Rapid detection of duplication/deletion of the PMP22 gene in patients with Charcot-Marie-Tooth disease Type 1A and hereditary neuropathy with liability to pressure palsy by real-time quantitative PCR using SYBR Green I dye

Mutations and altered gene dosage of the peripheral myelin protein (PMP22) gene in chromosome 17p11.2-12 are the main causes for hereditary neuropathies, accounting for approximately 70% of all cases. Patients with duplication of the PMP22 develop Charcot-Marie-Tooth disease type 1A (CMT1A) and deletion of one PMP22 allele leads to hereditary neuropathy with liability to pressure palsy (HNPP). Twenty patients with CMT1A, 17 patients with HNPP, and 18 normal family members and 28 normal controls were studied by real-time quantitative PCR using SYBR Green I on the ABI 7700 Sequence Detection System. The copy number of the PMP22 gene was determined by the comparative threshold cycle method and the albumin was used as a reference gene. The PMP22 duplication ratio ranged from 1.45 to 2.06 and the PMP22 deletion ratio ranged from 0.42 to 0.64. The PMP22 ratio in normal controls, including normal family members, ranged from 0.85 to 1.26. No overlap was found between patients with CMT1A or patients with HNPP and normal controls. This method is fast, highly sensitive, specific, and reproducible in detecting PMP22 duplication and deletion in CMT1A and HNPP patients, respectively.     

A rapid and reliable detection system for the analysis of PMP22 gene dosage by MP/DHPLC assay

Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are caused by a 1.5-Mb duplication and a deletion at chromosome 17p11.2–12 encompassing the peripheral myelin protein 22 gene (PMP22), respectively. We developed a rapid and reliable detection system for duplications/deletions of the PMP22 gene based on measurement of gene copy number. The method involves amplification of a test locus with unknown copy number and a reference locus of known copy number by multiplex PCR (MP), followed by denaturing high-performance liquid chromatography (DHPLC) or capillary electrophoresis detection to identify single copy changes. Thirty-two patients with CMT1A, 17 patients with HNPP, and 61 unaffected individuals were analyzed. Using the same competitive MP protocol, the measured PMP22 gene dosage revealed concordant results between DHPLC and capillary electrophoresis analysis. The results of the MP/DHPLC or the MP/capillary electrophoresis assay were all confirmed by PCR–restriction fragment length polymorphism analysis. We concluded that the MP/DHPLC assay is an efficient, accurate, and reliable technique for gene dosage determination of the PMP22 gene for CMT1A duplication and HNPP deletion. This technique further extends the application of DHPLC as an alternative method for the measurement of gene amplifications and heterozygous deletions in different genetic diseases.Chia-Yun Lin and Yi-Ning Su share the first authorship.     

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.     

Copy number variation upstream of PMP22 in Charcot�CMarie�CTooth disease

In several individuals with a Charcot–Marie–Tooth (CMT) phenotype, we found a copy number variation (CNV) on chromosome 17p12 in the direct vicinity of the peripheral myelin protein 22 (PMP22) gene. The exact borders and size of this CNV were determined by Southern blot analysis, MLPA, vectorette PCR, and microarray hybridization analyses. All patients from six apparently unrelated families carried an identical 186-kb duplication different from the commonly reported 1.5-Mb duplication associated with CMT1A. This ancestral mutation that was not reported in the human structural variation database was only detected in affected individuals and family members. It was absent in 2124 control chromosomes and 40 patients with a chronic inflammatory demyelinating polyneuropathy (CIDP) and therefore should be regarded as causative for the disease. This variant escapes most routine diagnostic screens for CMT1A, because copy numbers of PMP22 probes were all normal. No indications were found for the involvement of the genes that are located within this duplication. A possible association of this duplication with a mutation in the PMP22 coding regions was also excluded. We suggest that this CNV proximal of the PMP22 gene leads to CMT through an unknown mechanism affecting PMP22 expression.     

Asian hereditary neuropathy patients with peripheral myelin protein-22 gene aneuploidy

Japanese hereditary neuropathy with liability to pressure palsy (HNPP) patients have a deletion of one peripheral myelin protein-22 (PMP22) gene region in distal chromosome band 17p11.2 as do Caucasian patients. Japanese and Asiatic Indian CMT1A patients have a PMP22 gene duplication that results in Charcot-Marie-Tooth disease type IA (CMT1A; HMSNIA) in patients of European and Middle Eastern ancestry. About 70% of Japanese CMT1 patients have a PMP22 duplication as do Caucasians, while Japanese CMT1B, CMT2 and Dejerine-Sottas patients do not have PMP22 gene region aneuploidy. Although HNPP and CMT1A genotypes are generated simultaneously by unequal recombination that results in PMP22 gene aneuploidy in each daughter cell, only 3 Japanese HNPP probands with PMP22 deletion from a large patient population were referred to a single center compared to 18 referred CMT1A probands with PMP22 duplication. This lower HNPP frequency more likely reflects lower HNPP reproductive fitness than patient ascertainment bias because disease severity and variation in severity is about the same in CMT1A and HNPP patients and because all patients of both types were referred regardless of disease severity. These results, along with an apparently high de novo CMT1A mutation rate, suggest that common ancestors of Japanese, Asian Indians, and Caucasians carried PMP22 geneflanking sequences that enhance unequal crossing over. © 1995 Wiley-Liss, Inc.     

Rapid detection of 17p11.2 rearrangements by FISH without cell culture (direct FISH,DFISH): a prospective study of 130 patients with inherited peripheral neuropathies

Charcot-Marie-Tooth (CMT) disease and hereditary neuropathy with pressure palsies (HNPP) are two frequent hereditary motor and sensory neuropathies. CMT is characterized by slowly progressive weakness and atrophy, primarily in peroneal and distal leg muscles. The most frequent form, CMT1A, is due, in most cases, to the duplication of a 1.5 Mb region on chromosome 17p11.2 containing the peripheral myelin protein 22 gene (PMP22). The phenotype seems to result from dosage of the PMP22 gene. This hypothesis is reinforced by the existence of HNPP, which is clinically characterized by various recurrent truncular palsies or sensory loss precipitated by minor trauma, which is caused by deletion of the same 1.5 Mb region in 17p11.2. In clinical practice, the detection of the duplication or the deletion in 17p11.2, which permits a positive diagnosis, is still performed by time consuming methods (Southern blot or various combinations of molecular tools). We developed a method for the rapid detection of 17p11.2 rearrangements, using "direct FISH" and PRINS analyses, which does not require cell culture. In a prospective study of 92 patients with CMT and 38 with suspected HNPP, we compared this new technique to classical strategies like Southern blot. The results demonstrate the high sensitivity and specificity of the new FISH technique for the diagnosis of CMT1A and HNPP. Moreover, because of its simplicity and rapidity, this technique provides a useful alternative to the molecular approaches that have been used to diagnose segmental aneusomies, especially in the case of duplications that often go undetected.     

Detection of genomic rearrangements by DHPLC: a prospective study of 90 patients with inherited peripheral neuropathies associated with 17p11.2 rearrangements

Large genomic duplications and deletions are increasingly recognized as a cause of human disease. Charcot-Marie-Tooth type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsy (HNPP) result, respectively, from a duplication or deletion of a 1.5 Mb genomic region in 17p11.2-12, containing the PMP22 gene. In routine diagnostic analysis, CMT1A status is inferred from the detection of an imbalanced dosage of two alleles or the presence of three alleles of a polymorphic marker flanking the PMP22 gene. HNPP is suspected if only one allele is seen, but hemizygosity must be confirmed by analyzing allele segregation in the family or by other techniques such as Southern blotting or fluorescence in situ hybridization (FISH). PCR-based methodologies have also been developed that allow single-step determination of the PMP22 gene copy number, wherein amplicons are typically labeled and/or separated by gel electrophoresis. We describe here a fast and reliable PCR-based method for the diagnosis of CMT1A and HNPP in which the PMP22 gene is co-amplified with a reference gene, and the amplicons are separated according to their size and quantified by DHPLC. Our results suggest that this method for quantifying gene dosage could be applied to other genomic rearrangements.     

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.     

Strategies for clinical and molecular diagnosis of Charcot-Marie-Tooth 1A among Mexican patients

BACKGROUND: Charcot-Marie-Tooth (CMT) is the most common inherited disorder of the human peripheral nerve. The mos tfrequent subtype, CMT1A, is associated with duplication of approximately 1.5 Mb fragment in 17p11-p12, that includes the PMP22 gene. OBJECTIVE: The aim of this study was to describe different strategies used for clinical and molecular CNT1A diagnoses among patients attending the National Rehabilitation Institute of Mexico (INR). MATERIAL AND METHODS: 17 patients had clinical and electrophysiological features compatible with CMT1. A molecular study using capillary electrophoresis (CE) was performed and a PMP22 gene duplication was detected RESULTS: Clinical, biochemical and electrophysiological studies constituted the inclusion criteria to establish a CMT1 diagnosis. With CE the duplication of the PMP22 gene was observable and we established a possible CMT1A diagnosis in seven patients. All duplications detected by capillary electrophoresis were corroborated using FISH. CONCLUSION: CE is a feasible and reliable method to detect PMP22 gene duplication. Using different clinical, electrophysiological and molecular strategies in this patient population allowed us to establish an accurate diagnosis and offer suitable genetic counseling.     

Charcot-Marie-Tooth 1A Concurrent with Schwannomas of the Spinal Cord and Median Nerve

We identified Charcot-Marie-Tooth disease type 1A (CMT1A) in a family with schwannomas in the spinal cord and median nerve. The CMT1A in this family showed an autosomal dominant pattern, like other CMT patients with PMP22 duplication, and the family also indicated a possible genetic predisposition to schwannomas by ''mother-to-son'' transmission. CMT1A is mainly caused by duplication of chromosome 17p11.2-p12 (PMP22 gene duplication). A schwannoma is a benign encapsulated tumor originating from a Schwann cell. A case of hereditary neuropathy with liability to pressure palsies (HNPP) concurrent with schwannoma has been previously reported. Although it seems that the co-occurrence of CMT1A and schwannomas in a family would be the result of independent events, we could not completely ignore the possibility that the coincidence of two diseases might be due to a shared genetic background.     

A girl with duplication 17p10-p12 associated with a dicentric chromosome

We report a 7 1/2-year-old girl with an approximately 9.5 Mb duplication of proximal 17p. Her clinical features include moderately severe developmental delay, absence of speech, talipes, congenital dislocation of the hips, premature adrenarche, dysmorphic facial features, deep palmar creases, and signs and symptoms of peripheral neuropathy consistent with Charcot-Marie-Tooth disease type 1A (CMT1A). Chromosome analysis revealed a partially duplicated 17p with two centromeres on the derivative chromosome. Fluorescence in situ hybridization (FISH) analysis demonstrated the tandemly duplicated segment spans 17p10-p12, including the entire Smith-Magenis syndrome (SMS) critical region and a portion of the CMT1A critical region. One breakpoint mapped within the centromere and the second breakpoint mapped within the CMT1A critical region, distal to the PMP22 gene. Microsatellite polymorphism studies showed that the duplicated chromosome is of maternal origin. We compare the clinical features of our patient to those of individuals with partial trisomy of proximal 17p to further delineate the genotype-phenotype correlation associated with segmental duplication of this chromosomal region.     

Novel mutations in the Charcot-Marie-Tooth disease genes PMP22, MPZ,and GJB1

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous disorder of the peripheral nervous system. CMT type 1 is most frequently caused by a 1.4 Mb tandem duplication in chromosome 17p11.2 comprising the peripheral myelin protein 22 (PMP22) gene. Furthermore sequence variations of PMP22, myelin protein zero (MPZ) and the gap junction protein b 1 gene (GJB1 or Connexin 32) may cause a variety of distinct CMT phenotypes. In this study we screened DNA from 42 unrelated patients for mutations in the PMP22, MPZ and GJB1 genes. Four novel mutations were identified. A Val65Phe amino acid exchange in PMP22 causes CMT type 1 associated with deafness, in GJB1 Tyr7_Thr8delinsSer, Pro172Ala and Ser138Asn are causes of CMTX neuropathies".     

Effectiveness of real-time quantitative PCR compare to repeat PCR for the diagnosis of Charcot-Marie-Tooth Type 1A and hereditary neuropathy with liability to pressure palsies

The majority of cases of Charcot-Marie-Tooth type 1A (CMT1A) and of hereditary neuropathy with a liability to pressure palsies (HNPP) are the result of heterozygosity for the duplication or deletion of peripheral myelin protein 22 gene (PMP22) on 17p11.2. Southern blots, pulsed-field gel electrophoresis (PFGE), fluorescence in situ hybridization (FISH) and polymorphic marker analysis are currently used diagnostic methods. But they are time-consuming, labor-intensive and have some significant limitations. We describe a rapid real- time quantitative PCR method for determining gene copy number for the identification of DNA duplication or deletion occurring in CMT1A or HNPP and compare the results obtained with REP-PCR. Six patients with CMT1A and 14 patients with HNPP [confirmed by Repeat (REP)-PCR], and 16 patients with suspicious CMT1A and 13 patients with suspicious HNPP [negative REP-PCR], and 15 normal controls were studied. We performed REP-PCR, which amplified a 3.6 Kb region (including a 1.7Kb recombination hotspot), using specific CMT1A-REP and real-time quantitative PCR on the LightCycler system. Using a comparative threshold cycle (Ct) method and beta -globin as a reference gene, the gene copy number of the PMP22 gene was quantified. The PMP22 duplication ratio ranged from 1.35 to 1.74, and the PMP22 deletion ratio from 0.41 to 0.53. The PMP22 ratio in normal controls ranged from 0.81 to 1.12. All 6 patients with CMT1A and 14 patients with HNPP confirmed by REP-PCR were positive by real-time quantitative PCR. Among the 16 suspicious CMT1A and 13 suspicious HNPP with negative REP-PCR, 2 and 4 samples, respectively, were positive by real-time quantitative PCR. Real-time quantitative PCR is a more sensitive and more accurate method than REP-PCR for the detection of PMP22 duplications or deletions, and it is also faster and easier than currently available methods. Therefore, we believe that the real-time quantitative method is useful for diagnosing CMT1A and HNPP.     

Trisomy 17p10-p12 resulting from a supernumerary marker chromosome derived from chromosome 17: molecular analysis and delineation of the phenotype

We report a 5-year-old boy with a small de novo marker chromosome derived from the proximal short arm of chromosome 17. His clinical features include hypotonia, global developmental delay, oval face with large nose and prominent ears, and ligamentous laxity of the fingers. Magnetic resonance imaging of the brain demonstrated mildly delayed myelination. G-band chromosome analysis revealed mosaicism for a small marker chromosome in 85% of the peripheral blood cells analyzed. Fluorescence in situ hybridization and microsatellite polymorphism studies showed that the der(17) was of maternal origin and included genetic material from the 17p10-p12 region, but did not contain the PMP22 gene. One breakpoint mapped within the centromere and the second breakpoint mapped adjacent to the Charcot-Marie-Tooth disease type 1A proximal low-copy repeat (CMT1A-REP). We compare the clinical characteristics of our patient with those previously reported to have a duplication involving the proximal short arm region of chromosome 17 to further delineate the phenotype of trisomy 17pl0-p12.     

Prenatal detection of the 17p11.2 duplication in Charcot-Marie-Tooth disease type 1A: necessity of a multidisciplinary approach for heterogeneous disorders

Charcot-Marie-Tooth (CMT) disease is a typical example of a clinically and genetically heterogeneous disorder and, in most cases, is dominantly inherited and caused by a 1.5 megabase duplication on chromosome 17p11.2 containing the PMP22 gene. This is a non-lethal disease with a wide spectrum of severity, from asymptomatism to severe motor and sensory disability. Unpredictable degree of disability is usually the reason why prenatal diagnosis is required and must be addressed. Molecular procedures such as the use of polymorphic non microsatellite STRs, allowing very fast and reliable results even when requiring a gene dosage interpretation are now available and have been recently validated in post-natal diagnosis. Our results indicate that this approach is also the best-adapted method in case of prenatal diagnosis. Nevertheless, ethical considerations raised by prenatal diagnosis in CMT and more generally in non-lethal disorders remain to be actively considered. Here, we present our experience in genetic counselling, and address the psychological issues for 7 CMT at risk pregnancies. In five cases, a CMT1A duplication was evidenced; pregnancy was terminated in four of these cases and the parents from one affected foetus decided to pursue the pregnancy.     

Genetic basis of inherited peripheral neuropathies

Progress in the elucidation of the genetic basis for inherited peripheral neuropathies has been remarkable over the last years. In particular, the molecular mechanisms underlying the autosomal dominantly inherited disorders Charcot–Marie–Tooth disease type 1A (CMT1 A), Charcot–Marie–Tooth disease type 1B (CMT1B), and hereditary neuropathy with liability to pressure palsies (HNPP) have been determined. While mutation in the gene encoding the major myelin protein, P_o has been associated with CMT1B, CMT1A and HNPP have been shown to be associated with reciprocal recombination events leading either to a large submicroscopic duplication in CMT1 A, or the corresponding DNA deletion in HNPP. Available evidence is consistent with the hypothesis that one or more genes within the relevant rearranged segment of 1.5 Mb on chromosome 17 is sensitive to gene dosage providing a novel mechanism for inherited human disorders. It is likely that the gene encoding the peripheral myelin protein PMP22 is at least one of the genes involved since the PMP22 gene maps within the CMT1A duplication (or HNPP deletion), and point mutations within it have been shown to cause a CMT phenotype in humans and comparable neuropathies in rodents (trembler and trembler^J). The mechanism(s) by which gene dosage and point mutations affecting the same gene might lead to a similar phenotype are currently unknown but recent transgenic mouse experiments suggest that similar mechanisms may also underlie other genetic diseases. © 1994 Wiley-Liss, Inc.     

实时荧光定量PCR在周围髓鞘蛋白22基因重复或缺失检测中的应用

目的 应用实时荧光定量PCR在腓骨肌萎缩症和遗传性压力易感性神经病患者中检测周围髓鞘蛋白22基因（peripheral myelin protein 22，PMP22）重复或缺失。方法 采用实时荧光定量PCR检测113个腓骨肌萎缩症家系先证者、4个遗传性压力易感性神经病家系先证者和50名正常人PMP22基因重复或缺失突变。结果 113个腓骨肌萎缩症家系中发现有36个存在PMP22基因重复，4个遗传性压力易感性神经病先证者均存在PMP22基因缺失，50名正常人未发现异常。结论 我国PMP22基因重复突变的致病频率为31．9％（36／113），PMP22基因缺失突变是遗传性压力易感性神经病常见的致病原因。     

The clinical spectrum associated with a chromosome 17 short arm proximal duplication (dup 17p11.2) in three patients

The p11.2-p12 region of human chromosome 17 is gene rich and composed of at least two genomically unstable domains: the Smith-Magenis syndrome region (17p11.2) and the Charcot-Marie-Tooth region (17p12), both of which are flanked by several low-copy repeat sequences. Homologous recombination between these flanking repeats results in either deletion- or duplication-associated phenotypes caused by a gene dosage effect. We report on the clinical phenotype of three patients presenting with either a 17p11.2 or 17p11.2p12 duplication, revealed by chromosome analysis and confirmed by fluorescent in situ hybridization analysis, high resolution genomic analysis of the 17p region using oligonucleotide array comparative genomic hybridization, and molecular studies with microsatellite markers. Two patients carry the 17p11.2 duplication, while the third one shows a larger duplication including the 17p12 region. The facial features observed in our patients include triangular face, full cheeks, smooth philtrum, thin upper lip, dental malocclusion, irregular eyebrows, and sparse hair, all of which are consistent with the pure proximal dup 17p phenotype. The patients' other clinical features are compared with previously published cases.     

Diagnosis of CMT1A duplications and HNPP deletions by interphase FISH: Implications for testing in the cytogenetics laboratory

Charcot-Marie-Tooth (CMT) disease type 1A is an inherited peripheral neuropathy characterized by slowly progressive distal muscle wasting and weakness, decreased nerve conduction velocities, and genetic linkage to 17p12. Most (>98%) CMT1A cases are caused by a DNA duplication of a 1.5-Mb region in 17p12 containing the PMP22 gene. The reciprocal product of the CMT1A duplication is a 1.5-Mb deletion which causes hereditary neuropathy with liability to pressure palsies (HNPP). The most informative current diagnostic testing requires pulsed-field gel electrophoresis to detect DNA rearrangement-specific junction fragments. We investigated the use of interphase FISH for the detection of duplications and deletions for these disorders in the clinical molecular cytogenetics laboratory. Established cell lines or blood specimens from 23 individuals with known molecular diagnoses and 10 controls were obtained and scored using a two-color FISH assay. At least 70% of CMT1A cells displayed three signals consistent with duplications. Using this minimum expected percentile to make a CMT1A duplication diagnosis, all patients with CMT1A showed a range of 71–92% of cells displaying at least three signals. Of the HNPP cases, 88% of cells displayed only one hybridization signal, consistent with deletions. The PMP22 locus from normal control individuals displayed a duplication pattern in ∼9% of cells, interpreted as replication of this locus. The percentage of cells showing replication was significantly lower than in those cells displaying true duplications. We conclude that FISH can be reliably used to diagnose CMT1A and HNPP in the clinical cytogenetics laboratory and to readily distinguish the DNA rearrangements associated with these disorders from individuals without duplication or deletion of the PMP22 locus. Am. J. Med. Genet. 69:325–331, 1997. © 1997 Wiley-Liss, Inc.