The peripheral myelin protein gene PMP-22 is contained within the Charcot-Marie-Tooth disease type 1A duplication

A rapid and definitive test for Charcot-Marie-Tooth 1A and hereditary neuropathy with liability to pressure palsies using multiplexed real-time PCR

Alterations in gene copy number have been shown to cause disease in humans. Two of the most common inherited peripheral neuropathies, Charcot-Marie-Tooth 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP), are two such diseases resulting from alteration in gene copy number of the dosage sensitive peripheral myelin protein 22 (PMP22) gene. Many complicated and laborious diagnostic tests exist for the diagnosis of these diseases. The aim of our study was to develop the first quantitative multiplex real-time PCR assay for the diagnosis of CMT1A and HNPP. A total of 160 individuals who were known to have CMT1A, HNPP, or were normal from previous testing were assayed by our multiplex real-time PCR method. The results confirmed the previously determined gene copy number of all patient and control individuals tested. The range of ratio values between the disease and control groups were easily defined. The assay is accurate, simple, and cost effective and can detect a 50% change in gene copy number. This represents an ideal assay for any small diagnostic laboratory.

Ascorbic acid treatment corrects the phenotype of a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is the most common hereditary peripheral neuropathy, affecting 1 in 2,500 people. The only treatment currently available is rehabilitation or corrective surgery. The most frequent form of the disease, CMT-1A, involves abnormal myelination of the peripheral nerves. Here we used a mouse model of CMT-1A to test the ability of ascorbic acid, a known promoter of myelination, to correct the CMT-1A phenotype. Ascorbic acid treatment resulted in substantial amelioration of the CMT-1A phenotype, and reduced the expression of PMP22 to a level below what is necessary to induce the disease phenotype. As ascorbic acid has already been approved by the FDA for other clinical indications, it offers an immediate therapeutic possibility for patients with the disease.

A new locus for autosomal dominant Charcot-Marie-Tooth disease type 2 (CMT2L) maps to chromosome 12q24

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders with a prevalence estimated at 1/2500. The axonal form of this disorder is referred to as Charcot-Marie-Tooth type 2 disease (CMT2). Recently, a large Chinese family with CMT2 was found in the Hunan and Hubei provinces of China. The known loci for CMT1A, CMT2D, CMT1B (the same locus is also responsible for CMT2I and CMT2J), CMT2A, CMT2E, and CMT2F were excluded in this family by linkage analysis. A genome-wide screening was then carried out, and the results revealed linkage of CMT2 to a locus at chromosome 12q24. Haplotype construction and analyses localized this novel locus to a 6.8-cM interval between microsatellite markers D12S366 and D12S1611. The maximal two-point LOD score of 6.35 and multipoint LOD score of 8.08 for marker D12S76 at a recombination fraction (theta) of 0 strongly supported linkage to this locus. Thus, CMT2 neuropathy in this family represents a novel genetic entity that we have designated as CMT2L.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Autonomic and respiratory dysfunction in Charcot-Marie-Tooth disease due to Thr124Met mutation in the myelin protein zero gene

OBJECTIVE

To report the clinical and electrophysiological characteristics of a family presenting Charcot-Marie-Tooth disease (CMT) associated with autonomic nervous system disturbances.

METHODS

We studied nerve conduction values, postural adaptation, sympathetic skin reflex, the variation in heart rate by the Valsalva ratio and pupillometry in 7 members of a French family in which CMT due to a Thr124Met mutation in the myelin protein zero (MPZ) gene was diagnosed.

RESULTS

Clinical and laboratory evidence of autonomic nervous system disturbances were found in the affected individuals. The clinical phenotype was characterized by sensorimotor peripheral neuropathy, defined as axonal type by electrophysiological studies, and was associated with severe pain, bladder dysfunction, sudorimotor disturbances and abolished pupillary reflex to light. Moreover, two patients had severe restrictive respiratory insufficiency requiring noninvasive mechanical ventilation.

CONCLUSIONS

Our study demonstrates that autonomic disturbances may be one of the major clinical signs associated with CMT secondary to MPZ gene mutation in codon 124. Testing of pupillary reflex allows the discrimination of affected and unaffected subjects in our family. However, involvement of the autonomic nervous system in this type of neuropathy is unclear and further studies are required to elucidate the role of the MPZ gene in the autonomic nervous system.

A new quantitative PCR multiplex assay for rapid analysis of chromosome 17p11.2-12 duplications and deletions leading to HMSN/HNPP

A 1.4-Mb tandem duplication, including the gene for peripheral myelin protein 22 (PMP22) in chromosome 17p11.2-12 is responsible for 70% of the cases of the demyelinating type 1 of Charcot-Marie-Tooth disease or hereditary motor and sensory neuropathy I (CMT1A/HMSN I). A reciprocal deletion of this CMT1A region causes the hereditary neuropathy with liability to pressure palsies (HNPP). The CMT1A duplication increases the PMP22 gene dosage from two to three, the HNPP deletion reduces the gene dosage from two to one. Currently, routine diagnosis of HMSN/HNPP patients is mainly performed with polymorphic markers in-between the repetitive elements flanking the CMT1A region. These show quantitative and/or qualitative changes in case of a CMT1A duplication and a homozygous allele pattern in case of HNPP deletion. In HNPP patients the deletion is usually confirmed by fluorescence in situ hybridisation (FISH). We now developed a reliable, single tube real-time quantitative PCR assay for rapid determination of PMP22 gene dosage directly. This method involves a multiplex reaction using FAM labelled Taqman-probe with TAMRA quencher derived from PMP22 exon 3 and a VIC labelled probe with non-fluorescent quencher from exon 12 of the albumin gene as internal reference. Copy number of the PMP22 gene was determined by the comparative threshold cycle method (deltadeltaCt). Each sample was run in quadruplicate and analysed at two different threshold levels. The level giving the smallest standard deviation was scored. We evaluated this method through the retrospective analysis of 252 HMSN patients with known genotype and could confirm the previous findings in 99% of cases. Two patients were wrongly diagnosed with microsatellite analysis while quantitative real-time PCR identified the correct genotype, as confirmed by FISH. Thus, this method shows superior sensitivity to microsatellite analysis and has the additional advantage of being a fast and uniform assay for quantitative analysis of both CMT1A and HNPP.

Demyelinating and axonal features of Charcot-Marie-Tooth disease with mutations of myelin-related proteins (PMP22, MPZ and Cx32): a clinicopathological study of 205 Japanese patients

Three genes commonly causing Charcot-Marie-Tooth disease (CMT) encode myelin-related proteins: peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) and connexin 32 (Cx32). Demyelinating versus axonal phenotypes are major issues in CMT associated with mutations of these genes. We electrophysiologically, pathologically and genetically evaluated demyelinating and axonal features of 205 Japanese patients with PMP22 duplication, MPZ mutations or Cx32 mutations. PMP22 duplication caused mainly demyelinating phenotypes with slowed motor nerve conduction velocity (MCV) and demyelinating histopathology, while axonal features were variably present. Two distinctive phenotypic subgroups were present in patients with MPZ mutations: one showed preserved MCV and exclusively axonal pathological features, while the other was exclusively demyelinating. These axonal and demyelinating phenotypes were well concordant among siblings in individual families, and MPZ mutations did not overlap among these two subgroups, suggesting that the nature and position of the MPZ mutations mainly determine the axonal and demyelinating phenotypes. Patients with Cx32 mutations showed intermediate slowing of MCV, predominantly axonal features and relatively mild demyelinating pathology. These axonal and demyelinating features were present concomitantly in individual patients to a variable extent. The relative severity of axonal and demyelinating features was not associated with particular Cx32 mutations. Median nerve MCV and overall histopathological phenotype changed little with disease advancement. Axonal features of diminished amplitudes of compound muscle action potentials (CMAPs), axonal loss, axonal sprouting and neuropathic muscle wasting all changed as disease advanced, especially in PMP22 duplication and Cx32 mutations. Median nerve MCVs were well maintained independently of age, disease duration and the severity of clinical and pathological abnormalities, confirming that median nerve MCV is an excellent marker for the genetically determined neuropathic phenotypes. Amplitude of CMAPs was correlated significantly with distal muscle strength in PMP22 duplication, MPZ mutations and Cx32 mutations, while MCV slowing was not, indicating that clinical weakness results from reduced numbers of functional large axons, not from demyelination. Thus, the three major myelin-related protein mutations induced varied degrees of axonal and demyelinating phenotypic features according to the specific gene mutation as well as the stage of disease advancement, while clinically evident muscle wasting was attributable to loss of functioning large axons.

Molecular mechanisms for genomic disorders

Genomic rearrangements play a major role in the pathogenesis of human genetic diseases. Nonallelic homologous recombination (NAHR) between low-copy repeats (LCRs) that flank unique genomic segments results in changes of genome organization and can cause a loss or gain of genomic segments. These LCRs appear to have arisen recently during primate speciation via paralogous segmental duplication, thus making the human species particularly susceptible to genomic rearrangements. Genomic disorders are defined as a group of diseases that result from genomic rearrangements, mostly mediated by NAHR. Molecular investigations of genomic disorders have revealed genome architectural features associated with susceptibility to rearrangements and the recombination mechanisms responsible for such rearrangements. The human genome sequence project reveals that LCRs may account for 5% of the genome, suggesting that many novel genomic disorders might still remain to be recognized.

[Analysis of mutations in the chromosome 17p11.2 region in patients with Charcot-Marie-Tooth type 1 disease and in patients with tomaculous neuropathy]

Charcot-Marie-Tooth type 1A disease (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are common inherited disorders of the peripheral nervous system associated with duplication and deletion, respectively, of the 17p11.2 segment including the gene of peripheral myelin protein 22. We studied 48 subjects belonging to 29 families with clinical and electrophysiological signs of definite CMT1, 20 patients with suspected CMT phenotype, and 17 patients and healthy members of their families with HNPP. Blood sampling and DNA isolation, PCR, restriction analysis, southern blotting were performed using standard procedures. Of 48 patients with diagnosis of definite CMT1 in 25 (52%) we found a 1.5 Mb tandem duplication in chromosome 17p11.2. These duplications were not found in any of 20 sporadic cases with the clinical phenotype of CMT but without reliable electrophysiological data. Only 13 (44.8%) of 29 unrelated CMT1 patients from the first group had 17p11.2 duplications. Three of 4 sporadic cases (75%) with definite CMT1 had 17p11.2 duplications. Of 17 patients from 6 families with HNPP deletion of 17p11.2 segment was found in 15 (88.2%), as well as in 5 (83.3%) of six unrelated cases. Detection of CMT1A/HNPP recombination hotspot is a simple and reliable DNA diagnostic method, which is useful only for the patients with clinically already verified CMT1, and HNPP for further genetic counselling of patients and members of their families.

Mapping of Charcot-Marie-Tooth disease type 1C to chromosome 16p identifies a novel locus for demyelinating neuropathies

Charcot-Marie-Tooth (CMT) neuropathy represents a genetically heterogeneous group of diseases affecting the peripheral nervous system. We report genetic mapping of the disease to chromosome 16p13.1-p12.3, in two families with autosomal dominant CMT type 1C (CMT1C). Affected individuals in these families manifest characteristic CMT symptoms, including high-arched feet, distal muscle weakness and atrophy, depressed deep-tendon reflexes, sensory impairment, slow nerve conduction velocities, and nerve demyelination. A maximal combined LOD score of 14.25 was obtained with marker D16S500. The combined haplotype analysis in these two families localizes the CMT1C gene within a 9-cM interval flanked by markers D16S519 and D16S764. The disease-linked haplotypes in these two pedigrees are not conserved, suggesting that the gene mutation underlying the disease in each family arose independently. The epithelial membrane protein 2 gene (EMP2), which maps to chromosome 16p13.2, was evaluated as a candidate gene for CMT1C.

Phenotypic variation of a novel nonsense mutation in the P0 intracellular domain

Mutations in the gene for the peripheral myelin protein zero (P0, MPZ) cause type 1B of Charcot-Marie-Tooth sensorimotor neuropathy (CMT1B). Here we report a German family with a novel heterozygous P0 nonsense mutation (G206X) that supposedly removes four-fifths of the amino acid residues constituting the P0 intracellular domain. The 12-year-old propositus had childhood-onset CMT1B associated with bilateral pes cavus, moderate lower limb weakness, and mildly reduced sensory qualities in the distal legs. The electrophysiology was consistent with a demyelinating neuropathy. He inherited the mutation from his mother who had no complaints but slight pes cavus deformity and slow nerve conduction velocities (NCV). Conclusively, truncating mutations within the P0 intracellular domain do not necessarily cause a severe phenotype such as Dejerine-Sottas syndrome (DSS) or congenital hypomyelinating neuropathy (CHN), but can result in mild or moderate CMT1B with intrafamilial clinical variability.

Disease mechanisms and potential therapeutic strategies in Charcot-Marie-Tooth disease

Until 10 years ago, the genetic basis of Charcot-Marie-Tooth (CMT) disease was largely unknown. With the finding of an intrachromosomal duplication on chromosome 17 in 1991, associated with the most commonly found subtype CMT1A, and the discovery of a point mutation in the peripheral myelin protein-22 (pmp22) gene in the Trembler mouse in 1992, the groundwork was laid down for a novel chapter in the elucidation of the molecular basis of this large group of peripheral neuropathies. In the meantime, several different genes have been found to be associated with different forms of demyelinating and axonal forms of CMT. In this review, we will summarize what is known today about the genetics of this group of disease which constitute the most common known monogenetic disorder affecting the nervous system in man, the animal models that have been generated, and what we have learned about the underlying disease mechanisms. Furthermore, we will review how this gain of knowledge about CMT may open new avenues to the development of novel treatment strategies.

Dominant intermediate Charcot-Marie-Tooth neuropathy maps to chromosome 19p12-p13.2

The hereditary disorders of peripheral nerve form one of the most common groups of human genetic diseases, collectively called Charcot-Marie-Tooth (CMT) neuropathy. Using linkage analysis we have identified a new locus for a form of CMT that we have called "dominant intermediate CMT" (DI-CMT). A genomewide screen using 383 microsatellite markers showed strong linkage to the short arm of chromosome 19 (maximum LOD score 4.3, with a recombination fraction (straight theta) of 0, at D19S221 and maximum LOD score 5.28, straight theta=0, at D19S226). Haplotype analysis performed with 14 additional markers placed the DI-CMT locus within a 16.8-cM region flanked by the markers D19S586 and D19S546. Multipoint linkage analysis suggested the most likely location at D19S226 (maximum multipoint LOD score 6.77), within a 10-cM confidence interval. This study establishes the presence of a locus for DI-CMT on chromosome 19p12-p13.2.

CNS myelination and PLP gene dosage

The phenomenon of gene dosage effects demonstrates that the mechanisms of some genetic diseases are best recognised at the genomic level. Classical gene mutation screening approaches utilising PCR are unsuccessful in unravelling the basis of disease because the gene sequence is unaltered and only the copy number is different. Techniques for detecting DNA dosage are required. Examples of haploinsufficiency and gene deletions are well documented, but increased gene dosage is also an important genetic mechanism in disorders involving myelin proteins in the central (CNS) and peripheral nervous system (PNS). Here we review the dosage effects and mutations of the proteolipid protein (PLP) gene that causes Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia Type 2 (SPG2) disorders of CNS myelination. Similarities are drawn with the peripheral neuropathies Charcot-Marie-Tooth disease Type 1 (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) that are also caused by dosage effects and mutations in a single myelin protein gene (peripheral myelin protein 22, PMP-22). We compare the different mutational mechanisms in man and analogous mouse models that suggest a function for PLP beyond its structural role in myelin. We focus on the increased dosage of the PLP gene that is the major cause of PMD and results from a submicroscopic duplication of Xq22. Other clinical phenotypes may arise from gene dosage imbalance with the potential effect of submicroscopic duplications and deletions of the genome being underestimated. Genome sequencing may identify intrinsic structural properties of the DNA with greater susceptibility to these rearrangements and thereby reflect structural changes in the genome.

Recombination breakpoints in the Charcot-Marie-Tooth 1A repeat sequence in Norwegian families

OBJECTIVE

To investigate the recombination breakpoint in a 3.2 kb junction fragment of the 24 kb CMT1A repeat sequences (CMT1A-REPs) on chromosome 17p11.2-12.

MATERIALS AND METHODS

Thirty-eight Norwegian CMT1 patients and 15 asymptomatic family members of 15 separate families including 10 normal controls were investigated using repeat (REP)-PCR.

RESULTS

Twenty-six (68.4%) of the CMT1 patients from 9 (60%) families were positive for the CMT1A duplication which was not found in any of the controls. In 89.9% of the REP-PCR positive families the recombination breakpoint was mapped to a 1.7 kb "hot-spot" region, and in 11.1% of the families to a 1.5 kb region telomeric to the 1.7 kb region.

CONCLUSION

The frequency and regions for CMT1A-REPs crossover events in Norwegian CMT1A cases are similar to what is found in other populations. REP-PCR is not, however, as sensitive as other diagnostic methods to detect the CMT1A duplication.

Molecular genetic analysis of Charcot-Marie-Tooth 1A duplication in Norwegian patients by quantitative photostimulated luminescence imaging

Around 70% of Charcot-Marie-Tooth 1 (CMT1) cases are caused by a dominantly inherited 1.5-Mb duplication at 17p11.2-12 (CMT1A). Using photostimulated luminescence (PSL) imaging of MspI Southern blots, hybridization signals of the probe pVAW409R3a in relation to cohybridized probe SF85a, were densitometrically quantified and an RFLP allele-band ratio determined. A total of 55 Norwegian CMT patients and 16 asymptomatic family members from 26 separate families, clinically and neurophysiologically classified as CMT1 (n=46) and CMT2 (n=9), were studied. Thirty-two of 46 CMT1 cases (69.6%), all heterozygous but one homozygous for the pVAW409R3a MspI polymorphism, from 12 of 21 families (57.1%) were positive for the CMT1A duplication. In autosomal dominant familial cases (n=30), 26 of 30 cases (86.7%), all heterozygous, from six of seven families (85.7%) were positive for duplication. None of the CMT2 patients, asymptomatic family members or healthy controls were positive for duplication. The CMT1A frequency of duplication in Norwegian CMT1 patients is in general agreement with those reported in other European countries and the present results show that quantitative densitometric PSL imaging is a highly reliable test in diagnosing CMT1A duplication.

The 1.4-Mb CMT1A duplication/HNPP deletion genomic region reveals unique genome architectural features and provides insights into the recent evolution of new genes

Duplication and deletion of the 1.4-Mb region in 17p12 that is delimited by two 24-kb low copy number repeats (CMT1A-REPs) represent frequent genomic rearrangements resulting in two common inherited peripheral neuropathies, Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsy (HNPP). CMT1A and HNPP exemplify a paradigm for genomic disorders wherein unique genome architectural features result in susceptibility to DNA rearrangements that cause disease. A gene within the 1.4-Mb region, PMP22, is responsible for these disorders through a gene-dosage effect in the heterozygous duplication or deletion. However, the genomic structure of the 1.4-Mb region, including other genes contained within the rearranged genomic segment, remains essentially uncharacterized. To delineate genomic structural features, investigate higher-order genomic architecture, and identify genes in this region, we constructed PAC and BAC contigs and determined the complete nucleotide sequence. This CMT1A/HNPP genomic segment contains 1,421,129 bp of DNA. A low copy number repeat (LCR) was identified, with one copy inside and two copies outside of the 1.4-Mb region. Comparison between physical and genetic maps revealed a striking difference in recombination rates between the sexes with a lower recombination frequency in males (0.67 cM/Mb) versus females (5.5 cM/Mb). Hypothetically, this low recombination frequency in males may enable a chromosomal misalignment at proximal and distal CMT1A-REPs and promote unequal crossing over, which occurs 10 times more frequently in male meiosis. In addition to three previously described genes, five new genes (TEKT3, HS3ST3B1, NPD008/CGI-148, CDRT1, and CDRT15) and 13 predicted genes were identified. Most of these predicted genes are expressed only in embryonic stages. Analyses of the genomic region adjacent to proximal CMT1A-REP indicated an evolutionary mechanism for the formation of proximal CMT1A-REP and the creation of novel genes by DNA rearrangement during primate speciation.

Polymorphic Short Tandem Repeats for Diagnosis of the Charcot-Marie-Tooth 1A Duplication

Background: A 1.5-Mb microduplication containing the gene for peripheral myelin protein 22 (PMP22) on chromosome 17p11.2-12 is responsible for 75% of cases of the demyelinating form of Charcot-Marie-Tooth disease (CMT1A). Methods for molecular diagnosis of CMT1A use Southern blot and/or amplification by PCR of polymorphic poly(AC) repeats (microsatellites) located within the duplicated region, or the detection of junction fragments specific for the duplication. Difficulties with both strategies have led us to develop a new diagnostic strategy with highly polymorphic short tandem repeats (STRs) located inside the CMT1A duplicated region.

Methods: We tested 10 STRs located within the duplication for polymorphic behavior. Three STRs were selected and used to test a set of 130 unrelated CMT1A patients and were compared with nonduplicated controls. The study was then extended to a larger population of patients. Alleles of interest were sequenced. A manual protocol using polyacrylamide electrophoresis and silver staining and an automated capillary electrophoresis protocol to separate fluorescently labeled alleles were validated.

Results: We identified three new STRs covering 0.55 Mb in the center of the CMT1A duplication. One marker, 4A, is located inside the PMP22 gene. The two others, 9A and 9B, more telomerically positioned, have the highest observed heterozygosity reported to date for CMT1A markers: 0.80 for 9A, and 0.79 for 9B. Tetra- and pentanucleotide repeats offered clear amplification, accurate sizing, and easy quantification of intensities.

Conclusions: Combined use of the three STRs allows robust diagnosis with almost complete informativeness. In our routine diagnosis for CMT1A, they have replaced the use of other polymorphic markers, either in a manual adaptation or combined with fluorescence labeling and allele sizing on a DNA sequencer.

A duplication in chromosome 4q35 is associated with hereditary benign intraepithelial dyskeratosis

Hereditary benign intraepithelial dyskeratosis (HBID) is an autosomal dominant disorder characterized by elevated epithelial plaques on the ocular and oral mucous membranes. It has been reported primarily, but not exclusively, in individuals of American Indian heritage in North Carolina. We have examined and obtained DNA on two large families affected by HBID. Using genetic linkage analysis we have localized the HBID gene to chromosome 4 (4q35) with a peak LOD score of 8.97. Molecular analysis of these data reveals that all individuals affected with HBID in both families demonstrate the presence of three alleles for two tightly linked markers, D4S1652 and D4S2390, which map to the telomeric region of 4q35. This suggests the presence of a duplication segregating with the disease phenotype that is most likely involved in its causation.

A comparison of methods for gene dosage analysis in HMSN type 1

A number of different approaches are used in diagnostic laboratories to detect the 1.5 Mb duplication at 17p11.2 seen in approximately 70% of patients with hereditary motor and sensory neuropathy type 1 (HMSN1). Here we compare the methods used in UK diagnostic laboratories to detect the duplication. Samples referred to participating centres for HMSN testing were collected, randomised, and distributed for testing. One hundred samples were examined using five different methods; each method was tested by two independent laboratories. Identical results were obtained from all laboratories for 44 samples. The remaining samples were classified as duplication positive or duplication negative on the basis of the same result by two or more methods. A total of 95 samples were classified by more than one method, two were withdrawn from the study as the same result was not obtained by two methods, and three are thought to have a duplication smaller than 1.5 Mb. Seven of 49 duplications were not detected by methods used to detect the common junction fragment and the use of microsatellites failed to yield a result in four of 95 samples. Sequence tagged site (STS) dosage analysis was found to be the most sensitive of the methods tested, although this method was found to be the most likely to require repeat analysis. Eight samples gave discordant results between the two laboratories testing by the same method. Upon retesting, reasons for the initial incorrect result included processing and typographical errors.

Charcot-Marie-Tooth disease type I and related demyelinating neuropathies: Mutation analysis in a large cohort of Italian families

Charcot-Marie-Tooth neuropathy type 1 (CMT1), the most common hereditary neurological disorder in humans, is characterized by clinical and genetic heterogeneity. It is caused mainly by a 1.5 Mb duplication in 17p11.2, but also by mutations in the myelin genes PMP22 (peripheral myelin protein 22), MPZ (myelin protein zero), Cx32 (connexin 32; also called GJB1), and EGR2 (early growth response 2). In this study, we have screened 172 index cases of Italian families in which there was at least one subject with a CMT1 diagnosis for the duplication on 17p11.2 and mutations in these genes. Among 170 informative unrelated patients, the overall duplication frequency was 57.6%. A difference could be observed between the duplication frequency in familial cases (71.6%) and that observed in non-familial cases (36.8%). Among the non-duplicated patients, 12 were mutated in Cx32, four in MPZ, two in PMP22, and none in the EGR2. In the non-duplicated cases, the overall point mutation frequency for these genes was 25.0%. We describe the mutations identified, and consider possible genotype-phenotype correlation.

Mutational analysis and genotype/phenotype correlation in Turkish Charcot-Marie-Tooth Type 1 and HNPP patients

The major Charcot- Marie-Tooth Type 1 (CMT1) locus, CMT1A, and Hereditary neuropathy with liability to pressure palsies (HNPP) cosegregate with a 1.5-Mb duplication and a 1.5-Mb deletion, respectively, in band 17p11.2. Point mutations in peripheral myelin gene 22 (PMP22), myelin protein zero (MPZ), and connexin 32 (Cx32) have been reported in CMT1, and in PMP22 in HNPP patients without deletion. We have screened 54 CMT1 patients, of variable clinical severity, and 25 HNPP patients from Turkey, with no duplication or deletion, for mutations in the PMP22 and Cx32 genes. A novel frameshift mutation affecting the second extracellular domain of PMP22 was found in an HNPP patient, while a point mutation in the second transmembrane domain of the protein was detected in a CMT1 patient. Two point mutations affecting different domains of Cx32 were identified in two CMTX patients. Another patient was found to carry a polymorphism in a non-conserved codon of the Cx32 gene. The clinical phenotypes of the patients correlate well with the effect of the mutation on the protein.

Diagnosis of haploidy and triploidy based on measurement of gene copy number by real-time PCR

We report the development of a method for diagnosis of heterozygous deletions or duplications based on measurement of gene copy number. The method involves amplifications of a test locus with unknown copy number and a reference locus with known copy number using real-time PCR. Progress of the PCR reactions is monitored using fluorigenic probes and a real-time fluorescence detection system. For each reaction, the number of cycles is measured at which a defined threshold fluorescence emission is reached. Using standard curves, the copy number of the test DNA relative to a common standard DNA is determined for each locus. From the ratio of the relative copy numbers, the genomic copy number of the test locus is determined. In order to demonstrate the accuracy and reliability of the method for genetic testing, we analyzed 43 patients with hereditary neuropathy with liability to pressure palsies (HNPP), containing a heterozygous deletion of a 1.5 Mb region on chromosome 17p11.2-p12, eight patients with Charcot-Marie-Tooth disease, containing a heterozygous duplication of the same genomic region, and 50 normal control individuals. As a test locus we analyzed the PMP22 gene located within the 1.5 Mb region. The genomic copy number of the test locus was precisely measured, and the presence or absence of the genomic deletion or duplication was unambiguously diagnosed in all individuals.

A Ufd2/D4Cole1e chimeric protein and overexpression of Rbp7 in the slow Wallerian degeneration (WldS) mouse

Exons of three genes were identified within the 85-kilobase tandem triplication unit of the slow Wallerian degeneration mutant mouse, C57BL/Wld(S). Ubiquitin fusion degradation protein 2 (Ufd2) and a previously undescribed gene, D4Cole1e, span the proximal and distal boundaries of the repeat unit, respectively. They have the same chromosomal orientation and form a chimeric gene when brought together at the boundaries between adjacent repeat units in Wld(S). The chimeric mRNA is abundantly expressed in the nervous system and encodes an in-frame fusion protein consisting of the N-terminal 70 amino acids of Ufd2, the C-terminal 302 amino acids of D4Cole1e, and an aspartic acid formed at the junction. Antisera raised against synthetic peptides detect the expected 43-kDa protein specifically in Wld(S) brain. This expression pattern, together with the previously established role of ubiquitination in axon degeneration, makes the chimeric gene a promising candidate for Wld. The third gene altered by the triplication, Rbp7, is a novel member of the cellular retinoid-binding protein family and is highly expressed in white adipose tissue and mammary gland. The whole gene lies within the repeat unit leading to overexpression of the normal transcript in Wld(S) mice. However, it is undetectable on Northern blots of Wld(S) brain and seems unlikely to be the Wld gene. These data reveal both a candidate gene for Wld and the potential of the Wld(S) mutant for studies of ubiquitin and retinoid metabolism.

Rapid Real-Time Fluorescent PCR Gene Dosage Test for the Diagnosis of DNA Duplications and Deletions

Background: Current methods to determine gene dosage are time-consuming and labor-intensive. We describe a new and rapid method to assess gene copy number for identification of DNA duplications or deletions occurring in Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP), respectively.

Methods: We studied 16 patients with HNPP, 4 with CMT1A, and 49 control subjects. We used real-time PCR on the LightCycler system with use of a single capillary tube and no post-PCR handling. A polymorphic fragment of the PMP22 gene was amplified to determine gene dosage for heterozygous samples. The presence of two alleles was used to indicate that no deletion was present in HNPP samples. The ratio obtained between the areas under each allele melting curve of heterozygous CMT1A samples was used to determine whether the sequence was duplicated or normal. Homozygous samples required a competitive gene dosage test, where the ratio between the areas under the melting curves of the target DNA of samples and of the competitor molecule was used to determine whether the target sequence was duplicated, deleted, or normal. Samples from HNPP, CMT1A, and controls were analyzed.

Results: Area ratios were ∼0.6, 1.0, and 2.0 for HNPP, control, and CMT1A samples, respectively. The results agreed with those obtained by Southern blotting and microsatellite analysis in the same samples.

Conclusions: Direct and competitive real-time fluorescent PCR can differentiate one, two, or three copies of the target DNA. The method described is sensitive and accurate for detection of CMT1A duplications and HNPP deletions and is faster and easier than current methods.

Exposure at the cell surface is required for gas3/PMP22 To regulate both cell death and cell spreading: implication for the Charcot-Marie-Tooth type 1A and Dejerine-Sottas diseases

Gas3/PMP22 is a tetraspan membrane protein highly expressed in myelinating Schwann cells. Point mutations in the gas3/PMP22 gene account for the dominant inherited peripheral neuropathies Charcot-Marie-Tooth type 1A disease (CMT1A) and Dejerine-Sottas syndrome (DSS). Gas3/PMP22 can regulate apoptosis and cell spreading in cultured cells. Gas3/PMP22 point mutations, which are responsible for these diseases, are defective in this respect. In this report, we demonstrate that Gas3/PMP22-WT is exposed at the cell surface, while its point-mutated derivatives are intracellularly retained, colocalizing mainly with the endoplasmic reticulum (ER). The putative retrieval motif present in the carboxyl terminus of Gas3/PMP22 is not sufficient for the intracellular sequestration of its point-mutated forms. On the contrary, the introduction of a retrieval signal at the carboxyl terminus of Gas3/PMP22-WT leads to its intracellular accumulation, which is accompanied by a failure to trigger cell death as well as by changes in cell spreading. In addition, by substituting the Asn at position 41 required for N-glycosylation, we provide evidence that N-glycosylation is required for the full effect on cell spreading, but it is not necessary for triggering cell death. In conclusion, we suggest that the DSS and the CMT1A neuropathies derived from point mutations of Gas3/PMP22 might arise, at the molecular level, from a reduced exposure of Gas3/PMP22 at the cell surface, which is required to exert its biological functions.

Clinical predominance of proximal upper limb weakness in CMT1A syndrome

We report an Austrian family with proximal muscle weakness and wasting predominantly of the shoulder girdle musculature, normal or slightly reduced distal muscle power, mild foot deformity, absent or reduced tendon reflexes in the lower limbs, and normal or slightly diminished sensation. Electrophysiologically, motor nerve conduction velocities were slowed to less than 33 m/s, distal latencies were prolonged, and compound motor action potentials were low. Sensory nerve conduction velocities were extremely reduced or no sensory potentials were recordable. Genetic testing in three affected individuals revealed a duplication of the chromosomal region 17p11.2. In addition, genetic testing for facioscapulohumeral muscular dystrophy (FSHD) revealed a 33 kb EcoRI fragment on chromosome 4q35 in one affected individual and in the clinically normal parent, whereas in a second affected person normal DNA-sizes were observed. These clinical findings define a new phenotypic variant associated with the Charcot-Marie-Tooth 1A duplication. This may be due to a mutation in another gene contained in the 1.5 Mb duplication although mutations in the peripheral myelin protein 22 gene have been excluded. Alternatively, the genetic background of other genes in the family may modify the phenotypic expression, as found in other inherited diseases. The unusual phenotype cannot be explained by the concomitant presence of FSHD despite some evidence for coexistance in one individual.

N-myc downstream-regulated gene 1 is mutated in hereditary motor and sensory neuropathy-Lom

Hereditary motor and sensory neuropathies, to which Charcot-Marie-Tooth (CMT) disease belongs, are a common cause of disability in adulthood. Growing awareness that axonal loss, rather than demyelination per se, is responsible for the neurological deficit in demyelinating CMT disease has focused research on the mechanisms of early development, cell differentiation, and cell-cell interactions in the peripheral nervous system. Autosomal recessive peripheral neuropathies are relatively rare but are clinically more severe than autosomal dominant forms of CMT, and understanding their molecular basis may provide a new perspective on these mechanisms. Here we report the identification of the gene responsible for hereditary motor and sensory neuropathy-Lom (HMSNL). HMSNL shows features of Schwann-cell dysfunction and a concomitant early axonal involvement, suggesting that impaired axon-glia interactions play a major role in its pathogenesis. The gene was previously mapped to 8q24.3, where conserved disease haplotypes suggested genetic homogeneity and a single founder mutation. We have reduced the HMSNL interval to 200 kb and have characterized it by means of large-scale genomic sequencing. Sequence analysis of two genes located in the critical region identified the founder HMSNL mutation: a premature-termination codon at position 148 of the N-myc downstream-regulated gene 1 (NDRG1). NDRG1 is ubiquitously expressed and has been proposed to play a role in growth arrest and cell differentiation, possibly as a signaling protein shuttling between the cytoplasm and the nucleus. We have studied expression in peripheral nerve and have detected particularly high levels in the Schwann cell. Taken together, these findings point to NDRG1 having a role in the peripheral nervous system, possibly in the Schwann-cell signaling necessary for axonal survival.

A new variant of Charcot-Marie-Tooth disease type 2 is probably the result of a mutation in the neurofilament-light gene

Charcot-Marie-Tooth (CMT) disease is the most common inherited motor and sensory neuropathy. The axonal form of the disease is designated as "CMT type 2" (CMT2). Although four loci known to be implicated in autosomal dominant CMT2 have been mapped thus far (on 1p35-p36, 3q13. 1, 3q13-q22, and 7p14), no one causative gene is yet known. A large Russian family with CMT2 was found in the Mordovian Republic (Russia). Affected members had the typical CMT2 phenotype. Additionally, several patients suffered from hyperkeratosis, although the association, if any, between the two disorders is not clear. Linkage with the CMT loci already known (CMT1A, CMT1B, CMT2A, CMT2B, CMT2D, and a number of other CMT-related loci) was excluded. Genomewide screening pinpointed the disease locus in this family to chromosome 8p21, within a 16-cM interval between markers D8S136 and D8S1769. A maximum two-point LOD score of 5.93 was yielded by a microsatellite from the 5' region of the neurofilament-light gene (NF-L). Neurofilament proteins play an important role in axonal structure and are implicated in several neuronal disorders. Screening of affected family members for mutations in the NF-L gene and in the tightly linked neurofilament-medium gene (NF-M) revealed the only DNA alteration linked with the disease: a A998C transversion in the first exon of NF-L, which converts a conserved Gln333 amino acid to proline. This alteration was not found in 180 normal chromosomes. Twenty unrelated CMT2 patients, as well as 26 others with an undetermined form of CMT, also were screened for mutations in NF-L, but no additional mutations were found. It is suggested that Gln333Pro represents a rare disease-causing mutation, which results in the CMT2 phenotype.

Clustering of CMT1A duplication breakpoints in a 700 bp interval of the CMT1A-REP repeat

The CMT1A-REP repeat is proposed to mediate unequal crossover leading to a 1.5 Mb duplication in chromosome 17p11.2-12 associated with Charcot-Marie-Tooth neuropathy type 1A (CMT1A). There is an apparent recombinational "hotspot" in the CMT1A-REP repeat since the majority of crossover breakpoints for CMT1A are located within a 1.7 kb interval. Further to characterize the crossover breakpoint region, we constructed PCR primers that specifically amplify the duplication breakpoint junctions in a series of Japanese and Caucasian CMT1A patients. We mapped the breakpoints in 89% of patients within a 700 bp interval of the CMT1A-REP repeat. This 700 bp region is 1.3 kb telomeric to a previously described mariner-like transposable element. Our observations further define the location of crossovers for CMT1A and provide additional evidence that this region is a recombinational "hotspot" within the CMT1A-REP repeat.

Molecular dissection of the Schwann cell specific promoter of the PMP22 gene

PMP22, one of the major components of myelin, is overexpressed in Charcot-Marie-Tooth type 1A (CMT1A) patients. In an attempt to determine the mechanisms by which the expression of this gene is regulated (with a view to lowering its expression in CMT1A patients), we subcloned genomic fragments covering 6kb of the promoter region in an expression vector containing the beta-galactosidase gene as reporter, and used these in transfection assays. We show that the 300bp upstream of the transcription start contain the elements required for Schwann cell specific expression of the reporter gene. This minimal promoter activity appears to be under the control of a silencer element sensitive to cAMP, located between -0.3kb and -3. 5kb from the start of transcription. Computer analysis of 2kb of the promoter predicted the presence of transcription factor binding sites, including CREB (which may be involved in the response of PMP22 expression to cAMP stimulation) and steroid receptors. Using constructs with or without the CREB sites, we were able to demonstrate that these sites are involved in silencing the PMP22 promoter activity. Lastly, we identified a region containing blocks of polymorphic CA repeats, located close to the CREB binding site, which may further influence the transcriptional activity of PMP22.

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.

The peripheral myelin protein 22 and epithelial membrane protein family

The peripheral myelin protein 22 (PMP22) and the epithelial membrane proteins (EMP-1, -2, and -3) comprise a subfamily of small hydrophobic membrane proteins. The putative four-transmembrane domain structure as well as the genomic structure are highly conserved among family members. PMP22 and EMPs are expressed in many tissues, and functions in cell growth, differentiation, and apoptosis have been reported. EMP-1 is highly up-regulated during squamous differentiation and in certain tumors, and a role in tumorigenesis has been proposed. PMP22 is most highly expressed in peripheral nerves, where it is localized in the compact portion of myelin. It plays a crucial role in normal physiological and pathological processes in the peripheral nervous system. Progress in molecular genetics has revealed that genetic alterations in the PMP22 gene, including duplications, deletions, and point mutations, are responsible for several forms of hereditary peripheral neuropathies, including Charcot-Marie-Tooth disease type 1A (CMT1A), Dejerine-Sottas syndrome (DDS), and hereditary neuropathy with liability to pressure palsies (HNPP). The natural mouse mutants Trembler and Trembler-J contain a missense mutation in different hydrophobic domains of PMP22, resulting in demyelination and Schwann cell proliferation. Transgenic mice carrying many copies of the PMP22 gene and PMP22-null mice display a variety of defects in the initial steps of myelination and/or maintenance of myelination, whereas no pathological alterations are detected in other tissues normally expressing PMP22. Further characterization of the interactions of PMP22 and EMPs with other proteins as well as their regulation will provide additional insight into their normal physiological function and their roles in disease and possibly will result in the development of therapeutic tools.

Analysis of gap junction assembly using mutated connexins detected in Charcot-Marie-Tooth X-linked disease

The assembly of gap junction intercellular communication channels was studied by analysis of the molecular basis of the dysfunction of connexin 32 mutations associated with the X-linked form of Charcot-Marie-Tooth disease in which peripheral nervous transmission is impaired. A cell-free translation system showed that six recombinant connexin 32 mutated proteins-four point mutations at the cytoplasmic amino terminus, one at the membrane aspect of the cytoplasmic carboxyl terminus, and a deletion in the intracellular loop-were inserted into microsomal membranes and oligomerised into connexon hemichannels with varying efficiencies. The functionality of the connexons was determined by the ability of HeLa cells expressing the respective connexin cDNAs to transfer Lucifer yellow. The intracellular trafficking properties of the mutated connexins were determined by immunocytochemistry. The results show a relationship between intracellular interruption of connexin trafficking, the efficiency of intercellular communication, and the severity of the disease phenotype. Intracellular retention was explained either by deficiencies in the ability of connexins to oligomerise or by mutational changes at two targeting motifs. The results point to dominance of two specific targeting motifs: one at the amino terminus and one at the membrane aspect of the cytoplasmically located carboxyl tail. An intracellular loop deletion of six amino acids, associated with a mild phenotype, showed partial oligomerisation and low intercellular dye transfer compared with wild-type connexin 32. The results show that modifications in trafficking and assembly of gap junction channels emerge as a major feature of Charcot-Marie-Tooth X-linked disease.

Screening for mutations in the peripheral myelin genes PMP22, MPZ and Cx32 (GJB1) in Russian Charcot-Marie-Tooth neuropathy patients

Charcot-Marie-Tooth disease (CMT) and related inherited peripheral neuropathies, including Dejerine-Sottas syndrome, congenital hypomyelination, and hereditary neuropathy with liability to pressure palsies (HNPP), are caused by mutations in three myelin genes: PMP22, MPZ and Cx32 (GJB1). The most common mutations are the 1.5 Mb CMT1A tandem duplication on chromosome 17p11.2-p12 in CMT1 patients and the reciprocal 1.5 Mb deletion in HNPP patients. We performed a mutation screening in 174 unrelated CMT patients and three HNPP families of Russian origin. The unrelated CMT patients included 108 clinically and electrophysiologically diagnosed CMT1 cases, 32 CMT2 cases, and 34 cases with unspecified CMT. Fifty-nine CMT1A duplications were found, of which 58 belonged to the CMT1 patient group. We found twelve distinct mutations in Cx32, six mutations in MPZ, and two mutations in PMP22. Of these respectively, eight, five, and two lead to a CMT1 phenotype. Eight mutations (Cx32: Ile20Asn/Gly21Ser, Met34Lys, Leu90Val, and Phe193Leu; MPZ: Asp134Gly, Lys138Asn, and Thr139Asn; PMP22: ValSer25-26del) were not reported previously. Phenotype-genotype correlations were based on nerve conduction velocity studies and mutation type.

Peripheral myelin modification in CMT1B correlates with MPZ gene mutations

Morphological modifications were investigated in the peripheral nerve of three unrelated patients with CMT1B. In two patients, molecular genetic analysis showed an Arg98His mutation in the extracellular domain of MPZ, associated with irregularly uncompacted lamellae. This observation confirms previous studies of a well-defined correlation between mutations and morphological phenotypes. In the third patient, a de novo Asp109Asn mutation was associated with abnormally thick myelin sheaths. This adds to the known list of MPZ gene mutations associated with this morphological phenotype.

Distal axonopathy in peripheral nerves of PMP22-mutant mice

A partial duplication of chromosome 17 is associated with Charcot-Marie-Tooth disease type 1A (CMT1A), a demyelinating peripheral neuropathy that causes progressive distal muscle atrophy and sensory impairment. Trisomic expression of peripheral myelin protein 22 (PMP22) whose gene is contained within the duplicated region is considered to be responsible for the disease. By using recombinant gene technology in rodents, we had demonstrated previously that PMP22 is sensitive to gene dosage. Homozygous PMP22 knockout (PMP22(0/0)) mice and transgenic animals carrying additional copies of the PMP22 gene develop distinct peripheral polyneuropathies. We have now performed a detailed morphometrical analysis of the L3 roots, quadriceps and saphenous nerves of these PMP22-mutant mice to study whether the myelin and potential axonal deficits are evenly distributed. The L3 roots and the peripheral nerves were chosen as representatives of the proximal and distal segments of the peripheral nervous system. When the roots were compared with the peripheral nerves, myelin deficiencies appeared more severe at the radicular levels, in particular the ventral roots. Decreased numbers of large calibre axons were a prominent feature in the motor branches of both strains of PMP22-mutant mice, and these axonal deficits were more severe distally. Active axonal damage was only observed in the nerves of PMP22(0/0) mice. Despite the distinct effects on myelination and the Schwann cell phenotype that characterize the neuropathies of PMP22-mutant mice, both strains develop a distally accentuated axonopathy as a common disease mechanism which is likely to be responsible for the neurological deficits.

Rho-dependent regulation of cell spreading by the tetraspan membrane protein Gas3/PMP22

Gas3/PMP22 plays a crucial role in regulating myelin formation and maintenance, and different genetic alterations in gas3/PMP22 are responsible for a set of human peripheral neuropathies. We have previously demonstrated that Gas3/PMP22 could regulate susceptibility to apoptosis in NIH3T3 cells but not in REF 52 cells. In this report we demonstrate that when the apoptotic response triggered by gas3/PMP22 was counteracted by Bcl-2 coexpression, morphological changes were observed. Time-lapse analysis confirmed that Gas3/PMP22 can modulate cell spreading, and this effect was strengthened after inhibition of phosphoinositide 3-kinase. Using the active form of the small GTPase RhoA, we have been able to dissect the different Gas3/PMP22 biological activities. RhoA counteracted the Gas3/PMP22-dependent morphological response but was unable to neutralize the apoptotic response. Treatment of NIH3T3 cells with cytotoxic necrotizing factor 1, which activates endogenous Rho, also counteracted Gas3/PMP22-mediated cell shape and spreading changes. Treatment of REF 52 cells, which are unresponsive to Gas3/PMP22 overexpression, with the C3 exoenzyme, inhibiting Rho activity, renders REF 52 cells responsive to Gas3/PMP22 overexpression for cell shape and spreading changes. Finally, assembly of stress fibers and focal adhesions complexes, in response to lysophosphatidic acid-induced endogenous Rho activation, was impaired in Gas3/PMP22-overexpressing cells. We hypothesize that cell shape and spreading regulated by Gas3/PMP22 through the Rho GTPase might have an important role during Schwann cells differentiation and myelinization.

Prenatal diagnosis of Charcot-Marie-Tooth disease type 1A by interphase fluorescence in situ hybridization

Charcot-Marie-Tooth Disease (CMT) is the most common cause of peripheral neuropathy, with an incidence of 1:2500 persons affected. Previously, we reported the use of fluorescence in situ hybridization (FISH) to detect the common submicroscopic duplication of 17p12 found in more than 98 per cent of individuals with CMT1A. We found that FISH is a reliable means for the diagnosis of the duplication of 17p12 in peripheral blood and reported the validation of the FISH assay for amniotic fluid specimens. Herein, we report the validation of the FISH assay for use on chorionic villus samples (CVS) to prenatally diagnose CMT1A duplications and the testing of 17 prenatal specimens. Seven fetuses were found to carry the duplication and are predicted to be affected. FISH is a rapid assay in prenatal specimens, with a 9.3 day average turn-around time. Limited follow-up on pregnancies indicates that the duplication found in CMT1A is reliably diagnosed in the fetus, using FISH on either amniotic fluid specimens or CVS.

Charcot-Marie-Tooth disease type 1A duplication by PCR analysis

By generating new junctional fragments from the recombinant Charcot-Marie-Tooth (CMT) 1A-REPs in CMT1A patients, a 3.2-kb recombination hot spot is observed in three quarters of CMT1A patients. By a polymerase chain reaction (PCR) method the authors analyzed eight patients CMT1A duplication, confirmed by Southern blot, to detect a recombination hot spot. Four patients had a novel 3.2-kb junctional fragment by PCR analysis. These four patients with a novel 3.2-kb junctional fragment had an abnormal 1789-bp fragment in addition to 1986-bp fragment after NsiI digestion (type 1). One patient who demonstrated no novel 3.2-kb junctional fragment had an abnormal 336-bp fragment in addition to 265 bp (type 2). Three patients with CMT1A duplication were not diagnosed as having CMT1A on the basis of PCR analysis. The PCR-based DNA test is valuable for screening to detect CMT1A duplication.

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.

Sensorineural deafness in X-linked Charcot-Marie-Tooth disease with connexin 32 mutation (R142Q)

OBJECTIVE

To report a family with X-linked Charcot-Marie-Tooth disease (CMTX) with proven connexin 32 (Cx32) mutation associated with deafness.

METHODS

Twelve members of a CMTX family were examined clinically. Electromyography and sensory and motor conduction studies were performed in three men, two women, and a 7-year-old boy. Audiometric testing was carried out in the three men, one woman, and an 8-year-old girl. Molecular genetic analysis was performed in six men and five women.

RESULTS

The three men and the 7-year-old boy had the usual sensorimotor deficit and pronounced reduction of motor nerve conduction velocity. A 15-year-old boy was asymptomatic and had only areflexia. The women had impairment of vibratory sensation and slight slowing of nerve conduction velocities. Sensorineural deafness was observed in the three men and in an 8-year-old girl without any motor or sensory deficit. Molecular genetic analysis revealed a new missense mutation located in codon 142 of the Cx32 gene leading to the substitution of an arginine by a glutamine.

CONCLUSION

CMTX due to Cx32 mutations often shows interfamilial and intrafamilial phenotypic variation, which is also the hallmark of this family. The sensorineural deafness observed in this family suggests that Cx32 could play an important role in the auditory pathway.

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.

Pathogenesis of Charcot-Marie-Tooth 1A (CMT1A) neuropathy

The hereditary neuropathy Charcot-Marie-Tooth (CMT) type 1A is, in the majority of cases, caused by duplication of the gene for the peripheral myelin protein PMP22, which leads to abnormally increased PMP22 expression. Recent in vitro and in vivo data indicate a novel function of PMP22 in Schwann-cell growth and differentiation other than its role in myelination, and suggest that overproduction of PMP22 leads to a new Schwann-cell phenotype in CMT1A. Taking these data into account, we developed a new hypothesis on the pathogenesis of CMT1A neuropathy: that the defective myelin stability and turnover observed in the disease is caused by altered PMP22 gene dosage and its resultant effect on abnormal Schwann-cell growth and differentiation.

Mutation analysis of the nerve specific promoter of the peripheral myelin protein 22 gene in CMT1 disease and HNPP

We analysed the nerve specific promoter of the peripheral myelin protein 22 gene (PMP22) in a set of 15 unrelated patients with Charcot-Marie-Tooth type 1 disease (CMT1) and 16 unrelated patients with hereditary neuropathy with liability to pressure palsies (HNPP). In these patients no duplication/deletion nor a mutation in the coding region of the CMT1/ HNPP genes was detected. In one autosomal dominant CMT1 patient, we identified a base change in the non-coding exon 1A of PMP22 which, however, did not cosegregate with the disease in the family. This study indicates that mutations in the nerve specific PMP22 promoter and 5' untranslated exon will not be a common genetic cause of CMT1A and HNPP.

Pelizaeus-Merzbacher disease: identification of Xq22 proteolipid-protein duplications and characterization of breakpoints by interphase FISH

Pelizaeus-Merzbacher disease (PMD) is an X-linked, dysmyelinating disorder of the CNS. Duplications of the proteolipid protein (PLP) gene have been found in a proportion of patients, suggesting that, in addition to coding-region or splice-site mutations, overdosage of the gene can cause PMD. We show that the duplication can be detected by interphase FISH, using a PLP probe in five patients and their four asymptomatic carrier mothers. The extent of the duplication was analyzed in each family by interphase FISH, with probes from a 1. 7-Mb region surrounding the PLP gene between markers DXS83 and DXS94. A large duplication >=500 kb was detected, with breakpoints that differed, between families, at the proximal end. Distinct separation of the duplicated PLP signals could be seen only on metaphase chromosomes in one family, providing further evidence that different duplication events are involved. Quantitative fluorescent multiplex PCR was used to confirm the duplication in patients, by the detection of increased copy number of the PLP gene. Multiallelic markers from the duplicated region were analyzed, since the identification of two alleles in an affected boy would indicate a duplication. The majority of boys were homozygous for all four markers, compared with their mothers, who were heterozygous for one to three of the markers. These results suggest that intrachromosomal rearrangements may be a common mechanism by which duplications arise in PMD. One boy was heterozygous for the PLP marker, indicating a duplication and suggesting that interchromosomal rearrangements of maternal origin also can be involved. Since duplications are a major cause of PMD, we propose that interphase FISH is a reliable method for diagnosis and identification of female carriers.