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.
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.
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.
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.
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.
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.