Introduction to cancer biology

The cell is the fundamental unit of life. It is the smallest structure of the body capable of performing all of the processes that define life. Each of the organs in the body, such as the lung, breast, colon, and brain, consists of specialized cells that carry out the organ's functions such as the transportation of oxygen, digestion of nutrients, excretion of waste materials, locomotion, reproduction, thinking, etc.
To assure the proper performance of each organ, worn out or injured cells must be replaced, and particular types of cells must increase in response to environmental changes.
Cells divide only when they receive the proper signals from growth factors that circulate in the bloodstream or from a cell they directly contact. For example, if a person loses blood, a growth factor called erythropoietin, which is produced in the kidneys, circulates in the bloodstream and tells the bone marrow to manufacture more blood cells.
When a cell receives the message to divide, it goes through the cell cycle, which includes several phases for the division to be completed. Checkpoints along each step of the process make sure that everything goes the way it should.
Many processes are involved in cell reproduction and all these processes have to take place correctly for a cell to divide properly. If anything goes wrong during this complicated process, a cell may become cancerous.
A cancer cell is a cell that grows out of control. Unlike normal cells, cancer cells ignore signals to stop dividing, to specialize, or to die and be shed. Growing in an uncontrollable manner and unable to recognize its own natural boundary, the cancer cells may spread to areas of the body where they do not belong.
In a cancer cell, several genes change (mutate) and the cell becomes defective. There are two general types of gene mutations. One type, dominant mutation, is caused by an abnormality in one gene in a pair. An example is a mutated gene that produces a defective protein that causes the growth-factor receptor on a cell's surface to be constantly "on" when, in fact, no growth factor is present. The result is that the cell receives a constant message to divide. This dominant "gain of function gene" is often called an oncogene (onco = cancer).
The second general type of mutation, recessive mutation, is characterized by both genes in the pair being damaged. For example, a normal gene called p53 produces a protein that turns "off" the cell cycle and thus helps to control cell growth. The primary function of the p53 gene is to repair or destroy defective cells, thereby controlling potential cancerous cells. This type of gene is called an anti-oncogene or tumor suppressor gene. If only one p53 gene in the pair is mutated, the other gene will still be able to control the cell cycle. However, if both genes are mutated, the "off" switch is lost, and the cell division is no longer under control.
Abnormal cell division can occur either when active oncogenes are expressed or when tumor suppressor genes are lost. In fact, for a cell to become malignant, numerous mutations are necessary. In some cases, both types of mutations - dominant and recessive - may occur.
A gene mutation may allow an already abnormal cell to invade the normal tissue where the cancer started or to travel in the bloodstream (metastasize) to remote parts of the body, where it continues to divide.
A normal cell can become damaged in different ways. A cell can become abnormal when part of a gene is lost (deleted), when part of a chromosome is rearranged and ends up in the wrong place (translocation), or when an extremely small defect occurs in the DNA, which results in an abnormal DNA "blueprint" and production of a defective protein occurs.
Abnormal cell division can also be caused by viruses. In this case, genes may be normal, but the protein may not function normally because the cell contains a cancer-producing virus.