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

Novel MPZ mutations and congenital hypomyelinating neuropathy

We report two new MPZ mutations causing congenital hypomyelinating neuropathies; c.368_382delGCACGTTCACTTGTG (in-frame deletion of five amino acids) and c.392A>G, Asn131Ser. Each child had clinical and electrodiagnostic features consistent with an inherited neuropathy, confirmed by sural nerve biopsy. The cases illustrate the clinically heterogeneity that exists even within early-onset forms of this disease. They also lend additional support to the emerging clinical and laboratory evidence that impaired intracellular protein trafficking may represent the cause of some congenital hypomyelinating neuropathies.

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.

Peripheral neuropathies caused by mutations in the myelin protein zero

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

Dejerine-Sottas 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.

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

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

Steroid responsive polyneuropathy in a family with a novel myelin protein zero mutation

OBJECTIVE

To report a novel hereditary motor and sensory neuropathy (HMSN) phenotype, with partial steroid responsiveness, caused by a novel dominant mutation in the myelin protein zero (MPZ) gene. Most MPZ mutations lead to the HMSN type I phenotype, with recent reports of Déjérine-Sottas, congenital hypomyelination, and HMSN II also ascribed to MPZ mutations. Differing phenotypes may reflect the effect of particular mutations on MPZ structure and adhesivity.

METHODS

Clinical, neurophysiological, neuropathological, and molecular genetic analysis of a family presenting with an unusual hereditary neuropathy.

RESULTS

Progressive disabling weakness, with positive sensory phenomena and areflexia, occurred in the proband with raised CSF protein and initial steroid responsiveness. Nerve biopsy in a less severely affected sibling disclosed a demyelinating process with disruption of compacted myelin. The younger generation were so far less severely affected, becoming symptomatic only after 30 years. All affected family members were heterozygous for a novel MPZ mutation (Ile99Thr), in a conserved residue.

CONCLUSIONS

This broadens the range of familial neuropathy associated with MPZ mutations to include steroid responsive neuropathy, initially diagnosed as chronic inflammatory demyelinating polyneuropathy.

Molecular basis of inherited neuropathies

Considerable advances in our knowledge of the most frequently encountered group of inherited neuropathies, Charcot-Marie-Tooth neurpathy (CMT) and related disorders, have recently been made by genetic studies demonstrating that these disorders are caused by duplication, deletion or point mutations of specific genes of the peripheral myelin. The present classification of CMT and related disorders is based on a combination of clinical, neurophysiological, and genetic findings, and new genes and distinct mutations responsible for different clinical phenotypes are continuously being added. The genes that encode peripheral myelin protein of 22 kDa, protein zero, connexin-32 and early growth response-2 are the genes known to be involved in the pathogenesis of inherited neuropathies. Overexpression or underexpression of peripheral myelin protein of 22 kDa are causative for the most frequent forms of CMT-CMT1A and hereditary neuropathy with liability to pressure palsies--but the mechanisms that lead to incorrect myelin formation and maintenance are still unknown. Point mutations in the myelin genes can determine a loss of function, but in some cases an aberrant protein can act through a dominant negative or a toxic gain of function mechanism, disrupting the regular and precise relationship between the different myelin genes. Animal and in-vitro models of inherited neuropathies have been developed and will probably give the information that is necessary to clarify the pathogenetic mechanisms of demyelination.

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.