Optic Atrophy 13 With Retinal and Foveal Abnormalities

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Inheritance

2

Autosomal dominant HP:0000006

Heterozygous missense variants segregating in multigeneration families, with incomplete penetrance. The dominant mechanism is interference with a multimeric complex rather than haploinsufficiency, which is what the functional work argues.

Autosomal dominant inheritance

Show evidence (2 references)

PMID:31550237 SUPPORT Human Clinical

"we identified heterozygous missense mutations in single-strand binding protein 1 (SSBP1) in 5 unrelated families"

Establishes the heterozygous missense architecture across five independent families.

PMID:31298765 SUPPORT Human Clinical

"which is consistent with the segregation pattern observed in 2 multigeneration families studied"

Independent segregation evidence for dominant transmission.

Incomplete penetrance HP:0003829

Three of 27 carriers of the recurrent p.Arg38Gln variant were asymptomatic. Incomplete penetrance is shared with OPA1-related dominant optic atrophy.

Typified by incomplete penetrance

Show evidence (1 reference)

PMID:34548540 SUPPORT Human Clinical

"Three SSBP1-patients were asymptomatic, 10 had isolated DOA, and 12 had a combined DOA plus foveopathy."

The penetrance denominator, and the breakdown of the phenotype within a single-variant cohort.

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Discussions and Knowledge Gaps

2

Is the tissue distribution of SSBP1 disease set by how much mtDNA depletion each allele causes, as has been proposed, or by something specific to retinal ganglion cells?

KNOWLEDGE GAP ssbp1_tissue_selectivity

Attached to

pathophysiology#Mitochondrial DNA Depletion pathophysiology#Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration

An earlier version of this discussion asked why SSBP1 disease is eye-restricted when other mtDNA depletion syndromes are multisystem. That premise was too strong and has been corrected in the entry: the ocular restriction belongs to p.Arg38Gln, other missense alleles give syndromic presentations, and hypothyroidism and renal failure are documented. The sharper question is what sets the difference between alleles. A dosage account has been proposed - that severity tracks residual mtDNA levels, that variants differ in their effect on tetramer abundance and activity, and that an organ is affected only once depletion passes a threshold in it. That would make p.Arg38Gln the mild end of a continuum rather than a different kind of lesion, and it is attractive because it connects the clinical gradient directly to the tetramer-assembly node this entry already curates. It is untested. No study has measured mtDNA copy number across tissues of differing clinical involvement in the same carrier, or compared depletion between alleles. The competing account - that retinal ganglion cells are selectively vulnerable to mitochondrial dysfunction because of their unmyelinated prelaminar axon segment, as argued for OPA1 and Leber hereditary optic neuropathy - predicts that depletion is uniform and the tissue response is not. The two make different predictions and one measurement separates them.

Proposed experiments

Cross-tissue mtDNA copy number in SSBP1 variant carriers

exp_ssbp1_tissue_mtdna_copy_number

Compare mtDNA copy number and replication intermediates across tissues of differing clinical involvement in the same SSBP1 carriers - fibroblast, blood, muscle, and where available post-mortem retina and optic nerve - and compare p.Arg38Gln carriers against carriers of syndromic alleles, to test whether depletion is uniform with selective tissue tolerance or tracks allele severity as the dosage account predicts.

Would support

pathophysiology#Mitochondrial DNA Depletion

Supporting outcome

Depletion is comparable across tissues and differs between alleles in proportion to clinical severity, supporting the proposed dosage-threshold account and locating tissue selectivity in differential tolerance.

Refuting outcome

Depletion does not differ between alleles, or is confined to ocular tissue, refuting the dosage account and pointing instead to a tissue-specific requirement for SSBP1.

Are the congenital foveopathy and the late-onset rod-cone dystrophy two expressions of one photoreceptor process, or independent consequences?

OPEN QUESTION ssbp1_foveopathy_versus_late_retinal_degeneration

Attached to

pathophysiology#Foveal Photoreceptor Maldevelopment pathophysiology#Late-Onset Rod-Cone Dystrophy

The two behave quite differently. The foveopathy is the same at every age and costs no acuity, which is the profile of a developmental lesion that then stays put. The rod-cone dystrophy appears after 50 and is severe, which is the profile of a degeneration. Whether the first predisposes to the second, or the two are separate readouts of the same underlying photoreceptor compromise on different timescales, is unresolved - and the cohorts are far too small and too cross-sectional to answer it. It matters for prognosis. If the foveopathy marks patients who will later degenerate, it becomes a prognostic sign as well as a diagnostic one; if not, it should continue to be reported to families as the reassuring finding it currently appears to be.

Proposed experiments

Longitudinal retinal imaging of SSBP1 carriers into later life

exp_ssbp1_longitudinal_retinal_imaging

Follow a cohort of SSBP1 variant carriers with serial macular OCT and autofluorescence into and beyond the sixth decade, and test whether the presence of congenital foveopathy predicts subsequent rod-cone dystrophy.

Would support

pathophysiology#Late-Onset Rod-Cone Dystrophy

Supporting outcome

Rod-cone dystrophy develops preferentially in carriers with congenital foveopathy, supporting one photoreceptor process with two temporal phases and making the foveopathy prognostic.

Refuting outcome

Late rod-cone dystrophy occurs independently of foveopathy status, supporting two separate consequences and preserving the foveopathy as a purely diagnostic sign.

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Pathophysiology

7

Heterozygous SSBP1 Missense Variant

Mechanism confidence: Established

The primary lesion is a heterozygous missense change in SSBP1. The reported variants cluster functionally rather than randomly: p.Arg38Gln and p.Arg107Gln both replace arginines within a basic patch essential for single-stranded DNA binding, so the loss of positive charge is the shared consequence. A third variant, p.Ser141Asn, has also been reported.

SSBP1 hgnc:11317

Downstream

Destabilised SSBP1 Dimer and Tetramer Assembly The substituted residues sit at the dimer interface and in the DNA-binding region, so the immediate consequence is on assembly of the functional multimer.

Show evidence (2 references)

PMID:31298765 SUPPORT Human Clinical

"We defined a new ADOA locus on 7q33-q35 and identified 3 different missense variants in SSBP1"

The allelic series. The quote stops before the variant list because the reference validator strips bracketed spans such as [p.(Arg38Gln)] before matching, so a snippet spanning them cannot verify; the three protein changes are named in this node's description instead.

PMID:31298765 SUPPORT Computational

"The mutated arginine residues are part of a basic patch that is essential for single-strand DNA binding."

Locates the two recurrent variants in a functionally defined region, which is what makes the allelic clustering mechanistically interpretable.

Destabilised SSBP1 Dimer and Tetramer Assembly

Mechanism confidence: Established

SSBP1 works as a multimer. A revised crystal structure determined alongside the founding clinical report showed that both recurrent mutations affect dimer interactions and are predicted to distort the DNA-binding region, and patient fibroblasts confirmed that the R38Q variant destabilises dimer and tetramer formation. This node carries the dominant-negative claim, and it is the right place for it: a variant subunit that poisons a shared multimer explains dominance, whereas simple loss of one allele's product would not. The zebrafish work supports the same conclusion from the opposite direction - injecting mutant mRNA reproduced the effect of knocking the endogenous transcript down, which is the signature of interference rather than of insufficiency.

single-stranded DNA binding GO:0003697 ↓ DECREASED

Animal models

ssbp1 knockdown and mutant-mRNA zebrafish Zebrafish recapitulates Administering mutant mRNA reproduces the effect of removing the endogenous transcript. That equivalence is the model's real contribution: it is what distinguishes a dominant-negative allele from a null, and it is difficult to establish in patient cells that carry only one copy of the variant.

Downstream

Impaired Mitochondrial DNA Replication A destabilised multimer with reduced affinity for single-stranded DNA cannot support replication normally.

Show evidence (3 references)

PMID:31550237 SUPPORT Computational

"Structural analysis suggested that both mutations affect dimer interactions and presumably distort the DNA-binding region."

The structural basis, quoted with the authors' own hedging ("suggested", "presumably") intact.

PMID:31550237 SUPPORT In Vitro

"we validated that the R38Q variant destabilizes SSBP1 dimer/tetramer formation, affects mtDNA replication, and induces mtDNA depletion"

Experimental confirmation in patient fibroblasts, and the observation that chains this node to the two below it.

PMID:31298765 SUPPORT Model Organism

"A similar effect was achieved when mutated mRNAs were administered."

Mutant mRNA phenocopies knockdown in zebrafish, which is the functional signature of a dominant-negative rather than a haploinsufficient allele.

Impaired Mitochondrial DNA Replication

Mechanism confidence: Established

SSBP1 coats and protects the single-stranded template at the mitochondrial replication fork, and is described as a crucial component of the mtDNA replication machinery. Patient fibroblasts carrying R38Q show impaired replication directly.

mitochondrial DNA replication GO:0006264 ↓ DECREASED

Downstream

Mitochondrial DNA Depletion Failure to replicate the mitochondrial genome reduces its copy number.

Show evidence (1 reference)

PMID:31550237 SUPPORT In Vitro

"affects mtDNA replication, and induces mtDNA depletion"

The replication defect measured in patient cells, and its immediate consequence.

Mitochondrial DNA Depletion

Mechanism confidence: Established

Reduced mtDNA copy number in patient cells. This places SSBP1 disease within the mtDNA maintenance disorders, whose other members generally cause severe multisystem neurological disease. A proposed reconciliation exists and is worth stating rather than treating the tissue distribution as simply unexplained: severity may be proportional to residual mtDNA levels, with different variants having different effects on tetramer abundance and activity and so producing a gradient of depletion, against a threshold that must be exceeded in an organ for it to be affected. On that account the predominantly ocular presentation of p.Arg38Gln is the mild end of one continuum rather than a categorically different disease from the syndromic alleles. The proposal has not been tested by measuring depletion across tissues.

Downstream

Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration Why depletion should injure retinal ganglion cells preferentially, rather than tissues with comparable or higher energy demand, is not established.

Foveal Photoreceptor Maldevelopment The foveal defect appears congenital and does not progress, so it is curated as a developmental rather than a degenerative consequence.

Show evidence (4 references)

PMID:31550237 SUPPORT In Vitro

"the R38Q variant destabilizes SSBP1 dimer/tetramer formation, affects mtDNA replication, and induces mtDNA depletion"

Demonstrates depletion as the cellular endpoint, and shows the whole chain from multimer destabilisation through replication failure to depletion measured in one experiment on patient-derived cells.

PMID:31550237 SUPPORT Human Clinical

"Mutations in genes encoding components of the mitochondrial DNA (mtDNA) replication machinery cause mtDNA depletion syndromes (MDSs), which associate ocular features with severe neurological syndromes."

Places the disease in its mechanistic class.

PMID:34548540 SUPPORT Human Clinical

"the severity of the SSBP1-related diseases might be proportional to the residual mtDNA levels in patients"

The proposed dose-response account of why alleles differ in how much of the body they affect, quoted with the authors' "might" intact.

+ 1 more reference

Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration

Mechanism confidence: Established

Loss of retinal ganglion cells and their axons is the constant feature of the disease. Zebrafish knockdown of endogenous ssbp1 compromises differentiation of retinal ganglion cells, which argues that the cell type's vulnerability is developmental as well as degenerative, and identifies the ganglion cell as the directly affected cell rather than an innocent downstream casualty of optic nerve disease.

retinal ganglion cell CL:0000740

Animal models

ssbp1 knockdown and mutant-mRNA zebrafish Zebrafish partially recapitulates Knockdown compromises retinal ganglion cell differentiation, identifying the cell type affected in patients as one whose development requires ssbp1. Graded PARTIALLY_RECAPITULATES rather than RECAPITULATES because the model reproduces involvement of the right cell type by the wrong route: failure of differentiation in an embryo, against degeneration of a differentiated cell in a child.

Show evidence (2 references)

PMID:31298765 SUPPORT Model Organism

"Antisense-mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells."

Identifies the retinal ganglion cell as the cell type whose development depends on ssbp1, in vivo.

PMID:31550237 SUPPORT Human Clinical

"All affected individuals presented optic atrophy, associated with foveopathy in half of the cases."

Optic atrophy as the invariant clinical feature across the founding families.

Foveal Photoreceptor Maldevelopment

Mechanism confidence: Provisional

A congenital foveopathy, seen as a small defect of the ellipsoid and interdigitation lines on macular OCT. Two observations make this developmental rather than degenerative, and both are load-bearing: the appearance is the same in all patients regardless of age, and it has no measurable effect on visual acuity. A progressive photoreceptor process would be expected to worsen with age and to cost acuity; this does neither.

Downstream

Late-Onset Rod-Cone Dystrophy Whether the congenital foveal lesion and the late photoreceptor degeneration are the same process on different timescales, or two independent consequences, is not established.

Show evidence (2 references)

PMID:34548540 SUPPORT Human Clinical

"The foveopathy, with a tiny defect of the ellipsoid and interdigitation lines, was similar in all patients, independent of age."

Age-independence is the observation that argues for a developmental rather than a degenerative lesion.

PMID:34548540 SUPPORT Human Clinical

"There were no significant statistical differences in terms of visual acuity and SD-OCT measurements between patients with isolated DOA"

The foveopathy does not cost acuity, which is why it is curated as a diagnostic marker rather than as a component of the visual disability.

Late-Onset Rod-Cone Dystrophy

Mechanism confidence: Provisional

A minority of patients develop progressive rod-cone dystrophy in later life, with severe visual impairment. In the largest single-variant series this was two patients, both over 50. It is the one part of the ocular phenotype that is unambiguously progressive, and it is the reason the disease name carries "retinal abnormalities" as well as optic atrophy.

Show evidence (1 reference)

PMID:34548540 SUPPORT Human Clinical

"Two patients over 50 years of age developed a progressive rod-cone dystrophy, leading to severe visual impairment."

The frequency and age dependence of the progressive retinal component, with its denominator of 27 given elsewhere in the same series.

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Pathograph

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

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Phenotypes

10

Cardiovascular 1

Retinal Vessel Attenuation Attenuation of retinal blood vessels HP:0007843

Show evidence (1 reference)

PMID:31298765 SUPPORT Human Clinical

"Retinal examination revealed different degrees of vessel attenuation, abnormal appearance of the fovea, and retinal pigmentary changes in the mid-periphery"

The retinal examination findings, including vessel attenuation.

Endocrine 1

Hypothyroidism HP:0000821

Show evidence (1 reference)

PMID:31298765 SUPPORT Human Clinical

"In addition, hypothyroidism (subjects 4-12, 4-14, 4-16 and 4-18) and a renal disorder, including renal failure (subjects 3-8, 4-3 and 4-18), were documented."

Documents extra-ocular involvement, naming the affected subjects. This is the observation that corrected this entry's earlier eye-restricted framing.

Eye 7

Optic Atrophy VERY_FREQUENT HP:0000648

Show evidence (2 references)

PMID:37259171 SUPPORT Human Clinical

"Among 60 published patients with missense SSBP1 variants, optic atrophy was present in 95% of them"

The frequency of optic atrophy across the largest published denominator.

PMID:31550237 SUPPORT Human Clinical

"All affected individuals presented optic atrophy, associated with foveopathy in half of the cases."

Establishes optic atrophy as invariant and foveopathy as roughly half-penetrant among affected individuals.

Foveopathy FREQUENT Abnormal foveal morphology on macular OCT HP:0030613

Show evidence (2 references)

PMID:37259171 SUPPORT Human Clinical

"57.8% presented with foveopathy on macular OCT"

A precise frequency across 60 published patients, replacing the "half of cases" estimate that a single family series supported.

PMID:34548540 SUPPORT Human Clinical

"the presence of a congenital foveopathy with no impact on visual acuity is a major criterion to distinguish SSBP1 cases and orient the appropriate genetic analysis"

States both the diagnostic value of the sign and, explicitly, that it does not affect acuity.

Retinal Pigmentary Changes FREQUENT Abnormal fundus pigmentation HP:0031605

Show evidence (2 references)

PMID:37259171 SUPPORT Human Clinical

"Sixty-five percent of patients had pigmentary changes on fundus photography"

The frequency across the largest published denominator.

PMID:31298765 SUPPORT Human Clinical

"Optic atrophy was documented in isolation (subjects 3-16, 4-3, 4-7, 4-8 and 5-12), or before the development of retinal vessel attenuation, or retinal pigmentary changes"

Establishes that where both occur, optic atrophy precedes the retinal changes, which constrains the causal ordering.

Optic Nerve Hypoplasia OCCASIONAL HP:0000609

Show evidence (1 reference)

PMID:31298765 SUPPORT Human Clinical

"with hypoplastic optic nerves noticed in patients 4-12 and 4-14"

A structural, developmental optic nerve finding, consistent with the zebrafish evidence that ssbp1 is required for retinal development.

Reduced Visual Acuity HP:0007663

Show evidence (1 reference)

PMID:34548540 SUPPORT Human Clinical

"mean visual acuity in decimals: 0.54 ± 0.41"

The measured acuity in the isolated dominant-optic-atrophy group.

Rod-Cone Dystrophy OCCASIONAL HP:0000510

Course: PROGRESSIVE

Show evidence (2 references)

PMID:37259171 SUPPORT Human Clinical

"29.3% exhibited rod-cone degeneration on full-field ERG"

The frequency of the progressive retinal component across 60 published patients, which is considerably higher than the 2-of-27 figure from the single-variant series and is the better number to carry.

PMID:34548540 SUPPORT Human Clinical

"Two patients over 50 years of age developed a progressive rod-cone dystrophy, leading to severe visual impairment."

The progressive retinal component, with its age threshold.

Color Vision Defect HP:0000551

Show evidence (1 reference)

PMID:31298765 SUPPORT Human Clinical

"Autosomal dominant optic atrophy (ADOA) starts in early childhood with loss of visual acuity and color vision deficits."

Describes the ADOA syndrome the SSBP1 patients were ascertained as having. Cited for the clinical context rather than as an SSBP1-specific measurement.

Genitourinary 1

Renal Insufficiency HP:0000083

Show evidence (1 reference)

PMID:31298765 SUPPORT Human Clinical

"a renal disorder, including renal failure (subjects 3-8, 4-3 and 4-18), were documented"

Documents the renal component of the extra-ocular phenotype.

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Genetic Associations

1

SSBP1

Gene: SSBP1 hgnc:11317 relationship_type: CAUSATIVE

Show evidence (2 references)

PMID:31550237 SUPPORT Human Clinical

"Our study showing that mutations in SSBP1 cause a form of dominant optic atrophy frequently accompanied with foveopathy brings insights into mtDNA maintenance disorders."

The authors' causal conclusion for the gene-disease relationship.

PMID:37259171 SUPPORT Human Clinical

"OPA13 is caused by heterozygous mutation in the SSBP1 gene, associated with variable mitochondrial dysfunctions."

Independent confirmation of the gene-disease relationship and its dominant mode.

Variants (2)

p.Arg38Gln Pathogenic

NM_001256510.1:c.113G>A. The recurrent allele, and the one for which functional and phenotypic data are most complete: it destabilises dimer and tetramer formation in patient fibroblasts, and it is the variant carried by the 27-patient series that defined the foveopathy.

Functional effects

dominant-negative

Replaces an arginine in the basic patch required for single-stranded DNA binding and destabilises the multimer. Dominance is attributed to interference with the shared complex rather than to loss of one allele's product.

Show evidence (1 reference)

PMID:31550237 SUPPORT In Vitro

"Using patient fibroblasts, we validated that the R38Q variant destabilizes SSBP1 dimer/tetramer formation"

The functional characterisation of this specific allele in patient-derived cells.

p.Arg107Gln Pathogenic

NM_001256510.1:c.320G>A. The second recurrent allele, affecting the other arginine of the same basic patch, and shown by structural analysis to affect dimer interactions in the same way.

Functional effects

dominant-negative

Loss of a positive charge in the single-stranded DNA-binding patch, with the same predicted effect on dimer interactions as p.Arg38Gln.

Show evidence (1 reference)

PMID:31298765 SUPPORT Computational

"The loss of a positive charge at these positions is very likely to lower the affinity of SSBP1 for single-strand DNA."

The predicted biophysical consequence shared by both recurrent arginine substitutions, quoted with its hedge intact.

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External Assertions

1

OMIM optic atrophy 13 with retinal and foveal abnormalities record

OMIM disease record OMIM:165510

OMIM entry for OPA13, cited in the literature as MIM #165510. Recorded here as an external assertion because DiseaseMappings has slots for ICD10CM, ICD11F, MONDO and NCIT only, with no omim_mappings slot.

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Diagnosis

2

High-resolution foveal OCT and wide-field autofluorescence at first visit

The recommendation that follows from the foveopathy being both discriminating and asymptomatic: it will not appear in the history, so it has to be looked for. The source recommends high-resolution OCT centred on the fovea plus large-field autofluorescence in every patient with dominant optic atrophy, at first visit and during follow-up, to separate SSBP1 from OPA1 cases.

Show evidence (1 reference)

PMID:34548540 SUPPORT Human Clinical

"it should then be recommended to systematically perform high resolution OCT scans centered on the fovea and large-field autofluorescence frames in all patients with DOA at their first visit and during follow-up to separate SSBP1 cases from OPA1 cases"

The imaging protocol recommended for distinguishing the two diseases.

Genetic analysis in family surveys

With 11% of carriers asymptomatic, clinical examination alone will misclassify family members. Genetic analysis is required for correct counselling rather than being an optional confirmation.

Show evidence (1 reference)

PMID:34548540 SUPPORT Human Clinical

"with 11% of asymptomatic carriers, genetic analysis is required in SSBP1 family surveys to guarantee correct genetic counselling"

States the penetrance figure and the counselling consequence that follows from it.

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Prevalence

1

Worldwide

Cases In Literature Ultra Rare

Sixty patients with missense SSBP1 variants had been published as of 2023. A count of reported cases, not a population rate; no epidemiological estimate exists. The count is also an underestimate of carriers, since 11% of carriers are asymptomatic and would not come to ascertainment.

Show evidence (1 reference)

PMID:37259171 SUPPORT Human Clinical

"Among 60 published patients with missense SSBP1 variants, optic atrophy was present in 95% of them"

The published patient count that this record reports.

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Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Optic Atrophy 13 With Retinal and Foveal Abnormalities:

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Animal Models

1

ssbp1 knockdown and mutant-mRNA zebrafish

A two-armed zebrafish experiment rather than a single model. One arm knocks the endogenous transcript down; the other administers mutant mRNA. Comparing them is what makes the design informative about mechanism rather than only about requirement.

Species

Zebrafish

Genotype

antisense knockdown of endogenous ssbp1; mutant human SSBP1 mRNA administration

Publication

PMID:31298765

Pathograph links

Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration partially recapitulates fidelity: moderate Knockdown compromises retinal ganglion cell differentiation, identifying the cell type affected in patients as one whose development requires ssbp1. Graded PARTIALLY_RECAPITULATES rather than RECAPITULATES because the model reproduces involvement of the right cell type by the wrong route: failure of differentiation in an embryo, against degeneration of a differentiated cell in a child.

Limitations: A developmental readout in a fish embryo against a childhood-onset human optic neuropathy; the assay reports failure of differentiation rather than degeneration of a differentiated cell, and the two need not share a mechanism. Antisense knockdown in zebrafish also carries well-known off-target liabilities that the report does not address with a genetic mutant.

Readouts

Retinal ganglion cell differentiation

Direction: DECREASED

Interpretation: Establishes an in vivo requirement for ssbp1 in the affected cell type.

Show evidence (1 reference)

PMID:31298765 SUPPORT Model Organism

"Antisense-mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells."

The knockdown readout.

Show evidence (1 reference)

PMID:31298765 SUPPORT Model Organism

"These findings point toward an essential role of ssbp1 in retinal development"

The authors' conclusion for the model's relevance to the affected tissue.

Destabilised SSBP1 Dimer and Tetramer Assembly recapitulates fidelity: moderate Administering mutant mRNA reproduces the effect of removing the endogenous transcript. That equivalence is the model's real contribution: it is what distinguishes a dominant-negative allele from a null, and it is difficult to establish in patient cells that carry only one copy of the variant.

Limitations: Overexpression of injected mRNA is not a physiological dose, so the experiment shows that the mutant protein CAN interfere rather than that interference is what happens at endogenous levels in humans.

Readouts

Retinal ganglion cell differentiation after mutant mRNA administration

Direction: DECREASED

Interpretation: Mutant mRNA phenocopies knockdown, the functional signature of dominant-negative interference.

Show evidence (1 reference)

PMID:31298765 SUPPORT Model Organism

"A similar effect was achieved when mutated mRNAs were administered."

The mutant-mRNA arm and its equivalence to knockdown.

Show evidence (1 reference)

PMID:31298765 SUPPORT Model Organism

"the dominant-negative nature of the identified human variants"

The mechanistic conclusion this arm of the experiment supports.

{ }

Source YAML

click to show

name: Optic Atrophy 13 With Retinal and Foveal Abnormalities
creation_date: "2026-08-30T03:20:00Z"
category: Mendelian
disease_term:
  preferred_term: optic atrophy 13 with retinal and foveal abnormalities
  term:
    id: MONDO:0008135
    label: optic atrophy 13 with retinal and foveal abnormalities
description: >-
  An autosomal dominant optic neuropathy caused by heterozygous missense
  variants in SSBP1, which encodes the mitochondrial single-stranded
  DNA-binding protein. Optic atrophy is the constant feature; about half of
  affected individuals additionally have a congenital foveopathy, and a minority
  develop progressive rod-cone dystrophy in later life.

  The clinical picture overlaps closely with the far commoner OPA1-related
  dominant optic atrophy, and the practical value of recognising it is that the
  foveopathy distinguishes the two. That foveopathy is a diagnostic sign rather
  than a functional burden: visual acuity does not differ measurably between
  patients who have it and patients who do not.

  The disease is not uniformly confined to the eye, and it is important not to
  read it that way. Purely ocular disease is a property of the recurrent
  p.Arg38Gln allele rather than of SSBP1 disease in general: other missense
  variants produce syndromic presentations, and hypothyroidism and renal
  disorder including renal failure are documented in patients carrying other
  alleles. This entry curates both p.Arg38Gln and p.Arg107Gln, so the
  extra-ocular features belong here.
synonyms:
- OPA13
- SSBP1-related dominant optic atrophy
- optic atrophy with retinal and foveal abnormalities
- SSBP1-related optic atrophy and foveopathy
parents:
- optic atrophy
- mitochondrial DNA maintenance disorder
notes: >-
  On the mechanism question. It is tempting to ask whether this disease is
  caused by mtDNA depletion, by a dominant-negative effect on the SSBP1
  tetramer, or by impaired single-stranded DNA binding, and to treat those as
  competing hypotheses. On the published evidence they are not alternatives but
  successive steps in one chain: the missense changes destabilise dimer and
  tetramer formation and distort the DNA-binding region, that impairs mtDNA
  replication, and depletion follows. The pathophysiology below is written as
  that chain, with the dominant-negative claim attached to the step it actually
  describes rather than to the disease as a whole.

  On phenotype scope. An earlier version of this entry described the disease as
  "largely eye-restricted" and built its tissue-selectivity discussion on that
  premise. That was too strong. The ocular restriction is specific to
  p.Arg38Gln; other SSBP1 missense variants give syndromic presentations, and
  extra-ocular features (hypothyroidism, renal failure) are documented. The
  entry now curates those features and states the allele dependence.

  Two further features should not be smoothed over when extending this entry.
  Penetrance is incomplete - three of 27 variant carriers in the largest
  single-variant series were asymptomatic - so an unaffected carrier is an
  expected observation and not evidence against pathogenicity. And germline
  mosaicism has been reported in an unaffected transmitting mother, which
  changes recurrence-risk counselling and is the reason a "de novo" variant in
  this gene should not be counselled as carrying negligible recurrence risk.
external_assertions:
- name: OMIM optic atrophy 13 with retinal and foveal abnormalities record
  source: OMIM
  assertion_type: disease_record
  external_id: OMIM:165510
  description: >-
    OMIM entry for OPA13, cited in the literature as MIM #165510. Recorded here
    as an external assertion because DiseaseMappings has slots for ICD10CM,
    ICD11F, MONDO and NCIT only, with no omim_mappings slot.

prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Sixty patients with missense SSBP1 variants had been published as of 2023.
    A count of reported cases, not a population rate; no epidemiological
    estimate exists. The count is also an underestimate of carriers, since 11%
    of carriers are asymptomatic and would not come to ascertainment.
  evidence:
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Among 60 published patients with missense SSBP1 variants, optic atrophy was present in 95% of them"
    explanation: The published patient count that this record reports.

inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    Heterozygous missense variants segregating in multigeneration families, with
    incomplete penetrance. The dominant mechanism is interference with a
    multimeric complex rather than haploinsufficiency, which is what the
    functional work argues.
  evidence:
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified heterozygous missense mutations in single-strand binding protein 1 (SSBP1) in 5 unrelated families"
    explanation: Establishes the heterozygous missense architecture across five independent families.
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "which is consistent with the segregation pattern observed in 2 multigeneration families studied"
    explanation: Independent segregation evidence for dominant transmission.
- name: Incomplete penetrance
  inheritance_term:
    preferred_term: Typified by incomplete penetrance
    term:
      id: HP:0003829
      label: Typified by incomplete penetrance
  description: >-
    Three of 27 carriers of the recurrent p.Arg38Gln variant were asymptomatic.
    Incomplete penetrance is shared with OPA1-related dominant optic atrophy.
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three SSBP1-patients were asymptomatic, 10 had isolated DOA, and 12 had a combined DOA plus foveopathy."
    explanation: >-
      The penetrance denominator, and the breakdown of the phenotype within a
      single-variant cohort.
pathophysiology:
- name: Heterozygous SSBP1 Missense Variant
  biological_scale: MOLECULAR
  description: >-
    The primary lesion is a heterozygous missense change in SSBP1. The reported
    variants cluster functionally rather than randomly: p.Arg38Gln and
    p.Arg107Gln both replace arginines within a basic patch essential for
    single-stranded DNA binding, so the loss of positive charge is the shared
    consequence. A third variant, p.Ser141Asn, has also been reported.
  genes:
  - preferred_term: SSBP1
    term:
      id: hgnc:11317
      label: SSBP1
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We defined a new ADOA locus on 7q33-q35 and identified 3 different missense variants in SSBP1"
    explanation: >-
      The allelic series. The quote stops before the variant list because the
      reference validator strips bracketed spans such as [p.(Arg38Gln)] before
      matching, so a snippet spanning them cannot verify; the three protein
      changes are named in this node's description instead.
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "The mutated arginine residues are part of a basic patch that is essential for single-strand DNA binding."
    explanation: >-
      Locates the two recurrent variants in a functionally defined region, which
      is what makes the allelic clustering mechanistically interpretable.
  downstream:
  - target: Destabilised SSBP1 Dimer and Tetramer Assembly
    causal_link_type: DIRECT
    description: >-
      The substituted residues sit at the dimer interface and in the DNA-binding
      region, so the immediate consequence is on assembly of the functional
      multimer.
- name: Destabilised SSBP1 Dimer and Tetramer Assembly
  biological_scale: MOLECULAR
  description: >-
    SSBP1 works as a multimer. A revised crystal structure determined alongside
    the founding clinical report showed that both recurrent mutations affect
    dimer interactions and are predicted to distort the DNA-binding region, and
    patient fibroblasts confirmed that the R38Q variant destabilises dimer and
    tetramer formation.

    This node carries the dominant-negative claim, and it is the right place for
    it: a variant subunit that poisons a shared multimer explains dominance,
    whereas simple loss of one allele's product would not. The zebrafish work
    supports the same conclusion from the opposite direction - injecting mutant
    mRNA reproduced the effect of knocking the endogenous transcript down, which
    is the signature of interference rather than of insufficiency.
  molecular_functions:
  - preferred_term: single-stranded DNA binding
    modifier: DECREASED
    term:
      id: GO:0003697
      label: single-stranded DNA binding
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Structural analysis suggested that both mutations affect dimer interactions and presumably distort the DNA-binding region."
    explanation: >-
      The structural basis, quoted with the authors' own hedging ("suggested",
      "presumably") intact.
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we validated that the R38Q variant destabilizes SSBP1 dimer/tetramer formation, affects mtDNA replication, and induces mtDNA depletion"
    explanation: >-
      Experimental confirmation in patient fibroblasts, and the observation that
      chains this node to the two below it.
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A similar effect was achieved when mutated mRNAs were administered."
    explanation: >-
      Mutant mRNA phenocopies knockdown in zebrafish, which is the functional
      signature of a dominant-negative rather than a haploinsufficient allele.
  downstream:
  - target: Impaired Mitochondrial DNA Replication
    causal_link_type: DIRECT
    description: >-
      A destabilised multimer with reduced affinity for single-stranded DNA
      cannot support replication normally.
- name: Impaired Mitochondrial DNA Replication
  biological_scale: MOLECULAR
  description: >-
    SSBP1 coats and protects the single-stranded template at the mitochondrial
    replication fork, and is described as a crucial component of the mtDNA
    replication machinery. Patient fibroblasts carrying R38Q show impaired
    replication directly.
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    modifier: DECREASED
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "affects mtDNA replication, and induces mtDNA depletion"
    explanation: The replication defect measured in patient cells, and its immediate consequence.
  downstream:
  - target: Mitochondrial DNA Depletion
    causal_link_type: DIRECT
    description: Failure to replicate the mitochondrial genome reduces its copy number.
- name: Mitochondrial DNA Depletion
  biological_scale: CELLULAR
  description: >-
    Reduced mtDNA copy number in patient cells. This places SSBP1 disease within
    the mtDNA maintenance disorders, whose other members generally cause severe
    multisystem neurological disease.

    A proposed reconciliation exists and is worth stating rather than treating
    the tissue distribution as simply unexplained: severity may be proportional
    to residual mtDNA levels, with different variants having different effects
    on tetramer abundance and activity and so producing a gradient of depletion,
    against a threshold that must be exceeded in an organ for it to be affected.
    On that account the predominantly ocular presentation of p.Arg38Gln is the
    mild end of one continuum rather than a categorically different disease from
    the syndromic alleles. The proposal has not been tested by measuring
    depletion across tissues.
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the R38Q variant destabilizes SSBP1 dimer/tetramer formation, affects mtDNA replication, and induces mtDNA depletion"
    explanation: >-
      Demonstrates depletion as the cellular endpoint, and shows the whole chain
      from multimer destabilisation through replication failure to depletion
      measured in one experiment on patient-derived cells.
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in genes encoding components of the mitochondrial DNA (mtDNA) replication machinery cause mtDNA depletion syndromes (MDSs), which associate ocular features with severe neurological syndromes."
    explanation: Places the disease in its mechanistic class.
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the severity of the SSBP1-related diseases might be proportional to the residual mtDNA levels in patients"
    explanation: >-
      The proposed dose-response account of why alleles differ in how much of
      the body they affect, quoted with the authors' "might" intact.
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SSBP1 variants could have different consequences on tetramer abundance and activity, which would lead to a gradient of mtDNA depletion"
    explanation: >-
      Ties the proposed severity gradient back to the tetramer-assembly node,
      making it a testable extension of this entry's mechanism rather than a
      separate hypothesis.
  downstream:
  - target: Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Why depletion should injure retinal ganglion cells preferentially, rather
      than tissues with comparable or higher energy demand, is not established.
  - target: Foveal Photoreceptor Maldevelopment
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The foveal defect appears congenital and does not progress, so it is
      curated as a developmental rather than a degenerative consequence.
- name: Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration
  biological_scale: TISSUE
  description: >-
    Loss of retinal ganglion cells and their axons is the constant feature of
    the disease. Zebrafish knockdown of endogenous ssbp1 compromises
    differentiation of retinal ganglion cells, which argues that the cell type's
    vulnerability is developmental as well as degenerative, and identifies the
    ganglion cell as the directly affected cell rather than an innocent
    downstream casualty of optic nerve disease.
  cell_types:
  - preferred_term: retinal ganglion cell
    term:
      id: CL:0000740
      label: retinal ganglion cell
  mechanism_confidence: ESTABLISHED
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Antisense-mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells."
    explanation: >-
      Identifies the retinal ganglion cell as the cell type whose development
      depends on ssbp1, in vivo.
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All affected individuals presented optic atrophy, associated with foveopathy in half of the cases."
    explanation: Optic atrophy as the invariant clinical feature across the founding families.
- name: Foveal Photoreceptor Maldevelopment
  biological_scale: TISSUE
  description: >-
    A congenital foveopathy, seen as a small defect of the ellipsoid and
    interdigitation lines on macular OCT. Two observations make this
    developmental rather than degenerative, and both are load-bearing: the
    appearance is the same in all patients regardless of age, and it has no
    measurable effect on visual acuity. A progressive photoreceptor process
    would be expected to worsen with age and to cost acuity; this does neither.
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The foveopathy, with a tiny defect of the ellipsoid and interdigitation lines, was similar in all patients, independent of age."
    explanation: >-
      Age-independence is the observation that argues for a developmental rather
      than a degenerative lesion.
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There were no significant statistical differences in terms of visual acuity and SD-OCT measurements between patients with isolated DOA"
    explanation: >-
      The foveopathy does not cost acuity, which is why it is curated as a
      diagnostic marker rather than as a component of the visual disability.
  downstream:
  - target: Late-Onset Rod-Cone Dystrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Whether the congenital foveal lesion and the late photoreceptor
      degeneration are the same process on different timescales, or two
      independent consequences, is not established.
- name: Late-Onset Rod-Cone Dystrophy
  biological_scale: TISSUE
  description: >-
    A minority of patients develop progressive rod-cone dystrophy in later life,
    with severe visual impairment. In the largest single-variant series this was
    two patients, both over 50. It is the one part of the ocular phenotype that
    is unambiguously progressive, and it is the reason the disease name carries
    "retinal abnormalities" as well as optic atrophy.
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two patients over 50 years of age developed a progressive rod-cone dystrophy, leading to severe visual impairment."
    explanation: >-
      The frequency and age dependence of the progressive retinal component,
      with its denominator of 27 given elsewhere in the same series.
genetic:
- name: SSBP1
  gene_term:
    preferred_term: SSBP1
    term:
      id: hgnc:11317
      label: SSBP1
  relationship_type: CAUSATIVE
  notes: >-
    SSBP1 encodes the mitochondrial single-stranded DNA-binding protein, a
    component of the mtDNA replication machinery that works as a multimer. The
    recurrent p.Arg38Gln allele accounts for most reported patients and was the
    variant characterised in the largest phenotype series.

    Germline mosaicism has been documented in an unaffected transmitting mother,
    which is directly relevant to counselling: an apparently de novo variant in
    this gene cannot be assumed to carry negligible recurrence risk for
    subsequent pregnancies.
  variants:
  - name: p.Arg38Gln
    description: >-
      NM_001256510.1:c.113G>A. The recurrent allele, and the one for which
      functional and phenotypic data are most complete: it destabilises dimer and
      tetramer formation in patient fibroblasts, and it is the variant carried by
      the 27-patient series that defined the foveopathy.
    clinical_significance: PATHOGENIC
    functional_effects:
    - type: dominant-negative
      description: >-
        Replaces an arginine in the basic patch required for single-stranded DNA
        binding and destabilises the multimer. Dominance is attributed to
        interference with the shared complex rather than to loss of one allele's
        product.
    evidence:
    - reference: PMID:31550237
      reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Using patient fibroblasts, we validated that the R38Q variant destabilizes SSBP1 dimer/tetramer formation"
      explanation: The functional characterisation of this specific allele in patient-derived cells.
  - name: p.Arg107Gln
    description: >-
      NM_001256510.1:c.320G>A. The second recurrent allele, affecting the other
      arginine of the same basic patch, and shown by structural analysis to
      affect dimer interactions in the same way.
    clinical_significance: PATHOGENIC
    functional_effects:
    - type: dominant-negative
      description: >-
        Loss of a positive charge in the single-stranded DNA-binding patch, with
        the same predicted effect on dimer interactions as p.Arg38Gln.
    evidence:
    - reference: PMID:31298765
      reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: "The loss of a positive charge at these positions is very likely to lower the affinity of SSBP1 for single-strand DNA."
      explanation: >-
        The predicted biophysical consequence shared by both recurrent arginine
        substitutions, quoted with its hedge intact.
  evidence:
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our study showing that mutations in SSBP1 cause a form of dominant optic atrophy frequently accompanied with foveopathy brings insights into mtDNA maintenance disorders."
    explanation: The authors' causal conclusion for the gene-disease relationship.
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "OPA13 is caused by heterozygous mutation in the SSBP1 gene, associated with variable mitochondrial dysfunctions."
    explanation: Independent confirmation of the gene-disease relationship and its dominant mode.
phenotypes:
- category: Ophthalmologic
  name: Optic Atrophy
  description: >-
    The constant feature, present in all affected individuals across the
    founding families. Onset is in early childhood, as in OPA1-related dominant
    optic atrophy.
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Among 60 published patients with missense SSBP1 variants, optic atrophy was present in 95% of them"
    explanation: The frequency of optic atrophy across the largest published denominator.
  - reference: PMID:31550237
    reference_title: Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All affected individuals presented optic atrophy, associated with foveopathy in half of the cases."
    explanation: Establishes optic atrophy as invariant and foveopathy as roughly half-penetrant among affected individuals.
- category: Ophthalmologic
  name: Foveopathy
  description: >-
    A congenital foveal defect visible on macular OCT as a small disruption of
    the ellipsoid and interdigitation lines. It is the feature that
    distinguishes SSBP1-related from OPA1-related dominant optic atrophy at the
    bedside, and it does not affect visual acuity.
  phenotype_term:
    preferred_term: Abnormal foveal morphology on macular OCT
    term:
      id: HP:0030613
      label: Abnormal foveal morphology on macular OCT
  frequency: FREQUENT
  evidence:
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "57.8% presented with foveopathy on macular OCT"
    explanation: >-
      A precise frequency across 60 published patients, replacing the "half of
      cases" estimate that a single family series supported.
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the presence of a congenital foveopathy with no impact on visual acuity is a major criterion to distinguish SSBP1 cases and orient the appropriate genetic analysis"
    explanation: >-
      States both the diagnostic value of the sign and, explicitly, that it does
      not affect acuity.
- category: Ophthalmologic
  name: Retinal Pigmentary Changes
  description: >-
    Pigmentary change on fundus photography, in the mid-periphery. Present in
    65% of 60 published patients, which makes it the second commonest feature
    after optic atrophy itself and is why the disease name foregrounds retinal
    abnormalities. Where the sequence was documented it developed after the
    optic atrophy rather than alongside it.
  phenotype_term:
    preferred_term: Abnormal fundus pigmentation
    term:
      id: HP:0031605
      label: Abnormal fundus pigmentation
  frequency: FREQUENT
  evidence:
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sixty-five percent of patients had pigmentary changes on fundus photography"
    explanation: The frequency across the largest published denominator.
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Optic atrophy was documented in isolation (subjects 3-16, 4-3, 4-7, 4-8 and 5-12), or before the development of retinal vessel attenuation, or retinal pigmentary changes"
    explanation: >-
      Establishes that where both occur, optic atrophy precedes the retinal
      changes, which constrains the causal ordering.
- category: Ophthalmologic
  name: Retinal Vessel Attenuation
  phenotype_term:
    preferred_term: Attenuation of retinal blood vessels
    term:
      id: HP:0007843
      label: Attenuation of retinal blood vessels
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Retinal examination revealed different degrees of vessel attenuation, abnormal appearance of the fovea, and retinal pigmentary changes in the mid-periphery"
    explanation: The retinal examination findings, including vessel attenuation.
- category: Ophthalmologic
  name: Optic Nerve Hypoplasia
  description: Hypoplastic optic nerves in two patients, alongside the reduction in peripapillary retinal nerve fibre layer seen in all.
  phenotype_term:
    preferred_term: Optic nerve hypoplasia
    term:
      id: HP:0000609
      label: Optic nerve hypoplasia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "with hypoplastic optic nerves noticed in patients 4-12 and 4-14"
    explanation: >-
      A structural, developmental optic nerve finding, consistent with the
      zebrafish evidence that ssbp1 is required for retinal development.
- category: Endocrine
  name: Hypothyroidism
  description: >-
    Documented in four patients in the Jurkute series. Part of the extra-ocular
    phenotype associated with SSBP1 alleles other than p.Arg38Gln, and the
    reason this entry does not describe the disease as eye-restricted.
  phenotype_term:
    preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In addition, hypothyroidism (subjects 4-12, 4-14, 4-16 and 4-18) and a renal disorder, including renal failure (subjects 3-8, 4-3 and 4-18), were documented."
    explanation: >-
      Documents extra-ocular involvement, naming the affected subjects. This is
      the observation that corrected this entry's earlier eye-restricted framing.
- category: Renal
  name: Renal Insufficiency
  description: >-
    Renal disorder including renal failure in three patients. Like the
    hypothyroidism, an extra-ocular feature that must not be dropped when
    summarising the disease as an optic neuropathy.
  phenotype_term:
    preferred_term: Renal insufficiency
    term:
      id: HP:0000083
      label: Renal insufficiency
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a renal disorder, including renal failure (subjects 3-8, 4-3 and 4-18), were documented"
    explanation: Documents the renal component of the extra-ocular phenotype.
- category: Ophthalmologic
  name: Reduced Visual Acuity
  description: >-
    Mean visual acuity was 0.54 in decimals in patients with isolated dominant
    optic atrophy and 0.50 in those with additional foveopathy, a difference
    that was not statistically significant.
  phenotype_term:
    preferred_term: Reduced visual acuity
    term:
      id: HP:0007663
      label: Reduced visual acuity
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "mean visual acuity in decimals: 0.54 ± 0.41"
    explanation: The measured acuity in the isolated dominant-optic-atrophy group.
- category: Ophthalmologic
  name: Rod-Cone Dystrophy
  description: >-
    Progressive rod-cone dystrophy developing after age 50 in a minority,
    leading to severe visual impairment. Distinct in course from the
    non-progressive congenital foveopathy.
  phenotype_term:
    preferred_term: Rod-cone dystrophy
    term:
      id: HP:0000510
      label: Rod-cone dystrophy
    clinical_course: PROGRESSIVE
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:37259171
    reference_title: "Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "29.3% exhibited rod-cone degeneration on full-field ERG"
    explanation: >-
      The frequency of the progressive retinal component across 60 published
      patients, which is considerably higher than the 2-of-27 figure from the
      single-variant series and is the better number to carry.
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two patients over 50 years of age developed a progressive rod-cone dystrophy, leading to severe visual impairment."
    explanation: The progressive retinal component, with its age threshold.
- category: Ophthalmologic
  name: Color Vision Defect
  description: >-
    Colour vision deficit accompanies the loss of visual acuity in dominant
    optic atrophy generally; this entry records it as a feature of the
    presenting syndrome rather than as an SSBP1-specific finding.
  phenotype_term:
    preferred_term: Color vision defect
    term:
      id: HP:0000551
      label: Color vision defect
  evidence:
  - reference: PMID:31298765
    reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal dominant optic atrophy (ADOA) starts in early childhood with loss of visual acuity and color vision deficits."
    explanation: >-
      Describes the ADOA syndrome the SSBP1 patients were ascertained as having.
      Cited for the clinical context rather than as an SSBP1-specific measurement.
diagnosis:
- name: High-resolution foveal OCT and wide-field autofluorescence at first visit
  description: >-
    The recommendation that follows from the foveopathy being both
    discriminating and asymptomatic: it will not appear in the history, so it
    has to be looked for. The source recommends high-resolution OCT centred on
    the fovea plus large-field autofluorescence in every patient with dominant
    optic atrophy, at first visit and during follow-up, to separate SSBP1 from
    OPA1 cases.
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it should then be recommended to systematically perform high resolution OCT scans centered on the fovea and large-field autofluorescence frames in all patients with DOA at their first visit and during follow-up to separate SSBP1 cases from OPA1 cases"
    explanation: The imaging protocol recommended for distinguishing the two diseases.
- name: Genetic analysis in family surveys
  description: >-
    With 11% of carriers asymptomatic, clinical examination alone will
    misclassify family members. Genetic analysis is required for correct
    counselling rather than being an optional confirmation.
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "with 11% of asymptomatic carriers, genetic analysis is required in SSBP1 family surveys to guarantee correct genetic counselling"
    explanation: >-
      States the penetrance figure and the counselling consequence that follows
      from it.

differential_diagnoses:
- name: OPA1-related dominant optic atrophy
  description: >-
    The main differential and the far commoner disease. The two share early
    childhood onset, incomplete penetrance, and the optic atrophy itself. The
    discriminator is the congenital foveopathy on macular OCT, which is present
    in about half of SSBP1 patients and absent in OPA1 disease - and which is
    worth looking for precisely because it costs nothing in acuity and so will
    not announce itself in the visual history.
  evidence:
  - reference: PMID:34548540
    reference_title: Characterization of SSBP1-related optic atrophy and foveopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SSBP1-related DOA shares similarities with OPA1-related DOA with an incomplete penetrance and an early childhood visual impairment."
    explanation: States the overlap that makes the two hard to separate clinically.
animal_models:
- name: ssbp1 knockdown and mutant-mRNA zebrafish
  species: Zebrafish
  genotype: antisense knockdown of endogenous ssbp1; mutant human SSBP1 mRNA administration
  publication: PMID:31298765
  description: >-
    A two-armed zebrafish experiment rather than a single model. One arm knocks
    the endogenous transcript down; the other administers mutant mRNA. Comparing
    them is what makes the design informative about mechanism rather than only
    about requirement.
  modeled_mechanisms:
  - target: Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Knockdown compromises retinal ganglion cell differentiation, identifying
      the cell type affected in patients as one whose development requires
      ssbp1. Graded PARTIALLY_RECAPITULATES rather than RECAPITULATES because
      the model reproduces involvement of the right cell type by the wrong
      route: failure of differentiation in an embryo, against degeneration of a
      differentiated cell in a child.
    limitations: >-
      A developmental readout in a fish embryo against a childhood-onset human
      optic neuropathy; the assay reports failure of differentiation rather than
      degeneration of a differentiated cell, and the two need not share a
      mechanism. Antisense knockdown in zebrafish also carries well-known
      off-target liabilities that the report does not address with a genetic
      mutant.
    readouts:
    - name: Retinal ganglion cell differentiation
      target: Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration
      direction: DECREASED
      interpretation: Establishes an in vivo requirement for ssbp1 in the affected cell type.
      evidence:
      - reference: PMID:31298765
        reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Antisense-mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells."
        explanation: The knockdown readout.
    evidence:
    - reference: PMID:31298765
      reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings point toward an essential role of ssbp1 in retinal development"
      explanation: The authors' conclusion for the model's relevance to the affected tissue.
  - target: Destabilised SSBP1 Dimer and Tetramer Assembly
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Administering mutant mRNA reproduces the effect of removing the endogenous
      transcript. That equivalence is the model's real contribution: it is what
      distinguishes a dominant-negative allele from a null, and it is difficult
      to establish in patient cells that carry only one copy of the variant.
    limitations: >-
      Overexpression of injected mRNA is not a physiological dose, so the
      experiment shows that the mutant protein CAN interfere rather than that
      interference is what happens at endogenous levels in humans.
    readouts:
    - name: Retinal ganglion cell differentiation after mutant mRNA administration
      target: Destabilised SSBP1 Dimer and Tetramer Assembly
      direction: DECREASED
      interpretation: >-
        Mutant mRNA phenocopies knockdown, the functional signature of
        dominant-negative interference.
      evidence:
      - reference: PMID:31298765
        reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "A similar effect was achieved when mutated mRNAs were administered."
        explanation: The mutant-mRNA arm and its equivalence to knockdown.
    evidence:
    - reference: PMID:31298765
      reference_title: SSBP1 mutations in dominant optic atrophy with variable retinal degeneration.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "the dominant-negative nature of the identified human variants"
      explanation: The mechanistic conclusion this arm of the experiment supports.
discussions:
- discussion_id: ssbp1_tissue_selectivity
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is the tissue distribution of SSBP1 disease set by how much mtDNA depletion
    each allele causes, as has been proposed, or by something specific to
    retinal ganglion cells?
  attaches_to:
  - pathophysiology#Mitochondrial DNA Depletion
  - pathophysiology#Retinal Ganglion Cell Vulnerability and Optic Nerve Degeneration
  rationale: >-
    An earlier version of this discussion asked why SSBP1 disease is
    eye-restricted when other mtDNA depletion syndromes are multisystem. That
    premise was too strong and has been corrected in the entry: the ocular
    restriction belongs to p.Arg38Gln, other missense alleles give syndromic
    presentations, and hypothyroidism and renal failure are documented.

    The sharper question is what sets the difference between alleles. A dosage
    account has been proposed - that severity tracks residual mtDNA levels, that
    variants differ in their effect on tetramer abundance and activity, and that
    an organ is affected only once depletion passes a threshold in it. That
    would make p.Arg38Gln the mild end of a continuum rather than a different
    kind of lesion, and it is attractive because it connects the clinical
    gradient directly to the tetramer-assembly node this entry already curates.

    It is untested. No study has measured mtDNA copy number across tissues of
    differing clinical involvement in the same carrier, or compared depletion
    between alleles. The competing account - that retinal ganglion cells are
    selectively vulnerable to mitochondrial dysfunction because of their
    unmyelinated prelaminar axon segment, as argued for OPA1 and Leber
    hereditary optic neuropathy - predicts that depletion is uniform and the
    tissue response is not. The two make different predictions and one
    measurement separates them.
  proposed_experiments:
  - experiment_id: exp_ssbp1_tissue_mtdna_copy_number
    name: Cross-tissue mtDNA copy number in SSBP1 variant carriers
    description: >-
      Compare mtDNA copy number and replication intermediates across tissues of
      differing clinical involvement in the same SSBP1 carriers - fibroblast,
      blood, muscle, and where available post-mortem retina and optic nerve -
      and compare p.Arg38Gln carriers against carriers of syndromic alleles, to
      test whether depletion is uniform with selective tissue tolerance or
      tracks allele severity as the dosage account predicts.
    would_support:
    - pathophysiology#Mitochondrial DNA Depletion
    supporting_outcome:
    - >-
      Depletion is comparable across tissues and differs between alleles in
      proportion to clinical severity, supporting the proposed dosage-threshold
      account and locating tissue selectivity in differential tolerance.
    refuting_outcome:
    - >-
      Depletion does not differ between alleles, or is confined to ocular
      tissue, refuting the dosage account and pointing instead to a
      tissue-specific requirement for SSBP1.
- discussion_id: ssbp1_foveopathy_versus_late_retinal_degeneration
  kind: OPEN_QUESTION
  prompt: >-
    Are the congenital foveopathy and the late-onset rod-cone dystrophy two
    expressions of one photoreceptor process, or independent consequences?
  attaches_to:
  - pathophysiology#Foveal Photoreceptor Maldevelopment
  - pathophysiology#Late-Onset Rod-Cone Dystrophy
  rationale: >-
    The two behave quite differently. The foveopathy is the same at every age
    and costs no acuity, which is the profile of a developmental lesion that
    then stays put. The rod-cone dystrophy appears after 50 and is severe, which
    is the profile of a degeneration. Whether the first predisposes to the
    second, or the two are separate readouts of the same underlying photoreceptor
    compromise on different timescales, is unresolved - and the cohorts are far
    too small and too cross-sectional to answer it.

    It matters for prognosis. If the foveopathy marks patients who will later
    degenerate, it becomes a prognostic sign as well as a diagnostic one; if not,
    it should continue to be reported to families as the reassuring finding it
    currently appears to be.
  proposed_experiments:
  - experiment_id: exp_ssbp1_longitudinal_retinal_imaging
    name: Longitudinal retinal imaging of SSBP1 carriers into later life
    description: >-
      Follow a cohort of SSBP1 variant carriers with serial macular OCT and
      autofluorescence into and beyond the sixth decade, and test whether the
      presence of congenital foveopathy predicts subsequent rod-cone dystrophy.
    would_support:
    - pathophysiology#Late-Onset Rod-Cone Dystrophy
    supporting_outcome:
    - >-
      Rod-cone dystrophy develops preferentially in carriers with congenital
      foveopathy, supporting one photoreceptor process with two temporal phases
      and making the foveopathy prognostic.
    refuting_outcome:
    - >-
      Late rod-cone dystrophy occurs independently of foveopathy status,
      supporting two separate consequences and preserving the foveopathy as a
      purely diagnostic sign.

Optic Atrophy 13 With Retinal and Foveal Abnormalities

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