Linkage and mutation analysis in an extended family with Charcot-Marie-Tooth disease type 1B

Genetic spectrum of hereditary neuropathies with onset in the first year of life

Early onset hereditary motor and sensory neuropathies are rare disorders encompassing congenital hypomyelinating neuropathy with disease onset in the direct post-natal period and Dejerine–Sottas neuropathy starting in infancy. The clinical spectrum, however, reaches beyond the boundaries of these two historically defined disease entities. De novo dominant mutations in PMP22, MPZ and EGR2 are known to be a typical cause of very early onset hereditary neuropathies. In addition, mutations in several other dominant and recessive genes for Charcot–Marie–Tooth disease may lead to similar phenotypes. To estimate mutation frequencies and to gain detailed insights into the genetic and phenotypic heterogeneity of early onset hereditary neuropathies, we selected a heterogeneous cohort of 77 unrelated patients who presented with symptoms of peripheral neuropathy within the first year of life. The majority of these patients were isolated in their family. We performed systematic mutation screening by means of direct sequencing of the coding regions of 11 genes: MFN2, PMP22, MPZ, EGR2, GDAP1, NEFL, FGD4, MTMR2, PRX, SBF2 and SH3TC2. In addition, screening for the Charcot–Marie–Tooth type 1A duplication on chromosome 17p11.2-12 was performed. In 35 patients (45%), mutations were identified. Mutations in MPZ, PMP22 and EGR2 were found most frequently in patients presenting with early hypotonia and breathing difficulties. The recessive genes FGD4, PRX, MTMR2, SBF2, SH3TC2 and GDAP1 were mutated in patients presenting with early foot deformities and variable delay in motor milestones after an uneventful neonatal period. Several patients displaying congenital foot deformities but an otherwise normal early development carried the Charcot–Marie–Tooth type 1A duplication. This study clearly illustrates the genetic heterogeneity underlying hereditary neuropathies with infantile onset.

Correction of cavovarus foot deformity in Charcot-Marie-Tooth disease

Operative correction of cavovarus foot deformity in Charcot-Marie-Tooth disease (CMT) is challenging. This progressive peripheral sensory and motor neuropathy commonly involves the forefoot, midfoot, hindfoot, and toes. The authors present a new imaging technique that allows the surgeon to assess the flexibility of the hindfoot in patients with CMT to determine the best operative procedure to correct the deformity. Twenty-five patients (41 feet) with CMT and cavovarus foot deformity were evaluated and a new radiographic technique was studied in some of these patients to determine the usefulness of this imaging modality. The authors believe that this new imaging method will aid in determining the optimal operation for correcting this complex deformity.

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.

The Thr124Met mutation in the peripheral myelin protein zero (MPZ) gene is associated with a clinically distinct Charcot-Marie-Tooth phenotype

We observed a missense mutation in the peripheral myelin protein zero gene (MPZ, Thr124Met) in seven Charcot-Marie-Tooth (CMT) families and in two isolated CMT patients of Belgian ancestry. Allele-sharing analysis of markers flanking the MPZ gene indicated that all patients with the Thr124Met mutation have one common ancestor. The mutation is associated with a clinically distinct phenotype characterized by late onset, marked sensory abnormalities and, in some families, deafness and pupillary abnormalities. Nerve conduction velocities of the motor median nerve vary from <38 m/s to normal values in these patients. Clusters of remyelinating axons in a sural nerve biopsy demonstrate an axonal involvement, with axonal regeneration. Phenotype-genotype correlations in 30 patients with the Thr124Met MPZ mutation indicate that, based on nerve conduction velocity criteria, these patients are difficult to classify as CMT1 or CMT2. We therefore conclude that CMT patients with slightly reduced or nearly normal nerve conduction velocity should be screened for MPZ mutations, particularly when additional clinical features such as marked sensory disturbances, pupillary abnormalities or deafness are also present.

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.

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.

Correlation between the histopathologic, genotypic, and phenotypic features of hereditary peripheral neuropathies in childhood

In recent years, there have been remarkable advances in the understanding of the molecular genetic basis of the hereditary polyneuropathies. Linkage of the genes for Charcot-Marie-Tooth disease to chromosomes 1 and then 17 was followed by the discovery that the commonest form of Charcot-Marie-Tooth disease (CMT1A) was due to a duplication of DNA at 17p11.2-12. This duplication was shown to contain the gene for peripheral myelin protein PMP22. The finding that mutations of the myelin protein PMP22 gene were present in some Charcot-Marie-Tooth disease cases lacking the duplication confirmed the myelin protein PMP22 gene as the site of the defect in Charcot-Marie-Tooth disease. Similarly, defects of the myelin protein P0 gene on chromosome 1 have been demonstrated in a rarer form of Charcot-Marie-Tooth disease (CMT1B). A deletion of DNA at 17p11.2-12 results in the disorder hereditary neuropathy with liability to pressure palsies. Other mutations of the myelin protein PMP22 and myelin protein P0 genes have been associated with the clinical syndrome known as Dejerine-Sottas disease. An X-linked form of Charcot-Marie-Tooth disease (CMTX) has been characterized and shown to be due to mutations of the gap junction protein, connexin 32. Transgenic murine models with inactivated myelin protein PMP22 and myelin protein P0 genes have shown pathologic changes strinkingly similar to those seen in human patients with disturbances of those genes. In this paper, the clinical and histopathologic characteristics of these conditions are discussed in relation to the genotypic basis. It will be argued that there is still an important place for the clinician and nerve pathologist in a medical world immersed in the wonders of molecular genetics.

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.

Molecular genetic analysis of the 17p11.2 region in patients with hereditary neuropathy with liability to pressure palsies (HNPP)

Hereditary neuropathy with liability to pressure palsies (HNPP) is in most cases associated with an interstitial deletion of the same 1.5-Mb region at 17p11.2 that is duplicated in Charcot-Marie-Tooth type 1A (CMT1A) patients. Unequal crossing-over following misalignment at flanking repeat sequences (CMT1A-REP), either leads to tandem duplication in CMT1A patients or deletion in HNPP patients. With the use of polymorphic DNA markers located within the CMT1A/HNPP duplication/deletion region we detected the HNPP deletion in 16 unrelated HNPP patients, 11 of Belgian and 5 of French origin. In all cases, the 1.5-Mb size of the HNPP deletion was confirmed by EcoRI dosage analysis using a CMT1A-REP probe. In the 16 HNPP patients, the same 370/320-kb EagI deletion-junction fragments were detected with pulsed field gel electrophoresis (PFGE), while in CMT1A patients, a 150-kb EagI duplication-junction fragment was seen. Thus, PFGE analysis of EagI-digested DNA with a CMT1A-REP probe allows direct detection of the HNPP deletion or the CMT1A duplication for DNA diagnostic purposes.

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.

Prenatal diagnosis of Charcot-Marie-Tooth disease type 1A (CMT1A) using molecular genetic techniques

Charcot-Marie-Tooth disease type 1A (CMT1A) is a frequent hereditary motor and sensory neuropathy of the peripheral nerves. In most cases, the disease is associated with a 1.5 Mb tandem duplication at 17p11.2. A 42-year-old pregnant women requested prenatal diagnosis because of her age and since both her husband and two children were severely affected with CMT1. The CMT1A duplication was demonstrated in the father's, the two children's, and the fetus's DNA using different molecular genetic methods. Although cytogenetical analysis showed a normal female karyotype in the fetus, the parents decided to terminate the pregnancy because of the genetic risk associated with the CMT1A duplication.

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.

Mutations in the myelin protein zero gene associated with Charcot-Marie-Tooth disease type 1B

Charcot-Marie-Tooth type 1 (CMT1) disease is an autosomal dominant neuropathy of the peripheral nerve. The majority of CMT 1 cases are due to a duplication of an 1.5-Mb DNA fragment on chromosome 17p11.2 (CMT 1a). Micromutations were found in the gene for peripheral myelin protein 22 (PMP22) located in the duplicated region of CMT 1a, and in the peripheral myelin protein zero (PO) located on chromosome 1q21-q23 (CMT 1b). We have characterized two new mutations in the PO gene in two french families presenting CMT disease. Both mutations occur in the extracellular domain of the PO protein. One mutation is a de novo mutation and is from paternal origin.

Rapid screening of myelin genes in CMT1 patients by SSCP analysis: identification of new mutations and polymorphisms in the P0 gene

Charcot-Marie-Tooth type 1 (CMT1) disease or hereditary motor and sensory neuropathy type I (HMSNI) is an autosomal dominant peripheral neuropathy. In most CMT1 families, the disease cosegregates with a 1.5-Mb duplication on chromosome 17p11.2 (CMT1A). A few patients have been found with mutations in the peripheral myelin protein 22 (PMP-22) gene located in the CMT1A region. In other families mutations have been identified in the major peripheral myelin protein P0 gene localized on chromosome 1q21-q23 (CMT1B). We performed a rapid mutation screening of the PMP-22 and P0 genes in non-duplicated CMT1 patients by single-strand conformation polymorphism analysis followed by direct polymerase chain reaction sequencing of genomic DNA. Six new single base changes in the P0 gene were observed: two missense mutations in, respectively, exons 2 and 3, two nonsense mutations in exon 4, and two silent mutations or polymorphisms in, respectively, exons 3 and 6.