1. Age-pattern of cancer incidence rate - Both species exhibit low rates in youth, increased rates until old age, and decreased rates in the very old. Rates of tumor growth decline with age.
2. Many agents that are carcinogenic in mice are also carcinogenic in humans.
3. Several infectious agents are carcinogenic in both species.
4. Hormonal replacement therapy (estrogens) increases the risk of ovarian and endometrial cancers in humans. Estrogens are also carcinogenic in mice when administered at old age.
5. As mentioned above, the tumor suppressor genes p53 and retinoblastoma are important for both species. Several proto-oncogenes (normal genes that can be converted into cancer-causing genes) are similar in both species.
6. Sex differences - Cancer risk in females is higher than men before the female menopause, and lower thereafter.
7. Place differences - In humans, the overall cancer risk is higher in the more developed countries. In mice, the incidence of separate cancers varies among laboratories.
Improving the applicability of mouse studies to humans
Mouse models for human cancers generally begin by injecting human cancer cells under the skin and in the body cavity of the mouse. 5 If the injections "take," or result in tumor growth in the animal, studies are undertaken to study the cancer process. Mice that have developed a cancer condition can also be used to screen compounds for cancer treatments. Utilizing standard breeding techniques, specialized inbred strains of mice have been developed that are particularly sensitive to certain cancer conditions.
The use of mouse models in cancer research has been greatly enhanced with the advent of genetic engineering. 2 Genetic engineering is the technique of removing, modifying, or adding genes to the DNA molecule of a species in order to change the characteristics of the species. Two primary techniques are involved: the random insertion of a gene (transgenetic), and specific gene targeting to remove or replace a gene. Of great current interest is the replacement of certain mouse genes with human genes. The resultant "humanized" animals are better models for cancer research.
