Rinse and Spit: Saliva as a Cancer Biomarker Source
It's home to more than 700 types of bacteria (by current estimates, at least), can be a source of infection, but also has wound-healing properties. It's essential for swallowing and digestion, but, in many cultures, to expel it at somebody is the ultimate insult. And now this slimy body fluid - saliva - is gaining a reputation in biomedical research circles as an effective source for detecting the hidden presence of disease, including some types of cancer.
Most research into cancer biomarkers has focused on blood components, such as plasma or serum. Saliva, on the other hand, has been largely overlooked as a source of biomarkers. It has long been considered a hostile environment, riddled with bacteria and other detritus that would yield adulterated samples incapable of generating reliable and reproducible results.
But that perception is beginning to change. According to Dr. Sudhir Srivastava, director of NCI's Early Detection Research Network (EDRN), which focuses on identifying and validating novel biomarkers, recent data on saliva-based biomarkers - although preliminary - are promising.
"And, saliva-based technology is desirable," he says, "because it's a noninvasive means of detecting biomarkers."
Head and neck cancers have been the focus of most saliva-based biomarker research. These cancers typically are detected during clinical examinations, but often not until they have already progressed to late-stage disease - a big reason why 5-year survival rates have been mired in the 50 percent range for several decades.
Detecting these cancers at earlier stages, with the aid, for instance, of a saliva-based diagnostic test, could increase 5-year survival to 80 to 90 percent, according to Dr. Elizabeth Franzmann, of the Department of Otolaryngology at the University of Miami. This could help avoid some of the morbidity associated with treatment, including disfigurement and significant swallowing difficulties.
Public attention to saliva-based biomarker research received a significant boost last December with the publication of a pilot study conducted in the lab of Dr. David Wong, of the UCLA Jonsson Comprehensive Cancer Research Center. Elevated levels of seven different RNAs, they reported, could distinguish patients with oral squamous cell carcinoma (OSCC) from controls with 91 percent sensitivity and specificity.
Dr. Wong says that his lab has now performed 4 independent detection trials with 272 subjects and controls.
"These seven markers behave consistently throughout these trials, showing that they are significantly elevated in individuals with oral cancer compared to age- and gender-matched controls," he explains. "It really is quite an amazing observation."
Saliva-based detection methods don't have to be limited to head and neck cancers, Dr. Wong argues. As-yet-unpublished studies by his lab using the same RNA approach to detect early-stage breast cancer, he says, "have been very promising."
Dr. Wong's lab is working with newer testing technologies developed with funding from the National Institute of Dental and Craniofacial Research, which is investing significantly in this area. But other researchers are trying to tease out diagnostic clues from saliva using more conventional assays and are finding success.
Dr. Franzmann led a small study published earlier this year in which she used the conventional ELISA test to detect elevated levels of a soluble form of the protein CD44 (solCD44), which was found to reliably identify patients with head and neck squamous cell carcinoma (HNSCC), regardless of the tumor stage. The closer the cancer to the main oral cavity, the more sensitive the solCD44 levels.
"We've even had cancers where no tumor can be seen in the upper aerodigestive tract, but there is a metastasis to the lymph node," she says. "So that's telling us that it may be capable of picking up disease that we can't even see."
Like Dr. Wong's group, other researchers are also looking at more atypical markers. Dr. Joseph Califano of the Department of Otolaryngology-Head & Neck Surgery at Johns Hopkins Medical Institutions and colleagues recently reported that increased levels of mitochondrial DNA (mtDNA) in saliva also strongly correlated with HNSCC, particularly late-stage disease.
Based on this study and other work, Dr. Califano believes mtDNA has the potential to be most valuable as a surveillance tool in patients who have already been treated for HNSCC.
Research into saliva-based diagnostics definitely has a way to go, though, Dr. Califano stresses.
"Specificity is the real challenge. For screening, whether it's for modestly rare diseases or common diseases of any type, if you don't have high specificity, your false-positive rate becomes quickly, unacceptably very high," he says.
Most of the saliva studies to date, cautions Dr. Srivastava, have been pilots. The research is now at the point where, if it is to enter the clinical realm, "It needs to undergo rigorous validation studies," he says. "That means taking a broad spectrum of cases and controls and then seeing whether the markers consistently distinguish between the two."
EDRN is talking with Dr. Wong about the possibility of a national validation trial of his lab's RNA panel/assay for OSCC.
By Carmen Phillips