MicroRNA Expression Profiles as Biomarkers of Minor Salivary Gland Inflammation and Dysfunction in Sjögren's Syndrome

Sjogren's syndrome (SS) is characterized by features of systemic autoimmunity and exocrine gland dysfunction and inflammation. The exact cause of exocrine gland dysfunction in SS has not been delineated, but it is thought that both immunologically mediated and nonimmune mechanisms contribute significantly (1). The diagnosis of SS is based on the combination of symptoms and signs of dry mouth and/or dry eyes, the presence of autoantibodies, and an inflammatory infiltrate in the minor salivary glands (MSGs). The intensity of the infiltrate varies considerably and is described by the focus score, which can range from 0 to 12, with the highest score representing diffuse lymphocytic infiltrates. There is also a great deal of variation in salivary flow, ranging from essentially normal production to no production of saliva. The correlation between the focus score and salivary flow is poor, suggesting that these may be 2 independent processes. Alternatively, the poor correlation between the focus score and salivary flow may reflect the limitations of our current methods, which rely solely on quantitative assessments of inflammation and dysfunction. More sophisticated biomarkers that reliably describe the different pathophysiologic aspects of SS are needed to establish the diagnosis, predict the prognosis, characterize disease activity, and develop effective therapies. MicroRNA are a group of small RNAs, 21–24 nucleotides in length, involved in the regulation of a wide variety of cellular and physiologic processes (2–4). They exert their effects by 2 mechanisms: messenger RNA (mRNA) degradation and disruption of translation (5). A single mRNA is usually translated into a single protein; however, a single microRNA is capable of regulating the translation of a multitude of genes involved in a certain function. Changes in individual mRNA levels can be ultimately modulated or nullified by posttranscriptional regulation and thus may be less representative than microRNA of the physiologic status of the cell. A biomarker is defined as a characteristic that can be measured and evaluated objectively and reproducibly and serves as an indicator of normal or pathogenic biologic processes or pharmacologic responses to therapeutic interventions (6). MicroRNA fulfill all of these requirements and therefore are appealing biomarker candidates. It has been shown in both experimental and clinical settings that microRNA reflect important biologic processes (7–11). In addition, microRNA are remarkably stable, a characteristic that is extremely important in clinical settings and makes microRNA superior to other classes of biomarkers; when using other classes of biomarkers, minor differences in sample processing can have a major impact on outcomes (8). Finally, microRNA can be objectively measured by several methods. Single microRNA are best measured by real-time quantitative polymerase chain reaction (qPCR), whereas microRNA microarrays can provide a more global assessment of microRNA expression patterns. MicroRNA have been shown to be valuable biomarkers for classifying cancers and their prognosis (12–14). Associations of selected biomarkers with autoimmune diseases have also been described (15–20), but to date there have been no reports of the use of microRNA as biomarkers of the diagnosis and pathophysiologic processes in a systemic autoimmune disease. In this report, we present results of a proof-of-concept study of the feasibility of using microRNA as diagnostic and functional biomarkers in autoimmune diseases. More specifically, we used microRNA microarrays to profile MSGs from control subjects and patients with primary SS. To assess the biologic plausibility of differences in microRNA patterns among various groups, we identified the predicted target pathways of the differentially expressed microRNA, using the Ingenuity Pathways Analysis program.