Cancer | Research & Encyclopedia Articles

Cancer

Cancer may be as old as humankind. During the fifteenth century, what might now be considered a cancer-like growth was referred to as a scirrus, or scar. Environmental substances have long been associated with the disease. In 1775 Sir Percivall Pott connected frequent occurrences of scrotal cancer among chimney sweeps with their continual exposure to flue dust. Until the 1700s, Europeans treated cancer with crude methods like cauterization and bloodletting. Although cancer research has been continuous for centuries, the most important conclusions have been drawn in the twentieth century.

Cancer is a group of many diseases in which certain cells within the body lose their ability to regulate cell division. The cancerous cell multiplies uncontrollably, causing other normal cells to be crowded out and destroyed. If this growth takes place in a vital organ, malfunctions and death can result. The causes of cancer are diverse and the cure rates associated with different types of cancer vary. Scientists have long believed that cancer is linked to changes in the genetic material of the cell. It has been noted that the chromosomes of cancerous cells show abnormalities that may include deletions or translocations of certain genes. Many agents known to cause cancer (carcinogens) are also proven mutagens, or substances that cause atypical genetic changes. Ultraviolet radiation, x-rays, viruses, and some chemicals are carcinogens that cause genetic mutations.

These initial observations triggered the search for other causes of cancer. In 1950 scientists demonstrated that nucleic acids were important hereditary chemicals. Frank L. Horsfall, Jr. (1906-1971), a clinician and virologist, was interested in finding the causes of cancer. He knew that once cancerous cells appeared, these cells would produce more cells with similar cancerous properties. In cell culture, the cancerous alteration was passed on even when the carcinogen was removed. This clue lead Horsfall to believe that the changes in the cell must have been a result of changes inDNA. He also saw similarities in animal cancers and changes in virus-infected bacterial cells. Horsfall's discovery that all cancer is attributed to changes in the nucleic acid of the cell provided a unifying concept for studying cancer.

Nearly 50 years before Horsfall's observations, a physician named Peyton Rous (1879- 1970) began research in pathology at what is now Rockefeller University. Rous was interested in the physiology of cancer within mammals and birds and discovered the first virus-induced cancer. This connective tissue cancer in chickens, called Rous sarcoma, causes enlargement of the liver and is fatal. Rous's initial experiment included grafting sarcoma tumor cells from diseased hens into healthy hens. He noted that the healthy hens soon contracted the disease. Even filtered fluid extracts from diseased hens were contagious. Rous hypothesized that a virus may be the cause of the cancer. Unfortunately, other scientists were not able to duplicate Rous's experiments using different bird species and his work was ignored for several decades. Soon after Horsfall and other researchers, including Sarah Stewart and Bernice Eddy, established a better understanding of viruses and cancer, Rous's work was finally recognized when he received the Nobel Prize in 1966. Rous's viral theory of cancer engendered an entirely new perspective. It became accepted that some cancers are infectious, that is, able to spread from individual to individual via certain viruses. Today more than 24 oncogenic (cancer-causing) viruses are known. Several of these, including the Epstein-Barr and hepatitis viruses, cause other diseases in addition to cancer.

Because of our better understanding of the causes of cancer, various therapies have recently been introduced in an effort to improve cure rates for many cancers. Chemotherapy employs numerous chemical medications to attack and kill cancerous cells. Unfortunately, many of these compounds are toxic to the human body and cause severe side effects. One particularly inventive type of chemotherapy uses light sensitive compounds to deliver the active agent directly to the cancerous tumor. This is referred to as photodynamic chemotherapy. Radiotherapy uses radiation to shrink cancerous growths prior to or in place of surgical removal. Although radiation is site directed from an exterior source, or in some cases of breast cancer an implanted device, it also kills normal cells in massive numbers. Radiation therapy is frequently used in combination with chemotherapy. The side effects of both chemotherapy and radiotherapy, which include severe nausea and loss of hair, are well documented. Immunotherapy is yet another useful cancer treatment. This type of treatment is relatively new and appears to have great potential. Immunotherapy uses several different methods to bolster the patient's own immune system. The body is then better able to recognize and destroy the cancerous cells. Finally, herbal therapy has experienced a renewed interest in the medical community. Several herbs have been shown to contain cancer-inhibiting compounds.

The twentieth century's final decade heralded enormous advances in genetics research, including the ability to pinpoint genes and the role they play in disease. Scientists have identified mutations in three primary types of genes that can cause cancer. Mutations in tumor suppressor gene inhibit their ability to prevent the reproduction of damaged cells and tumor growth. Oncogenes, a mutated form of protooncogenes (before oncogenes), produce a dominant protein that encourages cell growth. DNA repair genes correct genetic errors; when they cannot perform their function due to a mutation, cancer occurs. Perhaps the most frequent genetic mutation associated with cancer occurs in the p53 gene and its encoded protein. Located on the short arm of the human chromosome 17, this tumor suppressor gene is often lost or mutated in tumors, leading to abnormal cell proliferation.

A growing number of genes have been identified as playing a major role in many types of cancer. A gene on chromosome 3 is thought to be a tumor suppressor gene associated with up to two-thirds of kidney cancer cases. A gene on the upper part of chromosome 2 causes colon cancer by fostering mutations in other genes. A mutation in a gene on chromosome 9 causes basal cell carcinoma, the most common type of skin cancer. A dysfunction in the p16 gene prevents it from producing a protein, leading to benign pituitary tumors, which can still cause vision problems and disrupt hormone balances.

While it is still uncertain whether many of these genetic mutations are inherited, the result of environmental influences, or a combination of both, researchers are closing in on many familial cancer-causing genetic mutations, including forms of colon and breast cancer. In 1995 the identification of the "breast cancer genes" BRCA1 and BRCA2 represented the first "susceptibility" genes found for a prevalent cancer. BRCA1, found on chromosomes 17, is responsible for a prevalent form of hereditary breast cancer. BRCA2 is found on chromosome 13 and appears to cause as many cases of breast cancer as BRCA-1, including rare forms of male breast cancer. Originally thought to raise a women's chances of getting breast cancer by 85%, follow-up research has indicated that women with either one of these mutations has an increased risk of closer to 50%. Nevertheless, these findings, as well as similar findings for other cancers, certainly will lead to tests that can identify individuals with genes that may predispose them to cancer. Understanding the genetic causes of cancer will also lead to gene therapy as a viable approach to curing and helping to prevent cancer.

Even with the confirmation of a genetic link to cancer, it will be many more decades before we fully understand this disease. Some forms of cancer are extremely rare and deadly, while others are common and may boast a cure rate of 90 percent or better. Every day potential carcinogens are discovered and novel therapies tested. Undoubtedly, much more information has yet to be learned before this feared disease is conquered.