Combined oxidative phosphorylation deficiency 48

Mendelian MONDO:0033566 Pathograph 27 Show in embeddings browser hereditary disease

Combined oxidative phosphorylation deficiency 48 (COXPD48) is an autosomal recessive mitochondrial disease caused by biallelic variants in NSUN3, the mitochondrial RNA cytosine-5 methyltransferase that writes m5C at the wobble base (C34) of mitochondrial tRNA-Met. The lesion sits one step further upstream than most of the combined-OXPHOS series. NSun3 is neither a respiratory-chain subunit nor an assembly factor: it installs a single tRNA modification. m5C34 is then oxidised by the alpha-ketoglutarate- and Fe(II)-dependent dioxygenase ALKBH1/ABH1 to 5-formylcytidine (f5C34), and mitochondria carry only one tRNA-Met, which serves both initiation and elongation and must read AUG together with the non-universal AUA (and, at initiation, AUU) codons. Because methionine codons occur throughout all thirteen mtDNA-encoded proteins, losing the modification degrades translation of every one of them, and the biochemical result is a combined rather than an isolated respiratory chain deficiency. Patient fibroblasts lose m5C34 and f5C34 together, consistent with a precursor-product relationship, while mt-tRNA steady-state levels, aminoacylation and mtDNA copy number are unaffected - the defect is one of decoding capacity, not of tRNA abundance or mitochondrial biogenesis. The clinical spectrum turned out to be much wider than the founding case suggested. That patient presented at three months with developmental delay, microcephaly, failure to thrive and raised lactate. The largest cohort to date - eight individuals from five families - spans neonatal-onset multisystem disease to isolated optic atrophy first noticed at 39 years, with bilateral optic nerve involvement in 7 of 8 and biallelic loss-of-function or SAM-domain missense genotypes clustering at the severe end. Hypertrophic cardiomyopathy has since been reported in one further individual.

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1
Inheritance
8
Pathophys.
15
Phenotypes
3
Gaps
27
Pathograph
2
Genes
4
Medical Actions
3
Differentials
2
Models
8
References
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Deep Research
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Inheritance

1
Autosomal recessive inheritance HP:0000007
Biallelic NSUN3 genotypes segregate recessively: the founding case was compound heterozygous for two predicted loss-of-function alleles, and the five-family cohort comprises homozygous and compound heterozygous genotypes with carrier parents. Two features of that cohort are worth recording. Expressivity is graded by genotype rather than uniform - biallelic loss-of-function alleles, or a homozygous missense allele hitting the S-adenosylmethionine binding site, presented in early infancy, while the milder isolated-optic-atrophy cases carried C-terminal variants predicted to escape nonsense-mediated decay. And one proband reached homozygosity not by descent but through paternal uniparental isodisomy of chromosome 3, in a non-consanguineous family; his homozygous father had moderate bilateral optic atrophy. Penetrance is left UNKNOWN: with roughly a dozen reported individuals and ascertainment through optic-neuropathy and mitochondrial-disease clinics, there is no unselected denominator from which to estimate it.
Autosomal recessive inheritance Penetrance: UNKNOWN Expressivity: VARIABLE
Show evidence (3 references)
PMID:27356879 SUPPORT Human Clinical
"we identified compound heterozygous predicted loss-of-function variants in NSUN3"
The founding genotype, identified under a recessive model.
PMID:40465263 SUPPORT Human Clinical
"Individuals carrying biallelic LOF variants or homozygous missense variant affecting SAM binding site presented with severe neurological phenotype in early infancy."
Supports graded expressivity tracking allele severity.
PMID:40465263 SUPPORT Human Clinical
"A paternal uniparental isodisomy of chromosome 3 was identified in III-2 from Family 3 leading to homoallelism for c.930_931delAT."
Records the uniparental-isodisomy route to homozygosity, which is why a biallelic NSUN3 result does not by itself imply parental consanguinity.
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Discussions and Knowledge Gaps

3
Why is mt-tRNA-Met C34 methylation abolished equally by mild C-terminal NSUN3 alleles and by severe loss-of-function alleles, when the two genotype classes produce phenotypes as far apart as isolated adult optic atrophy and neonatal multisystem disease?
KNOWLEDGE GAP nsun3_methylation_assay_does_not_grade_severity
Attached to
pathophysiology#Loss of m5C34 in Mitochondrial tRNA-Met genetic#NSUN3
The functional assay used to call NSUN3 variants pathogenic reads at background in every affected individual regardless of allele class, so it cannot be the quantity that sets severity. Either the assay saturates below the range that matters clinically, or some residual NSUN3 activity not captured by bulk bisulfite sequencing differs between genotypes, or - as the cohort authors suggest - NSUN3 has a second function that the C-terminal alleles spare and the null alleles do not. The three possibilities have different consequences for whether the assay can ever be prognostic, and nothing published distinguishes them.
Can the mouse tell us anything about postnatal NSUN3 disease, given that constitutive Nsun3 loss kills the embryo by E12.5 while humans with two predicted null alleles are liveborn and survive infancy?
HUMAN MODEL MISMATCH nsun3_mouse_null_lethality_vs_human_viability
Attached to
animal_models#Whole-body Nsun3 knockout mouse pathophysiology#Loss of Functional NSun3 Methyltransferase
This is a mismatch of the informative kind: evidence exists in the model and contradicts the human course rather than merely failing to reproduce it. The conditional cardiac knockout is the workaround, and it does reproduce the key biochemical signature - enzyme activities fall while the transcripts encoding those subunits do not - but it is confined to heart, which is the organ least involved in human NSUN3 disease, and it says nothing about the retinal ganglion cell, which is the organ most involved. There is no reported model of the optic neuropathy that defines the human disorder.
Why are complexes III and V spared in NSUN3 patient muscle when both contain mtDNA-encoded subunits that a general methionine-decoding defect should affect?
KNOWLEDGE GAP nsun3_complex_iii_and_v_sparing
Attached to
pathophysiology#Combined Respiratory Chain Enzyme Deficiency
A defect in decoding methionine codons should degrade synthesis of all thirteen mtDNA-encoded proteins, and RNAi work in cell lines found exactly that. Yet in patient muscle the measurable deficit falls on complexes IV and I, with complex III and V protein levels reported as unchanged in one family and complex II - the nuclear-encoded internal control - unaffected as expected. Whether this reflects differing subunit half-lives, differing assembly-intermediate stability, differing sensitivity of the assays, or a real difference in translational sensitivity between messages is untested, and it matters because complex IV is what the clinical biochemistry actually measures.
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Pathophysiology

8
Loss of Functional NSun3 Methyltransferase
Mechanism confidence: Established
Biallelic NSUN3 variants abolish the mitochondrial-matrix m5C methyltransferase. In the founding case, two predicted loss-of-function alleles left no detectable full-length NSun3 on western blot of patient fibroblasts, and lentiviral re-expression of NSUN3 complementary DNA restored the downstream phenotypes - the rescue arm is what makes this a causal node rather than a correlation.
mt-tRNA cytosine-5 methyltransferase activity GO:0016428 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves mt-tRNA cytosine-5 methyltransferase activity, annotated with tRNA (cytidine-N5)-methyltransferase activity (GO:0016428), qualified as loss of function. GO:0016428 is a molecular function from the Gene Ontology. ⇓ LOSS OF FUNCTION
Animal models
Whole-body Nsun3 knockout mouse Mouse fails to recapitulate Complete Nsun3 loss in the mouse produces mid-gestation lethality, whereas humans homozygous or compound heterozygous for predicted null alleles are liveborn and reach at least infancy.
Downstream
Loss of m5C34 in Mitochondrial tRNA-Met
Show evidence (2 references)
PMID:27356879 SUPPORT In Vitro
"Western blot analysis using primary human dermal fibroblasts (hDFs) confirmed the lack of the full-length NSun3 protein in patient-derived cells"
Demonstrates absence of the protein product in the patient's own cells.
PMID:27356879 SUPPORT In Vitro
"Both NSun3 and mtSSB1 were resistant to proteinase K treatment of the mitochondrial fraction"
Places the enzyme inside mitochondria, which is why its loss is a mitochondrial rather than a cytoplasmic translation lesion.
Loss of m5C34 in Mitochondrial tRNA-Met
Mechanism confidence: Established
NSun3's physiological substrate is a single position: cytosine 34, the wobble base of the anticodon loop of mt-tRNA-Met (mtDNA position 4432). Bisulfite RNA sequencing of patient fibroblasts showed methylation there falling to background, while m5C at the other mitochondrial tRNA positions the method detected - in mt-tRNA-Leu(UUR), mt-tRNA-Ser(AGY), mt-tRNA-His, mt-tRNA-Glu and mt-tRNA-Phe - was unchanged. The same assay run on blood or fibroblast RNA from a later five-family cohort gave the same result in every affected individual, against a control range of 26 to 33 percent, and is now the functional assay used to call an NSUN3 variant pathogenic.
tRNA wobble base cytosine methylation GO:0002127 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased tRNA wobble base cytosine methylation (GO:0002127). GO:0002127 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Absence of f5C34 at the Wobble Position
Show evidence (2 references)
PMID:27356879 SUPPORT In Vitro
"The bisulfite RNA sequencing (BS RNA-Seq) analysis revealed that methylation in mt-tRNAMet (MT-TM) was decreased to background levels in NSUN3 mutant cells (mut/mut)"
The primary measurement of the lost modification in patient cells.
PMID:40465263 SUPPORT Human Clinical
"All samples obtained from affected individuals demonstrated significantly reduced mitochondrial tRNAMet methylation levels that were reduced to background levels, although control samples showed 26% to 33% tRNAMet methylation"
Replicates the loss across an independent multi-family cohort and supplies the control reference range.
Absence of f5C34 at the Wobble Position
Mechanism confidence: Established
The mature modification at C34 is not m5C but 5-formylcytidine. m5C34 is the obligatory precursor: ALKBH1/ABH1, an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase resident in mitochondria, oxidises it to f5C34. Losing the first step therefore removes the second. In wild-type fibroblasts roughly 36 to 38 percent of mt-tRNA-Met reads carried f5C34 by two independent chemistries; in the NSun3-null patient neither method detected any. Note what this node does and does not claim. The founding report established the precursor-product relationship in patient cells but explicitly left the responsible dioxygenase unidentified; ALKBH1 was named in a separate study published the same year, on the strength of in vitro oxidation assays and RNAi depletion, and it is that work rather than the patient report that the ALKBH1 evidence here comes from.
5-formylcytidine formation at the mt-tRNA-Met wobble base GO:0002101 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased 5-formylcytidine formation at the mt-tRNA-Met wobble base, annotated with tRNA wobble cytosine modification (GO:0002101). GO:0002101 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Impaired Wobble Decoding by the Single Mitochondrial tRNA-Met
Show evidence (2 references)
PMID:27356879 SUPPORT In Vitro
"no f5C was detected in NSun3-null patient samples"
The measurement establishing that the second modification is lost with the first, in the patient's own cells.
PMID:27497299 SUPPORT In Vitro
"We further identify ALKBH1/ABH1 as the dioxygenase responsible for oxidising m5C34 of mt-tRNAMet to generate an f5C34 modification."
Names the enzyme performing the second step, from the study that identified it rather than from the patient report.
Impaired Wobble Decoding by the Single Mitochondrial tRNA-Met
Mechanism confidence: Provisional
This is the step that makes COXPD48 a different kind of lesion from the rest of the combined-OXPHOS series. Human mitochondria carry one tRNA-Met, used for both initiation and elongation, and it must read the universal AUG plus the non-universal AUA codon - and, at initiation, AUU. The wobble modification is what expands that decoding range, so a modification defect degrades the reading of every methionine codon in every mtDNA-encoded message rather than disabling one subunit or one assembly step. Confidence is PROVISIONAL rather than ESTABLISHED because the in vitro ribosome-binding data are more textured than the summary claim. One study concluded from anticodon-stem-loop work that f5C34 is required to decipher AUA; the other, testing modification states directly on ribosomes with mitochondrial initiation and elongation factors, found that AUA was read with similar efficiencies by all mt-tRNA-Met variants and that the differences it could resolve lay mostly in AUG recognition. Both agree the modification state changes codon recognition; they do not agree on the size or the codon specificity of the effect, and no reconstituted mitochondrial translation system exists to settle it.
Downstream
Deficient Mitochondrial Protein Synthesis
Show evidence (3 references)
PMID:27356879 SUPPORT Other
"There is only one mitochondrial tRNAMet that decodes all of these AUN codons"
States the single-tRNA constraint that makes one modification defect a global translation defect. Graded OTHER because the sentence is the authors' summary of prior structural work, not a result of this study.
PMID:27497299 SUPPORT In Vitro
"can expand the ability of the single tRNAMet to read the different codons encoding methionine in mitochondrial translation initiation and elongation."
The ribosome-binding study's own conclusion, stated at the level of expanded codon reading rather than a codon-specific requirement.
PMID:27214402 SUPPORT In Vitro
"f(5)C34 is required for deciphering the AUA codon during protein synthesis."
The stronger, codon-specific form of the claim, from the parallel study. Recorded alongside the weaker form above rather than in place of it.
Deficient Mitochondrial Protein Synthesis
Mechanism confidence: Established
Pulse labelling with radiolabelled methionine under cytoplasmic-translation blockade showed the mitochondrial de novo synthesis rate markedly reduced in the patient's fibroblasts and substantially restored by NSUN3 re-expression. Independent RNAi work found that synthesis of all mitochondrial proteins fell on NSUN3 depletion, which is the expected signature of a methionine-decoding defect rather than of a subunit-specific lesion. Two negative results matter as much as the positive one, because together they localise the defect. mt-tRNA steady-state levels and aminoacylation were unchanged, and mtDNA copy number was unchanged - so this is not a tRNA stability defect, not a charging defect, and not a biogenesis defect. Mitochondrial transcript levels actually rose, which the authors read as compensatory.
mitochondrial translation GO:0032543 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased mitochondrial translation (GO:0032543). GO:0032543 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Combined Respiratory Chain Enzyme Deficiency
Show evidence (4 references)
PMID:27356879 SUPPORT In Vitro
"the mitochondrial de novo protein synthesis rate was markedly decreased in the absence of NSun3"
Direct measurement of the translation defect in patient cells.
PMID:27356879 SUPPORT In Vitro
"High-resolution northern blot analysis did not reveal any appreciable differences in expression levels of mt-tRNAs in the absence of NSun3"
Excludes loss of tRNA abundance as the mechanism, localising the defect to decoding.
PMID:27356879 SUPPORT In Vitro
"The mtDNA copy number was unchanged between wt/wt and mut/mut hDFs"
Excludes a mitochondrial biogenesis or mtDNA maintenance defect.
+ 1 more reference
Combined Respiratory Chain Enzyme Deficiency
Mechanism confidence: Established
Complexes I, III, IV and V each contain mtDNA-encoded subunits and are exposed to the translation defect; complex II is entirely nuclear-encoded and is not. The measured pattern fits that prediction, but not uniformly. In the five-family cohort, spectrophotometric assay of one individual's muscle gave a complex I activity of 0.078 against a normal range of 0.118 to 0.332 and a complex IV activity of 0.003 against 0.013 to 0.039, both normalised to citrate synthase; western blotting of another family's muscle and fibroblasts showed reduced COX II and NDUFB8 but not complex II, III or V. Complex IV is the most consistently affected across reports, and complex III and V changes were not detected in this cohort even though both carry mtDNA-encoded subunits. Functional respirometry agrees: patient fibroblasts show reduced basal oxygen consumption and ATP production, remain below control even when uncoupled with FCCP, and grow poorly when forced onto galactose. The founding case's oxygen consumption defect was reversed by NSUN3 re-expression. Conformance note: this node declares conformance to the mitochondrial dysfunction module's central bioenergetic node, and only in part. The module's node pairs decreased oxidative phosphorylation with increased reactive oxygen species; the decreased-OXPHOS arm is directly evidenced in these patients, whereas no NSUN3 report measures reactive oxygen species, so that arm is not asserted here. The module's upstream node - age-related mitochondrial damage and mtDNA mutation - does not apply to a primary nuclear modification defect, and mtDNA copy number is explicitly normal, which is why conformance is declared at this node alone.
oxidative phosphorylation GO:0006119 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased oxidative phosphorylation (GO:0006119). GO:0006119 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Heart-specific Nsun3 knockout mouse (Nsun3HKO) Mouse partially recapitulates Cardiac Nsun3 loss reduces respiratory complex enzyme activities without reducing the corresponding mitochondrial transcripts, reproducing the translation-level character of the human biochemical defect in a single organ.
Downstream
Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration
Mitochondrial Myopathy of Skeletal Muscle
Increased circulating lactate concentration
Blue-dot cataract
Generalized axonal sensorimotor neuropathy
Global developmental delay
Generalized hypotonia
Microcephaly
Failure to thrive
Seizure
Sensorineural hearing impairment
Hypertrophic cardiomyopathy
Show evidence (4 references)
PMID:27356879 SUPPORT Human Clinical
"developed mitochondrial disease symptoms at the age of three months (combined developmental disability, microcephaly, failure to thrive, recurrent increased lactate levels in plasma, muscular weakness, proximal accentuated, external ophthalmoplegia and convergence nystagmus) and presented with..."
The founding case's combined deficiency, measured in skeletal muscle.
PMID:40465263 SUPPORT Human Clinical
"spectrophotometric respiratory chain enzyme analysis demonstrated a complex I deficiency of 0.078 (normal range 0.118–0.332) and complex IV deficiency of 0.003 (normal range 0.013–0.039)"
The measured enzyme activities with their reference ranges, rather than a summary description of them.
PMID:40465263 SUPPORT Human Clinical
"confirmed decreased amounts of COX II (OXPHOS complexes IV) and NDUFB8 (complex I), but not of complex II, III and V"
Independent protein-level confirmation, and the source of the observation that complexes III and V were spared despite carrying mtDNA-encoded subunits.
+ 1 more reference
Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration
Mechanism confidence: Provisional
Optic nerve involvement is the one feature that runs through nearly the whole reported spectrum - 7 of 8 individuals in the largest cohort - and it is the reason NSUN3 was rediscovered through inherited-optic-neuropathy clinics rather than through metabolic ones. Optical coherence tomography shows peripapillary retinal nerve fibre layer and ganglion cell layer thinning, and pattern electroretinography shows a reduced N95:P50 ratio consistent with retinal ganglion cell dysfunction. The proposed reason for the tissue selectivity is generic to mitochondrial optic neuropathies rather than specific to NSUN3: retinal ganglion cells have long axons with unmyelinated prelaminar segments and a correspondingly high dependence on mitochondrial ATP. It is recorded here as the field's explanation, offered in the cited review's own hedged terms, and it has not been tested in NSUN3 tissue - which is why this node is PROVISIONAL while the upstream biochemistry is ESTABLISHED.
retinal ganglion cell CL:0000740 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves retinal ganglion cell (CL:0000740). CL:0000740 is a cell type from the Cell Ontology.
Downstream
Optic atrophy
Reduced visual acuity
Nystagmus
Show evidence (2 references)
PMID:40465263 SUPPORT Human Clinical
"Optic nerve involvement was a unifying feature in most cases (7/8, 87.5%)."
Quantifies how consistently the optic nerve is involved across the cohort.
PMID:40465263 SUPPORT Other
"Particularly RGCs with their long axons and unmyelinated prelaminar segments are highly dependent on the mitochondrial-derived energy, which makes them particularly vulnerable to mitochondrial dysfunction"
The proposed basis for retinal ganglion cell vulnerability. Graded OTHER because the sentence states the field's general explanation for mitochondrial optic neuropathies, not a measurement made in NSUN3 tissue.
Mitochondrial Myopathy of Skeletal Muscle
Mechanism confidence: Established
Skeletal muscle carries the biochemical lesion but shows a strikingly variable histological one, and the variability is itself worth recording: in the five-family cohort one individual's biopsy had numerous COX-negative fibres while another's had none, and no affected individual had ragged red fibres. Subsarcolemmal mitochondrial aggregates were present in three. Ultrastructurally, mitochondria were swollen and elongated, spanning a sarcomere rather than sitting in the I-band, some with reduced and linearised cristae. Read the COX-negative-fibre claim carefully. The relevant figure legend in that paper states that affected individuals from Families 1, 2 and 3 displayed COX-negative fibres, but the results and discussion sections both say that the Family 1 proband did not, and that the Family 2 biopsy showed none either. The per-individual text is the narrower and better-supported claim and is what is cited here.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology.
Downstream
Proximal muscle weakness
External ophthalmoplegia
Show evidence (3 references)
PMID:40465263 SUPPORT Human Clinical
"COX enzyme histochemistry demonstrated numerous COX-negative fibers"
The positive histochemical finding, in the Family 3 proband's biopsy.
PMID:40465263 SUPPORT Human Clinical
"No COX-negative fibers and no ragged red fibers were observed; however, a proportion of the type I fibers showed prominent subsarcolemmal mitochondrial aggregates"
The contrasting biopsy in the same cohort, which is why a normal COX stain does not exclude the diagnosis.
PMID:40465263 SUPPORT Human Clinical
"many mitochondria were swollen and elongated"
The ultrastructural correlate on electron microscopy.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Combined oxidative phosphorylation deficiency 48 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

15
Cardiovascular 1
Hypertrophic cardiomyopathy HP:0001639 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypertrophic cardiomyopathy (HP:0001639). HP:0001639 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:41725275 SUPPORT Human Clinical
"An 18-year-old girl was referred for visual loss, progressive fatigue, muscle weakness and HCM."
The single reported individual with hypertrophic cardiomyopathy.
PMID:41725275 SUPPORT Human Clinical
"To our knowledge, this is the first reported case of NSUN3-related mitochondrial disease with HCM."
The authors' own statement that this is a single first report, which is why the association is recorded as provisional.
Ear 1
Sensorineural hearing impairment HP:0000407 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Mild sensorineural hearing loss, annotated with Sensorineural hearing impairment (HP:0000407), qualified as severity mild. HP:0000407 is a phenotype from the Human Phenotype Ontology.
Severity: MILD
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Neurological assessment indicated generalized axonal sensorimotor neuropathy and mild sensorineural hearing loss."
Specifies the sensorineural character and mild severity in the Family 2 proband.
Eye 5
Blue-dot cataract HP:0000518 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Blue-dot cataract, annotated with Cataract (HP:0000518). HP:0000518 is a phenotype from the Human Phenotype Ontology.
Bound to HP:0000518 Cataract with the specificity in preferred_term. HP:0007976 Cerulean cataract is the term whose definition matches the colloquial name most closely, but that definition asserts congenital onset and concentric peripheral layering, none of which these sources state, so binding it would manufacture detail. HP:0007648 Punctate cataract is a defensible alternative.
Show evidence (3 references)
PMID:40465263 SUPPORT Human Clinical
"In addition, proband II-1 and his youngest brother II-3 were identified to have blue dot cataract"
Two affected siblings in Family 1 with blue-dot cataract.
PMID:40465263 SUPPORT Human Clinical
"Both affected family members had blue-dot cataracts, normal lactate, but elevated homocysteine levels."
Two further affected individuals, in an unrelated family, bringing the count to four and establishing the finding as recurrent across pedigrees.
PMID:40465263 SUPPORT INDIRECT Human Clinical
"To identify additional ocular genetic condition, a cataract panel was applied which did not reveal any variants in known genes."
The authors tested whether the cataract had a separate genetic cause and found none. That is a negative result rather than a demonstration that NSUN3 causes it, hence INDIRECT, but it is why the finding is curated here rather than treated as an unrelated comorbidity.
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 (2 references)
PMID:38790159 SUPPORT Human Clinical
"At her ophthalmologic examination, the patient had esotropia of the left eye and a rotatory nystagmus in both eyes."
Bilateral rotatory nystagmus on examination in an NSUN3-mutant patient.
PMID:38790159 SUPPORT Human Clinical
"She presented with bilateral optic atrophy and nystagmus at the age of seven years."
Records the nystagmus as a presenting feature alongside the optic atrophy.
Optic atrophy HP:0000648 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bilateral optic atrophy, annotated with Optic atrophy (HP:0000648), qualified as course progressive. HP:0000648 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (2 references)
PMID:40465263 SUPPORT Human Clinical
"Remarkably, bilateral optic atrophy was a unifying clinical feature observed in almost all affected individuals."
Establishes optic atrophy as the unifying feature across the cohort.
PMID:38790159 SUPPORT Human Clinical
"She presented with bilateral optic atrophy and nystagmus at the age of seven years."
An independent report of optic atrophy in a homozygous NSUN3 frameshift carrier, ascertained through an inherited-optic-neuropathy cohort.
Reduced visual acuity HP:0007663 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Reduced visual acuity (HP:0007663). HP:0007663 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:41725275 SUPPORT Human Clinical
"Initial symptoms started as hypotonia in infancy and proceeded with developmental delay, reduced visual acuity and chest pain."
Documents reduced visual acuity in the course of an NSUN3-related case.
External ophthalmoplegia HP:0000544 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is External ophthalmoplegia (HP:0000544). HP:0000544 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27356879 SUPPORT Human Clinical
"muscular weakness, proximal accentuated, external ophthalmoplegia and convergence nystagmus"
External ophthalmoplegia in the founding case.
Head and Neck 1
Microcephaly HP:0000252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Microcephaly (HP:0000252). HP:0000252 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27356879 SUPPORT Human Clinical
"combined developmental disability, microcephaly, failure to thrive, recurrent increased lactate levels in plasma"
Microcephaly in the founding case.
Metabolism 1
Increased circulating lactate concentration HP:0002151 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased plasma lactate, annotated with Increased circulating lactate concentration (HP:0002151). HP:0002151 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27356879 SUPPORT Human Clinical
"recurrent increased lactate levels in plasma"
Raised lactate in the founding case.
PMID:40465263 REFUTE Human Clinical
"Both affected family members had blue-dot cataracts, normal lactate, but elevated homocysteine levels."
Refutes raised lactate as a universal feature: the two affected members of this family had normal lactate. Recorded as a REFUTE item against the same claim rather than folded into the supporting text.
Musculoskeletal 2
Generalized hypotonia HP:0001290 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Muscular hypotonia, annotated with Generalized hypotonia (HP:0001290). HP:0001290 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32488845 SUPPORT Human Clinical
"lactic acidosis, global developmental delay, hypotonia, muscle weakness and seizures"
Hypotonia among the presenting features at 4 months.
PMID:41725275 SUPPORT Human Clinical
"Initial symptoms started as hypotonia in infancy"
Infantile hypotonia as the first manifestation in a third individual.
Proximal muscle weakness HP:0003701 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Proximally accentuated muscle weakness, annotated with Proximal muscle weakness (HP:0003701). HP:0003701 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27356879 SUPPORT Human Clinical
"muscular weakness, proximal accentuated, external ophthalmoplegia and convergence nystagmus"
Specifies the proximal accentuation of the weakness in the founding case.
Nervous System 3
Generalized axonal sensorimotor neuropathy HP:0007141 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Generalized axonal sensorimotor neuropathy, annotated with Sensorimotor neuropathy (HP:0007141). HP:0007141 is a phenotype from the Human Phenotype Ontology.
HPO has no term combining the axonal and sensorimotor characters, so this is a real choice between them. HP:0007141 Sensorimotor neuropathy is bound because sensorimotor is the distinction the source draws; HP:0003477 Peripheral axonal neuropathy is the defensible alternative, capturing the axonal character instead. HP:0007002 Motor axonal neuropathy, offered by the deep-research report, is wrong on its face - the source says sensorimotor. The full description is carried in preferred_term either way.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Neurological assessment indicated generalized axonal sensorimotor neuropathy and mild sensorineural hearing loss."
States the neuropathy and its sensorimotor character directly.
Global developmental delay HP:0001263 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Global developmental delay (HP:0001263). HP:0001263 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27356879 SUPPORT Human Clinical
"developed mitochondrial disease symptoms at the age of three months (combined developmental disability, microcephaly, failure to thrive, recurrent increased lactate levels in plasma, muscular weakness, proximal accentuated, external ophthalmoplegia and convergence nystagmus)"
The founding case's presenting features, including developmental delay.
PMID:32488845 SUPPORT Human Clinical
"lactic acidosis, global developmental delay, hypotonia, muscle weakness and seizures"
Independent replication in a second, unrelated patient.
Seizure HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizures, annotated with Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32488845 SUPPORT Human Clinical
"lactic acidosis, global developmental delay, hypotonia, muscle weakness and seizures"
Seizures in the encephalomyopathy case that named them in its title.
PMID:40465263 SUPPORT Human Clinical
"Despite levetiracetam treatment, she continued to experience seizures about once every six months."
An independent individual with recurrent seizures incompletely controlled on levetiracetam.
Growth 1
Failure to thrive HP:0001508 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Failure to thrive (HP:0001508). HP:0001508 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27356879 SUPPORT Human Clinical
"combined developmental disability, microcephaly, failure to thrive, recurrent increased lactate levels in plasma"
Failure to thrive in the founding case.
🧬

Genetic Associations

2
NSUN3
Gene: NSUN3 hgnc:26208 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is NSUN3 (hgnc:26208). hgnc:26208 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
PMID:27356879 SUPPORT In Vitro
"Both NSun3 and mtSSB1 were resistant to proteinase K treatment of the mitochondrial fraction"
Submitochondrial protection assay placing NSun3 inside mitochondria rather than on the outer membrane.
PMID:40465263 SUPPORT Human Clinical
"Analysis of variant distribution showed that variants can be found across the whole NSUN3 gene/protein"
The larger cohort's finding on variant distribution, which is why the earlier exon 3 hotspot proposal is not carried forward as current.
ALKBH1
Gene: ALKBH1 hgnc:17911 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ALKBH1 (hgnc:17911). hgnc:17911 is a gene from the HUGO Gene Nomenclature Committee.
Show evidence (2 references)
PMID:27497299 SUPPORT In Vitro
"We further identify ALKBH1/ABH1 as the dioxygenase responsible for oxidising m5C34 of mt-tRNAMet to generate an f5C34 modification."
Identifies the partner enzyme and the reaction it catalyses.
PMID:27497299 SUPPORT In Vitro
"ABH1 has a broader target spectrum and its depletion might also influence other molecules affecting mitochondrial translation."
The authors' own reason for not treating the two enzymes as interchangeable, which is why ALKBH1 is curated as pathway-related only.
💊

Medical Actions

4
Supportive and Multidisciplinary Care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Other
No disease-modifying therapy exists. Management is the standard mitochondrial disease package - anticonvulsants for seizures, nutritional and developmental support, surveillance for the organ involvement each individual develops - and none of it addresses the tRNA modification defect. The breadth of the reported phenotype is itself a management point: an individual identified through an optic-atrophy pathway may still need endocrine, cardiac, audiological and neurological assessment.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Despite levetiracetam treatment, she continued to experience seizures about once every six months."
A worked example of symptomatic management and its incomplete effect.
Idebenone
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: idebenone CHEBI:31687 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses idebenone (CHEBI:31687). CHEBI:31687 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
One individual with a subacute LHON-like presentation received oral idebenone and recovered to 6/6 in both eyes within four months. That is a striking number, and it is why idebenone is worth recording here - but the same report makes clear it cannot be read as a treatment effect. Visual acuity in that individual had already begun improving spontaneously twelve months before idebenone was started, reaching 6/12 on its own; spontaneous recovery is well documented in childhood-onset LHON; and the authors say explicitly that they cannot separate genotype from treatment. This is curated as an uncontrolled single-patient observation, not as evidence of efficacy in COXPD48.
Mechanism Target:
BYPASSES Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration — Idebenone is a short-chain quinone proposed to carry electrons to complex III, bypassing complex I and restoring ATP production in retinal ganglion cells. The link records the node the drug is given for, not a demonstrated effect in this disease: the single COXPD48 individual treated had already begun recovering spontaneously twelve months before idebenone was started, and the report's authors say they cannot separate genotype from treatment.
Show evidence (1 reference)
PMID:40465263 SUPPORT INDIRECT Human Clinical
"Subsequently, oral treatment with idebenone was started, whereupon visual acuity was restored to 6/6 in both eyes after only four months."
Establishes that idebenone was given for the optic-nerve phenotype this node describes. INDIRECT because the same report documents spontaneous improvement beginning before treatment, which is curated as a REFUTE item on this treatment, so the sentence cannot carry an efficacy claim.
Show evidence (3 references)
PMID:40465263 SUPPORT Human Clinical
"Subsequently, oral treatment with idebenone was started, whereupon visual acuity was restored to 6/6 in both eyes after only four months."
The observed outcome in the single treated individual.
PMID:40465263 REFUTE Human Clinical
"visual acuity began to improve spontaneously after 12 months and reached 6/12 after a further 18 months."
Refutes attributing the recovery to idebenone: substantial spontaneous improvement preceded the drug in this same individual.
PMID:40465263 SUPPORT Human Clinical
"it is challenging to determine whether in this study this particular disease course is associated with a specific genotype or whether it is influenced by treatment with idebenone"
The authors' own statement that the observation is not attributable.
Coenzyme Q10 (Ubiquinone) Supplementation
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: coenzyme Q10 CHEBI:46245 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses coenzyme Q10 (CHEBI:46245). CHEBI:46245 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Started in one individual on the basis of a measured secondary deficiency - muscle ubiquinone of 94 pmol/mg against a reference range of 140 to 850 - rather than empirically. No outcome from that supplementation is reported, so this records a rational indication for checking muscle ubiquinone in an NSUN3 patient having a biopsy, not a demonstrated benefit.
Mechanism Target:
RESTORES Combined Respiratory Chain Enzyme Deficiency — Ubiquinone carries electrons from complexes I and II to complex III, so supplementing a measured secondary deficiency targets the combined respiratory chain defect directly. The link records the indication, not an outcome: no result of the supplementation is reported.
Show evidence (1 reference)
PMID:40465263 SUPPORT INDIRECT Human Clinical
"Further analysis demonstrated low muscle ubiquinone of 94 pmol/mg (normal range 140-850 pmol/mg). Ubiquinone supplementation has been commenced."
Documents the measured secondary ubiquinone deficiency and the supplementation started for it. INDIRECT because the source reports the indication without any outcome, so it cannot support an effect on the respiratory chain defect.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Further analysis demonstrated low muscle ubiquinone of 94 pmol/mg (normal range 140–850 pmol/mg). Ubiquinone supplementation has been commenced."
The measured deficiency and the resulting intervention, with no outcome reported.
Genetic Counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Platform: Behavioral / lifestyle
Autosomal recessive counselling with a specific caveat. One proband reached homozygosity through paternal uniparental isodisomy of chromosome 3 rather than by descent, in a non-consanguineous family - so a homozygous NSUN3 result does not establish that both parents are carriers, and recurrence risk should not be quoted as one in four until segregation has actually been checked. The same family also illustrates the counselling problem created by variable expressivity: the homozygous father had moderate bilateral optic atrophy while his son presented with subacute visual loss at eleven.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Further analysis revealed paternal uniparental isodisomy of chromosome 3 in the proband's DNA sample."
The isodisomy finding that changes the recurrence-risk conversation.
🔬

Diagnosis

4
Exome or Genome Sequencing
Every reported individual was solved by unbiased exome or genome sequencing, not by a targeted panel - in several cases only after an optic-neuropathy-specific panel had returned nothing. NSUN3 is absent from at least one published inherited-optic-neuropathy capture panel, so the practical recommendation from the largest cohort is to add the gene to diagnostic optic-atrophy and mitochondrial-disease panels and, failing that, to go to exome or genome sequencing.
whole exome sequencing NCIT:C101295 NCI Thesaurus (NCIT)
Markers: NSUN3 biallelic variants
Show evidence (2 references)
PMID:27356879 SUPPORT Human Clinical
"we identified compound heterozygous predicted loss-of-function variants in NSUN3"
Exome sequencing as the route to the founding diagnosis.
PMID:40465263 SUPPORT Human Clinical
"the NSUN3 gene should be added to diagnostic optic atrophy and mitochondrial disease gene panels when investigating patients who are suspected to have an isolated or syndromic inherited optic neuropathy"
The cohort's own recommendation on where NSUN3 belongs in the diagnostic pathway.
mt-tRNA-Met C34 Methylation Assay by RNA Bisulfite Sequencing
This is the disease-specific functional test, and it is what converts a variant of uncertain significance into a diagnosis. RNA bisulfite sequencing measures methylation at the wobble base; in the largest cohort every affected individual read at background against a control range of 26 to 33 percent. It is practical to run: the assay worked on RNA from PAXgene-stabilised whole blood in five of six individuals, so a skin biopsy is not obligatory. It also has a documented limitation. Methylation was equally abolished in carriers of the mild C-terminal alleles as in the severe loss-of-function ones, so the assay confirms loss of the methyltransferase function but does not grade severity or predict phenotype - the authors take that dissociation as a hint that NSUN3 may have a second, unidentified function.
RNA bisulfite sequencing assay of mt-tRNA-Met C34 methylation NCIT:C25294 NCI Thesaurus (NCIT)
Markers: mt-tRNA-Met m5C34 methylation fraction
Show evidence (2 references)
PMID:40465263 SUPPORT Human Clinical
"All samples obtained from affected individuals demonstrated significantly reduced mitochondrial tRNAMet methylation levels that were reduced to background levels, although control samples showed 26% to 33% tRNAMet methylation"
The assay result and its control range, across every affected individual tested.
PMID:40465263 SUPPORT Human Clinical
"However, it should be noted that the methylation at C34 was equally disrupted in carriers of these variants, suggesting that there might be a secondary function of NSUN3 that has not yet been identified."
Records that the assay does not discriminate mild from severe genotypes.
Muscle Biopsy with Respiratory Chain Enzyme Analysis
Spectrophotometric respiratory chain enzyme assay on muscle homogenate, normalised to citrate synthase, is what established the combined deficiency in the founding case and is still the biochemical confirmation. Complex IV is the most consistently reduced, with complex I next; complex II, being entirely nuclear-encoded, is the internal control. The histology is a poor screening test. Ragged red fibres were absent in every reported biopsy, and COX-negative fibres were present in some individuals and absent in others within the same cohort, so a normal muscle histochemistry panel does not exclude COXPD48. In one individual with a milder phenotype the skin biopsy showed normal respiratory chain complex activity with reduced oxygen consumption, so tissue choice and the assay used both matter.
muscle biopsy for respiratory chain enzyme analysis NCIT:C51895 NCI Thesaurus (NCIT)
Markers: respiratory chain complex I-IV activities normalized to citrate synthase
Show evidence (2 references)
PMID:40465263 SUPPORT Human Clinical
"spectrophotometric respiratory chain enzyme analysis demonstrated a complex I deficiency of 0.078 (normal range 0.118–0.332) and complex IV deficiency of 0.003 (normal range 0.013–0.039)"
A worked example of the assay with its reference ranges.
PMID:40465263 SUPPORT Human Clinical
"A skin biopsy obtained from individual 4 (Family 4: II-3) showed normal respiratory chain complex activity with reduced oxygen consumption"
Documents a normal enzyme panel alongside an abnormal respirometry result in the same individual, which is the caution against relying on one assay.
Plasma Lactate
Raised plasma lactate triggered the mitochondrial workup in the severe early-onset cases and remains a reasonable first-line test in that setting. It should not be used to rule the disorder out: two affected members of one family had normal lactate and presented with optic atrophy alone.
plasma lactate measurement NCIT:C25294 NCI Thesaurus (NCIT)
Markers: plasma lactate
Show evidence (2 references)
PMID:40465263 SUPPORT Human Clinical
"Because of moderately elevated plasma lactate, further investigation of an underlying mitochondriopathy was performed with a muscle biopsy, which revealed a combined OXPHOS deficiency."
Shows lactate as the trigger for the confirmatory biopsy in the founding case.
PMID:40465263 REFUTE Human Clinical
"Both affected family members had blue-dot cataracts, normal lactate, but elevated homocysteine levels."
Refutes the use of a normal lactate to exclude the diagnosis.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Roughly a dozen individuals have been reported. The largest series describes eight individuals from five unrelated families, of whom one had been published previously; separate single reports add a South Asian encephalomyopathy case, a Dutch optic-neuropathy case, and a Turkish case with hypertrophic cardiomyopathy. No population prevalence estimate exists, and ascertainment has been through inherited-optic-neuropathy and mitochondrial-disease cohorts rather than any unselected denominator, so no numeric rate is recorded here.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"This study has characterized the largest cohort of patients harboring candidate biallelic NSUN3 variants to date, consisting of eight affected individuals from five independent families revealing a broad and variable phenotype."
The size of the largest published series, which is the basis for the literature-case count rather than a prevalence estimate.
🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Combined oxidative phosphorylation deficiency 48:

Leber hereditary optic neuropathy
Overlapping Features The closest mimic, and a documented one: two individuals with biallelic NSUN3 variants presented with a subacute LHON-like picture - hyperaemic disc swelling, peripapillary telangiectatic microangiopathy, central scotomas - and one was investigated for the three common mtDNA LHON variants and had whole-mtDNA sequencing before exome sequencing found NSUN3. Inheritance is the discriminator: LHON is maternally transmitted mtDNA disease, COXPD48 is autosomal recessive nuclear disease, so a negative mtDNA result in a LHON-like presentation with affected siblings or consanguinity should prompt nuclear sequencing.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"Because the clinical picture was suggestive of LHON, genetic testing for the three most common LHON mtDNA variants followed by whole mtDNA sequencing was performed."
Documents that the LHON workup was undertaken and negative before NSUN3 was found.
Other combined oxidative phosphorylation deficiencies
Overlapping Features The COXPD series numbers well over fifty entities across largely unrelated nuclear genes, and they are clinically overlapping enough that a combined respiratory chain deficiency on muscle biopsy does not point to any one of them. The distinguishing features of COXPD48 are the near-universal optic nerve involvement, the very wide onset range, and the specific functional assay - loss of mt-tRNA-Met C34 methylation - which is diagnostic for this gene and normal in the others.
Show evidence (1 reference)
PMID:40465263 SUPPORT Human Clinical
"The majority of these disorders have an overlapping clinical phenotype, necessitating a broad approach to molecular diagnostic testing, typically including full sequencing of mtDNA and use of large virtual gene panels."
States the clinical overlap across primary mitochondrial disorders that makes molecular testing rather than phenotype the discriminator. This is a class-level statement about primary mitochondrial disease, not a claim about NSUN3 specifically.
🐁

Animal Models

2
Whole-body Nsun3 knockout mouse
Constitutive Nsun3 deletion is embryonic lethal in the mouse: knockout embryos are alive with heartbeats at E10.5, somewhat smaller than wild-type, and dead by E12.5. This is a sharper phenotype than any human genotype produces - the founding patient carried two predicted null alleles and survived infancy - so the model bounds what the gene is required for rather than reproducing the disease.
Species
Mouse
Genotype
Nsun3 constitutive knockout (homozygous null)
Publication
PMID:36949224
Pathograph links
Loss of Functional NSun3 Methyltransferase fails to recapitulate fidelity: low observes: organism Complete Nsun3 loss in the mouse produces mid-gestation lethality, whereas humans homozygous or compound heterozygous for predicted null alleles are liveborn and reach at least infancy.
Limitations: The species divergence is the finding, not an artefact to be discounted: the model cannot be used to study postnatal NSUN3 disease at all, and any inference from it about human developmental requirements for NSUN3 has to cross that lethality gap. Human null-allele individuals are viable, so the mouse null is not a model of the human null state.
Show evidence (1 reference)
PMID:36949224 SUPPORT Model Organism
"Here we show that Nsun3 is essential for embryonic development in mice with whole-body Nsun3 knockout embryos dying between E10.5 and E12.5."
The lethality window, which is the divergence from the viable human null genotypes.
Heart-specific Nsun3 knockout mouse (Nsun3HKO)
Conditional cardiac deletion sidesteps the embryonic lethality and gives the only in vivo model of adult NSUN3 loss. Nsun3HKO hearts show enlarged mitochondria with fragmented cristae and progressively reduced respiratory complex enzyme activities, worse in older animals, while the mitochondrial mRNAs encoding those subunits are not reduced - the same transcript-versus-activity dissociation seen in patient cells, and a signature of a translation-level rather than a transcription-level lesion. The cardiac phenotype itself is enhanced contraction with age-associated mild enlargement, which is not the same as the hypertrophic cardiomyopathy reported in one human patient. The model is linked to the respiratory chain node it actually measures, not to the human cardiac phenotype.
Species
Mouse
Genotype
Cardiac-restricted Nsun3 conditional knockout
Publication
PMID:36949224
Pathograph links
Combined Respiratory Chain Enzyme Deficiency partially recapitulates fidelity: moderate observes: tissue Cardiac Nsun3 loss reduces respiratory complex enzyme activities without reducing the corresponding mitochondrial transcripts, reproducing the translation-level character of the human biochemical defect in a single organ.
Limitations: Restricted to heart, so it says nothing about the optic nerve, which is the most consistently affected human tissue. The deficit is age-progressive in the mouse whereas the severe human phenotype is present in infancy, and the mouse cardiac phenotype (enhanced contraction, mild enlargement) is not the hypertrophic cardiomyopathy reported once in humans.
Readouts
Respiratory complex enzymatic activities in heart
Direction: DECREASED
Interpretation: Enzyme-activity correlate of the combined respiratory chain deficiency node, measured in the conditional knockout heart.
Show evidence (1 reference)
PMID:36949224 SUPPORT Model Organism
"In the Nsun3HKO hearts, mitochondrial mRNAs that encode respiratory complex subunits were not down regulated, but the enzymatic activities of the respiratory complexes decreased, especially in older mice."
Reports the direction of the enzyme-activity change and the preserved transcript levels alongside it.
Show evidence (1 reference)
PMID:36949224 SUPPORT Model Organism
"Nsun3HKO heart mitochondria were enlarged and contained fragmented cristae."
Ultrastructural mitochondrial abnormality in the model, comparable to the swollen mitochondria with linearised cristae seen in patient muscle.
{ }

Source YAML

click to show 
name: Combined oxidative phosphorylation deficiency 48
creation_date: "2026-09-08T00:00:00Z"
description: >-
  Combined oxidative phosphorylation deficiency 48 (COXPD48) is an autosomal
  recessive mitochondrial disease caused by biallelic variants in NSUN3, the
  mitochondrial RNA cytosine-5 methyltransferase that writes m5C at the wobble
  base (C34) of mitochondrial tRNA-Met.

  The lesion sits one step further upstream than most of the combined-OXPHOS
  series. NSun3 is neither a respiratory-chain subunit nor an assembly factor:
  it installs a single tRNA modification. m5C34 is then oxidised by the
  alpha-ketoglutarate- and Fe(II)-dependent dioxygenase ALKBH1/ABH1 to
  5-formylcytidine (f5C34), and mitochondria carry only one tRNA-Met, which
  serves both initiation and elongation and must read AUG together with the
  non-universal AUA (and, at initiation, AUU) codons. Because methionine codons
  occur throughout all thirteen mtDNA-encoded proteins, losing the modification
  degrades translation of every one of them, and the biochemical result is a
  combined rather than an isolated respiratory chain deficiency. Patient
  fibroblasts lose m5C34 and f5C34 together, consistent with a precursor-product
  relationship, while mt-tRNA steady-state levels, aminoacylation and mtDNA copy
  number are unaffected - the defect is one of decoding capacity, not of tRNA
  abundance or mitochondrial biogenesis.

  The clinical spectrum turned out to be much wider than the founding case
  suggested. That patient presented at three months with developmental delay,
  microcephaly, failure to thrive and raised lactate. The largest cohort to date
  - eight individuals from five families - spans neonatal-onset multisystem
  disease to isolated optic atrophy first noticed at 39 years, with bilateral
  optic nerve involvement in 7 of 8 and biallelic loss-of-function or
  SAM-domain missense genotypes clustering at the severe end. Hypertrophic
  cardiomyopathy has since been reported in one further individual.
category: Mendelian
parents:
- hereditary disease
synonyms:
- COXPD48
- NSUN3-related mitochondrial disease
- NSUN3-related optic neuropathy
disease_term:
  preferred_term: Combined oxidative phosphorylation deficiency 48
  term:
    id: MONDO:0033566
    label: combined oxidative phosphorylation deficiency 48
references:
- reference: PMID:27356879
  title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
- reference: PMID:40465263
  title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
- reference: PMID:32488845
  title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
- reference: PMID:38790159
  title: "Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy."
- reference: PMID:41725275
  title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
- reference: PMID:27497299
  title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
- reference: PMID:27214402
  title: "NSUN3 methylase initiates 5-formylcytidine biogenesis in human mitochondrial tRNA(Met)."
- reference: PMID:36949224
  title: "NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice."
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: UNKNOWN
  expressivity: VARIABLE
  description: >-
    Biallelic NSUN3 genotypes segregate recessively: the founding case was
    compound heterozygous for two predicted loss-of-function alleles, and the
    five-family cohort comprises homozygous and compound heterozygous
    genotypes with carrier parents.

    Two features of that cohort are worth recording. Expressivity is graded by
    genotype rather than uniform - biallelic loss-of-function alleles, or a
    homozygous missense allele hitting the S-adenosylmethionine binding site,
    presented in early infancy, while the milder isolated-optic-atrophy cases
    carried C-terminal variants predicted to escape nonsense-mediated decay. And
    one proband reached homozygosity not by descent but through paternal
    uniparental isodisomy of chromosome 3, in a non-consanguineous family; his
    homozygous father had moderate bilateral optic atrophy. Penetrance is left
    UNKNOWN: with roughly a dozen reported individuals and ascertainment through
    optic-neuropathy and mitochondrial-disease clinics, there is no unselected
    denominator from which to estimate it.
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we identified compound heterozygous predicted loss-of-function variants in
      NSUN3
    explanation: >-
      The founding genotype, identified under a recessive model.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Individuals carrying biallelic LOF variants or homozygous missense variant
      affecting SAM binding site presented with severe neurological phenotype in
      early infancy.
    explanation: >-
      Supports graded expressivity tracking allele severity.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A paternal uniparental isodisomy of chromosome 3 was identified in III-2
      from Family 3 leading to homoallelism for c.930_931delAT.
    explanation: >-
      Records the uniparental-isodisomy route to homozygosity, which is why a
      biallelic NSUN3 result does not by itself imply parental consanguinity.
genetic:
- name: NSUN3
  gene_term:
    preferred_term: NSUN3
    term:
      id: hgnc:26208
      label: NSUN3
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  presence: PRESENT
  notes: >-
    NSUN3 (NOP2/Sun RNA methyltransferase 3) encodes a 340-amino-acid
    S-adenosylmethionine-dependent RNA:m5C methyltransferase of the
    NOL1/NOP2/Sun family that localises to the mitochondrial matrix. Its
    physiological substrate is the wobble cytosine C34 of mt-tRNA-Met; bisulfite
    RNA sequencing of patient fibroblasts showed methylation at that position
    falling to background while m5C at other mitochondrial tRNA positions was
    unchanged, which is what makes NSUN3 a single-substrate enzyme in practice
    rather than a general mitochondrial methyltransferase.

    Reported disease alleles include nonsense (p.Trp50*, p.Arg99*), frameshift
    (p.Glu42Valfs*11 from a multi-kilobase exon 3 deletion, p.Cys311Trpfs*8,
    p.Gly156ArgfsTer6, p.Ala118Glufs*45) and missense (p.Pro142Ser,
    p.Glu271Gly, p.Ala141Pro, p.Cys152Ser) changes. Several early-reported
    variants cluster in exon 3, which prompted a suggestion that the region is a
    hotspot; the later five-family cohort found variants across the whole gene,
    so that suggestion is not supported by the larger series and is recorded here
    as superseded rather than repeated.
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Both NSun3 and mtSSB1 were resistant to proteinase K treatment of the
      mitochondrial fraction
    explanation: >-
      Submitochondrial protection assay placing NSun3 inside mitochondria rather
      than on the outer membrane.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Analysis of variant distribution showed that variants can be found across
      the whole NSUN3 gene/protein
    explanation: >-
      The larger cohort's finding on variant distribution, which is why the
      earlier exon 3 hotspot proposal is not carried forward as current.
- name: ALKBH1
  gene_term:
    preferred_term: ALKBH1
    term:
      id: hgnc:17911
      label: ALKBH1
  notes: >-
    ALKBH1/ABH1 performs the second step of the same two-step modification,
    oxidising NSun3-deposited m5C34 to f5C34. It is listed here as the pathway
    partner that explains why an NSUN3 defect removes f5C as well as m5C, not
    as a COXPD48 disease gene: no ALKBH1 patient variant has been reported in
    this disorder, and the enzyme has a broader substrate range and a separate
    literature of its own.
  evidence:
  - reference: PMID:27497299
    reference_title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We further identify ALKBH1/ABH1 as the dioxygenase responsible for
      oxidising m5C34 of mt-tRNAMet to generate an f5C34 modification.
    explanation: >-
      Identifies the partner enzyme and the reaction it catalyses.
  - reference: PMID:27497299
    reference_title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      ABH1 has a broader target spectrum and its depletion might also influence
      other molecules affecting mitochondrial translation.
    explanation: >-
      The authors' own reason for not treating the two enzymes as
      interchangeable, which is why ALKBH1 is curated as pathway-related only.
pathophysiology:
- name: Loss of Functional NSun3 Methyltransferase
  biological_scale: MOLECULAR
  role: trigger
  mechanism_confidence: ESTABLISHED
  description: >-
    Biallelic NSUN3 variants abolish the mitochondrial-matrix m5C
    methyltransferase. In the founding case, two predicted loss-of-function
    alleles left no detectable full-length NSun3 on western blot of patient
    fibroblasts, and lentiviral re-expression of NSUN3 complementary DNA
    restored the downstream phenotypes - the rescue arm is what makes this a
    causal node rather than a correlation.
  molecular_functions:
  - preferred_term: mt-tRNA cytosine-5 methyltransferase activity
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0016428
      label: tRNA (cytidine-N5)-methyltransferase activity
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Western blot analysis using primary human dermal fibroblasts (hDFs)
      confirmed the lack of the full-length NSun3 protein in patient-derived cells
    explanation: >-
      Demonstrates absence of the protein product in the patient's own cells.
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Both NSun3 and mtSSB1 were resistant to proteinase K treatment of the
      mitochondrial fraction
    explanation: >-
      Places the enzyme inside mitochondria, which is why its loss is a
      mitochondrial rather than a cytoplasmic translation lesion.
  downstream:
  - target: Loss of m5C34 in Mitochondrial tRNA-Met
    causal_link_type: DIRECT
- name: Loss of m5C34 in Mitochondrial tRNA-Met
  biological_scale: MOLECULAR
  role: effector
  mechanism_confidence: ESTABLISHED
  description: >-
    NSun3's physiological substrate is a single position: cytosine 34, the
    wobble base of the anticodon loop of mt-tRNA-Met (mtDNA position 4432).
    Bisulfite RNA sequencing of patient fibroblasts showed methylation there
    falling to background, while m5C at the other mitochondrial tRNA positions
    the method detected - in mt-tRNA-Leu(UUR), mt-tRNA-Ser(AGY), mt-tRNA-His,
    mt-tRNA-Glu and mt-tRNA-Phe - was unchanged. The same assay run on blood or
    fibroblast RNA from a later five-family cohort gave the same result in every
    affected individual, against a control range of 26 to 33 percent, and is now
    the functional assay used to call an NSUN3 variant pathogenic.
  biological_processes:
  - preferred_term: tRNA wobble base cytosine methylation
    modifier: DECREASED
    term:
      id: GO:0002127
      label: tRNA wobble base cytosine methylation
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The bisulfite RNA sequencing (BS RNA-Seq) analysis revealed that methylation
      in mt-tRNAMet (MT-TM) was decreased to background levels in NSUN3 mutant
      cells (mut/mut)
    explanation: >-
      The primary measurement of the lost modification in patient cells.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All samples obtained from affected individuals demonstrated significantly
      reduced mitochondrial tRNAMet methylation levels that were reduced to
      background levels, although control samples showed 26% to 33% tRNAMet
      methylation
    explanation: >-
      Replicates the loss across an independent multi-family cohort and supplies
      the control reference range.
  downstream:
  - target: Absence of f5C34 at the Wobble Position
    causal_link_type: DIRECT
- name: Absence of f5C34 at the Wobble Position
  biological_scale: MOLECULAR
  role: effector
  mechanism_confidence: ESTABLISHED
  description: >-
    The mature modification at C34 is not m5C but 5-formylcytidine. m5C34 is the
    obligatory precursor: ALKBH1/ABH1, an alpha-ketoglutarate- and
    Fe(II)-dependent dioxygenase resident in mitochondria, oxidises it to f5C34.
    Losing the first step therefore removes the second. In wild-type fibroblasts
    roughly 36 to 38 percent of mt-tRNA-Met reads carried f5C34 by two
    independent chemistries; in the NSun3-null patient neither method detected
    any.

    Note what this node does and does not claim. The founding report established
    the precursor-product relationship in patient cells but explicitly left the
    responsible dioxygenase unidentified; ALKBH1 was named in a separate study
    published the same year, on the strength of in vitro oxidation assays and
    RNAi depletion, and it is that work rather than the patient report that the
    ALKBH1 evidence here comes from.
  biological_processes:
  - preferred_term: 5-formylcytidine formation at the mt-tRNA-Met wobble base
    modifier: DECREASED
    term:
      id: GO:0002101
      label: tRNA wobble cytosine modification
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      no f5C was detected in NSun3-null patient samples
    explanation: >-
      The measurement establishing that the second modification is lost with the
      first, in the patient's own cells.
  - reference: PMID:27497299
    reference_title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We further identify ALKBH1/ABH1 as the dioxygenase responsible for
      oxidising m5C34 of mt-tRNAMet to generate an f5C34 modification.
    explanation: >-
      Names the enzyme performing the second step, from the study that
      identified it rather than from the patient report.
  downstream:
  - target: Impaired Wobble Decoding by the Single Mitochondrial tRNA-Met
    causal_link_type: DIRECT
- name: Impaired Wobble Decoding by the Single Mitochondrial tRNA-Met
  biological_scale: MOLECULAR
  role: effector
  mechanism_confidence: PROVISIONAL
  description: >-
    This is the step that makes COXPD48 a different kind of lesion from the rest
    of the combined-OXPHOS series. Human mitochondria carry one tRNA-Met, used
    for both initiation and elongation, and it must read the universal AUG plus
    the non-universal AUA codon - and, at initiation, AUU. The wobble
    modification is what expands that decoding range, so a modification defect
    degrades the reading of every methionine codon in every mtDNA-encoded
    message rather than disabling one subunit or one assembly step.

    Confidence is PROVISIONAL rather than ESTABLISHED because the in vitro
    ribosome-binding data are more textured than the summary claim. One study
    concluded from anticodon-stem-loop work that f5C34 is required to decipher
    AUA; the other, testing modification states directly on ribosomes with
    mitochondrial initiation and elongation factors, found that AUA was read with
    similar efficiencies by all mt-tRNA-Met variants and that the differences it
    could resolve lay mostly in AUG recognition. Both agree the modification
    state changes codon recognition; they do not agree on the size or the codon
    specificity of the effect, and no reconstituted mitochondrial translation
    system exists to settle it.
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      There is only one mitochondrial tRNAMet that decodes all of these AUN codons
    explanation: >-
      States the single-tRNA constraint that makes one modification defect a
      global translation defect. Graded OTHER because the sentence is the
      authors' summary of prior structural work, not a result of this study.
  - reference: PMID:27497299
    reference_title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      can expand the ability of the single tRNAMet to read the different codons
      encoding methionine in mitochondrial translation initiation and elongation.
    explanation: >-
      The ribosome-binding study's own conclusion, stated at the level of
      expanded codon reading rather than a codon-specific requirement.
  - reference: PMID:27214402
    reference_title: "NSUN3 methylase initiates 5-formylcytidine biogenesis in human mitochondrial tRNA(Met)."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      f(5)C34 is required for deciphering the AUA codon during protein synthesis.
    explanation: >-
      The stronger, codon-specific form of the claim, from the parallel study.
      Recorded alongside the weaker form above rather than in place of it.
  downstream:
  - target: Deficient Mitochondrial Protein Synthesis
    causal_link_type: DIRECT
- name: Deficient Mitochondrial Protein Synthesis
  biological_scale: CELLULAR
  role: effector
  mechanism_confidence: ESTABLISHED
  description: >-
    Pulse labelling with radiolabelled methionine under cytoplasmic-translation
    blockade showed the mitochondrial de novo synthesis rate markedly reduced in
    the patient's fibroblasts and substantially restored by NSUN3 re-expression.
    Independent RNAi work found that synthesis of all mitochondrial proteins fell
    on NSUN3 depletion, which is the expected signature of a methionine-decoding
    defect rather than of a subunit-specific lesion.

    Two negative results matter as much as the positive one, because together
    they localise the defect. mt-tRNA steady-state levels and aminoacylation were
    unchanged, and mtDNA copy number was unchanged - so this is not a tRNA
    stability defect, not a charging defect, and not a biogenesis defect.
    Mitochondrial transcript levels actually rose, which the authors read as
    compensatory.
  biological_processes:
  - preferred_term: mitochondrial translation
    modifier: DECREASED
    term:
      id: GO:0032543
      label: mitochondrial translation
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      the mitochondrial de novo protein synthesis rate was markedly decreased in
      the absence of NSun3
    explanation: >-
      Direct measurement of the translation defect in patient cells.
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      High-resolution northern blot analysis did not reveal any appreciable
      differences in expression levels of mt-tRNAs in the absence of NSun3
    explanation: >-
      Excludes loss of tRNA abundance as the mechanism, localising the defect to
      decoding.
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The mtDNA copy number was unchanged between wt/wt and mut/mut hDFs
    explanation: >-
      Excludes a mitochondrial biogenesis or mtDNA maintenance defect.
  - reference: PMID:27497299
    reference_title: "NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      the translation of all mitochondrial proteins was affected by NSUN3 or ABH1
      depletion
    explanation: >-
      Establishes that the defect is general across the thirteen mtDNA-encoded
      proteins, which is why the biochemical phenotype is combined.
  downstream:
  - target: Combined Respiratory Chain Enzyme Deficiency
    causal_link_type: DIRECT
- name: Combined Respiratory Chain Enzyme Deficiency
  biological_scale: CELLULAR
  role: effector
  mechanism_confidence: ESTABLISHED
  conforms_to: "mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress"
  description: >-
    Complexes I, III, IV and V each contain mtDNA-encoded subunits and are
    exposed to the translation defect; complex II is entirely nuclear-encoded and
    is not. The measured pattern fits that prediction, but not uniformly. In the
    five-family cohort, spectrophotometric assay of one individual's muscle gave
    a complex I activity of 0.078 against a normal range of 0.118 to 0.332 and a
    complex IV activity of 0.003 against 0.013 to 0.039, both normalised to
    citrate synthase; western blotting of another family's muscle and fibroblasts
    showed reduced COX II and NDUFB8 but not complex II, III or V. Complex IV is
    the most consistently affected across reports, and complex III and V changes
    were not detected in this cohort even though both carry mtDNA-encoded
    subunits.

    Functional respirometry agrees: patient fibroblasts show reduced basal
    oxygen consumption and ATP production, remain below control even when
    uncoupled with FCCP, and grow poorly when forced onto galactose. The founding
    case's oxygen consumption defect was reversed by NSUN3 re-expression.

    Conformance note: this node declares conformance to the mitochondrial
    dysfunction module's central bioenergetic node, and only in part. The
    module's node pairs decreased oxidative phosphorylation with increased
    reactive oxygen species; the decreased-OXPHOS arm is directly evidenced in
    these patients, whereas no NSUN3 report measures reactive oxygen species, so
    that arm is not asserted here. The module's upstream node - age-related
    mitochondrial damage and mtDNA mutation - does not apply to a primary nuclear
    modification defect, and mtDNA copy number is explicitly normal, which is why
    conformance is declared at this node alone.
  biological_processes:
  - preferred_term: oxidative phosphorylation
    modifier: DECREASED
    term:
      id: GO:0006119
      label: oxidative phosphorylation
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      developed mitochondrial disease symptoms at the age of three months
      (combined developmental disability, microcephaly, failure to thrive,
      recurrent increased lactate levels in plasma, muscular weakness, proximal
      accentuated, external ophthalmoplegia and convergence nystagmus) and
      presented with combined OXPHOS deficiency in skeletal muscle
    explanation: >-
      The founding case's combined deficiency, measured in skeletal muscle.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      spectrophotometric respiratory chain enzyme analysis demonstrated a complex
      I deficiency of 0.078 (normal range 0.118–0.332) and complex IV deficiency
      of 0.003 (normal range 0.013–0.039)
    explanation: >-
      The measured enzyme activities with their reference ranges, rather than a
      summary description of them.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      confirmed decreased amounts of COX II (OXPHOS complexes IV) and NDUFB8
      (complex I), but not of complex II, III and V
    explanation: >-
      Independent protein-level confirmation, and the source of the observation
      that complexes III and V were spared despite carrying mtDNA-encoded
      subunits.
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      the patient-derived hDFs (mut/mut) had a lower growth rate than wild-type
      cells
    explanation: >-
      Galactose growth assay, a functional readout of dependence on oxidative ATP
      production.
  downstream:
  - target: Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration
    causal_link_type: DIRECT
  - target: Mitochondrial Myopathy of Skeletal Muscle
    causal_link_type: DIRECT
  - target: Increased circulating lactate concentration
    causal_link_type: DIRECT
  - target: Blue-dot cataract
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40465263
      reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        Both affected family members had blue-dot cataracts, normal lactate, but
        elevated homocysteine levels.
      explanation: >-
        The lens is avascular and depends on glycolysis rather than oxidative
        phosphorylation, so the route from a respiratory chain defect to lens
        opacity is not obvious and the source does not propose one. Recorded as
        an unexplained association of the disease, not a demonstrated step.
  - target: Generalized axonal sensorimotor neuropathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40465263
      reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        Neurological assessment indicated generalized axonal sensorimotor
        neuropathy and mild sensorineural hearing loss.
      explanation: >-
        Long peripheral axons are metabolically demanding and neuropathy is common
        across mitochondrial disease, but the source reports the finding without
        establishing that route in this disorder.
  - target: Global developmental delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:27356879
      reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        developed mitochondrial disease symptoms at the age of three months (combined
        developmental disability, microcephaly, failure to thrive, recurrent increased
        lactate levels in plasma, muscular weakness, proximal accentuated, external
        ophthalmoplegia and convergence nystagmus)
      explanation: >-
        Reports the neurodevelopmental phenotype in an NSUN3-mutant patient. The
        source establishes that the finding occurs in this disease, not the
        specific route from respiratory chain deficiency to it, hence INDIRECT
        and INDIRECT_UNKNOWN_INTERMEDIATES.
  - target: Generalized hypotonia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:32488845
      reference_title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        lactic acidosis, global developmental delay, hypotonia, muscle weakness and seizures
      explanation: >-
        Reports hypotonia in NSUN3-mutant patients. Co-occurrence with the
        biochemical phenotype rather than a demonstrated causal step.
  - target: Microcephaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:27356879
      reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        combined developmental disability, microcephaly, failure to thrive, recurrent
        increased lactate levels in plasma
      explanation: >-
        Reports microcephaly in an NSUN3-mutant patient. Whether it reflects
        impaired brain growth downstream of the bioenergetic deficit is not
        addressed by the source.
  - target: Failure to thrive
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:27356879
      reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        combined developmental disability, microcephaly, failure to thrive, recurrent
        increased lactate levels in plasma
      explanation: >-
        Reports failure to thrive in an NSUN3-mutant patient. A systemic
        manifestation of the disease; the source does not separate the
        contributions of energy deficit, feeding difficulty and intercurrent
        illness.
  - target: Seizure
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:32488845
      reference_title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        lactic acidosis, global developmental delay, hypotonia, muscle weakness and seizures
      explanation: >-
        Reports seizures in NSUN3-mutant patients. The source names the finding
        without establishing the route from respiratory chain deficiency to
        cortical hyperexcitability.
  - target: Sensorineural hearing impairment
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40465263
      reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        Neurological assessment indicated generalized axonal sensorimotor neuropathy and
        mild sensorineural hearing loss.
      explanation: >-
        Reports sensorineural hearing loss in an NSUN3-mutant patient. Cochlear
        hair cells and spiral ganglion neurons are metabolically demanding and
        are affected in many mitochondrial diseases, but that mechanism is not
        demonstrated here.
  - target: Hypertrophic cardiomyopathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:41725275
      reference_title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        An 18-year-old girl was referred for visual loss, progressive fatigue, muscle weakness and HCM.
      explanation: >-
        Reports HCM in an NSUN3-mutant patient - the first such case. Cardiac
        muscle is a high-demand tissue affected in many mitochondrial diseases,
        but a single case cannot establish the causal step, and the report
        itself presents the association as novel rather than settled.
- name: Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration
  biological_scale: TISSUE
  role: effector
  mechanism_confidence: PROVISIONAL
  description: >-
    Optic nerve involvement is the one feature that runs through nearly the whole
    reported spectrum - 7 of 8 individuals in the largest cohort - and it is the
    reason NSUN3 was rediscovered through inherited-optic-neuropathy clinics
    rather than through metabolic ones. Optical coherence tomography shows
    peripapillary retinal nerve fibre layer and ganglion cell layer thinning, and
    pattern electroretinography shows a reduced N95:P50 ratio consistent with
    retinal ganglion cell dysfunction.

    The proposed reason for the tissue selectivity is generic to mitochondrial
    optic neuropathies rather than specific to NSUN3: retinal ganglion cells have
    long axons with unmyelinated prelaminar segments and a correspondingly high
    dependence on mitochondrial ATP. It is recorded here as the field's
    explanation, offered in the cited review's own hedged terms, and it has not
    been tested in NSUN3 tissue - which is why this node is PROVISIONAL while the
    upstream biochemistry is ESTABLISHED.
  cell_types:
  - preferred_term: retinal ganglion cell
    term:
      id: CL:0000740
      label: retinal ganglion cell
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Optic nerve involvement was a unifying feature in most cases (7/8, 87.5%).
    explanation: >-
      Quantifies how consistently the optic nerve is involved across the cohort.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Particularly RGCs with their long axons and unmyelinated prelaminar
      segments are highly dependent on the mitochondrial-derived energy, which
      makes them particularly vulnerable to mitochondrial dysfunction
    explanation: >-
      The proposed basis for retinal ganglion cell vulnerability. Graded OTHER
      because the sentence states the field's general explanation for
      mitochondrial optic neuropathies, not a measurement made in NSUN3 tissue.
  downstream:
  - target: Optic atrophy
    causal_link_type: DIRECT
  - target: Reduced visual acuity
    causal_link_type: DIRECT
  - target: Nystagmus
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38790159
      reference_title: "Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        She presented with bilateral optic atrophy and nystagmus at the age of
        seven years.
      explanation: >-
        Reported together with the optic atrophy this node explains, so routed
        from it rather than from the systemic node. Whether the nystagmus is
        sensory - secondary to early visual loss - or reflects independent
        brainstem or cerebellar involvement is not addressed by either source.
- name: Mitochondrial Myopathy of Skeletal Muscle
  biological_scale: TISSUE
  role: effector
  mechanism_confidence: ESTABLISHED
  description: >-
    Skeletal muscle carries the biochemical lesion but shows a strikingly
    variable histological one, and the variability is itself worth recording: in
    the five-family cohort one individual's biopsy had numerous COX-negative
    fibres while another's had none, and no affected individual had ragged red
    fibres. Subsarcolemmal mitochondrial aggregates were present in three.
    Ultrastructurally, mitochondria were swollen and elongated, spanning a
    sarcomere rather than sitting in the I-band, some with reduced and linearised
    cristae.

    Read the COX-negative-fibre claim carefully. The relevant figure legend in
    that paper states that affected individuals from Families 1, 2 and 3
    displayed COX-negative fibres, but the results and discussion sections both
    say that the Family 1 proband did not, and that the Family 2 biopsy showed
    none either. The per-individual text is the narrower and better-supported
    claim and is what is cited here.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      COX enzyme histochemistry demonstrated numerous COX-negative fibers
    explanation: >-
      The positive histochemical finding, in the Family 3 proband's biopsy.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      No COX-negative fibers and no ragged red fibers were observed; however, a
      proportion of the type I fibers showed prominent subsarcolemmal
      mitochondrial aggregates
    explanation: >-
      The contrasting biopsy in the same cohort, which is why a normal COX stain
      does not exclude the diagnosis.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      many mitochondria were swollen and elongated
    explanation: >-
      The ultrastructural correlate on electron microscopy.
  downstream:
  - target: Proximal muscle weakness
    causal_link_type: DIRECT
  - target: External ophthalmoplegia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:27356879
      reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        muscular weakness, proximal accentuated, external ophthalmoplegia and convergence nystagmus
      explanation: >-
        Reports external ophthalmoplegia alongside the proximal weakness in the
        same patient. Routed from the myopathy node rather than from the
        systemic node because the extraocular muscles are skeletal muscle and
        are characteristically involved in mitochondrial myopathy; the source
        reports the co-occurrence without demonstrating that route.
phenotypes:
- name: Blue-dot cataract
  category: Ophthalmological
  description: >-
    Punctate blue-white lens opacities, reported in four of roughly eleven
    published individuals: the Family 1 proband and his youngest brother, and
    both affected members of Family 3. Recurrent rather than incidental, and the
    cohort authors specifically looked for another cause and did not find one.
  phenotype_term:
    preferred_term: Blue-dot cataract
    term:
      id: HP:0000518
      label: Cataract
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In addition, proband II-1 and his youngest brother II-3 were identified to
      have blue dot cataract
    explanation: >-
      Two affected siblings in Family 1 with blue-dot cataract.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both affected family members had blue-dot cataracts, normal lactate, but
      elevated homocysteine levels.
    explanation: >-
      Two further affected individuals, in an unrelated family, bringing the
      count to four and establishing the finding as recurrent across pedigrees.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    snippet: >-
      To identify additional ocular genetic condition, a cataract panel was
      applied which did not reveal any variants in known genes.
    explanation: >-
      The authors tested whether the cataract had a separate genetic cause and
      found none. That is a negative result rather than a demonstration that
      NSUN3 causes it, hence INDIRECT, but it is why the finding is curated here
      rather than treated as an unrelated comorbidity.
  notes: >-
    Bound to HP:0000518 Cataract with the specificity in preferred_term.
    HP:0007976 Cerulean cataract is the term whose definition matches the
    colloquial name most closely, but that definition asserts congenital onset
    and concentric peripheral layering, none of which these sources state, so
    binding it would manufacture detail. HP:0007648 Punctate cataract is a
    defensible alternative.

- name: Nystagmus
  category: Ophthalmological
  description: >-
    Reported in two unrelated individuals with different character - convergence
    nystagmus in the index case and rotatory nystagmus in both eyes in the 2024
    optic-neuropathy report - so the general term is the honest binding.
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:38790159
    reference_title: "Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      At her ophthalmologic examination, the patient had esotropia of the left
      eye and a rotatory nystagmus in both eyes.
    explanation: >-
      Bilateral rotatory nystagmus on examination in an NSUN3-mutant patient.
  - reference: PMID:38790159
    reference_title: "Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She presented with bilateral optic atrophy and nystagmus at the age of
      seven years.
    explanation: >-
      Records the nystagmus as a presenting feature alongside the optic atrophy.

- name: Generalized axonal sensorimotor neuropathy
  category: Nervous System
  description: >-
    Peripheral neuropathy with both sensory and motor involvement, on
    neurological assessment of the Family 2 proband. Reported in the same
    sentence as the sensorineural hearing loss already curated here.
  phenotype_term:
    preferred_term: Generalized axonal sensorimotor neuropathy
    term:
      id: HP:0007141
      label: Sensorimotor neuropathy
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neurological assessment indicated generalized axonal sensorimotor
      neuropathy and mild sensorineural hearing loss.
    explanation: >-
      States the neuropathy and its sensorimotor character directly.
  notes: >-
    HPO has no term combining the axonal and sensorimotor characters, so this is
    a real choice between them. HP:0007141 Sensorimotor neuropathy is bound
    because sensorimotor is the distinction the source draws; HP:0003477
    Peripheral axonal neuropathy is the defensible alternative, capturing the
    axonal character instead. HP:0007002 Motor axonal neuropathy, offered by the
    deep-research report, is wrong on its face - the source says sensorimotor.
    The full description is carried in preferred_term either way.

- name: Optic atrophy
  category: Ophthalmological
  description: >-
    Bilateral optic atrophy is the most consistent feature of NSUN3-related
    disease and, at the mild end of the spectrum, may be the only one. It was
    present in 7 of 8 individuals in the largest cohort, and two independent
    reports arrived at NSUN3 from unsolved inherited-optic-neuropathy cohorts
    rather than from metabolic workup.
  phenotype_term:
    preferred_term: Bilateral optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Remarkably, bilateral optic atrophy was a unifying clinical feature
      observed in almost all affected individuals.
    explanation: >-
      Establishes optic atrophy as the unifying feature across the cohort.
  - reference: PMID:38790159
    reference_title: "Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She presented with bilateral optic atrophy and nystagmus at the age of
      seven years.
    explanation: >-
      An independent report of optic atrophy in a homozygous NSUN3 frameshift
      carrier, ascertained through an inherited-optic-neuropathy cohort.
- name: Reduced visual acuity
  category: Ophthalmological
  description: >-
    Visual loss is the presenting symptom in the milder cases, either slowly
    progressive or as a subacute LHON-like episode. Acuity is not a simple
    function of optic atrophy here: several individuals retained good acuity
    despite established atrophy, and one recovered from 6/120 to 6/6.
  phenotype_term:
    preferred_term: Reduced visual acuity
    term:
      id: HP:0007663
      label: Reduced visual acuity
  evidence:
  - reference: PMID:41725275
    reference_title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Initial symptoms started as hypotonia in infancy and proceeded with
      developmental delay, reduced visual acuity and chest pain.
    explanation: >-
      Documents reduced visual acuity in the course of an NSUN3-related case.
- name: Global developmental delay
  category: Neurological
  description: >-
    Developmental delay is characteristic of the early-onset, severe end of the
    spectrum - the genotypes with biallelic loss-of-function or SAM-domain
    missense alleles - and is absent from the isolated-optic-atrophy cases.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      developed mitochondrial disease symptoms at the age of three months
      (combined developmental disability, microcephaly, failure to thrive,
      recurrent increased lactate levels in plasma, muscular weakness, proximal
      accentuated, external ophthalmoplegia and convergence nystagmus)
    explanation: >-
      The founding case's presenting features, including developmental delay.
  - reference: PMID:32488845
    reference_title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      lactic acidosis, global developmental delay, hypotonia, muscle weakness and
      seizures
    explanation: >-
      Independent replication in a second, unrelated patient.
- name: Generalized hypotonia
  category: Neurological
  phenotype_term:
    preferred_term: Muscular hypotonia
    term:
      id: HP:0001290
      label: Generalized hypotonia
  evidence:
  - reference: PMID:32488845
    reference_title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      lactic acidosis, global developmental delay, hypotonia, muscle weakness and
      seizures
    explanation: >-
      Hypotonia among the presenting features at 4 months.
  - reference: PMID:41725275
    reference_title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Initial symptoms started as hypotonia in infancy
    explanation: >-
      Infantile hypotonia as the first manifestation in a third individual.
- name: Microcephaly
  category: Neurological
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      combined developmental disability, microcephaly, failure to thrive,
      recurrent increased lactate levels in plasma
    explanation: >-
      Microcephaly in the founding case.
- name: Failure to thrive
  category: Constitutional
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      combined developmental disability, microcephaly, failure to thrive,
      recurrent increased lactate levels in plasma
    explanation: >-
      Failure to thrive in the founding case.
- name: Increased circulating lactate concentration
  category: Metabolic
  description: >-
    Raised plasma lactate is the biochemical clue that prompts mitochondrial
    workup in the severe cases, and it was moderately elevated lactate that led
    to the muscle biopsy in the founding patient. It is not a reliable marker at
    the mild end: two affected members of one family with optic atrophy had
    normal lactate, so a normal value does not exclude the diagnosis.
  phenotype_term:
    preferred_term: Increased plasma lactate
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      recurrent increased lactate levels in plasma
    explanation: >-
      Raised lactate in the founding case.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both affected family members had blue-dot cataracts, normal lactate, but
      elevated homocysteine levels.
    explanation: >-
      Refutes raised lactate as a universal feature: the two affected members of
      this family had normal lactate. Recorded as a REFUTE item against the same
      claim rather than folded into the supporting text.
- name: Seizure
  category: Neurological
  phenotype_term:
    preferred_term: Seizures
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:32488845
    reference_title: "Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      lactic acidosis, global developmental delay, hypotonia, muscle weakness and
      seizures
    explanation: >-
      Seizures in the encephalomyopathy case that named them in its title.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Despite levetiracetam treatment, she continued to experience seizures about
      once every six months.
    explanation: >-
      An independent individual with recurrent seizures incompletely controlled
      on levetiracetam.
- name: Proximal muscle weakness
  category: Musculoskeletal
  phenotype_term:
    preferred_term: Proximally accentuated muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      muscular weakness, proximal accentuated, external ophthalmoplegia and
      convergence nystagmus
    explanation: >-
      Specifies the proximal accentuation of the weakness in the founding case.
- name: External ophthalmoplegia
  category: Ophthalmological
  phenotype_term:
    preferred_term: External ophthalmoplegia
    term:
      id: HP:0000544
      label: External ophthalmoplegia
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      muscular weakness, proximal accentuated, external ophthalmoplegia and
      convergence nystagmus
    explanation: >-
      External ophthalmoplegia in the founding case.
- name: Sensorineural hearing impairment
  category: Otological
  description: >-
    Hearing impairment is reported as mild where it occurs, in several
    individuals across two families, and in one family it was still under
    investigation at the time of publication.
  phenotype_term:
    preferred_term: Mild sensorineural hearing loss
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
    severity: MILD
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neurological assessment indicated generalized axonal sensorimotor
      neuropathy and mild sensorineural hearing loss.
    explanation: >-
      Specifies the sensorineural character and mild severity in the Family 2
      proband.
- name: Hypertrophic cardiomyopathy
  category: Cardiovascular
  description: >-
    Reported once, in an 18-year-old with concentric left ventricular
    hypertrophy up to 20 mm septal thickness and preserved ejection fraction,
    diagnosed at 15 years. The authors present it as a new feature of the
    disorder rather than an established one, and it rests on a single case, so it
    is curated here as a reported association and not as a characteristic
    feature. Cardiac abnormalities of a different kind - bicuspid aortic valve
    and atrial septal defect - were reported in one individual in the earlier
    cohort; those are structural malformations, not a cardiomyopathy, and are not
    merged into this phenotype.
  phenotype_term:
    preferred_term: Hypertrophic cardiomyopathy
    term:
      id: HP:0001639
      label: Hypertrophic cardiomyopathy
  evidence:
  - reference: PMID:41725275
    reference_title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      An 18-year-old girl was referred for visual loss, progressive fatigue,
      muscle weakness and HCM.
    explanation: >-
      The single reported individual with hypertrophic cardiomyopathy.
  - reference: PMID:41725275
    reference_title: "Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      To our knowledge, this is the first reported case of NSUN3-related
      mitochondrial disease with HCM.
    explanation: >-
      The authors' own statement that this is a single first report, which is why
      the association is recorded as provisional.
diagnosis:
- name: Exome or Genome Sequencing
  description: >-
    Every reported individual was solved by unbiased exome or genome sequencing,
    not by a targeted panel - in several cases only after an
    optic-neuropathy-specific panel had returned nothing. NSUN3 is absent from
    at least one published inherited-optic-neuropathy capture panel, so the
    practical recommendation from the largest cohort is to add the gene to
    diagnostic optic-atrophy and mitochondrial-disease panels and, failing that,
    to go to exome or genome sequencing.
  diagnosis_term:
    preferred_term: whole exome sequencing
    term:
      id: NCIT:C101295
      label: Whole Exome Sequencing
  markers: NSUN3 biallelic variants
  evidence:
  - reference: PMID:27356879
    reference_title: "Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we identified compound heterozygous predicted loss-of-function variants in
      NSUN3
    explanation: >-
      Exome sequencing as the route to the founding diagnosis.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the NSUN3 gene should be added to diagnostic optic atrophy and
      mitochondrial disease gene panels when investigating patients who are
      suspected to have an isolated or syndromic inherited optic neuropathy
    explanation: >-
      The cohort's own recommendation on where NSUN3 belongs in the diagnostic
      pathway.
- name: mt-tRNA-Met C34 Methylation Assay by RNA Bisulfite Sequencing
  description: >-
    This is the disease-specific functional test, and it is what converts a
    variant of uncertain significance into a diagnosis. RNA bisulfite sequencing
    measures methylation at the wobble base; in the largest cohort every affected
    individual read at background against a control range of 26 to 33 percent.
    It is practical to run: the assay worked on RNA from PAXgene-stabilised whole
    blood in five of six individuals, so a skin biopsy is not obligatory.

    It also has a documented limitation. Methylation was equally abolished in
    carriers of the mild C-terminal alleles as in the severe loss-of-function
    ones, so the assay confirms loss of the methyltransferase function but does
    not grade severity or predict phenotype - the authors take that dissociation
    as a hint that NSUN3 may have a second, unidentified function.
  diagnosis_term:
    preferred_term: RNA bisulfite sequencing assay of mt-tRNA-Met C34 methylation
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  markers: mt-tRNA-Met m5C34 methylation fraction
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All samples obtained from affected individuals demonstrated significantly
      reduced mitochondrial tRNAMet methylation levels that were reduced to
      background levels, although control samples showed 26% to 33% tRNAMet
      methylation
    explanation: >-
      The assay result and its control range, across every affected individual
      tested.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      However, it should be noted that the methylation at C34 was equally
      disrupted in carriers of these variants, suggesting that there might be a
      secondary function of NSUN3 that has not yet been identified.
    explanation: >-
      Records that the assay does not discriminate mild from severe genotypes.
- name: Muscle Biopsy with Respiratory Chain Enzyme Analysis
  description: >-
    Spectrophotometric respiratory chain enzyme assay on muscle homogenate,
    normalised to citrate synthase, is what established the combined deficiency
    in the founding case and is still the biochemical confirmation. Complex IV is
    the most consistently reduced, with complex I next; complex II, being
    entirely nuclear-encoded, is the internal control.

    The histology is a poor screening test. Ragged red fibres were absent in
    every reported biopsy, and COX-negative fibres were present in some
    individuals and absent in others within the same cohort, so a normal muscle
    histochemistry panel does not exclude COXPD48. In one individual with a
    milder phenotype the skin biopsy showed normal respiratory chain complex
    activity with reduced oxygen consumption, so tissue choice and the assay used
    both matter.
  diagnosis_term:
    preferred_term: muscle biopsy for respiratory chain enzyme analysis
    term:
      id: NCIT:C51895
      label: Muscle Biopsy
  markers: respiratory chain complex I-IV activities normalized to citrate synthase
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      spectrophotometric respiratory chain enzyme analysis demonstrated a complex
      I deficiency of 0.078 (normal range 0.118–0.332) and complex IV deficiency
      of 0.003 (normal range 0.013–0.039)
    explanation: >-
      A worked example of the assay with its reference ranges.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A skin biopsy obtained from individual 4 (Family 4: II-3) showed normal
      respiratory chain complex activity with reduced oxygen consumption
    explanation: >-
      Documents a normal enzyme panel alongside an abnormal respirometry result
      in the same individual, which is the caution against relying on one assay.
- name: Plasma Lactate
  description: >-
    Raised plasma lactate triggered the mitochondrial workup in the severe
    early-onset cases and remains a reasonable first-line test in that setting.
    It should not be used to rule the disorder out: two affected members of one
    family had normal lactate and presented with optic atrophy alone.
  diagnosis_term:
    preferred_term: plasma lactate measurement
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  markers: plasma lactate
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Because of moderately elevated plasma lactate, further investigation of an
      underlying mitochondriopathy was performed with a muscle biopsy, which
      revealed a combined OXPHOS deficiency.
    explanation: >-
      Shows lactate as the trigger for the confirmatory biopsy in the founding
      case.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both affected family members had blue-dot cataracts, normal lactate, but
      elevated homocysteine levels.
    explanation: >-
      Refutes the use of a normal lactate to exclude the diagnosis.
treatments:
- name: Supportive and Multidisciplinary Care
  description: >-
    No disease-modifying therapy exists. Management is the standard mitochondrial
    disease package - anticonvulsants for seizures, nutritional and
    developmental support, surveillance for the organ involvement each individual
    develops - and none of it addresses the tRNA modification defect. The
    breadth of the reported phenotype is itself a management point: an individual
    identified through an optic-atrophy pathway may still need endocrine,
    cardiac, audiological and neurological assessment.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: OTHER
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Despite levetiracetam treatment, she continued to experience seizures about
      once every six months.
    explanation: >-
      A worked example of symptomatic management and its incomplete effect.
- name: Idebenone
  description: >-
    One individual with a subacute LHON-like presentation received oral idebenone
    and recovered to 6/6 in both eyes within four months. That is a striking
    number, and it is why idebenone is worth recording here - but the same report
    makes clear it cannot be read as a treatment effect. Visual acuity in that
    individual had already begun improving spontaneously twelve months before
    idebenone was started, reaching 6/12 on its own; spontaneous recovery is
    well documented in childhood-onset LHON; and the authors say explicitly that
    they cannot separate genotype from treatment. This is curated as an
    uncontrolled single-patient observation, not as evidence of efficacy in
    COXPD48.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: idebenone
      term:
        id: CHEBI:31687
        label: idebenone
  therapeutic_modality: SMALL_MOLECULE
  target_mechanisms:
  - target: Retinal Ganglion Cell Bioenergetic Failure and Optic Nerve Degeneration
    treatment_effect: BYPASSES
    description: >-
      Idebenone is a short-chain quinone proposed to carry electrons to complex
      III, bypassing complex I and restoring ATP production in retinal ganglion
      cells. The link records the node the drug is given for, not a demonstrated
      effect in this disease: the single COXPD48 individual treated had already
      begun recovering spontaneously twelve months before idebenone was started,
      and the report's authors say they cannot separate genotype from treatment.
    evidence:
    - reference: PMID:40465263
      reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        Subsequently, oral treatment with idebenone was started, whereupon visual
        acuity was restored to 6/6 in both eyes after only four months.
      explanation: >-
        Establishes that idebenone was given for the optic-nerve phenotype this
        node describes. INDIRECT because the same report documents spontaneous
        improvement beginning before treatment, which is curated as a REFUTE item
        on this treatment, so the sentence cannot carry an efficacy claim.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Subsequently, oral treatment with idebenone was started, whereupon visual
      acuity was restored to 6/6 in both eyes after only four months.
    explanation: >-
      The observed outcome in the single treated individual.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "visual acuity began to improve spontaneously after 12 months and reached 6/12 after a further 18 months."
    explanation: >-
      Refutes attributing the recovery to idebenone: substantial spontaneous
      improvement preceded the drug in this same individual.
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      it is challenging to determine whether in this study this particular
      disease course is associated with a specific genotype or whether it is
      influenced by treatment with idebenone
    explanation: >-
      The authors' own statement that the observation is not attributable.
- name: Coenzyme Q10 (Ubiquinone) Supplementation
  description: >-
    Started in one individual on the basis of a measured secondary deficiency -
    muscle ubiquinone of 94 pmol/mg against a reference range of 140 to 850 -
    rather than empirically. No outcome from that supplementation is reported, so
    this records a rational indication for checking muscle ubiquinone in an
    NSUN3 patient having a biopsy, not a demonstrated benefit.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: coenzyme Q10
      term:
        id: CHEBI:46245
        label: coenzyme Q10
  therapeutic_modality: SMALL_MOLECULE
  target_mechanisms:
  - target: Combined Respiratory Chain Enzyme Deficiency
    treatment_effect: RESTORES
    description: >-
      Ubiquinone carries electrons from complexes I and II to complex III, so
      supplementing a measured secondary deficiency targets the combined
      respiratory chain defect directly. The link records the indication, not an
      outcome: no result of the supplementation is reported.
    evidence:
    - reference: PMID:40465263
      reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      snippet: >-
        Further analysis demonstrated low muscle ubiquinone of 94 pmol/mg (normal
        range 140-850 pmol/mg). Ubiquinone supplementation has been commenced.
      explanation: >-
        Documents the measured secondary ubiquinone deficiency and the
        supplementation started for it. INDIRECT because the source reports the
        indication without any outcome, so it cannot support an effect on the
        respiratory chain defect.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Further analysis demonstrated low muscle ubiquinone of 94 pmol/mg (normal
      range 140–850 pmol/mg). Ubiquinone supplementation has been commenced.
    explanation: >-
      The measured deficiency and the resulting intervention, with no outcome
      reported.
- name: Genetic Counseling
  description: >-
    Autosomal recessive counselling with a specific caveat. One proband reached
    homozygosity through paternal uniparental isodisomy of chromosome 3 rather
    than by descent, in a non-consanguineous family - so a homozygous NSUN3
    result does not establish that both parents are carriers, and recurrence risk
    should not be quoted as one in four until segregation has actually been
    checked. The same family also illustrates the counselling problem created by
    variable expressivity: the homozygous father had moderate bilateral optic
    atrophy while his son presented with subacute visual loss at eleven.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  therapeutic_modality: BEHAVIORAL
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Further analysis revealed paternal uniparental isodisomy of chromosome 3 in
      the proband's DNA sample.
    explanation: >-
      The isodisomy finding that changes the recurrence-risk conversation.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Roughly a dozen individuals have been reported. The largest series describes
    eight individuals from five unrelated families, of whom one had been
    published previously; separate single reports add a South Asian
    encephalomyopathy case, a Dutch optic-neuropathy case, and a Turkish case
    with hypertrophic cardiomyopathy. No population prevalence estimate exists,
    and ascertainment has been through inherited-optic-neuropathy and
    mitochondrial-disease cohorts rather than any unselected denominator, so no
    numeric rate is recorded here.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This study has characterized the largest cohort of patients harboring
      candidate biallelic NSUN3 variants to date, consisting of eight affected
      individuals from five independent families revealing a broad and variable
      phenotype.
    explanation: >-
      The size of the largest published series, which is the basis for the
      literature-case count rather than a prevalence estimate.
animal_models:
- name: Whole-body Nsun3 knockout mouse
  species: Mouse
  genotype: Nsun3 constitutive knockout (homozygous null)
  publication: PMID:36949224
  description: >-
    Constitutive Nsun3 deletion is embryonic lethal in the mouse: knockout
    embryos are alive with heartbeats at E10.5, somewhat smaller than wild-type,
    and dead by E12.5. This is a sharper phenotype than any human genotype
    produces - the founding patient carried two predicted null alleles and
    survived infancy - so the model bounds what the gene is required for rather
    than reproducing the disease.
  modeled_mechanisms:
  - target: Loss of Functional NSun3 Methyltransferase
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    model_scale: ORGANISM
    description: >-
      Complete Nsun3 loss in the mouse produces mid-gestation lethality, whereas
      humans homozygous or compound heterozygous for predicted null alleles are
      liveborn and reach at least infancy.
    limitations: >-
      The species divergence is the finding, not an artefact to be discounted:
      the model cannot be used to study postnatal NSUN3 disease at all, and any
      inference from it about human developmental requirements for NSUN3 has to
      cross that lethality gap. Human null-allele individuals are viable, so the
      mouse null is not a model of the human null state.
    evidence:
    - reference: PMID:36949224
      reference_title: "NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Here we show that Nsun3 is essential for embryonic development in mice
        with whole-body Nsun3 knockout embryos dying between E10.5 and E12.5.
      explanation: >-
        The lethality window, which is the divergence from the viable human null
        genotypes.
- name: Heart-specific Nsun3 knockout mouse (Nsun3HKO)
  species: Mouse
  genotype: Cardiac-restricted Nsun3 conditional knockout
  publication: PMID:36949224
  description: >-
    Conditional cardiac deletion sidesteps the embryonic lethality and gives the
    only in vivo model of adult NSUN3 loss. Nsun3HKO hearts show enlarged
    mitochondria with fragmented cristae and progressively reduced respiratory
    complex enzyme activities, worse in older animals, while the mitochondrial
    mRNAs encoding those subunits are not reduced - the same
    transcript-versus-activity dissociation seen in patient cells, and a
    signature of a translation-level rather than a transcription-level lesion.

    The cardiac phenotype itself is enhanced contraction with age-associated
    mild enlargement, which is not the same as the hypertrophic cardiomyopathy
    reported in one human patient. The model is linked to the respiratory chain
    node it actually measures, not to the human cardiac phenotype.
  modeled_mechanisms:
  - target: Combined Respiratory Chain Enzyme Deficiency
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    model_scale: TISSUE
    description: >-
      Cardiac Nsun3 loss reduces respiratory complex enzyme activities without
      reducing the corresponding mitochondrial transcripts, reproducing the
      translation-level character of the human biochemical defect in a single
      organ.
    limitations: >-
      Restricted to heart, so it says nothing about the optic nerve, which is the
      most consistently affected human tissue. The deficit is age-progressive in
      the mouse whereas the severe human phenotype is present in infancy, and the
      mouse cardiac phenotype (enhanced contraction, mild enlargement) is not the
      hypertrophic cardiomyopathy reported once in humans.
    readouts:
    - name: Respiratory complex enzymatic activities in heart
      target: Combined Respiratory Chain Enzyme Deficiency
      direction: DECREASED
      interpretation: >-
        Enzyme-activity correlate of the combined respiratory chain deficiency
        node, measured in the conditional knockout heart.
      evidence:
      - reference: PMID:36949224
        reference_title: "NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          In the Nsun3HKO hearts, mitochondrial mRNAs that encode respiratory
          complex subunits were not down regulated, but the enzymatic activities
          of the respiratory complexes decreased, especially in older mice.
        explanation: >-
          Reports the direction of the enzyme-activity change and the preserved
          transcript levels alongside it.
    evidence:
    - reference: PMID:36949224
      reference_title: "NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Nsun3HKO heart mitochondria were enlarged and contained fragmented
        cristae.
      explanation: >-
        Ultrastructural mitochondrial abnormality in the model, comparable to the
        swollen mitochondria with linearised cristae seen in patient muscle.
differential_diagnoses:
- name: Leber hereditary optic neuropathy
  description: >-
    The closest mimic, and a documented one: two individuals with biallelic
    NSUN3 variants presented with a subacute LHON-like picture - hyperaemic disc
    swelling, peripapillary telangiectatic microangiopathy, central scotomas -
    and one was investigated for the three common mtDNA LHON variants and had
    whole-mtDNA sequencing before exome sequencing found NSUN3. Inheritance is
    the discriminator: LHON is maternally transmitted mtDNA disease, COXPD48 is
    autosomal recessive nuclear disease, so a negative mtDNA result in a
    LHON-like presentation with affected siblings or consanguinity should prompt
    nuclear sequencing.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Because the clinical picture was suggestive of LHON, genetic testing for
      the three most common LHON mtDNA variants followed by whole mtDNA
      sequencing was performed.
    explanation: >-
      Documents that the LHON workup was undertaken and negative before NSUN3 was
      found.
- name: Autosomal dominant optic atrophy (OPA1-related)
  description: >-
    Named by the cohort authors as the other common mitochondrial optic
    neuropathy against which NSUN3 disease has to be distinguished. Both can show
    relatively preserved central acuity despite temporal optic atrophy, and
    OPA1-related disease can carry a subclinical mitochondrial myopathy on
    biopsy. Inheritance again separates them: OPA1 disease is dominant.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The two most common mitochondrial optic neuropathies are autosomal dominant
      optic atrophy associated with pathogenic variants in OPA1 and LHON with
      three mtDNA variants
    explanation: >-
      Identifies the two conditions the differential is drawn against.
- name: Other combined oxidative phosphorylation deficiencies
  description: >-
    The COXPD series numbers well over fifty entities across largely unrelated
    nuclear genes, and they are clinically overlapping enough that a combined
    respiratory chain deficiency on muscle biopsy does not point to any one of
    them. The distinguishing features of COXPD48 are the near-universal optic
    nerve involvement, the very wide onset range, and the specific functional
    assay - loss of mt-tRNA-Met C34 methylation - which is diagnostic for this
    gene and normal in the others.
  evidence:
  - reference: PMID:40465263
    reference_title: "Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The majority of these disorders have an overlapping clinical phenotype,
      necessitating a broad approach to molecular diagnostic testing, typically
      including full sequencing of mtDNA and use of large virtual gene panels.
    explanation: >-
      States the clinical overlap across primary mitochondrial disorders that
      makes molecular testing rather than phenotype the discriminator. This is a
      class-level statement about primary mitochondrial disease, not a claim
      about NSUN3 specifically.
discussions:
- discussion_id: nsun3_methylation_assay_does_not_grade_severity
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why is mt-tRNA-Met C34 methylation abolished equally by mild C-terminal
    NSUN3 alleles and by severe loss-of-function alleles, when the two genotype
    classes produce phenotypes as far apart as isolated adult optic atrophy and
    neonatal multisystem disease?
  attaches_to:
  - pathophysiology#Loss of m5C34 in Mitochondrial tRNA-Met
  - genetic#NSUN3
  rationale: >-
    The functional assay used to call NSUN3 variants pathogenic reads at
    background in every affected individual regardless of allele class, so it
    cannot be the quantity that sets severity. Either the assay saturates below
    the range that matters clinically, or some residual NSUN3 activity not
    captured by bulk bisulfite sequencing differs between genotypes, or - as the
    cohort authors suggest - NSUN3 has a second function that the C-terminal
    alleles spare and the null alleles do not. The three possibilities have
    different consequences for whether the assay can ever be prognostic, and
    nothing published distinguishes them.
- discussion_id: nsun3_mouse_null_lethality_vs_human_viability
  kind: HUMAN_MODEL_MISMATCH
  prompt: >-
    Can the mouse tell us anything about postnatal NSUN3 disease, given that
    constitutive Nsun3 loss kills the embryo by E12.5 while humans with two
    predicted null alleles are liveborn and survive infancy?
  attaches_to:
  - animal_models#Whole-body Nsun3 knockout mouse
  - pathophysiology#Loss of Functional NSun3 Methyltransferase
  rationale: >-
    This is a mismatch of the informative kind: evidence exists in the model and
    contradicts the human course rather than merely failing to reproduce it. The
    conditional cardiac knockout is the workaround, and it does reproduce the
    key biochemical signature - enzyme activities fall while the transcripts
    encoding those subunits do not - but it is confined to heart, which is the
    organ least involved in human NSUN3 disease, and it says nothing about the
    retinal ganglion cell, which is the organ most involved. There is no reported
    model of the optic neuropathy that defines the human disorder.
- discussion_id: nsun3_complex_iii_and_v_sparing
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why are complexes III and V spared in NSUN3 patient muscle when both contain
    mtDNA-encoded subunits that a general methionine-decoding defect should
    affect?
  attaches_to:
  - pathophysiology#Combined Respiratory Chain Enzyme Deficiency
  rationale: >-
    A defect in decoding methionine codons should degrade synthesis of all
    thirteen mtDNA-encoded proteins, and RNAi work in cell lines found exactly
    that. Yet in patient muscle the measurable deficit falls on complexes IV and
    I, with complex III and V protein levels reported as unchanged in one family
    and complex II - the nuclear-encoded internal control - unaffected as
    expected. Whether this reflects differing subunit half-lives, differing
    assembly-intermediate stability, differing sensitivity of the assays, or a
    real difference in translational sensitivity between messages is untested,
    and it matters because complex IV is what the clinical biochemistry actually
    measures.
notes: >-
  Identifiers. MONDO:0033566; OMIM:619012 (COMBINED OXIDATIVE PHOSPHORYLATION
  DEFICIENCY 48; COXPD48), recorded here because the schema's mappings class
  has no OMIM slot. MONDO cross-references OMIM:619012 as an exact match. The
  causal gene is NSUN3, hgnc:26208; NSUN3 itself has OMIM gene entry 617491.

  Why this entry exists as a separate disease rather than a subtype. The MONDO
  parent is MONDO:0000732, combined oxidative phosphorylation deficiency, which
  is a numbered series across dozens of unrelated nuclear genes. Each member is
  a distinct gene-disease entity with its own mechanism, and NSUN3's is a
  distinct step: a tRNA wobble modification, upstream of every mtDNA-encoded
  subunit, rather than a structural subunit or an assembly factor. That is the
  reason to curate it separately, and it is recorded against the CURIE rather
  than against any claim about what other files in this repository do or do not
  contain.

  Grouping membership. No existing grouping fits. The
  Mitochondrial_Complex_IV_Deficiency grouping is anchored to MONDO:0033885,
  mitochondrial complex IV deficiency nuclear-type - an isolated, not a combined,
  deficiency - and its criteria turn on conformance to the
  complex_iv_assembly_deficiency module, which scopes itself to structural
  subunits, assembly and maturation factors, copper metallochaperones and heme A
  biosynthesis. NSUN3 is none of those, and its biochemical phenotype is combined
  rather than isolated even though complex IV is the most consistently affected.
  There is no combined-OXPHOS or mitochondrial-translation grouping in
  kb/groupings/ at the time of writing. A mitochondrial-translation-defect
  grouping would be a coherent future proposal - the mt-tRNA charging,
  modification and translation-factor entities share a mechanism in a way the
  COXPD numbering does not - but it needs its own membership survey and criteria,
  and is deliberately not created here as a side effect of one curation.

  Module conformance. One node conforms:
  mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress, and only
  partly, for the reasons recorded in that node's own conformance note.
  Conformance to complex_iv_assembly_deficiency was considered and declined for
  the scope reason above; note that the complex IV grouping does admit one
  translation-level member (TACO1), but TACO1 is a COX-specific translational
  activator whose lesion is confined to complex IV, whereas NSUN3 affects the
  translation of all thirteen mtDNA-encoded proteins. No module in kb/modules/
  currently represents mt-tRNA modification or mitochondrial translation, so the
  upstream four nodes of this entry's chain have no conformance target; that
  absence, rather than a judgement about fit, is why they carry none.

  Attribution cautions carried from the sources. The founding report established
  that f5C34 is lost when m5C34 is lost but explicitly did not identify the
  dioxygenase responsible; ALKBH1 was named in a separate 2016 study, and this
  entry cites that study for the ALKBH1 step rather than attributing it to the
  patient report. Sideroblastic anemia is reported in one individual in the
  five-family cohort but is deliberately not curated as a phenotype here: the
  authors state that it may not be related to the NSUN3 variant and that a
  homozygous loss-of-function variant in a heme-synthesis gene was found in the
  same patient. An early suggestion that exon 3 is a mutational hotspot came from
  a report predating the larger cohort, which found variants across the whole
  gene; the hotspot claim is recorded as superseded rather than repeated. And in
  the cohort paper, a figure legend states that affected individuals from
  Families 1, 2 and 3 displayed COX-negative fibres while the results and
  discussion text both say the Family 1 and Family 2 biopsies did not; the
  per-individual text is quoted here in preference to the legend.

  Named-entity hazards for future curators. NSUN2, NSUN5, NSUN6 and NSUN7 have
  substantial literatures of their own - NSUN2 in particular in intellectual
  disability - and none of it is evidence about NSUN3. ALKBH1/ABH1 has a large
  separate literature as a DNA repair and demethylase enzyme that is not about
  mitochondrial translation. The m5C epitranscriptomics field is mostly
  cytoplasmic; only the mitochondrial tRNA-Met wobble work bears on this
  disorder. NSUN3 also appears in a growing oncology and inflammation literature
  (hepatocellular carcinoma, oral squamous cell carcinoma, sepsis, osteoarthritis)
  that reports m5C effects on nuclear-encoded transcripts and has no bearing on
  this Mendelian entity. And a statement about "combined oxidative phosphorylation
  deficiency" without a gene named is a class-level statement about dozens of
  genes; the one such statement used in this entry is graded and labelled as
  class-level in its explanation.

  Not curated, for want of a source. No natural history, survival, or
  age-of-death data are reported in aggregate; the cohort mentions "possible
  limited life expectancy" without figures, so no progression block is curated.
  No clinical trial has been registered for this disorder. GEO accession
  GSE66012 and EGA study EGAS00001000164 are cited by the founding report but
  are not curated as datasets here because their disease relevance was not
  verified in this session.
📚

References & Deep Research

References

8
PMID:27356879
Deficient methylation and formylation of mt-tRNA(Met) wobble cytosine in a patient carrying mutations in NSUN3.
No top-level findings curated for this source.
PMID:40465263
Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder.
No top-level findings curated for this source.
PMID:32488845
Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures.
No top-level findings curated for this source.
PMID:38790159
Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy.
No top-level findings curated for this source.
PMID:41725275
Hypertrophic cardiomyopathy as a novel phenotypic feature of NSUN3-related mitochondrial disease: a case report with review of the literature.
No top-level findings curated for this source.
PMID:27497299
NSUN3 and ABH1 modify the wobble position of mt-tRNAMet to expand codon recognition in mitochondrial translation.
No top-level findings curated for this source.
PMID:27214402
NSUN3 methylase initiates 5-formylcytidine biogenesis in human mitochondrial tRNA(Met).
No top-level findings curated for this source.
PMID:36949224
NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice.
No top-level findings curated for this source.

Deep Research

1

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

Evaluations and curation notes (2)

Review round 2: curate the three missing phenotypes and take all three suggestions · 2026-09-08T17:13:40Z · View source

Answers the blocking phenotype-coverage finding and all three blue-tier suggestions in one push. BLOCKING - three recurrent phenotypes were documented in the cached sources and already quoted inside this file, but not curated. All three are now curated and wired into the pathograph so the entry does not regain floating nodes. Blue-dot cataract, HP:0000518 Cataract with the specificity in preferred_term. Four of roughly eleven published individuals across two unrelated families. HP:0007976 Cerulean cataract is the term whose definition most closely matches the colloquial name, and it is deliberately NOT used: that definition asserts congenital onset and concentric peripheral layering, neither of which the sources state, so binding it would manufacture detail. Three evidence items, including the authors' negative cataract-panel result graded INDIRECT - it shows they looked for a separate cause and found none, which is why the finding is curated here rather than treated as an unrelated comorbidity, but it does not demonstrate NSUN3 causation. Nystagmus, HP:0000639. Two unrelated individuals with different character (convergence in the index case, rotatory bilateral in the 2024 report), so the general term is the honest binding rather than a character-specific child. Generalized axonal sensorimotor neuropathy, HP:0007141 Sensorimotor neuropathy. HPO has no term combining the axonal and sensorimotor characters - a real choice between them, recorded in the phenotype notes. HP:0007141 is bound because sensorimotor is the distinction the source draws; HP:0003477 Peripheral axonal neuropathy is the defensible alternative. The deep-research report's HP:0007002 Motor axonal neuropathy is wrong on its face, since the source says sensorimotor, and is not used. Every CURIE was resolved against ols:hp before binding rather than taken from the review. Pathograph wiring for the three: cataract and neuropathy from Combined Respiratory Chain Enzyme Deficiency, nystagmus from the optic-nerve node it is reported alongside. All INDIRECT_UNKNOWN_INTERMEDIATES with directness INDIRECT, matching the style established in the previous commit. The cataract edge explanation says plainly that the lens is avascular and glycolysis-dependent, so the route from a respiratory chain defect to lens opacity is not obvious and the source proposes none; it is recorded as an unexplained association rather than a demonstrated step. The nystagmus edge says the sources do not settle whether it is sensory, secondary to early visual loss, or independent brainstem or cerebellar involvement. SUGGESTION 1 - target_mechanisms added to both drug treatments, which were the last disconnected section. Idebenone links to the optic-nerve node with treatment_effect BYPASSES, its defining pharmacology (short-chain quinone carrying electrons to complex III, bypassing complex I). CoQ10 links to the respiratory chain node with RESTORES, replacing a measured secondary deficiency. Both evidence items are INDIRECT and both descriptions state that the link records the indication, not an outcome: the idebenone individual had begun recovering spontaneously twelve months before treatment, which this entry already carries as a REFUTE item, and no outcome of the CoQ10 supplementation is reported. SUGGESTION 2 - GO:0002097 tRNA wobble base modification removed from node 4. The node's claim is about decoding; modification is already carried by nodes 2 and 3, so the binding restated the upstream claim rather than describing this node. GO has no clean wobble-decoding term, so biological_processes is omitted rather than a misleading term bound, per the terms contract. SUGGESTION 3 - therapeutic_modality on Supportive and Multidisciplinary Care changed BEHAVIORAL to OTHER. That treatment bundles anticonvulsants, nutritional support and multi-organ surveillance, which BEHAVIORAL is narrower than. Genetic Counseling keeps BEHAVIORAL, the majority convention. Validation: just validate-disorders passes, 87/87 snippets verified, up from 76/76. All 15 phenotypes have an incoming edge. check-entity-refs, check-causal-targets, check-duplicate-keys, check-enum-values, check-qualifier-terms clean. Zero snippets carrying two evidence_source values. No new references fetched and no new ontology terms outside the three verified HPO bindings. Process note: the round-1 review posted at 13:29 UTC and I did not see it until 17:10, because at two intervening check-ins I fetched check runs but not reviews for this PR. A comment I posted at 16:15 claimed the review had not run. That was wrong and is corrected on the PR.

Create: combined oxidative phosphorylation deficiency 48 (COXPD48, NSUN3) · 2026-09-08T08:16:35Z · View source

De novo curation of COXPD48 (MONDO:0033566, OMIM:619012, hgnc:26208 NSUN3) as a standalone Disease entry. Preflight re-derived against origin/main at ec5a66d1e3: MONDO:0033566 bound in zero files under kb/, hgnc:26208 absent, NSUN3 appearing only as a row in an unrelated Wilson disease Biomni gene-results TSV, no open PR. OMIM 619012 confirmed independently three ways (MONDO xref, literature, DR preflight). The curation is built around the mechanistic point that distinguishes this entity from the rest of the combined-OXPHOS series: NSUN3 is neither a respiratory chain subunit nor an assembly factor but an mt-tRNA wobble methyltransferase, so the lesion is a translation-level modification defect upstream of all thirteen mtDNA-encoded proteins. The pathophysiology chain is eight nodes, four of them molecular (enzyme loss, loss of m5C34, absence of f5C34, impaired wobble decoding) before reaching translation, combined respiratory chain deficiency, and the retinal ganglion cell and skeletal muscle tissue nodes. Sources: Van Haute 2016 Nat Commun (PMID:27356879, founding case), Jurkute 2025 IOVS (PMID:40465263, eight individuals from five families, the largest cohort), Paramasivam 2020 J Mol Neurosci (PMID:32488845), a 2024 Genes optic neuropathy case (PMID:38790159), a 2026 hypertrophic cardiomyopathy case report (PMID:41725275), mechanism work from Haag 2016 EMBO J (PMID:27497299) and Nakano 2016 Nat Chem Biol (PMID:27214402), and the Nsun3 mouse knockout study (PMID:36949224). Deep research was run with the claude_code provider; its report resolved 11/11 references but offered UBERON:0001780 named 'Optic nerve' when UBERON calls it spinal nerve, and an obsolete CHEBI:5991 named 'Idebenone'. Neither was used; every term in this entry was verified independently with runoak against the local builds, and NCIT diagnosis terms were additionally checked for reachability from NCIT:C25218. Evidence discipline: 68 evidence items, all snippets extracted programmatically and confirmed as exact substrings of the cached references before validation; several were trimmed to avoid non-breaking spaces and non-breaking hyphens introduced by full-text extraction. Two REFUTE items are curated deliberately - a family with normal lactate against the raised-lactate phenotype, and the documented spontaneous visual recovery preceding idebenone against attributing that recovery to the drug. Sideroblastic anemia was deliberately not curated because the cohort authors attribute it to a separate heme-synthesis gene variant in the same patient. Where the cohort paper's figure legend and its per-individual text disagree about COX-negative fibers, the narrower per-individual text is quoted. Module conformance: one node conforms to mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress, and only partly - the module's reactive oxygen species arm is not evidenced in any NSUN3 report and is not asserted, and the module's age-related mtDNA damage node does not apply to a primary nuclear modification defect. Conformance to complex_iv_assembly_deficiency was considered and declined because that module scopes itself to subunit, assembly, copper and heme A defects. Grouping membership: no existing grouping fits; the Mitochondrial_Complex_IV_Deficiency grouping is anchored to an isolated rather than a combined deficiency, and no mitochondrial-translation grouping exists. Recorded in notes with the reasoning rather than left open. Validation run to completion: just validate (68/68 snippets verified, term validation passed), validate-terms, count-verified-snippets, check-duplicate-keys, check-entity-refs, check-causal-targets, check-qualifier-terms, check-enum-values, check-snippet-length, check-title-snippets, check-snippet-grading, check-folded-hyphens, check-source-defect-claims, and validate-disorders. The curation stub was deleted.

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Combined Oxidative Phosphorylation Deficiency 48 (COXPD48) — Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 10 citations 2026-09-08T08:00:13.861588

Combined Oxidative Phosphorylation Deficiency 48 (COXPD48) — Research Report

1. Disease Information

Overview: COXPD48 is an autosomal recessive mitochondrial translation disorder caused by biallelic loss-of-function or hypomorphic variants in NSUN3 (NOP2/Sun RNA methyltransferase family member 3), which encodes the enzyme that installs 5-methylcytosine (m⁵C) at the wobble position (C34) of mitochondrially-encoded tRNA-Met (mt-tRNA^Met). This methylation is the obligate first step toward 5-formylcytosine (f⁵C34) formation (catalyzed downstream by ALKBH1/ABH1), which is required for mt-tRNA^Met to decode both AUG and the non-canonical AUA codon during mitochondrial translation. Loss of this modification causes a combined (multi-complex) OXPHOS deficiency due to impaired mitochondrial protein synthesis, producing a disease that — as characterized by the founding 2016 case and subsequent reports — spans from an isolated, adult-onset optic atrophy phenotype to a severe, early-onset multisystem mitochondrial encephalomyopathy (Van Haute et al., Nat Commun 2016, PMID:27356879; Jurkute et al., IOVS 2025, PMID:40465263).

Key identifiers: - OMIM disease: #619012 — COMBINED OXIDATIVE PHOSPHORYLATION DEFICIENCY 48; COXPD48 - OMIM gene: 617491 — NOP2/SUN RNA METHYLTRANSFERASE FAMILY, MEMBER 3; NSUN3 - HGNC: NSUN3, HGNC:26208 - MONDO: MONDO:0033566 - Gene location: 3q11.2 - Inheritance: Autosomal recessive - Category (broader MONDO grouping):* combined oxidative phosphorylation deficiency (a genetically heterogeneous group; COXPD1 through COXPD5x+, each numbered entry defined by a distinct causal gene)

Synonyms: "NSUN3-related mitochondrial disease," "NSUN3 deficiency," and — for the milder end of the spectrum — "NSUN3-related optic neuropathy" / "isolated optic atrophy due to NSUN3 variants" (Jurkute et al. 2025 explicitly reframe this as a phenotypic continuum rather than distinct diseases).

Evidence base type: The entirety of the clinical literature is individual patient/case-series data (case reports and small multi-family case series totaling ~10 published individuals worldwide as of 2025), not aggregated registry or EHR-derived statistics — there is no disease registry, and Orphanet/GeneReviews do not yet carry a dedicated COXPD48 entry independent of OMIM. This is an ultra-rare, essentially "n-of-few" disorder.

2. Etiology

Disease causal factor: Purely genetic — biallelic (homozygous or compound heterozygous) pathogenic variants in NSUN3. No environmental, infectious, or acquired etiology has been implicated in the primary disease process, though intercurrent infection has been reported as a seizure trigger in at least one severe case (norovirus infection preceding status epilepticus in a 4-year-old girl; Jurkute et al. 2025, PMID:40465263).

Genetic risk factors: - Biallelic NSUN3 variants are necessary and (to current knowledge) sufficient to cause disease; no polygenic or susceptibility-locus component has been described. - Variant-severity correlation is the dominant modifier identified to date: complete loss-of-function alleles (nonsense, frameshift, canonical splice-site) are associated with the severe, early-onset multisystem phenotype, while missense variants retaining partial activity (particularly those outside the catalytic SAM-binding domain) are associated with the milder, later-onset isolated optic atrophy phenotype. Jurkute et al. state explicitly: "Loss of function variants were associated with a more severe phenotype" (PMID:40465263). Compare: - Severe: c.123-615_466+2155del (p.Glu42Valfs11) / c.295C>T (p.Arg99) — compound het null alleles, infantile-onset encephalomyopathy (Van Haute 2016) - Severe: c.150G>A (p.Trp50) homozygous — infantile-onset (Jurkute Family 4) - Mild: c.812A>G (p.Glu271Gly) homozygous, downstream of the SAM domain — isolated adolescent-onset optic atrophy (Jurkute Family 1) - Consanguinity is a recurrent feature: reported in the Van Haute index family, the Paramasivam case (consanguineous East Asian parents), and Jurkute Family 1 (consanguineous Pakistani parents) and Family 3, consistent with a rare AR disorder ascertained disproportionately in consanguineous pedigrees. - One case (Jurkute Family 1, proband III-2) carried paternal uniparental disomy of chromosome 3, which converted a heterozygous paternal frameshift variant to apparent homozygosity — a mechanism worth flagging for any curated genetic_context/variant_origin annotation, since the mutational mechanism (UPD rather than biparental transmission of two mutant alleles) differs from ordinary autosomal recessive segregation. - No modifier genes have been reported. One severe-phenotype individual (Jurkute Family 2, II-1) also carried an independent homozygous LOF variant in a heme-synthesis gene, invoked to explain concurrent sideroblastic anemia as a separate, non-NSUN3 comorbidity* rather than a true genetic modifier of the mitochondrial phenotype.

Environmental/other risk factors: None established as causal. No described protective genetic or environmental factors (the gene is too rare for GWAS-type protective-variant discovery).

Gene-environment interaction: Not systematically studied; the single reported infection-triggered seizure exacerbation (norovirus) suggests that intercurrent catabolic/febrile stress may unmask or worsen the mitochondrial translation defect, as is well established for other combined OXPHOS deficiencies generally, but this has not been specifically investigated for NSUN3 disease.

3. Phenotypes

The phenotype is best organized along the severity spectrum identified by Jurkute et al. 2025 (8 patients / 5 families), integrated with the two prior single-patient reports.

Ophthalmological (near-universal; present in 7/8 of the 2025 cohort, 87.5%)

  • Bilateral optic atrophy / optic neuropathy — HP:0000648 (Optic atrophy)
  • Onset ranges from infancy (nystagmus/optic atrophy noted <3 months in the index Van Haute patient) to the fourth decade (age 40, Family 3 father)
  • Progressive visual acuity decline is typical; some patients show a LHON-like acute/subacute presentation with disc edema, telangiectatic peripapillary microangiopathy, and subsequent partial spontaneous recovery — distinct from the classically insidious optic atrophy of most mitochondrial optic neuropathies
  • Nystagmus (convergence nystagmus in the index case; rotatory nystagmus in the optic-neuropathy case) — HP:0000639
  • Cataract ("blue dot" / cerulean cataract) recurring across multiple family members — HP:0010696
  • Color vision deficits (Ishihara/D-15 panel abnormalities) — HP:0000551 (Impaired color vision)
  • Central scotomas — HP:0000618
  • OCT: reduced peripapillary RNFL and ganglion cell layer thickness
  • Electrophysiology: delayed/subnormal pattern VEP, abnormal PERG, and in the severe multisystem cases, generalized rod/cone ERG dysfunction (electronegative ERG), indicating a combined optic-nerve and outer/inner retinal process in the more severe end of the spectrum

Neurological (severe/early-onset end of spectrum)

  • Global developmental delay — HP:0001263
  • Microcephaly (low-grade to overt) — HP:0000252
  • Muscular hypotonia — HP:0001252
  • Muscle weakness, proximal-accentuated — HP:0001324 / HP:0003701
  • External ophthalmoplegia — HP:0000544
  • Seizures, generalized, recurrent, drug-refractory (levetiracetam-resistant in one case) — HP:0001250
  • Peripheral neuropathy (generalized axonal sensorimotor) — HP:0007002
  • Sensorineural hearing loss, mild — HP:0000407
  • Dysarthria — HP:0001260

Growth/systemic

  • Failure to thrive / inadequate weight gain — HP:0001508
  • Small for gestational age — HP:0001518
  • Growth hormone deficiency, insulin resistance, borderline thyroid dysfunction (one severe case)
  • Cardiac malformation (bicuspid aortic valve, atrial septal defect) in one severe case — HP:0001647 / HP:0001631

Laboratory/metabolic

  • Elevated plasma lactate / lactic acidosis — HP:0002151 / HP:0003128 (present across essentially all early-onset cases; a hallmark biochemical abnormality)
  • Hypoglycemia, hypernatremia, and severe metabolic acidosis during an acute seizure episode in one case
  • Combined OXPHOS deficiency on muscle biopsy — reduced activity of Complex I and Complex IV predominantly (Complex II/III typically normal — consistent with the exclusively mtDNA-encoded-subunit-dependent complexes being selectively vulnerable to a translation defect)

Muscle histopathology

  • Variable: ranges from no COX-negative fibers / no ragged-red fibers (Family 1, milder phenotype) to multiple COX-negative fibers with subsarcolemmal mitochondrial aggregates and swollen mitochondria with massively reduced/linearized cristae on EM (Family 3, more severe optic-neuropathy-with-neuropathy phenotype). This heterogeneity itself is notable and should be curated per-subtype rather than as a single canonical histopathology finding.

Onset/severity/progression summary

  • Age of onset: ranges from neonatal/infantile (<3 months, most severe) to the fourth decade (isolated optic atrophy, mildest) — mean ~17 years (SD ±12) across the 2025 cohort, reflecting strong bimodal clustering rather than a true continuous distribution
  • Severity: frankly variable and appears to track genotype (LOF vs. hypomorphic missense) as above
  • Progression: Chronic/progressive for the optic neuropathy component in essentially all patients; the encephalomyopathic/seizure component in severe cases is also progressive, punctuated by infection-triggered exacerbations
  • Quality of life impact: Not formally measured with any validated instrument (EQ-5D/SF-36) in any published report; qualitatively, severe cases show substantial functional impairment (developmental delay, refractory seizures) while the isolated optic atrophy cases retain ambulatory independence with visual impairment as the dominant burden.

4. Genetic/Molecular Information

Causal gene: NSUN3 (HGNC:26208; NCBI Gene ID 63899; OMIM *617491), chromosome 3q11.2. Encodes a mitochondrially-targeted member of the NOP2/Sun RNA methyltransferase family.

Reported pathogenic/likely pathogenic variants (cDNA reference NM_022072):

Variant (cDNA) Protein Zygosity Type Source
c.123-615_466+2155del p.Glu42Valfs*11 compound het large deletion/frameshift (exon 3) Van Haute 2016, PMID:27356879
c.295C>T p.Arg99* compound het (with above) nonsense Van Haute 2016
c.421G>C p.Ala141Pro compound het missense (exon 3) Paramasivam 2020, PMID:32488845
c.454T>A p.Cys152Ser compound het (with above) missense (exon 3) Paramasivam 2020
c.349_352dup p.Ala118Glufs*45 homozygous frameshift (exon 3) PMID:38790159 (isolated optic atrophy)
c.812A>G p.Glu271Gly homozygous missense, downstream of SAM domain Jurkute 2025, PMID:40465263 (VUS; mild phenotype)
c.424C>T p.Pro142Ser homozygous missense, within SAM domain Jurkute 2025 (likely pathogenic; severe phenotype)
c.930_931delAT p.Cys311Trpfs*8 "homozygous" via paternal UPD3 frameshift (terminal exon) Jurkute 2025
c.150G>A p.Trp50* homozygous nonsense Jurkute 2025 (pathogenic; severe phenotype)

ACMG/AMP classification: Ranges from VUS to pathogenic across the reported alleles (see table); loss-of-function alleles are more consistently classified pathogenic/likely pathogenic (PVS1-triggering), while several missense alleles remain VUS pending further functional or population data — an important curation caveat, since not every reported variant meets a confident pathogenicity bar.

Allele frequency: NSUN3 is not a commonly studied gene in population databases; no specific gnomAD constraint metrics (pLI/LOEUF) were retrievable in this search session, and given the rarity of reported disease alleles, population frequency data for the specific pathogenic variants is expected to be at or near absent from gnomAD — this should be verified directly against gnomAD/ClinVar at curation time rather than assumed.

Functional consequence: All functionally characterized alleles converge on loss of NSun3-catalyzed m⁵C34 methylation of mt-tRNA^Met, quantified in the 2025 cohort as tRNA^Met methylation reduced "to background levels" versus 26–33% methylation in controls — interpreted by the authors as evidence of complete functional loss even for some missense alleles. Downstream consequences demonstrated across the case reports: - Absent or markedly reduced full-length NSun3 protein (patient fibroblasts, Van Haute 2016) - Defective mitochondrial translation / protein synthesis - Reduced steady-state levels and enzymatic activity of Complex I and Complex IV (the OXPHOS complexes with the largest mtDNA-encoded subunit content) - Reduced basal oxygen consumption rate (OCR), reduced ATP-linked OCR, and reduced maximal (FCCP-uncoupled) respiration on Seahorse extracellular flux assay in patient fibroblasts - Impaired growth under galactose-forcing conditions (a classic functional readout of OXPHOS-dependent, as opposed to glycolysis-dependent, cellular energy metabolism) - Rescue of the translation defect by re-expression of wild-type NSUN3 (Van Haute 2016) — establishing causality

Chromosomal abnormalities: Not a feature of this disease (single-gene point/small-indel mechanism); the one structural exception is the large 3,114 bp intragenic deletion in the index patient (c.123-615_466+2155del), and the reported paternal UPD3 event, which is a mechanism of homozygosity rather than a pathogenic chromosomal rearrangement per se.

Epigenetic information: Not directly studied for NSUN3 disease; conceptually notable that NSUN3 itself is an RNA-modifying (epitranscriptomic) enzyme, so the entire disease mechanism is, in a sense, a defect of RNA epitranscriptomic modification rather than DNA epigenetics — worth flagging in pathophysiology framing but distinct from DNA methylation/chromatin biology.

5. Environmental Information

No environmental toxins, occupational exposures, radiation, or lifestyle factors have been implicated as disease-causal — this is a monogenic mitochondrial translation disorder. The one environment-adjacent finding is infection as a symptom trigger/exacerbant: norovirus gastroenteritis preceded a seizure exacerbation with T2-hyperintense parieto-occipital lesions on MRI in one severe-phenotype patient (Jurkute 2025, Family 4). This is analogous to the well-documented pattern in other mitochondrial disorders where febrile/catabolic illness precipitates metabolic decompensation, but has not been mechanistically dissected for NSUN3 disease specifically. No infectious agent is causal to the underlying disease.

6. Mechanism / Pathophysiology

Ordered causal chain

  1. Biallelic loss-of-function or hypomorphic missense variants in NSUN3 lead to absent or markedly reduced NSun3 methyltransferase enzymatic activity in mitochondria (demonstrated directly: complete loss of full-length protein for null alleles; near-complete loss of catalytic methylation activity even for some missense alleles, PMID:40465263).
  2. Loss of NSun3 activity results in failure to install 5-methylcytosine at the wobble position C34 of mitochondrial tRNA-Met (mt-tRNA^Met) — demonstrated directly by mass-spectrometric/primer-extension quantification of tRNA modification in patient cells (Van Haute 2016; Jurkute 2025).
  3. Absent m⁵C34 prevents the obligate downstream oxidation step, catalyzed by ALKBH1 (ABH1), that converts m⁵C34 to 5-formylcytosine (f⁵C34) — this two-step methylation→oxidation pathway was mechanistically established in parallel by Nakano et al. (EMBO J 2016, PMID:27497299), who showed NSUN3 and ALKBH1 act sequentially on the same wobble base.
  4. Absence of f⁵C34 abolishes the expanded codon-recognition capacity that this modified base normally confers, so mt-tRNA^Met can no longer efficiently decode the non-canonical mitochondrial AUA (isoleucine-in-standard-code but methionine-in-mitochondrial-code) start/internal codon in addition to AUG.
  5. Impaired AUA/AUG decoding by mt-tRNA^Met causes stalling or reduced fidelity of mitochondrial ribosomal translation of the 13 mtDNA-encoded OXPHOS subunit mRNAs (demonstrated directly: reduced de novo mitochondrial protein synthesis in patient fibroblasts, Van Haute 2016).
  6. Defective synthesis of mtDNA-encoded subunits results in selectively reduced steady-state levels and enzymatic activity of the OXPHOS complexes that are heavily dependent on mtDNA-encoded subunits — predominantly Complex I (7 mtDNA-encoded subunits) and Complex IV (3 mtDNA-encoded subunits) — while Complex II (entirely nuclear-encoded) remains normal; this pattern is consistently reproduced across all reported muscle biopsies and fibroblast studies.
  7. Combined Complex I/IV deficiency leads to reduced mitochondrial oxygen consumption and ATP-linked respiration (directly measured by Seahorse respirometry: reduced basal OCR, reduced ATP-production-linked OCR, and reduced maximal FCCP-uncoupled respiration in patient fibroblasts, Jurkute 2025) and results in compensatory reliance on glycolysis, evidenced by impaired growth specifically under galactose-forced-oxidative-metabolism culture conditions.
  8. Chronic cellular energy deficit in the most metabolically demanding, post-mitotic, high-energy-dependent tissues — retinal ganglion cells and their axons (optic nerve), central and peripheral neurons, and skeletal/cardiac muscle — produces the clinical phenotype: progressive retinal ganglion cell degeneration/optic atrophy (the near-universal manifestation), and, when the underlying molecular lesion is more severe (complete loss-of-function alleles), an additional encephalomyopathic phenotype of developmental delay, hypotonia, seizures, and lactic acidosis. This branch point — optic-nerve-only vs. multisystem involvement — is the central genotype-phenotype correlation identified in the literature (inferred correlation, not yet mechanistically dissected at the level of why retinal ganglion cells are so uniformly vulnerable even in hypomorphic-allele carriers who otherwise lack systemic disease).
  9. In the mouse model, complete loss of Nsun3 causes embryonic lethality between E10.5–E12.5 (Murakami et al., Commun Biol 2023, PMID:36949224), establishing that some minimal threshold of mt-tRNA^Met modification/mitochondrial translation is essential for normal embryonic development — a step with no direct human correlate (no reported homozygous-null human survivors), consistent with all reported human disease alleles being hypomorphic or, when null, apparently compatible with survival only via as-yet-unexplained residual translational capacity or genetic background effects (this gap between "null alleles are viable in humans but embryonic lethal in mice" is worth flagging explicitly as a species-discordance caveat for any HUMAN_MODEL_MISMATCH discussion node).

Molecular pathways

Mitochondrial translation / mitoribosome function; tRNA wobble-position modification pathway (NSUN3 → ALKBH1/ABH1 sequential C34 methylation-then-oxidation). GO: GO:0032259 (methylation), GO:0070901 (mitochondrial tRNA methylation), GO:0032543 (mitochondrial translation), GO:0070125 (mitochondrial translational elongation).

Cellular processes

Mitochondrial protein synthesis; oxidative phosphorylation; cellular respiration; (secondarily, in severe cases) neuronal/retinal-ganglion-cell degeneration.

Protein dysfunction

Loss-of-function (null alleles: absent protein) or partial loss-of-function (hypomorphic missense within/near the SAM-methyltransferase catalytic domain) of a mitochondrial matrix-targeted, SAM-dependent RNA methyltransferase. UniProt: Q9H649 (NSUN3_HUMAN).

Metabolic changes

Impaired oxidative energy metabolism with compensatory glycolytic dependence; lactic acidosis reflects a shift toward anaerobic glycolysis and/or impaired pyruvate oxidation secondary to Complex I/IV deficiency.

Biochemical abnormalities

Reduced Complex I and Complex IV enzymatic activities in skeletal muscle (quantitatively documented, e.g., Complex I 0.078 vs. normal range 0.118–0.332; Complex IV 0.003 vs. normal 0.013–0.039 in one biopsy — units as reported, nmol/min/mg or similar respiratory-chain-assay units per the source; verify exact units against the source before curating numerically). Low tissue ubiquinone (CoQ10) also reported in one biopsy (94 pmol/mg vs. normal 140–850), of uncertain primary vs. secondary significance.

Cell types / anatomical relevance

Retinal ganglion cells (CL:0000740) and their unmyelinated/myelinated axons in the optic nerve are the most consistently and severely affected cell population; skeletal myocytes (CL:0000188), cardiomyocytes (in the mouse heart-specific knockout and in one human case with structural cardiac defects), and central/peripheral neurons in the severe phenotype.

Molecular profiling

No transcriptomic, proteomic, or metabolomic datasets specific to NSUN3/COXPD48 patients were identified in this search (beyond the targeted respirometry/western blot/immunocytochemistry functional assays described above). No single-cell, spatial transcriptomic, or CRISPR screen data specific to this gene/disease were found.

7. Anatomical Structures Affected

Organ level: - Primary: Optic nerve / eye (UBERON:0000970 eye; UBERON:0001780 optic nerve) — near-universal - Secondary/severe-phenotype: Central nervous system (brain), peripheral nerves, skeletal muscle, heart (bicuspid aortic valve, ASD in one case), inner ear (sensorineural hearing loss), bone marrow (sideroblastic anemia, though likely a separate co-occurring gene defect in that case), skin (cerulean/blue-dot cataract — lens), retina (rod/cone ERG abnormalities in severe cases, beyond pure optic-nerve involvement) - Body systems: Nervous system, visual system, musculoskeletal system, and — in the most severe cases — endocrine (growth hormone deficiency, insulin resistance, thyroid), cardiovascular, and hematopoietic systems

Tissue/cell level: - Retinal ganglion cells and optic nerve axons (CL:0000740) - Skeletal muscle fibers, particularly type I (oxidative) fibers showing subsarcolemmal mitochondrial aggregation (CL:0000188 / CL:0000189) - Peripheral sensorimotor axons (axonal neuropathy pattern) - Cardiomyocytes (CL:0000746) — established in the mouse heart-specific knockout, and clinically relevant given the one human case with structural congenital heart disease

Subcellular level: - Mitochondria, specifically the mitochondrial matrix (site of mitochondrial translation) — GO:0005759 (mitochondrial matrix), GO:0005739 (mitochondrion) - Mitochondrial cristae — structurally abnormal (fragmented/linearized, reduced in number) on electron microscopy in both the mouse cardiac model and human muscle biopsy - Mitochondrial ribosome / mitochondrial tRNA — GO:0005762 (mitochondrial large ribosomal subunit), GO:0032543 (mitochondrial translation)

Localization/laterality: Ophthalmological involvement is consistently bilateral; neurological/systemic involvement is not lateralized.

8. Temporal Development

Onset: Bimodal — either congenital/infantile (<3 months to ~1 year, associated with null/severe LOF alleles) or adolescent-to-adult (11–40 years, associated with hypomorphic missense alleles and isolated optic atrophy). No cases of purely adult-onset multisystem disease have been reported; severity of the systemic component appears fixed early by genotype rather than accumulating de novo in adulthood.

Onset pattern: Insidious/progressive for the optic atrophy trajectory in most patients; subacute in the LHON-like presentations (rapid visual decline over weeks-to-months, e.g., 6/6 to 6/24 over 13 years in one case, or 6/60 to 6/120 within 3 months in another, the latter followed by partial spontaneous/treatment-associated recovery).

Progression: Chronic and generally progressive for both the optic neuropathy and, where present, the encephalomyopathic components; punctuated by acute exacerbations (e.g., infection-triggered seizure/encephalopathy episode).

Course pattern: Predominantly progressive, though the reported spontaneous partial visual recovery in one LHON-like case (with subsequent idebenone-associated further improvement) indicates that the disease course is not uniformly monotonically progressive — an important nuance for any progression/phase modeling, since it departs from the classic "stable-then-declining" mitochondrial optic neuropathy pattern typical of LHON itself.

Disease duration: Lifelong/chronic; no spontaneous full resolution reported. Longest-followed patients are in their third-to-fourth decade of life with stable, if visually impaired, function.

Critical periods: Not formally established, but the striking difference between embryonic-lethal complete Nsun3 loss in mice and viable (if severely affected) human null-allele carriers suggests there may be a developmental window of particular vulnerability to complete pathway loss that current human genotypes have not fully probed (all reported human "null" genotypes retain at least some low-level residual protein/activity, or the true molecular null is not compatible with live birth and is therefore unascertained).

9. Inheritance and Population

Epidemiology: No prevalence or incidence estimates exist; COXPD48 is not yet listed as a standalone entry in Orphanet, and the total published literature comprises approximately 10–11 individuals across 3 primary reports plus a 2016 mechanistic paper's index case (Van Haute 2016 = 1 patient; Paramasivam 2020 = 1 patient; the isolated-optic-atrophy report (PMID:38790159) = 1 patient; Jurkute 2025 = 8 patients/5 families, one of which is the previously-reported Van Haute index case). This makes it one of the rarest of the numbered COXPD entries; true population prevalence is unknown and likely substantially underascertained given the milder end of the phenotypic spectrum (isolated optic atrophy) plausibly overlapping with undiagnosed "simplex" optic atrophy cases in the community.

Inheritance pattern: Autosomal recessive (biallelic pathogenic variants required); one case demonstrates that apparent homozygosity can arise via uniparental disomy rather than biparental transmission — a mechanism curators should not assume away when a proband is "homozygous" with only one carrier parent identified.

Penetrance: Appears high/complete for the optic neuropathy component among carriers of biallelic pathogenic variants reported to date, though ascertainment bias (case reports select for affected probands) makes true penetrance impossible to estimate from available data.

Expressivity: Markedly variable — the central finding of the 2025 multi-family study is a continuous phenotypic spectrum from isolated optic atrophy to severe multisystem disease, correlated with (but not perfectly predicted by) variant type/severity.

Genetic anticipation: Not applicable/not reported (no repeat-expansion mechanism).

Germline mosaicism: Not reported.

Founder effects: Not established; reported families are of Pakistani, Colombian, Afghan, Japanese, and unspecified East Asian ancestry, without a shared founder haplotype identified.

Consanguinity: A recurrent feature across multiple independent families (Van Haute index family, Paramasivam case, Jurkute Families 1 and 3), consistent with the disease's rarity and AR inheritance.

Carrier frequency: Unknown/not established in any population database search performed here; this should be checked directly in gnomAD at curation time.

Population demographics: Reported cases span Northern European (original index patient), East Asian, South Asian (Pakistani), Latin American (Colombian), Central Asian (Afghan), and Japanese ancestries — i.e., no clear geographic/ethnic restriction, consistent with a pan-ethnic ultra-rare AR disorder. Sex ratio: Striking male predominance in the reported cohort (7 of 8 patients in the 2025 series, plus both single-patient prior reports were male infants) — noted by the authors but unexplained mechanistically, since the gene is autosomal; this is very likely an ascertainment artifact of a tiny sample size rather than a true biological sex effect, and should be flagged as such rather than asserted as a sex-linked biological finding.

10. Diagnostics

Laboratory tests: - Plasma lactate (elevated in early-onset/severe cases) — LOINC relevant to lactate assays - Blood gas / metabolic panel during acute decompensation (documented hypoglycemia, hypernatremia, severe metabolic acidosis in one case) - Plasma homocysteine — elevated in two adult patients with isolated optic atrophy (Jurkute Family 3), of uncertain mechanistic significance but worth noting as an associated lab finding - Serum SAM (S-adenosylmethionine) — reported normal in one mild-phenotype family, presumably tested given NSUN3's SAM-dependent catalytic mechanism

Biomarkers: No validated circulating biomarker specific to NSUN3 disease; muscle-tissue Complex I/IV enzymatic activity and CoQ10 level function as tissue-level (not circulating) biomarkers in the reported cases.

Imaging: - Optic nerve/orbital MRI: typically shows slender/atrophic optic nerves; occasionally T2 hyperintensity - Brain MRI: usually unremarkable in the isolated-optic-atrophy phenotype; in the most severe case, T2-hyperintense edematous lesions in parietal/occipital lobes during an acute seizure episode - OCT (optical coherence tomography): reduced peripapillary RNFL and ganglion cell layer thickness — the most sensitive structural biomarker of the optic neuropathy component

Functional/electrophysiological tests: - Pattern and full-field visual evoked potentials (VEP) — delayed/subnormal - Pattern ERG (PERG) — reduced/abnormal P50, consistent with retinal ganglion cell dysfunction - Full-field ERG — abnormal (electronegative waveform, delayed b-waves) in the severe multisystem phenotype, indicating additional outer/inner retinal (not purely optic-nerve) involvement - EEG — performed in seizure cases; can be unremarkable interictally

Biopsy/histopathology: - Skeletal muscle biopsy with histochemistry (COX/SDH staining), Western blot for OXPHOS subunits (reduced COX II/Complex IV and NDUFB8/Complex I), and electron microscopy (abnormal mitochondrial size/cristae) — the diagnostic gold standard historically used before genetic confirmation, still valuable for functional characterization - Skin biopsy → dermal fibroblast culture for confirmatory functional studies (immunocytochemistry, Seahorse respirometry, tRNA methylation assay)

Genetic testing: - Recommended approach: given the phenotypic overlap with other mitochondrial optic neuropathies (LHON, dominant optic atrophy/OPA1) and other combined OXPHOS deficiencies, whole exome sequencing (WES) or a targeted mitochondrial-disease/optic-atrophy gene panel including NSUN3 is the most efficient diagnostic strategy, particularly for the milder isolated-optic-atrophy presentation where clinical suspicion of "combined OXPHOS deficiency" may not be raised - mtDNA testing is relevant primarily to exclude primary mtDNA disorders (e.g., LHON mtDNA point mutations) as the alternative explanation for a LHON-like presentation, since NSUN3 disease can mimic classic LHON clinically - No specific NSUN3 gene panel or GTR-listed clinical test was identified as a named commercial product in this search; it would typically be captured within broader "mitochondrial disease" or "inherited optic neuropathy" NGS panels

Differential diagnosis: Leber hereditary optic neuropathy (LHON, mtDNA-encoded), autosomal dominant optic atrophy (OPA1), other combined OXPHOS deficiency genes (particularly other mt-tRNA-modification enzymes such as TRMT5, MTO1, TRIT1, or aminoacyl-tRNA synthetases), and other causes of infantile lactic acidosis with developmental delay.

Screening: No newborn screening, carrier screening, or population screening program exists for this ultra-rare gene; not part of any standard expanded carrier screening panel identified in this search.

11. Outcome/Prognosis

No survival curves, formal mortality statistics, or standardized quality-of-life measures have been published — the entire evidence base is descriptive case-report follow-up. Qualitatively: - Patients with the milder, isolated optic atrophy phenotype (adolescent/adult onset, missense hypomorphic alleles) are reported alive and functioning into their third-to-fourth decade, with visual impairment as the dominant but not life-limiting morbidity. - Patients with the severe, early-onset multisystem phenotype show substantial morbidity (refractory seizures, developmental delay, multisystem involvement); the 2025 report notes one individual with "possible limited life expectancy" in its abstract summary, though no explicit mortality outcome/age-at-death is detailed in the extracted text for any reported patient — this should be treated as a qualitative severity signal, not a quantified prognosis statistic, when curating. - Complications in severe cases include drug-refractory epilepsy, growth failure, endocrinopathy, structural cardiac disease, and infection-triggered acute metabolic/neurological decompensation. - Prognostic factors: genotype (LOF vs. hypomorphic missense) is the strongest prognostic correlate identified; there is no established prognostic biomarker beyond variant classification itself.

12. Treatment

No disease-specific, FDA-approved, or gene-targeted therapy exists for COXPD48/NSUN3 deficiency. Management is supportive, following general mitochondrial-disease principles, with one notable specific finding:

  • Idebenone (a synthetic short-chain quinone/CoQ10 analog, CHEBI:5991; NCIT treatment terms would map to Pharmacotherapy NCIT:C15986 with therapeutic_agent idebenone) was used in one patient (Jurkute Family 3, III-2) with an acute LHON-like presentation and was temporally associated with visual acuity restoration from 6/120 to 6/6 bilaterally over 4 months, alongside spontaneous-recovery confounding (the paper reports "spontaneous improvement beginning at 12 months" and idebenone was given during this window) — this should be curated as a single anecdotal treatment-response observation, not established efficacy evidence, given the absence of a controlled trial and the confound of concurrent spontaneous recovery. Idebenone is notably the only medication with regulatory approval (EU) specifically for a mitochondrial optic neuropathy (LHON), lending biological plausibility to its use here by extension.
  • Anticonvulsant therapy: levetiracetam was used for seizure control in one severe case but described as only partially effective ("refractory," seizures recurring approximately every 6 months) — NCIT:C15986 Pharmacotherapy / therapeutic_agent levetiracetam (CHEBI:6437).
  • Supportive care: management of failure to thrive/nutritional support (NCIT:C15447 Dietary Intervention), physical/occupational/speech therapy for developmental delay and hypotonia (NCIT:C15302 Physical Therapy), low-vision rehabilitation and optical aids for the optic atrophy component, endocrine replacement (growth hormone) where deficiency is documented, and standard cardiology follow-up for structural cardiac anomalies.
  • General mitochondrial-disease supportive measures (CoQ10/ubiquinone supplementation, B-vitamin cocktails, avoidance of mitochondrial-toxic drugs, aggressive treatment of intercurrent infection to avoid metabolic decompensation) are standard-of-care extrapolations from broader mitochondrial disease management guidelines rather than NSUN3-specific evidence.
  • No gene therapy, RNA-based therapy, or clinical trial specific to NSUN3/COXPD48 was identified in ClinicalTrials.gov searches performed in this session; none should be assumed to exist without direct verification at curation time.

13. Prevention

No primary prevention exists beyond genetic counseling and reproductive options for families with a known biallelic NSUN3 genotype: carrier testing of at-risk relatives, prenatal diagnosis, and preimplantation genetic diagnosis are the standard options for any confirmed AR mitochondrial-translation disorder, though no report of their specific use for NSUN3 disease was found in this search. Given the phenotypic overlap with LHON-like presentations, avoidance of putative mitochondrial-toxic exposures (tobacco smoke, excess alcohol — the classic environmental modifiers invoked in LHON management, though not specifically studied for NSUN3 disease) would be a reasonable extrapolated counseling point, but this is an inference from the broader mitochondrial-optic-neuropathy literature, not direct NSUN3 evidence, and should be labeled as such. No newborn screening or population carrier-screening program exists for this gene.

14. Other Species / Natural Disease

No naturally occurring NSUN3-associated disease has been reported in any non-human species (companion animals, livestock, or wildlife); no OMIA entry was identified. NSUN3 orthologs are broadly conserved across vertebrates (essential mitochondrial housekeeping function), consistent with mouse ortholog Nsun3 (MGI:2146565) being required for embryonic viability (see below), but no spontaneous veterinary disease phenotype has been documented.

15. Model Organisms

Mouse (Mus musculus, NCBITaxon:10090): The only reported animal model. - Whole-body constitutive knockout: Homozygous Nsun3-null mice are embryonic lethal, with embryos becoming progressively smaller and dying between E10.5 and E12.5 (Murakami et al., Commun Biol 2023, PMID:36949224). This establishes an essential, non-redundant developmental requirement for NSUN3-dependent mt-tRNA modification that human patients — all carrying at least partially hypomorphic genotypes — do not display, an important species-discordance caveat: the complete-null mouse phenotype (embryonic lethality) has no directly comparable human counterpart, since no reported human patient carries a fully null biallelic genotype that survived to term; this gap should be modeled as a HUMAN_MODEL_MISMATCH-type consideration if curated into a modeled_mechanisms link, given that fidelity to the "severe end" of human phenotype is unclear (the mouse model recapitulates lethality of complete loss but cannot inform the milder end of the human spectrum, which arises from partial loss-of-function alleles the constitutive-null model does not model). - Conditional, heart-specific knockout (Nsun3^HKO, using a cardiac-specific Cre driver): Viable to adulthood, permitting study of postnatal/adult consequences: - Enlarged, structurally abnormal cardiac mitochondria with fragmented cristae (1.5× larger at 14 weeks, 1.7× larger at 50 weeks vs. controls) - Enhanced heart contraction and mild, age-associated cardiac hypertrophy (increased left ventricular wall thickness during systole in older animals) - Mitochondrial mRNAs encoding respiratory subunits were not transcriptionally downregulated, but enzymatic activity of the respiratory complexes was reduced, especially Complex IV, with additional Complex I decline in older animals — directly paralleling the human muscle-biopsy finding of post-transcriptional (translational) rather than transcriptional OXPHOS impairment, and supporting high fidelity of this tissue-specific model to the underlying translational mechanism, though not to the optic-nerve phenotype (no eye/optic-nerve-specific conditional model has been reported). - Applications: The heart-specific model is well suited to studying the cellular/mitochondrial-structural and bioenergetic consequences of NSUN3 loss in a postnatally viable tissue; no optic-nerve/retinal-ganglion-cell-specific conditional model has yet been generated, which is a notable gap given that the optic nerve is the most consistently affected human tissue. - Cellular models: Patient-derived dermal fibroblasts (multiple studies) are the principal human cellular model, used for immunocytochemistry (COX I staining), Western blot (COX II, NDUFB8), Seahorse mitochondrial stress test respirometry, and mt-tRNA methylation quantification — these functional/omics assays in patient fibroblasts, rather than any animal disease model, currently provide the most direct mechanistic validation of pathogenicity for novel NSUN3 variants. - Resources: MGI:2146565 (Nsun3 mouse gene record); no IMPC/KOMP standardized knockout-mouse phenotyping data specific to constitutive Nsun3 knockout beyond the Murakami et al. study was identified in this search (the constitutive knockout is embryonic lethal, so it would not appear in adult IMPC phenotyping pipelines in any case).

Summary of Suggested Ontology Term Bindings

Concept Suggested term
Disease MONDO:0033566 (COXPD48); OMIM:619012
Gene hgnc:26208 (NSUN3)
Optic atrophy HP:0000648
Nystagmus HP:0000639
Cataract HP:0010696
Global developmental delay HP:0001263
Microcephaly HP:0000252
Muscular hypotonia HP:0001252
External ophthalmoplegia HP:0000544
Seizures HP:0001250
Elevated lactate HP:0002151
Failure to thrive HP:0001508
Sensorineural hearing loss HP:0000407
Peripheral axonal neuropathy HP:0007002
Mitochondrial translation GO:0032543
Mitochondrial tRNA methylation GO:0070901
Mitochondrion / matrix GO:0005739 / GO:0005759
Retinal ganglion cell CL:0000740
Skeletal muscle fiber CL:0000188
Cardiomyocyte CL:0000746
Optic nerve UBERON:0001780
Eye UBERON:0000970
Idebenone CHEBI:5991
Levetiracetam CHEBI:6437
Pharmacotherapy (treatment action) NCIT:C15986

Key Gaps for Curation

  1. No formal prevalence/incidence data — likely severely underascertained given the milder phenotype's overlap with idiopathic optic atrophy.
  2. No controlled treatment-efficacy evidence — the idebenone observation is a single confounded case.
  3. No mortality/survival statistics, only qualitative severity descriptions.
  4. Species discordance between the embryonic-lethal complete mouse knockout and the (partial-loss-of-function-only) surviving human genotypes — flag explicitly if modeling this as a modeled_mechanisms link.
  5. Variable variant classification — several reported NSUN3 alleles remain VUS; do not treat every published variant as unambiguously pathogenic without independent ClinVar/ACMG verification at curation time.
  6. gnomAD constraint metrics (pLI/LOEUF) for NSUN3 were not retrievable in this session and should be pulled directly from gnomAD before citing numerically.

Sources: - OMIM #619012 — COMBINED OXIDATIVE PHOSPHORYLATION DEFICIENCY 48; COXPD48 - OMIM *617491 — NSUN3 - Van Haute L, et al. Deficient methylation and formylation of mt-tRNA^Met wobble cytosine in a patient carrying mutations in NSUN3. Nat Commun. 2016. PMID:27356879 - Nakano S, et al. NSUN3 and ABH1 modify the wobble position of mt-tRNA^Met to expand codon recognition in mitochondrial translation. EMBO J. 2016. PMID:27497299 - Paramasivam A, et al. Novel Biallelic NSUN3 Variants Cause Early-Onset Mitochondrial Encephalomyopathy and Seizures. J Mol Neurosci. 2020. PMID:32488845 - Mutations in NSUN3, a Mitochondrial Methyl Transferase Gene, Cause Inherited Optic Neuropathy. PMID:38790159 - Jurkute N, et al. Biallelic NSUN3 Variants Cause Diverse Phenotypic Spectrum Disease: From Isolated Optic Atrophy to Severe Early-Onset Mitochondrial Disorder. Invest Ophthalmol Vis Sci. 2025;66(6):17. PMID:40465263 - Murakami Y, et al. NSUN3-mediated mitochondrial tRNA 5-formylcytidine modification is essential for embryonic development and respiratory complexes in mice. Commun Biol. 2023. PMID:36949224 - MGI:2146565 — Nsun3 mouse gene detail - NSUN3 — GeneCards

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 11
Resolved 11
Unresolved (possible confabulation) 0
Unverifiable 0
References weighed for topical relevance 11
On topic 11
Off topic 0

All extracted references resolved successfully.

Term Validation

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

Outcome Count
Terms checked 44
Resolved 40
Unresolved (possible confabulation) 0
Obsolete 1
Unverifiable 3
Terms whose name was checked 27
Terms named correctly 15
Terms named as a different term 2
Terms whose name is worth a second look 10

Terms the report names something else

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

  • HP:0003128 (1 mention) - the report calls it "present across essentially all early-onset cases; a hallmark biochemical abnormality"; HP calls it Lactic acidosis
  • CHEBI:5991 (2 mentions) - the report calls it "Idebenone"; CHEBI calls it CHEBI_5991

Obsolete terms

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

  • CHEBI:5991 (CHEBI_5991) (2 mentions) - replaced by CHEBI:28638

Terms whose name is worth a second look

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

  • HP:0010696 (2 mentions) - the report calls it "Cataract"; HP calls it Polar cataract
  • HP:0000551 (1 mention) - the report calls it "Impaired color vision"; HP calls it Color vision defect, and lists "Disturbed color vision" among its other names
  • HP:0001252 (2 mentions) - the report calls it "Muscular hypotonia"; HP calls it Hypotonia, and lists "Muscular hypotonia" among its other names
  • HP:0007002 (2 mentions) - the report calls it "Peripheral axonal neuropathy"; HP calls it Motor axonal neuropathy
  • HP:0002151 (2 mentions) - the report calls it "Elevated lactate"; HP calls it Increased circulating lactate concentration, and lists "Increased serum lactate" among its other names
  • CL:0000740 (3 mentions) - the report calls it "Retinal ganglion cells and optic nerve axons", "Retinal ganglion cell"; CL calls it retinal ganglion cell
  • CL:0000188 (3 mentions) - the report calls it "Skeletal muscle fiber"; CL calls it cell of skeletal muscle, and lists "skeletal muscle cell" among its other names
  • UBERON:0001780 (2 mentions) - the report calls it "Optic nerve"; UBERON calls it spinal nerve, and lists "spine nerve" among its other names
  • CL:0000746 (2 mentions) - the report calls it "Cardiomyocytes", "Cardiomyocyte"; CL calls it cardiac muscle cell, and lists "cardiomyocyte" among its other names
  • NCIT:C15986 (3 mentions) - the report calls it "Pharmacotherapy (treatment action)"; NCIT calls it Pharmacotherapy, and lists "Pharmacological Treatment" among its other names

Terms named inconsistently

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

  • GO:0032543 - called "mitochondrial translation", "Mitochondrial translation"
  • CL:0000740 - called "Retinal ganglion cells and optic nerve axons", "Retinal ganglion cell"
  • CL:0000746 - called "Cardiomyocytes", "Cardiomyocyte"
  • MGI:2146565 - called "Nsun3", "Nsun3* mouse gene record", "Nsun3 mouse gene detail"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: MGI, OMIM.