Type II Collagenopathies (COL2A1-related disorders)

name: Type 2 Collagenopathies
display_name: Type II Collagenopathies (COL2A1-related disorders)
creation_date: "2026-08-17T00:00:00Z"
description: >-
  The type II collagenopathies are a spectrum of skeletal dysplasias and
  connective-tissue disorders caused by pathogenic variants in COL2A1, the gene
  encoding the alpha-1 chain of type II collagen — the principal structural
  protein of hyaline cartilage, the vitreous humor, and the inner ear. Because
  the same gene and the same proximal defect (impaired assembly or reduced
  quantity of the type II collagen triple helix, producing an abnormal cartilage
  and vitreous matrix) underlie every member, the group forms a single allelic
  continuum rather than a set of mechanistically unrelated diseases. That
  continuum spans a striking severity range, from the perinatal-lethal forms
  (achondrogenesis type II, hypochondrogenesis), through the intermediate
  short-trunk skeletal dysplasias (spondyloepiphyseal dysplasia congenita, Kniest
  dysplasia, spondyloepimetaphyseal dysplasia Strudwick type), to the mild,
  ocular-predominant Stickler syndrome type 1. Genotype largely tracks with this
  severity: dominant-negative glycine substitutions in the Gly-X-Y triple-helical
  domain give the more severe structural dysplasias, whereas COL2A1
  haploinsufficiency (premature-termination / null alleles) gives the milder,
  vitreoretinopathy-predominant Stickler type 1.
grouping_basis:
- SHARED_GENE_FAMILY
- SHARED_PATHWAY
grouping_rationale: >-
  Grouped on a single shared gene and pathway: every member is caused by
  pathogenic COL2A1 variants that disrupt type II collagen (the Gly-X-Y triple
  helix and its downstream cartilage/vitreous matrix). This is deliberately
  modeled as a Grouping over the already-curated, clinically distinct Disease
  entries rather than as one umbrella Disease with has_subtypes, so that each
  established clinical entity (with its own phenotype set, prevalence, and
  management) is preserved intact and not duplicated. NOTE FOR REVIEWERS — this
  is a genuine lump-vs-split judgment call: because the group is a single-gene
  allelic series (unlike a multi-gene convergence grouping such as the
  mucopolysaccharidoses or the heritable thoracic aortic diseases), a
  Disease-with-has_subtypes umbrella would also be defensible. The Grouping shape
  was chosen because the members already exist as separate entries; see the PR
  discussion for the full argument. The criteria are stated as NECESSARY: being a
  type II collagenopathy entails a COL2A1 defect, but a COL2A1 mention alone (e.g.
  in a differential-diagnosis context) does not make a disease a member. The
  members are ordered along the severity continuum, from perinatal-lethal
  achondrogenesis type II to mild adult-onset Stickler syndrome type 1.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0022800
      label: type 2 collagenopathy
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
    mapping_justification: >-
      MONDO:0022800 (type 2 collagenopathy) is the ontology grouping term for the
      COL2A1-related disorder spectrum this Grouping curates. closeMatch (not
      exactMatch) because the exact membership of the MONDO subtree and this
      curated union are not guaranteed to coincide leaf-for-leaf.
membership_criteria:
- description: >-
    A member is caused by pathogenic variants in COL2A1 (the type II collagen
    alpha-1 chain).
  criteria_semantics: NECESSARY
  logic:
    criterion_predicate: HAS_GENE
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
    description: >-
      COL2A1 pathogenic variant — the shared causal gene for every type II
      collagenopathy, encoding the alpha-1 chain of type II collagen.
members:
- member: Achondrogenesis Type II
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The perinatal-lethal, most severe end of the spectrum (Langer-Saldino
      achondrogenesis). Heterozygous, typically dominant-negative COL2A1
      glycine-substitution variants produce a near-complete failure of type II
      collagen triple-helix assembly, with virtually absent cartilage
      ossification, extreme micromelia, and a hypoplastic thorax incompatible
      with postnatal survival. Skeletal, not ocular, predominance (lethality
      precludes an ocular phenotype).
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
- member: Hypochondrogenesis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      A severe, usually perinatal-lethal skeletal dysplasia one step milder than
      achondrogenesis type II, with which it forms a continuum. Heterozygous
      COL2A1 glycine-substitution variants give better-preserved (though still
      markedly deficient) cartilage ossification than achondrogenesis, so
      vertebral and long-bone mineralization is present but abnormal.
      Skeletal-predominant.
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
- member: Spondyloepiphyseal Dysplasia Congenita
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      An intermediate-severity, non-lethal short-trunk skeletal dysplasia.
      Predominantly dominant-negative COL2A1 glycine substitutions in the Gly-X-Y
      repeat impair triple-helix folding, giving disproportionate short stature,
      flat vertebrae (platyspondyly), and delayed epiphyseal ossification.
      Skeletal predominance with variable ocular involvement (myopia, retinal
      detachment risk).
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
- member: Kniest Dysplasia
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      A distinctive intermediate short-trunk dysplasia set apart by its variant
      class: small in-frame exon-skipping deletions in the triple-helical domain
      of COL2A1 (rather than the point-substitution mechanism of the other
      forms), producing a shortened but assembly-competent, dominant-negative
      collagen chain and the characteristic "Swiss-cheese" cartilage. Combines
      skeletal (dumbbell-shaped long bones, kyphoscoliosis) with prominent ocular
      (high myopia, retinal detachment) and craniofacial features.
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
- member: Spondyloepimetaphyseal Dysplasia Strudwick Type
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      An intermediate short-trunk dysplasia distinguished from SEDC by additional
      metaphyseal involvement — the irregular, mottled ("dappled") metaphyseal
      changes that define the Strudwick type — on top of the spondyloepiphyseal
      pattern. Caused by COL2A1 glycine substitutions in the Gly-X-Y repeat, with
      ocular involvement (high myopia, retinal detachment) reflecting the
      vitreous role of type II collagen.
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1
- member: Stickler Syndrome Type 1
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The mild, ocular-predominant end of the spectrum, and the one mechanistic
      outlier in variant class: STL1 typically results from COL2A1
      haploinsufficiency (premature-termination / null alleles) rather than the
      dominant-negative glycine substitutions of the severe skeletal forms.
      Dominated by the membranous vitreous phenotype, high myopia, and retinal
      detachment risk, with sensorineural hearing loss, cleft palate, and only
      mild/early-onset osteoarthritis rather than a severe congenital dysplasia.
    gene:
      preferred_term: COL2A1
      term:
        id: hgnc:2200
        label: COL2A1