PLP1 duplication at the breakpoint regions of an apparently balanced t(X;22) translocation causes Pelizaeus–Merzbacher disease in a girl

Fonseca ACS, Bonaldi A, Costa SS, Freitas MR, Kok F, Vianna‐Morgante AM. PLP1 duplication at the breakpoint regions of an apparently balanced t(X;22) translocation causes Pelizaeus–Merzbacher disease in a girl. PLP1 (proteolipid protein1 gene) mutations cause Pelizaeus–Merzbacher disease (PMD), characterized by hypomyelination of the central nervous system, and affecting almost exclusively males. We report on a girl with classical PMD who carries an apparently balanced translocation t(X;22)(q22;q13). By applying array‐based comparative genomic hybridization (a‐CGH), we detected duplications at 22q13 and Xq22, encompassing 487–546 kb and 543–611 kb, respectively. The additional copies were mapped by fluorescent in situ hybridization to the breakpoint regions, on the derivative X chromosome (22q13 duplicated segment) and on the derivative 22 chromosome (Xq22 duplicated segment). One of the 14 duplicated X‐chromosome genes was PLP1.The normal X chromosome was the inactive one in the majority of peripheral blood leukocytes, a pattern of inactivation that makes cells functionally balanced for the translocated segments. However, a copy of the PLP1 gene on the derivative chromosome 22, in addition to those on the X and der(X) chromosomes, resulted in two active copies of the gene, irrespective of the X‐inactivation pattern, thus causing PMD. This t(X;22) is the first constitutional human apparently balanced translocation with duplications from both involved chromosomes detected at the breakpoint regions.     

High frequency of GJA12/GJC2 mutations in Turkish patients with Pelizaeus–Merzbacher disease

Bilir B, Yapici Z, Yalcinkaya C, Baris I, Carvalho CMB, Bartnik M, Ozes B, Eraksoy M, Lupski JR, Battaloglu E. High frequency of GJA12/GJC2 mutations in Turkish patients with Pelizaeus–Merzbacher disease. Pelizaeus–Merzbacher disease is an early onset dysmyelinating leukodystrophy. About 80% of PMD cases have been associated with duplications and mutations of the proteolipid protein 1 (PLP1) gene. Pelizaeus–Merzbacher‐like disease is a genetically heterogeneous autosomal recessive disease and rarely caused by mutations in gap junction protein α12 (GJA12/GJC2) gene. The molecular basis of the disease was investigated in a cohort of 19 Turkish families. This study identified novel chromosomal rearrangements proximal and distal to, and exclusive of the PLP1 gene, showed equal frequencies of PLP1 and GJA12/GJC2 mutations at least in our cohort, and suggested further genetic heterogeneity.     

Further genotype–phenotype correlation emerging from two families with PLP1 exon 4 skipping

Proteolipid protein 1 (PLP1) gene‐related disorders due to mutations in the PLP1 include a wide spectrum of X‐linked disorders ranging from severe connatal Pelizaeus–Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). Duplications, deletions or point mutations in coding and noncoding regions of the PLP1 gene may occur. We report the clinical, neuroradiologic and molecular findings in six patients from two unrelated families. The affected males showed severe mental retardation, spastic tetraparesis, inability of walking and pes cavus at onset in early infancy. Brain magnetic resonance imaging (MRI) showed hypomyelination and brain atrophy. Nystagmus was never observed. The affected females showed adult‐onset progressive spastic paraparesis leading to wheel‐chair dependency and subtle white matter changes on brain MRI. Molecular studies in the two families identified two different intronic mutations, the novel c.622+2T>C and the known c.622+1G>A, leading to the skipping of PLP1‐exon 4. The clinical presentation of the affected males did not consistently fit in any of the PLP1‐related disorder subtypes (i.e., connatal or classic PMD, SPG2 and ‘PLP1 null syndrome’), and in addition, the carrier females were symptomatic despite the severe clinical picture of their respective probands. This study provides new insight into the genotype–phenotype correlations of patients with PLP1 splice‐site mutations.     

Pelizaeus-Merzbacher disease caused by a duplication-inverted triplication-duplication in chromosomal segments including the PLP1 region

Pelizaeus-Merzbacher disease (PMD; MIM#312080) is a rare X-linked leukodystrophy presenting with motor developmental delay associated with spasticity and nystagmus. PMD is mainly caused by abnormalities in the proteolipid protein 1 gene (PLP1), most frequently due to duplications of chromosomal segments including PLP1. In this study, a 9-year-old male patient manifesting severe developmental delay and spasticity was analyzed for PLP1 alteration, and triplication of PLP1 was identified. Further examination revealed an underlying genomic organization, duplication-inverted triplication-duplication (DUP-TRP/INV-DUP), in which a triplicated segment was nested between 2 junctions. One of the 2 junctions was caused by inverted homologous regions, and the other was caused by non-homologous end-joining. PMD patients with PLP1 duplications usually show milder-classical forms of the disease compared with patients with PLP1 missense mutations manifesting severe connatal forms. The present patient showed severe phenotypic features that represent an intermediate form of PMD between classical and connatal forms. This is the first report of a patient with PLP1 triplication caused by a DUP-TRP/INV-DUP structure. This study adds additional evidence about the consequences of PLP1 triplication.     

X inactivation phenotype in carriers of Pelizaeus-Merzbacher disease: skewed in carriers of a duplication and random in carriers of point mutations

Pelizaeus-Merzbacher disease (PMD) is an X-linked recessive disease caused by coding sequence mutations in the PLP gene, sub-microscopic duplications of variable sizes including the PLP gene or very rarely deletions of the PLP gene. We analysed the X inactivation pattern in blood of PMD female carriers with duplications and with point mutations. In the majority of duplication carriers (7/11), the X chromosome bearing the duplication was preferentially inactivated, whereas a random pattern of X inactivation was detected in point mutation carriers (3/3), a deletion carrier (1/1), affected females (4/4) who did not have a recognised mutation and normal control females. However 2/5 non-carrier female relatives of patients with a duplication, had skewed X inactivation. The skewed pattern of inactivation observed in most duplication carriers and not in mutation carriers suggests a) that there is selection against those cells in which the duplicated X chromosome is active and b) other expressed sequences within the duplicated region rather than mutant PLP may be responsible. Since the skewed X inactivation did not segregate with the disease in two families and the pattern of X inactivation was variable among the duplication carriers, the pattern X inactivation is an unsuitable diagnostic tool for female carriers of PMD.     

A case of cerebral hypomyelination with spondylo‐epi‐metaphyseal dysplasia

We reported on a male patient with rare leukoencephalopathy and skeletal abnormalities. The condition was first noticed as a developmental delay, nystagmus and ataxia at 1 year of age. At 4 years of age, he was diagnosed as hypomyelination with skeletal abnormalities from clinical features, brain magnetic resonance imaging (MRI) and skeletal X‐rays. His brain MRI revealed diffuse hypomyelination. These findings suggested the classical type of Pelizaeus–Merzbacher disease (PMD) caused by proteolipid protein (PLP)‐1 gene or Pelizaeus–Merzbacher‐like disease (PMLD). However, we found neither mutation nor duplication of PLP‐1. The patient had severe growth retardation and general skeletal dysplasia compatible with spondylo‐epi‐metaphyseal dysplasia; however the mutation of discoidin domain receptor (DDR) 2 gene was absent. The co‐morbidity of hypomyelination with skeletal abnormalities is rare. We performed array CGH and no causal copy number variation was recognized. Alternatively, this condition may have been caused by a mutation of the gene encoding a molecule that functions in both cerebral myelination and skeletal development. © 2012 Wiley Periodicals, Inc.     

Mutation analysis of the M6b gene in patients with Pelizaeus-Merzbacher-like syndrome

"Pelizaeus-Merzbacher-like syndrome" is an undetermined leukodystrophy disorder of diffuse hypomyelination. The patients' clinical phenotype is indistinguishable from classical Pelizaeus-Merzbacher disease (PMD), but the patients lack PLP1 gene duplications or mutations. They represent about 20% of all cases with a clinical PMD phenotype. The M6b gene has been localized to Xp22.2. The encoded M6B protein is a member of a novel proteolipid family that also includes other major brain myelin components like the proteolipid protein (PLP). Recent cotransfection experiments suggest a protein-protein interaction of M6B and mutant PLP1 that may contribute to oligodendrocyte dysfunction in PMD. Therefore, M6b has been considered a good candidate gene for Pelizaeus-Merzbacher-like syndrome. However, our molecular analyses in eight thoroughly characterized patients make it unlikely that mutations in this gene are involved in this subgroup of human hypomyelination disorders.     

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

Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia type 2 (SPG2) are X-linked developmental defects of myelin formation affecting the central nervous system (CNS). They differ clinically in the onset and severity of the motor disability but both are allelic to the proteolipid protein gene (PLP), which encodes the principal protein components of CNS myelin, PLP and its spliced isoform, DM20. We investigated 52 PMD and 28 SPG families without large PLP duplications or deletions by genomic PCR amplification and sequencing of the PLP gene. We identified 29 and 4 abnormalities respectively. Patients with PLP mutations presented a large range of disease severity, with a continuum between severe forms of PMD, without motor development, to pure forms of SPG. Clinical severity was found to be correlated with the nature of the mutation, suggesting a distinct strategy for detection of PLP point mutations between severe PMD, mild PMD and SPG. Single amino-acid changes in highly conserved regions of the DM20 protein caused the most severe forms of PMD. Substitutions of less conserved amino acids, truncations, absence of the protein and PLP-specific mutations caused the milder forms of PMD and SPG. Therefore, the interactions and stability of the mutated proteins has a major effect on the severity of PLP-related diseases.     

Genetic diagnosis of PLP gene duplications/deletions in patients with Pelizaeus-Merzbacher disease

Genetic diagnosis of PLP gene duplications/deletions in patients with Pelizaeus-Merzbacher disease.PMD is an X-linked recessive disorder due to a proteolipid protein (PLP) deficiency. Duplications of PLP gene were shown to be the principle cause of the disorder, accounting for an estimated 50-70% of cases. To define a simple and reliable method for genetic diagnosis of PMD, a group of 42 patients with clinical manifestation of PMD was analyzed by means of real-time quantitative PCR. Parallel fluorescence in situ hybridization (FISH) analysis was performed on the same group of patients. Real-time PCR found seventeen samples had increased gene dosage, whereas FISH detected sixteen duplicated samples. Both methods identified a sample with PLP gene deletion. Our results indicate that real-time PCR is a sensitive and reliable method for the detection of gene duplications/deletions. We further discussed the advantages and limitations of each method in clinical diagnosis of PMD.     

A new polymorphism in the proteolipid protein (PLP1) gene and its use for carrier detection of PLP1 gene duplication in Pelizaeus-Merzbacher disease

Pelizaeus Merzbacher Disease (PMD) is an X-linked recessive dysmyelinating disorder of the central nervous system. Most patients have point mutations in exons of the proteolipid protein (PLP1) gene or duplication of a genomic region that includes the PLP1 gene. We identified a common MspI polymorphism in intron 1 of the PLP1 gene and used it to determine carrier status for PLP1 gene duplication in PMD by using a quantitative PCR approach.     

Novel PORCN mutations in focal dermal hypoplasia

Focal dermal hypoplasia (FDH), Goltz or Goltz–Gorlin syndrome, is an X‐linked dominant multisystem disorder characterized primarily by involvement of the skin, skeletal system and eyes. We screened for mutations in the PORCN gene in eight patients of Belgian and Finnish origin with firm clinical suspicion of FDH. First, we performed quantitative PCR (qPCR) analysis to define the copy number at this locus. Next, we sequenced the coding regions and flanking intronic sequences of the PORCN gene. Three de novo mutations were identified in our patients with FDH: a 150‐kb deletion removing six genes including PORCN, as defined by qPCR and X‐array‐CGH, and two heterozygous missense mutations; c.992T>G (p.L331R) in exon 11 and c.1094G>A (p.R365Q) in exon 13 of the gene. Both point mutations changed highly conserved amino acids and were not found in 300 control X chromosomes. The three patients in whom mutations were identified all present with characteristic dermal findings together with limb manifestations, which were not seen in our mutation‐negative patients. The clinical characteristics of our patients with PORCN mutations were compared with the previously reported mutation‐positive cases. In this report, we summarize the literature on PORCN mutations and associated phenotypes.     

NF1 microdeletions in neurofibromatosis type 1: from genotype to phenotype

In 5‐10% of patients, neurofibromatosis type 1 (NF1) results from microdeletions that encompass the entire NF1 gene and a variable number of flanking genes. Two recurrent microdeletion types are found in most cases, with microdeletion breakpoints located in paralogous regions flanking NF1 (proximal NF1‐REP‐a and distal NF1‐REP–c for the 1.4 Mb type‐1 microdeletion, and SUZ12 and SUZ12P for the 1.2 Mb type‐2 microdeletion). A more severe phenotype is usually associated with NF1 microdeletion patients than in those with intragenic mutations. We characterized NF1 microdeletions in 70 unrelated NF1 microdeleted patients using a high‐resolution NF1 custom array comparative genomic hybridization (CGH). Genotype‐phenotype correlations were studied in 58 of these microdeletion patients and compared to 389 patients with intragenic truncating NF1 mutations and phenotyped in the same standardized way. Our results confirmed in an unbiased manner the existence of a contiguous gene syndrome with a significantly higher incidence of learning disabilities and facial dysmorphism in microdeleted patients compared to patients with intragenic NF1 mutations. Microdeleted NF1 patients also showed a trend toward significance for childhood overgrowth. High‐resolution array‐CGH identified a new recurrent ～1.0 Mb microdeletion type, designated as type‐3, with breakpoints in the paralogous regions middle NF1‐REP‐b and distal NF1‐REP–c. © 2010 Wiley‐Liss, Inc.     

Golli-MBP Copy Number Analysis by FISH, QMPSF and MAPH in 195 Patients with Hypomyelinating Leukodystrophies

The inherited disorders of CNS myelin formation represent a heterogeneous group of leukodystrophies. The proteolipoprotein (PLP1) gene has been implicated in two X‐linked forms, Pelizaeus‐Merzbacher disease (PMD) and spastic paraplegia type 2, and the gap junction protein α12 (GJA12) gene in a recessive form of PMD. The myelin basic protein (MBP) gene, which encodes the second most abundant CNS myelin protein after PLP1, presents rearrangements in hypomyelinating murine mutants and is always included in the minimal region deleted in 18q‐ patients with an abnormal hypomyelination pattern on cerebral MRI. In this study, we looked at the genomic copy number at the Golli‐MBP locus in 195 patients with cerebral MRI suggesting a myelin defect, who do not have PLP1 mutation. Although preliminary results obtained by FISH suggested the duplication of Golli‐MBP in 3 out of 10 patients, no abnormal gene quantification was found using Quantitative Multiplex PCR of Short Fluorescent fragments (QMPSF), Multiplex Amplifiable Probe Hybridization (MAPH), or another FISH protocol using directly‐labelled probes. Pitfalls and interest in these different techniques to detect duplication events are emphasised. Finally, the study of this large cohort of patients suggests that Golli‐MBP deletion or duplication is rarely involved in inherited defects of myelin formation.     

Seventeen novel PLP1 mutations in patients with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a rare X-chromosomal neurodegenerative disorder that affects primarily the white matter of the central nervous system and is caused by mutations of the PLP1 (proteolipid protein 1) gene. We performed mutation analysis of 133 male patients with suspected PMD. Following SSCP analysis of all coding exons of PLP1, we found most likely pathogenic mutations (single base substitutions and small rearrangements) including 17 novel sequence variants in 21 (15.8%) patients. Most patients with missense mutations had a severe phenotype. Twelve patients (9.0%) carried a duplication of the entire gene, as demonstrated by quantitative real-time PCR, and presented with a variable clinical phenotype including mild, classical, and severe courses of disease. Two patients had large deletions, spanning approximately 115 kb, that included the PLP1 gene. In total, we identified pathogenic mutations involving PLP1 in 35 (26.3%) of the 133 patients analyzed.     

Genotype-phenotype correlation in five Pelizaeus-Merzbacher disease patients with PLP1 gene duplications

Pelizaeus–Merzbacher disease (PMD) is an X-linked myelination disorder most frequently caused by duplication of a genomic segment of variable length containing the PLP1 gene. We studied five PMD male patients affected by the classic PMD form carrying a PLP1 gene duplication. On the basis of clinical and neuroradiological features, two of the five patients appeared to be the most severely affected. In order to establish a possible genotype–phenotype correlation, the extent of the duplication was determined in each patient and in the respective mother by quantifying the copy number of genomic markers surrounding the PLP1 gene by a real-time PCR-based approach. Duplications, ranging in size from 167–195 to 580–700 kb, were in the same genomic interval of the majority of the reported duplications. The extent of the duplicated genomic segments does not correlate with the clinical severity. Interestingly enough, each duplication had one of the two breakpoints in or near to low copy repeats (LCRs), supporting recent evidence concerning a possible role of LCRs in the generation of the duplications in PMD.     

A novel insertional mutation at exon VII of the myelin proteolipid protein gene in Pelizaeus -- Merzbacher disease

Pelizaeus — Merzbacher disease (PMD) is an X-linked neurologicaldisorder characterized by dysmyelination in the central nervoussystem (CNS). Recently mutations of the myelln proteollpid protein(PLP) gene which encodes both PLP and Its Isoform, DM-20 generatedby alternative spllcing, have been demonstrated In PMD patients.We analyzed the seven exons of the PLP gene of a Japanese boyaffected with PMD by direct sequencing and identified an Insertionevent In exon Vll of the PLP gene. This mutation was also presentIn his carrier mother, but was absent In ninety-five X chromosomesof normal Japanese. The frame-shift mutation leads to the productionof truncated PLP with altered carboxyl terminal amlno acid sequences,resulting In conslderable change of the structure of PLP andDM-20 necessary for functional purposes. This is the first reportof a mutation In exon Vll of the PLP gene associated with PMD.     

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.     

Xq22 deletions and correlation with distinct neurological disease traits in females: Further evidence for a contiguous gene syndrome

Xq22 deletions that encompass PLP1 (Xq22‐PLP1‐DEL) are notable for variable expressivity of neurological disease traits in females ranging from a mild late‐onset form of spastic paraplegia type 2 (MIM# 312920), sometimes associated with skewed X‐inactivation, to an early‐onset neurological disease trait (EONDT) of severe developmental delay, intellectual disability, and behavioral abnormalities. Size and gene content of Xq22‐PLP1‐DEL vary and were proposed as potential molecular etiologies underlying variable expressivity in carrier females where two smallest regions of overlap (SROs) were suggested to influence disease. We ascertained a cohort of eight unrelated patients harboring Xq22‐PLP1‐DEL and performed high‐density array comparative genomic hybridization and breakpoint‐junction sequencing. Molecular characterization of Xq22‐PLP1‐DEL from 17 cases (eight herein and nine published) revealed an overrepresentation of breakpoints that reside within repeats (11/17, ~65%) and the clustering of ~47% of proximal breakpoints in a genomic instability hotspot with characteristic non‐B DNA density. These findings implicate a potential role for genomic architecture in stimulating the formation of Xq22‐PLP1‐DEL. The correlation of Xq22‐PLP1‐DEL gene content with neurological disease trait in female cases enabled refinement of the associated SROs to a single genomic interval containing six genes. Our data support the hypothesis that genes contiguous to PLP1 contribute to EONDT.