What Causes Anal Cancer
Although the exact cause of most anal cancers is not known, most doctors tie it to infection with HPV. A great deal of research is now under way to learn how HPV causes anal cancer.
It is important to remember that some patients with anal cancers do not have any known risk factors and the causes of their cancers are not known.
There is good evidence that the human papillomavirus (HPV) causes many squamous cell anal carcinomas and transitional cell (cloacogenic) anal carcinomas. But, the role of this virus in causing anal adenocarcinomas is less certain. More than 100 types of HPV have been found by scientists. The type known as HPV-16 is often found in anal squamous-cell carcinoma and is also found in some anal warts. Another type, HPV 18 is found less often. Most anal warts are caused by HPV-6 and HPV-11. Warts containing HPV-6 or HPV-11 are much less likely to become cancerous than those containing HPV-16.
Research has recently shown that HPV produces proteins called E6 and E7 that can inactivate two important tumor suppressor proteins in normal cells. These 2 proteins, p53 and Rb, normally work to keep cells from growing out of control. When they are inactive, cells can become cancerous.
HIV, the virus that causes AIDS, weakens the body’s immune system. The immune system of patients with kidney, heart, liver, or other organ transplants is purposely weakened by medication to prevent rejection of their new organs. When the body is less able to fight off infections, viruses such as HPV may become more active and thus trigger the development of anal cancer. Also, a weakened immune system allows cancer to grow more readily.
Most people know that smoking is the main cause of lung cancer. But few realize that the cancer-causing chemicals in tobacco smoke are absorbed by the lungs and can spread through the bloodstream to the rest of the body. Many studies have noted an increased rate of anal cancer in smokers, and the effect of smoking is especially important in people with other anal cancer risk factors.