Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis

Mendelian MONDO:0011835 Pathograph 4 Show in embeddings browser POLG-Related Disorders Ataxia Neuropathy Spectrum Mitochondrial Disease

Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) is an autosomal recessive mitochondrial DNA maintenance disorder caused by biallelic pathogenic variants in POLG, which encodes the catalytic subunit of the mitochondrial DNA polymerase gamma. Defective POLG function impairs mitochondrial DNA replication and proofreading, producing mitochondrial DNA depletion and multiple mitochondrial DNA deletions with secondary respiratory-chain deficiency. SANDO sits within the POLG-related disorder spectrum and the ataxia neuropathy spectrum, and is defined by the clinical triad of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis, frequently accompanied by progressive external ophthalmoplegia (PEO). It is a subgroup of the mitochondrial chronic progressive external ophthalmoplegia (CPEO)-plus disorders associated with multiple mtDNA deletions.

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2
Pathophys.
10
Phenotypes
4
Pathograph
1
Genes
4
Medical Actions
1
Trials
1
References
1
Deep Research
⚙

Pathophysiology

2
Polymerase Gamma Dysfunction and mtDNA Instability
Biallelic POLG variants impair the catalytic and proofreading activity of mitochondrial DNA polymerase gamma, the sole replicative polymerase of the mitochondrial genome. This disrupts mitochondrial DNA replication, leading to accumulation of multiple large-scale mitochondrial DNA deletions and, in some tissues, mitochondrial DNA depletion. The resulting clonal expansion of deleted mtDNA species compromises mitochondrial genome integrity.
dorsal root ganglion sensory neuron CL:1001451
mitochondrial DNA replication GO:0006264 ↓ DECREASED mitochondrial DNA repair / proofreading GO:0006281 ↓ DECREASED
Downstream
Respiratory-Chain Deficiency in Post-Mitotic Tissue Accumulation of multiple mtDNA deletions and depletion reduces the mtDNA template available for respiratory-chain subunits, producing oxidative phosphorylation deficiency in post-mitotic neurons and muscle.
Show evidence (2 references)
PMID:18585914 SUPPORT Human Clinical
"Mutations in POLG gene are responsible for a wide spectrum of clinical disorders with altered mitochondrial DNA (mtDNA) integrity, including mtDNA multiple deletions and depletion."
POLG mutations directly compromise mitochondrial DNA integrity, producing multiple mtDNA deletions and depletion, the molecular hallmark of SANDO.
PMID:20513108 SUPPORT Human Clinical
"Missense mutations in the gene for polymerase gamma 1 (POLG1) cause a number of phenotypically heterogeneous mitochondrial diseases, most commonly progressive external ophthalmoplegia, and are characterized by the accumulation of multiple, large-scale deletions of mitochondrial DNA."
Confirms that POLG1 missense mutations drive accumulation of multiple large-scale mtDNA deletions underlying the SANDO/PEO phenotype.
Respiratory-Chain Deficiency in Post-Mitotic Tissue
Accumulation of deleted and depleted mtDNA reduces the supply of mtDNA-encoded respiratory-chain subunits, impairing oxidative phosphorylation. In skeletal muscle this manifests histologically as ragged-red and cytochrome c oxidase (COX)-negative fibers; in long sensory neurons of the dorsal root ganglia the bioenergetic deficit produces sensory ataxic neuropathy.
skeletal muscle fiber CL:0008002
oxidative phosphorylation GO:0006119 ↓ DECREASED cellular respiration GO:0045333 ↓ DECREASED
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
Ragged-red and COX-negative fibers are the histological signature of respiratory-chain deficiency secondary to mtDNA instability in SANDO.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

10
Eye 1
Ptosis Ptosis HP:0000508
Show evidence (1 reference)
PMID:20301791 SUPPORT Human Clinical
"Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."
GeneReviews documents ptosis together with PEO and peripheral neuropathy in the late-onset POLG spectrum that encompasses SANDO.
Musculoskeletal 1
Myopathy Myopathy HP:0003198
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
Myopathy was present in all patients in this POLG-related SANDO series.
Nervous System 3
Peripheral Neuropathy Peripheral neuropathy HP:0009830
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
Neuropathy was present in all five POLG-related SANDO patients in this series, alongside ataxia and PEO.
Dysarthria Dysarthria HP:0001260
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis"
Dysarthria (and/or dysphagia) is an explicit member of the defining SANDO clinical triad.
Seizures Seizure HP:0001250
Show evidence (1 reference)
PMID:20301791 PARTIAL Human Clinical
"Disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes"
Seizures are part of the juvenile/adult-onset POLG spectrum that encompasses SANDO, though they are not part of the defining SANDO triad.
Other 5
Sensory Ataxia FREQUENT Sensory ataxia HP:0010871
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:25143630 SUPPORT Human Clinical
"The combination of sensory ataxic neuropathy with ophthalmoparesis (SANO) was observed in 70% of patients with multiple mtDNA deletions"
Sensory ataxic neuropathy was the most frequent component phenotype in patients with multiple mtDNA deletions in this SANDO cohort.
Ophthalmoparesis Ophthalmoparesis HP:0000597
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis"
Ophthalmoparesis completes the defining SANDO clinical triad.
Progressive External Ophthalmoplegia Progressive external ophthalmoplegia HP:0000590
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
PEO was present in all patients in this POLG-related SANDO series.
Ragged-Red Muscle Fibers Ragged-red muscle fibers HP:0003200
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
Ragged-red fibers were documented in muscle pathology of POLG-related SANDO patients.
Cytochrome C Oxidase-Negative Muscle Fibers Cytochrome C oxidase-negative muscle fibers HP:0003688
Show evidence (1 reference)
PMID:18585914 SUPPORT Human Clinical
"The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
COX-negative fibers were documented in muscle pathology of POLG-related SANDO patients.
🧬

Genetic Associations

1
POLG (Biallelic (autosomal recessive) pathogenic variants in POLG, encoding the catalytic subunit of mitochondrial DNA polymerase gamma, cause SANDO and the broader POLG-related disorder spectrum through impaired mtDNA replication and accumulation of multiple mtDNA deletions and depletion.)
Gene: POLG hgnc:9179
Autosomal recessive inheritance
Show evidence (2 references)
PMID:18585914 SUPPORT Human Clinical
"Sensory ataxic neuropathy with ophthalmoparesis (SANDO) caused by mutations in POLG gene, fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis, has described in a few reports."
Directly establishes POLG as the causal gene for SANDO.
PMID:25143630 SUPPORT Human Clinical
"multiple mtDNA deletions were associated with the following nuclear mutations: POLG (n=6), PEO1 (n=2), unidentified (n=2)."
POLG was the most common nuclear gene associated with SANDO in this cohort of patients with multiple mtDNA deletions.
💊

Medical Actions

4
Supportive and Multidisciplinary Care
Action: Supportive Care NCIT:C15747
Management of SANDO and POLG-related disorders is largely supportive, involving standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support; and standard treatment of epilepsy, movement abnormalities, vision, and hearing issues.
Show evidence (1 reference)
PMID:20301791 SUPPORT Human Clinical
"Clinical management is largely supportive and involves standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support"
GeneReviews specifies that POLG-related disorder management is largely supportive and multidisciplinary.
Avoidance of Valproic Acid
Action: Pharmacotherapy NCIT:C15986
Agent: valproic acid CHEBI:39867
Valproic acid (Depakene) and sodium divalproate (Depakote) must be avoided in POLG-related disorders, including SANDO, because they can precipitate and/or accelerate hepatotoxicity and liver failure. This is a critical drug-safety contraindication, particularly relevant when treating seizures.
Show evidence (1 reference)
PMID:20301791 SUPPORT Human Clinical
"Agents/circumstances to avoid: Valproic acid (Depakene®) and sodium divalproate (divalproex) (Depakote®) because of the risk of precipitating and/or accelerating liver disease."
GeneReviews explicitly lists valproic acid and divalproex as agents to avoid in POLG-related disorders due to hepatotoxicity risk.
Genetic Counseling
Action: Genetic Counseling NCIT:C15240
Genetic counseling is recommended given the autosomal recessive inheritance of SANDO; once biallelic POLG variants are identified, testing of at-risk family members and reproductive options become available.
Show evidence (1 reference)
PMID:20301791 SUPPORT Human Clinical
"Once the POLG pathogenic variants have been identified in an affected family member, testing for at-risk family members is possible."
GeneReviews supports genetic counseling and family testing once POLG variants are identified.
Deoxynucleoside (dC/dT) Substrate Supplementation
Action: Pharmacotherapy NCIT:C15986
Agent: deoxycytidine CHEBI:15698 deoxythymidine CHEBI:17748
Combination therapy with enteral deoxycytidine and deoxythymidine (dC/dT) aims to replenish mitochondrial dNTP pools and support mtDNA maintenance in POLG-related disorders. In an open-label phase 2 trial (NCT04802707) that included ataxia-neuropathy spectrum (SANDO-spectrum) patients, dC/dT was associated with improvement in the Newcastle Mitochondrial Disease Scale and reduction in the mitochondrial biomarker GDF-15. This is an investigational therapy, not an established standard of care.
Show evidence (1 reference)
PMID:39091670 SUPPORT Human Clinical
"we assessed the safety and efficacy of combination therapy with deoxycytidine and deoxythymidine (dC/dT) in children with POLG-related disorders."
Phase 2 open-label trial (NCT04802707) of enteral dC/dT in POLG-related disorders, including ataxia-neuropathy spectrum (SANDO-spectrum) patients.
🔬

Clinical Trials

1
NCT04802707 PHASE_II
Single-arm phase 2 trial of combined deoxycytidine (dC) and deoxythymidine (dT) nucleoside substrate supplementation as early treatment for mitochondrial DNA depletion syndromes, including POLG-related disease.
Show evidence (1 reference)
clinicaltrials:NCT04802707 SUPPORT Human Clinical
"In this phase II Trial a mix of Deoxynucleosides Pyrimidine (Deoxycytidine dC and Deoxythymidine dT) will be used as early treatment of MDS."
Phase II trial of dC/dT substrate supplementation enrolling mtDNA depletion disorders including POLG, directly relevant to the SANDO/POLG spectrum.
{ }

Source YAML

click to show 
name: Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis
category: Mendelian
creation_date: "2026-06-03T00:00:00Z"
synonyms:
- SANDO
- POLG-related sensory ataxic neuropathy
- mitochondrial recessive ataxia syndrome
description: >
  Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) is an
  autosomal recessive mitochondrial DNA maintenance disorder caused by biallelic
  pathogenic variants in POLG, which encodes the catalytic subunit of the
  mitochondrial DNA polymerase gamma. Defective POLG function impairs mitochondrial
  DNA replication and proofreading, producing mitochondrial DNA depletion and
  multiple mitochondrial DNA deletions with secondary respiratory-chain deficiency.
  SANDO sits within the POLG-related disorder spectrum and the ataxia neuropathy
  spectrum, and is defined by the clinical triad of sensory ataxic neuropathy,
  dysarthria, and ophthalmoparesis, frequently accompanied by progressive external
  ophthalmoplegia (PEO). It is a subgroup of the mitochondrial chronic progressive
  external ophthalmoplegia (CPEO)-plus disorders associated with multiple mtDNA
  deletions.
disease_term:
  preferred_term: sensory ataxic neuropathy, dysarthria, and ophthalmoparesis
  term:
    id: MONDO:0011835
    label: sensory ataxic neuropathy, dysarthria, and ophthalmoparesis
parents:
- POLG-Related Disorders
- Ataxia Neuropathy Spectrum
- Mitochondrial Disease
references:
- reference: PMID:20301791
  title: "POLG-Related Disorders."
  tags:
  - GeneReviews
pathophysiology:
- name: Polymerase Gamma Dysfunction and mtDNA Instability
  description: >
    Biallelic POLG variants impair the catalytic and proofreading activity of
    mitochondrial DNA polymerase gamma, the sole replicative polymerase of the
    mitochondrial genome. This disrupts mitochondrial DNA replication, leading to
    accumulation of multiple large-scale mitochondrial DNA deletions and, in some
    tissues, mitochondrial DNA depletion. The resulting clonal expansion of
    deleted mtDNA species compromises mitochondrial genome integrity.
  cell_types:
  - preferred_term: dorsal root ganglion sensory neuron
    term:
      id: CL:1001451
      label: sensory neuron of dorsal root ganglion
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
    modifier: DECREASED
  - preferred_term: mitochondrial DNA repair / proofreading
    term:
      id: GO:0006281
      label: DNA repair
    modifier: DECREASED
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in POLG gene are responsible for a wide spectrum of clinical disorders with altered mitochondrial DNA (mtDNA) integrity, including mtDNA multiple deletions and depletion."
    explanation: >
      POLG mutations directly compromise mitochondrial DNA integrity, producing
      multiple mtDNA deletions and depletion, the molecular hallmark of SANDO.
  - reference: PMID:20513108
    reference_title: "Sensory ataxic neuropathy with dysarthria and ophthalmoparesis (SANDO) in late life due to compound heterozygous POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Missense mutations in the gene for polymerase gamma 1 (POLG1) cause a number of phenotypically heterogeneous mitochondrial diseases, most commonly progressive external ophthalmoplegia, and are characterized by the accumulation of multiple, large-scale deletions of mitochondrial DNA."
    explanation: >
      Confirms that POLG1 missense mutations drive accumulation of multiple
      large-scale mtDNA deletions underlying the SANDO/PEO phenotype.
  downstream:
  - target: Respiratory-Chain Deficiency in Post-Mitotic Tissue
    description: >-
      Accumulation of multiple mtDNA deletions and depletion reduces the mtDNA
      template available for respiratory-chain subunits, producing oxidative
      phosphorylation deficiency in post-mitotic neurons and muscle.
- name: Respiratory-Chain Deficiency in Post-Mitotic Tissue
  description: >
    Accumulation of deleted and depleted mtDNA reduces the supply of mtDNA-encoded
    respiratory-chain subunits, impairing oxidative phosphorylation. In skeletal
    muscle this manifests histologically as ragged-red and cytochrome c oxidase
    (COX)-negative fibers; in long sensory neurons of the dorsal root ganglia the
    bioenergetic deficit produces sensory ataxic neuropathy.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  - preferred_term: cellular respiration
    term:
      id: GO:0045333
      label: cellular respiration
    modifier: DECREASED
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
    explanation: >
      Ragged-red and COX-negative fibers are the histological signature of
      respiratory-chain deficiency secondary to mtDNA instability in SANDO.
phenotypes:
- name: Sensory Ataxia
  description: >
    Sensory ataxic neuropathy is a defining feature of SANDO, reflecting
    degeneration of large sensory neurons and dorsal column/peripheral sensory
    pathways.
  phenotype_term:
    preferred_term: Sensory ataxia
    term:
      id: HP:0010871
      label: Sensory ataxia
    clinical_course: PROGRESSIVE
  frequency: FREQUENT
  evidence:
  - reference: PMID:25143630
    reference_title: "SANDO syndrome in a cohort of 107 patients with CPEO and mitochondrial DNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The combination of sensory ataxic neuropathy with ophthalmoparesis (SANO) was observed in 70% of patients with multiple mtDNA deletions"
    explanation: >
      Sensory ataxic neuropathy was the most frequent component phenotype in
      patients with multiple mtDNA deletions in this SANDO cohort.
- name: Peripheral Neuropathy
  description: >
    A sensory-predominant axonal peripheral neuropathy underlies the sensory
    ataxia of SANDO and is part of the defining clinical triad.
  phenotype_term:
    preferred_term: Peripheral neuropathy
    term:
      id: HP:0009830
      label: Peripheral neuropathy
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
    explanation: >
      Neuropathy was present in all five POLG-related SANDO patients in this
      series, alongside ataxia and PEO.
- name: Dysarthria
  description: >
    Dysarthria (with or without dysphagia) is one of the three defining clinical
    features of SANDO.
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis"
    explanation: >
      Dysarthria (and/or dysphagia) is an explicit member of the defining SANDO
      clinical triad.
- name: Ophthalmoparesis
  description: >
    Ophthalmoparesis, often progressing to progressive external ophthalmoplegia
    (PEO), is the third defining feature of the SANDO triad.
  phenotype_term:
    preferred_term: Ophthalmoparesis
    term:
      id: HP:0000597
      label: Ophthalmoparesis
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis"
    explanation: >
      Ophthalmoparesis completes the defining SANDO clinical triad.
- name: Progressive External Ophthalmoplegia
  description: >
    Progressive external ophthalmoplegia (PEO) is a frequent feature of SANDO and
    reflects the underlying mitochondrial myopathy with multiple mtDNA deletions.
  phenotype_term:
    preferred_term: Progressive external ophthalmoplegia
    term:
      id: HP:0000590
      label: Progressive external ophthalmoplegia
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
    explanation: >
      PEO was present in all patients in this POLG-related SANDO series.
- name: Ragged-Red Muscle Fibers
  description: >
    Skeletal muscle biopsy in SANDO frequently shows ragged-red fibers, a
    histological marker of mitochondrial proliferation in respiratory-chain
    deficient fibers.
  phenotype_term:
    preferred_term: Ragged-red muscle fibers
    term:
      id: HP:0003200
      label: Ragged-red muscle fibers
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
    explanation: >
      Ragged-red fibers were documented in muscle pathology of POLG-related SANDO
      patients.
- name: Cytochrome C Oxidase-Negative Muscle Fibers
  description: >
    COX-negative fibers on muscle histochemistry reflect focal respiratory-chain
    (Complex IV) deficiency secondary to clonally expanded mtDNA deletions.
  phenotype_term:
    preferred_term: Cytochrome C oxidase-negative muscle fibers
    term:
      id: HP:0003688
      label: Cytochrome C oxidase-negative muscle fibers
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients."
    explanation: >
      COX-negative fibers were documented in muscle pathology of POLG-related
      SANDO patients.
- name: Myopathy
  description: >
    A mitochondrial myopathy accompanies the neuropathy and ophthalmoplegia in
    SANDO, contributing to muscle weakness.
  phenotype_term:
    preferred_term: Myopathy
    term:
      id: HP:0003198
      label: Myopathy
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO)."
    explanation: >
      Myopathy was present in all patients in this POLG-related SANDO series.
- name: Ptosis
  description: >
    Ptosis frequently accompanies the external ophthalmoplegia in the
    POLG-related spectrum that includes SANDO, particularly in late-onset disease.
  phenotype_term:
    preferred_term: Ptosis
    term:
      id: HP:0000508
      label: Ptosis
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."
    explanation: >
      GeneReviews documents ptosis together with PEO and peripheral neuropathy in
      the late-onset POLG spectrum that encompasses SANDO.
- name: Seizures
  description: >
    Seizures occur in the broader POLG-related disorder spectrum and may be seen
    in SANDO-spectrum patients, particularly with juvenile/adult onset.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes"
    explanation: >
      Seizures are part of the juvenile/adult-onset POLG spectrum that
      encompasses SANDO, though they are not part of the defining SANDO triad.
genetic:
- name: POLG
  gene_term:
    preferred_term: POLG
    term:
      id: hgnc:9179
      label: POLG
  association: >-
    Biallelic (autosomal recessive) pathogenic variants in POLG, encoding the
    catalytic subunit of mitochondrial DNA polymerase gamma, cause SANDO and the
    broader POLG-related disorder spectrum through impaired mtDNA replication and
    accumulation of multiple mtDNA deletions and depletion.
  inheritance:
  - name: Autosomal recessive inheritance
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
    evidence:
    - reference: PMID:20301791
      reference_title: "POLG-Related Disorders."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Early-onset and juvenile/adult-onset POLG-related disorders are typically caused by biallelic pathogenic variants and inherited in an autosomal recessive manner."
      explanation: >
        SANDO is part of the juvenile/adult-onset POLG spectrum that follows
        autosomal recessive inheritance with biallelic POLG variants.
  evidence:
  - reference: PMID:18585914
    reference_title: "Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sensory ataxic neuropathy with ophthalmoparesis (SANDO) caused by mutations in POLG gene, fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis, has described in a few reports."
    explanation: >
      Directly establishes POLG as the causal gene for SANDO.
  - reference: PMID:25143630
    reference_title: "SANDO syndrome in a cohort of 107 patients with CPEO and mitochondrial DNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "multiple mtDNA deletions were associated with the following nuclear mutations: POLG (n=6), PEO1 (n=2), unidentified (n=2)."
    explanation: >
      POLG was the most common nuclear gene associated with SANDO in this cohort
      of patients with multiple mtDNA deletions.
  variants:
  - name: p.A467T and p.W748S
    description: >
      The recurrent POLG missense variants p.A467T and p.W748S are commonly
      implicated in the recessive POLG ataxia-neuropathy spectrum and have been
      reported, in compound heterozygous form, in patients with overlapping
      SANDO and mitochondrial recessive ataxia syndrome (MIRAS) features.
    evidence:
    - reference: PMID:22931735
      reference_title: "Early-onset ataxia with progressive external ophthalmoplegia associated with POLG mutation: autosomal recessive mitochondrial ataxic syndrome or SANDO?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "we present a case of progressive ataxia caused by A467T and W748S mutations of POLG gene, who presented with overlapping symptoms of autosomal recessive mitochondrial ataxic syndrome and SANDO"
      explanation: >
        Documents the recurrent p.A467T and p.W748S POLG variants in a patient
        with overlapping SANDO and recessive mitochondrial ataxic syndrome.
treatments:
- name: Supportive and Multidisciplinary Care
  description: >
    Management of SANDO and POLG-related disorders is largely supportive,
    involving standard approaches for associated complications including
    occupational, physical, and speech therapy; nutritional support; respiratory
    support; and standard treatment of epilepsy, movement abnormalities, vision,
    and hearing issues.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical management is largely supportive and involves standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support"
    explanation: >
      GeneReviews specifies that POLG-related disorder management is largely
      supportive and multidisciplinary.
- name: Avoidance of Valproic Acid
  description: >
    Valproic acid (Depakene) and sodium divalproate (Depakote) must be avoided in
    POLG-related disorders, including SANDO, because they can precipitate and/or
    accelerate hepatotoxicity and liver failure. This is a critical drug-safety
    contraindication, particularly relevant when treating seizures.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: valproic acid
      term:
        id: CHEBI:39867
        label: valproic acid
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Agents/circumstances to avoid: Valproic acid (Depakene®) and sodium divalproate (divalproex) (Depakote®) because of the risk of precipitating and/or accelerating liver disease."
    explanation: >
      GeneReviews explicitly lists valproic acid and divalproex as agents to
      avoid in POLG-related disorders due to hepatotoxicity risk.
- name: Genetic Counseling
  description: >
    Genetic counseling is recommended given the autosomal recessive inheritance of
    SANDO; once biallelic POLG variants are identified, testing of at-risk family
    members and reproductive options become available.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Once the POLG pathogenic variants have been identified in an affected family member, testing for at-risk family members is possible."
    explanation: >
      GeneReviews supports genetic counseling and family testing once POLG
      variants are identified.
- name: Deoxynucleoside (dC/dT) Substrate Supplementation
  description: >
    Combination therapy with enteral deoxycytidine and deoxythymidine (dC/dT)
    aims to replenish mitochondrial dNTP pools and support mtDNA maintenance in
    POLG-related disorders. In an open-label phase 2 trial (NCT04802707) that
    included ataxia-neuropathy spectrum (SANDO-spectrum) patients, dC/dT was
    associated with improvement in the Newcastle Mitochondrial Disease Scale and
    reduction in the mitochondrial biomarker GDF-15. This is an investigational
    therapy, not an established standard of care.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: deoxycytidine
      term:
        id: CHEBI:15698
        label: 2'-deoxycytidine
    - preferred_term: deoxythymidine
      term:
        id: CHEBI:17748
        label: thymidine
  evidence:
  - reference: PMID:39091670
    reference_title: "Safety and efficacy of deoxycytidine/deoxythymidine combination therapy in POLG-related disorders: 6-month interim results of an open-label, single arm, phase 2 trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we assessed the safety and efficacy of combination therapy with deoxycytidine and deoxythymidine (dC/dT) in children with POLG-related disorders."
    explanation: >
      Phase 2 open-label trial (NCT04802707) of enteral dC/dT in POLG-related
      disorders, including ataxia-neuropathy spectrum (SANDO-spectrum) patients.
clinical_trials:
- name: NCT04802707
  phase: PHASE_II
  description: >-
    Single-arm phase 2 trial of combined deoxycytidine (dC) and deoxythymidine
    (dT) nucleoside substrate supplementation as early treatment for
    mitochondrial DNA depletion syndromes, including POLG-related disease.
  evidence:
  - reference: clinicaltrials:NCT04802707
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this phase II Trial a mix of Deoxynucleosides Pyrimidine (Deoxycytidine dC and Deoxythymidine dT) will be used as early treatment of MDS."
    explanation: >-
      Phase II trial of dC/dT substrate supplementation enrolling mtDNA
      depletion disorders including POLG, directly relevant to the SANDO/POLG
      spectrum.
📚

References & Deep Research

References

1
PMID:20301791
POLG-Related Disorders.
No top-level findings curated for this source.

Deep Research

1
Falcon ▸
Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis (SANDO) — Disease Characteristics Research Report
Edison Scientific Literature 34 citations 2026-06-03T23:33:21.935243

Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis (SANDO) — Disease Characteristics Research Report

Target disease: Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) (POLG-related ataxia-neuropathy spectrum)
Category: Mendelian (nuclear gene defect affecting mtDNA maintenance)
MONDO / Orphanet / ICD / MeSH: Not captured in the retrieved sources used for this report (see “Evidence gaps”).

Executive summary

SANDO is a clinical syndrome defined by the triad of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis/ophthalmoplegia, and is best understood today as part of the broader POLG-related ataxia-neuropathy spectrum (ANS), an umbrella term that includes disorders previously referred to as MIRAS and SANDO. (specchio2020polg1relatedepilepsyreview pages 1-3, rahman2019polgrelateddisordersand pages 4-6) The dominant disease mechanism is impaired mitochondrial DNA (mtDNA) replication/repair due to pathogenic variants in POLG, leading to mtDNA deletions and/or depletion, mitochondrial respiratory chain dysfunction (often demonstrated as COX-deficient ragged-red fibers in muscle), and progressive neuro(myopathic) degeneration. (wong2008molecularandclinical pages 1-2, mchugh2010sensoryataxicneuropathy pages 1-2) A key real-world management point is that valproic acid is contraindicated in all patients with POLG mutations due to risk of precipitating liver failure. (rahman2019polgrelateddisordersand pages 4-6) The most notable 2023–2024 development relevant to disease modification is an open-label phase 2 trial of enteral deoxycytidine/deoxythymidine (dC/dT) in POLG-related disorders (ClinicalTrials.gov NCT04802707) with encouraging interim clinical and biomarker signals. (pekeles2024safetyandefficacy pages 1-2)

Evidence map (knowledge-base ready)

Category Key item Short notes
Definition/Identifiers Disease name Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO); defined within the POLG-related ataxia-neuropathy spectrum (ANS) (specchio2020polg1relatedepilepsyreview pages 1-3, wong2008molecularandclinical pages 1-2)
Definition/Identifiers Synonym/relationship ANS is an umbrella term that includes disorders previously referred to as MIRAS and SANDO (rahman2019polgrelateddisordersand pages 4-6)
Definition/Identifiers Identifier status OMIM for SANDO was not directly captured in readable evidence; MONDO/Orphanet/ICD/MeSH not captured in retrieved sources (rahman2019polgrelateddisordersand pages 4-6)
Etiology Causal gene POLG is the principal causal gene; it encodes mitochondrial DNA polymerase gamma required for mtDNA replication/repair (pekeles2024safetyandefficacy pages 1-2, specchio2020polg1relatedepilepsyreview pages 1-3)
Etiology Inheritance Biallelic pathogenic POLG variants are common in recessive POLG disease/SANDO-spectrum presentations; homozygous p.A467T and compound heterozygous cases are reported (mchugh2010sensoryataxicneuropathy pages 1-2, rotig2024distinctclinicalcourses pages 1-2)
Etiology Recurrent variants A467T, W748S, and G848S are major recurrent POLG variants across POLG-related disease cohorts; p.A467T is repeatedly linked to SANDO cases (specchio2020polg1relatedepilepsyreview pages 1-3, mchugh2010sensoryataxicneuropathy pages 3-4)
Core phenotypes Sensory ataxia/neuropathy Ataxia is often early and dominant, frequently driven by proprioceptive loss from sensory neuropathy; absent sensory nerve action potentials and sensory axonal neuropathy are typical (wei2023phenotypicvariabilityof pages 1-4, mchugh2010sensoryataxicneuropathy pages 1-2, lipponen2024mitataxhereditaryataxias pages 39-42)
Core phenotypes Dysarthria and ophthalmoparesis Progressive dysarthria with ophthalmoparesis/ophthalmoplegia and ptosis form the classic clinical triad; gaze limitation is often bilateral and progressive (wei2023phenotypicvariabilityof pages 1-4, ali2024mitochondrialchronicprogressive pages 1-3, mchugh2010sensoryataxicneuropathy pages 1-2)
Additional features Other neurologic features Dysphagia, exercise intolerance, seizures/epilepsy, hearing loss, myopathy, neuropathic pain, and encephalopathy may occur; PEO can appear late in some patients (wei2023phenotypicvariabilityof pages 1-4, rahman2019polgrelateddisordersand pages 4-6, lipponen2024mitataxhereditaryataxias pages 39-42)
Diagnostics Genetic testing Final diagnosis relies on identification of deleterious POLG variants; sequencing of mtDNA-maintenance genes and broader mitochondrial testing is recommended in appropriate phenotypes (wei2023phenotypicvariabilityof pages 1-4, kierdaszuk2020progressiveexternalophthalmoplegia pages 1-2, ali2024mitochondrialchronicprogressive pages 1-3)
Diagnostics Electrophysiology and imaging NCS/EMG commonly show sensory axonal neuropathy; EEG may show occipital slowing/epileptiform abnormalities in POLG disease; MRI can show cerebellar atrophy or thalamic/occipital/cerebellar lesions, though imaging may be noncontributory early (lipponen2024mitataxhereditaryataxias pages 39-42, wei2023phenotypicvariabilityof pages 1-4, specchio2020polg1relatedepilepsyreview pages 1-3)
Diagnostics Muscle pathology/biochemistry Muscle biopsy can show COX-deficient ragged-red fibers and multiple mtDNA deletions; however, biopsy may be normal and blood mtDNA can be less sensitive than muscle-derived material (mchugh2010sensoryataxicneuropathy pages 1-2, mchugh2010sensoryataxicneuropathy pages 3-4)
Pathophysiology Molecular chain POLG dysfunction impairs mtDNA replication/repair, causing mtDNA deletions and/or depletion, respiratory-chain/OXPHOS failure, reduced ATP production, and mitochondrial dysfunction in high-energy tissues (wei2023phenotypicvariabilityof pages 1-4, wong2008molecularandclinical pages 1-2, dombi2024nucleosidesupplementsas pages 1-2)
Pathophysiology Tissue-level consequences Post-mitotic tissues such as muscle and nervous system accumulate mtDNA defects; biopsy evidence includes COX deficiency and ragged-red fibers, consistent with mitochondrial myopathic/neurodegenerative injury (mchugh2010sensoryataxicneuropathy pages 1-2, wong2008molecularandclinical pages 1-2)
Natural history/outcomes Onset and progression Onset is variable from childhood to late adulthood; gait ataxia is a common initial symptom, and progression is chronic and degenerative with substantial morbidity (lipponen2024mitataxhereditaryataxias pages 39-42, pekeles2024safetyandefficacy pages 1-2)
Natural history/outcomes Outcome signals In broader POLG cohorts, epilepsy-associated disease can be severe: status epilepticus occurred in 46.4% of reviewed epilepsy cases and 5-year survival was 30.2%; childhood-onset POLG disease had only 6/40 survivors in one series (specchio2020polg1relatedepilepsyreview pages 1-3, rotig2024distinctclinicalcourses pages 1-2)
Treatments/management Supportive care and contraindications No definitive disease-modifying standard therapy is established; management is multidisciplinary and symptomatic. Valproic acid/valproate is contraindicated in POLG mutation carriers because it can precipitate liver failure (rahman2019polgrelateddisordersand pages 4-6, ali2024mitochondrialchronicprogressive pages 1-3)
Recent research 2023-2024 Nucleoside therapy trial Phase 2 open-label POLG trial NCT04802707 tested enteral deoxycytidine/deoxythymidine (dC/dT); in the first 10 participants, mean NMDS improved 27.3→20.7, mean GDF-15 1031→729 pg/mL, EEG improved in 5/8 abnormal baselines, with diarrhea in 2 patients (pekeles2024safetyandefficacy pages 1-2)
Recent research 2023-2024 Preclinical fibroblast findings In POLG1 fibroblasts, ATGC nucleoside supplementation increased mtDNA content and significantly improved mtDNA recovery after ddC-induced depletion in quiescent cells; lower-dose regimens reduced toxicity concerns (dombi2024nucleosidesupplementsas pages 1-2)
Models Experimental systems Yeast (Saccharomyces cerevisiae; MIP1 ortholog), patient fibroblasts, and broader POLG disease models are used to study mtDNA maintenance defects and therapeutic screening; yeast-based drug-drop tests are highlighted for mitochondrial disease discovery pipelines (magistrati2023drugdroptest pages 1-2, dombi2024nucleosidesupplementsas pages 1-2)

Table: This table condenses the main disease-knowledge-base attributes for SANDO from retrieved evidence, including etiology, phenotype, mechanisms, diagnosis, management, and recent 2023-2024 therapeutic developments. It is useful as a structured reference for curating core facts and citations.

1. Disease information

1.1 What is the disease?

SANDO is a clinical mnemonic for a progressive neurologic syndrome characterized by sensory ataxic neuropathy, dysarthria, and ophthalmoparesis/ophthalmoplegia, and is now typically classified within the POLG-related ataxia-neuropathy spectrum (ANS). (wong2008molecularandclinical pages 1-2, specchio2020polg1relatedepilepsyreview pages 1-3) In a POLG-associated ataxia case series, SANDO was used as a diagnostic label when sensory ataxia (driven by sensory neuropathy) co-occurred with ophthalmoplegia/ptosis and bulbar involvement (dysarthria/dysphagia). (wei2023phenotypicvariabilityof pages 1-4)

Direct abstract-quotable definitions/examples (for evidentiary traceability): - POLG-associated ataxia report: “Mutations in the mitochondrial DNA polymerase gamma (POLG) are causing a wide spectrum of overlapping disorders…” and patients were “identified as sensory ataxic neuropathy, dysarthria and ophthalmoparesis (SANDO)”. (wei2023phenotypicvariabilityof pages 1-4) - POLG epilepsy review: “The ataxia neuropathy spectrum (ANS) includes mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO).” (specchio2020polg1relatedepilepsyreview pages 1-3)

1.2 Key identifiers

  • OMIM / Orphanet / ICD-10/ICD-11 / MeSH / MONDO: Not captured in the retrieved evidence excerpts used here; therefore not reported to avoid fabrication. (rahman2019polgrelateddisordersand pages 4-6)

1.3 Synonyms and alternative names

  • Ataxia-neuropathy spectrum (ANS): umbrella term including disorders previously referred to as MIRAS and SANDO. (rahman2019polgrelateddisordersand pages 4-6)
  • SANDO is frequently discussed alongside the broader POLG-related disorder phenotypic spectrum (including PEO and epilepsy-dominant presentations). (rahman2019polgrelateddisordersand pages 4-6, pekeles2024safetyandefficacy pages 1-2)

1.4 Evidence source type

The SANDO-specific clinical characterization in the retrieved set is derived primarily from: - Human case reports/series (e.g., sibling pair with homozygous POLG p.A467T; muscle pathology and course). (mchugh2010sensoryataxicneuropathy pages 1-2) - Aggregated disease-level reviews and cohorts of POLG-related disorders, which provide the most robust statistics and management guidance (e.g., valproate contraindication; outcome statistics in POLG epilepsy). (rahman2019polgrelateddisordersand pages 4-6, specchio2020polg1relatedepilepsyreview pages 1-3)

2. Etiology

2.1 Disease causal factors

Primary cause: pathogenic variants in POLG, encoding the catalytic subunit of mitochondrial DNA polymerase gamma, essential for mtDNA replication and repair. (pekeles2024safetyandefficacy pages 1-2, specchio2020polg1relatedepilepsyreview pages 1-3)

Mechanistic causal chain (high-level): POLG dysfunction → impaired mtDNA replication/repair → mtDNA deletions and/or depletion → oxidative phosphorylation (OXPHOS) failure / reduced ATP production → progressive dysfunction and degeneration in high-energy tissues (nervous system, muscle). (wei2023phenotypicvariabilityof pages 1-4, wong2008molecularandclinical pages 1-2)

2.2 Risk factors

  • Genetic risk: carrying pathogenic POLG variants; in many SANDO/ANS cases this is biallelic (autosomal recessive). (mchugh2010sensoryataxicneuropathy pages 1-2, rotig2024distinctclinicalcourses pages 1-2)
  • Iatrogenic trigger: exposure to valproic acid (VPA) in individuals with POLG mutations can precipitate severe hepatic failure; it is explicitly contraindicated. (rahman2019polgrelateddisordersand pages 4-6)

2.3 Protective factors

No genetic or environmental protective factors specific to SANDO were identified in the retrieved evidence.

2.4 Gene–environment interactions

Strongest documented interaction in the retrieved evidence is drug-triggered toxicity: VPA exposure interacting with POLG genotype to precipitate liver failure. (rahman2019polgrelateddisordersand pages 4-6)

3. Phenotypes

3.1 Core phenotypes (with suggested HPO terms)

Below, phenotype type is indicated, followed by suggested HPO terms.

1) Sensory ataxia due to sensory neuropathy (symptom/sign)
- Key features: prominent proprioceptive loss and sensory neuropathy driving the ataxia. (wei2023phenotypicvariabilityof pages 1-4)
- Electrophysiology: absent sensory nerve action potentials (SNAPs) described in a SANDO sibling-pair case. (mchugh2010sensoryataxicneuropathy pages 1-2)
- Suggested HPO: HP:0001251 Ataxia, HP:0000762 Sensory ataxia, HP:0001278 Orthostatic hypotension (only if autonomic involvement), HP:0002936 Areflexia, HP:0003401 Axonal neuropathy, HP:0002497 Abnormal proprioception.

2) Dysarthria (symptom/sign)
- Reported as part of the defining triad and common associated feature in POLG ataxia syndromes. (specchio2020polg1relatedepilepsyreview pages 1-3, wei2023phenotypicvariabilityof pages 1-4)
- Suggested HPO: HP:0001260 Dysarthria.

3) Ophthalmoparesis / ophthalmoplegia / ptosis (clinical sign)
- CPEO-related definition: “bilateral symmetrical progressive ptosis and reduced ocular motility” (as a broader mitochondrial ophthalmoplegia construct relevant to SANDO). (ali2024mitochondrialchronicprogressive pages 1-3)
- Suggested HPO: HP:0000602 Ophthalmoplegia, HP:0000657 Oculomotor apraxia (only if present), HP:0000508 Ptosis.

3.2 Additional/common associated phenotypes (HPO suggestions)

  • Dysphagia (wei2023phenotypicvariabilityof pages 1-4): HP:0002015 Dysphagia.
  • Seizures / epilepsy (wei2023phenotypicvariabilityof pages 1-4, pekeles2024safetyandefficacy pages 1-2): HP:0001250 Seizures, HP:0002133 Status epilepticus.
  • Myopathy / exercise intolerance (wei2023phenotypicvariabilityof pages 1-4): HP:0003198 Myopathy, HP:0003546 Exercise intolerance.
  • Neuropathic pain noted as relatively frequent in POLG-related neuropathy cohorts (not necessarily SANDO-specific). (rahman2019polgrelateddisordersand pages 4-6): HP:0012531 Neuropathic pain.

3.3 Age of onset, severity, progression, frequency

  • Age of onset: variable; SANDO can be adult-onset (e.g., fifth decade onset in a sibling pair). (mchugh2010sensoryataxicneuropathy pages 1-2) POLG-related disease overall ranges from childhood to late adulthood. (rahman2019polgrelateddisordersand pages 4-6)
  • Progression: generally progressive; in the p.A467T sibling pair, severe disability developed over ~12 years. (mchugh2010sensoryataxicneuropathy pages 1-2)
  • Frequency among affected individuals: Specific frequency of individual SANDO features was not available in the retrieved sources; however, in a POLG cohort 6 of 11 (55%) SANDO patients had POLG mutations leading to multiple mtDNA deletions. (rahman2019polgrelateddisordersand pages 4-6)

3.4 Quality-of-life impact

Direct QoL instrument data (EQ-5D/SF-36/PROMIS) were not found in the retrieved evidence; functional decline to severe disability over years is documented in at least one SANDO family report. (mchugh2010sensoryataxicneuropathy pages 1-2)

4. Genetic / molecular information

4.1 Causal genes

  • POLG (primary): encodes mitochondrial DNA polymerase gamma. (pekeles2024safetyandefficacy pages 1-2)

4.2 Pathogenic variants and variant classes

  • Recurrent variants across POLG disease: A467T, W748S, G848S were prevalent in a POLG-epilepsy review (74.2% of patients carried one of these across aggregated cases). (specchio2020polg1relatedepilepsyreview pages 1-3)
  • SANDO-specific example (primary human case report): homozygous POLG p.A467T (c.1399G>A) in two siblings with classic SANDO triad. (mchugh2010sensoryataxicneuropathy pages 1-2)

Population frequency example (variant-specific): In a SANDO sibling-pair paper, p.A467T allele frequency in controls was reported as 0.6% (Belgian) and 0.7% (British). (mchugh2010sensoryataxicneuropathy pages 3-4)

Functional consequence example: p.A467T “results in a 96% reduction in the catalytic activity of the polymerase gamma protein.” (mchugh2010sensoryataxicneuropathy pages 3-4)

4.3 Modifier genes / epigenetics / chromosomal abnormalities

No SANDO-specific modifier gene or epigenetic signatures were identified in the retrieved evidence.

5. Environmental information

The most salient non-genetic factor in the retrieved evidence is drug exposure: - Valproic acid can precipitate liver failure in POLG-related disease and is contraindicated. (rahman2019polgrelateddisordersand pages 4-6) Other environmental/lifestyle risk factors were not identified as SANDO-specific.

6. Mechanism / pathophysiology

6.1 Molecular pathways and cellular processes

  • mtDNA replication/repair failure: POLG is essential for mtDNA replication and repair; pathogenic variants cause mtDNA maintenance defects. (pekeles2024safetyandefficacy pages 1-2)
  • mtDNA deletions/depletion → OXPHOS failure: POLG mutations can lead to mtDNA depletion and deletions with downstream defective oxidative phosphorylation and reduced ATP. (wei2023phenotypicvariabilityof pages 1-4)
  • Muscle mitochondrial pathology evidence: COX-deficient ragged-red fibers and multiple mtDNA deletions documented in SANDO siblings. (mchugh2010sensoryataxicneuropathy pages 1-2)

Suggested GO biological process terms: - GO:0006260 DNA replication (mitochondrial context), GO:0007005 mitochondrion organization, GO:0006119 oxidative phosphorylation, GO:0032543 mitochondrial translation (downstream consequences).

6.2 Cell types and tissue vulnerability

Evidence indicates vulnerability of high-energy tissues (nervous system, muscle) in mtDNA maintenance disorders; extraocular muscles are particularly affected in CPEO/ophthalmoplegia due to high mitochondrial demand. (ali2024mitochondrialchronicprogressive pages 1-3)

Suggested Cell Ontology (CL) terms: - CL:0000540 neuron, CL:0000100 motor neuron (as clinically relevant), CL:0000187 skeletal muscle cell.

6.3 Anatomical chain from trigger to manifestations (causal chain)

POLG pathogenic variant (germline) → reduced/aberrant pol γ activity → mtDNA deletions/depletion (especially in post-mitotic tissues) → respiratory chain dysfunction (COX deficiency) → impaired ATP generation → degeneration/dysfunction in peripheral sensory neurons (sensory neuropathy/ataxia), bulbar/brainstem circuits (dysarthria/dysphagia), and extraocular muscles (ophthalmoparesis). (wong2008molecularandclinical pages 1-2, mchugh2010sensoryataxicneuropathy pages 1-2, ali2024mitochondrialchronicprogressive pages 1-3)

7. Anatomical structures affected

7.1 Organ/system level

  • Nervous system: peripheral neuropathy and central manifestations (seizures in some POLG cases). (rahman2019polgrelateddisordersand pages 4-6, pekeles2024safetyandefficacy pages 1-2)
  • Skeletal muscle / extraocular muscle system: ophthalmoplegia/ptosis and myopathy features. (ali2024mitochondrialchronicprogressive pages 1-3, rahman2019polgrelateddisordersand pages 4-6)
  • Liver (in POLG spectrum): vulnerability especially with VPA exposure; hepatic failure prominent in severe POLG phenotypes (Alpers and others). (rahman2019polgrelateddisordersand pages 4-6, pekeles2024safetyandefficacy pages 1-2)

Suggested UBERON terms: - Peripheral nerve (UBERON:0001021), cerebellum (UBERON:0002037; if cerebellar component), extraocular muscle (UBERON:0001631), skeletal muscle tissue (UBERON:0001134), liver (UBERON:0002107).

7.2 Subcellular localization

  • Mitochondrion (matrix/nucleoid context): mtDNA maintenance and OXPHOS systems. (pekeles2024safetyandefficacy pages 1-2, zeviani2022mitochondrialneurodegeneration pages 1-2)

Suggested GO cellular component terms: - GO:0005739 mitochondrion, GO:0005759 mitochondrial matrix, GO:0005764 lysosome (only if evidence; not found here), GO:0005743 mitochondrial inner membrane.

8. Temporal development

  • Onset: from childhood to adult/late-life; case evidence includes fifth-decade onset. (mchugh2010sensoryataxicneuropathy pages 1-2, rahman2019polgrelateddisordersand pages 4-6)
  • Course: chronic progressive, degenerative. (pekeles2024safetyandefficacy pages 1-2)

9. Inheritance and population

9.1 Inheritance

  • Many POLG-related SANDO/ANS presentations are associated with biallelic pathogenic variants (autosomal recessive). (mchugh2010sensoryataxicneuropathy pages 1-2, rotig2024distinctclinicalcourses pages 1-2)

9.2 Epidemiology

  • SANDO-specific prevalence/incidence was not found in the retrieved sources.

10. Diagnostics

10.1 Clinical tests and biomarkers

  • Electrophysiology: sensory neuropathy patterns (e.g., absent SNAPs). (mchugh2010sensoryataxicneuropathy pages 1-2)
  • Muscle biopsy: COX-deficient ragged-red fibers; detection of multiple mtDNA deletions (long-range PCR). (mchugh2010sensoryataxicneuropathy pages 1-2)
  • Blood biomarkers: GDF-15 used as a “biomarker of mitochondrial dysfunction” in the 2024 POLG dC/dT trial. (pekeles2024safetyandefficacy pages 1-2)

10.2 Genetic testing

  • Gold standard: molecular confirmation of deleterious POLG variants; WES was used in a POLG-associated ataxia report and “The final diagnosis relies on the molecular finding of deleterious mutations in POLG.” (wei2023phenotypicvariabilityof pages 1-4)
  • Testing caveat: lymphocyte-derived mtDNA can be less sensitive than muscle-derived tissue for detecting mtDNA deletions in POLG disease. (mchugh2010sensoryataxicneuropathy pages 3-4)

10.3 Differential diagnosis

Not exhaustively enumerated in retrieved evidence; the main differential frame is within autosomal recessive ataxias with ocular involvement and mitochondrial CPEO+ phenotypes, where POLG is a key gene to test. (ali2024mitochondrialchronicprogressive pages 1-3, lopergolo2024autosomalrecessivecerebellar pages 6-7)

11. Outcome / prognosis

SANDO-specific survival estimates were not retrieved; prognosis is typically progressive disability.

For broader POLG-related disease (important contextual statistics): - POLG-related epilepsy review (195 patients): status epilepticus in 46.4%; 5-year survival 30.2%. (specchio2020polg1relatedepilepsyreview pages 1-3) - Pediatric biallelic POLG cohort (n=40): 24/40 required urgent neurointensive care for seizures/status epilepticus, and 6/40 survived (study-specific cohort). (rotig2024distinctclinicalcourses pages 1-2)

12. Treatment

12.1 Standard-of-care management (symptomatic/supportive)

No definitive established disease-modifying therapy exists for POLG-related disease; management is typically symptomatic and multidisciplinary. (pekeles2024safetyandefficacy pages 1-2, ali2024mitochondrialchronicprogressive pages 1-3)

Critical safety guidance: - Review-level expert statement: “valproic acid (VPA) is contra-indicated in all patients with POLG mutations,” and can precipitate liver failure. (rahman2019polgrelateddisordersand pages 4-6)

Suggested MAXO terms: - MAXO:0000058 genetic testing, MAXO:0000474 physical therapy, MAXO:0000934 speech therapy, MAXO:0000747 seizure management, MAXO:0001072 avoidance of harmful medication (for VPA avoidance).

12.2 Recent developments (prioritizing 2023–2024)

(A) Phase 2 open-label dC/dT trial in POLG disorders (2024)

Study: Pekeles et al., eClinicalMedicine (Aug 2024), “Safety and efficacy of deoxycytidine/deoxythymidine combination therapy in POLG-related disorders: 6-month interim results…”
URL: https://doi.org/10.1016/j.eclinm.2024.102740
ClinicalTrials.gov: NCT04802707 (trial registration cited in paper). (pekeles2024safetyandefficacy pages 1-2)

Key quantitative interim results in first 10 participants: - Mean NMDS improved from 27.3 to 20.7 at 6 months. (pekeles2024safetyandefficacy pages 1-2) - Mean GDF-15 decreased from 1031 pg/mL to 729 pg/mL. (pekeles2024safetyandefficacy pages 1-2) - EEG improved in 5/8 with abnormal baseline EEG. (pekeles2024safetyandefficacy pages 1-2) - Notable adverse event: diarrhea in 2 patients, self-resolving. (pekeles2024safetyandefficacy pages 1-2)

(B) POLG/TWNK fibroblast nucleoside supplementation study (2024)

Study: Dombi et al., Frontiers in Cell and Developmental Biology (Published 02 Apr 2024)
URL: https://doi.org/10.3389/fcell.2024.1260496
Key findings: In POLG1 cells, certain nucleoside combinations (notably ATGC) increased mtDNA content and significantly improved mtDNA recovery after ddC-induced depletion, with dose-dependent toxicity mitigated at lower concentrations. (dombi2024nucleosidesupplementsas pages 1-2)

13. Prevention

SANDO is genetic; therefore prevention is mainly via genetic counseling and avoidance of triggers. - Primary prevention (iatrogenic): avoid valproate in POLG mutation carriers. (rahman2019polgrelateddisordersand pages 4-6) - Reproductive prevention: not specifically described in retrieved evidence.

14. Other species / natural disease

No naturally occurring veterinary SANDO-like disease evidence was found in the retrieved sources.

15. Model organisms

  • Yeast (Saccharomyces cerevisiae): highlighted as an efficient model organism for mitochondrial disease variant validation and drug screening (“drug drop test”). (magistrati2023drugdroptest pages 1-2)
  • Patient-derived fibroblasts: used to assay mtDNA copy number and mitochondrial membrane potential and to test nucleoside combinations in POLG/TWNK deficiency. (dombi2024nucleosidesupplementsas pages 1-2)

Evidence gaps and limitations (important for knowledge-base curation)

1) Formal disease identifiers (MONDO, Orphanet, ICD-10/11, MeSH, and explicit OMIM for SANDO) were not present in the retrieved full-text excerpts available to the toolchain for this run; they are therefore intentionally omitted to avoid hallucination. (rahman2019polgrelateddisordersand pages 4-6) 2) Many quantitative statistics come from broader POLG cohorts (especially epilepsy-focused), not SANDO-only cohorts, reflecting the rarity of SANDO and limited cohort-size literature. (specchio2020polg1relatedepilepsyreview pages 1-3, rotig2024distinctclinicalcourses pages 1-2)

Key references (with publication dates and URLs)

  • Rahman S, Copeland WC. POLG-related disorders and their neurological manifestations. Nat Rev Neurol. (manuscript available in PMC 2022; published 2019). https://doi.org/10.1038/s41582-018-0101-0 (rahman2019polgrelateddisordersand pages 4-6)
  • Pekeles H, et al. Safety and efficacy of deoxycytidine/deoxythymidine combination therapy in POLG-related disorders… eClinicalMedicine. Aug 2024. https://doi.org/10.1016/j.eclinm.2024.102740 (pekeles2024safetyandefficacy pages 1-2)
  • Dombi E, et al. Nucleoside supplements as treatments for mitochondrial DNA depletion syndrome. Front Cell Dev Biol. 02 Apr 2024. https://doi.org/10.3389/fcell.2024.1260496 (dombi2024nucleosidesupplementsas pages 1-2)
  • McHugh JC, et al. SANDO in a sibling pair with a homozygous p.A467T POLG mutation. Muscle Nerve. Feb 2010. https://doi.org/10.1002/mus.21494 (mchugh2010sensoryataxicneuropathy pages 1-2)
  • Specchio N, et al. POLG1-Related Epilepsy: Review of Diagnostic and Therapeutic Findings. Brain Sciences. Oct 2020. https://doi.org/10.3390/brainsci10110768 (specchio2020polg1relatedepilepsyreview pages 1-3)
  • Wong LJC, et al. Molecular and clinical genetics of mitochondrial diseases due to POLG mutations. Human Mutation. Sep 2008. https://doi.org/10.1002/humu.20824 (wong2008molecularandclinical pages 1-2)
  • Ali A, et al. Mitochondrial Chronic Progressive External Ophthalmoplegia. Brain Sciences. Jan 2024. https://doi.org/10.3390/brainsci14020135 (ali2024mitochondrialchronicprogressive pages 1-3)

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Artifacts