Everything You Need To Know About How Does Breast Cancer Starts
Changes or mutations in DNA can cause normal breast cells to become cancer. Certain DNA
changes are passed on from parents (inherited) and can greatly increase the
risk for breast cancer. Other lifestyle-related risk factors, such as what you eat and how much you exercise, can increase your chance of developing breast cancer, but it’s not yet known exactly how some of these risk factors cause normal cells to become cancer. Hormones seem to play a role in many cases of breast cancer, but just how this happens is not fully understood.
Inherited versus acquired DNA mutations
Normal breast cells become cancer because of changes (mutations) in DNA. DNA is the chemical in each of our cells that makes up our genes. Genes have the instructions for how our cells function. Some DNA mutations are inherited or passed to you from your parents. This means the mutations are in every cell in your body and certain mutations can greatly increase the risk of certain cancers. They cause many of the cancers that run in some families.
But most DNA changes linked to breast cancer are acquired. This means the change takes place in breast cells during a woman's life rather than having been inherited. Acquired DNA changes take place over time and are only in the breast cancer cells, not in every cell in the body.
Mutated DNA can lead to mutated genes. Some genes control when our cells grow, divide into new cells, and die. Changes in these genes are linked to cancer.
Genes that speed up cell division are called oncogenes.
Proto-oncogenes are genes that normally help cells grow. When a proto-oncogene mutates
(changes) or there are too many copies of it, it becomes a "bad" gene that can stay turned on or activated when it’s not supposed to be. When this happens, the cell grows out of control and makes more cells that grow out of control. This can lead to cancer. This bad gene is called an oncogene.
Think of a cell as a car. For the car to work properly, there need to be ways to control how fast it goes. A proto-oncogene normally functions in a way that’s much like a gas pedal. It helps control how and when the cell grows and divides. An oncogene is like a gas pedal that’s stuck down, which causes the cell to divide out of control.
Tumor suppression genes
Tumor suppressor genes are normal genes that slow down cell division, repair DNA mistakes, or tell cells when to die (a process known as apoptosis or programmed cell death). When tumor suppressor genes don't work properly, cells can grow out of control and make more cells that grow out of control, which can lead to cancer.
A tumor suppressor gene is like the brake pedal on a car. It normally keeps the cell from dividing too quickly, just as a brake keeps a car from going too fast. When something goes wrong with the gene, such as a mutation, the “brakes” don’t work and cell division can get out of control.
Inherited gene changes
Certain inherited DNA mutations (changes) can dramatically increase the risk for
developing certain cancers and are linked to many of the cancers that run in
some families. For instance, the BRCA genes (BRCA1 and BRCA2) are tumor suppressor genes. When one of these genes changes, it no longer suppresses abnormal cell growth, and cancer is more likely to develop. A change in one of these genes can be passed on from a parent.
Women have already begun to benefit from advances in understanding the genetic basis of
breast cancer. Genetic testing can identify some women who have inherited
mutations in the BRCA1 or BRCA2 tumor suppressor genes (or less commonly in other genes such as PTEN or TP53). These women can then take steps to reduce their risk of breast cancers and make plans to look for changes in their breasts to help find cancer at an earlier, more treatable stage.
Learn more in our breast cancer risk and prevention section.
Mutations in tumor suppressor genes like the BRCA genes are considered “high-penetrance” because they often lead to cancer. Although many women with high-penetrance mutations develop cancer, most cases of cancer (including breast cancer) are not caused by this kind of mutation.
More often, low-penetrance mutations or gene variations are a factor in cancer development.
Each of these may have a small effect on cancer occurring in any one person, but the overall effect on the population can be large because the mutations are common, and people often have more than one at the same time. The genes involved may affect things like hormone levels, metabolism, or other things that impact risk factors for breast cancer. These genes may cause much of the risk of breast cancer that runs in families.
Acquired gene changes
Most DNA mutations related to breast cancer take place in breast cells during a woman's
life rather than having been inherited. These acquired mutations of oncogenes and/or tumor suppressor genes may result from other factors, like radiation or cancer-causing chemicals. But so far, the causes of most acquired mutations that could lead to breast cancer are still unknown. Most breast cancers have several acquired gene mutations.
Tests to spot acquired gene changes may help doctors more accurately predict the outlook
(prognosis) for some women with breast cancer. For example, tests can identify women whose breast cancer cells have too many copies of the HER2 oncogene. These cancers tend to grow and spread faster. There are drugs that target these cancer cell changes and improve outcomes for patients.
Learn more about genes changes and how they can affect cancer risk and treatment in our
section on breast cancer risk and prevention.
There are some things that might be risk factors for breast cancer, but the research is not yet clear about whether they really affect breast cancer risk. They include things like tobacco smoke and working at night.
Diet and vitamins
While being overweight or obese and not being physically active have been linked to breast cancer risk, the possible link between diet and breast cancer risk is less clear. Results of some studies have shown that diet may play a role, while others have not found that diet influences breast cancer risk.
Most studies of women in the United States have not found a link between breast cancer risk and fat in the diet. Still, studies have found that breast cancer is less common in countries where the typical diet is low in total fat, low in polyunsaturated fat, and low in saturated fat. Researchers are still not sure how to explain this. It may be at least partly due to the effect of diet on body weight. Also, studies comparing diet and breast cancer risk in different countries are complicated by other differences (such as activity level, intake of other nutrients, and genetic factors) that might also affect breast cancer risk.
We do know that high-fat diets can lead to being overweight or obese, which is a known breast cancer risk factor. A diet high in fat is also a risk factor for some other types of cancer. And intake of certain types of fat is clearly related to higher risk of heart disease.
Studies looking at vitamin levels have had inconsistent results. So far, no study has shown that taking vitamins reduces the risk of breast cancer (or any other cancer). But this does not mean that there’s no point in eating a healthy diet. A diet low in fat, low in red meat and processed meat, and high in fruits and vegetables can clearly have other health benefits, including lowering the risk of some other cancers.
Chemicals in the environment
A great deal of research has been reported and more is being done to understand possible environmental influences on breast cancer risk.
Compounds in the environment that have estrogen-like properties are of special interest. For example, substances found in some plastics, certain cosmetics and personal care products, pesticides, and PCBs (polychlorinated biphenyls) seem to have such properties. In theory, these could affect breast cancer risk.
This issue causes a great deal of public concern, but at this time research does not show a clear link between breast cancer risk and exposure to these substances. Studying such effects in humans is hard to do. More research is needed to better define the possible health effects of these substances and others like them.
For a long time, studies showed no link between cigarette smoking and breast cancer. But in recent years, more studies have shown that heavy smoking over a long time might be linked to a higher risk of breast cancer. In some studies, the risk has been highest in certain groups, such as women who started smoking before they had their first child. The 2014 US Surgeon General’s report on smoking concluded that there is “suggestive but not sufficient” evidence that smoking increases the risk of breast cancer.
Researchers are also looking at whether secondhand smoke increases the risk of breast cancer. Both mainstream and secondhand smoke contain chemicals that, in high concentrations, cause breast cancer in rodents. Studies have shown that chemicals in tobacco smoke reach breast tissue and are found in breast milk of rodents. In human studies, the evidence on secondhand smoke and breast cancer risk is not clear. Most studies have not found a link, but some studies have suggested it may increase risk, particularly in premenopausal women. The 2014 US Surgeon General’s report concluded that there is “suggestive but not sufficient” evidence of a link at this point. In any case, this possible link to breast cancer is yet another reason to avoid secondhand smoke.
Night shift work
Some studies have suggested that women who work at night, such as nurses on a night shift, might have an increased risk of breast cancer. This is a fairly recent finding, and more studies are looking at this. Some researchers think the effect may be due to changes in levels of melatonin, a hormone that’s affected by the body’s exposure to light, but other hormones are also being studied. Read More