Dyssegmental Dysplasia

Mendelian Pathograph 9 Show in embeddings browser hereditary disease Skeletal Dysplasia

Dyssegmental dysplasia (DD) is a rare autosomal recessive skeletal dysplasia caused by biallelic loss-of-function variants in HSPG2, the gene encoding perlecan — the major heparan sulfate proteoglycan of basement membranes and of the cartilage pericellular matrix. Its defining radiographic feature is anisospondyly: vertebral ossification centres of irregular size and shape, with defective segmentation and fusion of the vertebral bodies, from which the disorder takes its name. This is combined with micromelia, short thick bowed and dumbbell-shaped tubular bones, a narrow thorax, and a characteristic flat facies with short neck, micrognathia and cleft palate. Mutant perlecan is retained and degraded intracellularly rather than secreted, so the cartilage extracellular matrix is depleted of perlecan; the growth-plate columnar architecture and endochondral ossification then fail. DD is curated here as one entry spanning its two long-recognized clinical grades — the lethal Silverman-Handmaker type (DDSH) and the nonlethal Rolland-Desbuquois type (DDRD) — because both are now known to be caused by biallelic HSPG2 variants and share a single pathomechanism, differing in the amount of residual functional perlecan rather than in kind. Together with Schwartz-Jampel syndrome, which arises from hypomorphic alleles of the same gene, they form an allelic perlecan gene-dosage series.

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2
Mappings
1
Inheritance
7
Pathophys.
15
Phenotypes
1
Gaps
9
Pathograph
1
Genes
2
Subtypes
2
Differentials
1
Models
🏷

Classifications

ISDS Skeletal Nosology
proteoglycan core protein disorders
🔗

Mappings

MONDO
MONDO:0009140 Silverman-Handmaker type dyssegmental dysplasia Not Yet Curated
skos:narrowMatch MONDO (OMIM:224410, Orphanet:1865)
The lethal severity grade of the spectrum modeled here, curated as the DDSH subtype. narrowMatch rather than exactMatch because this entry also covers the nonlethal Rolland-Desbuquois grade.
MONDO:0009139 dyssegmental dysplasia, Rolland-Desbuquois type Not Yet Curated
skos:narrowMatch MONDO (OMIM:224400, Orphanet:156731)
The nonlethal severity grade of the spectrum modeled here, curated as the DDRD subtype. narrowMatch rather than exactMatch because this entry also covers the lethal Silverman-Handmaker grade.
👪

Inheritance

1
Autosomal recessive inheritance HP:0000007
Both clinical grades segregate as autosomal recessive traits. Affected individuals carry biallelic (homozygous or compound heterozygous) HSPG2 variants; parental consanguinity is frequent in DDSH but was absent in all five genotyped DDRD patients, who instead shared a founder allele.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:3605216 SUPPORT Human Clinical
"We conclude that there are at least two forms of dyssegmental dysplasia, each autosomal recessive, which can be delineated on clinical, radiographic and morphologic grounds."
The clinical-delineation study that separated the two grades states that both are autosomal recessive.
PMID:11279527 SUPPORT Human Clinical
"Here we report a homozygous, 89-bp duplication in exon 34 of HSPG2 in a pair of siblings with DDSH born to consanguineous parents"
Homozygosity in the offspring of consanguineous parents, consistent with the recessive mode recorded here.
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Subtypes

2
Dyssegmental dysplasia, Silverman-Handmaker type (lethal) MONDO:0009140
HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee. Autosomal recessive inheritance
The severe, lethal grade. Anisospondyly is marked, with rudimentary or absent vertebral bodies, and limb shortening and bowing are severe. Most affected pregnancies are terminated or end in stillbirth, and liveborn infants usually die within the first days of life from respiratory failure. Reported HSPG2 alleles are functional nulls — insertions, deletions and splice-site variants predicted to truncate the perlecan core protein.
Show evidence (2 references)
PMID:38424183 SUPPORT Human Clinical
"Dyssegmental dysplasia (DD) is a severe skeletal dysplasia comprised of two subtypes: lethal Silverman-Handmaker type (DDSH) and nonlethal Rolland-Desbuquois type (DDRD)."
Establishes the two-subtype structure used here and identifies DDSH as the lethal grade.
PMID:3605216 SUPPORT Human Clinical
"The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes."
The original delineation of the severe grade and its natural history. Note that the cited text misspells the eponym as "Silverman-Handmarker"; the snippet is quoted verbatim, as required.
Dyssegmental dysplasia, Rolland-Desbuquois type (nonlethal) MONDO:0009139
HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee. Autosomal recessive inheritance
The milder, nonlethal grade. Vertebral bodies are better ossified than in DDSH and long-bone shortening and bowing are milder, with radiographic changes that were originally described as resembling Kniest dysplasia. Survival beyond the newborn period is frequent, though respiratory support is commonly required. Attributed to HSPG2 only in 2024, when biallelic variants were found in five patients sharing a founder p.G3324R haplotype; mild myotonic features overlapping Schwartz-Jampel syndrome were noted in one of them.
Show evidence (2 references)
PMID:38424183 SUPPORT Human Clinical
"Here, we report that five patients with DDRD carried four pathogenic variants in HSPG2"
Establishes HSPG2 as the cause of the Rolland-Desbuquois grade, which is what makes curating DD as a single mechanism entry defensible.
PMID:9388290 SUPPORT Human Clinical
"The milder Rolland-Desbuquois type is frequently characterised by survival beyond the newborn period."
States the natural-history distinction that defines this grade.
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Discussions and Knowledge Gaps

1
What determines whether a given biallelic HSPG2 genotype produces Schwartz-Jampel syndrome, nonlethal Rolland-Desbuquois dyssegmental dysplasia, or lethal Silverman-Handmaker dyssegmental dysplasia?
KNOWLEDGE GAP OPEN ddsh_ddrd_severity_determinants
Attached to
pathophysiology#Biallelic HSPG2 Loss-of-Function Variation pathophysiology#Failure of Perlecan Secretion into the Extracellular Matrix pathophysiology#Narrow Thorax with Neonatal Respiratory Failure
The three perlecan disorders are allelic, and the intuitive account is a gene-dosage series in which residual extracellular perlecan sets severity. Three observations keep that account from being settled. First, no threshold separating the lethal from the nonlethal grade has been proposed. Second, the same p.G3324R allele has been reported homozygously in a patient labelled DDSH whose nonlethal course actually fitted DDRD, so published grade assignments are partly unreliable and identical genotypes have carried different labels. Third, two DDRD patients homozygous for that allele differed markedly in when skeletal disease became apparent, which the authors attribute to an unidentified modifier gene or environmental factor. Perlecan is also a five-domain multifunctional protein, so a domain-specific effect is a competing explanation to a purely quantitative one. Resolving this matters clinically, because prenatal counselling for a newly identified HSPG2 genotype currently cannot predict lethality.
Proposed experiments
Quantify secreted perlecan across genotypes of the HSPG2 series
ddsh_ddrd_residual_perlecan_quantification
Measure secreted perlecan in patient fibroblasts from genotype-matched SJS, DDRD and DDSH cases, and test whether residual extracellular perlecan orders with clinical grade independently of which domain is affected. Include the p.G3324R homozygotes, whose divergent courses would separate a dose effect from a modifier effect.
Domain IV missense knock-in mouse
ddsh_ddrd_domain_iv_knockin
Knock the DDRD founder allele p.G3324R into the mouse Hspg2 immunoglobulin-like domain IV repeat and compare it with the existing null and hypomorphic lines, to test whether a domain IV missense change produces an intermediate phenotype at a perlecan level that a hypomorph does not.
Show evidence (2 references)
PMID:38424183 SUPPORT Human Clinical
"No definite threshold has been proposed to delineate lethal and nonlethal phenotypes."
States directly that no threshold separating the two grades has been proposed — the gap recorded here.
PMID:38424183 SUPPORT Human Clinical
"Presence of an unidentified modifier gene or an unidentified environmental factor might have accounted for the different disease severities."
Records the authors' own reading that genotype alone does not account for the severity difference between two homozygotes for the same allele.
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Pathophysiology

7
Biallelic HSPG2 Loss-of-Function Variation
Mechanism confidence: Established
The initiating lesion is biallelic loss-of-function variation in HSPG2, which encodes perlecan. In the lethal Silverman-Handmaker grade the reported alleles are functional nulls — exonic duplications, deletions and splice-site variants that cause frameshifts and truncate the core protein. In the nonlethal Rolland-Desbuquois grade the recurrent allele is instead a missense change, p.G3324R in an immunoglobulin-like repeat of perlecan domain IV, carried either homozygously or in trans with a nonsense or splice-site allele.
HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee. variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION
Homozygous or compound heterozygous germline HSPG2 alleles. Homozygosity predominates in consanguineous DDSH families; compound heterozygosity is common in the DDRD series, where one allele is the founder missense variant.
Downstream
Failure of Perlecan Secretion into the Extracellular Matrix Truncated perlecan core protein is unstable and is retained and degraded inside the cell instead of being secreted.
Show evidence (3 references)
PMID:11279527 SUPPORT Human Clinical
"These mutations are predicted to cause a frameshift, resulting in a truncated protein core."
Gives the molecular consequence of the DDSH alleles that initiates this pathograph.
PMID:40503612 SUPPORT Human Clinical
"Silverman-Handmaker type dyssegmental dysplasia (DDSH) is a rare and lethal skeletal dysplasia caused by biallelic null variations in the HSPG2 gene, which encodes the extracellular matrix proteoglycan perlecan."
Contemporary confirmation that biallelic HSPG2 null variation is the cause of the lethal grade.
PMID:38424183 SUPPORT Human Clinical
"Here, we report that five patients with DDRD carried four pathogenic variants in HSPG2"
Extends the same initiating lesion to the nonlethal grade.
Failure of Perlecan Secretion into the Extracellular Matrix
Mechanism confidence: Established
Perlecan is the major heparan sulfate proteoglycan of basement membranes and is also a component of the cartilage pericellular matrix, even though cartilage has no basement membranes. Patient fibroblasts do not secrete the truncated protein; it is degraded intracellularly, and patient cartilage matrix stains only weakly for perlecan while intracellular inclusion bodies stain positively. The result is a functional null for extracellular perlecan — the distinction from Schwartz-Jampel syndrome, where hypomorphic alleles still permit secretion at a reduced level.
extracellular matrix GO:0031012 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves decreased extracellular matrix (GO:0031012). GO:0031012 is a cellular component from the Gene Ontology. basement membrane GO:0005604 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves decreased basement membrane (GO:0005604). GO:0005604 is a cellular component from the Gene Ontology.
Downstream
Cartilage Extracellular Matrix Disorganization Loss of extracellular perlecan disorganizes the collagen fibril and glycosaminoglycan architecture of the cartilage matrix.
Show evidence (3 references)
PMID:11279527 SUPPORT In Vitro
"Biochemically, truncated perlecan was not secreted by the patient fibroblasts, but was degraded to smaller fragments within the cells."
Direct biochemical demonstration of the secretion failure in patient cells.
PMID:11279527 SUPPORT Human Clinical
"The cartilage matrix from these patients stained poorly with antibody specific for perlecan, but there was staining of intracellular inclusion bodies."
Shows the deficit in the tissue that matters — patient cartilage matrix — together with the intracellular retention that causes it.
PMID:11279527 SUPPORT Human Clinical
"Thus, DDSH is caused by a functional null mutation of HSPG2."
States the functional-null character of the lesion, which is what separates this entry from Schwartz-Jampel syndrome.
Cartilage Extracellular Matrix Disorganization
Mechanism confidence: Established
Without perlecan the cartilage extracellular matrix loses its normal collagen fibril and glycosaminoglycan organization. That a chondrodysplasia should follow from loss of a basement-membrane proteoglycan was unexpected, and it is the finding that established perlecan as a structural requirement of cartilage rather than only of basement membranes.
chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
extracellular matrix organization GO:0030198 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased extracellular matrix organization (GO:0030198). GO:0030198 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Perlecan-null (Hspg2-/-) mouse Mouse recapitulates Null cartilage matrix shows reduced and disorganized collagen fibrils and glycosaminoglycans, the tissue-level content of this node.
Downstream
Growth-Plate Disorganization and Defective Endochondral Ossification A disorganized pericellular matrix removes the scaffold on which growth plate chondrocytes form proliferative columns and progress to hypertrophy.
Show evidence (2 references)
PMID:11891676 SUPPORT Human Clinical
"A chondrodysplasia phenotype caused by the loss of perlecan was unexpected, because cartilage does not have basement membranes."
Frames why this node is the mechanistically informative step rather than an obvious consequence of a basement-membrane defect.
PMID:11891676 SUPPORT Human Clinical
"These results indicate that DDSH is caused by functional null mutations of HSPG2 and that perlecan is essential for cartilage development."
States the requirement for perlecan in cartilage development that this node represents.
Growth-Plate Disorganization and Defective Endochondral Ossification
Mechanism confidence: Established
The growth plate loses its columnar chondrocyte organization, chondrocyte proliferation falls and the prehypertrophic zone is diminished, so endochondral ossification — the process that lengthens tubular bones and ossifies the vertebral bodies — proceeds abnormally. This single node is the branch point from which both cardinal radiographic features of the disorder follow: anisospondyly in the axial skeleton and micromelia with dumbbell-shaped, bowed tubular bones in the appendicular skeleton.
growth plate chondrocyte CL:1000217 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves growth plate chondrocyte, annotated with growth plate cartilage chondrocyte (CL:1000217). CL:1000217 is a cell type from the Cell Ontology.
endochondral ossification GO:0001958 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased endochondral ossification (GO:0001958). GO:0001958 is a biological process from the Gene Ontology. ↓ DECREASED chondrocyte proliferation GO:0035988 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased chondrocyte proliferation (GO:0035988). GO:0035988 is a biological process from the Gene Ontology. ↓ DECREASED chondrocyte differentiation GO:0002062 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased chondrocyte differentiation (GO:0002062). GO:0002062 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Perlecan-null (Hspg2-/-) mouse Mouse recapitulates The line reproduces the growth-plate failure: loss of chondrocyte columnar organization, reduced proliferation, a diminished prehypertrophic zone and defective endochondral ossification.
Downstream
Anisospondyly Abnormal ossification of the vertebral bodies produces ossification centres of irregular size and shape with defective segmentation and fusion.
Micromelia with Bowed, Dumbbell-Shaped Tubular Bones Defective endochondral growth shortens and deforms the tubular bones, giving the short, thick, bowed and dumbbell-shaped appearance.
Show evidence (2 references)
PMID:10545953 SUPPORT Model Organism
"In Hspg2-/- cartilage, proliferation of chondrocytes was reduced and the prehypertrophic zone was diminished."
Gives the cellular content of this node — reduced proliferation and a diminished prehypertrophic zone.
PMID:10545953 SUPPORT Model Organism
"Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification."
States the growth-plate architectural failure and the ossification defect.
Anisospondyly
Mechanism confidence: Established
Anisospondyly — vertebral ossification centres of irregular size and shape, with defective segmentation and fusion of the vertebral body components — is the feature the disorder is named for and the one that distinguishes it radiographically from other lethal short-limbed dysplasias. Its severity is what separates the two clinical grades: vertebral bodies may be rudimentary or absent in DDSH but are better ossified in DDRD.
Animal models
Perlecan-null (Hspg2-/-) mouse Mouse recapitulates The null mouse shows dyssegmental ossification of the spine, the animal counterpart of human anisospondyly.
Downstream
Narrow Thorax with Neonatal Respiratory Failure Vertebral malsegmentation, together with the short ribs of the appendicular arm, restricts thoracic volume.
Show evidence (2 references)
PMID:20542149 SUPPORT Human Clinical
"is an autosomal recessive form of lethal dwarfism characterized by a defect in segmentation and fusion of vertebral bodies components ("anisospondyly") and by severe limb shortening"
Defines anisospondyly in the terms used for this node.
PMID:23836246 SUPPORT Human Clinical
"The main prenatal ultrasound and the radiological features of DD-SH were severe limb shortening and vertebral segmentation and fusion defects (anisospondyly)."
Confirms anisospondyly as a main radiological feature, detectable prenatally.
Micromelia with Bowed, Dumbbell-Shaped Tubular Bones
Mechanism confidence: Established
Severe, proximally predominant limb shortening with short, thick, bowed long bones, the metaphyseal flaring of which gives the characteristic dumbbell shape. Rib shortening is part of the same appendicular arm and contributes to the narrow thorax.
Animal models
Perlecan-null (Hspg2-/-) mouse Mouse recapitulates Null animals surviving to birth show micromelia with broad and bowed long bones and a narrow thorax.
Downstream
Narrow Thorax with Neonatal Respiratory Failure Short ribs and a small chest wall limit lung growth and ventilation.
Show evidence (2 references)
PMID:3605216 SUPPORT Human Clinical
"The dyssegmental dysplasias are lethal forms of neonatal short-limbed dwarfism in which vertebral segmentation defects and short, thick, bowed long bones are the prominent radiographic features."
Names the appendicular features recorded at this node.
PMID:11891676 SUPPORT Human Clinical
"Dyssegmental dysplasia, Silverman-Handmaker type (DDSH), is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia."
Pairs the micromelia with the anisospondyly as the defining combination.
Narrow Thorax with Neonatal Respiratory Failure
Mechanism confidence: Established
The terminal common pathway and the proximate cause of death. A small, narrow thorax restricts lung growth and ventilation; liveborn infants present with respiratory failure. In the lethal grade this is fatal within days, while in the nonlethal grade prolonged respiratory support may allow survival for years. Cleft palate and micrognathia add an upper-airway component.
Show evidence (2 references)
PMID:3605216 SUPPORT Human Clinical
"The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes."
Gives the lethal outcome of the severe grade. The eponym is misspelled in the source and is quoted verbatim.
PMID:9388290 SUPPORT INDIRECT Human Clinical
"The case is unusual because although it was the milder form, the child died on the 10th day of life."
Shows that the nonlethal grade is not reliably nonlethal. Indirect for this node: the report describes one death in the milder grade, from which the framing of a shared terminal pathway differing in timing rather than presence follows by inference rather than being stated.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Dyssegmental Dysplasia Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

15
Head and Neck 4
Occipital Encephalocele HP:0002085 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Occipital encephalocele (HP:0002085). HP:0002085 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38424183 SUPPORT DIRECT Human Clinical
"DDSH is sometimes accompanied by hydrocephalus and occipital encephalocele that may manifest with defective calvarial ossification, while DDRD is not."
Directly attributes this finding to DDSH and explicitly excludes it from DDRD, which is the basis for scoping the phenotype to the DDSH subtype. No frequency is asserted: the source says "sometimes" without quantifying it.
Cleft Palate HP:0000175 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cleft palate (HP:0000175). HP:0000175 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen."
Lists cleft palate among the commonly seen clinical features.
Short Neck HP:0000470 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short neck (HP:0000470). HP:0000470 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen."
Lists short neck among the commonly seen clinical features.
Micrognathia HP:0000347 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Micrognathia (HP:0000347). HP:0000347 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:40503612 SUPPORT Human Clinical
"referred due to ultrasonographic findings of limb shortening, retrognathia, and irregularities in the lumbar vertebrae"
Prenatal report listing retrognathia alongside the skeletal findings.
Limbs 2
Micromelia HP:0002983 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Micromelia (HP:0002983). HP:0002983 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:11891676 SUPPORT Human Clinical
"Dyssegmental dysplasia, Silverman-Handmaker type (DDSH), is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia."
Names micromelia as characteristic of the disorder.
Bowed Long Bones Bowing of the long bones HP:0006487 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bowing of the long bones (HP:0006487). HP:0006487 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"The dyssegmental dysplasias are lethal forms of neonatal short-limbed dwarfism in which vertebral segmentation defects and short, thick, bowed long bones are the prominent radiographic features."
Names short, thick, bowed long bones as a prominent radiographic feature.
Musculoskeletal 6
Anisospondyly Vertebral segmentation defect HP:0003422 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Vertebral segmentation defect (HP:0003422). HP:0003422 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:23836246 SUPPORT Human Clinical
"The main prenatal ultrasound and the radiological features of DD-SH were severe limb shortening and vertebral segmentation and fusion defects (anisospondyly)."
Names vertebral segmentation and fusion defects as a main feature.
Coronal Cleft Vertebrae HP:0003417 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Coronal cleft vertebrae (HP:0003417). HP:0003417 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29526034 SUPPORT DIRECT Human Clinical
"Skeletal survey confirmed anisospondyly, coronal clefts and lack of ossification of several vertebrae, sagittal cleft in the vertebral bodies of the cervical and lumbar spine"
Radiographic confirmation of coronal clefting in a case of the severe grade.
PMID:38424183 SUPPORT DIRECT Human Clinical
"The mildest end of DDRD is associated with only large coronal clefts at the thoracolumbar spine"
Establishes coronal clefting as the defining spinal finding at the mild end of DDRD, where it may be the only one.
Deficient Vertebral Ossification Absent or minimally ossified vertebral bodies HP:0004599 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Absent or minimally ossified vertebral bodies (HP:0004599). HP:0004599 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29526034 SUPPORT DIRECT Human Clinical
"Skeletal survey confirmed anisospondyly, coronal clefts and lack of ossification of several vertebrae, sagittal cleft in the vertebral bodies of the cervical and lumbar spine"
Documents absent ossification of several vertebrae in the severe grade.
PMID:38424183 SUPPORT DIRECT Human Clinical
"Some vertebral bodies are rudimentary or even absent in DDSH, while all vertebral bodies are better ossified in DDRD."
States the ossification deficit and grades it between DDSH and DDRD, which is why this phenotype is curated at disease level rather than scoped to one subtype.
Narrow Thorax Narrow chest HP:0000774 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Narrow chest (HP:0000774). HP:0000774 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen."
Lists narrow thorax among the commonly seen clinical features.
Limitation of Joint Mobility HP:0001376 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Limitation of joint mobility (HP:0001376). HP:0001376 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen."
Lists reduced joint mobility among the commonly seen clinical features.
Myotonia HP:0002486 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Myotonia (HP:0002486). HP:0002486 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38424183 SUPPORT DIRECT Human Clinical
"She had mild bilateral blepharophimosis, mild pursed lips, and mild limb myotonia, which are commonly observed in SJS."
Direct observation of limb myotonia in a DDRD patient — the quote asserts the phenotype itself. It rests on one of five patients in the series and the other four were not examined for muscle hyperexcitability, so the frequency is unestablished even though the observation is direct.
Nervous System 1
Hydrocephalus HP:0000238 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hydrocephalus (HP:0000238). HP:0000238 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38424183 SUPPORT DIRECT Human Clinical
"DDSH is sometimes accompanied by hydrocephalus and occipital encephalocele that may manifest with defective calvarial ossification, while DDRD is not."
Directly attributes this finding to DDSH and explicitly excludes it from DDRD, which is the basis for scoping the phenotype to the DDSH subtype. No frequency is asserted: the source says "sometimes" without quantifying it.
Respiratory 1
Respiratory Insufficiency HP:0002093 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Respiratory insufficiency (HP:0002093). HP:0002093 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:10545953 SUPPORT INDIRECT Model Organism
"The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities."
Model-organism evidence for perinatal death with a narrow thorax. Indirect on two counts: the mouse study reports death rather than a measured respiratory phenotype, and it is a non-human model system, so respiratory insufficiency in the human disease follows by inference.
Growth 1
Severe Short Stature HP:0003510 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Severe short stature (HP:0003510). HP:0003510 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:11891676 SUPPORT Human Clinical
"is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia"
Characterizes the disorder as a form of dwarfism.
🧬

Genetic Associations

1
HSPG2
Gene: HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Autosomal recessive inheritance
Show evidence (3 references)
PMID:11279527 SUPPORT Human Clinical
"Thus, DDSH is caused by a functional null mutation of HSPG2."
The gene-identification report for the lethal grade.
PMID:38424183 SUPPORT Human Clinical
"SJS, DDRD, and DDSH are allelic disorders with pathogenic variants in HSPG2."
Extends HSPG2 causation to the nonlethal grade and places all three perlecan disorders in one allelic series.
PMID:38424183 SUPPORT Human Clinical
"Two patients were homozygous for p.G3324R, and three patients were heterozygous for p.G3324R."
Documents the recurrent founder allele and the homozygous versus compound heterozygous allele configurations recorded in the notes.
🔬

Diagnosis

2
Prenatal ultrasound and fetal imaging
The diagnosis is usually first raised prenatally on limb shortening well below the expected range, long-bone bowing, a small thorax and irregular vertebral ossification. Three-dimensional ultrasound and CT resolve the dumbbell deformity and anisospondyly that distinguish dyssegmental dysplasia from other lethal short-limbed dysplasias, and it has been detected as early as 13 weeks.
Show evidence (1 reference)
PMID:23836246 SUPPORT Human Clinical
"DD-SH can be diagnosed prenatally using fetal ultrasound as early as 13 weeks."
States the earliest reported gestational age at prenatal diagnosis.
HSPG2 sequencing
Molecular confirmation by targeted skeletal-dysplasia panel, exome or genome sequencing. Confirmation matters beyond diagnosis: it enables preimplantation and prenatal diagnosis in subsequent at-risk pregnancies, and it is the only way to separate this disorder from radiographically overlapping lethal dysplasias.
Show evidence (2 references)
PMID:23836246 SUPPORT Human Clinical
"Xrays and DNA analysis of the HSPG2 gene are important for the confirmation of the diagnosis and for the preimplantation and prenatal diagnosis in pregnancies at risk."
States the role of HSPG2 sequencing in confirmation and in reproductive counselling.
PMID:40503612 SUPPORT Human Clinical
"Targeted skeletal dysplasia panel testing via next-generation sequencing (NGS) revealed a novel homozygous splice site variant"
Worked example of panel-based NGS confirmation in a prenatal case.
📈

Progression

2
Perinatal course of the lethal grade
DDSH Age: Prenatal to first days of life
Most affected pregnancies are terminated after prenatal diagnosis or end in stillbirth; liveborn infants typically die within the first days of life from respiratory failure. Prolonged survival is exceptional.
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes."
States the natural history of the lethal grade. The eponym is misspelled in the source and is quoted verbatim.
Postnatal course of the nonlethal grade
DDRD Age: Neonatal period onward
Survival beyond the newborn period is frequent, commonly after a period of ventilatory support for neonatal respiratory insufficiency. The boundary with the lethal grade is not absolute: a case reported as the milder form nevertheless died on the tenth day of life.
Show evidence (2 references)
PMID:9388290 SUPPORT Human Clinical
"The milder Rolland-Desbuquois type is frequently characterised by survival beyond the newborn period."
States the natural history of the milder grade.
PMID:9388290 SUPPORT DIRECT Human Clinical
"The case is unusual because although it was the milder form, the child died on the 10th day of life."
Documents that the nonlethal designation is not absolute; the quote asserts that claim directly.
📊

Prevalence

2
Worldwide (published cases)
Cases In Literature Ultra Rare DDSH
Cumulative published case count for the lethal grade as of 2018. No population-based rate has been established for either grade.
Show evidence (1 reference)
PMID:29526034 SUPPORT Human Clinical
"Less than forty cases are reported in the literature, of which only four were prenatally detected."
Gives the cumulative reported case count, the appropriate occurrence measure for a disorder with no population-based ascertainment.
Worldwide (molecularly confirmed cases)
Cases In Literature Ultra Rare DDSH
Molecularly confirmed cases are a much smaller subset of the clinically reported ones, because most affected pregnancies are terminated or the neonate dies before genetic testing is undertaken.
Show evidence (1 reference)
PMID:29526034 SUPPORT Human Clinical
"To the best of our knowledge, the current report is only the seventh molecularly confirmed case of DDSH."
States how few cases had molecular confirmation at the time of report.
🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Dyssegmental Dysplasia:

Schwartz-Jampel syndrome
Overlapping Features The hypomorphic-allele end of the same HSPG2 series: a nonlethal chondrodysplasia with continuous muscle hyperactivity (myotonia) and a characteristic mask-like face. Distinguished from dyssegmental dysplasia by residual perlecan secretion, survival into adult life, and the dominance of the neuromuscular rather than the axial-ossification phenotype — though the boundary is not absolute, since mild myotonic features have been reported in DDRD.
Distinguishing Features
  • Hypomorphic HSPG2 alleles permitting reduced perlecan secretion, rather than functional nulls
  • Myotonia and mask-like facies dominate the presentation
  • No anisospondyly
  • Survival into adult life
Show evidence (1 reference)
PMID:38424183 SUPPORT Human Clinical
"Schwartz-Jampel syndrome (SJS) is also caused by biallelic pathogenic variants in HSPG2 and is an allelic disorder of DDSH."
Establishes the allelic relationship that makes SJS the primary differential.
Kniest dysplasia
Overlapping Features A COL2A1 type II collagenopathy whose radiographic changes — including large coronal clefts of the vertebral bodies — were the original comparator for the milder Rolland-Desbuquois grade. Distinguished molecularly (COL2A1 rather than HSPG2) and by its nonlethal, myopic and hearing-impaired phenotype.
Distinguishing Features
  • Causal gene COL2A1 rather than HSPG2
  • Coronal clefting without the irregular or absent vertebral ossification centres of DDSH
  • Myopia and sensorineural hearing loss
Show evidence (1 reference)
PMID:3605216 SUPPORT Human Clinical
"The milder form, "dyssegmental dysplasia, type Rolland-Desbuquois," is characterized clinically by frequent survival beyond the newborn period and by distinct radiographic changes resembling Kniest dysplasia."
Records the radiographic resemblance that makes Kniest dysplasia the relevant differential for the milder grade.
🐁

Animal Models

1
Perlecan-null (Hspg2-/-) mouse
Germline disruption of Hspg2. About 40% of nulls die at embryonic day 10.5 with defective cephalic development; the remainder die just after birth with a skeletal dysplasia. The radiographic, clinical and chondro-osseous resemblance of this line to human DDSH is what prompted the search for HSPG2 mutations in patients, so the model is historically upstream of the human gene identification rather than derived from it.
Species
Mouse
Genotype
Hspg2 homozygous null (Hspg2-/-)
Genes
HSPG2 hgnc:5273 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns HSPG2 (hgnc:5273). hgnc:5273 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:10545953
Pathograph links
Cartilage Extracellular Matrix Disorganization recapitulates fidelity: high Null cartilage matrix shows reduced and disorganized collagen fibrils and glycosaminoglycans, the tissue-level content of this node.
Limitations: The mouse is a constitutive whole-body null, whereas human DDSH alleles are functional nulls that still produce an intracellularly retained, degraded core protein; any gain-of-function or ER-stress contribution from the retained human protein is absent from this model.
Readouts
Cartilage collagen fibril and glycosaminoglycan organization
Direction: DECREASED
Interpretation: Ultrastructural correlate of the matrix-disorganization node.
Show evidence (1 reference)
PMID:10545953 SUPPORT Model Organism
"Hspg2-/- cartilage matrix contained reduced and disorganized collagen fibrils and glycosaminoglycans, suggesting that perlecan has an important role in matrix structure."
Reports the measured matrix abnormality.
Show evidence (1 reference)
PMID:11279527 SUPPORT Model Organism
"Mice lacking the perlecan gene (Hspg2) have a severe chondrodysplasia with dyssegmental ossification of the spine and show radiographic, clinical and chondro-osseous morphology similar to a lethal autosomal recessive disorder in humans termed dyssegmental dysplasia, Silverman-Handmaker type..."
States the chondro-osseous correspondence between the null mouse and human DDSH that makes this model informative.
Growth-Plate Disorganization and Defective Endochondral Ossification recapitulates fidelity: high The line reproduces the growth-plate failure: loss of chondrocyte columnar organization, reduced proliferation, a diminished prehypertrophic zone and defective endochondral ossification.
Limitations: Reported as a qualitative histological description rather than as quantified zone measurements, and the authors note the skeletal phenotype also resembles thanatophoric dysplasia type I, so growth-plate disorganization at this level of description is not specific to perlecan loss.
Readouts
Growth-plate chondrocyte proliferation
Direction: DECREASED
Interpretation: Cellular correlate of the growth-plate node.
Show evidence (1 reference)
PMID:10545953 SUPPORT Model Organism
"In Hspg2-/- cartilage, proliferation of chondrocytes was reduced and the prehypertrophic zone was diminished."
Reports the proliferation and prehypertrophic-zone deficits.
Show evidence (1 reference)
PMID:10545953 SUPPORT Model Organism
"Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification."
Supports treating this model as informative for the growth-plate node.
Anisospondyly recapitulates fidelity: moderate The null mouse shows dyssegmental ossification of the spine, the animal counterpart of human anisospondyly.
Limitations: Reported as dyssegmental ossification of the spine, without the vertebra-by-vertebra radiographic grading used to separate DDSH from DDRD in humans, so the model speaks to the presence of the axial defect rather than to its severity grading.
Show evidence (1 reference)
PMID:11279527 SUPPORT Model Organism
"Mice lacking the perlecan gene (Hspg2) have a severe chondrodysplasia with dyssegmental ossification of the spine"
Reports the axial ossification defect in the null animal.
Micromelia with Bowed, Dumbbell-Shaped Tubular Bones recapitulates fidelity: moderate Null animals surviving to birth show micromelia with broad and bowed long bones and a narrow thorax.
Limitations: Broad and bowed long bones are not the same observation as the dumbbell metaphyseal configuration used diagnostically in humans, and roughly 40% of nulls die at embryonic day 10.5 from a cephalic defect that has no counterpart in the human disorder, so the surviving fraction is a selected subset.
Show evidence (1 reference)
PMID:10545953 SUPPORT Model Organism
"The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities."
Reports the appendicular and thoracic phenotype of the null animal.
{ }

Source YAML

click to show 
name: Dyssegmental Dysplasia
creation_date: "2026-08-23T00:00:00Z"
category: Mendelian
description: >-
  Dyssegmental dysplasia (DD) is a rare autosomal recessive skeletal dysplasia
  caused by biallelic loss-of-function variants in HSPG2, the gene encoding
  perlecan — the major heparan sulfate proteoglycan of basement membranes and
  of the cartilage pericellular matrix. Its defining radiographic feature is
  anisospondyly: vertebral ossification centres of irregular size and shape,
  with defective segmentation and fusion of the vertebral bodies, from which
  the disorder takes its name. This is combined with micromelia, short thick
  bowed and dumbbell-shaped tubular bones, a narrow thorax, and a
  characteristic flat facies with short neck, micrognathia and cleft palate.
  Mutant perlecan is retained and degraded intracellularly rather than
  secreted, so the cartilage extracellular matrix is depleted of perlecan; the
  growth-plate columnar architecture and endochondral ossification then fail.
  DD is curated here as one entry spanning its two long-recognized clinical
  grades — the lethal Silverman-Handmaker type (DDSH) and the nonlethal
  Rolland-Desbuquois type (DDRD) — because both are now known to be caused by
  biallelic HSPG2 variants and share a single pathomechanism, differing in the
  amount of residual functional perlecan rather than in kind. Together with
  Schwartz-Jampel syndrome, which arises from hypomorphic alleles of the same
  gene, they form an allelic perlecan gene-dosage series.
parents:
- hereditary disease
- Skeletal Dysplasia
synonyms:
- DD
- dyssegmental dwarfism
- anisospondylic camptomicromelic dwarfism
- dyssegmental dysplasia, Silverman-Handmaker type
- dyssegmental dysplasia, Rolland-Desbuquois type
- DDSH
- DDRD
notes: >-
  ONTOLOGY-ANCHOR NOTE (deliberate; do not "fix" by grounding this entry on one
  of its own subtypes). MONDO has no umbrella class for dyssegmental dysplasia:
  it carries MONDO:0009140 (Silverman-Handmaker type) and MONDO:0009139
  (Rolland-Desbuquois type) as separate leaves, plus the unrelated
  MONDO:0011110 (dyssegmental dysplasia-glaucoma syndrome), but nothing for the
  spectrum concept the literature calls "dyssegmental dysplasia (DD)". Because
  this entry models the shared HSPG2/perlecan pathomechanism rather than either
  severity grade individually, no top-level disease_term is asserted; both leaf
  terms are recorded as skos:narrowMatch mappings and as subtype_term bindings
  on the two has_subtypes entries. Grounding the spectrum on MONDO:0009140
  would repeat the "spectrum entry grounded to one of its own subtypes" defect
  tracked in monarch-initiative/dismech#8997. A MONDO new-term request for the
  umbrella concept is the correct upstream fix and has not been filed.

  ENTITY-IDENTITY NOTE. Until 2024 the molecular basis of DDRD was formally
  undetermined and only DDSH was attributed to HSPG2. Yamaguchi et al.
  (PMID:38424183) identified biallelic HSPG2 variants in five DDRD patients
  sharing a founder p.G3324R haplotype, and concluded that SJS, DDRD and DDSH
  are allelic disorders. Sources published before 2024 — including the MONDO
  definitions and OMIM 224400 — may still describe DDRD as of unknown cause;
  that is stale, not a contradiction of this entry. The same paper reports that
  a patient previously published as DDSH homozygous for p.G3324R had a
  nonlethal course better fitting DDRD, so published type assignments are not
  fully reliable and the lethal/nonlethal boundary has no agreed threshold
  (see the ddsh_ddrd_severity_determinants discussion).

  Not to be confused with Schwartz-Jampel syndrome
  (kb/disorders/Schwartz-Jampel_Syndrome.yaml), the hypomorphic-allele end of
  the same HSPG2 series, which is a separate entry and a separate row of the
  ISDS nosology.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0009140
      label: Silverman-Handmaker type dyssegmental dysplasia
    mapping_predicate: skos:narrowMatch
    mapping_source: MONDO (OMIM:224410, Orphanet:1865)
    mapping_justification: >-
      The lethal severity grade of the spectrum modeled here, curated as the
      DDSH subtype. narrowMatch rather than exactMatch because this entry also
      covers the nonlethal Rolland-Desbuquois grade.
  - term:
      id: MONDO:0009139
      label: dyssegmental dysplasia, Rolland-Desbuquois type
    mapping_predicate: skos:narrowMatch
    mapping_source: MONDO (OMIM:224400, Orphanet:156731)
    mapping_justification: >-
      The nonlethal severity grade of the spectrum modeled here, curated as the
      DDRD subtype. narrowMatch rather than exactMatch because this entry also
      covers the lethal Silverman-Handmaker grade.
classifications:
  isds_skeletal_category:
  - classification_value: proteoglycan_core_protein_disorders
    notes: >-
      ISDS Nosology of Genetic Skeletal Disorders, 2023 revision (11th edition;
      Unger et al., PMID:36779427), group 7 "Proteoglycan core protein
      disorders", which lists "Dyssegmental dysplasia, Silverman-Handmaker and
      Rolland-Desbuquois types" with HSPG2 as a single Table 1 row — the same
      lumping decision this entry makes. In the 2019 revision
      (PMID:31633310) the disorder sat in the separate "Perlecan group"
      alongside Schwartz-Jampel syndrome.
    evidence:
    - reference: PMID:36779427
      reference_title: "Nosology of genetic skeletal disorders: 2023 revision."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        the previous “Perlecan group” and “Aggrecan group” were incorporated into the new group of “Proteoglycan core protein disorders”
      explanation: >-
        Supports the group's identity and its provenance from the merged 2019
        perlecan group. Note the scope of this citation: the consensus paper's
        text documents the restructuring, while the assignment of dyssegmental
        dysplasia specifically to that group comes from the revision's Table 1
        row for the disorder, which is not quotable from the cached record.
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    Both clinical grades segregate as autosomal recessive traits. Affected
    individuals carry biallelic (homozygous or compound heterozygous) HSPG2
    variants; parental consanguinity is frequent in DDSH but was absent in all
    five genotyped DDRD patients, who instead shared a founder allele.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We conclude that there are at least two forms of dyssegmental dysplasia, each autosomal recessive, which can be delineated on clinical, radiographic and morphologic grounds.
    explanation: >-
      The clinical-delineation study that separated the two grades states that
      both are autosomal recessive.
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we report a homozygous, 89-bp duplication in exon 34 of HSPG2 in a pair of siblings with DDSH born to consanguineous parents
    explanation: >-
      Homozygosity in the offspring of consanguineous parents, consistent with
      the recessive mode recorded here.
prevalence:
- population: Worldwide (published cases)
  subtype: DDSH
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Cumulative published case count for the lethal grade as of 2018. No
    population-based rate has been established for either grade.
  evidence:
  - reference: PMID:29526034
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: A challenging antenatal diagnosis in a dizygotic twin pregnancy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Less than forty cases are reported in the literature, of which only four were prenatally detected.
    explanation: >-
      Gives the cumulative reported case count, the appropriate occurrence
      measure for a disorder with no population-based ascertainment.
- population: Worldwide (molecularly confirmed cases)
  subtype: DDSH
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Molecularly confirmed cases are a much smaller subset of the clinically
    reported ones, because most affected pregnancies are terminated or the
    neonate dies before genetic testing is undertaken.
  evidence:
  - reference: PMID:29526034
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: A challenging antenatal diagnosis in a dizygotic twin pregnancy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      To the best of our knowledge, the current report is only the seventh molecularly confirmed case of DDSH.
    explanation: >-
      States how few cases had molecular confirmation at the time of report.
has_subtypes:
- name: DDSH
  display_name: Dyssegmental dysplasia, Silverman-Handmaker type (lethal)
  subtype_term:
    preferred_term: Silverman-Handmaker type dyssegmental dysplasia
    term:
      id: MONDO:0009140
      label: Silverman-Handmaker type dyssegmental dysplasia
  description: >-
    The severe, lethal grade. Anisospondyly is marked, with rudimentary or
    absent vertebral bodies, and limb shortening and bowing are severe. Most
    affected pregnancies are terminated or end in stillbirth, and liveborn
    infants usually die within the first days of life from respiratory
    failure. Reported HSPG2 alleles are functional nulls — insertions,
    deletions and splice-site variants predicted to truncate the perlecan core
    protein.
  genes:
  - preferred_term: HSPG2
    term:
      id: hgnc:5273
      label: HSPG2
  inheritance:
  - name: Autosomal recessive inheritance
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Dyssegmental dysplasia (DD) is a severe skeletal dysplasia comprised of two subtypes: lethal Silverman-Handmaker type (DDSH) and nonlethal Rolland-Desbuquois type (DDRD).
    explanation: >-
      Establishes the two-subtype structure used here and identifies DDSH as
      the lethal grade.
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes.
    explanation: >-
      The original delineation of the severe grade and its natural history.
      Note that the cited text misspells the eponym as "Silverman-Handmarker";
      the snippet is quoted verbatim, as required.
- name: DDRD
  display_name: Dyssegmental dysplasia, Rolland-Desbuquois type (nonlethal)
  subtype_term:
    preferred_term: dyssegmental dysplasia, Rolland-Desbuquois type
    term:
      id: MONDO:0009139
      label: dyssegmental dysplasia, Rolland-Desbuquois type
  description: >-
    The milder, nonlethal grade. Vertebral bodies are better ossified than in
    DDSH and long-bone shortening and bowing are milder, with radiographic
    changes that were originally described as resembling Kniest dysplasia.
    Survival beyond the newborn period is frequent, though respiratory support
    is commonly required. Attributed to HSPG2 only in 2024, when biallelic
    variants were found in five patients sharing a founder p.G3324R haplotype;
    mild myotonic features overlapping Schwartz-Jampel syndrome were noted in
    one of them.
  genes:
  - preferred_term: HSPG2
    term:
      id: hgnc:5273
      label: HSPG2
  inheritance:
  - name: Autosomal recessive inheritance
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report that five patients with DDRD carried four pathogenic variants in HSPG2
    explanation: >-
      Establishes HSPG2 as the cause of the Rolland-Desbuquois grade, which is
      what makes curating DD as a single mechanism entry defensible.
  - reference: PMID:9388290
    reference_title: "Dyssegmental dysplasia: a case report of a Rolland-Desbuquois type."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The milder Rolland-Desbuquois type is frequently characterised by survival beyond the newborn period.
    explanation: >-
      States the natural-history distinction that defines this grade.
  review_notes: >-
    The 2024 molecular series is five patients from a single Japanese founder
    haplotype, so the DDRD allelic spectrum outside that haplotype is
    essentially uncharacterized. Treat the p.G3324R-centred genotype picture as
    provisional.
pathophysiology:
- name: Biallelic HSPG2 Loss-of-Function Variation
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    The initiating lesion is biallelic loss-of-function variation in HSPG2,
    which encodes perlecan. In the lethal Silverman-Handmaker grade the
    reported alleles are functional nulls — exonic duplications, deletions and
    splice-site variants that cause frameshifts and truncate the core protein.
    In the nonlethal Rolland-Desbuquois grade the recurrent allele is instead a
    missense change, p.G3324R in an immunoglobulin-like repeat of perlecan
    domain IV, carried either homozygously or in trans with a nonsense or
    splice-site allele.
  genes:
  - preferred_term: HSPG2
    term:
      id: hgnc:5273
      label: HSPG2
  genetic_context:
    gene:
      preferred_term: HSPG2
      term:
        id: hgnc:5273
        label: HSPG2
    variant_origin: GERMLINE
    zygosity: HOMOZYGOUS
    functional_impact_category: LOSS_OF_FUNCTION
    description: >-
      Homozygous or compound heterozygous germline HSPG2 alleles. Homozygosity
      predominates in consanguineous DDSH families; compound heterozygosity is
      common in the DDRD series, where one allele is the founder missense
      variant.
  evidence:
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These mutations are predicted to cause a frameshift, resulting in a truncated protein core.
    explanation: >-
      Gives the molecular consequence of the DDSH alleles that initiates this
      pathograph.
  - reference: PMID:40503612
    reference_title: "Prenatal Diagnosis of HSPG2-Related Dyssegmental Dysplasia: The First Report From Turkey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Silverman-Handmaker type dyssegmental dysplasia (DDSH) is a rare and lethal skeletal dysplasia caused by biallelic null variations in the HSPG2 gene, which encodes the extracellular matrix proteoglycan perlecan.
    explanation: >-
      Contemporary confirmation that biallelic HSPG2 null variation is the
      cause of the lethal grade.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report that five patients with DDRD carried four pathogenic variants in HSPG2
    explanation: >-
      Extends the same initiating lesion to the nonlethal grade.
  downstream:
  - target: Failure of Perlecan Secretion into the Extracellular Matrix
    causal_link_type: DIRECT
    description: >-
      Truncated perlecan core protein is unstable and is retained and degraded
      inside the cell instead of being secreted.
    evidence:
    - reference: PMID:11891676
      reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Immunostaining and biochemical analysis revealed that the mutant perlecan molecules were unstable and not secreted into the extracellular matrix.
      explanation: >-
        States the direct consequence of the variant on perlecan secretion.
- name: Failure of Perlecan Secretion into the Extracellular Matrix
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Perlecan is the major heparan sulfate proteoglycan of basement membranes
    and is also a component of the cartilage pericellular matrix, even though
    cartilage has no basement membranes. Patient fibroblasts do not secrete the
    truncated protein; it is degraded intracellularly, and patient cartilage
    matrix stains only weakly for perlecan while intracellular inclusion bodies
    stain positively. The result is a functional null for extracellular
    perlecan — the distinction from Schwartz-Jampel syndrome, where hypomorphic
    alleles still permit secretion at a reduced level.
  gene_products:
  - preferred_term: perlecan
    term:
      id: NCIT:C95448
      label: Basement Membrane-Specific Heparan Sulfate Proteoglycan Core Protein
    modifier: DECREASED
  cellular_components:
  - preferred_term: extracellular matrix
    term:
      id: GO:0031012
      label: extracellular matrix
    modifier: DECREASED
  - preferred_term: basement membrane
    term:
      id: GO:0005604
      label: basement membrane
    modifier: DECREASED
  evidence:
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Biochemically, truncated perlecan was not secreted by the patient fibroblasts, but was degraded to smaller fragments within the cells.
    explanation: >-
      Direct biochemical demonstration of the secretion failure in patient
      cells.
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The cartilage matrix from these patients stained poorly with antibody specific for perlecan, but there was staining of intracellular inclusion bodies.
    explanation: >-
      Shows the deficit in the tissue that matters — patient cartilage matrix —
      together with the intracellular retention that causes it.
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Thus, DDSH is caused by a functional null mutation of HSPG2.
    explanation: >-
      States the functional-null character of the lesion, which is what
      separates this entry from Schwartz-Jampel syndrome.
  downstream:
  - target: Cartilage Extracellular Matrix Disorganization
    causal_link_type: DIRECT
    description: >-
      Loss of extracellular perlecan disorganizes the collagen fibril and
      glycosaminoglycan architecture of the cartilage matrix.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Hspg2-/- cartilage matrix contained reduced and disorganized collagen fibrils and glycosaminoglycans, suggesting that perlecan has an important role in matrix structure.
      explanation: >-
        The perlecan-null mouse supplies the causal direction from absent
        perlecan to disorganized cartilage matrix, which patient material
        cannot establish.
- name: Cartilage Extracellular Matrix Disorganization
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    Without perlecan the cartilage extracellular matrix loses its normal
    collagen fibril and glycosaminoglycan organization. That a chondrodysplasia
    should follow from loss of a basement-membrane proteoglycan was
    unexpected, and it is the finding that established perlecan as a structural
    requirement of cartilage rather than only of basement membranes.
  cell_types:
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: extracellular matrix organization
    term:
      id: GO:0030198
      label: extracellular matrix organization
    modifier: DECREASED
  evidence:
  - reference: PMID:11891676
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A chondrodysplasia phenotype caused by the loss of perlecan was unexpected, because cartilage does not have basement membranes.
    explanation: >-
      Frames why this node is the mechanistically informative step rather than
      an obvious consequence of a basement-membrane defect.
  - reference: PMID:11891676
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These results indicate that DDSH is caused by functional null mutations of HSPG2 and that perlecan is essential for cartilage development.
    explanation: >-
      States the requirement for perlecan in cartilage development that this
      node represents.
  downstream:
  - target: Growth-Plate Disorganization and Defective Endochondral Ossification
    causal_link_type: DIRECT
    description: >-
      A disorganized pericellular matrix removes the scaffold on which growth
      plate chondrocytes form proliferative columns and progress to hypertrophy.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification.
      explanation: >-
        Links matrix disorganization to failure of growth-plate architecture
        and ossification in the null animal.
- name: Growth-Plate Disorganization and Defective Endochondral Ossification
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    The growth plate loses its columnar chondrocyte organization, chondrocyte
    proliferation falls and the prehypertrophic zone is diminished, so
    endochondral ossification — the process that lengthens tubular bones and
    ossifies the vertebral bodies — proceeds abnormally. This single node is
    the branch point from which both cardinal radiographic features of the
    disorder follow: anisospondyly in the axial skeleton and micromelia with
    dumbbell-shaped, bowed tubular bones in the appendicular skeleton.
  cell_types:
  - preferred_term: growth plate chondrocyte
    term:
      id: CL:1000217
      label: growth plate cartilage chondrocyte
  biological_processes:
  - preferred_term: endochondral ossification
    term:
      id: GO:0001958
      label: endochondral ossification
    modifier: DECREASED
  - preferred_term: chondrocyte proliferation
    term:
      id: GO:0035988
      label: chondrocyte proliferation
    modifier: DECREASED
  - preferred_term: chondrocyte differentiation
    term:
      id: GO:0002062
      label: chondrocyte differentiation
    modifier: DECREASED
  evidence:
  - reference: PMID:10545953
    reference_title: Perlecan is essential for cartilage and cephalic development.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In Hspg2-/- cartilage, proliferation of chondrocytes was reduced and the prehypertrophic zone was diminished.
    explanation: >-
      Gives the cellular content of this node — reduced proliferation and a
      diminished prehypertrophic zone.
  - reference: PMID:10545953
    reference_title: Perlecan is essential for cartilage and cephalic development.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification.
    explanation: >-
      States the growth-plate architectural failure and the ossification defect.
  downstream:
  - target: Anisospondyly
    causal_link_type: DIRECT
    description: >-
      Abnormal ossification of the vertebral bodies produces ossification
      centres of irregular size and shape with defective segmentation and
      fusion.
    evidence:
    - reference: PMID:11279527
      reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Mice lacking the perlecan gene (Hspg2) have a severe chondrodysplasia with dyssegmental ossification of the spine
      explanation: >-
        Perlecan loss is sufficient to produce dyssegmental spinal ossification
        in the mouse, supporting this edge.
  - target: Micromelia with Bowed, Dumbbell-Shaped Tubular Bones
    causal_link_type: DIRECT
    description: >-
      Defective endochondral growth shortens and deforms the tubular bones,
      giving the short, thick, bowed and dumbbell-shaped appearance.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities.
      explanation: >-
        The null mouse reproduces the micromelic, bowed-bone and narrow-thorax
        phenotype downstream of the same lesion.
- name: Anisospondyly
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    Anisospondyly — vertebral ossification centres of irregular size and shape,
    with defective segmentation and fusion of the vertebral body components —
    is the feature the disorder is named for and the one that distinguishes it
    radiographically from other lethal short-limbed dysplasias. Its severity is
    what separates the two clinical grades: vertebral bodies may be rudimentary
    or absent in DDSH but are better ossified in DDRD.
  evidence:
  - reference: PMID:20542149
    reference_title: Phenotypic and molecular characterization of a novel case of dyssegmental dysplasia, Silverman-Handmaker type.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      is an autosomal recessive form of lethal dwarfism characterized by a defect in segmentation and fusion of vertebral bodies components ("anisospondyly") and by severe limb shortening
    explanation: >-
      Defines anisospondyly in the terms used for this node.
  - reference: PMID:23836246
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: prenatal ultrasound findings and molecular analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The main prenatal ultrasound and the radiological features of DD-SH were severe limb shortening and vertebral segmentation and fusion defects (anisospondyly).
    explanation: >-
      Confirms anisospondyly as a main radiological feature, detectable
      prenatally.
  downstream:
  - target: Narrow Thorax with Neonatal Respiratory Failure
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Vertebral malsegmentation, together with the short ribs of the
      appendicular arm, restricts thoracic volume.
    evidence:
    - reference: PMID:3605216
      reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen.
      explanation: >-
        Establishes that a narrow thorax co-occurs with the vertebral and limb
        features. Indirect for this causal edge: the source reports
        co-occurrence rather than testing the causal contribution of
        malsegmentation to thoracic restriction, so the edge follows by an
        inference step rather than being asserted.
- name: Micromelia with Bowed, Dumbbell-Shaped Tubular Bones
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    Severe, proximally predominant limb shortening with short, thick, bowed
    long bones, the metaphyseal flaring of which gives the characteristic
    dumbbell shape. Rib shortening is part of the same appendicular arm and
    contributes to the narrow thorax.
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The dyssegmental dysplasias are lethal forms of neonatal short-limbed dwarfism in which vertebral segmentation defects and short, thick, bowed long bones are the prominent radiographic features.
    explanation: >-
      Names the appendicular features recorded at this node.
  - reference: PMID:11891676
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Dyssegmental dysplasia, Silverman-Handmaker type (DDSH), is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia.
    explanation: >-
      Pairs the micromelia with the anisospondyly as the defining combination.
  downstream:
  - target: Narrow Thorax with Neonatal Respiratory Failure
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Short ribs and a small chest wall limit lung growth and ventilation.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities.
      explanation: >-
        In the null mouse the micromelic skeletal dysplasia, the narrow thorax
        and death just after birth occur together, supporting this edge.
- name: Narrow Thorax with Neonatal Respiratory Failure
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  description: >-
    The terminal common pathway and the proximate cause of death. A small,
    narrow thorax restricts lung growth and ventilation; liveborn infants
    present with respiratory failure. In the lethal grade this is fatal within
    days, while in the nonlethal grade prolonged respiratory support may allow
    survival for years. Cleft palate and micrognathia add an upper-airway
    component.
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes.
    explanation: >-
      Gives the lethal outcome of the severe grade. The eponym is misspelled in
      the source and is quoted verbatim.
  - reference: PMID:9388290
    reference_title: "Dyssegmental dysplasia: a case report of a Rolland-Desbuquois type."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The case is unusual because although it was the milder form, the child died on the 10th day of life.
    explanation: >-
      Shows that the nonlethal grade is not reliably nonlethal. Indirect for
      this node: the report describes one death in the milder grade, from which
      the framing of a shared terminal pathway differing in timing rather than
      presence follows by inference rather than being stated.
phenotypes:
- category: Skeletal
  name: Anisospondyly
  description: >-
    Irregular size and shape of single or multiple vertebral ossification
    centres with defective segmentation and fusion; the eponymous feature.
  phenotype_term:
    preferred_term: Vertebral segmentation defect
    term:
      id: HP:0003422
      label: Vertebral segmentation defect
  diagnostic: true
  evidence:
  - reference: PMID:23836246
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: prenatal ultrasound findings and molecular analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The main prenatal ultrasound and the radiological features of DD-SH were severe limb shortening and vertebral segmentation and fusion defects (anisospondyly).
    explanation: >-
      Names vertebral segmentation and fusion defects as a main feature.
- category: Skeletal
  name: Coronal Cleft Vertebrae
  description: >-
    Coronal clefting of the vertebral bodies, arising when a vertebral body
    forms from two ventral/dorsal primary ossification centres instead of one.
    At the mild end of DDRD it may be the only spinal finding, which makes it
    the feature the disorder is recognized by in its least severe presentation.
  phenotype_term:
    preferred_term: Coronal cleft vertebrae
    term:
      id: HP:0003417
      label: Coronal cleft vertebrae
  evidence:
  - reference: PMID:29526034
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: A challenging antenatal diagnosis in a dizygotic twin pregnancy."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Skeletal survey confirmed anisospondyly, coronal clefts and lack of ossification of several vertebrae, sagittal cleft in the vertebral bodies of the cervical and lumbar spine
    explanation: >-
      Radiographic confirmation of coronal clefting in a case of the severe
      grade.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The mildest end of DDRD is associated with only large coronal clefts at the thoracolumbar spine
    explanation: >-
      Establishes coronal clefting as the defining spinal finding at the mild
      end of DDRD, where it may be the only one.
- category: Skeletal
  name: Deficient Vertebral Ossification
  description: >-
    Reduced or absent mineralization of the vertebral bodies. This is the
    substrate of anisospondyly and the sharpest severity contrast between the
    two grades: vertebral bodies may be rudimentary or absent in DDSH, whereas
    in DDRD they are better ossified.
  phenotype_term:
    preferred_term: Absent or minimally ossified vertebral bodies
    term:
      id: HP:0004599
      label: Absent or minimally ossified vertebral bodies
  evidence:
  - reference: PMID:29526034
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: A challenging antenatal diagnosis in a dizygotic twin pregnancy."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Skeletal survey confirmed anisospondyly, coronal clefts and lack of ossification of several vertebrae, sagittal cleft in the vertebral bodies of the cervical and lumbar spine
    explanation: >-
      Documents absent ossification of several vertebrae in the severe grade.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Some vertebral bodies are rudimentary or even absent in DDSH, while all vertebral bodies are better ossified in DDRD.
    explanation: >-
      States the ossification deficit and grades it between DDSH and DDRD,
      which is why this phenotype is curated at disease level rather than
      scoped to one subtype.
- category: Neurologic
  name: Hydrocephalus
  subtype: DDSH
  description: >-
    Hydrocephalus reported in the severe grade and not in DDRD, one of the
    central nervous system features that separates the two grades.
  phenotype_term:
    preferred_term: Hydrocephalus
    term:
      id: HP:0000238
      label: Hydrocephalus
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      DDSH is sometimes accompanied by hydrocephalus and occipital encephalocele that may manifest with defective calvarial ossification, while DDRD is not.
    explanation: >-
      Directly attributes this finding to DDSH and explicitly excludes it from
      DDRD, which is the basis for scoping the phenotype to the DDSH subtype.
      No frequency is asserted: the source says "sometimes" without
      quantifying it.
- category: Neurologic
  name: Occipital Encephalocele
  subtype: DDSH
  description: >-
    Occipital encephalocele reported in the severe grade, which may manifest
    with defective calvarial ossification. Not reported in DDRD.
  phenotype_term:
    preferred_term: Occipital encephalocele
    term:
      id: HP:0002085
      label: Occipital encephalocele
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      DDSH is sometimes accompanied by hydrocephalus and occipital encephalocele that may manifest with defective calvarial ossification, while DDRD is not.
    explanation: >-
      Directly attributes this finding to DDSH and explicitly excludes it from
      DDRD, which is the basis for scoping the phenotype to the DDSH subtype.
      No frequency is asserted: the source says "sometimes" without
      quantifying it.
- category: Skeletal
  name: Micromelia
  description: >-
    Severe shortening of the limbs, recognizable prenatally as long-bone
    measurements far below the expected range.
  phenotype_term:
    preferred_term: Micromelia
    term:
      id: HP:0002983
      label: Micromelia
  diagnostic: true
  evidence:
  - reference: PMID:11891676
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Dyssegmental dysplasia, Silverman-Handmaker type (DDSH), is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia.
    explanation: >-
      Names micromelia as characteristic of the disorder.
- category: Skeletal
  name: Bowed Long Bones
  description: >-
    Short, thick, bowed tubular bones with metaphyseal flaring giving a
    dumbbell configuration.
  phenotype_term:
    preferred_term: Bowing of the long bones
    term:
      id: HP:0006487
      label: Bowing of the long bones
  diagnostic: true
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The dyssegmental dysplasias are lethal forms of neonatal short-limbed dwarfism in which vertebral segmentation defects and short, thick, bowed long bones are the prominent radiographic features.
    explanation: >-
      Names short, thick, bowed long bones as a prominent radiographic feature.
- category: Skeletal
  name: Severe Short Stature
  description: >-
    Disproportionate short-limbed dwarfism; in the survivors of the nonlethal
    grade, height remains far below the population range.
  phenotype_term:
    preferred_term: Severe short stature
    term:
      id: HP:0003510
      label: Severe short stature
  evidence:
  - reference: PMID:11891676
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia
    explanation: >-
      Characterizes the disorder as a form of dwarfism.
- category: Respiratory
  name: Narrow Thorax
  description: >-
    A small, narrow chest that restricts lung growth and ventilation and is the
    principal determinant of survival.
  phenotype_term:
    preferred_term: Narrow chest
    term:
      id: HP:0000774
      label: Narrow chest
  diagnostic: true
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen.
    explanation: >-
      Lists narrow thorax among the commonly seen clinical features.
- category: Respiratory
  name: Respiratory Insufficiency
  description: >-
    Neonatal respiratory failure from thoracic restriction, requiring
    ventilatory support in survivors and causing death in the lethal grade.
  phenotype_term:
    preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: PMID:10545953
    reference_title: Perlecan is essential for cartilage and cephalic development.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities.
    explanation: >-
      Model-organism evidence for perinatal death with a narrow thorax.
      Indirect on two counts: the mouse study reports death rather than a
      measured respiratory phenotype, and it is a non-human model system, so
      respiratory insufficiency in the human disease follows by inference.
- category: Craniofacial
  name: Cleft Palate
  description: >-
    Palatal clefting, frequently U-shaped, contributing to the upper-airway and
    feeding difficulties.
  phenotype_term:
    preferred_term: Cleft palate
    term:
      id: HP:0000175
      label: Cleft palate
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen.
    explanation: >-
      Lists cleft palate among the commonly seen clinical features.
- category: Craniofacial
  name: Short Neck
  description: >-
    A short neck, part of the recognizable gestalt together with the flat
    facial appearance and micrognathia.
  phenotype_term:
    preferred_term: Short neck
    term:
      id: HP:0000470
      label: Short neck
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen.
    explanation: >-
      Lists short neck among the commonly seen clinical features.
- category: Craniofacial
  name: Micrognathia
  description: >-
    A small mandible, noted prenatally as retrognathia and contributing to the
    upper-airway component of the respiratory phenotype.
  phenotype_term:
    preferred_term: Micrognathia
    term:
      id: HP:0000347
      label: Micrognathia
  evidence:
  - reference: PMID:40503612
    reference_title: "Prenatal Diagnosis of HSPG2-Related Dyssegmental Dysplasia: The First Report From Turkey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      referred due to ultrasonographic findings of limb shortening, retrognathia, and irregularities in the lumbar vertebrae
    explanation: >-
      Prenatal report listing retrognathia alongside the skeletal findings.
- category: Musculoskeletal
  name: Limitation of Joint Mobility
  description: >-
    Reduced joint mobility with contractures, overlapping the joint phenotype
    of the allelic Schwartz-Jampel syndrome.
  phenotype_term:
    preferred_term: Limitation of joint mobility
    term:
      id: HP:0001376
      label: Limitation of joint mobility
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinically, unusual facies, short neck, narrow thorax, cleft palate, and reduced joint mobility are commonly seen.
    explanation: >-
      Lists reduced joint mobility among the commonly seen clinical features.
- category: Neuromuscular
  name: Myotonia
  subtype: DDRD
  description: >-
    Mild limb myotonia with blepharophimosis and pursed lips — features typical
    of the allelic Schwartz-Jampel syndrome — observed in one genotyped DDRD
    patient. Muscle hyperexcitability was not assessed in the other four, so
    its frequency is unknown and may be underestimated in survivors whose
    disability is dominated by the skeletal disease.
  phenotype_term:
    preferred_term: Myotonia
    term:
      id: HP:0002486
      label: Myotonia
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She had mild bilateral blepharophimosis, mild pursed lips, and mild limb myotonia, which are commonly observed in SJS.
    explanation: >-
      Direct observation of limb myotonia in a DDRD patient — the quote asserts
      the phenotype itself. It rests on one of five patients in the series and
      the other four were not examined for muscle hyperexcitability, so the
      frequency is unestablished even though the observation is direct.
progression:
- phase: Perinatal course of the lethal grade
  subtype: DDSH
  age_range: Prenatal to first days of life
  notes: >-
    Most affected pregnancies are terminated after prenatal diagnosis or end in
    stillbirth; liveborn infants typically die within the first days of life
    from respiratory failure. Prolonged survival is exceptional.
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The severe form, "dyssegmental dysplasia, type Silverman-Handmarker," is characterized by stillbirth or death within the first few days of life and by distinct and more severe radiographic changes.
    explanation: >-
      States the natural history of the lethal grade. The eponym is misspelled
      in the source and is quoted verbatim.
- phase: Postnatal course of the nonlethal grade
  subtype: DDRD
  age_range: Neonatal period onward
  notes: >-
    Survival beyond the newborn period is frequent, commonly after a period of
    ventilatory support for neonatal respiratory insufficiency. The boundary
    with the lethal grade is not absolute: a case reported as the milder form
    nevertheless died on the tenth day of life.
  evidence:
  - reference: PMID:9388290
    reference_title: "Dyssegmental dysplasia: a case report of a Rolland-Desbuquois type."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The milder Rolland-Desbuquois type is frequently characterised by survival beyond the newborn period.
    explanation: >-
      States the natural history of the milder grade.
  - reference: PMID:9388290
    reference_title: "Dyssegmental dysplasia: a case report of a Rolland-Desbuquois type."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The case is unusual because although it was the milder form, the child died on the 10th day of life.
    explanation: >-
      Documents that the nonlethal designation is not absolute; the quote
      asserts that claim directly.
genetic:
- name: HSPG2
  gene_term:
    preferred_term: HSPG2
    term:
      id: hgnc:5273
      label: HSPG2
  presence: PRESENT
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  inheritance:
  - name: Autosomal recessive inheritance
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
  frequency: >-
    The only gene known to cause dyssegmental dysplasia; biallelic variants
    account for both the lethal and the nonlethal grade.
  notes: >-
    HSPG2 is a 97-exon gene, and reported alleles are distributed across it
    rather than clustering in a hotspot. DDSH alleles are insertions, deletions
    and splice-site variants predicted to truncate the perlecan core protein.
    The DDRD series is dominated by a single missense founder allele,
    p.G3324R, in an immunoglobulin-like repeat of perlecan domain IV, seen
    either homozygously or in trans with a nonsense or splice-site allele.
    Whether an allele pair produces the hypomorphic Schwartz-Jampel phenotype,
    DDRD, or lethal DDSH is not predictable from variant class alone; see the
    ddsh_ddrd_severity_determinants discussion.
  evidence:
  - reference: PMID:11279527
    reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Thus, DDSH is caused by a functional null mutation of HSPG2.
    explanation: >-
      The gene-identification report for the lethal grade.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SJS, DDRD, and DDSH are allelic disorders with pathogenic variants in HSPG2.
    explanation: >-
      Extends HSPG2 causation to the nonlethal grade and places all three
      perlecan disorders in one allelic series.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two patients were homozygous for p.G3324R, and three patients were heterozygous for p.G3324R.
    explanation: >-
      Documents the recurrent founder allele and the homozygous versus compound
      heterozygous allele configurations recorded in the notes.
diagnosis:
- name: Prenatal ultrasound and fetal imaging
  description: >-
    The diagnosis is usually first raised prenatally on limb shortening well
    below the expected range, long-bone bowing, a small thorax and irregular
    vertebral ossification. Three-dimensional ultrasound and CT resolve the
    dumbbell deformity and anisospondyly that distinguish dyssegmental
    dysplasia from other lethal short-limbed dysplasias, and it has been
    detected as early as 13 weeks.
  evidence:
  - reference: PMID:23836246
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: prenatal ultrasound findings and molecular analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      DD-SH can be diagnosed prenatally using fetal ultrasound as early as 13 weeks.
    explanation: >-
      States the earliest reported gestational age at prenatal diagnosis.
- name: HSPG2 sequencing
  description: >-
    Molecular confirmation by targeted skeletal-dysplasia panel, exome or
    genome sequencing. Confirmation matters beyond diagnosis: it enables
    preimplantation and prenatal diagnosis in subsequent at-risk pregnancies,
    and it is the only way to separate this disorder from radiographically
    overlapping lethal dysplasias.
  evidence:
  - reference: PMID:23836246
    reference_title: "Dyssegmental dysplasia, Silverman-Handmaker type: prenatal ultrasound findings and molecular analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Xrays and DNA analysis of the HSPG2 gene are important for the confirmation of the diagnosis and for the preimplantation and prenatal diagnosis in pregnancies at risk.
    explanation: >-
      States the role of HSPG2 sequencing in confirmation and in reproductive
      counselling.
  - reference: PMID:40503612
    reference_title: "Prenatal Diagnosis of HSPG2-Related Dyssegmental Dysplasia: The First Report From Turkey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Targeted skeletal dysplasia panel testing via next-generation sequencing (NGS) revealed a novel homozygous splice site variant
    explanation: >-
      Worked example of panel-based NGS confirmation in a prenatal case.
animal_models:
- name: Perlecan-null (Hspg2-/-) mouse
  species: Mouse
  genotype: Hspg2 homozygous null (Hspg2-/-)
  publication: PMID:10545953
  description: >-
    Germline disruption of Hspg2. About 40% of nulls die at embryonic day 10.5
    with defective cephalic development; the remainder die just after birth
    with a skeletal dysplasia. The radiographic, clinical and chondro-osseous
    resemblance of this line to human DDSH is what prompted the search for
    HSPG2 mutations in patients, so the model is historically upstream of the
    human gene identification rather than derived from it.
  genes:
  - preferred_term: HSPG2
    term:
      id: hgnc:5273
      label: HSPG2
  modeled_mechanisms:
  - target: Cartilage Extracellular Matrix Disorganization
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Null cartilage matrix shows reduced and disorganized collagen fibrils and
      glycosaminoglycans, the tissue-level content of this node.
    limitations: >-
      The mouse is a constitutive whole-body null, whereas human DDSH alleles
      are functional nulls that still produce an intracellularly retained,
      degraded core protein; any gain-of-function or ER-stress contribution
      from the retained human protein is absent from this model.
    readouts:
    - name: Cartilage collagen fibril and glycosaminoglycan organization
      target: Cartilage Extracellular Matrix Disorganization
      direction: DECREASED
      interpretation: >-
        Ultrastructural correlate of the matrix-disorganization node.
      evidence:
      - reference: PMID:10545953
        reference_title: Perlecan is essential for cartilage and cephalic development.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Hspg2-/- cartilage matrix contained reduced and disorganized collagen fibrils and glycosaminoglycans, suggesting that perlecan has an important role in matrix structure.
        explanation: Reports the measured matrix abnormality.
    evidence:
    - reference: PMID:11279527
      reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Mice lacking the perlecan gene (Hspg2) have a severe chondrodysplasia with dyssegmental ossification of the spine and show radiographic, clinical and chondro-osseous morphology similar to a lethal autosomal recessive disorder in humans termed dyssegmental dysplasia, Silverman-Handmaker type (DDSH; MIM 224410)
      explanation: >-
        States the chondro-osseous correspondence between the null mouse and
        human DDSH that makes this model informative.
  - target: Growth-Plate Disorganization and Defective Endochondral Ossification
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      The line reproduces the growth-plate failure: loss of chondrocyte
      columnar organization, reduced proliferation, a diminished
      prehypertrophic zone and defective endochondral ossification.
    limitations: >-
      Reported as a qualitative histological description rather than as
      quantified zone measurements, and the authors note the skeletal phenotype
      also resembles thanatophoric dysplasia type I, so growth-plate
      disorganization at this level of description is not specific to perlecan
      loss.
    readouts:
    - name: Growth-plate chondrocyte proliferation
      target: Growth-Plate Disorganization and Defective Endochondral Ossification
      direction: DECREASED
      interpretation: >-
        Cellular correlate of the growth-plate node.
      evidence:
      - reference: PMID:10545953
        reference_title: Perlecan is essential for cartilage and cephalic development.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          In Hspg2-/- cartilage, proliferation of chondrocytes was reduced and the prehypertrophic zone was diminished.
        explanation: Reports the proliferation and prehypertrophic-zone deficits.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification.
      explanation: >-
        Supports treating this model as informative for the growth-plate node.
  - target: Anisospondyly
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      The null mouse shows dyssegmental ossification of the spine, the animal
      counterpart of human anisospondyly.
    limitations: >-
      Reported as dyssegmental ossification of the spine, without the
      vertebra-by-vertebra radiographic grading used to separate DDSH from
      DDRD in humans, so the model speaks to the presence of the axial defect
      rather than to its severity grading.
    evidence:
    - reference: PMID:11279527
      reference_title: Dyssegmental dysplasia, Silverman-Handmaker type, is caused by functional null mutations of the perlecan gene.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Mice lacking the perlecan gene (Hspg2) have a severe chondrodysplasia with dyssegmental ossification of the spine
      explanation: >-
        Reports the axial ossification defect in the null animal.
  - target: Micromelia with Bowed, Dumbbell-Shaped Tubular Bones
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Null animals surviving to birth show micromelia with broad and bowed long
      bones and a narrow thorax.
    limitations: >-
      Broad and bowed long bones are not the same observation as the dumbbell
      metaphyseal configuration used diagnostically in humans, and roughly 40%
      of nulls die at embryonic day 10.5 from a cephalic defect that has no
      counterpart in the human disorder, so the surviving fraction is a
      selected subset.
    evidence:
    - reference: PMID:10545953
      reference_title: Perlecan is essential for cartilage and cephalic development.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities.
      explanation: >-
        Reports the appendicular and thoracic phenotype of the null animal.
differential_diagnoses:
- name: Schwartz-Jampel syndrome
  description: >-
    The hypomorphic-allele end of the same HSPG2 series: a nonlethal
    chondrodysplasia with continuous muscle hyperactivity (myotonia) and a
    characteristic mask-like face. Distinguished from dyssegmental dysplasia by
    residual perlecan secretion, survival into adult life, and the dominance of
    the neuromuscular rather than the axial-ossification phenotype — though the
    boundary is not absolute, since mild myotonic features have been reported
    in DDRD.
  distinguishing_features:
  - Hypomorphic HSPG2 alleles permitting reduced perlecan secretion, rather than functional nulls
  - Myotonia and mask-like facies dominate the presentation
  - No anisospondyly
  - Survival into adult life
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Schwartz-Jampel syndrome (SJS) is also caused by biallelic pathogenic variants in HSPG2 and is an allelic disorder of DDSH.
    explanation: >-
      Establishes the allelic relationship that makes SJS the primary
      differential.
- name: Kniest dysplasia
  description: >-
    A COL2A1 type II collagenopathy whose radiographic changes — including
    large coronal clefts of the vertebral bodies — were the original
    comparator for the milder Rolland-Desbuquois grade. Distinguished
    molecularly (COL2A1 rather than HSPG2) and by its nonlethal, myopic and
    hearing-impaired phenotype.
  distinguishing_features:
  - Causal gene COL2A1 rather than HSPG2
  - Coronal clefting without the irregular or absent vertebral ossification centres of DDSH
  - Myopia and sensorineural hearing loss
  evidence:
  - reference: PMID:3605216
    reference_title: "Dyssegmental dysplasias: clinical, radiographic, and morphologic evidence of heterogeneity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The milder form, "dyssegmental dysplasia, type Rolland-Desbuquois," is characterized clinically by frequent survival beyond the newborn period and by distinct radiographic changes resembling Kniest dysplasia.
    explanation: >-
      Records the radiographic resemblance that makes Kniest dysplasia the
      relevant differential for the milder grade.
discussions:
- discussion_id: ddsh_ddrd_severity_determinants
  prompt: >-
    What determines whether a given biallelic HSPG2 genotype produces
    Schwartz-Jampel syndrome, nonlethal Rolland-Desbuquois dyssegmental
    dysplasia, or lethal Silverman-Handmaker dyssegmental dysplasia?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Biallelic HSPG2 Loss-of-Function Variation
  - pathophysiology#Failure of Perlecan Secretion into the Extracellular Matrix
  - pathophysiology#Narrow Thorax with Neonatal Respiratory Failure
  rationale: >-
    The three perlecan disorders are allelic, and the intuitive account is a
    gene-dosage series in which residual extracellular perlecan sets severity.
    Three observations keep that account from being settled. First, no
    threshold separating the lethal from the nonlethal grade has been proposed.
    Second, the same p.G3324R allele has been reported homozygously in a
    patient labelled DDSH whose nonlethal course actually fitted DDRD, so
    published grade assignments are partly unreliable and identical genotypes
    have carried different labels. Third, two DDRD patients homozygous for that
    allele differed markedly in when skeletal disease became apparent, which
    the authors attribute to an unidentified modifier gene or environmental
    factor. Perlecan is also a five-domain multifunctional protein, so a
    domain-specific effect is a competing explanation to a purely quantitative
    one. Resolving this matters clinically, because prenatal counselling for a
    newly identified HSPG2 genotype currently cannot predict lethality.
  proposed_experiments:
  - experiment_id: ddsh_ddrd_residual_perlecan_quantification
    name: Quantify secreted perlecan across genotypes of the HSPG2 series
    description: >-
      Measure secreted perlecan in patient fibroblasts from genotype-matched
      SJS, DDRD and DDSH cases, and test whether residual extracellular
      perlecan orders with clinical grade independently of which domain is
      affected. Include the p.G3324R homozygotes, whose divergent courses
      would separate a dose effect from a modifier effect.
  - experiment_id: ddsh_ddrd_domain_iv_knockin
    name: Domain IV missense knock-in mouse
    description: >-
      Knock the DDRD founder allele p.G3324R into the mouse Hspg2
      immunoglobulin-like domain IV repeat and compare it with the existing
      null and hypomorphic lines, to test whether a domain IV missense change
      produces an intermediate phenotype at a perlecan level that a hypomorph
      does not.
  evidence:
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      No definite threshold has been proposed to delineate lethal and nonlethal phenotypes.
    explanation: >-
      States directly that no threshold separating the two grades has been
      proposed — the gap recorded here.
  - reference: PMID:38424183
    reference_title: Dyssegmental dysplasia Rolland-Desbuquois type is caused by pathogenic variants in HSPG2 - a founder haplotype shared in five patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Presence of an unidentified modifier gene or an unidentified environmental factor might have accounted for the different disease severities.
    explanation: >-
      Records the authors' own reading that genotype alone does not account for
      the severity difference between two homozygotes for the same allele.
tracked_issues:
- url: https://github.com/monarch-initiative/dismech/issues/7867
  title: "Coverage gap: 4 ISDS skeletal-nosology groups have zero dismech entries"
  tracked_issue_role: SOURCE
  notes: >-
    This entry is the second dismech member of the ISDS 2023 group 7
    (proteoglycan core protein disorders), after Schwartz-Jampel syndrome, and
    completes the HSPG2/perlecan arm of that group. The three ACAN disorders
    listed in that issue remain uncurated.
- url: https://github.com/monarch-initiative/dismech/issues/8997
  title: "CRB1_Retinal_Dystrophies: spectrum entry is grounded to one of its own subtypes"
  tracked_issue_role: RELATED
  notes: >-
    The reason this entry asserts no top-level disease_term rather than
    grounding the spectrum on MONDO:0009140. See the ontology-anchor note.
review_notes: >-
  Deliberately left open in this first pass:
  (1) no top-level disease_term — MONDO has no umbrella dyssegmental dysplasia
  class; a MONDO new-term request is the correct upstream fix and has not been
  filed (see notes);
  (2) no Orphanet provenance — ORPHA:1865 (DDSH) and ORPHA:156731 (DDRD) are
  not in references_cache and would need an Orphadata bulk refresh, which would
  also supply HPO frequency annotations;
  (3) no phenotype frequency: bands, per docs/frequency-evidence-guidelines.md,
  since no source in the evidence set quantifies them across a cohort;
  (4) no treatments block — management is supportive (respiratory support,
  tracheostomy) and is described in individual case reports rather than in any
  source that states an intervention-outcome relationship worth curating;
  (5) no histopathology block, although the chondro-osseous light- and
  electron-microscopic differences between the two grades reported in
  PMID:3605216 would support one;
  (6) no conforms_to — as with Schwartz-Jampel syndrome, there is no
  proteoglycan/pericellular-matrix module in kb/modules/ yet. With both HSPG2
  entries now present, and the three ACAN disorders of the same ISDS group
  still to come, a "proteoglycan core protein deficiency" module is the natural
  follow-up once there are enough conformers to justify it;
  (7) perinatal lethality is curated as a progression record rather than as a
  phenotype, because HPO's mortality terms (HP:0003826 Stillbirth,
  HP:0003811 Neonatal death) sit outside the Phenotypic abnormality branch the
  PhenotypeTerm dynamic enum is expanded from;
  (8) the DDRD genetic picture rests on a single Japanese founder haplotype
  (five patients) and should be revisited as further DDRD genotypes are
  published;
  (9) GeneReviews was searched on 2026-09-09 and has no chapter for this
  disease or for HSPG2, so no top-level references: entry carries a
  tags: [GeneReviews] item. Queries run against the PubMed and NCBI Books
  E-utilities: dyssegmental[TI] AND GeneReviews[TA], dyssegmental[TI] AND
  "GeneReviews"[Book], dyssegmental AND genereviews[All Fields] — all zero —
  and db=books term=dyssegmental, which returns 11 records that are all
  differential-diagnosis tables inside other chapters (type II collagen
  disorders, segmental dysplasia of the vertebrae), not a chapter of its own.
  db=books term=HSPG2 AND gene[Filter] returns zero. This closes the check
  rather than deferring it;
  (10) anisospondyly is deliberately left bound to HP:0003422 Vertebral
  segmentation defect rather than moved to the more general HP:0003468. The
  argument for moving it is that PMID:38424183 defines anisospondyly as
  irregular size and shape of vertebral ossification centres — an ossification
  claim — while HP:0003422 denotes a somite/segmentation-clock defect, and this
  entry's own pathograph descends the phenotype from defective endochondral
  ossification. The argument against is that two of the three cited sources
  define the term in explicitly segmentation language: PMID:20542149 ("a defect
  in segmentation and fusion of vertebral bodies components") and PMID:23836246
  ("vertebral segmentation and fusion defects (anisospondyly)"), so the current
  binding is the one the quoted evidence supports. The ossification component
  is now separately curated as the Deficient Vertebral Ossification phenotype,
  which carries that side of the concept on its own term. The real fix is an
  HPO new-term request for anisospondyly, which HPO lacks; that has not been
  filed. Recorded here so the binding is not silently changed in either
  direction without reading this.