Population prevalence of the MELAS A3243G mutation

A case with short stature and proteinuria: atypical presentation of a family with m.3243A>G mutation

BACKGROUND

The mitochondrial DNA (mtDNA) m.3243A>G mutation is one of the most common pathogenic mtDNA variants. The phenotypes associated with this mutation range from asymptomatic induviduals to well-defined clinical syndromes, or non-syndromic mitochondrial disorders. Variable clinical features in pediatric cases may cause difficulty in diagnosis. Kidney involvement in this mutation is uncommon and reported on a case-by-case basis. Here, we report on a patient with m.3243A>G mutation, who presented with short stature and proteinuria, and his family, who share the same genotype but exhibit different heteroplasmy levels in different tissues and variable phenotypes.

CASE PRESENTATION

A 15-year-old male patient was admitted to the pediatric endocrinology department with short stature. His examinations revealed nephrotic range proteinuria, hearing loss, impaired glucose tolerance, and Wolf-Parkinson-White syndrome. From family history, it was learned that diabetes mellitus (DM) and progressive sensorineural hearing loss were common in this family. The patient's mother, who had chronic kidney disease, DM, and hearing loss, had died suddenly for an unknown reason. Considering the family history, a genetic analysis was performed for mitochondrial disease. Mitochondrial DNA analysis revealed a m.3243A>G mutation with 47% heteroplasmy in blood, 62% heteroplasmy in buccal cells, and 96% heteroplasmy in urothelial cells in our patient.

CONCLUSIONS

Short stature without any other complaint and renal involvement are rare findings in m.3243A>G mutation. In patients presenting with proteinuria, in the presence of conditions affecting many systems such as endocrine system pathologies, hearing loss, and cardiac pathologies, and in the presence of individuals with a similar family history of multiple organ involvement, mitochondrial diseases should be considered, and examined from this perspective. Our case illustrates the value of a detailed medical and family history.

Unconstrained Precision Mitochondrial Genome Editing with αDdCBEs

DddA-derived cytosine base editors (DdCBEs) enable the targeted introduction of C•G-to-T•A conversions in mitochondrial DNA (mtDNA). DdCBEs work in pairs, with each arm composed of a transcription activator-like effector (TALE), a split double-stranded DNA deaminase half, and a uracil glycosylase inhibitor. This pioneering technology has helped improve our understanding of cellular processes involving mtDNA and has paved the way for the development of models and therapies for genetic disorders caused by pathogenic mtDNA variants. Nonetheless, given the intrinsic properties of TALE proteins, several target sites in human mtDNA are predicted to remain out of reach to DdCBEs and other TALE-based technologies. Specifically, due to the conventional requirement for a thymine immediately upstream of the TALE target sequences (i.e., the 5'-T constraint), over 150 loci in the human mitochondrial genome are presumed to be inaccessible to DdCBEs. Previous attempts at circumventing this requirement, either by developing monomeric DdCBEs or utilizing DNA-binding domains alternative to TALEs, have resulted in suboptimal specificity profiles with reduced therapeutic potential. Here, aiming to challenge and elucidate the relevance of the 5'-T constraint in the context of DdCBE-mediated mtDNA editing, and to expand the range of motifs that are editable by this technology, we generated DdCBEs containing TALE proteins engineered to recognize all 5' bases. These modified DdCBEs are herein referred to as αDdCBEs. Notably, 5'-T-noncompliant canonical DdCBEs efficiently edited mtDNA at diverse loci. However, they were frequently outperformed by αDdCBEs, which exhibited significant improvements in activity and specificity, regardless of the most 5' bases of their TALE binding sites. Furthermore, we showed that αDdCBEs are compatible with the enhanced DddAtox variants DddA6 and DddA11, and we validated TALE shifting with αDdCBEs as an effective approach to optimize base editing outcomes. Overall, αDdCBEs enable efficient, specific, and unconstrained mitochondrial base editing.

Neurological manifestations in adult patients with the m.3243A>G variant in mitochondrial DNA

ABSTRACT

Background

The m.3243A>G variant in mitochondrial DNA (mtDNA) is the most common cause of the MELAS (Mitochondrial encephalopathy, lactic acidosis and stroke-like episodes) syndrome usually commencing in childhood or adolescence. In adults, the variant presents with versatile and mostly neurological phenotypes, but MELAS may not be common.

Objective

To examine the frequency of phenotypes in adults with m.3243A>G in a population-based cohort and in a meta-analysis of reported case series.

Methods

We clinically examined 51 adult patients with m.3243A>G to determine the frequency of phenotypes and to analyse the contribution of variant heteroplasmy, age, sex and mtDNA haplogroup to the phenotypes. The frequencies of neurological features were also assessed in a meta-analysis on 25 published case series reporting 1314 patients.

Results

Sensorineural hearing impairment (HI), cognitive impairment and myopathy were the most common manifestations, whereas stroke-like episodes were infrequent. Variant heteroplasmy and age were only modest predictors of the phenotypes, although heteroplasmy correlated significantly with disability and Kaplan-Meier analysis showed progression of phenotypes with age. Male sex predicted more severe disability, whereas haplogroup UK was associated with no significant disability. Meta-analysis revealed substantial heterogeneity of phenotype frequencies and preferential inclusion of the MELAS phenotype.

Discussion

In adult patients with m.3243A>G sensorineural HI, cognitive impairment and myopathy are common manifestations with lifetime prevalences approaching unity. Stroke-like episodes are rare. Variant heteroplasmy, age, sex and mtDNA haplogroup contribute to the severity of the disease. Meta-analysis provided a solid estimate of the various neurological symptoms in adults with m.3243A>G.

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes with coexisting nemaline myopathy: a case report

BACKGROUND

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes and nemaline myopathy are two rare genetic conditions. We report the first case reported in world literature with coexistence of both these rare disorders.

CASE PRESENTATION

A 11-year-old previously healthy Sri Lankan male child, product of a nonconsanguineous marriage with normal development presented with acute onset short lasting recurring episodes of right-sided eye deviation with impaired consciousness. In between episodes he regained consciousness. Family history revealed a similar presentation in the mother at 36 years of age. Examination was significant for short stature and proximal upper and lower limb weakness. His plasma and cerebrospinal fluid lactate were elevated. Magnetic resonance imaging brain had evidence of an acute infarction in the right occipital territory. Sanger sequencing for common mitochondrial variants of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes confirmed this diagnosis. Whole exome sequencing revealed pathogenic compound heterozygous variants in NEB gene implicating in coexisting nemaline myopathy. Acute presentation was managed with supportive care, antiepileptics, and mitochondrial supplementation. Currently he is stable on daily supplementation of arginine and limb-strengthening physiotherapy. He is being monitored closely clinically and with serum lactate level.

CONCLUSION

Genetic diseases are rare. Coexistence of two genetic conditions is even rarer. Genetic confirmation of diagnosis is imperative for prediction of complications, accurate management, and genetic counseling.

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.

T cell activation contributes to purifying selection against the MELAS-associated m.3243A>G pathogenic variant in blood

T cells have been shown to maintain a lower percentage (heteroplasmy) of the pathogenic m.3243A>G variant (MT-TL1, associated with maternally inherited diabetes and deafness [MIDD] and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes [MELAS]). The mechanism(s) underlying this purifying selection, however, remain unknown. Here we report that purified patient memory CD4+ T cells have lower bulk m.3243A>G heteroplasmy compared to naïve CD4+ T cells. In vitro activation of naïve CD4+ m.3243A>G patient T cells results in lower bulk m.3243A>G heteroplasmy after proliferation. Finally, m.3243A>G patient T cell receptor repertoire sequencing reveals relative oligoclonality compared to controls. These data support a role for T cell activation in peripheral, purifying selection against high m.3243A>G heteroplasmy T cells at the level of the cell, in a likely cell-autonomous fashion.

Neovascular Glaucoma in MELAS syndrome

Purpose

To describe examination and findings in a case of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) with particular focus on the ocular sequelae from diabetes.

Observations

Neovascular glaucoma is not a common manifestation of MELAS.

Conclusions and Importance

We present a rare case of neovascular glaucoma in a patient with MELAS with a history of diabetes, hearing loss, and macular dystrophy. MELAS should be suspected in patients with this constellation of symptoms.

Unconstrained Precision Mitochondrial Genome Editing with αDdCBEs

DddA-derived cytosine base editors (DdCBEs) enable the targeted introduction of C•G-to-T•A conversions in mitochondrial DNA (mtDNA). DdCBEs are often deployed as pairs, with each arm comprised of a transcription activator-like effector (TALE), a split double-stranded DNA deaminase half, and a uracil glycosylase inhibitor. This pioneering technology has helped improve our understanding of cellular processes involving mtDNA and has paved the way for the development of models and therapies for genetic disorders caused by pathogenic mtDNA variants. Nonetheless, given the intrinsic properties of TALE proteins, several target sites in human mtDNA remain out of reach to DdCBEs and other TALE-based technologies. Specifically, due to the conventional requirement for a thymine immediately upstream of the TALE target sequences (i.e., the 5'-T constraint), over 150 loci in the human mitochondrial genome are presumed to be inaccessible to DdCBEs. Previous attempts at circumventing this constraint, either by developing monomeric DdCBEs or utilizing DNA-binding domains alternative to TALEs, have resulted in suboptimal specificity profiles with reduced therapeutic potential. Here, aiming to challenge and elucidate the relevance of the 5'-T constraint in the context of DdCBE-mediated mtDNA editing, and to expand the range of motifs that are editable by this technology, we generated αDdCBEs that contain modified TALE proteins engineered to recognize all 5' bases. Notably, 5'-T-noncompliant, canonical DdCBEs efficiently edited mtDNA at diverse loci. However, DdCBEs were frequently outperformed by αDdCBEs, which consistently displayed significant improvements in activity and specificity, regardless of the 5'-most bases of their TALE binding sites. Furthermore, we showed that αDdCBEs are compatible with DddA tox and its derivatives DddA6, and DddA11, and we validated TALE shifting with αDdCBEs as an effective approach to optimize base editing outcomes at a single target site. Overall, αDdCBEs enable efficient, specific, and unconstrained mitochondrial base editing.

Digenic Inheritance in Rare Disorders and Mitochondrial Disease-Crossing the Frontier to a More Comprehensive Understanding of Etiology

Our understanding of rare disease genetics has been shaped by a monogenic disease model. While the traditional monogenic disease model has been successful in identifying numerous disease-associated genes and significantly enlarged our knowledge in the field of human genetics, it has limitations in explaining phenomena like phenotypic variability and reduced penetrance. Widening the perspective beyond Mendelian inheritance has the potential to enable a better understanding of disease complexity in rare disorders. Digenic inheritance is the simplest instance of a non-Mendelian disorder, characterized by the functional interplay of variants in two disease-contributing genes. Known digenic disease causes show a range of pathomechanisms underlying digenic interplay, including direct and indirect gene product interactions as well as epigenetic modifications. This review aims to systematically explore the background of digenic inheritance in rare disorders, the approaches and challenges when investigating digenic inheritance, and the current evidence for digenic inheritance in mitochondrial disorders.

Mitochondrial disease in New Zealand: a nationwide prevalence study

BACKGROUND

The complexities of mitochondrial disease make epidemiological studies challenging, yet this information is important in understanding the healthcare burden and addressing service and educational needs. Existing studies are limited to quaternary centres or focus on a single genotype or phenotype and estimate disease prevalence at 12.5 per 100 000. New Zealand's (NZ) size and partially integrated national healthcare system make it amenable to a nationwide prevalence study.

AIM

To estimate the prevalence of molecularly confirmed and suspected mitochondrial disease on 31 December 2015 in NZ.

METHODS

Cases were identified from subspecialists and laboratory databases and through interrogation of the Ministry of Health National Minimum Dataset with a focus on presentations between 2000 and 2015. Patient records were reviewed, and those with a diagnosis of 'mitochondrial disease' who were alive and residing in NZ on the prevalence date were included. These were divided into molecularly confirmed and clinically suspected cases. Official NZ estimated resident population data were used to calculate prevalence.

RESULTS

Seven hundred twenty-three unique national health index numbers were identified. Five hundred five were excluded. The minimum combined prevalence for mitochondrial disease was 4.7 per 100 000 (95% confidence interval (CI): 4.1-5.4). The minimum prevalence for molecularly confirmed and suspected disease was 2.9 (95% CI 2.4-3.4) and 1.8 (95% CI 1.4-2.2) cases per 100 000 respectively.

CONCLUSIONS

Within the limitations of this study, comparison to similar prevalence studies performed by specialist referral centres suggests mitochondrial disease is underdiagnosed in NZ. This highlights a need for improved education and referral pathways for mitochondrial disease in NZ.

Late-onset mitochondrial encephalopathy with lactic acidosis and stroke-like episodes and the role of serial imaging

Mitochondria are essential for human metabolic function. Over 350 genetic mutations are associated with mitochondrial diseases, which are inherited in a matrilineal fashion. In mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), defective mitochondrial function and resultant impaired cellular energy production compromise vascular perfusion in affected tissues. Early diagnostic criteria suggested the diagnosis should be considered in those under 40. However, a broader range of phenotypes are now recognised, including those that present for the first time later in life. The primary presenting feature in MELAS is a stroke-like episode invariably resulting in patients undergoing neuroradiological imaging. We present a case of a woman with a first presentation of a stroke-like episode and seizures in her 40s who was eventually diagnosed with MELAS. We detail her clinical presentation, treatment and diagnosis, emphasising the role of serial imaging in her diagnosis.

Adult-onset mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS): a diagnostic challenge

Rare causes of stroke-like presentations can be difficult to diagnose. We report a case of a man in his 40s who first presented with stroke symptoms, but whose clinical course was not typical for a stroke. A detailed investigation of the patient's medical history revealed bilateral sensorineural hearing loss which prompted a wider diagnostic assessment.Furthermore, lack of vascular risk factors and a normal angiogram strengthened our suspicion of an unusual underlying condition. Raised lactic acid levels and genetic analysis confirmed a diagnosis of mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome.

Penetrance and expressivity of mitochondrial variants in a large clinically unselected population

Whole genome sequencing (WGS) from large clinically unselected cohorts provides a unique opportunity to assess the penetrance and expressivity of rare and/or known pathogenic mitochondrial variants in population. Using WGS from 179 862 clinically unselected individuals from the UK Biobank, we performed extensive single and rare variant aggregation association analyses of 15 881 mtDNA variants and 73 known pathogenic variants with 15 mitochondrial disease-relevant phenotypes. We identified 12 homoplasmic and one heteroplasmic variant (m.3243A>G) with genome-wide significant associations in our clinically unselected cohort. Heteroplasmic m.3243A>G (MAF = 0.0002, a known pathogenic variant) was associated with diabetes, deafness and heart failure and 12 homoplasmic variants increased aspartate aminotransferase levels including three low-frequency variants (MAF ~0.002 and beta~0.3 SD). Most pathogenic mitochondrial disease variants (n = 66/74) were rare in the population (<1:9000). Aggregated or single variant analysis of pathogenic variants showed low penetrance in unselected settings for the relevant phenotypes, except m.3243A>G. Multi-system disease risk and penetrance of diabetes, deafness and heart failure greatly increased with m.3243A>G level ≥ 10%. The odds ratio of these traits increased from 5.61, 12.3 and 10.1 to 25.1, 55.0 and 39.5, respectively. Diabetes risk with m.3243A>G was further influenced by type 2 diabetes genetic risk. Our study of mitochondrial variation in a large-unselected population identified novel associations and demonstrated that pathogenic mitochondrial variants have lower penetrance in clinically unselected settings. m.3243A>G was an exception at higher heteroplasmy showing a significant impact on health making it a good candidate for incidental reporting.

The West of Scotland Cohort of Mitochondrial Individuals with the m.3243A>G Variant: Variations in Phenotypes and Predictors of Disease Severity

BACKGROUND

The m.3243A>G variant is the commonest mitochondrial (mt) DNA pathogenic variant and a frequent cause of mitochondrial disease. Individuals present with a variety of clinical manifestations from diabetes to neurological events resembling strokes. Due to this, patients are commonly cared for by a multidisciplinary team.

OBJECTIVES

This project aimed to identify patients with confirmed mt.3243A>G-related mitochondrial disease attending the Muscle Clinic at Queen Elizabeth University Hospital in Glasgow. We explored potential correlates between clinical phenotypes and mtDNA heteroplasmy levels, HbA1c levels, body mass index, and specific clinical manifestations. We investigated if there were discrepancies between non-neurological speciality labelling in clinical records and individuals' phenotypes.

METHODS

Data were gathered from the West of Scotland electronic records. Phenotypes were ascertained by a clinician with expertise in mitochondrial disorders. Statistical analyses were applied to study relationships between tissue heteroplasmy, HbA1c and clinical phenotypes including body mass index (BMI).

RESULTS

Forty-six individuals were identified from 31 unrelated pedigrees. Maternally inherited diabetes and deafness was the prominent syndromic phenotype (48%). A significant association was found between overall number of symptoms and bowel dysmotility (p < 0.01). HbA1c was investigated as a predictor of severity with potential association seen. Although used widely as a prognosticator, neither corrected blood nor urine mtDNA heteroplasmy levels were associated with increased number of symptoms. In 74.1% of records, syndromic phenotypes were incorrectly used by non-neurological specialities.

CONCLUSIONS

This m.3243 A > G patient cohort present with marked clinical heterogeneity. Urine and blood heteroplasmy levels are not reliable predictors of disease severity. HbA1c may be a novel predictor of disease severity with further research required to investigate this association. We infer that prognosis may be worse in patients with low BMIs and in those with bowel dysmotility. These results underscore a multidisciplinary approach and highlight a problem with inaccurate use of the existing nomenclature.

MELAS-Derived Neurons Functionally Improve by Mitochondrial Transfer from Highly Purified Mesenchymal Stem Cells (REC)

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episode (MELAS) syndrome, caused by a single base substitution in mitochondrial DNA (m.3243A>G), is one of the most common maternally inherited mitochondrial diseases accompanied by neuronal damage due to defects in the oxidative phosphorylation system. There is no established treatment. Our previous study reported a superior restoration of mitochondrial function and bioenergetics in mitochondria-deficient cells using highly purified mesenchymal stem cells (RECs). However, whether such exogenous mitochondrial donation occurs in mitochondrial disease models and whether it plays a role in the recovery of pathological neuronal functions is unknown. Here, utilizing induced pluripotent stem cells (iPSC), we differentiated neurons with impaired mitochondrial function from patients with MELAS. MELAS neurons and RECs/mesenchymal stem cells (MSCs) were cultured under contact or non-contact conditions. Both RECs and MSCs can donate mitochondria to MELAS neurons, but RECs are more excellent than MSCs for mitochondrial transfer in both systems. In addition, REC-mediated mitochondrial transfer significantly restored mitochondrial function, including mitochondrial membrane potential, ATP/ROS production, intracellular calcium storage, and oxygen consumption rate. Moreover, mitochondrial function was maintained for at least three weeks. Thus, REC-donated exogenous mitochondria might offer a potential therapeutic strategy for treating neurological dysfunction in MELAS.

Efficient elimination of MELAS-associated m.3243G mutant mitochondrial DNA by an engineered mitoARCUS nuclease

Nuclease-mediated editing of heteroplasmic mitochondrial DNA (mtDNA) seeks to preferentially cleave and eliminate mutant mtDNA, leaving wild-type genomes to repopulate the cell and shift mtDNA heteroplasmy. Various technologies are available, but many suffer from limitations based on size and/or specificity. The use of ARCUS nucleases, derived from naturally occurring I-CreI, avoids these pitfalls due to their small size, single-component protein structure and high specificity resulting from a robust protein-engineering process. Here we describe the development of a mitochondrial-targeted ARCUS (mitoARCUS) nuclease designed to target one of the most common pathogenic mtDNA mutations, m.3243A>G. mitoARCUS robustly eliminated mutant mtDNA without cutting wild-type mtDNA, allowing for shifts in heteroplasmy and concomitant improvements in mitochondrial protein steady-state levels and respiration. In vivo efficacy was demonstrated using a m.3243A>G xenograft mouse model with mitoARCUS delivered systemically by adeno-associated virus. Together, these data support the development of mitoARCUS as an in vivo gene-editing therapeutic for m.3243A>G-associated diseases.

Negative correlation between organ heteroplasmy, particularly hepatic heteroplasmy, and age at death revealed by post-mortem studies of m.3243A > G cases

Mitochondrial DNA m.3243A > G mutation causes mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) and its associated multi-organ disorders, including diabetes. To clarify associations between m.3243A > G organ heteroplasmy and clinical phenotypes, including the age at death, we combined genetic and pathological examinations from seven unreported and 36 literature cases of autopsied subjects. Clinical characteristics of subjects were as follows: male, 13; female, 28; unknown, 2; the age at death, 36.9 ± 20.2 [4-82] years; BMI, 16.0 ± 2.9 [13.0-22.3]; diabetes, N = 21 (49%), diabetes onset age 38.6 ± 14.2 years; deafness, N = 27 (63%); stroke-like episodes (StLEp), N = 25 (58%); congestive heart failure (CHF), N = 15 (35%); CHF onset age, 51.3 ± 14.5 years. Causes of death (N = 32) were as follows: cardiac, N = 13 (41%); infection, N = 8 (25%); StLEp, N = 4 (13%); gastrointestinal, N = 4 (13%); renal, N = 2 (6%); hepatic, N = 1 (2%). High and low heteroplasmies were confirmed in non-regenerative and regenerative organs, respectively. Heteroplasmy of the liver, spleen, leukocytes, and kidney for all subjects was significantly associated with the age at death. Furthermore, the age at death was related to juvenile-onset (any m.3243A > G-related symptoms appeared before 20) and stroke-like episodes. Multiple linear regression analysis with the age at death as an objective variable showed the significant contribution of liver heteroplasty and juvenile-onset to the age at death. m.3243A > G organ heteroplasmy levels, particularly hepatic heteroplasmy, are significantly associated with the age at death in deceased cases.

Anesthetic Management of a Patient With Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes Syndrome During Extensive Spinal Surgery With Both Motor Evoked Potentials and Somatosensory Evoked Potentials: A Case Report

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is a complex and infrequently encountered mitochondrial cytopathy. Patients with MELAS often present with multi-systemic manifestations, making their anesthetic management particularly challenging. In this case report, we describe in detail our anesthetic approach for a 19-year-old male with confirmed MELAS linked to an m.3243A>G mutation. The patient had been diagnosed with MELAS at age 12 following a stroke-like episode and presented with progressive spinal deformities. He exhibited a 70° thoracic spine curvature and an 80° kyphosis, requiring a T1-L2 posterior spinal fusion. The surgical plan included neuromonitoring with both somatosensory and motor evoked potentials. Intravenous anesthetics such as propofol are typically preferred in this context due to their reduced interference with neuromonitoring compared to volatile anesthetics. Anticipating a surgical duration of six to seven hours, however, we hesitated to rely on propofol for this extended period due to its potential risks of lactic acidosis in the context of MELAS. Given that propofol infusion for extended periods (>48 hours) or at high doses (≥5 mg·kg-1·hour-1) is known to induce propofol-related infusion syndrome, and coupled with our concerns about the risk of lactic acidosis in this patient, we were compelled to design an anesthetic plan that avoided propofol altogether without excessive use of volatile anesthetics. This proactive approach ensured the maintenance of consistent neuromonitoring signals and the patient's safety, especially given his underlying mitochondrial dysfunction. Our primary rationale in presenting this case report is to highlight the challenges posed by MELAS in the setting of extended surgery, with a focus on anesthetic considerations during neuromonitoring. For prolonged surgeries that typically rely heavily on intravenous anesthetics, which interfere less with neuromonitoring than volatile anesthetics, the use of propofol should be approached with caution in MELAS contexts due to its associated risk of lactic acidosis. To our knowledge, this is the first case report that described the anesthetic management of a patient with MELAS undergoing a procedure of such duration, requiring both somatosensory and motor evoked potential neuromonitoring. We believe our experiences will serve as a reference for anesthesiologists and perioperative teams faced with similar challenging clinical situations.

The Mitochondrial m.3243A>G Mutation on the Dish, Lessons from In Vitro Models

The m.3243A>G mutation in the tRNA Leu(UUR) gene (MT-TL1) is one of the most common pathogenic point mutations in human mtDNA. Patient symptoms vary widely and the severity of the disease ranges from asymptomatic to lethal. The reason for the high heterogeneity of m.3243A>G-associated disease is still unknown, and the treatment options are limited, with only supportive interventions available. Furthermore, the heteroplasmic nature of the m.3243A>G mutation and lack of specific animal models of mtDNA mutations have challenged the study of m.3243A>G, and, besides patient data, only cell models have been available for studies. The most commonly used cell models are patient derived, such as fibroblasts and induced pluripotent stem cell (iPSC)-derived models, and cybrid models where the mutant DNA is transferred to an acceptor cell. Studies on cell models have revealed cell-type-specific effects of the m.3243A>G mutation and that the tolerance for this mutation varies between cell types and between patients. In this review, we summarize the literature on the effects of m.3243A>G in cell models.

Ischemic optic neuropathy as first presentation in patient with m.3243 A > G MELAS classic mutation

BACKGROUND

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is a systemic disorder in which multi-organ dysfunction may occur from mitochondrial metabolism failure. Maternally inherited mutations in the MT-TL1 gene are the most frequent causes for this disorder. Clinical manifestations may include stroke-like episodes, epilepsy, dementia, headache and myopathy. Among these, acute visual failure, usually in association with cortical blindness, can occur because of stroke-like episodes affecting the occipital cortex or the visual pathways. Vision loss due to optic neuropathy is otherwise considered a typical manifestation of other mitochondrial diseases such as Leber hereditary optic neuropathy (LHON).

CASE PRESENTATION

Here we describe a 55-year-old woman, sister of a previously described patient with MELAS harbouring the m.3243A > G (p.0, MT-TL1) mutation, with otherwise unremarkable medical history, that presented with subacute, painful visual impairment of one eye, accompanied by proximal muscular pain and headache. Over the next weeks, she developed severe and progressive vision loss limited to one eye. Ocular examination confirmed unilateral swelling of the optic nerve head; fluorescein angiography showed segmental perfusion delay in the optic disc and papillary leakage. Neuroimaging, blood and CSF examination and temporal artery biopsy ruled out neuroinflammatory disorders and giant cell arteritis (GCA). Mitochondrial sequencing analysis confirmed the m.3243A > G transition, and excluded the three most common LHON mutations, as well as the m.3376G > A LHON/MELAS overlap syndrome mutation. Based on the constellation of clinical symptoms and signs presented in our patient, including the muscular involvement, and the results of the investigations, the diagnosis of optic neuropathy as a stroke-like event affecting the optic disc was performed. L-arginine and ubidecarenone therapies were started with the aim to improve stroke-like episode symptoms and prevention. The visual defect remained stable with no further progression or outbreak of new symptoms.

CONCLUSIONS

Atypical clinical presentations must be always considered in mitochondrial disorders, even in well-described phenotypes and when mutational load in peripheral tissue is low. Mitotic segregation of mitochondrial DNA (mtDNA) does not allow to know the exact degree of heteroplasmy existent within different tissue, such as retina and optic nerve. Important therapeutic implications arise from a correct diagnosis of atypical presentation of mitochondrial disorders.

Stroke-like episodes in adult mitochondrial disease

Stroke-like episode is a paroxysmal neurological manifestation which affects a specific group of patients with mitochondrial disease. Focal-onset seizures, encephalopathy, and visual disturbances are prominent findings associated with stroke-like episodes, with a predilection for the posterior cerebral cortex. The most common cause of stroke-like episodes is the m.3243A>G variant in MT-TL1 gene followed by recessive POLG variants. This chapter aims to review the definition of stroke-like episode and delineate the clinical phenomenology, neuroimaging and EEG findings typically seen in patients. In addition, several lines of evidence supporting neuronal hyper-excitability as the key mechanism of stroke-like episodes are discussed. The management of stroke-like episodes should focus on aggressive seizure management and treatment for concomitant complications such as intestinal pseudo-obstruction. There is no robust evidence to prove the efficacy of l-arginine for both acute and prophylactic settings. Progressive brain atrophy and dementia are the sequalae of recurrent stroke-like episode, and the underlying genotype in part predicts prognosis.

Clinical features of epileptic seizures in patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes

BACKGROUND AND PURPOSE

This study aimed to characterize the clinical features of epilepsy in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and analyze the clinical determinants for drug-resistant epilepsy in MELAS.

METHODS

A single-center, retrospective study was conducted to investigate the clinical features of epilepsy in patients with MELAS. Collected variables included seizure semiology, electroencephalography (EEG), muscle biopsy, genetic testing, neuroimaging findings, resting serum lactic value and modified Rankin scale (mRS) of patients with MELAS. We also investigated the differences between the adult-onset group and the child-onset group and analyzed the risk factors for drug-resistant epilepsy in MELAS.

RESULTS

We studied 97 patients (56 males: 41 females) with confirmed MELAS. Epileptic seizure occurred in 100.0% of patients and the initial symptom of 69.1% patients was epileptic seizure. The average age of disease onset was 21.0 years, ranging from 2 to 60 years. The seizure types of these patients with MELAS were variable, with generalized onset (51.5%) to be the most common type. The EEG changes in the patients with MELAS were mainly slow wave (90.9%) and epileptiform discharge (68.2%). The child-onset group with earlier seizure onset presented significantly higher resting serum lactic value (p = 0.0048) and lower incidence of stroke-like lesion in the brain (p = 0.003), especially in the temporal lobe (p < 0.001), compared with the adult-onset group. Importantly, drug-resistant epilepsy in MELAS was demonstrated to be closely related to the earlier age of seizure onset (p = 0.013), as well as the higher mRS score (p < 0.001) and higher resting serum lactic value (p = 0.009).

CONCLUSION

Early identification of MELAS should be considered among individuals with recurrent epilepsy through clinical screening. Age of seizure onset and resting serum lactic value may predict the development of drug-resistant epilepsy in MELAS. Close observation and appropriate anti-epileptic treatment are indispensable for individuals with MELAS to improve the prognosis. Further studies with larger sample size are required to further evaluate the risk factors of drug-resistant epilepsy in MELAS and provide guidance on treatment of MELAS.

Low disease risk and penetrance in Leber hereditary optic neuropathy

The risk of Leber hereditary optic neuropathy (LHON) has largely been extrapolated from disease cohorts, which underestimate the population prevalence of pathogenic primary LHON variants as a result of incomplete disease penetrance. Understanding the true population prevalence of primary LHON variants, alongside the rate of clinical disease, provides a better understanding of disease risk and variant penetrance. We identified pathogenic primary LHON variants in whole-genome sequencing data of a well-characterized population-based control cohort and found that the prevalence is far greater than previously estimated, as it occurs in approximately 1 in 800 individuals. Accordingly, we were able to more accurately estimate population risk and disease penetrance in LHON variant carriers, validating our findings by using other large control datasets. These findings will inform accurate counseling in relation to the risk of vision loss in LHON variant carriers and disease manifestation in their family. This Matters Arising paper is in response to Lopez Sanchez et al. (2021), published in The American Journal of Human Genetics. See also the response by Mackey et al. (2022), published in this issue.

Bone Deformities and Kidney Failure: Coincidence of PHEX-Related Hypophosphatemic Rickets and m.3243A&gt;G Mitochondrial Disease

X-linked hypophosphatemic rickets (XLH) and m.3243A&gt;G mitochondrial disease share several clinical findings, including short stature, hearing impairment (HI), nephropathy, and hypertension. Here, we report on a case with the rare coincidence of these two genetic conditions. In early childhood, the patient presented with hypophosphatemia and bone deformities and was clinically diagnosed with XLH. This was genetically verified in adulthood with the identification of a de novo pathogenic deletion in phosphate-regulating endopeptidase homolog X-linked (PHEX). In addition, the patient developed HI and hypertension and when his mother was diagnosed with m.3243A&gt;G, subsequent genetic testing confirmed the patient to carry the same variant. Over the next two decades, the patient developed progressive renal impairment however without nephrocalcinosis known to associate with XLH which could indicate an m.3243A&gt;G-related kidney disease. Parallel with the progression of renal impairment, the patient developed hyperphosphatemia and secondary hyperparathyroidism. In conclusion, this case represents a complex clinical phenotype with the reversal of hypo- to hyperphosphatemia in XLH potentially mediated by the development of an m.3243A&gt;G-associated nephropathy.

Pathogenic mitochondrial DNA 3243A>G mutation: From genetics to phenotype

The mitochondrial DNA (mtDNA) m.3243A>G mutation is one of the most common pathogenic mtDNA variants, showing complex genetics, pathogenic molecular mechanisms, and phenotypes. In recent years, the prevention of mtDNA-related diseases has trended toward precision medicine strategies, such as preimplantation genetic diagnosis (PGD) and mitochondrial replacement therapy (MRT). These techniques are set to allow the birth of healthy children, but clinical implementation relies on thorough insights into mtDNA genetics. The genotype and phenotype of m.3243A>G vary greatly from mother to offspring, which compromises genetic counseling for the disease. This review is the first to systematically elaborate on the characteristics of the m.3243A>G mutation, from genetics to phenotype and the relationship between them, as well as the related influencing factors and potential strategies for preventing disease. These perceptions will provide clarity for clinicians providing genetic counseling to m.3243A>G patients.

Varied Responses to a High m.3243A&gt;G Mutation Load and Respiratory Chain Dysfunction in Patient-Derived Cardiomyocytes

The m.3243A&gt;G mutation in mitochondrial tRNA-Leu(UUR) is one of the most common pathogenic mitochondrial DNA mutations in humans. The clinical manifestations are highly heterogenous and the causes for the drastic clinical variability are unknown. Approximately one third of patients suffer from cardiac disease, which often increases mortality. Why only some patients develop cardiomyopathy is unknown. Here, we studied the molecular effects of a high m.3243A&gt;G mutation load on cardiomyocyte functionality, using cells derived from induced pluripotent stem cells (iPSC-CM) of two different m.3243A&gt;G patients, only one of them suffering from severe cardiomyopathy. While high mutation load impaired mitochondrial respiration in both patients' iPSC-CMs, the downstream consequences varied. mtDNA mutant cells from a patient with no clinical heart disease showed increased glucose metabolism and retained cellular ATP levels, whereas cells from the cardiac disease patient showed reduced ATP levels. In this patient, the mutations also affected intracellular calcium signaling, while this was not true in the other patient's cells. Our results reflect the clinical variability in mitochondrial disease patients and show that iPSC-CMs retain tissue specific features seen in patients.

Use of Whole-Genome Sequencing for Mitochondrial Disease Diagnosis

BACKGROUND AND OBJECTIVES

Mitochondrial diseases (MDs) are the commonest group of heritable metabolic disorders. Phenotypic diversity can make molecular diagnosis challenging, and causative genetic variants may reside in either mitochondrial or nuclear DNA. A single comprehensive genetic diagnostic test would be highly useful and transform the field. We applied whole-genome sequencing (WGS) to evaluate the variant detection rate and diagnostic capacity of this technology with a view to simplifying and improving the MD diagnostic pathway.

METHODS

Adult patients presenting to a specialist MD clinic in Sydney, Australia, were recruited to the study if they satisfied clinical MD (Nijmegen) criteria. WGS was performed on blood DNA, followed by clinical genetic analysis for known pathogenic MD-associated variants and MD mimics.

RESULTS

Of the 242 consecutive patients recruited, 62 participants had "definite," 108 had "probable," and 72 had "possible" MD classification by the Nijmegen criteria. Disease-causing variants were identified for 130 participants, regardless of the location of the causative genetic variants, giving an overall diagnostic rate of 53.7% (130 of 242). Identification of causative genetic variants informed precise treatment, restored reproductive confidence, and optimized clinical management of MD.

DISCUSSION

Comprehensive bigenomic sequencing accurately detects causative genetic variants in affected MD patients, simplifying diagnosis, enabling early treatment, and informing the risk of genetic transmission.

Dynamics of the most common pathogenic mtDNA variant m.3243A > G demonstrate frequency-dependency in blood and positive selection in the germline

The A-to-G point mutation at position 3243 in the human mitochondrial genome (m.3243A > G) is the most common pathogenic mtDNA variant responsible for disease in humans. It is widely accepted that m.3243A > G levels decrease in blood with age, and an age correction representing ~ 2% annual decline is often applied to account for this change in mutation level. Here we report that recent data indicate that the dynamics of m.3243A > G are more complex and depend on the mutation level in blood in a bi-phasic way. Consequently, the traditional 2% correction, which is adequate 'on average', creates opposite predictive biases at high and low mutation levels. Unbiased age correction is needed to circumvent these drawbacks of the standard model. We propose to eliminate both biases by using an approach where age correction depends on mutation level in a biphasic way to account for the dynamics of m.3243A > G in blood. The utility of this approach was further tested in estimating germline selection of m.3243A > G. The biphasic approach permitted us to uncover patterns consistent with the possibility of positive selection for m.3243A > G. Germline selection of m.3243A > G shows an 'arching' profile by which selection is positive at intermediate mutant fractions and declines at high and low mutant fractions. We conclude that use of this biphasic approach will greatly improve the accuracy of modelling changes in mtDNA mutation frequencies in the germline and in somatic cells during aging.

Mitochondrial Dysfunction in Cardiovascular Diseases: Potential Targets for Treatment

Cardiovascular diseases (CVDs) are serious public health issues and are responsible for nearly one-third of global deaths. Mitochondrial dysfunction is accountable for the development of most CVDs. Mitochondria produce adenosine triphosphate through oxidative phosphorylation and inevitably generate reactive oxygen species (ROS). Excessive ROS causes mitochondrial dysfunction and cell death. Mitochondria can protect against these damages via the regulation of mitochondrial homeostasis. In recent years, mitochondria-targeted therapy for CVDs has attracted increasing attention. Various studies have confirmed that clinical drugs (β-blockers, angiotensin-converting enzyme inhibitors/angiotensin receptor-II blockers) against CVDs have mitochondrial protective functions. An increasing number of cardiac mitochondrial targets have shown their cardioprotective effects in experimental and clinical studies. Here, we briefly introduce the mechanisms of mitochondrial dysfunction and summarize the progression of mitochondrial targets against CVDs, which may provide ideas for experimental studies and clinical trials.

Visual memory failure presages conversion to MELAS phenotype

OBJECTIVE

To examine the correlation between verbal and visual memory function and correlation with brain metabolites (lactate and N-Acetylaspartate, NAA) in individuals with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS).

METHODS

Memory performance and brain metabolites (ventricular lactate, occipital lactate, and occipital NAA) were examined in 18 MELAS, 58 m.3243A > G carriers, and 20 familial controls. Measures included the Selective Reminding Test (verbal memory), Benton Visuospatial Retention Test (visual memory), and MR Spectroscopy (NAA, Lactate). ANOVA, chi-squared/Fisher's exact tests, paired t-tests, Pearson correlations, and Spearman correlations were used.

RESULTS

When compared to carriers and controls, MELAS patients had the: (1) most impaired memory functions (Visual: p = 0.0003; Verbal: p = 0.02), (2) greatest visual than verbal memory impairment, (3) highest brain lactate levels (p < 0.0001), and (4) lowest brain NAA levels (p = 0.0003). Occipital and ventricular lactate to NAA ratios correlated significantly with visual memory performance (p ≤ 0.001). Higher lactate levels (p ≤ 0.01) and lower NAA levels (p = 0.0009) correlated specifically with greater visual memory dysfunction in MELAS. There was little or no correlation with verbal memory.

INTERPRETATION

Individuals with MELAS are at increased risk for impaired memory. Although verbal and visual memory are both affected, visual memory is preferentially affected and more clearly associated with brain metabolite levels. Preferential involvement of posterior brain regions is a distinctive clinical signature of MELAS. We now report a distinctive cognitive phenotype that targets visual memory more prominently and earlier than verbal memory. We speculate that this finding in carriers presages a conversion to the MELAS phenotype.

Prevalence and health care costs of mitochondrial disease in Ontario, Canada: A population-based cohort study

Background

Mitochondrial disease prevalence has been estimated at 1 in 4000 in the United States, and 1 in 5000 worldwide. Prevalence in Canada has not been established, though multi-linked health administrative data resources present a unique opportunity to establish robust population-based estimates in a single-payer health system. This study used administrative data for the Ontario, Canada population between April 1988 and March 2019 to measure mitochondrial disease prevalence and describe patient characteristics and health care costs.

Results

3069 unique individuals were hospitalized with mitochondrial disease in Ontario and eligible for the study cohort, representing a period prevalence of 2.51 per 10,000 or 1 in 3989. First hospitalization was most common between ages 0–9 or 50–69. The mitochondrial disease population experiences a high need for health care and incurred high costs (mean = CAD$24,023 in 12 months before first hospitalization) within the single-payer Ontario health care system.

Conclusions

This study provides needed insight into mitochondrial disease in Canada, and demonstrates the high health burden on patients. The methodology used can be adapted across jurisdictions with similar routine collection of health data, such as in other Canadian provinces. Future work should seek to validate this approach via record linkage of existing disease cohorts in Ontario, and identify specific comorbidities with mitochondrial disease that may contribute to high health resource utilization.

Gene Panel Sequencing of Patients With Monogenic Diabetes Brings to Light Genes Typically Associated With Syndromic Presentations

Gene panel sequencing (NGS) offers the possibility of analyzing rare forms of monogenic diabetes (MgD). To that end, 18 genes were analyzed in 1,676 patients referred for maturity-onset diabetes of the young genetic testing. Among the 307 patients with a molecular diagnosis of MgD, 55 (17.9%) had a mutation in a gene associated with a genetic syndrome. Of the patients with mutations, 8% (n = 25) carried the m.3243A>G variant associated with maternally inherited diabetes and deafness. At the time of referral very few had reported hearing loss or any other element of the typical syndromic presentation. Of the patients, 6% had mutation in HNF1B even though the typical extrapancreatic features were not known at the time of referral. Surprisingly, the third most prominent etiology in these rare forms was the WFS1 gene, accounting for 2.9% of the patients with pathogenic mutations (n = 9). None of them displayed a Wolfram syndrome presentation even though some features were reported in six of nine patients. To restrict the analysis of certain genes to patients with the respective specific phenotypes would be to miss those with partial presentations. These results therefore underlie the undisputable benefit of NGS strategies even though the situation implies cascade consequences both for the molecular biologist and for the clinician.

Neurological manifestations in m.3243A>G-related disease triggered by metformin

INTRODUCTION

m.3243A>G-related disease has multi-systemic manifestations including diabetes mellitus. It is uncertain whether metformin would trigger neurological manifestations of this disease. This study aims to review the diagnosis and management of m.3243A>G-related diabetes genetically confirmed by our laboratory and to evaluate the risk of metformin use triggering neurological manifestations.

METHODS

Cases with m.3243A>G detected between 2009 and 2020 were reviewed. Cases with diabetes mellitus were included. Cases with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) before diabetes onset were excluded. Odds ratio was calculated for association between metformin use and newly developed neurological manifestations.

RESULTS

Sixteen patients were identified. Odds ratio for metformin use was 3.50 [0.37-33.0; p = 0.3287]. One illustrative case with clear causal relationship between metformin use and neurological manifestations was described in detail.

CONCLUSION

m.3243A>G-related diabetes mellitus is underdiagnosed. Red flags including positive family history, short stature, low body weight and hearing loss are often overlooked. Early diagnosis allows regular systemic assessment. In the era of precision medicine and novel therapies, it is prudent to avoid metformin as it could trigger neurological manifestations in this condition. Coenzyme Q10, DPP-IV inhibitors, SGLT2 inhibitors and GLP-1 receptor agonists may be considered.

OUP accepted manuscript

Mutations of the mitochondrial DNA are an important cause of inherited diseases that can severely affect the tissue’s homeostasis and integrity. The m.3243A > G mutation is the most commonly observed across mitochondrial disorders and is linked to multisystemic complications, including cognitive deficits. In line with in vitro experiments demonstrating the m.3243A > G’s negative impact on neuronal energy production and integrity, m.3243A > G patients show cerebral grey matter tissue changes. However, its impact on the most neuron dense, and therefore energy-consuming brain structure—the cerebellum—remains elusive. In this work, we used high-resolution structural and functional data acquired using 7 T MRI to characterize the neurodegenerative and functional signatures of the cerebellar cortex in m.3243A > G patients. Our results reveal altered tissue integrity within distinct clusters across the cerebellar cortex, apparent by their significantly reduced volume and longitudinal relaxation rate compared with healthy controls, indicating macroscopic atrophy and microstructural pathology. Spatial characterization reveals that these changes occur especially in regions related to the frontoparietal brain network that is involved in information processing and selective attention. In addition, based on resting-state functional MRI data, these clusters exhibit reduced functional connectivity to frontal and parietal cortical regions, especially in patients characterized by (i) a severe disease phenotype and (ii) reduced information-processing speed and attention control. Combined with our previous work, these results provide insight into the neuropathological changes and a solid base to guide longitudinal studies aimed to track disease progression.

Mutational Analysis and mtDNA Haplogroup Characterization in Three Serbian Cases of Mitochondrial Encephalomyopathies and Literature Review

Mitochondrial encephalomyopathies (MEMP) are heterogeneous multisystem disorders frequently associated with mitochondrial DNA (mtDNA) mutations. Clinical presentation varies considerably in age of onset, course, and severity up to death in early childhood. In this study, we performed molecular genetic analysis for mtDNA pathogenic mutation detection in Serbian children, preliminary diagnosed clinically, biochemically and by brain imaging for mitochondrial encephalomyopathies disorders. Sanger sequencing analysis in three Serbian probands revealed two known pathogenic mutations. Two probands had a heteroplasmic point mutation m.3243A>G in the MT-TL1 gene, which confirmed mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episode syndrome (MELAS), while a single case clinically manifested for Leigh syndrome had an almost homoplasmic (close to 100%) m.8993T>G mutation in the MT-ATP6 gene. After full mtDNA MITOMASTER analysis and PhyloTree build 17, we report MELAS' association with haplogroups U and H (U2e and H15 subclades); likewise, the mtDNA-associated Leigh syndrome proband shows a preference for haplogroup H (H34 subclade). Based on clinical-genetic correlation, we suggest that haplogroup H may contribute to the mitochondrial encephalomyopathies' phenotypic variability of the patients in our study. We conclude that genetic studies for the distinctive mitochondrial encephalomyopathies should be well-considered for realizing clinical severity and possible outcomes.

Clinical Characteristics of Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, a maternally inherited mitochondrial disorder, is characterized by its genetic, biochemical and clinical complexity. The most common mutation associated with MELAS syndrome is the mtDNA A3243G mutation in the MT-TL1 gene encoding the mitochondrial tRNA-leu(UUR), which results in impaired mitochondrial translation and protein synthesis involving the mitochondrial electron transport chain complex subunits, leading to impaired mitochondrial energy production. Angiopathy, either alone or in combination with nitric oxide (NO) deficiency, further contributes to multi-organ involvement in MELAS syndrome. Management for MELAS syndrome is amostly symptomatic multidisciplinary approach. In this article, we review the clinical presentations, pathogenic mechanisms and options for management of MELAS syndrome.

The Genetic Landscape of Mitochondrial Diseases in Spain: A Nationwide Call

The frequency of mitochondrial diseases (MD) has been scarcely documented, and only a few studies have reported data in certain specific geographical areas. In this study, we arranged a nationwide call in Spain to obtain a global estimate of the number of cases. A total of 3274 cases from 49 Spanish provinces were reported by 39 centres. Excluding duplicated and unsolved cases, 2761 patients harbouring pathogenic mutations in 140 genes were recruited between 1990 and 2020. A total of 508 patients exhibited mutations in nuclear DNA genes (75% paediatric patients) and 1105 in mitochondrial DNA genes (33% paediatric patients). A further 1148 cases harboured mutations in the MT-RNR1 gene (56% paediatric patients). The number of reported cases secondary to nuclear DNA mutations increased in 2014, owing to the implementation of next-generation sequencing technologies. Between 2014 and 2020, excepting MT-RNR1 cases, the incidence was 6.34 (95% CI: 5.71–6.97) cases per million inhabitants at the paediatric age and 1.36 (95% CI: 1.22–1.50) for adults. In conclusion, this is the first study to report nationwide epidemiological data for MD in Spain. The lack of identification of a remarkable number of mitochondrial genes necessitates the systematic application of high-throughput technologies in the routine diagnosis of MD.

Sensitive quantification of m.3243A>G mutational proportion in non-retinal tissues and its relationship with visual symptoms

The m.3243A>G mutation in the mitochondrial genome commonly causes retinal degeneration in patients with maternally inherited diabetes and deafness (MIDD) and mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Like other mitochondrial mutations, m.3243A>G is inherited from the mother with a variable proportion of wild type and mutant mitochondrial genomes in different cells. The mechanism by which the m.3243A>G variant in each tissue relates to the manifestation of disease phenotype is not fully understood. Using a digital PCR assay we found that the % m.3243G in skin derived dermal fibroblasts was positively correlated with that of blood from the same individual. The % m.3243G detected in fibroblast cultures remained constant over multiple passages and was negatively correlated with mtDNA copy number. Although the % m.3243G present in blood was not correlated with severity of vision loss, as quantified by Goldmann visual field, a significant negative correlation between % m.3243G and the age of onset of visual symptoms was detected. Together, these results indicate that precise measurement of % m.3243G in clinically accessible tissues such as skin and blood may yield information relevant to the management of retinal m.3243A>G associated disease.

Sonlicromanol improves neuronal network dysfunction and transcriptome changes linked to m.3243A>G heteroplasmy in iPSC-derived neurons

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) is often caused by an adenine to guanine variant at m.3243 (m.3243A>G) of the MT-TL1 gene. To understand how this pathogenic variant affects the nervous system, we differentiated human induced pluripotent stem cells (iPSCs) into excitatory neurons with normal (low heteroplasmy) and impaired (high heteroplasmy) mitochondrial function from MELAS patients with the m.3243A>G pathogenic variant. We combined micro-electrode array (MEA) measurements with RNA sequencing (MEA-seq) and found reduced expression of genes involved in mitochondrial respiration and presynaptic function, as well as non-cell autonomous processes in co-cultured astrocytes. Finally, we show that the clinical phase II drug sonlicromanol can improve neuronal network activity when treatment is initiated early in development. This was intricately linked with changes in the neuronal transcriptome. Overall, we provide insight in transcriptomic changes in iPSC-derived neurons with high m.3243A>G heteroplasmy, and show the pathology is partially reversible by sonlicromanol.

Circulating markers of NADH-reductive stress correlate with mitochondrial disease severity

Mitochondrial disorders represent a large collection of rare syndromes that are difficult to manage both because we do not fully understand biochemical pathogenesis and because we currently lack facile markers of severity. The m.3243A>G variant is the most common heteroplasmic mitochondrial DNA mutation and underlies a spectrum of diseases, notably mitochondrial encephalomyopathy lactic acidosis and stroke-like episodes (MELAS). To identify robust circulating markers of m.3243A>G disease, we first performed discovery proteomics, targeted metabolomics, and untargeted metabolomics on plasma from a deeply phenotyped cohort (102 patients, 32 controls). In a validation phase, we measured concentrations of prioritized metabolites in an independent cohort using distinct methods. We validated 20 analytes (1 protein, 19 metabolites) that distinguish patients with MELAS from controls. The collection includes classic (lactate, alanine) and more recently identified (GDF-15, α-hydroxybutyrate) mitochondrial markers. By mining untargeted mass-spectra we uncovered 3 less well-studied metabolite families: N-lactoyl-amino acids, β-hydroxy acylcarnitines, and β-hydroxy fatty acids. Many of these 20 analytes correlate strongly with established measures of severity, including Karnofsky status, and mechanistically, nearly all markers are attributable to an elevated NADH/NAD+ ratio, or NADH-reductive stress. Our work defines a panel of organelle function tests related to NADH-reductive stress that should enable classification and monitoring of mitochondrial disease.

Patient Care Standards for Primary Mitochondrial Disease in Australia. An Australian adaptation of the Mitochondrial Medicine Society recommendations

This document provides consensus‐based recommendations for general physicians and primary care physicians who diagnose and manage patients with mitochondrial diseases (MD). It builds on previous international guidelines, with particular emphasis on clinical management in the Australian setting. This statement was prepared by a working group of medical practitioners, nurses and allied health professionals with clinical expertise and experience in managing Australian patients with MD. As new treatments and management plans emerge, these consensus‐based recommendations will continue to evolve, but current standards of care are summarised in this document.

Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like Episodes (MELAS) Syndrome: Frequency, Clinical Features, Imaging, Histopathologic, and Molecular Genetic Findings in a Third-level Health Care Center in Mexico

INTRODUCTION

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, is a multisystemic entity of mitochondrial inheritance. To date, there is no epidemiological information on MELAS syndrome in Mexico.

CASE SERIES

A retrospective, cross-sectional design was employed to collect and analyze the data. The clinical records of patients with mitochondrial cytopathies in the period ranging from January 2018 to March 2020 were reviewed. Patients who met definitive Yatsuga diagnostic criteria for MELAS syndrome were included to describe frequency, clinical, imaging, histopathologic, and molecular studies. Of 56 patients diagnosed with mitochondrial cytopathy, 6 patients met definitive Yatsuga criterion for MELAS (10.7%). The median age at diagnosis was 34 years (30 to 34 y), 2 females and the median time from onset of symptoms at diagnosis 3.5 years (1 to 10 y). The median of the number of stroke-like episodes before the diagnosis was 3 (range, 2 to 3). The main findings in computed tomography were basal ganglia calcifications (33%), whereas in magnetic resonance imaging were a lactate peak in the spectroscopy sequence in 2 patients. Five patients (84%) had red-ragged fibers and phantom fibers in the Cox stain in the muscle biopsy. Four patients (67%) had presence of 3243A>G mutation in the mitochondrial MT-TL1 gene. One patient died because of status epilepticus.

CONCLUSIONS

MELAS syndrome represents a common diagnostic challenge for clinicians, often delaying definitive diagnosis. It should be suspected in young patients with stroke of undetermined etiology associated with other systemic and neurological features.

m.3243A > G-Induced Mitochondrial Dysfunction Impairs Human Neuronal Development and Reduces Neuronal Network Activity and Synchronicity

Epilepsy, intellectual and cortical sensory deficits, and psychiatric manifestations are the most frequent manifestations of mitochondrial diseases. How mitochondrial dysfunction affects neural structure and function remains elusive, mostly because of a lack of proper in vitro neuronal model systems with mitochondrial dysfunction. Leveraging induced pluripotent stem cell technology, we differentiated excitatory cortical neurons (iNeurons) with normal (low heteroplasmy) and impaired (high heteroplasmy) mitochondrial function on an isogenic nuclear DNA background from patients with the common pathogenic m.3243A > G variant of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). iNeurons with high heteroplasmy exhibited mitochondrial dysfunction, delayed neural maturation, reduced dendritic complexity, and fewer excitatory synapses. Micro-electrode array recordings of neuronal networks displayed reduced network activity and decreased synchronous network bursting. Impaired neuronal energy metabolism and compromised structural and functional integrity of neurons and neural networks could be the primary drivers of increased susceptibility to neuropsychiatric manifestations of mitochondrial disease.

Sudden Unexpected Death in MELAS Syndrome Due to Diabetic Ketoacidosis

We present a case report of a 25 year-old man with MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes) syndrome, who died suddenly and unexpectedly from diabetic ketoacidosis. This case report illustrates why it is important for medical examiners to be familiar with the clinical and autopsy features of MELAS syndrome and to be aware of the common complications, which may lead to sudden unexpected death.

Mitochondrial DNA A3243G variant-associated retinopathy: a meta-analysis of the clinical course of visual acuity and correlation with systemic manifestations

PURPOSE

The mitochondrial DNA A3243G (m.3243A>G) variant causes a wide spectrum of phenotypes, with pigmentary retinopathy as the most common ocular finding. We undertook this meta-analysis to investigate the clinical course of visual acuity (VA) in patients with m.3243A>G variant and provide key clinical correlations with systemic manifestations.

METHODS

A PubMed literature search was performed and studies were selected after satisfying pre-set inclusion criteria. Demographic and clinical data, including retinal findings and systemic manifestations were recorded. Cross-sectional and linear regression analyses were used to investigate the relationship between VA and age, as well as between the age at diagnosis of retinopathy and the mean ages at diagnosis of sensorineural hearing loss or diabetes. The age and prevalence of systemic manifestations among patients with and without retinopathy were studied using t-tests and Mann-Whitney U-tests (performed on binarized data). Likelihood ratios were computed.

RESULTS

The mean VA (average of both eyes) of 90 patients (72.2% female; 65/90) were collected from 18 studies published between 1990 and 2018. The baseline mean age was 45.2 years (range 17 to 92). The mean logMAR VA was 0.10 (- 0.12 to 1.39). There was a statistically significant linear correlation between the logMAR VA and age (p = .008). The VA of patients less than or equal to 50 years of age was significantly better than that of patients older than 50 years (0.06 vs.0.18 logMAR, p = .002). 67 patients (74.4%) showed a characteristic pigmentary retinopathy with a mean age at diagnosis of 47.9 years (17 to 92) and VA of 0.14 logMAR (- 0.12 to 1.24). Age at diagnosis of retinopathy was linearly correlated with age at diagnosis of hearing loss or diabetes (p < .001). Patients with retinopathy were more likely to have hearing loss (83.6% vs. 56.5%, p = .03) or diabetes (56.7% vs. 17.4%, p = .001) than those without retinopathy. Those with both hearing loss and diabetes had an earlier onset of retinopathy than those without (46.4 vs. 60.4 years, p = .01). Patients without both hearing loss and diabetes were 5.3-fold less likely to develop a retinopathy.

CONCLUSIONS

Patients with m.3243A>G variant pigmentary retinopathy maintain highly functional VA until around the fifth decade of life, after which significant visual decline ensues. Patients without hearing loss and diabetes have a lower likelihood of exhibiting a retinopathy, which tends to appear about one decade after hearing loss and diabetes are diagnosed.

Adapting CRISPR/Cas9 System for Targeting Mitochondrial Genome

Gene editing of the mitochondrial genome using the CRISPR-Cas9 system is highly challenging mainly due to sub-efficient delivery of guide RNA and Cas9 enzyme complexes into the mitochondria. In this study, we were able to perform gene editing in the mitochondrial DNA by appending an NADH-ubiquinone oxidoreductase chain 4 (ND4) targeting guide RNA to an RNA transport-derived stem loop element (RP-loop) and expressing the Cas9 enzyme with a preceding mitochondrial localization sequence. We observe mitochondrial colocalization of RP-loop gRNA and a marked reduction of ND4 expression in the cells carrying a 11205G variant in their ND4 sequence coincidently decreasing the mtDNA levels. This proof-of-concept study suggests that a stem-loop element added sgRNA can be transported to the mitochondria and functionally interact with Cas9 to mediate sequence-specific mtDNA cleavage. Using this novel approach to target the mtDNA, our results provide further evidence that CRISPR-Cas9-mediated gene editing might potentially be used to treat mitochondrial-related diseases.

The molecular pathology of pathogenic mitochondrial tRNA variants

Mitochondrial diseases are clinically and genetically heterogeneous disorders, caused by pathogenic variants in either the nuclear or mitochondrial genome. This heterogeneity is particularly striking for disease caused by variants in mitochondrial DNA-encoded tRNA (mt-tRNA) genes, posing challenges for both the treatment of patients and understanding the molecular pathology. In this review, we consider disease caused by the two most common pathogenic mt-tRNA variants: m.3243A>G (within MT-TL1, encoding mt-tRNALeu(UUR) ) and m.8344A>G (within MT-TK, encoding mt-tRNALys ), which together account for the vast majority of all mt-tRNA-related disease. We compare and contrast the clinical disease they are associated with, as well as their molecular pathologies, and consider what is known about the likely molecular mechanisms of disease. Finally, we discuss the role of mitochondrial-nuclear crosstalk in the manifestation of mt-tRNA-associated disease and how research in this area not only has the potential to uncover molecular mechanisms responsible for the vast clinical heterogeneity associated with these variants but also pave the way to develop treatment options for these devastating diseases.

Clinical application of sequencing-based methods for parallel preimplantation genetic testing for mitochondrial DNA disease and aneuploidy

OBJECTIVE

To validate and apply a strategy permitting parallel preimplantation genetic testing (PGT) for mitochondrial DNA (mtDNA) disease and aneuploidy (PGT-A).

DESIGN

Preclinical test validation and case reports.

SETTING

Fertility centers. Diagnostics laboratory.

PATIENTS

Four patients at risk of transmitting mtDNA disease caused by m.8993T>G (Patients A and B), m.10191T>G (Patient C), and m.3243A>G (Patient D). Patients A, B, and C had affected children. Patients A and D displayed somatic heteroplasmy for mtDNA mutations.

INTERVENTIONS

Embryo biopsy, genetic testing, and uterine transfer of embryos predicted to be euploid and mutation-free.

MAIN OUTCOME MEASURES

Test accuracy, treatment outcomes, and mutation segregation.

RESULTS

Accuracy of mtDNA mutation quantification was confirmed. The test was compatible with PGT-A, and half of the embryos tested were shown to be aneuploid (16/33). Mutations were detected in approximately 40% of embryo biopsies from Patients A and D (10/24) but in none from Patients B and C (n = 29). Patients B and C had healthy children following PGT and natural conception, respectively. The m.8993T>G mutation displayed skewed segregation, whereas m.3243A>G mutation levels were relatively low and potentially impacted embryo development.

CONCLUSIONS

Considering the high aneuploidy rate, strategies providing a combination of PGT for mtDNA disease and aneuploidy may be advantageous compared with approaches that consider only mtDNA. Heteroplasmic women had a higher incidence of affected embryos than those with undetectable somatic mutant mtDNA but were still able to produce mutation-free embryos. While not conclusive, the results are consistent with the existence of mutation-specific segregation mechanisms occurring during oogenesis and possibly embryogenesis.

Mitochondrial DNA A3243G variant-associated retinopathy: Current perspectives and clinical implications

Cellular function and survival are critically dependent on the proper functionality of the mitochondrion. Neurodegenerative cellular processes including cellular adenosine triphosphate production, intermediary metabolism control, and apoptosis regulation are all mitochondrially mediated. The A to G transition at position 3243 in the mitochondrial MTTL1 gene that encodes for the leucine transfer RNA (m.3243A>G) causes a variety of diseases, including maternally inherited loss of hearing and diabetes syndrome (MIDD), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). Ophthalmological findings-including posterior sub-capsular cataract, ptosis, external ophthalmoplegia, and pigmentary retinopathy- have all been associated with the m.3243A>G variant. Pigmentary retinopathy is, however, the most common ocular finding, occurring in 38% to 86% of cases. To date, little is known about the pathogenesis, natural history, and heteroplasmic and phenotypic correlations of m.3243A>G-associated pigmentary retinopathy. We summarize the current understanding of mitochondrial genetics and pathogenesis of some associated diseases. We then review the pathophysiology, histology, clinical features, treatment, and important ocular and systemic phenotypic manifestations of m.3243A>G variant associated retinopathy. Mitochondrial diseases require a multidisciplinary team approach to ensure effective treatment, regular follow-up, and accurate genetic counseling.