Exfoliation Syndrome: A Disease of Autophagy and LOXL1 Proteopathy

Mitochondrial and microtubule defects in Exfoliation Glaucoma

Exfoliation Syndrome is an age-related systemic condition characterized by large aggregated fibrillar material deposition in the anterior eye tissues. This aggregate formation and deposition on the aqueous humor outflow pathway are significant risk factors for developing Exfoliation Glaucoma (XFG). XFG is a multifactorial late-onset disease that shares common features of neurodegenerative diseases, such as increased protein aggregation, impaired protein degradation, and oxidative and cellular stress. XFG patients display decreased mitochondrial membrane potential and mitochondrial DNA deletions. Here, using Tenon Capsule Fibroblasts (TFs) from patients without glaucoma (No Glaucoma, NG) and XFG patients, we found that XFG TFs have impaired mitochondrial bioenergetics and increased reactive oxygen species accumulation. These defects are associated with mitochondrial abnormalities as XFG TFs exhibit smaller mitochondria that contain dysmorphic cristae, with increased mitochondrial localization to lysosomes and slowed mitophagic flux. Mitochondrial dysfunction in the XFG TFs was associated with hyperdynamic microtubules, decreased acetylated tubulin, and increased HDAC6 activity. Treatment of XFG TFs with a mitophagy inducer, Urolithin A (UA), and a mitochondrial biogenesis inducer, Nicotinamide Ribose (NR), improved mitochondrial bioenergetics and reduced ROS accumulation. Our results demonstrate that XFG TFs have abnormal mitochondria and suggest that mitophagy inducers may represent a potential class of therapeutics for reversing mitochondrial dysfunction in XFG patients.

Zonulopathies as Genetic Disorders of the Extracellular Matrix

The zonular fibres are formed primarily of fibrillin-1, a large extracellular matrix (ECM) glycoprotein, and also contain other constituents such as LTBP-2, ADAMTSL6, MFAP-2 and EMILIN-1, amongst others. They are critical for sight, holding the crystalline lens in place and being necessary for accommodation. Zonulopathies refer to conditions in which there is a lack or disruption of zonular support to the lens and may clinically be manifested as ectopia lens (EL)-defined as subluxation of the lens outside of the pupillary plane or frank displacement (dislocation) into the vitreous or anterior segment. Genes implicated in EL include those intimately involved in the formation and function of these glycoproteins as well as other genes involved in the extracellular matrix (ECM). As such, genetic pathogenic variants causing EL are primarily disorders of the ECM, causing zonular weakness by (1) directly affecting the protein components of the zonule, (2) affecting proteins involved in the regulation of zonular formation and (3) causing the dysregulation of ECM components leading to progressive zonular weakness. Herein, we discuss the clinical manifestations of zonulopathy and the underlying pathogenetic mechanisms.

Mitochondrial and Microtubule Defects in Exfoliation Glaucoma

Exfoliation Syndrome (XFS) is an age-related systemic condition characterized by large aggregated fibrillar material deposition in the anterior eye tissues. This aggregate formation and deposition on the aqueous humor outflow pathway are significant risk factors for developing Exfoliation Glaucoma (XFG), a secondary open-angle glaucoma. XFG is a complex, multifactorial late-onset disease that shares common features of neurodegenerative diseases, such as altered cellular processes with increased protein aggregation, impaired protein degradation, and oxidative and cellular stress. XFG patients display decreased mitochondrial membrane potential and mitochondrial DNA deletions. Here, using Tenon Capsule Fibroblasts (TFs) from Normal (No Glaucoma) and XFG patients, we found that XFG TFs have impaired mitochondrial bioenergetics and increased reactive oxygen species (ROS) accumulation. These defects are associated with mitochondrial abnormalities as XFG TFs exhibit smaller mitochondria that contain dysmorphic cristae, with an increase in mitochondrial localization to lysosomes and slowed mitophagy flux. Mitochondrial dysfunction in the XFG TFs was associated with an increase in the dynamics of the microtubule cytoskeleton, decreased acetylated tubulin, and increased HDAC6 activity. Treatment of XFG TFs with a mitophagy inducer, Urolithin A, and a mitochondrial biogenesis inducer, NAD + precursor, Nicotinamide Ribose, improved mitochondrial bioenergetics and reduced ROS accumulation. Our results demonstrate abnormal mitochondria in XFG TFs and suggest that mitophagy inducers may represent a potential class of therapeutics for reversing mitochondrial dysfunction in XFG patients.

Massively parallel sequencing of mitochondrial genome in primary open angle glaucoma identifies somatically acquired mitochondrial mutations in ocular tissue

Glaucoma is a sight threatening neurodegenerative condition of the optic nerve head associated with ageing and marked by the loss of retinal ganglion cells. Mitochondrial dysfunction plays a crucial role in the pathogenesis of neurodegeneration in the most prevalent type of glaucoma: primary open angle glaucoma (POAG). All previous mitochondrial genome sequencing studies in POAG analyzed mitochondrial DNA (mtDNA) isolated from peripheral blood leukocytes and have not evaluated cells derived from ocular tissue, which better represent the glaucomatous disease context. In this study, we evaluated mitochondrial genome variation and heteroplasmy using massively parallel sequencing of mtDNA in a cohort of patients with POAG, and in a subset assess the role of somatic mitochondrial genome mutations in disease pathogenesis using paired samples of peripheral blood leukocytes and ocular tissue (Tenon's ocular fibroblasts). An enrichment of potentially pathogenic nonsynonymous mtDNA variants was identified in Tenon's ocular fibroblasts from participants with POAG. The absence of oxidative DNA damage and predominance of transition variants support the concept that errors in mtDNA replication represent the predominant mutation mechanism in Tenon's ocular fibroblasts from patients with POAG. Pathogenic somatic mitochondrial genome mutations were observed in people with POAG. This supports the role of somatic mitochondrial genome variants in the etiology of glaucoma.

Minimal phenotypes in transgenic mice with the human LOXL1/LOXL1-AS1 locus associated with exfoliation glaucoma

Exfoliation syndrome is a leading cause of secondary glaucoma worldwide. Among the risk-factors for exfoliation syndrome and exfoliation glaucoma that have been investigated, a genetic association with 15q24.1 is among the most striking. The leading candidates for the causal gene at this locus are LOXL1 and/or LOXL1-AS1, but studies have not yet coalesced in establishing, or ruling out, either candidate. Here, we contribute to studies of the 15q24.1 locus by making a partially humanized mouse model in which 166 kb of human genomic DNA from the 15q24.1 locus was introduced into the mouse genome via BAC transgenesis (B6-Tg(RP11-71M11)Andm). Transgenic expression of human genes in the BAC was only detectable for LOXL1-AS1. One cohort of 34 mice (21 experimental hemizygotes and 13 non-carrier control littermates) was assessed by slit-lamp exams and SD-OCT imaging at early (1-2 months) and mid (4-5 months) time points; fundus exams were performed at 5 months of age. A second smaller cohort (3 hemizygotes) were aged extensively (>12 months) to screen for overt abnormalities. Across all genotypes and ages, 136 slit-lamp exams, 128 SD-OCT exams, and 42 fundus exams detected no overt indices of exfoliation syndrome. Quantitatively, small, but statistically significant, age-related declines in ganglion cell complex thickness and total retinal thickness were detected in the hemizygotes at 4 months of age. Overall, this study demonstrates complexity in gene regulation from the 15q24.1 locus and suggests that LOXL1-AS1 is unlikely to be a monogenic cause of exfoliation syndrome but may contribute to glaucomatous retinal damage.

Glaucoma, Pseudoexfoliation and Hearing Loss: A Systematic Literature Review

Purpose: To investigate the relationship between glaucoma, pseudoexfoliation and hearing loss (HL). Methods: A systematic literature search following PRISMA guidelines was conducted using the PubMed, Embase, Scopus and Cochrane databases from 1995 up to 28 August 2023. Results: Thirty studies out of the 520 records screened met the inclusion criteria and were included. Most articles (n = 20) analysed the association between pseudoexfoliation syndrome (XFS) and HL, showing XFS patients to have higher prevalence of sensorineural hearing loss (SNHL) at both speech frequencies (0.25, 0.5, 1 and 2 kHz), and higher frequencies (4 and 8 kHz) compared to controls in most cases. No significant differences in prevalence or level of HL between XFS and pseudoexfoliative glaucoma (XFG) were detected in most studies. Eight articles analysed the relationship between primary open-angle glaucoma (POAG) and HL. Overall, a positive association between the two conditions was highlighted across all studies except for two cases. Similarly, articles focusing on NTG and HL (n = 4) showed a positive association in most cases. The role of autoimmunity and, in particular, the presence of antiphosphatidylserine antibodies (APSA) in patients with NTG and HL suggested an underlying autoimmune or vascular mechanism contributing to their pathogenesis. Only one study analysed the relationship between angle-closure glaucoma (ACG) and HL, showing higher incidence of ACG in patients with SNHL compared to normal hearing controls. Conclusions: Most studies detected an association between XFS and HL as well as POAG/NTG/ACG and HL, suggesting the presence of a similar pathophysiology of neurodegeneration. However, given the strength of the association of XFS with HL, it remains unclear whether the presence of XFG is further associated with SNHL. Further research specifically targeted to assess the correlation between glaucoma, XFS and HL is warranted to provide a more comprehensive understanding of this association.

A Study on the Candidate Gene Association and Interaction with Measures of UV Exposure in Pseudoexfoliation Patients from India

PURPOSE

Environmental and genetic factors are associated with development of Pseudoexfoliation syndrome (XFS). Here we intended to elucidate the association of candidate genes in relevance to UV exposure in these patients.

METHODS

This is a case-control study of 309 subjects (N = 219 controls and 90 XFS cases) from India. PCR based direct sequencing was performed for candidate genes (LOXL1, POMP and TMEM136) followed by genotype and haplotype analysis. The promoter methylation status was assessed by Methylation specific PCR based direct sequencing of genomic DNA for all samples. The methylation status was compared with that of primary fibroblasts cultures established from patient's Tenon's tissue samples in subset of these patients.

RESULTS

SNPs rs3825942, rs41435250, rs8818 (LOXL1) and rs3737528 (POMP) showed significant association with XFS. LOXL1 gene haplotype GAGC (rs1048661- rs3825942- rs41435250-rs8818) was associated with lower risk for XFS with a p value 4.1961 × 10-6 (OR =0; 95%CI, 0.000-0.003). POMP gene haplotypes for intronic SNPs (rs1340815- rs3737528- rs913797) TCC and TTC were associated with increased risk for the disease (OR > 1.0). Significant correlation for SNPs rs3825942 of LOXL1 (ρ= -0.132) and rs3737528 of POMP (ρ = 0.12) was observed with measure of lifetime UV exposure (CUVAF value). Reduced LOXL1 gene expression was observed in cultured tenon fibroblasts from the patients that correlated with differential methylation of the Sp-1 binding sites at -253, -243bp upstream to the transcription start site of LOXL1 promoter region.

CONCLUSION

Our results suggest a possible interaction for LOXL1 gene haplotype (GAGC) with the measure of ocular UV exposure in pseudoexfoliation syndrome.

Investigating the Impact of Sun/UV Protection and Ease of Skin Tanning on the Risk of Pseudoexfoliation Glaucoma: A Mendelian Randomization Study

Purpose

To investigate the potential causal associations between the use of sun/ultraviolet (UV) protection and ease of skin tanning and the risk of pseudoexfoliation glaucoma (PXG) in European populations.

Methods

Single nucleotide polymorphisms (SNPs) associated with the use of sun/UV protection and ease of skin tanning were selected from the UK Biobank genome-wide association study database consisting of 498,751 European participants. SNPs of PXG were obtained from the FinnGen study including 3424 PXG cases and 326,434 controls. Two-sample Mendelian randomization (MR) analyses were performed to assess the association between the use of sun/UV protection and ease of skin tanning and risk of PXG.

Results

Inverse variance weighted regression of genetic susceptibility predicted that both use of sun/UV protection and ease of skin tanning were potentially positively associated with the decreased risk of PXG in the European ancestry (use of sun/UV protection: odds ratio [OR] = 0.47; 95% confidence interval [CI], 0.24-0.92; P = 0.028; ease of skin tanning: OR = 0.81; 95% CI, 0.67-0.97; P = 0.025).

Conclusions

We found genetic evidence supporting a potential causal association between UV protection and a decreased risk of PXG in European population. Further research will help elucidate the underlying mechanisms and promote UV protection for eyes, especially in people with a high risk of PXG.

Deuterated polyunsaturated fatty acids provided protection against oxidative stress in ocular fibroblasts derived from glaucoma patients

Glaucoma is a complex neurodegenerative disease of the optic nerve that leads to irreversible sight loss. Lowering intraocular pressure (IOP) medically or surgically represents the mainstay of treatment but despite adequate treatment optic nerve function can continue to deteriorate leading to blindness. There is significant clinical and experimental evidence that oxidative stress is involved in the pathogenesis of glaucoma. Decreasing the formation of lipid peroxidation products or scavenging them chemically could be beneficial in limiting the deleterious effects of oxidative stress in glaucoma. A solution to control the susceptibility of PUFAs to noxious lipid peroxidation reactions is by regioselective deuteration. Deuterium incorporated into PUFAs at bis-allylic positions (D-PUFAs) inhibits the rate-limiting step of lipid peroxidation. In this study, we have shown that Tenon's ocular fibroblasts from glaucoma patients have significantly increased basal oxidative stress compared to non-glaucomatous control patients. Furthermore, we have shown that deuterated polyunsaturated fatty acids (D-PUFAs) provide an enhanced rescue of menadione induced lipid peroxidation in both non-glaucomatous and glaucomatous Tenon's ocular fibroblasts using malondialdehyde (MDA) levels as a marker. Our study suggests that D-PUFAs may provide a potentially safe and effective method to reduce cytotoxic oxidative stress in glaucoma.

Genome-Wide RNA Sequencing of Human Trabecular Meshwork Cells Treated with TGF-β1: Relevance to Pseudoexfoliation Glaucoma

Pseudoexfoliation glaucoma (XFG) is an aggressive form of secondary open angle glaucoma, characterised by the production of exfoliation material and is estimated to affect 30 million people worldwide. Activation of the TGF-β pathway by TGF-β1 has been implicated in the pathogenesis of pseudoexfoliation glaucoma. To further investigate the role of TGF-β1 in glaucomatous changes in the trabecular meshwork (TM), we used RNA-Seq to determine TGF-β1 induced changes in the transcriptome of normal human trabecular meshwork (HTM) cells. The main purpose of this study was to perform a hypothesis-independent RNA sequencing analysis to investigate genome-wide alterations in the transcriptome of normal HTMs stimulated with TGF-β1 and investigate possible pathophysiological mechanisms driving XFG. Our results identified multiple differentially expressed genes including several genes known to be present in exfoliation material. Significantly altered pathways, biological processes and molecular functions included extracellular matrix remodelling, Hippo and Wnt pathways, the unfolded protein response, oxidative stress, and the antioxidant system. This cellular model of pseudoexfoliation glaucoma can provide insight into disease pathogenesis and support the development of novel therapeutic interventions.

Dysregulated Retinoic Acid Signaling in the Pathogenesis of Pseudoexfoliation Syndrome

Pseudoexfoliation (PEX) syndrome, a stress-induced fibrotic matrix process, is the most common recognizable cause of open-angle glaucoma worldwide. The recent identification of PEX-associated gene variants uncovered the vitamin A metabolic pathway as a factor influencing the risk of disease. In this study, we analyzed the role of the retinoic acid (RA) signaling pathway in the PEX-associated matrix metabolism and evaluated its targeting as a potential candidate for an anti-fibrotic intervention. We provided evidence that decreased expression levels of RA pathway components and diminished RA signaling activity occur in an antagonistic crosstalk with TGF-β1/Smad signaling in ocular tissues and cells from PEX patients when compared with age-matched controls. Genetic and pharmacologic modes of RA pathway inhibition induced the expression and production of PEX-associated matrix components by disease-relevant cell culture models in vitro. Conversely, RA signaling pathway activation by natural and synthetic retinoids was able to suppress PEX-associated matrix production and formation of microfibrillar networks via antagonization of Smad-dependent TGF-β1 signaling. The findings indicate that deficient RA signaling in conjunction with hyperactivated TGF-β1/Smad signaling is a driver of PEX-associated fibrosis, and that restoration of RA signaling may be a promising strategy for anti-fibrotic intervention in patients with PEX syndrome and glaucoma.

RNA Sequencing of Lens Capsular Epithelium Implicates Novel Pathways in Pseudoexfoliation Syndrome

Purpose

Pseudoexfoliation syndrome (PEX) is a common systemic disease that results in severe and often irreversible vision loss. Despite considerable research effort, PEX remains incompletely understood. This study sought to perform the first RNAseq study in elucidate the pathophysiology of PEX, and contribute a publicly available transcriptomic data resource for future research.

Methods

Human ocular lens capsular epithelium samples were collected from 25 patients with PEX and 39 non-PEX controls undergoing cataract surgery. RNA extracted from these specimens was subjected to polyadenylated (mRNA) selection and deep bulk RNA sequencing. Differential expression analysis investigated protein-coding gene transcripts. Exploratory analyses used pathway analysis tools, and curated class- and disease-specific gene sets.

Results

Differential expression analysis demonstrated that 2882 genes were differentially expressed according to PEX status. Genes associated with viral gene expression pathways were among the most upregulated, alongside genes encoding ribosomal and mitochondrial respiratory transport chain proteins. Cell adhesion protein transcripts including type 4 collagen subunits were downregulated.

Conclusions

This comparative transcriptomic dataset highlights novel and previously recognized pathogenic pathways in PEX and provides the first comprehensive transcriptomic resource, adding an additional layer to build further understanding of PEX pathophysiology.

Monitoring basal autophagy in the retina utilizing CAG-mRFP-EGFP-MAP1LC3B reporter mouse: technical and biological considerations

We describe the utility of a tandem-tagged autophagy reporter mouse model (CAG-RFP-EGFP-MAP1LC3B) in investigating basal macroautophagic/autophagic flux in the neural retina. Western blot, in situ hybridization, immunohistochemistry, and confocal microscopy showed that CAG promoter-driven expression of RFP-EGFP-MAP1LC3B increased “cytosolic” RFP-EGFP-LC3B-I levels, whereas RFP-EGFP-LC3B-II decorates true phagosomes. We verified that the electroretinographic (ERG) responses of tandem-tagged LC3B mice were comparable to those of age-matched controls. Optimized microscope settings detected lipofuscin autofluorescence in retinas of abca4^−/- mice. The majority of retinal phagosomes in the reporter mice exhibited only RFP (not EGFP) fluorescence, suggesting rapid maturation of phagosomes. Only ~1.5% of the total phagosome population was EGFP-labeled; RFP-labeled (mature) phagosomes colocalized with lysosomal markers LAMP2 and CTSD. In the outer retina, phagosome sizes were as follows (in µm², ave ± SEM): RPE, 0.309 ± 0.015; photoreceptor inner segment-myoid, 0.544 ± 0.031; and outer nuclear layer, 0.429 ± 0.011. Detection of RPE phagosomes by fluorescence microscopy is challenging, due to the presence of melanin. Increased lipofuscin autofluorescence, such as observed in the abca4^−/- mouse model of Stargardt disease, is a strong confounding factor when attempting to study autophagy in the RPE. In addition to RPE and photoreceptor cells, phagosomes were detected in inner retinal cell types, microglia, astrocytes, and endothelial cells. We conclude that the tandem-tagged LC3B mouse model serves as a useful system for studying autophagy in the retina. This utility, however, is dependent upon several technical and biological factors, including microscope settings, transgene expression, choice of fluorophores, and lipofuscin autofluorescence.

Abbreviations: ACTB: actin, beta; AIF1: allograft inflammatory factor 1; ATG: autophagy related; CTSD: cathepsin D; DAPI: (4’,6-diamido-2-phenylindole); DIC: differential interference contrast; EGFP: enhanced green fluorescent protein; ELM: external limiting membrane; ERG: electroretinography; GCL: ganglion cell layer; GLUL: glutamine-ammonia ligase (glutamine synthetase); INL: inner nuclear layer; IS-E/M: inner segment – ellipsoid/myoid; ISH: in situ hybridization; LAMP2: lysosomal-associated membrane protein 2; L.I.: laser Intensity; MAP1LC3B: microtubule-associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; O.C.T.: optimal cutting temperature; OS: outer segment; ONL: outer nuclear layer; PE: phosphatidylethanolamine; RFP: red fluorescent protein; R.O.I.: region of interest; RPE: retinal pigment epithelium

A Case-Cohort Study of Exfoliation Risk Factors and Literature Review

The purpose of the study is to evaluate the risk factors associated with exfoliation in a case-cohort setting and literature review. This single-center, prospective, case-cohort study was carried out from January 2010 to April 2020 on patients operated for cataract surgery by a single surgeon in Lebanon. Forty-nine consecutive patients with exfoliation syndrome (XFS) and 62 consecutive control patients were identified and further investigated for selected systemic (diabetes mellitus, systemic hypertension, asthma, or atopy) and ocular variables (baseline vision, severity of nuclear sclerosis, glaucoma, eye rubbing, history of dry eye, or allergic eye disease). The mean baseline Snellen visual acuity was 20/283 in XFS versus 20/145 in control cases (P = 0.012). XFS also demonstrated significantly denser nuclear sclerosis than controls (P = 0.00958). By univariate analysis, allergic conjunctivitis (15 [30.6%] vs. 2 [3.2%]; P < 0.001), dry eye (20 [40.8%] vs. 13 [21.0%]; P = 0.0133), and habitual rubbing of the eyes (33 [67.3%] vs. 19 [30.6%]; P < 0.001) were associated with the presence of XFS. Habitual ocular rubbing was closely associated with allergic conjunctivitis (odds ratio [OR] = 13.0; 95% confidence interval [CI]: 2.8–58.8; P = 0.032). After multivariable analysis, the following variables showed significant results: glaucoma (OR = 34.5; 95% CI: 4.4–250; P = 0.010), duration of surgery (OR = 5.6; 95% CI 2.43–12.9; P < 0.001), and habitual ocular rubbing (OR = 4.42; 95% CI: 1.97–9.90; P = 0.029). This study shows a novel potential correlation between eye rubbing and XFS in a Lebanese cohort. Chronic eye rubbing induces or may exacerbate preexistent zonular damage in subjects with XFS, hence the need to better manage concurrent ocular surface disorder in these patients.

Early-onset Exfoliation Syndrome: A Literature Synthesis

PRECIS

We conducted a literature review of younger patients with exfoliation syndrome (XFS) in an attempt to identify case similarities and better understand disease etiology.

PURPOSE

XFS that predisposes to secondary glaucoma is a strongly age-related condition. We performed a literature review of XFS and exfoliation glaucoma (XFG) in patients aged younger than 40 years to examine potential common characteristics and gain clues to its etiology.

METHODS

We conducted a broad literature search with appropriate keywords and manually extracted key demographic and ocular features on younger XFS and XFG cases. Articles that did not provide past ocular history on early-onset XFS/XFG were excluded.

RESULTS

We identified 12 cases of XFS and XFG in patients from 13 to 40 years old (8 females; 11 White; 5 from Iran). All had past ocular history remarkable for intraocular surgery for other glaucoma conditions (7 cases), other ocular diseases (3 cases), or ocular trauma (2 cases).

CONCLUSIONS

All reported early-onset XFS and XFG cases arise in the setting of events that produced a significant disruption of the blood-aqueous barrier. Understanding the metabolic alterations of aqueous humor from such cases could provide clues regarding how exfoliation material forms.

Mitophagy: An Emerging Target in Ocular Pathology

Mitochondrial function is essential for the viability of aerobic eukaryotic cells, as mitochondria provide energy through the generation of adenosine triphosphate (ATP), regulate cellular metabolism, provide redox balancing, participate in immune signaling, and can initiate apoptosis. Mitochondria are dynamic organelles that participate in a cyclical and ongoing process of regeneration and autophagy (clearance), termed mitophagy specifically for mitochondrial (macro)autophagy. An imbalance in mitochondrial function toward mitochondrial dysfunction can be catastrophic for cells and has been characterized in several common ophthalmic diseases. In this article, we review mitochondrial homeostasis in detail, focusing on the balance of mitochondrial dynamics including the processes of fission and fusion, and provide a description of the mechanisms involved in mitophagy. Furthermore, this article reviews investigations of ocular diseases with impaired mitophagy, including Fuchs endothelial corneal dystrophy, primary open-angle glaucoma, diabetic retinopathy, and age-related macular degeneration, as well as several primary mitochondrial diseases with ocular phenotypes that display impaired mitophagy, including mitochondrial encephalopathy lactic acidosis stroke, Leber hereditary optic neuropathy, and chronic progressive external ophthalmoplegia. The results of various studies using cell culture, animal, and human tissue models are presented and reflect a growing awareness of mitophagy impairment as an important feature of ophthalmic disease pathology. As this review indicates, it is imperative that mitophagy be investigated as a targetable mechanism in developing therapies for ocular diseases characterized by oxidative stress and mitochondrial dysfunction.

Pseudoexfoliation syndrome and glaucoma: from genes to disease mechanisms

PURPOSE OF REVIEW

The genetic basis of pseudoexfoliation (PEX) syndrome, the most common identifiable cause of open-angle glaucoma, is steadily being elucidated. This review summarizes the recent advances on genetic risk factors for PEX syndrome/glaucoma and their potential functional implications in PEX pathophysiology.

RECENT FINDINGS

As of today, seven loci associated with the risk of PEX surpassing genome-wide significance have been identified by well-powered genome-wide association studies and sequencing efforts. LOXL1 (lysyl oxidase-like 1) represents the major genetic effect locus, although the biological role of common risk variants and their reversed effect in different ethnicities remain an unresolved problem. Rare protein-coding variants at LOXL1 and a single noncoding variant downstream of LOXL1 showed no allele effect reversal and suggested potential roles for elastin homeostasis and vitamin A metabolism in PEX pathogenesis. Other PEX-associated genetic variants provided biological insights into additional disease processes and pathways, including ubiquitin-proteasome function, calcium signaling, and lipid biosynthesis. Gene-environment interactions, epigenetic alterations, and integration of multiomics data have further contributed to our knowledge of the complex etiology underlying PEX syndrome and glaucoma.

SUMMARY

PEX-associated genes are beginning to reveal relevant biological pathways and processes involved in disease development. To understand the functional consequences and molecular mechanisms of these loci and to translate them into novel therapeutic approaches are the major challenges for the future.

Recent advances in risk factors associated with ocular exfoliation syndrome

Exfoliation syndrome is generally considered a progressive age-related systemic disorder of the extracellular matrix, which is clinically characterized through the observation of flaky white aggregates on ocular tissues. Exfoliation syndrome is directly linked to exfoliative glaucoma in elderly patients, where it is known as the most common identifiable cause of open-angle glaucoma. Despite the identification of various risk factors associated with exfoliation syndrome, the exact pathogenesis of this syndrome has not been fully elucidated. There is a growing number of genome-wide association studies in different populations around the world to identify genetic factors underlying exfoliation syndrome. Besides variants in LOXL1 and CACNA1A genes, new loci have been recently identified which are believed to be associated with exfoliation syndrome. Among different genetic factors, functional variants might help to better understand mechanisms underlying this systemic disorder. Besides genetic factors, epigenetic regulation of different gene expression patterns has been thought to play a role in its pathogenesis. Other factors have been also considered to be involved in the development of exfoliation syndrome at cellular organelles level where mitochondrial impairment and autophagy dysfunction have been suggested in relation to exfoliation syndrome. This review addresses the most recent findings on genetic factors as well as cellular and molecular mechanisms involved in both the development and progression of exfoliation syndrome.

Nailfold Capillaroscopy of Resting Peripheral Blood Flow in Exfoliation Glaucoma and Primary Open-Angle Glaucoma

Importance

Systemic blood flow alterations have been described using video nailfold capillaroscopy (NFC) in high-tension glaucoma (HTG) and normal-tension glaucoma (NTG) variants of primary open-angle glaucoma (POAG). To date, no previous studies have explored alterations in nailfold capillary blood flow in exfoliation glaucoma (XFG).

Objective

To investigate the measure of peripheral blood flow as a surrogate marker of systemic vascular involvement in patients with XFG, HTG, and NTG, as well as in individuals serving as controls, using NFC.

Design, Setting, and Participants

A cross-sectional clinic-based study at the New York Eye and Ear Infirmary of Mount Sinai was conducted from July 6, 2017, to May 18, 2018. A total of 111 participants (30 XFG, 30 NTG, 30 HTG, and 21 controls) received a comprehensive ophthalmic examination to confirm eligibility. Exclusion criteria were the presence of connective tissue disease, uncontrolled diabetes, history of bleeding disorders, and/or history of trauma or surgery to the nondominant hand.

Main Outcomes and Measures

Resting capillary blood flow at the nailfold of the fourth digit of the nondominant hand in patients with NTG, HTG, XFG, and controls, using NFC.

Results

Two participants were excluded owing to poor nailfold image quality, resulting in 109 participants. Sixty-two participants (57%) were women and 79 (72%) were white. Mean (SD) age of the participants was 67.9 (11.7) years. Mean (SD) resting peripheral capillary blood flow at the nailfold for controls was 70.9 (52.4) picoliters/s (pL/s); HTG, 47.5 (41.9) pL/s; NTG, 40.1 (16.6) pL/s; and XFG, 30.6 (20.0) pL/s. Multivariable analysis of the differences of flow in HTG vs control participants showed values of -18.97 (95% CI, -39.22 to 1.27; P = .07) pL/s, NTG vs controls of -25.17 (95% CI, -45.92 to -4.41; P = .02) pL/s, and XFG vs controls of -28.99 (95% CI, -51.35 to -6.63; P = .01) pL/s.

Conclusions and Relevance

Decreased resting peripheral capillary blood flow may occur in patients with XFG and NTG compared with individuals without glaucoma. These findings may contribute to understanding the possible systemic nature of glaucoma.

LOXL1 folding in exfoliation glaucoma

Exfoliation syndrome (XFS) is an age-related disease defined by the deposition of aggregated fibrous material (XFM) in the peri-cellular space. Principal morbidity occurs in the eye, where XFM accumulates on the anterior ocular tissues. GWAS have found that certain genetic variants of lysyl oxidase-like 1 (LOXL1), a matrix cross-linking enzyme that is required for elastic fiber formation confer risk for the development of XFS, but are not a single causative factor as many genetically affected individuals do not develop XFS or subsequent glaucoma (XFG). We have found that XFG cells display defects in lysosomes, microtubules, autophagy, and mitochondria resembling defects found in cells from age-related syndromes, such as the main neurodegenerative diseases. In the majority of these diseases, the determining cellular factor is a protein containing intrinsically disordered regions (IDRs) and displaying a high propensity for aggregation. We have found that in XFG patient-derived cells, LOXL1 protein is actively subjected to autophagic clearance, suggesting that LOXL1 is undergoing aggregation. In silico analysis demonstrates that LOXL1's first 369 aa constitute an IDR with the highest disorder probability peak centering around the known risk positions. Experimentally, we have found over-expression of either unmodified LOXL1 or fluorescent chimeras preserving the well-structured N-terminus cause copious intracellular aggregation and that aggregation wanes when the high IDR peaks are deleted. Overall, our work suggests that XFS/G results from the aggregation of the LOXL1 protein coupled with a reduction of cellular proteostasis capabilities in aging, resulting in a chronic build-up of LOXL1-containing protein aggregates.

The role of lysyl oxidase-like 1 (LOXL1) in exfoliation syndrome and glaucoma

Exfoliation syndrome (XFS) is an age-related systemic disease that affects the extracellular matrix. It increases the risk of glaucoma (exfoliation glaucoma, XFG) and susceptibility to diseases of elastin-rich connective tissues. LOXL1 (lysyl oxidase-like 1) is still recognized as the major genetic effect locus in XFS and XFG in all populations worldwide, although its genetic architecture is incompletely understood. LOXL1 is a key cross-linking enzyme in elastic fiber formation and remodeling, which is compatible with the pathogenetic concept of XFS as a specific type of elastosis. This review provides an overview on the current knowledge about the role of LOXL1 in the etiology and pathophysiology of XFS and XFG. It covers the known genetic associations at the LOXL1 locus, potential mechanisms of gene regulation, implications of LOXL1 in XFS-associated fibrosis and connective tissue homeostasis, its role in the development of glaucoma and associated systemic diseases, and the currently available LOXL1-based in vivo and in vitro models. Finally, it also identifies gaps in knowledge and suggests potential areas for future research.

Macula Vessel Density and Foveal Avascular Zone Parameters in Exfoliation Glaucoma Compared to Primary Open-Angle Glaucoma

Purpose

The purpose of this study was to explore macula vessel density and foveal avascular zone (FAZ) parameters in exfoliation glaucoma (XFG) compared to primary open-angle glaucoma (POAG) using optical coherence tomography angiography (OCTA).

Methods

This was a cross-sectional observational study. Twenty-six XFG and 28 POAG patients with comparable visual field defects on Humphrey 24-2 and 10-2 perimetries were recruited. OCTA scans (3 × 3 mm) centered on the fovea were obtained. Built-in software was used to measure superficial capillary plexus (SCP) vessel density at different quadrants of the macula. Custom software was then used to create a full-thickness image. The FAZ was manually delineated, and large vessels were removed. Vessel density in eight concentric rings with increments of 200-μm diameters from the delineated FAZ was measured. FAZ parameters were calculated using the custom software.

Results

SCP density was significantly lower in the superior (mean difference [MD] = -4.32, 95% confidence interval [CI] = -7.02, -1.61, P = 0.003) and nasal (MD = -3.00, 95% CI = 05.22, -0.77, P = 0.010) quadrants in XFG versus POAG. SCP vessel density using the concentric ring approach revealed significantly decreased values at all eight rings in XFG versus POAG. In the full-thickness analysis, density was significantly less in the XFG group in all rings except the initial 200 μm. No significant differences existed in FAZ parameters between the groups.

Conclusions

Despite the presence of comparable central visual field defects, the macula vessel density was predominately lower in XFG compared with POAG in our sample of patients. Further studies are warranted to investigate the consistency of our results.