PLP1 and Pelizeaus-Merzbacher spectrum disorder: Mutations in proteolipid protein gene (PLP1), GJC2, and MAG are responsible for a variety of phenotype ranging from hereditary spastic paraplegias (SPG2, SPG44, and SPG75, respectively) to neurodevelopmental disorders including Pelizaeus-Merzbacher disease, a hypomyelinating disorder of the central nervous system (Saugier-Veber et al., 1994; Orthmann-Murphy et al., 2009; Novarino et al., 2014; Lossos et al., 2015).