Clinical presentations of mitochondrial cardiomyopathies

Recognition, Diagnosis, and Treatment of Mitochondrial Myopathies in Endurance Athletes

Endurance athletes complaining of muscle pains concomitant with fatigue and exercise intolerance provide a diagnostic challenge. When the most common causes have been ruled out, the presence of metabolic myopathies, including mitochondrial myopathies (MMs), should be considered. MMs are a group of diseases characterized by inadequate mitochondrial ATP production needed for the energetic requirement of the exercising muscles. Athletes with myalgia, fatigue, dyspnea, and muscular cramping should be questioned for history of rhabdomyolysis or myoglobinuria as well as detailed family history, given the predominant matrilinear inheritance of MMs. In all suspected cases, blood lactate and ventilatory response on effort plus muscle biopsy for histologic and molecular studies are recommended. Therapeutic recommendations consist of a set of instructions including genetic counseling, awareness of possible myoglobinuric episodes, and controlled exercise training.

Molecular and functional characterization of a human frataxin mutation found in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy is associated with marked genetic and phenotypic heterogeneity. Pathogenic mutations in the 10 hypertrophic cardiomyopathy-associated sarcomeric genes cause autosomal dominant disease as a rule, although recessive disease has been reported. Cardiac hypertrophy is also a hallmark of Friedreich ataxia, an autosomal recessive disease caused by deficiency of the mitochondrial protein frataxin. We hypothesized that heterozygous mutations in frataxin may mimic or modify hypertrophic cardiomyopathy. Using DHPLC and DNA sequencing, we identified the novel R40C-frataxin mutation in a patient who also harbored a previously reported R810H-myosin binding protein C mutation. The R810H mutation is reported to cause hypertrophic cardiomyopathy only in the setting of homozygosity or compound heterozygosity with another sarcomeric mutation. Site-directed mutagenesis and in vitro and in vivo analysis enabled functional characterization of the mutant frataxin protein. R40C-frataxin protein is not cleaved to the mature form in vitro and shows delayed kinetics of cleavage by isolated mouse mitochondria. Yeast cells expressing R40C-frataxin demonstrated increased sensitivity to oxidative stress and abnormal accumulation of precursor frataxin protein. These data indicate that frataxin deficiency may have contributed to this patient's particular phenotype. Furthermore, these findings suggest that mutations altering myocyte energetics may act in synergy with sarcomeric mutations to cause hypertrophic cardiomyopathy.