating with other investigators at the Center for Interdisciplinary Health Disparities Research in Chicago who are conducting community-based participant research studies with black women in the neighborhood. By combining McClintock's genetic animal models with home visits, in-person interviews and neighborhood safety evaluations, the researchers hope to create a complete picture of how stress and social isolation are associated with both hypervigilance and mammary tumors in black women.