Office of Public Affairs
Baylor College of Medicine
One Baylor Plaza - 176B
Houston, TX 77030
Tel: (713) 798-4712
Fax: (713) 798-3692
email: pa@bcm.tmc.edu

Photos Available Upon Request

p53 - a gene with many faces
By Kate Ramsayer

Larry Donehower, PhD

With a few clicks of his mouse, Larry Donehower, PhD, pulls up a bar graph illustrating the quantitative history of p53 research. Beginning with the discovery of p53 in 1979, and continuing through the modest beginnings of the field in the early 1980's, during which fewer than 50 papers were published each year, the slope of the graph remains fairly flat. This was altered drastically in 1989, when p53 was identified as a tumor suppressor gene mutated in numerous human cancers. Interest in the gene skyrocketed. In 2001 alone, at least 3500 papers were published on p53.

The subject of all this excitement is a gene that is not even essential for the growth or development of an organism. When called upon, however, it can protect cells from a fate worse than death - the unregulated growth and incorrect gene expression that leads to cancer. It is a gene with the unassuming name of p53, which encodes for a set of instructions so essential that one tiny misstep can wreak havoc in a cell.

"Every year I update the graph, and so far it hasn't plateaued yet. It keeps getting bigger every year," remarks Donehower, a professor of molecular virology and microbiology at Baylor College of Medicine in Houston.

Researchers at Baylor College of Medicine are taking a wide range of approaches to elucidate the many pathways in which this multifaceted gene is involved, using model systems ranging from specialized cell cultures to mice that mimic human diseases.

Guardian of the genome

	Xiangwei Wu, PhD

One of the principle functions of p53 is to act as the "guardian of the genome." When a cell detects damage to its DNA, p53 will either cause the cell to stop growth or division, giving repair mechanisms time to correct the damage, or it will forgo repair and activate one of the pathways that lead to apoptosis, a complex process in which a cell undergoes programmed cell death. Xiangwei Wu, PhD, an assistant professor in the molecular and cellular biology department and the Huffington Center on Aging, is studying this central question concerning the p53 stress response: Why does the cell need both of these pathways, and how is the choice between the two made?

In a screen to discover genes expressed only in cells that opt for the apoptotic cell death response, Wu and his group identified the gene Peg3/Pw1. By expressing this gene in cell cultures, they found that Peg3/Pw1 is pro-apoptotic, which means that it can induce cell death. They also observed that the cellular location of the pro-apoptotic gene Bax, is altered in apoptotic cells; it is moved from the cytosol (the liquid in which all organelles reside and in which much metabolism takes place) to the mitochondria, the intercellular structure that regulates apoptosis.

"We realized that the Bax translocation is a very important step for the regulation of the differential effect of p53," explained Wu, "and it turns out the gene we identified functions right here. It induces Bax translocation."

Wu and his group now theorize that activated p53 induces genes that cause both growth arrest and apoptosis at equal levels, but that the pro-apoptotic Bax remains inactive unless the p53-independent "modulator proteins" such as Peg3/Pw1 are induced and cause Bax translocation. Without these proteins, and without activated Bax, the cell will stop growing. Wu also noted that if modulator proteins have a response that is specific to different forms or degrees of cellular stress, the cell could be able to judge the severity of the damage, and determine the cell fate accordingly.

Knocking out and revving up

While Wu conducts his experiments in cell culture lines, many other Baylor researchers use mouse models to study p53. Donehower was the first to create and characterize the knockout p53 mouse, in 1992. "Those mice get cancer very rapidly, demonstrating that in the absence of its (p53's) activity, you are highly cancer prone," said Donehower.

As a follow up to the original p53 null mouse, Donehower's lab recently attempted to introduce a less rapidly lethal p53 mutation into a mouse, with unexpected and novel results. "It's a good example of serendipity. We actually anticipated a 180 degree different outcome," said Donehower. "We wanted a subtle mutation and here we got a drastic mutation, and it really turned out to have a very interesting phenotype."

Mice with one copy of the shortened mutant gene, which the researchers believe binds to and "hyperactivates" the wild type p53 gene, exhibited many early aging phenotypes. Their muscles and skin cells were reduced in number, they were slow to respond to injury and they showed signs of osteoporosis. "There is a general reduction in cellularity in different organs, and that suggests to me a failure of the stem cells in these different organs to keep up the supply of mature cells to each organ," observed Donehower.

With this exciting new function associated with p53, Donehower's lab is now investigating the possibilities that p53 is involved in the regulation of stem cells, and that it is having an effect on the age-related hormonal environment of the cell. "My idea is that as organisms got longer lifespans, p53 came along to allow them to have those lifespans without getting cancer," said Donehower. These experiments will help to define the relationship between these two processes.

p53 in breast cancer

Two other researchers who have built on the original p53 knockout mouse model of cancer are Jeff Rosen, PhD, and Daniel Medina, PhD, both professors in Baylor's molecular and cellular biology department. In order to develop a mouse model that would resemble human breast cancers, Rosen created a transgenic mouse with a p53 mutation common in human tumors. When he treated these mice with cancer causing chemicals called carcinogens, or crossed them with mice carrying the Neu or erbB2 oncogene, they formed tumors quicker. "These tumors looked a lot like what we call Stage 3 breast cancer, which are the more aggressive, aneuploid, genetically unstable types of breast cancer," said Rosen. Mouse models that mimic cancer might be used to test preventive strategies and treatments before they are tried in people.

Because advanced human cancers are usually aneuploid, which means they have an incorrect number of chromosomes, the underlying issue of genomic instability is a major focus of Rosen's lab. When a cell that contains too many or too few chromosomes divides, the machinery that usually allocates the correct number of chromosomes to each daughter cell is disrupted. This leads to chromosomes that are joined together or broken into fragments. "Mutant p53 seems to predispose to that," said Rosen of genomic instability, "and it doesn't seem like it's just a loss of the wild type, or the dominant negative effect of the mutant on the wild type, so the question is how it does that."

A student in his lab conducted a screen to identify genes induced by the mutant p53, and experiments are underway to determine how these genes lead to genetic instability.

Medina is working on a different mouse model that enables him to study the hormone response of tumors in mice with selective p53 loss. Starting with the p53 null mouse that was developed by Donehower, Joe Jerry, a former postdoctoral student in the laboratories of Drs. Medina and Janet Butel, set up multiple genetic crosses designed to get the knockout p53 allele into a strain of mice that are susceptible to breast cancer. The mammary epithelium of the knockout mouse is then transplanted into a wild type mouse, allowing researchers to study the effect of p53 loss solely in the mammary tissue.

Medina is interested in the molecular changes involved with p53 loss. Specifically, he is looking at how p53 null cells respond to the hormone progesterone. "If p53 is present and functioning normally, the same degree of hormone stimulation will not have any deleterious effect on the cells. But if you knock out p53, even if you only give it a short-term hormone stimulation, you get genetic instability. If you give it longer term hormone stimulation, you get tumors very rapidly," observed Medina.

As with many scientific studies, the issue now is to elucidate the mechanisms and events that occur once p53 is missing. "What is it about p53 that allows progesterone to have such a dramatic effect? Probably some genes that are normally present, that now in the absence of p53 are no longer expressed," said Medina.

Powel Brown, MD, PhD

Medina is also collaborating with Powel Brown, MD, PhD, an associate professor of medicine and the director of breast cancer prevention at Baylor's Breast Center, on experiments involving the effects of antiestrogens on p53 null mice. They have found that the estrogen antagonist tamoxifen, which blocks the effects of the hormone estrogen, can postpone the formation of tumors in mice that are treated with progesterone/prolactin.

Brown not only conducts research in the laboratory, but also has a clinic in which he sees patients at high risk for breast cancer. Although the familial breast cancer patients he counsels predominantly have mutations in the BRCA1 or BRCA2 breast cancer susceptibility genes, he also sees rare families with a mutated copy of the p53 gene, which leads to a disease called Li Fraumeni syndrome.

There are currently no therapies specific for Li Fraumeni patients, and Brown cites the small number of patients as a reason that clinical trials testing possible drugs exclusively in Li Fraumeni patients is not feasible. However, clinical trials in other high-risk women suggest anti-estrogens are useful drugs to reduce breast cancer risk, he said.

The ability to do both clinical and laboratory research is what Brown enjoys. "The experience in the lab is what I can apply to helping manage patients better. The experience with cancer patients keeps me motivated to work hard in the lab to find better ways to treat or prevent cancer. That's what so exciting about doing "translational research" - taking findings from the laboratory to the clinic and then using the information from the clinic to guide our laboratory research to find even better therapies."

p53 and lung cancer

	Charles Densmore, PhD

Developing and testing treatments in the laboratory for later possible clinical use is also one goal of Vernon Knight, MD, and Charles Densmore, PhD, both of the molecular physiology and biophysics department. Knowing that disruptions in the p53 pathway are the major causes of lung cancer, they have developed a method of using aerosol gene therapy to deliver functional p53 to cancerous lung cells.

This technology involves combining a positively charged molecule called polyethyleneiminine with negatively charged DNA encoding for p53. The two form a complex that can survive the harsh physiological climate of the lungs, enter the cells lining the airways, and deposit the DNA in the nucleus where it is expressed. "We found two very different mouse models of lung cancer in which we've been able to fairly consistently show inhibition," said Densmore.

One of the models is human p53 null osteosarcoma cells that are injected into the lungs of immunodeficient mice, done in collaboration with Eugenie Kleinerman at M.D. Anderson, while the other model is of mouse melanoma. "One of the things we think is going on, is that as p53 is expressed in these cells very efficiently, it leads to the up-regulation and the down-regulation of some genes that have an effect on the process of angiogenesis," said Densmore. Without angiogenesis, in which new blood vessels are formed, the tumors will starve and die.

Not content with using only one agent for aerosol therapy, these researchers are also looking at the effect of combining p53 gene therapy with radiation therapy or aerosolized treatments of chemotherapeutic drugs, including 9-nitrocamptothecin, itself in the first stages of clinical trials. "We have to build a whole structure of cancer therapy," said Knight. "We have to determine usefulness of each product as well as combinations of products."

The heart and p53

Lawrence Chan, MB, DSc

While p53 is primarily known as a gene involved in cancer, it has also been found to have a role in heart disease. Lawrence Chan, MB, DSc, a professor in the departments of molecular and cellular biology and medicine, studies atherosclerosis, and sees a connection between the two diseases. "When you develop an atherosclerotic lesion, what happens is that the plaques are nothing more than almost a benign tumor that grows and grows and occludes the artery."

An interest in what causes this unrestrained growth led Chan to investigate the role of p53 in lesion formation. When he crossed p53 knockout mice with a mouse model for atherosclerosis, he found that the p53 null mice developed larger lesions, which he had logically attributed to a decrease in apoptotic cell death. Upon further investigation into the mechanism, he found out that instead of being apoptosis related, this increase in lesion growth was actually due to the proliferation rate. "So we started with the hypothesis and ended up with an observation that fit," said Chan, "but then when we looked deeper it didn't. But it's good, we like new things."

New things are necessities in a field as intensely studied as p53 research. "It turned out to be very tough, very competitive," said Wu of the p53 discipline. "In this very crowded field you have to have something of your own." But at least at Baylor College of Medicine there is a community of researchers that "interact well and have common interests," an attitude confirmed by Medina and evidenced by the number of collaborations among faculty members.

Does Donehower think that his graph of p53 journal articles will ever start to slope downwards? "I think it will plateau when the next big gene comes along, but so far it's been an uphill climb. It seems like every time we think interest may decline, p53 comes up in another role like aging." With a wide range of interests and a multitude of experimental systems, these researchers will be sure to contribute to this expanding and biomedically relevant field of knowledge.

(Kate Ramsayer, a graduate of Williams College in Massachusetts and currently in a science writing program at University of California at Santa Cruz, was a science-writing intern at Baylor College of Medicine in the summer of 2002.)

Back To Top

© Copyright 2002 Baylor College of Medicine. All Rights Reserved.