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Published in final edited form as: Curr Opin Ophthalmol. 2023 Dec 20;35(3):244–251. doi: 10.1097/ICU.0000000000001028

Leber Hereditary Optic Neuropathy Gene Therapy

Byron L Lam 1
PMCID: PMC10959684  NIHMSID: NIHMS1951515  PMID: 38117686

Abstract

Purpose of review:

To discuss relevant clinical outcomes, challenges, and future opportunities of gene therapy in Leber hereditary optic neuropathy (LHON).

Recent findings:

Results of G11778A LHON Phase 3 randomized clinical trials with unilateral intravitreal rAAV2/2-ND4 allotopic gene therapy show good safety and unexpected bilateral partial improvements of BCVA (best-corrected visual acuity) with mean logMAR BCVA improvements of up to near ~0.3 LogMAR (3 lines) in the treated eyes and ~0.25 LogMAR (2.5 lines) in the sham-treated or placebo-treated fellow eyes. Final mean BCVA levels after gene therapy were in the range of ~1.3 logMAR (20/400) bilaterally.

Summary:

Bilateral partial improvement with unilateral LHON gene therapy was unanticipated and may be due to treatment efficacy, natural history, learning effect, and other mediators. The overall efficacy is limited given the final BCVA levels. The sequential progressive visual loss and varied occurrence of spontaneous partial improvement in LHON confound trial results. Future clinical trials with randomization of patients to a group not receiving gene therapy in either eye would help to assess treatment effect. Promising future LHON gene therapy strategies include mitochondrially-targeted-sequence AAV (“MTS-AAV”) for direct delivery of the wild-type mitochondrial DNA into the mitochondria and CRISPR-free, RNA-free mitochondrial base editing systems. Signs of anatomical optic nerve damage and objective retinal ganglion cell dysfunction are evident in the asymptomatic eyes of LHON patients experiencing unilateral visual loss, indicating the therapeutic window is narrowing before onset of visual symptoms. Future treatment strategies utilizing mitochondrial base editing in LHON carriers without optic neuropathy holds the promise of a more advantageous approach to achieve optimal visual outcome by reducing disease penetrance and mitigating RGC loss when optic neuropathy develops.

Keywords: Leber hereditary optic neuropathy, gene therapy, mitochondrial DNA, allotopic expression, gene editing

INTRODUCTION

Leber hereditary optic neuropathy (LHON) is a rare maternally-inherited mitochondrial DNA (mtDNA) disorder associated with severe bilateral visual loss and has a prevalence of 1 in 30,000 to 50,000.1 Over 50 mtDNA genetic variants are associated with LHON with 95% of LHON cases being caused by one of three pathogenic point mtDNA mutations coding for the respiratory chain subunits of the nicotinamide adenine dinucleotide ubiquinone-oxidoreductase (complex I) genes: 3460G>A ND1, 11778G>A ND4, and 14484T>C ND6.2 Among the 3 common LHON types, the G11778A variant is the most common and has the worst prognosis.3 Only a portion of LHON genotype carriers will manifest the disease with a penetrance of 20% to 60% in males and 8% to 20% in females.1,4 Risks of disease development are in part related to genetic and environmental factors including mtDNA and nuclear gene variations, homoplasmy (>85% of cases), mtDNA copy number, tobacco smoking, excessive alcohol consumption, and toxic drug exposure.5,6 Affected persons are usually young adults who mostly develop sequential bilateral severe visual loss over weeks to a few months; simultaneous visual loss occurring in 25% of cases.7 During the acute phase, peripapillary nerve fiber swelling and telangiectatic microangiopathy are typically present followed by nerve fiber layer thinning and optic atrophy. The papillomacular retinal ganglion cell (RGC) bundles are preferentially affected owing to high metabolic demand and smaller axonal caliber with less mitochondrial functional reserve.8 Spontaneous partial visual improvement is observed in some patients and is more likely in patients under 12 years of age.9

The aim of this review is to discuss LHON gene therapy with an emphasis on insights gained from vector development, clinical trial results and challenges, and future opportunities. Of interest, among antioxidant and neuro-protective therapies for LHON, Idebenone is the most studied and is a synthetic hydrosoluble analogue of co-enzyme Q10 also known as ubiquinone that plays an important role in the mitochondrial respiratory chain by shuttling electrons from complexes I and II. Unlike ubiquinone, idebenone has the ability to cross the blood-brain barrier. Taken together, LHON idebenone clinical trials (RHODOS, RHODOS-OFU, LEROS) have demonstrated some limited efficacy given the severity of the vision loss, and the treatment may have best efficacy if started early and continued for 24 months.10–12 Idebenone (Raxone®) was approved by the European Medicines Agency in 2015 for treating LHON at 900 mg/day in three divided doses.

Natural History of LHON and Efficacy Endpoints

Prospective LHON natural history studies are scarce, and only one long-term prospective longitudinal study is available.13 Early Treatment Diabetic Retinopathy Study (ETDRS) best-corrected visual acuity (BCVA), automated visual field (VF), steady-state pattern electroretinogram (SS-PERG), and spectral-domain optical coherence tomography (SD-OCT) peripapillary retinal nerve fiber layer (RNFL) thickness were evaluated every 6 months for up to 36 months in 44 affected G11778A LHON patients with various time periods since the onset of vision loss. Worst BCVA levels in patients were reached 4 to 12 months after onset of vision loss. Thirteen eyes of 8 patients (18%) improved to 15 or more letters of ETDRS BCVA from baseline. Among these 8 patients whose vision improved, the delay after onset of acuity loss to when an eye first demonstrated a 15-letter ETDRS improvement ranged from 8.3 to 71.5 months (median, 27.5 months), and patients whose vision improved late tended to have lower BCVA. This partial BCVA improvement occurred in some patients despite continued marked RNFL thinning and persistent impaired SS-PERG indistinguishable from that in patients who did not recover BCVA. Improvements in VF in eyes with improved BCVA were not consistently observed.

Therefore, BCVA is the most suitable primary endpoint for LHON clinical trials with a lack of other available objective functional and anatomical measures that can reflect efficacy in affected LHON patients. Of interest, 45 asymptomatic G11778A LHON carriers were also followed for up to 36 months in the same long-term prospective study, and BCVA, VF, and RNFL remained stable and within the normal range.13 However, the SS-PERG amplitudes of asymptomatic carriers dropped by ~40% from baseline to 36 months suggesting progressive subclinical RGC dysfunction but the relationship of this finding to the risk of developing LHON is unclear.14

Of the remaining LHON natural history studies, all were cross-sectional15 except one study was short-term prospective with data from 12 months prior to the initiation of gene therapy16 and one study was retrospective longitudinal3. Of interest, although SS-PERG amplitude, as an objective functional measure of RGC, cannot distinguish between LHON disease stages, SS-PERG latency shortening occurs in patients with longer time of onset of visual loss and is consistent with primary damage of smaller/slower axons and sparing of larger/faster axons and may provide an objective staging of LHON.8

LHON has a significant acute negative impact on quality of life (QOL) and mental well-being measures followed by a gradual partial improvement over time particularly in persons with younger age of onset.17–19 Garcia and associates surveyed 103 LHON patients with an average age of 26.4 years and mean BCVA of 20/800. Nearly half (49.5%) had at least 5 of 9 criteria for major depressive disorder, despite nearly all (92.5%) indicating no pre-LHON mental health concerns.18 Older subjects reported lower levels of psychological well-being and the investigators attributed this to greater adaptability of younger people. Cui and colleagues prospectively assessed affected Chinese people with G11778A mutations with bilateral vision loss for at least 3 years and found higher mean visual QOL scores at the 3-year follow-up compared to the 6-month follow-up, and the visual QOL improvement was not significantly correlated with visual acuity of the better eye, but rather with younger age of onset.17

Gene Therapy Approaches

Gene therapy is a promising approach to treat LHON given the RGC layer is accessible to gene vectors delivered by an intravitreal injection. Gene therapy approaches include allotopic expression, mitochondrially-targeted vector, and mitochondrial base editing. Allotopic expression is utilized by LHON gene therapy clinical trials conducted thus far, and to overcome the barrier of the double membranes of the mitochondria that block the entry of DNA sequences, allotopic gene therapy was developed to deliver a nuclear-encoded version of the mitochondrial ND4 gene into the RGC by using an adeno-associated virus (AAV) vector. The ND4 gene enters the nucleus of the RGC and is transcribed into mRNA which is transported into the cytoplasm. The mRNA is translated to produce ND4 protein which is then transported into the mitochondria using a mitochondrial targeting sequence (MTS), and mitochondrial function is improved.20

A mitochondrially-targeted vector is generated when one of the AAV viral capsid proteins is modified to include an MTS (“MTS-AAV”) which directs the AAV to the mitochondria with delivery of the wild-type version of the mtDNA directly into the mitochondria. Therefore, MTS-AAV may be more efficient than allotopic expression, and MTS-AAV vector carrying ND1 has been shown to restore mitochondrial respiratory complex I activity and ATP synthesis rate and protect RGC from degeneration.21 Of interest, a mouse model with features of human LHON was created by using MTV-AAV to deliver the human mutant ND4 gene into the mitochondria.22

Gene editing of the mitochondrial genome using clustered regularly interspaced short palindromic repeats (CRISPR) systems with guide RNA are difficult given the inadequate delivery of guide RNA into the mitochondria. Gene editing with CRISPR–Cas9 system is possible by appending a targeting guide RNA to an RNA transport-derived stem loop element (RP-loop) and expressing the Cas9 enzyme with a preceding mitochondrial localization sequence.23 Manipulation of mtDNA using CRISPR has been limited to the targeted destruction of the mitochondrial genome by designer nucleases. Mitochondrial gene editing using targeted zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) cuts the segment of mtDNA containing the point mutation causing disease.24 While this approach shifts mtDNA heteroplasmy by lowering the level of pathogenic mtDNA, the application is limited in LHON given >85% of LHON patients are homoplasmy.

The development of a CRISPR-free mitochondrial base editing system with an interbacterial toxin DddA provides RNA-free DddA-derived cytosine base editors (DdCBEs) combined with transcription activator-like-effector (TALE) and enables the precise manipulation of mtDNA.25,26 Another mtDNA editing approach using zinc finger deaminases (ZFDs) has been developed.27 Additional bacterial deaminase toxin families and identified deaminases with different sequence contexts and substrates are expanding and are applicable to future LHON gene therapy.

G11778A LHON Gene Therapy Clinical Trials Using Allotopic Expression

Clinical trials of G11778A LHON have reported bilateral partial BCVA improvement with unilateral allotopic expression gene therapy.28–32 The BCVA improvement in the contralateral eyes was unanticipated and was not included in the design of the trials. An adequate comparison of the unexpected results to the natural history of LHON is difficult given the scarcity of prospective natural history studies. Despite the observed bilateral improvement, final mean BCVA levels after gene therapy were in the range of ~1.3 logMAR (20/400) bilaterally. Future clinical trials with randomization of patients to a group not receiving gene therapy in either eye would help to assess treatment effect and determine if reported bilateral changes in BCVA differ from natural history, sham treatment, or placebo treatment.

The natural course of LHON of sequential progressive bilateral vision loss with improvements in some patients poses challenges to assessing treatment efficacy. Progressive vision loss generally continues until BCVA reaches the worst levels at 4 to 12 months after onset of visual loss followed by variable partial BCVA improvements in some patients. Phase 3 trials have used changes in BCVA from “nadir”, defined as the worst BCVA during the study period to adjust for progressive visual loss from the natural history of LHON after trial entry.28–30 This strategy does not account for spontaneous improvement based on natural history, and given the definition of nadir BCVA, BCVA changes from nadir BCVA will always be an improvement. The analytical design of future clinical trials should consider that both worsening and improvement are possible from natural history and attempt to account for the variability of repeated testing of BCVA as a subjective measure.

The RESCUE phase 3 trial (NCT02652767) evaluated the efficacy of a single intravitreal injection of recombinant adeno-associated virus vector 2, serotype 2 (rAAV2/2-ND4) in 38 G11778A LHON subjects (mean age 36.8 years, 82% male) with vision loss within 6 months in at least 1 eye and vision loss of no longer than 6 months in both eyes.28 The primary endpoint was the between-eye change of logMAR BCVA from baseline to week 48 after treatment. A difference of 0.3 logMAR (15 ETDRS letters equivalent) between the change from baseline in the rAAV2/2-ND4-treated eyes and the sham-treated eyes was considered clinically significant. The right eye of each subject was randomized to rAAV2/2-ND4 intravitreal injection (9×1010 viral genomes (vg) in 90 ml) or a sham injection irrespective of which eye was first affected. The left eye received either treatment not allocated to the right eye in a 1:1 ratio. Sham injection consisted of the blunt end of a syringe applied on the eye at a typical injection site. Prior use of idebenone was required to have ceased at least 7 days before enrollment. Mean duration of vision loss at time of treatment was 3.6 and 3.9 months in the rAAV2/2-ND4-treated eyes and sham-treated eyes, respectively. At baseline, the mean (SD) logMAR BCVAs were 1.31 (0.52) and 1.26 (0.62) in rAAV2/2-ND4-treated and sham-treated eyes, respectively, and at week 48, a comparable deterioration in BCVA was observed in the treated eyes and the sham-treated eyes, with a least squares mean change from baseline of +0.38 (−19 ETDRS letters equivalent) and +0.39 logMAR (−20 ETDRS letters equivalent), respectively. The difference of the change in BCVA from baseline between rAAV2/2-ND4-treated and sham-treated eyes was −0.01 logMAR (+1 ETDRS letters equivalent) (P=0.89), and the primary endpoint of a difference of 0.3 logMAR was not met. The mean BCVA for both groups deteriorated over the initial weeks, reaching the worst levels at week 24, followed by a plateau phase, and then improving +10 and +9 letters equivalent from the plateau level in the rAAV2/2-ND4-treated and sham-treated eyes, respectively.

The methodology of the REVERSE phase 3 trial (NCT02652780, 37 subjects, mean age 34.2 years, 78% male) was similar to the RESCUE trial except the duration of vision loss was between 6 and 12 months.30 Mean duration of vision loss at time of treatment was 8.8 and 9.3 months in the rAAV2/2-ND4-treated eyes and sham-treated eyes, respectively. At baseline, the mean (SD) logMAR BCVA was 1.67 (0.50) and 1.55 (0.42) in the rAAV2/2-ND4-treated and sham-treated eyes, respectively. The primary endpoint difference of 0.3 logMAR BCVA at week 48 between rAAV2/2-ND4-treated and sham-treated eyes was not met (P=0.894). At week 96, rAAV2/2-ND4–treated eyes showed a mean BCVA improvement of −0.308 LogMAR (+15 ETDRS letters) and −0.259 LogMAR (+13 ETDRS letters) in the rAAV2/2-ND4-treated and sham-treated eyes, respectively.

Taken together subjects who received earlier treatment at a mean of 16 weeks after the onset of visual loss in the RESCUE trial had worse visual outcome at 96 weeks than subjects who received treatment later in the disease at a mean of 39 weeks in the REVERSE trial.30 This finding is somewhat counter-intuitive and may be related the wide spectrum of LHON natural history or variable efficacy effect. Other G11778A LHON clinical trials with similar intravitreal-injected allotopic-expression vectors have also found bilateral improvement with unilateral gene therapy.31,33

The REFLECT phase 3 trial (NCT03293524), evaluated the efficacy of bilateral intravitreal injection of rAAV2/2-ND4 in G11778A LHON subjects with vision loss duration of ≤12 months.29 The first-affected eye received gene therapy; the fellow (affected/not-yet-affected) eye was randomly injected with gene therapy or placebo (balanced salt solution). Forty-eight patients (mean age 32.4 years, 77% male) were treated bilaterally and 50 patients (mean age 31.9 years, 82% male) unilaterally. Mean duration of vision loss at time of treatment was 8.33 and 8.27 months in the bilaterally-treated and unilaterally-treated groups, respectively. The baseline mean (SD) logMAR BCVA’s of the first-affected eye were 1.59 (0.47) and 1.68 (0.43) in the bilaterally-treated and unilaterally-treated groups, respectively, and the baseline mean (SD) logMAR BCVA’s of the second/not-yet-effected eye were 1.44 (0.51) and 1.50 (0.46) in the bilaterally-treated and unilaterally-treated groups, respectively. REFLECT also showed bilateral BCVA improvement in placebo eyes of unilaterally treated patients with mean change from baseline of −0.22 and −0.05 LogMAR BCVA in the rAAV2/2-ND4-treated eyes and placebo eyes, respectively. A slightly larger BCVA improvements were found in patients who received bilateral treatment with a mean change −0.25 and −0.08 LogMAR BCVA for the first-affected and second-affected eye, respectively. The primary endpoint was the difference in change from baseline of BCVA in second-affected/not-yet-affected eyes treated with rAAV2/2-ND4 versus eyes treated with placebo at 1.5 years post-treatment. The mean (SD) changes in BCVA from baseline to 1.5 years was −0.09 (0.072) and −0.04 (0.071) LogMAR for the second/not-yet-affected rAAV2/2-ND4-treated eyes and second/not-yet-affected placebo-treated eyes, respectively. The least squares mean difference in the change of BCVA between these two treatment arms at 1.5 years was −0.05 LogMAR (P= 0.6080). The primary endpoint, defined as a difference of at least −0.3 LogMAR, was thus not met.

Gene therapy of asymptomatic fellow eyes of G11778A LHON patients with unilateral visual loss is ineffective in preventing vision loss. In a clinical trial of unilateral intravitreal AAV2(Y444,500,730F)-P1ND4v2 gene therapy, all 8 asymptomatic fellow eyes with mean LogMAR BCVA of 0.26 at baseline went on to progressive vision loss after intravitreal gene therapy to 1.5 LogMAR BCVA (20/640) at month 6.34 At baseline, the 8 asymptomatic eyes had a reduced mean (SD) GCIPL thickness of 73.4 (10.2) μm compared to the age-matched mean of 84.2 (5.3) μm35 as well as reduced mean SS-PERG amplitude of 0.47 μV compared to the age-expected amplitude of 1.12 μV.36 This indicated that despite good subjective visual function, signs of anatomical optic nerve damage and objective RGC dysfunction were already present. Of note, among the 4 eyes with preserved BCVA at baseline in the RESCUE trial, only 1 eye was treated with rAAV2/2-ND4, and early unilateral treatment also did not spare the second eye from worsening BCVA to 20/800 at week 72.28

The gene therapy clinical trials have shown good safety without prophylactic oral or topical corticosteroid.37 Transient increased intraocular pressure occur in some patients from the intravitreal injection and is manageable with topical intraocular pressure lowering eye drops. Incident uveitis is the main vector-related adverse event and is related to vector dose. Incident uveitis has resulted in no attributable vision sequelae and is manageable with topical and systemic corticosteroid and topic inflammatory treatment. Incident uveitis requiring treatment is associated with increased serum AAV2 neutralizing antibody titers (p = 0.007) but not serum AAV2 polymerase chain reaction, and in rare cases, mild uveitis may persist chronically and may not require treatment.31

Hypotheses of Bilateral BCVA Improvement with Unilateral Allotopic Expression Gene Therapy

The finding of transfer of viral vector DNA after unilateral intravitreal gene therapy to the retina of the untreated fellow eye in cynomolgus monkeys may support the finding of bilateral partial BCVA improvement observed in clinical trials.30 However, the biodistribution of rAAV2/2-ND4 DNA three months after a unilateral intravitreal injection in cynomolgus monkeys using the clinical trial vector showed a mean of 2.7×106 in the retina of the injected eye and only 5.99×103 viral copies in the retina of fellow eye which is only 0.2% of that of the injected eye and is likely below the therapy level.30 Other factors that may attribute to the bilateral effect may include cortical neuroplasticity, placebo effect, learning effect, and natural history.

Treatment Timing Options in LHON

Treatment timing options in LHON are shown in Figure 1 and include: 1) treat LHON genotype carriers before LHON, 2) treat asymptomatic eye after unilateral visual loss from LHON, and 3) treat LHON after onset of visual loss. The overview of the advantages, disadvantages, and status of the 3 therapeutic windows are provided in Table 1. The bilateral sequential visual loss in LHON suggests systemic mediators, and autophagy and mitophagy are pathologically increased in LHON and reflect a mitochondrial stress phenotype that activates the apoptotic response.38 While therapeutic strategies balancing the autophagy and mitophagy levels and the mitochondrial homeostasis reverses the pathologic phenotype of LHON-affected cells38, the window for treatment effect appears to have already narrowed in asymptomatic and symptomatic eyes of patients with LHON given the significant functional and anatomical RGC deficits even during the early course of the disease. Therefore, the treatment of LHON genotype carriers before LHON utilizing promising mitochondrial base editing techniques in development may potentially provide the best visual outcome by reducing disease penetrance and lessen RGC loss when LHON develops.

Figure 1.

Figure 1.

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Schematic diagram of visual loss over time in each eye in Leber hereditary optic neuropathy (LHON) showing treatment timing options. The advantages, disadvantages, and status of the 3 potential therapeutic windows are discussed in Table 1.

Table 1.

Treatment Timing Options in Leber Hereditary Optic Neuropathy

Treatment Timing Options
(see Figure 1)		 Main Advantages Main Disadvantages Status
A. Treat LHON genotype carriers before LHON

  1. Potential to decrease disease penetrance and reduce conversion of LHON genotype carrier to having LHON

  2. Potential to mitigate RGC loss when LHON develops given pre-treatment of healthy RGC

  3. Best treatment window to achieve optimal visual outcome

  1. Treating many LHON genotype carriers who would never develop LHON. One option could be to treat only carriers of pedigrees with relatives affected with LHON

  2. Challenges in conducting clinical trial to demonstrate efficacy. May need to utilize composite outcome criteria of at least one of several primary endpoints is sufficient (e.g. conversion rate, LHON visual outcome)

  1. CRISPR-free, RNA-free mitochondrial base editing systems in development
B. Treat asymptomatic eye after unilateral visual loss from LHON

  1. Providing treatment at the earliest stage of LHON when RGC number is not yet severely diminished and RGC cells are still functioning to provide good vision

  1. Signs of anatomical optic nerve damage and objective RGC dysfunction with reduced GCIPL and SS-PERG are evident in asymptomatic eyes of LHON, indicating limited potential of RGC response to therapy

  1. Allotopic gene therapy of asymptomatic fellow eyes of G11778A LHON patients with unilateral visual loss is ineffective in preventing vision loss
C. Treat LHON after onset of visual loss

  1. Ease of identifying subjects to test treatment in clinical trials

  2. Treatment may improve function and survival of retained RGCs

  1. Autophagic activity and apoptotic death of LHON affected RGCs are underway which limits the potential of rescue of RGCs and overall efficacy outcome

  2. Natural history of rapid bilateral visual loss and varied occurrence of spontaneous partial improvement confound clinical trial results

  1. Allotopic gene therapy of G11778A LHON patients showed unexpected bilateral partial improvement with unilateral treatment. Mean logMAR BCVA improved to ~0.3 LogMAR (3 lines) in treated eyes and ~0.25 LogMAR (2.5 lines) in sham-treated or placebo-treated fellow eyes. Overall efficacy limited with final mean BCVA levels of ~1.3 logMAR (20/400) bilaterally
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GCIPL, ganglion cell + inner plexiform layer; LHON, Leber hereditary optic neuropathy; RGC, retinal ganglion cell; SS-PERG, steady-state pattern electroretinogram.

Conclusion

Effective treatment development and the assessment of treatment efficacy in LHON pose significant challenges due to the natural history of rapid bilateral visual loss, the varied occurrence of spontaneous partial improvement, and the reliance on BCVA, a subjective measure, as the only currently suitable primary endpoint. G11778A LHON gene therapy clinical trials using allotopic expression found unanticipated bilateral partial BCVA improvement from unilateral treatment with overall limited efficacy given the final mean BCVA levels after gene therapy are in the range of ~1.3 logMAR (20/400) bilaterally. Future clinical trial designs to include randomization of patients to a group not receiving gene therapy in either eye will help to determine whether the reported bilateral partial BCVA improvement differ from natural history, sham treatment, placebo treatment, or learning effect. Promising LHON gene therapy approaches include mitochondrially-targeted-sequence AAV (“MTS-AAV”) for direct delivery of the wild-type mtDNA into mitochondria and CRISPR-free, RNA-free mitochondrial base editing systems. The bilateral sequential visual loss in LHON suggests systemic mediators that lead to subacute severe progressive RGC dysfunction and death. Signs of anatomical optic nerve damage and objective RGC dysfunction are evident in the asymptomatic eyes of LHON patients experiencing unilateral visual loss, indicating the therapeutic window has already narrowed before visual symptoms become noticeable. Future treatment strategies utilizing mitochondrial base editing in LHON carriers without optic neuropathy holds the promise of a more advantageous approach to achieve optimal visual outcome by reducing disease penetrance and mitigate RGC loss once optic neuropathy develops.

Key Points.

  • Assessing the efficacy of LHON treatment is challenging given the natural course of sequential progressive visual loss, varied occurrence of spontaneous partial improvement, and the only suitable primary endpoint being BCVA, a subjective measure.

  • LHON G11778A clinical trials have found unanticipated bilateral partial BCVA improvement resulting from unilateral allotopic gene therapy, an aspect not considered in the trials’ original design, and an adequate comparison of the unexpected results to the natural history of LHON is difficult given the scarcity of prospective natural history studies.

  • Future clinical trials with randomization of patients to a group not receiving gene therapy in either eye would help to assess treatment effect and determine if reported bilateral changes in BCVA differ from natural history, sham treatment, or placebo treatment.

  • Promising future LHON gene therapy strategies include mitochondrially-targeted-sequence AAV (“MTS-AAV”) for direct delivery of the wild-type mtDNA into mitochondria and CRISPR-free, RNA-free mitochondrial base editing systems.

  • Mitochondrial base editing in LHON carriers without optic neuropathy holds the promise of a more advantageous approach to achieve optimal visual outcome by reducing disease penetrance and mitigate RGC loss once optic neuropathy develops.

Acknowledgements

The author wishes to acknowledge the invaluable contributions of his colleagues in LHON research who played a role in crafting this paper, notably the late John Guy, MD, who invented LHON allotopic and MTS-AAV gene therapies and the late William J. Feuer, M.S., whose unwavering dedication to analytical thinking has enriched this work.

Financial support and sponsorship

B.L.L.: Supported by the National Eye Institute, National Institutes of Health, Bethesda, Maryland, 1U10EY023558–01A1 & UG1EY023558–01A1 and by the Mark J. Daily Inherited Retinal Disease Clinical and Research Center, Bascom Palmer Eye Institute, University of Miami.

Footnotes

Conflict of Interest

None.

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