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

Bilateral cochlear implants in a MELAS patient

BACKGROUND

Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is a maternally inherited mitochondrial disease that affects various systems in the body, particularly the brain, nervous system, and muscles. Among these systems, sensorineural hearing loss is a common additional symptom.

METHODS

A 42-year-old female patient with MELAS who experienced bilateral profound deafness and underwent bilateral sequential cochlear implantation (CIs). Speech recognition and subjective outcomes were evaluated.

RESULTS

Following the first CI follow-up, the patient exhibited improved speech recognition ability and decided to undergo the implantation of the second ear just two months after the initial CI surgery. The second CI also demonstrated enhanced speech recognition ability. Subjective outcomes were satisfactory for bilateral CIs.

CONCLUSIONS

MELAS patients receiving bilateral CIs can attain satisfactory post-CI speech recognition, spatial hearing, and sound qualities.

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.

Psychobiological regulation of plasma and saliva GDF15 dynamics in health and mitochondrial diseases

GDF15 (growth differentiation factor 15) is a marker of cellular energetic stress linked to physical-mental illness, aging, and mortality. However, questions remain about its dynamic properties and measurability in human biofluids other than blood. Here, we examine the natural dynamics and psychobiological regulation of plasma and saliva GDF15 in four human studies representing 4,749 samples from 188 individuals. We show that GDF15 protein is detectable in saliva (8% of plasma concentration), likely produced by salivary glands secretory duct cells. Using a brief laboratory socio-evaluative stressor paradigm, we find that psychosocial stress increases plasma (+3.5-5.9%) and saliva GDF15 (+43%) with distinct kinetics, within minutes. Moreover, saliva GDF15 exhibits a robust awakening response, declining by ~40-89% within 30-45 minutes from its peak level at the time of waking up. Clinically, individuals with genetic mitochondrial OxPhos diseases show elevated baseline plasma and saliva GDF15, and post-stress GDF15 levels in both biofluids correlate with multi-system disease severity, exercise intolerance, and the subjective experience of fatigue. Taken together, our data establish that saliva GDF15 is dynamic, sensitive to psychological states, a clinically relevant endocrine marker of mitochondrial diseases. These findings also point to a shared psychobiological pathway integrating metabolic and mental stress.

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.

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.

Endocrine features of primary mitochondrial diseases

PURPOSE OF REVIEW

Primary mitochondrial diseases are one of the most prevalent groups of multisystem genetic disorders. Endocrinopathies associated with mitochondrial diseases may have clinical features that are distinct from the more common forms. We provide an overview of mitochondrial disorder genetics and phenotypes, focusing on recent studies regarding identification and treatment of associated endocrinopathies.

RECENT FINDINGS

Known endocrine phenotypes of mitochondrial disorders continue to expand, and now include growth hormone deficiency, hypogonadism, precocious puberty, hypoparathyroidism, hypo- and hyperthyroidism, diabetes, and adrenal insufficiency. Recent studies suggest several genotype-phenotype correlations, including those related to nuclear variants. Diagnosis is important, as special considerations should be made in the management of endocrinopathies in mitochondrial patients. Finally, new mitochondrial replacement strategies may soon be available for women interested in preventing mitochondrial disease transmission to offspring.

SUMMARY

Patients with multiple endocrinopathies or atypical endocrinopathies should be evaluated for primary mitochondrial disease, as a diagnosis may impact management of these individuals.

Therapeutic Effects of Mesenchymal Stromal Cells Require Mitochondrial Transfer and Quality Control

Due to their beneficial effects in an array of diseases, Mesenchymal Stromal Cells (MSCs) have been the focus of intense preclinical research and clinical implementation for decades. MSCs have multilineage differentiation capacity, support hematopoiesis, secrete pro-regenerative factors and exert immunoregulatory functions promoting homeostasis and the resolution of injury/inflammation. The main effects of MSCs include modulation of immune cells (macrophages, neutrophils, and lymphocytes), secretion of antimicrobial peptides, and transfer of mitochondria (Mt) to injured cells. These actions can be enhanced by priming (i.e., licensing) MSCs prior to exposure to deleterious microenvironments. Preclinical evidence suggests that MSCs can exert therapeutic effects in a variety of pathological states, including cardiac, respiratory, hepatic, renal, and neurological diseases. One of the key emerging beneficial actions of MSCs is the improvement of mitochondrial functions in the injured tissues by enhancing mitochondrial quality control (MQC). Recent advances in the understanding of cellular MQC, including mitochondrial biogenesis, mitophagy, fission, and fusion, helped uncover how MSCs enhance these processes. Specifically, MSCs have been suggested to regulate peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1α)-dependent biogenesis, Parkin-dependent mitophagy, and Mitofusins (Mfn1/2) or Dynamin Related Protein-1 (Drp1)-mediated fission/fusion. In addition, previous studies also verified mitochondrial transfer from MSCs through tunneling nanotubes and via microvesicular transport. Combined, these effects improve mitochondrial functions, thereby contributing to the resolution of injury and inflammation. Thus, uncovering how MSCs affect MQC opens new therapeutic avenues for organ injury, and the transplantation of MSC-derived mitochondria to injured tissues might represent an attractive new therapeutic approach.

Deleterious heteroplasmic mitochondrial mutations are associated with an increased risk of overall and cancer-specific mortality

Mitochondria carry their own circular genome and disruption of the mitochondrial genome is associated with various aging-related diseases. Unlike the nuclear genome, mitochondrial DNA (mtDNA) can be present at 1000 s to 10,000 s copies in somatic cells and variants may exist in a state of heteroplasmy, where only a fraction of the DNA molecules harbors a particular variant. We quantify mtDNA heteroplasmy in 194,871 participants in the UK Biobank and find that heteroplasmy is associated with a 1.5-fold increased risk of all-cause mortality. Additionally, we functionally characterize mtDNA single nucleotide variants (SNVs) using a constraint-based score, mitochondrial local constraint score sum (MSS) and find it associated with all-cause mortality, and with the prevalence and incidence of cancer and cancer-related mortality, particularly leukemia. These results indicate that mitochondria may have a functional role in certain cancers, and mitochondrial heteroplasmic SNVs may serve as a prognostic marker for cancer, especially for leukemia.

Modeling-based prediction tools for preimplantation genetic testing of mitochondrial DNA diseases: estimating symptomatic thresholds, risk, and chance of success

PURPOSE

Preimplantation genetic testing (PGT) has become a reliable tool for preventing the germline transmission of mitochondrial DNA (mtDNA) variants. However, procedures are not standardized across mtDNA variants. In this study, we aim to estimate symptomatic thresholds, risk, and chance of success for PGT for mtDNA pathogenic variant carriers.

METHODS

We performed a systematic analysis of heteroplasmy data including 455 individuals from 187 familial pedigrees with the common m.3243A>G, m.8344A>G, or m.8993T>G pathogenic variants. We applied binary logistic regression for estimating symptomatic thresholds of heteroplasmy, simplified Sewell-Wright formula and Kimura equations for predicting the risk of disease transmission, and binomial distribution for predicting minimum oocyte numbers.

RESULTS

We estimated the symptomatic thresholds of m.8993T>G and m.8344A>G as 29.86% and 16.15%, respectively. We could not determine a threshold for m.3243A>G. We established models for mothers harboring common and rare mtDNA pathogenic variants to predict the risk of disease transmission and the number of oocytes required to produce an embryo with sufficiently low variant load. In addition, we provide a table allowing the prediction of transmission risk and the minimum required oocytes for PGT patients with different variant levels.

CONCLUSION

We have established models that can determine the symptomatic thresholds of common mtDNA pathogenic variants. We also constructed universal models applicable to nearly all mtDNA pathogenic variants which can predict risk and minimum numbers for PGT patients. These models have advanced our understanding of mtDNA disease pathogenesis and will enable more effective prevention of disease transmission using PGT.

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.

Hearing Impairment and Neuroimaging Results in Mitochondrial Diseases

Mitochondrial diseases (MDs) are heterogeneous genetic disorders characterized by mitochondrial DNA (mtDNA) defects, involving tissues highly dependent on oxidative metabolism: the inner ear, brain, eye, skeletal muscle, and heart. We describe adult patients with genetically defined MDs, characterizing hearing function and neuroimaging results. We enrolled 34 patients (mean age: 50.02 ± 15 years, range: 18-75 years; 20 females and 14 males) classified in four groups: MELAS, MIDD, PEO, and Encephalopathy/Polyneuropathy. Audiological evaluations included psychoacoustical tests (pure-tone and speech audiometry), electrophysiological tests (Auditory Brainstem Responses, ABRs), and Impedenzometry. Neuroimaging evaluations considered global MRI abnormalities or structural brain changes. In total, 19/34 patients carried the m.3243A > G mutation (6 affected by MELAS, 12 affected by MIDD, and 1 affected by PEO); 11 had an mtDNA deletion (all affected by PEO); 3 had nuclear genes associated with MDs (POLG1 and OPA1); and 1 patient had an mtDNA deletion without an identified nuclear gene defect (affected by PEO). Sensory neural, bilateral, and symmetrical hearing loss was present in 25 patients (73.5%) to different degrees: 9 mild, 9 moderate, 5 severe, and 2 profound. The severe/profound and mild hearing losses were associated with pantonal and high-frequency audiograms, respectively. Instead, moderate hearing losses were associated with both high-frequency (five cases) and pantonal (five cases) audiogram shapes. In addition, 21/25 patients showed a cochlear site of lesion (84%), and 4/25 (16%) showed a retrocochlear site. We found global MRI abnormalities or structural brain changes in 26/30 subjects (86.6%): 21 had white matter abnormalities, 15 had cortical atrophy, 10 had subcortical atrophy, 8 had basal nuclei involvement or cerebellar atrophy, 4 had stroke-like lesions or laminar necrosis, and 1 had cysts or vacuolated lesions. We concluded that genetic alterations are associated with different clinical presentations for both auditory function and neuroradiological findings. There is no fixed relationship between genotype and phenotype for the clinical conditions analyzed.

Treatment of a patient with severe lactic acidosis and multiple organ failure due to mitochondrial myopathy: A case report

BACKGROUND

Mitochondrial myopathy is a rare genetic disease with maternal inheritance that may involve multiple organ systems. Due to the lack of typical characteristics, its clinical diagnosis is difficult, and it is often misdiagnosed or even missed.

CASE SUMMARY

The patient was a young college student. When he presented at the hospital, he had severe lactic acidosis, respiratory failure, and shock with multiple organ dysfunction syndrome (MODS). He was treated by mechanical ventilation, veno-arterial extracorporeal membrane oxygenation, and other organ support. However, his condition continued to worsen. After a thorough and detailed medical and family history was taken, a mitochondrial crisis was suspected. A muscle biopsy was taken. Further genetic testing confirmed a mitochondrial gene mutation (TRNL1 3243A>G). The final diagnosis of mitochondrial myopathy was made. Although there is no known specific treatment, intravenous methylprednisone and intravenous immunoglobulin were started. The patient's shock eventually improved. The further course was complicated by severe infection in multiple sites, severe muscle weakness, and recurrent MODS. After 2 mo of multidisciplinary management and intensive rehabilitation, the patient could walk with assistance 4 mo after admission and walk independently 6 mo after admission.

CONCLUSION

More attention should be paid to mitochondrial myopathy to avoid missed diagnosis and misdiagnosis.

Treatment of a patient with severe lactic acidosis and multiple organ failure due to mitochondrial myopathy: A case report

BACKGROUND

Mitochondrial myopathy is a rare genetic disease with maternal inheritance that may involve multiple organ systems. Due to the lack of typical characteristics, its clinical diagnosis is difficult, and it is often misdiagnosed or even missed.

CASE SUMMARY

The patient was a young college student. When he presented at the hospital, he had severe lactic acidosis, respiratory failure, and shock with multiple organ dysfunction syndrome (MODS). He was treated by mechanical ventilation, veno-arterial extracorporeal membrane oxygenation, and other organ support. However, his condition continued to worsen. After a thorough and detailed medical and family history was taken, a mitochondrial crisis was suspected. A muscle biopsy was taken. Further genetic testing confirmed a mitochondrial gene mutation (TRNL1 3243A>G). The final diagnosis of mitochondrial myopathy was made. Although there is no known specific treatment, intravenous methylprednisone and intravenous immunoglobulin were started. The patient’s shock eventually improved. The further course was complicated by severe infection in multiple sites, severe muscle weakness, and recurrent MODS. After 2 mo of multidisciplinary management and intensive rehabilitation, the patient could walk with assistance 4 mo after admission and walk independently 6 mo after admission.

CONCLUSION

More attention should be paid to mitochondrial myopathy to avoid missed diagnosis and misdiagnosis.

Treatment of a patient with severe lactic acidosis and multiple organ failure due to mitochondrial myopathy: A case report

BACKGROUND

Mitochondrial myopathy is a rare genetic disease with maternal inheritance that may involve multiple organ systems. Due to the lack of typical characteristics, its clinical diagnosis is difficult, and it is often misdiagnosed or even missed.

CASE SUMMARY

The patient was a young college student. When he presented at the hospital, he had severe lactic acidosis, respiratory failure, and shock with multiple organ dysfunction syndrome (MODS). He was treated by mechanical ventilation, veno-arterial extracorporeal membrane oxygenation, and other organ support. However, his condition continued to worsen. After a thorough and detailed medical and family history was taken, a mitochondrial crisis was suspected. A muscle biopsy was taken. Further genetic testing confirmed a mitochondrial gene mutation (TRNL1 3243A>G). The final diagnosis of mitochondrial myopathy was made. Although there is no known specific treatment, intravenous methylprednisone and intravenous immunoglobulin were started. The patient’s shock eventually improved. The further course was complicated by severe infection in multiple sites, severe muscle weakness, and recurrent MODS. After 2 mo of multidisciplinary management and intensive rehabilitation, the patient could walk with assistance 4 mo after admission and walk independently 6 mo after admission.

CONCLUSION

More attention should be paid to mitochondrial myopathy to avoid missed diagnosis and misdiagnosis.

The Role of Primary Mitochondrial Disorders in Hearing Impairment: An Overview

Background. Defects of mitochondrial DNA (mtDNA) involved in the function of the mitochondrial electron transport chain can result in primary mitochondrial diseases (PMDs). Various features can influence the phenotypes of different PMDs, with relevant consequences on clinical presentation, including the presence of hearing impairment. This paper aims to describe the hearing loss related to different PMDs, and when possible, their phenotype. Methods. A systematic review was performed according to PRISMA guidelines, searching Medline until December 2022. A total of 485 papers were identified, and based on specified criteria, 7 were included in this study. Results. A total of 759 patients affected by PMDs and hearing loss were included. The age of patients ranged from 2 days to 78 years old, and the male-to-female ratio was 1.3:1. The percentage of subjects affected by hearing loss was 40.8%, (310/759), and in most cases, hearing impairment was described as sensorineural, bilateral, symmetrical, and progressive, with different presentations depending on age and syndrome severity. Conclusions. PMDs are challenging conditions with different clinical phenotypes. Hearing loss, especially when bilateral and progressive, may represent a red flag; its association with other systemic disorders (particularly neuromuscular, ocular, and endocrine) should alert clinicians, and confirmation via genetic testing is mandatory nowadays.

Case report: Transient lactate elevation by intravenous insulin infusion therapy for diabetic ketoacidosis in a patient with mitochondrial DNA 3243 A > G mutation: A glycolysis rebooting syndrome?

Mitochondrial disease, most cases of which are caused by mitochondrial DNA (mtDNA) mutation, is present with multiple phenotypes including diabetes mellitus, sensorineural hearing loss, cardiomyopathy, muscle weakness, renal dysfunction, and encephalopathy, depending on the degree of heteroplasmy. While mitochondria play an important role in intracellular glucose and lactate metabolism in insulin-sensitive tissues such as muscles, appropriate strategies for glycemic control have not yet been established in a patient with mitochondrial disease, which is often complicated by myopathy. Here, we describe the history of a 40-year-old man with mtDNA 3243A > G who had sensorineural hearing loss, cardiomyopathy, muscle wasting, and diabetes mellitus with stage 3 chronic kidney disease. He developed mild diabetic ketoacidosis (DKA) in the process of treatment for poor glycemic control with severe latent hypoglycemia. According to the standard therapy for DKA, he was treated with continuous intravenous insulin infusion therapy, which unexpectedly resulted in an abrupt and transient elevation in blood lactate levels without exacerbation of heart failure and kidney function. Since blood lactate levels are determined by the balance between lactate production and consumption, an abrupt and transient lactate elevation following intravenous insulin injection therapy may reflect not only enhanced glycolysis in insulin-sensitive tissues with mitochondrial dysfunction but also decreased lactate consumption in the sarcopenic skeletal muscle and failing heart. Intravenous insulin infusion therapy in patients with mitochondrial disease may unmask derangements of intracellular glucose metabolism in response to insulin signaling.