Thursday July 5 2012
Breast cancer risk has been 'linked to larger breast size'
“Women with bigger breasts do have higher risk of breast cancer,” the Daily Mail reports. The Mail goes on to say that this may be due to the effect of oestrogen on both breast size and tumour development.
The eye-catching, yet somewhat misleading, headline oversimplifies research that examined the genetic factors underlying breast development, and identified specific variations in genes that were associated with breast size. It compared these variants to several genetic patterns that are risk factors for breast cancer. Of the seven variants identified as being associated with breast size, three were also associated with an increased risk of breast cancer.
Breast cancer is a complex condition that is linked with multiple risk factors, such as age, obesity and oestrogen levels. It is unclear how these factors interact, and what role the genetic variations identified in this study may play in the development of breast cancer.
This study does not support headlines claiming that women with larger breasts have a higher risk of breast cancer. It can only tell us that some of the genes associated with breast size are also associated with breast cancer. It does not tell us whether these genetic variations translate into increased rates of the condition among women with large breasts.
Where did the story come from?
The study was carried out by researchers from the company 23andMe, a genetics company based in the US. The funding source was not reported in the newspapers or by the journal. The company provides genetic testing services. All study participants were customers of 23andMe, and the study authors were employees and stakeholders in the company.
The study was published in the peer-reviewed journal BioMed Central Medical Genetics, and its publication was accompanied by a press release from 23andMe.
This research does not directly support the headlines claiming that women with bigger breasts have a higher risk of breast cancer. Because risk of breast cancer itself was not directly studied, the research can only provide information on genes that are associated with both breast size and breast cancer. While the Daily Mail headline claims an increase in risk among women with bigger breasts, the article itself is a more appropriate review of the research, including cautions that more research is needed before the results “could be considered concrete”. The Daily Telegraph carries a similar report.
What kind of research was this?
This was a genome-wide association study. It examined the association between breast size and small variations in the DNA of 16,175 women. This type of study examines variations in the genomes of a large group of people to determine whether any variations are linked to specific traits.
Genome-wide association studies can provide useful information on the genetic features underlying certain conditions, but cannot tell us whether people with these genome variations will go on to develop the condition. There are multiple risk factors for breast cancer, ranging from genetic to environmental to lifestyle factors. This study cannot tell us how these factors interact to increase the risk of developing breast cancer.
What did the research involve?
Researchers recruited 16,175 women (all customers of 23andme) and mapped their genomes. Breast size was determined through an online questionnaire, which included questions on bra size. The questionnaire also collected information on factors that may affect reported breast size, including bra band size, which was used as an indicator of body size.
The women were grouped into 10 categories based on bra cup size (ranging from smaller than AAA to larger than DDD), and the researchers identified genome regions that were associated with differences in breast size. They then compared these regions of the genome to those that are known to be associated with an increased risk of breast cancer. The researchers conducted a secondary analysis of 29 genetic variations that have previously been found to be associated with breast cancer. They then determined whether they were also associated with breast size in the study group.
During the data analysis, the researchers controlled for possible confounding factors, including age, genetic ancestry, previous breast surgeries, previous or current pregnancy, breastfeeding status and body size.
What were the basic results?
The researchers identified seven unique variations in the women’s genomes that were significantly associated with breast size. Two of these were also associated with breast cancer.
In the second analysis, the researchers found that one of the 29 genetic variations associated with breast cancer had a possible association with breast size, but that this association did not reach statistical significance.
How did the researchers interpret the results?
The researchers conclude that their study “identified genetic variations that have an effect on both breast cancer and natural variation in breast size”.
This study suggests that there are genetic variations that are linked to both breast size and breast cancer. It does not show, however, that larger breast size increases a woman’s risk of developing breast cancer.
Genome-wide association studies can be useful in identifying genetic factors that may play a part in whether or not someone develops a condition. This type of study is only an initial step, however, and further research would be needed to confirm a plausible biological mechanism accounting for the association between a genetic variant and the development of a specific trait. Yet more studies would then be needed to work out if this association translates into an increase in cases among individuals with the variation.
This study has several limitations, chiefly that breast size, the main characteristic under investigation, may not have been measured accurately. Self-reported bra size may not accurately reflect the actual breast size of the participants. The researchers say that further research using more precise measures of breast size could help determine whether the genetic associations found in this study are truly linked to breast size.
Another limitation is that all the women who took part were white (defined as being of European ancestry) so the results may not necessarily translate to other ethnic groups.
Breast cancer is a complicated condition and there are multiple factors that increase a person’s risk of developing the cancer. These include:
- family history of breast cancer
- oestrogen levels
- whether a woman has been through the menopause
- history of breastfeeding
- body composition, such as obesity
- lifestyle factors
This study cannot tell us how the identified genetic variations interact with these risk factors. Importantly, major factors such as the participants’ weight and BMI are not reported.
Overall, this study provides more information about the genes that are associated with breast cancer, and how they are linked to genes associated with breast size. This information may be useful to researchers in identifying targets for further medical research, but has no direct impact on current breast cancer prevention or treatment efforts. Even if the link were more definite, there would be little that could be done to help women as, even if breast size were reduced through weight loss or surgery, this would not alter their genes.
This study cannot tell us whether or not breast size is a risk factor for breast cancer. Prospective cohort studies would be needed to establish that epidemiological link. The researchers who carried out the study were keen to stress that all women comply with the current recommendations for breast cancer screening whatever the size of their breasts.