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Researchers have long known that breast cancer affects African-American women differently than white women. Black women are less likely to get the disease, but more likely to die from it if they do. Black women also get the disease at younger ages than white women do.

But what accounts for these discrepancies? Social factors like poverty and access to care are certainly a large part of the problem. But a growing body of research suggests biological factors may also be at work.

A recent study led by Rowan Chlebowski, MD, PhD, of Harbor-UCLA Medical Center, found that breast cancer differences between white and black women persisted even after accounting for numerous risk factors that could influence the development of the disease, like age, body weight, family history of breast cancer, and whether the women got mammograms. The findings were published in the Journal of the National Cancer Institute.

The researchers looked at the records of more than 156,000 postmenopausal women in the Women's Health Initiative, a large study of lifestyle factors and disease.

RESOURCES: Closing the Black/White Cancer Gap

They compared breast cancer occurrence rates and deaths among American women of various ethnic groups, including white, African American, Hispanic, American Indian/Alaska Native, and Asian/Pacific Islander. They also looked at the differences in risk factors among the ethnic groups.

Women in all the minority groups were less likely to develop breast cancer than white women, the researchers found. For all groups except African Americans, those differences could be explained by differences in breast cancer risk factors. For instance, minority women were less likely to drink alcohol than white women, and less likely to use hormone therapy; both alcohol use and hormone therapy are known to raise a woman's risk of breast cancer.

African-American women were much more likely to be obese than white women (obesity raises the risk of breast cancer after menopause), and they got fewer mammograms than white women. But even these differences couldn't fully explain why African-American women had higher death rates from breast cancer, despite having a lower incidence of the disease.

The researchers looked at tumor characteristics and found that black women were more likely than women of all other races to have high-grade (aggressive) tumors, and tumors without estrogen receptors (ER-negative). Those characteristics make a tumor more difficult to treat. The differences between black and white women in this regard were especially great.

The different obesity rates might account for some of this gap, Chlebowski and his colleagues wrote, but not all of it.

"It remains to be determined whether differences in unidentified environmental exposures, genetic makeup, or other factors lead to the higher frequency of high-grade, ER-negative cancers in African Americans," they wrote.

There is some evidence to suggest genetic differences may play a role. Chlebowski and colleagues noted that breast tumors from African-American women are more likely than tumors from white women to have a particular gene, BP1, that is associated with aggressive breast cancer and ER-negative status.

Another study, published in the journal Cancer, found similarities between breast cancer in African-American women and in black women from sub-Saharan Africa. Like black women in the US, black women in Africa have a relatively low incidence of breast cancer, but high death rates from the disease. They are also more likely to develop breast cancer at younger ages, and to have advanced tumors.

To a large degree, the advanced stage and poor outlook for African breast cancer patients can be explained by poverty; African women have limited access to screening and are often unable to get timely, high-quality care.

But researchers Alero Fregene, MD, and Lisa Newman, MD, PhD, of the University of Michigan Comprehensive Cancer Center, noted that there may also be genetic similarities between African and African-American women, since many African Americans are descended from African slaves brought to the US centuries ago.

Some of these genetic factors may increase the risk of breast cancer, or make black women more likely to develop high-grade, hormone-receptor negative tumors. The researchers noted that further study will be needed to clarify the role of genetics in the development and progression of breast cancer in these women.

African-American families also tend to have different mutations in the BRCA1 and BRCA2 genes than families of other races. These gene mutations make women more susceptible to breast and ovarian cancer.

Genetic testing, which is available to women with a family history of either disease, can reveal whether a women carries a BRCA mutation. If she does, she can take steps to lower her risk of developing one of these cancers, such as taking tamoxifen, getting more frequent mammograms, or having her breasts or ovaries removed before disease can strike them.

Yet African-American women are much less likely to get tested for BRCA mutations than white women, researchers from the University of Pennsylvania reported in the Journal of the American Medical Association.

In their study of 408 women with a family history of the disease, only a handful of African-American women chose to get tested. The researchers couldn't fully explain why. None of the factors they looked at -- likelihood of having the mutation, socioeconomic status, worry about developing breast cancer, attitudes about the benefits and risks of testing, or talking to a doctor about testing -- seemed to play a major role.

The researchers speculated that distrust of the health care system, fear of discrimination based on the test results, and differences in doctors’ attitudes toward testing may be at least partly to blame (though they didn't measure these factors in this study).

In an editorial published with the study, University of Chicago genetics experts Michael Hall, MD, and Olufunmilayo Olopade, MBBS, added other possible reasons for the discrepancy.

"The average African-American woman underestimates her risk of breast cancer and African Americans are on the whole less aware of genetic testing technology as a means of assessing personal risk," they wrote.

All Americans, and especially ethnic and racial minorities, need to learn more about inherited cancer risks and the pros and cons of genetic testing, they said.

Citations:
"Ethnicity and Breast Cancer: Factors Influencing Differences in Incidence and Outcome." Published in the Journal of the National Cancer Institute (Vol. 97, No. 6: 439-448). First author: Rowan T. Chlebowski, MD, PhD, of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center.

"Breast cancer in sub-Saharan Africa: How does it relate to breast cancer in African-American women?" Published in Cancer (Vol. 103, No. 8: 1540-1550). First author: Alero Fregene, MD, University of Michigan Comprehensive Cancer Center.

"Racial Differences in the Use of BRCA1/2 Testing Among Women With a Family History of Breast or Ovarian Cancer." Published in the Journal of the American Medical Association (Vol. 293, No. 14: 1729-1736). First author: Katrina Armstrong, MD, University of Pennsylvania School of Medicine.

"Confronting Genetic Testing Disparities: Knowledge Is Power." Published in the Journal of the American Medical Association (Vol. 293, No. 14: 1783-1785). First author: Michael Hall, MD, University of Chicago.