Seeking toxic causes of breast cancer

[Guest guest]

Seeking toxic causes of breast cancer

Effort to find source of Bay Area's unusually high rate

 

Jane Kay, Chronicle Environment Writer

 

Monday, November 8, 2004

http://www.sfgate.com/cgi-bin/article.cgi?f=/chronicle/archive/2004/11/08/MNG4K9\

NRUB1.DTL & type=health

 

One of the most poignant questions that doctors hear is one that they

often can't answer: Why did I get breast cancer?

 

The variations are no easier: Why did my wife get it? My mother? My

daughter?

 

And more and more, doctors are struggling to answer another disturbing

question: Do exposures to toxic substances in the environment cause or

contribute to breast cancer?

 

The question is particularly crucial in the Bay Area, where white, non-

Latino women have breast cancer rates among the highest on the globe,

according to the World Health Organization.

 

While scientists can tick off several lifestyle and genetic factors that

appear to increase the risk of breast cancer in about 50 percent of the

211, 000 cases diagnosed in a year in the United States, they can only

speculate about the rest.

 

" We know enough to explain about half the causes,'' said Aaron Blair,

senior investigator in the National Cancer Institute's division of

cancer epidemiology and genetics. But that's the best they can do right

now, he said.

 

One of the known factors is an inherited predisposition -- including

where a woman carries one of two known mutations, BRCA1 or BRCA2.

Genetic predispositions are believed to account for less than 10 percent

of the total.

 

The known lifestyle factors -- which are probably shared by many women

in San Francisco and Marin, the Bay Area counties with the highest

breast cancer rates -- account for about 40 percent of the cases.

Several factors in a woman's reproductive life may increase risk:

starting menstruation before age 12, having no children or delaying

childbirth until after age 30, and reaching menopause after age 55.

 

These factors bear out that a prolonged exposure to estrogen, even one's

own, increases the risk of breast cancer, scientists say.

 

But for the other half of the cases -- those not connected to genetic or

reproductive factors -- the links are not so clear.

 

Exposure to radiation is well-documented as an environmental risk,

particularly X-rays at a young age. Consuming more than one or two

alcoholic drinks a day, and a poor diet of high fats and low fruits and

vegetables are fairly well documented as risks.

 

What's least understood are the risks presented by toxic chemicals that

may be present in the air, water, food and the workplace, scientists say.

 

One clue in particular, however, leads some scientists to believe that

there is a link between the rates of breast cancer and the widespread

use of man-made chemicals: The highest rates of breast cancer are found

in the industrialized nations of North America and northern Europe,

while the lowest rates are in the least developed nations of Asia and

Central Africa.

 

According to the National Toxicology Program at the U.S. Department of

Health and Human Services, there are 52 chemicals known to cause cancer

of all kinds in humans and 176 chemicals reasonably anticipated to be

human carcinogens. Lung and mammary cancers are the most common forms of

cancers that arise in animal studies looking at potential carcinogens.

 

" We know that mammary cancer occurs in lab animals, and that makes us

think maybe we ought to worry about humans,'' said Blair.

 

The search for the causes of breast cancer is made more difficult by the

complex nature of the disease. Researchers find it hard to design

studies that isolate a single culprit because so many elements are

intertwined: a woman's life history of exposure to toxic chemicals in

the environment and radiation, as well as exposure to her own natural

hormones.

 

After years of research, scientists generally agree that breast cancer

can arise from a variety of conditions -- genetic mutation, or damage to

the DNA; altered gene expression, or changing the signal that the gene

gives the body for its development; altered cell interaction, or

modifying how the cells message one another; and altering the body's

natural production of estrogen and other hormones.

 

" Depending on the individual, cancer might develop after just two

exposures, perhaps after dozens more, or may not develop at all,''

according to the third edition of an annual report that looks at the

connection between the environment and breast cancer, released last

month by the Breast Cancer Fund in San Francisco, an advocacy group

formed to encourage research on the subject.

 

" One exposure might occur prenatally, another during childhood and a

third during adolescence. Each of these exposures increases the risk of

breast cancer in later life,'' said the unpublished report, which was

reviewed by several leading cancer researchers.

 

There are a host of chemicals common in the environment that are

suspected of causing breast cancer and other serious illnesses, the

report said.

 

They include the pesticides endosulfan, toxaphene and dieldrin; the

solvents trichloroethylene, toluene and benzene; ingredients in

commercial products, including bisphenol-A in hard, clear plastic water

bottles and baby bottles, and polyvinyl chloride in food packaging,

medical products, appliances, cars and toys. Another is the group of

polycyclic aromatic hydrocarbons, or PAHs, found in charbroiled meat and

released in car exhaust and from cigarettes and burning fuel.

 

The report cites mounting evidence against excessive radiation exposure,

particularly to children and young adults from CT scans. Some

researchers say women who received radiation for treatment in the chest

and breast area for tuberculosis and other diseases as children are at

greater risk for breast cancer, and caution that the mammograms in the

early 1970s delivered a dose 10 times the current level.

 

In recent years, clinical trials and epidemiological and lab studies

have shown the risk of developing breast cancer from chemicals that

behave like estrogens -- the synthetic estrogens, or xenoestrogens, the

report said.

 

Certain industrial chemicals, many of which contain chlorine, begin to

promote tumors in test animals only in the presence of estrogen -- in

female animals, for example -- the report said.

 

Nancy Evans, a health consultant to the Breast Cancer Fund, said,

" Studies tell us that too much estrogen, either your own estrogen or

chemicals that act like estrogens, will cause the cells to proliferate.''

 

Scientists have found human evidence that bears out what is seen in

animal studies.

 

Pregnant women who were given a synthetic form of estrogen, DES, or

diethylstilbestrol, from the early 1940s to 1971 may have a slightly

higher risk for breast cancer. Their daughters have twice the risk of

breast cancer of unexposed women.

 

The recent Women's Health Initiative and the Million Women Study show

that estrogens prescribed after menopause increase the risk of breast

cancer.

 

There are many unknowns, and the answers lie in more research,

scientists agree. For example, U.S. health officials don't know how many

chemicals and which ones behave like estrogens. With some 80,000

chemicals in the marketplace, only 10 percent have been tested for human

health effects.

 

As it stands, only $1 out of every $9 of breast cancer research money

goes to studying environmental links to the disease, Evans said.

 

Last month, the National Institute of Environmental Health Sciences -- a

division of the National Institutes of Health -- announced an ambitious

study of the genetic and environmental causes of breast cancer. It will

enroll 50,000 sisters of women who already have been diagnosed with the

disease in order to better evaluate the factors that make women

susceptible to the cancer.

 

Christopher Portier, associate director of the National Toxicology

Program, said the government's mission for the next five years is to

" fill in the gaps.''

 

" You can think of our knowledge base as being like Swiss cheese. ...

Public health science needs a brick wall. We need to build that wall. If

we see the mechanism (for producing tumors), we can presume the disease

and ban the compounds.''

 

E-mail Jane Kay at jkay.