Tumor-Suppressor Gene Located in Liver Cancer Cells

A malfunctioning "traffic cop" gene apparently playsan important role in the formation of liver cancer, accordingto researchers from the Duke University Comprehensive Cancer Centerand Zeneca Pharmaceuticals of Chesire, United Kingdom.

The discovery that the gene-mannose 6-phosphate/insulin-like growthfactor II receptor (M6P/IGF2r)-acts as a tumor-suppressor genein human liver tumors could help researchers develop an earlydiagnostic test for liver cancer as well as new treatments, theresearchers said. The disease is often far advanced at detection,and the 5-year survival rate in the United States is only 4%.A report on this tumor-suppressor gene appears in the December1995 issue of Nature Genetics.

"Liver cancer is one of the most common cancers worldwide,particularly because of its association with viral hepatitis,"said Dr. Randy Jirtle, professor of radiation oncology at DukeUniversity Medical Center and prinicipal investigator of the study.The lack of effective treatments also makes it a particularlydeadly disease, he said.

"As a consequence, knowing something about liver tumor formationor having a handle on how one could detect these tumors when they'remuch smaller could have a significant impact on survival,"he said. Drs. Gerald R. Hankins and Mary K. Washington of Dukeand Drs. Angus T. De Souza and Terry C. Orton of Zeneca coauthoredthe study.

The protein receptor produced by M6P/IGF2r is an attractive targetbecause it is present on the cell surface and in the plasma, Jirtlesaid, making it readily accessible for use in both liver tumortherapy and diagnosis.

Researchers often work backwards from the biologic changes involvedin cancer to find the genes responsible for these alterations.In this case, however, the functions of an already identifiedgene led Jirtle and his team to hypothesize its involvement inliver cancer.

"This is an 'old' gene for which we have characterized animportant new role," Jirtle said.

Three Distinct Regulatory Roles Within Cells

When working properly, the M6P/IGF2r receptor protein has at leastthree distinct regulatory roles within a cell, said Jirtle. Itis involved in activating a very potent growth inhibitor, transforminggrowth factor beta, and it disables a positive growth factor,insulin-like growth factor II. The receptor also works insidethe cell as a shuttle craft, moving proteolytic enzymes to thelysosomes, a part of the cellular digestive system that breaksdown proteins into simpler compounds.

Because the receptor is involved both in switching on a growthinhibitor and inactivating a growth factor, the researchers hypothesizedthat losing it might well predispose a cell to cancerous growth.Their past studies showing that the protein was abundantly presentin normal liver cells but nearly absent in cancer cells strengthenedtheir suspicion.

In research published earlier last year in Oncogene, Jirtle'steam found that liver tumors from 64% of patients studied hadlost one copy of the gene. The deletion of one copy, or allele,means that a mutation in the remaining copy can limit or destroya cell's ability to produce functional protein.

Next, the researchers began the arduous process of screening thelarge gene for one or more mutations that might disable it. Usinga method to detect mismatches in genetic material, they comparedstrands of DNA from tumor cells and surrounding normal tissue,and discovered mutations in the tumor samples. The discovery ofidentical mutations in more than one tumor indicated potential"hot spots"-regions of the gene that may be more susceptibleto mutation.

"One mutation results in an altered protein that lacks theability to insert itself into the cell membrane," accordingto Dr. De Souza, who has spent three years at Duke Universityinvestigating this gene. "Instead of carrying out its normalregulatory functions, this shortened protein leaves the cell.Once outside, it can no longer regulate cell growth."

The gene is also interesting to researchers because it could helpexplain why mice appear to be more prone to liver tumor formationthan humans. Mice normally have only one active copy of this tumor-suppressorgene rather than the two working copies humans possess. This evolutionaryoddity could have important implications for species susceptibilityto liver tumors and human risk assessment.

Animals and humans have two functional copies of almost everygene, one from each parent. In rare cases, a gene is imprinted,meaning that only one of the parental copies is active. The M6P/IGF2rgene is especially unusual, because it is imprinted in mice butnot in humans. Lacking the back-up that humans have in case onecopy malfunctions, mice are more likely to lose complete functionof this gene, the researchers said. If further studies show thatloss of M6P/IGF2r function is important to the way that chemicalscause liver tumors in rats, the scientists explained, pharmaceuticalcompanies could take into account the decreased likelihood ofthis genetic loss occurring in humans when determining whetherto pursue the testing of a promising new drug.

Jirtle and colleagues are continuing their studies of M6P/IGF2rby investigating whether this gene also plays a role in the formationof other types of tumors. After 4 years of study, Jirtle said,"We are at the end of the beginning."