A 3-year-old child presents with a floppy infant history and now shows progressive limb and bulbar weakness, leading to respiratory failure requiring non-invasive ventilation. Laboratory workup reveals an elevated creatine kinase and a high plasma lactate level. A mitochondrial depletion syndrome is suspected. Pathogenic variants in which gene are associated with this specific myopathic subgroup of mtDNA maintenance defect, for which treatment with pyrimidine deoxynucleosides is a potential therapy?