SMAD6-related craniosynostosis - Disorder Mechanisms

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Mappings

MONDO

MONDO:0015338 syndromic craniosynostosis Not Yet Curated

ClinGen used syndromic craniosynostosis as the MONDO anchor for its lumped SMAD6 validity assertion spanning cranial, cardiovascular, and radioulnar phenotypes.

📘

Definitions

2

Clinical disease framing for SMAD6-related craniosynostosis

SMAD6-related craniosynostosis is a craniofacial developmental disorder in which pathogenic SMAD6 variants increase risk of premature cranial suture fusion, especially metopic and sagittal synostosis.

CASE_DEFINITION Disease-focused framing of the craniosynostosis branch of SMAD6 deficiency

Show evidence (1 reference)

PMID:32499606 SUPPORT Human Clinical

"Pathogenic SMAD6 variants substantially increase the risk of both nonsyndromic and syndromic presentations of craniosynostosis, especially metopic synostosis."

This cohort study directly defines the craniosynostosis branch as a SMAD6-associated disease risk concentrated in metopic and related sutures.

Molecular definition for SMAD6-related craniosynostosis

Molecularly, this entry captures heterozygous deleterious SMAD6 variants that impair inhibitory control of BMP-driven osteoblast differentiation in cranial suture development.

DIAGNOSTIC_CRITERIA Molecular anchoring of the craniosynostosis branch of SMAD6-related disease

Show evidence (1 reference)

PMID:27606499 SUPPORT Human Clinical

"Thirteen probands (7%) had damaging de novo or rare transmitted mutations in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20)."

This establishes SMAD6 as a recurrent craniosynostosis gene and directly links it to BMP-regulated osteoblast differentiation.

👪

Inheritance

1

Autosomal dominant HP:0000006

SMAD6-related craniosynostosis usually segregates as an autosomal dominant trait with marked incomplete penetrance and variable phenotypic manifestation.

Autosomal dominant inheritance Penetrance: INCOMPLETE Expressivity: VARIABLE

Show evidence (2 references)

PMID:27606499 SUPPORT Human Clinical

"SMAD6 mutations nonetheless showed striking incomplete penetrance (<60%)."

This directly supports reduced penetrance in SMAD6-related midline craniosynostosis.

PMID:32499606 SUPPORT Human Clinical

"Combined with eight additional variants, ≥20/26 were transmitted from an unaffected parent but rs1884302 genotype did not predict phenotype."

This provides direct evidence for dominant transmission with unaffected carriers and variable expression.

⚙

Pathophysiology

4

Germline SMAD6 loss with impaired BMP restraint

Heterozygous deleterious SMAD6 variants reduce inhibitory control of BMP signaling, forming the shared upstream lesion of the craniosynostosis branch.

Show evidence (2 references)

PMID:36414630 SUPPORT Human Clinical

"SMAD6 encodes an intracellular inhibitor of the bone morphogenetic protein (BMP) signalling pathway."

This review defines the shared upstream role of SMAD6 as an intracellular BMP inhibitor.

PMID:21681813 SUPPORT Model Organism

"Overall, our results show that Smad6 is required to limit BMP signaling during endochondral bone formation."

Skeletal model data support a general role for Smad6 in limiting BMP activity in developing bone-forming tissues.

Excess BMP signaling in cranial suture mesenchyme

Failure of SMAD6-mediated negative feedback permits excessive BMP-driven osteogenic signaling in the mesenchymal compartment that normally preserves suture patency.

Show evidence (1 reference)

PMID:36414630 SUPPORT Human Clinical

"Until now, SMAD6-deficiency has been associated with three distinctive human congenital conditions, i.e., congenital heart diseases, including left ventricular obstruction and conotruncal defects, craniosynostosis and radioulnar synostosis."

This supports SMAD6 deficiency as a discrete craniosynostosis-causing branch within a broader pleiotropic developmental spectrum.

Accelerated osteoblast differentiation and suture ossification

The immediate cranial mechanism is premature osteogenic maturation and ossification at cranial sutures, with strongest enrichment in midline sutures.

Show evidence (1 reference)

PMID:32499606 SUPPORT Human Clinical

"Pathogenic SMAD6 variants substantially increase the risk of both nonsyndromic and syndromic presentations of craniosynostosis, especially metopic synostosis."

This supports premature suture ossification as the dominant disease mechanism and metopic synostosis as the strongest phenotype.

Modifier-dependent penetrance and phenotype variability

The craniosynostosis phenotype is incompletely penetrant and variably expressed, implying strong influence from modifier loci or developmental context beyond the primary SMAD6 variant.

Show evidence (2 references)

PMID:27606499 SUPPORT Human Clinical

"SMAD6 mutations nonetheless showed striking incomplete penetrance (<60%)."

This supports a distinct modifier-dependent penetrance node in the craniosynostosis branch.

PMID:32499606 SUPPORT Human Clinical

"Genotyping of rs1884302 is not clinically useful."

This later cohort tempers the original two-locus model and supports the idea that phenotype variability remains only partly explained.

⬡

Pathograph

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Referential integrity issues (1):

Target 'Metopic and sagittal suture fusion' (from 'Accelerated osteoblast differentiation and suture ossification') not found in named elements

●

Phenotypes

2

Metopic synostosis HP:0011330

Show evidence (1 reference)

PMID:32499606 SUPPORT Human Clinical

"We found 18 (2.3%) different rare damaging SMAD6 variants, with the highest prevalence in metopic synostosis (5.8%)"

This directly supports metopic synostosis as the most characteristic cranial presentation.

Sagittal craniosynostosis HP:0004442

Show evidence (1 reference)

PMID:27606499 PARTIAL Human Clinical

"To identify mutations contributing to common non-syndromic midline (sagittal and metopic) craniosynostosis, we performed exome sequencing of 132 parent-offspring trios and 59 additional probands."

This partially supports sagittal craniosynostosis as part of the broader SMAD6-associated midline craniosynostosis cohort, rather than establishing sagittal-specific enrichment on its own.

🧬

Genetic Associations

1

SMAD6 (Causative (Primary))

Show evidence (2 references)

PMID:27606499 SUPPORT Human Clinical

"Thirteen probands (7%) had damaging de novo or rare transmitted mutations in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20)."

This establishes SMAD6 as a recurrent causative gene in midline craniosynostosis.

PMID:32499606 SUPPORT Human Clinical

"Pathogenic SMAD6 variants substantially increase the risk of both nonsyndromic and syndromic presentations of craniosynostosis, especially metopic synostosis."

This larger cohort confirms the same gene-disease relationship across a broader craniosynostosis case series.

{ }

Source YAML

click to show

name: SMAD6-related craniosynostosis
creation_date: "2026-03-26T20:45:00Z"
updated_date: "2026-04-07T16:11:29Z"
category: Mendelian
description: >-
  SMAD6-related craniosynostosis is an autosomal dominant, incompletely
  penetrant craniosynostosis spectrum caused by heterozygous deleterious SMAD6
  variants. It is enriched for metopic and sagittal synostosis and is driven by
  failure to restrain BMP-dependent osteogenic differentiation within cranial
  suture mesenchyme, causing premature suture ossification and fusion.
disease_term:
  preferred_term: craniosynostosis 7
  term:
    id: MONDO:0044315
    label: craniosynostosis 7
definitions:
- name: Clinical disease framing for SMAD6-related craniosynostosis
  definition_type: CASE_DEFINITION
  description: >-
    SMAD6-related craniosynostosis is a craniofacial developmental disorder in
    which pathogenic SMAD6 variants increase risk of premature cranial suture
    fusion, especially metopic and sagittal synostosis.
  scope: Disease-focused framing of the craniosynostosis branch of SMAD6 deficiency
  evidence:
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Pathogenic SMAD6 variants substantially increase the risk of both
      nonsyndromic and syndromic presentations of craniosynostosis, especially
      metopic synostosis.
    explanation: >-
      This cohort study directly defines the craniosynostosis branch as a
      SMAD6-associated disease risk concentrated in metopic and related sutures.
- name: Molecular definition for SMAD6-related craniosynostosis
  definition_type: DIAGNOSTIC_CRITERIA
  description: >-
    Molecularly, this entry captures heterozygous deleterious SMAD6 variants
    that impair inhibitory control of BMP-driven osteoblast differentiation in
    cranial suture development.
  scope: Molecular anchoring of the craniosynostosis branch of SMAD6-related disease
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Thirteen probands (7%) had damaging de novo or rare transmitted mutations
      in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20).
    explanation: >-
      This establishes SMAD6 as a recurrent craniosynostosis gene and directly
      links it to BMP-regulated osteoblast differentiation.
synonyms:
- craniosynostosis 7
- SMAD6-associated craniosynostosis
categories:
- Craniofacial Developmental Disorder
- Craniosynostosis Spectrum Disorder
- Developmental Bone Disorder
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0015338
      label: syndromic craniosynostosis
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
    mapping_justification: >-
      ClinGen used syndromic craniosynostosis as the MONDO anchor for its
      lumped SMAD6 validity assertion spanning cranial, cardiovascular, and
      radioulnar phenotypes.
external_assertions:
- name: ClinGen SMAD6 gene-disease validity assertion
  source: ClinGen
  assertion_type: gene_disease_validity_lumped_assertion
  external_id: CCID:009009
  url: https://search.clinicalgenome.org/CCID:009009
  description: >-
    ClinGen curated SMAD6 under a lumped disease entity anchored to syndromic
    craniosynostosis, incorporating aortic valve disease 2, craniosynostosis 7,
    and radioulnar synostosis.
  notes: >-
    This external assertion is broader than the present entry, which
    intentionally isolates the craniosynostosis mechanism branch for dismech.
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: INCOMPLETE
  expressivity: VARIABLE
  description: >-
    SMAD6-related craniosynostosis usually segregates as an autosomal dominant
    trait with marked incomplete penetrance and variable phenotypic
    manifestation.
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SMAD6 mutations nonetheless showed striking incomplete penetrance (<60%).
    explanation: >-
      This directly supports reduced penetrance in SMAD6-related midline craniosynostosis.
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Combined with eight additional variants, ≥20/26 were transmitted from an
      unaffected parent but rs1884302 genotype did not predict phenotype.
    explanation: >-
      This provides direct evidence for dominant transmission with unaffected
      carriers and variable expression.
progression:
- phase: Congenital suture fusion phase
  age_range: fetal development to infancy
  notes: >-
    Dysregulated cranial osteogenesis causes premature fusion of cranial sutures
    during skull growth, typically recognized in infancy.
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Premature fusion of the cranial sutures (craniosynostosis), affecting 1 in 2000
      newborns, is treated surgically in infancy to prevent adverse neurologic outcomes.
    explanation: >-
      This supports congenital onset and early infant recognition of the
      craniosynostosis phenotype.
- phase: Secondary cranial growth restriction phase
  age_range: infancy through childhood
  notes: >-
    Once a suture fuses prematurely, compensatory skull growth distortion and
    neurologic risk from constrained skull growth become the main downstream
    clinical consequences.
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Premature fusion of the cranial sutures (craniosynostosis), affecting 1 in 2000
      newborns, is treated surgically in infancy to prevent adverse neurologic outcomes.
    explanation: >-
      This supports a clinically important post-fusion phase in which surgery is
      used to mitigate neurologic consequences of constrained skull growth.
pathophysiology:
- name: Germline SMAD6 loss with impaired BMP restraint
  description: >-
    Heterozygous deleterious SMAD6 variants reduce inhibitory control of BMP
    signaling, forming the shared upstream lesion of the craniosynostosis
    branch.
  gene:
    preferred_term: SMAD6
    modifier: DECREASED
    term:
      id: hgnc:6772
      label: SMAD6
  downstream:
  - target: Excess BMP signaling in cranial suture mesenchyme
    description: Loss of inhibitory SMAD6 increases the osteogenic drive within cranial suture developmental programs.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - impaired inhibitory feedback on BMP signaling
    evidence:
    - reference: PMID:27606499
      reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Thirteen probands (7%) had damaging de novo or rare transmitted mutations
        in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20).
      explanation: >-
        This directly supports the edge from SMAD6 loss to disinhibited
        osteogenic BMP signaling in craniosynostosis.
  evidence:
  - reference: PMID:36414630
    reference_title: "SMAD6-deficiency in human genetic disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SMAD6 encodes an intracellular inhibitor of the bone morphogenetic protein
      (BMP) signalling pathway.
    explanation: >-
      This review defines the shared upstream role of SMAD6 as an intracellular
      BMP inhibitor.
  - reference: PMID:21681813
    reference_title: "Smad6 is essential to limit BMP signaling during cartilage development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Overall, our results show that Smad6 is required to limit BMP signaling
      during endochondral bone formation.
    explanation: >-
      Skeletal model data support a general role for Smad6 in limiting BMP
      activity in developing bone-forming tissues.
- name: Excess BMP signaling in cranial suture mesenchyme
  description: >-
    Failure of SMAD6-mediated negative feedback permits excessive BMP-driven
    osteogenic signaling in the mesenchymal compartment that normally preserves
    suture patency.
  downstream:
  - target: Accelerated osteoblast differentiation and suture ossification
    description: Excess BMP signaling pushes suture mesenchyme toward premature osteoblast maturation and bone deposition.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27606499
      reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Thirteen probands (7%) had damaging de novo or rare transmitted mutations
        in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20).
      explanation: >-
        This directly links the upstream SMAD6 lesion to dysregulated BMP-driven
        osteoblast differentiation.
  evidence:
  - reference: PMID:36414630
    reference_title: "SMAD6-deficiency in human genetic disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Until now, SMAD6-deficiency has been associated with three distinctive
      human congenital conditions, i.e., congenital heart diseases, including
      left ventricular obstruction and conotruncal defects, craniosynostosis and
      radioulnar synostosis.
    explanation: >-
      This supports SMAD6 deficiency as a discrete craniosynostosis-causing
      branch within a broader pleiotropic developmental spectrum.
- name: Accelerated osteoblast differentiation and suture ossification
  description: >-
    The immediate cranial mechanism is premature osteogenic maturation and
    ossification at cranial sutures, with strongest enrichment in midline
    sutures.
  downstream:
  - target: Metopic and sagittal suture fusion
    description: Early osteogenic closure produces the characteristic midline synostosis phenotype.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:32499606
      reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We found 18 (2.3%) different rare damaging SMAD6 variants, with the
        highest prevalence in metopic synostosis (5.8%) and an 18.3-fold
        enrichment of loss-of-function variants comparedwith gnomAD data (P < 10-7).
      explanation: >-
        This directly supports the preferential expression of the cranial
        mechanism in metopic and other midline sutures.
  evidence:
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Pathogenic SMAD6 variants substantially increase the risk of both
      nonsyndromic and syndromic presentations of craniosynostosis, especially
      metopic synostosis.
    explanation: >-
      This supports premature suture ossification as the dominant disease
      mechanism and metopic synostosis as the strongest phenotype.
- name: Modifier-dependent penetrance and phenotype variability
  description: >-
    The craniosynostosis phenotype is incompletely penetrant and variably
    expressed, implying strong influence from modifier loci or developmental
    context beyond the primary SMAD6 variant.
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SMAD6 mutations nonetheless showed striking incomplete penetrance (<60%).
    explanation: >-
      This supports a distinct modifier-dependent penetrance node in the craniosynostosis branch.
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genotyping of rs1884302 is not clinically useful.
    explanation: >-
      This later cohort tempers the original two-locus model and supports the
      idea that phenotype variability remains only partly explained.
phenotypes:
- name: Metopic synostosis
  description: Metopic synostosis is the most enriched cranial phenotype in SMAD6-related disease.
  diagnostic: true
  phenotype_term:
    preferred_term: Metopic synostosis
    term:
      id: HP:0011330
      label: Metopic synostosis
  evidence:
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We found 18 (2.3%) different rare damaging SMAD6 variants, with the
      highest prevalence in metopic synostosis (5.8%)
    explanation: >-
      This directly supports metopic synostosis as the most characteristic
      cranial presentation.
- name: Sagittal craniosynostosis
  description: Sagittal craniosynostosis is a recurrent midline phenotype within the SMAD6 craniosynostosis spectrum.
  phenotype_term:
    preferred_term: Sagittal craniosynostosis
    term:
      id: HP:0004442
      label: Sagittal craniosynostosis
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      To identify mutations contributing to common non-syndromic midline
      (sagittal and metopic) craniosynostosis, we performed exome sequencing of
      132 parent-offspring trios and 59 additional probands.
    explanation: >-
      This partially supports sagittal craniosynostosis as part of the broader
      SMAD6-associated midline craniosynostosis cohort, rather than establishing
      sagittal-specific enrichment on its own.
genetic:
- name: SMAD6
  gene_term:
    preferred_term: SMAD6
    term:
      id: hgnc:6772
      label: SMAD6
  association: Causative (Primary)
  notes: >-
    Heterozygous truncating and missense SMAD6 variants define the main
    molecular route. Midline craniosynostosis is the dominant cranial outcome,
    with metopic involvement most enriched. Early BMP2 modifier models remain
    biologically interesting but are not clinically definitive.
  evidence:
  - reference: PMID:27606499
    reference_title: "Two locus inheritance of non-syndromic midline craniosynostosis via rare SMAD6 and common BMP2 alleles."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Thirteen probands (7%) had damaging de novo or rare transmitted mutations
      in SMAD6, an inhibitor of BMP - induced osteoblast differentiation (p<10-20).
    explanation: >-
      This establishes SMAD6 as a recurrent causative gene in midline
      craniosynostosis.
  - reference: PMID:32499606
    reference_title: "SMAD6 variants in craniosynostosis: genotype and phenotype evaluation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Pathogenic SMAD6 variants substantially increase the risk of both
      nonsyndromic and syndromic presentations of craniosynostosis, especially
      metopic synostosis.
    explanation: >-
      This larger cohort confirms the same gene-disease relationship across a
      broader craniosynostosis case series.
  variants:
  - name: NM_005585.6(SMAD6):c.840dup (p.Arg281SerfsTer22)
    description: Truncating SMAD6 variant reported in a sagittal craniosynostosis proband.
    type: frameshift_variant
    gene:
      preferred_term: SMAD6
      term:
        id: hgnc:6772
        label: SMAD6
    external_assertions:
    - name: ClinGen Allele Registry record
      source: ClinGen Allele Registry
      assertion_type: allele_registry_record
      external_id: CA2573049045
      description: Allele Registry identifier for a representative sagittal craniosynostosis-associated SMAD6 variant.
  - name: NM_005585.6(SMAD6):c.1219G>T (p.Glu407Ter)
    description: Truncating SMAD6 variant reported in a metopic synostosis proband.
    type: nonsense_variant
    gene:
      preferred_term: SMAD6
      term:
        id: hgnc:6772
        label: SMAD6
    external_assertions:
    - name: ClinGen Allele Registry record
      source: ClinGen Allele Registry
      assertion_type: allele_registry_record
      external_id: CA7626756
      description: Allele Registry identifier for a representative metopic synostosis-associated SMAD6 variant.
notes: >-
  This entry intentionally models the craniosynostosis branch of SMAD6-related
  disease rather than the broader ClinGen-lumped spectrum. MONDO:0044315 is
  used as the formal disease-term binding here despite its current
  susceptibility/disposition semantics in MONDO, reflecting a dismech curation
  choice that this term is better treated as a disease-level anchor for the
  craniosynostosis branch pending parallel MONDO alignment.