In this issue of Circulation, Anan et al 1 describe cardiac abnormalities occurring in a small group of patients with well-defined defects in mitochondrial DNA (mtDNA). Abnormal cardiac findings, as assessed by ECG, chest radiograph, echocardiography, and His bundle electrograms, were detected in patients within each of four categories of mitochondrial diseases.

Conduction disturbances were observed uniformly in patients with Kearns-Sayre syndrome, while some patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) exhibited symmetric left ventricular hypertrophy with either normal or abnormal wall motion. Asymmetric septal hypertrophy with a hypokinetic ventricle was present in a majority of patients with MERRF (myoclonus epilepsy with ragged red fibers) and progressed over time to dilated cardiomyopathy in one individual. Cardiac abnormalities were less prevalent in patients with ocular myopathy, but diffuse hypokinesis was noted in 1 of 6 patients. Kearns-Sayre syndrome and ocular myopathy result from accumulation of deleted forms of mtDNA in affected tissues, whereas MERRF and MELAS are caused by single nucleotide substitutions in the mitochondrial tRNA Lys and tRNA Leu genes, respectively.