Identification of a 4 bp deletion (1560del4) in po gene in a family with severe Charcot-Marie-Tooth disease

Myelin protein zero: mutations in the cytoplasmic domain interfere with its cellular trafficking

The cytoplasmic domain of myelin protein zero (MPZ), the principal protein of peripheral myelin, undergoes phosphorylation on several serine residues and a tyrosine group that is maximal during peak nerve myelination. Mutations that could affect MPZ phosphorylation cause the inherited neuropathy, Charcot-Marie-Tooth disease Type 1B. To investigate a possible role for phosphorylation in regulation of MPZ trafficking within the cell, we expressed wild-type and mutated MPZ-enhanced green fluorescent protein (GFP) fusion proteins in cultured Schwann-like cells. Whereas wild-type protein is present almost entirely at the cell surface, mutation of serine 204 to alanine or at a nearby presumed PKC substrate motif (198RSTK201) causes 40-60% of protein to be retained in the cytoplasm. Mutation of S204 to aspartate, which introduces a permanent negative charge, also impairs MPZ movement to the plasma membrane. In contrast, tyrosine 191 mutation has no effect on MPZ cellular distribution. Simultaneous alteration of S204 and Y191 produces much less perturbation of MPZ trafficking than mutation of S204 alone. Colocalization studies showed that mutated MPZ-EGFP trapped in the cytoplasm associates with all organelles in the secretory pathway. Previous studies have shown that cytoplasmic mutations at serine, but not tyrosine phosphorylation sites, abolish MPZ adhesive properties. Our results suggest that this loss of adhesion may be due, at least in part, to a failure of sufficient MPZ to reach the cell surface and that this impaired trafficking is associated with deficient serine phosphorylation in the cytoplasmic domain.

Median nerve motor conduction velocity is concordant with myelin protein zero gene mutation

BACKGROUND

Myelin protein zero gene (MPZ) mutations may account for a small proportion of cases of Charcot-Marie-Tooth disease (CMT). Different MPZ mutations may be associated with different clinical and electrophysiological phenotypes.

OBJECTIVES

To expand our understanding of the characteristics of nerve conduction velocity (NCV) in patients with different MPZ mutations, the authors collected and analysed the NCV values from patients with MPZ mutations.

MATERIALS AND METHODS

The NCVs of fourteen patients from six families carrying MPZ mutations of Val58Asp, Ser63Phe, Thr65Ile,Arg98Cys, Arg98His, and Ser233fs were collected retrospectively. Five of them had received nerve conduction studies (NCS) twice. The mutations were verified by polymerase chain reaction (PCR) amplifications and nucleotide sequencing. Scatterplot analyses of median motor NCV (MNCV) versus specific MPZ mutation were performed.

RESULTS

The median MNCV varied widely, with a mean of 16.3 m/s (SD = 7.7 m/s) and a range of 5.1-32.9 m/s. Median MNCVs of patients with particular MPZ mutations were similar. Moreover, Median MNCV did not change significantly over time.

CONCLUSIONS

There was concordance between median MNCV and specific MPZ mutations. However, median MNCV is not an ideal measure with which to distinguish CMT1B patients with MPZ mutations from CMT1A patients with PMP22 mutations.

Does parkin play a role in the peripheral nervous system? A family report

Two genes were identified for autosomal recessive forms of early onset Parkinson's disease: parkin and DJ-1. We describe 2 siblings with EOPD due to parkin mutations and peripheral neuropathy, which presented as neuropathy with liability to pressure palsies (HNPP) in the index case. RT-PCR experiments revealed that the parkin gene is expressed in sural nerves from both controls and patient with parkin-related disease. Our findings support the view that parkin may play a role in the peripheral nervous system.

Myelin protein zero gene mutations in Taiwanese patients with Charcot-Marie-Tooth disease type 1

BACKGROUND

Charcot-Marie-Tooth disease type 1 (CMT1) is the most common inherited peripheral neuropathy and represents a genetically heterogeneous condition. In addition to the peripheral myelin protein 22 gene (PMP22) duplication (CMT1A), myelin protein zero gene (MPZ) mutations may account for a certain portion of CMT1 patients (CMT1B).

OBJECTIVES

The authors analyzed the MPZ mutations in Taiwanese patients who do not have PMP22 duplication. Specifically, their clinical and molecular features were characterized.

MATERIALS AND METHODS

Twenty-four of 57 unrelated Taiwanese patients with CMT1 were selected after excluding the CMT1A duplication. Subsequent analysis of the coding regions of the MPZ gene was performed with single-strand-conformation polymorphism (SSCP), which was then followed by nucleotide sequencing.

RESULTS

Four missense mutations and one 4-base pair (bp) deletion, respectively, were identified in five patients, of which one mutation, c.173 T>A, has never been previously reported. Three missense mutations were located in exon 2, the other one in exon 3, and the deletion in exon 6.

CONCLUSIONS

This study expands the number of CMT1 associated MPZ mutation and suggests that analysis of the coding sequence of MPZ should be performed in all CMT patients without CMT1A duplication to clarify their disease nature.

Charcot-Marie-Tooth neuropathy: clinical phenotypes of four novel mutations in the MPZ and Cx 32 genes

Charcot-Marie-Tooth Hereditary Neuropathy is a heterogeneous syndrome associated with mutations in several different genes including peripheral myelin protein 22, myelin P0, connexin 32, and early growth response 2. There is considerable variability in the phenotypic expression of this syndrome and the relationship of this variability to mutation genotypes requires extensive analysis. Here we describe the phenotypes and genotypes of four new mutations underlying the Charcot-Marie-Tooth syndrome and document segregation with disease. Four families with Charcot-Marie-Tooth were ascertained, examined, and evaluated electrophysiologically. Each family had peripheral blood DNA screened for mutations in myelin protein 22, myelin P0, and connexin 32. Two families were found with new mutations in the myelin P0 gene: S140T in the extracellular domain and K236del in the cytoplasmic domain. All families showed segregation of the mutations with the Charcot-Marie-Tooth phenotype as did a new family with the rare G163R mutation in the membrane domain. A 49-year-old man with the S140T mutation demonstrated conduction block on electrophysiological testing. A family with a novel S49P mutation in the connexin 32 gene had a neuropathy with very slow nerve conduction. These new mutations in the myelin P0 and connexin 32 genes help to clarify the pathophysiology of the clinical Charcot-Marie-Tooth syndrome. The S140T mutation in myelin P0 can be associated with conduction block and Charcot-Marie-Tooth should be part of the differential diagnosis of that phenomenon. Mutations in the cytoplasmic domain of myelin P0 can cause clinical neuropathy. The S49P mutation in the connexin 32 gene can produce aspects of a demyelinating type of X-linked hereditary neuropathy.

Phenotypic variation of a novel nonsense mutation in the P0 intracellular domain

Mutations in the gene for the peripheral myelin protein zero (P0, MPZ) cause type 1B of Charcot-Marie-Tooth sensorimotor neuropathy (CMT1B). Here we report a German family with a novel heterozygous P0 nonsense mutation (G206X) that supposedly removes four-fifths of the amino acid residues constituting the P0 intracellular domain. The 12-year-old propositus had childhood-onset CMT1B associated with bilateral pes cavus, moderate lower limb weakness, and mildly reduced sensory qualities in the distal legs. The electrophysiology was consistent with a demyelinating neuropathy. He inherited the mutation from his mother who had no complaints but slight pes cavus deformity and slow nerve conduction velocities (NCV). Conclusively, truncating mutations within the P0 intracellular domain do not necessarily cause a severe phenotype such as Dejerine-Sottas syndrome (DSS) or congenital hypomyelinating neuropathy (CHN), but can result in mild or moderate CMT1B with intrafamilial clinical variability.

A molecular basis for hereditary motor and sensory neuropathy disorders

Charcot-Marie-Tooth disease (CMT), or inherited peripheral neuropathies, is one of the most frequent genetically inherited neurologic disorders, with a prevalence of approximately one in 2500 people. CMT is usually inherited in an autosomal dominant fashion, although X-linked and recessive forms of CMT also exist. Over the past several years, considerable progress has been made toward understanding the genetic causes of many of the most frequent forms of CMT, particularly those caused by mutations in Schwann cell genes inducing the demyelinating forms of CMT, also known as CMT1. Because the genetic cause of these disorders is known, it is now possible to study how mutations in genes encoding myelin proteins cause neuropathy. Identifying these mechanisms will be important both for understanding demyelination and for developing future treatments for CMT.

Steroid responsive polyneuropathy in a family with a novel myelin protein zero mutation

OBJECTIVE

To report a novel hereditary motor and sensory neuropathy (HMSN) phenotype, with partial steroid responsiveness, caused by a novel dominant mutation in the myelin protein zero (MPZ) gene. Most MPZ mutations lead to the HMSN type I phenotype, with recent reports of Déjérine-Sottas, congenital hypomyelination, and HMSN II also ascribed to MPZ mutations. Differing phenotypes may reflect the effect of particular mutations on MPZ structure and adhesivity.

METHODS

Clinical, neurophysiological, neuropathological, and molecular genetic analysis of a family presenting with an unusual hereditary neuropathy.

RESULTS

Progressive disabling weakness, with positive sensory phenomena and areflexia, occurred in the proband with raised CSF protein and initial steroid responsiveness. Nerve biopsy in a less severely affected sibling disclosed a demyelinating process with disruption of compacted myelin. The younger generation were so far less severely affected, becoming symptomatic only after 30 years. All affected family members were heterozygous for a novel MPZ mutation (Ile99Thr), in a conserved residue.

CONCLUSIONS

This broadens the range of familial neuropathy associated with MPZ mutations to include steroid responsive neuropathy, initially diagnosed as chronic inflammatory demyelinating polyneuropathy.

Mutations in the peripheral myelin genes and associated genes in inherited peripheral neuropathies

The peripheral myelin protein 22 gene (PMP22), the myelin protein zero gene (MPZ, P0), and the connexin 32 gene (Cx32, GJB1) code for membrane proteins expressed in Schwann cells of the peripheral nervous system (PNS). The early growth response 2 gene (EGR2) encodes a transcription factor that may control myelination in the PNS. Mutations in the respective genes, located on human chromosomes 17p11.2, 1q22-q23, Xq13.1, and 10q21.1-q22.1, are associated with several inherited peripheral neuropathies. To date, a genetic defect in one of these genes has been identified in over 1,000 unrelated patients manifesting a wide range of phenotypes, i.e., Charcot-Marie-Tooth disease type 1 (CMT1) and type 2 (CMT2), Dejerine-Sottas syndrome (DSS), hereditary neuropathy with liability to pressure palsies (HNPP), and congenital hypomyelination (CH). This large number of genetically defined patients provides an exceptional opportunity to examine the correlation between phenotype and genotype.

Clinical phenotypes of different MPZ (P0) mutations may include Charcot-Marie-Tooth type 1B, Dejerine-Sottas, and congenital hypomyelination

Hereditary demyelinating peripheral neuropathies consist of a heterogeneous group of genetic disorders that includes hereditary neuropathy with liability to pressure palsies (HNPP), Charcot-Marie-Tooth disease (CMT), Dejerine-Sottas syndrome (DSS), and congenital hypomyelination (CH). The clinical classification of these neuropathies into discrete categories can sometimes be difficult because there can be both clinical and pathologic variation and overlap between these disorders. We have identified five novel mutations in the myelin protein zero (MPZ) gene, encoding the major structural protein (P0) of peripheral nerve myelin, in patients with either CMT1B, DSS, or CH. This finding suggests that these disorders may not be distinct pathophysiologic entities, but rather represent a spectrum of related "myelinopathies" due to an underlying defect in myelination. Furthermore, we hypothesize the differences in clinical severity seen with mutations in MPZ are related to the type of mutation and its subsequent effect on protein function (i.e., loss of function versus dominant negative).