Endophenotypes for Age-Related Macular Degeneration: Extending Our Reach into the Preclinical Stages of Disease

Current Views on Chr10q26 Contribution to Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in the global aging population. Familial aggregation and genome-wide association (GWA) studies have identified gene variants associated with AMD, implying a strong genetic contribution to AMD development. Two loci, on human Chr 1q31 and 10q26, respectively, represent the most influential of all genetic factors. While the role of CFH at Chr 1q31 is well established, uncertainty remains about the genes ARMS2 and HTRA1, at the Chr 10q26 locus. Since both genes are in strong linkage disequilibrium, assigning individual gene effects is difficult. In this chapter, we review current literature about ARMS2 and HTRA1 and their relevance to AMD risk. Future studies will be necessary to unravel the mechanisms by which they contribute to AMD.

Dark Adaptation and Its Role in Age-Related Macular Degeneration

Dark adaptation (DA) refers to the slow recovery of visual sensitivity in darkness following exposure to intense or prolonged illumination, which bleaches a significant amount of the rhodopsin. This natural process also offers an opportunity to understand cellular function in the outer retina and evaluate for presence of disease. How our eyes adapt to darkness can be a key indicator of retinal health, which can be altered in the presence of certain diseases, such as age-related macular degeneration (AMD). A specific focus on clinical aspects of DA measurement and its significance to furthering our understanding of AMD has revealed essential findings underlying the pathobiology of the disease. The process of dark adaptation involves phototransduction taking place mainly between the photoreceptor outer segments and the retinal pigment epithelial (RPE) layer. DA occurs over a large range of luminance and is modulated by both cone and rod photoreceptors. In the photopic ranges, rods are saturated and cone cells adapt to the high luminance levels. However, under scotopic ranges, cones are unable to respond to the dim luminance and rods modulate the responses to lower levels of light as they can respond to even a single photon. Since the cone visual cycle is also based on the Muller cells, measuring the impairment in rod-based dark adaptation is thought to be particularly relevant to diseases such as AMD, which involves both photoreceptors and RPE. Dark adaptation parameters are metrics derived from curve-fitting dark adaptation sensitivities over time and can represent specific cellular function. Parameters such as the cone-rod break (CRB) and rod intercept time (RIT) are particularly sensitive to changes in the outer retina. There is some structural and functional continuum between normal aging and the AMD pathology. Many studies have shown an increase of the rod intercept time (RIT), i.e., delays in rod-mediated DA in AMD patients with increasing disease severity determined by increased drusen grade, pigment changes and the presence of subretinal drusenoid deposits (SDD) and association with certain morphological features in the peripheral retina. Specifications of spatial testing location, repeatability of the testing, ease and availability of the testing device in clinical settings, and test duration in elderly population are also important. We provide a detailed overview in light of all these factors.

Macular Rod Function in Retinitis Pigmentosa Measured With Scotopic Microperimetry

Purpose

To investigate the validity and reliability of macular rod photoreceptor function measurement with a microperimeter.

Methods

Macular sensitivity in dark-adapted retinitis pigmentosa (RP) patients (22 eyes; 9–67 years of age) and controls (five eyes; 22–55 years of age) was assessed with a modified Humphrey field analyzer (mHFA), as well as a scotopic microperimeter (Nidek MP-1S). Sensitivity loss (SL) was estimated at rod-mediated locations. All RP eyes were re-evaluated at a second visit 6 months later. The dynamic range of the MP-1S was expanded with a range of neutral-density filters (NDFs).

Results

In controls, a 4 NDF was used at all macular locations tested. In patients with RP, 0 to 3 NDFs were used, depending on the local disease severity. At rod-mediated locations (n = 281), SL estimates obtained with the MP-1S were highly correlated (r = 0.80) with those of the mHFA. The inter-perimeter difference of SL averaged less than 3 decibels (dB) with all NDFs, except those with most severe locations evaluated with a 0 NDF, where the difference averaged more than 6 dB. The results were similar on the second visit.

Conclusions

The MP-1S estimates of SL are highly correlated with those of the mHFA over a wide range of disease severity replicated at two visits; however, there was an unexplained bias in the magnitude of SL estimated by the MP-1S especially at loci with severe disease.

Translational Relevance

MP-1S scotopic microperimetry can be used to evaluate changes to macular rod function, but evaluation of treatment potential by quantitative comparison of SL to retinal structure will be more challenging.

Predictive genetics for AMD: Hype and hopes for genetics-based strategies for treatment and prevention

Age-related macular degeneration (AMD) is a complex disease with multiple genetic and environmental risk factors. In the age of molecular genetics, many investigators have established a link between genes and development or progression of the disease. This later evolved to determine whether phenotypic features of AMD have distinct genetic profiles. Molecular genetics have subsequently been introduced as factors in risk assessment models, increasing the predictive value of these tools. Models seek to predict either development or progression of disease, and different AMD-related genes aid our understanding of these respective features. Several investigators have attempted to link molecular genetics with treatment response, but results and their clinical significance vary. Ocular and systemic biomarkers may interact with established genes, promising future routes of ongoing clinical assessment. Our understanding of AMD molecular genetics is not yet sufficient to recommend routine testing, despite its utility in the research setting. Clinicians must be wary of misusing population-based risk models from genetic and biomarker associations, as they are not necessarily relevant for individual counseling. This review addresses the known uses of predictive genetics, and suggests future directions.

Association of age-related macular degeneration with complement activation products, smoking, and single nucleotide polymorphisms in South Carolinians of European and African descent

Purpose

Smoking and the incidence of age-related macular degeneration (AMD) have been linked to an overactive complement system. Here, we examined in a retrospective cohort study whether AMD-associated single nucleotide polymorphisms (SNPs), smoking, ethnicity, and disease status are correlated with blood complement levels.

Methods

Population: The study involved 91 AMD patients and 133 controls, which included 73% Americans of European descent (EUR) and 27% Americans of African descent (AFR) in South Carolina. Readouts: Participants were genotyped for 10 SNPs and systemic levels of complement factor H (CFH) activity, and the complement activation products C3a, C5a, and Bb were assessed. Main Outcome Measures: Univariate and multivariable logistic regression models were used to examine associations between AMD status and distinct readouts.

Results

AMD affects EUR individuals more than AFRs. EUR but not AFR AMD subjects revealed higher levels of Factors C3a and Bb. In all subjects, a 10-unit increase in C3a levels was associated with an approximately 10% increase in the odds of being AMD-positive, and C3a and Bb were associated with smoking. While CFH activity levels were not correlated with AMD, a significant interaction was evident between patient age and CFH activity. Finally, EURs had lower odds of AMD with enhanced copies of rs1536304 (VEGFA) and higher odds with more copy numbers of rs3766404 (CFH).

Conclusions

Our results support previous studies of systemic complement components being potential biomarkers for AMD, but they suggest that smoking and disease do not synergistically affect complement levels. We also suggest a novel susceptibility and protective haplotypes in the South Carolinian AMD population. Our studies indicate that augmented complement activation associated with advanced AMD could be attributed to a decrease in CFH activity in younger patients.

HEALTH CONDITIONS LINKED TO AGE-RELATED MACULAR DEGENERATION ASSOCIATED WITH DARK ADAPTATION

PURPOSE

To determine the association between dark adaption (DA) and different health conditions linked with age-related macular degeneration (AMD).

METHODS

Cross-sectional study, including patients with AMD and a control group. Age-related macular degeneration was graded according to the Age-Related Eye Disease Study (AREDS) classification. We obtained data on medical history, medications, and lifestyle. Dark adaption was assessed with the extended protocol (20 minutes) of AdaptDx (MacuLogix). For analyses, the right eye or the eye with more advanced AMD was selected. Multivariate linear and logistic regressions were performed, accounting for age and AMD stage.

RESULTS

Seventy-eight subjects (75.6% AMD; 24.4% controls) were included. Multivariate assessments revealed that body mass index (BMI; β = 0.30, P = 0.045), taking AREDS vitamins (β = 5.51, P < 0.001), and family history of AMD (β = 2.68, P = 0.039) were significantly associated with worse rod intercept times. Abnormal DA (rod intercept time ≥ 6.5 minutes) was significantly associated with family history of AMD (β = 1.84, P = 0.006), taking AREDS supplements (β = 1.67, P = 0.021) and alcohol intake (β = 0.07, P = 0.017).

CONCLUSION

Besides age and AMD stage, a higher body mass index, higher alcohol intake, and a family history of AMD seem to impair DA. In this cohort, the use of AREDS vitamins was also statistically linked with impaired DA, most likely because of an increased severity of disease in subjects taking them.

Retinal Structure in Pre-Clinical Age-Related Macular Degeneration

PURPOSE

To determine, if there are identifiable retinal structural changes associated with genetic risk for age-related macular degeneration (AMD).

MATERIALS AND METHODS

Seventy-three subjects (range 51.5 to 68.9 years) participated in this prospective study. Subjects were recruited based on the presence of a family history of AMD in one or both parents. All participants underwent a complete ophthalmic exam and imagery for staging of disease severity and genetic testing to assess genetic risk for AMD development. Optical coherence tomography (OCT) imaging was performed on all participants. Semi-automated retinal layer segmentation was performed to assess retinal structural changes.

RESULTS

Of 73 subjects, 47 subjects had normal appearing retina with no evidence of drusen or other changes consistent with AMD, 16 subjects were classified as early AMD, and 13 were designated as intermediate AMD. Retinal volume measures of total retina, outer retina, outer nuclear layer and the retinal pigment epithelium, were not related to AMD classification, genetic risk scores, or age. The thickness of the outer retina showed statistically significant thickening in the foveal region in only the intermediate AMD group and a statistically significant thickening of the RPE in early and intermediate AMD groups in the central retina.

CONCLUSION

No consistent changes were observed in retinal structure at multiple locations that are associated with pre-clinical AMD, based on AMD genetic risk or with aging within the age range of our cohort.

Heritability of Choroidal Thickness in the Amish

PURPOSE

To evaluate the heritability of choroidal thickness and its relationship to age-related macular degeneration (AMD).

DESIGN

Cohort study.

PARTICIPANTS

Six hundred eighty-nine individuals from Amish families with early or intermediate AMD.

METHODS

Ocular coherence tomography was used to quantify choroidal thickness, and fundus photography was used to classify eyes into categories using a modified Clinical Age-Related Maculopathy Staging (CARMS) system. Repeatability and heritability of choroidal thickness and its phenotypic and genetic correlations with the AMD phenotype (CARMS category) were estimated using a generalized linear mixed model (GLMM) approach that accounted for relatedness, repeated measures (left and right eyes), and the effects of age, gender, and refraction.

MAIN OUTCOME MEASURES

Heritability of choroidal thickness and its phenotypic and genetic correlation with the AMD phenotype (CARMS category).

RESULTS

Phenotypic correlation between choroidal thickness and CARMS category was moderate (Spearman's rank correlation, r_s = -0.24; n = 1313 eyes) and significant (GLMM posterior mean, -4.27; 95% credible interval [CI], -7.88 to -0.79; P = 0.02) after controlling for relatedness, age, gender, and refraction. Eyes with advanced AMD had thinner choroids than eyes without AMD (posterior mean, -73.8; 95% CI, -94.7 to -54.6; P < 0.001; n = 1178 eyes). Choroidal thickness was highly repeatable within individuals (repeatability, 0.78; 95% CI, 0.68 to 0.89) and moderately heritable (heritability, 0.40; 95% CI, 0.14 to 0.51), but did not show significant genetic correlation with CARMS category, although the effect size was moderate (genetic correlation, -0.18; 95% CI, -0.49 to 0.16). Choroidal thickness also varied with age, gender, and refraction. The CARMS category showed moderate heritability (heritability, 0.49; 95% CI, 0.26 to 0.72).

CONCLUSIONS

We quantify the heritability of choroidal thickness for the first time, highlighting a heritable, quantitative trait that is measurable in all individuals regardless of AMD affection status, and moderately phenotypically correlated with AMD severity. Choroidal thickness therefore may capture variation not captured by the CARMS system. However, because the genetic correlation between choroidal thickness and AMD severity was not significant in our data set, genes associated with the 2 traits may not overlap substantially. Future studies should therefore test for genetic variation associated with choroidal thickness to determine the overlap in genetic basis with AMD.