What causes cancer?
We do not know the exact cause of many cancers. Many cancers like leukemia have no apparent cause. Some other types of cancers like lung cancers have identifiable causes like smoking in most cases. Lung cancer can occur in those who have never smoked in their life. It is hard to know in these cases what is causing that patients lung cancer. Factors that increase the risk of development of cancer is known as risk factors for cancer. These risk factors may be modifiable like smoking where a person can exert control or un-modifiable factors like age and gender. Smoking can cause cancers of the lungs, mouth, throat, bladder, kidneys and several other organs. Smoking is also associated with heart disease and stroke. While not everyone who smokes will get cancer, smoking increases a person's chance of getting the disease. Drinking a lot of alcohol has also been shown to increase a person's chance of getting cancer of the mouth cavity, throat, and some other organs. A combination of smoking and drinking is especially dangerous since the risk of development of cancer increases by multiplication rather than addition. Exposure to radiation (x-rays or radiation from radio-active materials) can cause cancer. Dosage of radiation need to cause cancer is much higher than those used in normal x-rays that are taken in doctor's or dentist's office and these procedures are generally safe. Too much exposure to sunlight can cause skin cancer.
Even though we know that smoking causes lung cancer, the exact mechanism by which smoke causes lung cancer is unknown. Nicotine and other chemicals present in the cigarettes plays an important role in the development of lung cancer. But how exactly the chemicals contained in the smoke brings changes that lead to the development of cancer is not clearly understood.
Cancer causing substances like nicotine in cigarette smoke are known as carcinogens. Carcinogens generally brings about transformation of a cell to cancer by altering the genetic material inside the nucleus (known as DNA) of the cell. This change in the genetic material causes what is called a mutation. Control mechanisms of the cells are located in these genetic materials.
Basics of cancer development
All living things are made up of cells. Cells are the basic living unit. Some living things like amoeba may have one cell only while a complex animal like a cat may have billions of cells in the body. Many cells divide to produce daughter cells. This is required for the replacement of cells lost during the course of life. The process of cell division is a very tightly controlled process and occurs in the body only to the extent needed in any particular situation. During the process of cell division one cell may acquire some genetic mutation that would alter the cell division control mechanisms of that cell. This altered cell no longer listens to the control signals for cell division and may continue to divide and multiply. This uncontrolled cell division and growth ultimately results in cancer. Multiple genetic events are involved in the development of most malignancies.
Physical agents that cause DNA mutation
Many physical agents can cause mutation of DNA, and these include radiation, ultraviolet light and mineral fibers like asbestos fiber, which causes mesothelioma. Radiation causes breaks in the DNA strands on which genetic information is located. Sources of radiation may include x-rays, cosmic rays (solar radiation) and indirectly from radon gas. Ultraviolet radiation (from sunlight) causes genetic mutation by making artificial bridges in the DNA strands. This will lead to mistakes in genetic code reading. Certain natural mineral fibers like asbestos, because of their size can case damage directly to DNA resulting in mesothelioma. Development of mesothelioma is a complex process involving the asbestos fiber and a virus known as simian virus 40 (SV40). The irritation caused by asbestos fiber also gives rise to various chemical substances called cytokines, which further contribute to the development of malignant mesothelioma.
Chemical agents that cause DNA mutation
Chemical mutation is caused by binding and chemical interaction of a chemical substance to the DNA. This binding may cause damage to DNA or may simply cause the genetic code to be misread. Examples or chemical carcinogens include benzopyrene (found in cigarette smoke) and vinyl chloride (found in the plastics industry), aflatoxin (found in certain moulds leading to liver cancer) and hetrocyclic amines (found in over-cooked food).
Biological agents that cause DNA mutation
There are several biological agents that can cause DNA mutation. This may include viruses and bacteria. Viruses may use a number of different complex mechanisms to cause a cell to become cancerous. Some of the viruses donate their DNA materials to the host cell and cause alteration in the genetic code. SV 40 virus causing malignant mesothelioma is an example. Here the SV virus works with the asbestos fiber to cause mutation which would ultimately result in transformation to mesothelioma. Other viruses that may cause DNA mutations include human papilloma virus (implicated in cervical cancer) the human T-cell lymphyocytic virus (implicated in lymphoma), the hepatitis B virus (implicated in liver cancer). Helicobacter pylori is a bacterial agent that may bring about DNA mutation. This bacterial agent is implicated in the development of stomach cancer and some type of lymphoma