Biology 207Spring 2004

Biology of Cancer

Lectures 4 and 5: Cancer and Environment: Chemical Carcinogens, Radiation

Readings: King Chaps. 4 and 7, Reserve reading, Carson, "One in Every Four, Reserve reading, Proctor, "Nuclear Nemesis"

Lecture Outline:

1. Epidemiology

2. How chemicals cause cancer

3. Testing for carcinogenicity

4. Effect of radiation on DNA

5. Radiation and cancer

Lecture Notes:

1. Epidemiology

Epidemiology=Study of diseases in human populations.

Observations based on epidemiology:

a. Individuals from the same ethnic or cultural background living in different countries have different incidences of cancer.

Fig. 4.2

b. Different cancers may be caused in different ways. Chemical exposure is a common cause of many cancers.

Table 4.3, Table 4.8

c. Cigarette smoking is a major factor in lung and other cancers.

Table 4.4

Fig. 4.8

d. High fat diet associated with some cancers.

Fig. 4.10 (breast cancer)

2. Chemical Carcinogens

  • Rachel Carson, a biologist and science writer, first brought the problem of environmental pollution into public view.
  • A chapter of her famous book Silent Spring is devoted to cancer.
  • In 1962, one in every four persons was affected by cancer in their lifetimes. Now one in three persons gets cancer.
  • Rachel was actually fighting breast cancer as she wrote Silent Spring.
  • Key points:
  • Industrial pollution and practices can increase cancer risk
  • Pesticides may be major contributors to cancer.
  • Combinations of chemicals have unknown effects.
  • "Human exposures to cancer producing chemicals (including pesticides) are uncontrolled and they are multiple." (p. 237 Silent Spring)

Carcinogen: Agent that promotes formation of cancer.

Carcinogens work in two major ways:

  • genotoxic
  • change DNA sequenceleads to mutation
  • initiate formation of cancer
  • examples: Benzo(a)pyrene (forms bulky adduct)

aflatoxin in peanut butter (attaches to guanine base on DNA)

Fig. 7.3 Action of Benzopyrene

  • non-genotoxic
  • promote cell growth and division
  • cannot initiate cancer, but can accelerate cancer in abnormal cells
  • examples: estrogen (female sex hormone),

xenoestrogen (estrogen mimic like the pesticide DDT)

3. Testing for carcinogenicity

  • Need to establish link between cause and effect

Approaches:

A. Epidemiology

  • Compare individuals exposed to those not exposed
  • Establish correlation and/or carry out statistical tests
  • Difficult to go back in time for data

B. Animal studies:

  • Compare animals exposed to those not exposed
  • Can test different dosages
  • Can combine treatments
  • Mice/rats not the same as humans

4. Effect of radiation on DNA

Radiation is a form of energy.

High energy Low energy

Atomic particles> X-rays> UV light > visible light > infrared radiation> microwaves

  • Elements: materials that can't be broken down further
  • Radioactive elements: unstable elements that release energy

238uranium222radon (gas)218 or 214polonium207lead (stable)

226radium

ultraviolet radiation (UV) / causes formation of thymine dimer, leads to point mutation
ionizing radiation (radioactive elements, X-rays) / cause major changes to the DNA such as deletions, rearrangements

5. Radiation and cancer

High risk of cancer:

  • X-ray injuries
  • Acute = radiation sickness
  • Chronic = cancer
  • Radium
  • Discovery: Marie Curie, twice Nobel laureate, died of cancer
  • Radium dial painters
  • Atomic bomb
  • Fallout increased cancer risk by 5% in A-bomb survivors
  • Uranium ore miners at high risk for cancers, especially lung cancer
  • Nuclear power plant accidents
  • Worst accident occurred in former Soviet Union: Chernobyl
  • Acute radiation poisoning
  • Predicted increase in cancer risk in local area of Chernobyl ~2%
  • Worst US nuclear accident: Three Mile Island
  • No increased incidence in cancer around TMI
  • Ultraviolet rays (sun)
  • Increased risk of skin cancer
  • Sunburns increase risk

Low level sources of radiation with some cancer risk

  • X-rays
  • Radon
  • Medical tests
  • Medical research
  • Radiation treatment for cancer

Minimal cancer risk (data weak, incomplete or contradictory)

  • High voltage wires
  • Microwaves
  • Household appliances
  • Cell phones

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