Tumor-suppressor Gene Is Critical For Placenta Development

Those conclusions come from a new study of the retinoblastoma (Rb) gene in mice. In humans, this gene, when mutated, raises the risk of a rare cancer of the eye called retinoblastoma. Two decades ago, it was identified as the first tumor-suppressor gene, a class of genes that protects cells from becoming cancerous. It has since been shown to be inactivated in many cancers.

In this study, researchers shut off the Rb gene in stem cells that give rise to most of the placenta, resulting in an abnormal placenta and death of the embryos.

The findings provide new insights into development of the placenta and into how the Rb gene blocks tumor growth.

They also raise the possibility that this important tumor-suppressor gene might play a role in miscarriages.

The study, led by researchers at the Ohio State University Comprehensive Cancer Center, is published in the January 2007 issue of the journal Genes and Development.

“Our findings strongly suggest that the Rb gene is important in the development of the placenta, but they have other important implications, as well,” says principal investigator Gustavo Leone, assistant professor of molecular virology, immunology and medical genetics and a researcher with Ohio State's Comprehensive Cancer Center and human cancer genetics program.

“People born with one mutated Rb gene have a higher risk of developing retinoblastoma. But are they also predisposed to miscarriage? Do Rb-related defects in the placenta cause learning or physical abnormalities? We are investigating these questions now.”

Scientists know that the protein encoded by the Rb gene, the Rb protein, plays an important role in regulating how cells grow. But exactly how it does this – the molecule interacts with 110 other proteins – remains unknown.

In 2003, Leone and a group of collaborators discovered that loss of the Rb gene caused abnormalities in the placenta, especially where it contacts the uterus. They published that research in the journal Nature.

“This was the first evidence that Rb had an important role in the placenta,” says Pamela Wenzel, a graduate student in Leone's laboratory and first author of the new paper.

For this study, Wenzel developed a transgenic mouse that makes an enzyme that deletes DNA. Specifically, it removes the DNA that lies between two gene markers. She then used a second transgenic mouse in which the gene markers were placed in the Rb gene.

When she crossed the two strains of mice, the resulting embryos had the DNA-destroying enzyme and the gene markers in their placenta stem cells. The enzyme removed the DNA between the two markers, leaving the embryos without a working Rb gene in the placental progenitor cells.

These stem cells divided far too often. They gave rise to too many cells and formed a defective placenta, especially where the placenta interacts with the uterus.

These embryos died about 15 days after fertilization, a time at which placenta function becomes critical for survival (the gestation period for mice is 19 days).

Interestingly, the researchers also produced some mice in which the Rb gene was present in the placenta stem cells but destroyed in cells produced by them. These cells formed a normal-looking placenta and the embryos survived.

“This suggests that Rb is important in maintaining placental stems cells, and that it plays a smaller or different role in the cells they give rise to,” Leone says.

Lastly, the researchers produced transgenic mice that lacked both the Rb gene and a second gene, E2f3. The Rb protein normally couples with, or binds to, the protein encoded by the E2f3 gene. Embryos missing both genes lived three days longer than the embryos missing Rb only, but all died before birth.

“This showed that the interaction of these two proteins is definitely important,” Leone says. “It strongly suggests that E2f3 is a key player in mediating Rb function in stem cells and possibly in its role as a tumor suppressor.”

Next, Leone and his colleagues want to know if the Rb gene is important in miscarriages.

“Miscarriages have never been linked to a gene defect, but understanding the genetic basis of miscarriage would be a hugely important,” Leone says. “It would be the first link between a gene mutation, placental function and development.”

Funding from the National Cancer Institute, the National Science Foundation, the Pew Charitable Trust and the Leukemia & Lymphoma Society supported this research.