Identification of a de novo insertional mutation in P0 in a patient with a Déjérine-Sottas syndrome (DSS) phenotype

Myelin protein zero mutation-related hereditary neuropathies: Neuropathological insight from a new nerve biopsy cohort

Myelin protein zero (MPZ/P0) is a major structural protein of peripheral nerve myelin. Disease-associated variants in the MPZ gene cause a wide phenotypic spectrum of inherited peripheral neuropathies. Previous nerve biopsy studies showed evidence for subtype-specific morphological features. Here, we aimed at enhancing the understanding of these subtype-specific features and pathophysiological aspects of MPZ neuropathies. We examined archival material from two Central European centers and systematically determined genetic, clinical, and neuropathological features of 21 patients with MPZ mutations compared to 16 controls. Cases were grouped based on nerve conduction data into congenital hypomyelinating neuropathy (CHN; n = 2), demyelinating Charcot-Marie-Tooth (CMT type 1; n = 11), intermediate (CMTi; n = 3), and axonal CMT (type 2; n = 5). Six cases had combined muscle and nerve biopsies and one underwent autopsy. We detected four MPZ gene variants not previously described in patients with neuropathy. Light and electron microscopy of nerve biopsies confirmed fewer myelinated fibers, more onion bulbs and reduced regeneration in demyelinating CMT1 compared to CMT2/CMTi. In addition, we observed significantly more denervated Schwann cells, more collagen pockets, fewer unmyelinated axons per Schwann cell unit and a higher density of Schwann cell nuclei in CMT1 compared to CMT2/CMTi. CHN was characterized by basal lamina onion bulb formation, a further increase in Schwann cell density and hypomyelination. Most late onset axonal neuropathy patients showed microangiopathy. In the autopsy case, we observed prominent neuromatous hyperinnervation of the spinal meninges. In four of the six muscle biopsies, we found marked structural mitochondrial abnormalities. These results show that MPZ alterations not only affect myelinated nerve fibers, leading to either primarily demyelinating or axonal changes, but also affect non-myelinated nerve fibers. The autopsy case offers insight into spinal nerve root pathology in MPZ neuropathy. Finally, our data suggest a peculiar association of MPZ mutations with mitochondrial alterations in muscle.

A dual tyrosine-leucine motif mediates myelin protein P0 targeting in MDCK cells

Differential targeting of myelin proteins to multiple, biochemically and functionally distinct Schwann cell plasma membrane domains is essential for myelin formation. In this study, we investigated whether the myelin protein P0 contains targeting signals using Madin-Darby canine kidney (MDCK) cells. By confocal microscopy, P0 was localized to MDCK cell basolateral membranes. C-terminal deletion resulted in apical accumulation, and stepwise deletions defined a 15-mer region that was required for basolateral targeting. Alanine substitutions within this region identified the YAML sequence as a functional tyrosine-based targeting signal, with the ML sequence serving as a secondary leucine-based signal. Replacement of the P0 ectodomain with green fluorescent protein altered the distribution of constructs lacking the YAML signal. Coexpression of the myelin-associated glycoprotein did not alter P0 distribution in MDCK cells. The results indicate that P0 contains a hierarchy of targeting signals, which may contribute to P0 localization in myelinating Schwann cells and the pathogenesis in human disease.

Curcumin treatment abrogates endoplasmic reticulum retention and aggregation-induced apoptosis associated with neuropathy-causing myelin protein zero-truncating mutants

Mutations in MPZ, the gene encoding myelin protein zero (MPZ), the major protein constituent of peripheral myelin, can cause the adult-onset, inherited neuropathy Charcot-Marie-Tooth disease, as well as the more severe, childhood-onset Dejerine-Sottas neuropathy and congenital hypomyelinating neuropathy. Most MPZ-truncating mutations associated with severe forms of peripheral neuropathy result in premature termination codons within the terminal or penultimate exons that are not subject to nonsense-mediated decay and are stably translated into mutant proteins with potential dominant-negative activity. However, some truncating mutations at the 3' end of MPZ escape the nonsense-mediated decay pathway and cause a mild peripheral neuropathy phenotype. We examined the functional properties of MPZ-truncating proteins that escaped nonsense-mediated decay, and we found that frameshift mutations associated with severe disease cause an intracellular accumulation of mutant proteins, primarily within the endoplasmic reticulum (ER), which induces apoptosis. Curcumin, a chemical compound derived from the curry spice tumeric, releases the ER-retained MPZ mutants into the cytoplasm accompanied by a lower number of apoptotic cells. Our findings suggest that curcumin treatment is sufficient to relieve the toxic effect of mutant aggregation-induced apoptosis and may potentially have a therapeutic role in treating selected forms of inherited peripheral neuropathies.

Clinical and electrophysiological characteristics of autosomal recessive axonal Charcot-Marie-Tooth disease (ARCMT2B) that maps to chromosome 19q13.3

Charcot-Marie-Tooth disease (CMT) comprises a heterogeneous group of hereditary motor and sensory peripheral neuropathies. The autosomal recessive axonal form of CMT (ARCMT2) is rare. Eight patients of a large consanguineous family of Spanish ancestry in Costa Rica were diagnosed with ARCMT2B; previous genetic studies of this family revealed linkage to chromosome 19q13.3. The clinical and electrophysiological features of these patients are reported. All patients presented with a symmetric motor and sensory neuropathy, which was more pronounced in the lower limbs. Further, distal muscle wasting and impaired deep tendon reflexes were found. Age at onset was between 26 and 42 years, and the disease duration ranged from 2 to 19 years. Electrophysiological studies revealed a primary axonal degenerative process. The clinical characteristics of this family differed in several aspects from previously reported families with ARCMT2.

Chapter 24 Genetic evaluation of inherited motor/sensory neuropathy

Inherited disorders of peripheral nerves represent a common group of neurologic diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is a genetically heterogeneous group of chronic demyelinating polyneuropathies with loci mapping to chromosome 17 (CMT1A), chromosome 1 (CMT1B), chromosome 16 (CMT1C) and chromosome 10 (CMT1D). CMT1A is most often associated with a tandem 1.5-megabase (Mb) duplication in chromosome 17p11.2-p12. In rare patients it may result from a point mutation in the peripheral myelin protein-22 (PMP22) gene. CMT1B is associated with point mutations in the myelin protein zero (Po or MPZ) gene. Mutations in the SIMPLE gene cause CMT1C, and CMT1D is the result of mutations in the early response 2 (ERG2 or Krox-20) gene. An X-linked form of CMT1 (CMT1X) maps to Xq13 and is associated with mutations in the connexin32 (Cx32) gene. Charcot-Marie-Tooth neuropathy type 2 (CMT2) is an axonal neuropathy that maps to chromosome 1p35-p36 (CMT2A), chromosome 3q13-q22 (CMT2B), chromosome 7p14 (CMT2D), chromosome 8p21 (CMT2E), chromosome 1q22-q23 (CMT2F) or chromosome 3q13 (CMT2G). Two X-linked forms of CMT2 have been reported (CMT2XA and CMT2XB), but the genes remain unidentified. An area that has recently expanded is the identification of autosomal recessive forms of CMT type 1 and 2. Of the eight recessive forms of CMT1 that have been identified to date, only two have been fully characterized at the molecular level (CMT1 AR B 1 and CMT1 AR D). Point mutations were found in the myotubularin-related protein-2 (MTM2) gene for CMT1 AR B1. CMT1 AR D is the result of point mutations in the N-myc downstream-regulated gene 1 (NDRG1). Dejerine-Sottas disease (DSD), also called hereditary motor and sensory neuropathy type III (HMSNIII), is a severe, infantile-onset demyelinating polyneuropathy syndrome that may be associated with point mutations in either the PMP22 gene, PO gene, EGR2 gene or the PRX gene (for the recessive form). It shares considerable clinical and pathological features with CMT1. Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder that results in a recurrent, episodic demyelinating neuropathy. HNPP is associated with a 1.5-Mb deletion in chromosome 17p11.2-p12 that results in reduced expression of the PMP22 gene. CMT1A and HNPP are reciprocal duplication/deletion syndromes that originate from unequal crossover during germ cell meiosis. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q and 11q.

Tracing myelin protein zero (P0) in vivo by construction of P0-GFP fusion proteins

BACKGROUND

Mutations in P0, the major protein of the myelin sheath in peripheral nerves, cause the inherited peripheral neuropathies Charcot-Marie-Tooth disease type 1B (CMT1B), Dejerine-Sottas syndrome (DSS) and congenital hypomyelination (CH). We reported earlier a de novo insertional mutation c.662_663GC (Ala221fs) in a DSS patient. The c.662_663GC insertion results in a frame shift mutation Ala221fs altering the C-terminal amino acid sequence. The adhesion-relevant intracellular RSTK domain is replaced by a sequence similar to Na+/K+ ATPase. To further clarify the molecular disease mechanisms in this sporadic patient we constructed wild type P0 and the c.662_663GC mutant expression cassettes by site-specific mutagenesis and transfected the constructs into insect cells (S2, High5). To trace the effects in live cells, green fluorescent protein (GFP) has been added to the carboxyterminus of the wild type and mutated P0 protein.

RESULTS

In contrast to the membrane-localized wild type P0-GFP the Ala221fs P0-GFP protein was detectable almost only in the cytoplasm of the cells, and a complete loss of adhesion function was observed.

CONCLUSIONS

The present study provides evidence that GFP is a versatile tool to trace in vivo effects of P0 and its mutations. Not only a loss of adhesion function as a result of the loss of the RSTK domain, but also altered intracellular trafficking indicated by a loss of membrane insertion are possible consequences of the Ala221fs mutation.

Dejerine-Sottas disease and hereditary demyelinating polyneuropathy of infancy

Dejerine-Sottas disease (DSD) was originally described as a hypertrophic polyneuropathy characterized by onset in infancy or early childhood in patients born to unaffected parents. The clinical features included distal sensory changes with ataxia; pes cavus, at times with kyphoscoliosis; motor deficit and atrophy predominating in the distal lower limbs and progressing toward the proximal limbs following a length-dependent pattern; palpable nerve hypertrophy; and Argyll-Robertson pupils. The morphological hallmark was the extensive nerve and root hypertrophy associated with demyelination-remyelination of surviving, originally myelinated axons and profuse Schwann-cell proliferation forming onion bulbs. Wide variations in clinical manifestations of chronic demyelinating polyneuropathies of early onset in children born to unaffected parents have now been reported, with only some of the characteristics required in the original study, and at least seven genes encoding the myelin proteins P0, PMP22, the transcriptional factor EGR2, and others have been implicated. Thus, DSD is now a component of the hereditary demyelinating polyneuropathies of infancy that also include subsets of the recently individualized CMT4 neuropathies. The presumed recessive transmission of patients with DSD should be confirmed by molecular genetic analysis, which is still negative in a significant proportion of patients. The nerve biopsy can be useful in patients in whom genealogical or DNA abnormalities in favor of a genetic disorder are missing, because in a few patients with a progressive or relapsing course the diagnosis of early-onset chronic inflammatory demyelinating polyneuropathy must be considered.

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.

Progress in clinical neurosciences: Charcot-Marie-Tooth disease and related inherited peripheral neuropathies

The classification of Charcot-Marie-Tooth disease and related hereditary motor and sensory neuropathies has evolved to incorporate clinical, electrophysiological and burgeoning molecular genetic information that characterize the many disorders. For several inherited neuropathies, the gene product abnormality is known and for others, candidate genes have been identified. Genetic testing can pinpoint a specific inherited neuropathy for many patients. However, clinical and electrophysiological assessments continue to be essential tools for diagnosis and management of this disease group. This article reviews clinical, electrophysiological, pathological and molecular aspects of hereditary motor and sensory neuropathies.

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.

Clinical syndromes associated with tomacula or myelin swellings in sural nerve biopsies

OBJECTIVES

To describe the neuropathological features of clinical syndromes associated with tomacula or focal myelin swellings in sural nerve biospies and to discuss possible common aetiopathological pathways leading to their formation in this group of neuropathies.

METHODS

Fifty two patients with sural nerve biopsies reported to show tomacula or focal myelin swellings were reviewed, light and electron microscopy were performed, and tomacula were analysed on teased fibre studies. Molecular genetic studies were performed on those patients who were available for genetic testing.

RESULTS

Thirty seven patients were diagnosed with hereditary neuropathy with liability to pressure palsies (HNPP), four with hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT1), four with HMSN with myelin outfolding (CMT4B), three with IgM paraproteinemic neuropathy, three with chronic inflammatory demyelinating polyneuropathy (CIDP), and one with HMSN III (CMT3).

CONCLUSIONS

Most of these syndromes were shown to be related to genetic or immunological defects of myelin components such as peripheral myelin protein 22 (PMP22), myelin protein zero (P0), or myelin associated glycoprotein (MAG). These proteins share the HNK-1 epitope which has been implicated in cell adhesion processes. Impaired myelin maintenance may therefore contribute to the formation of tomacula and subsequent demyelination.

Tyrosine phosphorylation of myelin P(0) and its implication in signal transduction

P(0), a major structural protein of peripheral myelin, belongs to the immunoglobulin superfamily. Sequence comparison of P(0) with PZR, a tyrosine phosphatase SHP-2 binding protein we recently cloned, revealed the presence of an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the intracellular portion of the P(0) molecule. To study the role of this putative ITIM in signal transduction, we have expressed P(0) in HT-1080 and 293 cells. Stimulation of the transfected cells with pervanadate, a powerful inhibitor of tyrosine phosphatases, resulted in tyrosine phosphorylation of P(0) and its association with several tyrosine-phosphorylated proteins. Mutation of Y(220) embedded in the ITIM to phenylalanine abolished the tyrosine phosphorylation and the association. Tyrosine phosphorylation of P(0) and its association with other signaling proteins were also observed in pervanadate-treated RN22 Schwannoma cells, which express endogenous P(0). Furthermore, injection of pervanadate induced tyrosine phosphorylation of P(0) in peripheral nerves of newborn but not adult mice. The physiological importance of the ITIM in P(0) is implied by the fact that a naturally occurred P(0) mutant with a disrupted ITIM has a dominant role in causing Dejerine-Scotts syndrome. Taken together, P(0) is phosphorylated on Try(220). The presence of an ITIM in P(0) and its ability to mediate protein-protein interaction through tyrosine phosphorylation indicate that P(0) is not merely a structural protein but may also be a crucial player in cell signaling.

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.

Inherited neuropathies: from gene to disease

Inherited disorders of peripheral nerves represent a common group of neurologic diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is a genetically heterogeneous group of chronic demyelinating polyneuropathies with loci mapping to chromosome 17 (CMT1A), chromosome 1 (CMT1B) and to another unknown autosome (CMT1C). CMT1A is most often associated with a tandem 1.5-megabase (Mb) duplication in chromosome 17p11.2-12, or in rare patients may result from a point mutation in the peripheral myelin protein-22 (PMP22) gene. CMT1B is associated with point mutations in the myelin protein zero (P0 or MPZ) gene. The molecular defect in CMT1C is unknown. X-linked Charcot-Marie-Tooth neuropathy (CMTX), which has clinical features similar to CMT1, is associated with mutations in the connexin32 gene. Charcot-Marie-Tooth neuropathy type 2 (CMT2) is an axonal neuropathy, also of undetermined cause. One form of CMT2 maps to chromosome 1p36 (CMT2A), another to chromosome 3p (CMT2B) and another to 7p (CMT2D). Dejerine-Sottas disease (DSD), also called hereditary motor and sensory neuropathy type III (HMSNIII), is a severe, infantile-onset demyelinating polyneuropathy syndrome that may be associated with point mutations in either the PMP22 gene or the P0 gene and shares considerable clinical and pathological features with CMT1. Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder that results in a recurrent, episodic demyelinating neuropathy. HNPP is associated with a 1.5-Mb deletion in chromosome 17p11.2-12 and results from reduced expression of the PMP22 gene. CMT1A and HNPP are reciprocal duplication/deletion syndromes originating from unequal crossover during germ cell meiosis. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q and 11q. Hereditary neuralgic amyotrophy (familial brachial plexus neuropathy) is an autosomal dominant disorder causing painful, recurrent brachial plexopathies and maps to chromosome 17q25.

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.

An adhesion test system based on Schneider cells to determine genotype-phenotype correlations for mutated P0 proteins

Myelin protein zero (MPZ, P0) is well known as the adhesion molecule responsible for the compaction of the myelin sheath of peripheral nerves. Mutations are linked to Charcot-Marie-Tooth syndrome type 1B (CMT1B) and the more severe Dejerine-Sottas syndrome (DSS). Three mutations leading to phenotypes of increasing severity (Ser34del/CMT1B, Ser34Cys/DSS, INS663GC/DSS) were expressed in S2 insect cells and resulted in a decreased adhesion capability in correlation with their respective phenotypes.

A small direct tandem duplication of the myelin protein zero gene in a patient with Dejerine-Sottas disease phenotype

We present a male patient with Dejerine-Sottas disease phenotype, who had a small direct tandem duplication of the Po gene. The pathology of the sural nerve showed hypomyelinated fibers with absence of active demyelination and onion-bulb formations composed of two parallel layers of basement membrane, consistent with congenital hypomyelination neuropathy (CHN). However, his clinical features were more severe than those of previously reported CHN patients. A GGCA insertion was identified at the position of nucleotide 560 in the myelin protein zero (Po) gene. This insertional mutation was located in exon 4 coding for the transmembrane domain of the Po gene and caused a shift of reading frame, creating a stop codon. The mutation of the transmembrane domain probably has the largest impact on Po function. The mutation was not identified in both parents.

De novo mutation (Arg98-->Cys) of the myelin P0 gene and uncompaction of the major dense line of the myelin sheath in a severe variant of Charcot-Marie-Tooth disease type 1B

A point mutation (Arg98-->Cys) of exon 3 coding for the extracellular domain of the myelin protein zero (P0) gene was found in a sporadic case of an eighteen year old Japanese man with a severe variant of Charcot-Marie-Tooth disease type 1B (CMT1B). A de novo mutation was established by parentage testing and analyses of the P0 gene in the family. This patient showed delayed motor development, nonprogressive limb weakness and kyphoscoliosis. In addition to the nerve biopsy findings typical of CMT1B, such as segmental demyelination, marked decrease in the density of myelinated fibers, and frequent onion-bulb formation, ultrastructural examination disclosed uncompaction of the major dense lines with slight widening of the intraperiod distance in the inner layers of the myelin sheath. Although mutations in the extracellular domain of P0 should affect homophilic adhesion between external surfaces of Schwann cell processes, resulting in the separation at the intraperiod lines, our study shows uncompacted major dense lines as a main myelin abnormality where the cytoplasmic domain of P0 resides.

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).

Charcot-Marie-Tooth disease and related inherited neuropathies

Charcot-Marie-Tooth disease (CMT) was initially described more than 100 years ago by Charcot, Marie, and Tooth. It was only recently, however, that molecular genetic studies of CMT have uncovered the underlying causes of most forms of the diseases. Most cases of CMT1 are associated with a 1.5-Mb tandem duplication in 17p11.2-p12 that encompasses the PMP22 gene. Although many genes may exist in this large duplicated region, PMP22 appears to be the major dosage-sensitive gene. CMT1A is the first autosomal dominant disease associated with a gene dosage effect due to an inherited DNA rearrangement. There is no mutant gene, but instead the disease phenotype results from having 3 copies of a normal gene. Furthermore, these findings suggest that therapeutic intervention in CMT1A duplication patients may be possible by normalizing the amount of PMP22 mRNA levels. Alternatively, CMT1A can be caused by mutations in the PMP22 gene. Other forms of CMT are associated with mutations in the MPZ (CMT1B) and Cx32 (CMTX) genes. Thus, mutations in different genes can cause similar CMT phenotypes. The related but more severe neuropathy, Dejerine-Sottas syndrome (DSS), can also be caused by mutations in the PMP22 and MPZ genes. All 3 genes thus far identified by CMT researchers appear to play an important role in the myelin formation or maintenance of peripheral nerves. CMT1A, CMT1B, CMTX, hereditary neuropathy with liability to pressure palsies (HNPP), and DSS have been called myelin disorders or "myelino-pathies." Other demyelinating forms, CMT1C and CMT-AR, may be caused by mutations of not yet identified myelin genes expressed in Schwann cells. The clinically distinct disease HNPP is caused by a 1.5-Mb deletion in 17p11.2-p12, which spans the same region duplicated in most CMT1A patients. Underexpression of the PMP22 gene causes HNPP just as overexpression of PMP22 causes CMT1A. Thus, 2 different phenotypes can be caused by dosage variations of the same gene. It is apparent that the CMT1A duplication and HNPP deletion are the reciprocal products of a recombination event during meiosis mediated through the CMT1A-REPs. CMT1A and HNPP could be thought of as a "genomic disease" more than single gene disorders. Other genetic disorders may also prove to arise from recombination events mediated by specific chromosomal structural features of the human genome (102). Further studies on the recombination mechanism of CMT and HNPP might reveal the causes of site specific homologous recombination in the human genome. The discovery of the PMP22 gene in the 1.5-Mb CMT1A duplication/HNPP deletion critical region also suggests that the clinical phenotype of chromosome aneuploid syndromes may result from the effect of a small subset of dosage-sensitive genes mapping within the region of aneuploidy. The understanding of the molecular basis of CMT1 and related disorders has allowed accurate DNA diagnosis and genetic counseling of inherited peripheral neuropathies and will make it possible to develop rational strategies for therapy. As several loci for CMT2 have been identified, the genes responsible for CMT2 will most likely be disclosed using positional cloning and candidate gene approaches in the near future.

Onion bulb cells in mice deficient for myelin genes share molecular properties with immature, differentiated non-myelinating, and denervated Schwann cells

Onion bulb formation is a pathological feature observed in peripheral nerves of patients suffering from inherited peripheral neuropathies such as Charcot-Marie-Tooth and Déjérine-Sottas diseases. An onion bulb consists of small circumferentially oriented (supernumerary) cells and their processes surrounding a large caliber axon. In the present study, we investigated the molecular phenotype of supernumerary cells at the light and electron microscopic levels. The major motor (quadriceps muscle) branch of the femoral nerve from 16- to 24-month-old mice with an inactivated allele of the myelin protein zero gene or deficient for myelin-associated glycoprotein (MAG; P0(+)- and MAG--mice, respectively), which have numerous onion bulbs, was teased to obtain single nerve fibers, which were then processed for immunocytochemistry. Corresponding nerves from wild-type mice served as controls. In both P0(+)- and MAG--mice, supernumerary cells expressed S-100, the low-affinity nerve growth factor receptor (p75, NGFr), the cell adhesion molecule L1, the neural cell adhesion molecule (N-CAM), and glial fibrillary acidic protein (GFAP). At the electron microscopic level, the cell surface of supernumerary cells was NGFr immunoreactive and L1 and N-CAM were expressed at points of contact between supernumerary cells. NGFr, L1, and N-CAM were also present in the basal lamina surrounding myelinated axons associated with onion bulbs. Both S-100 and GFAP immunoreactivities were seen in the cytoplasm of supernumerary cells. In contrast, in wild-type mice myelinating Schwann cells only expressed S-100 intracellularly and L1 and N-CAM in their basal lamina, whereas non-myelinating Schwann cells expressed all five molecules investigated. The present study indicates that supernumerary cells in onion bulbs have a molecular phenotype characteristic of immature, differentiated non-myelinating, and denervated Schwann cells, thus excluding the possibility that supernumerary cells are perineurial cells.

A new point mutation affecting the fourth transmembrane domain of PMP22 results in severe de novo Charcot-Marie-Tooth disease

A novel T-->G mutation in exon 4 of the PMP22 gene was identified heterozygously in a girl with severe, de novo CMT1A disease. Duplication of the chromosomal 17p11-12 region, encompassing the PMP22 gene, was ruled out. This is the only known mutation that specifically affects the human fourth transmembrane (TM) domain of PMP22. It results in a substitution of a non-polar amino acid by a polar one (Leu147-->Arg), similar to the nearby Gly150-->Asp substitution, underlying the severe Trembler phenotype in the mouse. These mutations suggest that the fourth TM domain plays a crucial role in the normal function of PMP22. The new mutation also augments previous observations that diseases caused by mutations in PMP22 are more severe than those caused by the duplication of 17p11-12.

Myelin protein zero (MPZ) gene mutations in nonduplication type 1 Charcot-Marie-Tooth disease

The myelin protein zero gene (MPZ) maps to chromosome 1q22-q23 and encodes the most abundant peripheral nerve myelin protein. The Po protein functions as a homophilic adhesion molecule in myelin compaction. Mutations in the MPZ gene are associated with the demyelinating peripheral neuropathies Charcot-Marie-Tooth disease type 1B (CMT1B), and the more severe Dejerine-Sottas syndrome (DSS). We have surveyed a cohort of 70 unrelated patients with demyelinating polyneuropathy for additional mutations in the MPZ gene. The 1.5-Mb DNA duplication on chromosome 17p11.2-p12 associated with CMT type 1A (CMT1A) was not present. By DNA heteroduplex analysis, four base mismatches were detected in three exons of MPZ. Nucleotide sequence analysis identified a de novo mutation in MPZ exon 3 that predicts an Ile(135)Thr substitution in a family with clinically severe early-onset CMT1, and an exon 3 mutation encoding a Gly(137)Ser substitution was identified in a second CMT1 family. Each predicted amino acid substitution resides in the extracellular domain of the Po protein. Heteroduplex analysis did not detect either base change in 104 unrelated controls, indicating that these substitutions are disease-associated mutations rather than common polymorphisms. In addition, two polymorphic mutations were identified in MPZ exon 5 and exon 6, which do not alter the codons for Gly(200) and Ser(228), respectively. These observations provide further confirmation of the role of MPZ in CMT1B and suggest that MPZ coding region mutations may account for a limited percentage of disease-causing mutations in nonduplication CMT1 patients.

A de novo duplication in 17p11.2 and a novel mutation in the Po gene in two Déjérine-Sottas syndrome patients

Déjérine-Sottas syndrome (DSS), or hereditary motor and sensory neuropathy (HMSN) type III, is a severe hypertrophic demyelinating neuropathy with infantile onset. The clinical symptoms are similar to those found in Charcot-Marie-Tooth disease type 1 (CMT1) or HMSN type I patients, but they are more severe. DSS is genetically heterogeneous. Dominant mutations in two major peripheral myelin protein genes, PMP22 and Po, are associated with a DSS phenotype. Mutations in the same genes are also responsible for the CMT1 phenotype. A 1.5-Mb duplication in 17p11.2 is the major mutation found in familial and sporadic CMT1 patients. We studied two genetically sporadic DSS patients. The presence of a de novo duplication in one patient was revealed by Southern blot analysis, using polymorphic markers located in the duplicated area. The 17p11.2 allele segregation in this patient and in her parents suggests that the duplication is of maternal origin. In the other patient, single-strand conformation polymorphism (SSCP) analysis of the 6 exons of the Po gene revealed two additional bands in exon 3. Sequencing of this exon identified a novel dominant mutation replacing a sequence of 8 bp by a mutated sequence of 5 bp. The mutation apparently leads to the replacement of 4 amino acids at positions 86-89 by three different amino acids, in an area that is part of a predicted beta-strand. Our findings support the suggestion that DSS and CMT1 disease should not be considered as two different clinical entities.

Widespread expression of the peripheral myelin protein-22 gene (PMP22) in neural and non-neural tissues during murine development

The gene encoding the peripheral myelin protein PMP22 is affected by various mutations in the hereditary peripheral neuropathies Charcot-Marie-Tooth disease type 1A (CMT1A), Déjérine-Sottas syndrome (DSS) and hereditary neuropathy with liability to pressure palsies (HNPP). In contrast to the recent remarkable progress in the genetics of the PMP22 gene, the biological function of PMP22 remains largely unknown. In this report, we have confirmed by using in situ hybridization techniques that high levels of PMP22 mRNA are present in maturing peripheral nerves of the 2-week-old mouse, a finding consistent with the PNS-specific defect observed in hereditary peripheral neuropathies. However, high levels of PMP22 transcripts were also found in the villi of the adult gut, and PMP22 expression was detected in various non-neural tissues during embryonic mouse development. In early embryogenesis (9.5 days postconception, dpc), PMP22 RNA expression appears restricted to the epithelial ectodermal layer. During early organogenesis (11.5 dpc), particularly high levels of expression are present in the capsule surrounding the liver and in the forming gut, while low levels of PMP22 mRNA can be found in precartilagous condensations forming the vertebrae and the ventricular layer of the myelencephalon. During midgestation development (14.5 dpc to 16.5 dpc), the number of PMP22-positive tissues increases, and high expression is detected in several mesoderm-derived tissues, in particular connective tissues of the face region, bones including the vertebrae, the lung mesenchym, and in muscles. In addition, high expression is also found in ectoderm-derived tissues, especially the epithelia of the lens and the skin. These findings strongly suggest that PMP22 serves not only a PNS-specific function but is also of broader biological significance in cell proliferation and/or differentiation.

Protein zero (P0)-deficient mice show myelin degeneration in peripheral nerves characteristic of inherited human neuropathies

Mutations in the human gene for the myelin recognition molecule protein zero (P0) give rise to severe and progressive forms of dominantly inherited peripheral neuropathies. We have previously reported that mice homozygous for a null mutation in P0 have severely hypomyelinated nerves ten weeks after birth. Here we show hypomyelination already exists at day four with subsequent demyelination and impaired nerve conduction. Furthermore, heterozygous mutants show normal myelination, but develop progressive demyelination after four months of age. Thus, the pathology of homo- and heterozygous P0 mutants resembles that of the severely affected Déjérine-Sottas and the more mildly affected Charcot-Marie-Tooth type 1B patients, respectively.

Molecular analysis of three cases with hereditary motor and sensory neuropathy with myelin outfolding

We describe three patients affected by a congenital motor and sensory neuropathy with excessive myelin outfoldings (MOs) [15]. Clinical and electrophysiological features supported the diagnosis of hereditary motor and sensory neuropathy. We previously reported a genetic study on these three patients, which failed to demonstrate either the duplication in chromosome 17p11.2 or the mutations at exons 1 and 2 of the peripheral myelin protein gene (PMP-22) and suggested an autosomal recessive (AR) inheritance. In this study we described the absence of the most common mutations, which characterized other forms of hereditary motor and sensory neuropathy (HMSN). In particular the absence of molecular changes in the PMP-22 gene definitively sets HMSN with MOs apart from the more common CMT1A, hereditary neuropathy with liability to pressure palsies (HNPP) and progressive sensory-motor polyneuropathy with tomaculous changes at sural nerve biopsy.

Molecular basis of common hereditary motor and sensory neuropathies in humans and in mouse models

The Hereditary Motor and Sensory Neuropathies (HMSNs) are well known to be clinically, morphologically, and genetically heterogeneous. Yet, recent advances in the cellular and molecular biology of the peripheral nervous system coupled with remarkable progress in human and mouse genetics have provided a framework that has profoundly changed our understanding of the pathogenesis of these diseases. It now appears that most of the HMSNs are related to mutations affecting genes encoding Schwann cell proteins, specifically the Peripheral Myelin Protein PMP22, Myelin Protein Zero, and one of the gap junction proteins, connexin-32. Accordingly, these findings are discussed in the context of the clinical and pathologic features of the human HMSNs, but are interpreted in the context of basic research findings on the cellular and molecular biology of the peripheral nervous system derived from in vivo and in vitro studies in spontaneously-occurring and genetically engineered animal models for the HMSNs.