Figure 20–28 Monitoring tumor growth and metastasis in a mouse with a luminescent reporter. A mouse was genetically engineered in a way that allows both copies of its PTEN tumor suppressor gene to be inactivated in the prostate gland, simultaneously with the prostate-specific activation of a gene engineered to produce the enzyme luciferase (derived from fireflies). After an injection of luciferin (the substrate molecule for luciferase) into the mouse’s bloodstream, the cells in the prostate emit light and can be detected by their bioluminescense in a live mouse, as seen in the 67-day-old animal at the left. Cells lacking the PTEN phosphatase enzyme contain elevated amounts of the Akt activator, PI(3,4,5)P3, and this causes the prostate cells to proliferate abnormally, progressing over time to form a cancer. In this way, the process of metastasis could be followed in the same animal over the course of a year. The light intensity in these experiments is proportional to the number of prostate-cell descendants, increasing from light blue to green, to yellow, to red in this representation. (Adapted from  C.-P. Liao et al., Cancer Res. 67:7525– 7533, 2007.)
Figure 20–29 Oncogene collaboration in transgenic mice. The graphs show the incidence of tumors in three types of transgenic mouse strains, one carrying a Myc oncogene, one carrying a Ras oncogene, and one carrying both oncogenes. For these experiments, two lines of transgenic mice were first generated. One carries an inserted copy of an oncogene created by fusing the proto-oncogene Myc with the mouse mammary tumor virus regulatory DNA (which then drives Myc overexpression in the mammary gland). The other line carries an inserted copy of the Ras oncogene under 50 control of the same regulatory element. Both strains of mice develop tumors much more frequently than normal, most often in the mammary or salivary glands. Mice that carry both oncogenes together are obtained by crossing the two strains. These hybrids develop tumors at a far higher rate still, much greater than the sum of the rates for the two oncogenes separately. 0 Nevertheless, the tumors arise only after a delay and only from a small percentage of tumor-free mice proportion of the cells in the tissues where the two genes are expressed. Further accidental changes, in addition to the two oncogenes, are apparently required for the development of cancer. (After E. Sinn et al., Cell 49:465–475, 1987. With permission from Elsevier.)
Transgenic mouse studies confirm, for example, that a single oncogene is generally not enough to turn a normal cell into a cancer cell. Thus, in mice engineered to express a Myc or Ras oncogenic transgene, some of the tissues that express the oncogene may show enhanced cell proliferation, and, over time, occasional cells will undergo further changes to give rise to cancers. Most cells expressing the oncogene, however, do not give rise to cancers. Nevertheless, from the point of view of the whole animal, the inherited oncogene is a serious menace because it creates a high risk that a cancer will arise somewhere in the body. Mice that express both Myc and Ras oncogenes (bred by mating a transgenic mouse carrying a Myc oncogene with one carrying a Ras oncogene) develop cancers earlier and at a much higher rate than either parental strain (Figure 20–29); but, again, the cancers originate as scattered, isolated tumors among noncancerous cells. Thus, even cells expressing these two oncogenes must undergo further, randomly generated changes to become cancerous. This strongly suggests that multiple mutations are required for tumorigenesis, as supported by a great deal of other evidence discussed earlier. Experiments using mice with deletions of tumor suppressor genes lead to similar conclusions.
Cancers Become More and More heterogeneous as They Progress