By David N. Leff
Ovarian cancer is the fourth leading cause of death from cancer among American women, and the top killer from gynecological malignancy. An estimated 14,500 U.S. victims of the disease will die of it in 1998. One reason for ovarian cancer's high mortality is its stealth - causing no symptoms until too late. By the time an ovarian tumor is diagnosed, it has likely spread far beyond the ovary into other parts of the body, beyond the reach of timely surgery.
Despite the best efforts of surgeons, plus an array of chemotherapeutic agents, more than half of all patients so treated are dead within five years of such combined treatment.
Ovarian cancer can strike women in the prime of reproductive life, but the majority of its victims are over 50. This age-related tendency has given rise to a theory that the cancer's likelihood increases with the number of ovulations experienced in a woman's life.
Biophysicist Daniel Pinkel, on the faculty of the University of California Cancer Center, in San Francisco (UCSF), suggested a mechanism: "I think it is related to the damage done to the ovary when ovulation takes place, and the healing of that damage," he told BioWorld Today.
Pinkel is a co-author of a report in the January 1999 issue of Nature Genetics, titled "PIK3CA is implicated as an oncogene in ovarian cancer." Its two co-senior authors are molecular geneticist Joe Gray, professor of laboratory medicine at UCSF and Gordon Mills, chairman of molecular oncology at M.D. Anderson Cancer Center, in Houston.
"This paper," Mills told BioWorld Today, "provides the first evidence that the PI3-kinase protein functions as an oncogene in a major cancer, in this case, ovarian carcinoma. And that finding suggests it as a new target for therapy in ovarian cancer, and potentially in other cancers."
The co-authors based their research, Mills continued, "on the hypothesis that if one studies genetic abnormalities in tumors, one will find the targets for therapy for patients. So, Joe Gray at UC-San Francisco studied the genetic abnormalities in ovarian cancer with a scanning technique called comparative genomic hybridization [CGH].
"The way it works is, first, you label DNA from a tumor cell with a green stain, and dye DNA from normal cells red. You then probe that on a normal genome, in this case a spread by fluorescence in situ hybridization [FISH]. And you look for areas where you have increased amounts of green fluorescence, which would mean more tumor, or areas where there's more red, which means you have less DNA present in the tumor than you had in your normal control.
"With that," Mills said, "Gray noted an area on the long arm of human chromosome 3 that was amplified frequently in ovarian cancer. He then showed that one of the genes in that area is a gene called PIK3CA, which encodes the catalytic subunit of the PI3-kinase protein."
Mills and his team in Houston then took that information and showed that RNA protein and enzyme activity were increased in ovarian cancer, and that the ovarian tumors were sensitive to the effect of a compound, LY294002, that inhibited the PI3-kinase protein in vitro.
"We have some brand new preliminary data from our collaborator in San Francisco, Robert Jaffe," Mills observed, "which says that this inhibitor [from Eli Lilly and Co., of Indianapolis] we used also works on ovarian cancers in vivo, in animal models. Jaffe did the mouse experiments, which showed that that inhibitor could block the growth of human ovarian cancer cells in nude mice."
More DNA = More Gene Expression = More Tumor
At the heart of this entire discovery was the amplification of DNA in ovarian tumor genomes.
"One of the threads in our research," Pinkel said, "was to look, with FISH and CGH, for regions of the ovarian cancer genome that had an abnormal gene copy number. The basic idea there is that regions of abnormal copy number tend to be regions where critical genes are located, and the amplified copy number alters their expression levels.
"What we're talking about here is a piece of DNA much larger than any one gene, being multiplied," he said. "For example, if there is a gene that contributes to cancer, then increases in copy number of that gene, and perhaps surrounding genes, will increase its expression, and thereby be selective for the development of the tumor. In this case, we found a region of repeatedly increased copy number on human chromosome 3. That region included several known genes - among them, this PIK3CA - and a whole bunch of unknown genes. Who knows what else is there?
"So, when copy-number changes were found in that part of the chromosome," Pinkel said, "there was a red-hot suspect waiting there. The idea is that those changes contribute to the health of the tumor, not to the health of the patient. And, in a Darwinian selection process, the tumor progressively selects cells that are more and more able to grow abnormally, so whatever random genetic changes are taking place, the ones helpful to the malignant growth are selected for."
An Unknown Gene A-Lurking?
"One of the things, here, that's open," Pinkel said, "is that there may be another gene in this same region of increased copy number, which could be critical for ovarian cancer, but which nobody knows about yet."
Mills made the point that "other tumors are going to have amplification in PI3-kinase. Breast [cancer] doesn't; melanoma doesn't. But there is an indication that the same areas of chromosome 3's long arm are amplified in small-cell lung cancer and cervix cancer, so it's likely that the same story is going on with PI3-kinase.
"Where that happens," he said, "in prostate, gliomas, endometrial cancer, they may act just as if they have an amplified PI3-kinase. They might also be more sensitive to this Lilly inhibitor. So one of our next steps is to identify better and more selective inhibitors of PI3-kinase, take them into animal trials, toxicity studies, and then to human therapy. It may be possible to take LY294002, and one other existing systemic inhibitor, wortmannin, to Phase I clinical trials within 18 months." N