Squalene Synthase Deficiency

Mendelian MONDO:0032566 Pathograph 34 Show in embeddings browser Cholesterol biosynthesis disorder Inborn Error of Metabolism

Squalene synthase deficiency (SQSD) is an ultra-rare autosomal recessive inborn error of cholesterol biosynthesis caused by biallelic FDFT1 variants. FDFT1 encodes squalene synthase, which condenses two molecules of farnesyl diphosphate into squalene, the first committed step of sterol synthesis. Loss of the enzyme reduces endogenous cholesterol synthesis and diverts the accumulating farnesyl diphosphate into farnesol and farnesol-derived dicarboxylic acids and glucuronides, which are excreted in urine and form the diagnostic signature. Clinically the disease resembles Smith-Lemli-Opitz syndrome: facial dysmorphism, neonatal-onset generalized seizures, nonspecific structural brain malformations, cortical visual impairment, optic nerve hypoplasia, profound developmental delay and intellectual disability, 2-3 toe syndactyly, dry skin with marked photosensitivity, and genital malformations in males.

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1
Inheritance
7
Pathophys.
18
Phenotypes
2
Hypotheses
34
Pathograph
1
Genes
6
Medical Actions
1
Differentials
5
Models
2
References
1
Deep Research
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Inheritance

1
Autosomal recessive HP:0000007
Autosomal recessive inheritance
Show evidence (3 references)
PMID:32027475 SUPPORT Human Clinical
"SQSD is inherited in an autosomal recessive manner."
States the inheritance pattern.
PMID:32027475 SUPPORT Human Clinical
"Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the FDFT1 pathogenic variants in the family are known."
GeneReviews genetic counseling statement on carrier and prenatal testing.
PMID:32027475 SUPPORT Human Clinical
"At conception, each sib of an affected individual has a 25% change of being affected, a 50% change of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier."
The GeneReviews transmission risks. The source text reads "change" for "chance"; the quote keeps it as written.
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Mechanistic Hypotheses

2
Developmental cholesterol deficit causes the malformation pattern
sqsd_cholesterol_deficit CANONICAL
The SLOS-like malformations follow from insufficient endogenous cholesterol during embryogenesis. Supported by the embryonic lethality and neural tube defects of the Fdft1 null mouse, which maternal cholesterol does not rescue, and by the shared phenotype across the Mendelian sterol synthesis defects.
Pathograph links
Reduced De Novo Cholesterol Synthesis -> Disrupted Cholesterol-Dependent Embryonic Development INDIRECT UNKNOWN INTERMEDIATES Cholesterol is needed for embryonic development, particularly of the nervous system; the knockout mouse shows the extreme of this dependency.
Impaired cholesterol-dependent hedgehog signaling mediates the malformations
sqsd_hedgehog_by_analogy EMERGING
Cholesterol deficiency is proposed to act through reduced sonic hedgehog signaling, as in experimental cholesterol-deficiency models of Smith-Lemli-Opitz syndrome. No study has tested hedgehog signaling in FDFT1-deficient cells or tissue, so for SQSD this remains an inference by analogy.
Pathograph links
Reduced De Novo Cholesterol Synthesis -> Impaired Cholesterol-Dependent Hedgehog Signaling DIRECT Sonic hedgehog is activated by covalent attachment of cholesterol, so a cholesterol shortfall is expected to impair the morphogen pathway. Inferred for SQSD by analogy with the post-squalene defects; not tested in SQSD.
Impaired Cholesterol-Dependent Hedgehog Signaling -> Disrupted Cholesterol-Dependent Embryonic Development DIRECT Impaired morphogen signaling disturbs embryonic patterning.
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Pathophysiology

7
Biallelic Hypomorphic FDFT1 Variants
Recessive FDFT1 variants identified by exome sequencing in the three index patients. The one variant characterized functionally acts through a partial splicing defect together with altered promoter and/or enhancer activity, so some normal transcript is retained. Partial rather than complete loss is what the mouse data predict for a viable human disease, since complete loss of the enzyme is embryonic lethal in the mouse.
FDFT1 hgnc:3629 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves FDFT1 (hgnc:3629). hgnc:3629 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context variant_origin: GERMLINE functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
squalene synthase activity GO:0051996 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased squalene synthase activity, annotated with squalene synthase [NAD(P)H] activity (GO:0051996). GO:0051996 is a molecular function from the Gene Ontology.
Downstream
Deficient Squalene Synthase Activity The variants reduce production of functional squalene synthase.
Show evidence (2 references)
PMID:29909962 SUPPORT Human Clinical
"Whole-exome sequencing was used to identify recessive disease-causing variants in FDFT1."
Establishes biallelic FDFT1 variants as the cause of the disease.
PMID:29909962 SUPPORT In Vitro
"Functional characterization of one variant demonstrated a partial splicing defect and altered promoter and/or enhancer activity"
The functional assays behind this sentence are cell-based, so this item is graded IN_VITRO; it shows that at least one allele reduces expression through splicing and regulatory effects.
Deficient Squalene Synthase Activity
Squalene synthase (farnesyl-diphosphate farnesyltransferase 1) catalyzes the reductive head-to-head condensation of two farnesyl diphosphate molecules to squalene, the first step committed to sterol rather than to other isoprenoid products. Its loss blocks the pathway at the branch point.
squalene synthase activity GO:0051996 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased squalene synthase activity, annotated with squalene synthase [NAD(P)H] activity (GO:0051996). GO:0051996 is a molecular function from the Gene Ontology.
endoplasmic reticulum membrane GO:0005789 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves endoplasmic reticulum membrane (GO:0005789). GO:0005789 is a cellular component from the Gene Ontology.
Downstream
Reduced De Novo Cholesterol Synthesis Flux from farnesyl diphosphate into squalene and on to cholesterol falls.
Farnesyl Diphosphate Accumulation The substrate upstream of the block accumulates.
Show evidence (4 references)
PMID:24531458 SUPPORT In Vitro
"Squalene synthase (SQS) is a divalent metal-ion-dependent enzyme that catalyzes the two-step reductive `head-to-head' condensation of two molecules of farnesyl pyrophosphate to form squalene using presqualene diphosphate (PSPP) as an intermediate."
Defines the reaction lost in the disease, from crystal structures of the human enzyme.
PMID:30578317 SUPPORT In Vitro
"Insertion of the weakly hydrophobic tail-anchor (TA) of SQS into the ER membrane by the EMC ensures sufficient flux through the sterol biosynthetic pathway"
Places the enzyme in the endoplasmic reticulum membrane and ties the amount of correctly inserted enzyme to sterol pathway flux.
PMID:29909962 SUPPORT BACKGROUND Other
"FDFT1 encodes for an evolutionarily conserved enzyme, squalene synthase (SS, farnesyl-pyrophosphate farnesyl-transferase 1), which catalyzes the first committed step in cholesterol biosynthesis."
Identifies the enzyme encoded by the disease gene and its position in the pathway.
+ 1 more reference
Reduced De Novo Cholesterol Synthesis
Endogenous cholesterol synthesis falls in every tissue that depends on it. The developing embryo and the brain, which cannot draw on circulating lipoprotein cholesterol, are the tissues where the shortfall is expected to matter most.
Developmental cholesterol deficit causes the malformation pattern Impaired cholesterol-dependent hedgehog signaling mediates the malformations
cholesterol biosynthetic process GO:0006695 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased cholesterol biosynthetic process (GO:0006695). GO:0006695 is a biological process from the Gene Ontology.
Animal models
Liver-specific Fdft1 knockout mouse (L-SSKO) Mouse recapitulates Loss of the enzyme abolishes hepatic cholesterol synthesis.
Shumiya cataract rat Rat partially recapitulates Partial Fdft1 loss, together with a second hit in the same pathway, lowers tissue cholesterol enough to cause organ-specific disease.
Downstream
Impaired Cholesterol-Dependent Hedgehog Signaling Sonic hedgehog is activated by covalent attachment of cholesterol, so a cholesterol shortfall is expected to impair the morphogen pathway. Inferred for SQSD by analogy with the post-squalene defects; not tested in SQSD. Impaired cholesterol-dependent hedgehog signaling mediates the malformations
Disrupted Cholesterol-Dependent Embryonic Development Cholesterol is needed for embryonic development, particularly of the nervous system; the knockout mouse shows the extreme of this dependency. Developmental cholesterol deficit causes the malformation pattern
Show evidence (1 reference)
PMID:25755092 SUPPORT Model Organism
"liver-specific ablation of SS inhibits hepatic cholesterol biosynthesis and induces hypolipidemia without increasing significant mortality"
Loss of squalene synthase reduces cholesterol synthesis in vivo.
Impaired Cholesterol-Dependent Hedgehog Signaling
Reduced sonic hedgehog signaling in the developing embryo secondary to insufficient cholesterol. Experimental cholesterol deficiency in rodents produces the limb, genital, brain and facial malformations of Smith-Lemli-Opitz syndrome with loss of ventral neural tube Shh expression. For SQSD this node is an inference by analogy: no study has measured hedgehog signaling in FDFT1-deficient tissue.
Impaired cholesterol-dependent hedgehog signaling mediates the malformations
sonic hedgehog signaling GO:0007224 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased sonic hedgehog signaling, annotated with smoothened signaling pathway (GO:0007224). GO:0007224 is a biological process from the Gene Ontology.
Downstream
Disrupted Cholesterol-Dependent Embryonic Development Impaired morphogen signaling disturbs embryonic patterning. Impaired cholesterol-dependent hedgehog signaling mediates the malformations
Show evidence (1 reference)
PMID:10564872 SUPPORT INDIRECT Model Organism
"They provide further evidence that cholesterol-deficiency-induced HPE originates from impaired Shh signaling activity in the ventral neural tube."
Links cholesterol deficiency to impaired Shh signaling in the embryonic brain, in a pharmacological model of a different (post-squalene) block.
Farnesyl Diphosphate Accumulation
Farnesyl diphosphate, the substrate of squalene synthase, builds up behind the block.
Downstream
Farnesol Shunt to Dicarboxylic Acids and Glucuronides Excess farnesyl diphosphate is dephosphorylated to farnesol and oxidized to farnesoic acid and farnesol-derived dicarboxylic acids, or conjugated.
Show evidence (1 reference)
PMID:9083051 SUPPORT Model Organism
"Farnesyl diphosphate, the substrate for squalene synthase, accumulates in the presence of zaragozic acid A, a squalene synthase inhibitor."
Direct statement that squalene synthase blockade causes substrate accumulation.
Farnesol Shunt to Dicarboxylic Acids and Glucuronides
Carbon diverted from sterol synthesis is converted to farnesol and then to saturated and unsaturated branched-chain dicarboxylic acids and farnesol glucuronides. In animals treated with a squalene synthase inhibitor this is the major fate of the diverted carbon, and the urinary profile it produces is the biochemical signature of the human disease. In inhibitor-treated mice farnesol formation was not detected, which the authors read as rapid onward conversion to farnesoic acid and dicarboxylic acids in the liver, so the measurable products are the acids and conjugates rather than farnesol itself.
farnesol metabolic process GO:0016487 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased farnesol metabolic process (GO:0016487). GO:0016487 is a biological process from the Gene Ontology.
Animal models
Liver-specific Fdft1 knockout mouse (L-SSKO) Mouse partially recapitulates The authors attribute the transient liver dysfunction to increased farnesol production behind the block.
Zaragozic acid A-treated rodents Rat and mouse recapitulates Blocking the same enzyme produces the same class of urinary metabolites seen in patients.
Show evidence (4 references)
PMID:9083051 SUPPORT Model Organism
"The high level of FDDCAs that were found suggests that their synthesis is the major metabolic fate for carbon diverted from cholesterol synthesis by a squalene synthase inhibitor."
Identifies the shunt as the dominant fate of the diverted carbon.
PMID:9647670 SUPPORT Model Organism
"These metabolites were found to be produced only in the liver and not in the kidney."
Localizes production of the shunt metabolites to the liver.
PMID:9647670 SUPPORT Model Organism
"We were unable to detect any farnesol formation in the zaragozic acid-treated animals which indicates that FPP is readily converted to farnesoic acid and dicarboxylic acids in the liver."
Supports the statement that free farnesol does not accumulate measurably.
+ 1 more reference
Disrupted Cholesterol-Dependent Embryonic Development
Developmental programs that require locally synthesized cholesterol are disturbed, producing the Smith-Lemli-Opitz-like pattern of brain, facial, digital and genital malformations. Which developmental pathway mediates this in SQSD has not been established; hedgehog signaling is the leading candidate by analogy with the post-squalene defects (see the hedgehog node).
Animal models
Fdft1 null mouse Mouse partially recapitulates Shows that endogenous squalene synthase activity is required for embryonic, particularly neural, development.
Downstream
Structural Brain Malformations
Facial Dysmorphism
2-3 Toe Syndactyly
Genital Malformations in Males
Optic Nerve Hypoplasia
Hypospadias
Cryptorchidism
Congenital Heart Defects
Show evidence (3 references)
PMID:10521476 SUPPORT Model Organism
"E9.5-10.5 SS-/- embryos exhibited severe growth retardation and defective neural tube closure."
Complete loss of the enzyme in the mouse disrupts early neural development, the severe end of the developmental phenotype.
PMID:29909962 SUPPORT Human Clinical
"We report three individuals with profound developmental delay, brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms, resembling Smith-Lemli-Opitz syndrome"
The human developmental phenotype in the three index patients.
PMID:9375918 SUPPORT INDIRECT Model Organism
"Although not previously directly linked to cholesterol deficiency in experimental animals, limb and external genital defects were a notable outcome in this multifactorially-based cholesterol deficiency model."
Experimental cholesterol deficiency reproduces the limb and genital arm of the malformation pattern shared by SQSD and Smith-Lemli-Opitz syndrome.
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Pathograph

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

18
Cardiovascular 1
Congenital Heart Defects Abnormal heart morphology HP:0001627 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Congenital heart defect, annotated with Abnormal heart morphology (HP:0001627). HP:0001627 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
A management instruction that presupposes congenital heart defects occur.
Digestive 2
Constipation HP:0002019 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Constipation (HP:0002019). HP:0002019 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
A management instruction that presupposes constipation occurs.
Dysphagia HP:0002015 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dysphagia (HP:0002015). HP:0002015 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"placement of a gastrostomy tube is recommended for those with dysphagia and/or poor growth"
Presupposes dysphagia in some affected individuals.
Eye 2
Cortical Visual Impairment Cerebral visual impairment HP:0100704 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cortical visual impairment, annotated with Cerebral visual impairment (HP:0100704). HP:0100704 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"nonspecific structural brain malformations, cortical visual impairment, optic nerve hypoplasia"
GeneReviews lists cortical visual impairment among the key features.
Optic Nerve Hypoplasia HP:0000609 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Optic nerve hypoplasia (HP:0000609). HP:0000609 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"nonspecific structural brain malformations, cortical visual impairment, optic nerve hypoplasia"
GeneReviews lists optic nerve hypoplasia among the key features.
Genitourinary 3
Genital Malformations in Males Abnormality of the male genitalia HP:0010461 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Genital malformations in males, annotated with Abnormality of the male genitalia (HP:0010461). HP:0010461 is a phenotype from the Human Phenotype Ontology.
Coarse binding: source unspecified
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"dry skin with photosensitivity, and genital malformations in males"
GeneReviews lists genital malformations in males among the key features.
Hypospadias HP:0000047 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypospadias (HP:0000047). HP:0000047 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
A management instruction that presupposes hypospadias occurs; it says nothing about how often.
Cryptorchidism HP:0000028 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cryptorchidism (HP:0000028). HP:0000028 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
A management instruction that presupposes cryptorchidism occurs; it says nothing about how often.
Head and Neck 1
Facial Dysmorphism Abnormal facial shape HP:0001999 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Facial dysmorphism, annotated with Abnormal facial shape (HP:0001999). HP:0001999 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"Key clinical features include facial dysmorphism, a generalized seizure disorder presenting in the neonatal period"
GeneReviews lists facial dysmorphism first among the key features.
PMID:29909962 SUPPORT Human Clinical
"We report three individuals with profound developmental delay, brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms, resembling Smith-Lemli-Opitz syndrome"
Facial dysmorphism in the three index patients.
Integument 2
Dry Skin HP:0000958 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dry skin (HP:0000958). HP:0000958 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"profound developmental delay / intellectual disability, dry skin with photosensitivity, and genital malformations in males"
GeneReviews lists dry skin among the key features.
Cutaneous Photosensitivity HP:0000992 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cutaneous photosensitivity (HP:0000992). HP:0000992 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"profound developmental delay / intellectual disability, dry skin with photosensitivity, and genital malformations in males"
GeneReviews lists photosensitivity among the key features.
PMID:32027475 SUPPORT Human Clinical
"skin photosensitivity has produced clinically significant UV-related sunburns within ten minutes of direct sunlight exposure"
Gives the clinical severity of the photosensitivity.
Limbs 1
2-3 Toe Syndactyly HP:0004691 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is 2-3 toe syndactyly (HP:0004691). HP:0004691 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29909962 SUPPORT Human Clinical
"brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms"
2-3 toe syndactyly in the index patients.
Musculoskeletal 1
Spasticity HP:0001257 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spasticity (HP:0001257). HP:0001257 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
A management instruction that presupposes spasticity occurs.
Nervous System 5
Neonatal-Onset Generalized Seizures Generalized-onset seizure HP:0002197 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Generalized seizures of neonatal onset, annotated with Generalized-onset seizure (HP:0002197), qualified as neonatal onset. HP:0002197 is a phenotype from the Human Phenotype Ontology.
Onset: NEONATAL
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"Key clinical features include facial dysmorphism, a generalized seizure disorder presenting in the neonatal period"
States both the generalized seizure type and the neonatal onset.
Structural Brain Malformations Abnormal brain morphology HP:0012443 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Structural brain malformation, annotated with Abnormal brain morphology (HP:0012443). HP:0012443 is a phenotype from the Human Phenotype Ontology.
Sequelae: Neonatal-Onset Generalized Seizures Profound Developmental Delay Cortical Visual Impairment Spasticity
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"a generalized seizure disorder presenting in the neonatal period, nonspecific structural brain malformations, cortical visual impairment"
GeneReviews describes the brain malformations as nonspecific.
PMID:29909962 SUPPORT Human Clinical
"We report three individuals with profound developmental delay, brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms"
Brain abnormalities in the index patients.
Profound Developmental Delay Profound global developmental delay HP:0012736 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Profound global developmental delay (HP:0012736). HP:0012736 is a phenotype from the Human Phenotype Ontology.
Sequelae: Profound Intellectual Disability
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"profound developmental delay / intellectual disability"
GeneReviews grades the developmental delay as profound.
PMID:29909962 SUPPORT Human Clinical
"We report three individuals with profound developmental delay"
Profound developmental delay in the index patients.
Profound Intellectual Disability HP:0002187 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Profound intellectual disability (HP:0002187). HP:0002187 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"profound developmental delay / intellectual disability"
GeneReviews pairs the profound delay with intellectual disability.
Sleep Disturbance HP:0002360 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sleep disturbance (HP:0002360). HP:0002360 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT INDIRECT Human Clinical
"In those with sleep disturbance, a trial of melatonin may be considered."
Presupposes sleep disturbance in some affected individuals.
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Genetic Associations

1
FDFT1
Gene: FDFT1 hgnc:3629 gene page HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is FDFT1 (hgnc:3629). hgnc:3629 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:29909962 SUPPORT Human Clinical
"Whole-exome sequencing was used to identify recessive disease-causing variants in FDFT1."
Identifies FDFT1 as the disease gene.
PMID:32027475 SUPPORT Human Clinical
"identification of biallelic pathogenic variants in FDFT1 by molecular genetic testing"
GeneReviews diagnostic criterion naming biallelic FDFT1 variants.
PMID:36180924 SUPPORT Computational
"FDFT1 | Squalene synthase deficiency, MIM# 618156 | 8p23.1 | 9.495x10-6 | 5.1 | 55, 53, 53 | 38 | 2.52x10-5"
A population-genetic model places FDFT1 inside the recurrent 8p23.1 NAHR deletion and predicts that this deletion is the leading contributor to SQSD burden, carried by about 55 percent of affected individuals (Table 2, column "Top 3 allele contribution to disease"). This is a modeled estimate from population allele frequencies, not an observed patient series, but it argues for copy-number analysis alongside sequencing.
💊

Medical Actions

6
Sun and UV Light Avoidance
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Behavioral / lifestyle
GeneReviews lists sun and ultraviolet light exposure as the circumstance to avoid, because the skin photosensitivity can produce clinically significant sunburn within ten minutes of direct sunlight.
Target Phenotypes: Cutaneous photosensitivity HP:0000992 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Cutaneous photosensitivity (HP:0000992). HP:0000992 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"Agents/circumstances to avoid: Sun and UV light exposure; skin photosensitivity has produced clinically significant UV-related sunburns within ten minutes of direct sunlight exposure."
The GeneReviews agents and circumstances to avoid statement.
Antiseizure Therapy
Action: Anticonvulsant therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Anticonvulsant therapy (NCIT:C64172). NCIT:C64172 is a clinical intervention from the NCI Thesaurus. Ontology label: Anticonvulsant Therapy NCIT:C64172
Platform: Small molecule
Standard treatment of epilepsy; no SQSD-specific antiseizure regimen has been described.
Mechanism Target:
INHIBITS Neonatal-Onset Generalized Seizures — Symptomatic seizure control.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"Currently there are no specific disease-modifying treatments. Standard treatment for epilepsy, congenital heart defects, constipation, cryptorchidism, hypospadias, spasticity, and developmental delay / intellectual disability is appropriate."
GeneReviews recommends standard epilepsy treatment and states that no disease-modifying therapy exists.
Gastrostomy Tube Feeding
Action: Gastrostomy tube placementNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Gastrostomy tube placement, annotated with Gastrostomy Tube Procedure (NCIT:C157864). NCIT:C157864 is a clinical intervention from the NCI Thesaurus. Ontology label: Gastrostomy Tube Procedure NCIT:C157864
Platform: Surgery
Feeding therapy may help; a gastrostomy tube is recommended for individuals with dysphagia or poor growth.
Mechanism Target:
BYPASSES Dysphagia — Provides enteral nutrition that does not depend on safe swallowing.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"Feeding therapy may be useful, although placement of a gastrostomy tube is recommended for those with dysphagia and/or poor growth."
GeneReviews feeding recommendation.
Early Visual Intervention
Action: Early interventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Early intervention (NCIT:C159524). NCIT:C159524 is a clinical intervention from the NCI Thesaurus. Ontology label: Early Intervention NCIT:C159524
Platform: Behavioral / lifestyle
Early intervention to stimulate visual development in children with visual impairment.
Mechanism Target:
MODULATES Cortical Visual Impairment — Aims to stimulate visual development; it does not reverse the lesion.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"In those with visual impairment, early intervention may help to stimulate visual development."
GeneReviews recommendation for visual impairment.
Melatonin for Sleep Disturbance
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: melatonin CHEBI:16796 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses melatonin (CHEBI:16796). CHEBI:16796 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
A trial of melatonin may be considered for sleep disturbance.
Mechanism Target:
MODULATES Sleep Disturbance — Symptomatic treatment of sleep disturbance.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"In those with sleep disturbance, a trial of melatonin may be considered."
GeneReviews recommendation.
Surveillance Schedule
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Other
At each visit, assessment for new seizures, changes in tone and movement disorder; developmental, educational and behavioral monitoring; assessment for aspiration, respiratory insufficiency and sleep disorder; and monitoring of growth, nutrition and constipation. Ophthalmology evaluation annually or as clinically indicated.
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"At each visit: assess for new manifestations such as seizures, changes in tone, and movement disorder; monitor developmental progress, educational needs, and behavior"
The GeneReviews surveillance recommendation.
PMID:32027475 SUPPORT Human Clinical
"Ophthalmology evaluation annually or as clinically indicated."
The GeneReviews ophthalmologic surveillance interval.
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Biochemical Markers

1
Urinary farnesol-derived dicarboxylic acids and glucuronides (Increased)
Context: The diagnostic analytes, detected on urine organic acid analysis. They report the farnesol shunt directly and distinguish SQSD from the post-squalene sterol defects, which are identified instead by the sterol that accumulates.
Pathograph Readouts
Readout Of Farnesol Shunt to Dicarboxylic Acids and Glucuronides Positive Diagnostic
The excreted acids and glucuronides are the end products of the shunt that opens when squalene synthase is absent.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"The diagnosis of squalene synthase deficiency is established in a proband with characteristic urine metabolites on urine organic acids analysis"
The urine profile alone is sufficient to establish the diagnosis.
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"Individuals with SQSD have a unique urine metabolic profile with increased saturated and unsaturated branched-chain dicarboxylic acids and glucuronides derived from farnesol."
GeneReviews description of the diagnostic urine profile.
🔬

Diagnosis

1
Urine organic acid analysis and FDFT1 molecular testing
Either the characteristic urine metabolite profile on organic acid analysis or biallelic pathogenic FDFT1 variants establishes the diagnosis. The urine route is the one that separates SQSD biochemically from Smith-Lemli-Opitz syndrome, whose clinical picture it closely resembles.
Genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Show evidence (1 reference)
PMID:32027475 SUPPORT Human Clinical
"The diagnosis of squalene synthase deficiency is established in a proband with characteristic urine metabolites on urine organic acids analysis or by the identification of biallelic pathogenic variants in FDFT1 by molecular genetic testing."
The GeneReviews diagnostic criterion, giving both routes.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Three affected individuals were reported in the paper that defined the disease in 2018.
Show evidence (1 reference)
PMID:29909962 SUPPORT Human Clinical
"We report three individuals with profound developmental delay"
The case count in the defining report.
🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Squalene Synthase Deficiency:

Smith-Lemli-Opitz syndrome MONDO:0010035
Overlapping Features The most common cholesterol biosynthesis defect and the main clinical mimic, sharing the dysmorphic facies, brain involvement, developmental delay and 2-3 toe syndactyly.
Distinguishing Features
  • Farnesol-derived dicarboxylic acids and glucuronides on urine organic acid analysis favor SQSD.
  • Elevated 7-dehydrocholesterol favors Smith-Lemli-Opitz syndrome.
  • Biallelic FDFT1 variants confirm SQSD; DHCR7 variants confirm Smith-Lemli-Opitz syndrome.
Show evidence (2 references)
PMID:32027475 SUPPORT Human Clinical
"Squalene synthase deficiency (SQSD) is a rare inborn error of cholesterol biosynthesis with multisystem clinical manifestations similar to Smith-Lemli-Optiz syndrome."
GeneReviews names the clinical resemblance to Smith-Lemli-Opitz syndrome.
PMID:20104611 SUPPORT Human Clinical
"The diagnosis of SLOS was confirmed by finding an elevated amniotic fluid 7-dehydrocholesterol level"
Illustrates the sterol marker that confirms Smith-Lemli-Opitz syndrome and that is not part of the SQSD profile.
🐁

Animal Models

5
Fdft1 null mouse
Constitutive squalene synthase knockout. Heterozygotes are normal with unchanged hepatic cholesterol synthesis despite halved enzyme activity; homozygotes die around midgestation with growth retardation and defective neural tube closure, and neither maternal dietary squalene nor cholesterol rescues them.
Species
Mouse
Genotype
Fdft1(-/-)
Publication
PMID:10521476
Pathograph links
Disrupted Cholesterol-Dependent Embryonic Development partially recapitulates fidelity: low observes: organism Shows that endogenous squalene synthase activity is required for embryonic, particularly neural, development.
Limitations: Complete loss of the enzyme is embryonic lethal, whereas the human patients survive with partial-function alleles, so the model shows the severe extreme rather than the human malformation pattern.
Divergences from the human mechanism
species mismatch qualifying
The authors attribute lethality partly to the immaturity of the murine chorioallantoic circulation at midgestation, a placental constraint that may not apply in the same way to the human embryo.
Show evidence (1 reference)
PMID:10521476 SUPPORT Model Organism
"The mice homozygous for the disrupted SS gene (SS-/-) were embryonic lethal around midgestation."
The null phenotype.
Show evidence (2 references)
PMID:10521476 SUPPORT Model Organism
"The lethal phenotype was not rescued by supplementing the dams either with dietary squalene or cholesterol."
Maternal sterol supplementation does not rescue the embryo.
PMID:21247357 SUPPORT REVIEW SYNTHESIS Model Organism
"The Mvk knockout is early embryonic lethal in the mouse, the absence of Fdft1 is lethal at E9.5-12.5 dpc, while the Cyp51 knockouts die at 15.0 dpc."
Places the Fdft1 null lethality within the pattern that earlier-acting sterol enzymes give earlier lethality.
Liver-specific Fdft1 knockout mouse (L-SSKO)
Hepatocyte squalene synthase ablation. Hepatic cholesterol synthesis is almost abolished, plasma cholesterol and triglyceride fall in the fed state, and a transient liver dysfunction with moderate hepatomegaly is attributed by the authors to increased farnesol production.
Species
Mouse
Genotype
Liver-specific Fdft1 deletion (Cre-loxP)
Publication
PMID:25755092
Pathograph links
Reduced De Novo Cholesterol Synthesis recapitulates fidelity: moderate observes: tissue Loss of the enzyme abolishes hepatic cholesterol synthesis.
Limitations: Liver only; the extrahepatic tissues that matter for the human phenotype retain the enzyme and supply cholesterol to the liver.
Show evidence (1 reference)
PMID:25755092 SUPPORT Model Organism
"Hepatic SS activity of L-SSKO mice was reduced by >90%. In addition, cholesterol biosynthesis in the liver slices was almost eliminated."
Direct measurement of the reduced synthesis.
Farnesol Shunt to Dicarboxylic Acids and Glucuronides partially recapitulates fidelity: low observes: organism The authors attribute the transient liver dysfunction to increased farnesol production behind the block.
Limitations: Increased farnesol is inferred by the authors as the presumed cause of the liver dysfunction rather than reported as a measured result in the abstract.
Show evidence (1 reference)
PMID:25755092 SUPPORT INDIRECT Model Organism
"L-SSKO mice showed a transient liver dysfunction with moderate hepatomegaly presumably secondary to increased farnesol production."
Links the block to farnesol excess in vivo.
Oligodendrocyte-specific Fdft1 knockout mouse
Mice unable to synthesize cholesterol in myelin-forming oligodendrocytes. The abstract does not name the targeted gene; the record's MeSH indexing lists Farnesyl-Diphosphate Farnesyltransferase deficiency.
Species
Mouse
Genotype
Oligodendrocyte-specific Fdft1 deletion
Publication
PMID:15793579
Show evidence (1 reference)
PMID:15793579 SUPPORT Model Organism
"Mutant oligodendrocytes survived, but CNS myelination was severely perturbed, and mutant mice showed ataxia and tremor."
Shows that cell-autonomous cholesterol synthesis is required for normal CNS myelination.
Shumiya cataract rat
A hereditary cataract strain in which hypomorphic lanosterol synthase and squalene synthase alleles combine to lower lens cholesterol. It is a partial loss model of Fdft1 acting with a second sterol enzyme, not a model of the human disease; cataract is not among the SQSD features listed by GeneReviews.
Species
Rat
Genotype
Hypomorphic Lss and Fdft1 alleles (naturally occurring)
Publication
PMID:16440058
Pathograph links
Reduced De Novo Cholesterol Synthesis partially recapitulates fidelity: low observes: tissue Partial Fdft1 loss, together with a second hit in the same pathway, lowers tissue cholesterol enough to cause organ-specific disease.
Limitations: The cholesterol deficit depends on the combination with Lss alleles, the major cataract locus, so the Fdft1 contribution alone is not isolated, and the affected tissue is the lens rather than the developing brain.
Show evidence (1 reference)
PMID:16440058 SUPPORT Model Organism
"Cataract onset was associated with the specific combination of Lss and Fdft1 mutant alleles that decreased cholesterol levels in cataractous lenses to about 57% of normal."
Quantifies the tissue cholesterol deficit produced by the allele combination.
Show evidence (1 reference)
PMID:16440058 SUPPORT Model Organism
"Hypomorphic mutations were identified in genes for lanosterol synthase (Lss) on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on chromosome 15, both of which function in the cholesterol biosynthesis pathway."
Identifies the naturally occurring hypomorphic Fdft1 allele in this strain.
Zaragozic acid A-treated rodents
Pharmacological squalene synthase inhibition in rats, dogs and mice. The urinary farnesol-derived dicarboxylic acids these animals excrete match those from farnesol feeding, and anticipated the urine profile later found in SQSD patients.
Species
Rat and mouse
Genotype
Wild type, treated with the squalene synthase inhibitor zaragozic acid A
Publication
PMID:9083051
Pathograph links
Farnesol Shunt to Dicarboxylic Acids and Glucuronides recapitulates fidelity: high observes: organism Blocking the same enzyme produces the same class of urinary metabolites seen in patients.
Limitations: Acute pharmacological inhibition in adult animals, not lifelong genetic deficiency from conception; developmental consequences are not modeled.
Show evidence (1 reference)
PMID:9647670 SUPPORT Model Organism
"trans-3,7-Dimethyl-2-octaen-1,8-dioic acid and 3, 7-dimethyloctan-1,8-dioic acid were identified as the major end products of farnesyl-diphosphate metabolism in the urine of mice treated with zaragozic acid A."
Identifies the specific dicarboxylic acids produced.
Show evidence (1 reference)
PMID:9083051 SUPPORT Model Organism
"When rats were treated with zaragozic acid A, large amounts of FDDCAs were excreted in the urine."
The urinary excretion phenotype of the pharmacological model.
{ }

Source YAML

click to show 
name: Squalene Synthase Deficiency
creation_date: '2026-10-14T12:36:44Z'
category: Mendelian
description: >-
  Squalene synthase deficiency (SQSD) is an ultra-rare autosomal recessive inborn
  error of cholesterol biosynthesis caused by biallelic FDFT1 variants. FDFT1
  encodes squalene synthase, which condenses two molecules of farnesyl diphosphate
  into squalene, the first committed step of sterol synthesis. Loss of the enzyme
  reduces endogenous cholesterol synthesis and diverts the accumulating farnesyl
  diphosphate into farnesol and farnesol-derived dicarboxylic acids and
  glucuronides, which are excreted in urine and form the diagnostic signature.
  Clinically the disease resembles Smith-Lemli-Opitz syndrome: facial dysmorphism,
  neonatal-onset generalized seizures, nonspecific structural brain malformations,
  cortical visual impairment, optic nerve hypoplasia, profound developmental delay
  and intellectual disability, 2-3 toe syndactyly, dry skin with marked
  photosensitivity, and genital malformations in males.
disease_term:
  preferred_term: squalene synthase deficiency
  term:
    id: MONDO:0032566
    label: squalene synthase deficiency
synonyms:
- SQSD
notes: >-
  Published human data are limited to the three index patients of the 2018
  report (PMID:29909962) and the GeneReviews chapter written by the same group,
  so no phenotype frequencies are recorded. Phenotypes listed only in the
  GeneReviews management section (hypospadias, cryptorchidism, congenital heart
  defects, spasticity, constipation, dysphagia, sleep disturbance) are carried
  with indirect evidence because the summary presupposes rather than reports them.
parents:
- Cholesterol biosynthesis disorder
- Inborn Error of Metabolism
references:
- reference: PMID:32027475
  title: Squalene Synthase Deficiency.
  tags:
  - GeneReviews
- reference: PMID:29909962
  title: >-
    Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
    Characterization of a Defect in Cholesterol Biosynthesis.

pathophysiology:
- name: Biallelic Hypomorphic FDFT1 Variants
  biological_scale: MOLECULAR
  description: >-
    Recessive FDFT1 variants identified by exome sequencing in the three index
    patients. The one variant characterized functionally acts through a partial
    splicing defect together with altered promoter and/or enhancer activity, so
    some normal transcript is retained. Partial rather than complete loss is
    what the mouse data predict for a viable human disease, since complete loss
    of the enzyme is embryonic lethal in the mouse.
  genes:
  - preferred_term: FDFT1
    term:
      id: hgnc:3629
      label: FDFT1
  molecular_functions:
  - preferred_term: squalene synthase activity
    modifier: DECREASED
    term:
      id: GO:0051996
      label: squalene synthase [NAD(P)H] activity
  genetic_context:
    variant_origin: GERMLINE
    functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
  evidence:
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole-exome sequencing was used to identify recessive disease-causing
      variants in FDFT1.
    explanation: Establishes biallelic FDFT1 variants as the cause of the disease.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Functional characterization of one variant demonstrated a partial splicing
      defect and altered promoter and/or enhancer activity
    explanation: >-
      The functional assays behind this sentence are cell-based, so this item is
      graded IN_VITRO; it shows that at least one allele reduces expression
      through splicing and regulatory effects.
  downstream:
  - target: Deficient Squalene Synthase Activity
    causal_link_type: DIRECT
    description: The variants reduce production of functional squalene synthase.

- name: Deficient Squalene Synthase Activity
  biological_scale: MOLECULAR
  description: >-
    Squalene synthase (farnesyl-diphosphate farnesyltransferase 1) catalyzes the
    reductive head-to-head condensation of two farnesyl diphosphate molecules to
    squalene, the first step committed to sterol rather than to other isoprenoid
    products. Its loss blocks the pathway at the branch point.
  molecular_functions:
  - preferred_term: squalene synthase activity
    modifier: DECREASED
    term:
      id: GO:0051996
      label: squalene synthase [NAD(P)H] activity
  cellular_components:
  - preferred_term: endoplasmic reticulum membrane
    term:
      id: GO:0005789
      label: endoplasmic reticulum membrane
  evidence:
  - reference: PMID:24531458
    reference_title: Structural insights into the catalytic mechanism of human squalene synthase.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Squalene synthase (SQS) is a divalent metal-ion-dependent enzyme that
      catalyzes the two-step reductive `head-to-head' condensation of two
      molecules of farnesyl pyrophosphate to form squalene using presqualene
      diphosphate (PSPP) as an intermediate.
    explanation: >-
      Defines the reaction lost in the disease, from crystal structures of the
      human enzyme.
  - reference: PMID:30578317
    reference_title: >-
      The ER membrane protein complex promotes biogenesis of sterol-related
      enzymes maintaining cholesterol homeostasis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Insertion of the weakly hydrophobic tail-anchor (TA) of SQS into the ER
      membrane by the EMC ensures sufficient flux through the sterol biosynthetic
      pathway
    explanation: >-
      Places the enzyme in the endoplasmic reticulum membrane and ties the amount
      of correctly inserted enzyme to sterol pathway flux.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      FDFT1 encodes for an evolutionarily conserved enzyme, squalene synthase (SS,
      farnesyl-pyrophosphate farnesyl-transferase 1), which catalyzes the first
      committed step in cholesterol biosynthesis.
    explanation: Identifies the enzyme encoded by the disease gene and its position in the pathway.
    quote_role: BACKGROUND
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The metabolite profile in plasma and urine suggested that their defect was
      at the level of squalene synthase.
    explanation: Places the block at squalene synthase from patient metabolite profiles.
  downstream:
  - target: Reduced De Novo Cholesterol Synthesis
    causal_link_type: DIRECT
    description: Flux from farnesyl diphosphate into squalene and on to cholesterol falls.
    evidence:
    - reference: PMID:25755092
      reference_title: >-
        Plasma cholesterol-lowering and transient liver dysfunction in mice lacking
        squalene synthase in the liver.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Hepatic SS activity of L-SSKO mice was reduced by >90%. In addition,
        cholesterol biosynthesis in the liver slices was almost eliminated.
      explanation: >-
        Tissue-specific ablation of the enzyme abolishes cholesterol synthesis in
        that tissue, which is the edge asserted here.
  - target: Farnesyl Diphosphate Accumulation
    causal_link_type: DIRECT
    description: The substrate upstream of the block accumulates.
    evidence:
    - reference: PMID:9083051
      reference_title: >-
        Farnesol-derived dicarboxylic acids in the urine of animals treated with
        zaragozic acid A or with farnesol.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Farnesyl diphosphate, the substrate for squalene synthase, accumulates in
        the presence of zaragozic acid A, a squalene synthase inhibitor.
      explanation: >-
        Pharmacological inhibition of the same enzyme in animals produces substrate
        accumulation; the genetic deficiency removes the same activity.
      directness: INDIRECT

- name: Reduced De Novo Cholesterol Synthesis
  biological_scale: CELLULAR
  description: >-
    Endogenous cholesterol synthesis falls in every tissue that depends on it.
    The developing embryo and the brain, which cannot draw on circulating
    lipoprotein cholesterol, are the tissues where the shortfall is expected to
    matter most.
  biological_processes:
  - preferred_term: cholesterol biosynthetic process
    modifier: DECREASED
    term:
      id: GO:0006695
      label: cholesterol biosynthetic process
  evidence:
  - reference: PMID:25755092
    reference_title: >-
      Plasma cholesterol-lowering and transient liver dysfunction in mice lacking
      squalene synthase in the liver.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      liver-specific ablation of SS inhibits hepatic cholesterol biosynthesis and
      induces hypolipidemia without increasing significant mortality
    explanation: Loss of squalene synthase reduces cholesterol synthesis in vivo.
  downstream:
  - target: Impaired Cholesterol-Dependent Hedgehog Signaling
    causal_link_type: DIRECT
    hypothesis_groups:
    - sqsd_hedgehog_by_analogy
    description: >-
      Sonic hedgehog is activated by covalent attachment of cholesterol, so a
      cholesterol shortfall is expected to impair the morphogen pathway. Inferred
      for SQSD by analogy with the post-squalene defects; not tested in SQSD.
    evidence:
    - reference: PMID:15539288
      reference_title: The function of cholesterol in embryogenesis.
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: >-
        The protein SHH is expressed in the notochord of the CNS in the early
        embryo and is activated by being cleaved autocatalytically, with
        simultaneous covalent attachment of cholesterol to the N-terminal fragment
      explanation: >-
        States the molecular dependency of hedgehog signaling on cholesterol that
        this edge relies on.
      directness: INDIRECT
  - target: Disrupted Cholesterol-Dependent Embryonic Development
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    hypothesis_groups:
    - sqsd_cholesterol_deficit
    description: >-
      Cholesterol is needed for embryonic development, particularly of the nervous
      system; the knockout mouse shows the extreme of this dependency.
    evidence:
    - reference: PMID:10521476
      reference_title: Embryonic lethality and defective neural tube closure in mice lacking squalene synthase.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        We speculate that cholesterol is required for the development, particularly
        of the nervous system, and that the chorioallantoic circulatory system is
        not mature enough to supply the rapidly growing embryos with maternal
        cholesterol at this developmental stage.
      explanation: >-
        The authors attribute the embryonic phenotype of the null mouse to absent
        endogenous cholesterol at a stage when maternal supply cannot compensate.
    - reference: PMID:29909962
      reference_title: >-
        Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
        Characterization of a Defect in Cholesterol Biosynthesis.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: BACKGROUND
      snippet: >-
        Mendelian disorders of cholesterol biosynthesis typically result in
        multi-system clinical phenotypes, underlining the importance of cholesterol
        in embryogenesis and development.
      explanation: >-
        Frames the human malformation phenotype as a consequence of insufficient
        cholesterol during development, as for the other sterol synthesis defects.

- name: Impaired Cholesterol-Dependent Hedgehog Signaling
  biological_scale: CELLULAR
  description: >-
    Reduced sonic hedgehog signaling in the developing embryo secondary to
    insufficient cholesterol. Experimental cholesterol deficiency in rodents
    produces the limb, genital, brain and facial malformations of
    Smith-Lemli-Opitz syndrome with loss of ventral neural tube Shh expression.
    For SQSD this node is an inference by analogy: no study has measured hedgehog
    signaling in FDFT1-deficient tissue.
  biological_processes:
  - preferred_term: sonic hedgehog signaling
    modifier: DECREASED
    term:
      id: GO:0007224
      label: smoothened signaling pathway
  evidence:
  - reference: PMID:10564872
    reference_title: >-
      Absence of ventral cell populations in the developing brain in a rat model of
      the Smith-Lemli-Opitz syndrome.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      They provide further evidence that cholesterol-deficiency-induced HPE
      originates from impaired Shh signaling activity in the ventral neural tube.
    explanation: >-
      Links cholesterol deficiency to impaired Shh signaling in the embryonic brain,
      in a pharmacological model of a different (post-squalene) block.
    directness: INDIRECT
  downstream:
  - target: Disrupted Cholesterol-Dependent Embryonic Development
    causal_link_type: DIRECT
    hypothesis_groups:
    - sqsd_hedgehog_by_analogy
    description: Impaired morphogen signaling disturbs embryonic patterning.

- name: Farnesyl Diphosphate Accumulation
  biological_scale: MOLECULAR
  description: >-
    Farnesyl diphosphate, the substrate of squalene synthase, builds up behind
    the block.
  chemical_entities:
  - preferred_term: farnesyl diphosphate
    modifier: INCREASED
    term:
      id: CHEBI:17407
      label: 2-trans,6-trans-farnesyl diphosphate
  evidence:
  - reference: PMID:9083051
    reference_title: >-
      Farnesol-derived dicarboxylic acids in the urine of animals treated with
      zaragozic acid A or with farnesol.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Farnesyl diphosphate, the substrate for squalene synthase, accumulates in
      the presence of zaragozic acid A, a squalene synthase inhibitor.
    explanation: Direct statement that squalene synthase blockade causes substrate accumulation.
  downstream:
  - target: Farnesol Shunt to Dicarboxylic Acids and Glucuronides
    causal_link_type: DIRECT
    description: >-
      Excess farnesyl diphosphate is dephosphorylated to farnesol and oxidized to
      farnesoic acid and farnesol-derived dicarboxylic acids, or conjugated.
    evidence:
    - reference: PMID:9083051
      reference_title: >-
        Farnesol-derived dicarboxylic acids in the urine of animals treated with
        zaragozic acid A or with farnesol.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The profile of urinary dicarboxylic acids from rats fed farnesol was
        virtually identical to that produced by treating with zaragozic acid A,
        establishing that these dicarboxylic acids are farnesol-derived.
      explanation: >-
        Shows that the acids excreted when squalene synthase is blocked come from
        farnesol, which places farnesol between the accumulated substrate and the
        excreted products.

- name: Farnesol Shunt to Dicarboxylic Acids and Glucuronides
  biological_scale: MOLECULAR
  description: >-
    Carbon diverted from sterol synthesis is converted to farnesol and then to
    saturated and unsaturated branched-chain dicarboxylic acids and farnesol
    glucuronides. In animals treated with a squalene synthase inhibitor this is
    the major fate of the diverted carbon, and the urinary profile it produces is
    the biochemical signature of the human disease. In inhibitor-treated mice
    farnesol formation was not detected, which the authors read as rapid onward
    conversion to farnesoic acid and dicarboxylic acids in the liver, so the
    measurable products are the acids and conjugates rather than farnesol itself.
  notes: >-
    Farnesol is not bound as an increased chemical entity: the mouse study found
    no detectable farnesol, and the cited human source reports farnesol-derived
    products rather than a farnesol concentration.
  biological_processes:
  - preferred_term: farnesol metabolic process
    modifier: INCREASED
    term:
      id: GO:0016487
      label: farnesol metabolic process
  chemical_entities:
  - preferred_term: farnesol-derived dicarboxylic acids
    modifier: INCREASED
    term:
      id: CHEBI:35692
      label: dicarboxylic acid
  evidence:
  - reference: PMID:9083051
    reference_title: >-
      Farnesol-derived dicarboxylic acids in the urine of animals treated with
      zaragozic acid A or with farnesol.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      The high level of FDDCAs that were found suggests that their synthesis is the
      major metabolic fate for carbon diverted from cholesterol synthesis by a
      squalene synthase inhibitor.
    explanation: Identifies the shunt as the dominant fate of the diverted carbon.
  - reference: PMID:9647670
    reference_title: >-
      Massive production of farnesol-derived dicarboxylic acids in mice treated with
      the squalene synthase inhibitor zaragozic acid A.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      These metabolites were found to be produced only in the liver and not in the
      kidney.
    explanation: Localizes production of the shunt metabolites to the liver.
  - reference: PMID:9647670
    reference_title: >-
      Massive production of farnesol-derived dicarboxylic acids in mice treated with
      the squalene synthase inhibitor zaragozic acid A.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We were unable to detect any farnesol formation in the zaragozic acid-treated
      animals which indicates that FPP is readily converted to farnesoic acid and
      dicarboxylic acids in the liver.
    explanation: Supports the statement that free farnesol does not accumulate measurably.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Individuals with SQSD have a unique urine metabolic profile with increased
      saturated and unsaturated branched-chain dicarboxylic acids and glucuronides
      derived from farnesol.
    explanation: Shows the same shunt operating in patients.

- name: Disrupted Cholesterol-Dependent Embryonic Development
  biological_scale: TISSUE
  description: >-
    Developmental programs that require locally synthesized cholesterol are
    disturbed, producing the Smith-Lemli-Opitz-like pattern of brain, facial,
    digital and genital malformations. Which developmental pathway mediates this
    in SQSD has not been established; hedgehog signaling is the leading candidate
    by analogy with the post-squalene defects (see the hedgehog node).
  evidence:
  - reference: PMID:10521476
    reference_title: Embryonic lethality and defective neural tube closure in mice lacking squalene synthase.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      E9.5-10.5 SS-/- embryos exhibited severe growth retardation and defective
      neural tube closure.
    explanation: >-
      Complete loss of the enzyme in the mouse disrupts early neural development,
      the severe end of the developmental phenotype.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report three individuals with profound developmental delay, brain
      abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms,
      resembling Smith-Lemli-Opitz syndrome
    explanation: The human developmental phenotype in the three index patients.
  - reference: PMID:9375918
    reference_title: >-
      Limb, genital, CNS, and facial malformations result from gene/environment-induced
      cholesterol deficiency: further evidence for a link to sonic hedgehog.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Although not previously directly linked to cholesterol deficiency in
      experimental animals, limb and external genital defects were a notable outcome
      in this multifactorially-based cholesterol deficiency model.
    explanation: >-
      Experimental cholesterol deficiency reproduces the limb and genital arm of
      the malformation pattern shared by SQSD and Smith-Lemli-Opitz syndrome.
    directness: INDIRECT
  downstream:
  - target: Structural Brain Malformations
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Facial Dysmorphism
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: 2-3 Toe Syndactyly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Genital Malformations in Males
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Optic Nerve Hypoplasia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Hypospadias
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cryptorchidism
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Congenital Heart Defects
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES

mechanistic_hypotheses:
- hypothesis_group_id: sqsd_cholesterol_deficit
  hypothesis_label: Developmental cholesterol deficit causes the malformation pattern
  status: CANONICAL
  description: >-
    The SLOS-like malformations follow from insufficient endogenous cholesterol
    during embryogenesis. Supported by the embryonic lethality and neural tube
    defects of the Fdft1 null mouse, which maternal cholesterol does not rescue,
    and by the shared phenotype across the Mendelian sterol synthesis defects.
- hypothesis_group_id: sqsd_hedgehog_by_analogy
  hypothesis_label: Impaired cholesterol-dependent hedgehog signaling mediates the malformations
  status: EMERGING
  description: >-
    Cholesterol deficiency is proposed to act through reduced sonic hedgehog
    signaling, as in experimental cholesterol-deficiency models of
    Smith-Lemli-Opitz syndrome. No study has tested hedgehog signaling in
    FDFT1-deficient cells or tissue, so for SQSD this remains an inference by
    analogy.

phenotypes:
- category: Craniofacial
  name: Facial Dysmorphism
  description: >-
    Dysmorphic facies resembling Smith-Lemli-Opitz syndrome. The GeneReviews
    abstract names facial dysmorphism without listing the individual features.
  phenotype_term:
    preferred_term: Facial dysmorphism
    term:
      id: HP:0001999
      label: Abnormal facial shape
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Key clinical features include facial dysmorphism, a generalized seizure
      disorder presenting in the neonatal period
    explanation: GeneReviews lists facial dysmorphism first among the key features.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report three individuals with profound developmental delay, brain
      abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms,
      resembling Smith-Lemli-Opitz syndrome
    explanation: Facial dysmorphism in the three index patients.

- category: Neurologic
  name: Neonatal-Onset Generalized Seizures
  description: >-
    A generalized seizure disorder presenting in the neonatal period.
  phenotype_term:
    preferred_term: Generalized seizures of neonatal onset
    term:
      id: HP:0002197
      label: Generalized-onset seizure
    onset:
      onset_category: NEONATAL
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Key clinical features include facial dysmorphism, a generalized seizure
      disorder presenting in the neonatal period
    explanation: States both the generalized seizure type and the neonatal onset.

- category: Neurologic
  name: Structural Brain Malformations
  description: >-
    Nonspecific structural brain malformations. The sources do not name a
    consistent malformation, so the binding is the general brain-morphology term.
  phenotype_term:
    preferred_term: Structural brain malformation
    term:
      id: HP:0012443
      label: Abnormal brain morphology
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      a generalized seizure disorder presenting in the neonatal period,
      nonspecific structural brain malformations, cortical visual impairment
    explanation: GeneReviews describes the brain malformations as nonspecific.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report three individuals with profound developmental delay, brain
      abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms
    explanation: Brain abnormalities in the index patients.
  sequelae:
  - target: Neonatal-Onset Generalized Seizures
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Profound Developmental Delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cortical Visual Impairment
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Spasticity
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES

- category: Ophthalmologic
  name: Cortical Visual Impairment
  description: >-
    Visual impairment of cerebral origin. GeneReviews recommends early
    intervention to stimulate visual development.
  phenotype_term:
    preferred_term: Cortical visual impairment
    term:
      id: HP:0100704
      label: Cerebral visual impairment
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      nonspecific structural brain malformations, cortical visual impairment,
      optic nerve hypoplasia
    explanation: GeneReviews lists cortical visual impairment among the key features.

- category: Ophthalmologic
  name: Optic Nerve Hypoplasia
  phenotype_term:
    preferred_term: Optic nerve hypoplasia
    term:
      id: HP:0000609
      label: Optic nerve hypoplasia
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      nonspecific structural brain malformations, cortical visual impairment,
      optic nerve hypoplasia
    explanation: GeneReviews lists optic nerve hypoplasia among the key features.

- category: Neurodevelopmental
  name: Profound Developmental Delay
  phenotype_term:
    preferred_term: Profound global developmental delay
    term:
      id: HP:0012736
      label: Profound global developmental delay
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: profound developmental delay / intellectual disability
    explanation: GeneReviews grades the developmental delay as profound.
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report three individuals with profound developmental delay
    explanation: Profound developmental delay in the index patients.
  sequelae:
  - target: Profound Intellectual Disability
    causal_link_type: DIRECT

- category: Neurodevelopmental
  name: Profound Intellectual Disability
  phenotype_term:
    preferred_term: Profound intellectual disability
    term:
      id: HP:0002187
      label: Profound intellectual disability
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: profound developmental delay / intellectual disability
    explanation: GeneReviews pairs the profound delay with intellectual disability.

- category: Skeletal
  name: 2-3 Toe Syndactyly
  description: >-
    The same digital sign that characterizes Smith-Lemli-Opitz syndrome.
  phenotype_term:
    preferred_term: 2-3 toe syndactyly
    term:
      id: HP:0004691
      label: 2-3 toe syndactyly
  evidence:
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms
    explanation: 2-3 toe syndactyly in the index patients.

- category: Dermatologic
  name: Dry Skin
  phenotype_term:
    preferred_term: Dry skin
    term:
      id: HP:0000958
      label: Dry skin
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      profound developmental delay / intellectual disability, dry skin with
      photosensitivity, and genital malformations in males
    explanation: GeneReviews lists dry skin among the key features.

- category: Dermatologic
  name: Cutaneous Photosensitivity
  description: >-
    Marked sensitivity to ultraviolet light, severe enough that sunburn has
    followed less than ten minutes of direct sun exposure. The mechanism has not
    been established.
  phenotype_term:
    preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      profound developmental delay / intellectual disability, dry skin with
      photosensitivity, and genital malformations in males
    explanation: GeneReviews lists photosensitivity among the key features.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      skin photosensitivity has produced clinically significant UV-related
      sunburns within ten minutes of direct sunlight exposure
    explanation: Gives the clinical severity of the photosensitivity.

- category: Genitourinary
  name: Genital Malformations in Males
  description: >-
    GeneReviews reports genital malformations in males as a key feature and, in
    its management section, names hypospadias and cryptorchidism among the
    manifestations to treat. The summary does not say which malformations occur
    in which patients, so the general term is bound here and the two named
    lesions are recorded separately.
  phenotype_term:
    preferred_term: Genital malformations in males
    term:
      id: HP:0010461
      label: Abnormality of the male genitalia
    coarse_binding_basis: SOURCE_UNSPECIFIED
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: dry skin with photosensitivity, and genital malformations in males
    explanation: GeneReviews lists genital malformations in males among the key features.

- category: Genitourinary
  name: Hypospadias
  description: >-
    Named in the GeneReviews management section; no frequency is given.
  phenotype_term:
    preferred_term: Hypospadias
    term:
      id: HP:0000047
      label: Hypospadias
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Standard treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: >-
      A management instruction that presupposes hypospadias occurs; it says
      nothing about how often.
    directness: INDIRECT

- category: Genitourinary
  name: Cryptorchidism
  description: >-
    Named in the GeneReviews management section; no frequency is given.
  phenotype_term:
    preferred_term: Cryptorchidism
    term:
      id: HP:0000028
      label: Cryptorchidism
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Standard treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: >-
      A management instruction that presupposes cryptorchidism occurs; it says
      nothing about how often.
    directness: INDIRECT

- category: Cardiovascular
  name: Congenital Heart Defects
  description: >-
    Named in the GeneReviews management section without a specific lesion or
    frequency.
  phenotype_term:
    preferred_term: Congenital heart defect
    term:
      id: HP:0001627
      label: Abnormal heart morphology
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Standard treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: >-
      A management instruction that presupposes congenital heart defects occur.
    directness: INDIRECT

- category: Neurologic
  name: Spasticity
  description: >-
    Named in the GeneReviews management section, and changes in tone are part of
    the recommended surveillance.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Standard treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: A management instruction that presupposes spasticity occurs.
    directness: INDIRECT

- category: Gastrointestinal
  name: Constipation
  phenotype_term:
    preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Standard treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: A management instruction that presupposes constipation occurs.
    directness: INDIRECT

- category: Gastrointestinal
  name: Dysphagia
  description: >-
    Dysphagia and poor growth are the indications GeneReviews gives for
    gastrostomy, and aspiration is part of the recommended surveillance.
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      placement of a gastrostomy tube is recommended for those with dysphagia
      and/or poor growth
    explanation: Presupposes dysphagia in some affected individuals.
    directness: INDIRECT

- category: Neurologic
  name: Sleep Disturbance
  phenotype_term:
    preferred_term: Sleep disturbance
    term:
      id: HP:0002360
      label: Sleep disturbance
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In those with sleep disturbance, a trial of melatonin may be considered.
    explanation: Presupposes sleep disturbance in some affected individuals.
    directness: INDIRECT

biochemical:
- name: Urinary farnesol-derived dicarboxylic acids and glucuronides
  presence: Increased
  context: >-
    The diagnostic analytes, detected on urine organic acid analysis. They report
    the farnesol shunt directly and distinguish SQSD from the post-squalene sterol
    defects, which are identified instead by the sterol that accumulates.
  biomarker_term:
    preferred_term: farnesol-derived dicarboxylic acids
    term:
      id: CHEBI:35692
      label: dicarboxylic acid
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Individuals with SQSD have a unique urine metabolic profile with increased
      saturated and unsaturated branched-chain dicarboxylic acids and glucuronides
      derived from farnesol.
    explanation: GeneReviews description of the diagnostic urine profile.
  readouts:
  - target: Farnesol Shunt to Dicarboxylic Acids and Glucuronides
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      The excreted acids and glucuronides are the end products of the shunt that
      opens when squalene synthase is absent.
    evidence:
    - reference: PMID:32027475
      reference_title: Squalene Synthase Deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The diagnosis of squalene synthase deficiency is established in a proband
        with characteristic urine metabolites on urine organic acids analysis
      explanation: The urine profile alone is sufficient to establish the diagnosis.

genetic:
- name: FDFT1
  gene_term:
    preferred_term: FDFT1
    term:
      id: hgnc:3629
      label: FDFT1
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    Biallelic germline variants cause SQSD. Somatic biallelic inactivation and
    promoter hypermethylation of FDFT1 in skin cause forms of porokeratosis, a
    separate disease curated in Porokeratosis; that association is not part of
    this entry. FDFT1 lies in the 8p23.1 interval affected by a recurrent
    NAHR-mediated deletion; a population-genetic model (PMID:36180924) predicts
    this deletion to be a common SQSD allele, so chromosomal microarray or genome
    sequencing can find an allele that exome sequencing misses. No ClinGen gene-disease validity assertion for FDFT1 was found in
    the committed ClinGen export (`grep -i FDFT1 cache/clingen/gene_validity.csv`
    returns nothing).
  evidence:
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole-exome sequencing was used to identify recessive disease-causing
      variants in FDFT1.
    explanation: Identifies FDFT1 as the disease gene.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: identification of biallelic pathogenic variants in FDFT1 by molecular genetic testing
    explanation: GeneReviews diagnostic criterion naming biallelic FDFT1 variants.
  - reference: PMID:36180924
    reference_title: >-
      Sequencing individual genomes with recurrent genomic disorder deletions: an
      approach to characterize genes for autosomal recessive rare disease traits.
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "FDFT1 | Squalene synthase deficiency, MIM# 618156 | 8p23.1 | 9.495x10-6 | 5.1 | 55, 53, 53 | 38 | 2.52x10-5"
    explanation: >-
      A population-genetic model places FDFT1 inside the recurrent 8p23.1
      NAHR deletion and predicts that this deletion is the leading contributor to
      SQSD burden, carried by about 55 percent of affected individuals (Table 2,
      column "Top 3 allele contribution to disease"). This is a modeled estimate
      from population allele frequencies, not an observed patient series, but it
      argues for copy-number analysis alongside sequencing.

inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: SQSD is inherited in an autosomal recessive manner.
    explanation: States the inheritance pattern.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Carrier testing for at-risk relatives and prenatal testing for pregnancies
      at increased risk are possible if the FDFT1 pathogenic variants in the
      family are known.
    explanation: GeneReviews genetic counseling statement on carrier and prenatal testing.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      At conception, each sib of an affected individual has a 25% change of being
      affected, a 50% change of being an asymptomatic carrier, and a 25% chance of
      being unaffected and not a carrier.
    explanation: >-
      The GeneReviews transmission risks. The source text reads "change" for
      "chance"; the quote keeps it as written.

prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Three affected individuals were reported in the paper that defined the disease
    in 2018.
  evidence:
  - reference: PMID:29909962
    reference_title: >-
      Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular
      Characterization of a Defect in Cholesterol Biosynthesis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report three individuals with profound developmental delay
    explanation: The case count in the defining report.

diagnosis:
- name: Urine organic acid analysis and FDFT1 molecular testing
  description: >-
    Either the characteristic urine metabolite profile on organic acid analysis or
    biallelic pathogenic FDFT1 variants establishes the diagnosis. The urine route
    is the one that separates SQSD biochemically from Smith-Lemli-Opitz syndrome,
    whose clinical picture it closely resembles.
  diagnosis_term:
    preferred_term: Genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnosis of squalene synthase deficiency is established in a proband
      with characteristic urine metabolites on urine organic acids analysis or by
      the identification of biallelic pathogenic variants in FDFT1 by molecular
      genetic testing.
    explanation: The GeneReviews diagnostic criterion, giving both routes.

differential_diagnoses:
- name: Smith-Lemli-Opitz syndrome
  description: >-
    The most common cholesterol biosynthesis defect and the main clinical mimic,
    sharing the dysmorphic facies, brain involvement, developmental delay and 2-3
    toe syndactyly.
  distinguishing_features:
  - Farnesol-derived dicarboxylic acids and glucuronides on urine organic acid analysis favor SQSD.
  - Elevated 7-dehydrocholesterol favors Smith-Lemli-Opitz syndrome.
  - Biallelic FDFT1 variants confirm SQSD; DHCR7 variants confirm Smith-Lemli-Opitz syndrome.
  disease_term:
    preferred_term: Smith-Lemli-Opitz syndrome
    term:
      id: MONDO:0010035
      label: Smith-Lemli-Opitz syndrome
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Squalene synthase deficiency (SQSD) is a rare inborn error of cholesterol
      biosynthesis with multisystem clinical manifestations similar to
      Smith-Lemli-Optiz syndrome.
    explanation: GeneReviews names the clinical resemblance to Smith-Lemli-Opitz syndrome.
  - reference: PMID:20104611
    reference_title: >-
      Cyclopia (synophthalmia) in Smith-Lemli-Opitz syndrome: First reported case
      and consideration of mechanism.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnosis of SLOS was confirmed by finding an elevated amniotic fluid
      7-dehydrocholesterol level
    explanation: >-
      Illustrates the sterol marker that confirms Smith-Lemli-Opitz syndrome and
      that is not part of the SQSD profile.

treatments:
- name: Sun and UV Light Avoidance
  description: >-
    GeneReviews lists sun and ultraviolet light exposure as the circumstance to
    avoid, because the skin photosensitivity can produce clinically significant
    sunburn within ten minutes of direct sunlight.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Agents/circumstances to avoid: Sun and UV light exposure; skin
      photosensitivity has produced clinically significant UV-related sunburns
      within ten minutes of direct sunlight exposure.
    explanation: The GeneReviews agents and circumstances to avoid statement.

- name: Antiseizure Therapy
  description: >-
    Standard treatment of epilepsy; no SQSD-specific antiseizure regimen has been
    described.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Anticonvulsant therapy
    term:
      id: NCIT:C64172
      label: Anticonvulsant Therapy
  target_mechanisms:
  - target: Neonatal-Onset Generalized Seizures
    treatment_effect: INHIBITS
    description: Symptomatic seizure control.
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Currently there are no specific disease-modifying treatments. Standard
      treatment for epilepsy, congenital heart defects, constipation,
      cryptorchidism, hypospadias, spasticity, and developmental delay /
      intellectual disability is appropriate.
    explanation: >-
      GeneReviews recommends standard epilepsy treatment and states that no
      disease-modifying therapy exists.

- name: Gastrostomy Tube Feeding
  description: >-
    Feeding therapy may help; a gastrostomy tube is recommended for individuals
    with dysphagia or poor growth.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: Gastrostomy tube placement
    term:
      id: NCIT:C157864
      label: Gastrostomy Tube Procedure
  target_mechanisms:
  - target: Dysphagia
    treatment_effect: BYPASSES
    description: Provides enteral nutrition that does not depend on safe swallowing.
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Feeding therapy may be useful, although placement of a gastrostomy tube is
      recommended for those with dysphagia and/or poor growth.
    explanation: GeneReviews feeding recommendation.

- name: Early Visual Intervention
  description: >-
    Early intervention to stimulate visual development in children with visual
    impairment.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Early intervention
    term:
      id: NCIT:C159524
      label: Early Intervention
  target_mechanisms:
  - target: Cortical Visual Impairment
    treatment_effect: MODULATES
    description: Aims to stimulate visual development; it does not reverse the lesion.
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In those with visual impairment, early intervention may help to stimulate
      visual development.
    explanation: GeneReviews recommendation for visual impairment.

- name: Melatonin for Sleep Disturbance
  description: >-
    A trial of melatonin may be considered for sleep disturbance.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: melatonin
      term:
        id: CHEBI:16796
        label: melatonin
  target_mechanisms:
  - target: Sleep Disturbance
    treatment_effect: MODULATES
    description: Symptomatic treatment of sleep disturbance.
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In those with sleep disturbance, a trial of melatonin may be considered.
    explanation: GeneReviews recommendation.

- name: Surveillance Schedule
  description: >-
    At each visit, assessment for new seizures, changes in tone and movement
    disorder; developmental, educational and behavioral monitoring; assessment
    for aspiration, respiratory insufficiency and sleep disorder; and monitoring
    of growth, nutrition and constipation. Ophthalmology evaluation annually or as
    clinically indicated.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  notes: >-
    Carries no target_mechanisms: surveillance detects the manifestations rather
    than acting on them.
  evidence:
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      At each visit: assess for new manifestations such as seizures, changes in
      tone, and movement disorder; monitor developmental progress, educational
      needs, and behavior
    explanation: The GeneReviews surveillance recommendation.
  - reference: PMID:32027475
    reference_title: Squalene Synthase Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Ophthalmology evaluation annually or as clinically indicated.
    explanation: The GeneReviews ophthalmologic surveillance interval.

animal_models:
- name: Fdft1 null mouse
  species: Mouse
  genotype: Fdft1(-/-)
  publication: PMID:10521476
  description: >-
    Constitutive squalene synthase knockout. Heterozygotes are normal with
    unchanged hepatic cholesterol synthesis despite halved enzyme activity;
    homozygotes die around midgestation with growth retardation and defective
    neural tube closure, and neither maternal dietary squalene nor cholesterol
    rescues them.
  modeled_mechanisms:
  - target: Disrupted Cholesterol-Dependent Embryonic Development
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: ORGANISM
    description: >-
      Shows that endogenous squalene synthase activity is required for embryonic,
      particularly neural, development.
    limitations: >-
      Complete loss of the enzyme is embryonic lethal, whereas the human patients
      survive with partial-function alleles, so the model shows the severe extreme
      rather than the human malformation pattern.
    divergences:
    - divergence_type: SPECIES_MISMATCH
      materiality: QUALIFYING
      description: >-
        The authors attribute lethality partly to the immaturity of the murine
        chorioallantoic circulation at midgestation, a placental constraint that
        may not apply in the same way to the human embryo.
    evidence:
    - reference: PMID:10521476
      reference_title: Embryonic lethality and defective neural tube closure in mice lacking squalene synthase.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The mice homozygous for the disrupted SS gene (SS-/-) were embryonic lethal
        around midgestation.
      explanation: The null phenotype.
  evidence:
  - reference: PMID:10521476
    reference_title: Embryonic lethality and defective neural tube closure in mice lacking squalene synthase.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      The lethal phenotype was not rescued by supplementing the dams either with
      dietary squalene or cholesterol.
    explanation: Maternal sterol supplementation does not rescue the embryo.
  - reference: PMID:21247357
    reference_title: >-
      Defects in cholesterol synthesis genes in mouse and in humans: lessons for
      drug development and safer treatments.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    quote_role: REVIEW_SYNTHESIS
    snippet: >-
      The Mvk knockout is early embryonic lethal in the mouse, the absence of Fdft1
      is lethal at E9.5-12.5 dpc, while the Cyp51 knockouts die at 15.0 dpc.
    explanation: >-
      Places the Fdft1 null lethality within the pattern that earlier-acting
      sterol enzymes give earlier lethality.

- name: Liver-specific Fdft1 knockout mouse (L-SSKO)
  species: Mouse
  genotype: Liver-specific Fdft1 deletion (Cre-loxP)
  publication: PMID:25755092
  description: >-
    Hepatocyte squalene synthase ablation. Hepatic cholesterol synthesis is almost
    abolished, plasma cholesterol and triglyceride fall in the fed state, and a
    transient liver dysfunction with moderate hepatomegaly is attributed by the
    authors to increased farnesol production.
  modeled_mechanisms:
  - target: Reduced De Novo Cholesterol Synthesis
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: TISSUE
    description: Loss of the enzyme abolishes hepatic cholesterol synthesis.
    limitations: >-
      Liver only; the extrahepatic tissues that matter for the human phenotype
      retain the enzyme and supply cholesterol to the liver.
    evidence:
    - reference: PMID:25755092
      reference_title: >-
        Plasma cholesterol-lowering and transient liver dysfunction in mice lacking
        squalene synthase in the liver.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Hepatic SS activity of L-SSKO mice was reduced by >90%. In addition,
        cholesterol biosynthesis in the liver slices was almost eliminated.
      explanation: Direct measurement of the reduced synthesis.
  - target: Farnesol Shunt to Dicarboxylic Acids and Glucuronides
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: ORGANISM
    description: >-
      The authors attribute the transient liver dysfunction to increased farnesol
      production behind the block.
    limitations: >-
      Increased farnesol is inferred by the authors as the presumed cause of the
      liver dysfunction rather than reported as a measured result in the abstract.
    evidence:
    - reference: PMID:25755092
      reference_title: >-
        Plasma cholesterol-lowering and transient liver dysfunction in mice lacking
        squalene synthase in the liver.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: INDIRECT
      snippet: >-
        L-SSKO mice showed a transient liver dysfunction with moderate hepatomegaly
        presumably secondary to increased farnesol production.
      explanation: Links the block to farnesol excess in vivo.

- name: Oligodendrocyte-specific Fdft1 knockout mouse
  species: Mouse
  genotype: Oligodendrocyte-specific Fdft1 deletion
  publication: PMID:15793579
  description: >-
    Mice unable to synthesize cholesterol in myelin-forming oligodendrocytes.
    The abstract does not name the targeted gene; the record's MeSH indexing
    lists Farnesyl-Diphosphate Farnesyltransferase deficiency.
  evidence:
  - reference: PMID:15793579
    reference_title: High cholesterol level is essential for myelin membrane growth.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Mutant oligodendrocytes survived, but CNS myelination was severely perturbed,
      and mutant mice showed ataxia and tremor.
    explanation: >-
      Shows that cell-autonomous cholesterol synthesis is required for normal CNS
      myelination.

- name: Shumiya cataract rat
  species: Rat
  genotype: Hypomorphic Lss and Fdft1 alleles (naturally occurring)
  publication: PMID:16440058
  description: >-
    A hereditary cataract strain in which hypomorphic lanosterol synthase and
    squalene synthase alleles combine to lower lens cholesterol. It is a partial
    loss model of Fdft1 acting with a second sterol enzyme, not a model of the
    human disease; cataract is not among the SQSD features listed by GeneReviews.
  modeled_mechanisms:
  - target: Reduced De Novo Cholesterol Synthesis
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: TISSUE
    description: >-
      Partial Fdft1 loss, together with a second hit in the same pathway, lowers
      tissue cholesterol enough to cause organ-specific disease.
    limitations: >-
      The cholesterol deficit depends on the combination with Lss alleles, the
      major cataract locus, so the Fdft1 contribution alone is not isolated, and
      the affected tissue is the lens rather than the developing brain.
    evidence:
    - reference: PMID:16440058
      reference_title: >-
        Lanosterol synthase mutations cause cholesterol deficiency-associated
        cataracts in the Shumiya cataract rat.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Cataract onset was associated with the specific combination of Lss and
        Fdft1 mutant alleles that decreased cholesterol levels in cataractous
        lenses to about 57% of normal.
      explanation: Quantifies the tissue cholesterol deficit produced by the allele combination.
  evidence:
  - reference: PMID:16440058
    reference_title: >-
      Lanosterol synthase mutations cause cholesterol deficiency-associated
      cataracts in the Shumiya cataract rat.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Hypomorphic mutations were identified in genes for lanosterol synthase (Lss)
      on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on
      chromosome 15, both of which function in the cholesterol biosynthesis
      pathway.
    explanation: Identifies the naturally occurring hypomorphic Fdft1 allele in this strain.

- name: Zaragozic acid A-treated rodents
  species: Rat and mouse
  genotype: Wild type, treated with the squalene synthase inhibitor zaragozic acid A
  publication: PMID:9083051
  description: >-
    Pharmacological squalene synthase inhibition in rats, dogs and mice. The
    urinary farnesol-derived dicarboxylic acids these animals excrete match those
    from farnesol feeding, and anticipated the urine profile later found in SQSD
    patients.
  modeled_mechanisms:
  - target: Farnesol Shunt to Dicarboxylic Acids and Glucuronides
    relationship: RECAPITULATES
    fidelity: HIGH
    model_scale: ORGANISM
    description: >-
      Blocking the same enzyme produces the same class of urinary metabolites seen
      in patients.
    limitations: >-
      Acute pharmacological inhibition in adult animals, not lifelong genetic
      deficiency from conception; developmental consequences are not modeled.
    evidence:
    - reference: PMID:9647670
      reference_title: >-
        Massive production of farnesol-derived dicarboxylic acids in mice treated
        with the squalene synthase inhibitor zaragozic acid A.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        trans-3,7-Dimethyl-2-octaen-1,8-dioic acid and 3, 7-dimethyloctan-1,8-dioic
        acid were identified as the major end products of farnesyl-diphosphate
        metabolism in the urine of mice treated with zaragozic acid A.
      explanation: Identifies the specific dicarboxylic acids produced.
  evidence:
  - reference: PMID:9083051
    reference_title: >-
      Farnesol-derived dicarboxylic acids in the urine of animals treated with
      zaragozic acid A or with farnesol.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      When rats were treated with zaragozic acid A, large amounts of FDDCAs were
      excreted in the urine.
    explanation: The urinary excretion phenotype of the pharmacological model.
📚

References & Deep Research

References

2
PMID:32027475
Squalene Synthase Deficiency.
No top-level findings curated for this source.
PMID:29909962
Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular Characterization of a Defect in Cholesterol Biosynthesis.
No top-level findings curated for this source.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Squalene Synthase Deficiency · 2026-10-14T13:03:58Z · View source

New entry for squalene synthase deficiency (MONDO:0032566, FDFT1, autosomal recessive), replacing stub stubs/Squalene_Synthase_Deficiency.yaml (entry_type DISEASE: one gene, one enzyme step, one recessive mechanism; no cholesterol-biosynthesis grouping exists in kb/groupings to join). Sources. The GeneReviews chapter (PMID:32027475, tagged GeneReviews) is the phenotype, diagnosis, management, agents-to-avoid (sun and UV exposure) and genetic counseling baseline; the 2018 defining report (PMID:29909962) supplies the gene, the three index patients and the metabolite localization. One OpenScientist deep-research report was used (research/Squalene_Synthase_Deficiency-deep-research-openscientist.md): 15/15 references resolved, 15/15 quotes valid, term validation needs_review for three mislabelled CURIEs (HP:0003231, HP:0012443 as named, NCBITaxon:10116) and obsolete GO:0004310, none of which were bound. just preflight-dr against MONDO:0032566 returned WARN: FDFT1 is mentioned 27 times and the OMIM identifier matches MONDO (618156); the warning is that SHH is the second most mentioned gene (17). Those SHH passages discuss cholesterol-dependent hedgehog signaling as a candidate developmental mechanism by analogy with Smith-Lemli-Opitz syndrome, which the entry records as an EMERGING hypothesis, not a second disease entity. Taken from the report and verified against cached abstracts: Fdft1 null mouse (PMID:10521476), human SQS structure (PMID:24531458), EMC-dependent ER insertion (PMID:30578317), Fdft1 null lethality review (PMID:21247357), Shumiya cataract rat (PMID:16440058), cholesterol and hedgehog signaling (PMID:15539288, 9375918, 10564872), SLOS 7-dehydrocholesterol (PMID:20104611). Added outside the report: zaragozic acid A and farnesol-derived dicarboxylic acid studies (PMID:9083051, 9647670), the liver-specific Fdft1 knockout (PMID:25755092), oligodendrocyte cholesterol and myelination (PMID:15793579), and a population-genetic model placing FDFT1 in the recurrent 8p23.1 NAHR deletion (PMID:36180924, Table 2 row). Dropped report claims that the cited abstracts do not contain: low total and LDL cholesterol, raised plasma farnesol, the specific FDFT1 alleles (120 kb deletion, intronic splice variant, 16 bp regulatory deletion), reduced SQS protein on Western blot, and statins being contraindicated. The Coman full text is not available through PMC efetch, so these could not be quoted. Not used: PMID:9575210 and 21098670 (enzymology detail beyond what the entry needs; CrtM is bacterial) and PMID:34939274 (colon adenocarcinoma, a different context). The hedgehog step is curated as an EMERGING hypothesis because it is inferred by analogy with post-squalene defects and has not been tested in FDFT1-deficient tissue. Validation: just validate, validate-terms, count-verified-snippets (all snippets verified), check-entity-refs, check-causal-targets, check-duplicate-keys, check-coarse-phenotypes, check-snippet-length, check-title-snippets, check-enum-values, check-block-scalar-comments, check-genereviews (TAGGED) and validate-disorders all pass.

OpenScientist ▸
Squalene Synthase Deficiency — Comprehensive Disease Characterization Report
openscientist-autonomous 14 citations 2026-10-14T12:56:31.589765

Squalene Synthase Deficiency — Comprehensive Disease Characterization Report

Disease: Squalene Synthase Deficiency (SQSD) MONDO ID: MONDO:0032566 · OMIM: 618156 · Gene: FDFT1 (farnesyl-diphosphate farnesyltransferase 1 / squalene synthase) Category: Mendelian, autosomal recessive inborn error of cholesterol biosynthesis

Summary

Squalene synthase deficiency (SQSD) is an ultra-rare autosomal recessive inborn error of the cholesterol (sterol) biosynthetic pathway caused by biallelic hypomorphic variants in FDFT1, the gene encoding squalene synthase (SS/SQS). Squalene synthase catalyzes the first committed step of cholesterol biosynthesis — the metal-dependent, head-to-head reductive condensation of two molecules of farnesyl pyrophosphate (FPP) into squalene, proceeding through a presqualene diphosphate (PSPP) intermediate. Because this step is the branch-point committing isoprenoid carbon to sterols, its disruption simultaneously (a) lowers downstream cholesterol synthesis and (b) diverts accumulating FPP into farnesol and its oxidation products. The disease was first defined clinically, biochemically, and molecularly by Coman and colleagues in 2018, who reported three affected individuals from two unrelated European families whose multisystem phenotype closely resembles Smith-Lemli-Opitz syndrome (SLOS), the most common cholesterol biogenesis defect.

The clinical picture is congenital and severe: profound developmental delay, structural brain malformations, neonatal-onset generalized seizures, cortical visual impairment and optic nerve hypoplasia, facial dysmorphism, 2–3 toe cutaneous syndactyly, male genital malformations (cryptorchidism, hypospadias), and dry, markedly photosensitive skin. The biochemical hallmark is a unique urinary organic-acid signature of saturated and unsaturated branched-chain dicarboxylic acids and glucuronides derived from farnesol, accompanied by low total and LDL cholesterol. This profile localizes the metabolic block precisely to the squalene synthase step and is mirrored in individuals treated with pharmacologic squalene synthase inhibitors or dosed with farnesol.

Mechanistically, the best-supported causal model runs from biallelic hypomorphic FDFT1 variants → reduced squalene synthase protein/activity → impaired embryonic cholesterol synthesis → defective cholesterol-dependent Sonic hedgehog (SHH) signaling → the SLOS-like pattern of CNS, limb, genital, and craniofacial malformations. The SHH limb of this chain is inferred by analogy to SLOS and to experimental cholesterol-deficiency models rather than directly demonstrated in FDFT1-deficient humans. Complete loss of squalene synthase is embryonic-lethal in mice (neural tube closure defects, death at ~E9.5–E12.5) and is not rescued by dietary squalene or cholesterol, which explains why viable human disease requires partial-function (hypomorphic) alleles that preserve residual activity. There is no disease-modifying therapy; management is entirely supportive, and HMG-CoA reductase inhibitors (statins) are mechanistically contraindicated.

Key Findings

Finding 1 — SQSD is an autosomal recessive cholesterol biosynthesis disorder caused by biallelic FDFT1 variants and resembling SLOS

The disease-defining report (Coman et al., 2018, American Journal of Human Genetics) used whole-exome sequencing to identify recessive disease-causing variants in FDFT1 in three individuals with "profound developmental delay, brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms, resembling Smith-Lemli-Opitz syndrome, the most common cholesterol biogenesis defect" [PMID: 29909962]. FDFT1 encodes "an evolutionarily conserved enzyme, squalene synthase (SS, farnesyl-pyrophosphate farnesyl-transferase 1), which catalyzes the first committed step in cholesterol biosynthesis." The plasma and urine metabolite profile localized the enzymatic block to the squalene synthase step, firmly establishing both the causal gene and the pathway-level lesion.

  • Evidence type: Human clinical + molecular genetics (n=3, 2 families).
  • Ontology: HGNC:3629 (FDFT1); MONDO:0032566; enzyme EC 2.5.1.21.

Finding 2 — Squalene synthase catalyzes the first committed, metal-dependent head-to-head condensation of two FPP molecules via a presqualene diphosphate intermediate

Human squalene synthase crystal structures (Liu et al., 2014) establish that "Squalene synthase (SQS) is a divalent metal-ion-dependent enzyme that catalyzes the two-step reductive 'head-to-head' condensation of two molecules of farnesyl pyrophosphate to form squalene using presqualene diphosphate (PSPP) as an intermediate" [PMID: 24531458]. The enzyme uses NADPH and follows an ordered sequential mechanism. Two conserved aspartate-rich DXXXD motifs coordinate FPP through Mg²⁺, and conserved aromatic residues (e.g., Phe288/Phe286 in rat squalene synthase) are essential for the second (PSPP→squalene) half-reaction [PMID: 9575210]. Mechanistic studies of the homologous dehydrosqualene synthase CrtM show the primary carbocation forms PSPP, whose diphosphate then returns to the Mg²⁺ cluster for the second ionization [PMID: 21098670]. Squalene synthase is a C-terminally tail-anchored ER membrane enzyme whose insertion depends on the ER membrane protein complex (EMC): "Insertion of the weakly hydrophobic tail-anchor (TA) of SQS into the ER membrane by the EMC ensures sufficient flux through the sterol biosynthetic pathway" [PMID: 30578317].

  • Evidence type: Structural biology, enzymology (human, rat, bacterial, in vitro).
  • Ontology: GO:0004310 (farnesyl-diphosphate farnesyltransferase activity); GO:0006695 (cholesterol biosynthetic process); GO:0005789 (endoplasmic reticulum membrane); CHEBI: farnesyl diphosphate, presqualene diphosphate, squalene.

Finding 3 — Complete squalene synthase loss is embryonic lethal with neural tube defects in mice; the enzyme is not rate-limiting

Tozawa et al. (1999) disrupted the mouse Fdft1 (SS) gene. "The mice homozygous for the disrupted SS gene (SS-/-) were embryonic lethal around midgestation. E9.5-10.5 SS-/- embryos exhibited severe growth retardation and defective neural tube closure" and "The lethal phenotype was not rescued by supplementing the dams either with dietary squalene or cholesterol" [PMID: 10521476]. Heterozygotes were phenotypically normal despite ~50% reduced hepatic/testis SS activity and 60% lower hepatic SS mRNA, with unchanged hepatic cholesterol synthesis — "indicating that SS is not a rate-limiting enzyme in the cholesterol biosynthetic pathway." This is corroborated by Horvat et al. (2011): "the absence of Fdft1 is lethal at E9.5-12.5 dpc" [PMID: 21247357]. These data explain two central features of the human disease: heterozygous carriers are asymptomatic, and viable disease requires partial-function alleles that retain residual enzymatic flux.

  • Evidence type: Mouse knockout, review.

Finding 4 — Diagnostic biochemical hallmark: farnesol-derived branched-chain dicarboxylic aciduria with low total/LDL cholesterol

Coman et al. report a unique urine profile: increased saturated and unsaturated branched-chain dicarboxylic acids and glucuronides derived from farnesol, elevated plasma total farnesol, and low total and LDL cholesterol (squalene reduced or normal). "The metabolite profile in plasma and urine suggested that their defect was at the level of squalene synthase" [PMID: 29909962]. Mechanistically, the enzymatic block causes accumulation of the substrate FPP, which is dephosphorylated to farnesol then glucuronidated/hydroxylated/ω-oxidized to shorter branched-chain dicarboxylic acids detectable by urine organic-acid GC-MS and NMR spectroscopy. The identical profile occurs in people given pharmacologic squalene synthase inhibitors or dosed with farnesol, confirming the metabolic localization.

  • Evidence type: Human clinical biochemistry + pharmacologic corroboration.
  • Ontology: HP:0003231 (organic aciduria family); CHEBI: farnesol, dicarboxylic acid; HP:0003146 (hypocholesterolemia).

Finding 5 — FDFT1 pathogenic variants are hypomorphic non-coding/regulatory and splicing variants, not simple coding loss-of-function

Consistent with the essentiality of residual activity, the reported variants are not simple coding null alleles. Two affected siblings were compound heterozygous for a ~120-kb deletion encompassing FDFT1 plus an intronic splice variant (c.880-24_880-23delinsAG) causing retention of 22 bases of intron 8 and a premature termination codon in exon 9 (predicted nonsense-mediated decay); minigene assays and fibroblast cDNA showed both normal and mis-spliced transcript (a leaky/partial splicing defect). A third, unrelated, non-consanguineous child was homozygous for a rare 16-bp deep-intronic deletion predicted to disrupt promoter/enhancer chromatin state, validated by luciferase reporter assay. "Functional characterization of one variant demonstrated a partial splicing defect and altered promoter and/or enhancer activity, reflecting essential mechanisms for regulating cholesterol biosynthesis/uptake in steady state" [PMID: 29909962]. Western blot showed markedly reduced SQS protein in patient cells, worsened in lipid-depleted media.

  • Evidence type: Human molecular genetics + functional validation (minigene, luciferase, Western blot).
  • Implication: Standard coding-exon sequencing may MISS these variants; WGS, CMA (for the CNV), and RNA/functional studies can be required.

Finding 6 — Clinical spectrum

Integrating the primary report and the GeneReviews chapter (Coman, Pitt et al.; PMID: 32027475), features across the ~3 characterized individuals include:

Phenotype Suggested HPO term Notes on onset/severity
Profound developmental delay / intellectual disability HP:0012736 / HP:0002187 Congenital; severe
Structural brain malformations (nonspecific) HP:0012443 Congenital
Generalized seizures HP:0002197 Neonatal onset
Cortical visual impairment HP:0100704 Congenital
Optic nerve hypoplasia HP:0000609 Congenital
Facial dysmorphism HP:0001999 Congenital
2–3 toe cutaneous syndactyly HP:0001770 Congenital; shared with SLOS
Cryptorchidism (male) HP:0000028 Males
Hypospadias (male) HP:0000047 Males
Dry skin with photosensitivity HP:0000958 / HP:0000992 Severe; clinically significant sunburn within ~10 min of sun
Congenital heart defects HP:0001627 Reported/managed
Constipation HP:0002019 Managed
Spasticity HP:0001257 Managed
Low total & LDL cholesterol HP:0003146 Biochemical hallmark

"We report three individuals with profound developmental delay, brain abnormalities, 2-3 syndactyly of the toes, and facial dysmorphisms, resembling Smith-Lemli-Opitz syndrome" [PMID: 29909962]. FDFT1 is ubiquitously expressed, consistent with the multisystem pattern.

Finding 7 — Diagnosis and differential diagnosis

Per GeneReviews (PMID: 32027475), diagnosis is established in a proband by (a) the characteristic urine organic-acid metabolites (GC-MS + NMR showing complex farnesol-derived branched-chain dicarboxylic acids/glucuronides), OR (b) identification of biallelic pathogenic FDFT1 variants. Because the causal variants include deep-intronic/regulatory changes and a large CNV, coding-only testing may be insufficient; WGS/CMA and RNA studies may be needed. SQSD sits within the ~10 characterized Mendelian cholesterol biosynthesis disorders; four overlap clinically — lanosterol synthase deficiency, lathosterolosis, Smith-Lemli-Opitz syndrome (DHCR7), and desmosterolosis (DHCR24). SQSD is differentiated by its unique farnesol-derived dicarboxylic acid urine profile; by contrast, SLOS shows elevated 7-dehydrocholesterol [PMID: 20104611]. "The metabolite profile in plasma and urine suggested that their defect was at the level of squalene synthase" [PMID: 29909962].

Finding 8 — Management is entirely supportive; statins contraindicated

GeneReviews states "there are no specific disease-modifying treatments." Management is symptom-directed: anti-seizure medication for the neonatal generalized seizure disorder; standard treatment of congenital heart defects, constipation, cryptorchidism, hypospadias, and spasticity; feeding therapy / gastrostomy for dysphagia and poor growth; early-intervention developmental therapies (OT, PT, speech); visual stimulation for cortical visual impairment; behavioral support (ABA). Rigorous sun protection is essential because photosensitivity has produced clinically significant UV sunburns within ~10 minutes of direct sunlight. No approved pharmacotherapy, gene therapy, or enzyme/substrate replacement exists, and no SQSD clinical trials are registered. Statins (HMG-CoA reductase inhibitors) are mechanistically contraindicated because they act upstream and would further reduce already-impaired sterol flux.

Finding 9 — Pathophysiology causal chain

See the Mechanistic Model section below. In brief: biallelic hypomorphic FDFT1 variants → reduced SS activity → impaired embryonic cholesterol synthesis (plus FPP→farnesol diversion) → defective cholesterol-dependent Sonic hedgehog signaling → SLOS-like CNS/limb/genital/craniofacial malformations. The SHH limb is supported by: "The protein SHH is expressed in the notochord of the CNS in the early embryo and is activated by being cleaved autocatalytically, with simultaneous covalent attachment of cholesterol to the N-terminal fragment" [PMID: 15539288]; "Limb, genital, CNS, and facial malformations result from gene/environment-induced cholesterol deficiency: further evidence for a link to sonic hedgehog" [PMID: 9375918]; and "They provide further evidence that cholesterol-deficiency-induced HPE originates from impaired Shh signaling activity in the ventral neural tube" [PMID: 10564872].

Finding 10 — Model organisms and comparative biology

System Organism / identifier Key observation PMID
Knockout mouse Mus musculus (NCBITaxon:10090); Fdft1, Gene 14137 Complete KO embryonic-lethal E9.5–10.5, neural tube defects, not rescued by dietary squalene/cholesterol; heterozygotes normal 10521476; 21247357
Natural rat model Rattus norvegicus (NCBITaxon:10116); Shumiya cataract rat Synergy of hypomorphic Lss + Fdft1 alleles lowers lens cholesterol to ~57% of normal → hereditary cataract 16440058; 33204712
Cellular/in vitro Patient fibroblasts, lymphoblasts; human cell lines Reduced SQS protein; EMC-dependent SQS biogenesis 29909962; 30578317
Oncology (gain) Human colon adenocarcinoma FDFT1 upregulated, predicts poor prognosis, promotes tumor progression (synergy with SQLE) 34939274

"Hypomorphic mutations were identified in genes for lanosterol synthase (Lss) on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on chromosome 15, both of which function in the cholesterol biosynthesis pathway" and "Cataract onset was associated with the specific combination of Lss and Fdft1 mutant alleles that decreased cholesterol levels in cataractous lenses to about 57% of normal" [PMID: 16440058]. "Squalene synthase (FDFT1) is an upstream enzyme of squalene epoxidase (SQLE) in cholesterol biosynthesis" [PMID: 34939274]. No viable mouse model of the human hypomorphic disease is yet reported — a key limitation.

Finding 11 — Epidemiology and inheritance

SQSD is inherited in an autosomal recessive manner (OMIM 618156; GeneReviews PMID: 32027475). Only ~3 affected individuals from 2 unrelated families of European descent have been reported (Coman 2018, PMID: 29909962); no formal prevalence/incidence has been established (ultra-rare). Onset is congenital/prenatal with neonatal presentation. Recurrence risk for carrier parents is the standard 25% affected / 50% carrier / 25% unaffected per pregnancy. Reported families were non-consanguineous; one proband was homozygous for a rare regulatory variant, two were compound heterozygous (120-kb deletion + intronic splice variant). Both sexes are affected (genital malformations noted in males; sex ratio not estimable at n~3). Pathogenic alleles are rare in gnomAD; complete-LoF FDFT1 alleles are depleted (constraint), consistent with embryonic lethality of the biallelic null. Penetrance of biallelic hypomorphic genotypes appears complete; expressivity is relatively consistent and severe. "Whole-exome sequencing was used to identify recessive disease-causing variants in FDFT1" [PMID: 29909962].

Mechanistic Model / Interpretation

Ordered causal chain (initiating lesion → clinical manifestation)

(1) Biallelic HYPOMORPHIC FDFT1 variants (deep-intronic splice; promoter/
    enhancer regulatory deletion; 120-kb CNV)                     [PMID 29909962]
│  leads to
▼
(2) Reduced squalene synthase protein & enzymatic activity
    (partial, not complete — complete loss is embryonic-lethal)   [PMID 10521476]
│  results in failure to condense 2× FPP → squalene
▼
(3) BRANCH A: reduced de novo cholesterol synthesis      BRANCH B: FPP accumulates
    → low total & LDL cholesterol                         → dephosphorylated to farnesol
│                                                  → ω-oxidation/glucuronidation
│                                                  → branched-chain dicarboxylic
│                                                     ACIDURIA (diagnostic marker)
▼                                                            [PMID 29909962]
(4) Embryonic cholesterol deficiency impairs covalent cholesterol
    modification & signaling of Sonic hedgehog (SHH)              [PMID 15539288]
│  (INFERRED for SQSD by analogy to SLOS / cholesterol-deficiency models)
▼
(5) Impaired ventral neural-tube SHH signaling; loss of ventral
    midline cell populations/patterning                           [PMID 10564872, 9375918]
│  leads to
▼
(6) SLOS-like MALFORMATIONS: structural brain malformation,
    profound developmental delay, cortical visual impairment /
    optic nerve hypoplasia, 2-3 toe syndactyly, male genital
    anomalies, facial dysmorphism, photosensitive dry skin        [PMID 29909962, 32027475]

Upstream vs downstream. The initiating molecular lesion (steps 1–2) and the immediate metabolic consequences (step 3) are directly demonstrated in patients. The SHH-mediated developmental steps (4–5) are downstream and inferred — robust for cholesterol deficiency in general (SLOS, teratogen models) but not yet shown directly in FDFT1-deficient human embryos.

Pathways / processes / compartments (ontology suggestions). - Molecular pathway: cholesterol/sterol biosynthesis (KEGG hsa00100; Reactome "Cholesterol biosynthesis"); Sonic hedgehog signaling (Reactome "Hedgehog ligand biogenesis"). - GO biological process: GO:0016126 (sterol biosynthetic process), GO:0006695 (cholesterol biosynthetic process); GO:0007224 (smoothened signaling pathway) for the downstream developmental step. - GO cellular component: GO:0005789 (ER membrane) — site of squalene synthase. - Cell types (CL): ventral-midline neuroepithelial/neural cells (CL:0000031 neuroblast; CL:0002319 neural cell); hepatocytes (CL:0000182) and fibroblasts (CL:0000057) as sterol-synthesizing cells. - Anatomy (UBERON): brain (UBERON:0000955), neural tube (UBERON:0001049), optic nerve (UBERON:0000941), skin (UBERON:0002097), external genitalia (UBERON:0000990), toes/autopod (UBERON:0002387). - Chemicals (CHEBI): farnesyl diphosphate, presqualene diphosphate, squalene, farnesol, cholesterol, NADPH; cofactor Mg²⁺/Mn²⁺.

Why the disease is hypomorphic, not null

The mouse knockout and the human genetics converge on one principle: some squalene synthase activity is required for embryonic survival because squalene/cholesterol cannot be supplied to the embryo from the diet (complete step-2/3 failure is lethal). The natural Shumiya cataract rat independently demonstrates that partial Fdft1 loss plus a second hit lowers tissue cholesterol enough to cause organ-specific disease (lens cholesterol ~57% of normal → cataract), providing comparative support for a dose-dependent cholesterol-deficiency mechanism.

Evidence Base

PMID Title (abbrev.) Evidence type Role in this report
29909962 Squalene Synthase Deficiency: Clinical, Biochemical, and Molecular Characterization Human clinical/molecular Disease-defining paper: gene, inheritance, phenotype, biomarker, variant types
32027475 Squalene Synthase Deficiency (GeneReviews) Clinical review Diagnosis, differential, management, inheritance
24531458 Structural insights into catalytic mechanism of human squalene synthase Structural biology Enzyme mechanism, Mg²⁺ dependence, PSPP intermediate
30578317 ER membrane protein complex promotes biogenesis of sterol enzymes Cell biology SQS is EMC-dependent tail-anchored ER enzyme
9575210 Function-structure studies of rat hepatic squalene synthase Enzymology Conserved Asp/aromatic residues; two half-reactions
21098670 Mechanism of action and inhibition of dehydrosqualene synthase Structural/enzymology Head-to-head mechanism, PSPP chemistry
10521476 Embryonic lethality and defective neural tube closure in mice lacking squalene synthase Mouse KO Complete loss embryonic-lethal; SS not rate-limiting
21247357 Defects in cholesterol synthesis genes in mouse and humans Review Fdft1 loss lethal E9.5–12.5; statin teratogenicity context
16440058 Lanosterol synthase mutations cause cataracts in Shumiya cataract rat Rat natural model Hypomorphic Fdft1+Lss lowers cholesterol → cataract
33204712 Differential gene expression in Shumiya cataract rat lens Rat model Confirms cholesterol-deficiency cataract mechanism
34939274 Squalene synthase predicts poor prognosis in colon adenocarcinoma Human oncology Contrasting gain-of-expression role of FDFT1
15539288 The function of cholesterol in embryogenesis Review Cholesterol required for SHH activation
10564872 Absence of ventral cell populations in rat SLOS model Rat model Cholesterol deficiency → impaired ventral SHH → HPE
9375918 Limb, genital, CNS, facial malformations from cholesterol deficiency Model/teratogen Same malformation pattern via cholesterol/SHH
20104611 Cyclopia in SLOS Human case SLOS differential (elevated 7-DHC); SHH-HPE severity

Convergence. The human report (29909962), the GeneReviews synthesis (32027475), the enzymology/structural papers (24531458, 9575210, 21098670, 30578317), the mouse/rat models (10521476, 21247357, 16440058, 33204712), and the cholesterol–SHH developmental literature (15539288, 10564872, 9375918, 20104611) form a self-consistent model. No retrieved paper contradicts the core claims; the principal uncertainty is that the SHH limb has not been directly tested in FDFT1-deficient human tissue.

Section-by-Section Summary (template coverage)

  • 1. Disease information: AR inborn error of cholesterol biosynthesis; OMIM 618156, MONDO:0032566, gene FDFT1. Synonyms: SQSD, squalene synthase deficiency, farnesyl-diphosphate farnesyltransferase 1 deficiency, SS deficiency. Information is disease-level/aggregated from ~3 individual patients.
  • 2. Etiology: Genetic — biallelic hypomorphic FDFT1 variants. No environmental cause; no established protective factors. Gene–environment note: lipid-depleted conditions further reduce SQS protein in patient cells (29909962). Statin exposure in pregnancy is a relevant teratogenic analogue (21247357).
  • 3. Phenotypes: see Finding 6 table with HPO terms, onset (congenital/neonatal), severity (severe), progression (static congenital malformations), frequency (qualitative given n~3). Severe QoL impact: profound disability.
  • 4. Genetic/molecular: FDFT1 (HGNC:3629, chr 15); variant classes = deep-intronic splice (NMD), regulatory promoter/enhancer deletion, 120-kb CNV; all germline, hypomorphic (partial LoF); rare in gnomAD; complete LoF depleted. No established human modifier genes (second-hit modifier shown in rat). No disease-specific epigenetic data.
  • 5. Environmental: None established; infectious agents N/A.
  • 6. Mechanism: see Mechanistic Model (causal chain; GO/CL/UBERON/CHEBI terms).
  • 7. Anatomy: brain/neural tube (UBERON:0000955/0001049), eye/optic nerve, skin, external genitalia, toes; ER subcellular compartment (GO:0005789); malformations largely bilateral.
  • 8. Temporal: congenital onset; static structural malformations; chronic lifelong; neonatal seizures; critical window = embryogenesis (sterol/SHH-dependent patterning).
  • 9. Inheritance/population: AR; ultra-rare (~3 individuals, 2 families, European); complete penetrance of biallelic hypomorphic genotype; standard 25% recurrence risk.
  • 10. Diagnostics: urine organic-acid GC-MS + NMR (farnesol-derived dicarboxylic acids/glucuronides); plasma farnesol ↑; total/LDL cholesterol ↓; molecular testing via multigene cholesterol-biosynthesis panel / WES / WGS / CMA / RNA studies. Differential: SLOS (7-DHC↑), lathosterolosis, desmosterolosis, lanosterol synthase deficiency.
  • 11. Prognosis: severe, lifelong profound disability; no formal survival data; no disease-modifying therapy.
  • 12. Treatment: supportive only (anti-seizure meds, cardiac/urologic/orthopedic management, feeding/gastrostomy, developmental therapies, sun protection); statins contraindicated. NCIT: supportive care; anticonvulsant therapy.
  • 13. Prevention: genetic counseling, carrier/cascade testing, prenatal/preimplantation testing for known familial variants; rigorous sun protection as tertiary prevention of UV injury.
  • 14. Other species: mouse Fdft1 (Gene 14137, NCBITaxon:10090) — embryonic-lethal KO; rat Fdft1 (NCBITaxon:10116) — Shumiya cataract natural model. No companion-animal natural disease reported; not zoonotic.
  • 15. Model organisms: embryonic-lethal mouse KO (no viable hypomorphic model yet); Shumiya cataract rat (partial-loss natural model); patient fibroblasts/lymphoblasts; EMC-biogenesis cell systems. Key limitation: no model recapitulating the full human hypomorphic multisystem phenotype.

Limitations and Knowledge Gaps

  1. Extremely small sample (n≈3, 2 families). All clinical/biochemical/molecular claims rest largely on a single defining report (29909962) plus its GeneReviews synthesis. Frequencies, penetrance, expressivity, and sex ratio are not statistically estimable.
  2. No prevalence/incidence data. The disease is ultra-rare with no registry.
  3. SHH mechanism is inferred, not demonstrated in SQSD. Steps 4–5 of the causal chain are supported only by analogy to SLOS and cholesterol-deficiency models; direct SHH assays in FDFT1-deficient human embryonic tissue are lacking.
  4. No viable animal model of the hypomorphic human disease. Complete-null mice die in utero; a hypomorphic/conditional mouse recapitulating the human phenotype has not been reported.
  5. No omics (transcriptomics/proteomics/broad metabolomics), no QoL instruments, no natural-history/survival data.
  6. No therapeutics. No approved or trial-stage disease-modifying treatment; no registered clinical trials.

Proposed Follow-up Experiments / Actions

  1. Case finding & registry. Deploy the characteristic urine organic-acid signature (farnesol-derived branched-chain dicarboxylic acids) as a biochemical screen across undiagnosed SLOS-like cohorts and metabolic biobanks to identify additional patients and enable natural-history study.
  2. Deep-intronic/regulatory variant discovery. Reanalyze coding-negative SLOS-like cases with WGS + RNA-seq + CMA, since pathogenic FDFT1 alleles are frequently non-coding (deep-intronic splice, promoter/enhancer, CNV).
  3. Direct test of the SHH limb. Generate patient-derived iPSCs and neural-tube/organoid or hypomorphic/conditional mouse models; quantify SHH processing (cholesterol-adducted N-terminal fragment), ventral patterning markers (Shh, Otx2), and sterol content to convert the inferred mechanism (steps 4–5) into direct evidence.
  4. Hypomorphic mouse model. Engineer knock-in of a human-equivalent leaky splice or regulatory allele (or conditional partial knockdown) to obtain a viable model recapitulating the multisystem phenotype for mechanism and preclinical therapeutics.
  5. Sterol-supplementation proof-of-concept. Evaluate whether postnatal dietary/pharmacologic cholesterol supplementation (as trialed in SLOS) offers any benefit; define biomarkers (plasma farnesol, cholesterol, urinary dicarboxylic acids) as pharmacodynamic endpoints.
  6. Variant functional pipeline. Standardize minigene splice assays and promoter/enhancer luciferase reporters for FDFT1 VUS classification per ACMG/AMP.

Report compiled from 12 confirmed findings and 25 reviewed papers over 5 investigation iterations. Evidence sources are labeled by type (human clinical, model organism, in vitro, computational/structural). Direct abstract quotations are reproduced verbatim and attributed by PMID.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.3.0.

Outcome Count
References checked 15
Resolved 15
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 15
Quoted claims found in source 15
Quoted claims not found in source 0
References weighed for topical relevance 15
On topic 9
Off topic 0

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome Count
Terms checked 36
Resolved 34
Unresolved (possible confabulation) 0
Obsolete 1
Unverifiable 1
Terms whose name was checked 20
Terms named correctly 9
Terms named as a different term 3
Terms whose name is worth a second look 8

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

  • HP:0003231 (1 mention) - the report calls it "organic aciduria family"; HP calls it Hypertyrosinemia
  • HP:0012443 (1 mention) - the report calls it "Structural brain malformations (nonspecific)"; HP calls it Abnormal brain morphology
  • NCBITaxon:10116 (2 mentions) - the report calls it "Rattus norvegicus", "Fdft1"; NCBITaxon calls it Rattus norvegicus

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

  • GO:0004310 (obsolete farnesyl-diphosphate farnesyltransferase activity) (1 mention) - replaced by GO:0051996

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

  • GO:0004310 (1 mention) - the report calls it "farnesyl-diphosphate farnesyltransferase activity"; GO calls it obsolete farnesyl-diphosphate farnesyltransferase activity
  • GO:0005789 (3 mentions) - the report calls it "endoplasmic reticulum membrane", "ER membrane"; GO calls it endoplasmic reticulum membrane, and lists "ER membrane" among its other names
  • HP:0003146 (2 mentions) - the report calls it "hypocholesterolemia", "Low total & LDL cholesterol"; HP calls it Hypocholesterolemia
  • HP:0100704 (1 mention) - the report calls it "Cortical visual impairment"; HP calls it Cerebral visual impairment, and lists "Cortical visual impairment" among its other names
  • HP:0001770 (1 mention) - the report calls it "2–3 toe cutaneous syndactyly"; HP calls it Toe syndactyly
  • HP:0000028 (1 mention) - the report calls it "Cryptorchidism (male)"; HP calls it Cryptorchidism
  • HP:0000047 (1 mention) - the report calls it "Hypospadias (male)"; HP calls it Hypospadias
  • HP:0001627 (1 mention) - the report calls it "Congenital heart defects"; HP calls it Abnormal heart morphology, and lists "Congenital heart defect" among its other names

Terms named inconsistently

The report gives these identifiers more than one name of its own:

  • GO:0005789 - called "endoplasmic reticulum membrane", "ER membrane"
  • HP:0003146 - called "hypocholesterolemia", "Low total & LDL cholesterol"
  • NCBITaxon:10116 - called "Rattus norvegicus", "Fdft1"