Scientists have created a new strain of mice lacking cyclin D1, a vital component of the growth machinery in all cells, and found that knocking out this important cog causes surprisingly little damage.
These results have implications for treating human breast cancer and should lead to a better understanding of the molecular basis of cancer. The study, reported in the August 25 issue of Cell, was carried out in the laboratory of Dr. Robert Weinberg, a cancer research pioneer at the Whitehead Institute for Biomedical Research and professor of biology at MIT.
"Cyclin D1 is overproduced in more than half of human breast cancer biopsies, but treatment strategies have not targeted this protein because of the damage they might cause to normal cells in other tissues," Dr. Weinberg said. "These new results suggest, however, that breast cancer therapies designed to block cyclin D1 action may prevent the growth of tumor cells without harming normal tissues."
Dr. Piotr Sicinski and his colleagues at the Whitehead Institute developed the mouse strain by replacing the normal mouse cyclin D1 gene with a nonfunctional version in embryonic stem cells. Because cyclin D1 is thought to be a vital component of the growth machinery in all cells, the researchers had expected the mutation to be lethal to embryos developing from these stem cells. Surprisingly, a number of embryos developed to term, and adult mice showed relatively few negative consequences: reduced body weight, mild neurological impairment and an underdeveloped retina.
Researchers noticed a striking difference in the mutant mice when females delivered pups. The mother mice, whose breast tissue was otherwise normal, were unable to nurse after giving birth. Without cyclin D1 their mammary cells failed to undergo the rapid growth that normally occurs during pregnancy. The researchers concluded that in adult female mice, the cyclin D1 molecule seems critical only for a specialized process in breast tissue-the rapid growth of mammary cells during pregnancy.
The finding sheds light on the observation that a majority of human breast cancer cells have higher than normal levels of cyclin D1. While normal levels of this protein are critical for normal mammary cell growth, excessive levels of cyclin D1 may lead to uncontrolled, cancerous growth.
The researchers also found that the new mouse strain had a strikingly low number of retinal cells, indicating that cyclin D1 is required for the growth and development of retinal cells during embryonic development. Collaborators in this study included researchers from the Baylor College of Medicine in Houston, Harvard Medical School and Michigan State University.
Several years ago, scientists discovered that each cell has its own internal clock mechanism-the biological equivalent of a mini-computer inside the cell nucleus-to control its cycles of growth and division. In a normal cell, the clock mechanism is well regulated; in many cancer cells, the cell cycle clock runs wild, causing these cells to grow even without the growth-promoting signals that are normally necessary to induce growth.
In 1986, Dr. Weinberg and his colleagues isolated the first known growth-suppressor gene, Rb, the retinoblastoma gene. Scientists now know that the retinoblastoma protein, pRB, is the vital part of the complex cell cycle clock that regulates cell growth by acting as a brake. Cancer cells lack the retinoblastoma protein required to brake the uncontrolled growth. Scientists have known that cyclin D1 plays an important role in inactivating the Rb protein brake and that a perturbation in cyclin D1 might represent one cause of the unbridled division that leads to cancer.
"To explore the role of cyclin D1 in the growth and differentiation of various cell types, we used transgenic technology to determine what happens when you knock out its function in mice," Dr. Sicinski said. Once he and his colleagues found that the mice lacking cyclin D1 showed certain physical characteristics, such as reduced body weight, the underdeveloped retina and the inability to nurse infants, they began to search for the biological basis of these characteristics.
Collaborators at the Baylor College of Medicine used state-of-the-art in situ hybridization techniques to gauge the levels of cyclin D1 in various cells. They found an extremely high level of cyclin D1 in the retina, suggesting that cyclin D1 plays a special role in the development of the retina. Collaborators at Michigan State University explored the possibility that inadequate levels of receptors for the ovarian steroid hormones, estrogen and progesterone, in the breast epithelium might explain why the mouse breast epithelial cells failed to grow during pregnancy. Their studies showed that the receptors are present at normal levels, leading the research team to conclude that the rapid growth of the mammary epithelium during pregnancy may be cyclin D1-driven.
This work was funded in part by grants from the National Institutes of Health, the Mathers Foundation, the Leukemia Society of America, the Medical Research Council of Canada, a Pew scholarship, the Howard Hughes Medical Institute and the American Cancer Society.
A version of this article appeared in MIT Tech Talk on September 13, 1995.