Saliva-based diagnostics: Part 2
There are many exciting developments in salivaomics. The UCLA Human Salivary Proteome Research project has identified more than 1,000 core salivary proteins and analyzed 165 million genetic sequences. There is verifiable evidence to substantiate the biological connection between systemic disease and the oral cavity, and to explore the use of salivary biomarkers for the detection of life-threatening disease in a way that is noninvasive and painless.
How do biomarkers for systemic disease enter saliva?
The salivary glands have a rich blood supply; subsequently, saliva is a filtrate of the blood via diffusion from the capillaries surrounding the salivary glands. DNA and RNA from tumors enter the circulation and modulate the activities and gene expression of distal organs.
Disease biomarkers enter saliva by means of exosomes. These cell-derived vesicles are present in most body fluids and contain lipids, messenger RNA, microRNA, DNA and proteins. Exosome vesicles are extracellular; they transfer molecules from one cell to another to influence the immune system, and may play a role in mediating immune responses to pathogens and tumors.
Breast cancer-derived, exosomelike microvesicles are capable of interacting with salivary gland cells. This has also been shown with lung, gastric and pancreatic cancers. Tumor-derived RNA molecules are transported via exosome vesicles from the source of the tumor and reprocessed into saliva as biomarkers.
Types of biomarkers in saliva
- The proteome is the entire set of expressed proteins in a given type of cell or organism, under defined conditions.
- The metabolome is the collection of all metabolites in a biological cell, which are the end products of cellular processes. Metabolic profiling can provide a snapshot of the physiology of that cell.
- The microbiome is the combined genetic material of the microorganisms in a particular part of the body.
- The genome is an organism’s complete set of DNA, including all of its genetic material. Each genome contains all of the information needed to build and maintain that organism.
- The transcriptome is the sum total of all of the messenger RNA molecules expressed from the genes of an organism. Messenger RNA is synthesized from a DNA template during transcription and mediates the transfer of genetic information from the cell nucleus to ribosomes in the cytoplasm, where it serves as a template for protein synthesis.
- MicroRNA describes short, single-stranded RNA molecules that bind to messenger RNA molecules to regulate gene expression.
Current research on salivary diagnostics
The National Cancer Institute’s Early Detection Research Network has validated proteomic and genomic salivary biomarkers. Three protein and seven messenger RNA markers were found to be feasible in distinguishing oral squamous cell carcinomas from healthy controls. Subjects with oral squamous cell carcinoma also demonstrated characteristic salivary metabolic signatures.
Lung cancer is the leading cause of cancer-related deaths worldwide, yet it is treatable in the early stages by medication such as thymidine kinase inhibitors. Testing saliva may be as successful in detecting early lung cancer as testing excised lung tissue. Researchers utilized electric field-induced release and measurement (EFIRM) to test lung cancer patients’ saliva for gene mutations of the epidermal growth factor receptor, which is a sign of lung cancer.
Research on saliva testing for pancreatic cancer has great significance, because most patients die within one year of diagnosis, and the five-year survival rate is just 3–5 percent. Current biopsy techniques are complicated and invasive. The salivary microbiome can distinguish individuals with early pancreatic cancer, with 96.4 percent sensitivity and 82.1 percent specificity. In addition, eight metabolites have been identified in saliva which are specific to pancreatic cancer, and the combination of four messenger RNA biomarkers can differentiate pancreatic cancer patients from cancer-free subjects with 90 percent sensitivity and 95 percent specificity.
Autopsies of Parkinson’s patients show that abnormal proteins associated with Parkinson’s disease are consistently found in the submandibular salivary glands; saliva is also being investigated for biomarkers of Parkinson’s disease. Metabolomes and Amyloid-β Protein 42 have been found in saliva that may identify those at risk of developing Alzheimer’s disease, and elevated salivary cortisol levels in seniors are also associated with cognitive impairment.
Deficits and elevated levels of specific salivary proteins may serve as biomarkers of autism spectrum disorder (ASD): The saliva of children with ASD had no detectable histatin-1 or statherin, and had elevated lactotransferrin.
New research shows that an enzyme-linked immunosorbent assay (ELISA) saliva test detects the protein huntintin (Htt),1 which plays a key role in Huntington's disease, a fatal neurodegenerative disease. This test may provide an early marker of onset and progression of the disorder.
An emerging saliva test identifies diagnostic markers of Zika virus more rapidly than current tests.
Xerox’s Parc development lab has created a smart mouthguard biosensor that can detect early signs of dehydration, exhaustion, and mental engagement levels, based on a sample of saliva (Fig. 1). The sensor is made from flexible electronic plastic foil with printed sensor electrodes, and fits into any mouthguard for varied uses, including workouts and military missions. It utilizes chronoamperometry to analyze the lactate, uric acid and glucose in saliva, and is able to send this information to a connected smartphone in close to real time.
The World Health Organization published its first Essential Diagnostics List in May 2018, which contains 113 in-vitro tests for detection and diagnosis of a wide range of common conditions and the detection, diagnosis and monitoring of global priority diseases such as HIV, tuberculosis, malaria, hepatitis B and C, human papillomavirus and syphilis. Several of these tests could be performed utilizing saliva instead of blood or urine.
The increasing worldwide prevalence of infectious diseases and rising focus on decentralized diagnostics is driving rapid growth of point-of-care molecular diagnostics. Dental professionals, particularly hygienists, are in an ideal position to participate in screening and secondary prevention of diseases that are often life-threatening. Interdisciplinary collaboration between the medical and dental health care professions facilitates early diagnosis, for enhanced comprehensive patient care and improved outcomes.
1. "Saliva Test May Flag Early Huntington's Disease.” Medscape - Oct 23, 2017.