THE LIVING WORLD
Unit Three. The Continuity of Life
8.6. What Is Cancer?
Cancer is a growth disorder of cells. It starts when an apparently normal cell begins to grow in an uncontrolled way, spreading out to other parts of the body. The result is a cluster of cells, called a tumor, that constantly expands in size. The cluster of pink lung cells in the photo in figure 8.12 have begun to form a tumor. Benign tumors are completely enclosed by normal tissue and are said to be encapsulated. These tumors do not spread to other parts of the body and are therefore noninvasive. Malignant tumors are invasive and not encapsulated. Because they are not enclosed by normal tissue, cells are able to break away from the tumor and spread to other areas of the body. The tumor you see in figure 8.12, called a carcinoma, grows larger and eventually begins to shed cells that enter the bloodstream. Cells that leave a tumor and spread throughout the body, forming new tumors at distant sites, are called metastases.
Figure 8.12 Lung cancer cells (300x).
These cells are from a tumor located in the alveolus (air sac) of a human lung.
Cancer is perhaps the most devastating and deadly disease. Most of us have had family or friends affected by the disease. In 2010, about 1.5 million American men and women were diagnosed with cancer. One in every two Americans born will be diagnosed with some form of cancer during their lifetime; nearly one in four are projected to die from cancer.
In the U.S., the three deadliest human cancers are lung cancer, cancer of the colon and rectum, and breast cancer. Lung cancer, responsible for the most cancer deaths, is largely preventable; most cases result from smoking cigarettes. Colorectal cancers appear to be fostered by the high-meat diets so favored in the United States. The cause of breast cancer is still a mystery.
Not surprisingly, researchers are expending a great deal of effort to learn the cause of cancer. Scientists have made considerable progress in the last 30 years using molecular biological techniques, and the rough outlines of understanding are now emerging. We now know that cancer is a gene disorder of somatic tissue, in which damaged genes fail to properly control cell growth and division. The cell division cycle is regulated by a sophisticated group of proteins called growth factors. Cancer results from the damage of these genes encoding these proteins. Damage to DNA, such as damage to these genes, is called mutation.
There are two general classes of growth factor genes that are usually involved in cancer: proto-oncogenes and tumor-suppressor genes. Genes known as proto-oncogenes encode proteins that stimulate cell division. Mutations to these genes can cause cells to divide excessively. Mutated protooncogenes become cancer-causing genes called oncogenes.
The second class of cancer-causing genes is called tumor-suppressor genes. Cell division is normally turned off in healthy cells by proteins encoded by tumor-suppressor genes. Mutations to these genes essentially “release the brakes,” allowing the cell containing the mutated gene to divide uncontrolled.
Key Learning Outcome 8.6. Cancer is unrestrained cell growth and division caused by damage to genes regulating the cell division cycle.