Identification and characterization of the novel m.8305C>T MTTK and m.4440G>A MTTM gene mutations causing mitochondrial myopathies.

•We describe two novel mtDNA mutations in two patients affected with myopathy.•These are the m.8305C>T mutation (MTTK gene) and the m.4440G>A mutation (MTTM gene).•Muscle biopsies showed ragged red fibers and COX-negative fibers in both patients.•These two mutations are heteroplasmic and only detected in muscle tissue.•These two mutations presented higher mutation load in COX-negative fibers.