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

C-terminal tail length guides insertion and assembly of membrane proteins

A large number of newly synthesized membrane proteins in the endoplasmic reticulum (ER) are assembled into multiprotein complexes, but little is known about the mechanisms required for assembly membrane proteins. It has been suggested that membrane chaperones might exist, akin to the molecular chaperones that stabilize and direct the assembly of soluble protein complexes, but the mechanisms by which these proteins would bring together membrane protein components is unclear. Here, we have identified that the tail length of the C-terminal transmembrane domains (C-TMDs) determines efficient insertion and assembly of membrane proteins in the ER. We found that membrane proteins with C-TMD tails shorter than ∼60 amino acids are poorly inserted into the ER membrane, which suggests that translation is terminated before they are recognized by the Sec61 translocon for insertion. These C-TMDs with insufficient hydrophobicity are post-translationally recognized and retained by the Sec61 translocon complex, providing a time window for efficient assembly with TMDs from partner proteins. Retained TMDs that fail to assemble with their cognate TMDs are slowly translocated into the ER lumen and are recognized by the ER-associated degradation (ERAD) pathway for removal. In contrast, C-TMDs with sufficient hydrophobicity or tails longer than ∼80 residues are quickly released from the Sec61 translocon into the membrane or the ER lumen, resulting in inefficient assembly with partner TMDs. Thus, our data suggest that C-terminal tails harbor crucial signals for both the insertion and assembly of membrane proteins.

Subcellular diversion of cholesterol by gain- and loss-of-function mutations in PMP22

Abnormalities of the peripheral myelin protein 22 (PMP22) gene, including duplication, deletion and point mutations are a major culprit in Type 1 Charcot-Marie-Tooth (CMT) diseases. The complete absence of PMP22 alters cholesterol metabolism in Schwann cells, which likely contributes to myelination deficits. Here, we examined the subcellular trafficking of cholesterol in distinct models of PMP22-linked neuropathies. In Schwann cells from homozygous Trembler J (TrJ) mice carrying a Leu16Pro mutation, cholesterol was retained with TrJ-PMP22 in the Golgi, alongside a corresponding reduction in its plasma membrane level. PMP22 overexpression, which models CMT1A caused by gene duplication, triggered cholesterol sequestration to lysosomes, and reduced ATP-binding cassette transporter-dependent cholesterol efflux. Conversely, lysosomal targeting of cholesterol by U18666A treatment increased wild type (WT)-PMP22 levels in lysosomes. Mutagenesis of a cholesterol recognition motif, or CRAC domain, in human PMP22 lead to increased levels of PMP22 in the ER and Golgi compartments, along with higher cytosolic, and lower membrane-associated cholesterol. Importantly, cholesterol trafficking defects observed in PMP22-deficient Schwann cells were rescued by WT but not CRAC-mutant-PMP22. We also observed that myelination deficits in dorsal root ganglia explants from heterozygous PMP22-deficient mice were improved by cholesterol supplementation. Collectively, these findings indicate that PMP22 is critical in cholesterol metabolism, and this mechanism is likely a contributing factor in PMP22-linked hereditary neuropathies. Our results provide a basis for understanding how altered expression of PMP22 impacts cholesterol metabolism.

A Computational Approach to Identify a Potential Alternative Drug With Its Positive Impact Toward PMP22

Mutations in the Peripheral Myelin Protein 22 (PMP22) leads to Charcot Marie Tooth type 1A (CMT1A, a subtype of CMT1) disease which is the most common inherited neuropathy of peripheral nervous system. In the present study, we used series of in silico prediction methods to screen and identify the most deleterious non-synonymous SNPs (nsSNPs) in PMP22 gene. Out of 48 nsSNPs, five nsSNPs (L16P, L19P, T23R, W28R, and L147R) associated with PMP22 were predicted to be highly deleterious and destabilizing the protein. To explore the possible structure-function relationship, we employed abinitio modeling strategy using the CABS-fold server to predict the three-dimensional structure models in the absence of crystallized structures in PMP22 protein. We used Cytoscape 3.4.0 plugin Integrated Complex Traits Networks interface (iCTNet) to identify the probable drug-gene interactions in PMP22 gene. A total of 22 chemical compounds yielded from the aforementioned tool was subjected to Molinspiration and OSIRIS program to screen and identify the potent drug molecules for further analysis. Five chemical compounds with excellent bioavailability and drug relevant property were selected for molecular docking simulation study. We modeled five mutant structures at their corresponding positions and performed molecular docking simulation analysis using AutoDock Tools (ADT) version 1.5.6 and ArgusLab 4.0.1 tools to analyze their interaction patterns and binding efficacy. Based on the results obtained from the computational study, we predict that estradiol could be a potential drug of choice for treating patients with CMT1A which needs larger attention from biologists in the near future. J. Cell. Biochem. 118: 3730-3743, 2017. © 2017 Wiley Periodicals, Inc.

Charcot-Marie-Tooth disease: an overview of genotypes, phenotypes, and clinical management strategies

Charcot-Marie-Tooth (CMT) disease, which encompasses several hereditary motor and sensory neuropathies, is one of the most common neuromuscular disorders. Our understanding of the molecular genotypes of CMT and the resultant clinical and electrophysiological phenotypes has increased greatly in the past decade. Characterized by electrodiagnostic studies into demyelinating (type 1) and axonal (type 2) forms, subsequent genetic testing often provides an exact diagnosis of a specific subtype of CMT. These advancements have made diagnostic paradigms fairly straightforward. Still, the nature and extent of neuromuscular disability is often complex in persons with CMT, and no curative treatments are yet available. Genotypically homologous animal models of CMT have improved exploration of disease-modifying treatments, of which molecular genetic manipulation and stem cell therapies appear to be the most promising. Research is also needed to develop better rehabilitative strategies that may limit disease burden and improve physical performance and psychosocial integration. Clinical management should be multidisciplinary, including neurologists, physiatrists, neurogeneticists, neuromuscular nurse practitioners, and orthopedists, along with physical and occupational therapists, speech-language pathologists, orthotists, vocational counselors, social workers, and other rehabilitation clinicians. Goals should include maximizing functional independence and quality of life while minimizing disability and secondary morbidity.

Update on Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) disease encompasses a genetically heterogeneous group of inherited neuropathies, also known as hereditary motor and sensory neuropathies. CMT results from mutations in more than 40 genes expressed in Schwann cells and neurons causing overlapping phenotypes. The classic CMT phenotype reflects length-dependent axonal degeneration characterized by distal sensory loss and weakness, deep tendon reflex abnormalities, and skeletal deformities. Recent articles have provided insight into the molecular pathogenesis of CMT, which, for the first time, suggest potential therapeutic targets. Although there are currently no effective medications for CMT, multiple clinical trials are ongoing or being planned. This review will focus on the underlying pathomechanisms and diagnostic approaches of CMT and discuss the emerging therapeutic strategies.

Therapeutic strategies for the inherited neuropathies

More than 30 genetic causes have been identified for the inherited neuropathies collectively referred to as Charcot-Marie-Tooth (CMT) disease. Previous therapies for CMT were limited to traditional approaches such as rehabilitation medicine, ambulation aids, and pain management. Identification of the genes causing CMT has led to improved genetic counseling and assistance in family planning. Identification of these genes is beginning to delineate common molecular pathways in multiple forms of CMT that can be exploited in future molecular therapies. Scientifically based clinical trials for CMT are currently being implemented. Techniques of gene therapy are advancing to the point that they may become feasible options for patients with CMT and other neurodegenerative diseases.

Recessive, but not dominant, mutations in peripheral myelin protein 22 gene show unique patterns of aggregation and intracellular trafficking

A characteristic feature of mouse models of the peripheral neuropathies caused by dominant mutations in peripheral myelin protein 22 (pmp22) is the appearance, in Schwann cells, of pmp22 aggregates. Using a set of dominant and recessive pmp22 mutations that cause human disease of varying degrees of severity, we compared their potential for aggregation and trafficking patterns with those of wild-type pmp22. The potential for aggregation was assessed by determining the size distribution of the various pmp22 mutant proteins under conditions where wild-type pmp22 showed little or no aggregation. All disease-causing dominant mutations showed significant aggregation and failed to traffic to the cell surface. Although the position of the dominant mutation in the pmp22 molecule determined both its potential for aggregation and how far it trafficked in the cell, there was no correlation between aggregation and the severity of the disease. On the other hand, recessive mutations were uniquely distinguished from dominant mutations by both the low potential for aggregation and their trafficking to the cell surface. In the course of these studies, it was also noted that the potential for aggregation and the trafficking of mutant pmp22s is influenced by the nature and/or location of the epitope tag.

A novel stop codon mutation in the PMP22 gene associated with a variable phenotype

The most frequent inherited peripheral neuropathy is the peripheral myelin protein 22 (PMP22) gene related disease. Duplication, deletion, and point mutations in that gene are associated with phenotypic variability. Here we report a family carrying a novel mutation in the PMP22 gene (c. 327C>A), which results in a premature stop codon (Cys109stop). The family members who carry this mutation have a Charcot-Marie-Tooth type 1 variable phenotype, ranging from asymptomatic to severely affected. These findings suggest that the fourth transmembrane domain of the PMP22 gene may play an important role, although the intrafamilial clinical variability reinforces the observation that pathogenic mutations are not always phenotype determinant and that other factors (genetic or epigenetic) modulate the severity of the clinical course.

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.

Matrix metalloproteinase mediated degradation of basement membrane proteins in Trembler J neuropathy nerves

A single point mutation in peripheral myelin protein 22 (pmp22) of the Trembler-J (TrJ) mouse models the human peripheral neuropathy, Charcot-Marie-Tooth disease type 1 A (CMT1A). An unexplored aspect of this disease is the gradual remodeling of the extracellular matrix in affected nerves. To elucidate the mechanism responsible for these changes, the levels of the extracellular matrix molecules laminin, collagen IV, and fibronectin were determined. In TrJ nerves, laminin is modestly increased while full-length forms of collagen IV and fibronectin are decreased. Matrix metalloproteinases (MMPs) are known to degrade multiple matrix molecules; therefore, nerves were assayed for MMP-2 and MMP-9 proteins. In neuropathy nerves, elevated levels of MMP-2 and MMP-9 were detected on western blots, and gelatin zymography confirmed the up-regulation of gelatinalytic activity in affected samples. Immunostaining studies revealed an increase in the numbers of MMP-2- and MMP-9-expressing cells in TrJ nerves. Cell type-specific immunolabeling showed that infiltrating macrophages are a significant source of both MMP-2 and MMP-9. Finally, the degradation of exogenous collagen IV by TrJ nerve lysates was prevented with a specific MMP inhibitor. Together these observations suggest that infiltration by MMP-expressing macrophages contributes to the remodeling of the TrJ nerve matrix.

Characterisation of autoantibodies to peripheral myelin protein 22 in patients with hereditary and acquired neuropathies

To investigate the possibility that an autoimmune mechanism may play a role in the hereditary neuropathy Charcot-Marie-Tooth type 1A (CMT1A), sera were analysed by Western blot for anti-peripheral myelin protein 22 (PMP22) autoantibodies. These sera were compared with sera from patients with CMT type 2 (CMT2), acquired peripheral neuropathies such as chronic inflammatory demyelinating neuropathy (CIDP), anti-MAG IgM neuropathy, Miller-Fisher syndrome (MFS), diabetic neuropathy and with control blood donors. Anti-PMP22 positive sera were detected in 70% of patients with CMT1 and unexpectedly in 60% of patients with CMT2. Interestingly, 44% of the patients with other peripheral neuropathies and 23% of the apparently healthy controls showed also anti-PMP22 antibody reactivity. Immunohistochemical analysis of the human anti-PMP22 antisera on healthy sural nerve sections and on PMP22-expressing COS cells revealed that these sera did not recognise endogenous PMP22. Our results indicate that anti-PMP22 autoantibodies are found in sera of patients with different types of peripheral neuropathies, but their role in the pathogenesis of these diseases remains to be determined.

A unique point mutation in the PMP22 gene is associated with Charcot-Marie-Tooth disease and deafness

Charcot-Marie-Tooth disease (CMT) with deafness is clinically distinct among the genetically heterogeneous group of CMT disorders. Molecular studies in a large family with autosomal dominant CMT and deafness have not been reported. The present molecular study involves a family with progressive features of CMT and deafness, originally reported by Kousseff et al. Genetic analysis of 70 individuals (31 affected, 28 unaffected, and 11 spouses) revealed linkage to markers on chromosome 17p11.2-p12, with a maximum LOD score of 9.01 for marker D17S1357 at a recombination fraction of .03. Haplotype analysis placed the CMT-deafness locus between markers D17S839 and D17S122, a approximately 0.6-Mb interval. This critical region lies within the CMT type 1A duplication region and excludes MYO15, a gene coding an unconventional myosin that causes a form of autosomal recessive deafness called DFNB3. Affected individuals from this family do not have the common 1.5-Mb duplication of CMT type 1A. Direct sequencing of the candidate peripheral myelin protein 22 (PMP22) gene detected a unique G-->C transversion in the heterozygous state in all affected individuals, at position 248 in coding exon 3, predicted to result in an Ala67Pro substitution in the second transmembrane domain of PMP22.

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.

Impaired intracellular trafficking is a common disease mechanism of PMP22 point mutations in peripheral neuropathies

The most common forms of hereditary motor and sensory neuropathies (HMSN) or Charcot-Marie-Tooth disease (CMT) are associated with mutations affecting myelin genes in the peripheral nervous system. A minor subgroup of CMT type 1A (CMT1A) is caused by point mutations in the gene encoding the peripheral myelin protein 22 (PMP22). To study the mechanisms by which these mutations cause the CMT pathology, we transiently transfected COS7 and Schwann cells with wild-type and PMP22 expression constructs carrying six representative dominant or de novo point mutations and one putative recessive point mutation. All but one of the first group of mutant PMP22 proteins failed to be incorporated into the plasma membrane and were retained in intracellular compartments of transfected cells. Surprisingly, the recessive PMP22 mutation produced a protein that was also mildly impaired in trafficking. Thus, our results suggest a common disease mechanism underlying the pathology of CMT1A due to PMP22 point mutations.

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.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.

Severe Charcot-Marie-Tooth neuropathy type 1A with 1-base pair deletion and frameshift mutation in the peripheral myelin protein 22 gene

A 27-year-old man with negative family history and both parents with normal neurological evaluation and motor nerve conduction velocities (MNCVs) showed onset of severe weakness of feet at 4 years of age. Subsequently he developed left equinovarus deformity, thoracic scoliosis, ulnar nerve enlargement, areflexia, distal hypesthesia and slowing of MNCVs for median and ulnar nerves (15-25 m/sec). Molecular genetic studies showed deletion of one nucleotide (G330) (codon 94) in exon 3 of the PMP22 gene associated with frameshift mutation.

Animal models for inherited peripheral neuropathies

Recent progress in human genetics and neurobiology has led to the identification of various mutations in particular myelin genes as the cause for many of the known inherited demyelinating peripheral neuropathies. Mutations in 3 distinct myelin genes, PMP22, P0, and connexin 32 cause the 3 major demyelinating subtypes of Charcot-Marie-Tooth (CMT) disease, CMT1A, CMT1B and CMTX, respectively. In addition, a reduction in the gene dosage of PMP22 causes hereditary neuropathy with liability to pressure palsies (HNPP), while particular point mutations in PMP22 and P0 cause the severe Dejerine-Sottas (DS) neuropathy. A series of spontaneous and genetically engineered rodent mutants for genes for the above-mentioned myelin constituents are now available and their suitability to serve as models for these still untreatable diseases is an issue of particular interest. The spontaneous mutants Trembler-J and Trembler, with point mutations in PMP22, reflect some of the pathological alterations seen in CMT1A and DS patients, respectively. Furthermore, engineered mutants that either over or underexpress particular myelin genes are suitable models for patients who are similarly compromised in the gene dosage of the corresponding genes. In addition, engineered mutants heterozygously or homozygously deficient in the myelin component P0 show the pathology of distinct CMT1B and DS patients, respectively, while Cx32 deficient mice develop pathological abnormalities similar to those of CMTX patients. Mutants that mimic human peripheral neuropathies might allow the development of strategies to alleviate the symptoms of the diseases, and help to define environmental risk factors for aggravation of the disease. In addition, such mutants might be instrumental in the development of strategies to cure the diseases by gene therapy.

Analysis of compound heterozygous mice reveals that the Trembler mutation can behave as a gain-of-function allele

The most common form of Charcot-Marie-Tooth disease, CMT1A, is correlated with a 1.5 megabase duplication on chromosome 17p.11.2 containing the peripheral myelin protein 22 (PMP22) gene. Deletion of the same region is associated with a second inherited neural disorder, the hereditary neuropathy with liability to pressure palsies (HNPP). Moreover, several distinct point mutations within the PMP22 coding region are associated with CMT1A and Dejerine-Sottas Syndrome in humans and the Trembler (Tr) and Trembler-J phenotypes in mice. Heterozygous Tr mutants (Tr/+) display severe hypomyelination of peripheral nerve fibers while heterozygous pmp22 knockout mice (pmp22+/0) are characterized by focal hypermyelination. These findings suggest that the Tr mutation does not generate a pmp22 null allele but rather produces its deleterious effects by either a dominant-negative or gain-of-function mechanism. To address this question in detail, we have compared various combinations of pmp22 alleles including Tr/+, Tr/Tr, Tr/0, pmp22+/0, and pmp22(0/0) mice with respect to the resulting myelin abnormalities. The combined analysis of these mutants demonstrates that the Tr allele can act as a true gain-of-function mutation in both, the heterozygous state on a null background (Tr/0) as well as in homozygous Tr animals (Tr/Tr). Furthermore, increasing the relative Tr gene dosage correlates with more pronounced myelin deficiencies and decreased levels of MBP and P0 in 18-day-old mice.

Aberrant protein trafficking in Trembler suggests a disease mechanism for hereditary human peripheral neuropathies

The naturally occurring mouse mutant Trembler (Tr) represents an animal model for inherited human neuropathies caused by point mutations affecting peripheral myelin protein 22 (PMP22). We describe the likely pathogenic cellular mechanism underlying the observed myelin deficiency. In Tr/+ animals, PMP22 immunoreactivity was found not only in compact myelin but also abundantly in the cytoplasm of Schwann cells. Based on these observations, the biosynthesis of wildtype and Tr protein was examined in transfected cells. While wildtype PMP22 was readily transported to the plasma membrane, Tr protein was localized mainly in the endoplasmic reticulum. Coexpression revealed a dominant effect of Tr on protein trafficking of wildtype PMP22. In agreement with the findings in vitro, Tr protein was not detectable in myelin of Tr/0 mice.