POLG and mitochondrial DNA depletion syndrome 4a: POLG-related disorders exist on a spectrum with at least five major phenotypes, including (1) Alpers-Huttenlocher syndrome (AHS), (2) childhood myocerebrohepatopathy spectrum (MCHS), (3) myoclonic epilepsy myopathy sensory ataxia (MEMSA), (4) the ataxia neuropathy spectrum (ANS), and (5) progressive external ophthalmoplegia (PEO) with or without sensory ataxic neuropathy, dysarthria, and ophthalmoplegia (SANDO) [9].