In this essay we will consider the underlying biology of the disease cancer and how it can be prevented.
Biology of Cancer
From a biological point of view, several broad questions quickly come to mind:
- What Kinds of Cellular Abnormalities Allow Cancers to Grow and Spread in an Unrestrained Fashion?
- What Causes Cancer?
- What Role do Genes Play in the Development of Cancer?
- How is Cancer Detected and Treated?
- Can Cancer be Prevented?
In the remainder of this essay, we will examine how this text is organized to address each of these questions in detail.
Essay # 1. What kinds of Cellular Abnormalities Allow Cancers to Grow and Spread in an Unrestrained Fashion?
Cancer is a disease of abnormal cells. We have already seen that a central malfunction is the ability of cancer cells to proliferate in an uncontrolled fashion, producing an ever- increasing number of cells without regard to the needs of the rest of the body.
To understand the mechanisms responsible for such behavior, you need to know how the proliferation and survival of normal cells are controlled. When these control mechanisms are compared in normal cells and cancer cells, a number of important differences become apparent.
Although uncontrolled proliferation is a defining feature of cancer cells, it is not the property that makes the disease so dangerous. After all, the cells of benign tumors also proliferate in an uncontrolled fashion, but benign tumors are rarely life threatening because the cells remain in their original location.
The hazards posed by cancer cells come from uncontrolled proliferation combined with the ability to spread throughout the body. Spreading of cancer is a complex process involving multiple steps. First, tumors must trigger the development of blood vessels that supply nutrients and oxygen to the tumor and remove waste products.
Without this step, tumors cannot grow beyond a tiny size. After tumors have triggered the formation of a blood supply, cancer cells invade through surrounding tissues, enter the circulatory system, travel throughout the body, and establish new tumors at distant sites.
Essay # 2. What Causes Cancer?
The uncontrolled proliferation of cancer cells, combined with their ability to spread throughout the body, makes cancer a potentially life-threatening disease. What causes the emergence of such cells that have the ability to destroy the organism of which they are a part?
The conversion of normal cells into cancer cells is a complex, multistep process that typically takes many years to unfold. Despite the complexity of this process, however, many of its initiating causes are known.
Researches done for the half of 20th century reveal several causes that induce cancer in experimental animals. The causal agents are certain chemicals, radiation and viruses that behave as mutagens by acting at the level of DNA.
However, it has also been proved that cancer is a genetic disease caused by multiple mutations within the DNA of a cell. Consequently, the cell loses its control over the growth.
Essay # 3. What Role do Genes Play in the Development of Cancer?
The causes of cancer are quite diverse, but they often lead to the same outcome, namely gene mutations. A large body of evidence points to the pivotal role played by mutations in the development of cancer.
These cancer- causing mutations can be triggered by chemicals, radiation, or infectious agents or they may arise spontaneously, arise from errors in DNA replication, or be inherited. But regardless of these differences in how they arise, cancer-related mutations affect the same two classes of genes- oncogenes and tumor suppressor genes.
Oncogenes are defined as genes whose presence can lead to cancer. They arise by mutation from normal genes that code for proteins involved in stimulating cell proliferation and survival. By producing abnormal forms or excessive quantities of these proteins, oncogenes contribute to the uncontrolled proliferation and survival of cancer cells.
In contrast to oncogenes, which are abnormal genes whose activity can lead to cancer, tumor suppressor genes are normal genes whose deletion or loss of function can likewise lead to cancer. Tumor suppressor genes produce proteins that either directly or indirectly exert a restraining influence on cell proliferation and survival. The loss of such proteins can therefore allow cell proliferation and survival to evade normal restraints and controls.
Understanding the behavior of cancer-related genes requires some familiarity with DNA structure and function. For review purposes, Figure 8 illustrates the building blocks of DNA. As shown in this figure, DNA chains are constructed from varying sequences of four building blocks called nucleotides. Each nucleotide contains a sugar, a phosphate group, and a nitrogen-containing base that may be adenine (A), guanine (G), cytosine (C), or thymine (T).
An intact DNA molecule consists of two intertwined DNA chains wound into a double helix that is held together by hydrogen bonds between the base adenine (A) in one chain and thymine (T) in the other, or between the base guanine (G) in one chain and cytosine (C) in the other. The base sequence of one chain therefore determines the base sequence of the opposing chain, and the two chains of the DNA double helix are said to be held together by complementary base pairing.
A gene is any nucleotide sequence in DNA that codes for a functional product, in most cases a protein chain. The flow of information from a DNA gene to a protein chain occurs in a two-step process called transcription and translation (Figure 9). During transcription, the base sequence in one strand of the DNA double helix serves as the template for the synthesis of a complementary molecule of RNA.
The base-pairing rules are similar to those used in making DNA except that RNA utilizes the base uracil (U) where DNA would use thymine (T).
The RNA molecules produced by protein-coding genes, function as messenger RNAs (mRNAs) that in turn guide the synthesis of protein chains in a process known as translation.
During translation, mRNA associates with ribosomes and the genetic information in the mRNA is read in units of three bases called codons. Most codons specify an amino acid, but a few functions as stop signals that mark the end of a protein chain.
The net effect of the two-step process is that the nucleotide sequence of DNA molecules is used to guide the production of protein molecules, which in turn perform most cellular functions. Disruptions in these proteins— caused by either DNA mutations or changes in the way DNA is expressed—lie at the heart of cancer cell behavior.
Essay # 4. How is Cancer Detected and Treated?
During the past several decades, great strides have been made in unraveling the molecular and genetic abnormalities exhibited by cancer cells. One of the hopes for such research is that our growing understanding of the mechanisms responsible for the development of cancer will eventually lead to improved strategies for diagnosing and treating the disease. The bottom line, of course, is the urgent desire for a “cure for cancer.”
Hardly a week goes by without another newspaper or television story reporting the latest research developments that might lead to the long-awaited cure. Although this highly publicized search seems to imply that we lack effective approaches for dealing with the disease, such a dire assessment is far from accurate and many people with cancer are already being cured.
People diagnosed with cancer have a variety treatment options available that depend both on the type of cancer involved and how far it has spread. Cancers are easiest to cure when they are detected before the primary tumor has begun to invade and metastasize, so there is great interest in finding better procedures for early cancer detection.
After a cancer has been diagnosed, the goal of traditional cancer treatment is the complete removal or destruction of cancer cells accompanied by minimal damage to normal tissues. This goal is usually pursued through a combination of surgery to remove the primary tumor, followed if necessary by the use of radiation or chemotherapy (drugs) or both to destroy any remaining cancer cells.
Newer approaches to cancer treatment involve using the immune system to fight cancer and developing drugs that target proteins known to be required for the proliferation and spread of cancer cells.
Essay # 5. Can Cancer be Prevented?
The practical importance of understanding the events that lead to cancer is not restricted to guiding the development of new treatment strategies. It also provides crucial information as to how cancer might be prevented, and as the old adage states, “An ounce of prevention is worth a pound of cure.”
One tactic for preventing the disease is based on the fact that many of the environmental agents that cause cancer have been identified. Avoiding these known risk factors can significantly decrease a person’s chances of developing the disease. For example, more than 30% of all cancer deaths could be prevented if people simply did not smoke cigarettes.
A second approach to cancer prevention is based on the discovery that cancers arise through a multistep process that unfolds over several decades rather than occurring as a single discrete event. So even after a person has been exposed to agents that “cause” cancer, there are opportunities to intervene and block one of the subsequent steps required for the progression to malignancy.
Current statistics suggest that if people adopt both prevention approaches—that is, if they reduce their exposure to cancer-causing agents and take steps to protect against the development of cancer after such agents have been encountered—more than half of all cancer deaths could be prevented.