Pelizaeus-Merzbacher disease: three novel mutations and implication for locus heterogeneity

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.

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.

Pelizaeus-Merzbacher disease: cellular pathogenesis and pharmacologic therapy

Pelizaeus-Merzbacher disease (PMD) is a rare leukodystrophy that causes severe dysmyelination in the central nervous system in infancy and early childhood. Many previous studies showed that various proteolipid protein 1 (plp1) mutations, including duplications, point mutations, and deletions, lead to oligodendrocyte dysfunction in patients with PMD. PMD onset and clinical severity range widely, depending on the type of plp1 mutation. Patients with PMD exhibit a delayed mental and physical development phenotype, but specific pharmacological therapy and clinical treatment for PMD are not yet well established. This review describes PMD pathology and establishment of new clinical treatment for PMD. These findings support the development of a new therapy for PMD and these treatments may improve the quality of life in patients with 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.

A novel proteolipid protein 1 gene mutation causing classical type Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a rare dysmyelinating disorder caused by mutations in the proteolipid protein 1 (PLP1) gene. PMD is generally classified according to its clinical or pathological features into classical or connatal forms. We describe here a 19-year-old male with classical form PMD who presented with stridor and nystagmus in early infancy and whose psychomotor development has been severely delayed. Brain magnetic resonance imaging revealed white matter abnormalities typical of PMD. Direct sequencing of the PLP1 gene identified two nucleotide substitutions. One was a C-to-T transition at -31 in the 5'-flanking region of exon 1; the other was a novel point mutation, T-to-C transition in exon 4, which led to substitution of cysteine for arginine at residue 184. Because Cys184 forms a disulphide bridge with Cys228, the Cys184Arg mutation probably removes the bridge and changes the tertiary structure of PLP protein. A defective disulfide bond in PLP protein could be important in the pathogenesis of PMD.

Mild phenotype in Pelizaeus-Merzbacher disease caused by a PLP1-specific mutation

We present the case of a 26 year-old man who developed normally until he began having difficulty walking at age 12. He subsequently became unable to stand at 15 years old and exhibited mental regression and generalized tonic convulsions by age 20. Magnetic resonance imaging revealed incomplete myelination of cerebral white matter, which resembled that of Pelizaeus-Merzbacher disease. By sequencing the proteolipid protein 1 (PLP1) gene, we found a novel mutation (c.352_353delAG (p.Gly130fs)) in the latter half of exon 3 (exon 3B) that is spliced out in the DM20 isoform. Exon 3B mutations are known to cause a mild phenotype since they do not disturb DM20 production. Mutations that truncate PLP1 correlate with a mild phenotype by activating the nonsense-mediated decay mechanism that specifically detects and degrades mRNAs containing a premature termination codon. This attenuates the production of toxic mutant PLP1. The very mild presentation in the present case seems to be derived from the unique nature of the mutation, which preserves DM20 production and decreases mutant PLP1.

A common mechanism of PLP/DM20 misfolding causes cysteine-mediated endoplasmic reticulum retention in oligodendrocytes and Pelizaeus-Merzbacher disease

A large number of mutations in the human PLP1 gene lead to abnormal myelination and oligodendrocyte death in Pelizaeus-Merzbacher disease (PMD). Here we show that a major subgroup of PMD mutations that map into the extracellular loop region of PLP/DM20 leads to the failure of oligodendrocytes to form the correct intramolecular disulfide bridges. This leads to abnormal protein cross-links and endoplasmic reticulum retention and activates the unfolded protein response. Importantly, surface expression of mutant PLP/DM20 can be restored and the unfolded protein response can be reverted by the removal of two cysteines. Thus, covalent protein cross-links emerge as a cause, rather than as a consequence, of endoplasmic reticulum retention.

Aberrant trafficking of a proteolipid protein in a mild Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked leukodystrophy caused by proteolipid protein 1 (PLP1) gene mutations. Previous studies indicated that proteolipid proteins (PLPs) with disease-associated mutations are misfolded and trapped in the endoplasmic reticulum (ER) during transportation to the cell surface, which eventually leads to oligodendrocyte cell death in PMD. Here we report a PMD patient with a very mild phenotype carrying a novel mutation (485G-->T) in exon 4 of the PLP1 gene that causes a Trp(162)Leu substitution in the protein. We also investigated intracellular trafficking of this mutant PLP in COS-7 cells. Transiently transfected mutant PLP(W162L) fused to an enhanced green fluorescent protein (EGFP) or a short peptide tag was not carried to the plasma membrane. However, in contrast to previous studies, this mutant PLP was not retained in the ER, indicating an escape of the newly translated protein from the quality control machinery. We also found that the mutant PLP accumulated in the nuclear envelope (NE) in a time-dependent manner. This mutant PLP, with its distribution outside the ER and a very mild phenotype, supports the idea that accumulation of misfolded mutant protein in the ER causes the severe phenotype of PMD.

Spastic paraplegia type 2 associated with axonal neuropathy and apparent PLP1 position effect

OBJECTIVE

To report an association between spastic paraplegia type 2 with axonal peripheral neuropathy and apparent proteolipid protein gene (PLP1) silencing in a family.

METHODS

Pulsed-field gel electrophoresis, custom array comparative genomic hybridization, and semi-quantitative multiplex polymerase chain reaction analyses were used to examine the PLP1 genomic region.

RESULTS

Electrodiagnostic studies and a sural nerve biopsy showed features of a dystrophic axonal neuropathy. Molecular studies identified a small duplication downstream of PLP1.

INTERPRETATION

We propose the duplication to result in PLP1 gene silencing by virtue of a position effect. Our observations suggest that genomic rearrangements that do not include PLP1 coding sequences should be considered as yet another potential mutational mechanism underlying PLP1-related dysmyelinating disorders.

PLP1-related inherited dysmyelinating disorders: Pelizaeus-Merzbacher disease and spastic paraplegia type 2

Pelizaeus-Merzbacher disease (PMD) and its allelic disorder, spastic paraplegia type 2 (SPG2), are among the best-characterized dysmyelinating leukodystrophies of the central nervous system (CNS). Both PMD and SPG2 are caused by mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major component of CNS myelin proteins. Distinct types of mutations, including point mutations and genomic duplications and deletions, have been identified as causes of PMD/SPG2 that act through different molecular mechanisms. Studies of various PLP1 mutants in humans and animal models have shed light on the genomic, molecular, and cellular pathogeneses of PMD/SPG2. Recent discoveries include complex mutational mechanisms and associated disease phenotypes, novel cellular pathways that lead to the degeneration of oligodendrocytes, and genomic architectural features that result in unique chromosomal rearrangements. Here, I review the previous and current knowledge of the molecular pathogenesis of PMD/SPG2 and delineate future directions for PMD/SPG2 studies.

Clinical findings in Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a rare X-linked disease characterized by defective central nervous system myelination owing to a mutation in the proteolipid protein 1 gene. Few studies report detailed clinical findings in children with genetic confirmation of mutations in the proteolipid protein 1 gene. We reviewed the records of 10 boys with Pelizaeus-Merzbacher disease and one symptomatic carrier girl. Their median age was 2 1/2 years (range 10 months to 20 years). Nine had proteolipid protein 1 gene duplications, one had a point mutation, and one had a single codon deletion. The families of eight patients reported perinatal complications, including maternal hypertension (three patients) and meconium aspiration (three patients). All of the patients were social and interactive, but all had difficulty with expressive speech. All patients presented with nystagmus and had hypotonia that progressed to spasticity, affecting the legs more than the arms; ataxia also contributed to motor impairment. Additional problems reported regarded feeding (eight patients) and sleep (three patients). Further work is needed to clarify the variations in disease course and the relationship of genotype to phenotype.

Genomic rearrangements resulting in PLP1 deletion occur by nonhomologous end joining and cause different dysmyelinating phenotypes in males and females

In the majority of patients with Pelizaeus-Merzbacher disease, duplication of the proteolipid protein gene PLP1 is responsible, whereas deletion of PLP1 is infrequent. Genomic mechanisms for these submicroscopic chromosomal rearrangements remain unknown. We identified three families with PLP1 deletions (including one family described elsewhere) that arose by three distinct processes. In one family, PLP1 deletion resulted from a maternal balanced submicroscopic insertional translocation of the entire PLP1 gene to the telomere of chromosome 19. PLP1 on the 19qtel is probably inactive by virtue of a position effect, because a healthy male sibling carries the same der(19) chromosome along with a normal X chromosome. Genomic mapping of the deleted segments revealed that the deletions are smaller than most of the PLP1 duplications and involve only two other genes. We hypothesize that the deletion is infrequent, because only the smaller deletions can avoid causing either infertility or lethality. Analyses of the DNA sequence flanking the deletion breakpoints revealed Alu-Alu recombination in the family with translocation. In the other two families, no homologous sequence flanking the breakpoints was found, but the distal breakpoints were embedded in novel low-copy repeats, suggesting the potential involvement of genome architecture in stimulating these rearrangements. In one family, junction sequences revealed a complex recombination event. Our data suggest that PLP1 deletions are likely caused by nonhomologous end joining.

Compensating for central nervous system dysmyelination: females with a proteolipid protein gene duplication and sustained clinical improvement

A submicroscopic duplication that contains the entire proteolipid protein gene is the major cause of Pelizaeus-Merzbacher disease, an X-linked central nervous system dysmyelinating disorder. Previous studies have demonstrated that carrier females for the duplication are usually asymptomatic. We describe 2 unrelated female patients who present with mild Pelizaeus-Merzbacher disease or spastic paraplegia. In 1 patient, clinical features as well as cranial magnetic resonance imaging and brainstem auditory evoked potential results have improved dramatically over a 10-year period. The other patient, who presented with spastic diplegia and was initially diagnosed with cerebral palsy, has also shown clinical improvement. Interphase fluorescent in situ hybridization identified a proteolipid protein gene duplication in both patients. Interphase fluorescent in situ hybridization analyses of the family members indicated that the duplication in both patients occurred as de novo events. Neither skewing of X inactivation in the peripheral lymphocytes nor proteolipid protein gene coding alterations were identified in either patient. These findings indicate that, occasionally, females with a proteolipid protein gene duplication can manifest an early-onset neurological phenotype. We hypothesize that the remarkable clinical improvement is a result of myelin compensation by oligodendrocytes expressing one copy of proteolipid protein gene secondary to selection for a favorable X inactivation pattern. These findings indicate plasticity of oligodendrocytes in the formation of central nervous system myelin and suggest a potential role for stem cell transplantation therapies.

Molecular pathways of oligodendrocyte apoptosis revealed by mutations in the proteolipid protein gene

A decade after the genetic link was established between mutations in the proteolipid protein gene and two leukodystrophies, Pelizaeus-Merzbacher disease and spastic paraplegia, the molecular mechanisms underlying pathogenesis are beginning to come to light. Data from animal models of these diseases suggest that the absence of proteolipid protein gene products in the central nervous system confers a relatively mild phenotype while missense mutations in and duplications of this gene give rise to mild or severe forms of disease. Previously, we have interpreted the disease process in terms of the accumulation of the mutant proteins in the secretory pathway and, herein, we review the evidence in favor of such a cellular mechanism. Furthermore, on the basis of recent data we suggest that the unfolded protein response may be involved in the pathogenesis of Pelizaeus-Merzbacher disease and spastic paraplegia through a kinase signaling cascade that links the accumulation of mutant proteins in the endoplasmic reticulum of oligodendrocytes with changes in gene regulation, protein synthesis, and possibly apoptosis.

Myelin deficiencies in both the central and the peripheral nervous systems associated with a SOX10 mutation

We describe an unique patient presenting with severe leukodystrophy compatible with Pelizaeus-Merzbacher disease and peripheral neuropathy consistent with Charcot-Marie-Tooth disease type 1 in addition to Waardenburg-Hirschsprung syndrome. A novel mutation was identified in her SOX10 gene, which encodes a transcription factor preferentially expressed in the late embryonic glial cell lineage and in mature myelin-forming cells of both the central nervous system and peripheral nervous system, as well as in the early neural crest cells. Heterozygous SOX10 loss-of-function mutations have been reported in patients with Waardenburg-Hirschsprung syndrome and its murine model, Dominant megacolon. However, neither Waardenburg-Hirschsprung syndrome patients nor Dominant megacolon mice have dysmyelinating features, suggesting the question of how SOX10 acts in the glial lineage in vivo. The novel mutation described herein does not disrupt the coding region but extends the peptide and hence is likely to act as a dominant-negative allele. Our findings indicate that dysfunction of SOX10 may lead to deficiency of myelination in the central nervous system and peripheral nervous system as well as hypopigmentation and enteric aganglionosis.

A de novo splice donor site mutation causes in-frame deletion of 14 amino acids in the proteolipid protein in Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a leukodystrophy associated with mutations in the proteolipid protein (PLP) gene. Jimpy is a mouse model of human PMD, and a splice site mutation in Jimpy causes the deletion of exon 5 from the PLP mRNA, producing a truncated form of PLP. We describe a de novo point mutation at the 5' splice donor site of exon 5 in a 17-year-old male with PMD, which results in the skipping of 42 base pairs of exon 5. The mutation removes only 14 amino acids in-frame of PLP. This is a novel splice donor site mutation in the human PLP gene. Moreover, the results indicate that the 14-amino acid deletion in the PLP is responsible for oligodendrocyte cell death and the development of PMD.