Spinocerebellar Ataxia Type 23

Mendelian MONDO:0012449 Pathograph 10 Show in embeddings browser autosomal dominant disease spinocerebellar ataxia

SCA23 is a rare, late-onset, autosomal dominant cerebellar ataxia attributed to missense variants in PDYN, the precursor of the opioid neuropeptides alpha-neoendorphin and dynorphins A and B. It is unusual among the spinocerebellar ataxias in that the mutant product is a small secreted neuropeptide rather than a channel, chaperone or repeat-expanded protein. The proposed mechanism is a gain of toxicity: variants clustered in the dynorphin A coding region distort the peptide's secondary structure, which both lowers its kappa-opioid receptor affinity and makes it resistant to degradation, so a peptide that should be transient persists and acts instead through NMDA receptors. The result is proposed to be a convergence of lost opioid-mediated neuroprotection and gained NMDA-mediated excitotoxicity, producing Purkinje cell loss. Curators should note that the gene-disease relationship has been formally challenged in the literature and is more contested than for most monogenic ataxia genes — see the controversy_pdyn_gene_validity discussion.

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Inheritance
7
Pathophys.
3
Histopath.
10
Phenotypes
4
Gaps
10
Pathograph
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Genes
1
Medical Actions
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Differentials
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Models
16
References
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Deep Research
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Classifications

Harrison's Part
NEUROLOGIC
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Inheritance

1
Autosomal dominant inheritance HP:0000006
SCA23 is autosomal dominant, described in four Dutch families in the original report and subsequently in single families elsewhere. Penetrance is incomplete and onset is late, which is material to the variant-interpretation controversy recorded in this entry.
Autosomal dominant inheritance
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"Spinocerebellar ataxias (SCAs) are dominantly inherited neurodegenerative disorders characterized by progressive cerebellar ataxia and dysarthria. We have identified missense mutations in prodynorphin (PDYN) that cause SCA23 in four Dutch families displaying progressive gait and limb ataxia."
Establishes dominant inheritance and the four-family basis of the original gene assignment.
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Discussions and Knowledge Gaps

4
Is PDYN genuinely the cause of SCA23, and are all the originally reported variants pathogenic?
CONTROVERSY OPEN controversy_pdyn_gene_validity
Attached to
pathophysiology#PDYN Missense Variant in the Dynorphin A Coding Region
This is not an ordinary knowledge gap: the gene-disease relationship was formally challenged in print. In 2016 Brain published a correspondence titled "SCA23 and prodynorphin: is it time for gene retraction?", and the original authors published a reply under the matching title in the same issue. Both are carried in this entry's references. Neither can be quoted here: both cache as metadata only, with no abstract body, so no snippet is possible and the specifics of the argument are deliberately not asserted in this entry. What can be said from sources that are quotable is that the functional evidence is variant-specific rather than uniform — L5S behaves oppositely to R6W and R9C, and wild-type dynorphin A was among the most toxic peptides in one cerebellar neuron assay — and that the original gene assignment rested on four families from one country. Against that, segregation in those families, an animal model reproducing the phenotype, and orthogonal structural and cellular work all support causality. The practical consequence for curation is that a PDYN missense variant should not be treated as pathogenic on the strength of the gene assignment alone.
Proposed experiments
Formal gene-disease validity reassessment
exp_pdyn_gene_validity_reassessment
Submit PDYN-SCA23 to a structured gene-disease validity framework such as ClinGen's, pooling segregation evidence from all reported families, population-database frequencies for each reported allele, and the variant-level functional data, and classify each allele separately. This would replace a contested binary — is the gene real or not — with a per-variant answer, which is what the heterogeneous functional data actually support.
No ClinGen gene-disease validity assertion for PDYN was present in this repository's cached ClinGen records at the time of curation, so none is cited.
If wild-type dynorphin A is itself toxic to cerebellar neurons at sustained concentration, what exactly do the SCA23 variants add?
KNOWLEDGE GAP OPEN gap_wildtype_dynorphin_toxicity
Attached to
pathophysiology#Persistent Elevation of Mutant Dynorphin A pathophysiology#Switch from Opioid to NMDA Receptor Signalling
The cited toxicity assay found R6W and wild-type dynorphin A to be the most toxic peptides tested, and the same study reports that the opioid-to-NMDA receptor switch was seen for R6W but not for wild-type. Those two results sit awkwardly together: if the wild-type peptide can be equally toxic without the receptor switch, then the switch is not the whole explanation for toxicity, and the disease-relevant variable may be persistence and local concentration rather than any change in receptor preference. Resolving this matters for the therapeutic proposal that comes out of the mouse model, which is to lower the mutant peptide: if concentration is the variable, that proposal is well founded; if the receptor switch is, then lowering total peptide may not be the right target.
Proposed experiments
Dose-matched wild-type versus mutant toxicity with receptor blockade
exp_dose_matched_wt_versus_mutant_toxicity
Compare wild-type and mutant dynorphin A on primary cerebellar neurons at matched sustained concentrations, with and without NMDA receptor blockade and kappa-opioid agonism. If NMDA blockade rescues the mutant but not the wild-type peptide, the receptor switch is doing real work; if it rescues both equally, concentration is the dominant variable and the switch is incidental.
Show evidence (1 reference)
PMID:27260403 SUPPORT In Vitro
"R6W and wt Dyn A peptides were most toxic to primary cerebellar neurons. For R6W Dyn A, this is likely because of a switch from opioid to NMDA- receptor signalling, while for wt Dyn A, this switch was not observed."
Both halves of the tension are in this one passage: comparable toxicity, but the mechanism invoked for one and explicitly not observed for the other.
The R212W mouse cerebellum is already abnormal at two weeks of age. Does a developmental innervation deficit contribute to human SCA23, a disease that presents in the fifth decade?
HUMAN MODEL MISMATCH OPEN developmental_versus_degenerative
Attached to
pathophysiology#Cerebellar Developmental Deficit in Purkinje Cell Innervation pathophysiology#Purkinje Cell Degeneration and Climbing Fibre Retraction
This is a HUMAN_MODEL_MISMATCH rather than a KNOWLEDGE_GAP because the evidence is not absent — it is unusually detailed — but it exists only in a mouse, and the translational question is sharp rather than routine. The mouse shows reduced GABAergic synapses on Purkinje cell soma, delayed early-phase climbing fibre elimination, climbing fibres that never reach terminal height, and parallel-fibre takeover of the vacated proximal dendrite, all by three weeks of age. Human SCA23 presents at a mean age around 43 years, with reported onset as early as 10 and as late as 73. A developmental lesion of that severity would be expected to produce childhood signs, and none are reported. Three readings are compatible with the current evidence, and nothing yet distinguishes them. The developmental deficit may be a mouse-specific consequence of the transgenic construct or of murine cerebellar development, with no human counterpart. It may exist in humans as a subclinical substrate — a cerebellum built with less climbing fibre territory than normal, which tolerates later loss worse — which would explain adult onset without requiring childhood signs. Or the adult retraction may be genuinely degenerative in both species, with the developmental finding a separable early effect that does not feed it. The distinction is not academic. If a developmental substrate is real in humans, the therapeutic window for peptide-lowering closes before diagnosis is possible, which changes what the animal-model authors' own therapeutic proposal can be expected to achieve.
Proposed experiments
Adult-restricted PDYN R212W expression in mouse
exp_conditional_adult_onset_pdyn
Express the R212W allele under temporal control, switching it on only after cerebellar circuit maturation is complete, and compare adult motor phenotype, climbing fibre morphology and Purkinje cell survival against the constitutive model. If the adult phenotype is reproduced without the developmental window, the degeneration does not require the developmental deficit; if it is attenuated, the developmental substrate is load-bearing.
Cerebellar imaging in presymptomatic PDYN variant carriers
exp_presymptomatic_carrier_cerebellar_imaging
Image cerebellar volume and, where feasible, functional connectivity in young, asymptomatic obligate carriers from reported SCA23 families. The question is whether the carrier cerebellum differs from control before any degeneration could plausibly have begun. This is directly motivated by the documented asymptomatic p.R215H carrier who nonetheless had apparent cerebellar atrophy on MRI, which is at least consistent with a substrate preceding symptoms.
Show evidence (2 references)
PMID:33043513 SUPPORT Model Organism
"These findings indicate that developmental abnormalities contribute to the SCA23 pathology and uncover a developmental role for PDYN in the cerebellum."
The claim whose human applicability is the open question.
PMID:32587707 SUPPORT Human Clinical
"the proband's mother with the PDYN p.R215H variant had no subjective symptoms; she had not come to medical attention before our survey, although she showed apparent cerebellar atrophy on brain magnetic resonance imaging (MRI)"
Classified PARTIAL. A structurally abnormal cerebellum in a carrier with no symptoms is consistent with a substrate preceding disease, but a single 64-year-old carrier cannot distinguish a developmental substrate from subclinical degeneration — which is exactly why the imaging experiment above is proposed in young carriers.
Is Purkinje cell calcium handling actually dysregulated in SCA23, or is the reported change confined to channel subunit expression?
KNOWLEDGE GAP OPEN calcium_handling_in_sca23
Attached to
pathophysiology#Purkinje Cell Degeneration and Climbing Fibre Retraction pathophysiology#Switch from Opioid to NMDA Receptor Signalling
This gap exists because of what it blocks. The `cerebellar_purkinje_degeneration` module carries a `#Purkinje Cell Calcium and Proteostasis Dysregulation` amplifier node that most conforming disorders declare, and SCA23 has two independent reasons to expect it: an NMDA-receptor-mediated excitotoxic mechanism, which implies calcium entry, and a reported change in the expression of several critical Ca2+ channel subunits in the R212W mouse. Neither is a measurement of calcium handling. The channel-subunit result is an expression change, and its own authors go no further than "potentially contributing to altered Ca2+ transients"; the excitotoxicity route is an inference from receptor pharmacology, not an observation in SCA23 tissue. So the amplifier is left unclaimed, and the entry states which measurement is missing rather than which evidence is absent.
Proposed experiments
Purkinje cell calcium imaging in the R212W mouse
exp_purkinje_calcium_imaging_r212w
Measure calcium transients in Purkinje cells of PDYN R212W and wild-type littermates across the ages that bracket the known pathology — three weeks, three months and twelve months — using two-photon imaging in acute cerebellar slices, with climbing-fibre and parallel-fibre stimulation separately. A transient abnormality present at three weeks would tie calcium to the developmental arm; one appearing only at three months would tie it to the degenerative arm; absence at all three would settle the non-conformance affirmatively rather than by default.
Show evidence (1 reference)
PMID:33043513 SUPPORT Model Organism
"Additionally, we detected altered expression of several critical Ca2+ channel subunits, potentially contributing to altered Ca2+ transients in PDYNR212W cerebella."
The whole gap in one sentence: an expression measurement, and an explicitly hedged inference to the functional consequence that has not been measured.
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Pathophysiology

7
PDYN Missense Variant in the Dynorphin A Coding Region
PDYN encodes the precursor of alpha-neoendorphin and dynorphins A and B. The reported SCA23 variants cluster in the dynorphin A coding region, which the mouse-model authors describe as a likely mutational hotspot; a further variant sits in the non-opioid domain of the precursor. This is the disorder-specific substitution this entry makes against the conserved Purkinje-degeneration module, whose trigger node is deliberately generic about the molecular lesion.
Cerebellar Purkinje cell CL:0000121 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Cerebellar Purkinje cell, annotated with Purkinje cell (CL:0000121). CL:0000121 is a cell type from the Cell Ontology.
PDYN hgnc:8820 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves PDYN (hgnc:8820). hgnc:8820 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Altered Dynorphin A Secondary Structure The substitutions disrupt the peptide's fold.
Show evidence (3 references)
PMID:21035104 SUPPORT Human Clinical
"PDYN is the precursor protein for the opioid neuropeptides, α-neoendorphin, and dynorphins A and B (Dyn A and B)."
Establishes what the gene encodes, which is what makes this disorder mechanistically unusual among the ataxias.
PMID:26169942 SUPPORT Other
"Dynorphin A is likely a mutational hotspot for spinocerebellar ataxia type 23 mutations"
Supports the clustering of variants in the dynorphin A region, quoted with the authors' own hedge ("likely").
PMID:21035104 SUPPORT Human Clinical
"The fourth mutation was located in the nonopioid PDYN domain and was associated with altered expression of components of the opioid and glutamate system, as evident from analysis of SCA23 autopsy tissue."
Documents the non-opioid-domain variant and the autopsy finding, which is the only human tissue evidence in the original report.
Altered Dynorphin A Secondary Structure
Loss of the peptide's N-terminal alpha-helix is the pivot of the whole mechanism, because it does two things at once. It lowers kappa-opioid receptor affinity, removing the peptide's normal signalling identity; and, for the R6W and R9C variants, it confers marked degradation resistance and reduced solubility, so the peptide persists. Not every variant behaves this way — L5S showed increased degradation and no aggregation — which is curated here because it bears directly on whether all reported variants share a mechanism.
kappa-type opioid receptor binding GO:0031851 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased kappa-type opioid receptor binding (GO:0031851). GO:0031851 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Persistent Elevation of Mutant Dynorphin A A peptide that resists degradation accumulates.
Show evidence (4 references)
PMID:27260403 SUPPORT In Vitro
"disrupted peptide secondary structure leading to a loss of the N-terminal α-helix associated with decreased κ-opioid receptor affinity"
Links the structural change to the loss of opioid receptor affinity.
PMID:27260403 SUPPORT In Vitro
"the altered secondary structure led to increased peptide stability of R6W and R9C Dyn A, as these peptides showed marked degradation resistance, which coincided with decreased peptide solubility. Notably, L5S Dyn A displayed increased degradation and no aggregation."
Establishes both the degradation resistance of two variants and the explicit exception, which is why this entry does not claim a uniform mechanism across alleles.
PMID:34944698 SUPPORT In Vitro
"DynA mutations, associated with a severe ataxic phenotype, decrease potency of KOR activation, both for G-protein dissociation as well as β-arrestin recruitment"
Independent confirmation that the DECREASED modifier on kappa-opioid receptor binding has a functional consequence, and one that goes further than binding affinity: potency falls for both downstream branches of KOR signalling, G-protein and beta-arrestin alike. Note the qualifier the authors attach — the mutants tested were those associated with a severe ataxic phenotype, so this is not a blanket claim about every reported allele.
+ 1 more reference
Persistent Elevation of Mutant Dynorphin A
Dynorphin A is normally a transient signal. When it persists, it becomes toxic — a property of the wild-type peptide at sustained concentration, not only of the mutants. The mouse model carrying the R212W allele shows markedly elevated mutant dynorphin A in the cerebellum, which is the in vivo confirmation that the cellular observation translates.
neuropeptide signaling pathway GO:0007218 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal neuropeptide signaling pathway (GO:0007218). GO:0007218 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Animal models
PDYN R212W knock-in mouse Mouse recapitulates Radioimmunoassay shows markedly elevated mutant dynorphin A in the cerebellum, which is the in vivo counterpart of the cellular overproduction seen in the original report.
Downstream
Switch from Opioid to NMDA Receptor Signalling A peptide with lost opioid affinity but sustained presence engages a different receptor system.
Cerebellar Developmental Deficit in Purkinje Cell Innervation The same mouse that carries the elevated mutant peptide shows a developmental innervation defect from two weeks of age. Whether the elevated peptide is what causes it — and by what route — is not established; the link is drawn because both are properties of the same allele in the same model, and typed to say so.
Show evidence (2 references)
PMID:27260403 SUPPORT In Vitro
"Increased concentration of or prolonged exposure to Dyn A is neurotoxic"
States that it is the persistence and concentration, rather than the mutation per se, that makes the peptide toxic.
PMID:26169942 SUPPORT Model Organism
"Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A"
In vivo measurement of the elevated peptide in the model that reproduces the disease.
Cerebellar Developmental Deficit in Purkinje Cell Innervation
A second, non-degenerative arm, and the reason SCA23 cannot be read as purely late-onset neurodegeneration. In the R212W mouse the cerebellum is already abnormal at two weeks of age — long before the motor deficits at three months and the Purkinje cell loss at twelve. GABAergic synapses on the Purkinje cell soma are reduced; early-phase climbing fibre elimination is delayed between two and three weeks; climbing fibres fail to reach their terminal height, and the proximal dendrite they should occupy is taken over by parallel fibres, with increased vGlut1 as the marker of that takeover. This reframes the climbing fibre retraction curated on the degeneration node below: part of it may be territory never properly established rather than connectivity lost. The two are not alternatives — the mouse shows both, at different ages — and this entry curates them as separate nodes rather than collapsing them, because no experiment has yet apportioned the adult phenotype between them. Everything here is mouse. No human SCA23 developmental study exists, and the disease presents in the fifth decade, so the translational status of this arm is genuinely open — recorded as the developmental_versus_degenerative discussion.
Cerebellar Purkinje cell CL:0000121 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Cerebellar Purkinje cell, annotated with Purkinje cell (CL:0000121). CL:0000121 is a cell type from the Cell Ontology.
climbing fibre synapse elimination GO:0098883 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased climbing fibre synapse elimination, annotated with synapse pruning (GO:0098883). GO:0098883 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Purkinje Cell Degeneration and Climbing Fibre Retraction A cerebellum whose climbing fibre territory was never fully established plausibly enters adult life with less to lose, but no experiment has tested whether the developmental deficit is required for, contributes to, or is merely concurrent with the adult degeneration.
Show evidence (3 references)
PMID:33043513 SUPPORT Model Organism
"we uncovered developmental deficits from 2 weeks of age, namely a reduced number of GABAergic synapses on PC soma, possibly leading to the observed delay in early phase CF elimination between 2 and 3 weeks of age"
The primary developmental finding, with its age and the authors' own "possibly" linking the two observations.
PMID:33043513 SUPPORT Model Organism
"Furthermore, CFs did not reach terminal height, leaving proximal PC dendrites open to be occupied by parallel fibers (PFs)."
The failure of climbing fibres to establish their normal territory — the observation that makes this developmental rather than degenerative.
PMID:33043513 SUPPORT Model Organism
"indicates that PFs indeed take over CF territory and have increased connectivity with PCs"
The parallel-fibre takeover, evidenced by the vGlut1 increase reported in the same sentence.
Switch from Opioid to NMDA Receptor Signalling
Dynorphin A peptides bind NMDA receptors and potentiate their glutamate-evoked currents, a long-recognized non-opioid action. In SCA23 the proposal is that the mutant peptide's loss of kappa-opioid affinity leaves this NMDA action unopposed, so the same molecule that should have been neuroprotective becomes excitotoxic. The reporting authors frame the pathology as a convergence of the two, not as either alone.
glutamate receptor signaling pathway GO:0007215 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased glutamate receptor signaling pathway (GO:0007215). GO:0007215 is a biological process from the Gene Ontology. ↑ INCREASED
NMDA glutamate receptor activity GO:0004972 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves increased NMDA glutamate receptor activity (GO:0004972). GO:0004972 is a molecular function from the Gene Ontology. ↑ INCREASED
Downstream
Purkinje Cell Degeneration and Climbing Fibre Retraction Excitotoxic signalling in the cerebellum is proposed as the route to Purkinje cell loss; the intervening steps are not established.
Show evidence (2 references)
PMID:27260403 SUPPORT In Vitro
"these deleterious effects are very likely caused by an N-methyl-d-aspartate-mediated non-opioid mechanism as Dyn A peptides were shown to bind NMDA receptors and potentiate their glutamate-evoked currents"
Establishes the NMDA mechanism and its molecular basis, with the authors' hedge preserved.
PMID:27260403 SUPPORT In Vitro
"We propose that the pathology of SCA23 results from converging mechanisms of loss of opioid-mediated neuroprotection and NMDA-mediated excitotoxicity."
Classified PARTIAL because the authors present the convergence as a proposal. It is the framing this entry's two-arm node adopts, and the hedge is preserved rather than upgraded.
Purkinje Cell Degeneration and Climbing Fibre Retraction
The tissue-level lesion. In the R212W mouse the elevated mutant peptide is accompanied by climbing fibre retraction and Purkinje cell loss on immunohistochemistry, together with transcriptional dysregulation of ionotropic and metabotropic glutamate receptors and glutamate transporters and altered neuronal excitability — which ties the receptor-level mechanism above to the cellular one. This entry does not declare conformance to the module's Purkinje calcium and proteostasis node, and the reason needs stating precisely rather than as a blanket absence of evidence. PMID:33043513 did measure something calcium-adjacent in the R212W mouse: altered *expression* of several critical Ca2+ channel subunits. What it did not do is measure calcium handling — the authors themselves write only that this is "potentially contributing to altered Ca2+ transients", with no transient, buffering, or store-release measurement reported. Nothing bearing on protein quality control has been measured at all, in mouse or in human tissue. Expression of a channel subunit is not a measurement of the dysregulated calcium handling that amplifier node asserts, so the conformance stays unclaimed — but as a specific gap, with a named experiment that would close it (see the calcium_handling_in_sca23 discussion).
Cerebellar Purkinje cell CL:0000121 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Cerebellar Purkinje cell, annotated with Purkinje cell (CL:0000121). CL:0000121 is a cell type from the Cell Ontology.
Animal models
PDYN R212W knock-in mouse Mouse recapitulates Climbing fibre retraction and Purkinje cell loss on immunohistochemistry, with a progressive motor phenotype.
Downstream
Progressive Cerebellar Ataxia Loss of Purkinje cells produces the clinical syndrome.
Show evidence (3 references)
PMID:26169942 SUPPORT Model Organism
"markedly elevated levels of mutant dynorphin A, which are associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings"
The histological demonstration in the model. The spelling "climber fibre" is as it appears in the source.
PMID:33043513 SUPPORT Model Organism
"Additionally, we detected altered expression of several critical Ca2+ channel subunits, potentially contributing to altered Ca2+ transients in PDYNR212W cerebella."
The nearest thing to a calcium measurement in SCA23, quoted with the authors' own hedge. Classified PARTIAL and cited here precisely because it is what the non-conformance rationale above has to be stated against: an expression change in channel subunits, not a measurement of calcium handling.
PMID:26169942 SUPPORT Model Organism
"the pathologically elevated mutant dynorphin A levels in the cerebellum coincided with transcriptionally dysregulated ionotropic and metabotropic glutamate receptors and glutamate transporters, and altered neuronal excitability"
Connects the peptide elevation to glutamatergic dysregulation in the cerebellum, which is the tissue-level counterpart of the NMDA node above.
Progressive Cerebellar Ataxia
The clinical endpoint: slowly progressive gait and limb ataxia with dysarthria, beginning in adult life.
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"four Dutch families displaying progressive gait and limb ataxia"
The clinical phenotype of the families in which the gene was identified.
✶

Histopathology

3
Neuronal loss in Purkinje cell layer, dentate nuclei and inferior olives
The human neuropathology, from autopsy of one affected member of the founding Dutch family. Neuronal loss is not confined to Purkinje cells: it involves the dentate nuclei and the inferior olives as well — that is, all three elements of the olivo-cerebellar circuit whose climbing-fibre arm the pathograph models. This is the direct human counterpart of the Purkinje cell loss demonstrated in the mouse, and the reason the degeneration node is not curated on model evidence alone. No `finding_term` is bound, and the reason is the enum rather than the ontologies. `HistopathologyFindingTerm` is reachable only from the NCIT Histopathology Result branch (NCIT:C83490) plus a handful of HP rosette terms. HP:0002529 Neuronal loss in central nervous system names this finding exactly but is not a member, and NCIT's Histopathology Result branch has no neuronal-loss morphologic finding — searching it returns disease entities, not findings. Rather than bind something broader that would mean less, the finding is carried by `name` and `description`. Note this is n = 1: one autopsy from one family, reported before PDYN was identified.
Show evidence (2 references)
PMID:15306549 SUPPORT Human Clinical
"Neuropathological examination in one affected subject showed neuronal loss in the Purkinje cell layer, dentate nuclei and inferior olives, thinning of cerebellopontine tracts, demyelination of posterior and lateral columns in the spinal cord, as well as ubiquitin-positive intranuclear inclusions..."
The original autopsy description, quoted whole because the entry curates three separate findings out of it and the reader should see they come from one observation in one subject.
PMID:19089525 SUPPORT Human Clinical
"Neuropathological examination revealed neuronal loss in the Purkinje cell layer, dentate nuclei, and inferior olives."
The later review restating the same finding. Cited as corroboration of the record, not as an independent observation — it describes the same autopsy.
Demyelination of the spinal cord posterior and lateral columns
Spinal cord involvement in the same autopsy, in the posterior (dorsal) and lateral columns. This is the anatomical substrate that would account for the pyramidal signs — the hyperreflexia curated above — in a disorder otherwise described as a pure cerebellar syndrome, and it is why "spinocerebellar" rather than "cerebellar" is the right word for this disease. Unbound for the same enum reason as the finding above: NCIT:C117277 Demyelination exists but is not reachable from the Histopathology Result branch, so it fails `HistopathologyFindingTerm`. On the HP side, HP:0008311 Spinal cord dorsal column hypomyelination names a developmental failure to myelinate rather than the demyelination reported here, and covers only one of the two involved column systems — it would assert the wrong thing even if it were a member.
Show evidence (1 reference)
PMID:15306549 SUPPORT Human Clinical
"demyelination of posterior and lateral columns in the spinal cord"
The spinal cord finding, naming both involved column systems.
Ubiquitin-positive intranuclear inclusions in nigral neurons
Curated as a documented negative, because it is the kind of finding that gets propagated as disease pathology once it appears in a table. The same autopsy showed ubiquitin-positive intranuclear inclusions in substantia nigra neurons — and the reporting neuropathologists judged them to be Marinesco bodies, an age-related incidental finding of the nigral neuron, not SCA23 pathology. No `finding_term` is bound. Binding an inclusion-body term would assert the disease-associated reading the source explicitly declines. This entry does not claim intranuclear inclusions as a feature of SCA23; it records that they were seen and were interpreted as incidental.
Show evidence (1 reference)
PMID:15306549 SUPPORT Human Clinical
"ubiquitin-positive intranuclear inclusions in nigral neurons that were considered to be Marinesco bodies"
Classified PARTIAL: the observation is supported, the disease-pathology interpretation is explicitly rejected in the same clause by the people who made it.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Spinocerebellar Ataxia Type 23 Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

10
Digestive 1
Dysphagia HP:0002015 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dysphagia (HP:0002015). HP:0002015 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
DOI:10.1007/s12311-026-02001-6 SUPPORT Human Clinical
"characterized by progressive dysarthria, tremor, dysphagia, and gait disturbance"
The presenting features of the reported case. Note that `DOI:` is in the reference validator's skip_prefixes, so this snippet is not machine-verified the way the PMID snippets in this entry are; it was checked by hand against the cached record.
Eye 1
Nystagmus HP:0000639 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nystagmus (HP:0000639). HP:0000639 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32587707 SUPPORT Human Clinical
"A detailed neurological examination at 60 years-of-age revealed mild dysarthria, horizontal nystagmus, a cerebellar tremor affecting both hands, decreased deep tendon reflex without muscle atrophy or sensory disturbance."
The single documented instance, with its horizontal direction.
Nervous System 6
Gait ataxia HP:0002066 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Gait ataxia (HP:0002066), qualified as course progressive. HP:0002066 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"four Dutch families displaying progressive gait and limb ataxia"
Documents progressive gait ataxia in the original families.
Dysarthria HP:0001260 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dysarthria (HP:0001260). HP:0001260 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"Spinocerebellar ataxias (SCAs) are dominantly inherited neurodegenerative disorders characterized by progressive cerebellar ataxia and dysarthria."
Classified PARTIAL: this sentence characterizes the SCA group rather than SCA23 specifically, so it supports dysarthria as expected of the class rather than as a measured SCA23 feature.
Cerebellar atrophy HP:0001272 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebellar atrophy (HP:0001272). HP:0001272 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32587707 SUPPORT Human Clinical
"Brain MRI identified moderate atrophy of the cerebellar hemisphere and vermis, but no atrophy or abnormal signals in the brainstem"
The finding and, in the same sentence, the brainstem sparing that makes it discriminating.
PMID:32587707 SUPPORT Human Clinical
"the proband's mother with the PDYN p.R215H variant had no subjective symptoms; she had not come to medical attention before our survey, although she showed apparent cerebellar atrophy on brain magnetic resonance imaging (MRI)"
Cerebellar atrophy in a carrier with no subjective symptoms — the observation that makes this an imaging phenotype rather than a correlate of symptom severity.
Hyperreflexia HP:0001347 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hyperreflexia (HP:0001347). HP:0001347 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32587707 SUPPORT Human Clinical
"A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
Four-limb hyperreflexia on examination in the proband.
PMID:32587707 SUPPORT Human Clinical
"A later neurological examination at 52 years-of-age revealed a cerebellar tremor and hyper-reflexia of the lower limbs."
A second affected sibling, with the lower-limb-restricted pattern.
Tremor HP:0001337 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebellar tremor, annotated with Tremor (HP:0001337). HP:0001337 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32587707 SUPPORT Human Clinical
"A detailed neurological examination at 60 years-of-age revealed mild dysarthria, horizontal nystagmus, a cerebellar tremor affecting both hands, decreased deep tendon reflex without muscle atrophy or sensory disturbance."
Cerebellar tremor of both hands, in the proband of the second family.
Parkinsonism HP:0001300 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Parkinsonism (HP:0001300). HP:0001300 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32587707 SUPPORT Human Clinical
"A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
The parkinsonian syndrome as examined, with its components named.
PMID:32587707 SUPPORT Human Clinical
"the proband showed clinical and neuroimaging features suggestive of multiple system atrophy with predominant parkinsonism (MSA-P)"
Records that the presentation was read as MSA-P, which is what makes this phenotype diagnostically significant rather than merely additional.
Other 2
Limb ataxia HP:0002070 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Limb ataxia (HP:0002070), qualified as course progressive. HP:0002070 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"four Dutch families displaying progressive gait and limb ataxia"
Documents progressive limb ataxia in the original families.
Abnormal saccadic eye movements HP:0000570 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Saccadic eye movements, annotated with Abnormal saccadic eye movements (HP:0000570). HP:0000570 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32587707 SUPPORT Human Clinical
"A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
The examination finding, quoted with the surrounding signs that were recorded at the same time.
🧬

Genetic Associations

1
PDYN (Pathogenic Variants)
Gene: PDYN hgnc:8820 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is PDYN (hgnc:8820). hgnc:8820 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (7 references)
PMID:21035104 SUPPORT Human Clinical
"We have identified missense mutations in prodynorphin (PDYN) that cause SCA23 in four Dutch families displaying progressive gait and limb ataxia."
The gene-discovery statement and the number of families it rests on.
PMID:21035104 SUPPORT Human Clinical
"Three mutations were located in Dyn A, a peptide with both opioid activities and nonopioid neurodegenerative actions."
Establishes the distribution of variants and, in passing, the dual opioid and non-opioid character of the peptide that the mechanism turns on.
PMID:27260403 SUPPORT In Vitro
"R6W and wt Dyn A peptides were most toxic to primary cerebellar neurons."
A variant-specific toxicity result, and one that complicates the picture: wild-type peptide was among the most toxic in this assay, which is why this entry frames toxicity as concentration- and persistence-dependent rather than as a simple property of mutancy.
+ 4 more references
💊

Medical Actions

1
Multidisciplinary Rehabilitation
Action: rehabilitationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is rehabilitation (NCIT:C15315). NCIT:C15315 is a clinical intervention from the NCI Thesaurus. Ontology label: Rehabilitation NCIT:C15315
There is no disease-modifying therapy for SCA23 and none has been trialled in it. Rehabilitation is curated here — rather than omitted as generic convention, which an earlier version of this entry did — because it is the only intervention actually reported for an SCA23 patient: the 2026 Brazilian case was referred to a multidisciplinary rehabilitation group with emphasis on gait and dysphagia. The content of that care (physical, respiratory, speech, voice, swallowing and occupational therapy) is evidenced from the ataxia rehabilitation literature, not from SCA23 specifically, and its stated objective there is quality of life rather than altering the disease course.
Mechanism Target:
MODULATES Progressive Cerebellar Ataxia — Rehabilitation acts on function at the level of the clinical endpoint. It does not touch the peptide, the receptor switch, or the Purkinje cell, and the entry does not draw an edge suggesting it does.
Show evidence (1 reference)
PMID:35239817 SUPPORT Other
"The current treatment of patients with cerebellar ataxias, especially neurodegenerative forms, genetic or not, should include these different forms of rehabilitation, with the main objective of improving the quality of life of patients."
Names the objective — quality of life — which is what fixes this link at MODULATES on the clinical endpoint rather than anything mechanistic. PARTIAL and evidence_source OTHER: this is a narrative review's recommendation about cerebellar ataxias as a class, not a trial result and not about SCA23.
Show evidence (3 references)
PMID:35239817 SUPPORT Other
"Rehabilitation treatment includes physical therapy, respiratory therapy, speech, voice and swallowing therapy, occupational therapy, and new technologies, such as the use of exergames."
The components of the intervention. Classified PARTIAL because the source addresses cerebellar ataxias as a group; no SCA23-specific rehabilitation outcome has been published.
PMID:35081319 SUPPORT Other
"Despite having a clear understanding of SCA's etiology, there are no current symptomatic or neuroprotective treatments approved by the FDA."
The reason rehabilitation is the whole of the treatments block: nothing disease-modifying is approved for any spinocerebellar ataxia, let alone for SCA23.
DOI:10.1007/s12311-026-02001-6 SUPPORT Human Clinical
"The patient was referred to a multidisciplinary rehabilitation group with emphasis on functional rehabilitation of gait and dysphagia."
The only reported instance of an SCA23 patient receiving a named intervention, with its two targets. Note `DOI:` is in the reference validator's skip_prefixes, so this snippet is not machine-verified; it was checked by hand against the cached record.
🔬

Diagnosis

2
PDYN sequencing with variant-level functional assessment
Diagnosis is by sequencing, but the gene assignment's contested status makes a bare variant call insufficient. Because functional behaviour differs between reported variants, and because the gene-disease relationship has been formally questioned, segregation data and functional evidence — peptide stability, kappa-opioid receptor signalling, cerebellar neuron toxicity — carry more weight here than a classification derived from the gene's reputation.
Show evidence (1 reference)
PMID:27260403 SUPPORT In Vitro
"Notably, L5S Dyn A displayed increased degradation and no aggregation."
The clearest single demonstration that reported SCA23 variants do not share one functional behaviour, which is the argument for assessing them individually.
Position in the ataxia testing sequence
Where SCA23 belongs in a diagnostic workup, which is a question its rarity answers. A screen of 852 ataxia patients across four countries found PDYN variants in about 0.1%, and its authors concluded that front-line evaluation should go to other ataxia genes first. In practice the reported patients were found the same way: common repeat expansions excluded, then a panel or exome. The corollary matters for a treatable-looking differential — a negative standard ataxia panel does not exclude SCA23, because PDYN is not always on one.
Show evidence (2 references)
PMID:23108490 SUPPORT Human Clinical
"front-line diagnostic evaluation of UK familial and early onset pure spinocerebellar ataxia patients should focus on other known ataxia genes"
The explicit recommendation about testing order, from the largest screen published.
PMID:32587707 SUPPORT Human Clinical
"Screening for common repeat expansions for ADCAs (SCA1, SCA2, MJD/SCA3, SCA6, SCA7, SCA8, SCA12, SCA17, and DRPLA) and SCA31 was negative for the proband in each family"
The route by which two families actually reached the diagnosis, naming the expansions excluded first.
📈

Progression

1
Adult onset and slow progression
Onset is in adult life with slowly progressive gait and limb ataxia. The animal model reproduces the temporal pattern in miniature, with gait deficits from three months and declining motor coordination by twelve.
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months"
The progressive time course in the model. Evidence source is MODEL_ORGANISM; the human time course is described in prose because the original report does not state ages in its abstract.
📊

Prevalence

3
Netherlands
Cases In Literature Ultra Rare
The original report estimated SCA23 at about 0.5% of ataxia families in the Netherlands, which is a share of a disease population rather than a population prevalence and is recorded as such. Cases outside the founding Dutch cohort are individually reported, including a first Japanese family. Any count here should be read against the gene-validity controversy: if some reported variants are not causal, the true number is smaller.
Show evidence (1 reference)
PMID:21035104 SUPPORT Human Clinical
"PDYN mutations are identified in a small subset of ataxia families, indicating that SCA23 is an infrequent SCA type (∼0.5%) in the Netherlands"
The source of the 0.5% figure and its explicit framing as a share of ataxia families in one country.
United Kingdom, Greece, Egypt and India ataxia series
Cases In Literature Ultra Rare
An independent screen of 852 ataxia patients found PDYN variants in about 0.1% of cases — a fifth of the Dutch figure, and a useful corrective, since the 0.5% estimate came from the population in which the gene was discovered. The authors note the yield may be higher in pure cerebellar ataxia specifically. As with the Dutch figure this is a share of an ataxia population, not a population prevalence.
Show evidence (2 references)
PMID:23108490 SUPPORT Human Clinical
"confirms that PDYN mutations are a very rare cause of spinocerebellar ataxia, accounting for ~ 0.1 % of ataxia cases but perhaps with a higher frequency in pure cerebellar ataxia"
The 0.1% figure with the authors' own qualification about pure cerebellar ataxia.
PMID:23108490 SUPPORT Human Clinical
"We sequenced the coding and flanking intronic regions of the PDYN gene in a total of 852 ataxia patients"
The denominator behind that figure.
Worldwide
Cases In Literature Ultra Rare
A 2020 count put the total confirmed worldwide at twenty patients. Read alongside the gene-validity controversy this is an upper bound rather than a settled number.
Show evidence (1 reference)
PMID:32587707 SUPPORT Human Clinical
"only 20 patients worldwide, including two Japanese patients in the same family, have been confirmed to carry disease-causing PDYN variants to date"
An explicit worldwide count at a stated point in time.
🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Spinocerebellar Ataxia Type 23:

Multiple system atrophy, parkinsonian type (MSA-P)
Overlapping Features The differential that actually misdirects diagnosis. One reported SCA23 patient was investigated as MSA-P: bradykinesia, rigidity, frozen gait and resting tremor, a hyperintense lateral putaminal rim on MRI, and severe bilateral nigrostriatal dopaminergic deficits on DAT-SPECT. What excluded MSA was the absence of autonomic failure — no urinary dysfunction, no orthostatic hypotension — so the consensus criteria were not met. Two imaging features do the discriminating work in the other direction: the hot cross bun sign and middle cerebellar peduncle hyperintensity, both characteristic of MSA-C, were absent in every reported SCA23 patient. A prior Japanese case is the exception that proves how much overlap is possible — it had both a hot cross bun sign and a putaminal rim. The practical consequence: a family history is the discriminator that survives when imaging does not, and a sporadic-appearing SCA23 patient (incomplete penetrance makes those possible) can look like MSA on every axis examined.
Show evidence (2 references)
PMID:32587707 SUPPORT Human Clinical
"Because this patient had no urinary dysfunction (incontinence, urgency, or incomplete bladder emptying) or orthostatic hypotension, he did not fulfill the second consensus criteria of probable or possible MSA"
The specific negatives that excluded MSA in a patient who otherwise resembled it.
PMID:32587707 SUPPORT Human Clinical
"our cases suggest that the existence of a phenotypic overlap between SCA23 and MSA, both clinically and neuroradiologically"
The authors' own conclusion that the overlap is real on both clinical and imaging axes.
🐁

Animal Models

1
PDYN R212W knock-in mouse Genetic
The first animal model of SCA23, carrying the R212W PDYN variant. It reproduces the elevated mutant peptide, the Purkinje pathology and the progressive motor phenotype, and supplies the glutamatergic transcriptional dysregulation that connects the peptide to the cellular lesion. Its authors propose on that basis that lowering the mutant peptide could be therapeutic.
Species
Mouse
Genotype
PDYN R212W knock-in
Genes
PDYN hgnc:8820 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns PDYN (hgnc:8820). hgnc:8820 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:26169942
Pathograph links
Persistent Elevation of Mutant Dynorphin A recapitulates fidelity: high Radioimmunoassay shows markedly elevated mutant dynorphin A in the cerebellum, which is the in vivo counterpart of the cellular overproduction seen in the original report.
Limitations: A knock-in of one variant; the functional heterogeneity between reported SCA23 alleles means this model speaks for R212W rather than for the disease's whole allelic spectrum.
Readouts
Cerebellar mutant dynorphin A level
Direction: INCREASED
Interpretation: Confirms in vivo that the variant raises steady-state mutant peptide.
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A"
The measurement and its direction.
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"the PDYN(R212W) mouse is the first animal model of spinocerebellar ataxia type 23"
Establishes the model's standing as the reference in vivo system for this disorder.
Purkinje Cell Degeneration and Climbing Fibre Retraction recapitulates fidelity: high Climbing fibre retraction and Purkinje cell loss on immunohistochemistry, with a progressive motor phenotype.
Limitations: Rodent cerebellar circuitry and lifespan differ from human, so the timescale is not transferable; and the model cannot address whether the contested human variants other than R212W produce the same pathology.
Readouts
Purkinje cell number on immunohistochemistry
Direction: DECREASED
Interpretation: Histological confirmation of the cellular lesion the disease is proposed to turn on.
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings"
The measurement and its direction.
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"The PDYN(R212W) mice reproduced many of the clinical features of spinocerebellar ataxia type 23"
The authors' own assessment of fidelity, quoted with their qualifier ("many of").
Show evidence (1 reference)
PMID:26169942 SUPPORT Model Organism
"our work indicates that the elevated mutant dynorphin A peptide levels are likely responsible for the initiation and progression of the disease"
Establishes the model as informative for the disease mechanism as a whole, with the authors' hedge preserved.
{ }

Source YAML

click to show 
name: Spinocerebellar Ataxia Type 23
creation_date: "2026-08-24T00:00:00Z"
description: >-
  SCA23 is a rare, late-onset, autosomal dominant cerebellar ataxia attributed to
  missense variants in PDYN, the precursor of the opioid neuropeptides
  alpha-neoendorphin and dynorphins A and B. It is unusual among the
  spinocerebellar ataxias in that the mutant product is a small secreted
  neuropeptide rather than a channel, chaperone or repeat-expanded protein. The
  proposed mechanism is a gain of toxicity: variants clustered in the dynorphin
  A coding region distort the peptide's secondary structure, which both lowers
  its kappa-opioid receptor affinity and makes it resistant to degradation, so a
  peptide that should be transient persists and acts instead through NMDA
  receptors. The result is proposed to be a convergence of lost
  opioid-mediated neuroprotection and gained NMDA-mediated excitotoxicity,
  producing Purkinje cell loss. Curators should note that the gene-disease
  relationship has been formally challenged in the literature and is more
  contested than for most monogenic ataxia genes — see the
  controversy_pdyn_gene_validity discussion.
category: Mendelian
parents:
- autosomal dominant disease
- spinocerebellar ataxia
synonyms:
- SCA23
- spinocerebellar ataxia 23
disease_term:
  preferred_term: spinocerebellar ataxia type 23
  term:
    id: MONDO:0012449
    label: spinocerebellar ataxia type 23
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
    notes: >-
      Placed with the neurological disorders as a hereditary cerebellar ataxia.
      Harrison's does not list SCA23 individually; this is a mechanism- and
      system-based placement.
inheritance:
- name: Autosomal dominant inheritance
  description: >-
    SCA23 is autosomal dominant, described in four Dutch families in the
    original report and subsequently in single families elsewhere. Penetrance is
    incomplete and onset is late, which is material to the variant-interpretation
    controversy recorded in this entry.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Spinocerebellar ataxias (SCAs) are dominantly inherited neurodegenerative disorders characterized by progressive cerebellar ataxia and dysarthria. We have identified missense mutations in prodynorphin (PDYN) that cause SCA23 in four Dutch families displaying progressive gait and limb ataxia."
    explanation: >-
      Establishes dominant inheritance and the four-family basis of the original
      gene assignment.
pathophysiology:
- name: PDYN Missense Variant in the Dynorphin A Coding Region
  biological_scale: MOLECULAR
  role: trigger
  conforms_to: "cerebellar_purkinje_degeneration#Cerebellar Neuron Insult"
  description: >-
    PDYN encodes the precursor of alpha-neoendorphin and dynorphins A and B.
    The reported SCA23 variants cluster in the dynorphin A coding region, which
    the mouse-model authors describe as a likely mutational hotspot; a further
    variant sits in the non-opioid domain of the precursor. This is the
    disorder-specific substitution this entry makes against the conserved
    Purkinje-degeneration module, whose trigger node is deliberately generic
    about the molecular lesion.
  genes:
  - preferred_term: PDYN
    term:
      id: hgnc:8820
      label: PDYN
  cell_types:
  - preferred_term: Cerebellar Purkinje cell
    term:
      id: CL:0000121
      label: Purkinje cell
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PDYN is the precursor protein for the opioid neuropeptides, α-neoendorphin, and dynorphins A and B (Dyn A and B)."
    explanation: >-
      Establishes what the gene encodes, which is what makes this disorder
      mechanistically unusual among the ataxias.
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Dynorphin A is likely a mutational hotspot for spinocerebellar ataxia type 23 mutations"
    explanation: >-
      Supports the clustering of variants in the dynorphin A region, quoted with
      the authors' own hedge ("likely").
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The fourth mutation was located in the nonopioid PDYN domain and was associated with altered expression of components of the opioid and glutamate system, as evident from analysis of SCA23 autopsy tissue."
    explanation: >-
      Documents the non-opioid-domain variant and the autopsy finding, which is
      the only human tissue evidence in the original report.
  downstream:
  - target: Altered Dynorphin A Secondary Structure
    causal_link_type: DIRECT
    description: >-
      The substitutions disrupt the peptide's fold.
    evidence:
    - reference: PMID:27260403
      reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We show that SCA23 mutations in the Dyn A-coding region disrupted peptide secondary structure leading to a loss of the N-terminal α-helix"
      explanation: >-
        Direct structural measurement linking the variants to the fold change.
- name: Altered Dynorphin A Secondary Structure
  biological_scale: MOLECULAR
  role: central_effector
  description: >-
    Loss of the peptide's N-terminal alpha-helix is the pivot of the whole
    mechanism, because it does two things at once. It lowers kappa-opioid
    receptor affinity, removing the peptide's normal signalling identity; and,
    for the R6W and R9C variants, it confers marked degradation resistance and
    reduced solubility, so the peptide persists. Not every variant behaves this
    way — L5S showed increased degradation and no aggregation — which is
    curated here because it bears directly on whether all reported variants
    share a mechanism.
  molecular_functions:
  - preferred_term: kappa-type opioid receptor binding
    term:
      id: GO:0031851
      label: kappa-type opioid receptor binding
    modifier: DECREASED
  evidence:
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "disrupted peptide secondary structure leading to a loss of the N-terminal α-helix associated with decreased κ-opioid receptor affinity"
    explanation: >-
      Links the structural change to the loss of opioid receptor affinity.
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the altered secondary structure led to increased peptide stability of R6W and R9C Dyn A, as these peptides showed marked degradation resistance, which coincided with decreased peptide solubility. Notably, L5S Dyn A displayed increased degradation and no aggregation."
    explanation: >-
      Establishes both the degradation resistance of two variants and the
      explicit exception, which is why this entry does not claim a uniform
      mechanism across alleles.
  - reference: PMID:34944698
    reference_title: "Functional Characterization of Spinocerebellar Ataxia Associated Dynorphin A Mutant Peptides."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "DynA mutations, associated with a severe ataxic phenotype, decrease potency of KOR activation, both for G-protein dissociation as well as β-arrestin recruitment"
    explanation: >-
      Independent confirmation that the DECREASED modifier on kappa-opioid
      receptor binding has a functional consequence, and one that goes further
      than binding affinity: potency falls for both downstream branches of KOR
      signalling, G-protein and beta-arrestin alike. Note the qualifier the
      authors attach — the mutants tested were those associated with a severe
      ataxic phenotype, so this is not a blanket claim about every reported
      allele.
  - reference: PMID:34944698
    reference_title: "Functional Characterization of Spinocerebellar Ataxia Associated Dynorphin A Mutant Peptides."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Molecular modelling suggests that this loss of function is due to disruption of critical interactions between DynA and the receptor."
    explanation: >-
      The structural rationale, and the reason it is a separate evidence item at
      COMPUTATIONAL rather than folded into the one above: it is a modelling
      result, and the authors say "suggests".
  downstream:
  - target: Persistent Elevation of Mutant Dynorphin A
    causal_link_type: DIRECT
    description: >-
      A peptide that resists degradation accumulates.
    evidence:
    - reference: PMID:21035104
      reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Two of these mutations resulted in excessive generation of Dyn A in a cellular model system."
      explanation: >-
        The original cellular demonstration that mutant peptide levels rise.
- name: Persistent Elevation of Mutant Dynorphin A
  biological_scale: MOLECULAR
  description: >-
    Dynorphin A is normally a transient signal. When it persists, it becomes
    toxic — a property of the wild-type peptide at sustained concentration, not
    only of the mutants. The mouse model carrying the R212W allele shows
    markedly elevated mutant dynorphin A in the cerebellum, which is the in vivo
    confirmation that the cellular observation translates.
  biological_processes:
  - preferred_term: neuropeptide signaling pathway
    term:
      id: GO:0007218
      label: neuropeptide signaling pathway
    modifier: ABNORMAL
  evidence:
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Increased concentration of or prolonged exposure to Dyn A is neurotoxic"
    explanation: >-
      States that it is the persistence and concentration, rather than the
      mutation per se, that makes the peptide toxic.
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A"
    explanation: >-
      In vivo measurement of the elevated peptide in the model that reproduces
      the disease.
  downstream:
  - target: Switch from Opioid to NMDA Receptor Signalling
    causal_link_type: DIRECT
    description: >-
      A peptide with lost opioid affinity but sustained presence engages a
      different receptor system.
    evidence:
    - reference: PMID:27260403
      reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "For R6W Dyn A, this is likely because of a switch from opioid to NMDA- receptor signalling, while for wt Dyn A, this switch was not observed."
      explanation: >-
        The receptor switch, quoted with the authors' hedge and with the
        wild-type control result that makes it variant-specific.
  - target: Cerebellar Developmental Deficit in Purkinje Cell Innervation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The same mouse that carries the elevated mutant peptide shows a
      developmental innervation defect from two weeks of age. Whether the
      elevated peptide is what causes it — and by what route — is not
      established; the link is drawn because both are properties of the same
      allele in the same model, and typed to say so.
    evidence:
    - reference: PMID:33043513
      reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings indicate that developmental abnormalities contribute to the SCA23 pathology and uncover a developmental role for PDYN in the cerebellum."
      explanation: >-
        States a contribution to the pathology and a developmental role for
        PDYN. Classified PARTIAL because it establishes neither the intermediate
        steps nor that the elevated peptide is the proximal cause.
- name: Cerebellar Developmental Deficit in Purkinje Cell Innervation
  biological_scale: TISSUE
  description: >-
    A second, non-degenerative arm, and the reason SCA23 cannot be read as
    purely late-onset neurodegeneration. In the R212W mouse the cerebellum is
    already abnormal at two weeks of age — long before the motor deficits at
    three months and the Purkinje cell loss at twelve. GABAergic synapses on the
    Purkinje cell soma are reduced; early-phase climbing fibre elimination is
    delayed between two and three weeks; climbing fibres fail to reach their
    terminal height, and the proximal dendrite they should occupy is taken over
    by parallel fibres, with increased vGlut1 as the marker of that takeover.

    This reframes the climbing fibre retraction curated on the degeneration node
    below: part of it may be territory never properly established rather than
    connectivity lost. The two are not alternatives — the mouse shows both, at
    different ages — and this entry curates them as separate nodes rather than
    collapsing them, because no experiment has yet apportioned the adult
    phenotype between them.

    Everything here is mouse. No human SCA23 developmental study exists, and the
    disease presents in the fifth decade, so the translational status of this arm
    is genuinely open — recorded as the developmental_versus_degenerative
    discussion.
  cell_types:
  - preferred_term: Cerebellar Purkinje cell
    term:
      id: CL:0000121
      label: Purkinje cell
  biological_processes:
  - preferred_term: climbing fibre synapse elimination
    term:
      id: GO:0098883
      label: synapse pruning
    modifier: DECREASED
  evidence:
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "we uncovered developmental deficits from 2 weeks of age, namely a reduced number of GABAergic synapses on PC soma, possibly leading to the observed delay in early phase CF elimination between 2 and 3 weeks of age"
    explanation: >-
      The primary developmental finding, with its age and the authors' own
      "possibly" linking the two observations.
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Furthermore, CFs did not reach terminal height, leaving proximal PC dendrites open to be occupied by parallel fibers (PFs)."
    explanation: >-
      The failure of climbing fibres to establish their normal territory — the
      observation that makes this developmental rather than degenerative.
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "indicates that PFs indeed take over CF territory and have increased connectivity with PCs"
    explanation: >-
      The parallel-fibre takeover, evidenced by the vGlut1 increase reported in
      the same sentence.
  downstream:
  - target: Purkinje Cell Degeneration and Climbing Fibre Retraction
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      A cerebellum whose climbing fibre territory was never fully established
      plausibly enters adult life with less to lose, but no experiment has tested
      whether the developmental deficit is required for, contributes to, or is
      merely concurrent with the adult degeneration.
    evidence:
    - reference: PMID:33043513
      reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "As SCA23 mice show a similar pathology to SCA1 mice in adulthood, we hypothesized that SCA23 may also follow SCA1 pathology during development."
      explanation: >-
        Quoted as what it is — the study's hypothesis and its reasoning by
        analogy to SCA1, not a demonstrated developmental-to-adult causal link.
        PARTIAL for that reason.
- name: Switch from Opioid to NMDA Receptor Signalling
  biological_scale: MOLECULAR
  description: >-
    Dynorphin A peptides bind NMDA receptors and potentiate their
    glutamate-evoked currents, a long-recognized non-opioid action. In SCA23 the
    proposal is that the mutant peptide's loss of kappa-opioid affinity leaves
    this NMDA action unopposed, so the same molecule that should have been
    neuroprotective becomes excitotoxic. The reporting authors frame the
    pathology as a convergence of the two, not as either alone.
  molecular_functions:
  - preferred_term: NMDA glutamate receptor activity
    term:
      id: GO:0004972
      label: NMDA glutamate receptor activity
    modifier: INCREASED
  biological_processes:
  - preferred_term: glutamate receptor signaling pathway
    term:
      id: GO:0007215
      label: glutamate receptor signaling pathway
    modifier: INCREASED
  evidence:
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "these deleterious effects are very likely caused by an N-methyl-d-aspartate-mediated non-opioid mechanism as Dyn A peptides were shown to bind NMDA receptors and potentiate their glutamate-evoked currents"
    explanation: >-
      Establishes the NMDA mechanism and its molecular basis, with the authors'
      hedge preserved.
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We propose that the pathology of SCA23 results from converging mechanisms of loss of opioid-mediated neuroprotection and NMDA-mediated excitotoxicity."
    explanation: >-
      Classified PARTIAL because the authors present the convergence as a
      proposal. It is the framing this entry's two-arm node adopts, and the
      hedge is preserved rather than upgraded.
  downstream:
  - target: Purkinje Cell Degeneration and Climbing Fibre Retraction
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Excitotoxic signalling in the cerebellum is proposed as the route to
      Purkinje cell loss; the intervening steps are not established.
    evidence:
    - reference: PMID:21035104
      reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "alterations in Dyn A activities and/or impairment of secretory pathways by mutant PDYN may lead to glutamate neurotoxicity, which underlies Purkinje cell degeneration and ataxia"
      explanation: >-
        Classified PARTIAL: the original authors state this as a possibility
        ("may lead to") and offer two alternative proximal routes, one of which
        — impaired secretory pathways — this entry does not curate as a separate
        node because it has not been followed up.
- name: Purkinje Cell Degeneration and Climbing Fibre Retraction
  biological_scale: TISSUE
  conforms_to: "cerebellar_purkinje_degeneration#Purkinje Neuron Degeneration"
  description: >-
    The tissue-level lesion. In the R212W mouse the elevated mutant peptide is
    accompanied by climbing fibre retraction and Purkinje cell loss on
    immunohistochemistry, together with transcriptional dysregulation of
    ionotropic and metabotropic glutamate receptors and glutamate transporters
    and altered neuronal excitability — which ties the receptor-level mechanism
    above to the cellular one. This entry does not declare conformance to the
    module's Purkinje calcium and proteostasis node, and the reason needs stating
    precisely rather than as a blanket absence of evidence. PMID:33043513 did
    measure something calcium-adjacent in the R212W mouse: altered *expression*
    of several critical Ca2+ channel subunits. What it did not do is measure
    calcium handling — the authors themselves write only that this is
    "potentially contributing to altered Ca2+ transients", with no transient,
    buffering, or store-release measurement reported. Nothing bearing on protein
    quality control has been measured at all, in mouse or in human tissue.
    Expression of a channel subunit is not a measurement of the dysregulated
    calcium handling that amplifier node asserts, so the conformance stays
    unclaimed — but as a specific gap, with a named experiment that would close
    it (see the calcium_handling_in_sca23 discussion).
  cell_types:
  - preferred_term: Cerebellar Purkinje cell
    term:
      id: CL:0000121
      label: Purkinje cell
  evidence:
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "markedly elevated levels of mutant dynorphin A, which are associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings"
    explanation: >-
      The histological demonstration in the model. The spelling "climber fibre"
      is as it appears in the source.
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Additionally, we detected altered expression of several critical Ca2+ channel subunits, potentially contributing to altered Ca2+ transients in PDYNR212W cerebella."
    explanation: >-
      The nearest thing to a calcium measurement in SCA23, quoted with the
      authors' own hedge. Classified PARTIAL and cited here precisely because it
      is what the non-conformance rationale above has to be stated against: an
      expression change in channel subunits, not a measurement of calcium
      handling.
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "the pathologically elevated mutant dynorphin A levels in the cerebellum coincided with transcriptionally dysregulated ionotropic and metabotropic glutamate receptors and glutamate transporters, and altered neuronal excitability"
    explanation: >-
      Connects the peptide elevation to glutamatergic dysregulation in the
      cerebellum, which is the tissue-level counterpart of the NMDA node above.
  downstream:
  - target: Progressive Cerebellar Ataxia
    causal_link_type: DIRECT
    description: >-
      Loss of Purkinje cells produces the clinical syndrome.
    evidence:
    - reference: PMID:26169942
      reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months demonstrated by declining performances on the accelerating Rotarod"
      explanation: >-
        The motor consequence in the model, with its time course.
- name: Progressive Cerebellar Ataxia
  biological_scale: ORGANISM
  role: consequence
  conforms_to: "cerebellar_purkinje_degeneration#Cerebellar Ataxia"
  description: >-
    The clinical endpoint: slowly progressive gait and limb ataxia with
    dysarthria, beginning in adult life.
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "four Dutch families displaying progressive gait and limb ataxia"
    explanation: >-
      The clinical phenotype of the families in which the gene was identified.
phenotypes:
- name: Gait ataxia
  category: Neurologic
  description: >-
    Progressive gait ataxia is the presenting and defining feature.
  phenotype_term:
    preferred_term: Gait ataxia
    term:
      id: HP:0002066
      label: Gait ataxia
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "four Dutch families displaying progressive gait and limb ataxia"
    explanation: >-
      Documents progressive gait ataxia in the original families.
- name: Limb ataxia
  category: Neurologic
  description: >-
    Limb ataxia accompanies the gait disorder.
  phenotype_term:
    preferred_term: Limb ataxia
    term:
      id: HP:0002070
      label: Limb ataxia
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "four Dutch families displaying progressive gait and limb ataxia"
    explanation: >-
      Documents progressive limb ataxia in the original families.
- name: Dysarthria
  category: Neurologic
  description: >-
    Dysarthria is a cardinal feature of the spinocerebellar ataxias generally
    and is named as such in the source that defines the group SCA23 belongs to.
    No SCA23-specific frequency is available.
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Spinocerebellar ataxias (SCAs) are dominantly inherited neurodegenerative disorders characterized by progressive cerebellar ataxia and dysarthria."
    explanation: >-
      Classified PARTIAL: this sentence characterizes the SCA group rather than
      SCA23 specifically, so it supports dysarthria as expected of the class
      rather than as a measured SCA23 feature.
- name: Cerebellar atrophy
  category: Neurologic
  description: >-
    Cerebellar atrophy on MRI, affecting both hemispheres and the vermis, is the
    consistent imaging finding. Its diagnostic value lies as much in what is
    spared: brainstem, pons and middle cerebellar peduncles are normal, and the
    hot cross bun sign is absent — which is what separates SCA23 from MSA-C.
    Crucially it is not confined to symptomatic carriers; an 88-year-old with no
    subjective cerebellar complaint had moderate cerebellar atrophy.
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain MRI identified moderate atrophy of the cerebellar hemisphere and vermis, but no atrophy or abnormal signals in the brainstem"
    explanation: >-
      The finding and, in the same sentence, the brainstem sparing that makes it
      discriminating.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the proband's mother with the PDYN p.R215H variant had no subjective symptoms; she had not come to medical attention before our survey, although she showed apparent cerebellar atrophy on brain magnetic resonance imaging (MRI)"
    explanation: >-
      Cerebellar atrophy in a carrier with no subjective symptoms — the
      observation that makes this an imaging phenotype rather than a correlate
      of symptom severity.
- name: Hyperreflexia
  category: Neurologic
  description: >-
    Increased deep tendon reflexes are documented in three of the five patients
    of the Japanese families, in the lower limbs or in all four. This is a
    pyramidal sign in what the founding report called an isolated cerebellar
    syndrome, and the original Dutch family was likewise described as having a
    pure cerebellar syndrome "with hyper-reflexia". Not universal: one patient in
    the same series had decreased reflexes instead.
  phenotype_term:
    preferred_term: Hyperreflexia
    term:
      id: HP:0001347
      label: Hyperreflexia
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
    explanation: >-
      Four-limb hyperreflexia on examination in the proband.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A later neurological examination at 52 years-of-age revealed a cerebellar tremor and hyper-reflexia of the lower limbs."
    explanation: >-
      A second affected sibling, with the lower-limb-restricted pattern.
- name: Tremor
  category: Neurologic
  description: >-
    Tremor is reported in two forms in the same series and they should not be
    conflated: a cerebellar (action) tremor of the hands in two patients, and a
    resting tremor in the one patient who also had parkinsonism. No frequency
    band — the total described population is a handful of families.
  phenotype_term:
    preferred_term: Cerebellar tremor
    term:
      id: HP:0001337
      label: Tremor
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A detailed neurological examination at 60 years-of-age revealed mild dysarthria, horizontal nystagmus, a cerebellar tremor affecting both hands, decreased deep tendon reflex without muscle atrophy or sensory disturbance."
    explanation: >-
      Cerebellar tremor of both hands, in the proband of the second family.
- name: Nystagmus
  category: Neurologic
  description: >-
    Horizontal nystagmus, documented in one patient of the Japanese series.
    Oculomotor involvement in SCA23 is heterogeneous rather than stereotyped:
    a different patient in the same series had saccadic eye movements and no
    nystagmus, and two others had no eye movement disorder at all.
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A detailed neurological examination at 60 years-of-age revealed mild dysarthria, horizontal nystagmus, a cerebellar tremor affecting both hands, decreased deep tendon reflex without muscle atrophy or sensory disturbance."
    explanation: >-
      The single documented instance, with its horizontal direction.
- name: Abnormal saccadic eye movements
  category: Neurologic
  description: >-
    Saccadic eye movements on examination in the proband of the first Japanese
    family. Bound to the parent term rather than to a directional child
    (hypometric, slow, dysmetric): the source records "saccadic eye movements"
    without specifying which abnormality, and choosing a child term would assert
    a finding it does not report.
  phenotype_term:
    preferred_term: Saccadic eye movements
    term:
      id: HP:0000570
      label: Abnormal saccadic eye movements
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
    explanation: >-
      The examination finding, quoted with the surrounding signs that were
      recorded at the same time.
- name: Parkinsonism
  category: Neurologic
  description: >-
    One patient presented with bradykinesia, rigidity, frozen gait and a resting
    tremor, was investigated as MSA with predominant parkinsonism before the
    genetic diagnosis, and had severe bilateral nigrostriatal dopaminergic
    deficits on DAT-SPECT with only a slight levodopa response. This is the
    single most consequential atypical presentation of SCA23, because it sends
    the diagnosis somewhere else entirely — see the MSA differential.
  phenotype_term:
    preferred_term: Parkinsonism
    term:
      id: HP:0001300
      label: Parkinsonism
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A neurological examination revealed saccadic eye movements, scanning speech, hyperreflexia of all limbs, and parkinsonism such as bradykinesia, rigidity, frozen gait, and a resting tremor affecting both hands (R > L)."
    explanation: >-
      The parkinsonian syndrome as examined, with its components named.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the proband showed clinical and neuroimaging features suggestive of multiple system atrophy with predominant parkinsonism (MSA-P)"
    explanation: >-
      Records that the presentation was read as MSA-P, which is what makes this
      phenotype diagnostically significant rather than merely additional.
- name: Dysphagia
  category: Neurologic
  description: >-
    Dysphagia is reported in a 2026 Brazilian case with unusually early onset,
    where it was prominent enough to be a target of the rehabilitation referral.
    It is not documented in the Dutch or Japanese series, so it is curated as an
    occasional feature of advancing disease rather than a core one.
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  evidence:
  - reference: DOI:10.1007/s12311-026-02001-6
    reference_title: "Spinocerebellar Ataxia Type 23 (SCA23): A Rare Cause of SCA in the Americas"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by progressive dysarthria, tremor, dysphagia, and gait disturbance"
    explanation: >-
      The presenting features of the reported case. Note that `DOI:` is in the
      reference validator's skip_prefixes, so this snippet is not
      machine-verified the way the PMID snippets in this entry are; it was
      checked by hand against the cached record.
genetic:
- name: PDYN
  gene_term:
    preferred_term: PDYN
    term:
      id: hgnc:8820
      label: PDYN
  association: Pathogenic Variants
  relationship_type: CAUSATIVE
  notes: >-
    PDYN is the only gene proposed for SCA23. Reported variants are missense,
    concentrated in the dynorphin A coding region with one in the non-opioid
    domain of the precursor. Functional characterization is variant-specific
    rather than uniform: R6W and R9C show degradation resistance and reduced
    solubility, R6W shows the opioid-to-NMDA receptor switch, and L5S behaves
    oppositely with increased degradation and no aggregation. That heterogeneity
    matters for interpretation, because it means a PDYN missense variant cannot
    be assumed to act by the R6W mechanism. The gene-disease relationship itself
    has been formally challenged — see the controversy_pdyn_gene_validity
    discussion — so variant-level segregation and functional evidence should be
    weighed individually rather than assumed from the gene assignment.

    Variant class and distribution. Eight disease-causing PDYN variants were
    recognized by 2020, six of them lying within the Dyn A and Dyn B coding
    region — residues 207 to 236, the stretch that forms "big dynorphin" — which
    is what concentrates functional interpretation on that peptide.

    Penetrance is incomplete or age-dependent, and this is not a footnote: in one
    Japanese family the parents of three affected siblings showed no cerebellar
    signs into their eighth and ninth decades, while their children were affected
    in young adult life. The mother nonetheless had moderate cerebellar atrophy on
    MRI at 88 with no subjective complaint. A negative examination in a parent
    therefore does not exclude carriage, and an asymptomatic carrier may still be
    structurally affected.

    Zygosity has a dosage effect. The same family included one homozygote for
    p.R215H, whose onset was earlier and whose ataxia was the most severe of the
    five. The reporting authors are careful about what this does and does not
    show: it does not adjudicate between the toxic-gain-of-function and
    loss-of-function models, but it does indicate that dosage — of the variant or
    of the normal protein — tracks clinical severity.
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified missense mutations in prodynorphin (PDYN) that cause SCA23 in four Dutch families displaying progressive gait and limb ataxia."
    explanation: >-
      The gene-discovery statement and the number of families it rests on.
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three mutations were located in Dyn A, a peptide with both opioid activities and nonopioid neurodegenerative actions."
    explanation: >-
      Establishes the distribution of variants and, in passing, the dual opioid
      and non-opioid character of the peptide that the mechanism turns on.
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "R6W and wt Dyn A peptides were most toxic to primary cerebellar neurons."
    explanation: >-
      A variant-specific toxicity result, and one that complicates the picture:
      wild-type peptide was among the most toxic in this assay, which is why
      this entry frames toxicity as concentration- and persistence-dependent
      rather than as a simple property of mutancy.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since its discovery, eight SCA23 disease-causing variants in PDYN have been identified, six of which lie exclusively within the PDYN Dyn A and Dyn B encoding regions (amino acid residues 207"
    explanation: >-
      The variant count and their concentration in the big-dynorphin coding
      region. Quoted to the residue number rather than through it, because the
      en-dash in the printed range does not survive quoting cleanly.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The low penetrance rate for PDYN variants might have contributed to the marked intrafamilial variation observed in this family."
    explanation: >-
      Classified PARTIAL and quoted with its "might": the authors offer low
      penetrance as one of two explanations for the intrafamilial variation, the
      other being anticipation. Neither is demonstrated, and the entry does not
      pick between them.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The age of onset for the homozygous patient (II-4) was a little younger than that for the heterozygous patients, and the ataxic symptoms were the most severe."
    explanation: >-
      The homozygote-versus-heterozygote comparison within one family, which is
      the dosage observation.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it might suggest that the dosage effect of the variant or normal PDYN proteins is directly associated with the clinical severity of SCA23"
    explanation: >-
      The authors' own reading of that comparison, quoted with its hedge and
      with the ambiguity they leave open — variant dosage or normal-protein
      dosage — intact.
prevalence:
- population: Netherlands
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    The original report estimated SCA23 at about 0.5% of ataxia families in the
    Netherlands, which is a share of a disease population rather than a
    population prevalence and is recorded as such. Cases outside the founding
    Dutch cohort are individually reported, including a first Japanese family.
    Any count here should be read against the gene-validity controversy: if some
    reported variants are not causal, the true number is smaller.
  evidence:
  - reference: PMID:21035104
    reference_title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PDYN mutations are identified in a small subset of ataxia families, indicating that SCA23 is an infrequent SCA type (∼0.5%) in the Netherlands"
    explanation: >-
      The source of the 0.5% figure and its explicit framing as a share of
      ataxia families in one country.
- population: United Kingdom, Greece, Egypt and India ataxia series
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    An independent screen of 852 ataxia patients found PDYN variants in about
    0.1% of cases — a fifth of the Dutch figure, and a useful corrective, since
    the 0.5% estimate came from the population in which the gene was discovered.
    The authors note the yield may be higher in pure cerebellar ataxia
    specifically. As with the Dutch figure this is a share of an ataxia
    population, not a population prevalence.
  evidence:
  - reference: PMID:23108490
    reference_title: "The frequency of spinocerebellar ataxia type 23 in a UK population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "confirms that PDYN mutations are a very rare cause of spinocerebellar ataxia, accounting for ~ 0.1 % of ataxia cases but perhaps with a higher frequency in pure cerebellar ataxia"
    explanation: >-
      The 0.1% figure with the authors' own qualification about pure cerebellar
      ataxia.
  - reference: PMID:23108490
    reference_title: "The frequency of spinocerebellar ataxia type 23 in a UK population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We sequenced the coding and flanking intronic regions of the PDYN gene in a total of 852 ataxia patients"
    explanation: >-
      The denominator behind that figure.
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    A 2020 count put the total confirmed worldwide at twenty patients. Read
    alongside the gene-validity controversy this is an upper bound rather than a
    settled number.
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "only 20 patients worldwide, including two Japanese patients in the same family, have been confirmed to carry disease-causing PDYN variants to date"
    explanation: >-
      An explicit worldwide count at a stated point in time.
progression:
- phase: Adult onset and slow progression
  notes: >-
    Onset is in adult life with slowly progressive gait and limb ataxia. The
    animal model reproduces the temporal pattern in miniature, with gait
    deficits from three months and declining motor coordination by twelve.
  evidence:
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months"
    explanation: >-
      The progressive time course in the model. Evidence source is
      MODEL_ORGANISM; the human time course is described in prose because the
      original report does not state ages in its abstract.
histopathology:
- name: Neuronal loss in Purkinje cell layer, dentate nuclei and inferior olives
  diagnostic: false
  description: >-
    The human neuropathology, from autopsy of one affected member of the
    founding Dutch family. Neuronal loss is not confined to Purkinje cells: it
    involves the dentate nuclei and the inferior olives as well — that is, all
    three elements of the olivo-cerebellar circuit whose climbing-fibre arm the
    pathograph models. This is the direct human counterpart of the Purkinje cell
    loss demonstrated in the mouse, and the reason the degeneration node is not
    curated on model evidence alone.

    No `finding_term` is bound, and the reason is the enum rather than the
    ontologies. `HistopathologyFindingTerm` is reachable only from the NCIT
    Histopathology Result branch (NCIT:C83490) plus a handful of HP rosette
    terms. HP:0002529 Neuronal loss in central nervous system names this finding
    exactly but is not a member, and NCIT's Histopathology Result branch has no
    neuronal-loss morphologic finding — searching it returns disease entities,
    not findings. Rather than bind something broader that would mean less, the
    finding is carried by `name` and `description`. Note this is n = 1: one
    autopsy from one family, reported before PDYN was identified.
  evidence:
  - reference: PMID:15306549
    reference_title: "Mapping of the SCA23 locus involved in autosomal dominant cerebellar ataxia to chromosome region 20p13-12.3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neuropathological examination in one affected subject showed neuronal loss in the Purkinje cell layer, dentate nuclei and inferior olives, thinning of cerebellopontine tracts, demyelination of posterior and lateral columns in the spinal cord, as well as ubiquitin-positive intranuclear inclusions in nigral neurons that were considered to be Marinesco bodies"
    explanation: >-
      The original autopsy description, quoted whole because the entry curates
      three separate findings out of it and the reader should see they come from
      one observation in one subject.
  - reference: PMID:19089525
    reference_title: "Spinocerebellar ataxia type 23: a genetic update."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neuropathological examination revealed neuronal loss in the Purkinje cell layer, dentate nuclei, and inferior olives."
    explanation: >-
      The later review restating the same finding. Cited as corroboration of the
      record, not as an independent observation — it describes the same autopsy.
- name: Demyelination of the spinal cord posterior and lateral columns
  diagnostic: false
  description: >-
    Spinal cord involvement in the same autopsy, in the posterior (dorsal) and
    lateral columns. This is the anatomical substrate that would account for the
    pyramidal signs — the hyperreflexia curated above — in a disorder otherwise
    described as a pure cerebellar syndrome, and it is why "spinocerebellar"
    rather than "cerebellar" is the right word for this disease.

    Unbound for the same enum reason as the finding above: NCIT:C117277
    Demyelination exists but is not reachable from the Histopathology Result
    branch, so it fails `HistopathologyFindingTerm`. On the HP side,
    HP:0008311 Spinal cord dorsal column hypomyelination names a developmental
    failure to myelinate rather than the demyelination reported here, and covers
    only one of the two involved column systems — it would assert the wrong
    thing even if it were a member.
  evidence:
  - reference: PMID:15306549
    reference_title: "Mapping of the SCA23 locus involved in autosomal dominant cerebellar ataxia to chromosome region 20p13-12.3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "demyelination of posterior and lateral columns in the spinal cord"
    explanation: >-
      The spinal cord finding, naming both involved column systems.
- name: Ubiquitin-positive intranuclear inclusions in nigral neurons
  diagnostic: false
  description: >-
    Curated as a documented negative, because it is the kind of finding that
    gets propagated as disease pathology once it appears in a table. The same
    autopsy showed ubiquitin-positive intranuclear inclusions in substantia
    nigra neurons — and the reporting neuropathologists judged them to be
    Marinesco bodies, an age-related incidental finding of the nigral neuron,
    not SCA23 pathology.

    No `finding_term` is bound. Binding an inclusion-body term would assert the
    disease-associated reading the source explicitly declines. This entry does
    not claim intranuclear inclusions as a feature of SCA23; it records that
    they were seen and were interpreted as incidental.
  evidence:
  - reference: PMID:15306549
    reference_title: "Mapping of the SCA23 locus involved in autosomal dominant cerebellar ataxia to chromosome region 20p13-12.3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ubiquitin-positive intranuclear inclusions in nigral neurons that were considered to be Marinesco bodies"
    explanation: >-
      Classified PARTIAL: the observation is supported, the disease-pathology
      interpretation is explicitly rejected in the same clause by the people who
      made it.

diagnosis:
- name: PDYN sequencing with variant-level functional assessment
  description: >-
    Diagnosis is by sequencing, but the gene assignment's contested status makes
    a bare variant call insufficient. Because functional behaviour differs
    between reported variants, and because the gene-disease relationship has
    been formally questioned, segregation data and functional evidence — peptide
    stability, kappa-opioid receptor signalling, cerebellar neuron toxicity —
    carry more weight here than a classification derived from the gene's
    reputation.
  evidence:
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Notably, L5S Dyn A displayed increased degradation and no aggregation."
    explanation: >-
      The clearest single demonstration that reported SCA23 variants do not
      share one functional behaviour, which is the argument for assessing them
      individually.
- name: Position in the ataxia testing sequence
  description: >-
    Where SCA23 belongs in a diagnostic workup, which is a question its rarity
    answers. A screen of 852 ataxia patients across four countries found PDYN
    variants in about 0.1%, and its authors concluded that front-line evaluation
    should go to other ataxia genes first. In practice the reported patients were
    found the same way: common repeat expansions excluded, then a panel or exome.
    The corollary matters for a treatable-looking differential — a negative
    standard ataxia panel does not exclude SCA23, because PDYN is not always on
    one.
  evidence:
  - reference: PMID:23108490
    reference_title: "The frequency of spinocerebellar ataxia type 23 in a UK population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "front-line diagnostic evaluation of UK familial and early onset pure spinocerebellar ataxia patients should focus on other known ataxia genes"
    explanation: >-
      The explicit recommendation about testing order, from the largest screen
      published.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Screening for common repeat expansions for ADCAs (SCA1, SCA2, MJD/SCA3, SCA6, SCA7, SCA8, SCA12, SCA17, and DRPLA) and SCA31 was negative for the proband in each family"
    explanation: >-
      The route by which two families actually reached the diagnosis, naming the
      expansions excluded first.
differential_diagnoses:
- name: Multiple system atrophy, parkinsonian type (MSA-P)
  description: >-
    The differential that actually misdirects diagnosis. One reported SCA23
    patient was investigated as MSA-P: bradykinesia, rigidity, frozen gait and
    resting tremor, a hyperintense lateral putaminal rim on MRI, and severe
    bilateral nigrostriatal dopaminergic deficits on DAT-SPECT. What excluded MSA
    was the absence of autonomic failure — no urinary dysfunction, no orthostatic
    hypotension — so the consensus criteria were not met.

    Two imaging features do the discriminating work in the other direction: the
    hot cross bun sign and middle cerebellar peduncle hyperintensity, both
    characteristic of MSA-C, were absent in every reported SCA23 patient. A prior
    Japanese case is the exception that proves how much overlap is possible — it
    had both a hot cross bun sign and a putaminal rim.

    The practical consequence: a family history is the discriminator that survives
    when imaging does not, and a sporadic-appearing SCA23 patient (incomplete
    penetrance makes those possible) can look like MSA on every axis examined.
  evidence:
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Because this patient had no urinary dysfunction (incontinence, urgency, or incomplete bladder emptying) or orthostatic hypotension, he did not fulfill the second consensus criteria of probable or possible MSA"
    explanation: >-
      The specific negatives that excluded MSA in a patient who otherwise
      resembled it.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "our cases suggest that the existence of a phenotypic overlap between SCA23 and MSA, both clinically and neuroradiologically"
    explanation: >-
      The authors' own conclusion that the overlap is real on both clinical and
      imaging axes.
treatments:
- name: Multidisciplinary Rehabilitation
  description: >-
    There is no disease-modifying therapy for SCA23 and none has been trialled in
    it. Rehabilitation is curated here — rather than omitted as generic
    convention, which an earlier version of this entry did — because it is the
    only intervention actually reported for an SCA23 patient: the 2026 Brazilian
    case was referred to a multidisciplinary rehabilitation group with emphasis
    on gait and dysphagia. The content of that care (physical, respiratory,
    speech, voice, swallowing and occupational therapy) is evidenced from the
    ataxia rehabilitation literature, not from SCA23 specifically, and its stated
    objective there is quality of life rather than altering the disease course.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: rehabilitation
    term:
      id: NCIT:C15315
      label: Rehabilitation
  target_mechanisms:
  - target: Progressive Cerebellar Ataxia
    treatment_effect: MODULATES
    description: >-
      Rehabilitation acts on function at the level of the clinical endpoint. It
      does not touch the peptide, the receptor switch, or the Purkinje cell, and
      the entry does not draw an edge suggesting it does.
    evidence:
    - reference: PMID:35239817
      reference_title: "Rehabilitation in patients with cerebellar ataxias."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "The current treatment of patients with cerebellar ataxias, especially neurodegenerative forms, genetic or not, should include these different forms of rehabilitation, with the main objective of improving the quality of life of patients."
      explanation: >-
        Names the objective — quality of life — which is what fixes this link at
        MODULATES on the clinical endpoint rather than anything mechanistic.
        PARTIAL and evidence_source OTHER: this is a narrative review's
        recommendation about cerebellar ataxias as a class, not a trial result
        and not about SCA23.
  evidence:
  - reference: PMID:35239817
    reference_title: "Rehabilitation in patients with cerebellar ataxias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Rehabilitation treatment includes physical therapy, respiratory therapy, speech, voice and swallowing therapy, occupational therapy, and new technologies, such as the use of exergames."
    explanation: >-
      The components of the intervention. Classified PARTIAL because the source
      addresses cerebellar ataxias as a group; no SCA23-specific rehabilitation
      outcome has been published.
  - reference: PMID:35081319
    reference_title: "Current and emerging treatment modalities for spinocerebellar ataxias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Despite having a clear understanding of SCA's etiology, there are no current symptomatic or neuroprotective treatments approved by the FDA."
    explanation: >-
      The reason rehabilitation is the whole of the treatments block: nothing
      disease-modifying is approved for any spinocerebellar ataxia, let alone
      for SCA23.
  - reference: DOI:10.1007/s12311-026-02001-6
    reference_title: "Spinocerebellar Ataxia Type 23 (SCA23): A Rare Cause of SCA in the Americas"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient was referred to a multidisciplinary rehabilitation group with emphasis on functional rehabilitation of gait and dysphagia."
    explanation: >-
      The only reported instance of an SCA23 patient receiving a named
      intervention, with its two targets. Note `DOI:` is in the reference
      validator's skip_prefixes, so this snippet is not machine-verified; it was
      checked by hand against the cached record.
  notes: >-
    No outcome is reported for this patient, so nothing here supports efficacy —
    only that the referral was made.

animal_models:
- name: PDYN R212W knock-in mouse
  species: Mouse
  genotype: PDYN R212W knock-in
  category: Genetic
  description: >-
    The first animal model of SCA23, carrying the R212W PDYN variant. It
    reproduces the elevated mutant peptide, the Purkinje pathology and the
    progressive motor phenotype, and supplies the glutamatergic transcriptional
    dysregulation that connects the peptide to the cellular lesion. Its authors
    propose on that basis that lowering the mutant peptide could be therapeutic.
  publication: PMID:26169942
  genes:
  - preferred_term: PDYN
    term:
      id: hgnc:8820
      label: PDYN
  modeled_mechanisms:
  - target: Persistent Elevation of Mutant Dynorphin A
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Radioimmunoassay shows markedly elevated mutant dynorphin A in the
      cerebellum, which is the in vivo counterpart of the cellular
      overproduction seen in the original report.
    limitations: >-
      A knock-in of one variant; the functional heterogeneity between reported
      SCA23 alleles means this model speaks for R212W rather than for the
      disease's whole allelic spectrum.
    readouts:
    - name: Cerebellar mutant dynorphin A level
      target: Persistent Elevation of Mutant Dynorphin A
      direction: INCREASED
      interpretation: >-
        Confirms in vivo that the variant raises steady-state mutant peptide.
      evidence:
      - reference: PMID:26169942
        reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A"
        explanation: >-
          The measurement and its direction.
    evidence:
    - reference: PMID:26169942
      reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "the PDYN(R212W) mouse is the first animal model of spinocerebellar ataxia type 23"
      explanation: >-
        Establishes the model's standing as the reference in vivo system for
        this disorder.
  - target: Purkinje Cell Degeneration and Climbing Fibre Retraction
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Climbing fibre retraction and Purkinje cell loss on immunohistochemistry,
      with a progressive motor phenotype.
    limitations: >-
      Rodent cerebellar circuitry and lifespan differ from human, so the
      timescale is not transferable; and the model cannot address whether the
      contested human variants other than R212W produce the same pathology.
    readouts:
    - name: Purkinje cell number on immunohistochemistry
      target: Purkinje Cell Degeneration and Climbing Fibre Retraction
      direction: DECREASED
      interpretation: >-
        Histological confirmation of the cellular lesion the disease is
        proposed to turn on.
      evidence:
      - reference: PMID:26169942
        reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings"
        explanation: >-
          The measurement and its direction.
    evidence:
    - reference: PMID:26169942
      reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The PDYN(R212W) mice reproduced many of the clinical features of spinocerebellar ataxia type 23"
      explanation: >-
        The authors' own assessment of fidelity, quoted with their qualifier
        ("many of").
  evidence:
  - reference: PMID:26169942
    reference_title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "our work indicates that the elevated mutant dynorphin A peptide levels are likely responsible for the initiation and progression of the disease"
    explanation: >-
      Establishes the model as informative for the disease mechanism as a whole,
      with the authors' hedge preserved.
discussions:
- discussion_id: controversy_pdyn_gene_validity
  kind: CONTROVERSY
  status: OPEN
  prompt: >-
    Is PDYN genuinely the cause of SCA23, and are all the originally reported
    variants pathogenic?
  attaches_to:
  - "pathophysiology#PDYN Missense Variant in the Dynorphin A Coding Region"
  rationale: >-
    This is not an ordinary knowledge gap: the gene-disease relationship was
    formally challenged in print. In 2016 Brain published a correspondence
    titled "SCA23 and prodynorphin: is it time for gene retraction?", and the
    original authors published a reply under the matching title in the same
    issue. Both are carried in this entry's references. Neither can be quoted
    here: both cache as metadata only, with no abstract body, so no snippet is
    possible and the specifics of the argument are deliberately not asserted in
    this entry. What can be said from sources that are quotable is that the
    functional evidence is variant-specific rather than uniform — L5S behaves
    oppositely to R6W and R9C, and wild-type dynorphin A was among the most
    toxic peptides in one cerebellar neuron assay — and that the original gene
    assignment rested on four families from one country. Against that,
    segregation in those families, an animal model reproducing the phenotype,
    and orthogonal structural and cellular work all support causality. The
    practical consequence for curation is that a PDYN missense variant should
    not be treated as pathogenic on the strength of the gene assignment alone.
  proposed_experiments:
  - experiment_id: exp_pdyn_gene_validity_reassessment
    name: Formal gene-disease validity reassessment
    description: >-
      Submit PDYN-SCA23 to a structured gene-disease validity framework such as
      ClinGen's, pooling segregation evidence from all reported families,
      population-database frequencies for each reported allele, and the
      variant-level functional data, and classify each allele separately. This
      would replace a contested binary — is the gene real or not — with a
      per-variant answer, which is what the heterogeneous functional data
      actually support.
  notes: >-
    No ClinGen gene-disease validity assertion for PDYN was present in this
    repository's cached ClinGen records at the time of curation, so none is
    cited.
- discussion_id: gap_wildtype_dynorphin_toxicity
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    If wild-type dynorphin A is itself toxic to cerebellar neurons at sustained
    concentration, what exactly do the SCA23 variants add?
  attaches_to:
  - "pathophysiology#Persistent Elevation of Mutant Dynorphin A"
  - "pathophysiology#Switch from Opioid to NMDA Receptor Signalling"
  rationale: >-
    The cited toxicity assay found R6W and wild-type dynorphin A to be the most
    toxic peptides tested, and the same study reports that the opioid-to-NMDA
    receptor switch was seen for R6W but not for wild-type. Those two results
    sit awkwardly together: if the wild-type peptide can be equally toxic
    without the receptor switch, then the switch is not the whole explanation
    for toxicity, and the disease-relevant variable may be persistence and local
    concentration rather than any change in receptor preference. Resolving this
    matters for the therapeutic proposal that comes out of the mouse model,
    which is to lower the mutant peptide: if concentration is the variable, that
    proposal is well founded; if the receptor switch is, then lowering total
    peptide may not be the right target.
  proposed_experiments:
  - experiment_id: exp_dose_matched_wt_versus_mutant_toxicity
    name: Dose-matched wild-type versus mutant toxicity with receptor blockade
    description: >-
      Compare wild-type and mutant dynorphin A on primary cerebellar neurons at
      matched sustained concentrations, with and without NMDA receptor blockade
      and kappa-opioid agonism. If NMDA blockade rescues the mutant but not the
      wild-type peptide, the receptor switch is doing real work; if it rescues
      both equally, concentration is the dominant variable and the switch is
      incidental.
  evidence:
  - reference: PMID:27260403
    reference_title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "R6W and wt Dyn A peptides were most toxic to primary cerebellar neurons. For R6W Dyn A, this is likely because of a switch from opioid to NMDA- receptor signalling, while for wt Dyn A, this switch was not observed."
    explanation: >-
      Both halves of the tension are in this one passage: comparable toxicity,
      but the mechanism invoked for one and explicitly not observed for the
      other.
- discussion_id: developmental_versus_degenerative
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    The R212W mouse cerebellum is already abnormal at two weeks of age. Does a
    developmental innervation deficit contribute to human SCA23, a disease that
    presents in the fifth decade?
  attaches_to:
  - "pathophysiology#Cerebellar Developmental Deficit in Purkinje Cell Innervation"
  - "pathophysiology#Purkinje Cell Degeneration and Climbing Fibre Retraction"
  rationale: >-
    This is a HUMAN_MODEL_MISMATCH rather than a KNOWLEDGE_GAP because the
    evidence is not absent — it is unusually detailed — but it exists only in a
    mouse, and the translational question is sharp rather than routine. The
    mouse shows reduced GABAergic synapses on Purkinje cell soma, delayed
    early-phase climbing fibre elimination, climbing fibres that never reach
    terminal height, and parallel-fibre takeover of the vacated proximal
    dendrite, all by three weeks of age. Human SCA23 presents at a mean age
    around 43 years, with reported onset as early as 10 and as late as 73. A
    developmental lesion of that severity would be expected to produce childhood
    signs, and none are reported.

    Three readings are compatible with the current evidence, and nothing yet
    distinguishes them. The developmental deficit may be a mouse-specific
    consequence of the transgenic construct or of murine cerebellar development,
    with no human counterpart. It may exist in humans as a subclinical
    substrate — a cerebellum built with less climbing fibre territory than
    normal, which tolerates later loss worse — which would explain adult onset
    without requiring childhood signs. Or the adult retraction may be genuinely
    degenerative in both species, with the developmental finding a separable
    early effect that does not feed it.

    The distinction is not academic. If a developmental substrate is real in
    humans, the therapeutic window for peptide-lowering closes before diagnosis
    is possible, which changes what the animal-model authors' own therapeutic
    proposal can be expected to achieve.
  proposed_experiments:
  - experiment_id: exp_conditional_adult_onset_pdyn
    name: Adult-restricted PDYN R212W expression in mouse
    description: >-
      Express the R212W allele under temporal control, switching it on only
      after cerebellar circuit maturation is complete, and compare adult motor
      phenotype, climbing fibre morphology and Purkinje cell survival against the
      constitutive model. If the adult phenotype is reproduced without the
      developmental window, the degeneration does not require the developmental
      deficit; if it is attenuated, the developmental substrate is load-bearing.
  - experiment_id: exp_presymptomatic_carrier_cerebellar_imaging
    name: Cerebellar imaging in presymptomatic PDYN variant carriers
    description: >-
      Image cerebellar volume and, where feasible, functional connectivity in
      young, asymptomatic obligate carriers from reported SCA23 families. The
      question is whether the carrier cerebellum differs from control before any
      degeneration could plausibly have begun. This is directly motivated by the
      documented asymptomatic p.R215H carrier who nonetheless had apparent
      cerebellar atrophy on MRI, which is at least consistent with a substrate
      preceding symptoms.
  evidence:
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "These findings indicate that developmental abnormalities contribute to the SCA23 pathology and uncover a developmental role for PDYN in the cerebellum."
    explanation: >-
      The claim whose human applicability is the open question.
  - reference: PMID:32587707
    reference_title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the proband's mother with the PDYN p.R215H variant had no subjective symptoms; she had not come to medical attention before our survey, although she showed apparent cerebellar atrophy on brain magnetic resonance imaging (MRI)"
    explanation: >-
      Classified PARTIAL. A structurally abnormal cerebellum in a carrier with
      no symptoms is consistent with a substrate preceding disease, but a single
      64-year-old carrier cannot distinguish a developmental substrate from
      subclinical degeneration — which is exactly why the imaging experiment
      above is proposed in young carriers.
- discussion_id: calcium_handling_in_sca23
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Is Purkinje cell calcium handling actually dysregulated in SCA23, or is the
    reported change confined to channel subunit expression?
  attaches_to:
  - "pathophysiology#Purkinje Cell Degeneration and Climbing Fibre Retraction"
  - "pathophysiology#Switch from Opioid to NMDA Receptor Signalling"
  rationale: >-
    This gap exists because of what it blocks. The
    `cerebellar_purkinje_degeneration` module carries a
    `#Purkinje Cell Calcium and Proteostasis Dysregulation` amplifier node that
    most conforming disorders declare, and SCA23 has two independent reasons to
    expect it: an NMDA-receptor-mediated excitotoxic mechanism, which implies
    calcium entry, and a reported change in the expression of several critical
    Ca2+ channel subunits in the R212W mouse. Neither is a measurement of
    calcium handling. The channel-subunit result is an expression change, and
    its own authors go no further than "potentially contributing to altered Ca2+
    transients"; the excitotoxicity route is an inference from receptor
    pharmacology, not an observation in SCA23 tissue. So the amplifier is left
    unclaimed, and the entry states which measurement is missing rather than
    which evidence is absent.
  proposed_experiments:
  - experiment_id: exp_purkinje_calcium_imaging_r212w
    name: Purkinje cell calcium imaging in the R212W mouse
    description: >-
      Measure calcium transients in Purkinje cells of PDYN R212W and wild-type
      littermates across the ages that bracket the known pathology — three
      weeks, three months and twelve months — using two-photon imaging in acute
      cerebellar slices, with climbing-fibre and parallel-fibre stimulation
      separately. A transient abnormality present at three weeks would tie
      calcium to the developmental arm; one appearing only at three months would
      tie it to the degenerative arm; absence at all three would settle the
      non-conformance affirmatively rather than by default.
  evidence:
  - reference: PMID:33043513
    reference_title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Additionally, we detected altered expression of several critical Ca2+ channel subunits, potentially contributing to altered Ca2+ transients in PDYNR212W cerebella."
    explanation: >-
      The whole gap in one sentence: an expression measurement, and an explicitly
      hedged inference to the functional consequence that has not been measured.
notes: >-
  On the treatments block. An earlier version of this entry omitted one, on the
  grounds that rehabilitative care is generic convention rather than SCA23
  evidence. That reasoning was half right and is now recorded rather than acted
  on: there is still no disease-modifying therapy for SCA23 and none has been
  trialled — the SCA treatment literature says plainly that no symptomatic or
  neuroprotective treatment is FDA-approved for any SCA — but one SCA23 patient
  was in fact referred to multidisciplinary rehabilitation for gait and
  dysphagia, which is a reported intervention and is now curated as one. Its
  target_mechanisms link is deliberately drawn to the clinical endpoint with
  MODULATES, not to any mechanistic node, and no outcome is claimed because none
  is reported. The mouse-model authors' proposal that lowering the elevated
  mutant peptide could be explored therapeutically stays in the animal-model
  section as a proposal, not a treatment.

  Phenotype scope. The phenotypes curated here are those evidenced in SCA23
  patients specifically. Several manifestations attributed to SCA23 in review
  and database sources — decreased vibratory sense, peripheral neuropathy,
  ocular dysmetria, Babinski sign, falls — are deliberately NOT curated, because
  the sources that name them describe the spinocerebellar ataxias as a class
  (PMID:33175256, PMID:23108490) or are OMIM/Orphanet summaries, and the one
  SCA23 series with a per-patient examination table records Babinski sign as
  absent in all five patients and sensory deficits as absent in all four assessed.
  Curating them from class-level text would assert of SCA23 something one of its
  few primary sources contradicts. Dysarthria is the single class-level phenotype
  retained, marked PARTIAL and explained on the entry itself.

  An attempt was made to ground those manifestations properly by citing
  Orphanet's own phenotype table (ORPHA:101108, which the deep-research report
  named as their source). It was abandoned: `just refresh-orphadata` failed a
  sha256 check on en_product1.xml — Orphadata has published a release newer than
  the one pinned in `data/orphadata/MANIFEST.yaml` — and the recipe aborts before
  fetching en_product4.xml, which is the file that carries phenotypes. A cache
  file built in that state contains definitions and cross-references but no
  phenotype table, and would have been derived from an unpinned snapshot, so it
  was discarded rather than committed. The manifest drift is a repository-level
  issue, not one this entry should work around.

  Module conformance. Three nodes conform to `cerebellar_purkinje_degeneration`
  — the trigger insult, Purkinje neuron degeneration, and the cerebellar ataxia
  endpoint. The module's `#Purkinje Cell Calcium and Proteostasis Dysregulation`
  amplifier is deliberately NOT claimed, on narrower grounds than an earlier
  draft of this entry stated. That draft said no calcium-handling measurement
  has been made in SCA23; PMID:33043513 reports altered expression of several
  critical Ca2+ channel subunits in the R212W mouse, so that was too broad. What
  is true, and what the non-conformance rests on, is that no calcium *transient*,
  buffering or store-release measurement has been reported — the authors state
  only that the expression change is "potentially contributing to altered Ca2+
  transients" — and that nothing bearing on protein quality control has been
  measured at all. NMDA-mediated excitotoxicity implying calcium entry remains an
  inference rather than a measurement in this disease. The gap and the experiment
  that would close it are recorded as the `calcium_handling_in_sca23` discussion
  rather than left as prose.

  The gene-disease relationship is contested. The description, the genetic
  notes, the diagnosis entry and a dedicated CONTROVERSY discussion all say so,
  because a reader who takes only one of those sections away should still know
  it. The two 2016 Brain correspondence pieces that constitute the challenge
  and the reply are listed in `references:` but cache as metadata only with no
  abstract body, so nothing is quoted from them and their specific arguments are
  not asserted here.

  No `datasets:` block: no disease-specific dataset was identified.
references:
- reference: PMID:15306549
  title: "Mapping of the SCA23 locus involved in autosomal dominant cerebellar ataxia to chromosome region 20p13-12.3."
- reference: PMID:19089525
  title: "Spinocerebellar ataxia type 23: a genetic update."
- reference: PMID:23108490
  title: "The frequency of spinocerebellar ataxia type 23 in a UK population."
- reference: PMID:32587707
  title: "Intrafamilial phenotypic variation in spinocerebellar ataxia type 23."
- reference: PMID:33043513
  title: "Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23."
- reference: PMID:34944698
  title: "Functional Characterization of Spinocerebellar Ataxia Associated Dynorphin A Mutant Peptides."
- reference: PMID:35081319
  title: "Current and emerging treatment modalities for spinocerebellar ataxias."
- reference: PMID:35239817
  title: "Rehabilitation in patients with cerebellar ataxias."
- reference: DOI:10.1007/s12311-026-02001-6
  title: "Spinocerebellar Ataxia Type 23 (SCA23): A Rare Cause of SCA in the Americas"
- reference: PMID:21035104
  title: "Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23."
- reference: PMID:26169942
  title: "Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23."
- reference: PMID:27260403
  title: "Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23."
- reference: PMID:27190015
  title: "SCA23 and prodynorphin: is it time for gene retraction?"
- reference: PMID:27190014
  title: "Reply: SCA23 and prodynorphin: is it time for gene retraction?"
- reference: PMID:33175256
  title: "Spinocerebellar ataxia type 23 (SCA23): a review."
- reference: PMID:25595316
  title: "The first Japanese familial case of spinocerebellar ataxia 23 with a novel mutation in the PDYN gene."
📚

References & Deep Research

References

16
PMID:15306549
Mapping of the SCA23 locus involved in autosomal dominant cerebellar ataxia to chromosome region 20p13-12.3.
No top-level findings curated for this source.
PMID:19089525
Spinocerebellar ataxia type 23: a genetic update.
No top-level findings curated for this source.
PMID:23108490
The frequency of spinocerebellar ataxia type 23 in a UK population.
No top-level findings curated for this source.
PMID:32587707
Intrafamilial phenotypic variation in spinocerebellar ataxia type 23.
No top-level findings curated for this source.
PMID:33043513
Cerebellar developmental deficits underlie neurodegenerative disorder spinocerebellar ataxia type 23.
No top-level findings curated for this source.
PMID:34944698
Functional Characterization of Spinocerebellar Ataxia Associated Dynorphin A Mutant Peptides.
No top-level findings curated for this source.
PMID:35081319
Current and emerging treatment modalities for spinocerebellar ataxias.
No top-level findings curated for this source.
PMID:35239817
Rehabilitation in patients with cerebellar ataxias.
No top-level findings curated for this source.
DOI:10.1007/s12311-026-02001-6
Spinocerebellar Ataxia Type 23 (SCA23): A Rare Cause of SCA in the Americas
No top-level findings curated for this source.
PMID:21035104
Prodynorphin mutations cause the neurodegenerative disorder spinocerebellar ataxia type 23.
No top-level findings curated for this source.
PMID:26169942
Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23.
No top-level findings curated for this source.
PMID:27260403
Altered secondary structure of Dynorphin A associates with loss of opioid signalling and NMDA-mediated excitotoxicity in SCA23.
No top-level findings curated for this source.
PMID:27190015
SCA23 and prodynorphin: is it time for gene retraction?
No top-level findings curated for this source.
PMID:27190014
Reply: SCA23 and prodynorphin: is it time for gene retraction?
No top-level findings curated for this source.
PMID:33175256
Spinocerebellar ataxia type 23 (SCA23): a review.
No top-level findings curated for this source.
PMID:25595316
The first Japanese familial case of spinocerebellar ataxia 23 with a novel mutation in the PDYN gene.
No top-level findings curated for this source.

Deep Research

1
Claude Code ▸
Spinocerebellar Ataxia Type 23 (SCA23): Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 10 citations 2026-08-24T16:36:34.980030

Spinocerebellar Ataxia Type 23 (SCA23): Comprehensive Research Report

1. Disease Information

Overview. Spinocerebellar ataxia type 23 (SCA23) is a rare, adult/late-onset, autosomal dominant neurodegenerative disorder belonging to type I autosomal dominant cerebellar ataxia (ADCA type I). It is characterized by slowly progressive gait and limb ataxia, dysarthria, slowed saccades, ocular dysmetria, hyperreflexia and extensor plantar (Babinski) responses, with variable additional features such as peripheral neuropathy (OMIM #610245; Orphanet ORPHA101108). The disease was first mapped as a distinct genetic locus in a large Dutch family in 2004 and the causal gene, PDYN (prodynorphin), was identified in 2010.

Key identifiers: - OMIM: #610245 (phenotype, SCA23); 131340 (gene, PDYN) - Orphanet: ORPHA101108 - MONDO: MONDO:0012449 (autosomal dominant SCA23; distinct from MONDO:0014846, the unrelated autosomal-recessive SCA23/SCAR23 caused by TDP2, chromosome 6p22.3 — a name collision curators must not conflate) - Disease Ontology: DOID:0050973 - Gene: HGNC symbol PDYN (HGNC:8820), chromosome 20p13 - MeSH/ICD-10/11:* Grouped under hereditary ataxia/spinocerebellar ataxia (ICD-10 G11.1, "Early-onset cerebellar ataxia" category is not precise for this late-onset entity; most registries code SCA23 under the general spinocerebellar ataxia ICD-10 G11 heading, as there is no SCA23-specific ICD code)

Synonyms: SCA23; Spinocerebellar Ataxia 23; Prodynorphin-related spinocerebellar ataxia; ADCA type I due to PDYN mutation.

Data provenance: Almost all published knowledge on SCA23 derives from aggregated disease-level resources — pedigree/linkage studies, case series, and cohort screening studies — rather than large-scale EHR data, reflecting its rarity (Verbeek et al., 2004, PMID:15306549; Bakalkin et al., 2010, PMID:21035104).

2. Etiology

Disease causal factor: SCA23 is caused by heterozygous (autosomal dominant) missense mutations in PDYN, which encodes prodynorphin, the precursor of the opioid neuropeptides α-neoendorphin, dynorphin A (Dyn A), and dynorphin B (Dyn B) (Bakalkin et al. 2010, PMID:21035104). The locus was first mapped by genome-wide linkage analysis in a large two-generation Dutch pedigree to chromosome 20p13–12.3 (max LOD score 3.46 at marker D20S199) before the gene itself was identified (Verbeek et al. 2004, PMID:15306549).

Genetic risk factors: - Eight disease-causing PDYN variants have been reported to date (Biomedicines 2021 review, PMC8698333). Six of the eight cluster exclusively within the dynorphin A/B-encoding region (residues ~207–236 of prodynorphin), the peptide segment sometimes called "Big Dynorphin" — establishing this region as a mutational hotspot. - Named pathogenic variants (protein/cDNA nomenclature from Bakalkin et al. 2010, PMID:21035104): - c.414G>T, p.R138S — in the non-opioid domain of prodynorphin (large original Dutch family, 10 affected individuals) - c.632T>C, p.L211S (Dyn A position 5, "L5S") — sporadic case, onset age 73 - c.634C>T, p.R212W (Dyn A position 6, "R6W") — onset age 54; most functionally severe variant - c.643C>T, p.R215C (Dyn A position 9, "R9C") — segregated in two affected siblings - c.644G>A, p.R215H — novel variant reported in two families with five affected/carrier individuals, likely pathogenic (Cerebellum & Ataxias 2020, PMC7310450) - Overall variant frequency is very low: PDYN variants account for only ~0.1% of screened ataxia cohorts (UK study of 852 ataxia patients plus 190 MSA-C patients found essentially no convincing pathogenic carriers beyond one novel early-onset case; Chan et al., PMID:23108490). PDYN mutations are essentially absent as a cause of multiple system atrophy (PMID:23355175) and were not found among 104 PDYN-negative German ADCA families, indicating population-specific rarity. - Inheritance modifier — zygosity effect: In the Cerebellum & Ataxias 2020 family series, the individual homozygous for p.R215H had the most severe phenotype (wheelchair-dependent by age 59), consistent with a gene-dosage effect on disease severity, while heterozygotes showed slower, more variable progression — some remaining asymptomatic into the 9th decade (PMC7310450).

Environmental/other risk factors: No environmental, infectious, or lifestyle risk factors have been established; SCA23 is a purely monogenic disorder to date, though modifier loci affecting severity/penetrance have not been systematically studied.

Protective factors: None specifically established. Loss of normal κ-opioid receptor (KOR)-mediated neuroprotective dynorphin signaling is part of the proposed mechanism (see Section 6), implying that pharmacological restoration of KOR signaling could theoretically be protective, but this remains untested in humans.

Gene-environment interaction: Not established; no CTD/GWAS gene-environment interaction data exist for PDYN and SCA23.

3. Phenotypes

Core motor phenotype (progressive, adult-onset): - Gait ataxia — nearly universal presenting feature (HP:0002066, Gait ataxia) - Limb/appendicular ataxia (HP:0002070, Limb ataxia) - Dysarthria (HP:0001260, Dysarthria) — common, often an early symptom - Slowed saccades (HP:0000514 approximates "Slow saccadic eye movements", more precisely HP:0000605, Ophthalmoparesis is not correct; the specific term is HP:0000514/"Nystagmus" is also distinct — the precise HPO term is HP:0001344, "Abnormal saccadic eye movements" or the more specific HP:0000514 is Nystagmus; slow saccades map best to HP:0001347 is ataxia-related — recommend HP:0000640 (Slow saccadic eye movements) for curation) - Ocular dysmetria (HP:0007874, Ocular dysmetria) - Hyperreflexia (HP:0001347, Hyperreflexia) - Extensor plantar response / Babinski sign (HP:0003487, Babinski sign) - Decreased vibratory sense, typically distal/below the knees, reflecting peripheral sensory neuropathy (HP:0007166, Decreased vibratory sense) - Peripheral neuropathy — variably present (HP:0009830) - Tremor — reported in some cases (HP:0001337) - Dysphagia — reported in later/severe disease (HP:0002015) - Parkinsonian features (bradykinesia, rigidity) — reported in a subset mimicking multiple system atrophy with predominant parkinsonism (MSA-P) (Cerebellum & Ataxias 2020, PMC7310450)

Phenotype characteristics: - Age of onset: Classically reported as 43–56 years in the original Dutch family (Verbeek et al. 2004), but subsequent series show a much broader range. The Cerebellum & Ataxias 2020 intrafamilial variation study reports mean onset 37.8 ± 5.5 years (range including one individual asymptomatic at 88). The 2026 Cerebellum case-report review cites a broader literature range of onset 10–73 years, mean ~43 ± 15 years, underscoring substantial phenotypic and age-of-onset heterogeneity even within the same pedigree/mutation. - Severity/progression: Slowly progressive in most cases; disease course is typically measured over decades. Severity correlates with genotype dosage (homozygotes more severe than heterozygotes in the R215H family) and possibly with specific variant (R212W/R215C show the most severe in vitro neurotoxicity). - Frequency (qualitative): Because SCA23 is exceedingly rare, formal frequency-of-symptom tables (as in HPO annotation) are not well established; gait/limb ataxia and dysarthria are reported in essentially all published cases (i.e., "obligate" features), while peripheral neuropathy, tremor, and parkinsonism are "occasional" in the published literature. - Marked intrafamilial and interfamilial variability: Some mutation carriers remain asymptomatic into advanced age (e.g., an 88-year-old asymptomatic carrier in one family), while others with the identical variant show severe, early progressive disease — indicating incomplete or age-dependent penetrance (PMC7310450).

Quality of life impact: Progressive gait and limb ataxia, dysarthria, and (in advanced/homozygous cases) wheelchair dependence and dysphagia substantially impair mobility, communication, nutrition, and independence, consistent with the general burden described for hereditary ataxias; disease-specific QOL instrument data for SCA23 specifically have not been published, but general SCA rehabilitation literature (PMC9648943) applies directly to management goals.

Suggested HPO terms: HP:0002066 (Gait ataxia), HP:0002070 (Limb ataxia), HP:0001260 (Dysarthria), HP:0007874 (Ocular dysmetria), HP:0001347 (Hyperreflexia), HP:0003487 (Babinski sign), HP:0007166 (Decreased vibratory sensation), HP:0009830 (Peripheral neuropathy), HP:0001337 (Tremor), HP:0002015 (Dysphagia), HP:0002527 (Falls), HP:0001272 (Cerebellar atrophy — imaging correlate).

4. Genetic/Molecular Information

Causal gene: PDYN (prodynorphin), OMIM 131340, chromosome 20p13, encoding the polyprotein precursor of α-neoendorphin, dynorphin A (Dyn A, 17 aa), dynorphin A-(1-8), and dynorphin B (Dyn B, 13 aa) — endogenous opioid peptides that are the primary endogenous ligands for the κ-opioid receptor (KOR, gene OPRK1*).

Pathogenic variants (missense, dominant, GAIN/altered-function — see Section 6):

Variant (protein) cDNA Position in Dyn A Reference
p.R138S c.414G>T Non-opioid domain (upstream of Dyn A) Bakalkin 2010, PMID:21035104
p.L211S ("L5S") c.632T>C Dyn A residue 5 Bakalkin 2010
p.R212W ("R6W") c.634C>T Dyn A residue 6 Bakalkin 2010
p.R215C ("R9C") c.643C>T Dyn A residue 9 Bakalkin 2010
p.R215H c.644G>A Dyn A residue 9 (alternative substitution) PMC7310450
(additional variants) — — 8 total reported per Biomedicines 2021 review (PMC8698333)
  • Variant classification: ClinVar lists multiple PDYN variants for SCA23, including p.R138S (RCV000018094, classified pathogenic/likely pathogenic by submitters) and additional missense/synonymous variants of uncertain significance (e.g., c.456C>T p.Asn152=, RCV001143591; c.405C>T p.Asp135=, RCV000392614).
  • Population frequency: Critically, a major controversy arose because 7 of 9 originally published SCA23 variants were subsequently found in the ExAC population database, raising doubt about pathogenicity for some variants and prompting the 2016 Brain correspondence "SCA23 and prodynorphin: is it time for gene retraction?" (Sailer et al., PMID:27190015). The precipitating case was a 64-year-old Brazilian man carrying a rare PDYN missense variant, and skepticism about ExAC-observed "pathogenic" alleles. Bakalkin and colleagues rebutted (Reply, Brain 2016) that segregation in a large Dutch pedigree, identification of the same/related mutations in multiple independent families, and orthogonal functional/cell/mouse-model validation supported causality, and that presence of rare variants in population databases does not by itself exclude pathogenicity for a dominant, incompletely penetrant, late-onset disorder. Curators should treat PDYN-SCA23 causality as more contested than most monogenic ataxia genes, and weigh variant-level evidence (segregation + functional data) individually rather than assuming class-wide pathogenicity.
  • Functional consequence: Missense variants in the Dyn A/B coding region alter processing and stability of the mature dynorphin A peptide (see Mechanism, Section 6) — a mechanism distinct from simple loss-of-function or classical gain-of-function; better classified as altered/aberrant peptide function (functional_impact_category candidates: PARTIAL_LOSS_OF_FUNCTION at the receptor level combined with a gain of toxic peptide stability/accumulation — a mixed mechanism).
  • Variant origin: All reported variants are germline; no somatic PDYN variants have been implicated.
  • Modifier genes: None formally established; zygosity (homozygous vs. heterozygous R215H) modifies severity within one reported family (PMC7310450).
  • Epigenetics/chromosomal abnormalities: Not implicated; SCA23 is a point-mutation disorder, not a repeat-expansion or copy-number disorder (distinguishing it from most other numbered SCAs, which are commonly CAG-repeat expansions).

Suggested gene/ontology annotations: HGNC:8820 (PDYN); GO:0007218 (neuropeptide signaling pathway); GO:0038193 (thromboxane receptor signaling — not applicable); more precisely GO:0038047 (opioid receptor activity) and GO:0016248 (opioid receptor binding); CHEBI compound-level term for dynorphin A is not standard (peptide, not small molecule) — use UniProt P01213 (Proenkephalin-B/Prodynorphin, human) as protein reference.

5. Environmental Information

No environmental toxins, occupational exposures, radiation, pollutants, or infectious agents have been implicated in SCA23 causation; this is a purely monogenic, autosomal dominant disorder. No lifestyle factor (diet, smoking, alcohol, exercise) has been shown to modify onset or progression in the literature reviewed. This section is essentially not applicable beyond noting the absence of reported gene-environment interaction data.

6. Mechanism / Pathophysiology

SCA23 pathophysiology centers on dysregulated processing and toxic accumulation of mutant dynorphin A peptide, converging on loss of neuroprotective κ-opioid receptor (KOR) signaling combined with NMDA-receptor-mediated excitotoxicity and direct membrane-disruptive peptide toxicity.

Causal chain (upstream → downstream):

  1. Molecular trigger: Missense mutation in the PDYN Dyn A/B coding region (e.g., p.R212W, p.L211S, p.R215C) (GO:0006508, proteolysis; the mutations affect proprotein convertase processing sites).
  2. Altered peptide processing/stability: Two of the mutations (p.L211S, p.R212W) cause 10- to 18-fold elevated Dyn A peptide levels in a transfected cellular model (RINm-5F cells) compared to wild type — i.e., a "gain" in peptide abundance due to impaired degradation/processing rather than increased transcription (Bakalkin et al. 2010, PMID:21035104).
  3. Altered secondary structure: Mutant Dyn A peptides show loss of the N-terminal α-helix, altered secondary structure, and increased peptide stability/resistance to degradation (Human Molecular Genetics 2016, PMID:27260403).
  4. Loss of KOR signaling (loss-of-function at the receptor level): Mutant Dyn A peptides show markedly reduced potency at the κ-opioid receptor (KOR) for both canonical G-protein dissociation and β-arrestin recruitment (EC50 shifts: R6W ~11-fold reduced potency, L5S ~7-fold, R9C ~3-fold) (Biomedicines 2021, PMC8698333). Structural modeling shows the wild-type Arg6 forms a critical ionic interaction with glutamate E297 on the KOR transmembrane helix, which is disrupted by the R6W substitution.
  5. Partial switch to NMDA-receptor-mediated excitotoxicity: Loss of normal opioid-receptor-mediated neuroprotective signaling is accompanied by a partial shift toward NMDA receptor-mediated excitotoxic signaling, contributing to neuronal injury (PMID:27260403).
  6. Direct membrane toxicity: Enhanced peptide stability of mutant Dyn A enables plasma membrane poration/bilayer penetration, causing membrane leakage and cellular dysfunction independent of receptor signaling — a proposed general mechanism for pathological neuropeptide signal transduction (Cell Death & Disease 2015).
  7. Cellular consequence — cytotoxicity: Mutant Dyn A peptides (particularly R212W and R215C) induce marked neuronal loss in cultured striatal neurons on time-lapse imaging (Bakalkin 2010) and cause Purkinje cell loss in vivo.
  8. Circuit-level consequence — climbing fiber/Purkinje cell pathology and developmental component: In the PDYN R212W knock-in mouse, developmental deficits are detectable as early as 2 weeks of age — a reduced number of GABAergic synapses on Purkinje cell somata and delayed climbing-fiber (CF) elimination between postnatal days 14–21, so that CFs fail to reach their normal terminal dendritic height, leaving proximal Purkinje dendrites available for parallel-fiber (PF) territory takeover (increased vGlut1, a PF-PC synapse marker). This establishes SCA23 as having a neurodevelopmental component superimposed on later neurodegeneration (Smeets et al., Brain Pathology 2021, PMID:33043513).
  9. Progressive degeneration: PDYN R212W mice show progressive motor deficits from 3 months of age, ongoing climbing-fiber deficits from 3 months, and overt Purkinje cell loss by 12 months of age, reproducing core features of the human disease (Jezierska/Bakalkin group, Brain 2015, PMID:26169942).
  10. End-organ/clinical manifestation: Progressive cerebellar (and variably brainstem/spinal cord) degeneration produces the gait/limb ataxia, dysarthria, oculomotor, and reflex abnormalities described in Section 3.

Cell types involved: Cerebellar Purkinje cells (CL:0000121), striatal neurons (used as the in vitro toxicity model; CL:0000393 medium spiny neuron), inferior olivary neurons (climbing fiber origin), dentate nucleus neurons, and glial cells (gliosis in affected white matter).

Molecular functions/processes (suggested GO terms): GO:0038047 (opioid receptor activity), GO:0007218 (neuropeptide signaling pathway), GO:0007269 (neurotransmitter secretion), GO:0006508 (proteolysis, neuropeptide processing), GO:0043524 (negative regulation of neuron apoptotic process — for the lost neuroprotective KOR arm), GO:0007268 (chemical synaptic transmission), GO:0021702 (cerebellar Purkinje cell differentiation, developmental component), GO:0021688 (cerebellar molecular layer formation).

Anatomical/cellular localization (suggested UBERON/CL/GO-CC): UBERON:0002037 (cerebellum), UBERON:0002190 (cerebellar vermis specifically implicated — rostral vermis Purkinje cell loss), UBERON:0002037 dentate nucleus, UBERON:0002037 inferior olivary nucleus, UBERON:0002298 brainstem (basis pontis atrophy), CL:0000121 (Purkinje cell), GO:0005886 (plasma membrane — site of peptide-membrane interaction and KOR signaling).

7. Anatomical Structures Affected

  • Primary organ: Central nervous system — predominantly the cerebellum (UBERON:0002037), with prominent rostral vermis involvement.
  • Secondary/associated regions: Brainstem (basis pontis atrophy, relatively small cerebellopontine tracts), spinal cord (atrophy reported at autopsy), dentate nuclei, and inferior olivary nuclei (marked neuronal loss). One autopsy case additionally noted frontotemporal cerebral atrophy and ubiquitin-positive intranuclear inclusions in nigral neurons (interpreted as incidental Marinesco bodies, not disease-specific pathology) (Genetic Update review, PMID:19089525).
  • Peripheral nervous system: Variable peripheral sensory neuropathy (decreased vibratory sense) implicates dorsal root ganglia/peripheral sensory axons, though this is less consistently reported than the core cerebellar phenotype.
  • Tissue/cell level: Purkinje cell layer of the cerebellar cortex (severe loss), surrounding cerebellar white matter (myelin loss and gliosis), climbing fiber–Purkinje cell synapses (developmentally and progressively disrupted), granule cell layer (parallel fiber territory expansion documented in mouse model).
  • Subcellular level: Plasma membrane (site of peptide-induced poration; GO:0005886), synaptic vesicles/dense-core granules (site of prodynorphin processing and dynorphin storage/release; GO:0030141 secretory granule).
  • Localization/laterality: Diffuse, bilateral cerebellar and brainstem involvement; no reported lateralization.

Suggested UBERON/CL terms: UBERON:0002037 (cerebellum), UBERON:0002190 (cerebellar vermis), UBERON:0002315 (cerebellar cortex), UBERON:0002038 (dentate nucleus), UBERON:0002298 (brainstem), CL:0000121 (Purkinje cell), CL:0000119 (cerebellar granule cell).

8. Temporal Development

  • Onset: Adult-onset, with substantial reported range — classic Dutch family: 43–56 years; broader literature range across all reported families/cases: 10–73 years (mean ~43 ± 15 years per the 2026 case-report review); one large intrafamilial series reports mean onset 37.8 ± 5.5 years. Onset is typically insidious, presenting with dysarthria, gait unsteadiness, or speech disturbance.
  • Progression: Slowly progressive over years to decades in most patients; a subset (notably a homozygous R215H carrier) shows more rapid, severe progression to wheelchair dependence (by age 59 in the reported case).
  • Disease course pattern: Chronic, progressive, non-remitting; no episodic or relapsing-remitting pattern has been described.
  • Penetrance/critical periods: Age-dependent, incomplete penetrance is documented — an obligate carrier remained asymptomatic at age 88 in one family, indicating either very-late or absent clinical penetrance is possible. This has direct implications for genetic counseling of at-risk relatives.
  • Neurodevelopmental component: Mouse model data (Smeets et al. 2021) indicate that pathological changes at the synaptic/circuit level (climbing fiber elimination delay, reduced GABAergic PC synapses) begin in early postnatal development (~2 weeks in mice), long before any measurable motor deficit (3 months) or Purkinje cell loss (12 months) — suggesting SCA23 may have a subclinical developmental substrate decades before human symptom onset, an important consideration for early-biomarker or preventive-intervention strategies.

9. Inheritance and Population

  • Epidemiology: SCA23 is exceptionally rare. Population prevalence/incidence figures are not established in standard registries (SEER, GBD) given its rarity; the best available quantitative estimate is that PDYN variants account for ~0.1% of screened ataxia cohorts (UK cohort of 852 ataxia patients plus 190 MSA-C patients; Chan et al., PMID:23108490). As of the most recent case reports (2026), SCA23 remains reportable as individual case series — e.g., a 2026 case report describes only the third reported case in the Americas and second in Brazil, underscoring how few confirmed cases exist worldwide outside the Netherlands.
  • Geographic distribution: Originally and predominantly described in Dutch/Netherlands families (the founding pedigree and several subsequent Dutch families/sporadic cases); subsequently identified sporadically in the UK, China (Han population, described as an "uncommon SCA subtype"), and most recently South America (Brazil). SCA23 appears to be essentially absent or exceptionally rare in central European (German) ADCA cohorts specifically screened and found negative.
  • Inheritance pattern: Autosomal dominant (AD); one reported instance of homozygosity for p.R215H associated with more severe disease, suggesting a semi-dominant, gene-dosage effect atop dominant inheritance.
  • Penetrance: Incomplete and age-dependent — asymptomatic carriers into the 9th decade have been documented.
  • Expressivity: Highly variable, both between and within families (intrafamilial phenotypic heterogeneity extending to an MSA-parkinsonism-mimicking presentation in one proband).
  • Genetic anticipation: Not established/reported for SCA23 (unlike CAG-repeat SCAs); SCA23 is a point-mutation disorder, so classical repeat-expansion anticipation is not expected.
  • Founder effects: The concentration of cases in Dutch families raises the possibility of a founder effect for specific variants (e.g., p.R138S in the original large pedigree), though this has not been formally established via haplotype analysis in the literature reviewed.
  • Consanguinity: The one reported homozygous case (p.R215H) arose in the context of two related branches of an extended family rather than documented parental consanguinity per se (PMC7310450); consanguinity is not established as a general risk factor.
  • Sex ratio / age distribution: No specific male:female skew has been reported; case reports include both sexes across a wide adult age range at onset.
  • Carrier frequency: Not established in large reference population databases beyond the ExAC observations that prompted the 2016 pathogenicity controversy (Section 4).

10. Diagnostics

Clinical tests: - Neurological examination: gait/limb ataxia assessment, saccadic eye movement testing, deep tendon reflex and plantar response testing, vibratory sense testing. - Imaging (MRI): Cerebellar atrophy is a consistent finding across reported cases, generally with vermian and hemispheric involvement; brainstem is relatively preserved in most cases (distinguishing from MSA-C, which characteristically shows pontine "hot cross bun" sign — absent in SCA23 per the intrafamilial variation study). One SCA23 case showed an atypical "hyperintense lateral putaminal rim," creating diagnostic overlap with MSA-P and illustrating why SCA23 can mimic MSA clinically and radiologically (PMC7310450). - Electrophysiology: Nerve conduction studies may show peripheral neuropathy in a subset of patients (not systematically characterized). - Neuropathology (autopsy, when available): Marked Purkinje cell loss in the rostral cerebellar vermis, neuronal loss in dentate nuclei and inferior olives, myelin loss/gliosis in adjacent white matter, relatively preserved basis pontis myelination but reduced cerebellopontine tract size, and brain weight reduction reflecting overall atrophy (PMID:19089525).

Genetic testing: - Recommended approach: Given extreme rarity (~0.1% of ataxia cohorts), PDYN sequencing is generally not first-line in undiagnosed ataxia and is more appropriately pursued after exclusion of the common repeat-expansion SCAs (SCA1, 2, 3, 6, 7) and other more prevalent causes, or via a broad ataxia gene panel or exome/genome sequencing, particularly in familial, autosomal-dominant, "pure" cerebellar ataxia cases without another identified cause. The UK screening study explicitly concludes that "front-line diagnostic evaluation... should focus on other known ataxia genes" given PDYN's rarity (PMID:23108490). - Single-gene testing: Direct Sanger sequencing of PDYN coding exons and flanking intronic regions, focused especially on the dynorphin A/B-encoding hotspot region (residues ~207–236), is used once other causes are excluded or when segregation analysis in a family with an existing candidate variant is needed. - Whole exome/genome sequencing (WES/WGS): Increasingly the diagnostic route by which sporadic/small-family SCA23 cases are now identified (e.g., the 2026 Brazilian case, diagnosed via next-generation sequencing). - Variant interpretation caution: Given the ExAC-based controversy over pathogenicity of several PDYN variants (Section 4), genetic counseling for a PDYN variant of uncertain significance should incorporate segregation data, population frequency, and — where possible — functional evidence (e.g., peptide stability/KOR signaling assays), rather than classification alone. - Chromosomal microarray/karyotyping/FISH/mitochondrial testing/repeat-expansion testing: Not applicable — SCA23 is not a copy-number, chromosomal, mitochondrial, or repeat-expansion disorder.

Differential diagnosis: Other autosomal dominant SCAs (especially repeat-expansion SCA1/2/3/6/7 and other ADCA type I subtypes), multiple system atrophy with cerebellar or parkinsonian features (MSA-C/MSA-P) — a particularly important mimic given overlapping late-onset ataxia/parkinsonism and (in one case) similar putaminal MRI signal change, though brainstem "hot cross bun" sign is typically absent in SCA23. PDYN mutations were specifically shown not to be a cause of sporadic MSA in a dedicated screening study (PMID:23355175), reinforcing that PDYN testing should be reserved for familial/dominant-inheritance-pattern cases rather than sporadic MSA-like presentations.

Screening: No population-based or newborn screening applies given adult onset and rarity; predictive/cascade testing in at-risk relatives of a confirmed proband is the relevant genetic-counseling application, tempered by the documented incomplete, age-dependent penetrance.

Suggested NCIT/LOINC terms: NCIT:C16809 (Magnetic Resonance Imaging), NCIT:C15709 (Genetic Testing), NCIT:C158748-type panel testing concepts, LOINC terms for targeted PDYN gene sequencing (locus-specific, no dedicated LOINC panel code identified in this review).

11. Outcome/Prognosis

  • Survival/mortality: No formal survival statistics (5-/10-year survival, life expectancy) have been published for SCA23 specifically, consistent with its rarity and the absence of large natural-history cohorts. The disease is understood as being slowly progressive but not rapidly fatal in most reported cases, in contrast to some other SCAs with earlier or more aggressive courses.
  • Morbidity/functional outcomes: Progressive gait and limb ataxia leading to increasing mobility impairment; the most severe reported outcome (homozygous R215H carrier) was wheelchair dependence by age 59. Dysarthria and (in advanced disease) dysphagia contribute to communication and nutritional morbidity.
  • Complications: Falls and fall-related injury (implied by progressive gait ataxia, though not separately quantified), aspiration risk with dysphagia in advanced cases, and functional decline requiring rehabilitative support.
  • Prognostic factors: Genotype dosage (homozygous vs. heterozygous) appears to modify severity; specific variant identity may modify severity given differential in vitro neurotoxicity (R6W/R212W and R9C/R215C show more severe cell-culture neurotoxicity than L5S/L211S). Age of onset varies widely and does not appear tightly predictable from genotype alone given documented intrafamilial variability, including near-complete non-penetrance in at least one elderly carrier.
  • Recovery potential: As a neurodegenerative disorder, spontaneous recovery is not expected; multidisciplinary rehabilitation (see Section 12) aims to maximize function rather than reverse disease.

12. Treatment

No disease-modifying or curative therapy exists for SCA23. As with other hereditary cerebellar ataxias, management is entirely symptomatic and supportive, following the general framework used across the SCA spectrum (PMC9048095 review of current/emerging SCA treatment modalities; PMC9648943 rehabilitation review):

  • Pharmacotherapy (symptomatic, non-SCA23-specific evidence base):
  • Agents such as riluzole and valproic acid have been trialed for cerebellar ataxia symptoms in SCAs broadly, though evidence quality is limited and none is SCA23-specific (NCIT:C15986, Pharmacotherapy; CHEBI riluzole CHEBI:8804).
  • Antispasmodic agents for spasticity if present.
  • Analgesics/psycholeptics for pain and mood symptoms as needed.
  • Rehabilitative/supportive care (the mainstay of documented SCA23 management):
  • Multidisciplinary rehabilitation — the 2026 Brazilian case report explicitly describes referral to a multidisciplinary rehabilitation team focused on functional gait rehabilitation and dysphagia management (NCIT:C15302, Physical Therapy; NCIT:C159273, Speech Therapy; NCIT:C121351, Occupational Therapy).
  • Physical therapy for gait and balance training.
  • Speech and swallowing therapy for dysarthria and dysphagia, critical for aspiration prevention.
  • Occupational therapy to support activities of daily living as disability progresses.
  • Genetic counseling (NCIT:C15240) is an essential component of care given autosomal dominant inheritance with incomplete, age-dependent penetrance, informing at-risk relatives about predictive testing options and the uncertainty around variant-level pathogenicity.
  • Experimental/investigational directions (mechanism-informed, not yet in human trials for SCA23): Given the proposed loss of neuroprotective KOR signaling and gain of NMDA-receptor-mediated excitotoxicity, KOR agonism or NMDA receptor modulation represent plausible but unvalidated therapeutic strategies suggested by the mechanistic literature (PMID:27260403); no clinical trials specifically targeting PDYN/KOR pathway modulation in SCA23 patients were identified in this search (ClinicalTrials.gov search recommended as a follow-up for currency).
  • Surgical/device interventions: Not applicable; no surgical treatment is described for SCA23.

Suggested NCIT terms for treatment annotation: NCIT:C15747 (Supportive Care), NCIT:C15302 (Physical Therapy), NCIT:C159273 (Speech Therapy), NCIT:C121351 (Occupational Therapy), NCIT:C15240 (Genetic Counseling), NCIT:C15986 (Pharmacotherapy, generic, for symptomatic drug trials with therapeutic_agent sub-binding to riluzole/CHEBI:8804 where used).

13. Prevention

  • Primary prevention: Not applicable in the traditional sense (no modifiable environmental cause); the only "primary prevention" avenue is reproductive genetic counseling for known carrier families, including discussion of preimplantation genetic diagnosis (PGD) or prenatal testing where desired, tempered by the challenge that penetrance is incomplete and age-dependent, complicating risk communication.
  • Secondary prevention/early detection: Predictive genetic testing of at-risk relatives in a family with a confirmed pathogenic PDYN variant allows early identification of carriers, though clinical utility is limited by the absence of any preventive or disease-modifying intervention to offer asymptomatic carriers at present.
  • Tertiary prevention: Multidisciplinary rehabilitative care (Section 12) functions as tertiary prevention — minimizing fall risk, aspiration, and functional decline complications once disease is manifest.
  • Screening programs: No population or newborn screening applies (adult-onset, rare, no actionable early intervention).
  • Public health/environmental interventions: Not applicable — no environmental risk factor has been identified to intervene upon.

14. Other Species / Natural Disease

  • Naturally occurring disease in other species: No naturally occurring PDYN-associated cerebellar ataxia has been reported in companion animals or wildlife (e.g., no OMIA entry identified for a PDYN-linked ataxia). SCA23 knowledge in non-human species derives exclusively from engineered/induced genetic mouse models (Section 15), not naturally occurring veterinary disease.
  • Comparative biology: Prodynorphin and the opioid peptide system (dynorphins, κ-opioid receptor) are highly conserved across mammals, supporting the translational validity of the mouse knock-in model for studying the human mutation's mechanistic consequences (Purkinje cell loss, climbing fiber pathology).
  • Zoonotic potential: Not applicable — SCA23 is a purely genetic, non-communicable, non-zoonotic disorder.

15. Model Organisms

Genetic mouse model — PDYN R212W knock-in mouse: - Model type: Genetically engineered mouse expressing human PDYN carrying the SCA23-causing p.R212W (R6W in Dyn A) variant. - Phenotype recapitulation: - Progressive motor deficits beginning at 3 months of age. - Climbing fiber (CF) synaptic deficits detectable from 3 months of age, with earlier developmental CF elimination delay and reduced Purkinje cell somatic GABAergic synapse number detectable as early as 2 weeks postnatal age (Smeets et al., Brain Pathology 2021, PMID:33043513). - Purkinje cell loss emerging by 12 months of age, together with elevated levels of mutant dynorphin A (Jezierska/Bakalkin group, Brain 2015, PMID:26169942). - Reproduces core disease features: gait deficits, climbing-fiber pathology, and Purkinje cell degeneration — providing strong (though not complete) fidelity to the human phenotype, and directly linking the biochemical mechanism (elevated/toxic mutant Dyn A) to the anatomical and behavioral phenotype. - Model limitations: As with most knock-in models of a late-onset human disease, the mouse model's timeline is compressed relative to the human decades-long course, and it captures only the R212W allele — not the full allelic spectrum (e.g., R138S in the non-opioid domain acts through a potentially distinct mechanism not yet separately modeled). The model's most novel finding — a neurodevelopmental synaptic phenotype at 2 weeks — has not yet been confirmed to have a human correlate, representing an open HUMAN_MODEL_MISMATCH-type question: whether human PDYN-mutation carriers have subclinical developmental cerebellar circuit abnormalities decades before overt ataxia. - Cellular/in vitro models: Transfected RINm-5F cells (rat insulinoma cell line) used to demonstrate elevated mutant Dyn A peptide production; cultured striatal neurons (primary rodent) used in time-lapse imaging to demonstrate direct neurotoxicity of mutant Dyn A peptides (R212W, R215C) (Bakalkin et al. 2010, PMID:21035104). Receptor pharmacology (KOR G-protein/β-arrestin signaling assays) performed in heterologous expression systems to quantify EC50 shifts for mutant peptides (Biomedicines 2021, PMC8698333). - Applications: The mouse model is used to study the temporal relationship between molecular pathology (mutant peptide accumulation), synaptic/circuit pathology (climbing fiber-Purkinje cell synapse abnormalities), and cell loss/behavioral phenotype — and represents the principal in vivo platform for any future preclinical therapeutic testing (e.g., KOR-targeted or NMDA-modulating compounds).

Suggested NCBITaxon/model resource terms: NCBITaxon:10090 (Mus musculus), NCBITaxon:9606 (Homo sapiens, for the knock-in human-mutant transgene context); model resource cross-reference would be to MGI once/if a formal MGI allele record is confirmed (not verified in this search).

Summary of Key Evidence Citations (PMID)

Topic Citation PMID
Locus mapping, chromosome 20p13-12.3, original Dutch family Verbeek et al., Brain 2004 15306549
Identification of PDYN as SCA23 gene, 4 mutations, functional data Bakalkin et al., Am J Hum Genet 2010 21035104
Elevated mutant Dyn A, Purkinje cell loss, R212W mouse model Jezierska et al./Bakalkin group, Brain 2015 26169942
Altered Dyn A secondary structure, loss of opioid signaling, NMDA excitotoxicity Hum Mol Genet 2016 27260403
Pathogenicity controversy ("gene retraction?") Sailer et al., Brain 2016 (letter) 27190015
UK cohort screening, frequency ~0.1% Chan et al., J Neurol 2012 23108490
PDYN not implicated in MSA J Neurol 2013 23355175
Genetic update review Cerebellum 2009 19089525
Cerebellar developmental deficits, R212W mouse (climbing fiber) Smeets et al., Brain Pathol 2021 33043513
Intrafamilial phenotypic variation, R215H variant Cerebellum Ataxias 2020 (PMC7310450)
Functional characterization of Dyn A mutant peptides (KOR EC50 data) Biomedicines 2021 (PMC8698333)
SCA23 review J Neurol 2021 33175256
Third case in the Americas (Brazil), NGS diagnosis Cerebellum 2026 (in press, DOI 10.1007/s12311-026-02001-6)

Note on evidence quality: Given SCA23's rarity, most quantitative claims (frequency ~0.1%, age-of-onset ranges, EC50 shifts) derive from small cohort/case-series/single-family studies rather than large epidemiological databases; the pathogenicity of several individual PDYN variants remains actively debated in the literature (Section 4), which curators should flag explicitly (e.g., via evidence_source classification and possibly a mechanistic_hypotheses/KNOWLEDGE_GAP discussion block) rather than treating PDYN→SCA23 causality as uniformly settled across all eight reported variants.

Reference Validation

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References weighed for topical relevance 15
On topic 11
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