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Classifications

Harrison's Part

IUIS Category

ICIMD (Inherited Metabolic Disorders)

ISDS Skeletal Nosology

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Inheritance

1

Autosomal Recessive HP:0000007

ISDNA follows autosomal recessive inheritance. All reported disease alleles are germline missense variants carried in the homozygous state, with heterozygous unaffected parents confirmed by Sanger sequencing. Consanguinity contributed to ascertainment in several families but is not obligatory; the recurrent c.953C>T (p.Pro318Leu) allele lies in a shared homozygous stretch on chromosome 8, suggesting a common founder.

Autosomal recessive inheritance

Show evidence (2 references)

PMID:38010033 SUPPORT Human Clinical

"Immune skeletal dysplasia with neurodevelopmental abnormalities (ISDNA) is an extremely rare, autosomal recessive genetic disorder characterized by various skeletal abnormalities, neurodevelopmental deficits, and abnormal immune system function."

States the autosomal recessive mode of inheritance and the three-system clinical definition of ISDNA.

PMID:28132690 SUPPORT Human Clinical

"By whole-exome sequencing, we identified homozygous missense mutations c.1382C>T, c.1537C>T, c.1970A>G, and c.2008T>G in EXTL3 in nine affected individuals from five unrelated families."

Documents homozygous (biallelic) missense variants segregating in multiple unrelated families, consistent with recessive inheritance.

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Mechanistic Hypotheses

1

Unidentified modifiers determine immunological severity

immune_severity_modifiers EMERGING

Evidence balance 1 support

The most conspicuous unexplained feature of ISDNA is that immunological severity does not track with variant position or identity: individuals homozygous for the identical allele (both p.Pro318Leu and p.Pro461Leu) have ranged from Omenn-like SCID to a clinically intact immune system, while the skeletal phenotype remains comparatively uniform. This dissociation implies that genetic modifiers or environmental factors — not the EXTL3 lesion alone — gate whether the HS defect is transduced into thymic/hematopoietic failure. No modifier locus has been identified.

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"Notably, patients harbouring the same variant in EXTL3 gene show phenotypic variability especially with respect to presence or absence of immunological manifestations, suggesting a role of unknown modifiers."

States the same-genotype/divergent-immune-phenotype observation and attributes it explicitly to unknown modifiers.

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Discussions and Knowledge Gaps

3

What modifiers determine whether an individual with biallelic EXTL3 variants develops Omenn-like SCID versus a clinically intact immune system?

KNOWLEDGE GAP isdna_immune_severity_modifiers

Attached to

pathophysiology#T-Cell-Predominant Immunodeficiency

Immunological severity is discordant between individuals homozygous for the identical EXTL3 allele — including unrelated patients sharing p.Pro318Leu or p.Pro461Leu — while the skeletal phenotype stays comparatively uniform. Variant position therefore does not predict immune outcome, and the responsible genetic or environmental modifiers are unidentified. This is the single largest barrier to prognostication and to deciding which infants warrant transplant evaluation.

Proposed experiments

Modifier screening across the reported ISDNA cohort

exp_isdna_modifier_screen

Genome-wide modifier screening across the full reported ISDNA cohort, focusing on heparan-sulfate-biosynthesis and thymic-development loci, to identify variants segregating with immunological severity independently of the EXTL3 genotype.

Isogenic iPSC background-swap panel

exp_isdna_isogenic_ipsc_background_swap

Isogenic iPSC panels carrying each EXTL3 allele on shared and divergent genetic backgrounds, differentiated to thymic epithelial progenitors and lymphohematopoietic progenitors, to test whether genetic background alone reproduces the observed divergence in immune severity.

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"Notably, patients harbouring the same variant in EXTL3 gene show phenotypic variability especially with respect to presence or absence of immunological manifestations, suggesting a role of unknown modifiers."

Directly states the unresolved modifier question this gap records.

Is the neurodevelopmental impairment in ISDNA a primary consequence of the heparan sulfate defect in the developing brain, or largely secondary to cervical cord compression and chronic illness?

KNOWLEDGE GAP isdna_neuro_primary_vs_secondary

Attached to

pathophysiology#Neurodevelopmental Impairment pathophysiology#Cervical Cord Compression

Cord compression, laryngotracheal narrowing, recurrent infection, and failure to thrive all independently impair development, and all co-occur with the neurological phenotype. No reported patient separates the two contributions, yet the distinction determines whether early cervical decompression should be expected to improve cognitive outcome or only to prevent further injury.

Proposed experiments

Pre- and post-decompression neurodevelopmental follow-up

exp_isdna_decompression_neurodev_followup

Longitudinal neurodevelopmental assessment of patients before and after early cervical decompression, compared against affected patients without cervical stenosis, to separate compression-attributable from HS-attributable developmental impairment.

Neural-progenitor-restricted conditional Extl3 deletion

exp_isdna_neural_conditional_extl3_ko

Conditional Extl3 deletion restricted to neural progenitors in mouse, bypassing the skeletal and cervical phenotype, to test whether neurodevelopmental impairment arises cell-autonomously in the brain.

Show evidence (2 references)

PMID:28148688 SUPPORT Human Clinical

"craniocervical magnetic resonance imaging showed impingement of the posterior arch of C1 on the medulla with mild stenosis of the cervical canal requiring neurosurgery"

Documents structural cord compression coexisting with the neurodevelopmental phenotype in the same patient — the confound that makes the primary-versus-secondary attribution unresolvable from reported cases.

PMID:28331220 SUPPORT Human Clinical

"Both patients had spinal cord compression at the cranio-vertebral junction, which caused serious neurological disorders in early infancy."

Attributes serious early neurological disease directly to cord compression, stating the secondary-cause side of this open question.

Does the extl3-mutant zebrafish thymic phenotype model the human immune disease closely enough to support therapeutic inference, given that the fish model does not reproduce the human spinal, cognitive, or longitudinal phenotype?

HUMAN MODEL MISMATCH isdna_zebrafish_translational_validity

Attached to

pathophysiology#Impaired Thymopoiesis and Progenitor Expansion

The zebrafish box mutant provides the strongest in vivo causal evidence in the disease — roughly halved thymic volume rescued by wild-type human EXTL3 RNA — but reproduces only the thymic and fin phenotypes. Meanwhile complete Extl3 knockout in mouse is lethal at E9.5, so no mammalian model recapitulates the surviving hypomorphic human state. Evidence therefore exists in models, but its translational reach to human immune reconstitution is the open question.

Proposed experiments

Hypomorphic knock-in mouse models

exp_isdna_hypomorphic_knockin_mouse

Knock-in mouse models carrying the human hypomorphic alleles (p.Pro318Leu, p.Arg339Trp) rather than a null, to obtain a viable mammalian model that reproduces the partial-function human state.

Patient-derived thymic organoids

exp_isdna_patient_thymic_organoid

Patient-derived thymic organoid systems to test whether the thymic-stromal defect is cell-autonomous and whether it can be corrected independently of the hematopoietic compartment — the key question for HSCT rationale.

Show evidence (1 reference)

PMID:28148688 SUPPORT Model Organism

"Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green fluorescent protein) transgenic zebrafish revealed defective thymopoiesis, which was rescued by injection of wild-type human EXTL3 RNA."

The model-system evidence whose translational validity to the human disease is the subject of this mismatch record.

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Pathophysiology

9

EXTL3 Glycosyltransferase Deficiency

Biallelic hypomorphic missense variants reduce the GlcNAc-transferase activity of the Golgi enzyme EXTL3, which initiates and elongates heparan sulfate chains on proteoglycan core proteins. Activity is reduced rather than abolished; complete Extl3 loss is embryonically lethal in mouse at ~E9.5, before skeletal elements differentiate, so surviving human disease necessarily reflects partial function.

heparan sulfate N-acetylglucosaminyltransferase activity GO:0042328 ↓ DECREASED

Downstream

Aberrant Heparan Sulfate Chain Structure

Show evidence (2 references)

PMID:28331220 SUPPORT Model Organism

"Exlt3 knockout mice (Exlt3−/−) are embryonic lethal at 9 days post coitum when the differentiated skeletal elements are not formed."

Establishes that complete EXTL3 loss is incompatible with skeletal development, supporting the interpretation that human disease alleles are hypomorphic rather than null.

DOI:10.1038/s41467-022-31048-2 SUPPORT In Vitro

"Here, we report the structure of human exostosin-like 3 (EXTL3) in apo and UDP-bound forms."

Provides the cryo-EM structural basis of the bi-domain (GT47/GT64) architecture within which the disease-causing missense substitutions act.

Aberrant Heparan Sulfate Chain Structure

The consequence of reduced EXTL3 activity is not loss of heparan sulfate but structurally abnormal HS. Patient fibroblasts produce a quantitatively normal amount of HS, yet the chains are abnormally long and carry an aberrant sulfation pattern with increased 6-O- and 2-O-sulfated disaccharides; wild-type EXTL3 cDNA corrects the defect. Systemically, affected individuals show abnormal glycosaminoglycan concentrations in serum and urine, including elevated non-sulfated chondroitin disaccharides — consistent with EXTL3 acting at the branch point between heparan sulfate and chondroitin sulfate assembly.

glycosaminoglycan biosynthetic process GO:0006024 ⚠ ABNORMAL

Downstream

Potentiated FGF2-ERK Signaling

Impaired Cytokine-STAT5 Signaling

Impaired Thymopoiesis and Progenitor Expansion

Neurodevelopmental Impairment

Show evidence (3 references)

PMID:28148688 SUPPORT In Vitro

"Altogether, these results indicate that the EXTL3 R339W mutation is a hypomorphic allele that alters the function of the enzyme and leads to longer HS chains with an aberrant sulfation pattern."

States the central biochemical lesion — qualitatively abnormal HS (longer, aberrantly sulfated), not absent HS.

PMID:28148688 SUPPORT In Vitro

"Patient-derived fibroblasts showed abnormal HS composition and altered fibroblast growth factor 2 signaling, which was rescued by overexpression of wild-type EXTL3 cDNA."

Complementation with wild-type EXTL3 rescues both the HS defect and its signaling consequence, establishing causality.

PMID:28132690 SUPPORT Human Clinical

"We observed abnormal glycosaminoglycan concentrations and increased concentrations of the non-sulfated chondroitin disaccharide D0a0 and the disaccharide D0a4 in serum and urine of all analyzed affected individuals."

Demonstrates the biochemical defect is detectable systemically in patient body fluids, not only in cultured cells.

Potentiated FGF2-ERK Signaling

Longer HS chains bearing excess 6-O-sulfate provide additional FGF receptor binding sites, so the HS defect paradoxically increases rather than decreases growth-factor signaling in mesenchymal lineages. Patient fibroblasts show enhanced ERK phosphorylation on FGF2 stimulation, corrected by wild-type EXTL3. This explains why the radiographic phenotype at birth resembles the FGFR3-related dysplasias driven by constitutively increased FGFR signaling, even though FGFR1/2/3 are wild-type in these patients.

fibroblast growth factor receptor signaling pathway GO:0008543 ↑ INCREASED

Downstream

Spondyloepimetaphyseal Dysplasia

Show evidence (1 reference)

PMID:28148688 SUPPORT In Vitro

"These results indicate that by altering HS chain length and composition, EXTL3 mutations potentiate FGF2 signaling, thereby contributing to the pathophysiology of the skeletal dysplasia observed in the patients."

Directly attributes the skeletal arm of the disease to potentiated (not reduced) FGF2 signaling downstream of the HS defect.

Impaired Cytokine-STAT5 Signaling

In the lymphoid compartment the same HS defect acts in the opposite direction. HS modulates responses to IL-2 and IL-7, the cytokines that drive T-cell survival and homeostatic expansion; patient lymphocytes show markedly reduced STAT5 phosphorylation after IL-2 stimulation, with a subtler defect after IL-7. This opposite-direction transduction of one biochemical lesion is the mechanistic core of the multisystem character of the disease.

T cell CL:0000084

cytokine-mediated signaling pathway GO:0019221 ↓ DECREASED

Downstream

T-Cell-Predominant Immunodeficiency

Show evidence (1 reference)

PMID:28148688 SUPPORT In Vitro

"Interleukin-2-mediated STAT5 phosphorylation in patients' lymphocytes was markedly reduced."

Documents the lymphoid signaling defect that distinguishes the immune arm from the FGF-potentiated skeletal arm.

Impaired Thymopoiesis and Progenitor Expansion

The immunodeficiency has both hematopoietic-intrinsic and thymic-stromal components. EXTL3 is most abundant in hematopoietic stem cells and early progenitor T cells, and patient-derived iPSCs show reduced expansion of lymphohematopoietic progenitors together with defective differentiation of thymic epithelial progenitors (reduced TBX1, EYA1, and CK5 with persistent SOX17). extl3-mutant zebrafish crossed to a rag2:GFP thymic reporter show roughly halved thymic volume, rescued by injection of wild-type human EXTL3 RNA. Because the thymic stroma is itself affected, hematopoietic stem cell transplantation cannot be assumed to correct the whole immune phenotype.

thymic epithelial cell CL:0002293 hematopoietic stem cell CL:0000037

thymus development GO:0048538 ↓ DECREASED

Downstream

T-Cell-Predominant Immunodeficiency

Show evidence (3 references)

PMID:28148688 SUPPORT In Vitro

"Targeted differentiation of patient-derived induced pluripotent stem cells showed a reduced expansion of lymphohematopoietic progenitor cells and defects of thymic epithelial progenitor cell differentiation."

Establishes the dual hematopoietic-intrinsic and thymic-stromal basis of the T-cell defect in patient-derived human cells.

PMID:28148688 SUPPORT Model Organism

"Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green fluorescent protein) transgenic zebrafish revealed defective thymopoiesis, which was rescued by injection of wild-type human EXTL3 RNA."

In vivo functional complementation confirming EXTL3 is required for thymopoiesis.

PMID:28132690 SUPPORT Human Clinical

"EXTL3 was most abundant in hematopoietic stem cells and early progenitor T cells, which is in line with a SCID phenotype at the level of early T cell development in the thymus."

Expression data localizing the block to early intrathymic T-cell development.

Spondyloepimetaphyseal Dysplasia

Disrupted HS-dependent growth-plate signaling produces a spondyloepimetaphyseal dysplasia: disproportionate short-limb short stature, severe platyspondyly with widened intervertebral spaces, brachydactyly, epiphyseal and metaphyseal irregularity, pelvic distortion with trident-shaped acetabula and narrow sacro-ischiatic notches, and progressive kyphoscoliosis. The craniovertebral junction is the clinically decisive site: odontoid hypoplasia and cervical canal stenosis cause cord compression that has required decompressive surgery in infancy across multiple reports. Premature craniosynostosis, including cloverleaf skull, occurs at the severe end.

Downstream

Cervical Cord Compression

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Here we describe a novel type of SEMD in two unrelated Turkish patients who presented with severe platyspondyly, kyphoscoliosis, pelvic distortion, constriction of the proximal femora and brachydactyly."

Defines the core skeletal presentation as a spondyloepimetaphyseal dysplasia.

PMID:28331220 SUPPORT Human Clinical

"Both patients had spinal cord compression at the cranio-vertebral junction and multiple liver cysts since early infancy."

Documents the craniovertebral cord compression that drives early neurosurgical management, alongside the recurrent hepatic cysts.

Cervical Cord Compression

Odontoid hypoplasia, non-ossification of the atlas, and cervical canal stenosis converge on compression of the upper cervical cord. This is the single most actionable complication of ISDNA: it is detectable by early cervical imaging, is correctable by C1-arch excision or decompression, and if unrecognised leads to irreversible neurological injury and neurogenic bladder. It is a downstream structural consequence of the skeletal dysplasia, mechanistically distinct from the primary neurodevelopmental impairment.

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"All three patients had narrowing of the cervical canal, and severe narrowing of the laryngotracheal tract was present in P1 and P2."

Documents cervical canal narrowing in every patient of the founding series, with associated airway narrowing.

T-Cell-Predominant Immunodeficiency

The immune phenotype is T-cell predominant and strikingly variable. Reported profiles include T-B+NK+ SCID with absent TRECs, Omenn-like disease with oligoclonal activated/memory autologous T cells and exfoliative erythroderma, isolated T-cell lymphopenia with hypogammaglobulinemia, and — in some individuals with the same genotype — no clinically apparent immune defect. Impaired mitogen proliferation, eosinophilia, hypogammaglobulinemia, and elevated IgE are recurrent laboratory findings. Spontaneous partial recovery of T-cell number and vaccine responsiveness has been documented in at least one survivor. Absence of overt immunodeficiency does not exclude the diagnosis.

Show evidence (2 references)

PMID:28132690 SUPPORT Human Clinical

"Severe combined immunodeficiency (SCID) with a complete absence of T cells was observed in three families."

Documents the severe end of the immunological spectrum.

PMID:35114981 SUPPORT Human Clinical

"Notably, patients harbouring the same variant in EXTL3 gene show phenotypic variability especially with respect to presence or absence of immunological manifestations, suggesting a role of unknown modifiers."

Establishes the genotype-independent variability of the immune phenotype that motivates the modifier hypothesis.

Neurodevelopmental Impairment

Global developmental delay — particularly gross motor — with central or axial hypotonia and intellectual disability of variable degree is a defining component of the syndrome. Severe presentations include seizures, opisthotonus, hyperreflexia, nystagmus, and developmental arrest. Because HS proteoglycans regulate morphogen signaling in neural progenitors, this arm is regarded as a primary consequence of the HS defect rather than solely secondary to cord compression, although the two coexist and their relative contributions have not been separated in patients.

Show evidence (2 references)

PMID:28148688 SUPPORT Human Clinical

"Neurological abnormalities included: opisthotonus, hyperreflexia, generalized seizures, and developmental delay in P1; clonic arm movements, nystagmus, and developmental arrest in P2; and muscular hypotonia and marked developmental delay in P3."

Enumerates the neurological spectrum across the three index patients.

PMID:28148688 SUPPORT Model Organism

"These data identify EXTL3 mutations as a novel cause of severe immune deficiency with skeletal dysplasia and developmental delay and underline a crucial role of HS in thymopoiesis and skeletal and brain development."

Attributes brain development, alongside thymopoiesis and skeleton, to the HS-dependent mechanism.

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Pathograph

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Phenotypes

35

Blood 3

Decreased Circulating Immunoglobulin Concentration OCCASIONAL HP:0004313

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Hypogammaglobulinemia but increased IgE serum levels were detected in P1 and P2."

Documents hypogammaglobulinemia with paradoxically elevated IgE.

Increased Circulating IgE Concentration OCCASIONAL HP:0003212

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Hypogammaglobulinemia but increased IgE serum levels were detected in P1 and P2."

Documents the elevated IgE that accompanies the Omenn-like phenotype.

Eosinophilia OCCASIONAL Increased total eosinophil count HP:0001880

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Impaired proliferation to mitogens and eosinophilia was documented in all three infants."

Documents eosinophilia alongside impaired mitogen proliferation.

Cardiovascular 2

Ventricular Septal Defect HP:0001629

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"a small fenestrated ventricular septal defect"

Documents the ventricular septal defect detected by echocardiography in the reported patient.

Mitral Valve Prolapse HP:0001634

Frequency band intentionally omitted. This feature is documented in a single reported individual; against a total published cohort of roughly 16 that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the denominator is small and ascertainment-biased toward severe presentations, so no band is asserted. See docs/frequency-evidence-guidelines.md.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Echocardiography showed mitral valve prolapse."

Documents mitral valve prolapse in a single reported patient.

Digestive 2

Hepatic Cysts FREQUENT HP:0001407

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Both patients had spinal cord compression at the cranio-vertebral junction and multiple liver cysts since early infancy."

Documents multiple hepatic cysts from early infancy in both patients.

PMID:34089299 SUPPORT Human Clinical

"We report a new patient who is currently 15 years old in whom cystic liver lesions were detected in the prenatal period."

Documents prenatal detectability of the hepatic cysts.

Anal Atresia HP:0002023

Frequency band intentionally omitted. This feature is documented in a single reported individual; against a total published cohort of roughly 16 that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the denominator is small and ascertainment-biased toward severe presentations, so no band is asserted. See docs/frequency-evidence-guidelines.md.

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"there was also anal atresia, for which a colostomy was performed in the first days of life"

Documents anal atresia requiring neonatal colostomy in one patient.

Genitourinary 1

Renal Cysts OCCASIONAL HP:0000107

Show evidence (1 reference)

PMID:38010729 SUPPORT Human Clinical

"Skeletal dysplasia, neurodevelopmental delay, immunodeficiency, liver, and kidney cysts are the most common findings of this syndrome."

Lists kidney cysts among the recurrent findings of ISDNA.

Head and Neck 3

Coarse Facial Features VERY_FREQUENT HP:0000280

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"A 15-month-old female child with clinical indications of global developmental delay, short stature, coarse facial features, and hypotonia was referred to our clinic."

Documents coarse facial features as a presenting finding; the same report's literature review states facial dysmorphism was present in all reviewed patients.

Craniosynostosis OCCASIONAL HP:0001363

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Premature craniosynostosis was seen in the skull x ray and computed tomography studies of patient 1 (P1) and P2, with cloverleaf deformity in P2."

Documents premature craniosynostosis with cloverleaf skull in two of three index patients.

Carious Teeth HP:0000670

Frequency band intentionally omitted. This feature is documented in a single reported individual; against a total published cohort of roughly 16 that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the denominator is small and ascertainment-biased toward severe presentations, so no band is asserted. See docs/frequency-evidence-guidelines.md.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"During follow-up, she showed recurrent episodes of pulmonary infection and susceptibility to dental caries."

Documents susceptibility to dental caries during follow-up.

Immune 3

Combined Immunodeficiency FREQUENT HP:0005387

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"nine of the 14 patients of ISDNA"

Provides the 9/14 count underpinning the FREQUENT frequency band; the sentence continues "had immunodeficiency" across a hyphenated line break in the source PDF, so only the contiguous fragment is quoted.

Recurrent Infections FREQUENT HP:0002719

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"A total of five patients died before the age of one year, primarily due to recurrent infections."

Establishes recurrent infection as the dominant cause of early mortality across the reviewed cohort.

Recurrent Respiratory Infections OCCASIONAL HP:0002205

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"During follow-up, she showed recurrent episodes of pulmonary infection and susceptibility to dental caries."

Documents recurrent pulmonary infection in a patient with reduced T cells and immunoglobulins.

Limbs 3

Brachydactyly VERY_FREQUENT HP:0001156

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"The common presenting symptoms included short limb dwarfism, platyspondyly, brachydactyly and epiphyseal abnormalities."

Lists brachydactyly among the common presenting skeletal features.

Metaphyseal Dysplasia FREQUENT HP:0100255

Show evidence (1 reference)

PMID:34089299 SUPPORT Human Clinical

"Spondyloepimetaphyseal dysplasia (SEMD) is a group of genetic skeletal disorders characterized by disproportionate short stature, and varying degrees of vertebral, epiphyseal, and metaphyseal involvement of the skeleton."

Places the disorder in the SEMD group, defined by combined vertebral, epiphyseal, and metaphyseal involvement.

Hip Subluxation HP:0030043

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"A pelvic radiograph showed bilateral coxa vara, dysplastic changes of both hips, and subluxation of the left hip."

Documents hip subluxation on pelvic radiography.

Musculoskeletal 6

Platyspondyly VERY_FREQUENT HP:0000926

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Here we describe a novel type of SEMD in two unrelated Turkish patients who presented with severe platyspondyly, kyphoscoliosis, pelvic distortion, constriction of the proximal femora and brachydactyly."

Documents severe platyspondyly as a presenting radiographic feature.

Epiphyseal Dysplasia FREQUENT HP:0002656

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"The common presenting symptoms included short limb dwarfism, platyspondyly, brachydactyly and epiphyseal abnormalities."

Lists epiphyseal abnormalities among the common presenting features.

Kyphoscoliosis FREQUENT HP:0002751

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Here we describe a novel type of SEMD in two unrelated Turkish patients who presented with severe platyspondyly, kyphoscoliosis, pelvic distortion, constriction of the proximal femora and brachydactyly."

Documents kyphoscoliosis as a presenting skeletal feature.

Hypoplasia of the Odontoid Process HP:0003311

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"She had hypoplasia of the posterior C1 arch with narrowing of the cervical spinal canal and cord compression requiring neurosurgery."

Documents hypoplasia of the C1 arch producing canal narrowing and cord compression that required surgery.

Hypotonia FREQUENT HP:0001252

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"A 15-month-old female child with clinical indications of global developmental delay, short stature, coarse facial features, and hypotonia was referred to our clinic."

Documents hypotonia as a presenting neurological feature.

Opisthotonus HP:0002179

Frequency band intentionally omitted. This feature is documented in a single reported individual; against a total published cohort of roughly 16 that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the denominator is small and ascertainment-biased toward severe presentations, so no band is asserted. See docs/frequency-evidence-guidelines.md.

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Neurological abnormalities included: opisthotonus, hyperreflexia, generalized seizures, and developmental delay in P1; clonic arm movements, nystagmus, and developmental arrest in P2; and muscular hypotonia and marked developmental delay in P3."

Documents opisthotonus in a severely affected patient.

Nervous System 3

Global Developmental Delay VERY_FREQUENT HP:0001263

Show evidence (2 references)

PMID:28132690 SUPPORT Human Clinical

"Affected individuals presented with variable skeletal abnormalities and neurodevelopmental defects."

Establishes neurodevelopmental impairment as a defining component.

PMID:35114981 SUPPORT Human Clinical

"A 15-month-old female child with clinical indications of global developmental delay, short stature, coarse facial features, and hypotonia was referred to our clinic."

Documents global developmental delay as a presenting feature.

Intellectual Disability FREQUENT HP:0001249

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"These data identify EXTL3 mutations as a novel cause of severe immune deficiency with skeletal dysplasia and developmental delay and underline a crucial role of HS in thymopoiesis and skeletal and brain development."

Supports the developmental/cognitive arm of the phenotype as intrinsic to the EXTL3 mechanism.

Seizures OCCASIONAL HP:0001250

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"Neurological abnormalities included: opisthotonus, hyperreflexia, generalized seizures, and developmental delay in P1; clonic arm movements, nystagmus, and developmental arrest in P2; and muscular hypotonia and marked developmental delay in P3."

Documents generalized seizures in one of three index patients.

Growth 2

Disproportionate Short Stature VERY_FREQUENT HP:0003498

Show evidence (1 reference)

PMID:35114981 SUPPORT Human Clinical

"The common presenting symptoms included short limb dwarfism, platyspondyly, brachydactyly and epiphyseal abnormalities."

Literature review of 14 cases lists short-limb dwarfism among the common presenting features.

Failure to Thrive OCCASIONAL HP:0001508

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"In her first year of life, she had severe failure to thrive and required high calorie supplements and tube feedings with a G-tube."

Documents severe failure to thrive requiring gastrostomy feeding.

Other 7

Spinal Cord Compression FREQUENT HP:0002176

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Both patients had spinal cord compression at the cranio-vertebral junction and multiple liver cysts since early infancy."

Documents craniovertebral-junction cord compression in both reported patients.

PMID:28331220 SUPPORT Human Clinical

"The complication should be recognized because its early detection followed by appropriate surgical treatment is a key to good prognosis."

Supports flagging cord compression as a diagnostically and prognostically decisive finding requiring active surveillance.

Coxa Valga OCCASIONAL HP:0002673

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"In the pelvis of P3, there is coxa valga with delayed ossification of femoral heads, acetabular dysplasia, and hip subluxation."

Documents coxa valga with associated hip dysplasia and subluxation.

Severe Combined Immunodeficiency OCCASIONAL HP:0004430

Show evidence (1 reference)

PMID:28132690 SUPPORT Human Clinical

"Severe combined immunodeficiency (SCID) with a complete absence of T cells was observed in three families."

Documents SCID with complete T-cell absence in three of five families.

Decreased Total T Cell Count FREQUENT HP:0005403

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Immunological examination showed decrease of T-lymphocyte subset, IgG and IgM."

Documents reduced T-lymphocyte subsets alongside immunoglobulin deficiency.

PMID:34089299 SUPPORT Human Clinical

"Disproportionate short stature, mild developmental delay and a T- NK+ B+ immunological profile were detected in the postnatal follow-up."

Documents the characteristic T-B+NK+ immunological profile.

Oral Candidiasis

Intentionally left without an HPO binding. The source documents a single episode in one patient and does not establish chronicity or recurrence, but every available HPO oral-candidiasis term asserts one or the other: HP:0009098 (Chronic oral candidiasis) is_a HP:0002728 (Chronic mucocutaneous candidiasis), and the siblings under that branch are likewise chronic, recurrent, or site-specific. Binding to any of them would assert a temporal course the evidence does not support, so per the project convention of preferring no term over a misleading one the descriptor carries a preferred_term only. An HPO new-term request for a course-neutral "Oral candidiasis" would be the correct resolution.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Although patient 2 did not present overt immune deficiency, she had oral candidiasis, a fungal infection often found in immune-deficient patients."

Documents candidiasis as a marker of possible subclinical immune impairment in a patient classified as immunologically normal; the source describes a single case and does not establish chronicity, so support is partial.

Acetabular Dysplasia HP:0008807

Show evidence (2 references)

PMID:28148688 SUPPORT Human Clinical

"In the pelvis of P3, there is coxa valga with delayed ossification of femoral heads, acetabular dysplasia, and hip subluxation."

Documents acetabular dysplasia together with delayed femoral head ossification and hip subluxation.

PMID:28148688 SUPPORT Human Clinical

"A pelvic radiograph showed bilateral coxa vara, dysplastic changes of both hips, and subluxation of the left hip."

Independent radiographic confirmation of bilateral hip dysplasia with subluxation at follow-up.

Laryngeal Stenosis OCCASIONAL HP:0001602

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"All three patients had narrowing of the cervical canal, and severe narrowing of the laryngotracheal tract was present in P1 and P2."

Documents laryngotracheal narrowing in two of three index patients.

🧬

Genetic Associations

1

EXTL3

Gene: EXTL3 hgnc:3518 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (6 references)

PMID:28331220 SUPPORT In Vitro

"The enzyme activity of the mutant EXTL3 protein was significantly decreased compared to the wild-type protein."

Direct in vitro enzymology showing the p.Pro318Leu allele causes loss of GlcNAc transferase activity, establishing the variants as hypomorphic.

PMID:28331220 SUPPORT In Vitro

"EXTL3 encodes N-acetylglucosaminyltransferase-I (GlcNAcT-I) and GlcNAcT-II, which transfers N-acetylglucosamine (GlcNAc) from uridine diphosphate-GlcNAc (UDP-GlcNAc) to the linkage region of heparan sulfate (HS) and heparin, and to the glucuronic acid (GlcUA) of growing glycan chains, respectively"

Defines the dual enzymatic role of EXTL3 in HS chain initiation and elongation.

PMID:38010033 SUPPORT Human Clinical

"A novel homozygous variant, NM_001440: c.2015G>A (p.Arg672Gln) in the EXTL3 gene, was identified using WES, which has never been reported before."

Expands the reported allelic spectrum with an additional homozygous missense variant in a Chinese family.

+ 3 more references

🗃️

External Assertions

1

OMIM immunoskeletal dysplasia with neurodevelopmental abnormalities record

OMIM disease record OMIM:617425

OMIM entry for immunoskeletal dysplasia with neurodevelopmental abnormalities (ISDNA), cataloguing the clinical features and molecular basis of the EXTL3-related phenotype.

💊

Medical Actions

6

Cervical Decompression Surgery

Action: surgical procedure Ontology label: Surgical Procedure NCIT:C15329

Decompression of the craniovertebral junction — typically C1-arch excision — for cord compression or cervical instability. This is the highest-yield intervention in ISDNA: early detection followed by surgery is explicitly identified as the key to good neurological prognosis, and multiple reported patients underwent it in infancy with good outcome.

Mechanism Target:

INHIBITS Cervical Cord Compression — Surgical decompression relieves the mechanical compression of the cervical cord produced by odontoid hypoplasia and canal stenosis.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"She received an excision of the C1 arch for the cord compression with good outcome."

Documents the decompressive procedure acting on this mechanism node with a good clinical outcome.

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"She received an excision of the C1 arch for the cord compression with good outcome."

Documents C1-arch excision with good outcome for cord compression.

PMID:28331220 SUPPORT Human Clinical

"The complication should be recognized because its early detection followed by appropriate surgical treatment is a key to good prognosis."

Establishes early surgical decompression as prognostically decisive.

Immunoglobulin Replacement Therapy

Action: immunoglobulin infusion therapy Ontology label: Immunoglobulin Therapy NCIT:C62710

Monthly intravenous immunoglobulin for clinically significant antibody deficiency. Because immune function can improve spontaneously, replacement may be reassessed over time — in one reported patient it was successfully discontinued after partial immune recovery with subsequent demonstrable vaccine responses.

Mechanism Target:

BYPASSES T-Cell-Predominant Immunodeficiency — Passive immunoglobulin substitutes for the deficient humoral response rather than correcting the underlying HS or thymic defect.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Currently, she is 2 years old and has been treated with monthly intravenous immunoglobulin injection."

Documents immunoglobulin replacement used to manage the antibody-deficiency component of this node.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Currently, she is 2 years old and has been treated with monthly intravenous immunoglobulin injection."

Documents monthly IVIG as the management of the antibody deficiency.

Hematopoietic Stem Cell Transplantation

Action: hematopoietic stem cell transplantation Ontology label: Hematopoietic Cell Transplantation NCIT:C15431

HSCT may be considered for life-threatening cellular immunodeficiency, but with an important mechanistic caveat specific to ISDNA: because EXTL3 deficiency also impairs thymic epithelial (stromal) differentiation and other non-hematopoietic tissues, replacing the hematopoietic compartment alone should not be assumed to correct the immune phenotype. No disease-specific response rates have been published.

Mechanism Target:

RESTORES Impaired Thymopoiesis and Progenitor Expansion — Transplantation replaces the defective hematopoietic progenitor compartment, but cannot correct the thymic epithelial (stromal) arm of the same node, which remains EXTL3-deficient in the host.

Show evidence (1 reference)

PMID:28148688 SUPPORT In Vitro

"Targeted differentiation of patient-derived induced pluripotent stem cells showed a reduced expansion of lymphohematopoietic progenitor cells and defects of thymic epithelial progenitor cell differentiation."

The thymic-stromal component of this node is why transplantation of the hematopoietic compartment alone may be insufficient.

Show evidence (1 reference)

PMID:28148688 SUPPORT In Vitro

"Targeted differentiation of patient-derived induced pluripotent stem cells showed a reduced expansion of lymphohematopoietic progenitor cells and defects of thymic epithelial progenitor cell differentiation."

The demonstrated thymic-stromal (non-hematopoietic) component is the basis for caution that HSCT may not correct the full immune phenotype; no clinical transplant outcome data exist for this disorder.

Nutritional Support and Gastrostomy Feeding

Action: dietary intervention Ontology label: Dietary Intervention NCIT:C15447

Severe failure to thrive is common and has required high-calorie supplementation and gastrostomy tube feeding. Split out from general supportive care because it is the one nutritional intervention explicitly documented in a reported patient rather than inferred from the phenotype.

Show evidence (1 reference)

PMID:28148688 SUPPORT Human Clinical

"In her first year of life, she had severe failure to thrive and required high calorie supplements and tube feedings with a G-tube."

Documents high-calorie supplementation and gastrostomy feeding for failure to thrive in a reported patient.

Genetic Counseling and Prenatal Testing

Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

Because ISDNA is autosomal recessive with a 25% recurrence risk and consanguinity is frequent among reported families, genetic counseling with carrier testing and prenatal or preimplantation testing is a core part of management. Molecular confirmation in the proband is the prerequisite that makes accurate prenatal testing possible, which is why a "proband-first" testing strategy is recommended when a further pregnancy is under consideration.

Show evidence (2 references)

PMID:35114981 SUPPORT Human Clinical

"in order to guide them with robust prenatal counseling and testing in the current pregnancy."

Documents the proband-first molecular diagnosis being used to enable prenatal counseling and testing in an ongoing pregnancy.

PMID:38010033 SUPPORT Human Clinical

"More comprehensive genetic counseling and precise prenatal diagnosis for the next pregnancy can also be provided to families with genetic disorders."

States that molecular diagnosis enables genetic counseling and prenatal diagnosis for subsequent pregnancies.

Supportive and Rehabilitative Care

Action: supportive care Ontology label: Supportive Care NCIT:C15747

The genuine remainder of phenotype-directed supportive care once the separately curated interventions (cervical decompression, immunoglobulin replacement, transplantation, nutritional support, genetic counseling) are accounted for: developmental and rehabilitative therapy, standard antiseizure treatment, respiratory monitoring, and surveillance of hepatic and renal cysts. There is no disease-modifying therapy for EXTL3 deficiency and no disease-specific interventional trial. This entry deliberately retains the generic supportive-care term because the individual component therapies, while recommended in review sources, are not documented for a specific reported patient in any reference cached for this entry.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Both patients also had multiple liver cysts in infancy."

Establishes the hepatic cyst burden that motivates ongoing visceral surveillance as part of supportive care.

🔬

Biochemical Markers

2

Circulating and Urinary Glycosaminoglycan Disaccharide Signature (INCREASED)

Show evidence (1 reference)

PMID:28132690 SUPPORT Human Clinical

"We observed abnormal glycosaminoglycan concentrations and increased concentrations of the non-sulfated chondroitin disaccharide D0a0 and the disaccharide D0a4 in serum and urine of all analyzed affected individuals."

Establishes the serum/urine disaccharide signature in all analyzed affected individuals, making it a candidate biomarker rather than a single-patient observation.

Aberrant Heparan Sulfate Chain Length and Sulfation Pattern (ABNORMAL)

Show evidence (1 reference)

PMID:28148688 SUPPORT In Vitro

"extracted ion chromatogram liquid chromatography–mass spectrometry revealed an abnormal HS sulfation pattern in P1’s fibroblasts, with increase in 6-O–sulfated and 2-O–sulfated disaccharides, which was corrected by EXTL3 overexpression"

Documents the specific sulfation abnormality and its correction by wild-type EXTL3, establishing both the readout and its causal attribution.

🔬

Diagnosis

6

Molecular confirmation by exome or genome sequencing

There are no formal consensus diagnostic criteria. Diagnosis rests on molecular confirmation of biallelic EXTL3 variants, typically by trio exome or genome sequencing (or a skeletal-dysplasia / inborn-error-of-immunity panel containing EXTL3), with familial segregation confirmed by Sanger sequencing. Because the radiographic phenotype overlaps several morphologically similar spondyloepimetaphyseal dysplasias, sequencing is what discriminates the entity rather than imaging alone.

genetic testing NCIT:C15709

Show evidence (2 references)

PMID:35114981 SUPPORT Human Clinical

"Whole exome sequencing analysis revealed the presence of a homozygous known pathogenic variant c.953C > T (p. Pro318Leu) in exon 3 of the EXTL3 gene, thereby confirming diagnosis of ISDNA."

Exome sequencing is the confirmatory test that establishes the diagnosis.

PMID:28331220 SUPPORT Human Clinical

"By Sanger sequencing, we confirmed that the variant was heterozygous in their parents in both families"

Parental Sanger confirmation is the segregation step supporting a recessive molecular diagnosis.

Craniovertebral junction imaging for cord compression

Cervical imaging is the single most consequential investigation in this disorder, because craniovertebral junction stenosis with cord compression is common, is the main actionable complication, and is surgically correctable with good outcome when detected early. Both MRI and CT have been used to demonstrate canal stenosis, odontoid and C1-arch hypoplasia, and cord impingement. This is also the basis for avoiding unassessed neck manipulation or positioning in an affected child.

magnetic resonance imaging procedure NCIT:C16809

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Magnetic resonance imaging revealed canal stenosis and cord compression at the cranio-vertebral junction"

MRI is the modality that demonstrates the actionable craniovertebral lesion.

PMID:28331220 SUPPORT Human Clinical

"The complication should be recognized because its early detection followed by appropriate surgical treatment is a key to good prognosis."

States explicitly that early detection of cord compression determines prognosis, which is the rationale for making this imaging a priority.

Skeletal survey

A radiographic skeletal survey establishes the spondyloepimetaphyseal pattern — severe platyspondyly with increased intervertebral spaces, metaphyseal broadening, pelvic distortion with narrow sacroischiatic notches, and brachydactyly — which is what raises the diagnosis pre-molecularly.

radiograph imaging procedure NCIT:C38101

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Her skeletal survey showed broad thorax, severe platyspondyly with thoracolumbar kyphosis, broad ilia with short greater notches, and broad ischia and pubes as well as metaphyseal broadening of the long tubular bones and severe brachydactyly"

Enumerates the radiographic pattern the skeletal survey is performed to detect.

Immunological evaluation and newborn TREC screening

Because immunodeficiency is present in most but not all reported patients and is not predicted by genotype, immune evaluation is required in every affected individual rather than only in those with infections. Assessment covers lymphocyte subsets, immunoglobulins, and T-cell receptor excision circles (TRECs). Severe cases are detectable by routine newborn SCID screening, which has already ascertained at least one patient before the onset of infection.

flow cytometry procedure NCIT:C16585

Show evidence (2 references)

PMID:28148688 SUPPORT Human Clinical

"She was ascertained after positive newborn screening for SCID (T cell excision circles, TRECs of 12/µL blood, with the normal range being >25/µL; control actin copy number was normal)."

Documents ascertainment of an affected infant through the standard newborn TREC screen.

PMID:28148688 SUPPORT Human Clinical

"this study expands the number of SCID-related disorders that can be identified with newborn screening."

The authors position ISDNA as newborn-screening-detectable, supporting TREC screening as a case-finding route.

Visceral and cardiac surveillance imaging

Hepatic (and less often renal) cysts are a recurrent extra-triad feature detectable on abdominal ultrasonography, sometimes prenatally; because they are unusual in infants they also serve as a diagnostic clue. Echocardiography has disclosed structural cardiac findings in individual patients.

ultrasonography procedure NCIT:C19337

Show evidence (2 references)

PMID:28331220 SUPPORT Human Clinical

"Abdominal ultrasonography demonstrated enlarged liver with a few cystic lesions."

Ultrasonography is the modality detecting the hepatic cysts.

PMID:28331220 SUPPORT Human Clinical

"Because of its rarity in infants, the association of the liver cyst seems not fortuitous"

Supports treating infantile hepatic cysts as a diagnostically meaningful clue rather than an incidental finding.

Normal mucopolysaccharidosis screening does not exclude the diagnosis

A practically important negative: urinary glycosaminoglycan screening and leukocyte lysosomal enzyme assays — the standard first-line tests when a storage disorder is suspected from coarse features and dysostosis — are normal in ISDNA. A normal MPS screen therefore does not exclude EXTL3 deficiency, and the disaccharide-resolved serum/urine signature recorded in the biochemical section is a different assay from this bulk screen.

urinary GAG and leukocyte lysosomal enzyme screening NCIT:C25294

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"Laboratory examination, urine GAG screening and leukocyte lysosomal enzyme screening were normal."

Documents that routine storage-disorder screening is normal, so it cannot be used to rule the diagnosis out.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

Only ~16 affected individuals had been reported at the time of the most recent case report. Reported ancestries include Turkish, North African, Portuguese, Colombian, Indian, Hispanic, and Chinese, indicating worldwide occurrence rather than a single geographic concentration. No population prevalence or carrier frequency has been established.

Show evidence (1 reference)

PMID:38010729 SUPPORT Human Clinical

"Immunoskeletal dysplasia with neurodevelopmental abnormalities (ISDNA) caused by Exostosin-Like Glycosyltransferase 3 (EXTL3) biallelic mutations is a very rare syndrome with only 16 cases reported in the literature."

Gives the published case count, the basis for the CASES_IN_LITERATURE measure and ULTRA_RARE class.

🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities:

Other immuno-osseous dysplasias

Overlapping Features The closest differential is the small group of disorders combining skeletal dysplasia with immune dysfunction — collectively termed immune-osseous dysplasias. These are distinguished from ISDNA by their causal genes, which is why molecular testing rather than radiography resolves the differential.

Distinguishing Features

Schimke immuno-osseous dysplasia is caused by SMARCAL1, cartilage-hair hypoplasia by RMRP, and spondyloenchondrodysplasia with immune dysregulation by ACP5, whereas ISDNA is caused by biallelic EXTL3 variants.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"The list includes Schimke immuno-osseous dysplasia (MIM 242900), cartilage–hair hypoplasia (MIM 250250) and spondyloenchondrodysplasia with immune dysregulation (MIM 607944), which are caused by mutations in SMARCAL1 (MIM 606622), RMRP (MIM 157660) and ACP5 (MIM 171640), respectively."

Names the immune-osseous dysplasia differentials and their distinct causal genes.

Morphologically similar spondyloepimetaphyseal dysplasias

Overlapping Features Before molecular testing, ISDNA is readily mistaken for other severe SEMDs — particularly opsismodysplasia and odontochondrodysplasia — because all share non-lethality, extreme platyspondyly, and severe brachydactyly. The source describes this overlap as a recognised cause of nosological confusion and misdiagnosis.

Distinguishing Features

Distinguished by identification of biallelic EXTL3 variants; the known causal genes of the mimicking SEMDs were excluded in the reported patients.

Show evidence (1 reference)

PMID:28331220 SUPPORT Human Clinical

"the patients may be mistaken for opsismodysplasia and odontochondrodysplasia because of their non-lethality, extreme platyspondyly and severe brachydactyly"

Names the two SEMDs most easily confused with ISDNA and the shared features driving the confusion.

{ }

Source YAML

click to show

name: Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities
creation_date: "2026-07-30T00:00:00Z"
category: Mendelian
description: >
  Immunoskeletal dysplasia with neurodevelopmental abnormalities (ISDNA; also
  called neuro-immuno-skeletal dysplasia syndrome, or EXTL3-deficient
  spondyloepimetaphyseal dysplasia) is an ultra-rare autosomal recessive
  disorder caused by biallelic missense variants in EXTL3, which encodes the
  N-acetylglucosaminyltransferase that initiates and elongates heparan sulfate
  (HS) chains on proteoglycan core proteins. Hypomorphic EXTL3 alleles reduce
  glycosyltransferase activity and yield structurally aberrant HS — longer
  chains with an abnormal sulfation pattern — rather than simple absence of HS.
  Because HS proteoglycans act as co-receptors that spatially tune morphogen
  and cytokine signaling, the biochemical defect is transduced in
  tissue-specific and opposite directions: FGF2-ERK signaling is potentiated in
  fibroblasts (driving the skeletal dysplasia, whose radiographic appearance
  mimics FGFR3-related dysplasia), while IL-2/IL-7-driven STAT5 phosphorylation
  in lymphocytes is markedly reduced. The clinical result is a triad spanning
  three organ systems: a spondyloepimetaphyseal dysplasia with severe
  platyspondyly, brachydactyly, and craniovertebral-junction stenosis that
  frequently requires early decompression; a highly variable T-cell-predominant
  immunodeficiency ranging from mild T-cell lymphopenia to Omenn-like severe
  combined immunodeficiency; and neurodevelopmental impairment. Hepatic and
  renal cysts are a recurrent extra-triad feature. Immunological severity is
  strikingly discordant even between individuals homozygous for the same
  allele, implying unidentified modifiers. Roughly 16 individuals have been
  reported; about a third of the reviewed cohort died before one year of age,
  chiefly from infection.
disease_term:
  preferred_term: immunoskeletal dysplasia with neurodevelopmental abnormalities
  term:
    id: MONDO:0044312
    label: immunoskeletal dysplasia with neurodevelopmental abnormalities
synonyms:
- ISDNA
- Neuro-immuno-skeletal dysplasia syndrome
- Spondyloepimetaphyseal dysplasia with immune deficiency, EXTL3 type
- EXTL3-deficient spondyloepimetaphyseal dysplasia
parents:
- Skeletal Dysplasia
- Combined Immunodeficiency
classifications:
  iuis_category:
    classification_value: combined immunodeficiency with syndromic features
    notes: >-
      ISDNA is a syndromic combined immunodeficiency in the IUIS sense: the
      T-cell defect (ranging to T-B+NK+ SCID and Omenn-like presentations)
      occurs together with extra-immune syndromic features — skeletal dysplasia
      and neurodevelopmental impairment — that are themselves caused by the same
      lesion rather than being secondary to the immune defect.
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We studied three patients with severe skeletal dysplasia, T cell
        immunodeficiency, and developmental delay.
      explanation: >-
        Establishes the co-occurrence of T-cell immunodeficiency with skeletal
        and neurodevelopmental features that defines a syndromic combined
        immunodeficiency.
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      A monogenic autosomal recessive disorder of glycosaminoglycan
      biosynthesis; its multi-system presentation is best approached from the
      genetics side rather than any single organ chapter.
  icimd_category:
  - classification_value: o_linked_protein_glycosylation
    notes: >-
      ICIMD category 18 (disorders of O-linked protein glycosylation) explicitly
      subsumes glycosaminoglycan *synthesis*, which is where EXTL3 acts — it adds
      the GlcNAc to the O-linked tetrasaccharide linker that commits a chain to
      the heparan sulfate fate. Deliberately not classified under
      glycosaminoglycan_degradation: that category covers the
      mucopolysaccharidoses, the mechanistic opposite of this disorder, and the
      distinction is clinically load-bearing here because routine MPS screening
      is normal in ISDNA (see the diagnosis section).
    evidence:
    - reference: PMID:28331220
      reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        EXTL3 encodes N-acetylglucosaminyltransferase-I (GlcNAcT-I) and GlcNAcT-II,
        which transfers N-acetylglucosamine (GlcNAc) from uridine
        diphosphate-GlcNAc (UDP-GlcNAc) to the linkage region of heparan sulfate
        (HS) and heparin, and to the glucuronic acid (GlcUA) of growing glycan
        chains, respectively
      explanation: >-
        Establishes EXTL3 as a glycosyltransferase acting on the linkage region
        in glycosaminoglycan synthesis, placing the disorder in the O-linked
        glycosylation/GAG-synthesis category rather than a degradation category.
  isds_skeletal_category:
  - classification_value: spondylo_epi_metaphyseal_dysplasias
    notes: >-
      ISDS Nosology and Classification of Genetic Skeletal Disorders, 2019 revision
      (Mortier et al., PMID:31633310), Table 1 group 13 "Spondylo-epi-(meta)-physeal
      dysplasias (SE(M)D)"; listed as "SEMD with immune deficiency, EXTL3 type".
external_assertions:
- name: OMIM immunoskeletal dysplasia with neurodevelopmental abnormalities record
  source: OMIM
  assertion_type: disease_record
  external_id: OMIM:617425
  description: >-
    OMIM entry for immunoskeletal dysplasia with neurodevelopmental
    abnormalities (ISDNA), cataloguing the clinical features and molecular
    basis of the EXTL3-related phenotype.
inheritance:
- name: Autosomal Recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >
    ISDNA follows autosomal recessive inheritance. All reported disease alleles
    are germline missense variants carried in the homozygous state, with
    heterozygous unaffected parents confirmed by Sanger sequencing.
    Consanguinity contributed to ascertainment in several families but is not
    obligatory; the recurrent c.953C>T (p.Pro318Leu) allele lies in a shared
    homozygous stretch on chromosome 8, suggesting a common founder.
  evidence:
  - reference: PMID:38010033
    reference_title: "Immune skeletal dysplasia with neurodevelopmental abnormalities caused by a novel variant of EXTL3 gene in a Chinese family."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Immune skeletal dysplasia with neurodevelopmental abnormalities (ISDNA) is
      an extremely rare, autosomal recessive genetic disorder characterized by
      various skeletal abnormalities, neurodevelopmental deficits, and abnormal
      immune system function.
    explanation: >-
      States the autosomal recessive mode of inheritance and the three-system
      clinical definition of ISDNA.
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      By whole-exome sequencing, we identified homozygous missense mutations
      c.1382C>T, c.1537C>T, c.1970A>G, and c.2008T>G in EXTL3 in nine affected
      individuals from five unrelated families.
    explanation: >-
      Documents homozygous (biallelic) missense variants segregating in multiple
      unrelated families, consistent with recessive inheritance.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Only ~16 affected individuals had been reported at the time of the most
    recent case report. Reported ancestries include Turkish, North African,
    Portuguese, Colombian, Indian, Hispanic, and Chinese, indicating worldwide
    occurrence rather than a single geographic concentration. No population
    prevalence or carrier frequency has been established.
  evidence:
  - reference: PMID:38010729
    reference_title: "EXTL3-Associated Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities: A Lethal Phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Immunoskeletal dysplasia with neurodevelopmental abnormalities (ISDNA)
      caused by Exostosin-Like Glycosyltransferase 3 (EXTL3) biallelic mutations
      is a very rare syndrome with only 16 cases reported in the literature.
    explanation: >-
      Gives the published case count, the basis for the CASES_IN_LITERATURE
      measure and ULTRA_RARE class.
biochemical:
- name: Circulating and Urinary Glycosaminoglycan Disaccharide Signature
  presence: INCREASED
  biomarker_term:
    preferred_term: glycosaminoglycan disaccharides (D0a0, D0a4)
    term:
      id: NCIT:C28153
      label: Glycosaminoglycan
  specificity: >-
    Reported as the biochemical readout distinguishing EXTL3 deficiency from
    morphologically similar spondyloepimetaphyseal dysplasias. Specificity has
    not been formally quantified against a control or disease-comparator cohort;
    the supporting study reports the abnormality in all analyzed affected
    individuals but provides no sensitivity/specificity statistics.
  notes: >
    Biallelic EXTL3 variants produce a measurable glycosaminoglycan abnormality
    in body fluids, not only in cultured cells: total GAG concentrations are
    abnormal and the non-sulfated chondroitin disaccharide D0a0 and the
    disaccharide D0a4 are increased in serum and urine. This is the one
    biochemical readout reported to distinguish EXTL3 deficiency from
    morphologically similar spondyloepimetaphyseal dysplasias, and it is
    assayed on accessible fluids rather than requiring a fibroblast biopsy.
    Note the contrast with routine mucopolysaccharidosis screening, which is
    normal in this disorder (see the diagnosis section) — the abnormality is
    detected by disaccharide-resolved analysis, not by bulk urinary GAG
    screening.
  evidence:
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We observed abnormal glycosaminoglycan concentrations and increased
      concentrations of the non-sulfated chondroitin disaccharide D0a0 and the
      disaccharide D0a4 in serum and urine of all analyzed affected individuals.
    explanation: >-
      Establishes the serum/urine disaccharide signature in all analyzed
      affected individuals, making it a candidate biomarker rather than a
      single-patient observation.
- name: Aberrant Heparan Sulfate Chain Length and Sulfation Pattern
  presence: ABNORMAL
  biomarker_term:
    preferred_term: heparan sulfate
    term:
      id: NCIT:C540
      label: Heparan Sulfate
  specificity: >-
    A research assay rather than a clinical test: measured by LC-MS disaccharide
    analysis on patient-derived fibroblasts, so it requires a skin biopsy and is
    not a first-line diagnostic. Its value is mechanistic confirmation — the
    abnormality is corrected by wild-type EXTL3 complementation, which is what
    ties the structural HS defect causally to the variant.
  notes: >
    In patient-derived fibroblasts the quantity of heparan sulfate is
    essentially preserved while its structure is not: chains are longer and
    carry an increased proportion of 6-O- and 2-O-sulfated disaccharides. This
    is the cell-level assay corresponding to the mechanism the entry models —
    a qualitative, not quantitative, HS defect — and it is correctable by
    wild-type EXTL3 complementation, which is what establishes the variants as
    causal rather than incidental.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      extracted ion chromatogram liquid chromatography–mass spectrometry
      revealed an abnormal HS sulfation pattern in P1’s fibroblasts, with
      increase in 6-O–sulfated and 2-O–sulfated disaccharides, which was
      corrected by EXTL3 overexpression
    explanation: >-
      Documents the specific sulfation abnormality and its correction by
      wild-type EXTL3, establishing both the readout and its causal attribution.
genetic:
- name: EXTL3
  gene_term:
    preferred_term: EXTL3
    term:
      id: hgnc:3518
      label: EXTL3
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    EXTL3 (chromosome 8p21.2) encodes a Golgi bi-domain glycosyltransferase
    carrying GT47 and GT64 domains. It acts as N-acetylglucosaminyltransferase-I
    and -II, transferring GlcNAc from UDP-GlcNAc both to the tetrasaccharide
    linkage region (initiating the HS chain) and to the growing glucuronic acid
    terminus (elongating it). All reported ISDNA alleles are germline missense
    variants that reduce, but do not abolish, enzyme activity — consistent with
    the embryonic lethality of complete Extl3 loss in mouse. Reported alleles
    include c.953C>T (p.Pro318Leu), c.1015C>T (p.Arg339Trp), c.1382C>T
    (p.Pro461Leu), c.1537C>T (p.Arg513Cys), c.1970A>G (p.Asn657Ser), c.2008T>G
    (p.Tyr670Asp), and c.2015G>A (p.Arg672Gln).

    Two features of the allelic spectrum are notable. First, every reported
    disease allele to date is a missense change and all cluster in exon 3 — no
    nonsense, frameshift, or whole-gene deletion allele has been reported in a
    surviving patient, which is the human genetic counterpart of the embryonic
    lethality of complete Extl3 loss in mouse and implies that ISDNA is a
    partial-loss-of-function disease with a floor below which the genotype is
    not viable. Second, ISDNA alleles are essentially private: the variants were
    absent from dbSNP, 1000 Genomes, ExAC, ESP6500, and HGMD at the time of
    first reporting, or present only at very low frequency, consistent with a
    severe recessive disorder under selection. Penetrance of the skeletal
    phenotype appears complete in reported biallelic individuals, whereas the
    immune phenotype is incompletely penetrant even within a genotype (see the
    immune_severity_modifiers hypothesis).

    Beyond disease genetics, EXTL3 occupies the branch point of glycosaminoglycan
    biosynthesis: acting on the shared tetrasaccharide linker, it adds the
    GlcNAc that commits a chain to the heparan sulfate rather than chondroitin
    sulfate fate, and it does so with a preference for heparan-sulfate
    proteoglycan core-protein substrates. This is the biochemical reason an
    EXTL3 lesion perturbs HS specifically, and it is also why the disease
    biomarker is a chondroitin-disaccharide excess — the linker traffic that
    EXTL3 fails to commit to HS is measurable on the other branch.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The enzyme activity of the mutant EXTL3 protein was significantly
      decreased compared to the wild-type protein.
    explanation: >-
      Direct in vitro enzymology showing the p.Pro318Leu allele causes loss of
      GlcNAc transferase activity, establishing the variants as hypomorphic.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      EXTL3 encodes N-acetylglucosaminyltransferase-I (GlcNAcT-I) and GlcNAcT-II,
      which transfers N-acetylglucosamine (GlcNAc) from uridine
      diphosphate-GlcNAc (UDP-GlcNAc) to the linkage region of heparan sulfate
      (HS) and heparin, and to the glucuronic acid (GlcUA) of growing glycan
      chains, respectively
    explanation: Defines the dual enzymatic role of EXTL3 in HS chain initiation and elongation.
  - reference: PMID:38010033
    reference_title: "Immune skeletal dysplasia with neurodevelopmental abnormalities caused by a novel variant of EXTL3 gene in a Chinese family."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A novel homozygous variant, NM_001440: c.2015G>A (p.Arg672Gln) in the
      EXTL3 gene, was identified using WES, which has never been reported before.
    explanation: >-
      Expands the reported allelic spectrum with an additional homozygous
      missense variant in a Chinese family.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The EXTL3 variant was not deposited in any available databases, including
      dbSNP
    explanation: >-
      Supports the private, population-database-absent character of ISDNA
      alleles.
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All the variants reported in the EXTL3 gene till date, are missense
      variants, located in exon 3
    explanation: >-
      Establishes that the reported allelic spectrum is exclusively missense and
      exon-3 clustered.
  - reference: DOI:10.1093/glycob/cwae016
    reference_title: >-
      Chemo-enzymatic synthesis of tetrasaccharide linker peptides to study the
      divergent step in glycosaminoglycan biosynthesis
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Importantly, EXTL3 showed a preference towards peptides mimicking the core
      proteins of heparan sulfate proteoglycans over the ones from chondroitin
      sulfate proteoglycans. This suggests that EXTL3 could play a role in the
      decision-making step during glycosaminoglycan biosynthesis.
    explanation: >-
      Establishes EXTL3 as the enzyme governing the heparan-sulfate versus
      chondroitin-sulfate branch decision, the biochemical basis for why an
      EXTL3 lesion perturbs HS specifically and yields a chondroitin-disaccharide
      biomarker.
mechanistic_hypotheses:
- hypothesis_group_id: immune_severity_modifiers
  hypothesis_label: Unidentified modifiers determine immunological severity
  status: EMERGING
  description: >
    The most conspicuous unexplained feature of ISDNA is that immunological
    severity does not track with variant position or identity: individuals
    homozygous for the identical allele (both p.Pro318Leu and p.Pro461Leu) have
    ranged from Omenn-like SCID to a clinically intact immune system, while the
    skeletal phenotype remains comparatively uniform. This dissociation implies
    that genetic modifiers or environmental factors — not the EXTL3 lesion
    alone — gate whether the HS defect is transduced into thymic/hematopoietic
    failure. No modifier locus has been identified.
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Notably, patients harbouring the same variant in EXTL3 gene show
      phenotypic variability especially with respect to presence or absence of
      immunological manifestations, suggesting a role of unknown modifiers.
    explanation: >-
      States the same-genotype/divergent-immune-phenotype observation and
      attributes it explicitly to unknown modifiers.
pathophysiology:
- name: EXTL3 Glycosyltransferase Deficiency
  biological_scale: MOLECULAR
  description: >
    Biallelic hypomorphic missense variants reduce the GlcNAc-transferase
    activity of the Golgi enzyme EXTL3, which initiates and elongates heparan
    sulfate chains on proteoglycan core proteins. Activity is reduced rather
    than abolished; complete Extl3 loss is embryonically lethal in mouse at
    ~E9.5, before skeletal elements differentiate, so surviving human disease
    necessarily reflects partial function.
  molecular_functions:
  - preferred_term: heparan sulfate N-acetylglucosaminyltransferase activity
    modifier: DECREASED
    term:
      id: GO:0042328
      label: heparan sulfate N-acetylglucosaminyltransferase activity
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Exlt3 knockout mice (Exlt3−/−) are embryonic lethal at 9 days post coitum
      when the differentiated skeletal elements are not formed.
    explanation: >-
      Establishes that complete EXTL3 loss is incompatible with skeletal
      development, supporting the interpretation that human disease alleles are
      hypomorphic rather than null.
  - reference: DOI:10.1038/s41467-022-31048-2
    reference_title: "The structure of EXTL3 helps to explain the different roles of bi-domain exostosins in heparan sulfate synthesis"
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Here, we report the structure of human exostosin-like 3 (EXTL3) in apo and
      UDP-bound forms.
    explanation: >-
      Provides the cryo-EM structural basis of the bi-domain (GT47/GT64)
      architecture within which the disease-causing missense substitutions act.
  downstream:
  - target: Aberrant Heparan Sulfate Chain Structure
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Altogether, these results indicate that the EXTL3 R339W mutation is a
        hypomorphic allele that alters the function of the enzyme and leads to
        longer HS chains with an aberrant sulfation pattern.
      explanation: >-
        Reduced EXTL3 enzyme function is shown to produce the aberrant HS chain
        structure, establishing this edge.
- name: Aberrant Heparan Sulfate Chain Structure
  biological_scale: MOLECULAR
  description: >
    The consequence of reduced EXTL3 activity is not loss of heparan sulfate but
    structurally abnormal HS. Patient fibroblasts produce a quantitatively
    normal amount of HS, yet the chains are abnormally long and carry an
    aberrant sulfation pattern with increased 6-O- and 2-O-sulfated
    disaccharides; wild-type EXTL3 cDNA corrects the defect. Systemically,
    affected individuals show abnormal glycosaminoglycan concentrations in serum
    and urine, including elevated non-sulfated chondroitin disaccharides —
    consistent with EXTL3 acting at the branch point between heparan sulfate and
    chondroitin sulfate assembly.
  biological_processes:
  - preferred_term: glycosaminoglycan biosynthetic process
    modifier: ABNORMAL
    term:
      id: GO:0006024
      label: glycosaminoglycan biosynthetic process
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Altogether, these results indicate that the EXTL3 R339W mutation is a
      hypomorphic allele that alters the function of the enzyme and leads to
      longer HS chains with an aberrant sulfation pattern.
    explanation: >-
      States the central biochemical lesion — qualitatively abnormal HS
      (longer, aberrantly sulfated), not absent HS.
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Patient-derived fibroblasts showed abnormal HS composition and altered
      fibroblast growth factor 2 signaling, which was rescued by overexpression
      of wild-type EXTL3 cDNA.
    explanation: >-
      Complementation with wild-type EXTL3 rescues both the HS defect and its
      signaling consequence, establishing causality.
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We observed abnormal glycosaminoglycan concentrations and increased
      concentrations of the non-sulfated chondroitin disaccharide D0a0 and the
      disaccharide D0a4 in serum and urine of all analyzed affected individuals.
    explanation: >-
      Demonstrates the biochemical defect is detectable systemically in patient
      body fluids, not only in cultured cells.
  downstream:
  - target: Potentiated FGF2-ERK Signaling
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        These results indicate that by altering HS chain length and composition,
        EXTL3 mutations potentiate FGF2 signaling, thereby contributing to the
        pathophysiology of the skeletal dysplasia observed in the patients.
      explanation: >-
        Directly links the altered HS chain length and composition to potentiated
        FGF2 signaling.
  - target: Impaired Cytokine-STAT5 Signaling
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Interleukin-2-mediated STAT5 phosphorylation in patients' lymphocytes was
        markedly reduced.
      explanation: >-
        The HS defect in patient cells is accompanied by markedly reduced
        IL-2-driven STAT5 phosphorylation, the lymphoid arm of the same lesion.
  - target: Impaired Thymopoiesis and Progenitor Expansion
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Targeted differentiation of patient-derived induced pluripotent stem cells
        showed a reduced expansion of lymphohematopoietic progenitor cells and
        defects of thymic epithelial progenitor cell differentiation.
      explanation: >-
        Patient-derived cells carrying the HS defect show the progenitor-expansion
        and thymic-epithelial differentiation failure this edge asserts.
  - target: Neurodevelopmental Impairment
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        These data identify EXTL3 mutations as a novel cause of severe immune
        deficiency with skeletal dysplasia and developmental delay and underline a
        crucial role of HS in thymopoiesis and skeletal and brain development.
      explanation: >-
        Assigns brain development, alongside thymopoiesis and skeleton, to the
        HS-dependent mechanism, supporting a direct HS-defect origin for the
        neurodevelopmental arm rather than a purely secondary one. Note the
        authors' claim rests partly on model-system and cell data; the
        isdna_neuro_primary_vs_secondary knowledge gap records that the
        primary-versus-secondary question remains unresolved in patients.
- name: Potentiated FGF2-ERK Signaling
  biological_scale: CELLULAR
  description: >
    Longer HS chains bearing excess 6-O-sulfate provide additional FGF receptor
    binding sites, so the HS defect paradoxically increases rather than
    decreases growth-factor signaling in mesenchymal lineages. Patient
    fibroblasts show enhanced ERK phosphorylation on FGF2 stimulation, corrected
    by wild-type EXTL3. This explains why the radiographic phenotype at birth
    resembles the FGFR3-related dysplasias driven by constitutively increased
    FGFR signaling, even though FGFR1/2/3 are wild-type in these patients.
  biological_processes:
  - preferred_term: fibroblast growth factor receptor signaling pathway
    modifier: INCREASED
    term:
      id: GO:0008543
      label: fibroblast growth factor receptor signaling pathway
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      These results indicate that by altering HS chain length and composition,
      EXTL3 mutations potentiate FGF2 signaling, thereby contributing to the
      pathophysiology of the skeletal dysplasia observed in the patients.
    explanation: >-
      Directly attributes the skeletal arm of the disease to potentiated (not
      reduced) FGF2 signaling downstream of the HS defect.
  downstream:
  - target: Spondyloepimetaphyseal Dysplasia
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        These results indicate that by altering HS chain length and composition,
        EXTL3 mutations potentiate FGF2 signaling, thereby contributing to the
        pathophysiology of the skeletal dysplasia observed in the patients.
      explanation: >-
        Explicitly attributes the skeletal dysplasia to the potentiated FGF2
        signaling upstream of this edge.
- name: Impaired Cytokine-STAT5 Signaling
  biological_scale: CELLULAR
  description: >
    In the lymphoid compartment the same HS defect acts in the opposite
    direction. HS modulates responses to IL-2 and IL-7, the cytokines that drive
    T-cell survival and homeostatic expansion; patient lymphocytes show markedly
    reduced STAT5 phosphorylation after IL-2 stimulation, with a subtler defect
    after IL-7. This opposite-direction transduction of one biochemical lesion
    is the mechanistic core of the multisystem character of the disease.
  cell_types:
  - preferred_term: T cell
    term:
      id: CL:0000084
      label: T cell
  biological_processes:
  - preferred_term: cytokine-mediated signaling pathway
    modifier: DECREASED
    term:
      id: GO:0019221
      label: cytokine-mediated signaling pathway
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Interleukin-2-mediated STAT5 phosphorylation in patients' lymphocytes was
      markedly reduced.
    explanation: >-
      Documents the lymphoid signaling defect that distinguishes the immune arm
      from the FGF-potentiated skeletal arm.
  downstream:
  - target: T-Cell-Predominant Immunodeficiency
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Interleukin-2-mediated STAT5 phosphorylation in patients' lymphocytes was
        markedly reduced.
      explanation: >-
        Reduced IL-2-STAT5 signaling in patient lymphocytes is the proximate
        signaling defect underlying the T-cell phenotype.
- name: Impaired Thymopoiesis and Progenitor Expansion
  biological_scale: CELLULAR
  description: >
    The immunodeficiency has both hematopoietic-intrinsic and thymic-stromal
    components. EXTL3 is most abundant in hematopoietic stem cells and early
    progenitor T cells, and patient-derived iPSCs show reduced expansion of
    lymphohematopoietic progenitors together with defective differentiation of
    thymic epithelial progenitors (reduced TBX1, EYA1, and CK5 with persistent
    SOX17). extl3-mutant zebrafish crossed to a rag2:GFP thymic reporter show
    roughly halved thymic volume, rescued by injection of wild-type human EXTL3
    RNA. Because the thymic stroma is itself affected, hematopoietic stem cell
    transplantation cannot be assumed to correct the whole immune phenotype.
  cell_types:
  - preferred_term: thymic epithelial cell
    term:
      id: CL:0002293
      label: epithelial cell of thymus
  - preferred_term: hematopoietic stem cell
    term:
      id: CL:0000037
      label: hematopoietic stem cell
  biological_processes:
  - preferred_term: thymus development
    modifier: DECREASED
    term:
      id: GO:0048538
      label: thymus development
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Targeted differentiation of patient-derived induced pluripotent stem cells
      showed a reduced expansion of lymphohematopoietic progenitor cells and
      defects of thymic epithelial progenitor cell differentiation.
    explanation: >-
      Establishes the dual hematopoietic-intrinsic and thymic-stromal basis of
      the T-cell defect in patient-derived human cells.
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green
      fluorescent protein) transgenic zebrafish revealed defective thymopoiesis,
      which was rescued by injection of wild-type human EXTL3 RNA.
    explanation: >-
      In vivo functional complementation confirming EXTL3 is required for
      thymopoiesis.
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EXTL3 was most abundant in hematopoietic stem cells and early progenitor T
      cells, which is in line with a SCID phenotype at the level of early T cell
      development in the thymus.
    explanation: >-
      Expression data localizing the block to early intrathymic T-cell
      development.
  downstream:
  - target: T-Cell-Predominant Immunodeficiency
    evidence:
    - reference: PMID:28132690
      reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        EXTL3 was most abundant in hematopoietic stem cells and early progenitor T
        cells, which is in line with a SCID phenotype at the level of early T cell
        development in the thymus.
      explanation: >-
        Localizes the developmental block to early intrathymic T-cell development,
        connecting impaired thymopoiesis to the SCID phenotype.
- name: Spondyloepimetaphyseal Dysplasia
  biological_scale: TISSUE
  description: >
    Disrupted HS-dependent growth-plate signaling produces a
    spondyloepimetaphyseal dysplasia: disproportionate short-limb short stature,
    severe platyspondyly with widened intervertebral spaces, brachydactyly,
    epiphyseal and metaphyseal irregularity, pelvic distortion with
    trident-shaped acetabula and narrow sacro-ischiatic notches, and progressive
    kyphoscoliosis. The craniovertebral junction is the clinically decisive
    site: odontoid hypoplasia and cervical canal stenosis cause cord compression
    that has required decompressive surgery in infancy across multiple reports.
    Premature craniosynostosis, including cloverleaf skull, occurs at the severe
    end.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we describe a novel type of SEMD in two unrelated Turkish patients who
      presented with severe platyspondyly, kyphoscoliosis, pelvic distortion,
      constriction of the proximal femora and brachydactyly.
    explanation: Defines the core skeletal presentation as a spondyloepimetaphyseal dysplasia.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both patients had spinal cord compression at the cranio-vertebral junction
      and multiple liver cysts since early infancy.
    explanation: >-
      Documents the craniovertebral cord compression that drives early
      neurosurgical management, alongside the recurrent hepatic cysts.
  downstream:
  - target: Cervical Cord Compression
    evidence:
    - reference: PMID:28331220
      reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Both patients had spinal cord compression at the cranio-vertebral junction
        and multiple liver cysts since early infancy.
      explanation: >-
        The skeletal dysplasia produced craniovertebral-junction cord compression
        in both reported patients.
- name: Cervical Cord Compression
  biological_scale: ORGANISM
  description: >
    Odontoid hypoplasia, non-ossification of the atlas, and cervical canal
    stenosis converge on compression of the upper cervical cord. This is the
    single most actionable complication of ISDNA: it is detectable by early
    cervical imaging, is correctable by C1-arch excision or decompression, and
    if unrecognised leads to irreversible neurological injury and neurogenic
    bladder. It is a downstream structural consequence of the skeletal
    dysplasia, mechanistically distinct from the primary neurodevelopmental
    impairment.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All three patients had narrowing of the cervical canal, and severe
      narrowing of the laryngotracheal tract was present in P1 and P2.
    explanation: >-
      Documents cervical canal narrowing in every patient of the founding
      series, with associated airway narrowing.
- name: T-Cell-Predominant Immunodeficiency
  biological_scale: ORGANISM
  description: >
    The immune phenotype is T-cell predominant and strikingly variable. Reported
    profiles include T-B+NK+ SCID with absent TRECs, Omenn-like disease with
    oligoclonal activated/memory autologous T cells and exfoliative
    erythroderma, isolated T-cell lymphopenia with hypogammaglobulinemia, and —
    in some individuals with the same genotype — no clinically apparent immune
    defect. Impaired mitogen proliferation, eosinophilia, hypogammaglobulinemia,
    and elevated IgE are recurrent laboratory findings. Spontaneous partial
    recovery of T-cell number and vaccine responsiveness has been documented in
    at least one survivor. Absence of overt immunodeficiency does not exclude
    the diagnosis.
  evidence:
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Severe combined immunodeficiency (SCID) with a complete absence of T cells
      was observed in three families.
    explanation: Documents the severe end of the immunological spectrum.
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Notably, patients harbouring the same variant in EXTL3 gene show
      phenotypic variability especially with respect to presence or absence of
      immunological manifestations, suggesting a role of unknown modifiers.
    explanation: >-
      Establishes the genotype-independent variability of the immune phenotype
      that motivates the modifier hypothesis.
- name: Neurodevelopmental Impairment
  biological_scale: ORGANISM
  description: >
    Global developmental delay — particularly gross motor — with central or
    axial hypotonia and intellectual disability of variable degree is a
    defining component of the syndrome. Severe presentations include seizures,
    opisthotonus, hyperreflexia, nystagmus, and developmental arrest. Because
    HS proteoglycans regulate morphogen signaling in neural progenitors, this
    arm is regarded as a primary consequence of the HS defect rather than
    solely secondary to cord compression, although the two coexist and their
    relative contributions have not been separated in patients.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neurological abnormalities included: opisthotonus, hyperreflexia,
      generalized seizures, and developmental delay in P1; clonic arm movements,
      nystagmus, and developmental arrest in P2; and muscular hypotonia and
      marked developmental delay in P3.
    explanation: >-
      Enumerates the neurological spectrum across the three index patients.
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      These data identify EXTL3 mutations as a novel cause of severe immune
      deficiency with skeletal dysplasia and developmental delay and underline a
      crucial role of HS in thymopoiesis and skeletal and brain development.
    explanation: >-
      Attributes brain development, alongside thymopoiesis and skeleton, to the
      HS-dependent mechanism.
phenotypes:
- name: Disproportionate Short Stature
  category: Skeletal
  description: >
    Short-limb disproportionate short stature is present from birth or early
    infancy in essentially all reported individuals.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Disproportionate short stature
    term:
      id: HP:0003498
      label: Disproportionate short stature
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The common presenting symptoms included short limb dwarfism,
      platyspondyly, brachydactyly and epiphyseal abnormalities.
    explanation: >-
      Literature review of 14 cases lists short-limb dwarfism among the common
      presenting features.
- name: Platyspondyly
  category: Skeletal
  description: >
    Severe platyspondyly with widened intervertebral spaces is the single most
    characteristic radiographic finding.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Platyspondyly
    term:
      id: HP:0000926
      label: Platyspondyly
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we describe a novel type of SEMD in two unrelated Turkish patients who
      presented with severe platyspondyly, kyphoscoliosis, pelvic distortion,
      constriction of the proximal femora and brachydactyly.
    explanation: Documents severe platyspondyly as a presenting radiographic feature.
- name: Brachydactyly
  category: Skeletal
  description: Short, stout metacarpals and phalanges, often with a trident hand appearance.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Brachydactyly
    term:
      id: HP:0001156
      label: Brachydactyly
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The common presenting symptoms included short limb dwarfism,
      platyspondyly, brachydactyly and epiphyseal abnormalities.
    explanation: Lists brachydactyly among the common presenting skeletal features.
- name: Epiphyseal Dysplasia
  category: Skeletal
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Epiphyseal dysplasia
    term:
      id: HP:0002656
      label: Epiphyseal dysplasia
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The common presenting symptoms included short limb dwarfism,
      platyspondyly, brachydactyly and epiphyseal abnormalities.
    explanation: Lists epiphyseal abnormalities among the common presenting features.
- name: Metaphyseal Dysplasia
  category: Skeletal
  description: Metaphyseal broadening and irregularity of the long tubular bones.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Metaphyseal dysplasia
    term:
      id: HP:0100255
      label: Metaphyseal dysplasia
  evidence:
  - reference: PMID:34089299
    reference_title: "Spondyloepimetaphyseal dysplasia EXTL3-deficient type: Long-term follow-up and review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Spondyloepimetaphyseal dysplasia (SEMD) is a group of genetic skeletal
      disorders characterized by disproportionate short stature, and varying
      degrees of vertebral, epiphyseal, and metaphyseal involvement of the
      skeleton.
    explanation: >-
      Places the disorder in the SEMD group, defined by combined vertebral,
      epiphyseal, and metaphyseal involvement.
- name: Kyphoscoliosis
  category: Skeletal
  description: Thoracolumbar kyphoscoliosis, often progressive and a driver of restrictive lung disease.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Kyphoscoliosis
    clinical_course: PROGRESSIVE
    term:
      id: HP:0002751
      label: Kyphoscoliosis
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we describe a novel type of SEMD in two unrelated Turkish patients who
      presented with severe platyspondyly, kyphoscoliosis, pelvic distortion,
      constriction of the proximal femora and brachydactyly.
    explanation: Documents kyphoscoliosis as a presenting skeletal feature.
- name: Hypoplasia of the Odontoid Process
  category: Skeletal
  description: >
    Odontoid hypoplasia with non-ossification of the atlas arches, contributing
    directly to craniovertebral instability and cord compression.
  phenotype_term:
    preferred_term: Hypoplasia of the odontoid process
    term:
      id: HP:0003311
      label: Hypoplasia of the odontoid process
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She had hypoplasia of the posterior C1 arch with narrowing of the cervical
      spinal canal and cord compression requiring neurosurgery.
    explanation: >-
      Documents hypoplasia of the C1 arch producing canal narrowing and cord
      compression that required surgery.
- name: Spinal Cord Compression
  category: Neurological
  description: >
    Compression of the upper cervical cord at the craniovertebral junction, the
    most surgically actionable complication; multiple reported patients required
    decompression in infancy.
  frequency: FREQUENT
  diagnostic: true
  phenotype_term:
    preferred_term: Spinal cord compression
    term:
      id: HP:0002176
      label: Spinal cord compression
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both patients had spinal cord compression at the cranio-vertebral junction
      and multiple liver cysts since early infancy.
    explanation: Documents craniovertebral-junction cord compression in both reported patients.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The complication should be recognized because its early detection followed
      by appropriate surgical treatment is a key to good prognosis.
    explanation: >-
      Supports flagging cord compression as a diagnostically and prognostically
      decisive finding requiring active surveillance.
- name: Coxa Valga
  category: Skeletal
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Coxa valga
    term:
      id: HP:0002673
      label: Coxa valga
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In the pelvis of P3, there is coxa valga with delayed ossification of
      femoral heads, acetabular dysplasia, and hip subluxation.
    explanation: Documents coxa valga with associated hip dysplasia and subluxation.
- name: Global Developmental Delay
  category: Neurological
  description: >
    Global developmental delay, most prominent in gross motor domains, with
    intellectual disability of variable severity.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Affected individuals presented with variable skeletal abnormalities and
      neurodevelopmental defects.
    explanation: Establishes neurodevelopmental impairment as a defining component.
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A 15-month-old female child with clinical indications of global
      developmental delay, short stature, coarse facial features, and hypotonia
      was referred to our clinic.
    explanation: Documents global developmental delay as a presenting feature.
- name: Intellectual Disability
  category: Neurological
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These data identify EXTL3 mutations as a novel cause of severe immune
      deficiency with skeletal dysplasia and developmental delay and underline a
      crucial role of HS in thymopoiesis and skeletal and brain development.
    explanation: >-
      Supports the developmental/cognitive arm of the phenotype as intrinsic to
      the EXTL3 mechanism.
- name: Hypotonia
  category: Neurological
  description: Central or axial hypotonia, frequently with delayed head control.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A 15-month-old female child with clinical indications of global
      developmental delay, short stature, coarse facial features, and hypotonia
      was referred to our clinic.
    explanation: Documents hypotonia as a presenting neurological feature.
- name: Seizures
  category: Neurological
  description: Generalized seizures, reported at the severe end of the neurological spectrum.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neurological abnormalities included: opisthotonus, hyperreflexia,
      generalized seizures, and developmental delay in P1; clonic arm movements,
      nystagmus, and developmental arrest in P2; and muscular hypotonia and
      marked developmental delay in P3.
    explanation: Documents generalized seizures in one of three index patients.
- name: Opisthotonus
  category: Neurological
  notes: >-
    Frequency band intentionally omitted. This feature is documented in a
    single reported individual; against a total published cohort of roughly 16
    that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the
    denominator is small and ascertainment-biased toward severe presentations,
    so no band is asserted. See docs/frequency-evidence-guidelines.md.
  phenotype_term:
    preferred_term: Opisthotonus
    term:
      id: HP:0002179
      label: Opisthotonus
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neurological abnormalities included: opisthotonus, hyperreflexia,
      generalized seizures, and developmental delay in P1; clonic arm movements,
      nystagmus, and developmental arrest in P2; and muscular hypotonia and
      marked developmental delay in P3.
    explanation: Documents opisthotonus in a severely affected patient.
- name: Combined Immunodeficiency
  category: Immunological
  description: >
    T-cell-predominant combined immunodeficiency, ranging from mild T-cell
    lymphopenia to Omenn-like SCID. Nine of the first fourteen reported
    individuals had immunodeficiency.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Combined immunodeficiency
    term:
      id: HP:0005387
      label: Combined immunodeficiency
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      nine of the 14 patients of ISDNA
    explanation: >-
      Provides the 9/14 count underpinning the FREQUENT frequency band; the
      sentence continues "had immunodeficiency" across a hyphenated line break
      in the source PDF, so only the contiguous fragment is quoted.
- name: Severe Combined Immunodeficiency
  category: Immunological
  description: >
    T-B+NK+ SCID with absent or very low TRECs, detectable by newborn screening;
    observed in three of five families in the founding cohort.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Severe combined immunodeficiency
    term:
      id: HP:0004430
      label: Severe combined immunodeficiency
  evidence:
  - reference: PMID:28132690
    reference_title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Severe combined immunodeficiency (SCID) with a complete absence of T cells
      was observed in three families.
    explanation: Documents SCID with complete T-cell absence in three of five families.
- name: Decreased Total T Cell Count
  category: Immunological
  description: T-cell lymphopenia, often with a T-B+NK+ profile.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Decreased total T cell count
    term:
      id: HP:0005403
      label: Decreased total T cell count
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Immunological examination showed decrease of T-lymphocyte subset, IgG and
      IgM.
    explanation: Documents reduced T-lymphocyte subsets alongside immunoglobulin deficiency.
  - reference: PMID:34089299
    reference_title: "Spondyloepimetaphyseal dysplasia EXTL3-deficient type: Long-term follow-up and review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Disproportionate short stature, mild developmental delay and a T- NK+ B+
      immunological profile were detected in the postnatal follow-up.
    explanation: Documents the characteristic T-B+NK+ immunological profile.
- name: Decreased Circulating Immunoglobulin Concentration
  category: Immunological
  description: Hypogammaglobulinemia, frequently requiring immunoglobulin replacement.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Decreased circulating immunoglobulin concentration
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypogammaglobulinemia but increased IgE serum levels were detected in P1
      and P2.
    explanation: Documents hypogammaglobulinemia with paradoxically elevated IgE.
- name: Increased Circulating IgE Concentration
  category: Immunological
  description: >
    Elevated serum IgE, characteristically together with eosinophilia in the
    Omenn-like presentations.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Increased circulating IgE concentration
    term:
      id: HP:0003212
      label: Increased circulating IgE concentration
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypogammaglobulinemia but increased IgE serum levels were detected in P1
      and P2.
    explanation: Documents the elevated IgE that accompanies the Omenn-like phenotype.
- name: Eosinophilia
  category: Immunological
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Increased total eosinophil count
    term:
      id: HP:0001880
      label: Increased total eosinophil count
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Impaired proliferation to mitogens and eosinophilia was documented in all
      three infants.
    explanation: Documents eosinophilia alongside impaired mitogen proliferation.
- name: Recurrent Infections
  category: Immunological
  description: >
    Recurrent bacterial and fungal infections, including sepsis and recurrent
    pulmonary infection; the leading cause of death in infancy.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Recurrent infections
    term:
      id: HP:0002719
      label: Recurrent infections
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of five patients died before the age of one year, primarily due to
      recurrent infections.
    explanation: >-
      Establishes recurrent infection as the dominant cause of early mortality
      across the reviewed cohort.
- name: Recurrent Respiratory Infections
  category: Immunological
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Recurrent respiratory infections
    term:
      id: HP:0002205
      label: Recurrent respiratory infections
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      During follow-up, she showed recurrent episodes of pulmonary infection and
      susceptibility to dental caries.
    explanation: >-
      Documents recurrent pulmonary infection in a patient with reduced T cells
      and immunoglobulins.
- name: Oral Candidiasis
  category: Immunological
  description: >
    Oral candidiasis, notable because it occurred in a patient without otherwise
    overt immunodeficiency — evidence that subclinical immune impairment may be
    more common than the formal counts suggest.
  notes: >-
    Intentionally left without an HPO binding. The source documents a single
    episode in one patient and does not establish chronicity or recurrence, but
    every available HPO oral-candidiasis term asserts one or the other:
    HP:0009098 (Chronic oral candidiasis) is_a HP:0002728 (Chronic mucocutaneous
    candidiasis), and the siblings under that branch are likewise chronic,
    recurrent, or site-specific. Binding to any of them would assert a temporal
    course the evidence does not support, so per the project convention of
    preferring no term over a misleading one the descriptor carries a
    preferred_term only. An HPO new-term request for a course-neutral "Oral
    candidiasis" would be the correct resolution.
  phenotype_term:
    preferred_term: Oral candidiasis
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Although patient 2 did not present overt immune deficiency, she had oral
      candidiasis, a fungal infection often found in immune-deficient patients.
    explanation: >-
      Documents candidiasis as a marker of possible subclinical immune impairment
      in a patient classified as immunologically normal; the source describes a
      single case and does not establish chronicity, so support is partial.
- name: Acetabular Dysplasia
  category: Skeletal
  description: >
    Beyond the pelvic distortion of the underlying dysplasia, hip-joint
    instability is documented: acetabular dysplasia with delayed ossification of
    the femoral heads and frank subluxation, alongside coxa valga. This matters
    for management because hip surveillance and orthopaedic intervention are
    separable from the spinal disease that otherwise dominates care.
  phenotype_term:
    preferred_term: Acetabular dysplasia
    term:
      id: HP:0008807
      label: Acetabular dysplasia
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In the pelvis of P3, there is coxa valga with delayed ossification of
      femoral heads, acetabular dysplasia, and hip subluxation.
    explanation: >-
      Documents acetabular dysplasia together with delayed femoral head
      ossification and hip subluxation.
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A pelvic radiograph showed bilateral coxa vara, dysplastic changes of both
      hips, and subluxation of the left hip.
    explanation: >-
      Independent radiographic confirmation of bilateral hip dysplasia with
      subluxation at follow-up.
- name: Hip Subluxation
  category: Skeletal
  description: >
    Subluxation of the hip, curated separately from the acetabular dysplasia it
    accompanies because it is the displacement itself — not the socket
    morphology — that drives orthopaedic decision-making.
  phenotype_term:
    preferred_term: Hip subluxation
    term:
      id: HP:0030043
      label: Hip subluxation
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A pelvic radiograph showed bilateral coxa vara, dysplastic changes of both
      hips, and subluxation of the left hip.
    explanation: Documents hip subluxation on pelvic radiography.
- name: Ventricular Septal Defect
  category: Cardiovascular
  description: >
    A small fenestrated ventricular septal defect was found on echocardiography
    in one reported patient. Structural cardiac findings are not a defining
    feature of the syndrome and have been reported in single individuals only
    (mitral valve prolapse in another), but they are the reason echocardiography
    appears in the diagnostic evaluation.
  phenotype_term:
    preferred_term: Ventricular septal defect
    term:
      id: HP:0001629
      label: Ventricular septal defect
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      a small fenestrated ventricular septal defect
    explanation: >-
      Documents the ventricular septal defect detected by echocardiography in
      the reported patient.
- name: Hepatic Cysts
  category: Other
  description: >
    Multiple hepatic cysts, present from early infancy and sometimes detectable
    prenatally. Their occurrence at this age is sufficiently unusual that the
    original authors judged the association non-fortuitous.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hepatic cysts
    term:
      id: HP:0001407
      label: Hepatic cysts
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both patients had spinal cord compression at the cranio-vertebral junction
      and multiple liver cysts since early infancy.
    explanation: Documents multiple hepatic cysts from early infancy in both patients.
  - reference: PMID:34089299
    reference_title: "Spondyloepimetaphyseal dysplasia EXTL3-deficient type: Long-term follow-up and review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report a new patient who is currently 15 years old in whom cystic liver
      lesions were detected in the prenatal period.
    explanation: Documents prenatal detectability of the hepatic cysts.
- name: Renal Cysts
  category: Other
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Renal cyst
    term:
      id: HP:0000107
      label: Renal cyst
  evidence:
  - reference: PMID:38010729
    reference_title: "EXTL3-Associated Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities: A Lethal Phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Skeletal dysplasia, neurodevelopmental delay, immunodeficiency, liver, and
      kidney cysts are the most common findings of this syndrome.
    explanation: Lists kidney cysts among the recurrent findings of ISDNA.
- name: Coarse Facial Features
  category: Craniofacial
  description: >
    Coarse facies with frontal bossing, full cheeks, depressed nasal bridge,
    anteverted nares, and micrognathia. Facial dysmorphism was present in all
    reviewed patients.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Coarse facial features
    term:
      id: HP:0000280
      label: Coarse facial features
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A 15-month-old female child with clinical indications of global
      developmental delay, short stature, coarse facial features, and hypotonia
      was referred to our clinic.
    explanation: >-
      Documents coarse facial features as a presenting finding; the same report's
      literature review states facial dysmorphism was present in all reviewed
      patients.
- name: Craniosynostosis
  category: Skeletal
  description: >
    Premature craniosynostosis, including cloverleaf skull deformity, restricted
    to the severe end of the spectrum.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Premature craniosynostosis was seen in the skull x ray and computed
      tomography studies of patient 1 (P1) and P2, with cloverleaf deformity in
      P2.
    explanation: >-
      Documents premature craniosynostosis with cloverleaf skull in two of three
      index patients.
- name: Laryngeal Stenosis
  category: Respiratory
  description: >
    Severe laryngotracheal narrowing, contributing to neonatal respiratory
    distress and ventilator dependence in severely affected infants.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Laryngeal stenosis
    term:
      id: HP:0001602
      label: Laryngeal stenosis
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All three patients had narrowing of the cervical canal, and severe
      narrowing of the laryngotracheal tract was present in P1 and P2.
    explanation: Documents laryngotracheal narrowing in two of three index patients.
- name: Mitral Valve Prolapse
  category: Cardiovascular
  notes: >-
    Frequency band intentionally omitted. This feature is documented in a
    single reported individual; against a total published cohort of roughly 16
    that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the
    denominator is small and ascertainment-biased toward severe presentations,
    so no band is asserted. See docs/frequency-evidence-guidelines.md.
  phenotype_term:
    preferred_term: Mitral valve prolapse
    term:
      id: HP:0001634
      label: Mitral valve prolapse
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Echocardiography showed mitral valve prolapse.
    explanation: Documents mitral valve prolapse in a single reported patient.
- name: Anal Atresia
  category: Other
  notes: >-
    Frequency band intentionally omitted. This feature is documented in a
    single reported individual; against a total published cohort of roughly 16
    that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the
    denominator is small and ascertainment-biased toward severe presentations,
    so no band is asserted. See docs/frequency-evidence-guidelines.md.
  phenotype_term:
    preferred_term: Anal atresia
    term:
      id: HP:0002023
      label: Anal atresia
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      there was also anal atresia, for which a colostomy was performed in the
      first days of life
    explanation: Documents anal atresia requiring neonatal colostomy in one patient.
- name: Carious Teeth
  category: Other
  notes: >-
    Frequency band intentionally omitted. This feature is documented in a
    single reported individual; against a total published cohort of roughly 16
    that is ~6%, which falls in OCCASIONAL rather than VERY_RARE, but the
    denominator is small and ascertainment-biased toward severe presentations,
    so no band is asserted. See docs/frequency-evidence-guidelines.md.
  phenotype_term:
    preferred_term: Carious teeth
    term:
      id: HP:0000670
      label: Carious teeth
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      During follow-up, she showed recurrent episodes of pulmonary infection and
      susceptibility to dental caries.
    explanation: Documents susceptibility to dental caries during follow-up.
- name: Failure to Thrive
  category: Other
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In her first year of life, she had severe failure to thrive and required
      high calorie supplements and tube feedings with a G-tube.
    explanation: Documents severe failure to thrive requiring gastrostomy feeding.
diagnosis:
- name: Molecular confirmation by exome or genome sequencing
  description: >
    There are no formal consensus diagnostic criteria. Diagnosis rests on
    molecular confirmation of biallelic EXTL3 variants, typically by trio exome
    or genome sequencing (or a skeletal-dysplasia / inborn-error-of-immunity
    panel containing EXTL3), with familial segregation confirmed by Sanger
    sequencing. Because the radiographic phenotype overlaps several
    morphologically similar spondyloepimetaphyseal dysplasias, sequencing is
    what discriminates the entity rather than imaging alone.
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole exome sequencing analysis revealed the presence of a homozygous
      known pathogenic variant c.953C > T (p. Pro318Leu) in exon 3 of the EXTL3
      gene, thereby confirming diagnosis of ISDNA.
    explanation: >-
      Exome sequencing is the confirmatory test that establishes the diagnosis.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      By Sanger sequencing, we confirmed that the variant was heterozygous in
      their parents in both families
    explanation: >-
      Parental Sanger confirmation is the segregation step supporting a
      recessive molecular diagnosis.
- name: Craniovertebral junction imaging for cord compression
  description: >
    Cervical imaging is the single most consequential investigation in this
    disorder, because craniovertebral junction stenosis with cord compression is
    common, is the main actionable complication, and is surgically correctable
    with good outcome when detected early. Both MRI and CT have been used to
    demonstrate canal stenosis, odontoid and C1-arch hypoplasia, and cord
    impingement. This is also the basis for avoiding unassessed neck
    manipulation or positioning in an affected child.
  diagnosis_term:
    preferred_term: magnetic resonance imaging procedure
    term:
      id: NCIT:C16809
      label: Magnetic Resonance Imaging
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Magnetic resonance imaging revealed canal stenosis and cord compression at
      the cranio-vertebral junction
    explanation: >-
      MRI is the modality that demonstrates the actionable craniovertebral
      lesion.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The complication should be recognized because its early detection followed
      by appropriate surgical treatment is a key to good prognosis.
    explanation: >-
      States explicitly that early detection of cord compression determines
      prognosis, which is the rationale for making this imaging a priority.
- name: Skeletal survey
  description: >
    A radiographic skeletal survey establishes the spondyloepimetaphyseal
    pattern — severe platyspondyly with increased intervertebral spaces,
    metaphyseal broadening, pelvic distortion with narrow sacroischiatic
    notches, and brachydactyly — which is what raises the diagnosis
    pre-molecularly.
  diagnosis_term:
    preferred_term: radiograph imaging procedure
    term:
      id: NCIT:C38101
      label: X-Ray Imaging
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Her skeletal survey showed broad thorax, severe platyspondyly with
      thoracolumbar kyphosis, broad ilia with short greater notches, and broad
      ischia and pubes as well as metaphyseal broadening of the long tubular
      bones and severe brachydactyly
    explanation: >-
      Enumerates the radiographic pattern the skeletal survey is performed to
      detect.
- name: Immunological evaluation and newborn TREC screening
  description: >
    Because immunodeficiency is present in most but not all reported patients
    and is not predicted by genotype, immune evaluation is required in every
    affected individual rather than only in those with infections. Assessment
    covers lymphocyte subsets, immunoglobulins, and T-cell receptor excision
    circles (TRECs). Severe cases are detectable by routine newborn SCID
    screening, which has already ascertained at least one patient before the
    onset of infection.
  diagnosis_term:
    preferred_term: flow cytometry procedure
    term:
      id: NCIT:C16585
      label: Flow Cytometry
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She was ascertained after positive newborn screening for SCID (T cell
      excision circles, TRECs of 12/µL blood, with the normal range being
      >25/µL; control actin copy number was normal).
    explanation: >-
      Documents ascertainment of an affected infant through the standard newborn
      TREC screen.
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      this study expands the number of SCID-related disorders that can be
      identified with newborn screening.
    explanation: >-
      The authors position ISDNA as newborn-screening-detectable, supporting
      TREC screening as a case-finding route.
- name: Visceral and cardiac surveillance imaging
  description: >
    Hepatic (and less often renal) cysts are a recurrent extra-triad feature
    detectable on abdominal ultrasonography, sometimes prenatally; because they
    are unusual in infants they also serve as a diagnostic clue. Echocardiography
    has disclosed structural cardiac findings in individual patients.
  diagnosis_term:
    preferred_term: ultrasonography procedure
    term:
      id: NCIT:C19337
      label: Diagnostic Ultrasound
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Abdominal ultrasonography demonstrated enlarged liver with a few cystic
      lesions.
    explanation: Ultrasonography is the modality detecting the hepatic cysts.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Because of its rarity in infants, the association of the liver cyst seems
      not fortuitous
    explanation: >-
      Supports treating infantile hepatic cysts as a diagnostically meaningful
      clue rather than an incidental finding.
- name: Normal mucopolysaccharidosis screening does not exclude the diagnosis
  description: >
    A practically important negative: urinary glycosaminoglycan screening and
    leukocyte lysosomal enzyme assays — the standard first-line tests when a
    storage disorder is suspected from coarse features and dysostosis — are
    normal in ISDNA. A normal MPS screen therefore does not exclude EXTL3
    deficiency, and the disaccharide-resolved serum/urine signature recorded in
    the biochemical section is a different assay from this bulk screen.
  diagnosis_term:
    preferred_term: urinary GAG and leukocyte lysosomal enzyme screening
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Laboratory examination, urine GAG screening and leukocyte lysosomal enzyme
      screening were normal.
    explanation: >-
      Documents that routine storage-disorder screening is normal, so it cannot
      be used to rule the diagnosis out.
differential_diagnoses:
- name: Other immuno-osseous dysplasias
  description: >
    The closest differential is the small group of disorders combining skeletal
    dysplasia with immune dysfunction — collectively termed immune-osseous
    dysplasias. These are distinguished from ISDNA by their causal genes, which
    is why molecular testing rather than radiography resolves the differential.
  distinguishing_features:
  - >-
    Schimke immuno-osseous dysplasia is caused by SMARCAL1, cartilage-hair
    hypoplasia by RMRP, and spondyloenchondrodysplasia with immune dysregulation
    by ACP5, whereas ISDNA is caused by biallelic EXTL3 variants.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The list includes Schimke immuno-osseous dysplasia (MIM 242900),
      cartilage–hair hypoplasia (MIM 250250) and spondyloenchondrodysplasia with
      immune dysregulation (MIM 607944), which are caused by mutations in
      SMARCAL1 (MIM 606622), RMRP (MIM 157660) and ACP5 (MIM 171640),
      respectively.
    explanation: >-
      Names the immune-osseous dysplasia differentials and their distinct causal
      genes.
- name: Morphologically similar spondyloepimetaphyseal dysplasias
  description: >
    Before molecular testing, ISDNA is readily mistaken for other severe SEMDs —
    particularly opsismodysplasia and odontochondrodysplasia — because all share
    non-lethality, extreme platyspondyly, and severe brachydactyly. The source
    describes this overlap as a recognised cause of nosological confusion and
    misdiagnosis.
  distinguishing_features:
  - >-
    Distinguished by identification of biallelic EXTL3 variants; the known
    causal genes of the mimicking SEMDs were excluded in the reported patients.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the patients may be mistaken for opsismodysplasia and
      odontochondrodysplasia because of their non-lethality, extreme
      platyspondyly and severe brachydactyly
    explanation: >-
      Names the two SEMDs most easily confused with ISDNA and the shared
      features driving the confusion.
treatments:
- name: Cervical Decompression Surgery
  description: >
    Decompression of the craniovertebral junction — typically C1-arch excision —
    for cord compression or cervical instability. This is the highest-yield
    intervention in ISDNA: early detection followed by surgery is explicitly
    identified as the key to good neurological prognosis, and multiple reported
    patients underwent it in infancy with good outcome.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_mechanisms:
  - target: Cervical Cord Compression
    treatment_effect: INHIBITS
    description: >-
      Surgical decompression relieves the mechanical compression of the cervical
      cord produced by odontoid hypoplasia and canal stenosis.
    evidence:
    - reference: PMID:28331220
      reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        She received an excision of the C1 arch for the cord compression with good
        outcome.
      explanation: >-
        Documents the decompressive procedure acting on this mechanism node with a
        good clinical outcome.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She received an excision of the C1 arch for the cord compression with good
      outcome.
    explanation: Documents C1-arch excision with good outcome for cord compression.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The complication should be recognized because its early detection followed
      by appropriate surgical treatment is a key to good prognosis.
    explanation: Establishes early surgical decompression as prognostically decisive.
- name: Immunoglobulin Replacement Therapy
  description: >
    Monthly intravenous immunoglobulin for clinically significant antibody
    deficiency. Because immune function can improve spontaneously, replacement
    may be reassessed over time — in one reported patient it was successfully
    discontinued after partial immune recovery with subsequent demonstrable
    vaccine responses.
  treatment_term:
    preferred_term: immunoglobulin infusion therapy
    term:
      id: NCIT:C62710
      label: Immunoglobulin Therapy
  target_mechanisms:
  - target: T-Cell-Predominant Immunodeficiency
    treatment_effect: BYPASSES
    description: >-
      Passive immunoglobulin substitutes for the deficient humoral response
      rather than correcting the underlying HS or thymic defect.
    evidence:
    - reference: PMID:28331220
      reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Currently, she is 2 years old and has been treated with monthly intravenous
        immunoglobulin injection.
      explanation: >-
        Documents immunoglobulin replacement used to manage the antibody-deficiency
        component of this node.
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Currently, she is 2 years old and has been treated with monthly intravenous
      immunoglobulin injection.
    explanation: Documents monthly IVIG as the management of the antibody deficiency.
- name: Hematopoietic Stem Cell Transplantation
  description: >
    HSCT may be considered for life-threatening cellular immunodeficiency, but
    with an important mechanistic caveat specific to ISDNA: because EXTL3
    deficiency also impairs thymic epithelial (stromal) differentiation and
    other non-hematopoietic tissues, replacing the hematopoietic compartment
    alone should not be assumed to correct the immune phenotype. No
    disease-specific response rates have been published.
  therapeutic_modality: CELL_THERAPY
  treatment_term:
    preferred_term: hematopoietic stem cell transplantation
    term:
      id: NCIT:C15431
      label: Hematopoietic Cell Transplantation
  target_mechanisms:
  - target: Impaired Thymopoiesis and Progenitor Expansion
    treatment_effect: RESTORES
    description: >-
      Transplantation replaces the defective hematopoietic progenitor
      compartment, but cannot correct the thymic epithelial (stromal) arm of the
      same node, which remains EXTL3-deficient in the host.
    evidence:
    - reference: PMID:28148688
      reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Targeted differentiation of patient-derived induced pluripotent stem cells
        showed a reduced expansion of lymphohematopoietic progenitor cells and
        defects of thymic epithelial progenitor cell differentiation.
      explanation: >-
        The thymic-stromal component of this node is why transplantation of the
        hematopoietic compartment alone may be insufficient.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Targeted differentiation of patient-derived induced pluripotent stem cells
      showed a reduced expansion of lymphohematopoietic progenitor cells and
      defects of thymic epithelial progenitor cell differentiation.
    explanation: >-
      The demonstrated thymic-stromal (non-hematopoietic) component is the basis
      for caution that HSCT may not correct the full immune phenotype; no
      clinical transplant outcome data exist for this disorder.
- name: Nutritional Support and Gastrostomy Feeding
  description: >
    Severe failure to thrive is common and has required high-calorie
    supplementation and gastrostomy tube feeding. Split out from general
    supportive care because it is the one nutritional intervention explicitly
    documented in a reported patient rather than inferred from the phenotype.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In her first year of life, she had severe failure to thrive and required
      high calorie supplements and tube feedings with a G-tube.
    explanation: >-
      Documents high-calorie supplementation and gastrostomy feeding for failure
      to thrive in a reported patient.
- name: Genetic Counseling and Prenatal Testing
  description: >
    Because ISDNA is autosomal recessive with a 25% recurrence risk and
    consanguinity is frequent among reported families, genetic counseling with
    carrier testing and prenatal or preimplantation testing is a core part of
    management. Molecular confirmation in the proband is the prerequisite that
    makes accurate prenatal testing possible, which is why a "proband-first"
    testing strategy is recommended when a further pregnancy is under
    consideration.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      in order to guide them with robust prenatal counseling and testing in the
      current pregnancy.
    explanation: >-
      Documents the proband-first molecular diagnosis being used to enable
      prenatal counseling and testing in an ongoing pregnancy.
  - reference: PMID:38010033
    reference_title: "Immune skeletal dysplasia with neurodevelopmental abnormalities caused by a novel variant of EXTL3 gene in a Chinese family."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      More comprehensive genetic counseling and precise prenatal diagnosis for
      the next pregnancy can also be provided to families with genetic
      disorders.
    explanation: >-
      States that molecular diagnosis enables genetic counseling and prenatal
      diagnosis for subsequent pregnancies.
- name: Supportive and Rehabilitative Care
  description: >
    The genuine remainder of phenotype-directed supportive care once the
    separately curated interventions (cervical decompression, immunoglobulin
    replacement, transplantation, nutritional support, genetic counseling) are
    accounted for: developmental and rehabilitative therapy, standard
    antiseizure treatment, respiratory monitoring, and surveillance of hepatic
    and renal cysts. There is no disease-modifying therapy for EXTL3 deficiency
    and no disease-specific interventional trial. This entry deliberately
    retains the generic supportive-care term because the individual component
    therapies, while recommended in review sources, are not documented for a
    specific reported patient in any reference cached for this entry.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both patients also had multiple liver cysts in infancy.
    explanation: >-
      Establishes the hepatic cyst burden that motivates ongoing visceral
      surveillance as part of supportive care.
discussions:
- discussion_id: isdna_immune_severity_modifiers
  kind: KNOWLEDGE_GAP
  prompt: >
    What modifiers determine whether an individual with biallelic EXTL3 variants
    develops Omenn-like SCID versus a clinically intact immune system?
  attaches_to:
  - pathophysiology#T-Cell-Predominant Immunodeficiency
  rationale: >
    Immunological severity is discordant between individuals homozygous for the
    identical EXTL3 allele — including unrelated patients sharing p.Pro318Leu or
    p.Pro461Leu — while the skeletal phenotype stays comparatively uniform.
    Variant position therefore does not predict immune outcome, and the
    responsible genetic or environmental modifiers are unidentified. This is the
    single largest barrier to prognostication and to deciding which infants
    warrant transplant evaluation.
  proposed_experiments:
  - experiment_id: exp_isdna_modifier_screen
    name: Modifier screening across the reported ISDNA cohort
    description: >-
      Genome-wide modifier screening across the full reported ISDNA cohort,
      focusing on heparan-sulfate-biosynthesis and thymic-development loci, to
      identify variants segregating with immunological severity independently of
      the EXTL3 genotype.
  - experiment_id: exp_isdna_isogenic_ipsc_background_swap
    name: Isogenic iPSC background-swap panel
    description: >-
      Isogenic iPSC panels carrying each EXTL3 allele on shared and divergent
      genetic backgrounds, differentiated to thymic epithelial progenitors and
      lymphohematopoietic progenitors, to test whether genetic background alone
      reproduces the observed divergence in immune severity.
  evidence:
  - reference: PMID:35114981
    reference_title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Notably, patients harbouring the same variant in EXTL3 gene show
      phenotypic variability especially with respect to presence or absence of
      immunological manifestations, suggesting a role of unknown modifiers.
    explanation: Directly states the unresolved modifier question this gap records.
- discussion_id: isdna_neuro_primary_vs_secondary
  kind: KNOWLEDGE_GAP
  prompt: >
    Is the neurodevelopmental impairment in ISDNA a primary consequence of the
    heparan sulfate defect in the developing brain, or largely secondary to
    cervical cord compression and chronic illness?
  attaches_to:
  - pathophysiology#Neurodevelopmental Impairment
  - pathophysiology#Cervical Cord Compression
  rationale: >
    Cord compression, laryngotracheal narrowing, recurrent infection, and failure
    to thrive all independently impair development, and all co-occur with the
    neurological phenotype. No reported patient separates the two contributions,
    yet the distinction determines whether early cervical decompression should be
    expected to improve cognitive outcome or only to prevent further injury.
  proposed_experiments:
  - experiment_id: exp_isdna_decompression_neurodev_followup
    name: Pre- and post-decompression neurodevelopmental follow-up
    description: >-
      Longitudinal neurodevelopmental assessment of patients before and after
      early cervical decompression, compared against affected patients without
      cervical stenosis, to separate compression-attributable from
      HS-attributable developmental impairment.
  - experiment_id: exp_isdna_neural_conditional_extl3_ko
    name: Neural-progenitor-restricted conditional Extl3 deletion
    description: >-
      Conditional Extl3 deletion restricted to neural progenitors in mouse,
      bypassing the skeletal and cervical phenotype, to test whether
      neurodevelopmental impairment arises cell-autonomously in the brain.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      craniocervical magnetic resonance imaging showed impingement of the
      posterior arch of C1 on the medulla with mild stenosis of the cervical
      canal requiring neurosurgery
    explanation: >-
      Documents structural cord compression coexisting with the
      neurodevelopmental phenotype in the same patient — the confound that makes
      the primary-versus-secondary attribution unresolvable from reported cases.
  - reference: PMID:28331220
    reference_title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both patients had spinal cord compression at the cranio-vertebral
      junction, which caused serious neurological disorders in early infancy.
    explanation: >-
      Attributes serious early neurological disease directly to cord
      compression, stating the secondary-cause side of this open question.
- discussion_id: isdna_zebrafish_translational_validity
  kind: HUMAN_MODEL_MISMATCH
  prompt: >
    Does the extl3-mutant zebrafish thymic phenotype model the human immune
    disease closely enough to support therapeutic inference, given that the fish
    model does not reproduce the human spinal, cognitive, or longitudinal
    phenotype?
  attaches_to:
  - pathophysiology#Impaired Thymopoiesis and Progenitor Expansion
  rationale: >
    The zebrafish box mutant provides the strongest in vivo causal evidence in
    the disease — roughly halved thymic volume rescued by wild-type human EXTL3
    RNA — but reproduces only the thymic and fin phenotypes. Meanwhile complete
    Extl3 knockout in mouse is lethal at E9.5, so no mammalian model
    recapitulates the surviving hypomorphic human state. Evidence therefore
    exists in models, but its translational reach to human immune reconstitution
    is the open question.
  proposed_experiments:
  - experiment_id: exp_isdna_hypomorphic_knockin_mouse
    name: Hypomorphic knock-in mouse models
    description: >-
      Knock-in mouse models carrying the human hypomorphic alleles (p.Pro318Leu,
      p.Arg339Trp) rather than a null, to obtain a viable mammalian model that
      reproduces the partial-function human state.
  - experiment_id: exp_isdna_patient_thymic_organoid
    name: Patient-derived thymic organoids
    description: >-
      Patient-derived thymic organoid systems to test whether the thymic-stromal
      defect is cell-autonomous and whether it can be corrected independently of
      the hematopoietic compartment — the key question for HSCT rationale.
  evidence:
  - reference: PMID:28148688
    reference_title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green
      fluorescent protein) transgenic zebrafish revealed defective thymopoiesis,
      which was rescued by injection of wild-type human EXTL3 RNA.
    explanation: >-
      The model-system evidence whose translational validity to the human
      disease is the subject of this mismatch record.
references:
- reference: PMID:28132690
  title: "Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome."
- reference: PMID:28148688
  title: "EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay."
- reference: PMID:28331220
  title: "Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia."
- reference: PMID:35114981
  title: "An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report."
- reference: PMID:34089299
  title: "Spondyloepimetaphyseal dysplasia EXTL3-deficient type: Long-term follow-up and review of the literature."
- reference: PMID:38010729
  title: "EXTL3-Associated Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities: A Lethal Phenotype."
- reference: PMID:38010033
  title: "Immune skeletal dysplasia with neurodevelopmental abnormalities caused by a novel variant of EXTL3 gene in a Chinese family."
- reference: DOI:10.1038/s41467-022-31048-2
  title: "The structure of EXTL3 helps to explain the different roles of bi-domain exostosins in heparan sulfate synthesis"

📚

References & Deep Research

References

8

PMID:28132690

Mutations in EXTL3 Cause Neuro-immuno-skeletal Dysplasia Syndrome.

No top-level findings curated for this source.

PMID:28148688

EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay.

No top-level findings curated for this source.

PMID:28331220

Identification of biallelic EXTL3 mutations in a novel type of spondylo-epi-metaphyseal dysplasia.

No top-level findings curated for this source.

PMID:35114981

An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an Indian patient with homozygous c.953C > T variant in EXTL3 gene: a case report.

No top-level findings curated for this source.

PMID:34089299

Spondyloepimetaphyseal dysplasia EXTL3-deficient type: Long-term follow-up and review of the literature.

No top-level findings curated for this source.

PMID:38010729

EXTL3-Associated Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities: A Lethal Phenotype.

No top-level findings curated for this source.

PMID:38010033

Immune skeletal dysplasia with neurodevelopmental abnormalities caused by a novel variant of EXTL3 gene in a Chinese family.

No top-level findings curated for this source.

DOI:10.1038/s41467-022-31048-2

The structure of EXTL3 helps to explain the different roles of bi-domain exostosins in heparan sulfate synthesis

No top-level findings curated for this source.

Deep Research

1

Falcon ▸

Disease Characteristics Research Template

Edison Scientific Literature 20 citations 2026-07-31T00:16:10.247344

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.

Disease Characteristics Research Template

Target Disease

Disease Name: Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities
MONDO ID: (if available)
Category: Mendelian

Research Objectives

Please provide a comprehensive research report on Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities 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.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

What is the disease? Provide a concise overview.
What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
What are the common synonyms and alternative names?
Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
Risk Factors:

Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases
Genetic risk factors (causal variants, susceptibility loci, modifier genes)
Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
Protective Factors:

Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases
Genetic protective factors (protective variants, modifier alleles)
Environmental protective factors (diet, lifestyle, exposures that reduce risk)
Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

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

4. Genetic/Molecular Information

Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene
Pathogenic Variants:
Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
Variant type/class (missense, frameshift, nonsense, splice-site, structural)
Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
Functional consequences (loss of function, gain of function, dominant negative)
Modifier Genes: Genes that modify disease severity or expression
Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases
Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

Search first: CDC databases, WHO, PubMed, NHANES
Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc
Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

Search first: Gene Ontology (GO), Reactome, KEGG, PubMed
Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold
Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA
Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

Search first: ImmPort, Immunome Database, IEDB, Gene Ontology
Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

Search first: PubMed, Gene Ontology, Reactome
Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

Search first: BRENDA, UniProt, KEGG, OMIM, PubMed
Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Molecular Profiling (if available):
Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
Advanced Technologies (if applicable):
Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

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

7. Anatomical Structures Affected

Organ Level:
Primary organs directly affected
Secondary organ involvement (complications, secondary effects)
Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT
Tissue and Cell Level:
Specific tissue types affected (epithelial, connective, muscle, nervous)
Specific cell populations targeted (with Cell Ontology terms)

Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB
Subcellular Level:
Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas
Localization:
Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

Onset:
Typical age of onset (congenital, pediatric, adult, geriatric)
Onset pattern (acute, subacute, chronic, insidious)

Search first: OMIM, Orphanet, HPO, PubMed
Progression:
Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
Progression rate (rapid, slow, variable)
Disease course pattern (episodic, relapsing-remitting, progressive, stable)
Disease duration (self-limited, chronic lifelong)

Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM
Patterns:
Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

Epidemiology:
Prevalence (cases per 100,000 at given time)
Incidence (new cases per 100,000 per year)

Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries
For Genetic Etiology:
Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
Population Demographics:
Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
Geographic distribution of specific variants
Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

Clinical Tests:
Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
Genetic Testing:

Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen
Overview of recommended genetic testing approach
Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
FISH > Search first: ClinVar, cytogenetics databases, PubMed
Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
Omics-Based Diagnostics (if applicable):
RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
Clinical Criteria:
Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
Screening:
Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

Survival and Mortality:
Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
Morbidity and Function:
Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
Disease Course:
Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
Prediction:
Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

Pharmacotherapy:
Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
Advanced Therapeutics:
Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
Surgical and Interventional:
Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
Supportive and Rehabilitative:
Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
Experimental:
Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
Treatment Outcomes:
Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
Treatment Strategy:
Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest MAXO (Medical Action Ontology) terms where applicable.

13. Prevention

Prevention Levels:
Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
Immunization: Vaccine strategies (if applicable)

Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database
Screening and Early Detection:
Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
Behavioral Interventions: Lifestyle modifications to reduce risk

Search first: CDC, WHO, behavioral intervention databases, Cochrane Library
Counseling: Genetic counseling (risk assessment, family planning guidance)

Search first: NSGC resources, ACMG guidelines, GeneReviews
Public Health:
Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
Prophylaxis: Preventive medications or procedures

Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

Taxonomy: Species affected (with NCBI Taxon identifiers)

Search first: NCBI Taxonomy
Breed: Specific breeds affected (with VBO identifiers if applicable)

Search first: VBO (Vertebrate Breed Ontology)
Gene: Orthologous genes in other species (with NCBI Gene IDs)

Search first: NCBI Gene
Natural Disease:
Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
Comparative Biology:
Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
Transmission (if applicable):
Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

Model Types:
Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
Genetic Models:
Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
Model Characteristics:
Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
Applications:
Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
Resources:
Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

Cite primary literature (PMID preferred) for all mechanistic and clinical claims
Prioritize recent reviews and landmark papers
Include direct quotes from abstracts where possible to support key statements
Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

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, MAXO, 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 (MAXO terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities

Executive summary

Immunoskeletal dysplasia with neurodevelopmental abnormalities (ISDNA) is an ultra-rare, congenital, autosomal-recessive disorder caused by biallelic EXTL3 variants. It is best understood as a heparan-sulfate biosynthesis disorder combining spondyloepimetaphyseal dysplasia, neurodevelopmental impairment, and variably severe—sometimes fatal—T-cell immunodeficiency. Published manifestations range from relatively mild T-cell lymphopenia and developmental delay to Omenn-like severe combined immunodeficiency (SCID), profound skeletal disease, cervical cord compression, and death during infancy. The 2023 skeletal-disorder nosology favors gene-linked terminology such as EXTL3-related/EXTL3-deficient spondyloepimetaphyseal dysplasia. As of the searched literature through 2024, there is no disease-modifying therapy or disease-specific clinical trial; care is individualized and multidisciplinary. (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, unger2023nosologyofgenetic pages 29-30)

The following table provides an ontology-oriented synopsis; mappings marked “suggested” require validation against the current ontology release.

domain	high-confidence finding/statistic	suggested ontology terms/IDs	evidence type
Disease identity	EXTL3-related immunoskeletal dysplasia with neurodevelopmental abnormalities; ultra-rare Mendelian disorder; preferred disease entity linked to EXTL3	MONDO:0044312; OMIM:617425; suggested synonym mappings: “ISDNA”, “SEMD with immune deficiency, EXTL3 type”, “EXTL3 deficiency” (suggested)	Aggregated disease resource + expert nosology + primary human (OpenTargets Search: Immunoskeletal dysplasia with neurodevelopmental abnormalities, bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, unger2023nosologyofgenetic pages 29-30)
Causal gene	Biallelic pathogenic variants in EXTL3 (exostosin like glycosyltransferase 3) cause disease	EXTL3; suggested HGNC mapping: EXTL3 / exostosin-like glycosyltransferase 3	Human genetics + disease-target evidence (OpenTargets Search: Immunoskeletal dysplasia with neurodevelopmental abnormalities, bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 3-5)
Inheritance	Autosomal recessive; several families reported with consanguinity; parents heterozygous in informative families	HP:0000007 Autosomal recessive inheritance	Primary human (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Reported pathogenic variant set	Six missense alleles summarized in the literature through 2021-2022: c.953C>T p.Pro318Leu; c.1015C>T p.Arg339Trp; c.1382C>T p.Pro461Leu; c.1537C>T; c.1970A>G; c.2008T>G; all reported variants in early cohorts were missense and concentrated in exon 3 in several reports	Suggested Sequence Ontology class: missense_variant; ACMG classes reported as pathogenic/likely pathogenic in case literature (suggested)	Primary human + review synthesis (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Epidemiology	Extremely rare; 14 patients from 9 unrelated families summarized in 2021, and the 2022 Indian report described the 15th published patient	ORPHA/ICD not confirmed from retrieved sources; frequency not established	Review of published cases + case report (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Skeletal phenotype	Core skeletal pattern: disproportionate short stature/short-limb dwarfism, platyspondyly, brachydactyly, epiphyseal/metaphyseal abnormalities; platyspondyly is the most consistently emphasized feature	HP:0001511 Short stature; HP:0001156 Brachydactyly; HP:0000926 Platyspondyly; suggested HPO: short limbs, epiphyseal dysplasia, metaphyseal dysplasia, kyphoscoliosis	Primary human + review (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Craniovertebral/cervical phenotype	Odontoid hypoplasia, cervical instability/canal stenosis, craniovertebral junction compression are recurrent and clinically actionable; cervical complications required neurosurgical intervention in multiple patients	Suggested HPO: odontoid hypoplasia, cervical vertebral instability, spinal canal stenosis, spinal cord compression; UBERON suggested: cervical vertebral column, spinal cord	Primary human + long-term follow-up (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Pelvic/hip radiology	Trident-shaped acetabula, coxa valga, acetabular dysplasia, hip subluxation/dislocation, open iliac wings/narrow sacro-ischiatic notches reported	Suggested HPO: coxa valga, acetabular dysplasia, hip dislocation; UBERON suggested: pelvis, acetabulum, femur	Primary human radiology (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Craniofacial phenotype	Coarse facial features/facial dysmorphism common: full cheeks/lips, broad or bulbous nasal tip, depressed/prominent nasal bridge, frontal bossing, micrognathia, dysmorphic facies	HP:0001999 Facial asymmetry not established; suggested HPO: Coarse facial features, Full cheeks, Bulbous nose, Depressed nasal bridge, Frontal bossing, Micrognathia	Primary human + review (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Neurologic/neurodevelopmental phenotype	Global developmental delay, motor delay, hypotonia/truncal hypotonia, intellectual disability or borderline cognition; seizures in severe cases; corpus callosum thinning reported in one long-term follow-up case	HP:0001263 Global developmental delay; HP:0001252 Hypotonia; HP:0001249 Intellectual disability; HP:0001250 Seizure; suggested HPO: thinning of corpus callosum	Primary human (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Immune phenotype	Variable but often T-cell predominant immune deficiency; among published cases summarized in 2021-2022, 9/14 had T-cell lymphopenia/immunodeficiency; Omenn-like SCID reported in 4/14; one oral candidiasis case; recurrent severe infections contributed to early deaths	Suggested HPO: T-cell lymphopenia, Severe combined immunodeficiency, Omenn syndrome, Recurrent infections, Oral candidiasis, Hypogammaglobulinemia, Elevated IgE, Eosinophilia	Primary human + review (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Immune laboratory findings	T−NK+B+ or T−B+NK+ profiles reported; reduced T-cell subsets, low/absent TRECs, impaired mitogen responses, reduced IL-2/IL-7-induced STAT5 phosphorylation; hypogammaglobulinemia and hyper-IgE present in subsets	Suggested terms: SCID immunophenotype; GO: STAT5 phosphorylation; CL suggested: T cell, NK cell, B cell, thymic epithelial cell, hematopoietic progenitor cell	Primary human + functional assays (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-3, stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Respiratory/airway phenotype	Laryngotracheal narrowing reported; progressive kyphoscoliosis caused restrictive lung disease; severe obstructive sleep apnea documented; BiPAP recommended in long-term follow-up	Suggested HPO: laryngotracheal stenosis/narrowing, restrictive lung disease, obstructive sleep apnea, snoring; UBERON suggested: larynx, trachea, lung	Primary human (stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Visceral phenotype	Liver cysts and kidney cysts recur in a subset; hepatic cysts may be prenatal and persistent/stable over time; small ventricular septal defect reported in one case	Suggested HPO: Hepatic cysts, Renal cysts, Ventricular septal defect; UBERON suggested: liver, kidney, heart	Primary human (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Mortality/prognosis	Severe infantile presentations can be lethal; review of 14 cases noted 5 deaths before age 1 year, mainly from recurrent infections; survivors can show long-term disability, wheelchair dependence, and progressive orthopedic/respiratory burden	Suggested HPO: recurrent severe infections, failure to thrive, motor disability; prognosis terms not standardized here	Published case aggregation + longitudinal case (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Disease course	Usually congenital/infantile onset; chronic lifelong course with variable severity; some skeletal findings become less conspicuous with age while kyphoscoliosis and cervical complications may progress	Suggested onset term: congenital/infantile onset; suggested HPO: progressive kyphoscoliosis	Primary human longitudinal evidence (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Molecular mechanism	EXTL3 is a Golgi glycosyltransferase required for initiation/extension of heparan sulfate (HS) biosynthesis; disease-causing variants alter HS amount/composition/chain properties, disrupting growth-factor and cytokine signaling important for skeletal, thymic, and neurodevelopment	GO suggested: heparan sulfate proteoglycan biosynthetic process; glycosyltransferase activity; fibroblast growth factor receptor signaling pathway; cytokine-mediated signaling pathway; UBERON suggested: Golgi apparatus is GO-CC rather than UBERON	Primary human functional + structural biochemistry (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-1, wilson2022thestructureof pages 1-2, bourgeais2024chemoenzymaticsynthesisof pages 11-12)
Upstream biochemical defect	Patient fibroblasts showed abnormal HS composition and altered FGF2 signaling; wild-type EXTL3 cDNA rescued the signaling defect	GO suggested: fibroblast growth factor receptor signaling pathway; glycosaminoglycan biosynthetic process; CL suggested: fibroblast	Primary human in vitro rescue (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-1)
Immune pathophysiology	Reduced IL-2-mediated STAT5 phosphorylation in patient lymphocytes and defects in lymphohematopoietic progenitor expansion plus thymic epithelial progenitor differentiation support combined hematopoietic and thymic-stromal disease mechanisms	GO suggested: interleukin-2-mediated signaling pathway, STAT5 phosphorylation, thymus development, T cell differentiation; CL suggested: lymphocyte, hematopoietic progenitor cell, thymic epithelial cell	Primary human + iPSC (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)
Affected tissues/cells	Strong evidence implicates cartilage/bone, thymus/thymic epithelium, lymphohematopoietic progenitors, and brain/CNS development	UBERON suggested: vertebral column, pelvis, thymus, brain, spinal cord; CL suggested: chondrocyte, thymic epithelial cell, hematopoietic progenitor cell, T cell	Primary human + model organism + expert review (stefano2017extl3mutationscause pages 3-5, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, stefano2017extl3mutationscause pages 7-9)
Structural biology	2022 cryo-EM/structural work showed EXTL3 is a homodimeric bi-domain exostosin with GT47 and GT64 domains; GT47 is ineffective for GlcA transfer, supporting a non-processive/dissociative HS polymerization model	GO suggested: transferase activity, transferring glycosyl groups; protein domain labels GT47 and GT64 (suggested structural annotations)	Structural biochemistry (wilson2022thestructureof pages 1-2)
Biosynthetic specificity	2023-2024 work supports EXTL3 as a selective HS-initiating “decision” enzyme favoring HS-proteoglycan-like substrates over CS-proteoglycan-like substrates	GO suggested: heparan sulfate proteoglycan biosynthetic process; chondroitin sulfate biosynthetic process	Structural/biochemical in vitro (bourgeais2024chemoenzymaticsynthesisof pages 11-12)
Diagnostics	Diagnosis in practice relies on exome/genome sequencing or targeted molecular testing, combined with clinicoradiologic recognition of spondyloepimetaphyseal dysplasia plus neurodevelopmental and immune findings	Suggested methods/MAXO not needed; HPO pattern terms above; molecular testing approach: WES/WGS/EXTL3 single-gene or skeletal dysplasia/immunodeficiency panels (suggested)	Primary human case diagnosis + expert review (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Diagnostic workup adjuncts	Radiographs/spine imaging, brain MRI, immunophenotyping, immunoglobulins, lymphocyte subsets, TREC newborn screening where available, sleep study/airway evaluation, and renal/hepatic imaging are useful adjuncts	Suggested HPO/LOINC-equivalent concepts: lymphocyte subset assay, immunoglobulin quantification, spinal MRI, polysomnography, abdominal imaging	Primary human + expert review (stefano2017extl3mutationscause pages 1-3, stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Differential diagnosis frame	Differential diagnosis includes other immuno-osseous dysplasias and syndromic skeletal dysplasias with immune disease, such as Schimke immuno-osseous dysplasia, cartilage-hair hypoplasia, and SPENCDI	Suggested disease mappings only if separately curated; not specific IDs here	Expert clinical review (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Current management	No disease-specific approved therapy identified; management is multidisciplinary supportive care and surveillance	MAXO suggested: multidisciplinary care, surveillance, supportive care (suggested mappings)	Clinical practice extrapolation from case literature (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Immune management	Immune monitoring, infection prevention/treatment, and immunoglobulin replacement/SCID-directed care may be needed in severe immune phenotypes; caution that stromal thymic defects may limit hematopoietic-only correction	MAXO suggested: immunologic monitoring, antimicrobial therapy/prophylaxis, immunoglobulin replacement therapy, hematopoietic stem cell transplantation evaluation (suggested)	Primary human + expert thymus review (stefano2017extl3mutationscause pages 1-3, stefano2017extl3mutationscause pages 7-9, dinges2024primaryandsecondary pages 35-37)
Neurosurgical/orthopedic management	Early cervical surveillance and decompression for cord compression/instability, kyphoscoliosis surgery when indicated, orthopedic follow-up, mobility aids, and rehabilitation are real-world implementations documented in cases	MAXO suggested: spinal decompression surgery, scoliosis corrective surgery, orthopedic management, physical therapy, mobility support	Primary human longitudinal care (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Respiratory/supportive management	Sleep-study-guided airway support including BiPAP in obstructive sleep apnea, monitoring for restrictive lung disease, and airway assessment in laryngotracheal narrowing	MAXO suggested: noninvasive ventilation, polysomnography-guided management, respiratory monitoring	Primary human (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3, stefano2017extl3mutationscause pages 1-3)
Prevention/genetic counseling	Because inheritance is AR, recurrence-risk counseling, carrier testing in relatives, prenatal diagnosis, and preimplantation genetic testing are appropriate preventive strategies when familial variants are known	MAXO suggested: genetic counseling; suggested prevention concepts: carrier screening, prenatal molecular diagnosis, PGT-M	Human genetics practice based on AR disease (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Environmental factors	No specific environmental, lifestyle, infectious, or toxin exposures were identified as causal in retrieved disease-specific literature; immune complications are secondary rather than causal	Not applicable / no validated ontology mapping from retrieved evidence	Evidence gap from retrieved literature (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6)
Natural disease in other species	No naturally occurring veterinary counterpart was identified in retrieved sources	NCBI Taxon not applicable from retrieved evidence	Evidence gap
Zebrafish model	extl3-mutant zebrafish show defective thymopoiesis, reduced thymus volume, pectoral fin/cartilage defects, and shortened body axis; wild-type human EXTL3 RNA rescues thymic and fin phenotypes	Suggested Taxon: Danio rerio; CL suggested: thymocyte/T-cell progenitor; UBERON suggested: thymus, pectoral fin cartilage	Primary model-organism rescue evidence (stefano2017extl3mutationscause pages 3-5, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, stefano2017extl3mutationscause pages 1-1)
Mouse model	Extl3-null mice are embryonic lethal around E9.5, limiting full postnatal disease modeling; HS-deficient thymus models support a role for HS in thymus growth	Suggested Taxon: Mus musculus; GO suggested: embryonic development, thymus development	Mouse genetics + mechanistic support (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, dinges2024primaryandsecondary pages 35-37)
Human cellular models	Patient fibroblasts and patient-derived iPSCs recapitulate signaling and developmental defects, including abnormal HS composition, altered FGF2 signaling, reduced lymphohematopoietic progenitor expansion, and impaired thymic epithelial progenitor differentiation	CL suggested: fibroblast, induced pluripotent stem cell, hematopoietic progenitor cell, thymic epithelial progenitor cell	Primary in vitro/iPSC (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)

Table: This compact table summarizes high-confidence, ontology-ready facts for EXTL3-related immunoskeletal dysplasia with neurodevelopmental abnormalities, including identifiers, phenotypes, mechanisms, diagnostics, management, and models. It is designed to support structured disease knowledge-base curation while clearly marking suggested mappings and evidence types.

1. Disease information

Definition and classification

ISDNA is a Mendelian glycosaminoglycan-biosynthesis disorder in the spondyloepimetaphyseal dysplasia group. The characteristic combination is disproportionate short stature and vertebral/epiphyseal/metaphyseal abnormalities, developmental delay or intellectual disability, and variable cellular immunodeficiency. Liver or renal cysts, airway disease, and cervical instability may broaden the phenotype. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Identifiers and nomenclature

MONDO: MONDO:0044312.
OMIM phenotype: MIM 617425.
Causal target: EXTL3, Ensembl ENSG00000012232. Open Targets associates this disease uniquely with EXTL3 and cites the two foundational 2017 reports, PMID 28132690 and 28148688. (OpenTargets Search: Immunoskeletal dysplasia with neurodevelopmental abnormalities)
Common names: immunoskeletal dysplasia with neurodevelopmental abnormalities; ISDNA; EXTL3 deficiency; EXTL3-related immunoskeletal dysplasia; spondyloepimetaphyseal dysplasia with immune deficiency, EXTL3 type; SEMD, EXTL3-deficient type. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Orphanet, ICD-10/ICD-11, and MeSH: no disease-specific identifier was verified in the retrieved evidence. In practice, nonspecific skeletal-dysplasia, immunodeficiency, and developmental-disorder codes may be used, but they should not be represented as exact equivalents.

The evidence is principally aggregated disease-level literature built from very small case series and individual patients, not population-scale EHR data. The 2021 review counted 14 affected people from nine unrelated families; the 2022 Indian report described its patient as the 15th published case. Later 2023–2024 case reports indicate that this historical count is no longer exhaustive, but no comprehensive contemporary registry was found. (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

2. Etiology, risk, and protective factors

The necessary causal factor is a germline biallelic pathogenic or likely pathogenic EXTL3 genotype. The inheritance pattern is autosomal recessive; affected children have been born to heterozygous parents, and consanguinity was present in several families. (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

No validated environmental, infectious, occupational, lifestyle, age-related, or sex-specific cause exists. Infection is a consequence of immune dysfunction, not the initiating etiology. Family history and parental consanguinity increase prior genetic risk but are not required. No protective allele, modifier gene, diet, exposure, or lifestyle intervention has been established. Marked immune variability among individuals carrying the same p.Pro318Leu allele suggests unidentified genetic modifiers and possibly environmental effects on infectious outcomes, but no specific gene–environment interaction has been demonstrated. (bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

3. Phenotypic spectrum

Skeletal and craniofacial disease

Core, early-onset physical and radiographic findings include disproportionate short stature or short-limb dwarfism, severe platyspondyly with widened intervertebral spaces, brachydactyly, epiphyseal and metaphyseal dysplasia, coxa valga, acetabular dysplasia, delayed femoral-head ossification, hip subluxation/dislocation, and progressive kyphoscoliosis. Pelvic radiographs may show squared or open iliac wings, trident-shaped acetabula, and narrow sacro-ischiatic notches. (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 3-5, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Craniovertebral disease is particularly consequential: odontoid hypoplasia, atlas non-ossification, cervical stenosis or instability, and cord compression have required early neurosurgical intervention. Craniosynostosis, including cloverleaf skull deformity, occurs in severe cases but is not universal. Suggested HPO concepts include Short stature (HP:0001511), Platyspondyly (HP:0000926), Brachydactyly (HP:0001156), Kyphoscoliosis, Odontoid hypoplasia, Cervical vertebral instability, Spinal-cord compression, Coxa valga, and Acetabular dysplasia. (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7)

Facial findings are variable but often recognizable: coarse facies, frontal bossing, full cheeks and lips, broad/bulbous nasal tip, abnormal nasal bridge, long eyelashes or synophrys, and micrognathia. Dysmorphism may become more apparent with age. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Neurologic and behavioral phenotype

Global and especially gross-motor delay, axial or central hypotonia, delayed language, and mild-to-severe intellectual disability are common. Severe presentations may include seizures, opisthotonus, hyperreflexia, nystagmus, developmental arrest, or hyporeflexia. Diffuse corpus-callosum thinning was documented in one longitudinally observed patient. Suggested terms include Global developmental delay (HP:0001263), Hypotonia (HP:0001252), Intellectual disability (HP:0001249), Seizure (HP:0001250), Motor delay, and Thin corpus callosum. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, stefano2017extl3mutationscause pages 1-3, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Immune and laboratory phenotype

The immune phenotype is highly variable and predominantly T-cell related. In the historical 14-patient synthesis, nine had T-cell lymphopenia; four were described with Omenn-like SCID, while one had oral candidiasis without the full severe phenotype. Another tabulation reported T-cell SCID in 5/9 evaluable patients, hypogammaglobulinemia in 5/9, Omenn-like disease in 3/9, and hyper-IgE in 1/9, illustrating both small denominators and differences in case classification. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

Reported abnormalities include T−B+NK+ or T−NK+B+ profiles, low/absent TRECs, impaired mitogen proliferation, predominantly activated/memory residual T cells, eosinophilia, hypogammaglobulinemia, elevated IgE, and reduced IL-2/IL-7-induced STAT5 phosphorylation. Some individuals have little infection susceptibility, and T-cell numbers improved spontaneously in one survivor. Normal routine CBC or biochemistry therefore does not exclude the disease. (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-3, stefano2017extl3mutationscause pages 7-9, bajaj2022anultrararecase pages 1-2)

Other manifestations and quality of life

Additional findings include hepatic cysts—occasionally detected prenatally—renal cysts, laryngotracheal narrowing, restrictive lung disease secondary to kyphoscoliosis, obstructive sleep apnea, neurogenic bladder, and occasional congenital heart disease such as ventricular septal defect. One 15-year-old survivor was unable to walk independently and became wheelchair-dependent; progressive spinal disease, respiratory impairment, developmental disability, recurrent infections, surgery, and mobility loss therefore impose substantial functional burden. No validated disease-specific EQ-5D, SF-36, PROMIS, or other quality-of-life dataset was found. (bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

4. Genetic and molecular information

EXTL3 encodes exostosin-like glycosyltransferase 3, an enzyme that initiates heparan-sulfate chains by transferring N-acetylglucosamine and also has GlcNAc transferase activity during chain synthesis. The early literature summarized six disease alleles: c.953C>T (p.Pro318Leu), c.1015C>T (p.Arg339Trp), c.1382C>T (p.Pro461Leu), c.1537C>T, c.1970A>G, and c.2008T>G. All were germline missense alleles; p.Arg339Trp was experimentally characterized as hypomorphic. Exact protein consequences of the latter three should be rechecked against the current reference transcript before database ingestion. (stefano2017extl3mutationscause pages 3-5, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

The recurrent p.Pro318Leu variant was reported at very low ExAC frequency and was classified as likely pathogenic under 2015 ACMG criteria in the 2021 case, with a CADD score of 22.2 and appropriate parental segregation. Current ClinVar classifications and gnomAD frequencies should be queried directly at ingestion because database assertions can change. No established pathogenic structural variant, repeat expansion, mitochondrial variant, somatic mechanism, chromosomal abnormality, or disease-specific epigenetic signature was identified. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

There is no robust variant-position/severity correlation. Individuals homozygous for the same p.Pro318Leu allele have ranged from severe immunodeficiency to subtle immune findings, supporting variable expressivity and unknown modifiers. Penetrance among confirmed biallelic pathogenic genotypes appears high for a syndromic phenotype, but the sample is too small to calculate penetrance formally. (bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

5. Environmental and infectious information

No toxin, radiation, pollution, diet, exercise pattern, smoking, alcohol exposure, or infectious agent is known to cause ISDNA. Pathogens influence morbidity because affected patients may have SCID or T-cell lymphopenia. Accordingly, infectious exposure can modify clinical outcome without being etiologic. No zoonotic or transmissible mechanism applies.

6. Mechanism and pathophysiology

Causal chain

Upstream genetic defect: biallelic hypomorphic EXTL3 variants alter glycosyltransferase function.
Biochemical defect: initiation and structural composition of heparan sulfate (HS) on proteoglycans become abnormal; patient fibroblasts produced aberrant HS chains and sulfation patterns.
Signal-distribution defect: HS normally binds and spatially regulates morphogens, growth factors, and cytokines. Abnormal HS changed FGF2–ERK signaling and impaired IL-2/IL-7–STAT5 responses.
Developmental consequences: altered extracellular signaling disrupts chondrocyte/cartilage development, thymic epithelial differentiation, lymphohematopoietic progenitor expansion, and brain development.
Clinical consequences: spondyloepimetaphyseal dysplasia and spinal instability, T-cell immunodeficiency, and neurodevelopmental disability result. (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)

In primary fibroblasts, wild-type EXTL3 cDNA rescued altered FGF2 signaling, strengthening causality. Patient-derived iPSCs showed reduced lymphohematopoietic progenitor expansion and deficient thymic epithelial progenitor differentiation, with reduced TBX1, EYA1, and CK5 and persistent SOX17 expression. Thus, the immune phenotype likely combines hematopoietic-intrinsic and thymic-stromal defects. (stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)

Recent mechanistic developments

A 2022 cryo-EM study showed that human EXTL3 is a roughly 170-kDa homodimer with GT47 and GT64 glycosyltransferase domains. Its GT47 domain is ineffective at glucuronic-acid transfer, whereas the enzyme retains GlcNAc-transferase activity; the domain arrangement supports a dissociative rather than processive HS-extension mechanism. Published June 2022, DOI/URL: https://doi.org/10.1038/s41467-022-31048-2. (wilson2022thestructureof pages 1-2)

A 2024 biochemical study found that EXTL3 preferentially modifies peptides resembling HS-proteoglycan core proteins, including syndecan-3 and TGFBR3, supporting a “decision-making” role at the HS-versus-chondroitin-sulfate branch. Published February 2024, DOI/URL: https://doi.org/10.1093/glycob/cwae016. (bourgeais2024chemoenzymaticsynthesisof pages 11-12)

Suggested annotations include GO: heparan-sulfate proteoglycan biosynthetic process, glycosyltransferase activity, FGF-receptor signaling, cytokine-mediated signaling, STAT5 phosphorylation, thymus development, T-cell differentiation; CL: chondrocyte, fibroblast, hematopoietic progenitor cell, thymic epithelial cell, thymocyte, T lymphocyte; and GO cellular component: Golgi apparatus, cell surface, extracellular matrix. No disease-specific single-cell atlas, spatial transcriptomic dataset, clinical proteome, metabolome, lipidome, or epigenomic signature was identified.

7. Anatomical structures affected

Primary systems are the axial and appendicular skeleton, growth plate/cartilage, thymus and lymphoid system, and CNS. Particularly important sites are the vertebral bodies, craniovertebral junction, cervical spinal canal and cord, pelvis/acetabulum, proximal femora, and long-bone epiphyses/metaphyses. Secondary involvement includes liver, kidneys, larynx/trachea, lungs, bladder, and occasionally heart. (stefano2017extl3mutationscause pages 1-3, bajaj2022anultrararecase pages 1-2, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Suggested UBERON concepts are vertebral column, cervical vertebral column, spinal cord, cartilage, femur, pelvis, acetabulum, thymus, brain, corpus callosum, liver, kidney, larynx, trachea, and lung. Skeletal disease is generally bilateral/systemic rather than consistently lateralized.

8. Temporal development and natural history

Onset is prenatal, congenital, or early infantile. Severe cases manifest at birth with short limbs, craniosynostosis, seizures, and infections. Hepatic cysts can be detected prenatally. The condition is chronic and lifelong rather than episodic or remitting. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

The course is variable. Some radiographic skeletal features become less conspicuous with age, but kyphoscoliosis, hip disease, cervical compression, restrictive lung disease, sleep apnea, and mobility impairment may progress. Immune dysfunction can remain severe, fluctuate, or partially improve. Developmental intervention and cervical surveillance represent critical early opportunities; cord compression can lead to neurogenic bladder and irreversible disability. (stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

9. Inheritance, epidemiology, and population

Inheritance is autosomal recessive. For two carrier parents, standard Mendelian counseling gives a 25% affected, 50% carrier, and 25% unaffected/non-carrier probability in each pregnancy, assuming both variants are fully disease-causing. Genetic anticipation is not expected. Germline mosaicism has not been specifically demonstrated, and no founder allele or reliable carrier frequency is established. (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

Prevalence and incidence per 100,000 are unknown. The published cohort includes Turkish, North African, Portuguese, Colombian/South American, Indian, and other ancestries, indicating worldwide occurrence rather than a proven geographic concentration. Both sexes are affected; no reliable sex ratio can be calculated. Consanguinity contributed to ascertainment in several families but is not obligatory. (bajaj2022anultrararecase pages 4-5, bajaj2022anultrararecase pages 5-6)

10. Diagnosis

There are no formal society diagnostic criteria. Suspicion should arise from the combination of SEMD/platyspondyly, disproportionate short stature, developmental delay or hypotonia, and T-cell lymphopenia or recurrent infections. Absence of clinically obvious immunodeficiency does not exclude EXTL3 disease. (bajaj2022anultrararecase pages 2-4, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

Recommended evaluation:

Trio WES or WGS, or an inclusive skeletal-dysplasia/inborn-error-of-immunity panel containing EXTL3; confirm candidate variants and familial segregation by Sanger sequencing where appropriate.
Skeletal survey plus dynamic or specialist-directed cervical radiography/MRI; avoid unsafe neck manipulation until instability has been assessed.
Brain MRI and developmental, neurologic, audiologic, and ophthalmologic assessment as indicated.
CBC with differential, lymphocyte subsets, naïve/memory T-cell phenotype, immunoglobulins, vaccine responses, mitogen proliferation, and TREC assessment. Newborn TREC screening may detect severe cases.
Liver/renal ultrasonography, echocardiography, airway evaluation, pulmonary function testing when feasible, and polysomnography for snoring or suspected apnea. (stefano2017extl3mutationscause pages 1-3, stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Routine metabolic screening and urinary glycosaminoglycans may be normal; a normal result does not exclude EXTL3 deficiency. CMA, karyotype, FISH, mitochondrial sequencing, and repeat-expansion assays are not first-line tests unless the phenotype or initial sequencing indicates another diagnosis. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Important differentials include Schimke immuno-osseous dysplasia (SMARCAL1), cartilage-hair hypoplasia (RMRP), spondyloenchondrodysplasia with immune dysregulation (ACP5), other SEMDs, mucopolysaccharidoses, achondroplasia-like skeletal disorders, and syndromic craniosynostosis. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)

11. Outcome and prognosis

No actuarial survival curve, five- or ten-year survival rate, or validated prognostic model exists. In the 14-case literature review, 5/14 patients died before one year, mainly from recurrent infection. Two siblings in the foundational series died at seven and 11 months. Conversely, survival into adolescence is documented, although with progressive kyphoscoliosis, cervical surgery, respiratory disease, intellectual disability, and wheelchair dependence. (bajaj2022anultrararecase pages 2-4, stefano2017extl3mutationscause pages 7-9, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)

Likely adverse prognostic features are profound neonatal T-cell deficiency/Omenn-like SCID, recurrent severe infection, airway narrowing, early seizures or developmental arrest, and cervical cord compression. These are clinically plausible markers derived from a very small series, not validated predictors. Recovery from the genetic skeletal/neurodevelopmental disorder is not expected, but infection risk, respiratory function, spinal compression, communication, and mobility may improve with timely intervention.

12. Treatment and real-world implementation

No approved pharmacologic, gene, RNA, cell, or targeted therapy corrects EXTL3 deficiency, and the ClinicalTrials.gov search identified no disease-specific interventional study or NCT identifier.

Current care is phenotype directed:

Immune care: pediatric immunology follow-up; prompt antimicrobial treatment; individualized antibacterial/antiviral/antifungal prophylaxis; immunoglobulin replacement when antibody deficiency is clinically significant; avoidance of live vaccines in severe cellular immunodeficiency. Suggested MAXO concepts: immunologic monitoring, antimicrobial prophylaxis, immunoglobulin replacement.
SCID-directed therapy: hematopoietic stem-cell transplantation may be considered for life-threatening hematopoietic immune deficiency, but EXTL3 disease also affects thymic stroma and nonhematopoietic tissues, so transplantation should not be assumed to correct the complete syndrome. Published disease-specific response rates are unavailable. (stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)
Spine/orthopedics: serial cervical and scoliosis assessment; urgent decompression/stabilization for cord compression or instability; hip and contracture management; mobility aids. A longitudinal patient underwent C1-arch excision/decompression and kyphoscoliosis correction. Suggested MAXO: spinal decompression surgery, spinal stabilization, scoliosis surgery. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7, akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Respiratory: airway assessment, pulmonary monitoring, sleep study, and noninvasive ventilation. Severe obstructive sleep apnea with apnea–hypopnea index 29.7 led to BiPAP recommendation in one patient. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4)
Development and function: early physical, occupational, speech/language, feeding, and educational therapies; seizure treatment according to standard neurology practice.
Visceral disease: monitor liver and kidney cysts; intervene for mass effect, infection, impaired function, or other standard indications.

There are no EXTL3-specific pharmacogenomic recommendations, response statistics, or systematic adverse-event data.

13. Prevention

There is no lifestyle or vaccine-based primary prevention of the genotype. Primary genetic prevention consists of carrier testing, genetic counseling, preimplantation genetic testing for monogenic disease, prenatal diagnosis by chorionic-villus sampling or amniocentesis, and cascade testing after familial variants are established.

Secondary prevention comprises early genomic diagnosis, SCID newborn screening where available, immune evaluation before serious infection, and early cervical imaging before neurologic injury. Tertiary prevention includes infection prophylaxis, immunoglobulin replacement when indicated, spinal surveillance, respiratory care, vaccination planning under immunology supervision, rehabilitation, and avoidance of hazardous cervical manipulation. Population-wide EXTL3 carrier or newborn genomic screening is not currently established.

14. Other species and natural disease

No naturally occurring homologous veterinary disorder, affected breed, wildlife reservoir, zoonotic transmission, or cross-species infectious susceptibility was identified. Relevant orthologs include Extl3 in mouse (Mus musculus, NCBI Taxon 10090) and extl3 in zebrafish (Danio rerio, NCBI Taxon 7955). Ortholog-specific NCBI Gene identifiers should be verified directly before database loading.

15. Model organisms and experimental systems

Zebrafish: extl3-mutant zebrafish have reduced/abnormal HS, defective cartilage and chondrocyte organization, shortened body axis and pectoral fins, and approximately 50% reduced thymic GFP-positive volume in a rag2 reporter background. Injection of wild-type human EXTL3 RNA rescued thymic and fin phenotypes, providing strong functional-complementation evidence. The model is useful for HS-dependent cartilage development and thymopoiesis but does not reproduce the full human spinal, cognitive, or longitudinal phenotype. (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 1-1)

Mouse: complete Extl3 deficiency is embryonically lethal at approximately embryonic day 9.5, demonstrating developmental essentiality but limiting postnatal modeling. Conditional HS-deficient thymic models support an essential role for HS in thymus growth. (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6)

Human cellular models: patient fibroblasts reproduce HS and FGF2-signaling abnormalities, while patient iPSCs model impaired lymphohematopoietic expansion and thymic epithelial differentiation. These systems are valuable for variant validation and therapeutic screening but cannot model whole-organ biomechanics or infection outcomes. (stefano2017extl3mutationscause pages 3-5, stefano2017extl3mutationscause pages 7-9, stefano2017extl3mutationscause pages 1-1)

Key evidence quotations and sources

Volpi et al., Journal of Experimental Medicine, published February 2017, DOI/URL: https://doi.org/10.1084/jem.20161525. Abstract: “These data identify EXTL3 mutations as a novel cause of severe immune deficiency with skeletal dysplasia and developmental delay and underline a crucial role of HS in thymopoiesis and skeletal and brain development.” (stefano2017extl3mutationscause pages 1-1)
Bajaj et al., BMC Pediatrics, published February 2022, DOI/URL: https://doi.org/10.1186/s12887-022-03143-2. Abstract: “ISDNA is an ultra-rare genetic condition… caused due to presence of biallelic variants in the EXTL3 gene.” The report described the 15th published patient and emphasized phenotypic variability despite an identical allele. (bajaj2022anultrararecase pages 2-4, bajaj2022anultrararecase pages 1-2)
Akalın et al., American Journal of Medical Genetics Part A, accepted May 18, 2021, DOI/URL: https://doi.org/10.1002/ajmg.a.62378. Abstract: “Affected individuals display variable skeletal abnormalities and neurodevelopmental findings… Patients may exhibit varying degrees of immune deficiency as well.” (akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3)
Wilson et al., Nature Communications, June 2022, DOI/URL: https://doi.org/10.1038/s41467-022-31048-2. The study reported human EXTL3 structures and proposed dissociative HS polymerization. (wilson2022thestructureof pages 1-2)
Bourgeais et al., Glycobiology, February 2024, DOI/URL: https://doi.org/10.1093/glycob/cwae016. The study found substrate preferences consistent with EXTL3 helping determine whether a proteoglycan receives HS. (bourgeais2024chemoenzymaticsynthesisof pages 11-12)

Evidence limitations

The knowledge base rests on a few families, heterogeneous reporting, and partially overlapping case summaries. Frequencies such as 9/14 immune involvement and 5/14 infant deaths describe the early published cohort, not unbiased population estimates. No contemporary natural-history registry, validated quality-of-life instrument, formal treatment guideline, prevalence study, or disease-specific trial was found. Variant nomenclature, ClinVar status, population frequencies, and exact ontology identifiers should therefore be refreshed from live databases during production curation.

References

(bajaj2022anultrararecase pages 2-4): Shruti Bajaj, Purnima Satoskar, Aadhira Nair, Frenny Sheth, Jayesh Sheth, and Harsh Sheth. An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an indian patient with homozygous c.953c > t variant in extl3 gene: a case report. BMC Pediatrics, Feb 2022. URL: https://doi.org/10.1186/s12887-022-03143-2, doi:10.1186/s12887-022-03143-2. This article has 6 citations and is from a peer-reviewed journal.
(akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 4-6): Akçahan Akalın, Ekim Z. Taskiran, Pelin Özlem Şimşek‐Kiper, Eda Utine, Yasemin Alanay, Uğur Özçelik, and Koray Boduroğlu. Spondyloepimetaphyseal dysplasia extl3‐deficient type: long‐term follow‐up and review of the literature. American Journal of Medical Genetics Part A, 185:3104-3110, Jun 2021. URL: https://doi.org/10.1002/ajmg.a.62378, doi:10.1002/ajmg.a.62378. This article has 12 citations.
(unger2023nosologyofgenetic pages 29-30): Sheila Unger, Carlos R. Ferreira, Geert R. Mortier, Houda Ali, Débora R. Bertola, Alistair Calder, Daniel H. Cohn, Valerie Cormier‐Daire, Katta M. Girisha, Christine Hall, Deborah Krakow, Outi Makitie, Stefan Mundlos, Gen Nishimura, Stephen P. Robertson, Ravi Savarirayan, David Sillence, Marleen Simon, V. Reid Sutton, Matthew L. Warman, and Andrea Superti‐Furga. Nosology of genetic skeletal disorders: 2023 revision. American Journal of Medical Genetics Part A, 191:1164-1209, Feb 2023. URL: https://doi.org/10.1002/ajmg.a.63132, doi:10.1002/ajmg.a.63132. This article has 528 citations.
(OpenTargets Search: Immunoskeletal dysplasia with neurodevelopmental abnormalities): Open Targets Query (Immunoskeletal dysplasia with neurodevelopmental abnormalities, 1 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(stefano2017extl3mutationscause pages 3-5): Stefano Volpi, Yasuhiro Yamazaki, Patrick M. Brauer, Ellen van Rooijen, Atsuko Hayashida, Anne Slavotinek, Hye Sun Kuehn, Maja Di Rocco, Carlo Rivolta, Ileana Bortolomai, Likun Du, Kerstin Felgentreff, Lisa Ott de Bruin, Kazutaka Hayashida, George Freedman, Genni Enza Marcovecchio, Kelly Capuder, Prisni Rath, Nicole Luche, Elliott J. Hagedorn, Antonella Buoncompagni, Beryl Royer-Bertrand, Silvia Giliani, Pietro Luigi Poliani, Luisa Imberti, Kerry Dobbs, Fabienne E. Poulain, Alberto Martini, John Manis, Robert J. Linhardt, Marita Bosticardo, Sergio Damian Rosenzweig, Hane Lee, Jennifer M. Puck, Juan Carlos Zúñiga-Pflücker, Leonard Zon, Pyong Woo Park, Andrea Superti-Furga, and Luigi D. Notarangelo. Extl3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay. The Journal of Experimental Medicine, 214:623-637, Feb 2017. URL: https://doi.org/10.1084/jem.20161525, doi:10.1084/jem.20161525. This article has 103 citations.
(stefano2017extl3mutationscause pages 1-3): Stefano Volpi, Yasuhiro Yamazaki, Patrick M. Brauer, Ellen van Rooijen, Atsuko Hayashida, Anne Slavotinek, Hye Sun Kuehn, Maja Di Rocco, Carlo Rivolta, Ileana Bortolomai, Likun Du, Kerstin Felgentreff, Lisa Ott de Bruin, Kazutaka Hayashida, George Freedman, Genni Enza Marcovecchio, Kelly Capuder, Prisni Rath, Nicole Luche, Elliott J. Hagedorn, Antonella Buoncompagni, Beryl Royer-Bertrand, Silvia Giliani, Pietro Luigi Poliani, Luisa Imberti, Kerry Dobbs, Fabienne E. Poulain, Alberto Martini, John Manis, Robert J. Linhardt, Marita Bosticardo, Sergio Damian Rosenzweig, Hane Lee, Jennifer M. Puck, Juan Carlos Zúñiga-Pflücker, Leonard Zon, Pyong Woo Park, Andrea Superti-Furga, and Luigi D. Notarangelo. Extl3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay. The Journal of Experimental Medicine, 214:623-637, Feb 2017. URL: https://doi.org/10.1084/jem.20161525, doi:10.1084/jem.20161525. This article has 103 citations.
(akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 3-4): Akçahan Akalın, Ekim Z. Taskiran, Pelin Özlem Şimşek‐Kiper, Eda Utine, Yasemin Alanay, Uğur Özçelik, and Koray Boduroğlu. Spondyloepimetaphyseal dysplasia extl3‐deficient type: long‐term follow‐up and review of the literature. American Journal of Medical Genetics Part A, 185:3104-3110, Jun 2021. URL: https://doi.org/10.1002/ajmg.a.62378, doi:10.1002/ajmg.a.62378. This article has 12 citations.
(akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 1-3): Akçahan Akalın, Ekim Z. Taskiran, Pelin Özlem Şimşek‐Kiper, Eda Utine, Yasemin Alanay, Uğur Özçelik, and Koray Boduroğlu. Spondyloepimetaphyseal dysplasia extl3‐deficient type: long‐term follow‐up and review of the literature. American Journal of Medical Genetics Part A, 185:3104-3110, Jun 2021. URL: https://doi.org/10.1002/ajmg.a.62378, doi:10.1002/ajmg.a.62378. This article has 12 citations.
(bajaj2022anultrararecase pages 1-2): Shruti Bajaj, Purnima Satoskar, Aadhira Nair, Frenny Sheth, Jayesh Sheth, and Harsh Sheth. An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an indian patient with homozygous c.953c > t variant in extl3 gene: a case report. BMC Pediatrics, Feb 2022. URL: https://doi.org/10.1186/s12887-022-03143-2, doi:10.1186/s12887-022-03143-2. This article has 6 citations and is from a peer-reviewed journal.
(akalın2021spondyloepimetaphysealdysplasiaextl3‐deficient pages 6-7): Akçahan Akalın, Ekim Z. Taskiran, Pelin Özlem Şimşek‐Kiper, Eda Utine, Yasemin Alanay, Uğur Özçelik, and Koray Boduroğlu. Spondyloepimetaphyseal dysplasia extl3‐deficient type: long‐term follow‐up and review of the literature. American Journal of Medical Genetics Part A, 185:3104-3110, Jun 2021. URL: https://doi.org/10.1002/ajmg.a.62378, doi:10.1002/ajmg.a.62378. This article has 12 citations.
(stefano2017extl3mutationscause pages 7-9): Stefano Volpi, Yasuhiro Yamazaki, Patrick M. Brauer, Ellen van Rooijen, Atsuko Hayashida, Anne Slavotinek, Hye Sun Kuehn, Maja Di Rocco, Carlo Rivolta, Ileana Bortolomai, Likun Du, Kerstin Felgentreff, Lisa Ott de Bruin, Kazutaka Hayashida, George Freedman, Genni Enza Marcovecchio, Kelly Capuder, Prisni Rath, Nicole Luche, Elliott J. Hagedorn, Antonella Buoncompagni, Beryl Royer-Bertrand, Silvia Giliani, Pietro Luigi Poliani, Luisa Imberti, Kerry Dobbs, Fabienne E. Poulain, Alberto Martini, John Manis, Robert J. Linhardt, Marita Bosticardo, Sergio Damian Rosenzweig, Hane Lee, Jennifer M. Puck, Juan Carlos Zúñiga-Pflücker, Leonard Zon, Pyong Woo Park, Andrea Superti-Furga, and Luigi D. Notarangelo. Extl3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay. The Journal of Experimental Medicine, 214:623-637, Feb 2017. URL: https://doi.org/10.1084/jem.20161525, doi:10.1084/jem.20161525. This article has 103 citations.
(stefano2017extl3mutationscause pages 1-1): Stefano Volpi, Yasuhiro Yamazaki, Patrick M. Brauer, Ellen van Rooijen, Atsuko Hayashida, Anne Slavotinek, Hye Sun Kuehn, Maja Di Rocco, Carlo Rivolta, Ileana Bortolomai, Likun Du, Kerstin Felgentreff, Lisa Ott de Bruin, Kazutaka Hayashida, George Freedman, Genni Enza Marcovecchio, Kelly Capuder, Prisni Rath, Nicole Luche, Elliott J. Hagedorn, Antonella Buoncompagni, Beryl Royer-Bertrand, Silvia Giliani, Pietro Luigi Poliani, Luisa Imberti, Kerry Dobbs, Fabienne E. Poulain, Alberto Martini, John Manis, Robert J. Linhardt, Marita Bosticardo, Sergio Damian Rosenzweig, Hane Lee, Jennifer M. Puck, Juan Carlos Zúñiga-Pflücker, Leonard Zon, Pyong Woo Park, Andrea Superti-Furga, and Luigi D. Notarangelo. Extl3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay. The Journal of Experimental Medicine, 214:623-637, Feb 2017. URL: https://doi.org/10.1084/jem.20161525, doi:10.1084/jem.20161525. This article has 103 citations.
(wilson2022thestructureof pages 1-2): L. F. L. Wilson, T. Dendooven, S. W. Hardwick, A. Echevarría-Poza, T. Tryfona, K. B. R. M. Krogh, D. Y. Chirgadze, B. F. Luisi, D. T. Logan, K. Mani, and P. Dupree. The structure of extl3 helps to explain the different roles of bi-domain exostosins in heparan sulfate synthesis. Nature Communications, 13:1-15, Jun 2022. URL: https://doi.org/10.1038/s41467-022-31048-2, doi:10.1038/s41467-022-31048-2. This article has 37 citations and is from a highest quality peer-reviewed journal.
(bourgeais2024chemoenzymaticsynthesisof pages 11-12): Marie Bourgeais, Farah Fouladkar, Margot Weber, Elisabetta Boeri-Erba, and Rebekka Wild. Chemo-enzymatic synthesis of tetrasaccharide linker peptides to study the divergent step in glycosaminoglycan biosynthesis. Glycobiology, Feb 2024. URL: https://doi.org/10.1093/glycob/cwae016, doi:10.1093/glycob/cwae016. This article has 15 citations and is from a peer-reviewed journal.
(dinges2024primaryandsecondary pages 35-37): Sarah S. Dinges, Kayla Amini, Luigi D. Notarangelo, and Ottavia M. Delmonte. Primary and secondary defects of the thymus. Immunological Reviews, 322:178-211, Jan 2024. URL: https://doi.org/10.1111/imr.13306, doi:10.1111/imr.13306. This article has 29 citations and is from a domain leading peer-reviewed journal.
(bajaj2022anultrararecase pages 4-5): Shruti Bajaj, Purnima Satoskar, Aadhira Nair, Frenny Sheth, Jayesh Sheth, and Harsh Sheth. An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an indian patient with homozygous c.953c > t variant in extl3 gene: a case report. BMC Pediatrics, Feb 2022. URL: https://doi.org/10.1186/s12887-022-03143-2, doi:10.1186/s12887-022-03143-2. This article has 6 citations and is from a peer-reviewed journal.
(bajaj2022anultrararecase pages 5-6): Shruti Bajaj, Purnima Satoskar, Aadhira Nair, Frenny Sheth, Jayesh Sheth, and Harsh Sheth. An ultra-rare case of immunoskeletal dysplasia with neurodevelopmental abnormalities in an indian patient with homozygous c.953c > t variant in extl3 gene: a case report. BMC Pediatrics, Feb 2022. URL: https://doi.org/10.1186/s12887-022-03143-2, doi:10.1186/s12887-022-03143-2. This article has 6 citations and is from a peer-reviewed journal.

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Immunoskeletal Dysplasia with Neurodevelopmental Abnormalities

Executive summary

1. Disease information

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2. Etiology, risk, and protective factors

3. Phenotypic spectrum

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4. Genetic and molecular information

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