Objective: Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is a clinically and genetically heterogeneous group of autosomal recessive diseases characterized by a reduction in mtDNA copy number. Several nuclear genes have been shown to account for these severe oxidative phosphorylation disorders, but the disease-causing mutations remain largely unknown.
Methods: By virtue of homozygosity mapping, we tested candidate genes involved in mtDNA maintenance in patients born to consanguineous parents.
Results: We found homozygosity for microsatellite markers flanking the PEO1 gene, encoding the mitochondrial Twinkle helicase, in two sibs presenting a hepatocerebral form of MDS. Sequencing the PEO1 gene showed a homozygous mutation at a conserved position of the protein in the two patients (T457I). The modeling of the Twinkle protein showed that T457 is located in the interface between two monomers of the hexameric enzyme. Finally, using purified recombinant protein, we demonstrated that the T457I mutant Twinkle has a defective helicase activity.
Interpretation: Although dominant Twinkle mutations have been previously reported in patients with autosomal dominant progressive external ophthalmoplegia and multiple mtDNA deletions, we report here the first recessive Twinkle mutation in patients with hepatocerebral form of MDS. Identifying other Twinkle mutations in MDS and/or autosomal dominant progressive external ophthalmoplegia and studying their impact on the isolated proteins should help in understanding why some mutations are recessive and others are dominant.