TWNK and mitochondrial DNA depletion syndrome: Missense mutations in TWINKLE are associated with a number of mitochondrial disorders, including adult-onset progressive external ophthalmoplegia (PEO), mtDNA depletion syndromes (MDSs), Perrault syndrome, infantile-onset spinocerebellar ataxia (IOSCA) and other ataxia neuropathies (Figure 4) [17,97,98,99,100,101].