HMSN III phenotype due to homozygous expression of a dominant HMSN II gene

CIDP, CMT1B, or CMT1B plus CIDP?

Charcot-Marie-Tooth disease type 1 (CMT1) and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) have distinct clinical and neurophysiological features that result from dysmyelination in CMT1 and macrophage-mediated segmental demyelination in CIDP. CMT1 may occur in genetically isolated cases with atypical presentations that converge phenotypically with CIDP; in rare cases, however, CMT1 may be complicated by superimposed CIDP. We report the case of a patient harboring a de novo heterozygous null mutation of the myelin protein zero (MPZ) gene and affected by subclinical CMT1B who became symptomatic due to superimposed CIDP. Peripheral nerve high-resolution ultrasound (HRUS) aided in establishing the coexistence of CMT1B and CIDP; the diagnosis was further supported by favorable clinical, neurophysiological, and ultrasound responses to immunoglobulin therapy.

Myelin protein zero gene dose dependent axonal ion-channel dysfunction in a family with Charcot-Marie-Tooth disease

OBJECTIVE

The myelin impairment in demyelinating Charcot-Marie-Tooth (CMT) disease leads to various degrees of axonal degeneration, the ultimate cause of disability. We aimed to assess the pathophysiological changes in axonal function related to the neuropathy severity in hypo-/demyelinating CMT patients associated with myelin protein zero gene (MPZ) deficiency.

METHODS

We investigated four family members (two parents and two sons) harboring a frameshift mutation (c.306delA, p.Asp104ThrfsTer14) in the MPZ gene, predicted to result in a nonfunctional P₀, by conventional conduction studies and multiple measures of motor axon excitability. In addition to the conventional excitability studies of the median nerve at the wrist, we tested the spinal accessory nerves. Control measures were obtained from 14 healthy volunteers.

RESULTS

The heterozygous parents (aged 56 and 63) had a mild CMT1B whereas their two homozygous sons (aged 31 and 39 years) had a severe Dejerine-Sottas disease phenotype. The spinal accessory nerve excitability could be measured in all patients. The sons showed reduced deviations during depolarizing threshold electrotonus and other depolarizing features which were not apparent in the accessory and median nerve studies of the parents. Mathematical modeling indicated impairment in voltage-gated sodium channels. This interpretation was supported by comparative modeling of excitability measurements in MPZ deficient mice.

CONCLUSION

Our data suggest that axonal depolarization in the context of abnormal voltage-gated sodium channels precedes axonal degeneration in severely hypo-/demyelinating CMT as previously reported in the mouse models.

SIGNIFICANCE

Measures of the accessory nerve excitability could provide pathophysiological markers of neurotoxicity in severe demyelinating neuropathies.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

SIGNIFICANCE

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

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

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

Molecular mechanisms of inherited demyelinating neuropathies

The past 15 years have witnessed the identification of more than 25 genes responsible for inherited neuropathies in humans, many associated with primary alterations of the myelin sheath. A remarkable body of work in patients, as well as animal and cellular models, has defined the clinical and molecular genetics of these illnesses and shed light on how mutations in associated genes produce the heterogeneity of dysmyelinating and demyelinating phenotypes. Here, we review selected recent developments from work on the molecular mechanisms of these disorders and their implications for treatment strategies.

Normal expression of myelin protein zero with frame-shift mutation correlates with mild phenotype

Mutations in the gene encoding for myelin protein zero (MPZ) cause inherited demyelinating peripheral neuropathies of different severity. The molecular and cellular mechanisms by which the MPZ mutations cause neuropathy are incompletely understood. We investigated MPZ, myelin basic protein, and peripheral myelin protein 22 (PMP22) protein expression levels in a nerve biopsy of a Charcot-Marie-Tooth type 1B patient heterozygous for the Val 102 frame-shift mutation. We demonstrate by quantitative immunohistochemical as well as by Western blot analyses that MPZ expression levels were not reduced in myelin membranes, a finding that is in accordance with the mild phenotype of this patient. Our data show that heterozygous 'loss-of-function' of MPZ may not necessarily lead to reduced protein levels. In conclusion, we demonstrate that careful analysis of protein expression levels in peripheral nerve tissues provides important information with respect to the understanding of the molecular basis of these neuropathies.

Floppy infant syndrome

Floppiness/hypotonia is a common neurologic symptom in infancy. A variety of neuromuscular disorders and central nervous system (CNS) disorders cause floppy infant syndrome (FIS). CNS disorders are the much more common causes of the syndrome than neuromuscular disorders. On long-term follow up, cerebral palsy and mental retardation turn out to be the 2 most common causes of FIS. This review focuses on neuromuscular causes of FIS. With the advent of molecular diagnosis, a few conditions can be diagnosed by DNA analysis of the peripheral lymphocytes (myotonic dystrophy, spinal muscular atrophy); however, for the most part, electrodiagnostic studies and muscle biopsy remain as essential diagnostic tools for FIS. Immunohistochemical study of the biopsied muscle also improves diagnostic capability. Management for most conditions remains supportive.

Inherited neuropathies

Inherited neuropathies are common and are usually caused by mutations in genes that are expressed by myelinating Schwann cells or neurons, which is the biological basis for long-standing distinction between primary demyelinating and axonal neuropathies. Neuropathies can be isolated, the primary manifestation of a more complex syndrome, or overshadowed by other aspects of the inherited disease. Increasing knowledge of the molecular-genetic causes of inherited neuropathies facilitates faster, more accurate diagnosis, and sets the stage for development of specific therapeutic interventions.

Animal models for inherited peripheral neuropathies: chances to find treatment strategies?

Mutations in four identified genes (peripheral myelin protein 22, P(0), connexin 32, and the early growth response 2 zinc finger protein) are the cause for several forms of inherited peripheral neuropathies that are still incurable disorders. Some forms of these disorders are well mimicked by engineered or spontaneous rodent mutants that might be instrumental for developing treatment strategies. This review focusses on common pathways of pathogenesis of the disorders and emphasizes strategies that might be suitable to ameliorate disease expression.

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

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

Loss of distal axons and sensory Merkel cells and features indicative of muscle denervation in hindlimbs of P0-deficient mice

Mice lacking the major Schwann cell myelin component P0 show a severe dysmyelination with pathological features reminiscent of the Déjérine-Sottas syndrome in humans. Previous morphological and electrophysiological studies on these mice did not only demonstrate a compromised myelination and myelin maintenance, but were suggestive of an impairment of axons as well. Here, we studied the axonal pathology in P0-deficient mice by quantitative electron microscopy. In addition, we investigated epidermal receptor end organs by immunocytochemistry and muscle pathology by histochemistry. In proximal sections of facial and femoral nerves, axon calibers were significantly reduced, whereas the number of myelin-competent axons was not diminished in 5- and 17-month-old P0-deficient mice. However, in distal branches of the femoral and sciatic nerve (digital nerves innervating the skin of the first toe) the numbers of myelin-competent axons were reduced by 70% in 6-month-old P0-deficient mice. Immunolabeling of foot pads revealed a corresponding loss of Merkel cells by 75%, suggesting that survival of these cells is dependent on the presence or maintenance of their innervating myelinated axons. In addition, quadriceps and gastrocnemius muscles showed pathological features indicative of denervation and axonal sprouting. These findings demonstrate that loss of an important myelin component can initiate degenerative mechanisms not only in the Schwann cell but also in the distal portions of myelinated axons, leading to the degeneration of specialized receptor end organs and impairment of muscle innervation.

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.

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.

Hereditary neuropathies in children: the contribution of the new genetics

Although the prevalence of the hereditary motor and sensory neuropathies in childhood is not clearly established and the age of presentation may overlap the arbitrary boundary between pediatric and adult neurology, the recent explosion of genetic information regarding these conditions has completely altered our understanding and classification of these diseases. The current status of our understanding of the molecular basis of the hereditary neuropathies which might present in childhood is reviewed. The impact of this information on our concepts of the mechanisms operative in the production of the clinical signs and symptoms in these diseases is discussed.

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.

Homozygous hypertrophic hereditary motor and sensory neuropathies

We compared 25 autosomal dominant hereditary motor and sensory neuropathy (HMSN) type I patients with 7 subjects affected by hypertrophic HMSN with non-dominant inheritance. All the autosomal dominant HMSN I cases carried the chromosome 17p11.2 duplication, providing evidence that it is widely represented in HMSN I families. The second group included: two siblings born to unrelated, unaffected parents and suffering from hypertrophic HMSN of strikingly different severity; two sisters with HMSN I phenotype, born to first-cousin unaffected parents; two brothers with HMSN III phenotype born to unrelated parents both showing HMSN II phenotype; a child with classic HMSN III phenotype, born to unrelated, unaffected parents. The 17p11.2 duplication was not found in any of the patients of the second series or in their parents. Our data provide further evidence that: HMSN III is heterogeneous and encompasses the homozygous expressions of different neuropathic genes; it is advisable to separate autosomal recessive hypertrophic HMSN from dominant HMSN Ia, because they appear to be due to different DNA mutations.

De novo mutation of the myelin P0 gene in Dejerine-Sottas disease (hereditary motor and sensory neuropathy type III)

We have investigated the myelin P0 gene on chromosome 1 as a candidate gene in two sporadic cases with Dejerine-Sottas disease or hereditary motor and sensory neuropathy (HMSN) type III. We found different mutations, a cysteine substitution for serine 63 in the extracellular domain and an arginine substitution for glycine 167 in the transmembrane domain. The patients were genetically heterozygous for the normal allele and the mutant allele, which was absent in their parents and in one hundred unrelated, healthy controls. The results strongly suggest that a de novo dominant mutation of the P0 gene is responsible for at least some sporadic cases of Dejerine-Sottas disease.