Progressive myofiber loss with extensive fibro-fatty replacement in a child with mitochondrial DNA depletion syndrome and novel thymidine kinase 2 gene mutations.

The mitochondrial DNA depletion syndromes (MDS) are autosomal recessive disorders with a decreased mitochondrial DNA copy number. Mutations in thymidine kinase 2 (TK2) have been responsible for the myopathic form of MDS. We describe a child with congenital muscle weakness who had a progressive mitochondrial myopathy associated with extensive fibro-fatty replacement of myofibers resembling muscular dystrophy. MDS was suspected based upon findings in the initial muscle biopsy. Sequence analysis of the TK2 gene revealed two novel heterozygous mutations: the frame shift mutation, c.255_c.258delAGAA, and the heterozygous missense mutation, c.515G>A, (p.R172Q). This report extends the phenotype and genotype of TK2 defects.