Hydrolethalus Syndrome 2

name: Hydrolethalus Syndrome 2
creation_date: "2026-06-16T20:00:00Z"
category: Mendelian
description: >-
  Hydrolethalus syndrome 2 (HLS2) is a rare, autosomal recessive, perinatally
  lethal multiple-malformation disorder caused by biallelic mutations in KIF7,
  the human ortholog of Drosophila Costal2 and a cilium-tip regulator of the
  Sonic hedgehog (SHH) pathway. It sits at the severe, frequently lethal end of
  the KIF7-related ciliopathy spectrum that also encompasses acrocallosal
  syndrome (a milder allelic disorder) and a subset of Joubert syndrome. HLS2 is
  characterized by a severe midline brain malformation — most often hydrocephaly
  with absent midline structures and corpus callosum agenesis — together with
  pre- and/or postaxial polydactyly, cleft palate, and other midline
  craniofacial defects. Mechanistically, KIF7 dysfunction deregulates GLI
  transcription-factor output and impairs GLI3 processing in the primary cilium,
  derailing the tightly dosed Hedgehog signaling that patterns the midline
  brain, limbs, and face; affected fetuses are stillborn or die shortly after
  birth.
disease_term:
  preferred_term: hydrolethalus syndrome 2
  term:
    id: MONDO:0013585
    label: hydrolethalus syndrome 2
parents:
- Ciliopathies
inheritance:
- name: Autosomal Recessive
  description: >-
    Hydrolethalus syndrome 2 is inherited in an autosomal recessive manner,
    caused by biallelic (homozygous or compound heterozygous) loss-of-function
    mutations in KIF7. Cases are frequently reported in consanguineous families,
    and recurrence risk for siblings is 25%.
  evidence:
  - reference: DOI:10.1186/1750-1172-7-27
    reference_title: "A mutation in KIF7 is responsible for the autosomal recessive syndrome of macrocephaly, multiple epiphyseal dysplasia and distinctive facial appearance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      a regulator of Hedgehog signaling that has been recently found to be
      causing Joubert syndrome, fetal hydrolethalus and acrocallosal syndromes
    explanation: >-
      Identifies KIF7 as the recessive disease gene causing fetal hydrolethalus
      syndrome, within the broader autosomal recessive KIF7 ciliopathy spectrum.
  - reference: PMID:27081521
    reference_title: "The many faces of KIF7."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      born to consanguineous parents
    explanation: >-
      Documents the consanguineous family background typical of the recessively
      inherited KIF7-related ciliopathies, consistent with autosomal recessive
      transmission of HLS2.
prevalence:
- population: Published clinical estimates
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  percentage: Very rare; fewer than 20 reported KIF7-mutant cases
  notes: >-
    Hydrolethalus syndrome 2 is an exceedingly rare disorder; the KIF7-mutant
    fetal hydrolethalus phenotype was delineated in 2011, and only a small
    number of cases have been reported since, typically diagnosed prenatally or
    at autopsy. The phenotype lies at the perinatally lethal end of the KIF7
    spectrum. Hydrolethalus syndrome 1 (HYLS1) is the historically Finnish,
    genetically distinct form; HLS2 denotes the KIF7-related form.
pathophysiology:
- name: KIF7 Cilium-Tip Dysfunction and Impaired Hedgehog Signal Transduction
  conforms_to: "ciliopathy_dysfunction#Impaired Hedgehog Signal Transduction"
  description: >-
    KIF7, the human ortholog of Drosophila Costal2, is an immotile kinesin-4
    protein and a component of the anterograde intraflagellar transport (IFT)
    machinery of the primary cilium, where it organizes the cilium-tip
    compartment and regulates the GLI transcription factors of the Hedgehog
    pathway. In the absence of Hedgehog ligand KIF7 promotes processing of GLI3
    into its transcriptional repressor form; upon pathway activation KIF7 and
    GLI accumulate at the cilium tip. Biallelic loss-of-function KIF7 mutations
    deregulate most GLI target genes and impair GLI3 processing, disrupting the
    tightly dosed Hedgehog output required for midline brain, limb, and
    craniofacial patterning. In HLS2 this lesion is severe enough to produce a
    lethal malformation complex.
  biological_processes:
  - preferred_term: Smoothened (Hedgehog) Signaling
    term:
      id: GO:0007224
      label: smoothened signaling pathway
    modifier: ABNORMAL
  - preferred_term: Regulation of Hedgehog Signaling
    term:
      id: GO:0008589
      label: regulation of smoothened signaling pathway
    modifier: DECREASED
  cellular_components:
  - preferred_term: Ciliary Tip
    term:
      id: GO:0097542
      label: ciliary tip
  - preferred_term: Primary Cilium
    term:
      id: GO:0005929
      label: cilium
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  downstream:
  - target: Lethal Midline Brain and Craniofacial Malformation
    description: >-
      Deregulated cilium-dependent Hedgehog/GLI signaling disrupts dorsoventral
      patterning and midline development of the brain and face, producing the
      severe hydrocephalic CNS malformation, corpus callosum agenesis, and cleft
      palate of HLS2.
  - target: Polydactyly
    description: >-
      Deregulated cilium-dependent Hedgehog/GLI3 output disrupts limb
      anterior-posterior patterning, producing pre- or postaxial polydactyly.
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      KIF7, the human ortholog of Drosophila Costal2, is a key component of the
      Hedgehog signaling pathway
    explanation: >-
      Identifies KIF7 as a core Hedgehog pathway component (Costal2 ortholog),
      establishing the molecular basis of the HLS2 mechanism.
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we show deregulation of most GLI transcription factor targets and impaired
      GLI3 processing in tissues from individuals with KIF7 mutations
    explanation: >-
      Demonstrates the downstream molecular consequence of KIF7 loss in patient
      tissues: deregulated GLI targets and impaired GLI3 processing.
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Our data show the role of KIF7 in human primary cilia, especially in the
      Hedgehog pathway through the regulation of GLI targets, and expand the
      clinical spectrum of ciliopathies
    explanation: >-
      Places HLS2 within the primary-cilium Hedgehog-signaling ciliopathy
      spectrum, supporting conformance to the ciliopathy module Hedgehog node.
  - reference: PMID:27081521
    reference_title: "The many faces of KIF7."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the gene that encodes a component of the kinesin complex of anterograde
      intraflagellar transport in the cilia
    explanation: >-
      Establishes KIF7 as a component of the anterograde intraflagellar
      transport machinery of the cilium, linking the molecular lesion to the
      ciliary compartment.
- name: Lethal Midline Brain and Craniofacial Malformation
  conforms_to: "ciliopathy_dysfunction#Cerebellar and CNS Malformation"
  description: >-
    Impaired cilium-dependent Hedgehog signaling during early neurodevelopment
    produces the hallmark CNS malformation of HLS2: a severe midline brain
    defect, most often hydrocephaly with absent midline structures and agenesis
    of the corpus callosum, frequently with cleft palate and other midline
    craniofacial defects. This malformation complex lies at the severe,
    perinatally lethal extreme of the KIF7 ciliopathy spectrum.
  biological_processes:
  - preferred_term: Corpus Callosum Development
    term:
      id: GO:0022038
      label: corpus callosum development
    modifier: ABNORMAL
  - preferred_term: Neural Tube Formation
    term:
      id: GO:0001841
      label: neural tube formation
    modifier: ABNORMAL
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      polydactyly, brain abnormalities and cleft palate
    explanation: >-
      Lists brain abnormalities and cleft palate among the overlapping
      malformation features of fetal hydrolethalus and acrocallosal syndromes in
      the original KIF7 discovery paper.
  - reference: PMID:27081521
    reference_title: "The many faces of KIF7."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      absent corpus callosum and hydrocephalus in addition to polydactyly
    explanation: >-
      Describes the CNS involvement of the KIF7 ciliopathy spectrum (absent
      corpus callosum and hydrocephalus), of which HLS2 is the severe lethal
      extreme.
  downstream:
  - target: Hydrocephalus
    description: >-
      The severe midline CNS malformation includes hydrocephalus.
  - target: Agenesis of Corpus Callosum
    description: >-
      Disrupted midline brain patterning includes agenesis or absence of the
      corpus callosum.
  - target: Cleft Palate
    description: >-
      Disrupted midline craniofacial patterning produces cleft palate.
phenotypes:
- name: Hydrocephalus
  category: Neurologic
  description: >-
    Severe hydrocephaly with absent midline structures is the defining CNS
    malformation of hydrolethalus syndrome and the central feature of the HLS2
    malformation complex.
  phenotype_term:
    preferred_term: Hydrocephalus
    term:
      id: HP:0000238
      label: Hydrocephalus
  evidence:
  - reference: PMID:27081521
    reference_title: "The many faces of KIF7."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      absent corpus callosum and hydrocephalus in addition to polydactyly
    explanation: >-
      Hydrocephalus is described as part of the CNS involvement of the KIF7
      ciliopathy spectrum, of which HLS2 is the severe lethal end.
- name: Agenesis of Corpus Callosum
  category: Neurologic
  description: >-
    Agenesis or absence of the corpus callosum is part of the severe midline
    brain malformation of HLS2.
  phenotype_term:
    preferred_term: Agenesis of corpus callosum
    term:
      id: HP:0001274
      label: Agenesis of corpus callosum
  evidence:
  - reference: PMID:27081521
    reference_title: "The many faces of KIF7."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      absent corpus callosum and hydrocephalus in addition to polydactyly
    explanation: >-
      Absent corpus callosum is a core CNS malformation of the KIF7 ciliopathy
      spectrum that includes HLS2.
- name: Polydactyly
  category: Skeletal
  description: >-
    Pre- and/or postaxial polydactyly is a cardinal limb feature of HLS2,
    reflecting deregulated Hedgehog/GLI3 anterior-posterior limb patterning.
  phenotype_term:
    preferred_term: Polydactyly
    term:
      id: HP:0010442
      label: Polydactyly
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      polydactyly, brain abnormalities and cleft palate
    explanation: >-
      Polydactyly is listed among the cardinal overlapping malformations of
      fetal hydrolethalus and acrocallosal syndromes.
- name: Cleft Palate
  category: Craniofacial
  description: >-
    Cleft palate is a recurrent midline craniofacial malformation in HLS2,
    reflecting shared deregulation of midline Hedgehog patterning.
  phenotype_term:
    preferred_term: Cleft palate
    term:
      id: HP:0000175
      label: Cleft palate
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      polydactyly, brain abnormalities and cleft palate
    explanation: >-
      Cleft palate is enumerated among the overlapping malformation features of
      fetal hydrolethalus and acrocallosal syndromes.
genetic:
- name: KIF7 Mutations
  association: Causative
  gene_term:
    preferred_term: KIF7
    term:
      id: hgnc:30497
      label: KIF7
  notes: >-
    Biallelic (homozygous or compound heterozygous) loss-of-function mutations
    in KIF7 (15q26.1) cause hydrolethalus syndrome 2. KIF7 encodes the human
    ortholog of Drosophila Costal2, a cilium-tip kinesin-4 regulator of the
    Sonic hedgehog pathway. Allelic KIF7 mutations cause the milder acrocallosal
    syndrome and a subset of Joubert syndrome; HLS2 represents the severe,
    frequently lethal extreme.
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we report mutations in KIF7 in individuals with hydrolethalus and
      acrocallosal syndromes, two multiple malformation disorders with
      overlapping features that include polydactyly, brain abnormalities and
      cleft palate
    explanation: >-
      Original report identifying KIF7 as the causative gene for fetal
      hydrolethalus syndrome (HLS2) and the allelic acrocallosal syndrome.
  - reference: DOI:10.1186/1750-1172-7-27
    reference_title: "A mutation in KIF7 is responsible for the autosomal recessive syndrome of macrocephaly, multiple epiphyseal dysplasia and distinctive facial appearance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the first missense homozygous disease-causing mutation in KIF7
      and expand the clinical spectrum associated with mutations in this gene
    explanation: >-
      Confirms KIF7 as a recessive disease gene with an expanding allelic
      ciliopathy spectrum that includes fetal hydrolethalus.
diagnosis:
- name: Prenatal and Molecular Diagnosis
  description: >-
    Hydrolethalus syndrome 2 is suspected prenatally from the combination of a
    severe midline brain malformation (hydrocephaly with absent midline
    structures), polydactyly, and cleft palate, and is confirmed by
    identification of biallelic KIF7 pathogenic variants. Diagnosis is commonly
    established prenatally by ultrasound and molecular testing or at autopsy.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: MAXO:0000533
      label: molecular genetic testing
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we report mutations in KIF7 in individuals with hydrolethalus and
      acrocallosal syndromes
    explanation: >-
      Identification of biallelic KIF7 variants is the basis of molecular
      confirmation of HLS2.
differential_diagnoses:
- name: Acrocallosal Syndrome
  description: >-
    Acrocallosal syndrome is the milder, viable allelic KIF7-related disorder,
    sharing corpus callosum agenesis, polydactyly, and craniofacial dysmorphism
    but lacking the lethal hydrocephalic malformation complex of HLS2.
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      hydrolethalus and acrocallosal syndromes, two multiple malformation
      disorders with overlapping features that include polydactyly, brain
      abnormalities and cleft palate
    explanation: >-
      Establishes acrocallosal syndrome as the allelic, phenotypically
      overlapping disorder distinguishing the milder end of the KIF7 spectrum.
- name: Hydrolethalus Syndrome 1
  description: >-
    Hydrolethalus syndrome 1 (HYLS1) is the genetically distinct, historically
    Finnish form of hydrolethalus with a near-identical lethal clinical
    malformation complex; it is distinguished from HLS2 by its causal gene.
  evidence:
  - reference: PMID:21552264
    reference_title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we report mutations in KIF7 in individuals with hydrolethalus and
      acrocallosal syndromes
    explanation: >-
      The KIF7-related (HLS2) form must be distinguished from the genetically
      distinct HYLS1 form, which shares the hydrolethalus clinical phenotype.
treatments:
- name: Supportive and Palliative Care
  description: >-
    There is no disease-modifying therapy for HLS2, which is perinatally lethal.
    Management is supportive and palliative, with genetic counseling for the
    family.
  treatment_term:
    preferred_term: supportive care
    term:
      id: MAXO:0000950
      label: supportive care
- name: Genetic Counseling
  description: >-
    Autosomal recessive inheritance entails a 25% recurrence risk for siblings.
    Once the biallelic KIF7 variants are identified, carrier testing, prenatal
    testing, and preimplantation genetic testing become available to families.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
references:
- reference: PMID:21552264
  title: "KIF7 mutations cause fetal hydrolethalus and acrocallosal syndromes."
- reference: PMID:27081521
  title: "The many faces of KIF7."
- reference: DOI:10.1186/1750-1172-7-27
  title: "A mutation in KIF7 is responsible for the autosomal recessive syndrome of macrocephaly, multiple epiphyseal dysplasia and distinctive facial appearance."