When you lick that envelope, you may be sending a more detailed message than you realize. Your saliva leaves a DNA fingerprint that not only says who you are, but also whether you have a genetic predisposition for certain diseases. This wealth of information contained in saliva makes it a promising alternative to blood as a source of DNA for genetic testing, according to a report in the October issue of the Journal of Immunological Methods.
In a study supported by the National Institute of Dental Research, scientists were able to use DNA from saliva to identify individuals who may be at increased risk of certain infectious and autoimmune diseases. The study focused on two genes that play a role in removing bacteria from the body. Drs. Rob van Schie and Mark Wilson at the State University of New York at Buffalo were able to detect person-to-person differences of as little as a single nucleotide, or structural unit, in the genes. This seemingly minor difference in gene structure is known to affect the proper functioning of the immune system. Diseases potentially linked to these genes include childhood respiratory infections, lupus, and juvenile periodontal disease (LJP), a particularly aggressive form of gum disease that strikes young adults.
The ability to detect disease-associated genes in saliva has very important implications, according to Dr. van Schie, who plans to screen large populations of children for susceptibility to LJP. "Being able to substitute saliva for blood opens the door to populations we would not normally have access to," said van Schie. "Drawing blood is very invasive and it is not a practical procedure for children or individuals that can't give blood for religious or medical reasons. It is also a terrifying prospect for most adults." Saliva has other obvious advantages over blood as a clinical tool, he noted. It is easy to collect, store, and ship and can be obtained at low cost in sufficient quantities for analysis.
This study is not the first to use saliva as a source of DNA. Forensic scientists can retrieve enough saliva from a postage stamp to identify the person that licked the stamp. Saliva has also been used to test for fragile X syndrome, a rare genetic disorder that causes mental retardation in children who carry the gene.
The technology that allows tiny amounts of salivary DNA to be examined in such detail is a procedure called polymerase chain reaction, or PCR. PCR can be used to replicate small pieces of DNA a billionfold, and with such accuracy that minor differences in gene structure are readily distinguishable in laboratory tests. The method is so sensitive that one milliliter of saliva (approximately 1/5 teaspoon) yields enough DNA to do over one hundred separate tests. PCR has also been used to identify miniscule amounts of DNA obtained from fossilized animals, forensic specimens, and infectious microorganisms.
The investigators caution that although saliva has the potential to reveal variations in any gene whose sequence is known, it is not yet proven to have universal application. The DNA in saliva comes from many sources, including blood, tissue cells, and non-human DNA from bacteria and food particles. Each human gene will have to be validated for accurate PCR identification-and the number of disease-related genes that have been identified is rapidly growing. Recent evidence has shown that adults may also have a genetic marker for periodontal disease and therefore may be candidates for saliva screening. Other notable possibilities would be the genes for Alzheimer's disease, cystic fibrosis, or breast cancer. As the structure of more genes becomes known, it may be possible to test for many genetic disorders from a single sample of saliva.
The investigators were Drs. Rob C.A.A. van Schie and Mark E. Wilson from the School of Dental Medicine, State University of New York at Buffalo. The study was supported by the National Institute of Dental Research, one of the National Institutes of Health located in Bethesda, Maryland.