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Mechanisms of Ophthalmologic Disease |

Current Concepts in the Pathogenesis of Age-Related Macular Degeneration FREE

Marco A. Zarbin, MD, PhD
[+] Author Affiliations

From the Institute of Ophthalmology and Visual Science at the New JerseyMedical School, University of Medicine and Dentistry of New Jersey, Newark.The author has no relevant financial interest in this article.

Section Editor: Leonard A. Levin, MD, PhD

More Author Information
Arch Ophthalmol. 2004;122(4):598-614. doi:10.1001/archopht.122.4.598.
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Published online

Objective  To review and synthesize information concerning the pathogenesis ofage-related macular degeneration (AMD).

Methods  Review of the English-language literature.

Results  Five concepts relevant to the cell biology of AMD are as follows: (1)AMD involves aging changes plus additional pathological changes (ie, AMD isnot just an aging change); (2) in aging and AMD, oxidative stress causes retinalpigment epithelial (RPE) and, possibly, choriocapillaris injury; (3) in AMD(and perhaps in aging), RPE and, possibly, choriocapillaris injury resultsin a chronic inflammatory response within the Bruch membrane and the choroid;(4) in AMD, RPE and, possibly, choriocapillaris injury and inflammation leadto formation of an abnormal extracellular matrix (ECM), which causes altereddiffusion of nutrients to the retina and RPE, possibly precipitating furtherRPE and retinal damage; and (5) the abnormal ECM results in altered RPE-choriocapillarisbehavior leading ultimately to atrophy of the retina, RPE, and choriocapillarisand/or choroidal new vessel growth. In this sequence of events, both the environmentand multiple genes can alter a patient's susceptibility to AMD. Implicit inthis characterization of AMD pathogenesis is the concept that there is linearprogression from one stage of the disease to the next. This assumption maybe incorrect, and different biochemical pathways leading to geographic atrophyand/or choroidal new vessels may operate simultaneously.

Conclusions  Better knowledge of AMD cell biology will lead to better treatmentsfor AMD at all stages of the disease. Many unanswered questions regardingAMD pathogenesis remain. Multiple animal models and in vitro models of specificaspects of AMD are needed to make rapid progress in developing effective therapiesfor different stages of the disease.

Figures in this Article

Age-related macular degeneration (AMD) is the leading cause of blindnessand visual disability in patients 60 years or older in the Western hemisphere.1 The clinical presentation of AMD includes drusen,hyperplasia of the retinal pigment epithelium (RPE), geographic atrophy, andchoroidal new vessels (CNVs).2 Only approximately10% to 15% of patients with AMD have severe central vision loss. AtrophicAMD, characterized by outer retinal and RPE atrophy and subjacent choriocapillarisdegeneration, accounts for approximately 25% of cases with severe centralvision loss.1 Exudative AMD is characterizedby CNV growth under the RPE and retina, with subsequent hemorrhage, exudativeretinal detachment, disciform scarring, and retinal atrophy. Serous or hemorrhagicpigment epithelial detachment also occurs. Exudative AMD accounts for approximately75% of cases with severe central vision loss.1 Mostpatients with subfoveal choroidal neovascularization develop profound centralvision loss regardless of whether the CNV has classic or occult morphologicfeatures on angiography.3,4 Rarely,patients have peripheral and central vision loss due to extensive subretinaland vitreous hemorrhage.

The prevalence of early AMD (ie, the presence of soft indistinct orreticular drusen or drusen with RPE degeneration or hyperpigmentation) is18% in the population aged 65 to 74 years and 30% in the population olderthan 74 years.5 Findings from 2 population-basedstudies6,7 indicate that the prevalenceof geographic atrophy is 3.5% in persons older than 75 years, or approximatelyhalf the prevalence of CNVs. Since the population older than 65 years is thefastest growing segment of our society, the burden of disease will increaseduring the 21st century.8 Considering the highsocial and financial cost of this problem, the need for new therapies to preventand treat exudative and atrophic maculopathy is pressing. Many different strategiesare being pursued, ranging from antiangiogenic therapy to transplantationsurgery. The purpose of this review is to summarize recently developed experimentaland clinical biological data relevant to the pathogenesis of AMD.

Aging is associated with biological changes in the eye. These featuresof aging are present in AMD eyes and may contribute to the pathogenesis ofAMD, but they do not lead inevitably to AMD. Thus, it is important to recognizeaging changes in the RPE–Bruch membrane–choriocapillaris complexthat occur in aged eyes without AMD.

In general, aging is associated with cumulative oxidative injury.9 For example, postmitotic cells such as RPE cells accumulatemitochondrial DNA deletions and rearrangements with aging.10 Verzar11 recognized that aging is associated with extracellularmatrix (ECM) alterations. These may include abnormalities of ECM biosynthesis;postsynthetic modifications of ECM, including degradation; altered interactionamong ECM components; and changes in cell-ECM adhesion.12 Agedhuman fibroblasts, for example, seem to produce structurally and functionallyabnormal fibronectin that exhibits reduced binding to native types I and IIcollagen.13 Changes in the extracellular environmentcan induce changes in the cell phenotype.12 Manyof these changes may be under genetic control. Fibroblasts from patients withWerner syndrome, for example, exhibit each of these abnormalities.1416 (Werner syndromeis a condition associated with premature aging that results from a loss offunction mutations in the WRN gene [which encodesa DNA helicase], which leads to rapid telomere shortening.17)Epigenetic reactions involved in aging include the Maillard reaction, uncontrolledproteolytic degradation, and free radical release. The Maillard reaction isthe reaction of free reducing sugars or reactive aldehydes with free aminogroups to form Schiff bases and, after Amadori rearrangements, polycyclicadvanced glycation end products.18 Advancedglycation end products induce cell injury (directly or through cell surfacereceptors) and can induce dysregulation of tissue remodeling with enhanceddeposition of ECM. Each of these features of aging is relevant when consideringthe aging of the retina–RPE–Bruch membrane–choriocapillariscomplex and the pathogenesis of AMD (Figure1).

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Figure 1.

Effects of aging on cells andthe extracellular matrix (ECM). Oxidative damage results in altered cell behavior,including decreased proliferation and overexpression of ECM components. Abnormalcell-ECM interactions result in programmed cell death (apoptosis), includingdeath initiated by separation of the cell from its basement membrane (anoikis).

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RPE Lipofuscin

Lipofuscin comprises a group of autofluorescent lipid-protein aggregatespresent in nonneuronal and neuronal tissues. As is the case for many postmitoticcells, lipofuscin accumulates in RPE cells during life. In one study,19 lipofuscin occupied 1% of the RPE cytoplasmic volumeduring the first decade of life and 19% of the cytoplasmic volume by age 80years. Reduction in functional cytoplasmic volume might compromise RPE function,for example, phagocytosis,20 which can leadto photoreceptor death. In the RPE, the major source of lipofuscin is theundegradable products of photoreceptor outer segment metabolism.21 Intralysosomaliron–catalyzed reactions generate lipofuscin. By producing reactiveoxygen species, lipofuscin may induce oxidative damage in the RPE and surroundingtissues and may inhibit RPE lysosomal enzyme activity (see the bulleted list).Okubo and coworkers22 found a linear relationshipbetween RPE autofluorescence and Bruch membrane thickness, which indicatesthat aging changes in the RPE and Bruch membrane may be related.

Bruch Membrane Thickness

Bruch membrane thickness seems to increase linearly with aging fromapproximately 2 µm at birth to approximately 4 to 6 µm in thetenth decade of life.23 Bruch membrane thickeningcan arise from increased production and decreased degradation of extracellularmaterial. As noted in the "Bruch Membrane Composition and Permeability" subsection,changes in thickness are associated with changes in protein composition, proteincross-linking, increased glycosaminoglycan size, and increased lipid content.Age-related thickening of the Bruch membrane is not confined to the innercollagenous layer. For example, native Bruch membrane collagen content increasesin the outer collagenous layer during the teens. By age 40 years, wide-spacedcollagen also accumulates in this layer.24 Periodicacid–Schiff–positive material that resembles the contents of RPEphagosomes accumulates in the inner collagenous layer and, later, in the elasticlayer.25 Thus, during aging, dysfunctionalRPE cells might produce abnormal quantities of ECM material, including cellfragments, collagen, and other basement membrane components.26,27

Impaired ability to degrade the ECM might contribute to age-relatedBruch membrane thickening. Matrix metalloproteinases (MMPs), for example,are zinc-dependent enzymes that catabolize ECM proteins, including collagenand elastin. Tissue inhibitors of metalloproteinases (TIMPs) regulate theactivity of MMPs. Retinal pigment epithelium cells produce MMPs and TIMP-3.2830 Inactive forms ofMMP-2 and MMP-9 increase in the Bruch membrane with aging, particularly inthe submacular Bruch membrane.31 Abnormalitiesin metalloproteinase activity can result in changes in Bruch membrane thickness.Mutations of TIMP-3, for example, can cause decreased TIMP-3 turnover andresult in Sorsby fundus dystrophy, a condition characterized by the accumulationof abnormal extracellular material between the RPE and the inner collagenouslayer of the Bruch membrane.32,33 Bruchmembrane TIMP-3 content increases with age.34 SinceTIMP-3 gene expression in the macular area does not increase substantiallywith aging,35 there may be altered TIMP-3 turnoverwith sustained MMP inhibition during aging.

Age-related declines in choriocapillaris density and lumen diameter(see the "Choroidal Blood Flow" subsection) might also decrease clearanceof debris from the Bruch membrane, which would contribute to thickening withage. In Sorsby fundus dystrophy, symptoms and retinal function improve withhigh-dose vitamin A therapy, suggesting that impaired diffusion across theBruch membrane can be a consequence of Bruch membrane thickening.36 (The material that accumulates in the Bruch membranein this condition resembles the abnormal extracellular debris deposited inthe Bruch membrane in AMD.)

Bruch Membrane Composition and Permeability

Various collagens, glycosaminoglycans, laminin, and fibronectin arenormal constituents of the Bruch membrane (Table 1). With aging, the Bruch membrane thickens, periodic acid–Schiffstaining increases, and Bruch membrane type I collagen increases.26,35,3942 Membranousdebris, filamentous material, and coated vesicles accumulate primarily inthe inner collagenous layer by early adulthood and continue to do so throughoutadult life.39 With aging, collagen cross-linkingseems to increase in the Bruch membrane, and there is a significant increasein the amount of noncollagen protein in the submacular Bruch membrane butnot in the periphery, which might mean that protein-containing debris is trappedin the Bruch membrane during aging.43 By latemiddle age, lipid deposition in the Bruch membrane is apparent.39 Basallaminar deposit, which comprises mostly wide-spaced collagen40 andother materials, including laminin, membrane-bound vesicles, and fibronectin,is present in the seventh decade of life during normal aging.4446 Pauleikhoffand coworkers47 reported an age-related declinein the presence of laminin, fibronectin, and type IV collagen in the RPE basementmembrane (especially over drusen). Basal linear deposit, consisting primarilyof granular and vesicular material with foci of wide-spaced collagen, appearsin older persons and is more specific for AMD.44,48 Duringaging, Bruch membrane glycosaminoglycans increase in size, and heparan sulfatecontent increases.38 Advanced glycation endproducts accumulate in the Bruch membrane during aging.49 Advancedglycation end products have been shown to promote trapping of macromolecules,50,51 and they might alter cellular traffickingthrough the Bruch membrane, particularly if the cells express receptors foradvanced glycation end products. Molecules present in the Bruch membrane imparta negative electrostatic charge at physiologic pH.38,52 Age-relatedchanges in glycosaminoglycans might alter this charge and, as a result, thepermeability properties of the Bruch membrane.38

Table Graphic Jump LocationTable 1. Distribution of Various Extracellular Matrix Molecules inthe Human Bruch Membrane37,38

Thus, the molecular composition of the Bruch membrane and the tightjunctions between RPE cells affect the movement of molecules between the choriocapillarisand the subretinal space. Most evidence indicates that the hydraulic conductivityof the Bruch membrane decreases exponentially with age in healthy individuals.41,53 At any given age, the submacularBruch membrane is affected to a greater degree than the peripheral Bruch membrane.Starita and coworkers42 used excimer laserablation of different layers of the Bruch membrane to demonstrate that mostof the resistance to water flow lies in the inner collagenous layer of theBruch membrane. These investigators suggested that a high-resistance barrierdevelops in the Bruch membrane in older eyes, probably due to lipid and vesicular-granulardebris entrapment in the Bruch membrane.

Most of the Bruch membrane hydraulic conductivity decrease occurs byage 40 years. Marshall and coworkers39 notedthe discrepancy between the early rapid decline in conductivity and the relativelyslower rate of increase in Bruch membrane thickness. Age-related changes inBruch membrane biochemical composition probably underlie the discrepancy.Specifically, there is increased lipidization, protein cross-linking, andprotein deposition in the Bruch membrane with aging. Lipid accumulation inthe Bruch membrane begins to increase substantially after age 40 years.54,55 The rate of lipid accumulation underthe macula may be higher than under the peripheral retina, perhaps due tothe greater density of photoreceptors in the macula and a greater susceptibilityof outer segment lipids to peroxidation in the posterior pole. Spaide andcoworkers56 found that the amount of peroxidizedlipids in the Bruch membrane increased exponentially with age. The lipidsseemed to be derived from long-chain polyunsaturated fatty acids normallyfound in the outer segments, for example, docosahexanoic acid and linoleicacid, providing support for the notion that at least some of the lipid inthe Bruch membrane is of cellular origin rather than derived from the blood.

Bruch membrane morphometry indicates that the elastin layer has thehighest porosity and that the inner collagenous layer has the lowest porosity.39 The elastin layer seems to become increasingly porouswith age.57 This layer might normally constitutea barrier to vessel growth between the choroid and the sub-RPE space, andthis age-related change might have a permissive effect on CNV growth. Marshalland coworkers39 proposed that from the lateteens to the late thirties, membranous debris, vesicles, and collagen accumulationcause a reduction in effective pore size in the inner collagenous layer. Fromthe forties to the sixties, this process continues, and, abetted by substantiallipid deposition, there is an accelerated decline in hydraulic conductivity.At older ages, the deposition of basal laminar and linear deposit furtherreduces functional pore size. In older persons, diffusion of small and largemolecules across the Bruch membrane is impaired.5860 Changesin protein cross-linking, noncollagenous protein deposition, and age-relatedlipid accumulation in the Bruch membrane may be the underlying cause.61 To the degree that hydrodynamic forces alter moleculartransport across the Bruch membrane, it seems possible that hypertension wouldexacerbate age-related trans–Bruch membrane transport problems.