Formation of a Tumor

FORMATION OF A TUMOR

The actual generation of a real tumorrequires several steps. In practice multiple somatic mutations are necessary for the production of most cancers ( Fig. 18.21 ). For example,

many colon cancers carry the following defects:

1 . Inactivation of the APC anti-oncogene (both copies)

2 . Activation of the Ras oncogene

3 . Inactivation of the DCC anti-oncogene (both copies)

4 . Mutation of a single copy of the p53 gene

Cancer development needs half a dozen mutational steps before a full-blown tumor results. Even then, the cancer cells will all stay in one place, and if the tumor is cut out by surgery, all may still be well. However, cancers do not stay put forever. Eventually they bud off cancer cells that travel around the body, settle down in other tissues, and grow into secondary tumors. This is known as metastasis , and once things reach this stage it is virtually impossible to find and remove all of the cancers. Other mutations, which aren’t fully understood, are necessary for cancer cells to start traveling. These include mutations that result in:

1 . Loss of adhesion to neighboring cells in the home tumor

2 . Ability to penetrate the membranes surrounding other tissues

3 . Vascularization of the tumor, which not only provides nutrients but also allows mobile cancer cells access to the circulatory system