Seizures-scoliosis-macrocephaly syndrome is the autosomal recessive disorder caused by biallelic hypomorphic EXT2 variants. It was defined in 2015 in four siblings of one consanguineous family who had seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction, and specifically no exostoses. Later families led to the alternative name AREXT2, autosomal recessive EXT2-related syndrome, which is the more accurate label because the three eponymous features are not all constant: one family had microcephaly rather than macrocephaly, and another had no scoliosis. EXT2 is far better known for something else. Heterozygous loss-of-function EXT2 variants cause autosomal dominant hereditary multiple exostoses, and the overwhelming majority of the EXT2 literature is about that disease. The two disorders are separated by allele dose and by allele type, not by gene. The dominant disease is haploinsufficiency for a null allele. This disorder is biallelic missense: every reported family carries missense variants that reduce EXT2 abundance and activity without abolishing them. That the disorder is a partial-loss disease rather than a null disease is not incidental - Ext2-null mice arrest at gastrulation, so complete loss of heparan sulfate synthesis is not a survivable state. The boundary between the two disorders is nonetheless not a wall. The third reported AREXT2 family developed multiple exostoses in addition to the neurodevelopmental phenotype, which the authors read as a possible genotype-phenotype correlation within the recessive disorder rather than as a reclassification. Exostoses are therefore not a defining feature here, and their absence is not a diagnostic requirement.
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Conditions with similar clinical presentations that must be differentiated from Seizures-scoliosis-macrocephaly syndrome:
name: Seizures-scoliosis-macrocephaly syndrome
creation_date: "2026-08-29T00:00:00Z"
category: Mendelian
synonyms:
- SSMS
- SSM syndrome
- seizures, scoliosis, and macrocephaly syndrome
- AREXT2
- autosomal recessive EXT2-related syndrome
description: >-
Seizures-scoliosis-macrocephaly syndrome is the autosomal recessive disorder
caused by biallelic hypomorphic EXT2 variants. It was defined in 2015 in four
siblings of one consanguineous family who had seizures, intellectual
disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal
dysfunction, and specifically no exostoses. Later families led to the
alternative name AREXT2, autosomal recessive EXT2-related syndrome, which is
the more accurate label because the three eponymous features are not all
constant: one family had microcephaly rather than macrocephaly, and another
had no scoliosis.
EXT2 is far better known for something else. Heterozygous loss-of-function
EXT2 variants cause autosomal dominant hereditary multiple exostoses, and the
overwhelming majority of the EXT2 literature is about that disease. The two
disorders are separated by allele dose and by allele type, not by gene. The
dominant disease is haploinsufficiency for a null allele. This disorder is
biallelic missense: every reported family carries missense variants that
reduce EXT2 abundance and activity without abolishing them. That the disorder
is a partial-loss disease rather than a null disease is not incidental -
Ext2-null mice arrest at gastrulation, so complete loss of heparan sulfate
synthesis is not a survivable state.
The boundary between the two disorders is nonetheless not a wall. The third
reported AREXT2 family developed multiple exostoses in addition to the
neurodevelopmental phenotype, which the authors read as a possible
genotype-phenotype correlation within the recessive disorder rather than as a
reclassification. Exostoses are therefore not a defining feature here, and
their absence is not a diagnostic requirement.
disease_term:
preferred_term: seizures-scoliosis-macrocephaly syndrome
term:
id: MONDO:0014731
label: seizures-scoliosis-macrocephaly syndrome
parents:
- Congenital disorder of glycosylation
- Hereditary disease
references:
- reference: PMID:26246518
title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
- reference: PMID:30075207
title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
- reference: PMID:30288735
title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
- reference: PMID:30997052
title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
- reference: PMID:16236767
title: Mice deficient in Ext2 lack heparan sulfate and develop exostoses.
classifications:
icimd_category:
- classification_value: o_linked_protein_glycosylation
notes: >-
ICIMD category 18 (disorders of O-linked protein glycosylation) explicitly
subsumes glycosaminoglycan synthesis. EXT2 is the co-polymerase subunit
that elongates the heparan sulfate chain on the O-linked tetrasaccharide
linker, so this disorder belongs there. Deliberately not placed under
glycosaminoglycan degradation: that category is the mucopolysaccharidoses,
whose biochemical signature is the opposite of this disorder's. The
distinction is clinically load-bearing here, because the coarse facies
prompts mucopolysaccharidosis screening and the heparan sulfate result
comes back low rather than high.
evidence:
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
AREXT2 syndrome can be considered as a multiorgan Congenital Disorder of Glycosylation caused by a significant, but non-lethal, decrease in EXT2 expression, thereby affecting the synthesis of the heparan sulfate proteoglycans, which is relevant in many physiological processes.
explanation: >-
States both the classification as a congenital disorder of glycosylation
and the partial-loss mechanism.
inheritance:
- name: Autosomal recessive inheritance
inheritance_term:
preferred_term: Autosomal recessive inheritance
term:
id: HP:0000007
label: Autosomal recessive inheritance
description: >-
Biallelic EXT2 variants, homozygous in the consanguineous families and
compound heterozygous elsewhere. Heterozygous carriers in these families are
unaffected by the neurodevelopmental phenotype. This is the point at which
the disorder is most often confused with hereditary multiple exostoses,
which is dominant: a heterozygous EXT2 null allele causes that disease, not
this one.
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In short, we have unravelled the genetic basis of a new recessive disorder, seizures-scoliosis-macrocephaly syndrome.
explanation: >-
The paper's own conclusion that the disorder is recessive.
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
To confirm the autosomal-recessive mode of inheritance, all available family members were genotyped.
explanation: >-
Documents the segregation analysis behind that conclusion.
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Segregation analysis by Sanger sequencing confirmed homozygous by descent autosomal recessive transmission of this mutation.
explanation: >-
Independent confirmation of recessive transmission in a second family.
prevalence:
- population: Worldwide
measure_type: CASES_IN_LITERATURE
prevalence_class: ULTRA_RARE
notes: >-
No population estimate exists. The disorder was defined in one family of
four affected siblings in 2015, and a handful of further families have been
reported since.
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
The size of the defining family, standing in for a population estimate
that has never been made.
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Here, we report the third family affected by AREXT2 syndrome, harboring compound missense variants in EXT2, p.Asp227Asn, and p.Tyr608Cys.
explanation: >-
Establishes that only a small number of families had been reported by
2019.
pathophysiology:
- name: Biallelic Hypomorphic EXT2 Missense Variants
biological_scale: MOLECULAR
description: >-
Every family reported with this disorder carries EXT2 missense alleles on
both chromosomes. In the defining family, patient cells showed reduced EXT2
expression and function, and an in vitro reconstruction found each of the
two substitutions lowered EXT2 abundance on its own, with a larger drop when
both were present together. The allele class matters as much as the allele
dose: the disorder is a partial-loss state, and a complete-loss state is not
survivable, so a biallelic null genotype would not produce this phenotype.
genes:
- preferred_term: EXT2
term:
id: hgnc:3513
label: EXT2
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified two homozygous mutations p.Met87Arg and p.Arg95 Cys in exostosin 2, EXT2, a ubiquitously expressed gene that encodes a glycosyltransferase required for heparan sulfate synthesis.
explanation: >-
Identifies the defining genotype and names the enzymatic role of the gene.
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
We also performed an in vitro assay to determine which mutation has a larger effect on protein expression and observed reduced EXT2 expression in constructs expressing either one of the mutations but a greater reduction when both residues were mutated.
explanation: >-
The reconstruction experiment establishing that the alleles are
hypomorphic and additive rather than null.
downstream:
- target: Reduced Heparan Sulfate Chain Polymerisation
description: >-
Less EXT2 protein with less activity means less of the glycosyltransferase
that elongates the heparan sulfate chain.
causal_link_type: DIRECT
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In patient cells, we observed diminished EXT2 expression and function.
explanation: >-
Measures the loss of both protein and activity in cells from affected
individuals.
- name: Reduced Heparan Sulfate Chain Polymerisation
biological_scale: MOLECULAR
description: >-
EXT2 is a Golgi glycosyltransferase of the exostosin family that carries out
the chain-elongation step of heparan sulfate biosynthesis. Reduced but
non-zero enzyme output shortens or reduces the heparan sulfate chains
assembled on proteoglycan core proteins.
cellular_components:
- preferred_term: Golgi apparatus
term:
id: GO:0005794
label: Golgi apparatus
biological_processes:
- preferred_term: heparan sulfate proteoglycan biosynthetic process
modifier: DECREASED
term:
id: GO:0015012
label: heparan sulfate proteoglycan biosynthetic process
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: OTHER
snippet: >-
EXT2 belongs to the exostosin family of glycosyltransferases involved in the chain elongation step of heparan sulfate biosynthesis,2
explanation: >-
Names the specific enzymatic step this node describes.
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
AREXT2 syndrome can be considered as a multiorgan Congenital Disorder of Glycosylation caused by a significant, but non-lethal, decrease in EXT2 expression, thereby affecting the synthesis of the heparan sulfate proteoglycans, which is relevant in many physiological processes.
explanation: >-
States the significant-but-non-lethal reduction and its consequence for
proteoglycan synthesis.
downstream:
- target: Systemic Heparan Sulfate Proteoglycan Deficiency
description: >-
Reduced chain synthesis lowers heparan sulfate throughout the body, which
is measurable in blood and urine.
causal_link_type: DIRECT
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biochemical screening for mucopolysaccharidoses was performed in view of the coarse facial features and revealed low levels of heparan sulfate in dried blood spot (12 nmol/L), serum (4.46 ng/mL), and urine (0.5 mg/mmol creatinine) specimens (Reference ranges given in Table 1).
explanation: >-
Direct measurement of the systemic heparan sulfate deficit in a patient.
- name: Systemic Heparan Sulfate Proteoglycan Deficiency
biological_scale: ORGANISM
description: >-
Heparan sulfate proteoglycans are structural components of the extracellular
matrix and cell surface, and they act as co-receptors for a large number of
growth factors and morphogens. A body-wide reduction in them is therefore a
multi-system lesion rather than a tissue-specific one, and the disorder is
described accordingly as a multiorgan congenital disorder of glycosylation.
Which of the many heparan-sulfate-dependent pathways is responsible for any
given clinical feature is not established.
chemical_entities:
- preferred_term: heparan sulfate
modifier: DECREASED
term:
id: CHEBI:28815
label: heparan sulfate
biological_processes:
- preferred_term: extracellular matrix organization
modifier: ABNORMAL
term:
id: GO:0030198
label: extracellular matrix organization
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
Heparan sulfate proteoglycans are vital components of the extracellular matrix and are essential for cellular homeostasis.
explanation: >-
States the general role that makes a systemic deficit a multi-system
lesion.
downstream:
- target: Neurodevelopmental Impairment and Epilepsy
description: >-
The neurological features are the most consistent part of the phenotype,
but no specific heparan-sulfate-dependent pathway has been shown to
mediate them.
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
Many genes are involved in modulating heparan sulfate synthesis, and when these genes are mutated, they can give rise to early-onset developmental disorders affecting multiple body systems.
explanation: >-
Places the neurodevelopmental phenotype in the class of heparan-sulfate
synthesis disorders without specifying an intermediate pathway, which is
why this edge is indirect with unknown intermediates.
- target: Axial Skeletal Involvement
description: >-
The spine and bone-density features arise from the same systemic deficit,
again without an identified intermediate.
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
thereby affecting the synthesis of the heparan sulfate proteoglycans, which is relevant in many physiological processes
explanation: >-
The authors' own framing, which names many processes rather than an
identified route to the skeletal phenotype.
- target: Craniofacial Dysmorphism and Abnormal Head Size
description: >-
The facial pattern and the head-size abnormality arise from the same
systemic deficit. They are separated from the axial skeletal branch
because head size runs in opposite directions between families while the
facial pattern does not, so the two are not one finding.
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
thereby affecting the synthesis of the heparan sulfate proteoglycans, which is relevant in many physiological processes
explanation: >-
The same general framing; no route from the proteoglycan deficit to the
craniofacial pattern has been identified.
- name: Neurodevelopmental Impairment and Epilepsy
biological_scale: ORGANISM
description: >-
Seizures, intellectual disability, developmental delay and hypotonia are the
most consistent features across the reported families. Developmental
regression has been reported in more than one family. Seizure onset is
variable, reported between two and five years in the earlier families and at
ten years in a later patient.
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
The defining feature list, of which the neurological components are the
most consistent.
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
Independent confirmation of the neurological core, and the report in which
head size ran the other way.
- name: Axial Skeletal Involvement
biological_scale: ORGANISM
description: >-
Scoliosis in the defining family, and osteopenia in the second. Neither is
invariant: a later patient had neither scoliosis nor reduced bone density.
Exostoses were absent in the defining family and are not a required feature,
but they were present in the third family - which is the observation the
open gap below is about.
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records scoliosis among the defining features.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
However, skeletal issues such as scoliosis and decreased bone density and muscular issues such as hypotonia, observed with this disorder, were not observed in our patient.
explanation: >-
Records that the axial skeletal features are not invariant, which is why
this node is described as variable.
- name: Craniofacial Dysmorphism and Abnormal Head Size
biological_scale: ORGANISM
description: >-
Hypertelorism with coarse facial features including a long hypoplastic
philtrum, and an abnormal head circumference. Head size is the least
reliable feature in the whole disorder: the defining family had
macrocephaly, which is in the syndrome's name, and a later consanguineous
family had microcephaly instead. That is one of the reasons the alternative
name AREXT2 was proposed.
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records macrocephaly and hypertelorism among the defining features.
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
The family in which head size ran the opposite way, which is why this node
does not assert a direction.
phenotypes:
- name: Seizure
category: Neurologic
diagnostic: true
description: >-
Seizures, one of the three eponymous features and present in every reported
family. Onset has ranged from early childhood to the second decade.
phenotype_term:
preferred_term: Seizure
term:
id: HP:0001250
label: Seizure
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records seizures among the defining features.
- name: Intellectual disability
category: Neurologic
diagnostic: true
description: >-
Intellectual disability with developmental delay, present in every reported
family and in some the presenting problem.
phenotype_term:
preferred_term: Intellectual disability
term:
id: HP:0001249
label: Intellectual disability
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records intellectual disability among the defining features.
- name: Scoliosis
category: Skeletal
diagnostic: true
description: >-
Scoliosis, the second eponymous feature. It was present in the defining
family but has not been present in every subsequent patient.
phenotype_term:
preferred_term: Scoliosis
term:
id: HP:0002650
label: Scoliosis
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records scoliosis among the defining features of the index family.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
However, skeletal issues such as scoliosis and decreased bone density and muscular issues such as hypotonia, observed with this disorder, were not observed in our patient.
explanation: >-
A biallelic EXT2 patient without scoliosis, which is why the feature is not
treated as obligate.
- name: Macrocephaly
category: Craniofacial
diagnostic: true
description: >-
Macrocephaly, the third eponymous feature. Head size is the least reliable
of the three: a later consanguineous family had microcephaly instead, which
is one of the reasons the alternative name AREXT2 was proposed.
phenotype_term:
preferred_term: Macrocephaly
term:
id: HP:0000256
label: Macrocephaly
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records macrocephaly in the family that named the syndrome.
- name: Microcephaly
category: Craniofacial
description: >-
Microcephaly rather than macrocephaly in a second consanguineous family. The
two head-size findings are recorded as separate phenotypes rather than
averaged, because the direction is opposite and the syndrome name asserts
only one of them.
phenotype_term:
preferred_term: Microcephaly
term:
id: HP:0000252
label: Microcephaly
evidence:
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
Documents microcephaly in a family with the same recessive disorder.
- name: Hypotonia
category: Neurologic
description: >-
Hypotonia, present in the defining family and in the second family, absent in
at least one later patient.
phenotype_term:
preferred_term: Hypotonia
term:
id: HP:0001252
label: Hypotonia
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records hypotonia among the defining features.
- name: Hypertelorism
category: Craniofacial
description: >-
Hypertelorism, part of the dysmorphic facial pattern together with coarse
features and a long hypoplastic philtrum.
phenotype_term:
preferred_term: Hypertelorism
term:
id: HP:0000316
label: Hypertelorism
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records hypertelorism among the defining features.
- name: Coarse facial features
category: Craniofacial
description: >-
Coarse facies with a long hypoplastic philtrum. This is the feature that
most often sends the patient for mucopolysaccharidosis screening, which is
where the low heparan sulfate is found.
phenotype_term:
preferred_term: Coarse facial features
term:
id: HP:0000280
label: Coarse facial features
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biochemical screening for mucopolysaccharidoses was performed in view of the coarse facial features and revealed low levels of heparan sulfate in dried blood spot (12 nmol/L), serum (4.46 ng/mL), and urine (0.5 mg/mmol creatinine) specimens (Reference ranges given in Table 1).
explanation: >-
Records the coarse facies and the screening it prompted in the same
sentence.
- name: Abnormal renal physiology
category: Renal
description: >-
Renal dysfunction in the defining family. It has not been prominent in the
later reports.
phenotype_term:
preferred_term: Abnormal renal physiology
term:
id: HP:0012211
label: Abnormal renal physiology
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Records renal dysfunction among the defining features.
- name: Developmental regression
category: Neurologic
description: >-
Loss of previously acquired skills, reported as neurological regression in
one family and as regression of verbal and social skills with cognitive
decline in a later patient.
phenotype_term:
preferred_term: Developmental regression
term:
id: HP:0002376
label: Developmental regression
evidence:
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
Records neurological regression in a reported family.
- name: Osteopenia
category: Skeletal
description: >-
Reduced bone density, reported in one family and explicitly absent in
another patient.
phenotype_term:
preferred_term: Osteopenia
term:
id: HP:0000938
label: Osteopenia
evidence:
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
Records osteopenia in a reported family.
- name: Ventricular septal defect
category: Cardiovascular
frequency: 2/4
description: >-
A ventricular septal defect in two of the four siblings of the defining
family. It is the clearest single piece of evidence that this is a
multiorgan disorder rather than a neuroskeletal one.
phenotype_term:
preferred_term: Ventricular septal defect
term:
id: HP:0001629
label: Ventricular septal defect
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
There is clinical heterogeneity in this disorder, with two of four patients with biallelic EXT2 alterations noted to have a ventricular septal defect.1
explanation: >-
The 2/4 frequency recorded here is the figure quoted.
- name: Autism
category: Behavioral
description: >-
Autism, with aggressive behaviour toward self and others, rocking
stereotypies and sleep difficulty in the patient in whom it is described in
detail. Her fraternal twin, who carries the same two EXT2 variants, has
Asperger syndrome.
phenotype_term:
preferred_term: Autism
term:
id: HP:0000717
label: Autism
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a patient with developmental delay, autism, epilepsy, macrocephaly, facial dysmorphism, gastrointestinal, and behavioral issues due to EXT2 compound heterozygous likely pathogenic variants.
explanation: >-
Records autism among the presenting features of a biallelic EXT2 patient.
- name: Gastroesophageal reflux
category: Gastrointestinal
description: >-
Gastro-oesophageal reflux, part of the gastrointestinal involvement reported
in more than one family.
phenotype_term:
preferred_term: Gastroesophageal reflux
term:
id: HP:0002020
label: Gastroesophageal reflux
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Additional features include sensitive skin prone to acne and gastro‐intestinal problems such as constipation and gastroesophageal reflux.
explanation: >-
Names both gastrointestinal problems in the patient described in detail.
- name: Constipation
category: Gastrointestinal
description: >-
Constipation, reported alongside the reflux.
phenotype_term:
preferred_term: Constipation
term:
id: HP:0002019
label: Constipation
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Additional features include sensitive skin prone to acne and gastro‐intestinal problems such as constipation and gastroesophageal reflux.
explanation: >-
Names constipation among the gastrointestinal features.
- name: Feeding difficulties
category: Gastrointestinal
description: >-
Feeding difficulties, which in the second reported family were severe enough
to be part of the paper's title. They are the indication for the nutritional
support in the management section.
phenotype_term:
preferred_term: Feeding difficulties
term:
id: HP:0011968
label: Feeding difficulties
evidence:
- reference: PMID:30075207
reference_title: A novel EXT2 mutation in a consanguineous family with severe developmental delay, microcephaly, seizures, feeding difficulties, and osteopenia extends the phenotypic spectrum of autosomal recessive EXT2-related syndrome (AREXT2).
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report a consanguineous family where 2 boys presented with developmental delay, hypotonia, microcephaly, seizures, gastro-intestinal abnormalities, osteopenia, and neurological regression.
explanation: >-
The gastrointestinal abnormalities recorded in that family; the paper's
own title names feeding difficulties among the features it reports.
- name: Multiple exostoses
category: Skeletal
frequency: 1 of 4 reported families
description: >-
Osteochondromas were specifically absent in the family that defined the
syndrome, and their absence is in the title of that paper. They were then
reported in the third family, whose members carried biallelic EXT2 missense
variants, and were explicitly absent again in the fourth. That is a
within-disorder observation, not a reclassification to hereditary multiple
exostoses: exostoses in a heterozygous carrier belong to the dominant
disease, exostoses in a biallelic patient with the neurodevelopmental
phenotype belong here.
phenotype_term:
preferred_term: Multiple exostoses
term:
id: HP:0002762
label: Multiple exostoses
evidence:
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In addition, our patients developed multiple exostoses, which were not observed in the previously described families.
explanation: >-
Records exostoses in biallelic patients and, in the same sentence, that
they were absent in the earlier families.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Heterozygous pathogenic variants within EXT2 are causative of an autosomal dominant disorder known as multiple Exostoses type 2 in which patients develop multiple benign (ie, non‐malignant) bone tumors called osteochondromas, not present in our patient.
explanation: >-
A biallelic EXT2 patient without osteochondromas, which is why this
phenotype is recorded as variable rather than characteristic.
biochemical:
- name: Heparan sulfate
context: >-
Heparan sulfate measured in dried blood spot, serum and urine was low in a
patient with biallelic EXT2 variants. The direction is the diagnostically
useful part. Coarse facies prompts mucopolysaccharidosis screening, and a
mucopolysaccharidosis raises glycosaminoglycan levels because degradation
fails; here synthesis fails, so the same assay returns a low result. A low
heparan sulfate on an assay run to exclude a mucopolysaccharidosis should
not be read as a normal or uninformative result.
biomarker_term:
preferred_term: heparan sulfate
term:
id: CHEBI:28815
label: heparan sulfate
presence: Decreased
notes: >-
Reported patient values were 12 nmol/L in dried blood spot, 4.46 ng/mL in
serum and 0.5 mg/mmol creatinine in urine. The source states that reference
ranges are given in its Table 1; that table is not reproduced in the cached
text of the article, so no reference_ranges block is curated here rather
than one being reconstructed.
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biochemical screening for mucopolysaccharidoses was performed in view of the coarse facial features and revealed low levels of heparan sulfate in dried blood spot (12 nmol/L), serum (4.46 ng/mL), and urine (0.5 mg/mmol creatinine) specimens (Reference ranges given in Table 1).
explanation: >-
The measured values and the clinical circumstance in which they were
obtained.
genetic:
- name: EXT2
association: Causal biallelic variant
gene_term:
preferred_term: EXT2
term:
id: hgnc:3513
label: EXT2
notes: >-
EXT2 causes two different diseases by two different genetic mechanisms, and
the literature is dominated by the other one. Heterozygous loss-of-function
alleles cause autosomal dominant hereditary multiple exostoses; biallelic
missense alleles cause this recessive neurodevelopmental syndrome. A
citation about exostoses in a heterozygous carrier is evidence about the
dominant disease and not about this one.
Reported alleles here are p.Met87Arg with p.Arg95Cys (homozygous, defining
family), p.Ser4Leu (homozygous), p.Asp227Asn with p.Tyr608Cys (compound
heterozygous, the family that also had exostoses), and p.Val373Asp with
p.Thr672Met (compound heterozygous). All are missense. No biallelic null
genotype has been reported, and the Ext2-null mouse suggests why: complete
loss of the enzyme arrests development at gastrulation.
Expressivity is variable even within a genotype. In one family the proband's
fraternal twin sister carried the same two compound heterozygous EXT2
variants and was only mildly affected. That patient also carried a
monoallelic NDST1 variant of uncertain significance, in a second
heparan-sulfate gene, with no second NDST1 hit found; that variant is
present in the mildly affected sister too, so it does not by itself explain
the difference.
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Two compound heterozygous VUSs (one paternally inherited—c.1118T>A p.(Val373Asp) and a maternally inherited—c.2015C>T p.(Thr672Met)) were identified within the EXT2 gene.
explanation: >-
The fourth reported allele pair, both missense, which is the pattern this
note describes.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
EXT2 is implicated in the autosomal dominant disorder multiple Exostoses type 2 (MIM: 133701). Biallelic EXT2 mutations, however, have also been reported in four siblings born to consanguineous parents, manifesting scoliosis, seizures, and macrocephaly (MIM: 616682) without exostosis, following an autosomal recessive pattern of inheritance.1
explanation: >-
States the two-disease, two-mechanism structure of this gene explicitly,
with both OMIM numbers.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Notably, the patient's fraternal twin sister who is mildly affected as compared to the proband, with some learning delays and speech concerns, was also found to have the two compound heterozygous EXT2 variants while the patient's unaffected brother was found to carry only the heterozygous paternally inherited EXT2 variant (Figure 1).
explanation: >-
Documents the within-genotype variability and confirms that the
heterozygous sibling was unaffected.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
However, no second mutation was identified in this patient within this gene, and the deletion/duplication analysis for this gene yielded normal/negative results.
explanation: >-
Establishes that the NDST1 variant in that patient was monoallelic with no
second hit, so the case is not a second-gene recessive disorder.
environmental: []
treatments:
- name: Symptom-directed supportive management
description: >-
No disease-modifying treatment exists. Management is symptom directed:
developmental and educational support, orthopaedic follow-up for the
scoliosis where present, and nutritional support for the feeding
difficulties. Antiseizure medication is the obvious mainstay for the
epilepsy, but no reported patient's antiseizure regimen is described in the
cited literature, so no drug-level treatment is curated here rather than one
being assumed. A therapeutic direction has been proposed - enzyme
replacement or administration of heparan sulfate - but it is a suggestion in
a discussion section, not an intervention anyone has tried.
therapeutic_modality: BEHAVIORAL
treatment_term:
preferred_term: supportive care
term:
id: NCIT:C15747
label: Supportive Care
target_phenotypes:
- preferred_term: Scoliosis
term:
id: HP:0002650
label: Scoliosis
- preferred_term: Feeding difficulties
term:
id: HP:0011968
label: Feeding difficulties
- preferred_term: Intellectual disability
term:
id: HP:0001249
label: Intellectual disability
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.
explanation: >-
Establishes the burden that symptom-directed management addresses.
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The findings described above could allow for a better characterization of distinct features of such clinically variable and nonspecific phenotypes and may also provide potential strategies for therapeutic intervention by enzyme replacement therapy or administration of heparan sulfate to the patients.
explanation: >-
The only therapeutic direction proposed for this disorder, stated as a
possibility rather than as an intervention that exists.
diagnosis:
- name: Exome sequencing with segregation analysis
description: >-
The diagnosis has been made by exome sequencing in every reported family,
with segregation confirming biallelic inheritance. Autozygosity mapping was
used alongside it in the consanguineous families. Segregation matters more
than usual here: a heterozygous EXT2 variant in a parent is a hereditary
multiple exostoses genotype, not a carrier finding to be dismissed, and the
two interpretations diverge completely.
results: Biallelic EXT2 missense variants segregating with the phenotype.
diagnosis_term:
preferred_term: genetic testing
term:
id: NCIT:C15709
label: Genetic Testing
evidence:
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Our application of autozygosity mapping and whole-exome sequencing allowed us to identify mutations in the patients.
explanation: >-
Documents the diagnostic route used in the defining family.
- name: Glycosaminoglycan measurement
description: >-
Heparan sulfate measurement in dried blood spot, serum or urine. In a
patient investigated for coarse facies the result was low rather than high,
which is the opposite of a mucopolysaccharidosis and is consistent with a
synthesis defect. The test is usually ordered to exclude a
mucopolysaccharidosis, so the low result is easily filed as negative.
results: >-
Low heparan sulfate in dried blood spot, serum and urine.
diagnosis_term:
preferred_term: laboratory procedure
term:
id: NCIT:C25294
label: Laboratory Procedure
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biochemical screening for mucopolysaccharidoses was performed in view of the coarse facial features and revealed low levels of heparan sulfate in dried blood spot (12 nmol/L), serum (4.46 ng/mL), and urine (0.5 mg/mmol creatinine) specimens (Reference ranges given in Table 1).
explanation: >-
The measurement, the indication, and the direction of the result.
differential_diagnoses:
- name: Hereditary multiple exostoses type 2
description: >-
The other EXT2 disease and the one the gene is known for. It is dominant and
caused by a heterozygous loss-of-function allele; the phenotype is
osteochondromas near the ends of long bones, without the neurodevelopmental
features. This is the confusion to guard against when reading the EXT2
literature: nearly all of it is about this disorder, not the recessive one.
disease_term:
preferred_term: exostoses, multiple, type 2
term:
id: MONDO:0007586
label: exostoses, multiple, type 2
distinguishing_features:
- Autosomal dominant with a heterozygous loss-of-function allele
- Osteochondromas as the defining and usually only feature
- No seizures, intellectual disability or hypotonia
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Heterozygous pathogenic variants within EXT2 are causative of an autosomal dominant disorder known as multiple Exostoses type 2 in which patients develop multiple benign (ie, non‐malignant) bone tumors called osteochondromas, not present in our patient.
explanation: >-
States the dominant disease, its allele dose, and its phenotype, and
contrasts it with the recessive patient in the same sentence.
- name: Autosomal recessive intellectual disability type 46
description: >-
The NDST1-related disorder. NDST1 acts on the same pathway, modifying the
glycosaminoglycan by N-deacetylation and N-sulfation, and biallelic NDST1
variants cause a recessive intellectual disability. It matters here because
a monoallelic NDST1 variant of uncertain significance was found alongside
the biallelic EXT2 variants in one reported patient, and because deep
research on this disorder returns NDST1 material.
disease_term:
preferred_term: intellectual disability, autosomal recessive 46
term:
id: MONDO:0014499
label: intellectual disability, autosomal recessive 46
distinguishing_features:
- Biallelic NDST1 rather than EXT2 variants
- Sulfation and modification defect rather than chain elongation defect
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biallelic pathogenic variants within NDST1 are implicated in autosomal recessive intellectual disability type 46 (MIM: 616116).
explanation: >-
Names the NDST1 disorder and its biallelic requirement.
- name: Mucopolysaccharidosis
description: >-
The differential that gets tested for, because of the coarse facies and
developmental regression. It is worth separating carefully: the
mucopolysaccharidoses are degradation defects and raise glycosaminoglycan
levels, whereas this disorder is a synthesis defect and lowers them, so the
same assay distinguishes them by direction.
disease_term:
preferred_term: mucopolysaccharidosis
term:
id: MONDO:0019249
label: mucopolysaccharidosis
distinguishing_features:
- Glycosaminoglycan levels raised rather than lowered
- Lysosomal degradation defect rather than a Golgi synthesis defect
evidence:
- reference: PMID:30997052
reference_title: Developmental delay, coarse facial features, and epilepsy in a patient with EXT2 gene variants.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Biochemical screening for mucopolysaccharidoses was performed in view of the coarse facial features and revealed low levels of heparan sulfate in dried blood spot (12 nmol/L), serum (4.46 ng/mL), and urine (0.5 mg/mmol creatinine) specimens (Reference ranges given in Table 1).
explanation: >-
Documents that this differential was actively tested and how the result
distinguished the two.
animal_models:
- name: Ext2-null mouse
species: Mouse
genotype: Ext2 homozygous null (gene targeting)
publication: PMID:16236767
description: >-
The Ext2 knockout mouse, generated to study hereditary multiple exostoses.
Homozygous embryos are growth arrested by embryonic day 6.0 and never
gastrulate. It models the dominant disease through its heterozygotes rather
than this disorder through its homozygotes. Other Ext2 mice exist but are
heterozygous, alone or compound heterozygous with an Ext1 allele, and were
built to study osteochondroma formation or heparan-sulfate physiology.
modeled_mechanisms:
- target: Biallelic Hypomorphic EXT2 Missense Variants
relationship: FAILS_TO_RECAPITULATE
fidelity: LOW
description: >-
A biallelic null is not the human genotype. Human patients carry biallelic
missense alleles that lower EXT2 abundance without abolishing it; the
mouse abolishes it, and the embryo dies before any organ this disorder
affects has formed.
limitations: >-
Homozygous mutant embryos arrest at embryonic day 6.0 and fail to
gastrulate, so no neurological, skeletal, craniofacial or renal phenotype
can be assessed. Nothing in this model corresponds to the human
neurodevelopmental syndrome. The heterozygous arm of the same mouse - one
third of which develop ectopic bone growths, and all of which show
abnormal cartilage differentiation - models hereditary multiple exostoses,
the dominant EXT2 disease, and is not evidence about this disorder. The
literature searched for this entry contains no mouse carrying two
hypomorphic Ext2 alleles of the kind patients have, so the human genotype
has not been modelled in an animal; the nearest partial-loss mice are
Ext1/Ext2 compound heterozygotes made to study sodium and water
homeostasis, which is neither the genotype nor the question.
evidence:
- reference: PMID:16236767
reference_title: Mice deficient in Ext2 lack heparan sulfate and develop exostoses.
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Homozygous mutant embryos developed normally until embryonic day 6.0, when they became growth arrested and failed to gastrulate, pointing to the early essential role for heparan sulfate in developing embryos.
explanation: >-
The gastrulation arrest that makes the biallelic null mouse unusable as
a model of this disorder, and that explains why every human allele
reported is hypomorphic.
- reference: PMID:16236767
reference_title: Mice deficient in Ext2 lack heparan sulfate and develop exostoses.
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Heterozygotes had a normal lifespan and were fertile; however, analysis of their skeletons showed that about one-third of the animals formed one or more ectopic bone growths (exostoses).
explanation: >-
The heterozygous phenotype, recorded here to mark it explicitly as the
dominant disease's model rather than this disorder's.
discussions:
- discussion_id: mismatch_ssms_no_hypomorphic_model
kind: HUMAN_MODEL_MISMATCH
status: OPEN
prompt: >-
Can any existing model system be used to study this disorder, given that the
only Ext2 mouse is a null that dies at gastrulation and that the human
disease is caused by partial loss?
attaches_to:
- pathophysiology#Biallelic Hypomorphic EXT2 Missense Variants
- pathophysiology#Neurodevelopmental Impairment and Epilepsy
rationale: >-
The one biallelic mouse answers a different question. Homozygous Ext2-null
embryos arrest at embryonic day 6.0, before the brain, skeleton or kidney
exists, so the model cannot show anything about a disorder defined by
seizures, intellectual disability, scoliosis and renal dysfunction. The
heterozygous arm of the same mouse is informative, but about hereditary
multiple exostoses, and the other Ext2 mice in the literature are
heterozygous too. The human alleles are missense and hypomorphic, and the
one experiment done on them - reconstruction in vitro, showing reduced EXT2
abundance for each substitution and a greater reduction for both - measures
protein level, not a physiological consequence. Beyond that assay and the
patient cells of the discovery paper, no cellular model has been reported
either. The mismatch is not that the mouse gives a misleading answer; it is
that the mouse cannot be asked the question, and nothing else has filled the
gap.
evidence:
- reference: PMID:16236767
reference_title: Mice deficient in Ext2 lack heparan sulfate and develop exostoses.
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Homozygous mutant embryos developed normally until embryonic day 6.0, when they became growth arrested and failed to gastrulate, pointing to the early essential role for heparan sulfate in developing embryos.
explanation: >-
The developmental arrest that puts every organ of interest out of reach in
the only available biallelic model.
- reference: PMID:26246518
reference_title: "Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
We also performed an in vitro assay to determine which mutation has a larger effect on protein expression and observed reduced EXT2 expression in constructs expressing either one of the mutations but a greater reduction when both residues were mutated.
explanation: >-
The extent of functional work done on the human alleles, which is a
protein-abundance measurement rather than a physiological one.
- discussion_id: gap_ssms_dominant_recessive_boundary
kind: KNOWLEDGE_GAP
status: OPEN
prompt: >-
What decides whether an EXT2 genotype produces hereditary multiple
exostoses, this recessive neurodevelopmental syndrome, or both at once?
attaches_to:
- genetic#EXT2
- phenotypes#Multiple exostoses
- pathophysiology#Axial Skeletal Involvement
rationale: >-
The textbook answer is allele dose: one null allele gives exostoses, two
hypomorphic alleles give the neurodevelopmental syndrome. The third reported
recessive family breaks it. Those patients had biallelic missense variants
and the neurodevelopmental phenotype, and they also developed multiple
exostoses, which the earlier families did not. Their authors read this as a
possible genotype-phenotype correlation within the recessive disorder,
implying that some hypomorphic combinations reach the threshold for
exostoses and others do not - but with three families there is no way to
test that. A related unknown sits underneath it: which of the many
heparan-sulfate-dependent pathways carries each clinical feature is not
known for any of them, so there is no mechanistic account that would predict
where the threshold should be. The practical consequence is that absence of
exostoses cannot be used to rule this disorder in, and their presence cannot
be used to rule it out.
evidence:
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Our finding expands the clinical and molecular spectrum of the AREXT2 syndrome and suggests a possible genotype/phenotype correlation in the development of the exostoses.
explanation: >-
The authors' own hypothesis about the boundary, stated as a suggestion.
- reference: PMID:30288735
reference_title: "Novel exostosin-2 missense variants in a family with autosomal recessive exostosin-2-related syndrome: further evidences on the phenotype."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In addition, our patients developed multiple exostoses, which were not observed in the previously described families.
explanation: >-
The observation that breaks the simple dose model.
clinical_trials: []
datasets: []
notes: >-
Two-arm gene, and the reason this entry exists. EXT2 is overwhelmingly known
for autosomal dominant hereditary multiple exostoses, caused by
haploinsufficiency for a null allele. This entry is the biallelic
hypomorphic-missense disorder. Every citation was checked for which arm it
describes. The one paper here that is substantially about the dominant arm,
PMID:16236767, is used deliberately and only in two places: its homozygous-null
gastrulation arrest, which is evidence about why human alleles must be
hypomorphic and why no model of this disorder exists; and its heterozygous
exostoses result, quoted inside the model's `limitations` for the express
purpose of marking it as the other disease's evidence.
Not curated from the EXT2 literature, deliberately: the zebrafish dackel
(ext2) mutants, which have cartilage defects resembling hereditary multiple
exostoses and were studied to explain that disease's aetiology; the
osteochondroma-to-chondrosarcoma progression literature; and the EXT-carrier
metabolic studies. All are about the dominant arm.
Second-locus caution. One reported patient carried a monoallelic NDST1 variant
of uncertain significance alongside the biallelic EXT2 variants. NDST1 acts on
the same pathway and its biallelic loss causes a different recessive disorder.
No second NDST1 hit was found, and the same NDST1 variant is present in that
patient's mildly affected twin sister, so it is recorded in genetic.notes and
in a differential rather than being folded into this disorder's mechanism.
Head size is recorded as two separate phenotypes, Macrocephaly and
Microcephaly, rather than as one averaged claim, because different families
went in opposite directions and the syndrome name asserts only one of them.
Evidence-source convention: patient clinical, genetic and biochemical findings
HUMAN_CLINICAL; the variant reconstruction assay IN_VITRO; mouse
MODEL_ORGANISM; background enzymology quoted from an introduction OTHER.
Question: You are an expert researcher providing comprehensive, well-cited information.
Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies
Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.
Please provide a comprehensive research report on seizures-scoliosis-macrocephaly syndrome (SSMS, biallelic EXT2 - the autosomal recessive phenotype, NOT dominant hereditary multiple exostoses) covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.
For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.
Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed
Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases
Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases
Search first: CTD, PubMed, PheGenI, GxE databases
Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC
For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities
For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype
Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene
Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser
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Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold
Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA
Search first: ImmPort, Immunome Database, IEDB, Gene Ontology
Search first: PubMed, Gene Ontology, Reactome
Search first: BRENDA, UniProt, KEGG, OMIM, PubMed
Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types
Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT
Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB
Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas
Search first: OMIM, Orphanet, HPO, PubMed
Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM
Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries
Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen
For each treatment, suggest NCIT (NCI Thesaurus) clinical-intervention terms where applicable.
Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database
Search first: CDC, WHO, behavioral intervention databases, Cochrane Library
Search first: NSGC resources, ACMG guidelines, GeneReviews
Search first: Clinical guidelines, FDA approvals, PubMed
Search first: NCBI Taxonomy
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Search first: NCBI Gene
Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, NCIT, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease
This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (NCIT terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details
SSMS is an ultra-rare, autosomal-recessive developmental disorder caused by biallelic hypomorphic EXT2 variants. It is characterized primarily by developmental/intellectual disability, epilepsy, abnormal head size, hypotonia, spinal deformity and dysmorphic features, with variable gastrointestinal, cardiac, renal, behavioral and growth abnormalities. “Autosomal-recessive EXT2-related syndrome” (AREXT2) is the broader and increasingly appropriate label because later families do not invariably have all three eponymous findings: microcephaly rather than macrocephaly and absence of scoliosis have both been reported. The defining distinction is from autosomal-dominant hereditary multiple exostoses type 2: osteochondromas are not a defining feature of AREXT2 and were specifically absent in the original family. (jaeken2020congenitaldisordersof pages 4-5, mizumoto2021congenitaldisordersof pages 12-13, gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 2-4)
The evidence base remains exceptionally small. The foundational report was Farhan et al., published August 2015 (J Med Genet 52:666–675; PMID 26246518; DOI/URL: https://doi.org/10.1136/jmedgenet-2015-103279). Subsequent informative reports include El-Bazzal et al. (2018; DOI: https://doi.org/10.1016/j.ejmg.2018.07.025), Gentile et al. (online 2018/issue 2019; DOI: https://doi.org/10.1111/cge.13458), Gupta et al. (accepted January 8, 2019; DOI: https://doi.org/10.1002/ccr3.2010), and Sabir et al. (2022; DOI: https://doi.org/10.1097/MCD.0000000000000406). No disease-specific clinical study published in 2023–2024 was identified in the searches performed for this report. The main relevant 2024 advance is structural work on the interacting HS-modification enzyme NDST1, not new SSMS natural-history or treatment evidence. (OpenTargets Search: seizures-scoliosis-macrocephaly syndrome-EXT2, gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 5-6)
Key primary-source abstract quotation (Gupta et al., 2019): “We report a patient with developmental delay, autism, epilepsy, macrocephaly, facial dysmorphism, gastrointestinal, and behavioral issues due to EXT2 compound heterozygous likely pathogenic variants.” (gupta2019developmentaldelaycoarse pages 1-2)
The available human cohorts and their evidentiary limitations are summarized below.
| Publication / cohort | Family / patients | EXT2 genotype | Key phenotype and onset | Major outcomes | Evidence caveats |
|---|---|---|---|---|---|
| Farhan et al., 2015; J Med Genet 52:666-675; PMID: 26246518; DOI: 10.1136/jmedgenet-2015-103279 | 1 Old Order Mennonite family; 4 affected siblings (3M, 1F), assessed ages reported in later summaries/thesis as ~10-19 years | Complex homozygous missense variants: p.Met87Arg and p.Arg95Cys; family segregated with recessive disease | Core SSMS phenotype: developmental delay/intellectual disability, hypotonia, seizures with onset ~2.5-5 years, macrocephaly, scoliosis/kyphosis, hypertelorism; additional renal/GI/cardiac findings in some; exostoses specifically absent (mizumoto2018defectsinbiosynthesis pages 13-14, mizumoto2021congenitaldisordersof pages 12-13, farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryina pages 110-114) | Severe variable course: one patient died after status epilepticus at 17; one had prolonged seizure/status with hemiplegic stroke and wheelchair dependence; one had renal failure; patient fibroblasts showed reduced EXT2 protein/transcript and abolished NDST1 protein (farhan2016genediscoveryina pages 123-130, farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryina pages 110-114) | Most granular clinical detail available from thesis/review excerpts rather than full 2015 paper text in retrieved context; some ages differ across excerpt types because thesis followed patients longer (farhan2016genediscoveryina pages 123-130, farhan2016genediscoveryina pages 131-135, farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryina pages 110-114) |
| El-Bazzal et al., 2018; Eur J Med Genet, cited in Gupta 2019; DOI reported as 10.1016/j.ejmg.2018.07.025 | 1 family; 2 brothers | Homozygous p.Ser4Leu (c.11C>T) | Severe developmental delay, seizures, feeding difficulties, microcephaly rather than macrocephaly/normal head size in later AREXT2 spectrum summaries; facial flattening/coarse features summarized in comparison table; exostoses absent in AREXT2 framing (gupta2019developmentaldelaycoarse pages 4-5, gupta2019developmentaldelaycoarse pages 5-6) | Expanded phenotype toward more severe neurodevelopmental disease; both boys alive at reported ages 6 and 8 in comparison table (gupta2019developmentaldelaycoarse pages 4-5) | Direct article text was not retrieved; details come from Gupta 2019 comparison table/citation trail, so onset specifics and full systems review should be treated as second-hand summary (gupta2019developmentaldelaycoarse pages 4-5, gupta2019developmentaldelaycoarse pages 5-6) |
| Gentile et al., 2019; Clin Genet 95:165-171; DOI: 10.1111/cge.13458 | 1 family; 2 affected relatives/patients (F15, M21 in Gupta comparison table) | Compound heterozygous missense variants p.Asp227Asn and p.Tyr608Cys | Intellectual disability/developmental delay, seizures in one or both, macrocephaly reported as “high” head size in comparison table, facial dysmorphism; phenotype milder/variable compared with founder family; exostoses not reported as present in AREXT2 summaries (gupta2019developmentaldelaycoarse pages 4-5, gupta2019developmentaldelaycoarse pages 5-6) | Survived into adolescence/adulthood (15 and 21 years in comparison table), supporting variable severity and nonlethal course in some genotypes (gupta2019developmentaldelaycoarse pages 4-5) | Full paper not retrieved; patient-level details limited to secondary table excerpt and review mentions, so seizure onset/treatment/systemic findings cannot be stated with confidence here (gupta2019developmentaldelaycoarse pages 4-5, gupta2019developmentaldelaycoarse pages 5-6) |
| Gupta et al., 2019; Clin Case Rep 7:632-637; DOI: 10.1002/ccr3.2010 | 1 nonconsanguineous family; proband F14 plus mildly affected fraternal twin sister; unaffected brother carried only one paternal EXT2 variant | Compound heterozygous EXT2 p.Val373Asp (c.1118T>A) and p.Thr672Met (c.2015C>T); both sisters also carried heterozygous NDST1 p.Arg454Cys VUS | Proband: developmental delay during first 2 years, regression at 27 months, autism, macrocephaly, hypertelorism, long philtrum, strabismus, GI issues, behavioral issues, seizures at ~10 years; EEG abnormal with occipital/midline spike-wave; brain MRI normal; no scoliosis/hypotonia/decreased bone density documented. Twin: milder Asperger syndrome/motor-cognitive-speech delay (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 2-4, gupta2019developmentaldelaycoarse pages 5-6) | Demonstrates intrafamilial variability and possible modifier effect; low heparan sulfate measured in dried blood spot, serum, and urine in proband and unaffected parents; variants submitted to ClinVar; no exostoses reported (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 2-4, gupta2019developmentaldelaycoarse pages 5-6) | EXT2 variants were classified as likely pathogenic/clinical-interest in context of emerging AREXT2 literature, but authors noted NDST1 contribution remained uncertain; unaffected parents also had low heparan sulfate, limiting biomarker specificity (gupta2019developmentaldelaycoarse pages 2-4, gupta2019developmentaldelaycoarse pages 5-6) |
| Sabir et al., 2022; Clin Dysmorphol 31:84-90; DOI: 10.1097/MCD.0000000000000406 | Reported as a further case / extending phenotype | Not extractable from retrieved context | Not extractable from retrieved context | Not extractable from retrieved context | Mentioned only as an unobtainable paper in search results; because no supporting details were retrieved, it is intentionally not summarized beyond bibliographic mention to avoid inventing facts (gupta2019developmentaldelaycoarse pages 4-5) |
Table: This table summarizes the main published human cohorts for autosomal recessive EXT2-related syndrome/SSMS, emphasizing genotype, core phenotype, outcomes, and limits of the available evidence. It is useful for quickly separating well-supported patient data from second-hand summaries and unretrieved reports.
Knowledge is derived mainly from individual patients and multiplex families published as case reports/series, then aggregated by OMIM/MONDO and reviews—not from EHR-scale cohorts, registries or population surveillance. The original report involved four siblings; subsequent publications added only small families or single cases. Accordingly, apparent frequencies are descriptive proportions among published cases and are highly vulnerable to ascertainment and publication bias. (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 4-5, farhan2016genediscoveryin pages 110-114)
The primary cause is germline biallelic EXT2 variation, usually missense alleles retaining partial function. In the original consanguineous Old Order Mennonite family, all affected siblings were homozygous for both NM_000401.3:c.260T>G, p.(Met87Arg) and c.283C>T, p.(Arg95Cys) on the same disease haplotype (“complex homozygosity”). Patient fibroblasts had significantly reduced EXT2 protein (P<0.001), modestly reduced transcript (P<0.05), and loss of detectable NDST1 protein despite preserved NDST1 transcript. Expression constructs showed that each variant reduced EXT2 abundance and that the combined changes had the greatest effect; p.Arg95Cys had the larger individual effect. This supports synergistic partial loss of protein stability/function rather than gain of function. (farhan2016genediscoveryina pages 123-130)
Other reported genotypes include homozygous c.11C>T, p.(Ser4Leu); compound-heterozygous c.679G>A, p.(Asp227Asn) plus c.1823A>G, p.(Tyr608Cys); and compound-heterozygous c.1118T>A, p.(Val373Asp) plus c.2015C>T, p.(Thr672Met). The last pair was considered likely pathogenic by the authors under ACMG/AMP reasoning, although originally returned as VUSs in a gene with an emerging disease relationship. (gupta2019developmentaldelaycoarse pages 2-4, gupta2019developmentaldelaycoarse pages 4-5, gupta2019developmentaldelaycoarse pages 5-6)
No toxin, diet, lifestyle, occupational exposure, radiation or infectious agent is known to cause SSMS. No protective lifestyle factor or reproducible gene–environment interaction has been identified. Fever, sleep loss or medication nonadherence may trigger seizures in any epilepsy, but there is no SSMS-specific evidence. Family history and consanguinity alter the probability of inheriting the genotype, not its molecular action.
The original four siblings all had developmental delay/intellectual disability, seizures beginning at approximately 2.5–5 years, hypotonia, macrocephaly and scoliosis/kyphosis. They had minimal expressive speech with relatively preserved comprehension. Additional findings included coarse/dysmorphic facies, hypertelorism, long hypoplastic philtrum, cryptorchidism in males, ventricular septal defects, gastrointestinal dysmotility/reflux, and renal abnormalities. No exostoses were found. (farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryina pages 110-114, farhan2016genediscoveryin pages 105-110)
Later cases broadened the spectrum to microcephaly, feeding difficulty, osteopenia, absent scoliosis, autism, behavioral dysregulation, sleep disturbance, normal brain MRI and later-onset focal epilepsy. The Gupta proband sat at 7–8 months, walked at 14 months, had language delay by age 2 and regression at 27 months; seizures began around age 10. EEG showed occipital-midline spike-and-wave discharges consistent with focal seizures, whereas MRI was normal. Her fraternal twin carrying the same EXT2 variants had much milder motor, speech and cognitive difficulties/Asperger syndrome. (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 2-4, gupta2019developmentaldelaycoarse pages 4-5)
| Clinical domain | Character/course | Published frequency signal | Suggested HPO term |
|---|---|---|---|
| Global developmental delay | Infancy/early childhood; variable, sometimes regression | Very common across reported cases | HP:0001263 Global developmental delay |
| Intellectual disability | Mild to severe; lifelong | Very common | HP:0001249 Intellectual disability |
| Speech/language delay | Often marked; minimal expressive speech in founder cohort | Common | HP:0000750 Delayed speech and language development |
| Seizures/epilepsy | Usually childhood; generalized tonic–clonic, focal or status epilepticus | Very common; onset 2.5–5 years in all four founder cases, age 10 in Gupta proband | HP:0001250 Seizure; HP:0002069 Generalized tonic-clonic seizure; HP:0002133 Status epilepticus |
| Hypotonia | Congenital or early childhood; variable | All four founder cases; absent in Gupta proband | HP:0001252 Muscular hypotonia |
| Macrocephaly | Congenital/postnatal; nonprogressive status unclear | All four founder cases and Gupta proband; not universal | HP:0000256 Macrocephaly |
| Microcephaly | Alternative head-size phenotype | Reported in the severe El-Bazzal family | HP:0000252 Microcephaly |
| Scoliosis/kyphoscoliosis | Childhood, potentially progressive and function-limiting | All four founder cases; absent in Gupta proband | HP:0002650 Scoliosis; HP:0002751 Kyphoscoliosis |
| Facial dysmorphism | Coarse facies, hypertelorism, tall/prominent forehead, long philtrum, broad/bulbous nose | Common but variable | HP:0000316 Hypertelorism; HP:0000343 Long philtrum; HP:0000280 Coarse facial features |
| Autism/behavioral abnormalities | Autism, stereotypies, aggression/self-injury, sleep difficulty | Variable | HP:0000717 Autism; HP:0000718 Aggressive behavior; HP:0002360 Sleep disturbance |
| GI dysfunction | GERD, constipation/diarrhea, dysmotility; volvulus/malrotation in founder family | Variable | HP:0002020 Gastroesophageal reflux; HP:0002019 Constipation; HP:0002566 Intestinal malrotation |
| Renal disease | Hematuria/proteinuria; renal failure in one founder sibling | Uncommon but clinically important | HP:0000093 Proteinuria; HP:0000790 Hematuria; HP:0000083 Renal insufficiency |
| Cardiac defect | Ventricular septal defect in members of founder family | Variable | HP:0001629 Ventricular septal defect |
| Cryptorchidism | Bilateral in affected males in founder family | Reported in founder males | HP:0000028 Cryptorchidism |
| Strabismus | Surgically treated in Gupta proband | Variable | HP:0000486 Strabismus |
| Osteopenia | Described in later severe cases | Variable | HP:0000938 Osteopenia |
| Osteochondroma/exostoses | Absent, an important discriminator | Absent in original family and reported AREXT2 cases | HP:0002859 Multiple exostoses—use as an excluded/negative phenotype |
These are suggested knowledge-base mappings, not a formally curated SSMS HPO disease model.
No EQ-5D, SF-36, PROMIS or disease-specific quality-of-life study exists. Nevertheless, case data show major functional burden: severe communication limitation, lifelong supervision, epilepsy risk, orthopedic restriction and multisystem surveillance. One sibling became wheelchair-dependent after prolonged status epilepticus with hemiplegic stroke; another died after status epilepticus at 17. Scoliosis/kyphosis, gastrointestinal disease and renal failure add substantial morbidity. Conversely, the mildly affected Gupta twin illustrates that biallelic disease can permit much greater independence. (farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryina pages 110-114)
Suggested GO annotations include GO:0000139 Golgi membrane, GO:0005794 Golgi apparatus, GO:0005783 endoplasmic reticulum, GO:0015012 heparan sulfate proteoglycan biosynthetic process, GO:0008375 acetylglucosaminyltransferase activity, and GO:0008194 UDP-glycosyltransferase activity.
| Variant(s) | State | Evidence/classification | Functional or population evidence |
|---|---|---|---|
| c.260T>G p.Met87Arg + c.283C>T p.Arg95Cys | Both homozygous on a complex allele/haplotype | Disease-associated in original family; functionally supported | Reduced EXT2 protein/transcript in fibroblasts; each construct reduced expression, combined strongest; historical ESP MAF 0.054% and 0.015% |
| c.11C>T p.Ser4Leu | Homozygous | Reported in severe AREXT2 family | Direct functional evidence not retrieved |
| c.679G>A p.Asp227Asn + c.1823A>G p.Tyr608Cys | Compound heterozygous | Reported AREXT2 missense pair | Direct assay not retrieved |
| c.1118T>A p.Val373Asp + c.2015C>T p.Thr672Met | Compound heterozygous | Authors’ final assessment: likely pathogenic collectively | gnomAD 3/246,244 and 20/276,874, respectively; no homozygotes; damaging in-silico predictions; segregation with two affected sisters |
All reported SSMS alleles are germline. No somatic SSMS mechanism is known. No recurrent pathogenic structural variant, aneuploidy, translocation or inversion defines SSMS. Heterozygous deletion of 11p11.2 including EXT2 instead produces Potocki–Shaffer syndrome, a distinct contiguous-gene condition. (mizumoto2021congenitaldisordersof pages 12-13, gupta2019developmentaldelaycoarse pages 2-4)
The original alleles are best interpreted as hypomorphic loss-of-function/protein-destabilizing missense changes. Complete EXT2 loss is probably incompatible with normal embryogenesis, consistent with animal null lethality. Variant classification should be performed allele-by-allele using current ClinVar submissions, segregation, phase, rarity, phenotype and functional evidence. The Gupta variants have ClinVar submissions SCV000782709 and SCV000782708; the NDST1 VUS was submitted as SCV000782711. (gupta2019developmentaldelaycoarse pages 2-4)
No SSMS-specific DNA-methylation signature, histone alteration, imprinting mechanism or epigenetic biomarker has been demonstrated. No transcriptomic, proteomic, metabolomic, lipidomic, single-cell, spatial or multi-omic patient cohort exists.
SSMS is a constitutional Mendelian disorder. No environmental exposure, lifestyle practice or pathogen is necessary or sufficient to produce it, and none has been shown to alter penetrance. Routine healthy diet, exercise within orthopedic/neurologic limits, avoidance of smoking/alcohol and vaccination are general-health measures, not disease-specific interventions. Environmental seizure-safety measures can reduce injury but do not prevent the molecular disease.
A 2024 cryo-EM study resolved human NDST1’s N-terminal, deacetylase and sulfotransferase architecture and proposed that substrate anchoring at the sulfotransferase domain initiates a catalytic cycle despite spatial separation of catalytic domains (Mycroft-West et al., published February 2024; DOI: https://doi.org/10.1038/s41467-024-45419-4). This refines understanding of the pathway downstream of EXT2 but did not study SSMS variants or patients.
2024 abstract wording: “Mature HS polysaccharides contain complex, non-templated patterns of sulfation and epimerization, which mediate interactions with diverse protein partners.” This helps explain why simply supplying nonspecific heparan sulfate may not reproduce the missing developmental signal.
No primary immune, inflammatory, oxidative-stress, mitochondrial, autophagic or apoptotic mechanism has been demonstrated in SSMS. Immune abnormalities reported in other HS-biosynthesis disorders should not be transferred to EXT2 disease without evidence.
No consistent lateralization is known. The hemiplegia in one patient followed a prolonged seizure/stroke and should be treated as an acquired complication, not a primary asymmetric malformation. Patient MRI can be normal, as in the Gupta proband; therefore normal structural imaging does not exclude the diagnosis. (gupta2019developmentaldelaycoarse pages 1-2)
SSMS is genetically present from conception, but clinical recognition is generally infantile or early-childhood. Hypotonia and motor delay may be evident in infancy; language delay appears in the first two years; seizures typically begin in early childhood but can emerge as late as approximately age 10. Spinal deformity may become more apparent with growth. (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 4-5, farhan2016genediscoveryin pages 105-110)
The course is chronic and lifelong, with markedly variable severity. Some developmental deficits are stable; regression, declining mobility and progressive orthopedic or renal morbidity can occur. Epilepsy is episodic and may include life-threatening status epilepticus. No recognized biochemical stages, remission pattern or validated critical-treatment window exists. Developmental biology suggests that prenatal/early-postnatal HS signaling is a critical period, but this remains an experimental inference. (farhan2016genediscoveryina pages 131-135)
Consider AREXT2 in a child or adult with otherwise unexplained developmental/intellectual disability plus epilepsy, abnormal head circumference, hypotonia, scoliosis/osteopenia or characteristic coarse facial features—particularly with consanguinity, similarly affected siblings and no osteochondromas. The phenotype is not specific enough for clinical diagnosis alone. (mizumoto2021congenitaldisordersof pages 12-13, gupta2019developmentaldelaycoarse pages 2-4)
The Gupta proband had low HS in dried blood spot (12 nmol/L), serum (4.46 ng/mL) and urine (0.5 mg/mmol creatinine). However, her clinically unaffected parents also had low values, so HS measurement is neither validated nor sufficiently specific/sensitive for diagnosis. It may remain a research assay. Conventional karyotype, array CGH, Fragile-X testing and Rett/Angelman/Prader–Willi testing were normal in that patient. (gupta2019developmentaldelaycoarse pages 1-2, gupta2019developmentaldelaycoarse pages 2-4)
CMA is appropriate when copy-number disease remains in the differential, particularly Potocki–Shaffer syndrome, but it will not detect most missense AREXT2 genotypes. Karyotype/FISH, mitochondrial sequencing, repeat-expansion tests, biopsy, proteomics, metabolomics, epigenomics and liquid biopsy have no SSMS-specific indication unless another diagnosis is suspected.
No standardized clinical diagnostic criteria or newborn-screening program exists.
No survival curve, median life expectancy, 5-/10-year survival, mortality rate or validated prognostic score exists. Published survival into adulthood—including age 21 and at least the mid/late twenties in follow-up—is compatible with long survival in some genotypes. Severe outcomes in the founder family included renal failure, loss of ambulation after status epilepticus with stroke, and death after status epilepticus at 17. (gupta2019developmentaldelaycoarse pages 4-5, farhan2016genediscoveryin pages 110-114, farhan2016genediscoveryin pages 105-110)
Likely prognostic factors, not formally validated, are epilepsy severity/status epilepticus, degree of developmental impairment and hypotonia, scoliosis progression, feeding/aspiration risk, renal involvement and congenital-heart disease. Neither HS concentration nor a molecular biomarker has been shown to predict course. Recovery of the underlying developmental phenotype is not documented, although skills, communication, seizure control and mobility may improve with supportive treatment.
There is no approved disease-modifying therapy and no SSMS-specific guideline.
The Gupta proband showed some language/social improvement after immunotherapy directed at neuronal potassium-channel antibodies but tolerated it poorly and treatment was discontinued. This was treatment of a possible comorbidity, not evidence for immunotherapy in SSMS. (gupta2019developmentaldelaycoarse pages 1-2)
Farhan and Gupta proposed raising HS levels through HS administration, enzyme replacement or gene replacement. These remain speculative: no animal efficacy study tailored to SSMS, dose, delivery strategy, safety dataset or human trial was identified. Farhan explicitly cautioned that nonspecific HS administration would be premature because genotype matters and toxicity from excess HS is unknown. Developmental timing and delivery across the blood–brain barrier are major obstacles. (gupta2019developmentaldelaycoarse pages 5-6, farhan2016genediscoveryina pages 131-135)
No relevant SSMS/AREXT2 interventional ClinicalTrials.gov study, gene therapy, cell therapy, RNA therapy, CRISPR trial, immunotherapy or targeted small molecule was identified. There are no treatment response percentages or SSMS-specific adverse-event datasets.
The genotype cannot be prevented by lifestyle or vaccination.
No naturally occurring companion-animal, livestock or wildlife syndrome convincingly homologous to human biallelic EXT2 SSMS was identified; no breed or VBO term can therefore be assigned. The disorder is not infectious and has no zoonotic or cross-species transmission potential.
Orthologous EXT-family function is highly conserved across Mus musculus (NCBI Taxon 10090), Danio rerio (7955), Drosophila melanogaster (7227) and Caenorhabditis elegans (6239). Conservation of HS-dependent morphogen signaling makes these species mechanistically relevant, but induced genetic phenotypes should not be represented as natural veterinary SSMS.
Complete Ext1/Ext2 deficiency markedly impairs HS synthesis, prevents normal gastrulation and causes embryonic death around E8.5; hypomorphic models survive longer and produce shorter HS chains. These models establish that residual activity is likely necessary for viability and support the classification of human missense alleles as hypomorphic. Heterozygous/truncating models can develop osteochondromas and are more directly models of dominant HME than of AREXT2. (farhan2016genediscoveryina pages 131-135)
Application: embryogenesis, neural patterning, HS chain length and skeletal growth. Limitation: complete-null lethality prevents recapitulation of the viable, chronic human syndrome; heterozygous tumor models emphasize exostoses absent from SSMS.
The dackel/ext2 mutant has reduced, abnormally sulfated HS; disordered chondrocyte intercalation/stacking and pharyngeal-cartilage morphology; abnormal optic-tract axon organization; and disturbed FGF/Wnt-dependent development. These observations directly connect Ext2 to neural guidance and cartilage organization. (mizumoto2021congenitaldisordersof pages 12-13, jankun2017thestudyof pages 24-28)
Application: live developmental imaging, morphogen gradients, cartilage and axon guidance, rapid genetic/drug screens. Limitation: larval craniofacial/fin phenotypes do not reproduce human epilepsy, macrocephaly or chronic scoliosis, and strong mutant alleles may be more severe than human hypomorphs.
Drosophila sister of tout-velu (sotv; Ext2 ortholog) and related HS-pathway mutants disrupt morphogen distribution and neuronal/dendritic development. Viable hypomorphic C. elegans rib-1/rib-2 exostosin-pathway mutants markedly reduce HS and cause selective cell/axon-migration and morphogenetic abnormalities; cell-specific rescue indicates requirements in both migrating neurons and neighboring cells. These systems demonstrate conserved, partly non-cell-autonomous HS function. (farhan2016genediscoveryina pages 131-135)
Application: genetic interaction screens and cell-specific pathway dissection. Limitation: neither organism models vertebral scoliosis, mammalian brain anatomy or human epilepsy adequately.
No retrieved publication described an SSMS-patient iPSC line, cerebral organoid, CRISPR knock-in of a human AREXT2 allele, or humanized mouse. Such isogenic hypomorphic models are a high-priority research need because they could measure variant-specific HS chain length/sulfation, neuronal network excitability and rescue by precisely timed gene or glycan-pathway correction.
The most authoritative direct evidence remains the 2015 founder-family study and later small case reports. Reviews correctly emphasize that recessive EXT2 disease is “without exostoses and with an autosomal recessive inheritance” and that the EXT1/EXT2 complex catalyzes HS synthesis, but reviews cannot resolve penetrance or phenotype frequencies in such a small population. (jaeken2020congenitaldisordersof pages 4-5, mizumoto2021congenitaldisordersof pages 12-13)
Recent 2024 HS literature advances structural and systems-level understanding—particularly NDST1 architecture and HSPG-mediated signaling—but it does not yet supply new AREXT2 patients, prospective natural history, validated biomarkers or therapeutic data. Consequently, all epidemiologic rates, genotype–phenotype rules, surveillance intervals and treatment algorithms should be recorded as unknown or expert-practice extrapolation, not established evidence.
Bottom line: SSMS/AREXT2 is a recessive, hypomorphic EXT2 glycosaminoglycan-biosynthesis disorder with a predominantly neurodevelopmental–epileptic phenotype and variable skeletal/multisystem involvement. The human evidence supports reduced EXT2 abundance and disturbed HS-pathway organization; downstream FGF/Wnt, axon-guidance and cartilage mechanisms are strongly supported by models but remain partly inferential in patients. Diagnosis is molecular, management is symptomatic and multidisciplinary, and prevention currently consists of genetic counseling and reproductive testing.
References
(jaeken2020congenitaldisordersof pages 4-5): Jaak Jaeken. Congenital disorders of glycosylation: a multi-genetic disease family with multiple subcellular locations. Journal of Mother and Child, 24:14-20, Jun 2020. URL: https://doi.org/10.34763/jmotherandchild.20202402si.2005.000004, doi:10.34763/jmotherandchild.20202402si.2005.000004. This article has 21 citations.
(mizumoto2021congenitaldisordersof pages 12-13): Shuji Mizumoto and Shuhei Yamada. Congenital disorders of deficiency in glycosaminoglycan biosynthesis. Frontiers in Genetics, Sep 2021. URL: https://doi.org/10.3389/fgene.2021.717535, doi:10.3389/fgene.2021.717535. This article has 60 citations and is from a peer-reviewed journal.
(gupta2019developmentaldelaycoarse pages 1-2): Aditi Gupta, Sarah A. Ewing, Deborah L. Renaud, Linda Hasadsri, Kimiyo M. Raymond, Eric W. Klee, and Ralitza H. Gavrilova. Developmental delay, coarse facial features, and epilepsy in a patient with ext2 gene variants. Clinical Case Reports, 7:632-637, Feb 2019. URL: https://doi.org/10.1002/ccr3.2010, doi:10.1002/ccr3.2010. This article has 9 citations.
(gupta2019developmentaldelaycoarse pages 2-4): Aditi Gupta, Sarah A. Ewing, Deborah L. Renaud, Linda Hasadsri, Kimiyo M. Raymond, Eric W. Klee, and Ralitza H. Gavrilova. Developmental delay, coarse facial features, and epilepsy in a patient with ext2 gene variants. Clinical Case Reports, 7:632-637, Feb 2019. URL: https://doi.org/10.1002/ccr3.2010, doi:10.1002/ccr3.2010. This article has 9 citations.
(OpenTargets Search: seizures-scoliosis-macrocephaly syndrome-EXT2): Open Targets Query (seizures-scoliosis-macrocephaly syndrome-EXT2, 1 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(gupta2019developmentaldelaycoarse pages 5-6): Aditi Gupta, Sarah A. Ewing, Deborah L. Renaud, Linda Hasadsri, Kimiyo M. Raymond, Eric W. Klee, and Ralitza H. Gavrilova. Developmental delay, coarse facial features, and epilepsy in a patient with ext2 gene variants. Clinical Case Reports, 7:632-637, Feb 2019. URL: https://doi.org/10.1002/ccr3.2010, doi:10.1002/ccr3.2010. This article has 9 citations.
(mizumoto2018defectsinbiosynthesis pages 13-14): Shuji Mizumoto. Defects in biosynthesis of glycosaminoglycans cause hereditary bone, skin, heart, immune, and neurological disorders. Trends in Glycoscience and Glycotechnology, 30:E67-E89, May 2018. URL: https://doi.org/10.4052/tigg.1812.2e, doi:10.4052/tigg.1812.2e. This article has 14 citations and is from a peer-reviewed journal.
(farhan2016genediscoveryin pages 110-114): S Farhan. Gene discovery in mendelian and complex diseases. Unknown journal, 2016.
(farhan2016genediscoveryina pages 110-114): S Farhan. Gene discovery in mendelian and complex diseases. Unknown journal, 2016.
(farhan2016genediscoveryina pages 123-130): S Farhan. Gene discovery in mendelian and complex diseases. Unknown journal, 2016.
(farhan2016genediscoveryina pages 131-135): S Farhan. Gene discovery in mendelian and complex diseases. Unknown journal, 2016.
(gupta2019developmentaldelaycoarse pages 4-5): Aditi Gupta, Sarah A. Ewing, Deborah L. Renaud, Linda Hasadsri, Kimiyo M. Raymond, Eric W. Klee, and Ralitza H. Gavrilova. Developmental delay, coarse facial features, and epilepsy in a patient with ext2 gene variants. Clinical Case Reports, 7:632-637, Feb 2019. URL: https://doi.org/10.1002/ccr3.2010, doi:10.1002/ccr3.2010. This article has 9 citations.
(farhan2016genediscoveryin pages 105-110): S Farhan. Gene discovery in mendelian and complex diseases. Unknown journal, 2016.
(mizumoto2021congenitaldisordersof pages 11-12): Shuji Mizumoto and Shuhei Yamada. Congenital disorders of deficiency in glycosaminoglycan biosynthesis. Frontiers in Genetics, Sep 2021. URL: https://doi.org/10.3389/fgene.2021.717535, doi:10.3389/fgene.2021.717535. This article has 60 citations and is from a peer-reviewed journal.
(jankun2017thestudyof pages 24-28): P Jankun. The study of molecular interactions during zebrafish tail regeneration for use in glycotherapeutics. Unknown journal, 2017.
Checked with linkml-reference-validator 0.2.1.
| Outcome | Count |
|---|---|
| References checked | 10 |
| Resolved | 10 |
| Unresolved (possible confabulation) | 0 |
| Unverifiable | 0 |
| References weighed for topical relevance | 10 |
| On topic | 5 |
| Off topic | 1 |
These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:
DOI:10.1038/s41467-024-45419-4 (2 mentions) - Structural and mechanistic characterization of bifunctional heparan sulfate N-deacetylase-N-sulfotransferase 1Weighed against this report's own most characteristic terms: disease, ext2, ssms, clinical, gene, phenotype, genetic, variant, developmental, human, scoliosis, epilepsy, model, affected, family, arext2, patient, gupta, seizure, syndrome.
All extracted references resolved successfully. Resolving is not the same as being relevant, though - see the references listed above as possibly off topic.
Checked with linkml-term-validator 0.4.5, through the ols: adapter.
| Outcome | Count |
|---|---|
| Terms checked | 54 |
| Resolved | 54 |
| Unresolved (possible confabulation) | 0 |
| Obsolete | 0 |
| Unverifiable | 0 |
| Terms whose name was checked | 1 |
| Terms named correctly | 0 |
| Terms named as a different term | 1 |
These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:
MONDO:0014731 (3 mentions) - the report calls it "if available"; MONDO calls it seizures-scoliosis-macrocephaly syndrome