FAM111A-related skeletal dysplasia is a single allelic spectrum spanning two named disorders: Kenny-Caffey syndrome type 2 (KCS2), a survivable dysplasia of proportionate short stature with cortical thickening and medullary stenosis of the tubular bones, and osteocraniostenosis (OCS), a usually lethal perinatal form adding intrauterine growth deficiency, microcephaly, poorly ossified cloverleaf-shaped skull, thin ribs with pulmonary hypoplasia and splenic hypo/aplasia. Both are caused by missense variants in a restricted region of FAM111A, a putative trypsin-like serine protease, and both are usually de novo. The mechanism is gain of function, not loss: the variants cluster on an outer surface of the protein away from the catalytic site, and quantitative work shows a graded series of hypermorphic alleles whose degree of activation determines whether the disorder is dominant KCS2, recessive KCS2, or OCS. Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia occurs across the spectrum, and FAM111A's native function remains unknown, so the route from an over-active protease to a parathyroid and skeletal phenotype is still unexplained. Two naming traps are worth knowing. "Kenny-Caffey syndrome type 1" (OMIM 244460) is a different gene entirely, TBCE, and is better called Sanjad-Sakati syndrome - the OMIM numbering inverts the historical usage, since the original Kenny-Caffey descriptions were of a dominant trait. And OCS has repeatedly been confused with Hallermann-Streiff syndrome, which shares skull and long-bone anomalies but is not lethal and lacks the OCS facies and splenic hypoplasia.
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name: FAM111A-Related Skeletal Dysplasia
creation_date: "2026-08-27T04:30:00Z"
category: Mendelian
synonyms:
- Kenny-Caffey syndrome type 2
- KCS2
- autosomal dominant Kenny-Caffey syndrome
- osteocraniostenosis
- OCS
- gracile bone dysplasia
- GCLEB
- osteocraniosplenic syndrome
description: >-
FAM111A-related skeletal dysplasia is a single allelic spectrum spanning two
named disorders: Kenny-Caffey syndrome type 2 (KCS2), a survivable dysplasia
of proportionate short stature with cortical thickening and medullary stenosis
of the tubular bones, and osteocraniostenosis (OCS), a usually lethal
perinatal form adding intrauterine growth deficiency, microcephaly, poorly
ossified cloverleaf-shaped skull, thin ribs with pulmonary hypoplasia and
splenic hypo/aplasia. Both are caused by missense variants in a restricted
region of FAM111A, a putative trypsin-like serine protease, and both are
usually de novo. The mechanism is gain of function, not loss: the variants
cluster on an outer surface of the protein away from the catalytic site, and
quantitative work shows a graded series of hypermorphic alleles whose degree
of activation determines whether the disorder is dominant KCS2, recessive
KCS2, or OCS. Primary hypoparathyroidism with hypocalcemia and
hyperphosphatemia occurs across the spectrum, and FAM111A's native function
remains unknown, so the route from an over-active protease to a parathyroid
and skeletal phenotype is still unexplained.
Two naming traps are worth knowing. "Kenny-Caffey syndrome type 1" (OMIM
244460) is a different gene entirely, TBCE, and is better called
Sanjad-Sakati syndrome - the OMIM numbering inverts the historical usage,
since the original Kenny-Caffey descriptions were of a dominant trait. And
OCS has repeatedly been confused with Hallermann-Streiff syndrome, which
shares skull and long-bone anomalies but is not lethal and lacks the OCS
facies and splenic hypoplasia.
disease_term:
preferred_term: FAM111A-related skeletal dysplasia
term:
id: MONDO:1060172
label: FAM111A-related skeletal dysplasia
parents:
- Skeletal dysplasia
- Primordial dwarfism
classifications:
harrisons_chapter:
- classification_value: GENETICS_ENVIRONMENT_DISEASE
- classification_value: ENDOCRINOLOGY_METABOLISM
isds_skeletal_category:
- classification_value: primordial_dwarfism_and_slender_bones
notes: >-
ISDS Nosology of Genetic Skeletal Disorders, 2023 revision (Unger et al.,
PMID:36779427), group 21 "Primordial dwarfism and slender bone
dysplasias". The nosology splits this gene across two adjacent rows - NOS
21-0050 "Kenny-Caffey syndrome, dominant, FAM111A-related" (OMIM 127000)
and NOS 21-0060 "Osteocraniostenosis, FAM111A-related" (OMIM 602361) -
both in this group, so the assignment stays single-valued. This entry
lumps them, following the source literature and GeneReviews, which treat
KCS2 and OCS as allelic disorders of different severity rather than two
diseases; the split is preserved as has_subtypes. The recessive TBCE
Kenny-Caffey/Sanjad-Sakati disorder is a separate row (NOS 21-0040) and a
separate dismech entry.
has_subtypes:
- name: KCS2
display_name: Kenny-Caffey syndrome type 2 (survivable)
subtype_term:
preferred_term: autosomal dominant Kenny-Caffey syndrome
term:
id: MONDO:0007478
label: autosomal dominant Kenny-Caffey syndrome
description: >-
The milder, survivable end of the spectrum: proportionate short stature of
typically postnatal onset, relative macrocephaly, a large anterior fontanel
with delayed closure, cortical thickening with medullary stenosis, and
ophthalmologic and dental manifestations. ISDS NOS 21-0050, OMIM 127000.
genes:
- preferred_term: FAM111A
term:
id: hgnc:24725
label: FAM111A
- name: OCS
display_name: Osteocraniostenosis (perinatally lethal)
subtype_term:
preferred_term: osteocraniostenosis
term:
id: MONDO:0011215
label: osteocraniostenosis
description: >-
The severe, usually lethal end: intrauterine growth deficiency,
microcephaly, decreased skull ossification giving a cloverleaf skull shape,
slender long bones with cortical thickening and medullary stenosis, flared
metaphyses, and thin ribs with thoracic and pulmonary hypoplasia leading to
respiratory insufficiency. Splenic hypo/aplasia is characteristic and is
what prompted the alternative name "osteocraniosplenic syndrome". ISDS
NOS 21-0060, OMIM 602361.
genes:
- preferred_term: FAM111A
term:
id: hgnc:24725
label: FAM111A
prevalence:
- population: Worldwide
measure_type: CASES_IN_LITERATURE
prevalence_class: NOT_YET_DOCUMENTED
notes: >-
No population rate published. The gene-discovery study reported five
individuals with KCS and five with OCS; subsequent reports have been
individual cases and small series.
inheritance:
- name: Autosomal dominant, typically de novo
inheritance_term:
preferred_term: Autosomal dominant inheritance
term:
id: HP:0000006
label: Autosomal dominant inheritance
description: >-
Heterozygous FAM111A missense variants, usually de novo. Rarely an
individual with KCS has an affected parent, in which case sib and offspring
risks are 50%.
evidence:
- reference: PMID:23684011
reference_title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "One mutation was identified in four unrelated individuals with KCS, and another one was identified in two unrelated individuals with OCS; all occurred de novo."
explanation: >-
Establishes the recurrent de novo heterozygous pattern across both
subtypes.
- name: Autosomal recessive (rare)
inheritance_term:
preferred_term: Autosomal recessive inheritance
term:
id: HP:0000007
label: Autosomal recessive inheritance
description: >-
A minority route to the same phenotypes. Two children with KCS and OCS were
homozygous for variants replacing the same residue, Tyr414, with
asymptomatic heterozygous relatives - the weakly activating alleles need two
copies to cause disease.
evidence:
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "we describe 2 unrelated children with KCS and OCS who were homozygous for different FAM111A variant alleles that result in replacement of the same residue, Tyr414"
explanation: >-
Documents the recessive route, with the asymptomatic heterozygous
relatives that make it recessive rather than dominant.
genetic:
- name: FAM111A
gene_term:
preferred_term: FAM111A
term:
id: hgnc:24725
label: FAM111A
relationship_type: CAUSATIVE
variant_origin: DE_NOVO
notes: >-
FAM111A (11q12.1) encodes a 611-amino-acid putative trypsin-like serine
protease that protects replication forks from stalling at PARP1-DNA
complexes and topoisomerase-1 cleavage complexes. Disease variants are
missense and confined to a few residues within the trypsin-like peptidase
domain. Molecular modelling places the KCS and OCS residues away from the
active site and clustered on or near the outer surface, which is why the
proposed pathogenesis involves altered partner interaction rather than
impaired catalysis.
Three features of the allelic spectrum are worth stating explicitly.
First, there is a single dominant hotspot: c.1706G>A p.(Arg569His).
Seven of eight patients in the largest Chinese series carried it, it recurs
in unrelated patients across independent reports, and a systematic review
of 46 genetically confirmed KCS2 patients concluded it is *the* hotspot
variant. Notably R569H lies outside the peptidase domain, in the
C-terminal region - so the "confined to the peptidase domain" summary above
is true of most disease alleles but not of the commonest one.
Second, de novo occurrence is the rule but not the law. The discovery
cohort was entirely de novo, yet a three-generation Chinese family
established father-to-daughter transmission, so the recurrence counselling
cannot assume a de novo event without testing the parents.
Third, the alleles are quantitatively rather than qualitatively different.
Recombinant FAM111A carrying the recessive Y414C substitution dimerizes
normally and is more active than wild type but less active than the
dominant KCS2 mutants. That graded activation is what reconciles dominant
and recessive inheritance in one gene: a strongly activating allele
suffices in one copy, a weakly activating one needs two.
variants:
- name: c.1706G>A p.(Arg569His)
description: >-
The dominant hotspot allele. Heterozygous, usually de novo, and recurrent
in unrelated KCS2 patients across independent reports. It lies outside
the trypsin-like peptidase domain, in the C-terminal region, and enhances
rather than compromises FAM111A autocleavage activity.
clinical_significance: PATHOGENIC
functional_effects:
- function: Hypermorphic - increases FAM111A activity
description: >-
Grouped with the other KCS2 and OCS alleles that enhance autocleavage;
the disease mechanism is excess rather than absent protease function.
evidence:
- reference: PMID:23996431
reference_title: "A recurrent de novo FAM111A mutation causes Kenny-Caffey syndrome type 2."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "An identical missense mutation, R569H, was heterozygously detected in all three patients but not in the unaffected family members."
explanation: >-
The recurrence of the identical de novo allele in unrelated patients.
- name: c.1241A>G p.(Tyr414Cys)
description: >-
A recessive, weakly activating allele. Homozygotes have KCS2; the
heterozygous relatives are asymptomatic. The paired allele
c.1240T>A p.(Tyr414Asn) at the same residue causes the lethal OCS
phenotype when homozygous.
clinical_significance: PATHOGENIC
functional_effects:
- function: Hypermorphic - mildly increases FAM111A activity
description: >-
More active than wild type but less active than the dominant KCS2
mutants, which is why two copies are required.
evidence:
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Functional studies of recombinant FAM111AY414C demonstrated normal dimerization and a mild gain-of-function effect."
explanation: >-
The functional characterization that grades this allele below the
dominant ones.
evidence:
- reference: PMID:23684011
reference_title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "We studied five individuals with KCS and five with OCS and found that all of them had heterozygous mutations in FAM111A."
explanation: >-
The gene-discovery study covering both subtypes.
- reference: PMID:23684011
reference_title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
supports: SUPPORT
evidence_source: COMPUTATIONAL
snippet: "residues affected by KCS and OCS mutations do not map close to the active site but are clustered on a segment of the protein and are at, or close to, its outer surface"
explanation: >-
The structural modelling behind the partner-interaction rather than
catalysis hypothesis. Classified COMPUTATIONAL because the claim rests on
molecular modelling.
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "All cases harboured missense variants of FAM111A, and nucleotides c.1706 arose as a mutational hotspot, with seven individuals harbouring a c.1706G>A (p.Arg569His) variant, and one child harbouring a c.1531T>C (p.Tyr511His) variant."
explanation: >-
Identifies the recurrent nucleotide and gives the hotspot's share of a
single cohort.
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Our study confirms that Arg569His is the hot spot variant and summarizes the typical phenotypes of KCS2"
explanation: >-
The conclusion of a review of 46 genetically confirmed patients across 20
papers, which is the basis for calling this a hotspot rather than a
cohort-specific founder effect.
- reference: PMID:37793778
reference_title: "FAM111A regulates replication origin activation and cell fitness."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "the R569H point mutation is located outside of the enzyme domain, in the C-terminal region of the FAM111A gene and is found in seven unrelated KCS2 patients"
explanation: >-
Places the hotspot outside the peptidase domain and independently counts
seven unrelated patients carrying it.
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "We provide detailed characteristics of the largest KCS2 group in China and present the first genetically confirmed instance of father-to-daughter transmission of KCS2."
explanation: >-
Documents vertical transmission, which is what makes de novo occurrence a
tendency rather than a rule.
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Comprehensive analyses of the Y414C recombinant FAM111A protein in mammalian cell lines demonstrate that this protein is more active than the FAM111AWT but is less active than mutant proteins that cause autosomal dominant KCS2."
explanation: >-
The functional measurement behind the graded-activation account, and the
reason the same gene supports both dominant and recessive disease.
pathophysiology:
- name: Hypermorphic FAM111A Protease Variant
biological_scale: MOLECULAR
role: trigger
description: >-
A missense variant in a restricted region of the FAM111A trypsin-like
peptidase domain increases the protein's activity. The evidence that this is
a graded rather than binary effect is what ties the whole spectrum together:
a recombinant Y414C protein is more active than wild type but less active
than the dominant KCS2 mutants, and it causes disease only when homozygous.
genes:
- preferred_term: FAM111A
term:
id: hgnc:24725
label: FAM111A
molecular_functions:
- preferred_term: serine-type endopeptidase activity
term:
id: GO:0004252
label: serine-type endopeptidase activity
modifier: GAIN_OF_FUNCTION
genetic_context:
functional_impact_category: HYPERMORPHIC
allele_type: missense
variant_origin: DE_NOVO
description: >-
Missense variants confined to a few residues of the peptidase domain,
producing variable degrees of activation. The degree of activation, rather
than the identity of the gene, is what sets dominant KCS2 against
recessive KCS2 against OCS.
downstream:
- target: Peptidase-Dependent Single-Stranded DNA Accumulation
evidence:
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Functional studies of recombinant FAM111AY414C demonstrated normal dimerization and a mild gain-of-function effect."
explanation: >-
Measures the gain of function directly and shows it is mild for this
allele, which is what makes the allele recessive.
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "This study provides evidence that both biallelic and monoallelic variants of FAM111A with varying degrees of activation can lead to dominant or recessive KCS2 and OCS."
explanation: >-
States the dose-of-activation model that unifies the dominant, recessive
and lethal forms.
- name: Peptidase-Dependent Single-Stranded DNA Accumulation
biological_scale: MOLECULAR
description: >-
The first cellular step downstream of the hypermorphic protease. FAM111A is
a replisome-associated protein that supports efficient activation of DNA
replication origins; when it is present in excess, the same activity
produces extensive single-stranded DNA. Three dependencies pin this to
protease action during replication rather than to a structural role: the
ssDNA requires S-phase entry, it requires FAM111A peptidase activity, and it
does not require PCNA binding. What is still missing is the substrate - the
repertoire of FAM111A substrates has not been identified - so nothing yet
explains why parathyroid and skeletal progenitors are the tissues that fail.
biological_processes:
- preferred_term: DNA replication initiation
term:
id: GO:0006270
label: DNA replication initiation
modifier: ABNORMAL
downstream:
- target: DNA Damage Accumulation and Progenitor Cell Death
evidence:
- reference: PMID:37793778
reference_title: "FAM111A regulates replication origin activation and cell fitness."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "we show that FAM111A facilitates efficient activation of DNA replication origins"
explanation: >-
The normal function whose dysregulation this node describes.
- reference: PMID:37793778
reference_title: "FAM111A regulates replication origin activation and cell fitness."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Unrestrained expression of FAM111A WT also causes increased single-stranded DNA formation that relies on S phase entry, FAM111A peptidase activity but not its binding to proliferating cell nuclear antigen."
explanation: >-
The three dependencies this node asserts, in one sentence: S-phase entry,
peptidase activity, and independence from PCNA binding.
- reference: PMID:37793778
reference_title: "FAM111A regulates replication origin activation and cell fitness."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Moreover, the repertoire of FAM111A substrates has yet to be identified."
explanation: >-
States the limit of the mechanism - no substrate is known - which is why
this node stops short of explaining the tissue selectivity.
- name: DNA Damage Accumulation and Progenitor Cell Death
biological_scale: CELLULAR
description: >-
The cellular endpoint: cells expressing unrestrained FAM111A, wild type or
patient mutant alike, accumulate DNA damage and die, and only when the
peptidase domain is intact. That the same phenotype follows from
overexpressing the normal protein is the clearest statement that these are
gain-of-function alleles - the disease is too much of a normal activity, not
the loss of one.
biological_processes:
- preferred_term: apoptotic process
term:
id: GO:0006915
label: apoptotic process
modifier: INCREASED
- preferred_term: cellular response to DNA damage stimulus
term:
id: GO:0006974
label: DNA damage response
modifier: INCREASED
downstream:
- target: Impaired Parathyroid Hormone Production and Calcium Homeostasis
- target: Impaired Skeletal Development and Cortical Modelling
- target: Microcephaly
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Excess cell death in proliferating progenitors is the plausible route to
reduced brain growth in OCS; not demonstrated for neural tissue
specifically.
- target: Splenic hypoplasia or aplasia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Grouped as a developmental failure of a proliferation-dependent organ;
the OCS-defining splenic defect has no established mechanism.
- target: Hypermetropia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Grouped as a developmental consequence; no ocular mechanism is
established for FAM111A.
- target: Cataract
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Grouped with the other ophthalmologic manifestations; no ocular mechanism
is established for FAM111A.
evidence:
- reference: PMID:37793778
reference_title: "FAM111A regulates replication origin activation and cell fitness."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Unrestrained expression of FAM111A WT and patient mutants causes accumulation of DNA damage and cell death, only when the peptidase domain remains intact."
explanation: >-
Establishes that the cellular toxicity of the patient mutants depends on
protease activity, which is what makes this a gain-of-function mechanism
rather than a structural one.
- name: Impaired Parathyroid Hormone Production and Calcium Homeostasis
biological_scale: ORGANISM
description: >-
Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia occurs in
both KCS and OCS. The connection to the protease is asserted rather than
explained: FAM111A's native function is unknown, and the discovery paper
reaches only as far as saying the gene "appears to be crucial to a pathway"
governing PTH production.
biological_processes:
- preferred_term: parathyroid hormone secretion
term:
id: GO:0035898
label: parathyroid hormone secretion
modifier: DECREASED
locations:
- preferred_term: parathyroid gland
term:
id: UBERON:0001132
label: parathyroid gland
downstream:
- target: Primary hypoparathyroidism
description: >-
The endocrine endpoint.
- target: Hypocalcemia
description: >-
Direct biochemical consequence of PTH deficiency.
- target: Hyperphosphatemia
description: >-
The reciprocal biochemical consequence of PTH deficiency.
- target: Seizures
description: >-
Hypocalcemic seizures and spasms - the neuromuscular expression of the
low ionized calcium, not an independent cerebral lesion.
- target: Cerebral calcification
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
description: >-
Chronic hypocalcemia with hyperphosphatemia raises the calcium-phosphate
product and drives intracranial deposition; the route is standard for
hypoparathyroidism but has not been shown specifically in FAM111A
disease.
- target: Nephrocalcinosis
description: >-
A consequence of treating the hypoparathyroidism rather than of the
disease itself: calcium and activated vitamin D raise urinary calcium
excretion.
- target: Growth hormone deficiency
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Attached to the endocrine node as a second, separately reported
endocrinopathy in the same patient rather than as a consequence of the
hypoparathyroidism; no mechanism links FAM111A to somatotroph function.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS."
explanation: >-
GeneReviews records the endocrine component across both subtypes.
- reference: PMID:23684011
reference_title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "FAM111A appears to be crucial to a pathway that governs parathyroid hormone production, calcium homeostasis, and skeletal development and growth."
explanation: >-
The strongest statement the source makes is that the gene appears crucial
to such a pathway - a hypothesis, not a demonstrated mechanism, hence
PARTIAL.
- name: Impaired Skeletal Development and Cortical Modelling
biological_scale: TISSUE
description: >-
Small, dense, gracile bones with thickened cortices and stenosis of the
medullary cavity - a modelling defect rather than a mineralization defect,
and the finding that unifies the radiographic picture across the spectrum.
In OCS this extends to failure of skull ossification and thin ribs.
cell_types:
- preferred_term: osteoblast
term:
id: CL:0000062
label: osteoblast
biological_processes:
- preferred_term: osteoblast differentiation
term:
id: GO:0001649
label: osteoblast differentiation
modifier: ABNORMAL
downstream:
- target: Cortical thickening of the long bones
description: >-
The defining radiographic expression of the modelling defect.
- target: Medullary stenosis of the long bones
description: >-
The reciprocal half of the same modelling defect - cortex encroaches on
the medullary cavity.
- target: Proportionate short stature
description: >-
Growth endpoint of impaired skeletal development, in the survivable KCS2
subtype.
- target: Intrauterine growth deficiency
description: >-
Prenatal growth endpoint, in the severe OCS subtype.
- target: Decreased skull ossification
description: >-
Failure of cranial bone formation, giving the cloverleaf skull shape in
OCS.
- target: Large anterior fontanel with delayed closure
description: >-
The milder KCS2 counterpart of the OCS skull-ossification failure.
- target: Relative macrocephaly
description: >-
Cranial vault growth is relatively spared against the body-size deficit
in KCS2.
- target: Thin ribs with thoracic hypoplasia
description: >-
Chest-wall expression of the skeletal defect in OCS.
- target: Perinatal fractures
description: >-
Consequence of the abnormal bone produced by the modelling defect.
- target: Dental manifestations
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Dental hard tissue shares the mineralized-tissue defect; the specific
route is not established.
- target: Carious teeth
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Grouped with the other dental manifestations of impaired
mineralized-tissue development; no specific route is established.
evidence:
- reference: PMID:23684011
reference_title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "genetic conditions characterized by impaired skeletal development with small and dense bones, short stature, and primary hypoparathyroidism with hypocalcemia"
explanation: >-
Defines the shared skeletal phenotype of the two subtypes.
phenotypes:
- category: Growth
name: Proportionate short stature
subtype: KCS2
phenotype_term:
preferred_term: Proportionate short stature
term:
id: HP:0003508
label: Proportionate short stature
frequency: VERY_FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "KCS is characterized by proportionate short stature (typically postnatal onset), relative macrocephaly, large anterior fontanel with delayed closure"
explanation: >-
GeneReviews defines the KCS growth phenotype and notes its postnatal
onset, which distinguishes it from OCS.
- category: Craniofacial
name: Relative macrocephaly
subtype: KCS2
phenotype_term:
preferred_term: Relative macrocephaly
term:
id: HP:0004482
label: Relative macrocephaly
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "proportionate short stature (typically postnatal onset), relative macrocephaly, large anterior fontanel with delayed closure"
explanation: >-
Listed by GeneReviews among the KCS features. Note that OCS has
microcephaly instead - the two subtypes diverge on this point.
- category: Craniofacial
name: Large anterior fontanel with delayed closure
subtype: KCS2
phenotype_term:
preferred_term: Wide anterior fontanel
term:
id: HP:0000260
label: Wide anterior fontanel
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "relative macrocephaly, large anterior fontanel with delayed closure, characteristic facial features"
explanation: >-
Listed by GeneReviews among the KCS features.
- category: Skeletal
name: Cortical thickening of the long bones
phenotype_term:
preferred_term: Cortical thickening of long bone diaphyses
term:
id: HP:0005791
label: Cortical thickening of long bone diaphyses
frequency: VERY_FREQUENT
diagnostic: true
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "cortical thickening of the long bones with stenosis of the medullary cavity"
explanation: >-
The radiographic hallmark, recorded by GeneReviews for KCS.
- category: Skeletal
name: Medullary stenosis of the long bones
phenotype_term:
preferred_term: Stenosis of the medullary cavity of the long bones
term:
id: HP:0100254
label: Stenosis of the medullary cavity of the long bones
frequency: VERY_FREQUENT
diagnostic: true
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "cortical thickening of the long bones with stenosis of the medullary cavity"
explanation: >-
Paired with cortical thickening as the defining radiographic finding.
- category: Endocrine
name: Primary hypoparathyroidism
phenotype_term:
preferred_term: Hypoparathyroidism
term:
id: HP:0000829
label: Hypoparathyroidism
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS."
explanation: >-
GeneReviews records hypoparathyroidism across both subtypes, with "can
occur" supporting FREQUENT rather than universal.
- category: Metabolic
name: Hypocalcemia
phenotype_term:
preferred_term: Hypocalcemia
term:
id: HP:0002901
label: Hypocalcemia
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS."
explanation: >-
The biochemical consequence of the hypoparathyroidism.
- category: Metabolic
name: Hyperphosphatemia
phenotype_term:
preferred_term: Hyperphosphatemia
term:
id: HP:0002905
label: Hyperphosphatemia
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS."
explanation: >-
Recorded by GeneReviews alongside hypocalcemia.
- category: Growth
name: Intrauterine growth deficiency
subtype: OCS
phenotype_term:
preferred_term: Intrauterine growth retardation
term:
id: HP:0001511
label: Intrauterine growth retardation
frequency: VERY_FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "OCS is characterized by intrauterine growth deficiency, microcephaly, characteristic facial features, decreased skull ossification"
explanation: >-
Prenatal onset is one of the features that separates OCS from KCS.
- category: Neurological
name: Microcephaly
subtype: OCS
phenotype_term:
preferred_term: Microcephaly
term:
id: HP:0000252
label: Microcephaly
frequency: VERY_FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "OCS is characterized by intrauterine growth deficiency, microcephaly, characteristic facial features, decreased skull ossification"
explanation: >-
Microcephaly in OCS, against relative macrocephaly in KCS.
- category: Skeletal
name: Decreased skull ossification
subtype: OCS
phenotype_term:
preferred_term: Decreased skull ossification
term:
id: HP:0004331
label: Decreased skull ossification
frequency: VERY_FREQUENT
description: >-
The reason the OCS skull radiographically resembles a cloverleaf; it has
been argued that this is hypomineralization rather than true
craniosynostosis, which is why "osteocraniosplenic syndrome" was proposed
as a less misleading name.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "characteristic facial features, decreased skull ossification, slender long bones with cortical thickening"
explanation: >-
GeneReviews records decreased skull ossification as an OCS feature.
- category: Skeletal
name: Thin ribs with thoracic hypoplasia
subtype: OCS
sequelae:
- target: Pulmonary hypoplasia
description: >-
A restrictive, hypoplastic chest wall constrains fetal lung growth; this
is the route by which the skeletal defect becomes lethal in OCS.
phenotype_term:
preferred_term: Thin ribs
term:
id: HP:0000883
label: Thin ribs
frequency: VERY_FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "flared metaphyses, and thin ribs with thoracic and pulmonary hypoplasia leading to respiratory insufficiency"
explanation: >-
The chest-wall finding that drives the lethality of OCS.
- category: Respiratory
name: Pulmonary hypoplasia
subtype: OCS
phenotype_term:
preferred_term: Pulmonary hypoplasia
term:
id: HP:0002089
label: Pulmonary hypoplasia
frequency: VERY_FREQUENT
description: >-
The proximate cause of death in OCS, via respiratory insufficiency.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "thin ribs with thoracic and pulmonary hypoplasia leading to respiratory insufficiency"
explanation: >-
GeneReviews links the chest-wall hypoplasia to respiratory insufficiency.
- category: Hematologic
name: Splenic hypoplasia or aplasia
subtype: OCS
phenotype_term:
preferred_term: Splenic hypo/aplasia
term:
id: HP:0006270
label: Hypoplastic spleen
frequency: FREQUENT
description: >-
Characteristic enough to have prompted the alternative name
"osteocraniosplenic syndrome", and one of the features that distinguishes
OCS from Hallermann-Streiff syndrome.
evidence:
- reference: PMID:35205306
reference_title: "Clinical and Molecular Diagnosis of Osteocraniostenosis in Fetuses and Newborns: Prenatal Ultrasound, Clinical, Radiological and Pathological Features."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "a severe, usually lethal condition characterized by gracile bones with thin diaphyses, a cloverleaf-shaped skull and splenic hypo/aplasia"
explanation: >-
Splenic hypo/aplasia is named as one of the three defining OCS features.
- category: Skeletal
name: Perinatal fractures
phenotype_term:
preferred_term: Recurrent fractures
term:
id: HP:0002757
label: Recurrent fractures
frequency: OCCASIONAL
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "thin ribs with thoracic and pulmonary hypoplasia leading to respiratory insufficiency. Perinatal fractures may occur."
explanation: >-
GeneReviews records fractures as possible rather than expected, supporting
OCCASIONAL.
- category: Ocular
name: Hypermetropia
subtype: KCS2
phenotype_term:
preferred_term: Hypermetropia
term:
id: HP:0000540
label: Hypermetropia
frequency: FREQUENT
description: >-
The commonest of the ophthalmologic manifestations GeneReviews groups
together; five of eight patients in the largest Chinese series.
evidence:
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Clinical manifestations included ocular defects such as hypermetropia (5/8), dental problems such as defective dentition (3/8) and dental caries (3/8)"
explanation: >-
Gives the specific refractive error and its frequency in the cohort,
replacing the general "ophthalmologic manifestations" grouping.
- category: Ocular
name: Cataract
subtype: KCS2
phenotype_term:
preferred_term: Cataract
term:
id: HP:0000518
label: Cataract
frequency: OCCASIONAL
description: >-
Named in the GeneReviews management section alongside refractive error as
an ophthalmologic manifestation requiring management; no frequency is
given, so it is curated conservatively.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "management of refractive errors and cataracts"
explanation: >-
GeneReviews requires cataract management, which establishes the finding
occurs but does not quantify it - hence PARTIAL and OCCASIONAL.
- category: Dental
name: Dental manifestations
phenotype_term:
preferred_term: Abnormality of the dentition
term:
id: HP:0000164
label: Abnormality of the dentition
frequency: FREQUENT
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "stenosis of the medullary cavity, and ophthalmologic and dental manifestations"
explanation: >-
GeneReviews records dental manifestations, and its surveillance schedule
calls for six-monthly dental examinations.
- category: Dental
name: Carious teeth
subtype: KCS2
phenotype_term:
preferred_term: Dental caries
term:
id: HP:0000670
label: Carious teeth
frequency: FREQUENT
description: >-
The specific dental finding behind the GeneReviews grouping, alongside
defective dentition; each in three of eight patients in the Chinese series.
evidence:
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "dental problems such as defective dentition (3/8) and dental caries (3/8)"
explanation: >-
Quantifies the dental manifestations that GeneReviews reports without a
frequency.
- category: Neurological
name: Seizures
subtype: KCS2
phenotype_term:
preferred_term: Seizures and spasms
term:
id: HP:0001250
label: Seizure
frequency: VERY_FREQUENT
description: >-
Present in over 70% of genetically confirmed KCS2 cases across the
published literature. Mechanistically these are hypocalcemic seizures - the
neurological expression of the hypoparathyroidism rather than an
independent cerebral phenotype - which is how the pathograph wires them.
evidence:
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Data analysis showed that short stature, hypoparathyroidism and hypocalcemia, ocular and dental defects, skeletal features including cortical thickening and medullary stenosis of tubular bones, and seizures/spasms were present in more than 70% of the reported KCS2 cases."
explanation: >-
A literature-wide review of 46 genetically confirmed patients placing
seizures/spasms above 70%.
- category: Neurological
name: Cerebral calcification
subtype: KCS2
phenotype_term:
preferred_term: Cerebral calcification
term:
id: HP:0002514
label: Cerebral calcification
frequency: FREQUENT
description: >-
A recognized consequence of chronic hypoparathyroidism with
hyperphosphatemia, seen in three of eight patients in the Chinese series.
evidence:
- reference: PMID:38591167
reference_title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "skeletal and brain anomalies such as delayed closure of anterior fontanelle (6/8), cerebral calcification (3/8), cortical thickening (3/8) and medullary stenosis (4/8) of tubular bones"
explanation: >-
Reports the intracranial calcification with its cohort frequency.
- category: Renal
name: Nephrocalcinosis
phenotype_term:
preferred_term: Nephrocalcinosis
term:
id: HP:0000121
label: Nephrocalcinosis
frequency: OCCASIONAL
description: >-
Curated as a treatment-associated finding rather than a primary feature of
the disease: calcium and activated vitamin D replacement for
hypoparathyroidism raises urinary calcium, and GeneReviews requires annual
renal ultrasound for nephrocalcinosis and nephrolithiasis specifically
while a patient is on treatment. It is included because it drives
management, not because FAM111A is known to act in the kidney.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "abdominal ultrasound to assess for nephrocalcinosis and/or nephrolithiasis annually while on treatment"
explanation: >-
Establishes nephrocalcinosis as an expected, surveilled risk of therapy;
PARTIAL because the source prescribes surveillance rather than reporting
a frequency.
- category: Endocrine
name: Growth hormone deficiency
subtype: KCS2
phenotype_term:
preferred_term: Growth hormone deficiency
frequency: OCCASIONAL
description: >-
Deliberately left unbound. HPO has no plain "growth hormone deficiency"
term: the near neighbours each assert something the source does not say -
HP:0008240 asserts a secondary (pituitary/hypothalamic) cause, and
HP:0000824 asserts a stimulation-test result. Needs term.
Reported alongside transient central hypothyroidism in a single
homozygous-Y414C patient, whose height velocity nonetheless did not improve
on recombinant growth hormone from age six - so the finding is a documented
additional endocrinopathy rather than a treatable cause of the short
stature.
evidence:
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "In addition to hypoparathyroidism, other endocrine defects included growth hormone deficiency and transient central hypothyroidism."
explanation: >-
The single reported instance, which is why the frequency is occasional.
diagnosis:
- name: Molecular genetic testing of FAM111A
diagnosis_term:
preferred_term: molecular genetic testing
term:
id: NCIT:C15709
label: Genetic Testing
presence: Heterozygous FAM111A pathogenic variant, most often c.1706G>A p.(Arg569His)
description: >-
GeneReviews requires a heterozygous pathogenic variant, which covers the
dominant presentations that account for nearly all cases. Two caveats it
predates: recessive disease exists - homozygosity for a weakly activating
Tyr414 allele, with unaffected heterozygous parents - so a homozygous
result in a consanguineous family is not a reporting error; and vertical
transmission has been documented, so an apparently de novo variant should
still prompt parental testing before recurrence counselling.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The diagnosis of a FAM111A-related skeletal dysplasia is established in a proband with suggestive findings and a heterozygous pathogenic variant in FAM111A identified by molecular genetic testing."
explanation: >-
The GeneReviews diagnostic criterion.
- reference: PMID:39932783
reference_title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "In the present study, we have identified 2 FAM111A missense variants that replace the same critical amino acid, p.Y414C and p.Y414N, that cause KCS2 and OCS, respectively, only when homozygous."
explanation: >-
Establishes the recessive route that the heterozygous-only criterion does
not cover; PARTIAL because it qualifies rather than supports that
criterion.
- name: Serum calcium, phosphate and parathyroid hormone
diagnosis_term:
preferred_term: serum biochemistry
term:
id: NCIT:C25294
label: Laboratory Procedure
presence: Hypocalcemia with hyperphosphatemia and inappropriately low PTH
description: >-
The biochemical signature of the primary hypoparathyroidism, which occurs
in both KCS and OCS. It is also the surveillance test: GeneReviews asks for
calcium, phosphate and vitamin D every three months until the calcium is
normalized on treatment, then every six months.
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS."
explanation: >-
The biochemical abnormality this test detects.
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "measurement of serum calcium, phosphate, and vitamin D every three months until calcium level is normalized on treatment and then subsequently every six months"
explanation: >-
The surveillance schedule this entry records.
treatments:
- name: Calcium and Activated Vitamin D
description: >-
Supplemental calcium and activated forms of vitamin D, managed by an
endocrinologist. GeneReviews sets an explicit monitoring cadence - serum
calcium, phosphate and vitamin D every three months until calcium is
normalized and every six months thereafter, with annual abdominal ultrasound
for nephrocalcinosis and nephrolithiasis while on treatment.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
target_phenotypes:
- preferred_term: Hypocalcemia
term:
id: HP:0002901
label: Hypocalcemia
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "for all affected individuals, supplemental calcium and activated forms of vitamin D per endocrinologist"
explanation: >-
GeneReviews management recommendation.
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "abdominal ultrasound to assess for nephrocalcinosis and/or nephrolithiasis annually while on treatment"
explanation: >-
The renal surveillance that GeneReviews attaches to this therapy.
- name: Respiratory Support in OCS Survivors
description: >-
Survivors with OCS require aggressive respiratory support and management of
restrictive lung disease with a respiratory specialist - the intervention
that determines whether an OCS infant survives at all.
therapeutic_modality: DEVICE
treatment_term:
preferred_term: Supportive Care
term:
id: NCIT:C15747
label: Supportive Care
target_phenotypes:
- preferred_term: Pulmonary hypoplasia
term:
id: HP:0002089
label: Pulmonary hypoplasia
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Survivors with OCS require aggressive respiratory support and management of restrictive lung disease with a respiratory specialist"
explanation: >-
GeneReviews management recommendation specific to the OCS subtype.
- name: Orthopedic Management of Scoliosis
description: >-
Conservative or surgical management per orthopedist and neurosurgeon, with
clinical examination for scoliosis at every visit.
therapeutic_modality: SURGERY
treatment_term:
preferred_term: orthopedic surgical procedure
term:
id: NCIT:C16186
label: Orthopedic Surgical Procedure
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "conservative or surgical management per orthopedist and/or neurosurgeon for scoliosis"
explanation: >-
GeneReviews management recommendation.
- name: Genetic Counseling
description: >-
Dominant counseling with a strong de novo prior. For KCS, most cases are de
novo and offspring risk is 50%; for OCS, essentially all tested probands
were de novo, so recurrence risk to family members is presumed low.
therapeutic_modality: BEHAVIORAL
treatment_term:
preferred_term: Genetic Counseling
term:
id: NCIT:C15240
label: Genetic Counseling
evidence:
- reference: PMID:37023242
reference_title: "FAM111A-Related Skeletal Dysplasias."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Each child of an individual with KCS has a 50% chance of inheriting the FAM111A pathogenic variant."
explanation: >-
GeneReviews genetic counseling guidance for the survivable subtype.
animal_models:
- name: Fam111a knockout mouse (Fam111a-/- C57BL/6N)
species: Mouse
genotype: Fam111a-/- (constitutive knockout), C57BL/6N
publication: PMID:35715480
description: >-
The first and so far only in vivo characterization of FAM111A, and a
comprehensively negative one: knockout mice have normal weight, normal
serum PTH, normal serum and 24-hour urinary magnesium, calcium and
phosphate, normal expression of the calciotropic, magnesiotropic and
phosphotropic transport genes in kidney, duodenum and colon, normal femoral
bone morphology and density, and normal kidney and parathyroid histology.
This is a model of the wrong lesion, and its negative result is the point.
Human disease alleles are hypermorphic missense variants that make FAM111A
more active, so deleting the gene is not a milder version of the disease -
it is the opposite manipulation. The absence of any phenotype in the null
is therefore consistent with, rather than contrary to, a gain-of-function
mechanism, and it means that no animal model of KCS2 or OCS currently
exists.
modeled_mechanisms:
- target: Impaired Parathyroid Hormone Production and Calcium Homeostasis
relationship: FAILS_TO_RECAPITULATE
fidelity: LOW
description: >-
The defining endocrine phenotype of the human disease is entirely absent:
serum PTH, calcium, phosphate and magnesium are all unaltered, and
parathyroid histology is normal.
limitations: >-
A constitutive null models loss of function, whereas human KCS2 and OCS
are caused by hypermorphic missense alleles - so the model tests the
wrong direction of effect and cannot bear on whether excess FAM111A
protease activity damages the parathyroid. The authors additionally note
the animals were on a standard diet, leaving open whether a challenge
would unmask a phenotype.
readouts:
- name: Serum PTH, calcium, phosphate and magnesium
target: Impaired Parathyroid Hormone Production and Calcium Homeostasis
direction: UNCHANGED
interpretation: >-
A genuine negative result - the electrolyte and PTH abnormalities that
define the human disease do not occur when Fam111a is deleted.
evidence:
- reference: PMID:35715480
reference_title: "FAM111A is dispensable for electrolyte homeostasis in mice."
supports: REFUTE
evidence_source: MODEL_ORGANISM
snippet: "Fam111a-/- mice displayed normal weight and serum parathyroid hormone (PTH) concentration and exhibited unaltered magnesium, calcium and phosphate levels in serum and 24-hour urine."
explanation: >-
The measurement itself, reported as unchanged.
evidence:
- reference: PMID:35715480
reference_title: "FAM111A is dispensable for electrolyte homeostasis in mice."
supports: REFUTE
evidence_source: MODEL_ORGANISM
snippet: "In conclusion, our study is the first to characterise the function of FAM111A in vivo and we report that mice lacking FAM111A exhibit normal electrolyte homeostasis on a standard diet."
explanation: >-
The authors' own summary: loss of Fam111a does not disturb electrolyte
homeostasis, so this model is not informative for the human
hypoparathyroidism node.
- target: Impaired Skeletal Development and Cortical Modelling
relationship: FAILS_TO_RECAPITULATE
fidelity: LOW
description: >-
Femoral morphology and bone density are unaffected, so the cortical
thickening and medullary stenosis that define the skeletal phenotype have
no counterpart in the null mouse.
limitations: >-
Same direction-of-effect problem as above: a null cannot model a
hypermorphic allele. Femoral morphometry and density were assessed, but
the specific human findings - cortical thickening with medullary stenosis
- are geometry rather than density measures, so a subtle change is not
formally excluded by the reported analysis.
readouts:
- name: Femoral bone morphology and density
target: Impaired Skeletal Development and Cortical Modelling
direction: UNCHANGED
interpretation: >-
No skeletal phenotype on deletion of the gene.
evidence:
- reference: PMID:35715480
reference_title: "FAM111A is dispensable for electrolyte homeostasis in mice."
supports: REFUTE
evidence_source: MODEL_ORGANISM
snippet: "Analysis of femurs showed unaffected bone morphology and density in Fam111a-/- mice."
explanation: >-
The skeletal measurement, reported as unchanged.
evidence:
- reference: PMID:35715480
reference_title: "FAM111A is dispensable for electrolyte homeostasis in mice."
supports: REFUTE
evidence_source: MODEL_ORGANISM
snippet: "Kidney and parathyroid histology were also normal in Fam111a-/- mice."
explanation: >-
Extends the negative result to tissue architecture in the two organs
most relevant to the human phenotype.
discussions:
- discussion_id: fam111a_protease_to_pth
kind: KNOWLEDGE_GAP
attaches_to:
- pathophysiology#Impaired Parathyroid Hormone Production and Calcium Homeostasis
prompt: >-
How does an over-active FAM111A protease impair parathyroid hormone
production and bone modelling? What are the physiological substrates?
rationale: >-
Part of the chain is now closed: the patient mutants kill cells through
peptidase activity, and FAM111A's normal role in replication origin
activation is established. What remains open is the part that would explain
the disease. No FAM111A substrate has been identified, so nothing accounts
for why parathyroid progenitors and skeletal cortical modelling are what
fail while most proliferating tissue does not - and the authors of the
origin-activation study say as much, that it is unclear how gain-of-function
mutations contribute to the etiology. The disease residues also sit away
from the catalytic site, so even the route from variant to hyperactivity is
inferred rather than shown.
references:
- reference: PMID:37023242
title: "FAM111A-Related Skeletal Dysplasias."
tags:
- GeneReviews
- reference: PMID:23684011
title: "FAM111A mutations result in hypoparathyroidism and impaired skeletal development."
- reference: PMID:39932783
title: "Quantitative hypermorphic FAM111A alleles cause autosomal recessive Kenny-Caffey syndrome type 2 and osteocraniostenosis."
- reference: PMID:37793778
title: "FAM111A regulates replication origin activation and cell fitness."
- reference: PMID:35205306
title: "Clinical and Molecular Diagnosis of Osteocraniostenosis in Fetuses and Newborns: Prenatal Ultrasound, Clinical, Radiological and Pathological Features."
- reference: PMID:38591167
title: "Further delineation of phenotype and genotype of Kenny-Caffey syndrome type 2 (phenotype and genotype of KCS type 2)."
- reference: PMID:23996431
title: "A recurrent de novo FAM111A mutation causes Kenny-Caffey syndrome type 2."
- reference: PMID:35715480
title: "FAM111A is dispensable for electrolyte homeostasis in mice."
FAM111A-related skeletal dysplasia is not a single entity but an allelic disease spectrum caused by variants in FAM111A (FAM111 trypsin-like peptidase A; HGNC:24151; chromosome 11q12.1), spanning from the relatively milder, non-lethal Kenny-Caffey syndrome type 2 (KCS2) (OMIM #127000) at one end to the perinatally lethal Gracile Bone Dysplasia / Osteocraniostenosis (GCLEB/OCS) (OMIM #602361) at the other, with a growing recognition of autosomal recessive forms at both ends of severity described in 2024–2025. The unifying molecular mechanism is gain-of-function (hyperactivation) of the FAM111A serine protease, a PCNA-associated enzyme that normally clears protein obstacles from DNA replication forks; disease-causing variants relieve autoinhibitory constraints on the protease, producing a dose-dependent, cytotoxic excess of proteolytic activity that impairs replication, triggers apoptosis, and secondarily disrupts skeletal mineralization and parathyroid gland development (Unger et al., Am J Hum Genet 2013, PMID: 23684011; Alabert lab and collaborators, EMBO Reports 2020, PMC: PMC7534640).
Overview. FAM111A-related skeletal dysplasia comprises a phenotypic continuum of rare, genetically defined skeletal/endocrine disorders unified by pathogenic variation in FAM111A:
| Entity | OMIM | Severity | Typical inheritance |
|---|---|---|---|
| Kenny–Caffey syndrome, type 2 (KCS2) | #127000 | Non-lethal; short stature + hypoparathyroidism | Autosomal dominant (usually de novo); recessive forms now reported |
| Gracile bone dysplasia (GCLEB) / Osteocraniostenosis (OCS) | #602361 | Perinatally lethal | Autosomal dominant (de novo) and recessive (biallelic hypomorphic/hypermorphic) forms |
| Gene | FAM111A | OMIM *615292 | 11q12.1 |
Osteocraniostenosis (OCS) and gracile bone dysplasia (GCLEB) are generally treated as the same entity/synonymous terminology in the literature (also called "osteocraniosplenic syndrome" when splenic hypoplasia is emphasized).
Key identifiers: - OMIM: 127000 (KCS2), 602361 (GCLEB/OCS), 615292 (FAM111A gene) - Orphanet: ORPHA2333 (Kenny-Caffey syndrome) - MONDO/MedGen: Osteocraniostenosis, MedGen concept C1865639 - MeSH: Kenny-Caffey Syndrome (D064878) - Gene: HGNC:24151, NCBI Gene 63901, Ensembl ENSG00000166801 - Contrast with the TBCE-associated, unrelated Kenny-Caffey syndrome type 1 (KCS1), OMIM #244460, an autosomal recessive disorder allelic to Sanjad-Sakati syndrome* — an important differential (see §10).
Synonyms: Kenny-Caffey syndrome, dominant type; Kenny-Caffey syndrome, autosomal dominant; osteocraniostenosis; osteocraniosplenic syndrome; gracile bone dysplasia with skull dysplasia.
Evidence basis. The knowledge base for this condition derives almost entirely from aggregated case reports and small case series/cohort reviews (total published cohort now approaching ~50 KCS2 patients and fewer than 30 GCLEB/OCS cases), rather than large EHR-derived or population-registry datasets, reflecting its extreme rarity (Chen & Zou, Mol Genet Genomic Med 2024, PMID: 38591167; malacards.org).
Disease causal factor. Monogenic — heterozygous (dominant) or, in newly described families, biallelic (recessive) pathogenic variants in FAM111A, essentially always missense variants clustering in and around the C-terminal trypsin-like serine protease domain (SPD). No environmental, infectious, or multifactorial contribution is documented; this is a purely genetic/mechanistic disease.
Genetic risk factors: - Dominant hotspot: c.1706G>A (p.Arg569His) accounts for ~70% (32/46) of reported KCS2 cases in one systematic literature review, and up to 87.5% of some regional cohorts (Chen & Zou 2024). It arises recurrently de novo (ClinVar RCV000050209). - Other recurrent dominant KCS2/OCS variants: p.Tyr511His (c.1531T>C), p.Ser541Pro, p.Asp528Gly (D528G), p.Thr338Ala (T338A), and others in the SPD (Isojima et al., J Bone Miner Res 2014; Chen & Zou 2024). - De novo rate: ~79.5% (31/39) of dominant cases arise de novo; parent-to-child (both mother-to-daughter and, newly documented, father-to-daughter) transmission has been reported, confirming full penetrance with dominant inheritance (Chen & Zou 2024). - Newly described recessive alleles (2024–2025): - Hypermorphic recessive: homozygous p.Tyr414Cys / p.Tyr414Asn — a "quantitative hypermorphic" mechanism where the homozygous state produces intermediate protease hyperactivation sufficient to cause KCS2/OCS, while heterozygous carriers are asymptomatic (Li et al., JCI Insight 2025, PMID: 39932783). - Hypomorphic recessive: homozygous synonymous variant c.81G>A (p.Pro27=) causing aberrant splicing, reduced mRNA, and near-absent protein — a partial loss-of-function mechanism in a consanguineous family (2 siblings) (Journal of Human Genetics 2024, DOI: 10.1038/s10038-024-01301-1, PMC: PMC11762410). - Compound heterozygous FAM111A variants have also been reported causing autosomal recessive KCS2 (PMID: 34382758). - No modifier genes, susceptibility loci, or GWAS hits are established — this is a fully penetrant Mendelian disease, not a complex/polygenic trait.
Environmental risk factors: None identified; not associated with teratogens, maternal exposures, or lifestyle factors.
Protective factors: None described; there is no known protective allele. Notably, mouse Fam111a knockout studies (below) show no overt skeletal or electrolyte phenotype, implying the human disease mechanism is a toxic gain-of-function rather than simple loss of a protective/essential activity — so there is no analogous "loss-of-function protects" relationship as seen in some other disorders.
Gene-environment interactions: None reported; disease expression appears to be driven purely by the quantitative level of FAM111A protease hyperactivation (a dose-dependent, cell-intrinsic mechanism), not by external modifiers.
Phenotype data below are drawn primarily from the 46-patient KCS2 literature synthesis (Chen & Zou 2024, PMC11002637) and OMIM.
| Phenotype | Frequency | Suggested HPO term |
|---|---|---|
| Proportionate short stature (severe, congenital-onset, often −3 to −10 SDS) | 95–100% | HP:0003508 (Proportionate short stature) |
| Hypoparathyroidism | 80–86.5% | HP:0000829 (Hypoparathyroidism) |
| Hypocalcemia | 82.9% | HP:0002901 (Hypocalcemia) |
| Medullary stenosis of tubular bones | 73% | HP:0003044 (Long bone bowing) / custom: medullary stenosis |
| Cortical thickening of long bones | 69.4% | HP:0002684 (Thickened cortex of long bones) |
| Delayed closure of anterior fontanelle | 67.6% | HP:0001005 (Large fontanelles) |
| Prominent/bossed forehead | 73.5% | HP:0000239 (Frontal bossing) |
| Ocular abnormalities (hypermetropia most common, 60%) | ~75% overall | HP:0000540 (Hypermetropia); HP:0000568 (Microphthalmia) |
| Dental abnormalities (defective dentition, caries, oligodontia) | ~72% | HP:0000692 (Abnormality of dental enamel) / HP:0000668 (Dental crowding) / HP:0000668 |
| Seizures/hypocalcemic spasms | ~70% | HP:0001250 (Seizure) |
| Micrognathia | 35.3% | HP:0000347 |
| Cerebral calcification (basal ganglia, secondary to chronic hypocalcemia) | 35.3% | HP:0002514 (Basal ganglia calcification) |
| Depressed nasal bridge | 38.2% | HP:0005280 |
| Small eyes | 41.2% | HP:0020045 |
| Micropenis / microorchidism (males) | reported in subset | HP:0000054 / HP:0000035 |
| Growth hormone deficiency | 37.5% of those tested | HP:0000824 |
| Intellectual disability | 11.6% (an emerging, previously atypical finding — historically KCS2 was defined by normal intelligence, distinguishing it from KCS1/Sanjad-Sakati) | HP:0001249 |
Onset/course: Prenatal ultrasound may show shortened long bones and IUGR in some cases; postnatal presentation is frequently a neonatal hypocalcemic seizure picture. Mean age at diagnosis across the literature is 13.2 years (range 20 weeks gestation to 66 years — reflecting both severe neonatal presentations and mild adult-diagnosed cases). Calcium control tends to stabilize with age in survivors, but long-term complications (chronic kidney disease, nephrocalcinosis from treatment-related hypercalciuria) are increasingly recognized (Chen & Zou 2024; case report literature).
Quality of life impact: For KCS2 survivors, QoL is driven chiefly by (1) seizure burden from labile hypocalcemia, (2) short stature and its psychosocial effects, (3) visual impairment from significant hypermetropia, and (4) dental morbidity requiring extensive restorative care. No formal EQ-5D/SF-36/PROMIS studies specific to KCS2 were identified in the literature (consistent with its extreme rarity) — this is a case-report-level knowledge base, not systematically measured QoL data.
Causal gene: FAM111A (HGNC:24151; NCBI Gene ID 63901; OMIM *615292), encoding a 611-amino-acid, ~70 kDa protein.
Protein domain architecture (Unravelling FAM111A/FAM111B review, PMC: PMC10931937): - N-terminal PCNA-interacting protein (PIP) box — tethers FAM111A to the DNA sliding clamp PCNA at replication forks - Two ubiquitin-like domains (UBL-1, UBL-2) - C-terminal trypsin-like serine protease domain (SPD), catalytic triad His385–Asp439–Ser541, structurally resembling Trypsin-2/PRSS2 but with chymotrypsin-like substrate specificity - The protein undergoes autocleavage between Phe334 and Gly335
Variant classification and type: Essentially all reported pathogenic variants are missense substitutions clustering in the serine protease domain (dominant hotspot p.Arg569His; other dominant sites p.Tyr511His, p.Ser541Pro, p.Asp528Gly, p.Thr338Ala; recessive sites p.Tyr414Cys/Asn); one recessive synonymous/splice-altering variant (p.Pro27=) has also been reported causing loss of function. No large structural rearrangements, frameshift, or nonsense variants have been reported to cause disease — consistent with a strict gain-of-function/toxic mechanism for the dominant/hypermorphic recessive forms, where haploinsufficiency (nonsense/frameshift null alleles) is apparently tolerated (as also suggested by the viable, largely unaffected Fam111a-null mouse — see §15).
Allele frequency: All known pathogenic missense variants are absent or present at only extremely low frequency in gnomAD population databases (consistent with a fully penetrant, mostly de novo dominant disease); specific gnomAD allele counts were not enumerated in the sources reviewed but rarity is implicit in ClinVar submissions for this gene.
Somatic vs. germline: Disease-causing variants are germline (constitutional); separately, FAM111A has been implicated in somatic contexts in cancer genomics (see below), which is a distinct, unrelated body of literature from the skeletal dysplasia.
Functional consequence — the core molecular mechanism: FAM111A pathogenic variants are gain-of-function (hyperactivating) with respect to its intrinsic serine protease activity. Two converging structural/functional studies establish this: 1. Kim et al./Alabert-lab-adjacent work (EMBO Reports 2020, PMC: PMC7534640) showed that "FAM111A proteolytic activity suppresses DNA replication and transcription by displacing key effectors of these processes from chromatin, triggering rapid programmed cell death by Caspase-dependent apoptosis... Patient-associated point mutations in FAM111A exacerbate these phenotypes by hyperactivating its intrinsic protease activity." 2. A 2024 X-ray crystallography study (Nature Communications, PMID: 38453899) solved the SPD structure (using a catalytically dead S541A mutant to avoid autocleavage) and showed that FAM111A is a dimerization-dependent protease: dimerization via an N-terminal helix triggers an allosteric activation cascade from a "dimerization sensor loop" to the oxyanion hole through disorder-to-order transitions, and this dimerization is essential for proteolytic activity against DNA-protein crosslink (DPC) substrates in cells (though dispensable for autocleavage). 3. The 2025 JCI Insight recessive-allele paper (PMID: 39932783) refined this to a quantitative, gene-dose-dependent model: Tyr414 sits in the dimer-sensing L4 loop, contacting Tyr359 in the central β-sheet; substitutions here partially disrupt the allosteric network, producing a mild gain-of-function that is subclinical in heterozygotes but disease-causing when homozygous — directly explaining how the same gene produces both dominant (single strong hypermorphic allele) and recessive (double weak hypermorphic allele, or biallelic hypomorphic-loss-of-function) inheritance patterns. Structural modeling also predicts that more severe positions (e.g., Tyr562Ser) are "far more destabilizing," correlating with the lethal OCS phenotype — an emerging genotype-severity correlation.
Epigenetic information / chromosomal abnormalities: No epigenetic (DNA methylation/histone) mechanism or chromosomal-scale abnormality (aneuploidy, translocation) has been implicated in FAM111A-related skeletal dysplasia; disease is driven by point-mutation-level protein hyperactivation, not epigenetic dysregulation of the locus itself.
No environmental, lifestyle, or infectious contributing factors are documented for FAM111A-related skeletal dysplasia — it is a purely monogenic disorder with full/near-full penetrance independent of exposure history. (Note: FAM111A itself has a documented role as a host antiviral restriction factor — see §6 — but this is a downstream consequence of its normal biology, not an environmental trigger of the skeletal dysplasia phenotype.)
The rhPTH treatment case report (PMC: PMC13025059) proposes specifically that "FAM111A dysfunction impairs parathyroid progenitor cell development during embryogenesis" — i.e., the parathyroid glands are structurally hypoplastic/dysfunctional from a developmental replication-stress insult, rather than merely under-secreting from an otherwise normal gland, which is the rationale for hormone-replacement (rather than secretagogue) therapy.
Not a loss-of-function/misfolding disease in the classical sense; rather, the disease protein is structurally intact but biochemically dysregulated — an unrestrained/hyperactive enzyme. This distinguishes FAM111A-related dysplasia mechanistically from most other skeletal dysplasias (e.g., collagenopathies), which are typically loss-of-function or dominant-negative structural-protein disorders.
No transcriptomic (GEO/ArrayExpress), proteomic (PRIDE), metabolomic, single-cell, or spatial-transcriptomic dataset specific to patient tissue in FAM111A-related skeletal dysplasia was identified in this search — molecular characterization to date rests on patient-derived fibroblast functional assays (chromatin fractionation, NanoBRET dimerization assays, apoptosis marker immunoblotting) and heterologous overexpression systems, not large-scale omics profiling. This is a notable knowledge gap given the rarity of patient material.
Organ level: - Skeletal system (primary): long bones (femur, tibia, radius, ulna — cortical thickening, medullary stenosis, gracile diaphyses in the lethal form), skull (delayed fontanelle closure in KCS2; cloverleaf, hypomineralized skull in OCS), ribs (gracile/irregular in OCS), hands/feet (short, overmodeled tubular bones; brachydactyly) - Endocrine system: parathyroid glands (hypoplasia/dysfunction → hypoparathyroidism); anterior pituitary/GH axis (deficiency in ~37.5% tested) - Eyes: hypermetropia, microphthalmia, papilledema - Teeth: enamel defects, oligodontia, caries - Spleen: hypoplasia/aplasia (OCS) - CNS: basal ganglia calcification (secondary to chronic hypocalcemia); seizures - Reproductive: micropenis/microorchidism reported in a subset of males - Respiratory: restrictive lung insufficiency from thoracic cage dysplasia — the proximate cause of death in lethal OCS
Body systems: Skeletal, endocrine (parathyroid/pituitary), ophthalmologic, dental, and (secondarily) neurologic systems are the principal ones involved; splenic and respiratory involvement is specific to the lethal OCS end of the spectrum.
Tissue/cell level: Growth-plate cartilage and cortical/trabecular bone-forming cells (chondrocytes, osteoblasts); parathyroid chief cells; corneal/lens tissue.
Subcellular level: The core lesion is nuclear/chromatin-level — FAM111A acts at replication forks and chromatin (GO Cellular Component: nucleus GO:0005634, replication fork GO:0005657); pathogenic hyperactivation displaces replication/transcription machinery from chromatin.
Lateralization: Not applicable — this is a systemic, bilateral/symmetric developmental disorder (e.g., bilateral ocular and dental findings), not a lateralized process.
Onset: - KCS2: Congenital/perinatal onset of skeletal findings (detectable on prenatal ultrasound as shortened long bones in some cases); classic clinical presentation is a neonatal or early-infancy hypocalcemic seizure. Diagnosis can, however, be delayed into adulthood in milder presentations (reported diagnostic ages up to 66 years), and late-middle-age case reports exist describing "KCS2-suggestive" features emerging over a long follow-up (PMC: PMC9846794). - GCLEB/OCS: Prenatal onset, detectable by ultrasound in the second/third trimester; presentation at birth or shortly after.
Progression: - KCS2 is generally non-progressive to slowly evolving in its skeletal features once established, but the endocrine (hypocalcemia control) and secondary complications (nephrocalcinosis from treatment, chronic kidney disease) can evolve over years — so "disease course" is better characterized as chronic/lifelong management of a static structural lesion with an evolving complication burden, rather than a classic progressive degenerative disease. - OCS is rapidly fatal, typically within hours to months of birth (respiratory failure).
Patterns: No remission pattern is described (this is a structural/developmental, not relapsing-immunologic, disease); calcium homeostasis can "stabilize" with age in survivors per the Chen & Zou 2024 cohort review, but this reflects treatment optimization and possibly some catch-up parathyroid function rather than true disease remission. Critical period: the parathyroid/skeletal developmental window in utero and early infancy is the critical period during which FAM111A hyperactivation produces its organ-level damage; there is no evidence of a postnatal "window of opportunity" for prevention once the germline variant is present, though early recognition and calcium/PTH management is critical to prevent secondary seizure-related and nephrocalcinosis-related morbidity.
Epidemiology: KCS2 is classified as ultra-rare, prevalence <1:1,000,000. Fewer than 50 KCS2 cases and fewer than 30 GCLEB/OCS cases have been published in total to date (search synthesis; malacards.org). No formal incidence/prevalence registry (e.g., GBD, national birth-defect registry) figure specific to this gene was identified — figures are literature-count-based estimates rather than population-ascertained rates, an inherent epistemic limitation for an ultra-rare Mendelian disease.
Inheritance pattern: - Autosomal dominant (most common) — typically de novo (~79.5% of cases); rare vertical transmission (mother-to-daughter and father-to-daughter both documented) confirms full penetrance and absence of imprinting effects. - Autosomal recessive (newly recognized, 2021–2025) — via (a) biallelic weakly-hypermorphic missense alleles (e.g., homozygous p.Tyr414Cys/Asn) or (b) biallelic hypomorphic/loss-of-function alleles (e.g., homozygous synonymous splice-altering p.Pro27=), the latter identified in a consanguineous family. - Penetrance: Complete/full penetrance for dominant pathogenic missense variants (no documented non-penetrant heterozygous carriers of the classical hotspot variants); by contrast, the recessive Tyr414 variant is fully non-penetrant in the heterozygous state — obligate carrier relatives in both reported families are asymptomatic — which is itself an important, quantifiable genotype-phenotype/dosage finding. - Expressivity: Variable — the literature documents a wide severity range even among KCS2 patients with the identical p.Arg569His hotspot variant, and Chen & Zou (2024) found no statistically significant phenotypic difference between hotspot-variant carriers and carriers of other FAM111A variants, suggesting expressivity is not strongly variant-position-dependent within the dominant-KCS2 range (though it clearly differs sharply between the dominant-KCS2 range and the more severely destabilizing OCS-associated variants). - Genetic anticipation, germline mosaicism, founder effects, consanguinity: No genetic anticipation is reported (not a repeat-expansion disorder). No specific founder-population variant or consanguinity-linked founder effect is documented for the dominant hotspot (it is recurrent de novo, not inherited from a founder chromosome), though the recessive forms have specifically been described in consanguineous families, consistent with a classical recessive-disease ascertainment pattern. No mosaicism data specific to FAM111A were identified in this search. - Carrier frequency: Not established/reported for recessive pathogenic alleles given the disease's ultra-rarity; not present in population databases at frequencies suggesting a common carrier state.
Population demographics: - Sex ratio: Approximately 1:1 (no sex predominance) — literature review found 25 females : 20 males among reported KCS2 cases (ratio ~1:1.25), not statistically distinct from unity. - Ethnic/geographic distribution: General search results (via malacards/secondary sources) suggested a higher reported incidence in Middle Eastern and Arabian Gulf populations — however, this may partly reflect ascertainment bias and the historical confusion with the TBCE-related KCS1/Sanjad-Sakati syndrome (which does have a well-established Middle Eastern/Bedouin founder-mutation epidemiology). Recent large single-country cohorts have also come from China (Chen & Zou 2024, 8 patients from 6 families), suggesting the true distribution is likely pan-ethnic with variable ascertainment. - Age distribution: Bimodal in effect — neonatal/infantile presentation (hypocalcemic seizures, most common ascertainment route) versus occasional adult incidental/delayed diagnosis.
Laboratory tests: - Serum calcium (low), phosphate (may be elevated), magnesium (may be low), PTH (low/inappropriately normal for the hypocalcemia — primary hypoparathyroidism pattern; note some "atypical" cases now reported with normal PTH, per Chen & Zou 2024) - 24-hour urinary calcium/creatinine ratio — critical for monitoring hypercalciuria risk during conventional (calcium/vitamin D) therapy (LOINC-codeable analytes) - Growth hormone axis testing when short stature is disproportionate to calcium control (GH deficiency found in ~37.5% tested)
Imaging: - Long-bone radiographs: cortical thickening + medullary stenosis (KCS2) vs. gracile/thin diaphyses (OCS) — the defining radiographic dichotomy across the disease spectrum - Skull imaging: delayed fontanelle closure (KCS2) vs. cloverleaf/hypomineralized skull (OCS) - Prenatal ultrasound: shortened long bones, IUGR; the first molecularly confirmed prenatal diagnosis of OCS was achieved via targeted whole-exome sequencing after ultrasound suggested a "serious but non-lethal" skeletal dysplasia (BMC Med Genet 2019 case report, PMC: PMC6947839) - Neuroimaging (CT/MRI): basal ganglia calcification secondary to chronic hypocalcemia (found in ~35% of cases)
Genetic testing: - Single-gene FAM111A sequencing or targeted skeletal-dysplasia/hypoparathyroidism gene panel is the recommended first-tier approach given the well-defined hotspot (p.Arg569His) and clustering of variants in the SPD. - Whole-exome sequencing (WES) has proven diagnostic utility, including for prenatal diagnosis and for identifying atypical/recessive presentations (e.g., the synonymous splice variant, which would likely be missed or misclassified by a naive coding-only variant filter and required RNA/splicing-level functional follow-up). - Chromosomal microarray/karyotype/FISH are not primary diagnostic tools here (this is not a copy-number or chromosomal disorder) but may be used to exclude differentials in an undiagnosed short-stature/skeletal-dysplasia workup.
Clinical/differential diagnosis (critical distinguishing step): - Kenny-Caffey syndrome type 1 (KCS1) and the allelic Sanjad-Sakati syndrome, both caused by biallelic TBCE variants (OMIM #244460), are autosomal recessive and are distinguished from FAM111A-related KCS2 by the presence of microcephaly and intellectual disability in KCS1/Sanjad-Sakati, which are absent (or only rarely present, ~11.6%) in classic FAM111A-KCS2. An overlapping/blended phenotype case has been reported, underscoring that clinical overlap exists and molecular confirmation is essential (PMID: 33010201). - Other causes of isolated/syndromic hypoparathyroidism (e.g., 22q11.2 deletion syndrome, autoimmune polyglandular syndrome type 1, HDR/Barakat syndrome via GATA3) should be excluded in an undiagnosed hypocalcemic infant without a clear skeletal-dysplasia radiographic picture. - Other lethal skeletal dysplasias with hypomineralized skull (e.g., osteogenesis imperfecta type II, hypophosphatasia) enter the differential for OCS on imaging alone, but splenic hypoplasia and the specific gracile-bone/cloverleaf-skull combination, plus molecular confirmation, are distinguishing.
Screening: No population/newborn screening program exists for this ultra-rare condition; case ascertainment is clinical (neonatal hypocalcemic seizure or dysmorphic skeletal survey) followed by confirmatory single-gene or exome sequencing. Cascade testing of relatives is warranted given documented vertical transmission of dominant alleles and the discovery of asymptomatic heterozygous carriers in recessive families.
Survival/mortality: - KCS2: Not classically lethal; reported patients survive into adulthood (up to 66 years at diagnosis in the literature), though long-term mortality/survival statistics (e.g., formal life-table or actuarial data) are not available given the small cohort size. Fatalities have, however, been reported among recessive-form family members with severe/hydropic presentations (e.g., a sibling in the Y414 kindred died at 14 months with hydrops fetalis and respiratory distress), indicating the dominant/recessive-hypermorphic spectrum blurs into life-threatening severity in some individuals. - GCLEB/OCS: Essentially uniformly perinatally lethal — stillbirth or death within hours to months after birth from restrictive respiratory failure; "no treatment currently exists, leading to a poor prognosis."
Morbidity/function (KCS2 survivors): - Chronic seizure risk (especially in early childhood, tied to labile calcium control) - Visual impairment from marked hypermetropia - Significant dental morbidity - Short stature with limited response to growth hormone therapy (see §12) - Emerging long-term renal morbidity: nephrocalcinosis and chronic kidney disease from hypercalciuric conventional (calcium + vitamin D) therapy — this is now recognized as a major long-term complication driver, motivating interest in rhPTH therapy (see §12)
Prognostic factors: Genotype-severity correlation is emerging but incomplete — structural destabilization modeling correlates variant position with severity (mild Y414 hypermorph → recessive KCS2/OCS spectrum; more destabilizing SPD positions like Y562S → lethal OCS), but Chen & Zou (2024) found no significant phenotype difference across dominant KCS2 variants including the p.Arg569His hotspot, so genotype is not yet a reliable individual-level prognostic tool within the dominant-KCS2 range.
No established prognostic biomarker beyond genotype/variant position and clinical calcium control status.
There is no curative or disease-modifying (protease-inhibitor) therapy for FAM111A-related skeletal dysplasia at present; all management is supportive/replacement-based for KCS2, and OCS has no effective treatment (uniformly lethal).
NCIT:C15986 (Pharmacotherapy); therapeutic agents e.g. calcitriol (CHEBI), calcium carbonate (CHEBI).NCIT:C15986), therapeutic agent teriparatide (recombinant PTH 1-34).NCIT:C16186, Orthopedic Surgical Procedure)NCIT:C15302-adjacent dental care terms)NCIT:C15240, Genetic Counseling)No registered clinical trials (ClinicalTrials.gov) specific to FAM111A-related skeletal dysplasia were identified. Given the established gain-of-function protease mechanism, a small-molecule FAM111A protease inhibitor is a logical future therapeutic direction, but no such agent has entered even early preclinical development based on available literature — this represents an open translational-research gap.
No naturally occurring FAM111A-associated disease in non-human animal species (companion animals, livestock, wildlife) was identified in the literature search — no OMIA (Online Mendelian Inheritance in Animals) entry or veterinary case series for a FAM111A-related skeletal dysplasia was found. This gene's disease relevance to date is human-specific in the clinical/veterinary literature reviewed; the "natural disease" comparative angle is essentially unexplored for this gene, unlike many other skeletal dysplasia genes.
FAM111A has clear orthologs in standard vertebrate model species (mouse Fam111a, used experimentally — see §15), but no report of spontaneous/natural veterinary disease.
Mouse (the only in vivo model identified): - A Fam111a knockout (Fam111a⁻/⁻) C57BL/6N mouse, reported as "the first to characterise the function of FAM111A in vivo" (Scientific Reports 2022, PMID: 35715480; PMC: PMC9205974). - Key, somewhat counterintuitive finding: the knockout is essentially phenotypically silent — normal body weight, normal serum PTH, unaltered serum/24h-urine magnesium, calcium, and phosphate, normal femur bone morphology/density, and normal kidney and parathyroid histology. - Interpretation/limitation: This result is consistent with the human disease mechanism being a toxic gain-of-function (not simple loss-of-function), so a null mouse would not be expected to recapitulate a hyperactivation phenotype — meaning the knockout mouse is not a disease model for KCS2/OCS and instead only demonstrates that FAM111A itself is dispensable for baseline electrolyte/skeletal homeostasis in mice. The authors also raise the possibility that redundant/compensating proteases mask a phenotype in mice that would be revealed by protein-level loss in humans. - Consequently, there is no validated knock-in point-mutation (patient-variant) mouse model, no zebrafish, Drosophila, C. elegans, or yeast model, and no organoid/iPSC-based whole-tissue model of FAM111A-related skeletal dysplasia identified in this search — this is a significant translational-model gap. All current mechanistic work is done in: - Patient-derived dermal fibroblasts (used to demonstrate reduced protein/aberrant splicing in the recessive hypomorphic family, and hyperactivation markers in other functional studies) - Heterologous cell-line overexpression systems (e.g., U2OS, HEK293-type systems) expressing wild-type vs. patient-mutant FAM111A to assay chromatin displacement, PCNA loading, DNA damage markers, apoptosis, and (via NanoBRET) dimerization behavior.
Applications/limitations of current "models": - The knockout mouse is useful for excluding an essential-housekeeping-loss explanation and for baseline in vivo electrolyte physiology, but cannot be used to study the actual gain-of-function disease mechanism, model therapeutic protease-inhibition strategies, or test rhPTH/other interventions in vivo. - A patient-variant knock-in mouse (e.g., R569H or Y414C) would be a high-value, currently unmet model-development need for this disease, given (a) the mechanism is now well-defined at the biochemical/structural level, (b) a monogenic gain-of-function point mutation is technically tractable to knock in, and (c) it would enable in vivo testing of hypothetical protease-inhibitor therapeutics.
Resources: MGI (Mouse Genome Informatics) carries the Fam111a gene/allele records referenced in the 2022 knockout study; no IMPC/KOMP conditional-allele-specific disease-modeling publication beyond the constitutive knockout was identified.
| Concept | Suggested term |
|---|---|
| Disease (KCS2) | MONDO (search "Kenny-Caffey syndrome type 2"); OMIM:127000 |
| Disease (GCLEB/OCS) | OMIM:602361; MedGen C1865639 |
| Gene | hgnc:24151 (FAM111A) |
| Short stature | HP:0003508 |
| Hypoparathyroidism | HP:0000829 |
| Hypocalcemia | HP:0002901 |
| Thickened long-bone cortex | HP:0002684 |
| Large/delayed fontanelle | HP:0001005 |
| Hypermetropia | HP:0000540 |
| Seizure | HP:0001250 |
| Basal ganglia calcification | HP:0002514 |
| Cloverleaf skull | HP:0002676 |
| Thin long bones | HP:0004970 |
| Splenic hypoplasia/aplasia | HP:0030068 / HP:0001743 |
| Hydrops fetalis | HP:0001789 |
| Serine-type endopeptidase activity (GO) | GO:0004252 |
| DNA replication (GO) | GO:0006260 |
| Apoptotic process (GO) | GO:0006915 |
| Parathyroid chief cell (CL) | CL:0000446 |
| Osteoblast (CL) | CL:0000062 |
| Chondrocyte (CL) | CL:0000138 |
| Growth plate cartilage (UBERON) | UBERON:0002229 |
| Parathyroid gland (UBERON) | UBERON:0001132 |
| Calcitriol (CHEBI) | CHEBI:17823 |
| Teriparatide/rhPTH pharmacotherapy (NCIT) | NCIT:C15986 (Pharmacotherapy) |
| Genetic counseling (NCIT) | NCIT:C15240 |
Checked with linkml-reference-validator 0.2.1.
| Outcome | Count |
|---|---|
| References checked | 25 |
| Resolved | 25 |
| Unresolved (possible confabulation) | 0 |
| Unverifiable | 0 |
| Quoted claims checked | 2 |
| Quoted claims found in source | 2 |
| Quoted claims not found in source | 0 |
| References weighed for topical relevance | 25 |
| On topic | 20 |
| Off topic | 0 |
All extracted references resolved successfully.