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In simple terms, cancer is a group of more than 100 diseases that develop across time and involve the uncontrolled division of the body's cells. Although cancer can develop in virtually any of the body's tissues, and each type of cancer has its unique features, the basic processes that produce cancer are quite similar in all forms of the disease.
Cancer begins when a cell breaks free from the normal restraints on cell division and begins to follow its own agenda for proliferation (Figure 3). All of the cells produced by division of this first, ancestral cell and its progeny also display inappropriate proliferation. A tumor, or mass of cells, formed of these abnormal cells may remain within the tissue in which it originated (a condition called in situ cancer), or it may begin to invade nearby tissues (a condition called invasive cancer). An invasive tumor is said to be malignant, and cells shed into the blood or lymph from a malignant tumor are likely to establish new tumors (metastases) throughout the body. Tumors threaten an individual's life when their growth disrupts the tissues and organs needed for survival.
 Figure 3 - The stages of tumor development. A malignant tumor develops across time, as shown in this diagram. This tumor develops as a result of four mutations, but the number of mutations involved in other types of tumors can vary. We do not know the exact number of mutations required for a normal cell to become a fully malignant cell, but the number is probably less than ten. a. The tumor begins to develop when a cell experiences a mutation that makes the cell more likely to divide than it normally would. b. The altered cell and its descendants grow and divide too often, a condition called hyperplasia. At some point, one of these cells experiences another mutation that further increases its tendency to divide. c. This cell's descendants divide excessively and look abnormal, a condition called dysplasia. As time passes, one of the cells experiences yet another mutation. d. This cell and its descendants are very abnormal in both growth and appearance. If the tumor that has formed from these cells is still contained within its tissue of origin, it is called in situ cancer. In situ cancer may remain contained indefinitely. e. If some cells experience additional mutations that allow the tumor to invade neighboring tissues and shed cells into the blood or lymph, the tumor is said to be malignant. The escaped cells may establish new tumors (metastases) at other locations in the body. |
What happens to cause a cell to become cancerous? Thirty years ago, scientists could not offer a coherent answer to this question. They knew that cancer arose from cells that began to proliferate uncontrollably within the body, and they knew that chemicals, radiation, and viruses could trigger this change. But exactly how it happened was a mystery.
Research across the last three decades, however, has revolutionized our understanding of cancer. In large part, this success was made possible by the development and application of the techniques of molecular biology, techniques that enabled researchers to probe and describe features of individual cells in ways unimaginable a century ago. Today, we know that cancer is a disease of molecules and genes, and we even know many of the molecules and genes involved. In fact, our increasing understanding of these genes is making possible the development of exciting new strategies for avoiding, forestalling, and even correcting the changes that lead to cancer.
People likely have wondered about the cause of cancer for centuries. Its name derives from an observation by Hippocrates more than 2,300 years ago that the long, distended veins that radiate out from some breast tumors look like the limbs of a crab. From that observation came the term karkinoma in Greek, and later, cancer in Latin.
With the work of Hooke in the 1600s, and then Virchow in the 1800s, came the understanding that living tissues are composed of cells, and that all cells arise as direct descendants of other cells. Yet, this understanding raised more questions about cancer than it answered. Now scientists began to ask from what kinds of normal cells cancer cells arise, how cancer cells differ from their normal counterparts, and what events promote the proliferation of these abnormal cells. And physicians began to ask how cancer could be prevented or cured.
Clues from epidemiology. One of the most important early observations that people made about cancer was that its incidence varies between different populations. For example, in 1775, an extraordinarily high incidence of scrotal cancer was described among men who worked as chimney sweeps as boys. In the mid-1800s, lung cancer was observed at alarmingly high rates among pitchblende miners in Germany. And by the end of the 19th century, using snuff and cigars was thought by some physicians to be closely associated with cancers of the mouth and throat.
These observations and others suggested that the origin or causes of cancer may lie outside the body and, more important, that cancer could be linked to identifiable and even preventable causes. These ideas led to a widespread search for agents that might cause cancer. One early notion, prompted by the discovery that bacteria cause a variety of important human diseases, was that cancer is an infectious disease. Another idea was that cancer arises from the chronic irritation of tissues. This view received strong support with the discovery of X-rays in 1895 and the observation that exposure to this form of radiation could induce localized tissue damage, which could lead in turn to the development of cancer. A conflicting view, prompted by the observation that cancer sometimes seems to run in families, was that cancer is hereditary.
Such explanations, based as they were on fragmentary evidence and incomplete understanding, helped create the very considerable confusion about cancer that existed among scientists well into the mid-twentieth century. The obvious question facing researchers—and no one could seem to answer it—was how agents as diverse as this could all cause cancer. Far from bringing science closer to understanding cancer, each new observation seemed to add to the confusion.
Yet each new observation also, ultimately, contributed to scientists' eventual understanding of the disease. For example, the discovery in 1910 that a defined, submicroscopic agent isolated from a chicken tumor could induce new tumors in healthy chickens showed that a tumor could be traced simply and definitively back to a single cause. Today, scientists know this agent as Rous sarcoma virus, one of several viruses that can act as causative factors in the development of cancer.
Although cancer-causing viruses are not prime agents in promoting most human cancers, their intensive study focused researchers' attention on cellular genes as playing a central role in the development of the disease.
Likewise, investigations into the association between cancer and tissue damage, particularly that induced by radiation, revealed that while visible damage sometimes occurs, something more subtle happens in cells exposed to cancer-causing agents. One clue to what happens came from the work of Herman Muller, who noticed in 1927 that X-irradiation of fruit flies often resulted in mutant offspring. Might the two known effects of X-rays, promotion of cancer and genetic mutation, be related to one another? And might chemical carcinogens induce cancer through a similar ability to damage genes?
Support for this idea came from the work of Bruce Ames and others who showed in 1975 that compounds known to be potent carcinogens (cancer causing agents) generally also were potent mutagens (mutation-inducing agents), and that compounds known to be only weak carcinogens were only weak mutagens. Although scientists know today that many chemicals do not follow this correlation precisely, this initial, dramatic association between mutagenicity and carcinogenicity had widespread influence on the development of a unified view of the origin and development of cancer.
Finally, a simple genetic model, proposed by Alfred Knudson in 1971, provided both a compelling explanation for the origins of retinoblastoma, a rare tumor that occurs early in life, and a convincing way to reconcile the view of cancer as a disease produced by external agents that damage cells with the observation that some cancers run in families. Knudson's model states that children with sporadic retinoblastoma (children whose parents have no history of the disease) are genetically normal at the moment of conception, but experience two somatic mutations that lead to the development of an eye tumor. Children with familial retinoblastoma (children whose parents have a history of the disease) already carry one mutation at conception and thus must experience only one more mutation to reach the doubly mutated configuration required for a tumor to form. In effect, in familial retinoblastoma, each retinal cell is already primed for tumor development, needing only a second mutational event to trigger the cancerous state. The difference in probabilities between the requirement for one or two mutational events, happening randomly, explains why in sporadic retinoblastoma, the affected children have only one tumor focus, in one eye, while in familial retinoblastoma, the affected children usually have multiple tumor foci growing in both eyes.
Although it was years before Knudson's explanation was confirmed, it had great impact on scientists' understanding of cancer. Retinoblastoma, and by extension, other familial tumors, appeared to be linked to the inheritance of mutated versions of growth-suppressing genes. This idea led to the notion that cells in sporadically arising tumors might also have experienced damage to these critical genes as the cells moved along the path from the normal to the cancerous state.
Clues from cell biology. Another field of study that contributed to scientists' growing understanding of cancer was cell biology. Cell biologists studied the characteristics of cancer cells, through observations in the laboratory and by inferences from their appearance in the whole organism. Not unexpectedly, these investigations yielded a wealth of information about normal cellular processes. But they also led to several key understandings about cancer, understandings that ultimately allowed scientists to construct a unified view of the disease.
One such understanding is that cancer cells are indigenous cells—abnormal cells that arise from the body's normal tissues. Furthermore, virtually all malignant tumors are monoclonal in origin, that is, derived from a single ancestral cell that somehow underwent conversion from a normal to a cancerous state. These insights, as straightforward as they seem, were surprisingly difficult to reach. How could biologists describe the cell pedigree of a mass of cells that eventually is recognized as a tumor?
One approach to identifying the origin of cancer cells came from attempts to transplant tissues from one person to another. Such transplants work well between identical twins, but less well as the people involved are more distantly related. The barrier to successful transplantation exists because the recipient's immune system can distinguish between cells that have always lived inside the self and cells of foreign origin. One practical application of this discovery is that tissues can be classified as matching or nonmatching before a doctor attempts to graft a tissue or organ into another person's body. Such tissue-typing tests, when done on cancer cells, reveal that the tumor cells of a particular cancer patient are always of the same transplantation type as the cells of normal tissues located elsewhere in the person's body. Tumors, therefore, arise from one's own tissues, not from cells introduced into the body by infection from another person.
How do we know that tumors are monoclonal? Two distinct scenarios might explain how cancers develop within normal tissues. In the first, many individual cells become cancerous, and the resulting tumor represents the descendants of these original cells. In this case, the tumor is polyclonal in nature (Figure 4). In the second scenario, only one cell experiences the original transformation from a normal cell to a cancerous cell, and all of the cells in the tumor are descendants of that cell.
Direct evidence supporting the monoclonal origin of virtually all malignant tumors has been difficult to acquire because most tumor cells lack obvious distinguishing marks that scientists can use to demonstrate their clonal relationship. There is, however, one cellular marker that scientists can use as an indication of such relationships: the inactivated X chromosome that occurs in almost all of the body cells of a human female. X-chromosome inactivation occurs randomly in all cells during female embryonic development. Because the inactivation is random, the female is like a mosaic in terms of the X chromosome, with different copies of the X turned on or off in different cells of the body. Once inactivation occurs in a cell, all of the future generations of cells coming from that cell have the same chromosome inactivated in them as well (either the maternal or the paternal X). The observation that all the cells within a given tumor invariably have the same X chromosome inactivated suggests that all cells in the tumor must have descended from a single ancestral cell.
Figure 4 - Two schemes by which
tumors can develop. Most—if not all—human cancer appears to
be monoclonal. D |
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