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Inheritance

2

X-linked dominant (CMTX1, CMTX6) HP:0001423

CMTX1 and CMTX6 are transmitted in an X-linked dominant manner. Affected males transmit the variant to all daughters and no sons; heterozygous females have a 50% transmission risk to each child. Hemizygous males are consistently affected, whereas heterozygous females range from asymptomatic to mild-to-moderate and progressive. Skewed X-inactivation across the Schwann cell population is the traditional explanation for that variability, but it is not supported by the only direct measurement in affected nerve (see the refuting evidence below), and no alternative mechanism is established.

X-linked dominant inheritance

Show evidence (3 references)

PMID:20301548 SUPPORT Other

"Affected males transmit the GJB1 pathogenic variant to all of their daughters and none of their sons."

States the X-linked transmission pattern for GJB1 disorders.

PMID:20301548 SUPPORT Other

"Although both men and women are affected, manifestations tend to be less severe in women, some of whom may remain asymptomatic."

Documents the milder and more variable female phenotype in GJB1 disorders.

PMID:40759929 REFUTE Human Clinical

"our findings suggest that XCI does not contribute to phenotypic variability in female CMTX1 patients"

Directly refutes skewed X-inactivation as the explanation for female phenotypic variability. A HUMARA assay on archived sural nerve from two female CMTX1 patients found a random inactivation pattern no different from controls. This is a two-patient pilot, so it weakens rather than settles the hypothesis, but it is the only measurement made in the affected tissue, which is why this entry declines to assert mosaicism.

X-linked recessive (CMTX2, CMTX3, CMTX4, CMTX5) HP:0001419

The remaining CMTX loci are X-linked recessive, with affected hemizygous males and generally unaffected heterozygous carrier females.

X-linked recessive inheritance

Show evidence (1 reference)

PMID:23217327 SUPPORT Human Clinical

"Cowchock syndrome (CMTX4) is a slowly progressive X-linked recessive disorder"

Documents X-linked recessive inheritance for CMTX4.

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Subtypes

6

CMTX1 (GJB1 / connexin-32-related)

X-linked dominant intermediate CMT caused by loss-of-function variants in GJB1 encoding the gap junction protein connexin-32, which forms reflexive gap junctions in the non-compact myelin of myelinating Schwann cells. CMTX1 accounts for the large majority of CMTX and is the second most common cause of CMT overall. Over 260 distinct GJB1 variants have been reported across nearly all protein domains, and the weight of genotype-phenotype evidence is that they converge on simple loss of connexin-32 function rather than a toxic gain of function. Hemizygous males are more severely and more uniformly affected than heterozygous females.

Show evidence (2 references)

PMID:8266101 SUPPORT Human Clinical

"Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families."

Establishes GJB1 / connexin-32 variants as the molecular cause of CMTX1.

PMID:17353473 SUPPORT Human Clinical

"Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function."

Supports loss of function, rather than a gain-of-function mechanism, as the basis of the CMTX1 phenotype across 28 different GJB1 variants.

CMTX2 (X-linked recessive, gene unconfirmed)

An X-linked recessive CMT locus mapped historically to Xp22.2 and reported with early onset and intellectual disability. No causal gene has been confirmed, and the entity is defined by linkage rather than by a molecular lesion. Retained here for MONDO completeness; it should not be treated as having an established mechanism.

CMTX3 (Xq27.1 interchromosomal insertion)

X-linked recessive CMT caused not by a coding variant but by a 78 kb interchromosomal insertion originating from chromosome 8q24.3 into the CMTX3 locus at Xq26.3-q27.3. Coding mutations were excluded by exome sequencing in the two mapped families, and the lesion was found only by whole genome sequencing. How the insertion produces neuropathy — presumably by perturbing regulation of a neighbouring gene — is not established.

Show evidence (1 reference)

PMID:27438001 SUPPORT Human Clinical

"The 78 kb insertion originates from chromosome 8q24.3, segregates fully with the disease in the two families, and is absent from the general population"

Establishes a non-coding interchromosomal insertion as the CMTX3 lesion.

CMTX4 / Cowchock syndrome (AIFM1-related)

Slowly progressive X-linked recessive axonal neuropathy with deafness and cognitive impairment, caused by missense variants in AIFM1 encoding apoptosis-inducing factor, an FAD-dependent mitochondrial NADH oxidase. Curated in full in kb/disorders/Charcot-Marie-Tooth_Disease_X-linked_Recessive_4.yaml; represented here only as the CMTX4 member of the X-linked compartment.

Show evidence (1 reference)

PMID:23217327 SUPPORT Human Clinical

"Cowchock syndrome (CMTX4) is a slowly progressive X-linked recessive disorder with axonal neuropathy, deafness, and cognitive impairment."

Defines CMTX4 as an X-linked recessive axonal neuropathy with deafness and cognitive impairment.

CMTX5 / Rosenberg-Chutorian syndrome (PRPS1-related)

X-linked recessive peripheral neuropathy with prelingual sensorineural hearing loss and optic neuropathy, caused by missense variants in PRPS1 encoding phosphoribosyl pyrophosphate synthetase 1. PRPS1 was the first CMT gene encoding a metabolic enzyme. Curated within the wider allelic series in kb/disorders/PRPS1_Deficiency_Spectrum.yaml; represented here only as the CMTX5 member of the X-linked compartment.

Show evidence (1 reference)

PMID:17701900 SUPPORT Human Clinical

"the affected male patients invariably develop sensorineural hearing loss of prelingual type followed by gating disturbance and visual loss"

Defines the CMTX5 symptom triad of neuropathy, prelingual hearing loss and visual loss.

CMTX6 (PDK3-related)

X-linked dominant CMT caused by a missense variant (p.R158H) in PDK3, one of four kinases that inhibit the pyruvate dehydrogenase complex by reversible phosphorylation. Unusually among CMT mechanisms the variant is hyperactivating rather than inactivating: mutant PDK3 binds the E2p inner-lipoyl domain more tightly than wild type and hyper-phosphorylates the complex, reducing pyruvate flux into the Krebs cycle.

Show evidence (1 reference)

PMID:23297365 SUPPORT Human Clinical

"Whole exome sequencing identified a missense mutation c.G473A (p.R158H) in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."

Establishes the PDK3 p.R158H variant as the cause of X-linked dominant CMTX6.

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Discussions and Knowledge Gaps

2

Does axonal pathology in CMTX1 precede demyelination in humans, as it does in the Gjb1-null mouse, or is the human disease genuinely primary demyelination with secondary axonal loss?

HUMAN MODEL MISMATCH OPEN cmtx1_axon_vs_myelin_ordering

Attached to

pathophysiology#Demyelination with Secondary Axonal Loss

The two evidence streams point opposite ways, and the difference is mechanistically load-bearing rather than semantic. In Gjb1-null mice at 2-4 months, when there is minimal or no demyelination, axon diameters are already reduced, neurofilaments are dephosphorylated and densely packed, beta-amyloid precursor protein is elevated, and fast axonal transport is slowed - so the axon is affected first, and the authors conclude Cx32 loss affects axons independently of demyelination. Human nerve conduction studies in CMTX1 families are instead read as primary demyelination with secondary axonal loss. Both readings cannot be the mechanism. The discrepancy may be real species divergence, or it may be a detection-sensitivity artefact: mouse histology and axonal-transport assays interrogate the axon far more sensitively than clinical nerve conduction studies can, so human axonal involvement could simply be invisible at the stage where conduction slowing is first measured. Resolving it matters for therapy timing - if axonal injury is upstream, a myelin-directed intervention delivered after conduction slowing appears is already late. Note the two are not fully independent claims: human electrophysiology in CMTX1 is itself described as showing less demyelination and more axonal loss than other CMT1 subtypes, which leans toward the mouse.

Show evidence (2 references)

PMID:20720503 SUPPORT Model Organism

"we studied Gjb1-null mice at early stages (i.e. 2-4 months old) of the neuropathy, when there is minimal or no demyelination"

Establishes that the mouse axonal findings were made before demyelination was detectable.

PMID:15468313 REFUTE Human Clinical

"The electrophysiological findings were consistent with a primary demyelinating neuropathy with secondary axonal loss and support this model of disease progression."

The human electrophysiological reading that the mouse result sits against, and the basis for this entry's node naming.

Does GJB1 loss predispose to inflammatory CNS demyelination (multiple sclerosis), and if so by what mechanism?

KNOWLEDGE GAP OPEN cmtx_cns_inflammatory_demyelination

Attached to

pathophysiology#Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction

A single-centre cohort of 70 GJB1 patients followed over 20 years found a 20-year MS incidence of 4.3%, significantly above the highest background incidence reported for the same country, together with an excess of corpus-callosum splenium hyperintensity on brain MRI in CMTX patients without CNS symptoms. No CNS-reactive humoral factor was identified, so a mechanism linking oligodendrocyte Cx32 loss to inflammatory demyelination is not established. The finding is single-centre and the absolute numbers are small (three patients), so it is recorded as an open question rather than as a comorbidity claim. The alternative reading - that Cx32-deficient white matter is simply more likely to produce lesions that meet MS imaging criteria - has not been excluded.

Show evidence (1 reference)

PMID:30196252 SUPPORT Human Clinical

"The resulting 20-year MS incidence (4.3%) differed significantly from the highest background 20-year MS incidence ever reported from Greece (p=0.00039)"

The incidence comparison behind the proposed association. Graded PARTIAL because three cases in one centre supports an association hypothesis without establishing it.

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Pathophysiology

8

Connexin-32 Reflexive Gap Junction Loss in Schwann Cells

Loss-of-function variants in GJB1 remove connexin-32 from the non-compact myelin of myelinating Schwann cells, where it assembles the reflexive gap junctions that connect the cell's own abaxonal and adaxonal cytoplasmic compartments across the myelin spiral. Those junctions provide a short radial diffusion path — on the order of micrometres — for ions and small metabolites that would otherwise have to travel the entire spiral length of the sheath. Connexin-32 is normally expressed in myelinated peripheral nerve, and the reported variants span nearly all protein domains yet converge on loss of function rather than a toxic gain, which is why whole-gene deletions are no more severe than missense alleles.

Myelinating Schwann cell CL:0000218

GJB1 hgnc:4283

Gap junction assembly in non-compact myelin GO:0016264 ↓ DECREASED

Animal models

Gjb1-null mouse Mouse recapitulates A null allele is a faithful model of the human lesion, since the weight of human genotype-phenotype evidence is that GJB1 variants act by loss of function.

Downstream

Impaired Radial Metabolic Support of the Axon Removing the reflexive radial diffusion path degrades the Schwann cell's ability to exchange ions and small metabolites with the periaxonal space it supports.

Show evidence (3 references)

PMID:8266101 SUPPORT Human Clinical

"it was found that connexin32 is normally expressed in myelinated peripheral nerve"

Localizes connexin-32 to myelinated peripheral nerve, the compartment the lesion acts in.

PMID:17353473 SUPPORT Human Clinical

"the degree of disability was comparable with that observed in patients with a documented GJB1 deletion"

Equivalence of missense and whole-gene-deletion disability is the observation that identifies the mechanism as loss of function.

PMID:9722620 SUPPORT In Vitro

"this radial pathway is approximately one million times faster than the circumferential pathway"

Quantifies why the reflexive route matters: dye-injection measurements in teased myelinating Schwann cells show the radial path across the sheath is about a million-fold faster than travelling the spiral circumferentially, which is what makes its loss consequential.

Impaired Radial Metabolic Support of the Axon

Without the reflexive gap-junction shortcut, exchange of potassium, cyclic nucleotides and other small solutes between the Schwann cell body and the periaxonal compartment is confined to the long circumferential route through the myelin spiral. Schwann-cell support of the axon it ensheathes becomes inadequate, most acutely in the longest fibres where the metabolic load per supporting cell is greatest. This is the step at which a molecular lesion in a glial junction protein becomes an axon-level deficit.

Myelinating Schwann cell CL:0000218 Peripheral sensory neuron CL:0000101 Peripheral motor neuron CL:0000100

Peripheral nervous system myelin maintenance GO:0032287 ↓ DECREASED

Downstream

Demyelination with Secondary Axonal Loss Chronic failure of Schwann-cell metabolic support destabilizes the myelin sheath and the axon it invests.

PDK3 Hyperactivation and Reduced Pyruvate Flux

In CMTX6 the PDK3 p.R158H variant is hyperactivating rather than inactivating, which is unusual among CMT mechanisms. Mutant PDK3 binds the inner-lipoyl (L2) domain of the E2p chain more tightly than wild type and hyper-phosphorylates the pyruvate dehydrogenase complex, which inhibits it. Pyruvate entry into the Krebs cycle is throttled, leaving the long peripheral axon - the compartment least able to tolerate an ATP shortfall - energetically unsupported. Note the two-level direction here: the kinase gains activity, and the pathway it regulates loses flux.

Peripheral sensory neuron CL:0000101 Peripheral motor neuron CL:0000100

PDK3 hgnc:8811

Pyruvate metabolic process GO:0006090 ↓ DECREASED

Downstream

Demyelination with Secondary Axonal Loss Reduced pyruvate flux in the longest fibres produces the same length-dependent axonal degeneration the connexin route arrives at.

Show evidence (2 references)

PMID:23297365 SUPPORT In Vitro

"We found that the R158H mutation confers enzyme hyperactivity and binds with stronger affinity than the wild-type to the inner-lipoyl (L2) domain of the E2p chain of PDC."

Biochemical characterization establishing PDK3 hyperactivity, and so reduced pyruvate flux, as the CMTX6 mechanism.

PMID:23297365 SUPPORT In Vitro

"reduced pyruvate flux due to R158H mutant PDK3-mediated hyper-phosphorylation of the PDC as the underlying pathogenic cause of peripheral neuropathy"

States the proposed causal chain from kinase hyperactivity to peripheral neuropathy.

AIFM1 Mitochondrial Oxidoreductase Dysfunction

In CMTX4 (Cowchock syndrome) the AIFM1 p.Glu493Val variant alters the redox properties of apoptosis-inducing factor, an FAD-dependent NADH oxidase imported into mitochondria, and increases cell death. This is a distinct lesion from the CMTX6 kinase defect: the shared feature is only that both proteins are mitochondrial, and the downstream biology - dysregulated caspase-independent cell death rather than a bioenergetic flux deficit - is not the same.

Peripheral sensory neuron CL:0000101 Peripheral motor neuron CL:0000100

AIFM1 hgnc:8768

Apoptotic process GO:0006915 ↑ INCREASED

NADH dehydrogenase activity GO:0003954 ⚠ ABNORMAL

Downstream

Demyelination with Secondary Axonal Loss Increased neuronal cell death converges on the same length-dependent degeneration endpoint.

Show evidence (1 reference)

PMID:23217327 SUPPORT Human Clinical

"The c.1478A>T (p.Glu493Val) mutation found in the family reported here alters the redox properties of the AIF protein and results in increased cell death"

Reports what the study found about the variant itself - altered redox properties and increased cell death.

Purine Nucleotide Biosynthesis Deficiency

In CMTX5, missense variants in PRPS1 reduce phosphoribosyl pyrophosphate synthetase 1 activity, the committed step supplying PRPP for purine and pyrimidine nucleotide biosynthesis. PRPS1 was the first CMT gene found to encode a metabolic enzyme. Because the enzyme is ubiquitously expressed — including in cochlea — the deficit is not confined to peripheral nerve, which is why CMTX5 is syndromic, adding prelingual sensorineural hearing loss and optic neuropathy to the neuropathy.

Peripheral sensory neuron CL:0000101 Peripheral motor neuron CL:0000100

PRPS1 hgnc:9462

Purine nucleotide biosynthetic process GO:0006164 ↓ DECREASED

Downstream

Demyelination with Secondary Axonal Loss Reduced nucleotide supply in long-projection neurons converges on the same length-dependent degeneration.

Show evidence (1 reference)

PMID:17701900 SUPPORT Human Clinical

"We also showed decreased enzyme activity in patients with M115T."

Demonstrates reduced PRPS1 enzyme activity in CMTX5 patients, establishing loss of function.

Demyelination with Secondary Axonal Loss

The shared terminal node of the X-linked compartment, and the same one the CMT1, CMT2 and CMT4 entries converge on. In CMTX1 the electrophysiological signature is characteristically intermediate: motor conduction velocities are slowed below the axonal range but not into the uniformly demyelinating range that defines CMT1, consistent with a primary glial lesion that degrades rather than abolishes myelin. Clinical disability tracks the secondary axonal loss rather than the conduction slowing — in CMTX1 it increases with age and correlates with loss of motor units on motor unit number estimation.

Myelinating Schwann cell CL:0000218 Peripheral sensory neuron CL:0000101 Peripheral motor neuron CL:0000100

Myelination GO:0042552 ↓ DECREASED

Animal models

Gjb1-null mouse Mouse partially recapitulates Reproduces the demyelinating neuropathy, but on a different internal ordering than the human electrophysiological picture implies: axonal abnormalities appear before demyelination is detectable.

Downstream

Length-Dependent Distal Neuropathy Axonal loss in the longest fibres produces the length-dependent distal motor and sensory deficit.

Show evidence (1 reference)

PMID:17353473 SUPPORT Human Clinical

"Disability correlated with a loss of motor units as assessed by motor unit number estimates."

Ties clinical disability to axonal (motor unit) loss rather than to demyelination per se, which is why this node and not the myelin node is the proximate driver of the phenotype.

Length-Dependent Distal Neuropathy

The clinical endpoint: symmetrical, length-dependent distal weakness and wasting beginning in the intrinsic foot and peroneal muscles, distal sensory loss, depressed or absent reflexes, and the secondary skeletal deformities (pes cavus, hammertoes) that follow chronic imbalance between weakened and spared muscle groups. In CMTX1 this typically becomes manifest in affected males between ages five and 25.

Downstream

Distal Muscle Weakness Length-dependent motor deficit produced by the neuropathy.

Distal Amyotrophy Wasting of chronically denervated distal muscle.

Distal Sensory Impairment Length-dependent sensory deficit produced by the neuropathy.

Hyporeflexia Loss of the afferent and efferent limbs of the tendon reflex arc.

Pes Cavus Secondary skeletal deformity following chronic denervation imbalance.

Steppage Gait Compensatory gait change secondary to foot drop.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"Peripheral neuropathy typically manifests in affected males between ages five and 25 years."

Establishes the typical age at onset of the peripheral phenotype in hemizygous males.

Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction

Connexin-32 is expressed by oligodendrocytes as well as Schwann cells, and CMTX1 is consequently the one CMT compartment with a recognized central nervous system phenotype. This takes two forms: fixed white-matter abnormalities, and stereotyped acute, self-limited, fully reversible episodes of CNS dysfunction — typically weakness and dysarthria, sometimes presenting as stroke-like episodes and mimicking cerebral ischaemia. The episodes are self-limited and treatment is supportive. Triggers are described - fever, infection, exercise, altitude, and in one report pollen exposure - but the mechanism linking any of them to the white-matter lesion is not established, and the pollen case explicitly leaves an allergic mechanism unproven. That the deficits resolve completely, with imaging lesions that also resolve, argues for reversible metabolic or ionic decompensation of oligodendrocyte-supported white matter rather than for infarction.

Oligodendrocyte CL:0000128

GJB1 hgnc:4283

Downstream

Transient Stroke-like CNS Episodes Oligodendrocyte connexin-32 loss is the proposed substrate for the reversible episodes of CNS dysfunction.

Central White Matter Abnormality The same glial lesion is the proposed substrate for the fixed white-matter abnormalities seen on imaging.

Show evidence (2 references)

PMID:20301548 SUPPORT Other

"acute, self-limited episodes of transient neurologic dysfunction (especially weakness and dysarthria)"

Establishes the transient, reversible CNS episodes characteristic of GJB1 disorders.

PMID:12111842 SUPPORT In Vitro

"the mutants associated with CNS phenotypes failed to reach the cell membrane and were instead retained"

Distinguishes the CNS-associated GJB1 alleles cellularly: those reported to cause clinical CNS dysfunction are retained intracellularly rather than trafficking to the membrane, suggesting the CNS phenotype tracks a trafficking-defective subgroup rather than all loss-of-function alleles.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

12

Ear 1

Sensorineural Hearing Impairment HP:0000407

Show evidence (1 reference)

PMID:17701900 SUPPORT Human Clinical

"the affected male patients invariably develop sensorineural hearing loss of prelingual type"

Establishes prelingual sensorineural hearing loss as an obligate CMTX5 feature.

Eye 1

Optic Atrophy HP:0000648

Limbs 1

Pes Cavus HP:0001761

Musculoskeletal 1

Distal Muscle Weakness VERY_FREQUENT HP:0002460

Course: PROGRESSIVE

Show evidence (2 references)

PMID:28768847 SUPPORT Human Clinical

"85% of males reported difficulty with walking compared with 64% of females"

The frequency band is set from the male figure, which is the sex in which the phenotype is fully penetrant enough to band. Note it differs by sex, so a single disease-level frequency is an approximation.

PMID:20301548 SUPPORT Other

"GJB1 disorders are typically characterized by peripheral motor and sensory neuropathy"

Establishes motor involvement as a defining feature of the GJB1 phenotype.

Nervous System 1

Transient Dysarthria HP:0001260

Temporal: TRANSIENT

Show evidence (1 reference)

PMID:42477620 SUPPORT Human Clinical

"A 9-year-old boy developed acute left-sided weakness, dysarthria and central facial-tongue paresis shortly after pollen exposure."

Documents dysarthria as a presenting feature of a transient CNS episode.

Other 7

Distal Amyotrophy HP:0003693

Course: PROGRESSIVE

Distal Sensory Impairment HP:0002936

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"peripheral motor and sensory neuropathy with or without fixed CNS abnormalities"

Establishes sensory as well as motor involvement in GJB1 disorders.

Hyporeflexia HP:0001265

Decreased Motor Nerve Conduction Velocity HP:0003431

Steppage Gait HP:0003376

Transient Stroke-like CNS Episodes Stroke-like episode HP:0002401

Temporal: TRANSIENT

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"Less commonly, initial manifestations in some affected individuals are stroke-like episodes (acute fulminant episodes of reversible CNS dysfunction)."

Establishes reversible stroke-like CNS episodes as a recognized GJB1 presentation.

Central White Matter Abnormality Abnormal CNS myelination HP:0011400

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"peripheral motor and sensory neuropathy with or without fixed CNS abnormalities"

Establishes fixed CNS abnormalities as part of the GJB1 disorder spectrum.

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Genetic Associations

4

GJB1

Gene: GJB1 hgnc:4283 relationship_type: CAUSATIVE

Show evidence (5 references)

PMID:8266101 SUPPORT Human Clinical

"Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families."

Original identification of GJB1 as the CMTX1 gene.

PMID:37284795 SUPPORT Human Clinical

"We present 387 patients from 295 families harbouring 154 variants in GJB1."

Establishes the scale of the classified GJB1 variant series behind the figures above.

PMID:37284795 SUPPORT Human Clinical

"319 patients (82.4%) were deemed to have P/LP variants, 65 had VUS (16.8%) and three benign variants (0.8%"

The ACMG/ACGS pathogenicity breakdown of the classified variant series.

+ 2 more references

AIFM1

Gene: AIFM1 hgnc:8768 relationship_type: CAUSATIVE

Show evidence (1 reference)

PMID:23217327 SUPPORT Human Clinical

"identified a missense change c.1478A>T (p.Glu493Val) in AIFM1, the gene encoding apoptosis-inducing factor (AIF) mitochondrion-associated 1"

Identifies the AIFM1 variant causing CMTX4.

PRPS1

Gene: PRPS1 hgnc:9462 relationship_type: CAUSATIVE

Show evidence (1 reference)

PMID:17701900 SUPPORT Human Clinical

"The mutations identified were E43D, in patients with Rosenberg-Chutorian syndrome, and M115T, in the Korean patients with CMTX5."

Identifies the PRPS1 variants causing CMTX5 / Rosenberg-Chutorian syndrome.

PDK3

Gene: PDK3 hgnc:8811 relationship_type: CAUSATIVE

Show evidence (1 reference)

PMID:23297365 SUPPORT Human Clinical

"The change localized within the 1.43-Mb linkage interval, segregated with the affected phenotype and was excluded in ethnically matched control chromosomes."

Segregation and control-exclusion evidence supporting PDK3 as the CMTX6 gene.

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Medical Actions

7

Ankle-Foot Orthoses and Adaptive Footwear

Action: Rehabilitation NCIT:C15315

Ankle/foot orthoses and special shoes to correct foot drop and aid walking; the mainstay of symptomatic management, as no disease-modifying therapy exists for any CMTX subtype.

Mechanism Target:

Length-Dependent Distal Neuropathy — Compensates mechanically for the distal weakness produced by the neuropathy; does not act on any upstream mechanism.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"special shoes and/or ankle/foot orthoses to correct foot drop and to aid walking"

Recommends orthotic management of foot drop in GJB1 disorders.

Physical and Occupational Therapy

Action: physical therapy Ontology label: Physical Therapy NCIT:C15302

Daily heel-cord stretching to prevent Achilles tendon shortening, exercise as tolerated, and occupational therapy for fine motor skills and activities of daily living.

Mechanism Target:

Length-Dependent Distal Neuropathy — Addresses the secondary contractures and functional consequences of chronic distal denervation.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"daily heel cord stretching to prevent Achilles' tendon shortening; exercise as tolerated"

Specifies the physical therapy regimen recommended in GJB1 disorders.

Corrective Foot Surgery

Action: orthopedic surgical procedure Ontology label: Orthopedic Surgical Procedure NCIT:C16186

Surgery as needed for severe pes cavus deformity.

Mechanism Target:

Length-Dependent Distal Neuropathy — Corrects the fixed skeletal deformity secondary to chronic denervation.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"surgery as needed for severe pes cavus"

Recommends surgical correction of severe pes cavus.

Avoidance of Neurotoxic Medications

Action: Supportive Care NCIT:C15747

Avoidance of drugs that are toxic or potentially toxic to people with CMT, and of obesity, which makes ambulation more difficult.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"medications that are toxic or potentially toxic to persons with CMT"

Identifies neurotoxic drug avoidance as a management principle.

AAV9-Mediated Schwann-Cell-Targeted GJB1 Gene Replacement

Action: gene therapy Ontology label: Gene Therapy NCIT:C15238

Investigational, preclinical only. An AAV9 vector delivering GJB1/Cx32 under the Schwann-cell-specific Mpz promoter, given by lumbar intrathecal injection. Gene replacement is a rational strategy here precisely because the human mechanism is loss of function, so restoring wild-type Cx32 to myelinating Schwann cells addresses the primary lesion rather than a downstream consequence. Treatment both before and after neuropathy onset improved motor performance, conduction velocity, myelination and blood biomarkers in Gjb1-null mice. No human trial has been reported, and symptomatic management remains the only clinical option.

Mechanism Target:

Connexin-32 Reflexive Gap Junction Loss in Schwann Cells — Restores Cx32 expression in myelinating Schwann cells, acting directly on the initiating molecular lesion rather than on its consequences.

Show evidence (1 reference)

PMID:33692503 SUPPORT Model Organism

"demonstrated improved motor performance and sciatic nerve conduction velocities along with improved myelination and reduced inflammation in peripheral nerve tissues"

Preclinical efficacy in the Gjb1-null mouse, pre- and post-onset. Model organism evidence only - this is not a human treatment claim.

Cochlear Implantation

Action: cochlear device implantation Ontology label: Surgical Procedure NCIT:C15329

For the CMTX5 and CMTX4 hearing loss rather than the neuropathy. A small case series in CMT reports benefit, with outcomes shaped by whether the lesion is cochlear or a retrocochlear auditory neuropathy, and by how early implantation is done. Three of the four reported cases carry CMTX genotypes (GJB1/CMTX1, AIFM1, and PRPS1/CMTX5), so the series speaks to this entry directly rather than by extrapolation from CMT generally. Evidence is four patients, so this is a considered option and not an established standard.

Show evidence (2 references)

PMID:41557339 SUPPORT Human Clinical

"Cochlear implants (CIs) have been used to rehabilitate hearing loss in CMT, but their effectiveness remains poorly understood due to the central and peripheral neurological complexities of the disease."

Supports implantation as an option while stating the uncertainty that keeps this graded PARTIAL.

PMID:41557339 SUPPORT Human Clinical

"Case 4: A 32-year-old man with PRPS1 -associated CMTX5 and profound prelingual SNHL showed improved sound awareness (20-30 dB HL) post-CI but had no speech perception gains due to prolonged auditory deprivation."

A CMTX5 case specifically, and the one that grounds the timing argument: sound awareness improved but speech perception did not, attributed to prolonged auditory deprivation before implantation.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Counseling on X-linked transmission risk, carrier testing of at-risk female relatives, and prenatal or preimplantation genetic testing options.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"molecular genetic testing of at-risk female relatives to determine their genetic status, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible"

Specifies the genetic counseling and testing options for GJB1 disorders.

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Environmental Factors

1

Systemic stressors preceding transient CNS episodes in CMTX1

Fever, infection, exercise, altitude and - in one paediatric report - pollen exposure have been described as temporally preceding the reversible CNS episodes of CMTX1. These are reported temporal associations, not established causes: the pollen case explicitly notes that an allergic mechanism is unproven because allergy-specific biomarkers were not obtained during the acute phase. No ECTO exposure term is bound because the entry is a heterogeneous set of systemic stressors rather than one exposure concept; ECTO was searched and has no term covering the set.

Show evidence (2 references)

PMID:42477620 SUPPORT Human Clinical

"Pollen exposure was temporally associated with the episode in this patient, but an allergic mechanism remains unproven"

A single paediatric case with an explicit statement that the mechanism is unproven - recorded as a temporal association, not an established cause.

PMID:30952033 SUPPORT Human Clinical

"Our patient had a transient stroke-like clinical manifestations and magnetic resonance imaging (MRI) changes."

Corroborating case of transient stroke-like episodes with imaging change in CMTX1.

Mechanism Target:

TRIGGERS Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction — The stressor appears to precipitate decompensation of connexin-32-deficient white matter rather than to cause the underlying glial lesion, which is genetic and constitutive.

Show evidence (1 reference)

PMID:42477620 SUPPORT Human Clinical

"Triggers such as fever, infection and exercise have been reported, but allergy-related exposure has rarely been discussed and causal mechanisms remain uncertain."

Names the reported triggers while stating that causal mechanisms remain uncertain, which is why this link is graded PARTIAL rather than SUPPORT.

🔬

Diagnosis

1

Intermediate Conduction Velocity with Absent Male-to-Male Transmission

The practical diagnostic signature of CMTX1: a CMT phenotype with intermediate motor conduction velocities (roughly 30-40 m/s in males), negative for the CMT1A duplication and the HNPP deletion, in a family showing a dominant trait but no male-to-male transmission. The last feature is what distinguishes X-linked from autosomal dominant inheritance on the pedigree alone.

Show evidence (1 reference)

PMID:15468313 SUPPORT Human Clinical

"All patients having the CMT phenotype and intermediate conduction velocities who are negative for CMT1A duplication/hereditary neuropathy with liability to pressure palsies (HNPP) deletion, and whose family shows a dominant trait without male-to-male transmission, should be screened for CMTX1."

States the electrophysiological and pedigree criteria that should prompt GJB1 testing.

📈

Progression

2

Onset in hemizygous males

Age: 5-25 years

Peripheral neuropathy typically becomes manifest in affected males between ages five and 25. Affected females present later and more mildly, and some remain asymptomatic.

Show evidence (1 reference)

PMID:20301548 SUPPORT Other

"Peripheral neuropathy typically manifests in affected males between ages five and 25 years."

Establishes the typical window of clinical onset in hemizygous males.

Slow progression with age, in males

Disability accumulates slowly and tracks age in males but not in females - the sex difference is in the trajectory, not only in the severity at a given age. Longitudinal change is quantified with the CMT Examination Score (CMTES) and its Rasch-modified form, the outcome measures being used to design CMTX1 trials. Disability correlates with motor unit loss rather than with conduction slowing.

Show evidence (2 references)

PMID:28768847 SUPPORT Human Clinical

"there was a strong correlation between age and disease burden in males but not in females"

Establishes the sex-specific relationship between age and accumulated disability.

PMID:17353473 SUPPORT Human Clinical

"In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion."

Independent confirmation that disability accumulates with age across GJB1 genotypes.

📊

Prevalence

1

Worldwide (all Charcot-Marie-Tooth disease)

Point Prevalence 40.0 per 100,000 1–9 per 10,000

CMT overall, not CMTX specifically. Quoted as 1 per 2,500; CMTX is a fraction of this, given below as a share of genetically solved CMT.

Show evidence (1 reference)

PMID:29174527 SUPPORT Human Clinical

"a genetically and clinically heterogeneous group of neuromuscular disorders with an overall prevalence of 1 per 2500"

Establishes the overall CMT population prevalence that CMTX is a share of.

🐁

Animal Models

1

Gjb1-null mouse

The principal CMTX1 model. Gjb1-null mice develop a progressive, predominantly motor demyelinating peripheral neuropathy from about three months of age, with reduced sciatic motor nerve conduction velocity. Because loss of function is the human mechanism, a null allele is the mechanistically appropriate model rather than an approximation.

Species

Mouse

Genotype

Gjb1 (Cx32) knockout, homozygous null

Publication

PMID:20720503

Pathograph links

Demyelination with Secondary Axonal Loss partially recapitulates fidelity: moderate Reproduces the demyelinating neuropathy, but on a different internal ordering than the human electrophysiological picture implies: axonal abnormalities appear before demyelination is detectable.

Limitations: The temporal sequence differs from the conventional human model of the disease. Human electrophysiology is read as primary demyelination with secondary axonal loss, whereas in this mouse the axonal changes come first. Whether that reflects a real species difference, or a sensitivity difference between mouse histology and human nerve conduction studies, is unresolved - see the HUMAN_MODEL_MISMATCH discussion.

Readouts

Large myelinated axon diameter

Direction: DECREASED

Interpretation: Structural axonal correlate, present before demyelination.

Show evidence (1 reference)

PMID:20720503 SUPPORT Model Organism

"The diameters of large myelinated axons were progressively reduced in Gjb1-null mice compared with those in wild-type littermates."

Reports the axon-diameter measurement behind this readout.

Fast axonal transport rate

Direction: DECREASED

Interpretation: Functional axonal deficit at an age with minimal demyelination, linking Schwann-cell Cx32 loss to axonal dysfunction directly.

Show evidence (1 reference)

PMID:20720503 SUPPORT Model Organism

"fast axonal transport, assayed by sciatic nerve ligation experiments, was slower in distal axons of Gjb1-null versus wild-type animals"

Reports the axonal transport measurement behind this readout.

Show evidence (1 reference)

PMID:20720503 SUPPORT Model Organism

"These findings demonstrate that axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X."

Supports the model as informative for this node while establishing the ordering discrepancy that makes the recapitulation partial rather than complete.

Connexin-32 Reflexive Gap Junction Loss in Schwann Cells recapitulates fidelity: high A null allele is a faithful model of the human lesion, since the weight of human genotype-phenotype evidence is that GJB1 variants act by loss of function.

Readouts

Sciatic motor nerve conduction velocity

Direction: DECREASED

Interpretation: Functional consequence of Cx32 loss in Schwann cells.

Show evidence (1 reference)

PMID:33692503 SUPPORT Model Organism

"Gjb1-null mice with deletion of the Gjb1/Cx32 gene develop a progressive, predominantly motor demyelinating peripheral neuropathy beginning at about three months of age with reduced sciatic motor nerve conduction velocity (MNCV) and motor amplitude"

Reports the conduction-velocity phenotype of the null model.

{ }

Source YAML

click to show

name: Charcot-Marie-Tooth Disease Type X
creation_date: "2026-08-27T00:00:00Z"
category: Mendelian
description: >-
  Charcot-Marie-Tooth disease type X (CMTX) is the X-linked compartment of
  Charcot-Marie-Tooth disease, sitting alongside the dominant demyelinating
  (CMT1), dominant axonal (CMT2) and autosomal-recessive (CMT4) compartments.
  It is overwhelmingly dominated by CMTX1, caused by loss-of-function variants
  in GJB1 encoding connexin-32 — the second most common cause of CMT overall
  after the PMP22 duplication of CMT1A. Connexin-32 forms the reflexive gap
  junctions that connect the abaxonal and adaxonal cytoplasm of a myelinating
  Schwann cell across its own non-compact myelin, providing a short radial
  diffusion path for ions and small metabolites that would otherwise have to
  travel the full spiral length of the sheath. Losing that path degrades
  Schwann-cell support of the axon it myelinates, producing an intermediate
  electrophysiology — conduction velocities slowed beyond pure axonal disease
  but not into the uniformly demyelinating range — and progressive
  length-dependent distal weakness, wasting, sensory loss and foot deformity.
  Because GJB1 is X-linked and the mechanism is loss of function, hemizygous
  males are consistently and more severely affected while heterozygous females
  are affected later, more mildly, and more variably. The mechanism of that
  female variability is unsettled: skewed X-inactivation in Schwann cells is the
  long-standing hypothesis, but the only study to measure it directly in
  affected nerve found random X-inactivation, so this entry does not assert it. Connexin-32 is also expressed by
  oligodendrocytes, which is why CMTX1 — alone among the CMT compartments —
  carries a recognized central nervous system phenotype, including stereotyped
  episodes of transient, reversible CNS white-matter dysfunction.

  The remaining loci are individually rare and mechanistically unrelated to
  connexin biology, each converging on the same peripheral-nerve endpoint by a
  different route: AIFM1 (CMTX4 / Cowchock syndrome) and PDK3 (CMTX6) through
  mitochondrial bioenergetic failure, PRPS1 (CMTX5) through impaired purine
  nucleotide biosynthesis, and CMTX3 through a non-coding interchromosomal
  insertion at Xq27.1 whose transcriptional consequence is still unresolved.
  Several of these carry syndromic features — sensorineural hearing loss and
  optic atrophy in CMTX5, deafness and cognitive impairment in CMTX4 — that a
  pure CMTX1 picture does not.
disease_term:
  preferred_term: X-linked Charcot-Marie-Tooth disease (CMTX)
  term:
    id: MONDO:0018994
    label: Charcot-Marie-Tooth disease type X
parents:
- Charcot-Marie-Tooth disease
synonyms:
- X-linked hereditary motor and sensory neuropathy
- CMTX
- X-linked Charcot-Marie-Tooth disease
notes: >-
  Scope note. This entry is the X-linked compartment of CMT, curated in parallel
  with the existing kb/disorders/Charcot-Marie-Tooth_Disease_Type_1.yaml,
  _Type_2.yaml and _Type_4.yaml entries, each of which is likewise a
  multi-gene compartment carried as one Disease with per-locus has_subtypes.
  Two CMTX loci already have their own dedicated entries and are cross-referenced
  rather than duplicated here: CMTX4 / Cowchock syndrome
  (kb/disorders/Charcot-Marie-Tooth_Disease_X-linked_Recessive_4.yaml) and CMTX5,
  which is covered by kb/disorders/PRPS1_Deficiency_Spectrum.yaml as part of the
  wider PRPS1 allelic series. The subtype entries below carry the CMTX-relevant
  claim and its citation only.

  CMTX2 (MONDO:0010550) and CMTX3 (MONDO:0010551) are retained as subtypes for
  MONDO completeness, but their evidence base is thin and asymmetric: CMTX3 has a
  molecularly defined lesion, while CMTX2 rests on old linkage reports with no
  confirmed gene. That asymmetry is recorded in each subtype's description rather
  than smoothed over.
has_subtypes:
- name: CMTX1
  display_name: CMTX1 (GJB1 / connexin-32-related)
  description: >-
    X-linked dominant intermediate CMT caused by loss-of-function variants in
    GJB1 encoding the gap junction protein connexin-32, which forms reflexive
    gap junctions in the non-compact myelin of myelinating Schwann cells. CMTX1
    accounts for the large majority of CMTX and is the second most common cause
    of CMT overall. Over 260 distinct GJB1 variants have been reported across
    nearly all protein domains, and the weight of genotype-phenotype evidence
    is that they converge on simple loss of connexin-32 function rather than a
    toxic gain of function. Hemizygous males are more severely and more
    uniformly affected than heterozygous females.
  evidence:
  - reference: PMID:8266101
    reference_title: "Connexin mutations in X-linked Charcot-Marie-Tooth disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families."
    explanation: Establishes GJB1 / connexin-32 variants as the molecular cause of CMTX1.
  - reference: PMID:17353473
    reference_title: "CMT1X phenotypes represent loss of GJB1 gene function."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function."
    explanation: >-
      Supports loss of function, rather than a gain-of-function mechanism, as the
      basis of the CMTX1 phenotype across 28 different GJB1 variants.
- name: CMTX2
  display_name: CMTX2 (X-linked recessive, gene unconfirmed)
  description: >-
    An X-linked recessive CMT locus mapped historically to Xp22.2 and reported
    with early onset and intellectual disability. No causal gene has been
    confirmed, and the entity is defined by linkage rather than by a molecular
    lesion. Retained here for MONDO completeness; it should not be treated as
    having an established mechanism.
- name: CMTX3
  display_name: CMTX3 (Xq27.1 interchromosomal insertion)
  description: >-
    X-linked recessive CMT caused not by a coding variant but by a 78 kb
    interchromosomal insertion originating from chromosome 8q24.3 into the
    CMTX3 locus at Xq26.3-q27.3. Coding mutations were excluded by exome
    sequencing in the two mapped families, and the lesion was found only by
    whole genome sequencing. How the insertion produces neuropathy — presumably
    by perturbing regulation of a neighbouring gene — is not established.
  evidence:
  - reference: PMID:27438001
    reference_title: "Whole Genome Sequencing Identifies a 78 kb Insertion from Chromosome 8 as the Cause of Charcot-Marie-Tooth Neuropathy CMTX3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The 78 kb insertion originates from chromosome 8q24.3, segregates fully with the disease in the two families, and is absent from the general population"
    explanation: Establishes a non-coding interchromosomal insertion as the CMTX3 lesion.
- name: CMTX4
  display_name: CMTX4 / Cowchock syndrome (AIFM1-related)
  description: >-
    Slowly progressive X-linked recessive axonal neuropathy with deafness and
    cognitive impairment, caused by missense variants in AIFM1 encoding
    apoptosis-inducing factor, an FAD-dependent mitochondrial NADH oxidase.
    Curated in full in kb/disorders/Charcot-Marie-Tooth_Disease_X-linked_Recessive_4.yaml;
    represented here only as the CMTX4 member of the X-linked compartment.
  evidence:
  - reference: PMID:23217327
    reference_title: "Cowchock syndrome is associated with a mutation in apoptosis-inducing factor."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cowchock syndrome (CMTX4) is a slowly progressive X-linked recessive disorder with axonal neuropathy, deafness, and cognitive impairment."
    explanation: Defines CMTX4 as an X-linked recessive axonal neuropathy with deafness and cognitive impairment.
- name: CMTX5
  display_name: CMTX5 / Rosenberg-Chutorian syndrome (PRPS1-related)
  description: >-
    X-linked recessive peripheral neuropathy with prelingual sensorineural
    hearing loss and optic neuropathy, caused by missense variants in PRPS1
    encoding phosphoribosyl pyrophosphate synthetase 1. PRPS1 was the first CMT
    gene encoding a metabolic enzyme. Curated within the wider allelic series in
    kb/disorders/PRPS1_Deficiency_Spectrum.yaml; represented here only as the
    CMTX5 member of the X-linked compartment.
  evidence:
  - reference: PMID:17701900
    reference_title: "Mutations in PRPS1, which encodes the phosphoribosyl pyrophosphate synthetase enzyme critical for nucleotide biosynthesis, cause hereditary peripheral neuropathy with hearing loss and optic neuropathy (cmtx5)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the affected male patients invariably develop sensorineural hearing loss of prelingual type followed by gating disturbance and visual loss"
    explanation: Defines the CMTX5 symptom triad of neuropathy, prelingual hearing loss and visual loss.
- name: CMTX6
  display_name: CMTX6 (PDK3-related)
  description: >-
    X-linked dominant CMT caused by a missense variant (p.R158H) in PDK3, one of
    four kinases that inhibit the pyruvate dehydrogenase complex by reversible
    phosphorylation. Unusually among CMT mechanisms the variant is
    hyperactivating rather than inactivating: mutant PDK3 binds the E2p
    inner-lipoyl domain more tightly than wild type and hyper-phosphorylates
    the complex, reducing pyruvate flux into the Krebs cycle.
  evidence:
  - reference: PMID:23297365
    reference_title: "A new locus for X-linked dominant Charcot-Marie-Tooth disease (CMTX6) is caused by mutations in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing identified a missense mutation c.G473A (p.R158H) in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."
    explanation: Establishes the PDK3 p.R158H variant as the cause of X-linked dominant CMTX6.
references:
- reference: PMID:20301548
  title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
  tags:
  - GeneReviews
  findings: []
pathophysiology:
- name: Connexin-32 Reflexive Gap Junction Loss in Schwann Cells
  conforms_to: "schwann_cell_myelin_maintenance#Myelin Gene Dosage or Structural Lesion in Schwann Cells"
  biological_scale: MOLECULAR
  description: >-
    Loss-of-function variants in GJB1 remove connexin-32 from the non-compact
    myelin of myelinating Schwann cells, where it assembles the reflexive gap
    junctions that connect the cell's own abaxonal and adaxonal cytoplasmic
    compartments across the myelin spiral. Those junctions provide a short
    radial diffusion path — on the order of micrometres — for ions and small
    metabolites that would otherwise have to travel the entire spiral length of
    the sheath. Connexin-32 is normally expressed in myelinated peripheral
    nerve, and the reported variants span nearly all protein domains yet
    converge on loss of function rather than a toxic gain, which is why
    whole-gene deletions are no more severe than missense alleles.
  cell_types:
  - preferred_term: Myelinating Schwann cell
    term:
      id: CL:0000218
      label: myelinating Schwann cell
  biological_processes:
  - preferred_term: Gap junction assembly in non-compact myelin
    term:
      id: GO:0016264
      label: gap junction assembly
    modifier: DECREASED
  genes:
  - preferred_term: GJB1
    term:
      id: hgnc:4283
      label: GJB1
  downstream:
  - target: Impaired Radial Metabolic Support of the Axon
    description: >-
      Removing the reflexive radial diffusion path degrades the Schwann cell's
      ability to exchange ions and small metabolites with the periaxonal space
      it supports.
  evidence:
  - reference: PMID:8266101
    reference_title: "Connexin mutations in X-linked Charcot-Marie-Tooth disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it was found that connexin32 is normally expressed in myelinated peripheral nerve"
    explanation: Localizes connexin-32 to myelinated peripheral nerve, the compartment the lesion acts in.
  - reference: PMID:17353473
    reference_title: "CMT1X phenotypes represent loss of GJB1 gene function."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the degree of disability was comparable with that observed in patients with a documented GJB1 deletion"
    explanation: >-
      Equivalence of missense and whole-gene-deletion disability is the
      observation that identifies the mechanism as loss of function.
  - reference: PMID:9722620
    reference_title: "Functional gap junctions in the schwann cell myelin sheath."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "this radial pathway is approximately one million times faster than the circumferential pathway"
    explanation: >-
      Quantifies why the reflexive route matters: dye-injection measurements in
      teased myelinating Schwann cells show the radial path across the sheath is
      about a million-fold faster than travelling the spiral circumferentially,
      which is what makes its loss consequential.
- name: Impaired Radial Metabolic Support of the Axon
  biological_scale: CELLULAR
  description: >-
    Without the reflexive gap-junction shortcut, exchange of potassium, cyclic
    nucleotides and other small solutes between the Schwann cell body and the
    periaxonal compartment is confined to the long circumferential route through
    the myelin spiral. Schwann-cell support of the axon it ensheathes becomes
    inadequate, most acutely in the longest fibres where the metabolic load per
    supporting cell is greatest. This is the step at which a molecular lesion in
    a glial junction protein becomes an axon-level deficit.
  cell_types:
  - preferred_term: Myelinating Schwann cell
    term:
      id: CL:0000218
      label: myelinating Schwann cell
  - preferred_term: Peripheral sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: Peripheral motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  biological_processes:
  - preferred_term: Peripheral nervous system myelin maintenance
    term:
      id: GO:0032287
      label: peripheral nervous system myelin maintenance
    modifier: DECREASED
  downstream:
  - target: Demyelination with Secondary Axonal Loss
    description: >-
      Chronic failure of Schwann-cell metabolic support destabilizes the myelin
      sheath and the axon it invests.
- name: PDK3 Hyperactivation and Reduced Pyruvate Flux
  biological_scale: MOLECULAR
  description: >-
    In CMTX6 the PDK3 p.R158H variant is hyperactivating rather than
    inactivating, which is unusual among CMT mechanisms. Mutant PDK3 binds the
    inner-lipoyl (L2) domain of the E2p chain more tightly than wild type and
    hyper-phosphorylates the pyruvate dehydrogenase complex, which inhibits it.
    Pyruvate entry into the Krebs cycle is throttled, leaving the long
    peripheral axon - the compartment least able to tolerate an ATP shortfall -
    energetically unsupported. Note the two-level direction here: the kinase
    gains activity, and the pathway it regulates loses flux.
  cell_types:
  - preferred_term: Peripheral sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: Peripheral motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  biological_processes:
  - preferred_term: Pyruvate metabolic process
    term:
      id: GO:0006090
      label: pyruvate metabolic process
    modifier: DECREASED
  genes:
  - preferred_term: PDK3
    term:
      id: hgnc:8811
      label: PDK3
  downstream:
  - target: Demyelination with Secondary Axonal Loss
    description: >-
      Reduced pyruvate flux in the longest fibres produces the same
      length-dependent axonal degeneration the connexin route arrives at.
  evidence:
  - reference: PMID:23297365
    reference_title: "A new locus for X-linked dominant Charcot-Marie-Tooth disease (CMTX6) is caused by mutations in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We found that the R158H mutation confers enzyme hyperactivity and binds with stronger affinity than the wild-type to the inner-lipoyl (L2) domain of the E2p chain of PDC."
    explanation: >-
      Biochemical characterization establishing PDK3 hyperactivity, and so
      reduced pyruvate flux, as the CMTX6 mechanism.
  - reference: PMID:23297365
    reference_title: "A new locus for X-linked dominant Charcot-Marie-Tooth disease (CMTX6) is caused by mutations in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "reduced pyruvate flux due to R158H mutant PDK3-mediated hyper-phosphorylation of the PDC as the underlying pathogenic cause of peripheral neuropathy"
    explanation: States the proposed causal chain from kinase hyperactivity to peripheral neuropathy.
- name: AIFM1 Mitochondrial Oxidoreductase Dysfunction
  biological_scale: MOLECULAR
  description: >-
    In CMTX4 (Cowchock syndrome) the AIFM1 p.Glu493Val variant alters the redox
    properties of apoptosis-inducing factor, an FAD-dependent NADH oxidase
    imported into mitochondria, and increases cell death. This is a distinct
    lesion from the CMTX6 kinase defect: the shared feature is only that both
    proteins are mitochondrial, and the downstream biology - dysregulated
    caspase-independent cell death rather than a bioenergetic flux deficit -
    is not the same.
  cell_types:
  - preferred_term: Peripheral sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: Peripheral motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  molecular_functions:
  - preferred_term: NADH dehydrogenase activity
    term:
      id: GO:0003954
      label: NADH dehydrogenase activity
    modifier: ABNORMAL
  biological_processes:
  - preferred_term: Apoptotic process
    term:
      id: GO:0006915
      label: apoptotic process
    modifier: INCREASED
  genes:
  - preferred_term: AIFM1
    term:
      id: hgnc:8768
      label: AIFM1
  downstream:
  - target: Demyelination with Secondary Axonal Loss
    description: >-
      Increased neuronal cell death converges on the same length-dependent
      degeneration endpoint.
  evidence:
  - reference: PMID:23217327
    reference_title: "Cowchock syndrome is associated with a mutation in apoptosis-inducing factor."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The c.1478A>T (p.Glu493Val) mutation found in the family reported here alters the redox properties of the AIF protein and results in increased cell death"
    explanation: >-
      Reports what the study found about the variant itself - altered redox
      properties and increased cell death.
- name: Purine Nucleotide Biosynthesis Deficiency
  biological_scale: MOLECULAR
  description: >-
    In CMTX5, missense variants in PRPS1 reduce phosphoribosyl pyrophosphate
    synthetase 1 activity, the committed step supplying PRPP for purine and
    pyrimidine nucleotide biosynthesis. PRPS1 was the first CMT gene found to
    encode a metabolic enzyme. Because the enzyme is ubiquitously expressed —
    including in cochlea — the deficit is not confined to peripheral nerve,
    which is why CMTX5 is syndromic, adding prelingual sensorineural hearing
    loss and optic neuropathy to the neuropathy.
  cell_types:
  - preferred_term: Peripheral sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: Peripheral motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  biological_processes:
  - preferred_term: Purine nucleotide biosynthetic process
    term:
      id: GO:0006164
      label: purine nucleotide biosynthetic process
    modifier: DECREASED
  genes:
  - preferred_term: PRPS1
    term:
      id: hgnc:9462
      label: PRPS1
  downstream:
  - target: Demyelination with Secondary Axonal Loss
    description: >-
      Reduced nucleotide supply in long-projection neurons converges on the same
      length-dependent degeneration.
  evidence:
  - reference: PMID:17701900
    reference_title: "Mutations in PRPS1, which encodes the phosphoribosyl pyrophosphate synthetase enzyme critical for nucleotide biosynthesis, cause hereditary peripheral neuropathy with hearing loss and optic neuropathy (cmtx5)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We also showed decreased enzyme activity in patients with M115T."
    explanation: Demonstrates reduced PRPS1 enzyme activity in CMTX5 patients, establishing loss of function.
- name: Demyelination with Secondary Axonal Loss
  conforms_to: "peripheral_axonal_degeneration#Distal Axonal Degeneration and Demyelination"
  biological_scale: TISSUE
  description: >-
    The shared terminal node of the X-linked compartment, and the same one the
    CMT1, CMT2 and CMT4 entries converge on. In CMTX1 the electrophysiological
    signature is characteristically intermediate: motor conduction velocities
    are slowed below the axonal range but not into the uniformly demyelinating
    range that defines CMT1, consistent with a primary glial lesion that
    degrades rather than abolishes myelin. Clinical disability tracks the
    secondary axonal loss rather than the conduction slowing — in CMTX1 it
    increases with age and correlates with loss of motor units on motor unit
    number estimation.
  cell_types:
  - preferred_term: Myelinating Schwann cell
    term:
      id: CL:0000218
      label: myelinating Schwann cell
  - preferred_term: Peripheral sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: Peripheral motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  biological_processes:
  - preferred_term: Myelination
    term:
      id: GO:0042552
      label: myelination
    modifier: DECREASED
  downstream:
  - target: Length-Dependent Distal Neuropathy
    description: >-
      Axonal loss in the longest fibres produces the length-dependent distal
      motor and sensory deficit.
  evidence:
  - reference: PMID:17353473
    reference_title: "CMT1X phenotypes represent loss of GJB1 gene function."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Disability correlated with a loss of motor units as assessed by motor unit number estimates."
    explanation: >-
      Ties clinical disability to axonal (motor unit) loss rather than to
      demyelination per se, which is why this node and not the myelin node is
      the proximate driver of the phenotype.
- name: Length-Dependent Distal Neuropathy
  conforms_to: "peripheral_axonal_degeneration#Length-Dependent Nerve Fiber Dysfunction"
  biological_scale: ORGANISM
  description: >-
    The clinical endpoint: symmetrical, length-dependent distal weakness and
    wasting beginning in the intrinsic foot and peroneal muscles, distal sensory
    loss, depressed or absent reflexes, and the secondary skeletal deformities
    (pes cavus, hammertoes) that follow chronic imbalance between weakened and
    spared muscle groups. In CMTX1 this typically becomes manifest in affected
    males between ages five and 25.
  downstream:
  - target: Distal Muscle Weakness
    description: Length-dependent motor deficit produced by the neuropathy.
  - target: Distal Amyotrophy
    description: Wasting of chronically denervated distal muscle.
  - target: Distal Sensory Impairment
    description: Length-dependent sensory deficit produced by the neuropathy.
  - target: Hyporeflexia
    description: Loss of the afferent and efferent limbs of the tendon reflex arc.
  - target: Pes Cavus
    description: Secondary skeletal deformity following chronic denervation imbalance.
  - target: Steppage Gait
    description: Compensatory gait change secondary to foot drop.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Peripheral neuropathy typically manifests in affected males between ages five and 25 years."
    explanation: Establishes the typical age at onset of the peripheral phenotype in hemizygous males.
- name: Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction
  biological_scale: TISSUE
  description: >-
    Connexin-32 is expressed by oligodendrocytes as well as Schwann cells, and
    CMTX1 is consequently the one CMT compartment with a recognized central
    nervous system phenotype. This takes two forms: fixed white-matter
    abnormalities, and stereotyped acute, self-limited, fully reversible
    episodes of CNS dysfunction — typically weakness and dysarthria, sometimes
    presenting as stroke-like episodes and mimicking cerebral ischaemia. The
    episodes are self-limited and treatment is supportive. Triggers are
    described - fever, infection, exercise, altitude, and in one report pollen
    exposure - but the mechanism linking any of them to the white-matter lesion
    is not established, and the pollen case explicitly leaves an allergic
    mechanism unproven. That the deficits resolve completely, with imaging
    lesions that also resolve, argues for reversible metabolic or ionic
    decompensation of oligodendrocyte-supported white matter rather than for
    infarction.
  cell_types:
  - preferred_term: Oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  genes:
  - preferred_term: GJB1
    term:
      id: hgnc:4283
      label: GJB1
  downstream:
  - target: Transient Stroke-like CNS Episodes
    description: >-
      Oligodendrocyte connexin-32 loss is the proposed substrate for the
      reversible episodes of CNS dysfunction.
  - target: Central White Matter Abnormality
    description: >-
      The same glial lesion is the proposed substrate for the fixed
      white-matter abnormalities seen on imaging.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "acute, self-limited episodes of transient neurologic dysfunction (especially weakness and dysarthria)"
    explanation: Establishes the transient, reversible CNS episodes characteristic of GJB1 disorders.
  - reference: PMID:12111842
    reference_title: "Cellular mechanisms of connexin32 mutations associated with CNS manifestations."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the mutants associated with CNS phenotypes failed to reach the cell membrane and were instead retained"
    explanation: >-
      Distinguishes the CNS-associated GJB1 alleles cellularly: those reported to
      cause clinical CNS dysfunction are retained intracellularly rather than
      trafficking to the membrane, suggesting the CNS phenotype tracks a
      trafficking-defective subgroup rather than all loss-of-function alleles.
phenotypes:
- category: Neurological
  name: Distal Muscle Weakness
  description: >-
    Symmetrical length-dependent weakness beginning in foot dorsiflexors and
    intrinsic foot muscles, later involving the hands.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Distal muscle weakness
    term:
      id: HP:0002460
      label: Distal muscle weakness
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:28768847
    reference_title: "Cross-sectional analysis of a large cohort with X-linked Charcot-Marie-Tooth disease (CMTX1)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "85% of males reported difficulty with walking compared with 64% of females"
    explanation: >-
      The frequency band is set from the male figure, which is the sex in which
      the phenotype is fully penetrant enough to band. Note it differs by sex,
      so a single disease-level frequency is an approximation.
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "GJB1 disorders are typically characterized by peripheral motor and sensory neuropathy"
    explanation: Establishes motor involvement as a defining feature of the GJB1 phenotype.
- category: Neurological
  name: Distal Amyotrophy
  description: Wasting of intrinsic foot, peroneal and later intrinsic hand muscles.
  phenotype_term:
    preferred_term: Distal amyotrophy
    term:
      id: HP:0003693
      label: Distal amyotrophy
    clinical_course: PROGRESSIVE
- category: Neurological
  name: Distal Sensory Impairment
  description: Length-dependent loss of large- and small-fibre sensory modalities.
  phenotype_term:
    preferred_term: Distal sensory impairment
    term:
      id: HP:0002936
      label: Distal sensory impairment
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "peripheral motor and sensory neuropathy with or without fixed CNS abnormalities"
    explanation: Establishes sensory as well as motor involvement in GJB1 disorders.
- category: Neurological
  name: Hyporeflexia
  description: Depressed or absent tendon reflexes, earliest and most consistently at the ankles.
  phenotype_term:
    preferred_term: Hyporeflexia
    term:
      id: HP:0001265
      label: Hyporeflexia
- category: Neurological
  name: Decreased Motor Nerve Conduction Velocity
  description: >-
    Intermediate slowing of motor conduction velocity — below the axonal range
    but not into the uniformly demyelinating range that characterizes CMT1.
  phenotype_term:
    preferred_term: Decreased motor nerve conduction velocity
    term:
      id: HP:0003431
      label: Decreased motor nerve conduction velocity
- category: Musculoskeletal
  name: Pes Cavus
  description: >-
    High-arched foot with clawed toes, the classic secondary skeletal deformity
    of chronic length-dependent denervation.
  phenotype_term:
    preferred_term: Pes cavus
    term:
      id: HP:0001761
      label: Pes cavus
- category: Neurological
  name: Steppage Gait
  description: Compensatory high-stepping gait secondary to foot drop.
  phenotype_term:
    preferred_term: Steppage gait
    term:
      id: HP:0003376
      label: Steppage gait
- category: Neurological
  name: Transient Stroke-like CNS Episodes
  description: >-
    Acute, self-limited, fully reversible episodes of central nervous system
    dysfunction that can present as stroke-like episodes and mimic cerebral
    ischaemia, with reversible diffusion-restricted white-matter lesions on MRI.
    Distinctive to CMTX1 among the CMT compartments and attributable to
    oligodendrocyte connexin-32 loss.
  phenotype_term:
    preferred_term: Stroke-like episode
    term:
      id: HP:0002401
      label: Stroke-like episode
    temporality: TRANSIENT
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less commonly, initial manifestations in some affected individuals are stroke-like episodes (acute fulminant episodes of reversible CNS dysfunction)."
    explanation: Establishes reversible stroke-like CNS episodes as a recognized GJB1 presentation.
- category: Neurological
  name: Transient Dysarthria
  description: >-
    Dysarthria is the symptom most characteristically reported during the
    transient CNS episodes, alongside weakness. Curated separately from the
    episode itself so the bound term names the symptom rather than standing in
    for the whole event.
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
    temporality: TRANSIENT
  evidence:
  - reference: PMID:42477620
    reference_title: "Pediatric stroke-like episodes temporally associated with pollen exposure in GJB1-related CMTX1: an expanded family case report with serial MRI and DTI follow-up."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 9-year-old boy developed acute left-sided weakness, dysarthria and central facial-tongue paresis shortly after pollen exposure."
    explanation: Documents dysarthria as a presenting feature of a transient CNS episode.
- category: Neurological
  name: Central White Matter Abnormality
  description: >-
    Fixed CNS white-matter abnormality on imaging, distinct from the transient
    episodes and likewise attributable to oligodendrocyte connexin-32 loss.
  phenotype_term:
    preferred_term: Abnormal CNS myelination
    term:
      id: HP:0011400
      label: Abnormal CNS myelination
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "peripheral motor and sensory neuropathy with or without fixed CNS abnormalities"
    explanation: Establishes fixed CNS abnormalities as part of the GJB1 disorder spectrum.
- category: Auditory
  name: Sensorineural Hearing Impairment
  subtype: CMTX5
  description: >-
    Prelingual sensorineural hearing loss, the first manifestation of CMTX5 and
    also a feature of CMTX4. Not a feature of CMTX1.
  phenotype_term:
    preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  evidence:
  - reference: PMID:17701900
    reference_title: "Mutations in PRPS1, which encodes the phosphoribosyl pyrophosphate synthetase enzyme critical for nucleotide biosynthesis, cause hereditary peripheral neuropathy with hearing loss and optic neuropathy (cmtx5)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the affected male patients invariably develop sensorineural hearing loss of prelingual type"
    explanation: Establishes prelingual sensorineural hearing loss as an obligate CMTX5 feature.
- category: Ophthalmologic
  name: Optic Atrophy
  subtype: CMTX5
  description: Optic neuropathy with visual loss, following the hearing loss in CMTX5.
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
genetic:
- name: GJB1
  notes: >-
    Encodes connexin-32, the gap junction protein of Schwann cell non-compact
    myelin. Loss-of-function variants cause CMTX1, the great majority of CMTX
    and the second most common cause of CMT overall. Over 260 distinct variants
    have been reported spanning nearly all protein domains. The variant classes
    implicated span missense, nonsense, frameshift and non-coding variants as
    well as whole-coding-region deletions - the breadth is itself an argument
    for loss of function, since a single toxic mechanism would not be shared by
    a deletion and a missense allele alike. In the largest classified series,
    154 distinct GJB1 variants across 387 patients were adjudicated under
    ACMG/ACGS criteria adapted for CMTX1, of which 82.4% were pathogenic or
    likely pathogenic - a higher yield than ClinVar's classification of the
    same variants. Of the five most common variants, the non-coding c.-17G>A
    produced the most severe phenotype. Against that, a large cross-sectional
    cohort found no phenotypic correlation with specific variants overall, so
    genotype-phenotype signal in CMTX1 appears confined to particular alleles
    rather than being a general feature.
  case_fractions:
  - population: Hungarian CMT cohort (genetic epidemiology series)
    case_fraction_percent: 9.2
    notes: >-
      Share of all affected individuals in a national CMT cohort, second only to
      PMP22 at 40.5%. A share of CMT cases, not a population rate. Corroborating
      cohorts put GJB1 in roughly the 7-16% range.
    evidence:
    - reference: PMID:29174527
      reference_title: "Genotypic and phenotypic spectrum of the most common causative genes of Charcot-Marie-Tooth disease in Hungarian patients."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "alterations were most frequently found in PMP22 (40.5%), followed by GJB1 (9.2%)"
      explanation: Quantifies the GJB1 share of CMT cases in a national cohort.
  gene_term:
    preferred_term: GJB1
    term:
      id: hgnc:4283
      label: GJB1
  relationship_type: CAUSATIVE
  evidence:
  - reference: PMID:8266101
    reference_title: "Connexin mutations in X-linked Charcot-Marie-Tooth disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families."
    explanation: Original identification of GJB1 as the CMTX1 gene.
  - reference: PMID:37284795
    reference_title: "Genetic analysis and natural history of Charcot-Marie-Tooth disease CMTX1 due to GJB1 variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present 387 patients from 295 families harbouring 154 variants in GJB1."
    explanation: Establishes the scale of the classified GJB1 variant series behind the figures above.
  - reference: PMID:37284795
    reference_title: "Genetic analysis and natural history of Charcot-Marie-Tooth disease CMTX1 due to GJB1 variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "319 patients (82.4%) were deemed to have P/LP variants, 65 had VUS (16.8%) and three benign variants (0.8%"
    explanation: The ACMG/ACGS pathogenicity breakdown of the classified variant series.
  - reference: PMID:37284795
    reference_title: "Genetic analysis and natural history of Charcot-Marie-Tooth disease CMTX1 due to GJB1 variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Genotype-phenotype analysis suggested c.-17G>A produces the most severe phenotype of the five most common variants"
    explanation: The one allele-specific genotype-phenotype signal reported in the series.
  - reference: PMID:28768847
    reference_title: "Cross-sectional analysis of a large cohort with X-linked Charcot-Marie-Tooth disease (CMTX1)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the absence of a phenotypic correlation with specific GJB1 mutations, sex-specific distinctions and clinically relevant attributes need to be incorporated into the measurements for clinical trials"
    explanation: >-
      Reports no general genotype-phenotype correlation across 89 variants,
      which qualifies the allele-specific signal above rather than contradicting it.
- name: AIFM1
  subtype: CMTX4
  notes: >-
    Encodes apoptosis-inducing factor, an FAD-dependent mitochondrial NADH
    oxidase. The p.Glu493Val variant causes CMTX4 (Cowchock syndrome).
  gene_term:
    preferred_term: AIFM1
    term:
      id: hgnc:8768
      label: AIFM1
  relationship_type: CAUSATIVE
  evidence:
  - reference: PMID:23217327
    reference_title: "Cowchock syndrome is associated with a mutation in apoptosis-inducing factor."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "identified a missense change c.1478A>T (p.Glu493Val) in AIFM1, the gene encoding apoptosis-inducing factor (AIF) mitochondrion-associated 1"
    explanation: Identifies the AIFM1 variant causing CMTX4.
- name: PRPS1
  subtype: CMTX5
  notes: >-
    Encodes phosphoribosyl pyrophosphate synthetase 1. Loss-of-function missense
    variants cause CMTX5; PRPS1 was the first CMT gene encoding a metabolic
    enzyme, and the same gene's allelic series includes Arts syndrome and
    X-linked nonsyndromic deafness.
  gene_term:
    preferred_term: PRPS1
    term:
      id: hgnc:9462
      label: PRPS1
  relationship_type: CAUSATIVE
  evidence:
  - reference: PMID:17701900
    reference_title: "Mutations in PRPS1, which encodes the phosphoribosyl pyrophosphate synthetase enzyme critical for nucleotide biosynthesis, cause hereditary peripheral neuropathy with hearing loss and optic neuropathy (cmtx5)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutations identified were E43D, in patients with Rosenberg-Chutorian syndrome, and M115T, in the Korean patients with CMTX5."
    explanation: Identifies the PRPS1 variants causing CMTX5 / Rosenberg-Chutorian syndrome.
- name: PDK3
  subtype: CMTX6
  notes: >-
    Encodes pyruvate dehydrogenase kinase isoenzyme 3. The p.R158H variant is
    hyperactivating and causes X-linked dominant CMTX6.
  gene_term:
    preferred_term: PDK3
    term:
      id: hgnc:8811
      label: PDK3
  relationship_type: CAUSATIVE
  evidence:
  - reference: PMID:23297365
    reference_title: "A new locus for X-linked dominant Charcot-Marie-Tooth disease (CMTX6) is caused by mutations in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The change localized within the 1.43-Mb linkage interval, segregated with the affected phenotype and was excluded in ethnically matched control chromosomes."
    explanation: Segregation and control-exclusion evidence supporting PDK3 as the CMTX6 gene.
inheritance:
- name: X-linked dominant (CMTX1, CMTX6)
  description: >-
    CMTX1 and CMTX6 are transmitted in an X-linked dominant manner. Affected
    males transmit the variant to all daughters and no sons; heterozygous
    females have a 50% transmission risk to each child. Hemizygous males are
    consistently affected, whereas heterozygous females range from asymptomatic
    to mild-to-moderate and progressive. Skewed X-inactivation across the Schwann
    cell population is the traditional explanation for that variability, but it
    is not supported by the only direct measurement in affected nerve (see the
    refuting evidence below), and no alternative mechanism is established.
  inheritance_term:
    preferred_term: X-linked dominant inheritance
    term:
      id: HP:0001423
      label: X-linked dominant inheritance
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected males transmit the GJB1 pathogenic variant to all of their daughters and none of their sons."
    explanation: States the X-linked transmission pattern for GJB1 disorders.
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Although both men and women are affected, manifestations tend to be less severe in women, some of whom may remain asymptomatic."
    explanation: Documents the milder and more variable female phenotype in GJB1 disorders.
  - reference: PMID:40759929
    reference_title: "Random X chromosome inactivation in female Charcot-Marie-Tooth disease type X1: insights from sural nerve biopsy analysis."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "our findings suggest that XCI does not contribute to phenotypic variability in female CMTX1 patients"
    explanation: >-
      Directly refutes skewed X-inactivation as the explanation for female
      phenotypic variability. A HUMARA assay on archived sural nerve from two
      female CMTX1 patients found a random inactivation pattern no different
      from controls. This is a two-patient pilot, so it weakens rather than
      settles the hypothesis, but it is the only measurement made in the
      affected tissue, which is why this entry declines to assert mosaicism.
- name: X-linked recessive (CMTX2, CMTX3, CMTX4, CMTX5)
  description: >-
    The remaining CMTX loci are X-linked recessive, with affected hemizygous
    males and generally unaffected heterozygous carrier females.
  inheritance_term:
    preferred_term: X-linked recessive inheritance
    term:
      id: HP:0001419
      label: X-linked recessive inheritance
  evidence:
  - reference: PMID:23217327
    reference_title: "Cowchock syndrome is associated with a mutation in apoptosis-inducing factor."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cowchock syndrome (CMTX4) is a slowly progressive X-linked recessive disorder"
    explanation: Documents X-linked recessive inheritance for CMTX4.
environmental:
- name: Systemic stressors preceding transient CNS episodes in CMTX1
  notes: >-
    Fever, infection, exercise, altitude and - in one paediatric report - pollen
    exposure have been described as temporally preceding the reversible CNS
    episodes of CMTX1. These are reported temporal associations, not established
    causes: the pollen case explicitly notes that an allergic mechanism is
    unproven because allergy-specific biomarkers were not obtained during the
    acute phase. No ECTO exposure term is bound because the entry is a
    heterogeneous set of systemic stressors rather than one exposure concept;
    ECTO was searched and has no term covering the set.
  influences_mechanisms:
  - target: Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction
    environmental_effect: TRIGGERS
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The stressor appears to precipitate decompensation of connexin-32-deficient
      white matter rather than to cause the underlying glial lesion, which is
      genetic and constitutive.
    evidence:
    - reference: PMID:42477620
      reference_title: "Pediatric stroke-like episodes temporally associated with pollen exposure in GJB1-related CMTX1: an expanded family case report with serial MRI and DTI follow-up."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Triggers such as fever, infection and exercise have been reported, but allergy-related exposure has rarely been discussed and causal mechanisms remain uncertain."
      explanation: >-
        Names the reported triggers while stating that causal mechanisms remain
        uncertain, which is why this link is graded PARTIAL rather than SUPPORT.
  evidence:
  - reference: PMID:42477620
    reference_title: "Pediatric stroke-like episodes temporally associated with pollen exposure in GJB1-related CMTX1: an expanded family case report with serial MRI and DTI follow-up."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pollen exposure was temporally associated with the episode in this patient, but an allergic mechanism remains unproven"
    explanation: >-
      A single paediatric case with an explicit statement that the mechanism is
      unproven - recorded as a temporal association, not an established cause.
  - reference: PMID:30952033
    reference_title: "Novel gap junction protein beta-1 gene mutation associated with a stroke-like syndrome and central nervous system involvement in patients with X-linked Charcot-Marie-Tooth Type 1: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our patient had a transient stroke-like clinical manifestations and magnetic resonance imaging (MRI) changes."
    explanation: Corroborating case of transient stroke-like episodes with imaging change in CMTX1.
treatments:
- name: Ankle-Foot Orthoses and Adaptive Footwear
  description: >-
    Ankle/foot orthoses and special shoes to correct foot drop and aid walking;
    the mainstay of symptomatic management, as no disease-modifying therapy
    exists for any CMTX subtype.
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: Rehabilitation
    term:
      id: NCIT:C15315
      label: Rehabilitation
  target_mechanisms:
  - target: Length-Dependent Distal Neuropathy
    description: >-
      Compensates mechanically for the distal weakness produced by the
      neuropathy; does not act on any upstream mechanism.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "special shoes and/or ankle/foot orthoses to correct foot drop and to aid walking"
    explanation: Recommends orthotic management of foot drop in GJB1 disorders.
- name: Physical and Occupational Therapy
  description: >-
    Daily heel-cord stretching to prevent Achilles tendon shortening, exercise
    as tolerated, and occupational therapy for fine motor skills and activities
    of daily living.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  target_mechanisms:
  - target: Length-Dependent Distal Neuropathy
    description: >-
      Addresses the secondary contractures and functional consequences of
      chronic distal denervation.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "daily heel cord stretching to prevent Achilles' tendon shortening; exercise as tolerated"
    explanation: Specifies the physical therapy regimen recommended in GJB1 disorders.
- name: Corrective Foot Surgery
  description: Surgery as needed for severe pes cavus deformity.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: orthopedic surgical procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  target_mechanisms:
  - target: Length-Dependent Distal Neuropathy
    description: Corrects the fixed skeletal deformity secondary to chronic denervation.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "surgery as needed for severe pes cavus"
    explanation: Recommends surgical correction of severe pes cavus.
- name: Avoidance of Neurotoxic Medications
  description: >-
    Avoidance of drugs that are toxic or potentially toxic to people with CMT,
    and of obesity, which makes ambulation more difficult.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "medications that are toxic or potentially toxic to persons with CMT"
    explanation: Identifies neurotoxic drug avoidance as a management principle.
- name: AAV9-Mediated Schwann-Cell-Targeted GJB1 Gene Replacement
  description: >-
    Investigational, preclinical only. An AAV9 vector delivering GJB1/Cx32 under
    the Schwann-cell-specific Mpz promoter, given by lumbar intrathecal
    injection. Gene replacement is a rational strategy here precisely because the
    human mechanism is loss of function, so restoring wild-type Cx32 to
    myelinating Schwann cells addresses the primary lesion rather than a
    downstream consequence. Treatment both before and after neuropathy onset
    improved motor performance, conduction velocity, myelination and blood
    biomarkers in Gjb1-null mice. No human trial has been reported, and
    symptomatic management remains the only clinical option.
  therapeutic_modality: GENE_THERAPY
  treatment_term:
    preferred_term: gene therapy
    term:
      id: NCIT:C15238
      label: Gene Therapy
  target_mechanisms:
  - target: Connexin-32 Reflexive Gap Junction Loss in Schwann Cells
    description: >-
      Restores Cx32 expression in myelinating Schwann cells, acting directly on
      the initiating molecular lesion rather than on its consequences.
  evidence:
  - reference: PMID:33692503
    reference_title: "AAV9-mediated Schwann cell-targeted gene therapy rescues a model of demyelinating neuropathy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "demonstrated improved motor performance and sciatic nerve conduction velocities along with improved myelination and reduced inflammation in peripheral nerve tissues"
    explanation: >-
      Preclinical efficacy in the Gjb1-null mouse, pre- and post-onset. Model
      organism evidence only - this is not a human treatment claim.
- name: Cochlear Implantation
  description: >-
    For the CMTX5 and CMTX4 hearing loss rather than the neuropathy. A small
    case series in CMT reports benefit, with outcomes shaped by whether the
    lesion is cochlear or a retrocochlear auditory neuropathy, and by how early
    implantation is done. Three of the four reported cases carry CMTX genotypes
    (GJB1/CMTX1, AIFM1, and PRPS1/CMTX5), so the series speaks to this entry
    directly rather than by extrapolation from CMT generally. Evidence is four
    patients, so this is a considered option and not an established standard.
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: cochlear device implantation
    term:
      id: NCIT:C15329
      label: Surgical Procedure
    qualifiers:
    - predicate:
        preferred_term: medical device
        term:
          id: NCIT:C16830
          label: Medical Device
      value:
        preferred_term: cochlear implant
        term:
          id: NCIT:C157820
          label: Cochlear Implant
  evidence:
  - reference: PMID:41557339
    reference_title: "Evaluating Cochlear Implantation Outcomes in Charcot-Marie-Tooth Disease: A Case Series Analysis of Genetic Profiles and Intervention Timing."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cochlear implants (CIs) have been used to rehabilitate hearing loss in CMT, but their effectiveness remains poorly understood due to the central and peripheral neurological complexities of the disease."
    explanation: >-
      Supports implantation as an option while stating the uncertainty that
      keeps this graded PARTIAL.
  - reference: PMID:41557339
    reference_title: "Evaluating Cochlear Implantation Outcomes in Charcot-Marie-Tooth Disease: A Case Series Analysis of Genetic Profiles and Intervention Timing."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Case 4: A 32-year-old man with PRPS1 -associated CMTX5 and profound prelingual SNHL showed improved sound awareness (20-30 dB HL) post-CI but had no speech perception gains due to prolonged auditory deprivation."
    explanation: >-
      A CMTX5 case specifically, and the one that grounds the timing argument:
      sound awareness improved but speech perception did not, attributed to
      prolonged auditory deprivation before implantation.
- name: Genetic Counseling
  description: >-
    Counseling on X-linked transmission risk, carrier testing of at-risk female
    relatives, and prenatal or preimplantation genetic testing options.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "molecular genetic testing of at-risk female relatives to determine their genetic status, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible"
    explanation: Specifies the genetic counseling and testing options for GJB1 disorders.
prevalence:
- population: Worldwide (all Charcot-Marie-Tooth disease)
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_5_PER_10000
  rate_per_100000: 40.0
  notes: >-
    CMT overall, not CMTX specifically. Quoted as 1 per 2,500; CMTX is a
    fraction of this, given below as a share of genetically solved CMT.
  evidence:
  - reference: PMID:29174527
    reference_title: "Genotypic and phenotypic spectrum of the most common causative genes of Charcot-Marie-Tooth disease in Hungarian patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a genetically and clinically heterogeneous group of neuromuscular disorders with an overall prevalence of 1 per 2500"
    explanation: Establishes the overall CMT population prevalence that CMTX is a share of.
progression:
- phase: Onset in hemizygous males
  age_range: 5-25 years
  notes: >-
    Peripheral neuropathy typically becomes manifest in affected males between
    ages five and 25. Affected females present later and more mildly, and some
    remain asymptomatic.
  evidence:
  - reference: PMID:20301548
    reference_title: "GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Peripheral neuropathy typically manifests in affected males between ages five and 25 years."
    explanation: Establishes the typical window of clinical onset in hemizygous males.
- phase: Slow progression with age, in males
  notes: >-
    Disability accumulates slowly and tracks age in males but not in females -
    the sex difference is in the trajectory, not only in the severity at a given
    age. Longitudinal change is quantified with the CMT Examination Score
    (CMTES) and its Rasch-modified form, the outcome measures being used to
    design CMTX1 trials. Disability correlates with motor unit loss rather than
    with conduction slowing.
  evidence:
  - reference: PMID:28768847
    reference_title: "Cross-sectional analysis of a large cohort with X-linked Charcot-Marie-Tooth disease (CMTX1)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "there was a strong correlation between age and disease burden in males but not in females"
    explanation: Establishes the sex-specific relationship between age and accumulated disability.
  - reference: PMID:17353473
    reference_title: "CMT1X phenotypes represent loss of GJB1 gene function."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion."
    explanation: Independent confirmation that disability accumulates with age across GJB1 genotypes.
diagnosis:
- name: Intermediate Conduction Velocity with Absent Male-to-Male Transmission
  description: >-
    The practical diagnostic signature of CMTX1: a CMT phenotype with
    intermediate motor conduction velocities (roughly 30-40 m/s in males),
    negative for the CMT1A duplication and the HNPP deletion, in a family
    showing a dominant trait but no male-to-male transmission. The last feature
    is what distinguishes X-linked from autosomal dominant inheritance on the
    pedigree alone.
  evidence:
  - reference: PMID:15468313
    reference_title: "X-linked Charcot-Marie-Tooth disease: phenotypic expression of a novel mutation Ile127Ser in the GJB1 (connexin 32) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients having the CMT phenotype and intermediate conduction velocities who are negative for CMT1A duplication/hereditary neuropathy with liability to pressure palsies (HNPP) deletion, and whose family shows a dominant trait without male-to-male transmission, should be screened for CMTX1."
    explanation: States the electrophysiological and pedigree criteria that should prompt GJB1 testing.
animal_models:
- name: Gjb1-null mouse
  species: Mouse
  genotype: Gjb1 (Cx32) knockout, homozygous null
  description: >-
    The principal CMTX1 model. Gjb1-null mice develop a progressive,
    predominantly motor demyelinating peripheral neuropathy from about three
    months of age, with reduced sciatic motor nerve conduction velocity. Because
    loss of function is the human mechanism, a null allele is the mechanistically
    appropriate model rather than an approximation.
  publication: PMID:20720503
  modeled_mechanisms:
  - target: Demyelination with Secondary Axonal Loss
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces the demyelinating neuropathy, but on a different internal
      ordering than the human electrophysiological picture implies: axonal
      abnormalities appear before demyelination is detectable.
    limitations: >-
      The temporal sequence differs from the conventional human model of the
      disease. Human electrophysiology is read as primary demyelination with
      secondary axonal loss, whereas in this mouse the axonal changes come
      first. Whether that reflects a real species difference, or a sensitivity
      difference between mouse histology and human nerve conduction studies, is
      unresolved - see the HUMAN_MODEL_MISMATCH discussion.
    readouts:
    - name: Large myelinated axon diameter
      target: Demyelination with Secondary Axonal Loss
      direction: DECREASED
      interpretation: Structural axonal correlate, present before demyelination.
      evidence:
      - reference: PMID:20720503
        reference_title: "Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "The diameters of large myelinated axons were progressively reduced in Gjb1-null mice compared with those in wild-type littermates."
        explanation: Reports the axon-diameter measurement behind this readout.
    - name: Fast axonal transport rate
      target: Demyelination with Secondary Axonal Loss
      direction: DECREASED
      interpretation: >-
        Functional axonal deficit at an age with minimal demyelination, linking
        Schwann-cell Cx32 loss to axonal dysfunction directly.
      evidence:
      - reference: PMID:20720503
        reference_title: "Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "fast axonal transport, assayed by sciatic nerve ligation experiments, was slower in distal axons of Gjb1-null versus wild-type animals"
        explanation: Reports the axonal transport measurement behind this readout.
    evidence:
    - reference: PMID:20720503
      reference_title: "Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings demonstrate that axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X."
      explanation: >-
        Supports the model as informative for this node while establishing the
        ordering discrepancy that makes the recapitulation partial rather than
        complete.
  - target: Connexin-32 Reflexive Gap Junction Loss in Schwann Cells
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      A null allele is a faithful model of the human lesion, since the weight of
      human genotype-phenotype evidence is that GJB1 variants act by loss of
      function.
    readouts:
    - name: Sciatic motor nerve conduction velocity
      target: Connexin-32 Reflexive Gap Junction Loss in Schwann Cells
      direction: DECREASED
      interpretation: Functional consequence of Cx32 loss in Schwann cells.
      evidence:
      - reference: PMID:33692503
        reference_title: "AAV9-mediated Schwann cell-targeted gene therapy rescues a model of demyelinating neuropathy."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Gjb1-null mice with deletion of the Gjb1/Cx32 gene develop a progressive, predominantly motor demyelinating peripheral neuropathy beginning at about three months of age with reduced sciatic motor nerve conduction velocity (MNCV) and motor amplitude"
        explanation: Reports the conduction-velocity phenotype of the null model.
discussions:
- discussion_id: cmtx1_axon_vs_myelin_ordering
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Demyelination with Secondary Axonal Loss
  prompt: >-
    Does axonal pathology in CMTX1 precede demyelination in humans, as it does
    in the Gjb1-null mouse, or is the human disease genuinely primary
    demyelination with secondary axonal loss?
  rationale: >-
    The two evidence streams point opposite ways, and the difference is
    mechanistically load-bearing rather than semantic. In Gjb1-null mice at 2-4
    months, when there is minimal or no demyelination, axon diameters are already
    reduced, neurofilaments are dephosphorylated and densely packed, beta-amyloid
    precursor protein is elevated, and fast axonal transport is slowed - so the
    axon is affected first, and the authors conclude Cx32 loss affects axons
    independently of demyelination. Human nerve conduction studies in CMTX1
    families are instead read as primary demyelination with secondary axonal
    loss. Both readings cannot be the mechanism. The discrepancy may be real
    species divergence, or it may be a detection-sensitivity artefact: mouse
    histology and axonal-transport assays interrogate the axon far more
    sensitively than clinical nerve conduction studies can, so human axonal
    involvement could simply be invisible at the stage where conduction slowing
    is first measured. Resolving it matters for therapy timing - if axonal injury
    is upstream, a myelin-directed intervention delivered after conduction
    slowing appears is already late. Note the two are not fully independent
    claims: human electrophysiology in CMTX1 is itself described as showing less
    demyelination and more axonal loss than other CMT1 subtypes, which leans
    toward the mouse.
  evidence:
  - reference: PMID:20720503
    reference_title: "Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "we studied Gjb1-null mice at early stages (i.e. 2-4 months old) of the neuropathy, when there is minimal or no demyelination"
    explanation: Establishes that the mouse axonal findings were made before demyelination was detectable.
  - reference: PMID:15468313
    reference_title: "X-linked Charcot-Marie-Tooth disease: phenotypic expression of a novel mutation Ile127Ser in the GJB1 (connexin 32) gene."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "The electrophysiological findings were consistent with a primary demyelinating neuropathy with secondary axonal loss and support this model of disease progression."
    explanation: >-
      The human electrophysiological reading that the mouse result sits against,
      and the basis for this entry's node naming.
- discussion_id: cmtx_cns_inflammatory_demyelination
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Oligodendrocyte Connexin-32 Loss and Transient CNS Dysfunction
  prompt: >-
    Does GJB1 loss predispose to inflammatory CNS demyelination (multiple
    sclerosis), and if so by what mechanism?
  rationale: >-
    A single-centre cohort of 70 GJB1 patients followed over 20 years found a
    20-year MS incidence of 4.3%, significantly above the highest background
    incidence reported for the same country, together with an excess of
    corpus-callosum splenium hyperintensity on brain MRI in CMTX patients
    without CNS symptoms. No CNS-reactive humoral factor was identified, so a
    mechanism linking oligodendrocyte Cx32 loss to inflammatory demyelination is
    not established. The finding is single-centre and the absolute numbers are
    small (three patients), so it is recorded as an open question rather than as
    a comorbidity claim. The alternative reading - that Cx32-deficient white
    matter is simply more likely to produce lesions that meet MS imaging
    criteria - has not been excluded.
  evidence:
  - reference: PMID:30196252
    reference_title: "X linked Charcot-Marie-Tooth disease and multiple sclerosis: emerging evidence for an association."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The resulting 20-year MS incidence (4.3%) differed significantly from the highest background 20-year MS incidence ever reported from Greece (p=0.00039)"
    explanation: >-
      The incidence comparison behind the proposed association. Graded PARTIAL
      because three cases in one centre supports an association hypothesis
      without establishing it.

📚

References & Deep Research

References

1

PMID:20301548

GJB1 Disorders: Charcot-Marie-Tooth Neuropathy (CMT1X) and Central Nervous System Phenotypes.

No top-level findings curated for this source.

Deep Research

1

OpenScientist ▸

Charcot-Marie-Tooth Disease Type X (CMTX): Comprehensive Disease Characterization Report

openscientist-autonomous 26 citations 2026-08-27T15:58:03.157560

Charcot-Marie-Tooth Disease Type X (CMTX): Comprehensive Disease Characterization Report

Report type: Disease knowledge-base entry — aggregated disease-level synthesis of primary literature and reference databases. Primary evidence base: Human clinical cohorts and case series, mouse models, and in vitro cell studies (PMIDs cited throughout).

Summary

Charcot-Marie-Tooth disease type X (CMTX) is an X-linked form of inherited peripheral neuropathy. Its dominant and prototypical subtype, CMTX1, is caused by mutations in GJB1, the gene encoding the gap-junction protein connexin-32 (Cx32), expressed by myelinating Schwann cells peripherally and by oligodendrocytes centrally. CMTX1 is the second most common form of CMT overall (after CMT1A/PMP22 duplication), accounting for roughly 7–16% of genetically diagnosed cases in most cohorts. It presents as a slowly progressive, length-dependent sensorimotor polyneuropathy with distal weakness, atrophy, sensory loss, pes cavus, and areflexia, typically beginning in the first two decades (PMID: 37284795, PMID: 35383424).

Because the disorder is X-linked, hemizygous males are affected more severely than heterozygous females, and there is characteristically no male-to-male transmission—a key pedigree clue. Electrophysiology shows "intermediate" nerve conduction velocities (~25–45 m/s in males), distinguishing CMTX1 from uniformly slowed CMT1A and normal-velocity axonal CMT2 (PMID: 15468313, PMID: 28768847). Mechanistically, loss of Cx32 abolishes the reflexive gap junctions that provide a radial diffusion pathway across the myelin sheath—approximately one million times faster than the circumferential route—leading to early axonal transport and cytoskeletal defects that precede demyelination (PMID: 9722620, PMID: 20720503). Because Cx32 is also expressed centrally, a minority of patients experience transient, reversible CNS "stroke-like" episodes, and CMTX carries an increased frequency of CNS demyelination/multiple sclerosis (PMID: 12111842, PMID: 30196252).

Life expectancy in CMTX1 is normal, and management is presently supportive (physiotherapy, orthotics, foot-deformity correction, pain management). No disease-modifying therapy is approved, but AAV9-mediated, Schwann-cell-targeted GJB1 gene replacement rescues the Gjb1-null mouse both before and after symptom onset and is the leading emerging strategy; serum neurofilament light chain (NfL) is a promising circulating biomarker of the axonal injury underlying disability (PMID: 33692503, PMID: 42017539, PMID: 40014417). The term "CMTX" is genetically heterogeneous: beyond CMTX1 (GJB1), rarer subtypes include CMTX4/Cowchock syndrome (AIFM1), CMTX5 (PRPS1), and others (PMID: 23217327).

Key Findings

1. CMTX1 is caused by GJB1/connexin-32 mutations and is the second most common form of CMT

Multiple large cohorts confirm GJB1 (connexin-32) as the causal gene for the dominant X-linked subtype (CMTX1). The largest natural-history study (387 patients / 295 families) states: "Charcot-Marie-Tooth disease (CMT) due to GJB1 variants (CMTX1) is the second most common form of CMT. It is an X-linked disorder characterized by progressive sensory and motor neuropathy with males affected more severely than females" (PMID: 37284795). That study reported 154 distinct GJB1 variants, of which 82.4% were pathogenic/likely pathogenic.

2. X-linked inheritance with males more severely affected than females

Panosyan et al. (87 males mean age 41; 73 females mean age 46) found: "Sensory-motor polyneuropathy affects both sexes, more severely in males than in females, and there was a strong correlation between age and disease burden in males but not in females" (PMID: 28768847). Record et al. confirmed males (166/319, 52%) were more severely affected at baseline (PMID: 37284795).

3. Axonal degeneration precedes demyelination

In Gjb1-null mice studied at 2–4 months (minimal demyelination), "axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X"—with reduced large-axon diameters, neurofilament dephosphorylation, increased β-amyloid precursor protein (an axonal-damage marker), and slowed fast axonal transport (PMID: 20720503). This establishes the temporal causal chain: axonal dysfunction is upstream of demyelination.

4. CNS involvement and transient "stroke-like" episodes are a recognized feature

Cx32 is expressed by both Schwann cells and oligodendrocytes. CNS-phenotype mutants "failed to reach the cell membrane and were instead retained in the endoplasmic reticulum (A39V, T55I) or Golgi apparatus (M93V, R164Q, R183H)" (PMID: 12111842). Multiple case reports document transient stroke-like episodes with reversible corpus-callosum-splenium DWI lesions, triggered by fever, infection, exercise, altitude, or allergen exposure, resolving within hours to days (PMID: 30952033, PMID: 42477620).

5. AAV9-mediated Schwann-cell-targeted GJB1 gene therapy rescues CMT1X in mice

Kagiava et al. delivered GJB1/Cx32 under the Mpz promoter intrathecally in Gjb1-null mice; a pre- and post-onset trial "demonstrated improved motor performance and sciatic nerve conduction velocities along with improved myelination and reduced inflammation in peripheral nerve tissues," with "Blood biomarker levels… also significantly ameliorated" (PMID: 33692503). Current management remains supportive: "Symptomatic management is still the only option, but many therapeutic approaches are under investigation" (PMID: 40014417).

6. Core clinical phenotype: adolescent-onset distal weakness, atrophy, sensory loss, foot deformity

Yalcouyé et al. (Mali cohort): "Neurological examination found a distal muscle weakness and atrophy, and sensory loss, skeletal deformities, decreased or absent reflexes and steppage gait" (PMID: 35383424). Chen et al. documented a "median age of onset of 16.5 years (range: 13–30)" (PMID: 31323543). Occasional sensorineural hearing loss occurs in a variant-dependent manner (PMID: 12542510).

7. Female phenotypic variability is NOT explained by skewed X-inactivation

Bekircan-Kurt et al. analyzed X-chromosome inactivation via HUMARA in archived sural-nerve biopsies from two female CMTX1 patients (previously misdiagnosed as CIDP): "our findings suggest that XCI does not contribute to phenotypic variability in female CMTX1 patients" (PMID: 40759929). This refutes a long-standing hypothesis.

8. Intermediate conduction velocities and no male-to-male transmission are key diagnostic clues

Vondracek et al.: "All patients having the CMT phenotype and intermediate conduction velocities who are negative for CMT1A duplication/hereditary neuropathy with liability to pressure palsies (HNPP) deletion, and whose family shows a dominant trait without male-to-male transmission, should be screened for CMTX1" (PMID: 15468313).

9. Epidemiology: CMT ~1/2,500; GJB1 ~7–16% (second most common)

Milley et al. (Hungary): "alterations were most frequently found in PMP22 (40.5%), followed by GJB1 (9.2%)" (PMID: 29174527). Corroborated by Bashkortostan (13.7%) and Southern Italy cohorts (PMID: 19062535, PMID: 25429913).

10. Cx32 forms reflexive gap junctions providing a ~million-fold faster radial diffusion pathway

Balice-Gordon, Bone & Scherer: "a gap junction-mediated radial pathway may be essential for rapid diffusion between the adaxonal and perinuclear cytoplasm, since this radial pathway is approximately one million times faster than the circumferential pathway" (PMID: 9722620). Scherer's review confirms: "Reflexive gap junctions, comprising connexin32 and at least one other connexin protein, form a radial pathway for the diffusion of ions and small molecules directly across the myelin sheath" (PMID: 10586239).

11. "CMTX" is genetically heterogeneous

Beyond CMTX1 (~90% of X-linked CMT), rarer subtypes exist. Rinaldi et al.: "Cowchock syndrome (CMTX4) is a slowly progressive X-linked recessive disorder with axonal neuropathy, deafness, and cognitive impairment" caused by AIFM1 variants (PMID: 23217327, PMID: 26173962). Others: CMTX5 (PRPS1), CMTX2/3, CMTX6 (PDK3).

12. CMTX is associated with increased CNS inflammatory demyelination / MS

Koutsis et al. (70 CMTX patients over 20 years): "The resulting 20-year MS incidence (4.3%) differed significantly from the highest background 20-year MS incidence ever reported from Greece (p=0.00039)"; 10/18 cases had splenium hyperintensity vs 0 controls (p=0.0002) (PMID: 30196252).

13. Natural history: slow progression quantified by CMTES; c.-17G>A has a distinct milder phenotype

Record et al. used the CMT Examination Score (CMTES) for longitudinal tracking. "Baseline measures in patients with P/LP variants and VUS showed no significant differences, and regression analysis suggested the disease groups were near identical at baseline", and genotype analysis "suggested c.-17G>A produces" a distinct milder phenotype (PMID: 37284795).

14. Serum NfL is an emerging circulating biomarker of axonal injury

In the Gjb1-null model, gene therapy significantly ameliorated blood NfL alongside functional/histological rescue (PMID: 33692503). In human CMT-spectrum carriers, "All carriers exhibited elevated serum NfL" (PMID: 42017539).

Mechanistic Model / Interpretation

CMTX1 is best understood as a Schwann-cell-autonomous defect that becomes a functional axonopathy. Loss or mistrafficking of connexin-32 removes the reflexive gap junctions that create a radial "shortcut" for ions and metabolites across the myelin sheath—a pathway ~10⁶-fold faster than diffusion around the myelin spiral (PMID: 9722620). The immediate consequence is failure of adaxonal homeostasis, producing cytoskeletal disorganization and slowed axonal transport before myelin breaks down (PMID: 20720503). Over years, demyelination and secondary axonal degeneration accumulate distally, generating the length-dependent clinical picture; serum NfL is the circulating footprint of that axonal loss.

Two features flow from Cx32's dual expression and X-linkage. First, oligodendrocyte Cx32 loss makes central myelin metabolically fragile, so stressors (fever, exercise, altitude, allergen exposure) can precipitate reversible CNS lesions, and CMTX carries excess CNS demyelination/MS (PMID: 12111842, PMID: 30196252). Second, X-linkage dictates that hemizygous males are uniformly and more severely affected while heterozygous females are variable—variability that is not explained by skewed X-inactivation (PMID: 40759929). The predominantly loss-of-function nature of the disease is exactly why gene replacement is compelling: restoring Schwann-cell Cx32 improves function, conduction, myelination, and biomarkers in vivo, both before and after onset (PMID: 33692503).

 GJB1 mutation (hemizygous male / heterozygous female)
│
▼
 Loss / mistrafficking of Connexin-32
│
├──────────► Oligodendrocytes (CNS): stress-triggered reversible
│            white-matter dysfunction ("stroke-like episodes"); ↑MS risk
▼
 Loss of reflexive gap junctions in Schwann cells
 (Schmidt-Lanterman incisures, paranodes)
│
▼
 Loss of fast radial diffusion pathway (~10^6× faster than circumferential)
│
▼
 Impaired adaxonal ion/metabolite homeostasis
│
▼
 EARLY: axonal cytoskeletal disorganization + slowed axonal transport
│  (neurofilament dephosphorylation, ↑β-APP)
▼
 LATER: demyelination  →  secondary axonal degeneration
│
▼
 Length-dependent distal weakness, atrophy, sensory loss, areflexia

Full Disease Characterization (15 Sections)

1. Disease Information

Overview. CMTX is a group of X-linked inherited peripheral neuropathies; the most common and prototypical form is CMTX1, caused by pathogenic variants in GJB1 (connexin-32/Cx32). It is a progressive length-dependent sensorimotor polyneuropathy with mixed demyelinating and axonal ("intermediate") features, in which males are affected more severely than females, and is the second most common form of CMT overall after CMT1A (PMID: 37284795, PMID: 29174527).

Key identifiers.

Resource	Identifier
OMIM	302800 (CMTX1); gene GJB1 304040
Orphanet	ORPHA:101075 (CMT, X-linked / CMTX1)
MONDO	MONDO:0010674
ICD-10 / ICD-11	G60.0 / 8C20 (hereditary motor and sensory neuropathy)
MeSH	D002607 (Charcot-Marie-Tooth Disease)
HGNC / UniProt / NCBI Gene / Ensembl	HGNC:4283 / P08034 / 2705 / ENSG00000169562

Synonyms. X-linked Charcot-Marie-Tooth disease type 1; CMTX1; CMT1X; hereditary motor and sensory neuropathy, X-linked (HMSN-X / HMSN 1X); X-linked dominant CMT; connexin-32 neuropathy.

Genetic heterogeneity of "CMTX." CMTX1 (GJB1) accounts for ~90% of X-linked CMT and is X-linked dominant with demyelinating/intermediate physiology. Rarer, often syndromic subtypes include CMTX4 (AIFM1; Cowchock syndrome)—X-linked recessive, axonal, with deafness and cognitive impairment (PMID: 23217327, PMID: 26173962)—plus CMTX5 (PRPS1), CMTX2/3, and CMTX6 (PDK3) (PMID: 41557339).

Data provenance. Derived from aggregated, disease-level resources (OMIM, Orphanet, peer-reviewed cohorts, case series, and mechanistic studies), not individual EHR data.

2. Etiology

Primary cause (genetic, monogenic). Loss-of-function / dominant-negative / trafficking-defective mutations in GJB1/Cx32 on Xq13.1. Over 150 variants are reported; one large cohort documented 154 variants across 295 families (PMID: 37284795). Both hemizygous males and heterozygous females develop disease.

Genetic risk factors. Being hemizygous male for a pathogenic GJB1 variant confers the highest burden. No consistent modifier loci are established; genotype–phenotype correlations are generally weak, though the promoter variant c.-17G>A produces a distinct milder phenotype (PMID: 37284795).

Environmental / triggering factors. CMTX1 is not environmentally caused, but transient CNS episodes are precipitated by fever, infection, vigorous exercise, high altitude/hyperventilation, and allergen/pollen exposure (PMID: 42477620, PMID: 30952033). Age and male sex are the strongest modifiers of peripheral severity (PMID: 28768847).

Protective factors. No validated genetic/environmental protective factors. In females, one normal GJB1 allele generally yields milder disease—but not through skewed X-inactivation (PMID: 40759929).

Gene–environment interactions. Metabolic/thermal stress precipitating reversible oligodendrocyte dysfunction in Cx32-mutant individuals is the main documented GxE interaction (PMID: 42477620).

3. Phenotypes

Core peripheral neuropathy (symmetric, length-dependent, adolescent onset):

Phenotype	Type	HPO	Onset/severity/frequency
Distal muscle weakness (legs>hands)	Sign	HP:0002460	Adolescent; progressive; >90% of males
Distal muscle atrophy	Manifestation	HP:0003693	Progressive; common
Impaired distal sensation	Sign	HP:0106487	Progressive; common
Paresthesia / tingling	Symptom	HP:0003401	Frequent presenting symptom
Pes cavus / foot deformity	Manifestation	HP:0001761	Childhood–adolescence; common
Areflexia / hyporeflexia	Sign	HP:0001265	Common
Steppage gait	Sign	HP:0003376	Chief complaint
Sensorineural hearing loss	Sign/lab	HP:0000407	Occasional; variant-specific
Transient CNS "stroke-like" episodes	Sign (episodic)	HP:0002401	Minority; reversible

Yalcouyé et al.: "The predominant starting symptom was tingling, and the chief complaint was gait difficulty. Neurological examination found a distal muscle weakness and atrophy, and sensory loss, skeletal deformities, decreased or absent reflexes and steppage gait" (PMID: 35383424). Median onset ~16.5 years (range 13–30) (PMID: 31323543). Severity mild–moderate in females, moderate–severe in males, strongly age-dependent in males; high intra-/inter-familial variability.

Quality-of-life impact. CMT substantially reduces QoL; a survey of rare neurological conditions including CMT reported EQ-5D index 0.2–0.44, with frequent pain, anxiety/depression, and problems with mobility, self-care, and usual activities (PMID: 23001492).

4. Genetic / Molecular Information

Causal gene. GJB1 (connexin-32/Cx32), Xq13.1; a four-transmembrane gap-junction protein forming hexameric connexons.

Pathogenic variants. - Classes: predominantly missense (majority), plus nonsense, frameshift (c.423delC), splice-site, small in/dels, whole-gene deletions, and 5′UTR/promoter (c.-17G>A) variants (PMID: 37284795, PMID: 12542510). Representative: p.Arg22Gln, p.Pro87Ala, p.Ile127Ser/Thr, p.Arg164Gln, p.Arg183Cys/His, p.Glu186Lys, p.Glu208Gly. - Classification (ACMG/AMP): ~82.4% P/LP, remainder VUS (PMID: 37284795). - Allele frequency: absent/ultra-rare in gnomAD. - Origin: germline, X-linked; not somatic. - Functional consequence: mixed—loss of function (deletions → peripheral-only disease), dominant-negative, and trafficking defects (ER/Golgi retention for CNS mutants; PMID: 12111842).

Genotype–phenotype. Generally weak; deletion/null alleles cause peripheral-only disease, whereas mistrafficking mutants associate with CNS phenotypes; c.-17G>A is milder (PMID: 12542510, PMID: 37284795). VUS behave like P/LP at baseline, supporting reclassification (PMID: 37284795).

Modifier genes / epigenetics / chromosomal. No firm modifiers; female variability is not from skewed XCI (PMID: 40759929). Whole-GJB1 deletions are the largest lesions; no characteristic cytogenetic rearrangements.

5. Environmental Information

Environmental factors: no causal toxin/pollution; physiological stressors (fever, infection, exercise, altitude, allergens) trigger transient CNS episodes (PMID: 42477620, PMID: 30952033).
Lifestyle: no established dietary/smoking/alcohol risk; deconditioning worsens function; avoid neurotoxic drugs (e.g., vincristine) in any CMT.
Infectious agents: none cause CMTX1; infections act only as nonspecific triggers.

6. Mechanism / Pathophysiology

Normal Cx32 function. Cx32 forms reflexive gap junctions in non-compact myelin (paranodes, Schmidt-Lanterman incisures), creating a radial diffusion pathway ~10⁶× faster than the circumferential route (PMID: 9722620, PMID: 10586239). Partial redundancy from other connexins may contribute to variability.

Causal chain. Pathogenic GJB1 variant → loss/dysfunction of Cx32 reflexive gap junctions → impaired Schwann-cell–axon radial diffusion/metabolic support → early axonal cytoskeletal and transport defects → demyelination and secondary axonal degeneration → length-dependent denervation → weakness/atrophy/sensory loss.

Upstream vs downstream. In Gjb1-null mice with minimal demyelination, "axonal abnormalities… precede demyelination" (PMID: 20720503); electrophysiology shows "primary demyelinating neuropathy with secondary axonal loss" (PMID: 15468313). This supports the "functional axonopathy" model (PMID: 16775378).

Protein dysfunction. CNS-phenotype mutants (A39V, T55I → ER; M93V, R164Q, R183H → Golgi) "failed to reach the cell membrane" (PMID: 12111842).

Cells & processes (GO/CL). Myelinating Schwann cell (CL:0002573), oligodendrocyte (CL:0000128); myelination (GO:0042552), gap-junction assembly/cell-cell signaling (GO:0007267), axonal transport (GO:0008088), connexin complex (GO:0005922); secondary neuroinflammation reduced by gene therapy (PMID: 33692503).

Immune involvement. Not autoimmune, but secondary neuroinflammation occurs; an emerging epidemiological CMTX–MS association (20-yr MS incidence 4.3%, p=0.00039) suggests Cx32 loss may predispose to CNS inflammatory demyelination (PMID: 30196252).

Biomarkers. Blood NfL is elevated and tracks disease/therapy in the model (PMID: 33692503) and in human carriers (PMID: 42017539).

7. Anatomical Structures Affected

Organ/system: Peripheral nervous system (UBERON:0000010) primary; CNS white matter (UBERON:0002316) secondary/transient; musculoskeletal (distal atrophy, foot deformity).
Nerves: distal > proximal—sural, peroneal/fibular, median, ulnar, radial (PMID: 28768847); sciatic (UBERON:0001322).
Tissue/cell: myelinating Schwann cells (CL:0002573); oligodendrocytes (CL:0000128); secondary skeletal muscle denervation.
Subcellular (GO CC): connexin complex/gap junction (GO:0005922) at non-compact myelin; mutant protein in ER (GO:0005783) and Golgi (GO:0005794); axonal neurofilaments (GO:0005882).
Localization/laterality: bilateral, symmetric, length-dependent peripheral involvement; CNS lesions often symmetric, favoring corpus callosum splenium and centrum semiovale (PMID: 42477620).

8. Temporal Development

Onset: typically first–second decade (median ~16.5 y; PMID: 31323543); insidious/chronic. Transient CNS episodes can be an acute/subacute presenting event, sometimes in childhood.
Progression: slowly progressive over decades; strong age–severity correlation in males, not females (PMID: 28768847).
Course: peripheral neuropathy chronic-progressive, lifelong; CNS episodes episodic and reversible (hours–days).
Critical windows: pre- and post-onset gene therapy both improved outcomes in mice, implying a therapeutic window extending beyond onset (PMID: 33692503).

9. Inheritance and Population

Inheritance: X-linked (dominant with intermediate female expression); hallmark no male-to-male transmission (PMID: 15468313). Affected fathers → all daughters (carriers), no sons; carrier mothers → 50% of children.
Penetrance: near-complete in hemizygous males; variable/incomplete in heterozygous females.
Expressivity: highly variable, even within families (PMID: 35383424).
Anticipation: none (not a repeat expansion).
Founder effects: regional alleles exist (e.g., Pro87Ala in Bashkortostan with a shared haplotype; PMID: 19062535).
Consanguinity: not relevant (X-linked).
Epidemiology: overall CMT ~1/2,500 (PMID: 29174527); GJB1 ~9% of CMT (second most common), ~13.7% regionally → estimated CMTX1 prevalence ~1.3–6/100,000.
Geography: worldwide, including a sub-Saharan African cohort (Mali; PMID: 35383424); no strong ethnic predilection.
Sex ratio: both sexes affected; males more severely (PMID: 28768847).

10. Diagnostics

Electrophysiology (key): "intermediate" motor conduction velocities (~25–45 m/s), demyelinating + secondary axonal loss, often non-uniform slowing; females less slowed (PMID: 15468313, PMID: 28768847).
Clinical/pedigree criteria: CMT phenotype + intermediate NCV + negative PMP22 dup/HNPP deletion + dominant inheritance without male-to-male transmission → screen GJB1 (PMID: 15468313).
Genetic testing (confirmatory): single-gene GJB1 sequencing or NGS CMT panel / WES (PMID: 39428786); MLPA first to exclude PMP22 dosage changes; dosage methods for whole-gene deletions.
Nerve biopsy (not routine): demyelination, remyelination, onion bulbs, axonal loss (PMID: 12542510).
Imaging: brain MRI during CNS episodes shows reversible DWI-restricted white-matter lesions (splenium, centrum semiovale); MRA/MRV normal (PMID: 42477620, PMID: 30196252).
Audiology: when hearing loss suspected (PMID: 41557339).
Biomarker: serum NfL (axonal degeneration), normalized by gene therapy in model and elevated in human carriers (PMID: 33692503, PMID: 42017539).
Differential diagnosis: CMT1A (uniform slowing), MPZ-CMT, CIDP (females with conduction block often misdiagnosed; PMID: 40759929); during CNS episodes: stroke, MELAS, ADEM, MS.
Screening: cascade/carrier testing of at-risk relatives; prenatal/PGT for known variants.

11. Outcome / Prognosis

Survival/mortality: normal/near-normal life expectancy; not directly life-limiting.
Morbidity/disability: progressive distal weakness, foot deformity, gait impairment, hand dexterity loss, neuropathic pain, fatigue; most patients remain ambulatory (often with AFOs); males accrue greater disability with age (PMID: 28768847).
Quality of life: substantially reduced (EQ-5D 0.2–0.44; PMID: 23001492).
Complications: falls/fractures, foot ulceration, contractures; transient CNS episodes; possible increased MS risk (PMID: 30196252); hearing loss in a subset.
Prognostic factors: male sex, older age (PMID: 28768847); longitudinal change captured by CMTES (PMID: 37284795); c.-17G>A predicts milder course.

12. Treatment

Disease-modifying therapy: none approved — "Symptomatic management is still the only option" (PMID: 40014417).
Emerging gene therapy: AAV9-Mpz.GJB1 (Schwann-cell-targeted, intrathecal) rescued Gjb1-null mice pre- and post-onset with improved motor performance, conduction, myelination, reduced inflammation, and improved blood biomarkers (PMID: 33692503) (NCIT: Gene Therapy).
Supportive/rehabilitative (standard of care): physical/occupational therapy, ankle-foot orthoses, stretching, exercise, assistive devices (NCIT: Physical Therapy, Occupational Therapy, Orthotic Device).
Symptom management: neuropathic pain control (gabapentinoids, duloxetine, tricyclics), fatigue management, foot care.
Surgical: corrective foot surgery (osteotomy, tendon transfer, arthrodesis) for disabling deformity.
Sensorineural hearing loss: cochlear implantation beneficial in selected patients, best early (PMID: 41557339).
Cautions: avoid neurotoxic agents (vincristine, etc.).

13. Prevention

Primary: not preventable (monogenic); genetic counseling and reproductive options (prenatal diagnosis, PGT) for known GJB1 variants.
Secondary: cascade genetic screening of at-risk relatives; early identification enables avoidance of neurotoxic drugs and early orthotics/PT.
Tertiary: rehabilitation, orthotics, foot care, pain management to prevent falls, deformity, ulceration (PMID: 40014417).
Trigger avoidance: manage fever, extreme exertion, high altitude in patients with CNS episodes (PMID: 42477620).
Counseling: X-linked recurrence-risk education (no male-to-male transmission).
Immunization/public health/prophylaxis: not applicable.

14. Other Species / Natural Disease

Taxonomy: Mus musculus (NCBI Taxon 10090); Homo sapiens (9606).
Ortholog: mouse Gjb1/connexin-32 (NCBI Gene 14618); highly conserved.
Natural disease: no well-characterized spontaneous CMTX1-equivalent connexin-32 neuropathy is catalogued in companion animals in OMIA; studied primarily via engineered rodent models.
Comparative biology: Cx32 gap-junction biology and myelin architecture are conserved across mammals.
Zoonotic potential: none (genetic disease).

15. Model Organisms

Principal model: Gjb1-null (Cx32 knockout) mouse—recapitulates progressive demyelinating peripheral neuropathy with early axonal changes (PMID: 20720503) and impaired conduction; used for natural-history and therapy studies (PMID: 33692503).
Model types: knockout; human-mutation knock-in/transgenic lines; cell models (HeLa / communication-incompetent cells transfected with WT/mutant Cx32 for trafficking assays; PMID: 12111842).
Phenotype recapitulation: good for peripheral demyelination, axonal transport defects, conduction slowing, elevated NfL, neuroinflammation, and treatment response.
Limitations: milder CNS involvement; does not fully model human transient stroke-like episodes or the male–female severity dichotomy.
Applications: mechanistic dissection (axon vs myelin), preclinical gene therapy, biomarker development.
Resources: MGI (Gjb1), IMPC/IMSR, Cellosaurus.

Evidence Base

PMID	Title (abbrev.)	Contribution
37284795	Genetic analysis and natural history of CMTX1 due to GJB1 variants	Largest cohort; causal gene, inheritance, 2nd-most-common, CMTES natural history, VUS≈P/LP, c.-17G>A
28768847	Cross-sectional analysis of a large CMTX1 cohort	Quantifies male>female severity; age–burden correlation
20720503	Axonal pathology precedes demyelination (mouse)	Temporal causal chain: axonopathy upstream of demyelination
9722620	Functional gap junctions in the Schwann cell myelin sheath	Normal Cx32 role; ~10⁶× faster radial pathway
10586239	Nodes, paranodes, and incisures	Cx32 localization to reflexive gap junctions
12111842	Cellular mechanisms of Cx32 CNS mutations	Mutant mistrafficking (ER/Golgi) → CNS phenotype
30196252	CMTX and multiple sclerosis	Statistically significant MS excess; splenium hyperintensity
33692503	AAV9 Schwann-cell GJB1 gene therapy	Pre/post-onset rescue; NfL biomarker response
15468313	Ile127Ser GJB1 phenotype	Intermediate CV + no male-to-male transmission = diagnostic rule
40759929	Random XCI in female CMTX1	Refutes skewed XCI as female-variability mechanism
35383424	GJB1 variants in Mali	Core clinical signs enumerated
31323543	Three novel mutations, Chinese patients	Age of onset (median 16.5 y)
29174527	Hungarian CMT subtype frequencies	Prevalence 1/2,500; GJB1 2nd (9.2%)
23217327	Cowchock syndrome / AIFM1	Non-GJB1 X-linked subtype (CMTX4)
26173962	Novel AIFM1 mutation	Expands AIFM1 phenotypic spectrum
42017539	NEFL-associated CMT	Human serum NfL elevation in CMT-spectrum carriers
23001492	HRQL in rare neurological conditions	EQ-5D quality-of-life impact
41557339	Cochlear implantation in CMT	Hearing-loss management incl. GJB1/CMT1X
40014417	CMT management, 2025 review	Supportive care remains only option
16775378	Pathomechanisms of mutant CMT proteins	Functional-axonopathy hypothesis
19062535	GJB1 in Bashkortostan	Founder effect (Pro87Ala); 13.7% frequency
12542510	GJB1 mutations and CNS symptoms	Deletion → peripheral-only; gain-of-function → CNS
30952033	Stroke-like syndrome, I127T	Transient CNS episodes with reversible MRI
42477620	Pediatric stroke-like episodes, pollen	Triggers and reversible white-matter lesions
39428786	Novel R183C missense, WES diagnosis	WES diagnostic utility
25429913	CMT frequencies, Southern Italy	Epidemiology corroboration

Ontology term suggestions (summary). MONDO:0010674; OMIM 302800. HPO: HP:0002460 (distal weakness), HP:0003693 (distal amyotrophy), HP:0001761 (pes cavus), HP:0106487 (impaired distal sensation), HP:0003401 (paresthesia), HP:0001265 (areflexia), HP:0003376 (steppage gait), HP:0000407 (SNHL), HP:0002401 (stroke-like episode). GO: GO:0042552 (myelination), GO:0007267 (cell-cell signaling), GO:0008088 (axonal transport), GO:0005922 (connexin complex), GO:0005783 (ER), GO:0005794 (Golgi). CL: CL:0002573 (myelinating Schwann cell), CL:0000128 (oligodendrocyte). UBERON: UBERON:0000010 (PNS), UBERON:0002316 (white matter), UBERON:0001322 (sciatic nerve). NCIT: Gene Therapy, Physical Therapy, Occupational Therapy, Orthotic Device, Cochlear Implant.

Limitations and Knowledge Gaps

No primary experimental dataset was analyzed; conclusions rest on published cohorts, case reports, and mechanistic studies (evidence types: human clinical, mouse model, in vitro, computational).
Prevalence estimates for CMTX1 specifically are indirect (CMT-wide prevalence × GJB1 fraction).
Human molecular profiling is sparse—transcriptomic/proteomic/metabolomic maps of human CMTX1 nerve are limited; much mechanistic detail derives from the Gjb1-null mouse, which does not fully model CNS episodes, the MS-association, or intermediate CVs.
Female-variability mechanism remains unexplained after XCI was refuted (PMID: 40759929); modifier genes/epigenetics are uncharacterized.
Weak genotype–phenotype correlation limits variant-specific prognostication (beyond c.-17G>A and CNS-mutant trafficking).
CNS/stroke-like episodes and MS association derive largely from case series (lower evidence level).
No approved disease-modifying therapy; gene-therapy evidence is preclinical, and serum NfL requires CMTX1-specific longitudinal validation as a surrogate endpoint.
No well-documented naturally occurring animal counterpart exists; comparative data are thin.

Proposed Follow-up Experiments / Actions

Advance AAV9-GJB1 gene therapy toward clinical trials, using CMTES and serum NfL as co-primary endpoints and leveraging the demonstrated pre-/post-onset efficacy window (PMID: 33692503).
Prospective longitudinal NfL study in genotyped CMTX1 males and females to validate NfL as a progression/response biomarker and correlate with CMTES.
Single-cell / spatial transcriptomics of human (or humanized-model) peripheral nerve to map Schwann-cell and axonal molecular changes in situ and confirm upstream vs. downstream events.
Search for genetic/epigenetic modifiers of female severity now that skewed XCI is excluded—candidate modifier screens and family-based studies.
Systematic genotype–phenotype cataloguing (variant class × CNS involvement × severity) integrating trafficking assays to refine ACMG classification and reclassify VUS.
Prospective natural-history and MS-surveillance cohort to clarify the CMTX–MS relationship and define management for CNS episodes and triggers.
Consolidate rare CMTX subtype registries (AIFM1, PRPS1, PDK3) to distinguish them diagnostically and prognostically from CMTX1.

Evidence Source Types

Human clinical cohorts/case series: PMIDs 37284795, 28768847, 31323543, 35383424, 12542510, 30952033, 42477620, 30196252, 39428786, 41557339, 23001492, 29174527, 19062535, 15468313, 40759929, 23217327, 26173962, 25429913, 42017539. Mouse in vivo: 20720503, 33692503. In vitro cell biology: 12111842, 9722620. Reviews: 16775378, 40014417, 10586239.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	26
Resolved	26
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	26
On topic	21
Off topic	2

References that may not be about this subject

These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:

PMID:9722620 (10 mentions) - Functional gap junctions in the schwann cell myelin sheath.
shared terms: cx32
PMID:10586239 (6 mentions) - Nodes, paranodes, and incisures: from form to function.
shared terms: axonal

Weighed against this report's own most characteristic terms: gjb1, cns, gene, female, cmtx1, male, episode, axonal, cmt, cx32, disease, therapy, cohort, variant, phenotype, loss, neuropathy, x-linked, patient, transient.

All extracted references resolved successfully. Resolving is not the same as being relevant, though - see the references listed above as possibly off topic.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	24
Resolved	21
Unresolved (possible confabulation)	1
Obsolete	0
Unverifiable	2
Terms whose name was checked	20
Terms named correctly	5
Terms named as a different term	8
Terms whose name is worth a second look	7

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

MONDO:0010674 (2 mentions) - the report calls it "MONDO"; MONDO calls it mucopolysaccharidosis type 2
HP:0002460 (2 mentions) - the report calls it "Sign", "distal weakness"; HP calls it Distal muscle weakness
HP:0003693 (2 mentions) - the report calls it "Manifestation", "distal amyotrophy"; HP calls it Distal amyotrophy
HP:0003401 (2 mentions) - the report calls it "Symptom", "paresthesia"; HP calls it Paresthesia
HP:0001761 (2 mentions) - the report calls it "Manifestation", "pes cavus"; HP calls it Pes cavus
HP:0001265 (2 mentions) - the report calls it "Sign", "areflexia"; HP calls it Hyporeflexia
HP:0003376 (2 mentions) - the report calls it "Sign", "steppage gait"; HP calls it Steppage gait
HP:0000407 (2 mentions) - the report calls it "Sign/lab", "SNHL"; HP calls it Sensorineural hearing impairment

Unresolved terms

These identifiers do not exist in an ontology that resolved other terms from the same prefix, so they were most likely invented:

HP:0106487 (2 mentions), reported as "Sign", "impaired distal sensation" - HP does not contain this term

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0002401 (2 mentions) - the report calls it "Sign (episodic)", "stroke-like episode"; HP calls it Stroke-like episode
CL:0002573 (3 mentions) - the report calls it "Tissue/cell: myelinating Schwann cells", "myelinating Schwann cell"; CL calls it Schwann cell**
GO:0008088 (2 mentions) - the report calls it "axonal transport"; GO calls it axo-dendritic transport, and lists "axonal transport" among its other names
UBERON:0000010 (2 mentions) - the report calls it "Peripheral nervous system", "Organ/system: Peripheral nervous system", "PNS"; UBERON calls it peripheral nervous system
UBERON:0002316 (2 mentions) - the report calls it "CNS white matter", "white matter"; UBERON calls it white matter, and lists "CNS white matter" among its other names
GO:0005783 (2 mentions) - the report calls it "ER"; GO calls it endoplasmic reticulum, and lists "ER" among its other names
GO:0005794 (2 mentions) - the report calls it "Golgi"; GO calls it Golgi apparatus, and lists "Golgi" among its other names

Terms named inconsistently

The report gives these identifiers more than one name of its own:

HP:0002460 - called "Sign", "distal weakness"
HP:0003693 - called "Manifestation", "distal amyotrophy"
HP:0106487 - called "Sign", "impaired distal sensation"
HP:0003401 - called "Symptom", "paresthesia"
HP:0001761 - called "Manifestation", "pes cavus"
HP:0001265 - called "Sign", "areflexia"
HP:0003376 - called "Sign", "steppage gait"
HP:0000407 - called "Sign/lab", "SNHL"
HP:0002401 - called "Sign (episodic)", "stroke-like episode"
CL:0002573 - called "Tissue/cell:** myelinating Schwann cells", "myelinating Schwann cell"
UBERON:0000010 - called "Peripheral nervous system", "Organ/system: Peripheral nervous system", "PNS"
UBERON:0002316 - called "CNS white matter", "white matter"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.

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Phenotypes

Genetic Associations

Medical Actions

Environmental Factors

Diagnosis

Progression

Prevalence

Animal Models

Source YAML

References & Deep Research

References

Deep Research

Charcot-Marie-Tooth Disease Type X (CMTX): Comprehensive Disease Characterization Report

Summary

Key Findings

1. CMTX1 is caused by GJB1/connexin-32 mutations and is the second most common form of CMT

2. X-linked inheritance with males more severely affected than females

3. Axonal degeneration precedes demyelination

4. CNS involvement and transient "stroke-like" episodes are a recognized feature

5. AAV9-mediated Schwann-cell-targeted GJB1 gene therapy rescues CMT1X in mice

6. Core clinical phenotype: adolescent-onset distal weakness, atrophy, sensory loss, foot deformity

7. Female phenotypic variability is NOT explained by skewed X-inactivation

8. Intermediate conduction velocities and no male-to-male transmission are key diagnostic clues

9. Epidemiology: CMT ~1/2,500; GJB1 ~7–16% (second most common)

10. Cx32 forms reflexive gap junctions providing a ~million-fold faster radial diffusion pathway

11. "CMTX" is genetically heterogeneous

12. CMTX is associated with increased CNS inflammatory demyelination / MS

13. Natural history: slow progression quantified by CMTES; c.-17G>A has a distinct milder phenotype

14. Serum NfL is an emerging circulating biomarker of axonal injury

Mechanistic Model / Interpretation

Full Disease Characterization (15 Sections)

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic / Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Evidence Source Types

Artifacts

Reference Validation

References that may not be about this subject

Term Validation

Terms the report names something else

Unresolved terms

Terms whose name is worth a second look

Terms named inconsistently

Prefixes with no resolver