Why is breast cancer common but heart cancer rare?
Malignant cancers strike certain organs, such as the colon or breast,
more often than others.
In an Opinion publishing in Trends in Cancer, researchers propose
that this vulnerability in some organs may be due to natural selection.
Humans can tolerate tumors in large or paired organs more easily than in
small, critical organs, such as the heart. Therefore, the larger organs
may have evolved fewer mechanisms to defend against cancerous cells.
"The organs that are the most important to keeping you alive and
capable of reproduction, such as the heart, brain, or uterus, may enjoy
a better protection against cancer, all other things being equal," says
Frédéric Thomas, an evolutionary biologist at the Center for Ecological
and Evolutionary Cancer Research in France. "We are not saying that this
is the main factor to explain the different susceptibility of organs to
cancer, but it is a factor that contributes with others." Many
oncologists have explained the difference in rates of organ cancer by
looking at either external risk factors, such as smoking or UV light
exposure, or internal factors, such as how often cells must divide in an
organ. Thomas and his coauthors, including senior author Beata Ujvari,
an evolutionary ecologist at Deakin University in Australia, now propose
this evolutionary theory to supplement the current understanding.
The team suggests that natural selection has favored strong
anti-cancer protection for small organs that are critical to human
survival and reproduction. "Organs that are large or in pairs could
potentially accumulate larger numbers of oncogenic manifestations
without being impaired, whereas small and important organs like the
pancreas could be easily compromised with only a few tumors inside,"
says Thomas. Therefore, so the theory goes, the pancreas should be
better at defending against cancer compared to an organ like the kidney,
if all other factors are equal. Anti-cancer protection mechanisms vary
from organ to organ, but in general, they make an organ resistant to
The researchers also recommend that cancer biologists think of
individual organs as specialized islands with their own environmental
conditions (such as the level of oxygen, acidity, or water), where the
survival of cancer cells depends on is the hospitality of the local
environment. "Malignant cells are living entities - it's just impossible
that they are not influenced by the ecological conditions," says Thomas.
"It clearly means that certain organs are more favorable than others to
Thomas, Ujvari, and their colleagues are now working to test their
hypothesis. "A complete analysis requires that we take into account all
the possible confounding factors," emphasizes Thomas. "We cannot just
look at existing statistics on cancer and the size of the organs and
make a correlation to see if it works or not." Currently, the team is
running a long-term experiment with mice to measure the accumulation of
cancerous and precancerous mutations inside different organs. The
research is part of a new international collaboration between Deakin
University and the French National Center for Scientific Research (CNRS).