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

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

Recombination hot spot in a 3.2-kb region of the Charcot-Marie-Tooth type 1A repeat sequences: new tools for molecular diagnosis of hereditary neuropathy with liability to pressure palsies and of Charcot-Marie-Tooth type 1A. French CMT Collaborative Research Group.

Charcot-Marie-Tooth type 1A (CMT1A) disease and hereditary neuropathy with liability to pressure palsies (HNPP) are autosomal dominant neuropathies, associated, respectively, with duplications and deletions of the same 1.5-Mb region on 17p11.2-p12. These two rearrangements are the reciprocal products of an unequal meiotic crossover between the two chromosome 17 homologues, caused by the misalignment of the CMT1A repeat sequences (CMT1A-REPs), the homologous sequences flanking the 1.5-Mb CMT1A/HNPP monomer unit. In order to map recombination breakpoints within the CMT1A-REPs, a 12.9-kb restriction map was constructed from cloned EcoRI fragments of the proximal and distal CMT1A-REPs. Only 3 of the 17 tested restriction sites were present in the proximal CMT1A-REP but absent in the distal CMT1A-REP, indicating a high degree of homology between these sequences. The rearrangements were mapped in four regions of the CMT1A-REPs by analysis of 76 CMT1A index cases and 38 HNPP patients, who where unrelated. A hot spot of crossover breakpoints, located in a 3.2-kb region, accounted for three-quarters of the rearrangements, detected after EcoRI/SacI digestion, by the presence of 3.2-kb and 7.8-kb junction fragments in CMT1A and HNPP patients, respectively. These junction fragments, which can be detected on classical Southern blots, permit molecular diagnosis. Other rearrangements can also be detected by gene dosage on the same Southern blots.     

Mutational analysis of the MPZ, PMP22 and Cx32 genes in patients of Spanish ancestry with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsies (HNPP) are two inherited peripheral neuropathies. The most prevalent mutations are a reciprocal 1.5-Mb duplication and 1.5-Mb deletion, respectively, at the CMT1A/HNPP locus on chromosome 17p11.2. Point mutations in the coding region of the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) or connexin 32 (Cx32) have been reported in CMT patients, including CMT type 1 (CMT1), CMT type 2 (CMT2) and Déjérine-Sottas neuropathy (DS) patients, and only in the coding region of PMP22 in HNPP families lacking a deletion. We have investigated point and small mutations in the MPZ, PMP22 and Cx32 genes in a series of patients of Spanish ancestry: 47 CMT patients without duplications, and 5 HNPP patients without deletions. We found 15 different mutations in 16 CMT patients (34%). Nine different mutations in ten patients were detected in the Cx32 gene, this being the most frequently involved gene in this series, whereas five mutations involved the MPZ gene and only one the PMP22 gene. Six out of nine nucleotide substitutions in the Cx32 gene involved two codons encoding arginine at positions 164 and 183, suggesting that these two codons may constitute two Cx32 regions prone to mutate in the Spanish population. Analysis of HNPP patients revealed a 5′ splicing mutation in intron 1 of the PMP22 gene in a family with autosomal dominance, which confirms allelic heterogeneity in HNPP. Ectopic mRNA analysis on leukocytes suggests that this mutation might behave as a null allele. Received: 25 July 1996 / Revised: 15 November 1996     

Molecular evolution of the CMT1A-REP region: a human- and chimpanzee-specific repeat.

The CMT1A-REP repeat consists of two copies of a 24-kb sequence on human chromosome 17p11.2-12 that flank a 1.5-Mb region containing a dosage-sensitive gene, peripheral nerve protein-22 (PMP22). Unequal meiotic crossover mediated by misalignment of proximal and distal copies of the CMT1A-REP in humans leads to a 1.5-Mb duplication or deletion associated with two common peripheral nerve diseases, Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP). Previous molecular hybridization studies with CMT1A-REP sequences suggested that two copies of the repeat are also found in the chimpanzee, raising the possibility that this unique repeat arose during primate evolution. To further characterize the structure and evolutionary synthesis of the CMT1A-REP repeat, fluorescent in situ hybridization (FISH) analysis and heterologous PCR-based assays were carried out for a series of primates. Genomic DNA was analyzed with primers selected to differentially amplify the centromeric and telomeric ends of the human proximal and distal CMT1A-REP elements and an associated mariner (MLE) sequence. All primate species examined (common chimpanzee, pygmy chimpanzee, gorilla, orangutan, gibbon, baboon, rhesus monkey, green monkey, owl monkey, and galago) tested positive for a copy of the distal element. In addition to humans, only the chimpanzee was found to have a copy of the proximal CMT1A-REP element. All but one primate species (galago) tested positive for the MLE located within the CMT1A-REP sequence. These observations confirm the hypothesis that the distal CMT1A-REP element is the ancestral sequence which was duplicated during primate evolution, provide support for a human-chimpanzee clade, and suggest that insertion of the MLE into the CMT1A-REP sequence occurred in the ancestor of anthropoid primates.     

Screening of the 17p11.2–p12 region in a large cohort of patients with Charcot-Marie-Tooth (CMT) disease or hereditary neuropathy with liability to pressure palsies (HNPP)

Within the last decade, numerous methods have been applied to detect the most common mutation in patients affected with Charcot-Marie-Tooth (CMT) disease, i.e. submicroscopic duplication in the 17p11.2–p12 region. In 1993, another neuropathy — known as hereditary neuropathy with liability to pressure palsies (HNPP) — has been shown to be caused by a 17p11.2–p12 deletion. Historically, Southern blot analysis was the first approach to identify CMT1A duplication or HNPP deletion. This time- and labor-consuming method requires prior selection of DNA samples. In fact, only CMT patients affected with the demyelinating form of CMT1 have been screened for CMT1A duplication. After the 17p11.2–p12 duplication was identified in the CMT1 families, subsequent studies revealed additional axonal features in the patients harboring the 17p11.2–p12 duplication. Thus it seems reasonable to test all patients affected with CMT for the presence of the 17p11.2–p12 duplication. To evaluate the utility of real-time polymerase chain reaction (Q-PCR) and restriction fragment length polymorphism PCR (RFLP-PCR), we screened a large group of 179 families with the diagnosis of CMT/HNPP for the presence of the 17p11.2–p12 duplication/deletion. Due to a high frequency of CMT1A duplication in familial cases of CMT, we propose (in contrast to the previous studies) to perform Q-PCR analysis in all patients diagnosed with CMT.     

Rapid diagnosis of CMT1A duplications and HNPP deletions by multiplex microsatellite PCR

Charcot-Marie-Tooth (CMT) disease and hereditary neuropathy with liability to pressure palsies (HNPP) are frequent forms of genetically heterogeneous peripheral neuropathies. Reciprocal unequal crossover between flanking CMT1A-REPs on chromosome 17p11.2-p12 is a major cause of CMT type 1A (CMT1A) and HNPP. The importance of a sensitive and rapid method for identifying the CMT1A duplication and HNPP deletion is being emphasized. In the present study, we established a molecular diagnostic method for the CMT1A duplication and HNPP deletion based on hexaplex PCR of 6 microsatellite markers (D17S921, D17S9B, D17S9A, D17S918, D17S4A and D17S2230). The method is highly time-, cost- and sample-saving because the six markers are amplified by a single PCR reaction and resolved with a single capillary in 3 h. Several statistical and forensic estimates indicated that most of these markers are likely to be useful for diagnosing the peripheral neuropathies. Reproducibility, as determined by concordance between independent tests, was estimated to be 100%. The likelihood that genotypes of all six markers are homozygous in randomly selected individuals was calculated to be 1.6 x 10(-4) which indicates that the statistical error rate for this diagnosis of HNPP deletion is only 0.016%.     

Human meiotic recombination products revealed by sequencing a hotspot for homologous strand exchange in multiple HNPP deletion patients.

The HNPP (hereditary neuropathy with liability to pressure palsies) deletion and CMT1A (Charcot-Marie-Tooth disease type 1A) duplication are the reciprocal products of homologous recombination events between misaligned flanking CMT1A-REP repeats on chromosome 17p11. 2-p12. A 1.7-kb hotspot for homologous recombination was previously identified wherein the relative risk of an exchange event is 50 times higher than in the surrounding 98.7% identical sequence shared by the CMT1A-REPs. To refine the region of exchange further, we designed a PCR strategy to amplify the recombinant CMT1A-REP from HNPP patients as well as the proximal and distal CMT1A-REPs from control individuals. By comparing the sequences across recombinant CMT1A-REPs to that of the proximal and distal CMT1A-REPs, the exchange was mapped to a 557-bp region within the previously identified 1.7-kb hotspot in 21 of 23 unrelated HNPP deletion patients. Two patients had recombined sequences suggesting an exchange event closer to the mariner-like element previously identified near the hotspot. Five individuals also had interspersed patches of proximal or distal repeat specific DNA sequence indicating potential gene conversion during the exchange of genetic material. Our studies provide a direct observation of human meiotic recombination products. These results are consistent with the hypothesis that minimum efficient processing segments, which have been characterized in Escherichia coli, yeast, and cultured mammalian cells, may be required for efficient homologous meiotic recombination in humans.     

Mutation analysis of PMP22 in Slovak patients with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and related peripheral neuropathies are the most commonly inherited neurological disorders in humans, characterized by clinical and genetic heterogeneity. The most prevalent clinical entities belonging to this group of disorders are CMT type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP). CMT1A and HNPP are predominantly caused by a 1.5 Mb duplication and deletion in the chromosomal region 17p11.2, respectively, and less frequently by other mutations in the peripheral myelin protein 22 (PMP22) gene. Despite being relatively common diseases, they haven't been previously studied in the Slovak population. Therefore, the aim of this study was to identify the spectrum and frequency of PMP22 mutations in the Slovak population by screening 119 families with CMT and 2 families with HNPP for causative mutations in this gene. The copy number determination of PMP22 resulted in the detection of CMT1A duplication in 40 families and the detection of HNPP deletion in 7 families, 6 of which were originally diagnosed as CMT. Consequent mutation screening of families without duplication or deletion using dHPLC and sequencing identified 6 single base changes (3 unpublished to date), from which only c.327C>A (Cys109X) present in one family was provably causative. These results confirm the leading role of PMP22 mutation analysis in the differential diagnosis of CMT and show that the spectrum and frequency of PMP22 mutations in the Slovak population is comparable to that seen in the global population.     

Polymorphisms in the PMP-22 gene region (17p11.2–12) are crucial for simplified diagnosis of duplications/deletions

DNA duplications and deletions of a 1.5-Mb region in chromosome 17p11.2–12 comprising the gene encoding peripheral myelin protein 22 (PMP-22) are the common mutations in Charcot-Marie-Tooth disease type 1 (CMT1) and hereditary neuropathy with liability to pressure palsies (HNPP). A 1.7-kb recombination hotspot region has been identified within misaligned flanking repeats (CMT1-REP elements) by detection of CMT- and HNPP-specific junction fragments in Southern blot analyses. In order to simplify routine diagnosis we introduce a polymerase chain reaction-based method to identify directly specific REP junction fragments. Using this test, specific fragments were detected in ∼ 67% of both CMT duplication and HNPP deletion cases. Polymorphism within a specific restriction enzyme recognition site is crucial for both Southern blot and PCR analyses of junction fragments. Received: 25 October 1996 / Revised: 16 December 1996     

A unique point mutation in the PMP22 gene is associated with Charcot-Marie-Tooth disease and deafness.

Charcot-Marie-Tooth disease (CMT) with deafness is clinically distinct among the genetically heterogeneous group of CMT disorders. Molecular studies in a large family with autosomal dominant CMT and deafness have not been reported. The present molecular study involves a family with progressive features of CMT and deafness, originally reported by Kousseff et al. Genetic analysis of 70 individuals (31 affected, 28 unaffected, and 11 spouses) revealed linkage to markers on chromosome 17p11.2-p12, with a maximum LOD score of 9.01 for marker D17S1357 at a recombination fraction of .03. Haplotype analysis placed the CMT-deafness locus between markers D17S839 and D17S122, a approximately 0.6-Mb interval. This critical region lies within the CMT type 1A duplication region and excludes MYO15, a gene coding an unconventional myosin that causes a form of autosomal recessive deafness called DFNB3. Affected individuals from this family do not have the common 1.5-Mb duplication of CMT type 1A. Direct sequencing of the candidate peripheral myelin protein 22 (PMP22) gene detected a unique G-->C transversion in the heterozygous state in all affected individuals, at position 248 in coding exon 3, predicted to result in an Ala67Pro substitution in the second transmembrane domain of PMP22.     

Study of the duplication of 17p11.2-12 chromosome region in the patients with hereditary motor and sensory neuropathy type 1A

Charcot-Marie-Tooth neuropathy (CMT) is one of the most common hereditary disorders, affecting 1:2500 individuals. CMT is a heterogeneous group of disorders characterized by chronic peripheral motor and sensory neuropathy. We have performed the detection of 1.5 Mb CMT1A tandem duplication in 17p11.2-12 chromosome region for autosome-dominant CMT1 patients and their relatives using the analysis of two (CA)n polymorphic microsatellite loci: 17S921 and 17S1358 localised in the duplication region. CMT1A duplication was found in three of five autosome-dominant CMT1 families. It has been shown that CMT1A duplication analysis is important for early differential diagnosis of CMT including prenatal diagnosis and genetic consulting in high risk families.     

Analysis of 17p11.2 chromosome region rearrangements in CMT1 patients from Ukraine

Two intercomplementary methods of 17p11.2 duplication/deletion identification have been elaborated: STR allelic variants analysis and direct PMP22 gene dosage measuring by means of quantitative Real-Time PCR. It has been carried out detection and analysis of 17p11.2 chromosome region rearrangements in CMT1 patients from Ukraine. It has been registered the high level of de novo cases with 17p11.2-duplication. It has been shown the 17p11.2 chromosome region duplication/deletion association with CMT1A and HNPP clinical phenotypes which may be used in differential diagnosis of this type of CMT polyneuropathy. The article is published in the original.     

DNA rearrangements on both homologues of chromosome 17 in a mildly delayed individual with a family history of autosomal dominant carpal tunnel syndrome.

Disorders known to be caused by molecular and cytogenetic abnormalities of the proximal short arm of chromosome 17 include Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), Smith-Magenis syndrome (SMS), and mental retardation and congenital anomalies associated with partial duplication of 17p. We identified a patient with multifocal mononeuropathies and mild distal neuropathy, growth hormone deficiency, and mild mental retardation who was found to have a duplication of the SMS region of 17p11.2 and a deletion of the peripheral myelin protein 22 (PMP22) gene within 17p12 on the homologous chromosome. Further molecular analyses reveal that the dup(17)(p11.2p11.2) is a de novo event but that the PMP22 deletion is familial. The family members with deletions of PMP22 have abnormalities indicative of carpal tunnel syndrome, documented by electrophysiological studies prior to molecular analysis. The chromosomal duplication was shown by interphase FISH analysis to be a tandem duplication. These data indicate that familial entrapment neuropathies, such as carpal tunnel syndrome and focal ulnar neuropathy syndrome, can occur because of deletions of the PMP22 gene. The co-occurrence of the 17p11.2 duplication and the PMP22 deletion in this patient likely reflects the relatively high frequency at which these abnormalities arise and the underlying molecular characteristics of the genome in this region.     

Polymorphic short tandem repeats for PCR-based diagnosis of the Charcot-Marie-Tooth 1A duplication in Ukraine

Charcot-Marie-Tooth neuropathy (CMT) is one of the most common hereditary disorders, affecting 1:2500 individuals. The major mutation--microduplication of 1.4 megabases in 17p11.2 region, which is responsible for 68-90 % of cases of CMT1, results in CMT1A. In the present article we provide the population genetic study in 52 unrelated non-CMT volunteers from population of Ukraine in three STRs (D17S921, D17S1358 and D17S122) from the 17p11.2 chromosomal region to determine their ability for the CMT1A-duplication detection using STR-PCR method in Ukraine. The informativity for the CMT1A detection in current use STR panel is calculated to be 93,6%. It has been shown that current use STR panel analysis is important for CMT1A duplication detection, early differential diagnosis of CMT including prenatal diagnosis and genetic consulting in high risk families.     

Evidence for genetic heterogeneity underlying hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (HNPP) is a disorder of the peripheral nervous system, the cause of which has recently been identified as a deletion on chromosome 17p. The deletion corresponds to the duplication that is commonly observed in patients with hereditary motor and sensory neuropathy type Ia (HMSNIa, 17p11.2–p12). Therefore, the gene for peripheral myelin protein 22 (PMP-22) is a candidate gene for both HMSNIa and HNPP. Here, we show that a similar deletion is present in one family with HNPP but is clearly absent in another family. Affected members of this family carry the expected two copies of the PMP-22 gene and the surrounding region. Furthermore, linkage analyses of this family exclude a large part of 17p, spanning the area deleted in other families with HNPP, as the location for the disease gene. These data strongly argue for the existence of genetic heterogeneity underlying HNPP. Results from two-point linkage analysis with markers on chromosome 1q are inconsistent with a possible involvement of the locus for HMSNIb in the present family.     

Assignment of the Charcot-Marie-Tooth neuropathy type 1 (CMT 1a) gene to 17p11.2-p12.

Charcot-Marie-Tooth disease type 1a (CMT 1a) is an autosomal dominant peripheral neuropathy linked to the DNA markers D17S58 and D17S71, located in the pericentromeric region of the chromosome 17p arm. We analyzed an extended 5-generation Belgian family, multiply affected with CMT 1a, for linkage with eight chromosome 17 markers. The results indicated that the CMT 1a mutation is localized in the chromosomal region 17p11.2-p12 between the marker D17S71 and the gene for myosin heavy polypeptide 2 of adult skeletal muscle.     

Prevalence and origin of de novo duplications in Charcot-Marie-Tooth disease type 1A: first report of a de novo duplication with a maternal origin.

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy. Sporadic cases of CMT have been described since the earliest reports of the disease. The most frequent form of the disorder, CMT1A, is associated with a 1.5-Mb DNA duplication on chromosome 17p11.2, which segregates with the disease. In order to investigate the prevalence of de novo CMT1A duplications, this study examined 118 duplication-positive CMT1A families. In 10 of these families it was demonstrated that the disease had arisen as the result of a de novo mutation. By taking into account the ascertainment of families, it can be estimated that > or = 10% of autosomal dominant CMT1 families are due to de novo duplications. The CMT1A duplication is thought to be the product of unequal crossing over between parental chromosome 17 homologues during meiosis. Polymorphic markers from within the duplicated region were used to determine the parental origin of these de novo duplications in eight informative families. Seven were of paternal and one of maternal origin. This study represents the first report of a de novo duplication with a maternal origin and indicates that it is not a phenomenon associated solely with male meioses. Recombination fractions for the region duplicated in CMT1A are larger in females than in males. That suggests that oogenesis may be afforded greater protection from misalignment during synapsis, and/or that there may be lower activity of those factors or mechanisms that lead to unequal crossing over at the CMT1A locus.     

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

Charcot-Marie-Tooth type (CMT1) disease or hereditary motor and sensory neuropathy type I (HMSNI) is an autosomal dominant peripheral neuropathy. In most CMT1 families, the disease cosegregates with a 1.5-Mb duplication on chromosome 17p11.2 (CMT1A). A few patients have been found with mutations in the peripheral myelin protein 22 (PMP-22) gene located in the CMT1A region. In other families mutations have been identified in the major peripheral myelin protein p_o gene localized on chromosome Iq21-q23 (CMT1B). We performed a rapid mutation screening of the PMP-22 and P₀ genes in non-duplicated CMT1 patients by single-strand conformation polymorphism analysis followed by direct polymerase chain reaction sequencing of genomic DNA. Six new single base changes in the P₀ gene were observed: two missense mutations in, respectively, exons 2 and 3, two nonsense mutations in exon 4, and two silent mutations or polymorphisms in, respectively, exons 3 and 6.     

Mosaicism for the Charcot-Marie-Tooth disease type 1A duplication suggests somatic reversion

A female patient with clinical signs and symptoms of a demyelinating neuropathy was shown to have a duplication of the 1.5-Mb region on chromosome 17p11.2, typical of the great majority of cases of Charcot-Marie-Tooth disease type 1A (CMT1A). However, analysis of DNA extracted from peripheral blood revealed a 2:2.4 instead of the usual 2:3 ratio between the 7.8- and 6.0-kb EcoRI fragments in the proximal and distal repetitive extragenic palindromic (REP) elements of CMT1A. Detection of a 3.2-kb EcoRI/SacI kb junction fragment with probe pLR7.8 confirmed the CMT1A duplication. The dosage of this junction fragment, compared with a 2.8-kb EcoRI/SacI fragment of the proximal REP elements of CMT1A, was 2:0.58 instead of the expected 2:1 dosage for heterozygous CMT1A duplications. We hypothesized that the lower dosages of these restriction fragments specific for the CMT1A duplication were due to mosaicism; this was confirmed by fluorescence in situ hybridization analysis with the D17S122-specific probe pVAW409R1. In peripheral blood lymphocytes the percentage of interphase nuclei with a duplication in 17p11.2 was 49%. In interphase nuclei extracted from buccal mucosa, hair-root cells or paraffin-embedded nervous tissue the duplication was detectable in 51%, 66% and 74%, respectively. This is the first report of mosaicism in a patient with a CMT1A duplication identified by three different and independent techniques. Received: 14 November 1995 / Revised: 13 February 1996     

Duplication of the PMP22 gene in 17p partial trisomy patients with Charcot-Marie-Tooth type-1A neuropathy

Autosomal dominant Charcot-Marie-Tooth type-1A neuropathy (CMT1A) is a demyelinating peripheral nerve disorder that is commonly associated with a submicroscopic tandem DNA duplication of a 1.5-Mb region of 17p11.2p12 that contains the peripheral myelin gene PMP22. Clinical features of CMT1A include progressive distal muscle atrophy and weakness, foot and hand deformities, gait abnormalities, absent reflexes, and the completely penetrant electrophysiologic phenotype of symmetric reductions in motor nerve conduction velocities (NCVs). Molecular and fluorescence in situ hybridization (FISH) analyses were performed to determine the duplication status of the PMP22 gene in four patients with rare cytogenetic duplications of 17p. Neuropathologic features of CMT1A were seen in two of these four patients, in addition to the complex phenotype associated with 17p partial trisomy. Our findings show that the CMT1A phenotype of reduced NCV is specifically associated with PMP22 gene duplication, thus providing further support for the PMP22 gene dosage mechanism for CMT1A. Received: 3 May 1995 / Revised: 1 August 1995