Recurrent rhabdomyolysis and exercise intolerance: A new phenotype of late-onset thymidine kinase 2 deficiency.

A 29-year-old man developed, since the age of 18, exercise intolerance and exercise-induced rhabdomyolysis, with myoglobinuria. Muscle biopsy showed ragged-red fibers. Multiple mitochondrial DNA deletions were detected. The previously reported pathogenic homozygous mutation c.323C>T (p.Thr108Met) in TK2 was identified. This case expands the phenotypic spectrum of TK2 deficiency and indicates that it should be considered in the differential diagnosis of episodic rhabdomyolysis and exercise intolerance, along with other metabolic and mitochondrial myopathies. Since a new treatment is under development, it is essential improving knowledge of the natural history of TK2 deficiency.