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Inheritance

1

Autosomal dominant inheritance HP:0000006

Autosomal dominant inheritance

Show evidence (1 reference)

PMID:23246003 SUPPORT Human Clinical

"We report the occurrence of three microcephalic patients with structural brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I, and E401K)."

The founding human case series reports de novo TUBB5 missense variants in three unrelated affected individuals, supporting dominant, typically sporadic inheritance.

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Discussions and Knowledge Gaps

3

Are the progenitor cell-cycle, spindle-orientation, apoptosis, and migration defects observed in mouse Tubb5 models conserved in human radial glia, outer-radial-glia-like cells, and cortical organoids carrying pathogenic TUBB/TUBB5 variants?

HUMAN MODEL MISMATCH OPEN gap_tubb_tubb5_human_organoid_translatability

Attached to

pathophysiology#Progenitor Cell-Cycle and Spindle Dysfunction pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion pathophysiology#Impaired Neurogenic Division and Migration

The strongest mechanistic evidence is mouse based, while the Falcon report did not identify a direct TUBB/TUBB5 human iPSC or organoid disease model. Human cortical expansion depends on outer radial glia and OSVZ biology, so a human organoid/iPSC test is needed before assuming the mouse progenitor apoptosis branch fully explains the human microcephaly phenotype.

Proposed experiments

TUBB/TUBB5 isogenic cortical-organoid progenitor and apoptosis experiment

isogenic cortical organoid progenitor perturbation experiment

exp_tubb_tubb5_isogenic_organoid_progenitor_apoptosis

Generate patient-derived or CRISPR-engineered human cortical organoids carrying M299V, V353I, E401K, or newer pathogenic TUBB/TUBB5 variants; compare mutant, isogenic-corrected, and knock-in organoids for microtubule dynamics, tubulin heterodimer assembly, progenitor cell-cycle timing, spindle orientation, p53 activation, apoptosis, neuronal output, and migration.

Model systems

TUBB/TUBB5 human iPSC-derived cortical organoid

Three-dimensional human cortical organoid carrying a pathogenic TUBB/TUBB5 variant, with matched isogenic correction and knock-in controls.

ORGANOID namo:Organoid link

human NCBITaxon:9606

cerebral cortex UBERON:0000956

neural progenitor cell CL:0011020 radial glial cell CL:0000681

Perturbations

Isogenic TUBB/TUBB5 variant correction or knock-in

pathophysiology#Altered TUBB/TUBB5 Beta-Tubulin Function

Correct a patient variant or introduce a pathogenic variant to separate variant effects from donor background.

Readouts

Progenitor mitosis, p53 activation, and apoptosis

pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion

Quantify cell-cycle length, spindle orientation, ectopic progenitor markers, p53 activation, apoptotic cells, and upper-layer neuron output.

mitotic cell cycle GO:0000278 ↕ DYSREGULATED apoptotic process GO:0006915 ↑ INCREASED

immunostaining single-cell transcriptomic profiling

Direction: POSITIVE

Controls

Isogenic corrected organoids

Patient-derived variant organoids corrected to the reference sequence.

Isogenic knock-in organoids

Control-background organoids carrying the introduced pathogenic variant.

Decision criterion

The mouse-derived mechanism is supported if human variant organoids show delayed progenitor cell cycle, spindle orientation defects, p53 activation, increased apoptosis, reduced upper-layer neuron output, and/or migration defects that are rescued by correction and reproduced by knock-in.

Would support

pathophysiology#Progenitor Cell-Cycle and Spindle Dysfunction pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion pathophysiology#Impaired Neurogenic Division and Migration

Show evidence (2 references)

PMID:26903504 SUPPORT Model Organism

"cause TUBB5-associated microcephaly by exploiting two new mouse models"

Identifies the main mechanistic evidence source as mouse models, which motivates a human/model translatability gap.

PMID:28111201 SUPPORT In Vitro

"Recent work has uncovered critical cellular and molecular differences between cortical development in humans and mice, further underscoring the need to develop human model systems."

Supports organoid/iPSC testing when mouse cortical-development mechanisms are being translated to human disease.

What is the full natural-history and phenotype range for TUBB/TUBB5-related microcephaly beyond the three original cases and small tubulinopathy cohort counts?

KNOWLEDGE GAP OPEN gap_tubb_tubb5_natural_history_and_phenotype_breadth

Attached to

phenotypes#Global Developmental Delay / Intellectual Disability phenotypes#Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern

The entry is supported by a strong founding case series and model mechanism, but TUBB/TUBB5-specific longitudinal outcomes, seizure frequency, genotype- phenotype correlations, and variant-class effects remain much thinner than for TUBA1A or TUBB2B. Two 2025-2026 reports narrow the gap without closing it: a Korean beta-tubulinopathy cohort places TUBB alongside the other beta-tubulin genes in a series of only twelve patients across five genes, and a single family shows complete penetrance with markedly variable expressivity - including a transmitting mother judged healthy until MRI - which means published TUBB case counts are a floor, not an estimate.

Show evidence (3 references)

PMID:41152456 SUPPORT Human Clinical

"Ten pathogenic or likely pathogenic missense variants were identified in five β-tubulin genes (TUBB2A, TUBB2B, TUBB3, TUBB4A, and TUBB5)."

Shows that even a contemporary multi-gene cohort contributes only a handful of TUBB/TUBB5 patients, so gene-specific natural history remains underpowered.

PMID:42015805 SUPPORT Human Clinical

"The variant showed complete penetrance but markedly variable expressivity, contributing to the broad clinical spectrum associated with TUBB-related disorders."

Documents complete penetrance with markedly variable expressivity within a single family, the specific reason gene-level severity cannot yet be predicted from genotype.

PMID:42015805 SUPPORT Human Clinical

"After identification of the TUBB variant in both siblings and a presumably healthy mother, maternal MRI also demonstrated features consistent with tubulinopathy."

A transmitting parent considered healthy was found on imaging to be affected, showing that ascertainment by severe presentation systematically undercounts the mild end of the TUBB phenotype range.

Does the Notch-dependent block of neural progenitor differentiation seen in tubb5-null zebrafish operate in human TUBB-related microcephaly, and is it pharmacologically reversible in a human model?

HUMAN MODEL MISMATCH OPEN gap_tubb_tubb5_notch_arm_human_translatability

Attached to

pathophysiology#Notch-Dependent Block of Neural Progenitor Differentiation pathophysiology#Microcephaly and Structural Brain Abnormalities

The zebrafish arm is mechanistically attractive because it is reversible: gamma-secretase inhibition with DAPT rescued both developmental and locomotor deficits. But three features do not transfer automatically to the human disease. First, the fish model is a homozygous knockout, whereas human disease alleles are heterozygous missense variants whose mechanism has been shown to be gain-of-function rather than loss-of-function - a null and a dose-dependent gain-of-function allele need not converge on the same downstream pathway. Second, the mammalian mechanism established to date is p53-associated apoptosis of upper-layer neurons, which removes neurons that were made, while the Notch arm prevents them being made; whether both operate in humans, or whether one is species-specific, is untested. Third, DAPT rescue in a larval fish says nothing about a therapeutic window in a human brain whose malformation is established prenatally. Resolving this requires a human neural model carrying a patient TUBB missense allele.

Show evidence (2 references)

PMID:42241496 SUPPORT Model Organism

"DAPT treatment rescued both developmental and locomotor deficits tubb5 knockout zebrafish."

The pharmacological rescue that makes this arm interesting, and the result whose translatability to human TUBB missense disease is the open question.

PMID:41309602 SUPPORT In Vitro

"Furthermore, we demonstrate that the disease mechanism is not haploinsufficiency and that other patient mutations in TUBB affect cilia formation, putting forward defective ciliogenesis as a contributing pathogenic factor in a subset of tubulinopathy patients."

Establishes that human TUBB disease is not a loss-of-function mechanism, which is precisely why a homozygous-null zebrafish phenotype cannot be assumed to model it.

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Pathophysiology

7

Altered TUBB/TUBB5 Beta-Tubulin Function

Pathogenic variants in the beta-tubulin isotype historically named TUBB5 alter chaperone-dependent tubulin heterodimer assembly and microtubule function. The upstream lesion is not simply a generic microcephaly label: different missense variants perturb the beta-tubulin folding/assembly machinery in different ways and feed both progenitor cell-cycle and neuronal migration branches.

neural progenitor cell CL:0011020 migrating cortical neuron CL:0000540

TUBB (formerly TUBB5) hgnc:20778

Genetic context allele_type: missense variant_origin: DE_NOVO zygosity: HETEROZYGOUS functional_impact_category: GAIN_OF_FUNCTION

Reported disease alleles are heterozygous de novo missense substitutions. Haploinsufficiency has been tested directly and excluded: TUBB-haploid hTERT-RPE1 cells ciliate normally, and the G308S, E108K and E401K patient variants only produce their ciliation defect when overexpressed in wild-type cells - the signature of a gain-of-function rather than a loss-of-function allele. GAIN_OF_FUNCTION is therefore recorded here instead of the DOMINANT_NEGATIVE category used for TUBA1A and TUBB3. Curators should note the qualification the same work makes: E401K may act through a dual mechanism, with loss-of-function effects contributing to microcephaly while gain-of-function effects drive defective neuronal positioning, so this category describes the best-resolved ciliation and positioning arm rather than every phenotype of every allele.

tubulin heterodimer assembly GO:0007021 ↕ DYSREGULATED microtubule cytoskeleton organization GO:0000226 ↕ DYSREGULATED

Downstream

Progenitor Cell-Cycle and Spindle Dysfunction Altered beta-tubulin function perturbs neural progenitor cell-cycle progression and spindle orientation.

Impaired Neurogenic Division and Migration Altered beta-tubulin function also changes the position of migrating neurons and disrupts migration-dependent cortical organization.

Impaired Ciliogenesis and Reduced Ciliation A subset of TUBB variants additionally impairs the early stages of primary cilium assembly, a branch that is parallel to - not downstream of - the progenitor and migration arms.

Notch-Dependent Block of Neural Progenitor Differentiation Loss of tubb5 function derepresses Notch target genes that hold neural progenitors in an undifferentiated state, an arm shown so far only in zebrafish.

Show evidence (3 references)

PMID:23246003 SUPPORT In Vitro

"These mutant proteins, which affect the chaperone-dependent assembly of tubulin heterodimers in different ways"

Establishes variant-specific disruption of chaperone-dependent tubulin heterodimer assembly as the upstream beta-tubulin lesion.

PMID:23246003 SUPPORT Model Organism

"Tubb5 is expressed in neurogenic progenitors in the mouse and that its depletion in vivo perturbs the cell cycle of progenitors and alters the position of migrating neurons."

Separately classifies the in vivo mouse depletion data supporting progenitor cell-cycle and neuronal-position effects.

PMID:41309602 SUPPORT In Vitro

"Furthermore, we demonstrate that the disease mechanism is not haploinsufficiency and that other patient mutations in TUBB affect cilia formation, putting forward defective ciliogenesis as a contributing pathogenic factor in a subset of tubulinopathy patients."

Direct test excluding haploinsufficiency as the TUBB disease mechanism, which is the basis for the GAIN_OF_FUNCTION functional_impact_category on this node, and the link to the ciliogenesis branch below.

Progenitor Cell-Cycle and Spindle Dysfunction

Tubb/TUBB5 dysfunction affects neurogenic progenitors during corticogenesis. In the founding study, Tubb5 depletion perturbed progenitor cell-cycle progression and altered neuron position; in the knock-in mouse model, the E401K allele produced delayed cell-cycle progression, ectopic progenitor markers, and spindle-orientation defects. This progenitor branch is the main reason to model TUBB/TUBB5-related microcephaly separately from beta-tubulin disorders whose central skeleton is postmitotic neuronal migration.

neural progenitor cell CL:0011020 radial glial cell CL:0000681

mitotic cell cycle GO:0000278 ↕ DYSREGULATED mitotic spindle organization GO:0007052 ↕ DYSREGULATED cell division GO:0051301 ↕ DYSREGULATED

Downstream

p53-Associated Apoptosis and Progenitor Pool Distortion Cell-cycle and spindle defects trigger cell death and change the progenitor and neuronal output of the developing cortex.

Show evidence (3 references)

PMID:23246003 SUPPORT Model Organism

"Tubb5 is expressed in neurogenic progenitors in the mouse and that its depletion in vivo perturbs the cell cycle of progenitors and alters the position of migrating neurons."

Shows that Tubb5 acts in neurogenic progenitors and that depletion alters progenitor cell cycle and migrating neuron position.

PMID:26903504 SUPPORT Model Organism

"associated with a delay in cell cycle progression and ectopic DNA elements in progenitors"

Supports delayed progenitor cell-cycle progression as a Tubb5-dependent mechanism in mouse.

PMID:26903504 SUPPORT Model Organism

"we report ectopic Sox2-positive progenitors and defects in spindle orientation"

Supports progenitor mislocalization and spindle-orientation defects in the Tubb5 E401K knock-in model.

p53-Associated Apoptosis and Progenitor Pool Distortion

Tubb5 mouse models show that the microcephaly branch is driven by abnormal cell survival and progenitor-output effects, not only by altered neuronal migration. The E401K knock-in and loss-of-function models produce profound microcephaly with loss of upper-layer neurons, massive apoptosis, and p53 upregulation, making p53-associated cell death a central mechanistic branch.

neural progenitor cell CL:0011020 cortical neuron CL:0000540

apoptotic process GO:0006915 ↑ INCREASED maintenance of cell number GO:0098727 ↓ DECREASED neurogenesis GO:0022008 ↕ DYSREGULATED

Downstream

Microcephaly and Structural Brain Abnormalities Progenitor-pool distortion and apoptosis reduce cortical neuron output and produce the small-brain phenotype with associated structural malformations.

Show evidence (2 references)

PMID:26903504 SUPPORT Model Organism

"present with profound microcephaly due to a loss of upper-layer neurons that correlates with massive apoptosis and upregulation of p53."

Demonstrates the mouse microcephaly mechanism: upper-layer neuron loss with apoptosis and p53 upregulation.

PMID:26903504 SUPPORT Model Organism

"demonstrates that the cellular pathology driving TUBB5-associated microcephaly is cell death."

Directly identifies cell death as the cellular pathology driving the TUBB5-associated microcephaly model.

Impaired Neurogenic Division and Migration

TUBB/TUBB5 disease also has a migration and cortical-organization branch: Tubb5 depletion alters the position of migrating neurons, and tubulinopathy cohorts place TUBB5 among genes whose malformations include focal or multifocal polymicrogyria-like cortical dysplasia and simplified gyration. This branch is secondary to the progenitor-centered microcephaly skeleton but remains part of the pathograph.

migrating cortical neuron CL:0000540

neuron migration GO:0001764 ↓ DECREASED cerebral cortex development GO:0021987 ↕ DYSREGULATED

Downstream

Microcephaly and Structural Brain Abnormalities Migration and cortical organization defects contribute to the cortical dysgyria and structural brain abnormalities observed in affected individuals.

Show evidence (2 references)

PMID:23246003 SUPPORT Model Organism

"disrupt neurogenic division and/or migration in vivo."

Links TUBB5 mutant proteins to disrupted neurogenic division and migration in vivo.

PMID:24860126 SUPPORT Human Clinical

"TUBB3 and TUBB5 mutations cause milder malformations with focal or multifocal polymicrogyria-like cortical dysplasia with abnormal and simplified gyral pattern."

Places TUBB5 in the tubulinopathy subgroup with focal/multifocal polymicrogyria-like cortical dysplasia and simplified gyration.

Impaired Ciliogenesis and Reduced Ciliation

A subset of TUBB variants impairs the early stages of primary cilium assembly. Because the ciliary axoneme is itself built from microtubules, the beta-tubulin lesion acts on ciliogenesis directly, without any basal body, transition zone or intraflagellar-transport gene defect. Patient-derived cells and gene-edited isogenic lines carrying G308S show reduced ciliation through altered microtubule dynamics and structure; knock-in mice show decreased ciliation in cerebellum and kidney; and E108K and E401K reproduce the defect. The mechanism is dose-dependent gain-of-function, not haploinsufficiency - TUBB-haploid cells ciliate normally - so this branch is allele-specific rather than a property of every TUBB variant. It is what connects this entry to the ciliopathy spectrum, and the patients carrying these alleles show ciliopathy-like brain features on MRI.

ciliated cell CL:0000064

cilium assembly GO:0060271 ↓ DECREASED microtubule cytoskeleton organization GO:0000226 ↕ DYSREGULATED

Downstream

Microcephaly and Structural Brain Abnormalities Reduced ciliation in the developing brain contributes to the structural brain malformations, which in the reported patients include ciliopathy-like features.

Show evidence (4 references)

PMID:41309602 SUPPORT In Vitro

"Using patient-derived cells and gene-edited isogenic cell lines, we show that the identified variant impairs the early stages of cilia formation by altering microtubule dynamics and structure."

Establishes impaired early ciliogenesis as a TUBB disease mechanism in human patient-derived and isogenic cell models.

PMID:41309602 SUPPORT Model Organism

"Consistent with this, mice carrying the variant display decreased ciliation in the cerebellum and kidney."

In vivo confirmation that the ciliation defect is reproduced in a mammalian knock-in model, in both a CNS and a non-CNS ciliated tissue.

PMID:41309602 SUPPORT Human Clinical

"Here, we present a de novo heterozygous missense variant (p.G308S) in Tubulin Beta Class I (TUBB) identified in a patient showing features of both ciliopathy and tubulinopathy."

The index patient shows a mixed ciliopathy/tubulinopathy presentation, which is why this branch is curated as part of the TUBB pathograph rather than as an unrelated observation.

+ 1 more reference

Notch-Dependent Block of Neural Progenitor Differentiation

In a zebrafish tubb5 knockout, loss of the beta-tubulin isotype upregulates the Notch effectors notch1a and her5, which hold neural progenitors in an undifferentiated state: neurogenin1- and huc-positive neurons are reduced while sox2-positive cells increase. Pharmacological Notch inhibition with DAPT rescues the developmental and locomotor deficits, so the arm is reversible in that model. This is curated as a separate branch from the p53-apoptosis arm because it removes neurons by failing to make them rather than by killing them, and the two have different therapeutic implications. The evidence is zebrafish-only; see the gap_tubb_tubb5_notch_arm_human_translatability discussion.

neural progenitor cell CL:0011020

Notch signaling pathway GO:0007219 ↑ INCREASED neuronal differentiation from neural progenitors GO:0030182 ↓ DECREASED

Animal models

tubb5 knockout zebrafish (CRISPR/Cas9) Zebrafish recapitulates The model is the sole source of evidence for this node: it establishes the Notch-dependent differentiation block and shows it is pharmacologically reversible.

Downstream

Microcephaly and Structural Brain Abnormalities Failure of neural progenitors to differentiate reduces the number of neurons produced, contributing to reduced brain size in the model.

Show evidence (2 references)

PMID:42241496 SUPPORT Model Organism

"Transcriptome analysis revealed that tubb5 knockout larvae showed upregulated expression of notch1a and her5, which inhibit neural progenitor differentiation."

Identifies Notch-pathway upregulation as the transcriptional signature of tubb5 loss and the mechanism holding progenitors undifferentiated.

PMID:42241496 SUPPORT Model Organism

"Whole-mount in situ hybridization demonstrated decreased neurogenin1 and huc positive neurons, while sox2 positive neurons were increased in tubb5 mutants, indicating disrupted neural progenitor differentiation."

Provides the cellular readout of the differentiation block - fewer differentiated neurons, more retained progenitors.

Microcephaly and Structural Brain Abnormalities

The clinical-anatomical endpoint is microcephaly with structural brain abnormalities: dysmorphic basal ganglia, corpus callosum anomalies, simplified or polymicrogyria-like gyral pattern, and variable hindbrain involvement. This node is the shared endpoint of the progenitor apoptosis branch and the microtubule-dependent neuronal migration branch.

cerebral cortex development GO:0021987 ↕ DYSREGULATED

cerebral cortex UBERON:0000956 basal ganglion UBERON:0002420

Animal models

tubb5 knockout zebrafish (CRISPR/Cas9) Zebrafish partially recapitulates Reproduces a broadly abnormal neurodevelopmental phenotype - developmental delay, craniofacial malformation, uncoordinated movement and increased seizure susceptibility - but not the defining human readout.

Downstream

Microcephaly The structural brain endpoint includes the defining microcephaly phenotype in TUBB/TUBB5 disease.

Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern The endpoint structural brain abnormality includes the simplified/polymicrogyria-like cortical malformation pattern.

Abnormal Basal Ganglia Morphology Dysmorphic basal ganglia are represented as an extracortical structural abnormality downstream of tubulinopathy brain malformation.

Show evidence (2 references)

PMID:23246003 SUPPORT Human Clinical

"We report the occurrence of three microcephalic patients with structural brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I, and E401K)."

Directly establishes the human endpoint: microcephaly with structural brain abnormalities due to de novo TUBB5 variants.

PMID:24860126 SUPPORT Human Clinical

"three in TUBB5 (2.8%)"

Shows that TUBB5 is a rare but recognized tubulinopathy gene in a large complex cortical malformation cohort.

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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

5

Head and Neck 1

Microcephaly HP:0000252

Show evidence (1 reference)

PMID:23246003 SUPPORT Human Clinical

"We report the occurrence of three microcephalic patients with structural brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I, and E401K)."

Establishes microcephaly as the defining phenotype in the human TUBB5 case series.

Nervous System 4

Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern HP:0002126

Show evidence (1 reference)

PMID:24860126 SUPPORT Human Clinical

"TUBB3 and TUBB5 mutations cause milder malformations with focal or multifocal polymicrogyria-like cortical dysplasia with abnormal and simplified gyral pattern."

Supports the cortical dysplasia/simplified gyral phenotype for TUBB5.

Abnormal Basal Ganglia Morphology HP:0002134

Show evidence (1 reference)

PMID:24860126 SUPPORT Human Clinical

"Dysmorphic basal ganglia are the hallmark of tubulinopathies (found in 75% of cases)"

Establishes dysmorphic basal ganglia as a tubulinopathy imaging hallmark.

Corpus Callosum Abnormality Abnormal corpus callosum morphology HP:0001273

Show evidence (1 reference)

PMID:24860126 SUPPORT Human Clinical

"Tubulinopathies are also characterized by a high prevalence of corpus callosum agenesis (32/80; 40%)"

Documents corpus callosum agenesis as a common tubulinopathy structural feature.

Global Developmental Delay / Intellectual Disability HP:0001263

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Genetic Associations

1

TUBB (Causative)

Gene: TUBB (formerly TUBB5) hgnc:20778

Show evidence (1 reference)

PMID:23246003 SUPPORT Human Clinical

"We report the occurrence of three microcephalic patients with structural brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I, and E401K)."

The historical TUBB5 symbol in the primary article maps to current HGNC symbol TUBB; the case series directly establishes causative variants in this gene.

💊

Medical Actions

3

Supportive and Rehabilitative Care

Action: supportive care Ontology label: Supportive Care NCIT:C15747

Management is supportive, including developmental, physical, occupational, speech/language, feeding, vision/oculomotor, and epilepsy surveillance services tailored to the individual's structural brain malformation and neurodevelopmental needs.

Anti-Seizure Medication

Action: pharmacotherapy Ontology label: Pharmacotherapy NCIT:C15986

Standard anti-seizure medication is used when seizures occur. No disease-modifying therapy for the beta-tubulin progenitor/migration mechanism is established.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Genetic counseling should use current TUBB nomenclature, explain the historical TUBB5 literature, and discuss de novo dominant inheritance with residual recurrence risk from possible parental mosaicism.

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Animal Models

1

tubb5 knockout zebrafish (CRISPR/Cas9)

A CRISPR/Cas9 tubb5-null zebrafish. Homozygotes die within two weeks post-fertilization with developmental delay, craniofacial malformation, uncoordinated movement, reduced locomotor activity, impaired photomotor response and increased seizure susceptibility. The model's distinctive contribution is a reversible mechanism: tubb5 loss derepresses Notch effectors that hold neural progenitors undifferentiated, and gamma-secretase inhibition rescues the phenotype.

Species

Zebrafish

Genotype

tubb5 homozygous CRISPR/Cas9 knockout

Publication

PMID:42241496

Pathograph links

Notch-Dependent Block of Neural Progenitor Differentiation recapitulates fidelity: moderate The model is the sole source of evidence for this node: it establishes the Notch-dependent differentiation block and shows it is pharmacologically reversible.

Limitations: A homozygous null, whereas human disease alleles are heterozygous missense variants shown to act by gain-of-function rather than loss-of-function; a null and a dose-dependent gain-of-function allele need not converge on the same downstream pathway. Zebrafish larval neurodevelopment also lacks the expanded outer subventricular zone of human cortex.

Readouts

Notch effector transcript levels (notch1a, her5)

Notch signaling pathway GO:0007219 ↑ INCREASED

Direction: INCREASED

Interpretation: Transcriptomic signature of Notch derepression in tubb5-null larvae.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"Transcriptome analysis revealed that tubb5 knockout larvae showed upregulated expression of notch1a and her5, which inhibit neural progenitor differentiation."

Reports the measured direction of change for this readout.

Differentiated neuron versus progenitor marker counts

Direction: DECREASED

Interpretation: Fewer neurogenin1/huc-positive differentiated neurons with more sox2-positive progenitors - the cellular readout of the block.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"Whole-mount in situ hybridization demonstrated decreased neurogenin1 and huc positive neurons, while sox2 positive neurons were increased in tubb5 mutants, indicating disrupted neural progenitor differentiation."

Reports the in situ measurement behind this readout.

Developmental and locomotor phenotype under gamma-secretase inhibition

Direction: RESTORED

Interpretation: DAPT rescue establishes that the differentiation block is Notch-dependent and reversible in this model.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"DAPT treatment rescued both developmental and locomotor deficits tubb5 knockout zebrafish."

The rescue arm of the model, measured as developmental and locomotor recovery.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"This study demonstrates that tubb5 regulates neural progenitor differentiation via the Notch signaling pathway"

Supports treating this model as informative for the Notch-dependent differentiation node.

Microcephaly and Structural Brain Abnormalities partially recapitulates fidelity: low Reproduces a broadly abnormal neurodevelopmental phenotype - developmental delay, craniofacial malformation, uncoordinated movement and increased seizure susceptibility - but not the defining human readout.

Limitations: No microcephaly, cortical dysgyria or basal ganglia dysmorphism is reported: zebrafish have no gyrencephalic cortex and no upper-layer neocortical neurons for the human mechanism to act on. Homozygotes are lethal within two weeks post-fertilization, so there is no adult course to compare with the human natural history.

Readouts

Larval seizure susceptibility and locomotor activity

Direction: ALTERED

Interpretation: A functional neurodevelopmental readout standing in for the human epilepsy and motor phenotype, not for the malformation itself.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"Homozygous mutants were lethality within two weeks post-fertilization along with developmental delay, craniofacial malformations, uncoordinated movement and increased seizure susceptibility."

Reports the phenotypes actually measured, which cover the functional but not the structural side of the human presentation.

Show evidence (1 reference)

PMID:42241496 SUPPORT Model Organism

"Mutations in TUBB5 cause complex cortical dysplasia with other brain malformations. However, the pathogenic mechanisms underlying TUBB5 mutations remain incompletely understood."

The model is offered as a mechanism-discovery system for TUBB5 disease; it is recorded here as only partially recapitulating the human malformation phenotype.

{ }

Source YAML

click to show

name: TUBB/TUBB5-related Microcephaly
creation_date: "2026-06-12T03:31:46Z"
category: Mendelian
disease_term:
  preferred_term: complex cortical dysplasia with other brain malformations 6
  term:
    id: MONDO:0014341
    label: complex cortical dysplasia with other brain malformations 6
description: >-
  TUBB/TUBB5-related microcephaly is a beta-tubulin neurodevelopmental disorder
  caused by heterozygous de novo missense variants in the gene historically
  reported as TUBB5 and currently represented by HGNC as TUBB. It is modeled as a
  distinct disease entry because its skeleton is centered on embryonic
  neurogenesis and cortical size: altered beta-tubulin folding, heterodimer
  assembly, and microtubule function perturb neural progenitor cell-cycle
  progression, spindle orientation, neurogenic division, and neuronal migration;
  in mouse models, the microcephaly mechanism includes p53-associated apoptosis
  and loss of upper-layer neurons. The human phenotype combines congenital or
  early microcephaly with structural brain abnormalities such as dysmorphic
  basal ganglia, corpus callosum abnormality, simplified or polymicrogyria-like
  cortical malformation, and variable brainstem/cerebellar involvement, together
  with developmental delay, intellectual disability, motor and speech delay, and
  occasional seizures. This is therefore split from TUBB2A/TUBB2B cortical
  malformation and TUBB3 disease, while sharing module-level microtubule
  apparatus biology with the broader tubulinopathy family.
parents:
- congenital nervous system disorder
- disorder of development or morphogenesis
- hereditary neurological disease
- primary microcephaly
references:
- reference: PMID:23246003
  title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
- reference: PMID:26903504
  title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
- reference: PMID:24860126
  title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
- reference: PMID:28111201
  title: Human iPSC-Derived Cerebral Organoids Model Cellular Features of Lissencephaly and Reveal Prolonged Mitosis of Outer Radial Glia.
- reference: PMID:41309602
  title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
- reference: PMID:42091926
  title: "Bridging the gap: an emerging link between tubulinopathies and ciliopathies."
- reference: PMID:42241496
  title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
- reference: PMID:41152456
  title: "Expanding genetic and clinical spectra of β-tubulinopathies: A Korean study."
- reference: PMID:42015805
  title: "Tubulinopathy Case Series: Marked Intrafamilial Phenotypic Variability Associated With a Novel Missense TUBB Variant."
- reference: PMID:42589608
  title: "Restoring the Balance: CRISPRa-Driven β-Tubulin Compensation as a Strategy for Tubulinopathy Treatment."

inheritance:
- name: Autosomal dominant inheritance
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the occurrence of three microcephalic patients with structural
      brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I,
      and E401K).
    explanation: >-
      The founding human case series reports de novo TUBB5 missense variants in
      three unrelated affected individuals, supporting dominant, typically
      sporadic inheritance.
pathophysiology:
- name: Altered TUBB/TUBB5 Beta-Tubulin Function
  conforms_to: microtubule_dependent_neuronal_migration_failure#Microtubule Apparatus Perturbation
  description: >-
    Pathogenic variants in the beta-tubulin isotype historically named TUBB5
    alter chaperone-dependent tubulin heterodimer assembly and microtubule
    function. The upstream lesion is not simply a generic microcephaly label:
    different missense variants perturb the beta-tubulin folding/assembly
    machinery in different ways and feed both progenitor cell-cycle and neuronal
    migration branches.
  biological_scale: MOLECULAR
  genetic_context:
    functional_impact_category: GAIN_OF_FUNCTION
    variant_origin: DE_NOVO
    zygosity: HETEROZYGOUS
    allele_type: missense
    description: >-
      Reported disease alleles are heterozygous de novo missense substitutions.
      Haploinsufficiency has been tested directly and excluded: TUBB-haploid
      hTERT-RPE1 cells ciliate normally, and the G308S, E108K and E401K patient
      variants only produce their ciliation defect when overexpressed in
      wild-type cells - the signature of a gain-of-function rather than a
      loss-of-function allele. GAIN_OF_FUNCTION is therefore recorded here
      instead of the DOMINANT_NEGATIVE category used for TUBA1A and TUBB3.
      Curators should note the qualification the same work makes: E401K may act
      through a dual mechanism, with loss-of-function effects contributing to
      microcephaly while gain-of-function effects drive defective neuronal
      positioning, so this category describes the best-resolved ciliation and
      positioning arm rather than every phenotype of every allele.
  genes:
  - preferred_term: TUBB (formerly TUBB5)
    term:
      id: hgnc:20778
      label: TUBB
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: migrating cortical neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: tubulin heterodimer assembly
    term:
      id: GO:0007021
      label: tubulin complex assembly
    modifier: DYSREGULATED
  - preferred_term: microtubule cytoskeleton organization
    term:
      id: GO:0000226
      label: microtubule cytoskeleton organization
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      These mutant proteins, which affect the chaperone-dependent assembly of
      tubulin heterodimers in different ways
    explanation: >-
      Establishes variant-specific disruption of chaperone-dependent tubulin
      heterodimer assembly as the upstream beta-tubulin lesion.
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Tubb5 is expressed in neurogenic progenitors in the mouse and that its
      depletion in vivo perturbs the cell cycle of progenitors and alters the
      position of migrating neurons.
    explanation: >-
      Separately classifies the in vivo mouse depletion data supporting
      progenitor cell-cycle and neuronal-position effects.
  - reference: PMID:41309602
    reference_title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Furthermore, we demonstrate that the disease mechanism is not
      haploinsufficiency and that other patient mutations in TUBB affect cilia
      formation, putting forward defective ciliogenesis as a contributing
      pathogenic factor in a subset of tubulinopathy patients.
    explanation: >-
      Direct test excluding haploinsufficiency as the TUBB disease mechanism,
      which is the basis for the GAIN_OF_FUNCTION functional_impact_category on
      this node, and the link to the ciliogenesis branch below.
  downstream:
  - target: Progenitor Cell-Cycle and Spindle Dysfunction
    description: >-
      Altered beta-tubulin function perturbs neural progenitor cell-cycle
      progression and spindle orientation.
  - target: Impaired Neurogenic Division and Migration
    description: >-
      Altered beta-tubulin function also changes the position of migrating
      neurons and disrupts migration-dependent cortical organization.
  - target: Impaired Ciliogenesis and Reduced Ciliation
    description: >-
      A subset of TUBB variants additionally impairs the early stages of primary
      cilium assembly, a branch that is parallel to - not downstream of - the
      progenitor and migration arms.
  - target: Notch-Dependent Block of Neural Progenitor Differentiation
    description: >-
      Loss of tubb5 function derepresses Notch target genes that hold neural
      progenitors in an undifferentiated state, an arm shown so far only in
      zebrafish.
- name: Progenitor Cell-Cycle and Spindle Dysfunction
  conforms_to: neural_progenitor_centrosome_spindle_dysfunction#Abnormal Progenitor Division and Fate Choice
  description: >-
    Tubb/TUBB5 dysfunction affects neurogenic progenitors during corticogenesis.
    In the founding study, Tubb5 depletion perturbed progenitor cell-cycle
    progression and altered neuron position; in the knock-in mouse model, the
    E401K allele produced delayed cell-cycle progression, ectopic progenitor
    markers, and spindle-orientation defects. This progenitor branch is the main
    reason to model TUBB/TUBB5-related microcephaly separately from beta-tubulin
    disorders whose central skeleton is postmitotic neuronal migration.
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: radial glial cell
    term:
      id: CL:0000681
      label: radial glial cell
  biological_processes:
  - preferred_term: mitotic cell cycle
    term:
      id: GO:0000278
      label: mitotic cell cycle
    modifier: DYSREGULATED
  - preferred_term: mitotic spindle organization
    term:
      id: GO:0007052
      label: mitotic spindle organization
    modifier: DYSREGULATED
  - preferred_term: cell division
    term:
      id: GO:0051301
      label: cell division
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Tubb5 is expressed in neurogenic progenitors in the mouse and that its
      depletion in vivo perturbs the cell cycle of progenitors and alters the
      position of migrating neurons.
    explanation: >-
      Shows that Tubb5 acts in neurogenic progenitors and that depletion alters
      progenitor cell cycle and migrating neuron position.
  - reference: PMID:26903504
    reference_title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      associated with a delay in cell cycle progression and ectopic DNA elements
      in progenitors
    explanation: >-
      Supports delayed progenitor cell-cycle progression as a Tubb5-dependent
      mechanism in mouse.
  - reference: PMID:26903504
    reference_title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      we report ectopic Sox2-positive progenitors and defects in spindle
      orientation
    explanation: >-
      Supports progenitor mislocalization and spindle-orientation defects in
      the Tubb5 E401K knock-in model.
  downstream:
  - target: p53-Associated Apoptosis and Progenitor Pool Distortion
    description: >-
      Cell-cycle and spindle defects trigger cell death and change the
      progenitor and neuronal output of the developing cortex.
- name: p53-Associated Apoptosis and Progenitor Pool Distortion
  conforms_to: neural_progenitor_centrosome_spindle_dysfunction#Progenitor Pool Distortion
  description: >-
    Tubb5 mouse models show that the microcephaly branch is driven by abnormal
    cell survival and progenitor-output effects, not only by altered neuronal
    migration. The E401K knock-in and loss-of-function models produce profound
    microcephaly with loss of upper-layer neurons, massive apoptosis, and p53
    upregulation, making p53-associated cell death a central mechanistic branch.
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: cortical neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: apoptotic process
    term:
      id: GO:0006915
      label: apoptotic process
    modifier: INCREASED
  - preferred_term: maintenance of cell number
    term:
      id: GO:0098727
      label: maintenance of cell number
    modifier: DECREASED
  - preferred_term: neurogenesis
    term:
      id: GO:0022008
      label: neurogenesis
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:26903504
    reference_title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      present with profound microcephaly due to a loss of upper-layer neurons
      that correlates with massive apoptosis and upregulation of p53.
    explanation: >-
      Demonstrates the mouse microcephaly mechanism: upper-layer neuron loss
      with apoptosis and p53 upregulation.
  - reference: PMID:26903504
    reference_title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      demonstrates that the cellular pathology driving TUBB5-associated
      microcephaly is cell death.
    explanation: >-
      Directly identifies cell death as the cellular pathology driving the
      TUBB5-associated microcephaly model.
  downstream:
  - target: Microcephaly and Structural Brain Abnormalities
    description: >-
      Progenitor-pool distortion and apoptosis reduce cortical neuron output and
      produce the small-brain phenotype with associated structural
      malformations.
- name: Impaired Neurogenic Division and Migration
  conforms_to: microtubule_dependent_neuronal_migration_failure#Microtubule-Based Neuronal Motility Failure
  description: >-
    TUBB/TUBB5 disease also has a migration and cortical-organization branch:
    Tubb5 depletion alters the position of migrating neurons, and tubulinopathy
    cohorts place TUBB5 among genes whose malformations include focal or
    multifocal polymicrogyria-like cortical dysplasia and simplified gyration.
    This branch is secondary to the progenitor-centered microcephaly skeleton
    but remains part of the pathograph.
  cell_types:
  - preferred_term: migrating cortical neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: neuron migration
    term:
      id: GO:0001764
      label: neuron migration
    modifier: DECREASED
  - preferred_term: cerebral cortex development
    term:
      id: GO:0021987
      label: cerebral cortex development
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      disrupt neurogenic division and/or migration in vivo.
    explanation: >-
      Links TUBB5 mutant proteins to disrupted neurogenic division and
      migration in vivo.
  - reference: PMID:24860126
    reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      TUBB3 and TUBB5 mutations cause milder malformations with focal or
      multifocal polymicrogyria-like cortical dysplasia with abnormal and
      simplified gyral pattern.
    explanation: >-
      Places TUBB5 in the tubulinopathy subgroup with focal/multifocal
      polymicrogyria-like cortical dysplasia and simplified gyration.
  downstream:
  - target: Microcephaly and Structural Brain Abnormalities
    description: >-
      Migration and cortical organization defects contribute to the cortical
      dysgyria and structural brain abnormalities observed in affected
      individuals.
- name: Impaired Ciliogenesis and Reduced Ciliation
  conforms_to: ciliopathy_dysfunction#Basal Body and Transition Zone Dysfunction
  biological_scale: CELLULAR
  description: >-
    A subset of TUBB variants impairs the early stages of primary cilium
    assembly. Because the ciliary axoneme is itself built from microtubules, the
    beta-tubulin lesion acts on ciliogenesis directly, without any basal body,
    transition zone or intraflagellar-transport gene defect. Patient-derived
    cells and gene-edited isogenic lines carrying G308S show reduced ciliation
    through altered microtubule dynamics and structure; knock-in mice show
    decreased ciliation in cerebellum and kidney; and E108K and E401K reproduce
    the defect. The mechanism is dose-dependent gain-of-function, not
    haploinsufficiency - TUBB-haploid cells ciliate normally - so this branch is
    allele-specific rather than a property of every TUBB variant. It is what
    connects this entry to the ciliopathy spectrum, and the patients carrying
    these alleles show ciliopathy-like brain features on MRI.
  cell_types:
  - preferred_term: ciliated cell
    term:
      id: CL:0000064
      label: ciliated cell
  biological_processes:
  - preferred_term: cilium assembly
    term:
      id: GO:0060271
      label: cilium assembly
    modifier: DECREASED
  - preferred_term: microtubule cytoskeleton organization
    term:
      id: GO:0000226
      label: microtubule cytoskeleton organization
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:41309602
    reference_title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Using patient-derived cells and gene-edited isogenic cell lines, we show
      that the identified variant impairs the early stages of cilia formation by
      altering microtubule dynamics and structure.
    explanation: >-
      Establishes impaired early ciliogenesis as a TUBB disease mechanism in
      human patient-derived and isogenic cell models.
  - reference: PMID:41309602
    reference_title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Consistent with this, mice carrying the variant display decreased ciliation
      in the cerebellum and kidney.
    explanation: >-
      In vivo confirmation that the ciliation defect is reproduced in a
      mammalian knock-in model, in both a CNS and a non-CNS ciliated tissue.
  - reference: PMID:41309602
    reference_title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we present a de novo heterozygous missense variant (p.G308S) in
      Tubulin Beta Class I (TUBB) identified in a patient showing features of
      both ciliopathy and tubulinopathy.
    explanation: >-
      The index patient shows a mixed ciliopathy/tubulinopathy presentation,
      which is why this branch is curated as part of the TUBB pathograph rather
      than as an unrelated observation.
  - reference: PMID:42091926
    reference_title: "Bridging the gap: an emerging link between tubulinopathies and ciliopathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Since cilia rely on microtubules for ciliogenesis, axoneme formation, and
      intraflagellar transport, defects in tubulin genes can directly disrupt
      ciliary function.
    explanation: >-
      Provides the general mechanistic rationale linking tubulin-gene defects to
      ciliary dysfunction, framing this branch within the wider
      tubulinopathy-ciliopathy overlap.
  downstream:
  - target: Microcephaly and Structural Brain Abnormalities
    description: >-
      Reduced ciliation in the developing brain contributes to the structural
      brain malformations, which in the reported patients include
      ciliopathy-like features.
- name: Notch-Dependent Block of Neural Progenitor Differentiation
  biological_scale: CELLULAR
  description: >-
    In a zebrafish tubb5 knockout, loss of the beta-tubulin isotype upregulates
    the Notch effectors notch1a and her5, which hold neural progenitors in an
    undifferentiated state: neurogenin1- and huc-positive neurons are reduced
    while sox2-positive cells increase. Pharmacological Notch inhibition with
    DAPT rescues the developmental and locomotor deficits, so the arm is
    reversible in that model. This is curated as a separate branch from the
    p53-apoptosis arm because it removes neurons by failing to make them rather
    than by killing them, and the two have different therapeutic implications.
    The evidence is zebrafish-only; see the
    gap_tubb_tubb5_notch_arm_human_translatability discussion.
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  biological_processes:
  - preferred_term: Notch signaling pathway
    term:
      id: GO:0007219
      label: Notch signaling pathway
    modifier: INCREASED
  - preferred_term: neuronal differentiation from neural progenitors
    term:
      id: GO:0030182
      label: neuron differentiation
    modifier: DECREASED
  evidence:
  - reference: PMID:42241496
    reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Transcriptome analysis revealed that tubb5 knockout larvae showed
      upregulated expression of notch1a and her5, which inhibit neural
      progenitor differentiation.
    explanation: >-
      Identifies Notch-pathway upregulation as the transcriptional signature of
      tubb5 loss and the mechanism holding progenitors undifferentiated.
  - reference: PMID:42241496
    reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Whole-mount in situ hybridization demonstrated decreased neurogenin1 and
      huc positive neurons, while sox2 positive neurons were increased in tubb5
      mutants, indicating disrupted neural progenitor differentiation.
    explanation: >-
      Provides the cellular readout of the differentiation block - fewer
      differentiated neurons, more retained progenitors.
  downstream:
  - target: Microcephaly and Structural Brain Abnormalities
    description: >-
      Failure of neural progenitors to differentiate reduces the number of
      neurons produced, contributing to reduced brain size in the model.
- name: Microcephaly and Structural Brain Abnormalities
  conforms_to: neural_progenitor_centrosome_spindle_dysfunction#Abnormal Cortical Neuron Output and Gyration
  description: >-
    The clinical-anatomical endpoint is microcephaly with structural brain
    abnormalities: dysmorphic basal ganglia, corpus callosum anomalies,
    simplified or polymicrogyria-like gyral pattern, and variable hindbrain
    involvement. This node is the shared endpoint of the progenitor apoptosis
    branch and the microtubule-dependent neuronal migration branch.
  locations:
  - preferred_term: cerebral cortex
    term:
      id: UBERON:0000956
      label: cerebral cortex
  - preferred_term: basal ganglion
    term:
      id: UBERON:0002420
      label: basal ganglion
  biological_processes:
  - preferred_term: cerebral cortex development
    term:
      id: GO:0021987
      label: cerebral cortex development
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the occurrence of three microcephalic patients with structural
      brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I,
      and E401K).
    explanation: >-
      Directly establishes the human endpoint: microcephaly with structural
      brain abnormalities due to de novo TUBB5 variants.
  - reference: PMID:24860126
    reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      three in TUBB5 (2.8%)
    explanation: >-
      Shows that TUBB5 is a rare but recognized tubulinopathy gene in a large
      complex cortical malformation cohort.
  downstream:
  - target: Microcephaly
    causal_link_type: DIRECT
    description: The structural brain endpoint includes the defining microcephaly phenotype in TUBB/TUBB5 disease.
    evidence:
    - reference: PMID:23246003
      reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We report the occurrence of three microcephalic patients with structural
        brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I,
        and E401K).
      explanation: This directly establishes microcephaly with structural brain abnormalities in human TUBB5 patients.
  - target: Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Abnormal cortical gyration downstream of beta-tubulin neurogenesis and migration defects
    description: The endpoint structural brain abnormality includes the simplified/polymicrogyria-like cortical malformation pattern.
    evidence:
    - reference: PMID:24860126
      reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        TUBB3 and TUBB5 mutations cause milder malformations with focal or
        multifocal polymicrogyria-like cortical dysplasia with abnormal and
        simplified gyral pattern.
      explanation: This supports the TUBB5 polymicrogyria-like/simplified gyral phenotype.
  - target: Abnormal Basal Ganglia Morphology
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Tubulinopathy-associated deep grey matter dysmorphogenesis
    description: Dysmorphic basal ganglia are represented as an extracortical structural abnormality downstream of tubulinopathy brain malformation.
    evidence:
    - reference: PMID:24860126
      reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Dysmorphic basal ganglia are the hallmark of tubulinopathies (found in
        75% of cases)
      explanation: This supports basal-ganglia dysmorphism as a tubulinopathy structural-brain endpoint.
phenotypes:
- name: Microcephaly
  description: >-
    Microcephaly is the defining head-size phenotype in the founding human
    TUBB5 series and is recapitulated as profound microcephaly in Tubb5 mouse
    models.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the occurrence of three microcephalic patients with structural
      brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I,
      and E401K).
    explanation: >-
      Establishes microcephaly as the defining phenotype in the human TUBB5
      case series.
- name: Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern
  description: >-
    TUBB5 is associated with milder focal or multifocal polymicrogyria-like
    cortical dysplasia and simplified gyration within the tubulinopathy cortical
    malformation spectrum.
  phenotype_term:
    preferred_term: Polymicrogyria
    term:
      id: HP:0002126
      label: Polymicrogyria
  evidence:
  - reference: PMID:24860126
    reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      TUBB3 and TUBB5 mutations cause milder malformations with focal or
      multifocal polymicrogyria-like cortical dysplasia with abnormal and
      simplified gyral pattern.
    explanation: >-
      Supports the cortical dysplasia/simplified gyral phenotype for TUBB5.
- name: Abnormal Basal Ganglia Morphology
  description: >-
    Dysmorphic basal ganglia are a hallmark of tubulinopathies and are part of
    the structural brain abnormality pattern that should be assessed in
    TUBB/TUBB5-related microcephaly.
  phenotype_term:
    preferred_term: Abnormal basal ganglia morphology
    term:
      id: HP:0002134
      label: Abnormal basal ganglia morphology
  evidence:
  - reference: PMID:24860126
    reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Dysmorphic basal ganglia are the hallmark of tubulinopathies (found in 75%
      of cases)
    explanation: >-
      Establishes dysmorphic basal ganglia as a tubulinopathy imaging hallmark.
- name: Corpus Callosum Abnormality
  description: >-
    Corpus callosum agenesis or dysmorphism is a frequent tubulinopathy
    structural brain abnormality and was part of the TUBB5 case-image summary in
    Falcon research.
  phenotype_term:
    preferred_term: Abnormal corpus callosum morphology
    term:
      id: HP:0001273
      label: Abnormal corpus callosum morphology
  evidence:
  - reference: PMID:24860126
    reference_title: "The wide spectrum of tubulinopathies: what are the key features for the diagnosis?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Tubulinopathies are also characterized by a high prevalence of corpus
      callosum agenesis (32/80; 40%)
    explanation: >-
      Documents corpus callosum agenesis as a common tubulinopathy structural
      feature.
- name: Global Developmental Delay / Intellectual Disability
  description: >-
    Developmental and cognitive impairment are expected clinical outcomes of
    TUBB/TUBB5-related microcephaly with structural brain abnormalities.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
genetic:
- name: TUBB
  association: Causative
  gene_term:
    preferred_term: TUBB (formerly TUBB5)
    term:
      id: hgnc:20778
      label: TUBB
  evidence:
  - reference: PMID:23246003
    reference_title: Mutations in the beta-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the occurrence of three microcephalic patients with structural
      brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I,
      and E401K).
    explanation: >-
      The historical TUBB5 symbol in the primary article maps to current HGNC
      symbol TUBB; the case series directly establishes causative variants in
      this gene.
treatments:
- name: Supportive and Rehabilitative Care
  description: >-
    Management is supportive, including developmental, physical, occupational,
    speech/language, feeding, vision/oculomotor, and epilepsy surveillance
    services tailored to the individual's structural brain malformation and
    neurodevelopmental needs.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
- name: Anti-Seizure Medication
  description: >-
    Standard anti-seizure medication is used when seizures occur. No
    disease-modifying therapy for the beta-tubulin progenitor/migration
    mechanism is established.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
- name: Genetic Counseling
  description: >-
    Genetic counseling should use current TUBB nomenclature, explain the
    historical TUBB5 literature, and discuss de novo dominant inheritance with
    residual recurrence risk from possible parental mosaicism.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
animal_models:
- name: tubb5 knockout zebrafish (CRISPR/Cas9)
  species: Zebrafish
  genotype: tubb5 homozygous CRISPR/Cas9 knockout
  publication: PMID:42241496
  description: >-
    A CRISPR/Cas9 tubb5-null zebrafish. Homozygotes die within two weeks
    post-fertilization with developmental delay, craniofacial malformation,
    uncoordinated movement, reduced locomotor activity, impaired photomotor
    response and increased seizure susceptibility. The model's distinctive
    contribution is a reversible mechanism: tubb5 loss derepresses Notch
    effectors that hold neural progenitors undifferentiated, and gamma-secretase
    inhibition rescues the phenotype.
  modeled_mechanisms:
  - target: Notch-Dependent Block of Neural Progenitor Differentiation
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      The model is the sole source of evidence for this node: it establishes the
      Notch-dependent differentiation block and shows it is pharmacologically
      reversible.
    limitations: >-
      A homozygous null, whereas human disease alleles are heterozygous missense
      variants shown to act by gain-of-function rather than loss-of-function; a
      null and a dose-dependent gain-of-function allele need not converge on the
      same downstream pathway. Zebrafish larval neurodevelopment also lacks the
      expanded outer subventricular zone of human cortex.
    readouts:
    - name: Notch effector transcript levels (notch1a, her5)
      target: Notch-Dependent Block of Neural Progenitor Differentiation
      direction: INCREASED
      interpretation: >-
        Transcriptomic signature of Notch derepression in tubb5-null larvae.
      biological_processes:
      - preferred_term: Notch signaling pathway
        term:
          id: GO:0007219
          label: Notch signaling pathway
        modifier: INCREASED
      evidence:
      - reference: PMID:42241496
        reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Transcriptome analysis revealed that tubb5 knockout larvae showed
          upregulated expression of notch1a and her5, which inhibit neural
          progenitor differentiation.
        explanation: Reports the measured direction of change for this readout.
    - name: Differentiated neuron versus progenitor marker counts
      target: Notch-Dependent Block of Neural Progenitor Differentiation
      direction: DECREASED
      interpretation: >-
        Fewer neurogenin1/huc-positive differentiated neurons with more
        sox2-positive progenitors - the cellular readout of the block.
      evidence:
      - reference: PMID:42241496
        reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Whole-mount in situ hybridization demonstrated decreased neurogenin1
          and huc positive neurons, while sox2 positive neurons were increased in
          tubb5 mutants, indicating disrupted neural progenitor differentiation.
        explanation: Reports the in situ measurement behind this readout.
    - name: Developmental and locomotor phenotype under gamma-secretase inhibition
      target: Notch-Dependent Block of Neural Progenitor Differentiation
      direction: RESTORED
      interpretation: >-
        DAPT rescue establishes that the differentiation block is Notch-dependent
        and reversible in this model.
      evidence:
      - reference: PMID:42241496
        reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          DAPT treatment rescued both developmental and locomotor deficits tubb5
          knockout zebrafish.
        explanation: The rescue arm of the model, measured as developmental and locomotor recovery.
    evidence:
    - reference: PMID:42241496
      reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        This study demonstrates that tubb5 regulates neural progenitor
        differentiation via the Notch signaling pathway
      explanation: >-
        Supports treating this model as informative for the Notch-dependent
        differentiation node.
  - target: Microcephaly and Structural Brain Abnormalities
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    description: >-
      Reproduces a broadly abnormal neurodevelopmental phenotype - developmental
      delay, craniofacial malformation, uncoordinated movement and increased
      seizure susceptibility - but not the defining human readout.
    limitations: >-
      No microcephaly, cortical dysgyria or basal ganglia dysmorphism is
      reported: zebrafish have no gyrencephalic cortex and no upper-layer
      neocortical neurons for the human mechanism to act on. Homozygotes are
      lethal within two weeks post-fertilization, so there is no adult course to
      compare with the human natural history.
    readouts:
    - name: Larval seizure susceptibility and locomotor activity
      target: Microcephaly and Structural Brain Abnormalities
      direction: ALTERED
      interpretation: >-
        A functional neurodevelopmental readout standing in for the human
        epilepsy and motor phenotype, not for the malformation itself.
      evidence:
      - reference: PMID:42241496
        reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Homozygous mutants were lethality within two weeks post-fertilization
          along with developmental delay, craniofacial malformations,
          uncoordinated movement and increased seizure susceptibility.
        explanation: >-
          Reports the phenotypes actually measured, which cover the functional
          but not the structural side of the human presentation.
    evidence:
    - reference: PMID:42241496
      reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Mutations in TUBB5 cause complex cortical dysplasia with other brain
        malformations. However, the pathogenic mechanisms underlying TUBB5
        mutations remain incompletely understood.
      explanation: >-
        The model is offered as a mechanism-discovery system for TUBB5 disease;
        it is recorded here as only partially recapitulating the human
        malformation phenotype.

discussions:
- discussion_id: gap_tubb_tubb5_human_organoid_translatability
  prompt: >-
    Are the progenitor cell-cycle, spindle-orientation, apoptosis, and migration
    defects observed in mouse Tubb5 models conserved in human radial glia,
    outer-radial-glia-like cells, and cortical organoids carrying pathogenic
    TUBB/TUBB5 variants?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Progenitor Cell-Cycle and Spindle Dysfunction
  - pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion
  - pathophysiology#Impaired Neurogenic Division and Migration
  rationale: >-
    The strongest mechanistic evidence is mouse based, while the Falcon report
    did not identify a direct TUBB/TUBB5 human iPSC or organoid disease model.
    Human cortical expansion depends on outer radial glia and OSVZ biology, so
    a human organoid/iPSC test is needed before assuming the mouse progenitor
    apoptosis branch fully explains the human microcephaly phenotype.
  evidence:
  - reference: PMID:26903504
    reference_title: Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      cause TUBB5-associated microcephaly by exploiting two new mouse models
    explanation: >-
      Identifies the main mechanistic evidence source as mouse models, which
      motivates a human/model translatability gap.
  - reference: PMID:28111201
    reference_title: Human iPSC-Derived Cerebral Organoids Model Cellular Features of Lissencephaly and Reveal Prolonged Mitosis of Outer Radial Glia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Recent work has uncovered critical cellular and molecular differences
      between cortical development in humans and mice, further underscoring the
      need to develop human model systems.
    explanation: >-
      Supports organoid/iPSC testing when mouse cortical-development mechanisms
      are being translated to human disease.
  proposed_experiments:
  - experiment_id: exp_tubb_tubb5_isogenic_organoid_progenitor_apoptosis
    name: TUBB/TUBB5 isogenic cortical-organoid progenitor and apoptosis experiment
    description: >-
      Generate patient-derived or CRISPR-engineered human cortical organoids
      carrying M299V, V353I, E401K, or newer pathogenic TUBB/TUBB5 variants;
      compare mutant, isogenic-corrected, and knock-in organoids for
      microtubule dynamics, tubulin heterodimer assembly, progenitor cell-cycle
      timing, spindle orientation, p53 activation, apoptosis, neuronal output,
      and migration.
    experiment_type:
      preferred_term: isogenic cortical organoid progenitor perturbation experiment
    model_systems:
    - name: TUBB/TUBB5 human iPSC-derived cortical organoid
      description: >-
        Three-dimensional human cortical organoid carrying a pathogenic
        TUBB/TUBB5 variant, with matched isogenic correction and knock-in
        controls.
      experimental_model_type: ORGANOID
      namo_type: namo:Organoid
      organism:
        preferred_term: human
        term:
          id: NCBITaxon:9606
          label: Homo sapiens
      tissue_term:
        preferred_term: cerebral cortex
        term:
          id: UBERON:0000956
          label: cerebral cortex
      cell_types:
      - preferred_term: neural progenitor cell
        term:
          id: CL:0011020
          label: neural progenitor cell
      - preferred_term: radial glial cell
        term:
          id: CL:0000681
          label: radial glial cell
      conditions:
      - TUBB/TUBB5-related microcephaly
      - progenitor cell-cycle dysfunction
      - p53-associated apoptosis
      cell_source: Patient-derived or CRISPR-engineered human induced pluripotent stem cells
      culture_system: Three-dimensional cortical organoid with live imaging, immunostaining, and single-cell profiling
    perturbations:
    - name: Isogenic TUBB/TUBB5 variant correction or knock-in
      target: pathophysiology#Altered TUBB/TUBB5 Beta-Tubulin Function
      description: >-
        Correct a patient variant or introduce a pathogenic variant to separate
        variant effects from donor background.
    readouts:
    - name: Progenitor mitosis, p53 activation, and apoptosis
      target: pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion
      description: >-
        Quantify cell-cycle length, spindle orientation, ectopic progenitor
        markers, p53 activation, apoptotic cells, and upper-layer neuron output.
      biological_processes:
      - preferred_term: mitotic cell cycle
        term:
          id: GO:0000278
          label: mitotic cell cycle
        modifier: DYSREGULATED
      - preferred_term: apoptotic process
        term:
          id: GO:0006915
          label: apoptotic process
        modifier: INCREASED
      assays:
      - preferred_term: immunostaining
      - preferred_term: single-cell transcriptomic profiling
      direction: POSITIVE
    controls:
    - name: Isogenic corrected organoids
      description: Patient-derived variant organoids corrected to the reference sequence.
    - name: Isogenic knock-in organoids
      description: Control-background organoids carrying the introduced pathogenic variant.
    decision_criterion: >-
      The mouse-derived mechanism is supported if human variant organoids show
      delayed progenitor cell cycle, spindle orientation defects, p53 activation,
      increased apoptosis, reduced upper-layer neuron output, and/or migration
      defects that are rescued by correction and reproduced by knock-in.
    would_support:
    - pathophysiology#Progenitor Cell-Cycle and Spindle Dysfunction
    - pathophysiology#p53-Associated Apoptosis and Progenitor Pool Distortion
    - pathophysiology#Impaired Neurogenic Division and Migration
- discussion_id: gap_tubb_tubb5_natural_history_and_phenotype_breadth
  prompt: >-
    What is the full natural-history and phenotype range for TUBB/TUBB5-related
    microcephaly beyond the three original cases and small tubulinopathy cohort
    counts?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - phenotypes#Global Developmental Delay / Intellectual Disability
  - phenotypes#Polymicrogyria-like Cortical Dysplasia / Simplified Gyral Pattern
  rationale: >-
    The entry is supported by a strong founding case series and model mechanism,
    but TUBB/TUBB5-specific longitudinal outcomes, seizure frequency, genotype-
    phenotype correlations, and variant-class effects remain much thinner than
    for TUBA1A or TUBB2B. Two 2025-2026 reports narrow the gap without closing
    it: a Korean beta-tubulinopathy cohort places TUBB alongside the other
    beta-tubulin genes in a series of only twelve patients across five genes,
    and a single family shows complete penetrance with markedly variable
    expressivity - including a transmitting mother judged healthy until MRI -
    which means published TUBB case counts are a floor, not an estimate.
  evidence:
  - reference: PMID:41152456
    reference_title: "Expanding genetic and clinical spectra of β-tubulinopathies: A Korean study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Ten pathogenic or likely pathogenic missense variants were identified in
      five β-tubulin genes (TUBB2A, TUBB2B, TUBB3, TUBB4A, and TUBB5).
    explanation: >-
      Shows that even a contemporary multi-gene cohort contributes only a
      handful of TUBB/TUBB5 patients, so gene-specific natural history remains
      underpowered.
  - reference: PMID:42015805
    reference_title: "Tubulinopathy Case Series: Marked Intrafamilial Phenotypic Variability Associated With a Novel Missense TUBB Variant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The variant showed complete penetrance but markedly variable expressivity,
      contributing to the broad clinical spectrum associated with TUBB-related
      disorders.
    explanation: >-
      Documents complete penetrance with markedly variable expressivity within a
      single family, the specific reason gene-level severity cannot yet be
      predicted from genotype.
  - reference: PMID:42015805
    reference_title: "Tubulinopathy Case Series: Marked Intrafamilial Phenotypic Variability Associated With a Novel Missense TUBB Variant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      After identification of the TUBB variant in both siblings and a presumably
      healthy mother, maternal MRI also demonstrated features consistent with
      tubulinopathy.
    explanation: >-
      A transmitting parent considered healthy was found on imaging to be
      affected, showing that ascertainment by severe presentation systematically
      undercounts the mild end of the TUBB phenotype range.
- discussion_id: gap_tubb_tubb5_notch_arm_human_translatability
  prompt: >-
    Does the Notch-dependent block of neural progenitor differentiation seen in
    tubb5-null zebrafish operate in human TUBB-related microcephaly, and is it
    pharmacologically reversible in a human model?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Notch-Dependent Block of Neural Progenitor Differentiation
  - pathophysiology#Microcephaly and Structural Brain Abnormalities
  rationale: >-
    The zebrafish arm is mechanistically attractive because it is reversible:
    gamma-secretase inhibition with DAPT rescued both developmental and
    locomotor deficits. But three features do not transfer automatically to the
    human disease. First, the fish model is a homozygous knockout, whereas human
    disease alleles are heterozygous missense variants whose mechanism has been
    shown to be gain-of-function rather than loss-of-function - a null and a
    dose-dependent gain-of-function allele need not converge on the same
    downstream pathway. Second, the mammalian mechanism established to date is
    p53-associated apoptosis of upper-layer neurons, which removes neurons that
    were made, while the Notch arm prevents them being made; whether both operate
    in humans, or whether one is species-specific, is untested. Third, DAPT
    rescue in a larval fish says nothing about a therapeutic window in a human
    brain whose malformation is established prenatally. Resolving this requires a
    human neural model carrying a patient TUBB missense allele.
  evidence:
  - reference: PMID:42241496
    reference_title: tubb5 knockout in zebrafish causes neurodevelopmental defects via notch pathways.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      DAPT treatment rescued both developmental and locomotor deficits tubb5
      knockout zebrafish.
    explanation: >-
      The pharmacological rescue that makes this arm interesting, and the result
      whose translatability to human TUBB missense disease is the open question.
  - reference: PMID:41309602
    reference_title: Mutations in the β-tubulin TUBB impair ciliogenesis and are associated with ciliopathy-like phenotypes.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Furthermore, we demonstrate that the disease mechanism is not
      haploinsufficiency and that other patient mutations in TUBB affect cilia
      formation, putting forward defective ciliogenesis as a contributing
      pathogenic factor in a subset of tubulinopathy patients.
    explanation: >-
      Establishes that human TUBB disease is not a loss-of-function mechanism,
      which is precisely why a homozygous-null zebrafish phenotype cannot be
      assumed to model it.
notes: >-
  Entry created from cortical-malformation epic 4098 (issue 4086), seeded from
  Romero, Bahi-Buisson & Francis 2018 and refined with Falcon deep research.
  The primary literature uses TUBB5; HGNC now resolves TUBB5 to current symbol
  TUBB (HGNC:20778), so this entry preserves both names in the title while using
  TUBB as the gene term. The disease is not lumped with generic primary
  microcephaly or generic tubulinopathy because its coherent skeleton combines
  beta-tubulin heterodimer/microtubule dysfunction with a progenitor cell-cycle,
  spindle, p53-apoptosis, and upper-layer neuron-output branch. It conforms
  mainly to the neural progenitor centrosome-spindle dysfunction module, with a
  secondary conformance to the microtubule-dependent neuronal migration module
  for the migration/cortical dysgyria branch. The Falcon report found no direct
  TUBB/TUBB5 organoid or iPSC disease model; the organoid experiment is included
  as a human/model mismatch knowledge gap, not as existing disease evidence.

📚

References & Deep Research