In exfoliation syndrome (XFS), fibrillar extracellular matrix material is produced by cells in the anterior segment, possibly in response to oxidative stress. Pertinent to glaucoma, excessive production and progressive accumulation of a fibrillar material occurs in various tissues including the anterior segment and outflow pathways.1 In addition, abnormal cross-linking of elastic microfibrils into fibrillar protein aggregates is a stress-induced fibrotic event in the pathogenesis of open-angle glaucoma.1 Central to these assertions was a comprehensive gene expression analysis performed in 2005, which attributes the overall pathophysiology of XFS to differential expression of genes associated with production of elastic microfibrils, cross-linking, matrix metalloproteinase action, and oxidative stress.2
As early diagnosis is key to preventing visual impairment in glaucoma, much recent work has focused on glaucoma biomarker discovery where a biomarker is a molecule/gene/clinical characteristic that allows the identification of a disease. Accepted characteristics for the “ideal” biomarker denote an agent of measureable entity with high sensitivity and specificity, which accurately predicts the presence, progression, or absence of a disease. For XFS, it is critical that a biomarker will predict progression from ocular hypertension to glaucoma and identify those at high risk of progressive damage. The role of “omic” techniques in the elucidation of glaucomatous biomarkers has recently been reviewed.3 Here we highlight some important findings based on proteomic and metabolomic techniques used to define possible biomarkers from the anterior lens capsule, blood or aqueous humor of exfoliation glaucoma (XFG) patients; these results are summarized in Table 1.
Exfoliative material isolated from the surgically removed anterior lens capsule has been analyzed to characterize resident protein constituents. Mass-spectrometry (LC-MS/MS) analysis has identified lysyl oxidase-like 1 (LOXL1) (involved in elastin cross-linking) to be genetically associated with the risk of XFS, as well as other proteins [apolipoproteinE (ApoE), latent-transforming growth factor β-binding protein-2 (LTBP-2), complement 3, and clusterin proteins] detected in the exfoliative material.4 An earlier study from 2007 utilized anterior lens capsules from patients with and without XFS which were homogenized, and protein fragments compared by SDS-PAGE after silver staining and liquid chromatography coupled to tandem MS.5 In addition to fibrillin-1, fibronectin, vitronectin, laminin, and amyloid P-component, proteomic approaches identified clusterin and TIMP-3 as well as novel molecules [among them fibulin-2, desmocollin-2, the glycosaminoglycans syndecan-3, and versican, membrane metalloproteases of the ADAM family (a disintegrin and metalloprotease), and the initiation component of the classic complement activation pathway C1q].
Spectrophotometric analysis of blood samples from primary open-angle glaucoma (POAG) and XFG patients reveals evidence of elevated oxidative stress when compared with control serum.6 To date, the literature reports that XFG patients may be characterized by an impaired oxidative-antioxidative balance in the anterior segment.13 In addition, raised levels of oxidative stress indicators nitric oxide and malondialdehyde have been detected in serum from POAG and XFG patients compared with controls.
In our laboratory, high-density protein array screening has been used previously to analyze and compare the serum autoantibody profiles of patients with and without XFG to identify glaucoma disease–associated autoantibodies.7 Protein arrays containing >10,000 different human proteins derived from a human fetal cDNA library identify unique antibody profiles that may discriminate between patients and controls.14 High-density colony protein macroarrays were used to characterize the IgG autoantibody profiles from patient sera samples. Comparison of the autoantibody profiles between XFG and control groups identified 7 proteins that were significantly more prevalent in the XFG group, including a previously uncharacterized protein C6orf129 that contains a transmembrane domain; current work is ongoing to further characterize C6orf129 and its role in XFG pathology.
A recent study from 2014 has analyzed proteins from blood serum of patients of POAG, XFG, and healthy controls to identify a candidate panel of glaucoma biomarkers.15 Comparative differential proteomic analysis has elucidated enrichment of immune and inflammatory network proteins (including members of the complement C pathway) from glaucoma patients.
Proteomic analysis of human aqueous humor has come to the fore in recent times as a mechanism of identifying proteins responsible for maintenance of homeostasis in the anterior chamber. A complex mixture of growth factors, cytokines, electrolytes, and organic solutes in human aqueous humor sustain cellular metabolism in the avascular tissues of the anterior segment. We have previously shown that levels of connective tissue growth factor (a matricellular protein that interacts with the profibrotic cytokine TGF-β) are significantly higher in aqueous humor of patients with PXFG than in both POAG and normal control subjects undergoing cataract surgery.8 Improved detection technologies have enabled investigations which, to date, have been hindered by the reality that the protein component of human aqueous humor is relatively low, containing between 120 and 500 ng/μL of protein.8,14 Antibody protein arrays and nanoflow liquid chromatography electrospray ionization tandem mass spectrometry (nano-LC-ESI-MS/MS) have been used to characterize the human aqueous humor proteome as a resevoir of 676 nonredundant proteins (cytokines, receptors, and chemokines with catalytic, enzymatic, and structural properties).16
Other groups have highlighted the aqueous humor as a pivotal site of oxidant-antioxidant balance in the distinction between XFS and XFG.9 In addition, the concentration of hydrogen peroxide has been found to be higher in both the serum and aqueous of XFG patients compared with controls.9 A 2003 study utilized commercially available in vitro assays to highlight elevated concentration of 8-isoprostaglandin F(2a) (an oxidative stress marker) versus decreased ascorbic acid concentration (a protectant of oxidative damage) in the aqueous humor of XFS patients compared with age-matched cataract controls.10 Inflammatory biomarkers may also pose attractive targets for the investigation of disease mechanisms given that aqueous humor from XFS patients versus controls is characterized by an elevated interleukin-6 (IL-6) and interleukin-8 (IL-8) signature when assessed by multibead immunoassay.11 A separate study from Japan also using the multiplex bead immunoassay technique found that aqueous humor from POAG and XFG patients display elevated TGF-β1, IL-8, and serum amyloid A levels when compared with cataract controls.12
Areas to “Keep an Eye on”
New approaches to elucidating disease mechanisms are ever evolving and rely heavily on improved proteomic and metabolomic strategies. Interestingly, novel “shot-gun” proteomics approaches (label-free proteomics analysis by nLC-MS) emerging in the field have identified differential expression of inflammatory and radical-scavenging proteins in the tears of POAG patients.17 Separately, metobolomics (which allows the study of metabolites in body fluids or tissues, usually by nuclear magnetic resonance or mass spectrometry) analyses in relation to human ocular disease has been reviewed by Young and Wallace.18 The power of metabolic profiling of vitreous fluid by NMR is described and highlights differential profiles of urea, oxaloacetate, and glucose in patients with lens-induced uveitis versus patients with chronic uveitis. It is clear that validated biomarkers that emerge from ongoing studies have long-term potential to highlight specific molecular pathways, which direct drug development and ultimately improve patient outcome. Figure 1 details how efforts from the clinic and the laboratory can yield insight into glaucoma disease phenotype and also demonstrates that there is an abundance of data to be mined from the proteomics/metabolomics spectrum of the “omics cascade” with relevance to XFG. The need to identify biomarkers that enhance our understanding of XFG pathogenesis is matched only by the need for elucidation of biomarkers that correlate with disease progression.
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