This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "cns_myelin_failure#Deficient or Unstable CNS Myelin Sheath").
SCOPE - this is the myelin-disorder arm only, and the boundary matters. Van der Knaap and Bugiani (PMID:28638987) partition the leukodystrophies into five pathogenetic categories: myelin disorders due to a primary defect in oligodendrocytes or myelin; astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Only the first is this module. The same review stresses that "only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes", so a white-matter disease is NOT a conformer merely because it has white-matter signal change on MRI. Alexander disease (GFAP astrocytopathy) and vanishing white matter disease (eIF2B astrocytopathy) are the canonical near-misses: their primary lesion is in the astrocyte, and they should NOT declare conformance at the trigger or oligodendrocyte nodes. A disease may still conform at the downstream "Deficient or Unstable CNS Myelin Sheath" node when its myelin loss is separately evidenced - node-level conformance is the unit, not the whole chain. Likewise, acquired demyelination (multiple sclerosis, MOGAD, ADEM, osmotic demyelination) is out of scope for the trigger node, which is a heritable lesion; those diseases carry their own immune or osmotic trigger.
PERIPHERAL NERVE IS A DIFFERENT MODULE. Several conformers (metachromatic leukodystrophy, Krabbe disease, PLP1-null Pelizaeus-Merzbacher) also have Schwann-cell demyelinating neuropathy. That arm belongs to peripheral_axonal_degeneration, whose "Distal Axonal Degeneration and Demyelination" node covers PNS myelin; do not route a peripheral neuropathy node here. A disorder with both arms conforms to both modules at different nodes.
Key disorder-specific substitutions for the trigger node: a myelin structural protein (PLP1 point mutation, duplication, or null allele in Pelizaeus-Merzbacher disease; GJC2 in hypomyelinating leukodystrophy 2) in the primary hypomyelinating disorders; an oligodendrocyte-essential transcription/biogenesis machinery (RNA polymerase III subunits POLR3A/POLR3B in POLR-related leukodystrophy, PYCR2, TUBB4A) in the POLR-type and allied hypomyelinating leukodystrophies; and a lipid-degradation enzyme whose undegraded substrate is enriched in myelinating cells (arylsulfatase A and sulfatide in metachromatic leukodystrophy; galactocerebrosidase and psychosine in Krabbe disease; ABCD1 and very-long-chain fatty acids in X-linked adrenoleukodystrophy; aspartoacylase and N-acetylaspartate in Canavan disease) in the demyelinating and vacuolating disorders. The oligodendrocyte (CL:0000128) and its precursor (CL:0002453) are the shared vulnerable cell types across all conforming disorders.
RELATIONSHIP TO lysosomal_substrate_accumulation. Metachromatic leukodystrophy and Krabbe disease already conform to that module for the enzyme-deficiency and substrate-accumulation steps. That is correct and should be kept: the lysosomal module explains how the toxic substrate arises, and this module explains what that substrate does to the oligodendrocyte lineage and the sheath. The two are complementary and a conformer declares both, at different nodes; this module does not re-derive the lysosomal chain.
Oligodendrocyte-Lineage or Myelin-Membrane Insult
trigger
A heritable lesion acts primarily on the oligodendrocyte or on the myelin membrane it produces. The molecular lesion is heterogeneous across disorders - a mutated myelin structural protein that misfolds or is expressed at the wrong dosage, a defect in transcriptional or biosynthetic machinery on which the oligodendrocyte lineage is disproportionately dependent, or a defect in lipid degradation whose undegraded substrate is concentrated in myelinating cells - but all converge on a stressed oligodendrocyte lineage. This is the defining entry criterion of the module: a disease whose primary lesion is in the astrocyte, the axon, the microglial cell, or the vessel wall enters the white-matter cascade elsewhere and is not a conformer here.
Downstream
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Oligodendrocyte Differentiation Arrest and Death
The primary lesion is delivered to the oligodendrocyte lineage, which responds by failing to complete terminal differentiation or by dying after myelination.
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Microglial Injury and Reactive Gliosis
In the lipid-degradation disorders, toxic substrate enriched in myelinating cells is transferred to microglia, injuring them early in lesion evolution.
Show evidence (2 references)
"Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies."
Establishes "a primary defect in oligodendrocytes or myelin" as a distinct pathogenetic category of leukodystrophy, which is exactly the trigger state this module models, and names the four sibling categories that are deliberately excluded from it. Evidence source is OTHER because this is a classification review synthesizing neuropathology across many disorders.
"Both PMD and SPG2 are caused by mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major component of CNS myelin proteins."
A worked instance of the trigger node in its myelin-structural-protein form: the lesion is in a gene encoding a major constituent of the CNS myelin membrane itself. Evidence source is OTHER because this is a review of PLP1 mutational mechanisms.
Oligodendrocyte Differentiation Arrest and Death
amplifier
The injured oligodendrocyte lineage takes one of two routes, and which route dominates is what separates hypomyelination from demyelination downstream. Oligodendrocyte precursors may arrest before terminal differentiation, so that myelin is never adequately laid down; or mature, already-myelinating oligodendrocytes may undergo apoptotic or other regulated cell death, so that myelin formed normally is subsequently lost. Both routes may operate in the same disorder, as they do in Krabbe disease, where accumulating psychosine simultaneously impedes differentiation and kills maturating oligodendrocytes.
Downstream
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Deficient or Unstable CNS Myelin Sheath
Failure of the lineage to differentiate leaves myelin unformed; death of myelinating cells leaves formed myelin unmaintained.
Show evidence (2 references)
"In this study, we report that accumulation of endogenous psychosine under GALC deficient Krabbe conditions impedes OL differentiation process both by decreasing the expression of myelin lipids and protein and by inducing the cell death of maturating OLs."
Demonstrates both routes of this node in one disorder - a toxic substrate arising from the trigger lesion arrests oligodendrocyte differentiation and also kills maturating oligodendrocytes. Evidence source is IN_VITRO because the differentiation and survival experiments are cell-culture based.
"Recent discoveries include complex mutational mechanisms and associated disease phenotypes, novel cellular pathways that lead to the degeneration of oligodendrocytes, and genomic architectural features that result in unique chromosomal rearrangements."
Confirms that in the PLP1 disorders the mutational lesion acts through cellular pathways whose endpoint is oligodendrocyte degeneration, the death route of this node. Evidence source is OTHER because this is a review across human and animal-model studies.
Microglial Injury and Reactive Gliosis
amplifier
Microglia are not passive spectators of the myelin lesion. In the lipid-degradation leukodystrophies, toxic substrate enriched in myelinating cells is transferred to microglia, whose immune phenotype is altered early and which are then lost by programmed cell death - and in both X-linked adrenoleukodystrophy and metachromatic leukodystrophy this microglial loss precedes full-blown myelin breakdown rather than following it. Reactive astrogliosis accompanies the process. This node is therefore positioned as a parallel amplifier feeding the sheath defect, not as a downstream reaction to it, and it is the mechanistic rationale for the efficacy of microglia-replenishing haematopoietic cell transplantation in these disorders.
Downstream
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Deficient or Unstable CNS Myelin Sheath
Loss of the microglial population that maintains white matter integrity compounds the oligodendrocyte-intrinsic sheath defect.
Show evidence (4 references)
"The immune-phenotype of microglia was altered already early in lesion evolution, and microglia loss preceded full-blown myelin degeneration both in X-ALD and MLD."
Establishes the causal ordering that justifies placing this node upstream of the sheath defect rather than downstream of it. Evidence source is HUMAN_CLINICAL because the observation is made in human autopsy material from X-ALD and MLD patients.
"In summary, the present study shows an early and severe damage to microglia in the pathogenesis of X-ALD and MLD. This hints at a central pathophysiologic role of these cells in the diseases and provides evidence for an ongoing transfer of toxic substrates primarily enriched in myelinating cells..."
Supplies the mechanism by which the trigger lesion reaches microglia - transfer of the toxic substrate from myelinating cells - which is the edge this module draws from the trigger node to this one.
"The frequent connection of microglia to leukodystrophies, genetic disorders of the white matter, has highlighted their involvement in the maintenance of white matter integrity."
Generalizes the microglial contribution beyond the two disorders of the autopsy study to the leukodystrophies as a class. Evidence source is OTHER because this is a review.
+ 1 more reference
Deficient or Unstable CNS Myelin Sheath
central effector
The rate-limiting, disorder-agnostic convergence point of the module and its key conformance target. Whatever the upstream lesion, the shared outcome is a CNS myelin sheath that is inadequate in amount, in composition, or in stability. It takes three morphological forms, which are the three myelin subcategories of the leukodystrophy classification and which conforming entries should name explicitly: hypomyelination, in which myelin is never adequately deposited; demyelination, in which a normally formed sheath is subsequently destroyed; and myelin vacuolization, in which the sheath is present but split by intramyelinic oedema. The distinction is prognostically and radiologically real - hypomyelinating disease shows only mildly abnormal T1 signal, demyelinating disease markedly hypointense T1 - but it is a difference in the route to the node, not a difference in the node itself.
Downstream
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Loss of Oligodendroglial Support for the Axon
A deficient sheath removes both the insulating structure and the oligodendroglial metabolic support that travels with it.
Show evidence (2 references)
"Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization)"
Names the three morphological forms this node asserts - hypomyelination, demyelination, and myelin vacuolization - as the subcategories of the myelin-disorder group. Cited here for the outcome forms; the same sentence is cited on the trigger node for the pathogenetic category it defines. Evidence source is OTHER because this is a classification review.
"Mildly hypo-, iso- or hyperintense T1 signal relative to the cortex may be consistent with a hypomyelinating leukodystrophy. Demyelinating leukodystrophy leads to significantly hypointense T1 signal."
Supports the module's insistence that hypomyelination and demyelination are distinguishable forms of this node rather than interchangeable words, and gives the radiological criterion conforming entries should use to say which form they mean.
Loss of Oligodendroglial Support for the Axon
effector
Myelin is not only insulation. Oligodendrocytes maintain the long-term integrity of the axons they ensheath through a metabolic route that is independent of myelination itself: they perform aerobic glycolysis and deliver lactate and pyruvate to the axon through monocarboxylate transporter 1, on which myelinated axons draw when energy-deprived. Loss or dysfunction of the oligodendrocyte therefore withdraws trophic and metabolic support from the axon at the same time as it withdraws saltatory conduction, and experimental disruption of the transporter alone is sufficient to damage axons and kill neurons. This node is why leukodystrophy is a neurodegenerative and not merely a conduction disease.
Downstream
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Axonal Degeneration and Progressive White Matter Dysfunction
An axon deprived of both insulation and metabolic support degenerates.
Show evidence (3 references)
"Oligodendroglia support axon survival and function through mechanisms independent of myelination, and their dysfunction leads to axon degeneration in several diseases."
States the claim of this node directly - oligodendroglial support of the axon is separable from myelination, and its failure causes axon degeneration. Evidence source is MODEL_ORGANISM because the supporting experiments are animal and cell-culture disruption models.
"Here we show that the most abundant lactate transporter in the central nervous system, monocarboxylate transporter 1 (MCT1, also known as SLC16A1), is highly enriched within oligodendroglia and that disruption of this transporter produces axon damage and neuron loss in animal and cell culture models."
Identifies the molecular route of the support - oligodendroglial MCT1 lactate transport - and shows that disrupting it alone is sufficient to damage axons, establishing sufficiency rather than mere association.
"This indicates that aerobic glycolysis products derived from oligodendrocytes are rapidly metabolized within white matter tracts."
Supplies the complementary in vivo evidence that oligodendrocyte-derived glycolytic products are consumed within white matter tracts, i.e. that the coupling this node depends on is real in the intact CNS. Evidence source is MODEL_ORGANISM because the work is done in conditional mutant mice.
Axonal Degeneration and Progressive White Matter Dysfunction
consequence
The clinical endpoint of the module. Axons within the affected tracts degenerate, and the selective, usually progressive white-matter disease that results produces the motor, cognitive, and visual deterioration characteristic of the leukodystrophies. Onset may be at any age from prenatal life to senescence, the course is mostly progressive though it may be static or even improving, and progressive forms are often fatal with no curative treatment known. Conforming entries specialize this node with their disorder-specific tract distribution and clinical tempo; the module deliberately does not assert a single age of onset or rate of decline.
Show evidence (3 references)
"Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence."
Establishes the selective white-matter involvement that defines this endpoint and the wide onset range that is why the module does not fix an age of onset. Evidence source is OTHER because this is a classification review.
"The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known."
Supports the qualified statement of tempo in this node - mostly but not invariably progressive - and the absence of curative therapy that motivates the substrate-reduction and cell-replacement strategies conforming entries curate.
"Glial cells that myelinate axons were found to have a dual role by also supporting the long-term integrity of those axons. This function may be independent of myelin itself. Myelin abnormalities cause a number of neurological diseases"
Links the preceding support-failure node to this degenerative endpoint and states that myelin abnormalities are a cause of neurological disease. Evidence source is OTHER because this is a review.