Molecular screening of 980 cases of suspected hereditary optic neuropathy with a report on 77 novel OPA1 mutations

Novel heterozygous OPA3 variant in a family with congenital cataracts, sensorineural hearing loss and neuropathy, without optic atrophy and comparison of pathogenic and population variants

Heterozygous mutations in the OPA3 gene are associated with autosomal dominant optic atrophy-3 (OPA3), whereas biallelic mutations cause autosomal recessive 3-methylglutaconic aciduria type III. To date, all cases with pathogenic variants in the gene OPA3 have presented with optic atrophy. We report a large family with congenital cataracts, hearing loss and neuropathy, with a likely pathogenic novel missense variant in OPA3, c.30G>C; p.(Lys10Asn) that segregates with disease in the family pedigree. The family's clinical presentation has significant phenotypic overlap with previously reported cases of OPA3, except for a notable lack of optic atrophy. The analysis of all known disease-associated variants in OPA3 revealed an enrichment in missense variants in patients with OPA3 phenotype compared with loss-of-function variants, which are more likely to be observed in individuals with 3-methylglutaconic aciduria type III, supporting different mechanisms of disease. This case broadens the clinical and genetic spectrum associated with OPA3 mutations and highlights that optic atrophy is not an obligate feature of OPA3-related disorders.

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

Advancements in ocular imaging have significantly broadened our comprehension of mitochondrial retinopathies and optic neuropathies by examining the structural and pathological aspects of the retina and optic nerve in these conditions. This article aims to review the prominent imaging characteristics associated with mitochondrial retinopathies and optic neuropathies, aiming to deepen our insight into their pathogenesis and clinical features. Preceding this exploration, the article provides a detailed overview of the crucial genetic and clinical features, which is essential for the proper interpretation of in vivo imaging. More importantly, we will provide a critical analysis on how these imaging modalities could serve as biomarkers for characterization and monitoring, as well as in guiding treatment decisions. However, these imaging methods have limitations, which will be discussed along with potential strategies to mitigate them. Lastly, the article will emphasize the potential advantages and future integration of imaging techniques in evaluating patients with mitochondrial eye disorders, considering the prospects of emerging gene therapies.

Creation of an Isogenic Human iPSC-Based RGC Model of Dominant Optic Atrophy Harboring the Pathogenic Variant c.1861C>T (p.Gln621Ter) in the OPA1 Gene

Autosomal dominant optic atrophy (ADOA) is a rare progressive disease mainly caused by mutations in OPA1, a nuclear gene encoding for a mitochondrial protein that plays an essential role in mitochondrial dynamics, cell survival, oxidative phosphorylation, and mtDNA maintenance. ADOA is characterized by the degeneration of retinal ganglion cells (RGCs). This causes visual loss, which can lead to legal blindness in many cases. Nowadays, there is no effective treatment for ADOA. In this article, we have established an isogenic human RGC model for ADOA using iPSC technology and the genome editing tool CRISPR/Cas9 from a previously generated iPSC line of an ADOA plus patient harboring the pathogenic variant NM_015560.3: c.1861C>T (p.Gln621Ter) in heterozygosis in OPA1. To this end, a protocol based on supplementing the iPSC culture media with several small molecules and defined factors trying to mimic embryonic development has been employed. Subsequently, the created model was validated, confirming the presence of a defect of intergenomic communication, impaired mitochondrial respiration, and an increase in apoptosis and ROS generation. Finally, we propose the analysis of OPA1 expression by qPCR as an easy read-out method to carry out future drug screening studies using the created RGC model. In summary, this model provides a useful platform for further investigation of the underlying pathophysiological mechanisms of ADOA plus and for testing compounds with potential pharmacological action.

Establishing induced pluripotent stem cell lines from two dominant optic atrophy patients with distinct OPA1 mutations and clinical pathologies

Dominant optic atrophy (DOA) is an inherited disease that leads to the loss of retinal ganglion cells (RGCs), the projection neurons that relay visual information from the retina to the brain through the optic nerve. The majority of DOA cases can be attributed to mutations in optic atrophy 1 (OPA1), a nuclear gene encoding a mitochondrial-targeted protein that plays important roles in maintaining mitochondrial structure, dynamics, and bioenergetics. Although OPA1 is ubiquitously expressed in all human tissues, RGCs appear to be the primary cell type affected by OPA1 mutations. DOA has not been extensively studied in human RGCs due to the general unavailability of retinal tissues. However, recent advances in stem cell biology have made it possible to produce human RGCs from pluripotent stem cells (PSCs). To aid in establishing DOA disease models based on human PSC-derived RGCs, we have generated iPSC lines from two DOA patients who carry distinct OPA1 mutations and present very different disease symptoms. Studies using these OPA1 mutant RGCs can be correlated with clinical features in the patients to provide insights into DOA disease mechanisms.

OPA1 Dominant Optic Atrophy: Diagnostic Approach in the Pediatric Population

A clinical and genetic study was conducted with pediatric patients and their relatives with optic atrophy 1 (OPA1) mutations to establish whether there is a genotype-phenotype correlation among the variants detected within and between families. Eleven children with a confirmed OPA1 mutation were identified during the study period. The main initial complaint was reduced visual acuity (VA), present in eight patients of the cohort. Eight of eleven patients had a positive family history of optic atrophy. The mean visual acuity at the start of the study was 0.40 and 0.44 LogMAR in the right and left eye, respectively. At the end of the study, the mean visual acuity was unchanged. Optical coherence tomography during the first visit showed a mean retinal nerve fiber layer thickness of 81.6 microns and 80.5 microns in the right and left eye, respectively; a mean ganglion cell layer of 52.5 and 52.4 microns, respectively, and a mean central macular thickness of 229.5 and 233.5 microns, respectively. The most common visual field defect was a centrocecal scotoma, and nine out of eleven patients showed bilateral temporal disc pallor at baseline. Sequencing of OPA1 showed seven different mutations in the eleven patients, one of which, NM_130837.3: c.1406_1407del (p.Thr469LysfsTer16), has not been previously reported. Early diagnosis of dominant optic atrophy is crucial, both for avoiding unnecessary consultations and/or treatments and for appropriate genetic counseling.

[Rare pathogenic nucleotide variants of mitochondrial DNA associated with Leber's hereditary optic neuropathy]

Patients with Leber Hereditary Optic Neuropathy (LHON) in most cases have one of the three most common mutations: m.11778G>A in the ND4 gene, m.3460G>A in the ND1 gene, or m.14484T>C in the ND6 gene. According to the international Mitomap database, in addition to these three most common mutations, there are 16 other primary mutations that are even more rare. There are nucleotide substitutions that are classified as candidate or conditionally pathogenic mutations. Their involvement in the disease development is not proven due to insufficient research. Moreover, in many publications, the authors describe new primary and potential mitochondrial DNA mutations associated with LHON, which are not yet included in the genetic data bases. This makes it possible to expand the diagnostic spectrum during genetic testing in the future. The advancements in genetic diagnostic technologies allow confirmation of the clinical diagnosis of LHON. The importance of genetic verification of the disease is determined by the existing problem of differential diagnosis of hereditary optic neuropathies with optic neuropathies of a different origin.

Combining genetic constraint with predictions of alternative splicing to prioritize deleterious splicing in rare disease studies

BACKGROUND

Despite numerous molecular and computational advances, roughly half of patients with a rare disease remain undiagnosed after exome or genome sequencing. A particularly challenging barrier to diagnosis is identifying variants that cause deleterious alternative splicing at intronic or exonic loci outside of canonical donor or acceptor splice sites.

RESULTS

Several existing tools predict the likelihood that a genetic variant causes alternative splicing. We sought to extend such methods by developing a new metric that aids in discerning whether a genetic variant leads to deleterious alternative splicing. Our metric combines genetic variation in the Genome Aggregate Database with alternative splicing predictions from SpliceAI to compare observed and expected levels of splice-altering genetic variation. We infer genic regions with significantly less splice-altering variation than expected to be constrained. The resulting model of regional splicing constraint captures differential splicing constraint across gene and exon categories, and the most constrained genic regions are enriched for pathogenic splice-altering variants. Building from this model, we developed ConSpliceML. This ensemble machine learning approach combines regional splicing constraint with multiple per-nucleotide alternative splicing scores to guide the prediction of deleterious splicing variants in protein-coding genes. ConSpliceML more accurately distinguishes deleterious and benign splicing variants than state-of-the-art splicing prediction methods, especially in "cryptic" splicing regions beyond canonical donor or acceptor splice sites.

CONCLUSION

Integrating a model of genetic constraint with annotations from existing alternative splicing tools allows ConSpliceML to prioritize potentially deleterious splice-altering variants in studies of rare human diseases.

Genetic spectrum and characteristics of autosomal optic neuropathy in Korean: Use of next-generation sequencing in suspected hereditary optic atrophy

Aims

To evaluate the clinical characteristics and causative genetic variants in autosomal optic atrophy diagnosed using next-generation sequencing (NGS).

Methods

A cohort of 57 unrelated families affected with bilateral optic atrophy were recruited from two university-based tertiary referral hospitals from May 2016 to April 2022. Genetic variants were detected using a target enrichment panel consisting of 429 or 595 genes and known deep intronic variants associated with inherited eye diseases, exome sequencing, or genome sequencing. The results of detailed clinical examinations, disease-causing variants, and clinical diagnoses were analyzed.

Results

Among the 57 probands, 33 (57.9%) were men, and the median age at genetic testing was 19.1 years (interquartile range, 7.6–42.5 years). We identified 22 likely causative variants in 18 families and corresponding diagnostic yields of 31.6% (95% confidence interval, 21.0–44.5%). The diagnostic rate of NGS was higher in patients with infantile or early childhood onset optic atrophy than in those with late-onset or unknown optic atrophy (18/39, 46.2% vs. 0/18, 0%, P < 0.001). Among the 22 variants, 15 were novel in our cohort. The OPA1 variants (n = 7) were found to be the major genetic causes, followed by the NR2F1 variant (n = 4). The causative variants in PTPN23, TMEM126A, NBAS, and WFS1 genes were identified in 4 probands with a recessive form of optic atrophy.

Conclusions

Based on the results of diagnostic NGS for optic atrophy, the causative variant could be detected in 31.6% of patients. Our study also demonstrated that NGS is unlikely to help identify molecular causes in late-onset unexplained optic atrophy.

Neurohormonal Connections with Mitochondria in Cardiomyopathy and Other Diseases

Neurohormonal signaling and mitochondrial dynamism are seemingly distinct processes that are almost ubiquitous among multicellular organisms. Both of these processes are regulated by GTPases, and disturbances in either can provoke disease. Here, inconspicuous pathophysiological connectivity between neurohormonal signaling and mitochondrial dynamism is reviewed in the context of cardiac and neurological syndromes. For both processes, greater understanding of basic mechanisms has evoked a reversal of conventional pathophysiological concepts. Thus, neurohormonal systems induced in, and previously thought to be critical for, cardiac functioning in heart failure are now pharmaceutically interrupted as modern standard of care. And, mitochondrial abnormalities in neuropathies that were originally attributed to an imbalance between mitochondrial fusion and fission are increasingly recognized as an interruption of axonal mitochondrial transport. The data are presented in a historical context to provided insight into how scientific thought has evolved and to foster an appreciation for how seemingly different areas of investigation can converge. Finally, some theoretical notions are presented to explain how different molecular and functional defects can evoke tissue-specific disease.

Biallelic Optic Atrophy 1 (OPA1) Related Disorder-Case Report and Literature Review

Dominant optic atrophy (DOA), MIM # 605290, is the most common hereditary optic neuropathy inherited in an autosomal dominant pattern. Clinically, it presents a progressive decrease in vision, central visual field defects, and retinal ganglion cell loss. A biallelic mode of inheritance causes syndromic DOA or Behr phenotype, MIM # 605290. This case report details a family with Biallelic Optic Atrophy 1 (OPA1). The proband is a child with a severe phenotype and two variants in the OPA1 gene. He presented with congenital nystagmus, progressive vision loss, and optic atrophy, as well as progressive ataxia, and was found to have two likely pathogenic variants in his OPA1 gene: c.2287del (p.Ser763Valfs*15) maternally inherited and c.1311A>G (p.lIle437Met) paternally inherited. The first variant is predicted to be pathogenic and likely to cause DOA. In contrast, the second is considered asymptomatic by itself but has been reported in patients with DOA phenotype and is presumed to act as a phenotypic modifier. On follow-up, he developed profound vision impairment, intractable seizures, and metabolic strokes. A literature review of reported biallelic OPA1-related Behr syndrome was performed. Twenty-one cases have been previously reported. All share an early-onset, severe ocular phenotype and systemic features, which seem to be the hallmark of the disease.

Clinical and genetic spectrums of 413 North African families with inherited retinal dystrophies and optic neuropathies

Background

Inherited retinal dystrophies (IRD) and optic neuropathies (ION) are the two major causes world-wide of early visual impairment, frequently leading to legal blindness. These two groups of pathologies are highly heterogeneous and require combined clinical and molecular diagnoses to be securely identified. Exact epidemiological studies are lacking in North Africa, and genetic studies of IRD and ION individuals are often limited to case reports or to some families that migrated to the rest of the world. In order to improve the knowledge of their clinical and genetic spectrums in North Africa, we reviewed published data, to illustrate the most prevalent pathologies, genes and mutations encountered in this geographical region, extending from Morocco to Egypt, comprising 200 million inhabitants.

Main body

We compiled data from 413 families with IRD or ION together with their available molecular diagnosis. The proportion of IRD represents 82.8% of index cases, while ION accounted for 17.8%. Non-syndromic IRD were more frequent than syndromic ones, with photoreceptor alterations being the main cause of non-syndromic IRD, represented by retinitis pigmentosa, Leber congenital amaurosis, and cone-rod dystrophies, while ciliopathies constitute the major part of syndromic-IRD, in which the Usher and Bardet Biedl syndromes occupy 41.2% and 31.1%, respectively. We identified 71 ION families, 84.5% with a syndromic presentation, while surprisingly, non-syndromic ION are scarcely reported, with only 11 families with autosomal recessive optic atrophies related to OPA7 and OPA10 variants, or with the mitochondrial related Leber ION. Overall, consanguinity is a major cause of these diseases within North African countries, as 76.1% of IRD and 78.8% of ION investigated families were consanguineous, explaining the high rate of autosomal recessive inheritance pattern compared to the dominant one. In addition, we identified many founder mutations in small endogamous communities.

Short conclusion

As both IRD and ION diseases constitute a real public health burden, their under-diagnosis in North Africa due to the absence of physicians trained to the identification of inherited ophthalmologic presentations, together with the scarcity of tools for the molecular diagnosis represent major political, economic and health challenges for the future, to first establish accurate clinical diagnoses and then treat patients with the emergent therapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02340-7.

First Description of Inheritance of a Postzygotic OPA1 Mosaic Variant

Optic atrophy 1 (MIM #165500) is caused by pathogenic variants in the gene OPA1 (OPA1 MITOCHONDRIAL DYNAMIN-LIKE GTPase, MIM *605290) and is inherited in an autosomal dominant manner. We describe a 6-year-old male patient with severe early onset manifestation of optic atrophy, whose parents are subjectively asymptomatic. OPA1-sequence analysis revealed the heterozygous missense variant NM_015560.3:c.806C>T, p.(Ser269Phe) in the patient. Segregation analysis of the parents showed that the mother carried a low-grade postzygotic mosaic of this variant, which apparently also involves germline cells. In line with this, ophthalmological investigation of the mother showed subclinical manifestation of optic atrophy 1. This is the first report of an OPA1 postzygotic mosaic that was inherited to offspring.

WFS1 protein expression correlates with clinical progression of optic atrophy in patients with Wolfram syndrome

BACKGROUND

Wolfram syndrome (WFS) is a rare disorder characterised by childhood-onset diabetes mellitus and progressive optic atrophy. Most patients have variants in the WFS1 gene. We undertook functional studies of WFS1 variants and correlated these with WFS1 protein expression and phenotype.

METHODS

9 patients with a clinical diagnosis of WFS were studied with quantitative PCR for markers of endoplasmic reticulum (ER) stress and immunoblotting of fibroblast protein extracts for WFS1 protein expression. Luciferase reporter assay was used to assess ATF-6 dependent unfolded protein response (UPR) activation.

RESULTS

6 patients with compound heterozygous nonsense mutations in WFS1 had no detectable WFS1 protein expression; 3 patients with missense variants had 4%, 45% and 48% WFS1 protein expression. One of these also had an OPA1 mutation and was reclassified as autosomal dominant optic atrophy-plus syndrome. There were no correlations between ER stress marker mRNA and WFS1 protein expression. ERSE-luciferase reporter indicated activation of the ATF6 branch of UPR in two patients tested. Patients with partial WFS1 expression showed milder visual acuity impairment (asymptomatic or colour blind only), compared with those with absent expression (registered severe vision impaired) (p=0.04). These differences remained after adjusting for duration of optic atrophy.

CONCLUSIONS

Patients with WFS who have partial WFS1 protein expression present with milder visual impairment. This suggests a protective effect of partial WFS1 protein expression on the severity and perhaps progression of vision impairment and that therapies to increase residual WFS1 protein expression may be beneficial.

Saccharomyces cerevisiae as a Tool for Studying Mutations in Nuclear Genes Involved in Diseases Caused by Mitochondrial DNA Instability

Mitochondrial DNA (mtDNA) maintenance is critical for oxidative phosphorylation (OXPHOS) since some subunits of the respiratory chain complexes are mitochondrially encoded. Pathological mutations in nuclear genes involved in the mtDNA metabolism may result in a quantitative decrease in mtDNA levels, referred to as mtDNA depletion, or in qualitative defects in mtDNA, especially in multiple deletions. Since, in the last decade, most of the novel mutations have been identified through whole-exome sequencing, it is crucial to confirm the pathogenicity by functional analysis in the appropriate model systems. Among these, the yeast Saccharomyces cerevisiae has proved to be a good model for studying mutations associated with mtDNA instability. This review focuses on the use of yeast for evaluating the pathogenicity of mutations in six genes, MPV17/SYM1, MRM2/MRM2, OPA1/MGM1, POLG/MIP1, RRM2B/RNR2, and SLC25A4/AAC2, all associated with mtDNA depletion or multiple deletions. We highlight the techniques used to construct a specific model and to measure the mtDNA instability as well as the main results obtained. We then report the contribution that yeast has given in understanding the pathogenic mechanisms of the mutant variants, in finding the genetic suppressors of the mitochondrial defects and in the discovery of molecules able to improve the mtDNA stability.

The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

Optic atrophy (OA) with autosomal inheritance is a form of optic neuropathy characterized by the progressive and irreversible loss of vision. In some cases, this is accompanied by additional, typically neurological, extra-ocular symptoms. Underlying the loss of vision is the specific degeneration of the retinal ganglion cells (RGCs) which form the optic nerve. Whilst autosomal OA is genetically heterogenous, all currently identified causative genes appear to be associated with mitochondrial organization and function. However, it is unclear why RGCs are particularly vulnerable to mitochondrial aberration. Despite the relatively high prevalence of this disorder, there are currently no approved treatments. Combined with the lack of knowledge concerning the mechanisms through which aberrant mitochondrial function leads to RGC death, there remains a clear need for further research to identify the underlying mechanisms and develop treatments for this condition. This review summarizes the genes known to be causative of autosomal OA and the mitochondrial dysfunction caused by pathogenic mutations. Furthermore, we discuss the suitability of available in vivo models for autosomal OA with regards to both treatment development and furthering the understanding of autosomal OA pathology.

Mitochondrial Mutations in Ethambutol-Induced Optic Neuropathy

Background: Ethambutol-induced optic neuropathy (EON) is a well-recognized ocular complication in patients who take ethambutol as a tuberculosis treatment. The aim of the current study was to investigate the presence of mitochondrial mutations, including OPA1 and Leber's hereditary optic neuropathy (LHON)-mitochondrial DNA (mtDNA), in patients with EON and to determine their effect on clinical features of these patients. Methods: All 47 patients underwent clinical evaluations, including best-corrected visual acuity, fundus examination, and color fundus photography; 37 patients were then followed up over time. Molecular screening methods, including PCR-based sequencing of the OPA1 gene and LHON-mtDNA mutations, together with targeted exome sequencing, were used to detect mutations. Results: We detected 15 OPA1 mutations in 18 patients and two LHON-mtDNA mutations in four patients, for an overall mutation detection rate of 46.8%. The mean presentation age was significantly younger in the patients with the mitochondrial mutations (27.5 years) than in those without mutations (48 years). Fundus examination revealed a greater prevalence of optic disc hyperemia in the patients with mutations (70.5%) than without mutations (48%). Half of the patients with mutations and 91% of the patients without mutations had improved vision. After adjusting for confounders, the logistic regression revealed that the patients with optic disc pallor on the first visit (p = 0.004) or the patients with the mitochondrial mutations (p < 0.001) had a poorer vision prognosis. Conclusion: Our results indicated that carriers with OPA1 mutations might be more vulnerable for the toxicity of EMB to develop EON.

Genetic Spectrum and Characteristics of Hereditary Optic Neuropathy in Taiwan

Hereditary optic neuropathy (HON) is a group of genetically heterogeneous diseases that cause optic nerve atrophy and lead to substantial visual impairment. HON may present with optic nerve atrophy only or in association with various systemic abnormalities. Although a genetic survey is indispensable for diagnosing HON, conventional sequencing techniques could render its diagnosis challenging. In this study, we attempted to explore the genetic background of patients with HON in Taiwan through capture-based next-generation sequencing targeting 52 HON-related genes. In total, 57 patients from 48 families were recruited, with 6 patients diagnosed as having Leber hereditary optic neuropathy through initial screening for three common variants (m.3460G>A, m.11778G>A, m.14484T>C). Disease-causing genotypes were identified in 14 (33.3%) probands, and OPA1 variants were the most prevalent cause of autosomal HON. Exposure to medications such as ethambutol could trigger an attack of autosomal dominant optic atrophy. WFS1 variants were identified in three probands with variable clinical features in our cohort. Hearing impairment could occur in patients with OPA1 or WFS1 variants. This is the first comprehensive study investigating the genetic characteristics of HON in Taiwan, especially for autosomal HON. Our results could provide useful information for clinical diagnosis and genetic counseling in this field.

Associations between OPA1, MFN1, and MFN2 polymorphisms and primary open angle glaucoma in Polish participants of European ancestry

BACKGROUND

Glaucomatous optic nerve damage is caused by selective death of retinal ganglion cells (RGCs). Another condition with underlying loss of RGCs is autosomal dominant optic atrophy (ADOA). Majority of ADOA patients have mutations in OPA1, gene responsible for mitochondrial fusion final steps. Clinical resemblance between the two diseases make genes involved in mitochondrial fusion good candidates as glaucoma genes. In this study, we investigated if selected polymorphisms of OPA1, MFN1, and MFN2 were associated with glaucoma in Polish population.

METHODS

Four OPA1 (rs166850, rs10451941, rs7624750, rs9851685), one MFN1 (rs2111534), and two MFN2 (rs873458, rs2295281) single nucleotide polymorphisms were investigated in 304 primary open angle glaucoma patients (204 with normal tension glaucoma, 100 with high-tension glaucoma) and 258 control subjects using RT-PCR method.

RESULTS

There was a significant difference in genotype frequencies of rs9851685 and rs2111534 polymorphisms between glaucoma patients and control subjects. Several genotype combinations comprising SNPs at OPA1 and MFN1 were significantly differently distributed in a three-way comparison between controls, patients with NTG and patients with HTG. None of the studied MFN2 polymorphisms was significantly associated with HTG or NTG.

CONCLUSIONS

In studied population, genotype CC and allele C of rs9851685 OPA1 polymorphism are NTG risk factors, whereas TT genotype and T allele of this polymorphism are protective factors against NTG. Genotype GA of rs2111534 MFN1 polymorphism is an HTG risk factor and AA genotype of this polymorphism is a protective factor against HTG. Several OPA1 and MFN2 genotype combinations are significantly associated with either increased or decreased risk of glaucoma in this population.

Mutation spectrum of the OPA1 gene in a large cohort of patients with suspected dominant optic atrophy: Identification and classification of 48 novel variants

Autosomal dominant optic atrophy is one of the most common inherited optic neuropathies. This disease is genetically heterogeneous, but most cases are due to pathogenic variants in the OPA1 gene: depending on the population studied, 32–90% of cases harbor pathogenic variants in this gene. The aim of this study was to provide a comprehensive overview of the entire spectrum of likely pathogenic variants in the OPA1 gene in a large cohort of patients. Over a period of 20 years, 755 unrelated probands with a diagnosis of bilateral optic atrophy were referred to our laboratory for molecular genetic investigation. Genetic testing of the OPA1 gene was initially performed by a combined analysis using either single-strand conformation polymorphism or denaturing high performance liquid chromatography followed by Sanger sequencing to validate aberrant bands or melting profiles. The presence of copy number variations was assessed using multiplex ligation-dependent probe amplification. Since 2012, genetic testing was based on next-generation sequencing platforms. Genetic screening of the OPA1 gene revealed putatively pathogenic variants in 278 unrelated probands which represent 36.8% of the entire cohort. A total of 156 unique variants were identified, 78% of which can be considered null alleles. Variant c.2708_2711del/p.(V903Gfs*3) was found to constitute 14% of all disease-causing alleles. Special emphasis was placed on the validation of splice variants either by analyzing cDNA derived from patients´ blood samples or by heterologous splice assays using minigenes. Splicing analysis revealed different aberrant splicing events, including exon skipping, activation of exonic or intronic cryptic splice sites, and the inclusion of pseudoexons. Forty-eight variants that we identified were novel. Nine of them were classified as pathogenic, 34 as likely pathogenic and five as variant of uncertain significance. Our study adds a significant number of novel variants to the mutation spectrum of the OPA1 gene and will thereby facilitate genetic diagnostics of patients with suspected dominant optic atrophy.

Dominant optic atrophy: Culprit mitochondria in the optic nerve

Dominant optic atrophy (DOA) is an inherited mitochondrial disease leading to specific degeneration of retinal ganglion cells (RGCs), thus compromising transmission of visual information from the retina to the brain. Usually, DOA starts during childhood and evolves to poor vision or legal blindness, affecting the central vision, whilst sparing the peripheral visual field. In 20% of cases, DOA presents as syndromic disorder, with secondary symptoms affecting neuronal and muscular functions. Twenty years ago, we demonstrated that heterozygous mutations in OPA1 are the most frequent molecular cause of DOA. Since then, variants in additional genes, whose functions in many instances converge with those of OPA1, have been identified by next generation sequencing. OPA1 encodes a dynamin-related GTPase imported into mitochondria and located to the inner membrane and intermembrane space. The many OPA1 isoforms, resulting from alternative splicing of three exons, form complex homopolymers that structure mitochondrial cristae, and contribute to fusion of the outer membrane, thus shaping the whole mitochondrial network. Moreover, OPA1 is required for oxidative phosphorylation, maintenance of mitochondrial genome, calcium homeostasis and regulation of apoptosis, thus making OPA1 the Swiss army-knife of mitochondria. Understanding DOA pathophysiology requires the understanding of RGC peculiarities with respect to OPA1 functions. Besides the tremendous energy requirements of RGCs to relay visual information from the eye to the brain, these neurons present unique features related to their differential environments in the retina, and to the anatomical transition occurring at the lamina cribrosa, which parallel major adaptations of mitochondrial physiology and shape, in the pre- and post-laminar segments of the optic nerve. Three DOA mouse models, with different Opa1 mutations, have been generated to study intrinsic mechanisms responsible for RGC degeneration, and these have further revealed secondary symptoms related to mitochondrial dysfunctions, mirroring the more severe syndromic phenotypes seen in a subgroup of patients. Metabolomics analyses of cells, mouse organs and patient plasma mutated for OPA1 revealed new unexpected pathophysiological mechanisms related to mitochondrial dysfunction, and biomarkers correlated quantitatively to the severity of the disease. Here, we review and synthesize these data, and propose different approaches for embracing possible therapies to fulfil the unmet clinical needs of this disease, and provide hope to affected DOA patients.

Pathogenicity evaluation and the genotype-phenotype analysis of OPA1 variants

Autosomal dominant optic atrophy (ADOA) is an important cause of irreversible visual impairment in children and adolescents. About 60-90% of ADOA is caused by the pathogenic variants of OPA1 gene. By evaluating the pathogenicity of OPA1 variants and summarizing the relationship between the genotype and phenotype, this study aimed to provide a reference for clinical genetic test involving OPA1. Variants in OPA1 were selected from the exome sequencing results in 7092 cases of hereditary eye diseases and control groups from our in-house data. At the same time, the urine cells of some optic atrophy patients with OPA1 variants as well as their family members were collected and oxygen consumption rates (OCR) were measured in these cells to evaluate the pathogenicity of variants. As a result, 97 variants were detected, including 94 rare variants and 3 polymorphisms. And the 94 rare variants were classified into three groups: pathogenic (33), variants of uncertain significance (19), and likely benign (42). Our results indicated that the frameshift variants at the 3' terminus might be pathogenic, while the variants in exon 7 and intron 4 might be benign. The penetrance of the missense variants was higher than that of truncation variants. The OCR of cells with pathogenic OPA1 variants were significantly lower than those without pathogenic variants. In conclusion, some variants might be benign although predicted pathogenic in previous studies while some might have unknown pathogenesis. Measuring the OCR in urine cells could be used as a method to evaluate the pathogenicity of some OPA1 variants.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Use of Next-Generation Sequencing for the Molecular Diagnosis of 1,102 Patients With a Autosomal Optic Neuropathy

Advances in next-generation sequencing (NGS) facilitate the diagnosis of genetic disorders. To evaluate its use for the molecular diagnosis of inherited optic neuropathy (ION), a blinding disease caused by the degeneration of retinal ganglion cells, we performed genetic analysis using targeted NGS of 22 already known and candidate genes in a cohort of 1,102 affected individuals. The panel design, library preparation, and sequencing reactions were performed using the Ion AmpliSeq technology. Pathogenic variants were detected in 16 genes in 245 patients (22%), including 186 (17%) and 59 (5%) dominant and recessive cases, respectively. Results confirmed that OPA1 variants are responsible for the majority of dominant IONs, whereas ACO2 and WFS1 variants are also frequently involved in both dominant and recessive forms of ION. All pathogenic variants were found in genes encoding proteins involved in the mitochondrial function, highlighting the importance of mitochondria in the survival of retinal ganglion cells.

Opa1 Deficiency Leads to Diminished Mitochondrial Bioenergetics With Compensatory Increased Mitochondrial Motility

Purpose

Retinal ganglion cells (RGCs) are susceptible to mitochondrial deficits and also the major cell type affected in patients with mutations in the OPA1 gene in autosomal dominant optic atrophy (ADOA). Here, we characterized mitochondria in RGCs in vitro from a heterozygous B6; C3-Opa1^Q285STOP (Opa1^+/−) mouse model to investigate mitochondrial changes underlying the pathology in ADOA.

Methods

Mouse RGCs were purified from wild-type and Opa1^+/− mouse retina by two-step immunopanning. The mitochondria in neurites of RGCs were labeled with MitoTracker Red for structure and motility measurement by time-lapse imaging. Mitochondrial bioenergetics were determined by the real-time measurement of oxygen consumption rate using a Seahorse XF^e 96 Extracellular Flux Analyzer.

Results

We observed a significant decrease in mitochondrial length in Opa1^+/− RGCs with a remarkably higher proportion and density of motile mitochondria along the neurites. We also observed an increased transport velocity with a higher number of contacts between mitochondria in Opa1^+/− RGC neurites. The oxygen consumption assays showed a severe impairment in basal respiration, Adenosine triphosphate-linked (ATP-linked) oxygen consumption, as well as reserve respiratory capacity, in RGCs from Opa1^+/− mouse retina.

Conclusions

Opa1 deficiency leads to significant fragmentation of mitochondrial morphology, activation of mitochondrial motility and impaired respiratory function in RGCs from the B6; C3-Opa1^Q285STOP mouse model. This highlights the significant alterations in the intricate interplay between mitochondrial morphology, motility, and energy production in RGCs with Opa1 deficiency long before the onset of clinical symptoms of the pathology.

First submicroscopic inversion of the OPA1 gene identified in dominant optic atrophy - a case report

Background

Dominant optic atrophy (DOA) is an inherited optic neuropathy that mainly affects visual acuity, central visual fields and color vision due to a progressive loss of retinal ganglion cells and their axons that form the optic nerve. Approximately 45–90% of affected individuals with DOA harbor pathogenic variants in the OPA1 gene. The mutation spectrum of OPA1 comprises nonsense, canonical and non-canonical splice site, frameshift and missense as well as copy number variants, but intragenic inversions have not been reported so far.

Case presentation

We report a 33-year-old male with characteristic clinical features of DOA. Whole-genome sequencing identified a structural variant of 2.4 kb comprising an inversion of 937 bp at the OPA1 locus. Fine mapping of the breakpoints to single nucleotide level revealed that the structural variation was an inversion flanked by two deletions. As this rearrangement inverts the entire first exon of OPA1, it was classified as likely pathogenic.

Conclusions

We report the first DOA case harboring an inversion in the OPA1 gene. Our study demonstrates that copy-neutral genomic rearrangements have to be considered as a possible cause of disease in DOA cases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12881-020-01166-z.

Mutation Screening of mtDNA Combined Targeted Exon Sequencing in a Cohort With Suspected Hereditary Optic Neuropathy

Purpose

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (ADOA) are the two commonest forms of hereditary optic neuropathy. The aim of this study was to comprehensively investigate the incidence and spectrum of mutations in patients with suspected hereditary optic neuropathy by combining mitochondrial DNA (mtDNA) genome-wide and targeted exon sequencing.

Methods

A cohort of 1101 subjects were recruited to participate in the study, comprising 177 families (177 probands and their family members, a total of 537 subjects, including 254 patients) and 164 sporadic cases with suspected hereditary optic neuropathy, and 400 unrelated control subjects for genetic analysis: all subjects (including control subjects) underwent a comprehensive ophthalmologic examination and were subjected to sequencing analysis of mtDNA genome-wide and targeted exon. Overall, targeted exon sequencing was used to screen 792 genes associated with common hereditary eye diseases, and the mtDNA genome-wide were screened by next-generation sequencing.

Results

We found variants detected in 168 (40.2%, 168/418) of the 418 patients screened. Among these, 132 cases (78.6%, 132/168) were detected with known LHON disease-causing mtDNA variants; 40 cases (23.8%, 40/168) were detected with nuclear DNA (ntDNA) variants, which included 36 cases (21.4%, 36/168) with detected OPA1 mutations, 4 patients (2.4%, 4/168) with detected OPA3 mutations, and 2 patients (1.2%, 2/168) with detected TMEM126A homozygous mutation. Coexistence variation (mtDNA/mtDNA [n = 16], ntDNA/ntDNA [n = 4], mtDNA/ntDNA [n = 7]) was found in 27 patients (16.4%, 27/165), including mtDNA/ntDNA coexistence variation that was detected in seven patients. Among these ntDNA mutations, 38 distinct disease-causing variants, including autosomal recessive heterozygous mutations, were detected, which included 22 novel variants and two de novo variants. Total haplogroup distribution showed that 34.5% (29/84) and 28.6% (24/84) of the affected subjects with m.11778G>A belonged to haplogroup D and M, with a high frequency of subhaplogroups D4, D5, and M7.

Conclusions

The LHON-mtDNA mutations are the commonest genetic defects in this Chinese cohort, followed by the OPA1 mutations. To our knowledge, this is the first comprehensive study of LHON, ADOA, and autosomal recessive optic atrophy combined with mtDNA genome-wide and targeted exon sequencing, as well as haplogroup analysis, in a large cohort of Chinese patients with suspected hereditary optic neuropathy. Our findings provide a powerful basis for genetic counseling in patients with suspected hereditary optic neuropathy.

Translational Relevance

We applied mtDNA genome-wide sequencing combined with panel-based targeted exon sequencing to explore the pathogenic variation spectrum and genetic characteristics of patients with suspected hereditary optic neuropathy, providing a comprehensive research strategy for clinical assistant diagnosis, treatment, and genetic counseling.

Auditory Neuropathy Spectrum Disorders: From Diagnosis to Treatment: Literature Review and Case Reports

Auditory neuropathy spectrum disorder (ANSD) refers to a range of hearing impairments characterized by deteriorated speech perception, despite relatively preserved pure-tone detection thresholds. Affected individuals usually present with abnormal auditory brainstem responses (ABRs), but normal otoacoustic emissions (OAEs). These electrophysiological characteristics have led to the hypothesis that ANSD may be caused by various dysfunctions at the cochlear inner hair cell (IHC) and spiral ganglion neuron (SGN) levels, while the activity of outer hair cells (OHCs) is preserved, resulting in discrepancies between pure-tone and speech comprehension thresholds. The exact prevalence of ANSD remains unknown; clinical findings show a large variability among subjects with hearing impairment ranging from mild to profound hearing loss. A wide range of prenatal and postnatal etiologies have been proposed. The study of genetics and of the implicated sites of lesion correlated with clinical findings have also led to a better understanding of the molecular mechanisms underlying the various forms of ANSD, and may guide clinicians in better screening, assessment and treatment of ANSD patients. Besides OAEs and ABRs, audiological assessment includes stapedial reflex measurements, supraliminal psychoacoustic tests, electrocochleography (ECochG), auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs). Hearing aids are indicated in the treatment of ANSD with mild to moderate hearing loss, whereas cochlear implantation is the first choice of treatment in case of profound hearing loss, especially in case of IHC presynaptic disorders, or in case of poor auditory outcomes with conventional hearing aids.

Optic atrophy in children: Current causes and diagnostic approach

BACKGROUND/AIMS

Optic atrophy is one of the leading causes of sight impairment in children. It frequently poses a diagnostic challenge, as it can be caused by many ocular and systemic conditions. We aimed to determine the current causes of optic atrophy at our centre and to describe the use of investigations, including molecular genetic testing.

METHODS

We reviewed the medical records of children with optic atrophy seen at Moorfields Eye Hospital between 2010 and 2015. We recorded demographic data, reason for referral, history, investigations and diagnosis.

RESULTS

We studied 143 cases aged below 16 years. A cause could be identified in all cases. Taking a full history was the most important part of the diagnostic workup, identifying a cause in 96 (67%) children. A developmental disorder of the brain and/or optic nerve, sometimes with retinal involvement, was the commonest cause (n = 33, 23%), followed by inheritable optic neuropathies (n = 27, 19%). Other causes included perinatal insults (n = 18, 13%), post-infectious or post-inflammatory conditions (n = 18, 13%), accidental or abusive trauma (n = 14, 10%) and inheritable retinal dystrophies (n = 13, 9%). Rare conditions included neurodegenerative disorders (n = 7, 5%), skeletal developmental disorders such as rickets (n = 4, 3%), tumours (n = 4, 3%), ischaemic events including large optic nerve head drusen (n = 4, 3%) and toxic events/metabolic conditions (n = 1, 0.7%).

CONCLUSION

In this series, an underlying cause could be identified in all cases. Taking a comprehensive antenatal, perinatal, postnatal and family history will indicate a probable diagnosis in two-thirds of children, and targeted ancillary tests may identify the cause in most remaining cases.

OPA1: 516 unique variants and 831 patients registered in an updated centralized Variome database

BACKGROUND

The dysfunction of OPA1, a dynamin GTPase involved in mitochondrial fusion, is responsible for a large spectrum of neurological disorders, each of which includes optic neuropathy. The database dedicated to OPA1 ( https://www.lovd.nl/OPA1 ), created in 2005, has now evolved towards a centralized and more reliable database using the Global Variome shared Leiden Open-source Variation Database (LOVD) installation.

RESULTS

The updated OPA1 database, which registers all the patients from our center as well as those reported in the literature, now covers a total of 831 patients: 697 with isolated dominant optic atrophy (DOA), 47 with DOA "plus", and 83 with asymptomatic or unclassified DOA. It comprises 516 unique OPA1 variants, of which more than 80% (414) are considered pathogenic. Full clinical data for 118 patients are documented using the Human Phenotype Ontology, a standard vocabulary for referencing phenotypic abnormalities. Contributors may now make online submissions of phenotypes related to OPA1 mutations, giving clinical and molecular descriptions together with detailed ophthalmological and neurological data, according to an international thesaurus.

CONCLUSIONS

The evolution of the OPA1 database towards the LOVD, using unified nomenclature, should ensure its interoperability with other databases and prove useful for molecular diagnoses based on gene-panel sequencing, large-scale mutation statistics, and genotype-phenotype correlations.

Genetic analysis in a cohort of patients with hereditary optic neuropathies in Southwest of China

We report the results of molecular screening in 121 patients with suspected hereditary optic neuropathies. The 34 primary and 9 secondary LHON mutations were screened in all the patients. In the familial cases, OPA1 was also tested when negative finding for the mtDNA mutations screening. Molecular defects were identified in 35 patients (28.9% of screened patients). Among these, 33 patients (94.3%) had an mtDNA mutation, including m.11778G > A (69.7%), m.14484 T > C, m.3460G > A, m.3635G > A, m.14502 T > C and three secondary mutations m.3316G > A, m.3394 T > C, m.3497C > T. Two novel OPA1 mutations, c.1301 T > G (p.Leu434Arg) and c.985-1G > A (IVS9-1G > A), were also detected in families with the evidence of father-to-son transmission. In conclusion, we reported the results of the molecular screening of 121 patients with hereditary optic neuropathies from southwest of China. Our results highlight the importance of investigating LHON-causing mtDNA mutations and OPA1 mutations in cases of suspected hereditary optic neuropathy.

A sibling study of isolated optic neuropathy associated with novel variants in the ACO2 gene

Inherited optic neuropathy is a rare cause of debilitating vision loss. It may occur in constellation with other syndromic features of neurological impairment, or present as an isolated finding. We describe a sibling pair, without a family history of vision loss, who developed visual impairment in early childhood consistent with optic neuropathy. Genetic testing identified novel compound heterozygous variants in the aconitase 2 (ACO2) gene. To date, seven families hosting ACO2 variants have been described in the literature. We describe the second family with ACO2 variants to have an isolated optic neuropathy highlighting the importance of including this gene in genomic panels assessing inherited optic neuropathies.

Deciphering OPA1 mutations pathogenicity by combined analysis of human, mouse and yeast cell models

OPA1 is the major gene responsible for Dominant Optic Atrophy (DOA) and the syndromic form DOA "plus". Over 370 OPA1 mutations have been identified so far, although their pathogenicity is not always clear. We have analyzed one novel and a set of known OPA1 mutations to investigate their impact on protein functions in primary skin fibroblasts and in two "ad hoc" generated cell systems: the MGM1/OPA1 chimera yeast model and the Opa1-/- MEFs model expressing the mutated human OPA1 isoform 1. The yeast model allowed us to confirm the deleterious effects of these mutations and to gain information on their dominance/recessivity. The MEFs model enhanced the phenotypic alteration caused by mutations, nicely correlating with the clinical severity observed in patients, and suggested that the DOA "plus" phenotype could be induced by the combinatorial effect of mitochondrial network fragmentation with variable degrees of mtDNA depletion. Overall, the two models proved to be valuable tools to functionally assess and define the deleterious mechanism and the pathogenicity of novel OPA1 mutations, and useful to testing new therapeutic interventions.

Clinical and genetic features of eight Chinese autosomal-dominant optic atrophy pedigrees with six novel OPA1 pathogenic variants

BACKGROUND

Autosomal-dominant optic atrophy (ADOA) is one of the most common types of inherited optic atrophy. We identify OPA1 pathogenic variants and assess the clinical features of a cohort of Chinese ADOA patients Materials and Methods: Detailed clinical evaluations were performed and genomic DNA was extracted from peripheral blood for all the participants. Sanger sequencing was used to analyze all exons and exon/intron junctions of OPA1 for eight pedigrees. Target exome capture plus next-generation sequencing (NGS) were applied for one atypical family with photophobia. Reverse transcription polymerase chain reaction was carried out to further characterize the mRNA change of selected splicing alteration.

RESULTS

All 17 patients had impaired vision and optic-disk pallor; however, the clinical severity varied markedly. Two patients complicated with hearing loss. Six novel and two reported pathogenic variants in OPA1 (GenBank Accession No. NM_130837.2) were identified including four nonsynonymous variants (c.2400T > G, c.1468T > C, c.1567A > G and c.1466T > C), two splicing variants (c.2984-1_2986delGAGA and c.2983 + 5G > A), one small deletion (c.2960_2968delGCGTTCAAC), and one small insertion (c.3009_3010insA). RNA analysis revealed the splicing variant c.2984-1_2986delGAGA caused small deletion of mRNA (r.2983_2988del).

CONCLUSIONS

ADOA patients presented variable clinical manifestations. Novel OPA1 pathogenic variants are the main genetic defect for Chinese ADOA cases. NGS may be a useful molecular testing tool for atypical ADOA.

Targeted next generation sequencing with an extended gene panel does not impact variant detection in mitochondrial diseases

Background

Since the advent of next generation sequencing (NGS), several studies have tried to evaluate the relevance of targeted gene panel sequencing and whole exome sequencing for molecular diagnosis of mitochondrial diseases. The comparison between these different strategies is extremely difficult. A recent study analysed a cohort of patients affected by a mitochondrial disease using a NGS approach based on a targeted gene panel including 132 genes. This strategy led to identify the causative mutations in 15.2% of cases. The number of novel genes responsible for respiratory chain deficiency increases very rapidly.

Methods

In order to determine the impact of larger panels used as a first screening strategy on molecular diagnosis success, we analysed a cohort of 80 patients affected by a mitochondrial disease with a first mitochondrial DNA (mtDNA) NGS screening and secondarily a targeted mitochondrial panel of 281 nuclear genes.

Results

Pathogenic mtDNA abnormalities were identified in 4.1% (1/24) of children and 25% (14/56) of adult patients. The remaining 65 patients were analysed with our targeted mitochondrial panel and this approach enabled us to achieve an identification rate of 21.7% (5/23) in children versus 7.1% (3/42) in adults.

Conclusions

Our results confirm that larger gene panels do not improve diagnostic yield of mitochondrial diseases due to (i) their very high genetic heterogeneity, (ii) the ongoing discovery of novel genes and (iii) mutations in genes apparently not related to mitochondrial function that lead to secondary respiratory chain deficiency.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0568-y) contains supplementary material, which is available to authorized users.

Genetic testing for optic atrophy

We studied the scientific literature and disease guidelines in order to summarize the clinical utility of the genetic test for optic atrophy (OA). OA is mostly inherited in an autosomal dominant manner, rarely in an autosomal recessive manner, with an overall prevalence of 3/100,000 live births. It is caused by mutations in the OPA1, OPA3 and TMEM126A genes. Clinical diagnosis is based on clinical findings, ophthalmological examination, OCT, visual evoked potentials (VEPs) and electroretinography. The genetic test is useful for confirming diagnosis, differential diagnosis, couple risk assessment and access to clinical trials.

Mitochondrial disorders of the retinal ganglion cells and the optic nerve

OBJECTIVES

To summarise and discuss recent findings and future perspectives concerning mitochondrial disorders (MIDs) affecting the retinal ganglion cells and the optic nerve (mitochondrial optic neuropathy. MON).

METHOD

Literature review.

RESULTS

MON in MIDs is more frequent than usually anticipated. MON may occur in specific as well as non-specific MIDs. In specific and non-specific MIDs, MON may be a prominent or non-prominent phenotypic feature and due to mutations in genes located either in the mitochondrial DNA (mtDNA) or the nuclear DNA (nDNA). Clinically, MON manifests with painless, bilateral or unilateral, slowly or rapidly progressive visual impairment and visual field defects. In some cases, visual impairment may spontaneously recover. The most frequent MIDs with MON include LHON due to mutations in mtDNA-located genes and autosomal dominant optic atrophy (ADOA) or autosomal recessive optic atrophy (AROA) due to mutations in nuclear genes. Instrumental investigations for diagnosing MON include fundoscopy, measurement of visual acuity, visual fields, and color vision, visually-evoked potentials, optical coherence tomography, fluorescein angiography, electroretinography, and MRI of the orbita and cerebrum. In non-prominent MON, work-up of the muscle biopsy with transmission electron microscopy may indicate mitochondrial destruction. Treatment is mostly supportive but idebenone has been approved for LHON and experimental approaches are promising.

CONCLUSIONS

MON needs to be appreciated, requires extensive diagnostic work-up, and supportive treatment should be applied although loss of vision, as the most severe outcome, can often not be prevented.

Processing of OPA1 with a novel N-terminal mutation in patients with autosomal dominant optic atrophy: Escape from nonsense-mediated decay

Autosomal Dominant Optic Atrophy (ADOA) is the most common dominantly inherited optic neuropathy. In the majority of patients it is caused by OPA1 mutations and those predicted to introduce a premature termination codon (PTC) are frequently detected. Transcripts containing PTC may be degraded by nonsense-mediated mRNA decay (NMD), however very little is known about an effect of OPA1 mutations on NMD activation. Here, using a combination of linkage analysis and DNA sequencing, we have identified a novel c.91C>T OPA1 mutation with a putative premature stop codon (Q31*), which segregated with ADOA in two Polish families. At the mRNA level we found no changes in the amount of OPA1 transcript among mutation carriers vs. non-carriers. Specific allele quantification revealed a considerable level of the OPA1 mutant transcript. Our study identifies a novel pathogenic OPA1 mutation and shows that it is located in the transcript region not prone for NMD activation. The data emphasizes the importance of analyzing how mutated genes are being processed in the cell. This gives an insight into the molecular mechanism of a genetic disease and promotes development of innovative therapeutic approaches.

Leigh-like neuroimaging features associated with new biallelic mutations in OPA1

Behr syndrome is characterized by the association of early onset optic atrophy, cerebellar ataxia, pyramidal signs, peripheral neuropathy and mental retardation. Recently, some cases were reported to be caused by biallelic mutations in OPA1. We describe an 11-year-old girl (Pt1) and a 7-year-old boy (Pt2) with cognitive delay, ataxic gait and clinical signs suggestive of a peripheral neuropathy, with onset in early infancy. In Pt1 ocular fundus examination revealed optic disk pallor whereas Pt2 exhibited severe optic atrophy. In both children neuroimaging detected a progressive cerebellar involvement accompanied by basal ganglia hyperintensities and pathological peak levels of lactate. In both patients, muscle biopsy showed diffuse reduction of cytochrome c oxidase stain, some atrophic fibers and type II fiber grouping. Using a targeted resequencing panel in next generation sequencing, we identified the homozygous c.1180G>A/p.Ala394Thr mutation in Pt1 and the c.2779-2A>C mutation in compound heterozygosity with the c.2809C>T/p.Arg937Cys mutation in Pt2. All variants were novel and segregated in the healthy parents. Expression of OPA1 protein was significantly reduced in muscle tissues of both patients by Western blotting. We also observed in patients' fibroblasts a higher proportion of fragmented and intermediate mitochondria upon galactose treatment compared to controls, as already seen in other patients harboring mutations in OPA1. The presence of Leigh-like neuroimaging features is a novel finding in Behr syndrome and further adds to the complex genotype-phenotype correlations in OPA1-associated disorders.

OPA1 analysis in an international series of probands with bilateral optic atrophy

PURPOSE

To determine the molecular genetic cause in previously unreported probands with optic atrophy from the United Kingdom, Czech Republic and Canada.

METHODS

OPA1 coding regions and flanking intronic sequences were screened by direct sequencing in 82 probands referred with a diagnosis of bilateral optic atrophy. Detected rare variants were assessed for pathogenicity by in silico analysis. Segregation of the identified variants was performed in available first degree relatives.

RESULTS

A total of 29 heterozygous mutations evaluated as pathogenic were identified in 42 probands, of these seven were novel. In two probands, only variants of unknown significance were found. 76% of pathogenic mutations observed in 30 (71%) of 42 probands were evaluated to lead to unstable transcripts resulting in haploinsufficiency. Three probands with the following disease-causing mutations c.1230+1G>A, c.1367G>A and c.2965dup were documented to suffer from hearing loss and/or neurological impairment.

CONCLUSIONS

OPA1 gene screening in patients with bilateral optic atrophy is an important part of clinical evaluation as it may establish correct clinical diagnosis. Our study expands the spectrum of OPA1 mutations causing dominant optic atrophy and supports the fact that haploinsufficiency is the most common disease mechanism.

Dominant Optic Atrophy and Leber's Hereditary Optic Neuropathy: Update on Clinical Features and Current Therapeutic Approaches

Dominant optic atrophy (DOA) and Leber hereditary optic neuropathy (LHON) are the two most common inherited optic neuropathies encountered in clinical practice. This review provides a summary of recent advances in the understanding of the clinical manifestations, current treatments, and ongoing clinical trials of these two optic neuropathies. Substantial progress has been made in the understanding of the clinical, genetic, and pathophysiological basis of DOA and LHON. Pathogenic OPA1 gene mutations in DOA and 3 primary mutations of mitochondrial DNA in LHON-induced mitochondrial dysfunction, which in turn leads to increased reactive oxygen species levels in mitochondria and possibly insufficient ATP production. The pathologic hallmark of these inherited optic neuropathies is primary degeneration of retinal ganglion cells, preferentially in the papillomacular bundle, which results in temporal optic disc pallor and central or cecocentral visual loss. There are no effective treatments for patients with LHON and DOA, although clinical trials are underway for the former. Translational research for these diseases is entering an accelerated phase with the availability of animal models, and a variety of pharmacological and genetic therapies are being developed.

Characterization of two novel intronic OPA1 mutations resulting in aberrant pre-mRNA splicing

BACKGROUND

We report two novel splice region mutations in OPA1 in two unrelated families presenting with autosomal-dominant optic atrophy type 1 (ADOA1) (ADOA or Kjer type optic atrophy). Mutations in OPA1 encoding a mitochondrial inner membrane protein are a major cause of ADOA.

METHODS

We analyzed two unrelated families including four affected individuals clinically suspicious of ADOA. Standard ocular examinations were performed in affected individuals of both families. All coding exons, as well as exon-intron boundaries of the OPA1 gene were sequenced. In addition, multiplex ligation-dependent probe amplification (MLPA) was performed to uncover copy number variations in OPA1. mRNA processing was monitored using RT-PCR and subsequent cDNA analysis.

RESULTS

We report two novel splice region mutations in OPA1 in two unrelated individuals and their affected relatives, which were previously not described in the literature. In one family the heterozygous insertion and deletion c.[611-37_611-38insACTGGAGAATGTAAAGGGCTTT;611-6_611-16delCATATTTATCT] was found in all investigated family members leading to the activation of an intronic cryptic splice site. In the second family sequencing of OPA1 disclosed a de novo heterozygous deletion c.2012+4_2012+7delAGTA resulting in exon 18 and 19 skipping, which was not detected in healthy family members.

CONCLUSION

We identified two novel intronic mutations in OPA1 affecting the correct OPA1 pre-mRNA splicing, which was confirmed by OPA1 cDNA analysis. This study shows the importance of transcript analysis to determine the consequences of unclear intronic mutations in OPA1 in proximity to the intron-exon boundaries.

Genetic and Clinical Analyses of DOA and LHON in 304 Chinese Patients with Suspected Childhood-Onset Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA), the most common forms of hereditary optic neuropathy, are easily confused, and it is difficult to distinguish one from the other in the clinic, especially in young children. The present study was designed to survey the mutation spectrum of common pathogenic genes (OPA1, OPA3 and mtDNA genes) and to analyze the genotype-phenotype characteristics of Chinese patients with suspected childhood-onset hereditary optic neuropathy. Genomic DNA and clinical data were collected from 304 unrelated Chinese probands with suspected hereditary optic neuropathy with an age of onset below 14 years. Sanger sequencing was used to screen variants in the coding and adjacent regions of OPA1, OPA3 and the three primary LHON-related mutation sites in mitochondrial DNA (mtDNA) (m.3460G>A, m.11778G>A and m.14484T>C). All patients underwent a complete ophthalmic examination and were compared with age-matched controls. We identified 89/304 (29.3%) primary mtDNA mutations related to LHON in 304 probands, including 76 mutations at m.11778 (76/89, 85.4% of all mtDNA mutations), four at m.3460 (4/89, 4.5%) and nine at m.14484 (9/89, 10.1%). This result was similar to the mutation frequency among Chinese patients with LHON of any age. Screening of OPA1 revealed 23 pathogenic variants, including 11 novel and 12 known pathogenic mutations. This study expanded the OPA1 mutation spectrum, and our results showed that OPA1 mutation is another common cause of childhood-onset hereditary optic neuropathy in Chinese pediatric patients, especially those with disease onset during preschool age.

Dominant optic atrophy: updates on the pathophysiology and clinical manifestations of the optic atrophy 1 mutation

PURPOSE OF REVIEW

Review recent advances in clinical and experimental studies of dominant optic atrophy (DOA) to better understand the complexities of pathophysiology caused by the optic atrophy 1 (OPA1) mutation.

RECENT FINDINGS

DOA is the most commonly diagnosed inherited optic atrophy, causing progressive bilateral visual loss that begins early in life. During the past 25 years, there has been substantial progress in the understanding of the clinical, genetic, and pathophysiological basis of this disease. The histopathological hallmark of DOA is the primary degeneration of retinal ganglion cells, preferentially in the papillomacular bundle, which results temporal optic disc pallor and cecocentral scotomata in patients with DOA. Loss of OPA1 protein function by OPA1 gene mutations causes mitochondrial dysfunction because of the loss of mitochondrial fusion, impaired mitochondrial oxidative phosphorylation, increases in reactive oxygen species, and altered calcium homeostasis. These factors lead to apoptosis of retinal ganglion cells by a haploinsufficiency mechanism.

SUMMARY

Improved understanding of the pathophysiology of DOA provides insights that can be used to develop therapeutic approaches to the DOA.