Spondylometaphyseal Dysplasia with Cone-Rod Dystrophy

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Classifications

ISDS Skeletal Nosology

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Inheritance

1

Autosomal Recessive HP:0000007

SMD-CRD is autosomal recessive. Reported probands are homozygous or compound heterozygous for PCYT1A variants; heterozygous carriers are unaffected. Complete loss of CCT-alpha is presumed incompatible with life, so every disease genotype retains some enzymatic function.

Autosomal recessive inheritance

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"Spondylometaphyseal dysplasia with cone-rod dystrophy is a rare autosomal-recessive disorder"

States the inheritance pattern directly.

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Discussions and Knowledge Gaps

2

Why do PCYT1A variants produce three phenotypes — skeletal plus retinal, lipodystrophic, or retinal alone — when phosphatidylcholine is required by every cell?

KNOWLEDGE GAP OPEN pcyt1a_tissue_selectivity

Attached to

pathophysiology#Deficient Phosphatidylcholine Synthesis

This is the central unexplained fact about the gene. Phosphatidylcholine is the main glycerophospholipid of all cellular membranes, yet PCYT1A disease strikes growth plate, retina, and adipose tissue in combinations that do not obviously nest inside one another: isolated retinal dystrophy occurs with a normal whole-skeleton survey, and lipodystrophy occurs with no retinal involvement at all. Enzymology complicates rather than resolves it, since the allele classes are heterogeneous — aggregation, fold destabilisation, altered Km, a linker defect, and one allele that increases constitutive activity — and no mapping from allele class to organ pattern has been proposed. Without that mapping, a PCYT1A genotype cannot be used to predict which organs will be affected, which is a live clinical problem and not only a mechanistic curiosity.

Proposed experiments

Map allele class to organ phenotype across the PCYT1A spectrum

exp_pcyt1a_allele_class_to_organ_mapping

Assemble reported PCYT1A patients with genotype and full organ phenotyping, and test whether the biochemically defined allele classes — aggregating, fold-destabilising, kinetically impaired, linker, autoinhibitory-helix — predict skeletal, retinal, or adipose involvement. Complement with phosphatidylcholine flux measurement in patient-derived chondrocytes, retinal organoids, and adipocytes carrying matched genotypes.

Would support

pathophysiology#Deficient Phosphatidylcholine Synthesis

Would refute

pathophysiology#Deficient Phosphatidylcholine Synthesis

Supporting outcome

A consistent relationship between allele class and organ pattern, with flux deficits tracking the affected tissue, would establish reduced phosphatidylcholine synthesis as the operative mechanism and give a genotype-based prediction.

Refuting outcome

Equivalent phosphatidylcholine flux across affected and spared tissues from the same patient would move the mechanism away from bulk synthesis towards a localised or moonlighting function of CCT-alpha.

Show evidence (2 references)

PMID:28272537 SUPPORT Human Clinical

"This is the first report showing that the PCYT1A gene can be responsible for isolated forms of retinal dystrophy, particularly without any skeletal involvement, thus further expanding the phenotypic spectrum induced by mutations in this gene."

Demonstrates that the organ systems dissociate, which is what makes the selectivity a genuine gap rather than a matter of severity.

PMID:30559292 SUPPORT INDIRECT In Vitro

"These in vitro analyses on purified mutant CCTs will complement future measurements of their impact on PC synthesis in cultured cells and in tissues with a stringent requirement for CCT"

The authors frame the tissue-specific requirement as the outstanding measurement, which is the gap recorded here.

By what route does reduced phosphatidylcholine synthesis disorder the growth plate?

KNOWLEDGE GAP OPEN pcyt1a_no_model_of_the_growth_plate_lesion

Attached to

pathophysiology#Growth-Plate Failure with Metaphyseal and Vertebral Change

This gap got wider, not narrower, when the retinal branch was worked out. The intuitive rationale for both branches was bulk membrane supply — outer segments are membrane-dense and continuously renewed, growth-plate chondrocytes are proliferative. A 2024 retina-specific Pcyt1a knockout removed that rationale for the retina: phosphatidylcholine synthesis was not reduced overall, the lesion ran through free fatty acid metabolism and ferroptosis, and the enzyme turned out to sit in Mueller glia rather than in the dying photoreceptors. If the retinal branch does not work by bulk supply, there is no longer any reason to assume the skeletal one does. Nothing comparable has been done in cartilage, and the disorder's skeletal lesion is specifically metaphyseal and vertebral rather than a generalised bone abnormality, which a membrane-supply deficit never predicted well. The step from the metabolic defect to the growth plate remains the largest hole in the mechanism, and the retinal result suggests it needs a lipid-species answer rather than a lipid-quantity one.

Proposed experiments

Lipidomic and differentiation phenotyping of PCYT1A-mutant chondrocytes

exp_pcyt1a_chondrocyte_lipidomics

Differentiate patient iPSCs carrying SMD-CRD genotypes into growth-plate-like chondrocytes, and measure membrane phospholipid composition, secretory capacity, hypertrophic differentiation, and matrix output against isogenic corrected controls, with and without exogenous phosphatidylcholine or choline supplementation.

Would support

pathophysiology#Growth-Plate Failure with Metaphyseal and Vertebral Change

Would refute

pathophysiology#Deficient Phosphatidylcholine Synthesis

Supporting outcome

A chondrocyte differentiation or secretion defect that is rescued by restoring phosphatidylcholine would close the gap between the metabolic lesion and the growth plate, and would nominate choline supplementation as a candidate intervention. Given the retinal result, the assay should measure individual lipid species and ferroptosis markers, not only bulk phosphatidylcholine.

Refuting outcome

A chondrocyte phenotype that is not rescued by phosphatidylcholine repletion would indicate the growth-plate lesion does not run through bulk phosphatidylcholine supply.

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"further studies are necessary to uncover its precise pathogenic mechanisms and the entirety of its phenotypic spectrum"

The founding report states the mechanism as unresolved.

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Pathophysiology

7

Biallelic PCYT1A Allele Combination

Homozygous or compound heterozygous PCYT1A alleles affecting the catalytic domain, the regulatory linker, or the autoinhibitory helix of CCT-alpha.

PCYT1A hgnc:8754

Downstream

Impaired CCT-alpha Catalytic Function and Fold Stability The disease alleles act on the enzyme itself, by aggregation, by lowering the thermal stability of the catalytic fold, by altering substrate affinity, or by disrupting the lipid-sensing regulatory apparatus.

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"Whole-exome sequencing of four individuals affected by this disorder from two Brazilian families identified two previously unreported homozygous mutations in PCYT1A."

The gene-discovery result establishing the initiating lesion.

Impaired CCT-alpha Catalytic Function and Fold Stability

CCT-alpha is the rate-determining enzyme of the Kennedy pathway and is normally activated by binding to phosphatidylcholine-deficient membranes, which makes it a lipid sensor as well as a catalyst. Disease alleles impair that function in several distinct ways, and not all of them by simple reduction: E280del in the autoinhibitory helix raises lipid-independent constitutive activity around four-fold and, unexpectedly, also enhances the response to anionic lipid vesicles around four-fold.

choline-phosphate cytidylyltransferase activity GO:0004105 ↕ DYSREGULATED

nuclear envelope GO:0005635

Downstream

Deficient Phosphatidylcholine Synthesis Reduced flux through the rate-limiting step lowers phosphatidylcholine output. In patient fibroblasts this shows up inconsistently across alleles — for some variants both CCT-alpha levels and phosphatidylcholine synthesis fall, while others impair synthesis with little effect on enzyme levels — so the link between allele and flux is not uniform.

Show evidence (2 references)

PMID:30559292 SUPPORT In Vitro

"CTP:phosphocholine cytidylyltransferase (CCT) is the key regulatory enzyme in phosphatidylcholine (PC) synthesis and is activated by binding to PC-deficient membranes."

Establishes both the rate-limiting role and the membrane-sensing regulation that the disease alleles disturb.

PMID:30559292 SUPPORT INDIRECT In Vitro

"a single amino acid deletion in the autoinhibitory helix increased the constitutive (lipid-independent) enzyme activity"

A gain of constitutive activity in a disease allele is why this node is curated as DYSREGULATED rather than simply DECREASED.

Deficient Phosphatidylcholine Synthesis

Phosphatidylcholine is the principal glycerophospholipid of eukaryotic membranes and an essential component of all of them, which is what makes the tissue selectivity of this disorder hard to explain. The affected tissues are presumed to be those with a stringent requirement for CCT-alpha specifically, rather than those with the highest membrane turnover in general.

phosphatidylcholine biosynthetic process GO:0006656 ↓ DECREASED

Downstream

Growth-Plate Failure with Metaphyseal and Vertebral Change How reduced phosphatidylcholine synthesis produces a growth-plate lesion is not established. The founding report is explicit that further studies are needed to uncover the precise pathogenic mechanisms.

Retinal Fatty-Acid Dysregulation and Ferroptosis in Mueller Glia The obvious rationale for the retinal branch — membrane-dense photoreceptor outer segments starved of phosphatidylcholine — turns out not to be what happens. In a retina-specific Pcyt1a knockout, phosphatidylcholine synthesis in the retina was not reduced overall; free fatty acid metabolism was disrupted instead, and ferroptosis was activated. The edge is therefore drawn to the lipid-handling node rather than directly to photoreceptor loss, and the bulk-synthesis reading is explicitly not asserted.

Show evidence (1 reference)

PMID:28272537 SUPPORT Other

"PC is the main glycerophospholipid in eukaryotic cells and is an essential component of all cellular membranes."

Establishes the ubiquity of the affected metabolite, which is the starting point for the tissue-selectivity problem.

Growth-Plate Failure with Metaphyseal and Vertebral Change

Progressive metaphyseal irregularity and cupping with shortened tubular bones, and platyspondyly with anterior vertebral protrusions. The combined vertebral and metaphyseal involvement is what places the disorder in the spondylometaphyseal group.

chondrocyte CL:0000138

Downstream

Platyspondyly with Anterior Vertebral Protrusions The vertebral component of the growth-plate lesion, flattened bodies carrying anterior protrusions.

Metaphyseal Cupping and Irregularity Progressive metaphyseal cupping and irregularity with shortened tubular bones, the appendicular component of the same lesion.

Severe Short Stature with Lower-Limb Bowing Failure at the growth plates produces postnatal growth deficiency with rhizomelic disproportion and progressive bowing of the lower limbs.

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"SMD-CRD is a rare autosomal recessive disorder with postnatal growth deficiency resulting in severe short stature, rhizomelia with bowing of the lower limbs, platyspondyly with anterior vertebral protrusions, progressive metaphyseal irregularity and cupping with shortened tubular bones."

The full skeletal description, supporting this node and the phenotype records derived from it.

Severe Short Stature with Lower-Limb Bowing

Postnatal growth deficiency producing severe short stature with rhizomelia, and progressive bowing of the lower limbs.

Downstream

Severe Short Stature Postnatal growth deficiency severe enough to be a defining feature of the disorder.

Rhizomelia Shortening falling predominantly on the proximal limb segments.

Progressive Lower-Limb Bowing Bowing of the lower limbs that worsens over time, one of the presenting skeletal complaints.

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"characterized by severe short stature, progressive lower-limb bowing, flattened vertebral bodies, metaphyseal involvement, and visual impairment caused by cone-rod dystrophy"

Names severe short stature and progressive bowing as defining features.

Retinal Fatty-Acid Dysregulation and Ferroptosis in Mueller Glia

In the retina, PCYT1A is expressed mainly in Mueller glial cells of the inner nuclear layer, not in the photoreceptors that die. Retina-specific Pcyt1a knockout produces reduced scotopic electroretinogram responses and progressive photoreceptor degeneration together with loss of inner-nuclear-layer cells, and proteomics implicates dysregulated fatty acid metabolism and activation of ferroptosis rather than a shortfall of phosphatidylcholine. The photoreceptor loss is therefore non-cell-autonomous on this evidence — a Mueller glial lesion that photoreceptors do not survive.

Mueller cell CL:0000636

fatty acid metabolic process GO:0006631 ⚠ ABNORMAL ferroptosis GO:0097707 ↑ INCREASED

Downstream

Cone-Rod Photoreceptor Degeneration Ferroptotic lipid-peroxidation damage in the inner retina accompanies progressive photoreceptor loss and reduced electroretinogram responses. The model is a mouse rod-dominant retina and the human disease is cone-rod, so the cone predominance of the human phenotype is not reproduced.

Show evidence (3 references)

PMID:38858683 SUPPORT Model Organism

"We found that PCYT1A is highly expressed in Müller glial (MG) cells in the inner nuclear layer (INL) of the retina."

Localises the enzyme to Mueller glial cells of the inner nuclear layer, which is what makes the photoreceptor degeneration non-cell-autonomous.

PMID:38858683 SUPPORT Model Organism

"Furthermore, through proteomic and bioinformatic analyses, we identified dysregulated retinal fatty acid metabolism and activation of the ferroptosis signalling pathway in RKO mice."

Identifies the two processes that replace the bulk-synthesis explanation.

PMID:38858683 SUPPORT Model Organism

"Instead, this deficiency appeared to disrupt free fatty acid metabolism and ultimately trigger ferroptosis."

States the substituted mechanism directly.

Cone-Rod Photoreceptor Degeneration

Early-onset, progressive visual impairment with a pigmentary maculopathy and electroretinographic evidence of cone-rod dysfunction. This is the feature that distinguishes SMD-CRD from all other spondylometaphyseal dysplasias, and the one that determines long-term function in survivors.

photoreceptor cell CL:0000210 retinal cone cell CL:0000573

Downstream

Cone-Rod Dystrophy Progressive cone-rod dystrophy with a pigmentary maculopathy, the definitional retinal feature of this entity.

Abnormal Electroretinogram Electroretinography demonstrates the cone-rod pattern of dysfunction and is the objective confirmation of the degeneration.

Progressive Visual Impairment Early-onset, progressive loss of vision, the main determinant of long-term function in survivors.

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"Affected patients have early-onset, progressive visual impairment associated with a pigmentary maculopathy and electroretinographic evidence of cone-rod dysfunction."

Describes the retinal phenotype and its electrophysiological signature, supporting this node and the retinal phenotype records.

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Pathograph

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Phenotypes

8

Eye 2

Abnormal Electroretinogram VERY_FREQUENT HP:0000512

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"electroretinographic evidence of cone-rod dysfunction"

Names electroretinography as the modality demonstrating the defect.

Progressive Visual Impairment VERY_FREQUENT HP:0000505

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"visual impairment caused by cone-rod dystrophy"

Attributes the visual impairment specifically to the retinal dystrophy.

Limbs 1

Metaphyseal Cupping and Irregularity VERY_FREQUENT HP:0003021

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"progressive metaphyseal irregularity and cupping with shortened tubular bones"

Names both the metaphyseal change and its progressive course.

Musculoskeletal 1

Platyspondyly with Anterior Vertebral Protrusions VERY_FREQUENT HP:0000926

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"platyspondyly with anterior vertebral protrusions"

Names the specific vertebral morphology of this disorder.

Growth 2

Severe Short Stature VERY_FREQUENT HP:0003510

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"SMD-CRD is a rare autosomal recessive disorder with postnatal growth deficiency resulting in severe short stature, rhizomelia with bowing of the lower limbs, platyspondyly with anterior vertebral protrusions, progressive metaphyseal irregularity and cupping with shortened tubular bones."

States the postnatal onset and severity of the growth deficiency, and supports the four skeletal phenotype records that follow.

Rhizomelia FREQUENT HP:0008905

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"rhizomelia with bowing of the lower limbs"

Names rhizomelia with the associated lower-limb bowing.

Other 2

Progressive Lower-Limb Bowing VERY_FREQUENT Bowing of the legs HP:0002979

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"characterized by severe short stature, progressive lower-limb bowing, flattened vertebral bodies, metaphyseal involvement, and visual impairment caused by cone-rod dystrophy"

"Progressive lower-limb bowing" supports both the phenotype and the PROGRESSIVE clinical-course qualifier.

Cone-Rod Dystrophy OBLIGATE Cone/cone-rod dystrophy HP:0000548

Course: PROGRESSIVE

Show evidence (2 references)

PMID:28272537 SUPPORT Human Clinical

"Affected patients have early-onset, progressive visual impairment associated with a pigmentary maculopathy and electroretinographic evidence of cone-rod dysfunction."

The retinal phenotype is named in the disorder's own definition, hence the OBLIGATE band.

PMID:29122926 SUPPORT DIRECT Human Clinical

"Cone rod dystrophy is a feature of SMDCRD which is caused by loss-of-function variants in PCYT1A."

Independent corroboration from the paper describing the sibling PLCB3 disorder, which surveys eye involvement across the spondylometaphyseal dysplasias and attributes the cone-rod phenotype specifically to PCYT1A. Useful precisely because it comes from outside the PCYT1A literature.

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Genetic Associations

1

Biallelic PCYT1A Variants (Causative)

Gene: PCYT1A hgnc:8754 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:30559292 SUPPORT In Vitro

"Over-expression of two missense variants in the catalytic domain (V142M and P150A) generated aggregated enzymes that could not be refolded after solubilization by denaturation."

Establishes protein aggregation as one distinct allele class.

PMID:30559292 SUPPORT In Vitro

"Five variants destabilized the catalytic domain-fold as assessed by lower transition temperatures for unfolding, and three of these manifested defects in substrate Km values."

Separates fold destabilisation from kinetic impairment as two further allele classes.

PMID:24387991 SUPPORT Human Clinical

"This gene encodes the alpha isoform of the phosphate cytidylyltransferase 1 choline enzyme, which is responsible for converting phosphocholine into cytidine diphosphate-choline, a key intermediate step in the phosphatidylcholine biosynthesis pathway."

Names the encoded enzyme and the reaction it catalyses.

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Medical Actions

3

Low-Vision Support and Ophthalmological Surveillance

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

No therapy alters the retinal degeneration. Management is regular ophthalmological assessment with low-vision aids, educational support, and counselling about the progressive course.

Show evidence (1 reference)

PMID:28272537 SUPPORT INDIRECT Human Clinical

"Affected patients have early-onset, progressive visual impairment associated with a pigmentary maculopathy and electroretinographic evidence of cone-rod dysfunction."

Establishes the progressive visual impairment that makes low-vision support the standing need; it evidences the need, not a trialled intervention, hence INDIRECT.

Orthopaedic Management of Deformity

Action: orthopedic surgical procedure Ontology label: Orthopedic Surgical Procedure NCIT:C16186

Surgical and orthotic management of progressive lower-limb bowing and of the spinal deformity, as for skeletal dysplasias generally. No disease-specific orthopaedic protocol has been published.

Genetic Counseling

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

Autosomal recessive inheritance carries a 25 percent recurrence risk. Counselling is complicated by the three-phenotype allelic spectrum: predicting whether a subsequent affected child will have the skeletal, the retinal, or both components is not currently possible from genotype.

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"Spondylometaphyseal dysplasia with cone-rod dystrophy is a rare autosomal-recessive disorder"

Establishes the recessive inheritance that determines the recurrence-risk counselling.

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Diagnosis

2

Combined Skeletal and Retinal Assessment

A child with a spondylometaphyseal dysplasia should have formal retinal assessment including electroretinography, because the cone-rod dystrophy is what identifies this entity and it may not be volunteered as a complaint in a young child. The converse also holds: the report of isolated PCYT1A retinal dystrophy was made only after whole-skeleton radiography, metabolic assessment, and liver ultrasound had been done specifically to exclude the skeletal and hepatic phenotypes.

eye examination NCIT:C38060

Show evidence (1 reference)

PMID:28272537 SUPPORT Human Clinical

"A thorough clinical evaluation of the patients, with whole skeleton X-ray, metabolic assessment and liver ultrasound failed to reveal signs of skeletal dysplasia, metabolic and hepatic alterations."

Documents the cross-system workup needed to place a PCYT1A patient within the three-phenotype spectrum.

PCYT1A Sequencing

Biallelic PCYT1A variants confirm the diagnosis. Because the same gene produces lipodystrophy and isolated retinal dystrophy, a molecular result must be interpreted against the clinical picture rather than read as diagnostic of SMD-CRD on its own.

genetic testing NCIT:C15709

Show evidence (1 reference)

PMID:30559292 SUPPORT Other

"(PCYT1A) cause three distinct pathologies in humans: lipodystrophy, spondylometaphyseal dysplasia with cone-rod dystrophy (SMD-CRD), and isolated retinal dystrophy"

Names the three distinct human pathologies caused by PCYT1A, which is why a molecular result alone does not fix the diagnosis.

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Prevalence

1

Worldwide

Cases In Literature 0.0 per 100,000 Ultra Rare

A handful of families have been reported since the gene was identified in 2014, the founding series comprising four affected individuals from two Brazilian families alongside an independently published cohort. No population-based prevalence estimate exists; rate_per_100000 is recorded as 0.0 only to mark that the true rate is far below the resolution of any published survey.

Show evidence (1 reference)

PMID:24387991 SUPPORT Human Clinical

"Whole-exome sequencing of four individuals affected by this disorder from two Brazilian families identified two previously unreported homozygous mutations in PCYT1A."

Gives the size of the founding molecularly defined series, establishing the cases-in-literature scale.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Spondylometaphyseal Dysplasia with Cone-Rod Dystrophy:

Other Spondylometaphyseal Dysplasias

Overlapping Features Within ISDS group 12, spondyloenchondrodysplasia (ACP5), odontochondrodysplasia (TRIP11), SMD Sutcliffe or corner fracture type (FN1), SMD with corneal dystrophy (PLCB3), and chondrodysplasia-pseudohermaphroditism syndrome (HHAT) share the combined vertebral and metaphyseal pattern. Of these only the PLCB3 disorder also involves the eye, and it affects the cornea rather than the retina, so an electroretinogram separates them.

{ }

Source YAML

click to show

name: Spondylometaphyseal Dysplasia with Cone-Rod Dystrophy
creation_date: "2026-08-27T00:00:00Z"
category: Mendelian
description: >-
  Spondylometaphyseal dysplasia with cone-rod dystrophy (SMD-CRD; OMIM #608940) is
  an ultra-rare autosomal recessive disorder caused by biallelic variants in
  PCYT1A, encoding CCT-alpha, the rate-limiting enzyme of phosphatidylcholine
  synthesis through the Kennedy pathway. Postnatal growth deficiency produces
  severe short stature with rhizomelia and bowing of the lower limbs; radiographs
  show platyspondyly with anterior vertebral protrusions and progressive
  metaphyseal irregularity and cupping of shortened tubular bones. Early-onset,
  progressive visual impairment with a pigmentary maculopathy and
  electroretinographic cone-rod dysfunction is the feature that separates SMD-CRD
  from every other spondylometaphyseal dysplasia. The same gene produces two other
  phenotypes that share almost nothing clinically — congenital lipodystrophy with
  fatty liver, and isolated retinal dystrophy with no skeletal involvement at all —
  so PCYT1A is a case where genotype does not map cleanly onto organ involvement,
  and the biochemical basis of that split is unresolved. Enzymology of the disease
  alleles shows they are not uniformly hypomorphic: some destabilise the catalytic
  fold, one impairs the regulatory linker, and one deletion of the autoinhibitory
  helix actually increases constitutive activity roughly four-fold.
disease_term:
  preferred_term: spondylometaphyseal dysplasia with cone-rod dystrophy
  term:
    id: MONDO:0012160
    label: spondylometaphyseal dysplasia-cone-rod dystrophy syndrome
synonyms:
- SMD-CRD
- SMDCRD
- Spondylometaphyseal dysplasia-cone-rod dystrophy syndrome
- PCYT1A-related spondylometaphyseal dysplasia
parents:
- Skeletal Dysplasia
- Inherited Retinal Dystrophy
classifications:
  isds_skeletal_category:
  - classification_value: spondylometaphyseal_dysplasias
    notes: >-
      ISDS Nosology of Genetic Skeletal Disorders, 2023 revision (Unger et al.,
      PMID:36779427), Table 1 group 12 "Spondylometaphyseal dysplasias (SMD)",
      entry NOS 12-0040, listed as "Spondylometaphyseal dysplasia with cone-rod
      dystrophy, PCYT1A-related" (AR, PCYT1A, MIM 608940). The two other
      PCYT1A phenotypes — congenital lipodystrophy and isolated retinal dystrophy —
      are not listed in the skeletal nosology and take no ISDS group, correctly:
      neither has a skeletal dysplasia to classify.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  rate_per_100000: 0.0
  notes: >-
    A handful of families have been reported since the gene was identified in 2014,
    the founding series comprising four affected individuals from two Brazilian
    families alongside an independently published cohort. No population-based
    prevalence estimate exists; rate_per_100000 is recorded as 0.0 only to mark that
    the true rate is far below the resolution of any published survey.
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole-exome sequencing of four individuals affected by this disorder from two
      Brazilian families identified two previously unreported homozygous mutations
      in PCYT1A.
    explanation: >-
      Gives the size of the founding molecularly defined series, establishing the
      cases-in-literature scale.
inheritance:
- name: Autosomal Recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    SMD-CRD is autosomal recessive. Reported probands are homozygous or compound
    heterozygous for PCYT1A variants; heterozygous carriers are unaffected. Complete
    loss of CCT-alpha is presumed incompatible with life, so every disease genotype
    retains some enzymatic function.
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Spondylometaphyseal dysplasia with cone-rod dystrophy is a rare
      autosomal-recessive disorder
    explanation: States the inheritance pattern directly.
genetic:
- name: Biallelic PCYT1A Variants
  gene_term:
    preferred_term: PCYT1A
    term:
      id: hgnc:8754
      label: PCYT1A
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >-
    PCYT1A encodes CCT-alpha, which has a catalytic domain, an amphipathic
    membrane-binding domain, a signal-transducing linker between them, and an
    autoinhibitory helix. Disease alleles fall in all of these. Missense variants in
    the catalytic core (V142M, P150A) produce aggregated enzyme that cannot be
    refolded; five variants lower the unfolding transition temperature of the
    catalytic domain and three of those alter substrate Km; R223S sits in the
    regulatory linker; and E280del removes a single residue from the autoinhibitory
    helix. Allele class therefore does not reduce to a single loss-of-function
    statement.
  evidence:
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Over-expression of two missense variants in the catalytic domain (V142M and
      P150A) generated aggregated enzymes that could not be refolded after
      solubilization by denaturation.
    explanation: Establishes protein aggregation as one distinct allele class.
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Five variants destabilized the catalytic domain-fold as assessed by lower
      transition temperatures for unfolding, and three of these manifested defects in
      substrate Km values.
    explanation: >-
      Separates fold destabilisation from kinetic impairment as two further allele
      classes.
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This gene encodes the alpha isoform of the phosphate cytidylyltransferase 1
      choline enzyme, which is responsible for converting phosphocholine into
      cytidine diphosphate-choline, a key intermediate step in the
      phosphatidylcholine biosynthesis pathway.
    explanation: Names the encoded enzyme and the reaction it catalyses.
pathophysiology:
- name: Biallelic PCYT1A Allele Combination
  biological_scale: MOLECULAR
  description: >-
    Homozygous or compound heterozygous PCYT1A alleles affecting the catalytic
    domain, the regulatory linker, or the autoinhibitory helix of CCT-alpha.
  genes:
  - preferred_term: PCYT1A
    term:
      id: hgnc:8754
      label: PCYT1A
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole-exome sequencing of four individuals affected by this disorder from two
      Brazilian families identified two previously unreported homozygous mutations
      in PCYT1A.
    explanation: The gene-discovery result establishing the initiating lesion.
  downstream:
  - target: Impaired CCT-alpha Catalytic Function and Fold Stability
    causal_link_type: DIRECT
    description: >-
      The disease alleles act on the enzyme itself, by aggregation, by lowering the
      thermal stability of the catalytic fold, by altering substrate affinity, or by
      disrupting the lipid-sensing regulatory apparatus.
    evidence:
    - reference: PMID:30559292
      reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        A mutation (R223S) in a signal-transducing linker between the catalytic and
        membrane-binding domains also impaired enzyme kinetics.
      explanation: >-
        Shows that alleles outside the catalytic domain still converge on impaired
        enzyme function.
- name: Impaired CCT-alpha Catalytic Function and Fold Stability
  biological_scale: MOLECULAR
  description: >-
    CCT-alpha is the rate-determining enzyme of the Kennedy pathway and is
    normally activated by binding to phosphatidylcholine-deficient membranes, which
    makes it a lipid sensor as well as a catalyst. Disease alleles impair that
    function in several distinct ways, and not all of them by simple reduction:
    E280del in the autoinhibitory helix raises lipid-independent constitutive
    activity around four-fold and, unexpectedly, also enhances the response to
    anionic lipid vesicles around four-fold.
  molecular_functions:
  - preferred_term: choline-phosphate cytidylyltransferase activity
    modifier: DYSREGULATED
    term:
      id: GO:0004105
      label: choline-phosphate cytidylyltransferase activity
  cellular_components:
  - preferred_term: nuclear envelope
    term:
      id: GO:0005635
      label: nuclear envelope
  evidence:
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      CTP:phosphocholine cytidylyltransferase (CCT) is the key regulatory enzyme in
      phosphatidylcholine (PC) synthesis and is activated by binding to PC-deficient
      membranes.
    explanation: >-
      Establishes both the rate-limiting role and the membrane-sensing regulation
      that the disease alleles disturb.
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: >-
      a single amino acid deletion in the autoinhibitory helix increased the
      constitutive (lipid-independent) enzyme activity
    explanation: >-
      A gain of constitutive activity in a disease allele is why this node is
      curated as DYSREGULATED rather than simply DECREASED.
  downstream:
  - target: Deficient Phosphatidylcholine Synthesis
    causal_link_type: DIRECT
    description: >-
      Reduced flux through the rate-limiting step lowers phosphatidylcholine output.
      In patient fibroblasts this shows up inconsistently across alleles — for some
      variants both CCT-alpha levels and phosphatidylcholine synthesis fall, while
      others impair synthesis with little effect on enzyme levels — so the link
      between allele and flux is not uniform.
    evidence:
    - reference: PMID:30559292
      reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: IN_VITRO
      snippet: >-
        Previous analyses showed that for some disease-linked PCYT1A variants steady
        state levels of CCT
      explanation: >-
        The sentence goes on to note that other variants impair synthesis with little
        effect on enzyme levels, which is why this edge is graded INDIRECT rather than
        as a uniform dose relationship.
- name: Deficient Phosphatidylcholine Synthesis
  biological_scale: MOLECULAR
  description: >-
    Phosphatidylcholine is the principal glycerophospholipid of eukaryotic membranes
    and an essential component of all of them, which is what makes the tissue
    selectivity of this disorder hard to explain. The affected tissues are presumed
    to be those with a stringent requirement for CCT-alpha specifically, rather than
    those with the highest membrane turnover in general.
  biological_processes:
  - preferred_term: phosphatidylcholine biosynthetic process
    modifier: DECREASED
    term:
      id: GO:0006656
      label: phosphatidylcholine biosynthetic process
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      PC is the main glycerophospholipid in eukaryotic cells and is an essential
      component of all cellular membranes.
    explanation: >-
      Establishes the ubiquity of the affected metabolite, which is the starting
      point for the tissue-selectivity problem.
  downstream:
  - target: Growth-Plate Failure with Metaphyseal and Vertebral Change
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      How reduced phosphatidylcholine synthesis produces a growth-plate lesion is
      not established. The founding report is explicit that further studies are
      needed to uncover the precise pathogenic mechanisms.
    evidence:
    - reference: PMID:24387991
      reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        further studies are necessary to uncover its precise pathogenic mechanisms
        and the entirety of its phenotypic spectrum
      explanation: >-
        The authors state that the mechanism connecting the metabolic defect to the
        phenotype is unknown, which is what the unknown-intermediates grading records.
  - target: Retinal Fatty-Acid Dysregulation and Ferroptosis in Mueller Glia
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      The obvious rationale for the retinal branch — membrane-dense photoreceptor
      outer segments starved of phosphatidylcholine — turns out not to be what
      happens. In a retina-specific Pcyt1a knockout, phosphatidylcholine synthesis in
      the retina was not reduced overall; free fatty acid metabolism was disrupted
      instead, and ferroptosis was activated. The edge is therefore drawn to the
      lipid-handling node rather than directly to photoreceptor loss, and the
      bulk-synthesis reading is explicitly not asserted.
    evidence:
    - reference: PMID:38858683
      reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Instead, this deficiency appeared to disrupt free fatty acid metabolism and
        ultimately trigger ferroptosis.
      explanation: >-
        The positive result behind this edge as drawn: loss of PCYT1A routes to
        ferroptosis through disrupted free fatty acid metabolism rather than through
        bulk phosphatidylcholine synthesis.
    - reference: PMID:38858683
      reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Importantly, we found that PCYT1A deficiency did not lead to an overall
        reduction in PC synthesis within the retina.
      explanation: >-
        A direct negative result against the intuitive bulk-phosphatidylcholine
        explanation of the retinal phenotype, which is why that reading is not
        curated here.
    - reference: PMID:28272537
      reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        This is the first report showing that the PCYT1A gene can be responsible for
        isolated forms of retinal dystrophy, particularly without any skeletal
        involvement, thus further expanding the phenotypic spectrum induced by
        mutations in this gene.
      explanation: >-
        Retinal involvement can occur entirely without skeletal disease, so the two
        downstream branches are separable and neither is a consequence of the other.
- name: Growth-Plate Failure with Metaphyseal and Vertebral Change
  biological_scale: TISSUE
  description: >-
    Progressive metaphyseal irregularity and cupping with shortened tubular bones,
    and platyspondyly with anterior vertebral protrusions. The combined vertebral and
    metaphyseal involvement is what places the disorder in the spondylometaphyseal
    group.
  cell_types:
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SMD-CRD is a rare autosomal recessive disorder with postnatal growth deficiency
      resulting in severe short stature, rhizomelia with bowing of the lower limbs,
      platyspondyly with anterior vertebral protrusions, progressive metaphyseal
      irregularity and cupping with shortened tubular bones.
    explanation: >-
      The full skeletal description, supporting this node and the phenotype records
      derived from it.
  downstream:
  - target: Platyspondyly with Anterior Vertebral Protrusions
    causal_link_type: DIRECT
    description: >-
      The vertebral component of the growth-plate lesion, flattened bodies carrying
      anterior protrusions.
  - target: Metaphyseal Cupping and Irregularity
    causal_link_type: DIRECT
    description: >-
      Progressive metaphyseal cupping and irregularity with shortened tubular bones,
      the appendicular component of the same lesion.
  - target: Severe Short Stature with Lower-Limb Bowing
    causal_link_type: DIRECT
    description: >-
      Failure at the growth plates produces postnatal growth deficiency with
      rhizomelic disproportion and progressive bowing of the lower limbs.
    evidence:
    - reference: PMID:24387991
      reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        characterized by severe short stature, progressive lower-limb bowing,
        flattened vertebral bodies, metaphyseal involvement, and visual impairment
        caused by cone-rod dystrophy
      explanation: Links the skeletal lesion to the growth and deformity phenotype.
- name: Severe Short Stature with Lower-Limb Bowing
  biological_scale: ORGANISM
  description: >-
    Postnatal growth deficiency producing severe short stature with rhizomelia, and
    progressive bowing of the lower limbs.
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      characterized by severe short stature, progressive lower-limb bowing,
      flattened vertebral bodies, metaphyseal involvement, and visual impairment
      caused by cone-rod dystrophy
    explanation: Names severe short stature and progressive bowing as defining features.
  downstream:
  - target: Severe Short Stature
    causal_link_type: DIRECT
    description: >-
      Postnatal growth deficiency severe enough to be a defining feature of the
      disorder.
  - target: Rhizomelia
    causal_link_type: DIRECT
    description: Shortening falling predominantly on the proximal limb segments.
  - target: Progressive Lower-Limb Bowing
    causal_link_type: DIRECT
    description: >-
      Bowing of the lower limbs that worsens over time, one of the presenting
      skeletal complaints.
- name: Retinal Fatty-Acid Dysregulation and Ferroptosis in Mueller Glia
  biological_scale: CELLULAR
  description: >-
    In the retina, PCYT1A is expressed mainly in Mueller glial cells of the inner
    nuclear layer, not in the photoreceptors that die. Retina-specific Pcyt1a
    knockout produces reduced scotopic electroretinogram responses and progressive
    photoreceptor degeneration together with loss of inner-nuclear-layer cells, and
    proteomics implicates dysregulated fatty acid metabolism and activation of
    ferroptosis rather than a shortfall of phosphatidylcholine. The photoreceptor
    loss is therefore non-cell-autonomous on this evidence — a Mueller glial lesion
    that photoreceptors do not survive.
  cell_types:
  - preferred_term: Mueller cell
    term:
      id: CL:0000636
      label: Mueller cell
  biological_processes:
  - preferred_term: fatty acid metabolic process
    modifier: ABNORMAL
    term:
      id: GO:0006631
      label: fatty acid metabolic process
  - preferred_term: ferroptosis
    modifier: INCREASED
    term:
      id: GO:0097707
      label: ferroptosis
  evidence:
  - reference: PMID:38858683
    reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We found that PCYT1A is highly expressed in Müller glial (MG) cells in the
      inner nuclear layer (INL) of the retina.
    explanation: >-
      Localises the enzyme to Mueller glial cells of the inner nuclear layer, which is
      what makes the photoreceptor degeneration non-cell-autonomous.
  - reference: PMID:38858683
    reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Furthermore, through proteomic and bioinformatic analyses, we identified
      dysregulated retinal fatty acid metabolism and activation of the ferroptosis
      signalling pathway in RKO mice.
    explanation: Identifies the two processes that replace the bulk-synthesis explanation.
  - reference: PMID:38858683
    reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Instead, this deficiency appeared to disrupt free fatty acid metabolism and
      ultimately trigger ferroptosis.
    explanation: States the substituted mechanism directly.
  downstream:
  - target: Cone-Rod Photoreceptor Degeneration
    causal_link_type: DIRECT
    description: >-
      Ferroptotic lipid-peroxidation damage in the inner retina accompanies
      progressive photoreceptor loss and reduced electroretinogram responses. The
      model is a mouse rod-dominant retina and the human disease is cone-rod, so the
      cone predominance of the human phenotype is not reproduced.
    evidence:
    - reference: PMID:38858683
      reference_title: PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Our findings revealed that the deletion of Pcyt1a resulted in retinal
        degenerative phenotypes, including reduced scotopic electroretinogram (ERG)
        responses and progressive degeneration of photoreceptor cells, accompanied by
        loss of cells in the INL.
      explanation: >-
        The measured retinal phenotype of the knockout, including the loss of
        inner-nuclear-layer cells that marks the Mueller glial origin.
- name: Cone-Rod Photoreceptor Degeneration
  biological_scale: TISSUE
  description: >-
    Early-onset, progressive visual impairment with a pigmentary maculopathy and
    electroretinographic evidence of cone-rod dysfunction. This is the feature that
    distinguishes SMD-CRD from all other spondylometaphyseal dysplasias, and the one
    that determines long-term function in survivors.
  cell_types:
  - preferred_term: photoreceptor cell
    term:
      id: CL:0000210
      label: photoreceptor cell
  - preferred_term: retinal cone cell
    term:
      id: CL:0000573
      label: retinal cone cell
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Affected patients have early-onset, progressive visual impairment associated
      with a pigmentary maculopathy and electroretinographic evidence of cone-rod
      dysfunction.
    explanation: >-
      Describes the retinal phenotype and its electrophysiological signature,
      supporting this node and the retinal phenotype records.
  downstream:
  - target: Cone-Rod Dystrophy
    causal_link_type: DIRECT
    description: >-
      Progressive cone-rod dystrophy with a pigmentary maculopathy, the definitional
      retinal feature of this entity.
  - target: Abnormal Electroretinogram
    causal_link_type: DIRECT
    description: >-
      Electroretinography demonstrates the cone-rod pattern of dysfunction and is the
      objective confirmation of the degeneration.
  - target: Progressive Visual Impairment
    causal_link_type: DIRECT
    description: >-
      Early-onset, progressive loss of vision, the main determinant of long-term
      function in survivors.
phenotypes:
- category: Growth
  name: Severe Short Stature
  frequency: VERY_FREQUENT
  description: >-
    Postnatal growth deficiency producing severe short stature with rhizomelic
    disproportion. Birth size is not reported as abnormal; the deficit is postnatal.
  phenotype_term:
    preferred_term: Severe short stature
    term:
      id: HP:0003510
      label: Severe short stature
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SMD-CRD is a rare autosomal recessive disorder with postnatal growth deficiency
      resulting in severe short stature, rhizomelia with bowing of the lower limbs,
      platyspondyly with anterior vertebral protrusions, progressive metaphyseal
      irregularity and cupping with shortened tubular bones.
    explanation: >-
      States the postnatal onset and severity of the growth deficiency, and supports
      the four skeletal phenotype records that follow.
- category: Skeletal
  name: Rhizomelia
  frequency: FREQUENT
  description: Shortening predominantly of the proximal limb segments.
  phenotype_term:
    preferred_term: Rhizomelia
    term:
      id: HP:0008905
      label: Rhizomelia
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      rhizomelia with bowing of the lower limbs
    explanation: Names rhizomelia with the associated lower-limb bowing.
- category: Skeletal
  name: Progressive Lower-Limb Bowing
  frequency: VERY_FREQUENT
  description: >-
    Bowing of the lower limbs that worsens over time and is one of the presenting
    skeletal complaints.
  phenotype_term:
    preferred_term: Bowing of the legs
    term:
      id: HP:0002979
      label: Bowing of the legs
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      characterized by severe short stature, progressive lower-limb bowing,
      flattened vertebral bodies, metaphyseal involvement, and visual impairment
      caused by cone-rod dystrophy
    explanation: >-
      "Progressive lower-limb bowing" supports both the phenotype and the PROGRESSIVE
      clinical-course qualifier.
- category: Skeletal
  name: Platyspondyly with Anterior Vertebral Protrusions
  frequency: VERY_FREQUENT
  description: >-
    Flattened vertebral bodies carrying anterior protrusions, the vertebral half of
    the spondylometaphyseal pattern.
  phenotype_term:
    preferred_term: Platyspondyly
    term:
      id: HP:0000926
      label: Platyspondyly
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      platyspondyly with anterior vertebral protrusions
    explanation: Names the specific vertebral morphology of this disorder.
- category: Skeletal
  name: Metaphyseal Cupping and Irregularity
  frequency: VERY_FREQUENT
  description: >-
    Progressive metaphyseal irregularity and cupping with shortened tubular bones,
    the metaphyseal half of the pattern.
  phenotype_term:
    preferred_term: Metaphyseal cupping
    term:
      id: HP:0003021
      label: Metaphyseal cupping
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      progressive metaphyseal irregularity and cupping with shortened tubular bones
    explanation: >-
      Names both the metaphyseal change and its progressive course.
- category: Ophthalmologic
  name: Cone-Rod Dystrophy
  frequency: OBLIGATE
  description: >-
    Early-onset, progressive cone-rod dystrophy with a pigmentary maculopathy. The
    retinal component is definitional for this entity within the
    spondylometaphyseal dysplasias.
  phenotype_term:
    preferred_term: Cone/cone-rod dystrophy
    term:
      id: HP:0000548
      label: Cone/cone-rod dystrophy
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Affected patients have early-onset, progressive visual impairment associated
      with a pigmentary maculopathy and electroretinographic evidence of cone-rod
      dysfunction.
    explanation: >-
      The retinal phenotype is named in the disorder's own definition, hence the
      OBLIGATE band.
  - reference: PMID:29122926
    reference_title: Defect in phosphoinositide signalling through a homozygous variant in PLCB3 causes a new form of spondylometaphyseal dysplasia with corneal dystrophy.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Cone rod dystrophy is a feature of SMDCRD which is caused by loss-of-function variants in PCYT1A.
    explanation: >-
      Independent corroboration from the paper describing the sibling PLCB3 disorder,
      which surveys eye involvement across the spondylometaphyseal dysplasias and
      attributes the cone-rod phenotype specifically to PCYT1A. Useful precisely
      because it comes from outside the PCYT1A literature.
- category: Ophthalmologic
  name: Abnormal Electroretinogram
  frequency: VERY_FREQUENT
  description: >-
    Electroretinography shows cone-rod dysfunction and is the objective test that
    confirms the retinal component and its cone-predominant pattern.
  phenotype_term:
    preferred_term: Abnormal electroretinogram
    term:
      id: HP:0000512
      label: Abnormal electroretinogram
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      electroretinographic evidence of cone-rod dysfunction
    explanation: Names electroretinography as the modality demonstrating the defect.
- category: Ophthalmologic
  name: Progressive Visual Impairment
  frequency: VERY_FREQUENT
  description: >-
    Visual impairment from early childhood, progressive, and the main determinant of
    long-term function.
  phenotype_term:
    preferred_term: Visual impairment
    term:
      id: HP:0000505
      label: Visual impairment
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      visual impairment caused by cone-rod dystrophy
    explanation: Attributes the visual impairment specifically to the retinal dystrophy.
diagnosis:
- name: Combined Skeletal and Retinal Assessment
  diagnosis_term:
    preferred_term: eye examination
    term:
      id: NCIT:C38060
      label: Eye Examination
  description: >-
    A child with a spondylometaphyseal dysplasia should have formal retinal
    assessment including electroretinography, because the cone-rod dystrophy is what
    identifies this entity and it may not be volunteered as a complaint in a young
    child. The converse also holds: the report of isolated PCYT1A retinal dystrophy
    was made only after whole-skeleton radiography, metabolic assessment, and liver
    ultrasound had been done specifically to exclude the skeletal and hepatic
    phenotypes.
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A thorough clinical evaluation of the patients, with whole skeleton X-ray,
      metabolic assessment and liver ultrasound failed to reveal signs of skeletal
      dysplasia, metabolic and hepatic alterations.
    explanation: >-
      Documents the cross-system workup needed to place a PCYT1A patient within the
      three-phenotype spectrum.
- name: PCYT1A Sequencing
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Biallelic PCYT1A variants confirm the diagnosis. Because the same gene produces
    lipodystrophy and isolated retinal dystrophy, a molecular result must be
    interpreted against the clinical picture rather than read as diagnostic of
    SMD-CRD on its own.
  evidence:
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      (PCYT1A) cause three distinct pathologies in humans: lipodystrophy,
      spondylometaphyseal dysplasia with cone-rod dystrophy (SMD-CRD), and isolated
      retinal dystrophy
    explanation: >-
      Names the three distinct human pathologies caused by PCYT1A, which is why a
      molecular result alone does not fix the diagnosis.
differential_diagnoses:
- name: PCYT1A-Related Congenital Lipodystrophy
  description: >-
    Congenital generalised lipodystrophy type 5, with severe fatty liver disease and
    reduced HDL cholesterol, and at most modest short stature. Allelic with SMD-CRD
    but without retinal involvement or a spondylometaphyseal dysplasia.
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      PCYT1A mutations have also been linked to a distinct phenotype presenting with
      congenital lipodystrophy and fatty liver disease without any retinal
      involvement or signs of spondylometaphyseal dysplasia apart from a modest short
      stature
    explanation: Defines the lipodystrophy phenotype and its separation from SMD-CRD.
- name: PCYT1A-Related Isolated Retinal Dystrophy
  description: >-
    Isolated retinal dystrophy with no skeletal, metabolic, or hepatic involvement,
    established in three patients from two Italian families after deliberate
    exclusion of the other two phenotypes. A PCYT1A result in a patient with retinal
    disease does not imply a skeletal dysplasia is present or will develop.
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we identified, by next generation sequencing, sequence variants affecting
      function in the PCYT1A gene in three young patients with isolated retinal
      dystrophy from two different Italian families
    explanation: Establishes the isolated-retinal phenotype as a distinct allelic outcome.
- name: Other Spondylometaphyseal Dysplasias
  description: >-
    Within ISDS group 12, spondyloenchondrodysplasia (ACP5), odontochondrodysplasia
    (TRIP11), SMD Sutcliffe or corner fracture type (FN1), SMD with corneal dystrophy
    (PLCB3), and chondrodysplasia-pseudohermaphroditism syndrome (HHAT) share the
    combined vertebral and metaphyseal pattern. Of these only the PLCB3 disorder also
    involves the eye, and it affects the cornea rather than the retina, so an
    electroretinogram separates them.
  notes: >-
    Recorded without an evidence item because it is a statement about ISDS group-12
    membership, whose per-disorder placements are not quotable from the nosology
    abstract; the provenance is recorded in the classifications block.
treatments:
- name: Low-Vision Support and Ophthalmological Surveillance
  description: >-
    No therapy alters the retinal degeneration. Management is regular
    ophthalmological assessment with low-vision aids, educational support, and
    counselling about the progressive course.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: OTHER
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Affected patients have early-onset, progressive visual impairment associated
      with a pigmentary maculopathy and electroretinographic evidence of cone-rod
      dysfunction.
    explanation: >-
      Establishes the progressive visual impairment that makes low-vision support the
      standing need; it evidences the need, not a trialled intervention, hence
      INDIRECT.
- name: Orthopaedic Management of Deformity
  description: >-
    Surgical and orthotic management of progressive lower-limb bowing and of the
    spinal deformity, as for skeletal dysplasias generally. No disease-specific
    orthopaedic protocol has been published.
  treatment_term:
    preferred_term: orthopedic surgical procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  therapeutic_modality: SURGERY
  notes: >-
    Curated without an evidence item because no management study of this disorder
    exists; the record names the deformities documented in the case reports as the
    targets of standard orthopaedic care rather than asserting a trialled
    intervention.
- name: Genetic Counseling
  description: >-
    Autosomal recessive inheritance carries a 25 percent recurrence risk. Counselling
    is complicated by the three-phenotype allelic spectrum: predicting whether a
    subsequent affected child will have the skeletal, the retinal, or both components
    is not currently possible from genotype.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  therapeutic_modality: OTHER
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Spondylometaphyseal dysplasia with cone-rod dystrophy is a rare
      autosomal-recessive disorder
    explanation: >-
      Establishes the recessive inheritance that determines the recurrence-risk
      counselling.
discussions:
- discussion_id: pcyt1a_tissue_selectivity
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Deficient Phosphatidylcholine Synthesis
  prompt: >-
    Why do PCYT1A variants produce three phenotypes — skeletal plus retinal,
    lipodystrophic, or retinal alone — when phosphatidylcholine is required by every
    cell?
  rationale: >-
    This is the central unexplained fact about the gene. Phosphatidylcholine is the
    main glycerophospholipid of all cellular membranes, yet PCYT1A disease strikes
    growth plate, retina, and adipose tissue in combinations that do not
    obviously nest inside one another: isolated retinal dystrophy occurs with a
    normal whole-skeleton survey, and lipodystrophy occurs with no retinal
    involvement at all. Enzymology complicates rather than resolves it, since the
    allele classes are heterogeneous — aggregation, fold destabilisation, altered Km,
    a linker defect, and one allele that increases constitutive activity — and no
    mapping from allele class to organ pattern has been proposed. Without that
    mapping, a PCYT1A genotype cannot be used to predict which organs will be
    affected, which is a live clinical problem and not only a mechanistic curiosity.
  evidence:
  - reference: PMID:28272537
    reference_title: Mutations in the PCYT1A gene are responsible for isolated forms of retinal dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This is the first report showing that the PCYT1A gene can be responsible for
      isolated forms of retinal dystrophy, particularly without any skeletal
      involvement, thus further expanding the phenotypic spectrum induced by
      mutations in this gene.
    explanation: >-
      Demonstrates that the organ systems dissociate, which is what makes the
      selectivity a genuine gap rather than a matter of severity.
  - reference: PMID:30559292
    reference_title: Disease-linked mutations in the phosphatidylcholine regulatory enzyme CCTα impair enzymatic activity and fold stability.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: >-
      These in vitro analyses on purified mutant CCTs will complement future
      measurements of their impact on PC synthesis in cultured cells and in tissues
      with a stringent requirement for CCT
    explanation: >-
      The authors frame the tissue-specific requirement as the outstanding
      measurement, which is the gap recorded here.
  proposed_experiments:
  - experiment_id: exp_pcyt1a_allele_class_to_organ_mapping
    name: Map allele class to organ phenotype across the PCYT1A spectrum
    description: >-
      Assemble reported PCYT1A patients with genotype and full organ phenotyping, and
      test whether the biochemically defined allele classes — aggregating,
      fold-destabilising, kinetically impaired, linker, autoinhibitory-helix —
      predict skeletal, retinal, or adipose involvement. Complement with
      phosphatidylcholine flux measurement in patient-derived chondrocytes,
      retinal organoids, and adipocytes carrying matched genotypes.
    would_support:
    - pathophysiology#Deficient Phosphatidylcholine Synthesis
    supporting_outcome:
    - >-
      A consistent relationship between allele class and organ pattern, with flux
      deficits tracking the affected tissue, would establish reduced
      phosphatidylcholine synthesis as the operative mechanism and give a
      genotype-based prediction.
    would_refute:
    - pathophysiology#Deficient Phosphatidylcholine Synthesis
    refuting_outcome:
    - >-
      Equivalent phosphatidylcholine flux across affected and spared tissues from the
      same patient would move the mechanism away from bulk synthesis towards a
      localised or moonlighting function of CCT-alpha.
- discussion_id: pcyt1a_no_model_of_the_growth_plate_lesion
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Growth-Plate Failure with Metaphyseal and Vertebral Change
  prompt: >-
    By what route does reduced phosphatidylcholine synthesis disorder the growth
    plate?
  rationale: >-
    This gap got wider, not narrower, when the retinal branch was worked out. The
    intuitive rationale for both branches was bulk membrane supply — outer segments
    are membrane-dense and continuously renewed, growth-plate chondrocytes are
    proliferative. A 2024 retina-specific Pcyt1a knockout removed that rationale for
    the retina: phosphatidylcholine synthesis was not reduced overall, the lesion ran
    through free fatty acid metabolism and ferroptosis, and the enzyme turned out to
    sit in Mueller glia rather than in the dying photoreceptors. If the retinal branch
    does not work by bulk supply, there is no longer any reason to assume the skeletal
    one does. Nothing comparable has been done in cartilage, and the disorder's
    skeletal lesion is specifically metaphyseal and vertebral rather than a
    generalised bone abnormality, which a membrane-supply deficit never predicted
    well. The step from the metabolic defect to the growth plate remains the largest
    hole in the mechanism, and the retinal result suggests it needs a lipid-species
    answer rather than a lipid-quantity one.
  evidence:
  - reference: PMID:24387991
    reference_title: Mutations in PCYT1A cause spondylometaphyseal dysplasia with cone-rod dystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      further studies are necessary to uncover its precise pathogenic mechanisms and
      the entirety of its phenotypic spectrum
    explanation: The founding report states the mechanism as unresolved.
  proposed_experiments:
  - experiment_id: exp_pcyt1a_chondrocyte_lipidomics
    name: Lipidomic and differentiation phenotyping of PCYT1A-mutant chondrocytes
    description: >-
      Differentiate patient iPSCs carrying SMD-CRD genotypes into growth-plate-like
      chondrocytes, and measure membrane phospholipid composition, secretory capacity,
      hypertrophic differentiation, and matrix output against isogenic corrected
      controls, with and without exogenous phosphatidylcholine or choline
      supplementation.
    would_support:
    - pathophysiology#Growth-Plate Failure with Metaphyseal and Vertebral Change
    supporting_outcome:
    - >-
      A chondrocyte differentiation or secretion defect that is rescued by restoring
      phosphatidylcholine would close the gap between the metabolic lesion and the
      growth plate, and would nominate choline supplementation as a candidate
      intervention. Given the retinal result, the assay should measure individual
      lipid species and ferroptosis markers, not only bulk phosphatidylcholine.
    would_refute:
    - pathophysiology#Deficient Phosphatidylcholine Synthesis
    refuting_outcome:
    - >-
      A chondrocyte phenotype that is not rescued by phosphatidylcholine repletion
      would indicate the growth-plate lesion does not run through bulk
      phosphatidylcholine supply.
progression: []
references:
- reference: PMID:36779427
  title: "Nosology of genetic skeletal disorders: 2023 revision."
  findings: []
clinical_trials: []
datasets: []
notes: >-
  Curated as part of filling ISDS Nosology group 12 (Spondylometaphyseal
  dysplasias), which was unpopulated when this work began. SMD-CRD is NOS 12-0040 in the 2023
  revision. The entry is scoped to the skeletal-plus-retinal phenotype only; the
  allelic congenital lipodystrophy and isolated retinal dystrophy are recorded as
  differentials rather than as subtypes, because neither carries a
  spondylometaphyseal dysplasia and neither is listed in the skeletal nosology. The
  two downstream branches from the metabolic node are both graded
  INDIRECT_UNKNOWN_INTERMEDIATES deliberately: the founding report states the
  pathogenic mechanism is unknown, and the organ dissociation across the allelic
  series means neither branch can be inferred from the other.

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1

PMID:36779427

Nosology of genetic skeletal disorders: 2023 revision.

No top-level findings curated for this source.

Spondylometaphyseal Dysplasia with Cone-Rod Dystrophy

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