Brittle Cornea Syndrome

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Inheritance

1

Autosomal recessive HP:0000007

Both genetic subtypes are autosomal recessive; several defining families were consanguineous and were solved by autozygosity mapping.

autosomal recessive inheritance

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"Extreme corneal fragility and thinning, which have a high risk of catastrophic spontaneous rupture, are the cardinal features of brittle cornea syndrome (BCS), an autosomal-recessive generalized connective tissue disorder."

States the inheritance mode and the systemic (not purely ocular) nature of the disorder.

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Subtypes

2

Brittle cornea syndrome 1 (ZNF469) MONDO:0024543

ZNF469 hgnc:23216

Caused by biallelic ZNF469 variants. ZNF469 encodes a zinc finger protein originally identified as a quantitative trait locus for central corneal thickness. It is the more frequently identified of the two genes: 14 families in the series that defined the molecular diagnosis, against 8 with PRDM5. Clinically it is not distinguishable from BCS2.

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"We have previously identified mutations in ZNF469 in 14 families (in addition to 6 reported by others in the literature), and in PRDM5 in 8 families (with 1 further family now published by others)."

Quantifies the relative contribution of the two genes in the largest ascertained series, supporting ZNF469 as the more commonly identified cause.

Brittle cornea syndrome 2 (PRDM5) MONDO:0013605

PRDM5 hgnc:9349

Caused by biallelic PRDM5 variants. PRDM5 is a PR-domain transcriptional repressor that recruits chromatin-modifying complexes to extracellular matrix target genes. Both truncating alleles (p.Arg590*, p.Glu134*, p.Cys325LeufsX2, deletion of exons 9-14) and at least one missense allele (p.Arg83Cys) are pathogenic, and they fail in mechanistically different ways - the truncations remove zinc finger and PR domains outright, while p.Arg83Cys leaves DNA binding intact and instead disrupts recruitment of repressive complexes.

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"Using autozygosity mapping, we identified mutations in PRDM5 in families with BCS."

Establishes PRDM5 as the second BCS gene.

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Discussions and Knowledge Gaps

1

Should brittle cornea syndrome be curated as one disease with two genotypic subtypes, as done here, or as two sibling Disease entries keyed on MONDO:0024543 and MONDO:0013605?

OPEN QUESTION OPEN split_bcs_one_disease_two_loci

Attached to

pathophysiology#Loss of PRDM5 or ZNF469 Transcriptional Regulator Function

This entry lumps, and the reasoning should be auditable rather than assumed. In favour of lumping: the two genes were shown to participate in the same regulatory pathway; the downstream lesion, transcriptional dysregulation of fibrillar collagens, is shared; the clinical feature list and the management plan in the definitive review are stated once for BCS rather than per gene; and dismech's own rule is that a disorder entry needs one reasonably conserved pathograph, which BCS has. In favour of splitting: MONDO models them as two distinct children, and ZNF469 and PRDM5 are not the same molecule, so a gene-level query answers differently depending on the choice. That objection is addressed here by giving each subtype its own subtype_term, so both MONDO IDs remain curated and queryable without duplicating a pathograph that does not differ. If future work demonstrates a genotype-phenotype difference between BCS1 and BCS2 - none is currently described - splitting would become the better model.

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Pathophysiology

5

Loss of PRDM5 or ZNF469 Transcriptional Regulator Function

The initiating lesion is biallelic loss of function in one of two transcriptional regulators that act in a shared pathway. PRDM5 is a PR-domain zinc finger repressor; ZNF469 is a zinc finger protein of previously undefined function identified as a quantitative trait locus for central corneal thickness. The demonstration that they participate in the same regulatory pathway is what licenses curating BCS as one disease with two genotypic subtypes rather than two sibling diseases.

PRDM5 hgnc:9349 ZNF469 hgnc:23216

regulation of DNA-templated transcription GO:0006355 ⇓ LOSS OF FUNCTION

Downstream

Failure of Repressive Chromatin Complex Recruitment at ECM Target Genes For PRDM5, loss of function is expressed as failure to recruit repressive chromatin machinery to its target loci.

Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression Both genes converge on transcriptional control of extracellular matrix components.

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"We show that ZNF469 and PRDM5, two genes that when mutated cause BCS, participate in the same regulatory pathway."

The pathway-convergence result. This is the specific evidence behind the lump decision recorded in this entry's description.

Failure of Repressive Chromatin Complex Recruitment at ECM Target Genes

PRDM5 acts by recruiting repressive complexes and depositing H3K9 di-methylation at target genes. The BCS2 missense allele p.Arg83Cys is mechanistically informative because it leaves the DNA-binding zinc fingers intact and instead diminishes interaction with the NuRD component CHD4 and with HP1BP3, showing that loss of the protein-protein interaction alone is sufficient to cause disease. Patient fibroblasts carrying truncating alleles show dysregulated H3K9me2, and ECM stability mediators are upregulated - the direction expected from loss of a repressor, and worth noting because the net downstream consequence is nonetheless a structurally deficient matrix.

dermal fibroblast CL:0000057

PRDM5 hgnc:9349

negative regulation of transcription by RNA polymerase II GO:0000122 ⇓ LOSS OF FUNCTION

Downstream

Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression Loss of repressive control at ECM loci produces the downstream matrix transcriptional lesion.

Show evidence (2 references)

PMID:26395458 SUPPORT In Vitro

"demonstrate that the BCS2 mutation p.Arg83Cys diminishes interaction of PRDM5 with repressive complexes, including NuRD complex protein CHD4, and the repressive chromatin interactor HP1BP3, by co-immunoprecipitation combined with mass spectrometry."

Identifies the specific protein-protein interactions lost, and shows a missense allele can be pathogenic without disrupting DNA binding.

PMID:26395458 SUPPORT In Vitro

"Gene ontology analysis of dysregulated PRDM5-target genes reveals enrichment for extracellular matrix (ECM) genes supporting vascular integrity and development."

Connects the chromatin lesion to the extracellular matrix target class that carries the phenotype.

Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression

The shared downstream lesion is loss of transcriptional control over extracellular matrix components, particularly the fibrillar collagens. This is the node the authors identify as the key molecular mechanism underlying corneal fragility, and it is the node at which the two genotypic subtypes become indistinguishable.

corneal keratocyte CL:0002363

extracellular matrix organization GO:0030198 ↓ DECREASED collagen biosynthetic process GO:0032964 ↓ DECREASED

substantia propria of cornea UBERON:0001777

Downstream

Corneal Stromal Thinning and Biomechanical Failure A deficient collagen matrix cannot maintain corneal stromal thickness or tensile strength.

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"We demonstrate that regulation of expression of extracellular matrix components, particularly fibrillar collagens, by PRDM5 is a key molecular mechanism that underlies corneal fragility in BCS and controls normal corneal development and maintenance."

The authors' own statement of the central mechanism, naming fibrillar collagens as the operative target class.

Corneal Stromal Thinning and Biomechanical Failure

The cornea becomes extremely thin - central corneal thickness below 400 micrometres in the large majority of patients, against a normal range of 515 to 575, with a thinnest point of 151 and 157 micrometres in two reported cases - and loses the tensile strength needed to contain intraocular pressure. Thinning is most pronounced centrally, with the periphery relatively preserved. The same matrix deficiency produces protrusion (keratoglobus, keratoconus), scleral thinning visible as blue sclerae, and axial elongation manifesting as high myopia.

corneal keratocyte CL:0002363

cornea UBERON:0000964

Downstream

Corneal Rupture A cornea below the thickness at which it can contain intraocular pressure ruptures spontaneously or after trivial trauma.

Show evidence (1 reference)

PMID:40647596 SUPPORT Human Clinical

"Two siblings-a 28-year-old male and a 25-year-old female-presented with progressive visual deterioration and marked corneal thinning"

Documents the degree of corneal thinning in genetically confirmed PRDM5 disease.

Corneal Rupture

Rupture is the event that defines the disease's severity and its management. It occurs spontaneously or after minimal trauma, and historically enucleation was frequently the only option, causing blindness. Because rupture can be the presenting feature in a child, it has been misattributed to non-accidental injury - a diagnostic error with obvious consequences, and a reason recognition matters beyond the eye clinic.

cornea UBERON:0000964

Show evidence (2 references)

PMID:21664999 SUPPORT Human Clinical

"Enucleation is frequently the only management option for this condition, resulting in blindness and psychosocial distress."

Establishes the severity and historical management of rupture.

PMID:23642083 SUPPORT Human Clinical

"Corneal rupture may be the presenting feature of BCS, and it is possible that this may be incorrectly attributed to non-accidental injury."

Records the safeguarding hazard, which is a recognition issue rather than a purely ophthalmic one.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

10

Eye 3

High Myopia HP:0011003

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Clinical features include extreme corneal thinning with rupture, high myopia, blue sclerae, deafness of mixed aetiology with hypercompliant tympanic membranes, and variable skeletal manifestations."

The consensus clinical feature list from the series that defined molecular diagnosis in BCS.

Keratoconus HP:0000563

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"Even when the cornea remains grossly intact, visual function could also be impaired by a high degree of myopia and keratoconus."

Records that vision is threatened by refractive and ectatic change even without rupture.

Blue Sclerae HP:0000592

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Clinical features include extreme corneal thinning with rupture, high myopia, blue sclerae, deafness of mixed aetiology with hypercompliant tympanic membranes, and variable skeletal manifestations."

Lists blue sclerae among the defining clinical features.

Musculoskeletal 2

Joint Hypermobility HP:0001382

Show evidence (1 reference)

PMID:35791165 SUPPORT Human Clinical

"A systemic evaluation revealed sensorineural hearing loss and hyperextensible joints."

Documents joint hyperextensibility on systemic evaluation in a BCS sibship. Note this report describes the hearing loss as sensorineural, where the definitive review describes deafness of mixed aetiology; the auditory phenotype is curated from the latter.

Developmental Dysplasia of the Hip Hip dysplasia HP:0001385

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Mainstays of management include the prevention of ocular rupture by provision of protective polycarbonate spectacles, careful monitoring of visual and auditory function, and assessment for skeletal complications such as developmental dysplasia of the hip."

Names developmental dysplasia of the hip as the skeletal complication requiring active assessment. This snippet previously sat on the joint hypermobility phenotype, which it does not mention.

Other 5

Extreme Corneal Thinning OBLIGATE Decreased corneal thickness HP:0100689

Show evidence (5 references)

PMID:23642083 SUPPORT Human Clinical

"Brittle cornea syndrome (BCS) is an autosomal recessive disorder characterised by extreme corneal thinning and fragility."

Corneal thinning is definitional for BCS, which is what supports the OBLIGATE band.

PMID:40647596 SUPPORT Human Clinical

"From a clinical standpoint, BCS is marked by severe corneal thinning, with central corneal thickness (CCT) often measuring below 400 µm and, in extreme cases, even below 300 µm"

Supplies the quantitative thresholds the description states, in central corneal thickness terms.

PMID:23642083 SUPPORT Human Clinical

"the large majority of patients with BCS have had central corneal thickness (CCT) of less than 400 μm (normal range 515– 575 μm"

Gives the normal central corneal thickness range the disease figure is measured against, so the comparison is cited rather than assumed.

+ 2 more references

Corneal Perforation HP:0100583

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Corneal rupture can therefore occur either spontaneously or following minimal trauma in affected patients."

Establishes rupture as a discrete event distinct from the thinning.

Keratoglobus HP:0001119

Show evidence (1 reference)

PMID:40647596 SUPPORT Human Clinical

"Detailed ophthalmologic evaluation revealed keratoglobus, high myopia, and irregular astigmatism."

Documents keratoglobus in genetically confirmed PRDM5 disease, alongside the refractive consequences.

Mixed Deafness Mixed hearing impairment HP:0000410

Show evidence (2 references)

PMID:23642083 SUPPORT Human Clinical

"Clinical features include extreme corneal thinning with rupture, high myopia, blue sclerae, deafness of mixed aetiology with hypercompliant tympanic membranes, and variable skeletal manifestations."

Records the mixed deafness and its characteristic tympanic finding.

PMID:21664999 SUPPORT Human Clinical

"Deafness is another common feature and results in combined sensory deprivation."

Supports deafness as a common feature and names the combined-deficit consequence.

Hypercompliant Tympanic Membranes Tympanic membrane hypermobility HP:6000015

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Clinical features include extreme corneal thinning with rupture, high myopia, blue sclerae, deafness of mixed aetiology with hypercompliant tympanic membranes, and variable skeletal manifestations."

Names hypercompliant tympanic membranes explicitly among the defining clinical features.

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Genetic Associations

2

PRDM5 (Biallelic loss-of-function variants in PRDM5, a PR-domain zinc finger transcriptional repressor, cause BCS2.)

Gene: PRDM5 hgnc:9349

Show evidence (1 reference)

PMID:40647596 SUPPORT Human Clinical

"Genetic testing identified the homozygous pathogenic variant c.974delG (p.Cys325LeufsX2) in the PRDM5 gene in both cases."

A recent genetically confirmed sibling pair, giving a specific homozygous truncating PRDM5 allele.

ZNF469 (Biallelic ZNF469 variants cause BCS1. ZNF469 was independently identified as a quantitative trait locus for central corneal thickness, which is consistent with a dose-sensitive role in setting corneal stromal thickness.)

Gene: ZNF469 hgnc:23216

Show evidence (1 reference)

PMID:21664999 SUPPORT Human Clinical

"ZNF469, encoding a zinc finger protein of hitherto undefined function, has been identified as a quantitative trait locus for central corneal thickness, and mutations in this gene have been demonstrated in Tunisian Jewish and Palestinian kindreds with BCS."

Establishes ZNF469 as a BCS gene and links it to normal variation in corneal thickness.

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Medical Actions

3

Protective Polycarbonate Spectacles

Action: protective eyewear Ontology label: Supportive Care NCIT:C15747

The primary preventive intervention. Because the treatable event is rupture rather than the matrix lesion, physical protection of the globe is the mainstay of management and is not a stopgap for something better.

Mechanism Target:

INHIBITS Corneal Rupture — Acts on the rupture event by excluding trauma, not on any upstream mechanism node.

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Mainstays of management include the prevention of ocular rupture by provision of protective polycarbonate spectacles"

Establishes protective spectacles as the mainstay of rupture prevention.

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Mainstays of management include the prevention of ocular rupture by provision of protective polycarbonate spectacles, careful monitoring of visual and auditory function, and assessment for skeletal complications such as developmental dysplasia of the hip."

The management standard, including the surveillance components.

Keratoplasty

Action: corneal transplantation Ontology label: Corneal Transplantation NCIT:C210959

Corneal transplantation, penetrating or deep anterior lamellar. In a recent genetically confirmed sibling pair both achieved good corrected acuity and remained complication-free at seven years, though one planned lamellar procedure had to be converted intraoperatively because of central endothelial perforation - which is the operative expression of the tissue fragility the disease is named for.

Show evidence (1 reference)

PMID:40647596 SUPPORT Human Clinical

"The female initially underwent deep anterior lamellar keratoplasty (DALK), which was converted to PKP intraoperatively due to central endothelial perforation, resulting in a BCVA of 20/25."

Documents both the achievable visual outcome and the intraoperative fragility that constrains surgical planning.

Corneal Collagen Cross-Linking

Action: corneal collagen cross-linking Ontology label: Therapeutic Procedure NCIT:C49236

Photochemical stiffening of the corneal stroma. It is the only reported intervention that acts on the biomechanical lesion itself rather than on the rupture event, but the evidence is two paediatric cases, not a series. The standard (Dresden) protocol is contraindicated below 200 micrometres, which excludes most brittle corneas; both reported patients were treated with a transepithelial protocol and UV fluence scaled to their thickness. Curated as an emerging option, not a standard of care.

Mechanism Target:

INHIBITS Corneal Stromal Thinning and Biomechanical Failure — Unlike spectacles, which exclude the trauma that triggers rupture, cross-linking targets the weakened stroma upstream of it.

Show evidence (1 reference)

PMID:40647596 SUPPORT Human Clinical

"Both cases achieved better visual acuity and maintained stable endothelial cell counts, indicating that, with protocol modifications, cross-linking may be a viable treatment even in ultra-thin corneas normally excluded by the standard Dresden criteria"

Reports improvement in the biomechanically determined outcome, but from two cases only, so the effect on the stromal node is PARTIAL rather than SUPPORT.

Show evidence (3 references)

PMID:40647596 SUPPORT Human Clinical

"Two pediatric patients with brittle cornea syndrome, each with central corneal thickness < 280 μm, underwent transepithelial (epithelium-on) corneal cross-linking with UV fluence scaled to corneal thickness."

The primary report of cross-linking in genetically relevant brittle corneas, and the source of the transepithelial, thickness-scaled protocol.

PMID:40647596 SUPPORT Human Clinical

"In these two cases, conventional cross-linking was not considered due to contraindications for corneal thickness less than 200 µm."

Records why the standard protocol is unavailable in this disease, which is what makes the transepithelial variant the relevant one.

PMID:40647596 SUPPORT Human Clinical

"There is currently no pharmacological treatment available to halt the progression of corneal thinning, making surgical intervention the primary treatment option for visual rehabilitation"

Places cross-linking in context: there is no drug for the matrix lesion, so procedural options carry the whole therapeutic burden.

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Diagnosis

2

ZNF469 and PRDM5 molecular genetic testing (Positive)

Molecular diagnosis is available and effective, because the two known genes together account for the condition in nearly all ascertained patients. It also identifies heterozygous carriers, which matters in the consanguineous kindreds in which BCS is most often seen. Whole-exome sequencing is the route used in recent genetically confirmed cases.

Show evidence (2 references)

PMID:23642083 SUPPORT Human Clinical

"Two genes, ZNF469 and PRDM5, have now been identified, in which causative pathogenic mutations collectively account for the condition in nearly all patients with BCS ascertained to date. Therefore, effective molecular diagnosis is now available for affected patients, and those at risk of being..."

Establishes that two-gene testing is diagnostically near-complete and extends to carrier identification.

PMID:40647596 SUPPORT Human Clinical

"This report presents two genetically confirmed cases of BCS in Albanian siblings, emphasizing the diagnostic value of whole-exome sequencing and individualized surgical management strategies."

Documents whole-exome sequencing as the practical diagnostic route.

Recognition against overlapping connective tissue disorders

The hard part of diagnosis is not the test but reaching it. BCS overlaps phenotypically with other heritable connective tissue disorders, and corneal rupture as a presenting event in a child has been misattributed to non-accidental injury. Early recognition is what prevents avoidable blindness.

Show evidence (1 reference)

PMID:23642083 SUPPORT Human Clinical

"Effective management depends upon appropriate identification of affected individuals, which may be challenging given the phenotypic overlap of BCS with other connective tissue disorders."

States the recognition problem explicitly and ties management outcomes to it.

{ }

Source YAML

click to show

name: Brittle Cornea Syndrome
creation_date: "2026-08-23T16:00:00Z"
category: Mendelian
synonyms:
- BCS
- Brittle cornea syndrome 1
- Brittle cornea syndrome 2
- BCS1
- BCS2
- Corneal fragility, keratoglobus, blue sclerae, joint hyperextensibility
- Fragilitas oculi with joint hyperextensibility
description: >-
  Brittle cornea syndrome (BCS) is an autosomal recessive generalized connective
  tissue disorder in which extreme corneal thinning leads to spontaneous or
  minimally traumatic corneal rupture, accompanied by high myopia, blue sclerae,
  keratoconus or keratoglobus, mixed deafness with hypercompliant tympanic
  membranes, and joint hypermobility. Two genes account for the condition in
  nearly all patients ascertained to date: ZNF469 (BCS1) and PRDM5 (BCS2).
  This entry is curated as one disease with two genotypic subtypes rather than
  as two sibling entries, because the two genes were shown to participate in the
  same regulatory pathway, the downstream lesion is the same - loss of
  transcriptional control over fibrillar collagen and other extracellular matrix
  genes - and the clinical picture and management are not separable by gene.
  Corneal rupture may be the presenting event and has been mistaken for
  non-accidental injury, which makes recognition a safeguarding matter as well
  as an ophthalmic one. BCS was historically classified as Ehlers-Danlos
  syndrome type VIB and is no longer an EDS subtype.
disease_term:
  preferred_term: Brittle Cornea Syndrome
  term:
    id: MONDO:0009242
    label: brittle cornea syndrome
parents:
- Connective Tissue Disease
has_subtypes:
- name: BCS1
  display_name: Brittle cornea syndrome 1 (ZNF469)
  subtype_term:
    preferred_term: brittle cornea syndrome 1
    term:
      id: MONDO:0024543
      label: brittle cornea syndrome 1
  description: >-
    Caused by biallelic ZNF469 variants. ZNF469 encodes a zinc finger protein
    originally identified as a quantitative trait locus for central corneal
    thickness. It is the more frequently identified of the two genes: 14
    families in the series that defined the molecular diagnosis, against 8 with
    PRDM5. Clinically it is not distinguishable from BCS2.
  genes:
  - preferred_term: ZNF469
    term:
      id: hgnc:23216
      label: ZNF469
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We have previously identified mutations in ZNF469 in 14 families (in
      addition to 6 reported by others in the literature), and in PRDM5 in 8
      families (with 1 further family now published by others).
    explanation: >-
      Quantifies the relative contribution of the two genes in the largest
      ascertained series, supporting ZNF469 as the more commonly identified
      cause.
- name: BCS2
  display_name: Brittle cornea syndrome 2 (PRDM5)
  subtype_term:
    preferred_term: brittle cornea syndrome 2
    term:
      id: MONDO:0013605
      label: brittle cornea syndrome 2
  description: >-
    Caused by biallelic PRDM5 variants. PRDM5 is a PR-domain transcriptional
    repressor that recruits chromatin-modifying complexes to extracellular
    matrix target genes. Both truncating alleles (p.Arg590*, p.Glu134*,
    p.Cys325LeufsX2, deletion of exons 9-14) and at least one missense allele
    (p.Arg83Cys) are pathogenic, and they fail in mechanistically different
    ways - the truncations remove zinc finger and PR domains outright, while
    p.Arg83Cys leaves DNA binding intact and instead disrupts recruitment of
    repressive complexes.
  genes:
  - preferred_term: PRDM5
    term:
      id: hgnc:9349
      label: PRDM5
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Using autozygosity mapping, we identified mutations in PRDM5 in families
      with BCS.
    explanation: Establishes PRDM5 as the second BCS gene.
inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    Both genetic subtypes are autosomal recessive; several defining families
    were consanguineous and were solved by autozygosity mapping.
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Extreme corneal fragility and thinning, which have a high risk of
      catastrophic spontaneous rupture, are the cardinal features of brittle
      cornea syndrome (BCS), an autosomal-recessive generalized connective
      tissue disorder.
    explanation: >-
      States the inheritance mode and the systemic (not purely ocular) nature of
      the disorder.
genetic:
- name: PRDM5
  subtype: BCS2
  gene_term:
    preferred_term: PRDM5
    term:
      id: hgnc:9349
      label: PRDM5
  association: >-
    Biallelic loss-of-function variants in PRDM5, a PR-domain zinc finger
    transcriptional repressor, cause BCS2.
  evidence:
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genetic testing identified the homozygous pathogenic variant c.974delG
      (p.Cys325LeufsX2) in the PRDM5 gene in both cases.
    explanation: >-
      A recent genetically confirmed sibling pair, giving a specific homozygous
      truncating PRDM5 allele.
- name: ZNF469
  subtype: BCS1
  gene_term:
    preferred_term: ZNF469
    term:
      id: hgnc:23216
      label: ZNF469
  association: >-
    Biallelic ZNF469 variants cause BCS1. ZNF469 was independently identified as
    a quantitative trait locus for central corneal thickness, which is
    consistent with a dose-sensitive role in setting corneal stromal thickness.
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      ZNF469, encoding a zinc finger protein of hitherto undefined function, has
      been identified as a quantitative trait locus for central corneal
      thickness, and mutations in this gene have been demonstrated in Tunisian
      Jewish and Palestinian kindreds with BCS.
    explanation: >-
      Establishes ZNF469 as a BCS gene and links it to normal variation in
      corneal thickness.
pathophysiology:
- name: Loss of PRDM5 or ZNF469 Transcriptional Regulator Function
  biological_scale: MOLECULAR
  description: >-
    The initiating lesion is biallelic loss of function in one of two
    transcriptional regulators that act in a shared pathway. PRDM5 is a
    PR-domain zinc finger repressor; ZNF469 is a zinc finger protein of
    previously undefined function identified as a quantitative trait locus for
    central corneal thickness. The demonstration that they participate in the
    same regulatory pathway is what licenses curating BCS as one disease with
    two genotypic subtypes rather than two sibling diseases.
  genes:
  - preferred_term: PRDM5
    term:
      id: hgnc:9349
      label: PRDM5
  - preferred_term: ZNF469
    term:
      id: hgnc:23216
      label: ZNF469
  biological_processes:
  - preferred_term: regulation of DNA-templated transcription
    term:
      id: GO:0006355
      label: regulation of DNA-templated transcription
    modifier: LOSS_OF_FUNCTION
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We show that ZNF469 and PRDM5, two genes that when mutated cause BCS,
      participate in the same regulatory pathway.
    explanation: >-
      The pathway-convergence result. This is the specific evidence behind the
      lump decision recorded in this entry's description.
  downstream:
  - target: Failure of Repressive Chromatin Complex Recruitment at ECM Target Genes
    causal_link_type: DIRECT
    description: >-
      For PRDM5, loss of function is expressed as failure to recruit repressive
      chromatin machinery to its target loci.
  - target: Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression
    causal_link_type: DIRECT
    description: >-
      Both genes converge on transcriptional control of extracellular matrix
      components.
- name: Failure of Repressive Chromatin Complex Recruitment at ECM Target Genes
  biological_scale: MOLECULAR
  description: >-
    PRDM5 acts by recruiting repressive complexes and depositing H3K9
    di-methylation at target genes. The BCS2 missense allele p.Arg83Cys is
    mechanistically informative because it leaves the DNA-binding zinc fingers
    intact and instead diminishes interaction with the NuRD component CHD4 and
    with HP1BP3, showing that loss of the protein-protein interaction alone is
    sufficient to cause disease. Patient fibroblasts carrying truncating alleles
    show dysregulated H3K9me2, and ECM stability mediators are upregulated -
    the direction expected from loss of a repressor, and worth noting because
    the net downstream consequence is nonetheless a structurally deficient
    matrix.
  genes:
  - preferred_term: PRDM5
    term:
      id: hgnc:9349
      label: PRDM5
  biological_processes:
  - preferred_term: negative regulation of transcription by RNA polymerase II
    term:
      id: GO:0000122
      label: negative regulation of transcription by RNA polymerase II
    modifier: LOSS_OF_FUNCTION
  cell_types:
  - preferred_term: dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:26395458
    reference_title: >-
      A role for repressive complexes and H3K9 di-methylation in
      PRDM5-associated brittle cornea syndrome.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      demonstrate that the BCS2 mutation p.Arg83Cys diminishes interaction of
      PRDM5 with repressive complexes, including NuRD complex protein CHD4, and
      the repressive chromatin interactor HP1BP3, by co-immunoprecipitation
      combined with mass spectrometry.
    explanation: >-
      Identifies the specific protein-protein interactions lost, and shows a
      missense allele can be pathogenic without disrupting DNA binding.
  - reference: PMID:26395458
    reference_title: >-
      A role for repressive complexes and H3K9 di-methylation in
      PRDM5-associated brittle cornea syndrome.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Gene ontology analysis of dysregulated PRDM5-target genes reveals
      enrichment for extracellular matrix (ECM) genes supporting vascular
      integrity and development.
    explanation: >-
      Connects the chromatin lesion to the extracellular matrix target class
      that carries the phenotype.
  downstream:
  - target: Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression
    causal_link_type: DIRECT
    description: >-
      Loss of repressive control at ECM loci produces the downstream matrix
      transcriptional lesion.
- name: Dysregulated Fibrillar Collagen and Extracellular Matrix Gene Expression
  biological_scale: CELLULAR
  description: >-
    The shared downstream lesion is loss of transcriptional control over
    extracellular matrix components, particularly the fibrillar collagens. This
    is the node the authors identify as the key molecular mechanism underlying
    corneal fragility, and it is the node at which the two genotypic subtypes
    become indistinguishable.
  biological_processes:
  - preferred_term: extracellular matrix organization
    term:
      id: GO:0030198
      label: extracellular matrix organization
    modifier: DECREASED
  - preferred_term: collagen biosynthetic process
    term:
      id: GO:0032964
      label: collagen biosynthetic process
    modifier: DECREASED
  cell_types:
  - preferred_term: corneal keratocyte
    term:
      id: CL:0002363
      label: keratocyte
  locations:
  - preferred_term: substantia propria of cornea
    term:
      id: UBERON:0001777
      label: substantia propria of cornea
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We demonstrate that regulation of expression of extracellular matrix
      components, particularly fibrillar collagens, by PRDM5 is a key molecular
      mechanism that underlies corneal fragility in BCS and controls normal
      corneal development and maintenance.
    explanation: >-
      The authors' own statement of the central mechanism, naming fibrillar
      collagens as the operative target class.
  downstream:
  - target: Corneal Stromal Thinning and Biomechanical Failure
    causal_link_type: DIRECT
    description: >-
      A deficient collagen matrix cannot maintain corneal stromal thickness or
      tensile strength.
- name: Corneal Stromal Thinning and Biomechanical Failure
  biological_scale: TISSUE
  description: >-
    The cornea becomes extremely thin - central corneal thickness below 400
    micrometres in the large majority of patients, against a normal range of 515
    to 575, with a thinnest point of 151 and 157 micrometres in two reported
    cases - and loses the tensile strength needed to contain intraocular
    pressure. Thinning is most pronounced centrally, with the periphery
    relatively preserved. The same matrix deficiency
    produces protrusion (keratoglobus, keratoconus), scleral thinning visible as
    blue sclerae, and axial elongation manifesting as high myopia.
  locations:
  - preferred_term: cornea
    term:
      id: UBERON:0000964
      label: cornea
  cell_types:
  - preferred_term: corneal keratocyte
    term:
      id: CL:0002363
      label: keratocyte
  evidence:
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two siblings-a 28-year-old male and a 25-year-old female-presented with
      progressive visual deterioration and marked corneal thinning
    explanation: >-
      Documents the degree of corneal thinning in genetically confirmed PRDM5
      disease.
  downstream:
  - target: Corneal Rupture
    causal_link_type: DIRECT
    description: >-
      A cornea below the thickness at which it can contain intraocular pressure
      ruptures spontaneously or after trivial trauma.
- name: Corneal Rupture
  biological_scale: ORGANISM
  description: >-
    Rupture is the event that defines the disease's severity and its management.
    It occurs spontaneously or after minimal trauma, and historically enucleation
    was frequently the only option, causing blindness. Because rupture can be the
    presenting feature in a child, it has been misattributed to non-accidental
    injury - a diagnostic error with obvious consequences, and a reason
    recognition matters beyond the eye clinic.
  locations:
  - preferred_term: cornea
    term:
      id: UBERON:0000964
      label: cornea
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Enucleation is frequently the only management option for this condition,
      resulting in blindness and psychosocial distress.
    explanation: Establishes the severity and historical management of rupture.
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Corneal rupture may be the presenting feature of BCS, and it is possible
      that this may be incorrectly attributed to non-accidental injury.
    explanation: >-
      Records the safeguarding hazard, which is a recognition issue rather than
      a purely ophthalmic one.
phenotypes:
- category: Ophthalmological
  name: Extreme Corneal Thinning
  description: >-
    The cardinal feature, and quantitatively extreme: central corneal thickness
    below 400 micrometres in the large majority of patients against a normal
    range of 515 to 575, and below 300 in extreme cases. The thinnest point,
    which is not the same measurement, reached 151 and 157 micrometres in two
    reported cases.
  phenotype_term:
    preferred_term: Decreased corneal thickness
    term:
      id: HP:0100689
      label: Decreased corneal thickness
  frequency: OBLIGATE
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Brittle cornea syndrome (BCS) is an autosomal recessive disorder
      characterised by extreme corneal thinning and fragility.
    explanation: >-
      Corneal thinning is definitional for BCS, which is what supports the
      OBLIGATE band.
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      From a clinical standpoint, BCS is marked by severe corneal thinning, with
      central corneal thickness (CCT) often measuring below 400 µm and, in
      extreme cases, even below 300 µm
    explanation: >-
      Supplies the quantitative thresholds the description states, in central
      corneal thickness terms.
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the large majority of patients with BCS have had central corneal thickness
      (CCT) of less than 400 μm (normal range 515– 575 μm
    explanation: >-
      Gives the normal central corneal thickness range the disease figure is
      measured against, so the comparison is cited rather than assumed.
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      On pachymetry, thinning has been most pronounced in the central cornea,
      with relative preservation of peripheral corneal thickness
    explanation: >-
      Establishes that the thinning is central rather than uniform, which is why
      central corneal thickness is the measurement the literature reports.
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      thinnest point 151 µm; (Case 2) (B) K-Max 73, AC depth 3.84 mm, thinnest
      point 157 µm.
    explanation: >-
      The thinnest-point figures from the two genetically confirmed cases. A
      thinnest point is not a central corneal thickness, so this is curated as a
      separate measurement rather than folded into the CCT figures above.
- category: Ophthalmological
  name: Corneal Perforation
  description: >-
    Rupture of the thinned cornea, spontaneously or after minimal trauma.
    Curated separately from the thinning that predisposes to it, because it is
    the discrete event that management is organised around and the one that
    ends vision.
  phenotype_term:
    preferred_term: Corneal perforation
    term:
      id: HP:0100583
      label: Corneal perforation
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Corneal rupture can therefore occur either spontaneously or following
      minimal trauma in affected patients.
    explanation: Establishes rupture as a discrete event distinct from the thinning.
- category: Ophthalmological
  name: High Myopia
  phenotype_term:
    preferred_term: High myopia
    term:
      id: HP:0011003
      label: High myopia
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinical features include extreme corneal thinning with rupture, high
      myopia, blue sclerae, deafness of mixed aetiology with hypercompliant
      tympanic membranes, and variable skeletal manifestations.
    explanation: >-
      The consensus clinical feature list from the series that defined molecular
      diagnosis in BCS.
- category: Ophthalmological
  name: Keratoconus
  description: >-
    Conical corneal protrusion. Curated separately from keratoglobus because the
    two are distinct ectatic morphologies and are separate major criteria in the
    2017 international EDS classification.
  phenotype_term:
    preferred_term: Keratoconus
    term:
      id: HP:0000563
      label: Keratoconus
  evidence:
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Even when the cornea remains grossly intact, visual function could also be
      impaired by a high degree of myopia and keratoconus.
    explanation: >-
      Records that vision is threatened by refractive and ectatic change even
      without rupture.
- category: Ophthalmological
  name: Keratoglobus
  description: >-
    Globular protrusion of the whole cornea, a distinct ectatic morphology from
    the conical deformation of keratoconus.
  phenotype_term:
    preferred_term: Keratoglobus
    term:
      id: HP:0001119
      label: Keratoglobus
  evidence:
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Detailed ophthalmologic evaluation revealed keratoglobus, high myopia, and
      irregular astigmatism.
    explanation: >-
      Documents keratoglobus in genetically confirmed PRDM5 disease, alongside
      the refractive consequences.
- category: Ophthalmological
  name: Blue Sclerae
  description: >-
    Scleral thinning allowing the underlying uvea to show through, the same
    matrix deficiency expressed outside the cornea.
  phenotype_term:
    preferred_term: Blue sclerae
    term:
      id: HP:0000592
      label: Blue sclerae
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinical features include extreme corneal thinning with rupture, high
      myopia, blue sclerae, deafness of mixed aetiology with hypercompliant
      tympanic membranes, and variable skeletal manifestations.
    explanation: Lists blue sclerae among the defining clinical features.
- category: Auditory
  name: Mixed Deafness
  description: >-
    Hearing loss is of mixed aetiology, and the hypercompliant tympanic membrane
    is the connective-tissue signature of the conductive component. Combined with
    visual loss this produces dual sensory deprivation.
  phenotype_term:
    preferred_term: Mixed hearing impairment
    term:
      id: HP:0000410
      label: Mixed hearing impairment
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinical features include extreme corneal thinning with rupture, high
      myopia, blue sclerae, deafness of mixed aetiology with hypercompliant
      tympanic membranes, and variable skeletal manifestations.
    explanation: Records the mixed deafness and its characteristic tympanic finding.
  - reference: PMID:21664999
    reference_title: >-
      Mutations in PRDM5 in brittle cornea syndrome identify a pathway
      regulating extracellular matrix development and maintenance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Deafness is another common feature and results in combined sensory
      deprivation.
    explanation: Supports deafness as a common feature and names the combined-deficit consequence.
- category: Auditory
  name: Hypercompliant Tympanic Membranes
  description: >-
    The connective-tissue signature of the conductive component of the hearing
    loss, and the same matrix deficiency expressed in the middle ear. Curated as
    its own phenotype rather than left asserted inside another phenotype's name,
    which is where it previously sat unmodeled.
  phenotype_term:
    preferred_term: Tympanic membrane hypermobility
    term:
      id: HP:6000015
      label: Tympanic membrane hypermobility
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinical features include extreme corneal thinning with rupture, high
      myopia, blue sclerae, deafness of mixed aetiology with hypercompliant
      tympanic membranes, and variable skeletal manifestations.
    explanation: >-
      Names hypercompliant tympanic membranes explicitly among the defining
      clinical features.
- category: Musculoskeletal
  name: Joint Hypermobility
  description: >-
    Part of the generalized connective tissue phenotype, and one of the features
    that placed BCS in the Ehlers-Danlos classification historically.
  phenotype_term:
    preferred_term: Joint hypermobility
    term:
      id: HP:0001382
      label: Joint hypermobility
  evidence:
  - reference: PMID:35791165
    reference_title: "Brittle cornea syndrome: A tale of three brothers."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A systemic evaluation revealed sensorineural hearing loss and
      hyperextensible joints.
    explanation: >-
      Documents joint hyperextensibility on systemic evaluation in a BCS
      sibship. Note this report describes the hearing loss as sensorineural,
      where the definitive review describes deafness of mixed aetiology; the
      auditory phenotype is curated from the latter.
- category: Musculoskeletal
  name: Developmental Dysplasia of the Hip
  description: >-
    The specific skeletal complication that management guidance singles out for
    active assessment, rather than a general statement about skeletal
    involvement.
  phenotype_term:
    preferred_term: Hip dysplasia
    term:
      id: HP:0001385
      label: Hip dysplasia
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mainstays of management include the prevention of ocular rupture by
      provision of protective polycarbonate spectacles, careful monitoring of
      visual and auditory function, and assessment for skeletal complications
      such as developmental dysplasia of the hip.
    explanation: >-
      Names developmental dysplasia of the hip as the skeletal complication
      requiring active assessment. This snippet previously sat on the joint
      hypermobility phenotype, which it does not mention.
treatments:
- name: Protective Polycarbonate Spectacles
  description: >-
    The primary preventive intervention. Because the treatable event is rupture
    rather than the matrix lesion, physical protection of the globe is the
    mainstay of management and is not a stopgap for something better.
  treatment_term:
    preferred_term: protective eyewear
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: DEVICE
  target_mechanisms:
  - target: Corneal Rupture
    treatment_effect: INHIBITS
    description: >-
      Acts on the rupture event by excluding trauma, not on any upstream
      mechanism node.
    evidence:
    - reference: PMID:23642083
      reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Mainstays of management include the prevention of ocular rupture by
        provision of protective polycarbonate spectacles
      explanation: Establishes protective spectacles as the mainstay of rupture prevention.
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mainstays of management include the prevention of ocular rupture by
      provision of protective polycarbonate spectacles, careful monitoring of
      visual and auditory function, and assessment for skeletal complications
      such as developmental dysplasia of the hip.
    explanation: The management standard, including the surveillance components.
- name: Keratoplasty
  description: >-
    Corneal transplantation, penetrating or deep anterior lamellar. In a recent
    genetically confirmed sibling pair both achieved good corrected acuity and
    remained complication-free at seven years, though one planned lamellar
    procedure had to be converted intraoperatively because of central
    endothelial perforation - which is the operative expression of the tissue
    fragility the disease is named for.
  treatment_term:
    preferred_term: corneal transplantation
    term:
      id: NCIT:C210959
      label: Corneal Transplantation
  therapeutic_modality: SURGERY
  evidence:
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The female initially underwent deep anterior lamellar keratoplasty (DALK),
      which was converted to PKP intraoperatively due to central endothelial
      perforation, resulting in a BCVA of 20/25.
    explanation: >-
      Documents both the achievable visual outcome and the intraoperative
      fragility that constrains surgical planning.
- name: Corneal Collagen Cross-Linking
  description: >-
    Photochemical stiffening of the corneal stroma. It is the only reported
    intervention that acts on the biomechanical lesion itself rather than on
    the rupture event, but the evidence is two paediatric cases, not a series.
    The standard (Dresden) protocol is contraindicated below 200 micrometres,
    which excludes most brittle corneas; both reported patients were treated
    with a transepithelial protocol and UV fluence scaled to their thickness.
    Curated as an emerging option, not a standard of care.
  treatment_term:
    preferred_term: corneal collagen cross-linking
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  therapeutic_modality: DEVICE
  target_mechanisms:
  - target: Corneal Stromal Thinning and Biomechanical Failure
    treatment_effect: INHIBITS
    description: >-
      Unlike spectacles, which exclude the trauma that triggers rupture,
      cross-linking targets the weakened stroma upstream of it.
    evidence:
    - reference: PMID:40647596
      reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Both cases achieved better visual acuity and maintained stable
        endothelial cell counts, indicating that, with protocol modifications,
        cross-linking may be a viable treatment even in ultra-thin corneas
        normally excluded by the standard Dresden criteria
      explanation: >-
        Reports improvement in the biomechanically determined outcome, but from
        two cases only, so the effect on the stromal node is PARTIAL rather than
        SUPPORT.
  evidence:
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two pediatric patients with brittle cornea syndrome, each with central
      corneal thickness < 280 μm, underwent transepithelial (epithelium-on)
      corneal cross-linking with UV fluence scaled to corneal thickness.
    explanation: >-
      The primary report of cross-linking in genetically relevant brittle
      corneas, and the source of the transepithelial, thickness-scaled protocol.
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In these two cases, conventional cross-linking was not considered due to
      contraindications for corneal thickness less than 200 µm.
    explanation: >-
      Records why the standard protocol is unavailable in this disease, which is
      what makes the transepithelial variant the relevant one.
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      There is currently no pharmacological treatment available to halt the
      progression of corneal thinning, making surgical intervention the primary
      treatment option for visual rehabilitation
    explanation: >-
      Places cross-linking in context: there is no drug for the matrix lesion,
      so procedural options carry the whole therapeutic burden.
diagnosis:
- name: ZNF469 and PRDM5 molecular genetic testing
  presence: Positive
  description: >-
    Molecular diagnosis is available and effective, because the two known genes
    together account for the condition in nearly all ascertained patients. It
    also identifies heterozygous carriers, which matters in the consanguineous
    kindreds in which BCS is most often seen. Whole-exome sequencing is the
    route used in recent genetically confirmed cases.
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two genes, ZNF469 and PRDM5, have now been identified, in which causative
      pathogenic mutations collectively account for the condition in nearly all
      patients with BCS ascertained to date. Therefore, effective molecular
      diagnosis is now available for affected patients, and those at risk of
      being heterozygous carriers for BCS.
    explanation: >-
      Establishes that two-gene testing is diagnostically near-complete and
      extends to carrier identification.
  - reference: PMID:40647596
    reference_title: "Brittle Cornea Syndrome: Molecular Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This report presents two genetically confirmed cases of BCS in Albanian
      siblings, emphasizing the diagnostic value of whole-exome sequencing and
      individualized surgical management strategies.
    explanation: Documents whole-exome sequencing as the practical diagnostic route.
- name: Recognition against overlapping connective tissue disorders
  description: >-
    The hard part of diagnosis is not the test but reaching it. BCS overlaps
    phenotypically with other heritable connective tissue disorders, and corneal
    rupture as a presenting event in a child has been misattributed to
    non-accidental injury. Early recognition is what prevents avoidable
    blindness.
  evidence:
  - reference: PMID:23642083
    reference_title: "Brittle cornea syndrome: recognition, molecular diagnosis and management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Effective management depends upon appropriate identification of affected
      individuals, which may be challenging given the phenotypic overlap of BCS
      with other connective tissue disorders.
    explanation: >-
      States the recognition problem explicitly and ties management outcomes to
      it.
discussions:
- discussion_id: split_bcs_one_disease_two_loci
  kind: OPEN_QUESTION
  status: OPEN
  attaches_to:
  - pathophysiology#Loss of PRDM5 or ZNF469 Transcriptional Regulator Function
  prompt: >-
    Should brittle cornea syndrome be curated as one disease with two genotypic
    subtypes, as done here, or as two sibling Disease entries keyed on
    MONDO:0024543 and MONDO:0013605?
  rationale: >-
    This entry lumps, and the reasoning should be auditable rather than assumed.
    In favour of lumping: the two genes were shown to participate in the same
    regulatory pathway; the downstream lesion, transcriptional dysregulation of
    fibrillar collagens, is shared; the clinical feature list and the management
    plan in the definitive review are stated once for BCS rather than per gene;
    and dismech's own rule is that a disorder entry needs one reasonably
    conserved pathograph, which BCS has. In favour of splitting: MONDO models
    them as two distinct children, and ZNF469 and PRDM5 are not the same
    molecule, so a gene-level query answers differently depending on the choice.
    That objection is addressed here by giving each subtype its own
    subtype_term, so both MONDO IDs remain curated and queryable without
    duplicating a pathograph that does not differ. If future work demonstrates a
    genotype-phenotype difference between BCS1 and BCS2 - none is currently
    described - splitting would become the better model.
notes: >-
  On the absent references: block. That block is GeneReviews-specific by
  schema - its tags slot permits exactly one value, GeneReviews - so it is
  correct for this entry not to carry one: no GeneReviews chapter appears to
  exist for brittle cornea syndrome. PMID:23642083 serves the equivalent role
  here, supplying the consensus clinical feature list, the two-gene molecular
  diagnostic statement and the management standard, and it is mined across the
  phenotypes, diagnosis and treatments sections rather than merely cited. This
  is recorded so the absence is a documented finding rather than an unexplained
  gap.

  Scope decision: this entry supersedes the separate BCS1 and BCS2 curation
  targets. Both stubs are retired by this PR, and both MONDO IDs are carried as
  has_subtypes terms, which counts as curated coverage.

  Nosology: BCS was historically classified as Ehlers-Danlos syndrome type VIB -
  the designation still appears among the MONDO synonyms for BCS1 - and it is
  not among the 13 subtypes recognised in the 2017 international EDS
  classification. That exclusion is recorded here as prose rather than as an
  evidence item because the 2017 classification's abstract does not itself
  mention brittle cornea syndrome, and quoting a title or fabricating a snippet
  to support it would be worse than leaving it uncited.

  Curation provenance: a Perplexity deep-research report was generated
  (research/Brittle_Cornea_Syndrome_2-deep-research-perplexity.md) and passed
  the NEC preflight against MONDO:0013605 - PRDM5 mentioned 169 times against
  ZNF469 23, with the report OMIM matching the MONDO xref. It was used as a
  lead-generation aid only; every snippet here was verified against the cached
  abstract. One of the report's own sources is a MalaCards page for brittle
  cornea syndrome 1 rather than 2, which in this instance is benign because the
  entry deliberately covers both, but is recorded for the same reason as
  elsewhere: source-level entity drift survives a report-level preflight PASS.

Brittle Cornea Syndrome

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