Kyphomelic dysplasia

name: Kyphomelic Dysplasia
display_name: Kyphomelic dysplasia
creation_date: "2026-08-22T00:00:00Z"
description: >-
  Kyphomelic dysplasia is a clinical-radiographic label for skeletal dysplasias
  characterized by severe bowing — sharp angulation, or kyphomelia — of the long
  bones, especially the femora, together with variable additional findings such
  as a narrow thorax and distinctive facies. First described nearly four decades
  ago, it remained molecularly undefined until two separate genes were
  identified within three years of each other: de novo heterozygous KIF5B
  variants, and biallelic CCN2 variants. Those two entities differ in gene,
  inheritance, protein class and mechanism, and are distinguishable clinically.
  This grouping therefore unions two distinct diseases that share a radiographic
  appearance, rather than describing one disease.
grouping_basis:
- SHARED_PHENOTYPE
- CLINICAL_CONVENTION
grouping_rationale: >-
  Grouped on a shared radiographic phenotype and on established clinical usage,
  explicitly NOT on a shared mechanism. The two members reach severe long bone
  bowing by unrelated routes: KIF5B encodes the kinesin-1 heavy chain, a
  microtubule motor, and its variants are de novo heterozygous missense alleles
  clustered in the catalytic motor domain; CCN2 encodes a matricellular protein
  required for chondrocyte proliferation and differentiation, and its variants
  are biallelic loss-of-function alleles. Inheritance differs (dominant de novo
  versus autosomal recessive), and the phenotypes are separable — the KIF5B form
  adds postnatal osteoporosis with fractures and optic atrophy, the CCN2 form
  adds cleft palate, micro-retrognathia and radial head dislocation.

  The members are kept as separate Disease entries rather than merged because
  merging them would produce a single pathograph in which a kinesin motor defect
  and a chondrocyte growth-factor defect were presented as one mechanism. Both
  papers that molecularly defined a form of this condition describe the clinical
  label itself as a heterogeneous group, which is the authors' own judgement that
  this is a grouping and not a disease. Historical case reports, including
  sibling recurrences and at least one patient whose radiographic changes largely
  regressed by age seven, suggest the label may still cover further entities that
  are not yet molecularly resolved.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0008881
      label: kyphomelic dysplasia
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
    mapping_justification: >-
      closeMatch rather than exactMatch, deliberately, because MONDO:0008881 is
      currently doing double duty and cannot be cleanly equated with either this
      grouping or either member. It carries the historical clinical-radiographic
      concept AND a causal-gene edge to CCN2 (RO:0004003), i.e. one of the two
      molecular forms. Its definition is also stale: it still states that
      kyphomelic dysplasia is no longer considered its own entity and that cases
      should be reclassified as other chondrodysplasias, a position from
      PMID:11038441 (2000) that predates the 2022 KIF5B and 2024 CCN2 gene
      discoveries by two decades. Asserting exactMatch against a term in that
      state would record more confidence than exists. Under the CLAUDE.md rule
      that only exactMatch and narrowMatch retire a concept from the curation
      queue, closeMatch also correctly leaves MONDO:0008881 open — which is the
      right outcome, since the MONDO record needs fixing before the mapping can
      be tightened.
    consistency:
    - reference: MONDO
      consistent: INCONSISTENT
      notes: >-
        MONDO records no subclasses of MONDO:0008881, so neither member has an
        is-a descendant term, and the term's own definition contradicts its own
        causal-gene assertion (definition says the entity has been dissolved; the
        RO:0004003 edge asserts CCN2 causes it). OMIM has already split the
        label into OMIM:211350 (KMD, the CCN2 form) and OMIM:621644 (KDII,
        Itai-Ikegawa type, the KIF5B form); MONDO covers only the former. Two
        upstream MONDO requests would resolve this: refresh the stale definition,
        and add a term for OMIM:621644. Neither has been filed with MONDO yet.
        Both are tracked on the dismech side at
        https://github.com/monarch-initiative/dismech/issues/9274, which the
        KIF5B member entry records structurally in its `tracked_issues` block
        (the Grouping class has no such slot, so the pointer is prose here).
membership_criteria:
- description: >-
    A disorder belongs to the kyphomelic dysplasias if it presents with severe
    bowing or sharp angulation of the long bones, characteristically the femora,
    as a defining radiographic feature of a congenital skeletal dysplasia.
  criteria_semantics: NECESSARY
  logic:
    criterion_predicate: HAS_PHENOTYPE
    description: Bowing or sharp angulation of the femora.
    phenotype_term:
      preferred_term: Femoral bowing
      term:
        id: HP:0002980
        label: Femoral bowing
members:
- member: KIF5B-Related Kyphomelic Dysplasia
  member_type: DISEASE
  display_name: KIF5B-Related Kyphomelic Dysplasia
  differentiating_mechanisms:
  - description: >-
      De novo heterozygous (dominant) missense variants in KIF5B, encoding the
      kinesin-1 heavy chain, clustered in or near the ATPase-related motifs of
      the catalytic motor domain — a microtubule motor defect rather than a
      cartilage growth-factor defect. Clinically distinguished by postnatal
      osteoporosis with subsequent fractures and by optic atrophy, neither of
      which is reported in the CCN2 form, and by sporadic de novo occurrence
      rather than sibling recurrence.
    gene:
      preferred_term: KIF5B
      term:
        id: hgnc:6324
        label: KIF5B
- member: CCN2-Related Kyphomelic Dysplasia
  member_type: DISEASE
  display_name: CCN2-Related Kyphomelic Dysplasia
  differentiating_mechanisms:
  - description: >-
      Biallelic (autosomal recessive) loss-of-function variants in CCN2, encoding
      a matricellular protein required for chondrocyte proliferation and
      differentiation — a growth plate cartilage defect. Clinically distinguished
      by cleft palate, micro-retrognathia and radial head dislocation, and
      supported by concordant skeletal phenotypes in Ccn2-null mice and ccn2a
      knockout zebrafish. Ascertained through consanguineous families with
      sibling recurrence.
    gene:
      preferred_term: CCN2
      term:
        id: hgnc:2500
        label: CCN2
notes: >-
  Why this is a grouping and not a disease entry. This concept entered the
  curation queue as a stub carrying a single causal gene (CCN2) from MONDO,
  which made it look like a leaf disease. The literature says otherwise: the
  2022 report establishing the KIF5B form and the 2024 report establishing the
  CCN2 form both open by describing kyphomelic dysplasia as a heterogeneous
  group of skeletal dysplasias. Curating it as one Disease would have merged a
  dominant kinesin-motor disorder with a recessive matricellular-protein
  disorder on the strength of a shared radiographic appearance — precisely the
  lumping error the grouping mechanism exists to avoid.

  The membership criterion is deliberately thin. Only femoral bowing is asserted
  as NECESSARY, because that radiographic feature is what the clinical label has
  always denoted and is the only property both members demonstrably share. No
  SUFFICIENT criterion is given: long bone bowing occurs in many skeletal
  dysplasias — campomelic dysplasia, osteogenesis imperfecta, thanatophoric
  dysplasia, Stuve-Wiedemann syndrome — so bowing alone does not make a disorder
  a kyphomelic dysplasia, and asserting sufficiency here would generate false
  candidate members.

  Upstream data issues this grouping records rather than papers over. MONDO's
  definition of MONDO:0008881 still declares kyphomelic dysplasia dissolved as a
  nosological entity, citing work that predates both gene discoveries, while the
  same record asserts CCN2 as a causal gene. OMIM has already split the label
  into two numbered entities (211350 and 621644) and MONDO has followed only one
  of them. The mapping is therefore closeMatch, which under the CLAUDE.md
  mapping-predicate rule deliberately leaves MONDO:0008881 un-retired from MONDO
  *coverage tooling* until the upstream record is corrected. That is a distinct
  register from the `stubs/` curation queue, which this curation does clear:
  `stubs/Kyphomelic_Dysplasia.yaml` is deleted because the lump/split question it
  posed has been answered.

  The label may still be incompletely resolved. Older case series describe
  presentations that neither molecular form obviously accounts for, including a
  patient whose severe radiographic changes almost completely regressed by age
  seven, and families in which affected sibs were born to a mother with
  brachydactyly type E. Whether these represent further unidentified genes,
  phenocopies, or misdiagnoses is unknown, and additional members should be
  expected rather than treated as a closed set of two.