Ever
marvel at someone who smoked and still lived to be 90? Just plain good
luck, researchers say. And those who live like Puritans and get cancer
anyway?
That's bad luck -- and it's the primary cause of most cancer cases, says a Johns Hopkins Medicine research study.
Roughly
two-thirds of cancers in adults can be attributed to random mutations
in genes capable of driving cancer growth, said two scientists who ran
statistics on cancer cases.
That may
sound jaw-dropping. And Johns Hopkins anticipates that the study will
change the way people think about cancer risk factors.
They
also believe it could lead to changes in the funding of cancer studies,
with a greater focus on finding ways to detect those cancers attributed
to random mutations in genes at early, curable stages.
Smoking can still kill you
But, no, that's not permission to smoke or to not use sunblock.
Some
forms of cancer are exceptions, where lifestyle and environment play a
big role. Lung cancer is one of them. So is skin cancer.
And,
if cancer runs in your family, this unfortunately doesn't mean you're
in the clear. Some cancers are more strongly influenced by genetic
heritage than others.
"The remaining third (of cancer cases) are due to environmental factors and inherited genes," the Kimmel Cancer Center said in a statement on the study published Friday in the magazine Science.
In fact, all three factors work together.
"All
cancers are caused by a combination of bad luck, the environment and
heredity, and we've created a model that may help quantify how much of
these three factors contribute to cancer development," said cancer
researcher Bert Vogelstein.
Compounding matters
An unhealthy lifestyle can compound matters, but more for some cancers than for others, the scientists said.
"Changing
our lifestyle and habits will be a huge help in preventing certain
cancers, but this may not be as effective for a variety of others,"
medical statistician Cristian Tomasetti said.
He placed heavy emphasis on early detection.
Stem
cells in our organs divide constantly to replenish damaged tissue.
Sometimes there are random mistakes in the replication of DNA, small
mutations, Vogelstein said.
Some genes, when they mutate, are more apt to promote cancer growth.
"The
more these mutations accumulate, the higher the risk that cells will
grow unchecked, a hallmark of cancer," Vogelstein said.
Scientist have known this for a long time, but what the study reveals was how big of an influence it is.
"The
actual contribution of these random mistakes to cancer incidence, in
comparison to the contribution of hereditary or environmental factors,
was not previously known," says Vogelstein.
Dr. Otis Brawley,
chief medical officer for the American Cancer Society, told CNN the
study was "good science" that backed up what many scientists already
thought.
"This is actually just confirmation of something that we have known for probably 20 years," he said.
"As
we have learned more and more about cancer ... we've come to realize
that a number of cancers start purely because of mutations that happen
that are just unexplainable. Bad luck is, unfortunately, the right way
to explain it."
Those cancers that
develop have escaped at least three fail-safe systems in the body that
deal with these cell mutations, he said; these are programmed cell
death, or apoptosis, DNA repair enzymes and certain cells in the immune
system.
Cell division and statistics
For
their study, the two scientists came up with an average total number of
cell divisions in 31 different tissues within a human lifetime. And
they looked at the cancer risk in each of those tissues.
They determined that the more a tissue's cells divide, the higher the chance cancer could develop in that tissue.
"Our
study shows, in general, that a change in the number of stem cell
divisions in a tissue type is highly correlated with a change in the
incidence of cancer in that same tissue," says Vogelstein.
Colon
tissue, for example, divides much more than other intestinal tissue,
and cancer in the colon is much more prevalent there, the study said.
With colon tissue, the scientists took environmental influences into account.
Doing
the math overall, the two scientists arrived a rate at which cancer
risk can be explained by the cell divisions. It was 65%, they said.
Lifestyle
But
the researchers drew a line between one group of cancers and another.
Of the 31 they looked at, they determined that 22 were basically "bad
luck" cancers.
But nine others appeared
at rates noticeably higher than could be expected from cell division
alone -- which the researchers said is probably due to habits, pollution
or genetics.
No surprise: Lung cancer
and skin cancer were two of them, they said. Smoking and too much sun
exposure are still strongly linked to those cancers.
Brawley said the study's findings should be no reason to alter behaviors shown to lessen the risk of cancer.
"We
have good epidemiological data to show that people can reduce their
risk of cancer and I would encourage them to do those things," he said.
They include not smoking, managing their weight so they don't become obese and taking physical exercise, he said.
As
for the suggestion the study's findings may prompt changes in funding,
Brawley said he would be pleased just to see more money go into
research.
"Only 10% of the grants
submitted to the NIH (National Institutes of Health) actually get funded
because we have such a shortage of money," he said.
"We invested in the United States last year $5 billion in cancer research. I would like to see more."
