Definition of Pelizaeus-Merzbacher disease Pelizaeus-Merzbacher
disease: A disorder of the central nervous system (CNS) in which
there is loss of myelin, the sheath around the nerves. The
disease is clinically characterized by nystagmus (rhythmical
oscillation of the eyes), impaired motor development, tremor,
progressive spasticity (increased muscle tone), ataxia
(wobbliness), choreoathetotic movements, and dysartria
(difficulty speaking). Pelizaeus-Merzbacher disease (PMD) in its
classical form manifests in infancy or early childhood and
progresses to severe spasticity and ataxia. The lifespan may be
shortened. PMD is due to mutation in the gene PLP1. This gene is
located on the X chromosome in band Xq22. The disease describes
an X-linked pattern of inheritance with boys who have the
mutation affected with the disease while females with the
mutation are carriers. The PLP1 gene encodes proteolipid protein
(PLP), the most abundant protein of the myelin sheath in the
CNS. The mutation in PLP1 in PMD results in