Amazing as it might seem, doctors can detect and monitor diseases using molecules found in a sample of spit
Mirror of the Body
Saliva is mostly water, but it also contains protein molecules that lubricate our wagging tongues, inhibit the growth of bacteria, prevent excessive swings in pH and begin the process of digestion. Unfortunately, the importance of saliva is often appreciated only when it's gone, as commonly happens in patients who get radiation treatments or have oral cancer. These individuals routinely face speech problems, and simply chewing and swallowing everyday foods becomes difficult. Without saliva, the mouth is vulnerable to bad breath, yeast infections, cavities and gum disease.
Saliva comes primarily from three distinct places—the parotid, the submandibular and the sublingual salivary glands, where specialized cells take up water, salts and macromolecules from the blood, mix them with a cocktail of saliva-specific proteins and secrete the resultant concoction. Some substances may also reach saliva by passing from blood through the spaces between cells. Hence, most compounds found in blood are also in saliva, leading to the aphorism among dentists that saliva is the "mirror of the body." It's a mirror that reflects the levels of natural and artificial substances, including drugs taken for therapeutic or recreational purposes. Saliva can also indicate emotional and hormonal status, the health of the immune system, neurological conditions, nutritional deficits and metabolic states.
Currently, most molecular diagnoses are based on blood samples, and for good reason: Serum, the cell-free, liquid component of whole blood, typically contains high concentrations of the molecules of interest. But newer, more sensitive tests enable the detection of very small amounts of material. This advance in technology has reduced the importance of having a high level of the target compound.
One of the most important reasons for developing saliva-based diagnostic tests is a matter of simple economics. In situations where saliva and blood can both serve, it might make sense, from the patient's perspective, to use blood—after all, the quantities of most biomarkers are higher in blood than in saliva. The momentary discomfort seems a small price to pay for ruling out some rare, undetected disease. But for insurance companies, it may be that only saliva-based tests are inexpensive enough to use in a large population. If the condition is sufficiently rare, treating the late stages in the few people that develop it costs less than checking everybody for early signs. This seemingly cold-hearted calculation is, of course, something insurers are very sensitive to.
Were cost not a consideration, it would always be smart to screen people for both common and uncommon diseases. After all, even with a physician's most astute observations and modern laboratory tools, rock-solid diagnoses early on are not the norm, and many diseases remain hidden until they become quite advanced. For this reason, medical researchers are keen to identify biomarkers of disease: molecules, usually DNA, RNA or protein, that act as proxies for particular physiological states. Physicians could then use these indicators to discover sicknesses, perhaps even before the onset of symptoms. The best biomarkers are both specific and reliable, meaning that they are uniquely indicative of a particular disease and that all the people with that illness have the markers.
Although the benefits of diagnostic molecules are obvious, relatively few have been approved for use in the clinic. The modest number is not from inattention—scientists in academia, government and industry have devoted significant resources to finding the molecules that signal disease. In particular, the National Institute of Dental and Craniofacial Research has invested heavily in research to ascertain which conditions might reveal themselves through components found in saliva. However, biological systems have proven more complex at the molecular level than I and other investigators thought even a few years ago. Specific physiological states seldom reveal themselves with a change in one protein or RNA. Instead, the molecular hallmark of a disease may consist of altered levels of RNA from many genes—more than a hundred in some of our preliminary studies. In such cases, no single marker can form the basis of a diagnosis, but many, taken together, can yield a more detailed picture of a person's physiology.
Looking for patterns among the variations of thousands of potential biomarkers is a daunting task and one that requires much basic study before approaching the point where disease signals can be routinely detected. For that reason, my colleagues and I at UCLA began a few years ago to catalog the entire collection of proteins and RNAs found in the cell-free portion of saliva. Fortunately, techniques refined in the past 10 years have made it possible for scientists to examine many RNA or protein molecules at once, making our ambitious quest feasible.
In 2003, with funding from the National Institute of Dental and Craniofacial Research, our team began studying the proteins in saliva from healthy individuals. We first split each of the samples and then studied the different parts using different techniques. In each case, we separated the proteins into ordered groups, or fractions, according to some physical attribute of the molecule, such as size or charge. Then we analyzed the fractions independently. This "divide and conquer" strategy reduced the complexity of the protein mixture and provided a measure of redundancy (and thus validation) for our results.
The goal of this study was twofold: first to identify the individual proteins present in the saliva, and second, to get an idea of how abundant they are. While some members of my laboratory are continuing to pursue this whole-proteome initiative, other team members are creating a catalog of glycoproteins, or proteins that have attached sugar molecules. Such sugars decorate many of the proteins most important for the proper functioning of saliva, including mucin, which protects and lubricates the lining of the mouth and throat, and amylase, which breaks starch molecules into glucose. In this case, an analytical technique called liquid chromatography-tandem mass spectrometry helps us to identify the specific types of sugars and their sites of attachment on the protein.
As of November 2007, we have cataloged more than 1,000 proteins in whole saliva. To help disseminate these results, we developed an online database that contains our accumulated knowledge of these proteins. This Web site is freely available to the public, provides a repository for data and allows us to compare our results with information from other protein databases. We've learned that many of the proteins we find in saliva have already been discovered elsewhere in the body. The investigators who carried out these earlier studies often assigned them names and described their functions.
As we have accumulated information about the proteins in saliva, an ongoing project of ours has been to examine carefully the differences between the human salivary proteome and the collection of proteins found in human plasma. The molecular constituents of these two fluids are not identical. Indeed, our preliminary efforts show that proteins found in saliva tend to be more hydrophilic (which is to say that they are attracted to water molecules), whereas the proteins in plasma were more often hydrophobic. When we looked at known proteins from each group, we noted that extracellular proteins—those that are normally excreted into the spaces between cells—are more abundant in saliva than they are in plasma. By contrast, proteins that are known to reside in or near the lipid membranes of cells are found more often in plasma than in saliva. These observations are consistent with the fact that saliva is a filtrate of blood, meaning that the contents of blood—water, salts and macromolecules—pass through capillary walls into the salivary glands. And they can sometimes provide hints about a molecule's function, because what a protein does is often tied to where in the cell it is found.
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