Abstract: Sjogren’s syndrome (SS), which was first described in 1933 by the Swedish physician Henrik Sjogren (1), is a chronic autoimmune disorder clinically characterized by a dry mouth (xerostomia) and dry eyes (keratoconjunctivitis sicca). The disease primarily affects women, with a ratio of 9:1 over the occurrence in men. While SS affects up to 4 million Americans, about half of the cases are primary SS. Primary SS occurs alone, whereas secondary SS presents in connection with another autoimmune disease, such as rheumatoid arthritis or systemic lupus erythematosus (SLE). Histologically, SS is characterized by infiltration of exocrine gland tissues by predominantly CD4 T lymphocytes. At the molecular level, glandular epithelial cells express high levels of HLA–DR, which has led to the speculation that these cells are presenting antigen (viral antigen or autoantigen) to the invading T cells. Cytokine production follows, with interferon (IFN) and interleukin-2 (IL-2) being especially important. There is also evidence of B cell activation with autoantibody production and an increase in B cell malignancy. SS patients exhibit a 40-fold increased risk of developing lymphoma.
SS is a complex disease that can go undiagnosed for several months to years. Although the underlying immune-mediated glandular destruction is thought to develop slowly over several years, a long delay from the start of symptoms to the final diagnosis has been frequently reported. SS presumably involves the interplay of genetic and environmental factors. To date, few of these factors are well understood. As a result, there is a lack of early diagnostic markers, and diagnosis usually lags symptom onset by years. A new international consensus for the diagnosis of SS requires objective signs and symptoms of dryness, including a characteristic appearance of a biopsy sample from a minor or major salivary gland and/or the presence of autoantibody such as anti-SSA (2–4). However, establishing the diagnosis of primary SS has been difficult in light of its nonspecific symptoms (dry eyes and mouth) and the lack of both sensitive and specific biomarkers, either body fluid– or tissue-based, for its detection. It is widely believed that developing molecular biomarkers for the early diagnosis of primary SS will improve the application of systematic therapies and the setting of criteria with which to monitor therapies and assess prognosis (e.g., lymphoma development).
Saliva is the product of 3 pairs of major salivary glands (the parotid, submandibular, and sublingual glands) and multiple minor salivary glands that lie beneath the oral mucosa. Human saliva contains many informative proteins that can be used for the detection of diseases. Saliva is an attractive diagnostic fluid because testing of saliva provides several key advantages, including low cost, noninvasiveness, and easy sample collection and processing. This biologic fluid has been used for the survey of general health and for the diagnosis of diseases in humans, such as human immunodeficiency virus, periodontal diseases, and autoimmune diseases (5–8). Our laboratory is active in the comprehensive analysis of the saliva proteome (for more information, see www.hspp.ucla.edu), thus providing the technologies and expertise to contrast proteomic constituents in primary SS with those in control saliva (9–11). Thus far, we have identified over 1,000 proteins in whole saliva (WS). In addition, we have recently identified and cataloged ~3,000 messenger RNAs (mRNA) in human WS (12). These studies have provided a solid foundation for the discovery of biomarkers in the saliva of patients with primary SS. We have previously demonstrated proteome- and genome-wide approaches to harnessing saliva protein and mRNA signatures for the detection of oral cancer in humans (13,14).
There have been continuous efforts in the search for biomarkers in human serum or saliva for the diagnosis of primary SS. Some gene products were found at elevated levels in SS patient sera or saliva, including β2-microglobulin (β2m), soluble IL-2 receptor, IL-6, anti-Ro/SSA, anti-La/SSB, and anti–α-fodrin autoantibodies (15–20). However, none of them individually is sensitive or specific enough to use for the confirmative diagnosis of SS (15). Therefore, it is crucial to use emerging proteome- and genome-wide approaches to discover a wide spectrum of informative and discriminatory biomarkers that can be combined to improve the sensitivity and specificity for the detection of primary SS.