The Search for a Cancer Cure

At present there are only three main ways of treating cancer, which will kill more than 335,000 in the U.S. alone this year. Doctors can cut tumors out with a knife, burn them out with radiation or kill them cell by cell with drugs. Though these treatments can be effective in combination, each has its drawbacks. Now, cancer researchers have turned to molecular biology, which shows promise of providing new and more effective means of dealing with the disease.

No one really knows what causes cancer, which is actually more than 100 distinct diseases, all snaring two common characteristics: rapid cell growth and a terrifying tendency to spread from one part of the body to another. Most researchers agree, however, that the villain is a virus, a miniature packet of nucleic acid with a membranous coat that was shown as early as 1911 to cause tumors in animals.

Unable to reproduce themselves, viruses invade normal cells and use their hosts' chemical mechanisms to produce more viruses. Eventually, the infected cell ruptures, releasing the newly formed viruses to infect other cells. Dr. Howard Temin of the University of Wisconsin has shown that some tumor viruses behave differently. They reverse the normal order of genetic transmission, and with the aid of a recently discovered enzyme, use their RNA messenger molecules to produce DNA, the double-helix master molecule. In a way not yet understood, this triggers the cellular genetic machinery to order cell division, causing the cancerous growth that is then perpetuated in succeeding cell generations.

The new enzyme associated with Temin's discovery was at first believed to be unique to cancer cells infected by viruses. Thus when Columbia University's Sol Spiegelman and the National Cancer Institute's Robert Gallo found high enzyme activity in the cells of leukemic patients, medical science had a solid clue that leukemia might be caused by a virus. Even more important, some researchers speculated that if the Temin enzyme was found only in cancer cells, the spread of cancer might be halted simply by inhibiting the enzyme.

Their hopes for an immediate cancer cure were short-lived. The NCI's George Todaro and other researchers have since found similar enzyme activity in normal cells as well. They have also found evidence of these enzymes in human and animal embryonic tissues, thus helping to confirm the views of many scientists who believe that cancer is probably an aberration of normal cellular growth.

If it is, Temin thinks he knows why it occurs. According to his hypothesis, normal cells manufacture RNA, which moves to neighboring cells in the form of a protovirus, or template, and stimulates the production of a new form of DNA. But, theorizes Temin, if this wandering RNA somehow transmits the wrong message after entering the cells, it can cause the production of altered DNA that orders the cells to grow abnormally.

Dr. Robert Huebner of the NCI speculates that cancer is caused by a noninfectious virus that is a normal part of every living thing. According to Huebner, the virus, which he has labeled the "C particle," is a part of everyone's genetic heritage, a tiny bit of RNA that is passed vertically from one generation to another and perhaps helps normal development by causing the cells of an embryo to grow. The C particle should become inactive as the fetus matures; if it fails to do so, the result is the rapid cell growth that characterizes cancer.

MANY RESEARCHERS believe that the best method of attacking cancer is to use the body's own genetic mechanism to order cancerous cells to stop growing. Transmitting such orders may be difficult. DNA programmed to carry the command would be digested almost immediately by the body's enzymes if it were injected into the bloodstream, thus making conventional methods of administering medication impractical. But RNA viruses, which produce DNA, have proven their ability to move directly into the cells and could easily carry such communications. Scientists speculate that benign viruses could be made in test tubes with proteins and synthesized RNA. The viruses, injected into the body, would home in on the cancerous growth and shut down the cells' runaway reproductive mechanism.

AVENUES OTHER THAN virology are also being explored in the search for a cancer cure. Researchers have long been aware that animal cells growing in a culture medium will stop multiplying once they come in contact with one another. But in some recent experiments at Princeton, Biochemist Max Burger found that when he stripped normal mouse cells of their membranes, they continued to grow wildly--as do cancer cells--even after they had touched. Burger thus speculates that the loss of a cell's protective coating, possibly as a result of viral infection, could lead to cancer by exposing a sensitive area that signals the cells to continue growth. If the protective covering could be restored, he suggests, it might be possible to stop the genes of cancer cells from ordering further growth.

It may even be possible to use the body's immunological mechanism, which now helps to protect it against other diseases, to combat cancer. Some researchers note that organ transplant recipients, who take large doses of drugs to suppress their immune reactions and prevent the rejection of foreign tissue, may develop cancer. Also, the immune system often fails to respond to many cancer cells, although they have unique antigens that should alert the body to their presence. Accordingly, doctors have begun exploring ways of beefing up the body's defenses and immunizing man against cancer in the same way that he can now be vaccinated against polio and other viral diseases.

In a unique series of experiments, Dr. Loren Humphrey of Atlanta's Emory University inoculated patients with a vaccine made, at least in part, with tissues taken from tumors similar to their own. He then followed up the inoculations by cross-injecting the patients with white blood cells from fellow patients who had presumably been sensitized to the tumor antigens. Though only long-term testing will tell if Humphrey's approach is effective, the preliminary results appear promising. One patient with bowel cancer has been free of the disease for three years, while three others have evidenced definite remissions.

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