| 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 (pqac-00000000, pqac-00000001, pqac-00000003, pqac-00000014) |
| 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 (pqac-00000000, pqac-00000001, pqac-00000002) |
| Inheritance | Autosomal recessive; several families reported with consanguinity; parents heterozygous in informative families | HP:0000007 Autosomal recessive inheritance | Primary human (pqac-00000001, pqac-00000004, pqac-00000016) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000016) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000007, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000008, pqac-00000017) |
| 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 (pqac-00000002, pqac-00000004, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000007, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000004, pqac-00000008, pqac-00000016) |
| 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 (pqac-00000002, pqac-00000004, pqac-00000005, pqac-00000016) |
| 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 (pqac-00000004, pqac-00000008, pqac-00000017) |
| 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 (pqac-00000003, pqac-00000007, pqac-00000017) |
| 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 (pqac-00000001, pqac-00000005, pqac-00000017) |
| 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 (pqac-00000003, pqac-00000007, pqac-00000017) |
| 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 (pqac-00000002, pqac-00000006, pqac-00000009, pqac-00000010) |
| 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 (pqac-00000002, pqac-00000006) |
| 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 (pqac-00000002, pqac-00000005, pqac-00000006) |
| 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 (pqac-00000002, pqac-00000003, pqac-00000005) |
| 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 (pqac-00000009) |
| 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 (pqac-00000010) |
| 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 (pqac-00000001, pqac-00000003, pqac-00000017) |
| 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 (pqac-00000004, pqac-00000005, pqac-00000017) |
| 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 (pqac-00000016) |
| 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 (pqac-00000001, pqac-00000017) |
| 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 (pqac-00000004, pqac-00000005, pqac-00000015) |
| 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 (pqac-00000008, pqac-00000017) |
| 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 (pqac-00000017, pqac-00000004) |
| 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 (pqac-00000001, pqac-00000016) |
| 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 (pqac-00000001, pqac-00000003) |
| 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 (pqac-00000002, pqac-00000003, pqac-00000006) |
| 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 (pqac-00000003, pqac-00000015) |
| 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 (pqac-00000002, pqac-00000005, pqac-00000006) |


*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.*