DURHAM, N.C. — Lung cancer is often deadly by the time doctors have detected it, but scientists at Duke University Medical Center are developing a non-invasive test that could detect lung cancer in its earliest stages, while it is still treatable.
Expanding on the MALDI-TOF MS technique, Duke radiologists have identified a specific protein, serum amyloid A, which is elevated in the blood of lung cancer patients but not in the blood of normal patients. While serum amyloid A has previously been shown to be elevated in cancers and other diseases, the Duke team is the first to use MALDI-TOF MS to identify this protein and others that may be involved in lung cancer, said Edward Patz, M.D., professor of radiology and pharmacology/cancer biology at Duke.
Based on his new findings, Patz plans to develop a blood test that will measure serum amyloid A and other, more specific proteins that can detect lung cancer in the blood before a tumor is clinically apparent.
"Our technique is a new paradigm for identifying protein targets in cancer, because we are zeroing in on the disease-causing protein itself rather than searching for a defective gene and then hunting down its relevant proteins," said Patz.
Patz described his methods and results using MALDI-TOF MS (matrix-assisted laser desorption/ionization time-of-flight mass spectrometry) in two studies published in the September 2003 issue of the journal Proteomics.
The Duke studies are proof of principle that MALDI-TOF MS can, in fact, pinpoint and identify proteins in blood that are elevated in cancer and other diseases, said Patz. Moreover, the Duke approach to MALDI-TOF is more sensitive than other diagnostic techniques, he said. It generates more precise information on protein expression because it can detect proteins of low molecular mass, acidic or basic, and at concentrations much lower than other methods.
"Using a MALDI-TOF MS platform is a particularly exciting advance because current diagnostic tools — such as PET and CT scans — have had no obvious impact on lung cancer mortality rate over the last several decades," said Patz. The overall five-year survival rate remains about 14 percent, despite major advances in genomics and drug discovery.
Patz’s approach is the reverse of how scientists generally discover the genetics that underlie a disease. Typically, researchers begin by isolating a defective gene, but a single gene can produce many different proteins — only one of which may be the culprit in a particular disease process. Identifying the relevant proteins in a disease puts scientists much closer to developing novel diagnostic and therapeutic targets, said Patz.
Once identified, the proteins can be used as biologic "markers" to diagnose the earliest stages of cancer, possibly before a PET scan or CT scan pick up the image of a tumor on the lungs. Moreover, researchers can develop new drugs designed to block a protein’s unique role in causing cancer.
"The biology of lung cancer may have been played out by the time we detect a tumor using imaging studies like PET and CT scans," said Patz. "This is why we’re trying to develop very sensitive biomarkers that can detect the disease in high-risk individuals early enough to treat them successfully."
While MALDI-TOF MS is not unique to Duke, Patz has expanded its use in novel ways. Not only does his team record the protein "peaks" in blood samples, he then applies a computer algorithm to each protein that identifies its biologic role in the disease process. Patz calls it "fingerprinting" the protein, and he will eventually use that information to elucidate potential ways of blocking the protein’s cancer-causing effects in the cell.
The process begins by "fractionating" (dividing) the sample, then feeding the samples into the mass spectrometer, which electrically charges or "ionizes" the blood proteins. The instrument then propels the proteins down a flight tube at high velocity. The manner in which each molecule lands determines its precise mass and hence its level of "expression" in the blood.
Next, Patz’s team performs an extra step: they take the most significant protein peaks recorded by the instrument and purify the samples repeatedly until they are able to determine each protein’s unique amino acid structure or fingerprint.
"It is useful to know that you have a protein marker for a disease, but it is far more useful to understand the biology of that protein and use that knowledge to develop strategies to combat the disease."
From a clinical perspective, the development of serum biomarkers has benefits, as well. The technique requires only a blood sample from the patient; hence, it is less invasive than tissue biopsies. It is also more cost efficient and may be much more accurate than CT and PET scans, said Patz.
Source: Duke University Medical Center. September 17, 2003.