Pelizaeus-Merzbacher disease presenting as spinal muscular atrophy: clinical and molecular studies

Central and peripheral dysmyelination in a 3-year-old girl with ring chromosome 18

Myelin basic protein (MBP) contributes to peripheral and central nervous system myelin. Developmental myelinopathies exist on a clinical spectrum, but MBP is not included on leukodystrophy or CMT gene panels. This ring chromosome 18 case presents serial MRI and EMG/NCS, shedding light on the early clinical course of the disorder.

Neurogenetics of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an X-linked disorder caused by mutations in the PLP1 gene, which encodes the proteolipid protein of myelinating oligodendroglia. PMD exhibits phenotypic variability that reflects its considerable genotypic heterogeneity, but all forms of the disease result in central hypomyelination associated with early neurologic dysfunction, progressive deterioration, and ultimately death. PMD has been classified into three major subtypes, according to the age of presentation: connatal PMD, classic PMD, and transitional PMD, combining features of both connatal and classic forms. Two other less severe phenotypes were subsequently described, including the spastic paraplegia syndrome and PLP1-null disease. These disorders may be associated with duplications, as well as with point, missense, and null mutations within the PLP1 gene. A number of clinically similar Pelizaeus-Merzbacher-like disorders (PMLD) are considered in the differential diagnosis of PMD, the most prominent of which is PMLD-1, caused by misexpression of the GJC2 gene encoding connexin-47. No effective therapy for PMD exists. Yet, as a relatively pure central nervous system hypomyelinating disorder, with limited involvement of the peripheral nervous system and little attendant neuronal pathology, PMD is an attractive therapeutic target for neural stem cell and glial progenitor cell transplantation, efforts at which are now underway in a number of centers internationally.

Concise Review: Stem Cell-Based Treatment of Pelizaeus-Merzbacher Disease

Pelizaeus-Merzbacher disease (PMD) is an X-linked disorder caused by mutation in the proteolipid protein-1 (PLP1) gene, which encodes the proteolipid protein of myelinating oligodendroglia. PMD exhibits phenotypic variability that reflects its considerable genotypic heterogeneity, but all forms of the disease result in central hypomyelination, associated in most cases with early neurological dysfunction, progressive deterioration, and ultimately death. PMD may present as a connatal, classic and transitional forms, or as the less severe spastic paraplegia type 2 and PLP-null phenotypes. These disorders are most often associated with duplications of the PLP1 gene, but can also be caused by coding and noncoding point mutations as well as full or partial deletion of the gene. A number of genetically-distinct but phenotypically-similar disorders of hypomyelination exist which, like PMD, lack any effective therapy. Yet as relatively pure CNS hypomyelinating disorders, with limited involvement of the PNS and relatively little attendant neuronal pathology, PMD and similar hypomyelinating disorders are attractive therapeutic targets for neural stem cell and glial progenitor cell transplantation, efforts at which are now underway in a number of research centers. Stem Cells 2017;35:311-315.

Variable expression of a novel PLP1 mutation in members of a family with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a rare X-linked disorder caused by mutations of the proteolipid protein 1 gene that encodes a structural component of myelin. It is characterized by progressive psychomotor delay, nystagmus, spastic quadriplegia, and cerebellar ataxia. Variable clinical expression was seen in 5 members of a family bearing a novel missense mutation in proteolipid protein 1, c.619T>C. Symptomatic patients included a 6-year-old girl, her younger brother, and their maternal uncle, a 29-year-old college graduate initially diagnosed with cerebral palsy; their brain magnetic resonance imaging studies showed diffuse dysmyelination. The mother had a history of delayed walking, achieved independently by age 3; she and the maternal grandmother were asymptomatic on presentation. Review of clinical information and family history led to consideration of Pelizaeus-Merzbacher disease. Subsequent identification of the causal mutation enabled preimplantation genetic diagnosis and the birth of an unaffected child.

The latest on leukodystrophies

PURPOSE OF REVIEW

Important advances in our understanding of genetic disorders of the white matter have been made and are discussed here.

RECENT FINDINGS

It has recently been discovered that mutations in the genes encoding the five subunits of eukaryocytic initiation factor 2B (eIF2B) are the cause of vanishing white-matter disease/childhood ataxia with central hypomyelination syndrome. The extension of the clinical features of the eIF2B-related disorders to encompass both infant- and adult-onset disorders is discussed. New clinico-imaging syndromes such as hypomyelination with atrophy of the basal ganglia and cerebellum and leukoencephalopathy with brain-stem and spinal cord involvement and elevated white-matter lactate are described. Recent findings include evidence that mitochondrial fat-oxidation abnormalities may be important in the pathogenesis of adrenoleukodystrophy, and that a mutant myelin protein can cause maldistribution of other myelin proteins, causing dysmyelination, axonal damage, or both.

SUMMARY

This review focuses on advances in the understanding of the role of eIF2B as a cause of a common leukodystrophy syndrome. eIF2B-related disorders have a clinical spectrum ranging from a severe, rapidly progressive congenital or early infantile encephalopathy to a slowly progressive cognitive and motor deterioration often associated with premature ovarian failure. Two newly recognized leukodystrophy syndromes are described: hypomyelination with atrophy of the basal ganglia and cerebellum, and leukoencephalopathy with brain-stem and spinal cord involvement and elevated white-matter lactate. An update is also given for adrenoleukodystrophy and myelin-protein-related disorders. This update demonstrates that an increasing number of genetic defects are being identified that may cause primary white-matter disorders.

Insertion of mutant proteolipid protein results in missorting of myelin proteins

Two brothers with a leukodystrophy, progressive spastic diplegia, and peripheral neuropathy were found to have proteinaceous aggregates in the peripheral nerve myelin sheath. The patients' mother had only subclinical peripheral neuropathy, but the maternal grandmother had adult-onset leukodystrophy. Sequencing of the proteolipid protein (PLP) gene showed a point mutation IVS4 + 1 G-->A within the donor splice site of intron 4. We identified one transcript with a deletion of exon 4 (Deltaex4, 169bp) encoding for PLP and DM20 proteins and lacking two transmembrane domains, and a second transcript with exon 4 + 10bp encoding three transmembrane domains. Immunohistochemistry showed abnormal aggregation in the myelin sheath of MBP and P0. Myelin-associated glycoprotein was present in the Schmidt-Lanterman clefts but significantly reduced in the periaxonal region. Using immunogold electron microscopy, we demonstrated the presence of mutated PLP/DM20 and the absence of the intact protein in the patient peripheral myelin sheath. We conclude that insertion of mutant PLP/DM20 with resulting aberrant distribution of other myelin proteins in peripheral nerve may constitute an important mechanism of dysmyelination in disorders associated with PLP mutations.

Update on genetic disorders affecting white matter

The classification of diseases affecting white matter has changed dramatically with the use of magnetic resonance imaging. Classical leukodystrophies, such as metachromatic leukodystrophy and Krabbe's disease, account for only a small number of inherited diseases that affect white matter. Magnetic resonance imaging has clarified genetic disorders that result in white matter changes or leukoencephalopathies. The term leukoencephalopathy is used to reflect the broader number of diseases that may cause as either primary or secondary changes in myelin development. This review attempts to categorize white matter disorders into classes such as lipid, myelin protein, organic acids, and defects in energy metabolism, in addition to other causes.

A cellular mechanism governing the severity of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a leukodystrophy linked to the proteolipid protein gene (PLP). We report a cellular basis for the distinction between two disease subtypes, classical and connatal, based on protein trafficking of the two PLP gene products (PLP and DM20). Classical PMD mutations correlate with accumulation of PLP in the ER of transfected COS-7 cells while the cognate DM20 traverses the secretory pathway to the cell surface. On the other hand, connatal PMD mutations lead to the accumulation of both mutant PLP and DM20 proteins in the ER of COS-7 cells with little of either isoform transported to the cell surface. Moreover, we show that transport-competent mutant DM20s facilitate trafficking of cognate PLPs and hence may influence disease severity.

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Spinal muscular atrophy with oculomotor palsy, epilepsy, and cerebellar hypoperfusion

We report a boy with spinal muscular atrophy (SMA) type 1, who developed novel clinical and neuroradiologic features, indicating the broad spectrum of this degenerative disease. Widespread lesions in the central nervous system were disclosed by magnetic resonance imaging and single photon emission computed tomography (SPECT) studies. In particular, scattered regions of hypoperfusion demonstrated by SPECT suggested multisystem involvement including the cerebellum. The novel features of this patient were complex partial seizures and oculomotor palsy.