Genetically Altered Mice Offer Clues to Cancer

While most physicians today have discontinued the practice of making house calls, one professor at Harvard Medical School still makes them.

Well, sort of.

"We make mouse calls," proclaims a sign on the desk of the secretary of Philip Leder '56, Andrus professor of genetics and chair of the Medical School's Department of Genetics.

Leder's genetic research gained worldwide attention in 1988 when he became the first to obtain a patent on a genetically altered animal.

With the ability to inject a mouse egg with a gene that may lead to cancer, Leder says he and his staff of researchers can test various hypotheses about cancers.

By looking at whether or not the mouse's offspring develop cancer, Leder can learn what secondary factors influence the development of malignant tumors, he says.

Leder says that with the aid of the genetically altered--or transgenic--mice, he has discovered much about cancer that could not be learned from just studying tissue cultures.

"It is a disease which occurs in an organism," he says. "If you want to understand a disease this complex which occurs in a living organism, you have to study that organism."

Genetics and Cancer Linked

In the last decade, researchers have learned that genetics and genes are to a large measure at the root of the cancer problem, he says.

But this doesn't mean that humans necessarily inherit the propensity to develop cancer from their parents, or pass it on to their offspring, he adds.

"This is not genetics as we conventionally think about it," Leder says. "When cancer occurs, it reflects something that has gone wrong on the level of gene mutation."

In other words, some oncogenes--or cancer-causing genes--may be necessary, but not sufficient, for the development of malignant tumors, Leder says.

Mice Are Good Models

Determining which of these genes are involved in cancer is made especially easy by the use of the transgenic mice, Leder says.

"These animals, since they spontaneously develop cancer, are good models for regiments, or testing procedures to prevent incidence of malignancy," Leder says.

"Using recombinant DNA technology, we can isolate genes which we think may be involved in cancer, we can alter them in accordance to a hypothesis we have about the development of that cancer, and then we can test the hypothesis by introducing the gene into the fertilized egg of a mouse," he continues.

The offspring of this mouse will have the gene which has been altered, he adds, and if the gene is involved in the development of that malignancy, then it would be expected that the offspring will develop the cancer.

For example, the mice could be used to test the theory that a high fat diet is a factor in the development of tumors. Another theory, that pregnancy at a young age in humans prevents cancer, can be easily tested.

"You can make any model system you want," says Leder, including mice to study colon cancer, leukemia, lung cancer or any other form of the disease.

The model system, he adds, is a good one to test agents which might be useful in the treatment of tumors because these are, in a sense, naturally occurring tumors in their native context.

The strains are kept breeding, Leder says, and can even be bred to one another so that one animal's genetic code includes two factors for the causation or prevention of cancer.

"Transgenic mice are used all over the world now in research," says Leder.

Initial Research

Leder says that studies done may years ago on concer known as Burkitt's lymphoma paved the way for his transgenic research.

The disease causes lymphomas, or tumors of the blood, in children. Leder says that upon characterization of the cancer, the genetic background of its development produced some interesting results.

"Invariably, it occurs as a particular kind of genetic damage called a translocation," he says. A translocation occurs when a piece of a chromosome is broken off and joined to a different chromosome abnormally.

"The reason we arrived at Burkitt's lymphoma was that the research was directed at the genetic basis for formation of antibodies," says Leder.

He says that researchers were asking where the genetic information necessary to create the body's millions of antibodies came from.

When the DNA locus of the encoding of these antibody producers was identified, he says, it was found that this locus was always involved in the same region as Burkitt's lymphoma. The insertion of the lymphoma abnormality, a gene known as c-myc, was responsible for the rise of the tumor.

At that point, his research team wanted to test the notion that the gene was actually involved in the development of cancer.

"We thought that the problem was that the fundamental control of that gene had been broken, or left behind, so that when it was brought into the new chromosome, it was brought in under the influence of a different control region," says Leder.

This different control region, he says, was found to be involved in the synthesis of antibodies.

The team cloned the normal gene and systematically removed what were thought to be the gene's regulatory sequences. In their place were inserted regulatory sequences borrowed from a virus which would induce irregularities in breast epithelial cells and would lead to breast cancer.

This gene was then injected into a mouse egg. After the altered gene was taken up by the nucleus of the egg, the egg was placed in the uterus of a 'sham' female mouse, one which had been mated top a sterile male.

Leder says the female would give birth to a litter of mice, some of whom will have taken up that gene.

"When that happens, we then ask whether or not the gene we produced will develop malignancy," he says.

Sure enough, after 300 days, Leder says, 50 percent of the mice had developed breast cancer.

Another important result, according to Leder, was that the tumor was only present in a certain part of the animal's breast.

"Since the tumor is the descendant of a single cell, only one cell, therefore, has changed enough to develop cancer," he says. This meant that some other cancer-causing event had taken place in that cell.

This led to the conclusion that the gene suspected as causal in Burkitt's lymphoma was necessary but not sufficient for cancer, Leder says. It was also concluded that the gene could cause cancer in other mammals, and in regions of the body other than the blood.

Current Uses of Technique

One of the issues that Leder's research team is currently trying to understand is how the immune system might provide protection against malignancy. One arm of this study is growth factors which affect the immune system, known as lymphokynes.

"One of these [lymphokynes] initializes a host response which causes the rejection of tumors. We are testing the notion that this might represent a natural host system to prevent development of malignancies," Leder says.

In other research, the group is studying problems of the development of the embryo.

"We created a mutation which affects development of limbs. This is a wonderful opportunity to understand what goes on when you shape a normal limb," says Leder.

By using the transgenic mouse, he says that the system of limb pattern development can be understood in animals ranging from the mouse to humans, since this system development is very well-preserved.