Free Sialic Acid Storage Disease

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Inheritance

1

Autosomal Recessive

FSASD is inherited in an autosomal recessive pattern caused by biallelic pathogenic variants in SLC17A5.

Show evidence (2 references)

PMID:20301643 SUPPORT Other

"FSASD is inherited in an autosomal recessive manner."

GeneReviews states the inheritance pattern.

PMID:34667062 SUPPORT Human Clinical

"revealed a novel homozygous deletion of exons 8 and 9, inherited from heterozygous carrier parents."

The reported affected newborn with carrier parents illustrates autosomal recessive inheritance.

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Subtypes

3

Infantile Free Sialic Acid Storage Disease MONDO:0010027

Severe infantile FSASD, historically called infantile free sialic acid storage disease or ISSD, is the most severe end of the spectrum and often presents with severe developmental delay, coarse facial features, hepatosplenomegaly, cardiomegaly, and early childhood mortality.

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."

GeneReviews summarizes the severe infantile form of the FSASD spectrum.

Salla Disease MONDO:0011449

Milder FSASD presentation, historically enriched in Finland, with normal appearance at birth followed by slowly progressive neurologic deterioration.

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."

GeneReviews defines the less severe Salla disease end of the spectrum.

Intermediate Severe Salla Disease MONDO:0017737

Intermediate presentation between classic Salla disease and infantile FSASD, with infantile onset and delayed development followed by later neurologic plateau or regression.

Show evidence (1 reference)

PMID:37713976 SUPPORT Human Clinical

"Biallelic pathogenic variants in SLC17A5 cause three forms of free sialic acid storage disease categorized based on severity from least to most severe: Salla disease, intermediate-severe Salla disease, and infantile free sialic acid storage disease."

This longitudinal report explicitly places intermediate-severe Salla disease within the FSASD severity spectrum.

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Pathophysiology

6

SLC17A5 Pathogenic Variation

Biallelic SLC17A5 variants reduce or abolish sialin transporter function across the free sialic acid storage disease spectrum. Genotype contributes to severity: homozygous R39C is associated with milder Salla disease, while other truncating, deletion, missense, and compound-heterozygous genotypes can produce severe infantile or intermediate phenotypes.

Downstream

Sialin Transport Dysfunction Pathogenic variants impair the encoded lysosomal membrane transporter.

Show evidence (2 references)

PMID:20301643 SUPPORT Other

"The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing."

GeneReviews directly links biallelic SLC17A5 pathogenic variants to FSASD diagnosis.

PMID:10947946 SUPPORT Human Clinical

"Our results indicate some genotype-phenotype correlation in free sialic acid-storage diseases, suggesting that the phenotype associated with the homozygote R39C mutation is milder than that associated with other mutations."

This cohort analysis supports genotype-severity correlation in FSASD.

Sialin Transport Dysfunction

Sialin is the lysosomal sialic acid exporter. Pathogenic variants impair proton-coupled sialic acid transport, with recent cryo-EM structures identifying residues and conformational states that explain mutation effects and substrate recognition.

sialic acid transport link ↓ DECREASED

sialic acid transmembrane transporter activity link ↓ DECREASED

lysosome link

Downstream

Lysosomal Free Sialic Acid Accumulation Reduced sialin export traps free sialic acid in lysosomes.

Show evidence (3 references)

PMID:37727271 SUPPORT In Vitro

"Loss or deficiency of sialin impairs FSA transport out of the lysosome, leading to cellular dysfunction and neurological impairment, with the most severe form of FSASD resulting in death during early childhood."

Patient-fibroblast and model-cell work directly supports impaired lysosomal free sialic acid transport as the proximal defect.

PMID:36662855 SUPPORT In Vitro

"By mapping known pathogenic mutations, we provide mechanistic explanations for corresponding functional defects."

Structural and functional analysis supports how pathogenic SLC17A5 variants impair sialin function.

PMID:38782953 SUPPORT In Vitro

"Thus, our findings uncover the essential conformational states in NAAG and sialic acid transport, demonstrating a working model of SLC17 transporters."

Cryo-EM structural work refines the sialin transport model relevant to FSASD mechanisms.

Lysosomal Free Sialic Acid Accumulation

Free N-acetylneuraminic acid accumulates in lysosomes and is measurable in urine, CSF, leukocytes, and fibroblasts. Higher urinary or intracellular free sialic acid burden predicts a more severe disease course.

lysosome link

Experimental models

Patient-derived iPSC neural cell models Patient-derived neural cells recapitulate elevated free sialic acid across differentiated neural cell types.

Downstream

Glycosphingolipid Metabolic Dysregulation Lysosomal sialic acid storage is associated with altered GSL profiles and lysosomal glycohydrolase activity.

White Matter Hypomyelination Lysosomal storage and downstream neural-cell dysfunction contribute to central white matter disease.

Coarse Facial Features Severe infantile lysosomal storage disease is linked to coarse facial features as part of the multisystem FSASD phenotype.

Hepatosplenomegaly Severe infantile lysosomal storage disease is connected to hepatosplenomegaly as part of the multisystem storage phenotype.

Cardiomegaly Severe infantile lysosomal storage disease is connected to cardiomegaly as part of the multisystem FSASD phenotype.

Nonimmune Hydrops Fetalis Severe infantile SLC17A5 loss can present prenatally with hydrops fetalis.

Show evidence (3 references)

PMID:33862140 SUPPORT Human Clinical

"SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid."

The FSASD consortium review summarizes lysosomal and urinary free sialic acid accumulation.

PMID:40529477 SUPPORT Human Clinical

"Individuals with FSASD exhibited significantly higher levels of free Neu5Ac compared to their unaffected biological parents (36-fold), LSD-negative subjects (22-fold), and individuals with other LSDs (49-fold)."

Leukocyte measurements provide human clinical evidence of markedly elevated free Neu5Ac.

PMID:29875421 SUPPORT Human Clinical

"The biochemical phenotype clearly predicted the disease course: patients with a urinary free sialic acid excretion below 6.37-fold or an intracellular free sialic acid storage in fibroblasts below 7.37-fold of the mean of normal survived longer than patients with biochemical values above these..."

The natural-history study supports biochemical burden as a prognostic marker.

Glycosphingolipid Metabolic Dysregulation

Human CSF/plasma and patient-derived iPSC neural-cell studies show secondary glycosphingolipid dysregulation, especially in mature astrocytes, together with altered lysosomal glycohydrolase activity.

Astrocyte link Neuron link

glycosphingolipid metabolic process link ↕ DYSREGULATED

Experimental models

Patient-derived iPSC neural cell models Patient-derived neural cells capture altered GSL abundance, especially in mature astrocytes.

Downstream

White Matter Hypomyelination Altered neural lipid metabolism plausibly contributes to white matter vulnerability.

Neurodevelopmental and Neurodegenerative Dysfunction Neural-cell metabolic dysfunction contributes to developmental delay and progressive neurologic impairment.

Show evidence (2 references)

PMID:39991440 SUPPORT Human Clinical

"In CSF, total GSLs, GM1a, GM3, GD3, GD1a, and GD1b were significantly elevated compared to comparison samples."

Human CSF data support altered glycosphingolipid composition in FSASD.

PMID:40804080 SUPPORT In Vitro

"We observed significant alterations in the abundance of specific GSL species, predominantly in mature astrocytes, with fewer changes in other cell types."

Patient-derived iPSC neural-cell data support cell-type-specific GSL alterations.

White Matter Hypomyelination

Oligodendroglial and broader neural-cell vulnerability produces central hypomyelination, thinning of the corpus callosum, and cerebellar atrophy, which contribute to motor and developmental impairment.

Oligodendrocyte link

central nervous system myelination link ↓ DECREASED

Downstream

Corpus Callosum Hypoplasia Thinning of the corpus callosum is a structural neuroimaging manifestation of the same central white matter disease process.

Cerebellar Atrophy Cerebellar atrophy is a neuroimaging component of FSASD central white matter/CNS involvement.

Global Developmental Delay White matter disease contributes to developmental delay.

Ataxia Cerebellar and white matter involvement contribute to impaired coordination.

Spasticity Central motor pathway involvement contributes to spasticity.

Show evidence (2 references)

PMID:33862140 SUPPORT Human Clinical

"Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."

The FSASD consortium review supports central hypomyelination and associated neuroimaging abnormalities.

PMID:40804080 PARTIAL In Vitro

"While the involvement of oligodendroglia in FSASD pathogenesis is established, the roles of other neural cell types remain elusive."

The iPSC study supports oligodendroglial involvement but focuses experimentally on broader neural-cell models, so the support is partial.

Neurodevelopmental and Neurodegenerative Dysfunction

CNS involvement causes delayed development, cognitive impairment, ataxia, hypotonia, spasticity, athetosis, seizures, and behavioral or psychiatric manifestations.

Neuron link

Downstream

Intellectual Disability Cognitive impairment is a core manifestation of FSASD neurodevelopmental and neurodegenerative dysfunction.

Hypotonia Muscular hypotonia is a neurologic manifestation of FSASD neurodevelopmental involvement.

Nystagmus Nystagmus is connected as part of the Salla disease neurologic phenotype cluster.

Athetosis Athetosis is connected as part of the Salla disease neurologic movement phenotype cluster.

Seizure Epileptic seizures are connected as a downstream manifestation of progressive neurologic disease.

Psychosis Psychosis is modeled as a later neuropsychiatric manifestation in the Salla disease spectrum.

Sleep Disturbance Sleep disturbance is modeled as a psychiatric/behavioral downstream manifestation in Salla disease.

Aggressive Behavior Aggressive behavior or restlessness is modeled as a behavioral downstream manifestation in Salla disease.

Show evidence (2 references)

PMID:33862140 SUPPORT Human Clinical

"Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."

The FSASD consortium review links the disease to progressive neurodegenerative clinical manifestations.

PMID:35796883 SUPPORT Human Clinical

"Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents."

The Salla disease cohort supports neuropsychiatric involvement in the milder FSASD spectrum.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

18

Cardiovascular 2

Hepatosplenomegaly Hepatosplenomegaly (HP:0001433)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."

GeneReviews lists hepatosplenomegaly in severe infantile FSASD.

Cardiomegaly Cardiomegaly (HP:0001640)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."

GeneReviews lists cardiomegaly in severe infantile FSASD.

Eye 1

Nystagmus Nystagmus (HP:0000639)

Show evidence (1 reference)

PMID:35796883 SUPPORT Human Clinical

"Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination."

Salla disease cohort paper lists nystagmus among characteristic features.

Head and Neck 1

Coarse Facial Features Coarse facial features (HP:0000280)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."

GeneReviews lists coarse facial features in severe infantile FSASD.

Metabolism 1

Nonimmune Hydrops Fetalis Nonimmune hydrops fetalis (HP:0001790)

Show evidence (1 reference)

PMID:34667062 SUPPORT Human Clinical

"We describe a case of fetal hydrops due to a novel homozygous deletion in the SLC17A5 gene."

The case report directly supports fetal hydrops as an infantile FSASD presentation.

Musculoskeletal 2

Hypotonia Hypotonia (HP:0001252)

Show evidence (2 references)

PMID:33862140 SUPPORT Human Clinical

"Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."

The consortium review lists muscular hypotonia among FSASD clinical features.

PMID:28662915 SUPPORT Human Clinical

"PATIENT DESCRIPTION: This five-year-old girl presented with infantile-onset severe global developmental delay, truncal hypotonia, and generalized dystonia following normal development during her first six months of life."

The intermediate-severe case provides patient-level evidence of infantile-onset hypotonia.

Spasticity Spasticity (HP:0001257)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."

GeneReviews lists spasticity as part of less severe FSASD.

Nervous System 10

Global Developmental Delay Global developmental delay (HP:0001263)

Show evidence (2 references)

PMID:28662915 SUPPORT Human Clinical

"Likely pathogenic variants in SLC17A5 results in allelic disorders of free sialic acid metabolism including (1) infantile free sialic acid storage disease with severe global developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; (2) intermediate severe Salla disease..."

The case report and review supports developmental delay across infantile and intermediate-severe FSASD.

PMID:37713976 SUPPORT Human Clinical

"Disease onset occurred within the first six months of life in both patients. Early childhood development was delayed"

The longitudinal intermediate-severe report supports early developmental delay.

Intellectual Disability Intellectual disability (HP:0001249)

Show evidence (1 reference)

PMID:35796883 SUPPORT Human Clinical

"Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination."

Salla disease cohort paper lists intellectual disability among characteristic features.

Ataxia Ataxia (HP:0001251)

Show evidence (1 reference)

PMID:33862140 SUPPORT Human Clinical

"Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."

The consortium review lists cerebellar ataxia among FSASD manifestations.

Seizure Seizure (HP:0001250)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."

GeneReviews lists epileptic seizures as part of less severe FSASD.

White Matter Hypomyelination Cerebral hypomyelination (HP:0006808)

Show evidence (1 reference)

PMID:33862140 SUPPORT Human Clinical

"Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."

The consortium review lists central hypomyelination as a prominent disease feature.

Corpus Callosum Hypoplasia Hypoplasia of the corpus callosum (HP:0002079)

Show evidence (1 reference)

PMID:33862140 SUPPORT Human Clinical

"Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."

The consortium review lists thinning of the corpus callosum as a prominent feature.

Cerebellar Atrophy Cerebellar atrophy (HP:0001272)

Show evidence (1 reference)

PMID:33862140 SUPPORT Human Clinical

"Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."

The consortium review lists cerebellar atrophy as a prominent disease feature.

Psychosis Psychosis (HP:0000709)

Show evidence (1 reference)

PMID:35796883 SUPPORT Human Clinical

"Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents."

The Salla disease cohort supports psychosis as part of the phenotypic spectrum.

Sleep Disturbance Sleep disturbance (HP:0002360)

Show evidence (1 reference)

PMID:35796883 SUPPORT Human Clinical

"Other psychiatric factors associated with SD were sleeping disorders (8/24, 32%)"

The Salla disease cohort reports sleep disorders in 32% of reviewed patients.

Aggressive Behavior Aggressive behavior (HP:0000718)

Show evidence (1 reference)

PMID:35796883 SUPPORT Human Clinical

"aggressive behaviour disorders or restlessness (6/24, 25%)"

The Salla disease cohort reports aggressive behavior or restlessness in 25% of reviewed patients.

Other 1

Athetosis Athetosis (HP:0002305)

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."

GeneReviews lists athetosis as part of less severe FSASD.

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Genetic Associations

1

SLC17A5 (Causative)

Show evidence (2 references)

PMID:20301643 SUPPORT Other

"The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing."

GeneReviews supports SLC17A5 as the causative gene for FSASD.

PMID:10947946 SUPPORT Human Clinical

"Ten different mutations, including deletions, insertions, and missense and nonsense mutations, were identified in patients with the most severe ISSD phenotype, most of whom were compound heterozygotes."

Variant-series evidence supports multiple SLC17A5 variant classes and severe genotype associations.

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Treatments

3

Supportive Care

Action: supportive care MAXO:0000950

Current care is symptomatic and supportive, including seizure management, developmental and educational support, rehabilitation, nutrition and feeding support, ophthalmologic care, cardiology and nephrology surveillance when needed, and family support.

Mechanism Target:

Neurodevelopmental and Neurodegenerative Dysfunction — Supportive care manages seizures, developmental impairment, rehabilitation needs, and neurobehavioral manifestations downstream of CNS dysfunction.

White Matter Hypomyelination — Rehabilitation and developmental supports address motor and functional consequences of central white matter disease.

Lysosomal Free Sialic Acid Accumulation — Supportive care is used because no approved therapy currently reverses the underlying lysosomal storage burden.

Show evidence (2 references)

PMID:20301643 SUPPORT Other

"Treatment of manifestations: Management is symptomatic and supportive: standard treatment of seizures; developmental and educational support; rehabilitation to optimize mobility; supplementation of calcium and vitamin D for low bone density; feeding therapy and provision of adequate nutrition;..."

GeneReviews gives the current management strategy.

PMID:33862140 SUPPORT Human Clinical

"There is no approved therapy for FSASD."

The consortium review supports the absence of approved disease-modifying therapy.

Genetic Counseling

Action: genetic counseling MAXO:0000079

Carrier testing and prenatal or preimplantation testing are relevant once familial SLC17A5 variants are known.

Mechanism Target:

SLC17A5 Pathogenic Variation — Counseling and reproductive testing address familial biallelic SLC17A5 pathogenic variants.

Show evidence (1 reference)

PMID:20301643 SUPPORT Other

"Once the SLC17A5 pathogenic variants have been identified in an affected family member, molecular genetic carrier testing and prenatal/preimplantation genetic testing are possible."

GeneReviews supports reproductive genetic counseling and testing once familial variants are known.

Base Editing Gene Therapy

Action: gene therapy MAXO:0001001

Investigational adenine base editing has corrected the common SLC17A5 c.115C>T variant and reduced free sialic acid in patient-derived fibroblasts and mouse embryonic fibroblasts, but it is not an approved therapy.

Mechanism Target:

RESTORES SLC17A5 Pathogenic Variation — Adenine base editing is designed to correct the SLC17A5 c.115C>T founder variant.

Show evidence (2 references)

PMID:37727271 PARTIAL In Vitro

"Furthermore, ABE treatment of either homozygous or compound heterozygous SLC17A5 c.115C>T human dermal fibroblasts demonstrated significant FSA reduction, supporting amelioration of disease pathology."

The evidence supports preclinical correction and biochemical improvement, but not clinical efficacy in patients.

PMID:37727271 PARTIAL In Vitro

"Our study demonstrates the feasibility of base editing as a therapeutic approach for the FSASD variant SLC17A5 c.115C>T and highlights the usefulness of base editing in monogenic diseases where transmembrane protein function is impaired."

The authors frame base editing as a feasible therapeutic approach in preclinical models, so support remains partial.

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Biochemical Markers

3

Free Sialic Acid (Elevated)

Pathograph Readouts

Readout Of Lysosomal Free Sialic Acid Accumulation Positive Diagnostic

Elevated free sialic acid reports the upstream lysosomal N-acetylneuraminic acid storage burden.

Show evidence (2 references)

PMID:20101035 SUPPORT Human Clinical

"We found a significant increase of free sialic acid in CSF or urine in 6 of 41 patients presenting with hypomyelination of unknown etiology."

CSF/urine testing in a leukodystrophy cohort supports increased free sialic acid as a diagnostic biochemical feature.

PMID:40529477 SUPPORT Human Clinical

"Individuals with FSASD exhibited significantly higher levels of free Neu5Ac compared to their unaffected biological parents (36-fold), LSD-negative subjects (22-fold), and individuals with other LSDs (49-fold)."

Leukocyte data support elevated free Neu5Ac as a minimally invasive biomarker.

N-acetylaspartylglutamate (Elevated)

Pathograph Readouts

Readout Of Sialin Transport Dysfunction Positive Diagnostic

Elevated CSF NAAG supports SLC17A5-related sialin transport dysfunction in hypomyelinating disease.

Show evidence (1 reference)

PMID:20101035 SUPPORT Human Clinical

"H-NMRS revealed an increase of N-acetylaspartylglutamate in the CSF of all patients with SLC17A5 mutation (range 13-114 micromol/L, reference <12 micromol/L)."

The leukodystrophy cohort supports CSF NAAG elevation in SLC17A5-mutated FSASD patients.

Glycosphingolipids (Altered)

Pathograph Readouts

Readout Of Glycosphingolipid Metabolic Dysregulation Positive Diagnostic

Altered glycosphingolipid levels report secondary GSL metabolic dysregulation in FSASD.

Show evidence (1 reference)

PMID:39991440 SUPPORT Human Clinical

"These results reveal dysregulated GSL metabolism and suggest the potential of gangliosides as biomarkers."

Human CSF/plasma data support GSL metabolism as an altered biochemical domain and possible biomarker class.

🧫

Experimental Models

1

Patient-derived iPSC neural cell models

iPSC-derived radial glial cells, immature and mature astrocytes, and cortical neurons from individuals with FSASD model free sialic acid accumulation and neural-cell GSL dysregulation.

Pathograph links

Lysosomal Free Sialic Acid Accumulation Patient-derived neural cells recapitulate elevated free sialic acid across differentiated neural cell types.

Show evidence (1 reference)

PMID:40804080 SUPPORT In Vitro

"Our findings revealed significant elevations in free sialic acid levels across all FSASD cell types, indicating that iPSCs and derived iRGCs, iIAs, iMAs and iCNs may be used to model FSASD in vitro."

The iPSC study shows the model recapitulates the free sialic acid storage phenotype.

Glycosphingolipid Metabolic Dysregulation Patient-derived neural cells capture altered GSL abundance, especially in mature astrocytes.

Show evidence (1 reference)

PMID:40804080 SUPPORT In Vitro

"We observed significant alterations in the abundance of specific GSL species, predominantly in mature astrocytes, with fewer changes in other cell types."

The iPSC study shows the model captures neural-cell GSL dysregulation.

Show evidence (1 reference)

PMID:40804080 SUPPORT In Vitro

"Our findings revealed significant elevations in free sialic acid levels across all FSASD cell types, indicating that iPSCs and derived iRGCs, iIAs, iMAs and iCNs may be used to model FSASD in vitro."

The iPSC study explicitly supports these neural-cell types as FSASD in vitro models.

{ }

Source YAML

click to show

name: Free Sialic Acid Storage Disease
creation_date: "2026-05-10T16:49:07Z"
updated_date: "2026-05-18T10:32:18Z"
category: Mendelian
description: >
  Free sialic acid storage disease (FSASD) is an autosomal recessive lysosomal
  storage disease spectrum caused by SLC17A5 pathogenic variation. Impaired
  sialin-mediated lysosomal export causes free N-acetylneuraminic acid
  accumulation and a neurodevelopmental/neurodegenerative phenotype ranging
  from severe infantile FSASD to milder Salla disease.
synonyms:
- Free SASD
- FSASD
- Free Sialic Acid Storage Disorder
- Lysosomal Free Sialic Acid Storage Disorder
- Sialic Acid Storage Disease
disease_term:
  preferred_term: free sialic acid storage disease
  term:
    id: MONDO:0019366
    label: free sialic acid storage disease
references:
- reference: PMID:20301643
  title: "Free Sialic Acid Storage Disorder."
  tags:
  - GeneReviews
  findings: []
parents:
- Inborn Disorder Of Lysosomal Amino Acid Transport
- Lysosomal Storage Disease
has_subtypes:
- name: Infantile Free Sialic Acid Storage Disease
  subtype_term:
    preferred_term: infantile free sialic acid storage disease
    term:
      id: MONDO:0010027
      label: free sialic acid storage disease, infantile form
  description: >
    Severe infantile FSASD, historically called infantile free sialic acid
    storage disease or ISSD, is the most severe end of the spectrum and often
    presents with severe developmental delay, coarse facial features,
    hepatosplenomegaly, cardiomegaly, and early childhood mortality.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
    explanation: GeneReviews summarizes the severe infantile form of the FSASD spectrum.
- name: Salla Disease
  subtype_term:
    preferred_term: Salla disease
    term:
      id: MONDO:0011449
      label: Salla disease
  description: >
    Milder FSASD presentation, historically enriched in Finland, with normal
    appearance at birth followed by slowly progressive neurologic deterioration.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
    explanation: GeneReviews defines the less severe Salla disease end of the spectrum.
- name: Intermediate Severe Salla Disease
  subtype_term:
    preferred_term: intermediate severe Salla disease
    term:
      id: MONDO:0017737
      label: intermediate severe Salla disease
  description: >
    Intermediate presentation between classic Salla disease and infantile FSASD,
    with infantile onset and delayed development followed by later neurologic
    plateau or regression.
  evidence:
  - reference: PMID:37713976
    reference_title: "Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biallelic pathogenic variants in SLC17A5 cause three forms of free sialic acid storage disease categorized based on severity from least to most severe: Salla disease, intermediate-severe Salla disease, and infantile free sialic acid storage disease."
    explanation: This longitudinal report explicitly places intermediate-severe Salla disease within the FSASD severity spectrum.
prevalence:
- population: Global
  percentage: Ultra-rare
  notes: >
    FSASD is extremely rare, likely underdiagnosed, and has panethnic
    distribution. A natural-history synthesis found substantial diagnostic
    delay and median survival of 11 years across published postnatal cases.
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Around 200 FSASD cases are reported worldwide, with the clinical spectrum ranging from a severe infantile onset form, often lethal in early childhood, to a mild, less severe form with subjects living into adulthood, also called Salla disease."
    explanation: The consortium review supports global rarity and the broad clinical spectrum.
  - reference: PMID:29875421
    reference_title: "A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease-an ultra-orphan multisystemic lysosomal storage disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Median age at disease onset was 0.17 years. Median age at diagnosis was 3 years with a median diagnostic delay of 2.5 years. Median survival was 11 years."
    explanation: The natural-history study quantifies age at onset, diagnostic delay, and survival.
- population: Finland
  percentage: Rare; founder-enriched
  notes: >
    Salla disease is part of the Finnish disease heritage. The p.Arg39Cys
    founder variant accounts for most Finnish Salla disease cases, while other
    variant classes are often associated with more severe phenotypes.
  evidence:
  - reference: PMID:10947946
    reference_title: "The spectrum of SLC17A5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All Finnish patients with SD (n=80) had a missense mutation changing a highly conserved arginine to cysteine (R39C); 91% of them were homozygotes for this old founder mutation."
    explanation: This genotype-phenotype study documents the Finnish founder variant distribution.
progression:
- phase: Infantile and Severe FSASD
  age_range: Prenatal to early childhood
  notes: >
    Severe infantile FSASD can present prenatally with nonimmune hydrops fetalis
    or in infancy with severe neurodevelopmental and multisystem disease.
  evidence:
  - reference: PMID:34667062
    reference_title: "Prenatal hydrops fetalis associated with infantile free sialic acid storage disease due to a novel homozygous deletion in the SLC17A5 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Infantile free sialic acid storage disease (ISSD) (OMIM #269920) is a severe form of autosomal recessive sialic acid storage disease."
    explanation: This prenatal case report supports severe infantile FSASD as an autosomal recessive disease that can present with fetal hydrops.
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
    explanation: GeneReviews supports early severe multisystem disease and shortened survival in infantile FSASD.
- phase: Less Severe FSASD / Salla Disease
  age_range: Infancy to adulthood
  notes: >
    Milder FSASD often lacks neonatal abnormalities but evolves through
    psychomotor delay, spasticity, movement disorder, seizures, and possible
    psychiatric features, with survival into adulthood in some individuals.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
    explanation: GeneReviews describes the typical less severe disease course.
  - reference: PMID:37713976
    reference_title: "Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Disease onset occurred within the first six months of life in both patients. Early childhood development was delayed with achievement of some milestones followed by a developmental plateau in late childhood. After this, both patients began a slow and progressive neurological regression in adolescence."
    explanation: Longitudinal follow-up supports delayed development followed by plateau and adolescent regression in intermediate-severe disease.
inheritance:
- name: Autosomal Recessive
  description: >
    FSASD is inherited in an autosomal recessive pattern caused by biallelic
    pathogenic variants in SLC17A5.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "FSASD is inherited in an autosomal recessive manner."
    explanation: GeneReviews states the inheritance pattern.
  - reference: PMID:34667062
    reference_title: "Prenatal hydrops fetalis associated with infantile free sialic acid storage disease due to a novel homozygous deletion in the SLC17A5 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "revealed a novel homozygous deletion of exons 8 and 9, inherited from heterozygous carrier parents."
    explanation: The reported affected newborn with carrier parents illustrates autosomal recessive inheritance.
pathophysiology:
- name: SLC17A5 Pathogenic Variation
  description: >
    Biallelic SLC17A5 variants reduce or abolish sialin transporter function
    across the free sialic acid storage disease spectrum. Genotype contributes
    to severity: homozygous R39C is associated with milder Salla disease, while
    other truncating, deletion, missense, and compound-heterozygous genotypes
    can produce severe infantile or intermediate phenotypes.
  gene:
    preferred_term: SLC17A5
    term:
      id: hgnc:10933
      label: SLC17A5
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing."
    explanation: GeneReviews directly links biallelic SLC17A5 pathogenic variants to FSASD diagnosis.
  - reference: PMID:10947946
    reference_title: "The spectrum of SLC17A5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our results indicate some genotype-phenotype correlation in free sialic acid-storage diseases, suggesting that the phenotype associated with the homozygote R39C mutation is milder than that associated with other mutations."
    explanation: This cohort analysis supports genotype-severity correlation in FSASD.
  downstream:
  - target: Sialin Transport Dysfunction
    causal_link_type: DIRECT
    description: Pathogenic variants impair the encoded lysosomal membrane transporter.
- name: Sialin Transport Dysfunction
  description: >
    Sialin is the lysosomal sialic acid exporter. Pathogenic variants impair
    proton-coupled sialic acid transport, with recent cryo-EM structures
    identifying residues and conformational states that explain mutation effects
    and substrate recognition.
  molecular_functions:
  - preferred_term: sialic acid transmembrane transporter activity
    term:
      id: GO:0015136
      label: sialic acid transmembrane transporter activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: sialic acid transport
    term:
      id: GO:0015739
      label: sialic acid transport
    modifier: DECREASED
  cellular_components:
  - preferred_term: lysosome
    term:
      id: GO:0005764
      label: lysosome
  evidence:
  - reference: PMID:37727271
    reference_title: "Base editing corrects the common Salla disease SLC17A5 c.115C>T variant."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Loss or deficiency of sialin impairs FSA transport out of the lysosome, leading to cellular dysfunction and neurological impairment, with the most severe form of FSASD resulting in death during early childhood."
    explanation: Patient-fibroblast and model-cell work directly supports impaired lysosomal free sialic acid transport as the proximal defect.
  - reference: PMID:36662855
    reference_title: "The molecular mechanism of sialic acid transport mediated by Sialin."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "By mapping known pathogenic mutations, we provide mechanistic explanations for corresponding functional defects."
    explanation: Structural and functional analysis supports how pathogenic SLC17A5 variants impair sialin function.
  - reference: PMID:38782953
    reference_title: "Structure and inhibition of the human lysosomal transporter Sialin."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Thus, our findings uncover the essential conformational states in NAAG and sialic acid transport, demonstrating a working model of SLC17 transporters."
    explanation: Cryo-EM structural work refines the sialin transport model relevant to FSASD mechanisms.
  downstream:
  - target: Lysosomal Free Sialic Acid Accumulation
    causal_link_type: DIRECT
    description: Reduced sialin export traps free sialic acid in lysosomes.
- name: Lysosomal Free Sialic Acid Accumulation
  description: >
    Free N-acetylneuraminic acid accumulates in lysosomes and is measurable in
    urine, CSF, leukocytes, and fibroblasts. Higher urinary or intracellular
    free sialic acid burden predicts a more severe disease course.
  chemical_entities:
  - preferred_term: N-acetylneuraminic acid
    modifier: INCREASED
    term:
      id: CHEBI:17012
      label: N-acetylneuraminic acid
  cellular_components:
  - preferred_term: lysosome
    term:
      id: GO:0005764
      label: lysosome
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid."
    explanation: The FSASD consortium review summarizes lysosomal and urinary free sialic acid accumulation.
  - reference: PMID:40529477
    reference_title: "Investigating the Utility of Leukocyte Sialic Acid Measurements in Lysosomal Free Sialic Acid Storage Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with FSASD exhibited significantly higher levels of free Neu5Ac compared to their unaffected biological parents (36-fold), LSD-negative subjects (22-fold), and individuals with other LSDs (49-fold)."
    explanation: Leukocyte measurements provide human clinical evidence of markedly elevated free Neu5Ac.
  - reference: PMID:29875421
    reference_title: "A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease-an ultra-orphan multisystemic lysosomal storage disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The biochemical phenotype clearly predicted the disease course: patients with a urinary free sialic acid excretion below 6.37-fold or an intracellular free sialic acid storage in fibroblasts below 7.37-fold of the mean of normal survived longer than patients with biochemical values above these thresholds."
    explanation: The natural-history study supports biochemical burden as a prognostic marker.
  downstream:
  - target: Glycosphingolipid Metabolic Dysregulation
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Lysosomal sialic acid storage is associated with altered GSL profiles and lysosomal glycohydrolase activity.
  - target: White Matter Hypomyelination
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Lysosomal storage and downstream neural-cell dysfunction contribute to central white matter disease.
  - target: Coarse Facial Features
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Severe infantile lysosomal storage disease is linked to coarse facial features as part of the multisystem FSASD phenotype.
    evidence:
    - reference: PMID:20301643
      reference_title: Free Sialic Acid Storage Disorder.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
      explanation: GeneReviews lists coarse facial features in the severe infantile FSASD phenotype.
  - target: Hepatosplenomegaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Severe infantile lysosomal storage disease is connected to hepatosplenomegaly as part of the multisystem storage phenotype.
    evidence:
    - reference: PMID:20301643
      reference_title: Free Sialic Acid Storage Disorder.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
      explanation: GeneReviews lists hepatosplenomegaly in the severe infantile FSASD phenotype.
  - target: Cardiomegaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Severe infantile lysosomal storage disease is connected to cardiomegaly as part of the multisystem FSASD phenotype.
    evidence:
    - reference: PMID:20301643
      reference_title: Free Sialic Acid Storage Disorder.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
      explanation: GeneReviews lists cardiomegaly in the severe infantile FSASD phenotype.
  - target: Nonimmune Hydrops Fetalis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Severe infantile SLC17A5 loss can present prenatally with hydrops fetalis.
    evidence:
    - reference: PMID:34667062
      reference_title: "Prenatal hydrops fetalis associated with infantile free sialic acid storage disease due to a novel homozygous deletion in the SLC17A5 gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We describe a case of fetal hydrops due to a novel homozygous deletion in the SLC17A5 gene."
      explanation: This prenatal case report connects severe SLC17A5-related FSASD to fetal hydrops.
- name: Glycosphingolipid Metabolic Dysregulation
  description: >
    Human CSF/plasma and patient-derived iPSC neural-cell studies show secondary
    glycosphingolipid dysregulation, especially in mature astrocytes, together
    with altered lysosomal glycohydrolase activity.
  cell_types:
  - preferred_term: Astrocyte
    term:
      id: CL:0000127
      label: astrocyte
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: glycosphingolipid metabolic process
    term:
      id: GO:0006687
      label: glycosphingolipid metabolic process
    modifier: DYSREGULATED
  chemical_entities:
  - preferred_term: glycosphingolipid
    term:
      id: CHEBI:24402
      label: glycosphingolipid
    modifier: INCREASED
  evidence:
  - reference: PMID:39991440
    reference_title: "Changes in glycosphingolipid levels in plasma and cerebrospinal fluid of individuals with Lysosomal Free Sialic Acid Storage Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In CSF, total GSLs, GM1a, GM3, GD3, GD1a, and GD1b were significantly elevated compared to comparison samples."
    explanation: Human CSF data support altered glycosphingolipid composition in FSASD.
  - reference: PMID:40804080
    reference_title: "Lysosomal free sialic acid storage disorder iPSC-derived neural cells display altered glycosphingolipid metabolism."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We observed significant alterations in the abundance of specific GSL species, predominantly in mature astrocytes, with fewer changes in other cell types."
    explanation: Patient-derived iPSC neural-cell data support cell-type-specific GSL alterations.
  downstream:
  - target: White Matter Hypomyelination
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Altered neural lipid metabolism plausibly contributes to white matter vulnerability.
  - target: Neurodevelopmental and Neurodegenerative Dysfunction
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Neural-cell metabolic dysfunction contributes to developmental delay and progressive neurologic impairment.
- name: White Matter Hypomyelination
  description: >
    Oligodendroglial and broader neural-cell vulnerability produces central
    hypomyelination, thinning of the corpus callosum, and cerebellar atrophy,
    which contribute to motor and developmental impairment.
  cell_types:
  - preferred_term: Oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  biological_processes:
  - preferred_term: central nervous system myelination
    term:
      id: GO:0022010
      label: central nervous system myelination
    modifier: DECREASED
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
    explanation: The FSASD consortium review supports central hypomyelination and associated neuroimaging abnormalities.
  - reference: PMID:40804080
    reference_title: "Lysosomal free sialic acid storage disorder iPSC-derived neural cells display altered glycosphingolipid metabolism."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "While the involvement of oligodendroglia in FSASD pathogenesis is established, the roles of other neural cell types remain elusive."
    explanation: The iPSC study supports oligodendroglial involvement but focuses experimentally on broader neural-cell models, so the support is partial.
  downstream:
  - target: Corpus Callosum Hypoplasia
    causal_link_type: DIRECT
    description: Thinning of the corpus callosum is a structural neuroimaging manifestation of the same central white matter disease process.
    evidence:
    - reference: PMID:33862140
      reference_title: "Free sialic acid storage disorder: Progress and promise."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
      explanation: The consortium review directly links central hypomyelination with corpus callosum thinning.
  - target: Cerebellar Atrophy
    causal_link_type: DIRECT
    description: Cerebellar atrophy is a neuroimaging component of FSASD central white matter/CNS involvement.
    evidence:
    - reference: PMID:33862140
      reference_title: "Free sialic acid storage disorder: Progress and promise."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
      explanation: The consortium review directly links central hypomyelination with cerebellar atrophy.
  - target: Global Developmental Delay
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: White matter disease contributes to developmental delay.
  - target: Ataxia
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Cerebellar and white matter involvement contribute to impaired coordination.
  - target: Spasticity
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Central motor pathway involvement contributes to spasticity.
- name: Neurodevelopmental and Neurodegenerative Dysfunction
  description: >
    CNS involvement causes delayed development, cognitive impairment, ataxia,
    hypotonia, spasticity, athetosis, seizures, and behavioral or psychiatric
    manifestations.
  cell_types:
  - preferred_term: Neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."
    explanation: The FSASD consortium review links the disease to progressive neurodegenerative clinical manifestations.
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents."
    explanation: The Salla disease cohort supports neuropsychiatric involvement in the milder FSASD spectrum.
  downstream:
  - target: Intellectual Disability
    causal_link_type: DIRECT
    description: Cognitive impairment is a core manifestation of FSASD neurodevelopmental and neurodegenerative dysfunction.
    evidence:
    - reference: PMID:33862140
      reference_title: "Free sialic acid storage disorder: Progress and promise."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."
      explanation: The consortium review lists cognitive impairment as part of the progressive neurodegenerative FSASD phenotype.
  - target: Hypotonia
    causal_link_type: DIRECT
    description: Muscular hypotonia is a neurologic manifestation of FSASD neurodevelopmental involvement.
    evidence:
    - reference: PMID:33862140
      reference_title: "Free sialic acid storage disorder: Progress and promise."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."
      explanation: The consortium review lists muscular hypotonia as a clinical feature of FSASD.
  - target: Nystagmus
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Nystagmus is connected as part of the Salla disease neurologic phenotype cluster.
    evidence:
    - reference: PMID:35796883
      reference_title: "Psychiatric symptoms in Salla disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination."
      explanation: The Salla disease cohort paper lists nystagmus among characteristic neurologic features.
  - target: Athetosis
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Athetosis is connected as part of the Salla disease neurologic movement phenotype cluster.
    evidence:
    - reference: PMID:20301643
      reference_title: Free Sialic Acid Storage Disorder.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
      explanation: GeneReviews lists athetosis as part of less severe FSASD.
  - target: Seizure
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: Epileptic seizures are connected as a downstream manifestation of progressive neurologic disease.
    evidence:
    - reference: PMID:20301643
      reference_title: Free Sialic Acid Storage Disorder.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
      explanation: GeneReviews lists epileptic seizures as part of less severe FSASD.
  - target: Psychosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Psychosis is modeled as a later neuropsychiatric manifestation in the Salla disease spectrum.
    evidence:
    - reference: PMID:35796883
      reference_title: "Psychiatric symptoms in Salla disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents."
      explanation: The Salla disease cohort reports psychosis in adolescent patients.
  - target: Sleep Disturbance
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Sleep disturbance is modeled as a psychiatric/behavioral downstream manifestation in Salla disease.
    evidence:
    - reference: PMID:35796883
      reference_title: "Psychiatric symptoms in Salla disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Other psychiatric factors associated with SD were sleeping disorders (8/24, 32%)"
      explanation: The Salla disease cohort reports sleep disorders in 32% of reviewed patients.
  - target: Aggressive Behavior
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Aggressive behavior or restlessness is modeled as a behavioral downstream manifestation in Salla disease.
    evidence:
    - reference: PMID:35796883
      reference_title: "Psychiatric symptoms in Salla disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "aggressive behaviour disorders or restlessness (6/24, 25%)"
      explanation: The Salla disease cohort reports aggressive behavior or restlessness in 25% of reviewed patients.
phenotypes:
- name: Global Developmental Delay
  description: >
    Developmental delay is a core presentation across the FSASD spectrum and can
    be severe in infantile disease.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:28662915
    reference_title: "A New Patient With Intermediate Severe Salla Disease With Hypomyelination: A Literature Review for Salla Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Likely pathogenic variants in SLC17A5 results in allelic disorders of free sialic acid metabolism including (1) infantile free sialic acid storage disease with severe global developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; (2) intermediate severe Salla disease with moderate to severe global developmental delay, hypotonia, and hypomyelination with or without coarse facial features, and (3) Salla disease with normal appearance, mild cognitive dysfunction, and spasticity."
    explanation: The case report and review supports developmental delay across infantile and intermediate-severe FSASD.
  - reference: PMID:37713976
    reference_title: "Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Disease onset occurred within the first six months of life in both patients. Early childhood development was delayed"
    explanation: The longitudinal intermediate-severe report supports early developmental delay.
- name: Intellectual Disability
  subtype: Salla Disease
  description: Cognitive impairment is common in the Salla disease end of the FSASD spectrum.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination."
    explanation: Salla disease cohort paper lists intellectual disability among characteristic features.
- name: Hypotonia
  description: Muscular hypotonia is part of both infantile and intermediate/severe presentations.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."
    explanation: The consortium review lists muscular hypotonia among FSASD clinical features.
  - reference: PMID:28662915
    reference_title: "A New Patient With Intermediate Severe Salla Disease With Hypomyelination: A Literature Review for Salla Disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PATIENT DESCRIPTION: This five-year-old girl presented with infantile-onset severe global developmental delay, truncal hypotonia, and generalized dystonia following normal development during her first six months of life."
    explanation: The intermediate-severe case provides patient-level evidence of infantile-onset hypotonia.
- name: Ataxia
  subtype: Salla Disease
  description: Cerebellar ataxia contributes to motor impairment in FSASD, especially Salla disease.
  phenotype_term:
    preferred_term: Ataxia
    term:
      id: HP:0001251
      label: Ataxia
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia."
    explanation: The consortium review lists cerebellar ataxia among FSASD manifestations.
- name: Nystagmus
  subtype: Salla Disease
  description: Nystagmus is a characteristic neurologic feature of Salla disease.
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination."
    explanation: Salla disease cohort paper lists nystagmus among characteristic features.
- name: Spasticity
  subtype: Salla Disease
  description: Slowly progressive neurologic deterioration can lead to limb spasticity.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
    explanation: GeneReviews lists spasticity as part of less severe FSASD.
- name: Athetosis
  subtype: Salla Disease
  description: Athetotic movements are a characteristic movement phenotype in Salla disease.
  phenotype_term:
    preferred_term: Athetosis
    term:
      id: HP:0002305
      label: Athetosis
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
    explanation: GeneReviews lists athetosis as part of less severe FSASD.
- name: Seizure
  subtype: Salla Disease
  description: Epileptic seizures can occur with progressive neurologic disease.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures."
    explanation: GeneReviews lists epileptic seizures as part of less severe FSASD.
- name: White Matter Hypomyelination
  description: Brain MRI often shows central hypomyelination and related white matter abnormalities.
  phenotype_term:
    preferred_term: Cerebral hypomyelination
    term:
      id: HP:0006808
      label: Cerebral hypomyelination
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
    explanation: The consortium review lists central hypomyelination as a prominent disease feature.
- name: Corpus Callosum Hypoplasia
  description: Thinning or hypoplasia of the corpus callosum is a recurrent neuroimaging feature.
  phenotype_term:
    preferred_term: Hypoplasia of the corpus callosum
    term:
      id: HP:0002079
      label: Hypoplasia of the corpus callosum
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
    explanation: The consortium review lists thinning of the corpus callosum as a prominent feature.
- name: Cerebellar Atrophy
  description: Cerebellar atrophy contributes to ataxia and progressive neurologic impairment.
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
    explanation: The consortium review lists cerebellar atrophy as a prominent disease feature.
- name: Coarse Facial Features
  subtype: Infantile Free Sialic Acid Storage Disease
  description: Coarse facial features are most prominent in severe infantile FSASD.
  phenotype_term:
    preferred_term: Coarse facial features
    term:
      id: HP:0000280
      label: Coarse facial features
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
    explanation: GeneReviews lists coarse facial features in severe infantile FSASD.
- name: Hepatosplenomegaly
  subtype: Infantile Free Sialic Acid Storage Disease
  description: Hepatosplenomegaly is a severe infantile FSASD manifestation.
  phenotype_term:
    preferred_term: Hepatosplenomegaly
    term:
      id: HP:0001433
      label: Hepatosplenomegaly
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
    explanation: GeneReviews lists hepatosplenomegaly in severe infantile FSASD.
- name: Cardiomegaly
  subtype: Infantile Free Sialic Acid Storage Disease
  description: Cardiomegaly can occur in severe infantile FSASD.
  phenotype_term:
    preferred_term: Cardiomegaly
    term:
      id: HP:0001640
      label: Cardiomegaly
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood."
    explanation: GeneReviews lists cardiomegaly in severe infantile FSASD.
- name: Nonimmune Hydrops Fetalis
  subtype: Infantile Free Sialic Acid Storage Disease
  description: Nonimmune hydrops fetalis is a prenatal presentation of severe infantile FSASD.
  phenotype_term:
    preferred_term: Nonimmune hydrops fetalis
    term:
      id: HP:0001790
      label: Nonimmune hydrops fetalis
  evidence:
  - reference: PMID:34667062
    reference_title: "Prenatal hydrops fetalis associated with infantile free sialic acid storage disease due to a novel homozygous deletion in the SLC17A5 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe a case of fetal hydrops due to a novel homozygous deletion in the SLC17A5 gene."
    explanation: The case report directly supports fetal hydrops as an infantile FSASD presentation.
- name: Psychosis
  subtype: Salla Disease
  description: Psychotic symptoms have been reported in adolescent Salla disease patients.
  phenotype_term:
    preferred_term: Psychosis
    term:
      id: HP:0000709
      label: Psychosis
  evidence:
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents."
    explanation: The Salla disease cohort supports psychosis as part of the phenotypic spectrum.
- name: Sleep Disturbance
  subtype: Salla Disease
  description: Sleep disorders are relatively common psychiatric/behavioral manifestations in Salla disease.
  phenotype_term:
    preferred_term: Sleep disturbance
    term:
      id: HP:0002360
      label: Sleep disturbance
  evidence:
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other psychiatric factors associated with SD were sleeping disorders (8/24, 32%)"
    explanation: The Salla disease cohort reports sleep disorders in 32% of reviewed patients.
- name: Aggressive Behavior
  subtype: Salla Disease
  description: Aggressive behavior or restlessness can occur as a behavioral manifestation in Salla disease.
  phenotype_term:
    preferred_term: Aggressive behavior
    term:
      id: HP:0000718
      label: Aggressive behavior
  evidence:
  - reference: PMID:35796883
    reference_title: "Psychiatric symptoms in Salla disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "aggressive behaviour disorders or restlessness (6/24, 25%)"
    explanation: The Salla disease cohort reports aggressive behavior or restlessness in 25% of reviewed patients.
genetic:
- name: SLC17A5
  gene_term:
    preferred_term: SLC17A5
    term:
      id: hgnc:10933
      label: SLC17A5
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing."
    explanation: GeneReviews supports SLC17A5 as the causative gene for FSASD.
  - reference: PMID:10947946
    reference_title: "The spectrum of SLC17A5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten different mutations, including deletions, insertions, and missense and nonsense mutations, were identified in patients with the most severe ISSD phenotype, most of whom were compound heterozygotes."
    explanation: Variant-series evidence supports multiple SLC17A5 variant classes and severe genotype associations.
  variants:
  - name: c.115C>T (p.Arg39Cys)
    description: Finnish founder variant associated with milder Salla disease when homozygous.
  - name: SLC17A5 exon 8-9 deletion
    description: Reported homozygous deletion causing infantile FSASD with prenatal hydrops.
biochemical:
- name: Free Sialic Acid
  presence: Elevated
  biomarker_term:
    preferred_term: N-acetylneuraminic acid
    term:
      id: CHEBI:17012
      label: N-acetylneuraminic acid
  notes: >
    Free sialic acid is elevated in lysosomes and body fluids/cells; higher
    urinary or intracellular levels predict shorter survival.
  readouts:
  - target: Lysosomal Free Sialic Acid Accumulation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated free sialic acid reports the upstream lysosomal N-acetylneuraminic acid storage burden.
  evidence:
  - reference: PMID:20101035
    reference_title: "Elevated CSF N-acetylaspartylglutamate in patients with free sialic acid storage diseases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found a significant increase of free sialic acid in CSF or urine in 6 of 41 patients presenting with hypomyelination of unknown etiology."
    explanation: CSF/urine testing in a leukodystrophy cohort supports increased free sialic acid as a diagnostic biochemical feature.
  - reference: PMID:40529477
    reference_title: "Investigating the Utility of Leukocyte Sialic Acid Measurements in Lysosomal Free Sialic Acid Storage Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with FSASD exhibited significantly higher levels of free Neu5Ac compared to their unaffected biological parents (36-fold), LSD-negative subjects (22-fold), and individuals with other LSDs (49-fold)."
    explanation: Leukocyte data support elevated free Neu5Ac as a minimally invasive biomarker.
- name: N-acetylaspartylglutamate
  presence: Elevated
  biomarker_term:
    preferred_term: N-acetylaspartylglutamate
    term:
      id: CHEBI:73688
      label: Ac-Asp-Glu
  notes: >
    Elevated CSF N-acetylaspartylglutamate is a supportive marker in a subset of
    hypomyelinating disorders that includes FSASD.
  readouts:
  - target: Sialin Transport Dysfunction
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated CSF NAAG supports SLC17A5-related sialin transport dysfunction in hypomyelinating disease.
  evidence:
  - reference: PMID:20101035
    reference_title: "Elevated CSF N-acetylaspartylglutamate in patients with free sialic acid storage diseases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "H-NMRS revealed an increase of N-acetylaspartylglutamate in the CSF of all patients with SLC17A5 mutation (range 13-114 micromol/L, reference <12 micromol/L)."
    explanation: The leukodystrophy cohort supports CSF NAAG elevation in SLC17A5-mutated FSASD patients.
- name: Glycosphingolipids
  presence: Altered
  biomarker_term:
    preferred_term: glycosphingolipids
    term:
      id: CHEBI:24402
      label: glycosphingolipid
  notes: >
    Ganglioside and broader GSL abnormalities are emerging secondary biomarkers
    and mechanistic readouts in CSF/plasma and neural-cell models.
  readouts:
  - target: Glycosphingolipid Metabolic Dysregulation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Altered glycosphingolipid levels report secondary GSL metabolic dysregulation in FSASD.
  evidence:
  - reference: PMID:39991440
    reference_title: "Changes in glycosphingolipid levels in plasma and cerebrospinal fluid of individuals with Lysosomal Free Sialic Acid Storage Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These results reveal dysregulated GSL metabolism and suggest the potential of gangliosides as biomarkers."
    explanation: Human CSF/plasma data support GSL metabolism as an altered biochemical domain and possible biomarker class.
diagnosis:
- name: SLC17A5 Molecular Genetic Testing
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: MAXO:0000533
      label: molecular genetic testing
  description: Molecular testing confirms biallelic pathogenic SLC17A5 variants.
  results: Biallelic pathogenic SLC17A5 variants establish the diagnosis.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing."
    explanation: GeneReviews states molecular confirmation of the diagnosis.
- name: Urine or CSF Free Sialic Acid Testing
  description: >
    Quantification of free sialic acid in urine or CSF supports FSASD diagnosis,
    especially in patients with unexplained hypomyelination.
  results: Increased free sialic acid in CSF or urine supports FSASD.
  evidence:
  - reference: PMID:20101035
    reference_title: "Elevated CSF N-acetylaspartylglutamate in patients with free sialic acid storage diseases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In patients with undiagnosed leukodystrophies, increased free sialic acid in CSF or urine is a marker for free sialic acid storage disorder and facilitates the identification of the underlying genetic defect."
    explanation: The leukodystrophy cohort directly supports urine/CSF free sialic acid testing in diagnosis.
- name: Brain MRI
  diagnosis_term:
    preferred_term: magnetic resonance imaging procedure
    term:
      id: MAXO:0000424
      label: magnetic resonance imaging procedure
  description: MRI can identify hypomyelination, thin corpus callosum, and cerebellar atrophy.
  results: Central hypomyelination and associated white matter abnormalities support FSASD in the appropriate clinical context.
  evidence:
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features."
    explanation: The consortium review supports MRI-detectable central hypomyelination and structural abnormalities.
treatments:
- name: Supportive Care
  description: >
    Current care is symptomatic and supportive, including seizure management,
    developmental and educational support, rehabilitation, nutrition and feeding
    support, ophthalmologic care, cardiology and nephrology surveillance when
    needed, and family support.
  treatment_term:
    preferred_term: supportive care
    term:
      id: MAXO:0000950
      label: supportive care
  target_mechanisms:
  - target: Neurodevelopmental and Neurodegenerative Dysfunction
    description: Supportive care manages seizures, developmental impairment, rehabilitation needs, and neurobehavioral manifestations downstream of CNS dysfunction.
  - target: White Matter Hypomyelination
    description: Rehabilitation and developmental supports address motor and functional consequences of central white matter disease.
  - target: Lysosomal Free Sialic Acid Accumulation
    description: Supportive care is used because no approved therapy currently reverses the underlying lysosomal storage burden.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Treatment of manifestations: Management is symptomatic and supportive: standard treatment of seizures; developmental and educational support; rehabilitation to optimize mobility; supplementation of calcium and vitamin D for low bone density; feeding therapy and provision of adequate nutrition; treatment of ophthalmologic manifestations per ophthalmologist with low vision services as needed; treatment of cardiomegaly per cardiologist; treatment of nephropathy / nephrotic syndrome per nephrologist; surgical treatment of hernia as needed; family and social support."
    explanation: GeneReviews gives the current management strategy.
  - reference: PMID:33862140
    reference_title: "Free sialic acid storage disorder: Progress and promise."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is no approved therapy for FSASD."
    explanation: The consortium review supports the absence of approved disease-modifying therapy.
- name: Genetic Counseling
  description: >
    Carrier testing and prenatal or preimplantation testing are relevant once
    familial SLC17A5 variants are known.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: MAXO:0000079
      label: genetic counseling
  target_mechanisms:
  - target: SLC17A5 Pathogenic Variation
    description: Counseling and reproductive testing address familial biallelic SLC17A5 pathogenic variants.
  evidence:
  - reference: PMID:20301643
    reference_title: Free Sialic Acid Storage Disorder.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Once the SLC17A5 pathogenic variants have been identified in an affected family member, molecular genetic carrier testing and prenatal/preimplantation genetic testing are possible."
    explanation: GeneReviews supports reproductive genetic counseling and testing once familial variants are known.
- name: Base Editing Gene Therapy
  description: >
    Investigational adenine base editing has corrected the common SLC17A5
    c.115C>T variant and reduced free sialic acid in patient-derived fibroblasts
    and mouse embryonic fibroblasts, but it is not an approved therapy.
  treatment_term:
    preferred_term: gene therapy
    term:
      id: MAXO:0001001
      label: gene therapy
  target_mechanisms:
  - target: SLC17A5 Pathogenic Variation
    treatment_effect: RESTORES
    description: Adenine base editing is designed to correct the SLC17A5 c.115C>T founder variant.
  evidence:
  - reference: PMID:37727271
    reference_title: "Base editing corrects the common Salla disease SLC17A5 c.115C>T variant."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Furthermore, ABE treatment of either homozygous or compound heterozygous SLC17A5 c.115C>T human dermal fibroblasts demonstrated significant FSA reduction, supporting amelioration of disease pathology."
    explanation: The evidence supports preclinical correction and biochemical improvement, but not clinical efficacy in patients.
  - reference: PMID:37727271
    reference_title: "Base editing corrects the common Salla disease SLC17A5 c.115C>T variant."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Our study demonstrates the feasibility of base editing as a therapeutic approach for the FSASD variant SLC17A5 c.115C>T and highlights the usefulness of base editing in monogenic diseases where transmembrane protein function is impaired."
    explanation: The authors frame base editing as a feasible therapeutic approach in preclinical models, so support remains partial.
experimental_models:
- name: Patient-derived iPSC neural cell models
  description: >
    iPSC-derived radial glial cells, immature and mature astrocytes, and
    cortical neurons from individuals with FSASD model free sialic acid
    accumulation and neural-cell GSL dysregulation.
  modeled_mechanisms:
  - target: Lysosomal Free Sialic Acid Accumulation
    description: Patient-derived neural cells recapitulate elevated free sialic acid across differentiated neural cell types.
    evidence:
    - reference: PMID:40804080
      reference_title: "Lysosomal free sialic acid storage disorder iPSC-derived neural cells display altered glycosphingolipid metabolism."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Our findings revealed significant elevations in free sialic acid levels across all FSASD cell types, indicating that iPSCs and derived iRGCs, iIAs, iMAs and iCNs may be used to model FSASD in vitro."
      explanation: The iPSC study shows the model recapitulates the free sialic acid storage phenotype.
  - target: Glycosphingolipid Metabolic Dysregulation
    description: Patient-derived neural cells capture altered GSL abundance, especially in mature astrocytes.
    evidence:
    - reference: PMID:40804080
      reference_title: "Lysosomal free sialic acid storage disorder iPSC-derived neural cells display altered glycosphingolipid metabolism."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We observed significant alterations in the abundance of specific GSL species, predominantly in mature astrocytes, with fewer changes in other cell types."
      explanation: The iPSC study shows the model captures neural-cell GSL dysregulation.
  evidence:
  - reference: PMID:40804080
    reference_title: "Lysosomal free sialic acid storage disorder iPSC-derived neural cells display altered glycosphingolipid metabolism."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Our findings revealed significant elevations in free sialic acid levels across all FSASD cell types, indicating that iPSCs and derived iRGCs, iIAs, iMAs and iCNs may be used to model FSASD in vitro."
    explanation: The iPSC study explicitly supports these neural-cell types as FSASD in vitro models.
datasets:
notes: >-
  The existing Salla Disease page captures the classic Salla subtype in more
  detail. This page models the broader MONDO:0019366 FSASD parent spectrum and
  therefore emphasizes shared SLC17A5/sialin biology, severity spectrum,
  biochemical diagnosis, and cross-spectrum phenotypes.

📚

References & Deep Research

References

1

PMID:20301643

Free Sialic Acid Storage Disorder.

No top-level findings curated for this source.

Deep Research

1

Falcon ▸

1. Disease Information

Edison Scientific Literature 32 citations 2026-05-10T13:05:27.483233

1. Disease Information

1.1 What is the disease?

FSASD is a group of lysosomal storage disorders in which defective lysosomal export of free sialic acid results in abnormal retention of free sialic acid in lysosomes and elevated free sialic acid in urine (often also CSF) with a clinical spectrum ranging from Salla disease to ISSD. (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2, aulanko2023psychiatricsymptomsin pages 1-2, hu2023themolecularmechanism pages 1-2, verheijen1999anewgene pages 1-3)

1.2 Key identifiers

OMIM
Salla disease: OMIM 604369 (aulanko2023psychiatricsymptomsin pages 1-2, mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)
Infantile free sialic acid storage disease (ISSD): OMIM 269920 (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)
Causal gene: SLC17A5 (OMIM 604322) (zielonka2019acrosssectionalquantitative pages 3-4)
MONDO / Orphanet / MeSH / ICD-10/ICD-11: not retrieved from the accessible full-text sources in this run; should be added from OMIM/Orphanet/MONDO database pages during curation.

1.3 Synonyms / alternative names used in the literature

Free sialic acid storage diseases/disorders (FSASD/FSASDs) (hu2023themolecularmechanism pages 1-2, schmiege2023structuresandmechanisms pages 31-36)
Sialic acid storage disease (SSD) (valianpour2004quantificationoffree pages 1-2)
Salla disease; “Finnish disease heritage” disorder (aulanko2023psychiatricsymptomsin pages 1-2)
Infantile sialic acid storage disease (ISSD) (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2, valianpour2004quantificationoffree pages 1-2)

1.4 Evidence source type

Most clinical characterization derives from aggregated case reports/case series and literature-based cohorts (e.g., cross-sectional natural history synthesis) rather than EHR-based population studies. (zielonka2019acrosssectionalquantitative pages 3-4, zielonka2019acrosssectionalquantitative pages 2-3)

2. Etiology

2.1 Disease causal factors

Primary cause (genetic, Mendelian): biallelic pathogenic variants in SLC17A5, encoding the lysosomal membrane transporter sialin, cause FSASD. (aulanko2023psychiatricsymptomsin pages 1-2, hu2023themolecularmechanism pages 1-2, verheijen1999anewgene pages 1-3)

Foundational discovery: The causal gene was identified in 1999: “A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases,” establishing SLC17A5/sialin as the disease gene. (verheijen1999anewgene pages 1-1)

2.2 Risk factors

Genetic risk: inheritance is autosomal recessive; Finnish founder effect is prominent for Salla disease.
In Finland, “More than 90% of Finnish SD patients are homozygous for the pathogenic SLC17A5 c.115C>T; p.(Arg39Cys) … founder variant.” (aulanko2023psychiatricsymptomsin pages 1-2)
Environmental risk factors / protective factors / gene–environment: no specific environmental modifiers were identified in the retrieved sources; disease is primarily monogenic with severity driven largely by genotype and biochemical burden. Genotype–phenotype correlation has been discussed historically. (aula2000thespectrumof pages 9-9, aulanko2023psychiatricsymptomsin pages 1-2)

3. Phenotypes

3.1 Core phenotype spectrum (human)

Salla disease (milder end of spectrum): intellectual disability, ataxia, athetosis, nystagmus, and CNS demyelination/dysmyelination, with typical symptom detection in infancy. * “Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination.” (aulanko2023psychiatricsymptomsin pages 1-2) * “The first symptoms are typically detected between the ages of 3–12 months.” (aulanko2023psychiatricsymptomsin pages 1-2)

ISSD (severe infantile form): multisystem involvement with visceromegaly and coarse facial features; poor survival. * ISSD is described as “clinically distinct and very serious… visceromegaly, coarse facial features, failure to thrive, and early death.” (valianpour2004quantificationoffree pages 1-2)

Neuroimaging phenotype: in FSASD/Salla disease, MRI frequently shows hypomyelination and atrophy. * In undiagnosed leukodystrophy presentations, symptoms are “associated with diffuse supratentorial hypomyelination, thin corpus callosum, and cortical and cerebellar atrophy.” (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2) * In a literature cohort, MRI abnormalities included brain atrophy (19.8%), hypomyelination (19%), and corpus callosum thinning (13.8%). (zielonka2019acrosssectionalquantitative pages 4-6)

3.2 Psychiatric/behavioral features (recent quantitative data)

A 24-patient Salla disease cohort review found psychiatric symptoms are common: * “Psychiatric symptoms were frequently associated with SD (10/24, 42%),” with sleeping disorders 8/24 (32%), aggressive behavior/restlessness 6/24 (25%), and off-label antipsychotic medication 4/24 (17%). (aulanko2023psychiatricsymptomsin pages 1-2)

3.3 Suggested HPO terms (non-exhaustive; for knowledge-base mapping)

Intellectual disability (HP:0001249)
Global developmental delay (HP:0001263)
Nystagmus (HP:0000639)
Cerebellar ataxia (HP:0001251)
Hypotonia (HP:0001252)
Spasticity (HP:0001257)
Seizures (HP:0001250)
Hypomyelination (HP:0003429)
Thin corpus callosum / corpus callosum hypoplasia (HP:0002079 / HP:0002079-related)
Cerebellar atrophy (HP:0001272)
Hepatosplenomegaly (HP:0001433)
Coarse facial features (HP:0000280)
Psychosis (HP:0000709)
Sleep disturbance (HP:0002360)

3.4 Quality-of-life impact

Direct QoL instrument data (EQ-5D/SF-36/PROMIS) were not located in retrieved texts; functional dependence and behavioral symptoms indicate high caregiver burden, and some individuals require assistance with activities of daily living. (aulanko2023psychiatricsymptomsin pages 1-2)

4. Genetic / Molecular Information

4.1 Causal gene

SLC17A5 (solute carrier family 17 member 5), encoding sialin—a transporter within the SLC17 family / major facilitator superfamily fold. (hu2023themolecularmechanism pages 1-2, schmiege2024structureandinhibition pages 1-2, verheijen1999anewgene pages 1-3)

4.2 Pathogenic variants and variant classes (examples from primary sources)

Founder variant in Finnish Salla disease: c.115C>T, p.Arg39Cys. (aulanko2023psychiatricsymptomsin pages 1-2, verheijen1999anewgene pages 1-3)
ISSD-associated alleles include truncating/frameshift and other missense variants in SLC17A5 (examples shown in the original gene discovery table). (verheijen1999anewgene pages 1-3)

Because ClinVar/gnomAD/HGMD were not directly queried via tools in this run, allele frequencies and ClinVar pathogenicity classifications are not provided here; they should be filled from ClinVar/gnomAD during curation.

4.3 Functional consequences

Pathogenic SLC17A5 variants impair lysosomal export of free sialic acid, causing storage. * FSASD are characterized by “enlarged cellular lysosomes and elevated levels of free sialic acids in urine.” (hu2023themolecularmechanism pages 1-2)

Recent structural work provides mechanistic explanations for pathogenic mutations: * 2023 cryo-EM work: “By mapping known pathogenic mutations, we provide mechanistic explanations for corresponding functional defects.” (hu2023themolecularmechanism pages 1-2)

4.4 Suggested GO terms (biological process; examples)

Lysosomal transport (GO:0007040 / related)
Carbohydrate derivative transport (GO:1901264)
Lysosome organization (GO:0007040-related)
Myelination (GO:0042552)

4.5 Suggested GO cellular component terms

Lysosome (GO:0005764)
Lysosomal membrane (GO:0005765)
Synaptic vesicle (GO:0008021) (given sialin’s neurotransmitter transport roles) (schmiege2024structureandinhibition pages 1-2)

5. Environmental Information

No disease-specific environmental, lifestyle, or infectious triggers/protectors were identified in the retrieved sources; FSASD is primarily a monogenic lysosomal transporter disorder. (aulanko2023psychiatricsymptomsin pages 1-2, verheijen1999anewgene pages 1-3)

6. Mechanism / Pathophysiology

6.1 Core causal chain (current understanding)

Biallelic SLC17A5 variants →
Defective sialin function (lysosomal sialic acid exporter / proton-coupled transporter) →
Retention/accumulation of free sialic acid in lysosomes, with elevated free sialic acid in urine and/or CSF →
Downstream CNS pathology including hypomyelination, thin corpus callosum, cortical/cerebellar atrophy, and progressive neurodevelopmental impairment. (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2, hu2023themolecularmechanism pages 1-2, schmiege2024structureandinhibition pages 1-2)

6.2 Biochemical abnormalities and diagnostic markers

“The increase of free sialic acid in urine has been considered the biochemical hallmark.” (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)
In a leukodystrophy workup study, CSF NAAG was elevated in all SLC17A5-mutated patients (see §10 Diagnostics for numeric ranges). (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2, mochel2010elevatedcsfnacetylaspartylglutamate media ffd5dd78)

6.3 Recent mechanistic developments (prioritize 2023–2024)

2023 (Science Advances; published 20 Jan 2023; URL in BibTeX): first high-resolution human SLC17 family structure. * Abstract quote: “we present the structure of human Sialin… determined by cryo–electron microscopy, representing the first high-resolution structure of any human SLC17 member.” (hu2023themolecularmechanism pages 1-2) * Abstract quote: “The H+ coupling/sensing requires two highly conserved Glu residues (E171 and E175)…” (hu2023themolecularmechanism pages 1-2)

2024 (Nature Communications; accepted 3 May 2024; DOI URL): multiple conformational states and inhibitor binding. * Abstract quote: “we present the cryogenic electron-microscopy structures of human Sialin: apo cytosol-open, apo lumen-open, NAAG–bound, and inhibitor–bound.” (schmiege2024structureandinhibition pages 1-2) * The work identifies an inhibitor-binding mode (“Sialin inhibitor Fmoc-Leu-OH”) and residues governing substrate recognition, supporting structure-guided drug discovery. (schmiege2024structureandinhibition pages 1-2)

6.4 Molecular profiling (available evidence)

Although beyond 2024, recent disease-modeling indicates glycosphingolipid (GSL) metabolism perturbation in patient iPSC-derived neural cells with striking astrocyte changes. * Free sialic acid elevations vs controls were reported across cell types, including mature astrocytes (830% of healthy donors). (sabir2025lysosomalfreesialic pages 2-3)

6.5 Suggested cell types (CL terms; examples)

Oligodendrocyte (CL:0000128) (white-matter pathology)
Astrocyte (CL:0000127) (recent iPSC modeling suggests mature astrocyte vulnerability) (sabir2025lysosomalfreesialic pages 2-3)
Neuron (CL:0000540)

7. Anatomical Structures Affected

7.1 Organ/system level

Primary: central nervous system (brain white matter, cerebellum) with hypomyelination and atrophy. (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)

Secondary/multisystem (especially ISSD): visceromegaly/hepatosplenomegaly and coarse facial features. (valianpour2004quantificationoffree pages 1-2, verheijen1999anewgene pages 1-1)

7.2 Suggested UBERON terms (examples)

Brain (UBERON:0000955)
Cerebellum (UBERON:0002037)
Corpus callosum (UBERON:0002330)
Liver (UBERON:0002107)
Spleen (UBERON:0002106)

7.3 Subcellular localization

Lysosomal lumen/membrane involvement is central (lysosomal free sialic acid accumulation; lysosomal membrane transporter). (schmiege2024structureandinhibition pages 1-2)

8. Temporal Development

8.1 Onset

In a large literature-based cohort (postnatally diagnosed), median onset was 0.17 years (~2 months). (zielonka2019acrosssectionalquantitative pages 1-2)

In Salla disease specifically, first symptoms are “typically detected between the ages of 3–12 months.” (aulanko2023psychiatricsymptomsin pages 1-2)

8.2 Progression and disease course

FSASD demonstrates a continuous phenotypic spectrum; ISSD is rapidly progressive with early mortality, while Salla disease can allow survival into adulthood. (hu2023themolecularmechanism pages 1-2, schmiege2023structuresandmechanisms pages 31-36, zielonka2019acrosssectionalquantitative pages 1-2)

8.3 Critical periods

Prenatal and perinatal manifestations occur in a substantial subset: * Hydrops fetalis occurred as the first clinical sign in 20.7% of cases in the literature cohort. (zielonka2019acrosssectionalquantitative pages 3-4, zielonka2019acrosssectionalquantitative pages 2-3)

9. Inheritance and Population

9.1 Inheritance

Autosomal recessive inheritance is consistently reported for Salla disease/FSASD. * “Salla disease… is a rare autosomal recessive lysosomal storage disorder…” (aulanko2023psychiatricsymptomsin pages 1-2)

9.2 Epidemiology

Robust prevalence/incidence estimates were not retrieved from Orphanet/registry pages in this run; however: * Literature synthesis notes Orphanet-listed case counts (~130 known cases) and panethnic distribution. (zielonka2019acrosssectionalquantitative pages 1-2, zielonka2019acrosssectionalquantitative pages 4-6) * Salla disease is enriched in Finland (“Finnish disease heritage”), with the p.Arg39Cys founder variant accounting for >90% of Finnish SD cases. (aulanko2023psychiatricsymptomsin pages 1-2)

9.3 Sex ratio and demographics

A Finnish hospital-based cohort of 24 Salla disease patients was 75% male (18/24). This may reflect ascertainment/registry bias rather than true sex skew given autosomal recessive inheritance. (aulanko2023psychiatricsymptomsin pages 1-2)

10. Diagnostics

10.1 Biochemical testing (real-world implementations)

Urine free sialic acid quantification is central. * A 2004 Clinical Chemistry method paper states: “Defective free sialic acid transport can be established by quantification of free sialic acid in urine.” (valianpour2004quantificationoffree pages 1-2) * Age-dependent control ranges were established (n=72 controls). For example, ages 0–1 year: mean 31.3 mmol/mol creatinine (range 0.7–56.9; n=20). (valianpour2004quantificationoffree pages 1-2) * In three SSD patients aged 1.2, 3.9, 12 years, urinary free sialic acid concentrations were 111.5, 54.2, and 36.1 mmol/mol creatinine. (valianpour2004quantificationoffree pages 1-2)

CSF/urine screening in leukodystrophy workups: * In a cross-sectional leukodystrophy cohort (41 patients with hypomyelination of unknown etiology), 6/41 had increased free sialic acid in CSF or urine and all had SLC17A5 mutations. (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)

CSF NAAG as supportive biomarker: * “H-NMRS revealed an increase of N-acetylaspartylglutamate in the CSF of all patients with SLC17A5 mutation (range 13–114 µmol/L, reference <12 µmol/L).” (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)

Visual evidence for the Mochel et al. biochemical table and MR spectrum is available in the extracted figure/table crops. (mochel2010elevatedcsfnacetylaspartylglutamate media ffd5dd78, mochel2010elevatedcsfnacetylaspartylglutamate media de78242d)

10.2 Genetic testing

Diagnosis is confirmed by identifying biallelic pathogenic variants in SLC17A5. * Foundational mutation discovery and examples (including R39C) were reported in 1999, with a mutation table for SD and ISSD. (verheijen1999anewgene pages 1-3)

For leukodystrophy presentations, genome/exome-first strategies are increasingly implemented in clinical programs that include Salla disease among target conditions. * Clinical WGS study (LeukoSEQ; completed) enrolled 236 participants; interim analysis described diagnostic efficacy of WGS+standard care at 26/34 (76.5%; 95% CI 58.8–89.3%) within <4 months and faster time to diagnosis than standard care alone. (NCT02699190 chunk 1)

10.3 Differential diagnosis

Not comprehensively enumerated in retrieved sources; key differentials in hypomyelinating leukodystrophy evaluations include other genetic white-matter disorders. (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2, NCT02699190 chunk 1)

10.4 Screening

Not a standard newborn screening condition in the retrieved sources; prenatal presentation (hydrops fetalis) supports consideration in prenatal/NIHF workups with biochemical/genetic testing. (zielonka2019acrosssectionalquantitative pages 3-4)

11. Outcome / Prognosis

A 2019 quantitative natural-history analysis (116 published cases; 106 postnatally diagnosed) reported: * Median survival 11 years (postnatally diagnosed subset). (zielonka2019acrosssectionalquantitative pages 3-4, zielonka2019acrosssectionalquantitative pages 2-3) * Median age at diagnosis 3 years and median diagnostic delay 2.5 years (subset N=90 with data). (zielonka2019acrosssectionalquantitative pages 3-4, zielonka2019acrosssectionalquantitative pages 2-3)

Biomarker severity correlates with outcome: * Urinary free sialic acid excretion <6.37-fold of normal (and fibroblast storage <7.37-fold) was associated with significantly longer survival in recursive partitioning analyses. (zielonka2019acrosssectionalquantitative pages 3-4)

12. Treatment

12.1 Current standard of care

No disease-modifying therapy was identified in the retrieved sources; management is primarily symptomatic/supportive. * A lysosomal transporter review/thesis summary notes: “Currently, there are only symptomatic treatments for these FSASDs.” (schmiege2023structuresandmechanisms pages 31-36) * 2019 natural-history synthesis reported no clinical trials or orphan drug designations as of 31 Dec 2017. (zielonka2019acrosssectionalquantitative pages 1-2, zielonka2019acrosssectionalquantitative pages 4-6)

12.2 Experimental/forward-looking approaches (expert analysis)

Structural biology now enables rational exploration of small-molecule modulation and mutation-specific rescue. * 2024 Nature Communications work reports an “inhibitor–bound” structure and identifies a sialin inhibitor (Fmoc-Leu-OH), providing a foothold for structure-guided ligand development (though not yet a therapy). (schmiege2024structureandinhibition pages 1-2)

12.3 Suggested MAXO terms (examples)

Supportive care (MAXO:0000004)
Physical therapy (MAXO:0000014)
Occupational therapy (MAXO:0000015)
Genetic counseling (MAXO:0000071)
Whole genome sequencing (MAXO:0000126)

13. Prevention

Primary prevention is not applicable in the usual sense for a recessive Mendelian disorder; prevention focuses on genetic counseling and reproductive options. * Given autosomal recessive inheritance, family-based carrier testing and prenatal diagnosis are standard preventive strategies, especially in founder populations; prenatal/pregnancy presentations (hydrops fetalis) reinforce this clinical need. (aulanko2023psychiatricsymptomsin pages 1-2, zielonka2019acrosssectionalquantitative pages 3-4)

14. Other Species / Natural Disease

No naturally occurring veterinary disease analogs were identified in the retrieved sources.

15. Model Organisms

A sialin-deficient mouse model is referenced in natural-history synthesis: * “sialin-/- mice show progressive leukoencephalopathy with decreased myelinated axons, oligodendrocyte apoptosis… coordination defects, seizures, and premature death.” (zielonka2019acrosssectionalquantitative pages 1-2, zielonka2019acrosssectionalquantitative pages 4-6)

Cellular models include patient-derived iPSC neural differentiation platforms (two FSASD lines vs two controls) enabling mechanistic studies of cell-type vulnerability (notably astrocytes). (sabir2025lysosomalfreesialic pages 1-2, sabir2025lysosomalfreesialic pages 2-3)

Notes on evidence limitations (important for knowledge-base curation)

PMIDs were not available in the tool-returned full texts for most citations in this run; DOIs/URLs and publication timing are provided from the retrieved PDFs/records. Curators should augment with PMIDs from PubMed.
MONDO/Orphanet/MeSH/ICD identifiers were not retrievable from the accessible texts; these should be populated directly from the corresponding ontology/database pages.

Key primary sources (with URLs and dates as available from retrieved texts)

Verheijen et al. Nature Genetics (Dec 1999). “A new gene… mutated in sialic acid storage diseases.” https://doi.org/10.1038/70585 (verheijen1999anewgene pages 1-3)
Valianpour et al. Clinical Chemistry (Feb 2004). Urine free sialic acid quantification by HPLC–MS/MS. https://doi.org/10.1373/clinchem.2003.027169 (valianpour2004quantificationoffree pages 1-2)
Mochel et al. Neurology (Jan 2010). CSF NAAG elevation and 6/41 leukodystrophy screening yield. https://doi.org/10.1212/WNL.0b013e3181cbcdc4 (mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2)
Zielonka et al. Genetics in Medicine (Feb 2019). Natural history synthesis (116 cases; median survival 11 years). https://doi.org/10.1038/s41436-018-0051-3 (zielonka2019acrosssectionalquantitative pages 3-4)
Hu et al. Science Advances (20 Jan 2023). First high-resolution human sialin structure. https://doi.org/10.1126/sciadv.ade8346 (hu2023themolecularmechanism pages 1-2)
Aulanko et al. European Child & Adolescent Psychiatry (published online 7 Jul 2022; journal issue 2023). Psychiatric symptoms (10/24, 42%). https://doi.org/10.1007/s00787-022-02031-5 (aulanko2023psychiatricsymptomsin pages 1-2)
Schmiege et al. Nature Communications (May 2024). Multiple-state structures and inhibition. https://doi.org/10.1038/s41467-024-48535-3 (schmiege2024structureandinhibition pages 1-2)

References

(aulanko2023psychiatricsymptomsin pages 1-2): Ida Aulanko, Elisa Rahikkala, and Jukka Moilanen. Psychiatric symptoms in salla disease. European Child & Adolescent Psychiatry, 32:2043-2047, Jul 2023. URL: https://doi.org/10.1007/s00787-022-02031-5, doi:10.1007/s00787-022-02031-5. This article has 4 citations and is from a domain leading peer-reviewed journal.
(hu2023themolecularmechanism pages 1-2): Wenxin Hu, Congwu Chi, Kunhua Song, and Hongjin Zheng. The molecular mechanism of sialic acid transport mediated by sialin. Science Advances, Jan 2023. URL: https://doi.org/10.1126/sciadv.ade8346, doi:10.1126/sciadv.ade8346. This article has 27 citations and is from a highest quality peer-reviewed journal.
(schmiege2024structureandinhibition pages 1-2): Philip Schmiege, Linda Donnelly, Nadia Elghobashi-Meinhardt, Chia-Hsueh Lee, and Xiaochun Li. Structure and inhibition of the human lysosomal transporter sialin. Nature Communications, May 2024. URL: https://doi.org/10.1038/s41467-024-48535-3, doi:10.1038/s41467-024-48535-3. This article has 13 citations and is from a highest quality peer-reviewed journal.
(schmiege2023structuresandmechanisms pages 31-36): PFP Schmiege. Structures and mechanisms of lysosomal transporters. Unknown journal, 2023.
(mochel2010elevatedcsfnacetylaspartylglutamate pages 1-2): F. Mochel, U.F.H. Engelke, J. Barritault, B. Yang, N. H. McNeill, J. N. Thompson, A. Vanderver, N. I. Wolf, M. A. Willemsen, F. W. Verheijen, F. Seguin, R. A. Wevers, and R. Schiffmann. Elevated csf n-acetylaspartylglutamate in patients with free sialic acid storage diseases. Neurology, 74:302-305, Jan 2010. URL: https://doi.org/10.1212/wnl.0b013e3181cbcdc4, doi:10.1212/wnl.0b013e3181cbcdc4. This article has 32 citations and is from a highest quality peer-reviewed journal.
(valianpour2004quantificationoffree pages 1-2): Fredoen Valianpour, Nicolaas G G M Abeling, Marinus Duran, Jan G M Huijmans, and Willem Kulik. Quantification of free sialic acid in urine by hplc-electrospray tandem mass spectrometry: a tool for the diagnosis of sialic acid storage disease. Clinical chemistry, 50 2:403-9, Feb 2004. URL: https://doi.org/10.1373/clinchem.2003.027169, doi:10.1373/clinchem.2003.027169. This article has 58 citations and is from a highest quality peer-reviewed journal.
(verheijen1999anewgene pages 1-3): Frans W. Verheijen, Elly Verbeek, Nina Aula, Cecile E.M.T. Beerens, Adrie C. Havelaar, Marijke Joosse, Leena Peltonen, Pertti Aula, Hans Galjaard, Peter J. van der Spek, and Grazia M.S. Mancini. A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases. Nature Genetics, 23:462-465, Dec 1999. URL: https://doi.org/10.1038/70585, doi:10.1038/70585. This article has 351 citations and is from a highest quality peer-reviewed journal.
(verheijen1999anewgene pages 1-1): Frans W. Verheijen, Elly Verbeek, Nina Aula, Cecile E.M.T. Beerens, Adrie C. Havelaar, Marijke Joosse, Leena Peltonen, Pertti Aula, Hans Galjaard, Peter J. van der Spek, and Grazia M.S. Mancini. A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases. Nature Genetics, 23:462-465, Dec 1999. URL: https://doi.org/10.1038/70585, doi:10.1038/70585. This article has 351 citations and is from a highest quality peer-reviewed journal.
(mochel2010elevatedcsfnacetylaspartylglutamate media ffd5dd78): F. Mochel, U.F.H. Engelke, J. Barritault, B. Yang, N. H. McNeill, J. N. Thompson, A. Vanderver, N. I. Wolf, M. A. Willemsen, F. W. Verheijen, F. Seguin, R. A. Wevers, and R. Schiffmann. Elevated csf n-acetylaspartylglutamate in patients with free sialic acid storage diseases. Neurology, 74:302-305, Jan 2010. URL: https://doi.org/10.1212/wnl.0b013e3181cbcdc4, doi:10.1212/wnl.0b013e3181cbcdc4. This article has 32 citations and is from a highest quality peer-reviewed journal.
(hu2023themolecularmechanism pages 6-7): Wenxin Hu, Congwu Chi, Kunhua Song, and Hongjin Zheng. The molecular mechanism of sialic acid transport mediated by sialin. Science Advances, Jan 2023. URL: https://doi.org/10.1126/sciadv.ade8346, doi:10.1126/sciadv.ade8346. This article has 27 citations and is from a highest quality peer-reviewed journal.
(NCT02699190 chunk 1): Adeline Vanderver, MD. LeukoSEQ: Whole Genome Sequencing as a First-Line Diagnostic Tool for Leukodystrophies. Children's Hospital of Philadelphia. 2017. ClinicalTrials.gov Identifier: NCT02699190
(NCT03047369 chunk 1): Adeline Vanderver, MD. The Myelin Disorders Biorepository Project. Children's Hospital of Philadelphia. 2016. ClinicalTrials.gov Identifier: NCT03047369
(zielonka2019acrosssectionalquantitative pages 3-4): Matthias Zielonka, Sven F. Garbade, Stefan Kölker, Georg F. Hoffmann, and Markus Ries. A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease—an ultra-orphan multisystemic lysosomal storage disorder. Genetics in Medicine, 21:347-352, Feb 2019. URL: https://doi.org/10.1038/s41436-018-0051-3, doi:10.1038/s41436-018-0051-3. This article has 31 citations and is from a highest quality peer-reviewed journal.
(zielonka2019acrosssectionalquantitative pages 2-3): Matthias Zielonka, Sven F. Garbade, Stefan Kölker, Georg F. Hoffmann, and Markus Ries. A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease—an ultra-orphan multisystemic lysosomal storage disorder. Genetics in Medicine, 21:347-352, Feb 2019. URL: https://doi.org/10.1038/s41436-018-0051-3, doi:10.1038/s41436-018-0051-3. This article has 31 citations and is from a highest quality peer-reviewed journal.
(aula2000thespectrumof pages 9-9): Nina Aula, Pirjo Salomäki, Ritva Timonen, Frans Verheijen, Grazia Mancini, Jan-Eric Månsson, Pertti Aula, and Leena Peltonen. The spectrum of slc17a5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation. American journal of human genetics, 67 4:832-40, Oct 2000. URL: https://doi.org/10.1086/303077, doi:10.1086/303077. This article has 144 citations and is from a highest quality peer-reviewed journal.
(zielonka2019acrosssectionalquantitative pages 4-6): Matthias Zielonka, Sven F. Garbade, Stefan Kölker, Georg F. Hoffmann, and Markus Ries. A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease—an ultra-orphan multisystemic lysosomal storage disorder. Genetics in Medicine, 21:347-352, Feb 2019. URL: https://doi.org/10.1038/s41436-018-0051-3, doi:10.1038/s41436-018-0051-3. This article has 31 citations and is from a highest quality peer-reviewed journal.
(sabir2025lysosomalfreesialic pages 2-3): M. Sabir, V. Jovanovic, Seungmi Ryu, Chaitali Sen, Pinar Ormanoglu, Laura Pollard, Richard Steet, William A. Gahl, M. Huizing, Carlos A. Tristan, Frances M. Platt, and M. Malicdan. Lysosomal free sialic acid storage disorder ipsc-derived neural cells display altered glycosphingolipid metabolism. Scientific Reports, Aug 2025. URL: https://doi.org/10.1038/s41598-025-12682-4, doi:10.1038/s41598-025-12682-4. This article has 2 citations and is from a peer-reviewed journal.
(zielonka2019acrosssectionalquantitative pages 1-2): Matthias Zielonka, Sven F. Garbade, Stefan Kölker, Georg F. Hoffmann, and Markus Ries. A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease—an ultra-orphan multisystemic lysosomal storage disorder. Genetics in Medicine, 21:347-352, Feb 2019. URL: https://doi.org/10.1038/s41436-018-0051-3, doi:10.1038/s41436-018-0051-3. This article has 31 citations and is from a highest quality peer-reviewed journal.
(mochel2010elevatedcsfnacetylaspartylglutamate media de78242d): F. Mochel, U.F.H. Engelke, J. Barritault, B. Yang, N. H. McNeill, J. N. Thompson, A. Vanderver, N. I. Wolf, M. A. Willemsen, F. W. Verheijen, F. Seguin, R. A. Wevers, and R. Schiffmann. Elevated csf n-acetylaspartylglutamate in patients with free sialic acid storage diseases. Neurology, 74:302-305, Jan 2010. URL: https://doi.org/10.1212/wnl.0b013e3181cbcdc4, doi:10.1212/wnl.0b013e3181cbcdc4. This article has 32 citations and is from a highest quality peer-reviewed journal.
(sabir2025lysosomalfreesialic pages 1-2): M. Sabir, V. Jovanovic, Seungmi Ryu, Chaitali Sen, Pinar Ormanoglu, Laura Pollard, Richard Steet, William A. Gahl, M. Huizing, Carlos A. Tristan, Frances M. Platt, and M. Malicdan. Lysosomal free sialic acid storage disorder ipsc-derived neural cells display altered glycosphingolipid metabolism. Scientific Reports, Aug 2025. URL: https://doi.org/10.1038/s41598-025-12682-4, doi:10.1038/s41598-025-12682-4. This article has 2 citations and is from a peer-reviewed journal.

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Inheritance

Subtypes

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Treatments

Biochemical Markers

Experimental Models

Source YAML

References & Deep Research

References

Deep Research

1. Disease Information

1.1 What is the disease?

1.2 Key identifiers

1.3 Synonyms / alternative names used in the literature

1.4 Evidence source type

2. Etiology

2.1 Disease causal factors

2.2 Risk factors

3. Phenotypes

3.1 Core phenotype spectrum (human)

3.2 Psychiatric/behavioral features (recent quantitative data)

3.3 Suggested HPO terms (non-exhaustive; for knowledge-base mapping)

3.4 Quality-of-life impact

4. Genetic / Molecular Information

4.1 Causal gene

4.2 Pathogenic variants and variant classes (examples from primary sources)

4.3 Functional consequences

4.4 Suggested GO terms (biological process; examples)

4.5 Suggested GO cellular component terms

5. Environmental Information

6. Mechanism / Pathophysiology

6.1 Core causal chain (current understanding)

6.2 Biochemical abnormalities and diagnostic markers

6.3 Recent mechanistic developments (prioritize 2023–2024)

6.4 Molecular profiling (available evidence)

6.5 Suggested cell types (CL terms; examples)

7. Anatomical Structures Affected

7.1 Organ/system level

7.2 Suggested UBERON terms (examples)

7.3 Subcellular localization

8. Temporal Development

8.1 Onset

8.2 Progression and disease course

8.3 Critical periods

9. Inheritance and Population

9.1 Inheritance

9.2 Epidemiology

9.3 Sex ratio and demographics

10. Diagnostics

10.1 Biochemical testing (real-world implementations)

10.2 Genetic testing

10.3 Differential diagnosis

10.4 Screening

11. Outcome / Prognosis

12. Treatment

12.1 Current standard of care

12.2 Experimental/forward-looking approaches (expert analysis)

12.3 Suggested MAXO terms (examples)

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Notes on evidence limitations (important for knowledge-base curation)

Key primary sources (with URLs and dates as available from retrieved texts)