POLG and mitochondrial disease: Liver associated with central nervous system dysfunction is often involved in mitochondrial disease especially those involving defects of mitochondrial DNA maintenance caused by nuclear gene mutations i.e. mitochondrial DNA depletion syndromes (resulting from POLG, DGUOK or MPV17 mutations for instance (Al-Hussaini et al., 2014; Naviaux and Nguyen, 2004).