Identifying trajectories of fatigue in patients with primary mitochondrial disease due to the m.3243A > G variant.

Severe fatigue is a common complaint in patients with primary mitochondrial disease. However, less is known about the course of fatigue over time. This longitudinal observational cohort study of patients with the mitochondrial DNA 3243 A>G variant explored trajectories of fatigue over 2 years, and characteristics of patients within these fatigue trajectories. Fifty-three adult patients treated at the Radboud University Medical Center Nijmegen were included. The majority of the patients reported consistent, severe fatigue (41%), followed by patients with a mixed pattern of severe and mild fatigue (36%). Then, 23% of patients reported stable mild fatigue levels. Patients with a stable high fatigue trajectory were characterized by higher disease manifestations scores, more clinically relevant mental health symptoms, and lower psychosocial functioning and quality of life compared to patients reporting stable low fatigue levels. Fatigue at baseline and disease manifestation scores predicted fatigue severity at the 2-year assessment (57% explained variance). This study demonstrates that severe fatigue is a common and stable complaint in the majority of patients. Clinicians should be aware of severe fatigue in patients with moderate to severe disease manifestation scores on the Newcastle Mitochondrial Disease Scale, the high prevalence of clinically relevant mental health symptoms and overall impact on quality of life in these patients. Screening of fatigue and psychosocial variables will guide suitable individualized treatment to improve the quality of life.

Companies Committed to Responsible AI: From Principles towards Implementation and Regulation?

The term 'responsible AI' has been coined to denote AI that is fair and non-biased, transparent and explainable, secure and safe, privacy-proof, accountable, and to the benefit of mankind. Since 2016, a great many organizations have pledged allegiance to such principles. Amongst them are 24 AI companies that did so by posting a commitment of the kind on their website and/or by joining the 'Partnership on AI'. By means of a comprehensive web search, two questions are addressed by this study: (1) Did the signatory companies actually try to implement these principles in practice, and if so, how? (2) What are their views on the role of other societal actors in steering AI towards the stated principles (the issue of regulation)? It is concluded that some three of the largest amongst them have carried out valuable steps towards implementation, in particular by developing and open sourcing new software tools. To them, charges of mere 'ethics washing' do not apply. Moreover, some 10 companies from both the USA and Europe have publicly endorsed the position that apart from self-regulation, AI is in urgent need of governmental regulation. They mostly advocate focussing regulation on high-risk applications of AI, a policy which to them represents the sensible middle course between laissez-faire on the one hand and outright bans on technologies on the other. The future shaping of standards, ethical codes, and laws as a result of these regulatory efforts remains, of course, to be determined.

One mutation, three phenotypes: novel metabolic insights on MELAS, MIDD and myopathy caused by the m.3243A > G mutation.

INTRODUCTION: The m.3243A > G mitochondrial DNA mutation is one of the most common mitochondrial disease-causing mutations, with a carrier rate as high as 1:400. This point mutation affects the MT-TL1 gene, ultimately affecting the oxidative phosphorylation system and the cell's energy production. Strikingly, the m.3243A > G mutation is associated with different phenotypes, including mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), maternally inherited diabetes and deafness (MIDD) and myopathy. OBJECTIVES: We investigated urine metabolomes of MELAS, MIDD and myopathy patients in order to identify affected metabolic pathways and possible treatment options. METHODS: A multiplatform metabolomics approach was used to comprehensively analyze the metabolome and compare metabolic profiles of different phenotypes caused by the m.3243A > G mutation. Our analytical array consisted of NMR spectroscopy, LC-MS/MS and GC-TOF-MS. RESULTS: The investigation revealed phenotypic specific metabolic perturbations, as well as metabolic similarities between the different phenotypes. We show that glucose metabolism is highly disturbed in the MIDD phenotype, but not in MELAS or myopathy, remodeled fatty acid oxidation is characteristic of the MELAS patients, while one-carbon metabolism is strongly modified in both MELAS and MIDD, but not in the myopathy group. Lastly we identified increased creatine in the urine of the myopathy patients, but not in MELAS or MIDD. CONCLUSION: We conclude by giving novel insight on the phenotypes of the m.3243A > G mutation from a metabolomics point of view. Directives are also given for future investigations that could lead to better treatment options for patients suffering from this debilitating disease.

Six-year prospective follow-up study in 151 carriers of the mitochondrial DNA 3243 A>G variant.

BACKGROUND: The mitochondrial DNA (mDNA) 3243A>G variant is the most common pathogenic variant of the mDNA. To interpret results of clinical trials in mitochondrial disease, it is important to have a clear understanding of the natural course of disease. To obtain more insight into the disease burden and the progression of disease in carriers of the mDNA 3243 A>G variant, we followed a cohort of 151 carriers from 61 families prospectively for up to 6 years. METHODS: The disease severity was scored using the Newcastle Mitochondrial Disease Adult Scale (NMDAS), including SF-36 quality of life (QoL) scores. Heteroplasmy levels were measured in urinary epithelial cells (UEC), leucocytes and saliva. The progression of the disease was studied using linear mixed model analysis. RESULTS: One hundred twenty-four carriers (out of 151) were symptomatic. Four clinical groups were identified: 1) classical mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome (n=7), 2) maternally inherited diabetes deafness syndrome (n=60), 3) 'other' (n=57) and 4) dormant carriers (n=27). A yearly increase of NMDAS score of 0.47 point was measured in the total group. Heteroplasmy levels in both leucocytes and UEC were only weakly correlated with disease severity. Physical QoL declined with age. The most important determinants of QoL decline were hearing loss, speech problems, exercise intolerance, gait instability, psychiatric problems and gastrointestinal involvement. CONCLUSION: The mDNA 3243 A>G variant causes a slowly progressive disease, with a yearly increase of NMDAS score of ~0.5 point overall with the clinical phenotype being the only determinant of disease progression.

Five non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype adult patients with m.3243A>G mutation after kidney transplantation: follow-up and review of the literature.

BACKGROUND: Renal involvement in patients with the m.3243A>G mutation may result in end-stage renal disease (ESRD) requiring renal replacement therapy. Although kidney transplantations have been performed in a small number of patients, short- and long-term follow-up data are lacking. METHODS: We describe five patients with the m.3243AG carriers described in the literature. RESULTS: Proteinuria with or without renal failure was the first clinical presentation of renal involvement in 13 of 18 (72%) patients. Focal segmental glomerulosclerosis (FSGS) was found in 9 of 13 (69%) biopsies. Sixteen of 18 (84%) patients developed hearing loss. All patients were diagnosed with diabetes mellitus, of whom eight (44%) developed the disease after transplantation. All patients with reported follow-up data (13/18) had stable kidney function from 6 months to 13 years of follow-up after transplantation. CONCLUSIONS: Renal involvement in carriers of the m.3243A>G mutation most commonly leads to proteinuria and FSGS and may lead to ESRD. Proper recognition of the mitochondrial origin of the renal disease in these patients is important for adequate treatment selection and suitable supportive care. This case series and review of the available literature on long-term follow-up after kidney transplantation shows it is feasible for non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype carriers of the m.3243A>G mutation to be considered for kidney transplantation in case of ESRD. These patients should not be excluded from transplant solely for their mitochondrial diagnosis.

A urinary biosignature for mitochondrial myopathy, encephalopathy, lactic acidosis and stroke like episodes (MELAS).

We used a comprehensive metabolomics approach to study the altered urinary metabolome of two mitochondrial myopathy, encephalopathy lactic acidosis and stroke like episodes (MELAS) cohorts carrying the m.3243A>G mutation. The first cohort were used in an exploratory phase, identifying 36 metabolites that were significantly perturbed by the disease. During the second phase, the 36 selected metabolites were able to separate a validation cohort of MELAS patients completely from their respective control group, suggesting usefulness of these 36 markers as a diagnostic set. Many of the 36 perturbed metabolites could be linked to an altered redox state, fatty acid catabolism and one-carbon metabolism. However, our evidence indicates that, of all the metabolic perturbations caused by MELAS, stalled fatty acid oxidation prevailed as being particularly disturbed. The strength of our study was the utilization of five different analytical platforms to generate the robust metabolomics data reported here. We show that urine may be a useful source for disease-specific metabolomics data, linking, amongst others, altered one-carbon metabolism to MELAS. The results reported here are important in our understanding of MELAS and might lead to better treatment options for the disease.

The KHENERGY Study: Safety and Efficacy of KH176 in Mitochondrial m.3243A>G Spectrum Disorders.

KH176 is a potent intracellular reduction-oxidation-modulating compound developed to treat mitochondrial disease. We studied tolerability, safety, pharmacokinetics, pharmacodynamics, and efficacy of twice daily oral 100 mg KH176 for 28 days in a double-blind, randomized, placebo-controlled, two-way crossover phase IIA study in 18 adult m.3243A>G patients without cardiovascular involvement. Efficacy parameters included clinical and functional outcome measures and biomarkers. The trial was registered within ClinicalTrials.gov (NCT02909400), the European Clinical Trials Database (2016-001696-79), and ISRCTN (43372293) (The KHENERGY study). Twice daily oral 100 mg KH176 was well tolerated and appeared safe. No serious treatment-emergent adverse events were reported. No significant improvements in gait parameters or other outcome measures were obtained, except for a positive effect on alertness and mood, although a coincidence due to multiplicity cannot be ignored. The results of the study provide first data on safety and efficacy of KH176 in patients with mitochondrial disease and will be instrumental in designing future clinical trials.

Intra-patient variability of heteroplasmy levels in urinary epithelial cells in carriers of the m.3243A>G mutation.

BACKGROUND: The mitochondrial DNA m.3243A>G mutation is one the most prevalent mutation causing mitochondrial disease in adult patients. Several cohort studies have used heteroplasmy levels in urinary epithelial cells (UEC) to correlate the genotype of the patients to the clinical severity. However, the interpretation of these data is hampered by a lack of knowledge on the intra-patient variability of the heteroplasmy levels. The goal of this study was to determine the day-to-day variation of the heteroplasmy levels in UEC. METHODS: Fifteen carriers of the m.3243A>G mutation collected five urine samples in a 14-day window. Heteroplasmy levels of the m.3243A>G mutation were determined in these samples. Data from the national cohort study, including Newcastle Mitochondrial Disease Adult Scale scores and clinical diagnosis, were used. RESULTS: In the samples of six patients, heteroplasmy levels were within a 5% margin. In the samples collected from five patients, the margin was >20%. CONCLUSION: Heteroplasmy levels of UEC in carriers of the m.3243A>G mutation have a significant day-to-day variation. The interpretation of a correlation between heteroplasmy levels in urine and disease severity is therefore not reliable. Therefore, heteroplasmy levels in UEC should not be used as a prognostic biomarker in these patients.

Fear of disease progression in carriers of the m.3243A > G mutation.

BACKGROUND: Being diagnosed with mitochondrial disease due to the m.3243A > G mutation is frequently preceded by a long diagnostic process. The disease itself is characterized by heterogeneous course and expression, so leaving patients with considerable uncertainty regarding their prognosis and treatment possibilities. This could easily result in fear of disease progression. This study investigated the presence of this fear and its correlates with genetic characteristics and clinical disease severity in m.3243A > G carriers. METHODS: In total 125 eligible m.3243A > G mutation carriers were invited to participate in this cross-sectional study. After informed consent, participants completed questionnaires including items on socio-demographics, fear of progression, depression, anxiety, and quality of life. Clinical disease severity was assessed by the NMDAS questionnaire. Heteroplasmy levels were assessed in leucocytes, urine epithelial cells and buccal mucosa. RESULTS: Seventy-six carriers participated in this study. Results showed that 18% reported high fear of progression. Fear of progression was significantly related to all domains of quality of life. Furthermore, fear of progression was moderately correlated with feelings of depression (r = .37), and anxiety (r = .44). Patients with moderate or severe clinical symptoms on the NMDAS experienced more fear of progression than patients with mild clinical symptoms. Fear of progression was weakly correlated with heteroplasmy in leucocytes (r = .27) and buccal mucosa (r = .31). CONCLUSIONS: A substantial part of m.3243A > G mutation carriers experience high levels of fear of progression which coincide with significantly lower quality of life. Only a small relation with disease characteristics was found. The impact of receiving a diagnosis without therapeutic possibilities on fear is important to consider.

Algorithmic Decision-Making Based on Machine Learning from Big Data: Can Transparency Restore Accountability?

Decision-making assisted by algorithms developed by machine learning is increasingly determining our lives. Unfortunately, full opacity about the process is the norm. Would transparency contribute to restoring accountability for such systems as is often maintained? Several objections to full transparency are examined: the loss of privacy when datasets become public, the perverse effects of disclosure of the very algorithms themselves ("gaming the system" in particular), the potential loss of companies' competitive edge, and the limited gains in answerability to be expected since sophisticated algorithms usually are inherently opaque. It is concluded that, at least presently, full transparency for oversight bodies alone is the only feasible option; extending it to the public at large is normally not advisable. Moreover, it is argued that algorithmic decisions preferably should become more understandable; to that effect, the models of machine learning to be employed should either be interpreted ex post or be interpretable by design ex ante.

Quantification of gait in mitochondrial m.3243A > G patients: a validation study.

BACKGROUND: More than half of the patients harbouring the m.3243A > G mutation were found to have trouble maintaining balance when walking in a recent study by our group. Others demonstrated that these patients had an abnormal gait pattern, as quantified by gait analysis. Gait analysis is an emerging method to quantify subtle changes in walking pattern, also during therapeutic interventions. Therefore, we aimed to test the reliability and reproducibility of gait analysis and select the most suitable protocol for this group of patients using a GAITRite electronic walkway. Four different protocols were tested: normal walking, dual task, post exercise and after a ten minutes of rest. RESULTS: In total 36 patients with the m.3243A > G mutation and 50 healthy controls were enrolled in this study. Overall high intra class correlation coefficients were found in all experimental conditions for both patients and healthy controls indicating good reproducibility. Marked differences in gait between patients and controls were observed and were in line with the only available exploratory study performed. There was a good correlation between both the overall NMDAS score, NMDAS subscale scores, both markers for disease severity, and specific gait parameters. CONCLUSIONS: The observed reliability of the test makes GAITRite a suitable instrument for intervention studies in patients with mitochondrial disease.

Three families with 'de novo' m.3243A > G mutation.

The m.3243A > G mutation is the most prevalent, disease-causing mitochondrial DNA (mtDNA) mutation. In a national cohort study of 48 families harbouring the m.3243A > G mutation, we identified three families in which the mutation appeared to occur sporadically within these families. In this report we describe these three families. Based on detailed mtDNA analysis of three different tissues using two different quantitative pyrosequencing assays with sensitivity to a level of 1% mutated mtDNA, we conclude that the m.3243A > G mutation has arisen de novo in each of these families. The symptomatic carriers presented with a variety of symptoms frequently observed in patients harbouring the m.3243A > G mutation. A more severe phenotype is seen in the de novo families compared to recent cohort studies, which might be due to reporting bias. The observation that de novo m.3243A > G mutations exist is of relevance for both diagnostic investigations and genetic counselling. Firstly, even where there is no significant (maternal) family history in patients with stroke-like episodes, diabetes and deafness or other unexplained organ dysfunction, the m.3243A > G mutation should be screened as a possible cause of the disease. Second, analysis of maternally-related family members is highly recommended to provide reliable counselling for these families, given that the m.3243A > G mutation may have arisen de novo.

Quality of life, fatigue and mental health in patients with the m.3243A > G mutation and its correlates with genetic characteristics and disease manifestation.

BACKGROUND: Mitochondrial disorders belong to the most prevalent inherited metabolic diseases with the m.3243A > G mutation reflecting being one of the most common mutations in mitochondrial DNA. Previous studies showed little relationship between mitochondrial genetics and disease manifestation. Relationship between genotype and disease manifestation with patient reported quality of life and other patient reported outcomes is still unexplored. METHODS: Seventy-two out of the 122 invited adult patients with m.3243A > G mutation completed online standardized questionnaires on quality of life, functional impairment, fatigue and mental health as assessed by the RAND-SF36, the Sickness Impact Profile (SIP), the Checklist Individual Strength (CIS) and the Hospital Anxiety and Depression scale (HADS). Data were related to clinical manifestation reflected by the Newcastle Mitochondrial Disease Adult Scale (NMDAS) score and heteroplasmy levels of the mutation in urine epithelial cells. RESULTS: Patients reported impaired quality of life. Sixty percent showed severe levels of fatigue, and 37% showed clinical relevant mental health problems, which was significantly more than healthy norms. These patient reported health outcomes showed negligible relationship with levels of heteroplasmy (r = <.30) and weak (.30 < r < .50) to moderate (.50 < r < .70) relationship with clinical manifestation. CONCLUSIONS: Patient reported outcomes on quality of life, fatigue and mental health problems, are only partly reflected by clinical assessments. In order to support patients more effectively, integration of patient reported outcomes, alongside symptoms of their disease, in clinical practice is warranted.

Obstetric complications in carriers of the m.3243A>G mutation, a retrospective cohort study on maternal and fetal outcome.

INTRODUCTION: The mitochondrial DNA m.3243A>G mutation is the most prevalent mutation causing mitochondrial disease in adult patients. Aside from some case reports, there are no studies on obstetric complications in a cohort of m.3243A>G carriers. We aimed to identify the prevalence of obstetric complications in a cohort of women carrying the m.3243A>G mutation. METHODS: All female carriers of the m.3243A>G mutation known from our previous national inventory were sent a questionnaire regarding their obstetric history. Data were compared to national references. Data from the national inventory, including NMDAS (disease severity) scores and heteroplasmy levels in urinary epithelial cells (UEC) were used to stratify women. RESULTS: Sixty women participated, the mean age was 47 years (range 20-70), mean NMDAS was 14.6 (range 0-46), and mean heteroplasmy percentage in UEC was 19.9% (range 5-85%). Ninety-eight pregnancies in 46 women were reported. Twenty-three (25.3%) had a premature delivery and five of them (5.5%) had a gestation of ≤ 32 weeks and eleven of the women (12%) suffered from preeclampsia. No different heteroplasmy level was found in the women with preeclampsia. Nine pregnancies (11%) were complicated by gestational diabetes. DISCUSSION: Obstetric complications occur frequently in carriers of the m.3243A>G mutation. Proper guidance during pregnancies and early detection of possible obstetric complications are needed. As techniques to prevent transmission of mitochondrial mutations are studied it is important to know the possible complications patients may experience from the ensuing pregnancy.

Serum GDF15 Levels Correlate to Mitochondrial Disease Severity and Myocardial Strain, but Not to Disease Progression in Adult m.3243A>G Carriers.

In this observational cohort study, we examined the prognostic value of growth and differentiation factor 15 (GDF15) in indicating and monitoring general mitochondrial disease severity and progression in adult carriers of the m.3243A>G mutation.Ninety-seven adult carriers of the m.3243A>G mutation were included in this study. The Newcastle mitochondrial disease adult scale was used for rating mitochondrial disease severity. In parallel, blood was drawn for GDF15 analysis by ELISA. Forty-nine carriers were included in a follow-up study. In a small subset of subjects of whom an echocardiogram was available from general patient care, myocardial deformation was assessed using two-dimensional speckle-tracking strain analysis.A moderate positive correlation was found between the concentration of GDF15 and disease severity (r = 0.59; p < 0.001). The concentration of serum GDF15 was higher in m.3243A>G carriers with diabetes mellitus, cardiomyopathy, and renal abnormalities. After a 2-year follow-up, no significant correlation was found between the change in disease severity and the change in the concentration of GDF15 or between the GDF15 level at the first assessment and the change in disease severity. In the subcohort of patients of whom an echocardiogram was available, the concentration of GDF15 correlated moderately to longitudinal global strain (r = 0.55; p = 0.006; n = 23) but not to circumferential or radial strain.Our results indicate that serum GDF15 is not a strong surrogate marker for general mitochondrial disease severity. Its value in indicating myocardial deformation should be confirmed in a prospective longitudinal study.

Migratory polyarthritis as an adverse effect of thiamazole use in a 13-year-old girl with Graves' disease.

Graves' disease is the most prevalent cause of hyperthyroidism in children. The treatment commonly involves antithyroid therapy using a thionamide. We present a case of a 13-year-old girl with the antithyroid arthritis syndrome, presenting as a migratory polyarthritis, after the initiation of thionamide treatment for Graves' disease. Antithyroid arthritis syndrome warranted immediate cessation of thionamide. Improvement of the arthritis was seen in subsequent days. As there are no other reversible treatment modalities for Graves' disease in children, definitive treatment with radioactive iodine was needed to control the hyperthyroidism in this child. Antithyroid arthritis syndrome presenting as a migratory polyarthritis is a severe adverse effect of a common pediatric disease and should therefore be recognized by pediatricians.

Serum FGF21 levels in adult m.3243A>G carriers: clinical implications.

OBJECTIVES: To determine the value of fibroblast growth factor 21 (FGF21), a recently discovered biomarker for mitochondrial disease, in predicting clinical disease severity and disease progression in adult carriers of the m.3243A>G mutation. METHODS: In the context of a national inventory, the heteroplasmy levels of the m.3243A>G mutation were measured in leukocytes and urinary epithelial cells. The Newcastle Mitochondrial Disease Adult Scale score was determined and blood was drawn for measuring FGF21 concentration. Twenty-five of the included initial patients studied were then selected randomly for a follow-up study. RESULTS: This prognostic study included 99 adult carriers of the m.3243A>G mutation. Our analysis revealed a moderate, significant correlation between FGF21 concentration and disease severity (r = 0.49; p = <0.001). No significant correlations were found between disease severity and the heteroplasmy percentage determined in urinary epithelial cells or the heteroplasmy percentage determined in leukocytes. Weak but significant correlations were also found between FGF21 concentration and the severity of the myopathy (r = 0.38; p = <0.001) and between the concentration of FGF21 and the severity of the encephalopathy (r = 0.30; p = <0.001). Repeated measurements following 25 subjects for 2 years revealed no significant correlation between FGF21 concentration and disease progression. CONCLUSIONS: Measuring FGF21 concentration had little added value in monitoring and predicting the disease course in this specific patient group.

Mitochondrial retinal dystrophy associated with the m.3243A>G mutation.

OBJECTIVE: To describe the spectrum of retinal abnormalities associated with the m.3243A>G mutation in the mitochondrial MTTL1 gene and to analyze putative correlations among the severity of retinal abnormalities, disease severity in other organ systems, and heteroplasmy levels. DESIGN: Observational, cohort-based, cross-sectional study. PARTICIPANTS: Twenty-nine patients carrying the m.3243A>G mutation. METHODS: Extensive clinical examinations, including visual acuity testing, indirect ophthalmoscopy, color fundus photography, fundus autofluorescence (FAF), high-resolution optical coherence tomography (OCT), and central visual field analysis. In selected patients, Goldmann perimetry, fluorescein angiography, full-field electroretinography (ERG) and electro-oculography (EOG), and color vision testing were performed. Heteroplasmy levels of the m.3243A>G mutation were measured in leukocytes, urinary epithelial cells (UECs), and buccal mucosa. The Newcastle Mitochondrial Disease Adult Scale (NMDAS) score was measured for all patients. MAIN OUTCOME MEASURES: Age at onset, visual acuity, fundus appearance, FAF, OCT findings, systemic disease features, heteroplasmy levels, and NMDAS scores. RESULTS: Twenty-five of the 29 mutation carriers (86%) had retinal abnormalities that could be classified into 4 grades. Six patients (21%) had grade 1 retinal dystrophy with fine pigment abnormalities that were clearly visible with FAF and fluorescein angiography. Eleven patients (38%) had grade 2 abnormalities that were characterized by yellowish or mildly pigmented deposits in the early stage; in advanced grade 2, these pigment changes encompassed the entire macula and often encircled the optic disc. Six patients (21%) had grade 3 disease in which profound chorioretinal atrophy was present outside the fovea. Two patients (7%) with retinal abnormalities had grade 4 disease, in which the fovea was affected by atrophy, with marked loss of visual acuity. The grade of mitochondrial retinal dystrophy correlated significantly with both age (r = -0.483, P = 0.008) and visual acuity (r = -0.614, P < 0.001), whereas no correlation was observed with heteroplasmy level or overall disease involvement. CONCLUSIONS: Mitochondrial retinal dystrophy associated with the m.3243A>G mutation has specific characteristics that can be classified into 4 grades based on the findings on ophthalmoscopy, FAF, and OCT. However, because the maternal inheritance pattern can be masked and the systemic disease associations can be variable or even absent, the disease may be misdiagnosed.

Inheritance of the m.3243A>G mutation.

The m.3243A>G is the most prevalent pathogenic mtDNA mutation but little is known about its inheritance. We studied 34 families containing 56 mother-child relations and 82 intersibling relations to investigate its transmission. We found a significant correlation between mother and child heteroplasmy levels (r = 0.679, p < 0.001). In mothers with a heteroplasmy level of below 25% we found 30% offspring without detectable mutation, while in mothers with a heteroplasmy level of above 25%, 100% of the offspring showed the m.3243A>G mutation. Heteroplasmy levels between siblings also correlated (r = 0.512, p < ;0.001), but had limited extra predictive value because of outliers. These new data on inheritance of the m.3243A>G mutation might be of value in counseling patients and preventing transmission of the mutation.