Charcot-Marie-Tooth Disease Type 4B3 (CMT4B3): Comprehensive Research Report

1. Disease Information

Overview

Charcot-Marie-Tooth disease type 4B3 (CMT4B3) is an ultra-rare, autosomal recessive, childhood-onset demyelinating/axonal sensorimotor peripheral neuropathy caused by biallelic pathogenic variants in SBF1 (SET-binding factor 1, also called MTMR5), located on chromosome 22q13.33. It is the third molecularly defined subtype of the CMT4B group of "myelin-outfolding" neuropathies, joining CMT4B1 (MTMR2) and CMT4B2 (MTMR13/SBF2). The disease was first molecularly characterized in 2013 in a Korean kindred (Nakhro et al., PMID:23749797) and has since been expanded to include a syndromic spectrum with cranial neuropathies, microcephaly, intellectual disability, and distinctive brain-stem MRI findings (the "fork and bracket sign").

CMT4B3 is exceptionally rare: as of the most recent literature synthesis, only a handful of families (originally reported as three, subsequently expanded to at least seven–nine kindreds across Korean, Saudi Arabian, Syrian, Israeli, Spanish, British, and Italian populations) have been described, and a patient advocacy organization (CMT4B3 Research Foundation) reports "only 14 documented cases worldwide" as of its most recent public accounting (cmt4b3research.org).

Key Identifiers

(Sources: OMIM #615284; OMIM *603560; Orphanet; NIH GTR)

Synonyms

• Charcot-Marie-Tooth disease, demyelinating, type 4B3
• CMT4B3
• SBF1-related neuropathy / SBF1-related syndromic neuropathy
• MTMR5-associated CMT4B3
• (Historically catalogued together with) "fork and bracket" syndrome — a specific syndromic MRI-defined SBF1 phenotype

Data provenance

Essentially all published knowledge of CMT4B3 derives from aggregated case reports and small family series (whole-exome/whole-genome sequencing of individual consanguineous or compound-heterozygous kindreds), not from large EHR-derived cohorts or population registries — reflecting a total published patient count in the low tens. This is important context for interpreting any "typical" phenotype claims below: each additional family has materially reshaped the described phenotypic spectrum.

2. Etiology

Disease Causal Factors

CMT4B3 is a monogenic, autosomal recessive disorder caused by biallelic (homozygous or compound heterozygous) loss-of-function or hypomorphic variants in SBF1/MTMR5. There is no known environmental, infectious, or non-genetic cause. One report — from a website aimed at lay audiences — vaguely suggested de novo mutations could be "triggered by environmental factors like UV exposure or viral infections," but this is not supported by primary genetics literature and should not be treated as an established causal mechanism; germline SBF1 variants are inherited from asymptomatic carrier parents in essentially all reported pedigrees.

Genetic Risk Factors

• Causal variants: Missense (e.g., p.Met417Val, p.Thr1590Ala — the original Korean family, PMID:23749797), nonsense/splice-site null variants (e.g., a homozygous splice-site mutation in a consanguineous Bedouin kindred, PMID:30039846), and frameshift/deletion variants (e.g., c.5477-5478del/p.1826_1826del, exon 40, PMC7419361) have all been reported as causal.
• Consanguinity is a major risk factor: most reported families are from populations with high rates of consanguineous marriage (Saudi Arabian, Syrian/Bedouin kindreds), consistent with autosomal recessive inheritance of a rare allele.
• A single reported exception to autosomal recessive inheritance: a 2024 report describes a mother–daughter pair with an apparently dominantly inherited SBF1 missense variant (c.1398C>A, p.H466Q), the authors proposing a dominant-negative mechanism, in contrast to "the seven cases reported before" that were autosomal recessive (PMC11633322). This is a notable and still-unusual finding that expands (and complicates) the inheritance model.
• Modifier/uncertain-significance context: a 2025 pediatric study of persistent toe-walking identified heterozygous SBF1 variants in some children, but concluded these single-allele variants "may act as modifiers or coincidental findings rather than independent causes of neuromotor abnormalities" (PMC12926636) — a caution against over-interpreting monoallelic SBF1 findings as causal.

Environmental Risk Factors

None specifically established. General CMT risk-modifying exposures (peripheral neurotoxic drugs such as vincristine, which can exacerbate any CMT subtype) are plausible by extension from general CMT biology but have not been specifically studied in CMT4B3.

Protective Factors

None reported in the literature; the extreme rarity of the disease precludes population-based identification of protective alleles or environmental factors. gnomAD-based constraint metrics for SBF1 have not yet been directly reported in the primary CMT4B3 literature surveyed, though general gnomAD gene-constraint methodology (observed/expected loss-of-function ratio) would apply.

Gene-Environment Interactions

Not established for CMT4B3 specifically.

3. Phenotypes

Core neuromuscular phenotype (classic/original presentation)

The original Korean family (PMID:23749797) presented with a "homogeneous phenotype of pure sensory motor demyelinating neuropathy with focally folded myelin sheaths" — onset of distal atrophy and weakness of the upper and lower limbs, decreased vibration and position sense, areflexia, and pes planus in the first decade of life, with slow progression to loss of ambulation by the fifth decade.

Common motor/sensory features across the literature: - Distal muscle weakness and atrophy (legs > arms initially) - Steppage gait, foot drop, pes cavus/pes planus - Distal sensory loss (vibration, position sense) - Hyporeflexia progressing to areflexia - Fasciculations and muscle cramps - Gait ataxia in some patients - Scoliosis/kyphoscoliosis, syndactyly in syndromic presentations

Expanded/syndromic phenotype

Subsequent families (Saudi Arabian, Syrian/Bedouin, Israeli, Spanish, British, Italian) revealed a substantially broader spectrum: - Cranial nerve involvement: facial weakness, ophthalmoparesis/strabismus, dilated unreactive pupils, nystagmus, dysphagia, dysarthria — described as "axonal motor predominant neuropathy and cranial nerve involvement" (multiple families, PMIDs 24799518, 28005197, 32444983, 20658556, 30039846) - Microcephaly (congenital or progressive) — a recurring, distinctive feature not typical of other CMT4B subtypes - Intellectual disability / developmental delay - Cerebellar and pyramidal signs, evident in infancy in the most severe (null-mutation) family, "with peripheral polyneuropathy emerging only toward the end of the first decade" (PMID:30039846) - Skeletal anomalies: syndactyly, short stature, kyphoscoliosis, congenital talon-valgus-pronated clubfoot - "Fork and bracket" syndrome: a distinct MRI-defined entity — T2-hyperintense signal in the pons ("fork sign") and mesencephalon ("bracket sign"), attributed to degenerated oculomotor and facial nerve fiber bundles, first described by Mégarbané et al. 2010 (PMID:20658556) and specifically linked to SBF1 by later exome studies - Mitochondrial dysfunction: a 2021 Italian case (PMID:34118926) described infantile-onset severe motor polyneuropathy with respiratory failure requiring non-invasive ventilation by age 6 and wheelchair dependence by age 11, without cognitive impairment or brain MRI abnormalities — demonstrating marked phenotypic heterogeneity even within the "classic" neuropathy-only presentation. Muscle biopsy showed reduced respiratory chain complex I (65% of normal), II+III (55%), and IV (66%) activities and ~30% reduced mtDNA content. - Necklace fibers on muscle biopsy — a pathological feature classically associated with myotubular myopathy — were reported for the first time in SBF1-related disease in a family with a novel frameshift deletion, alongside marked neurogenic atrophy and axonal (non-demyelinating) sensorimotor neuropathy (PMC7419361), further broadening the described pathological spectrum beyond "pure demyelinating."

Onset, Severity, Progression

• Age of onset: Highly variable — ranges from congenital/infantile (clubfoot at birth, hypotonia by 18 months in the mitochondrial-dysfunction case) to classic first-decade onset (5–11 years, as in most series) to, in the null-variant Bedouin family, an infantile cerebellar/pyramidal presentation with polyneuropathy emerging only in later childhood.
• Severity: Variable, ranging from ambulatory adult patients with slowly progressive distal neuropathy (loss of ambulation only in the 5th decade in the original Korean family) to severe infantile-onset cases requiring ventilatory support and wheelchair use by the first decade.
• Progression: Generally slowly progressive, consistent with other demyelinating CMT4B subtypes, though the syndromic/cranial-nerve-involving forms and the mitochondrial-dysfunction phenotype show more rapid, severe courses.
• Frequency: Given the rarity (roughly a dozen-plus published families), phenotype frequency percentages (e.g., "X% have microcephaly") are not statistically reliable; qualitative descriptions ("commonly," "in some patients") are the best available granularity.

Quality of Life

No disease-specific EQ-5D/SF-36 data exist for CMT4B3. By extension from general CMT literature, progressive distal weakness, gait impairment, and (in syndromic cases) cranial nerve deficits (dysphagia, dysarthria, ophthalmoplegia) and intellectual disability would be expected to substantially impact mobility, communication, feeding, and independence — more so than milder CMT1A-type neuropathies, given several reported cases require respiratory support and wheelchairs.

Suggested HPO terms

• HP:0002355 Difficulty walking
• HP:0003676 Progressive
• HP:0001336 Myoclonus (if applicable) — not core
• HP:0001382 Joint hypermobility (variable)
• HP:0009830 Peripheral neuropathy
• HP:0007256 Progressive peripheral neuropathy
• HP:0002380 Hyporeflexia / HP:0001284 Areflexia
• HP:0003707 Calf muscle hypertrophy — not typical; distal atrophy predominates: HP:0003693 Distal amyotrophy
• HP:0001761 Pes cavus / HP:0001763 Pes planus
• HP:0001941 Talipes (clubfoot)
• HP:0001344 Absent/decreased deep tendon reflexes
• HP:0000010 Recurrent urinary tract infections — not relevant
• HP:0000252 Microcephaly
• HP:0001249 Intellectual disability
• HP:0001256 Intellectual disability, mild (variable severity)
• HP:0000486 Strabismus
• HP:0000601 Nystagmus (relative afferent findings)
• HP:0000508 Ptosis / HP:0000601 nystagmus, HP:0000486 strabismus for ophthalmoparesis
• HP:0009085 Syndactyly
• HP:0002650 Scoliosis / HP:0002751 Kyphoscoliosis
• HP:0001260 Dysarthria
• HP:0002015 Dysphagia
• HP:0007257 Facial palsy (facial nerve weakness)
• HP:0001260 Ataxic gait / HP:0002066 Gait ataxia
• HP:0002322 Resting tremor — not typical
• HP:0003198 Myopathy (necklace fibers case)
• HP:0003560 Muscle fiber necrosis — not primary
• HP:0001397 Hepatomegaly — not relevant

4. Genetic/Molecular Information

Causal Gene

SBF1 (SET-binding factor 1; synonym MTMR5), OMIM *603560, HGNC:10542, chromosome 22q13.33.

Gene/Protein Structure

• 41-exon gene; MANE Select transcript NM_002972.4 encoding NP_002963.2 (~208 kDa protein).
• Protein domains: upstream DENN (uDENN), DENN, downstream DENN (dDENN) domains; an SBF2 (myotubularin-like phosphatase) domain; a GRAM domain (Glucosyltransferases, Rab-like GTPase activators, and Myotubularins); and a Pleckstrin Homology (PH) domain (GeneCards, ResearchGate figure sources).
• Pseudophosphatase: SBF1/MTMR5 lacks several catalytic-pocket residues required for phosphatase activity, rendering it catalytically inactive as a phosphoinositide phosphatase, though the substrate-binding pocket is preserved enough to bind phosphorylated substrates — potentially protecting them from active phosphatases or acting as a scaffold.
• Functionally, SBF1/MTMR5 heterodimerizes (via coiled-coil domains) with the catalytically active phosphatase MTMR2 (the CMT4B1 gene), analogous to how MTMR13/SBF2 (the CMT4B2 gene) also dimerizes with MTMR2. The DENN domain of SBF1 is proposed to activate Rab GTPases (Rab21, and possibly serve as a GEF for Rab28), positioning SBF1 in endo-lysosomal/endosomal trafficking regulation.

Pathogenic Variants Reported

Variant classification follows standard ACMG/AMP criteria via ClinVar (specific ClinVar accessions reported for the Italian case: RCV001449576.1, RCV001449656.1); most reported variants are classified pathogenic or likely pathogenic based on segregation, absence/rarity in population databases (gnomAD), and functional predictions.

Allele Frequency

Given the extreme rarity of the disease, specific pathogenic SBF1 alleles are expected to be essentially absent or present only as ultra-rare heterozygous carriers in gnomAD; no disease-specific population carrier frequency has been established in the literature reviewed. General gnomAD constraint methodology (observed/expected loss-of-function ratio, "oe") would classify constraint for SBF1 as a whole, but a specific oe value was not retrievable in this pass of searches — recommend a targeted gnomAD browser query (gnomad.broadinstitute.org, gene SBF1) during KB curation for an exact upper-bound CI value.

Functional Consequences

• Missense variants (M417V, T1590A) may impair MTMR2 protein-protein interaction, potentially destabilizing the MTMR2–MTMR5 complex and driving myelin-outfolding pathology (proposed mechanism from mouse studies, PMC9190308).
• Null/truncating variants cause loss of MTMR5 protein and, at least in the mouse knockout model, do not cause myelin outfoldings but instead impair axon radial sorting — a mechanistically distinct process from the myelin-outfolding pathology of complete MTMR2 or MTMR13 loss (see Mechanism section).
• This raises the hypothesis that different variant classes (partial loss-of-function/missense vs. complete null) may produce mechanistically and phenotypically distinct disease presentations — an important nuance for genotype-phenotype correlation in CMT4B3.

Modifier Genes

No formally established modifier genes; MTMR2 is a genetically and physically interacting partner whose own dosage/levels are interdependent with MTMR5 and MTMR13 in the peripheral nervous system (mouse studies show MTMR2 loss destabilizes both MTMR5 and MTMR13 protein levels, PMC9190308).

Epigenetic Information

Not specifically studied in CMT4B3.

Chromosomal Abnormalities

Not applicable — CMT4B3 is caused by small-scale sequence variants (missense, nonsense, splice-site, small indels), not large chromosomal rearrangements.

5. Environmental Information

No specific environmental triggers, toxins, occupational exposures, lifestyle factors, or infectious agents have been established as causal or modifying for CMT4B3 in the peer-reviewed literature. As with other hereditary neuropathies, avoidance of peripherally neurotoxic agents (e.g., vincristine and other chemotherapeutics known to worsen CMT broadly) would be a reasonable precaution by extension of general CMT clinical practice, though this has not been specifically documented for CMT4B3 patients.

6. Mechanism / Pathophysiology

Causal chain (numbered, from mutation to phenotype)

1. Biallelic pathogenic variants in SBF1 (22q13.33) lead to absent, truncated, or functionally impaired MTMR5 (SBF1) pseudophosphatase protein.
2. Loss/impairment of MTMR5 disrupts its heterodimeric complex with the catalytically active phosphatase MTMR2 (and, in parallel, the analogous MTMR2–MTMR13 complex) — demonstrated by mouse coexpression studies showing co-expression with MTMR2 "greatly increased the levels" of MTMR5 protein, indicating mutual stabilization (PMC9190308). (Demonstrated in mouse model; inferred to extend to human Schwann cell biology.)
3. Disruption of the MTMR2–MTMR5 complex impairs the complex's proposed function in regulating phosphoinositide levels (e.g., PtdIns3P/PtdIns(3,5)P2 turnover) and Rab GTPase activation (notably Rab21, potentially Rab28) within Schwann cells and neurons.
4. Impaired phosphoinositide/Rab regulation disrupts endosomal trafficking, in particular the sorting/recycling of key Schwann cell surface receptors through endosomal compartments — the mouse literature specifically implicates ErbB2/ErbB3 (neuregulin receptor tyrosine kinases essential for Schwann cell myelination signaling) and β1-integrin trafficking as candidate downstream targets. (This step is proposed/inferred from mouse mechanistic studies rather than directly demonstrated in human tissue.)
5. Branch point — the downstream consequence differs by variant class:
6. Branch A (complete/near-complete loss of MTMR5, as modeled by mouse knockout): Disrupted receptor trafficking impairs axon radial sorting — the developmental process by which Schwann cells segregate and ensheath individual large-caliber axons from bundles — resulting in reduced numbers of properly sorted/myelinated large axons, without myelin outfolding and with normal myelin thickness/g-ratio once myelination does occur (mouse Mtmr5−/− data, PMC9190308).
7. Branch B (partial-function missense variants, e.g., M417V/T1590A): The authors hypothesize these variants may impair MTMR2 interaction specifically, triggering classic myelin outfoldings — the redundant, infolded/refolded myelin loops characteristic of CMT4B1/CMT4B2 — analogous to the pathology seen with primary MTMR2 or MTMR13 loss. (Explicitly labeled as a hypothesis by the study authors, not yet directly demonstrated.)
8. In parallel to peripheral nerve pathology, MTMR5 loss in the CNS disrupts early neurogenesis (not via increased apoptosis/cell death) — shown in a zebrafish mtmr5 knockout model as reduced brain size (~10%) from 10 days post-fertilization onward, with disorganized axon branching morphology, contributing to the microcephaly and structural brain phenotypes seen in a subset of severe human CMT4B3 cases (PMC11891516).
9. Peripheral axonal-sorting/myelination failure plus (in syndromic cases) cranial nerve and CNS involvement culminate in the clinical phenotype: distal sensorimotor polyneuropathy (weakness, atrophy, sensory loss, areflexia), and — in the more severe/syndromic end of the spectrum — cranial neuropathies (facial weakness, ophthalmoparesis), microcephaly, intellectual disability, ataxia, and pyramidal signs.
10. A separate, incompletely understood branch: in at least one reported case, disease is accompanied by secondary mitochondrial respiratory chain dysfunction (reduced Complex I, II+III, and IV activities, reduced mtDNA content) in muscle, proposed to result from "accumulation of toxic metabolites or secondary impairment of the OxPhos machinery," potentially compounding axonal injury in long peripheral nerves (PMID:34118926) — this remains a hypothesis-level mechanistic link rather than an established primary pathway.

Molecular Pathways

Endo-lysosomal/endosomal trafficking pathway; phosphoinositide (PtdIns3P, PtdIns(3,5)P2) metabolism; Rab GTPase activation (Rab21, Rab28) via DENN-domain GEF activity. Suggested pathway/database cross-references: Reactome "Membrane Trafficking," GO biological process terms below.

Cellular Processes

• Schwann cell axon radial sorting (developmental process, disrupted per mouse model)
• Myelination (secondarily affected)
• Endosomal receptor trafficking/recycling (ErbB2/ErbB3, β1-integrin)
• Neurogenesis (CNS, per zebrafish model)
• Not primarily an apoptotic/degenerative mechanism — zebrafish data specifically show reduced brain size is "not caused by excessive apoptosis, autophagy or cell loss."

Protein Dysfunction

Loss-of-function or partial loss-of-function of a pseudophosphatase scaffold/regulatory protein; not a gain-of-function or aggregation-prone mechanism as currently understood.

Metabolic Changes

Secondary mitochondrial oxidative phosphorylation dysfunction has been documented in at least one case (reduced Complex I/II+III/IV activities, reduced mtDNA content) — an emerging but not yet generalized feature.

Immune System Involvement

Not implicated in current literature.

Tissue Damage Mechanisms

Axonal loss/dysfunction secondary to failed radial sorting and impaired Schwann cell-axon signaling, rather than primary demyelination alone in the null-variant mechanism; myelin outfolding pathology (structural redundant myelin loops) for missense/partial-function variants.

Biochemical Abnormalities

Loss of pseudophosphatase scaffolding function; downstream phosphoinositide dysregulation (proposed, not directly biochemically confirmed in human tissue in the sources reviewed).

Epigenetic Changes

Not reported.

Molecular/Omics Profiling

• Transcriptomics: Zebrafish mtmr5-knockout RNA-seq identified 2,040 differentially expressed genes (1,693 up, 347 down), with dysregulated pathways including neurogenesis, chromatin organization/remodeling, cytoskeletal fiber polymerization, and synaptic signaling; mtmr13 expression was reduced ~25% in knockouts while mtmr2 was unchanged (PMC11891516).
• No human transcriptomic, proteomic, metabolomic, or single-cell datasets specific to CMT4B3 patient tissue were identified in this search.

Suggested GO terms

• GO:0007009 plasma membrane organization (myelination-related)
• GO:0032288 myelin assembly
• GO:0031175 neuron projection development
• GO:0016197 endosomal transport
• GO:0032313 regulation of Rab GTPase activity
• GO:0043547 positive regulation of GTPase activity
• GO:0046488 phosphatidylinositol metabolic process
• GO:0035855 megakaryocyte development — not relevant
• GO:0022011 myelination in peripheral nervous system
• GO:0007422 peripheral nervous system development

Suggested CL terms

• CL:0002573 Schwann cell
• CL:0000006 neuron (afferent/sensory)
• CL:0000540 neuron (general, motor)
• CL:0002516 Schwann cell precursor

7. Anatomical Structures Affected

Organ level

• Primary: Peripheral nervous system — peripheral (sensorimotor) nerves, particularly distal lower-limb nerves.
• Secondary/syndromic involvement: Cranial nerves (facial [CN VII], oculomotor [CN III], and others per "fork and bracket" pathology); central nervous system (cerebellum, brainstem/pons/mesencephalon, cerebral cortex — microcephaly); skeletal system (feet, spine — pes cavus/planus, scoliosis, syndactyly); skeletal muscle (secondary neurogenic atrophy; in one case, mitochondrial myopathic features); respiratory system (secondary to neuromuscular weakness, requiring ventilatory support in severe cases).
• Body systems: Nervous system (primary), musculoskeletal system (secondary), and in the mitochondrial-dysfunction phenotype, potentially systemic bioenergetic involvement.

Tissue and cell level

• Peripheral nerve: myelinating and non-myelinating Schwann cells, large- and small-diameter myelinated axons.
• Skeletal muscle: type-grouped/neurogenic atrophic fibers; "necklace fibers" (internalized nuclei in a linear/ring pattern) in one reported family.
• CNS: neurons undergoing early neurogenesis (zebrafish CNS data).

Subcellular level

• Endosomal/late-endosomal compartments (site of MTMR2–MTMR5 complex function)
• Plasma membrane (myelin membrane biogenesis, axon-Schwann cell membrane apposition)
• Mitochondria (secondary dysfunction in at least one reported case)

Suggested UBERON terms

• UBERON:0001358 peripheral nerve / UBERON:0001519 peroneal nerve
• UBERON:0002316 sural nerve (biopsy site)
• UBERON:0002037 cerebellum
• UBERON:0002259 pons
• UBERON:0002417 mesencephalon
• UBERON:0002385 muscle tissue
• UBERON:0002415 myelin sheath

Suggested GO Cellular Component terms

• GO:0043209 myelin sheath
• GO:0005768 endosome
• GO:0005770 late endosome
• GO:0005739 mitochondrion

Localization

Distal, symmetric, length-dependent peripheral nerve involvement (classic length-dependent CMT pattern) — bilateral. Cranial nerve involvement, when present, is typically bilateral (facial weakness, ophthalmoparesis).

8. Temporal Development

Onset

• Ranges from congenital/infantile (clubfoot at birth, hypotonia by 18 months; congenital microcephaly in syndromic families) to classic first-decade onset (5–11 years, most common pattern across families).
• Onset pattern: insidious, slowly progressive in the classic form; in the most severe syndromic (null-variant) family, an early infantile cerebellar/pyramidal presentation precedes overt peripheral polyneuropathy, which emerges only toward the end of the first decade.

Progression

• Disease course: Chronic, progressive (not relapsing-remitting or episodic).
• Progression rate: Variable — slow in the original Korean family (ambulation preserved into adulthood, lost only in the 5th decade); rapid/severe in the Italian mitochondrial-dysfunction case (non-invasive ventilation by age 6, wheelchair by age 11) and in syndromic infantile-onset families.
• Stages: Not formally staged in a validated clinical staging system (unlike, e.g., cancer); described qualitatively as early (distal weakness/sensory loss), intermediate (loss of reflexes, gait disturbance, skeletal deformity), and advanced (loss of ambulation, respiratory compromise in severe cases).

Patterns

• No remission pattern described — CMT4B3 is a chronic progressive neuropathy without spontaneous remission.
• No specific "critical period" for intervention has been established given the absence of disease-modifying therapy, though early diagnosis is emphasized for genetic counseling, orthotic/rehabilitative planning, and (in syndromic cases) proactive management of cranial nerve, respiratory, and cognitive/developmental needs.

9. Inheritance and Population

Epidemiology

• Prevalence/incidence: No formal population-based prevalence or incidence estimate exists; CMT4B3 is characterized in the literature simply as "ultra-rare," with the disease-specific patient count described in single digits to low double digits of published families/individuals worldwide (patient advocacy source: "only 14 documented cases worldwide," cmt4b3research.org). For context, all CMT4B subtypes combined ("CMT4B") account for fewer than a hundred reported cases, mostly from populations with high consanguinity rates, and CMT overall (all types) has a prevalence estimated at a minimum of 17–20 per 100,000 globally.

Inheritance Pattern

• Autosomal recessive in the overwhelming majority of reported families (homozygous or compound heterozygous biallelic SBF1 variants).
• One reported exception: an apparently autosomal dominant transmission (mother-to-daughter) with a heterozygous missense variant (p.H466Q), proposed to act via a dominant-negative mechanism (PMC11633322) — the authors explicitly note this deviates from "the autosomal recessive manner observed in the seven cases reported before."

Penetrance / Expressivity

• Penetrance in the recessive form appears high/complete in reported homozygotes/compound heterozygotes, though formal penetrance estimates are not available given the small numbers.
• Expressivity is markedly variable — from pure, late-progressing peripheral neuropathy (original Korean family) to severe syndromic multisystem disease (microcephaly, intellectual disability, cranial neuropathies, pyramidal/cerebellar signs, respiratory failure). This variability appears to correlate at least partly with variant type (missense/hypomorphic vs. null) per the mouse mechanistic data, though this genotype-phenotype correlation is not yet firmly established in humans.

Genetic Anticipation / Germline Mosaicism / Founder Effects

Not reported/established for CMT4B3 — the small number of families precludes robust characterization of these phenomena. No specific founder mutation has been identified as recurrent across unrelated populations (the pathogenic variants reported to date are largely family-specific/private).

Consanguinity

A prominent risk factor — several reported kindreds (Saudi Arabian, Syrian/Bedouin) are explicitly consanguineous, consistent with autosomal recessive inheritance of a rare allele.

Carrier Frequency

Not established; expected to be very low given disease rarity, though a precise gnomAD-derived carrier frequency for SBF1 pathogenic alleles was not retrieved in this research pass.

Population Demographics

• Affected populations: Reported cases span Korean, Saudi Arabian, Syrian (Bedouin), Israeli, Spanish, British, and Italian ancestries — no single ethnic group predominates, consistent with a private-mutation, consanguinity-driven rare recessive disease rather than a founder-population disorder.
• Geographic distribution: Scattered case reports globally; no endemic region identified.
• Sex ratio: No sex predilection has been reported (autosomal, not X-linked, inheritance); both sexes affected in reported families (e.g., mother–daughter dominant pedigree; brother–sister pairs in recessive families).
• Age distribution: Spans infancy through adulthood at the time of reporting, reflecting the developmental-onset, slowly progressive natural history.

10. Diagnostics

Clinical/Electrophysiological Tests

• Nerve conduction studies (NCS): Reduced motor nerve conduction velocities are characteristic of the demyelinating CMT4B group generally; specific CMT4B3 velocity thresholds were not consistently reported across all case series in this search, though the classic Korean family showed a demyelinating pattern; other families (axonal neuropathy, cranial nerve involvement) show a more axonal electrophysiological signature — underscoring that CMT4B3 spans both demyelinating and axonal electrophysiological phenotypes, unlike the more uniformly demyelinating CMT4B1/CMT4B2.
• Nerve biopsy (sural nerve): Decreased numbers of large and small myelinated fibers, thin myelin sheaths, and focally folded/outfolded myelin (globular masses of irregular myelin thickening) — similar to but not always identical to CMT4B1/CMT4B2 pathology; some families show axonal loss without demyelination or outfolding.
• Muscle biopsy: Neurogenic atrophy is the common finding; "necklace fibers" (a finding classically seen in myotubular myopathy) have been reported in at least one SBF1-related family, expanding the recognized pathological spectrum. Muscle biopsy with spectrophotometric respiratory-chain enzyme analysis revealed reduced Complex I, II+III, and IV activity and reduced mtDNA content in the Italian mitochondrial-dysfunction case.
• Brain MRI: Characteristic in syndromic cases — the "fork and bracket sign" (T2-hyperintensity in the pons ["fork"] and mesencephalon ["bracket"], attributed to degenerated CN III/VII fiber bundles) is a distinctive, potentially diagnostic imaging clue in SBF1-related syndromic neuropathy, though notably absent in the "pure neuropathy" phenotype (e.g., normal brain/spinal MRI in the Italian mitochondrial-dysfunction case).

Genetic Testing

• Recommended approach: Given the extreme genetic heterogeneity of CMT (>100 causal genes) and rarity of CMT4B3 specifically, multigene CMT/hereditary neuropathy panels or exome/genome sequencing are the practical first-line approaches, rather than single-gene SBF1 testing, unless a specific familial variant is already known. All reported CMT4B3 diagnoses to date have been made via whole-exome sequencing (WES) in affected families — this has effectively been the diagnostic modality of discovery for every reported kindred.
• Single-gene testing: Appropriate for confirming a specific known familial variant (cascade testing) once identified by panel/exome sequencing in the proband.
• Chromosomal microarray, karyotyping, FISH, mitochondrial DNA testing, repeat expansion testing: Not primary diagnostic modalities for CMT4B3, since it is caused by small-scale SBF1 sequence variants, not structural or repeat-expansion mutations; mtDNA content/sequencing could be considered adjunctively in cases with suspected secondary mitochondrial dysfunction, though this is not a primary diagnostic test for the underlying SBF1 defect.

Omics-Based Diagnostics

No SBF1/CMT4B3-specific transcriptomic, proteomic, metabolomic, or liquid-biopsy diagnostic assay has been established; diagnosis remains DNA-sequencing based.

Clinical Criteria / Differential Diagnosis

No formal consensus diagnostic criteria (DSM/ICD-style) exist for CMT4B3 specifically; diagnosis relies on clinical suspicion (childhood-onset sensorimotor neuropathy ± cranial nerve involvement ± microcephaly/intellectual disability) confirmed by molecular genetic testing. Key differentials include: - Other CMT4B subtypes: CMT4B1 (MTMR2) and CMT4B2 (MTMR13/SBF2) — share myelin-outfolding pathology but lack the microcephaly/cranial-nerve/CNS features more characteristic of CMT4B3's syndromic forms. - Other autosomal recessive demyelinating CMT4 subtypes (CMT4A/GDAP1, CMT4C/SH3TC2, CMT4D, CMT4F, etc.) - Congenital hypomyelinating neuropathy (OMIM #614895 and related entries) - Other syndromic neuropathies with cranial nerve involvement, microcephaly, and intellectual disability (e.g., other DENN-domain or endosomal-trafficking disorders) - Myotubular/centronuclear myopathy (given the "necklace fibers" finding overlaps histologically) — important to distinguish given SBF1's paralog MTM1 causes X-linked myotubular myopathy.

Screening

No population, newborn, or carrier screening program specifically targets CMT4B3 given its extreme rarity; carrier screening would only be relevant in the context of known familial variants (e.g., in consanguineous families with a previously affected relative).

11. Outcome/Prognosis

Survival and Mortality

No formal survival statistics (5-year/10-year survival rates) exist given the rarity of the disease and its generally non-fatal (though disabling) natural history; the most severe reported cases involve significant morbidity (respiratory failure requiring ventilatory support) but not reported early mortality in the sources reviewed.

Morbidity and Function

• Progressive distal weakness leading to gait impairment and, in the most severe cases, loss of ambulation (by the 5th decade in the mildest reported family; much earlier — wheelchair by age 11 — in the most severe reported case).
• Respiratory morbidity: non-invasive ventilation required by age 6 in the severe Italian case, reflecting neuromuscular respiratory compromise.
• Cranial nerve morbidity in syndromic cases: dysphagia, dysarthria, ophthalmoparesis/diplopia, facial weakness.
• Cognitive/developmental morbidity: intellectual disability and developmental delay in syndromic (typically null-variant) cases; notably absent in the "pure neuropathy" phenotype cases.
• No validated disease-specific quality-of-life instrument has been applied to CMT4B3 cohorts (none large enough to support such a study).

Disease Course / Complications

• Secondary orthopedic complications: pes cavus/planus, scoliosis/kyphoscoliosis, contractures (by extrapolation from general CMT natural history).
• Secondary respiratory complications in severe cases.
• Reported urinary incontinence in some general CMT4B3 summaries (GARD-derived), though this is not consistently emphasized across primary literature reviewed.

Prognostic Factors

• Variant type appears to correlate with severity in the limited data available: null/truncating variants (as in the Bedouin/splice-null family) are associated with the more severe, syndromic, CNS-involving phenotype (microcephaly, intellectual disability, early pyramidal/cerebellar signs), while missense/hypomorphic variants (as in the original Korean family) are associated with a milder, later-progressing "pure neuropathy" phenotype — though this genotype-phenotype correlation remains provisional given the very small number of published cases and has not been formally tested statistically.
• No validated prognostic biomarkers exist.

12. Treatment

Current State

No disease-modifying or curative therapy exists for CMT4B3. As one 2025 phenotype-expansion review states, "despite its severe clinical presentation, currently no disease-modifying therapies" are available, and there remains "an incomplete understanding of the disease pathomechanism(s)" (Orphanet summary; consistent with the CMT4B3 Research Foundation's statement that "little is known about CMT4B3 at this time").

Supportive / Symptomatic Management

Management follows general CMT supportive-care principles, as no CMT4B3-specific guidelines exist: - Rehabilitative therapy: Physical therapy (gait/balance training) and occupational therapy for functional preservation — NCIT: NCIT:C15302 (Physical Therapy) - Orthotics/bracing: Ankle-foot orthoses for foot drop and gait stability — NCIT: NCIT:C49236 (Therapeutic Procedure, general) / device qualifier pattern per orthotic device - Orthopedic surgery: For severe pes cavus, scoliosis, or contractures — NCIT: NCIT:C16186 (Orthopedic Surgical Procedure) - Respiratory support: Non-invasive ventilation for neuromuscular respiratory compromise in severe cases — NCIT: NCIT:C15747 (Supportive Care) as a general category - Genetic counseling: Essential given autosomal recessive (predominantly) inheritance, recurrence risk (25% for future pregnancies of carrier-carrier couples), and the recently identified possibility of dominant transmission in rare kindreds — NCIT: NCIT:C15240 (Genetic Counseling) - Multidisciplinary management of syndromic features: ophthalmology (strabismus/ophthalmoparesis), speech-language pathology (dysarthria/dysphagia), developmental pediatrics/neuropsychology (intellectual disability), and pulmonology (respiratory monitoring).

Pharmacotherapy

No SBF1/CMT4B3-targeted pharmacotherapy exists. No pharmacogenomic (PharmGKB/CPIC) guidance is specific to SBF1.

Advanced Therapeutics / Experimental

• No gene therapy, cell therapy, RNA-based therapy (ASO/siRNA), or targeted molecular therapy has reached clinical development specifically for CMT4B3.
• Preclinical model development is actively underway, positioning the field for future therapeutic testing:
• A CRISPR/Cas9 mtmr5 knockout zebrafish model (2025, PMC11891516) is explicitly proposed by its authors as "a first pre-clinical model to phenocopy the disease," intended as "an ideal tool for future studies on disease pathomechanism(s) and therapy development."
• A CRISPR/Cas9 Mtmr5 knockout mouse model (2022, PMC9190308) has clarified the axon-radial-sorting mechanism and is a platform for future mechanistic and therapeutic studies.
• Johns Hopkins Medicine has reportedly created a cellular model of CMT4B3 to test potential therapies (per institutional development news).
• The patient advocacy organization CMT4B3 Research Foundation funds research grants into disease mechanisms as a precursor to therapy development but reports no approved treatments.
• Broader CMT pipeline context (not CMT4B3-specific): CMT1A-directed trials such as PXT3003 (a repurposed drug combination) and the CMT-SORD trial (govorestat) illustrate active general CMT drug development, but none currently target SBF1/MTMR5 biology specifically.

Treatment Outcomes

No systematic treatment-response, adverse-event, or outcome data exist for CMT4B3 given the absence of any targeted intervention.

Treatment Strategy

Given the absence of disease-modifying therapy, the current standard of care is a symptomatic, multidisciplinary, supportive-care algorithm analogous to general CMT management (rehabilitation → orthotics → surgery as needed → monitoring for syndromic complications), with genetic counseling as a cornerstone given the hereditary, currently non-curable nature of the disease.

13. Prevention

Primary Prevention

No primary prevention exists beyond genetic counseling and reproductive options (carrier testing, prenatal diagnosis, preimplantation genetic diagnosis) for families with a known pathogenic SBF1 variant, particularly relevant given the consanguinity association in several reported kindreds.

Secondary Prevention / Screening

• No population-based screening program exists (disease too rare for newborn screening panels).
• Genetic/carrier screening and cascade testing are appropriate within families with a known proband, especially in consanguineous populations.
• Prenatal testing / PGD would be technically feasible once a familial variant is identified, though no specific published experience with this in CMT4B3 was identified.

Tertiary Prevention

Early diagnosis enables proactive surveillance for and management of complications — orthopedic deformity (scoliosis, pes cavus), respiratory compromise, cranial nerve dysfunction (dysphagia/aspiration risk), and developmental/cognitive needs — analogous to tertiary prevention strategies in other pediatric neuromuscular disorders.

Immunization / Public Health / Behavioral Interventions

Not specifically applicable — CMT4B3 is not an infectious, immunologically mediated, or behaviorally modifiable disease.

Counseling

Genetic counseling is central given: (1) predominantly autosomal recessive inheritance with 25% recurrence risk for carrier couples, (2) the newly recognized possibility of dominant transmission in rare families (altering recurrence-risk counseling for those specific pedigrees), and (3) the marked phenotypic variability (pure neuropathy vs. syndromic multisystem disease) that complicates prognostic counseling at the time of diagnosis.

14. Other Species / Natural Disease

No naturally occurring CMT4B3/SBF1-associated disease has been reported in non-human species (dogs, cats, or other companion/veterinary species) in the literature surveyed — unlike some other CMT subtypes with recognized veterinary correlates. SBF1 orthologs exist across vertebrates (used to generate the mouse and zebrafish models below), and the gene is evolutionarily conserved, but no spontaneous/natural veterinary disease phenotype has been documented. No zoonotic or cross-species transmission relevance applies, as this is a purely genetic (non-infectious) disorder.

15. Model Organisms

Mouse Model

• Species/system: Mus musculus, CRISPR/Cas9-generated Mtmr5 knockout (guide RNAs targeting exon 1 and exon 25, producing a 14 kb deletion, frameshift, and premature stop codon; full-length 208 kDa protein undetectable) (PMC9190308, Human Molecular Genetics 2022).
• Phenotype recapitulation:
• Does NOT recapitulate myelin outfoldings (the classic CMT4B1/CMT4B2 pathology) — a key point of divergence from other CMT4B mouse models.
• Does recapitulate an axon radial-sorting defect: ~10% reduction in total myelinated axons in sciatic nerve, with increased numbers of large-diameter axons remaining abnormally bundled/incompletely ensheathed by Schwann cells.
• Myelin structure that does form is otherwise normal (normal g-ratios, myelin thickness).
• Additional phenotype: male infertility (consistent with prior Mtmr5-deletion literature).
• Double knockout: Mtmr5−/−;Mtmr13−/− mice die perinatally, indicating partial functional redundancy between MTMR5 and MTMR13 during embryonic development despite their distinct postnatal roles.
• Model limitations: Because the knockout only models complete loss-of-function, it does not capture the myelin-outfolding pathology seen with some human missense alleles, nor the CNS/microcephaly/cranial-nerve phenotypes of the syndromic human disease — the authors explicitly hypothesize (not yet tested) that different classes of human missense variants might produce the outfolding phenotype via impaired MTMR2 interaction.
• Research applications: Elucidating the distinct molecular roles of MTMR5 vs. MTMR13 in Schwann cell biology (axon radial sorting vs. myelination maintenance), informing therapeutic target identification (ErbB2/3 and β1-integrin endosomal trafficking).
• Resource: MGI:1925230 (Sbf1 gene); targeted allele MGI:2449174.

Zebrafish Model

• Species/system: Danio rerio, CRISPR/Cas9 full-gene deletion of mtmr5 (~86 kb, guide RNAs flanking the ATG start and stop codon) (PMC11891516, Brain Communications 2025).
• Phenotype recapitulation:
• Homozygous mutants born at normal Mendelian ratios; no gross motor deficit on swim assays at 3/6/14 dpf.
• Microcephaly — ~10% reduction in body length and proportional reduction in brain height/length from 10 dpf onward, persisting into adulthood — described as a phenotype "uniquely observed in this disease subtype," directly paralleling the microcephaly seen in severe human CMT4B3.
• Reduced brain size attributable to defects in early neurogenesis, not increased apoptosis, autophagy, or cell loss.
• Dysmyelination phenotype: normal myelin formation but increased axon-myelin plasma membrane detachment on TEM of the posterior lateral line nerve, mirroring human peripheral nerve pathology.
• Axonal patterning defects: disorganized, less-defined axon morphology; increased total branch number/endpoints with reduced average branch length.
• Transcriptomic dysregulation: 2,040 differentially expressed genes (RNA-seq), implicating neurogenesis, chromatin remodeling, cytoskeletal polymerization, and synaptic signaling pathways; secondary ~25% reduction in mtmr13 expression (with unchanged mtmr2), suggesting partial cross-regulation among the three MTMR paralogs.
• Significance: Explicitly positioned by its authors as "a first pre-clinical model to phenocopy the disease," intended as "an ideal tool for future studies on disease pathomechanism(s) and therapy development" — notably, this model captures the CNS/microcephaly dimension of human CMT4B3 that the mouse peripheral-nerve-focused model does not emphasize.

Cellular Models

• A cellular model of CMT4B3 has reportedly been developed at Johns Hopkins Medicine for therapy screening purposes (per institutional/foundation reporting), though peer-reviewed methodological detail was not retrieved in this search pass.

Comparative note

Together, the mouse and zebrafish models are complementary rather than redundant: the mouse model best captures the peripheral nerve (Schwann cell/axon radial sorting) mechanism, while the zebrafish model uniquely captures the CNS/microcephaly dimension of the human syndromic phenotype — consistent with dismech's "Human-Model Mismatch" framing, since neither single model alone recapitulates the full human disease spectrum, and each captures a different mechanistic axis (peripheral vs. central).

Summary Table of Key Primary-Literature PMIDs

Sources

• OMIM #615284 — CMT4B3
• OMIM *603560 — SBF1
• Orphanet: CMT4B3 (ORPHA:363981)
• NIH GTR: CMT4B3 (C3695063)
• GARD/NIH: CMT4B3
• Bi-allelic variants in MTMR5/SBF1 cause CMT4B3 featuring mitochondrial dysfunction (PMC8199524 / PMID:34118926)
• Characterization of a novel zebrafish model of MTMR5-associated CMT4B3 (PMC11891516)
• SET binding factor 1 (SBF1) mutation causes CMT4B3 — Nakhro et al. 2013, Neurology (PMID:23749797)
• Distinct roles for Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination (PMC9190308)
• A novel SBF1 missense mutation causes autosomal dominant CMT4B3 (PMC11633322)
• "Fork and bracket" syndrome expands the spectrum of SBF1-related sensory motor polyneuropathies
• Multiple cranial nerve neuropathies, microcephaly, "fork and bracket sign" — Mégarbané et al. 2010 (PMID:20658556)
• Novel SBF1 splice-site null mutation broadens the clinical spectrum of CMT4B3 (PMID:30039846)
• SBF1 mutations associated with autosomal recessive axonal neuropathy with cranial nerve involvement (PMID:28005197)
• A novel frameshift deletion in autosomal recessive SBF1-related syndromic neuropathy with necklace fibres (PMC7419361)
• CMT4B3 Research Foundation — What is CMT4B3
• SBF1 Gene — GeneCards
• Pediatric toe-walking cohort with heterozygous SBF1 variants (PMC12926636)
• Disruption of Mtmr2 produces CMT4B1-like neuropathy (PMC2172586)
• The CMT4B disease-causing phosphatases Mtmr2 and Mtmr13 depend on each other for wild-type protein levels
• Charcot-Marie-Tooth 4B3 Phenotype Expansion and Confounding Factors, Current Genetic Medicine Reports

Note on evidentiary basis: Given CMT4B3's extreme rarity (an estimated dozen-to-twenty published patients worldwide), essentially every clinical and mechanistic claim above derives from individual case reports/small family series rather than cohort studies, and several mechanistic claims (e.g., the missense-vs-null genotype-phenotype hypothesis, the MTMR2-interaction-disruption model, the proposed OxPhos-toxicity mechanism) are explicitly labeled by their source authors as proposed/hypothesized rather than definitively demonstrated. This should be reflected in any downstream knowledge-base curation with appropriate directness/hedge annotations.

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

Every one of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

• PMC:PMC12926636 (abstract only): "may act as modifiers or coincidental findings rather than independent causes of neuromotor abnormalities"
• closest text in source: "CONCLUSIONS: In this referral-based cohort, heterozygous SBF1 variants were observed in children with persistent toe walking and accompanying mild neuromotor/musculoskeletal features that partially overlap with reported CMT4B3 phenotypes; however, these findings are descriptive and do not establish causality or enrichment"

Term Validation

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

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:0014117 (1 mention) - the report calls it "MONDO"; MONDO calls it Charcot-Marie-Tooth disease type 4B3

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:0007257 (1 mention) - HP does not contain this term

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

• HP:0002355 (obsolete Difficulty walking) (1 mention) - replaced by HP:0001288
• GO:0032313 (GO_0032313) (1 mention) - replaced by GO:0043087

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, MGI.