Etiology: Finding the Cause
Identifying and understanding the factors that lead to breast cancer in individual women and in the population as a whole are crucial. Through this work, we can learn how to prevent breast cancer, for example, by discovering causative factors that can be eliminated or reduced, or by identifying conditions that predispose a person to breast cancer and for which there may be preventive treatments.
Research Conclusions
Peggy Reynolds, Ph.D. of the Public Health Institute investigated whether California flight crews on American airline carriers might have an elevated breast cancer risk as a result of their possible increased exposure to chemicals, cosmic radiation, electric and magnetic fields. This collaborative project between the Association of Flight Attendants (AFA) and the California Department of Health Services' environmental and occupational health programs, found that the two most common invasive cancer types in this group, female breast cancer and malignant melanoma of the skin, occurred substantially more frequently among flight attendants than would be expected from the rate of the general population. Breast cancer incidence was over 30% higher than expected, and melanoma incidence was roughly twice that expected. These findings were consistent with the results from a much smaller European study of cabin crews, and suggest that follow-up investigations should focus on the potential relative contribution of workplace exposures and lifestyle characteristics to the higher rates of disease for these two cancers.
Margaret Wrensch, Ph.D. of the University of California, San Francisco and Mary Gould of the Marin Breast Cancer Watch (MBCW), a grass–roots organization of approximately 350 members, were funded to do a pilot study regarding the high incidence of breast cancer in Marin County. They are investigating the question: did adolescent and pre–adolescent experiences differ between women with and without breast cancer in Marin County? They completed comprehensive literature reviews into the role of pre–adolescent and adolescent factors in breast cancer development; the effects of recall bias (differential and non–differential) on interpretation of results of case–control studies; and on mechanisms for enhancing recall and minimizing effects of recall bias. Focus groups were conducted, one aim being to develop methods to help women remember accurately their pre–adolescent and adolescent experiences and exposures, with the aim of using these methods in a population–based research project. The success of this pilot enabled the collaborative team to obtain a full CBCRP research grant to pursue these questions further.
Linda Kingsbury, Ph.D. of the University of California, Berkeley completed a two–year postdoctoral fellowship entitled "Bovine Leukemia Virus and Mammary Cell Infection" that investigated whether bovine leukemia virus (BLV) was able to infect human breast cells. The theory that BLV infection can lead to breast cancer in humans is based on the observation that infection by a virus (MMTV) can lead to breast cancer in mice. BLV, a virus similar to MMTV, has been found in milk from cows but can only cause breast cancer if it has the ability to get inside human cells. Viruses enter cells through specialized proteins called receptors. Dr. Kingsbury performed studies to determine whether these receptors exist in human breast cells. She employed five different methods to detect BLV receptors. She was able to identify BLV receptors in several human breast cancer cell lines as well as in human fibroblasts. This supports the idea that the BLV will be able to enter human breast cells and infect them — the first step toward the development of cancer.
Donna Williams–Hill, Ph.D. of the University of Southern California completed a 3–year New Investigator award entitled "BRCA1 Regulation in Breast Cancer: A Rat Mammary Model". Dr. Williams–Hill examined the role of the breast cancer gene, BRCA–1 in the development of the rat mammary gland. She found that BRCA1 is present in the breast tissues of adult rats proportional to their genetic susceptibility to induced breast cancer. Levels of BRCA1 increase as the rat matures. BRCA1 levels are lowest in the mammary gland of 3–week old rats and increases gradually as the rat ages to 5 weeks (pre–puberty) and 8 weeks (puberty). As BRCA1 increases, the susceptibility to breast cancer decreases. These observations are consistent with the reported role of BRCA1 in DNA repair. With this information, we could learn about what BRCA1 is doing in normal human breast cells and extrapolate what could be going wrong in cancer cells with the BRCA1 mutation.
Sue Ingles, Dr.P.H. of the University of Southern California was funded to investigate the possibility that vitamin D may reduce the risk of breast cancer by interacting with vitamin D receptors (which occur in at least two different genetic "types") in the breast and other tissues. Specifically, her goal was to develop genetic "markers" that could be used to measure vitamin D receptor types, and to determine whether women with specific vitamin D receptor types are relatively protected against breast cancer. Results indicate that nearly a quarter of women in California may be at increased risk of breast cancer, either because they have inherited two copies of a non–protective–type vitamin D receptor gene or because they have vitamin D deficiency. Therefore, development of effective interventions targeted to these at–risk women could significantly impact breast cancer incidence or mortality in California. Furthermore, she found a third vitamin D receptor genetic type and found that women who inherited this third type from both parents appear to have the highest levels of vitamin D in the blood. These results, however, need to be confirmed in larger studies.
Hormones and Nutrition: Understanding the Modern Woman's Lifestyle
Anna H. Wu, Ph.D. at the University of Southern California was funded to investigate the causes of Asian–American's increased breast cancer rates as compared to the rates of breast cancer in Asia, by testing the hypothesis that a diet rich in soy products reduces the risk of breast cancer. Preliminary analysis shows that the mean intake of soy (in milligrams of isoflavones per day) among Asian–American women without breast cancer in Los Angeles was one–fifth to one–half of that reported in recent Asian studies, with the highest intake of soy among the Chinese. In case–control comparisons, her findings showed a 30% reduction in risk in association with the highest level of isoflavone intake after adjustment for other factors that might also affect risk. When risk patterns in relation to intake of individual soy foods were examined, statistically significant reductions in risk were observed in association with the highest intake level of fresh tofu, eaten alone or in mixed dishes.
Breast cancer risk has been hypothesized to increase with exposure to carcinogens formed when meat is cooked at high temperature. Ralph Delfino, M.D., Ph.D. at the University of California, Irvine examined the role of certain of these compounds (HAAs, heterocyclic aromatic amines). HAAs require an enzyme produced by the body to become potentially cancer causing. The rate of this enzyme's activity is determined by a gene called NAT2. Dr. Delfino and his group examined the effect of variations of the NAT2 gene on breast cancer risk from exposure to meat comparing cooking method, doneness, and estimated intake of three important HAAs. There were no significant associations of breast cancer with red meat for any doneness category. However, white meat seemed to be significantly protective. Women eating over 67 grams per day had half the risk of breast cancer as compared with women eating less than 26 grams per day. The results were also similar for well–done pan–fried or barbecued chicken, which is known to have higher concentrations of HAAs. Therefore, these findings do not support a role for HAAs from meat or NAT2 in the etiology of breast cancer. Results are still preliminary, and further research is needed to explain the apparent protective effect of white meat intake.
PhIP is a cancer–causing agent that can be generated by cooked meats and has been show to cause breast cancer in rats. James Felton, Ph.D. of Lawrence Livermore National Laboratory completed a study investigating this compound entitled "Linking a Dietary Carcinogen to Breast Cancer Susceptibility". PhIP is broken down by the body into different chemicals (metabolites), which have different abilities to cause breast cancer. The ultimate goal of this project was to determine the PhIP metabolite profile that is associated with breast cancer susceptibility, and thereby generate a measure for determining a woman's relative risk of developing breast cancer. Dr. Felton developed a method to detect PhIP metabolites in human urine. Although he was unable to identify a specific profile for susceptibility, Dr. Felton was able to establish that individual metabolism can affect the amounts and rates of carcinogen excretion and that other dietary components (such as fruits and vegetables) can modulate it.
Heather Feigelson, Ph.D., M.P.H. of the University of Southern California expanded her earlier work on two genes, CYP17 and HSD17B1, involved in the synthesis of estrogens and plasma levels of hormones in healthy, postmenopausal women. (Estrogens and other hormones play an important role in breast cancer.) This work indicated that a certain form of CYP17 is associated with a 2.5 times increased risk for advanced breast cancer. Preliminary evidence suggested that both CYP17and HSD17B1 influence body hormone levels. On average, levels of androstenedione and estrone are 5–9% higher in women with the "higher risk" type of either of these two genes (results did not reach statistical significance due to small number of samples). CYP17 also influences a modifiable breast cancer risk factor: depending on CYP17 genotype, some women were twice as likely to be current users of hormone replacement therapy (HRT), suggesting that CYP 17 genotype may be an important piece of information for women seeking advice on HRT use, and that the actual risk of breast cancer associated with HRT use may be higher than previously reported. These results were consistent across all four ethnic groups (African–American, Japanese, Latina and White) included in the study.
Ann M. Geiger, Ph.D. of the Kaiser Foundation Research Institute investigated the possible relationships between commonly understood breast cancer risk factors and the presence of structures (receptors) in breast tumors that bind to the hormones estrogen and progesterone. These hormones can enter a tumor and influence the tumor's growth. She looked at the records of 2,441 women with breast cancer in a large HMO whose tumors had positive test results for estrogen and/or progesterone receptors. She then obtained information from the women on their risk factors for breast cancer. Analysis to date is focussing on postmenopausal women and has found that, for women with breast cancer, the presence of estrogen receptors on the tumor is less common in blacks and Hispanics than in white women; and more common in women whose mothers had breast cancer than in women whose mothers did not. Another preliminary analysis suggests that women who have used hormone replacement therapy are diagnosed with cancers at an earlier stage.
Other Searches for the Causes
Thomas Balon, Ph.D. of City of Hope National Medical Center completed his IDEA grant entitled “Nitric Oxide's Role in Breast Cancer Metabolism and Growth”. Dr. Balon's goal was to determine whether chemicals that regulate the cell's ability to take up and metabolize fuel behave differently in normal and tumor breast cells and whether any differential regulation is influenced by estrogen. Nitric oxide (NO) is a chemical that is intimately involved in the transport of sugars (the source of energy) into the cell. Breast cells are able to produce nitric oxide, but when this production is inhibited, there is no effect on the growth of normal breast cells, hormone–responsive breast cancer cells or non–hormone responsive breast cells. However, Dr. Balon found that if he exposed the different cell lines to compounds that caused the cells to release nitrous oxide into the cell's surrounding environment, there was a differential effect. The normal breast cells and hormone responsive breast cancer cells did not respond, whereas the growth of the non–hormone–responsive cancer cell lines decreased. In the future, Dr. Balon plans to make compounds that will take advantage of this differential effect and inhibit the growth of tumors cells while not affecting the growth of normal cells.
Although it has been hypothesized that lesbians are at greater risk of breast cancer than heterosexual women are; no studies have yet been reported comparing breast cancer risk factors among these groups. Suzanne L. Dibble, D.N. Sc. of the University of California, San Francisco and Stephanie Roberts, M.D. of the Lyon–Martin Women's Health Services undertook such a comparison among women, aged 35 or older, seen at Lyon–Martin. There were no significant differences between lesbian and heterosexual women in occurrence of family history of breast cancer, current alcohol usage or history of alcohol problems, age at menarche and menopause, or use of hormone replacement therapy. However, lesbians reported more breast biopsies, less use of birth control pills, higher body mass index, fewer pregnancies and fewer biological children. These results suggest that, since lesbians have a higher rate of some known breast cancer risk factors, they may be at higher risk of breast cancer. Future studies are needed, and if these differences are replicated in a larger study, the next step will be to inform both lesbians and their health care providers of the potential breast cancer risks. Studies are also needed to find the best ways to reach lesbians for breast cancer screening.
Ronald Ross, M.D. at the USC/Norris Breast Cancer Research Center was funded to implement a formal, focused, interdisciplinary graduate research training program devoted to breast cancer research. The goal was to train graduate students in research to better understand the reasons for the underlying differences in breast cancer incidence, mortality and survival among the numerically most important racial–ethnic groups comprising Los Angeles County, the most populous and ethnically diverse County in the United States. The program is multi–faceted and involves epidemiologists and prevention scientists, behavioral scientists, tumor biologists, and molecular geneticists, and radiation, surgical and medical oncologists. The training program has been highly successful. Five trainees have been supported from among 10 highly qualified applicants each year, after careful review and scoring by senior scientists at the Center. Trainees are highly multi–disciplinary in their interests, which include pathology, molecular biology, cell biology, and cancer control. Trainees have been matched to an appropriate faculty mentor with an active breast cancer research program. This work is continuing through another three years of funding begun in June of 1998.
Research In Progress
Environment and gene/environment Interactions: Nature and Nurture
Deirdre Hill, Ph.D. of the University of Southern California has been funded to investigate several factors that may be related to breast cancer risk among women 40 and younger and has completed work on the first research question: do women who received diagnostic X rays or radiation therapy prior to age 20 have an increased risk of developing breast cancer at age 40 or younger? Focusing on women with a family history of breast cancer or a personal history of benign breast disease (some studies suggest a greater susceptibility to ionizing radiation among these women), they found that breast cancer risk was elevated following medical radiation received prior to age 20 primarily among women with benign breast disease. Women who reported benign breast disease, and who received moderate radiation exposure prior to age 20, had a breast cancer risk that was 2.1 times greater than women who reported neither.
The Bovine Leukemia Virus (BLV) provides an interesting possible cause of breast cancer. The virus is present in cow's milk, but additional research needs to be performed in order to determine whether humans are actually exposed to the virus. Gertrude Buehring, Ph.D. of the University of California, Berkeley has found evidence of the presence of the virus protein in a proportion of breast tissues as well as an antibody response to the virus. This indicates that humans are indeed exposed to BLV.
Hormones and Nutrition: Understanding the Modern Woman's Lifestyle
While the exact causes of breast cancer remain unknown, certain aspects of a woman's life appear to increase her chances of breast cancer. A woman who has her first menstrual period at an early age is more likely to be diagnosed with breast cancer than a woman who has her first menstrual period at a later age. Carol Koprowski, Ph.D. of the University of Southern California analyzed data previously gathered on a total of 751 girls (4th through 7th grade) who provided information on diet and physical activity at least one time prior to experiencing their first menstrual periods. The results of this study indicated that girls who spent more hours in physical activity (13 or more hours per week) were more likely to have their first menstrual periods at later ages when compared to girls who spent fewer hours performing physical activity (less than five hours per week). Current analyses are examining the relationship between diet, physical activity and ovulatory status of a menstrual cycle (i.e., whether an egg is released during the menstrual cycle).
Recently Initiated Research
Eight projects funded in 1999 focus on the causes of breast cancer. Given the evidence that there is a strong association between lifetime exposure to estrogen and increasing breast cancer risk, it is not surprising that six of the eight grants funded under this priority issue are aimed at understanding the role of estrogen, though their approaches differ widely. Leslie Bernstein, Ph.D. of the University of Southern California will investigate leptin, a newly discovered hormone produced by fat cells. Large amounts of adipose (fat) tissue in obese women increase circulating estrone (a form of estrogen) levels in proportion to the amount of adipose tissue. Leptin could be a factor in breast cancer etiology because of the important role that it plays in fat storage and metabolism. Donna–Williams Hill, Ph.D. of the University of Southern California will undertake to isolate the mechanisms by which exercise produces protective factor(s) against breast cancer. Many studies have been conducted, but it is not yet known what types, duration, intensity of exercise is likely to be best and for whom. By using a rat mammary model, Dr. Hill will be able to control for several of the factors (confounding variables) which to date have made interpretation of exercise studies in humans difficult. Brian Henderson, M.D., also of the University of Southern California will look at several molecules, called insulin–like growth factors or IGFs, that collaborate in stimulating cell division by mediating the effect of estrogen. He will analyze blood samples from postmenopausal African–American, Japanese, Latina, and non–Latina White women to determine if circulating levels of IGFs and their binding proteins differ by race or ethnicity. His results will enlarge those of a previous study by a colleague showing strong positive association between IGF–1 and breast cancer in a small group of premenopausal White women. Vicki Davis, Ph.D. of Cedars–Sinai Medical Center will investigate whether it is possible to obtain protective effects against breast cancer by inhibiting estrogen action in the breast, while maintaining the important beneficial effects of estrogen such as protecting against heart disease and osteoporosis, by looking closely at a natural variant of the estrogen receptor on cells called estrogen receptor alpha.
There is strong evidence that certain dietary patterns are associated with lower breast cancer risk. Most notable of these is a diet rich in soy products which seems to lessen the impact of estrogen on breast cells. Anna H. Wu, Ph.D. of the University of Southern California will conduct a case–control study to investigate soy–estrogen metabolizing gene interaction, specifically variants of the CYP17 and COMT genes. This large study (which will result in a sample size of approximately 1300 cancer cases) is necessary to sort out the complex and interrelated lifestyle/ environmental and genetic factors in breast cancer development.
Ann Hamilton, Ph.D. also of the University of Southern California will probe the genetic basis of estrogen's role in breast cancer from yet another direction by looking at pairs of identical twins. Using tissue from 200 pairs of identical twins, both with breast cancer, 200 pairs, one with breast cancer, and 100 pairs, neither with breast cancer, she will compare the frequency of one genetic polymorphism among these groups, called the A2 allele on the CYP 17 gene, which has been shown to be related to higher levels of estrogen.
Two studies undertake other etiologic investigations: adolescent risk factor identification, and testing hypotheses about a mechanism by which cell growth may become uncontrolled. Georgianna Farren of Marin Breast Cancer Watch and Margaret Wrensch, Ph.D. of the University of California, San Francisco (co–PIs), heading a community–university collaboration, will investigate possible risk factors encountered by adult women residents of Marin County during their adolescence. An important methodological innovation is the development of survey instruments that may prove very useful in overcoming one of the most intractable barriers to such research, namely the accuracy and completeness of recall by women of events which occurred decades previously. Another study will investigate the role of the enzyme ornithine decarboxylase. Based on previous research, it appears that ornithine decarboxylase exists in rapidly growing and cancerous tissue at high levels. Craig Byus, Ph.D. of the University of California, Riverside will attempt to identify in model cell lines of human mammary tissue and carcinomas where there are genes that are turned on and off when ornithine decarboxylase is present at these high levels, as a preparatory step to understanding their function.
