The peripheral myelin protein 22 (PMP22) is a tetraspan membrane protein which is localised in the compact myelin of the peripheral nerves. In fibroblasts, where it was originally identified as growth arrest related factor 3 (Gas3), PMP22 has been shown to modulate cell proliferation; in the peripheral nervous system its roles are still debated. The duplication of PMP22 is the most common cause of the demyelinating form of the autosomal dominant Charcot-Marie-Tooth neuropathy (CMT1A); rarer missense mutations of PMP22 also cause CMT1A or severe dehypomyelinating neuropathies of infancy grouped under the heading of Dejerine-Sottas syndrome (DSS). Here, a sporadic patient affected with DSS is described; nerve biopsy disclosed a picture of hypomyelination/amyelination with basal laminae onion bulbs and no florid demyelination and it was consistent with congenital hypomyelination neuropathy (CHN); molecular analysis disclosed a novel point mutation of PMP22 that causes a non-conservative arginine for cysteine substitution at codon 109, in the third transmembrane domain. CHN is the rarest and severest form of DSS and it is thought to reflect dysmyelination rather than demyelination. The reported case suggests that missense point mutations may alter a putative role of PMP22 in modulating Schwann cell growth and differentiation.
- Dejerine-Sottas syndrome
- congenital hypomyelination neuropathy
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The peripheral myelin protein 22 (PMP22) is a tetraspan membrane glycoprotein expressed in myelinating Schwann cells as well as in other neural and non-neural tissues. In fibroblasts, it plays a part in the regulation of the cell cycle. In peripheral nerves, PMP22 localises in the compact myelin, accounts for 2%-5% of the total myelin proteins, and has a still unclear function.1 PMP22 has four transmembrane domains (TM1–4), two short cytoplasmic tails, a short intracellular loop, and two extracellular loops; the first loop bears the carbohydrate epitope L2/HNK1, a mediator of adhesive processes.1
The heterozygous 1.5 megabases (Mb) duplication at chromosome 17p11.2-p12 which encompasses PMP22 causes the autosomal dominant Charcot-Marie-Tooth neuropathy type 1A (CMT1A), characterised by decreased nerve conduction velocities (NCVs) and a well known picture of deremyelination with onion bulbs. Very rare heterozygous missense point mutations that affect one of the four TMs of PMP22 also cause dominant or sporadic forms of CMT1A or severer neuropathies of infancy characterised by extremely decreased NCVs and by dehypomyelination2 3; the last are still classified as Dejerine-Sottas syndrome (DSS)2-3 although DSS was long considered to be strictly autosomal recessive.4 The histopathology of missense mutations is poorly characterised. The 1.5 Mb deletion at chromosome 17p11.2-p12 and rare nonsense mutations cause the mild hereditary neuropathy with liability to pressure (HNPP) which is associated with the pathological phenotype of tomaculous neuropathy.2
Here, we describe a sporadic patient harbouring a novel heterozygous Cys109Arg substitution of PMP22 associated with congenital hypomyelination neuropathy (CHN), the rarest and severest form of DSS.4
Our patient, a 14 year old girl, is the first child of healthy non-consanguineous parents; her 12 year old brother is healthy. Early motor milestones were delayed: she could sit at 9 months and walk with orthoses at 4 years. At the age of 3 years she could neither stand nor walk unaided; there were generalised hypotonia and weakness, mainly of the legs, and areflexia. At the median nerve, motor nerve conduction velocity (MNCV) was 3.6 m/s, distal latency (DL) 40 ms, and compound motor action potentials (cMAPs) 200 μV; MNCV at the peroneal nerve was 2.4 m/s. Sensory action potentials (SAPs) were diffusely not evokable. At the age of 7, examination disclosed an ataxic gait with bilateral foot drop, scoliosis, pes cavus, generalised muscle weakness, and wasting prevalent in the legs, areflexia, and distal sensory impairment. Nerve conduction studies showed no modifications compared with the previous examination. Brain stem acoustic evoked potentials (BAEPs) showed bilateral increase of the interpeak latency between waves I and III, indicating an involvement of the peripheral acoustic pathways. Brain MRI was normal.
A left sural nerve biopsy performed at the age of 3 was processed for light and electron microscopy as described elsewhere.5 The total transverse fascicular area was increased (1.25 mm2; age matched control=0.37 mm2). Myelinated fibres were markedly decreased (density=1690 fibres/mm2; normal values=10–13 000 fibres/mm2). The histogram of the presumptive myelinated fibres was skewed to the left, 94% of fibres having a diameter smaller than 6 μm. All fibres had no myelin or extremely thin myelin sheaths and were surrounded by Schwann cell processes made of empty basal membranes (fig 1). There was no evidence of active demyelination. The g ratio (axon diameterv total fibre diameter) was markedly increased (0.92; age matched control=0.73).
Genomic DNA was extracted from blood samples of the proband, parents, and 50 healthy controls. The duplication at chromosome 17p11.2-p12 encompassing the PMP22 gene was ruled out by PFGE analysis as described elsewhere.6 The automated direct sequencing of the PMP22 gene and of the myelin protein zero gene (MPZ/P0) was performed as done previously.6 In P0, we did not find any mutation. In PMP22, we detected a heterozygous T to C transition at nucleotide (nt) 374 (exon 4) that causes an arginine for cysteine109 substitution in the third transmembrane domain (TM3) (fig 2). Rapid detection of the mutation was performed by restriction fragment length polymorphism analysis because the T374C transition introduces aCfoI restriction site. Briefly, a 356 base pair (bp) long fragment spanning exon 4 was amplified by polymerase chain reaction (PCR) according to Roa et al,7 and digested withCfoI (New England Biolabs); restriction products were electrophoresed on a 12% acrylamide gel (acrylamide/bis-acrylamide=19/1). CfoIdigestion of the 356 bp fragment generated two bands of 212 and 144 bp in the wild type allele and three bands of 161, 144, 51 bp in the mutant allele. The T374C transition was absent in the healthy parents as well as in the 50 healthy controls.
In a sporadic patient who met the clinical criteria of DSS, we identified a novel heterozygous Cys109Arg substitution of PMP22. The mutation seemed to be originated de novo because it was absent in the healthy parents. Because the Cys109Arg substitution changes a neutral polar aminoacid into a basic hydrophilic aminoacid and it was absent in 50 healthy controls the mutation is pathogenic. Furthermore, cysteine 109 is likely crucial for the PMP22's structure and function(s) because it is conserved in rat and mouse orthologues, as well as in other growth arrest related molecules belonging to the same family of tetraspan membrane proteins— namely, the human epithelial membrane protein-1 (EMP1) and epithelial membrane protein-2 (EMP-2).8 9
Nerve biopsy was consistent with CHN, disclosing hypomyelination/amyelination and basal laminae onion bulbs in all fibres and no florid demyelination. Because of the pathological picture and the non-progressive evolution, CHN is envisaged as a defect of myelin formation rather than a disorder of myelin maintenance and stability.4
Although current reviews link CHN with rare mutations of the myelin protein zero (MPZ/P0) gene, which codes the major structural component in the compact myelin of peripheral nerves, or with the early growth response 2 (EGR2) gene, which codes a transcription factor crucial for the Schwann cell differentiation,2 3 we think that genetic lesions of PMP22 are a relevant cause of CHN. By reviewing the available pathological phenotypes of missense mutations of PMP22, seven mutations out of 25 (for a review of the mutations see Haiteset al 2) actually seem to be associated with CHN no matter what the original diagnosis was. The seven mutations included His12Gln (TM1),10 Leu16Pro (TM1),11-13 Ser72Leu (TM2),5 14 Ser72Trp (TM2),15 Ser76Ile (TM2),15 Gly100Glu (TM3),16 and Leu105Arg (TM3).4 13 For istance, Leu16Pro was identified in three young patients belonging to the same family who were diagnosed as CMT1 because of autosomal dominant inheritance but who really met the clinical, electrophysiological, and pathological criteria of CHN.11-13 Furthermore, Vandenbergheet al communicated a patient with CHN who was a genetic compound for a nonsense mutation of PMP22 and the 17p11.2-p12 deletion.2 The relation between CHN and genetic lesions of PMP22 seems even stronger when animal models of inherited neuropathies are considered. The spontaneous murine mutants Trembler-J and Trembler that carry respectively the “human” mutations Leu16Pro (TM1) and Gly150Asp (TM4), also show a severe congenital deficiency of the myelin sheath.1
Phenotype to genotype correlations of missense mutations may shed light on the parts that PMP22 plays in peripheral nerves. PMP22 might play a structural, possibly adhesive, function or it might modulate Schwann cell growth and differentiation, as it does in fibroblasts.1 Some evidence in vivo and in vitro support the idea that the more common duplication of the entire PMP22 gene alters the Gas3 function and induces an abnormal Schwann cell differentiation.1 The pathogenesis of missense mutations is less known. Divergent sites and types of the aminoacid substitutions seem to affect differently the structure and function of PMP22. We recently identified a unique Asp37Val substitution occurring in the first extracellular domain that was associated with a peculiar form of CMT1A characterised by prominent uncompaction of the myelin sheath; this finding suggested that PMP22 has a structural role in the myelin sheath although it is a low-abundant component and has an unusual structure for an adhesive molecule.6 On the other hand, the association between CHN and missense mutations distributed in TM1–4 indicates that PMP22 is also involved in Schwann cell differentiation and myelin formation. Thus the pathological phenotypes of missense mutations suggest that PMP22 may have at least a dual function in the peripheral nervous system.
We thank Dr Thomas Pawelzik for editing the manuscript. This work was supported by Telethon-Italy (grant No 1089 to NR) and by the Ministero dell' Universita' e della Ricerca Scientifica (MURST grant No 9806409869).
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