Two-Hit Tumor Suppressor Cancer Predisposition Syndromes (Knudson Syndromes)

name: Two-Hit Tumor Suppressor Cancer Predisposition Syndromes
display_name: Two-Hit Tumor Suppressor Cancer Predisposition Syndromes (Knudson Syndromes)
creation_date: "2026-08-23T00:00:00Z"
description: >-
  The Mendelian cancer predisposition syndromes in which a constitutional
  heterozygous tumor-suppressor lesion is carried by every somatic cell, and
  tumors arise where a stochastic somatic second event removes the retained
  wild-type allele. This is the group Knudson's analysis of retinoblastoma
  defined and the reason these syndromes share a clinical signature that is
  recognizable before any gene is sequenced: earlier onset than the sporadic
  counterpart tumor, multifocality, and bilaterality in paired organs. The
  members differ in almost everything else — the suppressor may govern the cell
  cycle (RB1, TP53, CDKN2A), genome maintenance (BRCA1/2, the mismatch-repair
  genes), a signalling pathway (NF1, PTEN, STK11, PTCH1, VHL, PRKAR1A), cell
  adhesion (CDH1), nutrient sensing (FLCN), or intermediary metabolism (FH,
  SDHx) — and the tumor spectrum each produces is narrow and gene-specific in a
  way the shared mechanism does not predict.
grouping_basis:
- SHARED_MECHANISM
grouping_rationale: >-
  Grouped on a shared allelic architecture rather than a shared pathway,
  phenotype or gene family: every member conforms to
  germline_two_hit_tumor_predisposition, the module that models the
  constitutional first hit, the rate-limiting somatic second hit, and the
  epidemiology that follows from them. That is a genuinely unifying mechanism
  and it is what the members have in common; the downstream pathway each
  disabled suppressor governs is not, which is why the members stay separate
  Disease entries and additionally conform to whichever hallmark or
  organ-specific module fits their own biology (Von Hippel-Lindau to
  tumor_angiogenesis, HLRCC and hereditary paraganglioma-pheochromocytoma to
  oncometabolite_dioxygenase_inhibition, HBOC to dna_repair_synthetic_lethality,
  Neurofibromatosis Type 1 to sustaining_proliferative_signaling, and so on).
  Two exclusions define the boundary and are deliberate. Dominantly acting
  proto-oncogene predisposition syndromes are out of scope: in Multiple Endocrine
  Neoplasia Type 2 a germline activating RET variant needs no second hit at the
  same locus, so the allelic architecture that defines this grouping is absent.
  Recessive genome-maintenance syndromes are also out of scope: MUTYH-Associated
  Polyposis carriers inherit two damaged alleles of a base-excision-repair gene
  and the resulting constitutional mutator phenotype inactivates APC somatically
  — a different architecture that reaches a similar clinical picture, and one the
  entry itself is explicit about. Excluding it here rather than stretching the
  criteria is the point of writing the criteria down.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0015356
      label: hereditary neoplastic syndrome
    mapping_predicate: skos:narrowMatch
    mapping_source: MONDO
    mapping_justification: >-
      narrowMatch: this grouping is the two-hit tumor-suppressor subset of the
      broader MONDO hereditary neoplastic syndrome class. MONDO:0015356 also
      covers proto-oncogene predisposition syndromes and recessive
      genome-instability syndromes, both of which are explicitly excluded here,
      so its descendant set must not be read as the curation gap list for this
      grouping.
    consistency:
    - reference: MONDO
      consistent: CONSISTENT
      notes: >-
        MONDO has no term for the two-hit allelic architecture as such, which is
        the axis this grouping is built on.
membership_criteria:
- description: >-
    Every member is a Mendelian cancer predisposition syndrome whose
    pathophysiology conforms to the germline two-hit tumor predisposition module
    — a constitutional heterozygous tumor-suppressor lesion whose tumors require
    a somatic second event at the same locus.
  criteria_semantics: NECESSARY
  logic:
    criterion_predicate: CONFORMS_TO_MODULE
    module: germline_two_hit_tumor_predisposition
    description: >-
      Conforms to at least one node of the germline two-hit tumor predisposition
      module.
members:
- member: Retinoblastoma
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The founding member and the paradigm case. Germline RB1 loss plus somatic
      loss of heterozygosity at 13q14; bilateral and multifocal disease in
      carriers, unilateral and unifocal when both hits are somatic.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: RB1
      term:
        id: hgnc:9884
        label: RB1
- member: Li-Fraumeni Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline TP53 loss with somatic loss of the wild-type allele; uniquely
      broad tumor spectrum for this grouping (sarcoma, breast, adrenocortical,
      brain, leukaemia) and marked radiation hypersensitivity.
    module: germline_two_hit_tumor_predisposition
- member: Hereditary Breast and Ovarian Cancer Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline BRCA1/BRCA2 loss; the disabled function is homologous-recombination
      repair, so biallelic loss additionally creates the PARP/platinum synthetic
      lethality that defines treatment.
    module: dna_repair_synthetic_lethality
- member: Lynch Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline mismatch-repair gene loss producing microsatellite instability and
      a hypermutated, neoantigen-rich tumor that is checkpoint-inhibitor
      sensitive — the only member whose mechanism makes immunotherapy the
      rational first-line treatment.
    module: genome_instability_mutation
- member: Classic Familial Adenomatous Polyposis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline APC loss with somatic second hits clustering in the mutation
      cluster region; second hits occur across the entire colorectal epithelium,
      producing hundreds to thousands of adenomas.
    module: germline_two_hit_tumor_predisposition
- member: Familial Adenomatous Polyposis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The general APC-related polyposis entry, retained alongside the classic-form
      entry; same two-hit APC architecture, curated at a lower level of
      genotype-phenotype detail.
    module: germline_two_hit_tumor_predisposition
- member: Peutz-Jeghers syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline STK11/LKB1 loss; the disabled suppressor is an AMPK-activating
      kinase, so the phenotype is hamartomatous polyposis with mucocutaneous
      pigmentation rather than adenomatous polyposis.
    module: germline_two_hit_tumor_predisposition
- member: Juvenile Polyposis Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline SMAD4 or BMPR1A loss; BMP/TGF-beta signalling failure produces
      hamartomatous juvenile polyps, and SMAD4 carriers additionally develop
      hereditary haemorrhagic telangiectasia.
    module: germline_two_hit_tumor_predisposition
- member: Cowden Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline PTEN loss driving PI3K/AKT/mTOR activation; hamartomas across
      multiple organs with high breast, thyroid and endometrial cancer risk.
    module: germline_two_hit_tumor_predisposition
- member: PTEN Hamartoma Tumor Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The umbrella PTEN entry spanning Cowden, Bannayan-Riley-Ruvalcaba and the
      PTEN-related overgrowth phenotypes; same nutrient-sensing suppressor,
      broader clinical scope.
    module: germline_two_hit_tumor_predisposition
- member: Von Hippel-Lindau Disease
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline VHL loss removes the E3 ligase that degrades HIF, so the tumors
      are constitutively pseudohypoxic and hypervascular — reaching the same
      state as the SDHx and FH syndromes by losing the ligase rather than
      inhibiting the hydroxylase.
    module: tumor_angiogenesis
- member: Neurofibromatosis Type 1
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline NF1 loss removes a RAS GTPase-activating protein, so biallelic
      loss in Schwann cells produces constitutive RAS-MAPK signalling and
      neurofibromas; the only member of this grouping that is also a RASopathy.
    module: sustaining_proliferative_signaling
- member: Gorlin Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline PTCH1 (or SUFU) loss removes negative regulation of Hedgehog
      signalling; second-hit loss of heterozygosity at 9q22 in basal cell
      carcinomas, with developmental as well as neoplastic features.
    module: hedgehog_pathway_activation
- member: Schwannomatosis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline SMARCB1 or LZTR1 loss on 22q, with an unusual composite four-hit,
      three-event mechanism that additionally inactivates NF2 in cis — the most
      complex allelic architecture in this grouping.
    module: germline_two_hit_tumor_predisposition
- member: Multiple Endocrine Neoplasia Type 1
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline MEN1 loss with 11q13 loss of heterozygosity; menin is a nuclear
      scaffold with no catalytic activity, so the tumors are lineage-defined
      (parathyroid, enteropancreatic, pituitary) rather than pathway-defined.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: MEN1
      term:
        id: hgnc:7010
        label: MEN1
- member: Carney Complex
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline PRKAR1A loss with locus loss of heterozygosity; the suppressor is
      the PKA regulatory subunit, so tumors show decreased basal but increased
      cAMP-stimulated kinase activity — the biochemical basis of the syndrome's
      paradoxical endocrine responses.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: PRKAR1A
      term:
        id: hgnc:9388
        label: PRKAR1A
- member: Birt-Hogg-Dube Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline FLCN loss; folliculin acts in the FNIP1-AMPK-mTOR nutrient-sensing
      network, and the renal tumors are oncocytic and chromophobe rather than
      clear-cell, distinguishing it from Von Hippel-Lindau disease.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: FLCN
      term:
        id: hgnc:27310
        label: FLCN
- member: Hereditary Leiomyomatosis and Renal Cell Cancer
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline FH loss; the suppressor is a TCA-cycle enzyme and the oncogenic
      signal is its accumulated substrate, which both competitively inhibits
      2-oxoglutarate-dependent dioxygenases and covalently succinates KEAP1.
    module: oncometabolite_dioxygenase_inhibition
    gene:
      preferred_term: FH
      term:
        id: hgnc:3700
        label: FH
- member: Hereditary Pheochromocytoma-Paraganglioma Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline succinate dehydrogenase subunit loss; accumulated succinate drives
      both pseudohypoxia and a hypermethylator phenotype, with SDHB carrying the
      highest metastatic risk and SDHD showing a paternal-transmission effect.
    module: oncometabolite_dioxygenase_inhibition
    gene:
      preferred_term: SDHB
      term:
        id: hgnc:10681
        label: SDHB
- member: CDH1-Related Hereditary Diffuse Gastric Cancer
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Germline CDH1 loss; the suppressor is an adhesion molecule, so the tumor is
      diffusely infiltrative and endoscopically occult — the reason prophylactic
      total gastrectomy rather than surveillance is the recommended risk
      management.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: CDH1
      term:
        id: hgnc:1748
        label: CDH1
- member: DICER1 Tumor Predisposition Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The partial member. Germline DICER1 truncating first hit is classical, but
      the somatic event is an RNase IIIb missense change altering strand-selective
      processing rather than abolishing the enzyme, and in pleuropulmonary
      blastoma the compartment losing DICER1 is not the compartment that
      transforms. Conforms at the first-hit and tumor-spectrum nodes only.
    module: germline_two_hit_tumor_predisposition
    gene:
      preferred_term: DICER1
      term:
        id: hgnc:17098
        label: DICER1