BIO372

Cancer

TERMS

GLOSSARY

Precursor DNA changes: One of the hypotheses of the origin of cancer is that the evolution of a normal cell into a tumor cell depends on-
  1. A mutation that makes the normal cells susceptible to further genetic change.
  2. A second mutation that transforms the mutant susceptible cell into a tumor cell.

Cancer: An abnormal and unrestrained new growth (neoplasm) in cells and tissues that produces deleterious and often fatal effects. Cancerous cells grow more rapidly than normal, assume abnormal shapes and sizes and cease functioning in a normal manner. The distinction of cancerous tissues is the partial or complete loss of growth restraint in cancer cells and their failure to differentiate into a useful, limited tissue of the type that characterizes the functional equilibrium of growth of normal tissue. Cancerous lesions probably do not grow independently, but are influenced by the host's susceptibility and immunity. For example, cancers of the human reproductive system are considered dependent on specific hormones. Other cancers may be dependent on viruses.

Metastasis: Cancerous cells break off from the original growth and are carried in the blood or lymph systems to distant parts of the body where they begin new growths.

Carcinoma: A cancer of epithelial tissue.

Sarcoma: A cancer of tissues of the structural framework of the body, e.g., muscle, fat, connective tissue, bone, etc.

Primary cancer: A cancer which originated in the tissue, rather than having been established as a secondary site through metastasis from another lesion.

Racial association of cancer: Some cancers seem to occur with relatively high frequency in certain races -

  1. Cancer of nasopharynx - Chinese, much more frequent in males
  2. Cancer of the liver - Malay groups & west African Bantu - may result from severe nutritional deficiencies
  3. Kaposi's hemorrhagic sarcoma of the skin - Chinese, Jews, and Bantu peoples in the Mediterranean basin, almost exclusively in males
  4. Cancer of the stomach - Japanese and Icelanders, 60% occurrence in males

Cancers selectively rare: Some cancers are present in the world population at large, but are extremely rare in certain groups -

  1. Cancer of the breast - seldom in women who nurse their children
  2. Malignant melanoma (which develops from preexisting moles) - rarely in Negroes
  3. Cancer of the cervix - extremely rare in Jewish women
  4. Cancer of the penis - infrequent in peoples who practice circumcision in childhood

Cancer incidence and age: Some cancers occur at different rates in different age groups. In general, cancer is a disease of middle and old age but can occur at any age. Children 5-14 years have greater freedom from malignant tumors than any other age group. Some cancers are found more frequently in children, e.g., glioma of the eye, embryonal adenosarcoma of the kidney and neuroblastoma. Cancer of the testicle primarily affects males under 30. Carcinomas of the breast and uterus occur most often in women between 45-65 and then decrease in relative frequency. Connective tissue sarcomas occur at the same rate at all ages.

Necessary factors: As a rule, 2 factors must be present for human cancer to develop -

  1. the genetic background or susceptibility, which is not the same for every variety of cancer.
  2. an inciting or causal factor that precipitates the onset of the cancer in the predisposed tissue. The distribution of certain cancers within families and among relatives indicates that a genetic predisposition to certain cancers can be inherited. Glioma of the eye in childhood is inherited.

Trauma initiating cancer: It is unlikely that a single trauma plays an important role in causing cancer. But when the injury consists of a chronic irritation, sometimes occurring daily for many years, cancer may result. Common examples are irritation of the tongue or mucosa of the cheek by ill-fitting dental plates, pressure of hot pipestems on the lip and daily irritation of the esophagus from swallowing hot soups or other liquids.

Occupational cancers: Cancers whose cause is associated with the workplace. Continued exposure to small amounts of irritant gradually produces tissue changes that ultimately become cancerous. The precancerous change may be so permanent that a malignant tumor will develop many years after an occupation has been abandoned. Examples are -

  1. X-ray technicians who get X-ray or radium cancer;
  2. workers with dyestuffs exposed to aniline, benzidine, or beta-naphthylamine, who develop papillomas or bladder cancer;
  3. asbestos exposure leads to lung cancer;
  4. benzene exposure leads to leukemia
  5. exposure to coal tar and crude mineral oils leads to cancer of the skin, mainly hands, arms, and scrotum

Cancer diagnosis: Identification of cancer relies on 3 steps -

  1. The history of the patient's complaints relating to abnormalities in the functioning of a specific organ
  2. Physical examination with the hand, with special examining instruments, with X-rays, and sometimes by chemical analysis
  3. Biopsy, in which a portion of the suspected cancerous tissue is removed for microscopic study and identification

Carcinogens: Substances which stimulate a cancer, probably by inducing a change in the protein and/or nucleic acid control of the regulation of cell growth. They fall into 3 major classes -

  1. Physical - radiation, both electromagnetic (ultraviolet, X-rays, gamma rays, magnetic fields) and particulate (alpha, beta, protons, neutrons).
  2. Chemical - cyclic hydrocarbons in petroleum products, amines from the dye industry, estrogens affecting tissues highly responsive to the physiological action of these hormones, carbon tetrachlo- ride, tannic acid and alkaloids, polymers and plastics used for implants, metals such as arsenic and asbestos, and alkylating compounds such as nitrogen mustards.
  3. Oncogenic viruses - such as the polyoma virus.

Sir Percival Pott: In 1775 he suggested that the cancers commonly found among chimney sweeps in England resulted from excessive exposure to soot, an environmental agent.

Risk factor: An activity that is associated with a high incidence of cancer relative to the incidence associated with other activities. For example, smoking is an important risk factor in lung disease.

Stages in carcinogenesis: It is thought that cancer develops in discrete stages, each of which is regulated independently at different times by different agents.

  1. Initiation - a brief and irreversible interaction between a carcinogen and the genetic material of its target tissue. The reaction changes the DNA (a mutation), but no tumor results unless the modified DNA interacts with another class of agents, called promoters.
  2. Promotion - causes transformed cells (containing modified DNA) to proliferate and form a tumor. The promoter by itself is neither mutagenic nor carcinogenic. It promotes cancer only while it is present - the effect disappears if the promoter is removed. An example of a promoter is dietary fat - its presence seems to increase tumor development.

Latent period: The time between an initiation event (when the DNA is modified by a carcinogen) and the appearance of a tumor.

Cancer preventative strategy:

  1. Identify and eliminate initiating factors - the initiating factors for the most common cancers are unknown.
  2. Identify cancer promoters, then reduce or eliminate exposure to them. If a promoter is removed, a lesion may remain in the latent phase forever, or progress to the tumor stage at a slower rate than if the promoter continued to be present.
  3. Identify cancer antipromoters, then increase exposure to them.

Cancer antipromoters: Substances that seem to inhibit the onset of cancer.

  1. Dietary fiber - hypothesized to counteract the cancer promoting effects of bile acids in the colon, and to decrease the time of exposure to bile acids in the colon by moving feces rapidly out of the bowels.
  2. Vitamin C - hypothesized to block the conversion of nitrate to nitrite in the stomach, preventing the formation of the powerful carcinogens, nitrosamines and nitroamides.
  3. Complex carbohydrates - such as starches common in potatoes and wheat, are hypothesized to produce a gradual effect on the insulin-producing cells in the pancreas by relatively slow digestion compared to the "shock" of digesting the simple, refined sugars in candy.
  4. Cabbage family - such as broccoli, cabbage, and cauliflower, contain compounds that are hypothesized to block the activation of hydrocarbon agents into carcinogens.

Dietary fat: A high fat diet is associated with cancer of the breast, colon, and pancreas. Diets contain various types of fatty acids -

  1. Linoleic acid - common plant oils (corn, safflower, sunflower, etc.) act as promoters. It is hypothesized that they contribute to increased hormone production and/or an increased production of prostaglandins.
  2. Oleic acid - from olive oil. It does not act as a promoter.
  3. Fish oil (and the oil from marine mammals) - hypothesized to interfere with the conversion of linoleic acid into prosta- glandins.

Dietary supplements: Some antipromoters, such as vitamin A and selenium, must be taken in abnormally high doses to be effective against cancer. Sometimes such high doses can lead to other medical problems. Even these supplements cannot completely prevent the development of a tumor.

Stomach cancer: Tends to be high in those areas where food is preserved by salting, pickling, or smoking. When these procedures have been replaced by refrigeration, there has been a large drop in the incidence of gastric cancer.

Apoptosis: Normal cell death in which the cell nucleus condenses and the cell shrivels. Neighboring cells then engulf and digest the remnant of the cell. Apoptosis is genetically controlled.

Cyclin: Proteins that help control the cell cycle. They build up to a threshold level at which the cell then enters the mitotic phase. At the end of mitosis, cyclin is destroyed and the buildup begins again.

Transcription factor: A protein needed for the expression of a gene. There are a number of transcription factors that attach to the DNA and cause the gene to be expressed.

Oncogene: A gene that has been altered in such a way as to produce cancer. In its normal condition, such a gene is called a proto-oncogene.

Tumor suppressor gene: A gene that inhibits cell proliferation. One example is the p53 gene. Another is the Rb gene, first identified from retinoblastoma.

Kinase: An enzyme that transfers a phosphate group from ATP to another molecule, e.g., a protein. Phosphorylation in this way controls the activity of the protein. Kinases play a central role in intracellular signaling.

Epidemiology: The study of the relationship between the incidence of disease and other factors, e.g., location, exposure to environmental substances, occupation, etc., to find the statistical associations between the disease and possible causes.

Cancer incidence and survival data are provided by cancer registries, which document cases of cancers within a particular region, and mortality statistics are produced by national statistics systems.

Last updated on January 30, 2004

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