Genes and Cancer: Discovering their Related Secrets, Step by Step
Geoff Wahl will never forget the day in 1989 when he got a crazy idea for a new experiment. He had already been at home for some time when it hit him just before 9 p.m. He rushed back to his lab at Salk that night hoping somebody would be there. He was in luck. He found his postdoc, Yuxin Yin.
Click here to listen to Geoff Wahl discuss p53.
"This is what's great about science. I told him, 'I've got this crazy idea, but it's a good crazy idea because it suggests an experiment,' " Wahl says.
For more than a decade, Wahl's lab had been working to understand the mechanisms behind genetic instability and its link to cancer, and by this point they suspected the p53 gene, which is now known to be mutated in half of all human cancers, played a key role.
Wahl's crazy idea came at a time when his lab was studying an anti-proliferating compound. When administered to cancer cells, most would die off, with the exception of a few resistant ones that continued to divide. But what if the drug was applied to noncancerous cells with normal p53 genes?
As Wahl predicted, the cells stopped dividing completely, which led him to test one final idea: replace the mutant p53 gene of a cancerous cell with one from a normal cell. To his team's relief, the cancer cells stopped dividing.
Published in 1992, the series of experiments was among the first to demonstrate p53's role in maintaining genomic stability – leading many in the field to regard p53 as the Guardian of the Genome.
It is in fact at the genetic level where cancer begins. To be precise, it is the mutation of specific types of genes that leads to the development of the disease, which the National Cancer Institute (NCI) estimates will claim more than 562,000 lives this year (1,540 per day) in the United States. It also estimates that nearly 1.5 million more will be diagnosed with cancer in 2009.
Mutations in our genes take place every day of our lives. The simplest examples are DNA damage to skin cells caused by the sun's ultraviolet rays, or in the lining of our gut from toxins in food that we ingest, or in the lung as a consequence of smoking cigarettes. Despite the common frequency of these biological events, many of us may never develop cancer.
Why? Cells are equipped with mechanisms -- p53 chief among them – that detect and either repair DNA damage or signal the cell to self-destruct if the damage is too great.
"These genes are part of the normal growth process for human beings. This is how we have evolved," says Reuben Shaw, Hearst Endowment assistant professor in the Molecular and Cell Biology Laboratory, who, like Wahl, is one of 30 faculty members in the Salk Institute's NCI-designated Cancer Center. "Every species has evolved a mechanism to get rid of the cells that are chronically exposed to something that could be bad for them."
But the machinery isn't fool proof. For reasons not yet completely known, cells with DNA damage sometimes go unrepaired and continue to divide as mutant copies of their original self.
These mutant cells are prime candidates for cancer development in cases where the affected genes are normally designed to regulate cell growth (oncogenes) or cell death (tumor suppressor genes, such as p53). In the unfortunate event a second or third mutation occurs to multiple genes, cancer will always develop. The process is expedited if an oncogene is mutated in combination with damage to p53 in the same cell, Shaw says.
"P53's main function is to serve as a sensor for cells to make sure it is safe for them to reproduce their genetic dowry, or to prevent them from attempting to do so when conditions are suboptimal," says Wahl. "If p53 function is lost because of damage to its gene, then the cell has no ability to sense impending dangers, which enables the cell to divide and leads to production and perpetuation of mutations. This creates opportunities for additional mutation to form, and that's when things really start going haywire. The problem just compounds itself."
Scientists were first clued into cancer's genetic nature after Salk's Distinguished Professor and Nobel Laureate Renato Dulbecco collaborated on a study that explained how oncogene carrying tumor viruses interact with host cell chromosomes to replicate themselves and induce cancer. The work led to Dulbecco's Nobel Prize in 1975 and revolutionized the way scientists think of the disease.
The discovery opened the door to new areas of cancer research, which has led to a deeper understanding of the disease over the last three decades. Today, scientists can now identify and quantify the steps that lead to cancer in the body. There is also growing evidence that chronic inflammation can also induce cancer, especially in the liver and pancreas, the latter of which claimed the life of actor Patrick Swayze in September.
Interestingly, some of the rare cases in which patients inherit defective copies of either oncogenes or tumor suppressors have helped scientists zero in on genes that are involved in specific types of cancer in the rest of the population.
"The genes shared by members of a single family are 99 percent identical so you can pinpoint the key genetic difference between the family member who got cancer and the one who didn't," Shaw explains. "Then you study the genes of a completely unrelated cancer patient and trace the mutation back to that same spot in their DNA and bingo, you've found the cancer-causing gene."
There are approximately 22,000 genes in the human genome. To date scientists have identified and decoded more than 300 cancer-causing genes, which has enabled researchers to match them up with the types of cancer they induce. This in turn has led to the development of drugs to counteract the activity of these mutant genes.
Researchers at the Salk's Cancer Center have identified a number of them. Oncogenes FOS and MOS, which are mutated in fibrosarcomas, and REL, which leads to lymphosarcomas, were identified by Inder Verma, an American Cancer Society professor in the Laboratory of Genetics.
Work by Ronald M. Evans, professor in the Gene Expression Laboratory, led to the discovery of a family of nuclear hormone receptors, a member of which – the estrogen receptor (ER) – is now known to contribute to the development of breast cancers when expressed at high levels. Another nuclear hormone receptor, the retinoic acid receptor (RAR), plays a key role in promyelocytic leukemia when joined to the product of a second gene.
"The fundamental progress in cancer research over the last 30 years has been to identify specific genes that cause cancer," Dulbecco says. "Once the genes responsible for cancer have been identified, you can move forward to therapy."
Click here to listen to Reuben Shaw discuss his latest experiments using compounds to treat cancer.
Founded in 1970 by Jonas Salk and designated an NCI Center in 1973, the Salk Institute Cancer Center is led by Tony Hunter, an American Cancer Society professor in the Molecular and Cell Biology Laboratory. His discovery of tyrosine phosphorylation, a key event in normal cell growth which can drive tumor cell proliferation when unregulated, eventually led other scientists to develop a new generation of cancer drugs, including Gleevec – a leukemia drug whose developers received the Lasker Prize for their work in October.
The Cancer Center is made up of three distinct research programs: Metabolism and Cancer, Mouse Models and Stem Cells, and Growth Control and Genomic Stability. Thirty of Salk's 57 principal investigators, 161 postdoctoral researchers, and 70 graduate students are part of the Center, which continues to make new breakthroughs with potential for new therapeutic strategies.
Like Wahl's, some discoveries are born from unorthodox ideas.
This year, Shaw published a study that demonstrated how he used rapamycin, an immunosuppressant drug normally prescribed to prevent organ transplant rejection, to drastically reduce tumors in lab mouse models developed to mirror Peutz-Jeghers syndrome, a rare cancer of the colon in which patients inherit a mutated copy of the LKB1 gene.
He got the idea after first linking LKB1, a tumor suppressor also commonly mutated in lung cancer, to AMPK, a protein involved with glucose production. Further studies by his team revealed that AMPK regulates TOR, an oncogene that is highly expressed in cancer (its name is short for Target of Rapamycin).
Shaw then asked the question: Will rapamycin work on LKB1-deficient tumors to block TOR activity? He found out that it does. The massive tumors that had developed in the mice's colons drastically shrunk in size and were stabilized in the experiments.
"This is a wonderful example of being able to use basic research and genetic modeling to treat human disease," Shaw says. "We basically put the mice through a clinical trial and test them with different doses of rapamycin."
Current studies in Shaw's lab are examining whether these same therapeutic approaches will work in a mouse model they have developed for lung cancer.
Other strategies being tested in the Cancer Center seem born out of science fiction, but show great promise. Clodagh O'Shea, assistant professor in the Molecular and Cell Biology Laboratory, and her team have developed a modified adenovirus, normally associated with upper respiratory infections in humans, to undergo selective lytic replication in p53-deficient cancer cells while leaving all other tissues intact.
Once introduced into the tumors, the virus multiplies and causes the cancer cells to implode – releasing thousands of viral offspring in the process to seek out more cancer cells. Initial clinical trials in the United States using an early version of the modified virus showed encouraging results when it was injected directly into the tumor. However, O'Shea believes the latest modified virus will be highly effective.
"In an ideal world, it would be really amazing if we could inject it systemically so it could spread throughout the body and find distant micro metastases in places where we don't even know the cancer has spread," O'Shea says.
Assistant Professor Clodagh O'Shea (right) with UCSD graduate student Kristen Espantman.
"However, the virus we work with is not a naturally blood-bourne virus, so there are factors that can limit its activity. That's why we are also turning to different serotypes/subgroups of virus that naturally infect different tissues, such as the colon and the kidney."
In order to really understand cancer, however, Dulbecco suggested in a 1986 Science article that the human genome would first need to be sequenced so that it could be compared to its cancer counterpart. Four years later the Human Genome Project was initiated by an international group of scientists and completed in 2000.
"Now that it's cheaper to sequence, it's clear that cancer cells have hundreds of mutations, which makes it more difficult to know which genes are important," Hunter says. "But in the long run, what we hope is that if enough cancer genomes are sequenced, commonalities will begin to emerge for particular cancer types so that they can be correlated with more effective treatments."
Most of the cancer-therapy drugs available today work to counteract the uncontrolled activity of oncogenes as a result of p53 mutation. But more is being done to find treatments for the other 50 percent of cancers where p53 is not damaged. In such cases, there are two proteins that are working in conjunction to degrade or completely inactivate the p53 gene: Mdmx2 and Mdmx.
"These are proteins that are working like anti-tumor suppressors," Wahl explains. "When they bind to p53, the cell can no longer sense DNA damage or initiate the cell death program."
His lab is now working on developing new compounds that impede Mdm2 and Mdmx's ability to bind to p53 and restore the gene's normal protective function.
"It's rare for academia to go into drug discovery experiments, but we felt it was important for us to do so, and we also feel these compounds will be very valuable research tools," Wahl says.
The most recent studies at the Salk involve research of stem cell-like cells and their relationship to cancer. It's a very new field that may provide more answers and possibly new treatments in the future.
Teams of scientists in the Verma and Fred H. Gage labs, for example, are using a model for glioblastoma, the deadliest and most common brain cancer in humans, to uncover the nature of specific cancer cells that are capable of spawning new tumors.
"There is increasing evidence that there is a population of cells in most cancers that have stem cell-like properties," Hunter says. "It's a field that's in flux at the moment because how these cells acquire these properties is unclear. There is a school of thought that you have to target these tumor-initiating cells because if you only kill off the rest of the tumor, then it doesn't matter because they simply regrow.
"Chromatin reprogramming is a critical step in the genesis of stem cells and this appears to be true of the stem cell-like cells in tumors," Hunter says.
A recent finding from the Wahl and Juan Carlos Izpisúa Belmonte groups has shed light on this idea. They found that p53 itself acts as a block to chromatin reprogramming in somatic cells, suggesting that the lack of p53 in most tumors may predispose cells in these tumors to undergo reprogramming and adopt stem-cell like fates. (See related story in this issue.)
As a result, there is now a heightened interest in using new-generation inhibitors that will block chromatin reprogramming as new types of cancer therapy. The Evans lab is currently testing one novel inhibitor developed by his team on colitis-associated colon cancer in lab mouse models.
"The Salk Cancer Center has a history of significant contributions that have led to a deeper understanding and treatments for cancer," Hunter says. "Over the next few years, new findings hold the promise to lead to new types of cancer drugs, and to alleviate the mortality and suffering from this terrible disease."