Mitochondrial retinopathies and optic neuropathies: The impact of retinal imaging on modern understanding of pathogenesis, diagnosis, and management

En face OCT: Breakthroughs in understanding the pathoanatomy of retinal disease and clinical applications

En face optical coherence tomography (OCT) is a practical and informative imaging modality to noninvasively visualize distinct retinal and choroidal layers by providing coronal images using boundary-specific segmentation. Ongoing research with this method is generating breakthroughs in the illustration of new perspectives of retinal disease. The clinical value of en face OCT as an advanced retinal imaging tool is growing steadily and it has unveiled many new insights into the pathoanatomy of retinal disorders. Moreover, this modality can capture various en face OCT biomarkers that correspond to different cell or tissue subtypes, which were previously only identified through histological or electron microscopy methods, underscoring the significance of this technique in providing valuable pathoanatomical information. In this comprehensive review, we will systematically summarize the en face OCT findings across a broad spectrum of retinal diseases, including disorders of the vitreoretinal interface and retinal vascular system (e.g. paracentral acute middle maculopathy or PAMM and diabetic retinopathy), in addition to the en face OCT features of other conditions such as age-related macular degeneration, pachychoroid disease spectrum, myopic degeneration, uveitis and inflammatory disorders, inherited retinal dystrophies, and drug toxicity. We will discuss and highlight the unique clinical and pathoanatomical findings uncovered with en face OCT of each these diseases mentioned above.

Discrepancies Between Autofluorescence Imaging Modalities in CNGB3-Associated Achromatopsia and Correlation With Ellipsoid Zone Continuity

Purpose

To explore discrepancies on fundus autofluorescence (FAF) obtained with two widely used devices in patients with CNGB3-associated achromatopsia, with respect to the central foveal signal. Secondly, to explore continuity of the foveal ellipsoid zone (EZ) in these patients.

Methods

Patients who had undergone blue (488 nm; Heidelberg Spectralis) and green (532 nm; Optos) FAF imaging during the same visit were included. The central foveal signal was graded qualitatively as brighter or darker compared to the wider foveal/parafoveal region. Optical coherence tomography images from the same visit were also graded (masked to FAF grading) with respect to foveal EZ continuity.

Results

Forty-one patients (24 females; mean age, 32 ± 19 years) were included. For blue FAF, the central foveal signal was graded darker in all cases. For green FAF, the central fovea was brighter in 11 patients (27%), indicating discordance with blue FAF. The discordant group were significantly younger (P = 0.022). The EZ line was gradable in 40 patients: 22 (55%) had continuous foveal EZ in both eyes; these were younger than those with an interrupted EZ in one or both eyes (P < 0.0001). All patients discordant for FAF images had continuous foveal EZ.

Conclusions

Discordance occurred between the FAF modalities in more than one-quarter of patients; these patients were significantly younger, and all had a continuous EZ line. Investigating mechanisms of discordance could yield pathophysiological insights.

Translational Relevance

FAF platforms are not interchangeable; these findings could inform the design of natural history studies and therapeutic trials for this condition.

Vitamin A treatment restores vision failures arising from Leber's hereditary optic neuropathy-linked mtDNA mutation

Leber hereditary optic neuropathy (LHON) is a paradigm for mitochondrial retinopathy due to mitochondrial DNA (mtDNA) mutations. However, the mechanism underlying retinal cell-specific effects of LHON-linked mtDNA mutations remains poorly understood, and there has been no effective treatment or cure for this disorder. Using a mouse model bearing an LHON-linked ND6P25L mutation, we demonstrated that the mutation caused retinal cell-specific deficiencies, especially in retinal ganglion cells (RGCs), rods, and Müller cells. Single-cell RNA sequencing revealed cell-specific dysregulation of oxidative phosphorylation and visual signaling pathways in the mutant retina. Strikingly, ND6 mutation-induced dysfunctions caused abnormal vitamin A (VA) metabolism essential for visual function. VA supplementation remarkably alleviated retinal deficiencies, including reduced fundus lesion and retinal thickness and increased numbers of RGCs, photoreceptors, and Müller cell neurites. The restoration of visual functions with VA treatment were further evidenced by correcting dysregulations of phototransduction cascade and neurotransmitter transmission and restoring electrophysiological properties. Interestingly, VA supplementation markedly rescued the abnormal mitochondrial morphologies and functions in the mutant retina. These findings provide insight into retina-specific pathophysiology of mitochondrial retinopathy arising from VA deficiency and mitochondrial dysfunction induced by mtDNA mutation and a step toward therapeutic intervention for LHON and other mitochondrial retinopathies.

Surface moieties drive the superior protection of curcumin-derived carbon quantum dots against retinal ischemia-reperfusion injury

Despite the recognized neuroprotective benefits of curcumin, its clinical utility is constrained by poor bioavailability and high cytotoxicity at effective doses. This study evaluates the therapeutic potential of curcumin-derived carbon quantum dots (Cur-CQDs) for retinal protection against ischemia-reperfusion (IR) injury in rats. Cur-CQDs were synthesized via mild pyrolysis at varying temperatures and assessed for efficacy in rat retinal ganglion cells and a model of retinal IR injury. The Cur-CQDs, particularly those synthesized at 150 °C, displayed significant reductions in apoptosis in retinal tissues, as indicated by TUNEL assays, immunofluorescence localization of HIF-α, CD68, BCL-2, and Grp78, and Western blot analysis for HO-1, Grp78, CHOP, caspase 3, and Nrf2. These results suggest that Cur-CQDs not only enhance cell survival and reduce inflammation but also decrease oxidative and endoplasmic reticulum stress markers. Mechanistic insights reveal that Cur-CQDs modulate pathways involved in oxidative stress, apoptosis, and inflammation, specifically through the upregulation of BCL-2 and HO-1 and the downregulation of CHOP, caspase-3, and endoplasmic reticulum stress markers. The identification of cinnamic acid-, anisole-, guaiacol, and ferulic acid-like structures on Cur-CQDs' surfaces may contribute to their superior antioxidative and anti-inflammatory activities. Collectively, these findings position Cur-CQDs as a promising approach for treating retinal IR injuries, enhancing curcumin's bioavailability and therapeutic efficacy, and paving new pathways in ocular neuroprotection research and potential clinical applications.

Identification of IDH3G, encoding the gamma subunit of mitochondrial isocitrate dehydrogenase, as a novel candidate gene for X-linked retinitis pigmentosa

PURPOSE

Retinitis pigmentosa (RP) is a genetically heterogeneous group of retinal degenerative disorders characterized by the loss of rod and cone photoreceptors, leading to visual impairment and blindness. To date, to our knowledge, X-linked RP has been associated with variants in 3 genes (RPGR, RP2, and OFD1), whereas genetic defects at 3 loci (RP6, RP24, and RP34) are yet unidentified. The aim of this study was to identify a novel candidate gene underlying X-linked RP.

METHODS

Participants were identified from cohorts of genetically unsolved male individuals affected by RP, who underwent genome sequencing, exome sequencing, or candidate gene screening via direct Sanger sequencing at 3 referral centers. Specifically, 2 probands were identified at the National Reference Centre for Rare Retinal Diseases (Paris, France), 2 at the Massachusetts Eye and Ear Hospital (Boston, MA), and 1 at the National Reference Centre for Inherited Sensory Diseases (Montpellier, France). The pathogenicity of the identified variants was assessed using bioinformatic predictions, protein expression analyses, and mitochondrial function assays.

RESULTS

We identified 4 rare single-nucleotide variants in IDH3G (HGNC:5386), located at the RP34 locus on the X chromosome, and a complete gene deletion, in 5 unrelated male individuals affected with nonsyndromic RP. The variants segregated with the phenotype in all available family members. In all cases, the disease severity was intermediate. None had high myopia. IDH3G encodes the γ subunit of mitochondrial isocitrate dehydrogenase (IDH3), an enzyme involved in the citric acid cycle, which is expressed in the inner segments of photoreceptors. Variants in IDH3A and IDH3B, encoding the other subunits of IDH3, have already been associated with nonsyndromic autosomal recessive RP. Bioinformatic predictions and functional assays support a pathogenic role for the variants identified in this study, possibly through partial loss of enzymatic activity and mitochondrial function.

CONCLUSION

Our findings suggest that variants in IDH3G are a novel cause of X-linked RP.

Syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) is a progressive inherited retinal dystrophy, characterized by the degeneration of photoreceptors, presenting as a rod-cone dystrophy. Approximately 20-30% of patients with RP also exhibit extra-ocular manifestations in the context of a syndrome. This manuscript discusses the broad spectrum of syndromes associated with RP, pathogenic mechanisms, clinical manifestations, differential diagnoses, clinical management approaches, and future perspectives. Given the diverse clinical and genetic landscape of syndromic RP, the diagnosis may be challenging. However, an accurate and timely diagnosis is essential for optimal clinical management, prognostication, and potential treatment. Broadly, the syndromes associated with RP can be categorized into ciliopathies, inherited metabolic disorders, mitochondrial disorders, and miscellaneous syndromes. Among the ciliopathies associated with RP, Usher syndrome and Bardet-Biedl syndrome are the most well-known. Less common ciliopathies include Cohen syndrome, Joubert syndrome, cranioectodermal dysplasia, asphyxiating thoracic dystrophy, Mainzer-Saldino syndrome, and RHYNS syndrome. Several inherited metabolic disorders can present with RP, including Zellweger spectrum disorders, adult Refsum disease, α-methylacyl-CoA racemase deficiency, certain mucopolysaccharidoses, ataxia with vitamin E deficiency, abetalipoproteinemia, several neuronal ceroid lipofuscinoses, mevalonic aciduria, PKAN/HARP syndrome, PHARC syndrome, and methylmalonic acidaemia with homocystinuria type cobalamin (cbl) C disease. Due to the mitochondria's essential role in supplying continuous energy to the retina, disruption of mitochondrial function can lead to RP, as seen in Kearns-Sayre syndrome, NARP syndrome, primary coenzyme Q10 deficiency, SSBP1-associated disease, and long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Lastly, Cockayne syndrome and PERCHING syndrome can present with RP, but they do not fit the abovementioned hierarchy and are thus categorized as miscellaneous. Several first-in-human clinical trials are underway or in preparation for some of these syndromic forms of RP.

Gene therapy for Leber hereditary optic neuropathy

INTRODUCTION

Leber hereditary optic neuropathy (LHON) is among the most frequent inherited mitochondrial disease, causing a severe visual impairment, mostly in young-adult males. The causative mtDNA variants (the three common are m.11778 G>A/MT-ND4, m.3460 G>A/MT-ND1, and m.14484T>C/MT-ND6) by affecting complex I impair oxidative phosphorylation in retinal ganglion cells, ultimately leading to irreversible cell death and consequent functional loss. The gene therapy based on allotopic expression of a wild-type transgene carried by adeno-associated viral vectors (AVV-based) appears a promising approach in mitochondrial disease and its efficacy has been explored in several large clinical trials.

AREAS COVERED

The review work employed basic concepts in mitochondrial diseases, LHON, and gene therapy procedures. Reports from completed trials in LHON (i.e. RESCUE) were reviewed and critically compared.

EXPERT OPINION

New challenges, as the improvement of the contralateral untreated eye or the apparently better outcome in patients treated in later stages (6-12 months), were highlighted by the latest gene therapy trials. A better understanding of the pathogenetic mechanisms of the disease together with combined therapy (medical and gene therapy) and optimization in genetic correction approaches could improve the visual outcome of treated eyes.