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Chapter 47

Proteomic and Genomic Biomarkers for Age-Related Macular Degeneration

Jiayin Gu, Gayle J.T. Pauer, Xiuzhen Yue, Umadevi Narendra, Gwen M. Sturgill, James Bena, Xiaorong Gu, Neal S. Peachey, Robert G. Salomon, Stephanie A. Hagstrom, John W. Crabb, and The Clinical Genomic, and Proteomic AMD Study Group

Abstract Toward early detection of susceptibility to age-related macular degeneration (AMD), we quantified plasma carboxyethylpyrrole (CEP) oxidative protein modifications and CEP autoantibodies by ELISA in 916 AMD and 488 control donors. Mean CEP adduct and autoantibody levels were elevated in AMD plasma by60 and 30%, respectively, and the odds ratio for both CEP markers elevated was3-fold greater in AMD than in control patients. Genotyping was performed for AMD risk polymorphisms associated with age-related maculopathy susceptibility 2 (ARMS2), high-temperature requirement factor A1 (HTRA1), complement factor H (CFH), and complement C3. The AMD risk predicted for those exhibiting elevated CEP markers and risk genotypes was 2- to 3-fold greater than the risk based on genotype alone. AMD donors carrying the ARMS2 and HTRA1 risk alleles were the most likely to exhibit elevated CEP markers. Receiver operating characteristic curves suggest that CEP markers alone can discriminate between AMD and control plasma donors with 76% accuracy and in combination with genomic markers, provide up to 80% discrimination accuracy. CEP plasma biomarkers, particularly in combination with genomic markers, offer a potential early warning system for predicting susceptibility to this blinding disease.

47.1 Introduction

A direct molecular link between oxidative damage and AMD was established by the finding that carboxyethylpyrrole (CEP), an oxidative protein modification generated from docosahexaenoate (DHA) – containing phospholipids, is elevated in Bruch’s membrane and drusen from AMD patients (Crabb et al. 2002). Subsequently, CEP adducts as well as CEP autoantibodies were found to be elevated in plasma from

J.W. Crabb (B)

Cole Eye Institute, Cleveland, OH, USA e-mail: crabbj@ccf.org

R.E. Anderson et al. (eds.), Retinal Degenerative Diseases, Advances in Experimental

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Medicine and Biology 664, DOI 10.1007/978-1-4419-1399-9_47,C Springer Science+Business Media, LLC 2010

412

J. Gu et al.

AMD donors (Gu et al. 2003a) and CEP adducts were found to stimulate neovascularization in vivo, suggesting a role in the induction of CNV (Ebrahem et al. 2006). From such observations, oxidative protein modifications were hypothesized to serve as catalysts of AMD pathology (Crabb et al. 2002; Gu et al. 2003a; Renganathan et al. 2008). Support for this hypothesis was provided by showing that mice immunized with CEP adducted mouse albumin develop a dry AMD-like phenotype (Hollyfield et al. 2008). While identified AMD susceptibility genes account for over half of AMD cases (Fritsche et al. 2008), many individuals carrying AMD risk genotypes may never develop the disease. Likewise, only a fraction of those diagnosed with early AMD progress to advanced stage disease with severe visual loss (AREDs 2001).

Toward the discovery of better methods to predict susceptibility to advanced AMD, we quantified CEP adducts and autoantibodies in over 1,400 plasma donors and also genotyped many of these donors for AMD risk polymorphisms in complement factor H (CFH) (Edwards et al. 2005; Hageman et al. 2005; Haines et al. 2005; Klein et al. 2005), complement C3 (Maller et al. 2007; Yates et al. 2007), agerelated maculopathy susceptibility 2 (ARMS2, also known as LOC387715) (Fritsche et al. 2008; Jakobsdottir et al. 2005; Maller et al. 2006; Rivera et al. 2005) and hightemperature requirement factor A1 (HTRA1) (Dewan et al. 2006; Yang et al. 2006). Here we review the results that demonstrate combined CEP proteomic and genomic biomarker measurements are more effective in predicting AMD than either method alone (Gu et al. 2009).

47.2 Methods

Clinically documented AMD and control blood donors were recruited from the Cole Eye Institute, Cleveland Clinic Foundation and the Eye Clinic, Louis Stokes Cleveland VA Medical Center. AMD disease progression was categorized based on fundus examination and patients were included in the study from AREDS AMD categories 2, 3 and 4 (AREDs, 2001). Nonfasting blood specimens were collected and plasma was prepared within 6 h and aliquotted to vials containing butylated hydroxytoluene and a protease inhibitor cocktail (Gu et al. 2003a), flushed with argon, quench-frozen in liquid nitrogen and stored at –80C. DNA was isolated from blood using standard procedures. CEP adducts and CEP autoantibodies were measured by ELISA (Gu et al. 2003a) and Western blot analysis of plasma proteins from AMD and normal donors was performed as previously described (Crabb et al. 2002). AMD risk polymorphisms were genotyped in HTRA1 (rs11200638 in the promoter region), C3 (rs2230199 encoding a R102G interchange), and CFH (rs1061170 encoding a Y402H interchange) by restriction analysis. Those in ARMS2 (LOC387715 rs10490924 encoding a A69S interchange) were determined by direct DNA sequence analysis. Differences in plasma CEP adduct concentration and CEP autoantibody titer between control and AMD patients were evaluated using two sample t-tests in Minitab Release 15 (Minitab Inc., PA). To evaluate a