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Corresponding author at: Department of Internal Medicine, Asahi University School of Dentistry, 1851 Hozumi, Mizuho 501-0296, Japan. Fax: +81 58 329 1415.
Department of Internal Medicine, Asahi University School of Dentistry, Mizuho, JapanDepartment of Cardiology, Gifu University Graduate School of Medicine, Gifu, Japan
Department of Internal Medicine, Asahi University School of Dentistry, Mizuho, JapanDepartment of Cardiology, Gifu University Graduate School of Medicine, Gifu, JapanDepartment Emergency and Disaster Medicine, Gifu University Graduate School of Medicine, Gifu, Japan
Department of Internal Medicine, Asahi University School of Dentistry, Mizuho, JapanDepartment of Cardiology, Gifu University Graduate School of Medicine, Gifu, Japan
During electron microscopic examination of 156 consecutive human endomyocardial biopsy specimens, we found marked mitochondrial deformity within a single cardiomyocyte in each of 4 specimens. The deformed mitochondria were unevenly distributed, but the deformities were confined to the one cardiomyocyte. Those affected cardiomyocytes were accompanied by nonspecific degenerative changes such as nuclear hypertrophy and/or rarefaction of the myofibrils. Mitochondria in all other cells within the specimens appeared normal. Such an abnormality has never been reported to date. Each of the four cases was diagnosed with a different ailment: post-myocarditis, dilated cardiomyopathy, amyloidosis, and tachycardia-induced heart failure. However, all four cases were accompanied by left ventricular systolic dysfunction at biopsy. The very limited mitochondrial deformation may thus reflect a type of degenerative change that accompanies heart failure.
<Learning objective: A marked mitochondrial deformity must have been overlooked to date, which is confined to a single cardiomyocyte in an endomyocardial biopsy specimen. Its etiology is still unknown but may reflect a type of degenerative change that accompanies heart failure.>
Mitochondria are the primary source of energy, in the form of ATP, that fuels the contractile apparatus within cardiomyocytes, and are thus essential for the heart’s pumping activity. Accumulation of disorganized small mitochondria (mitochondriosis) is observed in a variety of cardiac conditions associated with heart failure, including dilated cardiomyopathy [
], and may be responsible for mitochondriosis, although the significance of these changes to the pathogenesis of heart failure is unclear if they are secondary to metabolic changes. Heart failure is indeed associated with reduced mitochondrial oxidative phosphorylation and production of oxidative stress, but it is unknown whether changes in mitochondrial morphology contribute to the pathogenesis of mitochondrial dysfunction during heart failure [
]. Within failing hearts, mitochondria with subtle abnormal morphologies can be observed in most cardiomyocytes, but it is unusual for the abnormality to be confined to a single cardiomyocyte.
Presentation of the cases
Over a period of 3 years (2013–2015), 156 consecutive endomyocardial biopsy specimens were collected and examined in an electron microscope (H700 or HT7700, Hitachi, Tokyo, Japan) for routine diagnostic purposes at Gifu University School of Medicine, Kizawa Memorial Hospital, Nara Medical College, Yamaguchi University School of Medicine, Nagoya University School of Medicine, Niigata University School of Medicine, and Gifu Heart Center. The specimens were obtained from 80 patients with dilated cardiomyopathy (including post-myocarditis), 34 with hypertrophic cardiomyopathy, 8 with amyloidosis, 8 with mitochondrial cardiomyopathy, 7 with sarcoidosis, 4 with Fabry disease, 4 with hypertensive heart disease, 3 with adriamycin cardiomyopathy, 2 with acute myocarditis, 2 with arrhythmogenic right ventricular cardiomyopathy, 2 with Sjogren’s syndrome combined with pulmonary hypertension, 1 with peripartum cardiomyopathy, and 1 with unsustained ventricular tachycardia.
Endomyocardial biopsy specimens were immediately fixed for 4 h in 2.5% glutaraldehyde in 0.1 mol/l phosphate buffer. The specimens were then postfixed in 1% osmium tetroxide for 1 h, dehydrated through graded ethanol and propylene oxide series and embedded in epon. They were sectioned at 0.7-μm thickness (semithin sections) and stained with toluidine blue dye and thereafter thin-sectioned (70 nm) using an ultramicrotome, mounted on plain copper grids, stained with uranyl acetate and lead citrate, and examined by an H700 or HT7700 transmission electron microscope (Hitachi). We counted the number of cardiomyocytes contained in each specimen using toluidine blue-stained semithin sections under a light microscope.
During these examinations, we observed marked mitochondrial deformities within cardiomyocytes. Moreover, the abnormal morphology was unevenly distributed within the cells and showed different patterns of deformity. Notably, in four cases the mitochondrial deformity was confined to a single cardiomyocyte; all the mitochondria in the other cardiomyocytes in each specimen appeared normal. These four cases are presented here.
Case 1
Case 1 was a 91-year-old man admitted to the hospital because of faintness due to pacing failure of his pacemaker. The permanent pacemaker was implanted 25 years earlier to treat complete atrioventricular block of unknown origin. Pacing failure had occurred several times in the past due to increases in the sensing threshold. The heart was neither hypertrophied nor dilated, but did show mild systolic hypofunction; the left ventricular ejection fraction (EF) was 42%. An endomyocardial biopsy was collected from the right ventricle to search for basic heart disease. Marked fibrosis was observed, with interstitial, perivascular replacement, and with myofibroblasts scattered in the endocardium. Cardiomyocytes were not hypertrophied [the mean diameter, 12.4 ± 4.1 (mean ± SD) μm], and there was no conspicuous myofibrillar rarefaction. Post-myocarditis was thus suspected based on the histologic findings. In an electron microscope, one cardiomyocyte attracted attention because the mitochondria were strangely shaped (Fig. 1A and B). Nearly all the mitochondria in that cell were abnormal; some were gigantic with abnormally running cristae and others had cristae that appeared to dissolve. In addition to such mitochondrial deformity, the nucleus of the cell was hypertrophic, i.e. bizarre shaped with chromatin clumping (Fig. 1A). Nonetheless, the remaining 108 cardiomyocytes in the specimen contained normal mitochondria (Fig. 1C).
Fig. 1Electron micrographs of Case 1 (A–C) and Case 2 (D–F). Panels A and B show mitochondria with unusual shapes in one cardiomyocyte; panel B is the highly magnified photograph of the square portion of panel A. Nearly all mitochondria in this cell are abnormal; some are gigantic and have abnormal running cristae or cristae that dissolve. In addition, the nucleus of the cell was hypertrophic, i.e. bizarre shaped with chromatin clumping. All other cardiomyocytes in the specimen had normal mitochondria (C). Panels D and E show amyloid fibrils diffusely distributed in the myocardial interstitium and apparently abnormal mitochondria in one cardiomyocyte. The mitochondria are donut-shaped with glycogen granules in the center, or they are abnormally elongated. In this cell, rarefaction of the myofibrils is apparent due to marked accumulation of glycogen granules. No mitochondrial abnormality was detected in the other cardiomyocytes in the specimen (F).
Case 2 was a 49-year-old man who suffered from severe heart failure and was suspected of having cardiac amyloidosis. His left ventricle was not dilated, but was markedly hypertrophied and hypokinetic (EF, 34%). The cardiomyocytes were hypertrophic (the mean diameter, 19.1 ± 3.2 μm) and the interstitium of a left ventricular endomyocardial specimen was wide and weakly positive for Congo red stain. Electron microscopy revealed amyloid fibrils diffusely distributed in the myocardial interstitium and apparently abnormal mitochondria in one cardiomyocyte. Those mitochondria were donut-shaped and contained glycogen granules in the center, or they were abnormally elongated (Fig. 1D and E). We speculate the donut-shaped ones may actually be cup-shaped but appear donut-like in the sections. In this cell, myofibrillar rarefaction was apparent mainly due to marked accumulation of glycogen granules. No mitochondrial abnormalities were noted in the other 97 cardiomyocytes (Fig. 1F).
Case 3
Case 3 was an 81-year-old man admitted to the hospital because of uncontrolled heart failure. This patient had suffered from heart failure for more than 20 years and had a history of repeated admission to hospitals. The left ventricle was dilated but not hypertrophied and showed diffuse severe hypokinesis (EF, 30%). An endomyocardial biopsy was collected from the right ventricle and examined for secondary myocardial disease, such as cardiac sarcoidosis. The cardiomyocytes were slightly hypertrophied (the mean diameter, 15.6 ± 2.6 μm) and showed myofibrillar rarefaction. Slight interstitial and endocardial fibrosis was noted. Electron microscopy revealed no accumulation of abnormal metabolites in the cytoplasm of the cardiomyocytes. This case was therefore judged compatible with dilated cardiomyopathy. In one cardiomyocyte, however, nearly all the mitochondria showed structural abnormalities. We observed mitochondria that were gigantic in size and had abnormally running cristae, vague cristae, or cristae with areas of dissolution (Fig. 2). This cardiomyocyte also showed nonspecific degenerative changes including rarefaction of the myofibrils and abundant accumulation of lipofuscin and glycogen granules (Fig. 2E). Mitochondria in the other 112 cardiomyocytes in the specimen appeared normal.
Fig. 2Electron micrographs of Case 3, showing that nearly all the mitochondria in one cardiomyocyte present structural abnormalities (A–E). These include gigantic mitochondria with abnormal running cristae, vague cristae, or dissolved cristae. The cardiomyocyte also showed nonspecific degenerative changes including rarefaction of the myofibrils and abundant accumulation of lipofuscin and glycogen granules.
Case 4 was a 69-year-old man who had planned cataract surgery but was found be in heart failure with accompanying pleurisy. This patient had atrial flutter with a rapid ventricular response. The left ventricle was not hypertrophic, but was dilated and diffusely hypokinetic; EF was 27% at biopsy. An endomyocardial biopsy specimen from the right ventricle showed mild cardiomyocyte hypertrophy (mean diameter, 17.8 ± 2.5 μm) and mild interstitial and endocardial fibrosis. Electron microscopy revealed one cardiomyocyte containing mitochondria with various deformities (Fig. 3A–F). Some presented with abnormally running cristae or apparently dissolved cristae; others were elongated or contained electron-dense material in their center. The concerned cardiomyocyte was also affected by severe rarefaction of the myofibrils. Marked deformities of mitochondria were seen in only one cardiomyocyte and never in the 151 others (Fig. 3G). Catheter ablation was performed to treat the atrial flutter, after which his cardiac function dramatically improved; the ejection fraction reached 64%. The patient was thus ultimately diagnosed with tachycardia-induced heart failure.
Fig. 3Electron micrographs of Case 4, showing a cardiomyocyte (panel A) containing mitochondria with various deformities, including abnormal running cristae (B and C), dissolved cristae (D), elongation (E), and containing electron-dense material in the center (panel F). This cardiomyocyte was also affected by severe rarefaction of the myofibrils. No such deformities were seen in the other cardiomyocytes in the specimen (G).
We report a marked mitochondrial deformity confined to a single cardiomyocyte within a human endomyocardial biopsy specimen. The deformed mitochondria were unevenly distributed, but the deformities were confined to the one cardiomyocyte. Such an abnormality has never been reported until now, to our knowledge. From among 156 consecutive cases, moreover, we detected 4 such cases (2.6%). This is an unexpectedly high incidence, and we are somewhat surprised that there have been no earlier reports of similar cases. We suppose such findings may have been overlooked because they entailed only subtle changes confined in a single cell within a specimen. Although we noted only one affected cardiomyocyte in each biopsy specimen, it is supposed that multiple cardiomyocytes, not many though, must be affected in the whole heart.
All four cases suffered from heart failure, but each with a different etiology. In addition, the affected cardiomyocytes showed degenerative changes of the subcellular organelles nonspecifically observed in hypertrophic or failing hearts [
]. These suggest that the mitochondrial abnormality is not disease-specific and may be reflecting a type of degenerative change that accompanies heart failure, the nature of which is entirely unknown at present and will require future clarification. Although it is difficult to explain why only one cardiomyocyte in the specimen, i.e. limited numbers of cardiomyocytes in the whole heart, was affected, such phenotype expression may depend on difference of the strength of stress or overload on each cardiomyocyte. This could be compared to an extreme variation in the degree of cardiomyocyte hypertrophy in the same heart.
Mitochondrial gene mutation can cause cardiomyopathy characterized by hypertrophy, atrioventricular block, or congestive heart failure [
]. From their electron microscopic examination of endomyocardial biopsy specimens, Albustini et al. reported mitochondrial structural abnormalities that included giant organelles; angulated, tubular, and concentric cristae; and crystalloid or osmiophilic inclusion bodies in 85 of 601 patients clinically diagnosed with dilated cardiomyopathy (14.1%) [
]. They also noted mitochondrial gene mutations in 19 of those 85 cases (22.4%), indicating that 3.2% of clinically diagnosed dilated cardiomyopathy was in fact mitochondrial cardiomyopathy. Needless to say, however, none of the present cases is comparable to mitochondrial cardiomyopathy because the mitochondrial deformity was extremely confined.
Funding
None.
Conflict of interest
The authors confirm that this article content has no conflict of interest.
Acknowledgments
We thank Yasuaki Hotta, Chihiro Takada, and Akiko Niwa (Asahi University) for technical advice and Rieko Hori and Norie Soga (Asahi University) for the secretarial assistance.
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Impairment of the myocardial ultrastructure and changes of the cytoskeleton in dilated cardiomyopathy.
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