What Causes Cancer?
There is no one single cause for cancer. Scientists believe that it
is the interaction of many factors together that produces cancer. The
factors involved may be genetic, environmental, or constitutional
characteristics of the individual.
Diagnosis, treatment, and prognosis for childhood cancers are
different than for adult cancers. The main differences are the
survival rate and the cause of the cancer. The overall five-year
survival rate for childhood cancer is about 80%, while in adult
cancers the survival rate is 68%. This difference is thought to be
because childhood cancer is more responsive to therapy and a child can
tolerate more aggressive therapy.
Childhood cancers often occur or begin in the stem cells, which are
simple cells capable of producing other types of specialized cells
that the body needs. A sporadic (occurs by chance) cell change or
mutation is usually what causes childhood cancer. In adults, the type
of cell that becomes cancerous is usually an epithelial cell.
Epithelial cells line the body cavity and cover the body surface.
Cancer occurs from environmental exposures to these cells over time.
Adult cancers are sometimes referred to as acquired for this reason.
Cancer Risk Factors
As mentioned, some cancers, particularly in adults, have been
associated with repetitive exposures or risk factors. A risk factor is
anything that may increase a person's chance of developing a disease.
A risk factor does not necessarily cause the disease, but it may make
the body less resistant to it. The following risk factors and
mechanisms have been proposed as contributing to cancer:
-
Lifestyle factors. Smoking, a high-fat diet, and working with
toxic chemicals are examples of lifestyle choices that may be risk
factors for some adult cancers. Most children with cancer, however,
are too young to have been exposed to these lifestyle factors for
any extended time.
-
Family history, inheritance, and genetics may play an important
role in some childhood cancers. It is possible for cancer of
varying forms to be present more than once in a family. It is
unknown in these circumstances if the disease is caused by a genetic
mutation, exposure to chemicals near a family's residence, a
combination of these factors, or simply coincidence.
-
Some genetic disorders. For example, Wiskott-Aldrich and
Beckwith-Wiedemann syndrome are known to alter the immune system.
The immune system is a complex system that functions to protect our
bodies from infection and disease. The bone marrow produces cells
that later mature and function as part of the immune system. One
theory suggests that the cells in the bone marrow, the stem cells,
become damaged or defective, so when they reproduce to make more
cells, they make abnormal cells or cancer cells. The cause of the
defect in the stem cells could be related to an inherited genetic
defect or exposure to a virus or toxin.
-
Exposures to certain viruses. Epstein-Barr virus and HIV, the
virus that causes AIDS, have been linked to an increased risk of
developing certain childhood cancers, such as Hodgkin and
non-Hodgkin lymphoma. Possibly, the virus alters a cell in some way.
That cell then reproduces an altered cell and, eventually, these
alterations become a cancer cell that reproduces more cancer
cells.
-
Environmental exposures. Pesticides, fertilizers, and power
lines have been researched for a direct link to childhood cancers.
There has been evidence of cancer occurring among nonrelated
children in certain neighborhoods and/or cities. Whether prenatal or
infant exposure to these agents causes cancer, or whether it is a
coincidence, is unknown.
-
Some forms of high-dose chemotherapy and radiation. In some
cases, children who have been exposed to these agents may develop a
second malignancy later in life. These strong anticancer agents can
alter cells and/or the immune system. A second malignancy is a
cancer that appears as a result from treatment of a different
cancer.
Cancer Genes
How do genes affect cancer growth?
The discovery of certain types of genes that contribute to cancer
has been an extremely important development for cancer research. Over
90% of cancers are observed to have some type of genetic alteration.
Some of these alterations are inherited, while others are sporadic,
which means they occur by chance or occur from environmental exposures
(usually over many years).
Types of cancer genes
There are three main types of genes that can affect cell growth and
are altered (mutated) in certain types of cancers, including the following:
-
Oncogenes: These genes regulate the normal growth of cells.
Scientists commonly describe oncogenes as similar to a cancer
"switch" that most people have in their bodies. What
"flips the switch" to make these oncogenes suddenly become
unable to control the normal growth of cells and allowing abnormal
cancer cells to begin to grow, is unknown.
-
Tumor suppressor genes: These genes are able to recognize
abnormal growth and reproduction of damaged cells, or cancer cells,
and can interrupt their reproduction until the defect is corrected.
If the tumor suppressor genes are mutated, however, and they do not
function properly, tumor growth may occur.
-
Mismatch-repair genes: These genes help recognize errors when
DNA is copied to make a new cell. If the DNA does not
"match" perfectly, these genes repair the mismatch and
correct the error. If these genes are not working properly, however,
errors in DNA can be transmitted to new cells, causing them to be
damaged.
Usually the number of cells in any of our body tissues is tightly
controlled so that new cells are made for normal growth and
development, as well as to replace dying cells. Ultimately, cancer is
a loss of this balance due to genetic alterations that "tip the
balance" in favor of excessive cell growth.