Cancer is caused by a number of factors, some of which we can control, and some we cannot. One of the uncontrollable factors is the presence of gene mutations. One type of gene that plays a role in normal cell growth -- an oncogene -- can be altered to contribute to the uncontrolled growth of a tumor. Oncogenes affect the way cells use energy and multiply. For example, in some cancers, the ras gene (an oncogene) is mutated, and produces a protein that stimulates cells to divide prematurely. Other oncogenes, such as C-myc and C-erb B-2, when amplified, are implicated in small cell lung cancer and breast cancer, respectively.
Mutations in tumor suppressor genes are another common cause of cancer. As you might expect, a tumor suppressor gene is supposed to prevent tumors. But when these genes are damaged, they can allow cancer to develop instead of preventing it. One of these genes, p53, normally prevents cells with abnormal DNA from surviving. When p53 is defective, these cells with abnormal DNA survive and can multiply, increasing the probability of developing cancer.
Certain cancers are associated with chromosomal abnormalities. Chromosomes are located within the nucleus of our cells, and contain our genes. When genes are missing, duplicated, or rearranged, a predisposition to develop a tumor is increased. Certain leukemias, sarcomas, lymphomas, and others tumors are associated with chromosomal abnormalities.
There are also viruses associated with cancer. The human papillomavirus (HPV) that causes genital warts is associated with carcinoma of the cervix, and the Epstein-Barr virus that causes infectious mononucleosis, is associated with Burkitt's lymphoma. Diseases or drugs that affect the immune system can also increase the risk for certain cancers. The disease AIDS, for instance, is associated with a high risk of two types of cancer, namely, Kaposi's sarcoma and lymphoma.
Exposure to ionizing radiation can increase the risk of certain cancers. X-rays used to treat disorders such as acne or adenoid enlargement can increase the risk of certain types of leukemias and lymphomas.
Fortunately, there are also factors under our control that can increase the risk of getting cancer, and can therefore be avoided. There are substances called carcinogens (cancer-forming agents) that can increase the risk of getting cancer. Some common carcinogens include:
- Arsenic, asbestos, and nickel, which can cause lung and other cancers
- Benzene, which can cause leukemia
- Formaldehyde, which can cause nasal and nasopharyngeal cancer
- and many others...
Carcinogens that are associated with a person's lifestyle include alcohol, which increases the risk of oral, esophageal, and oropharyngeal cancer, and tobacco, which causes lung, head and neck, esophageal, and bladder cancer. Smokeless or chewing tobacco can also increase the risk of oral cancer.
Unprotected exposure to sunlight (ultraviolet radiation) is associated with skin cancer. The main cancers caused by sunlight are basal cell carcinoma, squamous cell carcinoma and malignant melanoma.
We'll define some terminology that doctors use when they talk about cancer in the next section.