Age-related Macular Degeneration Loci Research Articles

A Genome-Wide Association Study for Primary Open Angle Glaucoma and Macular Degeneration Reveals Novel Loci

Glaucoma and age-related macular degeneration (AMD) are the two leading causes of visual loss in the United States. We utilized a novel study design to perform a genome-wide association for both primary open angle glaucoma (POAG) and AMD. This study design utilized a two-stage process for hypothesis generation and validation, in which each disease cohort was utilized as a control for the other. A total of 400 POAG patients and 400 AMD patients were ascertained and genotyped at 500,000 loci. This study identified a novel association of complement component 7 (C7) to POAG. Additionally, an association of central corneal thickness, a known risk factor for POAG, was found to be associated with ribophorin II (RPN2). Linked monogenic loci for POAG and AMD were also evaluated for evidence of association, none of which were found to be significantly associated. However, several yielded putative associations requiring validation. Our data suggest that POAG is more genetically complex than AMD, with no common risk alleles of large effect.

DNA Sequence Variants in PPARGC1A, a Gene Encoding a Coactivator of the ω-3 LCPUFA Sensing PPAR-RXR Transcription Complex, Are Associated with NV AMD and AMD-Associated Loci in Genes of Complement and VEGF Signaling Pathways

BackgroundIncreased intake of ω-3 long-chain polyunsaturated fatty acids (LCPUFAs) and use of peroxisome proliferator activator receptor (PPAR)-activating drugs are associated with attenuation of pathologic retinal angiogenesis. ω-3 LCPUFAs are endogenous agonists of PPARs. We postulated that DNA sequence variation in PPAR gamma (PPARG) co-activator 1 alpha (PPARGC1A), a gene encoding a co-activator of the LCPUFA-sensing PPARG-retinoid X receptor (RXR) transcription complex, may influence neovascularization (NV) in age-related macular degeneration (AMD).MethodsWe applied exact testing methods to examine distributions of DNA sequence variants in PPARGC1A for association with NV AMD and interaction of AMD-associated loci in genes of complement, lipid metabolism, and VEGF signaling systems. Our sample contained 1858 people from 3 elderly cohorts of western European ancestry. We concurrently investigated retinal gene expression profiles in 17-day-old neonatal mice on a 2% LCPUFA feeding paradigm to identify LCPUFA-regulated genes both associated with pathologic retinal angiogenesis and known to interact with PPARs or PPARGC1A.ResultsA DNA coding variant (rs3736265) and a 3'UTR-resident regulatory variant (rs3774923) in PPARGC1A were independently associated with NV AMD (exact P = 0.003, both SNPs). SNP-SNP interactions existed for NV AMD (P<0.005) with rs3736265 and a AMD-associated variant in complement factor B (CFB, rs512559). PPARGC1A influences activation of the AMD-associated complement component 3 (C3) promoter fragment and CFB influences activation and proteolysis of C3. We observed interaction (P≤0.003) of rs3736265 with a variant in vascular endothelial growth factor A (VEGFA, rs3025033), a key molecule in retinal angiogenesis. Another PPARGC1A coding variant (rs8192678) showed statistical interaction with a SNP in the VEGFA receptor fms-related tyrosine kinase 1 (FLT1, rs10507386; P≤0.003). C3 expression was down-regulated 2-fold in retinas of ω-3 LCPUFA-fed mice – these animals also showed 70% reduction in retinal NV (P≤0.001).ConclusionLigands and co-activators of the ω-3 LCPUFA sensing PPAR-RXR axis may influence retinal angiogenesis in NV AMD via the complement and VEGF signaling systems. We have linked the co-activator of a lipid-sensing transcription factor (PPARG co-activator 1 alpha, PPARGC1A) to age-related macular degeneration (AMD) and AMD-associated genes.

Genome-wide association study identifies two susceptibility loci for exudative age-related macular degeneration in the Japanese population

Age-related macular degeneration (AMD), the leading cause of irreversible blindness in the world, is a complex disease caused by multiple environmental and genetic risk factors. To identify genetic factors that modify the risk of exudative AMD in the Japanese population, we conducted a genome-wide association study and a replication study using a total of 1,536 individuals with exudative AMD and 18,894 controls. In addition to CFH (rs800292, P = 4.23 × 10(-15)) and ARMS2 (rs3750847, P = 8.67 × 10(-29)) loci, we identified two new susceptibility loci for exudative AMD: TNFRSF10A-LOC389641 on chromosome 8p21 (rs13278062, combined P = 1.03 × 10(-12), odds ratio = 0.73) and REST-C4orf14-POLR2B-IGFBP7 on chromosome 4q12 (rs1713985, combined P = 2.34 × 10(-8), odds ratio = 1.30). Fine mapping revealed that rs13278062, which is known to alter TNFRSF10A transcriptional activity, had the most significant association in 8p21 region. Our results provide new insights into the pathophysiology of exudative AMD.

Common variants near FRK/COL10A1 and VEGFA are associated with advanced age-related macular degeneration

Despite significant progress in the identification of genetic loci for age-related macular degeneration (AMD), not all of the heritability has been explained. To identify variants which contribute to the remaining genetic susceptibility, we performed the largest meta-analysis of genome-wide association studies to date for advanced AMD. We imputed 6 036 699 single-nucleotide polymorphisms with the 1000 Genomes Project reference genotypes on 2594 cases and 4134 controls with follow-up replication of top signals in 5640 cases and 52 174 controls. We identified two new common susceptibility alleles, rs1999930 on 6q21-q22.3 near FRK/COL10A1 [odds ratio (OR) 0.87; P = 1.1 × 10−8] and rs4711751 on 6p12 near VEGFA (OR 1.15; P = 8.7 × 10−9). In addition to the two novel loci, 10 previously reported loci in ARMS2/HTRA1 (rs10490924), CFH (rs1061170, and rs1410996), CFB (rs641153), C3 (rs2230199), C2 (rs9332739), CFI (rs10033900), LIPC (rs10468017), TIMP3 (rs9621532) and CETP (rs3764261) were confirmed with genome-wide significant signals in this large study. Loci in the recently reported genes ABCA1 and COL8A1 were also detected with suggestive evidence of association with advanced AMD. The novel variants identified in this study suggest that angiogenesis (VEGFA) and extracellular collagen matrix (FRK/COL10A1) pathways contribute to the development of advanced AMD.

Analysis of SERPING1 and its association with age-related macular degeneration

Purpose The genetics of age-related macular degeneration (AMD) pathogenesis is a growing area of interest. In the last year a new potential association locus has been reported; SERPING1, a C1 inhibitor of the compliment pathway. We present our own assessment of the SERPING1 AMD-risk locus. Methods Patients with exudative AMD (n=94) and age-matched controls (n=95) were selected for study. All subjects were genotyped for the SERPING1 polymorphism identified as a new risk locus for AMD; rs2511989, IVS6-865 g>a. Genotyping was determined by direct sequencing. Statistical analysis was attained using SPSS v15 (SPSS Inc.). Results Carriage of the G allele was raised in AMD group vs. controls (61.2% vs. 57.6%), but was not statistically significant (p=0.48). No significant associations were seen in genotype carriage between AMD and control groups (e.g. GG genotype, 40.4% vs. 30.4%, p=0.22). Conclusion No significant associations were seen between SERPING1 and AMD in this study. Only two published analyses have examined this potential association, and these studies produced conflicting results. While it appears SERPING1 mRNA is produced in the eye, the functional effects of the rs2511989 locus have yet to be determined. More work is needed to identify the biological role, if any, SERPING1 plays in the development of AMD.

Analysis of SERPING1 and its association with age‐related macular degeneration

Abstract Purpose The genetics of age‐related macular degeneration (AMD) pathogenesis is a growing area of interest. In the last year a new potential association locus has been reported; SERPING1, a C1 inhibitor of the compliment pathway. We present our own assessment of the SERPING1 AMD‐risk locus. Methods Patients with exudative AMD (n=94) and age‐matched controls (n=95) were selected for study. All subjects were genotyped for the SERPING1 polymorphism identified as a new risk locus for AMD; rs2511989, IVS6‐865 g&gt;a. Genotyping was determined by direct sequencing. Statistical analysis was attained using SPSS v15 (SPSS Inc.). Results Carriage of the G allele was raised in AMD group vs. controls (61.2% vs. 57.6%), but was not statistically significant (p=0.48). No significant associations were seen in genotype carriage between AMD and control groups (e.g. GG genotype, 40.4% vs. 30.4%, p=0.22). Conclusion No significant associations were seen between SERPING1 and AMD in this study. Only two published analyses have examined this potential association, and these studies produced conflicting results. While it appears SERPING1 mRNA is produced in the eye, the functional effects of the rs2511989 locus have yet to be determined. More work is needed to identify the biological role, if any, SERPING1 plays in the development of AMD.

Recent Patents Relating to Diagnostic Advances in Age Related Macular Degeneration (AMD)

Age Related Macular Degeneration (AMD) is a complex neurodegenerative disorder accounting for 50% of blind registrations in the western world. Its substantial impact on quality of life has been a main driver in research to understand its etiology, which up until recently was mostly unknown. In the last three years our understanding of the molecular pathology of AMD has increased dramatically with the identification of two major AMD loci comprising of, Complement Factor H (CFH) and a chromosome 10q26 locus consisting of the Heat Shock Serine Protease (HTRA1) and LOC387715 genes. These two loci have been described as associated with over 50% of disease in certain ethnicities. The rapid pace in our understanding of the complex biology of this disease has placed a large emphasis on gene patenting, especially with the licensing of the CFH and chromosome 10 patents to a private life science company called Optherion Inc. The patents discussed in this review highlight the important discoveries that have contributed to our understanding of AMD and provide valuable information as to where research in this area will be heading in the future.

Genetic variants in three genes and smoking show strong associations with susceptibility to exudative age-related macular degeneration in a Chinese population

The present study was undertaken to replicate the associations of representative polymorphisms in three genes (complement factor H (CFH), complement factor B (BF) and HtrA serine peptidase 1 (HTRA1)) with exudative age-related macular degeneration (AMD) in a Han Chinese population, and to test if the modifiable environmental factors affect AMD susceptibility associated with different type of genotype in these genes. An age, gender and ethnicity matched case-control study was conducted to genotype the representative single neucleotide polymorphisms (SNPs) loci including rs1061170 and rs1410996 in CFH, rs641153 and rs4151667 in BF and rs11200638 in HTRA1 gene in 144 exudative AMD patients and 126 normal controls using PCR-RFLP and direct resequencing. The demographic characteristics and behavioral risk factors were also recorded. Allelic and genotypic associations for individual SNP and joint associations with two loci were performed. The gene-gene and gene-environment interactions were analyzed using multivariate non-conditional Logistic regression analysis. The C risk allele frequencies for CFH Y402H (rs1061170) in cases and controls were 12.5% and 5.4% respectively, which were much lower than those in Caucasians (P < 0.001). Compared with TT homozygous genotype, the CT heterozygous genotype was positively associated with AMD with odds ratio (OR) of 3.23 (1.36 - 5.07). However, the population attributable risk (PAR) of C allele was only 3.3% (1.4% - 4.3%). rs1410996 was also associated with AMD independent of Y402H. The ORs of exudative AMD for individuals carrying one copy risk allele and two copy risk alleles were 2.57 (1.21 - 5.45) and 4.76 (2.15 - 10.55) respectively, with correspondent PARs of 28.3% (2.0% - 40.5%) and 38.2% (21.8% - 45.4%). rs11200638 in HTRA1 was another susceptible locus for AMD and the risk homozygotes were significantly susceptible for exudutive AMD (OR = 3.98, 1.88 - 8.43) with PAR of 38.9% (24.3% - 45.8%). Education status and cigarette smoking were also related to exudative AMD. After controlling for environmental risk factors, CFH and HTRA1 SNPs were independently associated with exudative AMD, with OR of 3.50 (1.45 - 8.45) for CT genotype in Y402H, 3.34 (1.33 - 8.36) for GG genotype in rs1410996 and 3.85 (1.58 - 9.42) for AA genotype in rs11200638 respectively. The interaction analysis between gene and environmental factors showed that smoking synergistically increased susceptibility of AMD for heterozygotes of rs1410996, with OR(interaction) of 7.33 (P(interaction) = 0.029). In a Han Chinese population, CFH and HTRA1 polymorphisms appear to be independently and possibly additively hereditary contributors to exudative AMD. Y402H polymorphism conferred a significant but relatively lower contribution in Chinese than in Caucasians with a low frequency of risk allele. The gene-environment interaction may be a best way to encourage those with a high genetic risk to prevent AMD by avoiding modifiable factors until there is effective treatment for AMD.

CFH and LOC387715/ARMS2 Genotypes and Antioxidants and Zinc Therapy for Age-Related Macular Degeneration

EVALUATION OF: Klein ML, Francis PJ, Rosner B et al.: CFH and LOC387715/ARMS2 genotypes and treatment with antioxidants and zinc for age-related macular degeneration. Ophthalmology 115(6), 1019-1025 (2008). The late form of age-related macular degeneration (AMD) is a leading cause of irreversible vision loss in the elderly, with a complex etiology involving genetic and environmental factors. Recently, multiple distinct genetic susceptibility loci for AMD have been identified. Specifically, variations in the Complement Factor H (CFH) gene as well as the LOC387715/ARMS2 gene have been shown to be strongly associated with AMD. The Age-Related Eye Disease Study (AREDS) is a large multicenter, placebo-controlled, randomized clinical trial that showed that a combination of zinc and antioxidants reduced progression to late-stage AMD. In the present study, the authors found that within AREDS there was a significant interaction between zinc and CFH genotypes, indicating that CFH genotypes may be predictive of treatment response to zinc supplementation.

Oxidative Stress Modulates Complement Factor H Expression in Retinal Pigmented Epithelial Cells by Acetylation of FOXO3

Age-related macular degeneration (AMD), the leading cause of severe vision loss in the elderly, is a complex disease that results from genetic modifications that increase susceptibility to environmental exposures. Smoking, a major source of oxidative stress, increases the incidence and severity of AMD, and antioxidants slow progression, suggesting that oxidative stress plays a major role. Polymorphisms in the complement factor H (CFH) gene that reduce activity of CFH increase the risk of AMD. In this study we demonstrate an interaction between these two risk factors, because oxidative stress reduces the ability of an inflammatory cytokine, interferon-gamma, to increase CFH expression in retinal pigmented epithelial cells. The interferon-gamma-induced increase in CFH is mediated by transcriptional activation by STAT1, and its suppression by oxidative stress is mediated by acetylation of FOXO3, which enhances FOXO3 binding to the CFH promoter, reduces its binding to STAT1, inhibits STAT1 interaction with the CFH promoter, and reduces expression of CFH. Expression of SIRT1, a mammalian homolog of NAD-dependent protein deacetylase sir2, attenuated FOXO3 recruitment to the CFH regulatory region and reversed the H(2)O(2)-induced repression of CFH gene expression. These data suggest an important interaction between environmental exposure and genetic susceptibility in the pathogenesis of AMD and, by elucidating molecular signaling involved in the interaction, provide potential targets for therapeutic intervention.

A polymorphism of LOC387715 gene is associated with age-related macular degeneration in the Japanese population

Age-related macular degeneration (AMD) is one of the leading causes of blindness among older adults in developed countries and also in Japan. Previous research suggests that AMD is etiologically a complex disease, caused by multiple genes and environmental factors. Association studies have identified that a complement factor H gene (CFH) variant is a major risk factor for AMD in Caucasians. However, we and two other groups have reported no association between CFH and AMD in the Japanese population. Recent studies have suggested that LOC387715 on chromosome 10q26 may be the second major risk loci for AMD in Caucasians. In this study, we examined the association between LOC387715 and AMD in Japanese, and our results show that polymorphism of the LOC387715 gene is associated with AMD in Japanese as well as in Caucasians. Our data show a disease odds ratio of 6.20 (95% CI: 2.87–13.40) conferred by homozygosity for risk alleles at LOC387715 compared with the non-risk genotype. A polymorphism of LOC387715 gene is associated with AMD in the Japanese population.

The Genetics of Age-Related Macular Degeneration: A Review of Progress to Date

Age-related macular degeneration (AMD) is the leading cause of vision loss and blindness among older adults in the USA and throughout the developed world. Etiological research suggests that AMD is a complex disease, caused by the actions and interactions of multiple genes and environmental factors. Familial aggregation studies, twin studies, and segregation analyses have provided strong evidence for the heritability of AMD, and linkage and association studies have been conducted to localize the disease-causing genes. Whole genome linkage scans have implicated nearly every chromosome in the human genome, with the most replicated signals residing on 1q25-31 and 10q26. Association studies have identified a major risk variant within the complement factor H gene (CFH), and recent reports suggest that PLEKHA1/LOC387715 and the BF/C2 regions may be major risk loci for AMD as well. Several other genes have had at least one positive association finding and deserve further exploration. Among these, apolipoprotein E (APOE) may be a minor risk locus. Additional genes will likely be identified, and future studies should explore the potential interactions of these genes with other genes as well as environmental factors.

Meta-analysis of genome scans of age-related macular degeneration

A genetic contribution to the development of age-related macular degeneration (AMD) is well established. Several genome-wide linkage studies have identified a number of putative susceptibility loci for AMD but only a few of these regions have been replicated in independent studies. Here, we perform a meta-analysis of six AMD genome screens using the genome-scan meta-analysis method, which allows linkage results from several studies to be combined, providing greater power to identify regions that show only weak evidence for linkage in individual studies. Results from non-parametric analysis for a broad AMD clinical phenotype (including two studies with quantitative traits) were extracted. For each study, 120 genomic bins of approximately 30 cM were defined and ranked according to maximum evidence for linkage within each bin. Bin ranks were weighted according to study size and summed across all studies; the summed rank (SR) for each bin was assessed empirically for significance using permutation methods. A high SR indicates a region with consistent evidence for linkage across studies. The strongest evidence for an AMD susceptibility locus was found on chromosome 10q26 where genome-wide significant linkage was observed (P=0.00025). Several other regions met the empirical significance criteria for bins likely to contain linked loci including adjacent pairs of bins on chromosomes 1q, 2p, 3p and 16. Several of the regions identified here showed only weak evidence for linkage in the individual studies. These results will help prioritize regions for future positional and functional candidate gene studies in AMD.

Complement Factor H Variant Increases the Risk of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of visual impairment and blindness in the elderly whose etiology remains largely unknown. Previous studies identified chromosome 1q32 as harboring a susceptibility locus for AMD. We used single-nucleotide polymorphisms to interrogate this region and identified a strongly associated haplotype in two independent data sets. DNA resequencing of the complement factor H gene within this haplotype revealed a common coding variant, Y402H, that significantly increases the risk for AMD with odds ratios between 2.45 and 5.57. This common variant likely explains approximately 43% of AMD in older adults.

Complement Factor H Polymorphism and Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a common, late-onset, and complex trait with multiple risk factors. Concentrating on a region harboring a locus for AMD on 1q25-31, the ARMD1 locus, we tested single-nucleotide polymorphisms for association with AMD in two independent case-control populations. Significant association (P = 4.95 x 10(-10)) was identified within the regulation of complement activation locus and was centered over a tyrosine-402 --> histidine-402 protein polymorphism in the gene encoding complement factor H. Possession of at least one histidine at amino acid position 402 increased the risk of AMD 2.7-fold and may account for 50% of the attributable risk of AMD.

A Discordant Sib-Pair Linkage Analysis of Age-Related Macular Degeneration

Background:Age-related macular degeneration (AMD) is the leading cause of visual impairment and blindness among older adults in the United States and throughout the developed world. Etiological research implicates both genetic and environmental components. Our prior genome scan in 511 affected sib-pairs and other relative pairs identified significant or suggestive linkage signals on chromosomes 1, 2, 3, 6, 8, 10, 12, 16, and 22. Purpose:To search for genetic loci for AMD using the extremely discordant sib-pair (EDSP) method of linkage analysis, which until now has never been applied to the study of AMD. Methods: The EDSP method is a more powerful approach than standard methods which rely on relative pairs selected at random or pairs concordant for the phenotype. The EDSP approach has also been characterized as the only design that is uniformly powerful in nearly all genetic situations. Thus, substantial reductions in sample size can be achieved. Study population:The study sample for analysis included 110 EDSPs from 40 families that comprise a subset of the 158 families studied in a prior genome-wide scan using affected relative pairs. Results: Evidence for linkage was found on chromosomes 1q, 2q, 6q, 19p, and 20q. The regions identified on chromosomes 1q and 2q were the same regions identified in our prior analysis, whereas the identified region on 6q was approximately 80 cM distal to our previous signal. Discussion: Within this study population, we have narrowed the focus to chromosomes 1q, 2q, 6q, 19p, and 20q in our search for AMD loci. However, given the fact that a gene was recently identified on chromosome 1q, future family- and population-based analyses should concentrate on testing for associations with candidate gene variants in the other identified chromosomal regions in searches for additional AMD loci.

Ordered subset linkage analysis supports a susceptibility locus for age-related macular degeneration on chromosome 16p12

BackgroundAge-related macular degeneration (AMD) is a complex disorder that is responsible for the majority of central vision loss in older adults living in developed countries. Phenotypic and genetic heterogeneity complicate the analysis of genome-wide scans for AMD susceptibility loci. The ordered subset analysis (OSA) method is an approach for reducing heterogeneity, increasing statistical power for detecting linkage, and helping to define the most informative data set for follow-up analysis. OSA assesses the linkage evidence in subsets of potentially more homogeneous families by rank-ordering family-specific lod scores with respect to trait-associated covariates or phenotypic features. Here, we present results of incorporating five continuous covariates into our genome-wide linkage analysis of 389 microsatellite markers in 62 multiplex families: Body mass index (BMI), systolic (SBP) and diastolic (DBP) blood pressure, intraocular pressure (IOP), and pack-years of cigarette smoking. Chromosome-wide significance of increases in nonparametric multipoint lod scores in covariate-defined subsets relative to the overall sample was assessed by permutation.ResultsUsing a correction for testing multiple covariates, statistically significant lod score increases were observed for two chromosomal regions: 14q13 with a lod score of 3.2 in 28 families with average IOP ≤ 15.5 (p = 0.002), and 6q14 with a lod score of 1.6 in eight families with average BMI ≥ 30.1 (p = 0.0004). On chromosome 16p12, nominally significant lod score increases (p ≤ 0.05), up to a lod score of 2.9 in 32 families, were observed with several covariate orderings. While less significant, this was the only region where linkage evidence was associated with multiple clinically meaningful covariates and the only nominally significant finding when analysis was restricted to advanced forms of AMD. Families with linkage to 16p12 had higher averages of SBP, IOP and BMI and were primarily affected with neovascular AMD. For all three regions, linkage signals at or very near the peak marker have previously been reported.ConclusionOur results suggest that a susceptibility gene on chromosome 16p12 may predispose to AMD, particularly to the neovascular form, and that further research into the previously suggested association of neovascular AMD and systemic hypertension is warranted.

Further Evidence for an Association of ABCR Alleles with Age-Related Macular Degeneration

Age-related macular degeneration (AMD) accounts for >50% of the registered visual disability among North American and Western European populations and has been associated both with environmental factors, such as smoking, and with genetic factors. Previously we have reported disease-associated variants in the ABCR (also called ABCA4) gene in a subset of patients affected with this complex disorder. We have now tested our original hypothesis, that ABCR is a dominant susceptibility locus for AMD, by screening 1,218 unrelated AMD patients of North American and Western European origin and 1,258 comparison individuals from 15 centers in North America and Europe for the two most frequent AMD-associated variants found in ABCR. These two sequence changes, G1961E and D2177N, were found in one allele of ABCR in 40 patients ( approximately 3.4%), and in 13 control subjects ( approximately 0.95%). Fisher's two-sided exact test confirmed that these two variants are associated with AMD at a statistically significant level (P<.0001). The risk of AMD is elevated approximately threefold in D2177N carriers and approximately fivefold in G1961E carriers. The identification of a gene that confers risk of AMD is an important step in unraveling this complex disorder.

The rod photoreceptor ATP-binding cassette transporter gene, ABCR, and retinal disease: from monogenic to multifactorial

The ABCR gene encodes a rod photoreceptor specific ATP-binding cassette transporter. Mutations in ABCR are associated with at least four inherited retinal dystrophies: Stargardt disease, Fundus Flavimaculatus, cone-rod dystrophy, and retinitis pigmentosa. A statistically significant increase in heterozygous ABCR alterations has been identified in patients with age-related macular degeneration (AMD). A pedigree is described which manifests both Stargardt disease and AMD in which an ABCR mutation cosegregates with both disease phenotypes. These data from this case report support the hypothesis that ABCR is a dominant susceptibility locus for AMD. Recent work regarding ABCR is reviewed and a model is presented in which decreased ABCR function correlates with severity of retinal disease.