TRIO-Related Neurodevelopmental Disorder

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Mappings

MONDO

MONDO:0014892 micrognathia-recurrent infections-behavioral abnormalities-mild intellectual disability syndrome Not Yet Curated

MONDO:0032939 intellectual developmental disorder, autosomal dominant 63, with macrocephaly Not Yet Curated

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Inheritance

1

Autosomal dominant HP:0000006

Both MRD44 and MRD63 are inherited in an autosomal dominant manner. Pathogenic TRIO variants are most often de novo, though inherited cases (including a multigenerational family) have been reported.

Autosomal dominant inheritance

Show evidence (1 reference)

PMID:27418539 SUPPORT Human Clinical

"We identified a frameshift deletion in TRIO that segregated autosomal"

Documents autosomal dominant segregation of a pathogenic TRIO variant in a family.

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Subtypes

2

MRD44 (microcephaly, GEFD1 / loss-of-function) MONDO:0014892

TRIO hgnc:12303

Intellectual developmental disorder, autosomal dominant 44, with microcephaly. Associated with missense variants in the RAC1-activating GEFD1 domain and with protein-truncating (nonsense/frameshift) variants spread across TRIO, both producing a loss-of-function effect and reduced TRIO-mediated RAC1 activation. Phenotype is typically milder intellectual disability with microcephaly. OMIM: 617061.

Show evidence (1 reference)

PMID:27418539 SUPPORT Human Clinical

"intellectual disability, microcephaly and dysmorphism"

Establishes the microcephaly-associated GEFD1 (Rac-GEF domain) TRIO phenotype that corresponds to MRD44.

MRD63 (macrocephaly, seventh spectrin repeat / gain-of-function) MONDO:0032939

TRIO hgnc:12303

Intellectual developmental disorder, autosomal dominant 63, with macrocephaly. Associated with missense variants clustering in the seventh spectrin repeat domain that release autoinhibition and hyperactivate TRIO-mediated RAC1 signaling (gain-of-function). Phenotype is typically more severe intellectual disability with macrocephaly. A single recent case proposes p.Ile2580Phe, reported by its authors in the second SH3 domain, as an extension of this arm; the report supplies no functional assay and does not establish a new domain-level rule. OMIM: 618825.

Show evidence (2 references)

PMID:32109419 SUPPORT Human Clinical

"Within group 1, we demonstrate a consistent phenotype of severe intellectual disability and macrocephaly"

Establishes the seventh-spectrin-repeat TRIO phenotype of severe ID and macrocephaly that corresponds to MRD63.

PMID:42421094 SUPPORT Human Clinical

"Our findings broaden the variant landscape of TRIO, establish a correlation between macrocephaly and TRIO variants within the second SH3 domain"

A single case proposes a second-SH3-domain extension of the MRD63 arm. PARTIAL reflects the absence of replication or a functional assay and the report's own statement that the genotype-phenotype correlation remains unclear.

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Discussions and Knowledge Gaps

2

Do heterozygous mouse models of NDD-associated TRIO variants faithfully reproduce the human variant-class-specific, RAC1-direction-dependent head-size phenotypes (microcephaly for GEFD1 loss-of-function vs macrocephaly for spectrin-repeat gain-of-function)?

HUMAN MODEL MISMATCH OPEN gap_trio_hetero_model_headsize_mismatch

Attached to

pathophysiology#Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation

Complete Trio knockout is embryonic lethal and nervous-system-specific knockout mice show reduced brain size, so null models capture the loss-of-function/microcephaly arm but not the gain-of-function/macrocephaly arm. Recent heterozygous knock-in models of specific NDD variants show distinct, variant-dependent effects on behavior, brain size, and dendritic arbors, but their fidelity to the human genotype-phenotype correlation across both arms remains to be established.

Proposed experiments

Allelic heterozygous knock-in comparison of TRIO GEFD1 vs spectrin variants

exp_trio_allelic_knockin_headsize

Compare occipitofrontal/brain size, cortical neuron output, dendritic arborization, and RAC1 activation across allelic heterozygous knock-in mice carrying GEFD1 loss-of-function versus seventh spectrin repeat gain-of-function TRIO variants, benchmarked against the human head-size and ID severity data.

Why is intellectual disability consistently more severe in individuals with seventh spectrin repeat (gain-of-function) variants than in those with GEFD1 (loss-of-function) variants, despite both perturbing the same TRIO-RAC1 axis?

KNOWLEDGE GAP OPEN gap_trio_id_severity_asymmetry

Attached to

pathophysiology#Impaired Dendritic and Synaptic Morphogenesis

The cohort literature reports that ID severity is greater with spectrin variants than with GEFD1 variants, but the mechanistic basis for the asymmetry between hyper- and hypo-activation of RAC1 is not resolved.

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Pathophysiology

4

TRIO GEF Regulation of RAC1 Signaling

TRIO is a Dbl-family Rho guanine nucleotide exchange factor and a major regulator of neuronal development. Through its first GEF domain (GEFD1) it catalyzes GDP-to-GTP exchange on RAC1 (and RHOG), while its second GEF domain (GEFD2) activates RHOA. By controlling RAC1 activity and actin cytoskeleton remodeling, TRIO governs neuronal migration, axon outgrowth and guidance, dendritic arborization, and excitatory synaptogenesis. Tight bidirectional control of TRIO-RAC1 signaling is required for normal cortical development, and pathogenic TRIO variants perturb this axis in opposite directions depending on the affected domain.

Neuron CL:0000540

TRIO hgnc:12303

Rac protein signal transduction GO:0016601 ⚠ ABNORMAL Actin filament organization GO:0007015 ⚠ ABNORMAL

Downstream

GEFD1 Loss-of-Function and RAC1 Hypoactivation

Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation

Show evidence (2 references)

PMID:32109419 SUPPORT Human Clinical

"The first GEF domain (GEFD1) regulates RAC1 and RHOG activity, and the second GEF domain (GEFD2) regulates RHOA activity."

Establishes the domain architecture and the RAC1/RHOA GEF activities of TRIO.

PMID:27418539 SUPPORT Human Clinical

"Trio is a Dbl family guanine nucleotide exchange factor (GEF) and a major regulator of neuronal development, controlling actin cytoskeleton dynamics by activating the GTPase Rac1."

Confirms TRIO controls actin cytoskeleton dynamics by activating RAC1 in neuronal development.

GEFD1 Loss-of-Function and RAC1 Hypoactivation

Missense variants in the RAC1-activating GEFD1 domain, and protein-truncating variants across TRIO, reduce TRIO-mediated RAC1 binding and activation. In neuronal and heterologous models these loss-of-function variants impair neurite outgrowth and lamellipodia formation and reduce excitatory synaptic transmission. The hypoactivation of RAC1 is proposed to underlie the microcephaly and milder intellectual disability of the MRD44 arm.

Neuron CL:0000540

Rac protein signal transduction GO:0016601 ↓ DECREASED Dendritic spine morphogenesis GO:0060997 ⚠ ABNORMAL

Downstream

Impaired Dendritic and Synaptic Morphogenesis

Show evidence (2 references)

PMID:27418539 SUPPORT Human Clinical

"Trio mutations led to a markedly reduced Rac1 activation."

Demonstrates that GEFD1/Rac-GEF-domain TRIO mutations markedly reduce RAC1 activation (loss-of-function).

PMID:33167890 SUPPORT Human Clinical

"it is hypothesized that variants in the GEF1 domain lead to loss of normal RAC1 activation and thus the observed microcephaly"

Links GEF1-domain loss-of-function and reduced RAC1 activation to the microcephaly phenotype.

Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation

Missense variants clustering in the seventh spectrin repeat domain of TRIO disrupt the intramolecular autoinhibition of the GEF1 domain by the spectrin repeats, producing increased RAC1 activation with enhanced neurite outgrowth and lamellipodia formation. This TRIO-mediated RAC1 hyperactivation is proposed to underlie the macrocephaly and the more severe intellectual disability of the MRD63 arm, with a striking correlation observed between RAC1 activation level and head size.

Neuron CL:0000540

Rac protein signal transduction GO:0016601 ↑ INCREASED Neuron projection development GO:0031175 ⚠ ABNORMAL

Downstream

Impaired Dendritic and Synaptic Morphogenesis

Show evidence (2 references)

PMID:33167890 SUPPORT Human Clinical

"Several missense variants that cluster in the spectrin repeat domain have been functionally shown to lead to increased RAC1 activation and enhanced neurite outgrowth and lamellipodia formation"

Establishes spectrin-repeat missense variants as gain-of-function with increased RAC1 activation.

PMID:32109419 SUPPORT Human Clinical

"In contrast, individuals in group 1 presented with macrocephaly."

Ties the seventh-spectrin-repeat (group 1) gain-of-function variant class to macrocephaly.

Impaired Dendritic and Synaptic Morphogenesis

Both the hypo- and hyper-activation of RAC1 converge on abnormal actin-based remodeling of dendritic and synaptic structures. Dysregulated RAC1 control of actin polymerization at glutamatergic synapses disturbs dendritic arborization, spine morphogenesis, and excitatory synaptic transmission, providing a shared cellular substrate for the intellectual disability, speech delay, and neurobehavioral phenotype seen across TRIO-related disease despite the opposing head-size outcomes.

Neuron CL:0000540

Dendritic spine morphogenesis GO:0060997 ⚠ ABNORMAL Regulation of dendrite development GO:0050773 ⚠ ABNORMAL

Show evidence (1 reference)

PMID:28928363 SUPPORT In Vitro

"directly activating the small GTPase Rac1."

Establishes that TRIO promotes actin polymerization by directly activating RAC1, the process dysregulated at glutamatergic synapses.

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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

14

Head and Neck 3

Microcephaly HP:0000252

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"Microcephaly, which is defined by a head circumference two SD below the mean, was seen in 100% of group 2 patients"

Quantifies microcephaly in the GEFD1 (group 2) subtype corresponding to MRD44.

Macrocephaly HP:0000256

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"In contrast, individuals in group 1 presented with macrocephaly."

Documents macrocephaly in the spectrin-repeat (group 1) subtype corresponding to MRD63.

Recognizable Facial Dysmorphism Abnormal facial shape HP:0001999

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"Individuals with spectrin variants show a more severe developmental phenotype, macrocephaly, and statistically significant clustering of facial dysmorphism, whereas individuals with GEFD1 variants have a less severe developmental phenotype and microcephaly."

Supports a recognizable facial gestalt in the spectrin-repeat MRD63 arm.

Limbs 1

Toe Syndactyly HP:0001770

Show evidence (1 reference)

PMID:27418539 SUPPORT Human Clinical

"Common features among studied individuals include microcephaly (seen in patients 1–5), mild dysmorphic facies, tapering fingers with prominent proximal interphalangeal joints, fifth finger clinodactyly and 2/3 toe syndactyly."

Documents 2/3 toe syndactyly among the recurrent distal-limb findings.

Nervous System 10

Intellectual Disability HP:0001249

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"These individuals, except individuals 15 and 16, define patient group 2 and present with milder ID and microcephaly."

Documents the milder intellectual disability of the GEFD1 arm, contrasted with the more severe spectrin-repeat arm.

Global Developmental Delay HP:0001263

Show evidence (1 reference)

PMID:27418539 SUPPORT Human Clinical

"global developmental delay, microcephaly and mild dysmorphism"

Documents global developmental delay in the presenting TRIO family.

Delayed Speech and Language Development HP:0000750

Autistic Behavior HP:0000729

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"A neurobehavioral phenotype was observed in 19/24 (79%) individuals with a TRIO variant"

Documents the high frequency of a neurobehavioral phenotype (including autistic traits) across the cohort.

Aggressive Behavior HP:0000718

Hyperactivity HP:0000752

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"The reported clinical phenotypes of affected individuals with TRIO mutations include intellectual disability, behavioral difficulties such as hyperactivity or aggression, autism or autistic behavioral tendencies, skeletal hand anomalies, and microcephaly."

Hyperactivity is explicitly included in the reported TRIO neurobehavioral phenotype.

Stereotypy OCCASIONAL Motor stereotypy HP:0000733

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"Recurrent behavioral features seen across all individuals included stereotypies (27%), poor attention (70%), obsessive compulsive traits (45%), autistic traits (31%), and aggression (36%)."

Stereotypies occurred in 27% of the reported cohort.

Short Attention Span FREQUENT HP:0000736

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"Recurrent behavioral features seen across all individuals included stereotypies (27%), poor attention (70%), obsessive compulsive traits (45%), autistic traits (31%), and aggression (36%)."

Poor attention occurred in 70% of the reported cohort.

Obsessive-Compulsive Behavior FREQUENT Compulsive behaviors HP:0000722

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"Recurrent behavioral features seen across all individuals included stereotypies (27%), poor attention (70%), obsessive compulsive traits (45%), autistic traits (31%), and aggression (36%)."

Obsessive-compulsive traits occurred in 45% of the reported cohort.

Seizures OCCASIONAL HP:0001250

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"There was a diagnosis of epilepsy or evidence of seizure activity in five individuals (24%)."

Quantifies seizure occurrence in the cohort (5/24, 24%).

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Genetic Associations

1

TRIO (Causal)

Gene: TRIO hgnc:12303 relationship_type: CAUSATIVE

Show evidence (2 references)

PMID:32109419 SUPPORT Human Clinical

"The first GEF domain (GEFD1) regulates RAC1 and RHOG activity, and the second GEF domain (GEFD2) regulates RHOA activity."

Establishes TRIO domain architecture underlying the variant-class genotype-phenotype correlation.

PMID:33167890 SUPPORT Human Clinical

"it is hypothesized that missense variants in this domain cause hyperactivation of RAC1 leading to the observed macrocephaly phenotype"

Documents the spectrin-domain gain-of-function hyperactivation-macrocephaly correlation.

Variants (1)

NM_007118.4:c.7738A>T (p.Ile2580Phe), reported second SH3 domain Likely Pathogenic

Gene: TRIO hgnc:12303 missense variant

Novel missense variant reported in a male infant with developmental delay and macrocephaly and positioned by the authors in the second SH3 domain of TRIO. It is curated as a proposed extension of the MRD63/macrocephaly arm beyond the established seventh-spectrin-repeat hotspot, not as evidence for a new domain-level mechanism. The cached report does not document de novo occurrence and supplies no functional assay. Pathogenicity rests on bioinformatic prediction and three-dimensional protein modelling, so the classification is LIKELY_PATHOGENIC and no functional effect is asserted.

Show evidence (2 references)

PMID:42421094 SUPPORT Human Clinical

"We identified a novel TRIO missense variant (NM_007118.4: c.7738 A > T, p.I2580F; chr5:14492781 A > T/hg19) in the patient, and the variant was positioned in the second Src homology 3 (SH3) domain."

Primary report of the variant, its transcript-level and genomic coordinates, and the authors' domain assignment.

PMID:42421094 SUPPORT Computational

"Bioinformatic and three-dimensional protein modeling evidence all support the p.I2580F variant as likely pathogenic."

Basis for the LIKELY_PATHOGENIC call. The assessment is in silico; no functional assay of GEF activity or Rho GTPase signalling was performed.

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Medical Actions

2

Supportive Care

Action: Supportive Care NCIT:C15747

Symptomatic, multidisciplinary management of developmental delay, intellectual disability, speech impairment, behavioral difficulties, and seizures. No disease-modifying therapy exists.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Counseling regarding autosomal dominant inheritance, the high proportion of de novo variants, and recurrence risk, including for reported inherited cases.

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Prevalence

1

Worldwide

Cases In Literature Ultra Rare

A rare disorder; the largest cohort to date reported 24 individuals from 22 families with confirmed pathogenic TRIO variants, in addition to smaller series and case reports. No population-based prevalence estimate is established.

Show evidence (1 reference)

PMID:32109419 SUPPORT Human Clinical

"we report the largest international cohort of 24 individuals with"

Documents the size of the largest reported TRIO cohort, supporting rarity.

{ }

Source YAML

click to show

name: TRIO-Related Neurodevelopmental Disorder
creation_date: "2026-07-11T18:00:00Z"
category: Mendelian
description: >-
  TRIO-related neurodevelopmental disorder is an autosomal dominant
  neurodevelopmental condition caused by heterozygous pathogenic variants in
  TRIO (trio Rho guanine nucleotide exchange factor). TRIO is a large, highly
  conserved RhoGEF that uses its first GEF domain (GEFD1) to activate the small
  GTPase RAC1 (and RHOG) and its second GEF domain (GEFD2) to activate RHOA,
  thereby controlling actin cytoskeleton dynamics during neuronal migration,
  axon outgrowth and guidance, dendritic arborization, and synaptogenesis. The
  entry is curated as a single lumped disorder with two head-size subtypes
  because the opposing phenotypes are explained by one shared axis: the
  direction in which a variant perturbs TRIO-mediated RAC1 activity. Missense
  variants cluster into two hotspots with opposite functional consequences.
  Loss-of-function variants in the RAC1-activating GEFD1 (and protein-truncating
  variants across the gene) reduce RAC1 activation and are associated with
  milder intellectual disability and microcephaly (MRD44). Missense variants in
  the seventh spectrin repeat release autoinhibition and hyperactivate RAC1,
  producing more severe intellectual disability with macrocephaly (MRD63). A
  striking correlation is observed between RAC1 activation level and head size,
  so the same gene gives rise to two seemingly divergent clinical syndromes.
  Shared features across the spectrum include developmental delay, speech and
  language delay, a neurobehavioral phenotype (attention problems, autistic
  traits, aggression, stereotypies), and variable skeletal/hand and dental
  anomalies; seizures occur in a minority.
parents:
- Intellectual disability
- Neurodevelopmental disorder
notes: >-
  No single disease-level disease_term is assigned because TRIO-related disease
  is curated as a lumped entry spanning two distinct OMIM/MONDO entities that
  differ in the direction of effect on RAC1 and in head-size phenotype: MRD44
  (intellectual developmental disorder, autosomal dominant 44, with
  microcephaly; OMIM 617061; MONDO:0014892) and MRD63 (intellectual
  developmental disorder, autosomal dominant 63, with macrocephaly; OMIM 618825;
  MONDO:0032939). Both MONDO terms are recorded in mappings and on the
  corresponding subtypes. Initial curation scaffold created in response to issue
  issue 6107 (routed from lit-scan issue 6041); anchored on the genotype-phenotype cohort
  literature (Pengelly 2016, Schultz-Rogers 2020, Barbosa 2020) rather than the
  single-case seed lead.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0014892
      label: micrognathia-recurrent infections-behavioral abnormalities-mild intellectual disability syndrome
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
  - term:
      id: MONDO:0032939
      label: intellectual developmental disorder, autosomal dominant 63, with macrocephaly
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
has_subtypes:
- name: MRD44
  display_name: MRD44 (microcephaly, GEFD1 / loss-of-function)
  description: >-
    Intellectual developmental disorder, autosomal dominant 44, with
    microcephaly. Associated with missense variants in the RAC1-activating GEFD1
    domain and with protein-truncating (nonsense/frameshift) variants spread
    across TRIO, both producing a loss-of-function effect and reduced
    TRIO-mediated RAC1 activation. Phenotype is typically milder intellectual
    disability with microcephaly. OMIM: 617061.
  subtype_term:
    preferred_term: intellectual developmental disorder, autosomal dominant 44, with microcephaly
    term:
      id: MONDO:0014892
      label: micrognathia-recurrent infections-behavioral abnormalities-mild intellectual disability syndrome
  genes:
  - preferred_term: TRIO
    term:
      id: hgnc:12303
      label: TRIO
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "intellectual disability, microcephaly and dysmorphism"
    explanation: Establishes the microcephaly-associated GEFD1 (Rac-GEF domain) TRIO phenotype that corresponds to MRD44.
- name: MRD63
  display_name: MRD63 (macrocephaly, seventh spectrin repeat / gain-of-function)
  description: >-
    Intellectual developmental disorder, autosomal dominant 63, with
    macrocephaly. Associated with missense variants clustering in the seventh
    spectrin repeat domain that release autoinhibition and hyperactivate
    TRIO-mediated RAC1 signaling (gain-of-function). Phenotype is typically more
    severe intellectual disability with macrocephaly. A single recent case
    proposes p.Ile2580Phe, reported by its authors in the second SH3 domain, as
    an extension of this arm; the report supplies no functional assay and does
    not establish a new domain-level rule. OMIM: 618825.
  subtype_term:
    preferred_term: intellectual developmental disorder, autosomal dominant 63, with macrocephaly
    term:
      id: MONDO:0032939
      label: intellectual developmental disorder, autosomal dominant 63, with macrocephaly
  genes:
  - preferred_term: TRIO
    term:
      id: hgnc:12303
      label: TRIO
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Within group 1, we demonstrate a consistent phenotype of severe intellectual disability and macrocephaly"
    explanation: Establishes the seventh-spectrin-repeat TRIO phenotype of severe ID and macrocephaly that corresponds to MRD63.
  - reference: PMID:42421094
    reference_title: "Identification and growth features of developmental delay with macrocephaly caused by a novel TRIO variant affecting the second SH3 domain."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Our findings broaden the variant landscape of TRIO, establish a
      correlation between macrocephaly and TRIO variants within the second SH3
      domain
    explanation: >-
      A single case proposes a second-SH3-domain extension of the MRD63 arm.
      PARTIAL reflects the absence of replication or a functional assay and the
      report's own statement that the genotype-phenotype correlation remains
      unclear.
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    Both MRD44 and MRD63 are inherited in an autosomal dominant manner.
    Pathogenic TRIO variants are most often de novo, though inherited cases
    (including a multigenerational family) have been reported.
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified a frameshift deletion in TRIO that segregated autosomal"
    explanation: Documents autosomal dominant segregation of a pathogenic TRIO variant in a family.
pathophysiology:
- name: TRIO GEF Regulation of RAC1 Signaling
  description: >-
    TRIO is a Dbl-family Rho guanine nucleotide exchange factor and a major
    regulator of neuronal development. Through its first GEF domain (GEFD1) it
    catalyzes GDP-to-GTP exchange on RAC1 (and RHOG), while its second GEF
    domain (GEFD2) activates RHOA. By controlling RAC1 activity and actin
    cytoskeleton remodeling, TRIO governs neuronal migration, axon outgrowth and
    guidance, dendritic arborization, and excitatory synaptogenesis. Tight
    bidirectional control of TRIO-RAC1 signaling is required for normal cortical
    development, and pathogenic TRIO variants perturb this axis in opposite
    directions depending on the affected domain.
  genes:
  - preferred_term: TRIO
    term:
      id: hgnc:12303
      label: TRIO
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: Rac protein signal transduction
    term:
      id: GO:0016601
      label: Rac protein signal transduction
    modifier: ABNORMAL
  - preferred_term: Actin filament organization
    term:
      id: GO:0007015
      label: actin filament organization
    modifier: ABNORMAL
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The first GEF domain (GEFD1) regulates RAC1 and RHOG activity, and the second GEF domain (GEFD2) regulates RHOA activity."
    explanation: Establishes the domain architecture and the RAC1/RHOA GEF activities of TRIO.
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Trio is a Dbl family guanine nucleotide exchange factor (GEF) and a major regulator of neuronal development, controlling actin cytoskeleton dynamics by activating the GTPase Rac1."
    explanation: Confirms TRIO controls actin cytoskeleton dynamics by activating RAC1 in neuronal development.
  downstream:
  - target: GEFD1 Loss-of-Function and RAC1 Hypoactivation
  - target: Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation
- name: GEFD1 Loss-of-Function and RAC1 Hypoactivation
  description: >-
    Missense variants in the RAC1-activating GEFD1 domain, and protein-truncating
    variants across TRIO, reduce TRIO-mediated RAC1 binding and activation. In
    neuronal and heterologous models these loss-of-function variants impair
    neurite outgrowth and lamellipodia formation and reduce excitatory synaptic
    transmission. The hypoactivation of RAC1 is proposed to underlie the
    microcephaly and milder intellectual disability of the MRD44 arm.
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: Rac protein signal transduction
    term:
      id: GO:0016601
      label: Rac protein signal transduction
    modifier: DECREASED
  - preferred_term: Dendritic spine morphogenesis
    term:
      id: GO:0060997
      label: dendritic spine morphogenesis
    modifier: ABNORMAL
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Trio mutations led to a markedly reduced Rac1 activation."
    explanation: Demonstrates that GEFD1/Rac-GEF-domain TRIO mutations markedly reduce RAC1 activation (loss-of-function).
  - reference: PMID:33167890
    reference_title: "Novel loss-of-function variants in TRIO are associated with neurodevelopmental disorder: case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it is hypothesized that variants in the GEF1 domain lead to loss of normal RAC1 activation and thus the observed microcephaly"
    explanation: Links GEF1-domain loss-of-function and reduced RAC1 activation to the microcephaly phenotype.
  downstream:
  - target: Impaired Dendritic and Synaptic Morphogenesis
- name: Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation
  description: >-
    Missense variants clustering in the seventh spectrin repeat domain of TRIO
    disrupt the intramolecular autoinhibition of the GEF1 domain by the spectrin
    repeats, producing increased RAC1 activation with enhanced neurite outgrowth
    and lamellipodia formation. This TRIO-mediated RAC1 hyperactivation is
    proposed to underlie the macrocephaly and the more severe intellectual
    disability of the MRD63 arm, with a striking correlation observed between
    RAC1 activation level and head size.
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: Rac protein signal transduction
    term:
      id: GO:0016601
      label: Rac protein signal transduction
    modifier: INCREASED
  - preferred_term: Neuron projection development
    term:
      id: GO:0031175
      label: neuron projection development
    modifier: ABNORMAL
  evidence:
  - reference: PMID:33167890
    reference_title: "Novel loss-of-function variants in TRIO are associated with neurodevelopmental disorder: case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Several missense variants that cluster in the spectrin repeat domain have been functionally shown to lead to increased RAC1 activation and enhanced neurite outgrowth and lamellipodia formation"
    explanation: Establishes spectrin-repeat missense variants as gain-of-function with increased RAC1 activation.
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In contrast, individuals in group 1 presented with macrocephaly."
    explanation: Ties the seventh-spectrin-repeat (group 1) gain-of-function variant class to macrocephaly.
  downstream:
  - target: Impaired Dendritic and Synaptic Morphogenesis
- name: Impaired Dendritic and Synaptic Morphogenesis
  description: >-
    Both the hypo- and hyper-activation of RAC1 converge on abnormal actin-based
    remodeling of dendritic and synaptic structures. Dysregulated RAC1 control
    of actin polymerization at glutamatergic synapses disturbs dendritic
    arborization, spine morphogenesis, and excitatory synaptic transmission,
    providing a shared cellular substrate for the intellectual disability,
    speech delay, and neurobehavioral phenotype seen across TRIO-related disease
    despite the opposing head-size outcomes.
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: Dendritic spine morphogenesis
    term:
      id: GO:0060997
      label: dendritic spine morphogenesis
    modifier: ABNORMAL
  - preferred_term: Regulation of dendrite development
    term:
      id: GO:0050773
      label: regulation of dendrite development
    modifier: ABNORMAL
  evidence:
  - reference: PMID:28928363
    reference_title: "An autism spectrum disorder-related de novo mutation hotspot discovered in the GEF1 domain of Trio."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "directly activating the small GTPase Rac1."
    explanation: Establishes that TRIO promotes actin polymerization by directly activating RAC1, the process dysregulated at glutamatergic synapses.
phenotypes:
- category: Neurologic
  name: Microcephaly
  description: >-
    Reduced occipitofrontal head circumference (two or more SD below the mean),
    characteristic of the GEFD1/loss-of-function (MRD44) arm; microcephaly was
    seen in essentially all individuals with GEFD1 missense variants.
  subtype: MRD44
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microcephaly, which is defined by a head circumference two SD below the mean, was seen in 100% of group 2 patients"
    explanation: Quantifies microcephaly in the GEFD1 (group 2) subtype corresponding to MRD44.
- category: Neurologic
  name: Macrocephaly
  description: >-
    Enlarged occipitofrontal head circumference, characteristic of the seventh
    spectrin repeat/gain-of-function (MRD63) arm.
  subtype: MRD63
  phenotype_term:
    preferred_term: Macrocephaly
    term:
      id: HP:0000256
      label: Macrocephaly
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In contrast, individuals in group 1 presented with macrocephaly."
    explanation: Documents macrocephaly in the spectrin-repeat (group 1) subtype corresponding to MRD63.
- category: Neurologic
  name: Intellectual Disability
  description: >-
    Cognitive impairment across the spectrum; typically milder in the GEFD1
    (MRD44) arm and more severe (moderate to severe) in the spectrin-repeat
    (MRD63) arm.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These individuals, except individuals 15 and 16, define patient group 2 and present with milder ID and microcephaly."
    explanation: Documents the milder intellectual disability of the GEFD1 arm, contrasted with the more severe spectrin-repeat arm.
- category: Neurologic
  name: Global Developmental Delay
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "global developmental delay, microcephaly and mild dysmorphism"
    explanation: Documents global developmental delay in the presenting TRIO family.
- category: Neurologic
  name: Delayed Speech and Language Development
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
- category: Neurologic
  name: Autistic Behavior
  description: >-
    Autistic traits are part of the recurrent neurobehavioral phenotype;
    ASD-related de novo variants also cluster in the GEF1 domain.
  phenotype_term:
    preferred_term: Autistic behavior
    term:
      id: HP:0000729
      label: Autistic behavior
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A neurobehavioral phenotype was observed in 19/24 (79%) individuals with a TRIO variant"
    explanation: Documents the high frequency of a neurobehavioral phenotype (including autistic traits) across the cohort.
- category: Neurologic
  name: Aggressive Behavior
  phenotype_term:
    preferred_term: Aggressive behavior
    term:
      id: HP:0000718
      label: Aggressive behavior
- category: Neurologic
  name: Hyperactivity
  phenotype_term:
    preferred_term: Hyperactivity
    term:
      id: HP:0000752
      label: Hyperactivity
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The reported clinical phenotypes of affected individuals with TRIO
      mutations include intellectual disability, behavioral difficulties such
      as hyperactivity or aggression, autism or autistic behavioral tendencies,
      skeletal hand anomalies, and microcephaly.
    explanation: Hyperactivity is explicitly included in the reported TRIO neurobehavioral phenotype.
- category: Neurologic
  name: Stereotypy
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Stereotypy
    term:
      id: HP:0000733
      label: Motor stereotypy
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Recurrent behavioral features seen across all individuals included
      stereotypies (27%), poor attention (70%), obsessive compulsive traits
      (45%), autistic traits (31%), and aggression (36%).
    explanation: Stereotypies occurred in 27% of the reported cohort.
- category: Neurologic
  name: Short Attention Span
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Short attention span
    term:
      id: HP:0000736
      label: Short attention span
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Recurrent behavioral features seen across all individuals included
      stereotypies (27%), poor attention (70%), obsessive compulsive traits
      (45%), autistic traits (31%), and aggression (36%).
    explanation: Poor attention occurred in 70% of the reported cohort.
- category: Neurologic
  name: Obsessive-Compulsive Behavior
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Obsessive-compulsive behavior
    term:
      id: HP:0000722
      label: Compulsive behaviors
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Recurrent behavioral features seen across all individuals included
      stereotypies (27%), poor attention (70%), obsessive compulsive traits
      (45%), autistic traits (31%), and aggression (36%).
    explanation: Obsessive-compulsive traits occurred in 45% of the reported cohort.
- category: Musculoskeletal
  name: Toe Syndactyly
  phenotype_term:
    preferred_term: 2-3 toe syndactyly
    term:
      id: HP:0001770
      label: Toe syndactyly
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Common features among studied individuals include microcephaly (seen in
      patients 1–5), mild dysmorphic facies, tapering fingers with prominent
      proximal interphalangeal joints, fifth finger clinodactyly and 2/3 toe
      syndactyly.
    explanation: Documents 2/3 toe syndactyly among the recurrent distal-limb findings.
- category: Craniofacial
  name: Recognizable Facial Dysmorphism
  subtype: MRD63
  phenotype_term:
    preferred_term: Abnormal facial shape
    term:
      id: HP:0001999
      label: Abnormal facial shape
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Individuals with spectrin variants show a more severe developmental
      phenotype, macrocephaly, and statistically significant clustering of
      facial dysmorphism, whereas individuals with GEFD1 variants have a less
      severe developmental phenotype and microcephaly.
    explanation: Supports a recognizable facial gestalt in the spectrin-repeat MRD63 arm.
- category: Neurologic
  name: Seizures
  frequency: OCCASIONAL
  description: >-
    Epilepsy or seizure activity occurred in a minority of individuals; within
    the cohort seizures were seen only in individuals with spectrin-domain or
    truncating variants.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was a diagnosis of epilepsy or evidence of seizure activity in five individuals (24%)."
    explanation: Quantifies seizure occurrence in the cohort (5/24, 24%).
genetic:
- name: TRIO
  gene_term:
    preferred_term: TRIO
    term:
      id: hgnc:12303
      label: TRIO
  association: Causal
  relationship_type: CAUSATIVE
  notes: >-
    Heterozygous pathogenic variants in TRIO cause both MRD44 and MRD63. A
    genotype-phenotype correlation is defined by variant class and its
    direction of effect on RAC1 activity: (1) GEFD1 missense variants and
    protein-truncating (nonsense/frameshift) variants act by loss-of-function
    (reduced RAC1 activation) and associate with microcephaly and milder ID
    (MRD44); (2) seventh spectrin repeat missense variants act by
    gain-of-function (increased RAC1 activation via released autoinhibition) and
    associate with macrocephaly and more severe ID (MRD63). The reported
    p.Ile2580Phe second-SH3 case is retained as a proposed, unreplicated extension
    of the MRD63 arm rather than being folded into the established
    spectrin-repeat mechanism.
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The first GEF domain (GEFD1) regulates RAC1 and RHOG activity, and the second GEF domain (GEFD2) regulates RHOA activity."
    explanation: Establishes TRIO domain architecture underlying the variant-class genotype-phenotype correlation.
  - reference: PMID:33167890
    reference_title: "Novel loss-of-function variants in TRIO are associated with neurodevelopmental disorder: case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it is hypothesized that missense variants in this domain cause hyperactivation of RAC1 leading to the observed macrocephaly phenotype"
    explanation: Documents the spectrin-domain gain-of-function hyperactivation-macrocephaly correlation.
  variants:
  - name: "NM_007118.4:c.7738A>T (p.Ile2580Phe), reported second SH3 domain"
    description: >-
      Novel missense variant reported in a male infant with developmental delay
      and macrocephaly and positioned by the authors in the second SH3 domain of
      TRIO. It is curated as a proposed extension of the MRD63/macrocephaly arm
      beyond the established seventh-spectrin-repeat hotspot, not as evidence for
      a new domain-level mechanism. The cached report does not document de novo
      occurrence and supplies no functional assay. Pathogenicity rests on
      bioinformatic prediction and three-dimensional protein modelling, so the
      classification is LIKELY_PATHOGENIC and no functional effect is asserted.
    clinical_significance: LIKELY_PATHOGENIC
    type: missense variant
    gene:
      preferred_term: TRIO
      term:
        id: hgnc:12303
        label: TRIO
    evidence:
    - reference: PMID:42421094
      reference_title: "Identification and growth features of developmental delay with macrocephaly caused by a novel TRIO variant affecting the second SH3 domain."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We identified a novel TRIO missense variant (NM_007118.4: c.7738 A >
        T, p.I2580F; chr5:14492781 A > T/hg19) in the patient, and the variant
        was positioned in the second Src homology 3 (SH3) domain.
      explanation: >-
        Primary report of the variant, its transcript-level and genomic
        coordinates, and the authors' domain assignment.
    - reference: PMID:42421094
      reference_title: "Identification and growth features of developmental delay with macrocephaly caused by a novel TRIO variant affecting the second SH3 domain."
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: >-
        Bioinformatic and three-dimensional protein modeling evidence all
        support the p.I2580F variant as likely pathogenic.
      explanation: >-
        Basis for the LIKELY_PATHOGENIC call. The assessment is in silico; no
        functional assay of GEF activity or Rho GTPase signalling was performed.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    A rare disorder; the largest cohort to date reported 24 individuals from 22
    families with confirmed pathogenic TRIO variants, in addition to smaller
    series and case reports. No population-based prevalence estimate is
    established.
  evidence:
  - reference: PMID:32109419
    reference_title: "Opposite Modulation of RAC1 by Mutations in TRIO Is Associated with Distinct, Domain-Specific Neurodevelopmental Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we report the largest international cohort of 24 individuals with"
    explanation: Documents the size of the largest reported TRIO cohort, supporting rarity.
treatments:
- name: Supportive Care
  description: >-
    Symptomatic, multidisciplinary management of developmental delay,
    intellectual disability, speech impairment, behavioral difficulties, and
    seizures. No disease-modifying therapy exists.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
- name: Genetic Counseling
  description: >-
    Counseling regarding autosomal dominant inheritance, the high proportion of
    de novo variants, and recurrence risk, including for reported inherited
    cases.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
discussions:
- discussion_id: gap_trio_hetero_model_headsize_mismatch
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Do heterozygous mouse models of NDD-associated TRIO variants faithfully
    reproduce the human variant-class-specific, RAC1-direction-dependent
    head-size phenotypes (microcephaly for GEFD1 loss-of-function vs
    macrocephaly for spectrin-repeat gain-of-function)?
  rationale: >-
    Complete Trio knockout is embryonic lethal and nervous-system-specific
    knockout mice show reduced brain size, so null models capture the
    loss-of-function/microcephaly arm but not the gain-of-function/macrocephaly
    arm. Recent heterozygous knock-in models of specific NDD variants show
    distinct, variant-dependent effects on behavior, brain size, and dendritic
    arbors, but their fidelity to the human genotype-phenotype correlation
    across both arms remains to be established.
  attaches_to:
  - pathophysiology#Seventh Spectrin Repeat Gain-of-Function and RAC1 Hyperactivation
  proposed_experiments:
  - experiment_id: exp_trio_allelic_knockin_headsize
    name: Allelic heterozygous knock-in comparison of TRIO GEFD1 vs spectrin variants
    description: >-
      Compare occipitofrontal/brain size, cortical neuron output, dendritic
      arborization, and RAC1 activation across allelic heterozygous knock-in
      mice carrying GEFD1 loss-of-function versus seventh spectrin repeat
      gain-of-function TRIO variants, benchmarked against the human head-size
      and ID severity data.
- discussion_id: gap_trio_id_severity_asymmetry
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Why is intellectual disability consistently more severe in individuals with
    seventh spectrin repeat (gain-of-function) variants than in those with GEFD1
    (loss-of-function) variants, despite both perturbing the same TRIO-RAC1
    axis?
  rationale: >-
    The cohort literature reports that ID severity is greater with spectrin
    variants than with GEFD1 variants, but the mechanistic basis for the
    asymmetry between hyper- and hypo-activation of RAC1 is not resolved.
  attaches_to:
  - pathophysiology#Impaired Dendritic and Synaptic Morphogenesis

TRIO-Related Neurodevelopmental Disorder

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