Blood-Screening Technique Detects Early-Stage Liver Cancer

Hepatocellular carcinoma (HCC) is one of the deadliest forms of cancer in China and throughout Asia. In the West, liver cancer is usually a secondary cancer, caused by the spread of tumor cells from elsewhere in the body; in China, liver cancer mainly manifests as HCC, a primary cancer, which has been linked to hepatitis B and C infection and cirrhosis. Noticeable symptoms do not usually appear until the cancer has progressed, so it is rarely caught early, when intervention would be most effective, and survival rates are typically low.

Due to widespread hepatitis B virus (HBV) infection, nearly 10% of China's population is at high risk for hepatocellular carcinoma (HCC). The new blood-screening technique, developed by scientists from the Chinese University of Hong Kong (Hong Kong, China), could make it possible to detect early-stage liver cancer and predict how well a patient will do following treatment. The scientists presented their data at the American Association for Cancer Research's Second International Conference on Molecular Diagnostics in Cancer Therapeutic Development, in Atlanta, Georgia (USA) in September 2007.

According to their report, the Chinese team has detected an altered version of RASSF1A, a tumor-suppressing gene, in the blood of HCC patients and in 58 % of HBV-infected test subjects. Healthy subjects showed no signs of the altered gene. They also found that patients treated for HCC with high blood levels of the gene were more likely to have a relapse of the disease.

Currently, ultrasound and computed tomography (CT) scans are the gold standard for detecting HCC. However, they are too expensive to be an effective mass-screening tool. About 70% of patients exhibit a detectable increase in bloodstream amounts of alphafetoprotein, but a screen for this protein would miss many potential patients. "We need a new biomarker for hepatocellular carcinoma, something that can be used to screen large populations of at-risk people for follow-up studies,” said Prof. Chan, MBBS, a professor at the Chinese University of Hong Kong.

It is known that the DNA of HCC tumor cells lack a functioning copy of RASSF1A. In these cells, RASSF1A is "hypermethylated,” meaning the RASSF1A gene has been physically altered by cancer-related processes that added methyl groups, to portions of the DNA within the gene. Since the cell's protein making system cannot access the gene, hypermethylation effectively knocks out the tumor-suppressing RASSF1A gene, which is then unable to stop cells from becoming cancerous.

While hypermethylated RASSF1A would make a useful biomarker for HCC, methylation-specific polymerase chain reaction (PCR), which is used to specifically amplify and detect methylated DNA, destroys about 85-93% of the DNA in a blood sample. Together with the fact that tumoral DNA is only present at very low concentrations in blood during early stages of HCC, this method has not been sensitive enough to detect altered RASSF1A in blood for the purpose of early cancer detection.

To compensate, the scientists invented a new technique that they call "methylation-sensitive enzyme-mediated real-time polymerase chain reaction (PCR),” which combines real-time PCR, a technique that enables scientists to simultaneously detect and amplify a given gene, with an enzyme that breaks unmethylated DNA apart. With this new technique, Prof. Chan's team was able to separate out the altered methylated DNA, thus developing a more sensitive technique for detecting and quantifying hypermethylated RASSF1A derived from cancer cells in blood.

"The respective levels of the gene for HCC patients and HBV carriers are consistent with what we already know about the progression of the disease,” Prof. Chan said. "The gene is altered very early in the procession of malignant transformation, and so we can see that the levels of the altered gene increase as the cancer process progresses.”


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