Triple-A Syndrome in Morocco: Founder Effect, Age Estimation of the AAAS c.1331+1G>A Variant, and Implications for Genetic Diagnosis.

Introduction: Triple-A syndrome (Triple-A) is an autosomal recessive disorder characterized by alacrimia, achalasia, and adrenal insufficiency. Several variants on the AAAS gene have been described, and some variants are clustered in particular geographical areas, such as the c.1331+1G>A variant which is very frequent in North Africa. Here, we describe the genetic features of Triple-A in a series of unrelated families from Morocco. Methods: Screening for the AAAS c.1331+1G>A variant was performed by direct sequencing or by PCR-RFLP. Haplotype analysis using Single Tandem Repeat (STR) markers flanking AAAS gene was performed in order to evaluate the founder effect and estimate the age of the c.1331+1G>A variant. Results: Seven unrelated families with ten individuals clinically diagnosed with Triple-A were evaluated for sequence variations in the AAAS gene. The median age at diagnosis was 3 years, with a range between 2 and 11 years. Molecular analysis revealed that all patients were homozygous for the c.1331+1G>A variant. This variant was not found in 200 healthy controls, indicating that carriers are very rare in the general Moroccan population. Subsequently, STR marker analysis revealed a founder effect and that the most recent common ancestor of Triple-A patients in Morocco would have lived 125 years ago. Conclusion: This is the largest series of Triple-A in Morocco. The same AAAS c.1331+1G>A variant was found in all patients, suggesting a founder effect in Morocco which was subsequently confirmed by microsatellite marker analysis. Therefore, this variant should be systematically investigated to diagnose Triple-A in Morocco.

The human OPA1delTTAG mutation induces adult onset and progressive auditory neuropathy in mice.

Dominant optic atrophy (DOA) is one of the most prevalent forms of hereditary optic neuropathies and is mainly caused by heterozygous variants in OPA1, encoding a mitochondrial dynamin-related large GTPase. The clinical spectrum of DOA has been extended to a wide variety of syndromic presentations, called DOAplus, including deafness as the main secondary symptom associated to vision impairment. To date, the pathophysiological mechanisms underlying the deafness in DOA remain unknown. To gain insights into the process leading to hearing impairment, we have analyzed the Opa1delTTAG mouse model that recapitulates the DOAplus syndrome through complementary approaches combining morpho-physiology, biochemistry, and cellular and molecular biology. We found that Opa1delTTAG mutation leads an adult-onset progressive auditory neuropathy in mice, as attested by the auditory brainstem response threshold shift over time. However, the mutant mice harbored larger otoacoustic emissions in comparison to wild-type littermates, whereas the endocochlear potential, which is a proxy for the functional state of the stria vascularis, was comparable between both genotypes. Ultrastructural examination of the mutant mice revealed a selective loss of sensory inner hair cells, together with a progressive degeneration of the axons and myelin sheaths of the afferent terminals of the spiral ganglion neurons, supporting an auditory neuropathy spectrum disorder (ANSD). Molecular assessment of cochlea demonstrated a reduction of Opa1 mRNA level by greater than 40%, supporting haploinsufficiency as the disease mechanism. In addition, we evidenced an early increase in Sirtuin 3 level and in Beclin1 activity, and subsequently an age-related mtDNA depletion, increased oxidative stress, mitophagy as well as an impaired autophagic flux. Together, these results support a novel role for OPA1 in the maintenance of inner hair cells and auditory neural structures, addressing new challenges for the exploration and treatment of OPA1-linked ANSD in patients.

First report on chromosomal abnormalities in Eastern Morocco: Identification of a new case of a de novo partial trisomy 13q using single-nucleotide polymorphism array.

BACKGROUND: Chromosomal abnormalities are the main cause of birth defects, intellectual disability, and miscarriages. They contribute to significant human morbidity and infant mortality. Here we report for the first time the chromosomal abnormalities encountered in the population of Eastern Morocco. Furthermore, we describe a new case of a de novo partial trisomy 13q combined with a terminal deletion in an 11-day-old girl. METHODS: From November 2015 to March 2022, 195 patients from the BRO Biobank who were clinically suspected of having chromosomal abnormalities were referred to the cytogenetics laboratory of the Genetics Unit of the Faculty of Medicine and Pharmacy of Oujda for cytogenetic study. Karyotyping analysis was performed on peripheral blood samples using standard R banding techniques. To identify single-nucleotide polymorphism (SNP) and copy number variants (CNVs), Illumina SNP array was used. RESULTS: Among 195 studied cases, 32 (16.4 %) had abnormal karyotypes, of which 12 cases had numerical aberrations while 20 cases had structural aberrations. The most common numerical aberrations were Turner syndrome and Down syndrome followed by Edward, Patau, and Klinefelter syndromes. For structural aberrations, translocations were the most common, followed by derivative chromosomes, inversions, deletions, and an addition on chromosome 13 identified in an 11-day-old girl. To further characterize this addition, SNP array was carried out and revealed a 58.8-Mb duplication in region 13q14.3q34 associated with a 1-Mb deletion in region 13q34. Follow-up parental chromosomes analysis showed normal karyotypes for the parents, confirming that this partial trisomy 13q was de novo. Comparison of the phenotype associated with this novel duplication on chromosome 13q with those previously reported confirmed the considerable variability in the phenotype of the patients with partial trisomy 13q. CONCLUSION: This study provided the first report on chromosomal abnormalities in Eastern Morocco and it enriched the phenotype spectrum of partial trisomy 13q and further confirmed the genotype-phenotype correlations. Furthermore, these findings justify the need to set up microarray comparative genomic hybridization techniques in Morocco for better genetic diagnosis.

Vitamin B1 deficiency leads to high oxidative stress and mtDNA depletion caused by SLC19A3 mutation in consanguineous family with Leigh syndrome.

Leigh syndrome (LS) and Leigh-like spectrum are the most common infantile mitochondrial disorders characterized by heterogeneous neurologic and metabolic manifestations. Pathogenic variants in SLC carriers are frequently reported in LS given their important role in transporting various solutes across the blood-brain barrier. SLC19A3 (THTR2) is one of these carriers transporting vitamin-B1 (vitB1, thiamine) into the cell. Targeted NGS of nuclear genes involved in mitochondrial diseases was performed in a patient belonging to a consanguineous Tunisian family with LS and revealed a homozygous c.1264 A > G (p.T422A) variant in SLC19A3. Molecular docking revealed that the p.T422A aa change is located at a key position interacting with vitB1 and causes conformational changes compromising vitB1 import. We further disclosed decreased plasma antioxidant activities of CAT, SOD and GSH enzymes, and a 42% decrease of the mtDNA copy number in patient blood.Altogether, our results disclose that the c.1264 A > G (p.T422A) variant in SLC19A3 affects vitB1 transport, induces a mtDNA depletion and reduces the expression level of oxidative stress enzymes, altogether contributing to the LS phenotype of the patient.

A novel variant in BMPR1B causes acromesomelic dysplasia Grebe type in a consanguineous Moroccan family: Expanding the phenotypic and mutational spectrum of acromesomelic dysplasias.

Acromesomelic dysplasia Grebe type (AMD Grebe type) is an autosomal recessive trait characterized by short stature, shortened limbs and malformations of the hands and feet. It is caused by variants in the growth differentiation factor 5 (GDF5) or, in rare cases, its receptor, the bone morphogenetic protein receptor-1B (BMPR1B). Here, we report a novel homozygous BMPR1B variant causing AMD Grebe type in a consanguineous Moroccan family with two affected sibs from BRO Biobank. Remarkably, the affected individuals showed additional features including bilateral simian creases, lumbar hyperlordosis, as well as lower limb length inequality and dislocated hips in one of them, which were never reported previously for AMD Grebe type patients. The identified novel BMPR1B variant (c.1201C>T, p.R401*) is predicted to result in loss of function of the BMPR1B protein either by nonsense-mediated mRNA decay or production of a truncated BMPR1B protein. Thus, these findings expand the phenotypic and mutational spectrum of AMD, and may improve the diagnosis of AMD and enable appropriate genetic counselling to be offered to patients.

Genotype-phenotype correlations in RHOBTB2-associated neurodevelopmental disorders.

PURPOSE: Missense variants clustering in the BTB domain region of RHOBTB2 cause a developmental and epileptic encephalopathy with early-onset seizures and severe intellectual disability. METHODS: By international collaboration, we assembled individuals with pathogenic RHOBTB2 variants and a variable spectrum of neurodevelopmental disorders. By western blotting, we investigated the consequences of missense variants in vitro. RESULTS: In accordance with previous observations, de novo heterozygous missense variants in the BTB domain region led to a severe developmental and epileptic encephalopathy in 16 individuals. Now, we also identified de novo missense variants in the GTPase domain in 6 individuals with apparently more variable neurodevelopmental phenotypes with or without epilepsy. In contrast to variants in the BTB domain region, variants in the GTPase domain do not impair proteasomal degradation of RHOBTB2 in vitro, indicating different functional consequences. Furthermore, we observed biallelic splice-site and truncating variants in 9 families with variable neurodevelopmental phenotypes, indicating that complete loss of RHOBTB2 is pathogenic as well. CONCLUSION: By identifying genotype-phenotype correlations regarding location and consequences of de novo missense variants in RHOBTB2 and by identifying biallelic truncating variants, we further delineate and expand the molecular and clinical spectrum of RHOBTB2-related phenotypes, including both autosomal dominant and recessive neurodevelopmental disorders.

Autosomal recessive Leber hereditary optic neuropathy, a new neuro-ophthalmo-genetic paradigm.

Leber hereditary optic neuropathy (LHON) is a primary inherited neurodegenerative disorder of the optic nerve. It has been ascribed to variants in the mitochondrial genome, mainly the m.3460G>A, m.11778G>A and m.14484T>C mutations in ND1, ND4 and ND6, respectively. Nonetheless, inconclusive molecular diagnosis is not uncommon. Recently, biallelic mutations in the NDUFS2, DNAJC30, MCAT and NDUFA12 nuclear genes have been identified in unresolved LHON cases, identifying an autosomal recessive LHON (arLHON, OMIM:619382). The clinical presentation of arLHON copies that of typical LHON due to mtDNA mutations (mtLHON), with an acute phase of sudden and severe vision loss, telangiectatic and tortuous vessels around the optic nerve and swelling of the retinal nerve fibre layer. This is followed by a chronic phase of retinal nerve fibre layer loss, but eventually affected individuals recover partial or full visual acuity. Idebenone treatment significantly improved vision recovery in DNAJC30-associated patients. As for mtLHON, arLHON predominantly affected male compared with female carriers. The discovery of arLHON cases breaks with the dogma of exclusive maternal inheritance. It defines a new neuro-ophthalmo-genetic paradigm, which should be considered in individuals manifesting a LHON phenotype but with an inconclusive molecular diagnosis. NDUFS2, DNAJC30, MCAT and NDUFA12 should be investigated in these individuals, knowing that other arLHON genes might exist.

The top 10 most frequently involved genes in hereditary optic neuropathies in 2186 probands.

Hereditary optic neuropathies are caused by the degeneration of retinal ganglion cells whose axons form the optic nerves, with a consistent genetic heterogeneity. As part of our diagnostic activity, we retrospectively evaluated the combination of Leber hereditary optic neuropathy mutations testing with the exon sequencing of 87 nuclear genes on 2186 patients referred for suspected hereditary optic neuropathies. The positive diagnosis rate in individuals referred for Leber hereditary optic neuropathy testing was 18% (199/1126 index cases), with 92% (184/199) carrying one of the three main pathogenic variants of mitochondrial DNA (m.11778G>A, 66.5%; m.3460G>A, 15% and m.14484T>C, 11%). The positive diagnosis rate in individuals referred for autosomal dominant or recessive optic neuropathies was 27% (451/1680 index cases), with 10 genes accounting together for 96% of this cohort. This represents an overall positive diagnostic rate of 30%. The identified top 10 nuclear genes included OPA1, WFS1, ACO2, SPG7, MFN2, AFG3L2, RTN4IP1, TMEM126A, NR2F1 and FDXR. Eleven additional genes, each accounting for less than 1% of cases, were identified in 17 individuals. Our results show that 10 major genes account for more than 96% of the cases diagnosed with our nuclear gene panel.

Expanding the genetic spectrum of mitochondrial diseases in Tunisia: novel variants revealed by whole-exome sequencing.

Introduction: Inherited mitochondrial diseases are the most common group of metabolic disorders caused by a defect in oxidative phosphorylation. They are characterized by a wide clinical and genetic spectrum and can manifest at any age. In this study, we established novel phenotype-genotype correlations between the clinical and molecular features of a cohort of Tunisian patients with mitochondrial diseases. Materials and methods: Whole-exome sequencing was performed on five Tunisian patients with suspected mitochondrial diseases. Then, a combination of filtering and bioinformatics prediction tools was utilized to assess the pathogenicity of genetic variations. Sanger sequencing was subsequently performed to confirm the presence of potential deleterious variants in the patients and verify their segregation within families. Structural modeling was conducted to study the effect of novel variants on the protein structure. Results: We identified two novel homozygous variants in NDUFAF5 (c.827G>C; p.Arg276Pro) and FASTKD2 (c.496_497del; p.Leu166GlufsTer2) associated with a severe clinical form of Leigh and Leigh-like syndromes, respectively. Our results further disclosed two variants unreported in North Africa, in GFM2 (c.569G>A; p.Arg190Gln) and FOXRED1 (c.1261G>A; p.Val421Met) genes, and we described the first case of fumaric aciduria in a Tunisian patient harboring the c.1358T>C; p.Leu453Pro FH variant. Conclusion: Our study expands the mutational and phenotypic spectrum of mitochondrial diseases in Tunisia and highlights the importance of next-generation sequencing to decipher the pathomolecular mechanisms responsible for these disorders in an admixed population.

Next-generation sequencing of Tunisian Leigh syndrome patients reveals novel variations: impact for diagnosis and treatment.

Mitochondrial cytopathies, among which the Leigh syndrome (LS), are caused by variants either in the mitochondrial or the nuclear genome, affecting the oxidative phosphorylation process. The aim of the present study consisted in defining the molecular diagnosis of a group of Tunisian patients with LS. Six children, belonging to five Tunisian families, with clinical and imaging presentations suggestive of LS were recruited. Whole mitochondrial DNA and targeted next-generation sequencing of a panel of 281 nuclear genes involved in mitochondrial physiology were performed. Bioinformatic analyses were achieved in order to identify deleterious variations. A single m.10197G>A (p.Ala47Thr) variant was found in the mitochondrial MT-ND3 gene in one patient, while the others were related to autosomal homozygous variants: two c.1412delA (p.Gln471ArgfsTer42) and c.1264A>G (p.Thr422Ala) in SLC19A3, one c.454C>G (p.Pro152Ala) in SLC25A19 and one c.122G>A (p.Gly41Asp) in ETHE1. Our findings demonstrate the usefulness of genomic investigations to improve LS diagnosis in consanguineous populations and further allow for treating the patients harboring variants in SLC19A3 and SLC25A19 that contribute to thiamine transport, by thiamine and biotin supplementation. Considering the Tunisian genetic background, the newly identified variants could be screened in patients with similar clinical presentation in related populations.

First characterization of LTBP3 variants in two Moroccan families with hypoplastic amelogenesis imperfecta.

OBJECTIVES: To decipher and improve the molecular diagnosis of Hypoplastic Amelogenesis Imperfecta in Morocco. DESIGN: Using whole exome sequencing, we analyzed two Moroccan families with Hypoplastic Amelogenesis Imperfecta. The 2 patients from the first family had dental anomalies and short stature syndrome, brachyolmia and nephrocalcinosis with difference in severity, while the proband of the second family had Hypoplastic Amelogenesis Imperfecta with a suspicion of brachyolmia. RESULTS: We identified two novel LTBP3 homozygous variants, the c.2495delT deletion (p.Phe832SerfsTer36) and the c.3716 G>A (p.Cys1239Tyr) missense variant, respectively. Molecular modelling and stability analyses of the missense variant disclosed a possible destabilization of the wild-type structure. CONCLUSION: Although LTBP3 variants were related to this phenotype in various populations, we report the first LTBP3 variants in the Moroccan population, in families with Hypoplastic Amelogenesis Imperfecta.

Next-Generation Sequencing Identifies Novel PMPCA Variants in Patients with Late-Onset Dominant Optic Atrophy.

Dominant Optic Atrophy (DOA) is one of the most common inherited mitochondrial diseases, leading to blindness. It is caused by the chronic degeneration of the retinal ganglion cells (RGCs) and their axons forming the optic nerve. Until now, DOA has been mainly associated with genes encoding proteins involved in mitochondrial network dynamics. Using next-generation and exome sequencing, we identified for the first time heterozygous PMPCA variants having a causative role in the pathology of late-onset primary DOA in five patients. PMPCA encodes an α subunit of the mitochondrial peptidase (MPP), responsible for the cleavage and maturation of the mitochondrial precursor proteins imported from the cytoplasm into mitochondria. Recently, PMPCA has been identified as the gene responsible for Autosomal Recessive Cerebellar Ataxia type 2 (SCAR2) and another severe recessive mitochondrial disease. In this study, four PMPCA variants were identified, two are frameshifts (c.309delA and c.820delG) classified as pathogenic and two are missenses (c.1363G>A and c.1547G>A) classified with uncertain pathological significance. Functional assays on patients' fibroblasts show a hyperconnection of the mitochondrial network and revealed that frameshift variants reduced α-MPP levels, while not significantly affecting the respiratory machinery. These results suggest that alterations in mitochondrial peptidase function can affect the fusion-fission balance, a key element in maintaining the physiology of retinal ganglion cells, and consequently lead to their progressive degeneration.

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.

Molecular heterogeneity of ß-thalassemia variants in the Eastern region of Morocco.

BACKGROUND: ß-thalassemia syndromes are the most common hereditary blood disorders in the world and are recognized as a major health problem in Morocco. They are characterized by the reduction or the absence of ß-globin chain synthesis. The severity of the disease depends on the nature of the variants affecting the ß-globin gene (HBB), and each ethnic group has its own mutation spectrum. Hereby, we present, for the first time, the molecular profile of ß-thalassemia in the Eastern region of Morocco. METHODS: This study concerns 39 cases from 33 families who were enrolled in the BRO Biobank. Nineteen were diagnosed with ß-thalassemia major and 20 with ß-thalassemia minor. To detect mutations of the ß-globin gene, we have used RFLP-PCR and Sanger sequencing. RESULTS: Nine known ß-thalassemia variants have been identified. Among these, we reported, for the first time in the Moroccan population, the Czechoslovakian variant C38/39(-C) at homozygous state. The C39(C > T) was the most frequent variant (72.54%), followed by FSC5(-CT) (5.88%), FSC6(-A), IVS-1-110(G > A), -29(A > G), C38/39(-C) (3.92% each), and finally by IVS-I-1(G > A), IVS-II-1(G > A), and -56(G > C) (1.96%). Of particular interest this mutational spectrum of ß-thalassemia is very different from that found in previous studies in Morocco or in other North African countries. CONCLUSION: This study is the first contribution to the description of the molecular profile of ß-thalassemia in the Eastern region of Morocco. It shows the high molecular heterogeneity of ß-thalassemia in our country. Therefore, these results can be valuable for the implementation of carrier screening, genetic counseling, and prenatal diagnosis programs.

A homozygous nonsense HECW2 variant is associated with neurodevelopmental delay and intellectual disability.

Intellectual disability is characterized by a significant impaired intellectual and adaptive functioning, affecting approximately 1-3% of the population, which can be caused by a variety of environmental and genetic factors. In this respect, de novo heterozygous HECW2 variants were associated recently with neurodevelopmental disorders associated to hypotonia, seizures, and absent language. HECW2 encodes an E3 ubiquitin-protein ligase that stabilizes and enhances transcriptional activity of p73, a key factor regulating proliferation, apoptosis, and neuronal differentiation, which are together essential for proper brain development. Here, using whole exome sequencing, we identified a homozygous nonsense HECW2 variant: c.736C > T; p.Arg246* in a proband from a Moroccan consanguineous family, with developmental delay, intellectual disability, hypotonia, generalized tonico-clonic seizures and a persistent tilted head. Thus this study describes the first homozygous HECW2 variant, inherited as an autosomal recessive pattern, contrasting with former reported de novo variants found in HECW2 patients.

Cancer/Testis Antigen 55 is required for cancer cell proliferation and mitochondrial DNA maintenance.

Cancer/Testis Antigens (CTAs) represent a group of proteins whose expression under physiological conditions is restricted to testis but activated in many human cancers. Also, it was observed that co-expression of multiple CTAs worsens the patient prognosis. Five CTAs were reported acting in mitochondria and we recently reported 147 transcripts encoded by 67 CTAs encoding for proteins potentially targeted to mitochondria. Among them, we identified the two isoforms encoded by CT55 for whom the function is poorly understood. First, we found that patients with tumors expressing wild-type CT55 are associated with poor survival. Moreover, CT55 silencing decreases dramatically cell proliferation. Second, to investigate the role of CT55 on mitochondria, we first show that CT55 is localized to both mitochondria and endoplasmic reticulum (ER) due to the presence of an ambiguous N-terminal targeting signal. Then, we show that CT55 silencing decreases mtDNA copy number and delays mtDNA recovery after an acute depletion. Moreover, demethylation of CT55 promotor increases its expression, which in turn increases mtDNA copy number. Finally, we measured the mtDNA copy number in NCI-60 cell lines and screened for genes whose expression is strongly correlated to mtDNA amount. We identified CT55 as the second highest correlated hit. Also, we show that compared to siRNA scrambled control (siCtrl) treatment, CT55 specific siRNA (siCT55) treatment down-regulates aerobic respiration, indicating that CT55 sustains mitochondrial respiration. Altogether, these data show for first time that CT55 acts on mtDNA copy number, modulates mitochondrial activity to sustain cancer cell proliferation.

Pathogenic NR2F1 variants cause a developmental ocular phenotype recapitulated in a mutant mouse model.

Pathogenic NR2F1 variants cause a rare autosomal dominant neurodevelopmental disorder referred to as the Bosch-Boonstra-Schaaf Optic Atrophy Syndrome. Although visual loss is a prominent feature seen in affected individuals, the molecular and cellular mechanisms contributing to visual impairment are still poorly characterized. We conducted a deep phenotyping study on a cohort of 22 individuals carrying pathogenic NR2F1 variants to document the neurodevelopmental and ophthalmological manifestations, in particular the structural and functional changes within the retina and the optic nerve, which have not been detailed previously. The visual impairment became apparent in early childhood with small and/or tilted hypoplastic optic nerves observed in 10 cases. High-resolution optical coherence tomography imaging confirmed significant loss of retinal ganglion cells with thinning of the ganglion cell layer, consistent with electrophysiological evidence of retinal ganglion cells dysfunction. Interestingly, for those individuals with available longitudinal ophthalmological data, there was no significant deterioration in visual function during the period of follow-up. Diffusion tensor imaging tractography studies showed defective connections and disorganization of the extracortical visual pathways. To further investigate how pathogenic NR2F1 variants impact on retinal and optic nerve development, we took advantage of an Nr2f1 mutant mouse disease model. Abnormal retinogenesis in early stages of development was observed in Nr2f1 mutant mice with decreased retinal ganglion cell density and disruption of retinal ganglion cell axonal guidance from the neural retina into the optic stalk, accounting for the development of optic nerve hypoplasia. The mutant mice showed significantly reduced visual acuity based on electrophysiological parameters with marked conduction delay and decreased amplitude of the recordings in the superficial layers of the visual cortex. The clinical observations in our study cohort, supported by the mouse data, suggest an early neurodevelopmental origin for the retinal and optic nerve head defects caused by NR2F1 pathogenic variants, resulting in congenital vision loss that seems to be non-progressive. We propose NR2F1 as a major gene that orchestrates early retinal and optic nerve head development, playing a key role in the maturation of the visual system.

Characterization of SSBP1-related optic atrophy and foveopathy.

Dominant optic atrophy (DOA) is genetically heterogeneous and most commonly caused by mutations in OPA1. To distinguish between the classical OPA1-related and the recently identified SSBP1-related DOAs, the retina and fovea of 27 patients carrying the SSBP1 p.Arg38Gln variant were scrutinized using 20° × 20° macular cube and 30° and 55° field fundus autofluorescence photographs. Age of onset, visual acuity, retinal nerve fiber layer and macular thicknesses were recorded. Three SSBP1-patients were asymptomatic, 10 had isolated DOA, and 12 had a combined DOA plus foveopathy. The foveopathy, with a tiny defect of the ellipsoid and interdigitation lines, was similar in all patients, independent of age. There were no significant statistical differences in terms of visual acuity and SD-OCT measurements between patients with isolated DOA (mean visual acuity in decimals: 0.54 ± 0.41) and those with combined foveopathy (0.50 ± 0.23). Two patients over 50 years of age developed a progressive rod-cone dystrophy, leading to severe visual impairment. SSBP1-related DOA shares similarities with OPA1-related DOA with an incomplete penetrance and an early childhood visual impairment. Nevertheless, the presence of a congenital foveopathy with no impact on visual acuity is a major criterion to distinguish SSBP1 cases and orient the appropriate genetic analysis.

ACO2 clinicobiological dataset with extensive phenotype ontology annotation.

Pathogenic variants of the aconitase 2 gene (ACO2) are responsible for a broad clinical spectrum involving optic nerve degeneration, ranging from isolated optic neuropathy with recessive or dominant inheritance, to complex neurodegenerative syndromes with recessive transmission. We created the first public locus-specific database (LSDB) dedicated to ACO2 within the "Global Variome shared LOVD" using exclusively the Human Phenotype Ontology (HPO), a standard vocabulary for describing phenotypic abnormalities. All the variants and clinical cases listed in the literature were incorporated into the database, from which we produced a dataset. We followed a rational and comprehensive approach based on the HPO thesaurus, demonstrating that ACO2 patients should not be classified separately between isolated and syndromic cases. Our data highlight that certain syndromic patients do not have optic neuropathy and provide support for the classification of the recurrent pathogenic variants c.220C>G and c.336C>G as likely pathogenic. Overall, our data records demonstrate that the clinical spectrum of ACO2 should be considered as a continuum of symptoms and refines the classification of some common variants.

Dominant ACO2 mutations are a frequent cause of isolated optic atrophy.

Biallelic mutations in ACO2, encoding the mitochondrial aconitase 2, have been identified in individuals with neurodegenerative syndromes, including infantile cerebellar retinal degeneration and recessive optic neuropathies (locus OPA9). By screening European cohorts of individuals with genetically unsolved inherited optic neuropathies, we identified 61 cases harbouring variants in ACO2, among whom 50 carried dominant mutations, emphasizing for the first time the important contribution of ACO2 monoallelic pathogenic variants to dominant optic atrophy. Analysis of the ophthalmological and clinical data revealed that recessive cases are affected more severely than dominant cases, while not significantly earlier. In addition, 27% of the recessive cases and 11% of the dominant cases manifested with extraocular features in addition to optic atrophy. In silico analyses of ACO2 variants predicted their deleterious impacts on ACO2 biophysical properties. Skin derived fibroblasts from patients harbouring dominant and recessive ACO2 mutations revealed a reduction of ACO2 abundance and enzymatic activity, and the impairment of the mitochondrial respiration using citrate and pyruvate as substrates, while the addition of other Krebs cycle intermediates restored a normal respiration, suggesting a possible short-cut adaptation of the tricarboxylic citric acid cycle. Analysis of the mitochondrial genome abundance disclosed a significant reduction of the mitochondrial DNA amount in all ACO2 fibroblasts. Overall, our data position ACO2 as the third most frequently mutated gene in autosomal inherited optic neuropathies, after OPA1 and WFS1, and emphasize the crucial involvement of the first steps of the Krebs cycle in the maintenance and survival of retinal ganglion cells.