Search for "Essential Factors" In Causes of Human Cancer

Researchers who are probing the mysteries of the relationship between viruses and human cancer may be tackling the most difficult job in all of medicine. It would be tough enough if their task involved whole viruses, most of which can not be seen and can be photographed only with the electron microscope. But cancer research must make even more minute explorations inside viruses; it must chart the behavior of molecules in a no man's land between the living and the nonliving.

A few elements in the mystery have been clarified, says ''Viruses and Cancer." a progress report published this week by Manhattan's Sloan-Kettering Institute for Cancer Research. In many of the cancers, including leukemias of domestic fowl and laboratory animals, a virus is an essential factor. But to say that a virus causes the cancer may be an oversimplification. The tubercle bacillus is the one essential factor in tuberculosis, but mil lions of people carry the bacillus without ever developing the disease. By analogy, researchers argue, it may be that viruses, or viruslike particles of whatever origin, are essential factors in human as well as in animal cancers. But it takes something else as well to bring on the disease, even though the virus particles may have been harbored for half a lifetime.

Not from Air Alone. Another seeming certainty is that no matter how viruses may be involved, cancer is not an infectious disease in the ordinary sense. Nobody catches lung cancer because a victim of the disease coughs in his face. From animals it appears that something like a virus, plus some sort of physical or chemical irritant, may be needed to bring on the disease. Mice do not get lung cancer from polluted air alone, nor from influenza virus. But they may develop something remarkably like human lung cancer if they are both infected with flu virus and exposed to air-polluting chemicals.

A dozen different viruses have been found to cause cancer in mice, and they show a bewildering variety of behavior. Some are clearly inherited. One is passed on from generation to generation in mouse-mothers' milk, so daughter mice develop breast cancer. A male mouse may be a healthy carrier of this virus and infect a female with which he is mated.

Mice that show no signs of harboring a leukemia virus may develop the fullblown disease, and produce the virus plentifully, after they are exposed to X rays.

Still more confusing are the crossovers between species. Millions of monkeys carry a virus which apparently does them no harm. But this virus, known as SV-40, may cause tumors if injected into hamsters. Viruses found in human tissues and in some rat cancers make hamsters bear deformed young with features resembling human mongolism. One virus that normally causes only grippe in man will cause cancer in hamsters.

Packaged Virus. Most baffling of all are the disappearing tactics of viruses involved in cancer. In one rabbit tumor, the virus cannot be detected in new, dividing cells, but is readily found in old cells where it has already done its damage. This seems to be a case, says the Sloan-Kettering report, of "the more active, the less evident." And it is the opposite of the situation in most viral infectious diseases, in which the virus abounds and is easily detectable as the fever approaches its climax, because infected cells are then mass-producing new virus particles.

The structure of many viruses in their conventional forms is well known. They consist of a core of nucleic acid--either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA)--wrapped in a protein overcoat. It is in this form that they are most readily detectable. And also, it appears, most active: the naked nucleic acid alone (stripped of its overcoat by delicate chemical means) can produce most of the effects of the whole virus, but it is a thousand times less powerful. Evidently, the researchers suggest, the virus needs to be "carefully packaged for safe transmission." One effective package design is like a tadpole: the virus uses its tail as a stinger to pierce the cell and inject the nucleic acid. Researchers at the National Cancer Institute have just reported photographic evidence that this is the mode of attack used by a mouse leukemia virus.

Foiled so far in their efforts to find viruses that can be indicted as essential factors in human cancer, the researchers are looking for guidelines in viruses that parasitize lower forms of life. They have a suggestive clue in diphtheria bacilli. All the microbes of this species can cause infection in man. but only a few have the dread power to manufacture the poison that leads to the formation of a dead ly, strangling membrane across the victim's throat. And this power depends on the microbes' being infected, in their turn, with a tiny particle of nucleic acid--the core of a virus, which has penetrated the bacterial cell. Why should not human cells become cancerous when a similar fragment of viral nucleic acid gets into their chromosomes and causes them to reproduce abnormally? By this reasoning, viruses have been called "bits of heredity in search of a chromosome."

From Plant to Man. It may be, according to Sloan-Kettering's Director Frank L. Horsfall Jr., that there are no special cancer-inducing viruses, but that in the appropriate host and in the appropriate circumstances perhaps any virus can invade the chromosomes of a cell and start the process of abnormal reproduction which we call cancer. A bit of evidence in support of this view came from Sweden's famed Geneticist Albert Levan. He has found breaks or changes in the chromosomes of children recover ing from measles. Though he still has no proof that such changes lead to cancer in later life. Dr. Levan is checking the effects of other common viruses.

It was in the tobacco plant that viruses were first shown to be capable of causing disease of any kind. Then came foot-and-mouth disease. Only after that was a disease of man--yellow fever--attributed to a virus that nobody had yet seen. Now, though words such as virus, gene, mutation, and even infection are taking on new meanings, medical history may be repeating itself as the cancer studies advance from plants to animals to man.

This file is automatically generated by a robot program, so reader's discretion is required.