This is a mechanism module, not a specific disease. It is deliberately the Schwann-cell-intrinsic *complement* of `peripheral_axonal_degeneration`, not a duplicate of it: that module models the axon-centred final common pathway shared by metabolic, toxic, inflammatory and axonal-genetic neuropathy, and bundles demyelination into a single node; this module unpacks the glial arm that the demyelinating hereditary neuropathies actually funnel through, where the primary lesion is in the Schwann cell and the axon is injured only secondarily. A demyelinating disorder normally conforms to BOTH — to this module for the myelin lesion and to `peripheral_axonal_degeneration#Distal Axonal Degeneration and Demyelination` for the shared downstream axonal course.
A note on trigger granularity: the five lesion classes could each have been a separate trigger node. They are deliberately kept as one because the module's claim is about where they converge, not about how they differ, and because a conforming entry names its own gene and mechanism in its node text. If a future conformer needs to distinguish the trafficking arm downstream — for instance to model myelin outfoldings as a distinct remodeling signature — that argues for extending the remodeling branch rather than splitting this node.
Disorder-specific substitutions at the trigger node: PMP22 duplication (CMT1A) and PMP22 deletion (HNPP) are reciprocal dosage lesions of the same gene; MPZ missense variants (CMT1B) act by protein misfolding and ER retention; GJB1/connexin-32 variants (CMT1X) remove the reflexive gap junctions of non-compact myelin; EGR2/KROX20 variants disable the transcriptional myelination program (congenital hypomyelinating neuropathy, Dejerine-Sottas); the recessive CMT4 genes (MTMR2, SBF2, SH3TC2, NDRG1, PRX, FIG4, GDAP1) disrupt Schwann cell membrane trafficking and phosphoinositide handling. Conformers should specialize the remodeling branch to the signature their disorder actually shows — tomacula for the PMP22-haploinsufficiency and MPZ-related tomaculous neuropathies, onion bulbs for the hypertrophic de-remyelinating forms, myelin outfoldings for the MTMR2/SBF2 forms.
Cell-type convention: nodes bind the generic CL:0002573 Schwann cell, which is what the existing conformers use. A conforming entry may specialize to CL:0000218 myelinating Schwann cell where its evidence is specific to the myelinating lineage; the module deliberately does not bind the narrower term, so that conformance runs generic-to-specific rather than the reverse.
Not an Xogenesis module: the terminal output is failure of a normal developmental and homeostatic process (myelination), not the formation of a pathological material anatomical entity. The tomacula/onion-bulb node is a reactive remodeling of existing myelin, not a derived pathological structure, and is deliberately curated as a branch rather than as the module's terminus.
Scope boundary: acquired demyelinating neuropathy (CIDP, Guillain-Barre) is out of scope here — the primary lesion there is immune attack on myelin from outside the Schwann cell, which is modelled by `molecular_mimicry_autoimmunity` plus `peripheral_axonal_degeneration`. Central (oligodendrocyte) hypomyelination is likewise out of scope; this module is peripheral-nerve-specific.
Does relieving Schwann cell ER stress improve myelination and disability in human demyelinating CMT, or is the UPR-modulation result confined to transgenic mouse models?
HUMAN MODEL MISMATCH
schwann_upr_mouse_only
Attached to:
Schwann Cell Proteostatic Stress and Myelin Protein Mistrafficking
The whole proteostatic-stress arm of this module, and the drug-target pattern attached to it, rests on transgenic mouse models (MpzS63del, MpzR98C/+, C3-PMP22) in which the mutant transgene is expressed at levels that may not match the human heterozygous state. Human demyelinating CMT disability is driven by secondary axonal loss accumulated over decades, which a mouse study running months cannot reproduce; a myelin-thickness (g-ratio) rescue in mouse is therefore not evidence of a disability benefit in patients. Conformers must not curate UPR modulation as an effective human treatment on the strength of this module.
Proposed experiments:
Controlled trial of UPR modulation in genetically defined CMT1B
Myelin Gene Dosage or Structural Lesion in Schwann Cells
trigger
A germline lesion in a gene the Schwann cell needs to build compact myelin. Five lesion classes enter here, and they are not variations on one theme — they are distinct cell-biological failures that converge on the same downstream node:
(1) gene dosage — PMP22 duplication in CMT1A and the reciprocal PMP22 deletion in HNPP; (2) a misfolding-prone missense change in a myelin structural protein — MPZ in CMT1B; (3) loss of a non-compact myelin gap-junction channel — GJB1/connexin-32 in CMT1X; (4) loss of the transcription factor driving the myelination program itself — EGR2/KROX20; and (5) disrupted Schwann cell membrane trafficking and phosphoinositide handling — the recessive CMT4 genes MTMR2, SBF2, SH3TC2, NDRG1, PRX and FIG4.
Class (5) was added after review: three CMT4 nodes were attaching to this node while its enumeration covered only the first four, so the module documented a narrower trigger than it was actually receiving. All five act cell-autonomously in the Schwann cell rather than in the axon, which is the property that makes them one trigger rather than five.
Downstream
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Schwann Cell Proteostatic Stress and Myelin Protein Mistrafficking
Excess, misfolded, or mislocalized myelin protein overwhelms the Schwann cell secretory pathway.
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Dysmyelination and Segmental Demyelination of Peripheral Nerve
Loss of the transcriptional or structural program can compromise myelin directly, without requiring an intervening proteostatic-stress step.
Show evidence (4 references)
"DNA markers within this interval revealed a duplication that is completely linked and associated with CMT1A."
Establishes the archetypal trigger lesion of this module, a gene-dosage duplication segregating with the most common demyelinating hereditary neuropathy.
"and includes the gene for peripheral myelin protein 22 (PMP-22), suggesting that underexpression of PMP-22 may cause HNPP."
Shows the reciprocal dosage lesion at the same locus, establishing that both over- and under-expression of a myelin gene enter this module at the trigger node.
"Mutations in the human EGR2 gene have recently been associated with the inherited peripheral neuropathies Charcot-Marie-Tooth type 1, Dejerine-Sottas syndrome and congenital hypomyelinating neuropathy."
Establishes an alternative trigger class, loss of the transcriptional myelination program, converging on the same Schwann cell myelination failure.
+ 1 more reference
Schwann Cell Proteostatic Stress and Myelin Protein Mistrafficking
amplifier
Overexpressed, misfolded, or mistrafficked myelin membrane protein accumulates in the Schwann cell endoplasmic reticulum and Golgi, exceeds the degradative capacity of the secretory quality-control system, and activates the unfolded protein response. The protein never reaches its destination in the myelin membrane, so the same lesion causes both a toxic gain (ER burden and a chronic UPR that suppresses myelination) and a loss (absence of the protein from compact or non-compact myelin).
Downstream
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Dysmyelination and Segmental Demyelination of Peripheral Nerve
Sustained ER stress and failed delivery of myelin proteins block myelination and destabilize existing internodes.
Show evidence (2 references)
"Altered proteostasis, ER stress and activation of the UPR have been demonstrated in mice carrying Pmp22 mutations."
Establishes that the proteostatic-stress amplifier is shared by the PMP22 and MPZ trigger arms rather than being specific to one gene.
"All three mutants were localized in the perinuclear compartment of myelinating Schwann cells consistent with retention in the ER (T55I) or Golgi (R75W, N175D) and loss of physiological expression in the non-compact myelin."
Shows the mistrafficking half of this node for the connexin-32 arm: the mutant protein is retained in the secretory pathway and is absent from the myelin compartment where it is needed.
Dysmyelination and Segmental Demyelination of Peripheral Nerve
central effector
The rate-limiting, disorder-agnostic step of this module and its key conformance target. Peripheral nerve fibers are inadequately myelinated from the outset (dysmyelination/hypomyelination) and lose myelin segmentally thereafter, with internodal myelin thinned, unstable, or absent. This is the lesion every demyelinating hereditary neuropathy converges on, whatever its trigger gene, and it is what distinguishes this module from primarily axonal neuropathy.
Downstream
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Tomaculous and Onion-Bulb Myelin Remodeling
Cycles of demyelination and attempted repair leave a characteristic structural signature on nerve biopsy.
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Nerve Conduction Slowing and Conduction Block
Loss of internodal myelin degrades saltatory conduction along the fiber.
Show evidence (1 reference)
"Charcot-Marie-Tooth disease type 1A is a hypertrophic de-remyelinating neuropathy manifesting with peroneal muscular atrophy and uniform, marked, slowing of nerve conduction velocities."
Characterizes the central effector directly: repeated de- and remyelination of peripheral nerve as the defining lesion of the prototypical conformer.
Tomaculous and Onion-Bulb Myelin Remodeling
effector
The histopathological signature of chronic Schwann cell myelin failure, and a branch of the module rather than a step on its causal spine: conformers reach the conduction and axonal-loss nodes whether or not they show this remodeling. Reduced PMP22 dosage yields focal sausage-like myelin thickenings (tomacula) at paranodes; repeated cycles of demyelination and remyelination yield concentric Schwann cell process wrapping (onion bulbs) and clinically palpable hypertrophic nerves. Which signature predominates is disorder-specific and is one of the substitutions a conforming entry makes.
Show evidence (2 references)
"Hereditary neuropathy with liability to pressure palsies is a recurrent focal neuropathy with sausage-like myelin thickening (tomacula) and non-uniform nerve conduction velocity changes."
Establishes the tomaculous arm of this remodeling node and ties it to the reduced-dosage trigger.
"Sural nerve biopsies revealed a chronic demyelinating and remyelinating process with onion bulb formations in the proband."
Documents the onion-bulb arm of this node in a genetically confirmed PMP22-duplication patient, tying the remodeling directly to the repeated demyelination-remyelination cycles and to a trigger lesion inside this module's scope.
Nerve Conduction Slowing and Conduction Block
effector
Loss and thinning of internodal myelin degrades saltatory conduction, slowing motor and sensory conduction velocity and, where demyelination is focal, producing frank conduction block across the affected segment. The pattern is diagnostically informative and disorder-specific: uniform, diffuse slowing in the PMP22-duplication forms; non-uniform, asymmetric slowing in the connexin-32 and reduced-dosage forms, where it can mimic acquired inflammatory neuropathy.
Downstream
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Secondary Axonal Loss and Length-Dependent Deficit
Chronically demyelinated axons lose trophic support from their Schwann cells and degenerate.
Show evidence (2 references)
"Nerve conduction slowing is uniform and diffuse in the most common CMT1A associated with the 17p12 duplication, whereas it is often asymmetric and nonhomogeneous in CMTX"
Establishes conduction slowing as the shared electrophysiological output of this module while recording the disorder-specific pattern conformers substitute. Evidence source is OTHER because this is a review.
"Patients with CMT1A typically have slowed nerve conduction velocities (NCVs), reduced compound motor and sensory nerve action potentials (CMAPs and SNAPs), distal weakness, sensory loss and decreased reflexes."
Documents the conduction abnormality alongside the reduced response amplitudes that mark the transition to the axonal-loss node.
Secondary Axonal Loss and Length-Dependent Deficit
consequence
The clinical terminus of the module, and the point at which it hands off to `peripheral_axonal_degeneration`. Chronically demyelinated axons lose the trophic and metabolic support their Schwann cells normally provide and degenerate in a length-dependent manner, producing distal weakness, wasting, sensory loss and areflexia. The mechanistically important asymmetry, and one conformers must preserve, is that disability tracks this secondary axonal loss rather than the conduction slowing that precedes it — which is why therapies aimed only at conduction velocity have not translated.
Show evidence (2 references)
"these data strongly support the hypothesis that neurological dysfunction and clinical disability in CMT1A are caused by loss or damage to large calibre motor and sensory axons."
Directly supports the claim that disability is driven by the secondary axonal loss rather than by demyelination itself, the asymmetry this node encodes.
"Mutations of genes involved in different functions eventually lead to a length-dependent axonal degeneration, which is the likely basis of the distal predominance of the CMT phenotype."
Establishes the length-dependent distribution of the terminal deficit and the hand-off to the shared axonal-degeneration pathway. Evidence source is OTHER because this is a review.
Unfolded Protein Response Modulation (IFB-088 / Sephin1 / icerguestat)
Platform:
Small molecule
An investigational small-molecule modulator of the integrated stress response that prolongs eIF2-alpha phosphorylation, reducing the load of newly synthesized protein entering the Schwann cell endoplasmic reticulum so that quality control can cope with the misfolded myelin protein and myelination can proceed. It improved motor function, neurophysiology, myelin thickness and UPR markers in mouse models of both the MPZ and the PMP22 trigger arms. This is the module's drug-target pattern, not a treatment recommendation, and conforming disorder entries do not inherit it: the evidence is mouse-only.
Evidence: 1