Severe Hypomyelination Research Articles

A novel heterozygous TMEM63A variant in a familial case with early onset nystagmus, severe hypomyelination, and a favorable adult prognosis.

Heterozygous transmembrane protein 63A (TMEM63A) variants cause transient infantile hypomyelinating leukodystrophy-19, which features remarkable natural resolution of clinical and imaging findings during childhood. Previous reports have mainly described de novo variants lacking detailed familial cases. Herein, we describe the clinical course of familial cases with a TMEM63A variant. A 5-month-old girl presented with nystagmus, global hypotonia, and difficulty swallowing since birth. Brain magnetic resonance imaging at 1.5 and 5 months revealed diffuse hypomyelination. Her mother, maternal aunt, and grandfather had nystagmus and motor developmental delays in infancy, which resolved spontaneously during childhood. Compared with these cases, the proband's motor developmental delay was profound, and she was the only one with feeding difficulties, necessitating nasogastric tube feeding. Genetic testing revealed a heterozygous TMEM63A variant (NM_014698.3:c.1658G>A, p.(Gly553Asp)) in the proband and her family. This is the first three-generation familial report of a TMEM63A variant that provides insight into its history and heterogeneity.

PAK2 is necessary for myelination in the peripheral nervous system.

Myelination enables electrical impulses to propagate on axons at the highest speed, encoding essential life functions. The Rho family GTPases, RAC1 and CDC42, have been shown to critically regulate Schwann cell myelination. P21-activated kinase 2 (PAK2) is an effector of RAC1/CDC42, but its specific role in myelination remains undetermined. We produced a Schwann cell-specific knockout mouse of Pak2 (scPak2-/-) to evaluate PAK2's role in myelination. Deletion of Pak2, specifically in mouse Schwann cells, resulted in severe hypomyelination, slowed nerve conduction velocity and behaviour dysfunctions in the scPak2-/- peripheral nerve. Many Schwann cells in scPak2-/- sciatic nerves were arrested at the stage of axonal sorting. These abnormalities were rescued by reintroducing Pak2, but not the kinase-dead mutation of Pak2, via lentivirus delivery to scPak2-/- Schwann cells in vivo. Moreover, ablation of Pak2 in Schwann cells blocked the promyelinating effect driven by neuregulin-1, prion protein and inactivated RAC1/CDC42. Conversely, the ablation of Pak2 in neurons exhibited no phenotype. Such PAK2 activity can also be either enhanced or inhibited by different myelin lipids. We have identified a novel promyelinating factor, PAK2, that acts as a critical convergence point for multiple promyelinating signalling pathways. The promyelination by PAK2 is Schwann cell-autonomous. Myelin lipids, identified as inhibitors or activators of PAK2, may be utilized to develop therapies for repairing abnormal myelin in peripheral neuropathies.

TMEM63 mechanosensitive ion channels: Activation mechanisms, biological functions and human genetic disorders

The transmembrane 63 (TMEM63) family of proteins are originally identified as homologs of the osmosensitive calcium-permeable (OSCA) channels in plants. Mechanosensitivity of OSCA and TMEM63 proteins are recently demonstrated in addition to their proposed activation mechanism by hyper/hypo-osmolarity. TMEM63 proteins exist in all animals, with a single member in Drosophila (TMEM63) and three members in mammals (TMEM63 A/B/C). In humans, monoallelic variants of TMEM63A have been reported to cause transient hypomyelination during infancy, or severe hypomyelination and global developmental delay. Heterozygous variants of TMEM63B are found in patients with intellectual disability and abnormal motor function and brain morphology. Biallelic variants of TMEM63C are associated with hereditary spastic paraplegias accompanied by mild or no intellectual disability. Physiological functions of TMEM63 proteins clearly recognized so far include detecting food grittiness and environmental humidity in Drosophila, and supporting hearing in mice by regulating survival of cochlear hair cells. In this review, we summarize current knowledge about the activation mechanisms and biological functions of TMEM63 channels, and provide a concise reference for researchers interested in investigating more physiological and pathogenic roles of this family of proteins with ubiquitous expression in the body.

Further Characterization of the Neuroendocrine Phenotype Associated With the PPOX-Related Variegate Porphyria

BackgroundVariegate porphyria is caused by mutations in the PPOX gene; it usually presents in adolescents and adults as an autosomal dominant condition, with cutaneous features or acute peripheral and/or central nervous system crises. A rarer variant, homozygous variegate porphyria, presents in childhood with cutaneous manifestations as well as neurophenotypes. This study sought to further characterize the homozygous PPOX-related neuroendocrine phenotype. MethodsThis study is a retrospective review of the patients’ charts, including their clinical evaluation and molecular genetics, neurodiagnostic, and neuroradiological investigations. ResultsWe describe here three children from a consanguineous family who presented with nystagmus, developmental delay and ataxia, photosensitive skin manifestations, and adrenal insufficiency. Analysis of porphyrins in plasma, urine, and stool together with a genetic study of the PPOX gene confirmed the diagnosis. Interestingly, brain MRI showed severe hypomyelination, a finding rarely reported in variegate porphyria, together with adrenal insufficiency. ConclusionWe recommend analysis of porphyrins in unexplained hypomyelination disorders. Patients with variegate porphyria should be tested for adrenal insufficiency.

Hypomyelination, hypodontia and craniofacial abnormalities in a Polr3b mouse model of leukodystrophy.

RNA polymerase III (Pol III)-related hypomyelinating leukodystrophy (POLR3-HLD), also known as 4H leukodystrophy, is a severe neurodegenerative disease characterized by the cardinal features of hypomyelination, hypodontia and hypogonadotropic hypogonadism. POLR3-HLD is caused by biallelic pathogenic variants in genes encoding Pol III subunits. While approximately half of all patients carry mutations in POLR3B encoding the RNA polymerase III subunit B, there is no in vivo model of leukodystrophy based on mutation of this Pol III subunit. Here, we determined the impact of POLR3BΔ10 (Δ10) on Pol III in human cells and developed and characterized an inducible/conditional mouse model of leukodystrophy using the orthologous Δ10 mutation in mice. The molecular mechanism of Pol III dysfunction was determined in human cells by affinity purification-mass spectrometry and western blot. Postnatal induction with tamoxifen induced expression of the orthologous Δ10 hypomorph in triple transgenic Pdgfrα-Cre/ERT; R26-Stopfl-EYFP; Polr3bfl mice. CNS and non-CNS features were characterized using a variety of techniques including microCT, ex vivo MRI, immunofluorescence, immunohistochemistry, spectral confocal reflectance microscopy and western blot. Lineage tracing and time series analysis of oligodendrocyte subpopulation dynamics based on co-labelling with lineage-specific and/or proliferation markers were performed. Proteomics suggested that Δ10 causes a Pol III assembly defect, while western blots demonstrated reduced POLR3BΔ10 expression in the cytoplasm and nucleus in human cells. In mice, postnatal Pdgfrα-dependent expression of the orthologous murine mutant protein resulted in recessive phenotypes including severe hypomyelination leading to ataxia, tremor, seizures and limited survival, as well as hypodontia and craniofacial abnormalities. Hypomyelination was confirmed and characterized using classic methods to quantify myelin components such as myelin basic protein and lipids, results which agreed with those produced using modern methods to quantify myelin based on the physical properties of myelin membranes. Lineage tracing uncovered the underlying mechanism for the hypomyelinating phenotype: defective oligodendrocyte precursor proliferation and differentiation resulted in a failure to produce an adequate number of mature oligodendrocytes during postnatal myelinogenesis. In summary, we characterized the Polr3bΔ10 mutation and developed an animal model that recapitulates features of POLR3-HLD caused by POLR3B mutations, shedding light on disease pathogenesis, and opening the door to the development of therapeutic interventions.

A Novel Constitutively Active c . 98 G > C , p.(R33P) Variant in RAB11A Associated with Intellectual Disability Promotes Neuritogenesis and Affects Oligodendroglial Arborization

Whole exome sequencing/whole genome sequencing has accelerated the identification of novel genes associated with intellectual disabilities (ID), and RAB11A which encodes an endosomal small GTPase is among them. However, consequent neural abnormalities have not been studied, and pathophysiological mechanisms underlying the ID and other clinical features in patients harboring RAB11A variants remain to be clarified. In this study, we report a novel de novo missense variant in RAB11A, NM_004663.5: c . 98 G > C , which would result in NP_004654.1: p.(R33P) substitution, in a Japanese boy with severe ID and hypomyelination. Biochemical analyses indicated that the RAB11A-R33P is a gain-of-function, constitutively active variant. Accordingly, the introduction of the RAB11A-R33P promoted neurite extension in neurons like a known constitutively active variant Rab11A-Q70L. In addition, the RAB11A-R33P induced excessive branching with thinner processes in oligodendrocytes. These results indicate that the gain-of-function RAB11A-R33P variant in association with ID and hypomyelination affects neural cells and can be deleterious to them, especially to oligodendrocytes, and strongly suggest the pathogenic role of the RAB11A-R33P variant in neurodevelopmental impairments, especially in the hypomyelination.

DST variants are responsible for neurogenic arthrogryposis multiplex congenita enlarging the spectrum of type VI hereditary sensory autonomic neuropathy.

Arthrogryposis multiplex congenita (AMC) is a developmental condition characterized by multiple joint contractures resulting from reduced or absent fetal movements. Through whole-exome sequencing combined with arrayCGH from DNA of a fetus presenting with early onset AMC, we identified biallelic loss of function variants in Dystonin (DST): a stop gain variant (NM_001144769.5:c.12208G > T:p.(Glu4070Ter)) on the neuronal isoform and a 175 kb microdeletion including exons 25-96 of this isoform on the other allele [NC_000006.11:g.(56212278_56323554)_(56499398_56507586)del]. Transmission electron microscopy of the sciatic nerve revealed abnormal morphology of the peripheral nerve with severe hypomyelination associated with dramatic reduction of fiber density which highlights the critical role of DST in peripheral nerve axonogenesis during development in human. Variants in the neuronal isoforms of DST cause hereditary sensory and autonomic neuropathy which has been reported in several unrelated families with highly variable age of onset from fetal to adult onset. Our data enlarge the disease mechanisms of neurogenic AMC.

Pdgfrα-dependent Polr3b Exon Loss Recapitulates POLR3-related Hypomyelinating Leukodystrophy Phenotypes in vivo (S2.004)

<h3>Objective:</h3> To develop the first model of RNA Polymerase III (Pol III)-Related Hypomyelinating Leukodystrophy (POLR3-HLD) based on a <i>POLR3B</i> mutation. <h3>Background:</h3> POLR3-HLD is a devastating neurological disease characterized by severe diffuse hypomyelination and progressive functional decline leading to early death. It is caused by biallelic pathogenic variants in genes encoding Pol III subunits. Patients are most commonly affected by mutations in <i>POLR3A</i> or <i>POLR3B</i>, but there are currently no disease models based on <i>POLR3B</i>. Furthermore, existing models of the disease based on <i>POLR3A</i> mutation fail to replicate the hypomyelination severity commonly observed in patients. We hypothesize that mutations in Pol III subunits such as <i>POLR3B</i> reduce enzyme function during a critical developmental period causing defective myelinogenesis and hypomyelination. <h3>Design/Methods:</h3> We characterized the impact of a <i>POLR3B</i> mutant lacking exon 10 (<i>POLR3BΔ10</i>) on Pol III complex assembly, nuclear import, and protein expression in human cells. We developed an inducible/conditional animal model using the cre/lox system to express the orthologous mutation in a <i>Pdgfrα-</i>dependent manner during postnatal development in mice. The animal model was characterized using a variety of techniques including tissue biochemistry, histology, and advanced imaging (microCT, <i>ex vivo</i> MRI). <h3>Results:</h3> <i>POLR3BΔ10</i> expression was shown to cause a severe Pol III assembly defect accompanied by reduced expression and nuclear import of the mutant protein in human cells. Inducing <i>Pdgfrα-</i>dependent expression of orthologous <i>Polr3bΔ10</i> during postnatal development in mice causes severe hypomyelination, craniofacial defects, and hypodontia. The hypomyelination phenotype was caused by proliferation and maturation defects in oligodendrocyte-lineage cells carrying homozygous <i>Polr3bΔ10</i>, which preceded hypomyelination and led to non-apoptotic cell loss from the brain parenchyma. <h3>Conclusions:</h3> We describe the first severe model of POLR3-HLD and the first working disease model based on mutation of <i>Polr3b</i>. This work advances our understanding of POLR3-HLD and implicates defective proliferation and differentiation of oligodendrocyte-lineage cells as key features of POLR3-HLD pathogenesis. <b>Disclosure:</b> Mackenzie A. Michell-Robinson has nothing to disclose. Dr. Watt has received research support from National Institute of Dental and Craniofacial Research. Mr. Grouza has nothing to disclose. Ms. Macintosh has nothing to disclose. Dr. Pinard has nothing to disclose. Mr. Tuznik has nothing to disclose. Dr. Chen has nothing to disclose. Lama Darbelli has nothing to disclose. Chia-Lun Wu has nothing to disclose. Dr. Perrier has received research support from the Fonds de Recherche du Quebec-Santé (FRQS) Doctoral Scholarship, the Fondation du Grand défi Pierre Lavoie Doctoral Scholarship, the McGill Faculty of Medicine F.S.B. Miller Fellowship, and the Research Institute of the McGill University Health Centre Desjardins Studentship in Child Health Research. Mr. Chitsaz has received research support from Multiple Sclerosis Society of Canada, Fonds de Recherche du Quebec. Dr. Uccelli has received research support from Hoffmann-La Roche Limited. Dr. Liu has nothing to disclose. The institution of Prof. Cox has received research support from NIH. Dr. Mueller has nothing to disclose. Timothy Kennedy has received research support from Canadian Institutes of Health Research. Benoit Coulombe has nothing to disclose. David Rudko has nothing to disclose. Dr. Trainor has received personal compensation for serving as an employee of Stowers Institute for Medical Researh. Dr. Trainor has received personal compensation in the range of $10,000-$49,999 for serving as an Editor, Associate Editor, or Editorial Advisory Board Member for Developmental Dynamics. The institution of Dr. Trainor has received research support from Stowers Institute for Medical Research. Geneviève Bernard has nothing to disclose.

MTORC2 Loss in Oligodendrocyte Progenitor Cells Results in Regional Hypomyelination in the Central Nervous System.

In the CNS, oligodendrocyte progenitor cells (OPCs) differentiate into mature oligodendrocytes to generate myelin, an essential component for normal nervous system function. OPC differentiation is driven by signaling pathways, such as mTOR, which functions in two distinct complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), containing Raptor or Rictor, respectively. In the current studies, mTORC2 signaling was selectively deleted from OPCs in PDGFRα-Cre X Rictorfl/fl mice. This study examined developmental myelination in male and female mice, comparing the impact of mTORC2 deletion in the corpus callosum and spinal cord. In both regions, Rictor loss in OPCs resulted in early reduction in myelin RNAs and proteins. However, these deficits rapidly recovered in spinal cord, where normal myelin was noted at P21 and P45. By contrast, the losses in corpus callosum resulted in severe hypomyelination and increased unmyelinated axons. The hypomyelination may result from decreased oligodendrocytes in the corpus callosum, which persisted in animals as old as postnatal day 350. The current studies focus on uniquely altered signaling pathways following mTORC2 loss in developing oligodendrocytes. A major mTORC2 substrate is phospho-Akt-S473, which was significantly reduced throughout development in both corpus callosum and spinal cord at all ages measured, yet this had little impact in spinal cord. Loss of mTORC2 signaling resulted in decreased expression of actin regulators, such as gelsolin in corpus callosum, but only minimal loss in spinal cord. The current study establishes a regionally specific role for mTORC2 signaling in OPCs, particularly in the corpus callosum.SIGNIFICANCE STATEMENT mTORC1 and mTORC2 signaling has differential impact on myelination in the CNS. Numerous studies identify a role for mTORC1, but deletion of Rictor (mTORC2 signaling) in late-stage oligodendrocytes had little impact on myelination in the CNS. However, the current studies establish that deletion of mTORC2 signaling from oligodendrocyte progenitor cells results in reduced myelination of brain axons. These studies also establish a regional impact of mTORC2, with little change in spinal cord in these conditional Rictor deletion mice. Importantly, in both brain and spinal cord, mTORC2 downstream signaling targets were impacted by Rictor deletion. Yet, these signaling changes had little impact on myelination in spinal cord, while they resulted in long-term alterations in myelination in brain.

CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

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.

Chronic stress disrupts the homeostasis and progeny progression of oligodendroglial lineage cells, associating immune oligodendrocytes with prefrontal cortex hypomyelination.

Major Depressive Disorder (MDD) is a chronic debilitating illness affecting yearly 300 million people worldwide. Oligodendrocyte-lineage cells have emerged as important neuromodulators in synaptic plasticity and crucial components of MDD pathophysiology. Using the repeated social defeat (RSDS) mouse model, we demonstrate that chronic psychosocial stress induces long-lasting losses and transient proliferation of oligodendrocyte-precursor cells (OPCs), aberrant differentiation into oligodendrocytes, and severe hypomyelination in the prefrontal cortex. Exposure to chronic stress results in OPC morphological impairments, excessive oxidative stress, and oligodendroglial apoptosis, implicating integrative-stress responses in depression. Analysis of single-nucleus transcriptomic data from MDD patients revealed oligodendroglial-lineage dysregulation and the presence of immune-oligodendrocytes (Im-OL), a novel population of cells with immune properties and myelination deficits. Im-OL were also identified in mice after RSDS, where oligodendrocyte-lineage cells expressed immune-related markers. Our findings demonstrate cellular and molecular changes in the oligodendroglial lineage in response to chronic stress and associate hypomyelination with Im-OL emergence during depression.

Growth and neurodevelopmental disorder with arthrogryposis, microcephaly and structural brain anomalies caused by Bi-allelic partial deletion of SMPD4 gene.

Neutral sphingomyelinases have an important role in generation of ceramide and phosphorylcholine from sphingomyelins which then act as secondary messengers in various signaling pathways of the cellular machinery. They function ubiquitously with a predominant role in the central nervous system. Neutral sphingomyelinase type 3, encoded by SMPD4 gene has recently been reported to cause a severe autosomal recessive neurodevelopmental disorder with congenital arthrogryposis and microcephaly. We report a 22-month-old girl having characteristic features of neurodevelopmental delay, prenatal onset growth failure, arthrogryposis, microcephaly and brain anomalies including severe hypomyelination, simplified gyral pattern and hypoplasia of corpus callosum and brainstem. Additionally, she was noted to have nystagmus and visual impairment secondary to macular dystrophy and retinal pigment epithelial stippling at posterior pole. Copy number variant analysis from trio whole exome sequencing (ES) enabled identification of a homozygous 11kb deletion encompassing exons 18 −20 of SMPD 4 gene, confirming the diagnosis of SMPD4-related disorder in her.

Characterizing the molecular etiology of arthrogryposis multiplex congenita in patients with LGI4 mutations.

Disruption of axon-glia interactions in the peripheral nervous system has emerged as a major cause of arthrogryposis multiplex congenita (AMC), a condition characterized by multiple congenital postural abnormalities involving the major joints. Several genes crucially important to the biology of Schwann cells have now been implicated with AMC. One such gene is LGI4 which encodes a secreted glycoprotein. LGI4 is expressed and secreted by Schwann cells and binds its receptor ADAM22 on the axonal membrane to drive myelination. Homozygous mutations in LGI4 or ADAM22 results in severe congenital hypomyelination and joint contractures in mice. Recently bi-allelic LGI4 loss of function mutations has been described in three unrelated families with severe AMC. Two individuals in a fourth, non-consanguineous family were found to be compound heterozygous for two LGI4 missense mutations. It is not known how these missense mutations affect the biology of LGI4. Here we investigated whether these missense mutations affected the secretion of the protein, its ADAM22 binding capacity, or its myelination-promoting function. We demonstrate that the mutations largely affect the progression of the mutant protein through the endomembrane system resulting in severely reduced expression. Importantly, binding to ADAM22 and myelination-promoting activity appear largely unaffected, suggesting that treatment with chemical chaperones to improve secretion of the mutant proteins might prove beneficial.

Migrating Focal Seizures and Myoclonic Status in ARV1-Related Encephalopathy.

ObjectiveTo report longitudinal clinical, EEG, and MRI findings in 2 sisters carrying compound heterozygous ARV1 mutations and exhibiting a peculiar form of developmental and epileptic encephalopathy (DEE). Neuropathologic features are also described in one of the sisters.MethodsClinical course description, video-EEG polygraphic recordings, brain MRI, skin and muscle biopsies, whole-exome sequencing (WES), and brain neuropathology.ResultsSince their first months of life, both girls exhibited severe axial hypotonia, visual inattention, dyskinetic movements, severe developmental delay, and slow background EEG activity. Intractable nonmotor seizures started in both at the eighth month of life, exhibiting the electroclinical characteristics of epilepsy of infancy with migrating focal seizures (EIMFS). In the second year of life, continuous epileptiform EEG activity of extremely high amplitude appeared in association with myoclonic status, leading to severely impaired alertness and responsiveness. Repeated brain MRI revealed progressive atrophic changes and severe hypomyelination. WES identified a compound heterozygous in the ARV1 gene [(p.Ser122Glnfs*7) and (p.Trp163*)] in one patient and was subsequently confirmed in the other. Both sisters died prematurely during respiratory infections. Postmortem neuropathologic examination of the brain, performed in one, revealed atrophic brain changes, mainly involving the cerebellum.ConclusionsThis report confirms that biallelic ARV1 mutations cause a severe form of DEE and adds epilepsy with migrating focal seizures and myoclonic status to the spectrum of epilepsy phenotypes. Considering the potential role of human ARV1 in glycosylphosphatidylinositol (GPI) anchor biosynthesis, this severe syndrome can be assigned to the group of inherited GPI deficiency disorders, with which it shares remarkably similar clinical and neuroimaging features. ARV1 should be considered in the genetic screening of individuals with EIMFS.

Hypomyelinating leukodystrophies - unravelling myelin biology.

Hypomyelinating leukodystrophies constitute a subset of genetic white matter disorders characterized by a primary lack of myelin deposition. Most patients with severe hypomyelination present in infancy or early childhood and develop severe neurological deficits, but the clinical presentation can also be mild with onset of symptoms in adolescence or adulthood. MRI can be used to visualize the process of myelination in detail, and MRI pattern recognition can provide a clinical diagnosis in many patients. Next-generation sequencing provides a definitive diagnosis in 80-90% of patients. Genes associated with hypomyelination include those that encode structural myelin proteins but also many that encode proteins involved in RNA translation and some lysosomal proteins. The precise pathomechanisms remain to be elucidated. Improved understanding of the process of myelination, the metabolic axonal support functions of myelin and the proposed contribution of myelin to CNS plasticity provide possible explanations as to why almost all patients with hypomyelination experience slow clinical decline after a long phase of stability. In this Review, we provide an overview of the hypomyelinating leukodystrophies, the advances in our understanding of myelin biology and of the genes involved in these disorders, and the insights these advances have provided into their clinical presentations and evolution.

Foetal onset of EIF2B related disorder in two siblings: cerebellar hypoplasia with absent Bergmann glia and severe hypomyelination

Bi-allelic pathogenic variants in genes of the EIF2B family are responsible for Childhood Ataxia with Central nervous system Hypomyelination/Vanishing White Matter disease, a progressive neurodegenerative disorder of the central white matter. Only seven molecularly proven cases with antenatal onset have been reported so far. We report for the first time the neuropathological findings obtained from two foetuses harbouring deleterious variants in the EIF2B5 gene who presented in utero growth retardation and microcephaly with simplified gyral pattern that led to a medical termination of the pregnancy at 27 and 32 weeks of gestation. Neuropathological examination confirmed microcephaly with delayed gyration, periventricular pseudo-cysts and severe cerebellar hypoplasia. Histologically, the cerebellar cortex was immature, the dentate nuclei were fragmented and myelin stains revealed almost no myelination of the infratentorial structures. Bergmann glia was virtually absent associated to a drastic decreased number of mature astrocytes in the cerebellar white matter, multiple nestin-positive immature astrocytes as well as increased numbers of PDGRFα-positive oligodendrocyte precursors. Whole exome sequencing performed in the two foetuses and their parents allowed the identification of two EIF2B5 compound heterozygous variants in the two foetuses: c.468C > G p.Ile156Met and c.1165G > A p.Val389Met, the parents being heterozygous carriers. These variants are absent in the genome Aggregation Database (gnomAD r2.0.2). Contrary to the variant Ile156Met already described in a patient with CACH syndrome, the variant p.Val389Met is novel and predicted to be deleterious using several softwares. Neuropathological findings further expand the phenotypic spectrum of the disease that very likely occurs during early gestation and may manifest from the second half of pregnancy by a severe impairment of cerebral and cerebellar development.

Oligodendrogenesis and Myelin Formation in the Forebrain Require Platelet-derived Growth Factor Receptor-alpha

The platelet-derived growth factor receptor-α (PDGFRα) principally mediates growth factor signals in oligodendroglial progenitors and is involved in oligodendrogenesis and myelinogenesis in the developing spinal cord. However, the role of PDGFRα in the developing forebrain remains relatively unknown. We established a conditional knockout mouse for the Pdgfra gene (N-PRα-KO) using a Nestin promoter/enhancer-driven Cre recombinase and examined forebrain development. The expression of PDGFRα was efficiently suppressed in the Olig2+ cells in N-PRα-KO mice. In these mice, Olig2+ cells were slightly decreased during embryonic periods. The decrease was particularly striking during the postnatal period. The commitment of Pdgfra-inactivated Olig2+ cells to Sox10+ oligodendroglial-lineage was largely suppressed. Surviving Olig2+ cells and Sox10+ cells were distributed widely in the N-PRα-KO mouse brain, similarly to those in control mice until the early neonatal period. After that, these cells were drastically depleted in the forebrain during the second postnatal week. The brains of N-PRα-KO mice were severely hypomyelinated, and these mice died on approximately P17 with motor disturbances. Disturbed axonal fibers and extensively aberrant vascular formations appeared in the postnatal N-PRα-KO mouse brains. After the defective PDGFRα signal in the forebrain, these phenotypes were clearly different from those in the spinal cord that showed defective populations expansion and migration of oligodendroglial lineage and premature myelination, as previously described. In contrast, areas of severe hypomyelination were common to both anatomical sites. PDGFRα was critically involved in the myelination of the forebrain and may differently regulate oligodendroglial lineage between the forebrain and spinal cord.

Wwox deficiency leads to neurodevelopmental and degenerative neuropathies and glycogen synthase kinase 3\u03b2-mediated epileptic seizure activity in mice

Human WWOX gene resides in the chromosomal common fragile site FRA16D and encodes a tumor suppressor WW domain-containing oxidoreductase. Loss-of-function mutations in both alleles of WWOX gene lead to autosomal recessive abnormalities in pediatric patients from consanguineous families, including microcephaly, cerebellar ataxia with epilepsy, mental retardation, retinal degeneration, developmental delay and early death. Here, we report that targeted disruption of Wwox gene in mice causes neurodevelopmental disorders, encompassing abnormal neuronal differentiation and migration in the brain. Cerebral malformations, such as microcephaly and incomplete separation of the hemispheres by a partial interhemispheric fissure, neuronal disorganization and heterotopia, and defective cerebellar midline fusion are observed in Wwox−/− mice. Degenerative alterations including severe hypomyelination in the central nervous system, optic nerve atrophy, Purkinje cell loss and granular cell apoptosis in the cerebellum, and peripheral nerve demyelination due to Schwann cell apoptosis correspond to reduced amplitudes and a latency prolongation of transcranial motor evoked potentials, motor deficits and gait ataxia in Wwox−/− mice. Wwox gene ablation leads to the occurrence of spontaneous epilepsy and increased susceptibility to pilocarpine- and pentylenetetrazol (PTZ)-induced seizures in preweaning mice. We determined that a significantly increased activation of glycogen synthase kinase 3β (GSK3β) occurs in Wwox−/− mouse cerebral cortex, hippocampus and cerebellum. Inhibition of GSK3β by lithium ion significantly abolishes the onset of PTZ-induced seizure in Wwox−/− mice. Together, our findings reveal that the neurodevelopmental and neurodegenerative deficits in Wwox knockout mice strikingly recapitulate the key features of human neuropathies, and that targeting GSK3β with lithium ion ameliorates epilepsy.

Myelin Deficits Caused by Olig2 Deficiency Lead to Cognitive Dysfunction and Increase Vulnerability to Social Withdrawal in Adult Mice.

Oligodendrocyte (OL) and myelin development are crucial for network integration and are associated with higher brain functions. Accumulating evidence has demonstrated structural and functional impairment of OLs and myelin in serious mental illnesses. However, whether these deficits contribute to the brain dysfunction or pathogenesis of such diseases still lacks direct evidence. In this study, we conditionally deleted Olig2 in oligodendroglial lineage cells (Olig2 cKO) and screened the behavioral changes in adult mice. We found that Olig2 ablation impaired myelin development, which further resulted in severe hypomyelination in the anterior cingulate cortex. Strikingly, Olig2 cKO mice exhibited an anxious phenotype, aberrant responses to stress, and cognitive deficits. Moreover, Olig2 cKO mice showed increased vulnerability to social avoidance under the mild stress of social isolation. Together, these results indicate that developmental deficits in OL and myelin lead to cognitive impairment and increase the risk of phenotypes reminiscent of mental illnesses.