Calvarial Doughnut Lesions-Bone Fragility Syndrome

Mendelian MONDO:0007470 Pathograph 12 Show in embeddings browser hereditary disease Skeletal Dysplasia Bone Fragility Disorder

Calvarial doughnut lesions with bone fragility (CDL) is an autosomal dominant skeletal disorder caused by heterozygous variants in SGMS2, the gene for sphingomyelin synthase 2 (SMS2). It pairs childhood-onset osteoporosis with recurrent spinal and peripheral fractures against a paradoxical local finding: sclerotic, ring-shaped ("doughnut") lesions in the cranial bones. The radiological entity was described by Keats and Holt in 1969 and stayed a purely clinical diagnosis for fifty years, until SGMS2 was identified in 2019. The mechanism is not a simple enzyme deficiency, and this is the entry's central point. SMS2 makes sphingomyelin - the most abundant sphingolipid of the plasma membrane - by transferring phosphocholine from phosphatidylcholine onto ceramide. It is synthesised in the endoplasmic reticulum and normally trafficked to the plasma membrane, where it does its work. The disease alleles fall into two classes with opposite biochemistry but a shared consequence, and the biochemically MILDER class is clinically the SEVERER one. The recurrent nonsense allele p.Arg50* yields a catalytically inactive enzyme; the missense alleles p.Ile62Ser and p.Met64Arg yield a FUNCTIONAL enzyme that is retained in the ER, and they actually increase the rate of de novo sphingomyelin production - yet they cause neonatal fractures, severe short stature and spondylometaphyseal dysplasia, while p.Arg50* carriers usually have osteoporosis with normal growth. The unifying reading is that what matters is sphingomyelin synthesised in the RIGHT PLACE: both allele classes misdirect SMS2 away from the plasma membrane, and it is the loss of plasma-membrane sphingomyelin and its asymmetry, not the total amount of sphingomyelin made, that breaks bone mineralization. The bone lesion has been characterised in unusual depth. Transiliac biopsies show normal bone turnover and normal osteoclast function but a specific defect of matrix mineralization - thin osteoid with a greatly prolonged mineralization lag time, low and heterogeneous matrix mineral, chaotically arranged collagen fibrils, and grossly abnormal osteocyte lacunae with a disrupted canalicular network. The disease also has an under-explained neurological arm: recurrent peripheral facial and other cranial nerve palsies, plus ocular findings, in a substantial minority of patients.

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1
Inheritance
9
Pathophys.
11
Phenotypes
6
Gaps
12
Pathograph
1
Genes
5
Medical Actions
2
Subtypes
2
Models
10
References
1
Deep Research
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Inheritance

1
Autosomal Dominant HP:0000006
Heterozygous SGMS2 variants transmitted dominantly, with documented segregation over as many as six generations in one family. Expressivity is markedly variable both between and within families carrying the identical recurrent allele, and penetrance of individual features (cranial lesions, nerve palsies) is incomplete.
Autosomal dominant inheritance
Show evidence (2 references)
PMID:34504906 SUPPORT Human Clinical
"These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings."
Documents dominant transmission, the recurrent allele, and the variable penetrance recorded in this block.
PMID:37175737 SUPPORT Human Clinical
"Our results show wide interfamilial and intrafamilial phenotypic variability in patients with a detected recurrent variant in the SGMS2 gene, the presence of which must be taken into consideration in the diagnosis of the disease."
Establishes that variability is not explained by allele identity, since it occurs within families sharing one recurrent variant.
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Subtypes

2
Osteoporosis with calvarial doughnut lesions (p.Arg50* class)
The commoner and milder pole, caused by the recurrent nonsense allele c.148C>T (p.Arg50*), which produces a catalytically inactive enzyme. Childhood-onset osteoporosis with spinal and peripheral fractures, with or without cranial sclerosis, and generally normal growth.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis."
Defines the milder subtype and ties it to the recurrent nonsense allele.
CDL with spondylometaphyseal dysplasia (p.Ile62Ser / p.Met64Arg class)
The severe pole, caused by the missense alleles c.185T>G (p.Ile62Ser) and c.191T>G (p.Met64Arg), which retain catalytic activity but are trapped in the endoplasmic reticulum. Neonatal fractures, severe short stature, marked cranial sclerosis and spondylometaphyseal dysplasia.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
Defines the severe subtype and ties it to the two missense alleles.
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Discussions and Knowledge Gaps

6
Why does a disease that under-mineralises the skeleton produce focal HYPER-sclerotic ring lesions in the calvaria?
KNOWLEDGE GAP cdl_calvarial_sclerosis_unexplained
Attached to
pathophysiology#Calvarial Sclerotic Doughnut Lesions
The doughnut lesions are the pathognomonic feature and the reason the entity was recognised in 1969, yet no mechanism connects them to the demonstrated mineralization defect - they point the opposite way. Candidate explanations (a site-specific difference between intramembranous calvarial and endochondral long-bone ossification, a reactive response to microdamage, a local difference in membrane lipid requirement) have not been tested. The node is deliberately left without an incoming causal edge from the mineralization branch, because drawing one would assert a mechanism nobody has proposed.
Why are the missense alleles, which retain and even increase catalytic activity, more severe than the catalytically dead nonsense allele?
OPEN QUESTION cdl_missense_severity_inversion
Attached to
pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
This inverts the usual dose-response intuition and is the single most informative fact in the entry. The proposed reading - that ER-retained active SMS2 does harm beyond what simply losing the enzyme does, whether by consuming ceramide in the wrong compartment, generating diacylglycerol where it should not be, or acting as a dominant-negative on trafficking - has not been distinguished experimentally. Distinguishing them matters therapeutically: a substrate or enzyme-replacement strategy would help one model and worsen the other.
Are the recurrent cranial nerve palsies caused by compression at sclerotic cranial foramina, or by a primary sphingomyelin defect in nerve or myelin?
OPEN QUESTION cdl_neuropathy_compressive_vs_lipid
Attached to
pathophysiology#Cranial Neuropathy
Both are plausible and they predict different management. Compression would make the palsies a mechanical consequence of the calvarial lesions and potentially surgically addressable; a primary lipid effect on neuron-supporting cells would make them an independent arm of the disease. The transient, recurrent, fully recovering character of the palsies fits a lipid or conduction mechanism better than progressive compression, and one patient with palsies had no reported cranial sclerosis - but no study has imaged the foramina during an episode.
Are antiresorptives the right drug class for a disease with normal osteoclast function and a primary matrix mineralization defect?
KNOWLEDGE GAP cdl_bisphosphonate_mechanism_mismatch
Attached to
treatments#Bisphosphonate Therapy
Osteoclast formation, morphology and resorptive capacity were all measured and found normal, and histomorphometry showed normal turnover. Bisphosphonates nonetheless are what patients receive, by analogy with osteoporosis generally, and uncontrolled reports describe benefit. Whether that benefit reflects a real effect on this mechanism, a nonspecific density gain, or the natural fall in fracture rate with age is untested. A bisphosphonate that further slows mineralization is at least mechanistically conceivable in a disease whose lesion is a prolonged mineralization lag time. The comparison that would answer this is already partly available and has never been made: reported patients have also received TERIPARATIDE, an anabolic agent that drives bone formation rather than suppressing resorption, and formation is the arm this disease actually impairs. No outcome is reported for it, and the one patient documented as receiving it had already sustained about 40 fractures by then - but an anabolic-versus-antiresorptive comparison in SGMS2 disease is the obvious next question, and the mechanism predicts which way it should go.
Why does Sgms2 loss cause a skeletal disease in humans but no bone phenotype in the mouse null?
HUMAN MODEL MISMATCH cdl_mouse_isoform_divergence
Attached to
pathophysiology#Defective Bone Matrix Mineralization
Sms2-deficient mice have no apparent bone abnormality; the murine bone-formation deficit requires additional osteoblast-specific Sms1 deletion, so SMS1 appears to be the limiting isoform in mouse osteoblasts while SGMS2 is the disease gene in humans. This is the reason CDL has no mouse model and drove the move to zebrafish. There is a second reading that is at least as likely and is more useful: since the SEVERE human alleles retain full enzymatic activity and act by ER retention, they may be dominant and gain-of-mislocalisation rather than loss of function - in which case a mouse null models the wrong lesion entirely, and the informative experiment is a knock-in of p.Ile62Ser or p.Met64Arg, which has not been reported.
Proposed experiments
Knock-in mouse carrying a human SGMS2 severe missense allele
cdl_sgms2_missense_knockin_mouse
Generate a mouse carrying the orthologous p.Ile62Ser or p.Met64Arg substitution rather than an Sgms2 null, and assess bone mass, mineral apposition rate, matrix mineralization and osteocyte canalicular architecture against wild-type and Sgms2-null littermates.
Would support
pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
Would refute
pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
Supporting outcome
  • A skeletal phenotype in the missense knock-in that is absent in the Sgms2-null littermates, establishing the lesion as mislocalisation rather than loss of enzyme.
Refuting outcome
  • No skeletal phenotype in the missense knock-in either, indicating the species difference is in isoform dependence rather than in allele mechanism.
Is the osteocyte lacunocanalicular defect present in the severe missense subtype, or only in p.Arg50* carriers?
KNOWLEDGE GAP cdl_lacunocanalicular_single_allele
Attached to
pathophysiology#Osteocyte Lacunocanalicular Network Disruption
The lacunocanalicular finding comes from transiliac biopsies of two adult males, both carrying p.Arg50*. Since the two allele classes mislocalise SMS2 to different compartments and produce clinically opposite severities, it cannot be assumed the tissue lesion is the same in both. No biopsy data from a missense carrier has been published.
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Pathophysiology

9
Heterozygous SGMS2 Variants
The primary lesion is a heterozygous variant in SGMS2 on chromosome 4, encoding sphingomyelin synthase 2. The published allelic series was for years only three variants - the recurrent nonsense c.148C>T (p.Arg50*) and two missense alleles at adjacent residues, c.185T>G (p.Ile62Ser) and c.191T>G (p.Met64Arg) - which is a remarkably tight series for a dominant skeletal disease and is what made the allele-class comparison below possible. A frameshift allele with a high-bone-mass presentation has since been reported. SGMS2 is prominently expressed in cortical bone.
SGMS2 hgnc:28395 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves SGMS2 (hgnc:28395). hgnc:28395 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Mislocalisation of SMS2 Away from the Plasma Membrane Both allele classes displace SMS2 from its normal plasma-membrane site of action, by different routes.
Show evidence (2 references)
PMID:30779713 SUPPORT Human Clinical
"In all the families, we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane-resident sphingomyelin synthase SMS2."
The discovery finding, establishing the gene, its heterozygous dominant action, its bone expression, and the normal plasma-membrane residence of its product.
PMID:30779713 SUPPORT Human Clinical
"Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg)."
Names the three founding alleles and the two allele classes the mechanism turns on.
Mislocalisation of SMS2 Away from the Plasma Membrane
The convergence point, and the reason two biochemically opposite allele classes cause one disease. Normal SMS2 is made in the ER and trafficked to the plasma membrane, where it synthesises sphingomyelin and maintains membrane lipid asymmetry. The missense alleles hold a catalytically competent SMS2 in the ER by blocking its export; the p.Arg50* product is reported to be dislocated into the cis/medial-Golgi. In both cases sphingomyelin synthesis is misdirected away from the plasma membrane. The striking corollary is that the missense alleles INCREASE total de novo sphingomyelin production while causing the more severe disease, which rules out a simple sphingomyelin-deficiency model.
SGMS2 hgnc:28395 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves SGMS2 (hgnc:28395). hgnc:28395 is a gene from the HUGO Gene Nomenclature Committee.
protein localization to plasma membrane GO:0072659 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased protein localization to plasma membrane (GO:0072659). GO:0072659 is a biological process from the Gene Ontology. ↓ DECREASED
sphingomyelin synthase activity GO:0033188 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves abnormal sphingomyelin synthase activity (GO:0033188). GO:0033188 is a molecular function from the Gene Ontology. ⚠ ABNORMAL
Downstream
Disrupted Secretory-Pathway Lipid Landscape Sphingomyelin made in the wrong compartment perturbs the sphingomyelin/sterol gradient that organises the whole secretory pathway.
Defective Bone Matrix Mineralization Loss of plasma-membrane sphingomyelin impairs mineralization. The steps between the membrane lipid defect and the mineral defect are not established.
Show evidence (5 references)
PMID:30779713 SUPPORT In Vitro
"While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum."
The functional experiment establishing the two allele classes and, crucially, that the severe alleles increase rather than decrease sphingomyelin synthesis.
PMID:40978119 SUPPORT In Vitro
"The missense variants uphold SMS2 in the ER, while the p.Arg50* variant dislocates SMS2 into the cis/medial-Golgi."
States the specific mislocalisation destination for each allele class, which is what makes mislocalisation rather than activity the shared lesion.
PMID:40978119 SUPPORT In Vitro
"However, SGMS2 variants affect the cellular localization of SMS2 resulting in misdirected SM synthesis and disruption of plasma membrane SM asymmetry."
Names the two proximate consequences - misdirected synthesis and loss of plasma-membrane asymmetry - that this node asserts.
+ 2 more references
Disrupted Secretory-Pathway Lipid Landscape
The step that converts a mislocalised enzyme into a cell-biological defect. Sphingomyelin made in the trans-Golgi normally traps ER-synthesised cholesterol and builds a sphingomyelin/sterol gradient along the secretory pathway - a gradient that marks the physical transition in membrane properties defining organelle identity. Pathogenic SMS2 makes sphingomyelin in the ER instead, and the consequence is not confined to sphingomyelin: cholesterol organisation, glycerophospholipid profiles and membrane lipid order are all disturbed, in patient-derived fibroblasts as well as in engineered cells. The authors' proposal is that osteogenic cells cannot sustain the non-random lipid distributions their bone-forming activity requires.
sphingomyelin biosynthetic process GO:0006686 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal sphingomyelin biosynthetic process (GO:0006686). GO:0006686 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Defective Bone Matrix Mineralization How a disturbed membrane lipid landscape translates into a prolonged mineralization lag time is not established.
Osteocyte Lacunocanalicular Network Disruption Canaliculi are thin cytoplasmic processes whose formation and maintenance depend on plasma-membrane composition, so a disturbed membrane lipid landscape is the natural upstream candidate for the disrupted network. The edge is INDIRECT_UNKNOWN_INTERMEDIATES because nothing establishes the steps between them - this is a coherence argument about what membranes are needed for, not a measured pathway.
Show evidence (2 references)
PMID:36102623 SUPPORT In Vitro
"These aberrant sphingomyelin distributions also occur in patient-derived fibroblasts and are accompanied by imbalances in cholesterol organization, glycerophospholipid profiles, and lipid order in the secretory pathway."
Establishes that the lipid disturbance extends beyond sphingomyelin and is present in patient material, not only in engineered cell systems.
PMID:36102623 SUPPORT In Vitro
"We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity."
Graded PARTIAL and quoted for its hedge. The authors postulate the link to bone-forming activity rather than demonstrate it, and this entry does not upgrade their claim.
Defective Bone Matrix Mineralization
The core skeletal lesion, and it is a MATERIAL defect rather than a turnover defect. Transiliac biopsies from two adult p.Arg50* carriers showed largely normal structural and turnover histomorphometry, but reduced osteoid thickness and mineralizing surface with increased osteoid surface - giving a greatly prolonged mineralization lag time - and low, heterogeneous matrix mineral on quantitative backscattered electron imaging, with collagen fibrils chaotically arranged under polarised light. Bone is being made at a normal rate and then mineralised badly.
osteoblast CL:0000062 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteoblast (CL:0000062). CL:0000062 is a cell type from the Cell Ontology.
bone mineralization GO:0030282 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased bone mineralization (GO:0030282). GO:0030282 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Sms2-deficient mouse (SMS1f/f;SMS2-/-) Mouse fails to recapitulate Sms2 deficiency alone does not reduce bone formation in mice. The bone-formation deficit appears only when Sms1 is additionally deleted in osteoblasts, implicating SMS1 rather than SMS2 as the isoform limiting for murine osteoblast differentiation.
sgms2a/sgms2b CRISPR-Cas13d knockdown zebrafish Zebrafish partially recapitulates Knockdown compromises notochord and craniofacial skeletal formation, and single-cell RNA sequencing localises sgms2a expression to osteogenic cells in juvenile calvaria, which supports a cell-autonomous skeletal role for the gene.
Downstream
Bone Fragility and Fractures Poorly and heterogeneously mineralised matrix with disordered collagen is mechanically weak.
Show evidence (3 references)
PMID:30779713 SUPPORT Human Clinical
"Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization."
The discovery cohort's biopsy finding of a bone material and mineralization defect.
PMID:34761145 SUPPORT Human Clinical
"qBEI showed low and heterogeneous matrix mineralization (CaPeak -2.41 SD, -3.72 SD; CaWidth +7.47 SD, +4.41 SD) with a chaotic arrangement of collagenous fibrils under polarized light."
Quantifies the mineral defect and the collagen disorganisation by qBEI and polarised light microscopy.
PMID:34761145 SUPPORT Human Clinical
"Bone histomorphometry portrayed largely normal values for structural and turnover parameters, but in both patient 1 and patient 2, respectively, osteoid thickness (-1.80 SD, -1.37 SD) and mineralizing surface (-1.03 SD, -2.73 SD) were reduced and osteoid surface increased (+9.03 SD, +0.98 SD),..."
The histomorphometric result showing normal turnover with a specifically prolonged mineralization lag time - the finding that makes this a material rather than a remodelling defect.
Osteocyte Lacunocanalicular Network Disruption
A second, independent tissue lesion. Osteocyte lacunae are abnormally large and round, and the canalicular network is severely disturbed, with short canaliculi lacking orderliness or continuity. This matters because the lacunocanalicular network is how bone senses mechanical load and coordinates remodelling; a plasma-membrane lipid defect impairing the formation of these fine cell processes is a mechanistically coherent - though not directly demonstrated - explanation.
osteocyte CL:0000137 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteocyte (CL:0000137). CL:0000137 is a cell type from the Cell Ontology.
Downstream
Bone Fragility and Fractures A disordered osteocyte network degrades bone quality independently of matrix mineral content.
Show evidence (2 references)
PMID:34761145 SUPPORT Human Clinical
"Last, osteocyte lacunae appeared abnormally large and round in shape and the canalicular network severely disturbed with short-spanned canaliculi lacking any orderliness or continuity."
The direct microscopic observation of the lacunocanalicular defect.
PMID:34761145 SUPPORT Human Clinical
"Taken together, these data underline a central role for functional SMS2 in bone matrix organization and mineralization, lacunocanalicular network, and in maintaining skeletal strength and integrity."
The authors' own synthesis linking SMS2 function to the three tissue-level defects this entry models.
Preserved Osteoclast Function
A deliberate negative node. Bone volume is reduced and the histology could superficially suggest increased resorption, but osteoclast formation and function in vitro were normal, and functional analysis found no difference in osteoclast morphology or resorptive capacity. This is what excludes the high-resorption model that would otherwise be the default reading of an osteoporosis phenotype - and it has a direct therapeutic implication, since antiresorptives are the standard osteoporosis drugs.
osteoclast CL:0000092 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteoclast (CL:0000092). CL:0000092 is a cell type from the Cell Ontology.
Show evidence (2 references)
PMID:30779713 SUPPORT In Vitro
"Osteoclast formation and function in vitro was normal."
The negative result excluding an osteoclast-driven mechanism.
PMID:40978119 SUPPORT In Vitro
"Although these findings suggest increased osteoclast numbers, functional analysis found no differences in osteoclast morphology or resorptive capacity."
States explicitly that the histological appearance suggests increased osteoclasts but function is normal, which is why this node exists.
Bone Fragility and Fractures
The clinical endpoint: low bone mineral density with recurrent low-trauma spinal and peripheral fractures from childhood, ranging from vertebral compression fractures to dozens of long-bone fractures over a lifetime. The fracture burden can be extreme - one proband had sustained about 40 fractures by age 29.
Show evidence (1 reference)
PMID:34761145 SUPPORT Human Clinical
"The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions."
States the clinical fragility phenotype and its severity range.
Calvarial Sclerotic Doughnut Lesions
The pathognomonic finding and the entry's most conspicuous unexplained feature: discrete ring-shaped sclerotic lesions in the cranial bones, coexisting with generalised low bone mineral density elsewhere. A disease that under-mineralises the skeleton produces focal HYPER-sclerosis in the calvaria, and no mechanism has been proposed for that inversion. The lesions are what distinguish CDL from other primary osteoporoses, including osteogenesis imperfecta.
Show evidence (2 references)
PMID:34761145 SUPPORT Human Clinical
"The cranial sclerotic, circular lesions differentiate this skeletal disorder from other types of primary osteoporosis."
Establishes the cranial lesions as the discriminating feature of the entity.
PMID:34504906 SUPPORT Human Clinical
"Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
States the coexistence of generalised low BMD with focal cranial sclerosis that this node records as unexplained.
Cranial Neuropathy
Recurrent transient peripheral facial nerve palsy is the commonest, together with trochlear, oculomotor and abducens palsies, transient diplopia and ptosis in individual patients. Two candidate explanations exist and have not been separated: mechanical compression of nerves in sclerotic cranial foramina, or a primary effect of disordered sphingomyelin metabolism on nerve or myelin - sphingomyelin being a major myelin lipid. A transcriptomic study found the differentially expressed genes enriched for neurological as well as skeletal processes and suggested an effect on neuron-supporting cells, which favours but does not establish the second.
Show evidence (3 references)
PMID:30779713 SUPPORT Human Clinical
"Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations."
Establishes the cranial neuropathy as part of the phenotype in the discovery cohort.
PMID:40978119 SUPPORT In Vitro
"These findings suggest that SGMS2 variants modulate circadian rhythm and gap junction assembly, adversely affecting bone health and homeostasis, and may affect neuron-supporting cells in SGMS2-related osteoporosis."
Graded PARTIAL and cited for the hedge it contains - the authors say "may affect", from fibroblast transcriptomics rather than nerve tissue, so it supports a neural arm as a hypothesis and not as a demonstrated mechanism.
PMID:37886644 SUPPORT Human Clinical
"several patients experience neurological symptoms, the most frequent being transitory, spontaneously resolving, and recurrent cranial nerve palsies"
Characterises the palsies as transient, self-resolving and recurrent - the temporal profile this entry argues fits a conduction or lipid mechanism better than progressive foraminal compression.
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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 Calvarial Doughnut Lesions-Bone Fragility Syndrome 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

11
Ear 1
Sensorineural Hearing Impairment HP:0000407 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sensorineural hearing impairment (HP:0000407). HP:0000407 is a phenotype from the Human Phenotype Ontology.
No frequency asserted - individual patients across two families. Worth noting that a sensorineural rather than conductive deficit argues against simple compression by sclerotic temporal bone, which would be the obvious skeletal explanation.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Additional findings included sensorineural hearing loss, myopia, dilated aortic root, and hypotonia with abnormal myopathic electromyography but normal muscle biopsy."
Reports sensorineural hearing loss alongside other extra-skeletal findings in the most severely affected patient. Graded PARTIAL - one patient, and the listed findings are heterogeneous.
Eye 1
Glaucoma HP:0000501 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Glaucoma (HP:0000501). HP:0000501 is a phenotype from the Human Phenotype Ontology.
No frequency asserted, but the association has replicated: the Finnish discovery family and an independent Filipino family, both carrying the same recurrent p.Arg50* allele. Two unrelated families with the same allele and the same extra-skeletal finding is a materially stronger claim than one, and it strengthens the argument that sphingomyelin metabolism matters outside bone. The mechanism connecting SGMS2 to the eye remains unknown.
Show evidence (2 references)
PMID:30779713 SUPPORT Human Clinical
"recurrent idiopathic peripheral facial nerve palsies (Bell’s palsy), and congenital bilateral glaucoma"
Records congenital bilateral glaucoma in the index patient of the Finnish family. Graded PARTIAL - one family, and no proposed mechanism linking SGMS2 to the eye.
PMID:37886644 SUPPORT Human Clinical
"the association of glaucoma with SGMS2 was further strengthened when Collantes and coworkers described a Filipino family harboring the SGMS2 p.Arg50* mutation, with characteristic skull lesions and juvenile onset open angle glaucoma"
The independent replication - a second, unrelated family carrying the same recurrent allele with juvenile open-angle glaucoma. This is what upgrades glaucoma from a single-family observation to a replicated association.
Head and Neck 1
Facial Nerve Palsy OCCASIONAL Facial palsy HP:0010628 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Facial palsy (HP:0010628), qualified as temporality recurrent. HP:0010628 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations."
Reports the facial nerve palsy in the discovery cohort.
Limbs 1
Long Bone Deformity 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 (2 references)
PMID:34761145 SUPPORT Human Clinical
"The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions."
Lists long-bone deformity among the defining features of the severe end of the spectrum. Note this snippet says "deformities" generically while the binding is the narrower `Bowing of the long bones`; the next item supplies the bowing-specific observation.
PMID:30779713 SUPPORT Human Clinical
"Bisphosphonate treatment was commenced at 2.6 years because of a radius fracture, lower limb bowing, and platyspondyly and discontinued at age 6 years; no new fractures have occurred thereafter."
Documents lower limb bowing specifically, which is what the HP:0006487 binding asserts.
Musculoskeletal 4
Recurrent Fractures VERY_FREQUENT HP:0002757 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent fractures (HP:0002757). HP:0002757 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:34504906 SUPPORT Human Clinical
"Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
States recurrent childhood-onset fractures as a defining feature.
Osteoporosis VERY_FREQUENT HP:0000939 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Osteoporosis (HP:0000939). HP:0000939 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis."
Ties childhood-onset osteoporosis to the p.Arg50* subtype.
Vertebral Compression Fracture FREQUENT HP:0002953 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Vertebral compression fracture (HP:0002953). HP:0002953 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:34504906 SUPPORT Human Clinical
"The proband presents with bone in bone vertebral appearance that had been defined uniquely in the first cases described in the same family."
Documents the distinctive vertebral radiographic appearance in this kindred.
Scoliosis HP:0002650 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Scoliosis (HP:0002650), qualified as course progressive. HP:0002650 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"The radiographic features evolved over time to spondylometaphyseal dysplasia with marked short stature and marked scoliosis."
Documents the scoliosis and, in the same sentence, that the radiographic picture EVOLVED - the severe subtype's features develop rather than being present from birth.
Growth 1
Short Stature 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:30779713 SUPPORT Human Clinical
"Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
Ties severe short stature specifically to the missense subtype.
Other 2
Cranial Hyperostosis with Doughnut Lesions FREQUENT Craniofacial hyperostosis HP:0004493 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Craniofacial hyperostosis (HP:0004493). HP:0004493 is a phenotype from the Human Phenotype Ontology.
HPO has no term for a calvarial doughnut lesion specifically. `Craniofacial hyperostosis` (HP:0004493) is the closest accurate ancestor and is used rather than manufacturing a narrower match. The ring morphology, which is the discriminating feature, is therefore not captured by the binding and is carried by `preferred_term` and this note. A term request to HPO would be the right fix.
Show evidence (1 reference)
PMID:34761145 SUPPORT Human Clinical
"The cranial sclerotic, circular lesions differentiate this skeletal disorder from other types of primary osteoporosis."
Establishes the cranial sclerotic lesions as the distinguishing phenotype.
Metaphyseal Dysplasia Spondylometaphyseal dysplasia HP:0002657 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spondylometaphyseal dysplasia (HP:0002657). HP:0002657 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
Ties spondylometaphyseal dysplasia to the missense subtype.
🧬

Genetic Associations

1
SGMS2
Gene: SGMS2 hgnc:28395 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is SGMS2 (hgnc:28395). hgnc:28395 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:30779713 SUPPORT Human Clinical
"Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg)."
Names the three founding alleles recorded in this block.
PMID:37175737 SUPPORT Human Clinical
"It has thereafter been reported that about two additional families had this variant, confirming it to be the "hot spot" in the SGMS2 gene"
Establishes c.148C>T as a recurrent hot spot rather than a private family allele.
PMID:40123745 SUPPORT Human Clinical
"He was found to have a heterozygous frameshift variant c.1052_1074dup in the SGMS2 gene."
Adds a fourth allele class. Graded PARTIAL because it is a single case report and its high-bone-mass phenotype is the opposite of the established low-BMD presentation, so it broadens the series without being established.
💊

Medical Actions

5
Bisphosphonate Therapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: pamidronate CHEBI:7903 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses pamidronate (CHEBI:7903). CHEBI:7903 is a therapeutic agent from Chemical Entities of Biological Interest. alendronic acid CHEBI:2567 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses alendronic acid (CHEBI:2567). CHEBI:2567 is a therapeutic agent from Chemical Entities of Biological Interest. risedronic acid CHEBI:8869 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses risedronic acid (CHEBI:8869). CHEBI:8869 is a therapeutic agent from Chemical Entities of Biological Interest.
Bisphosphonates are the treatment actually used - pamidronate and alendronate in the French-Canadian proband from childhood, pamidronate courses in several of the discovery families and in the Belarusian kindred, with reported improvement in back pain, quality of life, bone mineral density, and in some patients cessation of further long-bone fractures. There is a mechanistic tension worth stating: bisphosphonates are ANTIRESORPTIVE, and osteoclast function in this disease was measured and found normal. So the drug is not correcting the demonstrated lesion, which is defective matrix mineralization on a normal-turnover background. The reported benefits are real but come from uncontrolled clinical observation in small families, not from a trial.
Mechanism Target:
Bone Fragility and Fractures — Treatment is directed at the fracture endpoint rather than at the mineralization defect that produces it.
Show evidence (3 references)
PMID:30779713 SUPPORT Human Clinical
"Bisphosphonate treatment resulted in notable improvement in back pain and quality of life."
Reports symptomatic benefit in one family of the discovery cohort. Graded PARTIAL because it is uncontrolled clinical observation in a single patient within a case series, reporting symptoms rather than fracture rate.
PMID:30779713 SUPPORT Human Clinical
"Bisphosphonate treatment was commenced at 2.6 years because of a radius fracture, lower limb bowing, and platyspondyly and discontinued at age 6 years; no new fractures have occurred thereafter."
A fracture-free interval after treatment in one child. Graded PARTIAL - an uncontrolled single-patient observation, and fracture rates in this disease fall with age anyway, so the comparison is confounded.
PMID:34504906 SUPPORT Human Clinical
"Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
Records risedronate among the agents given to one patient, which is why it sits in this entry rather than with the anabolic agent - it is a bisphosphonate and shares their mechanism. Exposure only, no outcome.
Teriparatide
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: teriparatide CHEBI:135983 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses teriparatide (CHEBI:135983). CHEBI:135983 is a therapeutic agent from Chemical Entities of Biological Interest.
The mechanistically interesting option, and the one this entry's therapeutic argument turns on. Teriparatide is an ANABOLIC agent that stimulates bone formation, not an antiresorptive. Since the demonstrated lesion here is defective matrix mineralization on a normal-turnover background with measured-normal osteoclast function, an agent that drives formation is better matched to the mechanism than one that suppresses resorption. No outcome is reported for it in CDL. The patient documented as receiving it had already sustained about 40 fractures by then, so this records what was tried, not what worked.
Mechanism Target:
Defective Bone Matrix Mineralization — Teriparatide targets bone FORMATION, which is the arm this disease impairs - in contrast to the antiresorptives, which target an arm measured to be normal. This link is asserted for teriparatide alone and deliberately not for the antiresorptive entries.
Show evidence (1 reference)
PMID:34504906 SUPPORT Human Clinical
"Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
Records teriparatide exposure in one patient. Graded PARTIAL - the sentence reports what was given with no outcome measure attached, so it establishes use and nothing about efficacy.
Denosumab
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: denosumab NCIT:C61313 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses denosumab (NCIT:C61313). NCIT:C61313 is a therapeutic agent from the NCI Thesaurus.
A RANKL-targeting monoclonal antibody, and an antiresorptive. It is recorded because it was given, not because the mechanism recommends it: like the bisphosphonates it acts on osteoclast-mediated resorption, which was measured and found normal in this disease. No outcome is reported. It carries no `target_mechanisms` link for exactly that reason - drawing one to the mineralization node would assert the anabolic rationale that belongs to teriparatide.
Show evidence (1 reference)
PMID:34504906 SUPPORT Human Clinical
"Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
Records denosumab exposure in the same patient. PARTIAL for the same reason - exposure without an outcome measure.
Vitamin D Supplementation
Action: nutritional supportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is nutritional support (NCIT:C15433). NCIT:C15433 is a clinical intervention from the NCI Thesaurus. Ontology label: Nutritional Support NCIT:C15433
Agent: vitamin D CHEBI:27300 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses vitamin D (CHEBI:27300). CHEBI:27300 is a therapeutic agent from Chemical Entities of Biological Interest.
Vitamin D supplementation alongside bisphosphonate therapy, and low 25-hydroxyvitamin D has been documented in a reported patient. This is adjunctive and standard-of-care for any osteoporosis rather than disease-specific - it corrects a coexisting deficiency, it does not address the sphingomyelin lesion.
Show evidence (1 reference)
PMID:34504906 SUPPORT Human Clinical
"She was prescribed vitamin D supplementation and started Pamidronate treatment (Aredia) until the age of 14 years, when she was switched to Alendronate (Fosamax) for 4–5 months."
Records vitamin D given alongside bisphosphonates. PARTIAL - one patient, no separable outcome for the vitamin D arm.
Orthopedic Surgery
Action: orthopedic surgical procedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is orthopedic surgical procedure (NCIT:C16186). NCIT:C16186 is a clinical intervention from the NCI Thesaurus. Ontology label: Orthopedic Surgical Procedure NCIT:C16186
Corrective and stabilising surgery for long-bone deformity and recurrent fracture - femoral osteotomy and intramedullary rodding are both reported. Notably the patient who had both continued to fracture afterwards, which is consistent with surgery addressing deformity and mechanical support rather than the underlying bone material defect.
Mechanism Target:
Bone Fragility and Fractures — Mechanical correction and stabilisation of deformed or repeatedly fractured long bones, without altering matrix mineralization.
Show evidence (1 reference)
PMID:30779713 SUPPORT Human Clinical
"She underwent femoral osteotomy at 2.5 years and intramedullary rodding at 15 years but continued to have multiple fractures."
Records both procedures and, in the same sentence, that fractures continued - which is why the description presents surgery as mechanical management rather than disease modification.
🔬

Biochemical Markers

1
Alkaline phosphatase and 25-hydroxyvitamin D
Show evidence (1 reference)
PMID:37175737 SUPPORT Human Clinical
"The laboratory study of calcium phosphate metabolism revealed an increased level of alkaline phosphatase activity and decreased level of 25-hydroxyvitamin D (25(OH)D)."
The reported biochemical abnormalities. Graded PARTIAL - a single child in one family, and neither finding is specific to CDL.
🔬

Diagnosis

3
Skull radiography for calvarial doughnut lesions
The defining diagnostic modality. Ring-shaped sclerotic lesions on skull radiographs are what distinguishes CDL from other primary osteoporoses and from osteogenesis imperfecta, and the entity was originally defined radiologically in 1969. Two practical caveats follow from the natural history: the lesions are not fully penetrant, so their absence does not exclude the diagnosis in a fracturing child; and they can develop over time, so a normal skull film in infancy does not settle the question - one severely affected boy had normal skull radiographs at birth and later developed marked cranial sclerosis.
`diagnosis_term` is deliberately left unbound. NCIT has no plain skull- or general-radiography clinical-action term that accurately covers this; NCIT:C38103 is Chest Radiography specifically. Per the terms contract, no term beats a term that is nearly right.
Show evidence (2 references)
PMID:34504906 SUPPORT Human Clinical
"Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
States the radiological finding that defines the entity and is the basis of the clinical diagnosis.
PMID:34504906 SUPPORT Human Clinical
"These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings."
Supports the caveat that individual findings, the cranial lesions included, are variably penetrant.
Densitometry and spinal imaging
DXA quantifies the low bone mineral density, and spinal imaging identifies vertebral compression fractures, which are frequently the presenting problem and can be extensive before they are looked for - one 6-year-old proband had compression fractures of C7 and T2-T8 found on MRI performed for back pain. Densitometry is also how treatment response has been followed in the reported cases.
dual-energy X-ray absorptiometry NCIT:C48789 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:37175737 SUPPORT Human Clinical
"an MRI of the thoracolumbar spine was performed at the age of 4 years, which revealed signs of compression fractures of the C7, Th 2-8 vertebral bodies (Figure 2)."
A worked example of the extent of vertebral involvement detectable on spinal imaging in a young child.
PMID:37175737 SUPPORT Human Clinical
"The main clinical manifestations in the examined patients included multiple fractures, low bone mineral density (BMD) revealed on densitometry, and the presence of palpable indurations of the skull bones."
Names the three diagnostic pillars - fractures, densitometric low BMD, and cranial lesions, which can be PALPABLE as well as radiographic.
SGMS2 sequencing
Molecular confirmation. The recurrent nonsense allele c.148C>T (p.Arg50*) is a hot spot found in most reported families, so targeted analysis of that variant is a cheap first-line test before broader panels. The diagnostic-odyssey point is worth recording: because CDL is grouped with osteogenesis imperfecta among the bone fragility disorders and presents as childhood osteoporosis, patients are routinely worked up as OI first. One proband carried a working diagnosis of OI type I and was offered four sequential gene panels before SGMS2 was identified. Including SGMS2 in first-line early-onset-osteoporosis panels is the practical fix.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:37175737 SUPPORT Human Clinical
"The primary analysis of this variant will contribute to optimal molecular genetic diagnostics, which can reduce diagnostic costs and time."
The authors' own recommendation that targeted analysis of the recurrent variant comes first.
PMID:34504906 SUPPORT Human Clinical
"she was offered four sequential genetic testing panels before identifying the right molecular diagnosis."
Documents the diagnostic odyssey that motivates including SGMS2 in first-line panels.
📊

Prevalence

1
Worldwide, published cases
Cases In Literature Ultra Rare
Thirteen affected subjects in six families in the 2019 discovery series; a 2023 report counted 15 patients from eight families in the literature and added 11 more from three families. No population prevalence estimate exists.
Show evidence (1 reference)
PMID:37175737 SUPPORT Human Clinical
"To date, 15 patients from eight families with CDL with bone fragility have been described in the literature, and a recurrent variant c.148C>T (p.Arg50Ter) in the SGMS2 gene has been identified, which was found in patients from six families."
Gives the published case count and the dominance of the recurrent allele among reported families.
🐁

Animal Models

2
Sms2-deficient mouse (SMS1f/f;SMS2-/-)
The germline Sms2-null mouse - the obvious model for a human SGMS2 disease, and the reason a zebrafish model had to be built instead.
Species
Mouse
Genotype
SMS1-floxed, SMS2-null (SMS1f/f;SMS2-/-), without osteoblast Cre
Publication
PMID:31847800
Pathograph links
Defective Bone Matrix Mineralization fails to recapitulate fidelity: low Sms2 deficiency alone does not reduce bone formation in mice. The bone-formation deficit appears only when Sms1 is additionally deleted in osteoblasts, implicating SMS1 rather than SMS2 as the isoform limiting for murine osteoblast differentiation.
Limitations: This is a species-level divergence in isoform dependence, and it has a specific consequence for how human data should be read: the human severe alleles are NOT nulls but ER-retained active enzyme, so a mouse null was arguably never the right model of the human lesion in the first place. A knock-in of a human missense allele has not been reported.
Show evidence (1 reference)
PMID:31847800 REFUTE Model Organism
"we found that although cartilage formation is normal, Sp7-Cre;SMS1f/f;SMS2-/- mice showed reduced trabecular and cortical bone mass, had lower bone mineral density, and had a slower mineral apposition rate than control mice"
Graded REFUTE against the Sms2-null mouse as a model of this disease. The bone phenotype required osteoblast Sms1 deletion on top of the Sms2-null background; the Sms2-null control mice, which are the direct analogue of the human genotype, were among the unaffected comparators.
sgms2a/sgms2b CRISPR-Cas13d knockdown zebrafish
Transient knockdown of the two zebrafish SGMS2 orthologues, developed specifically because Sgms2 knockout mice have no skeletal phenotype at all - the model of first resort fails for this disease.
Species
Zebrafish
Genotype
sgms2a, sgms2b and sgms2a+b transient CRISPR-Cas13d knockdown
Publication
PMID:41143154
Pathograph links
Defective Bone Matrix Mineralization partially recapitulates fidelity: low Knockdown compromises notochord and craniofacial skeletal formation, and single-cell RNA sequencing localises sgms2a expression to osteogenic cells in juvenile calvaria, which supports a cell-autonomous skeletal role for the gene.
Limitations: The readout is embryonic craniofacial and notochord development, not adult bone matrix mineralization, so it does not model the human lesion. It is a transient knockdown rather than a stable genetic model, it is a loss-of-expression model while the severe human alleles are mislocalising rather than null, and the authors describe the models explicitly as preliminary platforms.
Show evidence (2 references)
PMID:41143154 SUPPORT Model Organism
"Knockdown of sgms2a and sgms2b in zebrafish had detrimental effect on embryonic development and compromised notochord and craniofacial formation."
The skeletal phenotype obtained, which is developmental rather than a model of adult bone fragility.
+ 1 more reference
{ }

Source YAML

click to show 
name: Calvarial Doughnut Lesions-Bone Fragility Syndrome
creation_date: "2026-08-28T00:00:00Z"
category: Mendelian
disease_term:
  preferred_term: calvarial doughnut lesions with bone fragility, with or without spondylometaphyseal dysplasia (SGMS2-related)
  term:
    id: MONDO:0007470
    label: calvarial doughnut lesions-bone fragility syndrome
description: >-
  Calvarial doughnut lesions with bone fragility (CDL) is an autosomal dominant
  skeletal disorder caused by heterozygous variants in SGMS2, the gene for
  sphingomyelin synthase 2 (SMS2). It pairs childhood-onset osteoporosis with
  recurrent spinal and peripheral fractures against a paradoxical local finding:
  sclerotic, ring-shaped ("doughnut") lesions in the cranial bones. The
  radiological entity was described by Keats and Holt in 1969 and stayed a purely
  clinical diagnosis for fifty years, until SGMS2 was identified in 2019.
  The mechanism is not a simple enzyme deficiency, and this is the entry's central
  point. SMS2 makes sphingomyelin - the most abundant sphingolipid of the plasma
  membrane - by transferring phosphocholine from phosphatidylcholine onto
  ceramide. It is synthesised in the endoplasmic reticulum and normally trafficked
  to the plasma membrane, where it does its work. The disease alleles fall into two
  classes with opposite biochemistry but a shared consequence, and the biochemically
  MILDER class is clinically the SEVERER one. The recurrent nonsense allele
  p.Arg50* yields a catalytically inactive enzyme; the missense alleles p.Ile62Ser
  and p.Met64Arg yield a FUNCTIONAL enzyme that is retained in the ER, and they
  actually increase the rate of de novo sphingomyelin production - yet they cause
  neonatal fractures, severe short stature and spondylometaphyseal dysplasia, while
  p.Arg50* carriers usually have osteoporosis with normal growth. The unifying
  reading is that what matters is sphingomyelin synthesised in the RIGHT PLACE:
  both allele classes misdirect SMS2 away from the plasma membrane, and it is the
  loss of plasma-membrane sphingomyelin and its asymmetry, not the total amount of
  sphingomyelin made, that breaks bone mineralization.
  The bone lesion has been characterised in unusual depth. Transiliac biopsies show
  normal bone turnover and normal osteoclast function but a specific defect of
  matrix mineralization - thin osteoid with a greatly prolonged mineralization lag
  time, low and heterogeneous matrix mineral, chaotically arranged collagen fibrils,
  and grossly abnormal osteocyte lacunae with a disrupted canalicular network. The
  disease also has an under-explained neurological arm: recurrent peripheral facial
  and other cranial nerve palsies, plus ocular findings, in a substantial minority
  of patients.
parents:
- hereditary disease
- Skeletal Dysplasia
- Bone Fragility Disorder
synonyms:
- CDL
- calvarial doughnut lesions with bone fragility
- calvarial doughnut lesions with bone fragility with or without spondylometaphyseal dysplasia
- doughnut lesions of skull, familial
- familial doughnut lesions of skull
- osteoporosis with calvarial doughnut lesions
- OP-CDL
- SGMS2-related osteoporosis
notes: >-
  Nosology. In the current classification of skeletal disorders CDL is grouped
  with osteogenesis imperfecta among the bone fragility disorders, and the
  differential in practice is OI - the French-Canadian proband carried a working
  diagnosis of OI type I for years and was offered four sequential gene panels
  before SGMS2 was found. The distinguishing feature is the cranial sclerotic
  lesion, which OI does not produce.

  Allele-severity inversion. The nonsense allele is the mild one and the missense
  alleles are the severe ones. Anyone reasoning from a loss-of-function default
  will get the genotype-phenotype prediction backwards here, which is why the
  description states it explicitly rather than leaving it to be inferred from the
  subtype blocks.

  Naming. The MONDO label is "calvarial doughnut lesions-bone fragility syndrome";
  the literature more often writes "calvarial doughnut lesions with bone fragility
  (CDL)" or "osteoporosis with calvarial doughnut lesions (OP-CDL)". OMIM #126550.
inheritance:
- name: Autosomal Dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    Heterozygous SGMS2 variants transmitted dominantly, with documented segregation
    over as many as six generations in one family. Expressivity is markedly
    variable both between and within families carrying the identical recurrent
    allele, and penetrance of individual features (cranial lesions, nerve palsies)
    is incomplete.
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings."
    explanation: Documents dominant transmission, the recurrent allele, and the variable penetrance recorded in this block.
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our results show wide interfamilial and intrafamilial phenotypic variability in patients with a detected recurrent variant in the SGMS2 gene, the presence of which must be taken into consideration in the diagnosis of the disease."
    explanation: Establishes that variability is not explained by allele identity, since it occurs within families sharing one recurrent variant.
prevalence:
- population: Worldwide, published cases
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Thirteen affected subjects in six families in the 2019 discovery series; a
    2023 report counted 15 patients from eight families in the literature and
    added 11 more from three families. No population prevalence estimate exists.
  evidence:
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date, 15 patients from eight families with CDL with bone fragility have been described in the literature, and a recurrent variant c.148C>T (p.Arg50Ter) in the SGMS2 gene has been identified, which was found in patients from six families."
    explanation: Gives the published case count and the dominance of the recurrent allele among reported families.
has_subtypes:
- name: CDL-Osteoporosis
  display_name: Osteoporosis with calvarial doughnut lesions (p.Arg50* class)
  description: >-
    The commoner and milder pole, caused by the recurrent nonsense allele
    c.148C>T (p.Arg50*), which produces a catalytically inactive enzyme.
    Childhood-onset osteoporosis with spinal and peripheral fractures, with or
    without cranial sclerosis, and generally normal growth.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis."
    explanation: Defines the milder subtype and ties it to the recurrent nonsense allele.
- name: CDL-SMD
  display_name: CDL with spondylometaphyseal dysplasia (p.Ile62Ser / p.Met64Arg class)
  description: >-
    The severe pole, caused by the missense alleles c.185T>G (p.Ile62Ser) and
    c.191T>G (p.Met64Arg), which retain catalytic activity but are trapped in the
    endoplasmic reticulum. Neonatal fractures, severe short stature, marked
    cranial sclerosis and spondylometaphyseal dysplasia.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
    explanation: Defines the severe subtype and ties it to the two missense alleles.
pathophysiology:
- name: Heterozygous SGMS2 Variants
  biological_scale: MOLECULAR
  description: >-
    The primary lesion is a heterozygous variant in SGMS2 on chromosome 4,
    encoding sphingomyelin synthase 2. The published allelic series was for years
    only three variants - the recurrent nonsense c.148C>T (p.Arg50*) and two
    missense alleles at adjacent residues, c.185T>G (p.Ile62Ser) and c.191T>G
    (p.Met64Arg) - which is a remarkably tight series for a dominant skeletal
    disease and is what made the allele-class comparison below possible. A
    frameshift allele with a high-bone-mass presentation has since been reported.
    SGMS2 is prominently expressed in cortical bone.
  genes:
  - preferred_term: SGMS2
    term:
      id: hgnc:28395
      label: SGMS2
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In all the families, we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane-resident sphingomyelin synthase SMS2."
    explanation: The discovery finding, establishing the gene, its heterozygous dominant action, its bone expression, and the normal plasma-membrane residence of its product.
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg)."
    explanation: Names the three founding alleles and the two allele classes the mechanism turns on.
  downstream:
  - target: Mislocalisation of SMS2 Away from the Plasma Membrane
    causal_link_type: DIRECT
    description: >-
      Both allele classes displace SMS2 from its normal plasma-membrane site of
      action, by different routes.
- name: Mislocalisation of SMS2 Away from the Plasma Membrane
  biological_scale: MOLECULAR
  description: >-
    The convergence point, and the reason two biochemically opposite allele classes
    cause one disease. Normal SMS2 is made in the ER and trafficked to the plasma
    membrane, where it synthesises sphingomyelin and maintains membrane lipid
    asymmetry. The missense alleles hold a catalytically competent SMS2 in the ER
    by blocking its export; the p.Arg50* product is reported to be dislocated into
    the cis/medial-Golgi. In both cases sphingomyelin synthesis is misdirected away
    from the plasma membrane. The striking corollary is that the missense alleles
    INCREASE total de novo sphingomyelin production while causing the more severe
    disease, which rules out a simple sphingomyelin-deficiency model.
  genes:
  - preferred_term: SGMS2
    term:
      id: hgnc:28395
      label: SGMS2
  molecular_functions:
  - preferred_term: sphingomyelin synthase activity
    modifier: ABNORMAL
    term:
      id: GO:0033188
      label: sphingomyelin synthase activity
  biological_processes:
  - preferred_term: protein localization to plasma membrane
    modifier: DECREASED
    term:
      id: GO:0072659
      label: protein localization to plasma membrane
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum."
    explanation: The functional experiment establishing the two allele classes and, crucially, that the severe alleles increase rather than decrease sphingomyelin synthesis.
  - reference: PMID:40978119
    reference_title: Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The missense variants uphold SMS2 in the ER, while the p.Arg50* variant dislocates SMS2 into the cis/medial-Golgi."
    explanation: States the specific mislocalisation destination for each allele class, which is what makes mislocalisation rather than activity the shared lesion.
  - reference: PMID:40978119
    reference_title: Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "However, SGMS2 variants affect the cellular localization of SMS2 resulting in misdirected SM synthesis and disruption of plasma membrane SM asymmetry."
    explanation: Names the two proximate consequences - misdirected synthesis and loss of plasma-membrane asymmetry - that this node asserts.
  - reference: PMID:36102623
    reference_title: Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we show that SMS2 variants linked to the most severe bone phenotypes retain full enzymatic activity but fail to leave the ER owing to a defective autonomous ER export signal"
    explanation: >-
      The dedicated mechanistic study, and the primary source for this node's
      central claim: the severe alleles are fully active and the lesion is a
      defective ER export signal, not a loss of catalysis.
  - reference: PMID:36102623
    reference_title: Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Cells harboring pathogenic SMS2 variants accumulate sphingomyelin in the ER and display a disrupted transbilayer sphingomyelin asymmetry."
    explanation: Demonstrates the two lipid consequences - ectopic ER sphingomyelin and loss of transbilayer asymmetry - that follow from the export failure.
  downstream:
  - target: Disrupted Secretory-Pathway Lipid Landscape
    causal_link_type: DIRECT
    description: >-
      Sphingomyelin made in the wrong compartment perturbs the sphingomyelin/sterol
      gradient that organises the whole secretory pathway.
  - target: Defective Bone Matrix Mineralization
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Loss of plasma-membrane sphingomyelin impairs mineralization. The steps
      between the membrane lipid defect and the mineral defect are not established.
- name: Disrupted Secretory-Pathway Lipid Landscape
  biological_scale: CELLULAR
  description: >-
    The step that converts a mislocalised enzyme into a cell-biological defect.
    Sphingomyelin made in the trans-Golgi normally traps ER-synthesised cholesterol
    and builds a sphingomyelin/sterol gradient along the secretory pathway - a
    gradient that marks the physical transition in membrane properties defining
    organelle identity. Pathogenic SMS2 makes sphingomyelin in the ER instead, and
    the consequence is not confined to sphingomyelin: cholesterol organisation,
    glycerophospholipid profiles and membrane lipid order are all disturbed, in
    patient-derived fibroblasts as well as in engineered cells. The authors'
    proposal is that osteogenic cells cannot sustain the non-random lipid
    distributions their bone-forming activity requires.
  biological_processes:
  - preferred_term: sphingomyelin biosynthetic process
    modifier: ABNORMAL
    term:
      id: GO:0006686
      label: sphingomyelin biosynthetic process
  mechanism_confidence: ESTABLISHED
  notes: >-
    The final inferential step - from disturbed lipid landscape to failed
    mineralization - is explicitly a postulate in the source, not a demonstration.
    The wording of the evidence explanation preserves that.
  evidence:
  - reference: PMID:36102623
    reference_title: Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These aberrant sphingomyelin distributions also occur in patient-derived fibroblasts and are accompanied by imbalances in cholesterol organization, glycerophospholipid profiles, and lipid order in the secretory pathway."
    explanation: >-
      Establishes that the lipid disturbance extends beyond sphingomyelin and is
      present in patient material, not only in engineered cell systems.
  - reference: PMID:36102623
    reference_title: Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity."
    explanation: >-
      Graded PARTIAL and quoted for its hedge. The authors postulate the link to
      bone-forming activity rather than demonstrate it, and this entry does not
      upgrade their claim.
  downstream:
  - target: Defective Bone Matrix Mineralization
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      How a disturbed membrane lipid landscape translates into a prolonged
      mineralization lag time is not established.
  - target: Osteocyte Lacunocanalicular Network Disruption
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Canaliculi are thin cytoplasmic processes whose formation and maintenance
      depend on plasma-membrane composition, so a disturbed membrane lipid
      landscape is the natural upstream candidate for the disrupted network.
      The edge is INDIRECT_UNKNOWN_INTERMEDIATES because nothing establishes the
      steps between them - this is a coherence argument about what membranes are
      needed for, not a measured pathway.
- name: Defective Bone Matrix Mineralization
  biological_scale: TISSUE
  description: >-
    The core skeletal lesion, and it is a MATERIAL defect rather than a turnover
    defect. Transiliac biopsies from two adult p.Arg50* carriers showed largely
    normal structural and turnover histomorphometry, but reduced osteoid thickness
    and mineralizing surface with increased osteoid surface - giving a greatly
    prolonged mineralization lag time - and low, heterogeneous matrix mineral on
    quantitative backscattered electron imaging, with collagen fibrils chaotically
    arranged under polarised light. Bone is being made at a normal rate and then
    mineralised badly.
  cell_types:
  - preferred_term: osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  biological_processes:
  - preferred_term: bone mineralization
    modifier: DECREASED
    term:
      id: GO:0030282
      label: bone mineralization
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization."
    explanation: The discovery cohort's biopsy finding of a bone material and mineralization defect.
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "qBEI showed low and heterogeneous matrix mineralization (CaPeak -2.41 SD, -3.72 SD; CaWidth +7.47 SD, +4.41 SD) with a chaotic arrangement of collagenous fibrils under polarized light."
    explanation: Quantifies the mineral defect and the collagen disorganisation by qBEI and polarised light microscopy.
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bone histomorphometry portrayed largely normal values for structural and turnover parameters, but in both patient 1 and patient 2, respectively, osteoid thickness (-1.80 SD, -1.37 SD) and mineralizing surface (-1.03 SD, -2.73 SD) were reduced and osteoid surface increased (+9.03 SD, +0.98 SD), leading to elevated mineralization lag time (+8.16 SD, +4.10 SD)."
    explanation: The histomorphometric result showing normal turnover with a specifically prolonged mineralization lag time - the finding that makes this a material rather than a remodelling defect.
  downstream:
  - target: Bone Fragility and Fractures
    causal_link_type: DIRECT
    description: >-
      Poorly and heterogeneously mineralised matrix with disordered collagen is
      mechanically weak.
- name: Osteocyte Lacunocanalicular Network Disruption
  biological_scale: CELLULAR
  description: >-
    A second, independent tissue lesion. Osteocyte lacunae are abnormally large
    and round, and the canalicular network is severely disturbed, with short
    canaliculi lacking orderliness or continuity. This matters because the
    lacunocanalicular network is how bone senses mechanical load and coordinates
    remodelling; a plasma-membrane lipid defect impairing the formation of these
    fine cell processes is a mechanistically coherent - though not directly
    demonstrated - explanation.
  cell_types:
  - preferred_term: osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  mechanism_confidence: PROVISIONAL
  notes: >-
    Two limits on this node. It rests on transiliac biopsies from exactly two
    adult males, both carrying the same p.Arg50* allele, so it is not established
    for the missense class. And the link from plasma-membrane sphingomyelin to
    canalicular architecture is a proposal about membrane-shaping requirements,
    not a measured pathway.
  evidence:
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Last, osteocyte lacunae appeared abnormally large and round in shape and the canalicular network severely disturbed with short-spanned canaliculi lacking any orderliness or continuity."
    explanation: The direct microscopic observation of the lacunocanalicular defect.
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Taken together, these data underline a central role for functional SMS2 in bone matrix organization and mineralization, lacunocanalicular network, and in maintaining skeletal strength and integrity."
    explanation: The authors' own synthesis linking SMS2 function to the three tissue-level defects this entry models.
  downstream:
  - target: Bone Fragility and Fractures
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      A disordered osteocyte network degrades bone quality independently of matrix
      mineral content.
- name: Preserved Osteoclast Function
  biological_scale: CELLULAR
  description: >-
    A deliberate negative node. Bone volume is reduced and the histology could
    superficially suggest increased resorption, but osteoclast formation and
    function in vitro were normal, and functional analysis found no difference in
    osteoclast morphology or resorptive capacity. This is what excludes the
    high-resorption model that would otherwise be the default reading of an
    osteoporosis phenotype - and it has a direct therapeutic implication, since
    antiresorptives are the standard osteoporosis drugs.
  cell_types:
  - preferred_term: osteoclast
    term:
      id: CL:0000092
      label: osteoclast
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Osteoclast formation and function in vitro was normal."
    explanation: The negative result excluding an osteoclast-driven mechanism.
  - reference: PMID:40978119
    reference_title: Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Although these findings suggest increased osteoclast numbers, functional analysis found no differences in osteoclast morphology or resorptive capacity."
    explanation: States explicitly that the histological appearance suggests increased osteoclasts but function is normal, which is why this node exists.
- name: Bone Fragility and Fractures
  biological_scale: ORGANISM
  description: >-
    The clinical endpoint: low bone mineral density with recurrent low-trauma
    spinal and peripheral fractures from childhood, ranging from vertebral
    compression fractures to dozens of long-bone fractures over a lifetime. The
    fracture burden can be extreme - one proband had sustained about 40 fractures
    by age 29.
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions."
    explanation: States the clinical fragility phenotype and its severity range.
- name: Calvarial Sclerotic Doughnut Lesions
  biological_scale: TISSUE
  description: >-
    The pathognomonic finding and the entry's most conspicuous unexplained
    feature: discrete ring-shaped sclerotic lesions in the cranial bones,
    coexisting with generalised low bone mineral density elsewhere. A disease that
    under-mineralises the skeleton produces focal HYPER-sclerosis in the calvaria,
    and no mechanism has been proposed for that inversion. The lesions are what
    distinguish CDL from other primary osteoporoses, including osteogenesis
    imperfecta.
  mechanism_confidence: PROVISIONAL
  notes: >-
    Modelled as a node with no incoming causal edge from the mineralization
    branch, because asserting one would claim a mechanism that does not exist in
    the literature. The lesions are also not fully penetrant and can appear over
    time - one severely affected boy had normal skull radiographs at birth and
    developed progressive severe cranial sclerosis - so they are a developing
    feature rather than a congenital one.
  evidence:
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The cranial sclerotic, circular lesions differentiate this skeletal disorder from other types of primary osteoporosis."
    explanation: Establishes the cranial lesions as the discriminating feature of the entity.
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
    explanation: States the coexistence of generalised low BMD with focal cranial sclerosis that this node records as unexplained.
- name: Cranial Neuropathy
  biological_scale: ORGANISM
  description: >-
    Recurrent transient peripheral facial nerve palsy is the commonest, together
    with trochlear, oculomotor and abducens palsies, transient diplopia and ptosis
    in individual patients. Two candidate explanations exist and have not been
    separated: mechanical compression of nerves in sclerotic cranial foramina, or
    a primary effect of disordered sphingomyelin metabolism on nerve or myelin -
    sphingomyelin being a major myelin lipid. A transcriptomic study found the
    differentially expressed genes enriched for neurological as well as skeletal
    processes and suggested an effect on neuron-supporting cells, which favours
    but does not establish the second.
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations."
    explanation: Establishes the cranial neuropathy as part of the phenotype in the discovery cohort.
  - reference: PMID:40978119
    reference_title: Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These findings suggest that SGMS2 variants modulate circadian rhythm and gap junction assembly, adversely affecting bone health and homeostasis, and may affect neuron-supporting cells in SGMS2-related osteoporosis."
    explanation: >-
      Graded PARTIAL and cited for the hedge it contains - the authors say "may
      affect", from fibroblast transcriptomics rather than nerve tissue, so it
      supports a neural arm as a hypothesis and not as a demonstrated mechanism.
  - reference: PMID:37886644
    reference_title: SGMS2 in primary osteoporosis with facial nerve palsy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "several patients experience neurological symptoms, the most frequent being transitory, spontaneously resolving, and recurrent cranial nerve palsies"
    explanation: >-
      Characterises the palsies as transient, self-resolving and recurrent - the
      temporal profile this entry argues fits a conduction or lipid mechanism
      better than progressive foraminal compression.
diagnosis:
- name: Skull radiography for calvarial doughnut lesions
  notes: >-
    `diagnosis_term` is deliberately left unbound. NCIT has no plain skull- or
    general-radiography clinical-action term that accurately covers this;
    NCIT:C38103 is Chest Radiography specifically. Per the terms contract, no
    term beats a term that is nearly right.
  description: >-
    The defining diagnostic modality. Ring-shaped sclerotic lesions on skull
    radiographs are what distinguishes CDL from other primary osteoporoses and
    from osteogenesis imperfecta, and the entity was originally defined
    radiologically in 1969. Two practical caveats follow from the natural history:
    the lesions are not fully penetrant, so their absence does not exclude the
    diagnosis in a fracturing child; and they can develop over time, so a normal
    skull film in infancy does not settle the question - one severely affected boy
    had normal skull radiographs at birth and later developed marked cranial
    sclerosis.
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
    explanation: States the radiological finding that defines the entity and is the basis of the clinical diagnosis.
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings."
    explanation: Supports the caveat that individual findings, the cranial lesions included, are variably penetrant.
- name: Densitometry and spinal imaging
  diagnosis_term:
    preferred_term: dual-energy X-ray absorptiometry
    term:
      id: NCIT:C48789
      label: Dual X-ray Absorptiometry
  description: >-
    DXA quantifies the low bone mineral density, and spinal imaging identifies
    vertebral compression fractures, which are frequently the presenting problem
    and can be extensive before they are looked for - one 6-year-old proband had
    compression fractures of C7 and T2-T8 found on MRI performed for back pain.
    Densitometry is also how treatment response has been followed in the reported
    cases.
  evidence:
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "an MRI of the thoracolumbar spine was performed at the age of 4 years, which revealed signs of compression fractures of the C7, Th 2-8 vertebral bodies (Figure 2)."
    explanation: A worked example of the extent of vertebral involvement detectable on spinal imaging in a young child.
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The main clinical manifestations in the examined patients included multiple fractures, low bone mineral density (BMD) revealed on densitometry, and the presence of palpable indurations of the skull bones."
    explanation: >-
      Names the three diagnostic pillars - fractures, densitometric low BMD, and
      cranial lesions, which can be PALPABLE as well as radiographic.
- name: SGMS2 sequencing
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Molecular confirmation. The recurrent nonsense allele c.148C>T (p.Arg50*) is a
    hot spot found in most reported families, so targeted analysis of that variant
    is a cheap first-line test before broader panels.
    The diagnostic-odyssey point is worth recording: because CDL is grouped with
    osteogenesis imperfecta among the bone fragility disorders and presents as
    childhood osteoporosis, patients are routinely worked up as OI first. One
    proband carried a working diagnosis of OI type I and was offered four
    sequential gene panels before SGMS2 was identified. Including SGMS2 in
    first-line early-onset-osteoporosis panels is the practical fix.
  evidence:
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The primary analysis of this variant will contribute to optimal molecular genetic diagnostics, which can reduce diagnostic costs and time."
    explanation: The authors' own recommendation that targeted analysis of the recurrent variant comes first.
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "she was offered four sequential genetic testing panels before identifying the right molecular diagnosis."
    explanation: Documents the diagnostic odyssey that motivates including SGMS2 in first-line panels.
genetic:
- name: SGMS2
  gene_term:
    preferred_term: SGMS2
    term:
      id: hgnc:28395
      label: SGMS2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    SGMS2 on chromosome 4 encodes sphingomyelin synthase 2. Inheritance is
    autosomal dominant through heterozygous variants. Three founding alleles
    dominate the literature: the recurrent nonsense c.148C>T (p.Arg50*), which is
    a hot spot found across most reported families and produces a catalytically
    inactive enzyme, and the missense c.185T>G (p.Ile62Ser) and c.191T>G
    (p.Met64Arg), which produce active enzyme trapped in the ER. A frameshift
    allele c.1052_1074dup has since been reported with a high-bone-mass
    presentation.

    Functional impact does not reduce to a single category, which is why it is
    described here rather than asserted in a slot: p.Arg50* is loss of function,
    while the missense alleles are better described as mislocalising - they retain
    and even increase catalytic output while acting dominantly and causing the
    more severe disease.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg)."
    explanation: Names the three founding alleles recorded in this block.
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It has thereafter been reported that about two additional families had this variant, confirming it to be the \"hot spot\" in the SGMS2 gene"
    explanation: Establishes c.148C>T as a recurrent hot spot rather than a private family allele.
  - reference: PMID:40123745
    reference_title: A novel SGMS2 mutation associated with high bone mass; description of an affected family with recurrent fragility fractures.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "He was found to have a heterozygous frameshift variant c.1052_1074dup in the SGMS2 gene."
    explanation: >-
      Adds a fourth allele class. Graded PARTIAL because it is a single case
      report and its high-bone-mass phenotype is the opposite of the established
      low-BMD presentation, so it broadens the series without being established.
phenotypes:
- category: Skeletal
  name: Recurrent Fractures
  description: >-
    Low-trauma spinal and peripheral fractures from childhood; neonatal fractures
    in the severe missense subtype. Fracture burden can reach several dozen over a
    lifetime.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones."
    explanation: States recurrent childhood-onset fractures as a defining feature.
- category: Skeletal
  name: Osteoporosis
  description: Low bone mineral density, childhood-onset, in the p.Arg50* subtype with otherwise normal growth.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Osteoporosis
    term:
      id: HP:0000939
      label: Osteoporosis
  subtype: CDL-Osteoporosis
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis."
    explanation: Ties childhood-onset osteoporosis to the p.Arg50* subtype.
- category: Skeletal
  name: Vertebral Compression Fracture
  description: >-
    Vertebral compression fractures, in severe cases involving nearly all
    vertebrae, and a "bone in bone" vertebral appearance described in the
    French-Canadian kindred.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Vertebral compression fracture
    term:
      id: HP:0002953
      label: Vertebral compression fracture
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The proband presents with bone in bone vertebral appearance that had been defined uniquely in the first cases described in the same family."
    explanation: Documents the distinctive vertebral radiographic appearance in this kindred.
- category: Skeletal
  name: Cranial Hyperostosis with Doughnut Lesions
  description: >-
    Sclerotic, ring-shaped lesions of the calvaria - the pathognomonic finding.
    Not fully penetrant, and can develop progressively rather than being present
    at birth.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Craniofacial hyperostosis
    term:
      id: HP:0004493
      label: Craniofacial hyperostosis
  notes: >-
    HPO has no term for a calvarial doughnut lesion specifically.
    `Craniofacial hyperostosis` (HP:0004493) is the closest accurate ancestor and
    is used rather than manufacturing a narrower match. The ring morphology, which
    is the discriminating feature, is therefore not captured by the binding and is
    carried by `preferred_term` and this note. A term request to HPO would be the
    right fix.
  evidence:
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The cranial sclerotic, circular lesions differentiate this skeletal disorder from other types of primary osteoporosis."
    explanation: Establishes the cranial sclerotic lesions as the distinguishing phenotype.
- category: Skeletal
  name: Short Stature
  description: Severe short stature in the missense (spondylometaphyseal dysplasia) subtype; growth is generally normal in the p.Arg50* subtype.
  phenotype_term:
    preferred_term: Severe short stature
    term:
      id: HP:0003510
      label: Severe short stature
  subtype: CDL-SMD
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
    explanation: Ties severe short stature specifically to the missense subtype.
- category: Skeletal
  name: Metaphyseal Dysplasia
  description: Spondylometaphyseal dysplasia in the severe missense subtype.
  phenotype_term:
    preferred_term: Spondylometaphyseal dysplasia
    term:
      id: HP:0002657
      label: Spondylometaphyseal dysplasia
  subtype: CDL-SMD
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia."
    explanation: Ties spondylometaphyseal dysplasia to the missense subtype.
- category: Skeletal
  name: Scoliosis
  description: Marked scoliosis, developing over time in the severe spondylometaphyseal subtype.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
    clinical_course: PROGRESSIVE
  subtype: CDL-SMD
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The radiographic features evolved over time to spondylometaphyseal dysplasia with marked short stature and marked scoliosis."
    explanation: >-
      Documents the scoliosis and, in the same sentence, that the radiographic
      picture EVOLVED - the severe subtype's features develop rather than being
      present from birth.
- category: Skeletal
  name: Long Bone Deformity
  description: >-
    Bowing and deformity of the long bones, severe enough in some patients to
    require femoral osteotomy and intramedullary rodding.
  phenotype_term:
    preferred_term: Bowing of the long bones
    term:
      id: HP:0006487
      label: Bowing of the long bones
  evidence:
  - reference: PMID:34761145
    reference_title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions."
    explanation: >-
      Lists long-bone deformity among the defining features of the severe end of
      the spectrum. Note this snippet says "deformities" generically while the
      binding is the narrower `Bowing of the long bones`; the next item supplies
      the bowing-specific observation.
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bisphosphonate treatment was commenced at 2.6 years because of a radius fracture, lower limb bowing, and platyspondyly and discontinued at age 6 years; no new fractures have occurred thereafter."
    explanation: Documents lower limb bowing specifically, which is what the HP:0006487 binding asserts.
- category: Ophthalmologic
  name: Glaucoma
  description: >-
    Glaucoma, reported across three generations of the founding Finnish family -
    congenital bilateral in the index, chronic congestive in her grandmother.
    Ocular involvement is one of the extra-skeletal features that argues
    sphingomyelin metabolism matters outside bone.
  phenotype_term:
    preferred_term: Glaucoma
    term:
      id: HP:0000501
      label: Glaucoma
  notes: >-
    No frequency asserted, but the association has replicated: the Finnish
    discovery family and an independent Filipino family, both carrying the same
    recurrent p.Arg50* allele. Two unrelated families with the same allele and
    the same extra-skeletal finding is a materially stronger claim than one, and
    it strengthens the argument that sphingomyelin metabolism matters outside
    bone. The mechanism connecting SGMS2 to the eye remains unknown.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "recurrent idiopathic peripheral facial nerve palsies (Bell’s palsy), and congenital bilateral glaucoma"
    explanation: >-
      Records congenital bilateral glaucoma in the index patient of the Finnish
      family. Graded PARTIAL - one family, and no proposed mechanism linking
      SGMS2 to the eye.
  - reference: PMID:37886644
    reference_title: SGMS2 in primary osteoporosis with facial nerve palsy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the association of glaucoma with SGMS2 was further strengthened when Collantes and coworkers described a Filipino family harboring the SGMS2 p.Arg50* mutation, with characteristic skull lesions and juvenile onset open angle glaucoma"
    explanation: >-
      The independent replication - a second, unrelated family carrying the same
      recurrent allele with juvenile open-angle glaucoma. This is what upgrades
      glaucoma from a single-family observation to a replicated association.
- category: Otologic
  name: Sensorineural Hearing Impairment
  description: >-
    Sensorineural hearing loss, reported in the most severely affected patient of
    the discovery cohort and as progressive hearing loss after age 25 in another
    family.
  phenotype_term:
    preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  notes: >-
    No frequency asserted - individual patients across two families. Worth noting
    that a sensorineural rather than conductive deficit argues against simple
    compression by sclerotic temporal bone, which would be the obvious
    skeletal explanation.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional findings included sensorineural hearing loss, myopia, dilated aortic root, and hypotonia with abnormal myopathic electromyography but normal muscle biopsy."
    explanation: >-
      Reports sensorineural hearing loss alongside other extra-skeletal findings
      in the most severely affected patient. Graded PARTIAL - one patient, and the
      listed findings are heterogeneous.
- category: Neurologic
  name: Facial Nerve Palsy
  description: >-
    Recurrent transient peripheral facial nerve palsy, often recovering over weeks,
    and recurring across decades in individual patients.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Facial palsy
    term:
      id: HP:0010628
      label: Facial palsy
    temporality: RECURRENT
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations."
    explanation: Reports the facial nerve palsy in the discovery cohort.
biochemical:
- name: Alkaline phosphatase and 25-hydroxyvitamin D
  notes: >-
    Calcium-phosphate metabolism is broadly unremarkable in CDL, which is itself
    diagnostically useful - it separates this disease from rickets and from
    hypophosphatasia, where the same panel is grossly abnormal. What has been
    reported is raised alkaline phosphatase activity with a low 25-hydroxyvitamin
    D, in a child in whom ionised calcium and inorganic phosphorus were both
    within reference limits. Neither finding is specific, and low vitamin D is
    common enough in children generally that it should not be over-read as part
    of the disease.
  evidence:
  - reference: PMID:37175737
    reference_title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The laboratory study of calcium phosphate metabolism revealed an increased level of alkaline phosphatase activity and decreased level of 25-hydroxyvitamin D (25(OH)D)."
    explanation: >-
      The reported biochemical abnormalities. Graded PARTIAL - a single child in
      one family, and neither finding is specific to CDL.
treatments:
- name: Bisphosphonate Therapy
  description: >-
    Bisphosphonates are the treatment actually used - pamidronate and alendronate
    in the French-Canadian proband from childhood, pamidronate courses in several
    of the discovery families and in the Belarusian kindred, with reported
    improvement in back pain, quality of life, bone mineral density, and in some
    patients cessation of further long-bone fractures.
    There is a mechanistic tension worth stating: bisphosphonates are
    ANTIRESORPTIVE, and osteoclast function in this disease was measured and found
    normal. So the drug is not correcting the demonstrated lesion, which is
    defective matrix mineralization on a normal-turnover background. The reported
    benefits are real but come from uncontrolled clinical observation in small
    families, not from a trial.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: pamidronate
      term:
        id: CHEBI:7903
        label: pamidronate
    - preferred_term: alendronic acid
      term:
        id: CHEBI:2567
        label: alendronic acid
    - preferred_term: risedronic acid
      term:
        id: CHEBI:8869
        label: Risedronic acid
  target_mechanisms:
  - target: Bone Fragility and Fractures
    description: >-
      Treatment is directed at the fracture endpoint rather than at the
      mineralization defect that produces it.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bisphosphonate treatment resulted in notable improvement in back pain and quality of life."
    explanation: >-
      Reports symptomatic benefit in one family of the discovery cohort. Graded
      PARTIAL because it is uncontrolled clinical observation in a single patient
      within a case series, reporting symptoms rather than fracture rate.
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bisphosphonate treatment was commenced at 2.6 years because of a radius fracture, lower limb bowing, and platyspondyly and discontinued at age 6 years; no new fractures have occurred thereafter."
    explanation: >-
      A fracture-free interval after treatment in one child. Graded PARTIAL - an
      uncontrolled single-patient observation, and fracture rates in this disease
      fall with age anyway, so the comparison is confounded.
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
    explanation: >-
      Records risedronate among the agents given to one patient, which is why it
      sits in this entry rather than with the anabolic agent - it is a
      bisphosphonate and shares their mechanism. Exposure only, no outcome.
- name: Teriparatide
  description: >-
    The mechanistically interesting option, and the one this entry's therapeutic
    argument turns on. Teriparatide is an ANABOLIC agent that stimulates bone
    formation, not an antiresorptive. Since the demonstrated lesion here is
    defective matrix mineralization on a normal-turnover background with
    measured-normal osteoclast function, an agent that drives formation is
    better matched to the mechanism than one that suppresses resorption.
    No outcome is reported for it in CDL. The patient documented as receiving it
    had already sustained about 40 fractures by then, so this records what was
    tried, not what worked.
  therapeutic_modality: PEPTIDE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: teriparatide
      term:
        id: CHEBI:135983
        label: teriparatide
  target_mechanisms:
  - target: Defective Bone Matrix Mineralization
    description: >-
      Teriparatide targets bone FORMATION, which is the arm this disease
      impairs - in contrast to the antiresorptives, which target an arm measured
      to be normal. This link is asserted for teriparatide alone and deliberately
      not for the antiresorptive entries.
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
    explanation: >-
      Records teriparatide exposure in one patient. Graded PARTIAL - the sentence
      reports what was given with no outcome measure attached, so it establishes
      use and nothing about efficacy.
- name: Denosumab
  description: >-
    A RANKL-targeting monoclonal antibody, and an antiresorptive. It is recorded
    because it was given, not because the mechanism recommends it: like the
    bisphosphonates it acts on osteoclast-mediated resorption, which was measured
    and found normal in this disease. No outcome is reported. It carries no
    `target_mechanisms` link for exactly that reason - drawing one to the
    mineralization node would assert the anabolic rationale that belongs to
    teriparatide.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: denosumab
      term:
        id: NCIT:C61313
        label: Denosumab
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since the age of 20 years, she received multiple types osteoporosis treatment including; Teriparatide (Forteo), Denosumab (Prolia) and Risedronate (Actonel)."
    explanation: >-
      Records denosumab exposure in the same patient. PARTIAL for the same
      reason - exposure without an outcome measure.
- name: Vitamin D Supplementation
  description: >-
    Vitamin D supplementation alongside bisphosphonate therapy, and low
    25-hydroxyvitamin D has been documented in a reported patient. This is
    adjunctive and standard-of-care for any osteoporosis rather than
    disease-specific - it corrects a coexisting deficiency, it does not address
    the sphingomyelin lesion.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: nutritional support
    term:
      id: NCIT:C15433
      label: Nutritional Support
    therapeutic_agent:
    - preferred_term: vitamin D
      term:
        id: CHEBI:27300
        label: vitamin D
  evidence:
  - reference: PMID:34504906
    reference_title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She was prescribed vitamin D supplementation and started Pamidronate treatment (Aredia) until the age of 14 years, when she was switched to Alendronate (Fosamax) for 4–5 months."
    explanation: >-
      Records vitamin D given alongside bisphosphonates. PARTIAL - one patient,
      no separable outcome for the vitamin D arm.
- name: Orthopedic Surgery
  description: >-
    Corrective and stabilising surgery for long-bone deformity and recurrent
    fracture - femoral osteotomy and intramedullary rodding are both reported.
    Notably the patient who had both continued to fracture afterwards, which is
    consistent with surgery addressing deformity and mechanical support rather
    than the underlying bone material defect.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: orthopedic surgical procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  target_mechanisms:
  - target: Bone Fragility and Fractures
    description: >-
      Mechanical correction and stabilisation of deformed or repeatedly fractured
      long bones, without altering matrix mineralization.
  evidence:
  - reference: PMID:30779713
    reference_title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She underwent femoral osteotomy at 2.5 years and intramedullary rodding at 15 years but continued to have multiple fractures."
    explanation: >-
      Records both procedures and, in the same sentence, that fractures continued
      - which is why the description presents surgery as mechanical management
      rather than disease modification.
discussions:
- discussion_id: cdl_calvarial_sclerosis_unexplained
  kind: KNOWLEDGE_GAP
  attaches_to:
  - pathophysiology#Calvarial Sclerotic Doughnut Lesions
  prompt: >-
    Why does a disease that under-mineralises the skeleton produce focal
    HYPER-sclerotic ring lesions in the calvaria?
  rationale: >-
    The doughnut lesions are the pathognomonic feature and the reason the entity
    was recognised in 1969, yet no mechanism connects them to the demonstrated
    mineralization defect - they point the opposite way. Candidate explanations
    (a site-specific difference between intramembranous calvarial and endochondral
    long-bone ossification, a reactive response to microdamage, a local difference
    in membrane lipid requirement) have not been tested. The node is deliberately
    left without an incoming causal edge from the mineralization branch, because
    drawing one would assert a mechanism nobody has proposed.
- discussion_id: cdl_missense_severity_inversion
  kind: OPEN_QUESTION
  attaches_to:
  - pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
  prompt: >-
    Why are the missense alleles, which retain and even increase catalytic
    activity, more severe than the catalytically dead nonsense allele?
  rationale: >-
    This inverts the usual dose-response intuition and is the single most
    informative fact in the entry. The proposed reading - that ER-retained active
    SMS2 does harm beyond what simply losing the enzyme does, whether by consuming
    ceramide in the wrong compartment, generating diacylglycerol where it should
    not be, or acting as a dominant-negative on trafficking - has not been
    distinguished experimentally. Distinguishing them matters therapeutically: a
    substrate or enzyme-replacement strategy would help one model and worsen the
    other.
- discussion_id: cdl_neuropathy_compressive_vs_lipid
  kind: OPEN_QUESTION
  attaches_to:
  - pathophysiology#Cranial Neuropathy
  prompt: >-
    Are the recurrent cranial nerve palsies caused by compression at sclerotic
    cranial foramina, or by a primary sphingomyelin defect in nerve or myelin?
  rationale: >-
    Both are plausible and they predict different management. Compression would
    make the palsies a mechanical consequence of the calvarial lesions and
    potentially surgically addressable; a primary lipid effect on
    neuron-supporting cells would make them an independent arm of the disease.
    The transient, recurrent, fully recovering character of the palsies fits a
    lipid or conduction mechanism better than progressive compression, and one
    patient with palsies had no reported cranial sclerosis - but no study has
    imaged the foramina during an episode.
- discussion_id: cdl_bisphosphonate_mechanism_mismatch
  kind: KNOWLEDGE_GAP
  attaches_to:
  - treatments#Bisphosphonate Therapy
  prompt: >-
    Are antiresorptives the right drug class for a disease with normal osteoclast
    function and a primary matrix mineralization defect?
  rationale: >-
    Osteoclast formation, morphology and resorptive capacity were all measured and
    found normal, and histomorphometry showed normal turnover. Bisphosphonates
    nonetheless are what patients receive, by analogy with osteoporosis generally,
    and uncontrolled reports describe benefit. Whether that benefit reflects a real
    effect on this mechanism, a nonspecific density gain, or the natural fall in
    fracture rate with age is untested. A bisphosphonate that further slows
    mineralization is at least mechanistically conceivable in a disease whose
    lesion is a prolonged mineralization lag time.
    The comparison that would answer this is already partly available and has
    never been made: reported patients have also received TERIPARATIDE, an
    anabolic agent that drives bone formation rather than suppressing resorption,
    and formation is the arm this disease actually impairs. No outcome is reported
    for it, and the one patient documented as receiving it had already sustained
    about 40 fractures by then - but an anabolic-versus-antiresorptive comparison
    in SGMS2 disease is the obvious next question, and the mechanism predicts which
    way it should go.
- discussion_id: cdl_mouse_isoform_divergence
  kind: HUMAN_MODEL_MISMATCH
  attaches_to:
  - pathophysiology#Defective Bone Matrix Mineralization
  prompt: >-
    Why does Sgms2 loss cause a skeletal disease in humans but no bone phenotype in
    the mouse null?
  rationale: >-
    Sms2-deficient mice have no apparent bone abnormality; the murine bone-formation
    deficit requires additional osteoblast-specific Sms1 deletion, so SMS1 appears
    to be the limiting isoform in mouse osteoblasts while SGMS2 is the disease gene
    in humans. This is the reason CDL has no mouse model and drove the move to
    zebrafish. There is a second reading that is at least as likely and is more
    useful: since the SEVERE human alleles retain full enzymatic activity and act by
    ER retention, they may be dominant and gain-of-mislocalisation rather than loss
    of function - in which case a mouse null models the wrong lesion entirely, and
    the informative experiment is a knock-in of p.Ile62Ser or p.Met64Arg, which has
    not been reported.
  proposed_experiments:
  - experiment_id: cdl_sgms2_missense_knockin_mouse
    name: Knock-in mouse carrying a human SGMS2 severe missense allele
    description: >-
      Generate a mouse carrying the orthologous p.Ile62Ser or p.Met64Arg
      substitution rather than an Sgms2 null, and assess bone mass, mineral
      apposition rate, matrix mineralization and osteocyte canalicular
      architecture against wild-type and Sgms2-null littermates.
    would_support:
    - pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
    supporting_outcome:
    - >-
      A skeletal phenotype in the missense knock-in that is absent in the
      Sgms2-null littermates, establishing the lesion as mislocalisation rather
      than loss of enzyme.
    would_refute:
    - pathophysiology#Mislocalisation of SMS2 Away from the Plasma Membrane
    refuting_outcome:
    - >-
      No skeletal phenotype in the missense knock-in either, indicating the
      species difference is in isoform dependence rather than in allele mechanism.
- discussion_id: cdl_lacunocanalicular_single_allele
  kind: KNOWLEDGE_GAP
  attaches_to:
  - pathophysiology#Osteocyte Lacunocanalicular Network Disruption
  prompt: >-
    Is the osteocyte lacunocanalicular defect present in the severe missense
    subtype, or only in p.Arg50* carriers?
  rationale: >-
    The lacunocanalicular finding comes from transiliac biopsies of two adult
    males, both carrying p.Arg50*. Since the two allele classes mislocalise SMS2 to
    different compartments and produce clinically opposite severities, it cannot be
    assumed the tissue lesion is the same in both. No biopsy data from a missense
    carrier has been published.
animal_models:
- name: Sms2-deficient mouse (SMS1f/f;SMS2-/-)
  species: Mouse
  genotype: SMS1-floxed, SMS2-null (SMS1f/f;SMS2-/-), without osteoblast Cre
  publication: PMID:31847800
  description: >-
    The germline Sms2-null mouse - the obvious model for a human SGMS2 disease, and
    the reason a zebrafish model had to be built instead.
  modeled_mechanisms:
  - target: Defective Bone Matrix Mineralization
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    description: >-
      Sms2 deficiency alone does not reduce bone formation in mice. The
      bone-formation deficit appears only when Sms1 is additionally deleted in
      osteoblasts, implicating SMS1 rather than SMS2 as the isoform limiting for
      murine osteoblast differentiation.
    limitations: >-
      This is a species-level divergence in isoform dependence, and it has a
      specific consequence for how human data should be read: the human severe
      alleles are NOT nulls but ER-retained active enzyme, so a mouse null was
      arguably never the right model of the human lesion in the first place. A
      knock-in of a human missense allele has not been reported.
    evidence:
    - reference: PMID:31847800
      reference_title: Deficiency of sphingomyelin synthase 1 but not sphingomyelin synthase 2 reduces bone formation due to impaired osteoblast differentiation.
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "we found that although cartilage formation is normal, Sp7-Cre;SMS1f/f;SMS2-/- mice showed reduced trabecular and cortical bone mass, had lower bone mineral density, and had a slower mineral apposition rate than control mice"
      explanation: >-
        Graded REFUTE against the Sms2-null mouse as a model of this disease. The
        bone phenotype required osteoblast Sms1 deletion on top of the Sms2-null
        background; the Sms2-null control mice, which are the direct analogue of
        the human genotype, were among the unaffected comparators.
- name: sgms2a/sgms2b CRISPR-Cas13d knockdown zebrafish
  species: Zebrafish
  genotype: sgms2a, sgms2b and sgms2a+b transient CRISPR-Cas13d knockdown
  publication: PMID:41143154
  description: >-
    Transient knockdown of the two zebrafish SGMS2 orthologues, developed
    specifically because Sgms2 knockout mice have no skeletal phenotype at all -
    the model of first resort fails for this disease.
  modeled_mechanisms:
  - target: Defective Bone Matrix Mineralization
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    description: >-
      Knockdown compromises notochord and craniofacial skeletal formation, and
      single-cell RNA sequencing localises sgms2a expression to osteogenic cells in
      juvenile calvaria, which supports a cell-autonomous skeletal role for the
      gene.
    limitations: >-
      The readout is embryonic craniofacial and notochord development, not adult
      bone matrix mineralization, so it does not model the human lesion. It is a
      transient knockdown rather than a stable genetic model, it is a
      loss-of-expression model while the severe human alleles are mislocalising
      rather than null, and the authors describe the models explicitly as
      preliminary platforms.
    evidence:
    - reference: PMID:41143154
      reference_title: Utilizing CRISPR-Cas13d-knockdown in zebrafish to study a rare monogenic bone fragility syndrome.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Knockdown of sgms2a and sgms2b in zebrafish had detrimental effect on embryonic development and compromised notochord and craniofacial formation."
      explanation: The skeletal phenotype obtained, which is developmental rather than a model of adult bone fragility.
    - reference: PMID:41143154
      reference_title: Utilizing CRISPR-Cas13d-knockdown in zebrafish to study a rare monogenic bone fragility syndrome.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Single-cell RNA sequencing of juvenile WT zebrafish calvaria cells detected high sgms2a expression in osteogenic cells."
      explanation: Localises the orthologue's expression to osteogenic cells in calvaria, supporting a cell-autonomous role in the tissue the human disease affects.
references:
- reference: PMID:30779713
  title: Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
- reference: PMID:37175737
  title: Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
- reference: PMID:34761145
  title: Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
- reference: PMID:34504906
  title: Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
- reference: PMID:40978119
  title: Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
- reference: PMID:41143154
  title: Utilizing CRISPR-Cas13d-knockdown in zebrafish to study a rare monogenic bone fragility syndrome.
- reference: PMID:40123745
  title: A novel SGMS2 mutation associated with high bone mass; description of an affected family with recurrent fragility fractures.
- reference: PMID:36102623
  title: Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
- reference: PMID:31847800
  title: Deficiency of sphingomyelin synthase 1 but not sphingomyelin synthase 2 reduces bone formation due to impaired osteoblast differentiation.
- reference: PMID:37886644
  title: SGMS2 in primary osteoporosis with facial nerve palsy.
📚

References & Deep Research

References

10
PMID:30779713
Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.
No top-level findings curated for this source.
PMID:37175737
Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant.
No top-level findings curated for this source.
PMID:34761145
Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations.
No top-level findings curated for this source.
PMID:34504906
Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature.
No top-level findings curated for this source.
PMID:40978119
Transcriptomic and lipidomic profiling provide novel insight into the pathogenesis of monogenic SGMS2-related osteoporosis.
No top-level findings curated for this source.
PMID:41143154
Utilizing CRISPR-Cas13d-knockdown in zebrafish to study a rare monogenic bone fragility syndrome.
No top-level findings curated for this source.
PMID:40123745
A novel SGMS2 mutation associated with high bone mass; description of an affected family with recurrent fragility fractures.
No top-level findings curated for this source.
PMID:36102623
Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway.
No top-level findings curated for this source.
PMID:31847800
Deficiency of sphingomyelin synthase 1 but not sphingomyelin synthase 2 reduces bone formation due to impaired osteoblast differentiation.
No top-level findings curated for this source.
PMID:37886644
SGMS2 in primary osteoporosis with facial nerve palsy.
No top-level findings curated for this source.

Deep Research

1
OpenScientist ▸
Calvarial Doughnut Lesions–Bone Fragility Syndrome (CDL/CDL-SMD): Comprehensive Disease Report
openscientist-autonomous 8 citations 2026-08-28T10:33:25.182993

Calvarial Doughnut Lesions–Bone Fragility Syndrome (CDL/CDL-SMD): Comprehensive Disease Report

Disease: Calvarial Doughnut Lesions with Bone Fragility, with or without Spondylometaphyseal Dysplasia OMIM: #126550 · Gene: SGMS2 (SMS2) · MONDO: MONDO:0007470 / MONDO:0007926 · ORPHA: 85192 Category: Mendelian (autosomal dominant)

Summary

Calvarial Doughnut Lesions–Bone Fragility Syndrome (CDL; OMIM #126550) is an ultra-rare autosomal-dominant skeletal dysplasia caused by heterozygous pathogenic variants in SGMS2, the gene encoding the plasma-membrane–resident enzyme sphingomyelin synthase 2 (SMS2) on chromosome 4q25. The disease is defined clinically by childhood-onset low bone mineral density, recurrent spinal and peripheral fragility fractures, and its pathognomonic radiographic hallmark: multiple ring-like ("doughnut-shaped") sclerotic/hyperostotic lesions of the calvarium, often palpable as cranial lumps. A subset of patients also has spondylometaphyseal dysplasia (CDL-SMD), representing the severe end of the disease spectrum.

The central mechanistic insight is that CDL results from two distinct molecular mechanisms operating along a genotype–phenotype gradient. The recurrent nonsense variant c.148C>T (p.Arg50*) produces a catalytically inactive enzyme (loss of function) and is associated with the milder, childhood-onset osteoporosis end of the spectrum. In contrast, the N-terminal missense variants c.185T>G (p.Ile62Ser) and c.191T>G (p.Met64Arg) produce a fully active but ER-retained enzyme — a "toxic gain-of-mislocalization" mechanism — and cause the severe CDL-SMD phenotype with neonatal fractures, severe short stature, and long-bone deformities. Because SMS2 normally acts at the plasma membrane and trans-Golgi to establish a sphingomyelin/sterol gradient along the secretory pathway, mislocalized enzyme disrupts membrane lipid organization in osteogenic cells and impairs the matrix mineralization that osteoblasts and osteocytes carry out. Osteoclast formation and function remain normal, distinguishing CDL from high-turnover bone disease and from osteogenesis imperfecta.

All three canonical pathogenic variants are absent from gnomAD (~1.6M alleles surveyed), and SGMS2 shows only moderate loss-of-function constraint (pLI ≈ 0.01, LOEUF ≈ 0.69). This population-genetic signature supports the interpretation that the severe phenotype depends on the mislocalization/gain mechanism rather than on simple gene-dosage loss. Management is currently symptomatic, centered on bisphosphonates plus calcium and vitamin D, which improve bone mineral density and reduce fractures; no curative therapy exists. This report synthesizes 13 confirmed findings across all 15 requested disease-characteristic domains.

Key Findings

1. Disease Information

CDL is a rare autosomal-dominant skeletal disorder characterized by low bone mineral density, spinal and peripheral fractures, and specific sclerotic lesions of the cranial bones (Merkuryeva et al. 2023). As stated verbatim: "Calvarial doughnut lesions (CDL) with bone fragility with or without spondylometaphyseal dysplasia (MIM: #126550) is a rare autosomal dominant skeletal disorder characterized by low bone mineral density, spinal and peripheral fractures, and specific sclerotic lesions of the cranial bones" (PMID: 37175737).

Key identifiers:

Resource Identifier
OMIM (disease) #126550
OMIM (gene SGMS2) 611574
Chromosomal locus 4q25
Orphanet ORPHA:85192
MONDO MONDO:0007470 (also mapped MONDO:0007926)
SNOMED CT 720598005
Disease Ontology DOID:0080721
UMLS / GTR C1852022
HGNC gene SGMS2

Synonyms / alternative names: familial calvarial doughnut lesions; CDL; CDL with bone fragility; CDL with spondylometaphyseal dysplasia (CDLSMD/CDL-SMD).

The information is derived from aggregated disease-level resources and small clinical case series/family studies (the entire literature comprises roughly 8–11 families), rather than from EHR-scale patient datasets.

2. Etiology

The primary cause is genetic: heterozygous pathogenic variants in SGMS2. There is no known environmental, infectious, or acquired etiology. Pekkinen et al. 2019 evaluated six families with rare skeletal phenotypes and osteoporosis by next-generation sequencing and identified in all families a heterozygous SGMS2 variant: "we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane-resident sphingomyelin synthase SMS2" (PMID: 30779713).

  • Genetic risk factors: The causal alleles are the disease-defining variants themselves (p.Arg50*, p.Ile62Ser, p.Met64Arg). No independent susceptibility loci or modifier genes have been established.
  • Environmental / lifestyle risk factors: None identified; the disease is monogenic and highly penetrant for the genetic lesion, though clinical expressivity is variable.
  • Protective factors / gene–environment interactions: No genetic or environmental protective factors have been reported. Supportive measures (calcium, vitamin D, bisphosphonates) mitigate the phenotype but are treatments rather than etiologic protective factors.

3. Phenotypes

CDL is a multi-system skeletal phenotype with variable severity. The following table consolidates the reported features with suggested HPO terms.

Phenotype Type Onset / severity Suggested HPO
Doughnut-shaped sclerotic calvarial lesions (palpable cranial lumps) Physical manifestation / imaging Childhood; pathognomonic HP:0002683 (Abnormal skull morphology); HP:0002684 (Thickened calvaria)
Low bone mineral density / osteoporosis Laboratory / imaging Childhood-onset HP:0000939 (Osteoporosis); HP:0004349 (Reduced bone mineral density)
Recurrent fragility fractures (spinal + peripheral) Clinical sign Childhood; severe in CDL-SMD (neonatal) HP:0002659 (Increased susceptibility to fractures); HP:0002757 (Recurrent fractures)
Vertebral compression fractures / "bone-in-bone" vertebrae Imaging Childhood–adult HP:0002953 (Vertebral compression fractures)
Spondylometaphyseal dysplasia (severe subset) Physical manifestation Neonatal/infantile HP:0002656 (Metaphyseal dysplasia); HP:0002655 (Spondylometaphyseal dysplasia)
Severe short stature (severe subset) Physical manifestation Congenital/infantile HP:0004322 (Short stature)
Long-bone deformity / undermodeling of tubular bones Imaging Childhood HP:0000924 (Abnormal skeletal morphology)
Peripheral facial nerve palsy Neurological sign Variable HP:0010628 (Facial palsy)
Elevated serum alkaline phosphatase Laboratory abnormality Variable HP:0003155 (Elevated alkaline phosphatase)
Dental caries / tooth hypoplasia Physical manifestation Childhood HP:0000670 (Carious teeth); HP:0006297 (Hypoplasia of teeth)
Glaucoma (occasional) Clinical sign Variable HP:0000501 (Glaucoma)
Scoliosis Physical manifestation Childhood HP:0002650 (Scoliosis)

Severity gradient: Subjects with p.Arg50* present at the milder end — "childhood-onset osteoporosis with or without cranial sclerosis" — whereas patients with p.Ile62Ser or p.Met64Arg have "a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia" (PMID: 30779713). Progression is chronic and lifelong, with fracture susceptibility being the dominant morbidity. There is wide interfamilial and intrafamilial phenotypic variability, even among individuals sharing the identical p.Arg50* variant (Merkuryeva 2023; Basalom 2021).

Neurological involvement: "Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations" (PMID: 30779713), attributed to the role of sphingomyelin in neural tissue. A dedicated 2023 review (Pihlström et al., PMID 37886644) specifically links SGMS2 primary osteoporosis with facial nerve palsy.

Quality-of-life impact: Recurrent fractures, chronic back pain from vertebral compressions, scoliosis, short stature, and facial nerve palsy collectively impair mobility, cause chronic pain, and reduce daily functioning. No formal EQ-5D/SF-36/PROMIS studies exist for this ultra-rare disease.

4. Genetic / Molecular Information

Causal gene: SGMS2 (sphingomyelin synthase 2 / SMS2), OMIM 611574, chromosome 4q25. The three canonical pathogenic variants (RefSeq NM_001375905.1) are:

Variant (cDNA) Protein Type ClinVar classification ClinVar VCV gnomAD
c.148C>T p.Arg50* (Arg50Ter) Nonsense Pathogenic/Likely pathogenic VCV000635285 Absent (AC=0)
c.185T>G p.Ile62Ser Missense Pathogenic VCV000635286 Absent (AC=0)
c.191T>G p.Met64Arg Missense Pathogenic VCV000635287 Absent (AC=0)

"Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg)" (PMID: 30779713).

Functional consequences: "While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum" (PMID: 30779713). Thus p.Arg50* = loss of function; the missense alleles = ER-retention / toxic gain-of-mislocalization.

Population genetics and constraint: In gnomAD v4 (GRCh38, ~1.61M alleles), all three canonical variants are absent (exome AC=0, genome AC=0; 95% upper-bound AF ≈ 1.9×10⁻⁶). By contrast, the adjacent benign-leaning VUS c.149G>A/p.Arg50Gln is observed (AC=15, AF≈9.3×10⁻⁶), reinforcing the specificity of the causal alleles. SGMS2 gene-level constraint is only moderate — pLI = 0.010, LOEUF (oe_lof_upper) = 0.69, observed/expected LOF = 21/43.8 = 0.48, mis_z = 1.20 — meaning the gene tolerates heterozygous LOF reasonably well. This is a key clue that the severe phenotype is not driven by simple haploinsufficiency/dosage but by the gain-of-mislocalization mechanism of the missense alleles.

Modifier genes / epigenetics / chromosomal abnormalities: No disease modifiers have been established. Some "pathogenic" SGMS2-region ClinVar entries are large chromosome-4q copy-number gains unrelated to CDL and should not be confused with the point-variant allelic series. Of ~200 SGMS2 ClinVar submissions, the vast majority are VUS from population/panel screening; only the three point variants above are disease-causing for CDL.

5. Environmental Information

No environmental factors, toxins, radiation, occupational exposures, lifestyle factors, or infectious agents are implicated in CDL. It is a purely Mendelian, monogenic disorder. (Sphingomyelin biology is relevant to other membrane-stress and infection contexts — e.g. SMPDL3B in cGAS-STING signaling, PMID 41175872 — but these are unrelated to CDL pathogenesis.)

6. Mechanism / Pathophysiology

Molecular pathway — sphingolipid metabolism. SMS2 catalyzes the transfer reaction phosphatidylcholine + ceramide → sphingomyelin + diacylglycerol at the plasma membrane and trans-Golgi. Sphingomyelin is the "main lipid component of the plasma membrane essential for bone mineralization" (PMID: 37175737). Normally, sphingomyelin production in the trans-Golgi traps ER cholesterol to build a sphingomyelin/sterol gradient along the secretory pathway.

Core pathomechanism (severe missense alleles). Sokoya et al. 2022 showed that "SMS2 variants linked to the most severe bone phenotypes retain full enzymatic activity but fail to leave the ER owing to a defective autonomous ER export signal. Cells harboring pathogenic SMS2 variants accumulate sphingomyelin in the ER and display a disrupted transbilayer sphingomyelin asymmetry" (PMID: 36102623). This ectopic ER sphingomyelin production produces imbalances in cholesterol organization, glycerophospholipid profiles, and membrane lipid order along the secretory pathway (also observed in patient-derived fibroblasts). The authors conclude: "We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity" (PMID: 36102623).

Causal chain (severe CDL-SMD):

Missense SGMS2 (p.Ile62Ser / p.Met64Arg)
   │  (disrupts N-terminal autonomous ER-export signal)
   ▼
Active SMS2 retained in the ER  ──► ectopic SM synthesis in ER
   ▼
Disrupted transbilayer SM asymmetry + altered cholesterol/
glycerophospholipid distribution + abnormal membrane lipid order
   ▼
Loss of nonrandom secretory-pathway lipid landscape in osteoblasts/osteocytes
   ▼
Defective bone-matrix mineralization (normal osteoclasts)
   ▼
Low BMD, fragile bone, doughnut calvarial lesions, SMD

Bone tissue-level pathology. Mäkitie et al. 2021 analyzed transiliac biopsies from two adult males with p.Arg50*. Histomorphometry showed reduced osteoid thickness and mineralizing surface, increased osteoid surface, and markedly elevated mineralization lag time (+8.16 SD, +4.10 SD). Quantitative backscattered electron imaging (qBEI) showed low, heterogeneous matrix mineralization (CaPeak −2.41/−3.72 SD; CaWidth +7.47/+4.41 SD) with chaotic collagen fibril arrangement under polarized light; osteocyte lacunae were abnormally large/round and the canalicular network severely disturbed (PMID: 34761145). Independent biopsy data: "Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization. Osteoclast formation and function in vitro was normal" (PMID: 30779713).

Upstream vs downstream: The upstream trigger is the mislocalized/inactive SMS2 enzyme; the downstream endpoint is impaired osteoblast/osteocyte matrix mineralization. Osteoclasts are not the effectors — resorption is normal — so the disease is a bone-formation/mineralization defect, not a resorption defect.

Ontology suggestions: GO:0006686 (sphingomyelin biosynthetic process); GO:0006665 (sphingolipid metabolic process); GO:0030282 (bone mineralization); GO:0001503 (ossification); GO:0006888 (ER-to-Golgi vesicle-mediated transport). Cell types: CL:0000062 (osteoblast); CL:0000137 (osteocyte). CHEBI:17636 (sphingomyelin); CHEBI:16113 (cholesterol); CHEBI:17761 (ceramide).

7. Anatomical Structures Affected

  • Primary organ/system: Skeletal system (UBERON:0001474 bone element; UBERON:0002481 bone tissue). Directly affected sites include the calvaria/skull (UBERON:0011618 calvaria; UBERON:0000209 cranial bone), vertebral column (UBERON:0001130), and long/tubular bones of the limbs (UBERON:0002495 long bone) with metaphyseal involvement in the severe subset.
  • Secondary involvement: Peripheral nervous system — facial nerve (UBERON:0001647), causing facial palsy; eye (glaucoma, UBERON:0000970); teeth/dentition (UBERON:0001091), with caries and hypoplasia.
  • Tissue/cell level: Connective tissue (bone). Target cells are osteoblasts (CL:0000062) and osteocytes (CL:0000137); osteoclasts (CL:0000092) are spared functionally. SGMS2 is "prominently expressed in cortical bone."
  • Subcellular level: Endoplasmic reticulum (GO:0005783) — site of pathogenic SMS2 retention and ectopic sphingomyelin accumulation; Golgi apparatus/trans-Golgi network (GO:0005802); plasma membrane (GO:0005886) — the normal SMS2 site of action; the osteocyte lacunocanalicular network is structurally disrupted.
  • Localization / lateralization: Calvarial lesions are typically multiple and bilateral; skeletal fragility is generalized/systemic.

8. Temporal Development

  • Onset: Childhood-onset in the milder (p.Arg50*) form; neonatal/congenital onset (neonatal fractures) in the severe CDL-SMD (missense) form. Onset pattern is insidious/chronic.
  • Progression: Chronic and lifelong. Fracture burden accrues over childhood and adulthood; vertebral compression fractures and scoliosis can progress. Progression rate is variable and correlates with genotype (severe in missense alleles).
  • Disease course: Progressive/stable skeletal fragility rather than episodic or relapsing–remitting; not self-limited.
  • Critical periods / intervention windows: Childhood/adolescence, during peak bone accrual, is the key window for anti-osteoporotic intervention (bisphosphonates) to prevent fractures and permit vertebral remodeling — as demonstrated in a pediatric case where compressed vertebrae partially recovered after 2 years of therapy (Zhang 2025).

9. Inheritance and Population

  • Inheritance: Autosomal dominant (heterozygous SGMS2 variants).
  • Epidemiology: Ultra-rare. By 2023, ~15 patients from 8 families had been described in the literature; Merkuryeva et al. added 11 more patients from three families (all p.Arg50*), for a total of only a few dozen reported cases worldwide. No population prevalence/incidence estimates are available due to rarity.
  • Penetrance / expressivity: "These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings" (PMID: 34504906). Expressivity is highly variable, both between and within families.
  • Founder effects: The recurrent c.148C>T (p.Arg50*) variant appears in French-Canadian and French families of shared ancestry, consistent with a founder allele; the French-Canadian pedigree spans six generations (Basalom 2021).
  • Carrier frequency / consanguinity: As a dominant disorder, carrier-frequency and consanguinity concepts do not apply in the recessive sense; the three causal alleles are absent from gnomAD, indicating they are not present at appreciable frequency in the general population.
  • Population demographics / sex ratio: No sex predilection has been established (both sexes affected); the best-documented cluster is of French/French-Canadian ancestry, but the disorder is otherwise pan-ethnic.

10. Diagnostics

  • Imaging (cornerstone): Skeletal radiography reveals the pathognomonic multiple doughnut-shaped (ring-like) sclerotic/hyperostotic calvarial lesions, generalized osteopenia/low BMD, vertebral compression fractures, "bone-in-bone" vertebrae, metaphyseal undermodeling, and squaring of metacarpals/metatarsals. DXA quantifies low BMD (e.g., whole-body BMD Z-score −2.8 in a pediatric case; Zhang 2025).
  • Laboratory: Serum alkaline phosphatase may be elevated. Bone-turnover markers and calcium/phosphate are generally used to exclude other metabolic bone disease.
  • Bone biopsy / histomorphometry: Transiliac biopsy shows defective mineralization (increased osteoid, prolonged mineralization lag time), low/heterogeneous matrix mineralization on qBEI, chaotic collagen, and disrupted osteocyte lacunocanalicular network — with normal osteoclast function (PMID: 34761145; PMID: 30779713).
  • Genetic testing (definitive): Targeted SGMS2 single-gene testing or osteoporosis/skeletal-dysplasia gene panels; whole-exome sequencing (WES) was the discovery method (Pekkinen 2019). Because the allelic series is very small and clustered, sequencing the exon encoding the N-terminal region of SGMS2 is high-yield. ClinVar (NM_001375905.1) confirms the three canonical variants as Pathogenic.
  • Clinical criteria / differential diagnosis: Diagnosis rests on the combination of characteristic calvarial doughnut lesions + bone fragility + SGMS2 variant. Key differentials: osteogenesis imperfecta (CDL is distinct — see below), other early-onset osteoporoses (e.g., LRP5, WNT1, PLS3), and sclerosing bone dysplasias. Nishimura et al. concluded such patients "may represent a group of fragile bone syndromes which differ from osteogenesis imperfecta" (PMID: 8958616).
  • Screening: Cascade genetic testing of at-risk relatives is appropriate given autosomal-dominant inheritance and the recurrent founder allele.

11. Outcome / Prognosis

  • Survival/mortality: CDL is not life-limiting; no disease-specific mortality is reported. Morbidity, not mortality, dominates the prognosis.
  • Morbidity/disability: Recurrent fractures, chronic back pain, vertebral compression and scoliosis, short stature (severe subset), facial nerve palsy, and dental problems drive long-term disability and reduced mobility/quality of life.
  • Disease course/complications: Chronic, lifelong skeletal fragility; complications include vertebral deformity, long-bone deformity requiring surgery in severe cases, and glaucoma/facial palsy requiring specialist care.
  • Recovery potential / prognostic factors: Genotype is the principal prognostic factor — missense (ER-retention) alleles predict severe CDL-SMD, whereas p.Arg50* predicts milder disease. With bisphosphonate therapy, BMD improves and fractures decrease; vertebral remodeling can occur in growing children (Zhang 2025).

12. Treatment

There is no curative/pathogenetic therapy; management is symptomatic and aimed at preventing osteoporosis progression and fractures.

Modality Details Suggested NCIT
Bisphosphonates (first-line) Pamidronate/other bisphosphonates to increase BMD and reduce fractures NCIT:C1876 (Bisphosphonate); NCIT:C1350 (Pamidronate)
Calcium supplementation Adjunct to bisphosphonate NCIT:C376 (Calcium)
Vitamin D Adjunct to support mineralization NCIT:C904 (Vitamin D)
Orthopedic surgery Management of fractures/deformities in severe cases NCIT:C15329 (Surgery)
Specialist care Ophthalmology (glaucoma), neurology/ENT (facial palsy), dentistry —

Representative outcome (pediatric case, Zhang 2025, PMID: 40393762): A 7.4-year-old boy with SGMS2 c.148C>T (p.Arg50*), scoliosis, multiple vertebral compressions, and whole-body BMD 0.664 g/cm² (Z-score −2.8) was treated with pamidronate disodium + calcium + vitamin D for 2 years. Outcome: back pain improved, no new fractures, BMD Z-score rose from −2.8 to +1.3, and compressed vertebrae partially recovered/remodeled.

Advanced/experimental therapeutics: No gene, cell, RNA, targeted, or immunotherapies are approved or in trials for CDL. The gain-of-mislocalization mechanism of the missense alleles suggests that, in principle, allele-specific silencing or strategies to restore ER export could be rational future targets — but none exist today. No pharmacogenomic guidance is established.

13. Prevention

  • Primary prevention: Not applicable in the classic sense (monogenic disease). Genetic counseling for affected families is central, given autosomal-dominant inheritance (50% transmission risk) and the recurrent founder allele.
  • Secondary prevention: Early diagnosis (radiographic + genetic) and early anti-osteoporotic therapy to prevent fractures during the childhood bone-accrual window.
  • Tertiary prevention: Fracture prevention (bisphosphonates, fall precautions), management of scoliosis/vertebral deformity, and monitoring/treatment of glaucoma and facial nerve palsy.
  • Genetic screening: Cascade testing of relatives; prenatal/preimplantation genetic testing is theoretically possible for known familial variants. No population newborn/carrier screening is warranted for this ultra-rare dominant disorder.

14. Other Species / Natural Disease

No naturally occurring CDL analog has been documented in companion animals or wildlife (no OMIA entry identified). SGMS2 is evolutionarily conserved; the mouse ortholog is Sgms2 (see model-organism section). No zoonotic or cross-species transmission applies (this is a genetic, non-communicable disorder). Comparative biology is informative chiefly through mouse genetics of the sphingomyelin-synthase family.

15. Model Organisms

The most relevant models are mouse (Mus musculus, NCBI Taxon:10090) knockouts of the sphingomyelin-synthase family. A key study revealed an important isoform-specificity caveat: Matsumoto et al. 2019 showed that osteoblast-specific Sms1 deletion on an Sms2-null background (Sp7-Cre;SMS1^f/f;SMS2^−/−) reduced trabecular and cortical bone mass, lowered BMD, and slowed mineral apposition, and impaired BMP2-induced Smad1/5/8 and p38 signaling during osteoblast differentiation, whereas plain Sms2-deficient mice did not show this bone-formation deficit (PMID: 31847800).

Model limitation / interpretation: This means simple Sms2 knockout mice do not faithfully recapitulate human CDL. The discrepancy is mechanistically consistent with the human genetics: human CDL is caused by SGMS2 (SMS2), and the severe alleles act by ER-retention (gain of mislocalized activity), not by loss of SMS2 function. A faithful mouse model would therefore likely require knock-in of the missense alleles (e.g., Ile62Ser/Met64Arg) rather than simple gene deletion. Cellular models exist — patient-derived fibroblasts reproduce the disrupted secretory-pathway lipid landscape (Sokoya 2022). Model resources: MGI (mouse Sgms2), IMPC/IMSR for knockout lines.

Mechanistic Model / Interpretation

CDL is best understood as a sphingolipid-membrane–organization disorder of bone-forming cells, with two mechanistic arms converging on defective mineralization:

    SGMS2 / SMS2 (sphingomyelin synthase 2, chr4q25)
                   │
┌──────────────────────────┴───────────────────────────┐
│                                                        │
  p.Arg50*  (nonsense)                              p.Ile62Ser / p.Met64Arg (missense)
  Loss of function                                  ER-export signal disrupted
  → catalytically inactive enzyme                   → ACTIVE enzyme trapped in ER
│                                                        │
  Reduced SM synthesis                              Ectopic SM synthesis in ER;
  (dosage effect; gene is                           disrupted transbilayer SM asymmetry,
   only moderately LOF-constrained,                 altered cholesterol/glycerophospholipid
   pLI≈0.01)                                        distribution & membrane lipid order
│                                                        │
   MILDER end:                                       SEVERE end (CDL-SMD):
   childhood osteoporosis                            neonatal fractures, severe
   ± cranial sclerosis                               short stature, SMD
└──────────────────────────┬───────────────────────────┘
                    ▼
Loss of the nonrandom secretory-pathway lipid landscape
that osteoblasts/osteocytes require for bone formation
                    ▼
Defective matrix mineralization + chaotic collagen +
disrupted osteocyte lacunocanalicular network
(osteoclasts NORMAL)
                    ▼
Low BMD · fragile bone · doughnut calvarial lesions

The population-genetic evidence ties the model together: because SGMS2 tolerates heterozygous LOF fairly well (pLI≈0.01, LOEUF≈0.69), pure haploinsufficiency produces the milder end, while the severe phenotype requires the toxic, active-but-mislocalized enzyme. The absence of all three causal alleles from ~1.6M gnomAD alleles confirms their pathogenic, non-polymorphic nature and distinguishes them from nearby tolerated variants (e.g., p.Arg50Gln, present at AF≈9×10⁻⁶). This model explains the genotype–phenotype gradient, the normal osteoclast biology, and why simple Sms2-knockout mice fail to reproduce the disease.

Evidence Base

PMID Paper (abbrev.) Role / support
30779713 Pekkinen 2019 — Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2 Landmark discovery. Establishes SGMS2 as causal gene, the three canonical variants, the LOF-vs-ER-retention dichotomy, genotype–phenotype gradient, normal osteoclasts, and facial-nerve palsy. Supports F001, F002, F003, F004.
36102623 Sokoya 2022 (eLife) — Pathogenic variants of SMS2 disrupt lipid landscapes in the secretory pathway Core mechanism. Shows severe alleles retain activity but are ER-retained; disrupted SM asymmetry and secretory-pathway lipid landscape; links to impaired osteogenic bone formation. Supports F005.
34761145 Mäkitie 2021 (JBMR Plus) — bone tissue organization/osteocyte network Tissue-level pathology. Quantitative histomorphometry/qBEI documenting the mineralization defect, chaotic collagen, disrupted lacunocanalicular network in p.Arg50* patients. Supports F006.
37175737 Merkuryeva 2023 — three families with recurrent variant Disease definition & recurrence. Provides disease definition, MIM number, inheritance, phenotypic variability, and SM's role in mineralization. Supports F007, F006.
34504906 Basalom 2021 — French-Canadian family Founder allele & penetrance. Documents recurrence of p.Arg50* in shared ancestry and variable penetrance. Supports F007.
8958616 Nishimura 1996 — original clinical description Historical/differential. Establishes CDL as distinct from osteogenesis imperfecta. Supports F003.
40393762 Zhang 2025 — pamidronate pediatric case Treatment outcome. Documents bisphosphonate efficacy (BMD Z-score −2.8→+1.3, no new fractures, vertebral remodeling). Supports F008.
31847800 Matsumoto 2019 (Mol Med) — mouse Sms1/Sms2 Model organism caveat. SMS1 (not SMS2) loss impairs osteoblast differentiation via BMP2–Smad/p38; explains why Sms2-KO mice don't model CDL. Supports F009.
38388831 Hu 2024 (Nat Struct Mol Biol) — SMSr cryo-EM Protein structure. Multi-TM fold with catalytic pentad; contextualizes clustering of disease residues near N-terminal ER-export region. Supports F011.

The evidence base is internally consistent: independent human genetic, cellular (patient fibroblast/heterologous expression), and bone-histology studies all converge on a mineralization defect driven by mislocalized/lost sphingomyelin-synthase activity, with population-genetic constraint data corroborating the mechanistic interpretation.

Limitations and Knowledge Gaps

  1. Ultra-rarity limits epidemiology. Only a few dozen patients from ~8–11 families are reported; there are no reliable prevalence/incidence figures, no sex-ratio data, and no formal quality-of-life (EQ-5D/SF-36/PROMIS) studies.
  2. Small allelic series. Essentially three pathogenic variants define the disease; genotype–phenotype conclusions rest on limited numbers, and the ~200 SGMS2 ClinVar entries are dominated by VUS.
  3. No faithful animal model. Sms2-knockout mice do not recapitulate CDL; the required knock-in missense models (Ile62Ser/Met64Arg) have not been reported, limiting mechanistic and preclinical therapeutic work.
  4. Mechanistic granularity. How disrupted secretory-pathway lipid distribution mechanistically produces both osteopenia (fragility) and focal calvarial hyperostosis (doughnut lesions) in the same patient remains incompletely explained.
  5. Treatment evidence is anecdotal. Bisphosphonate efficacy is supported by case reports, not controlled trials; long-term outcomes and optimal regimens are undefined. No disease-modifying therapy exists.
  6. Neurological and ocular features (facial palsy, glaucoma) are described but their frequency, natural history, and mechanistic link to sphingomyelin biology are not quantified.

Proposed Follow-up Experiments / Actions

  1. Generate knock-in mouse models of p.Ile62Ser and p.Met64Arg (and a p.Arg50* LOF line) to test the ER-retention/gain-of-mislocalization hypothesis in vivo and provide a preclinical platform. Compare to conditional osteoblast/osteocyte-specific lines.
  2. Osteoblast/osteocyte-specific lipidomics and imaging (patient iPSC-derived osteogenic cells and organoids) to map how ER-retained SMS2 remodels the secretory-pathway lipid gradient and to identify the mineralization step that fails.
  3. Establish an international CDL registry to aggregate genotype, radiographic phenotype, fracture history, treatment response, and neurological/ocular features — enabling proper penetrance/expressivity and natural-history quantification.
  4. Controlled/observational treatment studies of bisphosphonates (± other anti-osteoporotics) in CDL to define efficacy, dosing, and long-term skeletal outcomes.
  5. Allele-specific therapeutic exploration for the toxic missense alleles (e.g., allele-selective ASO/siRNA silencing, or chemical chaperones/ER-export modulators to relieve mislocalization).
  6. Functional reclassification of SGMS2 VUS using the ER-export/enzymatic assays established by Sokoya 2022, to triage the many VUS in ClinVar and refine diagnostic yield.
  7. Deep phenotyping of facial nerve palsy and glaucoma across the cohort to determine frequency, onset, and whether these track with genotype or sphingomyelin dysregulation in neural/ocular tissue.

Report compiled from 13 confirmed findings across 5 investigation iterations; 10 primary papers reviewed. Evidence types span human clinical genetics, patient-derived cellular assays, bone histomorphometry, mouse genetics, and population-genomic constraint analysis.

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