| Summary: | In this study, we report on a novel heteroplasmic pathogenic variant in mitochondrial DNA (mtDNA). The studied patient had myoclonus, epilepsy, muscle weakness, and hearing impairment and harbored a heteroplasmic m.8315A>C variant in the <i>MTTK</i> gene with a mutation load ranging from 71% to >96% in tested tissues. In muscle mitochondria, markedly decreased activities of respiratory chain complex I + III and complex IV were observed together with mildly reduced amounts of complex I and complex V (with the detection of V*- and free F1-subcomplexes) and a diminished level of complex IV holoenzyme. This pattern was previously seen in other <i>MTTK</i> pathogenic variants. The novel variant was not present in internal and publicly available control databases. Our report further expands the spectrum of <i>MTTK</i> variants associated with mitochondrial encephalopathies in adults.
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