HUMAN BIOLOGY

Genes and disease: CanCer 429

What triggers cancer?

• Cancer typically develops due to mutations in oncogenes or
  proto-oncogenes.


• Carcinogenesis usually also requires the absence or mutation of
  at least one tumor suppressor gene.


• Inherited susceptibility, viruses, chemical carcinogens, radiation,
  and weakened immunity all may trigger carcinogenesis.

taKe-Home messaGe

carcinogens Substances

that cause cancer by mutat-

ing DNA.

Viruses Viruses cause some cancers. For example, a
viral infection may switch on a proto-oncogene when
the viral DNA is inserted at a certain location in the host
cell’s DNA. Other viruses carry oncogenes as part of their
genetic material and insert them into the host’s DNA.

Chemical carcinogens Thousands of carcinogens—
cancer-causing substances—can lead to a mutation in
DNA. The list of known carcinogens includes many chemi-
cals that are by-products of industrial activities, such as
asbestos, vinyl chloride, and benzene. The list also includes
hydrocarbons in cigarette smoke and on the surfaces of
charred meats, and substances in dyes and pesticides.
Some plants and fungi also produce carcinogens. Afla-
toxin, which is produced by a fungus that attacks stored
grain, peanuts, and other seeds, causes liver cancer. For this
reason, some authorities advise against eating unprocessed
peanut butter.

radiation Section 18.5 noted that radiation can cause
cancer-related mutations in DNA. Common sources include
ultraviolet radiation from sunlight and tanning lamps,
medical and dental X-rays, and some radioactive materials
used to diagnose diseases. Other sources are radon gas in
soil and water, background radiation from cosmic rays, and
the gamma rays emitted from nuclear reactors and radioac-
tive wastes. Sun exposure is probably the greatest radiation
risk factor for most people (Figure 22.7).

Breakdowns in immunity When a normal cell turns
cancerous, altered proteins at its surface function like
foreign antigens—the “nonself” tags that mark a cell for
destruction by cytotoxic T cells and NK (natural killer)
cells. A healthy immune system can destroy some types
of cancer cells, but this protection wanes as a person ages.
This is why the risk of cancer rises with age.
A person’s cancer risk may rise whenever the immune
system is suppressed for a long time. In addition to factors
such as infection by HIV, anxiety and severe depression
can suppress immunity. So can some therapeutic drugs,
such as the glucocorticoids discussed in Section 15.7.

Figure 22.6 A common type of colorectal cancer may develop by these steps. (© Cengage Learning)

Normal

colon cells

Small adenoma

(benign growth)

Large adenoma

(benign growth)

Malignant tumor

with metastasis

Loss of a tumor

suppressor

gene and other

DNA changes

Oncogene

activated; loss

of second tumor

suppressor gene

Loss of a third

tumor suppres-

sor gene and

other mutations

Figure 22.7 Tanning can lead to malignant melanoma.

A Former Miss Maryland Brittany Lietz Cicala developed

malignant melanoma at age 20. Before her cancer was

diagnosed she had regularly used a tanning bed since the

age of 16. Since her diagnosis she has had more than twenty-

five tumors or precancerous lesions removed and must be

checked for new lesions every 3 months. B A scar on

Brittany’s back where a cancerous tumor was removed.

B

Nancy Lietz

A

AP Photo/

Herald-Mail

, Kevin G. Gilbert

Finally, for various reasons, the

cells of a growing cancer may not

trigger an immune response. When

this happens, the immune system is

“blind” to the cancer threat.

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