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Background Acquired human mitochondrial genome (mtDNA) deletions are symptoms and drivers of focal mitochondrial respiratory deficiency, a pathological hallmark of aging and late-onset mitochondrial disease. Results To decipher connections between these processes, we create LostArc, an ultrasensitive method for quantifying deletions in circular mtDNA molecules. LostArc reveals 35 million deletions (~ 470,000 unique spans) in skeletal muscle from 22 individuals with and 19 individuals without pathogenic variants in POLG. This nuclear gene encodes the catalytic subunit of replicative mitochondrial DNA polymerase γ. Ablation, the deleted mtDNA fraction, suffices to explain skeletal muscle phenotypes of aging and POLG-derived disease. Unsupervised bioinformatic analyses reveal distinct age- and disease-correlated deletion patterns. Conclusions These patterns implicate replication by DNA polymerase γ as the deletion driver and suggest little purifying selection against mtDNA deletions by mitophagy in postmitotic muscle fibers. Observed deletion patterns are best modeled as mtDNA deletions initiated by replication fork stalling during strand displacement mtDNA synthesis.

The reproductive success of men with mitochondrial disease is to date unreported. We compared the age- and era-adjusted reproductive success of 94 British male patients with mitochondrial disease to that of the UK general male population. The reproductive success of men with mitochondrial disease was 65% of that in the general population (95% confidence interval: 53%; 79%), and the effect magnitude was related to the disease severity. This contrasts with the two previous studies finding that the reproductive success of women with mitochondrial DNA disease is not impaired. We conclude that the reproductive health of men with mitochondrial disease merits increased clinical attention.

Mitochondrial disease is complex and variable, making diagnosis and management challenging. The situation is complicated by lack of sensitive outcomes of disease severity, progression, contributing pathology and clinical efficacy. Gait is emerging as a sensitive marker of pathology; however, to date, no studies have quantified gait in mitochondrial disease. In this cross-sectional study, we quantified gait characteristics in 24 patients with genetically confirmed mitochondrial disease (m.3243A>G and m.8344A>G) and 24 controls. Gait was measured using an instrumented walkway according to a predefined model with five domains hypothesised to reflect independent features of the neural control of gait in mitochondrial disease, including: pace (step velocity and step length); rhythm (step time); variability (step length and step time variability); asymmetry (step time asymmetry); and postural stability (step width, step width variability and step length asymmetry). Gait characteristics were compared with respect to controls and genotype. Additional measures of disease severity, pathophysiology and imaging were also compared to gait to verify the validity of gait characteristics. Discrete gait characteristics differed between controls and mitochondrial disease groups, even in relatively mildly affected patients harbouring the m.3243A>G mutation. The pattern of gait impairment (increased variability and reduced postural control) was supported by significant associations with measures of disease severity, progression, pathophysiology and radiological evidence of cerebellar atrophy. Discrete gait characteristics may help describe functional deficits in mitochondrial disease, enhance measures of disease severity and pathology, and could be used to document treatment effects of novel therapies.

Mitochondrial functions are intrinsically linked to their morphology and membrane ultrastructure. Characterizing abnormal mitochondrial structural features may thus provide insight into the underlying pathogenesis of inherited and acquired mitochondrial diseases. Following a systematic literature review on ultrastructural defects in mitochondrial myopathy, we investigated skeletal muscle biopsies from seven subjects with genetically defined mtDNA mutations. Mitochondrial ultrastructure and morphology were characterized using two complimentary approaches: transmission electron microscopy (TEM) and serial block face scanning EM (SBF-SEM) with 3D reconstruction. Six ultrastructural abnormalities were identified including i) paracrystalline inclusions, ii) linearization of cristae and abnormal angular features, iii) concentric layering of cristae membranes, iv) matrix compartmentalization, v) nanotunelling and vi) donut-shaped mitochondria. In light of recent molecular advances in mitochondrial biology, these findings reveal novel aspects of mitochondrial ultrastructure and morphology in human tissues with implications for understanding the mechanisms linking mitochondrial dysfunction to disease.

Mitochondrial disorders are a heterogeneous group of diseases affecting different organs (brain, muscle, liver, and heart), and the severity of the disease is highly variable. The chronicity and heterogeneity, both clinically and genetically, means that many patients require surveillance follow-up over their lifetime, often involving multiple disciplines. Although our understanding of the genetic defects and their pathological impact underlying mitochondrial diseases has increased over the past decade, this has not been paralleled with regards to treatment. Currently, no definitive pharmacological treatment exists for patients with mitochondrial dysfunction, except for patients with primary deficiency of coenzyme Q10. Pharmacological and nonpharmacological treatments increasingly being investigated include ketogenic diet, exercise, and gene therapy. Management is aimed primarily at minimizing disability, preventing complications, and providing prognostic information and genetic counseling based on current best practice. Here, we evaluate therapies used previously and review current and future treatment modalities for both adults and children with mitochondrial disease.