Heterozygous Mutations In OPA1 Research Articles

Human retinal organoids with an OPA1 mutation are defective in retinal ganglion cell differentiation and function

Autosomal dominant optic atrophy (ADOA), mostly caused by heterozygous OPA1 mutations and characterized by retinal ganglion cell (RGC) loss and optic nerve degeneration, is one of the most common types of inherited optic neuropathies. Previous work using a two-dimensional (2D) differentiation model of induced pluripotent stem cells (iPSCs) has investigated ADOA pathogenesis but failed to agree on the effect of OPA1 mutations on RGC differentiation. Here, we use 3D retinal organoids capable of mimicking invivo retinal development to resolve the issue. We generated isogenic iPSCs carrying the hotspot OPA1 c.2708_2711delTTAG mutation and found that the mutant variant caused defective initial and terminal differentiation and abnormal electrophysiological properties of organoid-derived RGCs. Moreover, this variant inhibits progenitor proliferation and results in mitochondrial dysfunction. These data demonstrate that retinal organoids coupled with gene editing serve as a powerful tool to definitively identify disease-related phenotypes and provide valuable resources to further investigate ADOA pathogenesis and screen for ADOA therapeutics.

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.

A novel AFG3L2 mutation close to AAA domain leads to aberrant OMA1 and OPA1 processing in a family with optic atrophy

Autosomal dominant optic atrophy (ADOA) is a neuro-ophthalmic condition characterized by bilateral degeneration of the optic nerves. Although heterozygous mutations in OPA1 represent the most common genetic cause of ADOA, a significant number of cases remain undiagnosed.Here, we describe a family with a strong ADOA history with most family members spanning three generation having childhood onset of visual symptoms. The proband, in addition to optic atrophy, had neurological symptoms consistent with relapsing remitting multiple sclerosis. Clinical exome analysis detected a novel mutation in the AFG3L2 gene (NM_006796.2:c.1010G > A; p.G337E), which segregated with optic atrophy in family members. AFG3L2 is a metalloprotease of the AAA subfamily which exerts quality control in the inner mitochondrial membrane. Interestingly, the identified mutation localizes close to the AAA domain of AFG3L2, while those localized in the proteolytic domain cause dominant spinocerebellar ataxia type 28 (SCA28) or recessive spastic ataxia with epilepsy (SPAX5). Functional studies in patient fibroblasts demonstrate that the p.G337E AFG3L2 mutation strongly destabilizes the long isoforms of OPA1 via OMA hyper-activation and leads to mitochondrial fragmentation, thus explaining the family phenotype. This study widens the clinical spectrum of neurodegenerative diseases caused by AFG3L2 mutations, which shall be considered as genetic cause of ADOA.

Analysis of Inherited Optic Neuropathies.

Inherited optic neuropathies (IONs) cover a spectrum of clinically and genetically heterogenic conditions. Genetic evaluation of patients with IONs may enable their better clinico-diagnostic assessment and management of the disease. The aim of the present study was to determine the genetic condition related to the phenotype in patients with diverse inherited optic neuropathies. A retrospective study was performed in 12 adults and 8 children of 8 non-related families. Clinical phenotyping, supported by color fundus, FAF, and OCT imaging, was performed. Genetic testing was obtained for all family members suspected for ION. Identification of pathogenic mutations in eight non-related families helped to confirm the diagnosis of ION. Affected from ION were ten patients (eight adults and two children; four women and six men). Bilateral Leber's hereditary optic neuropathy (LHON) was linked to the m.11778G>A mutation in two families (two affected and five carriers). Secondary homoplasmic LHON mutations in MT-ND1 (m.4216T>C) and MT-CO3 genes (m.9804G>A) were confirmed in two families (each one subject, three eyes affected), without detection of a primary LHON mutation. One member presented a picture of right-sited optic neuropathy associated with a c.220C>G mutation in the ACO2 gene and a heterozygous c.185C>T mutation in the LDLR gene. Autosomal dominant optic atrophy was confirmed in three non-related families (five subjects with bilateral ION), where molecular genetic analyses confirmed four different heterozygous mutations in OPA1: c.1847+1G>T; c.2497-1G>A, 297A>G and c.(2983+1_2984-1)_(c.*3211) (2 splicing mutations, 1 missense mutation, and 1 gross deletion encompassing exons 30 and 31). Combining clinics and molecular genetics when evaluating patients with IONs helps in characterizing disease and, therefore, is strongly recommended for such patients.

Not only dominant, not only optic atrophy: expanding the clinical spectrum associated with OPA1 mutations

BackgroundHeterozygous mutations in OPA1 are a common cause of autosomal dominant optic atrophy, sometimes associated with extra-ocular manifestations. Few cases harboring compound heterozygous OPA1 mutations have been described manifesting complex neurodegenerative disorders in addition to optic atrophy.ResultsWe report here three patients: one boy showing an early-onset mitochondrial disorder with hypotonia, ataxia and neuropathy that was severely progressive, leading to early death because of multiorgan failure; two unrelated sporadic girls manifesting a spastic ataxic syndrome associated with peripheral neuropathy and, only in one, optic atrophy. Using a targeted resequencing of 132 genes associated with mitochondrial disorders, in two probands we found compound heterozygous mutations in OPA1: in the first a 5 nucleotide deletion, causing a frameshift and insertion of a premature stop codon (p.Ser64Asnfs*7), and a missense change (p.Ile437Met), which has recently been reported to have clinical impact; in the second, a novel missense change (p.Val988Phe) co-occurred with the p.Ile437Met substitution. In the third patient a homozygous mutation, c.1180G > A (p.Ala394Thr) in OPA1 was detected by a trio-based whole exome sequencing approach. One of the patients presented also variants in mitochondrial DNA that may have contributed to the peculiar phenotype.The deleterious effect of the identified missense changes was experimentally validated in yeast model. OPA1 level was reduced in available patients’ biological samples, and a clearly fragmented mitochondrial network was observed in patients’ fibroblasts.ConclusionsThis report provides evidence that bi-allelic OPA1 mutations may lead to complex and severe multi-system recessive mitochondrial disorders, where optic atrophy might not represent the main feature.

ID 55 – Sensory neuropathy in children presenting with Behr syndrome due to OPA1 mutations

Behr syndrome is a complex genetic disorder occurring in children. It associates optic nerve atrophy and spinocerebellar degeneration (ataxia, pyramidal signs), some patients also presenting with peripheral neuropathy. Transmission is generally autosomal recessive and in some cases is related to mutation in OPA1 , a gene encoding a dynamin-related GTPase. 4 children (age 3–14 y-o) with severe visual loss and balance impairment due to OPA1 gene mutations underwent electrodiagnostic (EDX) examination. They presented with early visual loss, mixed cerebellar and proprioceptive ataxia. Some also had pyramidal signs. Compound heterozygous OPA1 mutations were found in all cases. EDX examination demonstrated a severe sensory neuropathy with absent sensory potentials or dramatic decrease of the amplitude, in contrast with normal motor nerve conduction parameters and needle EMG. To date, compound heterozygosity for OPA1 has been proven in only three patients from two unrelated families and neuropathy was not specifically assessed. This is therefore the first report demonstrating the occurrence of severe sensory neuropathy in such cases. Peripheral neuropathy sometimes represents a useful help in the diagnosis of a neurological or metabolic disease occurring in children. The search for sensory neuropathy in children with optic atrophy can be helpful in orientating the genetic diagnosis.

Early‐onset Behr syndrome due to compound heterozygous mutations in OPA1

SummaryBehr syndrome is an early‐onset and severe syndromic optic atrophy which is probably heterogeneous. Recently, a heterozygous mutation in OPA1 was reported in an adult‐onset Behr‐like syndrome. Heterozygous mutations in OPA1 are the main causes of autosomal dominant optic atrophy (DOA). As many as 20% of patients with DOA exhibit extra‐ocular signs including deafness, external ophthalmoplegia, ataxia, peripheral neuropathy and, myopathy. Aside from these autosomal dominant forms, only few syndromic cases have so far been linked to compound heterozygous OPA1 mutations suggestive of either recessive or semi‐dominant inheritance. However, the clinical spectrum of these emerging double‐mutant OPA1‐related disorders remains to be characterized. We report on four children affected with Behr syndrome associated with compound heterozygous OPA1 mutations. These children were similarly affected with an early‐onset neurological syndrome associating a severe optic atrophy (4/4), cerebellar ataxia (4/4), peripheral neuropathy (4/4), digestive involvement (2/4) and deafness (1/4). This report confirms the importance of searching an OPA1 compound heterozygosity in paediatric cases of syndromic optic neuropathy.

Reply: Early-onset Behr syndrome due to compound heterozygous mutations in OPA1

Reply: Early-onset Behr syndrome due to compound heterozygous mutations in OPA1

Defective mitochondrial fusion, altered respiratory function, and distorted cristae structure in skin fibroblasts with heterozygous OPA1 mutations

Deleterious consequences of heterozygous OPA1 mutations responsible for autosomal dominant optic atrophy remain a matter of debate. Primary skin fibroblasts derived from patients have shown diverse mitochondrial alterations that were however difficult to resolve in a unifying scheme. To address the potential use of these cells as disease model, we undertook parallel and quantitative analyses of the diverse reported alterations in four fibroblast lines harboring different OPA1 mutations, nonsense or missense, in the guanosine triphosphatase or the C-terminal coiled-coil domains. We tackled several factors potentially underlying discordant reports and showed that fibroblasts with heterozygous OPA1 mutations present with several mitochondrial alterations. These included defective mitochondrial fusion during pharmacological challenge with the protonophore carbonyl cyanide m‐chlorophenyl hydrazone, significant mitochondrial elongation with decreased OPA1 and DRP1 proteins, and abnormal mitochondrial fragmentation during glycolysis shortage or exogenous oxidative stress. Respiratory complex IV activity and subunits steady-state were decreased without alteration of the mitochondrial deoxyribonucleic acid size, amount or transcription. Physical link between OPA1 protein and oxidative phosphorylation was shown by reciprocal immunoprecipitation. Altered cristae structure coexisted with normal response to pro-apoptotic stimuli and expression of Bax or Bcl2 proteins. Skin fibroblasts with heterozygous OPA1 mutations thus share significant mitochondrial remodeling, and may therefore be useful for analyzing disease pathophysiology. Identifying whether the observed alterations are also present in ganglion retinal cells, and which of them underlies their degeneration process remains however an essential goal for therapeutic strategy.

Reply: Heterozygous OPA1 mutations in Behr syndrome

Sir, We were very interested to read the fascinating clinical descriptions of two brothers who presented with childhood-onset optic atrophy and subsequently developed progressive spastic paraparesis and mild ataxia (Marelli et al ., 2010). We agree that the heterozygous c.1652G>A (p.C551Y) missense mutation in exon 17 of OPA1 is likely to be the cause of their neurological disorder, consistent with a dominant optic atrophy ‘plus’ (DOA+) phenotype. This mutation was not identified in the brothers’ mother; and the father, who is deceased, was not thought to be visually or neurologically affected. De novo OPA1 mutations are rare, accounting for <4% of all reported cases (Cohn et al ., 2007). It is therefore much more likely that the father was either non-penetrant, a phenomenon observed in up to 20% of OPA1 carriers, or visually asymptomatic, …

Heterozygous OPA1 mutations in Behr syndrome

ARTICLE Sir, We read with interest the paper by Yu-Wai-Man et al. (2010) on patients with dominant optic atrophy-plus with OPA1 mutations, which follows on previous reports by the same group (Amati-Bonneau et al. , 2008; Hudson et al. , 2008), reporting the relatively high frequency (∼20%) and the large spectrum of extra-ocular neurological involvement in OPA1 disease. Here, we extend the clinical spectrum of OPA1 mutations to Behr syndrome, characterized by early-onset optic atrophy associated with pyramidal signs, ataxia and sometimes, posterior column sensory loss and mental retardation (Behr 1909; Thomas et al. , 1984). This clinical entity is probably a heterogeneous group of disorders, whose real causes are mostly unknown. In most cases, autosomal recessive inheritance has been suspected. A few patients with typical Behr syndrome and several with Costeff syndrome, a Behr syndrome-like disease associating optic atrophy with extrapyramidal signs, were shown to have type III 3-methylglutaconic aciduria (MGA type III), a recessive disease characterized …

Fussy mitochondria fuse in response to stress

Fussy mitochondria fuse in response to stress