Defective biosynthesis of proteolipid protein in Pelizaeus-Merzbacher disease

The neurovascular unit in leukodystrophies: towards solving the puzzle

The neurovascular unit (NVU) is a highly organized multicellular system localized in the brain, formed by neuronal, glial (astrocytes, oligodendrocytes, and microglia) and vascular (endothelial cells and pericytes) cells. The blood-brain barrier, a complex and dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma, is a component of the NVU. In a variety of neurological disorders, including Alzheimer's disease, multiple sclerosis, and stroke, dysfunctions of the NVU occurs. There is, however, a lack of knowledge regarding the NVU function in leukodystrophies, which are rare monogenic disorders that primarily affect the white matter. Since leukodystrophies are rare diseases, human brain tissue availability is scarce and representative animal models that significantly recapitulate the disease are difficult to develop. The introduction of human induced pluripotent stem cells (hiPSC) now makes it possible to surpass these limitations while maintaining the ability to work in a biologically relevant human context and safeguarding the genetic background of the patient. This review aims to provide further insights into the NVU functioning in leukodystrophies, with a special focus on iPSC-derived models that can be used to dissect neurovascular pathophysiology in these diseases.

Pathology of the neurovascular unit in leukodystrophies

The blood-brain barrier is a dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma. Specialized brain endothelial cells, astrocytes, neurons, microglia and pericytes together compose the neurovascular unit and interact to maintain blood-brain barrier function. A disturbed brain barrier function is reported in most common neurological disorders and may play a role in disease pathogenesis. However, a comprehensive overview of how the neurovascular unit is affected in a wide range of rare disorders is lacking. Our aim was to provide further insights into the neuropathology of the neurovascular unit in leukodystrophies to unravel its potential pathogenic role in these diseases. Leukodystrophies are monogenic disorders of the white matter due to defects in any of its structural components. Single leukodystrophies are exceedingly rare, and availability of human tissue is unique. Expression of selective neurovascular unit markers such as claudin-5, zona occludens 1, laminin, PDGFRβ, aquaporin-4 and α-dystroglycan was investigated in eight different leukodystrophies using immunohistochemistry. We observed tight junction rearrangements, indicative of endothelial dysfunction, in five out of eight assessed leukodystrophies of different origin and an altered aquaporin-4 distribution in all. Aquaporin-4 redistribution indicates a general astrocytic dysfunction in leukodystrophies, even in those not directly related to astrocytic pathology or without prominent reactive astrogliosis. These findings provide further evidence for dysfunction in the orchestration of the neurovascular unit in leukodystrophies and contribute to a better understanding of the underlying disease mechanism.

Glial cells in the driver seat of leukodystrophy pathogenesis

Glia cells are often viewed as support cells in the central nervous system, but recent discoveries highlight their importance in physiological functions and in neurological diseases. Central to this are leukodystrophies, a group of progressive, neurogenetic disease affecting white matter pathology. In this review, we take a closer look at multiple leukodystrophies, classified based on the primary glial cell type that is affected. While white matter diseases involve oligodendrocyte and myelin loss, we discuss how astrocytes and microglia are affected and impinge on oligodendrocyte, myelin and axonal pathology. We provide an overview of the leukodystrophies covering their hallmark features, clinical phenotypes, diverse molecular pathways, and potential therapeutics for clinical trials. Glial cells are gaining momentum as cellular therapeutic targets for treatment of demyelinating diseases such as leukodystrophies, currently with no treatment options. Here, we bring the much needed attention to role of glia in leukodystrophies, an integral step towards furthering disease comprehension, understanding mechanisms and developing future therapeutics.

Clinical implications of myelin regeneration in the central nervous system

INTRODUCTION

Amongst strategies to repair the brain, myelin repair offers genuine cause for optimism. Myelin, which sheaths most axons in the central nervous system (CNS), is vital for normal neurological function, as demonstrated by the functional deficits that accrue when it is absent in a range of debilitating myelin diseases. Following demyelination, post-mortem and imaging studies have shown that extensive regeneration of myelin is possible in the human brain. Over recent decades preclinical research has given us a strong understanding of the biology of myelin regeneration, opening up several exciting therapeutic opportunities that are on the cusp of clinical translation. Areas covered: This review discusses diseases that compromise the function of myelin, the endogenous capacity of the CNS to regenerate myelin, and why this sometimes fails. We then outline the extensive progress that has been made towards therapies that promote the regeneration of myelin. Expert commentary: Finally, a commentary on the first examples of these therapies to reach human patients and the evidence base that supports them, giving our opinion on where attention should be focused going forward is provided.

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies.

Modeling the Mutational and Phenotypic Landscapes of Pelizaeus-Merzbacher Disease with Human iPSC-Derived Oligodendrocytes

Pelizaeus-Merzbacher disease (PMD) is a pediatric disease of myelin in the central nervous system and manifests with a wide spectrum of clinical severities. Although PMD is a rare monogenic disease, hundreds of mutations in the X-linked myelin gene proteolipid protein 1 (PLP1) have been identified in humans. Attempts to identify a common pathogenic process underlying PMD have been complicated by an incomplete understanding of PLP1 dysfunction and limited access to primary human oligodendrocytes. To address this, we generated panels of human induced pluripotent stem cells (hiPSCs) and hiPSC-derived oligodendrocytes from 12 individuals with mutations spanning the genetic and clinical diversity of PMD-including point mutations and duplication, triplication, and deletion of PLP1-and developed an in vitro platform for molecular and cellular characterization of all 12 mutations simultaneously. We identified individual and shared defects in PLP1 mRNA expression and splicing, oligodendrocyte progenitor development, and oligodendrocyte morphology and capacity for myelination. These observations enabled classification of PMD subgroups by cell-intrinsic phenotypes and identified a subset of mutations for targeted testing of small-molecule modulators of the endoplasmic reticulum stress response, which improved both morphologic and myelination defects. Collectively, these data provide insights into the pathogeneses of a variety of PLP1 mutations and suggest that disparate etiologies of PMD could require specific treatment approaches for subsets of individuals. More broadly, this study demonstrates the versatility of a hiPSC-based panel spanning the mutational heterogeneity within a single disease and establishes a widely applicable platform for genotype-phenotype correlation and drug screening in any human myelin disorder.

Reprint of "Hypomyelinating disorders: An MRI approach

In recent years, the concept of hypomyelinating disorders has been proposed as a group of disorders with varying systemic manifestations that are identified by MR findings of absence or near absence of the T2 hypointensity that develops in white matter as a result of myelination. Initially proposed as a separate group because they were the largest single category of undiagnosed leukodystrophies, their separation as a distinct group that can be recognized by looking for a specific MRI feature has resulted in a marked increase in their diagnosis and a better understanding of the different causes of hypomyelination. This review will discuss the clinical presentations, imaging findings on standard MRI, and new MRI-related techniques that allow a better understanding of these disorders and proposed methods for quantifying the myelination as a potential means of assessing disease course and the effects of proposed treatments. Disorders with hypomyelination of white matter, or hypomyelinating disorders (HMDs), represent the single largest category among undiagnosed genetic leukoencephalopathies (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010). This group of inborn errors of metabolism is characterized by a magnetic resonance imaging (MRI) appearance of reduced or absent myelin development: delay in the development of T2 hypointensity and, often, T1 hyperintensity in the white matter of the brain. The concept of hypomyelination was first conceptualized by (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010; Schiffmann et al., 1994) in a series of papers that showed that these MRI characteristics were easily recognized, were different from the MRI characteristics of dysmyelinating and demyelinating disorders, and that the combination of these imaging findings with specific other clinical and imaging features could be used to make diagnoses with some confidence. In this manuscript, we will discuss the physiologic and genetic bases of hypomyelinating disorders, as well as their classification, clinical manifestations and imaging characteristics.

Hypomyelinating disorders: An MRI approach

In recent years, the concept of hypomyelinating disorders has been proposed as a group of disorders with varying systemic manifestations that are identified by MR findings of absence or near absence of the T2 hypointensity that develops in white matter as a result of myelination. Initially proposed as a separate group because they were the largest single category of undiagnosed leukodystrophies, their separation as a distinct group that can be recognized by looking for a specific MRI feature has resulted in a marked increase in their diagnosis and a better understanding of the different causes of hypomyelination. This review will discuss the clinical presentations, imaging findings on standard MRI, and new MRI-related techniques that allow a better understanding of these disorders and proposed methods for quantifying the myelination as a potential means of assessing disease course and the effects of proposed treatments. Disorders with hypomyelination of white matter, or hypomyelinating disorders (HMDs), represent the single largest category among undiagnosed genetic leukoencephalopathies (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010). This group of inborn errors of metabolism is characterized by a magnetic resonance imaging (MRI) appearance of reduced or absent myelin development: delay in the development of T2 hypointensity and, often, T1 hyperintensity in the white matter of the brain. The concept of hypomyelination was first conceptualized by (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010; Schiffmann et al., 1994) in a series of papers that showed that these MRI characteristics were easily recognized, were different from the MRI characteristics of dysmyelinating and demyelinating disorders, and that the combination of these imaging findings with specific other clinical and imaging features could be used to make diagnoses with some confidence. In this manuscript, we will discuss the physiologic and genetic bases of hypomyelinating disorders, as well as their classification, clinical manifestations and imaging characteristics.

Pelizaeus Merzbacher disease: morphological analysis of the vestibulo-cochlear system

CONCLUSION

In agreement with previously published findings, our results demonstrate that Pelizaeus Merzbacher disease (PMD) does not affect the development and morphology of the peripheral vestibulo-cochlear system.

OBJECTIVE

PMD is a consequence of X-linked mutation of the main central nervous system (CNS) myelin protein resulting in a complex neurological syndrome. Otorhinolaryngological symptoms include nystagmus and alterations of auditory-evoked brainstem responses. To date no histopathological analysis of the inner ear has been performed.

MATERIALS AND METHODS

The temporal bone morphology of an affected fetus was examined with light microscopy and synchrotron radiation-based micro computed tomography.

RESULTS

The regular structure of the vestibulo-cochlear system was shown in this multi-modular analysis.

Neuronal loss in Pelizaeus-Merzbacher disease differs in various mutations of the proteolipid protein 1

Mutations affecting proteolipid protein 1 (PLP1), the major protein in central nervous system myelin, cause the X-linked leukodystrophy Pelizaeus-Merzbacher disease (PMD). We describe the neuropathologic findings in a series of eight male PMD subjects with confirmed PLP1 mutations, including duplications, complete gene deletion, missense and exon-skipping. While PLP1 mutations have effects on oligodendrocytes that result in mutation-specific degrees of dysmyelination, our findings indicate that there are also unexpected effects in the central nervous system resulting in neuronal loss. Although length-dependent axonal degeneration has been described in PLP1 null mutations, there have been no reports on neuronal degeneration in PMD patients. We now demonstrate widespread neuronal loss in PMD. The patterns of neuronal loss appear to be dependent on the mutation type, suggesting selective vulnerability of neuronal populations that depends on the nature of the PLP1 disturbance. Nigral neurons, which were not affected in patients with either null or severe misfolding mutations, and thalamic neurons appear particularly vulnerable in PLP1 duplication and deletion patients, while hippocampal neuronal loss was prominent in a patient with complete PLP1 gene deletion. All subjects showed cerebellar neuronal loss. The patterns of neuronal involvement may explain some clinical findings, such as ataxia, being more prominent in PMD than in other leukodystrophies. While the precise pathogenetic mechanisms are not known, these observations suggest that defective glial functions contribute to neuronal pathology.

Hearing profile and MRI myelination of auditory pathway in Pelizaeus-Merzbacher disease

CONCLUSIONS

This study showed that delayed auditory pathway myelination is common in Pelizaeus-Merzbacher disease (PMD), but this delay does not necessarily indicate poor hearing function.

OBJECTIVE

PMD is a rare recessively inherited X-linked leukodystrophy characterized by defective central nervous system myelination owing to a mutation in the proteolipid protein gene (PLP). The aims of this study were to evaluate the hearing function and auditory brain response (ABR) findings of patients with PMD and relate these findings to MRI-assessed myelination in the central auditory pathway.

PATIENTS AND METHODS

We retrospectively studied eight male pediatric patients with PMD. Serial auditory examinations included audiometry, behavior audiometry, distortion product otoacoustic emission (DPOAE), and ABR. MRI-assessed myelination in the auditory pathway was evaluated in the PMD patients and in 23 normal young children as a control group.

RESULTS

Audiometry showed normal to moderate hearing impairment and the hearing threshold improved with age and became almost normal over time. DPOAEs positivity and only ABR wave I or waves I and II were found in all the patients. MRI showed delayed myelination in all the patients and the auditory pathway was myelinated up to the inferior colliculus in four cases and up to the medial geniculate body in four cases. Serial MRIs showed no progression in myelination. No clear relation was found between hearing threshold and MRI-assessed myelination in the auditory pathway.

Leukodystrophies: clinical and genetic aspects

The leukodystrophies comprise an ever-expanding group of rare central nervous system disorders with defined clinical, pathological, and genetic characteristics. The broader term, leukoencephalopathy, is applied to all brain white matter diseases, whether their molecular cause is known. Magnetic resonance imaging has helped to elucidate new forms of leukodystrophy as well as to permit longitudinal studies of disease progression. The white matter abnormality may appear similar in different forms of leukodystrophy so that in most cases, further studies such as magnetic resonance spectroscopy, tissue biopsies, enzyme studies, and molecular DNA analyses are needed to pinpoint the specific diagnosis. The primary inherited leukoencephalopathies include dysmyelinating, hypomyelinative, and vacuolating forms. Metabolic and vascular causes account for most of the secondary forms, but other inherited syndromes are recognized that have their onset in childhood or adult life and are characterized by distinctive clinical and neuropathologic features. This review discusses some of the mechanisms that have been proposed to explain deficiencies of myelin and the molecular genetic bases underlying these disorders.

Three young adult patients with Pelizaeus-Merzbacher disease who showed only waves I and II in auditory brainstem responses but had good auditory perception

Three young adult males with Pelizaeus-Merzbacher disease have been followed up since childhood. This disease is thought to be a dysmyelinating disorder of the brain during the prenatal period caused by gene mutations. The patients manifested horizontal nystagmus and severe rigidity of the extremities. Although the patients showed only waves I and II in auditory brainstem responses, they had relatively good hearing ability at approximately equal to dB. They could not speak words at all but could hear well and enjoy listening to conversation and music. One of them had a normal hearing threshold in pure-tone audiometry and a normal speech discrimination rate in speech audiometry. This can be explained by a nerve conduction blockade through dysmyelinated axons or the desynchronization of neurons and nerves responsible for the waves following waves I and II. At present, all three patients are living with their families. We report their present hearing, speech and language abilities.

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.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.

Magnetic resonance of metabolic and degenerative diseases in children

Cerebral magnetic resonance imaging and spectroscopy form an integral part in the diagnosis and management of the vast spectrum of metabolic and degenerative disorders in children. These varied disorders have been classified in many different ways, according to anatomic location, head size, enzyme disorder, or cellular morphology and function. The clinical features and magnetic resonance imaging appearances of the most common disorders are discussed.

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.

Late-onset neurodegeneration in mice with increased dosage of the proteolipid protein gene

Mutations of the proteolipid protein (Plp) gene cause a generalized central nervous system (CNS) myelin deficit in Pelizaeus-Merzbacher disease of man and various tremor syndromes in animal models. X-linked spastic paraplegia is also due to Plp gene mutations but has a different clinical profile and more restricted pathology involving specific tracts and regions. We have shown previously that PLP overexpression in mice homozygous for a Plp transgene results in premature arrest of CNS myelination and premature death. Here, we demonstrate that a low-level increase in Plp gene expression in transgenic mice causes significant axonal degeneration and demyelination with predilection for specific tracts. Following normal motor development, aged mice develop progressive myelin loss, axonal swellings with resultant Wallerian degeneration, and marked vacuolation of the neuropil associated with ataxia, tremor, and seizures. The age of onset and severity of the phenotype is a function of Plp gene dosage. The corticospinal tracts, optic nerve, fasciculus gracilis cerebellum, and brainstem are particularly involved. Although oligodendrocyte cell bodies show little abnormality, their inner adaxonal tongue is often abnormal, suggesting a perturbation of the axon/glial interface that may underlie the axonal changes. We conclude that abnormal expression of an oligodendrocyte-specific gene can cause axonal damage, a finding that is relevant to the pathogenesis of PLP-associated disorders and probably to other myelin-related diseases.

A history of proteolipids: a personal memoir

This review is a personal memoir of the history of proteolipids and is limited to aspects of the field with which the author has been involved in one way or another. The discovery of proteolipids was a serendipitous observation made in the course of the study of sulfatides. Initial focus was on the chemical characterization of brain proteolipids, their behavior under different conditions and their identification as the major protein of CNS myelin. The sequence of PLP was obtained using solid phase protein sequencing techniques. This, in turn, made possible a new era in which biochemical, cellular and molecular approaches could be applied to address new questions about PLP. Identification of genetic defects in the PLP molecule and its regulation has contributed to understanding myelin biology. Studies of the encephalitogenic activity of PLP in animal models have influenced the views of inflammatory processes in multiple sclerosis. Despite remarkable progress, much remains to be learned about the structure and function of PLP.

Proteolipid protein is necessary in peripheral as well as central myelin

Alternative products of the proteolipid protein gene (PLP), proteolipid protein (PLP) and DM20, are major components of compact myelin in the central nervous system, but quantitatively minor constituents of Schwann cells. A family with a null allele of PLP has a less severe CNS phenotype than those with other types of PLP mutations. Moreover, individuals with PLP null mutations have a demyelinating peripheral neuropathy, not seen with other PLP mutations of humans or animals. Direct analysis of normal peripheral nerve demonstrates that PLP is localized to compact myelin. This and the clinical and pathologic observations of the PLP null phenotype indicate that PLP/DM20 is necessary for proper myelin function both in the central and peripheral nervous systems.

Identification and characterization of a B-cell determinant within the amphipathic domain (residues 178-238) of the myelin proteolipid protein

Pooled polyclonal rabbit anti-rat myelin and mouse anti-human proteolipid protein (PLP) antisera were screened against a panel of PLP synthetic peptides spanning residues 178-238 of the protein. Cross-reactivity against one determinant defined by PLP(200-219) was particularly prominent in both the anti-myelin and anti-PLP antisera and was chosen for further study. Competitive inhibition studies, utilizing a panel of overlapping synthetic peptides, demonstrated that the C-terminal portion of PLP(200-219), specifically residues comprising PLP(200-217), was important for antibody recognition of this region. Immunohistochemical analyses with an affinity-purified rabbit anti-PLP(200-219) antiserum demonstrated antibody cross-reactivity with PLP in both paraffin- and gelatin-embedded brain sections and immunocytochemical staining of mouse oligodendrocyte-enriched cultures demonstrated antibody binding with native PLP in situ. Staining of living non-permeabilized cells localized binding to the extracellular face of the myelin membrane. Collectively, these data argue for the presence of an immunodominant B-cell determinant defined by PLP residues 200-219. Furthermore, the structural conformation of this determinant in native PLP can be mimicked by the synthetic peptide, resulting in the generation of an antibody reagent that has considerable utility for immunohistochemical and immunocytochemical investigations of PLP expression and localization within the central nervous system myelin membrane.

Pelizaeus-Merzbacher-like disease: female case report

We experienced a 15-year-old female, whose healthy parents were second cousins, who was suspected of having dysmyelinating disease involving only the central nervous system (CNS). She was noticed to have congenital pendula nystagmus, and spastic gait disturbance developed at the age of 10 years. Mild athetosis of the upper limbs and ataxia were recognized at age 13 years, and dysarthria presented at age 15. MRI and electrophysiological findings showed the characteristics of Pelizaeus-Merzbacher disease (PMD), although the extensive nerve conduction slowing of the CNS was less severe than that in male patients with PMD. No promoter or exonic mutations of proteolipid protein (PLP) gene were detected. Although this patient might be heterozygous for a mutation of the extraexonic PLP gene sequences or of other unknown X-linked PLP associated genes, we speculate that this case had a dysmyelinating disease with an autosomal recessive trait characterized by the same phenotype as that of PMD.

X-Linked Developmental Defects of Myelination: From Mouse Mutants to Human Genetic Diseases

Molecular cloning of the major myelin-specific genes and a systematic analysis of mouse mutants have led to the identification of molecular defects in human genetic diseases that affect myelination. In the central nervous system, Pelizaeus-Merzbacher disease (PMD) and X-linked spastic paraplegia (SPG-2) are clinically distinct with respect to the severity of motor dysfunction but involve the same gene for myelin proteolipid protein (PLP). Spontaneous mouse mutants of the PLP gene, such as jimpy and rumpshaker, provide faithful models of these human diseases and allow a detailed analysis of PLP dysfunction. Hypomyelination in jimpy and, presumably, in PMD is largely the result of abnormally increased oligodendrocyte death and a lack of terminal differentiation. In rumpshaker, a model for X-linked spastic paraplegia, myelinating oligodendrocytes appear normal in number but fail to assemble myelin correctly. Recently, PLP-transgenic mice have provided experimental evidence that increasing the normal PLP gene dosage (e.g., by a gene duplication) is by itself sufficient to cause PMD. The latter is strikingly similar to the peripheral neuropathy Charcot-Marie-Tooth disease frequently associated with a duplication of the myelin protein gene PMP-22.

Overexpression of DM20 messenger RNA in two brothers with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a rare, sex-linked recessive, dysmyelinating disease of the central nervous system that has been associated with mutations in the myelin proteolipid protein (PLP) gene. Only 25% of patients studied with Pelizaeus-Merzbacher disease have exonic mutations in this gene, the underlying cause of the disease in the remaining patients is unknown. The PLP gene encodes two major alternatively spliced transcripts called PLP and DM20. PLP messenger RNA is specifically expressed in central nervous system tissue, whereas DM20 messenger RNA is found in central nervous system, cardiac, and other tissues. We studied cultured skin fibroblasts from 2 brothers with Pelizaeus-Merzbacher disease who exhibited no detectable exonic mutation of the PLP gene. Examination of RNA from these cells showed that the level of DM20 messenger RNA is elevated sixfold relative to male control skin fibroblasts. An unrelated female carrier, also with no detectable exonic mutation, showed a threefold increase in DM20 messenger RNA in cultured skin fibroblasts. Our findings suggest that in some patients, Pelizaeus-Merzbacher disease is caused by overexpression of PLP gene transcripts, and that in these families a 50% increase of DM20 messenger RNA in females, relative to the increase in affected males, can identify a female carrier.

Selected leukodystrophies

The clinicopathologic profiles of the major leukodystrophies (adreno-leukodystrophy, metachromatic leukodystrophy, globoid cell leukodystrophy or Krabbe's disease, Pelizaeus-Merzbacher disease, and spongy degeneration of infancy or Canavan's disease) are reviewed. Particular attention is paid to distinctive imaging characteristics, molecular advances, pathogeneses, and potential therapies.

Ludwig Merzbacher (1875-1942): the man behind the disease

Ludwig Merzbacher (1875-1942) is widely known for his seminal work on the pathology of the dysmyelinating CNS disease named for the clinician Friedrich Pelizaeus and himself. Yet his training, his scientific achievements and his list of publications suggest a scientist with broad interests in neuropathology, neuroscience, neurology and psychiatry. Among several studies in experimental and clinical neuropathology, Merzbacher's work on scavenger cells is the most outstanding. While working in Alois Alzheimer's laboratory in Munich in 1906/1907, Ludwig Merzbacher analyzed in great detail the reaction patterns of these cells, which are nowadays known as reactive microglia, and already attempted to elucidate their function in brain pathology.

A sporadic case of very slow progressive leukodystrophy involving the cerebellar peduncles

We describe a patient with infantile onset leukodystrophy involving the cerebellar peduncles. She had mild mental retardation, spastic diplegia and mild cerebellar ataxia. The peripheral nerves seemed to be normal. The characteristic MRI findings in this case were extensive lesions of the white matter involving the cerebellar peduncles. In addition there was ventricular enlargement with a markedly decreased volume of the white matter and a hypoplastic corpus callosum. The clinical and laboratory findings imply that the white matter lesions in this patient were the result of delayed myelination rather than demyelination. The patient was evaluated for known metabolic and degenerative diseases, but no abnormalities were observed. Her symptoms and neuroimaging findings did not fit the criteria for any defined leukodystrophy.

Neuropathology and genetics of Pelizaeus-Merzbacher disease

The recent history of Pelizaeus-Merzbacher Disease (PMD) demonstrates paradigmatically the impact of basic biological research on clinical neurology and brain pathology: this rare and peculiar hereditary disease has become one of the best known disorders of its kind, through a cooperative research effort in neuropathology, human genetics, neurochemistry and molecular biology. PMD, a genetic dysmyelination restricted to the CNS, has been identified as a disease that involves the X chromosome-linked gene for myelin proteolipid protein (PLP), a major structural myelin component. Today more than 30 different mutations in this gene have been defined and associated with PMD or the clinically distinct form X-linked spastic paraplegia type-2 (SPG-2). Improved scanning techniques, specifically the non-invasive magnetic resonance imaging (MRI), allow its early diagnosis in the heterogeneous group of CNS myelin deficiencies. These remarkable achievements have, at the same time, caused a problem for disease classification. Myelin disorders have been grouped in the past on the basis of clinical and neuropathological criteria, creating a system that has now to be reconciled with molecular-genetic data.

Spinocerebellar degeneration and cerebral hypomyelination in a family

The proband is a 24-year-old woman who developed symptoms of a spinocerebellar degeneration in early childhood. Neurological examination revealed normal cognitive function, optic atrophy, dysarthria, titubation, action tremors, increased deep tendon reflexes, Babinski's signs, and a spastic scissoring gait. The magnetic resonance imaging (MRI) showed an abnormal increased signal on long TR images involving white matter throughout the cerebral hemispheres, most striking in the subcortical white matter, and to a lesser degree in the brainstem, compatible with diffuse hypomyelinating or dysmyelinating diseases. Metabolic and chromosomal studies were normal. Her 49-year-old mother developed similar symptoms in her 20s and is now wheelchair-bound. Findings on neurological examination and MRI were similar to her daughter but more severe. The proband's maternal grandfather had a female cousin who had a neurological illness beginning in her 20s with similar symptoms and signs and died at the age of 44 years. Spinocerebellar degenerations are a group of syndromes with similar clinical manifestations but heterogeneous etiology. We report a family with spinocerebellar degeneration with distinct MRI findings compatible with hypomyelination or dysmyelination which has not heretofore been described. This family may represent a new spinocerebellar syndrome due to an abnormality of as yet an undetermined gene.

Neurophysiological study in Pelizaeus-Merzbacher disease

The Neurophysiological characteristics of Pelizaeus-Marzbacher disease (PMD) were studied in four Japanese patients aged between 5 and 13 years. Pendular spontaneous nystagmus was always recorded with a frequency between 2.5 and 4 Hz, and abnormal saccades with an almost twofold prolongation in onset time and 50% decrease in velocity were noted. Brainstem auditory evoked potentials consistently demonstrated severely altered waves II to V, following a normal wave I, despite normal hearing acuity. Somatosensory evoked potentials (SEPs) were always absent between brainstem components and early cortical responses. Late cortical components of SEPs and visual evoked potentials with significantly prolonged latencies were recorded in the three younger cases having normal sensory and visual acuity (N35 of SEP, 73.1 +/- 2.1 ms; N75 of VEP, 129.0 +/- 12.7 ms; mean +/- S.D.), while these peaks were absent in the oldest case having the most severe handicap. In motor evoked potentials (MEPs), R1 of blink reflex with significantly prolonged latency (14.9 +/- 1.48ms) was always obtained, and no subsequent R2 was elicited. Magnetic transcortical stimulation elicited no MEPs of the thenar even in the facilitating condition on voluntary contraction despite mild weakness of the thenar, while normal MEPs were always elicited on cervical stimulation. These electrophysiological findings were consistent with extensive conduction slowing involving the brainstem to the cerebrum, which seemed to be accompanied by conduction block in motor systems rather than sensory systems. Although each of the results was not specific, in combination they suggested the characteristics of diffuse brain dysmyelination in PMD.

Neurophysiologic studies and MRI in Pelizaeus-Merzbacher disease: comparison of classic and connatal forms

Four patients with the classic form and 1 patient with the connatal form of Pelizaeus-Merzbacher disease were studied with magnetic resonance imaging, electroencephalography, and multimodal evoked potentials, including brainstem auditory evoked potentials, somatosensory evoked potentials, and visual evoked potentials. Comparisons between these findings were made. It was determined that the neurophysiologic studies, particularly brainstem auditory evoked potentials, are of value in early diagnosis of Pelizaeus-Merzbacher disease; brainstem auditory evoked potentials with only normal wave I may be a relatively reliable clue suggesting the classic form of Pelizaeus-Merzbacher disease in patients with nystagmus and chronic progressive encephalopathy. Magnetic resonance imaging allows an accurate assessment of the degree of hypomyelination; however, the clinical severity of various forms of Pelizaeus-Merzbacher disease seemed to be independent of the age of onset and the amount of residual myelin. The following may be distinguishing features between the connatal and classic forms of Pelizaeus-Merzbacher disease: hypoplasia of the cerebellum and brainstem, and diffuse brain atrophy on magnetic resonance imaging; optic atrophy with abnormal visual evoked potential; seizure disorder with abnormal electroencephalography, and/or auditory nerve impairment with abnormal wave I of brainstem auditory evoked potentials in the early stage of the disease.

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

Two brothers with profound neonatal hypotonia and hyporeflexia and electrodiagnostic testing consistent with lower motor neuron pathology were found to have a leukodystrophy. Using single-strand conformational polymorphism analysis and direct sequencing, a mutation within exon 3 of the gene encoding proteolipid protein (Gly73Arg substitution) was previously detected in both brothers and their mother, establishing the diagnosis of Pelizaeus-Merzbacher disease. Despite reported sparing of the peripheral nervous system in Pelizaeus-Merzbacher disease, we suggest that proteolipid protein gene products may influence the development of anterior horn cells or peripheral nervous system myelin and that some individuals affected with this disease may present with clinical and electromyographic features suggestive of neonatal spinal muscular atrophy.

Delayed myelination in a patient with 18q- syndrome

A Japanese boy with the typical manifestations of 18q-syndrome and delayed myelination on magnetic resonance imaging is described. Cytogenetic investigation revealed a deletion at 18q21.3. Three serial magnetic resonance images demonstrated that myelination in the central nervous system was delayed except for the corpus callosum and brainstem. This pattern of delayed myelination appears to be peculiar to the 18q- syndrome. Because the gene for myelin basic protein has been localized to the distal end of the long arm of chromosome 18, we speculate that the abnormal myelination in our patient was partly due to the failure of expression of the myelin basic protein gene.

MR diffusion imaging in Pelizaeus-Merzbacher disease

A case of classical type Pelizaeus-Merzbacher disease was reported. This patient exhibited marked motor and mental developmental delay, and nystagmus, with a positive familial history. Electrophysiological studies, such as on brainstem auditory evoked potentials, blink reflex and somatosensory evoked potentials, suggested marked disturbance of nerve conduction in CNS. T2-weighted magnetic resonance (MR) images revealed non-progressive diffuse T2 prolongation of cerebral white matter after a 2-year interval, indicating congenital hypomyelination in CNS. A newly developed magnetic resonance diffusion imaging method demonstrated the existence of diffusional anisotropy in the corpus callosum, internal capsule, and white matter of the frontal lobe. Although the diffusional anisotropy was considered to depend on the well-developed multiple layers of myelin around the axons, the imaging data of this patient demonstrated that the diffusional anisotropy did not necessarily depend on those multiple layers. These results may indicate the potential usefulness of MR diffusion imaging, combined with electrophysiological studies and conventional MR imaging, for analyzing the lesions of the cerebral white matter.

Childhood ataxia with diffuse central nervous system hypomyelination

A significant number of patients with progressive leukodystrophy do not have a definitive diagnosis. This report describes the clinical, morphological, and biochemical characteristics of 4 unrelated girls with progressive ataxic diplegia of unknown etiology. These patients had normal development until the ages of 1.5 to 5 years. A diffuse confluent abnormality of the white matter of the central nervous system was present on computed tomography and magnetic resonance scans obtained early in the course of the illness. Dementia was not present and peripheral nerves were normal. All patients were evaluated for known metabolic and degenerative diseases and no abnormalities were observed. Light and electron microscopy of open-brain biopsy specimens from 2 girls showed selective white matter abnormalities including hypomyelination, demyelination, and gliosis. Myelin-specific proteins in the subcortical white matter were examined immunocytochemically and by Western blot analysis. They were of normal molecular size but were markedly reduced in quantity in both patients compared to control subjects. Lipid analysis revealed decreased levels of characteristic myelin lipids. When examined by magnetic resonance spectroscopic imaging, all patients showed a marked decrease of N-acetylaspartic acid, choline, and creatine in white matter only. The magnetic resonance spectroscopic imaging profile is a unique diagnostic feature of this clinicopathological syndrome.

The inherited leukodystrophies: a clinical overview

The leukodystrophies are degenerative diseases that involve primarily the white matter of the brain. The most common leukodystrophies result from known disturbances in the synthesis or catabolism of myelin such as a block in the catabolism of sulphatides and of galactocerebrosides, respectively, in metachromatic leukodystrophy and in Krabbe disease, or from synthesis of an abnormal proteolipid protein in Pelizaeus-Merzbacher disease. The cause of white matter involvement in other leukodystrophies remains unknown even though metabolic anomalies, such as accumulation of acetylaspartic acid in Canavan disease, have been demonstrated. Common clinical features of the leukodystrophies include neurological deterioration following a period of normal development, predominant involvement of motor function at least initially, and absence of convulsions or myoclonus. Imaging-especially magnetic resonance-shows changes in density or signal from central white matter. Most leukodystrophies feature suggestive symptoms and signs such as effects on peripheral nerves' myelin in Krabbe disease and metachromatic leukodystrophy, or X-linked inheritance and slow deterioration in Pelizaeus-Merzbacher disease. Therapy of the leukodystrophies is purely symptomatic in most cases. Trials of bone marrow transplantation are being pursued for metachromatic leukodystrophy and adrenoleukodystrophy.

Oculomotor and vestibular anomalies in Pelizaeus-Merzbacher disease: a study on a kindred with 2 affected and 3 normal males, 3 obligate and 8 possible carriers

Two males suffering from Pelizaeus-Merzbacher disease were examined, one at the age of 1 year 4 months and at the age of 7 years, and the other at the age of 7 years 8 months. The former had spontaneous vertical pendular nystagmus. He also showed horizontal "micronystagmus", present only at the age of 1 year, which might be similar to "voluntary nystagmus". Both males had jerky bilateral gaze-evoked nystagmus, defective smooth pursuit and optokinetic responses and a hyporeactive vestibulo-ocular reflex (VOR). All three obligate carriers exhibited typical VOR disinhibition in the two horizontal nystagmus directions, which may be a distinctive feature. This feature was also found in one of the 7 possible carriers examined and was not observed in the 3 non-affected males, who had normal oculomotor responses.

Myelin deficiency in female rats due to a mutation in the PLP gene

Myelin deficiency (md) in female rats due to a mutation in the X-linked proteolipid protein (PLP) gene is caused by X-chromosome monosomy. Cytogenetic analysis revealed a single X karyotype [41,X(md/0)]. An immunocytochemical, electron microscopic, and biochemical study was performed on male and female md rats. The central nervous system (CNS) of the female md rat [41,X(md/0)] revealed the same total lack of PLP as the CNS of the affected male littermate [42,XY(md/Y)]. Immunocytochemistry for myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNP) revealed "islands" of myelin sheath-like reaction product in both. Electron microscopy showed great paucity of compact myelin sheaths in 41,X(md/0) and 42,XY(md/Y). Reduced levels of MPB, MAG, and CNP were confirmed for both sexes but MAG and CNP were substantially higher in 41,X(md/0). Sexual differentiation of the brain may account for the observed differences since normal female reproductive organs are present in the md female rat.

Pelizaeus-Merzbacher disease: a valine to phenylalanine point mutation in a putative extracellular loop of myelin proteolipid

In the central nervous system, myelin proteolipid protein isoforms (PLP and DM20) play an essential structural role in myelination. It has been shown in several species that myelination is impaired by molecular defects resulting from single base mutations in the PLP gene. We have used DNA amplification by polymerase chain reaction to study the PLP gene coding regions from 17 patients in 15 unrelated families with similar Pelizaeus-Merzbacher phenotype. In one case amplification of peripheral nerve PLP/DM20 cDNAs revealed that a silent T----C transition was unrelated to the disease. In one family a nonsilent mutation was identified that leads to a phenylalanine substitution for valine-218 in PLP/DM20 proteins. We investigated the inheritance of the mutant allele in 19 subjects of this four-generation family and found a strict cosegregation of the Phe218 substitution with transmission and expression of the disease. The effect of the Val218----Phe mutation is discussed in the frame of a recently suggested topological model of PLP/DM20, according to which Val218 is part of an extracellular loop that connects the last two of four membrane-spanning alpha-helices.

Carrier detection and prenatal diagnosis of Pelizaeus-Merzbacher disease using a combination of anonymous DNA polymorphisms and the proteolipid protein (PLP) gene cDNA

We report carrier identification and a prenatal diagnosis using DNA polymorphisms in 2 families with X-linked Pelizaeus-Merzbacher disease (PMD). In both families, the proteolipid protein (PLP) gene in the single affected male could be traced back to his unaffected maternal grandfather. Therefore, each family contains a new mutation. In the case of the prenatal diagnosis, the fetus was shown by cytogenetic analysis to be a female, who we predict will be a noncarrier of PMD based on her genotype with the PLP intragenic polymorphism.

Myelination patterns on magnetic resonance of children with developmental delay

Magnetic resonance (MR) imaging was performed in 30 children with unexplained developmental delay who had associated neurological abnormalities such as seizures, spasticity, hypotonia, ataxia or poor vision. No child had a history of regression, preterm birth or neonatal cerebral injury. CT scans were performed before MR in all cases and were either normal or showed only mild atrophy. At least two MR sequences were obtained for all patients. Nine children had delayed or absent myelination on MR, one had patchy white-matter abnormalities, and in one patient myelination was topographically normal, but of inappropriately low signal intensity. MR was abnormal in six of seven children who had abnormal brainstem auditory evoked potentials (BAEP), and was normal in nine of 11 patients who had a normal BAEP. MR may have a useful rôle in demonstrating abnormal white-matter maturation in children with unexplained neurodevelopmental delay, particularly when abnormalities are found on BAEP studies.