COA3-related COX deficiency (mitochondrial complex IV deficiency nuclear type 14, MC4DN14) is a rare autosomal recessive nuclear form of isolated cytochrome c oxidase (COX, Complex IV) deficiency caused by biallelic variants in COA3 (also known as CCDC56). COA3 encodes a small inner-mitochondrial-membrane Complex IV assembly factor that, together with COX14, couples synthesis of the mtDNA-encoded core catalytic subunit COX1 with its incorporation into the nascent holoenzyme. Loss of COA3 destabilizes COX1, stalls an early COX assembly intermediate, and produces an isolated Complex IV deficiency. Distinctively, the founding patient followed an unusually mild, adult-compatible course — sensorimotor peripheral neuropathy, exercise intolerance, obesity, and short stature — in marked contrast to the early-fatal encephalopathies seen with most severe COX assembly defects, underscoring the tissue-specific expression of mitochondrial disease. It conforms to the conserved Complex IV assembly-deficiency mechanism, with the lesion localized to a destabilized COX1-coupling assembly factor that is functionally interdependent with COX14.
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name: COA3-Related COX Deficiency
category: Mendelian
creation_date: "2026-06-21T00:00:00Z"
synonyms:
- COA3 deficiency
- Mitochondrial complex IV deficiency, nuclear type 14
- MC4DN14
- CCDC56 deficiency
- COA3-related cytochrome c oxidase deficiency
description: >
COA3-related COX deficiency (mitochondrial complex IV deficiency nuclear
type 14, MC4DN14) is a rare autosomal recessive nuclear form of isolated
cytochrome c oxidase (COX, Complex IV) deficiency caused by biallelic variants
in COA3 (also known as CCDC56). COA3 encodes a small inner-mitochondrial-membrane
Complex IV assembly factor that, together with COX14, couples synthesis of the
mtDNA-encoded core catalytic subunit COX1 with its incorporation into the
nascent holoenzyme. Loss of COA3 destabilizes COX1, stalls an early COX
assembly intermediate, and produces an isolated Complex IV deficiency.
Distinctively, the founding patient followed an unusually mild, adult-compatible
course — sensorimotor peripheral neuropathy, exercise intolerance, obesity, and
short stature — in marked contrast to the early-fatal encephalopathies seen with
most severe COX assembly defects, underscoring the tissue-specific expression of
mitochondrial disease. It conforms to the conserved Complex IV assembly-deficiency
mechanism, with the lesion localized to a destabilized COX1-coupling assembly
factor that is functionally interdependent with COX14.
disease_term:
preferred_term: COA3-related COX deficiency (MC4DN14)
term:
id: MONDO:0033649
label: mitochondrial complex IV deficiency, nuclear type 14
parents:
- Mitochondrial Disease
- Inborn Error of Metabolism
references:
- reference: PMID:25604084
title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
pathophysiology:
- name: COA3 Assembly-Factor Loss and Failed Complex IV Assembly
conforms_to: "complex_iv_assembly_deficiency#Complex IV Biogenesis Failure"
description: >
Biallelic COA3 variants reduce the steady-state level of COA3, a small inner
membrane Complex IV assembly factor. COA3 normally stabilizes the
mtDNA-encoded core catalytic subunit COX1 and couples its synthesis with
holoenzyme assembly. Loss of COA3 leaves COX1 destabilized and stalls an
early COX assembly intermediate, so the mature enzyme fails to form and an
isolated Complex IV biogenesis failure results. COA3 and COX14 are mutually
interdependent — each is undetectable when the other is lost — situating
this lesion within the COX1-coupling arm of the assembly pathway.
biological_processes:
- preferred_term: mitochondrial respiratory chain complex IV assembly
term:
id: GO:0033617
label: mitochondrial respiratory chain complex IV assembly
modifier: DECREASED
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Whole exome sequencing identified compound heterozygous mutations (c.199dupC, c.215A>G) in COA3, a small inner membrane COX assembly factor"
explanation: Identifies biallelic COA3 variants in a small inner-membrane COX assembly factor as the causal lesion.
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "analysis showed an almost complete lack of COX assembly in subject fibroblasts, consistent with the very low enzymatic activity"
explanation: BN-PAGE in patient fibroblasts shows almost complete loss of assembled Complex IV, the biogenesis-failure phenotype.
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "showed a specific decrease in synthesis of the COX1 subunit, the core catalytic subunit that nucleates assembly of the holoenzyme"
explanation: A specific decrease in COX1 synthesis identifies the assembly step that COA3 couples and that is lost in the disorder.
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "COX14 was undetectable in COA3 subject fibroblasts, and that COA3 was undetectable in fibroblasts from a COX14 subject, demonstrating the interdependence of these two COX assembly factors"
explanation: Demonstrates the functional interdependence of COA3 and COX14 in the COX1-coupling assembly arm.
downstream:
- target: Impaired Terminal Electron Transfer and ATP Synthesis
causal_link_type: DIRECT
description: Failure to assemble Complex IV yields a catalytically deficient holoenzyme.
- name: Impaired Terminal Electron Transfer and ATP Synthesis
conforms_to: "complex_iv_assembly_deficiency#Impaired Terminal Electron Transfer and ATP Synthesis"
description: >
The near-absence of assembled Complex IV blocks electron transfer from
cytochrome c to molecular oxygen and the coupled proton pumping, collapsing
oxidative ATP synthesis. In COA3-deficient patient fibroblasts the COX
enzymatic activity is very low, consistent with the assembly defect.
biological_processes:
- preferred_term: mitochondrial electron transport, cytochrome c to oxygen
term:
id: GO:0006123
label: mitochondrial electron transport, cytochrome c to oxygen
modifier: DECREASED
- preferred_term: ATP synthesis coupled electron transport
term:
id: GO:0042775
label: mitochondrial ATP synthesis coupled electron transport
modifier: DECREASED
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "analysis showed an almost complete lack of COX assembly in subject fibroblasts, consistent with the very low enzymatic activity"
explanation: Very low COX enzymatic activity establishes the functional terminal-electron-transfer deficit downstream of failed assembly.
downstream:
- target: High-Energy Tissue Dysfunction
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: The oxidative-phosphorylation deficit injures high-energy-demand tissues, here predominantly peripheral nerve and skeletal muscle.
- name: High-Energy Tissue Dysfunction
conforms_to: "complex_iv_assembly_deficiency#High-Energy Tissue Dysfunction"
description: >
The bioenergetic deficit manifests in high-demand tissues. In COA3-related
disease the dominant phenotype is sensorimotor peripheral neuropathy and
exercise intolerance, accompanied by obesity and short stature. The clinical
course is unusually mild compared with the early-fatal encephalopathies of
most severe COX assembly defects, illustrating the marked tissue-specific
involvement characteristic of mitochondrial disease.
biological_processes:
- preferred_term: aerobic respiration
term:
id: GO:0009060
label: aerobic respiration
modifier: DECREASED
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "presenting with an unusual phenotype characterised by neuropathy, exercise intolerance, obesity, and short stature"
explanation: The high-energy-tissue manifestations of the bioenergetic deficit in the founding patient.
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The mild clinical course in this patient contrasts with nearly all other cases of severe COX assembly defects that are usually fatal early in life"
explanation: Documents the unusually mild, tissue-specific course distinguishing this disorder from other nuclear COX deficiencies.
downstream:
- target: Peripheral neuropathy
causal_link_type: DIRECT
description: The bioenergetic deficit in peripheral nerve manifests as sensorimotor peripheral neuropathy.
- target: Exercise intolerance
causal_link_type: DIRECT
description: Impaired oxidative ATP synthesis in skeletal muscle manifests as exercise intolerance.
phenotypes:
- name: Peripheral neuropathy
description: >
Significant sensorimotor peripheral neuropathy, a dominant feature of
COA3-related Complex IV deficiency in the founding patient.
phenotype_term:
preferred_term: Peripheral neuropathy
term:
id: HP:0009830
label: Peripheral neuropathy
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "presenting with an unusual phenotype characterised by neuropathy, exercise intolerance, obesity, and short stature"
explanation: Neuropathy was a presenting feature of the founding patient.
- name: Exercise intolerance
description: >
Exercise intolerance reflecting the skeletal-muscle bioenergetic deficit.
phenotype_term:
preferred_term: Exercise intolerance
term:
id: HP:0003546
label: Exercise intolerance
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "presenting with an unusual phenotype characterised by neuropathy, exercise intolerance, obesity, and short stature"
explanation: Exercise intolerance was a presenting feature of the founding patient.
- name: Obesity
description: >
Obesity, part of the unusual phenotype of the founding patient.
phenotype_term:
preferred_term: Obesity
term:
id: HP:0001513
label: Obesity
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "presenting with an unusual phenotype characterised by neuropathy, exercise intolerance, obesity, and short stature"
explanation: Obesity was a presenting feature of the founding patient.
- name: Short stature
description: >
Short stature, part of the unusual phenotype of the founding patient.
phenotype_term:
preferred_term: Short stature
term:
id: HP:0004322
label: Short stature
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "presenting with an unusual phenotype characterised by neuropathy, exercise intolerance, obesity, and short stature"
explanation: Short stature was a presenting feature of the founding patient.
genetic:
- name: COA3 pathogenic variants causing MC4DN14
gene_term:
preferred_term: COA3
term:
id: hgnc:24990
label: COA3
inheritance:
- name: Autosomal recessive
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Whole exome sequencing identified compound heterozygous mutations (c.199dupC, c.215A>G) in COA3, a small inner membrane COX assembly factor"
explanation: Compound heterozygous COA3 variants establish autosomal recessive inheritance.
features: >
Biallelic COA3 variants cause MC4DN14. The founding patient carried compound
heterozygous variants (c.199dupC, c.215A>G), reducing COA3 protein levels.
Retroviral re-expression of wild-type COA3 cDNA in patient fibroblasts rescued
the COX assembly and mitochondrial translation defects, confirming pathogenicity.
evidence:
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Whole exome sequencing identified compound heterozygous mutations (c.199dupC, c.215A>G) in COA3, a small inner membrane COX assembly factor"
explanation: Identifies the compound heterozygous COA3 variants as the causal lesion.
- reference: PMID:25604084
reference_title: "Mutations in COA3 cause isolated complex IV deficiency associated with neuropathy, exercise intolerance, obesity, and short stature."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Retroviral expression of a wild-type COA3 cDNA completely rescued the COX assembly and mitochondrial translation defects, confirming the pathogenicity of the mutations"
explanation: Complementation rescue in patient fibroblasts confirms causality of the COA3 variants.
treatments:
- name: Supportive and Metabolic Care
description: >
No curative therapy exists; management is supportive, addressing the
peripheral neuropathy, exercise intolerance, and associated features with
multidisciplinary supportive care.
treatment_term:
preferred_term: supportive care
term:
id: MAXO:0000950
label: supportive care