We report here on a patient who presented with myasthenia gravis type symptoms (fatigable ptosis, increased jitter on single fiber EMG, and a thymic mass) who was subsequently diagnosed with a mitochondrial myopathy. Sequencing of the mitochondrial genome (mtDNA) identified a transition variant in the tRNA asparagine gene (MT-TN) (m.5728T>C) at in 41% of mtDNA molecules in muscle tissue. The variant was not detectable in blood. The m.5728T>C variant has been reported previously in a ten year old male with global developmental delays, failure to thrive, ataxia, weakness, cognitive regression, seizures, and glomerulosclerosis. The variant was seen in 97% of mtDNA molecules in muscle and 50% in blood. This case report supports the pathogenicity of the m.5728T>C and helps to establish the phenotypic spectrum of this condition at a lower heteroplasmy.
