Mutations in the early growth response 2 (EGR2) gene are associated with hereditary myelinopathies.

The early growth response 2 gene (EGR2) is part of a multigene family encoding Cys2His2 type zinc-finger proteins and may play a role in the regulation of cellular proliferation. Egr2, (also known as Krox20) is the mouse orthologue of human EGR2 and was first identified as an immediate-early response gene, encoding a protein that binds DNA in a sequence-specific manner and acts as a transcription factor. Stable expression of Egr2 is specifically associated with the onset of myelination in the peripheral nervous system (PNS). Egr2(-/-) mice display disrupted hindbrain segmentation and development, and a block of Schwann-cell differentiation at an early stage. We hypothesized that Egr2 may be a transcription factor affecting late myelin genes and that human myelinopathies of the PNS may result from mutations in EGR2. In support of this hypothesis, we have identified one recessive and two dominant missense mutations in EGR2 (within regions encoding conserved functional domains) in patients with congenital hypomyelinating neuropathy (CHN) and a family with Charcot-Marie-Tooth type 1 (CMT1).

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).

Comparison of single-strand conformation polymorphism and heteroduplex analysis for detection of mutations in Charcot-Marie-Tooth type 1 disease and related peripheral neuropathies.

To compare the sensitivity of the mutation detection techniques single-strand conformation polymorphism analysis (SSCP) and heteroduplex analysis (HA), we analyzed a cohort of 73 patients with a diagnosis of a demyelinating neuropathy, but without the CMT1A duplication, for mutations in the coding region of the myelin genes PMP22, MPZ and Cx32. In total, 21 samples showed 13 distinct altered migration patterns by one or both methods. Ten altered patterns were detected by both SSCP and HA, two were false negative by HA, and one was false negative by SSCP. Our results suggest that either technique can be useful for mutation detection, but a combination of factors appears to affect the sensitivity of both techniques.

Novel missense mutation in the early growth response 2 gene associated with Dejerine-Sottas syndrome phenotype.

BACKGROUND: Mutations in the early growth response 2 (EGR2) gene have recently been found in patients with congenital hypomyelinating neuropathy and Charcot-Marie-Tooth type 1 (CMT1) disease. OBJECTIVE: To determine the frequency of EGR2 mutations in patients with a diagnosis of CMT1, Dejerine-Sottas syndrome (DSS), or unspecified peripheral neuropathies. METHODS: Fifty patients and 70 normal control subjects were screened. RESULTS: A de novo missense mutation (Arg359Trp) in the alpha-helix of the first zinc-finger domain of the EGR2 transcription factor was identified in a patient diagnosed with a clinical phenotype consistent with DSS. This patient had a motor median nerve conduction velocity of 8 m/s. A sural nerve biopsy showed a severe loss of myelinated and unmyelinated fibers, evidence for demyelination, numerous classic onion bulbs, and focally folded myelin sheaths. DSS is a severe, childhood-onset demyelinating peripheral neuropathy initially thought to be inherited as an autosomal recessive trait. However, several dominant heterozygous mutations in the peripheral myelin protein 22 (PMP22) gene and dominant mutations in the peripheral myelin protein zero (MPZ) gene, both in the heterozygous and homozygous state, have been reported in patients with DSS. CONCLUSIONS: Hereditary peripheral neuropathies represent a spectrum of disorders due to underlying defects in myelin structure or formation.

Molecular mechanisms for CMT1A duplication and HNPP deletion.

As the best characterized human genomic disorders, CMT1A and HNPP illustrate several common mechanistic features of genomic rearrangements. These features include the following: (1) Recombination occurs between homologous sequences flanking the duplicated/deleted genomic segment. (2) The evolution of the mammalian genome may result in an architecture consisting of region-specific low-copy repeats that predispose to rearrangement secondary to providing homologous regions as substrate for recombination. (3) Strand exchange occurs preferentially in a region of perfect sequence identity within the flanking repeat sequences. (4) Double-strand breaks likely initiate recombination between the flanking repeats. (5) The mechanism and rate of homologous recombination resulting in DNA rearrangement may differ for male and female gametogenesis. (6) Homologous recombination resulting in DNA rearrangement occurs with high frequency in the human genome. (7) Genomic disorders result from structural features of the human genome and not population specific alleles or founder effects; therefore, genomic disorders appear to occur with equal frequencies in different world populations.

Congenital hypomyelinating neuropathy: two patients with long-term follow-up.

The authors report the long-term prospective follow-up of two unrelated females with congenital hypomyelinating neuropathy (CHN) and review previously reported cases. The authors' first patient presented with neonatal hypotonia and extremely slow nerve conduction velocities. Sural nerve biopsy revealed profound hypomyelination, without inflammation or evidence of myelin breakdown. She is now 9 years of age, and her motor function has continued to improve. Follow-up nerve-conduction velocities are unchanged. The authors' second patient presented at 5 months with hypotonia. Nerve-conduction velocities were extremely slow, and sural nerve biopsy revealed severe hypomyelination, with no inflammation or evidence of myelin breakdown. She is now 5 years of age and has also demonstrated improved motor function. Repeated nerve-conduction velocities are unchanged. Both patients have normal cognitive development. Molecular genetic analysis in Patient 2 disclosed a point mutation in the myelin protein zero gene; this same point mutation has been reported in three other patients diagnosed with Dejerine-Sottas syndrome (DSS) but has never been reported in a patient with CHN. Although CHN is a distinct clinical entity, it may share similar genetic features with DSS.

Hereditary peripheral neuropathies: clinical forms, genetics, and molecular mechanisms.

Hereditary peripheral neuropathies, among the most common genetic disorders in humans, are a complex, clinically and genetically heterogeneous group of disorders that produce progressive deterioration of the peripheral nerves. This group of disorders includes hereditary neuropathy with liability to pressure palsies, Charcot-Marie-Tooth disease, Dejerine-Sottas syndrome, and congenital hypomyelinating neuropathy. Our understanding of these disorders has progressed from the description of the clinical phenotypes and delineation of the electrophysiologic and pathologic features to the identification of disease genes and elucidation of the underlying molecular mechanisms.

Multiple de novo MPZ (P0) point mutations in a sporadic Dejerine-Sottas case.

Dejerine-Sottas syndrome (DSS), a severe demyelinating peripheral neuropathy with onset in infancy, has been associated with mutations in either PMP22 or MPZ. Most cases of DSS are caused by a single heterozygous dominant point mutation. We identified three de novo point mutations in MPZ exon 3 in a sporadic DSS patient. These three point mutations occur on the same allele and result in three novel amino acid substitutions: Ile(85)Thr, Asn(87)His, and Asp(99)Asn. Our data raise the question as to the potential mechanism(s) involved in the formation of multiple point mutations at a given locus.

Functional consequences of mutations in the early growth response 2 gene (EGR2) correlate with severity of human myelinopathies.

The early growth response 2 gene ( EGR2 ) is a Cys2His2zinc finger transcription factor which is thought to play a role in the regulation of peripheral nervous system myelination. This idea is based partly on the phenotype of homozygous Krox20 ( Egr2 ) knockout mice, which display hypomyelination of the PNS and a block of Schwann cells at an early stage of differentiation. Mutations in the human EGR2 gene have recently been associated with the inherited peripheral neuropathies Charcot-Marie-Tooth type 1, Dejerine-Sottas syndrome and congenital hypomyelinating neuropathy. Three of the four EGR2 mutations are dominant and occur within the zinc finger DNA-binding domain. The fourth mutation is recessive and affects the inhibitory domain (R1) that binds the NAB transcriptional co-repressors. A combination of DNA-binding assays and transcriptional analysis was used to determine the functional consequences of these mutations. The zinc finger mutations affect DNA binding and the amount of residual binding directly correlates with disease severity. The R1 domain mutation prevents interaction of EGR2 with the NAB co-repressors and thereby increases transcriptional activity. These data provide insight into the possible disease mechanisms underlying EGR2 mutations and the reason for varying severity and differences in inheritance patterns.

Molecular Mechanisms for CMT1A Duplication and HNPP Deletion.

As the best characterized human genomic disorders, 118 CMT1A and HNPP illustrate several common mechanistic features of genomic rearrangements. These features include the following: (1) Recombination occurs between homologous sequences flanking the duplicated/deleted genomic segment. (2) The evolution of the mammalian genome may result in an architecture consisting of region-specific low-copy repeats that predispose to rearrangement secondary to providing homologous regions as substrate for recombination. (3) Strand exchange occurs preferentially in a region of perfect sequence identity within the flanking repeat sequences. (4) Double-strand breaks likely initiate recombination between the flanking repeats. (5) The mechanism and rate of homologous recombination resulting in DNA rearrangement may differ for male and female gametogenesis. (6) Homologous recombination resulting in DNA rearrangement occurs with high frequency in the human genome. (7) Genomic disorders result from structural features of the human genome and not population specific alleles or founder effects; therefore, genomic disorders appear to occur with equal frequencies in different world populations.

Unusual electrophysiological findings in X-linked dominant Charcot-Marie-Tooth disease.

X-linked Charcot-Marie-Tooth disease (CMTX) is the second most common form of Charcot-Marie-Tooth disease. Variable histopathological and nerve conduction velocity (NCV) results have suggested either a primary demyelinating or axonal polyneuropathy. We identified five individuals across three generations in a family with CMTX associated with a mutation in the gene coding for connexin 32. All individuals were studied by clinical neurological examination, DNA analysis, and nerve conduction studies. The proband (1174/KD) also underwent a sural nerve biopsy. As expected, all the affected males were more clinically affected than the females. All affected males and obligate female carriers exhibited some electrophysiological characteristics of demyelination. However, striking heterogeneity of nerve conduction velocities was seen. This family shows that CMTX is a heterogeneous and distinctly nonuniform demyelinating polyneuropathy, the severity of which varies with sex and age. Such electrophysiological variability is unique among hereditary neuropathies.

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.