Lentiviral mediated delivery of CRISPR/Cas9 reduces intraocular pressure in a mouse model of myocilin glaucoma

Mutations in myocilin (MYOC) are the leading known genetic cause of primary open-angle glaucoma, responsible for about 4% of all cases. Mutations in MYOC cause a gain-of-function phenotype in which mutant myocilin accumulates in the endoplasmic reticulum (ER) leading to ER stress and trabecular meshwork (TM) cell death. Therefore, knocking out myocilin at the genome level is an ideal strategy to permanently cure the disease. We have previously utilized CRISPR/Cas9 genome editing successfully to target MYOC using adenovirus 5 (Ad5). However, Ad5 is not a suitable vector for clinical use. Here, we sought to determine the efficacy of adeno-associated viruses (AAVs) and lentiviruses (LVs) to target the TM. First, we examined the TM tropism of single-stranded (ss) and self-complimentary (sc) AAV serotypes as well as LV expressing GFP via intravitreal (IVT) and intracameral (IC) injections. We observed that LV_GFP expression was more specific to the TM injected via the IVT route. IC injections of Trp-mutant scAAV2 showed a prominent expression of GFP in the TM. However, robust GFP expression was also observed in the ciliary body and retina. We next constructed lentiviral particles expressing Cas9 and guide RNA (gRNA) targeting MYOC (crMYOC) and transduction of TM cells stably expressing mutant myocilin with LV_crMYOC significantly reduced myocilin accumulation and its associated chronic ER stress. A single IVT injection of LV_crMYOC in Tg-MYOCY437H mice decreased myocilin accumulation in TM and reduced elevated IOP significantly. Together, our data indicates, LV_crMYOC targets MYOC gene editing in TM and rescues a mouse model of myocilin-associated glaucoma.


Introduction
Glaucoma is the second leading cause of irreversible blindness worldwide affecting about 70 million people [1][2][3] .Primary open angle glaucoma (POAG), the most common form of glaucoma is associated with progressive loss of retinal ganglion cell (RGC) axons and optic nerve degeneration [4][5][6] .Elevated intraocular pressure (IOP), a major risk factor for glaucoma is caused by increased resistance to aqueous humor (AH) out ow through the trabecular meshwork (TM) [7][8][9] .Despite TM being the major site of glaucomatous pathology 10 , mechanisms regulating out ow resistance in TM are poorly understood 11 .
MYOC is abundantly expressed in TM cells and other ocular and non-ocular tissues [26][27][28] .However, the exact function of MYOC is still not clear, although there are suggestions that it may function as a matricellular protein [29][30][31][32][33][34] .Various studies have demonstrated that WT MYOC is not required for the regulation of IOP, however mutations in MYOC lead to a gain-of-function phenotype [35][36][37][38][39][40] .Overexpression or knockout of WT MYOC exhibited no ocular changes in mice 39,40 indicating that the WT MYOC is not required for homeostasis of IOP.This is further supported by the ndings that homozygous or heterozygous deletion of myocilin in humans is not associated with glaucoma [40][41][42][43] .Mutant MYOC forms detergent insoluble aggregates and accumulates in the endoplasmic reticulum (ER) causing ER stress 30,35,36,38,44,45 .The insu ciency of TM cells to resolve chronic ER stress results in cell death, leading to IOP elevation 44,[46][47][48] .Since myocilin is not required for IOP regulation and mutant myocilin acquires toxic gain-of-function phenotype leading to TM cell death, knocking out myocilin at the genomic level becomes an attractive strategy for developing a novel therapy for MYOC-associated glaucoma.
The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) in association with CRISPRassociated systems (Cas) is a powerful and widely used tool for genomic research 49,50 .It has two major components: an endonucleases enzyme Cas9 that cuts DNA and a gRNA that guides Cas9 to speci c DNA sites.Together, they form a ribonucleoprotein (RNP) complex that can identify and cut DNA at the speci c site.Once bound, Cas9 introduces a double strand break in the DNA.Gene knockouts can be generated due to indels incorporated by non-homologous end joining (NHEJ) or a homologous sequence can be simultaneously introduced for homology-directed repair (HDR) 49,50 .
Previously, our group has demonstrated the successful gene editing of MYOC using the CRISPR/Cas9 system in mice and human donor eyes 51 .In this study, the knockout of MYOC was targeted by designing gRNA targeting exon 1.The Cas9 + guide RNA was delivered using the adenovirus (Ad)-5, which has speci c tropism toward the TM 52 .Although, Ad5 is a highly e cient system, Ad5 is in ammatory and induces a strong immune response in transduced tissues 53 .Considering our goal of clinical development, we sought to investigate other viral vectors including adeno-associated viruses (AAVs) and lentiviral (LV) particles to deliver Cas9 targeting MYOC to TM in vitro and in vivo models 54,55 .These viruses hold potential for clinical application due to robust delivery with long-term transgene expression, e cient transduction in post-mitotic cells, low immunogenicity, and minimal toxicity 56,57 .In the present study, we rst explored whether various AAVs or LV particles have speci c tropism to TM in in vitro and in vivo models.We further examined whether selected AAV or LV expressing Cas9 and gRNA targeting MYOC (crMYOC) reduce myocilin misfolding and rescue glaucomatous phenotypes in in vitro and in vivo models.

Methods
Viral Vector Constructs: AAV 2, self-complementary AAV2 (scAAV2) and Trp-Mutant scAAV2 (scAAV2 Trp- Mut ) were selected for the study based on previous studies that show tropism toward the trabecular out ow pathway 58 .Ready to use AAV2, scAAV2 and ScAAV2 Trp-Mut expressing GFP under the control of the CMV promoter were purchased from the Viral Vector Core at the University of Florida, Gainesville, FL.LV expressing GFP under the control of the CMV promoter (LV_GFP) was purchased from Vector Builder, Inc (Product ID: LVMP-VB160109-10005).
Guide RNA (gRNA) targeting MYOC (GGCCTGCCTGGTGTGGGATG) published in the previous study, had the highest e ciency and selectivity in targeting human MYOC 51 .In our current study, this same gRNA was cloned with spCas9 in the shuttle vector for generating LV constructs.LV particles expressing Cas9 + gMYOC, LV expressing GFP and LV expressing Cas9 + scrambled gRNA were manufactured by Vector Builder, Inc.The LV_Cas9 + scrambled gRNA expresses spCas9 with non-speci c gRNA sequence that does not target any genomic DNA.A different gRNA (GACCAGCTGGAAACCCAAACCA) was designed for cloning into ssAAV2 vectors using saCas9 (AAV2_crMYOC; Product ID: AAV2 MP (VB 200728-1179 bqW)) as the packaging capacity of AAV is comparatively small.The e ciency of this gRNA to selectively target human MYOC was found to be equivalently high.We have utilized AAV2 expressing an empty cassette as a control (AAV2_Null; Viral Gene Core, University of Iowa).
Mouse Husbandry: All mice were housed and bred in a research facility at the University of North Texas Health Science Center (UNTHSC, Fort Worth, TX, USA).Animals were fed standard chow ad libitum and housed in cages with dry bedding.The animals were maintained in a 12 h light:12 h dark cycle (lights on at 0630hrs) under a controlled environment of 21-26 o C with 40-70% humidity.C57BL/6J (male) mice were obtained from the Jackson Laboratories (Bar Harbor, ME, USA).We have utilized Tg-MYOC Y437H mice that express mutant MYOC and develop ocular hypertension by the age of 3-months as described previously 46, 59, 60 .Tg-MYOC Y437H mice on a pure C57BL/6J strain were utilized for this study.These mice were genotyped by PCR using primers speci c to human MYOC as described previously 46,59,60 .Animal studies were executed in agreement with the guidelines and regulations of the UNTHSC Institutional Animal Care and Use Committee (IACUC) and the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research.This study is reported in accordance with ARRIVE guidelines (https://arriveguidelines.org).Experimental protocols were approved by UNTHSC IACUC and Biosafety o ce under the approved protocol.At the end of experiment, mice will be sacri ed by CO 2 inhalation followed by cervical dislocation.
TM cell culture and in vitro transduction: TM3 cells were transfected with pDsRed2-MYOC plasmids to generate stable cells expressing WT or mutant (Y437H or G364V) MYOC using Lipofectamine 3000 TM transfection kit (Invitrogen, Life Technologies, Grand Island, NY, USA).These plasmids express MYOC tagged with DsRed at the C-terminus.The con uent transfected cells were then treated with G418 antibiotic (0.6 mg/mL; Gibco, Life Technologies, Grand Island, NY, USA) for 7-10 days and individual colonies were selected and expanded.The cells stably expressing DsRed-tagged MYOC (with or without mutations) were characterized as described previously 61 , and maintained in DMEM media (Sigma-Aldrich Corp, St. Louis, MO, USA)) supplemented with G418 antibiotics, 10% FBS (Gibco), and streptomycin (Gibco).For viral transduction, TM3 cells were plated at 30-40% con uency.The following day, cells were incubated with AAV (5000 MOI/mL) or LV (10 MOI/mL) in antibiotic free and low serum (6%) media.30 hours post viral treatment, cells were switched back to regular maintenance medium.
Intraocular Injections: Viral deliveries were performed via intravitreal (IVT) and intracameral (IC) routes.Mouse eyes were anesthetized before injections by topical administration of proparacaine HCl drops (0.5%) (Akorn Inc., Lake Forest, IL, USA).Both IVT and IC bolus injections were performed on mice anesthetized intranasally with iso urane (2.5%; with 0.8 L/min oxygen).However, in case of slow-IC infusion protocol, mice were anesthetized using xylazine/ketamine (10/100 mg/kg; Vetus; Butler Animal Health Supply, Westbury, NY/Fort Dodge Animal Health, Fort Dodge, IA, USA) cocktail administered intraperitoneally.As required, additional one-quarter to one-half of the initial dose was provided for continuous maintenance of the surgical anesthetic state.LV particles (2.5 x 10 6 TU/eyes and 2.5 mL/eye) or various AAV2 (2 x 10 10 GC/eye) were injected via IVT or IC route.Hamilton's (Reno, NV, USA) glass micro-syringe (10 mL capacity) attached with a 33 gauge 1-inch-long needle was used for IVT injections as described previously 62 .For IC route, mouse eyes were treated topically with 1% cyclopentolate (Mydriacyl®, Alcon Laboratories, Fort Worth, TX) to dilate the pupils.Using the same micro-syringe system, the 33-gauge needle was inserted through the cornea 1-2 mm from the limbus, positioned parallel to the iris, and pushed towards the chamber angle opposite to the cannulation point.Care was taken to not touch the iris, corneal endothelium, or the anterior lens capsule.The viral solution was slowly released into the anterior chamber over a period of 30s, after which the needle was kept inside for a further 1 min, before being rapidly withdrawn.For slow infusion, the glass micropipette system was loaded onto a micro-dialysis infusion pump (SP101I Syringe Pump; WPI) that delivered the viral solution at a ow rate of 0.083 mL/min over the course of 30 mins (total volume delivered, 2.5 mL).A drop of ltered saline was also applied through this procedure to prevent corneal drying.IOP measurements: A TonoLab impact tonometer (Colonial Medical Supply, Londonderry, NH, USA) was used for IOP measurements on mice as previously described 63 .Baseline IOPs for C57BL/6J and Tg-MYOC Y437H mice were measured during dark conditions (between 6:00-8:00 AM).The mice were anesthetized via intranasal iso urane (2.5%; 0.8 L/min oxygen) delivery and readings were noted within 3 min of iso urane in uence to avoid any of its side effects on IOP 64 .Post-injections, IOPs were monitored weekly (daylight and dark) in a masked manner.The average value of six individual IOP readings were represented.
Slit lamp imaging: A slit lamp (SL-D7, Topcon Corporation, Tokyo, Japan) was used to determine in ammation and ocular abnormalities in the anterior segment, including corneal edema, and photodocumented with a digital camera (DC-4; Topcon) as described earlier 46 .
Histology and immuno uorescence staining: Following viral transduction, mice were euthanized at speci ed timepoints, and eyes were carefully enucleated and placed in 4% paraformaldehyde (PFA, Electron Microscopy Sciences, Hat eld, PA, USA) overnight at 4 o C. The next day, eyes were washed with 1x PBS (Sigma-Aldrich) and cryopreserved using increasing concentration of sucrose (10% and 20%), followed by OCT compound embedding and sectioning.For hematoxylin and eosin (H&E) staining, the eyes were dehydrated in ethanol, and embedded in para n wax for sectioning.The para n-embedded mouse eyes were sectioned (sagittal) at 5 mm thickness, followed by depara nization in xylene, rehydration with gradual 5 min washes in each 100, 95, 70, and 50% ethanol solution and ending with a 10 min wash in 1x PBS.These sections were later stained with H&E.The general morphology of the anterior segment was assessed including the TM structure at iridocorneal angle and corneal thickness by light microscopy.Images were captured using a Keyence microscope (Itasca, IL, USA).
The OCT-embedded sections from mouse eyes were incubated with 10% goat serum (EMD Millipore Corp) in 0.2% Triton X-100 (diluted in PBS; Fisher BioReagents, Fair Lawn, NJ, USA) for 2 hours.For in vitro studies, TM cells were plated in 8-well chamber slides (Lab-Tek Nunc Brand Products, Rochester, NY, USA) and xed with 4% PFA for 20 mins, followed by PBS washes.Fixed cells or sections were then incubated with 10% goat serum in 0.1% Triton X-100 for 2 hours.The slides were incubated with primary antibody (MYOC, catalog # 60357: Proteintech Group Inc, Rosemont, IL, USA; or GRP78, Catalog# ab21685: Abcam, Cambridge, MA, USA).The slides were washed 4 times with 1x PBS before incubating with Alexa Fluor secondary antibody (1:500; Invitrogen, Life Technologies, Grand Island, NY, USA) at room temperature for 2 hours.The slides were washed again and mounted with DAPI antifade mounting medium (Vectashield, Vector Laboratories Inc., Burlingame, CA, USA) as described previously 51,59,62,65 .
For evaluating GFP expression in mice, the OCT sections were washed once with PBS and mounted with DAPI medium.Fluorescent images were captured, processed, and quanti ed using a Leica SP8 confocal microscope and LAS-X software (Leica Microsystems Inc., Buffalo Grove, IL, USA).Tissue sections and TM cells incubated without primary antibodies served as a negative control and were used to normalize the uorescent intensities by background elimination.Sections of non-injected eyes served as a background control for GFP uorescence.For quantifying staining speci c to the mouse TM, a region of interest was drawn around the TM area and represented as the unit of uorescence intensity per mm 2 .MYOC uorescent intensity in TM3 cells stably expressing mutant MYOC was quanti ed by imaging thirteen to fteen different non-overlapping areas of each treated wells.The uorescent intensity was normalized using number of cells per image as determined by DAPI staining.
CRISPR-Cas9 off-target effects by whole genome sequencing (WGS): TM3 cells were transduced with lentivirus expressing Cas9 only (gScr), or Cas9 with gRNA against myocilin (gMYOC).48 hours after infection, genomic DNA was extracted from gScr, gMYOC, and parental TM3 (NT) cells.Samples were sequenced on a Novaseq 6000 system at 30x coverage.The FASTQ les for all three samples (gMYOC, gScr, NT) were aligned to the human reference genome (GRCh37) with BWA-mem and sorted with SAMtools 67 .The resulting BAM les were processed to remove duplicate reads with Picard Tools ( http://broadinstitute.github.io/picard/).Local realignment and base quality recalibration were performed with Genome Analysis Tollkit (GATK) 68 .The most-likely off-target sites were determined using Cas-OFFinder 69 based upon the human reference genome (GRCh37), allowing the alignment of the gRNA to the genome to have up to 3 mismatches, DNA bulge size less than or equal to 1, and an RNA bulge size less than or equal to 1.The resulting 1214 unique sites were prioritized using the crisprScore package in Bioconductor, with the CFD algorithm 70 .The top 100 sites were selected based upon their crisprScore.
Each site was inspected visually using the Integrated Genome Viewer 71 with the analysis-ready BAM les for all three samples loaded.Sites were judged to be off target if indels were observed within 20 nt of the target in the gMYOC sample and not in any of the other samples.
Statistics: Statistical analyses were performed using Prism 9.0 software (GraphPad, San Diego, CA, USA).A P value of <0.05 was considered signi cant.Data was represented as mean ± SEM.An unpaired Student's t test (two-tailed) was used for comparing data with two-groups.The IOP results that comprise more than two groups were analyzed by repeated-measures two-way ANOVA followed by a Bonferroni post-hoc correction.

Results
Ocular transduction patterns of various AAV2 serotypes and lentiviral particles in mouse eyes: Selective AAV serotypes were shown to have tropism towards the TM of mice, rats, and monkeys in previous studies [72][73][74] .These studies suggest that single-stranded (ss) 74 and self-complimentary (sc) AAV2 e ciently transduces TM 72,75 .However, AAV2 also exhibits strong tropism to other ocular tissues.Our recent study also showed robust tropism of LV to the mouse TM 62 .We therefore compared the TM speci c tropism of various AAV2 capsid variants and LV particles expressing GFP.To select which AAV serotypes has the best tropism towards TM, we rst screened several AAV2 serotypes in primary human TM cells (SI I).Human primary TM cells (n = 2 strains) were grown to con uency in 12-well plates and treated with AAV2/2, AAV2/4, AAV2/5 and AAV2/8 at multiplicities of infection (MOI) of 2.5×10 1 to 2.5×10 3 viral genomes (VG)/cell as described previously 74 .GFP expression was examined by uorescent microscopy after 72 hours of transduction (SI I).No AAVs caused GFP expression at MOI of 2.5×10 1 VG/cell (not shown), but we observed robust AAV2-GFP expression at MOI of 2.5×10 3 VG/cell.Note that these high MOI are consistent with other cell types 72,74,76 .Based on these data, we chose to further investigate whether various AAV2 capsids produce robust tropism in mouse TM.These viruses were injected via IVT or IC (bolus or slow perfusion) routes to determine GFP expression in ocular tissues (n = 3 for each vector per route of injections) and GFP was examined by confocal imaging 2-weeks postinjection.We chose to perform slow perfusion IC injection because bolus IC injections may wash out quickly through the out ow pathway, which can limit the viruses' ability to transduce TM cells.None of the three capsid variants, ssAAV2_GFP, scAAV2_GFP and scAAV2 Trp-Mut _GFP showed GFP expression in the TM region delivered by bolus IVT or IC injections (Figure 1A).Consistent with previous studies, we observed a robust GFP expression in the retina (data not shown).Slow IC infusion of scAAV2 Trp-Mut induced robust GFP expression in TM and other tissues at the iridocorneal angle compared to the ssAAV2 vector (Figure 1A).However, no GFP uorescence was observed in the eyes of scAAV2 slow IC-infused eyes.Irrespective of their differences in TM transduction e ciency, these AAV2 variants were also found to transduce retina, optic nerve, and optic nerve head regions robustly in slow IC-treated eyes (Figure 1B).Since AAVs did not show selective and robust tropism to TM, we next evaluated the selective tropism of LV.Consistent with our previous study 62 , IVT bolus injections of LV_GFP induced GFP expression in mouse TM (Figure 2A).This GFP uorescence was seen throughout the TM.Minor expression was also observed in ciliary body region.Importantly, no GFP expression was observed in the retina con rming the speci city of our LV to the TM (Figure 2B).For comparative purposes, the e ciency of LV vectors was also examined via slow IC infusion.We observed robust and more e cient TM transduction via the slowinfused IC route (Figure 2A-B).However, signi cant LV_GFP transduction was also observed in the inner corneal endothelium layer (Figure 2B).Since IVT delivery of LV demonstrated most selective and e cient tropism to TM, we utilized this approach to deliver Cas9 to TM in our subsequent studies.

Comparison of AAV2 and LV-mediated MYOC editing in vitro:
We have previously demonstrated CRISPR/Cas9-mediated knockout of the MYOC gene using the Ad5 delivery system 51 .Here, we examined whether AAV2 and LV expressing crMYOC e ciently edit the MYOC gene in human TM3 cells.TM3 cells stably expressing DsRed tagged human MYOC with Y437H or G364V mutations (DsRed-MYOC Mut ) exhibit reduced secretion and intracellular accumulation of mutant-MYOC 35,44,46,51,59,65 .Overall, a decrease in DsRed puncta was observed in TM3 cells transduced with AAV2_crMYOC and LV_crMYOC compared to TM3 cells transduced with controls viral particles (Figure 3A).We next quanti ed MYOC accumulation using Image J, which revealed that LV-crMYOC reduced MYOC signi cantly by 62% while AAV2_crMYOC-mediated reduction was 34% (Figure 3B).The decrease in MYOC accumulation in LV_crMYOC treated cells was also re ected on GRP78 uorescence, reduced signi cantly by 38% compared to the control cells.
We further determined genome editing e ciency of AAV_crMYOC or LV_crMYOC in TM3 cells stably expressing mutant MYOC using Western blot analysis (Figure 4A-B).Western blot and its densitometric analysis demonstrated signi cant reduction in MYOC and ER stress markers (GRP78, CHOP and GRP94) in LV_crMYOC-treated cells compared to cells transduced by LV-null.Although AAV2-crMYOC reduced MYOC and ER stress markers, this reduction was not statistically signi cant compared to cells treated with AAV2-null.Using the Alt-R TM Genome Editing Detection Kit, we further con rmed T7 endonuclease (T7E1) induced cleaved product in both our LV_crMYOC and AAV2_crMYOC treated DNA samples (SI II).No cleaved product was observed in untreated control, LV_Null and AAV2_Null treated DNA samples.
These data indicate that LV_crMYOC edits MYOC and reduces its intracellular accumulation, relieving ER stress in human TM cells.
LV_crMYOC decreases mutant myocilin in TM and reduces elevated IOP in mouse model of MYOCassociated glaucoma: Since LV_crMYOC disrupts MYOC e ciently in vitro, we further determined whether LV_crMYOC rescues Tg-MYOC Y437H mice, which expresses human MYOC with the Y437H mutation.As shown in Figure 1, intravitreal injection of LV targets mouse TM.We therefore performed a single intravitreal injection of LV_crMYOC in adult ocular hypertensive Tg-MYOC Y437H mice.Before injections (0 day), we observed that Tg-MYOC Y437H mice show IOP elevation compared to WT mice (Figure 5A).Ocular hypertensive Tg-MYOC Y437H mice were injected intravitreally with LV_crMYOC or LV_Cas9-Null (2.5 x 10 6 TU/eyes; Figure 4).While LV_Null treated Tg-MYOC Y437H mice exhibited signi cant IOP elevation compared to age-matched C57BL/6J mice, LV_crMYOC mice demonstrated a signi cant reduction of IOP 3-weeks after injection and IOPs in LV_crMYOC injected mice were similar to WT mice 3-weeks after injection (Figure 5A).The mean dark-adapted IOP was ~17.75 mmHg in LV_crMYOC-injected Tg-MYOC Y437H mice compared to ~21.07 mmHg in LV_Null treated Tg-MYOC Y437H mice and ~18.4 mmHg in control WT mice.We next determined whether LV_crMYOC reduced mutant myocilin accumulation in Tg-MYOC Y437H mice by immunolabeling of xed anterior segments with MYOC antibody (Figure 5B).Immunostaining data revealed that LV_crMYOC treatment reduced MYOC labeling in the TM region of Tg-MYOC Y437H mice compared to LV_null treated Tg-MYOC Y437H mice.These data indicate that LV_crMYOC edits the MYOC gene and prevents IOP elevation in Tg-MYOC Y437H mice.Since viral vectors including Ad5 tend to cause ocular in ammation, we next examined ocular structures in LV_crMYOC injected Tg-MYOC Y437H mice compared to Ad5_crMYOC injected Tg-MYOC Y437H mice using slit lamp imaging and histological analysis of anterior segments (Figure 5C and SI III).The eyes injected with Ad5_crMYOC (2 x 10 6 pfu/eyes) developed acute in ammation determined by an opaque white appearance of the anterior segments (Figure 5C).Moreover, H&E staining of the anterior segment revealed increased corneal thickness in Ad5 injected eyes (SI III).In contrast, both slit lamp imaging and H&E staining revealed that LV_crMYOC injected eyes showed no abnormalities in the anterior segments.These data indicate that an IVT injection of LV_crMYOC causes minimal ocular toxicity in mice.
One of the major concerns with CRISPR-Cas9 based genome editing is off-target effects.To determine the off-target effects due to LV_crMYOC, we performed WGS on TM3 cells transfected with LV_crMYOC or LV_crScrambled.The most obvious change is at the myocilin genome locus (SI IV and SI V ).Out of the top 100 predicted off-target sites based on crisprScore, 2 sites (MLLT3 with 7 reads, and FAM19A5 with one read) were identi ed with potential changes in TM3 cells treated with LV_crMYOC, but not in cells treated with LV_crScrambled gRNA or parental TM3 (NT) samples.These sites had crisprScores of 0.75 and 0.50 respectively.Both observed changes are in deep intronic regions, located more than 10,000 nt from the nearest exon.A total of nine off-target sites for LV_crMYOC were considered most likely (crisprScore ³ 0.5), of which only one falls in a coding region.That site is within SLC2A10 and has a crisprScore of 0.53 requiring 3 mismatches and a bulge.We detected the change at the MLLT3 site by the T7 endonuclease 1 assay (T7E1).We cannot detect the change at the FAM19A5 site by the T7E1 assay.This may be due to the lack of sensitivity of the T7E1 assay or sequencing error.Together, these data indicate that LV_crMYOC edits MYOC with high e ciency with only limited off-target effects.

Discussion
Recent advances in genome editing technologies allow investigators to directly alter the genes associated with disease pathology.The gain-of-function mutation of the MYOC gene serves as a direct target for gene editing without the need for gene replacement.Knocking down MYOC expression in the eye does not compromise any normal ocular physiological function and it is relatively easy to knock out the gene compared to correcting its mutations 25,39,40 .The eye is a favorable target to develop gene therapy attributed to its ease of accessibility for routine clinic-based applications and the fact that it is an isolated immune privileged compartment separated by the blood-retinal barrier 54,56 .Importantly, long duration of e cacy can be obtained from a single dose of gene delivery, thus eliminating the requirement for patient compliance with routine eye drop application 77 .We have previously demonstrated that Ad5_crMYOC decreases mutant MYOC in TM and rescues glaucoma in transgenic mice.Although Ad5 was used experimentally due to its tropism for the TM, Ad5 is not a suitable viral vector for clinical use due to its immunogenic response 56 .Here, we show that lentiviral particles mediate optimum and e cient MYOC editing in TM and prevent IOP elevation in a mouse model of MYOC-associated POAG.For clinical application, the selectivity to transduce and target transgene expression in a speci c cell region is important to avoid off-site gene editing 77 .The modi cations of viral serotypes or capsid can alter the cellular tropism of the viral vector 72,73,78,79 .In addition, the route of vector delivery, the intraocular environment and proximity of the target tissue to the delivery site help determine the e ciency and selectivity of the transduction 79,80 .Based on the anatomy, the IC route provides the most e cient TM transduction in several studies using AAV or LV vectors 54,81 .Most of the anterior segment aqueous humor ow exits via the TM, which is known for its phagocytic property 82 .This further promotes the viral vectors to have high a nity for TM transduction compared to cornea, lens or ciliary body.However, due to the constant AH out ow, IC bolus injection leads to rapid washout of the viral vectors, with limited exposure to the target tissue, especially in the mouse which has a very small eye.Hence, we employed slow-IC infusing that delivers the virus for a extended period.In contrast, the IVT injection route provides a longer-lasting depot effect for sustained release of the injected vectors, proving to be an e cient route for gene therapy application with single dose administration.Our ndings indicate that slow-IC infusion is the most e cient route for inducing robust transgene expression in the TM via both AAV2 capsid variants and LV vectors.However, LV vectors induced GFP expression in the corneal endothelium, which is consistent with a previous study 83 .Nonetheless, the IVT route for LV_GFP proved to be more speci c and selective in transducing the mouse TM, with minor GFP expression noted in the ciliary body region.The slow and smaller release of the virus particles from the posterior vitreous, prevent their proximity and exposure to corneal endothelial, enough to reduce the propensity to transduce.
The AAV vectors are well known for their safety and e cacy in clinical application and are the preferred option for retinal gene therapy 56,77 .This nonpathogenic ssDNA and replication de cient parvovirus provides long-term transgene expression, with only a mild immunogenic response.However, they are limited by their ability to transduce the tissues of the anterior segments.Several studies have emphasized the use of scAAV capsids or their mutant forms for e cient transduction of TM cells as they facilitate the generation of dsDNA 72,73,84 .However, the size of the transgene cassette that can be inserted is very limited.In contrast, the capsid mutation of AAV serotype 2 (AAV2) have better TM transduction via the intracameral route in rodents, perfused anterior chamber, and cultured human TM cells, thus resolving the issue associated with transgene insertion size 74 .While evaluating cellular tropism of AAV2 serotype capsid variants via GFP expression, the scAAV2 Trp − Mut induced prominent expression of GFP in TM via the slow-IC infusion route.TM transduction was also observed with our ssAAV2 capsid variant.However, we demonstrate that AAV2 is not selective to TM, with robust GFP expression observed in retina and ONH.The selectivity of transgene expression can also be determined by use of tissue speci c promoters.The CMV promoters used in our vector constructs promotes ubiquitous transgene expression in a majority of ocular tissues including corneal endothelium, non-pigmentary epithelial cells and retinal tissues 85 .A few studies have reported TM preferential promoters such as matrix Gla protein and chitinase-3-like-1 promoter 79,86 .This non-speci city of AAVs to ocular tissues can increase Cas9-associated off-target effects, thus limiting its clinical applications for the treatment of glaucoma.
Lentiviruses are known for their capacity to induce sustained transgene expression with low immunogenic response.Both FIV and HIV based LV are used in ocular research 81 .LV vector e ciency is currently being investigated in two macular degeneration clinical trials 77,80 .Our HIV based VSV-G pseudotyped vector proved to be selective towards the mouse TM via the IVT route.The ssRNA genome of lentivirus is reverse transcribed into dsDNA that becomes integrated into the host genome via integrase enzyme activity.This is one of the major limitations of using LV in clinical applications.Based on the recent advancement, our LV vectors are designed to avert insertional mutagenesis by inhibiting integrase.
These integrase-de cient lentivirus vectors can be generated by introducing non-pleiotropic mutations within the open reading frame that speci cally targets the integration function without affecting the life cycle of the virus 87 .
LVs are known for their high transgene loading capacity (7 kb), which is a major advantage over the AAV vectors (~ 4.6 kb).Therefore, they are more suitable for packaging gene editing constructs such as CRISPR/Cas9.Although scAAV vectors have higher TM transduction e ciency as reported by a previous study 84 , they are limited by the capacity for packaging the cargo gene.Therefore, we used ssAAV2 variants to determine the e ciency of CRISPR/Cas9 based MYOC gene editing.Moreover, we used SaCas9 for ssAAV2 based CRISPR assembly, as it is smaller in size compared to the SpCas9 88-90 .Both LV and AAV2 expressing Cas9 were able to edit the MYOC gene in human TM cells.However, the overall effect of MYOC gene editing on MYOC protein levels and ER stress was more signi cantly pronounced in LV-treated cells compared to AAV2-treated cells.Comparable to our previous study 51 , our LV_crMYOC was able to knock down MYOC expression in transduced TM of Tg-MYOC Y437H mice, resulting in signi cantly reduced IOP independent of any immunogenic response.
One serious concern with traditional Cas9 is off-target effects, which occur due to non-selectively of Cas9 to similar genomic regions.Traditional nuclease CRISPR/Cas9-based gene knockouts also introduce DNA double-strand breaks (DSBs), which pose serious risks such as large deletions, translocations, and chromosomal abnormalities.In addition, this effect can be more pronounced when Cas9 is expressed for longer period as in the case when delivered using viral vectors.WGS revealed that our LV_crMYOC targets MYOC in TM cells with high e ciency but we have also observed limited off-target effects in LV_crMYOC treated TM cells.We utilized in silico tools to select our gRNA targeting MYOC.These in silico tools search for potential off-target sites in the whole genome and calculate the likelihood of off-target editing.
Most off-target effects are often gRNA dependent and selecting another gRNA may reduce these offtarget effects.In addition,viral vectors tend to cause prolonged expression of Cas9, which can increase off-target effects 91 .To overcome these concerns, our future studies will be directed towards utilizing base editors and non-viral delivery approaches.Recent advances made in precision genome editing offers better promise in reducing these off-target effects [92][93][94] .Specially, adenine base editors, comprise a catalytically impaired Cas9 (nCas9) with adenosine deaminase (TadA) and enable the conversion of A•T to G•C or vice versa with high precision and e ciency without causing DNA double strand breaks 95 .Base editors may exhibit some bystander effect in nearby regions with little or no off-target effects.Since we are knocking out MYOC, this may not cause any serious issues.Our future experiments will be directed towards adapting precision genome editing for glaucoma.Several studies have recently utilized non-viral delivery platforms such as lipid nanoparticles to deliver Cas9 mRNA or protein for optimum gene editing with minimum off-target effects 94,[96][97][98] .These non-viral deliveries of base editors provide a promising lead for e cient gene editing in ocular diseases with minimum off-target effects.
In conclusion, our studies show that LVs are highly e cient in delivering Cas9 to TM without any ocular toxicity and LV-mediated gene editing is highly e cient in reducing mutant myocilin and lowering elevated IOP in mouse model of glaucoma.Importantly, our studies lay the foundation for further development of gene editing methods to cure glaucoma.

Declarations
Authors Contributions: GSZ, VCS and AFC designed research studies.SVP, BK, SR, YS, JCM, QZ, and CCS.performed experiments and analyzed data.SVP, and GSZ wrote the manuscript.VCS, AFC, TES, and QZ assisted in conducting key experiments, provided reagents, and assisted in manuscript preparation.All authors discussed the results and implications and commented on the manuscript at all stages.

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Figure 3 Comparison
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Figure 4 Effect
Figure 4