Cancer geneticists at the Kimmel Cancer Center of Thomas Jefferson University, Philadelphia, studying specially bred "knockout" mice, have found that two genes that normally protect against cancer may play a greater role than previously suspected in female development.
In the September issue of Nature Genetics , Richard Fishel, Ph.D., and postdoctoral fellows Aaron Cranston, Ph.D. and Tina Bocker, M.D., report that female mice that lack a pair of tumor-suppressor genes, p53 and MSH2, stop growing and die a little more than a week after conception.
Fishel, professor of microbiology and immunology at Jefferson Medical College, and co-discoverer of the human MSH2 colon cancer gene, thinks the answer lies with the X chromosome, one of two sex chromosomes responsible for sexual development. Somehow, he theorizes, the missing genes throw off normal cell proliferation and development. He believes that by getting a better handle on the mechanisms by which these genes actually can affect female development, scientists may better understand the genes' roles in both normal and abnormal cancer development, as well as lead to important new therapeutic strategies.
"The observation goes to the heart of how tumors develop and to tumor genetics," he points out. "It was previously found that this particular combination of altered genes is significantly lower in human tumors--our results may suggest why that is the case. These are two of the most commonly altered genes in colorectal cancer and understanding their mechanism in carcinogenesis is crucial to the development of therapeutic strategies."
When one cancer-protecting--so-called tumor suppressor--gene is missing, mice--and people--are much more likely than normal to develop cancer. When two such genes are absent or defective, the thinking goes, the likelihood of cancer development would be greater still.
P53 is the most common known genetic defect in human cancers. It may contribute to the development of several cancers, such as breast, colon, and lung, among others. Normally, p53 is a kind of genetic guardian. If genes become damaged, p53 shuts down everything until the damage can be fixed. MSH2 is a gene that helps cells' DNA spell-check and repair itself during replication. In 1993, Fishel and colleague Richard Kolodner showed that when MSH2 is altered, it accounts for about half of all cases of genetically linked hereditary non-polyposis colorectal cancer (HNPCC)--one of the most common human cancer predisposition syndromes (Fishel, R. et al., Cell, 74:1027, 1993).
Knockout mice lack a working gene or genes and are used as models to study the effects of cancer gene alterations, often helping scientists understand cancer mechanisms and develop effective therapies.
The researchers found that male knockout mice died from cancer at an average of 273 days, which is perhaps two to three times as quickly as they might die with only one missing gene. They expected that. But they didn't expect to find that the combination of missing genes was lethal to the female mouse embryos. By day 9.5 of gestation the female mice stopped developing and died.
"The embryo was undergoing global apoptosis, or programmed cell death," he said. "These embryos are self-destructing. As many as 60 to 90 percent of the cells underwent apoptosis."
Men have an X and a Y chromosome; women carry two Xs. During normal embryonic development, certain genes of one of the X chromosomes may be turned off, a process called X-inactivation. Normally, there is some damage to the X and other chromosomes when cells duplicate and go through a "cell cycle." Cells have an innate repair mechanism to fix the problem; both p53 and MSH2 are involved in regulating this cycle as well as controlling the genetic repair mechanism. Without these two genes, the result is "global, catastrophic cell death in developing females," according to Fishel.
One mystery that remains is the precise role of the two genes in preventing such mass cell death and allowing normal development. The p53 gene has been thought to have a critical role in apoptosis.
Fishel sees several possibilities. "The interesting observation is that they are undergoing global apoptosis independent of p53," he noted. "Many studies suggest that cells that decide to undergo apoptosis do it in a p53 dependent pathway. That presents two intriguing questions: what is this p53 independent pathway, and why are they [cells] dying? The only differences between males and females are the extra X or a Y chromosome. It might be that one of the X chromosomes is damaged [beyond repair] or the Y chromosome provides protection," he said.
"At the moment we favor the 'excessive X chromosome damage' argument since both MSH2 and p53 are involved in managing DNA repair."
Colleagues at both Jefferson and at the University of Toronto also contributed to the work.