Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum.

OBJECTIVE: In this study we aim to demonstrate the occurrence of adult forms of TK2 mutations causing progressive mitochondrial myopathy with significant muscle mitochondrial DNA (mtDNA) depletion.
METHODS: Patients' investigations included serum creatine kinase, blood lactate, electromyographic, echocardiographic, and functional respiratory analyses as well as TK2 gene sequencing and TK2 activity measurement. Mitochondrial activities and mtDNA were analyzed in the patients' muscle biopsy.
RESULTS: The 3 adult patients with TK2 mutations presented with slowly progressive myopathy compatible with a fairly normal life during decades. Apart from its much slower progression, these patients' phenotype closely resembled that of pediatric cases including early onset, absence of CNS symptoms, generalized muscle weakness predominating on axial and proximal muscles but affecting facial, ocular, and respiratory muscles, typical mitochondrial myopathy with a mosaic pattern of COX-negative and ragged-red fibers, combined mtDNA-dependent respiratory complexes deficiency and mtDNA depletion. In accordance with the disease's relatively slow progression, the residual mtDNA content was higher than that observed in pediatric cases. That difference was not explained by the type of the TK2 mutations or by the residual TK2 activity.
CONCLUSION: TK2 mutations can cause mitochondrial myopathy with a slow progression. Comparison of patients with similar mutations but different disease progression might address potential mechanisms of mtDNA maintenance modulation.