Increased SPARC expression in primary angle closure glaucoma iris

Abstract

Purpose: SPARC (secreted protein, acidic, and rich in cysteine) is involved in extracellular matrix (ECM) organization. The purpose of this study was to evaluate the expression of SPARC in iris tissue from primary angle closure glaucoma (PACG) eyes.

Methods: Iris tissue was obtained from peripheral iridectomies performed during trabeculectomy surgery in nine PACG and 16 primary open-angle glaucoma (POAG) eyes at the Singapore National Eye Centre. Three non-glaucoma control iris specimens were obtained from patients who underwent Descemet's stripping automated endothelial keratoplasty (DSAEK) procedure. SPARC and collagen I expression were quantified by real-time polymerase chain reaction (PCR). The histological distribution of collagen I and III in the iris stroma was determined using picrosirius red polarization. Density of the iris stromal vasculature was also calculated.

Results: The mean age was 68.9+/-10.9 years and 65.7+/-12.2 years in POAG and PACG groups, respectively. The PACG iris expressed SPARC 13.6-fold more and collagen I 5.2 fold more compared to non-glaucoma control iris. The PACG iris also demonstrated 3.3 fold higher SPARC and 2.0 fold higher collagen I expression relative to the POAG iris. The density of collagen I was greater in PACG eyes than in POAG and control eyes (p<0.001). The mean density of iris stromal blood vessels per micron square area was similar in all three groups.

Conclusions: SPARC was significantly increased in the PACG iris. The data suggest that SPARC could play a role in the development of PACG by influencing the biomechanical properties of the iris through a change in ECM organization.

Figure 1

SPARC and collagen I mRNA expression measured by quantitative real-time RT–PCR. Total RNA was purified from the iris stroma of PACG (n=9), POAG (n=16), and normal (n=3) subjects and analyzed by real-time quantitative RT–PCR. A: SPARC mRNA expression in PACG, POAG, and non-glaucomatous iris specimens is shown in the chart. The y axis (ΔC_t) represents SPARC expression normalized against β-actin expression. The data revealed an overexpression of SPARC in the iris tissue from PACG and POAG individuals relative to the iris tissue from non-glaucomatous subjects. B: Fold expression of SPARC mRNA in PACG and POAG relative to non-glaucomatous specimens is shown. The data illustrated on the graph represent the mean±SEM of fold expression (2^{-∆Ct,PACG/∆Ct,POAG-∆Ct,normal}) of SPARC in PACG and POAG specimens relative to that in non-glaucomatous specimens (fold expression=1). The PACG iris contained a mean 13.6 fold and a mean 3.3 fold more SPARC than non-glaucomatous and POAG iris (p<0.001), respectively. The POAG iris contained a mean 4.1 fold more SPARC than non-glaucomatous iris. C: Collagen I mRNA expression in PACG, POAG, and non-glaucomatous iris specimens is shown. The y axis (ΔC_t) represents collagen I expression normalized against β-actin expression. The data revealed an overexpression of collagen I in the iris from PACG and POAG individuals relative to the iris from normal subjects. D: The fold expression of collagen I mRNA in PACG and POAG relative to non-glaucomatous specimens is given in the graph. The data illustrated on the graph represent the mean±SEM of fold expression (2^{-∆Ct,PACG/∆Ct,POAG-∆Ct,normal}) of collagen I in PACG and POAG relative to that in non-glaucomatous specimens (fold expression=1). The PACG iris contained a mean 5.2 fold and a mean 2.0 fold more collagen I than non-glaucomatous and POAG iris (p<0.001), respectively, and the POAG iris contained a mean 2.6 fold more collagen I than non-glaucomatous iris.

Figure 2

Analysis of iris sections by Sirius red staining. Iris sections from PACG (A), POAG (B) as well as normal (C) patients were stained with Picrosirius red. When viewed with a polarized light, mature type I collagen fibers appear bright yellow or orange. Magnification x20.