Pelizaeus-Merzbacher disease: on the cusp of myelin medicine.

Pelizaeus-Merzbacher disease (PMD) is caused by mutations in the proteolipid protein 1 (PLP1) gene encoding proteolipid protein (PLP). As a major component of myelin, mutated PLP causes progressive neurodegeneration and eventually death due to severe white matter deficits. Medical care has long been limited to symptomatic treatments, but first-in-class PMD therapies with novel mechanisms now stand poised to enter clinical trials. Here, we review PMD disease mechanisms and outline rationale for therapeutic interventions, including PLP1 suppression, cell transplantation, iron chelation, and intracellular stress modulation. We discuss available preclinical data and their implications on clinical development. With several novel treatments on the horizon, PMD is on the precipice of a new era in the diagnosis and treatment of patients suffering from this debilitating disease.

Generation of Pelizaeus-Merzbacher disease (PMD) mutant (PLP1-C33Y) in induced pluripotent stem cell (iPSC) by CRISPR/Cas9 genome editing.

Genetic alterations in the PLP1 gene, i.e. point mutations and duplications, are associated with demyelinating disease Pelizaeus-Merzbacher. Here, we describe the generation of a human iPSC line harboring a PLP1 variant in codon 33 which leads to an amino acid change from cysteine to tyrosine. The established PLP1C33Y iPSC line enables the study of PMD pathophysiology by investigating various cell types and -characteristics in our developed protocol for bioengineered neuronal organoids (BENOs)1.

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.

Pelizaeus-Merzbacher Disease: A Caregiver Assessment of Disease Impact.

Pelizaeus-Merzbacher disease is a rare X-linked leukodystrophy accompanied by central nervous system hypomyelination with a spectrum of clinical phenotypes. This is the first survey of caregivers of individuals with Pelizaeus-Merzbacher disease to investigate the presenting symptoms, path to diagnosis, identity and impact of most bothersome symptoms, and needs that future treatment should address. One hundred participants completed the survey. Results from this survey demonstrate that the majority of Pelizaeus-Merzbacher disease symptoms manifest before 2 years of age and commonly include deficits in gross and fine motor skills, speech, and communication. Caregivers rated difficulty crawling, standing, or walking as the most bothersome symptoms due to Pelizaeus-Merzbacher disease, with constipation and difficulty with sleep, manual dexterity, and speech and communication rated nearly as high. The most important treatment goals for caregivers were improved mobility and communication. The survey findings present a caregiver perspective of the impact of symptoms in Pelizaeus-Merzbacher disease and provide helpful guidance to affected families, physicians, and drug developers on the often-long path to diagnosis and the unmet medical needs of this patient population.

Clinical application of long-read nanopore sequencing in a preimplantation genetic testing pre-clinical workup to identify the junction for complex Xq chromosome rearrangement-related disease.

OBJECTIVE: Xq chromosome duplication with complex rearrangements is generally acknowledged to be associated with neurodevelopmental disorders, such as Pelizaeus-Merzbacher disease (PMD) and MECP2 duplication syndrome. For couples who required a PGT-M (pre-implantation genetic testing for monogenic disease) for these disorders, junction-specific PCR is useful to directly detect pathogenic variants. Therefore, pre-clinical workup for PGT-M requires the identification of the junction of duplicated segments in PMD and MECP2 duplication syndrome, which is generally difficult. METHODS: In this report, we used nanopore long-read sequencing targeting the X chromosome using an adaptive sampling method to identify breakpoint junctions in disease-causing triplications. RESULTS: By long-read sequencing, we successfully identified breakpoint junctions in one PMD case with PLP1 triplication and in another MECP2 triplication case in a single sequencing run. Surprisingly, the duplicated region involving MECP2 was inserted 45 Mb proximal to the original position. This inserted region was confirmed by FISH analysis. With the help of precise mapping of the pathogenic variant, we successfully re-established STR haplotyping for PGT-M and avoided any potential misinterpretation of the pathogenic allele due to recombination. CONCLUSION: Long-read sequencing with adaptive sampling in a PGT-M pre-clinical workup is a beneficial method for identifying junctions of chromosomal complex structural rearrangements.

Identification of GJC2 gene mutations in Chinese patients with Pelizaeus-Merzbacher-like disease.

BACKGROUND: Clinical and genetic features were analyzed in five pedigrees with Pelizaeus-Merzbacher-like disease (PMLD) to provide bases for genetic counseling and prenatal diagnosis. CONCLUSIONS: Six patients from five pedigrees were diagnosed with PMLD based on their clinical data. Six GJC2 novel mutations were found in this study, expanding the spectrum of GJC2 mutations. This is the second group of GJC2 mutations reported from six Chinese patients with PMLD. METHODS: Clinical data including medical history, physical signs, and auxiliary examinations were collected from six patients and their family numbers in five pedigrees with PMLD. Polymerase chain reaction and sequence analysis were used to amplify GJC2 and PLP1 alterations, while multiplex ligation-dependent probe amplification (MLPA) was performed to detect PLP1 dosage changes. The gene mutations were diagnosed for further analysis of the genetic features. RESULTS: A total of seven GJC2 mutations were identified in these patients, including two novel missense mutations (c.217C>T, p.Pro73Ser; c.1199C>A, p.Ala400Glu), one nonsense mutation (c.735C>A, p.Cys245X), three novel frameshift mutations (c.579delC, p.Gly193fsX17 and c.1296_1297insG, p.Gly433fsX59; c.689delG, p.Gly230AlafsX241), and one known missense mutation (c.814T>G, p.Tyr272Asp). Compound heterozygotes were found for P1-3, while homozygotes were found for P4-6 that were inherited from their parents with normal phenotypes except for P5 and P6, respectively. The c.814T>G (p.Tyr272Asp) mutation in P5 was de novo. A c.1199C>A (p.Ala400Glu) homozygous mutation in GJC2 was identified in P6. A heterozygous variation was found in his father and the wild type was seen in his mother.

Variants in the zinc transporter TMEM163 cause a hypomyelinating leukodystrophy.

Hypomyelinating leukodystrophies comprise a subclass of genetic disorders with deficient myelination of the CNS white matter. Here we report four unrelated families with a hypomyelinating leukodystrophy phenotype harbouring variants in TMEM163 (NM_030923.5). The initial clinical presentation resembled Pelizaeus-Merzbacher disease with congenital nystagmus, hypotonia, delayed global development and neuroimaging findings suggestive of significant and diffuse hypomyelination. Genomic testing identified three distinct heterozygous missense variants in TMEM163 with two unrelated individuals sharing the same de novo variant. TMEM163 is highly expressed in the CNS particularly in newly myelinating oligodendrocytes and was recently revealed to function as a zinc efflux transporter. All the variants identified lie in highly conserved residues in the cytoplasmic domain of the protein, and functional in vitro analysis of the mutant protein demonstrated significant impairment in the ability to efflux zinc out of the cell. Expression of the mutant proteins in an oligodendroglial cell line resulted in substantially reduced mRNA expression of key myelin genes, reduced branching and increased cell death. Our findings indicate that variants in TMEM163 cause a hypomyelinating leukodystrophy and uncover a novel role for zinc homeostasis in oligodendrocyte development and myelin formation.

Generation of functional human oligodendrocytes from dermal fibroblasts by direct lineage conversion.

Oligodendrocytes, the myelinating cells of the central nervous system, possess great potential for disease modeling and cell transplantation-based therapies for leukodystrophies. However, caveats to oligodendrocyte differentiation protocols ( Ehrlich et al., 2017; Wang et al., 2013; Douvaras and Fossati, 2015) from human embryonic stem and induced pluripotent stem cells (iPSCs), which include slow and inefficient differentiation, and tumorigenic potential of contaminating undifferentiated pluripotent cells, are major bottlenecks towards their translational utility. Here, we report the rapid generation of human oligodendrocytes by direct lineage conversion of human dermal fibroblasts (HDFs). We show that the combination of the four transcription factors OLIG2, SOX10, ASCL1 and NKX2.2 is sufficient to convert HDFs to induced oligodendrocyte precursor cells (iOPCs). iOPCs resemble human primary and iPSC-derived OPCs based on morphology and transcriptomic analysis. Importantly, iOPCs can differentiate into mature myelinating oligodendrocytes in vitro and in vivo. Finally, iOPCs derived from patients with Pelizaeus Merzbacher disease, a hypomyelinating leukodystrophy caused by mutations in the proteolipid protein 1 (PLP1) gene, showed increased cell death compared with iOPCs from healthy donors. Thus, human iOPCs generated by direct lineage conversion represent an attractive new source for human cell-based disease models and potentially myelinating cell grafts.

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.

Activation of the unfolded protein response by Connexin47 mutations associated with Pelizaeus-Merzbacher-like disease.

Pelizaeus-Merzbacher-like disease type 1 (PMLD1) is a hypomyelinating disorder arising in patients with mutations in GJC2, encoding Connexin47 (Cx47). PMLD1 causes nystagmus, cerebellar ataxia, spasticity and changes in CNS white matter detected by MRI. At least one mutation (p.I33M) yields a much milder phenotype, spastic paraplegia type 44 (SPG44). Cx47 contributes to gap junction communication channels between oligodendrocytes (OLs), the myelinating cells in the central nervous system (CNS), and between OLs and astrocytes. Prior studies in cell lines have shown that PMLD1 mutants such as p.P87S display defective protein trafficking, intracellular retention in the ER and loss-of-function. Here we show that when expressed in primary OLs, three PMLD1 associated mutants (p.P87S, p.Y269D and p.M283T) show ER retention of Cx47 and evidence of activation of the cellular stress (unfolded protein response, UPR) and apoptotic pathways. On the other hand, the milder SPG44 associated mutation p.I33M shows a wild-type-like subcellular distribution and no activation of the UPR or apoptotic pathways. These studies provide new insight into a potential element of toxic gain of function underlying the mechanism of PMLD1 that should help guide future therapeutic approaches.

Missense mutation of MAL causes a rare leukodystrophy similar to Pelizaeus-Merzbacher disease.

Leukodystrophies are a heterogenous group of genetic disorders, characterised by abnormal development of cerebral white matter. Pelizaeus-Merzbacher disease is caused by mutations in PLP1, encoding major myelin-resident protein required for myelin sheath assembly. We report a missense variant p.(Ala109Asp) in MAL as causative for a rare, hypomyelinating leukodystrophy similar to Pelizaeus-Merzbacher disease. MAL encodes a membrane proteolipid that directly interacts with PLP1, ensuring correct distribution during myelin assembly. In contrast to wild-type MAL, mutant MAL was retained in the endoplasmic reticulum but was released following treatment with 4-phenylbutyrate. Proximity-dependent identification of wild-type MAL interactants implicated post-Golgi vesicle-mediated protein transport and protein localisation to membranes, whereas mutant MAL interactants suggested unfolded protein responses. Our results suggest that mislocalisation of MAL affects PLP1 distribution, consistent with known pathomechanisms for hypomyelinating leukodystrophies.

Pelizaeus-Merzbacher-Like Disease 1 Caused by a Novel Mutation in GJC2 Gene: A Case Report.

Pelizaeus-Merzbacher-Like Disease 1 is a genetic disorder affecting the central nervous system with an autosomal recessive inheritance pattern. It is a rare genetic disorder that affects the central nervous system. In this report, we demonstrated the clinical and paraclinical features of an Iranian consanguine pedigree with suspected hypomyelinating leukodystrophy, without any defined diagnosis. The proband, a 15-month-old girl, visited the Razi pathobiology and medical genetic laboratory of Karaj, where the study was conducted in 2020. Following whole-exome sequencing analysis of the proband and segregation analysis, a novel pathogenic mutation was discovered. GJC2 (NM_020435.4):c.1096dupG was found to be homozygous in the proband and heterozygous in both parents. This mutation was in the coding region of the protein, which results in D366Gfs*126 (p.Asp366GlyfsTer126). The site of mutation was at the 3' region of the connexin superfamily domain. The frameshift results in a different peptide sequence of the C-terminal and extended protein. Our findings led to the diagnosis of the proband's disease as Pelizaeus-Merzbacher-Like Disease 1 and led to the end of the diagnostic odyssey. We provided effective genetic counseling through the identification of a novel pathogenic mutation in gap junction protein C2 in this family and suggested preimplantation genetic diagnosis for the next pregnancy. Furthermore, our findings confirmed the association of GJC2 mutations with PMLD1. This discovery added to the repertoire of genetic mutations of Pelizaeus-Merzbacher-Like Disease 1. This knowledge could be applied for expanded carrier screening of other families, especially for Iranian consanguine marriages.

Novel Insight into the Potential Pathogenicity of Mitochondrial Dysfunction Resulting from PLP1 Duplication Mutations in Patients with Pelizaeus-Merzbacher Disease.

Among the hypomyelinating leukodystrophies, Pelizaeus-Merzbacher disease (PMD) is a representative disorder. The disease is caused by different types of PLP1 mutations, among which PLP1 duplication accounts for ∼70% of the mutations. Previous studies have shown that PLP1 duplications lead to PLP1 retention in the endoplasmic reticulum (ER); in parallel, recent studies have demonstrated that PLP1 duplication can also lead to mitochondrial dysfunction. As such, the respective roles and interactions of the ER and mitochondria in the pathogenesis of PLP1 duplication are not clear. In both PLP1 patients' and healthy fibroblasts, we measured mitochondrial respiration with a Seahorse XF Extracellular Analyzer and examined the interactions between the ER and mitochondria with super-resolution microscopy (spinning-disc pinhole-based structured illumination microscopy, SD-SIM). For the first time, we demonstrated that PLP1 duplication mutants had closer ER-mitochondrion interfaces mediated through structural and morphological changes in both the ER and mitochondria-associated membranes (MAMs). These changes in both the ER and mitochondria then led to mitochondrial dysfunction, as reported previously. This work highlights the roles of MAMs in bridging PLP1 expression in the ER and pathogenic dysfunction in mitochondria, providing novel insight into the pathogenicity of mitochondrial dysfunction resulting from PLP1 duplication. These findings suggest that interactions between the ER and mitochondria may underlie pathogenic mechanisms of hypomyelinating leukodystrophies diseases at the organelle level.

Regulation of ClC-2 Chloride Channel Proteostasis by Molecular Chaperones: Correction of Leukodystrophy-Associated Defect.

The ClC-2 channel plays a critical role in maintaining ion homeostasis in the brain and the testis. Loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the white matter disease leukodystrophy. Clcn2-deficient mice display neuronal myelin vacuolation and testicular degeneration. Leukodystrophy-causing ClC-2 mutant channels are associated with anomalous proteostasis manifesting enhanced endoplasmic reticulum (ER)-associated degradation. The molecular nature of the ER quality control system for ClC-2 protein remains elusive. In mouse testicular tissues and Leydig cells, we demonstrated that endogenous ClC-2 co-existed in the same protein complex with the molecular chaperones heat shock protein 90ß (Hsp90ß) and heat shock cognate protein (Hsc70), as well as the associated co-chaperones Hsp70/Hsp90 organizing protein (HOP), activator of Hsp90 ATPase homolog 1 (Aha1), and FK506-binding protein 8 (FKBP8). Further biochemical analyses revealed that the Hsp90ß-Hsc70 chaperone/co-chaperone system promoted mouse and human ClC-2 protein biogenesis. FKBP8 additionally facilitated membrane trafficking of ClC-2 channels. Interestingly, treatment with the Hsp90-targeting small molecule 17-allylamino-17-demethoxygeldanamycin (17-AAG) substantially boosted ClC-2 protein expression. Also, 17-AAG effectively increased both total and cell surface protein levels of leukodystrophy-causing loss-of-function ClC-2 mutant channels. Our findings highlight the therapeutic potential of 17-AAG in correcting anomalous ClC-2 proteostasis associated with leukodystrophy.

Pathology of the neurovascular unit in leukodystrophies.

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

Spinal cord involvement and paroxysmal events in "Infantile Onset Transient Hypomyelination" due to TMEM63A mutation.

Monoallelic mutations on TMEM63A have been recently reported as cause of a previously unrecognized disorder named "infantile-onset transient hypomyelination". Clinical and neuroradiological presentation is described as highly similar to Pelizaeus-Merzbacher Disease but evolution over time was surprisingly benign with a progressive spontaneous improving course. We report on a new TMEM63A-mutated girl. The clinical picture was similar to the one already described except for the presence of recurrent episodes of unilateral eyelid twitching, and for the evidence of spinal cord involvement on MRI. These are interesting findings helping in distinguishing this condition from classic PMD since early disease stages. However, additional observations are needed to confirm if these are common features of this condition.

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.

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.

Functional characterization of Polr3a hypomyelinating leukodystrophy mutations in the S. cerevisiae homolog, RPC160.

Mutations in RNA polymerase III (Pol III) cause hypomeylinating leukodystrophy (HLD) and neurodegeneration in humans. POLR3A and POLR3B, the two largest Pol III subunits, together form the catalytic center and carry the majority of disease alleles. Disease-causing mutations include invariant and highly conserved residues that are predicted to negatively affect Pol III activity and decrease transcriptional output. A subset of HLD missense mutations in POLR3A cluster in the pore region that provides nucleotide access to the Pol III active site. These mutations were engineered at the corresponding positions in the Saccharomyces cerevisiae homolog, Rpc160, to evaluate their functional deficits. None of the mutations caused a growth or transcription phenotype in yeast. Each mutation was combined with a frequently occurring pore mutation, POLR3A G672E, which was also wild-type for growth and transcription. The double mutants showed a spectrum of phenotypes from wild-type to lethal, with only the least fit combinations showing an effect on Pol III transcription. In one slow-growing temperature-sensitive mutant the steady-state level of tRNAs was unaffected, however global tRNA synthesis was compromised, as was the synthesis of RPR1 and SNR52 RNAs. Affinity-purified mutant Pol III was broadly defective in both factor-independent and factor-dependent transcription in vitro across genes that represent the yeast Pol III transcriptome. Thus, the robustness of yeast Rpc160 to single Pol III leukodystrophy mutations in the pore domain can be overcome by a second mutation in the domain.

[Analysis of genomic copy number variants in a patient with congenital type Pelizaeus-Merzbacher disease].

OBJECTIVE: To explore the genotypic and phenotypic characteristics of a child with congenital Pelizaeus-Merzbacher disease. METHODS: Clinical, imaging and genetic characteristics of the child were retrospectively analyzed. RESULTS: The patient manifested significantly reduced muscle tension, apparent tremor of eyeballs, and retardation of motor development after birth. Cranial MRI at 6-month-old showed no myelinization of brain white matter, though no pathogenic variant was detected by clinical exome sequencing. A copy number variation was found at chrX: 102 192 246-103 045 526. The duplication has spanned approximately 853 kb and was recorded in the Decipher database to be associated with Pelizaeus-Merzbacher disease. Quantitative PCR confirmed that the duplication has derived from his mother. CONCLUSION: Reduced muscle tension, nystagmus, poor motor development, and absence of myelinization of white matter should alert the diagnosis of congenital type Pelizaeus-Merzbacher disease. Both sequence variant and copy number variation should be searched upon genetic testing. A clear diagnosis is required for genetic counseling.