Clinically distinct codon 69 mutations in major myelin protein zero in demyelinating neuropathies

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

The spectrum of Charcot-Marie-Tooth disease due to myelin protein zero: An electrodiagnostic, nerve ultrasound and histological study

OBJECTIVE

Nerve ultrasound (US) data on myelin protein zero (MPZ)-related Charcot-Marie-Tooth disease (CMT) are lacking. To offer a comprehensive perspective on MPZ-related CMTs, we combined nerve US with clinics, electrodiagnosis and histopathology.

METHODS

We recruited 36 patients (12 MPZ mutations), and correlated nerve US to clinical, electrodiagnostic measures, and sural nerve biopsy.

RESULTS

According to motor nerve conduction velocity (MNCV) criteria, nine patients were categorized as "demyelinating" CMT1B, 17 as "axonal" CMT2I/J, and 10 as dominant "intermediate" CMTDID. Sural nerve biopsy showed hypertrophic de-remyelinating neuropathy with numerous complex onion bulbs in one patient, de-remyelinating neuropathy with scanty/absent onion bulbs in three, axonal neuropathy in two, mixed demyelinating-axonal neuropathy in five. Electrodiagnosis significantly differed in CMT1B vs. CMT2I/J and CMTDID subgroups. CMT1B had slightly enlarged nerve cross sectional area (CSA) especially at proximal upper-limb (UL) sites. CSA was negatively correlated to UL MNCV and not increased at entrapment sites. Major sural nerve pathological patterns were uncorrelated to UL nerve US and MNCV.

CONCLUSIONS

Sural nerve biopsy confirmed the wide pathological spectrum of MPZ-CMT. UL nerve US identified two major patterns corresponding to the CMT1B and CMT2I/J-CMTDID subgroups.

SIGNIFICANCE

Nerve US phenotype of MPZ-CMT diverged from those in other demyelinating peripheral neuropathies and may have diagnostic value.

Genotype-phenotype characteristics and baseline natural history of heritable neuropathies caused by mutations in the MPZ gene

We aimed to characterize genotype-phenotype correlations and establish baseline clinical data for peripheral neuropathies caused by mutations in the myelin protein zero (MPZ) gene. MPZ mutations are the second leading cause of Charcot-Marie-Tooth disease type 1. Recent research makes clinical trials for patients with MPZ mutations a realistic possibility. However, the clinical severity varies with different mutations and natural history data on progression is sparse. We present cross-sectional data to begin to define the phenotypic spectrum and clinical baseline of patients with these mutations. A cohort of patients with MPZ gene mutations was identified in 13 centres of the Inherited Neuropathies Consortium - Rare Disease Clinical Research Consortium (INC-RDCRC) between 2009 and 2012 and at Wayne State University between 1996 and 2009. Patient phenotypes were quantified by the Charcot-Marie-Tooth disease neuropathy score version 1 or 2 and the Charcot-Marie-Tooth disease paediatric scale outcome instruments. Genetic testing was performed in all patients and/or in first- or second-degree relatives to document mutation in MPZ gene indicating diagnosis of Charcot-Marie-Tooth disease type 1B. There were 103 patients from 71 families with 47 different MPZ mutations with a mean age of 40 years (range 3-84 years). Patients and mutations were separated into infantile, childhood and adult-onset groups. The infantile onset group had higher Charcot-Marie-Tooth disease neuropathy score version 1 or 2 and slower nerve conductions than the other groups, and severity increased with age. Twenty-three patients had no family history of Charcot-Marie-Tooth disease. Sixty-one patients wore foot/ankle orthoses, 19 required walking assistance or support, and 10 required wheelchairs. There was hearing loss in 21 and scoliosis in 17. Forty-two patients did not begin walking until after 15 months of age. Half of the infantile onset patients then required ambulation aids or wheelchairs for ambulation. Our results demonstrate that virtually all MPZ mutations are associated with specific phenotypes. Early onset (infantile and childhood) phenotypes likely represent developmentally impaired myelination, whereas the adult-onset phenotype reflects axonal degeneration without antecedent demyelination. Data from this cohort of patients will provide the baseline data necessary for clinical trials of patients with Charcot-Marie-Tooth disease caused by MPZ gene mutations.

Nerve biopsy: requirements for diagnosis and clinical value

In many instances, nerve biopsy is not necessary in the diagnostic work-up of a peripheral neuropathy. However, histological examination of a tissue sample is still mandatory to show specific lesions in various conditions involving peripheral nerves. As there are fewer laboratories that examine human nerve samples, practitioners including neurologists and general pathologists may not be completely aware of the technical issues and data that are provided by nerve biopsy. Nerve biopsy is considered an invasive diagnostic method, although, its complications are by far less disabling than most of the disorders that lead to its indications. Nevertheless, the decision to perform a nerve biopsy has to be made on a case-by-case basis, and its results must be discussed between the pathologist and the clinician who is in charge of the patient's care. In this paper, we review the minimal technical requirements for proper peripheral nerve tissue analysis. Moreover, we provide data on the usefulness of nerve biopsy in various situations including abnormal deposits, cell infiltrates, link between peripheral neuropathy and monoclonal gammopathy, and numerous hereditary disorders.

Contribution of electron microscopy to the study of neuropathies associated with an IgG monoclonal paraproteinemia

A typical monoclonal IgG dysglobulinemia whether benign (monoclonal gammopathy of undetermined significance, MGUS) or malignant can give rise to peripheral neuropathy by damaging nerves. At first, neurotoxicity of the chemotherapy if the patient is treated must be ruled out in such cases. Indeed, a variety of other mechanisms have been described: endoneurial deposits of immunoglobulin, infiltration of the immunoglobulin within myelin sheaths, POEMS syndrome, deposits of amyloid, chronic inflammatory demyelinating polyradiculoneuropathy and infiltration of malignant cells. Ultrastructural examination of a nerve biopsy can be decisive in combination with routine histological and immunopathological examinations. Characterization of the mechanism of the neuropathy in a dysglobulinemic context is important as it governs therapeutic options, which in certain cases are particularly beneficial.

Diagnostic value of ultrastructural nerve examination in Charcot-Marie-Tooth disease: two CMT 1B cases with pseudo-recessive inheritance

We report two sporadic patients of CMT disease in different consanguineous families. The electrophysiological examination led to the diagnosis of a severe demyelinating neuropathy. The nerve biopsies exhibited numerous outfoldings of the myelin sheaths and onion-bulb proliferations. The consanguinity and the histological findings pointed to a diagnosis of CMT 4B. However, the detection of abnormal and regular widenings between the major dense lines of the myelin lamellae by electron microscopy led us to search for a P0 gene mutation. Two heterozygous mutations of this gene were identified: S63F and N131Y. Different aspects of uncompacted myelin lamellae have been described in some cases of P0 mutations and a few now appear to be quite specific to it. More than 30 genes are implicated in CMT and as mutation search is time- and money-consuming, we believe that in some selected patients ultrastructural examination of nerves, among other criteria, helps orientate the molecular diagnosis of CMT.

Major myelin protein gene (P0) mutation causes a novel form of axonal degeneration

Mutations in the major peripheral nervous system (PNS) myelin protein, myelin protein zero (MPZ), cause Charcot-Marie-Tooth Disease type 1B (CMT1B), typically thought of as a demyelinating peripheral neuropathy. Certain MPZ mutations, however, cause adult onset neuropathy with minimal demyelination but pronounced axonal degeneration. Mechanism(s) for this phenotype are unknown. We performed an autopsy of a 73-year-old woman with a late-onset neuropathy caused by an H10P MPZ mutation whose nerve conduction studies suggested severe axonal loss but no demyelination. The autopsy demonstrated axonal loss and reorganization of the molecular architecture of the axolemma. Segmental demyelination was negligible. In addition, we identified focal nerve enlargements containing MPZ and ubiquitin either in the inner myelin intralaminar and/or periaxonal space that separates axons from myelinating Schwann cells. Taken together, these data confirmed that a mutation in MPZ can cause axonal neuropathy, in the absence of segmental demyelination, thus uncoupling the two pathological processes. More important, it also provided potential molecular mechanisms as to how the axonal degeneration occurred: either by disruption of glial-axon interaction by protein aggregates or by alterations in the molecular architecture of internodes and paranodes. This report represents the first study in which the molecular basis of axonal degeneration in the late-onset CMT1B has been explored in human tissue.

Peripheral neuropathies caused by mutations in the myelin protein zero

Charcot-Marie-Tooth disease type 1B (CMT1B) is caused by mutations in the major PNS myelin protein myelin protein zero (MPZ). MPZ is a member of the immunoglobulin supergene family and functions as an adhesion molecule helping to mediate compaction of PNS myelin. Mutations in MPZ appear to either disrupt myelination during development, leading to severe early onset neuropathies, or to disrupt axo-glial interactions leading to late onset neuropathies in adulthood. Identifying molecular pathways involved in early and late onset CMT1B will be crucial to understand how MPZ mutations cause CMT1B so that rational therapies for both early and late onset neuropathies can be developed.

Dejerine-Sottas syndrome grown to maturity: overview of genetic and morphological heterogeneity and follow-up of 25 patients

Dejerine-Sottas syndrome (DSS) is an early onset demyelinating motor and sensory neuropathy with motor nerve conduction velocities below 12 m s(-1). The phenotype is genetically heterogeneous, and autosomal dominant (AD) as well as autosomal recessive (AR) inheritance is described. Nerve pathology is highly variable. It is generally presumed that clinical course is severe, leading to wheelchair dependency at an early age. In this study we documented the clinical and pathological features in 25 patients with a DSS and we evaluated the clinical course. In our series 14 patients had an AD mutation and six were probably affected by an AR disorder. In three patients inheritance mode was unknown and two patients obviously suffered from an acquired disorder. The clinical course in all patients was documented. Nine of the 25 patients showed a moderate handicap in adult life; walking distance was still at least 1 km. Age at last investigation of the ambulant patients ranged from 22 to 62 years (mean 38.6 years), and ambulant patients were found in all genetic subgroups. We conclude that DSS, although in general denoting a more serious neuropathy than CMT1, does not imply a severe disability or wheelchair dependency in adult life.

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.

Myelinated fibers in Charcot-Marie-Tooth disease type 1B with Arg98His mutation of Po protein

This study was undertaken to characterize the clinical, electrophysiologic, and histopathologic features of five presumably unrelated Japanese patients with Charcot-Marie-Tooth (CMT) disease type 1B and Arg98His substitution of Po protein and, in particular, to correlate Arg98His substitution to the ultrastructural abnormalities of the myelin sheath. Systematic morphometric studies of the sural nerve, where the CMT type 1B gene abnormality is expressed, have not been performed, especially on the basis of the type of mutation causing CMT type 1B. Electrophysiologic evaluation of limb nerves and morphometric analysis of sural nerves obtained at biopsy were performed. Ultrastructural myelin abnormalities were precisely examined. Clinical symptoms appeared from the second to the fifth decade. All probands presented with gait disturbance. Motor and sensory conduction velocities in the median and ulnar nerves ranged from 10 to 30 m/s. Segmental demyelination and remyelination and marked loss of myelinated fibers were the main findings. On electron microscopy, widening between major dense lines was found between the paired intraperiod lines, where the extramembranous portion of the Po protein resides. This widening is probably directly related to Arg98His substitution. Focal uncompaction of major dense lines coexisted with this widening. This uncompaction, which directly decreases the number of myelin lamellae, may be a secondary effect of Arg98His substitution on the intramembranous domain of Po protein. In conclusion, myelin changes at both extracellular and cytoplasmic appositions of Schwann cell membranes were found in association with Arg98His substitution of Po protein. This study contributes to a better understanding of myelin abnormalities in patients with CMT type 1B and Arg98His or other similar extramembranous amino acid substitutions of Po protein.

Charcot-marie-tooth disease and related neuropathies: molecular basis for distinction and diagnosis

Great advances have been made in understanding the molecular basis of Charcot-Marie-Tooth disease (CMT) and related neuropathies, namely Dejerine-Sottas disease (DSD), hereditary neuropathy with liability to pressure palsies (HNPP) and congenital hypomyelination (CH). The number of newly uncovered mutations and identified genetic loci is rapidly increasing, and, as a consequence, the classification of these disorders is becoming more complicated. Molecular genetics, animal models, and transfected cell studies are shedding light on function and dysfunction of proteins involved in hereditary myelinopathies-peripheral myelin protein 22 (PMP22), myelin protein zero (PO), connexin 32 (Cx32), and early growth response 2 (EGR2). Gene dosage effect, loss of function, gain of toxic function, and dominant negative effect are possible mechanisms whereby different gene mutations may exert their detrimental action on peripheral nerves. A tentative rational approach to clinical and molecular diagnosis based on genotype-phenotype correlation analysis is described.

Myelin uncompaction in Charcot-Marie-Tooth neuropathy type 1A with a point mutation of peripheral myelin protein-22

BACKGROUND

The peripheral myelin protein-22 (PMP22) gene has four transmembrane domains, two extracellular loops, and a short cytoplasmic tail. Its roles in the peripheral nervous system remain unclear. The most common cause of Charcot-Marie-Tooth neuropathy type 1A (CMT1A) is a PMP22 gene duplication. Missense point mutations in the transmembrane domains are rare alternative causes that have undetermined pathogenetic mechanisms.

OBJECTIVE

To investigate the phenotype-to-genotype correlations in a pedigree with unusual CMT1A.

METHODS

We identified a pedigree with an autosomal dominant motor-sensory neuropathy and severely reduced nerve conduction velocities who did not have the PMP22 duplication. Specimens from sural nerve biopsies from two patients of different ages were evaluated morphometrically. By automated direct nucleotide sequencing we analyzed PMP22 and the gene of the major structural myelin protein zero (P0).

RESULTS

Nucleotide 159 of PMP22 showed an A-to-T heterozygous mutation, predicted to cause an aspartate-to-valine substitution at codon 37 in the first extracellular loop of the protein. The mutation co-segregated with the disease in the pedigree and was absent in 80 healthy controls. The histopathologic phenotype was a de-remyelinating neuropathy with onion bulb formations, characterized by prominent uncompaction of the myelin sheath in the majority of fibers and by frequent tomacula.

CONCLUSION

We have described a novel mutation in the first extracellular loop of PMP22 associated with an atypical CMT1A that overlaps pathologically with CMT1B caused by point mutations in the extracellular domain of P0.

Novel mutation in the myelin protein zero gene in a family with intermediate hereditary motor and sensory neuropathy

OBJECTIVES

To determine the molecular basis for autosomal dominant intermediate hereditary motor and sensory neuropathy (HMSN) in a four generation family. The gene defects in families with intermediate HMSN are not known, but it has been suggested that most have X linked HMSN.

METHODS

All participating family members were examined clinically. Genomic DNA was obtained from 10 affected and seven unaffected members. Linkage analysis for the known HMSN loci was first performed. Mutations in the peripheral myelin protein zero gene (PMP0) were sought in two affected members, using one unaffected member for comparison, by amplification of the six exons of the gene followed by single strand conformation polymorphism (SSCP) analysis, dideoxy fingerprinting (ddF), and sequencing. Subsequently, the mutation was screened for in all affected and unaffected members in the family using Alu I digestion and in 100 unrelated control subjects using "snap back" SSCP analysis. Sequencing of cDNA from a sural nerve biopsy from an affected member was also performed.

RESULTS

The clinical phenotype was of variable severity, with motor nerve conduction velocities in the intermediate range. Linkage to PMP0 was demonstrated. Analysis of genomic DNA and cDNA for PMP0 identified a novel codon 35 GAC to TAC mutation. The mutation produces an inferred amino acid change of aspartate to tyrosine at codon six of the processed protein (Asp6Tyr) in the extracellular domain and was present in all affected family members but not in 100 unrelated controls.

CONCLUSIONS

The present findings further extend the range of phenotypes associated with PMP0 mutations and indicate that families with "intermediate" HMSN need not necessarily be X-linked as previously suggested.

Congenital hypomyelinating neuropathy: two patients with long-term follow-up

The authors report the long-term prospective follow-up of two unrelated females with congenital hypomyelinating neuropathy (CHN) and review previously reported cases. The authors' first patient presented with neonatal hypotonia and extremely slow nerve conduction velocities. Sural nerve biopsy revealed profound hypomyelination, without inflammation or evidence of myelin breakdown. She is now 9 years of age, and her motor function has continued to improve. Follow-up nerve-conduction velocities are unchanged. The authors' second patient presented at 5 months with hypotonia. Nerve-conduction velocities were extremely slow, and sural nerve biopsy revealed severe hypomyelination, with no inflammation or evidence of myelin breakdown. She is now 5 years of age and has also demonstrated improved motor function. Repeated nerve-conduction velocities are unchanged. Both patients have normal cognitive development. Molecular genetic analysis in Patient 2 disclosed a point mutation in the myelin protein zero gene; this same point mutation has been reported in three other patients diagnosed with Dejerine-Sottas syndrome (DSS) but has never been reported in a patient with CHN. Although CHN is a distinct clinical entity, it may share similar genetic features with DSS.

Ultrastructural protein zero expression in Charcot-Marie-Tooth type 1B disease

Charcot-Marie-Tooth type 1B (CMT 1B) disease, an inherited demyelinating peripheral neuropathy, results from different point mutations located in the P0 gene on chromosome 1 q21-23. We have quantified, at the ultrastructural level, the immunocytochemical expression of the P0 protein in two unrelated CMT 1B patients with mutations (Ser 78 to Leu and Asn 122 to Ser) located in two different exons in the extracellular domain of the protein. A twofold decrease in P0 expression was observed in compact myelin in each case, compared with age-matched controls. The severity of the phenotypes showed no direct relationship to the levels of P0 protein expression in these 2 patients.