Genome editing in the treatment of ocular diseases

Genome-editing technologies have ushered in a new era in gene therapy, providing novel therapeutic strategies for a wide range of diseases, including both genetic and nongenetic ocular diseases. These technologies offer new hope for patients suffering from previously untreatable conditions. The unique anatomical and physiological features of the eye, including its immune-privileged status, size, and compartmentalized structure, provide an optimal environment for the application of these cutting-edge technologies. Moreover, the development of various delivery methods has facilitated the efficient and targeted administration of genome engineering tools designed to correct specific ocular tissues. Additionally, advancements in noninvasive ocular imaging techniques and electroretinography have enabled real-time monitoring of therapeutic efficacy and safety. Herein, we discuss the discovery and development of genome-editing technologies, their application to ocular diseases from the anterior segment to the posterior segment, current limitations encountered in translating these technologies into clinical practice, and ongoing research endeavors aimed at overcoming these challenges.

Coats Plus Syndrome in a Premature Infant, With a Focus on Management

Purpose

A premature infant was diagnosed with Coats plus syndrome based on a genetic evaluation showing biallelic heterozygous pathogenic CTC1 variants.

Methods

A case study was performed, including findings and interventions.

Results

A premature infant born 30 weeks gestational age weighing 817 g was evaluated for retinopathy of prematurity at 35 weeks corrected gestational age. An initial dilated fundus examination showed an exudative retinal detachment (RD) in the right eye and avascularity post-equatorially in the left eye with telangiectasias and aneurysmal dilations. Genetic evaluation showed biallelic heterozygous pathogenic CTC1 variants, diagnostic of Coats plus syndrome. Sequential examination under anesthesia with fluorescein showed progressive ischemia despite confluent photocoagulation.

Conclusions

CTC1 gene variants manifest as Coats plus syndrome, which has a clinical appearance consistent with retinovascular ischemia, capillary remodeling, aneurysmal dilation, and exudative RD. Systemic and local corticosteroids in conjunction with peripheral laser ablation decreased vascular exudation and avoided intraocular intervention.

Nano-scale resolution of native retinal rod disk membranes reveals differences in lipid composition

Photoreceptors rely on distinct membrane compartments to support their specialized function. Unlike protein localization, identification of critical differences in membrane content has not yet been expanded to lipids, due to the difficulty of isolating domain-specific samples. We have overcome this by using SMA to coimmunopurify membrane proteins and their native lipids from two regions of photoreceptor ROS disks. Each sample's copurified lipids were subjected to untargeted lipidomic and fatty acid analysis. Extensive differences between center (rhodopsin) and rim (ABCA4 and PRPH2/ROM1) samples included a lower PC to PE ratio and increased LC- and VLC-PUFAs in the center relative to the rim region, which was enriched in shorter, saturated FAs. The comparatively few differences between the two rim samples likely reflect specific protein-lipid interactions. High-resolution profiling of the ROS disk lipid composition gives new insights into how intricate membrane structure and protein activity are balanced within the ROS, and provides a model for future studies of other complex cellular structures.

Single particle cryo-EM of the complex between interphotoreceptor retinoid-binding protein and a monoclonal antibody

Interphotoreceptor retinoid‐binding protein (IRBP) is a highly expressed protein secreted by rod and cone photoreceptors that has major roles in photoreceptor homeostasis as well as retinoid and polyunsaturated fatty acid transport between the neural retina and retinal pigment epithelium. Despite two crystal structures reported on fragments of IRBP and decades of research, the overall structure of IRBP and function within the visual cycle remain unsolved. Here, we studied the structure of native bovine IRBP in complex with a monoclonal antibody (mAb5) by cryo‐electron microscopy, revealing the tertiary and quaternary structure at sufficient resolution to clearly identify the complex components. Complementary mass spectrometry experiments revealed the structure and locations of N‐linked carbohydrate post‐translational modifications. This work provides insight into the structure of IRBP, displaying an elongated, flexible three‐dimensional architecture not seen among other retinoid‐binding proteins. This work is the first step in elucidation of the function of this enigmatic protein.

Two-photon imaging of the mammalian retina with ultrafast pulsing laser

Noninvasive imaging of visual system components in vivo is critical for understanding the causal mechanisms of retinal diseases and for developing therapies for their treatment. However, ultraviolet light needed to excite endogenous fluorophores that participate in metabolic processes of the retina is highly attenuated by the anterior segment of the human eye. In contrast, 2-photon excitation fluorescence imaging with pulsed infrared light overcomes this obstacle. Reducing retinal exposure to laser radiation remains a major barrier in advancing this technology to studies in humans. To increase fluorescence intensity and reduce the requisite laser power, we modulated ultrashort laser pulses with high-order dispersion compensation and applied sensorless adaptive optics and custom image recovery software and observed an over 300% increase in fluorescence of endogenous retinal fluorophores when laser pulses were shortened from 75 fs to 20 fs. No functional or structural changes to the retina were detected after exposure to 2-photon excitation imaging light with 20-fs pulses. Moreover, wide bandwidth associated with short pulses enables excitation of multiple fluorophores with different absorption spectra and thus can provide information about their relative changes and intracellular distribution. These data constitute a substantial advancement for safe 2-photon fluorescence imaging of the human eye.

Towards Treatment of Stargardt Disease: Workshop Organized and Sponsored by the Foundation Fighting Blindness

Accumulation of fluorescent metabolic byproducts of the visual (retinoid) cycle is associated with photoreceptor and retinal pigment epithelial cell death in both Stargardt disease and atrophic (nonneovascular) age-related macular degeneration (AMD). As a consequence of this observation, small molecular inhibitors of enzymes in the visual cycle were recently tested in clinical trials as a strategy to protect the retina and retinal pigment epithelium in patients with atrophic AMD.

To address the clinical translational needs for therapies aimed at both diseases, a workshop organized by the Foundation Fighting Blindness was hosted by the Department of Pharmacology at Case Western Reserve University on February 17, 2017, at the Tinkham Veale University Center, Cleveland, OH, USA. Invited speakers highlighted recent advances in the understanding of the pathophysiology of Stargardt disease, in terms of its clinical characterization and the development of endpoints for clinical trials, and discussed the comparability of therapeutic strategies between atrophic age-related macular degeneration (AMD) and Stargardt disease. Investigators speculated that reducing the concentrations of visual cycle precursor substances and/or their byproducts may provide valid therapeutic options for the treatment of Stargardt disease. Here we review the workshop's presentations in the context of published literature to help shape the aims of ongoing research endeavors and aid the development of therapies for Stargardt disease.

Lecithin:Retinol Acyltransferase: A Key Enzyme Involved in the Retinoid (visual) Cycle

Lecithin:retinol acyltransferase (LRAT) catalyzes the acyl transfer from the sn-1 position of phosphatidylcholine (PC) to all-trans-retinol, creating fatty acid retinyl esters (palmitoyl, stearoyl, and some unsaturated derivatives). In the eye, these retinyl esters are substrates for the 65 kDa retinoid isomerase (RPE65). LRAT is well characterized biochemically, and recent structural data from closely related family members of the NlpC/P60 superfamily and a chimeric protein have established its catalytic mechanism. Mutations in the LRAT gene are responsible for approximately 1% of reported cases of Leber congenital amaurosis (LCA). Lack of functional LRAT, expressed in the retinal pigmented epithelium (RPE), results in loss of the visual chromophore and photoreceptor degeneration. LCA is a rare hereditary retinal dystrophy with an early onset associated with mutations in one of 21 known genes. Protocols have been devised to identify therapeutics that compensate for mutations in RPE65, also associated with LCA. The same protocols can be adapted to combat dystrophies associated with LRAT. Improvement in the visual function of clinical recipients of therapy with recombinant adeno-associated virus (rAAV) vectors incorporating the RPE65 gene provides a proof of concept for LRAT, which functions in the same cell type and metabolic pathway as RPE65. In parallel, a clinical trial that employs oral 9-cis-retinyl acetate to replace the missing chromophore in RPE65 and LRAT causative disease has proven to be effective and free of adverse effects. This article summarizes the biochemistry of LRAT and examines chromophore replacement as a treatment for LCA caused by LRAT mutations.

LRAT-specific domain facilitates vitamin A metabolism by domain swapping in HRASLS3

Cellular uptake of vitamin A, production of visual chromophore, and triglyceride homeostasis in adipocytes depend on two representatives of the vertebrate N1pC/P60 protein family, lecithin:retinol acyltransferase (LRAT) and HRAS-like tumor suppressor 3 (HRASLS3). Both proteins function as lipid-metabolizing enzymes but differ in their substrate preferences and dominant catalytic activity. The mechanism of this catalytic diversity is not understood. Here, by using a gain-of-function approach, we identified a specific sequence responsible for the substrate specificity of N1pC/P60 proteins. A 2.2 Å crystal structure of HRASLS3/LRAT chimeric enzyme in a thioester catalytic intermediate state revealed a major structural rearrangement accompanied by 3D-domain swapping dimerization not observed in native HRASLS proteins. Structural changes affecting the active site environment contributed to slower hydrolysis of the catalytic intermediate supporting efficient acyl transfer. These findings reveal structural adaption that facilitates selective catalysis and mechanism responsible for diverse substrate specificity within the LRAT-like enzyme family.

Structural basis for the acyltransferase activity of lecithin:retinol acyltransferase-like proteins

Lecithin:retinol acyltransferase-like proteins, also referred to as HRAS-like tumor suppressors, comprise a vertebrate subfamily of papain-like or NlpC/P60 thiol proteases that function as phospholipid-metabolizing enzymes. HRAS-like tumor suppressor 3, a representative member of this group, plays a key role in regulating triglyceride accumulation and energy expenditure in adipocytes and therefore constitutes a novel pharmacological target for treatment of metabolic disorders causing obesity. Here, we delineate a catalytic mechanism common to lecithin:retinol acyltransferase-like proteins and provide evidence for their alternative robust lipid-dependent acyltransferase enzymatic activity. We also determined high resolution crystal structures of HRAS-like tumor suppressor 2 and 3 to gain insight into their active site architecture. Based on this structural analysis, two conformational states of the catalytic Cys-113 were identified that differ in reactivity and thus could define the catalytic properties of these two proteins. Finally, these structures provide a model for the topology of these enzymes and allow identification of the protein-lipid bilayer interface. This study contributes to the enzymatic and structural understanding of HRAS-like tumor suppressor enzymes.

A herpes simplex virus 1 recombinant lacking the glycoprotein G coding sequences is defective in entry through apical surfaces of polarized epithelial cells in culture and in vivo

During infection of a new host, the first surfaces encountered by herpes simplex viruses are the apical membranes of epithelial cells of mucosal surfaces. These cells are highly polarized, and the protein composition of their apical and basolateral membranes are very different, so that different viral entry pathways have evolved for each surface. To determine whether the viral glycoprotein G (gG) is specifically required for efficient infection of a particular surface of polarized cells, apical and basal surfaces were infected with wild-type virus or a gG deletion mutant. After infection of polarized cells in culture, the gG(-) virus was deficient in infection of apical surfaces but was able to infect cells through basal membranes, replicate, and spread into surrounding cells. The gG-dependent step in apical infection was a stage beyond attachment. After in vivo infection of apical surfaces of epithelial cells of nonscarified mouse corneas, infection by glycoprotein C(-) or gG(-) virus was considerably reduced as compared with that observed after infection with wild-type virus. In contrast, when corneas were scarified, allowing virus access to other cell surfaces, the gG and glycoprotein C deletion mutants infected eyes as efficiently as wild-type viruses. A secondary mutation allowing infection of apical surfaces by gG(-) virus arose readily during passage of the virus in nonpolarized cells, indicating that either the gG-dependent step of apical infection can be bypassed or that another viral protein can acquire the same function.

Herpes simplex virus inhibits apoptosis through the action of two genes, Us5 and Us3

Apoptosis of virus-infected cells occurs either as a direct response to viral infection or upon recognition of infection by the host immune response. Apoptosis reduces production of new virus from these cells, and therefore viruses have evolved inhibitory mechanisms. We previously showed that laboratory strains of herpes simplex virus type 1 (HSV-1) protect infected cells from apoptosis induced by cytotoxic T lymphocytes or ethanol. We have now evaluated the ability of HSV-1 and HSV-2 laboratory and clinical isolates to inhibit apoptosis induced by anti-Fas antibody or UV irradiation and explored the genetic basis for this inhibition. HSV-1 isolates inhibited apoptosis induced by UV or anti-Fas antibody. In contrast, HSV-2 clinical isolates failed to inhibit apoptosis induced by either stimulus, although the HSV-2 laboratory strain 333 had a partial inhibitory effect on UV-induced apoptosis. Inhibition of apoptosis by HSV was accompanied by marked reduction of caspase-3 and caspase-8 activity. Deletion of the HSV-1 Us3 gene markedly reduced inhibition of UV-induced apoptosis and partially abrogated inhibition of Fas-mediated apoptosis. Conversely, deletion of the HSV-1 Us5 gene markedly reduced protection from Fas-mediated apoptosis and partially abrogated protection from UV. The Us11 and Us12 genes were not necessary for protection from apoptosis induced by either stimulus. The differences between HSV-1 and HSV-2 in the ability to inhibit apoptosis may be factors in the immunobiology of HSV infections.

Nuclear localization of the C1 factor (host cell factor) in sensory neurons correlates with reactivation of herpes simplex virus from latency

After a primary infection, herpes simplex virus is maintained in a latent state in neurons of sensory ganglia until complex stimuli reactivate viral lytic replication. Although the mechanisms governing reactivation from the latent state remain unknown, the regulated expression of the viral immediate early genes represents a critical point in this process. These genes are controlled by transcription enhancer complexes whose assembly requires and is coordinated by the cellular C1 factor (host cell factor). In contrast to other tissues, the C1 factor is not detected in the nuclei of sensory neurons. Experimental conditions that induce the reactivation of herpes simplex virus in mouse model systems result in rapid nuclear localization of the protein, indicating that the C1 factor is sequestered in these cells until reactivation signals induce a redistribution of the protein. The regulated localization suggests that C1 is a critical switch determinant of the viral lytic-latent cycle.

The null mutant of the U(L)31 gene of herpes simplex virus 1: construction and phenotype in infected cells

Earlier studies have shown that the U(L)31 protein is homogeneously distributed throughout the nucleus and cofractionates with nuclear matrix. We report the construction from an appropriate cosmid library a deletion mutant which replicates in rabbit skin cells carrying the U(L)31 gene under a late (gamma1) viral promoter. The mutant virus exhibits cytopathic effects and yields 0.01 to 0.1% of the yield of wild-type parent virus in noncomplementing cells but amounts of virus 10- to 1,000-fold higher than those recovered from the same cells 3 h after infection. Electron microscopic studies indicate the presence of small numbers of full capsids but a lack of enveloped virions. Viral DNA extracted from the cytoplasm of infected cells exhibits free termini indicating cleavage/packaging of viral DNA from concatemers for packaging into virions, but analyses of viral DNAs by pulsed-field electrophoresis indicate that at 16 h after infection, both the yields of viral DNA and cleavage of viral DNA for packaging are decreased. The repaired virus cannot be differentiated from the wild-type parent. These results suggest the possibility that U(L)31 protein forms a network to enable the anchorage of viral products for the synthesis and/or packaging of viral DNA into virions.

Infection of polarized MDCK cells with herpes simplex virus 1: two asymmetrically distributed cell receptors interact with different viral proteins

Herpes simplex virus 1 attaches to at least two cell surface receptors. In polarized epithelial (Madin-Darby canine kidney; MDCK) cells one receptor is located in the apical surface and attachment to the cells requires the presence of glycoprotein C in the virus. The second receptor is located in the basal surface and does not require the presence of glycoprotein C. Exposure of MDCK cells at either the apical or basal surface to wild-type virus yields plaques and viral products whereas infection by a glycoprotein C-negative mutant yields identical results only after exposure of MDCK cells to virus at the basal surface. Multiple receptors for viral entry into cells expand the host range of the virus. The observation that glycoprotein C-negative mutants are infectious in many nonpolarized cell lines suggests that cells in culture may express more than one receptor and explains why genes that specify the viral proteins that recognize redundant receptors, like glycoprotein C, are expendable.

Expression of the herpes simplex virus 1 alpha transinducing factor (VP16) does not induce reactivation of latent virus or prevent the establishment of latency in mice

A feature of the cascade regulation of herpes simplex virus 1 gene expression in productive infection is that the first genes to be expressed, the alpha genes, are transactivated by a structural component of the virion designated as the alpha transinducing factor (alpha TIF). In this study, we have tested the hypothesis that latent infection of sensory neurons results from the failure of alpha TIF, a tegument protein, to be transported from the nerve endings to the nucleus of the sensory neuron. Two viruses were constructed. The first recombinant virus (R6003) contained a second copy of the alpha TIF gene placed under the control of a metallothionein promoter. The second recombinant virus (R6004) is identical to R6003 except for the presence of a stop codon inserted at amino acid 70 of the second alpha TIF gene. The metallothionein promoter inserted into the viral genome was shown to be expressed, and alpha TIF mRNA was detected by in situ hybridization of sections of trigeminal ganglia of mice infected with R6003, both untreated and those given cadmium injections. In all experiments, there were no significant differences in the recovery of latent virus from mice infected with R6003 or R6004, whether injected with cadmium or not. Cadmium administration at the time of infection and at intervals thereafter did not preclude establishment of latency. In another series of experiments, transgenic mice expressing the metallothionein-driven alpha TIF did not differ from nontransgenic siblings with respect to the incidence of latent virus in trigeminal ganglia. We conclude that the absence of alpha TIF cannot alone account for the establishment of latency.

Amplification by host cell factors of a sequence contained within the herpes simplex virus 1 genome

We report that a cloned 1620-base-pair (bp) DNA fragment mapping in the BamHI O fragment of herpes simplex virus 1 DNA is amplified after transfection into uninfected cells. The DNA fragment maps entirely within a portion of the open reading frame encoding the large subunit of the viral ribonucleotide reductase and does not contain any of the known lytic origins of viral DNA synthesis. Amplification of this sequence in transfected cells results in accumulation of full-sized Dpn I-resistant plasmids containing the sequence in Hirt extracts of low molecular weight DNA. Subfragments of the 1620-bp fragment were not amplified, whereas larger fragments containing the intact 1620-bp fragment were amplified. The amplification of the fragment in MCF7 cells, which express steroid receptors, was stimulated by the addition of estrogen to the medium. Addition of progesterone, dexamethasone, or testosterone was ineffective. The viral genome therefore contains at least one origin of DNA synthesis capable of supporting replication of viral DNA by cellular factors. The existence of such a host origin of DNA replication in the viral genome was predicted by the hypothesis that viral DNA is amplified by cellular enzymes in sensory neurons harboring latent virus; the link between these sequences and amplification of viral DNA during latency remains to be proven.

Virulence of and establishment of latency by genetically engineered deletion mutants of herpes simplex virus 1

We report the results of studies on the biologic properties of seven deletion mutants of herpes simplex virus 1 (HSV-1). The genes deleted from six of these mutants map in the S component of HSV-1 DNA and include those specifying the alpha protein 47, the glycoproteins G and E, the viral protein kinase, and two proteins whose functions are not yet known (open reading frames US2 and US11). The seventh virus [HSV-1(F) delta 305] contained a 700-bp deletion in the thymidine kinase gene. The results of intracerebral inoculation of Balb/c mice indicated that all but one of the deletion mutants in the S component were significantly attenuated. The PFU/LD50 ratios for these mutants ranged from 10(4)- to 10(5)-fold higher than that of the wild-type, HSV-1(F). The PFU/LD50 for mutant R7032, from which the glycoprotein E gene had been deleted, was less than 100-fold higher than that of the parent virus. All of the mutants, with one exception, were able to establish latency in mice; the exception, HSV-1(F) delta 305, was able to establish latency in rabbits.

Cell-specific selection of mutants of a herpes simplex virus recombinant carrying deletions

Herpes simplex virus 1 (HSV-1) recombinant R316 was constructed so as to convert the thymidine kinase (TK), a beta gene, into an alpha-regulated gene by insertion of the BamHI N fragment in the proper transcriptional orientation into the BglII cleavage site of the TK gene (L. E. Post, S. Mackem, and B. Roizman, Cell 24, 555-565 (1981).) The BamHI N fragment contains the promoter and regulatory domains of the alpha 4 gene in addition to an origin of viral DNA synthesis and the complete domain of the alpha 22 gene. Passage of the R316 virus in HEp-2 or in human embryonic lung (HEL) cells resulted in rapid accumulation of mutants carrying approximately 4.4-kbp deletions in the insert. No appreciable accumulation of the deletions was observed upon passage of R316 virus in Vero cells. The accumulation of deletions in HEp-2 and HEL cells could not be attributed to the fusion of the TK gene with the alpha 4 gene promoter, to the presence of an origin of DNA replication, or to overexpression of any of the genes whose domain is contained entirely in the HSV-1 Bam HI N fragment; these conclusions are based on the observations that deletions did not accumulate in HEL or HEp-2 cells infected with recombinant R315, containing BamHI N inserted in an inverted orientation, or with recombinant R314, carrying an alpha 4-TK chimera constructed by insertion of the HSV-1 BamHI Z fragment into the BglII cleavage site in the TK gene. The BamHI Z fragment also contains a functional origin of DNA synthesis. The hypothetical models which could explain the host range-specific accumulation of deletions are discussed.