CAUSES OF CANCER

EL FAHRY BIMA

Definition
Cancer is the second leading cause of death in the United States, with one out of every three Americans falling victim to it at some point in their lives. It is a disease of unregulated cell growth. The knowledge gained in cancer biology over the past 20 years has allowed for the discovery of new, highly targeted drugs to treat cancer.

Causes of cancer
The molecular cause of cancer involves mutations in the nuclear DNA (the genetic material in cells) that can be caused by chemicals, viruses, radiation or spontaneous mutations. Although much importance has been put on chemicals and environmental pollutants as carcinogens (agents that cause cancer), it actually turns out that the predominant factors in determining cancer are associated with lifestyle. For instance, cigarette smoking accounts for 30% of cancers in males. Dietary factors are associated with another 35% of all human cancers. It is estimated that with dietary improvements there could be a 50% reduction in colon and rectal cancers, a 25% reduction in breast cancer and 15% reductions each in prostate, endometrial and gallbladder cancers. Other cancers that might be decreased by dietary improvements include cancer of the stomach, esophagus, pancreas, ovaries, liver, lung and urinary bladder. This adds up to 9% reduction in overall deaths. It is estimated that if Americans doubled their intake of fruits and vegetables and fiber and decreased their fat intake by 25%, significant advances could be made. Obesity also puts an individual at an increased risk of death for uterus, gallbladder, kidney, stomach, colon, and breast cancers. Obese women have a 55% greater risk of mortality from cancer than women of normal weight, while men are at a 33% greater risk of mortality. Alcohol and lack of exercise are also associated with increased risk for cancer.

Cell growth
Normal growth of cells is a highly regulated cellular function. The stimulus to begin cell division comes from growth factors that react with growth factor receptors on the surface of the cell. After the binding of growth factor to a growth factor receptor, the growth message is carried from the surface of the cell to the nucleus through a cascade of biochemical reactions referred to as signal transduction. Once the signal reaches the nucleus, transcription factors bind to the DNA, which turns on the production of proteins involved in growth and division of the cells.
DNA contains genetic information that encodes proteins involved in all aspects of cell metabolism. If a gene is damaged or mutated, the protein it encodes will be affected. DNA mutations can result in an altered expression of protein; either too much or too little, or in altered forms of a protein that either do not perform their function or perform it differently. Damage to genes that encode for proteins regulating cell growth such as oncogenes, tumor suppressor genes and DNA repair genes can result in alterations in cell growth and thus cancer.

Oncogenes
Oncogenes are altered forms of normal genes called proto-oncogenes. There have been over 100 oncogenes identified so far. Their primary role in the cell is in regulation of growth. They encode growth factors, growth factor receptors, transcription factors that regulate the manufacturing of new proteins and signal transduction proteins. Signal transduction refers to the process of transmitting a signal from the outside layer of the cell, through the cytoplasm into the nucleus of the cell and begins with a growth factor and receptor interaction. Cancer cells sometimes have altered levels of growth factors or their receptors or factors involved in signal transduction. For example, the K-ras oncogene is an example of a mutated signal transduction protein involved in cancers such as colon and lung cancer, and the HER2/neu oncogene is a mutated receptor associated with breast cancer. Finally, this series of biochemical reactions reaches the nucleus to affect gene transcription, or the reading of genes into RNA and protein. This occurs via transcription factors. Mutations in transcription factors result in abnormal levels of certain proteins that can result in cancer. Myc is an example of a transcription factor mutated in lung cancer.

Tumor suppressor genes
Also called anti-oncogenes, tumor suppressor genes code for proteins that halt cell growth. In the normal cell, when DNA has become damaged, the cell stops growing to devote time to repairing DNA. Factors responsible for allowing this repair to take place are tumor suppressor genes. If tumor suppressor genes malfunction, the cells do not stop dividing when DNA is damaged and the mutation is then carried over to the daughter cells after cell division. This increases the risk of developing cancer. In hereditary cancers it is often a malfunctioning tumor suppressor gene that is inherited. Although there are two copies of each tumor suppressor gene, the second gene can take over the role if it is not mutated. A mutation in the second copy of the gene is required for total loss of tumor suppressor function. There are dozens of tumor suppressor genes identified that are involved in cancer including p53 (identified with many cancers) and APC in colon cancer, and BRCA-1 in breast cancer.

Characteristics of cancer cells
Cancer cells appear differently than normal cells do under the microscope. Their nucleus is much larger than in normal cells, their chromosomes are irregular in distribution and the nucleoli in the nucleus are very prominent. When cancer cells are grown in culture in the lab they also appear different than normal cells. Rather than growing in neat single-layer sheets with one next to the other they grow more haphazardly. They have long processes that extend from the cells, they overlap one another and their shape is more rounded. Normal cells will continue to divide and grow in a culture plate until they touch a neighboring cell where they receive a signal to stop growing. Cancer cells, on the other hand, do not receive this signal and grow on top of each other forming piles of growing cells that resemble a tumor.
Normal cells require growth factors added to their growth medium to enable them to grow in culture. Cancer cells do not require the same amount of growth factors, possibly because they are able make their own growth factors. Normal human cells will grow for a short amount of time in culture and then die, while cancer cells tend to keep on growing. The term given for this ability is immortalization. Cancer cells in culture are immortalized or have unlimited growth potential.

Cancer cells also have a more immature appearance compared to normal cells. This is referred to as dedifferentiation, or they lack differentiation. As an embryo matures and develops, its cells differentiate. This means they take on more specific roles that are reflected in their appearance—kidney cells begin to look different than skin cells or breast cells. Cancer cells look less and less like the tissue they are part of and more like embryonic cells. They also produce embryonic proteins that are used as tumor markers such as carcinoembryonic antigen (CEA) and alpha fetoprotein (AFP).

Pathology
Tumors are either malignant or benign depending upon their invasiveness. Benign tumors are less aggressive, less likely to invade the surrounding tissue, less likely to metastasize (spread) and are slower growing. Although it sounds as if they pose no threat to the individual, this is not always the case. A tumor in the brain especially can be life threatening and put pressure on the brain as it grows. A benign tumor may also secrete hormones that in high levels can be toxic to the individual.

A malignant tumor is more aggressive, more invasive into the surrounding tissue, faster growing and more likely to metastasize. Malignant tumors usually kill the individual if they are not removed. The diagnosis as to whether a tumor is benign or malignant is done on a small sample of the tumor, called a biopsy. A pathologist will microscopically examine a thin, stained slice of the tissue. The tumor is graded, or given a number from 1 -4 that corresponds to its degree of malignancy, with 4 being the most malignant and 1 being benign. The more malignant the tumor, the less organized the cells of the tissue are and the more anaplastic or dedifferentiated they appear.
The tumor is also staged which refers to the amount it has spread. This is done both by gross examination of the patient and by microscopic examination of the tissue. The staging relates to the patient's prognosis. The best prognosis is if the tumor is confined to the epithelial layer of an organ and not spread into the basement membrane. The prognosis is worse if the tumor cells have spread to adjacent lymph nodes. As a tumor grows, it becomes capable of both invasion and metastasis.
An organ of the body consists of epithelial cells that are supported by a basement membrane. (Epithelial cells are cells that form the tissue that covers internal and external surfaces of the body and is found on skin and mucosal surfaces.) The basement membranes separate the epithelial cells from connective tissues that are rich in blood vessels. As the tumor enlarges, it can grow into the surrounding tissue, through the basement membrane and into blood or lymph vessels. This is a critical point in the growth of a tumor. Now, a small piece of tumor can break off and travel through the circulation until it receives a signal to attach to the vessel wall. It can then move through the vessel, into the tissue bed, where it grows to become a secondary tumor. This is termed metastasis. Two common locations for metastasizing tumors are the lungs and the liver.

Cancer can also involve the immune system and individuals with weakened immune systems are often at increased risk for developing cancer. AIDS patients, for example, are at increased risk for developing some cancers, such as Kaposi's sarcoma. As a cell becomes cancerous, it develops different antigens on its cell surface that should be recognized by the immune system and removed. For some reason, the immune system does not remove tumors. Probably, many cancer cells do develop in the body that are identified and removed by the immune system. It is not understood why this happens occasionally but not consistently. There have been documented cases of spontaneous tumor regression which may be due to activation of the immune system.
These unique antigens expressed on the surface of cancer cells can be used to the patient's advantage in treating cancer. Monoclonal antibodies are proteins produced in the laboratory from a single clone of a B cell, the type of cells of the immune system that make antibodies. Antibodies, also known as immunoglobulins, are proteins that help identify foreign substances to the immune system, such as bacteria or a virus. Antibodies work by binding to the foreign substance to mark it as foreign. The substance that the antibody binds to is called an antigen. Monoclonal antibodies that can recognize and attach to the specific antigens found on cancer cells are now being used to target cancer cells directly.

Unfortunately, cancer may go undiagnosed until it is quite advanced. This is because the body has many ways to adapt itself to damage and so symptoms are reduced for some time. Metastasis may be present by the time cancer is diagnosed. Symptoms of cancer include pain emanating from the organ being stretched, as well as fever and weakness. As the disease progresses, cachexia (the wasting that occurs due to starvation and debilitation caused by the cancer) may occur. The patient becomes unable to mount an anti-inflammatory response and infections occur. These infections become the cause of death in most cancer patients.

Carcinogenesis
How does a cancer cell become a cancer cell? Most scientists agree that cancer is a "multi-hit" process—a process that requires a series of genetic mutations that occur either spontaneously, are inherited or are caused by specific carcinogens. There are several stages in the development of cancer: initiation, promotion and progression.
•Initiation. During initiation, a carcinogen interacts with and damages the DNA. Repair can occur after this point and the process can be reversed.
•Promotion. Promotion causes reproduction or proliferation of these damaged cells, forming a mass of cells or a benign adenoma. This stage is still reversible and removal of the promoting agent can stop the expansion of the tumor mass.
•Progression. Progression, however, is irreversible and involves a number of sequential mutations in genes including oncogenes and tumor suppressor genes. The end result of progression is a late adenoma that eventually converts to a malignant carcinoma.

This entire process can take 20 years or more.
A specific multistep carcinogenesis scheme has been outlined for colon cancer that involves the following sequence of events:
•mutation of the APC tumor suppressor gene causing loss of its function
•activation of the K-ras oncogene
•loss of function of the DCC ("Deleted for Colon Cancer") tumor suppressor gene followed by loss of function of the p53 tumor suppressor gene
Such defining of the process of carcinogenesis can identify tumor markers used for diagnosis and monitoring of cancers. Also, an understanding of pathways involved in carcinogenesis can provide information to better design and target drugs, making them more specific to decrease their effects on normal cells. Information gained on carcinogenesis pathways can also be used to research targets for gene replacement therapy.

Angiogenesis
As the tumor grows, its need for a blood supply increases. A tumor larger than 1 mm diameter (0.03 in) cannot continue to grow without access to circulation. Blood supplies the nutrients the tumor requires and can remove the toxic metabolites that are built up in the tumor tissue. To keep up with the demand for blood vessels, the tumor releases factors called angiogenic factors. One such factor is termed vascular endothelial growth factor (VEGF). These factors initiate the growth of more blood vessels into the tumor. This process of increased blood vessel growth is termed angiogenesis. The rich supply of blood vessels also gives the tumor more opportunity to metastasize to distant sites by traveling through the blood. Using drugs that can block angiogenesis is a newly developing field of cancer treatment.

Apoptosis
Apoptosis is a way the organism has of removing genetically damaged cells from itself to prevent cancer. It is different from another process of death called necrosis where damaged tissue dies for various reasons. When DNA damage occurs to the cell, the body has many opportunities to repair this damage and thus to prevent cancer. If the DNA repair does not occur, however, the last chance the organism has to protect itself from cancer is to eradicate the entire cell. This programmed cell death, or selective destruction of a cell, is called apoptosis. Precancerous cells receive signals that activate this self-destruct program. Genes that are involved in apoptosis include bcl-2 (breast cancer tumor suppressor genes 1 and 2) and p53. When these genes are mutated, apoptosis is limited and the risk of a cell becoming cancerous is increased. Some anti-cancer drugs act by stimulating the apoptotic pathway.

Tumor markers
Tumor markers may be soluble factors secreted by cancer cells, altered proteins retained by cancer cells, or mutated genes in the cancer cells. They are typically identified in the blood of patients but sometimes tissue from a biopsy is necessary. Tumor markers can be used as an aid in diagnosing cancer but, more importantly, they can give information on the prognosis of the cancer and aid the clinician in determining appropriate treatment. For instance, if the tumor marker HER-2/neu associated with breast cancer is identified in a patient, specific chemotherapy that is directly targeted to the HER-2/neu protein can be used giving the patient a better prognosis. Some tumors secrete high levels of hormones that are used as tumor markers to help identify the cancer. For instance, choriocarcinoma (a malignancy that originates inside the uterus) produces large amounts of human chorionic gonadotropin (hCG). The presence of hCG in the blood helps identify the tumor. Other common tumor markers include prostate-specific antigen associated with prostate cancer and CA 125 associated with ovarian cancer.

Genetics
Cancer is basically thought of as a genetic disease. (That is not to say, however, that all cancers are inherited.) Genes are sequences of DNA located on chromosomes within the nucleus. The genes contain information that encodes proteins involved in all aspects of cell metabolism. Genes involved in cell growth and division are the most important in regards to cancer. These genes are oncogenes, tumor suppressor genes and DNA repair genes. Before science had the ability to identify specific genes it was noted that some cancers were associated with chromosomal abnormalities. For instance, chronic myeloid leukemia (CML) is associated with a fragmented chromosome termed the Philadelphia chromosome. The Philadelphia chromosome results from a translocation of part of chromosome 22 to chromosome 9. Using newer molecular techniques, we now know that this translocation results in a fusion between two oncogenes: bcr and abl.
We now know of many hereditary conditions that result in an increased risk of cancer. Although hereditary cancers are actually quite rare, they are notable because individuals who inherit a predisposition for cancer become afflicted with cancer at a much earlier age than those without an inherited susceptibility. Cancer results from a series of genetic mutations that may take 20 years or more to accumulate. If one of these mutations is inherited, it shortens the time for cancer to develop. Individuals with hereditary predispositions tend to get cancer at a much earlier age than those who get non-hereditary cancer.
Familial colon cancer and breast cancer are two widely known examples of inherited cancers. One percent of individuals with colon cancer has an inherited condition called familial adenomatous polyposis (FAP). Individuals with FAP inherit a mutation of the adenomatous polyposis coli (APC) gene, a tumor suppressor gene that is involved in apoptosis. Patients with FAP tend to have many benign polyps of the colon and are likely to develop colon cancer by the age of 40. Mutation of the APC gene is an early event in non-hereditary colon cancer as well. The difference is that in non-hereditary colon cancer, this mutation is a spontaneous event, and the accumulation of mutations that result in cancer occur later in life than with hereditary colon cancer.
There are two genes associated with hereditary breast cancer, the BRCA1 gene and the BRCA2 gene. About 80% of families with cases of early onset breast cancer have mutations in the BRCA1 gene. This gene is also associated with increased risk of ovarian cancer. The BRCA2 gene, associated with hereditary breast cancer is related to ovarian cancer to a lesser extent than BRCAI. Both genes act as tumor suppressor genes.

The Li-Fraumeni syndrome is a hereditary syndrome that puts individuals at an increased risk for a number of cancers including breast cancer, soft tissue sarcomas, osteosarcomas, brain tumors, leukemias and adrenocortical carcinomas. Individuals with this syndrome have mutations in the tumor suppressor gene, p53. Mutations in this gene are associated with 50% of all cancers. The protein associated with p53 is found in the nucleus of the cell and regulates cell functions such as cell cycle, DNA repair, and apoptosis. Mutations in p53 are also noted in colon cancer.
Other hereditary cancer syndromes include retinoblastoma in which a mutated Rb gene is inherited and neurofibromatosis in which a mutated NF1 gene is inherited. The K-ras gene is an oncogene that is commonly mutated in many types of cancer including colon and lung cancer. Individuals who are part of families with high rates of these types of cancers can choose to be genetically tested to determine if they are at an elevated risk for developing cancer.

Treatment
Most cancer treatments center around three modalities, surgical removal of the tumor, radiotherapy, and chemotherapy to kill the cancer cells. If the cancer has not spread and is isolated, surgery is the best option as it can physically remove the entire tumor. The location of all tumor tissue must be able to be identified for this procedure, however. There are risks of surgery that include those associated with anesthesia and infection.
Radiation therapy uses x rays directed at the tumor to cause damage that kills the cells. Radiation therapy will also affect normal tissue that lies in the radiation field. These side effects will vary depending upon the part of the body undergoing treatment.

Chemotherapy involves using drugs that circulate through the body to affect the tumor. The first drugs that were used to treat cancer are the antimetabolite drugs such as methotrexate and mercaptopurine. These drugs were designed to interfere in cell division and kill rapidly dividing cells. Unfortunately, they cannot differentiate between rapidly dividing tumor cells and rapidly dividing normal cells. The toxic effects on non-tumor cells account for many of the side effects of chemotherapy, including loss of hair and gastrointestinal problems. Calculating the correct dosage of these drugs is very important to minimize side effects. Although these drugs cause more side effects because they are delivered to the entire body, this is the only way to treat tumors that have metastasized from the main tumor.
Knowledge gained about cancer over the past 20 years or so have brought about cancer therapies more directly targeted at proteins known to be involved in carcinogenesis. For instance, small molecules that can specifically inhibit signal transduction proteins can slow cancer growth. A new drug known as imatinib mesylate (formerly known as STI-571) deactivates the enzyme called tyrosine kinase, which allows the growth of chronic myelocytic leukemia cells.
Monoclonal antibodies can be made in the lab that are able to recognize specific antigens on cancer cells. These antibodies can in turn be joined to cancer drugs and be used to deliver the drug directly to the cancer cell. Being able to deliver the cytotoxic drug to the cancer cell decreases the side effects on normal tissue.
New categories of cancer treatment have also evolved including hormone therapy and immunotherapy, also called biological therapy or biological response modifiers. Biological therapy takes advantage of the body's own immune system to recognize the cancer and remove it. Cytokines are immunoregulatory substances secreted by the cells of the immune system. Immunotherapy can use cytokines that are naturally produced by the body and affect immune cells and blood cells. These cytokines include interferons, interleukins and colony stimulating factors, such as filgrastim and sargramostim. For instance, interferon-alpha and interleukin-2 are now used to treat metastatic melanoma.

Hormonal treatment of cancer aims to interfere with some hormonal action on cancer cells. This therapy is mostly used on breast cancer and prostate cancer. Some breast cancers grow in response to estrogen. The antiestrogen tamoxifen can reduce the amount of growth in breast cancer. Prostate cancer grows in response to testosterone. Drugs can be used to decrease the amount of testosterone produced by the testes.
Finally, gene therapy has potential to directly target genetic abnormalities found in cancer cells. The tumor suppressor protein p53 is found mutated in a large number of cancers. By introducing the appropriate DNA sequence into a cell, this protein can be replaced bringing back the ability of the cancer cell to undergo apoptosis.
Tumor growth
A tumor that can be clinically identified is typically at least one gram in size and has undergone 30 population doublings. It is before this point that tumor growth has been its fastest and now growth has slowed down significantly. This growth curve of cancer is expressed mathematically by the Gompertzian equation. According to this growth curve, most tumors originate two years before detection. Why does tumor cell growth slow down after it reaches the one-gram size? Factors that contribute to this decline in growth include lack of oxygen, decreased availability of nutrients, accumulation of toxic metabolites and lack of communication between cells.
Understanding the Gompertzian growth curve mathematics can help with decisions about cancer treatment. Most chemotherapy drugs target fast-growing cells and so they work best when a tumor is growing quickly. If at the time a tumor is detected, its cells are growing slower, then chemotherapy is less effective. If initial treatment involves surgery or radiation therapy, then the number of tumor cells will be decreased enough that cells will begin to reproduce again at a faster rate of growth. This makes chemotherapy more effective following surgery or radiation therapy to reduce the tumor load. This might also explain why some patients seem to go into remission only to have their cancer recur later. During remission, the cell number was too low to be detected, but cell growth was rapid during that period.
Many times a patient's response to chemotherapy is very good only to be followed by a relapse with a drug-resistant tumor. Combination chemotherapy, or the use of more than one drug at a time, is often more effective that single drug therapy. This is because cancer cells can spontaneously mutate and become resistant to drugs. This ability to mutate was mathematically explained by Goldie and Coldman and named the Goldie-Coldman model. This model predicts that spontaneous mutations in cancer cells that are capable of leading to drug resistance occur every 10, 000 to 1, 000, 000 cell divisions. Basically this model implies that smaller tumors are less likely to be drug-resistant and are easier to cure. By treating tumors early and aggressively, the chance of recurrence with a drug-resistant cancer decreases. The combination of active drugs is also more effective in reducing the initial cancer.

This model of combination chemotherapy has proven useful in treating childhood acute lymphocytic leukemia, Hodgkin's disease, and testicular cancer. However, it has not proven useful in treating some solid tumors. These tumors seem to have a much higher capacity to develop drug resistance. The drugs used to treat these cancers are actually capable themselves of promoting resistance in the tumors. Resistance to one of these drugs often results in resistance to another drug and is referred to as multi-drug resistance. Multi-drug resistance is due to a decreased uptake and increased elimination of the drugs from the tumor cells. A specific pump has been identified in cancer cells that is responsible for multi-drug resistance.

Resources

BOOKS

American Cancer Society, ed: Osteen, Robert T. Cancer Manual. Framingham, MA: The American Cancer Society, 1996.
McKinnell, Robert G, Ralph E. Parchment, Alan O. Perantoni, and G. Barry Pierce. The Biological Basis of Cancer. New York: Cambridge University Press, 1998.
Templeton, Dennis J., and Robert A. Weinberg. "Principles of Cancer Biology." In Clinical Oncology, edited by Gerald P. Murphy, Walter Lawrence, and Raymond E. Lenhard. Atlanta, GA: The American Cancer Society, 1995.
Weinberg Robert A. One Renegade Cell: How Cancer Begins. New York: Basic Books, 1998.

PERIODICALS
Lowe Scott W. and Athena W. Lin. "Apoptosis in Cancer." Carcinogenesis 2000; 21:485-495.
Gibbs, Jackson B. "Mechanism-Based Target Identification and Drug Discovery in Cancer Research." Science 2000; 287:1969-1973.
Caldas, Carlos. "Science, Medicine, and the Future: Molecular Assessment of Cancer." British Medical Journal 1998; 316:1360-1363.

ORGANIZATIONS
American Cancer Society. 1599 Clifton Road NE, Atlanta, GA, 30329. (800) 227-2345. .
American Society of Clinical Oncology. 225 Reinekers Lane, Suite 650, Alexandria, VA 22314. (703) 299-0150. .
National Cancer Institute. 9000 Rockville Pike, Building 31, Bethesda, MD 20892.(800) 4-CANCER. .

OTHER
DeNittis, Albert, Thomas J. Dilling, Joel W. Goldwein. "The Biology of Cancer." OncoLink, University of Pennsylvania December 21, 2000,
Mellors, Robert C. "Etiology of Cancer: Carcinogenesis." Neoplasia. Weill Medical College of Cornell University, July 1999.
"Oncogenes and Tumor Suppressor Genes." Cancer Resource Center, American Cancer Society January 19, 2001,
Cindy L. A. Jones, Ph.D.

Anaplastic
—The undifferentiated appearance common to a cancer cell.
Antisense
—An RNA sequence that can prevent the synthesis of a specific protein.
Chromosomes
—Structures within the nucleus of the cell that contain DNA.
Genome
—All the genetic information of an organism.
Immortal
—To grow and divide indefinitely.
Metastasis
—Cancer growth at a secondary site.
Nucleus
—The cellular compartment containing the chromosomes.
Nucleoli
—Structures within the nucleus of the cell that are associated with chromosomes.
Phenotype
—The physical expression of the DNA, or the appearance of an organism.

Factors that favor the progression of cancer
Genetic susceptibility
Age
Mutations
Abnormal cell growth
Aneuploidy (abnormal numbers of chromosomes)
Growth and survival factors
Angiogenesis
Loss of heterozygosity (Having only one version of a gene instead of the usual two different versions)
Gene amplification
Factors that help to protect the body from cancer
Genetic resistance
Nutrition
Metabolism
DNA repair
Tumor suppressor genes
Cell differentiation
Death of damaged tissue
Programmed cell death
Immune system