Smith-Lemli-Opitz syndrome

Mendelian MONDO:0010035 Pathograph 8 Cholesterol biosynthesis disorder Inborn Error of Metabolism

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive developmental and metabolic disorder caused by biallelic pathogenic variants in DHCR7, which reduce 7-dehydrocholesterol reductase activity at the terminal step of cholesterol biosynthesis. The defining biochemical signature is decreased cholesterol with accumulation of 7-dehydrocholesterol. Clinical severity spans fetal-lethal multiple-malformation disease to milder neurodevelopmental phenotypes; characteristic findings include microcephaly, intellectual disability, 2-3 toe syndactyly, cleft palate, postaxial polydactyly, and male genital underdevelopment. The main mechanistic story is a dual sterol defect: cholesterol deficiency perturbs sterol-dependent developmental signaling, while excess 7-dehydrocholesterol and its oxidized derivatives add toxic effects on cellular membranes and neural development.

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Mappings
0
Definitions
1
Inheritance
7
Pathophysiology
0
Histopathology
6
Phenotypes
8
Pathograph
2
Genes
3
Treatments
0
Subtypes
0
Differentials
0
Datasets
0
Trials
0
Models
1
Literature
👪

Inheritance

1
Autosomal recessive HP:0000007
Biallelic pathogenic DHCR7 variants are required for disease expression.
Autosomal recessive inheritance
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"SLOS is inherited in an autosomal recessive pattern."
The review explicitly states the inheritance pattern for Smith-Lemli-Opitz syndrome.
⚙

Pathophysiology

7
DHCR7 Deficiency
Biallelic pathogenic DHCR7 variants reduce 7-dehydrocholesterol reductase activity, impairing the terminal conversion of 7-dehydrocholesterol to cholesterol.
DHCR7 link
cholesterol biosynthetic process link ↓ DECREASED
7-dehydrocholesterol reductase activity link ↓ DECREASED
Show evidence (1 reference)
PMID:9653161 SUPPORT Human Clinical
"Here we identify the SLOS gene as a Delta7-sterol reductase (DHCR7, EC 1.3.1. 21) required for the de novo biosynthesis of cholesterol."
The causative molecular lesion in SLOS is loss of DHCR7-mediated terminal cholesterol synthesis.
7-Dehydrocholesterol Accumulation
7-Dehydrocholesterol accumulates in plasma and tissues because the final reductive step of cholesterol synthesis is blocked.
Show evidence (2 references)
PMID:7706951 SUPPORT Human Clinical
"It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
Human sterol measurements identify elevated 7-dehydrocholesterol as a defining biochemical abnormality.
PMID:23059950 SUPPORT Human Clinical
"The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations."
Elevated 7DHC in serum or tissues is the diagnostic biochemical signature of SLOS.
Relative Cholesterol Deficiency
Cholesterol levels fall in plasma and tissues because terminal sterol biosynthesis is impaired.
cholesterol homeostasis link ↕ DYSREGULATED
Show evidence (1 reference)
PMID:7706951 SUPPORT Human Clinical
"These results suggest that a defect in 3 beta-hydroxysterol delta 7-reductase leads to both a profound lack of cholesterol and its replacement by dehydrocholesterols."
Human tissue sterol analysis supports a true cholesterol-deficient state rather than isolated precursor accumulation.
Impaired Smoothened Activation
Reduced cholesterol availability impairs sterol-sensitive Smoothened activation and its trafficking to the primary cilium.
smoothened signaling pathway link ↓ DECREASED
Show evidence (1 reference)
PMID:26685159 SUPPORT In Vitro
"Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium."
This study directly identifies Smoothened activation as a cholesterol-sensitive mechanistic defect in SLOS models.
7-DHC-Derived Oxysterol Toxicity
Oxidative metabolites generated from excess 7-dehydrocholesterol perturb neural development and add toxicity beyond cholesterol deficiency alone.
Show evidence (1 reference)
PMID:22182693 SUPPORT Model Organism
"The overall results suggest that 7-DHC oxidative metabolites are critical contributors to altered neural development in SLOS."
Dhcr7-deficient model work supports toxic oxysterol intermediates as an independent mechanistic arm in SLOS.
Oxidative Stress-Linked Autophagy
SLOS fibroblasts show increased autophagy together with oxidative stress and mitochondrial dysfunction.
fibroblast link
response to oxidative stress link ↑ INCREASED autophagy link ↑ INCREASED mitophagy link ↑ INCREASED
Show evidence (2 references)
PMID:25405082 SUPPORT In Vitro
"Further, the increased basal LC3B-II levels were decreased significantly by pretreating the cells with antioxidants implicating a role for oxidative stress in elevating autophagy in SLOS cells."
Antioxidant rescue in patient fibroblasts ties elevated autophagy to oxidative stress.
PMID:25405082 SUPPORT In Vitro
"Considering the possible source of oxidative stress, we examined mitochondrial function in the SLOS cells using JC-1 assay and found significant mitochondrial dysfunction compared to mitochondria in control cells."
The same fibroblast study documents mitochondrial dysfunction as part of this oxidative-stress-associated cellular state.
Premature Neurogenesis
Cortical neuronal differentiation occurs prematurely, depleting precursor pools and distorting normal neural development.
neurogenesis link ↑ INCREASED neuron differentiation link ↑ INCREASED
Show evidence (1 reference)
PMID:22182693 SUPPORT Model Organism
"We also report that neurons are ten fold more susceptible to a 7-DHC-derived oxysterol mixture than glial cells, and that DHCEO accelerates differentiation and arborization of cortical neurons."
Dhcr7-related oxysterol exposure drives premature neuronal differentiation in model systems.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Smith-Lemli-Opitz 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

6
Head and Neck 2
Microcephaly Microcephaly (HP:0000252)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"It is characterized by prenatal and postnatal growth retardation, microcephaly, a variable degree of intellectual disability that encompasses normal intelligence to severe intellectual deficiency, and multiple major and minor malformations."
The same clinical review lists microcephaly among defining SLOS features.
Cleft palate Cleft palate (HP:0000175)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
The review lists cleft palate among the characteristic external malformations of SLOS.
Limbs 1
Postaxial polydactyly Postaxial polydactyly (HP:0100259)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
Clinical review evidence identifies postaxial polydactyly as part of the SLOS malformation spectrum.
Nervous System 1
Intellectual disability Intellectual disability (HP:0001249)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"It is characterized by prenatal and postnatal growth retardation, microcephaly, a variable degree of intellectual disability that encompasses normal intelligence to severe intellectual deficiency, and multiple major and minor malformations."
The review explicitly identifies intellectual disability as a core part of the syndrome spectrum.
Other 2
2-3 toe syndactyly 2-3 toe syndactyly (HP:0004691)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
The review identifies 2-3 toe syndactyly as a characteristic external malformation.
Underdeveloped external genitalia in males Abnormal male external genitalia morphology (HP:0000032)
Show evidence (1 reference)
PMID:23059950 SUPPORT Human Clinical
"External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
The review directly supports male external genital underdevelopment as part of the characteristic phenotype.
🧬

Genetic Associations

2
DHCR7 (Causative)
Autosomal recessive
Show evidence (2 references)
PMID:9653161 SUPPORT Human Clinical
"Our results strongly suggest that defects in the DHCR7 gene cause the SLOS."
Landmark human genetic study identifies DHCR7 as the causative SLOS gene.
PMID:17965227 SUPPORT Human Clinical
"In 263 SLOS patients 10 common alleles (c.964-1G>C, p.Trp151X, p.Thr93Met, p.Val326Leu, p.Arg352Trp, p.Arg404Cys, p.Phe302Leu, p.Leu157Pro, p.Gly410Ser, p.Arg445Gln) were found to constitute approximately 80% of disease-causing mutations."
Cohort-level evidence documents the recurrent DHCR7 variant spectrum in SLOS.
ABCA1 (Modifier)
Show evidence (1 reference)
PMID:22929031 SUPPORT Human Clinical
"Maternal ABCA1 genotypes show a highly significant correlation with clinical severity in SLOS patients (P=0.007)."
This cohort study supports maternal ABCA1 as a true modifier of SLOS severity.
💊

Treatments

3
Dietary cholesterol supplementation
Action: dietary intervention MAXO:0000088
Dietary cholesterol supplementation can raise plasma cholesterol and lower circulating 7DHC over time, although it does not fully correct all disease manifestations.
Show evidence (1 reference)
PMID:10951458 SUPPORT Human Clinical
"These results support the hypothesis that over time dietary cholesterol supplementation from egg yolk increases the plasma cholesterol levels and decreases levels of 7-DHC which may be toxic."
Human intervention data support dietary cholesterol supplementation as a biochemical treatment strategy in SLOS.
Simvastatin
Action: pharmacotherapy MAXO:0000058
Agent: simvastatin ↗
Adjunctive simvastatin can improve the serum dehydrocholesterol-to-total sterol ratio and irritability in mild to classic SLOS with residual DHCR7 activity.
Show evidence (1 reference)
PMID:27513191 SUPPORT Human Clinical
"Simvastatin seems to be relatively safe in patients with SLOS, improves the serum dehydrocholesterol-to-total sterol ratio, and significantly improves irritability symptoms in patients with mild to classic SLOS."
Placebo-controlled trial evidence supports simvastatin as a pharmacologic adjunct in milder SLOS.
Cholic acid
Action: pharmacotherapy MAXO:0000058
Agent: cholic acid ↗
Pilot bile-acid replacement with cholic acid increased plasma cholesterol in a small SLOS cohort, but current evidence is biochemical and preliminary.
Show evidence (1 reference)
PMID:38077958 SUPPORT Human Clinical
"In this pilot study, cholic acid supplementation was well tolerated and safe and resulted in an increase in plasma cholesterol in most SLOS subjects."
Pilot human data support a preliminary biochemical benefit of cholic acid supplementation.
🔬

Biochemical Markers

2
Serum 7-dehydrocholesterol (INCREASED)
Context: Diagnostic sterol abnormality detected in serum and other tissues.
Show evidence (2 references)
PMID:23059950 SUPPORT Human Clinical
"The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations."
Elevated serum or tissue 7DHC is the core biochemical marker used to confirm SLOS.
PMID:7706951 SUPPORT Human Clinical
"It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
Independent human sterol measurements confirm marked dehydrocholesterol elevation.
Plasma cholesterol (DECREASED)
Context: Characteristically low, especially in more severely affected individuals.
Show evidence (2 references)
PMID:7706951 SUPPORT Human Clinical
"It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
The defining biochemical profile includes decreased plasma cholesterol.
PMID:23059950 SUPPORT Human Clinical
"The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth."
Review evidence confirms cholesterol deficiency across developmental stages.
📚

Literature Summaries

1
Smith-Lemli-Opitz syndrome deep research notes ▸

Smith-Lemli-Opitz syndrome deep research notes

Scope and disease framing

  • Curated disease: MONDO:0010035 Smith-Lemli-Opitz syndrome.
  • Framing choice: treat SLOS as one disease-level DHCR7 deficiency spectrum rather than splitting historical "type I/type II" labels into separate disease entries. The literature consistently describes a continuous severity range from fetal-lethal multiple malformations to relatively mild neurodevelopmental disease.
  • Do not lump with other sterol-biosynthesis disorders such as desmosterolosis or lathosterolosis. The mechanistic overlap is real, but the causal gene, sterol block, and downstream evidence base are disease-specific.
  • Main encoded mechanistic story: DHCR7 deficiency -> 7-DHC accumulation + relative cholesterol deficiency -> Smoothened/Hedgehog signaling impairment + oxysterol toxicity -> abnormal neurodevelopment, with a separate fibroblast-supported oxidative-stress/autophagy branch.

Why the pathophysiology graph was split this way

1. Core lesion

  • PMID:9653161
  • Quote: "Here we identify the SLOS gene as a Delta7-sterol reductase (DHCR7, EC 1.3.1. 21) required for the de novo biosynthesis of cholesterol."
  • Use: anchors the disease to loss of DHCR7 activity, not just a descriptive sterol pattern.

2. Separate cholesterol deficiency from precursor accumulation

  • PMID:23059950
  • Quote: "The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth."
  • Use: justifies two sibling downstream nodes rather than a bundled "abnormal sterol metabolism" node.
  • PMID:7706951
  • Quote: "It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
  • Use: strong human biochemical support for both branches.

3. Cholesterol deficiency branch

  • PMID:26685159
  • Quote: "Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium."
  • Use: this is the cleanest evidence that the sterol-deficiency arm should point specifically to impaired Smoothened activation, rather than vaguely to "abnormal Hedgehog signaling."
  • Curation decision:
  • Encoded Relative Cholesterol Deficiency -> Impaired Smoothened Activation.
  • Did not overextend this paper into specific human malformations; the abstract supports the signaling defect, not a specific phenotype edge.

4. 7-DHC toxicity branch

  • PMID:22182693
  • Quote: "The overall results suggest that 7-DHC oxidative metabolites are critical contributors to altered neural development in SLOS."
  • Use: supports a distinct oxysterol-toxicity node rather than treating 7-DHC excess as inert storage.
  • PMID:22182693
  • Quote: "We also report that neurons are ten fold more susceptible to a 7-DHC-derived oxysterol mixture than glial cells, and that DHCEO accelerates differentiation and arborization of cortical neurons."
  • Use: supports a downstream Premature Neurogenesis node.
  • PMID:36111785
  • Quote: "Here, we show that loss of DHCR7 causes accumulation of 7-DHC-derived oxysterol metabolites, premature neurogenesis from murine or human cortical neural precursors, and depletion of the cortical precursor pool, both in vitro and in vivo."
  • Use: reinforces the same causal branch and adds a modern mechanistic refinement through GR/TrkB signaling.
  • Curation decision: kept this paper in research notes but not as primary YAML evidence because its abstract mixes human precursor cultures and in vivo mouse work in one sentence, which complicates clean evidence_source tagging.

5. Oxidative stress / autophagy branch

  • PMID:25405082
  • Quote: "We also show that the LC3B-II concentration in SLOS cells is directly proportional to the cellular concentration of 7DHC, suggesting that the increased autophagy is caused by 7DHC accumulation secondary to defective DHCR7."
  • Quote: "Further, the increased basal LC3B-II levels were decreased significantly by pretreating the cells with antioxidants implicating a role for oxidative stress in elevating autophagy in SLOS cells."
  • Use: strong patient-fibroblast evidence for a cellular-stress branch that is mechanistically distinct from the developmental signaling branch.
  • Curation decision:
  • Encoded Oxidative Stress-Linked Autophagy as a separate node.
  • Did not claim this branch fully explains the congenital malformation phenotype.

Phenotype selection

  • Kept phenotype set focused on repeatedly cited, disease-defining human findings from PMID:23059950.
  • Encoded:
  • Intellectual disability
  • Microcephaly
  • 2-3 toe syndactyly
  • Cleft palate
  • Postaxial polydactyly
  • Underdeveloped external genitalia in males
  • Left out for now:
  • Autism spectrum disorder / behavioral phenotype as a standalone phenotype, because the strongest easily quotable support in the current source set comes from mechanistic papers or treatment trials rather than a clean phenotype-defining clinical abstract.
  • Organ-system-specific internal malformations beyond the general statement that they may affect every organ system, because I did not want to over-encode unspecific phenotype atoms without dedicated evidence.

Genetics and modifier curation

Causative gene

  • PMID:9653161
  • Quote: "Our results strongly suggest that defects in the DHCR7 gene cause the SLOS."
  • PMID:17965227
  • Quote: "In 263 SLOS patients 10 common alleles ... were found to constitute approximately 80% of disease-causing mutations."
  • Curation decision:
  • Encoded DHCR7 as causative.
  • Added the recurrent-allele note because it is clinically useful and cohort-backed.

Modifier

  • PMID:22929031
  • Quote: "Maternal ABCA1 genotypes show a highly significant correlation with clinical severity in SLOS patients (P=0.007)."
  • Quote: "ABCA1 is an additional genetic modifier in SLOS."
  • Curation decision:
  • Encoded ABCA1 as a modifier with an explicit note that the effect is maternal and plausibly mediated by placental cholesterol transfer.
  • Did not also encode maternal ApoE because I did not fetch a comparably clean primary abstract in this pass.

Treatment curation choices

Included

  • PMID:10951458
  • Quote: "These results support the hypothesis that over time dietary cholesterol supplementation from egg yolk increases the plasma cholesterol levels and decreases levels of 7-DHC which may be toxic."
  • Encoded as dietary cholesterol supplementation.
  • PMID:27513191
  • Quote: "Simvastatin seems to be relatively safe in patients with SLOS, improves the serum dehydrocholesterol-to-total sterol ratio, and significantly improves irritability symptoms in patients with mild to classic SLOS."
  • Encoded as adjunctive pharmacotherapy for mild-to-classic SLOS.
  • PMID:38077958
  • Quote: "In this pilot study, cholic acid supplementation was well tolerated and safe and resulted in an increase in plasma cholesterol in most SLOS subjects."
  • Encoded cautiously as a preliminary pilot therapy with biochemical benefit.

Not encoded

  • Prenatal therapy concepts based on placental cholesterol transfer.
  • Reason: mechanistically interesting, but still not an established clinical treatment.
  • Antioxidant therapy.
  • Reason: promising model-based rationale, but the current source set is still preclinical.

Human biochemical anchors used in the YAML

  • PMID:23059950
  • Quote: "The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations."
  • PMID:7706951
  • Quote: "These results suggest that a defect in 3 beta-hydroxysterol delta 7-reductase leads to both a profound lack of cholesterol and its replacement by dehydrocholesterols."

Things intentionally left for a future pass

  • Prenatal biomarkers and maternal serum/urine steroid signatures.
  • Adrenal insufficiency and bile-acid physiology as complication-specific subgraphs.
  • More granular internal-organ malformations with dedicated heart, renal, and genital-development mechanism edges.
  • Explicit behavioral/psychiatric phenotype expansion once a clean human-clinical source set is assembled.

Source set fetched into cache for this curation

  • PMID:9653161 DHCR7 causation
  • PMID:7706951 defining human sterol biochemistry
  • PMID:23059950 phenotype / natural history / diagnosis
  • PMID:17965227 prevalence and common allele spectrum
  • PMID:10951458 dietary cholesterol supplementation
  • PMID:27513191 simvastatin placebo-controlled trial
  • PMID:26685159 cholesterol deficiency impairs Smoothened activation
  • PMID:25405082 oxidative stress, autophagy, mitochondrial dysfunction
  • PMID:22182693 DHCEO / oxysterol neurodevelopmental toxicity
  • PMID:36111785 premature neurogenesis and antioxidant rescue
  • PMID:22929031 maternal ABCA1 modifier
  • PMID:38077958 cholic acid pilot study
{ }

Source YAML

click to show 
name: Smith-Lemli-Opitz syndrome
creation_date: '2026-04-13T01:17:48Z'
updated_date: '2026-04-13T01:17:48Z'
category: Mendelian
synonyms:
- SLOS
- RSH syndrome
description: >-
  Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive developmental and
  metabolic disorder caused by biallelic pathogenic variants in DHCR7, which
  reduce 7-dehydrocholesterol reductase activity at the terminal step of
  cholesterol biosynthesis. The defining biochemical signature is decreased
  cholesterol with accumulation of 7-dehydrocholesterol. Clinical severity spans
  fetal-lethal multiple-malformation disease to milder neurodevelopmental
  phenotypes; characteristic findings include microcephaly, intellectual
  disability, 2-3 toe syndactyly, cleft palate, postaxial polydactyly, and male
  genital underdevelopment. The main mechanistic story is a dual sterol defect:
  cholesterol deficiency perturbs sterol-dependent developmental signaling,
  while excess 7-dehydrocholesterol and its oxidized derivatives add toxic
  effects on cellular membranes and neural development.
disease_term:
  preferred_term: Smith-Lemli-Opitz syndrome
  term:
    id: MONDO:0010035
    label: Smith-Lemli-Opitz syndrome
parents:
- Cholesterol biosynthesis disorder
- Inborn Error of Metabolism
inheritance:
- name: Autosomal recessive
  description: Biallelic pathogenic DHCR7 variants are required for disease expression.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLOS is inherited in an autosomal recessive pattern."
    explanation: The review explicitly states the inheritance pattern for Smith-Lemli-Opitz syndrome.
prevalence:
- population: Populations of European origin
  percentage: 1 in 15,000-60,000
  notes: >-
    Published prevalence estimates vary across European-ancestry populations
    and likely remain influenced by under-recognition of fetal-lethal cases.
  evidence:
  - reference: PMID:17965227
    reference_title: "Age and origin of major Smith-Lemli-Opitz syndrome (SLOS) mutations in European populations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prevalence of SLOS has been estimated to range between 1:15000 and 1:60000 in populations of European origin."
    explanation: Population-genetic review provides the commonly cited prevalence range for SLOS.
pathophysiology:
- name: DHCR7 Deficiency
  description: >-
    Biallelic pathogenic DHCR7 variants reduce 7-dehydrocholesterol reductase
    activity, impairing the terminal conversion of 7-dehydrocholesterol to
    cholesterol.
  genes:
  - preferred_term: DHCR7
    term:
      id: hgnc:2860
      label: DHCR7
    modifier: DECREASED
  molecular_functions:
  - preferred_term: 7-dehydrocholesterol reductase activity
    term:
      id: GO:0047598
      label: 7-dehydrocholesterol reductase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: cholesterol biosynthetic process
    term:
      id: GO:0006695
      label: cholesterol biosynthetic process
    modifier: DECREASED
  evidence:
  - reference: PMID:9653161
    reference_title: "Mutations in the Delta7-sterol reductase gene in patients with the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we identify the SLOS gene as a Delta7-sterol reductase (DHCR7, EC 1.3.1. 21) required for the de novo biosynthesis of cholesterol."
    explanation: The causative molecular lesion in SLOS is loss of DHCR7-mediated terminal cholesterol synthesis.
  downstream:
  - target: 7-Dehydrocholesterol Accumulation
    description: Loss of DHCR7 blocks consumption of 7-dehydrocholesterol.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23059950
      reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth."
      explanation: Review evidence directly links enzymatic DHCR7 deficiency to increased 7DHC levels.
  - target: Relative Cholesterol Deficiency
    description: Loss of DHCR7 lowers endogenous cholesterol synthesis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23059950
      reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth."
      explanation: The same review directly supports reduced cholesterol downstream of DHCR7 deficiency.
- name: 7-Dehydrocholesterol Accumulation
  description: >-
    7-Dehydrocholesterol accumulates in plasma and tissues because the final
    reductive step of cholesterol synthesis is blocked.
  chemical_entities:
  - preferred_term: 7-dehydrocholesterol
    term:
      id: CHEBI:17759
      label: cholesta-5,7-dien-3beta-ol
    modifier: INCREASED
  evidence:
  - reference: PMID:7706951
    reference_title: "Markedly increased tissue concentrations of 7-dehydrocholesterol combined with low levels of cholesterol are characteristic of the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
    explanation: Human sterol measurements identify elevated 7-dehydrocholesterol as a defining biochemical abnormality.
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations."
    explanation: Elevated 7DHC in serum or tissues is the diagnostic biochemical signature of SLOS.
  downstream:
  - target: 7-DHC-Derived Oxysterol Toxicity
    description: Excess 7-dehydrocholesterol provides substrate for formation of reactive oxysterol metabolites.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22182693
      reference_title: "DHCEO accumulation is a critical mediator of pathophysiology in a Smith-Lemli-Opitz syndrome model."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The overall results suggest that 7-DHC oxidative metabolites are critical contributors to altered neural development in SLOS."
      explanation: Dhcr7-deficient model data support a direct transition from excess 7DHC to pathogenic oxysterol burden.
  - target: Oxidative Stress-Linked Autophagy
    description: High cellular 7-dehydrocholesterol burden is linked to oxidative-stress-associated autophagy.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:25405082
      reference_title: "Elevated Autophagy and Mitochondrial Dysfunction in the Smith-Lemli-Opitz Syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We also show that the LC3B-II concentration in SLOS cells is directly proportional to the cellular concentration of 7DHC, suggesting that the increased autophagy is caused by 7DHC accumulation secondary to defective DHCR7."
      explanation: Patient fibroblast data link increasing 7DHC burden to increased autophagy.
- name: Relative Cholesterol Deficiency
  description: >-
    Cholesterol levels fall in plasma and tissues because terminal sterol
    biosynthesis is impaired.
  chemical_entities:
  - preferred_term: cholesterol
    term:
      id: CHEBI:16113
      label: cholesterol
    modifier: DECREASED
  biological_processes:
  - preferred_term: cholesterol homeostasis
    term:
      id: GO:0042632
      label: cholesterol homeostasis
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:7706951
    reference_title: "Markedly increased tissue concentrations of 7-dehydrocholesterol combined with low levels of cholesterol are characteristic of the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These results suggest that a defect in 3 beta-hydroxysterol delta 7-reductase leads to both a profound lack of cholesterol and its replacement by dehydrocholesterols."
    explanation: Human tissue sterol analysis supports a true cholesterol-deficient state rather than isolated precursor accumulation.
  downstream:
  - target: Impaired Smoothened Activation
    description: Cholesterol deficiency reduces sterol-dependent Smoothened activation within Hedgehog signaling.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:26685159
      reference_title: "Reduced cholesterol levels impair Smoothened activation in Smith-Lemli-Opitz syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium."
      explanation: In vitro SLOS modeling shows that cholesterol deficiency directly impairs Smoothened activation.
- name: Impaired Smoothened Activation
  description: >-
    Reduced cholesterol availability impairs sterol-sensitive Smoothened
    activation and its trafficking to the primary cilium.
  biological_processes:
  - preferred_term: smoothened signaling pathway
    term:
      id: GO:0007224
      label: smoothened signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:26685159
    reference_title: "Reduced cholesterol levels impair Smoothened activation in Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium."
    explanation: This study directly identifies Smoothened activation as a cholesterol-sensitive mechanistic defect in SLOS models.
- name: 7-DHC-Derived Oxysterol Toxicity
  description: >-
    Oxidative metabolites generated from excess 7-dehydrocholesterol perturb
    neural development and add toxicity beyond cholesterol deficiency alone.
  chemical_entities:
  - preferred_term: oxysterol
    term:
      id: CHEBI:53030
      label: oxysterol
    modifier: INCREASED
  evidence:
  - reference: PMID:22182693
    reference_title: "DHCEO accumulation is a critical mediator of pathophysiology in a Smith-Lemli-Opitz syndrome model."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The overall results suggest that 7-DHC oxidative metabolites are critical contributors to altered neural development in SLOS."
    explanation: Dhcr7-deficient model work supports toxic oxysterol intermediates as an independent mechanistic arm in SLOS.
  downstream:
  - target: Premature Neurogenesis
    description: Oxysterol exposure accelerates neuronal differentiation and perturbs normal cortical development.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22182693
      reference_title: "DHCEO accumulation is a critical mediator of pathophysiology in a Smith-Lemli-Opitz syndrome model."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "We also report that neurons are ten fold more susceptible to a 7-DHC-derived oxysterol mixture than glial cells, and that DHCEO accelerates differentiation and arborization of cortical neurons."
      explanation: Model-organism neuronal data connect oxysterol exposure to aberrantly accelerated neuronal differentiation.
- name: Oxidative Stress-Linked Autophagy
  description: >-
    SLOS fibroblasts show increased autophagy together with oxidative stress and
    mitochondrial dysfunction.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: response to oxidative stress
    term:
      id: GO:0006979
      label: response to oxidative stress
    modifier: INCREASED
  - preferred_term: autophagy
    term:
      id: GO:0006914
      label: autophagy
    modifier: INCREASED
  - preferred_term: mitophagy
    term:
      id: GO:0000423
      label: mitophagy
    modifier: INCREASED
  evidence:
  - reference: PMID:25405082
    reference_title: "Elevated Autophagy and Mitochondrial Dysfunction in the Smith-Lemli-Opitz Syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Further, the increased basal LC3B-II levels were decreased significantly by pretreating the cells with antioxidants implicating a role for oxidative stress in elevating autophagy in SLOS cells."
    explanation: Antioxidant rescue in patient fibroblasts ties elevated autophagy to oxidative stress.
  - reference: PMID:25405082
    reference_title: "Elevated Autophagy and Mitochondrial Dysfunction in the Smith-Lemli-Opitz Syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Considering the possible source of oxidative stress, we examined mitochondrial function in the SLOS cells using JC-1 assay and found significant mitochondrial dysfunction compared to mitochondria in control cells."
    explanation: The same fibroblast study documents mitochondrial dysfunction as part of this oxidative-stress-associated cellular state.
- name: Premature Neurogenesis
  description: >-
    Cortical neuronal differentiation occurs prematurely, depleting precursor
    pools and distorting normal neural development.
  biological_processes:
  - preferred_term: neurogenesis
    term:
      id: GO:0022008
      label: neurogenesis
    modifier: INCREASED
  - preferred_term: neuron differentiation
    term:
      id: GO:0030182
      label: neuron differentiation
    modifier: INCREASED
  evidence:
  - reference: PMID:22182693
    reference_title: "DHCEO accumulation is a critical mediator of pathophysiology in a Smith-Lemli-Opitz syndrome model."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We also report that neurons are ten fold more susceptible to a 7-DHC-derived oxysterol mixture than glial cells, and that DHCEO accelerates differentiation and arborization of cortical neurons."
    explanation: Dhcr7-related oxysterol exposure drives premature neuronal differentiation in model systems.
phenotypes:
- name: Intellectual disability
  category: Neurologic
  description: Cognitive impairment ranges from mild to severe across the SLOS severity spectrum.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized by prenatal and postnatal growth retardation, microcephaly, a variable degree of intellectual disability that encompasses normal intelligence to severe intellectual deficiency, and multiple major and minor malformations."
    explanation: The review explicitly identifies intellectual disability as a core part of the syndrome spectrum.
- name: Microcephaly
  category: Growth
  description: Reduced head circumference is a common growth and neurodevelopmental manifestation.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized by prenatal and postnatal growth retardation, microcephaly, a variable degree of intellectual disability that encompasses normal intelligence to severe intellectual deficiency, and multiple major and minor malformations."
    explanation: The same clinical review lists microcephaly among defining SLOS features.
- name: 2-3 toe syndactyly
  category: Skeletal
  description: Cutaneous syndactyly of the second and third toes is a classic external malformation in SLOS.
  phenotype_term:
    preferred_term: 2-3 toe syndactyly
    term:
      id: HP:0004691
      label: 2-3 toe syndactyly
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
    explanation: The review identifies 2-3 toe syndactyly as a characteristic external malformation.
- name: Cleft palate
  category: Craniofacial
  description: Orofacial clefting is part of the external malformation spectrum in SLOS.
  phenotype_term:
    preferred_term: Cleft palate
    term:
      id: HP:0000175
      label: Cleft palate
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
    explanation: The review lists cleft palate among the characteristic external malformations of SLOS.
- name: Postaxial polydactyly
  category: Skeletal
  description: Postaxial extra digits occur in a subset of affected individuals as part of the congenital malformation pattern.
  phenotype_term:
    preferred_term: Postaxial polydactyly
    term:
      id: HP:0100259
      label: Postaxial polydactyly
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
    explanation: Clinical review evidence identifies postaxial polydactyly as part of the SLOS malformation spectrum.
- name: Underdeveloped external genitalia in males
  category: Genitourinary
  description: Male genital underdevelopment is a characteristic external malformation in affected males.
  phenotype_term:
    preferred_term: underdeveloped external genitalia in males
    term:
      id: HP:0000032
      label: Abnormal male external genitalia morphology
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "External malformations include distinctive facial features, cleft palate, postaxial polydactyly, 2-3 syndactyly of the toes, and underdeveloped external genitalia in males, while internal anomalies may affect every organ system."
    explanation: The review directly supports male external genital underdevelopment as part of the characteristic phenotype.
biochemical:
- name: Serum 7-dehydrocholesterol
  presence: INCREASED
  context: Diagnostic sterol abnormality detected in serum and other tissues.
  evidence:
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical diagnosis of SLOS is confirmed by demonstrating an abnormally elevated concentration of the cholesterol precursor, 7DHC, in serum or other tissues, or by the presence of two DHCR7 mutations."
    explanation: Elevated serum or tissue 7DHC is the core biochemical marker used to confirm SLOS.
  - reference: PMID:7706951
    reference_title: "Markedly increased tissue concentrations of 7-dehydrocholesterol combined with low levels of cholesterol are characteristic of the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
    explanation: Independent human sterol measurements confirm marked dehydrocholesterol elevation.
- name: Plasma cholesterol
  presence: DECREASED
  context: Characteristically low, especially in more severely affected individuals.
  evidence:
  - reference: PMID:7706951
    reference_title: "Markedly increased tissue concentrations of 7-dehydrocholesterol combined with low levels of cholesterol are characteristic of the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized biochemically by low plasma cholesterol and greatly elevated levels of two dehydrocholesterols, one of which is the cholesterol precursor 7-dehydrocholesterol."
    explanation: The defining biochemical profile includes decreased plasma cholesterol.
  - reference: PMID:23059950
    reference_title: "Smith-Lemli-Opitz syndrome: phenotype, natural history, and epidemiology."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The enzymatic deficiency results in decreased cholesterol and increased 7DHC levels, both during embryonic development and after birth."
    explanation: Review evidence confirms cholesterol deficiency across developmental stages.
genetic:
- name: DHCR7
  gene_term:
    preferred_term: DHCR7
    term:
      id: hgnc:2860
      label: DHCR7
  association: Causative
  inheritance:
  - name: Autosomal recessive
  features: >-
    Biallelic pathogenic DHCR7 variants cause SLOS by reducing activity of
    7-dehydrocholesterol reductase. In large European cohorts, 10 recurrent
    alleles account for most disease-causing variants.
  evidence:
  - reference: PMID:9653161
    reference_title: "Mutations in the Delta7-sterol reductase gene in patients with the Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our results strongly suggest that defects in the DHCR7 gene cause the SLOS."
    explanation: Landmark human genetic study identifies DHCR7 as the causative SLOS gene.
  - reference: PMID:17965227
    reference_title: "Age and origin of major Smith-Lemli-Opitz syndrome (SLOS) mutations in European populations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In 263 SLOS patients 10 common alleles (c.964-1G>C, p.Trp151X, p.Thr93Met, p.Val326Leu, p.Arg352Trp, p.Arg404Cys, p.Phe302Leu, p.Leu157Pro, p.Gly410Ser, p.Arg445Gln) were found to constitute approximately 80% of disease-causing mutations."
    explanation: Cohort-level evidence documents the recurrent DHCR7 variant spectrum in SLOS.
- name: ABCA1
  gene_term:
    preferred_term: ABCA1
    term:
      id: hgnc:29
      label: ABCA1
  association: Modifier
  notes: Maternal ABCA1 genotype modifies fetal SLOS severity, plausibly through placental cholesterol transfer.
  evidence:
  - reference: PMID:22929031
    reference_title: "Maternal ABCA1 genotype is associated with severity of Smith-Lemli-Opitz syndrome and with viability of patients homozygous for null mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Maternal ABCA1 genotypes show a highly significant correlation with clinical severity in SLOS patients (P=0.007)."
    explanation: This cohort study supports maternal ABCA1 as a true modifier of SLOS severity.
treatments:
- name: Dietary cholesterol supplementation
  description: >-
    Dietary cholesterol supplementation can raise plasma cholesterol and lower
    circulating 7DHC over time, although it does not fully correct all disease
    manifestations.
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: MAXO:0000088
      label: dietary intervention
  evidence:
  - reference: PMID:10951458
    reference_title: "Cholesterol supplementation with egg yolk increases plasma cholesterol and decreases plasma 7-dehydrocholesterol in Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These results support the hypothesis that over time dietary cholesterol supplementation from egg yolk increases the plasma cholesterol levels and decreases levels of 7-DHC which may be toxic."
    explanation: Human intervention data support dietary cholesterol supplementation as a biochemical treatment strategy in SLOS.
- name: Simvastatin
  description: >-
    Adjunctive simvastatin can improve the serum dehydrocholesterol-to-total
    sterol ratio and irritability in mild to classic SLOS with residual DHCR7
    activity.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: MAXO:0000058
      label: pharmacotherapy
    therapeutic_agent:
    - preferred_term: simvastatin
      term:
        id: CHEBI:9150
        label: simvastatin
  evidence:
  - reference: PMID:27513191
    reference_title: "A placebo-controlled trial of simvastatin therapy in Smith-Lemli-Opitz syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Simvastatin seems to be relatively safe in patients with SLOS, improves the serum dehydrocholesterol-to-total sterol ratio, and significantly improves irritability symptoms in patients with mild to classic SLOS."
    explanation: Placebo-controlled trial evidence supports simvastatin as a pharmacologic adjunct in milder SLOS.
- name: Cholic acid
  description: >-
    Pilot bile-acid replacement with cholic acid increased plasma cholesterol in
    a small SLOS cohort, but current evidence is biochemical and preliminary.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: MAXO:0000058
      label: pharmacotherapy
    therapeutic_agent:
    - preferred_term: cholic acid
      term:
        id: CHEBI:16359
        label: cholic acid
  evidence:
  - reference: PMID:38077958
    reference_title: "Cholic acid increases plasma cholesterol in Smith-Lemli-Opitz syndrome: A pilot study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this pilot study, cholic acid supplementation was well tolerated and safe and resulted in an increase in plasma cholesterol in most SLOS subjects."
    explanation: Pilot human data support a preliminary biochemical benefit of cholic acid supplementation.