Washington, Jan 2 (IANS) The majority of cancer cases
can be explained by "bad luck" rather than the result of environmental
factors and inherited genes, a US study said Thursday.
The study,
published in the US journal Science, found that two-thirds of adult
cancer incidence across tissues might be caused by random mutations that
occur in dividing healthy stem cells.
The findings, based on a
statistical model that quantified how much of three factors -- bad luck,
the environment and heredity -- contribute to cancer development, might
help researchers design more effective prevention strategies for
different cancer types.
"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," Xinhua quoted co-author Cristian
Tomasetti, assistant professor of oncology at the Johns Hopkins
University School of Medicine and Bloomberg School of Public Health, as
saying. "We should focus more resources on finding ways to detect such
cancers at early, curable stages."
It was well-known that cancer
arises when tissue-specific stem cells make random mistakes, or
mutations, when one chemical letter in DNA is incorrectly swapped for
another during the replication process in cell division.
The more these mutations accumulate, the higher the risk that cells will grow unchecked, a hallmark of cancer.
The
actual contribution of these random mistakes to cancer incidence, in
comparison to the contribution of hereditary or environmental factors,
was not previously known.
In the new study, researchers analysed
published data on stem cell divisions in 31 different human tissues and
compared the data to the lifetime incidence of cancer in those tissues.
They determined the correlation between the total number of stem cell divisions and cancer risk to be 0.804.
"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," said co-author Bert
Vogelstein, professor of oncology at the Johns Hopkins University School
of Medicine.
One example, he said, is in colon tissue, which
undergoes four times more stem cell divisions than small intestine
tissue in humans. Likewise, colon cancer is much more prevalent than
small intestinal cancer.
Mice, by contrast, have a lower number
of stem cell divisions in their colons than in their small intestines.
Similarly, cancer incidence in mice is lower in the colon than in the
small intestine.
Using statistical theory, the pair calculated
how much of the variation in cancer risk can be explained by the number
of stem cell divisions, which is 0.804 squared, or, in percentage form,
about 65 percent.
Further, research found that 22 cancer types in
31 tissues could be largely explained by the "bad luck" factor of
random DNA mutations during cell division.
The other nine cancer
types had incidences higher than predicted by "bad luck" and were
presumably due to a combination of bad luck plus environmental or
inherited factors, they said.
"We found that the types of cancer
that had higher risk than predicted by the number of stem cell divisions
were precisely the ones you'd expect, including lung cancer, which is
linked to smoking; skin cancer, linked to sun exposure; and forms of
cancers associated with hereditary syndromes," said Vogelstein.
He
claimed that cancer-free longevity in people exposed to cancer-causing
agents, such as tobacco, may not be due to their "good genes".
"The
truth is that most of them simply had good luck," said Vogelstein,
cautioning that poor lifestyles can add to the bad luck factor in the
development of cancer.
"However, many forms of cancer are due
largely to the bad luck of acquiring a mutation in a cancer driver gene
regardless of lifestyle and heredity factors," he said. "The best way to
eradicate these cancers will be through early detection, when they are
still curable by surgery."
The researchers noted that some
cancers, such as breast and prostate cancer, were not included in the
study because of their inability to find reliable stem cell division
rates in the scientific literature.