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Juvenile idiopathic arthritis (JIA) is one of those relatively rare diseases that affect children for which there is no cure. It attacks about one child in every 1000 in the UK with worldwide incidences ranging from 0.7-4.0 per 1000 and does not discriminate between boys and girls. It targets the joints, as do other forms of arthritis, but it can also target internal organs and cause inflammation of the eyes. The disease follows no typical course. It can manifest itself in individuals in different ways, as one or two flare ups before disappearing for good, or as repeated flare ups extending into adulthood. Various medications can be administered to alleviate systems and slow down or stop progression of the disease but relapses can occur once treatment is discontinued. The study of arthritis by proteomics techniques is in its early stages, with extremely little work on the juvenile form. One research team undertook a pilot study of joint inflammation in JIA, published in 2006, and identified proteins which are associated with cartilage breakdown and T-cell infiltration. This preliminary work, along with the observation that several molecular processes are probably implicated in the disease, prompted the team to carry out a broader proteomics study of JIA. David Gibson and colleagues from the Arthritis Research Group at Queen's University Belfast, UK and University College Dublin collected the synovial fluid and plasma from 32 JIA patients in the early stages of disease. The proteins from each sample were extracted and subjected to 2D difference gel electrophoresis to highlight the differences between them. In the first instance, synovial fluid was compared with plasma. Proteins from the former were labelled with Cy5 fluorescent dye and those from the latter with Cy3 dye. A third sample was prepared as an internal standard by pooling proteins from all matched synovial fluid and plasma samples and labelling with Cy3 dyes. After electrophoresis and image analysis, about 900 synovial fluid spots and 800 protein spots were detected but differences in their expression levels between the fluids were found. A total of 52 proteins, 26 plasma and 26 synovial, which had more than two-fold higher levels than in the other fluid, were selected for further study. The synovial fluid-specific proteins all had molecular masses <55,000 while plasma ones covered a broader range at 10,000-200,000. The significance of some of these proteins was discussed by the authors. Many of them have been associated previously with inflammation, such as transthyretin, transferrin and the apolipoproteins. The researchers found a significant inter-patient variability in the patient proteomes. Of the 52 proteins highlighted, a hierarchical cluster analysis revealed that only 18 from synovial fluid and 12 from plasma were consistently expressed in all patients. The variation was tentatively attributed to "the dilution of a protein into the larger volume of plasma, thus generating a narrower concentration interval and ... the heterogeneous degrees of joint inflammation which may result in differential rates of protein turnover across the study patients." These 30 proteins, identified by mass spectrometry, are capable of distinguishing synovial fluid from plasma. In a second experiment, master gels were constructed for each patient subgroup according to diagnosis as oligo-articular arthritis or poly-articular arthritis. Another cluster analysis revealed a set of 40 synovial fluid proteins which appeared to distinguish between the disease subtypes. This raises the possibility of using synovial fluid to classify patient subtypes in the early stages of inflammation (these samples were all taken at the time of initial knee inflammation) and possibly to predict disease evolution thereafter. Such a strategy would be a powerful tool in treating JIA, flagging particular patients for more extended and aggressive therapy earlier in the disease before it progressed. If the protein profiles were defined for different stages of the disease and treatment, then it might also be possible to monitor the effects of drug treatment. The proteome studies here measured biomarkers over a dynamic range of 5 orders of magnitude. There may be less abundant proteins in synovial fluid that are better disease predictors but a more refined approach such as protein depletion is required to reveal them. This work has illustrated the capacity of proteomics to define and subtype juvenile idiopathic arthritis by testing the synovial fluid of sufferers at the early stages of disease and shows great potential for predicting the course of the disease. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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