BioWorld International Correspondent

LONDON - The discovery of a gene that is switched off in the majority of ovarian cancers could lead to new strategies for early diagnosis of the disease and, researchers hope, powerful new drugs to treat the disease.

When scientists manipulated the DNA of ovarian cancer cell lines that did not express the gene, to make the cells manufacture the protein the gene encodes, they found that the cells' growth was dramatically suppressed, relative to that of the original ovarian cancer cell lines. When, in mice, the genetically manipulated cells were delivered to the peritoneal cavity - where ovarian cancers grow - there was, likewise, dramatic suppression of tumor growth.

Development of a mouse lacking the gene already is under way, and will help researchers to understand how the gene has an effect in suppressing tumor growth.

The work is reported in the June 22, 2003, Nature Genetics in a paper by Hani Gabra, clinical scientist and consultant in medical oncology at the Cancer Research UK Edinburgh Oncology Unit at the Western General Hospital in Scotland, and colleagues. Its title is "OPCML at 11q25 is epigenetically inactivated and has tumor suppressor function in epithelial ovarian cancer."

"One of the important implications of this finding is that clinicians may, in the future, be able use this completely new avenue in cancer research to improve diagnosis for predicting response to chemotherapy, or even to develop tumor suppressing mimetics," Gabra told BioWorld International.

He cautioned, however, that more basic laboratory work needed to be done before scientists will know the exact implications of the finding. "We now need to work on understanding more about this gene and exactly how it works and what makes it switch off," he said.

Since the early 1990s, Gabra and his team have worked on identifying regions of chromosomes missing in ovarian cancers in the hope of finding tumor suppressor genes. The theory behind this search for "loss of heterozygosity" is that if one allele of a tumor suppressor gene is missing, then a point mutation or other damage in the other allele will give rise to cancer.

About 10 years ago, they identified a region of chromosome 11 as possibly harboring a tumor suppressor gene with a role in ovarian cancer. Later, when the genetic information from the Human Genome Project began to flood in, they were able to narrow the region affected by loss of heterozygosity to a section that contained only a single gene, called OPCML.

An examination of samples of ovarian cancers taken from patients showed that more than 80 percent contained no messenger RNA derived from OPCML, whereas normal ovarian surface epithelium (the tissue in which ovarian cancer arises) contained generous amounts of that RNA.

"When we saw that there was no transcription of the gene in the cancer tissue, it was obvious that we had to look for a regulatory mechanism that was affecting transcription," Gabra said.

Experiments showed that cytosine bases in the DNA encompassing the promoter region that lies upstream of OPCML were densely covered with methyl groups, a phenomenon known as somatic methylation. Gabra said, "We found evidence of this methylation in more than 80 percent of patients' tumors, a figure which fitted nicely with non-expression of this gene in tumors, whereas in normal DNA there was no evidence of methylation."

When the group took ovarian cancer cell lines that did not express the gene, and in which the cytosine bases of the regulatory region upstream of OPCML were methylated, and added a drug that has a demethylating action, they were able to switch OPCML back on again and detect messenger RNA derived from it.

Next, Gabra and his colleagues embarked on a search for mutations in OPCML in tissue samples from patients. From 200 samples, they found only one that had a somatic missense mutation that resulted in loss of function of the gene.

"This gave us proof that the gene is disrupted during the process of cancer generation," Gabra said, "although this is clearly a very rare event, and in most cases the gene is inactivated in other ways, by loss of heterozygosity and somatic methylation."

To provide additional proof that OPCML is a tumor suppressor gene, they transfected it back into ovarian cancer cells and overexpressed it. The growth of the cells slowed dramatically.

"We also decided to inject the transfected cells overexpressing OPCML into the peritoneal cavity of mice that provide a model for ovarian cancer - and we saw a dramatic and powerful suppression of tumor growth in these animals," Gabra said.

He added that the discovery of OPCML could allow the development of new methods of early diagnosis for ovarian cancer. It is possible to detect the methylated strands of the OPCML promoter in the blood, for example. One day, it may be feasible to carry out molecular imaging scans that show tissues lacking OPCML protein in a particular color.

There also is the potential to develop new drugs to treat ovarian cancer. Gabra suggested that those might mimic the function of the OPCML protein, or interfere with alterations to the downstream pathway that result following loss of the gene.