Mitochondrial m.13513G>A Point Mutation in ND5 in a 16-Year-Old Man with Leber Hereditary Optic Neuropathy Detected by Next-Generation Sequencing

Exploring the Phenotypic Heterogeneity and Bioenergetic Profile of the m.13513G>A mtDNA Substitution: A Heteroplasmy Perspective

The m.13513G>A (p.Asp393Asn) substitution in the MT-ND5 (Mitochondrially Encoded NADH/Ubiquinone Oxidoreductase Core Subunit 5) gene is a common pathogenic variant associated with primary mitochondrial disorders. It frequently causes Leigh syndrome and mitochondrial encephalomyopathy with lactate acidosis and stroke-like episodes (MELAS). In this study, we present clinical data, heteroplasmy levels in various tissues (blood, urine, and skin fibroblasts), and bioenergetic characteristics from a cohort of 20 unrelated patients carrying the m.13513G>A mutation, classified according to the following phenotypes: Leigh syndrome (n = 12), MELAS (n = 2), and Leber's hereditary optic neuropathy (LHON, n = 6). We observed a significant correlation between high respiratory ratios and heteroplasmy levels in fibroblast cell lines of the patients. Furthermore, fibroblast cell lines with heteroplasmy levels exceeding 55% exhibited markedly reduced mitochondrial membrane potential. These findings contribute to a better understanding of the clinical and bioenergetic profiles of patients with m.13513G>A-variant-related phenotypes across different heteroplasmy levels, based on data from a single genetic center. Our data suggest that even a slight shift in heteroplasmy can improve cellular function and, consequently, the patients' phenotype, providing a solid foundation for the development of future gene therapies for mtDNA diseases.

Phenotypic Heterogeneity of the Mitochondrial DNA Variant m.13513 G > A

The mitochondrial DNA (mtDNA) variant m.13513G > A is increasingly recognized as a cause of syndromic and nonsyndromic mitochondrial disorders (MIDs). This minireview aims a summarizing and discussing recent and previous findings about the phenotypic heterogeneity of this variant. A systematic literature review using the databases PubMed and Google Scholar by application of specific search terms was performed. As per the end of July 2021, at least 50 patients carrying the mtDNA variant m.13513G > A have been reported. Age ranged between 0 and 63 years, and of these patients, 28 were male and 22 were female. The phenotype was highly variable. The most common phenotypes were Leigh syndrome (LS; n  = 25), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS; n  = 11), Leigh-like syndrome ( n  = 4), MELAS/LS ( n  = 3), progressive external ophthalmoplegia ( n  = 3), MELAS/Leber's hereditary optic neuropathy (LHON; n  = 1), and LHON ( n  = 1). More rarely are nonsyndromic phenotypes. Heteroplasmy rates were highly variable ranging from 0 to 86% depending on the investigated tissue. The outcome was reported in only a few cases but was worse in patients with LS compared with those with MELAS. The variant m.13513G > A is responsible for syndromic or nonsyndromic MIDs. Syndromic MIDs in which this variant should be particularly considered include LS, MELAS, and LHON and their overlaps. Patients with suspected MID and maternal inheritance should undergo sequencing of the entire mtDNA not to miss rare mtDNA variants.

Impact of the m.13513G>A Variant on the Functions of the OXPHOS System and Cell Retrograde Signaling

Mitochondria are involved in many vital functions in living cells, including the synthesis of ATP by oxidative phosphorylation (OXPHOS) and regulation of nuclear gene expression through retrograde signaling. Leigh syndrome is a heterogeneous neurological disorder resulting from an isolated complex I deficiency that causes damage to mitochondrial energy production. The pathogenic mitochondrial DNA (mtDNA) variant m.13513G>A has been associated with Leigh syndrome. The present study investigated the effects of this mtDNA variant on the OXPHOS system and cell retrograde signaling. Transmitochondrial cytoplasmic hybrid (cybrid) cell lines harboring 50% and 70% of the m.13513G>A variant were generated and tested along with wild-type (WT) cells. The functionality of the OXPHOS system was evaluated by spectrophotometric assessment of enzyme activity and high-resolution respirometry. Nuclear gene expression was investigated by RNA sequencing and droplet digital PCR. Increasing levels of heteroplasmy were associated with reduced OXPHOS system complex I, IV, and I + III activities, and high-resolution respirometry also showed a complex I defect. Profound changes in transcription levels of nuclear genes were observed in the cell lines harboring the pathogenic mtDNA variant, indicating the physiological processes associated with defective mitochondria.

Leber Mitochondrial Optic Neuropathy in Pediatric Females With Focus on Very Early Onset Cases

The aim of this study was to describe the phenotype of Leber hereditary optic neuropathy occurring in pediatric females. This disease generally affects young adult males, but it can occur also in females, and research data in this population is lacking. The very early onset can challenge the diagnosis and delay treatment. We searched PubMed through February 2021 and identified 226 pediatric females with genetically confirmed Leber hereditary optic neuropathy and added a new case of a 3-year-old female. The male-female ratio was 1.8:1; the mean onset age in females was 11 years with the onset at 3 years of age occurring in 3 females only. Acute onset with mild visual impairment was the most common presentation, associated with optic disc edema in 16%. Differential diagnoses are pseudotumor cerebri, optic nerve drusen and optic neuritis. The outcome is poor with partial recovery in 50%, despite some receiving Idebenone therapy.

Mitochondrial diseases mimicking autoimmune diseases of the CNS and good response to steroids initially

INTRODUCTION

Neuroimmunological diseases such as autoimmune encephalitis (AE) or acquired demyelinating syndromes (ADS), can present with neurological symptoms and imaging features that are indistinguishable from mitochondrial diseases (MD) in particular at initial presentation.

METHODS

Retrospective analysis of the clinical, laboratory and neuroimaging features of five patients who presented with signs of a neuroimmunological disease but all had pathological pathogenic variants in genes related to mitochondrial energy metabolism.

RESULTS

Four patients presented with an acute neurological episode reminiscent of a possible AE and one patient with a suspected ADS at initial presentation. MRI findings were compatible with neuroimmunological diseases in all patients. In two children cerebrospinal fluid (CSF) studies revealed a mildly elevated cell count, two had elevated CSF lactate, none had oligoclonal bands (OCBs). All patients improved rapidly with intravenous steroids or immunoglobulins. Four patients had one or more relapses. Three patients showed worsening of their neurological symptoms with subsequent episodes and one patient died. Relapses in conjunction with new and progressive neurological symptoms, led to additional work-up which finally resulted in different genetic diagnosis of MD in all patients (MT-TL1, MT-ND5, APOA1-BP, HPDL, POLG).

DISCUSSION

We would like to draw attention to a subset of patients with MD initially presenting with signs and symptoms mimicking neuroimmunological. Absence of CSF pleocytosis, elevated CSF lactate and progressive, relapsing course should trigger further (genetic) investigations in search of a MD even in patients with good response initially to immunomodulating therapies.

Mitochondrial Encephalopathy, Lactic Acidosis and Stroke-Like Episodes/Leigh Overlap Syndrome Due to Variant m.13513G>A in MT-ND5

Mitochondrial disorders are caused due to variants in genes located on the mitochondrial DNA or the nuclear DNA. Here, we report a case with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)/Leigh overlap syndrome due to variant m.13513G>A in ND5. A 60-month-old female with a congenital, complex, multisystem phenotype was diagnosed with MELAS/Leigh overlap syndrome due to variant m.13513G>A in ND5. Brainstem involvement resulted in dysphagia, dysarthria, and respiratory failure with recurrent episodes of aspiration, respiratory insufficiency, desaturations, lack of respiratory drive, hypercapnia, and pneumonia. Treatment was symptomatic and included non-invasive ventilation, antibiotics, implantation of a percutaneous endoscopic gastrostomy, anti-seizure drugs, anti-dystonia medication, a cocktail of vitamins and antioxidants, and analgesics. Despite these measures, the outcome was poor as the patient died at the age of 62 months after being discharged to home palliative care. In summary, the m.13513G>A variant can manifest as MELAS/Leigh overlap syndrome with Leigh syndrome dominating. Because of brainstem involvement leading to respiratory dysfunction, dysarthria, and dysphagia, the outcome is poor, despite symptomatic measures.

MELAS with multiple stroke-like episodes due to the variant m.13513G>A in MT-ND5

Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is characterized by metabolic stroke, seizures, cognitive decline, lactic acidosis, ragged-red fibers, headache, and vomiting, and in 80% of cases due to the mtDNA variant m.3243A>G. We report the case of a MELAS patient carrying a variant in subunit-5 of the respiratory chain (MT-ND5), rarely reported in MELAS. The patient is a 33-year-old male, who experienced a series of stroke-like episodes (StLEs) since age 23 years, which manifested clinically as seizures transient sensory disturbances, weakness, and visual or cognitive impairment. Over 9 years, these StLEs were misinterpreted as ischemic strokes, respectively, as cerebral vasculitis. He presented with mild, recurrent elevations of the creatine kinase. Initially, anti-seizure drugs and steroids appeared to be beneficial. Despite good recovery of each single StLE, the patient experienced a progressive decline of cognitive functions and activities of daily living. Cerebral imaging showed corresponding stroke-like lesions in changing locations. At age 32y, genetic work-up revealed the variant m.13513G>A in MT-ND5. The patient profited significantly from a cocktail with anti-oxidants/cofactors. This case shows that the variant m.13513G>A in MT-ND5 can manifest as MELAS that StLEs recover spontaneously and that the course of MELAS is slowly progressive.

Mitochondrial 13513G>A Mutation With Low Mutant Load Presenting as Isolated Leber's Hereditary Optic Neuropathy Assessed by Next Generation Sequencing

Objective: Mitochondrial 13513G>A mutation presenting as isolated Leber's hereditary optic neuropathy (LHON) without any extraocular pathology has not been reported in literature. We herein evaluate the clinical characteristics and heteroplasmy of m.13513G>A mutation manifesting as isolated LHON. Methods: Seven members of a Chinese family were enrolled in this study. All subjects underwent detailed systemic and ophthalmic examinations. Mitochondrial DNA in their blood was assessed by targeted PCR amplifications, next generation sequencing (NGS), and pyrosequencing. One hundred of blood samples from ethnic-matched healthy volunteers were tested by NGS and pyrosequencing as normal controls. Results: Isolated LHON without any other ocular or extraocular pathology was identified in a 16 year old patient in this family. Heteroplasmic m.13513G>A mutation was detected by NGS of the full mtDNA genome in the patient with mutant load of 33.56%, and of 26% 3 months and 3 years after the onset of LHON, respectively. No m.13513G>A mutation was detected in all his relatives by NGS. Pyrosequencing revealed the mutant load of m.13513G>A mutation of the LHON patient, his mother, father and sister were 22.4, 1.9, 0, and 0%, respectively. None of 100 healthy control subjects was detected to harbor m.13513G>A mutation either by NGS or by pyrosequencing of the full mt DNA genome. Conclusions: We first report m.13513G>A mutation with low mutant load presenting as isolated LHON. NGS of the full mitochondrial DNA genome is highly recommended for LHON suspects when targeted PCR amplification for main primary point mutations of LHON was negative.