Genotype-phenotype correlation in inherited brain myelination defects due to proteolipid protein gene mutations. Clinical European Network on Brain Dysmyelinating Disease

Terra incognita of glial cell dynamics in the etiology of leukodystrophies: Broadening disease and therapeutic perspectives

Neuroglial cells, also known as glia, are primarily characterized as auxiliary cells within the central nervous system (CNS). The recent findings have shed light on their significance in numerous physiological processes and their involvement in various neurological disorders. Leukodystrophies encompass an array of rare and hereditary neurodegenerative conditions that were initially characterized by the deficiency, aberration, or degradation of myelin sheath within CNS. The primary cellular populations that experience significant alterations are astrocytes, oligodendrocytes and microglia. These glial cells are either structurally or metabolically impaired due to inherent cellular dysfunction. Alternatively, they may fall victim to the accumulation of harmful by-products resulting from metabolic disturbances. In either situation, the possible replacement of glial cells through the utilization of implanted tissue or stem cell-derived human neural or glial progenitor cells hold great promise as a therapeutic strategy for both the restoration of structural integrity through remyelination and the amelioration of metabolic deficiencies. Various emerging treatment strategies like stem cell therapy, ex-vivo gene therapy, infusion of adeno-associated virus vectors, emerging RNA-based therapies as well as long-term therapies have demonstrated success in pre-clinical studies and show promise for rapid clinical translation. Here, we addressed various leukodystrophies in a comprehensive and detailed manner as well as provide prospective therapeutic interventions that are being considered for clinical trials. Further, we aim to emphasize the crucial role of different glial cells in the pathogenesis of leukodystrophies. By doing so, we hope to advance our understanding of the disease, elucidate underlying mechanisms, and facilitate the development of potential treatment interventions.

Diet and Nutrients in Rare Neurological Disorders: Biological, Biochemical, and Pathophysiological Evidence

Background/Objectives: Rare diseases are a wide and heterogeneous group of multisystem life-threatening or chronically debilitating clinical conditions with reduced life expectancy and a relevant mortality rate in childhood. Some of these disorders have typical neurological symptoms, presenting from birth to adulthood. Dietary patterns and nutritional compounds play key roles in the onset and progression of neurological disorders, and the impact of alimentary needs must be enlightened especially in rare neurological diseases. This work aims to collect the in vitro, in vivo, and clinical evidence on the effects of diet and of nutrient intake on some rare neurological disorders, including some genetic diseases, and rare brain tumors. Herein, those aspects are critically linked to the genetic, biological, biochemical, and pathophysiological hallmarks typical of each disorder. Methods: By searching the major web-based databases (PubMed, Web of Science Core Collection, DynaMed, and Clinicaltrials.gov), we try to sum up and improve our understanding of the emerging role of nutrition as both first-line therapy and risk factors in rare neurological diseases. Results: In line with the increasing number of consensus opinions suggesting that nutrients should receive the same attention as pharmacological treatments, the results of this work pointed out that a standard dietary recommendation in a specific rare disease is often limited by the heterogeneity of occurrent genetic mutations and by the variability of pathophysiological manifestation. Conclusions: In conclusion, we hope that the knowledge gaps identified here may inspire further research for a better evaluation of molecular mechanisms and long-term effects.

An Open-Label Administration of Bioavailable-Form Curcumin in Patients With Pelizaeus-Merzbacher Disease

BACKGROUND

Two preclinical studies using mouse models of Pelizeaus-Merzbacher disease (PMD) have revealed the potential therapeutic effects of curcumin. In this study, we examined the effects of curcumin in patients with PMD.

METHODS

We conducted a study administering an open-label oral bioavailable form of curcumin in nine patients genetically confirmed to have PMD (five to 20 years; mean 11 years) for 12 months (low doses for two months followed by high doses for 10 months). We evaluated changes in clinical symptoms as the primary end point using two scales, Gross Motor Function Measure (GMFM) and the PMD Functional Disability Score (PMD-FDS). The level of myelination by brain magnetic resonance imaging (MRI) and the electrophysiological state by auditory brainstem response (ABR) were evaluated as secondary end points. The safety and tolerability of oral curcumin were also examined.

RESULTS

Increase in GMFM and PMD-FDS were noted in five and three patients, respectively, but overall, no statistically significant improvement was demonstrated. We found no clear improvement in their brain MRI or ABR. No adverse events associated with oral administration of curcumin were observed.

CONCLUSIONS

Although we failed to demonstrate any significant therapeutic effects of curcumin after 12 months, its tolerability and safety were confirmed. This study does not exclude the possibility of therapeutic effects of curcumin, and a trial of longer duration should be considered to compare the natural history of the disease with the effects of curcumin.

Inherited white matter disorders: Hypomyelination (myelin disorders)

Hypomyelinating leukodystrophies are a subset of genetic white matter diseases characterized by insufficient myelin deposition during development. MRI patterns are used to identify hypomyelinating disorders, and genetic testing is used to determine the causal genes implicated in individual disease forms. Clinical course can range from severe, with patients manifesting neurologic symptoms in infancy or early childhood, to mild, with onset in adolescence or adulthood. This chapter discusses the most common hypomyelinating leukodystrophies, including X-linked Pelizaeus-Merzbacher disease and other PLP1-related disorders, autosomal recessive Pelizaeus-Merzbacher-like disease, and POLR3-related leukodystrophy. PLP1-related disorders are caused by hemizygous pathogenic variants in the proteolipid protein 1 (PLP1) gene, and encompass classic Pelizaeus-Merzbacher disease, the severe connatal form, PLP1-null syndrome, spastic paraplegia type 2, and hypomyelination of early myelinating structures. Pelizaeus-Merzbacher-like disease presents a similar clinical picture to Pelizaeus-Merzbacher disease, however, it is caused by biallelic pathogenic variants in the GJC2 gene, which encodes for the gap junction protein Connexin-47. POLR3-related leukodystrophy, or 4H leukodystrophy (hypomyelination, hypodontia, and hypogonadotropic hypogonadism), is caused by biallelic pathogenic variants in genes encoding specific subunits of the transcription enzyme RNA polymerase III. In this chapter, the clinical features, disease pathophysiology and genetics, imaging patterns, as well as supportive and future therapies are discussed for each disorder.

Molecular Pathogenic Mechanisms of Hypomyelinating Leukodystrophies (HLDs)

Hypomyelinating leukodystrophies (HLDs) represent a group of congenital rare diseases for which the responsible genes have been identified in recent studies. In this review, we briefly describe the genetic/molecular mechanisms underlying the pathogenesis of HLD and the normal cellular functions of the related genes and proteins. An increasing number of studies have reported genetic mutations that cause protein misfolding, protein dysfunction, and/or mislocalization associated with HLD. Insight into the mechanisms of these pathways can provide new findings for the clinical treatments of HLD.

Microdeletion in distal PLP1 enhancers causes hereditary spastic paraplegia 2

OBJECTIVES

Hereditary spastic paraplegia (HSP) is a genetically heterogeneous disease caused by over 70 genes, with a significant number of patients still genetically unsolved. In this study, we recruited a suspected HSP family characterized by spasticity, developmental delay, ataxia and hypomyelination, and intended to reveal its molecular etiology by whole exome sequencing (WES) and long-read sequencing (LRS) analyses.

METHODS

WES was performed on 13 individuals of the family to identify the causative mutations, including analyses of SNVs (single-nucleotide variants) and CNVs (copy number variants). Accurate circular consensus (CCS) long-read sequencing (LRS) was used to verify the findings of CNV analysis from WES.

RESULTS

SNVs analysis identified a missense variant c.195G>T (p.E65D) of MORF4L2 at Xq22.2 co-segregating in this family from WES data. Further CNVs analysis revealed a microdeletion, which was adjacent to the MORF4L2 gene, also co-segregating in this family. LRS verified this microdeletion and confirmed the deletion range (chrX: 103,690,507-103,715,018, hg38) with high resolution at nucleotide level accuracy.

INTERPRETATIONS

In this study, we identified an Xq22.2 microdeletion (about 24.5 kb), which contains distal enhancers of the PLP1 gene, as a likely cause of SPG2 in this family. The lack of distal enhancers may result in transcriptional repression of PLP1 in oligodendrocytes, potentially affecting its role in the maintenance of myelin, and causing SPG2 phenotype. This study has highlighted the importance of noncoding genomic alterations in the genetic etiology of SPG2.

Optical genome mapping in an atypical Pelizaeus-Merzbacher prenatal challenge

Pathogenic genetic variants represent a challenge in prenatal counseling, especially when clinical presentation in familial carriers is atypical. We describe a prenatal case involving a microarray-detected duplication of PLP1 which causes X-linked Pelizaeus-Merzbacher disease, a progressive hypomyelinating leukodystrophy. Because of atypical clinical presentation in an older male child, the duplication was examined using a novel technology, optical genome mapping, and was found to be an inverted duplication, which has not been previously described. Simultaneously, segregation analysis identified another healthy adult male carrier of this unique structural rearrangement. The novel PLP1 structural variant was reclassified, and a healthy boy was delivered. In conclusion, we suggest that examining structural variants with novel methods is warranted especially in cases with atypical clinical presentation and may in these cases lead to improved prenatal and postnatal genetic counseling.

Focused ion beam-scanning electron microscopy links pathological myelin outfoldings to axonal changes in mice lacking Plp1 or Mag

Healthy myelin sheaths consist of multiple compacted membrane layers closely encasing the underlying axon. The ultrastructure of CNS myelin requires specialized structural myelin proteins, including the transmembrane-tetraspan proteolipid protein (PLP) and the Ig-CAM myelin-associated glycoprotein (MAG). To better understand their functional relevance, we asked to what extent the axon/myelin-units display similar morphological changes if PLP or MAG are lacking. We thus used focused ion beam-scanning electron microscopy (FIB-SEM) to re-investigate axon/myelin-units side-by-side in Plp- and Mag-null mutant mice. By three-dimensional reconstruction and morphometric analyses, pathological myelin outfoldings extend up to 10 μm longitudinally along myelinated axons in both models. More than half of all assessed outfoldings emerge from internodal myelin. Unexpectedly, three-dimensional reconstructions demonstrated that both models displayed complex axonal pathology underneath the myelin outfoldings, including axonal sprouting. Axonal anastomosing was additionally observed in Plp-null mutant mice. Importantly, normal-appearing axon/myelin-units displayed significantly increased axonal diameters in both models according to quantitative assessment of electron micrographs. These results imply that healthy CNS myelin sheaths facilitate normal axonal diameters and shape, a function that is impaired when structural myelin proteins PLP or MAG are lacking.

PLP1 gene mutations cause spastic paraplegia type 2 in three families

OBJECTIVE

Spastic paraplegia type 2 (SPG2) is an X-linked recessive (XLR) form of hereditary spastic paraplegia (HSP) caused by mutations in proteolipid protein 1 (PLP1) gene. We described the clinical and genetic features of three unrelated families with PLP1 mutations and reviewed PLP1-related cases worldwide to summarize the genotype-phenotype correlations.

METHODS

The three probands were 23, 26, and 27 years old, respectively, with progressively aggravated walking difficulty as well as lower limb spasticity. Detailed physical examination showed elevated muscle tone, hyperreflexia, and Babinski signs in lower limbs. Brain MRI examinations were investigated for all cases. PLP1 mutations were identified by whole exome sequencing, followed by Sanger sequencing, family co-segregation, and phenotypic reevaluation.

RESULTS

A total of eight patients with SPG2 were identified in these three families. The probands additionally had cognitive impairment, urinary or fecal incontinence, ataxia, and white matter lesions (WML) in periventricular regions, with or without kinetic tremor. Three hemizygous mutations in PLP1 were identified, including c.453+159G>A, c.834A>T (p.*278C), and c.434G>A (p.W145*), of which c.834A>T was first associated with HSP.

INTERPRETATION

We identified three families with complicated SPG2 due to three PLP1 mutations. Our study supports the clinically inter-and intra-family heterogeneity of SPG2. The periventricular region WML and cognitive impairment are the most common characteristics. The kinetic tremor in upper limbs was observed in 2/3 families, suggesting the spectrum of PLP1-related disorders is still expanding.

Nonhuman primate genetic models for the study of rare diseases

Pre-clinical research and development relies heavily upon translationally valid models of disease. A major difficulty in understanding the biology of, and developing treatments for, rare disease is the lack of animal models. It is important that these models not only recapitulate the presentation of the disease in humans, but also that they share functionally equivalent underlying genetic causes. Nonhuman primates share physiological, anatomical, and behavioral similarities with humans resulting from close evolutionary relationships and high genetic homology. As the post-genomic era develops and next generation sequencing allows for the resequencing and screening of large populations of research animals, naturally occurring genetic variation in nonhuman primates with clinically relevant phenotypes is regularly emerging. Here we review nonhuman primate models of multiple rare genetic diseases with a focus on the similarities and differences in manifestation and etiologies across species. We discuss how these models are being developed and how they can offer new tools and opportunities for researchers interested in exploring novel therapeutics for these and other genetic diseases. Modeling human genetic diseases in translationally relevant nonhuman primates presents new prospects for development of therapeutics and a better understanding of rare diseases. The post-genomic era offers the opportunity for the discovery and further development of more models like those discussed here.

In Silico Structural Analysis Predicting the Pathogenicity of PLP1 Mutations in Multiple Sclerosis

The X chromosome gene PLP1 encodes myelin proteolipid protein (PLP), the most prevalent protein in the myelin sheath surrounding the central nervous system. X-linked dysmyelinating disorders such as Pelizaeus-Merzbacher disease (PMD) or spastic paraplegia type 2 (SPG2) are typically caused by point mutations in PLP1. Nevertheless, numerous case reports have shown individuals with PLP1 missense point mutations which also presented clinical symptoms and indications that were consistent with the diagnostic criteria of multiple sclerosis (MS), a disabling disease of the brain and spinal cord with no current cure. Computational structural biology methods were used to assess the impact of these mutations on the stability and flexibility of PLP structure in order to determine the role of PLP1 mutations in MS pathogenicity. The analysis showed that most of the variants can alter the functionality of the protein structure such as R137W variants which results in loss of helix and H140Y which alters the ordered protein interface. In silico genomic methods were also performed to predict the significance of these mutations associated with impairments in protein functionality and could suggest a better definition for therapeutic strategies and clinical application in MS patients.

Identification of PMD subgroups using a myelination score for PMD

BACKGROUND

The clinical spectrum of Pelizaeus-Merzbacher disease (PMD), a common hypomyelinating leukodystrophy, ranges between severe neonatal onset and a relatively stable presentation with later onset and mainly lower limb spasticity. In view of emerging treatment options and in order to grade severity and progression, we developed a PMD myelination score.

METHODS

Myelination was scored in 15 anatomic sites (items) on conventional T2-and T1w images in controls (n = 328) and 28 PMD patients (53 MRI; n = 5 connatal, n = 3 transitional, n = 10 classic, n = 3 intermediate, n = 2 PLP0, n = 3 SPG2, n = 2 female). Items included in the score were selected based on interrater variability, practicability of scoring and importance of scoring items for discrimination between patients and controls and between patient subgroups. Bicaudate ratio, maximal sagittal pons diameter, and visual assessment of midsagittal corpus callosum were separately recorded.

RESULTS

The resulting myelination score consisting of 8 T2-and 5 T1-items differentiates patients and controls as well as patient subgroups at first MRI. There was very little myelin and early loss in severely affected connatal and transitional patients, more, though still severely deficient myelin in classic PMD, ongoing myelination during childhood in classic and intermediate PMD. Atrophy, present in 50% of patients, increased with age at imaging.

CONCLUSIONS

The proposed myelination score allows stratification of PMD patients and standardized assessment of follow-up. Loss of myelin in severely affected and PLP0 patients and progressing myelination in classic and intermediate PMD must be considered when evaluating treatment efficacy.

Mutation of Proteolipid Protein 1 Gene: From Severe Hypomyelinating Leukodystrophy to Inherited Spastic Paraplegia

Pelizaeus-Merzbacher Disease (PMD) is an inherited leukodystrophy affecting the central nervous system (CNS)-a rare disorder that especially concerns males. Its estimated prevalence is 1.45-1.9 per 100,000 individuals in the general population. Patients affected by PMD exhibit a drastic reduction or absence of myelin sheaths in the white matter areas of the CNS. The Proteolipid Protein 1 (PLP1) gene encodes a transmembrane proteolipid protein. PLP1 is the major protein of myelin, and it plays a key role in the compaction, stabilization, and maintenance of myelin sheaths. Its function is predominant in oligodendrocyte development and axonal survival. Mutations in the PLP1 gene cause the development of a wide continuum spectrum of leukopathies from the most severe form of PMD for whom patients exhibit severe CNS hypomyelination to the relatively mild late-onset type 2 spastic paraplegia, leading to the concept of PLP1-related disorders. The genetic diversity and the biochemical complexity, along with other aspects of PMD, are discussed to reveal the obstacles that hinder the development of treatments. This review aims to provide a clinical and mechanistic overview of this spectrum of rare diseases.

Genotype-phenotype correlation and natural history analyses in a Chinese cohort with pelizaeus-merzbacher disease

BACKGROUND

The natural history and genotype-phenotype correlation of Pelizaeus-Merzbacher disease (PMD) of Chinese patients has been rarely reported.

METHOD

Patients who met the criteria for PMD were enrolled in our study. Genomic analysis was conducted by multiplex ligation probe amplification (MLPA) and Sanger or whole-exome sequencing (WES). Natural history differences and genotype-phenotype correlations were analyzed.

RESULT

A total of 111 patients were enrolled in our follow-up study. The median follow-up interval was 53 m (1185). Among PMD patients, developmental delay was the most common sign, and nystagmus and hypotonia were the most common initial symptoms observed. A total of 78.4% of the patients were able to control their head, and 72.1% could speak words. However, few of the patients could stand (9.0%) or walk (4.5%) by themselves. Nystagmus improved in more than half of the patients, and hypotonia sometimes deteriorated to movement disorders. More PLP1 point mutations patients were categorized into severe group, while more patients with PLP1 duplications were categorized into mild group (p < 0.001). Compared to patients in mild groups, those in the severe group had earlier disease onset and had acquired fewer skills at a later age.

CONCLUSION

PMD patients have early disease onset with nystagmus and hypotonia followed by decreased nystagmus and movement disorders, such as spasticit. Patients with PLP1 duplication were more likely to be categorized into the mild group, whereas patients with point mutations were more likely to be categorized into the severe group.

One-step Reprogramming of Human Fibroblasts into Oligodendrocyte-like Cells by SOX10, OLIG2, and NKX6.2

Limited access to human oligodendrocytes impairs better understanding of oligodendrocyte pathology in myelin diseases. Here, we describe a method to robustly convert human fibroblasts directly into oligodendrocyte-like cells (dc-hiOLs), which allows evaluation of remyelination-promoting compounds and disease modeling. Ectopic expression of SOX10, OLIG2, and NKX6.2 in human fibroblasts results in rapid generation of O4⁺ cells, which further differentiate into MBP⁺ mature oligodendrocyte-like cells within 16 days. dc-hiOLs undergo chromatin remodeling to express oligodendrocyte markers, ensheath axons, and nanofibers in vitro, respond to promyelination compound treatment, and recapitulate in vitro oligodendroglial pathologies associated with Pelizaeus-Merzbacher leukodystrophy related to PLP1 mutations. Furthermore, DNA methylome analysis provides evidence that the CpG methylation pattern significantly differs between dc-hiOLs derived from fibroblasts of young and old donors, indicating the maintenance of the source cells’ “age.” In summary, dc-hiOLs represent a reproducible technology that could contribute to personalized medicine in the field of myelin diseases.

•

SOX10, OLIG2, and NKX6.2 directly convert human fibroblasts into dc-hiOLs in 16 days

•

dc-hiOLs express key oligodendrocyte markers

•

dc-hiOLs preserve the epigenetic age of donor cells

•

dc-hiOLs from PMD patients show maturation deficit and vulnerability to cell death

Expanding the Clinical and Mutational Spectrum of the PLP1-Related Hypomyelination of Early Myelinated Structures (HEMS)

The PLP1 gene, located on chromosome Xq22, encodes the proteolipid protein 1 and its isoform DM20. Mutations in PLP1 cause a spectrum of white matter disorders of variable severity. Here we report on four additional HEMS patients from three families harboring three novel PLP1 mutations in exon 3B detected by targeted next-generation sequencing. Patients experienced psychomotor delay or nystagmus in the first year of age and then developed ataxic-spastic or ataxic syndrome, compatible with a phenotype of intermediate severity in the spectrum of PLP1-related disorders. Regression occurred at the beginning of the third decade of the eldest patient. Extrapyramidal involvement was rarely observed. Brain MRI confirmed the involvement of structures that physiologically myelinate early, although the pattern of abnormalities may differ depending on the age at which the study is performed. These new cases contribute to expanding the phenotypic and genotypic spectrum of HEMS. Additional studies, especially enriched by systematic functional evaluations and long-term follow-up, are welcome to better delineate the natural history of this rare hypomyelinating leukodystrophy.

Prenatal diagnosis of PLP1 duplication by single nucleotide polymorphism array in a family with Pelizaeus-Merzbacher disease

A family with a history of Pelizaeus-Merzbacher disease (PMD) received prenatal diagnosis of PLP1 gene duplication in a fetus using a single nucleotide polymorphism (SNP) array. A 27-year-old pregnant woman was referred for genetic counseling due to her four-year-old son being diagnosed with a suspected classic type of PMD. Amniocentesis was performed at 18 and 3/7 weeks of gestation, and the SNP array was carried out on DNA from the mother, her affected son, and fetus, then further confirmed by multiplex ligation-dependent probe amplification (MLPA). Cytogenetic analysis of the fetus showed 46,XY. SNP array analysis revealed that the male fetus did not carry PLP1 gene duplication but the affected boy did, and the mother was a carrier for the duplication of the PLP1 gene. All SNP array results were further confirmed by MLPA. SNP array and MLPA analyses of peripheral blood verified the nonduplication of the PLP1 gene in the infant after birth. At present, the child (without PLP1 duplication) is developing normally. This study preliminarily suggests that SNP array is a sensitive and accurate technology for identifying PLP1 duplication and is feasible for reliable diagnosis, including for the prenatal diagnosis of PMD resulting from PLP1 duplication.

The role of oligodendrocyte gap junctions in neuroinflammation

Gap junctions (GJs) provide channels for direct cell-to-cell connectivity serving the homeostasis in several organs of vertebrates including the central (CNS) and peripheral (PNS) nervous systems. GJs are composed of connexins (Cx), which show a highly distinct cellular and subcellular expression pattern. Oligodendrocytes, the myelinating cells of the CNS, are characterized by extensive GJ connectivity with each other as well as with astrocytes. The main oligodendrocyte connexins forming these GJ channels are Cx47 and Cx32. The importance of these channels has been highlighted by the discovery of human diseases caused by mutations in oligodendrocyte connexins, manifesting with leukodystrophy or transient encephalopathy. Experimental models have provided further evidence that oligodendrocyte GJs are essential for CNS myelination and homeostasis, while a strong inflammatory component has been recognized in the absence of oligodendrocyte connexins. Further studies revealed that connexins are also disrupted in multiple sclerosis (MS) brain, and in experimental models of induced inflammatory demyelination. Moreover, induced demyelination was more severe and associated with higher degree of CNS inflammation in models with oligodendrocyte GJ deficiency, suggesting that disrupted connexin expression in oligodendrocytes is not only a consequence but can also drive a pro-inflammatory environment in acquired demyelinating disorders such as MS. In this review, we summarize the current insights from human disorders as well as from genetic and acquired models of demyelination related to oligodendrocyte connexins, with the remaining challenges and perspectives.

Pathology of myelinated axons in the PLP-deficient mouse model of spastic paraplegia type 2 revealed by volume imaging using focused ion beam-scanning electron microscopy

Advances in electron microscopy including improved imaging techniques and state-of-the-art detectors facilitate imaging of larger tissue volumes with electron microscopic resolution. In combination with genetic tools for the generation of mouse mutants this allows assessing the three-dimensional (3D) characteristics of pathological features in disease models. Here we revisited the axonal pathology in the central nervous system of a mouse model of spastic paraplegia type 2, the Plp-/Y mouse. Although PLP is a bona fide myelin protein, the major hallmark of the disease in both SPG2 patients and mouse models are axonal swellings comprising accumulations of numerous organelles including mitochondria, gradually leading to irreversible axonal loss. To assess the number and morphology of axonal mitochondria and the overall myelin preservation we evaluated two sample preparation techniques, chemical fixation or high-pressure freezing and freeze substitution, with respect to the objective of 3D visualization. Both methods allowed visualizing distribution and morphological details of axonal mitochondria. In Plp-/Y mice the number of mitochondria is 2-fold increased along the entire axonal length. Mitochondria are also found in the excessive organelle accumulations within axonal swellings. In addition, organelle accumulations were detected within the myelin sheath and the inner tongue. We find that 3D electron microscopy is required for a comprehensive understanding of the size, content and frequency of axonal swellings, the hallmarks of axonal pathology.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Ketogenic diet ameliorates axonal defects and promotes myelination in Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an untreatable and fatal leukodystrophy. In a model of PMD with perturbed blood-brain barrier integrity, cholesterol supplementation promotes myelin membrane growth. Here, we show that in contrast to the mouse model, dietary cholesterol in two PMD patients did not lead to a major advancement of hypomyelination, potentially because the intact blood-brain barrier precludes its entry into the CNS. We therefore turned to a PMD mouse model with preserved blood-brain barrier integrity and show that a high-fat/low-carbohydrate ketogenic diet restored oligodendrocyte integrity and increased CNS myelination. This dietary intervention also ameliorated axonal degeneration and normalized motor functions. Moreover, in a paradigm of adult remyelination, ketogenic diet facilitated repair and attenuated axon damage. We suggest that a therapy with lipids such as ketone bodies, that readily enter the brain, can circumvent the requirement of a disrupted blood-brain barrier in the treatment of myelin disease.

Practical approach to the diagnosis of adult-onset leukodystrophies: an updated guide in the genomic era

Adult-onset leukodystrophies and genetic leukoencephalopathies comprise a diverse group of neurodegenerative disorders of white matter with a wide age of onset and phenotypic spectrum. Patients with white matter abnormalities detected on MRI often present a diagnostic challenge to both general and specialist neurologists. Patients typically present with a progressive syndrome including various combinations of cognitive impairment, movement disorders, ataxia and upper motor neuron signs. There are a number of important and treatable acquired causes for this imaging and clinical presentation. There are also a very large number of genetic causes which due to their relative rarity and sometimes variable and overlapping presentations can be difficult to diagnose. In this review, we provide a structured approach to the diagnosis of inherited disorders of white matter in adults. We describe clinical and radiological clues to aid diagnosis, and we present an overview of both common and rare genetic white matter disorders. We provide advice on testing for acquired causes, on excluding small vessel disease mimics, and detailed advice on metabolic and genetic testing available to the practising neurologist. Common genetic leukoencephalopathies discussed in detail include CSF1R, AARS2, cerebral arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and mitochondrial and metabolic disorders.

Pelizaeus-Merzbacher Disease: Molecular and Cellular Pathologies and Associated Phenotypes

Pelizaeus-Merzbacher disease (PMD) represents a group of disorders known as hypomyelinating leukodystrophies, which are characterized by abnormal development and maintenance of myelin in the central nervous system. PMD is caused by different types of mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major myelin membrane lipoprotein. These mutations in the PLP1 gene result in distinct cellular and molecular pathologies and a spectrum of clinical phenotypes. In this chapter, I discuss the historical aspects and current understanding of the mechanisms underlying how different PLP1 mutations disrupt the normal process of myelination and result in PMD and other disorders.

Exome sequencing reveals a novel PLP1 mutation in a Moroccan family with connatal Pelizaeus-Merzbacher disease: a case report

Background

Epilepsy regroups a common and diverse set of chronic neurological disorders that are characterized by spontaneous, unprovoked, and recurrent epileptic seizures. Epilepsies have a highly heterogeneous background with a strong genetic contribution and various mode of inheritance. X-linked epilepsy usually manifests as part of a syndrome or epileptic encephalopathy. The variability of clinical manifestations of X-linked epilepsy may be attributed to several factors including the causal genetic mutation, making diagnosis, genetic counseling and treatment decisions difficult. We report the description of a Moroccan family referred to our genetic department with X-linked epileptic seizures as the only initial diagnosis.

Case presentation

Knowing the new contribution of Next-Generation Sequencing (NGS) for clinical investigation, and given the heterogeneity of this group of disorders we performed a Whole-Exome Sequencing (WES) analysis and co-segregation study in several members of this large family. We detected a novel pathogenic PLP1 missense mutation c.251C > A (p.Ala84Asp) allowing us to make a diagnosis of Pelizaeus-Merzbacher Disease for this family.

Conclusion

This report extends the spectrum of PLP1 mutations and highlights the diagnostic utility of NGS to investigate this group of heterogeneous disorders.

PLP1 Mutations in Patients with Multiple Sclerosis: Identification of a New Mutation and Potential Pathogenicity of the Mutations

PLP1 is located on the X-chromosome and encodes myelin proteolipid protein (PLP), the most abundant protein in central nervous system myelin. Generally, point mutations in PLP1 result in X-linked dysmyelinating disorders, such as Pelizaeus-Merzbacher disease (PMD) or spastic paraplegia type 2 (SPG2). However, several case studies have identified patients with missense point mutations in PLP1 and clinical symptoms and signs compatible with a diagnosis of multiple sclerosis (MS). To investigate if PLP1 mutations occur relatively frequently in MS, we sequenced the coding regions of PLP1 in 22 female MS patients who had developed disease after the age of 40 and in 42 healthy women, and identified a missense mutation in exon 2 of PLP1 resulting in a Leu30Val mutation in the protein in one of the MS patients. mCherry-tagged plasmids containing wild type or mutant PLP1 sequences of PLP, including two known PMD/SPG2-related mutations as positive controls, were constructed and transfected into Cos-7 cells. In comparison with cells transfected with wild type PLP1, all mutations caused significant accumulation of PLP in the endoplasmic reticulum of the cells and induction of the unfolded protein response-a mechanism that leads to apoptosis of cells expressing mutant proteins. Additionally, in silico analysis of the binding of peptides containing the Leu30Val mutation to the human leukocyte antigen (HLA) molecules carried by the patient harboring this mutation suggested that the mutation could produce several novel immunogenic epitopes in this patient. These results support the idea that mutations in myelin-related genes could contribute to the development of MS in a small proportion of patients.

Morpholino Antisense Oligomers as a Potential Therapeutic Option for the Correction of Alternative Splicing in PMD, SPG2, and HEMS

DNA variants of the proteolipid protein 1 gene (PLP1) that shift PLP1/DM20 alternative splicing away from the PLP1 form toward DM20 cause the allelic X-linked leukodystrophies Pelizaeus-Merzbacher disease (PMD), spastic paraplegia 2 (SPG2), and hypomyelination of early myelinating structures (HEMS). We designed a morpholino oligomer (MO-PLP) to block use of the DM20 5' splice donor site, thereby shifting alternative splicing toward the PLP1 5' splice site. Treatment of an immature oligodendrocyte cell line with MO-PLP significantly shifted alternative splicing toward PLP1 expression from the endogenous gene and from transfected human minigene splicing constructs harboring patient variants known to reduce the amount of the PLP1 spliced product. Additionally, a single intracerebroventricular injection of MO-PLP into the brains of neonatal mice, carrying a deletion of an intronic splicing enhancer identified in a PMD patient that reduces the Plp1 spliced form, corrected alternative splicing at both RNA and protein levels in the CNS. The effect lasted to post-natal day 90, well beyond the early post-natal spike in myelination and PLP production. Further, the single injection produced a sustained reduction of inflammatory markers in the brains of the mice. Our results suggest that morpholino oligomers have therapeutic potential for the treatment of PMD, SPG2, and HEMS.

Novel PLP1 Mutations Identified With Next-Generation Sequencing Expand the Spectrum of PLP1-Associated Leukodystrophy Clinical Phenotypes

Next-generation sequencing was performed for 2 families with an undiagnosed neurologic disease. Analysis revealed X-linked mutations in the proteolipid protein 1 (PLP1) gene, which is associated with X-linked Pelizaeus-Merzbacher disease and Spastic Paraplegia type 2. In family A, the novel PLP1 missense mutation c.617T>A (p.M206K) was hemizygous in the 2 affected male children and heterozygous in the mother. In family B, the novel de novoPLP1 frameshift mutation c.359_369del (p.G120fs) was hemizygous in the affected male child. Although PLP1 mutations have been reported to cause an increasingly wide range of phenotypes inclusive of the dystonia, spastic paraparesis, motor neuronopathy, and leukodystrophy observed in our patients, atypical features included the cerebrospinal fluid deficiency of neurotransmitter and pterin metabolites and the delayed appearance of myelin abnormalities on neuroimaging studies. Next-generation sequencing studies provided a diagnosis for these families with complex leukodystrophy disease phenotypes, which expanded the spectrum of PLP1-associated leukodystrophy clinical phenotypes.

Pelizaeus-Merzbacher disease can be a differential diagnosis in males presenting with severe neonatal respiratory distress and hypotonia

Pelizaeus-Merzbacher disease (PMD; MIM #312080) is a rare X-linked recessive disorder. A male neonate presented with severe respiratory distress that required tracheostomy. After the appearance of nystagmus, PMD was suspected as a diagnosis for the patient, and a missense mutation, p.Phe51Val, was identified in PLP1, the gene responsible for PMD. PMD can be a differential diagnosis in a male neonate presenting severe respiratory distress.

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.

Oxidative stress and mitochondrial dynamics malfunction are linked in Pelizaeus-Merzbacher disease

Pelizaeus‐Merzbacher disease (PMD) is a fatal hypomyelinating disorder characterized by early impairment of motor development, nystagmus, choreoathetotic movements, ataxia and progressive spasticity. PMD is caused by variations in the proteolipid protein gene PLP1, which encodes the two major myelin proteins of the central nervous system, PLP and its spliced isoform DM20, in oligodendrocytes. Large duplications including the entire PLP1 gene are the most frequent causative mutation leading to the classical form of PMD. The Plp1 overexpressing mouse model (PLP‐tg^66/66) develops a phenotype very similar to human PMD, with early and severe motor dysfunction and a dramatic decrease in lifespan. The sequence of cellular events that cause neurodegeneration and ultimately death is poorly understood. In this work, we analyzed patient‐derived fibroblasts and spinal cords of the PLP‐tg^66/66 mouse model, and identified redox imbalance, with altered antioxidant defense and oxidative damage to several enzymes involved in ATP production, such as glycolytic enzymes, creatine kinase and mitochondrial proteins from the Krebs cycle and oxidative phosphorylation. We also evidenced malfunction of the mitochondria compartment with increased ROS production and depolarization in PMD patient's fibroblasts, which was prevented by the antioxidant N‐acetyl‐cysteine. Finally, we uncovered an impairment of mitochondrial dynamics in patient's fibroblasts which may help explain the ultrastructural abnormalities of mitochondria morphology detected in spinal cords from PLP‐tg^66/66 mice. Altogether, these results underscore the link between redox and metabolic homeostasis in myelin diseases, provide insight into the pathophysiology of PMD, and may bear implications for tailored pharmacological intervention.

An Xq22.1q22.2 nullisomy in a male patient with severe neurological impairment

The proteolipid protein 1 gene (PLP1) is located on chromosome Xq22.2 and is related to X-linked recessive leukoencephalopathy (Pelizaeus-Merzbacher disease: PMD). Compared to PLP1 duplications, which are a major contributor to PMD, chromosomal deletions in this region are rare and only a few PMD patients with small deletions have been reported, suggesting that large deletions of this region would cause embryonic lethality. Previously, we have reported female patients, with chromosomal deletions in this region, who showed severe developmental delays and behavioral abnormalities. In this study, we identified the first case of a male patient associated with an Xq22 nullisomy in a region proximal to PLP1. The patient showed severe neurological impairment and was bedridden. Brain magnetic resonance imaging revealed a severely reduced cerebral volume. The chromosomal region proximal to PLP1 was considered to be significantly important for brain development.

Efficient CNV breakpoint analysis reveals unexpected structural complexity and correlation of dosage-sensitive genes with clinical severity in genomic disorders

Genomic disorders are the clinical conditions manifested by submicroscopic genomic rearrangements including copy number variants (CNVs). The CNVs can be identified by array-based comparative genomic hybridization (aCGH), the most commonly used technology for molecular diagnostics of genomic disorders. However, clinical aCGH only informs CNVs in the probe-interrogated regions. Neither orientational information nor the resulting genomic rearrangement structure is provided, which is a key to uncovering mutational and pathogenic mechanisms underlying genomic disorders. Long-range polymerase chain reaction (PCR) is a traditional approach to obtain CNV breakpoint junction, but this method is inefficient when challenged by structural complexity such as often found at the PLP1 locus in association with Pelizaeus-Merzbacher disease (PMD). Here we introduced 'capture and single-molecule real-time sequencing' (cap-SMRT-seq) and newly developed 'asymmetry linker-mediated nested PCR walking' (ALN-walking) for CNV breakpoint sequencing in 49 subjects with PMD-associated CNVs. Remarkably, 29 (94%) of the 31 CNV breakpoint junctions unobtainable by conventional long-range PCR were resolved by cap-SMRT-seq and ALN-walking. Notably, unexpected CNV complexities, including inter-chromosomal rearrangements that cannot be resolved by aCGH, were revealed by efficient breakpoint sequencing. These sequence-based structures of PMD-associated CNVs further support the role of DNA replicative mechanisms in CNV mutagenesis, and facilitate genotype-phenotype correlation studies. Intriguingly, the lengths of gained segments by CNVs are strongly correlated with clinical severity in PMD, potentially reflecting the functional contribution of other dosage-sensitive genes besides PLP1. Our study provides new efficient experimental approaches (especially ALN-walking) for CNV breakpoint sequencing and highlights their importance in uncovering CNV mutagenesis and pathogenesis in genomic disorders.

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.

A novel PLP1 mutation associated with optic nerve enlargement in two siblings with Pelizaeus-Merzbacher disease: A new MRI finding

Pelizaeus-Merzbacher disease (PMD) is a rare, X-linked disorder characterized by hypomyelination of the Central Nervous System due to mutations in the PLP1 gene. Certain mutations of the PLP1 gene correlate with specific clinical phenotypes and neuroimaging findings. We herein report a novel mutation of the PLP1 gene in two siblings with PMD associated with a rare and protean neuroimaging finding of optic nerve enlargement. To the best of our knowledge this is the first time that this novel mutation H133P of PLP1 gene is identified and clinically associated with optic nerve enlargement in PMD patients.

Acquired pendular nystagmus

Acquired pendular nystagmus is comprised of quasi-sinusoidal oscillations of the eyes significantly affecting gaze holding and clarity of vision. The most common causes of acquired pendular nystagmus include demyelinating disorders such as multiple sclerosis and the syndrome of ocular palatal tremor. However, several other deficits, such as pharmacological intoxication, metabolic and genetic disorders, and granulomatous disorders can lead to syndromes mimicking acquired pendular nystagmus. Study of the kinematic features of acquired pendular nystagmus has suggested a putative pathophysiology of an otherwise mysterious neurological disorder. Here we review clinical features of neurological deficits that co-occur with acquired pendular nystagmus. Subsequent discussion of the pathophysiology of individual forms of pendular nystagmus speculates on mechanisms of the underlying disease while providing insights into pharmacotherapy of nystagmus.

A novel PLP1 mutation F240L identified in a patient with connatal type Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an X-linked recessive hypomyelination disorder caused by mutations in the proteolipid protein 1 gene (PLP1) located on chromosome Xq22. A male patient showed severe developmental delay, pendular nystagmus and laryngeal wheezing. The auditory brain stem response showed only the first wave and brain magnetic resonance imaging showed white matter hypomyelination, suggesting typical PMD. A novel PLP1 mutation, F240L, which was inherited from his mother, was identified.

Time-course of myelination and atrophy on cerebral imaging in 35 patients with PLP1-related disorders

AIM

Brain magnetic resonance imaging (MRI) motor development score (MDS) correlations were used to analyze the natural time-course of hypomyelinating PLP1-related disorders (Pelizaeus-Merzbacher disease [PMD] and spastic paraplegia type 2).

METHOD

Thirty-five male patients (ranging from 0.7-43.5y at the first MRI) with PLP1-related disorder were prospectively followed over 7 years. Patients were classified according to best motor function acquired before 5 years (MDS) into five categories (from PMD0 without motor acquisition to PMD4 with autonomous walking). We determined myelination and atrophy scores and measured corpus callosum area, volume of cerebellum, white matter and grey matter on 63 MRI.

RESULTS

Age-adjusted multivariate analysis revealed that patients with PMD0-1 had higher-severity atrophy scores and smaller corpus callosum area than did patients with PMD2 and PMD3-4. Myelination score increased until 12 years. There was evidence that the mean myelination differed in frontal white matter, arcuate fibres, and internal capsules among the groups. Most patients showed worsening atrophy (brain, cerebellum, corpus callosum), whereas grey matter and white matter proportions did not change.

INTERPRETATION

Brain atrophy and myelination of anterior cerebral regions appear to be pertinent biomarkers of motor development. The time-course of inter- and intra-individual cerebral white matter and grey matter atrophy suggests that both oligodendrocytes and neurons are involved in the physiopathology of PLP1-related disorders.

A splicing mutation of proteolipid protein 1 in Pelizaeus-Merzbacher disease

A patient with an unusually mild form of Pelizaeus-Merzbacher disease was studied. Clinically, mild developmental delay with acquisition of assisted walking at 16months and mild spastic tetraplegia were evident, but no nystagmus, cerebellar, or extra-pyramidal signs were present. PLP1 mutation analysis revealed a nucleotide substitution adjacent to the acceptor site of intron 3, NM_000533.4:c.454-9T>G. Expression analysis using the patient's leukocytes demonstrated an additional abnormal transcript including the last 118bp of intron 3. In silico prediction analysis suggested the reduction of wild-type acceptor activity, which presumably evokes the cryptic splicing variant. Putative cryptic transcript results in premature termination, which may explain the mild clinical phenotype observed in this patient.

TUBB4A-related hypomyelinating leukodystrophy: New insights from a series of 12 patients

BACKGROUND

Hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) was first described in 2002. After the recent identification of TUBB4A mutation as the genetic basis of the disease, the clinical and neuroimaging phenotype related to TUBB4A mutations expanded, ranging from primary dystonia type 4 with normal MRI to severe H-ABC cases.

PATIENTS AND METHODS

The study included patients referred to us for an unclassified hypomyelinating leukodystrophy. We selected patients with deleterious heterozygous TUBB4A mutations. Molecular analysis of TUBB4A was performed on genomic DNA extracted from peripheral blood.

RESULTS

The series included 12 patients (5 females and 7 males). Five patients carried the common mutation c.745G > A (p.Asp249Asn), while the remaining harbored different mutations. Three new mutations were found in 5 patients. Clinical and neuroimaging observations are described. A clear correlation between the clinical presentation and the genotype seems to be absent in our group of 12 patients.

CONCLUSIONS

TUBB4A-mutated patients manifest a comparable clinical and neuroimaging picture but they can differ from each other in terms of rate of disease progression. Extrapyramidal signs can be absent in the first stages of the disease, and a careful evaluation of MRI is fundamental to obtain the final diagnosis. From a therapeutic perspective a trial with l-dopa should be considered in all patients presenting extrapyramidal symptoms.

Identification and functional study of novel PLP1 mutations in Chinese patients with Pelizaeus-Merzbacher disease

PURPOSE

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked recessive hypomyelination disorder characterized by nystagmus, ataxia, impaired motor development, and progressive spasticity. Identification of proteolipid protein 1 (PLP1) mutations in Chinese patients with Pelizaeus-Merzbacher disease (PMD) and confirmation of the biological impacts of the identified mutations are the aims of this study.

METHODS

An analysis of clinical materials and a follow-up study were conducted for the patients with PMD. Sequencing and immunofluorescence were applied for molecular analysis of the causative gene PLP1.

RESULTS

We identified PLP1 mutations in seven male patients with PMD. Three novel missense mutations (c.353C>G, p.T118R; c.623G>T, p.G208V; c.709T>G, p.F237V) and three reported missense mutations (c.467C>T, p.T156I; c.517C>T, p.P173S; c.646C>T, p.P216S) of PLP1 were identified from seven Chinese PMD patients. The three mutations (F237V in patient 2, P216S in patient 5 and T156I in patient 6) were de novo. Mutant proteins were trapped in the lumen of endoplasmic reticulum.

CONCLUSION

We have identified six pathogenic mutations, enriching the specific spectrum of missense mutations in the patients with PMD. The six PLP1 mutations are probably pathogenic. By reviewing the known PLP1 mutations, we have preliminarily revealed the position of missense mutation may be associated with the severity of PMD.

Curcumin therapy in a Plp1 transgenic mouse model of Pelizaeus-Merzbacher disease

OBJECTIVE

Pelizaeus-Merzbacher disease (PMD) is a progressive and lethal leukodystrophy caused by mutations affecting the proteolipid protein (PLP1) gene. The most common cause of PMD is a duplication of PLP1 and at present there is no curative therapy available.

METHODS

By using transgenic mice carrying additional copies of Plp1, we investigated whether curcumin diet ameliorates PMD symptoms. The diet of Plp1 transgenic mice was supplemented with curcumin for 10 consecutive weeks followed by phenotypical, histological and immunohistochemical analyses of the central nervous system. Plp1 transgenic and wild-type mice fed with normal chow served as controls.

RESULTS

Curcumin improved the motor phenotype performance of Plp1 transgenic mice by 50% toward wild-type level and preserved myelinated axons by 35% when compared to Plp1 transgenic controls. Furthermore, curcumin reduced astrocytosis, microgliosis and lymphocyte infiltration in Plp1 transgenic mice. Curcumin diet did not affect the pathologically increased Plp1 mRNA abundance. However, high glutathione levels indicating an oxidative misbalance in the white matter of Plp1 transgenic mice were restored by curcumin treatment.

INTERPRETATION

Curcumin may potentially serve as an antioxidant therapy of PMD caused by PLP1 gene duplication.

Altered PLP1 splicing causes hypomyelination of early myelinating structures

Objective

The objective of this study was to investigate the genetic etiology of the X-linked disorder “Hypomyelination of Early Myelinating Structures” (HEMS).

Methods

We included 16 patients from 10 families diagnosed with HEMS by brain MRI criteria. Exome sequencing was used to search for causal mutations. In silico analysis of effects of the mutations on splicing and RNA folding was performed. In vitro gene splicing was examined in RNA from patients’ fibroblasts and an immortalized immature oligodendrocyte cell line after transfection with mutant minigene splicing constructs.

Results

All patients had unusual hemizygous mutations of PLP1 located in exon 3B (one deletion, one missense and two silent), which is spliced out in isoform DM20, or in intron 3 (five mutations). The deletion led to truncation of PLP1, but not DM20. Four mutations were predicted to affect PLP1/DM20 alternative splicing by creating exonic splicing silencer motifs or new splice donor sites or by affecting the local RNA structure of the PLP1 splice donor site. Four deep intronic mutations were predicted to destabilize a long-distance interaction structure in the secondary PLP1 RNA fragment involved in regulating PLP1/DM20 alternative splicing. Splicing studies in fibroblasts and transfected cells confirmed a decreased PLP1/DM20 ratio.

Interpretation

Brain structures that normally myelinate early are poorly myelinated in HEMS, while they are the best myelinated structures in Pelizaeus–Merzbacher disease, also caused by PLP1 alterations. Our data extend the phenotypic spectrum of PLP1-related disorders indicating that normal PLP1/DM20 alternative splicing is essential for early myelination and support the need to include intron 3 in diagnostic sequencing.

A novel PLP1 frameshift mutation causing a milder form of Pelizaeus-Merzbacher disease

BACKGROUND

Pelizaeus-Merzbacher disease (PMD), a hypomyelinating leukodystrophy, and the related but less severe allelic spastic paraplegia 2 (SPG2) are caused by mutations in the proteolipid protein 1 (PLP1) gene. Magnetic resonance imaging (MRI) is pivotal for diagnosing these disorders. The severity of PMD/SPG2 varies, and for a milder form of PMD, there have been some reports of near-normal findings in T1-weighted images but abnormal findings in T2-weighted images.

PATIENT

We report the case of a 5-year-old boy diagnosed with a milder form of PMD caused by a novel PLP1 mutation in exon 3: c.300delC (p.I100IfsX13). He had delayed development from several months of age and was able to walk with support at 19 months in spite of the spasticity in his lower extremities. Hypomyelination was noted at 12 months by brain MRI. Motor nerve conduction studies showed decreased velocities with reduced amplitudes. Follow-up MRI at 1-year intervals from 18 months until 55 months of age showed gradual myelination progress.

DISCUSSION

The single nucleotide deletion identified in this patient can cause a frameshift and premature termination of PLP1. Via the nonsense-mediated mRNA decay mechanism of this mutation will result in loss-of-function, leading to a milder form of PMD. The present case is compatible with previously reported cases of milder form of PMD. We incidentally identified progressive myelination in this patient by T1-weighted images obtained by serial MRI. This finding adds to our understanding of the pathological stages of a milder form of PMD.

Modeling the natural history of Pelizaeus-Merzbacher disease

Major gaps in our understanding of the leukodystrophies result from their rarity and the lack of tissue for the interdisciplinary studies required to extend our knowledge of the pathophysiology of the diseases. This study details the natural evolution of changes in the CNS of the shaking pup (shp), a model of the classical form of the X-linked disorder Pelizaeus-Merzbacher disease, in particular in glia, myelin, and axons, which is likely representative of what occurs over time in the human disease. The mutation in the proteolipid protein gene, PLP1, leads to a delay in differentiation, increased cell death, and a marked distension of the rough endoplasmic reticulum in oligodendrocytes. However, over time, more oligodendrocytes differentiate and survive in the spinal cord leading to an almost total recovery of myelination, In contrast, the brain remains persistently hypomyelinated. These data suggest that shp oligodendrocytes may be more functional than previously realized and that their early recruitment could have therapeutic value.

Brain magnetic resonance imaging findings and auditory brainstem response in a child with spastic paraplegia 2 due to a PLP1 splice site mutation

A boy with spastic paraplegia type 2 (SPG2) due to a novel splice site mutation of PLP1 presented with progressive spasticity of lower limbs, which was first observed during late infancy, when he gained the ability to walk with support. His speech was slow and he had dysarthria. The patient showed mildly delayed intellectual development. Subtotal dysmyelination in the central nervous system was revealed, which was especially prominent in structures known to be myelinated during earlier period, whereas structures that are myelinated later were better myelinated. These findings on the brain magnetic resonance imaging were unusual for subjects with PLP1 mutations. Peaks I and II of the auditory brainstem response (ABR) were normally provoked, but peaks III-V were not clearly demarcated, similarly to the findings in Pelizaeus-Merzbacher disease. These findings of brain MRI and ABR may be characteristic for a subtype of SPG2 patients.

Umbilical cord blood transplantation to treat Pelizaeus-Merzbacher Disease in 2 young boys

Pelizaeus-Merzbacher Disease (PMD) is a rare X-linked recessive leukodystrophy caused by mutations in the proteolipid protein 1 gene on the Xq22 chromosome. PMD is a dysmyelinating disorder characterized by variable clinical presentation and course. Symptoms range from mild motor deficits to progressive spasticity and neurologic decline resulting in death at an early age. There is no definitive curative treatment. This report presents the clinical course of 2 young boys with PMD who are the first known patients to receive umbilical cord blood transplantation as a therapeutic intervention to stabilize disease progression. Pretransplantation evaluation revealed that both patients had significant motor deficits as well as delayed cognitive function as compared with age-matched peers. Brain imaging revealed varying degrees of hypomyelination. Both patients received myeloablative chemotherapy followed by an unrelated donor umbilical cord blood infusion, which they tolerated well with no major transplantation-related complications. At 7-years and 1-year posttransplantation, respectively, both boys are making slow neurocognitive improvements and show no evidence of functional decline. Imaging results show stable or improving myelination. Although the results of unrelated donor umbilical cord blood transplantation in these 2 boys with PMD are encouraging, longer-term follow-up will be necessary to assess the effect of this treatment on the variable natural disease course.