Electron Microscopic Findings Are an Important Aid for Diagnosing Mitochondrial Cardiomyopathy With Mitochondrial DNA Mutation 3243A>G

Methodologies in Mitochondrial Testing: Diagnosing a Primary Mitochondrial Respiratory Chain Disorder

BACKGROUND

Mitochondria are cytosolic organelles within most eukaryotic cells. Mitochondria generate the majority of cellular energy in the form of adenosine triphosphate (ATP) through oxidative phosphorylation (OxPhos). Pathogenic variants in mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) lead to defects in OxPhos and physiological malfunctions (Nat Rev Dis Primer 2016;2:16080.). Patients with primary mitochondrial disorders (PMD) experience heterogeneous symptoms, typically in multiple organ systems, depending on the tissues affected by mitochondrial dysfunction. Because of this heterogeneity, clinical diagnosis is challenging (Annu Rev Genomics Hum Genet 2017;18:257-75.). Laboratory diagnosis of mitochondrial disease depends on a multipronged analysis that can include biochemical, histopathologic, and genetic testing. Each of these modalities has complementary strengths and limitations in diagnostic utility.

CONTENT

The primary focus of this review is on diagnosis and testing strategies for primary mitochondrial diseases. We review tissue samples utilized for testing, metabolic signatures, histologic findings, and molecular testing approaches. We conclude with future perspectives on mitochondrial testing.

SUMMARY

This review offers an overview of the current biochemical, histologic, and genetic approaches available for mitochondrial testing. For each we review their diagnostic utility including complementary strengths and weaknesses. We identify gaps in current testing and possible future avenues for test development.

Electron microscopy of cardiac 3D nanodynamics: form, function, future

The 3D nanostructure of the heart, its dynamic deformation during cycles of contraction and relaxation, and the effects of this deformation on cell function remain largely uncharted territory. Over the past decade, the first inroads have been made towards 3D reconstruction of heart cells, with a native resolution of around 1 nm³, and of individual molecules relevant to heart function at a near-atomic scale. These advances have provided access to a new generation of data and have driven the development of increasingly smart, artificial intelligence-based, deep-learning image-analysis algorithms. By high-pressure freezing of cardiomyocytes with millisecond accuracy after initiation of an action potential, pseudodynamic snapshots of contraction-induced deformation of intracellular organelles can now be captured. In combination with functional studies, such as fluorescence imaging, exciting insights into cardiac autoregulatory processes at nano-to-micro scales are starting to emerge. In this Review, we discuss the progress in this fascinating new field to highlight the fundamental scientific insight that has emerged, based on technological breakthroughs in biological sample preparation, 3D imaging and data analysis; to illustrate the potential clinical relevance of understanding 3D cardiac nanodynamics; and to predict further progress that we can reasonably expect to see over the next 10 years.

Superiority of focused ion beam-scanning electron microscope tomography of cardiomyocytes over standard 2D analyses highlighted by unmasking mitochondrial heterogeneity

BACKGROUND

Cardioprotection by preventing or repairing mitochondrial damage is an unmet therapeutic need. To understand the role of cardiomyocyte mitochondria in physiopathology, the reliable characterization of the mitochondrial morphology and compartment is pivotal. Previous studies mostly relied on two-dimensional (2D) routine transmission electron microscopy (TEM), thereby neglecting the real three-dimensional (3D) mitochondrial organization. This study aimed to determine whether classical 2D TEM analysis of the cardiomyocyte ultrastructure is sufficient to comprehensively describe the mitochondrial compartment and to reflect mitochondrial number, size, dispersion, distribution, and morphology.

METHODS

Spatial distribution of the complex mitochondrial network and morphology, number, and size heterogeneity of cardiac mitochondria in isolated adult mouse cardiomyocytes and adult wild-type left ventricular tissues (C57BL/6) were assessed using a comparative 3D imaging system based on focused ion beam-scanning electron microscopy (FIB-SEM) nanotomography. For comparison of 2D vs. 3D data sets, analytical strategies and mathematical comparative approaches were performed. To confirm the value of 3D data for mitochondrial changes, we compared the obtained values for number, coverage area, size heterogeneity, and complexity of wild-type cardiomyocyte mitochondria with data sets from mice lacking the cytosolic and mitochondrial protein BNIP3 (BCL-2/adenovirus E1B 19-kDa interacting protein 3; Bnip3^-/- ) using FIB-SEM. Mitochondrial respiration was assessed on isolated mitochondria using the Seahorse XF analyser. A cardiac biopsy was obtained from a male patient (48 years) suffering from myocarditis.

RESULTS

The FIB-SEM nanotomographic analysis revealed that no linear relationship exists for mitochondrial number (r = 0.02; P = 0.9511), dispersion (r = -0.03; P = 0.9188), and shape (roundness: r = 0.15, P = 0.6397; elongation: r = -0.09, P = 0.7804) between 3D and 2D results. Cumulative frequency distribution analysis showed a diverse abundance of mitochondria with different sizes in 3D and 2D. Qualitatively, 2D data could not reflect mitochondrial distribution and dynamics existing in 3D tissue. 3D analyses enabled the discovery that BNIP3 deletion resulted in more smaller, less complex cardiomyocyte mitochondria (number: P < 0.01; heterogeneity: C.V. wild-type 89% vs. Bnip3^-/- 68%; complexity: P < 0.001) forming large myofibril-distorting clusters, as seen in human myocarditis with disturbed mitochondrial dynamics. Bnip3^-/- mice also show a higher respiration rate (P < 0.01).

CONCLUSIONS

Here, we demonstrate the need of 3D analyses for the characterization of mitochondrial features in cardiac tissue samples. Hence, we observed that BNIP3 deletion physiologically acts as a molecular brake on mitochondrial number, suggesting a role in mitochondrial fusion/fission processes and thereby regulating the homeostasis of cardiac bioenergetics.

Comparison of electron microscopic findings and clinical presentation in three patients with mitochondrial cardiomyopathy caused by the mitochondrial DNA mutation m.3243A > G

Mitochondrial cardiomyopathy can be described as a condition characterized by abnormal heart-muscle structure and/or function, secondary to mutations in nuclear or mitochondrial DNA. Its severity can range from subclinical to critical conditions. We presented three cases of mitochondrial cardiomyopathy with m.3243A > G mutation and compared the clinical manifestations with the histological findings for each of these cases. All cases showed cardiac hypertrophy, juvenile-onset diabetes mellitus, and hearing loss. Case 1 (43-year-old male) showed less cardiac involvement and shorter duration of mitochondrial disease-related symptoms than case 2 (67-year-old female) and case 3 (51-year-old male), who showed the most advanced cardiac condition and longest duration from the manifestation of heart failure. The histological findings revealed that cardiomyocytes from case 1 showed no hypertrophy and mitochondrial degeneration in electron microscopy. Alternatively, cases 2 and 3 showed hypertrophy in their cardiomyocytes, and mitochondrial degeneration (e.g. onion-like lesions, swollen cristae, and lamellar bodies) was most apparent in case 3. These results suggested that mitochondrial degeneration, as evaluated by electron microscopy, might be correlated with impaired heart function in patients with mitochondrial cardiomyopathy.

Mitochondrial Cardiomyopathy Presenting as Dilated Phase of Hypertrophic Cardiomyopathy Diagnosed with Histological and Genetic Analyses

We report a case with 46-year-old man diagnosed with mitochondrial cardiomyopathy in the dilated phase of hypertrophic cardiomyopathy. Since cardiac magnetic resonance imaging, beta-methyl-p-123I-iodophenyl-pentadecanoic myocardial scintigraphy, and positron emission tomography/computed tomography revealed no remarkable findings, we performed electron microscopic examination, which aided in diagnosing mitochondrial cardiomyopathy. Muscle biopsy was also compatible with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes and DNA analysis also concluded it. Since muscle biopsy is less invasive for patients compared to endomyocardial biopsy, cardiologists need to consider it. The diagnosis of mitochondrial cardiomyopathy is helpful because it is a genetic condition and also for consideration of device therapy, as well as management for acute crisis.