Characterization of the 10q26-orthologue in rhesus monkeys corroborates a functional connection between ARMS2 and HTRA1.

Age-related macular degeneration, which is the leading cause of blindness in industrialized countries, is a multifactorial, degenerative disorder of the macula with strong heritability. For age-related macular degeneration in humans, the genes ARMS2 and HTRA1 in the region 10q26 are both promising candidates for being involved in pathogenesis. However, the associated variants are located in a region of strong linkage disequilibrium and so far, the identification of the causative gene in humans was not yet possible. This dilemma might be solved using an appropriate model organism. Rhesus monkeys suffer from drusen, a major hallmark of age-related macular degeneration, and the drusen-phenotype shares susceptibility factors with human macular degeneration. Thus, the rhesus monkey represents a natural animal model to uncover genetic factors leading to macular degeneration. Moreover, the existence of genetically homogenous cohorts offers an excellent opportunity to determine risk factors. However, the 10q26-orthologue genomic region in rhesus monkeys is not characterized in detail so far. Therefore, the aim of this study is to analyze the rhesus linkage disequilibrium structure and to investigate whether variants in ARMS2 or HTRA1 are associated with the drusen-phenotype as well. We sequenced parts of a 20 kb region around ARMS2 and HTRA1 in a genetically homogeneous cohort of 91 rhesus monkeys descending from the CPRC rhesus cohort on Cayo Santiago and currently housed in the German Primate Centre in Göttingen. Within this group, ophthalmoscopic examinations revealed a naturally high drusen prevalence of about 47% in monkeys >5 years. We detected 56 genetic variants within and around ARMS2 and HTRA1 and, as one deviates from Hardy-Weinberg-Equilibrium, 55 polymorphisms were used to generate a linkage disequilibrium-Plot and to perform association studies. We observed strong linkage disequilibrium between the markers and were able to define two haplotype blocks. One of these blocks spanned the whole ARMS2 locus and the 5' part of HTRA1 - almost perfectly resembling the situation found in humans. Tests for association revealed a variant in the promoter region of HTRA1 and two variants in the 5'-UTR of ARMS2 to be associated with drusen. The strong linkage disequilibrium inhibits - as in humans - a determination of the risk gene using statistical methods only. However, the conserved linkage disequilibrium structure in humans and macaques goes in line with the recently emerged dual causality model proposing that ARMS2 and HTRA1 are functionally connected and that both genes contribute to the disease pathology. Moreover, the characterization of the 10q26-orthologue genomic region of the rhesus monkey provides a basis for now needed functional investigations in a well-characterized model organism.

Bestrophin detected in the basal membrane of the retinal epithelium and drusen of monkeys with drusenoid maculopathy.

PURPOSE: To determine if bestrophin is present in the basal membrane of macular retinal pigment epithelium (RPE) and in drusen of rhesus monkeys with age-related drusenoid maculopathy. METHODS: The macular region of three rhesus monkeys (Macaca mulatta), 23-24 years of age, with drusenoid maculopathy was dissected from eyes fixed with 4% paraformaldehyde. The macula was sectioned into rectangular pieces. The sclera was removed from each segment and the remainder separated into segments of neural retina with retinal epithelium or choroid with retinal epithelium. These segments were incubated with a goat polyclonal antibody to human bestrophin 1, reacted with gold-labeled rabbit antibody to goat IgG, silver-enhanced, and processed for transmission electron microscopy. RESULTS: Bestrophin-labeled gold particles were found in quasi-linear arrays on the basal surface of the macular RPE and also within drusen where bestrophin was found in segments of membranous-like material. The array density of the bestrophin-linked gold particles on the basal membrane of the epithelium had a maximal value of about 5-100 bestrophin molecules/micron(2). Immuno-detection of bestrophin was most effective when examined in an RPE layer that remained attached to the neural retina, where the basal surface of the epithelium is more directly exposed to the antibodies. CONCLUSION: Bestrophin is present on the basal membrane of macular RPE of rhesus monkeys with age-related drusenoid maculopathy, and also found in the membranous-like structures of drusen. The latter finding provides insight into the pathogenesis of drusen by indicating that segments of the basal membrane of RPE contribute to the material that accumulates within drusen.

Drusenoid maculopathy in rhesus monkeys: autofluorescence, lipofuscin and drusen pathogenesis.

PURPOSE: To examine patterns of retinal pigment epithelial autofluorescence and lipofuscin accumulation in relation to drusen and to explore the pathogenesis of drusen in rhesus monkeys. METHODS: The macular areas of six rhesus monkeys, euthanized at 19 to 28 years of age, were studied by bright field and fluorescence light microscopy and transmission electron microscopy. RESULTS: There was strong autofluorescence in the retinal epithelium that tended to diminish over drusen. Electron microscopy revealed that all retinal epithelial cells had large concentrations of lipofuscin bodies. The epithelial cells overlying drusen, however, tended to have less lipofuscin than epithelial cells not associated with drusen. Electron microscopy revealed that the epithelial cells overlying drusen were losing segments of cytoplasm containing lipofuscin bodies. Macrophage-like cells were consistently present in Bruch's membrane microns away from this lipofuscin-containing cytoplasmic material. CONCLUSIONS: Retinal epithelial cells overlying drusen have less lipofuscin than neighboring epithelial cells. The loss of lipofuscin seems due to a loss of cytoplasm containing lipofuscin that contributes to drusen formation. Macrophages in Bruch's membrane may be responsible for removing this lipofuscin debris. The results support in vivo studies showing reduced autofluorescence over drusen and support the "budding" of epithelial cytoplasm as a source of drusen material.

Drusenoid maculopathy in rhesus monkeys (Macaca mulatta): effects of age and gender.

PURPOSE: To compare drusenoid maculopathy in monkeys with human age-related macular degeneration, and evaluate the influence of age, gender and caloric restriction. METHODS: Examination by indirect ophthalmoscopy, slit-lamp biomicroscopy and fundus photography, including in some cases fluorescein angiography, was performed on 61 male and 60 female rhesus macaques of ages 10-39 years. Fifty-four of the monkeys were maintained on a calorically restricted diet (approximately 30% lower than control levels) and 67 on an approximately ad libitum diet for 2-19 years, with all other environmental factors held constant. Maculopathies were graded on a 5-point scale and the effects of age, sex, and diet on prevalence and severity were examined. The retinas of six monkeys with macular drusen, 19-28 years old, were examined histologically. RESULTS: Rhesus monkeys showed a high prevalence (61%) of drusenoid maculopathy. The prevalence and severity of the maculopathy increased with age (p = 0.012). Fully half of all monkeys aged 10-12 years had some detectable degree of drusen. This high prevalence in young adulthood indicates that drusen develop much earlier in rhesus monkeys than in humans, who develop early maculopathy most rapidly at 50-60 years of age, even when correcting for the 3-fold difference in lifespan. No neovascularization or geographic atrophy was found. Females had a higher prevalence and severity than males (p = 0.019). Calorically restricted monkeys had a slightly lower prevalence and severity at 10-12 years than controls, but the difference was not statistically significant. This is an on-going project, and differences between the caloric restricted and ad-lib groups may emerge as the animals age. Some monkeys developed severe maculopathy in their 20s, with others unaffected in their 30s. The histology of drusen resembled those in human retina. CONCLUSION: Drusenoid maculopathy is common in rhesus monkeys, even in young adult life. Half of the rhesus monkeys examined have drusen at a much younger age than in humans. Severity of maculopathy was greater in female monkeys, a gender difference not consistently found in humans. No differences were detected due to caloric restriction, but a definitive test of this intervention will require a larger sample, longer period of observation, and/or an earlier institution of caloric restriction. Genetic factors are implied because with similar environments, some monkeys are affected at an early age, while older ones are not.

IMPG1 gene variation in rhesus macular drusen.

Drusen is a hallmark of human age-related maculopathy. Rhesus macaques (Macaca mulatta) represent a natural model of age-related maculopathy with drusen. We have already mapped the macular drusen susceptibility locus in rhesus macaques to the homolog of human chromosome 6q14-15 and shown that a particular IMPG1 gene SNP haplotype was apparently associated with drusen formation in the rhesus macaques maintained by the Caribbean Primate Research Center (CPRC), Puerto Rico, USA. The aim of the present study was to verify this finding in the macaques kept at the German Primate Research Center (DPZ), Germany. The study group comprised 64 animals (34 affected, 30 unaffected). These monkeys were genotyped for all known variations in the IMPG1 gene and haplotype analysis was performed. A total absence of the previously identified risk haplotype of the IMPG1 gene, and a much lower drusen prevalence in comparison to the CPRC group, was observed in the DPZ samples. This prompted a re-analysis of the original disease association in the CPRC, which revealed that the implied risk haplotype was in fact a sequencing artifact. Taken together, the data highlight that additional factors, other than IMPG1 variation, must play a role in drusen pathogenesis in rhesus macaques.

Drusen and lipid-filled retinal pigment epithelium cells in a rhesus macula.

A middle-aged rhesus monkey with detailed clinical history exhibited progression of a macular abnormality with a variety of clinical drusen and pigment changes typical of the Cayo Santiago phenotype. Numerous frozen sections of one sample of the macular retina/retinal pigment epithelium (RPE)/choroid showed a single classical druse but extensive single and clustered lipid-filled RPE cells. The monkey exhibited functional outer retinal decline and an insignificant number of 'window defects' as found among signs of relatively benign human macular aging. The clinical and histologically defined results agree if lipid-filled RPE cells are included among the clinically apparent signs of drusen.

Histologic localization of indocyanine green dye in aging primate and human ocular tissues with clinical angiographic correlation.

OBJECTIVE: This study aimed to histologically localize indocyanine green (ICG) dye in the geriatric primate and human eye and to correlate these findings with clinical ICG angiography. DESIGN: The study design was a clinicopathologic correlation. PARTICIPANTS: Six eyes of three geriatric monkeys (Maccaca mulatta) with macular drusen, 19 to 29 years of age, housed at the California Primate Research Center and an enucleated human eye from a 66-year-old patient with choroidal melanoma were examined. INTERVENTION: All six monkey eyes and the human eye underwent clinical ICG angiography. Five monkey eyes were enucleated at varying intervals after intravenous ICG dye injection for histologic examination. One monkey eye was removed without prior ICG injection as an age-matched control. The human eye was enucleated after intravenous injection of ICG dye. MAIN OUTCOME MEASURES: Infrared fluorescence microscopy of freeze-dried tissue sections was performed to detect ICG fluorescence. Histologic sections were stimulated with an 810-nm diode laser, and the fluorescence emitted was detected with a Hamamatsu infrared camera. The images were digitally recorded. The distribution of fluorescence on histologic examination was correlated with the fluorescence of the clinical ICG angiogram. RESULTS: Infrared fluorescence microscopy of monkey sections localized fluorescence within retinal and choroidal vessels early after injection of ICG dye. The ICG fluorescence was seen in the extravascular choroidal stroma within 10 minutes after injection. The stromal fluorescence persisted in sections obtained 50 minutes after injection of ICG. The retinal pigment epithelium (RPE)-Bruch's membrane complex was brightly fluorescent in the middle- and late-stage histologic sections. Drusen deposits were brightly fluorescent at all timepoints examined. Similar findings were observed in freeze-dried tissue sections of the human eye. The fluorescence detected on histologic sections correlated closely with the fluorescence of the clinical ICG angiograms for the same interval. CONCLUSIONS: The ICG dye does not remain solely within the choroidal intravascular space but extravasates into the choroidal stroma and accumulates within the RPE. Extravascular ICG binds to drusen material. These findings will enhance the interpretation of clinical ICG angiography.