Alternative titles; symbols
HGNC Approved Gene Symbol: PMP22
SNOMEDCT: 230558006, 40632002, 45853006; ICD10CM: G60.0;
Cytogenetic location: 17p12 Genomic coordinates (GRCh38): 17:15,229,779-15,265,326 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 17p12 | ?Neuropathy, inflammatory demyelinating | 139393 | ?Autosomal dominant | 3 |
| Charcot-Marie-Tooth disease, type 1A | 118220 | Autosomal dominant | 3 | |
| Charcot-Marie-Tooth disease, type 1E | 118300 | Autosomal dominant | 3 | |
| Dejerine-Sottas disease | 145900 | Autosomal dominant; Autosomal recessive | 3 | |
| Neuropathy, recurrent, with pressure palsies | 162500 | Autosomal dominant | 3 | |
| Roussy-Levy syndrome | 180800 | Autosomal dominant | 3 |
The PMP22 gene encodes a 22-kD protein that comprises 2 to 5% of peripheral nervous system myelin. It is produced primarily by Schwann cells and expressed in the compact portion of essentially all myelinated fibers in the peripheral nervous system (Snipes et al., 1992).
Schneider et al. (1988) cloned a family of growth arrest-specific (Gas) genes in the mouse by subtractive hybridization based on differential expression in quiescent and growing NIH 3T3 fibroblasts. Using a cDNA of the mouse Gas3 gene to screen a human lung fibroblast cDNA library, Martinotti et al. (1992) isolated a partial human cDNA homolog. By screening a human fetal spinal cord cDNA library, Hayasaka et al. (1992) isolated a full-length cDNA of human PMP22 encoding a deduced 160-amino acid protein with high sequence similarity to the mouse, rat, and bovine proteins. Patel et al. (1992) cloned the human PMP22 gene and showed 87% and 86% amino acid identity with rat and mouse PMP22, respectively. An N-linked glycosylation sequence and membrane-associated regions of the protein were especially highly conserved. Three transcripts of 1.8, 1.3, and 0.8 kb were detected at high levels in the spinal cord and femoral nerve (peripheral nerve). Edomi et al. (1993) also cloned and determined the sequence of the human PMP22 gene. Manfioletti et al. (1990) determined that the Gas3 protein product is a transmembrane glycoprotein.
Ben-Porath and Benvenisty (1996) reported that the amino acid identity among PMP22, epithelial membrane protein-1 (EMP1; 602333), EMP2 (602334), and EMP3 (602335) ranges from 33 to 43%, with highest homology in the transmembrane regions. In addition, all 4 proteins contain 1 to 3 potential N-linked glycosylation sites in the first extracellular loop. The authors stated that these proteins comprise a novel family and also noted that the lens-specific membrane protein MP20 is distantly related.
Colombo et al. (1992) determined the chromosomal localization of 6 Gas genes in the mouse, and found that Gas3 was localized on mouse chromosome 11, about 44 cM proximal to the gene for p53. Martinotti et al. (1992) demonstrated that the human homolog of Gas3 (PMP22) maps to 17p13-p12 by analysis of human-rodent somatic cell hybrids and in situ hybridization to human metaphases. Patel et al. (1992) isolated cDNA and genomic clones for human PMP22 and showed by Southern analysis of somatic cell hybrids that the gene maps to 17p12-p11.2. Takahashi et al. (1992) mapped the PMP22 gene to 17p11.2 by FISH.
PMP22 is a major component of myelin expressed in the compact portion of essentially all myelinated fibers in the peripheral nervous system and is produced predominantly by Schwann cells. Studies in injured nerve suggested a role during Schwann cell growth and differentiation (Spreyer et al., 1991, Snipes et al., 1992).
Martinotti et al. (1992) suggested a possible role of the PMP22 gene in the development of neoplasia in patients with neurofibromatosis (162200), and in the myelin degenerative Charcot-Marie-Tooth disease linked to chromosome 17p (CMT1A; 118220).
PMP22 is expressed in cranial nerves but not in the mature central nervous system; however, during development it is expressed initially in all 3 germ layers and subsequently in migratory neural crest cells (Hagedorn et al., 1999; Wulf et al., 1999). These observations suggested that mutations in PMP22 might cause sensorineural deafness by demyelination of the eighth cranial nerve or by maldevelopment of the inner ear, which is a neural crest derivative, or by a combination of the 2. The rarity of severe deafness among families with PMP22 mutations suggests that most PMP22 mutations have minimal effects on inner ear development or cranial nerve myelination (Boerkoel et al., 2002).
Fontanini et al. (2005) showed that wildtype human GAS3 transiently associated with calnexin (CANX; 114217) during its path to the cell surface, whereas the misfolded GAS3 L16P mutant (601097.0002) formed a stable complex with calnexin, aggregated, and was retained in the ER. Calnexin bound to the first transmembrane domain of wildtype GAS3 in a manner dependent on N-glycosylation. In contrast, interaction of calnexin with the L16P mutant, retention of GAS3 in the ER, and assembly into high molecular weight oligomers were glycan independent. Wildtype mature GAS3 associated with membrane rafts at the cell surface, whereas the misfolded L16P mutant was largely soluble, and its oligomeric forms in the ER were stabilized by disulfide bonds. Formation of disulfide bonds was common among other misfolded GAS3 point mutants, and photobleaching experiments showed that these misfolded mutants were retained in the ER due to reduced diffusional mobility.
Pantera et al. (2018) identified an approximately 37-kb super enhancer upstream of the rat Pmp22 gene. Deletion of the major enhancers within the super enhancer in S16 rat Schwann cells resulted in reduced overall Pmp22 expression. Pmp22 is primarily transcribed using 1 of 2 alternative promoters, P1 and P2, with P1 being Schwann cell specific. Deletion of the super enhancer had differential effects on transcripts from each promoter, as the P1 promoter was disproportionately more sensitive to loss of the super enhancer, which was critical for usage of P1 in Schwann cells. Knockdown of transcription factor Yy1 (YY1; 600013), a regulator of Pmp22 expression, reduced Pmp22 transcription independent of the super enhancer, indicating that Yy1 does not bridge the distal super enhancer and the Pmp22 gene.
PMP22 Duplication and Deletion
Lupski et al. (1991) and Raeymaekers et al. (1991) found that a DNA duplication on chromosome 17p11 was the apparent basis of Charcot-Marie-Tooth disease type 1A (CMT1A; 118220) (see 601097.0001 for a full discussion). Patel et al. (1992) showed that the PMP22 gene is located entirely within the CMT1A duplication region and that PMP22 is duplicated, but not disrupted, in CMT1A patients. They suggested that a gene dosage effect underlies, at least partially, the demyelinating neuropathy in CMT1A. Valentijn et al. (1992) likewise showed that the PMP22 gene is located within the CMT1A duplication and concluded that increased gene dosage may be responsible for the disorder in CMT1A. Using pulsed field gel electrophoresis and YACs, Timmerman et al. (1992) and Matsunami et al. (1992) also demonstrated that the PMP22 gene is contained within the CMT1A duplication.
In patients with hereditary neuropathy with liability to pressure palsies (HNPP; 162500), Chance et al. (1993) identified an interstitial deletion of distal 17p11.2 which included the PMP22 gene (601097.0004). Umehara et al. (1995) found deletions in 17p11.2 in 2 unrelated Japanese families with HNPP. Gonnaud et al. (1995) found interstitial deletions of the 17p11.2 region in affected and unaffected members of 4 unrelated families, including an affected woman who did not receive the paternal allele for PMP22.
In studying the duplication, Pentao et al. (1992) identified several low copy number repetitive elements (REP) in 17p12-p11.2, including a large (more than 17 kb) CMT1A-REP unit. They determined that the PMP22 gene is located between 2 homologous CMT1A-REPs, and that the CMT1A duplication is a tandem repeat of 1.5 Mb of DNA. CMT1A-REP flanked the 1.5-Mb CMT1A monomer unit on normal chromosome 17 and was present in an additional copy on the CMT1A duplicated chromosome. Pentao et al. (1992) proposed that the de novo CMT1A duplication arose from unequal crossing over due to misalignment at the CMT1A-REP repeat sequences during meiosis. Palau et al. (1993) proved this was indeed the case. They studied the parental origin of the duplication in 9 genetically sporadic CMT1A patients and demonstrated that in all cases the mutation was the product of an unequal nonsister chromatid exchange during spermatogenesis. They suggested that male-specific factors may be operating during spermatogenesis to help in the formation of the duplication and/or stabilization of the duplicated chromosome.
Lopes et al. (1996) developed a restriction map of the proximal and distal CMT1A-REPs. The combined use of cloned EcoRI fragments of the CMT1A-REPs and 3 different restriction enzymes permitted localization of the crossover breakpoints in 38 unrelated cases of HNPP and 76 unrelated cases of CMT1A that they analyzed. In 75% of patients the recombination breakpoint was localized within a 3.2-kb fragment of the CMT1A-REP. The remaining patients, with 1 exception, exhibited crossovers within a more telomeric 4.6-kb fragment. Lopes et al. (1996) noted that the strict coincidence of breakpoints in HNPP and CMT1A patients reinforces the hypothesis that duplication and deletion in the 17p11.2 region are mirror consequences of the same molecular event, namely, unequal recombination. They concluded that the 3.2-kb region where most recombination events occur probably contains sequences that promote recombination. Lopes et al. (1996) further reported that there are expressed sequences encoded in the CMT1A-REPs. They emphasized that close to 100% of the rearrangements present in HNPP and CMT1A can be detected with classic Southern blot methodology and appropriate probes, and that the extensive homology between distal and proximal CMT1A imposes constraints on PCR-based diagnosis. Ikegami et al. (1997) described a useful method for rapid diagnosis of the DNA duplication associated with CMT1A. They used a 1.0-kb EcoRI-PstI DNA fragment from the proximal CMT1A-REP repeat as a probe for Southern blot analysis and detected gene dosage in CMT1A by measuring radioactivity ratios with a photostimulated luminescence imaging plate.
Reiter et al. (1996) identified the molecular etiology of the homologous recombination event that is responsible for the unequal crossing-over resulting in either duplication or deletion of the CMT1A gene in Charcot-Marie-Tooth disease and HNPP. Through the detection of novel junction fragments from the recombinant CMT1A-REPs in both CMT1A and HNPP patients, they identified a 1.7-kb recombination hotspot within the 30-kb CMT1A-REPs. This hotspot showed 98% identity between the 2 CMT1A-REPs, indicating that sequence identity is probably not the sole factor involved in promoting crossover events. Sequence analysis revealed a 'mariner' transposon-like element near the hotspot of homologous recombination, which the authors referred to as MITE, for 'mariner insect transposon-like element.' Reiter et al. (1996) hypothesized that the MITE could mediate strand exchange events via cleavage by a transposase at or near the 3-prime end of the element. Hartl (1996) reviewed the molecular biology of the MITE in relation to CMT1A.
Reiter et al. (1998) followed up on their previous observations (Reiter et al., 1996) and found that in this hotspot, the relative risk of an exchange event was 50 times higher than in the surrounding 98.7% identical sequence shared by the 2 repeats. To refine the region of exchange further, they 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 REPs to that of the proximal and distal REPs, the exchange was mapped to a 557-basepair 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 MITE previously identified near the hotspot. The studies provided direct observation of human meiotic recombination products. These results were considered consistent with the hypothesis that the minimum efficient processing segments, which have been characterized in E. coli, yeast, and cultured mammalian cells, may be required for efficient homologous meiotic recombination in humans.
Lopes et al. (1999) reported a series of CMT1A patients in whom 50 of 59 chromosomal rearrangements were of paternal origin, and 54 of 59 were interchromosomal in nature. By sequencing the crossover hotspot in 28 patients with CMT1A or HNPP, the authors discovered chimeric sequences between proximal and distal repeat sequences in the region (CMT1A-REPs), suggesting conversion of DNA segments associated with the crossing-over. The finding of rearrangements supported a double-strand break (DSB) repair model, which was first described in yeast (Szostak et al., 1983). Successive steps of this model are heteroduplex DNA formation, mismatch correction, and gene conversion. The authors hypothesized that the DSB repair model of DNA exchange may apply universally from yeasts to humans.
Studies of CMT1A patients showed that the majority of unequal crossovers occurred within a small region (less than 1 kb) of the 24-kb repeats (REPs), suggesting the presence of a recombination hotspot. Han et al. (2000) directly measured the frequency of unequal recombination in the hotspot region using sperm from 4 normal individuals. Surprisingly, unequal recombination between the REPs occurred at a rate no greater than the average rate for the male genome (approximately 1 cM/Mb) and was the same as that expected for equally aligned REPs. The authors remarked that a similar finding is seen in yeast, where recombination between repeated sequences far apart on the same chromosome may occur at similar frequencies to allelic recombination. The CMT1A hotspot appears to stand in sharp contrast to the human MS32 minisatellite-associated hotspot, which exhibits highly enhanced recombination initiation in addition to positional specificity. The authors hypothesized that the CMT1A hotspot may consist of a region with genome-average recombination potential embedded within a recombination coldspot.
King et al. (1998) described a patient with CMT1A caused by duplication of the PMP22 gene through an unusual mechanism: unbalanced translocation of 17p to the X chromosome. This finding further supported the hypothesis of gene dosage as the basis of CMT1A.
Matise et al. (1994) referred to the tandem duplication underlying CMT1A as resulting in segmental trisomy. The search for the CMT1A disease gene was misdirected and impeded because some chromosome 17 genetic markers that are linked to CMT1A lie within the duplication. Matise et al. (1994) demonstrated that the undetected presence of a duplication distorts transmission ratios, hampers fine localization of the disease gene, and increases false evidence of linkage heterogeneity. They devised a likelihood-based method for detecting the presence of a tandemly duplicated marker when one is suspected.
Aarskog and Vedeler (2000) described a quantitative PCR method for detecting both duplication and deletion of the PMP22 gene in CMT1A and HNPP, respectively. Their method of real-time quantitative PCR is a sensitive, specific, and reproducible method allowing 13 patients to be diagnosed in 2 hours. It involves no radioisotopes and requires no post-PCR handling.
Korn-Lubetzki et al. (2002) identified the deletion in the PMP22 gene (601097.0004) typical of HNPP in 3 members of family with inflammatory demyelinating polyneuropathy (see 139393).
Zhang et al. (2009) provided evidence that human genomic rearrangements ranging in size from several megabases to a few hundred basepairs can be generated by FoSTeS (fork stalling and template switching)/MMBIR (microhomology-mediated break-induced replication). Furthermore, they showed that FoSTeS/MMBIR-mediated rearrangements can occur mitotically and can result in duplication or triplication of individual genes or even rearrangements of single exons. Zhang et al. (2009) concluded that the FoSTeS/MMBIR mechanism can explain both the gene duplication-divergence hypothesis and exon shuffling, suggesting an important role in both genome and single-gene evolution. The authors examined the underlying mechanisms of potentially pathogenic copy number variations (CNVs) involving PMP22 and detected an apparently mitotically generated FoSTeS/MMBIR-mediated complex PMP22 rearrangement in the unaffected mother of 2 children with neuropathy. In a follow-up article, Zhang et al. (2010) studied a total of 21 individuals with rare CNVs of atypical sizes associated with CMT1A or HNPP by oligonucleotide-based comparative genomic hybridization microarrays and breakpoint sequence analysis. Seventeen unique CNVs, including 2 genomic deletions, 10 genomic duplications, 2 complex rearrangements, and 3 small exonic deletions were identified. Each of the CNVs included either the entire PMP22 gene, certain exons only, or ultraconserved potential regulatory sequences upstream of the PMP22 gene. Breakpoint sequence analysis revealed various molecular mechanisms, including nonhomologous end joining, Alu-Alu-mediated recombination, and replication-based mechanisms such as FoSTeS and/or MMBIR that generated nonrecurrent rearrangements associated with neuropathy. Zhang et al. (2010) concluded that rare CNVs may potentially represent an important portion of missing heritability for human diseases, and confirmed that it is dosage alteration of the PMP22 gene that results in the neuropathy phenotypes associated with CNVs at chromosome 17p11.
Choi et al. (2011) reported 3 Korean families with CMT1A due to 3 different nonrecurrent partial duplications of chromosome 17p12 involving the PMP22 gene. One family (FC116) was very large with multiple affected individuals spanning several generations, another (FC388) consisted of an affected mother and her 2 affected children, and the third (FC85) was a patient with sporadic disease. The phenotype was similar to other patients with CMT1A, although there was broad intrafamilial variability in family FC116. The duplications ranged in size from 465 to 725 kb. The duplications in the 2 smaller families were shown to occur de novo, as the unaffected parents did not carry the duplication. The breakpoint regions of 2 of the duplications could be assessed by PCR. In family FC116, the breakpoints occurred within 2 different Alu sequence families with a 34-bp exact microhomology. The haplotypes of the duplicated region were the same in all affected family members except 1. In family FC388, the breakpoints were within a long terminal repeat (LTR) sequence and an intron of the CDRT4 gene with a 3-bp 'TCA' microhomology and the duplication was associated with an 11-bp deletion. The putative mechanism in family FC116 was FoSTeS/MMBIR as described by Zhang et al. (2009), whereas the duplicated region in family FC388 may have had a more complex etiology also involving recombination during meiosis.
Point Mutations
Nelis et al. (1998) analyzed the nerve-specific promoter and the noncoding exon 1A of the PMP22 gene in 15 unrelated patients with CMT1A and 16 unrelated patients with HNPP and found only 1 base change in exon 1A. In 1 autosomal dominant CMT1A patient, however, this base change did not cosegregate with the disease in the family (see 608236).
Roa et al. (1993) identified point mutations in the PMP22 gene in patients with Dejerine-Sottas syndrome (DSS; 145900), a severe form of peripheral neuropathy with congenital, infantile, or juvenile onset (601097.0006). Although the change in PMP22 in Dejerine-Sottas syndrome is usually a point mutation or deletion, Silander et al. (1996) described duplication in PMP22 in patients who seemed to fit the clinical description of Dejerine-Sottas syndrome.
Kleopa et al. (2004) reported a family from Cyprus in which 4 affected individuals had features of HNPP and/or CMT1A. One patient presented with typical HNPP, which later progressed to severe CMT1, 2 patients had HNPP with features of CMT1, and 1 patient had a chronic asymptomatic CMT1 phenotype. All 4 patients had the same heterozygous point mutation in the PMP22 gene (601457.0019). Kleopa et al. (2004) emphasized the broad phenotypic spectrum resulting from mutations in the PMP22 gene, as well as the phenotypic overlap of HNPP and CMT1A.
Nelis et al. (1999) tabulated 27 distinct mutations in the PMP22 gene causing CMT1A. In general, the phenotype of the PMP22 missense mutations tended to be more severe than that of the CMT1A duplication. All but 1 (a frameshift) of these mutations were localized in the putative transmembrane domains of PMP22, indicating the functional importance of these domains.
Boerkoel et al. (2002) pointed out that 2 mutations in the PMP22 gene that cause CMT1 with deafness (118300), W28R (601097.0014) and A67P (601097.0010), are located at the base of the first extracellular loop; thus, these mutations might be adjacent in the protein and effect hearing loss through a common mechanism. Sambuughin et al. (2003) identified a 12-bp deletion (601097.0015) in the PMP22 gene in a family with CMT1 and deafness, and noted that the mutation, like W28R and A67P, is located at the border of a transmembrane domain and an adjacent extracellular component.
Sanders et al. (2001) reviewed single missense mutations in the PMP22 gene that cause CMT1A. Loss-of-function PMP22 mutants fail to traffic beyond the ER or intermediate compartment to reach the plasma membrane. The authors noted that heterozygous PMP22 mutations often produce a more severe phenotype than wildtype/null hemizygotes because the coexpressed mutant PMP22 in heterozygotes interferes with normal trafficking of wildtype PMP22 to the cell surface.
Hodapp et al. (2006) reported 3 unrelated families in which individuals had mutations in the PMP22 gene and another neurogenetic disease mutation. In 1 family, 2 brothers had duplication of the PMP22 gene, inherited from their father, and a missense mutation in the GJB1 gene (304040) inherited from their mother. The resulting CMT phenotype was severe in the 2 brothers, with one dying at age 11 years. In the second family, a woman had a PMP22 duplication and a repeat CTG expansion in the DMPK gene (605377.0001), with a severe phenotype comprising both CMT and myotonic dystrophy (DM1; 160900). In the third family, a man had a PMP22 deletion and a mutation in the ABCD1 gene (300371), with a severe phenotype comprising HNPP and spasticity associated with adrenoleukodystrophy (ALD; 300100). Hodapp et al. (2006) noted that the 2 simultaneous mutations were additive, leading to neurologic phenotypes in these families that were more severe than expected for each individual disease.
In a review of hereditary motor and sensory neuropathies, Vance (1991) pointed to the autosomal dominant 'Trembler' mutation (Tr) in the mouse as a possibly homologous condition. A hypomyelin neuropathy with onion bulb formation develops in older animals. In 2 allelic forms of the Trembler mouse, Suter et al. (1992, 1992) demonstrated point mutations in 2 distinct putative membrane-associated domains of the PMP22 gene.
Sereda et al. (1996) generated a transgenic rat model of CMT1A and provided experimental evidence that CMT1A is caused by increased expression of PMP22. PMP22 transgenic rats developed gait abnormalities caused by a peripheral hypomyelination, Schwann cell hypertrophy (onion bulb formation), and muscle weakness. Reduced nerve conduction velocities closely resembled recordings in human patients with CMT1A. When bred to homozygosity, transgenic animals completely failed to elaborate myelin.
Sahenk et al. (1999) grafted sural nerve segments from patients with PMP22 duplications (CMT1A) or deletions (HNPP) into the cut ends of sciatic nerves of nude mice. Both grafts showed delayed onset of myelination compared to controls. PMP duplication xenografts showed proximal axonal enlargement with an increase in neurofilament and mitochondria density, suggesting an impairment of axonal transport. Distally, there was a decrease in myelin thickness with evidence of axonal loss, axonal degeneration and regeneration, and onion bulb formation. Changes from HNPP xenografts were similar, but more modest. Sahenk et al. (1999) concluded that PMP22 mutations in Schwann cells cause perturbations in the normal axonal cytoskeletal organization that underlie the pathogenesis of these hereditary disorders.
Using the rat transgenic model for CMT1A, Niemann et al. (2000) showed that Schwann cells segregated with axons in the normal 1:1 ratio but remained arrested at the promyelinating stage, apparently unable to elaborate myelin sheaths. Niemann et al. (2000) examined gene expression of these dysmyelinating Schwann cells using semiquantitative RT-PCR and immunofluorescence analysis. Unexpectedly, Schwann cell differentiation appeared to proceed normally at the molecular level when monitored by the expression of mRNAs encoding major structural proteins of myelin. Furthermore, an aberrant coexpression of early and late Schwann cell markers was observed. PMP22 itself acquired complex glycosylation, suggesting that trafficking of the myelin protein through the endoplasmic reticulum is not significantly impaired. Niemann et al. (2000) suggested that PMP22, when overexpressed, accumulates in a late Golgi-cell membrane compartment and uncouples myelin assembly from the underlying program of Schwann cell differentiation.
Using a genomewide, phenotype-driven, large-scale N-ethyl-N-nitrosourea (ENU) mutagenesis screen, Isaacs et al. (2000) identified 2 mutant mice with marked resting tremor. Backcross animals were generated using in vitro fertilization, and genome scans performed on DNA pools derived from multiple mutant mice. The mutation in each mouse was mapped to a region on chromosome 11 containing Pmp22. One Pmp22 mutation, his12 to arg, altered the same amino acid as in the severe human peripheral neuropathy Dejerine-Sottas syndrome (see 601097.0008), while the other mutation, tyr153 to ter, truncated the Pmp22 protein by 7 amino acids. Histologic analysis of both lines revealed hypomyelination of peripheral nerves.
Tobler et al. (2002) noted that common Pmp22 point mutations include L16P (601097.0002) in Trembler J (TrJ) mice and G150D in Tr mice. The same mutations have been found in humans. The Tr and TrJ phenotypes are not identical. In mice, the Tr mutation is dominant, and the TrJ mutation is semidominant over the wildtype allele. The homozygous TrJ genotype leads to a more severe peripheral myelin deficiency and a much shorter life span compared with the long-living homozygous Tr mice. Moreover, because the heterozygous Tr and TrJ mice display a more severe disease phenotype than the heterozygous Pmp22 knockout mice, both mutant alleles must act via gain-of-function or dominant-negative mechanisms. Tobler et al. (2002) found that all 3 Pmp22s (wildtype, Tr, and TrJ) formed complexes larger than dimers, with Tr Pmp22 especially prone to aggregate into high molecular weight complexes. Despite differences in aggregation of Tr and TrJ Pmp22, these 2 mutant Pmp22s sequestered the same amount of wildtype Pmp22 in heterodimers and heterooligomers. Thus, the differences in the phenotypes of Tr and TrJ mice may depend more on the ability of the mutant protein to aggregate than on the dominant-negative effect of the mutant Pmp22 on wildtype Pmp22 trafficking.
Saporta et al. (2011) observed that complete absence of Pmp22 had a differential effect on myelination between motor and sensory nerve fibers in young mice. Whereas axonal loss affected both ventral motor and dorsal sensory roots equally at ages 10 to 13 months, younger mice had immature Schwann cells that did not form myelin at ventral roots, but there were fully differentiated Schwann cells at the dorsal roots. These data suggested that complete Pmp22 deficiency delays maturation of Schwann cells particularly in motor nerve fibers.
Lupski et al. (1991) found a DNA duplication on chromosome 17p as the apparent basis of Charcot-Marie-Tooth disease type 1A (CMT1A; 118220). They showed complete linkage and association of this duplication in 7 multigenerational CMT1A pedigrees and in several isolated, unrelated patients. Pulsed field gel electrophoresis of genomic DNA from CMT1 patients of different ethnic origins showed a novel SacII fragment of 500 kb, and this fragment showed mendelian inheritance. The duplication was also directly visualized by 2-color FISH in interphase nuclei. Lupski et al. (1991) found that a severely affected person, the product of a first-cousin marriage (Killian and Kloepfer, 1979), was homozygous for the duplication. Onset was before age 1 year and reduction in motor nerve conduction velocity was severe. A less severely affected sister was heterozygous for the duplication. The finding implicated a local DNA duplication, a segmental trisomy, as a novel mechanism for an autosomal dominant human disease. The classic example of a DNA duplication is the Bar locus in Drosophila melanogaster as described by Bridges (1936). Lupski et al. (1991) noted that failure to recognize the molecular duplication could lead to misinterpretation of marker genotypes for affected persons with identification of false recombinance and incorrect localization of the disease locus. The duplication was likewise demonstrated by Raeymaekers et al. (1991) who, like Lupski et al. (1991), concluded that the duplication is probably the mutation responsible for the disease. The duplication was demonstrated in locus D17S122 (probe VAW409R3).
Using pulsed field gel electrophoresis analysis, Hoogendijk et al. (1991) estimated the minimal size of the duplicated region in CMT1A patients to be 1,100 kb.
While trying to determine the size of the chromosome 17 duplication, Raeymaekers et al. (1992) showed that on the genetic map the duplicated markers span a minimal distance of 10 cM, while on the physical map they are present in the same NotI restriction fragment of 1,150 kb. The discrepancy between the genetic and physical map distances suggests that the 17p11.2 region is highly prone to recombination. The authors suggested that the high recombination rate may be a contributing factor to the genetic instability of the region.
Valentijn et al. (1992) used 2-color fluorescence in situ hybridization (FISH) on interphase nuclei of fibroblasts to demonstrate that the duplication is a direct tandem repeat: they observed red-green for the normal chromosome and red-green-red-green for the chromosome with the duplication; in none of the nuclei analyzed was the order red-green-green-red or green-red-red-green, compatible with an inverted repeat. The authors suggested that those affected families in which there is no duplication of the PMP22 gene likely represent intragenic mutations comparable to those in the Trembler mouse.
Hoogendijk et al. (1992) found the chromosome 17 duplication as a de novo mutation in 9 of 10 sporadic patients with HMSN I. During a population survey of CMT1 in south Wales, MacMillan et al. (1992) found duplication of locus D17S122, recognized by a DNA probe that detects an MspI polymorphism, in 10 of 11 families selected only by clinical criteria. Trisomy for this chromosome region is demonstrated either by the presence of 3 alleles or a dosage effect when only 2 of the alleles are present. The 1 family without trisomy did not differ in type or severity of disease from the other families. Lupski et al. (1992) described a patient with a cytogenetically visible duplication, dup(17)(p11.2p12). Molecular analysis demonstrated that this patient had duplications of all the DNA markers duplicated in other cases of CMT1A as well as of markers both proximal and distal to the CMT1A duplication. Upadhyaya et al. (1993) reported another instance of a microscopically visible duplication of 17p12-p11.2 in association with CMT1A.
Wise et al. (1993) used 3 molecular methods to search for the CMT1A DNA duplication in 75 unrelated patients diagnosed clinically with CMT and evaluated by electrophysiologic methods. The CMT1A duplication was found in 68% of the 63 unrelated CMT patients with electrophysiologic studies consistent with CMT type 1. The CMT1A duplication was detected as a de novo event in 2 CMT1 families. Twelve CMT patients who did not have decreased nerve conduction velocities consistent with a diagnosis of CMT type 2 were found not to have the CMT1A duplication. The most informative molecular method was the detection of the CMT1A duplication-specific junction fragment by pulsed field gel electrophoresis. Given the high frequency of the CMT1A duplication in CMT patients and the high frequency of new mutations, Wise et al. (1993) concluded that a molecular test for the CMT1A DNA duplication is useful in the differential diagnosis of patients with peripheral neuropathies.
In a 2-year-old boy with severe demyelinating CMT, Meggouh et al. (2005) identified compound heterozygosity for 2 mutations: the PMP22 duplication and a mutation in the LITAF gene (G112S; 603795.0001), which causes CMT1C (601098). Each parent was heterozygous for 1 of the mutations, and each had pes cavus and reduced nerve conduction velocities consistent with mild CMT. Meggouh et al. (2005) concluded that the cooccurrence of both mutations resulted in the more severe phenotype in the proband.
In 3 members of a 4-generation family with Roussy-Levy syndrome (180800), Auer-Grumbach et al. (1998) identified the CMT1A PMP22 duplication.
Miltenberger-Miltenyi et al. (2009) identified the CMT1A PMP22 1.4-Mb duplication in 79 (31.6%) of 250 unrelated Austrian patients with CMT.
Valentijn et al. (1992) demonstrated a mutation leading to the substitution of proline for leucine in the first putative transmembrane domain of PMP22 as the cause of Charcot-Marie-Tooth disease type 1A (CMT1A; 118220) in a Dutch kindred. A T-to-C transition at position 96 was responsible for the leu16-to-pro (L16P) substitution. The identical mutation had been identified in the 'Trembler-J' mouse, a homolog of the human disease. Thus, either duplication or point mutation in the PMP22 gene can result in CMT1A. Hoogendijk et al. (1993) had previously shown that the clinical disorder in this family was tightly linked to a probe on 17p11.2. The histopathologic abnormalities in nerve biopsies of patients from this family were unusually severe (Gabreels-Festen et al., 1992). Hoogendijk et al. (1993) commented that, according to the clinical, neurophysiologic, and morphologic criteria used by some investigators, most of the patients in this family would individually be given a diagnosis of hereditary motor and sensory neuropathy type III (HMSN3; 145900).
Roa et al. (1993) analyzed DNA samples from 32 unrelated Charcot-Marie Tooth disease type 1A (CMT1A; 118220) patients who did not have the 1.5-Mb tandem duplication in 17p12-p11.2. Searching for mutations within the PMP22 region, they found in 1 family a C-to-G transversion, corresponding to the substitution of cysteine for serine in the 79th codon (S79C) of PMP22. The substitution occurred in the second putative transmembrane domain of PMP22.
Using DNA markers, Chance et al. (1993) demonstrated a large interstitial deletion in distal 17p11.2 in persons with hereditary neuropathy with liability to pressure palsies (HNPP; 162500), also called 'bulb diggers' palsy,' in 3 unrelated kindreds. In 1 pedigree, de novo genesis of the deletion was documented. The deletion spanned approximately 1.5 Mb and included all markers that were known to be duplicated in Charcot-Marie-Tooth disease type 1A (CMT1A; 118220). The deleted region appeared uniform in all pedigrees and included the PMP22 gene. Since the breakpoints in hereditary neuropathy with pressure palsy and CMT1A map to the same intervals in 17p11.2, one can conclude that these genetic disorders may be the result of reciprocal products of unequal crossingover. Lorenzetti et al. (1995) studied 9 unrelated Italian families with HNPP identified on the basis of clinical, electrophysiologic, and histologic evaluations. In all 9 families, Southern blot analysis indicated deletion of 1 copy of the probe used in the HNPP patients. Deletion was also indicated by typing with a polymorphic (CA)n repeat and with 3 RFLPs, all known to map within the deleted region. These findings suggested that a 1.5-Mb deletion is the most common mutation associated with HNPP.
To evaluate the frequency of 17p11.2 deletion involving a rearrangement in the CMT1A-REP (the 2 homologous sequences flanking the 1.5-Mb CMT1A/HNPP monomer unit), LeGuern et al. (1995) analyzed EcoRI-digested DNA from 30 unrelated patients by hybridization with appropriate probes. In this large series, the HNPP phenotype, determined by clinical and electrophysiologic criteria, was associated with a 17p11.2 deletion in 90% (27/30) of the patients. The 3 patients who did not carry the CMT1A/HNPP monomer unit deletion may have had mutations in the PMP22 gene or possibly in the P-zero gene (159440).
Inflammatory demyelinating polyneuropathy (see 139393), a putative autoimmune disorder, presents in an acute (AIDP; Guillain-Barre syndrome) or chronic form (CIDP). Korn-Lubetzki et al. (2002) described a father and 2 daughters of Jewish Kurdish origin who developed inflammatory demyelinating polyneuropathy within 10 years of each other. DNA analysis in the father and 1 daughter who was available for study revealed a 1.5-kb deletion of the PMP22 gene. The father presented at the age of 50 years with asymmetric distal involvement of the legs and right hand following surgery. Demyelination was demonstrated by sural nerve biopsy. Each of his 2 daughters presented at the age of 24 years with asymmetric distal involvement of the legs and left hand. Neither the father nor the daughters had evidence of preceding trauma or compression.
Al-Thihli et al. (2008) reported a 7-year-old boy with autosomal recessive Dejerine-Sottas disease (145900) associated with compound heterozygous deletions in the PMP2 gene: the common 1.5-Mb deletion and a deletion encompassing exons 2 and 3 (601097.0020). The nonconsanguineous parents were each heterozygous for a deletion and showed an HNPP phenotype. Al-Thihli et al. (2008) used multiplex ligation probe-dependent amplification (MLPA) to determine the breakpoints of the deletions. The 1.5-Mb deletion, which the authors stated was the 'typical' HNPP-associated deletion, included the neighboring TEKT3 (612683) and COX10 (602125) genes.
Saporta et al. (2011) reported a 7-year-old boy, born of consanguineous parents, with a homozygous 1.1-Mb deletion of chromosome 17p including all 5 exons of the PMP22 gene, the TEKT3 gene, and the FLJ gene, but not the COX10 gene. The deletion was identified by MLPA. Each unaffected parent was heterozygous for the deletion and had electrophysiologic features of HNPP. The boy was first noted to have hypotonia at age 4 months, and later showed delayed walking with an unsteady sensory ataxic gait requiring a walker. He had distal weakness of the hands and distal sensory impairment with areflexia. He also had bilateral facial weakness, mild ptosis, and mild hammertoes, but no pes cavus. Interestingly, he had no atrophy or weakness of the muscles in the limbs, suggesting normal motor function, although electrophysiologic studies showed slowed peroneal motor conduction velocities. Sural nerve sensory responses were unobtainable. Skin biopsy showed a reduction in myelinated fiber density, with noncompact myelin, axonal loss, and redundancy of the basal lamina around or near Schwann cells. Saporta et al. (2011) noted that this was the first reported patient with complete homozygous deletion of the PMP22 gene. The authors suggested that lack of PMP22's normal differential expression in motor and sensory axons during development contributed to the patient's phenotype of predominantly large fiber sensory loss with nonlength-dependent mild motor impairment.
Roa et al. (1993) identified a patient with severe Charcot-Marie-Tooth disease type 1A (CMT1A; 118220) who was compound heterozygous for 2 mutations in the PMP22 gene: a 353C-T transition, resulting in a thr118-to-met (T118M) substitution, and a 1.5-Mb deletion (601097.0004). The findings were consistent with autosomal recessive inheritance. A son heterozygous for the T118M mutation had no signs of neuropathy, while 2 other sons heterozygous for the 1.5-Mb deletion had hereditary neuropathy with liability to pressure palsies (HNPP; 162500). The deletion was demonstrated by FISH in the severely affected patient and in her affected sons.
Bathke et al. (1996) reported a man with CMT1A who was compound heterozygous for T118M and a 1.5-Mb deletion in the PMP22 gene. His unaffected mother was heterozygous for the T118M substitution. The T118M substitution was not identified in 104 healthy control individuals.
Nelis et al. (1997) presented evidence suggesting that the T118M substitution is not pathogenic. Although they identified heterozygosity for T118M in a single patient with CMT1, the patient's unaffected father also carried the substitution. The T118M substitution was also identified in the unaffected father of another family with CMT1, whereas the affected patient in that family did not have the substitution. The T118M substitution was also identified in the heterozygous state in 10 of 262 controls from northern Sweden, yielding an allele frequency of 1.9%.
Niedrist et al. (2009) identified a T118M substitution in cis with a truncating PMP22 mutation (601097.0021) in a 20-year-old man with severe CMT1A. The phenotype was attributed to the truncating mutation because the truncated protein would not contain the downstream T118M substitution. Analysis of the parents showed that the clinically unaffected father was heterozygous for the T118M substitution, which suggested that the T118M substitution may not be pathogenic, although electrophysiologic studies were not performed on the father.
Shy et al. (2006) reported 3 unrelated individuals with a mild demyelinating neuropathy similar to HNPP who were heterozygous for the T118M substitution. Two members of a fourth kindred with mild CMT1A and electrophysiologic features of HNPP had the T118M substitution and a duplication of the PMP22 gene (601097.0001). In a fifth family, a child with early-onset severe axonal neuropathy was found to be homozygous for the T118M mutation. Although she had severe denervation, she did not have overt demyelination. Her unaffected parents, who had electrophysiologic features consistent with HNPP, were both heterozygous for T118M. Shy et al. (2006) concluded that the T118M substitution is a pathogenic mutation resulting in a partial loss of protein function. The authors suggested that the corresponding phenotypes are due to a PMP22 dosage effect; T118M may thus act as a dominant allele with reduced penetrance.
Dejerine-Sottas syndrome (DSS; 145900) is characterized by hypertrophic, demyelinating neuropathy. Clinical symptoms are similar to but more severe than those of Charcot-Marie-Tooth disease type 1A (CMT1A; 118220). By mutation analysis of the PMP22 coding region in 2 unrelated Dejerine-Sottas patients, Roa et al. (1993) identified individual missense point mutations present in the heterozygous state. In 1 family, both parents were negative for the mutations, suggesting that it was de novo in origin. One patient had a T-to-A transversion predicting a met69-to-lys (M69K) substitution, whereas the other had a C-to-T transition predicting a ser72-to-leu (S72L; 601097.0007) substitution. The patient with the M69K substitution had no detectable abnormality at birth but did not begin walking until age 15 months and did so with an abnormal gait. Bilateral pes cavus was noted at age 6, and delayed nerve conduction velocity in the left ulnar nerve was measured at age 7. By age 18, she had severe lower limb weakness necessitating the use of a wheelchair and severe distal sensory loss in all 4 limbs. No other family member was known to be similarly affected. Electron microscopy of sural nerve biopsy demonstrated hypertrophy of the nerve with marked loss or abnormality of myelinated fibers.
The patient in whom Roa et al. (1993) demonstrated the ser72-to-leu (S72L) substitution was an 8-year-old male who had severe hypotonia and weakness at birth, delayed motor milestones with normal speech development, and gradual improvement in motor abilities. He walked with the aid of leg braces and a walker at 7 years of age. There was marked distal atrophy of the lower limbs, mild weakness of the intrinsic hand muscles, and absent deep tendon reflexes in all 4 limbs. Sensory examination showed distal decrease in sensation to pinprick and temperature in all limbs. Motor nerve conduction velocity and sural nerve biopsy were typical of Dejerine-Sottas syndrome (DSS; 145900). The mother of the patient, who died at 30 years of age from respiratory failure, had a history of similar neuromuscular problems. DNA was not available from that patient.
Ionasescu et al. (1996) found the same mutation in a patient with Dejerine-Sottas syndrome who also showed sensorineural hearing loss, nystagmus, and peripheral facial nerve weakness. The S72L mutation had occurred de novo. The authors stated that nystagmus and peripheral facial nerve weakness had not previously been reported in Dejerine-Sottas syndrome.
Marques et al. (1998) detected the S72L mutation in a 7-year-old girl with Dejerine-Sottas syndrome. The authors proposed that ser72 may be a hotspot for mutation.
Valentijn et al. (1995) identified a de novo mutation in the PMP22 gene of a patient with Dejerine-Sottas neuropathy (DSS; 145900). Single-strand conformation analysis of PCR-amplified DNA fragments showed an additional fragment for exon 1 in the patient, which was absent in the unaffected parents. Sequence analysis showed a de novo C-to-A transversion at nucleotide 85 that resulted in an amino acid substitution his12-to-gln (H12Q) in the first transmembrane domain of PMP22. The patient had been described as case 13 by Ouvrier et al. (1987). At 4 years of age, the child's height and weight were below the 3rd centile. There was generalized weakness of mild to moderate severity. All tendon reflexes were absent, except the triceps. Peripheral nerves were clinically enlarged. There was moderate truncal ataxia. Sensation was normal, except for mild loss of vibration sensation and diminished 2-point discrimination on the feet. Sensory action potentials could not be recorded from the right median or sural nerves. Motor nerve conduction velocity in the median nerve was 7 m/sec. Sural nerve biopsy at 2 years of age had shown reduced density of myelinated fibers, and all fibers were thinly myelinated and frequently surrounded by onion bulbs.
Nicholson et al. (1994) used SSCP analysis to study the PMP22 gene in an Australian family that was transmitting hereditary neuropathy with liability to pressure palsies (HNPP; 162500) but did not show a 1.5-Mb deletion (118220.0004). The affected individuals were heterozygous for a 2-bp deletion (207delTC) in exon 1 of the PMP22 gene, resulting in a frameshift at ser7 and premature stop at codon 36. The authors stated that this mutation provided further evidence that absence of one copy of the PMP22 gene is sufficient to cause liability to pressure palsies.
Li et al. (2007) found a 24% reduction of PMP22 levels in myelinated fibers from dermal nerves of affected members of the HNPP family reported by Nicholson et al. (1994). Electrophysiologic studies showed a pattern similar to HNPP resulting from the classic PMP22 deletion, with accentuated distal slowing occurring at sites subject to nerve compression. Three patients older than age 65 years had clinical and electrophysiologic evidence of length-dependent axonal loss. Li et al. (2007) concluded that the phenotype of HNPP due to a PMP22 truncating mutation (which they referred to as a leu7fs) is indistinguishable from that due to the PMP22 1.5-Mb deletion.
In a family with progressive features of Charcot-Marie-Tooth disease (CMT) and deafness (118300) originally reported by Kousseff et al. (1982), Kovach et al. (1999) found linkage to markers on 17p12-p11.2. Direct sequencing of the PMP22 gene detected a unique G-to-C transversion at nucleotide 248 in coding exon 3, predicting an ala67-to-pro (A67P) substitution in the second transmembrane domain of PMP22. The mutation was present in heterozygous state in all affected individuals. In light of the high levels of PMP22 transcript detected in the cochlea (Robertson et al., 1994), Kovach et al. (1999) suggested that this mutation may be responsible for deafness with the CMT phenotype. They speculated that the VIIIth nerve, which is surrounded by Schwann cells, was the most likely site of auditory neuropathy in this family. The prolongation of interpeak latencies in ABR (auditory brainstem responses) and/or the absence of ABR waveforms would generally be consistent with this putative site of the lesion. Unlike CMT associated with the PMP22 duplication and a gene dosage effect, the A67P mutation was thought to cause a dominant-negative effect, like the majority of point mutations causing CMT and Dejerine-Sottas syndrome (145900).
Whereas most cases of hereditary neuropathy with liability to pressure palsies (HNPP; 162500) are caused by a 1.5-Mb deletion in 17p of the PMP22 gene, Young et al. (1997) identified a family with clinical and electrophysiologic features of HNPP, in which all affected members were heterozygous for a single base (G) insertion within a polyguanosine tract (nucleotides 325-330) in exon 3 (325insG). This mutation was predicted to result in a reading frameshift, starting at amino acid 95 and including 127 random amino acids.
In a family with Dejerine-Sottas syndrome (DSS; 145900), Ikegami et al. (1998) identified a de novo mutation in the PMP22 gene. An abnormal fragment was seen on SSCP analysis of exon 4, and sequencing revealed a 2-bp deletion (CT) at nucleotides 426 and 427. Analysis of mRNA revealed a 2-bp deletion with no splicing abnormalities. This would suggest that the reading frame would be altered at leucine-80 and would result in a protein that was longer by 49 amino acids.
In a family with Dejerine-Sottas syndrome (DSS; 145900), Ikegami et al. (1998) identified a mutation in the PMP22 gene using SSCP analysis. A G-to-T transversion was identified at nucleotide 636, which resulted in a glycine-to-cysteine substitution at codon 150 (G150C). The mutation created a new PvuII site.
In a family in which a father and 2 daughters had Charcot-Marie-Tooth disease (CMT) type 1 with sensorineural deafness (118300), Boerkoel et al. (2002) identified an 82T-to-C transition in the PMP22 gene, resulting in a trp28-to-arg (W28R) substitution.
In 3 affected members of a family with autosomal dominant Charcot-Marie-Tooth disease (CMT) with deafness (118300), Sambuughin et al. (2003) identified a 12-bp deletion in exon 4 of the PMP22 gene, resulting in the deletion of 4 amino acids: ala, ile, tyr, and thr, at positions 115-118. The deletion occurs at the border of the third transmembrane domain and extracellular component of the protein.
Fabrizi et al. (1999) reported a family in which 4 individuals over 4 generations had severe Charcot-Marie-Tooth disease type 1A with focal myelin thickenings (CMT1A; 118220) with a regular fusiform contour (tomacula) or a coarsely granular appearance. Ultrastructural examination disclosed uncompacted myelin and redundant irregular myelin loops. All affected patients had a heterozygous 159A-T mutation in the PMP22 gene, resulting in an asp37-to-val (D37V) substitution in the first extracellular loop of the protein.
In a girl with hereditary neuropathy with liability to pressure palsies (HNPP; 162500), Nodera et al. (2003) identified a 199G-A transition in the PMP22 gene, resulting in an ala67-to-thr (A67T) substitution. The patient first developed symptoms at age 9 years and was examined again at age 17 years. She had ulnar neuropathy at the wrist and a diffuse distal sensorimotor demyelinative polyneuropathy. Her mother, who had a subclinical demyelinating polyneuropathy, also had the mutation. The authors noted that mutation in the same codon (A67P; 601097.0010) had been reported in patients with Charcot-Marie-Tooth disease type 1A and deafness (118300).
In 3 sibs with Dejerine-Sottas syndrome (DSS; 145900), Parman et al. (1999) identified a homozygous 518C-T change in the PMP22 gene, resulting in an arg157-to-trp (R157W) substitution. The unaffected parents were related as first cousins and both were heterozygous for the mutation. All 3 sibs showed a classic DSS phenotype, with delayed milestones, ataxia, distal muscle weakness and wasting, impaired sensation, pes cavus, and scoliosis. Nerve biopsy in 1 patient showed demyelination and onion bulb formation. The mutation occurred in the intracellular domain of PMP22. Parman et al. (1999) commented that DSS caused by mutation in the PMP22 gene is usually autosomal dominant, caused by a heterozygous mutation, and that the findings in this family demonstrate autosomal recessive inheritance.
In affected members of a family from Cyprus with hereditary neuropathy with liability to pressure palsies (HNPP; 162500) and/or Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), Kleopa et al. (2004) identified a 65C-T transition in exon 1 of the PMP22 gene, resulting in a ser22-to-phe (S22F) substitution. One patient presented with typical HNPP, which later progressed to severe CMT1A, 2 patients had HNPP with features of CMT1A, and 1 patient had a chronic asymptomatic CMT1A phenotype. Kleopa et al. (2004) emphasized the broad phenotypic spectrum resulting from mutations in the PMP22 gene, as well as the phenotypic overlap of HNPP and CMT1A.
Al-Thihli et al. (2008) reported a 7-year-old boy with autosomal recessive Dejerine-Sottas disease (DSS; 145900) associated with compound heterozygous deletions in the PMP2 gene: the common 1.5-Mb deletion (601097.0004), inherited from the mother, and a deletion encompassing exons 2 and 3, inherited from the father. The nonconsanguineous parents were each heterozygous for a deletion and showed a hereditary neuropathy (HNPP; 162500) phenotype. The boy had a severe phenotype with significantly delayed motor development, pes cavus, scoliosis, hyporeflexia, hearing deficits, severe demyelination on sural nerve biopsy, and gastroesophageal reflux. Al-Thihli et al. (2008) commented that the deletions in this patient were the largest compound heterozygous PMP22 deletions reported in the literature.
In a 20-year-old man with severe Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), Niedrist et al. (2009) identified a heterozygous 1-bp deletion (281delG) in the last exon (exon 5) of the PMP22 gene, resulting in a C-terminally truncated protein. The transcript escapes nonsense-mediated mRNA decay. The mutant allele also carried the T118M (601097.0005) substitution in cis. Analysis of the parents showed that the 281delG mutation occurred de novo on the paternal allele, because the unaffected father was heterozygous for the T118M substitution. The findings suggested that the T118M substitution may not be pathogenic, although electrophysiologic studies were not performed on the father. The patient had delayed walking and initially walked on tiptoes. On examination at age 20 years, he had weakness in the legs while walking, pes cavus, kyphoscoliosis, hammertoes, and gait disturbance. There was atrophy of the lower leg muscles and intrinsic plantar feet muscles, as well as distal sensory vibratory loss in the lower limbs. Median nerve conduction velocities were not obtainable.
In 2 unrelated patients with a severe form of Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), Liu et al. (2014) identified a 1.4-Mb triplication of the PMP22 gene. Each individual was part of a family with autosomal dominant CMT1A in which the other affected family members had a 1.4-Mb duplication (601097.0001) and a more typical CMT1A phenotype that was less severe. In both families, molecular analysis of the triplication indicated that it occurred on the chromosome with the duplication and arose from the duplication during meiosis in the affected mother. Haplotype analysis indicated 2 different mechanisms: in 1 family, the triplication arose via intrachromosomal nonallelic homologous recombination (NAHR), whereas in the other family it arose from intrachromosomal NAHR followed by a gene-conversion event that most likely exchanged alleles between the maternal homologous chromosomes. A review of a database for CMT1A duplication testing identified 13% with duplication and 0.024% with a duplication-to-triplication event. These findings suggested that the rate of duplication to triplication is higher than that of de novo duplication. Liu et al. (2014) proposed that individuals with duplications are predisposed to acquiring triplications and that the population prevalence of triplication may be underestimated. The inheritance pattern in this scenario resembles genetic anticipation and has implications for genetic counseling.
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