The identification of a specific pattern of gene expression in hepatocellular carcinoma cells capable of metastasis may help development of improved treatment plans, according to an article in the March 17th issue of Nature Medicine.

In addition, researchers discovered that expression of a specific gene is required for a tumor cell to be capable of metastasis. The gene, known as osteopontin, may be a useful diagnostic marker. The protein it produces is also a potential target for therapeutic agents for metastatic disease.

Hepatocellular carcinoma is one of the most common and aggressive malignant tumors. Although incidence rates are relatively low in the U.S., they are higher in Asia and Africa, where risk factors such as hepatitis infection and aflatoxin contamination are more prevalent. Poor survival among patients is attributed primarily to the high rate of metastasis after treatment.

The initial goal of the current study was to identify genes that would distinguish the original hepatocellular carcinoma tumor from metastastic tumors. Surprisingly, researchers found that genes expressed in metastatic tumors were identical to those in the original tumor, a finding that challenges the current model of metastasis progression. Instead, the researchers found differences in gene expression between tumors that would metastasize over time and tumors that would not metastasize.

"The fact that gene activity in metastatic tumors is identical to that in the tumors from which they originated, but metastasis-free tumors are distinct, suggests that changes favoring metastasis occur in the original tumor," said Xin Wei Wang, Ph.D., lead author of the study. "If we can identify in advance patients whose tumors are likely to metastasize, it will improve our ability to individualize treatment of their disease."

All of the tumor samples studied by the research team were taken from the original tumor regardless of whether or not the disease had metastasized. The investigators used DNA microarray technology to evaluate gene expression. Microarrays, or gene chips, are slides that have been coated with thousands of spots of DNA, each representing a different gene. When a gene is active, it produces RNA transcripts. To measure gene activity, researchers tagged the RNA transcripts in the tumor cell with a fluorescent marker. When the transcripts became bound to the corresponding DNA spots, the pattern and intensity of light emitted by each DNA spot reflected the gene’s relative activity.

After analyzing the activity of more than 9,000 genes within the tumor cells, a computer algorithm determined that a group of 153 genes could be used to distinguish between the two groups of tumors. The activity of the 153 genes differed significantly depending on whether or not a tumor had the potential to metastasize.

After investigators identified the set of genes that distinguished between hepatocellular tumors with and without the ability to metastasize, they used the gene set to see if they could correctly identify 40 blinded tumor samples. Of the 40 samples, the test correctly identified 82 percent of tumors with metastatic potential and 67 percent of
tumors that had not metastasized.

Researchers identified one gene whose activity was particularly high in tumors with metastatic potential. The gene produces a protein known as osteopontin, which plays an important role in metastasis. Increased gene activity leads to an abnormally high level of osteopontin protein, which appears to promote metastasis.

Blocking osteopontin activity prevented tumor cells from spreading in mice and in a cell culture model. Thus, osteopontin may be useful not only for predicting which tumors are likely to metastasize but also as a potential therapeutic target for hepatocellular carcinoma.

"Osteopontin can be found in all body fluids, which makes it an excellent diagnostic marker," said Wang. Because osteopontin protein is located on the outside of cells, it may be more easily reached by pharmaceutical agents than drug targets inside the cell, making researchers optimistic about its potential role in treatment.