By David N. Leff
Cancer researchers, say some, fail to see the forest for the trees. That is, they focus on the few mutated genes in tumor cells while overlooking many healthy genes and their protein products.
Molecular geneticist Ruth Sager, of the Boston-based Dana Farber Cancer Institute, puts it this way:
"Cancer geneticists have ignored the productive potential of screening for differential gene expression between tumor cells and normal counterparts." She added: "Conventionally, only mutated genes have been considered as candidate cancer genes," while in fact the shape and behavior of tumor cells result from the altered expression of non-malignant cells.
She told BioWorld Today that a typical cancer cell contains 10,000 to 15,000 proteins, of which perhaps 1 percent are mutated.
Sager is sole author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Feb. 4, 1997. Its title: "Expression genetics in cancer: Shifting the focus from DNA to RNA."
"I follow the principle of 'divide and conquer,'" Sager said, alluding to her separation of cancer genes into mutated or deleted, and non-mutated or normal. She identifies them as 'Class I' and 'Class II,' respectively.
Instead of screening genomically for mutational differences between DNA in normal and tumor cells, Sager screens for differences in gene expression, "by differential display [DD], a powerful and novel procedure."
That procedure was invented by Sager's close collaborator at Dana-Farber, molecular biologist Arthur Pardee.
"It's a way of finding which genes are active," Pardee told BioWorld Today, "that is, making messenger RNA in a given cell. For example, in a cancer cell, DD enables you to readily compare many gene products, in two or more kinds of cells, by side-by-side inspection in a sequencing gel."
In cancer and normal cells alike, he pointed out, "the same kinds of housekeeping genes will be turned on * to control nutrient intake, respiration and so on. But many other genes, related for example to growth control, adhesiveness and invasiveness," Pardee went on, "are turned on differently in cancer cells and normal cells * for that matter, in heart cells vs. lung cells."
Drugs To Turn On Turned-Off Genes
He made the point that, "If you know which of these Class II genes is turned on, and if you can find out what they do, hopefully, you can make drugs against them."
Using DD, Sager has discovered 100 such Class II genes in breast cancer cells. Among them is maspin (mammary serine protease inhibitor), which sits on the long arm of human chromosome 18. It expresses a protein, maspin, that remains 'on' in normal mammary epithelial cells, but switched 'off' in metastatic breast cancers.
"The recombinant maspin protein," she reported, "is inhibitory in the invasion assay, and this inhibition was reversed by an anti-maspin antibody preparation that recognized the reactive center of the protein."
Sager has proposed a small molecule that acts like the maspin protein, as a bioavailable drug to slow tumor growth.
"There are certainly industrial applications of this DD technology," Pardee observed, "which I think are very interesting indeed." He is co-inventor, with molecular biologist Peng Liang, of issued and pending DD patents, assigned to Dana-Farber.
"Therapeutic approaches based on maspin have attracted interest from both biotechnology and pharmaceutical companies," Dana-Farber's director of technology transfer, Vincent Miles, told BioWorld Today. "The maspin technology," he added, "has been licensed exclusively to LXR Biotechnology Inc., in Richmond, Calif., which is actively exploring its therapeutic potential.
"Separately," Miles added, "three licenses have been granted for the differential display technology, and further licenses are available to interested parties."
"Many companies," Pardee added, "are using these DD techniques. Several of them are doing so under license to Dana-Farber. Some are not -- but they should be."
These firms, he said, "are looking for * and finding * genes that are differently turned on in various diseases, including mental and neurobiological disorders, viral infections, etc. This gives them a possible target for therapy."
Breast Tumors Share Genes With Prostate
Pardee has been applying the DD method to prostate cancer genes, and finding overlaps with Sager's 100 Class II breast cancer sequences.
"We've looked now at about 25 different genes," Sager said, "which were originally found as genes related to breast cancer. Almost all of them show very similar behavior, at the expression level, in prostate cells. There's a big overlap in the regulation of prostate compared with breast. And even though they're two different systems, and different genes, it's possible that the same genes may be involved.
"The genes that show the largest effects in common," she surmised," are good candidates for manipulation by a particular drug.
"The great majority of the genes that Sager has found," Pardee pointed out, "are suppressed, but not because they are mutated. The difference is," he explained, "that the usual tumor suppressor genes * like the BRCA1 gene, or retinoblastoma * are mutated in the gene. But Class IIs are not. Those far more numerous genes can be perfectly OK, but not expressed, presumably, because some upstream regulatory mutation has altered their controls."
Both Sager and Pardee emphasize that cancer is not necessarily irreversible. "You have a chance of reversing those turned-off Class II genes with drugs," Sager pointed out, "precisely because they're not mutated."
"If you have a mutation," Pardee observed, "you almost have to do some sort of gene therapy to restore the damaged gene. Which is very difficult indeed, despite a lot of hype about gene therapy."
He sees DD "as an alternative, for this purpose, to the Human Genome Project " It is much quicker, and at the moment, I believe, is going to be more useful. Eventually the Genome Project will have everything mapped, and even then you would have to compare data bases for, say, various cancer cells. They won't have everything for a long, long time."
Sager concluded: "It's a big expansion of the whole world of opportunities for bringing into the disease research market genes that were never previously considered in this way." *